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In supercapacitors - Bài tập Cơ Nhiệt | Trường Đại học Khánh Hòa

54 27 lượt tải Tải xuống

H O S T E D B Y

Progress in Natural

Science

Materials International

Available online at www.sciencedirect.com

Progress in Natural Science: Materials International ] (]]]]) ]]] ]]]–

Review

Preparation of 3D graphene-based architectures and their applications

in supercapacitors

Zhuxian Yang, Sakineh Chabi, Yongde Xia, Yanqiu Zhu

College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter EX4 4QF, United Kingdom

Received 20 September 2015; accepted 5 November 2015

Abstract

Three dimensional (3D) graphene-based architectures such as 3D graphene-based hydrogels, aerogels, foams, and sponges have attracted huge

attention owing to the combination of the structural interconnectivities and the outstanding properties of graphene which offer these interesting

structures with low density, high porosity, large surface area, stable mechanical properties, fast mass and electron transport. They have been

extensively studied for a wide range of applications including capacitors, batteries, sensors, catalyst, etc. There are several reviews focusing on

the 3D graphene-based architectures and their applications. In this work, we only summarise the latest development on the preparation of 3D

graphene-based architectures and their applications in supercapacitors, with emphasis on the preparation strategies.

Crown Copyright & 2015 Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: 3D-graphene; Supercapacitor; Electrode architecture

1. Introduction

The exceptional properties of graphene, including outstanding

electron mobility up to 200,000 cm

 1

[1], excellent

thermal conductivity up to 5000 W m

 1

[2], extremely

high mechanical strength (Young's modulus  1.0 TPa) [3],

high optical transparency of 97.7% [4], and large theoretical

speciﬁc surface area of 2630 m

 1

, have drawn tremendous

research attention from scientists in chemistry, physics, materi-

als science and energy etc. Graphene-based thin ﬁlm composites

and freestanding nanosheets have been explored in high

performance nanocomposites [5], catalysis [6], energy storage

devices [7], electronics and optoelectronics [8], biological and

chemical sensors etc. [9] In some cases, 2D graphene sheets

suffered from aggregation or restacking in macroscopic scale,

which led to the loss of effective accessible surface areas [10].

To date, different strategies have been developed to achieve

3 dimensionally (3D) assembled macroscopic structures, in an

effort to eliminate the above mentioned issues. Examples

include adding spacers [11,12], crumpling the graphene sheets

[13], and creating freestanding 3D graphene-based architectures.

Recently, the creation of freestanding 3D graphene-based

architectures, such as 3D graphene-based hydrogels [14],

aerogels [15], foams [16 17], , and sponges [18], has been

demonstrated as an effective way to tackle the aforementioned

restacking issues [19]. These 3D architectures can offer high

electrical conductivity and improved structural stability, due to

the absence of defects and reduced intersheet junction contact

resistance compared with those 2D graphene-based thin lmﬁ

composites and freestanding nanosheets [16 17], . The low

density, high porosity, large surface area, excellent electrical

conductivity and stable mechanical properties render these 3D

architectures having potential applications in many ﬁelds such

as capacitors, batteries, sensors, catalysts, as absorbents, etc.

There have been a number of reviews on the preparation and the

applications of 3D graphene-based architectures [7,19 29]– , so

we will only focus in the recent advances on the preparation of

3D graphene-based architectures and their applications in super-

capactiors, especially on those controlled synthesis strategies.

www.elsevier.com/locate/pnsmi

www.sciencedirect.com

http://dx.doi.org/10.1016/j.pnsc.2015.11.010

1002-0071/ Crown Copyright & 2015 Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Corresponding author.

E-mail address: y.zhu@exeter.ac.uk (Y. Zhu).

Peer review under responsibility of Chinese Materials Research Society.

Please cite this article as: Z. Yang, et al., Preparation of 3D graphene-based architectures and their applications in supercapacitors, Progress in Natural Science:

Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.11.010

2. Preparation of 3D graphene-based architectures

The widely explored preparation methods are the assembly

of graphene oxide (GO) or graphene sheets for 3D graphene-

based sponges [18 30] [15 31 34], , foams , – , hydrogels and

aerogels [14 35 38], – , and the direct synthesis via CVD

(chemical vapour deposition) for 3D graphene based foams

[16 17 39 41], , – . Other methods, such as the sugar blowing

approach [42] [43 44], the 3D printing technique , , and the

commercial graphite paper (GP) technique [45], have also been

reported. According to the topology of the resulting 3D

structures, they can broadly be classiﬁed into macroscopically

uniform and random architectures [14 18 30 41]– , – , and macro-

scopically symmetric and well-controlled structures [43 44 46], , .

They can be produced by different strategies, and obviously

can be used for different applications. Since there have been

some excellent up to date reviews [19 25 26 29], , , , our attention

in this context will be focused on the controlled preparation of

3D graphene-based architectures.

2.1. The assembly method

Since the early report on self-assembled graphene hydrogel

prepared by chemical reduction of the aqueous GO dispersion

with sodium ascorbate demonstrated by Shi's group [14], there

have been many studies on the self-assembly of GO/graphene

sheets. It is generally accepted that the force balance between

the van der Waals attractions from the basal planes of GO

sheets and the electrostatic repulsions from the functional

groups of GO sheets maintains the GO sheets well-dispersed in

an aqueous solvent. Once this balance is broken, gelation of

the GO dispersion will take place, leading to the formation of

3D GO hydrogels that can be further reduced to produce 3D

graphene-based architectures [19 29], . Methods based on the

self-assembly of GO sheets include: chemical reduction

[47 49] [50]– , cross-linking agent (including metal ions ,

biomolecules [51] [52, polymers ,53] etc.), hydrothermal pro-

cess [14 54] [55], , sol–gel reaction , freeze-drying [30,56], and

so on. Apart from self-assembly, template-assisted assembly of

3D macroporous graphene ﬁlms have been demonstrated with

spherical polystyrene (PS) balls [37], and silica nanoparticles

(NPs) [31], which offer some control over the resulting 3D

architectures.

2.2. The CVD method

3D reduced GO architectures obtained by the assembly

strategies starting from GO sheets will inevitably contain some

defects introduced during exfoliation and reduction processes

and exhibit low electrical conductivity. On the contrary, the

CVD method can produce free-standing graphene foam with

high electrical conductivity and improved structural stability,

superior to those fabricated using chemically derived graphene

sheets, due to the absence of defects and intersheet junction

contact resistance [16]. Chen's group pioneered the synthesis

of 3D graphene foams using nickel foam as the template via

CVD [16]. Brieﬂy, graphene growth was carried out via CVD

at 1000 1C with nickel foam as the template and CH

as the

carbon source. This Ni foam template-CVD strategy has been

extensively studied for the preparation of 3D graphene foams

as scaffold for the creation of diverse functionalised materials

[17 19 26 39 57 62], , , , – . For example, a number of chemicals

including NiO [17], Co

[57], LiFePO

[58], Pt nanoparti-

cles [63], Si [64] [63], multi-walled CNTs , MnO

[63], Fe

[65] and MoS

[66] have been added to the 3D graphene foams

to create 3D graphene based composites. The macroscopic

randomly uniform feature and the high integrity allows them to

act a strong matrix to accommodate these new elements

uniformly, thereby the resulting new composite have the

potential to exhibit very interesting properties, and some of

which have been explored in these reports.

2.3. Preparation of 3D graphene-based architectures with

controlled manner

In most of the aforementioned cases, the pore shapes, pore

sizes, truss length, wall thickness and overall geometries of the

3D graphene-based architectures are randomly arranged. For

example, the dimensions of the structures fabricated by the

CVD method using Ni foam templates are limited by the

dimensions of the nickel foams, and the pores are on a scale of

a few hundred micrometres based on commercially available

foams. To maximise their potential applications, it is highly

desirable to design and control the 3D architectures with

preferred sizes and shapes of the pore, lengths and diameters of

the trusses, etc. to have preferentially enhanced properties.

This is a step forward towards speciﬁc applications. We will

summarise some recent reports below describing the controlled

preparation of 3D graphene-based architectures.

2.3.1. Assembly based strategies

Huang et al. have reported a hydrophobic interaction driven

hard templating approach to synthesise nanoporous graphene

foams with controllable pore size (30–120 nm) and ultra-high

pore volumes ( 4.3 cm

 1

) [31]. As shown in Scheme 1, in

a typical synthesis, they ﬁrst used methyl group grafted silica

spheres with a uniform particle size of 27.7, 60 or 120 nm as

the templates for the self-assembly of the GO sheets. The

resulting GO/silica sphere composites were further calcined at

900 1C for 5 h under argon atmosphere, followed by HF

(5 wt%) wash to remove the templates and obtain the 3D

materials [31]. In addition, Xie et al. developed a strategy

which allowed large-range tailoring of the porous architecture

and its properties by a modiﬁed freeze casting process [56]. In

their process, a GO dispersion obtained from the oxidation of

graphite was ﬁrst hydrothermally reduced to hydrogel, and

then treated with freeze drying under different temperatures.

This innovative technique is able to tailor the pore sizes and

wall thicknesses of the porous graphene from 10 to 800 mμ

and from 20 nm to 80 μm, respectively. As a result, these

structural changes have brought to property changes from

hydrophilic to hydrophobic with the Young's Modulus varying

by 15 times [56].

Z. Yang et al. / Progress in Natural Science: Materials International ] (]]]]) ]]] ]]]–2

Please cite this article as: Z. Yang, et al., Preparation of 3D graphene-based architectures and their applications in supercapacitors, Progress in Natural Science:

Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.11.010

Bi et al. have developed a pH-mediated hydrothermal reduction

technique to obtain high compressive strength 3D graphene

structures using a low temperature casting at 180 1C, combined

with various moulding [67]. In their method, the casting and the

shaping took place while reducing the GO at different reaction

times. This technique, by tailoring the pH values, allows the

controllable fabrication of compact high density graphene macro-

structures with various shapes including triangular prism, quad-

rangular prism, joint ring, crucible, screw stem, and gear. The

dimension of castings ranges from sub-millimetre to centimetre,

depending on the mould and the concentration of GO. They

claimed that as long as the concentration of the GO dispersions is

above 0.5 mg mL

 1

, the resulting graphene gel can be cast into

any complicated macroscopic shape [67]. It is obvious that the

overall shape of the product can be well-controlled by this strategy,

however the microscale dispersion of the graphene sheets within

the moulded and reduced product remains completely random, lack

of control.

Very recently, Barg et al. have explored a self-assembly

strategy for the fabrication of complex chemically modi edﬁ

graphene cellular networks (CMG-CNs) via a multi-step

soft/hard template mechanism that combines emulsion and

ice templating, which is illustrated in Fig. 1 [34]. This

approach allows the manipulation of the structure at multiple

levels from the densities (over two orders of magnitude from

1 to 200 mg cm

 3

), cell shape (polyhedral to spherical) and

sizes (  7 to 60 μm) at the micro-level to the cell walls

topography, porosity and chemistry at the micro-to-nano-level

[34]. These networks contain much more pores and are much

lighter than those of Bai et al.'s architectures, and they have

shown higher level of control over the microscopic features.

2.3.2. CVD based strategies

In the case of Ni foam templated growth, the pore shape and

geometries of the foam are arranged randomly, so are the

resulting 3D graphene foams. To counter this intrinsic dis-

advantage, Ito et al have recently produced a novel nanoscale

porous Ni-template, which has a high level of control of pore

size distribution, using the CVD approach to fabricate high

quality 3D nanoporous graphene with tailored pore sizes [41].

The nanoporous Ni with a thickness of 30 μm was prepared by

electrochemically leaching Mn from a Ni

precursor in a

weak acid solution. By controlling the size of the Ni ligaments,

they have realised 3D graphene with pores of 100 nm to

2.0 μm by controlling the CVD time and temperatures [41].

This indeed improved the pore size distribution of the resulting

3D graphene, with the normal features of CVD graphene,

however the electrochemical process on which the strategy is

based for the template creation makes this strategy unsuitable

for large scale production or for large dimensional structures.

Very recently, Yang et al. have demonstrated a conceptual

design and practical synthesis of 3D graphene networks [46],

whereas the CVD process combined with a 3D-printed

periodic Ni template, in an effort to introduce advanced

geometric control at both macroscopic and microscopic levels.

High resolution 3D-printing of Ni or Cu scaffolds allows the

templates to have the desired geometrical features, such as

truss diameters and lengths, pore shapes and sizes, symmetric

or non-symmetric, etc.; whereas the CVD enables the control

of the layer numbers in the ﬁnal products. Although the 3D

graphene networks demonstrated in the report were 3D

structures shown in Fig. 2, it is anticipated that by taking

advantage of the new development of 3D printing technologies

for Ni or Cu template creation, and of the versatile feature of

the CVD for graphene growth, customised 3D graphene

Scheme 1. Schematic illustration of the synthesis procedures of the nanoporous graphene foams (NGFs). I) The self-assembly occurs between GOs and

Z. Yang et al. / Progress in Natural Science: Materials International ] (]]]]) ]]]–]]] 3

Please cite this article as: Z. Yang, et al., Preparation of 3D graphene-based architectures and their applications in supercapacitors, Progress in Natural Science:

Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.11.010

networks with desired trusses, pores and special structural

features could be achieved. Although the overall dimension of

the 3D-printed templates can be very large (up to metres), the

effective of CVD for large porous samples to achieve uniform

number of graphene layers across the entire samples appears to

be a practical challenge. Further, the proper process for the

removal of the template at a controlled manner without

damaging the 3D graphene network, particularly when the

size is large, needs dedicated experience. Nevertheless, a few

centimetres of such networks could suit for speciﬁc applica-

tions in acoustic and optical, electromagnetic, photonic and

mechanical ﬁeld [46].

2.3.3. Direct 3D printing methods

Different to Yang's approach, 3D printing technique has

been directly attempted to produce 3D graphene architectures

by Seol's group [43]. Using reduced graphene oxide ink for a

nanometre-scale 3D printer, by exploiting a size-controllable

liquid meniscus, they succeeded in writing freestanding

reduced graphene oxide nanowires without any supporting

materials [43]. Similarly, Zhu et al. have fabricated periodic

graphene aerogel microlattices shown in Fig. 3 via 3D printing,

which is based on the precise deposition of GO ink lamentsﬁ

on a pre-deﬁned tool path to create architected 3D GO

structures [44]. The key for this fabrication process is the ink

preparation. They ﬁrst added fumed silica powders and catalyst

Fig. 1. Microstructural architecture of CMG-CNs. (a) Overview of the CMG-CN architecture, composed of nearly spherical pores in the micrometre scale, designed

by the emulsion droplet templates. (b) At the triple junction between adjacent cells, whose arrangement is template by ice. (c) Cell walls surface micro to

nanorugosity patterned by the ice crystals during unidirectional freezing. (d) SEM images of 5.6 mg cm

-3

GO-CN (a–d) that after thermal reduction at 1000 C1

results in rGO-CN of 2.2 mg cm

 3

Chemistry.

Z. Yang et al. / Progress in Natural Science: Materials International ] (]]]]) ]]] ]]]–4

Please cite this article as: Z. Yang, et al., Preparation of 3D graphene-based architectures and their applications in supercapacitors, Progress in Natural Science:

Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.11.010

into the as-prepared aqueous GO suspensions to generate a

homogeneous GO ink with designed rheological properties.

The GO ink was then extruded through a micronozzle

immersed in isooctane to prevent drying during printing. The

printed microlattice structure was supercritically dried to

remove the liquid, followed by carbonization at 1050 1C under

. Finally, the silica ﬁller was etched using HF acid .[44]

Both of these direct 3D-printing strategies realised high

level of shape and geometry control over the resulting 3D

graphene-based architectures, however it seems that the

process worked better with GO/rGO than graphene sheets.

Therefore the electric conductivity of the structures may be

inferior to those produced by CVD. The microscopic GO

dispersion/orientation within the printed objects remains ran-

dom. Therefore, to realise macroscopic and microscopic

control of 3D graphene-based structures is still challenging.

3. Applications of 3D graphene-based architectures in

supercapacitors

Depending on the electrode materials, two types of energy

storage can occur in supercapacitors. The ﬁrst such as double

layer capacitors (e.g. activated carbon) store charges electro-

statically via the physisorption of electrolyte ions, and the

second in the case of pseudocapacitive materials e.g. metal

oxides or conductive polymers, involves a fast surface redox

reaction [68,69]. Thus, a good control over the structure and

morphology of electrode materials is critical to obtain high

surface area and efﬁcient paths for ion diffusion. Despite the

excellent properties of graphene, still much in-depth research

and further modiﬁcations are needed to allow this truly 2D

material to replace the conventional electrode architecture.

It has been discussed that highly efﬁcient supercapacitor

materials should have high accessible surface areas, hierarch-

ical pores, high conductivity and robust structures [70 71], .

Although pristine 2D graphene does not meet these criteria,

such as limited accessible surface areas or hardly any pores,

the 3D architectures based on the 2D graphene can be designed

and created, to allow for improved and efﬁcient ion accessi-

bility and transport.

Fig. 3. Morphology and structure of graphene aerogels. (a) Optical image of a 3D printed graphene aerogel microlattice, (b–d) SEM images, (b) A 3D printed

graphene aerogel microlattice, (c) Graphene aerogel without R–F after etching, (d) Graphene aerogel with 4 wt% R–F after etching. (e–f) Optical images, (e) 3D

printed graphene aerogel microlattices with varying thickness, and (f) A 3D printed graphene aerogel honeycomb. Scale bars, 5 mm (a), 200 mm (b), 100 nm (c, d),

Z. Yang et al. / Progress in Natural Science: Materials International ] (]]]]) ]]]–]]] 5

Please cite this article as: Z. Yang, et al., Preparation of 3D graphene-based architectures and their applications in supercapacitors, Progress in Natural Science:

Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.11.010

3.1. 3D graphene-based architectures directly used in

supercapacitors

Due to the combination of the 3D interconnection structure

and the excellent electron conductivity of graphene, 3D

graphene architectures are promising for use as electrodes in

supercapacitors. For example, the graphene hydrogel early

reported by Shi's group has a well-deﬁned and cross-linked 3D

porous structure with pore sizes in the range of submicrom-

eters to several micrometres, and the speciﬁc capacitance of

this graphene hydrogel was demonstrated to be up to about

240 F g

 1

at a discharge current density of 1.2 A g

 1

, in 1 M

aqueous solution of H

[14]. In their following study, they

have achieved high speciﬁc capacitance of 220 F g

 1

at a

current density of 1 A g

–1

, which can be maintained for 74% at

a discharge current density of 100 A g

–1

[72]. The exceptional

electrical conductivity and mechanical robustness of graphene

hydrogel make it an excellent material for ﬂexible energy

storage devices. As demonstrated by Xu et al., a exibleﬂ

supercapacitor with a 120 μm thick graphene hydrogel thin

ﬁlm exhibited excellent capacitive characteristics, including a

high gravimetric speciﬁc capacitance of 186 F g

 1

(up to

196 F g

 1

for a 42 μm thick electrode), an unprecedented areal

speciﬁc capacitance of 372 mF cm

 2

(up to 402 mF cm

 2

for

a 185 μm thick electrode), low leakage current (10.6 A),μ

excellent cycling stability, and extraordinary mechanical ex-ﬂ

ibility [73]. In addition, In a very successful attempt, using

chemical activation of exfoliated GO, Ruoff’s group reported

the synthesis of a graphene-derived 3D network with extre-

mely high surface area of up to  3100 m

 1

[74]. The

unique hierarchically porous structures in the produced 3D

graphene-derived carbon structures give rise to a high gravi-

metric (174 F g

 1

) and volumetric (  100 F cm

 3

) speci cﬁ

capacitance in 1-ethyl-3-methylimidazolium bis (triﬂuorosul-

fonyl) imide acetonitrile electrolyte. The assembled two

electrode symmetric cell delivered gravimetric energy density

and power density of 74 W h kg

 1

and 338 kW kg

 1

and

volumetric values of 44 W h L

 1

and 199 kW L

 1

, which are

quite high [74]. The high performance was attributed to the

large fraction of micro- and mesopores that provide large and

accessible surface areas for charge accommodation.

Regarding graphene aerogels, Zhang and co-workers has

demonstrated graphene aerogels with a speci c capacitanceﬁ

128 F g

 1

(at a constant current density 50 mA g

 1

) prepared

by either CO

drying or freeze drying of graphene hydrogel

precursors obtained from heating the aqueous mixture of

graphene oxide with L-ascorbic acid without stirring [75].

As for graphene foams, Zhao and co-workers have reported

a versatile, N-doped, ultralight 3D GF, which exhibits a

signiﬁcantly high capacitance of 484 F g

 1

, approaching to

the theoretical electric double-layer (EDL) capacitance of

graphene. It is believed that the combination of the unique

3D porous structures of few-layer graphene which maximises

the exposure of their surfaces to electrolyte and effective

N-doping could lead to high capacitance performance [15].

3.2. 3D graphene-based architectures as scaffolds in

supercapacitors

The low density, high porosity, large surface area, excellent

electrical conductivity and stable mechanical properties make

these 3D architectures ideal scaffolds to host metal oxide

nanoparticles and conductive polymers, for the creation of

highly efﬁcient 3D graphene composite/hybrid electrodes

[76 79]– . Zhang et al. developed a scalable approach to prepare

honeycomb-like CoMoO

–3D graphene hybrid (NSCGH)

(NHC) electrode [80]. They reported speciﬁc capacitances as

high as 2741, 1585, and 1101 F g

 1

at current densities of

1.43, 22.85 and 85.71 A g

 1

(aqueous solution within the

potential range of 0 to 0.9 V (vs. standard Hg/HgO electrode)),

respectively. The aqueous symmetric (with NSCGH as two

electrodes) supercapacitors deliver a high energy density of

14.5 W h kg

 1

at a power density of 18,000 W kg

 1

, which

can power a 5 mm-diameter LED for more than 3 min with a

charging time of only 2 s [80].

Another advantage of 3D graphene-based composite electrodes

lies in the ability of the 3D network to accommodate large mass

loading for the active materials. For example, a MnO

mass

loading of 9.8 mg cm

 2

(92.9% of the mass of the entire

electrode) leads to a high area capacitance of 1.42 F cm

 2

at a

scan rate of 2 mV s

 1

, and a maximum speciﬁc capacitance of

130 F g

 1

[81]. Further development of a free-standing

Ni(OH)

/UGF composite electrode by another group has elimi-

nated the need for addition of either binder or metal-based current

collector [80]. The highly conductive 3D UGF network facilitates

electron transport, and the porous Ni(OH)

thin ﬁlm structure

shortens ion diffusion paths and facilitates the rapid migration of

electrolyte ions. They further demonstrated an asymmetric super-

capacitor using Ni(OH)

/UGF as the positive electrode and

activated microwave exfoliated graphite oxide as the negative

electrode. They have found that this asymmetric supercapacitor

had a capacitance retention of 63.2% after 10,000 cycles [80].

Using 3D graphene/Co

composite electrodes, the same group

reported a speciﬁc capacitance as high as 1100 F g

 1

at a

current density of 10 A g

 1

[80]. These high performances show

excellent synergetic effect between graphene and the electroactive

metal oxide materials, and the 3D graphene-based composite

electrode could be a game change element in supercapacitors.

On the conductive polymer front, 3D graphene networks also

showed very promising characteristics as an effective host [82–84].

A maximum speciﬁc capacitance of the PANI/3D graphene

electrode has been documented as high as 1024 F g

 1

10 mV s

 1

scan rate, and  1002 F g

 1

at 1 mA cm

 2

respec-

tively, in 1 M H

[85]. The high surface area offered by the

conducting, porous 3D graphene framework stimulates effective

utilisation of the deposited PANI and improves electrochemical

charge transport and storage. PPY-Graphene 3D composite

electrodes prepared by Chabi et al. using both chemical and

electrochemical deposition [85], showed a high capacity of 660 F

 1

with an excellent capacity retention of 100% after 6000

cycles. In contrast to the bare PPY electrode, which lost its initial

capacitance signiﬁcantly after few hundreds of charge–discharges

due to the mechanical degradation of the electrode materials, both

Z. Yang et al. / Progress in Natural Science: Materials International ] (]]]]) ]]] ]]]–6

Please cite this article as: Z. Yang, et al., Preparation of 3D graphene-based architectures and their applications in supercapacitors, Progress in Natural Science:

Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.11.010

graphene foam (GF) and PPY-GF showed 100% capacitance

retention due to the high mechanical strength and ﬂexibility of the

GF. As shown in Fig. 4, the post-tested GF and PPY-GF also

retained good interconnected 3D structure and integrity after

thousands of charge discharge cycles.

In the case of the composite electrode, Fig. 4c and d, the

PPY chains are still closely attached to the GF scaffold after

those cycles, revealing a strong interaction between the GF and

the PPY. The above images indicate clearly the excellent

mechanical integrity of the produced graphene-based 3D

materials. These results further veriﬁed the successful role of

the GF as a holder and stabiliser for the electroactive materials.

4. Summary

In summary, most of the open literatures using 3D

graphene-based electrodes have shown that they are probably

the closest to realise an ideal 3D architectured electrode, and

allow efﬁcient charge storage process. The high mechanical

stability and integrity of the graphene network improve

signiﬁcantly the integrity of composite electrodes, and extend

the device cycling life dramatically. Therefore, if these ideal

3D architectures could be designed and produced in a

controllable fashion, with desired density, pore features and

precise arrangement for the 2D graphene within the 3D

network, we would be able to harness the full potential of

graphene. Not only they will bring unprecedented performance

in the energy storage applications, but also many critical and

new applications in mechanical, optical and electromechanical

areas can be explored.

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Yanqiu Zhu, FRSC, FIMMM, is Chair of Functional

Materials at University of Exeter since 2010. He

obtained his BSc and MSc from Harbin Institute of

Technology (China) in 1989 and 1992, and PhD

from Tsinghua University (China) in 1996. After a

short period at NIMS (Tsukuba, Japan), he worked

at the Fullerene Science Centre at University of

Sussex (UK) for 7 years before he moved to the

University of Nottingham initially as an EPSRC

Advanced Research Fellow then a Reader in Nano-

materials, until he took the professorship at Exeter in 2010. His research covers

carbon-related nanomaterials and inorganic nanomaterials, focusing on their

synthesis and functionality investigation.

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Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.11.010

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H O S T E D B Y
Available online at www.sciencedirect.com Progress in Natural Science Materials International
Progress in Natural Science: Materials International ] (]]]]) ]]] ]]] – www.elsevier.com/locate/pnsmi www.sciencedirect.com Review
Preparation of 3D graphene-based architectures and their applications in supercapacitors
Zhuxian Yang, Sakineh Chabi, Yongde Xia, Yanqiu Zhun
College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter EX4 4QF, United Kingdom
Received 20 September 2015; accepted 5 November 2015 Abstract
Three dimensional (3D) graphene-based architectures such as 3D graphene-based hydrogels, aerogels, foams, and sponges have attracted huge
attention owing to the combination of the structural interconnectivities and the outstanding properties of graphene which offer these interesting
structures with low density, high porosity, large surface area, stable mechanical properties, fast mass and electron transport. They have been
extensively studied for a wide range of applications including capacitors, batteries, sensors, catalyst, etc. There are several reviews focusing on
the 3D graphene-based architectures and their applications. In this work, we only summarise the latest development on the preparation of 3D
graphene-based architectures and their applications in supercapacitors, with emphasis on the preparation strategies.
Crown Copyright & 2015 Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: 3D-graphene; Supercapacitor; Electrode architecture 1. Introduction
include adding spacers [11,12], crumpling the graphene sheets
[13], and creating freestanding 3D graphene-based architectures.
The exceptional properties of graphene, including outstanding
Recently, the creation of freestanding 3D graphene-based
electron mobility up to 200,000 cm2 V1 s 1 [1], excellent
architectures, such as 3D graphene-based hydrogels [14],
thermal conductivity up to 5000 W m1 K1 [2], extremely
aerogels [15], foams [16,17], and sponges [18], has been
high mechanical strength (Young's modulus 1.0 TPa) [3],
high optical transparency of 97.7% [4], and large theoretical
demonstrated as an effective way to tackle the aforementioned
speciﬁc surface area of 2630 m2 g1, have drawn tremendous
restacking issues [19]. These 3D architectures can offer high
research attention from scientists in chemistry, physics, materi-
electrical conductivity and improved structural stability, due to
als science and energy etc. Graphene-based thin ﬁlm composites
the absence of defects and reduced intersheet junction contact
and freestanding nanosheets have been explored in high
resistance compared with those 2D graphene-based thin ﬁlm
performance nanocomposites [5], catalysis [6], energy storage
composites and freestanding nanosheets [16,17]. The low
devices [7], electronics and optoelectronics [8], biological and
density, high porosity, large surface area, excellent electrical
chemical sensors etc. [9] In some cases, 2D graphene sheets
conductivity and stable mechanical properties render these 3D
suffered from aggregation or restacking in macroscopic scale,
which led to the loss of effective accessible surface areas [10].
architectures having potential applications in many ﬁelds such
To date, different strategies have been developed to achieve
as capacitors, batteries, sensors, catalysts, as absorbents, etc.
3 dimensionally (3D) assembled macroscopic structures, in an
There have been a number of reviews on the preparation and the
effort to eliminate the above mentioned issues. Examples
applications of 3D graphene-based architectures [7,19–29], so
we will only focus in the recent advances on the preparation of n Corresponding author.
3D graphene-based architectures and their applications in super-
E-mail address: y.zhu@exeter.ac.uk (Y. Zhu).
Peer review under responsibility of Chinese Materials Research Society.
capactiors, especially on those controlled synthesis strategies.
http://dx.doi.org/10.1016/j.pnsc.2015.11.010
1002-0071/ Crown Copyright & 2015 Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
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Z. Yang et al. / Progress in Natural Science: Materials International ] (]]]]) ]]] ]]] –
2. Preparation of 3D graphene-based architectures
at 1000 1C with nickel foam as the template and CH4 as the
carbon source. This Ni foam template-CVD strategy has been
The widely explored preparation methods are the assembly
extensively studied for the preparation of 3D graphene foams
of graphene oxide (GO) or graphene sheets for 3D graphene-
as scaffold for the creation of diverse functionalised materials
based sponges [18,30], foams [15,31–34], hydrogels and
[17,19,26,39,57–62]. For example, a number of chemicals
aerogels [14,35–38], and the direct synthesis via CVD
including NiO [17], Co3O4 [57], LiFePO4 [58], Pt nanoparti-
(chemical vapour deposition) for 3D graphene based foams
cles [63], Si [64], multi-walled CNTs [63], MnO2 [63], Fe3O4
[16,17,39–41]. Other methods, such as the sugar blowing
[65] and MoSx [66] have been added to the 3D graphene foams
approach [42], the 3D printing technique [43,44], and the
to create 3D graphene based composites. The macroscopic
commercial graphite paper (GP) technique [45], have also been
randomly uniform feature and the high integrity allows them to
reported. According to the topology of the resulting 3D
act a strong matrix to accommodate these new elements
structures, they can broadly be classiﬁed into macroscopically
uniformly, thereby the resulting new composite have the
uniform and random architectures [14–18,30–41], and macro-
potential to exhibit very interesting properties, and some of
scopically symmetric and well-controlled structures [43,44,46].
which have been explored in these reports.
They can be produced by different strategies, and obviously
can be used for different applications. Since there have been
2.3. Preparation of 3D graphene-based architectures with
some excellent up to date reviews [19,25,26,29], our attention controlled manner
in this context will be focused on the controlled preparation of
3D graphene-based architectures.
In most of the aforementioned cases, the pore shapes, pore
sizes, truss length, wall thickness and overall geometries of the 2.1. The assembly method
3D graphene-based architectures are randomly arranged. For
example, the dimensions of the structures fabricated by the
Since the early report on self-assembled graphene hydrogel
CVD method using Ni foam templates are limited by the
prepared by chemical reduction of the aqueous GO dispersion
dimensions of the nickel foams, and the pores are on a scale of
with sodium ascorbate demonstrated by Shi's group [14], there
a few hundred micrometres based on commercially available
have been many studies on the self-assembly of GO/graphene
foams. To maximise their potential applications, it is highly
sheets. It is generally accepted that the force balance between
desirable to design and control the 3D architectures with
the van der Waals attractions from the basal planes of GO
preferred sizes and shapes of the pore, lengths and diameters of
sheets and the electrostatic repulsions from the functional
the trusses, etc. to have preferentially enhanced properties.
groups of GO sheets maintains the GO sheets well-dispersed in
This is a step forward towards speciﬁc applications. We will
an aqueous solvent. Once this balance is broken, gelation of
summarise some recent reports below describing the controlled
the GO dispersion will take place, leading to the formation of
preparation of 3D graphene-based architectures.
3D GO hydrogels that can be further reduced to produce 3D
graphene-based architectures [19,29]. Methods based on the
self-assembly of GO sheets include: chemical reduction
2.3.1. Assembly based strategies
[47–49], cross-linking agent (including metal ions [50],
Huang et al. have reported a hydrophobic interaction driven
biomolecules [51], polymers [52,53] etc.), hydrothermal pro-
hard templating approach to synthesise nanoporous graphene
cess [14,54], sol–gel reaction [55], freeze-drying [30,56], and
foams with controllable pore size (30–120 nm) and ultra-high
so on. Apart from self-assembly, template-assisted assembly of
pore volumes ( 4.3 cm3 g1) [31]. As shown in Scheme 1, in
3D macroporous graphene ﬁlms have been demonstrated with
a typical synthesis, they ﬁrst used methyl group grafted silica
spherical polystyrene (PS) balls [37], and silica nanoparticles
spheres with a uniform particle size of 27.7, 60 or 120 nm as
(NPs) [31], which offer some control over the resulting 3D
the templates for the self-assembly of the GO sheets. The architectures.
resulting GO/silica sphere composites were further calcined at
900 1C for 5 h under argon atmosphere, followed by HF 2.2. The CVD method
(5 wt%) wash to remove the templates and obtain the 3D
materials [31]. In addition, Xie et al. developed a strategy
3D reduced GO architectures obtained by the assembly
which allowed large-range tailoring of the porous architecture
strategies starting from GO sheets will inevitably contain some
and its properties by a modiﬁed freeze casting process [56]. In
defects introduced during exfoliation and reduction processes
their process, a GO dispersion obtained from the oxidation of
and exhibit low electrical conductivity. On the contrary, the
graphite was ﬁrst hydrothermally reduced to hydrogel, and
CVD method can produce free-standing graphene foam with
then treated with freeze drying under different temperatures.
high electrical conductivity and improved structural stability,
This innovative technique is able to tailor the pore sizes and
superior to those fabricated using chemically derived graphene
wall thicknesses of the porous graphene from 10 to 800 μm
sheets, due to the absence of defects and intersheet junction
and from 20 nm to 80 μm, respectively. As a result, these
contact resistance [16]. Chen's group pioneered the synthesis
structural changes have brought to property changes from
of 3D graphene foams using nickel foam as the template via
hydrophilic to hydrophobic with the Young's Modulus varying
CVD [16]. Brieﬂy, graphene growth was carried out via CVD by 15 times [56].
Please cite this article as: Z. Yang, et al., Preparation of 3D graphene-based architectures and their applications in supercapacitors, Progress in Natural Science:
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Z. Yang et al. / Progress in Natural Science: Materials International ] (]]]]) ]]]–]]] 3
Bi et al. have developed a pH-mediated hydrothermal reduction 2.3.2. CVD based strategies
technique to obtain high compressive strength 3D graphene
In the case of Ni foam templated growth, the pore shape and
structures using a low temperature casting at 180 1C, combined
geometries of the foam are arranged randomly, so are the
with various moulding [67]. In their method, the casting and the
resulting 3D graphene foams. To counter this intrinsic dis-
shaping took place while reducing the GO at different reaction
advantage, Ito et al have recently produced a novel nanoscale
times. This technique, by tailoring the pH values, allows the
porous Ni-template, which has a high level of control of pore
controllable fabrication of compact high density graphene macro-
size distribution, using the CVD approach to fabricate high
structures with various shapes including triangular prism, quad-
quality 3D nanoporous graphene with tailored pore sizes [41].
rangular prism, joint ring, crucible, screw stem, and gear. The
The nanoporous Ni with a thickness of 30 μm was prepared by
dimension of castings ranges from sub-millimetre to centimetre,
electrochemically leaching Mn from a Ni30Mn70 precursor in a
depending on the mould and the concentration of GO. They
weak acid solution. By controlling the size of the Ni ligaments,
claimed that as long as the concentration of the GO dispersions is
they have realised 3D graphene with pores of 100 nm to
above 0.5 mg mL1, the resulting graphene gel can be cast into
2.0 μm by controlling the CVD time and temperatures [41].
any complicated macroscopic shape [67]. It is obvious that the
This indeed improved the pore size distribution of the resulting
overall shape of the product can be well-controlled by this strategy,
3D graphene, with the normal features of CVD graphene,
however the microscale dispersion of the graphene sheets within
however the electrochemical process on which the strategy is
the moulded and reduced product remains completely random, lack
based for the template creation makes this strategy unsuitable of control.
for large scale production or for large dimensional structures.
Very recently, Barg et al. have explored a self-assembly
Very recently, Yang et al. have demonstrated a conceptual
strategy for the fabrication of complex chemically modiﬁed
design and practical synthesis of 3D graphene networks [46],
graphene cellular networks (CMG-CNs) via a multi-step
whereas the CVD process combined with a 3D-printed
soft/hard template mechanism that combines emulsion and
periodic Ni template, in an effort to introduce advanced
ice templating, which is illustrated in Fig. 1 [34]. This
geometric control at both macroscopic and microscopic levels.
approach allows the manipulation of the structure at multiple
High resolution 3D-printing of Ni or Cu scaffolds allows the
levels from the densities (over two orders of magnitude from
templates to have the desired geometrical features, such as
1 to 200 mg cm3), cell shape (polyhedral to spherical) and
truss diameters and lengths, pore shapes and sizes, symmetric
sizes ( 7 to 60 μm) at the micro-level to the cell walls
or non-symmetric, etc.; whereas the CVD enables the control
topography, porosity and chemistry at the micro-to-nano-level
of the layer numbers in the ﬁnal products. Although the 3D
[34]. These networks contain much more pores and are much
graphene networks demonstrated in the report were 3D
lighter than those of Bai et al.'s architectures, and they have
structures shown in Fig. 2, it is anticipated that by taking
shown higher level of control over the microscopic features.
advantage of the new development of 3D printing technologies
for Ni or Cu template creation, and of the versatile feature of
the CVD for graphene growth, customised 3D graphene
Scheme 1. Schematic illustration of the synthesis procedures of the nanoporous graphene foams (NGFs). I) The self-assembly occurs between GOs and
hydrophobic silica templates; II) Calcination and silica etching to produce NGFs [31]. Copyright 2012, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Please cite this article as: Z. Yang, et al., Preparation of 3D graphene-based architectures and their applications in supercapacitors, Progress in Natural Science:
Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.11.010 4
Z. Yang et al. / Progress in Natural Science: Materials International ] (]]]]) ]]] ]]] –
Fig. 1. Microstructural architecture of CMG-CNs. (a) Overview of the CMG-CN architecture, composed of nearly spherical pores in the micrometre scale, designed
by the emulsion droplet templates. (b) At the triple junction between adjacent cells, whose arrangement is template by ice. (c) Cell walls surface micro to
nanorugosity patterned by the ice crystals during unidirectional freezing. (d) SEM images of 5.6 mg cm-3 GO-CN (a–d) that after thermal reduction at 1000 1C
results in rGO-CN of 2.2 mg cm 3 [34]. Copyright 2014, Macmillan Publishers Limited.
Fig. 2. SEM images of 3D graphene networks at various magniﬁcations: 1 mm (A), 100 mm (B), and 10 mm (C) [46]. Copyright 2015, The Royal Society of Chemistry.
networks with desired trusses, pores and special structural
2.3.3. Direct 3D printing methods
features could be achieved. Although the overall dimension of
Different to Yang's approach, 3D printing technique has
the 3D-printed templates can be very large (up to metres), the
been directly attempted to produce 3D graphene architectures
effective of CVD for large porous samples to achieve uniform
by Seol's group [43]. Using reduced graphene oxide ink for a
number of graphene layers across the entire samples appears to
nanometre-scale 3D printer, by exploiting a size-controllable
be a practical challenge. Further, the proper process for the
liquid meniscus, they succeeded in writing freestanding
removal of the template at a controlled manner without
reduced graphene oxide nanowires without any supporting
damaging the 3D graphene network, particularly when the
materials [43]. Similarly, Zhu et al. have fabricated periodic
size is large, needs dedicated experience. Nevertheless, a few
graphene aerogel microlattices shown in Fig. 3 via 3D printing,
centimetres of such networks could suit for speciﬁc applica-
which is based on the precise deposition of GO ink ﬁlaments
tions in acoustic and optical, electromagnetic, photonic and
on a pre-deﬁned tool path to create architected 3D GO mechanical ﬁeld [46].
structures [44]. The key for this fabrication process is the ink
preparation. They ﬁrst added fumed silica powders and catalyst
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Z. Yang et al. / Progress in Natural Science: Materials International ] (]]]]) ]]]–]]] 5
into the as-prepared aqueous GO suspensions to generate a
layer capacitors (e.g. activated carbon) store charges electro-
homogeneous GO ink with designed rheological properties.
statically via the physisorption of electrolyte ions, and the
The GO ink was then extruded through a micronozzle
second in the case of pseudocapacitive materials e.g. metal
immersed in isooctane to prevent drying during printing. The
oxides or conductive polymers, involves a fast surface redox
printed microlattice structure was supercritically dried to
reaction [68,69]. Thus, a good control over the structure and
remove the liquid, followed by carbonization at 1050 1C under
morphology of electrode materials is critical to obtain high
N2. Finally, the silica ﬁller was etched using HF acid [44].
surface area and efﬁcient paths for ion diffusion. Despite the
Both of these direct 3D-printing strategies realised high
excellent properties of graphene, still much in-depth research
level of shape and geometry control over the resulting 3D
and further modiﬁcations are needed to allow this truly 2D
graphene-based architectures, however it seems that the
material to replace the conventional electrode architecture.
process worked better with GO/rGO than graphene sheets.
It has been discussed that highly efﬁcient supercapacitor
Therefore the electric conductivity of the structures may be
materials should have high accessible surface areas, hierarch-
inferior to those produced by CVD. The microscopic GO
ical pores, high conductivity and robust structures [70,71].
dispersion/orientation within the printed objects remains ran-
Although pristine 2D graphene does not meet these criteria,
dom. Therefore, to realise macroscopic and microscopic
such as limited accessible surface areas or hardly any pores,
control of 3D graphene-based structures is still challenging.
the 3D architectures based on the 2D graphene can be designed
and created, to allow for improved and efﬁcient ion accessi-
3. Applications of 3D graphene-based architectures in bility and transport. supercapacitors
Depending on the electrode materials, two types of energy
storage can occur in supercapacitors. The ﬁrst such as double
Fig. 3. Morphology and structure of graphene aerogels. (a) Optical image of a 3D printed graphene aerogel microlattice, (b–d) SEM images, (b) A 3D printed
graphene aerogel microlattice, (c) Graphene aerogel without R–F after etching, (d) Graphene aerogel with 4 wt% R–F after etching. (e–f) Optical images, (e) 3D
printed graphene aerogel microlattices with varying thickness, and (f) A 3D printed graphene aerogel honeycomb. Scale bars, 5 mm (a), 200 mm (b), 100 nm (c, d),
1 cm (f) [44]. Copyright 2015, Macmillan Publishers Limited.
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Z. Yang et al. / Progress in Natural Science: Materials International ] (]]]]) ]]] ]]] –
3.1. 3D graphene-based architectures directly used in
3.2. 3D graphene-based architectures as scaffolds in supercapacitors supercapacitors
Due to the combination of the 3D interconnection structure
The low density, high porosity, large surface area, excellent
and the excellent electron conductivity of graphene, 3D
electrical conductivity and stable mechanical properties make
graphene architectures are promising for use as electrodes in
these 3D architectures ideal scaffolds to host metal oxide
supercapacitors. For example, the graphene hydrogel early
nanoparticles and conductive polymers, for the creation of
reported by Shi's group has a well-deﬁned and cross-linked 3D
highly efﬁcient 3D graphene composite/hybrid electrodes
porous structure with pore sizes in the range of submicrom-
[76–79]. Zhang et al. developed a scalable approach to prepare
eters to several micrometres, and the speciﬁc capacitance of
honeycomb-like CoMoO4–3D graphene hybrid (NSCGH)
this graphene hydrogel was demonstrated to be up to about
(NHC) electrode [80]. They reported speciﬁc capacitances as
240 F g1 at a discharge current density of 1.2 A g 1, in 1 M
high as 2741, 1585, and 1101 F g1 at current densities of
aqueous solution of H2SO 4 [14]. In their following study, they
1.43, 22.85 and 85.71 A g 1 (aqueous solution within the
have achieved high speciﬁc capacitance of 220 F g1 at a
potential range of 0 to 0.9 V (vs. standard Hg/HgO electrode)),
current density of 1 A g–1, which can be maintained for 74% at
respectively. The aqueous symmetric (with NSCGH as two
a discharge current density of 100 A g–1 [72]. The exceptional
electrodes) supercapacitors deliver a high energy density of
electrical conductivity and mechanical robustness of graphene
14.5 W h kg1 at a power density of 18,000 W kg 1, which
hydrogel make it an excellent material for ﬂexible energy
can power a 5 mm-diameter LED for more than 3 min with a
storage devices. As demonstrated by Xu et al., a ﬂexible
charging time of only 2 s [80].
supercapacitor with a 120 μm thick graphene hydrogel thin
Another advantage of 3D graphene-based composite electrodes
ﬁlm exhibited excellent capacitive characteristics, including a
lies in the ability of the 3D network to accommodate large mass
high gravimetric speciﬁc capacitance of 186 F g1 (up to
loading for the active materials. For example, a MnO2 mass
196 F g1 for a 42 μm thick electrode), an unprecedented areal
loading of 9.8 mg cm2 (92.9% of the mass of the entire
speciﬁc capacitance of 372 mF cm2 (up to 402 mF cm2 for
electrode) leads to a high area capacitance of 1.42 F cm 2 at a
a 185 μm thick electrode), low leakage current (10.6 μA),
scan rate of 2 mV s1, and a maximum speciﬁc capacitance of
excellent cycling stability, and extraordinary mechanical ﬂex- 130 F g1
[81]. Further development of a free-standing
ibility [73]. In addition, In a very successful attempt, using
Ni(OH)2/UGF composite electrode by another group has elimi-
chemical activation of exfoliated GO, Ruoff’s group reported
nated the need for addition of either binder or metal-based current
the synthesis of a graphene-derived 3D network with extre-
collector [80]. The highly conductive 3D UGF network facilitates
mely high surface area of up to 3100 m2 g1 [74]. The
electron transport, and the porous Ni(OH)2 thin ﬁlm structure
unique hierarchically porous structures in the produced 3D
shortens ion diffusion paths and facilitates the rapid migration of
graphene-derived carbon structures give rise to a high gravi-
electrolyte ions. They further demonstrated an asymmetric super-
metric (174 F g1) and volumetric (  100 F cm3) speciﬁc
capacitor using Ni(OH)2/UGF as the positive electrode and
capacitance in 1-ethyl-3-methylimidazolium bis (triﬂuorosul-
activated microwave exfoliated graphite oxide as the negative
fonyl) imide acetonitrile electrolyte. The assembled two
electrode. They have found that this asymmetric supercapacitor
electrode symmetric cell delivered gravimetric energy density
had a capacitance retention of 63.2% after 10,000 cycles [80].
and power density of 74 W h kg1 and 338 kW kg 1 and
Using 3D graphene/Co3O4 composite electrodes, the same group
volumetric values of 44 W h L1 and 199 kW L 1, which are
reported a speciﬁc capacitance as high as 1100 F g1 at a
quite high [74]. The high performance was attributed to the
current density of 10 A g1 [80]. These high performances show
large fraction of micro- and mesopores that provide large and
excellent synergetic effect between graphene and the electroactive
accessible surface areas for charge accommodation.
metal oxide materials, and the 3D graphene-based composite
Regarding graphene aerogels, Zhang and co-workers has
electrode could be a game change element in supercapacitors.
demonstrated graphene aerogels with a speciﬁc capacitance
On the conductive polymer front, 3D graphene networks also
128 F g1 (at a constant current density 50 mA g 1) prepared
showed very promising characteristics as an effective host [82–84].
by either CO2 drying or freeze drying of graphene hydrogel
A maximum speciﬁc capacitance of the PANI/3D graphene
precursors obtained from heating the aqueous mixture of
electrode has been documented as high as 1024 F g1 at
graphene oxide with L-ascorbic acid without stirring [75].
10 mV s1 scan rate, and 1002 F g1 at 1 mA cm2 respec-
As for graphene foams, Zhao and co-workers have reported
tively, in 1 M H2SO4 [85]. The high surface area offered by the
a versatile, N-doped, ultralight 3D GF, which exhibits a
conducting, porous 3D graphene framework stimulates effective
signiﬁcantly high capacitance of 484 F g1, approaching to
utilisation of the deposited PANI and improves electrochemical
the theoretical electric double-layer (EDL) capacitance of
charge transport and storage. PPY-Graphene 3D composite
graphene. It is believed that the combination of the unique
electrodes prepared by Chabi et al. using both chemical and
3D porous structures of few-layer graphene which maximises
electrochemical deposition [85], showed a high capacity of 660 F
the exposure of their surfaces to electrolyte and effective
g1 with an excellent capacity retention of 100% after 6000
N-doping could lead to high capacitance performance [15].
cycles. In contrast to the bare PPY electrode, which lost its initial
capacitance signiﬁcantly after few hundreds of charge–discharges
due to the mechanical degradation of the electrode materials, both
Please cite this article as: Z. Yang, et al., Preparation of 3D graphene-based architectures and their applications in supercapacitors, Progress in Natural Science:
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Fig. 4. SEM images of the GF and PPY-GF. (a, b) SEM images of the GF after 10,000 cycles of CV test, and (c, d) SEM images of PPY-GF after 6000 cycles of
CV test [79]. White spots on the image are electrolyte ions. Reproduced by permission of The Royal Society of Chemistry.
graphene foam (GF) and PPY-GF showed 100% capacitance
new applications in mechanical, optical and electromechanical
retention due to the high mechanical strength and ﬂexibility of the areas can be explored.
GF. As shown in Fig. 4, the post-tested GF and PPY-GF also
retained good interconnected 3D structure and integrity after
thousands of charge discharge cycles.
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Please cite this article as: Z. Yang, et al., Preparation of 3D graphene-based architectures and their applications in supercapacitors, Progress in Natural Science:
Materials International (2015), http://dx.doi.org/10.1016/j.pnsc.2015.11.010