ReviewAppl. Chem. Eng., Vol. 26, No. 5, October 2015, 521-525

http://dx.doi.org/10.14478/ace.2015.1101

521

슈퍼커패시터 응용을 위한 3차원 그래핀/금속 산화물 나노복합체 제조

김정원⋅최봉길†

강원대학교 화학공학과(2015년 9월 14일 접수, 2015년 9월 17일 심사, 2015년 9월 17일 채택)

Preparation of Three-Dimensional Graphene/Metal Oxide Nanocomposites for Application of Supercapacitors

Jung Won Kim and Bong Gill Choi†

Department of Chemical Engineering, Kangwon National University, 346 Joongang-ro, Samcheok, Gangwon-do 25913, South Korea(Received September 14, 2015; Revised September 17, 2015; Accepted September 17, 2015)

록

2차원 구조와 우수한 물성을 지닌 그래핀 기반의 전극 재료들은 슈퍼커패시터에 많이 응용되어 왔다. 특히 3차원 구조의 그래핀 소재들은 전극 제조에 매우 중요한데 이는 3차원 구조가 넓은 표면적, 효과적이고 빠른 전기 및 이온 전달, 우수한 기계적 물성을 제공하기 때문이다. 최근에는 3차원 하이브리드 구조를 가지는 그래핀/금속 산화물 재료들이 슈퍼커패시터의 에너지와 파워 밀도를 동시에 증가시키고자 개발되어 왔다. 본 논문은 그래핀과 금속 산화물로 이루어진 3차원 나노복합체의 최근 연구 경향을 논하고자 한다. 3차원 나노복합체의 제조와 구조 및 이를 이용한 슈퍼커패시터의 응용을 다룬다.

AbstractGraphene-based electrode materials have been widely explored for supercapacitor applications due to their unique two-dimen-sional structure and properties. In particular, Three-dimensional (3D) graphene materials are of great importance for preparing electrode materials because they can provide large surface area, efficient and rapid electron and ion transfer, and mechanical stability. Recently, a number of 3D hybrid architecture of graphene/metal oxides have been developed to increase simulta-neously energy and power densities of supercapacitors. This review presents the recent progress of 3D nanocomposites based on graphene and metal oxides. Preparation methods and structures of these 3D nanocomposites and their great potential in supercapacitor applications have been summarized.

Keywords: supercapacitor, metal oxide, graphene, nanocomposite

1. Introduction1)

Supercapacitors (SCs) have been widely used in consumer elec-

tronics, memory back-up devices, and industrial power and energy ve-

hicles and managements, owing to their many advantages of high pow-

er performance, excellent rate capability, long-term cycle life, and sim-

ple operation, etc[1-4]. As a main component of SCs, the electrode ma-

terials hold a key to achieving high-electrochemical performance based

on transport of electron and ion for energy storage[1-4]. So far, tre-

mendous efforts have been attempted to synthesis of electrode materi-

als, including activated carbons, carbon nanotubes, graphene, metal ox-

† Corresponding Author: Kangwon National University, Department of Chemical Engineering, 346 Joongang-ro, Samcheok, Gangwon-do 25913, South KoreaTel: +82-33-570-6545, e-mail: bgchoi@kangwon.ac.kr

pISSN: 1225-0112 eISSN: 2288-4505 @ 2014 The Korean Society of Industrial and Engineering Chemistry. All rights reserved.

ide, metal hydroxides, and conducting polymers[5-10]. Although a num-

ber of manufacturers have produced commercial SCs by Maxwell

NESSCAP, Nippon ESMA, Ioxus, Cap-xx, and so on, the most com-

mercial SCs suffer from the limited energy density less than 10 W h/kg,

which is still lower than lithium-ion batteires.

Among the various candidates of electrode materials, graphene, a

two-dimensional (2D) carbon sheet, has been considered to be a prom-

ising material for replacement of activated carbons in SCs, because of

it’s a large theoretical specific surface area (2630 m2/g), excellent elec-

trical conductivity (106 S/cm), and high Young’s modulus (~1.0

Tpa)[11-13]. Scalable production of graphene sheets is of great im-

portance for application of supercapacitive electrode materials. Although

various approaches have been developed for preparation of graphene,

including a Scotch tape, epitaxial growth, liquid phase exfoliation, and

electrochemical synthesis, the preparation of graphene oxide (GO) fol-

lowed by a reduction process is more suitable for massive production

of electrode materials in SCs because of its large-scale production, low

522 김정원⋅최봉길

공업화학, 제 26 권 제 5 호, 2015

Figure 1. (a) Schematic illustration of preparation GO materials based on modified Hummers method. (b) Photograph image of GO dispersed in water. (c) TEM image of GO sheet.

Figure 2. (a) and (b) SEM images of 3D CMG/MnO2 hydrogel. Inset is photograph image of 3D CMG/MnO2 hydrogel.

cost, and simple synthetic process (Figure 1)[14]. Ruoff and co-work-

ers firstly reported that chemically modified graphene (CMG) materials

have a great potential as electrode materials for SCs, showing specific

capacitances of 135 and 99 F/g[15]. However, most works of gra-

phene-based materials suffer from serious aggregation of graphene

sheets, which causes poor ion diffusion behavior and thus reduction of

SC performance.

In order to address this issue, incorporation of CMGs into three-di-

mensional (3D) structures could be a breakthrough for improvement of

SC performance, because 3D structure provides a large surface area,

easy accessibility of electrolyte ions, and increased mechanical proper-

ties[14,16]. In particular the construction of 3D heterogeneous structure

composited of more than two components has been proposed as an ap-

pealing strategy for improving the energy and power performance of

SCs. In this review, we mainly focus on the preparation and structure

of 3D graphene/metal oxide materials, based on the strategies of

self-assembly, template-assisted method, and direct approach. In addi-

tion, the application of 3D graphene/metal oxide materials for super-

capacitors is summarized.

2. Preparation Methods

CMG nanomaterials have been recognized as ideal for preparation of

electrode materials in SC systems because of their unique properties.

However, during the reduction process of GOs, the irreversible ag-

gregation or restacking of CMG sheets significantly reduced nano-

scopic properties of graphene[17,18]. In this regard, a big challenge in

fabrication of bulk electrode is how to transfer the unique properties

(i.e., specific surface area, electrical and ionic conductivity, and me-

chanical properties) of individual graphene sheet into the macroscopic

properties of the bulk electrodes[17]. In this section, the main strat-

egies for the construction of 3D graphene/metal oxide structures are

briefly summarized.

2.1. Self-assembly

Self-assembly has been recognized as one of the most powerful

methods for preparation of 3D CMG structures. In a typical method,

3D CMG structures could be synthesized through the gelation process

of GO dispersion in aqueous solution followed by reduction process of

GO to CMG. During this process, hydrophobic and hydrophilic force

balance between the CMG sheets induced self-assembly of GO and

CMG in aqueous media. Shi et al. reported that the stability and con-

centration of GO dispersion significantly influenced gelation of GO dis-

persion[19]. So far, many approaches have been attempted to achieving

3D CMG structures, including addition of cross-linkers (e.g., DNA, pol-

ymers, metal ions, organic molecules, and so on), changing the pH val-

ue of the GO dispersion, or ultrasonication of GO dispersion[20-24].

Two types of 3D CMG-based structures, hydrogels and aerogels, have

been developed by various approaches, direct freeze-drying, tape cast-

ing, controlled centrifugation, electrochemical deposition, sol-gel re-

action, and hydrothermal methods[25-29]. Besides the preparation of

3D CMG structures, further efforts have been attempted to fabricating

the 3D CMG/metal oxide heterogeneous structures. Wei et al. described

that the 3D CMG/MnO2 structures were prepared by gelation of GO

dispersion followed by post deposition of MnO2[30]. Figure 2 shows

photograph image of 3D CMG/MnO2 with a cylindrical morphology. A

thin layer of MnO2 nanoparticles was deposited on the surface of CMG

sheets, while the interconnected porous structure maintained. Yu’s

group reported one-step fabrication of 3D CMG/Fe3O4 (or FeOOH) hy-

drogels by control of pH value and addition of metal ions[24]. In addi-

tion, Choi et al. prepared macroscopic CMG/RuO2 spheres by means of

intermolecular interactions between the CMG sheets and RuO2 nano-

particles (Figure 3)[31].

2.2. Template-assisted method

The employment of templates is also effective for the preparation of

3D CMG-based structures with much more controlled morphologies

and properties. These approaches have been well developed by sacrifi

cial templates, such as polystyrene (PS) and silica colloidal nano-

particles[32-35]. Choi and co-workers reported that 3D CMG structures

could be prepared by strong electrostatic interactions between the neg-

523슈퍼커패시터 응용을 위한 3차원 그래핀/금속 산화물 나노복합체 제조

Appl. Chem. Eng., Vol. 26, No. 5, 2015

Figure 3. (a) and (b) SEM images of hierarchical microspheres of CMG/RuO2 samples.

Figure 4. (a) and (b) SEM images of 3D macroporous CMG film. (c) TEM image of 3D CMG/MnO2 film. (d) TEM image of 3D CMG/Co3O4 film.

atively charged CMG sheets and positively charged PS particles fol-

lowed by removing the sacrificial PS template[32,33]. In particular,

they showed free-standing 3D films, which include two experimental

steps; (1) fabrication of free-standing PS-embedded CMG films by

vacuum filtration using a mixture of CMG and PS and (2) removal

process of PS template to completely prepare 3D CMG structures. The

pore size could be controlled by the use of different size of PS par-

ticles as sacrificial templates ranging from 100 nm to 2 µm. As-pre-

pared 3D CMG films served as 3D templates for the post-deposition

of metal oxides such as MnO2 and Co3O4, as shown in Figure 4. They

demonstrated the superior electrochemical performance of 3D

CMG/metal oxide films as electrode materials for energy storage de-

vices of lithium-ion batteries and supercapacitors. Although the use of

PS or silica templates is powerful method for the construction of 3D

pore structure, post process should be included in order to prepare 3D

CMG/metal oxide hybrid structures. Giannelis et al. reported ice tem-

plate-assisted method for the preparation of 3D CMG/Pt/Nafion hybrid

films[36]. They firstly fabricated ice templating of a mixture of Nafion,

GO, and chloroplatinic acid for iced porous scaffold. After then, mild

chemical reduction of GO and chloroplatinic acid triggered to generate

CMG/Pt/Nafion hybrid films. Another approach is direct deposition on

3D supports or current collectors, including textile, Ni foam, and car-

bon fiber paper[16,36]. In particular, the employment of 3D current

collectors enabled a binder-free from of electrodes than reduce the total

mass of electrode and thus increasing specific performances. Li et al. reported MnO2/CMG/nickel foam composite through electrochemical

deposition strategy[37].

3. Application of Supercapacitors

Supercapacitors are generally divided into electric double-layer ca-

pacitors (EDLCs) and pseudocapacitors based on the different charge

storage mechanisms[1-5]. As electrode materials of EDLCs, carbona-

ceous materials have been widely used in preparation of electrode for

SCs, in which charges can be stored at the interface of the electrolyte

and electrode by electrostatic forces[7]. Metal oxides (or hydroxides)

and conducting polymers have been extensively explored as active ma

terials for pseudocapacitors because of their Faradic charge storage re-

actions on their surface[3]. These pseudocapacitive materials could pro-

vide much higher capacitance (300-1200 F/g) than those of carbona-

ceous materials (60-300 F/g)[1-5]. However, they have significant

drawbacks of capacitance fading at a high rate and long term cycling

measurements due to their intrinsic low electrical and ionic con-

ductivity and thus causing the irreversible redox reactions[3]. In con-

trast, carbon-based materials could provide high rate capability and

long-term cycling performance, but suffer from the limited specific ca-

pacitance (60-300 F/g). In this regard, heteogeneous nanostructures

have been proposed as a promising method, particularly carbon-based

hybrid materials[32]. To date, numerous hybrids or composites includ-

ing graphene/MnO2, graphene/Co3O4, graphene/polyaniline, and gra-

phene/Fe2O3, have been extensively exlored as electrode materials for

supercapacitors[37-41]. However, graphene-based materials should

solve the big issue of the strong aggregation of the individual graphene

sheets. Hence, 3D CMG/metal oxide heterogeneous structures is one of

the most promising architectures for ideal desing and fabrication of

electrodes for high-performance supercapacitors. Yan et al. Prepared a

hydrogel of 3D CMG/MnO2 composites by in-situ self-assembly of

MnO2 nanoparticle-deposited CMG sheets[42]. The hydrogel of 3D

CMG/MnO2 exhibited a specific capacitance of 242 F/g at a current

density of 1 A/g. Wang et al. reported one-step hydrothermal method

for preparation of 3D CMG/VO2 composites[43]. This composites

showed as high as specific capacitance of 426 F/g at a constant current

density of 1 A/g, which is higher than those of individual components

of VO2 (191 F/g) and CMG hydrogel (243 F/g). Chen and co-workers

successfully synthesized 3D CMG/ Co3O4 aerogel composites through

solvothermal process and subsequent freeze-drying and thermal reduc-

tion processes[44]. As-obtained 3D CMG/Co3O4 aerogels provided a

high specific capacitance of 660 F/g at current density of 0.5 A/g.

Another type of 3D CMG/MnO2 aerogel was prepared by Xue’s group

through solution-phase assembly process[45]. The 3D CMG/MnO2 aer-

ogel served as binder-and carbon-free electrode materials and showed

specific capacitance of 236 F/g at current density of 0.25 A/g. Choi

and co-workers reported that RuO2 nanoparticles triggered direct as

sembly of CMG sheets into hierarchical microspheres with high specfi-

cic surface area of 494.05 m2/g[31]. As-prepared 3D RuO2/CMG mi-

524 김정원⋅최봉길

공업화학, 제 26 권 제 5 호, 2015

Materials Synthetic method Cs (F/g) Electrolyte Ref.

3D CMG/MnO2 hydrogel hydrothermal method 242 (at 1 A/g) - 42

3D CMG/VO2 hydrogel Hydrothermal method 426 (at 1 A/g) 0.5 M K2SO4 43

3D CMG/Co3O4 aerogel Solvothermal and free-drying method 660 F/g (at 0.5 A/g) 6 m KOH 44

3D CMG/MnO2 aerogel Solution-phase assembly 236 (at 0.25 A/g)1-ethyl-3-methylimidazolium

tetrafluoroborate/acetonitrile

45

3D CMG/RuO2 microsphere Self-assembly 160 F/g (at 1 A/g) 1 M H2SO4 31

3D CMG/MnO2 film PS-template method 389 F/g (at 1 A/g) 1 M Na2SO4 32

3D CMG/MnO2 textile Solution coating method 315 F/g (2 mV/s) 0.5 M Na2SO4 46

Table 1. Comparison of Various Structures of 3D CMG/Metal Oxides with their Preparation and Specific Capacitance (Cs) Value with Electrolyte

crospheres exhibited specific capacitance of 160 F/g at 1 A/g. The use

of sacrificial template provided conrollable pore size and structure in

preparation of 3D structures. Choi and co-workers prepared 3D macro-

porous films of MnO2/CMG composites[32]. The 3D CMG/MnO2

films were prepared in two steps of fabrication of free-standing

PS/CMG films by vacuum filtration, followed by removal of the PS

to generate 3D hierarchical structure. The thin layer of MnO2 was in-

corporated into the 3D CMG film by self-limiting reaction of Mn pre-

cusors under neutral pH condition. The 3D macroporous CMG/MnO2

film reached as high as specific capacitance of 389 F/g at 1 A/g. In

particular, the 3D CMG/MnO2 film was combined with 3D CMG film

to generate asymmetric supercapacitors. They exhibtied maximum en-

ergy density of 44 Wh/kg and power density of 25 kW/kg. Bao et al. described that the use of 3D scafold of textile can serve 3D network

for hybrid structure of MnO2 and CMG[46]. First, CMG sheets dis-

persed in aqueous solution were coated into the surface of texile fibers.

And then, MnO2 nanoparticles were electrodeposited onto the surface

of CMG-wrapped texile fibers. The CMG/MnO2 textile exhibited spe-

cific capacitance of 315 F/g at 2 mV/s. Table 1 summarizes the typical

electrochemical performance of 3D CMG/metal oxide-based electrode

materials along with their specific capacitance, electrolyte, and prepara-

tion method.

4. Conclusions and Outlook

3D heterogeneous architectures based on graphene materials have

been proposed as attractive design for addressing the electrochemical

issues (e.g., electron and ion transfer, charge transfer, and mechanical

stability) in many electrochemical devices. In this regard, numerous 3D

graphene/metal oxide nanocomposites have been reported with different

structures and compositions, including self assembly and template-as-

sisted methods. Although these studies enhanced the electrochemical

performance in supercapacitors, they still remained challenges to more

increase the device performance. Most of 3D graphene/metal oxide ma-

terials showed a wide pore size distribution and large pore sizes rang-

ing from a few hundred nanometers to several hundred micrometers.

The controllable pore size with meso- or micrometers is still challenge.

In addition, hybridization of graphene and metal oxides is still

unskilled. To obtain high energy and power densities of super-

capacitors, more efforts and experiments for fabrication of 3D gra-

phene/metal oxide nanocomposites are required in terms of scalable,

controllable, and efficient methods.

Acknowledgement

This work was supported by the National Research Foundation of

Korea (NRF) Grant funded by the Korean Government (MSIP) (No.

2015R1C1A1A02036556).

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