![a) The basic configurations and developments of aqueous supercapacitors: type I: EDLC electrodes‐based symmetric device; type II, pseudocapacitive electrodes‐based symmetric device; type III, EDLC//pseudocapacitive electrodes‐based asymmetric device; type IV, all pseudocapacitive electrodes‐based asymmetric device. b) The electrochemical voltage and energy density profiles of aqueous symmetric supercapacitor and wide voltage aqueous asymmetric supercapacitor. c) Schematic representation of the electrochemical stability range of water and potential windows versus the standard hydrogen electrode (SHE) for different pseudocapacitor materials in an aqueous electrolyte from previous reports (carbon,[⁶⁷] MnO2,[⁶⁸] RuO2,[⁶⁹] MoO3,[⁷⁰] TiO2,[⁷¹] WO3‐x,[⁷²] V2O5,[⁷³] PANI,[⁷⁴] PPy,[⁷⁵] Na0.5MnO2,[⁷⁶] Na0.5Mn0.75O,[⁷⁷] Nb2O5,[⁷⁸] LiMn2O4,[⁷⁹] 2NbOPO4·H3PO4·H2O,[⁸⁰] Ti2Nb10O2‐x,[⁸¹] VOPO4,[⁸²] SnO2,[⁸³] Bi2O3,[⁸⁴] VN,[⁸⁵] Fe3O4,[⁸⁶] In2O3,[⁸⁷] MoS2,[⁸⁸] MoPO[⁸⁹]). Note that the potential windows could change with different electrolytes (pH or ion types) and active material structures (crystal phases or particle sizes). Note the following potentials were used for normalizing the potential differences of common reference electrodes with respect to SHE: mercury/mercury oxide (Hg/HgO, E = +0.098 V), silver/silver chloride electrode (Ag/AgCl, saturated, E = +0.197 V), and saturated calomel electrode (SCE, E = +0.241 V).](https://www.researchgate.net/profile/Jun-Huang-47/publication/355182523/figure/fig2/AS:11431281173211970@1688784176872/a-The-basic-configurations-and-developments-of-aqueous-supercapacitors-type-I-EDLC_Q320.jpg)
![a) RuO2·xH2O nanotubular arrayed electrode. b) SEM and c) TEM images of the RuO2·xH2O NTs arrayed electrode. d) CV curves at different scan rates and e) specific capacitances curves at different current densities of RuO2·xH2O NTs arrayed electrode. f) Frequency dependence of specific capacitance of RuO2·xH2O NTs arrayed electrode. Reproduced with permission.[⁶⁹] Copyright 2006, American Chemical Society. g) Schematic representation illustrating the fabrication process of RuO2//Ti3C2Tx asymmetric supercapacitor. h) CV curves of RuO2/CF cathode, Ti3C2Tx/CF anode, and the asymmetric device at a scan rate of 50 mV s–1. i) CV curves of RuO2//Ti3C2Tx asymmetric device at different scan rates. Reproduced with permission.[¹⁷²] Copyright 2018, Wiley‐VCH.](https://www.researchgate.net/profile/Jun-Huang-47/publication/355182523/figure/fig5/AS:11431281173202898@1688784177308/a-RuO2xH2O-nanotubular-arrayed-electrode-b-SEM-and-c-TEM-images-of-the-RuO2xH2O-NTs_Q320.jpg)
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2108107 (1 of 68)
Review
Wide Voltage Aqueous Asymmetric Supercapacitors:
Advances, Strategies, and Challenges
Jun Huang, Kai Yuan,* and Yiwang Chen*
Asymmetric supercapacitors (ASCs) can substantially broaden their working
voltage range, benefiting from the advantages of both cathode and anode
while breaking through the energy storage limitations of corresponding
symmetric cells. Wide voltage aqueous ASCs hold great promise for future
electronic systems that require satisfied energy density, power density, and
cycle life, due to the advantages of aqueous electrolyte in terms of low cost,
operational safety, facile manufacture, environment-friendly, and high ionic
conductivity. This review will first briefly present an overview of the historical
developments, charge storage mechanisms, and matching principles of wide
voltage aqueous ASCs. Then, the cathode and anode materials with wide
potential windows for building wide voltage aqueous ASCs over the last few
decades are summarized. The next section details the optimization methods
of aqueous electrolyte related to wide voltage aqueous ASCs. In addition, the
basic device configurations of wide voltage aqueous ASCs are classified and
discussed. Furthermore, several strategies are proposed for achieving high-
performance wide voltage aqueous ASCs in terms of voltage window, specific
capacitance, rate performance, and electrochemical stability. Finally, to moti-
vate further research and development, several key scientific challenges and
the perspectives are discussed.
DOI: 10.1002/adfm.202108107
J. Huang, Y. Chen
Institute of Advanced Scientific Research (iASR)/Key Laboratory
of Functional Organic Small Molecules for Ministry of Education
Jiangxi Normal University
Ziyang Avenue, Nanchang , China
J. Huang, K. Yuan, Y. Chen
College of Chemistry/Institute of Polymers and Energy Chemistry (IPEC)
Nanchang University
Xuefu Avenue, Nanchang , China
E-mail: kai.yuan@ncu.edu.cn; ywchen@ncu.edu.cn
through the storage and shuttling of ions
between two electrodes along with the
flow of electrons in an external circuit.[,]
Therefore, the electrode must eectively
transfer the suciency of ions into the
electrode and a large number of electrons
to the external circuit.[–] Theoretically, an
ideal EES device should meet the merits
of storing a great quantity of energy (i.e.,
high specific energy) and could be charged
and discharged in a very short time (i.e.,
high specific power).[–]
Among various EES devices, as shown
in Figure 1a, fuel cells possess ultrahigh
specific energy but suer from slow
kinetics (i.e., low power density) and
need expensive metal catalysts.[–] As
for capacitors, the ultralow energy density
greatly limits their applications in mobile
power supplies.[] Thus, batteries and
supercapacitors have become the main
EES technologies in today’s market. Bat-
teries exhibit high energy density but
relatively low power density, while super-
capacitors oer ultrahigh power density
but inferior energy density.[–] The huge dierence between
the two types of devices is derived from fundamentally dierent
energy storage mechanisms.[,] Battery materials can store a
large number of energy (≈Wh kg–) by diusion-controlled
redox reactions, resulting in slow charging process on the order
of hours with a low power density (≤ kW kg–).[,] On the
contrary, capacitive materials store much lower energy density
(≈ Wh kg–) with fast charging process on the order of sec-
onds and a superior high power density (≈kW kg–) through
the electrical double layers mechanism.[,] Fundamentally,
despite the remarkable dierences, both batteries and superca-
pacitors involve ion and electron transport and storage in active
electrode materials.[–]
Supercapacitors (SCs), also called electrochemical capaci-
tors, have received great attention in both academia and
industry due to their ultrahigh power density, fast charge/dis-
charge rate, and ultralong cycle life when compared with fuel
cells and batteries.[,] In SCs, the ions are rapidly delivered to
the surface of electrode through a liquid electrolyte, while the
electrons are rapidly transported through the highly conduc-
tive carbon electrode to an external circuit.[,] Theoretically,
there are two energy storage mechanisms in SCs systems,
those are electric double-layer capacitors (EDLCs, adsorption,
and desorption of electrolyte ions on the surface of electrodes)
and pseudocapacitors (PCs, rapid and reversible surface redox
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/./adfm..
1. Introduction
With the great progress of science and technology, energy
usage is undergoing a fast and large shift toward electricity as
the main power source.[,] The growing demand for portable
power supplies in mobile electronics, electric vehicles, and the
Internet of Things has promoted great research eorts in devel-
oping ecient electrochemical energy storage (EES) devices
with reversible storage and release of electricity.[–] All the evi-
dence shows that the growth of EES technologies will be almost
unabated in the near future. Fundamentally, all EES devices go
Adv. Funct. Mater. 2021,
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2108107 (2 of 68) © Wiley-VCH GmbH
reactions between active materials and electrolyte ions).[] In
the both cases, the energy storage is happened at the interface
of the electrode and electrolyte (Figureb). Consequently, SCs
can deliver an outstanding power density with a rapid charge/
discharge behavior. Thus, SCs exhibit huge application pros-
pect where maintenance-free, fast charging, and high power is
required. Nowadays, SCs are widely used in light rail, heavy-
duty vehicles, load-leveling systems for intermittent renew-
able energy sources, hybrid platforms for trucks and buses,
and storing the regenerative braking energy of electric vehi-
cles. However, the overall energy density of a SC cell is low
because the total number of ions that can be stored on the
electrode surface is limited.[] Although commercial SCs can
yield much higher specific energy (≈Wh kg–) than traditional
dielectric capacitors, this is still much lower than fuel cells
and batteries.[,] As a result, the wide usage of SCs has been
seriously limited, and intense research eorts are underway
to achieve high energy density of SCs, to be close to or even
beyond batteries, without cutting down their excellent power
density and cycling stability.[,,]
Generally, the energy density (E) of SCs is in proportion
to the specific capacitance (C) and the square of cell potential
window (V) according to the E= / CV.[] Thus, broadening
the working voltage window is a straightforward and eective
method to boost the energy density of an SC device. Techni-
cally, reasonably designing asymmetric supercapacitors (ASCs)
provides an opportunity to eectively widen the working
voltage range by using the potential windows of cathode and
anode materials, resulting in increased energy density for
potential applications where energy needs to be stored and
delivered with high power. As shown in Figure c, SCs could
be divided into three types: EDLCs, PCs, and ASCs.[] Theoreti-
cally, ASCs cover a wide range, which could include two kinds
of electrode materials (or the same EDLC material with dif-
ferent surface functional groups), or dierent redox-active elec-
trolytes.[] Generally, ASCs can be classified into two dierent
types: capacitive ASCs (both cathode and anode are capacitive
electrodes) and hybrid capacitors (one is the capacitive type and
another one is battery-type Faradaic materials).[–] During the
charging/discharging processes, ASCs can take full advantage
of cathode and anode with separated potential windows to max-
imize the working voltage range of the full device. Owning the
wide voltage range, the energy density of ASCs is much higher
than EDLCs and PCs. For instance, the operating voltage of
an aqueous symmetric SC is limited to ≈.V because of the
thermodynamic breakdown potential of water molecules, but
the working voltage of an ASC cell could be expanded beyond
.V.[] Therefore, building wide voltage ASCs is a promising
technology to closely achieve the goal of high energy density
and promote the further applications of SCs in the future.[]
In this review, for better understanding the advantages of
wide voltage aqueous ASCs, we summarized a large number
Figure 1. a) Ragone plots of dierent kinds of energy storage technologies. b) The basic configuration and working mechanism of a supercapacitor.
c) Historic timeline for the development of supercapacitors.
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2108107 (3 of 68) © Wiley-VCH GmbH
of literature about aqueous ASCs devices with working voltage
beyond .V, and defined them as wide voltage ASCs.
As we know, using organic electrolytes or ionic liquids (ILs)
is also an ecient method to extend the working voltage.[]
Unfortunately, numerous unsatisfied drawbacks seriously
restrict the wide usage of organic electrolyte and ILs. Organic
electrolytes and ILs showcase much lower ionic conductivity
and higher viscosity when compared with aqueous electrolytes,
which hinders ion penetration and leads to higher internal
resistances.[] Consequently, the specific capacitance value of
an organic or IL electrolyte-based SC device is generally lower
than F g– and displays a relatively low power density.
These SCs are also usually limited by high safety concerns and
high costs because of their high volatility, flammability, and
toxicity.[] In addition, to guarantee the normal operation of
an organic electrolyte-based SC, a complex and strict assembly
environment with limited amounts of moisture impurities and
oxygen are required.[] Comparatively, aqueous electrolytes
trigger much more research interest thanks to their low-cost,
high ionic conductivity, earth-abundant, and environmental
friendly.[] An aqueous SC cell can achieve higher capacitance
value and power density due to the smaller ionic size and faster
ionic conductivity of aqueous electrolytes.[] In the field of
supercapacitors, extensive research eorts are underway with
the goal to boost energy density without sacrificing their long-
term life and power density. Building wide voltage aqueous
ASCs is a promising strategy to break the energy storage limita-
tions of traditional aqueous symmetric and asymmetric devices.
More importantly, these aqueous devices can be assembled in
the air without the need for costly “dry rooms,” which is ben-
eficial for practical applications. But so far, the energy storage
mechanisms, new electrode materials, ideal electrolytes, and
stable current collectors still remain to be investigated for the
practical applications of wide voltage aqueous ASCs. In gen-
eral, the technology of wide voltage aqueous ASCs oers the
opportunity for wide applications where energy needs to be
stored and delivered with high power, and represents the cur-
rent developing trend of the clean energy future. That will
promote the integrated applications of supercapacitors with bat-
teries, catalysis, photovoltaic cells, nanogenerator, etc. for next-
generation clean energy and appeal extensive attentions among
researchers, engineers, developers, and under/postgraduate
students who are working and studying in areas such as nano-
materials, composites, and energy-related applications.
Currently, numerous excellent reviews have overviewed SCs
with electrode materials or structures, electrolytes, cell voltage,
charge storage mechanisms, and smart devices.[,,,–]
Although there have many inspiring reviews on SCs, there is
still no systematic review of recent research progresses on the
achievements of wide voltage aqueous ASCs with both high
energy and power densities. A comprehensive review on wide
voltage aqueous ASCs will be very helpful for researchers to
know what have been achieved and what they can do in this
promising field, and this review will guide the specific design
of wide voltage aqueous ASCs in electrode materials, electro-
lytes, device configurations, and smart strategies for improving
specific capacitance, voltage window, rate performance, and sta-
bility in order to further cater the practical demands. Therefore,
we believe this review will be very valuable for the development
and exploration of high-performance SCs and energy-related
devices. Accordingly, this review provides the insights and
updates related to the following knowledge gaps which have
not received the coverage they merit in the previously available
reviews:
• Sketched the historical development and evolution of device
configurations for wide voltage aqueous ASCs.
• Explained the charge storage mechanisms of dierent kinds
of SCs and their dierences with batteries, and proposed
the matching principles for rationally designing and fabri-
cating high-performance ASC devices from the perspective
of theory and reality.
• Comprehensively summarized typical cathode and anode
materials for building wide voltage aqueous ASCs over the
past few decades.
• Analyzed the compatibility between electrolytes and elec-
troactive materials, and summarized the advanced aqueous
electrolytes for building aqueous ASCs with high capacitance
and wide operating voltage.
• Divided two main device configurations for building wide
voltage aqueous ASCs, and discussed their advantages and
drawbacks.
• Systematically summarized numerous methods for
improving the electrochemical performance of ASCs in
voltage window, specific capacitance, rate performance, and
electrochemical stability.
• Discussed the challenges facing wide voltage aqueous ASCs
and our perspectives in future research of SCs field: )
deeper understanding of the electrochemical charge storage
mechanisms; ) developing new electroactive materials;
) designing rational electrode structure with high mass
loading; ) optimizing the electrolytes; ) compatibility with
current collectors; ) further fundamental understanding via
both advanced characterization techniques and theoretical
researches; ) device innovation and integration with multi-
functionality; and ) building standard methods to evaluate
the performance of SCs.
This review will provide readers with a comprehensive
insight into the fundamental understanding, electrode mate-
rials, electrolytes, device configurations, and strategies for
achieving high energy density wide voltage aqueous ASCs
without sacrificing their existing advantages. The present
review is expected to serve as one-stop reference on wide
voltage aqueous ASCs for next-generation clean energy applica-
tions and be of interest to the broad advanced energy storage
devices research community.
2. Brief Historical Development of Wide Voltage
Aqueous Asymmetric Supercapacitors
The first capacitor (“Leyden jar”) was invented by Ewald Georg
von Kleist and Pieter van Musschenbroek in the middle of the
eighteenth century.[] This built the initial concept of an elec-
tric double layer, which is earlier over years than the inven-
tion of the battery in . Von Helmholtz is the first one to
study the electrical charge storage mechanism in capacitors and
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2108107 (4 of 68) © Wiley-VCH GmbH
established the first electric double layer model in .[] The
modern theory of EDLC was developed by some pioneering
interfacial electrochemists, including Gouy,[] Chapman,[]
Stern,[] and Grahame[] during the nineteenth and early twen-
tieth centuries. In , the pseudocapacitor, a new kind of elec-
trochemical capacitor, was discovered in RuO, which involves
Faradaic processes.[,] The discovery of pseudocapacitance
provided a new and great opportunity to boost the energy den-
sity of electrochemical capacitors. In , the U.S. Department
of Energy (DOE) started to subsidize an over a long period of
time SC research aiming at high energy density SCs for electric
drivelines applications.[] Then, the SCs were gradually used
in the energy load leveling system for electric vehicles, where
energy can be collected from braking and releasing the elec-
tric energy for acceleration. Since then, dierent types of SCs
become available, including EDLCs, PCs, and ASCs. Each kind
of SC has its own marked features and target market.[,] Since
, with the rapid development of nanoscience and advanced
characterization techniques, the amount of research for SCs
has significantly and continuously increased and numerous
new electrochemical and physical phenomena have been dis-
covered for both EDLC and PC systems.
In the past decades, to enhance the energy density of aqueous
SCs, dierent types of SCs devices have been built.[,] As
illustrated in Figure 2a, the basic configurations of aqueous
SCs include: type I, EDLC electrodes-based symmetric device
(narrow operating voltage window, high-power, very stable
cycle life, and low energy density); type II, pseudocapacitive
electrodes-based symmetric device (narrow operating voltage
window, high-capacitance, low energy density); type III, EDLC//
pseudocapacitive electrodes-based asymmetric device (wide
operating voltage window, high-power, high-capacitance, mod-
erate energy density); type IV, all pseudocapacitive electrodes-
based asymmetric device with two dierent pseudocapacitive
electrodes (wide operating voltage window, moderate-power and
high energy density). The working voltage window of aqueous
SCs was enlarged from .V to near .V and the energy density
was pushed to Wh kg– benefitting from the development of
wide potential electrodes and asymmetric device configurations
(Figure b).[,] In recent years, various cathode and anode
materials with wide potential window have been discovered and
fabricated to build wide voltage ASCs (Figurec).[–] It should
be noted that the potential windows may change with dierent
kinds of electrolytes (ion types or pH) and electroactive materials
(particle sizes, crystal phases, or microstructures).
3. Charge Storage Mechanisms and Matching
Principles
3.1. Charge Storage Mechanisms
The discovering charge storage mechanisms of SC is the story
of developmental history about itself as mentioned above. The
Figure 2. a) The basic configurations and developments of aqueous supercapacitors: type I: EDLC electrodes-based symmetric device; type II, pseu-
docapacitive electrodes-based symmetric device; type III, EDLC//pseudocapacitive electrodes-based asymmetric device; type IV, all pseudocapacitive
electrodes-based asymmetric device. b) The electrochemical voltage and energy density profiles of aqueous symmetric supercapacitor and wide
voltage aqueous asymmetric supercapacitor. c) Schematic representation of the electrochemical stability range of water and potential windows
versus the standard hydrogen electrode (SHE) for dierent pseudocapacitor materials in an aqueous electrolyte from previous reports (carbon,[]
MnO,[] RuO,[] MoO,[] TiO,[] WO-x,[] VO,[] PANI,[] PPy,[] Na.MnO,[] Na.Mn.O,[] NbO,[] LiMnO,[] NbOPO·HPO·HO,[]
TiNbO-x,[] VOPO,[] SnO,[] BiO,[] VN,[] FeO,[] InO,[] MoS,[] MoPO[]). Note that the potential windows could change with dierent
electrolytes (pH or ion types) and active material structures (crystal phases or particle sizes). Note the following potentials were used for normalizing
the potential dierences of common reference electrodes with respect to SHE: mercury/mercury oxide (Hg/HgO, E=+.V), silver/silver chloride
electrode (Ag/AgCl, saturated, E=+.V), and saturated calomel electrode (SCE, E=+.V).
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2108107 (5 of 68) © Wiley-VCH GmbH
main electrochemical characteristic that distinguishes SCs from
batteries is that the voltage is always linear with current in SC
electrodes.[,] Specifically, during charge/discharge processes,
the cyclic voltammogram (CV) curve of the SC should keep
a rectangular shape, and the current is almost constant.[–]
However, a battery exhibits separated and sharp peaks with
obvious Faradaic reactions (Figure 3a).[,] Besides, the gal-
vanostatic charge/discharge (GCD) curve of the SC shows a
smooth oblique line with a constant slope, while the battery
displays a relatively flat charge/discharge plateau at a constant
voltage stage (Figureb).
In recent years, dierent types of energy-storage mecha-
nisms have been developed based on various kinds of energy-
storage materials in SCs systems.[] Specifically, according to
the dierent electrochemical features (typical CV and GCD
curves), energy-storage materials could be divided into EDLC
materials, pseudocapacitive materials (surface redox, interca-
lation, and partial redox-intercalation) and Faradaic materials
(Faradaic dominated and battery-like).[] In response to poten-
tial scanning, the EDLC-based materials exhibit a rectangular-
shaped CV curve (Figurec) and a linear voltage response (a
triangular-shaped curve) during constant current charge/dis-
charge (Figure d).[] Pseudocapacitive electrode materials
like RuO and MnO can store charge in two ways: ) Faradaic
electron transfer, by accessing two or more redox states of the
metal centers in these oxides (e.g., Mn(III) and Mn(IV))[,]
and ) non-Faradaic charge storage in the electrical double layer
present at the surfaces of these materials.[,] Their current–
voltage relationship is similar with EDLC materials taking a
pseudo-rectangular CV shape (Figuree) and an analogous tri-
angular-shaped GCD curve (Figuref). Some typical materials
such as nickel/cobalt oxides or hydroxides, due to their electro-
chemical features with prominent and widely separated peaks
in CV curve (Figureg) and a relatively flat charge/discharge
plateau in GCD curve (Figureh). Thus, the cobalt and nickel-
based oxides and hydroxides stand for typical battery-type Fara-
daic materials and should not be regarded as pseudocapacitive
electrodes.[–]
As mentioned above, pseudocapacitive materials store
charge through a Faradaic mechanism but whose electro-
chemical feature showcases a capacitive signature. There is a
linear relationship between the stored charge and the charging
potential. Nevertheless, cobalt and nickel-based oxides and
hydroxides do not show this capacitive feature and cannot be
regarded as pseudocapacitive materials owing to the phase tran-
sitions and the Faradaic charge storage mechanism.[–] To
correctly describe the energy storage characteristics of such bat-
tery-type materials, mAh g– or C g– should be used to quan-
tify their capacitive performance.[–] Thus, when evaluating
a new electrode material, the first question for researchers is
whether this material is capacitor-like or battery-like species.[]
Any material exhibiting intense, clearly separated oxidative
and reductive peaks in CV curves or obvious plateaus in GCD
curves, should be categorized as a battery-type electrode.[–]
On the other hand, a capacitor-like material owns a rectan-
gular CV and a linear voltage response during constant-current
charge/discharge.[,] The peak current (i) response of a bat-
tery-type material will be proportional to the square root of the
scan rate (i∼ v/), while a capacitor-type electrode will exhibit a
linear current response dependency on the scan rate (i∼ v). In
this way, the dierence between capacitor-type and battery-type
materials will be understood easily and acceptably.
3.2. Matching Principles
As discussed above, based on the charge storage mechanisms
of the cathode and anode, ASCs can theoretically be divided
Figure 3. Comparison of the electrochemical behavior for a typical battery and supercapacitor. Typical CV and GCD curves of a,b) battery and super-
capacitor: c,d) EDLC materials. e,f) Pseudocapacitive materials (surface redox, intercalation, and partial redox-intercalation). g,h) Faradic materials
(Faradic dominated and battery-like).
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2108107 (6 of 68) © Wiley-VCH GmbH
into capacitive-type and hybrid-type capacitors. Generally, the
two electrodes in a hybrid capacitor own two dierent charge
storage mechanisms with either capacitive- or battery-type Fara-
daic materials.[,] For example, a typical hybrid capacitor cell
was built with a Faradaic material (such as CoO or Ni(OH))
and a carbon material.[,] Usually, the Faradaic electrode
materials (such as cobalt and nickel-based compounds) have
high theoretical specific capacitances, but their low poten-
tial windows in aqueous electrolytes always limit the working
voltage ranges of hybrid capacitor devices.[,] Thus, in this
review, we mainly focus on the capacitive ASCs. For the capaci-
tive ASCs, both cathode and anode exhibit capacitive features,
leading to an ideal rectangular-shaped CV curve (Figure 4a) and
a triangular-shaped GCD curve (Figureb) for the full cell.[,]
Generally, in capacitive ASCs, EDLC-based materials with wide
potential window and inferior theoretical specific capacitance
were used as negative electrodes, pseudocapacitive materials
with moderate potential window and high theoretical specific
capacitance were used as positive electrodes.[,] As a result,
both working voltage and energy density of ASC devices were
greatly improved than those of symmetric devices.[] However,
limited by the out-o-balance of charge storage between EDLC-
based materials and pseudocapacitive materials, the working
voltage of this kind of ASC is usually lower than V and yield
an inferior energy density.[] Thus, a reasonable selection of
positive and negative materials with wide potential window and
high capacitance is very important to build high-performance
ASCs with wide working voltage and superior energy density.
It is known that the improvement of the overall performance
of ASCs is not only dependent on the intrinsic properties of
cathode and anode, or electrolytes, but also on the reasonable
design, mass and kinetics match, and compatible combination
of each isolated device component in one single device.[,] In
most cases, the overall performance of an ASC cell is not equal
to the sum of cathode and anode, especially for the voltage
widow. In particular, in order to make full use of potential
range and capacitance of both positive and negative electrodes,
some principles should be kept in mind during assembling and
testing a wide voltage aqueous ASC cell.
) For electrode materials, it is important to choose and fab-
ricate cathode and anode with wide potential windows, which
have high oxygen evolution reaction (OER) and hydrogen evo-
lution reaction (HER) over-potentials in a specific aqueous
electrolyte, respectively.[,] In this regard, some EDLC- and
pseudocapacitive-based materials are appropriate for positive
electrodes (e.g., carbon-based materials,[] RuO,[] MnO,[]
VO,[] MoO,[] PANI,[] etc.) or negative electrodes (e.g.,
carbon-based materials,[] FeO,[] SnO,[] BiO,[] VN,[]
etc.), rather than Faradic materials (e.g., CoO,[] NiO,[]
Co(OH),[] Ni(OH),[] NiCoS,[] etc.). However, the wide
voltage materials usually yield inferior specific capacitances
when compared with Faradic materials, especially for the
EDLC-based materials.[,] Thus, the construction of wide
voltage electrodes with superior conductivity, high specific
surface area, rational porous structure, high surface reactivity,
and robust electrochemically active sites is the main target
for high-performance ASCs.[,,] In other words, both wide
potential range and superb specific capacitance are essential
for cathode and anode to build a high-performance aqueous
ASC with wide operating voltage and satisfied energy density
(Figurec,d).[,]
) In order to obtaining optimal performance of a full ASC
device, a charge balance between cathode and anode should
be kept. According to Qelectrode= Celectrode× m×ΔE, the total
Figure 4. a,b) Schematic illustration of the typical charge and discharge characteristics of an asymmetric supercapacitor. c,d) Schematic illustration of
the matching principles of anode and cathode in an asymmetric supercapacitor by typical charge and discharge curves.
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2108107 (7 of 68) © Wiley-VCH GmbH
charge storage in an electrode is limited by its specific capaci-
tance (Celectrode), active mass (m), and potential window (ΔE).[]
To realize the charge balance (Q+= Q–), prior to the fabrication
of an ASC cell, the mass loadings of the cathode and anode
should be balanced according to the following equation:
electrode–
electrode
m
m
CE
CE
=
∆
∆
+
−
−
++
()
Before testing the electrochemical performance of an
ASC device, CV and GCD measurements should do first by
employing a three-electrode cell to investigate the stably poten-
tial windows and specific capacitances for both cathode and
anode.[] Furthermore, for achieving wide voltage ASCs, both
potential windows and specific capacitances of the cathode and
anode should be precisely evaluated and balanced to yield a
theoretical maximum of operating voltage and energy density
(Figurec,d).
) Reasonably selecting and optimizing the matching between
the electrode and electrolyte is also critical to enhance the overall
electrochemical performance of wide voltage aqueous ASCs. In
general, an ideal electrolyte should own the merits of wide elec-
trochemical stability window, high ionic conductivity, wide oper-
ating temperature range, high electrochemical and chemical
inertness to device components (e.g., electrodes, current collec-
tors, and packaging materials), low volatility and flammability,
well-matched with the electrode materials, low cost, and envi-
ronmental friendly.[] Unfortunately, till now, there is no elec-
trolyte that can meet all of these requirements, and each kind
of electrolyte owns its own merits and drawbacks. Numerous
theoretical and experimental researches have proved that the
nature of an electrolyte, including the ion size and type, the sol-
vent and ion concentration, the interaction between the solvent
and ion, the interaction between the electrode material and elec-
trolyte, and the stable potential window, all have an eect on the
energy and power densities as well as the cycling life of ASCs.[]
Therefore, except for matching the charge storage kinetics and
mass of cathode and anode, it is also important for reasonably
choosing a complemented electrolyte to achieve high-perfor-
mance ASCs. At present, the electrolytes applied in ASCs are
mainly divided into aqueous and nonaqueous electrolytes. For a
typical class of non-aqueous electrolytes, organic electrolytes are
widely used in commercial SCs with a wide voltage range of up
to V or even larger.[,] But their low ionic conductivity, high
flammability, and easy short-circuiting seriously increase safety
risks and decrease the capacitance and power density of SCs.[]
Compared with organic electrolytes, aqueous electrolytes (e.g.,
NaSO, LiCl) used in wide voltage aqueous ASCs exhibit the
superiority of non-flammability, low cost, high ionic conduc-
tivity, good safety, and convenient assembly in air.[] What is
more, aqueous ASCs can deliver much higher capacitance and
power density than organic ASCs due to their suitable size of
ions and high ionic conductivity.[–]
) To guarantee the wide voltage aqueous ASCs devices can
continuously and normally work, the compatibility of current
collector with electrode and electrolyte is vital. In wide voltage
aqueous ASCs, the current collector must bear the eects
of aqueous electrolytes themselves and the high current/
voltage environment. Thus, the current collector should own
reasonably chemical and electrochemical stability in a specific
aqueous electrolyte. For example, when a strong acid solution
(e.g., HSO) with high corrosive nature is employed as the elec-
trolyte, some corrosion-resistant materials (e.g., Au, indium tin
oxide (ITO), carbon-based materials, and conducting polymers)
are usually used as current collectors for supporting the active
materials. Fortunately, some self-standing carbon-based mate-
rials (e.g., carbon clothes, electrospun carbon fiber films, gra-
phene films, CNT films, etc.) could directly serve as the current
collectors in wide voltage aqueous ASCs thanks to their high
conductivity, light weight, corrosion resistance, high voltage
resistance, outstanding mechanical strength, and good flex-
ibility. Neutral aqueous electrolytes exhibit better compatibility
with dierent current collectors for aqueous ASCs because of
their much less corrosive nature. Nevertheless, under the high
working voltage environment, most of metallic current collec-
tors suer from corrosion and gas production while carbon-
based current collectors can relatively keep these devices work
steadily. Therefore, the development of current collector with
high electronic conductivity, high voltage resistance, corrosion
resistance, low cost, high industrial processability is highly
desired to build high-performance wide voltage aqueous ASCs.
4. The Big Family of Wide Potential Electrode
Materials
In the past decades, as the “engine” of the SC device, dif-
ferent kinds of electrode materials including carbon mate-
rials (e.g., activated carbon, graphene,[] carbon nanotube,[]
carbon onion,[] quantum dot,[] carbon particle,[] carbon
nanowire,[] and other carbon derivatives[]), pseudoca-
pacitive materials or battery-type materials (e.g., conducting
polymers,[] transition metal oxides,[] -hydroxides,[]
-sulfides,[] -phosphides,[] -selenides,[] -nitrides,[] -car-
bides,[] and -borides,[] etc.) have been widely studied to
produce high capacitive performance. For an aqueous ASC,
as discussed above, the cathode and anode should meet the
merits of both high potential range and comparable capacitance
to maximize its operating voltage window as well as energy
density. In addition, from the electrodes’ view, the ideal mate-
rial should have the following properties: ) large specific sur-
face area, ) good electronic and/or ionic conductivity, ) high
mechanical and/or electrochemical stability, ) low toxicity and
cost.[,,] Other features such as temperature dependence,
self-discharge and safety should also be considered in prac-
tical applications. In order to provide some insights for future
research on wide voltage ASCs, we summarized the redox
potential windows and the electrochemical performances of
some commonly investigated wide voltage capacitive electrode
materials in Table 1. We hope this will help researchers choose
satisfied electroactive materials and appropriate electrolytes to
build ecient wide voltage aqueous ASCs.
As shown in Table, these capacitive materials always exhibit
wider voltage ranges than battery-type electrode materials (e.g.,
CoO, Ni(OH), etc.), which have the very narrow Faradaic elec-
tron transfer potential window. Dierent electrode materials
have dierent voltage ranges, which is caused by the dierence
in the inherent catalytic activity of materials.[] As we know, the
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thermodynamically stable window of an aqueous electrolyte is
determined by the energy gap between the lowest unoccupied
molecular orbital (LUMO) and the highest occupied molecular
orbital (HOMO).[] For electrode materials, the cathode and
anode own their unique electrode potentials, which correspond to
their Fermi energies.[] In a Faradaic reaction system, the energy
levels at or near the Fermi level of cathode and anode should
be matched with a suitable vacant (LUMO) or occupied orbital
(HOMO) in the electrolyte. At the same time, in order to achieve
the condition of balance for facile electron transfer, an applied
potential is needed to modify the work function of the electrode
material to some value. The work function of metal oxides is
directly related to electrochemical oxidation/reduction potential.
If the applied potential of cathode or anode is higher than HOMO
or lower than LUMO of electrolyte, a passivation layer will usually
form to prevent the oxidation or reduction of electrolyte, respec-
tively. That is one reason for these capacitive materials and super-
capacitors they assembled exhibit wide voltage range in a specific
aqueous electrolyte. Besides, generally, the operating voltage of
an asymmetric supercapacitor could be widened by choosing dif-
ferent electrode materials with a large work function dierence.
4.1. Typical Cathode Materials
Recently, various cathode materials with wide potential range
have been developed to restrain the OER reaction of aqueous
electrolyte. These materials mainly process pseudocapacitive
or Faradaic mechanisms by fast and reversible redox reactions,
electrosorption and desorption, or intercalation and deintercala-
tion between the electrolyte and electroactive materials.[–]
However, although dierent types of cathodes have been reported,
great challenges remain to be solved to guarantee more reliable
cathodes for wide voltage aqueous ASCs. In this section, we
summarized and discuss some typical cathodes for wide voltage
aqueous ASCs, including the types, issues, and feasible solutions.
4.1.1. Metal Oxides Composite Electrodes
Ruthenium oxide (RuO) is a classical and promising electro-
active material for aqueous ASCs because of its high theoret-
ical specific capacitance ( F g–), reversible redox reaction
(RuOx(OH)y+ zH++ ze–↔ RuOx-z(OH)y+z (Max(z) = )) with
wide potential window (.-.V (vs RHE) in . HSO elec-
trolyte), and long cycle life.[] There are two phases that exist
in nature of amorphous hydrous phase (RuO·xHO) and the
crystalline phase (rutile RuO).[,] The RuO·xHO phase
was investigated to have better electrochemical performance
owing to high electron/proton conductivities, large active
reaction site, and ultrahigh capacitance. In , Hu and co-
workers reported the high-performance electrochemical capaci-
tors by using hydrous RuO as electroactive material.[] As
shown in Figure 5a, hydrous RuO nanotubular array electrodes
were fabricated through the anodic deposition method with the
membrane-template synthesis route. After removing the anodic
aluminum oxide (AAO) template, from the scanning electron
microscope image, the homogeneous nanotube architecture
Table 1. Summary of the electrochemical properties of cathode and anode materials for wide voltage aqueous asymmetric supercapacitors.
Materials Voltage window Electrolytes Specific capacitance Stability (cycles) Refs.
MnO–.V (Ag/AgCl) NaSO. F g-– []
RuO–.V (Ag/AgCl) HSO. F g-– []
MoO–.V (Ag/AgCl) HSO. F g-– []
TiO–.V (Ag/AgCl) . NaSO. mF cm-.% ( ) []
WO-x –.–.V (SCE) HSO. F cm-.% () []
VO–.V (Ag/AgCl) . KSO. F g-.% () []
PANI –.V (SCE) HSO. F g-.% () []
PPy –.V (SCE) LiCl . F g-.% ( ) []
Na.MnO–.V (Ag/AgCl) NaSO. F g-.% ( ) []
Na.Mn.O –.V (SCE) LiCl . mF cm-.% () []
NbO–.V (SCE) LiSO. F g-– []
LiMnO–.V (SCE) LiSO. mAh g-– []
NbOPO·HPO·HO –.V (Ag/AgCl) HCl . F g-.% ( ) []
TiNbO-x -.–.V (SCE) HSO. F g-.% ( ) []
VOPO–.V (SCE) HSO. F g-.% () []
SnO-.–V (Ag/AgCl) LiCl . F g-.% () []
BiO-.–V (Ag/AgCl) NaSO. F g-≈.% () []
VN -.–V (Ag/AgCl) NaSO. F g-.% ( ) []
FeO-.–V (Ag/AgCl) NaSO. F g-.% () []
InO-.–.V (Ag/AgCl) NaSO. F g-– []
MoS-.–V (Ag/AgCl) KOH . F g-.% () []
MoPO -.–V (SCE) KCl . F g-≈% ( ) []
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was built (Figureb). From the transmission electron micros-
copy (TEM) image, the thickness of these nanotubular and
the outer diameter are about and nm, respectively
(Figure c). The mesopores nanostructure, metallic conduc-
tivity, and hydrous nature provide the electron and proton
“superhighways” for the fast charge and discharge behaviors.
The as-prepared RuO nanotubes exhibited rectangle-like elec-
trochemical characteristics in HSO electrolyte for CV
curves at dierent scan rates (Figure d). After calcining at
°C for h in air, a high specific capacitance with F
g– of RuO nanotubes can be achieved (Figuree). Besides,
the capacitive performance of the annealed RuO nanotubes
was also measured by high-frequency capacitive responses. As
shown in Figuref, when the annealed RuO nanotubes were
tested at .V, the specific power and specific energy were
and .Wh kg– at Hz, as well as and .Wh kg– at
kHz, respectively, which exhibits the RuO nanotubes owing
ultrahigh power nature and ideal capacitive performances for
SCs applications. Recently, Jiang et al. demonstrated an aqueous
ASC with RuO/carbon fabric as cathode and TiCTx/carbon
fabric as anode (Figureg).[] The RuO/carbon fabric cathode
exhibited a pair of broad redox peaks under the voltage range
of –. V (vs Ag/AgCl), which originating from pseudoca-
pacitive behavior (Figureh). Combining with TiCTx/carbon
fabric anode (-.-.V vs Ag/AgCl), a .V asymmetric cell
was built and showed excellent capacitive performance even at
a high scan rate of mV s– (Figurei). The asymmetric cell
can yield a large energy density of μWh cm– and keep %
of initial capacitance after cycles.
Among the dierent kinds of pseudocapacitive materials,
MnO was considered as one of the most promising cathode
candidate for replacing RuO due to its wide potential window
(-. V vs Ag/AgCl in NaSO), high theoretical capaci-
tance ( F g–), fast and reversible redox reactions (MnO+
xC++ xe–↔ MnOOCx, Max(x) = , C can be H+, Li+, Na+, K+),
low cost, and abundance.[] Qu and co-workers studied the
electrochemical characteristics of MnO material as a cathode
for ASCs in dierent neutral electrolytes.[] The MnO
Figure 5. a) RuO·xHO nanotubular arrayed electrode. b) SEM and c) TEM images of the RuO·xHO NTs arrayed electrode. d) CV curves at dierent
scan rates and e) specific capacitances curves at dierent current densities of RuO·xHO NTs arrayed electrode. f ) Frequency dependence of specific
capacitance of RuO·xHO NTs arrayed electrode. Reproduced with permission.[] Copyright , American Chemical Society. g) Schematic represen-
tation illustrating the fabrication process of RuO//TiCTx asymmetric supercapacitor. h) CV curves of RuO/CF cathode, TiCTx/CF anode, and the
asymmetric device at a scan rate of mV s–. i) CV curves of RuO//TiCTx asymmetric device at dierent scan rates. Reproduced with permission.[]
Copyright , Wiley-VCH.
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nanorods were fabricated by a precipitation method and meas-
ured in . KSO, NaSO, and LiSO aqueous electrolytes,
respectively. The hydrated ionic radiuses of K+, Na+, and Li+ were
., ., and . Å, respectively. Thus, at slow scan rates, the
capacitance value of MnO nanorods in the three electrolytes
increases in the order of KSO< NaSO< LiSO, because of
the reversible intercalation/deintercalation of Li+ in MnO pro-
viding an additional capacitance. With the increasing of scan
rates, the capacitance of MnO nanorods in KSO showed the
biggest value due to the smallest resistance and the highest ionic
conductivity. They proved that the alkaline cations are indeed
involved in the reaction of the MnO electrode, which provides
useful information for studies on the charge storage mechanism
of MnO. Besides, a .V ASC was prepared through employing
MnO as cathode and activated carbon as anode in . the
KSO electrolyte. The cell exhibited a superior energy density of
.Wh kg– and retained Wh kg– at a high power density of
kW kg–, as well as an outstanding cycling durability with %
capacitance retention after cycles.
Although MnO has high theoretical capacitance, its high
capacitance can only be obtained in low-dimensional thin films
(tens of nanometers) and small nanoparticles with low mass
loading (<mg cm–).[] With the increasing of mass loadings,
the specific capacitance will sharply loss because of the intrin-
sically low conductivity (––– S cm–) of MnO. Kang et al.
developed a nonequilibrium doping with free-electron metal
atoms (e.g., Au, Cu, and Ag) to enhance the capacitive perfor-
mance of thick MnO film by improving its intrinsic conduc-
tivity.[] As shown in Figure 6a, the porous MnO layer with
nm thickness was firstly electrodeposited on a nanoporous
gold substrate. Then Au atoms were doped in porous MnO by
physical vapor deposition. A thick Au-doped MnO film was
prepared by repeating the procedures. The electronic structure
of Au-doped MnO was investigated by the first principle calcu-
lations with two possible atomic configurations: Au-interstitial
and Au-substituted spinel MnO. The electrons were found to
transfer from the doped Au atoms to the surrounding O and Mn
atoms in both configurations. Thus, the oxidation state of the
Mn atoms decreases because of the gain of electrons, which is
proved to improve the conductivity of MnO. The electrochem-
ical properties of pure MnO and dierent amounts of Au-doped
MnO with a similar thickness (about nm) were also evalu-
ated. The specific capacitance of the Au-doped MnO is much
larger than that of pure MnO (Figureb). The highest specific
capacitance of F g– was produced for the . at% Au-doped
MnO, which exhibited a % increment than that of pure
MnO at mV s–. In addition, after cycles, the capaci-
tance of the pure MnO lost about % while the capacitance of
the Au-doped MnO decreased first and then slightly increased.
Another strategy for achieving high capacitance of MnO
electrode is to construct hierarchical nanostructure for higher
specific surface area and utilization rate. Zhu et al. fabri-
cated β-MnO/birnessite core–shell structure with β-MnO
as the core and ultrathin birnessite sheets as the shell, which
exhibited the structural superiority to boost the utilization
rate of MnO from the bulk.[] The preparation process of
β-MnO/parallel birnessite core/shell nanorod is illustrated
in Figure c. First, the pure MnOOH nanowires were syn-
thesized through a simple hydrothermal method. Then, the
parallel birnessite sheets were grown on MnOOH nanowires.
Finally, the MnOOH core was completely transformed into β-
MnO during the high-temperature hydrothermal process. The
ultrathin (≈nm) and highly ordered birnessite sheets with an
interlayer spacing of . nm are parallel to the c-axis of the
Figure 6. Enhanced supercapacitor performance of MnO by atomic doping: a) Fabrication process, b) capacitive performance of Au-doped MnO.
Reproduced with permission.[] Copyright , Wiley-VCH. Structural directed growth of ultrathin parallel birnessite on β-MnO for high-performance
asymmetric supercapacitors: c) Schematic representation, d) variations of the capacitance with the shell thickness of birnessite for β-MnO/parallel
birnessite core/shell nanorod electrode. Reproduced with permission.[] Copyright , American Chemical Society.
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β-MnO nanorods, forming a fin-like nanostructure, which
enhanced the eective contact area between the active material
and electrolyte. The specific capacitances of β-MnO/birnessite
core-shell structure is variable with the dierent thickness of
birnessite sheets (Figured). A superb specific capacitance of
F g– was obtained with nm birnessite sheet at -.V
(vs SCE) in NaSO electrolyte. A . V aqueous asym-
metric cell was built by using β-MnO/birnessite nanosheets
as cathode and activated microwave expanded graphene oxide
as anode in NaSO. The device has shown a superior
energy density of .Wh kg– at a power density of W kg–
and well electrochemical stability.
Dier from improving conductivity and constructing hierar-
chical nanostructure, both potential range and specific capaci-
tance of MnO materials can be significantly improved by
the pre-insertion of cations like Na+ and K+ into MnO.[,]
Jabeen and co-workers reported a high Na content Birnes-
site Na.MnO nanosheet via electrochemical oxidation as a
high-performance cathode for aqueous ASCs.[] The struc-
tural evolution process from spinel MnO to Birnessite
Na.MnO during the electrochemical oxidation was illus-
trated in Figure 7a. The high-resolution transmission electron
microscopy (HRTEM) image of MnO particle exhibits well-
defined lattice fringes with an interplanar spacing of .nm,
Figure 7. High-performance .V aqueous asymmetric supercapacitors based on in situ formed Na.MnO nanosheet assembled nanowall arrays:
a) Structural evolution process during the electrochemical oxidation of the MnO and Na.MnO. b) HRTEM images of (i) MnO, (ii) Na.MnO
nanowalls and (iii) STEM image of the Na.MnO nanowall with corresponding EDS elemental mappings of Mn, O, and Na. c) The electrochemical
of Na.MnO electrode and the aqueous asymmetric supercapacitor. (i) CV curves in dierent potential windows, (ii) CV curves of the Na.MnO
and the FeO@C electrodes in separate potential windows at mV s– and (iii) CV curves of the Na.MnO//FeO@C device in dierent voltage
windows at mV s–. Reproduced with permission.[] Copyright , Wiley-VCH.
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corresponding to the () plane of tetrahedral MnO. The
selected area electron diraction (SAED) pattern of the MnO
indicates a polycrystalline nature for MnO spinel (Figurebi).
As for Birnessite Na.MnO, well-resolved lattice fringes with
an interplanar spacing of .nm could be detected from the
HRTEM image, which agrees with ≈.nm for () plane of
Birnessite MnO (Figure bii). The larger interlayer distance
demonstrates the existence of crystal water between the MnO
octahedral layers, which is beneficial for superior capacitive
performance. The scanning transmission electron microscopy
(STEM) image displays the homogenous distribution of Mn, O,
and Na elements in Na.MnO nanosheet (Figurebiii). Ben-
efiting from the thin nanosheet and large interlayer distance,
the Birnessite Na.MnO exhibited a wide potential window
of -. V (vs Ag/AgCl) with a high specific capacitance
of F g– in NaSO) (Figure ci). The charge storage
mechanism of Na.MnO was also evaluated by using Dunn’s
method. The surface capacitive contribution was ≈% of the
whole charge storage at mV s–, which is much higher than
the diusion-controlled contribution, demonstrating eective
surface charge storage for the Na.MnO electrode. To fabri-
cating an asymmetric device, the FeO@C anode was prepared
with a potential window of -.-V (vs Ag/AgCl) and a large
specific capacitance of F g– (Figure cii). Based on the
matchable potential windows and capacitances of Na.MnO
cathode and FeO@C anode, a .V asymmetric cell was fab-
ricated (Figureciii). The Na.MnO//FeO@C cell achieved
a superior specific capacitance of F g– and a maximum
energy density of Wh kg–, which largely surpasses previ-
ously reported MnO-based SCs and even comparable to those
of organic electrolyte-based SCs systems. This research repre-
sents a promising strategy for developing wide voltage aqueous
ASCs with outstanding energy density.
Vanadium oxide, as an intercalation compound, has received
a great attention in aqueous ASCs benefitting from its unique
layered structure with multiple oxidation states (V+, V+, V+,
and V+), versatility in various cations intercalation (VO+
xM++ xe–↔ MxVO (Max(x) = , M can be H+, Li+, Na+,
and K+)), low cost and easy to preparation.[,] Attribute to
its unique structure and electrochemical features, VO owns
a high theoretical capacitance of F g– with a wide poten-
tial range of -.V (vs Ag/AgCl) in a . KSO electrolyte,
which exhibits a great application prospect in wide voltage
aqueous ASCs.[] However, the poor electrical conductivity (–
to – S cm–) and high solubility in aqueous solutions of VO
weakened its capacitive performance.[] Various strategies have
been developed to overcome these drawbacks by engineering
VO into nanostructures and constructing composites with
high conductivity media. Wu et al. fabricated VO nanowires/
CNTs composite film by blade coating to enhance the electronic
conductivity and specific surface area of VO (Figure 8a).[]
After annealing, a scalable, free-standing VO/CNTs composite
film with a length of .cm and the thickness of μm was
obtained. From the cross-sectional energy-dispersive X-ray
Figure 8. a) Fabrication process, optical image, and EDX mappings of VO/CNT free-standing film. b) CV curves of h-VCNT under dierent voltage
windows. c) Atomic schematic illustration for the isomeric vanadium oxides produced by in situ corundum-to-rutile-phase transformation, which
consist of corundum-type VO and rutile r-VO-x core/shell structure. Reproduced with permission.[] Copyright , Wiley-VCH. d) Bright-field
TEM and HR-TEM images of c-VO/r-VO-x core/shell structure (scale bar, nm). e) Gravimetric and volumetric capacitances for c-VO, r-VO and
c-VO/r-VO-x electrodes at various scan rates, comparing with the previously reported typical materials. Reproduced with permission.[] Copyright
, Springer Nature.
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spectroscopy (EDX) mapping of VO/CNTs film, the C, V, O
elements are uniformly distributed throughout the film. The
VO/CNTs film exhibited high electronic conductivity for eec-
tive electron transport and its D network architecture with
porous channels as the “pool” of electrolyte for facile ions dif-
fusion, leading to faster reaction kinetics and sucient utiliza-
tion of active materials. A .V ASC was prepared by using the
VO/CNTs film with LiSO electrolyte (Figure b). The
cell delivered a large energy density of Wh L– at a power
density of kW L–. Liu and co-workers presented a smart
strategy through in situ corundum-to-rutile phase transforma-
tion to enhance redox and intercalation kinetics in vanadium
oxides to significantly improve its specific capacitance and
charge–discharge capability.[] As shown in Figurec, the hier-
archically nanoporous architecture (c-VO/r-VO-x) composite
consisted of vacancy ordered rutile VO-x phases (r-VO-x) and
corundum VO (c-VO) core by a thermal-oxidation-triggered
corundum-to-rutile (CTR)-phase transformation. Dier from
the rhombohedral c-VO, the r-VO owns a tunnel structure,
in which cations can fast transport along the z-axis tunnel while
electrons along the shortly V–V bonded walls. These merits
ensure the c-VO/r-VO-x composite with superior capacitive
behaviors. The nanostructure of c-VO/r-VO-x composite was
studied by bright-field HRTEM, the metastable r-VO-x ()
layer is perfectly integrated with the c-VO () core skeletons
through short V–V bonds (Figured). In the core–shell c-VO/
r-VO-x composite, the c-VO was served as the conductor to
promote electron transport along with the D vanadium-atom
framework and r-VO-x regulating cation storage in the tunnels
as the active intermediate. Benefiting from both intercalation
and redox pseudocapacitance, the core–shell c-VO/r-VO-x
composite electrode achieved ultrahigh specific and volumetric
pseudocapacitance (≈ F g– and ≈ F cm–) with excel-
lent rate performance in -.V (vs Ag/AgCl) with NaSO
electrolyte (Figure e), which outperformed most of typical
pseudocapacitive electrodes (e.g., nanotubular arrayed RuO,[]
hydrogenated TiO/MnO,[] MnO,[] TiCTx clay,[]
N-doped mesopores carbon nanosheets[] and nanostructured
hexagonal WO[]). This work opens a door for developing a
class of bipolar transition-metal oxides electrode with fast ions
accessibility and diusion as well as electrons transfer for
achieving ecient energy storage with rapid charge/discharge
rates in SCs.
4.1.2. Conductive Polymers Composite Electrodes
Conducting polymers (CPs) are considered as another sub-
class of attractive materials for SCs due to their high conduc-
tivity, high charge storage capacity, easy synthesis, low cost,
and environmental friendly since discovered in .[,] The
capacitive performance of CPs normally originates from the
reversible redox reaction of the π-conjugated double bonds in
polymer networks through doping/dedoping processes.[] In
recent years, various CPs, including polypyrrole (PPy), poly-
aniline (PANI),[] polythiophene (PTh), poly(,-ethylenedi-
oxythiophene) (PEDOT),[] and their derivatives,[–] have
been developed and investigated in SCs. However, due to the
poor conductivity, the PANI,[] PPy,[] and PTh[] processed
low specific capacitances of –, –, and – F g–
in both aqueous and non-aqueous electrolytes, respectively.
Additionally, their inferior electrochemical stability owing to
the structural degradation due to the shrinking and swelling of
CPs during the intercalating/deintercalating process seriously
limits their applications for high-performance SCs.[] Rational
structure design and modification are given great expectations
to achieve a stable and high capacitive performance conducting
polymer electrode. Yu et al. fabricated hierarchically porous
nitrogen-doped carbon (HPC)/PANI composites through in
situ polymerization with a greatly improved capacitive perfor-
mance.[] As shown in Figure 9a, the HPC with a superb spe-
cific surface area ( m g–) was used as a substrate to grow
PANI nanowire arrays via a facile in situ polymerization. The
electrochemical properties of the HPC and HPC/PANI com-
posite electrodes were measured in HSO electrolyte. From
CV curves (Figureb), the nearly symmetric rectangular shape
of HPC exhibits its ideal double-layer capacitor behavior. For
HPC/PANI electrodes, the two pair peaks of P/P and P/P
belong to the redox of PANI molecules (leucoemeraldine and
pernigraniline species) and the peaks P/P are ascribed to ben-
zoquinone/hydroquinone redox transitions. The HPC/PANI
electrode had a wide voltage window of -.-. V (vs SCE)
with the highest specific capacitance of F g– at A g–,
which is much higher than previously reported PANI hybrid
materials. Besides, the HPC/PANI electrode also exhibited an
outstanding electrochemical stability of % capacitance reten-
tions after cycles. Based on HPC anode and HPC/PANI
cathode, an ASC was fabricated in NaSO (Figurec). The
aqueous asymmetric cell has shown a stable voltage window
of to . V (Figure d). The specific capacitance rises from
to F g– with the voltage window increasing from
. to .V (Figuree). In order to improve the rate performance
of PANI, Wu et al. proposed a new self-assembly approach to
fabricate PANI/rGO D porous hybrid gels with molecular-
level uniformity even at a very high PANI content (>%).
The PANI@RGO nanosheets electrode exhibited a superior
specific capacitance of F g– ( mF cm–) at . A g–
(.mA cm–) in HSO electrolyte, and an outstanding
rate capability (% capacitance retention from . A g– to
. A g– (.mA cm–)).
The capacitive performance and multifunction of conducting
polymers could be engineered by designing rational nanostruc-
ture. Zhao et al. fabricated foam-like PPy/graphene (G) com-
posite for assembly of highly compression-tolerant SCs, which
possess high specific capacitances without various decreasing
under long-term compressive loading and unloading pro-
cess.[] The PPy/G foam has a high electrical conductivity of
× S m–, which is two folds of magnitude higher than D
graphene-based composites. In addition, the compressible SC
cell exhibited a wide voltage window of -.V (vs Ag/AgCl) in
NaClO and yield a large specific capacitance of F g–
at . A g–. To boost the cycling stability and broaden operating
potential windows, Ma et al. synthesized porous perchlorate-
doped PPy on nickel nanotube arrays (NiNTAs@PPy) via an
electropolymerization method.[] The NiNTAs@PPy electrode
displayed a superb specific capacitance of ≈. F g– with a
wide voltage range of -.–. V (vs SCE) in LiCl electrolyte,
owing to essential electrical and mechanical support of the
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NiNTAs for electroactive PPy. The NiNTAs@PPy displayed
strong electrochemical durability with ≈.% capacitance
retention after cycles, which is much better than most
reported PPy-based electrodes. The as-prepared NiNTAs@
PPy//MnO ASC demonstrated a satisfied energy density of
.Wh kg– in a stable voltage range from . to . V. This
work provides a smart approach for building hollow nanotubes
and porous wave-superposed nano-coatings to accommodate
volumetric deformation of electrodes caused by repeated inser-
tion/ejection of Li+ ions, and consequently guarantee the struc-
ture durability.
4.1.3. Other Cathode Materials
To further boosting the working voltage and specific capacitance
of aqueous ASCs, some other positive materials were also devel-
oped in recent years.[,,,,] Lu et al. fabricated hydrogenated
TiO nanotube arrays (H-TiO NTAs) by annealing anodized TiO
NTAs on Ti fiber in hydrogen atmosphere from to °C
(Figure 10a).[] The H-TiO NTAs were vertically aligned on
the surface of Ti fiber with a uniform diameter and length of
about nm and .μm, respectively (Figureb). The capaci-
tive properties of H-TiO NTAs were evaluated in . NaSO,
and processed in a large voltage range of -.V (vs Ag/AgCl)
(Figure c). In comparison to its counter samples, benefit-
ting from the oxidation/reduction of surface hydroxyl groups,
H-TiO NTAs electrode exhibited an obvious pseudocapacitive
feature with high capacity. Besides, the H-TiO NTAs electrode
shown a maximum energy density of . mWh cm– and a high
power density of . mW cm–. Zhang et al. reported that the
oxygen defect modulated TiNbO-x electrode can break the
limitation of low working voltage (< V) for SCs in an acidic
aqueous electrolyte.[] Titanium niobium oxide (TiNbO)
has the unique WadsleyeRoth shear structure with MO
(M = Ti, Nb) octahedral, which exhibits fast and massive ion
diusion as well as excellent chemical durability even in strong
acidic solution, are regarded as a promising candidate elec-
troactive material for SCs. In addition, the TiNbO owns
high pseudocapacitive capacitance due to multiple electron
reactions of Ti+/Ti+ and Nb+/Nb+/Nb+. However, the poor
electronic conductivity and insucient nanoarchitecture of
TiNbO hindered its additional applications. This group
well addressed these issues by inducing oxygen vacancies into
TiNbO (TiNbO-x) via thermal treatment with NH at
°C and using interlinked graphene arrays as supporting
for loading TiNbO-x (Figure d). The TiNbO-x nano-
particles with small size were uniformly coated on graphene
nanosheet arrays with open structure (Figure e). Impor-
tantly, the TiNbO-x/graphene nanosheets yielded a large
potential range of -.-.V (vs SCE) in HSO electrolyte
and a high specific capacitance of . F g– at . A g– as
well as superior rate capability (Figuref). Impressively, the
composite exhibited an outstanding electrochemical durability
with no capacitance decay after cycles in a large voltage
window of .V in an acidic electrolyte. This finding proposes
a new view for building wide voltage aqueous symmetric or
asymmetric SCs. Wu and co-workers prepared a layered acid
niobium phosphate (NbOPO·HPO·HO) nanosheet arrays
with sub-nm on conductive carbon fiber cloth in a mild oxalic
Figure 9. a) Schematic illustration for the preparation of HPC/PANI composites. b) Comparison of CV curves of HPC and HPC/PANI electrodes at a
scan rate of mV s–. c) Schematic diagram of the structure of ASC device based on HPC and HPC/PANI electrodes. d) CV curves of HPC/PANI//
HPC ASC measured at dierent potential windows at mV s–. e) Specific capacitance of HPC/PANI//HPC ASC at dierent potential windows.
Reproduced with permission.[] Copyright , Wiley-VCH.
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acid system.[] In NbOPO·HPO·HO crystal, the in-plane
atomic arrangement of layers consists the chains of corner-
shared, slightly distorted [NbO] octahedron and [PO] tetra-
hedron that run parallel to the c-axis. The site assignments of
[NbO] octahedron and [PO] tetrahedron produce alternating
rows, which form layer parallel to the ab plane (Figureg). In
a mild-acid synthetic environment, the NbOPO·HPO·HO
nanosheet can be in situ grown on a flexible carbon fiber cloth
with dense and ultrathin nanosheet array structure (Figureh).
The hybrid exhibited a large potential window of -V (vs Ag/
AgCl) and an outstanding specific capacitance of . F g–
in HCl electrolyte (Figure i). A . V asymmetric cell
was fabricated based on NbOPO·HPO·HO cathode and
carbon anode. The device achieved an excellent energy density
of .Wh kg– and a satisfied cycle stability with .% capac-
itance retention after cycles.
4.2. Typical Anode Materials
With the development of cathode, numerous anode materials
have been discovered and investigated with high HER overpo-
tentials for wide voltage aqueous ASCs.[] Except for the clas-
sical carbon materials, some kinds of wide potential window
and high capacitance anode materials such as FeO,[] VN,[]
InO[] etc. were also built for boosting the energy density of
SCs. In this section, dierent kinds of anode materials for wide
voltage aqueous ASCs are discussed in detail.
4.2.1. Carbons and Derived Composite Electrodes
Carbon-based materials are the most promising and classic
negative electrodes in ASCs due to low specific weight, low cost,
high abundance, nontoxicity, environmentally friendly, and
high electronic conductivity as well as outstanding mechanical
stability.[] Carbon is invaluable and versatile for nearly all
energy storage devices, including capacitors, supercapacitors,
batteries, and hydrogen-storage devices, and so on.[,] Thus,
the twenty-first century is universally regarded as the carbon
age. The capacitive mechanism of carbon materials stems from
surface physical adsorption/desorption.[] Therefore, the spe-
cific surface area and pore sizes are critical to the capacitance
of carbon electrodes. Previously research proved that carbon
materials with pore sizes within . to .nm are available for
adsorption with aqueous electrolytes, while a pore size of about
.nm is appropriate for most of organic electrolytes.[] Zhang
and co-workers fabricated a D porous carbon foam consisting
of multi-size pores including macro-, meso-, and micropores
with a superb surface area of m g–.[] The D carbon
Figure 10. a) Schematic diagram showing the fabrication of H-TiO NTAs. b) SEM image of H-TiO NTAs. c) CV curves of the untreated TiO,
air-TiO, and H-TiO NTAs at mV s–. Reproduced with permission.[] Copyright , American Chemical Society. d) Schematic diagram illus-
trating the growth processes for G, TNOG, and TNOxG on carbon cloth. e) High magnification SEM image of TNOxG. f) CV curves of G, TNOG,
and TNOxG electrodes at mV s–. Reproduced with permission.[] Copyright , Wiley-VCH. g) The local coordination of Nb and P atoms in
the NbOPO·HPO·HO crystal (NPene). h) SEM image of NPene@CFC. i) CV curves of the NPenes@CFC electrode in various electrolytes at
mV s–. Reproduced with permission.[] Copyright , Wiley-VCH.
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foam electrode yielded a high specific capacitance of . F g–
and excellent rate performance. This work proves that the hier-
archical porous structure with rational pore size distribution
can enhance the ecient ion diusion and charge storage for
high-performance SCs. Chen et al. prepared nitrogen-doped
hierarchically porous nanofibers by embedding ZIF- nano-
particles into electrospun polyacrylonitrile nanofibers.[] The
nitrogen-doped porous nanofibers displayed a specific surface
area of . m g– with pore size of to nm and exhibited a
high specific capacitance of . F g– and a remarkable cycling
durability of .% capacitance retention after cycles.
This research exhibits that heteroatom doping combining with
high surface area can boost the capacitive behaviors of carbon
electrodes. Qin et al. fabricated a wide potential carbon fiber
cloth electrode by modified with Na-containing functional
groups.[] Generally, carbon materials can be used as both
cathode and anode with voltage ranges of -.V and -.-V,
respectively. In this work, the Na-containing functional groups
modified carbon fiber fabric (Na-CC) electrode exhibited an
expanded voltage window of .- V (vs SCE) with enhanced
areal specific capacitance of mF cm–. A . V asym-
metric aqueous cell were prepared based on Na-CC as anode
and CC as cathode, and achieved the maximum specific energy
of mWh cm– with a high specific power of mW cm–
and an excellent electrochemical stability of ≈% capacitance
retention after cycles. This work presents a new insight
to build the safe and cost-eective aqueous high-voltage SCs.
Besides, the capacitive performance of carbon material can
be enhanced by designing D arrangement structure with
higher available surface area. Zhang et al. designed a scalable
method to prepare hierarchically aligned porous carbon nano-
tube arrays (PCNTAs) on carbon fibers.[] The PCNTAs have
an average diameter and thickness of about and . nm,
respectively, which exhibited a large specific capacitance of
F g– at A g– in KOH electrolyte. Among the dif-
ferent kinds of carbon-based materials, activated carbons (ACs)
with high specific surface area (≈ m g–), low cost, wide
pore-size distribution (from macropores (>nm) to nanopores
(< nm)) are doubtlessly the first choice in academia and
industry over the past decades.
Carbon-based materials are a big family consisting of D
(carbon onion,[] nanoparticles,[] quantum dots[]), D
(SWCNTs,[] MWCNTs,[] nanowires[]), D (graphene,[]
MXenes,[] nanoflakes[]), and D (pillared graphene,[]
MOF,[] aerogels[]) nanostructures (Figure 11). The advan-
tages and limitations of these materials are listed as follows:
D nanostructure owns small size in all dimensions; no bulk
solid-state diusion; surfaces on all sites are accessible to elec-
trolytes; could be used in stable inks for printing; can be inte-
grated into multiple systems.[] However, they are limited by
agglomeration; poor chemical stability; numerous points of
contact lead to high resistance; do not densify and form only
low-density nonuniform structures.[] D nanostructure ben-
efits from mechanical reliability; porous flexible free-standing
films; the possibility to integrate with wearable devices.[]
Also, they were hindered by low packing density; low yield
and high cost of synthesis; low volumetric performance; rela-
tively long diusion pathways.[] D nanostructure exhibits
advantages of open D channels for ion transport; compatible
with flexible devices; all surface is accessible ensuring fast
charge storage; small nanosheets could be used in inks for
printing.[] However, the limitation of D nanostructure
including re-stacking; high cost of synthesis; low out-of-plane
electronic and ionic conductivity.[] D nanostructure could be
used to create thick electrodes with large areal and volumetric
storage properties, while showcase some limitations of design,
stability, and manufacturing.[] In comparison with electrode
bulk materials, nanomaterials own nanosized structural fea-
tures and high electrochemical active area could change the
paradigm for energy storage with several orders of magnitude
high surface reaction rates, resulting in significantly improved
power density and cycle life.[] Low-dimensional materials
with intrinsic flexibility and strength combined with high ionic
and electronic conductivities enable a solution for flexible,
ultrathin, and structural energy storage. Carbon-based mate-
rials exhibit unique advantages and huge potential for devel-
oping high performance and multifunctional SCs, which will
trigger widely research in future.[,]
4.2.2. Metal Oxides (Iron Oxide) Composite Electrodes
Although carbon-based materials exhibit great potential as
anodes for aqueous ASCs, their low capacitances derived
from the electric double layer mechanism have seriously lim-
ited their energy densities. Thus, ever-increasing research
items have been triggered to develop new anode materials,
which simultaneously own high conductivity, wide voltage
window, and high capacitance.[] Thus, metal oxides, espe-
cially iron oxide (FeO or FeO), which stems from pseudo-
capacitive energy storage mechanism with rapidly reversible
redox reactions, showcase its superiority with high theo-
retical capacitance, large HER overpotential, low cost, and
natural abundance.[] However, their inferior conductivity
(≈– S cm–) and insucient ionic diusion rate lead to low
specific capacitance and power capability. In order to over-
come these issues, plenty of works have been carried out to
synthesize iron oxide-based composites with high conductive
media and construct rational nanostructures with high elec-
trochemical active area and short ion transfer paths. Zhou et
al. prepared a spindle-like a-FeO@C on oxidized carbon
fibers (a-FeO@C/OCNTF) (Figure 12a,b).[] The spindle-
like a-FeO@C shown a specific surface area of . m g–
and a mesoporous (.nm) nature. The a-FeO@C/OCNTF
electrode has a large voltage range of --V (vs Ag/AgCl) in
NaSO electrolyte (Figure c) and shows high areal
and gravimetric specific capacitance of . mF cm– and
F g–, respectively. A .V asymmetric cell was assembled by
Na-MnO/CNTF cathode and a-FeO@C/OCNTF anode, which
displayed a superior areal energy density of . μWh cm–
and a maximum power density of . μW cm–. Liang and
co-workers proposed a phosphine plasma activation method
to enhancing the capacitive performance of FeO nanorods
(FeO-P).[] The dense FeO-P nanorods with about nm
in diameter are vertically aligned on carbon cloth surface
(Figured). From the TEM image, the FeO-P nanorods have
a porous morphology with about .nm thick amorphous layer
on the surface and uniformly distribution of Fe, O, P elements
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(Figure e). The FeO-P hybrid electrode produced a large
areal capacitance of mF cm– at mA cm–, which is much
larger than that of pristine FeO with only mF cm– within
-.-V (vs Ag/AgCl) in NaSO electrolyte (Figure f ).
Xia et al. prepared FeO quantum dots (QDs) (≈nm) grew
on functionalized graphene sheets (FGS) (FeO QDs/FGS) by
one-step thermal decomposition strategy at a low temperature
of °C.[] The FeO QDs/FGS electrode showed a superb
specific capacitance of F g– between --V (vs Ag/AgCl)
in NaSO. An aqueous ASC with a high working voltage of
V was assembled by using MnO/FGS as cathode and FeO
QDs/FGS as anode (Figure g). The asymmetric cell exhib-
ited an outstanding energy density of .Wh kg– with a high
power density of W kg– and a superior cycling durability
(Figureh).
4.2.3. Metal Nitrides Composite Electrodes
Metal nitrides are another type of valuable anode material,
which exhibit outstanding pseudocapacitive characteristics and
electrical conductivity ( S cm–) outperforming metal
oxides for ASCs.[,] Among them, titanium nitride (TiN) is
the most popular anode material thanks to its superb electrical
conductivity and high mechanical stability.[,] However, pre-
vious studies found that TiN electrode suered from dramatic
capacitance loss with only % capacitance retention after only
cycles in alkaline electrolyte.[] The mechanism of insta-
bility for TiN material urgently needs to reveal and provide
guidance to design high-performance TiN-based electrodes.
Lu et al. prepared self-supporting TiN nanowires on carbon
cloth by hydrothermal combining with calcination under NH
Figure 11. Overview of D, D, D, and D carbon and derived nanomaterials. (a) D carbon onion (Reproduced with permission.[] Copyright
, Elsevier), nanoparticles (Reproduced with permission.[] Copyright , Wiley-VCH) and quantum dots (Reproduced with permission.[]
Copyright , Wiley-VCH). b) D SWCNTs, MWCNTs (Reproduced with permission.[] Copyright , American Chemical Society) and nanowires
(Reproduced with permission.[] Copyright , Wiley-VCH). c) D graphene (Reproduced with permission.[] Copyright , Wiley-VCH), MXenes
(Reproduced with permission.[] Copyright , Wiley-VCH) and nanoflakes (Reproduced with permission.[] Copyright , Wiley-VCH). d) D
pillared graphene (Reproduced with permission.[] Copyright , Wiley-VCH), MOF (Reproduced with permission.[] Copyright , Wiley-VCH)
and aerogels (Reproduced with permission.[] Copyright , American Chemical Society).
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atmosphere (Figure 13a).[] The TiN nanowires electrode
exhibited a specific capacitance of F g- within -.- V
(vs Ag/AgCl) in KOH electrolyte (Figureb,c). Moreover, the
capacitance retention of TiN nanowires is strikingly poor with
.% after cycles in KOH electrolyte. This group proved
that the electrochemical instability of TiN electrode in alkaline
electrolyte suers from the irreversible electrochemical oxi-
dation of TiN during the charge/discharge process by X-ray
photoelectron spectroscopy analyses. Most importantly, they
proposed that TiN can work steadily in a poly(vinyl alcohol)/
KOH gel electrolyte, in which the polymer electrolyte will sup-
press the oxidation reaction on the surface of the electrode.
Vanadium nitride (VN) is considered as a great potential
anode material because of its superior theoretical capacitance
( F g–), wide voltage window (-.-V vs Hg/HgO in
KOH electrolyte), reversible and fast redox reaction (VNxOy+
OH–↔ VNxOy||OH–+ VNxOy-OH + e–), and excellent electrical
conductivity (≈ S cm–).[,] However, VN exhibited poor
electrochemical stability due to the formation of vanadium
oxide (VOx) on its surface during electrochemical reaction in
KOH electrolyte.[] In addition, the inecient nanostructure
of previously reported VN-based electrodes leads to a low spe-
cific capacitance.[,] Our group fabricated a hierarchical
hybrid electrode with carbon-coated porous VN nanosheet ver-
tically aligned on carbon walls (CW/p-VN@C) (Figure d).[]
Benefiting from the hierarchically porous structure and high
mass loading with high surface area carbon walls as substrates,
the hybrid electrode exhibited a wide and stably potential range
Figure 12. a) Schematic and b) SEM image of S-α-FeO@C/OCTNF electrode. c) Comparison of CV curves of pristine CNTF, OCNTF, S-α-FeO@C/
CTNF, and S-α-FeO@C/OCTNF electrodes at mV s–. Reproduced with permission.[] Copyright , American Chemical Society. d) SEM image
and digital photograph of FeO nanorods. e) TEM images and EDS element mappings of FeO-P nanorod. f) Comparison of CV curves of FeO
and FeO-P nanorods at mV s–. Reproduced with permission.[] Copyright , Elsevier. g) Schematic illustration of the FGS/FeO and FGS/
MnO electrodes design and the construction of asymmetric supercapacitor. h) CV curves of the FGS/FeO//FGS/MnO asymmetric supercapacitor
at dierent scan rates from to mV s–. Reproduced with permission.[] Copyright , Wiley-VCH.
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of -.-V (vs Ag/AgCl) in NaSO electrolyte (Figuree)
and yielded an outstanding specific capacitance of F g–.
Besides, the hybrid electrode also shown a fantastic cycling
stability with .% capacitance retention after cycles,
due to the thin carbon layer coating on VN surface. A . V
asymmetric cell was fabricated based on CW/p-VN@C anode
and CW/Na.MnO cathode (Figure f), and displayed an
ultrahigh energy density of .Wh kg– and a satisfied cycling
stability (.% retention after cycles), which is much
higher than most of previously reported asymmetric devices
and even comparable to those of organic electrolyte SCs.
Except for TiN and VN, tungsten- and molybdenum-based
nitrides have also been investigated as anode materials for
wide voltage aqueous ASCs.[–] In cubic tungsten oxynitride
(WON), nonmetal atoms (e.g., N, O) occupy interstitial sites
in the metal lattice, resulting in maximizing the valence elec-
trons in the nonmetal atoms. Such a unique crystal structure
ensures WON with numerous advantages such as high con-
ductivity, excellent thermal stability, high hardness, and high
melting point. Besides, in comparison with other metal nitrides
and oxynitrides, the remarkably chemical inertia endows WON
great potential as a durable anode for SCs. Yu and co-workers
synthesized WON nanowires on carbon fibers by nitrogenizing
WO precursor nanowires (Figure g).[] The WON anode
showed a wide potential range of -.- V (vs SCE) in
LiCl with a large volumetric specific capacitance of . F cm–
at .mA cm– and excellent rate performance (Figure h).
Furthermore, an excellent electrochemical stability with %
capacitance retention was achieved after cycles, out-
performing most of previously reported metal nitrides or
oxynitride-based electrodes. In addition, the as-fabricated
MnO//WON ASCs operated a high working voltage of
. V (Figure i), and yielded a maximum energy density of
. mWh cm– and a maximum power density of . W cm–.
4.2.4. Other Anode Materials
In recent years, several anode materials have also been devel-
oped for making up the drawbacks of metal oxides and nitrides-
based materials for further boosting the working voltage
and energy density of aqueous SCs. Chen et al. synthesized
indium oxide (InO) nanowires/SWCNT hybrid films anode
and MnO nanowire/SWNT hybrid films cathode to build an
Figure 13. a) Schematic diagram illustrates the two-step growth process for preparing TiN NWs on carbon cloth substrate. b) CV curves and c) specific
capacitances of TiO and TiO NWs nitridized at various temperatures. Reproduced with permission.[] Copyright , American Chemical Society.
d) Schematic illustration of the synthesis procedure for hierarchical nanosheets/walls structured CC/CW/p-VN@C anode and CC/CW/Na.MnO
cathode. e) CV curves of CC/CW/p-VN@C anode between -. and V at dierent scan rates. f) CV curves of as-assembled aqueous ASC at dierent
scan rates from to mV s–. Reproduced with permission.[] Copyright , Wiley-VCH. g) HRTEM and HAADF-STEM images of a WON
nanowire with corresponding element mappings. h) CV curves of WON electrode at dierent scan rates. i) CV curves of the as-fabricated MnO//WON
ASC at dierent voltage windows at mV s–. Reproduced with permission.[] Copyright , Wiley-VCH.
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ASC (Figure 14a).[] The InO NWs/SWCNT processed a
voltage window of -.- V (vs Ag/AgCl) in NaSO elec-
trolyte (Figureb) and achieved a high specific capacitance of
F g–. A V asymmetric cell yielded an energy density of
. Wh kg–and power density of . kW kg–, respectively
(Figurec). Wang et al. produced a VOx@MoO heterogeneous
composite with a cross-linked nanorods structure by a facile elec-
trochemical method (Figured).[] The chemical environment
and electronic structure of VOx are modified upon MoO coating
and the electrochemical performance is largely improved. Thus,
the VOx@MoO electrode showed an outstanding areal specific
capacitance of mF cm– between -. and -.V (vs SCE) in
LiCl electrolyte (Figuree) and an exciting cycling stability
with % capacitance retained after cycles. A V asym-
metric device was built by using VOx@MoO anode and MnO/
CEG cathode (Figuref), and yielded a superior energy density
of . mWh cm– at a power density of . W cm–. Song
et al. prepared a new molybdenophosphate material as a prom-
ising anode for SCs.[] The molybdenophosphate (MoPO) was
first deposited on exfoliated graphene sheet (EG) and then modi-
fied by an electrochemical activation process (A-MoPO/EG) to
enlarge the lattice and introduce more oxygen vacancies, which
synergistically enhance the charge transfer kinetics (Figureg).
The A-MoPO/EG electrode exhibited an ultrahigh potential range
of -.- V (vs SCE) in KCl electrolyte (Figureh) and a
superb specific capacitance of F g–. Noteworthy, a .V ASC
was fabricated based on A-MoPO/EG anode and MnOx/EG as
cathode (Figurei), which delivered a fantastic energy density of
.Wh kg– at a high power density of W kg–.
5. Advanced Aqueous Electrolytes
The nature of the electrolyte has a significant influence on the
overall electrochemical performance and safety of EES devices.
Especially, electrolyte is one of the significant factors to aect the
operating voltage of the supercapacitor device. Besides, the ionic
conductivity and electrochemical stability of the electrolyte also
Figure 14. a) Schematic diagram of an ASC composed with a MnO nanowire/SWNT cathode and InO nanowire/SWNT anode. b) CV curves of InO
nanowire/SWNT anode at dierent scan rates. c) CV curves of MnO NW/SWNT//InO NW/SWNT device at mV s–. Reproduced with permis-
sion.[] Copyright , American Chemical Society. d) TEM image and EDS mapping of VOx@MoO heterogeneous nanorods. e) CV curves of the
MoO, VOx and heterogeneous VOx@MoO electrodes at mV s–. f) CV profiles of the VOx@MoO/FEG//MnO/CEG ASC device at dierent scan
rates. Reproduced with permission.[] Copyright , Wiley-VCH. g) Fabrication process, HRTEM image (scale bar: nm) and EDS elemental map-
pings of A-MoPO/EG electrodes. (h) CV curves of A-MoPO/EG, MoPO/EG, MoPO/G and A-EG electrodes. i) CV curves of the A-MoPO/EG//MnOx/
EG device at dierent potential windows. Reproduced with permission.[] Copyright , Elsevier.
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can partly determine the rate performance and cycling stability
of the supercapacitor cell. For now, the organic electrolytes are
the first choice for commercial SCs products instead of aqueous
electrolytes due to their wide voltage windows.[,] Neverthe-
less, in recent years, aqueous electrolytes have been extensively
researched and used in the laboratory, and most of the published
literatures about SCs were based on aqueous electrolytes.[,]
This is mainly because of the fact that aqueous electrolytes own
much higher ionic conductivity, higher ionic concentration, and
smaller solvated ions than organic electrolytes.[] Most impor-
tantly, aqueous electrolytes are low cost, easy to prepare, envi-
ronment friendly, and can be easily handled in air, which greatly
simplify the assembly and fabrication procedures.
Table 2 summarizes the sizes of hydrated ions and ionic
conductivity values of dierent kinds of cations and anions in
aqueous electrolytes. It is worth to note that the selection cri-
teria of aqueous electrolytes are not only aect the ionic con-
ductivity but also the specific capacitances. Generally, aqueous
electrolytes have at least one order of magnitude higher ionic
conductivity than those of organic electrolytes and ionic liq-
uids.[] Thus, the aqueous electrolytes exhibit low equiva-
lent series resistance, which is beneficial for achieving higher
power density for SCs. In general, aqueous electrolytes could
be divided into acids (e.g., HSO), alkali (e.g., NaOH), and
neutral electrolytes (e.g., NaSO). Generally, SCs employed
aqueous electrolytes usually can yield higher capacitive perfor-
mance than corresponding nonaqueous electrolytes ones due to
their higher dielectric constant, better electronic conductivity,
and larger accessible surface area for smaller aqueous electro-
lyte ions.[] Unfortunately, the main challenge is their relatively
narrow voltage window caused by the decomposition of water
of .V. Breaking this limitation, the gas evolution will rup-
ture the SC cells, seriously threatening the safety and losing
the performance.[] Besides, the compatibility with current col-
lectors should be attached importance to avoid corrosion when
using acid or alkaline electrolytes. To eectively solve this issue,
numerous eorts have been carried out to extend the poten-
tial ranges of aqueous electrolytes.[] Fortunately, building
ASCs with two dierent type electrodes in neutral electro-
lytes can eectively address this problem. Recent research
had proved that a large voltage window of ≈.-.V could be
achieved for ASCs in neutral aqueous electrolytes, which eec-
tively restrain the decomposition of water.[,,] Such a high
working voltage is owing to OH– ion generation potential and
the high hydrogen evolution overpotential in a neutral aqueous
electrolyte. Theoretically, on the basis of the Nernst equation
(Ered= -.pH), when the pH value increases, the potential
will move to a lower value. In this section, the influence factors
and strategies for improving the electrochemical performance
of aqueous electrolytes will be discussed in detail.
5.1. The pH of Electrolytes
The pH value of electrolyte represents the concentration of H+
and OH– ions in this system, which serves a key role to aect
the kinetics of redox reactions involving H+ and OH–. In this
point, the pH of electrolyte has a greatly influence on electro-
lyte decomposition including HER and OER and some Faradic
redox reactions.[] Thus, the pH of the electrolyte is an essential
factor for both stable potential range of electrolyte and capaci-
tive potential windows of electrodes. For example, carbon-based
SCs, which only exhibit electrical double-layer capacitance, have
been proved to be polarized only to . to .V in acidic or alka-
line electrolytes, while display extended working voltages of .
to .V in neutral electrolytes.[] This fact is mainly due to the
much higher overpotential for HER and OER in neutral electro-
lytes than those in acidic and alkaline electrolytes.[]
MXene, as a new family of D materials, owns hydro-
philic surfaces and metallic conductivity, which are synthe-
sized through selectively etching the A (ΙΙΙA or ΙVA element)
atoms from the MAX precursors in an aqueous acidic fluoride
(Figure 15a).[] The thickness of monolayer MXene is less than
nm, but the lateral dimension can reach tens of microns. It
is predicted that the bare MXene monolayer is metallic and the
electron density is close to Fermi level. The structural advantages
endow MXenes with outstanding electrochemical performance
in SCs.[–] In addition, the MXene-based materials also
exhibited dierent capacitive behaviors in dierent electrolytes.
Zhao et al. prepared sandwich-like MXene/SWCNT composite
electrodes (Figureb).[] The MXene/SWCNT hybrid electrode
exhibited a large potential window of -.-.V (vs Ag/AgCl) in
MgSO electrolyte and yielded a high specific capacitance of
F g– with no degradation after cycles. Hu et al. pre-
pared dierent concentrations HF-etched MXene (TiCTx-M
and TiCTx-M) electrodes with dierent components of the
O functional group (Figurec).[] The as-fabricated TiCTx-
M electrode produced a high specific capacitance of about
F g– within a potential window between -. to -.V
(vs Ag/AgCl) in HSO electrolyte. Besides, Yoon and co-
workers fabricated a flexible and high electrical conductivity
TiCTx/PVA-KOH composite film electrode (Figure d).[]
They proved that the confinement and intercalation of polymer
between the MXene sheets not only enhanced cationic intercala-
tion but also increased flexibility, achieving a superior volumetric
capacitance of ≈ F cm– for the MXene/PVA-KOH hybrid
film within a potential window from -. to -.V (vs Ag/AgCl)
in KOH electrolyte. Furthermore, Hu et al. investigated
the capacitive behaviors of TiCTx in HSO, (NH)SO, and
MgSO electrolytes.[] As shown in Figuree, the TiCTx elec-
trode exhibited a much higher capacitance in HSO electrolyte
than those in its counterparts of (NH)SO and MgSO within a
Table 2. The sizes of hydrated ions and ionic conductivity values of dif-
ferent kinds of cations and anions in aqueous electrolytes.
Ions Hydrated ion
sizes [Å]
Ionic conductivity
[S cm mo–]
Ions Hydrated ion
sizes [Å]
Ionic conductivity
[S cm mo–]
H+. . OH–. .
Li+. . Cl–. .
Na+. . SO– . .
K+. . NO–. .
NH+. . PO– . .
Mg+. . CO– . .
Ca+. . ClO–. .
Ba+. . – – –
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same potential window from -. to -.V (vs Ag/AgCl). The
dramatic dierence in capacitance for the three dierent electro-
lytes with the same anion witnesses that the cation and pH value
play critical roles in charge storage.
In an electrochemical reaction system, the increasing of pH
value could be interpreted that the H+ is consumed or OH– is
generated in the region nearby the interface of electrode and
electrolyte, following with the emerge of nascent hydrogen. But
the pH value will have a negligible change in a strong acidic or
alkaline condition, due to the number of H+ or OH– is excessive,
respectively. On the contrary, in neutral aqueous electrolyte, the
slightly reducing of water will induce a suddenly increasing of
local pH value, owing to the concentration of H+ or OH– is too
low. Therefore, according to the classical Pourbaix behavior, the
change of stable potential will lead to a high overpotential for
HER.[] Similarly, in neutral electrolytes, the upper limit of elec-
trochemical stable voltage window for electrolyte is also expanded
by the concentration overpotential for OER.[,] This eect is
believed to be more profound in porous electrodes, within which
both the electro-migration and ion-diusion would be limited.
Besides, once the applied potential is below the thermodynamic
potential of HER, nascent hydrogen is generated and stored on
the electrode surface through adsorption, chemisorption, and
intercalation. When the applied potential can provide enough
activation energy, the nascent hydrogen will combine with each
other to form H. In neutral condition, the change of pH value
hinders the electrolysis process, which means that hydrogen
adsorption is more favorable than hydrogen evolution, and
hydrogen evolution is decided by the relative magnitude of acti-
vation energy. The hydrogen desorption peak observed on the CV
curve indicates that the hydrogen storage process on the elec-
trode is reversible. Importantly, the increasing hydrogen storage
eciency induced by neutral pH also contributes to the capaci-
tance because of its pseudocapacitance-like nature.
Previous studies have elucidated the significance of pH value
from the perspective of overpotential for electrolyte decomposi-
tion. But in a Faradic redox reaction involving H+ and OH–, the
pH of the electrolyte likely also holds an important role. Thus,
more attentions should be paid to understand and clarify the
influence of electrolyte pH on the overpotential. The probability
of changing the capacitive potential ranges of electrodes by
adjusting the pH value of electrolyte needs further study.
5.2. Redox-Active Additives
Lately, an eective strategy has been developed to further boost
the capacitances of SCs by introducing additional capacitive
Figure 15. MXene-based electrodes in dierent pH electrolytes. a) Structure of MAX phases and the corresponding MXenes. Reproduced with permis-
sion.[] Copyright , Wiley-VCH. b) CV profiles of the TiCTx and TiCTx/CNT composite electrodes at mV s– in MgSO electrolyte. Repro-
duced with permission.[] Copyright , Wiley-VCH. c) CV curves of TiCTx electrodes in HSO electrolyte. Reproduced with permission.[]
Copyright , American Chemical Society. d) CV curves of TiCTx, TiCTx/PDDA, and TiCTx/PVA-KOH films at mV s– in KOH electrolyte.
Reproduced with permission.[] Copyright , Wiley-VCH. e) CV curves and gravimetric capacitances of TiCTx thin films in HSO, (NH)SO,
and MgSO electrolytes at room temperature, respectively. Reproduced with permission.[] Copyright , American Chemical Society.
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contribution from the redox-active electrolytes, in which the
capacitance is not only originated from the electroactive mate-
rials but can also from the Faradaic reactions in electrolytes
(Figure 16a).[] There are three major kinds of redox-active
electrolytes: redox-active aqueous electrolytes, redox-active
nonaqueous electrolytes and redox-active solid electrolytes.
In this section, for aqueous SC application, we will focus on
the redox-active aqueous electrolytes and discuss their electro-
chemical behaviors in detail. Typically, Lota et al. reported an
iodide/iodine redox pair based aqueous electrolyte ( KI) for
carbon electrode, which displayed a superior specific capaci-
tance of F g– with a very narrow positive potential window
(≈.V).[] Besides, the kind of the alkali metal counter-ion
in the iodides also plays an important role on the capacitive
characteristic of the electrode, which the specific capacitance of
the carbon electrode decreases with the reducing van der Waals
radius of alkali metal ( RbI ( F g–) > KI ( F g–) >
NaI ( F g–) > LiI ( F g–), respectively). To improve the
capacitance of full cell, the specific capacitance of anode should
be also considered. Frackowiak et al. assembled an AC-based
cell by using a VOSO for the anode and KI for the
cathode, which was divided by a glassy paper and a Nafions
membrane, respectively.[] The device can yield a maximum
specific capacitance of F g– within a voltage range of V as
well as a high energy density of Wh kg– with a maximum
power density of kW kg–.
Chun and co-workers studied a series of redox pairs and
their functions on the capacitive properties. As shown in
Figure b, the thermodynamic reduction potentials of redox
pairs are schematically illustrated.[] In order to avoid the
rapid self-discharge eect, the negative charge pairs are pre-
ferred for the redox-active additive of cathode. For this purpose,
I–/I–, Br–/Br–, and [Fe(CN)]–/[Fe(CN)]– redox pairs are
chosen to restrain the OER on cathode, resulting in expanding
the upper limit of the available voltage window. In addition,
EV+/EV+, MV+/MV+, HV+/HV+, and BV+/BV+ are expected
to broaden the lower limit of the available voltage window,
because their reduction potentials are near or slightly more
negative than the hydrogen evolution potential. In this work,
a .V symmetric SC was prepared by using active carbon as
electrodes with a mixed electrolyte of KBr/. MVCl.
Hwang and co-authors further broaden the working voltage of
activated carbon-based SCs to .V by adding . [Fe(CN)]–/
[Fe(CN)]– into . NaSO electrolyte.[] The high voltage
Figure 16. a) Schematic showing capacitive and faradaic charge-storage processes. The redox couple used at the positive electrode (which is oxidized
on charging, and reduced on discharge) is labeled as Op/Rp (catholyte), and the couple used at the negative electrode (which is reduced on charging,
and oxidized on discharge) as On/Rn (anolyte). b) Reduction potentials of the couples considered relative to the thermodynamic stability window of
water at neutral pH (white region). The lines colored in red, green, and blue are for couples stable in acidic ( acid), neutral, and basic ( base)
conditions, respectively. benzyl viologen (BV); ethyl viologen (EV); heptyl viologen (HV); methyl viologen (MV); standard calomel electrode (SCE).
Reproduced with permission.[] Copyright , Springer Nature.
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was attributed to the high solvation energy of Na+ and SO–,
and the fast redox kinetic of [Fe(CN)]–/[Fe(CN)]–. But SCs
cannot be charged to .V because of the large leaking current
when the concentration of [Fe(CN)]–/[Fe(CN)]– increased to
. . Thus, the concentration of redox pairs is the key factor
that leads to the shuttle of redox species and the self-discharge
eect. Lee et al. proposed a high-performance EES system with
nanoporous AC as the electrode and a hybrid aqueous electro-
lyte containing vanadyl sulfate (VOSO) and tin sulfate (SnSO)
as the electrolyte.[] In this system, the energy storage origi-
nates from the synergistic eect of EDLC, redox reactions of
tin, and redox reactions of vanadium. In this kind of device,
theoretically, the charge storage mechanisms may come from
several aspects: ) EDLC, ) typical batteries, ) pseudocapaci-
tors, and/or ) electrochemical reactions of soluble redox cou-
ples (redox electrolytes) (Figure 17a). They proved that the large
standard potential dierence between V+/V+ and VO+/VO+
reactions along with the reversible Sn+/Sn phase transition
inside carbon nanopores yielded superior energy. As shown
in Figure b, the system of aqueous VOSO/SnSO redox
electrolyte in . HSO stores maximum energy under the
entire accessible cell voltage range. More importantly, the
VOSO/ SnSO system achieved the highest energy density
and stable voltage window of Wh kg– and .V, respectively,
than others (Figure c). In addition, its electrochemical per-
formance could be optimized by increasing the VOSO/SnSO
ratio. As a result, the . SnSO/ VOSO system exhib-
ited the best performance of .Wh kg–, while only Wh kg–
for the SnSO/ VOSO system (Figured).
What is more, some organic redox mediators have also been
developed for boosting the pseudocapacitive,[] such as hyd-
roquinone (HQ),[] methylene blue (MB),[] indigo carmine
(IC),[] p-phenylenediamine (PPD),[] m-phenylenediamine
(MPD),[] sulfonated polyaniline (SPAni),[] and humic
acids.[] It is worth noting that SCs employed HQ-based
redox-active electrolytes exhibited much faster self-discharge
process. For example, Chen et al. demonstrated that the major
factor for this rapid self-discharge is the migration of the redox-
active electrolyte between the cathode and anode.[]
Except for the contribution of redox-active additives to the
capacitance, it also has a great impact on the operating voltage
of SCs. The working voltage of SCs can be extended by choosing
one-electron redox pairs whose redox potentials are close to
the decomposition potentials of electrolyte. This is due to the
rapid kinetics of HER and OER, which can inhibit the decom-
position of electrolyte. Table 3 summarizes some representative
Figure 17. a) Schematic illustration of various electrochemical energy storage mechanisms. b) CV curves of various redox electrolytes at mV s–.
c) Specific energies were calculated from the CV curves at mV s– (the colored area indicates the maximum stability windows of each system).
d) A full cell is charged up to .V while observing the potential of the negative electrode through an Ag/AgCl reference electrode. Reproduced with
permission.[] Copyright , Elsevier.
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Table 3. Some typical redox couples for dierent kinds of redox active electrolytes and their capacitive performance (Abbreviations: HQ: hydroqui-
none; IC: indigo carmine; MB: methylene blue; SPAni: sulfonated polyaniline; PPD: p-phenylenediamine; MPD: m-phenylenediamine).
Redox mediator Redox reactions Supporting
electrolyte
Capacitance without redox
mediator
Capacitance with
redox mediator
Stability
(cycles)
Refs.
I–/I– HSO. F g-
(mA cm-)
. F g-
(mA cm-)
– []
Br–/Br– HSO. F g-
(mA cm-)
. F g-
(mA cm-)
– []
[Fe(CN)]–/[Fe(CN)]– KOH . F g-
( A g-)
. F g-
( A g-)
.%
()
[]
Cu+/Cu+ HSO. F g-
( A g-)
. F g-
( A g-)
– []
Fe+/Fe+ HSO–. mAh g-
(. A g-)
– []
VO+/VO+ HSO. F g-
(mA cm-)
. F g-
(mA cm-)
.%
()
[]
HQ HSO. F g-
(.mA cm-)
. F g-
(.mA cm-)
– []
IC HSO. F g-
(mA g-)
. F g-
(mA g-)
.%
( )
[]
MB HSO. F g-
(.mA cm-)
. F g-
(.mA cm-)
.%
()
[]
SPAni HSO. F g-
(. A g-)
. F g-
(. A g-)
.%
()
[]
PPD KOH . F g-
( A g-)
. F g-
( A g-)
.%
()
[]
MPD – KOH . F g-
(. A g-)
. F g-
(. A g-)
.%
( )
[]
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redox-active electrolytes with corresponding redox processes,
and their capacitive performance.
5.3. Water-in-Salt Electrolytes
Although the voltage range of aqueous SCs could be enlarged
by adjusting the pH value of electrolyte with higher overpoten-
tials for HER and OER, the electrochemical stability windows
of aqueous electrolytes are still much narrower when compared
to organic electrolytes. Recently, Suo and co-workers reported a
high concentration aqueous electrolyte can expand the working
voltage of SCs to ≈.V due to the formation of an electrode–
electrolyte interphase.[] This highly concentrated aqueous
electrolyte, called “water-in-salt,” was prepared through dis-
solving lithium bis(trifluoromethane sulfonyl)imide (LiTFSI)
with a greatly high concentration (molality > ) in water,
which both the weight and volume of salt outweigh those of
solvent. More importantly, WIS electrolyte with a concentra-
tion of was proved to display a wide and stable potential
window of V via employing stainless steel as working elec-
trodes. In this system, the potentials for both HER and OER
were pushed beyond their thermodynamic potentials. These
favorable features could be attributed to two aspects. First,
the average number of water molecule used to solvate each
ion in a WIS electrolyte is much fewer than that of a normal
salt-in-water electrolyte, leading to the obvious cationic solva-
tion sheath structure. In a WIS electrolyte, the electrochemical
activity of water is remarkably reduced due to the formation of
a strong coordination between the high concentration Li-ion
solvated sheath and water molecules. Besides, the crystal-like
model of WIS electrolyte makes the reduction potential of TFSI
more positive than that of isolated TFSI– and hydrogen evolu-
tion. As a result, much higher voltage window was achieved by
using WIS electrolytes than that of pure water and traditional
aqueous electrolytes.
Due to the incredible physicochemical features with mois-
ture-tolerant, nonflammable, and large electrochemical stability
windows, WIS electrolyte was also shown a great opportunity
for aqueous SCs applications.[,] Nevertheless, the rate
capability of WIS electrolytes-based EES devises, especially
SCs usually stay low due to their intrinsically high viscosity
and low conductivity.[] In addition, using WIS electrolytes
will narrow down EES’s working temperature range caused by
the inevitable salt precipitation at low temperatures.[,] Dou
and co-workers introduced acetonitrile as a co-solvent into a
typical WIS electrolyte to form an “acetonitrile/water in salt”
(AWIS) hybrid electrolyte that exhibits greatly reduced vis-
cosity, enhanced conductivity, and extended applicable temper-
ature range while holding their outstanding physicochemical
characteristics of WIS electrolytes.[] As shown in Figure 18a,
dierent kinds of AWIS electrolytes displayed similarly wide
Figure 18. a) Electrochemical stability windows for various concentrations of AWIS electrolytes determined by CV tests at mV s–, b) CV curves of
a SC coin cell using YP-F carbon electrodes at dierent scan rates in AWIS electrolyte. Reproduced with permission.[] Copyright , Royal
Society of Chemistry. c) Electrochemical stability windows of various NaClO-based solutions determined by CV tests at mV s–. d) CV curves of a SC
coin cell using YP-F carbon electrodes at dierent scan rates in NaClO/(HO).(AN). electrolyte. Reproduced with permission.[] Copyright
, Elsevier.
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stability windows of ≈.V with the WIS electrolyte, dem-
onstrating that the introduction of acetonitrile into the
WIS electrolyte do not have an eect on the electrochemical
stability window. Correspondingly, the symmetric SCs assem-
bled with commercial activated carbon electrodes in the
AWIS electrolyte exhibited a wide operating voltage of . V
with a long cycling life (over cycles) and large tempera-
ture range from - °C to °C (Figure b). Besides, they
also fabricated a water/organic (acetonitrile) hybrid electrolyte
based on a low cost sodium perchlorate (NaClO) salt (NaClO/
(HO)x(AN).), which exhibited an improved ionic conduc-
tivity, wettability, moisture-tolerant and flame-retardant prop-
erties.[] As shown in Figurec, higher salt-to-water molar
ratio corresponds to wider electrochemical stable window of
electrolyte. Specifically, NaClO/(HO).(AN). hybrid
electrolyte exhibited the widest voltage window of ≈.V. The
symmetric SCs assembled with commercial activated carbon
electrodes in the NaClO/(HO).(AN). electrolyte shown
a superb operating voltage of . V and approximately rec-
tangular shape under the increasing of scan rates from to
mV s– (Figured). Furthermore, the .V cell exhibited
outstanding electrochemical stability with % capacitance
retention after cycles at A g– with nearly % Cou-
lombic eciency.
However, to date, still rare investigation was conducted to
extend the applications of WIS electrolytes in aqueous SCs. The
main challenge of using WIS electrolyte in aqueous SCs is the
low ionic conductivity for cutting down the rate performance
and power density of SCs.[,] Thus, more fundamentally
understanding and research for the electrochemical charac-
teristics of electrode materials in WIS electrolyte is still very
appealing, and needs more attractions.
6. Fundamental Device Engineering to Wide
Voltage Aqueous Asymmetric Supercapacitors
The critical issues for boosting the energy and power den-
sity of SCs involve broadening the working voltage window
and increasing the specific capacitances. Constructing an
ASC device can eectively solve this situation.[] The perfor-
mance of ASCs is mainly limited by the electrode materials,
electrolytes, and cell configurations.[] The electrode mate-
rials working for ASCs could be classified by dierent energy
storage mechanisms, such as EDLC and pseudocapacitive
electrodes, which can reversibly work under dierent voltage
windows.[] The energy and power density of ASCs could
be significantly improved by choosing and matching appro-
priate electrodes and building rational device configurations.
The basic device configurations of wide voltage aqueous ASCs
could be distinguished into two types by dierent charge
storage mechanisms of anode and cathode, including pseu-
docapacitive materials//carbon electrodes-based and all pseu-
docapacitive materials electrodes-based ASCs. It should be
noted that the Zn-ion hybrid capacitor, as an emerging EES
type (which generally use capacitive carbon-based materials as
cathodes and battery-type Zn as anodes in specific Zn-ion elec-
trolytes), could also be regarded as one kind of wide voltage
aqueous ASCs due to their wide working voltage beyond .V
in aqueous ZnSO-based electrolytes.[] But, as mentioned
in Section , in this review, we mainly focus on the capaci-
tive ASCs, which both cathode and anode materials exhibit
capacitive features. Thus, in this review, we will not discuss the
aqueous Zn-ion capacitors in detail.
6.1. Pseudocapacitive Materials//Carbon Electrodes-Based
Asymmetric Supercapacitors
Wide voltage ASCs based on pseudocapacitive cathode and
carbon anode in aqueous electrolyte are extensively investigated
thanks to their high energy density and low cost (Figure 19a).
As previously mentioned, the energy storage mechanism of
pseudocapacitive materials is derived from a Faradaic mecha-
nism but whose electrochemical characteristic shows a capac-
itive-like behavior.[] The carbon anode owning a wide voltage
window and moderate specific capacitance is easy to match
with a wide voltage pseudocapacitive cathode, which can
broaden the working voltage of an ASC over .V and achieve
higher energy density than that of a symmetric cell.[–]
Bao and co-workers fabricated an ASCs in NaSO elec-
trolyte with MnO/activated carbon textile (ACT) as cathode
and ACT as anode, respectively (Figure b).[] The MnO/
ACT//ACT device can steadily work in a wide voltage range of
to V (Figurec). The CV curves at various scan rates with
nearly rectangular shape exhibit an ideal capacitive feature and
high stability of the ASC. The MnO/ACT//ACT ASC achieved
a satisfied specific capacitance of F g– at mA cm– and
maximum energy density of . Wh kg–. El-Kady et al.
developed an ASC by laser scribing GO films (LSG) as the
anode and LSG–MnO as the cathode.[] It should be noted
that when the mass in a thin film electrode is negligible for
the total mass of the device, using gravimetric capacitance and
energy density to evaluate its performance is unreasonable. In
this case, it is suggested to evaluate its capacitive performance
with areal capacitance, volumetric capacitance, volumetric
energy, and power densities rather than gravimetric ones. In
such systems, their gravimetric capacitance, energy, and power
densities will be unrealistically high, yet those features will
not linearly increase with the increasing of the electrode thick-
ness. The flexible asymmetric cell processes a wide and stable
working voltage window up to .V in NaSO electrolyte
and exhibits high volumetric energy densities between and
Wh L–, which can store about six times higher capacitance
than those of commercial EDLC SCs. Surprisingly, the LSG-
MnO SC can provide ultrahigh power density of ≈kW L–,
which is times higher than lead acid batteries and
times higher than lithium thin-film batteries. Similarly,
Hwang et al. prepared LSG/RuO electrodes by the direct laser
writing of graphene and RuO nanoparticles.[] The assembled
asymmetric supercapacitor with LSG/RuO cathode and AC
anode exhibited a wide voltage window of to .V with a high
energy density of Wh kg– at a power density of kW kg–
(Figured–f).
Distinguishing from conventional aqueous symmetric
SCs (carbon//carbon electrodes-based and all pseudocapaci-
tive electrodes-based), the pseudocapacitive materials//carbon
electrodes-based aqueous ASCs can perform outstanding
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electrochemical characteristics, including working voltage
window, specific capacitance, and energy density by introducing
pseudocapacitive materials as cathode or anode. Table 4 sum-
marizes the wide voltage aqueous pseudocapacitive materials//
carbon electrodes-based ASCs with the corresponding elec-
trochemical properties of cathode materials, anode materials,
voltage windows, energy densities, power densities, and sta-
bility, respectively. In these systems, the cathodes mainly origi-
nate from wide potential metal oxides (such as MnO, RuO,
VO, etc.) and their composites, while anodes are EDLC-based
materials. The working voltage window and specific energy of
pseudocapacitive materials//carbon electrodes-based aqueous
ASCs can easily reach .V and Wh kg–, respectively. These
systems eectively cover the drawbacks of traditional aqueous
symmetric SCs and open the door to build high-performance
SCs through choosing suitable anode and cathode, and opti-
mizing their nanostructures as well as surface reactivity.
6.2. All Pseudocapacitive Material-Based Asymmetric
Supercapacitors
Another kind of wide voltage aqueous ASC is where both
anode and cathode are pseudocapacitive materials-based elec-
trodes, which process dierent redox reactions (Figure 20a).
Unlike the pseudocapacitive materials//carbon electrode ASCs,
the all pseudocapacitive materials-based ASCs exhibit higher
energy densities derived from higher specific capacitance of
pseudocapacitive materials than that of carbon electrodes.
According to the calculating formula of the total capacitance
(CT) of the ASC:
=+
111
TPN
CC
C ()
where CP and CN are the specific capacitances of the positive
and negative electrodes, respectively.[] Therefore, the electrode
with lower capacitance value plays as a decisive role to the total
capacitance of an ASC. That is the reason of pseudocapacitive
materials//carbon electrode ASCs showing inferior energy
density value. Thus, building an all pseudocapacitive materials-
based ASC system put forward an eective method to greatly
improve the total energy density of SCs. Lee et al. prepared
an ASC using MoO@CNT as anode and MnO@CNT as
cathode with a wide potential range from -. to V and -.
to .V (vs Ag/AgCl), respectively (Figureb).[] The specific
capacitances of MoO@CNT anode and MnO@CNT cathode
are and F g– at mV s–, respectively. As shown in
Figurec, a V ASC was achieved with a high specific capaci-
tance of F g– at . A g– and energy density of .Wh kg–.
Choi and co-workers prepared porous FeO nanoclusters on
graphene (p-FG) and porous MnO nanoparticles on graphene
(p-MG). An aqueous ASC was assembled by p-FG anode and
p-MG cathode (Figure d).[] The device can stably work
within a wide voltage window of V and exhibit the rectangular
CV feature even at a high scan rate of mV s– (Figuree).
A satisfied energy density of . Wh kg– was obtained from
the full cell, outperforming some aqueous sodium-ion batteries
Figure 19. a) The basic configuration and working mechanism of asymmetric supercapacitors assembly by pseudocapacitive materials//carbon elec-
trodes-based asymmetric supercapacitors. b) Schematic of the assembled asymmetric supercapacitor with MnO/ACT cathode and ACT anode, respec-
tively. c) CV curves of the MnO/ACT//ACT asymmetric supercapacitor at dierent scan rates with a potential window of V in NaSO electrolyte.
Reproduced with permission.[] Copyright , Wiley-VCH. d) Schematic of the assembled asymmetric supercapacitor with LSG/RuO cathode and
AC anode. e) CV curves of LSG/RuO cathode and AC anode at mV s–. f) CV curves of the asymmetric supercapacitor at increasing voltage window
from . to .V. Reproduced with permission.[] Copyright , Elsevier.
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and sodium-ion capacitors. Furthermore, the p-FG//p-MG
device delivered excellent electrochemical stability with %
capacitance retention after cycles at A g– and nearly
% Coulombic eciency (Figuref). Additionally, a .V
blue LED can be operated by two p-FG//p-MG devices in series
with ultrafast charging within several seconds, outperforming
those of typical aqueous batteries by about -fold.
Table 5 summarizes the electrochemical features of all pseu-
docapacitive material-based aqueous ASCs. In these cases, the
anodes also process redox reactions, which exhibit wide potential
widow and high specific capacitance than that of EDLC mate-
rials. Thus, the pseudocapacitive anode can match well with
the pseudocapacitive cathode according to the charge matching
principles mentioned in Section . With suitable anode and
cathode, the new aqueous ASC systems can achieve high oper-
ating voltage range over .V and ultrahigh energy density up to
Wh kg–, which are comparable to and even outperforming
those of organic SCs and other energy storage devices.[,]
7. Strategies for Achieving High-Performance Wide
Voltage Aqueous Asymmetric Supercapacitors
The electrochemical properties of a SC could be evaluated by
numerous key parameters, including the operating voltage, spe-
cific capacitance, rate performance, electrochemical stability,
time constant, equivalent series resistance, power density, and
energy density.[] Various materials have been developed to
meet these features for high-performance SCs. To build an ideal
SC device, the electrode materials should meet high charge–
discharge rate and long cycling stability while holding out-
standing charge storage characteristics.[] To eectively optimize
Table 4. Comparison of cathodes, anodes, voltage windows, energy densities, power densities, and stability for pseudocapacitive materials//carbon
electrodes-based wide voltage aqueous asymmetric supercapacitors (Abbreviations: CNTs: carbon nanotubes; AC: activated carbon; CNFs: carbon
nanofibers; GHCS: graphitic hollow carbon spheres; aMEGO: activated microwave expanded graphite oxide; p-BC: bacterial cellulose pellicles; GNR:
graphene nanoribbon; NCs: N-rich carbon nanosheets; rGO: reduced graphene oxide; LSG: laser-scribed graphene; DGS: densely stacked graphene;
ERPC: electrochemically reduced porous carbon; GQD: graphene quantum dot).
Cathode Anode Voltage window Energy density Power density Stability (cycles) Refs.
MnO/CNTs/graphene CNTs/AC .V ( NaSO).Wh kg-.kW kg-.% () []
MnOAC .V ( KNO).Wh kg-.kW kg-– []
MnOCNTs .V ( NaSO).Wh kg-. W kg-.% () []
MnOGraphene hydrogel .V (. NaSO).Wh kg-.kW kg-.% () []
MnOAC .V (. KSO).Wh kg-.kW kg-.% ( ) []
VO·.HO AC .V (. KSO).Wh kg-.kW kg-.% () []
NaMnOAC .V (. NaSO).Wh kg-. W kg-.% ( ) []
MnO/graphene Activated CNFs .V ( NaSO).Wh kg-. W kg-.% () []
MoOAC .V (. LiSO).Wh kg-. W kg-– []
GHCS–MnOGHCS .V ( KSO).Wh kg-.kW kg-.% () []
MnO/AC AC .V (. KSO).Wh kg-.kW kg-.% () []
aMEGO/MnOaMEGO .V ( HSO).Wh kg-.kW kg-.% () []
p-BC@MnOp-BC/N .V ( NaSO).Wh kg-.kW kg-.% () []
MnO/CNF Activated CNF .V (. NaSO).Wh kg-.kW kg-.% () []
MnOGraphene .V ( NaSO).Wh kg-. W kg-.% () []
Graphene/MnOGraphene/Ag .V ( NaSO).Wh kg-. W kg-– []
D MnO/graphene Graphene .V (. NaSO). mWh cm-–.% ( ) []
Carbon/MnOAC .V ( NaSO).Wh kg-. W kg-.% () []
CoMnO/GNR GNR .V (. NaSO).Wh kg-.kW kg-.% () []
MnO@NCs AC .V ( NaSO).Wh kg-. W kg-.% () []
MnO/C AC .V ( LiNO).Wh kg-. W kg-.% () []
MnO/C nanobox N-doped carbon .V ( NaSO).Wh kg-. W kg-.% () []
a-MnOx/rGO–CNT AC .V (. NaSO).Wh kg-. W kg-– []
LSG/RuOAC .V ( HSO).Wh kg-.kW kg-.% () []
T-NbOAC .V ( NaSO).Wh kg-. W kg-.% () []
Graphene-MnODSG .V ( NaSO).Wh L-. W L-.% () []
Na.MnOERPC .V ( NaSO).Wh kg-.kW kg-.% ( ) []
GQD/MnON-doped graphene .V ( NaSO).Wh kg-. W kg-.% ( ) []
rGO hydrogel FeCO/rGO hydrogel .V ( NaSO).Wh kg-. W kg-.% () []
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Figure 20. a) The basic configuration and working mechanism of asymmetric supercapacitors assembled by all pseudocapacitive materials-based
electrodes. b) Schematic illustration and (c) CV curves of MoO@CNT//MnO@CNT aqueous asymmetric supercapacitors. Reproduced with per-
mission.[] Copyright , Elsevier. d) Schematic illustration of an asymmetric supercapacitor with p-MG cathode and p-FG anode. e) CV curves of
the p-MG//p-FG asymmetric supercapacitor at various scan rates in a voltage window of V. f) Cycling stability of the asymmetric supercapacitor at
A g– and the inset image is blue LED light powered by two asymmetric supercapacitor coin cells, charged by rapid USB charger for s. Reproduced
with permission.[] Copyright , Wiley-VCH.
Table 5. Comparison of cathodes, anodes, voltage windows, energy densities, power densities, and stability for all pseudocapacitive materials-based
wide voltage aqueous asymmetric supercapacitors.
Cathode Anode Voltage window Energy density Power density Stability (cycles) Refs.
MnO/SWNTs InO/SWNTs .V ( NaSO).Wh kg-.kW kg-– []
MnO/graphene MoO/graphene .V ( NaSO).Wh kg-. W kg-– []
MnO/CNF BiO/CNF .V ( NaSO).Wh kg-. W kg-.% () []
MnO/FGS FeO/FGS .V ( NaSO).Wh kg-. W kg-.% () []
MnO/GNS FeOOH/GNS/CNTs .V ( LiSO).Wh kg-. W kg-.% () []
MnHCF FeO/rGO .V (. NaSO).Wh kg-. W kg-.% () []
MnO-PPy VO/PANI .V ( LiCl) . mWh cm-.mW cm-– []
NaxMnO/CC MoO/CC .V ( NaSO). mWh cm-.mW cm-.% () []
MnO/carbon aerogel FeO/carbon aerogel .V ( NaSO).Wh kg-. W kg-.% () []
MnOFeO/PPy .V (. NaSO).Wh kg-.kW kg-.% () []
Na.MnOFeO@C .V ( NaSO).Wh kg-. W kg-.% ( ) []
CNF/MnOCNF/MoO.V ( NaSO). μWh cm-.mW cm-.% () []
rGO/MnOrGO/VO.V ( NaSO).Wh kg-.kW kg-.% ( ) []
MnO/CEG VOx@MoO/FEG .V ( LiCl) . mWh cm-. W cm-.% () []
MnOHydrogenated TiO.V (. NaSO).Wh kg-. W kg-.% ( ) []
CNTs/MnOMoO/PPy .V ( NaSO).Wh kg-.kW kg-.% ( ) []
Na.MnOp-VN@C .V ( NaSO).Wh kg-. W kg-.% ( ) []
MnOx/EG A-MoPO/EG .V ( KCl) .Wh kg-. W kg-– []
MnOSnO-x@SnO-x.V ( LiCl) .Wh kg-.kW kg-.% () []
MnO/SWNTs InO/SWNTs .V ( NaSO).Wh kg-.kW kg-– []
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these targets, various strategies have been developed and used.
In this section, the strategies for achieving high-performance
wide voltage aqueous ASCs are discussed in detail as described
below: ) Broadening the operating voltage window, which can
straightforwardly improve the energy density of a SC device as
“fuel” according to the key equation E= / CV. ) Improving
the specific capacitance, which is the most popular strategy to
enhance the energy density of SCs as “engine” by improve the
specific capacitance of electrodes. ) Enhancing the rate perfor-
mance, which holds a key factor to ensure the advantage of SCs
as “wheel” with fast charge and discharge capacity at high cur-
rent. ) Boosting the electrochemical stability, which guarantees
the service life of a SC device as “insurance” for a long life. An
overview of the various strategies for achieving high-performance
wide voltage aqueous ASCs is provided in Figure 21.
7.1. Broadening the Operating Voltage Window
7.1.1. The Influence Factors of Cell Voltage Window
According to the key equation E= / CV, the operating voltage
window plays a determinable role to the energy density of a SC
like the “fuel” of a car. As discussed above, the electrode mate-
rial, electrolyte, and device type are three major factors to aect
the working voltage of an aqueous SC, resulting in judging
its energy density. Therefore, in this section, the strategies for
broadening the operating voltage window of aqueous SCs will
be discussed in detail. The relationship of the voltage window
for an aqueous ASC between electrodes and electrolyte is sche-
matically illustrated in Figure 22.[] The voltage window of full
SC cell is theoretically calculated by the following formula:[]
=−
cell positive negative
EE
E ()
Therefore, the full cell voltage depends on the stable poten-
tial windows of cathode and anode, which process reversible
charge–discharge procedure with stable nanostructure, chem-
ical composition, and electrochemical behaviors. If the cell
was charged over its capacitive voltage window, the electrode
materials will be overoxidized or overreduced with the result
of dramatic loss of its capacitive performance.[] Thus, before
charging a SC, the upper potential limit of the cathode and the
lower potential limit of the anode should be precisely evaluated.
Except for electrodes, the stably potential window of the aqueous
electrolyte is also important for the aqueous asymmetric cell.
As mentioned in Section , the energy gap between HER
(also called the lowest unoccupied molecular orbital, LUMO)
and OER (also called the highest occupied molecular orbital,
HOMO) is regarded as the thermodynamically stable window
of an aqueous electrolyte.[] As we all know, the cathode and
anode own their unique electrode potentials, which correspond
to their Fermi energies.[] In a Faradaic reaction, the energy
levels at or near the Fermi level of cathode and anode should
be matched with a suitable vacant (LUMO) or occupied orbital
(HOMO) in the electrolyte system. If the applied potential of
cathode is higher than HOMO of electrolyte, a passivation
layer will usually form to prevent the oxidation of electrolyte
by delaying the transportation of electron from the HOMO of
the electrolyte to the cathode. Similarly, if the applied potential
of anode is lower than LUMO of electrolyte, a passivation layer
will usually form to prevent the reduction of electrolyte by hin-
dering the transportation of electron from the anode to LUMO
of the electrolyte. That is one reason for the working voltage of
aqueous SCs can extend over the thermodynamic steady voltage
window of water (.V) in a practical electrochemical system.
In other words, once the formation of a passivation layer at the
interface of electrode and electrolyte, a kinetic stability will be
provided to broaden the working voltage range of the full cell.
For instance, the hydrous RuO,[] Na.MnO[] and MoPO[]
exhibit stable electrochemical behaviors at potential windows of
.-.V (vs RHE), -.V (vs SCE) and -.-V (vs SCE),
respectively, which break the limitation of the decomposition
potential of pure water. Besides, such a high potential is owing
Figure 21. Strategies for achieving high-performance wide voltage aqueous asymmetric supercapacitors.
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to OH– ion generation potential and the high hydrogen evolu-
tion overpotential in a neutral aqueous electrolyte. Theoretically,
on the basis of the Nernst equation (Ered= -.pH), when
the pH value increases, the potential will move to a lower value.
7.1.2. Adjusting Zero Voltage Potential
Another key parameter to aect the potential range of elec-
trodes is zero voltage potential (P V), which represents the
potential position of cathode and anode when their potential
contributions are equal in the system. Previous research shows
the P V takes a momentous responsibility on the cell voltage
and Coulombic eciency of a SC. In other words, when one
of the electrodes reaches its limit potential, the voltage window
of the cell will touch its maximum. Once one of the electrodes
crosses its limited potential position, the electrolyte will decom-
pose in the system. Therefore, adjusting the P V and potential
of both electrodes is critical for broadening the working voltage
of SCs. As shown in Figure 23a, Yu and co-workers presented a
controllable approach to tune the potential window of the elec-
trode, named surface charge control strategy, which can simul-
taneously increase the operating range and energy density of
SCs.[] The multiscale porous carbon (MSPC) showed a stably
wide voltage range of -. to .V (Figureb). Theoretically,
the working voltage range of the cell can reach .V. However,
when the cell was charged to . V, the negative potential is
only .V (vs SCE) while the positive potential has reached the
upper limit potential (Figurec). As discussed above, the cell
voltage is trapped in .V by cathode, resulting in an available
potential of .V is wasted. They pointed out that P V is related
to the open-circuit potentials (OCP) of anode and cathode.
According to their research, the P V can be tuned down by pre-
charging cathode with a low potential below its OCP. As shown
in Figured, the potential window between upper limit poten-
tial and lower limit potential was fully used by pre-charging
the cathode with .V (vs SCE), leading to the practical voltage
range successfully extended from . to .V. As a result, the
optimized MSPC-based SCs achieved a high energy density of
. mWh cm–, which is two-fold high than the original one.
7.1.3. Charge Balance Between Cathode and Anode
As mentioned in Section, the device structure also shows its
important status in voltage window and energy density of a SC.
As illustrated in Figure 24a, for a SC cell, the voltages of anode
and cathode expand in opposite directions during charging until
two electrodes achieve nearly the same charge (Q).[] Therefore,
for an ASC, the value of Q between anode and cathode should
be balanced before charging by evaluating their potentials (U),
specific capacitance (C), and mass loading (m).[] Specifically,
if the specific capacitance of one electrode is lower than that of
the other, this electrode will process higher potential to balance
the charge between them. For a symmetric device assembled
by the same electrode materials with the same mass loading,
the stable voltage range only exhibits a narrow voltage window
of a single electrode.[] Besides, the PV of a symmetric device
can be easily detected, which is approximately regarded as
the OCP of the electrode. Limited by the working voltage, the
energy density of a symmetric SC always stays at a low level.[]
Thus, building an ASC was regarded as one eective method to
Figure 22. Schematic illustration showing the influence factors of device voltage window. Reproduced with permission.[] Copyright , Wiley-VCH.
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cover the drawbacks of a symmetric cell. Taking the advantages
of dierent anode and cathode, the practical voltage window of
an ASC can break the limitation of the decomposition potential
of water, even up to .V. The ASC can achieve much larger
energy density than that of a symmetric SC in an aqueous elec-
trolyte with the same weight, area, or volume.
7.1.4. Optimizing Electrode Materials
Another promising strategy to broaden the voltage range of
ASC is optimizing electrode materials by combining with
pesudocapacitive materials whose redox potentials are close to
the decomposition potentials of electrolytes. The Faradic reac-
tions could preferentially carry on than the decomposition of
aqueous electrolyte when the Faradic redox reactions of elec-
trode is faster than HER or OER of electrolyte. The operating
voltage can be evaluated from the following equation:[]
()
p0 12
A
12
EE
EE N
F
EE
ωω
=+∆+∆= −+∆ +∆
βα
()
where ΔE and ΔE are the potential ranges of cathode and anode,
ωα and ωα represent their work functions, and NA is Avogadro’s
Figure 23. Adjusting zero voltage potential (P V): a) Illustration of precharging strategy to expand the operating voltage of the symmetric SCs. b) CV
curve of the MSPC electrode tested in three-electrode system at mV s–. c) The potential variation of the anode and the cathode when the assembled
symmetric SC was charged/discharged at mV s– with a voltage of .V. d) The potential variation of the anode and the cathode when the symmetric
SCs with the cathode pre-charged with .V versus SCE was charged/discharged at mV s– with a voltage of .V. Reproduced with permission.[]
Copyright , Wiley-VCH.
Figure 24. Charge balance between anode and cathode: a) Illustration of charge balance strategy to increase the operating voltage of ASCs. Reproduced
with permission.[] Copyright , Wiley-VCH. Optimizing the electrode material: b) EDX element mapping images of the electro-reduced porous
carbon. c) CV curves of electrochemically reduced porous carbon at various scan rates in a three-electrode configuration with NaSO electrolyte.
Reproduced with permission.[] Copyright , Wiley-VCH.
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number. Therefore, the cathode and anode with the biggest gap
in work functions can exhibit the widest voltage range in an
ASC cell.[] The work function of metal oxides is directly related
to electrochemical oxidation/reduction potential. Thus, the work
function can be eectively adjusted by modifying the redox
potential to improve the potential range of the electrode. For
metal oxides with higher work function, oxygen vacancy defects
are easier to reduce the work function of the oxide, because they
act as n-type dopants to make the Fermi level closer to the edge
of the conduction band. There are various methods to produce
oxygen vacancy defects, such as vacuum annealing, thermal
treatment in a reducing gas (e.g., H or NH), treatment with
reductively chemical agents (e.g., NaBH or hydrazine), etc.
Besides, using cations (e.g., Na+, Li+, K+) to pre-intercalate metal
oxides and carbon materials also an eective method to improve
their potential windows.[,,] Recently, some conventional
electrode materials exhibit greatly potential for higher electro-
chemical window after modification. As shown in Figureb,c,
Xiong et al. reported a Na+ functionalized porous carbon mate-
rial with extended potential range of -.-V (vs SCE) in
NaSO by an electroreduction technique.[] The Na+ was
adsorbed on the carbon electrode, which acts as a physical bar-
rier to retard the HER reaction. A . V aqueous asymmetric
device was built via using Na.MnO as cathode and the Na+
functionalized porous carbon as anode. The cell can achieve a
superior energy density of .Wh kg– with a high power den-
sity of W kg–, which showcases an exciting strategy for
developing high-performance SCs.
7.2. Improving the Specific Capacitance
In the past decades, improving the capacitance of SC is
the most popular strategy to enhance its energy density by
improving the specific capacitance of electrode materials.[–]
The capacitance of electrode materials serves as the “engine” for
driving a high-performance SC device. As mentioned above, in
a SC system, the charges are theoretically stored at the surface/
near surface on electroactive materials. Thus, the specific sur-
face area (SSA) is the most concerned and investigated element
for most of researches.[,] Generally, the electrode materials
with larger exposed SSA are expected to yield a higher specific
capacitance, because of the more materials availability for both
carbonaceous and pseudocapacitive materials. Nanomaterials
exhibit unique advantages for SCs applications when compared
with their bulk materials, especially, nanomaterials own ultra-
high surface area to volume ratio, resulting in higher utiliza-
tion rate of electroactive materials with the electrolyte for the
formation of electric double layer and/or reversible Faradaic
reactions.[] Early strategies to improve the SSA are focused
on activation and/or etching the carbon-based materials, and
then synthesized dierent nanostructured carbonaceous and
pseudocapacitive materials with D (carbon onion, nanopar-
ticle, quantum dot), D (nanowires, nanotubes, nanorods), D
(nanoflakes, nanosheets, nanowalls), D (carbon frameworks,
graphene aerogels, graphene foam, D nanofibers, graphene
microlattices, etc.) morphologies as well as hollow, porous,
core-shell, hierarchical structures for high SSA, tunable pore
sizes, and pore size distributions.[,] With the development
of new charge storage mechanism, researchers discovered that
creating rapid transmission channel for electrons and ions is
necessary for further improving the specific capacitance, espe-
cially for the pseudocapacitive materials.[,] Thus, the con-
struction of composites with the pseudocapacitive materials and
highly conductive substrates (CNT, graphene, carbon nanofiber,
etc.) has been widely studied with lower resistance and higher
specific capacitance than those of pure pseudocapacitive mate-
rials.[–] Besides, although the powdery nanomaterials are
easy to synthesize in a large scale, the conductive additives
and binders are essential for fabricating electrodes for SCs by
a traditional slurry-casting method. In this way, the availability
of active material is limited and some “dead volume” will gen-
erate, resulting in poor permeation of electrolyte for inferior
capacitive performance.[] In addition, the high capacitances
of composites are usually received only in ultrathin electrodes
with low mass loadings (≤mg cm–) and are hard to achieve
for commercial electrodes with higher mass loadings (>mg
cm–). In order to solve these awkward situations, self-standing
hierarchical D structure electrodes have been developed for
carbon-based electrodes (graphene film, carbon nanofiber film,
metal foam, etc.) and pseudocapacitive materials-based elec-
trodes with active materials growth on a self-standing conduc-
tive substrate, which simultaneously serve as the supporters
and current collectors.[–] In contrast to powdery nanoma-
terials, the binder-free electrodes provide unique benefits with
strengthened charge/ion transmission eciency and amelio-
rated utilization of electroactive materials for better capacitive
behaviors. Over the past few years, many noteworthy achieve-
ments in SCs have been realized by rationally designed and fab-
ricated homogeneous and/or heterogeneous D free-standing
electrodes thanks to a continuous conductive network for elec-
tron transport and a fully interconnected hierarchical porosity
for ion diusion.[–]
Except for increasing SSA and designing various nanostruc-
tures, other strategies, like doping, functionalization, surface
modification, creating defects and vacancies have been devel-
oped to boost the electrochemical activity of electrode mate-
rials for ecient capacitive performances.[] Moreover, the
crystal structure of nanomaterial is also an important factor
to aect its capacitive behavior.[] But in a SC system, it is
proved that the regulation of morphology and microstructure
is relatively more eective to improve the specific capacitance
than that of crystal structure. Furthermore, the discovery and
creation of new types of electroactive materials with adjustable
architectures are essential to build high-performance SCs with
improved capacitive performance.[,,] In this section, we
will specifically discuss the strategies to improve the specific
capacitance of electrode for achieving high-performance SCs.
It should be noted that these strategies could also simultane-
ously enhance rate capability and electrochemical stability as
well.
7.2.1. Specific Surface Area and Pore Structure
As we all know, pore structure holds a key influence on SSA
resulting in aecting the specific capacitance especially for
nanostructured electrode materials. In , the dierent kinds
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of pores were classified into macropores (>nm), mesopores
(between and nm), and micropores (<nm) by the Inter-
national Union of Pure and Applied Chemistry (IUPAC). With
the development of nanomaterials and nanotechnologies, in
, the IUPAC defined three new kinds of pores: nanopores
(<nm), supermicropores (between . and nm), and ultra-
micropores (<.nm). According to the new definitions, nano-
pores contain micropores, mesopores, and macropores with an
upper limit of pore width of nm, while micropores include
two subtypes of supermicropores and ultramicropores.[]
SCs have the remarkable ability of storing and releasing
energy rapidly. The critical issue of SCs is to increase the
number of charges that could be stored during fast charge/
discharge process. The capacitance of EDLC is directly propor-
tional to the accessible electrode surface area with the electro-
lyte, thus increasing SSA and porosity is a promising approach
to boost the EDLC capacitance. For example, Li et al. pre-
pared mesopores carbon nanofibers (CNFs) by a self-template
strategy.[] The CNFs displayed a high SSA of m g–, a D
interconnected pore texture, numerous micropores (.nm), a
large mesopores size of . and to nm. The mesoporous
CNFs yielded a large specific capacitance of F g– at . A g–,
which is higher than F g– for commercial activated carbons
(with a pore diameter of .nm, a surface area of ≈ m g–
and the micropore surface area of ≈ m g–). Wang et al. syn-
thesized a flower-like and hierarchically porous carbon mate-
rial (FHPC) with a high SSA of . m g– and delivered a
high specific capacitance of F g– at mV s–, which is
much higher than F g– for flower-like porous carbon mate-
rial (FPC) electrode with a SSA of . m g–.[] However,
in most cases, the improvement of capacitance by creating
micropores cannot maintain under high current density. For
instance, Li et al. prepared a hierarchically porous and hollow
carbon textile with a high specific capacitance of F g– at
. A g–, while only F g– was retained at A g–.[] Xiao
et al. fabricated a porous coaxial carbon nanofiber with large
SSA of m g– and exhibited a high specific capacitance of
F g– at A g–, but the capacitance decreased rapidly to
F g– with the increasing of current density to A g–.[]
The capacitance loss at a high rate may cause by the sluggish
ion transmission within micropores, which is dicult to catch
up under the rapid charge/discharge.
In , Chmiola et al. proposed that the pore size below
nm in carbon can provide an unusual capacitance, and dem-
onstrated that the highest specific capacitance will be obtained
when the pore size is close to the ion size.[] The results show
that reducing the pore size distribution could boost the energy
density. Subsequently, Kondrat and co-workers confirmed this
deduction.[] Based on a classical density functional theory,
they predicted that when an ionic liquid electrolyte is located
in a nanopore, the capacitance will decrease with the increase
of the pore size. This phenomenon is related to the utilization
of the inner pore surface when the diameter is comparable to
the size of electrolyte ions. Recently researches indicate that
the volume of micropore determines the specific capacitance of
activated carbons rather than the volume of mesopores.[,] It
is suggested that small pore with size around or below nm can
provide high specific capacitance, while large pore can boost
the diusion of ion leading to higher rate performance.[,]
Merlet et al. proved that the capacitance will increase when the
size of pores in nanoporous carbon is smaller than ion size
by removal (or partially) of solvent shell.[] Therefore, better
capacitive performance with high capacitance and rate capa-
bility may be achieved when carbon-based materials own hierar-
chically porous structure with micro-, meso-, and macropores.
In , Black and Andreas employed a transmission
line model to simulate the charge accumulation behavior of
dierent pore sizes and confirmed that the electrode with
multiscale pores is favorable for charge accumulation.[]
Then, Ervin proved that the formation of multiscale pores can
boost ion transmission in a thick graphene film.[] Xu and
co-workers proposed that ordered pore size distribution can
reduce structural tortuosity, resulting in strengthening ion
transport based on the following equations:[]
ε
=
eff
Dk
D
()
τ
=
2
eff
kL
D
()
where De is the diusion coecient of a fluid diuses through
a porous structure, D is the diusion coecient in pores, k is
the porosity, ε is the tortuosity, τ is the diusion time, L is the
ion diusion length. Thanks to the well-aligned transport chan-
nels, ordered porous structures exhibit much smaller ε values
than randomly distributed pores structures. Consequently,
ordered porous structures have much small τ value, indicating
the ion diusion in ordered channels is faster than their sto-
chastically porous networks. These theoretical and experimental
results proved that the electrode materials owned multiple-scale
pores are expected to yield high capacitance even at an ultra-
high current density. For example, Xu et al. synthesized hier-
archically hollow carbon nanospheres with ultrahigh SSA of
m g– and yielded a satisfied specific capacitance of F
g– at mV s– and retained F g– even at a high scan rate of
mV s– with a capacitance retention ration of %.[] Zhang
et al. demonstrated that the hierarchically porous structure can
allow ecient charge transfer and ion diusion, leading to a
high specific capacitance even at an ultrahigh current density
(Figure 25a).[] The hierarchically porous carbon foam is taken
an ultra-high SSA of m g– with pore size covering from
subnanometer to submicron and achieved a superior specific
capacitance of . F g– at A g– as well as an outstanding
capacitance retention rate of % at an ultrahigh current den-
sity of A g– (Figureb). In addition, Fei and co-workers
synthesized ordered porous carbon materials with tailored
pore sizes from to nm through using bottlebrush block
copolymers as templates (Figure c).[] The hysteresis loop
at . < p/p<.represents the existence of small mesopores
due to the template degradation and gas evolution during the
pyrolysis (Figured). The PC- and PC- exhibited high SSAs
of and m g–, respectively. These hierarchical pores are
beneficial for improving the interconnectivity of the structure
and increasing the total SSA of the porous carbon. As a result,
the specific capacitance of the PC- electrode can reach F
g– at A g– and still retained F g– even at a high current
density of A g– (Figuree).
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Except for carbon-based materials, SSA also has an impor-
tant influence for the pseudo-capacitor materials. For example,
the hierarchically porous MnO spheres with high SSA of
m g– can achieve a high specific capacitance of F g–,
while only F g– for MnO nanoneedles with much smaller SSA
of m g–.[] Salle et al. prepared dierent structured MnO
and evaluated the relationship between the electrochemical and
physicochemical characteristics.[] When the SSA stays at a low
level, the specific capacitance evidently rises with the increasing
of SSA. Yet, when the SSA is beyond m g–, the SSA exhibits
poor influence on the specific capacitance. Meanwhile, the porous
microstructure of the material can provide more diusion chan-
nels for the electrolyte ions, resulting in improved electrochemical
polarization and decreased dissolution of MnO.[] In addition,
the porous structure can eectively relieve the internal stress
under rapid charge/discharge, leading to better physical and elec-
trochemical stability.[] Thus, it is important to adjust the mor-
phology and microstructure of active materials and the nanostruc-
ture of electrodes. Moreover, it is worth noting that the contact
resistance between the structural unit in nanostructured hybrid
electrodes is usually high because of the large contact interfaces.
Thus, more attention should be paid to build eective channels
for ions and electrons transport to achieve better SCs cells.[,]
7.2.2. Nanostructure Tailoring
Dierent kinds of nanomaterials including carbon-based mate-
rials, pseudocapacitive materials and their hybrid materials with
various nanostructure, such as D (nanoparticles, nano-onions,
quantum dots), D (nanowires, nanotubes, nanofibers), D
(nanoflakes, nanosheets), D (aerogels, hierarchical structures,
various complex nanostructures) have been widely explored for
SCs.[] The main purpose of designing electroactive materials
into various nanostructures is to maximize their active sur-
face area for increasing the utilization rate. At the same time,
the nanostructure and micromorphology also have an impor-
tant influence on the diusion of electrons and ions.[,] As
an example, Peng et al. synthesized dierent structured CoS
spheres via a hydrothermal method by using soft templates
(Figure 26a).[] They demonstrated that the interior structures
of CoS spheres could be tuned with solid, yolk–shell, double-
shell and hollow structures through adding dierent amounts
of CS (Figureb). It is found that the hollow CoS spheres
own a mesopore structure and the highest surface area, which
enhance the redox reaction kinetics and improve the mass
transfer of electrolytes. As a result, the hollow CoS spheres
showcase superior capacitive performance than its counterparts
(Figurec). Liu et al. demonstrated that in situ corundum-to-
rutile phase transformation in electron-correlated vanadium
sesquioxide can obtain nonstoichiometric rutile vanadium
dioxide layers that are composed of highly sodium ion acces-
sible oxygen-deficiency quasi-hexagonal tunnels sandwiched
between conductive rutile slabs.[] The special structure can
strengthen the redox and intercalation kinetics for high pseudo-
capacitive energy storage in hierarchically isomeric vanadium
oxides (c-VO/r-VO-x), resulting in a satisfied specific capaci-
tance of ≈ F g– (almost six times higher than those of the
Figure 25. a) Schematic illustration of the synthesis of CF-MSP and corresponding SEM images. b) Gravimetric capacitance of CF-MSP electrode
measured at dierent current densities. Reproduced with permission.[] Copyright , American Chemical Society. c) Schematic illustration of the
additive-driven assembly of BBCPs and subsequent carbonization leading to nanoporous carbon. d) Nitrogen absorption/desorption isotherms of
PC- and PC- obtained at K. e) Gravimetric capacitance of PC- and PC- measured at dierent current densities. Reproduced with permission.[]
Copyright , American Chemical Society.
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pristine dioxide (r-VO) and vanadium sesquioxide (c-VO)) at
mV s–. More importantly, it still can retain F g– with a
high scan rate of mV s–, which is times higher than
the values of the r-VO ( F g–). Besides, the c-VO/r-VO-x
electrode also exhibited an excellent cycling durability of about
% capacitance remained after cycles.
7.2.3. Composite Construction
Nanomaterials generally have small thickness or diameters,
leading to large contact interfaces between component units in
nanostructures. This will cause high contact resistance especially
for electrode materials with poor conductivity, such as transition
metal oxides (except RuOx), resulting in unsatisfied capacitive
performance.[] Thus, dierent materials with high conduc-
tivity, such as graphene, CNTs, carbon nanoparticles, carbon
fiber, and various metallic media have been used to integrate
with pseudocapacitive materials and exhibited improved capaci-
tive properties.[,,] For example, our group prepared highly
crystalline NiCoO nanoparticles on N-doped reduced graphene
(NRGO-NiCoO) by a simple one-step hydrothermal method
(Figured).[] Benefitting from the favorable interfacial contact
and synergistic eect between NiCoO nanoparticles and NRGO
substrates (Figuree), the NRGO-NiCoO composites yielded
a superb specific capacitance of F g–, which is five times
than that of NiCoO nanoparticles (Figure f). Furthermore,
the specific capacitance of NRGO-NiCoO composite material is
still four times of NiCoO nanoparticles at a high current den-
sity of A g–. Shen and co-workers fabricated mesoporous
NiCoO nanowires on carbon textiles.[] The mesoporous
NiCoO nanowires are directly grown on the highly conduc-
tive carbon textiles, which provided facile ion diusion path
and fast electron transport among the abundant mesopores in
nanowires and neighboring nanowires, which enhance the utili-
zation of nanowires during fast electrochemical reactions. At the
same time, the mesoporous NiCoO nanowires are favorable
for alleviating the volume change under the rapid charge/dis-
charge process. Attributing to the advantages of the composite
structure, the free-standing NiCoO/carbon textiles exhibited an
outstanding specific capacitance of F g– at A g–, while
only F g– was achieved for NiCoO microsphere. Once the
discharging rate was set to A g–, the capacitance retentions
of NiCoO/carbon textiles and NiCoO microsphere were %
and %, respectively. Besides, the NiCoO/carbon textiles
also showed excellent cycling stability with a negligible specific
capacitance decay and only % of the initial capacitance for
NiCoO microsphere was maintained after cycles.
7.2.4. Hierarchical 3D Structure
Over the past few decades, dierent types of electroactive
materials and electrode structures have been discovered and
Figure 26. a) The schematic illustration of the formation process of dierent structured CoS: solid, yolk–shell, double-shell, and hollow spheres.
b) SEM images of solid, yolk–shell, double-shell, and hollow CoS spheres. c) Specific capacitance as a function of current density for the solid, yolk–
shell, double-shell, hollow CoS spheres and CoS bulk, respectively. Reproduced with permission.[] Copyright , Wiley-VCH. d) Schematic illus-
tration for the formation processes of NiCoO nanoparticles on NRGO substrate. e) High-magnification TEM of NRGO-NiCoO composite materials.
f) Specific capacitance of pure NiCoO nanoparticles and NRGO-NiCoO composites at various current densities from . A g– to A g–. Reproduced
with permission.[] Copyright , Royal Society of Chemistry.
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developed for achieving satisfied energy or power density.
Unfortunately, the high capacitances of composites are usually
received only in ultrathin electrodes with low mass loadings
(≤mg cm–) and are dicult to achieve for commercial elec-
trodes with higher mass loadings (>mg cm–). In traditional
electrode structures, high mass loadings usually cause sluggish
electron and ion transport because of the thick pseudocapaci-
tive layers and clogged pores. Thus, it is urgent to develop new
electrode architectures with more ecient charge storage and
transport beyond the limits of traditional electrodes to fully uti-
lize the potentiality of these electrode materials. As mentioned
previously, the construction of self-supporting composites with
active materials directly connect with the high electric conduc-
tive substrates without the need for binder or conductive addi-
tives is a promising strategy to improve the electrochemical
performance thanks to the contact resistance is highly sup-
pressed.[,] Recently, binder-free D architectures are consid-
ered as a solution to realize high capacitive performance with
both fast charge transport and high mass loadings for SCs.[]
Liu and co-workers prepared block copolymer-derived porous
carbon fibers (PCF) with uniform mesopores of .nm, which
can partially fill with MnO nanosheets of <nm in thickness
(Figure 27a).[] The uniform mesopores of PCF and ultrathin
MnO nanosheets (< nm in thickness) ensure rapid elec-
tron and ion transport. Surprisingly, the specific capacitance
of PCF@MnO-h electrode at mV s– reached F g–,
which is ≈% of the theoretical gravimetric capacitance of
MnO ( F g–) under a voltage of .V, indicating almost
all the MnO nanosheets on PCFs participated in the electro-
chemical reactions (Figure b). Besides, the PCF@MnO-h
electrode with a high mass loading of . mg cm– exhibited
an ultrahigh areal capacitance of . mF cm– owing to the
unique D structure. Remarkably, the gravimetric capacitance
and geometric areal capacitance of PCF@MnO- h electrode
are much higher than the previously reported MnO-based
electrodes at similar mass loadings (Figurec). They empha-
sized that hierarchical D structure and uniform mesopores are
important for the ecient transport of electrons/ions and the
high loading of guest materials.
With the rapid development of nanomaterials, the elec-
trochemical performance of EES devices have been greatly
improved. For instance, compared with their bulk materials,
nanosized silicon shows a -fold increase in capacity and nano-
sized niobia (NbO) has a –-fold increase in rate capa-
bility.[,] Nevertheless, the outstanding performance of these
materials is only achieved in laboratory cells with ultrathin
Figure 27. a) Schematic illustration of the synthesis of PCF and PCF@MnO. b) The radar chart compares the six figure-of-merits of PCF (black),
PCF@MnO-h (blue), and PCF@MnO-h (red). c) Mass loading, gravimetric capacitance, and geometric areal capacitance of PCF-based electrodes
in comparison with other reported electrodes. Reproduced with permission.[] Copyright , Springer Nature.
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electrodes and very low mass loadings (≤ mg cm–).[,]
The ultrahigh performance in laboratory devices only calcu-
lates the mass of electroactive materials and solely represents
the inherent material properties, yet the actual device should
include the separator, electrolyte, current collectors at each
electrode, and packaging. And these components do not par-
ticipate in charge storage without capacity contribution for
the full device.[] Therefore, once the areal mass loading of
the electroactive material is lower than that of these compo-
nents, the performance of the device is controlled by the mass
of these components and may be orders of magnitude lower
than the performance of the intrinsic material.[] As a result,
in order to reach the demands of commercial devices with a
mass loading of ≈mg cm– or higher, sucient electrons and
ions should be transferred and fully used to energy storage in a
thick electrode of ≈ to μm. Generally, to achieve a similar
capacity and current density in high mass loading electrodes as
those in low mass loading electrodes, proportionally more elec-
tron and ion currents go through a proportionally longer charge
transfer distance are required.[] In order to meet this require-
ment, nanostructured electrode materials are challenging with
an intrinsically higher capacity or rate performance, which
need much more charges to be transferred in a given time
than traditional electrode materials.[] Thus, with the increase
of mass loadings, the resistance for electron transport and the
mass transfer limit for ions become very important.[] Such
factors greatly increase the overpotential of the cell and reduce
the capacity during long-term charge/discharge.[] At a higher
current density, the problem of capacity degradation becomes
more obvious due to the need to transport enough charge more
quickly in a given time. These issues indicate that to achieve
high-performance nanostructured electrode materials with
high mass loadings in cells is not only a simple engineering
scale problem, but a basic scientific challenge.
Due to the limitation of charge transfer, D electrodes with
planar current collectors can provide enough charge to meet
the charge requirements of electrode materials but suer
from limited depths.[,] When the thickness of electrode
with a high mass loading exceeds such depth limits, the uti-
lization of the active material stays at a low level due to the
insucient charge delivery.[] From this point, the D nano-
structured electrodes own D conductive supports serving
as D current collectors for ecient electrons transport and
D porous networks for ecient ions transmission.[] Such
architecture enables ecient charge transfer over the entire
volume of the thick electrode, which is ideal for the use of all
electrode materials regardless of electrode thickness and for
high rate and large capacity energy storage. In this respect, in
comparison with other conductive materials, D carbon net-
works are promising supports for the ecient loading of elec-
troactive materials thanks to their low density, excellent con-
ductivity, high surface area, and outstanding electrochemical
durability.[–]
7.2.5. Doping, Defect, and Vacancy
With the purpose to produce more electrochemical active
sites for enhancing the capacitance of electroactive materials,
numerous dopants, containing metal elements, nonmetal
elements, and multi-elements, are employed to promote the
capacitive characteristics of electrode materials. In the past two
decades, heteroatom-doped carbon materials have been received
significant attentions for SCs due to their high electrochemical
activity, excellent stability, and sustainability.[] In comparison
to pure carbon, the presence of heteroatoms has led to faster
charge transfer rate and extra pseudocapacitance. Investigations
have illustrated that only appropriate types (such as N,[] S,[]
P,[] O,[] B,[] F[]) and degrees of heteroatom doping can
shed the optimal surface functionalities and exceptional elec-
trochemical performances. For instance, nitrogen can modu-
late the electronic structure of sp-hybridized carbon because
of its larger electronegativity (.) than carbon (.), which
thus improves surface wettability, shifts the Fermi level, and
modulates electronic structure of carbon materials, achieving
the ultimate enhancement of charge storage abilities.[] Wu
et al. prepared a N-doped porous carbon (NPC) electrode with
a large number of micropores and mesopores.[] The NPC
electrode exhibited a high specific capacitance of . F g–
at A g–, which is about times higher than that of un-
doped porous carbon thanks to the strong C-N covalent bonds
enhance the free charge carrier density of this NPC electrode.
In addition, S-doping also can improve the diusion rate of
electrolyte ions and reduce the resistance of charge transfer.[]
Compared with the single-element doping, multiple-element
doping shows better advantages due to the synergistic eects of
these functional elements. For example, Wu et al. synthesized
a D N and B co-doped graphene aerogels (BN-GAs) compos-
ites. The BN-GAs yielded a much improved specific capaci-
tance of F g–, which outperformed the un-doped GA with
F g–.[] Except for heteroatoms, the metal elements
doping could also improve the electrochemical features of elec-
trode materials.[,]
Metal oxides owned ultrahigh theoretical specific capaci-
tances are considered as promising electrode materials for SCs,
yet their capacitive performance is limited by their essentially
poor conductivities. In order to handle this issue, creating
defects into metal oxides such as oxygen and sulfur vacan-
cies have been proved to be an ecient strategy to boost their
electrochemical performances.[–] Such intrinsic defects
can induce impurity states in the band-gap of metal oxides,
resulting in enhancing the electrical conductivity. At the same
time, they can also play as the active centers of surface reac-
tions. For example, Ling et al. reported the Zn-doped CoO by
the atomic-level structure engineering to adjust the pseudo-
capacitive behaviors from surface redox reactions to the inter-
calation of ions into the bulk material (Figure 28a).[] Both
electronic and ionic conductivity of ZnxCo-xO electrodes were
strengthened through the atomic-level engineering due to the
atomically uniform Zn doping and the introduction of oxygen
vacancy. Consequently, the ZnxCo-xO electrode exhibited a
superior capacitance of F g– at mV s– and excellent rate
capability with a high capacitance retention of F g– at an
extremely high scan rate of V s–, which is also comparable
with the state-of-the-art transition metal carbides (Figureb).
Furthermore, the improved density of states also has a posi-
tive influence on the carrier density in ZnxCo-xO electrode for
much better conductivity than pristine CoO, which significantly
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enhances the capacitive performance of Zn-doped CoO
(Figurec).
Generally, oxygen vacancies are intrinsic and funda-
mental defects in transition-metal oxides including FeO,[]
MnO,[] TiO,[] and BiO.[] As an example, in rutile
TiO, the energy levels of the oxygen vacancies are located at
≈. to .eV, which is lower than its conduction bands.[]
The adjustment of oxygen vacancies could eectively control
the donor density of metal oxides and enhance the surface
redox reactions kinetics, resulting in improved capacitive per-
formance. Oxygen can escape from metal oxides by annealing
at a certain temperature under a vacuum condition to obtain
non-stoichiometric metal oxides containing oxygen vacancies.
Besides, thermal treatment of metal oxides in a reducing gas
(e.g., H or NH) can also introduce the oxygen vacancies.[]
As a strong reducing agent, hydrogen can convert various
metal oxides into corresponding low valence metal oxides at
high temperature. Except for thermal treatments, the oxygen
vacancies can also be produced by using reductively chemical
agents (e.g., NaBH or hydrazine) to reduce metal oxides.[]
In addition, the introduction of other defects (e.g., S or
P vacancies) is also a useful method to enhance the conduc-
tivity of metal compounds, leading to better electrochemical
performance.[]
7.2.6. Surface Functionalization
As we all know, in the field of SCs, the surface redox reac-
tions need a high surface reactivity and sucient electroac-
tive sites. Thus, the capacitive performance of SCs is directly
interrelated with the interface state between electrodes and
electrolytes. To improve the interfacial reactivity, numerous
strategies have been developed including surface modifica-
tion (coating with conductive polymers (e.g., PPy,[] PANI,[]
and PEDOT[]), carbon,[] and some metal oxides[–]),
surface-functionalization,[] surface activation[] to boost
their electrochemical performance. Conductive polymers are
widely employed to modify electrodes owning to their high
conductivity (> S cm–), controllable resistance, light weight,
flexibility and outstanding electrochemical properties.[] In
particular, conductive polymers could be directly served as
pseudocapacitive materials for SCs.[]
Carbon cloth with high conductivity and mechanical strength
triggers considerable attentions for serving as supports and/or
current collectors for building free-standing electrodes. But it
only acts as a chemically inert component in these composites
due to its poor electrochemical activity and low surface area.[]
Recently, some achievements have been made for optimizing
the carbon cloth surface with the purpose to directly use it as an
Figure 28. a) Schematic diagram of the atomic-level structure engineering of ZnxCo-xO. b) Specific capacity of ZnxCo-xO electrodes as a function
of sweep rate between and mV s–. c) The DOS on pristine and Zn-doped CoO. Reproduced with permission.[] Copyright , American
Association for the Advancement of Science.
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active electrode by chemical exfoliation or electrochemical mod-
ification with surface functional groups such as COOH, CO,
and others.[,,] It is worth noting that the surface func-
tional groups can not only serve as active sites but also aect
the physical and chemical properties of electrodes. For example,
oxygen-containing groups could not only provide pseudoca-
pacitance but also improve the surface wettability, leading to
better capacitive performance. Yan and co-workers fabricated
an oxygen-functionalized graphene nanosheets (FGN) by low
temperature ( °C, FGN-) treatment of graphite oxide.[]
With the increasing of calcination temperature, the amounts
of oxygen-containing functionalities decrease. The CV curve of
FGN- electrode exhibits a nearly rectangle shape, indicating
that its capacitance mainly comes from EDLC (Figure 29a). As
for FGN- electrode, the large pseudocapacitance contribu-
tion originates from the reversible redox reactions among the
carbonyl, hydroxyl, carboxyl, and lactone groups. Consequently,
the FGN- electrode yield both high specific gravimetric and
volumetric capacitances of F g– and F cm–, respec-
tively, which are almost . and . times higher than those of
hydrazine reduced graphene oxide (Figure b,c). This work
proposes a simple and eective low temperature strategy for
large-scale production of graphene-based materials that it is
expected to significantly boost the development of high volu-
metric performance SCs where space is limited.
7.2.7. Crystal Structure
The crystal structure also has an important influence on the elec-
trochemical performance of electroactive materials. Dierent
crystal structural materials display greatly dierent physical and
chemical properties like the electrical conductivity, electrochem-
ical activity, and ion diusion coecient.[–] The micromor-
phology of electrode materials is also partly decided by the crystal
structure. For instance, dierent stable phases of manganese
oxides (e.g., MnO, MnO, MnO, and MnO) own dierent
types of crystal structures.[] Among them, MnO, a complex
and nonstoichiometric oxide with average valence of Mn below
, is extensively investigated as an electroactive material for SCs.
The octahedral MnO is the basic unit of MnO crystal structure.
The octahedral MnO can pile up with dierent orders to form
D, D, and D tunnel structures, leading to derive α-, β-, γ-, λ-,
and δ-MnO (Figure d).[] It has been found that the spinel
λ-MnO with D tunnels is favorable for cations intercalation.
Ghodbane et al. demonstrated that the spinel MnO owns the
highest capacitive performance among dierent stable phases of
manganese oxides.[] Furthermore, the α-MnO shows better
electrochemical behaviors than other crystal structured MnO.
The pure powdery MnO normally yields a specific capacitance
less than F g–. But numerous works have achieved much
high specific capacitances by building dierent nanostructures
and their composites.[–] It is worthy to note that the control-
ling of morphology and microstructure has a more important
influence on the specific capacitances than that of crystal struc-
tures.[,] Besides, the structural defects could also aect the
intrinsic properties like bandgap, electronic conductivity, and dif-
fusion coecient of the electroactive materials. For instance, Fang
and co-workers reported that the insucient oxygen in NiO-Cu
nanowires can enhance its electronic conductivity.[] Wang et
al. proposed that the S vacancies could produce deep acceptor
levels in the single-layered MoS, resulting in improved electrons
Figure 29. a) Illustration of the relationship between thermal temperature and pseudocapacitance of FGN electrode. b) Gravimetric capacitance of
FGN-, FGN- , FGN-, TEGS, and RGO electrodes at the current densities from . to A g–. c) Comparison of the volumetric and gravi-
metric capacitances of FGN- electrode with other carbon electrodes in aqueous electrolytes at various particle densities. Reproduced with permis-
sion.[] Copyright , American Chemical Society. d) Crystal structures of α-, β-, γ-, δ-, and λ-MnO. Reproduced with permission.[] Copyright
, American Chemical Society.
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mobility.[] The similar phenomenon can also be discovered in
NiSe with Se vacancies.[] Yet, it is dicult to accurately esti-
mate the contribution to the improvement of electrochemical
performance. For MnO, during the charge/discharge, it often
exhibits the evolution of morphology and structure. Such defects
may not be stable in the long-term cycling test.[]
7.2.8. Developing New Materials
The development of EES performance is greatly dependent
on the discovery of new electrode materials. Over the last
few decades, a large number of new electroactive materials
have been discovered and studied for SCs, including metal-
organic frameworks (MOFs),[] covalent organic frameworks
(COFs),[] MXenes,[] black phosphorus (BP)[] and transi-
tion-metal dichalcogenides.[]
In , Yaghi defined the MOFs, which are extremely light
with well definite pore sizes, large pore volumes, ultra-high SSA
up to m g–.[] The whole framework of MOFs is built by
coordination bonds and/or other weak cooperative interactions
(e.g., van der Waals interactions, π–π stacking, H-bonding).
The MOF provides an opportunity to adjust mono atomic active
metal centers, which can decrease mass consumption and
increase electrode/electrolyte interface. MOFs exhibit unique
advantages for SCs due to their abilities to incorporate pseudo-
capacitive redox metal centers and the controllable pore size
in the micropore range of . to nm. In , the Co-based
MOFs were firstly studied as electroactive materials for SCs.[]
Specifically, the Co-MOF- (Zn.Co.O(BDC)(DEF).)
yielded a poor specific capacitance below F g– but exhibited a
high capacitance retention after cycles in a . TBAPF/
acetonitrile electrolyte.[] Nevertheless, the Co-based MOF dis-
played a high specific capacitances of ≈ F g– in an aqueous
electrolyte.[] Then, reversible redox reactions were detected in
Ni-based MOFs with a higher specific capacitance of F g–[]
(Figure 30a,b). Subsequently, numerous dierent kinds of
MOFs were synthesized as electroactive materials for SCs.[]
Unfortunately, most MOF materials suer from poor conduc-
tivities, resulting in unsatisfied electrochemical performance
for SCs. Therefore, researchers combined MOFs with highly
conductive carbon-based materials such as RGO, CNT, and
CN.[–] What is more, in , Sheberla and co-workers
prepared a highly electrical conductivity MOF of Ni(HITP)
(Ni(,,,,,-hexaiminotriphenylene)), which can directly
be used as an EDLC electrode material. The Ni(HITP) is built
of stacked π-conjugated D layers, which are penetrated by D
cylindrical channels of ≈.nm diameter.[] The open channels
Figure 30. The wireframe view of D Ni-DMOF-ADC along a) the () surface and b) the () surface. Blue: N; Green: Ni; Gray: C and H atoms
are omitted for clarity. Reproduced with permission.[] Copyright , Elsevier. c) Synthesis of DAB- and DAAQ-TFP COF. Reproduced with permis-
sion.[] Copyright , American Chemical Society. d) Atomistic model of the layer structure and HRTEM image of bilayer MXene. Reproduced with
permission.[] Copyright , Wiley-VCH. e) TEM image of black phosphorus nanosheet. Reproduced with permission.[] Copyright , Royal
Society of Chemistry.
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of Ni(HITP) are spacious enough to embrace large electro-
lyte ions for EDLCs energy storage, such as tetraethylammo-
nium tetrafluoroborate (TEABF), even with their first solvation
sphere in acetonitrile (ACN). The Ni(HITP)-based SC device
exhibits a superior areal capacitance of ≈ μF cm– that out-
performs most of carbon-based materials, and an outstanding
capacity retention of % after cycles. This is the first
work of pure MOFs as electroactive materials without binders
and conductive additives for SCs.
In , Yaghi et al. successfully synthesized the first so-
called COFs material, in which the porous organic frame-
works were connected by covalent bonds.[] COFs stand for an
exciting new type of covalent porous crystalline polymers, which
can integrate organic components into ordered structures with
atomic precision.[] COFs have clear and predictableD or D
crystal structures through various synthetic organic reactions
between building units to form strong covalent bonds.[] The
hydrolysis and oxidation stability of boronate esterlinked frame-
work in early COFs are poor, which hinder their applications in
the field of energy storage. Until , when the redox active
,-diaminoanthraquinone (DAAQ) was partially incorporated
into a D COF linked by b-ketoenamine, the COF displayed a
well-defined redox behavior with a potential at .V (vs Ag/
AgCl) in a HSO electrolyte (Figurec).[] The modified
COF yielded a high specific capacitance of ± F g–, and
exhibited no obvious decrease after cycles.
MXenes, a new family of D transition metal carbides and
carbonitrides, are discovered by Gogotsi and colleagues in
(Figure d).[] The general chemical formula of MXenes is
Mn+XnTx, where M is an early transition metal, X is carbon
and/or nitrogen, T stands for surface termination groups (e.g.,
OH, F, O) and n= , or . Since , MXenes have
received extensive attention in the field of EES because of
their remarkable physical and chemical properties such as well
mechanical properties, high conductivity, and strong hydro-
philicity.[] For example, TiC-based MXenes have a high
electronic conductivity up to S cm–.[,] The unique
structure of MXene triggers a new research boom for energy
storage applications, due to the following merits: ) a conduc-
tive inner transition metal carbide layer boosts rapid electron
supply to electrochemically active sites; ) a transition metal
oxide-like surface generated during the synthesis serve as redox
active; ) a D morphology and preintercalated water enable
fast ion transport.[]
In the large family of D materials, BP has attracted much
attentions due to its unique structure with corrugated planes
of P atoms, which are connected by strong interlayer P-P
bond and weak interlayer van der Waals forces.[] The thin
BP sheets with a few layers or even a monolayer can be pre-
pared by decomposing the weak interlayer interaction in bulk
BP (Figuree).[] The bandgap of few layers BP sheets (also
called phosphorene) could be tunable between . and .eV
by controlling the number of layers. Furthermore, BP owns a
large spacing of . Å between adjacent puckered layers, which
is larger than the . Å of graphite and comparable to the . Å
of the T MoS phase.[] Generally, the diusion of water into
BP nanosheets leads to irreversible reactions between water
and phosphorus. Interestingly, such reactions can produce
complex oxygen functional groups (like POx) on the exfoliated
BP nanosheets due to solvent molecules contaminating the
surface.[] These oxygen functional groups formed on the sur-
face of BP nanosheets can be used as reversible adsorption and
desorption sites for H+ ions, which is conducive to the great
increase of the specific capacitance.[] In addition, the exist-
ence of oxygen functional groups and HPO on BP nanosheets
surface can eectively hinder the direct contact between diu-
sion water and BP nanosheets, thus decreasing the reaction-
induced damage of BP nanosheets.
7.3. Enhancing the Rate Performance
As a prominent index of SCs, the rate performance plays like
the “wheel” for fast charging and discharging capacity at a high
current. As we know, the power density of a SC is inversely
proportional to the equivalent series resistance (ESR). The ESR
contains the transport resistance of ions and electrons. Specifi-
cally, many factors are related to ESR: ) the inherent electronic
resistance of the electrode material and current collector; )
the contact resistance between the electrode material and cur-
rent collector; ) the resistance of electrolyte ions, especially the
transport resistance of ions inside confined pores; and ) the
ions transport resistance pass through the separator.[] In gen-
eral, there are two ways to determine the ESR: ) use EIS and
) measure the IR drop at the initiation of a GCD curve. In a
SC system, its rate performance is mainly limited by the trans-
port of electrons and ions due to the rapid electrochemical reac-
tions. Thus, two directions should be seriously considered to
improve the rate capability. One is to decrease the resistance of
electrons transport pathway and the other one is to strengthen
the transmission of ions in the electrolyte and electroactive
materials, especially for pseudocapacitors. Fundamentally,
in a SC system, the surface capacitive contribution of electro-
active material is the key for evaluating rate performance. In
other words, assuming that the current follows the power-law
dependence on the sweep rate: i = avb, where a and b are adjust-
able parameters.[] A b-value of . is the characteristic of slow
diusion-controlled process, while a value of corresponds to a
high-rate capacitive storage behavior. In this way, the capacitive
behavior of electrode material could be precisely evaluated and
distinguished into surface capacitance and diusion-controlled
capacitance, resulting in judging the energy storage mecha-
nism and capacitive performance of electrodes.
It is worthy to note that the kinetic matching of cathode and
anode is also important for the rate capability of ASCs devices.
If the rate performance of two electrodes exists huge dierence,
the voltage and capacitance of one electrode will be damaged,
resulting in inferior device performance. Thus, before assem-
bling an ASC device, except for voltage, capacitance, mass
loading, the rate capability of cathode and anode should also be
adjusted to a similar degree. For example, when two electrodes
with matching voltage and capacitance are used to fabricate an
ASC device, their conductivity, surface reactivity, pore structure,
dimension or morphology, etc. should be kept at a similar level.
In this section, we summarized and discussed the strategies for
enhancing the rate performance of electrode materials in detail
from two aspects: improving electrons transport and control of
ion migration kinetics.
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7.3.1. Improving Electrons Transport
The fast electron transport mainly depends on improving the
conductivity of electrodes. Typically, for the powdery electroac-
tive materials, binders and conductive additives usually need
to coat electroactive materials on the current collector.[] The
resistance contains many components, such as the electrode
matrix resistance (including the active materials resistance,
the conductive additives resistance, the binder’s resistance,
the contact resistance between these additives and/or contact
resistance between active materials), the current collector resist-
ance, and the contact resistance between the electrode and the
current collector.[–] If the electroactive materials own high
electrical conductivity, the total resistance of the electrode will
stay low, resulting in high rate performance.[] That is the
reason why carbon-based EDLCs and RuO-based pseudoca-
pacitors normally display high power densities.[] However, a
large number of pseudomaterials exhibit a high resistivity.[]
Thus, it is necessary to combine active materials with binder
and conductive additives to make a more conductive matrix.
Generally, conductive additives include super-p, Ketjen black,
and acetylene black, while binders are poly(vinylidene fluoride)
or poly(tetrafluoroethylene).[–] Normally, when the content
of conductive additive increases, the equivalent resistance will
decrease and the rate capability of electrode will be improved.
Besides, combining pseudo-materials with high conductive
materials (e.g., CNTs, graphene, CNFs) is an eective strategy
to enhance the rate capability of electrodes.[–] When this
kind of powdery composites use as electrodes, only binder is
necessary. The more eective strategy for achieving high rate
performance of electrode is to construct of self-supporting
electrode.[,] The integration of electrode structure can sig-
nificantly reduce the ESR and improve the utilization of active
materials, without adding any conductive additives and binders.
Some typical methods for improving the rate performance of
electrodes by improving electrons transport are presented
below.
Metal and Heteroatom Doping: Heteroatom doping has been
demonstrated to be an eective strategy to improve the rate per-
formance of carbon-based materials. Generally, N, O, and S ele-
ments are the most well-studied dopants for carbon-based mate-
rials.[] The eects of these dopants depend on their special
chemical environments in the carbon matrix structure and they
can enhance the capacitive properties of carbon-based mate-
rials in dierent ways. It has been reported that the negatively
charged pyrrolic N and pyridinic N can act as Faradaic reaction
sites and contribute to the pseudocapacitance, while the posi-
tively charged quaternary N can promote the electron transport
in the carbon lattice.[] Besides, the introduction of N and O ele-
ments can simultaneously increase pseudocapacitance and boost
the wettability of electrode surface.[] For instance, Zhang et al.
prepared a N-superdoped D graphene network structure (D
GF-NG) with an N-doping level up to . at% (Figure 31a).[]
The D GF-NG exhibited a high specific capacitance of
F g– at . A g– and maintained F g– even at a high
current density of A g–. Lately, dual and multiple heteroatom
doped carbon-based materials have been developed and exhib-
ited improved capacitive performance.[] Besides, the S doping
metal oxides is proved to be a superior strategy to enhance
the rate performance with low electronegativity, high electron
conductivity and high electrochemical activity. Tang et al. pre-
pared hierarchical FeCoS nanotube arrays on D porous Ni
backbone.[] The FeCoS composite exhibited a high specific
capacitance of F g– at mA cm–, and still has F g–
at mA cm–, which outperformed the corresponding NiCoO
electrode. In addition, FeCoS electrode exhibits much smaller
resistances (Rct= . Ω, Rs= . Ω) than the FeCoO electrode
(Rct= . Ω, Rs= . Ω). This is expected due to sulfidation
leading to S– instead of O–. The highly enhanced pseudoca-
pacitances are attributed to the FeCoS electrodes with high
electrical conductivity and the large porosity providing more
electrolyte access. Metal doping can also improve the rate capa-
bility of metal oxide electrodes by increasing the ion diusion
rate and reducing the charge transfer resistance. Our previous
work displayed that the rate capability of cobalt sulfide could
be significantly enhanced through Al-doping cobalt sulfide
nanosheets on Ni nanotube arrays (Figure b).[] The com-
positional and structural advantages ensure the hybrid elec-
trode with an excellent capacitive performance of a superior
specific capacitance ( F g–/ F g– at mV s–/ A g–)
and an outstanding rate performance (.%/.% retention
at mV s–/ A g–) (Figurec). Ye et al. prepared a Co-
doped Ni(OH)/NiS composite material on Ni foam. The
free-standing electrode yielded a superb specific capacitance of
. F g– at A g– and a high rate performance with .%
capacitance retention at A g–.[]
2D Transmission Channel: Dimensionality acts a key role in
determining the physicochemical properties of functional mate-
rials in addition to the atom arrangement and elemental com-
position. D or pseudo-D electrode materials have aroused
tremendous interest in the past decade since their sheet-like
structures render the sucient exposure of the active mate-
rials and the anisotropic transportation of mass and charge
carriers.[] For instance, D materials like graphene, transi-
tion metal dichalcogenides, phosphorene, and MXenes exhibit
unique properties absent in their bulk materials.[–] Their
fast in-plane ionic diusion and high conductivity endow high
rate capability of D or pseudo-D electrode materials. Xi et al.
fabricated an ordered mesoporous carbon sheets (OMCSs) with
uniform hexagonal morphology by a soft-hard template-assisted
method.[] The D carbon sheets showed a high specific
capacitance of F g– at . A g– as well as a superior rate
performance ( F g– at A g–). Furthermore, the in-plane
microsupercapacitor also delivered an impressive specific capac-
itance of F cm– at mV s– and still maintained . F cm–
even at mV s–. Zhou et al. fabricated TiCTx nanoflakes
together with CNTs on polycaprolactone (PCL) nanofiber net-
works.[] The open structure of PCL networks and the employ
of CNTs as interlayer not only relieve the restacking of TiCTx
sheets, but also increase the contact surface of active mate-
rials, and promote the fast diusion of electrolyte ions in the
electrode. As a result, this hybrid electrode exhibited a mod-
erate areal specific capacitance of – mF cm–, which is
comparable to other similar electrodes, but exhibited a greatly
enhanced rate capability of –% capacitance retention even
at an extreme scan rate of V s–.
Pure pseudocapacitive materials are usually limited by
the low rate performance because of the inferior electron
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conductivity and ineective nanostructure.[,] Coating
pseudocapacitive materials on D nanomaterials to produce
sandwich-like heterostructure nanohybrids has proven to be
a promising strategy for preventing the stacking of D nano-
materials and improving the rate performance of pseudocapaci-
tive materials.[] Wu et al. proposed a self-assembly strategy
for fabricating PANI/rGO D porous gels with molecular-level
uniformity even at a high PANI content (>%) (Figured).[]
Because of this favorable microstructure with facilitating the
diusion of the electrolyte ions and transport of the electrons
in the composites, the hybrid electrode yielded a superb spe-
cific capacitance of F g– ( mF cm–) at . A g–
(. mA cm–), and an outstanding rate capability of %
capacitance retention from . to . A g–) (Figuree). Liu
et al. developed a novel MnO/TiC hybrid with a molecularly
stacked structure by a simple and scalable mixing and filtra-
tion method.[] For the neat MnO electrode, even at a low
scan rate of mV s–, only a limited capacity of F g– was
obtained, and the capacity quickly shrank to F g– with the
scan rate increased to mV s–, corresponding to only % of
its initial capacitance. In contrast, the capacity of MnO/TiC
hybrid electrode achieved F g– at mV s–, and a high
capacity of F g– was still maintained at a high scan rate
of mV s–, corresponding to % of the capacity retention,
which reflected the improved rate performance of the obtained
molecularly stacked electrode.
3D Current Collectors: Compared with the powdery and
dense film electrodes, the construction of active material on
D conductive current collector is a general strategy to achieve
high capacitive performance electrodes with a superior rate
performance. For example, Chen and co-workers fabricated ter-
nary Ni–Co–S nanosheets arrays on a commercial carbon cloth
by a simple one-step electrochemical deposition technique
(Figure 32a,b).[] The hybrid electrode shown a high specific
capacitance of F g– at A g–, and the capacitance retention
stayed .% ( F g–) even at A g– (Figurec). The out-
standing rate capability attributes to four points. First, the Ni–
Co–S nanosheets directly contact with highly conductive carbon
cloths to form an integrated electrode with high-speed elec-
trons transport and electrolyte ions diusion channels, which
largely improves the charging rate of the electrode. Second, the
interconnected Ni–Co–S nanosheets with mesoporous nature
grown on macroporous carbon clothes act as an ideal plat-
form for energy storage. Third, the porous D electrode allows
facile electrolyte ion access for a fast and reversible redox reac-
tion, which greatly improves the energy storage capacity. More
importantly, the contact resistance of the interface between
Ni–Co–S nanosheets and D current collector could be fur-
ther reduced through introducing a high conductive “second
substrate.” Liao and co-workers prepared a hybrid electrode of
CoO nanoparticles on vertically aligned graphene nanosheets
(VAGNs) supported by carbon clothes (Figure d,e).[]
The hybrid electrode achieved an ultrahigh specific capaci-
tance of F g– at mV s–, approaching the theoretical
value of CoO ( F g–) (Figuref). Besides, the hybrid
electrode also exhibited a superb rate capability of F g–
Figure 31. a) Schematic representation of N-superdoped rGO aerogels. Reproduced with permission.[] Copyright , Wiley-VCH. b) Schematic
illustration displaying the merits of the core-branch CC/H-Ni@Al-Co-S electrode for energy storage. c) Specific capacitances of CC/Co-S, CC/H-Ni@
Co-S, CC/Al-Co-S, and CC/H-Ni@Al-Co-S electrodes calculated from CV and GCD curves. Reproduced with permission.[] Copyright , American
Chemical Society. d) Schematic illustration of solution-based self-assembly method for preparation of PANI/RGO composite gels (PGG). e) Specific
capacitances of PGG- versus current densities. Reproduced with permission.[] Copyright , Royal Society of Chemistry.
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even at mV s–. In a typical D architecture, the D conduc-
tive support serving as the current collector and the D porous
network acting as rapid ion transport channels. Such a unique
structure enables ecient charge delivery across the entire bulk
volume of the thick electrode, which can make full use of all
electrode materials independent of the thickness of the elec-
trode to achieve high rate and superior energy storage.
7.3.2. Control of Ion Migration Kinetics
The ion migration kinetics also has a key influence on the
rate performance of SCs. Generally, this process includes two
aspects: one is the ion diusion in the electrolyte, the other
one is the ion diusion in the electroactive materials espe-
cially for pseudocapacitive-type materials and/or battery-type
materials. In most cases, the electroactive materials are in the
form of thin nanostructures, in which the ion diusion usually
is rapid. Thus, the ion diusion in the electrolyte normally is
the limited step. Therefore, in a SC system, the ionic conduc-
tivity of the electrolyte is very important, which mainly depends
on the species, concentration, and temperature of the electro-
lyte.[] It has been proved that increasing the concentration
of NaSO electrolyte from . to can achieve a better rate
capability. However, it does not mean that the electrolyte with
higher concentration always received a better ionic conductivity.
It should be noted that the specific capacitances in dierent
concentrations of electrolytes are almost the same when the
scan rate stays a low level.
In addition, the ion migration kinetics are also aected by
the length and cross-section area of the ion diusion channel
inside the electrode. Therefore, the thickness, tortuosity, and
porosity (including pore size, pore volume, and pore size dis-
tribution) are significant for the rate capability of the electrode.
Short diusion length is beneficial for the fast transport of
ions in a given time. Normally, electroactive materials in the
form of D nanoparticles, D nanowires, and D nanostruc-
tures with high porosity can accelerate the ion diusion, while
the layered D nanosheets are easy to stack together, resulting
in a sluggish ion transport rate. Unlike the electron transport,
the ion transport is also limited by electrode area and the dis-
charging time. When evaluating the rate performance of a SC
device, the current density may reach a high level of mA
cm– in a very short time. Thus, the mass loading of the elec-
trode should have several mg cm– in a SC for a high power
density. If you want to achieve a high energy density, the mass
loading of the electrode should be higher. Generally, high spe-
cific energy and high specific power can’t be achieved at the
same time at present. However, if the porosity of electrode is
very high, high energy density and high power density may
be obtained simultaneously at the cost of large volume of the
device. Some typical methods for improving the rate perfor-
mance of electrodes by improving ions diusion are presented
as below.
Figure 32. a) Schematic of the design of asymmetric supercapacitors by applying interconnected Ni-Co-S nanosheet arrays on carbon cloth as a posi-
tive electrode and porous graphene film as a negative electrode. b) SEM images of Ni-Co-S nanosheet arrays. c) Specific capacitances of dierent
Ni-Co-S nanosheet electrodes at dierent current densities. Reproduced with permission.[] Copyright , American Chemical Society. d) Schematic
illustration of the fabrication process of CoO nanoparticles onto the carbon fabric with VAGN. e) High-resolution TEM images of the VAGN and
CoO particles. (f) Specific capacitance versus scan rate for various CoO composites. Reproduced with permission.[] Copyright , American
Chemical Society.
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Pore Structure: Pore engineering not only can eectively
improve the specific capacitance but also improve the rate capa-
bility of carbon-based materials. First, introducing the pore
structure, especially the micropores, could greatly increase
the SSA of carbon-based materials. Second, the pores can
serve as a pool for electrolytes, resulting in decreasing the dif-
fusion length of ions. Third, the reasonable construction of
the interconnection network composed of multi-scale pores
can promote the mass transfer of ions.[,] The high SSA
combines with eective ion diusion will boost the eective
ion accessible surface area and the specific capacitance.[,]
Such structure is very important for electrodes stably oper-
ating under high charge/discharge rates. Our group reported
a diblock copolymer micelle-derived nitrogen-doped hierar-
chically porous carbon spheres (N-HPCSs).[] As shown in
Figure 33a,b, the N-HPCSs showcase a porous structure with
higher SSA of . m g– than that of N-CSs (. m g–).
Pore size distribution demonstrated that the N-HPCS has hier-
archically porous nature with three dierent kinds of pore sizes
centered at about ., ., and . nm. It was found that the
reasonable pore size distribution is a benefit for mass trans-
portation by reducing and smoothing the diusion routes,
resulting in higher rate performance. As a result, the N-HPCS-
based SC exhibits a high specific capacitance of . F g– at
. A g–, which is . times higher than that of the N-CS-based
SC (Figurec). Especially, the N-HPCS-based SC owns supe-
rior rate capability of . F g– at A g– and even . F g– at
A g–, which only F g– at A g– for the N-CS-based SC.
Peng and co-workers prepared a hierarchically porous N, O,
S-enriched carbon foam by the KOH activation (KNOSC) of N,
O, S-enriched carbon foam (NOSC).[] After the KOH activa-
tion, the SSA of KNOSC is increased up to m g–, that is
about . times higher than that of NOSC ( m g–). Both
porous carbon foams have a large number of mesopores with
a narrow pore size distribution of –nm, while the KNOSC
also has numerous mesopores with pore sizes of . and nm,
and micropores with average size about .nm. When the scan
rate increased from to mV s–, the KNOSC electrode dis-
played much higher rate performance (% capacitance reten-
tion) than the NOSC electrode (% capacitance retention).
Moreover, the EIS measurement was also carried out to inves-
tigate the electrochemical kinetics of KNOSC electrode. As a
result, the characteristic time constant (τ) was calculated to be
. s, which is much lower than those of reported carbon-
based materials (such as activated carbon ( s), holey graphene
film (. s), D graphene scaold (. s), MWCNTs (. s)).
This ultrafast frequency response of KNOSC further demon-
strates the highly ecient ion transmission in the multiscale
pore network for the high rate capability.
The transition metal compounds usually suer from the
poor electronic conductivity and the limited accessible surface
areas, resulting in low electrode utilizations for SCs. Similarly,
pore engineering can also increase the accessible surface areas,
shorten ions diusion pathways, and reduce the structural failure
during rapid redox reactions. For example, Chen et al. proposed
a novel intrinsic pillar eect of metaborate and porous structure
to improve the rate performance of nickel-cobalt hydroxide.[]
For the initial electrode, a specific capacitance of F g–
could be achieved at . A g– and maintained F g– at
A g–. After the introduction of porous structure, the specific
Figure 33. TEM images of a) N-CS and b) N-HPCS. c) Comparison of specific capacitances versus dierent current densities for N-CS and N-HPCS.
Reproduced with permission.[] Copyright , Royal Society of Chemistry. SEM images of CoO nanowires with d) brush-like and e) flower-like
morphologies. f) Specific capacitances of CoO nanowires with brush-like and flower-like morphologies at dierent current densities. Reproduced with
permission.[] Copyright , American Chemical Society.
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capacitance was greatly improved to and F g– at .
and A g–, respectively. That is because the porous structure
can increase the reaction active sites accessibility and enhance
ions transport for better rate performance. Besides, when a cer-
tain mass of graphene was added, the rate performance could
be further boosted with % retention of initial capacitance at a
high current density of A g–. These results indicated that the
ecient ion migration and high conductivity are essentially key
for achieving a high rate performance.
Morphology Control: Control of morphologies and micro-
structures of nanomaterials at the mesoscopic level is very
important, as with dierent micromorphologies and micro-
structures, they exhibit great dierences in electrochemical
features because of dissimilarities in the interface properties
of the electrode/electrolyte and ion transport rates during the
charge storage processes. Many reports have reported and
evaluated the SC properties of active materials with D (such
as nanowires, nanotubes, and nanorods),[] D (such as
nanosheets and nanoflakes)[] and D (such as nanobowls,
nanoflowers, and nanonets) morphologies.[] Specifically,
Rakhi et al. prepared CoO nanowires on carbon clothes
with flower- and brush-like morphologies (Figure d,e).[]
The brush-like CoO achieved a large specific capacitance of
F g– at . A g–, and retained % ( F g–) at A g–,
while that of flower-like CoO only maintained % ( F g–
at . A g– and F g– at A g–) (Figuref). The excel-
lent capacitive performance of CoO nanowires with brush-
like morphology is attributed to the homogeneous dispersion
of CoO nanowires around carbon microfibers to result in the
brush like morphology without altering the microstructure of
the substrate, promoting the rapid transmission of the elec-
trolyte ions and increasing the utilization of CoO. Thus, the
hybrid electrode with macroporosity allows facile electrolyte
flow to reduce the resistance and nanoporosity with a high sur-
face area to ensure faster reaction kinetics. In addition, the rate
performance of electrode can also be enhanced by introducing
a highly conductive media to support the active materials. For
example, Li et al. demonstrated the crystalline metal Mn layer
in the MnO/Mn/MnO sandwich-like nanotubes (SNTAs) can
increase the electrical conductivity and accessible surface area
with higher capacitive performance.[] Specifically, the MnO/
Mn/MnO SNTAs exhibited significantly enhanced capacitance
performance with F g– at . A g–, which is about three
times higher than that of MnO NTAs. With the current density
increasing from . to A g–, the MnO/Mn/MnO electrode
only exhibited ≈ % capacitance loss, which is much lower
than the ≈ % capacitance loss of MnO NTAs, indicating a
better rate capability of MnO/Mn/MnO SNTAs. The out-
standing rate performance of MnO/Mn/MnO electrode could
be attributed to the highly accessible SSA, the short diusion
length of electrolyte ions, and the high electrical conductivity.
Surface Modification: In recent years, huge works have been
carried out to develop various nanostructures for pseudoca-
pacitive materials to enlarge the surface area, aiming to achieve
improved pseudocapacitive performance.[] The performance
improvement by enlarging the surface area alone, however, is
not satisfied due to the low electrochemical activity of the sur-
face with limited electroactive sites. As we know, the redox
reactions between the interface of electrode/electrolyte require
numerous active sites and high surface reactivity. Therefore,
surface engineering is particularly important for pseudoca-
pacitive materials to obtain the desired surface for highly e-
cient and fast faradaic reactions. Several recent works have
demonstrated that surface modification is very eective in
improving the capacitive performance, especially rate capability
for pseudocapacitive materials. Recently, Zhai et al. anticipated
that functionalizing the surface of CoO with phosphate ions
could stimulate high chemical reactivity for fast and ecient
faradaic reactions (Figure 34a,b).[] It was found that phos-
phate ion functionalization on CoO (PCO) could greatly
reduce the charge transfer resistance and increase the active
sites, leading to greatly improved surface reactivity and capaci-
tive performance with a large specific capacitance and an excel-
lent rate performance. As a result, the PCO electrode achieved
a high specific capacitance of F g– at mV s–, and still
retained F g– at mV s–, which is obviously much
larger than the Ar-CoO electrode ( F g–) and the pristine
CoO electrode ( F g–) (Figure c). Besides, a remark-
able rate capability has also been achieved by the ASCs device,
which retained about .% of the initial capacitance as the
charge/discharge rate increased from to A g–.
Conductive polymers, as previously discussed, are prom-
ising active materials for SCs thanks to their high conductivity
(> S cm–), lightweight, flexibility, controllable resistance over
a wide range, and superior electrochemical characteristics.[–]
In particular, conductive polymers could be directly incorporated
with other electrode materials for achieving high-performance
SCs with rapid charge/discharge rates. For example, Zhou and
co-workers prepared a high-performance core-shell electrode by
coating PPy on mesopores CoO nanowires (CoO@PPy) with
a D nickel foam support (Figure d,e).[] The CoO@PPy
hybrid electrode exhibited higher areal capacitance of . F
cm– at mA cm–, which is about times higher than that of
the pristine CoO electrode (. F cm–) (Figuref). The CoO@
PPy electrode still displayed large areal capacitances of .,
., ., and . F cm–, at , , , and mA cm–, respec-
tively. The great improvement after PPy coating is due to the
higher electrical conductivity and smaller diusion resistance of
the CoO@PPy electrode than the pure CoO electrode. Specially,
the coating of PPy strengthens the electrical conductivity of each
CoO nanowire and the electrical contact of CoO with the cur-
rent collector, resulting in higher utilization of CoO nanowires
during the charge/discharge process. Besides, the improved
electrical conductivity from PPy coating combining with fast
ions transmission pathways from both the D nickel foam sub-
strate and the mesoporous nanowires, significantly enhance the
rate performance of the hybrid electrode.
7.4. Boosting the Electrochemical Stability
With the increasing energy density of SCs by optimizing elec-
trodes, electrolytes, and device configurations, the cycling life is
received much attention like a “insurance” for a car. Normally,
EDLCs show an ultrahigh electrochemical stability up to mil-
lion cycles due to its energy storage mechanism is an essentially
physical process. The capacitance degradation of pseudoca-
pacitive materials is caused by many factors. For instance, the
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volume and structure change of active materials during the
long-term charge/discharge process could lead to the structural
collapse and/or loss of electronic contact of the electrode.[]
The irreversible oxidation or reductions may weaken the activity
of electrodes. The impurity in the electrode or electrolyte may
cause side reactions leading to poor electrochemical stability. A
lot of papers reported well cycling stabilities.[] It is found that
the electrochemical stability of electroactive materials is closely
related to the preparation method and the posttreatment. Better
understanding of the relationship between electrochemical sta-
bility and physicochemical properties of electroactive materials
is very important for better capacitive performances. Because
the practical applications of SCs require long life, the electro-
chemical stability is another key factor to evaluate the perfor-
mance of SCs. The cycling stability is highly dependent on the
nature of electrode materials because the degradation mecha-
nisms of dierent active materials are quite dierent. In this
section, the typical strategies for boosting the electrochemical
stability are discussed in detail, especially for pseudocapacitive
materials.
7.4.1. Surface Coating and Encapsulation
Pseudocapacitive materials usually suer from low electro-
chemical and structural stability due to the volume change
during the long-term cycling process, dissolution of active
materials into electrolyte, mechanical degradation, irrevers-
ible reactions like gas evolutions, and so on.[–] Numerous
works have proved that surface coating and encapsulation
strategy can eectively restrain these drawbacks. For example,
TiN, possessing excellent conductivity (– S cm–)
and mechanical stability, has attracted increasing interest as an
electroactive material for SCs.[] However, TiN can be easily
converted into an irreversible manner to TiO by electrochemical
oxidation in the condition of water and/or oxygen. Lu et al.
successfully boosted the electrochemical stability of TiN by
applying an ultrathin amorphous carbon coating on their sur-
face (Figure 35a).[] The stable and highly conductive carbon
layer can prevent the metal nitride from oxidizing and changing
morphology. Obviously, the TiN@C NW electrode exhibited
remarkable long-term stability, with about .% of its initial
capacitance retained after cycles at a high scan rate of
mV s–, while TiN suered from a severe capacitance loss
with only .% retention of its initial capacitance (Figureb).
Besides, the cycling stability and capacitance of electrode could
be simultaneously increased by using conducting polymers
coating. Wang et al. prepared hierarchical a-FeO/PPy nanoar-
rays (T-FeO/PPy NAs) by an in situ vapor-phase polymeriza-
tion method (Figure c).[] The as-fabricated T-FeO NAs
electrode exhibited a high electrical conductivity by introducing
the PPy. As a result, the T-FeO/PPy NAs electrode exhibited
a large areal capacitance of . mF cm– at . mA cm–
and a high reversibility with .% capacitance retention after
cycles, which is much higher than those of T-FeO elec-
trode (Figured). Similar results were also confirmed by our
previously work.[]
Unfortunately, the electrochemical stability of the conducting
polymer usually is poor, due to the volume change caused
swelling and shrinking during the cycling process, resulting
in mechanical degradation of electrode and polymer chain
breaking. Besides, over oxidation and reduction can also cause
irreversible loss of capacitance. Recently, Liu and co-workers
proved that the electrochemical stability of conductive polymers
can be largely improved by a simple and general method with a
thin carbon shell coating on their surface.[] Remarkably, after
coating with a thin carbon shell, the PPy and PANI electrodes
can achieve satisfied capacitance retentions of ≈% and ≈%
Figure 34. a) Schematic illustration of the procedure for preparing phosphate ion functionalization on CoO (PCO). b) HAADF-STEM image of PCO
and EDS mapping images of Co, O, and P. c) Specific capacitances of the PCO, Ar-CoO, and pristine CoO electrodes calculated based on CV curves
as a function of scan rate. Reproduced with permission.[] Copyright , Wiley-VCH. d) The representative synthetic procedure and structure details
of the D CoO@PPy hybrid nanowire electrode. e) HRTEM image of the surface of individual CoO@PPy hybrid nanowires. f) Capacitance retention
rate as a function of current density. The inset is current density dependence of the areal capacitance and specific capacitance for both the CoO@PPy
hybrid and pristine CoO nanowire electrodes. Reproduced with permission.[] Copyright , American Chemical Society.
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after cycles, respectively. It is found that the presence of
≈nm thick carbon shell could eectively prevent the structural
damage of PPy and PANI electrodes during long-term charge/
discharge process. More importantly, the PPy and PANI elec-
trodes with a ≈nm thick carbon shell have a negligible impact
on their specific capacitances and capacitive behaviors with
their corresponding bare polymer electrode.
7.4.2. Composite Formation
Currently, the cycling life of commercially available carbon-
based SCs can reach up to million cycles, while those of the
reported metal oxides electrodes are normally less than
cycles.[] The reason for the poor cycling ability may be
the long-term Faraday reaction of metal oxides bringing con-
siderable physical and chemical changes/strains on the elec-
trode materials, resulting in insucient electrochemical active
states for a long period of time.[,] Although nanostructured
materials have exhibited potential improvement, there is still
a big challenge to develop metal oxides electrodes with satis-
fied electrochemical ability for long-term practical usage. The
combination with some metal oxides/hydroxides can eec-
tively improve the energy density of carbon-based electrode,
but many of them are at the cost of power density and long-
term cycling stability.[,] In order to obtain an optimized
overall electrochemical performance, it is necessary to reason-
ably design electrode structure with proper control in metal
oxide/carbon. For instance, Guan et al. prepared a novel FeO
growth on the D graphite foam CNT forest (GF-CNT@FeO)
(Figure e,f).[] Benefitting from the porous structure and
high conductivity of the CNT foam and the high density of
FeO, the GF-CNT@FeO electrode displayed an ultrahigh
specific capacitance. Importantly, the GF-CNT@FeO electrode
also showed a superior electrochemical stability with .%
capacitance retention even after cycles at mA cm–
(Figure g). The outstanding cycling durability is mainly
because of the following factors. First, the nanostructure of
FeO nanoparticles on CNT foam is mechanically stable, which
can relieve the structural collapse during cycling. Second, these
FeO nanoparticles are refined nanocrystallites with shorter
ion/proton diusion path and lower charge transfer resistance
than large nanowires or nanoparticles, which can better keep
the long-time reversible reactions and maintain the electrode
integrity. Third, there is enough free space between FeO nano-
particles, which can eectively buer the mechanical strain and
volume expansion caused by the fast and long-term Faradaic
reaction. Furthermore, the GF-CNT substrate is electrochemi-
cally and mechanically stable and the D structure can keep the
electrochemical ability during the long-time cycling test.
7.4.3. Electrochemical Treatment
In recent years, some electrochemical treatments (such as
electrochemical activation, oxidation, reduction, intercala-
tion, and converse voltage process) have been developed to
improve the structural and electrochemical stability of elec-
trode for SCs. For example, Wang et al. reported a novel one-
step electrochemical activation method to greatly enhance
the capacitive performance of the commercial carbon cloth
under mild conditions.[] Significantly, the as-prepared
Figure 35. a) HRTEM image of a TiN@C nanowire. b) Cycling performance of the TiN and TiN@C NW electrodes at a scan rate of mV s–
for cycles. Inset: The first and th CV curves of TiN@C NW electrode. Reproduced with permission.[] Copyright , Wiley-VCH.
c) HRTEM image and corresponding SAED pattern of T-FeO/PPy composites. d) Cycling performance of the pristine T-FeO and T-FeO/PPy-.
electrodes collected at a scan rate of mV s– for cycles. Reproduced with permission.[] Copyright , Wiley-VCH. e) Growth procedure of
GF-CNT@FeO starting from graphite foam. f) HRTEM images of GF-CNT@FeO. g) Cycling performance of GF-CNT@-FeO electrode. Repro-
duced with permission.[] Copyright , American Chemical Society.
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electrode exhibited a large areal capacitance of mF cm–
at mA cm– with high rate performance and outstanding
cycling stability without any decay in capacitance after
cycles. The improvement of the electrochemical performance
of carbon cloth is ascribed to introduce functional groups onto
its surface for producing pseudocapacitance and increase its
surface area simultaneously during the electrochemically oxi-
dizing process. Additionally, this fabricated asymmetric device
also has a superior cycling stability with no loss of capacitance
after cycles. Guo and co-workers developed an ultrafast
and low-energy-cost universal converse voltage process to sig-
nificantly boost the electrochemical performance of transition
metal compounds (including Mn-, Ni-, Co-, Fe-, and Cr-based
hybrids) (Figure 36a,b).[] For example, such a process causes
a phase conversion from cobalt hydroxide (Co-OH) to electric-
field-activated CoOOH (EA-CoOOH), resulting in forming
molecular structure with lattice disorders, abundant defects,
and connecting holes. As a result, the EA-CoOOH electrode
yields a much larger specific capacitance of F g– at A g–
than that of Co-OH ( F g–). Furthermore, the EA-CoOOH
hybrids can achieve a high capacitance retention up to %
after cycles at A g–, while only % maintained for
Co-OH electrode (Figurec).
As we know, the amorphous or low-crystalline metal oxides
are favorable for achieving better cycling stability than their
high-crystalline counterparts due to their more structural
disorder and defects. As an example, Owusu et al. reported
a low-crystalline FeOOH nanoparticles anode produced by
electrochemical transformation of iron oxide (a-FeO) nano-
particles.[] The FeOOH anode manifests a high specific capac-
itance of F g– at A g–, a high rate performance ( F g–
at A g–) and an outstanding electrochemical stability of %
capacitance retention after cycles under a wide voltage
range of -.-V (vs SCE). The surface capacitance contribu-
tions of the FeOOH anode are .%, .%, and .% of
the total capacitances at , , and mV s–, respectively, indi-
cating the dominant capacitive charge-storage mechanism in
such an electrode. That is the reason for fast the charge storage
kinetics and the superior cycling performance. Furthermore,
the FeOOH-based aqueous SC exhibited a high cycling stability
during float voltage test for h and achieved a high energy
density of Wh kg–.
Figure 36. a) Schematic illustration for the fabrication of EA-CoOOH by the converse voltage method and the molecular structure of EA-CoOOH
with Co+- and defects-enriched properties. b) Schematic for the adopted voltage and time at electrodeposition stage and converse voltage process
stage. c) Cycle performance of as-made EA-CoOOH, Co-OH, and O-CoOOH hybrids at a current density of A g–. Reproduced with permission.[]
Copyright , Wiley-VCH. d) The proposed layered structure of Ni.Co.(BO)y(OH)-y·xHO with BO “pillar” to stabilize the metal hydroxide lay-
ered structure. e) TEM image of metaborate stabilized α-Ni.Co.(OH). f) Cycling performance of α-Ni.Co.(OH) and metaborate stabilized
α-Ni.Co.(OH) at a current density of A g–. Reproduced with permission.[] Copyright , American Chemical Society.
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7.4.4. Chemical Treatment
For simplifying the experimental conditions, several investiga-
tions attempt to develop safer and simpler chemical treatments
by using chemical agents like HO,[] KBrO,[] KBH[] and
NaBH.[] For example, Chen et al. prepared a layered structure
of Ni.Co.(BO)y(OH)-y·xHO with BO “pillar” to stabilize
the metal hydroxide layered structure via the chemical treat-
ment of α-Ni.Co.(OH) with . KBH (Figure d,e).[]
The Ni.Co.(BO)y(OH)-y·xHO electrode exhibited much
higher electrochemical stability than that of α-Ni.Co.(OH),
due to the intrinsic pillar eect of the metaborate for high
structural stability (Figuref). The high electrochemical and
structural stability of Ni.Co.(BO)y(OH)-y·xHO is due to the
strong BO bonds and the van der Waals forces or hydrogen
bonds between the hydroxide ions and the water molecules.
Wang et al. demonstrated the capacitive performance of CoO
nanowires can be enhanced through a simple treatment in
M NaBH solution for h.[] In comparison with the pristine
CoO nanowires, the NaBH reduced CoO nanowires own
new defect states caused by the oxygen vacancies and located in
the bandgaps. Besides, the calculated formation energy shows
that the two electrons in the oxygen vacancy defect could be
easily excited into the conduction band, thus improving the
conductivity and electrochemical activity of CoO. As a result,
the reduced CoO nanowires yielded a much higher specific
capacitance of F g–, while only F g– for the pristine
CoO nanowires. Besides, the reduced CoO nanowires dis-
played an excellent electrochemical stability of over % capaci-
tance retention after cycles.
7.4.5. Doping and Functionalization
Element doping can not only improve the specific capacitance
but also boost the electrochemical stability of electrodes by
enhancing the wettability, conductivity, creating defects and
vacancies, and reducing crystallinity. For example, Zhou et
al. developed a N and F co-doped carbon spheres (CM-NF)
by a low-temperature solvothermal route.[] The CM-NF
electrode displayed a superior volumetric capacitance
of F cm– at . A g–, and exhibited an outstanding
cycling durability with no obvious loss of capacitance after
cycles in HSO and cycles in KOH.
The doping of F largely decreases the charge transfer resist-
ance of the electrode, which is similar to the enhancement
of electrical conductivity due to the high electronegativity of
the F atom. From the DFT calculation, the F atom can cause
charge redistribution of the N atom, reduce the gap between
the HOMO and the LUMO levels of CM-NF, thus reducing
the charge transfer resistance of the electrode. Besides, the
high graphitization of CM-NF can also greatly promote the
transport of electrolyte ions and conductivity in the process
of charge/discharge. Furthermore, the F atom doping can
also improve the cycling stability of metal oxide/hydroxide.
Hussain et al. fabricated F-doped α-Ni(OH) mesoporous
nanosheets by using one-step solvothermal process.[] Ben-
efitting from ecient electron transport, favorable electrical
conductivity, and water adsorption, the F-doped α-Ni(OH)
reflected a superb cycling stability of % capacitance reten-
tion after cycles.
In addition to the nonmetallic elements, the metal elements
doping could also strengthen the electrochemical stability of
electrode materials. For example, Yang et al. prepared a fractal
(NixCo-x)Se nanodendrite arrays on Ni foam ((NixCo-x)Se@
NF) (Figure 37a,b).[] The electrical conductivity and capaci-
tive performance of (NixCo-x)Se can be regulated by adjusting
the values of x. Specifically, the equivalent series resistance
of this hybrid electrode is in the order of (Ni.Co.)Se@NF
< CoSe@NF < NiSe@NF, indicating the highest electrical
conductivity of (Ni.Co.)Se@NF electrode. As a result, the
(Ni.Co.)Se@NF electrode remained .% capacitance
retention after cycles at A g–, which is much higher
than its (NixCo-x)Se counterparts (x= ., .) of . and
.%, respectively (Figure c). Hu and co-workers dem-
onstrated the Al-doped α-MnO can optimize the specific
capacitance and stability of α-MnO even under a high mass
loading.[] The density of states (DOS) of Al-doped α-MnO
proved that the Fermi energy lever increases and gets into
the conduction-band minimum comparing with pristine α-
MnO, indicating that the electrical conductivity of α-MnO
was improved by Al doping. As a result, the Al doped α-MnO
electrode exhibited a high specific capacitance of F g– and
F cm– at a high mass loading of mg cm– and an out-
standing electrochemical stability of % maintained after
cycles. Hao et al. presented a low crystalline oxygen-
vacancy-rich CoO electrode by introducing the Pd+ into
CoO under the hydrothermal process.[] The introduction
of Pd atoms will lead to the disorder of lattice orientation of
CoO nanoparticles, resulting in the decrease of crystallinity.
A large number of lattice boundaries caused by the poor crys-
tallinity can promote the permeation of electrolyte, and help to
generate oxygen vacancies, thus enhancing the redox reaction
and improving the conductivity. Consequently, the Pd-CoO
electrode delivered a large specific capacitance of F g–
at mA cm–, which is about two times than that of pristine
CoO. Besides, the Pd-CoO electrode also has a better cycling
stability of % capacitance retention after cycles, while
the CoO are only remained at %. The superior electrochem-
ical durability could be attributed to the self-adaptive strain
relaxation ability of the low crystal structure in the process of
frequent charge/discharge.
7.4.6. Nanostructure Tailoring
As mentioned above, the micromorphology and nanostruc-
ture serve as significant roles to the capacitive performance
for SCs. Recent researches pointed out the morphology and
nanostructure also have a nonignorable eect on the elec-
trochemical stability of electrodes. For example, Huang et
al. synthesized nano-sized structurally NixFeyOz@rGO with
highly disordered crystal structure and crystalline structure
(Figure d).[] The structurally disordered NixFeyOz@rGO
delivered a high capacity of mAh g– at A g–, which was
times higher than that of the crystalline one. More impor-
tantly, the NixFeyOz@rGO aerogel oered ultrahigh stability,
and a capacity of mAh g– was maintained after cycles
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at A g–, while the crystalline one can only yield mAh g–
(Figure e). The structurally disordered samples showed
much better capacitive performance, which can be attributed
to the following factors: ) the structurally disordered samples
show lower polarization during cycling, which can be due to
the isotropic structure of disordered nanoparticles providing
random orientation of channels for faster lithium diusion; )
according to DFT calculations, the volume expansion during
cycling for the structurally disordered nanoparticles is smaller
than that for the crystalline ones; ) the higher capacitive con-
tribution in the total capacity makes the disordered sample
more tolerant to faster charge/discharge rates. This work dis-
played that the degree of disorder in the crystal structure plays
a vital role for the electrochemical performance, oering an
alternative strategy to design of high-power and high stability
electrodes for hybrid SCs.
Except for crystal structure, the morphology structure of
active materials also aects the cycling stability of electrodes.
Hu et al. fabricated a template engaged formation of com-
plex CoS hollow structures with CoS-nanosheet-organized
single-shelled nanoboxes (CoS-NS SSNBs), CoS-nanoparticle-
assembled single-shelled nanoboxes (CoS-NP SSNBs), and
hierarchical double-shelled hollow structures consisting of
CoS-NP nanoboxes surrounded by CoS nanosheets (CoS-NP/
CoS-NS DSNBs).[] The CoS-NP/CoS-NS DSNBs yielded
a much larger specific capacitance of F g– and better
rate performance than those of CoS-NS SSNBs and CoS-NP
SSNBs. Furthermore, the CoS-NP/CoS-NS DSNBs exhibited an
outstanding electrochemical stability of % retention of the
initial capacitance after cycles, while only % and %
for CoS-NS SSNBs and CoS-NP SSNBs. The enhanced perfor-
mance of CoS-NP/CoS-NS DSNBs could greatly be attributed to
the hierarchical architecture. Specifically, the unique assembly
of dissimilar subunits enables the complex hollow structure
with a high SSA and suitable mesopores, which not only boost
the transport of electrolyte but also provide more electroactive
sites for electrochemical reactions. Besides, the double-shelled
structure may allow the confinement of electrolyte between
shells, thus oering a higher driving force to boost the redox
reactions. Furthermore, the inner shells and outer shells are
interconnected, which can achieve good structural robustness
to improve the electrochemical stability.
7.4.7. Activation
The activation of electrode materials, especially for carbon-
based materials, can create numerous and multiscale pores,
enhance the wettability of active materials, shorten ion and
electron transmission path, improve the electron conductivity,
resulting in the significantly optimized capacitive perfor-
mance of electrode as well as electrochemical durability. In
the past few decades, various activation strategies have been
developed including physical activation (steam and CO) and
chemical activation (KOH, NaOH, ZnCl, HPO, NaCO,
KCO, molten salt etching, decomposable salts etching,
Figure 37. a) Schematic diagram of the virtues of the interconnected stem-branch (NixCo-x)Se@NF. b) The atomic configuration of the exposed
() surface in (NixCo-x)Se. c) Cycle performances of CoSe@NF, (Ni.Co.)Se@NF, and NiSe@NF. Reproduced with permission.[] Copyright
, Wiley-VCH. d) Structural volume changes in crystalline and structurally disordered NiFeO during the insertion of Li+ per unit cell calculated
with DFT. e) Long-term stability performance of the structurally disordered NixFeyOz@rGO and the crystalline NiFeO@rGO at A g– and Coulombic
eciency of structurally disordered NixFeyOz@rGO. Reproduced with permission.[] Copyright , American Chemical Society.
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oxidative salt etching).[] Physical activation permits tailoring
of the pore-size distribution more accurately and narrowly,
which results in more micropores than the chemical activa-
tion.[] Besides, physical activation reduces both the particle
and microdomain sizes in the resultant activated carbons.
Otherwise, chemical activation has unique advantages of
high carbon yields, relatively low-temperature processes, and
high mesopore ratios in the resultant porous carbon.[] As
an example, Pham et al. synthesized porous carbon nanotube
bridged graphene D building blocks with KOH activation
(ac-Gr/SWCNT) (Figure 38a,b).[] The specific surface area of
the ac-Gr/SWCNT film increased to m g– from m g–
of the non-activated Gr/SWCNT sample. Besides, the ac-Gr/
SWCNT electrode has a superior electrical conductivity of
S m– and a high mass loading of . g cm– and
yielded a large volumetric and gravimetric capacitance of
F cm– and F g– at . A g–, respectively. Further-
more, a high capacitance retention of .% was observed
for ac-Gr/SWCNT after cycles at a discharge current
density of A g–, which is much higher than that of Gr/
SWCNT (.%) (Figurec).
Activation with dierent chemical agents also has dif-
ferent influences on the electrochemical stability of electrodes.
Elmouwahidi and co-workers fabricated various activated car-
bons from KOH and HPO-activation of olive residues (OR)
by three dierent activation procedures.[] The first one is an
impregnation of OR with KOH using two dierent KOH/OR
weight ratios of / and /, followed by annealing (named
as AK and AK). The second one is the physical mixture of
carbonized OR with solid KOH in a / or / mass ratio, fol-
lowed by annealing (named as CK and CK). The third one
is an impregnation of OR with HPO in a mass ratio of /,
followed by annealing (named as AP). The KOH activation led
to a higher specific surface area than that of HPO activation,
but more mesopores and phosphorus surface groups existed in
HPO activation sample. As a result, these samples exhibited
low capacitances fading after cycles with capacity reten-
tion of % for AK, % for AK, % for CK, % for CK
and % for AP, respectively.
7.4.8. Superstructure
Recently, D superlattices composed of nanoparticles (NPs)
are an important class of new nanostructured materials, the
properties of which could be reasonably adjusted by control-
ling the shape, size, and composition of constituent NPs.[,]
Particularly, the interaction between NPs and superlattices
is quite dierent from the collective properties of isolated
NPs.[] The controllable assembly of complex superstruc-
tures from micro- and nanoparticles as building blocks is elic-
iting widespread interest for particle engineering.[] Due to
their enhanced and synergistic performances, such materials
have vast application prospects in drug delivery, photonics,
Figure 38. a) Schematic for fabricating the ac-Gr/SWCNT hybrid nanostructure. b) Cross-sectional SEM image of ac-Gr/SWCNT. c) Cyclic stability of
Gr/SWCNT and ac-Gr/SWCNT at A g–. Reproduced with permission.[] Copyright , American Chemical Society. d) HRTEM image of the region
between CNF and CoO supraparticle. e) Cycling performance of CoO, CNF/H-CoO, and CNF/HSP-CoO electrodes at a current density of A g–,
respectively, and the corresponding Coulombic eciency of CNF/HSP-CoO electrode. Reproduced with permission.[] Copyright , Wiley-VCH.
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chemical sensing, energy storage, gas adsorption, and
catalysis.[–] To date, as much more progress has been made
on both physical technique and chemical synthesis, two major
assembly mechanisms for superstructures through the densest
packing of particles and the self-assembly of functionalized
blocks via interactions become mature gradually. For example,
Ji et al. synthesized centimeter-scale, free-standing thin films
of ordered mesoporous graphene frameworks (MGFs) from
D nanocrystal superlattices self-assembled at the solid- or
liquid-air interface.[] The resultant MGF films possess uni-
form thicknesses tunable in the range from several hundred
nanometers to few tens of micrometers, highly ordered and
interconnected mesopores, ultrathin pore walls comprising
few-layer graphene, and high surface areas ( m g–). The
MGF films yielded a large specific capacitance of F g– at
. A g–, which outperformed most D graphene materials
reported previously. Impressively, the MGF films exhibited
superior long-term cycling stabilities, as indicated by the nearly
constant capacitance of .% retention for up to cycles
at A g–.
Hollow nanostructures based on transition metal oxides
have been widely used in the field of energy-related applica-
tions due to their low density, high specific surface area, and
high load capacity.[] Designing superstructure electrode with
hollow interior show a promising strategy to preparing superior
capacitive performance electrodes with high SSA and surface
activities.[] Our previous work demonstrated that the bubble-
nanofiber-structured and bubble-nanosheet-structured CoO
hollow supraparticle (named as CNF/HSP-CoO and RGO/
HSP-CoO, respectively) exhibited excellent capacitive behav-
iors as well as good electrochemical stability.[] The CNF/HSP-
CoO and RGO/HSP-CoO composites were prepared by a
simple and scalable self-assembly of polydopamine (PDA) and
CoO nanoparticles. In comparison with solid bulk materials
and conventional hollow structure nanoparticles, hierarchically
structural supraparticles have been proved to be more suit-
able electrode materials for energy storage. Because the hollow
structure can provide more accessible reaction sites, resulting
in higher energy density, the “porous shell” composed of indi-
vidual ultrasmall NPs can promote the transmission of elec-
trolyte to the active surface, thus shortening the transmission
length of ions and charges, thus obtaining higher power den-
sity. Besides, after heat treatment of organic PDA ligands, the
thin carbon layer plays as a “bridge” between these individual
CoO nanoparticles, which significantly reduces the interpar-
ticle resistance of electron and ion transport and improves the
structural stability (Figured). Moreover, such a hierarchical
structure can reduce the damage of CoO in the process of
long-time electrochemical reactions, resulting in better elec-
trochemical stability. Consequently, the RGO/HSP-CoO
electrode yielded a superior specific capacitance of F g–
at A g– and retained F g– even at A g– as well as
an outstanding electrochemical stability (.% retention after
cycles at A g–) (Figuree). Furthermore, the RGO/
HSP-CoO-based ASC device exhibited a high energy density
of W h kg– and remarkable cycling performance of .%
capacitance retention after cycles. The finding demon-
strated that the reasonable design of electrode materials with a
bubble-like superstructure presents a chance for the realization
of high-performance electrode materials for advanced energy
storage devices.
8. Conclusion and Perspectives
Developing safe and sustainable clean energy for the future
is one of the greatest scientific and societal challenges. Large-
scale, inexpensive, and safe electrical energy storage cells
are highly desired for dierent applications such as con-
sumer electronics, power tools, and transportation. SCs are
promising charge storage cells with higher power densities,
shorter charging times, and longer cycle lives than those
of lithium-ion batteries. Given the rapid improvements of
SCs technologies, wide voltage aqueous ASCs present a
promising opportunity to complement or even replace bat-
teries in energy-storage related applications. Developing
wide voltage aqueous ASCs can achieve the purpose of sig-
nificantly increasing the specific energy without sacrificing
their high power density and rapid charge/discharge rate.
This review provides a comprehensive overview of recent
advances, strategies, and challenges for wide voltage aqueous
ASCs, as shown in Figure 39. Designing and fabricating
high-performance electrode materials and wide voltage cells
in ASCs are currently undergoing a remarkable develop-
ment with growing achievements. Discovering and creating
wide potential and high capacitance of positive and nega-
tive materials with tailored structures present an opportu-
nity to enhance the electrochemical performance with both
large energy and power densities for ASCs. Dierent types
of electrode materials studied and reported in literatures
are summarized. The eects of the aqueous electrolyte on
the working voltage of asymmetric device are discussed. The
strategies for improving the electrochemical performance of
SCs by designing and optimizing electroactive materials with
working voltage window, specific capacitance, rate capability,
and electrochemical stability are reviewed and discussed in
depth. Despite the numerous achievements in this field,
some main challenges still exist such as further boosting the
specific energy without cutting down the high power density
and long life, especially for large-scale energy-storage appli-
cations, unoptimized matching between newly developed
electroactive materials and electrolytes, compatibility with
current collectors and aqueous electrolytes, the insucient
fundamental understanding of charge storage mechanisms,
and the lack of standard methods to evaluate the performance
of SCs. To overcome these challenges, some perspectives and
opportunities for future research directions are suggested as
follows:
8.1. Deeper Understanding of the Electrochemical
Charge Storage Mechanisms
The interfacial interactions of the physical and chemical pro-
cesses at electrode and electrolyte need further understanding
to enhance the electrochemical performance of aqueous ASCs.
Fortunately, over the past decade, some new findings on mecha-
nisms for EDLCs and PCs have been developed, which present
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some new perspectives and guidelines for researchers to build
high-performance SCs. For EDLCs, in the past few decades,
the “common sense” in researchers’ mind for enhancing the
specific capacitance of EDLCs was to boost the SSA of carbon-
based materials. Specifically, the pore size in the porous carbon-
based materials should be about twice the size of the ions to
cover the pore walls. In other words, if the size of solvated ions
exceeded the pore dimensions, the specific capacitance will
decrease due to fewer electrolyte ions can enter these pores.
However, such “common sense” cognition was experimentally
denied by the report of a great rise of the specific capacitance
when the pore size of active material approached the size of
the desolvated ions. Recently, researchers gradually realize that
the nanostructure and the pore size of carbon-based materials
play a more key role than the SSA in improving the specific
capacitance. Besides, the energy storage in SCs was tradition-
ally attributed to a simple mechanism of ions with opposite
charges were adsorbed onto the surface of the porous carbon
to form EDLCs. But, for the positive polarization, the charging
mechanism is not only a purely adsorption process. Its charge
storage is usually driven by the exchange of anions and cat-
ions, that is, anions are adsorbed into micropores and cations
Figure 39. Summary of the advances, strategies, and challenges for wide voltage aqueous asymmetric supercapacitors.
Adv. Funct. Mater. 2021,
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are simultaneously ejected. Therefore, in the negative voltage
range, the charge storage is mainly controlled by counter-ion
adsorption, that is to say, the negative polarization is controlled
by cation adsorption. Thus, the charge storage mechanisms of
positive and negative electrodes are dierent. In addition, the
adsorbed ions are only partially solvated and will be partially
desolvated when they enter the microporous carbon pores. The
molecular dynamics (MD) simulation also confirmed the phe-
nomenon of ion confinement in carbon nanopores. The ions in
the electrolyte partially lose their solvated shell and enter these
small nanopores. The desolvation and the local charge stored
on the electrode increase with the degree of confinement. The
confinement degree has a great influence on the local charge
stored on the electrode surface. In fact, a high specific capaci-
tance could be expected if the ionic solvation is weak or if the
electrification of the pore destroys the internal strong ion sol-
vation structure. When the pore size is comparable to the ion
diameter, the maximum capacitance will be achieved due to the
ion adsorption in the most eective way.
In order to achieve higher energy SCs while retaining the
high power density and long cycling life, the growing reports of
combining rechargeable batteries and SCs are creating electrode
materials with new reaction mechanisms for rapid charge/dis-
charge and high rate capability. Through the physical control of
electrode materials (including SSA, particle size, conductivity,
crystalline, and phase structure), scientists have found that
there are pseudocapacitance contributions in some electrode
materials of metal ion batteries, which is called “intercalation
pseudocapacitance.” At present, the most widely studied elec-
trode materials for intercalation pseudocapacitance are mainly
nanostructured MoO, NbO, and TiO in organic electrolytes.
However, up to now, their molecular level processes still need
further research especially for the active material/electrolyte
interface. More attention should be paid to investigate the for-
mation of the passivation layer on the surface of conventional
pseudocapacitive electrode materials like MnO and RuO in
aqueous electrolytes. Such studies are of great significance
to improve the stability and broaden the voltage window of
aqueous ASCs. Moreover, in order to reveal the potential elec-
trochemical mechanisms on the nanoscale, advanced modeling
and simulation are required. A large number of theoretical and
computational researches on EDLCs have been reported. How-
ever, due to the complexity and diculty to simulate the surface
redox and ion-intercalation pseudocapacitance, the theoretical
understanding of pseudocapacitance is very limited. In addi-
tion, the state-of-the-art in situ spectroscopy and microscopy
techniques are significant for providing directly and convinc-
ingly experimental evidence.
8.2. Developing New Electroactive Materials
Discovering and creating new electroactive materials with tai-
lored structures provides an opportunity to boost the perfor-
mance with both large energy and power density. In recent
years, some new electroactive materials including MOFs,
COFs, MXenes, and phosphorene have been developed and
investigated for SCs applications. MOFs and COFs exhibit sig-
nificant potential in SCs due to their controllable pore sizes,
high SSAs, and highly flexible molecular designs. MXenes
possess high electronic conductivity, fine hydrophilicity, and
good mechanical properties, have displayed great potential
for improving the volumetric energy density for SCs. Other
layer-structured materials (such as phosphorene, transition
metal nitrides) have broad prospects in the application of SCs
because of their high-density structure and reduced transport
pathways which induce intercalation chemistry. The discov-
ering and fabricating of wide potential window, redox-active
electrode materials coupled with controllable pore struc-
ture presents a great chance to build high-performance wide
voltage aqueous ASCs.
8.3. Designing Rational Electrode Structure with
High Mass Loading
The discovery and development of electroactive materials
indicate higher energy or power density. Yet, high perfor-
mance can only be obtained in the ultrathin electrodes
with low mass loadings (≤ mg cm–), and it is hard to
achieve in commercial electrodes with high mass loadings
(>mg cm–). In order to make full use of the potential of
these electroactive materials, new electrode structures are
required to make the charge transfer eciency exceed the
limits of traditional electrodes. Particularly, when the elec-
troactive material is combined with the conductive supports
with controllable structures, the much higher electrochem-
ical utilization will be achieved. Unfortunately, the capaci-
tive performance is still not satisfied or the cost usually is
too high at the device level. From the perspective of practical
applications, these technologies are still far from the market
due to the factors of cost, safety, processability, mass produc-
tion, and environmental compatibility.
Recently, designing and synthesizing D electrodes are
considered as an ecient strategy to solve the limitations
of charge transfer in thick electrodes. The D porous con-
ductive networks provide interpenetrating transport paths
for ions and electrons, ensuring ecient charge transfer
between electrodes. Such transport pathways are necessary
to transform the extraordinary properties of nanomaterials
into macroscopic electrodes with high mass loadings. The
ability to increase the mass loading of electroactive mate-
rials without decreasing the energy storage performance is
not only to capture the advantages of a new generation of
ecient electrode materials in practical devices, but also to
break through the limitations of traditional electrode mate-
rials by reducing the relative ratio of the passive overhead.
Such improvements will enhance the total energy and power
densities of the device regardless of the kind of electroac-
tive materials. Except for the fast charge transport kinetics,
such D architectures usually have ductile frames with flex-
ible mechanical properties, which can bear large volume
changes and stresses during long-time charging/discharging
to ensure a high cycling stability. For the future, to boost the
energy density (specific capacitance) of the ASC while main-
taining their high power density, further research is required
to develop electrode materials with a unique structure which
can oer a high interfacial area for a high capacitance even
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2108107 (58 of 68) © Wiley-VCH GmbH
with a high mass loading while retaining ecient ions trans-
mission for a high power density.
8.4. Optimizing the Electrolytes
Electrolyte stands a key determining component in capacitive
performance of the wide voltage aqueous ASC. Specifically,
the electrochemical stable potential window (ESPW), ionic
conductivity, operating temperature range, and thermal and
chemical stabilities, have great influences on both device’s
performance and practical applications. Although significant
progresses have been made in the field of aqueous electrolytes
for ASCs, there still several challenges hinder the maturation
of these technologies and their commercial applications: ) The
ESPW of the electrolyte has a determining eect on device’s
operating voltage, leading to limit both the power and energy
densities; ) High equivalent series resistance of water-in-salt or
ionic liquid-based electrolytes with high viscosity and low ionic
conductivity will decrease the rate and power performance; )
Impurities in the electrolyte also take a huge influence on the
ESPW and self-discharge; ) Unreasonable matching between
the electrode materials and electrolytes always aect the total
cell performance; ) Unfavorable temperature range of aqueous
electrolytes for ASC usage under dierent practical application
conditions; ) High cost of electrolytes (e.g., water-in-salt elec-
trolytes) may hinder the applications of wide voltage aqueous
ASCs; ) Lack of fundamental understanding of the electrolyte
process in wide voltage aqueous ASCs for the selection and
design of new electrolyte materials and the optimization of the
interaction between electrodes and electrolytes. Especially, the
capacitance and stability of an ASC device will be improved
when the electroactive material and electrolyte are well
matched. However, there is no fixed combination between the
electrode material and the electrolyte. The electroactive material
has no consistent HER and OER potentials in various aqueous
electrolytes, and dierent electroactive materials own their
unique work functions, leading to dierent operating voltage
ranges of ASCs. Thus, the deep understanding of the structural
evolution of electrode, the interfacial reactions between the
electrode and electrolyte, and the electrode potential distribu-
tion is key to fabricate advanced ASCs devices. In this regard,
advanced in situ characterization techniques, theoretical and
computational simulation, and modeling are urgent to deter-
mine and explicate these uncertainties on the electrochemical
characteristics of wide voltage aqueous ASCs.
8.5. Compatibility with Current Collectors
Generally, the chemical and electrochemical stability of the cur-
rent collector in a specific electrolyte also have a strong eect
on ASCs’ lifetime and electrochemical performance (especially
for the working voltage). Besides, the structure or morphology
of the current collector has a huge influence on the utiliza-
tion rate of electroactive materials. Some corrosion-resistant
materials (e.g., Au, indium tin oxide (ITO), carbon-based
materials, and conducting polymers) are usually used as cur-
rent collectors when a strong acid electrolyte (e.g., HSO) is
employed because of its high corrosive nature. Besides, some
self-supporting, high conductive materials (e.g., carbon-based
composite films) have also been developed for SCs, which can
directly serve as the current collectors. Dierent current col-
lectors may own their unique advantages in dierent applica-
tions. For instance, ITO-based current collectors are favorable
to preparing the transparent devices because of their high
transparencies. The flexible cells usually employ self-standing
carbon- or conducting polymers-based materials as current col-
lectors. The Ni-based current collectors are commonly used in
alkaline electrolyte-based SCs owing to their high chemical and
good electrochemical stability in alkaline electrolytes. It should
be noted that the Ni-based current collectors may provide an
additional pseudocapacitive contribution due to the formation
of Ni oxides/hydroxides on their surface. Other metallic mate-
rials like the Inconel and stainless steel were also used
as current collectors in alkaline electrolyte-based SCs. Some
free-standing carbon-based materials like carbon clothes, elec-
trospun carbon fiber films, graphene films, CNT films, etc.
have received widely attentions thanks to their high conduc-
tivity, lightweight, outstanding mechanical strength, and good
flexibility.
Neutral aqueous electrolytes exhibited better compatibility
with dierent current collectors for SCs because of their much
less corrosive nature. However, with the increasing of working
voltage, especially for wide voltage ASCs, only carbon-based
current collectors can relatively keep these devices work stably
while metallic current collectors suer from corrosion and
gas production. Therefore, to avoid such issues from metallic
current collectors and simplify the fabrication process, a large
number of researches directly used self-supporting carbon-
based materials as current collectors. Especially in wide voltage
aqueous ASCs, the current collector must bear the eects of
aqueous electrolytes themselves and the high current/voltage
environment. However, their high cost and poor industrial pro-
cessability for a large scale may slow down the practical applica-
tions of wide voltage aqueous ASCs. Thus, the stable current
collector with low cost, high electronic conductivity, strong
corrosion-resistance, high industrial processability, or current
collector-free technology is highly desired to develop for pro-
moting the application process of wide voltage aqueous ASCs.
8.6. Further Fundamental Understanding through Both
Advanced Characterization Techniques and Theoretical
Investigations
“‘Technology is the first factor to promote the development of
science”’ and this holds the true whether it is in the past, or
present, or the future. Advanced in situ characterization tech-
niques are very important to investigate the complex interfacial
processes for both EDLC and pseudocapacitance. By combining
the coupling model with some advanced scattering methods
like small-angle X-ray scattering (SAXS) or neutron scattering
(SANS), the carbon-based materials have been explored with
more realistic structural models. Molecular dynamics (MD)
simulations and nuclear magnetic resonance (NMR) could
provide the ions distributions in confined carbon pores with
or without an applied potential, which significantly strengthen
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2108107 (59 of 68) © Wiley-VCH GmbH
the fundamental understanding of electrochemical behaviors
and ion dynamics. Advanced in-situ/operando experimental
methods like in-situ X-ray diraction, electrochemical quartz
crystal microbalance (EQCM), spectral characterizations can
oer deeper understandings of electrolyte distributions and ion
dynamics in carbon-confined pores.
For pseudocapacitive materials, in situ synchrotron X-ray
diraction is a powerful technique to investigate the structural
behaviors during charge storage. The applications of in situ
characterization technologies will be key and unique means to
study the microscopic pseudocapacitance mechanism of pseu-
docapacitive materials and for boosting their energy densities.
In addition, it is necessary to study deeply about the nature on
the dierence and similarity between pseudocapacitive mate-
rials and battery-type materials. It is no doubt that in situ meth-
odologies and theoretical calculation will promote the research
process in the field of SCs.
8.7. Device Innovation and Integration with Multifunctionality
So far, a variety of multifunctional SCs have been created and
developed. For example, alternating current (AC) line-filtering
SCs oer a possibility to replace aluminum electrolytic capaci-
tors to significantly decrease the size of future electronic cir-
cuits and electronic devices. Flow SCs have a great application
prospect in large-scale fixed energy storage systems like smart
grids. Metal ion hybrid SCs may hold the key to bridge the big
gap between SCs and conventional metal ion batteries. It should
be noted that the current ways for building metal ion capaci-
tors to enhance the energy density are usually achieved at the
cost of power density. As we all know, the insertion process of
metal ions into the bulk of negative electrodes is much slower
than that of EDLC in positive electrodes, which brings great
challenges to the searching of metal ion intercalation pseudo-
capacitive electrode materials. For MSCs, the unique structure
shortens the distance for electrolyte ions between the electrodes
and thus improves the capacitive performance. Furthermore,
dierent functions such as self-healing, electrochromism,
thermal-responsive, shape-memory, and light have been suc-
cessfully introduced into SCs. This is an important step to
realize smart SCs devices. Although some gratifying achieve-
ments have been made, the development of the new generation
of SCs is still at its early stage. There is still a lot of work that
need to be done before the real applications. There are several
fundamentally technical obstacles. Specially, the low energy
density always limits the development of SCs. In order to boost
the energy density of multifunctional SCs, the mass loading of
the electroactive material in per footprint area need to be con-
sidered. In general, the weight percentage of electroactive mate-
rials in a typical flexible SC is usually less than %, which will
lead to low energy density of the full device. Besides, the self-
discharge, mechanical reliability, and other factors should also
be paid attentions. Moreover, the cost of multifunctional SCs is
usually high. The multifunctional SCs with simple packaging
architectures, using cheap raw materials, and facile fabrication
processes should be preferentially developed. Furthermore, the
nanostructured materials’ design, optimization, and system
integration for large-scale production should also keep in mind.
8.8. Development of Standard Methods to Evaluate the
Performance of Supercapacitors
With the growing research in this field, it is very urgent to estab-
lish an appropriate and standardized method to evaluate and
compare the electrochemical performance of ASCs. Currently,
even for the same electrode material, it is dicult to compare
the performances reported in dierent literatures. A standard
method is required to fabricate electrodes and devices. Thus,
the details on active materials contents, area of electrodes,
loading density, and other conditions need to be described
minutely. Key parameters representing electrochemical perfor-
mance should also be standardized. For example, the mass in a
thin film electrode usually is negligible for the total mass of the
device, thus using gravimetric capacitance and energy density
to evaluate its performance is unreasonable. In these cases, it
is suggested to evaluate their performance with areal capaci-
tance, volumetric capacitance, volumetric energy, and power
densities rather than gravimetric ones. In such systems, their
gravimetric capacitance, energy, and power densities will be
unrealistically high, yet those features will not linearly increase
with the increasing of the electrode thickness. Recently, volu-
metric characteristics have become more and more important
than gravimetric ones, because future energy storage devices
normally need to store the maximum energy power in a very
limited space. Among the volumetric parameters, the volu-
metric capacitance is regarded as a key indicator to reflect the
intrinsic properties of the electrode material and compare the
performance with other materials.
It is believed that the wide voltage aqueous ASC is a very
emerging and environment-friendly technology, which is
expected to become a green and alternative solution to the
existing energy storage devices of commercial SCs and lithium-
ion batteries, and will make a great contribution to the current
energy crisis and environmental problems. In addition, the
preparation of high-performance wide voltage aqueous ASCs
from laboratory research to industrial production is highly
desirable, due to their largely improved energy density while
maintaining the intrinsic advantages of SCs and the simple,
green and uncomplicated fabricating environment. We sin-
cerely hope and believe that the existing and future eorts in
wide voltage aqueous ASCs can realize the ambitious goal of
improving their energy density close to that of thin-film bat-
teries, and applying as a new competitive pure power source
or a complement alternative with batteries to power consumer
electronics, electric vehicles, and other portable electronics in
the future.
Acknowledgements
This work was financially supported by the National Natural Science
Foundation of China (, , ), the NSFC-DFG
Joint Research Project (), and the Natural Science Foundation
of Jiangxi Province (BCB, ZDB).
Conflict of Interest
The authors declare no conflict of interest.
Adv. Funct. Mater. 2021,
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2108107 (60 of 68) © Wiley-VCH GmbH
Keywords
aqueous asymmetric supercapacitors, high energy density, matching
principles, optimization strategies, wide voltage
Received: August ,
Revised: September ,
Published online:
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Jun Huang is currently a Ph.D. student under the supervision of Prof. Yiwang Chen in Nanchang
University, Nanchang, China. He obtained his master’s degree from Nanchang University in ,
majoring in polymer chemistry. His current scientific interests mainly focus on advanced material
design for supercapacitors, micro-supercapacitors and hybrid ion capacitors.
Kai Yuan is a professor of Chemistry at Nanchang University. He received his first Ph.D. from
Nanchang University in under the supervision of professor Yiwang Chen. In , he joined
professor Ullrich Scherf’s group at the University of Wuppertal in Germany, where he obtained
his second Ph.D. (summa cum laude) in . His current scientific interests include graphene,
porous polymer networks, semiconducting (conjugated) polymers, D materials, and carbon
nanomaterials as well as corresponding hybrids for electronic and energy-related applications,
e.g., supercapacitors, metal–air batteries, and electrocatalysis, etc.
Yiwang Chen is a full professor of Chemistry at Nanchang University and Jiangxi Normal
University. He received his Ph.D. from Peking University in and conducted his postdoctoral
work at Johannes Gutenberg-Universität Mainz and Philipps-Universität Marburg in Germany
as awarded an Alexander von Humboldt fellowship. He joined the Nanchang University in .
He has been honored by the National Science Fund for Distinguished Young Scholars in .
Currently he is serving as a vice-president of Jiangxi Normal University since . He has ever
been Dean of the College of Chemistry at Nanchang University since –. His research
interests include polymer solar cells, perovskite solar cells, supercapacitor, electrocatalysis for
zinc–air batteries, and intelligent elastomer.
Adv. Funct. Mater. 2021,
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