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Review Article pubs.acs.org/JPCC

Flexible Graphene-Based Supercapacitors: A Review W. K. Chee,† H. N. Lim,†,‡,* Z. Zainal,† N. M. Huang,§ I. Harrison,∥ and Y. Andou⊥ †

Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Functional Device Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia § Low Dimension Materials Research Centre, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia ∥ Faculty of Engineering, The University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor, Malaysia ⊥ Graduate School of Life Science and Systems Engineering, Eco-Town Collaborative R&D Center for the Environment and Recycling, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu-city, Fukuoka 808-0196, Japan

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ABSTRACT: The recent rapid growth in graphene-based supercapacitors has reached the point where there is a need for solid-state devices with physical flexibility, which will be a crucial advantage in modern electronic devices. Herein, we summarize recent developments toward an all solid-state graphene-based flexible supercapacitor. The routes to produce graphene-based electrode materials, along with the typical fabrication techniques for flexible devices, are thoroughly discussed. Furthermore, the structural morphology of the electrode materials is closely related to the electrochemical performance, and the influence of the electrode components on the mechanical flexibility of the fabricated devices is examined. Lastly, a summary of the overall electrochemical properties and current development of the reported devices is presented progressively to predict the future trends toward the realization of an ultimate-performance graphene-based flexible supercapacitor.

1. INTRODUCTION In recent years, numerous studies have been conducted to investigate flexible energy storage devices, with the goal of applying flexible electronics to devices such as flexible displays, mobile phones, and computers.1 The development of flexible supercapacitors has become a crucial task because supercapacitors are equipped with the advantages of high-power and the high-energy density of batteries.2 Thus, mechanical flexibility would be an extra advantage. In general, supercapacitors are divided into two main categories: electric double-layer capacitors (EDLCs) and pseudocapacitors. An EDLC primarily utilizes the charges accumulated on the interfacial electrolyte/electrode surface, which mainly involves carbon-based materials with a high specific surface area. The latter utilizes conductive polymers or metal oxides as electrode materials, which use faradaic mechanisms to store charges. Numerous conductive materials have been reported that were expected to have higher capacitances than EDLC-based materials, including polyaniline (PANI), polypyrrole (PPy), and poly(3,4-ethylenedioxythiophene) (PEDOT);3 however, most of these had poor cycling stability due to structural degradation during the charge/discharge process.4 1.1. Performance of Supercapacitors. The EDLC concept is based on the utilization of the charge storage mechanism at the © 2016 American Chemical Society

electrode/electrolyte interface. The positive and negative charges are arrayed opposite each other at an extremely small distance at the contact interface between the different phases (the solid active material and liquid electrolyte interface). An EDLC system comprises two polarizable electrodes, and the charge accumulated (capacitance) at the electrode/electrolyte interface is calculated following eq 15 ε C= dS (1) 4πδ



where ε is the dielectric constant of the electrolyte used, δ is the distance between the electrolyte interface and the center of an ion, and S is the surface area of the electrode interface. Recently, the calculation of the specific capacitance was carried out via integration on the area under a cyclic voltammetry (CV) curve according to eq 2.6,7

Cm = k

∫i ms

(2)

where Cm is the specific capacitance in farads per gram, ∫ i is the integrated area of the CV curve, m is the mass of the electrode Received: October 18, 2015 Revised: December 21, 2015 Published: January 29, 2016 4153

DOI: 10.1021/acs.jpcc.5b10187 J. Phys. Chem. C 2016, 120, 4153−4172

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The Journal of Physical Chemistry C materials in grams, S is the scan rate of the CV conducted, and k is a constant multiplier (k = 2 if the mass of a single electrode is used and k = 4 if the mass of both electrodes is taken into account).8 Similarly, the specific capacitance could be obtained from the slope of the discharge curve of the galvanostatic charging/discharging mechanism according to eq 3.9−11 it Cm = k (3) Δv ·m

of micropores, which prevent electrolyte ions from wetting the surface of the active material. CNTs showed a moderate capacitance of ∼92.7 F g−1, as reported in the literature.15 Therefore, they are commonly modified by conductive polymers or metal oxides to form a hybrid combination of EDLC and pseudocapacitance. Nevertheless, the slow response performance and huge production cost of CNTs cause them to be less applicable in real-life industrial fields. Recently, the choice of active materials has mainly focused on graphene, a one-atomiclayer 2D hexagonal lattice of sp2 carbon atoms covalently bonded in two plane directions.16,17 The unique structure possesses excellent physical, chemical, and mechanical properties.18−20 In addition, it has a superb theoretical specific capacitance of 2675 m2g−1 and specific capacitance of 21 μF cm−2, which give it an EDL capacitance value as high as 550 F g−1 if the whole 2675 m2g−1 is fully utilized; however, there is a strong interest in increasing the energy density of a graphene-based supercapacitor closer to those provided by batteries. 1.3. Pseudocapacitance. Previously, the search for the active materials for a supercapacitor focused on those with a high specific capacitance and cycling stability as well as a high power density; however, the trend was expanding toward a combination of pseudoactive materials. Transition-metal oxides remain as one of the most promising choices because of their pseudocapacitive behavior, which provides a very high energy density by utilizing the redox reactions within the bulk material itself, along with conductive polymers, which typically show a redox mechanism;21 however, pseudocapacitors suffer the major drawbacks of having a low power density and little cycle stability as a result of the continuous volume expansion/contraction that occurs during the charge/discharge process.22 Therefore, they are often utilized as a subsidiary reinforcement to enhance the overall energy density of the main system.

where i is the current, t is the elapsed time during the discharge process, Δv is the total working potential (minus the IR/voltage drop), and m is the mass of the electrode materials. Commonly, the formula includes the mass of the active materials. Thus, the calculated capacitances are relatively comparable. Moreover, the area9,12 and volume of the device itself have been reported to be variables in the calculation of the specific capacitance. The calculation formula is very similar to that reported for the mass of the electrode, except for a direct modification by normalizing the capacitance toward the area or total volume of the fabricated device, as shown in eqs 4 and 5 areal capacitance: Cm = k

∫i As

and k

it ΔVxA

(4)

where A = geometrical area of the supercapacitor electrode. volumetric capacitance: Cm = k

∫i Vs

and k

It ΔVxV

(5)

where V is the total volume of the fabricated supercapacitor device. On the contrary, the actual performance of a supercapacitor device is frequently being measured by the power density and energy density of the system. In general, the specific energy density (E) is defined as the amount of energy stored per unit weight of a particular device, while the specific power density (P) is directly related to the rate at which energy can be transferred from the device. Both are commonly used to characterize and compare the electrochemical performances of different supercapacitor devices.13,14 The energy density and power density of an electrochemical system/device could be calculated from the cyclic voltammetry profiles and charge/discharge profiles using eqs 6 and 7 Ecell (Wh/kg) = 1/2CCV/C − DCV 2

(6)

Pcell (W/kg) = Ecell /Δt

(7)

2. FLEXIBLE GRAPHENE-BASED NANOCOMPOSITES The recent research on supercapacitor devices has heavily focused on the mechanical flexibility of solid-state devices, with the goal of maintaining their high electrochemical performance while following the significant trend of portable and wearable electronics becoming small, thin, lightweight, and flexible, which brings new challenges for energy-storage systems.23,24 Numerous attempts to fabricate a graphene-based flexible supercapacitor system have been reported, with and without the addition of binders, as summarized in Table 1. At a glance, the fabrication concept comes to a similar point with the utilization of a flexible supercapacitor electrode and a polymer gel electrolyte. Graphene has become one of the essential components in the fabrication of flexible electrodes because of its exceptionally high mechanical strength, excellent surface area, and good conductivity,8,25,26 which add extra advantages in preventing the electrode framework from being ruptured as a result of the mechanical bending/twisting of the flexible device. On the contrary, a polymer gel electrolyte is often utilized as both an ion porous separator and electrolyte reservoir because of its semisolid feature of microchannels or pores throughout the structure, which efficiently facilitate the flow of electrolyte ions while ensuring no physical contact between the electrodes and interface. Furthermore, the semisolid framework provides a maximum adhesion with minimal distance between the electrode/electrolyte/electrode interfaces, thus efficiently enhancing the charge-storage mechanism. Commonly, poly(vinyl alcohol) (PVA)-based gel electrolytes have been reported in the literatures,27−31 although other polymers such as potassium

where CCV/C‑DC is the specific capacitance of a cell calculated from cyclic voltammetry profile or charge/discharge measurements, V is the working potential window, and Δt is the discharge time. 1.2. Electric Double-Layer Capacitance (EDLC). In an early stage, EDLC is mostly related to activated carbon, as a result of which the specific capacitance is strongly affected by the specific surface area and pore structure possessed.5 Alternately, the total capacitance recorded is very proportional to the physical surface area of the active material itself. Furthermore, the ability of electrolyte ions to infuse into the electrode interface is strongly influenced by the microstructures present. Carbon-based materials, including activated carbon (AC) and carbon nanotubes (CNTs), possess superior stability; however, they have limited capacitance because of the purely interfacial mechanisms of EDLC, as previously discussed. Typically, AC possessed 10−20% of the theoretical capacitance4 because of the presence 4154

DOI: 10.1021/acs.jpcc.5b10187 J. Phys. Chem. C 2016, 120, 4153−4172

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The Journal of Physical Chemistry C Table 1. Summary on the Fabricated Graphene-Based Flexible Supercapacitor Devices electrode material(s)

current collector

binder

highest specific capacitance

current density (Ag−1)

capacitance basis

areal mass loading (cm2)

79 F g−1

1

whole electrode

1

100−250 F g−1

1

261 F g−1 210 F g−1

0.38 0.30

237 F g−1 82.4 F g−1

types of electrolyte

ref

PVA/H2SO4

27 4

active material whole electrode

celgard membrane/ EMIMBF4 PVA/H3PO4 filter paper/H2SO4

33 34

0.01 V s−1 (CV) 0.5

whole electrode active material

KCl PAAK/KCl

28 35

247.3 F g−1

0.176

whole electrode

PVA/H3PO4

36

9.1−9.6 mF cm−2

2 × 10−4 μA

whole electrode

PVA/H2SO4

37

3304 μF cm−2

0.1/m2

PVA/KOH

38

laser-scribed rGO graphene-coated MnO2

4.04 mF cm−2 29.8 F g−1

1 1.5 mA/cm2

area of single electrode whole electrode whole electrode

39 40

CNT-Mn3O4/ graphene IL-CMG/RuO2-ILCMG graphene-cellulose nanofibers aerogel MnO2/rGO

72.6 F g−1 167 F g−1

0.5 1

active material active material

PVA/H2SO4 membrane separator/ Na2SO4 PAAK/KCL PVA/H2SO4

32 29

203 F g−1

0.7

active material

0.02

PVA/H2SO4

30

active material

28

PVA/NaNO3

41

rGO/carbon black mesoporous graphene/ PTFE graphene/PANI graphene/PANI nanofibers PPy/graphene rGO/cMWCNT−CFP/ PPy multilayer rGO MnO2-coated graphene fiber β-Ni(OH)2/ graphene

graphene hydrogel OMC-graphene/Ag NWs CNT-graphene/Fe3O4 nanoparticle

Au-coated PET coin cell

PTFE

Ausputtered

Au-coated PET

CNF Au-coated PI

Au-coated PET

ethylene glycol

1.3 (coin cell)

0.4

1

186 F g−1

1

active material

PVA/H2SO4

42

213 F g−1

1

filter paper/KOH

43

200.4 F g−1

0.1 V s−1 (CV)

electrode material electrode material

filter paper/Na2SO4

44

1

Figure 1. Schematic diagram of fabrication of flexible supercapacitor device.

polyacrylate (PAAK)32 or a commercially available membrane separator (Celgard)4 have also been utilized. The fabrication technique for a flexible solid-state supercapacitor device is summarized in Figure 1. 2.1. Synthesis Approach for Electrode Materials. 2.1.1. Electro-Polymerization/Electrodeposition. The

electrodeposition technique is one of the most common techniques for the preparation of supercapacitor electrode materials. This facile and straightforward synthesis technique provides precise control of the thickness of the resulting films as well as the rate of the polymerization, which could be easily controlled by the current density applied.45 Therefore, this 4155

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2.1.5. Vacuum Filtration Technique. This fast yet efficient technique utilizes the simplest concept of vacuum filtration to obtain a nanocomposite from physical combination of different materials. In general, a mixture of graphene and other active materials is premixed, followed by a simple vacuum filtration process. The active materials undergo layer-by-layer stacking at the interface between the nanocomposite and the filter membrane by means of excessive water filtration toward the membrane itself.27 After drying, the nanocomposite is ready to be detached from the filter membrane as a free-standing film. Therefore, the composition of the nanocomposite could be altered by simply adjusting the concentration or weight percentage of each component during the mixture preparation. 2.2. Substrate Materials for Flexible Supercapacitors. The mechanical properties of the substrate materials play a crucial role in the fabrication of a flexible supercapacitor electrode. Often, it delivers mechanical flexibility and strength to the electrode components. Thus far, various substrate materials have been applied, including the most commonly reported, nickel foam,7,33,40 along with aluminum foam or foil, graphite sheets, and carbon cloths,9,53 mainly because of their exceptionally high conductivity, flexibility, and high porosity. The presence of a stable mechanical structure provides a conductive framework for the adhesion of the electrode materials, while also serving a purpose as a current collector. Recently, the trend has focused on the utilization of carbon nanofibers (CNFs) in the fabrication of supercapacitor electrodes.10,35,41 Undeniably, CNF provides excellent properties such as a high conductivity, very high specific surface area, and mechanical flexibility. In general, CNF has been synthesized via a polymer carbonization process. Numerous types of polymers can be used to form carbon nanofibers; however, poly(acrylonitrile) (PAN) and poly(vinyl alcohol) (PVA) are the most common polymers used in the synthesis, mainly because of their high carbon content upon carbonization. Initially, the polymer is dissolved in solvent to form a polymer solution. It is then subjected to an electrospinning process, during which the high potential applied (10−15 kV) eventually forces the polymer solution to form continuous fibers with an average diameter of several microns to nanometers, depending on the potential applied. The polymerized nanofibers are then introduced to a carbonization process, where the polymer matrix is subjected to a very high temperature in an inert atmosphere. Eventually, other organic residues will be degraded, leaving mainly carbon structures behind, therefore forming continuous carbon nanofibers. Various papers have also reported the use of CNF in the fabrication of supercapacitor electrodes.34,41,54

technique involves only mild processing conditions at room temperature without toxic or excess chemicals being used throughout the process. This method is usually utilized in the synthesis of common conductive polymers, including PANI, PPy, PEDOT, and polyacetylene; however, these conductive polymers have the same limitation of poor mechanical strength, which results in low and limited cycle stability.46,47 Numerous modifications to the synthesis method and prepared materials have been reported to overcome this limitation including the introduction of EDLCbased materials such as activated carbons, CNTs, and graphene, which have been reported to improve the specific capacitance and overall cycle stability performance. For instance, the fabrication of PANI nanowire with the addition of SWCNT resulted in a significant increase in the capacitive behavior of the modified electrode.48 Recently, the modification trend focused on the utilization of graphene to enhance the conductivity of conductive polymers. Graphene is a one-layer-thick carbon planar sheet arranged in a hexagonal manner, which offers numerous advantages such as a superb conductivity, large area, and high capacity.45 2.1.2. In Situ Polymerization. In situ polymerization often refers to a common chemical polymerization technique that has been widely reported. In general, the polymerization technique involves a reaction in an aqueous solution, whereby the monomers usually disperse into an aqueous solution with the aid of a sonication process. A specific oxidizing agent is added to initiate the polymerization process within the solution, and the sample is obtained via a direct filtration technique. Previously, this specific method was reported to yield irregular granular aggregates with only small portions of nanofibers; however, with certain modifications, it was recently reported that nanostructures such as nanoparticles, nanorods, and nanofibers were obtained with improved solution processability, along with better physical and chemical properties than their bulk counterparts.49 In addition, improvements and modifications of this conventional method have also been reported elsewhere.31,49,50 2.1.3. Direct Coating. Direct coating is one of the most common techniques utilized in the fabrication of supercapacitor electrodes. Because the active materials are usually applied directly to the surface of the substrate itself, this method heavily relies on the physical adhesion of the electrode materials on the substrate. Often, additive or binder such as carbon black and polyvinylidine fluoride (PVDF) was used as an to provide maximum adhesion, while maintaining the electrical conductivity at the electrode material/substrate interface.51 This was prepared in a slurry form and physically coated directly onto the substrate surface. 2.1.4. Chemical Vapor Deposition (CVD). CVD is commonly applied when the porosity of the final product is very crucial. In the graphene synthesis approach, CVD is often described as being able to produce a defect-free graphene structure that possesses a highly conductive network for charge transfer, with the absence of intersheet junction resistance in the continuous network;50,52 however, this method usually requires a template or growth substrate to provide a surface for the attachment of the newly deposited graphene layers. As described by its name, the process is carried out under a vapor phase, whereby the starting material of graphene is initially prepared in vapor form, flowed, and subjected to a very high temperature (800−1000 °C), along with the targeted substrate. The sample will therefore grow on the substrate with a very fine and even morphology.

3. GRAPHENE NANOCOMPOSITE BASED ON TYPES OF ELECTRODE MATERIALS 3.1. Additives/Binder Added Electrodes. 3.1.1. Additives/Graphene Electrodes. In the fabrication of a supercapacitor electrode, carbon black (CB) has often been used as a conductive additive, mainly because of its high conductivity and low cost.27 As reported, a graphene/CB hybrid film was fabricated and utilized directly as a supercapacitor electrode, for which reduced graphene oxide (rGO) was mixed with different amounts of CB and subjected to vacuum filtration to obtain the nanocomposite film. The rGO/CB film was free-standing and possessed flexible properties, as shown in Figure 2a. The presence of CB effectively acted as spacer, which prevented the restacking of rGO layers; however, higher CB content resulted in agglomeration (compare Figure 2c,e). 4156

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also improved the conductivity within the nanocomposite film. Moreover, a solid-state supercapacitor was built based on a hybrid rGO/CB-1.5 composite film. Two identical pieces of film were rolled onto an Au-coated PET substrate, followed by peeling off the membrane. A prepared PVA/H2SO4 polymer gel electrolyte was dripped onto both the electrodes, after which the two electrodes were pressed together for 10 h. The polymer gel electrolyte acted as both the electrolyte reservoir and a medium that enabled the electrodes to remain intact. Remarkably, the CV curve of the fabricated device showed an excellent CV rectangular shape without any interference in both the bent and normal states (Figure 3a), indicating the perfect structural integrity of the fabricated device. A specific capacitance as high as 79.6 F g−1 was recorded at a fast scan rate of 1 V s−1. The cycling stability of the device was tested at a current density of 2 A g−1, with the first 3000 cycles tested in the normal state and the subsequent 2000 cycles in the bent state. The capacitance retention remained at 94%, even after the total of 5000 cycles (Figure 3b). Its performance as a supercapacitor was further confirmed by successfully lighting an LED circuit, as shown in Figure 3c. 3.1.2. Binder/Graphene Electrodes. In a fabrication of a binder-added electrode, mesoporous graphene was utilized by mixing 10 wt % PTFE binder and 5 wt % super-p, which was then made into coin-size capacitor cells.4 The electrodes were separated via a Celgard porous membrane and 1-ethyl-3methyldizolium tetrafluoroborate (EMIMBF4) as the organic electrolyte. The morphology of the curved graphene sheets indicated an ability to prevent close stacking. Therefore, the mesoporous structure was maintained even after being compressed into an electrode structure. The pore size of the graphene structure was within the range of 2−25 nm. In contrast, graphene sheets prepared via conventional chemical reduction methods were said to have a higher tendency to restack with one another, which significantly reduced the effective surface area because the intergraphene gaps were smaller than 1 nm, therefore resulting in a significantly smaller capacitance of