Porous monoliths of 3D graphene for electric double‐layer supercapacitors

Abstract

For delivering the nanoscaled extraordinary characteristics in macroscopical bulk, it is essential to integrate two‐dimensional nanosheets into three‐dimensional (3D) porous monoliths, alternatively called as 3D architectures, 3D networks, or aerogels. The intersupported structure of porous monolithic 3D graphene (3DG) can prevent aggregation or restacking of graphene individuals, and the interconnected sp² network of 3DG not only can provide the highway for the transport of electron/phonon but also can present continual cavities/channels for mass transfer. This review summarizes the synthesis methodology of 3DG porous monoliths and highlights the application for electric double‐layer capacitors. Present challenges and future prospects about the manufacture and application of 3DG are also discussed.

Full text

Carbon Energy. 2021;3:193–224. wileyonlinelibrary.com/journal/cey2
|
193
Received: 8 December 2020
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Revised: 20 January 2021
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Accepted: 25 January 2021
DOI: 10.1002/cey2.107
REVIEW
Porous monoliths of 3D graphene for electric double‐layer
supercapacitors
Jinjue Zeng
1
|Chenyang Xu
1
|Tian Gao
1
|Xiangfen Jiang
1,2
|
Xue‐Bin Wang
1
1
National Laboratory of Solid State Microstructures (NLSSM), Jiangsu Key Laboratory of Artificial Functional Materials, Collaborative Innovation
Center of Advanced Microstructures, College of Engineering and Applied Sciences, Nanjing University, Nanjing, China
2
WPI Center for Materials Nanoarchitectonics, National Institute for Materials Science (NIMS), Tsukuba, Japan
Correspondence
Xue‐Bin Wang, National Laboratory of
Solid State Microstructures (NLSSM),
Collaborative Innovation Center of
Advanced Microstructures, Jiangsu Key
Laboratory of Artificial Functional
Materials, College of Engineering and
Applied Sciences, Nanjing University,
210093 Nanjing, China.
Email: wangxb@nju.edu.cn
Funding information
National Natural Science Foundation of
China, Grant/Award Numbers: 51972168,
51672124, 21603096
Abstract
For delivering the nanoscaled extraordinary characteristics in macroscopical
bulk, it is essential to integrate two‐dimensional nanosheets into three‐
dimensional (3D) porous monoliths, alternatively called as 3D architectures,
3D networks, or aerogels. The intersupported structure of porous monolithic
3D graphene (3DG) can prevent aggregation or restacking of graphene in-
dividuals, and the interconnected sp
2
network of 3DG not only can provide the
highway for the transport of electron/phonon but also can present continual
cavities/channels for mass transfer. This review summarizes the synthesis
methodology of 3DG porous monoliths and highlights the application for
electric double‐layer capacitors. Present challenges and future prospects about
the manufacture and application of 3DG are also discussed.
KEYWORDS
3D graphene, electric double‐layer capacitor, graphene aerogel, porous monolith,
supercapacitor
1|INTRODUCTION
Severe energy and environmental crisis requires the de-
velopment of sustainable energy storage technologies. In
principle, supercapacitors can deliver very high power
with the long lifetime as compared with batteries.
Supercapacitors are classified into three types: electric
double‐layer capacitors (EDLCs), pseudocapacitors, and
asymmetric supercapacitors.
1
EDLCs possess the highest
stability, power, lifetime, and frequency, benefiting from
the non‐Faradic electric double‐layer (EDL) energy
storage mechanism and the absent volume change.
EDLCs, normally based on porous carbon electrodes, are
greener than batteries, as they contain no heavy metal
elements. EDLCs also have nearly infinite lifetime and
the ease of maintenance. These aspects make EDLCs
complement and even replace batteries in some appli-
cations including grid regulators and capacitor vehicles,
for example, Shanghai capabus, Nanjing and Guangzhou
supercapacitor‐powered trams.
2–4
The low energy density (2–8 Wh·kg
−1
) is the biggest
bottleneck of current EDLCs, hindering their large‐scale
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
© 2021 The Authors. Carbon Energy published by Wenzhou University and John Wiley & Sons Australia, Ltd.

implementation. Essentially, it also results from the EDL
mode, where the energy storage density of the electro-
static field is not high. Every coin has two sides, so it is
with EDL. The opposite polar charges, that is, the built
electrostatic field, are stored only over the interface be-
tween electrodes and electrolytes. The energy density Eis
determined as follows:
E
CV=1
2
,
2
(1)
CCA=
,
dl
(2)
where Cis capacitance, Vis operable voltage, C
dl
is areal
capacitance, and Ais specific surface area (SSA) of
electrode materials. Vis determined by the electro-
chemical window of electrolytes as well as by the stability
and inhibitivity of electrodes. C
dl
is determined more by
hydrated ion radius of electrolytes than electrodes, for
example, typically 21 μF·cm
−2
of BMIMPF
6
ionic liquid
measured on a single‐layer graphene.
5
Most importantly,
increasing SSA is the first practicable way for increasing
energy density, without any compromise of other per-
formances. Therefore, a high‐SSA conductive cellular
monolith—that is a graphenic three‐dimensional (3D)
network basically—is the most desired electrode to ex-
tract the highest performances of energy, power, and
lifetime of EDLCs.
6
Graphene is the representative of two‐dimensional
(2D) materials, which possesses a high theoretical SSA
up to 2630 m
2
·g
−1
(geometric surface) or 2965 m
2
·g
−1
(Connolly surface),
7
high electrical conductivity, and
chemical inertness. The merits can contribute to energy,
power, and lifetime of supercapacitors. However, such
intrinsic advantages are suppressed by severe face‐to‐face
restacking and deficient hand‐in‐hand interconnection of
graphene individuals for macroscopical usage in practice.
To take full advantage of the remarkable properties, 2D
graphene sheets should be integrated into the porous
monolithic 3D graphene (3DG). A 3DG monolith is the
covalently interconnected network structure with the
mono‐/few‐layered graphene as the geometric unit. It can
also be regarded as the ultimate version of porous carbon
with the wall thickness down to the sp
2
atomic monolayer.
6
3DG exhibits multiple functionalities relying on three merits:
(i) the intersupported 3D structure prevents the restacking or
agglomeration of graphene sheets; (ii) the sp
2
carbon net-
work provides excellent highway for electron transport,
phonon transport, force transfer, and so forth; and (iii) the
designed continual cavity/channel inside the 3DG monoliths
enhances the mass transfer such as electrolyte and gas. 3DG
can, thus, deliver the excellent nanoscaled advantages in
macroscopical practical applications. The research on 3DG is
still ongoing and quite conceptual, yet ca. 15,000 publica-
tions have emerged for 10 years since its birth in about 2010.
This review summarizes the synthesis progress of 3DG
porousmonolithsaswellastheirstate‐of‐the‐art EDLCs, for
inspiring and promoting practicable graphene materials
(Figure 1).
2|SYNTHESIS OF 3D GRAPHENE
POROUS MONOLITHS
Since the tape exfoliation of graphene from graphite in
2004,
8
numerous methods have been developed for the
production of graphene powders and films. Typically, the
top‐down approaches exfoliate graphite into graphene
FIGURE 1 Advanced three‐dimensional graphene (3DG) porous monoliths: design, synthesis methods, and electric double‐layer
capacitor (EDLC) application. The dual networks are the solid carbon network and the pore cavity network
194
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ZENG ET AL.

powders by mechanical,
8,9
gaseous,
10
chemical,
11
or
electrochemical
12,13
means. The bottom‐up approaches
transform simple molecules into graphene films, for ex-
ample, chemical vapor deposition (CVD) on substrates
including metals,
14
metal oxides,
15
salts,
16
and so on.
When applying normal graphene powders to electro-
chemical electrodes, it is found that the face‐to‐face re-
stacking decreases the accessible SSA, and that the
deficient hand‐in‐hand interconnection leads to a high
contact resistance.
17–19
3DG is, thus, designed to over-
come such problems, for attaining higher energy, power,
and lifetime of electrodes. 3DG porous monoliths are
fabricated via the assembly of graphene platelets or via
the pyroconversion of gaseous/liquid/solid organics into
a graphenic network. In the former, graphene oxide (GO)
is typically used as building blocks to construct 3DG.
Concerning the latter method, CVD on a parental porous
template can yield 3DG using gaseous carbon sources.
20
Noticeably, solid/liquid raw materials are also available
to make 3DG via diverse smart approaches; however,
they are not easy, no matter with or without templates.
21
In this review, we summarize the synthesis methodology
of 3DG as follows: liquid‐based assembly, CVD, and
pyrolysis of solid/liquid carbon sources.
2.1 |Liquid‐based assembly
GO is produced by oxidizing and then exfoliating graphite,
which contains a lot of functional groups and defects.
22
The
oxygenic groups render GO platelets hydrophilic, which are
well‐dispersed in water.
23
It allows the assembly of platelets
into GO hydrogels and further reduces graphene oxide
(RGO) hydrogels, that is, a type of 3DG porous monolith.
The main assembly routes include gelation without tem-
plates, gelation assisted by templates, and liquid‐based
manufacturing techniques.
2.1.1 |Gelation based on GO platelets
for 3DG
GO is naturally decorated with abundant oxygenic groups
on graphitic basal and edge planes, which exhibits hy-
drophilicity and electrostatic repulsion effect.
24
The degree
of reduction from GO to RGO can adjust the balance be-
tween hydrophilicity and hydrophobicity. The reduction
process can induce the assembly of the 2D sheets into the
3D network. Li et al.
25
reported that the reduction of GO
dispersions with a concentration larger than 0.5 mg·mL
−1
led to gelation. Xu et al.
26
developed an RGO hydrogel via
one‐step hydrothermal process with GO aqueous disper-
sion (Figure 2A–C). As the reduction reaction proceeded,
the hydrophobicity and the π‐conjugated structure of GO
sheets increased, which enhanced the strength of π–π
stacking, so the derived RGO sheets were noncovalently
linked to 3DG hydrogel via π–πstacking, hydrogen bond,
and hydrophobic interaction. The self‐assembled hydrogel
(SGH) contained about 2.6 wt% RGO and 97.4 wt% water,
which could be used directly for EDLCs. The freeze‐dried
SGH was an aerogel. The weak intersheet connection and
the defects of RGO damage the electrical conduction,
showing a low conductivity of 5 × 10
−3
S·cm
−1
.Considering
the limited reduction degree by hydrothermal process, var-
ious reducing agents were added to fully eliminate oxygenic
groups and to restore sp
2
hybridization. Ascorbic acid,
27
hydrazine hydrate,
30,31
hypophosphorous acid–iodine,
32
and
ethylenediamine
28,33
were used as the reducing agents.
Zhang et al.
27
reported that the reduction of GO with
L‐ascorbic acid resulted in the 3DG monolith after additional
supercritical drying or freeze‐drying (Figure 2D–F). It
showed a large SSA of 512 m
2
·g
−1
and a conductivity about
1S·cm
−1
.Chenetal.
28
synthesized macroscopic N‐doped
graphene (NG) hydrogels with hydrothermal treatment
upon GO and ethylenediamine. The amine acted as a
structural modifier to adjust the microscopic structure and as
a nitrogen source for nitrogen doping (Figure 2G).
Besides the direct gelation discussed above, the ge-
lation assisted by cross‐linking agents was another
route that appeared early historically. Cross‐linking
regents can be polymers, metal ions, and so on. Worsley
et al.
34
reported 3DG aerogel monoliths via the sol–gel
polymerization of resorcinol and formaldehyde in GO
dispersion, where the wet gels were washed with
acetone, then dried with supercritical CO
2
, and finally
pyrolyzed at 1050°C. Thermal reduction of GO and
carbonization of resorcinol–formaldehyde contributed
to a conductivity of ~1 S·cm
−1
.Theyalsoreportedthat
the resorcinol–formaldehyde content affected SSA, pore
size, and the ratio of sp
2
carbon to sp
3
carbon.
35
Jiang
et al.
29
reported that divalent ions (Ca
2+
,Ni
2+
,Co
2+
)
linked GO into a gel‐like 3DG structure (Figure 2H).
They inferred that the interconnection of RGO relied
on chemical and hydrogen bonds generated from water
interlinkage, metal ions, and oxygenic groups. Gen-
erally, metal ions linkers were contained within the
final product to form RGO–metal hybrid materials. For
example, Fe
2+
could reduce GO and generate α‐FeOOH
or Fe
3
O
4
nanoparticles on RGO sheets to form hybrid
aerogels.
36
Tang et al.
37
reported palladium salt and glu-
cose together for the assembly of GO. Pd
2+
ions were re-
duced to nanoparticles by glucose and were anchored to
GO, acting as active sites for catalytic Heck reaction.
Glucose‐derived polymers can enhance the mechanical
strength of 3DG to some extent. It could load a weight of
330 g; however, it weighed only 80 mg. More than polymers
ZENG ET AL.
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195

and ions, carbon nanotubes (CNTs) are also considered as
the bridge coupler showing flexibility under stress.
38
Sun
et al.
39
reported the all‐carbon aerogel by freeze‐drying the
mixed solution of GO and CNTs. Giant RGO sheets were
used to construct a framework with macropores, and their
surface was tightly coated with CNT ribs to reinforce flex-
ibility of aerogels.
2.1.2 |Gelation assisted by templates
for 3DG
Templates can be combined into the gelation process of
GO sheets to tailor the porous structure of 3DG, for ex-
ample, in situ ice template,
40,41
additive nanoparticle,
42,43
emulsion,
44
and gases.
45–49
FIGURE 2 Direct gelation based on suspension of graphene oxide (GO) platelets for three‐dimensional graphene (3DG). (A) Photos of
GO aqueous dispersion before and after the hydrothermal process. (B) Photos of self‐assembled hydrogel (SGH) allowing the handling and
the loading. (C) A scanning electron microscopy image of microstructures of SGH. Reproduced with permission: Copyright 2010, American
Chemical Society.
26
(D) Photo of GO suspension. (E) Reduced graphene oxide (RGO) hydrogel prepared by heating the mixture of GO and
L‐ascorbic acid. (F) RGO aerogel supporting a counterpoise. Reproduced with permission: Copyright 2011, Royal Society of Chemistry.
27
(G) Illustration of an assembly of hydrogel: the distance enhancement between RGO layers by ethylenediamine and the possible reaction
pathways. Reproduced with permission: Copyright 2012, Elsevier.
28
(H) A schematic of formation of gel‐like RGO bulk via the linkage of
divalent ion. Reproduced with permission: Copyright 2010, American Chemical Society.
29
196
|
ZENG ET AL.

Similar to freeze‐casting technique in the field of
ceramic foam, Qiu et al.
40
reported the ice‐templating
method for cork‐like RGO monoliths. The mixture of GO
and ascorbic acid solution was preheated to obtain par-
tially reduced GO, which was then frozen and finally
thawed to get a cellular monolith of RGO. Upon freezing,
the partially reduced GO sheets were entrapped between
ice crystals to form a network structure. The RGO
monolith showed a low density of 0.5 mg·cm
−3
, a good
conductivity of 0.12 S·cm
−1
, and a high mechanical en-
ergy loss coefficient of 82.5%. Zhang et al.
41
reported
vertically aligned RGO with antifreeze‐assisted freeze‐
casting (Figure 3A–D). The mixture of GO and ethanol
was put into a mold, which was then placed on the
surface of liquid nitrogen for directional freeze‐casting
from bottom to the top, resulting in run‐through
channels.
Hard templates, for example polystyrene (PS), poly-
methyl methacrylate (PMMA), or silica nanospheres, can
guide to build a porous 3DG. It involves the assembly of
GO/RGO on hard templates, followed by removal of the
templates. For example, the negatively charged PS
sulfonate‐stabilized RGO (PSS‐G) sheets were electro-
statically induced into the positively charged PS beads. It
led to micron‐sized PSS‐G spheres after removing PS. A
sponge‐like macroporous scaffold was finally fabricated
by ice segregation‐induced self‐assembly of the mixture
of PSS‐G and poly(vinylalcohol) (PVA), as shown in
Figure 4A–C.
42
Apart from polymer templates, the silica
spheres were also employed to fabricate 3DG. Huang
et al.
43
reported that the hydrophobic nature of methyl‐
modified silica templates induced a self‐assembled
lamellar‐like structure of GO. RGO foams were then
obtained after calcination and subsequent hydrofluoric
acid (HF) etching (Figure 4D,E).
Soft templates are also effective for building 3DG,
such as emulsion,
44
gases,
45
and so forth. Li et al.
44
de-
veloped the modified hydrothermal method with hexane
emulsion as the soft template (Figure 5A–C). The hexane
droplets were homogeneously dispersed in GO, and they
induced GO sheets to form a 3D network during the
hydrothermal process. Laser can reduce GO into con-
ducting RGO.
46
In situ generated gases could create pores
between GO/RGO sheets. El‐Kady et al.
45
reported the
laser irradiation reduction method for converting a
stacked GO film into the well‐exfoliated porous structure
by gases generated in the rapid reduction. Yeo et al.
47
reported a plesiohedral cellular network from bubbling
(Figure 5D–G). They used microfluidic fabrication of gas‐
in‐oil‐in‐water compound bubbles to derive the solid‐
shelled bubbles. With packing bubbles driven by buoy-
ancy, they obtained the interconnected network structure
of RGO after additional thermal reduction. The resulting
structure was a space‐filling cellular foam with a rhombic
dodecahedral honeycomb lattice, which showed high
Young's modulus and elasticity. Zhang et al.
48
designed a
multistep strategy combining bubbles and ice templates.
F127 was selected as the bubbling agent to produce air
bubbles with whisk, in which bubbles were coated with
GO sheets. The structure was frozen before its break‐up,
FIGURE 3 Ice‐templating gelation for 3DG. (A) A schematic of fabrication of VA‐GSM. (B) A photo of VA‐GSM. (C, D) SEM images of
VA‐GSM. Reproduced with permission: Copyright 2017, American Chemical Society.
41
3DG, three‐dimensional graphene; GO, graphene
oxide; SEM, scanning electron microscopy; VA‐GSM, vertically aligned graphene sheet membrane
ZENG ET AL.
|
197

and the 3DG monolith was yielded after the thermal
reduction of GO and the removal of F127. A similar
strategy was also reported by Yang et al.
49
for elastic RGO
aerogel. It should be noted that bubble drainage and
Ostwald ripening can lead to nonuniformity of the bub-
ble size.
51
Recently, Yang et al.
50
reported a direct slurry
casting method to prepare the 3DG monoliths by
retarding Ostwald ripening of the GO wet foam
(Figure 5H). It was tuned from open pores to closed pores
by changing the concentration of GO slurry.
2.1.3 |Liquid‐based manufacturing
techniques for 3DG
Manufacturing technologies based on GO/RGO suspen-
sion or slurry have been widely studied, for example, the
wet spinning toward RGO hydrogel fibers, the filtration
toward films, the plastic foam‐assisted casting toward
monoliths, and the 3D printing toward monoliths. It is
generally acknowledged that the microscopic structure
depends on the gelation nature of GO/RGO sheets as
well as the templating sometimes.
Wet spinning can be utilized to fabricate RGO fibers
or films.
52–54
Xu and Gao
55
reported that GO could be
spun into meters of GO fibers. Neat “core‐shell”struc-
tured RGO fibers and cylinders with aligned pores could
be formed from the GO liquid crystals (Figure 6A–C).
54
After chemical reduction, the RGO sheets were densely
stacked on the surface but porously interconnected in-
side. The fibers were further transformed into non‐woven
fabrics by wet‐fusing assembly for supercapacitors.
59,60
Filtration has been used for making paper throughout
history. In 2007, Dikin et al.
61
reported the GO paper
fabricated by filtrating GO colloidal dispersions. The ge-
lation of RGO occurred at the interface of liquid and filter
membrane rather than bulk liquid during filtration
(Figure 6D).
56
The mechanical stiffness and the fracture
strength of GO papers were enhanced after modification
with Mg
2+
or Ca
2+
for cross‐linking.
62
Usually, GO platelets
are apt for dense packing within the GO paper, whereas the
enlargement of pores is preferred. Yang et al.
63
revealed that
FIGURE 4 Gelation assisted by hard templates for 3DG. (A) A scheme for fabricating porous PSS‐G/PVA monoliths using ice
segregation‐induced assembly. (B) A scheme of preparing hollow PSS‐G microspheres via the colloidal templating approach. (C) An SEM
image of PSS‐G/PVA. Reproduced with permission: Copyright 2009, Wiley.
42
(D) A schematic of formation of NGFs. (E) A scanning
transmission electron microscopy image of NGFs. Reproduced with permission: Copyright 2012, Wiley.
43
3DG, three‐dimensional graphene;
GO, graphene oxide; HF, hydrofluoric acid; NGF, nanoporous graphene foam; PSS‐G, polystyrene sulfonate‐stabilized reduced graphene
oxide; PVA, poly(vinylalcohol); SEM, scanning electron microscopy
198
|
ZENG ET AL.

FIGURE 5 Gelation assisted by soft templates for 3DG. (A) A schematic of preparation of MGM assisted by hexane emulsion. (B) An
optical image of the emulsion made from mixture of hexane and GO dispersion. (C) A scanning electron microscopy (SEM) image of MGM.
Reproduced with permission: Copyright 2014, Wiley.
44
(D) Design strategy of GO‐based microbubble assembly. (E, F) An optical image and
a schematic illustration of the microfluidic process, resulting in the formation of spherical microbubbles, for the assembled heavily alkylated
GO. (G) An SEM image of the 3D rhombic dodecahedral honeycomb structure. Reproduced with permission: Copyright 2018, Wiley.
47
(H) A
scheme of bubble evolution of a densely foamed GO coating when drying in air. Reproduced with permission: Copyright 2020, American
Chemical Society.
50
3DG, three‐dimensional graphene; CCS, closed cellular structure; DFGO, dual‐functionalized graphene oxide;
FCC, face‐centered cubic; GO, graphene oxide; MGM, macroporous graphene monolith
ZENG ET AL.
|
199

the severe restacking did not occur in the filtration process,
because the absorbed water induced repulsive hydration
force between GO platelets. The removal of water caused
restacking and pore collapse. Other strategies, for example,
ice crystals,
57
gas,
31
colloid particles,
58
dopamine,
64
and ex-
foliated graphene,
65
were incorporated together with filtra-
tion. Shao et al.
57
reported the honeycomb‐like RGO film via
combination of the vacuum filtration and the freeze‐casting
(Figure 6E). Niu et al.
31
reported the porous RGO foam via
vacuum filtration and then leavening by the self‐released
gas. Choi et al.
58
reported a 3D macroporous RGO frame-
work with PS spheres as sacrificial templates via the filtra-
tion process (Figure 6F,G).
Plastic foam‐assisted casting of GO suspension can
provide diverse inorganic foams. Yao et al.
66
reported an
RGO‐wrapped polyurethane (PU) sponge by dipping a
FIGURE 6 Wet spinning and filtration of GO/RGO suspension. (A, B) A schematic for preparation of GO porous fibers and GO porous
cylinders and a scheme of the core–shell structure model. Different colors represent GO lamellar regions with different director vectors. (C)
An SEM image of GO porous fibers. Reproduced with permission: Copyright 2012, American Chemical Society.
54
(D) A sketch of the
formation of GO hydrogel film by vacuum filtration. Reproduced with permission: Copyright 2011, Wiley.
56
(E) A schematic illustration of
the formation of porous RGO film through filtration and freeze‐casting. The phase diagram shows the aqueous solution during the different
procedures. Reproduced with permission: Copyright 2016, Wiley.
57
(F, G) A scheme and an SEM image of 3D macroporous RGO films via
an embossing process with filtration. Reproduced with permission: Copyright 2012, American Chemical Society.
58
CMG, chemically
modified graphene; e‐CMG, embossed chemically modified graphene; GO, graphene oxide; PS, polystyrene; RGO, reduced graphene oxide;
SEM, scanning electron microscopy
200
|
ZENG ET AL.

PU sponge in GO, followed by a reduction reaction. Du
et al.
67
immersed a PU sponge into GO ethanol disper-
sion and then pyrolyzed in ethanol flame to achieve
N‐doped RGO foams with open cells (Figure 7A–C).
Zhao et al.
68
reported the O and N co‐doped holey 3DG
aerogel via the hydrothermal treatment of a PU sponge in
the GO aqueous dispersion (Figure 7D). They pointed out
that PU‐derived hard carbon was burnt away in air for
2 h, which endowed 3DG the elastic characteristic. Other
polymeric templates such as cigarette filters were also
applied to cast 3DG porous monoliths.
69
3D printing, that is, additive manufacturing, em-
ploys computer‐controlled pattern generation to create
an architectural structure.
70
It is based on the con-
tinuous extrusion of ink, whose extrudate maintains its
shape and adheres to the previous extrudate layer.
Garcia‐Tuñon et al.
71
reported that GO functionalized
by branched copolymer surfactant (BCS) resulted in a
printable GO/BCS suspension. The addition of gluconic‐
δ‐lactone triggered the pH drop to protonate GO, en-
abling the formation of multiple hydrogen bonds
(Figure 8A,B). Zhu et al.
72
reported that hydrophilic
silica powders served as a viscosifier to enhance the
printability of GO ink (Figure 8C,D), in which basic
solution is adopted. Another typical strategy is the Ca
2+
ion‐induced gelation of GO. Through adding trace of
Ca
2+
as a cross‐linker, the printable GO gel ink showed
optimal storage modulus and shear‐thinning behavior,
avoiding the requirement of high‐concentration GO or
other additives (Figure 8E).
73
This Ca
2+
ion‐induced
strategy was also suitable for GO/CNTs, for fabricating
highly stretchable carbon aerogels.
74
2.2 |CVD
Large‐area graphene films had been grown via CVD
using a gaseous precursor on copper or nickel substrates
typically.
75–77
A 3DG porous monolith should be ac-
cordingly produced via CVD on the porous parent tem-
plate, for example, metals, metal oxides, salts, and
combined templates.
FIGURE 7 Plastic foam‐assisted casting of graphene oxide/reduced graphene oxide (GO/RGO) suspension. (A) A photo of N‐doped
graphene foams (NGFs) via polyurethane (PU)‐assisted casting, which were self‐adhered to a plastic board. (B, C) An X‐ray radiographic
image and a scanning electron microscopy image of NGF. Reproduced with permission: Copyright 2016, American Chemical Society.
67
(D)
A scheme of preparation of O and N co‐doped holey RGO via PU‐assisted casting and selective etching. Reproduced with permission:
Copyright 2020, Wiley.
68
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201

2.2.1 |CVD on metal templates for 3DG
Chen et al.
14
designed CVD growth of macroscopic 3DG
foams on nickel foam templates (Figure 9A). Graphene
was deposited on nickel foams via decomposing CH
4
at
1000°C, in which graphene was interconnected into a
network. The PMMA was then coated on the surface of
graphene as a support to prevent from collapsing during
wet etching. After etching away nickel with HCl solu-
tion and removing PMMA with acetone, the highly
conductive graphene foams were obtained. Limited by
commercial nickel foams, the produced graphene foams
FIGURE 8 Three‐dimensional (3D) printing of graphene oxide/reduced graphene oxide (GO/RGO) slurry. (A) A schematic depicting
branched copolymer surfactant (BCS) and GO structures at basic pH. (B) A sketch of assembly of noncovalent GO/BCS network via 3D
printing. Reproduced with permission: Copyright 2015, Wiley.
71
(C) A schematic of fabrication of RGO aerogel via printing. (D) Morphology
and structure of RGO aerogels via printing. Reproduced under the terms of the Creative Commons Attribution 4.0 International license:
Copyright 2015, Springer Nature.
72
(E) A schematic of a 3D printing process where a trace amount of CaCl
2
was added into the GO
dispersion to form the printable ink. Reproduced with permission: Copyright 2018, Wiley.
73
202
|
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had to inherit a large cell size about 200–500 μm. Ito
et al.
78
produced the small‐pore graphene using a na-
noporous nickel (np‐Ni) template (Figure 9B). The np‐
Ni was made by dealloying an Ni
30
Mn
70
precursor in
acidic solutions.
79
The np‐Ni was heated at 900°C under
the atmosphere of H
2
, Ar, and benzene, to grow gra-
phene on its surface. Nanoporous 3DG was finally ob-
tained by removing np‐Ni templates in the HCl
solution. On the basis of the coarsening of nickel liga-
ment, the pore size of nanoporous graphene could be
tailored from 100 nm to 2.0 μm by controlling the CVD
time and temperature.
78
N‐doped nanoporous graphene
was also obtained with pyridine as a carbon source
similarly.
80
2.2.2 |CVD on oxide templates for 3DG
Various oxides have the ability to act as substrates for
growing few‐layer graphitic or graphene under CVD con-
ditions, such as MgO,
81
ZnO,
82
CaO,
83–85
SiO
2
,
86–90
and so
forth. The oxides can prompt the decomposition of the
carbon source and the deposition of graphitic carbon on
their surface. They can achieve a 3DG product if they are
assisted by the 3D‐structured oxide template. For example,
Jiang et al. developed carbon nanocages (CNCs) using
MgO templates from the in situ decomposition of basic
magnesium carbonate (Figure 10A).
92,93
Large‐size MgO particles were yielded at higher
growth temperatures for larger CNCs, and N‐doped
FIGURE 9 CVD on metal templates for 3DG. (A) A scheme of 3D interconnected graphene foams using a nickel foam template via
CVD. (B) Fabrication of nanoporous graphene via CVD on np‐Ni. Reproduced with permission: Copyright 2014, Wiley.
78
3DG, three‐
dimensional graphene; CVD, chemical vapor deposition; GF, graphene foam; np‐Ni, nanoporous nickel; PDMS, polydimethylsiloxane;
PMMA, polymethyl methacrylate
ZENG ET AL.
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203

CNCs were also prepared with pyridine as the pre-
cursor.
91,94‐97
Hence, the template of 3D hierarchical basic mag-
nesium carbonate could result in 3D N‐doped CNCs.
97
Mecklenburg et al.
82
reported a hierarchical aerographite
consisting of interconnected close‐shell microtubes, via
CVD synthesis based on adjustable ZnO network
(Figure 10B–D).
CaO is a low‐cost oxide, which is extracted from
Ca(OH)
2
,
83
seashells,
84
eggshells,
85
and so forth. Tang
et al.
83
reported a CaO‐templated growth of hierarchical
porous graphene. Using porous CaO as templates, the re-
plication of its structural hierarchy resulted in a porous
graphene framework. Shi et al.
84
reported a scalable
seashell‐based template for the CVD growth of graphene
foams (Figure 10E). When heated in air, the organic
FIGURE 10 Chemical vapor deposition on metal oxide templates for three‐dimensional graphene. (A) A scheme and scanning
electron microscopy (SEM) images of the formation of carbon nanocages via in situ MgO template method. Reproduced with permission:
Copyright 2015, Elsevier.
91
(B) The growth model of aerographite, showing carbon nucleation belts (green) to closed sheets on a ZnO
template. (C) The proposed structure of aerographite shells and possible pathways for diffusion. (D) Morphologies of three types of
aerographites. Reproduced with permission: Copyright 2012, Wiley.
82
(E) A scheme and SEM images of fabrication of scallop‐derived 3DG
foams. Reproduced with permission: Copyright 2016, American Chemical Society.
84
204
|
ZENG ET AL.

molecules and CaCO
3
in seashells decomposed, forming an
interconnected porous structure. Utilizing the lime slaking
of CaO, graphene foams were molded into arbitrary shapes
macroscopically. Han et al.
85
reported that CaO derived
from eggshells transformed alcohols into nanoporous gra-
phene at the temperature as low as 500°C.
SiO
2
is also a suitable substrate for nucleating and
growing graphene.
86
Chen et al.
87
reported 3D biomorphic
graphene powders using diatomite templates. The product
inherited the naturally curved surface of diatomite
(Figure 11A–C), overcoming the interlayer stacking. Bi
et al.
88
reported a 3DG monolith of tetrahedrally con-
nected cellular structure with SiO
2
aerogels template. The
3DG showed a network of tubular few‐layer graphene with
hollow tetrahedral joints (Figure 11D–G). They then fab-
ricated another graphene porous monolith of mesoporous
graphene‐filled tubes using SiO
2
aerogels as a template.
89
Efforts have been made on fabricating graphene monoliths
with a finer interconnected network. Xu et al.
90
reported
3D free‐formable graphene foams by combination of 3D
printing and CVD (Figure 11H). Porous silica templates
were prepared by digital light process method with pho-
topolymerization of resin. The porosity of templates was
controlled by the debinding and sintering process. The
final 3DG inherited an interconnected network with
excellent SSA, conductivity, and mechanical properties.
2.2.3 |CVD on salt templates for 3DG
Salts can also template the interconnected graphene net-
work. The solubility of NaCl is 26.5 g at 20°C,
98
so it can be
removed via dissolving in water. Li et al.
99
developed a free‐
standing graphene paper using a microwave plasma‐
enhanced CVD process with NaCl as templates. NaCl
powders were melted on the top of graphene base layer and
recrystallized into prism‐like crystals, acting as templates
for growth of prism‐like graphene building blocks. The final
flexible transparent graphene paper was peeled from the
substrate (Figure 12A–D). Wei et al.
100
reported a scalable
NaCl‐templated synthesis of NG. The recrystallized NaCl
powders were selected as templates. Pyridine precursor was
converted to NG cages on the surface of NaCl. A printable
ink was further prepared with NG and GO for making
different patterns (Figure 12E–H).
2.2.4 |CVD on combined templates for 3DG
Considering the functions of various templates, the com-
bined templates may assist the fabrication of some unique
structures. Colloidal particles with better uniformity can
be used as good morphological regulators, whereas the
catalytic metals can be used to deposit graphene. Lin
et al.
101
reported an N‐doped mesoporous graphene‐like
carbon with the combined templates of silica and nickel,
where both could be considered as catalysts for growing
graphene. Liu et al.
102
reported a free‐standing 3DG por-
ous monolith via CVD with porous PS template and nickel
catalysts. Moreover, monodisperse colloidal microspheres
can be packed into the opal template, constructing the
desired symmetry. Xu et al.
103
reported an inverse opal
graphene. Aluminum nitrate was infiltrated into the in-
terstice of PMMA opal crystal to construct the oxide
template after heating. Through CVD of graphene on the
aforementioned template, the 3DG was obtained with a
inverse opal structure.
2.3 |Pyrolysis of solid/liquid carbon
sources
From industrialization perspective, the bottom‐up
synthesis starting from solid/liquid carbon sources in-
stead of gaseous ones is more efficient toward tonne‐level
production of 3DG,
104
especially using renewable bio-
mass or discarded organics for sustainability. The strat-
egy includes non‐templating pyrolysis and templating
pyrolysis of solid/liquid carbon sources. The former uti-
lizes chemical blowing,
105
optothermal reaction,
106
and
spatially separated carbonization,
107
to create pores. The
latter uses templates to assist the pore formation.
2.3.1 |Non‐templating pyrolysis for 3DG
Blowing is a general strategy using gases to blow the flow-
able matter into large variety of polymeric, metallic, and
ceramic foams. The chemical blowing strategy was first re-
ported for manufacturing the foam of BN mono‐/few‐
layered nanosheets as early as 2011 by Wang et al.
108
In
2013, Wang et al.
105
developed the ammonium‐assisted
chemical blowing route, that is, sugar blowing, to produce
3D strutted graphene (SG) foams, as shown in
Figure 13A–E.InheatingglucoseandNH
4
Cl, the molten
syrup was gradually polymerized to melanoidin, which was
blown into the polymeric bubble network by released gases
from NH
4
Cl. The polymeric bubble walls were thinned,
owing to the blowing dilation, the drainage, and the che-
mical elimination reaction. The polymeric walls were finally
graphitized into ultrathin graphene membranes at high
temperature. Diverse sugars were available in the
ammonium‐assisted chemical blowing method for making
SG foams, and the additional (NH
4
)
2
CO
3
promoted the
dispersion of NH
4
Cl in a syrup during heating
(Figure 13F–I).
109
The match between the decomposition of
ZENG ET AL.
|
205

ammonium salts and the polymerization curing of polymers
affects the structure of SG, whereas their decoupling results
in the undeveloped or the broken bubbles.
110
Geometrically,
SGs were the packing of polyhedrons, whose facets were
mainly pentagons and minorly quadrangles, hexagons, and
heptagons, so SG showed high conductivity, compressible
mechanics, and large SSA. The geometric constrain, the
thermodynamic and kinetic controls, and the static and
dynamic evolution govern the pore structure of foams,
which further determines the properties of foams.
111
The
blowing strategy is recently used to foam other organics,
such as hexamethylene tetraamine,
112
ferric citrate,
113
organic acid–ethylene glycol,
114–116
citrate,
117
and so on.
In another template‐free route, fast energy input is
applied to generate the gas to form pores. Tour's group
reported a laser‐induced graphene (LIG) with 3D net-
works from a commercial polyimide film with CO
2
laser
irradiation.
106
The laser irradiation led to extremely high
FIGURE 11 CVD on silica templates for 3DG. (A) A schematic diagram showing graphene growth on a diatom frustule and its hierarchical
structures. (B, C) SEM images of a diatom frustule and the derived 3DG. Reproduced under the terms of the Creative Commons Attribution 4.0
International license: Copyright 2016, Springer Nature.
87
(D) A schematic of 3DG via CVD on mesoporous SiO
2
template.
(E, F) SEM images of 3DG and its individualhollowtube.(G)ATEMimageoftwofew‐layer graphene walls. Reproduced with permission:
Copyright 2019, Wiley.
88
(H) A schematic of 3DG assembly via CVD on porous SiO
2
template designed in digital light process. I presents an SEM
image of a silica green body via digital light processing, II shows debinding of polymer additives and sintering procedure for porosity formation with
its micro‐CT scanning image, and III and IV present SEM and TEM images of 3DG assembly after CVD and wet etching, respectively. Reproduced
with permission: Copyright 2020, American Chemical Society.
90
3DG, three‐dimensional graphene; CT, computed tomography; CVD, chemical
vapor deposition; HF, hydrofluoric acid; SEM, scanning electron microscopy; TEM, transmission electron microscopy
206
|
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localized temperature (>2500°C), so it could break the
C–O, C═O, and N–C bonds of polyimide. These atoms
would be recombined and released as gases, and aro-
matic compounds were rearranged to form a graphitic
structure. They also reported a 3D LIG foam based on
laminated object manufacturing. The LIG foam can be
refined into more complex 3D shapes by using a fiber
laser (Figure 14A–D).
118
Besides, B‐doped LIG was
synthesized by introducing boron precursor into a
polymer.
119
The precursors of LIG have been expanded to
wood,
120
cloth,
121
food,
121
Teflon,
122
carbon nanodots,
123
and so on.
Considering cost control and sustainability, it is va-
luable to convert biomass into 3DG. The avoidance of
bulk carbon might be the key in the carbonization of
biomass. Gao et al.
107
reported an oxidation–aminolysis
treatment of cellulose for preparing 3D porous graphene‐
like paper. The cellulose paper was oxidized at 250°C in
FIGURE 12 CVD on salt templates for 3DG. (A) A scheme for synthesis of FFT‐GP via CVD. (B) A photo of FFT‐GP on a logo with a
dihedral angle of 60°. (C, D) SEM images as well as Raman profile of an FFT‐GP sheet. Reproduced with permission: Copyright 2015,
American Chemical Society.
99
(E) A schematic illustration of salt‐templated CVD direct growth of NG on NaCl crystal. (F) A TEM image of
NG cages. (G) A photo of different patterns via NG‐based ink printing. (H) A print of NG‐based ink with well‐defined shapes on a variety of
substrates. Reproduced with permission: Copyright 2019, American Chemical Society.
100
3DG, three‐dimensional graphene; CVD, chemical
vapor deposition; FFT‐GP, free‐standing flexible transparent graphene paper; NG, N‐doped graphene; PI, polyimide; SEM, scanning electron
microscopy; TEM, transmission electron microscopy
ZENG ET AL.
|
207

air and then heated in NH
3
at 1000°C. The graphene‐like
paper was directly obtained, consisting of ribbon‐like
ultrathin graphenic sheets (Figure 14E,F). They re-
vealed two exceptional effects: the spatially separated
charring below 500°C and the etching above 500°C. The
oxidation and the subsequent amidation could detach
the glucan chains and break the hydrogen bonding
network, avoiding the formation of solid/unhollow/
dense carbon. Ammonia would further etch away dis-
continuous or amorphous carbons. Further combina-
tion of the chemical vapor etching and the doping
yielded a flexible B, N, and O co‐doped 3D nanoporous
carbon paper.
124
2.3.2 |Templating pyrolysis for 3DG
When pyrolyzing solid/liquid organics to prepare 3DG,
templates may be added to regulate the morphology.
Sun et al.
21
reported that both pristine graphene and
doped graphene were fabricated with metal catalyst and
solid carbon sources. Zhao et al.
125
developed a 3DG by in‐
situ decomposition of basic copper carbonate‐derived
porous Cu template, which converted PMMA to few‐
layered graphene and micro–meso–macropore network.
Yoon et al.
126
reported multilayer graphene balls by the
carburization of nickel nanoparticles with polyol solution.
The formed nickel carbide would decompose into a phase‐
separated mixture of nickel and graphene. Han et al.
127
also reported a 3DG with the np‐Ni template by nickel
carbide‐mediated strategy (Figure 15A,B). Xiao et al.
128
reported the conversion of predefined 3D pyrolyzed pho-
toresist films into 3DG (Figure 15C–G). Interference li-
thography was implemented to fabricate 3D porous
carbon, which was sputtered with nickel on the surface.
When heated, amorphous carbon would diffuse into
nickel and concomitantly graphitize. Sha et al.
129
reported
a free‐standing 3DG foam by the powder metallurgy
FIGURE 13 Non‐templating pyrolysis of solid carbon for three‐dimensional graphene via blowing route. (A) Scanning electron
microscopy (SEM) images of ammonium‐assisted chemical blowing route, that is, sugar blowing, showing glucose and NH
4
Cl raw crystals,
subsequent melanoidin bubbles, and final strutted graphene (SG). (B) An SEM image of an SG piece. (C) Changes of the specific surface area
and the ratio of cell perimeter to strut width versus heating rates. (D) Three‐dimensional optical photos of SG. (E) High‐resolution
transmission electron microscopy images of a one‐to two‐layered graphene and a three‐to four‐layered graphene of SG. Reproduced under
the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs 3.0 Unported license: Copyright 2013, Springer Nature.
105
(F) An
optical image of a graphene face braced by graphitic struts, showing their intimate connection in SG. (G–I) SEM images of intermediate
polymeric bubbles and their thin walls in the chemical blowing of sugars. Reproduced with permission: Copyright 2015, Elsevier.
109
208
|
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method (Figure 15H). Nickel powders and sucrose were
mixed and heated, and graphene grew on the surface of
nickel powders. They also reported a powder‐bed 3D
printing method for producing 3DG porous monoliths by
feeding the mixture of nickel and sucrose powder, with
each layer treated by CO
2
laser (Figure 15I).
130
In addition to aforementioned metal templates, oxide
and salt templates are also used for the preparation of
3DG, such as MgO,
131
Fe
3
O
4
,
132
Zn(NO
3
)
2
,
133
Na
2
CO
3
,
134
NaCl,
135,136
KOH,
137
and so forth. Qian et al.
131
reported a
porous few‐layered graphene via the Pechini combustion
method, with salicylic acid as a complexing agent and
Mg‐(NO
3
)
2
for providing complexing cations and com-
bustion fuel. The homogeneous dispersion of Mg
2+
de-
termined the monodispersity of MgO nanocrystals, which
acted as the substrate for graphene growth. Ji et al.
132
re-
ported that the oleic acid ligands attached at the surface of
Fe
3
O
4
nanocrystals were carbonized into few‐layered gra-
phene. Wang et al.
133
reported the solution‐based spray
pyrolysis of the mixture of sugar and salt for fabrication of
porous carbon or graphene nanospheres. Qin et al.
135
re-
ported the 3D self‐assembly of NaCl templates for
fabrication of 3DG porous networks (Figure 16A). Citric
acid was chosen as the carbon source to restrain the hy-
drolysis of SnCl
2
for obtaining uniform dispersion of Sn.
Sn and NaCl catalyze the growth of graphene shells and
walls, respectively. The graphene shells were pinned on
the graphene walls to construct the 3D porous graphene
networks. Li et al.
136
reported that glucose on the surface
of NaCl crystals converted to graphitic walls when un-
dergoing carbonization. Wang et al.
137
reported a 3DG by
alkaline‐assisted pyrolysis of waste tires. Waste tires were
ground and mixed with KOH. By heating above 1000°C,
the generated potassium vapor induced the carbon atom
arrangement into the porous graphene (Figure 16B–E).
2.3.3 |Zinc‐tiering pyrolysis for 3DG
Considering that the removal of most templates includes
the use of wet etching, which injures surface‐grown gra-
phene and increases cost, the low‐boiling‐point metals are
explored for some special pyrolysis synthesis. Jiang et al.
6
established a zinc‐assisted solid‐state pyrolysis (ZASP) for
FIGURE 14 Non‐templating pyrolysis of solid carbon for three‐dimensional graphene via laser induction and spatially separated
charring, respectively. (A) A schematic of laminated object manufacturing (LOM) process for manufacturing laser‐induced graphene (LIG)
foam. (B) “R”shape of the LIG foam. (C) A schematic of fiber laser milling process. (D) 3D LIG foam printed by the combination of LOM
and fiber laser milling. Reproduced with permission: Copyright 2018, Wiley.
118
(E) A scheme of spatially separated charring via
oxidation–aminolysis route of cellulose for making 3D graphene‐like paper. (F) A scanning electron microscopy image and photo of the 3D
graphene‐like paper. Reproduced with permission: Copyright 2019, American Chemical Society.
107
ZENG ET AL.
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209

producing 3DG porous monoliths (Figure 17A–M). The
powders of glucose and zinc were compacted into an in-
got and then heated to straightforwardly produce graphene
aerogel. The zinc‐guided graphene aerogel was named
asZNG.Zincintroducedatieringprocess(anew
carbon–metal interaction) beyond the normal templating,
which guided the delamination of bulky organics or solid
chars. The glucose‐derived char was fully impregnated with
zinc at around 480°C (Figure 17G). With heating to 800°C,
the embedded zinc was gradually extruded from the
FIGURE 15 Metal‐assisted pyrolysis of solid/liquid carbon sources for 3DG. (A, B) A schematic and TEM image of preparation of MG as
well as N‐doped MG via a carbide‐mediated graphene growth method. Reproduced with permission: Copyright 2018, Wiley.
127
(C) A schematic of 3DG via CVD on lithographically defined porous nickel. (D) An SEM image of porous carbon. (E) Porous carbon coated
with nickel. (F, G) 3DG. Reproduced with permission: Copyright 2012, American Chemical Society.
128
(H) A schematic of powder
metallurgy method to prepare 3DG foam. Reproduced with permission: Copyright 2015, American Chemical Society.
129
(I) A schematic of
synthesis of 3DG foam using a simulated 3D printing process. Reproduced with permission: Copyright 2017, American Chemical Society.
130
3DG, three‐dimensional graphene; CVD, chemical vapor deposition; MG, mesoporous graphene; np‐Ni, nanoporous nickel; PMT‐GF,
powder metallurgy templates for 3DG foam; SEM, scanning electron microscopy; TEM, transmission electron microscopy
210
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char–zinc matrix, resulting in the lamellar texture of
char–zinc sandwich (Figure 17H). The extrudation is driven
by the diminishing surface energy of the char and the
coarsening of zinc. The lamellar texture relies on the ani-
sotropy of sp
2
carbon. After natural vaporization of zinc, the
monolith of all‐membrane 3DG was obtained (Figure 17I).
Additionally, zinc promoted the carbonization–
graphitization of organics, precluded the troublesome wet
etching, and realized the easy recycling of metal agents. It is
easy to be scaled up with the compatible establishments of
industrial powder metallurgy and sintering.
3|SUPERCAPACITORS BASED
ON 3DG POROUS MONOLITHS
3DG porous monoliths have been widely studied as
supporters, elastomers,
138–140
electrical conductor,
141
adsorbent,
142
and electrochemical electrodes.
143–145
As
advanced electrochemical porous electrodes, 3DG has
been applied to supercapacitors, electrocatalysis, and
batteries.
Among the three types of supercapacitors (EDLCs,
pseudocapacitors, and asymmetric supercapacitors),
1
the
EDL mechanism guarantees high power, operable fre-
quency, safety, millions of life cycles, and effortless
maintenance. Historically, graphene‐based EDLCs were
pioneered by Vivekchand et al.
146
and Stoller et al.
147
in
2008. Their capacitances were 117 and 135 F·g
−1
in 1 M
H
2
SO
4
and 5.5 M KOH, respectively, based on the elec-
trodes of RGO powders. Graphene‐based EDLCs per-
formed normally better than those of activated carbon,
but they were still far from the theoretical prospect, re-
sulting from the irreversible restacking of graphene
flakes during manufacturing and cycling.
3DG‐based EDLCs were attempted to mitigate such
issue. In principle, 3DG monoliths can increase the en-
ergy density beneficial from the high SSA, because the
FIGURE 16 Salt‐assisted pyrolysis of solid carbon sources for 3DG. (A) A schematic of synthesis of Sn@3DG using NaCl template.
Reproduced with permission: Copyright 2014, American Chemical Society.
135
(B) A schematic of formation of 3DG structure from waste
tires. (C–E) SEM, TEM, and HRTEM microscopy images of the 3DG structure. Reproduced with permission: Copyright 2019, Elsevier.
137
3DG, three‐dimensional graphene; CVD, chemical vapor deposition; HRTEM, high‐resolution transmission electron microscopy;
SEM, scanning electron microscopy; TEM, transmission electron microscopy
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211

intersupported structure prevents restacking. 3DG can
also powerfully improve power density and operable
frequency relying on the dually interconnected electron
highway and ion highway via respective sp
2
carbon
network and continual cavity. Plenty of efforts have
been made concerning the design of 3DG
toward high‐performance electrodes for EDLCs.
Besides the electrode, the electrolyte is another im-
portant component in supercapacitors. It is assorted as
aqueous, organic, and ionic liquid systems.
148
Ionic
conductivity and electrochemical stability are two vital
properties of electrolytes, which influence the rate cap-
ability and the electrochemical windows as well as cy-
cling performance.
149
Aqueous electrolytes provide high
ionic conductivity (up to ~1 S·cm
−1
) and nonflammable
safety but low operable voltage (1.23 V thermo-
dynamically).
149,150
Organic and ionic liquid systems
have high electrochemical windows (up to 4 V) for
increasing energy density at a cost of low ionic con-
ductivity (typically ~0.06 S·cm
−1
) and flammability.
148,149
Organic electrolytes are preferred commercially, con-
sidering the high operable voltage, whereas aqueous
electrolytes are popularly studied owing to the con-
venience without waterproof processing requirements.
3.1 |Aqueous EDLCs based on 3DG
Conway
151
put forward that carbon materials employed
as EDLCs electrodes should satisfy three characteristics:
high SSA, good conductivity, and excellent accessibility
for electrolyte to intrapore space. Therefore, the design
of 3DG focuses first on SSA, conductivity, and pore
distribution.
The high SSA provides large electrode–electrolyte in-
terface, to improve the gravimetric capacitance of EDLCs.
FIGURE 17 Zinc‐tiering pyrolysis for three‐dimensional graphene. (A) A sketch of zinc‐assisted solid‐state pyrolysis (ZASP) route
for producing zinc‐guided graphene aerogel (ZNG). (B, C) The feedstock of glucose–zinc workpiece and the corresponding ZNG
product in ZASP. (D, E) A scanning electron microscopy image and high‐resolution transmission electron microscopy image of ZNG and
an individual graphene membrane as well as the thickness statistics of graphene. (F–I) The zinc‐tiering effect in ZASP: (F) a sectional
diagram of glucose passing one face, two edges, and one vertex among four zinc microspheres; (G) zinc impregnates char via dynamic
evaporation–condensation in heating; (H) zinc delaminates char into sp
2
carbon lamella; (I) lamellar char forms graphene membranes, so
that the edge/vertex is a loose bundle of membranes. Iand II denote vertical and tilted double‐layered graphene, respectively. (J–L) Pyrolysis
of glucose among non‐tiering metals, yielding inevitable non‐membrane morphological impurities. III and IV mark the solid graphitic strut
and tetrapod joint, respectively. (M) Geometry of model of glucose syrup among four metal microspheres. Reproduced with permission:
Copyright 2019, Wiley
6
212
|
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Early studies indicated a linear dependence between spe-
cific capacitance and SSA.
152,153
It should be noted that
carbon nanomaterials often contain heteroatomic oxygen,
which contributed pseudocapacitance leading to some
unusual areal capacitance of >0.25 F·m
−2
in literature.
The interaction between pore wall and solvent varies
greatly in different pores.
154
For example, macropores are
large enough so that the pore curvature is no longer sig-
nificant, which can be simply modeled by the Helmholtz
model. In mesopores, solvated ions approach the pores
wall and form a double‐cylinder‐like capacitor, so the ef-
fect of pore curvature becomes prominent, because the
pore size is comparable with the distance between ions
and pore wall. In the case of micropores, the desolvated
ions form a wire‐in‐cylinder‐like capacitor, with increasing
capacitance. In addition, in case of a single‐or few‐layered
graphene electrode (i.e., a low‐density‐of‐state system), the
interfacial EDL capacitance was found to be dominated by
the quantum capacitance of the solid side at the
electrode–electrolyte interface.
5,155
It implied the capaci-
tance might be reduced if the pore wall was too thin.
The high‐SSA materials without a continuous con-
ductive network result in poor capacitance, whereas the
combination of high SSA and high conductivity leads to a
high gravimetric capacitance.
156
For example, activated
carbon (AC) has high SSA, but its conductivity is much
worse than that of graphene. There are high contact re-
sistances resulting from binders. The capacitance of AC‐
based EDLCs at low current density is acceptable, but it
is poor at high current density, that is, poor rate cap-
ability. Both the ohmic resistance and the pore effect
hereinafter are responsible for such phenomenon.
Pore structure can also influence specific capacitance
and rate capability. Generally, pores with different sizes
function variedly. Macropores can act as the buffering
reservoirs of electrolyte, mesopores are beneficial for ion
diffusion, and micropores can offer a large surface
area.
157–159
As mentioned before, the pores with size
smaller than 1 nm could lead to the desolvation or the
distortion of the ion solvation shell, anomalously increas-
ing capacitance at a cost of sluggish kinetics.
160
The pores
with size smaller than 0.5 nm become inaccessible to
electrolyte ions due to the sieving effect.
161,162
Hence, the
pore size close to solvated ion size might yield the optimal
EDL capacitance.
163,164
In addition, the capacitance decays
at high current or frequency. The distributed charge sto-
rage in electrode pores is equivalent to a resistor–capacitor
network, so the penetration depth into a certain pore de-
creases at high frequency. With increasing frequency,
smaller pores are blocked earlier, which decreases the
capacitance.
165
To sum up, a large‐SSA conductive elec-
trode with rational trade‐off pore structure may confer the
optimal capacitance and rate of EDLC devices.
These three properties—SSA, conductivity, and pore
structure—are key to EDLCs. Numerous 3DG materials
have been studied to obtain the better characteristics for
the higher performances of EDLCs. Xu et al.
26
employed
the assembled RGO hydrogel, showing a capacitance of
175 F·g
−1
in 5 M KOH. With a modified hydrothermal
reduction method, Xu et al.
166
established a 3DG hy-
drogel with exceptional electrical conductivity, and they
obtained a capacitance of 186 F·g
−1
in 1 M H
2
SO
4
. Shao
et al.
57
combined freeze‐casting and filtration to prepare
3D cellular RGO films. With the assistance of open por-
osity, high conductivity, and mechanical strength, the
electrode exhibited a capacitance of 284.2 F·g
−1
in 1 M
H
2
SO
4
. Bi et al.
89
adopted silica templates to construct
graphene monoliths, which possessed a conductivity of
32 S·cm
−1
, an SSA of 1590 m
2
·g
−1
, and finally a capaci-
tance of 303 F·g
−1
in 1 M H
2
SO
4
. Jiang et al.
6
used ZASP
to create all‐membrane 3DG monoliths, that is, ZNG.
ZNG showed the temperature‐invariant conductivity of
1.6 S·cm
−1
and the tunable SSA at 1390–2020 m
2
·g
−1
.It
showed a high capacitance of 336 F·g
−1
and an ultralong
lifetime of 1,300,000 cycles in 1 M H
2
SO
4
(Figure 18A–F).
3.2 |Aqueous EDLCs based on
doped 3DG
More than the structural design of 3DG for EDLCs, an-
other strategy for increasing capacitance is to introduce
heteroatoms providing pseudocapacitance.
150
The pseu-
docapacitance resulted from local changes in the electron
density of the carbon matrix or the redox reaction of
functional groups.
167,168
The heteroatom doping can also
improve wettability and conductivity of carbon.
169
Var-
ious heteroatoms, for example, B, N, O, P, and S, have
been adopted for doped 3DG materials for EDLCs.
B atoms can be doped into graphene without sig-
nificant lattice distortion, showing electron‐accepting
behavior.
156
Zuo et al.
170
reported a B‐doped 3DG via
freeze‐drying and thermal treatment of GO gel with boric
acid. The B‐doped 3DG showed a rectangular‐like curve
with one pair of broad Faradaic peaks in cyclic voltam-
metry (CV), and the capacitance was 281 F·g
−1
in 2 M
H
2
SO
4
. Li et al.
171
also fabricated B‐doped RGO aerogels
with GO and boric acid, showing a capacitance of
308.3 F·g
−1
in 6 M KOH.
N might be the most widely used doping element. N
has one more outermost electron than C, and it exhibits
the electron‐donating behavior.
156
There are three usual
configurations of N within graphitic lattice, that is,
pyrrolic‐N, pyridinic‐N, and graphitic‐N.
101,172
Litera-
ture indicated that the former two could enhance the
pseudocapacitance through reversible redox reactions,
ZENG ET AL.
|
213

whereas graphitic‐N could improve the conductivity of
graphene and promote its interaction with the anions
of electrolyte.
173–175
Zhang et al.
169
fabricated an
N‐superdoped 3DG (GF‐NG), where graphene foam
acted as the framework and N‐superdoped RGO aero-
gels served as the filler. N content of the hybrid reached
15.8%, dominated by pyridinic‐N and pyrrolic‐N. The
GF‐NG presented capacitances of 380, 332, and
245 F·g
−1
in 6 M KOH, 1 M H
2
SO
4
,and1MKCl,re-
spectively (Figure 19A–C). Lin et al.
101
reported that the
mixed three‐type N‐doped few‐layered graphene ex-
hibited a rectangular CV curve with a capacitance of
855 F·g
−1
in 0.5 M H
2
SO
4
(Figure 19D–F).
P has similar properties as N, except the larger atomic
radius and lower electronegativity.
176,177
Fan et al.
178
synthesized a P‐doped RGO hydrogel via one‐step hy-
drothermal method with phytic acid and GO. It gained a
capacitance of 388.5 F·g
−1
in 1 M H
2
SO
4
. They put for-
ward that the pseudocapacitance caused by P–O bonding
gave a greater boost than P–C bonding regarding capa-
citance. Yu et al.
179
combined spectroelectrochemical
methods with theoretical calculations to demonstrate
that pseudocapacitive feature was associated with
C–P═O bonding. The fabrication of P‐incorporated RGO
achieved a capacitance of 352 F·g
−1
in 1 M H
2
SO
4
.
O atoms are always easily attached to the carbon
matrix, resulting from the oxygenic precursors or the
calcination processes. In other words, it is not easy to
completely eliminate O heteroatoms or to get pure
carbon materials at the usually used experimental
conditions (e.g., heating around 1000°C). This leads to
the suspicious reports of EDLCs tests and the difficult
establishment of the SSA–capacitance relationship, as
mentioned previously. The O‐doping could increase the
interlayer distance of graphene, and it could provide
active sites for the pseudocapacitive contribution by
redox reactions. The O‐doping might cooperate with
other elements to confer carbon materials with the
minimum adsorption energy, facilitating more cations
stored on the surface. These adsorbed cations could
effectively inhibit the hydrogen evolution, extending
the voltage window.
180
S doping is also used in carbon materials for EDLCs.
Due to the electron‐withdrawing property of S, S–carbon
FIGURE 18 Aqueous electric double‐layer capacitors (EDLCs) based on three‐dimensional graphene (3DG). (A) Cyclic voltammetry of
symmetric electrodes of zinc‐guided graphene aerogel (ZNG) and references at a scan rate of 100 mV·s
−1
in 1 M H
2
SO
4
. (B) Accelerated
power cycling of ZNG and three‐dimensional reduced graphene oxide (3DRGO) electrodes at 1.4 V at room temperature. Inset is
galvanostatic charging–discharging at 5 A·g
−1
. (C) Terminal voltage of ZNG electrodes following time, as measured by the temporally
continuous monitoring in potentiostatic charging and galvanostatic discharging. (D) The ratio of the voltage drop to the discharging current.
(E) Internal resistance (IR) and its components. IR difference is dominated by equivalent series resistance (ESR), whereas in‐pore ionic
resistances are similar and self‐discharge contributes variable residues. (F) Life‐cycle stored energy of ZNG‐based supercapacitors (red line is
that of ZNG). Reproduced with permission: Copyright 2019, Wiley.
6
214
|
ZENG ET AL.

network possessed a wider band gap and exhibited
metallic‐like properties.
181
Yu et al.
182
synthesized an
S‐incorporated RGO aerogel through freeze‐drying and
thermal treatment, which showed a capacitance of
445.6 F·g
−1
in 1 M H
2
SO
4
. Islam et al.
183
also reported a
cellular 3D S‐doped RGO foam through self‐assembly
and thermal treatment, which presented a capacitance of
367 F·g
−1
in 6 M KOH.
Lastly, the electronic structure of graphene could be
regulated by the binary and ternary doping, for example,
B, N co‐doped,
184,185
N, P co‐doped,
186
N, S co‐
doped,
187–189
S, P co‐doped,
190
B, P co‐doped,
191
B, N, P
co‐doped,
192
and so forth. Wu et al.
184
reported a B, N co‐
doped monolithic RGO aerogel by hydrothermal assem-
bly and freeze‐drying. It showed a capacitance of
239 F·g
−1
in 1 M H
2
SO
4
. Zou et al.
185
fabricated a B,
N‐doped holey RGO aerogel with ammonia borane as the
precursor. It had a B content of 9.17% and an N content
of 4.15%, furnishing a capacitance of 456 F·g
−1
in 1 M
H
2
SO
4
. Pan et al.
192
reported a B, N, P co‐doped holey
RGO hydrogel (BNP‐HGH) via mild defect‐etching re-
action and hydrothermal process. It showed a capaci-
tance of 362 F·g
−1
in 1 M H
2
SO
4
. Literature indicated the
synergistic effect of multiple doping, which might tune
the properties of graphene versatility; however, more
efforts are needed to thoroughly understand the sy-
nergistic mechanism.
3.3 |Nonaqueous EDLCs based on 3DG
As mentioned above, the second way to increase the
energy density of EDLCs is to raise the operable voltage.
Acidic and alkaline aqueous electrolytes are commonly
used for studying EDLCs, owing to high ionic con-
ductivity, yet they suffer from narrow electrochemical
windows. Neutral electrolytes can achieve the higher
operable voltage on account of high overpotentials of
hydrogen and oxygen evolutions; nevertheless, the
voltage is still lower than that of nonaqueous systems.
Organic electrolytes and ionic liquids can be used at a
voltage up to 4 V, so they are currently applied to
commercial carbon‐based EDLCs.
193
Organic electrolytes are constituted by conductive salts
dissolved in organic solvents.
148
Considering the larger
solvated ion sizes in organic electrolytes, it is vital to avoid
FIGURE 19 Aqueous EDLCs based on N‐doped 3DG. (A) SEM images of GF‐NG. (B) The capacitance of GF, GF‐rGO, GF‐NGs, and NG
powder calculated by the CV. (C) Capacitance of GF‐rGO and GF‐NGs at various current densities in 6 M KOH, 1 M KCl, and 1 M H
2
SO
4
.
Reproduced with permission: Copyright 2017, Wiley.
169
(D) Possible locations for N incorporation into a graphene layer. (E, F) CV and GCD
from the different ordered mesoporous carbon. Reproduced with permission: Copyright 2015, AAAS.
101
3DG, three‐dimensional graphene;
CV, cyclic voltammetry; EDLC, electric double‐layer capacitor; GCD, galvanostatic charge–discharge; GF, graphene foam; NG, N‐doped
graphene; rGO, reduced graphene oxide; SEM, scanning electron microscopy
ZENG ET AL.
|
215

the loss of capacitance caused by inaccessible small pores.
Besides, ion conductivity and viscosity of solvents need
careful consideration. Typical solvents employed in organic
electrolytes are acetonitrile (AN) and propylene carbonate
(PC). The former possesses higher conductivity and lower
viscosity, whereas the latter displays lower toxicity.
194
Zhu
et al.
195
reported activated microwave‐expanded graphite
oxide via chemical activation of microwave‐exfoliated GO,
which showed 166 F·g
−1
in BMIMBF
4
/AN, and the energy
density was 70 Wh·kg
−1
.Jiangetal.
109
reported an SG via
ammonium‐assisted chemical blowing, which realized a
capacitance of 190 F·g
−1
, and the energy density was
50 Wh·kg
−1
in TEABF
4
/AN (Figure 20A–C). Xu et al.
196
reported a holey 3DG framework (HGF) through the hy-
drothermal process, which showed a capacitance of
298 F·g
−1
in EMIMBF
4
/AN. The gravimetric and volu-
metric energy density were 35 Wh·kg
−1
and 49 Wh·L
−1
,
respectively. Nishihara et al.
197
fabricated a sponge‐like
graphene mesoporous framework (GMS) via oxide‐assisted
CVD, which exhibited a capacitance of 125 F·g
−1
in 1 M
Et
4
NBF
4
/PC. Meanwhile, high oxidation resistance of GMS
allowed an operable voltage up to 4 V, achieving an energy
density of 59.3 Wh·kg
−1
.
Heteroatom‐doped 3DG materials presented an en-
hanced energy density in organic electrolytes.
198
Pan
et al.
192
tested the BNP‐GH in 1 M EMIMBF
4
/AN,
which obtained gravimetric and volumetric energy
densities of 38.5 Wh·kg
−1
and 57.4 Wh·L
−1
, respectively
(Figure 20D–F). The redox peaks seemed unobvious in
organic electrolytes. The origin of the pseudocapaci-
tance brought by heteroatoms in organic electrolytes
still needs an in‐depth study. Heteroatom doping might
affect the electrode behavior in organic electrolytes by
advancing wettability, conductivity, and so on.
199–201
Ionic liquids (ILs) are low‐melting‐point salts con-
stituted solely by ions.
202
The solvent‐free ILs are non-
flammable owing to the low vapor pressure, showing
superiority in safety.
149
ThecapacitanceoftheirEDLCs
could be enhanced along with the increasing temperature
due to the improved conductivity and the decreased
viscosity.
203
ILs could help attain a wider operable voltage
window.
204
El‐Kady et al.
45
reported that laser‐scribed
porous graphene (LSG) offered a capacitance of
4.82 mF·cm
−2
and an energy density of 1.36 mWh·cm
−3
in
EMIMBF
4
.Xuetal.
205
reported an N‐doped holey RGO
aerogel via hydrothermal treatments, which showed an
FIGURE 20 Nonaqueous electric double‐layer capacitors (EDLCs) based on three‐dimensional graphene. (A) A photo and scanning
electron microscopy (SEM) images of strutted graphene. (B) Cyclic voltammetry (CV) curves under different scan rates. (C) The change
of specific capacitances along with discharge current density. The inset is the cycling performance at the current density of 10 A·g
−1
.
Reproduced with permission: Copyright 2015, Elsevier.
109
(D) SEM image of B, N, P co‐doped holey reduced graphene oxide hydrogel
(BNP‐HGH). (E) CV curve of symmetric EDLCs of different graphene hydrogels. (F) CV curves of BNP‐HGHs in 1 M EMIMBF
4
/AN.
Reproduced with permission: Copyright 2018, Elsevier.
192
216
|
ZENG ET AL.

energy density of 60.3 Wh·kg
−1
in EMIMTFSI‐80. Li
et al.
206
synthesized porosity‐adjustable 3DG as ultrathick
bulk electrodes. Eventually, the 3DG electrode exhibited a
maximum energy density of 64.7 Wh·L
−1
in BMIMBF
4
.
Chen et al.
207
prepared the N, S co‐doped RGO aerogel
through a one‐pot solvothermal method, which attained an
energy density of 100.7 Wh·kg
−1
in EMIMBF
4
.
More efforts are needed on nonaqueous EDLCs based
on 3DG. It is worth noting that the capacitances of ma-
terials in organic electrolytes are commonly smaller than
those in aqueous electrolytes. It can be attributed to the
larger solvated ion sizes, lower ionic conductivity, higher
viscosity, and so forth. Besides, the aging process and the
consequent failure of EDLCs need deep studies.
1,148
In
addition, very few types of ILs have been explored, and
most ILs suffer from low ionic conductivity and high
cost.
208,209
Plenty of research studies remain waiting to
be carried out to overcome the bottlenecks.
3.4 |Practical usage of EDLCs
Supercapacitors are featured by high power density, long
cycle lifetime, and low maintenance cost. Commercially
available supercapacitors are EDLCs composed of AC
and organic electrolytes, whereas pseudocapacitors are
not industrialized at present due to the limited lifetime.
EDLCs can be used for four occasions in our opinion:
fast charge and fast discharge, normal charge and fast
discharge, fast charge and normal discharge, and normal
charge and normal discharge.
(i) Concerning fast‐charge‐and‐fast‐discharge applica-
tions, EDLCs can reduce the voltage fluctuation as
the grid power buffers. Similarly, they can shave the
power pulses to fill the power gaps in the grid of
renewable energy. Regarding the capture and reuse
of waste energy, EDLCs can equip forklifts, ex-
cavators, and cranes, to reduce the fuel consump-
tion during frequent loading and unloading.
(ii) Regarding normal‐charge‐and‐fast‐discharge appli-
cations, EDLCs can supply the high‐power output
for electromagnetic launch, motor startup, flash-
light, defibrillator, array radar, and so on. EDLCs
can incorporate with batteries to answer the high‐
rate usage. Similarly, EDLCs can serve as the un-
interruptible power supply for fast‐response power
supply with low maintenance cost.
(iii) Concerning fast‐charge‐and‐normal‐discharge ap-
plications, EDLCs are recently used for electric bu-
ses, where they are recharged in tens of seconds
when stopping at each station. EDLCs can be used
as the energy storage reservoir for hand‐held
devices, for example, electric screwdriver, imagers,
thermometers, and so forth. Another example is the
kinetic energy recovery system of city buses and
electric trains.
(iv) Regarding normal‐charge‐and‐normal‐discharge
applications, EDLCs can be used to provide power
for volatile memory, clock chips, and so on.
4|SUMMARY AND
PERSPECTIVES
3DG porous monoliths are advanced materials formed by
constructing a highly conductive porous network struc-
ture with 2D graphene building blocks. They not
only can deliver the excellent properties of 2D graphene
at macroscale, but also can bring some unique ad-
vantages, such as hierarchical porous network, tunable
porosity, and outstanding mechanical character. All of
these make them high‐performance electrical con-
ductors, elastomers, adsorbents, and electrochemical
electrodes, where the electrode attracts especially tre-
mendous attention. This review paper summarizes the
fabrication of 3DG with various strategies, including
liquid‐based assembly of GO, CVD growth on 3D porous
templates, and pyrolysis of solid/liquid carbon sources. It
also surveys the advanced EDLCs applications based on
the 3DG materials, including aqueous, organic, and ILs
systems.
Despite the prosperous progress, the 3DG‐based
EDLCs are still far away from the expectation. Regard-
ing the 3DG material, liquid‐based methods have ad-
vantages of high yield and so on, yet the hand‐in‐hand
intersheet junction of RGO is often insufficient and
thermally labile due to weak van der Waals adhesion or
noncovalent bonding. CVD can generate high‐quality
3DG by duplicating the porous monolithic template, yet
the cycling of parent templates should be considered in
future large‐scale industrialization. The pyrolysis of so-
lid/liquid carbon sources using templates will face the
same issue. The nontemplating pyrolysis is cost‐effective,
yet full of challenges, for example, morphological im-
purities. In addition, systematical study about the growth
mechanism may benefit the control of the internal net-
work structure and the pore structure of 3DG.
Regarding EDLCs, the energy density is urgently re-
quired to be improved. Fundamental studies about 3DG
electrode materials will be helpful to adjusting the electrode/
electrolyte interfaces and to enhancing the performance. The
high‐voltage‐durable electrode and electrolyte can effectively
improve the energy density. The highly graphitized 3DG
might be a suitable electrode candidate. It is noted that the
volumetric performance is a crucial indicator for some
ZENG ET AL.
|
217

compact devices, so the trade‐off and optimization of the
pore size should be deliberated. In addition, the suitable pore
structure and the electrode–electrolyte match need to be
explored. The rational design of electrodes and electrolytes
will enable the high‐performance EDLCs to finally reach
heavyweight industrialization and commercialization.
ACKNOWLEDGMENTS
The authors acknowledge the support from National
Natural Science Foundation of China (51972168,
51672124, 21603096), Program for Innovative Talents
and Entrepreneur in Jiangsu, State Key Laboratory of
Catalytic Materials and Reaction Engineering (RIPP,
SINOPEC), and Technical Center of Nano Fabrication
and Characterization of Nanjing University.
CONFLICT OF INTEREST
The authors declare no conflict of interest. [Correction
added on 15 June 2021, after first online publication:
Conflict of Interest section has been added.]
ORCID
Xue‐Bin Wang https://orcid.org/0000-0002-7894-1922
REFERENCES
1. Shao Y, El‐Kady MF, Sun J, et al. Design and mechanisms of
asymmetric supercapacitors. Chem Rev. 2018;118:9233‐9280.
2. Lu M. Supercapacitors: Materials, Systems, and Applications.
Wiley‐VCH Verlag GmbH & Co. KGaA; 2013.
3. Miller JR, Simon P. Electrochemical capacitors for energy
management. Science. 2008;321:651‐652.
4. Simon P, Gogotsi Y, Dunn B. Where do batteries end and
supercapacitors begin? Science. 2014;343:1210‐1211.
5. Xia JL, Chen F, Li JH, Tao NJ. Measurement of the quantum
capacitance of graphene. Nat Nanotechnol. 2009;4:505‐509.
6. Jiang XF, Li R, Hu M, et al. Zinc‐tiered synthesis of 3D
graphene for monolithic electrodes. Adv Mater. 2019;31:
1901186.
7. Chae HK, Siberio‐Pérez DY, Kim J, et al. A route to high
surface area, porosity and inclusion of large molecules in
crystals. Nature. 2004;427:523‐527.
8. Novoselov KS. Electric field in atomically thin carbon films.
Science. 2004;306:666‐669.
9. Hernandez Y, Nicolosi V, Lotya M, et al. High‐yield pro-
duction of graphene by liquid‐phase exfoliation of graphite.
Nat Nanotechnol. 2008;3:563‐568.
10. Pu NW, Wang CA, Sung Y, Liu YM, Ger MD. Production of
few‐layer graphene by supercritical CO
2
exfoliation of gra-
phite. Mater Lett. 2009;63:1987‐1989.
11. Dresselhaus MS, Dresselhaus G. Intercalation compounds of
graphite. Adv Phys. 1981;30:139‐326.
12. WuL,LiW,LiP,etal.Powder,paperandfoamoffew‐layer
graphene prepared in high yield by electrochemical intercalation
exfoliation of expanded graphite. Small. 2014;10:1421‐1429.
13. Zhou F, Huang H, Xiao C, et al. Electrochemically scalable
production of fluorine‐modified graphene for flexible and
high‐energy ionogel‐based microsupercapacitors. J Am Chem
Soc. 2018;140:8198‐8205.
14. Chen ZP, Ren WC, Gao LB, Liu BL, Pei SF, Cheng HM.
Three‐dimensional flexible and conductive interconnected
graphene networks grown by chemical vapour deposition.
Nat Mater. 2011;10:424‐428.
15. Zhao J, Zhu G, Huang W, et al. Synthesis of large‐scale un-
doped and nitrogen‐doped amorphous graphene on MgO
substrate by chemical vapor deposition. J Mater Chem. 2012;
22:19679‐19683.
16. Shi L, Chen K, Du R, et al. Direct synthesis of few‐layer
graphene on NaCl crystals. Small. 2015;11:6302‐6308.
17. Pandolfo AG, Hollenkamp AF. Carbon properties and their
role in supercapacitors. J Power Sources. 2006;157:11‐27.
18. Raccichini R, Varzi A, Passerini S, Scrosati B. The role of
graphene for electrochemical energy storage. Nat Mater.
2015;14:271‐279.
19. Brownson DC, Kampouris DK, Banks CE. Graphene elec-
trochemistry: fundamental concepts through to prominent
applications. Chem Soc Rev. 2012;41:6944‐6976.
20. Li X, Cai W, An J, et al. Large‐area synthesis of high‐quality
and uniform graphene films on copper foils. Science. 2009;
324:1312‐1314.
21. Sun ZZ, Yan Z, Yao J, Beitler E, Zhu Y, Tour JM. Growth of
graphene from solid carbon sources. Nature. 2010;468:
549‐552.
22. Kauling AP, Seefeldt AT, Pisoni DP, et al. The worldwide
graphene flake production. Adv Mater. 2018;30:1803784.
23. Paredes JI, Villar‐Rodil S, Martinez‐Alonso A, Tascon JMD.
Graphene oxide dispersions in organic solvents. Langmuir.
2008;24:10560‐10564.
24. BaiH,LiC,WangXL,ShiGQ.ApH‐sensitive graphene oxide
composite hydrogel. Chem Commun. 2010;46:2376‐2378.
25. Li D, Muller MB, Gilje S, Kaner RB, Wallace GG. Processable
aqueous dispersions of graphene nanosheets. Nat
Nanotechnol. 2008;3:101‐105.
26. Xu Y, Sheng KX, Li C, Shi GQ. Self‐assembled graphene
hydrogel via a one‐step hydrothermal process. ACS Nano.
2010;4:4324‐4330.
27. Zhang X, Sui Z, Xu B, et al. Mechanically strong and highly
conductive graphene aerogel and its use as electrodes for
electrochemical power sources. J Mater Chem. 2011;21:
6494‐6497.
28. Chen P, Yang JJ, Li SS, et al. Hydrothermal synthesis of
macroscopic nitrogen‐doped graphene hydrogels for ultrafast
supercapacitor. Nano Energy. 2013;2:249‐256.
29. Jiang X, Ma YW, Li JJ, Fan QL, Huang W. Self‐assembly of
reduced graphene oxide into three‐dimensional architecture
by divalent ion linkage. J Phys Chem C. 2010;114:
22462‐22465.
30. Gao XF, Jang J, Nagase S. Hydrazine and thermal reduction
of graphene oxide: reaction mechanisms, product structures,
and reaction design. J Phys Chem C. 2010;114:832‐842.
31. Niu ZQ, Chen J, Hng HH, Ma J, Chen XD. A leavening
strategy to prepare reduced graphene oxide foams. Adv
Mater. 2012;24:4144‐4150.
32. Pham HD, Pham VH, Cuong TV, et al. Synthesis of the
chemically converted graphene xerogel with superior elec-
trical conductivity. Chem Commun. 2011;47:9672‐9674.
218
|
ZENG ET AL.

33. Hu H, Zhao ZB, Wan WB, Gogotsi Y, Qiu JS. Ultralight and
highly compressible graphene aerogels. Adv Mater. 2013;25:
2219‐2223.
34. Worsley MA, Pauzauskie PJ, Olson TY, Biener J, Satcher JH,
Baumann TF. Synthesis of graphene aerogel with high elec-
trical conductivity. J Am Chem Soc. 2010;132:14067‐14069.
35. Worsley MA, Olson TY, Lee JRI, et al. High surface area, sp
2
‐
crosslinked three‐dimensional graphene monoliths. J Phys
Chem Lett. 2011;2:921‐925.
36. CongHP,RenXC,WangP,YuSH.Macroscopicmultifunctional
graphene‐based hydrogels and aerogels by a metal ion induced
self‐assembly process. ACS Nano. 2012;6:2693‐2703.
37. Tang ZH, Shen SL, Zhuang J, Wang S. Noble‐metal‐promoted
three‐dimensional macroassembly of single‐layered graphene
oxide. Angew Chem Int Ed. 2010;49:4603‐4607.
38. Falvo MR, Clary GJ, Taylor RM II, et al. Bending and
buckling of carbon nanotubes under large strain. Nature.
1997;389:582‐584.
39. Sun HY, Xu Z, Gao C. Multifunctional, ultra‐flyweight, sy-
nergistically assembled carbon aerogels. Adv Mater. 2013;25:
2554‐2560.
40. Qiu L, Liu JZ, Chang SLY, Wu YZ, Li D. Biomimetic su-
perelastic graphene‐based cellular monoliths. Nat Commun.
2012;3:1241.
41. Zhang PP, Li J, Lv LX, Zhao Y, Qu LT. Vertically aligned
graphene sheets membrane for highly efficient solar thermal
generation of clean water. ACS Nano. 2017;11:5087‐5093.
42. Vickery JL, Patil AJ, Mann S. Fabrication of graphene‐
polymer nanocomposites with higher‐order three‐
dimensional architectures. Adv Mater. 2009;21:2180‐2184.
43. Huang X, Qian K, Yang J, et al. Functional nanoporous
graphene foams with controlled pore sizes. Adv Mater. 2012;
24:4419‐4423.
44. LiYR,ChenJ,HuangL,LiC,HongJD,ShiGQ.Highly
compressible macroporous graphene monoliths via an
improved hydrothermal process. Adv Mater. 2014;26:
4789‐4793.
45. El‐Kady MF, Strong V, Dubin S, Kaner RB. Laser scribing of
high‐performance and flexible graphene‐based electro-
chemical capacitors. Science. 2012;335:1326‐1330.
46. Gao W, Singh N, Song L, et al. Direct laser writing of micro‐
supercapacitors on hydrated graphite oxide films. Nat
Nanotechnol. 2011;6:496‐500.
47. Yeo SJ, Oh MJ, Jun HM, et al. A plesiohedral cellular net-
work of graphene bubbles for ultralight, strong, and super-
elastic materials. Adv Mater. 2018;30:1802997.
48. Zhang R, Hu R, Li X, et al. A bubble‐derived strategy to
prepare multiple graphene‐based porous materials. Adv
Funct Mater. 2018;28:1705879.
49. Yang H, Li Z, Sun G, et al. Superplastic air‐dryable graphene
hydrogels for wet‐press assembly of ultrastrong superelastic
aerogels with infinite macroscale. Adv Funct Mater. 2019;29:
1901917.
50. Yang H, Jin X, Sun G, et al. Retarding Ostwald ripening to
directly cast 3D porous graphene oxide bulks at open ambi-
ent conditions. ACS Nano. 2020;14:6249‐6257.
51. Huang Z, Su M, Yang Q, et al. A general patterning approach
by manipulating the evolution of two‐dimensional liquid
foams. Nat Commun. 2017;8:14110.
52. Kou L, Liu Z, Huang T, et al. Wet‐spun, porous, orientational
graphene hydrogel films for high‐performance super-
capacitor electrodes. Nanoscale. 2015;7:4080‐4087.
53. Salman M, Chu X, Huang T, et al.Functionalizationofwet‐spun
graphene films using aminophenol molecules for high perfor-
mance supercapacitors. Mater Chem Front. 2018;2:2313‐2319.
54. Xu Z, Zhang Y, Li PG. Gao C. Strong, conductive, light-
weight, neat graphene aerogel fibers with aligned pores. ACS
Nano. 2012;6:7103‐7113.
55. Xu Z, Gao C. Graphene chiral liquid crystals and macro-
scopic assembled fibres. Nat Commun. 2011;2:571.
56. Yang XW, Qiu L, Cheng C, Wu YZ, Ma ZF, Li D. Ordered
gelation of chemically converted graphene for next‐
generation electroconductive hydrogel films. Angew Chem
Int Ed. 2011;50:7325‐7328.
57. Shao Y, El‐Kady MF, Lin CW, et al. 3D freeze‐casting of
cellular graphene films for ultrahigh‐power‐density super-
capacitors. Adv Mater. 2016;28:6719‐6726.
58. Choi BG, Yang M, Hong WH, Choi JW, Huh YS. 3D mac-
roporous graphene frameworks for supercapacitors with high
energy and power densities. ACS Nano. 2012;6:4020‐4028.
59. Li Z, Xu Z, Liu YJ, Wang R, Gao C. Multifunctional non‐woven
fabrics of interfused graphene fibres. Nat Commun. 2016;7:13684.
60. Li Z, Huang T, Gao W, et al. Hydrothermally activated gra-
phene fiber fabrics for textile electrodes of supercapacitors.
ACS Nano. 2017;11:11056‐11065.
61. Dikin DA, Stankovich S, Zimney EJ, et al. Preparation and
characterization of graphene oxide paper. Nature. 2007;448:
457‐460.
62. Park S, Lee KS, Bozoklu G, Cai WW, Nguyen ST, Ruoff RS.
Graphene oxide papers modified by divalent ions‐enhancing
mechanical properties via chemical cross‐linking. ACS Nano.
2008;2:572‐578.
63. Yang XW, Zhu JW, Qiu L, Li D. Bioinspired effective pre-
vention of restacking in multilayered graphene films: to-
wards the next generation of high‐performance
supercapacitors. Adv Mater. 2011;23:2833‐2838.
64. Flouda P, Shah SA, Lagoudas DC, Green MJ, Lutkenhaus JL.
Highly multifunctional dopamine‐functionalized reduced
graphene oxide supercapacitors. Matter. 2019;1:1532‐1546.
65. Li Z, Gadipelli S, Li H, et al. Tuning the interlayer spacing of
graphene laminate films for efficient pore utilization towards
compact capacitive energy storage. Nat Energy. 2020;5:
160‐168.
66. Yao HB, Ge J, Wang CF, et al. A flexible and highly pressure‐
sensitive graphene‐polyurethane sponge based on fractured
microstructure design. Adv Mater. 2013;25:6692‐6698.
67. Du XS, Liu HY, Mai YW. Ultrafast synthesis of multi-
functional N‑doped graphene foam in an ethanol flame. ACS
Nano. 2016;10:453‐462.
68. Zhao J, Zhang YZ, Chen J, et al. Codoped holey graphene
aerogel by selective etching for high‐performance sodium‐ion
storage. Adv Energy Mater. 2020;10:2000099.
69. Zhang RJ, Chen Q, Zhen Z, Jiang X, Zhong ML, Zhu HW.
Cellulose‐templated graphene monoliths with anisotropic
mechanical, thermal, and electrical properties. ACS Appl
Mater Interfaces. 2015;7:19145‐19152.
70. Lewis JA. Direct ink writing of 3D functional materials. Adv
Funct Mater. 2006;16:2193‐2204.
ZENG ET AL.
|
219

71. García‐Tuñon E, Barg S, Franco J, et al. Printing in three
dimensions with graphene. Adv Mater. 2015;27:1688‐1693.
72. Zhu C, Han TYJ, Duoss EB, et al. Highly compressible 3D
periodic graphene aerogel microlattices. Nat Commun. 2015;
6:6962.
73. Jiang Y, Xu Z, Huang T, et al. Direct 3D printing of ultralight
graphene oxide aerogel microlattices. Adv Funct Mater. 2018;
28:1707024.
74. Guo F, Jiang Y, Xu Z, et al. Highly stretchable carbon aero-
gels. Nat Commun. 2018;9:881.
75. KimKS,ZhaoY,JangH,etal.Large‐scale pattern growth
of graphene films for stretchable transparent electrodes.
Nature. 2009;457:706‐710.
76. Bae S, Kim H, Lee Y, et al. Roll‐to‐roll production of 30‐
inch graphene films for transparent electrodes. Nat
Nanotechnol. 2010;5:574‐578.
77. Cao X, Shi Y, Shi W, et al. Preparation of novel 3D graphene
networks for supercapacitor applications. Small. 2011;7:
3163‐3168.
78. Ito Y, Tanabe Y, Qiu HJ, et al. High‐quality three‐
dimensional nanoporous graphene. Angew Chem Int Ed.
2014;53:4822‐4826.
79. QiuHJ,KangJL,LiuP,HirataA,FujitaT,ChenMW.
Fabrication of large‐scale nanoporous nickel with a tun-
able pore size for energy storage. JPowerSources. 2014;
247:896‐905.
80. Ito Y, Qiu HJ, Fujita T, Tanabe Y, Tanigaki K, Chen MW.
Bicontinuous nanoporous N‐doped graphene for the oxygen
reduction reaction. Adv Mater. 2014;26:4145‐4150.
81. Wang XZ, Qian MJ, Xiao P, Jiang XF, Jian GQ, Hu Z.
Chinese patent CN101284663A; 2008.
82. Mecklenburg M, Schuchardt A, Mishra YK, et al. Aero-
graphite: ultra lightweight, flexible nanowall, carbon micro-
tube material with outstanding mechanical performance.
Adv Mater. 2012;24:3486‐3490.
83. Tang C, Li BQ, Zhang Q, et al. CaO‐templated growth of
hierarchical porous graphene for high‐power lithium‐sulfur
battery applications. Adv Funct Mater. 2016;26:577‐585.
84. Shi L, Chen K, Du R, et al. Scalable seashell‐based chemical
vapor deposition growth of three‐dimensional graphene
foams for oil‐water separation. J Am Chem Soc. 2016;138:
6360‐6363.
85. Han GF, Chen ZW, Jeon JP, et al. Low‐temperature con-
version of alcohols into bulky nanoporous graphene and pure
hydrogen with robust selectivity on CaO. Adv Mater. 2019;31:
1807267.
86. Chen J, Wen Y, Guo Y, et al. Oxygen‐aided synthesis of
polycrystalline graphene on silicon dioxide substrates. JAm
Chem Soc. 2011;133:17548‐17551.
87. Chen K, Li C, Shi L, et al. Growing three‐dimensional bio-
morphic graphene powders using naturally abundant diato-
mite templates towards high solution processability. Nat
Commun. 2016;7:13440.
88. Bi H, Chen IW, Lin TQ, Huang FQ. A new tubular graphene
form of a tetrahedrally connected cellular structure. Adv
Mater. 2015;27:5943‐5949.
89. Bi H, Lin TQ, Xu F, Tang YF, Liu ZQ, Huang FQ. New
graphene form of nanoporous monolith for excellent energy
storage. Nano Lett. 2016;16:349‐354.
90. Xu X, Guan C, Xu L, et al. Three dimensionally free‐formable
graphene foam with designed structures for energy and en-
vironmental applications. ACS Nano. 2020;14:937‐947.
91. Lyu Z, Xu D, Yang L, et al. Hierarchical carbon nanocages
confining high‐loading sulfur for high‐rate lithium‐sulfur
batteries. Nano Energy. 2015;12:657‐665.
92. Wang XZ, Qian MJ, Jiang XF, Wu Q, Yang LJ, Hu Z. Chinese
patent CN102600835A; 2008.
93. Jiang X, Wang X, Shen L, et al. High‐performance Pt catalysts
supported on hierarchical nitrogen‐doped carbon nanocages
for methanol electrooxidation. Chinese J Catal. 2016;37:
1149‐1155.
94. Rümmeli MH, Bachmatiuk A, Scott A, et al. Direct low‐
temperature nanographene CVD synthesis over a dielectric
insulator. ACS Nano. 2010;4:4206‐4210.
95. Xie K, Qin X, Wang X, et al. Carbon nanocages as super-
capacitor electrode materials. Adv Mater. 2012;24:347‐352.
96. Chen S, Bi J, Zhao Y, et al. Nitrogen‐doped carbon nanocages
as efficient metal‐free electrocatalysts for oxygen reduction
reaction. Adv Mater. 2012;24:5593‐5597.
97. Zhao J, Lai H, Lyu Z, et al. Hydrophilic hierarchical nitrogen‐
doped carbon nanocages for ultrahigh supercapacitive per-
formance. Adv Mater. 2015;27:3541‐3545.
98. Pinho PS, Macedo EA. Solubility of NaCl, NaBr, and KCl in
water, methanol, ethanol, and their mixed solvents. J Chem
Eng Data. 2005;50:29‐32.
99. Li N, Yang G, Sun Y, et al. Free‐standing and transparent
graphene membrane of polyhedron box‐shaped basic build-
ing units directly grown using a NaCl. Nano Lett. 2015;15:
3195‐3203.
100. Wei N, Yu L, Sun Z, et al. Scalable salt‐templated synthesis of
nitrogen‐doped graphene nanosheets toward printable en-
ergy storage. ACS Nano. 2019;13:7517‐7526.
101. Lin T, Chen IW, Liu F, et al. Nitrogen‐doped mesoporous
carbon of extraordinary capacitance for electrochemical en-
ergy storage. Science. 2015;350:1508‐1513.
102. Liu KW, Chen YM, Policastro GM, Becker ML, Zhu Y. Three‐
dimensional bicontinuous graphene monolith from polymer
templates. ACS Nano. 2015;9:6041‐6049.
103. Xu F, Lin TQ, Bi H, Huang FQ. Graphene‐like carbon with
three‐dimensional periodicity prepared from organic‐
inorganic templates for energy storage application. Carbon.
2017;111:128‐132.
104. Luong DX, Bets KV, Algozeeb WA, et al. Gram‐scale bottom‐
up flash graphene synthesis. Nature. 2020;577:647‐651.
105. Wang X, Zhang Y, Zhi C, et al. Three‐dimensional strutted
graphene grown by substrate‐free sugar blowing for high‐
power‐density supercapacitors. Nat Commun. 2013;4:2905.
106. Lin J, Peng Z, Liu Y, et al. Laser‐induced porous graphene
films from commercial polymers. Nat Commun. 2014;5:5714.
107. Gao T, Xu C, Li R, et al. Biomass‐derived carbon paper to
sandwich magnetite anode for long‐life Li‐ion battery. ACS
Nano. 2019;13:11901‐11911.
108. Wang X, Zhi C, Li L, et al. “Chemical blowing”of thin‐walled
bubbles: high‐throughput fabrication of large‐area, few‐
layered BN and C
x
‐BN nanosheets. Adv Mater. 2011;23:
4072‐4076.
109. Jiang XF, Wang XB, Dai P, et al. High‐throughput fabrication
of strutted graphene by ammonium‐assisted chemical
220
|
ZENG ET AL.

blowing for high‐performance supercapacitors. Nano Energy.
2015;16:81‐90.
110. Gao T, Xiao QL, Xu CY, Wang XB. Blowing route to fabricate
foams of 2D materials. J Inorg Mater. 2020;35:1315‐1326.
111. Wang XB, Jiang XF, Bando Y. Blowing route towards ad-
vanced inorganic foams. Bull Chem Soc Jpn. 2019;92:245‐263.
112. Talapaneni SN, Lee JH, Je SH, et al. Chemical blowing ap-
proach for ultramicroporous carbon nitride frameworks and
their applications in gas and energy storage. Adv Funct
Mater. 2017;27:1604658.
113. Zhu JY, Xu D, Wang CC, Qian WJ, Guo J, Yan F. Ferric
citrate‐derived N‐doped hierarchical porous carbons for
oxygen reduction reaction and electrochemical super-
capacitors. Carbon. 2017;115:1‐10.
114. Wang D, Zhou W, Zhang R, et al. Mass production of large‐
sized, nonlayered 2D nanosheets: their directed synthesis by
a rapid “gel‐blowing”strategy, and applications in Li/Na
storage and catalysis. Adv Mater. 2018;30:1803569.
115. Zhou WW, Du Y, Zeng JJ, Liu F, Zhu YM. A modified “gel‐
blowing”strategy toward the one‐step mass production of a
3D N‐doped carbon nanosheet@carbon nanotube hybrid
network for supercapacitors. Nanoscale. 2019;11:7624‐7633.
116. Qi S, Wang D, Li W, et al. Mass production of nitrogen and
oxygen codoped carbon nanotubes by a delicately‐designed
Pechini method for supercapacitors and electrocatalysis.
Nanoscale. 2019;11:17425‐17435.
117. Jiang H, Gu J, Zheng X, et al. Defect‐rich and ultrathin N
doped carbon nanosheets as advanced trifunctional metal‐
free electrocatalysts for ORR, OER and HER. Energy Environ
Sci. 2019;12:322‐333.
118. Luong DX, Subramanian AK, Silva GAL, et al. Laminated
object manufacturing of 3D‐printed laser‐induced graphene
foams. Adv Mater. 2018;30:1707416.
119. Peng Z, Ye R, Mann JA, et al. Flexible boron‐doped laser‐
induced graphene microsupercapacitors. ACS Nano. 2015;9:
5868‐5875.
120. Ye R, Chyan Y, Zhang J, et al. Laser‐induced graphene for-
mation on wood. Adv Mater. 2017;29:1702211.
121. Chyan Y, Ye R, Li Y, Singh SP, Arnusch CJ, Tour JM. Laser‐
induced graphene by multiple lasing: toward electronics on
cloth, paper, and food. ACS Nano. 2018;12:2176‐2183.
122. Ye R, Han X, Kosynkin DV, et al. Laser‐induced conversion
of teflon into fluorinated nanodiamonds or fluorinated gra-
phene. ACS Nano. 2018;12:1083‐1088.
123. Strauss V, Marsh K, Kowal, EI‐Kady M, Kaner RB. A simple
route to porous graphene from carbon nanodots for super-
capacitor applications. Adv Mater. 2018;30:1704449.
124. Dai P, Xue Y, Zhang S, et al. Paper‐derived flexible 3D in-
terconnected carbon microfiber networks with controllable
pore sizes for supercapacitors. ACS Appl Mater Interfaces.
2018;10:37046‐37056.
125. Zhao J, Jiang Y, Fan H, et al. Porous 3D few‐layer graphene‐
like carbon for ultrahigh‐power supercapacitors with well‐
defined structure‐performance relationship. Adv Mater. 2017;
29:1604569.
126. Yoon SM, Choi WM, Baik H, et al. Synthesis of multilayer
graphene balls by carbon segregation from nickel nano-
particles. ACS Nano. 2012;6:6803‐6811.
127. Han J, Huang G, Wang Z, et al. Low‐temperature carbide‐
mediated growth of bicontinuous nitrogen‐doped mesopor-
ous graphene as an efficient oxygen reduction electrocatalyst.
Adv Mater. 2018;30:1803588.
128. Xiao X, Beechem TE, Brumbach MT, et al. Lithographically
defined three‐dimensional graphene structures. ACS Nano.
2012;6:3573‐3579.
129. Sha JW, Gao CT, Lee SK, Li YL, Zhao NQ, Tour JM. Pre-
paration of three‐dimensional graphene foams using powder
metallurgy templates. ACS Nano. 2016;10:1411‐1416.
130. Sha J, Li Y, Villegas Salvatierra R, et al. Three‐dimensional
printed graphene foams. ACS Nano. 2017;11:6860‐6867.
131. Qian M, Wang Y, Xu F, Zhao W, Lin TQ, Huang FQ. Ex-
traordinary porous few‐layer carbons of high capacitance
from Pechini combustion of magnesium nitrate gel. ACS
Appl Mater Interfaces. 2018;10:381‐388.
132. Ji L, Guo G, Sheng H, et al. Free‐standing, ordered meso-
porous few‐layer graphene framework films derived from
nanocrystal superlattices self‐assembled at the solid–or
liquid–air interface. Chem Mater. 2016;28:3823‐3830.
133. Wang CW, Wang Y, Graser J, Zhao R, Gao F, O'Connell MJ.
Solution‐based carbohydrate synthesis of individual solid,
hollow, and porous carbon nanospheres using spray pyr-
olysis. ACS Nano. 2013;7:11156‐11165.
134. Cui HJ, Zheng JF, Zhu YY, Wang ZJ, Jia SP, Zhu ZP. Gra-
phene frameworks synthetized with Na
2
CO
3
as a renewable
water‐soluble substrate and their high rate capability for
supercapacitors. J Power Sources. 2015;293:143‐150.
135. Qin J, He C, Zhao N, et al. Graphene networks anchored
with Sn@graphene as lithium ion battery anode. ACS Nano.
2014;8:1728‐1738.
136. Li GX, Sun JH, Hou WP, Jiang SD, Huang Y, Geng JX. Three‐
dimensional porous carbon composites containing high sul-
fur nanoparticle content for high‐performance lithium‐sulfur
batteries. Nat Commun. 2016;7:10601.
137. Wang C, Li D, Zhai TY, Wang HW, Sun QF, Li HQ. Direct
conversion of waste tires into three‐dimensional graphene.
Energy Stor Mater. 2019;23:499‐507.
138. Jeong YR, Park H, Jin SW, Hong SY, Lee SS, Ha JS. Highly
stretchable and sensitive strain sensors using fragmentized
graphene foam. Adv Funct Mater. 2015;25:4228‐4236.
139. Qin Y, Peng Q, Ding Y, et al. Lightweight, superelastic, and
mechanically flexible graphene/polyimide nanocomposite foam
forstrainsensorapplication.ACS Nano. 2015;9:8933‐8941.
140. Huang K, Dong S, Yang J, et al. Three‐dimensional printing
of a tunable graphene‐based elastomer for strain sensors with
ultrahigh sensitivity. Carbon. 2019;143:63‐72.
141. Moon IK, Yoon S, Chun KY, Oh J. Highly elastic and con-
ductive N‐doped monolithic graphene aerogels for multi-
functional applications. Adv Funct Mater. 2015;25:6976‐6984.
142. Li C, Jiang D, Liang H, et al. Superelastic and arbitrary‐
shaped graphene aerogels with sacrificial skeleton of mela-
mine foam for varied applications. Adv Funct Mater. 2018;28:
1704674.
143. Li RQ, Wang BL, Gao T, et al. Monolithic electrode in-
tegrated of ultrathin NiFeP on 3D strutted graphene for bi-
functionally efficient overall water splitting. Nano Energy.
2019;58:870‐876.
ZENG ET AL.
|
221

144. Li RQ, Xu CY, Jiang XF, Bando Y, Wang XB. Porous
monolithic electrode of Ni
3
FeN on 3D graphene for efficient
oxygen evolution. J Nanosci Nanotechnol. 2020;20:5175‐5181.
145. He JR, Hartmann G, Lee M, Hwang GS, Chen YF,
Manthiram A. Freestanding 1T MoS
2
/graphene hetero-
structures as a highly efficient electrocatalyst for lithium
polysulfides in Li–S batteries. Energy Environ Sci. 2019;12:
344‐350.
146. Vivekchand SRC, Rout CS, Subrahmanyam KS,
Govindaraj A, Rao CNR. Graphene‐based electrochemical
supercapacitors. JChemSci. 2008;120:9‐13.
147. Stoller MD, Park S, Zhu YW, An J, Ruoff RS. Graphene‐based
ultracapacitors. Nano Lett. 2008;8:3498‐3502.
148. ZhongC,DengYD,HuWB,QiaoJL,ZhangL,ZhangJJ.
A review of electrolyte materials and compositions for
electrochemical supercapacitors. Chem Soc Rev. 2015;44:
7484‐7539.
149. Beguin F, Presser V, Balducci A, Frackowiak E. Carbons and
electrolytes for advanced supercapacitors. Adv Mater. 2014;
26:2219‐2251.
150. Fic K, Lota G, Meller M, Frackowiak E. Novel insight into
neutral medium as electrolyte for high‐voltage super-
capacitors. Energy Environ Sci. 2012;5:5842‐5850.
151. Conway BE. Electrochemical Supercapacitors: Scientific Fun-
damentals and Technological Applications. New York, NY:
Kluwer Academic/Plenum Publisher; 1999.
152. Morimoto T, Hiratsuka K, Sanada Y, Kurihara K. Electric
double‐layer capacitor using organic electrolyte. J Power
Sources. 1996;60:239‐247.
153. Lozano‐Castelló D, Cazorla‐Amorós D, Linares‐Solano A,
Shiraishi S, Kurihara H, Oya A. Influence of pore structure
and surface chemistry on electric double layer capacitance in
non‐aqueous electrolyte. Carbon. 2003;41:1765‐1775.
154. Huang JS, Sumpter BG, Meunier V. Theoretical model for
nanoporous carbon supercapacitors. Angew Chem Int Ed.
2008;120:530‐534.
155. Stoller MD, Magnuson CW, Zhu Y, et al. Interfacial capaci-
tance of single layer graphene. Energy Environ Sci. 2011;4:
4685‐4689.
156. Chen KF, Song SY, Liu F, Xue DF. Structural design of
graphene for use in electrochemical energy storage devices.
Chem Soc Rev. 2015;44:6230‐6257.
157. Shao Y, El‐Kady MF, Wang LJ, et al. Graphene‐based materials
for flexible supercapacitors. Chem Soc Rev. 2015;44:3639‐3665.
158. Huang JL, Wang JY, Wang CW, Zhang HN, Lu CX, Wang JZ.
Hierarchical porous graphene carbon‐based supercapacitors.
Chem Mater. 2015;27:2107‐2113.
159. Wang DW, Li F, Liu M, Lu GQ, Cheng HM. 3D aperiodic
hierarchical porous graphitic carbon material for high‐rate
electrochemical capacitive energy storage. Angew Chem Int
Ed. 2007;47:373‐376.
160. Chmiola J, Yushin G, Gogotsi Y, Portet C, Simon P, Taberna
PL. Anomalous increase in carbon capacitance at pore sizes less
than 1 nanometer. Science. 2006;313:1760‐1763.
161. Liu HJ, Wang J, Wang CX, Xia YY. Ordered hierarchical me-
soporous/microporous carbon derived from mesoporous
titanium‐carbide/carbon composites and its electrochemical
performance in supercapacitor. Adv Energy Mater. 2011;1:
1101‐1108.
162. Wang YG, Song YF, Xia YY. Electrochemical capacitors:
mechanism, materials, systems, characterization and appli-
cations. Chem Soc Rev. 2016;45:5925‐5950.
163. Largeot C, Portet C, Chmiola J, Taberna PL, Gogotsi Y, Simon
P. Relation between the ion size and pore size for an electric
double‐layer capacitor. JAmChemSoc. 2008;130:2730‐2731.
164. Jiang DE, Jin ZH, Henderson D, Wu JZ. Solvent effect on the
pore‐size dependence of an organic electrolyte super-
capacitor. J Phys Chem Lett. 2012;3:1727‐1731.
165. Dutta S, Bhaumik A, Wu KCW. Hierarchically porous carbon
derived from polymers and biomass: effect of interconnected
pores on energy applications. Energy Environ Sci. 2014;7:
3574‐3592.
166. Xu YX, Lin ZY, Huang XQ, Liu Y, Huang Y, Duan XF. Flexible
solid‐state supercapacitors based on three‐dimensional gra-
phene hydrogel films. ACS Nano. 2013;7:4042‐4049.
167. Zhao L, Fan LZ, Zhou MQ, et al. Nitrogen‐containing hy-
drothermal carbons with superior performance in super-
capacitors. Adv Mater. 2010;22:5202‐5206.
168. Jin Z, Yao J, Kittrell C, Tour JM. Large‐scale growth and
characterizations of nitrogen‐doped monolayer graphene
sheets. ACS Nano. 2011;5:4112‐4117.
169. Zhang W, Xu C, Ma C, et al. Nitrogen‐superdoped 3D gra-
phene networks for high‐performance supercapacitors. Adv
Mater. 2017;29:1701677.
170. Zuo ZC, Jiang ZQ, Manthiram A. Porous B‐doped graphene
inspired by fried‐ice for supercapacitors and metal‐free cat-
alysts. J Mater Chem A. 2013;1:13476‐13483.
171. Li JW, Li XF, Xiong DB, Wang LZ, Li DJ. Enhanced capa-
citance of boron‐doped graphene aerogels for aqueous sym-
metric supercapacitors. Appl Surf Sci. 2019;475:285‐293.
172. Biddinger EJ, Deak DV, Ozkan US. Nitrogen containing
carbon nanostructures as oxygen‐reduction catalysts. Top
Catal. 2009;52:1566‐1574.
173. Wang D, Hu W, Ma Q, et al. Nitrogen‐containing graphene
networks with high volumetric capacitance and exceptional
rate capability. Carbon. 2019;154:13‐23.
174. Ornelas O, Sieben JM, Ruiz‐Rosas R, et al. On the origin of
the high capacitance of nitrogen‐containing carbon nano-
tubes in acidic and alkaline electrolytes. Chem Commun.
2014;50:11343‐11346.
175. Deng YF, Xie Y, Zou KX, Ji XL. Review on recent advances in
nitrogen‐doped carbons: preparations and applications in
supercapacitors. J Mater Chem A. 2016;4:1144‐1173.
176. Yang DS, Bhattacharjya D, Inamdar S, Park J, Yu JS.
Phosphorus‐doped ordered mesoporous carbons with differ-
ent lengths as efficient metal‐free electrocatalysts for oxygen
reduction reaction in alkaline media. J Am Chem Soc. 2012;
134:16127‐16130.
177. Paraknowitsch JP, Thomas A. Doping carbons beyond ni-
trogen: an overview of advanced heteroatom doped carbons
with boron, sulphur and phosphorus for energy applications.
Energy Environ Sci. 2013;6:2839‐2855.
178. Fan X, Xu H, Zuo SS, Liang ZP, Yang SH, Chen Y. Preparation
and supercapacitive properties of phosphorus‐doped reduced
graphene oxide hydrogel. Electrochim Acta. 2020;330:135207.
179. Yu X, Kim HJ, Hong JY, et al. Elucidating surface redox
charge storage of phosphorus‐incorporated graphenes with
hierarchical architectures. Nano Energy. 2015;15:576‐586.
222
|
ZENG ET AL.

180. Cui C, Gao Y, Li J, et al. Origins of boosted charge storage on
heteroatom‐doped carbons. Angew Chem Int Ed. 2020;132:
8002‐8007.
181. Kicinski W, Szala M, Bystrzejewski M. Sulfur‐doped porous
carbons: synthesis and applications. Carbon. 2014;68:1‐32.
182. Yu X, Park SK, Yeon SH, Park HS. Three‐dimensional,
sulfur‐incorporated graphene aerogels for the enhanced
performances of pseudocapacitive electrodes. J Power
Sources. 2015;278:484‐489.
183. Islam MM, Subramaniyam CM, Akhter T, et al. Three di-
mensional cellular architecture of sulfur doped graphene:
self‐standing electrode for flexible supercapacitors, lithium
ion and sodium ion batteries. J Mater Chem A. 2017;5:
5290‐5302.
184. Wu ZS, Winter A, Chen L, et al. Three‐dimensional nitrogen
and boron co‐doped graphene for high‐performance all‐solid‐
state supercapacitors. Adv Mater. 2012;24:5130‐5135.
185. Zou X, Wu D, Mu Y, et al. Boron and nitrogen co‐doped
holey graphene aerogels with rich B–N motifs for flexible
supercapacitors. Carbon. 2020;159:94‐101.
186. Wang XW, Liu Y, Wu PY. Water‐soluble triphenylphosphine‐
derived microgel as the template towards in‐situ nitrogen,
phosphorus co‐doped mesoporous graphene framework for
supercapacitor and electrocatalytic oxygen reduction. Chem
Eng J. 2017;328:417‐427.
187. Wang T, Wang LX, Wu DL, Xia W, Jia DZ. Interaction be-
tween nitrogen and sulfur in co‐doped graphene and syner-
getic effect in supercapacitor. Sci Rep. 2015;5:9591.
188. Li JH, Zhang GP, Fu CP, Deng LB, Sun R, Wong CP. Facile
preparation of nitrogen/sulfur co‐doped and hierarchical
porous graphene hydrogel for high‐performance electro-
chemical capacitor. J Power Sources. 2017;345:146‐155.
189. Zhang W, Chen Z, Guo X, et al. N/S co‐doped three‐
dimensional graphene hydrogel for high performance su-
percapacitor. Electrochim Acta. 2018;278:51‐60.
190. Yu X, Kang YB, Park HS. Sulfur and phosphorus co‐doping
of hierarchically porous graphene aerogels for enhancing
supercapacitor performance. Carbon. 2016;101:49‐56.
191. Yuan B, Xing W, Hu Y, et al. Boron/phosphorus doping for
retarding the oxidation of reduced graphene oxide. Carbon.
2016;101:152‐158.
192. Pan Z, Zhi H, Qiu Y, et al. Achieving commercial‐level mass
loading in ternary‐doped holey graphene hydrogel electrodes
for ultrahigh energy density supercapacitors. Nano Energy.
2018;46:266‐276.
193. Simon P, Gogotsi Y. Materials for electrochemical capacitors.
Nat Mater. 2008;7:845‐854.
194. Liu CF, Liu YC, Yi TY, Hu CC. Carbon materials for high‐
voltage supercapacitors. Carbon. 2019;145:529‐548.
195. Zhu Y, Murali S, Stoller MD, et al. Carbon‐based super-
capacitors produced by activation of graphene. Science. 2011;
332:1537‐1541.
196. Xu Y, Lin Z, Zhong X, et al. Holey graphene frameworks for
highly efficient capacitive energy storage. Nat Commun.
2014;5:4554.
197. Nishihara H, Simura T, Kobayashi S, et al. Oxidation‐
resistant and elastic mesoporous carbon with single‐layer
graphene walls. Adv Funct Mater. 2016;26:6418‐6427.
198. Kota M, Jana M, Park HS. Improving energy density of
supercapacitors using heteroatom‐incorporated three‐
dimensional macro‐porous graphene electrodes and or-
ganic electrolytes. JPowerSources. 2018;399:83‐88.
199. Barranco V, Lillo‐Rodenas MA, Linares‐Solano A, et al.
Amorphous carbon nanofibers and their activated carbon
nanofibers as supercapacitor electrodes. J Phys Chem C. 2010;
114:10302‐10307.
200. VaqueroS,DíazR,AndersonM,PalmaJ,MarcillaR.
Insights into the influence of pore size distribution and
surface functionalities in the behaviour of carbon su-
percapacitors. Electrochim Acta. 2012;86:241‐247.
201. Jeong HM, Lee JW, Shin WH, et al. Nitrogen‐doped graphene
for high‐performance ultracapacitors and the importance of
nitrogen‐doped sites at basal planes. Nano Lett. 2011;11:
2472‐2477.
202. Watanabe M, Thomas ML, Zhang SG, Ueno K, Yasuda Y,
Dokko K. Application of ionic liquids to energy storage and
conversion materials and devices. Chem Rev. 2017;117:
7190‐7239.
203. Lin XR, Salari M, Arava LMR, Ajayan PM, Grinstaff MW.
High temperature electrical energy storage: advances,
challenges, and frontiers. Chem Soc Rev. 2016;45:
5848‐5887.
204. Lian C, Liu K, Van Aken KL, et al. Enhancing the capa-
citive performance of electric double‐layer capacitors
with ionic liquid mixtures. ACS Energy Lett. 2016;1:21‐26.
205. Xu P, Gao Q, Ma L, et al. A high surface area N‐doped holey
graphene aerogel with low charge transfer resistance as high
performance electrode of non‐flammable thermostable su-
percapacitors. Carbon. 2019;149:452‐461.
206. Li H, Tao Y, Zheng X, et al. Ultra‐thick graphene bulk
supercapacitor electrodes for compact energy storage.
Energy Environ Sci. 2016;9:3135‐3142.
207. Chen YJ, Liu ZE, Sun L, Lu ZW, Zhuo KL. Nitrogen and
sulfur co‐doped porous graphene aerogel as an efficient
electrode material for high performancesupercapacitor
in ionic liquid electrolyte. JPowerSources. 2018;390:
215‐223.
208. Jonsson E. Ionic liquids as electrolytes for energy storage
applications—a modelling perspective. Energy Stor Mater.
2020;25:827‐835.
209. Eftekhari A. Supercapacitors utilising ionic liquids. Energy
Stor Mater. 2017;9:47‐69.
AUTHOR BIOGRAPHIES
Jinjue Zeng received his B.S. degree
from Harbin Institute of Technology in
2018. He is a Ph.D. candidate in Materi-
als Science in Nanjing University, under
the supervision of Prof. Xue‐Bin Wang.
He is currently researching the syntheses
and applications of porous monolithic
3D graphene materials.
ZENG ET AL.
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Xue‐Bin Wang received B.S. and M.S.
degrees from Nanjing University and
Ph.D. degree from Waseda University. He
worked as junior researcher, postdoc
researcher, and independent researcher at
International Center for Young Scientists
(ICYS) in World Premier International Center for
Materials Nanoarchitectonics (WPI‐MANA), National
Institute for Materials Science (NIMS) in 2010–2016. He
then became a 1000‐Talents Young Scholar and a full
professor in Nanjing University. Wang's group has been
pursuing the designed syntheses, novel properties, and
practical applications of porous 2D materials. Wang's
group recently focuses on the growth of 3D‐designed
graphene and boronitrene for applications to electro-
lysis, thermocatalysis, supercapacitors, batteries, poly-
meric composites, and so forth.
How to cite this article: Zeng J, Xu C, Gao T,
Jiang X, Wang X‐B. Porous monoliths of 3D
graphene for electric double‐layer supercapacitors.
Carbon Energy. 2021;3:193–224.
https://doi.org/10.1002/cey2.107
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