Biomass-Derived Carbon Materials for High-Performance Supercapacitors: Current Status and Perspective

Abstract

Supercapacitors are electrochemical energy storage systems that depend on high-surface-area electrodes and can play a dominant role in areas that require high power delivery or uptake. And of various electrodes, biomass-derived carbona-ceous electrodes have recently shown impressive promise in high-performance supercapacitors because of their widespread availability, renewable nature and low-cost electricity storage. Based on this, this review will discuss the current status of biomass-derived carbon materials in supercapacitors and highlight current research with a specific emphasis on the influences of structure and elemental doping on the electrochemical performance of corresponding carbon electrodes. This review will also discuss the gap between laboratory achievements and practical utilization in terms of these biomass-derived carbon materials and outline practical strategies for future improvement.

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Electrochemical Energy Reviews
https://doi.org/10.1007/s41918-020-00090-3
REVIEW ARTICLE
Biomass‑Derived Carbon Materials forHigh‑Performance
Supercapacitors: Current Status andPerspective
JiangqiZhou1· ShilinZhang3· Ya‑NanZhou1· WeiTang1 · JunheYang2· ChengxinPeng2· ZaipingGuo3
Received: 11 July 2020 / Revised: 2 September 2020 / Accepted: 6 December 2020
© Shanghai University and Periodicals Agency of Shanghai University 2021
Abstract
Supercapacitors are electrochemical energy storage systems that depend on high-surface-area electrodes and can play a
dominant role in areas that require high power delivery or uptake. And of various electrodes, biomass-derived carbona-
ceous electrodes have recently shown impressive promise in high-performance supercapacitors because of their widespread
availability, renewable nature and low-cost electricity storage. Based on this, this review will discuss the current status of
biomass-derived carbon materials in supercapacitors and highlight current research with a specific emphasis on the influences
of structure and elemental doping on the electrochemical performance of corresponding carbon electrodes. This review will
also discuss the gap between laboratory achievements and practical utilization in terms of these biomass-derived carbon
materials and outline practical strategies for future improvement.
Keywords Biomass-derived carbonaceous· Electrodes· Structural engineering· Doping effects· Supercapacitors
1 Introduction
The depletion of fossil fuels and corresponding environmental
issues have spurred the intense development of sustainable
and high-performance energy storage technologies to meet
global needs [1, 2]. Of these technologies, supercapacitors
(SCs) show great promise for application in portable devices,
stationary energy storage systems and electric vehicles (EVs)
due to merits including high specific power (10kWkg–1), long
cycle life (> 105) and ultrafast charge/discharge (within several
seconds) [3–5]. In general, two types of storage mechanisms
exist for SCs in which one involves electrical double-layer
capacitors (EDLCs) as represented by carbon-based materials
and is dependent on physical electrostatic attraction between
ions and electrode surfaces, whereas the other is based on the
pseudocapacitance principle in which energy storage is gen-
erated through fast and reversible redox reactions or Faradic
charge transfer reactions (e.g., in transition metal oxides and
hydroxides) [6–8]. Regardless of the mechanism, however,
electrode materials in SCs play a significant role in deter-
mining electrochemical performance [9–11]. Because of this,
tremendous efforts are being made to develop novel electrode
materials for high-performance SCs to meet the increasing
needs of emerging technologies [12, 13]. Among these novel
electrode materials, carbon materials have attracted intense
attention due to unique properties such as functional surfaces,
cost-effective nature, superior electronic conductivity and
excellent cycling stability [14, 15]. Despite this, the wide-
spread development of functional carbonaceous materials is
limited by corresponding fossil fuel-based precursors (e.g.,
coal, cinder and asphalt) and inferior rate capabilities as well
as complex and energy-intensive synthesis processes [16, 17].
To address these issues, biomasses have recently gained
attention as viable feedstocks in the synthesis of carbon
Jiangqi Zhou, Shilin Zhang and Ya-Nan Zhou have contributed
equally to this work.
* Wei Tang
tangw2018@mail.xjtu.edu.cn
* Chengxin Peng
cxpeng@usst.edu.cn
* Zaiping Guo
zguo@uow.edu.au
1 School ofChemical Engineering andTechnology, Xi’an
Jiaotong University, Xi’an710049, Shaanxi, China
2 School ofMaterials Science andEngineering, University
ofShanghai forScience andTechnology, Shanghai200093,
China
3 School ofMechanical, Materials, Mechatronic,
andBiomedical Engineering, Faculty ofEngineering
andInformation Sciences, University ofWollongong,
Wollongong, NSW2522, Australia

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materials for SC electrodes due to their renewable nature, pro-
ductivity and environmentally friendly characteristic [18, 19].
In addition, these biomass-derived carbonaceous materials can
be produced from various sources in daily life, such as flax,
lignin, peanut shells, cornstarch and ox horns [20–22]. And
unlike other carbon materials, biomass-derived carbon materi-
als can readily inherit the unique structure, defects and chemi-
cal composition of biomass precursors [23, 24] during carboni-
zation, allowing resulting biomass-derived carbon materials
and corresponding composites to deliver high capacitance,
improved rate stability and excellent cycling performance at
low cost in high-performance SCs (Fig.1). Here, electrochemi-
cal performance is mainly related to the physicochemical prop-
erties of carbon materials. For example, only suitable pore dis-
tributions can enhance capacitance and rate cyclability because
narrow pore distributions can result in large inaccessible areas
for ion transportation, whereas broad porous structures can
induce possible structural collapse [25, 26]. Dimensional
variations (e.g., spheres, tubes, sheets, hierarchical structures)
[27–30] can also lead carbon materials to exhibit different elec-
trochemical behaviors due to different physicochemical prop-
erties. In addition, the influence of dopant dosages needs to be
carefully studied for optimized electrochemical capacitance
because doping is not always effective in the improvement of
electrochemical performance. And although various aspects
have been discussed by researchers, the relationship between
the physicochemical properties of biomass-derived carbon
materials and the electrochemical performance of SCs needs
to be further investigated.
Based on this, this review will summarize the recent
advancements of biomass-derived carbon materials and
their composites in SC applications with a focus on spe-
cific characteristics including diverse morphology, surface
properties, synthesis protocols and electrochemical perfor-
mances (Fig.2). This review will also provide insights into
current challenges and propose future perspectives for the
better development of SCs. Overall, this review can serve as
a research bridge between the laboratory and industry with
regard to renewable material-based electrodes in SCs.
2 Strategies toImprove thePerformance
ofBiomass‑Derived Carbon Electrodes
inSCs
In terms of electrode materials, the search for carbon elec-
trodes with high surface areas, suitable pore size distri-
butions and heteroatom dopant incorporation to optimize
capacitance and conductivity without sacrificing stability
is gaining increasing attention from researchers [31]. And
among various energy storage materials, carbon materials
derived from biomass precursors have attracted significant
Fig. 1 Current progress of
biomass-derived carbon materi-
als for SC applications

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interest due to competitive advantages including abundance
and environmental-friendliness (Table1). In this section,
the influences of morphology, dimension and heteroatom
doping on the electrochemical properties of biomass-derived
carbon electrodes are fully addressed. A summary of carbon
materials derived from biomass precursors as SC electrodes
Fig. 2 Proposed strategies based
on biomass-derived carbon
materials for SC application
improvement
Table 1 Summary of biomass-derived carbon materials as SC electrodes
Biomass SSA (m2g−1) Heteroatoms Specific capaci-
tance (Fg−1)
Measurement condition Electrolyte Reference
Plants
Zucchini 185 I 347 2mVs−1 1M (1M = 1molL−1)
KOH
[45]
Kapok 1600 N, O 141 1Ag−1 TEABF4/PC [62]
Salvia-splendens 1051 N, O 294 1Ag−1 6M KOH [73]
Petals
Bagasse 1892.4 O 268 2mVs−1 6M KOH [97]
Industrial products
Chitosan 2905 N, O 375 1Ag−1 6M KOH [106]
Ants 2650 N, S, O 576 1Ag−1 6M KOH [93]
Gelatin 393 B, N, O 358 0.1Ag−1 6M KOH [122]
Chitin 1000 N, O 110 10000mVs−1 6M KOH [41]
Agricultural products
Popcorn 3301 N, S, O 286 90Ag−1 6M KOH [81]
Flour 1313 N, O 473 0.5Ag−1 6M KOH [124]
Rape pollen 2488 N, O 290 0.5Ag−1 6M KOH [107]
Domestic residues
Waste newspapers 2812 N, S, O 308 1Ag−1 6M KOH [136]
Walnut shells 3577 O 330 0.1Ag−1 6M KOH [77]
Shrimp shells 1946 N, O 322 0.5Ag−1 6M KOH [78]
Fungus or algae
β-cyclodextrin 661 O 322 0.5Ag−1 6M KOH [115]
Red algae 4037 S 335 1Ag−1 6M KOH [135]

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is provided in Table1. Corresponding typical synthesis and
applicable activation processes are summarized in Table2
and are discussed in detail.
2.1 Diverse Morphology
Biomass materials naturally possess abundant and diverse
macrostructures ranging from zero to three dimensions that
can be inherited and evolved by corresponding biomass-
derived carbon materials and to date, various nanostruc-
tured carbon materials with zero-dimensional (0D) spheri-
cal structures, one-dimensional (1D) nanofibers/nanotubes,
two-dimensional (2D) nanosheets and three-dimensional (3D)
porous structures have been synthesized. The main motiva-
tions to create electrode materials with different dimensional
nanostructures are to enlarge exposed active surface areas,
broaden ionic channels and accelerate electron conductivity,
all of which can significantly promote SC electrochemical
performance [32].
2.1.1 Zero‑Dimensional Structures
0D carbon-based materials generally include microstructures
such as carbon dots (CDs), solid nanoparticles and hollow
nanoparticles, and as a representative 0D material, biomass-
derived carbon spheres possess great advantages including
space-efficient packing, high tap density and large specific
surface area (SSA). According to the literature, most bio-
mass-derived carbon spheres possess smooth surfaces and
can be synthesized through general hydrothermal reactions
without further treatment from biomass precursors such as
sugar [33, 34], oatmeal [35] and bamboo [36]. In addition,
the size distribution of these biomass-derived carbon spheres
varies from hundreds of nanometers to several micrometers
and is predominantly decided by the treatment procedure.
Notably, however, most prepared spherical carbon materials
exhibit dense characteristics, which can hinder ion diffu-
sion and transportation across interfaces between electrode
materials and electrolytes [37]. Based on this, porous bio-
mass-derived carbon nanospheres have attracted increasing
attention in the past few decades [38–40]. For example, Liu
etal. [39] prepared high-porosity glucose-derived nanopo-
rous carbon spheres through facile KOH activation as an
SC electrode and obtained a high specific capacitance of
405Fg–1 at a current density of 0.5Ag–1 with outstanding
cycling stability, resulting in a high-energy-density output
of 53.5Whkg–1. Zhang etal. [41] also prepared 3D inter-
connected microspheres with a carbon nanofiber framework
from the chitin precursor as an SC electrode and obtained
an optimal deliverable energy density of 58.7Whkg−1 in
a typical organic electrolyte, which they attributed to the
promising structure of the electrode with high porosity and
high specific surface area.
The development of suitable hollow structures for car-
bon-based electrodes can also enhance SC electrochemical
performance [42, 43]. For example, Tang etal. [42] synthe-
sized hollow carbon spheres ~ 100nm in diameter through a
template method in which glucose was used as the biomass
precursor and latex as the template and reported that the
as-obtained hollow structure effectively enhanced charge
transport and reduced diffusion distance. Moreover, Wei
etal. [43] prepared carbon nanorings from batatas leaves
and stalks through carbonization in a tube furnace under
inert atmosphere and attributed the preparation mechanism
to depolymerization and dehydration in the lignin or cellu-
lose under high temperature (Fig.3a) in which the release of
volatile gas during subsequent decomposition can allow for
the formation of interconnected carbon nanorings for further
application in SCs (Fig.3b, c). As a result, this microstruc-
tured electrode material provided a specific capacitance of
350Fg–1 at a current density of 1 A g–1 along with a remark-
able energy density of 24.5Whkg–1.
Carbon quantum dots (CQDs) are another typical 0D
material that consists of sp3 hybridized carbon ranging in
size between 2 and 10nm. As green and sustainable carbon
precursors, the development of biomass-derived CQDs for
carbon-based electrodes is also promising for practical SC
application. In addition, CQDs can serve as spacers interca-
lated into sheet layers to further enhance performance [44].
For example, Gomes etal. [45] prepared relatively uniform
GQDs with a size distribution of 3–10nm from waste zuc-
chini through a one-step hydrothermal process (Fig.3d) and
reported that the insertion of CQDs into graphene layers can
restrain the restacking of graphene layers (Fig.3e), result-
ing in the composite displaying a specific capacitance of
374Fg−1 at 2mVs−1 and 71.7% capacitive retention at a
high scan rate of 1000mVs−1.
Overall, biomass-derived carbon nanospheres with
porous structures can not only ensure convenient and effi-
cient electron transport, but also provide sufficient electrode/
electrolyte contact due to porous and interconnected struc-
tures. In addition, corresponding rich channels or active
sites as provided by high surface areas in biomass-derived
carbon structures are beneficial for rapid charge transfer as
compared with dense carbon spheres. Moreover, the perfor-
mance of obtained CQDs can be regulated by fine-tuning
diameters, developing facile dispersions, enlarging surface
areas and so on. Despite this, the template-assisted method
used to synthesize these spherical porous structures is com-
plex considering large-scale production for practical applica-
tion. Therefore, the further optimization of biomass-derived
carbon spheres needs to focus on facile synthesis methods.

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Table 2 Strategies to improve the electrochemical performance of biomass-derived carbon electrodes
Strategy Advantages Examples Disadvantages
Nanostructure design
0D Space-efficient packing
Reduced diffusion distance
Porous carbon spheres [38–41]
Hollow carbon spheres [42, 43]
Carbon quantum dots [45]
Complex synthesis
1D Convenient ion transport pathways
Rich surface functional groups
Excellent mechanical properties
Carbon fibers [56–59]
Porous carbon fibers [60]
Hollow carbon fibers [65, 66]
Limited tap density
2D High electric conductivity
High contact area
Rich active sites
Graphene-like carbon nanosheets
[73, 76]
Thin carbon flakes [78, 79]
Possible restacking
3D Effectively enlarged active surface
area
Improved electrochemical perfor-
mance
Hierarchically porous carbon
[91–94]
3D carbon aerogel [96, 97]
Low tap density
High surface area More exposed surface area for
charge storage
Physical activation [109]
Chemical activation [103–105]
Template-based activation [106]
Serious environmental concerns
Multiple ionic transportation chan-
nels
Hard template methods [112, 113]
Soft template methods [114, 115]
No template methods [82]
Complex, expensive and corrosive
Possible structural collapse after
pyrolysis
Doped with heteroatoms Increased active sites
Enhanced pseudocapacitance
N-doped carbon [120–129]
P-doped carbon [134]
S-doped carbon [135]
N-, S-doped carbon [136]
N-, P-doped carbon [138]
Complex post-/pretreatment
Construction of flexible or binder-free electrodes Accelerated ion diffusion
Reduced electrolyte diffusion resist-
ance
Flexible carbon fibers [139]
Flexible carbon films [122, 140]
Carbon sponge/self-supporting
electrodes [141]
Low energy density
Composite electrode High energy density Carbon/MnO2 [55, 141, 142, 153,
154]
Gr/lignin [159]
MWNTs/cellulose composites [161]
Interfacial interaction issues
Low mass loading

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2.1.2 One‑Dimensional Structures
With advancements in the field of biomass-derived carbon
materials, the physicochemical properties of 1D materials
have increasing influences on the electrochemical proper-
ties of SCs in which based on measurements and simula-
tions, researchers have found that ions and electrons can
move through carbon tubes at exceptionally high rates due
to direct channels functioning as convenient pathways [46,
47]. In addition, functional groups (e.g., hydroxyl groups)
anchored on 1D carbon structure surfaces can enhance wet-
ting behaviors between electrodes and electrolytes to signifi-
cantly influence the fill level of electrodes with electrolytes
[48]. The excellent mechanical stability provided by unique
1D structures can further prevent structural damage under
harsh testing conditions, especially in flexible SC systems
[49, 50]. Because of all of this, 1D biomass-derived carbon
materials are extremely promising and numerous biomass
candidates have been investigated and developed to satisfy
the requirement for tubelike morphology and in general,
these biomass precursors include Artemia cyst shells [51],
wood fibers [52], bacterial cellulose [53], cotton fibers [54]
and willow catkin [55].
As an ideal building block to develop fibrous carbon
materials for advanced SC systems, cellulose has been
widely studied as a biomass precursor due to its thermal
Fig. 3 a Schematic of the
synthesis of nanofibrous carbon
microspheres. b, c SEM images
of carbon microspheres. d
Ragone plot of nanofibrous
carbon microspheres in ionic
liquid. Reprinted with permis-
sion from Ref. [43]. Copyright
© 2017, Royal Society of
Chemistry. TEM images of
d CQDs and e after inserting
CQDs into graphene layers.
Reprinted with permission from
Ref. [45]. Copyright © 2019,
Elsevier

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stability, rich chemical composition and natural abundance
[56, 57]. For example, Cai etal. [57] prepared functional
N-doped carbon fibers (N-CNFs) through the electrospin-
ning of cellulose acetate nanofibers followed by deacetyla-
tion and reported that as assembled as an SC electrode, a
specific capacitance of ~ 236F g−1 was achievable as well
as ~ 1045F g−1 if Ni(OH)2 was mixed with the N-CNFs,
indicating that nanofibers derived from cellulose are promis-
ing electrode materials for high-performance SCs. Despite
this and the fact that cellulose is renewable and abundant
biomass for the synthesis of carbon nanofibers, popular cel-
lulosic precursors [e.g., (C6H10O5)n] suffer from low car-
bonization yields (10%–15%), whereas the economically
viable production of carbon fibers requires precursors that
are easily converted to carbon at higher yields. In addition,
intermolecular hydrogen bonding and stacking interac-
tions between cellulose sheets in cellulose result in poor
solubility in common solvents, making the construction of
nanofibrous assembly from cellulose difficult. To overcome
these issues, Kuzmenko etal. [58] synthesized N-doped
CNFs through the regeneration of cellulose impregnated
with NH4Cl solution to result in high yields that improved
from 13% to 20% as well as distinct fiber diameters between
70 and 400nm. Lignin as a by-product of paper produc-
tion is another viable biomass precursor for the synthesis
of fibrous carbon materials in which Berenguer etal. [59]
successfully achieved interconnected and porous carbon
fibers (CFs) through the use of electrospun lignin/ethanol
solutions with subsequent thermal carbonization, resulting
in a flexible SC assembled by using the interconnected and
porous CFs demonstrating superior power and energy densi-
ties of 61kWkg−1 and 10Whkg−1, respectively, as well as
long-term stability for 100000 charge–discharge cycles at
a current density of 5Ag−1. Here, these researchers found
that the structure of CFs was effectively controlled by heat-
ing rates and that only slightly fast heating rates can result
in the special structure.
Aside from conventional fibrous structures, the introduc-
tion of suitable pore distributions into 1D carbon materi-
als can further regulate electrochemical performance. For
example, Li etal. [60] extracted sisal-derived activated
carbon fibers (Fig.4a) from sisal through pretreatment fol-
lowed by high-temperature carbonization and KOH activa-
tion and found through TEM that the sisal-derived activated
carbon fibers preserved the fiber shape along with abundant
nanopores, including mesopores and a multilayer structure
(Fig.4b, c). This porous structure shortened ion transport
pathways and strengthened inner pore conductivity, allowing
the optimized sample to exhibit a capacitance of 415Fg−1 at
0.5Ag−1 in 6M (1M = 1molL−1) KOH. Dong etal. [61]
Fig. 4 a Schematic of the synthesis of porous activated carbon fibers.
b, c TEM images of the carbon fiber. Reprinted with permission from
Ref. [60]. Copyright © 2019, American Chemical Society. d Sche-
matic of the synthesis of hollow carbon microtubes (HCMTs). SEM
images of e KFs and f an HCMT. Reprinted with permission from
Ref. [62]. Copyright © 2018, Royal Society of Chemistry

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also prepared carbon plates containing 3D interpenetrating
rich porosity and binary channels through carbonization
using pomelo peel as the biomass precursor in which the
as-obtained material was composed of hollow intercon-
nected carbon tubes 20–50μm in diameter. And as an SC
electrode, this material provided large specific capacitances
of 197Fg–1 at 1Ag–1 and 172Fg–1 at 10Ag–1 along with
a capacitance decay rate of only 8.3% after continuous 5000
cycles at a current density of 5Ag–1, indicating excellent
cycling stability under high current density.
Researchers have also devoted efforts to prepare 1D
biomass-derived carbon materials with hollow structures
because they can not only serve as ion buffering reservoirs
to improve charge storage, but also reduce electrolyte ion
diffusion pathways [62–64]. For example, Wang etal. [65]
prepared a hollow carbon material through the carboniza-
tion of cotton and reported that a corresponding SC using
this hollow carbon material displayed an enhanced electrical
double-layer capacitance of 175Fg–1 at 1Ag–1 with out-
standing rate capability (107Fg–1 at 60Ag–1). Similar to
cotton, Li etal. [66] reported that natural kapok with a high
hollow ratio of 97% was another perfect template to produce
1D hollow carbon materials, but that the narrow tube thick-
ness of natural kapok (~ 0.74mm) was difficult to retain
during processing at high temperatures above 500°C. To
address this problem, Cao etal. [62] developed a novel pre-
stabilization method involving (NH4)2HPO4 before carboni-
zation and post-activation (Fig.4d) in which (NH4)2HPO4
can act as an important cross-linking agent to stabilize the
natural hollow structure of kapok (Fig.4e, f). As a result,
this special hollow carbon tube material exhibited a high
specific capacitance of 140Fg–1 at 1.0Ag–1 with remark-
able energy density.
Based on research into traditional 1D carbon materials
derived from biomass, their improved capacitance, high rate
performance and long cycling stability can be attributed to
the unique morphology of 1D structures. And although
cellulose and lignin are widely used biomass precursors,
corresponding complex treatment processes (e.g., long ice
water bath or electrospinning) as well as unsatisfactory car-
bon yields limit potential for industrial application. In addi-
tion, possible restacking or aggregation in 1D structures as
induced by strong van der Waals interactions hinders ion and
electrolyte diffusion, which leads to rapid capacitance decay.
Therefore, the search for suitable biomass precursors and
proper treatment methods to withstand deformation during
synthesis is vital.
Compared with traditional fibrous carbon materials, how-
ever, porous and hollow 1D carbon materials are more prom-
ising in terms of practical application due to their ability to
shorten ion diffusion pathways and enable rapid electrolyte
ion transport. And although the low tap density of porous
or hollow carbon tubelike structures is associated with low
energy density, the suitable preparation of composite elec-
trodes or heterostructures can fully overcome this limitation.
In addition, many biomass-based carbon materials with 1D
structures are promising building blocks for flexible elec-
trodes that can display excellent electrochemical perfor-
mance in laboratory settings despite challenges in scale-up
production to satisfy practical requirements.
2.1.3 Two‑Dimensional Sheet Structures
2D carbon materials have drawn increasing attention from
researchers as compared with 0D and 1D carbon materi-
als due to unique advantages including: (i) strong in-plane
covalent bonds that can provide high electronic conductiv-
ity [66, 67], (ii) suitable layer thicknesses and broad lateral
dimensions that can shorten ion transport pathways [68, 69]
and (iii) large exposed surface atoms on both sides that can
provide sufficient electrolyte contact areas [70], all of which
can enable the high-performance of carbon-based electrodes
in SCs [71]. And among various 2D carbon materials, gra-
phene has been widely studied for application in SCs due to
its superior conductivity as compared with other 2D carbon
materials. However, the production of graphene-based mate-
rials requires chemical vapor deposition (CVD), mechanical
stripping, epitaxial growth, etc. [72], all of which are com-
plex, energy-intensive and expensive. In addition, possible
restacking or aggregation during the synthesis of graphene-
based materials severely hinders SC electrochemical per-
formance [31], especially rate capability. To resolve these
issues, researchers have successfully synthesized graphene-
like carbon materials with suitable surface areas and pore
distributions from renewable and cheap biomass precur-
sors through efficient and environmentally friendly routes
[73–75].
For example, Liu etal. [73] prepared graphene-like
porous carbon nanosheets (GPCNs) through a mixture strat-
egy involving salt sealing combined with the post-thermal
treatment of Salvia-splendens petals and reported that the as-
prepared GPCN-SS-800 electrode (GPCNs thermally treated
at 800°C) possessed a graphene-like sheet structure and
demonstrated superior rate capability at a specific current
of 100Ag–1 in both basic (capacity retention of 88.6% in
6M KOH) and neutral (capacity retention of 83.4% in 1M
Na2SO4 solution) media as well as a high energy density
of 20.9Whkg–1 in neutral electrolytes. Liu etal. [76] also
reported that a typical porous carbon nanosheet-like struc-
ture can be easily achieved through the direct pyrolysis of
Perilla-frutescens leaves (PFCs) in which an assembled sym-
metric SC using a PFC-700 (acquired at 700°C) electrode
achieved a high volumetric energy density of 14.8WhL–1
and a long cycling life (capacitance retention of 96.1% after
10000 cycles at 2Ag–1). Here, the outstanding electrochem-
ical performances of these carbon nanosheets were mainly

Electrochemical Energy Reviews
1 3
attributed to the special 2D structure and the wide distribu-
tion of pore sizes from micropores to mesopores.
Apart from the direct pyrolysis of biomass precursors
to synthesize carbon sheet structures with special surface
properties (e.g., surface area and pore distribution), post-
activation is also an effective method to engineer carbon
sheet surface properties. For example, Yang etal. [77] pre-
pared sheetlike porous carbon nanosheets with thin carbon
walls from walnut shells (Fig.5a) in which with the help
of post-activation, these researchers were able to achieve
high yields reaching 15%–23%, which was much higher
than that of traditional biomass-based activated carbons
(~ 5%–10%). In addition, the as-prepared carbon material
possessed a sheet structure with thin pore walls consisting
of 1–2 carbon layers (Fig.5b), and as an electrode mate-
rial, demonstrated a specific capacitance of 330Fg–1 at
0.1Ag–1 and high capacitance retention of 81% from 0.1 to
100Ag–1. More remarkable, a corresponding SC demon-
strated ultrahigh power and energy densities of 100kWkg–1
and 120Whkg–1 in 1-ethyl-3-methylimidazolium tetrafluor-
oborate (EMIMBF4).
Thin 2D carbon sheets with controllable morphology
and porous structures are also pivotal to the development
of high-performance SCs. For example, Tian etal. [78] fac-
ilely fabricated porous carbon nanosheets from shrimp shells
through ultrasonically assisted liquid exfoliation followed by
activated carbonization and reported that the as-designed
hierarchical structure with graphene-like morphology dem-
onstrated a large SSA value of 1946m2g–1, a rich nitro-
gen content (8.75wt%) and a high electronic conductiv-
ity (7.8Scm–1), resulting in an assembled SC achieving
superior capacitance performance (322Fg–1 at 0.5Ag–1),
Fig. 5 a Schematic of the synthesis of carbon sheets from walnut
shells. b TEM images of the obtained carbon sheets. Reprinted with
permission from Ref. [77]. Copyright © 2020, Elsevier. c Schematic
of the synthesis of ultrathin porous carbon nanosheets. d SEM and e
TEM images of the carbon nanosheets. f Volumetric capacitance of
an optimal supercapacitor cell at different current densities. Reprinted
with permission from Ref. [79]. Copyright © 2018, Elsevier

Electrochemical Energy Reviews
1 3
outstanding rate capability (241F g–1 at 100A g–1) and
long-term stability (98.3% capacitance retention after 20000
cycles at 10Ag–1). Similarly, Liu etal. [79] reported that
fish scales can serve as a suitable precursor for the devel-
opment of multilayer carbon materials in which N, S-dual-
doped ultrathin porous carbon nanosheets (PCNSs) were
prepared through an activation-assisted method by using
fish scales (Fig.5c). Here, the ultrathin nanosheet structure
possessed a thickness of only 3–5nm along with a unique
microporous distribution (Fig.5d, e) and with further treat-
ment at 600°C, the resulting nanosheets were able to pro-
vide a high gravimetric/volumetric capacitive performance
of 306F g–1/324Fcm–3 at a current density of 1 Ag–1
(Fig.5f) with an energy density of 12.8Wh L–1 for an
assembled symmetric SC.
Agricultural or industrial product-based biomass
resources such as agaric [80], popcorn [81], chitosan [82],
eggplant [83], biomass-based waste [84, 85] and silk [86]
are also options for the synthesis of carbon sheets due to
rich functional groups. For example, Wang etal. [87] syn-
thesized 2D carbon sheets with highly porous structures
from sulfonated pitch through interfacial self-assembly fol-
lowed by activation in which during the preparation process,
the physical/chemical force fields of sulfonated pitch can
be altered by P123 through the formation of firm hydrogen
bonding due to rich oxygen groups. Here, these researchers
reported that increasing temperatures can result in the rear-
rangement of the carbon skeleton through the aromatization
of the carbon material, resulting in the final sample showing
a high specific capacitance of 90.1Fcm–3 at 0.5Ag–1 and
a volumetric energy density of 39WhL–1.
2.1.4 Three‑Dimensional Porous Structures
In terms of improving contact areas between electrolyte
solutions and electrodes, 3D porous structures with intercon-
nected ion pathways are promising due to unique structural
merits [88, 89]. Simulations also indicate that better electro-
chemical performance can be obtained by applying carbon
materials with hierarchical porous structures as electrodes
for SC applications [90] in which the interconnected porous
structure of 3D carbon materials can generally endow high
electrical conductivity, superior structural stability and rapid
diffusion kinetics for ion transfer. In addition, large electro-
chemically active surface areas can provide abundant active
sites for charge storage, all of which make 3D porous carbon
materials suitable for widespread application in energy stor-
age applications such as high-performance electrodes.
For example, Zhou etal. [91] successfully prepared a
3D hierarchically interconnected porous carbon material
through the pyrolysis of Rhus typhina fruit followed by
post-activation in which the pores inside the Rhus typhina
fruit precursor were well maintained without significant
collapse during preparation, resulting in widespread porous
distribution in the as-acquired carbon material. Ma etal.
[92] also easily mass-produced polyhedral macrotube car-
bon arrays with a hierarchical porous structure using lotus
stems and reported that benefitting from a unique tertiary
porous structure with hierarchical porosity and large SSA
(> 2500m2g–1), the sample exhibited a high specific capaci-
tance of > 370Fg–1 at 1Ag–1 in 6M KOH and achieved an
energy density of 9.3Whkg−1 in a symmetric SC device.
The direct carbonization of biomass precursors without pre-
treatment is another method to obtain 3D structures with
abundant pores. For example, Zhao etal. [93] obtained hier-
archical porous carbons (HPCs) through the one-step car-
bonization of powdered ants (Fig.6a) in which the resulting
3D scaffolding framework, heteroatom-doped carbon and
uniform pore distribution (Fig.6b, c) were mainly attributed
to the exoskeleton of the ant precursor, which is comprised
of various chitin and protein/fatty acid sources. And owing
to the remarkable surface structure, an ultrahigh specific
capacitance of 576Fg–1 was obtained at a high current den-
sity of 1Ag–1 in the 6M KOH electrolyte as well as a high
energy density of 107Whkg−1 in a corresponding symmet-
ric SC with the EMIMBF4 ionic liquid electrolyte (Fig.6d).
Yu etal. [94] further applied the direct carbonization of
urine to synthesize a porous carbon structure in which the
various mineral salts within the urine backbone can serve as
a template to be evaporated during carbonization to result in
a rich porous carbon framework after acid etching.
As another type of hierarchically structured porous car-
bon material, carbon-based aerogels have attracted intense
interest for application in SCs because of their unique 3D
structure, chemical stability and flexibility [31], and to date,
various synthesis techniques have been explored, including
self-assembly, template assembly and solvent exchange
[95]. Alternatively, facile and low-cost approaches derived
from biomass precursors have been proposed to prepare 3D
carbon aerogels. For example, Zu etal. [96] synthesized a
carbon aerogel with a high SSA of 1873m2g–1 and a high
pore volume of 2.65cm3g–1 following two steps includ-
ing the pyrolysis of cellulose aerogel and post-activation by
carbon dioxide. Hao etal. [97] also fabricated a hierarchical
porous carbon aerogel using bagasse aerogel as the biomass
precursor using multiple steps including freeze-drying, car-
bonization and NaOH activation (Fig.6e), resulting in a
porous carbon aerogel with high SSA and suitable pore size
distribution to facilitate ion transport and improve electro-
chemical performance.
Overall, biomass materials have been demonstrated to
be appropriate precursors in terms of dimensional benefits
for the facile preparation of carbon-based electrodes due to
advantages such as the use of green precursors and a sim-
ple equipment setup in which different carbon materials
with varying dimensions derived from biomass precursors

Electrochemical Energy Reviews
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Fig. 6 a Schematic of the synthesis of 3D hierarchical porous carbon.
b SEM image of the porous carbon. c Nitrogen adsorption–desorption
isotherm of the porous carbon. d Ragone plot of the porous carbon
electrode in EMIMBF4. Reprinted with permission from Ref. [93].
Copyright © 2018, Elsevier. e Schematic of the synthesis of carbon
aerogel. Reprinted with permission from Ref. [97]. Copyright ©
2014, Royal Society of Chemistry

Electrochemical Energy Reviews
1 3
possess intriguing physicochemical properties that can
enhance the rate capacity and cycling performance of cor-
responding SCs. For example, rate capability can be signifi-
cantly enhanced in 1D carbon structures due to direct chan-
nels acting as convenient pathways for both ion and electron
transport, whereas typical 2D carbon electrodes can enhance
electronic conductivity and their large lateral dimensions can
be easily applied to further improve the capacitance and rate
capability of corresponding SCs. Alternatively, 3D hierar-
chical carbon structures with greater exposed facets and tun-
able porous structures are ideal choices in high-performance
SC applications. Despite this, most reported electrochemi-
cal performances of biomass-derived carbon structures are
closely related to corresponding physicochemical proper-
ties such as modifications (suitable pore distributions, het-
eroatom doping, enlarged surface area, etc.) and not only to
dimensional differences. Because of this, the elucidation of
the relationship between structure and electrochemical per-
formance in the design of high-performance electrodes from
biomass precursors is difficult, and therefore, more efforts
are needed. Moreover, although pretreatment can enhance
carbon yields, most carbon yields from biomass precursors
are low and are not suitable for large-scale SC application.
In addition, interfacial issues between carbon electrodes and
electrolytes need to be further discussed because SC perfor-
mances can easily be modulated by increasing the wettabil-
ity of carbon frameworks.
2.2 Surface Area andPore Size Distribution
Intriguing carbon nanostructures with large specific sur-
face areas can enhance the electrochemical performance
of SCs. In particular, carbon materials with suitable pore
distributions can shorten diffusion path lengths for rapid
ionic transport to provide excellent rate cyclability in cor-
responding SCs. For example, Huo etal. [98] reported
through experimental testing and simulations that hierar-
chically porous structures can enhance the capacitance
of carbon-based materials in which hierarchical porosity
within carbon materials can accelerate electrolyte infiltra-
tion and ion transport and therefore enhance the ion acces-
sibility of the overall material. According to the Interna-
tional Union of Pure and Applied Chemistry (IUPAC),
pores can be classified into three categories, including:
macropores (pore widths > 50nm), mesopores (2nm < pore
widths < 50nm) and micropores (pore widths < 2nm) [31].
And with the rapid development of nanomaterials, IUPAC
further divided micropores in 2015 into supermicropores
(0.7nm < pore widt hs < 2nm) and ultramicropores (pore
widths < 0.7nm) [25]. Here, micropores can provide large
surface areas and rich active sites to promote electrochemi-
cal performance; however, pore sizes must match ion sizes in
order for micropores to be able to positively affect capacitive
performance in which both larger and smaller pores can
lead to significant drops in capacitance due to ion sieving
effects [27]. In addition, macropores can act as ion buffer
reservoirs to boost ion transport, whereas mesopores can
provide low-resistance pathways for electrolyte ions to accel-
erate ion diffusion kinetics, leading to high rate capability
and high power density [99]. Because of all of this, carbon
materials with multiscale pores can demonstrate outstanding
capacitive performances through the integration of various
advantages.
2.2.1 Activation Methods
Activation including physical and chemical activation can
allow for the effective control of pore size distribution and
surface area in carbon materials through the tuning of acti-
vation agents, temperatures, time and heating rates. Here,
physical activation refers to the activation of carbon materi-
als involving gaseous agents such as O2, CO2 or hot steam
to generate porous structures [99], whereas chemical activa-
tion is associated with typical chemical activators (e.g., acid,
alkali or salt) operating at high temperatures ranging from
450 to 900°C [100, 101]. Because of this, pore distribu-
tions and electrochemical performances are predominantly
influenced by varying activation processes.
For example, Wu etal. [102] fabricated four types of
activated carbon materials (denoted as ACs) with hierarchi-
cally porous structures from fir wood and pistachio shell
using steam activation and KOH activation, respectively, and
reported that the resulting ACs possessed a similar surface
area of ~ 1050m2g−1, but different pore distributions, and
that among these, the mesopore ratio in the AC generated
by KOH activation (~ 780°C for 1h) ranged from 9.2% to
15.3%, whereas 33.3%–49.5% were obtained in the steam-
activated AC (~ 900°C for several hours). These researchers
also reported that the AC derived from fir wood through
KOH activation (F-KOH-AC) showed the highest specific
capacitance of 180Fg−1 at 10mVs−1 and attributed this to
the special pore structure created by KOH activation at mod-
est temperatures over a short period of time. And although
the ACs obtained at high temperatures exhibited good elec-
tronic conductivity, excessive temperatures experienced in
the steam-activated AC can induce the destruction of the
biomass precursor, decrease surface areas and lower the pore
content in the generated AC material. Based on this, chemi-
cal activation is more suitable for the effective regulation of
pore distribution to avoid the collapse of pristine biomass
precursor structures.
Carbon material pore morphology as a result of the acti-
vation agent is also related to electrochemical behavior in
which it is believed that plane-shaped pores are more ben-
eficial for ion transport than disordered or curved struc-
tures [103–105]. For example, Lee etal. [105] synthesized

Electrochemical Energy Reviews
1 3
a series of carbon materials derived from cinnamon sticks
with variable pore morphology using various activation
agents including KOH, H3PO4 and ZnCl2 and reported
that although all the ACs showed similar capacitances of
200Fg−1 at 0.5Ag–1, the KOH- and H3PO4-treated ACs
displayed better rate performances as compared with the
ZnCl2-treated AC. Here, these researchers attributed this to
the long and tortuous pore structure of the AC as a result of
ZnCl2, which can limit electrolyte ion transport and there-
fore hinder performance at high rates.
Combined methods consisting of template-assisted
approaches and chemical activation processes are also prom-
ising in the construction of hierarchically porous structures
[5]. For example, Zhang etal. [106] synthesized a chitosan-
derived 3D carbon foam (CF-MSP) with a multiscale pore
network in which a mixture of silica spheres with chitosan
solution was acquired, gelled with glutaraldehyde, carbon-
ized and finally activated (Fig.7a) and reported that the
resulting CF-MSP possessed a hierarchical porous structure
with ~ 200nm cavities after the successful removal of the
silica template, leaving a hierarchical porous structure con-
taining macro-, meso- and micropores (Fig.7b, c) as well as
a surface area as high as 2905m2g–1 and resulting in a high
capacitance of (374.7 ± 7.7)F g–1 at 1Ag–1. More impor-
tantly, the CF-MSP retained a remarkable capacitance of
(235.9 ± 7.5)Fg–1 (60% of its capacitance at 1Ag–1) at an
ultrahigh current density of 500Ag–1. The specific surface
area and the carbon yield of biomass-derived carbon mate-
rials can further be regulated by using chemical activation.
For example, Liu etal. [107] developed a rape pollen car-
bon (RPC) material by applying different activation agents
(Fig.7d) and reported that the as-obtained C-RPC (activated
by CuCl2) exhibited a microporous structure with a high-
est specific surface area of 2488m2g–1 (Fig.7e, f) along
with a carbon yield of up to 37.6%, which was much higher
than the 28.3% for Z-RPC (activated by ZnCl2) and 1.4% for
Fig. 7 a Schematic of the
synthesis of hierarchical porous
carbon; b nitrogen adsorp-
tion–desorption isotherms; and
c pore size distribution curves
(enlargement in the inset) of
CF-MSP and the CF. The CF
is the sample with the silica
template removed. Reprinted
with permission from Ref.
[106]. Copyright © 2017,
American Chemical Society. d
Schematic of the preparation
of RPC. e N2 adsorption–des-
orption curves and f pore size
distribution curves of RPC,
Z-RPC and K-RPC. RPC is
the carbon material derived
from CuCl2-impregnated
rape pollen. Z-RPC is the
carbon material derived from
ZnCl2-impregnated rape pollen.
K-RPC is the carbon material
derived from KOH-impregnated
rape pollen. Reprinted with
permission from Ref. [107].
Copyright © 2018, Royal Soci-
ety of Chemistry

Electrochemical Energy Reviews
1 3
K-RPC (activated by KOH). And benefiting from its unique
advantages in terms of surface area and pore distribution,
the C-RPC showed a specific capacitance of 390Fg–1 at
0.5Ag–1 and good stability with 92.9% capacitance reten-
tion after 10000 cycles at 20Ag–1.
2.2.2 Methods withNo Activation
Although the utilization of activation reagents can greatly
optimize the surface area and pore size distribution of car-
bon materials, serious environmental concerns make them
unsuitable for green and facile production on a practical
scale. Alternatively, biomass materials that can self-acti-
vate to improve surface properties are attracting widespread
attention [108, 109]. For example, Raymundo-Piñero etal.
[110]. reported a porous carbon material generated from sea-
weed through carbonization under a nitrogen atmosphere
between 600 and 900°C followed by rinsing in slightly
acidic water in which the synthesis process can be adjusted
to result in a tuned micro/mesoporous carbon material. The
chemical composition of seaweed is naturally optimized,
allowing it to be directly heat-treated for the production of
nanoporous carbons. In addition, porous carbon with a large
SSA of 1230m2g–1 can be readily prepared through the one-
step carbonization of withered neem leaves and can exhibit
a remarkable specific capacitance of 400Fg–1 and a high
energy density of 55Whkg–1 [108]. Interestingly, cotton
fabric can also be converted into porous carbon materials
without further activation due to special structural merits
[111].
Hard template-assisted methods can also be used to syn-
thesize porous carbon materials without activation. For
example, Wahid etal. [112] used a mesoporous silica hard
template (SBA-15) and pectin to produce N-doped nanocar-
bon threads and reported that after the removal of the silica
template by using HF solution, the hierarchically porous
structure was able to exhibit a high SSA of 873m2g–1 along
with outstanding electrochemical performances (285Fg–1
at 1Ag–1 and 211Fg–1 at 10Ag–1 in 1M H2SO4). Chen
etal. [113] also synthesized a carbon material using gelatin
as the biomass precursor and MgO and ZnO as dual tem-
plates in which after the successful removal of the template
in dilute HCl solution, the obtained electrode delivered a
specific capacitance of 284.1Fg–1 at 1Ag–1 with good rate
performance, showing 31.2% capacitance retention, even at a
current density of 150Ag–1. Despite hard template-assisted
methods being effective in the construction of porous carbon
structures, the need for post-treatment under harsh condi-
tions is complex and costly. Alternatively, soft template-
assisted approaches involving organic species can circum-
vent these issues. For example, Zhou etal. [114] applied
polytetrafluoroethylene (PTFE) in a soft template-assisted
method to produce porous carbon, whereas Zhao etal.
[115] reported the reproducible synthesis of highly ordered
mesoporous carbons (OMCs) using a hydrothermal process
involving β-cyclodextrin (β-CD) as the biomass precursor
and triblock poly(ethylene oxide)-poly(propylene oxide)-
poly(ethylene oxide) (PEO-PPO-PEO) copolymers as the
soft template (Fig.8a) in which after calcination at 700°C
for 3h under inert flowing N2, the as-prepared carbon elec-
trode showed a uniform 2D hexagonal mesoporous structure
(Fig.8b, c) and allowed a corresponding SC to display high
specific capacitance (~ 157Fg–1 at 0.5Ag–1), excellent rate
capability and prolonged cycling stability.
Other non-template methods such as the sugar-blowing
technique [24] and the additive-assisted method [82, 116,
117] are also viable routes to prepare high-performance bio-
mass-based porous carbon electrode materials with distinct
porous structures. For example, Qiu etal. [82] synthesized a
typical N-doped carbon cryogel derived from chitosan (CS)
using a combination of dilute hydrochloric acid-assisted
freeze-drying and high-temperature carbonization without
the addition of activators (Fig.8d) in which CS was dis-
solved and freeze-dried to construct a CS cryogel that was
composed of sheetlike CS layers with interconnected pores
among the layers. As a result, the obtained carbon cryogel
showed a sheet-shaped structure with a large specific surface
area of 1025m2g−1, a high nitrogen content of 5.98wt% and
significantly enhanced electrochemical performance in terms
of capacity and rate capability.
Overall, activation as a common method to introduce suit-
able specific surface areas and hierarchical pore distributions
in carbon materials plays a significant role in the fabrication
of biomass-derived carbon materials. In terms of physical
and chemical activation processes, although the applied
high temperatures in physical activation processes over long
periods of time can enhance electron conductivity, concerns
over biomass microstructural collapse increase, resulting in
narrow pore distributions. This effect can further induce
blocked ionic channels and low capacitance contributions.
Because of this, the optimization of related parameters such
as the activation agent, temperature and duration is key to
the efficient engineering of pore distribution and volume as
well as specific surface area. In addition, although high sur-
face areas can result in better capacitive performance, there
is no linear relationship. This is because small micropores
(< 0.5nm) cannot be fully utilized by electrolyte ions, which
results in the deceased electrochemical performance of car-
bon electrodes. Moreover, ultrarich porosity in carbon mate-
rials can result in low packing density along with restrained
electrical conductivity and volumetric capacitance. Based on
all of this, the connection between related parameters and
surface properties needs to be further investigated.
Alternatively, the preparation of biomass-derived porous
carbon materials with the aid of pre-/post-treatment pro-
cesses but without activation may be safer and greener

Electrochemical Energy Reviews
1 3
because activation under continuous high temperature can
generate large amounts of volatile gases (such as CO2 or
H2) and can increase environment pollution and instrumen-
tal failure during synthesis. Nevertheless, surface areas in
current porous carbon materials developed through non-
activation processes are lower than those prepared through
activation processes. In addition, corresponding harsh condi-
tions (e.g., HF solution) still raise associated environmental
problems.
In terms of chemical activation, effective improvements
in pore distribution and specific surface area make chemical
activation more promising in future large-scale production
processes despite existing issues. And to produce porous
carbon materials in a safe, green and facile manner with
suitable specific areas by using chemical activation, the
search for proper biomass precursors is of great importance.
In addition, more attention needs to be focused on novel
activator agents that can be used under milder conditions.
Moreover, pretreatment and template-assisted methods can
be combined with chemical activation (such as freeze-drying
and chemical activation) to reduce safety risks and environ-
mental problems, not only at the laboratory scale, but for
practical development as well. Furthermore, the basic under-
standing of the relationship between surface properties (pore
distribution and specific surface area) and electrochemical
performance in biomass-derived carbonaceous materials
remains lacking and requires more attention.
2.3 Doping Effect
Doping as induced by heteroatoms (N, O, S, F, P, B, etc.) in
carbon materials is considered to be an effective approach
to create more active sites, enhance electronic conductiv-
ity and improve surface wettability in the preparation of
Fig. 8 a Schematic of the formation of ordered mesoporous car-
bon materials derived from β-CD. TEM b, c images of ordered
mesoporous carbon. Reprinted with permission from Ref. [115].
Copyright © 2014, Royal Society of Chemistry. d Schematic of the
formation of chitosan-derived N-doped carbon. Reprinted with per-
mission from Ref. [82]. Copyright © 2015, IOP publication

Electrochemical Energy Reviews
1 3
high-performance electrodes. And due to rich and plentiful
heteroatoms in biomass precursor skeletons, self-doped car-
bon materials derived from biomass precursors are expected
to be promising enough that they can be used directly as
high-performance electrode materials [5, 118] without fur-
ther modification.
Because N is one of the most widely distributed ele-
ments in biomass precursors, N-doped carbon electrodes
have received much attention for SC applications. Currently,
popular biomass precursors containing N atoms include silk
[86], chitosan [23], tofu [119], cysteine [120], algae [121]
and gelatin [122]. Due to the strong electronegativity and
lone pair electrons of N atoms, N can easily be hybridized
with carbon π electrons to effectively boost the electronic
conductivity of carbon electrodes. In addition, the existence
of rich defects or active sites in N-doped carbon materials
can further enhance SC pseudocapacitance. For example,
Yang etal. [123] reported that okara-derived N-doped car-
bon sheets were able to hold a higher N content of 9.89at%
than most N-doped carbon materials and that the signifi-
cantly increased wettability of the material surface and its
sufficient active sites can significantly improve rate per-
formances. Other types of biomass materials such as flour
[124], tobacco rods [125] and pine needles [126] produced
carbon materials with a low N content after direct pyrolysis,
however, meaning that these precursors should be mixed
with N-containing reagents (urea [23], NH3 [127], melamine
[128], etc.) to further increase the N content. For example,
Zhang etal. [129] reported that an agar and urea mixture
was a good source to synthesize N-doped carbon aerogels
through a typical sol–gel route in which in their preparation
process, KOH was used as the activator reagent to generate
rich porosity, whereas urea acted as an additional N source.
As a result, the obtained hierarchical porous N-doped car-
bon provided a 3D interconnected network with a higher N
content than comparison groups without the addition of urea
and therefore exhibited a higher gravimetric capacitance of
400 F g–1 at a current density of 0.5Ag–1.
Although recent progress indicates that N doping can
enhance the electronic conductivity of carbon materials, an
increased N content doped into carbon does not necessarily
translate to improved electronic conductivity. For example,
Mousavi etal. [130] theoretically simulated the relation-
ship between the N content and N-doped graphene or CNTs
conductivity recently and reported that electronic conduc-
tivity was a function of processing temperature in which
the electronic conductivity of carbon materials increased as
N dopant concentrations increased after low-temperature
treatment, whereas the electronic conductivity of carbon
materials decreased as N dopant concentrations increased
after high-temperature treatment, meaning that N concen-
tration can induce significant change in the intrinsic proper-
ties of CNTs or graphene sheets. Here, these researchers
attributed this to the local restriction of electrons by thermal
fluctuations in the N-doped products. In another example,
Ismagilov etal. [131] carefully studied the effects of N
concentration on the structure and electronic conductivity
of carbon nanofibers in which a series of herringbone-like
CNFs with different N concentrations varying from 0 to
8.2wt% (N-CNFs) was systematically investigated by using
XPS, FTIR and Raman spectroscopy. Here, these research-
ers compared the different types of graphite-like lattice dis-
tortions and structural defects as induced by N doping in
these carbon nanofibers and reported that optimal electrical
conductivity among the N-CNFs samples occurred at a N
concentration of 3.1wt% in which an increased N content
increased defect concentration in the graphite structure and
hindered electron migration in the carbon material.
O doping is another popular method to enhance the sur-
face wettability and pseudocapacitance of carbonaceous
electrodes [23, 132]. For example, Cao etal. [132] designed
an O-doped hierarchical porous carbon through the facile
thermal treatment of starch–magnesium nitrate as the raw
material with subsequent acid etching and reported that the
as-designed material exhibited a high specific capacitance of
229Fg–1 at 1Ag–1 and good rate capability (211Fg–1 even
at 10Ag–1) along with outstanding cycling stability that
was maintained for over 10000 cycles at 2Ag–1 with ~ 94%
capacitance retention. Despite this, the electrochemical per-
formances of oxygen-rich carbon structures are not compa-
rable to those of N-rich carbonaceous materials [32, 133].
For example, Elmouwahidi etal. [133] prepared activated
carbon materials through the KOH activation of argan seed
shells in which oxygen and nitrogen functionalities were
introduced onto the surface of the activated carbon. Here, the
O-rich carbon material with an O concentration of 23.32at%
showed the lowest capacitance (259Fg–1 at 0.125Ag–1) and
the lowest capacity retention (52% at 1Ag−1) as compared
with the N-rich porous carbon (355Fg–1 at 0.125Ag–1, 93%
at 1Ag−1), which these researchers attributed to sluggish
diffusion kinetics as caused by surface carboxyl groups as
well as the well-developed porosity and pseudocapacitance
effects of the N functionalities.
In addition to N- and S-doped carbon materials, other
single heteroatom (e.g., P, S, F)-doped carbon materials can
also be obtained from biomass-derived carbon structures to
improve electrochemical performance. For example, Yi etal.
[134] successfully synthesized a typical P-doped porous car-
bon through the direct pyrolysis of an aqueous solution of
lignocellulose and ZnCl2/NaH2PO4 and reported that the
resulting P-doped porous carbon showed a high capaci-
tance of 133Fg–1 (146mFcm−2) at a current density of
10Ag–1 and an energy density of 4.7Whkg–1, which these
researchers attributed to the high electron-donating nature of
Li etal. [135] also fabricated a typical S-doped carbon aero-
gel with a 3D hierarchical interconnected porous structure

Electrochemical Energy Reviews
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using carrageenan-Fe hydrogel as a precursor (Fig.9a) in
which according to theoretical calculations (Fig.9b–e), the
increasing binding energy of TEA+ and BF4
− on graphene
was obtained after S doping and demonstrated enhanced
chemisorption between electrolyte ions and active materi-
als. As a result, a corresponding SC demonstrated superior
capacitances of 335 and 217Fg–1 at a current density of
1Ag–1 in aqueous and organic electrolytes, respectively.
Moreover, the SC showed outstanding cycling stability with
capacitance retention of 93.2% after 10000 cycles in aqueous
electrolytes and 89.2% after 5000 cycles in organic electro-
lytes (Fig.9f, g).
To further improve the electron conductivity and wet-
tability of electrodes, dual-atom or multiple-atom doping in
carbon materials is promising due to synergistic effects such
as strong electronegativity, tunable electron-donor charac-
teristics and wide working windows [136–138]. As a repre-
sentative example, Fan etal. [136] facilely obtained a N, S
Fig. 9 a Synthesis process for hierarchical S-doped carbon aerogel.
Enhancement of electrochemical performance by S doping can be
demonstrated by DFT by using Gaussian 09: optimized configuration
of tetraethylammonium ions (TEA+) adsorbed on b pristine graphene
and c S-doped graphene; optimized configuration of BF4
− adsorbed
on d pristine graphene and e S-doped graphene. f Capacitance reten-
tion ratio for S-doped carbon aerogel at a charge–discharge current
density of 5Ag–1 in 6 M KOH for 10000 cycles and g in the 1M
tetraethylammonium tetrafluoroborate/acetonitrile (TEABF4/AN)
electrolyte for 5000 cycles. Reprinted with permission from Ref.
[135]. Copyright © 2019, Elsevier

Electrochemical Energy Reviews
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co-doped porous carbon material through the combination
of waste newspapers and cellulose as a precursor in which
cellulose can simultaneously serve as the solvent, the activa-
tor and the dopant, whereas appropriate porosity along with
large surface area in the as-obtained carbon material can eas-
ily be achieved through triple heteroatom doping (N-, S- and
O-doped carbon) to demonstrate superior electrochemical
performance in terms of cycling and rate capability. Zhao
etal. [138] also prepared a typical N, P co-doped carbon
material through the hydrothermal carbonization of chitosan
followed by H3PO4 activation and reported that the obtained
carbon possessed well-defined hierarchical porosity and high
N and P contents to allow for a high specific capacitance of
312.4 and 204.4Fg–1 at 0.2 and 10Ag–1, respectively. Wang
etal. [137] further investigated the origins of enhanced stor-
age capability in heteroatom-doped carbon materials through
experimental evidence and theoretical simulation and found
that heteroatoms can inhibit oxygen evolution reaction to
broaden operating voltages and reduce adsorption energy to
accommodate more ions on the electrode.
Based on this, heteroatom-doped carbon electrode materi-
als can readily be prepared from biomass precursors through
facile routes in which induced structural distortions/defects
as well as enlarged interlayer distances can enhance cor-
responding electrode wettability, electron conductivity and
ionic transfer kinetics. Despite this, insufficient studies exist
that can establish clear correlations between dopant concen-
tration and electrochemical performance based on existing
reports due to difficulties in the accurate control of heter-
oatom concentrations in biomass-derived carbon materials.
In addition, although elemental N is widely distributed in
many biomass materials, the identification of optimally effi-
cient heteroatoms for improvements in the electrochemical
performance of biomass-derived carbon materials is diffi-
cult. However, this type of understanding can assist in the
search for more suitable biomass precursors. Moreover, elec-
trical conductivity improvements through heteroatom doping
are often insufficient and further optimizations to biomass-
derived carbon matrix pore structures, tap densities and sur-
face areas are required to meet electrochemical performance
demands. Because of this, suitable production routes that
can maintain the pristine structure of biomass precursors
to the greatest extent need to be developed. And although
heteroatom concentration improvements in biomass-derived
carbon materials without additional dopants are more chal-
lenging, the careful design and application of mixed biomass
precursors are a promising method.
2.4 Flexible andBinder‑Free Electrodes
Extra insulating binders such as polytetrafluoroethylene
(PTFE) or polyvinylidene fluoride (PVDF) are involved in
the traditional preparation of working electrodes for SCs,
but can hinder ion diffusion and increase electrolyte ion
diffusion resistance, leading to the eventual decrease in SC
electrochemical performance. More importantly, inactive
binders in electrodes can reduce overall gravimetric/volu-
metric energy density. To resolve this issue, the development
of biomass-based carbon materials that are binder-free or
self-supporting has attracted much attention in the develop-
ment of flexible SCs and has shown great promise in flexible
energy storage applications.
Due to advantages in terms of renewability, cost and
mechanical strength, bacterial cellulose is considered to be
a promising biomass precursor to obtain flexible electrodes
[31, 139]. For example, Yu etal. [139] initially generated
flexible carbon nanofibers from bacterial cellulose through
hydrothermal synthesis followed by pyrolysis and reported
that the as-obtained carbon material with rich interconnected
pores inherited the merits of doped N and the unique 3D net-
work nanostructure of the precursor, resulting in a pliable SC
device reversibly delivering a maximum power density of
390.53Whkg−1 and good cycling durability with ∼95.9%
capacitance retention after 5000 cycles. These researchers
further reported that even after bending at 180° for one hun-
dred times, the electrode still demonstrated structural integ-
rity without significant performance decay. Yu etal. [140]
also developed an activated and wrinkled carbon membrane
(AWCM) from flower petals through simple thermal pyroly-
sis followed by activation (Fig.10a) in which the sheetlike
electrode with a thickness of 10–20mm was assembled into
a flexible all-solid SC (Fig.10b, c) and demonstrated high
specific capacitance (154F g–1 at 10mVs–1) with great
bending stability. Ling etal. [122] further fabricated B, N
dual-doped carbon nanosheets (B/N-CSs) using gelatine
molecules as a biomass precursor and typical 2D boric acid
as the template (Fig.10d) in which after subsequent anneal-
ing, the as-prepared carbon material was assembled into a
freestanding flexible film without the addition of binders
(Fig.10e–g) and resulted in exceptional high-rate perfor-
mances with high capacitance retention of 70% over a wide
range of current densities from 0.2 to 100A g–1. These
researchers also reported that even after cycling for 15,000
cycles, the capacitance retention of the material was over
113% of initial capacitance, demonstrating promising appli-
cation in flexible energy storage devices (Fig.10h, i). More-
over, Hu etal. [141] prepared a flexible electrode using acti-
vated wood carbon (AWC) derived from natural wood fibers
for application in asymmetric SCs (Fig.11a) and reported
that the uniform AWC material was able to retain the special
anisotropic structure of the wood framework and was rich
in open channels along the growth direction (Fig.11b–g),
which provided straight ion diffusion pathways with low tor-
tuosity and efficient immersion by electrolyte ions. And as a
flexible anode in SCs without binders or additives, the uni-
form AWC material maintained an extraordinarily high areal

Electrochemical Energy Reviews
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capacitance of 3204mFcm−2 (~ 118.7F g–1) at a current
density of 1mAcm–2. These researchers further reported
that even as current densities increased to 30mAcm–2, a
maximal areal capacitance of 2800mFcm–2 (~ 103.7Fg–1)
was achieved with ~ 87.3% of initial capacitance retained.
Due to the unique structure of typical biomass precursors,
they can also serve as natural frameworks in the synthe-
sis of flexible electrodes with good structural integrity and
excellent bending properties. Although achievements have
recently been made in flexible SC devices, issues remain
that need to be addressed, however. For example, the fac-
ile preparation of flexible electrodes with robust mechani-
cal properties and superior electrochemical performances
remains challenging. In addition, specific capacitance and
energy density of overall SC devices are much lower than
those of composite electrodes due to the inferior capaci-
tance of carbon materials. And although additional pseu-
docapacitive materials in flexible composite electrodes can
improve electrochemical performance, problems arising
from inferior interfaces between carbon materials and pseu-
docapacitive materials also need to be investigated. Here,
the use of solid/gel electrolytes with high ionic/electronic
conductivity and good mechanical stability is promising in
addressing these issues; however, corresponding electronic
conductivity needs to be further improved in terms of practi-
cal production.
Fig. 10 a Schematic of the synthesis of AWCM. b Schematic of
a flexible supercapacitor. c Optical image of the as-prepared super-
capacitor. Reprinted with permission from Ref. [140]. Copyright ©
2017, Macmillan. d Schematic of the synthesis of the B/N-CS (left)
and optical image of the B/N-CS (right). e, f Optical images of a
freestanding B/N-CS film. g Cross-sectional SEM image of a B/N-
CS film. h Specific capacitance of an electrode composed of B/N-CS
powders and B/N-CS films at different current densities. i Cycling
stability of B/N-CS powders and B/N-CS films at 5Ag−1. Reprinted
with permission from Ref. [122]. Copyright © 2016, Wiley–VCH

Electrochemical Energy Reviews
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2.5 Biomass Carbon‑Based Composite Electrodes
In biomass-derived carbonaceous materials, facilitated
ion diffusion as enabled by rich porosity and excellent
electronic conductivity due to carbon properties combined
with rich active sites generated by suitable surface areas
can significantly increase rate performance and cycling
stability. However, the low energy density of carbon leads
Fig. 11 a Schematic of the design concept and construction of an all-
wood-structured SC. WC stands for wood carbon. b, c SEM images
of AWC at different magnifications. d Cross-sectional SEM image
of AWC. Detailed morphology of e the open pore inner walls and f
nanopores on the inner walls. g SEM image of the hierarchical porous
structure of AWC. h, i SEM images of wood carbon/MnO2. j TEM
image of wood carbon/MnO2. Reprinted with permission from Ref.
[141]. Copyright © 2017, Royal Society of Chemistry

Electrochemical Energy Reviews
1 3
to inadequate performance in practical SC applications.
Because of this, the fabrication of conductive electrodes
with high energy densities is needed, and currently, com-
posite electrodes that combine carbonaceous and pseu-
docapacitive materials can display the comprehensive
advantages of each component to result in improved elec-
trochemical performance [142–144].
Among pseudocapacitive materials, conducting poly-
mers and metallic oxides are particularly promising and can
easily be combined with various carbon materials such as
graphene (Gr) [145], carbon nanotubes (CNTs) [146] and
porous carbon [147] to fabricate composite materials with
suitable physicochemical properties to satisfy the require-
ments of high-energy-density SCs. Traditional approaches
to the facile production of composite electrodes are too
complex and expensive, however, and hinder further appli-
cation on a practical scale, and therefore, the combination
of biomass-based carbon materials with pseudocapacitive
materials through green, facile and renewable methods is
more promising.
2.5.1 Oxide‑Based andPolymer‑Based Composites
Biomass materials with different microstructures have been
well developed as precursors to fabricate composite elec-
trodes such as metal hydroxide/carbon (Ni–Al layered dou-
ble hydroxide/C) [148], metal oxide or hydroxide/carbon
(MnO2/C, NiO/C, Co3O4/C, etc.) [149, 150] and conductive
polymer/carbon electrodes [polyaniline (PANI)/C, polypyr-
role (PPy)/C] [151, 152]. Among these, MnO2 is the most
popular due to its high capacitance and is always used in
composite electrodes in combination with biomass-derived
carbon materials such as glucose [153], willow catkin [55],
flour [142] and hemp [154]. For example, Yang etal. [154]
prepared MnO2/activated carbon composites using MnO2
and hemp-derived honeycomb-like carbon and reported that
the as-obtained composite displayed a specific capacitance
of 340Fg–1 at 1Ag–1 and a moderate cycling life (98%
initial specific capacitance retention after 3000 cycles),
which these researchers attributed to the well-dispersed
MnO2 and the honeycomb-like activated carbon material.
By using typical electrodeposition, Chen etal. [141] also
introduced MnO2 nanosheets easily onto the surface and
inside the channels of wood-derived carbon (MnO2/wood
carbon composites, Fig.11h–j) and reported that an assem-
bled asymmetric SC based on the composite cathode was
able to exhibit a high energy density of 1.6mWhcm–2 and a
maximum power density of 24Wcm–2, presenting a promis-
ing energy storage device with high energy/power density.
Here, these researchers attributed the superior electrochemi-
cal performance to accelerated ion penetration throughout
the entire surface and channels as provided by the uniformly
distributed porous structure and good biocompatibility.
Conducting polymers are also suitable candidates in the
preparation of electrodes to obtain high-energy-density
SCs due to ultrahigh theoretical specific capacitances. For
example, Fang etal. [155] fabricated hierarchically porous
N-doped carbon/polyaniline (HPC/PANI) nanowire arrays
through facile in situ polymerization (Fig.12a, b) in which
an assembled asymmetric SC exhibited a remarkable spe-
cific capacitance of 134Fg–1 at 1Ag−1 (Fig.12c) and a
high energy density of 60.3Whkg−1 in 1M Na2SO4. More
interestingly, the capacitive performance of biomass-derived
carbon materials can be notably enhanced if combined with
conducting polymers or metallic oxides. Xiong etal. [156]
also used hemp-derived carbon (HDC) fibers as a scaffold
to synthesize a composite HDC@NiCo2O4@polypyrrole
(HDC@NiCo2O4@PPy) electrode that possessed a high
energy density of 17.5Whkg–1 with outstanding mechani-
cal flexibility if assembled in a solid-state SC (Fig.12d–f).
Biomass-based carbon aerogels with interconnected
porous structures are also suitable freestanding scaffolds
that can be combined with metallic oxides or conducting
polymers to achieve high-performance SCs. For example,
Wu etal. [157] easily prepared watermelon-derived carbon
aerogels with a sponge-like structure using a simple, green
and template-free method that featured a special porous
structure, good chemical activity and perfect mechanical
flexibility in which if combined with Fe3O4 nanoparticles,
the resulting Fe3O4/carbon aerogel composite can exhibit
outstanding specific capacitance (333Fg–1 at 1Ag–1).
2.5.2 Graphene andCNTs Combined withBiomass‑Derived
Carbon Composites
Although additional metal oxides and conducting polymers
can positively enhance the pseudocapacitive performance of
SCs through faradaic reactions, concerns over heavy metal
ions and potential environmental issues limit further applica-
tion as green SC electrodes. Alternatively, all-carbon com-
posites possess not only good stability and security, but also
rich surface functional groups that can provide extra capaci-
tance in addition to faradaic redox reactions. For example,
Song etal. [158] constructed a binder-free electrode from
reduced graphene oxide (RGO) aerogel and biomass-derived
carbon and reported that the combination of the N-doped
carbon network and the unique structure of the RGO aero-
gel in this all-carbon composite resulted in a hierarchical
3D porous electrode that featured synergetic improvements
to chemical composition, surface area and pore size dis-
tribution, thus improving corresponding electrochemical
properties. Kim etal. [159] also combined a lignin precur-
sor with Gr nanosheets through strong π-π interactions to
prepare a Gr/lignin composite material and reported that
the as-designed material was able to provide a high specific
capacitance output of 432Fg–1 at 10mVs–1, excellent rate

Electrochemical Energy Reviews
1 3
performance and superior cycling stability (4% capacitance
loss over 3000 cycles). Here, these researchers attributed
this outstanding electrochemical performance to the strong
attachment of Gr nanosheets to the lignin surface as well as
functional sites offered by the quinone groups from lignin.
Apart from Gr nanosheets, CNTs with 1D morphology,
flexibility and good mechanical properties are also consid-
ered to be promising candidates in SC devices. And due to
strong van der Waals and electrostatic interactions between
CNTs and biomass materials (such as stable hydrogen
bonds) [160], CNTs can firmly attach to biomass material
surfaces to form stabilized structures even under harsh test-
ing conditions in SCs. For example, Deng etal. [161] syn-
thesized a multiwalled carbon nanotube (MWCNT)/cellu-
lose composite through the electrospinning of an MWCNT/
cellulose acetate blended solution followed by deacetylation
and reported that after carbonization under an inert atmos-
phere, the incorporated MWCNTs and cellulose-derived
carbon material could easily attain a stable carbon–carbon
composite structure that not only reduced the activation
energy for the oxidative stabilization of cellulose nanofibers
but also increased the surface area and electrical conductiv-
ity. And as employed as an SC electrode, this composite
showed a high specific capacitance of 145Fg–1 at 10Ag–1.
Similarly, Hu etal. [160] generated a flexible electrode con-
taining CNTs and cotton paper by coating acid-treated CNTs
onto porous absorbent cotton paper and reported that the
resulting flexible electrode was able to exhibit a high specific
capacitance of 115.83Fg–1 and a remarkable energy density
of 48.86Whkg–1.
Overall, various composite electrodes containing carbon
and pseudocapacitive components with high capacitive per-
formances have been investigated. However, issues concern-
ing poor interfacial interactions between pseudocapacitive
materials and biomass-derived carbon as well as the low
loading mass of electrodes need to be fully addressed before
Fig. 12 a Schematic of the preparation of the HPC/PANI composite.
b SEM image of HPC/PANI. c Capacitances of an HPC/PANI//HPC
asymmetric SC at different current densities. Reprinted with permis-
sion from Ref. [155]. Copyright © 2016, Wiley–VCH. d Schematic
of a fiber-shaped symmetric SC based on the HDC@NiCo2O4@PPy
electrode. e Digital image of a fiber-shaped SC 9cm in length (right).
f Typical Ragone plot of the symmetric SC. Reprinted with permis-
sion from Ref. [156]. Copyright © 2015, Royal Society of Chemistry

Electrochemical Energy Reviews
1 3
large-scale industrial production. In addition, further opti-
mizations to carbon ratios in composites can provide oppor-
tunities to continuously improve the gravimetric/volumetric
energy density of SCs. The exploration of suitable carbon
structures that can match well with pseudocapacitive com-
ponents should also be conducted to enhance the durability
of composite electrodes. In terms of carbon–carbon com-
posite electrodes, the complex synthesis of graphene and
carbon nanotubes remains time-consuming and expensive.
And although excellent ionic and electronic conductivity can
be achieved with corresponding carbon–carbon electrodes,
energy densities remain unsuitable for practical production.
Based on this, combinations of pseudocapacitive electrodes
with biomass-derived carbon are potentially more viable for
large-scale production in the near future.
3 Conclusion andOutlook
In this review, the correlation between biomass-derived car-
bon material physicochemical property and SC electrode
electrochemical performance has been discussed in detail
and suggests that naturally abundant, green, inexpensive and
sustainable biomass precursors are excellent resources for
the large-scale synthesis of biomass-derived carbon mate-
rials with specific functional merits for SC applications
in which the dimensionality, chemical composition and
microstructural characteristics of biomass precursors play
a key role in determining the texture, surface properties and
final electrochemical performance of biomass-derived car-
bon materials. Because of this, the rational design of optimal
biomass-derived carbon materials through the careful selec-
tion of precursors, carbonization/activation processes and
structures can allow for the synthesis of carbon electrodes
with suitable SSA, pore size distribution and composition
to satisfy high-performance SC requirements. And although
significant progress has been made toward biomass-derived
carbon materials for high-performance SCs, challenges
remain and can be summarized as follows (Fig.13):
Precursor selection. The in-depth understanding of simi-
larities and differences between various biomass precursors
is essential, especially among microorganisms, biological
minerals, plant organs and bio-macromolecule-derived pre-
cursors. And with further insights into the original structural
and chemical properties of biomass precursors, the produc-
tion of low-cost, high-quality carbon materials is easily
achievable through the appropriate selection of precursors.
Carbonization and activation. The structural evolution
of organic precursors to inorganic carbon materials during
carbonization needs to be further studied and the funda-
mental relationship between chemical additions and carbon
structural parameters during carbon activation needs to be
fully established. In addition, instruments that can insitu
observe structural evolution should be employed to elucidate
Fig. 13 Outlook for biomass-
derived porous carbon materials

Electrochemical Energy Reviews
1 3
corresponding processes. Furthermore, carbon yields, heter-
oatom contents and tap densities should not be ignored, and
more importantly, cost-effective methods suitable for scale-
up production need to be applied to improve the conversion
efficiency of biomass precursors.
Composite electrodes. The selection of SC electrode
components with high theoretical capacity, suitable inter-
facial carbon compatibility and/or good structural stabil-
ity to improve gravimetric and volumetric energy density
is important. In addition, novel solutions such as aqueous
or highly concentrated electrolytes can be useful to resolve
interfacial problems.
Theoretical simulation. The construction of proper theoret-
ical models to address capacitor mechanisms, molecular inter-
actions and chemistry can guide capacitance enhancement and
determine corresponding dynamics and ion diffusion.
Performance evaluation. Previous data on gravimetric/
areal/volumetric capacitances are meaningless as they ignore
the mass loading, electrolytes and volume of overall SCs.
Because of this, the investigation of high-mass loading
biomass-derived carbon electrodes is urgently needed for
high-performance SCs and requires effective performance
evaluation.
Environmental effect. The construction of clean and effi-
cient SCs is an effective and feasible solution to address the
current energy crisis in which optimizations to devices to
achieve stable, smart and efficient SCs coupled with enhance-
ments in the disposal of waste SCs in the current SC industry
can promote waste management, carbon emission reduction
and energy security, all of which can positively contribute to
global carbon management goals and clean air plans.
Acknowledgements This work was financially supported by the
National Natural Science Foundation of China (21905220, 21875141,
51772240, 21503158), the Key Research and Development Plan of
Shanxi Province (China, Grant No. 2018 ZDXM-GY-135), the Fun-
damental Research Funds for the “Young Talent Support Plan” of
Xi’an Jiaotong University (HG6J003) the Shanghai Pujiang Program
(18PJ1409000) and the Shanghai Scientific and Technological Inno-
vation Project (19JC1410400). Financial support provided by the
Australian Research Council (ARC) (DP200101862) is also gratefully
acknowledged. We would further like to thank Dr. T. Silver for her
critical reading of this manuscript.
Compliance with Ethical Standards
Conflict of Interest There are no conflicts to declare.
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Jiangqi Zhou is currently a Ph.D.
candidate under the supervision
of Prof. Wei Tang at Xi’an Jiao-
tong University. His research
interest is to design novel elec-
trode materials for the applica-
tions in energy storage and con-
version, such as supercapacitors
and Li-/Na-/K-ion batteries.
Shilin Zhang is currently a Ph.D.
candidate at the Institute for
Superconducting and Electronic
Materials (ISEM), University of
Wollongong (UOW), Australia,
under the supervision of Prof.
Zaiping Guo. His research inter-
est is to design novel anode
materials for the applications in
energy storage and conversion.
Ya‑Nan Zhou received her M.Eng.
degree from Beijing University
of Chemical Technology in
2019. She is currently a Ph.D.
candidate under the supervision
of Prof. Wei Tang at Xi’an Jiao-
tong University. Her research
focuses on advanced electrode
materials for advanced energy
storage devices.

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1 3
Wei Tang is now a professor in
Xi’an Jiaotong University. He
received his B.S. from Nanjing
University of Science and Tech-
nology, China, in 2009 and his
Ph.D. from the Department of
Chemistry in the National Uni-
versity of Singapore in 2016. He
was awarded the NUS Graduate
School for Integrative Sciences
and Engineering (NGS) scholar-
ship. Dr. Tang’s research is
focused on design and fabrica-
tion of novel nanomaterials for
the applications in new genera-
tion energy storage and conver-
sion, such as supercapacitors and Li-/Na-/K-ion batteries. He has pub-
lished about 40 papers in peer-reviewed international journals.
Junhe Yang is now a professor in
the University of Shanghai for
Science and Technology. He
received his Ph.D. in Northeast-
ern University in 2000. He
worked as a visiting scholar at
the University of Pittsburgh,
USA, from 2005 to 2006. He
joined the University of Shang-
hai for Science and Technology
as a full professor in 2008. His
research interest focuses on the
chemical utilization of new car-
bon materials and nanostruc-
tured energy materials for energy
storage and conversion.
Chengxin Peng earned his Ph.D.
degree in Applied Chemistry
from Shanghai Jiao Tong Uni-
versity in 2008. He conducted
his postdoc research at Tongji
University, Xiamen University
and the National University of
Singapore. Currently he is an
Associate Professor at the Uni-
versity of Shanghai for Science
and Technology. His research
interest focuses on rational
design and synthesis of nanoma-
terials/organic materials/hybrid
materials, the fundamental stud-
ies of basic structure-property
correlations at atomic and nanometric scales, and further implementa-
tion of electrochemical principles with basic material chemistry to
explore advanced materials toward high-performance, durable and
cost-effective energy storage devices including supercapacitors, aque-
ous batteries and others.
Zaiping Guo is now working at
the Institute of Superconducting
and Electronic Materials, Uni-
versity of Wollongong (Aus-
tralia). She received her Ph.D.
degree from the University of
Wollongong in 2003, and her
BSc and MSc in 1993 and 1996
from Xinjiang University
(China), respectively. Her cur-
rent research interests mainly
focused on the energy storage
such as supercapacitors, Li-/Na-
/K-ion batteries and hydrogen
storage, as well as electrochem-
istry characterization and com-
puter modeling.