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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 ‡

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 © XXXX 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

A

DOI: 10.1021/acs.jpcc.5b10187 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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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 B

DOI: 10.1021/acs.jpcc.5b10187 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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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 C

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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). D

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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 <50 F g−1. The significance of mesoporous structure possessed by the curved graphene sheets was shown by a high capacitance value of 100−250 F g−1 under a potential window of 4 V at a current density of 1 A g−1. 3.2. Binder-Less Electrodes. Recently, the synthesis of binder-less electrodes was reported in numerous paper. The binder-less properties offer various advantages, including a cost reduction (from eliminating the binder cost) and minimization of the electrical interference between the components at the interface. 3.2.1. Pure Graphene Electrode. A direct fabrication of 2D “in-plane” pristine graphene and reduced multilayer graphene

Figure 2. (a) Digital image of rGO/CB-2. Cross-sectional SEM images of (b) rGO, (c) rGO/CB-1, (d) rGO/CB-1.5, and (e) rGO/CB-2, respectively. Reprinted with permission from ref 27. © 2014 Elsevier.

In terms of electrochemical performance, the hybrid film was tested using a two-electrode configuration, whereby two pieces of identical film and a cellulose ester filter film were soaked in 1 M H2SO4 for 2 h. The ester filter film was then sandwiched between the hybrid film and clamped by titanium (Ti) plates, which acted as current collectors. As expected, typical rectangular-shape cyclic voltammetry (CV) profiles were recorded, which were responsible for the electric double-layer capacitance (EDLC). Interestingly, the rectangular CV shape remained even at a high scan rate of 5 V s−1, therefore showing the high rate capabilities. Similarly, a symmetrical triangular charge/discharge profile attributed to the EDLC was recorded. The fabricated supercapacitor showed a remarkable cycle stability by showing a 100% capacitance retention up to 2000 cycles. The improvement was mainly attributed to the presence of CB, which effectively acted as spacers and prevented the restacking of graphene layers and

Figure 3. (a) CV curves of solid-state supercapacitor on bare PET tested in bent and normal states (5 mV s−1). (b) Cycle life of solid-state supercapacitor on bare PET. (c) LED lighted by three solid-state supercapacitors on bare PET in series. Adopted with permission from ref 27. © 2014 Elsevier. E

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Figure 4. (a) Design and fabrication of flexible, all-solid-state LSG electrochemical capacitor. (b) Bending the device had almost no effect on its performance, as seen in these CVs collected at a scan rate of 1000 mV s−1. Adopted with permission from ref 39. © 2012 AAAS.

100−10 000 mV s−1, with a nearly triangular shape for the CC curves obtained at a high current density of 10 A g−1. In 1 M H2SO4 electrolyte, the supercapacitor showed an area capacitance of 4.04 mF cm−2 at 1 A g−1 and maintained an area capacitance of 1.84 mF cm−2 even at 1000 A g−1. Furthermore, the LSG supercapacitor retained 96.5% of its initial capacitance after 10 000 charge/discharge cycles. The fabricated supercapacitor showed remarkable flexibility, with the bending of the device having almost no effect on its electrochemical performance, as denoted by the CV curve shown in Figure 4b. Instead of utilizing pure solid film, a rather unique approach was used to fabricate a graphene hydrogel for application as a flexible electrode in a solid-state supercapacitor.42 The graphene hydrogel was prepared via a modified hydrothermal reduction method and had a thickness of ∼3 mm. It was immersed in a 1 M H2SO4 aqueous solution overnight. The graphene hydrogel was specifically cut and pressed on a gold-coated polyimide substrate with a pressure of ∼1 MPa to obtain a thin film with a real mass of ∼2 mg/cm2. Similar to previous reports, a solid-state supercapacitor was fabricated by sandwiching a layer of PVA/H2SO4 polymer electrolyte between identical electrodes made of the graphene hydrogel/polyimide film, with an additional mechanical pressure of 1 MPa for ∼30 min, resulting in a very thin layer of separator being formed between the electrodes. The schematic diagram and images of the fabricated supercapacitor device are shown in Figure 5a,b, respectively. The morphology of the 3D graphene framework was well maintained, with a continuous porous network still visible (Figure 5c−f), even after mechanical pressing, which resulted in a decrease in the overall thickness of the film from approximately 3 mm to 120 μm; however, a large difference in the electrical conductivities was reported before and after pressing (from 7.8 to 192 S cm−1). Similar to previous reports, a CV loop close to the ideal rectangular shape was recorded for the as-fabricated supercapacitor, even at a high scan rate of 200 mV s−1, which reflected an excellent capacitive behavior with a low contact resistance. The highest specific capacitance was calculated to be 187 F g−1 at a scan rate of 10 mV s−1. An almost symmetrical triangular shape for the galvanostatic charge/discharges curve was also reported, for which the specific capacitance was calculated to be ∼186 F g−1 at a current density of 1 A g−1 and dropped to ∼130 F g−1 at a fast current of 20 A g−1. The findings further confirmed the excellent conductive network of graphene hydrogel with large specific surface area, while infiltrating the electrolyte into the electrode network. On the contrary, the fabricated device also demonstrated a remarkable flexibility at various bending angles, as shown by the CV curves in Figure 6a. The device suffered a decay of only 8.4% in specific

oxide (RMGO) was reported,36 in which the pristine graphene was synthesized via the CVD technique and the multilayer graphene films were synthesized via the chemical reduction of graphene oxide. The in-plane design involved isolating a large planar sheet of graphene into two electrodes by physically creating a micrometer-sized gap via a scission on the graphene layer and then sputtering the external edges of the two electrodes with gold to allow them to perform as current collectors. An acidic polymer electrolyte (PVA/H3PO4) was spread across the active surface and the gap between the electrodes to serve as an electrolyte reservoir. The combination resulted in a fabricated device with ultrathin, flexible, and optically transparent features. The mass of the G electrode was too small to the point that it has the tendency to cause inaccuracy in the determination of specific capacitance. Therefore, the surface area of the electrodes was taken into account, and the G electrode showed a normalize capacitance of 80 μF cm−2 based on the CV curve; however, the capacitance of the RMGO device was five times higher (390 μF−2), which was attributed to the multiple layered structure of reduced graphene that possessed sufficient interlayer distance for optimal counterions intercalation. Neither device showed a performance degradation, even after 1500 charge/ discharge cycles. A similar supercapacitor was fabricated with the conventional RMGO geometry used for the electrode material. The stacked devices showed a much lower specific capacitance of 140 μF−2, which highlights that a favorable component placement was essential to maximize the effective electrochemical area of the multilayer graphene surface. Meanwhile, a much simpler technique involving the direct laser reduction of graphite oxide films to graphene has been demonstrated.39 A thin film of GO was initially cast onto a flexible substrate, which was followed by direct laser scribing inside a commercially available LightScribe CD/DVD optical drive. The laser-reduced graphene film (LSG) possessed excellent electrical conductivity (1738 Sm−1) and mechanical strength, with only a 1% change in the electrical resistivity after 1000 bending cycles. Therefore, it is directly applied as a supercapacitor electrode without the need of binders. Additionally, the laser scribing technique caused the simultaneous reduction/exfoliation of the graphene sheets, which produced an open network of LSG that effectively prevented agglomeration, while the existing open pores help to facilitate the electrolyte’s access to the electrode’s surface. A typical supercapacitor device was fabricated by sandwiching an ion porous separator or a polymer gel electrolyte between two identical LSG electrodes (Figure 4a), which resulted in a very thin device with a total thickness of <100 μm. The device showed an ideal rectangular CV shape over a wide scan rate of F

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Figure 5. (a) Schematic diagram and (b) photographs of fabrication process for flexible solid-state supercapacitors based on graphene hydrogel films. (c) Low- and (d) high-magnification SEM images of the interior microstructure of the graphene hydrogel before pressing. (e) Low- and (f) highmagnification SEM images of the interior microstructure of the graphene hydrogel film after pressing. Adopted with permission from ref 41. © 2013 Elsevier.

Figure 6. (a) CV curves of flexible solid-state device at 10 mV s−1 for different bending angles. (b) Cycling stability of the device at a current density of 10 A g−1 and (c) Ragone plots comparing the graphene hydrogel film-based supercapacitors to previously reported solid-state devices made of CNTs and graphene thin films. Adopted with permission from ref 41. © 2013 Elsevier.

capacitance after 10 000 charge/discharge cycles at 10 A g−1 under a 150° bending angle (refer Figure 6b), attributed to the flexible framework provided by the graphene hydrogel. Furthermore, a Ragone chart was plotted to compare the performances of solid-state devices with selected examples reported in the literature, as shown in Figure 6c. Similarly, a graphene-cellulose nanofiber (CNF) hybrid aerogel was synthesized and utilized as the electrode material of a flexible supercapacitor.30 The aerogel was produced via the initial acidization of a mixture of CNF, GO nanosheets, and VC-Na by hydrochloric acid vapor. The mixture was stored at 80 °C for 24 h, followed by supercritical CO2 drying to produce the final product. The hybrid aerogel had a 3D porous web-like structure with more wrinkles and rougher structures compared with an ordinary aerogel. After a high electrical conductivity was recorded (100 Sm−1), the RGO−CNF (20%) aerogel film was directly applied as a supercapacitor electrode without the incorporation of a binder or any additives. The identical electrodes were attached and separated via a PVA/H2SO4 polymer electrolyte. The fabricated supercapacitor had a rectangular CV curve at a high scan rate of 100 mV s−1, indicating a typical EDLC property with a rapid current response. The specific capacitance was 207 F g−1 at a scan rate of 5 mV s−1. The electrochemical performance of the hybrid aerogel (A-SC) was compared with that of an ordinary film-based device (F-SC), which only had a capacitance of 188 F g−1 at the same scan rate.

From the discharge curves, A-SC recorded a specific capacitance of 203 F g−1 at a current density of 0.7 mA cm−2. The superior performance of A-SC was mainly attributed to the presence of CNF, which effectively prevented aggregation of the graphene nanosheets while retaining hydroplilicity that promoted accessible specific surface area and facilitating the influx/outflow of electrolyte ions. Furthermore, the fabricated device showed high stability during bending, almost without changing the CV curves upon 100 bending cycles at 180° of bending, and a capacitance retention of 99.1% after 5000 charge/discharge cycles. 3.2.2. Symmetrical Supercapacitor. 3.2.2.1. Conductive Polymers/Graphene Composites Electrode. In one study, graphene/polyaniline (PANI) with a wavy shape was fabricated as a stretchable supercapacitor electrode33 via a series of chemical growth methods, during which the nickel foam template was completely removed, leaving only the porous graphene template behind, and the graphene sheet was subsequently pulseelectrodeposited with PANI. A stretchable supercapacitor device was then fabricated by immersing the identical electrodes in PVA/H3PO4 polymer electrolyte, followed by stacking the electrodes together. Upon completion of the solidification of the polymer gel, the device was embedded into an Ecoflex aqueous solution and cured. The porous graphene possessed a wrinkled structure, with some cracks identified, as shown in Figure 7a,b. As the function of pulse electro-polymerization, the PANI/graphene G

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which complimented the capability of the wavy shape electrode to tolerate with large scale mechanical deformation. On the contrary, a much simpler technique involving filtration was used to produce paper-like composite films of chemically converted graphene (CCG)/PANI nanofiber (PANI-NF).34 In this study, it was claimed that the PANI-NF was uniformly sandwiched between CCG layers, as shown in Figure 9.

Figure 7. (a) Cross-sectional SEM image of the device. (b) Stretchable supercapacitors encapsulated in Ecoflex and Ecoflex/fabric. SEM images of (c) porous graphene and (d) PANI/graphene. Adopted with permission from ref 33. © 2014 RSC.

Figure 9. Cross-sectional SEM images of G-PNF30 (a,b). Adopted with permission from ref 34.

The prepared PANI was reported positively charged in the emeraldine salt form; therefore, it forms a stable composite dispersion with the negatively charged CCG sheets. Additionally, the layered structure of G-PNF30 was attributed to the flow assembly effect of graphene sheets during the filtration process. In addition, the mechanical properties and conductivity of the composite film containing 44 wt % CCG were greatly improved compared with the pure PANI-NF film, with a high capacitance and high cycling stability when fabricated into a supercapacitor. The large rectangular CV areas indicated that the fabricated supercapacitor was dominated by double-layer capacitances. At 0.3 A g−1, the G-PNF30 showed a specific capacitance of 210 F g−1, whereas PANI-NF showed a slightly higher value (214 F g−1); however, the G-PNF30 performed better in relation to the long-term charge/discharge cycle stability, with a retention of 21%, compared with that of PANI-NF (28%), probably because the presence of CCG layers provided a framework for the PANI-NF in sustaining and preventing the fibers from swelling or shrinking during the vigorous cycling. In addition to PANI, polypyrrole (PPy) was directly pulseelectropolymerized onto a graphene surface, forming a uniform dispersion of PPy nanoparticles throughout the graphene structure.28 The electrochemical setup was prepared so that the graphene suspended on a polycarbonate membrane was applied to an aluminum disk as a working electrode, platinum was used as the counter electrode, and SCE was used as the reference electrode. Instead of a continuous polymerization technique, pulse polymerization was applied by varying the pulse

electrode retained its porosity, along with a high specific area. The cross-sectional image shows the total thickness of the fabricated supercapacitor, with ∼100 μm for each electrode and ∼100 μm for the thin electrolyte layer. On the basis of an electrochemical analysis, a huge difference in the current response was noticed for PANI/graphene electrode compared with the bare graphene sample. A pseudocapacitive peak for PANI was noticeable in the CV curve (Figure 8a), indicating that the electrochemical redox reaction occurred. The highest specific capacitance calculated from the charge/discharge curve was 261.24 F g−1 at a current density of 0.38 A g−1 (Figure 8b), complying the pseudocapacitive of PANI. Furthermore, a capacitance retention of up to 89% was recorded after 1000 charge−discharge cycles, tested under 1 mA−2, as shown in Figure 8c. The high performance and stability of charge/ discharge exhibited by the system was closely related to the dense PANI thin film being supported by the porous graphene as a stable and highly conductive structure while contributing the EDLC response for the overall capacitive performance. Nyquist plot showed a near-90° slope behavior, with a very low equivalent series resistance recorded (6.4 Ohm). Furthermore, the stretchable device exhibited an energy density of 23.2 Wh kg−1 at a power density of 399 W kg−1 and potential window of 0.8 V. Interestingly, the PANI/graphene supercapacitor showed only negligible differences upon bending and stretching, indicating that the fabricated device was not susceptible to large bending stress,

Figure 8. (a) Porous graphene electrodes with and without PANI at scan rate of 10 mV s−1. (b) Galvanostatic charge/discharge curves of the stretchable supercapacitor with PANI/graphene electrodes at different current densities. (c) Cycling performance of the stretchable supercapacitor with PANI/ graphene electrodes for charging and discharging at a current density of 1 mA cm−2. Adopted with permission from ref 33. © 2014 RSC. H

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aggregation. The intercalation of the cMWCNT resulted in a sandwich structure, with the cMWCNT uniformly incorporated into RGO nanosheets (Figure 12) with a significantly larger

potential from 0 to 1.05 V at 0.1s intervals for a predefined time. The pulse polymerization technique resulted in small particles of PPy being uniformly deposited throughout the graphene film, as shown in Figure 10. These clusters of PPy maximized the

Figure 12. Cross-sectional FE-SEM images of (a) RGO and (b) RGO/ cMWCNT (scale bars: 500 nm). Adopted with permission from ref 35. © 2014 RSC.

specific surface area of 910 m2 g−1 compared with the pristine RGO (790 m2g−1). For the CNP/PPy composites, it was found that the surface of the carbon fiber was coated with PPy, with the thickness of the coating heavily dependent on the deposition time frame. The surface of the PPy layer possessed a rugged morphology with nanoparticles of PPy observed (20−50 nm in diameter); however, increasing the deposition time resulted in the aggregation of the nanoparticles into bigger particles with a scale of 0.1 to 1 μm. In addition, both of the prepared electrodes were evaluated using a three-electrode configuration to estimate their effective potential windows. The RGO/cMWCNT electrode showed a close-rectangular CV profile from −0.9 to −0.1 V, whereas the CFP/PPy-900s electrode showed a closerectangular CV curve at a potential window of −0.1 to 0.7 V. A detailed description of the fabricated asymmetric supercapacitor device was discussed further in the Section 3.2.3. 3.2.2.2. Metal/Metal Oxides Composite Electrode. Besides the combination of graphene and conductive polymers, a rather unique method of preparing MnO2-coated graphene fiber was carried out using the direct electrochemical deposition of MnO2 onto a graphene fiber network.37 The 3D nanostructure of graphene that served as highly conductive backbones with high surface area was further enhanced with the deposition of nanoflowers (Figure 13). Therefore, the overall network possessed exceptionally large surface area that facilitated electrolyte ions with high accessibility. A flexible solid-state device was then fabricated by intertwining two MnO2/G/GF electrodes separated by a PVA/H2SO4 polymer gel electrolyte.

Figure 10. SEM images of (a) pure G and G/PPy after electrodeposition for (b) 60, (c) 120, and (d) 360 s. The white particles are the PPy, and the white bar is 1 μm. Reprinted with permission from ref 28.

exposure of the surface area on the graphene sheets and significantly improved the pseudocapacitive contribution to the composites, a part of the graphene film responsible toward the large rectangular curve. It was reported that a G/PPy composite prepared using a deposition time of 120 s showed the best performance by recording a specific capacitance of 237 F g−1 based on the CV graph (tabulated in Figure 11a), attributed to a high particle density, while maintaining a relatively low particle size, leading to a large redox-active surface area. Similar performance was also recorded in the charge/discharge analysis (Figure 11b). On the contrary, an asymmetric supercapacitor was fabricated by preparing reduced graphene oxide/carboxylated multiwalled carbon nanotube (cMWCNT) as a negative electrode using a chemical reduction technique, whereas the positive one was replaced by carbon fiber paper electrodeposited with PPy.35 Graphene remained the candidate of the negative electrode because of the large surface area it possessed, high electrical conductivity, and superior flexibility; however, because of its restacking behavior, it was added with CNT to inhibit

Figure 11. (a) Specific capacitances of all G/PPy electrodes by electrodeposition time, as determined by CV with different scan rates. (b) Galvanostatic charge/discharge curves for the G, G/PPy 60, G/PPy 120, and G/PPy 360 electrodes for comparison at a current density of 1 A g−1 between 0.4 and 0.6 V versus SCE in 1 M KCl. Reprinted with permission from ref 28. I

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Figure 13. (a) SEM images of graphene fiber (GF), (b) G/GF, and (c) MnO2/G/GF with MnO2 deposition time of 40 min. Adopted with permission from ref 37. © 2014 Elsevier.

Figure 14. (a) CV at a scan rate of 10 mV s−1 and (b) charge/discharge curves of the fiber capacitor at a current of 1 mA, with an effective length of 0.5 cm under straight and bent conditions. (c) CV at a scan rate of 25 mV s−1 and (d) charge/discharge curves of 1.3 cm long MnO2/G/GF fiber supercapacitor at 2 mA of applied current, with different straight/bending cycles. Reprinted with permission from ref 37. © 2014 Elsevier.

equally spaced (0 0 L) reflections, indicating an ordered layer-bylayer stacking (Figure 15C). In addition, the curled fringe of individual sheets showed parallel-aligned dark lines with a spacing of 5.15 Å (Figure 15D,E). An all solid-state flexible supercapacitor was fabricated by transferring the β-Ni(OH)2/ graphene film to a gold-coated PET substrate, followed by covering the surface of the working electrode with the PVA/ KOH polymer electrolyte and then laying another identical electrode on the top. Very distinctive redox current peaks were recorded on the CV curves over the potential of −0.6 to 0.6 V, regardless of the scan rates applied (see Figure 16a), corresponding to the reversible pseudocapacitive reaction of Ni(OH)2, as shown in eq 8.

Besides acting as the separator, the coating of the gel electrolyte provided a supporting structure to allow the rapid bending and twisting of the fabricated device without suffering an undesirable short circuit of the two electrodes. Moreover, when deformation was applied to the fiber capacitor, the lack of interference was shown by the nearly rectangular shape of the CV, without any distortion, with similar results obtained in the galvanostatic charge/ discharge analysis (Figure 14a,b). Remarkably, the fabricated device retained a stable capacitance of approximately 70−73 μF after 1000 cycles of the straightening−bending−straightening process, as shown in both the CV profiles and charge/discharge curves (Figure 14c,d). On the basis of the surface area, the device showed a specific capacitance of 9.1 to 9.6 mF cm−2; however, an asymmetric device fabricated utilizing MnO2/G/GF as the positive electrode and G/GF as the negative electrode did not show a promising specific capacitance value in either an acidic (PVA/H2SO4) or neutral electrolyte (PVA/NaCl), with values of 1.6 and 0.1 mF cm−2, respectively. Nanohybrid of β-Ni(OH)2/graphene was prepared via a solvothermal reaction, with a unique layer-by-layer characteristic structure obtained.38 The composite consists of nanosheet graphene morphology, with the scale of the sheets in the range of several micrometers (Figure 15A,B). The unique structure was further confirmed by an XRD analysis, with a new series of peaks emerging with 2θ values of 8.61, 17.31, and 26.01°, which are

Ni(OH)2 + OH− ⇌ NiOOH + H 2O + e−

(8)

The fabricated device had a specific capacitance of up to 3304 μF cm−2 calculated from the discharge slope at a current density of 0.1 Am−2. Increasing the current density by ten times resulted in a capacitance retention of 64.2% of its original value, 2120 μF cm−2. Furthermore, the measured galvanostatic charge/ discharge curve for the bending configuration did not change obviously compared with the extending configuration, which showed a remarkable flexibility performance. The continuous bending/extending of the fabricated device did not degrade the J

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each, separated by a polymer separator (Figure 17a). The as-prepared device was lightweight (<10 mg), thin (∼0.8 mm), and had high mechanical flexibility. It showed a rectangular CV curve, even at a high scan rate of 1000 mV s−1. Furthermore, the CV curve at 10 mV s−1 was retained even upon bending the flexible device, which significantly demonstrated the flexible properties of the prepared electrodes (see Figure 17b). The device possessed a low series resistance at 6.4 ohm (Figure 17c), with energy and power densities of 6.8 Wh kg−1 and 62 W kg−1 for a 1 V potential. Only a minor drop in specific capacitance was detected after 500 charge/discharge cycles, with the Coulombic efficiency remained at 93%. Interestingly, the supercapacitor was able to retain 92% of its initial capacitance even, after 200 times of bending cycles at a constant bending angle of 90° (Figure 17d). Another attempt at fabricating an MnO2-rGO nanocomposite on a flexible carbon fiber paper (CNP) was reported in the literature.41 rGO nanosheets were prepared using a commonly applied chemical reduction method, whereby MnO2 nanoparticles were produced via ethylene-glycol-assisted synthesis, followed by hydrolysis and condensation reactions.55 The rGO and MnO2 produced were premixed in an ethylene glycol solution, stirred, and coated on the CNP using a simple spraycoating technique. A flexible supercapacitor was fabricated by having the MnO2-rGO/CFP act as symmetrical electrodes, sandwiching NaNO3/PVA, which acted as the polymer gel electrolyte and separator itself; however, the electrochemical performance of the fabricated device was not determined. As shown in the TEM images, the MnO2-rGO composite was composed of tiny MnO2 nanoparticles sitting on an rGO surface (Figure 18a) with high porosity among the adjacent MnO2-rGO nanosheets, which were coated on CFP, as depicted in Figure 18b. The electrochemical performance of the electrode material was tested in 0.5 M Na2SO4 electrolyte at a potential of 0 to 1 V versus an Ag/AgCl electrode, and the MnO2-rGO electrode showed a box-shaped CV curve at a scan rate of 10 mV s−1, indicating excellent capacitive behavior supported by a very high specific capacitance of 393 F g−1 based on the discharge curve at a current density of 0.1 A g−1. The excellent capacitance performance was attributed to the combination of large surface area provided by the MnO2 particles, along with the high conductivity of the rGO nanosheets.56 The high conductivity of the MnO2-rGO electrode was further confirmed by a low polarization resistance (∼2 Ω cm−2), as evaluated via an EIS measurement. The performance of the solid-state as-fabricated supercapacitor was demonstrated by its ability to spin up a 3 V motor; however, no flexibility demonstration was conducted to fully describe its bent capability.

Figure 15. (A) Panoramic FE-SEM image of as-prepared b-Ni(OH)2/ graphene nanohybrids. (B) TEM image of the as-prepared nanohybrids confirming the nanosheet morphology. (C) XRD pattern of the as-prepared b-Ni(OH)2/graphene nanohybrids. (D) Cross-sectional HR-TEM image of the curled fringe of the nanohybrid sheet giving the interlayer spacing of 5.15 Å. (E) Enlarged view of the HR-TEM image and the structural model of the layer-by-layer nanohybrids. Reprinted with permission from ref 38. © 2013 Elsevier.

overall electrochemical performance, even after 500 cycles (Figure 16b,c). While some active materials did not possess excellent conductivities for direct application as current collectors, a flexible substrate such as nickel foam has often been utilized both as a flexible framework and the current collector itself. For example, a graphene-based flexible electrode coated with MnO2 was fabricated on a mechanically pressed nickel foam.40 A uniform graphene layer was initially deposited on the nickel surface via the atmospheric pressure CVD method. A foam-like graphene structure (thickness <200 μm) was then obtained after the removal of the nickel foam, which was further electrodeposited with MnO2. A flexible supercapacitor was fabricated using two graphene/MnO2 composite electrodes with 0.4 mg/cm2

Figure 16. (a) CV curves of ASSTFS measured at different scan rates of 100, 200, and 500 mV s−1. (b) Galvanostatic charge/discharge curve of the ultraflexible ASSTFS measured under bending configuration. (c) Cycling stability of the ultraflexible ASSTFS measured after repeated bending/ extending deformation. Reprinted with permission from ref 38. © 2013 Elsevier. K

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Figure 17. (a) Schematic of structure of our flexible supercapacitors. The two digital photographs show the flexible supercapacitors when bent. (b) CVs of the flexible supercapacitors at various scan rates. Inset shows the CVs of the flexible supercapacitors with bending angles of 0 and 90° at a fixed scan rate of 10 mV s−1. (c) Nyquist plot of the flexible supercapacitor. (d) Cycling performance of the flexible supercapacitors. Inset shows the cycling performance of the flexible supercapacitors for bending cycles with a bending angle of 90°. Reprinted with permission from ref 40.

macropores (Figure 19a); however, it had a flat morphology with randomly crumpled Ag NWs dispersed over the surface of the OMC film (Figure 19a, inset). The TEM image revealed that highly ordered hexagonal patterns could be found on the graphene sheets (Figure 19b), directly indicating the uniform coating of OMC on the graphene network. An electrochemical capacitor was fabricated by sandwiching a filter paper (soaked in 6 M KOH) between identical pieces of the flexible electrode, encapsulated by two pieces of PET substrate. The device showed a nearly rectangular CV curve over a wide range of scan rates, and even at a high scan rate of 0.1 V s−1 there was very little distortion (Figure 19c), which agreed with the low ESR value by the EIS measurement (ESR = 0.7 Ω). The galvanostatic charge/ discharge profile also showed a symmetrical characteristic with a very quick I−V response, with the specific capacitance for Ag-GF-OMC as high as 213 F g−1, which was higher than those of GF-OMC (139 F g−1) and pristine OMC (36 F g−1) (Figure 19d). The flexibility performance of the device was demonstrated by CV measurements at a scan rate of 0.05 V s−1 and a bending angle of 90°, where no significant difference could be detected from that without bending (Figure 19e). Furthermore, 92% of its initial capacitance was retained after 200 bending actions, indicating its superb flexibility. A Ragone plot also indicated that the device exhibited an energy density of 4.5 W h kg−1 at a power density of 250 W kg−1. Moreover, hybrid fibers that combined CNTs and 2D graphene (CNT/G) were achieved by the initial intercalating of graphene with Fe3O4 nanoparticles, followed by CNT growth using a CVD technique.44 The prepared CNT/G fibers

Figure 18. TEM images of (a) MnO2-rGO nanocomposite and (b) lowmagnification SEM image of MnO2-rGO coated on CFP. Adopted with permission from ref 42.

An ordered mesoporous carbon-based electrode was fabricated by the direct coating of an ordered mesoporous carbon (OMC) precursor on a 3D graphene foam with the addition of silver nanowire (Ag NW) to obtain an Ag NW/3D-graphene foam/OMC (Ag-GF-OMC)-based electrode.43 The fabricated electrode was said to combine the advantages of a 3D graphene foam and Ag NWs to overcome the poor electrical conductivity and rigidity of raw OMC, with the synthesized electrode showing a conductivity of up to 741 S cm−1. The 3D graphene foam was synthesized via CVD on a nickel foam, followed by the removal of the nickel substrate. The growth of OMC on the graphene foam was done through a solvent evaporation induced selfassembly process, followed by carbonization at 350 °C. The final step involved the decoration of Ag NWs on the surface of the carbon networks.57 The SEM image of the Ag-GF-OMC was composed of a 3D interconnected framework with random open L

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Figure 19. (a) SEM of Ag-GF-OMC, with inset illustrating morphology of surface of Ag-GF-OMC. (b) TEM of OMC film on 3D graphene foam. (c) CVs of Ag-GF-OMC flexible supercapacitor at scan rates of 0.03, 0.05, 0.07, and 0.1 V s−1. (d) Galvanostatic charging/discharging curves of the flexible supercapacitor device at different current densities. (e) Comparison of CV curves at 0.05 V s−1 for Ag-GF-OMC flexible supercapacitor tested under normal and bent conditions. Adopted with permission from ref 43. © 2014 Wiley.

Figure 20. (a,b) SEM images of G/CNT fiber and enlarged surface, respectively. (c,d) Cross-sectional view of the CNT/G fiber with different magnifications. Adopted with permission from ref 44. © 2013 RSC.

possessed a fluffy surface and were composed of highly entangled CNTs (Figure 20a,b). Furthermore, the CNT growth occurred on both the surface and within the fiber itself with increasing fiber diameter, as denoted by the cross-sectional view of the deliberately broken CNT/G fibers (Figure 20c,d). In the construction of a flexible device, the fibers were knit into textile electrodes, and a thin flexible supercapacitor was fabricated by sandwiching a filter paper (soaked in 1-M Na2SO4) between identical knitted CNT/G textile electrodes (Figure 20a,b). A thin gold-coated PET acted as the flexible substrate and also as a current collector. A near-rectangular CV shape was recorded for the device at various scan rates (0.01 to 10 V s−1), with straight triangular charge/discharge curves indicating an excellent EDLC behavior (Figure 21c,d). The weight-specific capacitance was calculated to be ∼200.4 F g−1, while the area capacitance was 0.98 mFcm−2, both at a current density of 20 μAcm−2. Interestingly, the near-rectangular CV curves of the fabricated device did not change between the bent and flat states (Figure 21e); however, the device somehow showed an initial decrease in specific capacitance and leveled off at a stable value of ∼0.4 mF cm−2 up to 1000 cycles, similar to that of the flat-tobending cycles (see Figure 21f).

3.2.3. Asymmetric Supercapacitor. A combination of CNT and Mn3O4 nanoparticles with graphene was fabricated as an electrode paper,32 whereby the CNT or Mn3O4 nanoparticles were uniformly intercalated within the graphene nanosheets, giving rise to a composite paper possessing excellent mechanical stability, greatly improved active surface areas, and enhanced ion transportation. rGO nanosheets were produced via a conventional hydrothermal reaction at 180 °C for 12 h, whereas the CNT was functionalized by refluxing in concentrated HNO3 for 6 h to introduce the carboxylic groups.58 Mn3O4 nanoparticles were directly synthesized via the chemical oxidation of a manganese acetate precursor in NaOH solution in the presence of polyethylene glycol.59 The electrode paper was fabricated via a conventional vacuum filtration technique involving the mixture of the active materials through a cellulose acetate membrane, followed by washing, air drying, and peeling off from the filters to obtain the free-standing composite paper (refer to Figure 22a,c). In addition to the findings on symmetrical devices, an asymmetric solid-state supercapacitor was fabricated by applying both the (CNT-graphene) CNTG and (Mn3O4-graphene) MG paper electrodes with a potassium polyacrylate (PAAK)/potassium M

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Figure 21. (a) Schematic illustration of flexible supercapacitor using CNT/G textile fibers as electrodes. (b) Photograph of the fabricated textile supercapacitor. (c) CV curves of the supercapacitor with the CNT/G textile fibers under various scan rates. (d) Galvanostatic charge/discharge curves at various current densities. (e) Influence of flat and bending state on the CV curves of the fabricated supercapacitor at a scan rate of 100 mV s−1. (d) Durability test of the textile supercapacitor undergoing repeated flat-to-bending cycles. Adopted with permission from ref 44. © 2013 RSC.

after 10 000 continuous charge/discharge cycles and was not even altered by being repeatedly bent and twisted at various angles (Figure 23b,c). Meanwhile, another asymmetrical flexible device was fabricated by utilizing an ionic liquid functionalized-chemically modified graphene film (IL-CMG) as the negative electrode, while the positive electrode comprised a hydrous RuO2-IL-CMG composite film, in which both electrodes were prepared by filtering a homogeneous solution of active materials through an anodizc membrane filter. The resulting films were air-dried and peeled off to form the self-supporting films.29 The electrodes were separated by a PVA/H2SO4 polymer gel electrolyte, as shown in Figure 24a. The graphene film was modified by adding GO to the ionic liquid via an ordinary solution mixing technique, aided by sonication for 60 min, followed by hydrazine reduction at 95 °C for 6 h. The RuO2-IL-CMG was prepared using a sol− gel process with an additional controlled thermal treatment (Figure 24b). It was proven via TEM images that the RuO2 NPs were uniformly distributed along the IL-CMG surface (Figure 25a,b (before and after, respectively)), which was further supported by an XPS study of the composite itself. Because of the application of asymmetric electrodes, the cell voltage was expanded up to a potential window of 1.8 V (initially approximately 1 V for each electrode), with a very stable galvanostatic charge/discharge cycle. Furthermore, a slight curvature of the slopes was noticed as an indication of the pseudocapacitive effects of RuO2. The specific capacitance calculated for the asymmetric device was up to 167 F g−1 at 1 A g−1, taken from the discharge cycles. The value recorded was about twice that of a symmetric device (85 F g−1), which was ascribed to the pseudocapacitive contribution of RuO2 to the overall CMGbased EDLC and the expanded working voltage. Maximum energy and power densities of 19.7 W h kg−1 and 0.5 kW kg−1, respectively, were recorded at a current density of 0.5 A g−1. Interestingly, the fabricated asymmetric supercapacitor retained 97% of its initial capacitance over 600 charge/discharge cycles.

Figure 22. (a) Photograph showing the free-standing CNTG-40 paper and (b) cross-sectional SEM image of rGO paper. (c) Photograph of MG-50 paper flexible enough to be folded up. (d) Cross-sectional SEM images of MG-50 paper. Reprinted with permission from ref 32.

chloride (KCL)-based polymer gel electrolyte. The combination using MG as the positive electrode and CNTG as the negative electrode successfully resulted in a supercapacitor device with a much wider operating voltage range of up to 1.8 V (0.8 V for both symmetric devices). Furthermore, its excellent capacitive behavior was indicated by an almost symmetric triangular shape for the charge/discharge curves within the potential range, with a specific capacitance of 72.6 F g−1 at 0.5 A g−1 (Figure 23a). The capacitance value of 50.8 F g−1 was retained even at a high current density of 10 A g−1. The energy density of the asymmetric device was reported to be 22.9 Wh/kg when the power density was up to 9.0 kW kg−1, signifying that the device could simultaneously provide a high energy density and high power density. The device retained up to 86.0% of its initial capacitance N

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Figure 23. (a) Galvanostatic charge/discharge curves and (b) specific capacitance retention ratio of asymmetric supercapacitor at normal, bent, and twisted states and after being bent repeatedly, as shown in (c) (number of times indicated). Adopted with permission from ref 32.

Figure 24. Schematic diagrams of (a) all-solid-state flexible thin a-SC and (b) experimental procedure for synthesis of water-soluble IL-CMG and RuO2-IL-CMG hybrids. Reprinted with permission from ref 29. © 2012 RSC.

Another type of asymmetric supercapacitor was reported at which it utilizes a reduced graphene oxide/carboxylated multiwalled carbon nanotube (rGO/CMWCNT) hybrid film and also a carbon fiber paper-supported polypyrrole (CFP/PPy) composite film, which were prepared using a vacuum filtration technique and electrochemical deposition, respectively.35 Then, an asymmetric solid-state supercapacitor device was fabricated using the RGO/CMWCNT as the negative electrode, with CFP/ PPy as the positive electrode. The electrodes were separated by a PAAK/KCL polymer gel electrolyte. Because of the application of asymmetric electrodes, the potential window of the solidstate device was extended up to 1.8 V (Figure 27a), at which a stable CV curve with a nearly rectangular shape was recorded. Furthermore, a stable linear relation was observed for the galvanostatic charge/discharge curve; however, the potential

Figure 25. (a) TEM image of IL-CMG sheets. (b) TEM image of RuO2−ILCMG hybrids. Reprinted with permission from ref 29 © 2012 RSC.

The same device was then twisted, and, remarkably, 95% of the capacitance was still retained even after 2000 cycles, as illustrated in Figure 26. O

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device was demonstrated by the CV curves remaining almost unaltered when collected under various curvatures, indicating that bending the device had nearly no effect on its capacitive performance, as shown in Figure 27f.

4. TRENDS, CHALLENGES, AND FUTURE TASKS Graphene-based materials with various microstructures signify very promising materials for energy-storage purpose. The crucial factors that dictate the selection of electrode materials for flexible supercapacitor applications include high surface area, controlled pore size for accessibility of electrolyte ions, excellent electrical conductivity, and high mechanical strength to maintain a stable framework upon external bending stress. Numerous efforts were accomplished to improve the electrode materials including the combination of EDL capacitance with reversible pseudocapacitance active materials, utilizing organic electrolyte or ionic liquids to achieve higher stability potential window or employing hybrid asymmetric system to optimize the overall working potential. Polymeric gel electrolyte was often employed in deriving a high flexibility device, complimenting to the ability of gel structure to penetrate efficiently to the porous electrode to the nearest contact, while providing a stable structure in maintaining the dimension that is not susceptible to the physical bending force. Nevertheless, it must be kept in mind that with the excellent flexibility achieved, the crucial advantages of supercapacitor with high rate capability and long life cycle must be well maintained, while the effort in the development of a high energy system is continuous. Despite all of the achievements and excellent progress, one of the main obstacles in supercapacitor technology is the high production cost when compared with other energy-storage devices. Therefore, the future tasks should focus toward the

Figure 26. Long-term cycling stability of a-SC devices under normal, twisted, and bent states with a constant current density of 1 A g−1 over 2000 cycles. Adopted with permission from ref 29. © 2012 RSC.

window was not more than 1.6 V, with a slight deviation of the triangle occurred when the cell voltage was increased to 1.8 V (Figure 27b). The fabricated device was reported to retain up to 93% of its initial specific capacitance after 2000 cycles at a current density of 1 A g−1, directly implying its electrochemical stability (Figure 27c). The specific capacitance was calculated to be 82.4 F g−1 at 0.5 A g−1, while the energy density was 28.6 W h kg−1 at a power density of 395 W kg−1. Furthermore, the device was able to retain its specific capacitance of 57.1 F g−1 at a high current density of 10 A g−1, with the energy density remaining at 15.1 W h kg−1 at a power density of 6.9 kW kg−1. The trends for both the specific capacitance and energy and power densities are illustrated in Figure 27d,e, respectively. The flexibility of the

Figure 27. (a) Cyclic voltammograms at different potential windows at scan rate of 20 mV s−1. (b) Galvanostatic charge/discharge curves at different potential windows at a current density of 1 A g−1. (c) Cycle performance. (d) Specific capacitance as a function of current density. (e) Ragone plot and (f) cyclic voltammetry curves of CFP/PPy//RGO/cMWCNT ASC at different curvatures of 0, 30, 60, and 90°. Adopted with permission from ref 35. © 2014 RSC. P

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development of graphene materials with high capacitance performance in a cost-effective route. One-step synthesis without any additional treatment would be favorable toward practicality on an industrial scale. On the contrary, although the reported devices possessed excellent flexibility with undisrupted performance, they relied heavily on polymeric electrolyte in maintaining the overall device workability. Thus, the ability of the fabricated supercapacitors to perform at elevated temperature environment remains doubted as the gel separator eventually degrades. Further work should focus on overcoming the structural collapse of polymeric gel separators for the application of high-temperature operating devices.

5. CONCLUSIONS Despite the rapid development of flexible supercapacitors, numerous challenges remain to be tackled and taken into consideration to develop high-performance and cost-effective flexible devices. For example, although a graphene-hydrogelbased solid-state supercapacitor has been successfully developed without the application of binders or additives,35 the need for a flexible substrate coated with a high-conductivity component (Au-coated) indirectly increased the production cost. On the contrary, numerous attempts to fabricate composite electrodes have been reported, with the successful fabrication of flexible devices via the direct utilization of a free-standing composite film, without the addition of binders or additives.35,37,40,42,45,46 The workability of this technique heavily relies on the electrical conductivity of the composite material itself, which is essential to ensure that the maximum current with minimal resistance flows through the electrode interfaces. Additionally, the mechanical strength of the films is of significance to provide a strong structure to withstand the stress load on the fabricated devices during bending or twisting. Third, the inclusion of graphene as one of the electrode components has successfully prevented mechanical rupture as a consequence of the expansion and contraction of the electrode materials during the heavy inflow/outflow of electrolyte ions during continuous charge/discharge cycles. Most of the fabricated solid-state devices demonstrated excellent cycle stability, from a minimum of 1000 up to 5000 continuous cycles. Furthermore, the presence of a ternary component such as carbon black,27,41,51 CNT,35,60 or metal oxides,40,41,61,62 which were described to aid in maintaining a 3D framework of graphene, provided a maximum surface area for the infusion of electrolyte ions and maximized the capacitance achieved. Overall, this Review Article of graphene-based flexible solidstate supercapacitor devices illustrated their electrochemical performance in various physical bending states. Their application in future electronic devices requires fabricated supercapacitors that are thin, flexible, lightweight, and capable of being rolled up. Furthermore, this Review Article emphasized the significance and uniqueness of graphene in the fabrication of flexible components, indirectly generating a new research goal involving the fabrication of ultimate performance flexible energy-storage devices.



W. K. Chee received his B.Sc. degree in Industrial Chemistry from Universiti Putra Malaysia in 2010. He received his M.Sc. degree in polymer chemistry from the same institution in 2012. He is currently a full-time Ph.D. student under the supervision of Dr. H. N. Lim, focusing on the development of flexible graphene-based energy storage devices.

H. N. Lim received her B.Sc. and M.Sc. degrees from Universiti Kebangsaan Malaysia in 2002 and 2004, respectively. She was awarded a Ph.D. degree in chemistry from Universiti Putra Malaysia in 2010. She was an Assistant Professor at the University of Nottingham Malaysia Campus and a Senior Lecturer at the University of Malaysia before joining Universiti Putra Malaysia as a Senior Lecturer. Since 2009, she has been actively involved in graphene-related research, encompassing synthesis of graphene-based nanomaterials and their applications.

AUTHOR INFORMATION

Corresponding Author

Z. Zainal received a B.Sc. degree in Chemistry from Universiti Kebangsaan Malaysia in 1985. He received his Ph.D. degree from UMIST, U.K. He is appointed as the Deputy Dean, School of Graduate studies of Universiti Putra Malaysia since 2009. He is a professor at the

*E-mail: [email protected] Notes

The authors declare no competing financial interest. Q

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Department of Chemistry in Universiti Putra Malaysia. His research interests includes synthesis of semiconducting materials for energy storage and photocatalysis.

REFERENCES

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N. M. Huang received his B.Sc., M.Sc., and Ph.D. degrees from Universiti Kebangsaan Malaysia. He joined the University of Malaya in 2009 as a Senior Lecturer at the Department of Physics, Faculty of Science. He started working on graphene and graphene-related materials in the same year and has since applied graphene in various fields such as solar energy conversion, energy storage, and sensors.

I. Harrison graduated from the University of Nottingham in 1982 with a degree in Physics. He then received his Ph.D. at the University of Manchester. He is a professor at the Faculty of Engineering in the University of Nottingham Malaysia Campus. He is currently the Dean of Engineering at the same institution. His research has focused on materials, devices, and circuits for radio frequency and microwave applications. More recently, he has been working on materials for electrical energy storage. Y. Andou received his B.Eng. and M.Sc. in polymer chemistry from Miyazaki University, Japan in 1996 and 1998. He received his Ph.D. in organic photochemistry from the same institution. He is currently an associate professor at the Kyushu Institute of Technology, Japan. His research interests include organic synthesis, polymeric composites, and functionalization of graphene materials.



Review Article

ACKNOWLEDGMENTS

This work was supported by a Newton-Ungku Omar Fund (6386300-13501) from British Council and MIGHT, and a High Impact Research Grant (UM.C/625/1/HIR/MOHE/05) from the Ministry of Higher Education. R

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