Recent progress on freestanding carbon electrodes for flexible supercapacitors

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NEW CARBON MATERIALS
Volume 37, Issue 5, Oct. 2022
Online English edition of the Chinese language journal
Cite this article as: New Carbon Materials, 2022, 37(5): 875-897
Received date: 16 Jun. 2022; Revised date: 01 Aug. 2022
*Corresponding author. E-mail: q.lu@ifw-dresden.de; xipan@lzu.edu.cn
Copyright©2022, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
DOI: 10.1016/S1872-5805(22)60637-1
REVIEW
Recent progress on freestanding carbon electrodes for
flexible supercapacitors
Yi-rong Zhao1,2, Cong-cong Liu2, Qiong-qiong Lu2,*, Omar Ahmad2, Xiao-jun Pan1,*, Mikhailova Daria2
1School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China;
2Leibniz Institute for Solid State and Materials Research (IFW) Dresden e.V., Helmholtzstr. 20, 01069 Dresden, Germany
Abstract: The construction of flexible supercapacitors with high electrochemical performance and excellent mechanical properties to power
flexible electronics and sensors is very important. Freestanding electrodes play a crucial role in flexible supercapacitors, and carbon has been
widely used in this role because of its high electronic conductivity, tunable porosity, adjustable surface area, excellent mechanical properties, low
density and easy functionalization. It is also abundant and cheap. Recent progress on the fabrication of freestanding carbon electrodes based on
various carbon materials for use in flexible supercapacitors is summarized, and remaining challenges and future opportunities are discussed.
Key Words: Carbon materials; Pseudocapacitive materials; Freestanding electrode; Flexible supercapacitors
1 Introduction
The burgeoning implementation of flexible electronics
and sensors in everyday life has led to a strong thrust in the
development of next-generation flexible energy-storage
devices (supercapacitors (SCs) and batteries)[1-5]. Among those,
flexible SCs are a promising choice due to their
high-power-density and long cycling life[6-10]. However, the
practical application of flexible SCs is hindered due to
challenges of exploring electrode materials with high
electrochemical performance as well as maintaining their high
electrochemical performance under large strain. Since the
electrochemical performance of SCs rely heavily on the
electrode materials, the electrode materials with high
electrochemical performance and excellent mechanical
property are highly desired for flexible SCs. However,
traditional electrode materials suffer from inferior
electrochemical performance. In addition, most of the
electrode materials are in powder form and additional
non-active materials such as binder, conductive additives, and
metal current collectors, are required for the fabrication of
electrodes. The usage of these non-active materials
significantly reduces the energy densities for devices as well
as involves significant additional processing costs (Fig. 1a)[11].
SCs store charge via forming an electrical double-layer
due to the electrolyte ions adsorption on the electrode surface
or pseudocapacitive properties of electrode surface induced by
redox reaction (Fig. 1b)[12-13]. Electrical double-layer
capacitance of the electrode is determined by its accessible
surface area, the electrolyte, and thickness of the double layer
(the Debye length), which enables a fast charging/discharge
ability and a high cycling stability of the electrode[14]. On the
other hand, the pseudocapacitance of the electrode is related to
a redox reaction between the electrode materials and
electrolyte on the electrode surface. As compared to
electrochemical double-layer capacitance, pseudocapacitance
possesses a higher energy density, but comes with lower
power density and an inferior cycling stability.
Various carbon materials, including active carbon, carbon
nanotubes (CNTs), and graphene, have been widely used in
energy storage devices especially for electrical double-layer
capacitors, due to their high electronic conductivity and large
surface area, along with lightweight and low cost[15-23]. The
lightweight nature of carbon materials enables a high
gravimetric energy density, the large surface area provides
more active sites and the porous conductive structure
facilitates the electron and ion transport. Furthermore,
freestanding carbon electrodes, such as carbon cloth, CNTs
film, and graphene film, combined with solid-state electrolytes
have demonstrated their success in flexible SCs benefiting
from their interconnected conductive structure and excellent
mechanical properties[24]. Moreover, the various macroscopic
forms of freestanding carbon (yarn, film, foam) make it
possible to fabricate diverse configurations of flexible SCs
(fiber-shaped SCs, paper-shaped SCs, sponge-like SCs)[25-26].
Unfortunately, despite the advantages of freestanding carbon
materials, they normally exhibit a low electrochemical activity
owing to small surface area and low capacitance. Micropores
(<2 nm) accessible to the electrolyte are essential for charge
storage. Mesopores (2-50 nm) working as diffusion channels
facilitate the ion diffusion, thus improving the power
performance, while macropores (>50 nm) work as reservoir s

Yi-rong Zhao et al. / New Carbon Materials, 2022, 37(5): 875-897
of electrolyte ions[27]. The wettability toward electrolyte favors
electrolyte ions transport, and pseudocapacitance active sites
or materials contribute to capacitance. Accordingly, strategies
of optimizing pore structure, adjusting surface properties, and
integrating with pseudocapacitance materials have been
adopted to improve capacitance.
Although many reviews on carbon materials for
supercapacitor have been published, only a few reviews focus
on freestanding carbon electrodes for flexible
supercapacitor[27-35]. In this review, diverse freestanding
carbon electrodes based on various carbon materials such as
commercial carbon cloth/felt, biomass-derived carbon,
polymer-derived carbon, CNTs, graphene, etc. for flexible
SCs are summarized (Fig. 2). Additionally, various strategies
including activation approaches (wet chemical oxidation,
electrochemical oxidation, thermal activation, plasma
treatment) and integration routes (heteroatom doping and
hybridization with pseudocapacitance materials) for
improving the electrochemical performance as well as the
structure-performance correlation are discussed in detail.
Furthermore, the remaining challenges and future
development of these freestanding carbon electrodes for
flexible SCs are presented to fasten the development of
practical flexible SCs.
2 Freestanding carbon electrodes for flexible SCs
Freestanding carbon materials are promising electrodes
for flexible SCs due to their high electronic conductivity,
lightweight, tunable porosity, adjustable surface area, low cost,
and excellent mechanical properties. Thus, diverse
freestanding carbon electrodes, including commercial carbon
cloth/felt, biomass-derived carbon, polymer-derived carbon,
CNTs, and graphene, have been used and demonstrated
success in flexible SCs. To further improve the
electrochemical performance, activation approaches (wet
chemical oxidation, electrochemical oxidation, thermal
activation, plasma treatment, etc.) and integration routes
(heteroatom doping and hybridization with
pseudo-capacitance materials) have been developed[47].
2.1 Commercial carbon cloth/felt/yarn-based electrode
Commercial carbon fiber cloth has a high potential
towards freestanding electrodes for flexible SCs owing to its
high electronic conductivity and excellent mechanical
properties. Carbon fiber cloth consists of bundle carbon fibers,
which are prepared by carbonizing polyacrylonitrile and
mesophase pitch[48]. However, the pristine carbon fiber cloth
suffers from a low surface area, low porosity, and poor
electrochemical activity, resulting in a poor electrochemical
performance. Various approaches have been proposed to
improve electrochemical performance, such as wet chemical
oxidation[49], electrochemical oxidation[40], thermal
activation[50], plasma modification[ 51], heteroatom doping[52],
and hybridization with pseudo-capacitance materials[53].
To increase the surface area and to introduce
electrochemically active groups into carbon cloth, a wet
chemical oxidation using a strong acid is commonly applied.
Similar to the exfoliation of multiwalled CNTs by modified
Hummer’s method, the exfoliation and oxidation of carbon
cloth was realized by using a solution consisted of H2SO4,
HNO3 and KMnO4 with high oxidizing properties[4 9]. The
Fig. 1 (a) Schematic illustration of the structural and features of conventional electrode (upper case) and freestanding
carbon electrode (lower case). (b) Schematics of charge storage in SCs.
Fig. 2 Freestanding carbon electrodes based on various carbon
materials for flexible SCs[36-46]. (Reprinted with permission)

Yi-rong Zhao et al. / New Carbon Materials, 2022, 37(5): 875-897
obtained oxidized carbon fiber showed 50 nm thick porous
shells with a large amount of mesoporous pores, and a larger
surface area of 61.2 m2 g−1 as compared to untreated carbon
fiber (5.3 m2 g−1) as well as numerous oxygen-containing
functional groups (Fig. 3a-c). Although oxygen functional
groups are favored to boost the performance, their excess
would decrease electronic conductivity. Therefore, the
reduction was further conducted by hydrazine gas and thermal
reduction in an NH3 atmosphere, and the porous surface
morphology of oxidized carbon fiber was maintained after
reduction process. As a result, a capacitance of 88 mF cm−2
(8.8 mF g−1) at a scan speed of 10 mV s−1 was achieved in 1
mol L−1 H2SO4 electrolyte, while a capacitance of 31 mF cm−2
(1.55 mF g−1) at a scan speed of 10 mV s−1 and a high rate
capability (i.e., a capacitance retention of 73% with the scan
rate from 10 to 10 000 mV s−1) were realized for a flexible
symmetric solid-state SCs based on modified carbon fiber and
a poly(vinyl alcohol) (PVA)/H2SO4 gel electrolyte (Fig. 3d).
Because only the exfoliated carbon shell with a thickness of
50 nm works as “active” part, the capacitance of the activated
carbon fiber cloth is relatively low. Besides, different
reduction methods were developed based on acid-oxidation
process using modified Hummer’s method. For example,
Jiang et al.[55] adopted a thermally reduction method to treat
oxidized carbon fiber at 1 000 °C in a N2/H2 atmosphere for 3
h. Consequently, a capacitance of 485 mF cm−2 at a current
density of 2 mA cm−2 was achieved in 1 mol L−1 H2SO4
electrolyte, and the flexible asymmetric solid-state SC using
PVA/H2SO4 as electrolyte displayed a capacitance of 161.28
mF cm−2 at 12.5 mA cm−2 and an exceptional cycling stability
for 30 000 cycles. Additionally, hydrothermal approach was
also used to reduce the oxidized carbon fiber, while
simultaneously the oxygen species were replaced by nitrogen
dopants using hydrazine and ammonia[52] . The roles of
different nitrogen species in capacitance were investigated by
X-ray photoelectron spectroscopy and electrochemical tests. It
has been found out that the formation of pyridinic nitrogen
species is beneficial for fast electron transfer. Consequently,
the obtained N-doped carbon fiber showed a high capacitance
of 136 mF cm−2 at 0.5 mA cm−2 and a capacitance retention of
81% with the current density from 0.5 to 15 mA cm−2 in 1 mol
L−1 H2SO4 electrolyte. Furthermore, Miao et al.[56] tailored the
oxygen functional groups by adjusting the acid-oxidation
treatment time, and investigated the correlation between
functional species and capacitive performance. They found
out that the ―C―O species gradually increased with the
increase of treatment time, while the ―C＝O and ―COOH
species firstly increased with the increase of treatment time
and reached a maximum value at 12 h. The ―C―O and ―C
＝O groups contribute to capacitance due to a reversible redox
reaction, while ―COOH increases charge transfer resistance
leading to a reduced electrochemical performance. As a result,
the active carbon fiber with 12 h treatment time displayed the
highest areal capacitance of 5 310 mF cm−2 at a current
density of 5 mA cm−2 and a low capacitance decay of 7% after
5 000 cycles in 1 mol L−1 H2SO4 electrolyte. In addition,
solid-state symmetric SCs using PVA/H2SO4 as electrolyte
showed a high energy density of 4.27 mWh cm−3 at a power
density of 1.32 W cm−3 and excellent mechanical flexibility.
Another common method to activate carbon fiber is
electrochemical oxidation, which is more facile and
cost-effectiv e[4 0]. For instance, Wang et al. [4 0] p roposed an
Fig. 3 SEM images of carbon cloth (a) before and (b) after oxidation. (c) TEM image of oxidized carbon fiber. (d) Schematic diagram and
the digital picture of solid-state symmetric SCs with PVA/H2SO4 gel electrolyte[49]. (e) Schematic diagram of the carbon cloth after
activation. (f) A LED light was lit by SCs devices[40]. (g) Schematic diagram of preparing activated carbon felts with graphene nanosheets.
(h) The specific capacitance at different bending states in PVA/H2SO4 electrolyte. (i) Capacitance retention of the flexible device using
PVA/H2SO4 electrolyte at different bending angles test [50]. (j) Schematic diagram of the fabrication procedure of fiber-shaped SCs. (k)
Digital images of fiber-shaped SCs woven into a glove at different bending states. (l) Digital images of fiber-shaped SCs before and after
stretching [54]. (Reprinted with permission).

Yi-rong Zhao et al. / New Carbon Materials, 2022, 37(5): 875-897
electrochemical oxidization in a mixed acid solution. After
oxidation in H2SO4/HNO3 electrolyte at 3 V for 10 min,
core-shell carbon fiber with abundant oxygen-containing
groups (―C―OH, ―C＝O and ―COOH) and high surface
area of 88.4 m2 g−1 was obtained (Fig. 3e). Benefiting from
abundant electrochemically active oxygen-containing groups
and high accessible surface area, an areal capacitance of 756
mF cm−2 at 6 mA cm−2 was achieved in 5 mol L−1 LiCl
electrolyte, while a high energy density of 1.5 mW h cm−3 and
a stable cycling over 70 000 cycles were realized in
MnO2@TiN//active carbon fiber flexible asymmetric SCs
using 5 mol L−1 LiCl as electrolyte at the voltage window of 2
V. More importantly, LED indicators were powered for 10
min by two asymmetric SCs devices, demonstrating the
potential for practical use (Fig. 3f). Furthermore,
electrochemical activation in a mild neutral electrolyte
solution was explored[57]. After oxidation in 0.1 mol L−1
(NH4)2SO4 aqueous solution at 10 V for 15 min and then
reduction in 1.0 mol L−1 NH4Cl aqueous solution at −1.2 V for
30 min, the active carbon fiber cloth with micro-crack,
exfoliated carbon fiber shells, and abundant
oxygen-containing functional groups was obtained.
Consequently, a capacitance of 505.5 mF cm−2 at 6 mA cm−2
and a high capacitance retention of 88% at a scan rate varying
from 6 to 48 mA cm−2 was achieved in 1 mol L−1 H2SO4
electrolyte. In addition, other aqueous inorganic salt
electrolytes were also explored, such as, KNO3 solution[58],
and Na2SO4 solution[59]. Moreover, electrochemical oxidation
combined with the chemical oxidation was found to further
boost the performance[60]. The NiOOH decorated carbon fiber
was annealed and residual Ni was removed by acid.
Subsequently, the carbon fiber was oxidized by
electrochemical oxidation, resulting in the formation of
hierarchical pores and abundant pseudocapacitive oxygenic
groups. As a result, the obtained active carbon fiber cloth
exhibited a large areal capacitance of 1.2 F cm−2 at 4 mA cm−2
and stable cycling for 25 000 cycles in 5 mol L−1 LiCl
electrolyte. Remarkably, an exceptional energy density of 4.7
mWh cm−3 was realized in flexible solid-state asymmetric SCs
using 5 mol L−1 LiCl in PVA as gel electrolyte.
Thermal activation is another effective method to
increase the surface area of commercial carbon cloth[50].
Chemical activating agents (KOH, ZnCl2, etc.) are mostly
used as additives in the thermal activation process[61-62]. For
example, Zhang et al.[62] proposed KOH as activation agent
along with thermal annealing to improve the specific area and
electrical conductivity of carbon fiber cloth. After treatment,
the obtained activated carbon fiber not only maintained its
excellent mechanical property, it also showed a much higher
surface area of 2 780 m2 g−1 and an improved electrical
conductivity of 320 S m−1 as compared to pristine carbon fiber
cloth (~1 000 m2 g−1; ~100 S m−1, respectively). Thanks to the
improved surface area and electrical conductivity, the
obtained carbon fiber cloth exhibited a high capacitance of
~197 F g−1/~0.5 F cm−2 at 0.1 A g−1 and a high capacitance
retention of 99% after 15 000 cycles at 3.3 A g−1 in 6 mol L−1
KOH electrolyte. Furthermore, the symmetric solid-state SCs
using PVA/H3PO4 as gel electrolyte demonstrated excellent
flexibility and performance stability. Due to the corrosion of
used chemical activating agents and additional wash step, an
activation without additive is preferred. Zhao et al.[ 50] adopted
thermal treatment at 400 °C to activate commercial carbon
felts and treatment was optimized. After treatment, graphene
nanosheets along with abundant micropores and
oxygen-containing functional groups were formed on the
surface of carbon fiber, which endowed an improved rate
capacitance (i.e., 188 F g−1 at 0.1 A g−1 and 122 F g−1 at 5 A
g−1) and an improved cycling stability with a capacitance
retention of 99% after 1 000 cycles at 2 A g−1 in 1 mol L−1
H2SO4 electrolyte (Fig. 3g). More importantly, the assembled
flexible symmetric solid-state SCs using PVA/H2SO4 as
electrolyte maintains a stable performance even under
different bending states (Fig. 3h-i).
Plasma treatment is another facile method to modify the
surface of carbon fiber[51] . For example, Ouyang et al.[51] used
nitrogen plasma to treat commercial carbon cloth. As a result
of the reaction between N2+ species and carbon fiber,
nitrogen-containing functional groups were formed along with
nano-structuring, leading to a high surface area and improved
liquid electrolyte wettability. The resultant carbon fiber
showed a high capacitance of 391 mF cm−2 at 4 mA cm−2 in 1
mol L−1 KOH electrolyte, which is ascribed to the interaction
between the electrolyte and N species in addition to a large
surface area.
Combining commercial carbon cloth/felt with other
materials with pseudocapacitance is an effective method to
obtain satisfactory capacitance, in which carbon cloth/fiber
serves as flexible conductive substrate. Transition metal
oxides (RuO2, MnO2, Fe3O4, etc.) are a kind of desired
pseudocapacitance materials to combine with the carbon fiber
cloth due to their high capacitance[63]. A carbon cloth
decorated with hierarchical carbon-coated Fe3O4 nanorods
derived from metal-organic framework (MOF) grown on
carbon cloth, which was fabricated as an electrode for SCs by
Peng et al.[63]. Benefiting from the conductive scaffold of
carbon fiber cloth, large surface area of nanorod-shaped Fe3O4,
and carbon coating protection, a high capacitance (i.e., 463 F
g−1 at 1.5 mA cm−2) and a fast reaction kinetics were realized
in 1 mol L−1 Na2SO4 electrolyte. Furthermore, the flexible
asymmetric solid-state SCs combining MnO2 coated on
carbon cloth as cathode using carboxymethyl cellulose sodium
(CMC)/Na2SO4 was as a gel electrolyte displayed a high
energy density of 74.6 W h kg−1 at a high power density of
1 249.1 W kg−1 and a superior rate performance (i.e., 258 mF
cm−2 at 3 mA cm−2), as well as stable capacitance under
different bending conditions.
Polymers (polyaniline (PANI), polypyrrole (PPy),
polythiophene (PTP), etc.) are another kind of promising
pseudocapacitance materials to composite with carbon fiber
cloth due to their low cost and easy preparation procedure[64 ].

Yi-rong Zhao et al. / New Carbon Materials, 2022, 37(5): 875-897
Xiong et al.[64] combined nanoscale thin PANI with graphitic
petals (GF) grown on carbon cloth (CC) by electrochemical
polymerization. The CC provides a conductive and flexible
substrate, and conductive GF with large surface area facilitates
electron transport to PANI. Consequently, PANI displayed a
high capacitance up 2 000 F g−1 at 1 A g−1, and excellent rate
performance with a capacitance of 1 200 F g−1 at 100 A g−1 as
well as exceptional cycling stability with a capacitance
retention of 93% after 2 000 cycles in 1 mol L−1 H2SO4
electrolyte. In addition, solid-state SCs using PVA/H2SO4 as
electrolyte also showed an excellent flexibility and superior
electrochemical performance.
Except for the commercial carbon fiber macroscopic
cloth for planar flexible SCs, commercial carbon fiber
thread/yarn is also widely used for flexible fiber/yarn-shaped
SCs[54,65-66]. Jin et al.[54] fabricated PANI coated carbon fiber
thread (CFT@PANI) via electrochemical deposition as
positive electrode and functionalized carbon fiber thread
(FCFT) with oxygen-containing groups (―C―O, ―C ＝
O, ―COOH) by electrochemical oxidation after annealing for
solid-state fiber-shaped SCs (Fig. 3j). Benefiting from the
high electrical conductivity of CFT, the porous networks of
PANI, and abundant oxygen functional groups of FCFT, the
assembled flexible asymmetric SCs using PVA/H3PO4 as
electrolyte delivered a high energy density of 2 mWh cm−3
and a power density of 11 W cm−3. More importantly, the
fiber-shaped solid-state SCs maintained their capacitance
under bending and stretching states, while still showed an
excellent capacitance stability and stretchability when knitted
into textile (Fig. 3k, l).
2.2 Biomass-derived carbon electrode
To achieve sustainable development, it is necessary to
maximize the utilization of biomass materials due to their
merits of environmental friendliness, widespread availability,
renewable nature, and low cost. Biomass has been widely used
in various applications, and especially in SCs as source for
carbon materials. For example, shrimp skin, shaddock peel,
corn cob, and wood have been used in SCs[32,67]. Some
biomass materials (cotton, silk, bacterial cellulose, chitosan,
etc.) can also be used for fabricating freestanding carbon
electrodes for flexible SCs[68].
Cotton is the most common biomass material, and a good
template for freestanding carbon electrode[69-71]. Zhang et al.[72]
fab ricat ed a freestanding carbon electrode by pyroly zing
Fig. 4 (a) Schematic diagram of the synthesis process of activated textile carbon. (b) Electrochemical performance under different bending
angles and (c) performance retention of a device under bending of symmetric SCs device for 200 cycles[72]. (d) Schematic illustration of the
synthesis of CNF/GN composite materials. (e) Optical photo and (f) CV curves under different bending angles of CNF/GN composite
electrodes[73]. (g) Schematic illustration of the synthesis process for CS-MnO2 electrode. (h) Electrochemical performance of a symmetric
SCs device under different bending angles. (i) A watch calculator powered by three symmetric SC devices connected in series[74]. (j) Optical
images of the carbonaceous hydrogel and aerogel. (k) SEM image of the carbonaceous gels. (l) Optical image of carbonaceous aerogel. (m)
Optical images of hydrogel and aerogel mechanical properties. (n) Electrochemical performance of the electrode[75]. (Reprinted with
permission).

Yi-rong Zhao et al. / New Carbon Materials, 2022, 37(5): 875-897
cotton cloth followed by an activation using KOH (Fig. 4a).
The obtained activated carbon cloth shows a hollow tubular
structure, a high conductivity of 1 506 S m−1, and a high
surface area of 1 075 m2 g−1. The electrode, applied as the
electrode for SCs, demonstrated a higher capacitance of
1 026 mF cm−2 at 5 mA cm−2 than that of carbon cloth without
activation (741 mF cm−2) and a long-term stable cycling with
a capacitance retention of 97% after 10 000 cycles at 10 mA
cm−2 in 6 mol L−1 KOH electrolyte. More importantly, the
assembled flexible symmetric solid-state SCs using
PVA/KOH as electrolyte exhibited an energy density of 0.42
mWh cm−3 at a power density of 10.9 mW cm−3 and only 8%
capacitance loss after 200 cycles at a maximal bending angle
of 180° (Fig. 4b,c). Besides, Zhao's group developed
nitrogen-doped graphene complexed carbonized cotton (NGCs)
for flexible SCs using freeze-drying technique and
ammonia-assistant thermal activation processes[70].
Freeze-drying technique made the graphene oxide (GO)
nanosheets well mixed with carbonized cotton skeleton fibers,
and a cross-linked structure between GO and carbonized
cotton was formed. The NH3 not only has an etching effect on
carbon fibers to form a porous structure during the activation
process, but also enables nitrogen doping on the surface of
graphene oxide and cotton-derived carbon fiber. Thanks to the
synergy effect of graphene nanosheets and cotton fibers, the
obtained NGCs have a high capacitance of 291 F g−1 at
1.0 A g−1 in 1 mol L−1 H2SO4 along with high flexibility.
Moreover, the assembled lightweight and flexible symmetric
SCs based on the NGCs electrodes showed excellent
capacitance stability under bending in 1 mol L−1 H2SO4
electrolyte. In addition, pseudocapacitance materials grown on
carbon fiber derived from cotton were also developed for
flexible SCs and showed promising performance[71] . Jiang et
al.[71] fabricated Ni0.54Co0.16O nanosheets grown on carbon
fiber derived from cotton cloth (NCO-NSs/CFC) for flexible
SCs. Thanks to the ultrathin and porous Ni0.54Co0.16O
nanosheets with high utilization, interconnected conductive
carbon fiber cloth as current collector, and hierarchical
structure for fast ion and electron transport, the
NCO-NSs/CFC showed a high capacitance of 438 μAh cm−2
at 1 mA cm−2 and a capacitance retention of 70% after 10 000
cycles at 10 mA cm−2 in 2 mol L−1 KOH electrolyte. Moreover,
the flexible symmetric solid-state SCs using PVA/KOH as
electrolyte showed an energy density of 92.4 Wh Kg−1 at a
power density of 207.2 W kg−1 and an excellent flexibility.
Bacterial cellulose (BC) is another commonly used
biomass material, and is widely used as carbon electrode
material precursor because of its advantages of high
production, environmental friendliness, non-toxic nature, low
cost, and a good mechanical strength. The carbon materials
derived from BC possesses a conductive network connected
by nanofibers and a large specific surface area, which
facilitates charge transfer and storage properities[41,73]. Hao et
al[76]. prepared a carbon nanofiber network from BC (CN-BC)
via silica-assisted strategy. Silica shell is served as a
nanoreactor creating a confined environment during the
pyrolysis. The generated CO2 and H2O functioned as
activation agents inside the nanoreactor, leading to an
improved surface area and porosity. The obtained carbon
nanofiber shows three-dimensional (3D) interconnected
structure, a surface area of 624 m2 g−1, and
mesopore-dominated hierarchical porosity. The CN-BC
delivered a capacitance of 302 F g−1 in 6 mol L−1 KOH
electrolyte, and the symmetric SCs displayed a capacitance of
184 F g−1 at 0.25 A g−1 as well as a capacitance retention of 78%
at 10 A g−1 in 6 mol L−1 KOH electrolyte. Besides, Luo et al.[73]
prepared freestanding carbon nanofiber/graphene nanosheet
(CNF/GN) composite films by a membrane–liquid interface
culture method followed by carbonization. GNs was put into
the culture medium of BC, and BC was in situ grown on the
surface of GNs. After carbonization, CNF/GN composite
films show excellent flexibility, mechanical robustness, good
structure stability, and high specific surface area (Fig. 4d,e).
Consequently, the symmetric SCs using 1 mol L−1 Na2SO4
electrolyte based on CNF/GN electrodes offered a high energy
density of 20 Wh kg−1 and a high power density of 900 W kg−1,
and kept a stable capacitance at different bending states (Fig.
4f).
A freestanding carbon electrode derived from silk,
composed of silk fibroin and sericin, is also a promising
electrode for flexible SCs due to abundant nitrogen doping
and high flexibility[77]. Li et al.[78] fabricated heteroatom (N, O
and S) co-doped carbonized silk as a flexible carbon electrode
by dyeing silk with heteroatom-enriched dye, followed by
pyrolysis. The heteroatoms (N, O and S) co-doped carbonized
silk shows a higher surface area of 256.6 m2 g−1 than that of
carbonized silk (16.3 m2 g−1), a more defective structure, and
hierarchical porosity. In addition, the heteroatoms (N, O and S)
doping contributes extra pseudocapacitance and improves the
wettability toward electrolyte. As a result, the heteroatoms (N,
O and S) co-doped carbonized silk delivered a higher
capacitance of 255.95 F g−1 at 2 mV s−1 as compared to
carbonized silk (45.69 F g−1), and a good cycling stability with
8% capacitance decay over 5 000 cycles at 100 mV s−1 in 1
mol L−1 Na2SO4 electrolyte. Additionally, Xia et al.[74]
fabricated MnO2 coated carbonized silk fabric (CS-MnO2)
for flexible and shape-editable SCs (Fig. 4g). The freestanding
carbonized silk fabric serves as substrate, and the introduction
of MnO2 improves the hydrophilicity as well as capacitance
performance. As a result, the CS-MnO2 delivered a
capacitance of 105 F g−1 at 0.25 A g−1 in 6 mol L−1 KOH
electrolyte. More interestingly, the assembled SCs using 6 mol
L−1 KOH as electrolyte showed 101.34% of initial capacitance
under a 180° bending angle and can be modifited to desired
shape (Fig. 4h,i).
Carbon gels derived from biomass are also promising as
electrodes for flexible SCs due to high surface area, high
porosity, and low density[68]. Wu et al.[75] developed a
sponge-like carbonaceous aerogels using watermelon. The
obtained carbona ceou s gels have a micro -structure with

Yi-rong Zhao et al. / New Carbon Materials, 2022, 37(5): 875-897
carbonaceous nanofibers and nanospheres, and show an
excellent mechanical flexibility (Fig. 4j-m). Meanwhile, Fe3O4
was incorporated into the porous carbonaceous gels (CGs) to
get magnetite carbon aerogels (MCAs), in which CGs with
porous 3D conductive structure as scaffold and Fe3O4 provides
an additional pseudocapacitance. The MCAs keeps the porous
structure of the original CGs, which facilitates both ion and
electron transportation to the electrode surface. As a result, a
high capacitance of 333.1 F·g−1 at 1 A·g−1 and excellent
cycling stability with a capacitance retention of 96% after
1 000 cycles were realized in 6 mol L−1 KOH electrolyte (Fig.
4n).
2.3 Polymer-derived carbon electrode
Melamine sponge consisting of a formaldehyde-
melamine-sodium bisulfite copolymer is a good template for
freestanding carbon electrodes in flexible SCs owing to its
light weight, high nitrogen content, and high porosity[ 79]. For
example, Xiao et al.[45] fabricated a N-doped carbon foam as
electrode for compressible SCs by carbonization of melamine
sponges. The obtained N-doped carbon foams show features
of a lightweight, interconnected network, robustness, and
extraordinary electrolyte wettability (Fig. 5a,b). As a result,
N-doped carbon foam displayed a high capacitance of 52 F g−1
at 1 mA cm−2 in 5 mol L−1 LiCl electrolyte. Moreover, it can
bear a high compressive strain of 80% and a durability over
100 cycles with a negligible volume change under a constant
strain of 55%. Furthermore, the assembled symmetric
solid-state SCs based on carbon foam using PVA/LiCl as
electrolyte showed an excellent compressible and stable
electrochemical performance under different strains (Fig. 5c).
Normally, the freestanding carbon electrodes from direct
carbonized melamine sponge shows an uneven pore size
distribution and a low specific surface area. Activation process
is necessary to further improve the surface area and porosity.
Zhang et al.[80] adopted ZnCl2 additive for activation treatment
during pyrolysis process of melamine sponges. Because the
ZnCl2 could dehydrate carbon atoms, the porous structure was
formed on the surface of the carbon sponges. The mass ratio
of ZnCl2 and melamine was optimized and resultant carbon
sponge showed a highest capacitance of 242 F g−1 at 0.5 A g−1
and a high capacitance retention of 97% after 10 000 cycles at
5 A g−1 with the ratio of 1:10 for 6 mol L−1 KOH electrolyte.
More importantly, a solid-state SCs using PVA/KOH as
electrolyte showed an energy density of 4.33 W h kg−1 at a
power density of 250 W kg−1. Other additives (KOH, K2CO3,
etc.) were also applied for activation, and an improved
electrochemical performance was also realized[81-82].
Introducing the heteroatom in the carbon materials is other
most used approach to improving the capacitance via the
pseudocapacitive effect. Yang et al.[83] developed a 3D
B-doped N-containing carbon foams as electrode for SCs by
annealing boric-acid-infused melamine sponge. They found
out that the porosity of mesoporous structure and
concentration of B doping increased with the increase of
temperature from 500 to 700 °C, while the collapse of pore
structure and the formation of insulating boron nitride were
caused with the temperature increasing from 700 to 900 °C.
As a result, the sample annealed at 700 °C showed the highest
capacitance of 462 mF cm−2 at 0.2 mA cm−2 in 6 mol L−1 KOH
electrolyte. Furthermore, the fabricated solid-state symmetric
SCs using PVA/KOH electrolyte showed a capacitance
retention of 77% after 2 000 cycles at 6 mA cm−2. Moreover,
the two solid-state symmetric SCs can power a light consisting
of 43 LEDs for 5 min. Integration with other pseudocapacitive
Fig. 5 (a) Optical photos of melamine foam and N-doped carbon foam. (b) SEM image of N-doped carbon foam. (c) Digital photographs of
a device with the compressing and recovering processes[45]. (d) Schematic illustration of preparation of the flexible Ti3C2Tx/ANF electrodes.
(e) Optical photos of fiber SCs with different curve radii[84]. (Reprinted with permission)

Yi-rong Zhao et al. / New Carbon Materials, 2022, 37(5): 875-897
materials (metal oxides, metal sulfides, polymers) to boost the
capacitance was explored here as well. Shen et al.[85]
fabricated NiCo2S4 nanosheet decorated melamine sponge
derived N-doped carbon foam (NiCo2S4/NCF) as a flexible
electrode for SCs. The interconnected framework and good
adhesion enabled a fast electron and ion transport, a high
capacitance of 877 F g−1 at 20 A g−1 and an extraordinary
cycling stability in 6 mol L−1 KOH electrolyte. More
importantly, the assembled flexible asymmetric solid-state
SCs using 6 mol L−1 KOH as electrolyte based on
NiCo2S4/NCF and order mesoporous carbon decorated
N-doped carbon foam (OMC/NCF) displayed a high energy
density of 45.5 Wh kg−1 at 512 W kg−1.
Electrospun polymer nanofibers or micro-fiber is also a
good template for freestanding carbon electrode, which is
fabricated by electrospinning. Electrospinning is a simple,
low-cost, and high efficiency technology, which allows to
fabricate polymer nanofibers under electric field using
polymer solutions, such as polyacrylonitrile (PAN), polyimide
(PI), polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA),
poly (vinyl fluoride) (PVDF) etc.[86-92]. The obtained
electrospun polymer nanofibers or micro-fiber can be
transformed into carbon nanofibers and mats after
carbonization, and these electrospun-derived carbon materials
are lightweight, free-standing, and flexible, making them ideal
electrode candidates for flexible SCs[93-94]. A large variety of
electrospun-derived carbons have been developed as electrode
materials for flexible SCs. However, most of
electrospun-derived carbon materials exhibit small accessible
surface area leading to an inferior electrochemical
performance[95]. To improve the surface area, activation of
CNF with additional various types of oxidizing gases (steam,
CO2, etc.) or various chemical activators (KOH, H3PO4,
NaOH, etc.) is developed[96-100]. Ma et al.[99] reported a porous
electrospun phenolic-based carbon nanofibers paper with a
large surface area by coating KOH on electrospun fiber for
activation. The surface area and the concentration of
micropores of the obtained CNFs gradually increased with the
increase of KOH. When 20% of KOH was used, a high
specific surface area for CNFs of 1 317 m2 g−1 with an average
pore width of 2.12 nm was obtained. When electrode materials
for SCs is employed, the obtained CNFs achieved high energy
densities of 7.1 W h kg−1 and 27.9 W h kg−1 in aqueous
electrolyte (6 mol L−1 KOH) and organic electrolyte (1 mol
L−1 tetraethylammonium tetrafluoroborate/propylene
carbonate ((C2H5)4NBF4/PC)), respectively.
Expect for conventional surface activation process to
improve the porosity and surface area, the morphology,
composition, and structure (e.g. hollow, core-shell,
multi-channel) of carbon nanofibers can be tuned and
designed by easily adjusting polymer blends or using
additives[101-102]. The thermolabile polymers can be serveved
as sacrificial polymer and decomposed during carbonization to
create pores or hollow structure, such as PVP and
polymethylmethacrylate (PMMA), which allows to design
nanofibers with a large surface area and porous
structure[103-106]. The porosity provides channel for quick
transfer of electrolyte ions thus improving the charge storage
capacities and rate capability. Kim et al.[107] reported a unique
N-doped hierarchical porous core–shell CNFs (PHCNFs)
using a PVP/PAN as the shell material and
polystyrene-acrylonitrile (SAN) as the sacrificial core polymer
by co-axial electrospinning. The PVPs were etched by
immersing the electrospun fibers in N, N-dimethylformamide
(DMF)/water solution generating pores and texture surface,
and the residual PVP decomposes to nitrogen species under
carbonization at N2 atmosphere, while the SAN decomposes
during carbonization forming hollow structure. The hollow
core and porous structure facilitates rapid adsorption and
desorption of electrolyte ions, and abundant nitrogen species
contribute to pseudocapacitance and improve surface
wettability as well as electronic conductivity. As a result, a
high energy density of 4.12 Wh kg−1 at powder density of 15
kW kg−1 and a high capacitance retention of 92.33% after
10 000 cycles were achieved for optimized structure in 2 mol
L−1 KOH electrolyte. Similarly, the use of additional hard
sacrificial templates like metal oxides (SiO2, ZnO, etc.), slat
(ZnCl2, NaCl, etc.) and MOFs into the electrospun polymeric
solutions also can control over the porosity and surface area of
CNFs[108-112].
Pseudocapacitive materials, such as metal oxides and
conductive polymers, can be grown on prepared electrospun
fibers or carbon nanofibers by chemical synthesis methods
(electrodeposition, hydrothermal processes, co-precipitation
processes, etc.), giving rise to an improved electrochemical
performance as compared to carbon nanofibers[113-116]. For
instance, MnO-carbon nanofibers (MnO/CNF) composite was
prepared by adding manganese acetylacetonate to PAN
precursor for electrospinning and calcination process [117]. The
synergistic effect of pseudocapacitive MnO and porous carbon
nanofibers endows the assembled fiber SCs (FSC) with
improved specific capacitance of 200 F g−1 as compared to
pure PAN-based FSC (90 F g−1) and a stable electrochemical
performance over a wide bending range in 0.25 mol L−1
aqueous bis(trifluoromethane)sulfonimide lithium (LiTFSI)
electrolyte. Besides the conventional pseudocapacitive
materials, pseudocapacitive two-dimensional (2D) layered
nanomaterials (MXenes, black phosphorus, etc.), have been
intensely explored for flexible SCs because of their high
specific area, and excellent electrical and electrochemical
properties[118-120]. MXenes regarding as a large family of
two-dimensional transition metal carbides and nitrides, it can
be easily processed into various structures by forming stable
colloidal solutions due to their hydrophilic property[ 121].
Ti3C2Tx/ANF hybrid fibers were prepared by mixing
ultra-high mechanical strength aramid nanofibers (ANFs) with
Ti3C2Tx suspensions as electrospinning precursors by
solidification baths injected with FeCl2 solution (Fig. 5d)[84].
ANFs not only improved the flexibility and mechanical
pro perties of Ti3C2Tx-based fibers, but also prevented th e

Yi-rong Zhao et al. / New Carbon Materials, 2022, 37(5): 875-897
re-stacking of Ti3C2Tx nanosheets during the wet spinning
process. The prepared Ti3C2Tx/ANF hybrid fibers have
relatively regular cylindrical shapes, and the hybrid fibers
inherit the MXene microstructure and show an expanded layer
spacing. As the Ti3C2Tx/ANFs were employed as electrode of
flexible solid-state symmetric SCs, they delivered a high
specific capacitance of 295 F cm−3 at 0.25 A cm−3, and a high
energy density of 26.2 mWh cm−3, benefiting from rapid
carrier mobility in 3 mol L−1 H2SO4 electrolyte (Fig. 5e).
2.4 Carbon nanotubes-based electrode
CNTs, as a one-dimensional (1D) nanocarbon material, is
a research hotspot because of its high theoretical specific
surface area, extraordinary electronic conductivity, excellent
thermo conductivity, high Young’s modulus, and admirable
tensile strength[24]. Owing to the intrinsically flexible merits,
CNTs can be used as building blocks to assemble freestanding
macroscopic structures with different dimensions, such as 1D
fibers, 2D films, and 3D foams[33,124-125]. These macroscopic
CNTs forms with light weight and superior mechanical
properties play a vital role in high-performance flexible
electrodes for flexible SCs[126-127]. Wang et al.[122] reported the
single-walled carbon nanotubes (SWCNTs) as electrode
materials and cellulose nanofiber as separator materials to
fabricate SCs via a consecutive spray printing strategy (Fig.
6a). This technique could build all SCs components
consecutively and controllably, and the SCs can be printed as
different shapes on various rough or smooth surface.
Moreover, this kind of flexible SC device demonstrated a
stable capacitance performance at different bending states (Fig.
6b-e). In addition, CNTs films fabricated by vacuum filtration
using CNT suspension is often used for SCs[128-129]. However,
the CNTs randomly cross and bundle with each other,
resulting in a lower specific surface area. To overcome this, Li
et al.[130] designed a CNTs-aerogel electrode prepared by
electrochemical activation and freeze-drying method.
Comparing with conventional vacuum filtration method, the
CNT-aerogel possesses a large number of small pores
maintained by freeze-drying method and CNT fibers inside
connected each other with Y-shaped junctions forming a 3D
conductive network, endowing the CNT-aerogel with high
specific surface area and strong mechanical strength. The
micro-morphological interlinked CNT-aerogel offers fast and
continuous electron transport paths, thus greatly enhancing the
electrochemical performance of the assembled SCs. When
fibrous CNT-aerogel electrode was assembled as solid-state
fibrous SC using P(VDF-HFP)/EMIMBF4 as electrolyte, it
showed a high specific capacitance of 11.3 F g−1 at 2 mA and
a high energy density of 29.6 W h kg−1. Moreover, the
mechanical performance test was performed at 0.5 mA, and
the 92.9% capacitance could still be maintained even after
being bent for 2 000 times.
Introducing multidimensional carbon with rational design
can make CNTs showing a higher electrical conductivity and
mechanical strength[131- 134]. Xue et al.[135] reported a one-step
Fig. 6 (a) Schematic diagram of fabrication process of a fully printable SCs via consecutive spray printing. SCs could integrate on (b) PET,
(c) cloth, and (d) paper. (e) Normalized specific capacitance of SCs at different bending states[122]. (f) Schematic illustration of fabrication of
asymmetric SCs based on CNT-MnO2 film. (g) Optical pictures and schematics of the SCs before and after being cut[123]. (Reprinted with
permission)

Yi-rong Zhao et al. / New Carbon Materials, 2022, 37(5): 875-897
chemical vapor deposition (CVD) method to obtain 3D
graphene–CNTs hollow fibers, which shows cylindrical
structure with seamless graphene sheath and radially aligned
CNTs. The seamlessly joined 3D structure endows graphene–
CNTs hollow fibers with efficient isotropic electrical transport
capability. When the fibers were woven to electrode for
flexible SCs using PVA/H2SO4 as electrolyte, it showed a high
specific capacitance of 89.4 mF cm−2, and excellent cycling
stability under repeated bending up to 90°. In addition, a
partially unzipped carbon nanotube/reduced graphene oxide
(PUCNT/RGO) hybrid fiber with low level of "dead volume"
and well-ordered porous structure was fabricated by wet
spinning and chemical reduction from partially unzipped
oxidized carbon nanotube/graphene oxide (PUOCNT/GO)
solution[136]. PUOCNT with high hydrophilic property and
large surface area served as spacer to inhibit the restacking of
GO and reduced the spacer itself, leading to reduced “dead
volume” and large pores. As a consequence, the assembled
solid-state fiber-shaped SCs constructed from hybrid fibers
using PVA/H3PO4 as electrolyte showed excellent volumetric
capacitance of 62.1 F cm−3 and energy density of 8.63 mWh
cm−3 as well as a high capacitance of 97.8% after 2 000 cycles,
along with good mechanical stability with a capacitance
retention of 98.2% over 1 000 bending cycles. Besides, the
introduction of heteroatoms (e.g., N, B) can further improve
the polarizability of the carbon network and electrical
conductivity, leading to an improved capacitance[137-139].
Zhang et al.[140] showed that N-doped core-shell CNTs array
can be synthesized for highly stretchable SCs. CNT arrays
were synthesized from ethylene by CVD at 740 °C, and then
the N-doped layers were coaxially regrown from acetonitrile
by chemical vapor deposition at 1 060 °C. The introduction of
N-containing species can significantly enhance the specific
capacitance by providing pseudocapacitance, enhancing
specific capacitance by over 80 times compared with the
original CNT array. As a result, the assembled solid-state
symmetric SCs using PVA/H3PO4 as electrolyte displayed a
specific capacitance of 30.8 mF cm−2 under a strain of 400%,
and a 96% capacitance retention after stretching for 1 000
cycles.
To further improve the electrochemical performance,
CNTs can be combined with pseudocapacitive materials
(metal oxides, conducting polymers) to form a hybrid
electrode. In hybrid electrodes, the CNT network provides a
robust substrate, which effectively buffers the volume change
of the supported high-capacitance pseudocapacitive materials,
thus extending the cycling stability of the electrode. Various
metal oxides, such as MnO2, Co3O4, NiO, have been studied
as CNTs hybrid electrode for flexible SCs[123,141-145]. For
instance, MnO2 nanosheets were in situ grown on CNTs films
via a hydrothermal reaction, forming a thin and tough
CNTs-MnO2 composite film[123]. The active sites of redox
reactions, the electron transfer efficiency, and MnO2
utilization efficiency increased and thus the capacitance,
energy density, and rate performance improved. Moreover, the
flexible CNTs-MnO2 electrodes can not only be assembled
into flexible planar SCs, but also can be rolled and twisted to
build yarns that can be reconfigured into stretchable wire SCs
(Fig. 6f). The assembled yarn asymmetric SCs based on
CNTs-MnO2 and carbon fiber (CF) decorated with FeSe2
nanonuts (CF@FeSe2) using PVA/LiCl as electrolyte showed
a very high energy density of 29.84 Wh kg−1 at a power
density of 571.3 W kg−1 and good capacitance retention after
8000 cycles, as well as excellent flexibility (Fig. 6g). In
addition to hybridization with inorganic pseudocapacitive
materials, conducting polymers were electrochemically
deposited on CNTs. The conformal coating enhances
charge-storage capability, while nanotubes accelerates the
ionic diffusion, leading to a high power density[146-148].
However, wet-based electrochemical deposition processes
may introduce solvents that interfere with the nanostructure
and reduce the compatibility of the electrode with the
device[149]. Wardel et al.[150] reported poly(3-methylthiophene)
deposited on horizontally aligned carbon nanotubes
(P3MT/HACNT) flexible electrode via a solvent-free
oxidative CVD method. HACNT endows an excellent
mechanical support and enables a high ion and electron
transport. The P3MT/HACNT flexible electrode displayed a
high area capacitance of 3.1 F cm−2 at 5 mA cm−2 and retained
1.8 F cm−2 even at 200 mA cm−2 in 1 mol L−1 Et4NBF4/PC
electrolyte, while the flexible asymmetric SCs from
P3MT/HACNT and HACNT electrode using Et4NBF4/PC as
electrolyte exhibited maximum energy densities of 1.08 mWh
cm−2 and 1.75 W cm−2. In addition, electrochemical
performance remained unchanged under bending, showing a
high electrochemical and mechanical stability.
2.5 Graphene-based electrode
Graphene, as a kind of 2D nanosheet materials, has
becomes one of the most promising carbon materials used in
SCs due to its large theoretical specific surface area, excellent
electrical, thermal conductivity, as well as high chemical
stability. Owing to facile dispersion of graphene oxide (GO)
and reduced graphene oxide (rGO) without surfactants, they
are widely employed for fabricating freestanding graphene
electrodes in SCs. Moreover, GO or rGO have excellent
ability to be self-assembled into different dimensional
macroscopic forms (1D fibers, 2D films, 3D aerogels) with
extremely predominant mechanical properties by spinning[151],
filtration[152], printing[153], hydrothermal[154] approach, which
provides the chance to build different types of flexible
SCs[155].
Niu and co-authors combined photolithography with
electrophoretic buildup to fabricate a new kind of lateral
ultrathin rGO interdigitated microelectrodes (Fig. 7a)[156]. The
H3PO4/PVA gel electrolyte was used, and the flexible,
compact, ultrathin, and solid-state micro-SCs was successfully
fabricated (Fig. 7b-d). The design of lateral interdigitated
microelectrodes shortens the diffusion distance of the
electrolyte, and shows effectiveness in using the
electrochemical surface area of graphene layers. As a result,

Yi-rong Zhao et al. / New Carbon Materials, 2022, 37(5): 875-897
the micro-SCs showed a high power density and high stability
under bending (Fig. 7e, f). Besides, Zhao et al.[154] fabricated a
rGO electrode for flexible micro-SCs (MSCs) using
hydrothermal method with GO dispersion. The obtained rGO
sponge possesses 3D porous structure with conductive
interconnected networks, which endows an outstanding
mechanical properties and superior electrochemical
performance. As a result, electrode with a high graphene
loading of 21.1 mg cm−2 reached a capacitance of 569.5 mF
cm–2 at 0.5 mA cm–2 in 1 mol L−1 Na2SO4 electrolyte.
Furthermore, the capacitance showed no attenuation after a
bending test from the angle from 0o to 180o, and it also kept
98.4% of capacitance after 2 000 repetitious of fatigue test at
the bending angle of 90o using PVA/LiCl as electrolyte.
Besides, He et al.[151] fabricated rGO fiber for flexible SCs via
a wetting-spinning processing of GO dispersions with
different coagulants. The morphologies and properties were
investigated when different cations (Ca2+, Fe3+, Al3+) are used.
They found that solid-state flexible SCs based on
Al3+-containing rGO using PVA/H2SO4 as electrolyte showed
the highest capacitance of 148.5 mF cm−2 at 40 mV s−1 and
fiber-shaped flexible SCs demonstrated high capacitance
retention after 1 000 bending cycles. This excellent
electrochemical performance is ascribed to the highest
electrical conductivity and higher toughness due to highest
binding energy between Al3+ and GO, which facilitates the
electron transport, electrolyte ions diffusion, and adsorption.
Heteroatom doping, such as N, can be introduced to improve
the conductivity and contribute to pseudocapacitance. Jin et
al.[152] fabricated a free-standing N-doped graphene aerogel
(NG) film via hydrogel method. Firstly, the GO suspension
was mixed with pyrrole (Py) monomer, the formed hydrogel
Fig. 7 (a-b) Schematic process of preparing rGO micro-SCs. (c-d) Optical images of micro-SCs. (e-f) The electrochemical performance of
the micro- SCs based on rGO film[156]. (g) Schematic illustration of fabricating process of rGO/PVA/H2SO4 electrolyte. (h) CV curves of
rGO@PVA composite films with different PVA contents at 50 mV s−1. (i-j) Electrochemical performance of the flexible SCs at different
bending states and with loading. (k) Optical images of a LED light lighted by 2 SC devices before and after bending[39]. (Reprinted with
permission).

Yi-rong Zhao et al. / New Carbon Materials, 2022, 37(5): 875-897
was further mixed with GO suspension, followed by vacuum
filtration, freeze drying, and pyrolysis. The obtained NG film
possesses a macro-porous structure, which ensures fast ion
adsorption. In addition, nitrogen species converted from
pyrrole endows better conductivity, while pyridinic and
pyrrolic N contribute to pseudocapacitance. Benefiting from
these outstanding properties, the NG film reached a
capacitance of 455.4 F g−1 at 1 A g−1 and no obvious
capacitance loss after 5 000 cycles in 0.5 mol L−1 H2SO4
electrolyte, while the asymmetric SCs based on NG and
rGO/PPy electrode exhibited energy density of 34.51 Wh kg−1
at a powder density of 849.77 W kg−1 in 0.5 mol L−1 H2SO4
electrolyte.
However, graphene nanosheets are prone to
self-aggregation stack through a strong π–π bond during the
preparation of the electrode, which would reduce the surface
area and thus restrict the electrochemical performance of
capacitance. One of the strategies is introducing other guests.
Zhang et al.[157] introduced layered double hydroxide (LDH)
nanoplatelets into rGO layers by filtrating the LDH and GO
solution, followed by reduction. The LDHs not only create
mesopore for high accessible surface area, but also contribute
to extra pseudocapacitance. The rGO with a conductive and
robust network facilitates fast transport of electron. As a result,
the obtained electrode showed a capacitance of 122 F g−1 at
0.5 A g−1 and a high volumetric capacitance of 43.5 F cm−3 at
1 A g−1 in 1 mol L−1 KOH electrolyte, while the assembled
flexible asymmetric solid-state SCs based on rGO/LDH and
rGO using PVA/KOH as electrolyte exhibited maximum
energy densities 22.6 Wh kg−1 and 1.5 kW kg−1 as well as a
Table 1 Comparison of comprehensive performance of freestanding carbon electrodes based on various carbon materials
for flexible SCs.
Type
Material
Electronic
conductivity
Flexibility/
Mechanical
strength
Capacitance and
cycling stability in
flexible SC
Advantage
Disadvantage
Commercial
carbon cloth
Carbon fiber textile
actived by
electrochemical
oxidation and
chemical oxidation[60]
Excellent
17.4 MPa
0.67 F cm−2 at 2
mA cm−2 89.5%
retentation after
30000 cycles
Commercially
available, high
conductivity, high
tensile strength,
moderate cost.
High weight, small
surface area.
Carbon fiber cloth
actived by KOH and
annealing [62]
3.2 S cm−1
Excellent
0.5 F cm−2 at 5 mV
s−1
Biomass-derived
carbon
Cotton-derivered
carbon[72]
15.06 S cm−1
0.3 MPa with 6%
strain
153 mF cm−2 at 1
mA cm−2 92%
retentation after
200 cycles
Renewable raw
materials, low cost.
Weak mechanical
strength, low
conductivity, low surface
area.
Bacterial cellulose/graphene
composite[73]
0.37 S
cm−1
0.67 MPa with
60% strain
178 F g−1 at 1 A g−1
~100% retentation
after 1000 cycles
Silk-derived carbon
decorated with MnO2[74]
Good
550 kPa with 7%
strain
Polymer-derived
carbon
Electrospun-derivecarbon
23 S
cm−1
-
-
Raw material
commercially
available,
chemically dopant
rich.
Low conductivity.
CNTs
CNTs film fabricated by
spay[122]
624 S
cm−1
-
120-129 F g−1 at
0.5 A g−1
High conductivity,
excellent
mechanical
property.
Catalyst residue, high
dispersing difficulty, high
production cost.
Partially unzipped CNTs/rGO
fiber[136]
28.2 S
cm−1
134.4 MPa with
3.1% strain
45.6 F g–1 at 0.1 A
g−1 97.8%
retentation after
20000 cycles
Graphene
rGO fiber[136]
25.1 S
cm−1
151.2 MPa with
5.3% strain
12.3 F g–1 at 0.1 A
g−1
High conductivity
and high surface
area (CVD
method), functional
group rich and high
dispersion ability
(Hummer’s
method), scalable
(mechanical
exfoliation).
Low density, weak
mechanical strength and
high production difficult
(CVD method), low
conductivity and layers
stacking (Hummer’s
method), layers stacking
and high difficulty of
dispersing (mechanical
exfoliation).
rGO fiber coagulated with
Ca2+[151]
171.3 S
cm−1
164.9 MPa with
10.3 strain
286.59 mF cm−2 at
0.53 mV s−1
rGO/PVA composite film[39]
Excellent
283 MPa with
0.9% strain
184.6 F g−1 at 1 A
g−1

Yi-rong Zhao et al. / New Carbon Materials, 2022, 37(5): 875-897
stable cycling over 5 000 cycles with a capacitance retention
of 94%. Polymer coupled with GO sheet was also used to
inhibit the aggregation of rGO during reduction process. For
example, Cao et al.[39] prepared flexible rGO@PVA film by
simultaneously reducing GO/PVA solution and assembling
rGO@PVA with molecular level coupling using Zn foil as
reductant. Due to the introduction of PVA, rGO and
neighboring rGO were separated and linked by PVA via
hydrogen bonding, which endow a high strength and Young’s
modulus. In addition, after immersing rGO@PVA into H2SO4,
the PVA/H2SO4 electrolyte between rGO sheets provide
transport channels for fast ion transport (Fig. 7g). As a result,
the composite film displayed a high energy density of 7.18
mWh cm−3 and power density of 2.92 W cm−3. Moreover, the
symmetric SC exhibits a stable capacitance under different
bending states and with loading, and two assembled SCs could
power LED before and after bending (Fig. 7h-k).
A porous and dense rGO film with a highly accessible
surface area and continuous ion transport network is desired
for high volumetric energy densities electrodes[158]. Yang et
al.[159] fabricated a compact graphene electrode by controlled
removal of a volatile solvent trapped in the gel. The rGO
hydrogel films obtained by filtration were exchanged with a
mixture of volatile (deionized water) and nonvolatile liquids
(liquid electrolyte, sulfuric acid, 1-ethyl-3-methylimidazolium
tetrafluoroborate (EMIMBF4)). After removing the volatile
liquid by vacuum evaporation, the thickness of rGO film was
reduced, and nonvolatile liquid remained trapped in the rGO
sheets forming ion transport channels. Benefiting from the
highly accessible surface area, network for fast ion transport,
high electrolyte wettability, and a high packing density, the
rGO electrode trapped EMIMBF4 between the layers shows a
maximum power density of ~ 75 kW L−1. In addition, Li et
al.[160] prepared a monolithic ultra-thick and dense rGO
electrode by combining capillary drying and ZnCl2 as pore
former. The rGO shows a maximum surface area of 1 000 m2
g−1 and maintains a high density of 0.6 g cm−3. As a result, the
symmetric SCs based on rGO with a thickness of 400 µm
using BMIMBF4 as electrolyte exhibited a capacitance of 150
F cm−3 and ~65 W h L−1. Although a good balance between
the porosity and density was achieved, the flexibility is
sacrificed to some extent.
To further improve the mechanical and electrochemical
performance, rGO was integrated with other materials. Zou et
al.[161] fabricated cellulose fibers/rGO/Ag nanoparticles film
via vacuum filtration and reduction by a hot hydrazine vapor.
Thanks to the high electronic conductivity (sheet resistance:
0.17 Ω sq−1) and excellent flexible, the composite film with a
thickness of 400 µm delivered a maximum capacitance of
1 242.7 mF cm−2 at 2 mA cm−2 and high cycling stability with
a capacitance retention of 99.6% after 10 000 cycles at 60 mA
cm−2 in 2 mol L−1 KCl electrolyte, while the assembled
flexible symmetric solid-state SCs using PVA/KCl as
electrolyte showed a high areal energy density of 95 µWh
cm−2 and stable performance under bending. In addition, Du et
al.[162] prepared ultraflexible MnO2@rGO film for in-plane
quasi-solid-state micro-SCs. The rGO film was firstly
fabricated by electrophoretic deposition of GO, followed by
HI reduction, and the MnO2 was in situ growth on rGO via a
hydrothermal reaction. The rGO film treated with HI shows an
excellent electronic conductivity and outstanding mechanical
properties due to highly stacked layers, which acted as a
subtract and current collector for MnO2. The assembled
symmetric MSCs using PVA/LiCl as electrolyte exhibited a
high capacitance of 31.5 mF cm−2 at 0.2 mA cm−2, a high
capacitance retention of 77.0% after 6 000 cycles.
3 Summary, present challenges and outlook
So far, recent advances of diverse freestanding carbon
electrodes for flexible SCs are summarized systematically.
However, further work still remains to do for development of
flexible carbon electrodes in flexible SCs. We outline some
key challenges below which need to be focused on for pushing
the flexible SCs towards practical realization.
Key advantages and disadvantages of these freestanding
carbon electrodes based on various carbon materials are
outlined in Table 1. Due to the advantages of high electronic
conductivity, excellent mechanical property and moderate cost,
the commercial carbon cloth/felt seems promising for practical
application in flexible SCs, but it faces the disadvantage of
high weight and low surface area. Biomass-derived materials
have the merits of renewable raw materials and low cost,
while their weak mechanical properties and inferior
conductivity limit their practical application in flexible SCs.
Electrospun fiber derived materials possesses the advantage of
simple fabrication and structure tunable, it also suffers from
the shortcoming of weak mechanical strength and low
conductivity. Large-scale application of high-quality carbon
nanotube and graphene-based materials by the CVD method is
still challenging considering the high fabrication cost.
Industrial CNTs with low cost normally shows unsatisfactory
properties owing to moderate purity. Other graphene materials
fabricated by large-scalable fabrication method have
individual limitations. For example, the graphene prepared by
reduction of graphene oxide has shortcoming of low
conductivity and many layers stacking, the graphene prepared
by mechanical exfoliation faces the disadvantage of high
dispersing difficulty and many layers stacking. Additional
efforts are needed to develop high electronic conductivity, low
cost, and excellent mechanical freestanding carbon electrodes.
Activation is required to improve the surface area and
porosity, and chemical activation using KOH and NaOH as
activation agents was mostly used for activation. However,
this method suffers from corrosive and pollution issues, and
post-treatment is required. Green activation method is
essential to develop. In addition, the activation processes may
reduce mechanical stability or destroy the fibrous structure.
Thus, the balance of porous and mechanical properties is
needed to realize for flexible electrodes.

Yi-rong Zhao et al. / New Carbon Materials, 2022, 37(5): 875-897
To hybridize pseudocapacitance materials with
freestanding carbon materials, chemical methods such as
hydrothermal synthesis and CVD, were widely used. However,
the hydrothermal possesses the shortcoming of long reaction
time and harsh reaction conditions. CVD has drawbacks of
special equipment required and high cost. The method for
large-scale practical application is needed to explored. In
addition, additional efforts are also needed to achieve the good
adhesion between the carbon electrode and additional
pseudocapacitive materials as well as homogeneous
distribution of pseudocapacitive materials.
Further advances can be achieved by using appropriate
geometrical design and rational structure design. Appropriate
geometrical design strategies have demonstrated high
potential for robust flexibility and stretchability, such as the
wavy-shaped design, winding/spring-shaped design, as well as
“origami” and “kirigami” inspired structures. These
geometrically designed structures can release external stress
through geometrical transformation. The all-in-one structured
flexible supercapacitor possesses advantages of high
resistance to deformation and low interface resistance.
Further advances can also be achieved by using new
emerging technologies. For example, 3D printing techniques
including direct ink writing printing, inkjet printing,
stereolithography, and fused deposition modeling printing, are
promising for the fabrication of various architectures of the
freestanding electrodes. Moreover, diverse materials (polymer,
metals, metal oxides, carbon, ceramics, etc.) can be used for
the fabrication of freestanding electrodes by 3D printing
technology. More importantly, 3D printing also possesses the
advantages of a highly automatic manufacturing process,
facile route, and low cost for practical application.
To fasten the development of flexible SCs, other
components (electrolytes, separators, substrate, encapsulating
materials, etc.) need to be further explored. The novel
configurations of flexible SCs are also need to designed to
match well with other electric devices.
For a better understanding, investigation of mechanical
behavior of a flexible electrode and the whole device under
bending deformation by the finite element method is needed.
Acknowledgements
The authors acknowledge the financial support from the
China Scholarship Council (CSC, 202006180045,
202108080263), the financial support from German Research
Foundation (DFG) under the joint German-Russian DFG
project “KIBSS” (448719339), and Federal Ministry of
Education and Research (BMBF) under the projects “HeNa”
(03XP0390C) and “KaSiLi” (03XP0254D).
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