Graphene-based materials for flexible supercapacitors.

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Graphene-based materials for flexible supercapacitors Yuanlong Shao,ab Maher F. El-Kady,ac Lisa J. Wang,a Qinghong Zhang,b Yaogang Li,*d Hongzhi Wang,*b Mir F. Mousaviae and Richard B. Kaner*af The demand for flexible/wearable electronic devices that have aesthetic appeal and multi-functionality has stimulated the rapid development of flexible supercapacitors with enhanced electrochemical performance and

Received 20th September 2014

mechanical flexibility. After a brief introduction to flexible supercapacitors, we summarize current progress

DOI: 10.1039/c4cs00316k

made with graphene-based electrodes. Two recently proposed prototypes for flexible supercapacitors, known as micro-supercapacitors and fiber-type supercapacitors, are then discussed. We also present our perspective

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on the development of graphene-based electrodes for flexible supercapacitors.

a

Department of Chemistry and Biochemistry and California NanoSystems Institute, University of California, Los Angeles (UCLA), Los Angeles, California 90095, USA b State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Material Science and Engineering, Donghua University, Shanghai, 201620, China. E-mail: [email protected]; Fax: +86-021-67792855; Tel: +86-021-67792881 c Department of Chemistry, Faculty of Science, Cairo University, Giza 12613, Egypt d Engineering Research Center of Advanced Glasses Manufacturing Technology, Ministry of Education, Donghua University, Shanghai, 201620, China. E-mail: [email protected]; Fax: +86-021-67792855; Tel: +86-021-67792526 e Department of Chemistry, Tarbiat Modares University, Tehran, Iran f Department of Materials Science and Engineering, UCLA, Los Angeles, California 90095, USA. E-mail: [email protected]; Fax: +1-310-2064038; Tel: +1-310-8255346

Yuanlong Shao received his BS degree from Changsha University of Science and Technology majoring in Inorganic Materials Science and Engineering in 2010. He is now a joint PhD candidate in the Department of Chemistry and Biochemistry at UCLA and in Materials Science and Engineering at Donghua University. His current research interests focus on graphene-based materials for energy storage devices. Yuanlong Shao

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1. Introduction The emergence of a myriad of recently launched flexible electronic concepts, prototypes, and products (Fig. 1), implies that flexible devices are leading the next revolution in electronics. This has sparked a lot of research efforts on flexible electronics,1–5 including flexible displays,6–8 electronic skins,9–11 curved smart phones,12–14 and implantable medical devices15 for wearable and multifunctional applications. The rapid development of flexible mobile phones exemplify the growing field of flexible electronics. As a result of the invention of flexible organic light emitting

Maher F. El-Kady received his PhD degree in Chemistry from the Kaner Lab at UCLA in 2013 where he is currently pursuing postdoctoral research. His work involves exploration of new materials for energy storage applications. His research was highlighted in Scientific American, USA Today, National Geographic, The Daily Mail and named the KCET most popular story of 2013. He has received numerous awards for his research Maher F. El-Kady work including the Herbert Newby McCoy Award from UCLA, the Cairo University Presidential Award for Excellence in Research, the 2012 Award for Excellence in Research from the Ministry of Higher Education of Egypt, and participant in the 59th Lindau Nobel Laureates Meeting. He spoke at TEDxCairo on the future of supercapacitors and how they could revolutionize the next generation of electronics. He was nominated by OnIslam.net as the Muslim Star of the year 2013 in the field of science.

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diode (OLED) displays, companies such Samsung, LG, and Apple, are all promoting their new designs for bendable mobile phones. LG electronics launched what they call the world’s first flexible smartphone: the six-inch, gently-curving G Flex.12 The almost crescent-shaped design of LG’s G Flex is based on curving its horizontal axis at a gentle angle. Despite these advances, the real bottleneck hindering flexible electronics from becoming ubiquitous in practical products is in constructing flexible and deformable energy storage devices.

Yaogang Li obtained his PhD in Materials Science and Technology from the Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS) in 2003. Prior to earning his PhD, he worked as an Associate Professor in Taiyuan University of Technology between 1991 and 2000. In 2003, he joined the College of Materials Science and Engineering, Donghua University and has been working as a full professor since then. Currently, he Yaogang Li is also the Vice President of the Shanghai Education Evaluation Institute (SEEI) and the Secretary of the Asia-Pacific Quality Network (APQN). His current research interests include low-dimensional carbon materials (including carbon nanotubes and graphene) and their composites for novel magnetic, electronic and energy storage devices.

Hongzhi Wang received his PhD in 1998 from the Shanghai Institute of Ceramics, Chinese Academy of Science (SICCAS) and joined the State Key Lab of High Performance Ceramics and Superfine Microstructure as an assistant researcher for two years. Afterwards, he spent two years as a post-doctoral researcher in Kyushu Center of the National Institute of Advanced Science and Technology (AIST) in Japan Hongzhi Wang with a Science and Technology Agency (STA) fellowship. He then worked as an AIST fellow in the Micro-space Chemistry Lab. Since 2005, he has been working as a full professor and vice-dean of the college of Materials Science and Engineering at Donghua University. He leads a research group of over 30 graduate students and his current research interests include graphene and its composites for electronic and energy storage devices, electrochromic devices based on transition metal oxides and conducting polymers and novel functional fibers.

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Current electronic devices are still too heavy, thick and bulky to match flexibility requirements. In order to realize the needed mechanical properties, energy storage devices that are light, thin and flexible must be developed. For this reason, various energy storage and power sources, such as flexible supercapacitors,22–39 and thin lithium-ion batteries34,40–46 with diverse sizes, shapes, and mechanical properties are being developed.47,48 These energy storage devices are required to keep high power and energy deliverability under continuous mechanical deformation, such as bending, twisting, stretching, even for long time cycles. The key challenge is to design and fabricate electrode materials with robust mechanical flexibility, high energy density, power density and excellent cycling stability, combined with compatible electrolyte and separator in a flexible assembly. In addition, safety is another important consideration, especially for wearable devices. Supercapacitors, also known as ultracapacitors, are energy storage devices that can often be safely charged or discharged in seconds with extremely long cycle life (4100 000 cycles).49–51 Combined with the properties of high power density (often 410 000 W kg 1) and simple structures, supercapacitors are one of the most promising candidates for flexible energy storage devices. As shown in Fig. 2A, a conventional supercapacitor typically consists of a positive and a negative electrode kept apart by an electrically insulating separator with ion conducting electrolyte. The electrodes are one of the key components of a supercapacitor. For commercial supercapacitors, they are generally fabricated with a slurry casting method that requires mixing active material in powder form with polymer binders and conductive additives.52,53 The additives are needed to improve the electrical conductivity between particles of the active material

Richard B. Kaner received a PhD in inorganic chemistry from the University of Pennsylvania in 1984. After carrying out postdoctoral research at UC Berkeley, he joined UCLA in 1987 as an Assistant Professor, earned tenure in 1991, became a Full Professor in 1993 and was given the title Distinguished Professor in 2012. Professor Kaner is a Fellow of the Royal Society of Chemistry, Materials Research Society and Richard B. Kaner the American Association for the Advancement of Science. According to the 2014 Thomas-Reuters rankings, he is among the world’s most highly cited authors. Professor Kaner has received awards from the Dreyfus, Fulbright, Guggenheim, Packard and Sloan Foundations, as well as the Exxon Fellowship in Solid State Chemistry, the Tolman Medal and the Award for the Chemistry of Materials from the American Chemical Society for his work on refractory materials, including new synthetic routes to ceramics, superhard metals, conducting polymers, and nanostructured carbon.


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Fig. 1 A timeline for recent innovative flexible electronics concepts (Nokia Morph,13 Philips Fluid16), prototypes (Nokia,17 Samsung14), and products (Google Glass,18 Samsung Galaxy Gear,19 LG G Flex,12 Intel Jarvis,20 and LG life Band Touch21).

Fig. 2

Schematic showing the structural differences between (A) conventional and (B, C) flexible electrodes in supercapacitors.

and to connect the electrode materials electronically with the current collectors. Both conductive additives and insulating binders are essential ingredients in traditional supercapacitors. However, they clearly have disadvantages as far as electrochemical performance is concerned. For example, after mixing the polymer binder and conductive additives with the active material, some aggregation inevitably results which limits capacitance and often creates ‘‘dead volume’’ in the electrodes, where electrolyte can’t reach all the active material. Because the additives contribute a much lower capacitance (if any) relative to the active materials, these additives can significantly decrease both the volumetric and gravimetric capacitance of the electrodes. Furthermore, the most commonly used commercial active material for supercapacitors are activated carbons. The rigid microstructure of activated carbons restrict the flexibility of the electrodes. When the electrodes are bent, twisted, or stretched frequently, the electrode materials could easily crack or get peeled off the current collector and are difficult to recover to the original architecture. Therefore, a novel configuration design of the mechanically flexible electrodes without binder and inert conductive additives is required for the development of high performance flexible supercapacitors.54 Recently, considerable effort has been dedicated to preparing flexible supercapacitor electrodes based on different types of carbon materials including activated carbon,56 carbon nanotubes,30,32–34,57–60 carbon fiber,61–63 graphene,22,55,64–73 and


their composites.74–78 Graphene, a two dimensional defect-free carbon monolayer has generated growing interest since Novoselov and Geim isolated a single layer of graphitic film in 2004.79 Due to its unique properties, including a large theoretical specific surface area (B2630 m2 g 1),80 high conductivity,79 good chemical and thermal stability,81 wide potential window,82 and excellent mechanical flexibility,83 graphene-based materials have been extensively explored as electrode materials for flexible supercapacitors.79,80,84–88 The intrinsic electrochemical double layer capacitance of single-layer graphene was measured to be B21 mF cm 2.89 Therefore, if its entire surface area could be fully utilized, a supercapacitor based on graphene is capable of achieving a theoretical electrochemical double layer capacitance up to B550 F g 1, which essentially sets the upper limit capacitance for all carbon materials. These aforementioned outstanding characteristics of graphene are rather sensitive to and primarily determined by the size of the sheets, the number of layers and the presence of defects.72 Graphene fabricated by methods of mechanical exfoliation or chemical vapor deposition (CVD), generally exhibit almost ideal properties because of the large crystal domains, single or bilayer structure and few defects of graphene sheets, leading to high carrier mobility for electronics applications. However, due to their high cost and low yield, graphene produced by these techniques seems unsuitable for the mass production of electrode materials for flexible supercapacitors. Over the past few years, some low cost and high

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yield two-dimensional carbon forms, such as reduced graphene oxide (RGO), have been developed as members of the ‘graphene family’.90 RGO are commonly produced by reduction of graphene oxide (GO) with chemical, thermal or electrochemical methods.91–93 Although they can be only classified as moderate quality graphene materials according to their defects (residual oxygen-containing groups, vacancy defects, edges and deformations) even after reduction, the RGO allows for continuous large-scale production with low cost and effective functionalization for further enhancement of the electrochemical performance of flexible supercapacitors. Some studies have even shown that the residual oxygen can be very beneficial and generate pseudocapacitance. In this feature article, we review the recent research activities carried out with graphene-based materials for flexible supercapacitors with an emphasis on the preparation methods for electrodes, their electrochemical performance, and mechanical flexibility of the devices. In addition, we provide a summary of recently proposed prototypes for flexible supercapacitors known as micro-supercapacitors, and cable-type supercapacitors. These recent advances in materials preparation and configuration design bring new insights into the future of graphene-based flexible supercapacitors.

2. Types of graphene-based materials for flexible supercapacitor electrodes 2.1

Freestanding graphene-based electrodes

Graphene is a promising material for flexible supercapacitors due to its unusual characteristics. Graphene is composed of pure carbon in a single layer two-dimensional structure that provides a very large surface area. In theory, the parallel plates could provide extensive channels through which different types of electrolytes can easily access the surface of each graphene sheet with low diffusion resistance. The high electrical conductivity of graphene sheets could eliminate the need for conductive additives, enabling graphene-based electrodes with increased energy density. Furthermore, the two-dimensional layer structure and high aspect ratio endow the graphene electrodes with excellent mechanical flexibility, which makes the graphene sheets easy to assemble into freestanding films with robust mechanical stability. Therefore, these intriguing features make graphene an attractive freestanding electrode film for flexible supercapacitors. 2.1.1 2D Graphene-based flexible electrodes. Freestanding two-dimensional graphene-based film, also known as graphene paper, has attracted much attention because it is ultrathin, flexible and lightweight, which are the essential qualities needed for flexible supercapacitors. Recently, many effective solution processing methods have been presented to fabricate graphene-based films, such as spin-coating,94 Langmuir–Blodgett,95 layer-by-layer deposition,96 interfacial self-assembly97 and vacuum filtration.98,99 Wallace and coworkers fabricated graphene films by a simple vacuum filtration assembly method and first investigated it as flexible electrode materials.100 However, during the fabrication process, the individual graphene sheets begin to aggregate and restack owing to the interplanar p–p interactions and

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van der Waals forces between the graphene layers. This agglomeration reduces the surface area of the graphene films and the diffusion of electrolyte ions, which results in a decrease in the electrochemical performance. Therefore, a number of strategies have been developed to prevent aggregation of graphene sheets in order to increase surface area and promote transport of electrolyte ions including adding spacers,101–103 template-assisted growth,104 and crumpling the graphene sheets.69 Wang et al. reported a flexible pillared-type graphene paper created by incorporating small amounts of carbon black nanoparticles to serve as spacers.101 The restacking of individual graphene sheets was reduced by the pillared carbon black, resulting in a significant increase in electrochemical performance due to the open structure for charge storage and ion transport. The pillared graphene paper electrode exhibited a specific capacitance of 138 F g 1 in aqueous electrolyte at a scan rate of 10 mV s 1 and 3.85% degradation after 2000 cycles at a current density of 10 A g 1. However, carbon black is one kind of conductive additive that only contributes a very small amount of capacitance to the whole device. Furthermore, compared with the intercalation method, solution based strategies could be a better way to reduce the agglomeration of graphene sheets in terms of simplicity, effectiveness, processing and materials cost. Park et al. prepared functionalized RGO films by functionalizing graphene with Nafion using a supramolecular assembly approach.66 The tight integration of Nafion, an amphiphilic molecule, not only prevents the re-stacking of graphene sheets, but also improves the interfacial wettability between the electrodes and electrolyte (Fig. 3B and C). Consequently, the interconnected functionalized RGO networks provide continuous transport pathways for fast ion transport. As shown in Fig. 3A, all-solid-state flexible supercapacitors have been fabricated by the assembly of functionalized RGO thin films (as electrodes) and solvent-cast Nafion electrolyte membranes (as electrolyte and separator). The specific capacitance measured for functionalized RGO is 118.5 F g 1, about two times higher than the 62.3 F g 1 reported for RGO. The capacitance value for functionalized RGO remained almost constant (90% retention at 30 A g 1) with respect to changes in current density from 1 to 30 A g 1. Furthermore, flexibility was demonstrated by bending the supercapacitor with high tensile strain during operation. After bending at a radius of 2.2 mm, the cyclic voltammetry (CV) curves of the functionalized RGO displayed almost the same rectangular shape and specific capacitances as those before the bending test (Fig. 3D and E), indicating good mechanical stability for this all solid-state supercapacitor. Analogously, Li and coworkers demonstrated that water can also serve as an effective ‘‘spacer’’ to prevent the restacking of graphene sheets.105 In contrast to the typical graphene paper that restacks to graphite when placed face-to-face, hydrated graphene sheets in a self-stacked, solvated graphene film can remain reasonably well separated when combined together in a nearly parallel manner. As described in their research report, the formation of this special graphene– water hybrid structure is ascribed to the balance between repulsive interactions and inter-sheet p–p attractions between the solvated graphene layers. Electrochemical characterization


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Fig. 3 (A) Photograph and schematic diagram of an all-solid-state flexibleRGO-supercapacitor. (B, C) SEM images of the surface and cross-section of RGO films. (D) Photos of a flexible-RGO-supercapacitor before and after bending. (E) Cyclic voltammograms at a 100 mV s 1 scan rate of a flexibleRGO-supercapacitor before and after bending. Reproduced with permission.66 Copyright 2011, American Chemical Society.

indicated that a self-stacked, solvated graphene film-based supercapacitor has a high specific capacitance (215 F g 1) in an aqueous electrolyte. Significantly, a capacitance of 156.5 F g 1 is maintained even when the supercapacitor is operated at an ultrafast charge–discharge rate of 1080 A g 1. This film can provide a maximum power density of 414.0 kW kg 1 and retains 497% of its capacitance after 10 000 cycles under a current density of 100 A g 1. Furthermore, a flexible chemically converted graphene hydrogel film was created by exchanging water with a miscible mixture of volatile (water) and nonvolatile liquid electrolytes (sulfuric acid, and 1-ethyl-3-methylimidazolium tetrafluoroborate – EMIMBF4) followed by removal of the volatile liquid by vacuum evaporation (Fig. 4A).106 Interestingly, the packing density of these flexible electrolyte-mediated chemically converted graphene (EM-CCG) films can be controlled from 0.13 to 1.33 g cm 3 by changing the ratio of volatile and nonvolatile liquids, while the density of fully dried CCG films is 1.49 g cm 3. Cross-section scanning electron microscopy (SEM) images (Fig. 4B and C) reveal that the EM-CCG films have a rather uniform face-to-face separated multilayered microstructure. After being fabricated into symmetric supercapacitors, the highly compact EM-CCG films (density from 1.25 to 1.33 g cm 3) exhibit a high specific capacitance of 255.5 F cm 3 in an aqueous electrolyte and 261.3 F cm 3 in an organic electrolyte at a current density of 0.1 A g 1 with a maximum energy density of 59.9 W h L 1 (Fig. 4D and E). These are among the highest capacitances


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Fig. 4 (A) A photo showing the flexibility of an electrolyte-mediated chemically converted graphene (EM-CCG) film. (B, C) SEM cross-sectional images of EM-CCG films with mass loadings of 0.42 and 1.33 mg cm 3. Volumetric performance of capacitors based on EM-CCG film electrodes. (D) Volumetric capacitance and (E) energy density as a function of the areal mass loading of an EM-CCG film (1.25 g cm 3) and a dried CCG film (1.49 g cm 3) at a current density of 0.1 A g 1. Reproduced with permission.106 Copyright 2013, American Association for the Advancement of Science.

achieved with pure carbon materials indicating that the EM-CCG films retain highly efficient electrolyte infiltration and transport, even though they have been subject to dense compression. Because of the electrical double-layer capacitor energy storage mechanism, the reversible capacitance of pure graphene films is generally limited by the electroactive surface, the pore size distribution within the graphene films and the transport resistance of electrolyte ions. As previous mentioned, supercapacitors based on graphene could achieve an electrical double layer capacitance as high as B550 F g 1, if their entire surface area is used.22 The actual observed gravimetric specific capacitance is usually lower than 300 F g 1, which leads to lower energy density. In order to increase the energy densities needed for many practical applications, pseudo-capacitive materials with much higher capacitances than graphene have attracted great interest as potential substitutes for carbonaceous materials. The most widely explored pseudo-capacitor materials include transition metal oxides and hydroxides such as RuO2,107–110 Fe3O4,111,112 CuO,113 NiO,114–116 MnO2,117–122 Co3O4,123–125 and Ni(OH)2,126,127 and conductive polymers such as polyaniline (PANi),23,128–131 polypyrrole (PPy),29,75 and polythiophene (PT).132–135 However, these materials often suffer from particle aggregation, poor electrical conductivities (especially for metal oxides), inherent rigidity and structural degradation, leading to low power density, poor cycling stability and marginal flexibility. Since graphene possesses a large surface area, high electrical conductivity, light-weight, and flexibility, it can be considered as a suitable scaffold to support these pseudocapacitive materials. Hence, it is anticipated that a combination

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of these high capacitance materials with flexible graphene films could provide better performance. PANi is a well-studied conducting polymer for pseudo-capacitors because of the low cost of aniline monomer, its straightforward synthesis, environmental stability, and relatively high specific pseudo-capacitance.130,136 PANi is often combined with graphene materials to enhance the electrochemical performance. Cheng et al. demonstrated an in situ anodic electropolymerization method to deposit PANi sheets on graphene films to fabricate graphene– PANi composite paper.137 This composite produced a relatively high gravimetric capacitance of 233 F g 1 and a volumetric capacitance of 135 F cm 3. Furthermore, the mechanical properties of this composite paper revealed a tensile strength of 12.6 MPa with a strain of 0.11. Another strategy to fabricate a composite with graphene and PANi nanofibers was explored by Wu et al. who vacuum filtered the mixed dispersions of both components.130 The PANi nanofibers were homogeneously sandwiched between graphene sheets producing a layered structure with good mechanical flexibility. Supercapacitor devices based on this flexible composite film showed an increased electrochemical capacitance (210 F g 1) at a current density of 0.3 A g 1 and maintained 94% of its capacitance (194 F g 1) as the current density was increased from 0.3 to 3 A g 1. Nevertheless, the electrochemical performance of these hybrid graphene films based on PANi are still quite far from its maximum theoretical specific capacitance.128,136,138,139 This could be caused by aggregation of graphene sheets and PANi on a microscopic level, thus limiting the transport of electrolyte. Li and coworkers developed an in situ polymerization method to deposit PANi onto chemically converted graphene sheets in an oriented graphene hydrogel film.140 The deposited ratio of PANi in the films is effectively controlled by adjusting the polymerization time. Because of the highly accessible electrolyte ions in the porous structure, these PANi–graphene hydrogel films gave better specific capacitance (530 F g 1 at a current density of 10 A g 1) compared to the previously mentioned graphene–PANi hybrid films. If the capacitance contribution of graphene is excluded, the resulting specific capacitance attributed to PANi is between 938 F g 1 and 1104 F g 1. The hydrated films maintained 96% of their specific capacitance when the current density was increased from 10 A g 1 to 100 A g 1. Very good cycling performance was reported with 93% of the initial specific capacitance retained after 10 000 cycles. Li et al. concluded that the ‘‘soft’’ and self-adaptive nature of the oriented graphene hydrogel film and strong adhesion of PANi on chemically converted graphene sheets through p–p interactions are the reasons why mechanical degradation is circumvented. In addition to good electrochemical performance, the ability to scale-up materials is crucial if practical applications of these flexible supercapacitors are to be realized. Yu et al. reported a one-step method to fabricate freestanding graphene films on a large scale and subsequently electrochemically deposited PANi nanorods to construct hybrid graphene–PANi films.141 The composite exhibited a specific capacitance of 763 F g 1 at a current density of 1 A g 1 with reasonable cycling stability (B82% capacitance retention after 1000 cycles). Zhang et al. designed graphene-hollow

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PPy nanostructures by inserting PPy hollow spheres between graphene layers. Based on the synergetic effects of reduced graphite oxide and PPy, the nanocomposites exhibited enhanced electrochemical performance.75 The specific capacitance rose to as high as 500 F g 1 with a charging–discharging current density of 5 A g 1. Apart from conductive polymers, transition metal oxides are another kind of commonly used pseudo-capacitor material.142 Among different metal oxides, manganese dioxide (MnO2), a traditional battery material, has received tremendous attention as a promising electrode material with great potential in view of its low cost, environmentally friendly nature, and large theoretical specific capacitance (1370 F g 1).117,143 However, poor conductivity of MnO2 often leads to high internal resistance of the electrodes and poor electrochemical performance. Thus, combining highly conductive materials such as graphene has proved to be an effective approach to improve the performance of MnO2. According to the equation E = 1/2CV2, besides increasing the capacitance (C), improvement of energy density (E) can also be achieved by maximizing the potential window (V). A promising way to increase the potential window is to develop asymmetric supercapacitors, which make full use of the different potential windows of the positive and negative electrodes to increase the operating voltage in the full cell system. Sumboja et al. synthesized a large areal mass, flexible, freestanding RGO– MnO2 paper by a template-free process.144 The large areal mass electrodes exhibited remarkable flexibility and high conductivity. An asymmetric supercapacitor was fabricated based on RGO– MnO2 and RGO paper as positive and negative electrodes, respectively. The bent asymmetric supercapacitor exhibited an area specific capacitance of 90.5 mF cm 2 with an active mass of 15 mg at 100 mA g 1. Similarly, Shao et al. demonstrated a filtration assembly method to prepare graphene–MnO2 nanorods and graphene–Ag hybrid thin-film electrodes.74 The unique sheet-nanorod structure of the graphene–MnO2 film and the porous architecture of the graphene–Ag film provided a high specific surface area and more porous structure than those of traditional graphene paper. An asymmetric supercapacitor with a graphene–MnO2 film as the positive electrode and graphene– Ag film as the negative electrode has been fabricated and could charge an LED light and power a fan under bent conditions. Duan et al. reported an asymmetric supercapacitor based on a free-standing Mn3O4 nanoparticle–graphene electrode and a carbon nanotube–graphene electrode.145 This combination produced an asymmetric aqueous-based supercapacitor with an increased cell voltage of 1.8 V, a relatively stable cycling performance of 86.0% retention after 10 000 cycles, and a 42-fold increase in energy density (to 32.7 W h kg 1) when compared with a symmetric supercapacitor. Choi et al. fabricated a solid-state flexible asymmetric supercapacitor with an ionic liquid functionalizedchemically modified graphene (IL-CMG) film as the negative electrode and a hydrous RuO2–IL-CMG composite film as the positive electrode (Fig. 5).110 The asymmetric supercapacitor operated at a cell voltage up to 1.8 V and delivered an energy density of 19.7 W h kg 1 and power density of 6.8 kW kg 1. Furthermore, the device showed excellent flexibility and cycling


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Fig. 5 Schematic diagrams of (A) an all-solid-state flexible thin supercapacitor and (B) an experimental procedure for the synthesis of water-soluble ionic liquid functionalized, chemically modified graphene (IL-CMG) and RuO2–ILCMG hybrids. Reproduced with permission.110 Copyright 2013, the Royal Society of Chemistry.

stability, with 95% capacitance retention after 2000 cycles under either bent or twisted conditions. As previously discussed, graphene-based composite films can be generally fabricated by decorating pseudo-capacitor materials on graphene sheets. Sometimes, the addition of pseudo-capacitor material leads to the destruction of the graphene conductive networks and mechanical flexibility. Some researchers proposed adding a third phase (carbon nanotubes (CNTs)) to solve this problem. For instance, Liu et al. presented a method to fabricate flexible supercapacitor electrodes based on interpenetrating nanocomposites of graphene–MnO2 and CNTs.146 The CNTs could enable synergistic effects from both graphene and nanotubes by using graphene as a high surface-area substrate for the direct growth of MnO2 nanoparticles and using CNTs as additives to provide electron conductance and mechanical reinforcement. The resulting ternary composite electrode film showed outstanding mechanical properties (Young’s modulus 2.3 GPa), low sheet resistance (5 O & 1), high gravimetric specific capacitance (326 F g 1) and volumetric specific capacitance (130 F cm 3). Furthermore, the combined contribution from graphene and CNTs in the system endowed this composite with only B55% capacitance loss (326 to 148 F g 1) when the scan rate was increased from 10 to 500 mV s 1, which is much lower than that of a graphene–MnO2 composite supported on a gold electrode (90% loss). In order to further promote the capacitance, they tried to fabricate asymmetric cells by replacing the negative electrode with an AC–CNT hybrid film.147 The energy density and rate stability were reasonably increased. Similarly, Lou et al. demonstrated a hydrothermal method to prepare flexible and freestanding Co3O4–RGO–CNTs ternary hybrid graphene films.148 The hydrothermal treatment in their preparation process realized the deposition of Co3O4 microspheres and the reduction of the GO sheets at the same time. The flexible electrode


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exhibited a high specific capacitance of 378 F g 1 at a current density of 2 A g 1. In addition to intercalating different kinds of spacers to prevent the aggregation of graphene sheets, some researchers focused on adjusting the microstructure of the graphene sheets. For instance, Ruoff and coworkers demonstrated a chemical activation process to decorate the surface of graphene sheets to produce a continuous porous freestanding, and flexible RGO paper.55 Inspired by their excellent research work on KOH activation of microwave exfoliated graphite oxide,149 they also prepared activated RGO films with a similar method. These flexible and freestanding activated RGO films exhibit specific surface areas of up to 2400 m2 g 1 and very high in-plane electrical conductivity up to 5880 S m 1. They contain highly porous and interconnected three-dimensional (3D) microstructures with a substantial amount of micro-and mesopores. The well-defined porous microstructures promote the transfer of ions and charge subsequently increasing the electrochemical performance. A twoelectrode supercapacitor using these RGO films showed a high energy density of 26 W h kg 1 (specific capacitance of 120 F g 1 at a current density of 10 A g 1). Alternatively, Liu et al. demonstrated a strategy to make a folded structured graphene paper by mechanically pressing a graphene aerogel.41 After experiencing a fast deformation under pressure, the wrinkles and curves in the graphene aerogel were transformed into folds to prevent the restacking of graphene sheets. Due to the nanopores and infiltration active sites provided by the folded structure, the graphene paper gives a specific capacitance up to 172 F g 1 at a current density of 1 A g 1, and capacitance of 110 F g 1 even at a fast rate of 100 A g 1. In addition, it also exhibits excellent cycling stability retaining 99% capacitance over 5000 cycles under a current density of 20 A g 1. 2.1.2 3D Graphene-based flexible electrodes. As mentioned above, supercapacitor performance is significantly affected by the transport rates of electrons and ions, which usually depend on the microstructure and conductive characteristics of the electrode materials. These can be improved by fabricating interconnected electrode materials with a high conductivity and porous structure.150 To address these issues, graphene-based macrostructures with 3D porous networks,151 such as aerogels,152,153 foams,65,154,155 frameworks156–158 and sponges,31 have recently attracted considerable interest. These 3D graphene materials, consisting of micro-, meso- and even macroporous networks, can provide high surface area, light-weight, and fast ion/electron transport. For example, a 3D macroporous bubble graphene film was fabricated by a hard template-directed assembly strategy.159 The monodisperse polymethyl methacrylate (PMMA) spheres were used as the hard templates, and subsequently removed by calcination at 800 1C. These mediated 3D macropores in the graphene film provided ion-buffering reservoirs and lowresistant channels for ion diffusion. The as-obtained graphene film showed high capacitance retention (67.9%) when scanned from 3 to 1000 mV s 1. Nevertheless, the air drying and annealing procedure inevitably introduced aggregation of graphene sheets in micro regions that decreased the specific area to 128.2 m2 g 1. Therefore, the utilizable specific capacitance of the macroporous

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bubble graphene film was only 92.7 F g 1. Choi et al. fabricated 3D macroporous chemically modified graphene films using polystyrene colloidal particles as templates and subsequently removed them with toluene (Fig. 6A).156 As shown in Fig. 6B and C, this low-temperature solution method resulted in well-defined interconnected pore networks of the chemically modified graphene films without any collapse of the porous structures. The films also showed good electrical conductivity (1204 S m 1). In order to further increase the capacitance, a thin layer of MnO2 was additionally deposited onto the porous films. Energy-dispersive X-ray spectroscopic (EDS) elemental maps of C, O, and Mn (Fig. 6D–G) provided clear evidence for the formation of a homogeneous coating of amorphous MnO2 throughout the 3D macroporous framework. Due to the fast ionic transport and high electrical conductivity within the electrode, the porous MnO2/chemically modified graphene hybrid films exhibited a high specific capacitance of 389 F g 1 at 1 A g 1 and superior capacitance retention of 97.7% when scanned from 1 A g 1 to 35 A g 1. An asymmetric supercapacitor device was also prepared by assembling a chemically modified graphene film as a negative electrode and a MnO2/chemically modified graphene film as a positive electrode. A maximum energy density of 44 W h kg 1 with a power density of 11.2 kW kg 1 and power density of 25 kW kg 1 with an energy density of 39.1 W h kg 1 were achieved by the asymmetric supercapacitors using a potential window of 2.0 V. Apart from fabrication using a polymer sphere template, 3D macroporous graphene films or graphene foam can also be synthesized by using a Ni foam template-directed CVD method.160

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The graphene foam consists of an interconnected graphene network, which gives it a high conductivity due to few defects and low contact resistance, as well as a high specific surface area. Based on this approach, Zhang et al. reported a method to prepare 3D porous graphene networks by using Ni foam as a sacrificial template in a CVD process with ethanol as the carbon source.161 Moreover, they electrochemically deposited nickel oxide (NiO) onto the 3D graphene networks to fabricate graphene–NiO composite electrodes for supercapacitors. The unique 3D porous structure of the graphene network with a large specific surface area allows rapid access of electrolyte ions to the NiO surfaces. The obtained NiO–graphene composite exhibits a high specific capacitance of 816 F g 1 at a scan rate of 5 mV s 1. Dong et al. synthesized a similar hybrid structure by in situ growth of Co3O4 nanowires on a 3D macroporous CVD graphene foam.162 Interestingly, the graphene–Co3O4 composite achieved a specific capacitance of 768 F g 1 at a current density of 10 A g 1, which increased to 1100 F g 1 after 500 cycles and remained stable up to 1000 cycles. More recently, Xie et al. electrochemically deposited a large mass load of MnO2 (9.8 mg cm 2) on a Ni foam template 3D graphene to fabricate graphene–MnO2 composite networks.163 This composite electrode exhibited a high areal capacitance of 1.42 F cm 2 and good mechanical flexibility. Although these 3D porous graphene-based electrodes show great electrochemical performance, when compared with one-step synthetic methods, the template-assisted fabrication is complicated, expensive, and time-consuming. Leavening is the process of adding gas to make bread more easily chewable by adding porosity. Inspired by this baking procedure,

Fig. 6 (A) Schematic illustrating the fabrication of 3D macroporous MnO2/chemically modified graphene films. (B, C) Low-magnified cross-sectional SEM and TEM images of chemically modified graphene film. (D–G) EDS mapping of C, O, Mn, and overlay elements on a segment of MnO2/chemically modified graphene film. Reproduced with permission.156 Copyright 2012, American Chemical Society.

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Chen et al. reported a ‘‘leavening’’ process to transform compact graphene paper to porous graphene film (Fig. 7A).65 They used a filtration assembled graphene paper as dough and hydrazine vapor as foaming and reducing agent. Cross-sectional SEM images (Fig. 7B and C) indicate the formation of an open porous graphene foam network with pore sizes in the range of sub-micrometer to several micrometers. They concluded that the hydrazine vapor induced gaseous species (such as H2O and CO2) were responsible for the formation of the porous structure. They built flexible supercapacitors by using freestanding RGO foams as both current collectors and electrodes (Fig. 7D and E). The specific capacitance of the resulting RGO foam is 110 F g 1. It was found that there was only a very slight difference in the CV curves (Fig. 7F) when the distance between the two sides of the supercapacitor was changed from 3 to 2 cm under bending. Gelation is another simple, effective, and straightforward route to fabricate 3D porous macroscopic materials. Kim et al. presented an interfacial gelation process to fabricate large-scale 3D porous graphene hydrogel films with simple immersion of arbitrary Zn objects in aqueous dispersions of GO (Fig. 8A and B).164 Interestingly, this gelation method enables a wide range of controllable 3D gel structures as well as macroscopic objects based on any desired structure. Cross-sectional SEM images of freeze-dried graphene aerogel films (Fig. 8C and D) revealed that graphene sheets were interconnected in a quasiparallel manner to form an open porous morphology with a large range of pore sizes from tens of nanometers to several micrometers. Interestingly, introduction of various template structures at the Zn surface facilitates a straightforward route for fabricating 3D macroscopic graphene gels with any arbitrary shape (Fig. 8E–G). Accordingly, the graphene hydrogel films were found to exhibit high specific areas (up to 778.5 m2 g 1). The supercapacitor performance was analyzed in a symmetric two-electrode configuration

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with two graphene hydrogel films. Although the gravimetric specific capacitance (76.8 F g 1 at a current density of 2.3 A g 1) is not particularly attractive, according to the large mass loading of 0.44 mg cm 2, the areal specific capacitance (33.8 mF cm 2 at a current density of 1 mA cm 2) exceeds most other reported values based on pure carbon materials. This cell also exhibited outstanding cycling stability with retention of 97.8% of its initial capacitance after 4000 cycles. 3D graphene macrostructures such as graphene hydrogels or aerogels can also be readily prepared by a one-step hydrothermal reduction process.153,165–167 Due to an inflexible structure and low mechanical strength, a bulk graphene hydrogel cannot easily be used directly as a flexible electrode. However, Duan and coworkers simply pressed the hydrothermal reduced graphene hydrogel onto a flexible current collector to assemble all-solidstate flexible supercapacitors, with a polyvinyl alcohol (PVA) gel electrolyte.64,168 The graphene hydrogel film in the solidstate device showed a high specific capacitance of 186 F g 1 at a current density of 1 A g 1. Based on thick electrodes with high mass loading, this 3D graphene hydrogel film achieved a superior area-specific capacitance of 372 mF cm 2. In order to further increase the specific capacitance, they prepared functionalized graphene hydrogels by incorporating hydroquinones into the high-surface-area 3D graphene framework via p–p interactions as a pseudo-capacitive component.168 As shown in Fig. 9A–C, the functionalized graphene hydrogel films maintain a 3D continuous porous network and high flexibility, even after physical pressing onto the current collectors. A high specific capacitance of 412 F g 1 at a current density of 1 A g 1 was recorded with 74% capacitance retention at 20 A g 1. The all-solid-state supercapacitors were also fabricated with functionalized graphene hydrogels films using a PVA gel electrolyte (see Fig. 9D and E). The device was then electrochemically measured while bending. Similar CV curves were obtained at different bending angles,

Fig. 7 (A) Schematic drawings illustrating the process to prepare RGO foams. (B and C) Cross-sectional SEM images of RGO foams. (D and E) Schematic diagram and optical image of the flexible RGO foam supercapacitor. (F) CV curves of the RGO foam supercapacitor before bending and while bent. Reproduced with permission.65 Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.


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Fig. 8 (A) Illustration of graphene gelation fabrication procedure. (B) Scalability of gelation. (C and D) SEM images of freeze-dried graphene aerogels. (E) Schematic illustration of templated gelation. (F) Local gelation at pre-patterned Zn substrate. (G) 3D graphene gel architecture grown from Zn wires plate assembly. Reproduced with permission.164 Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 9 (A) Photograph of a flexible functionalized graphene hydrogel thin film electrode. (B) Low- and (C) high-magnification SEM images of the interior microstructures of the functionalized graphene hydrogel film. (D) Photograph of a functionalized graphene hydrogel-based flexible solid-state supercapacitor. (E) The schematic diagram of the solid-state device with H2SO4–PVA polymer gel as the electrolyte and separator. Reproduced with permission.168 Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

demonstrating excellent mechanical robustness for the devices. Furthermore, the all-solid-state supercapacitors showed 87% capacitance retention after 10 000 cycles under a 1501 bending angle. These results demonstrate the excellent cycling stability and extraordinary mechanical flexibility of the all-solid-state functionalized graphene hydrogel supercapacitors. Although high electrochemical performance was achieved by the hydrothermal reduced graphene hydrogel, the relative harsh preparation environment and limited amount of products

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may restrict practical applications of the hydrothermal method. In order to further improve the energy density and large-scale application of flexible supercapacitors, our group reported a simple solid-state laser scribed strategy to fabricate 3D porous graphene electrodes by using an inexpensive commercially available LightScribe CD/DVD optical drive (Fig. 10A–D).22 During the preparation process, the laser scribed graphene (LSG) is reduced and exfoliated into 3D nanostructures simultaneously. The open network structure and the large accessible


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Fig. 10 (A–D) Schematic illustration of the fabrication of laser-scribed graphene-based supercapacitor. (E) Photograph of golden brown color of GO and black color of LSG. (F) Schematic of symmetric supercapacitor constructed from two identical LSG electrodes, ion-porous separator, and electrolyte. (G) A schematic diagram of the all-solid-state LSG-supercapacitor illustrates that the gelled electrolyte can serve as both the electrolyte and separator. (Inset) A digital photograph showing the flexibility of the device. (H) Bending the device has almost no effect on its performance. Reproduced with permission.22 Copyright 2012, American Association for the Advancement of Science.

specific surface area (1520 m2 g 1) is optimized for electrolyte ionic diffusion in LSG electrodes, which results in sizeable charge storage capacity. All-solid-state flexible supercapacitors were assembled by using two identical LSG electrodes and combined with a PVA–H3PO4 polymer gel electrolyte. In order to evaluate under real conditions the potential of this all-solidstate LSG supercapacitor for flexible energy storage, we placed the device under mechanical strain and tested the performance. The CV curves of this device when tested under different bending conditions is shown in Fig. 10H. The bending had almost no effect on the capacitive behavior; it can be bent arbitrarily without degrading performance. Moreover, the stability of the device was tested for more than 1000 cycles while in the bent state, with only B5% change in the device capacitance. The flexible all-solid-state LSG supercapacitor maintained 97% of the initial capacitance after 10 000 cycles. Moreover, the ionic liquid EMIMBF4 was used as the electrolyte, in order to broaden the potential window. As a result, the LSG supercapacitor exhibited an area specific capacitance as high as 5.02 mF cm 2 (gravimetric specific capacitance of 276 F g 1) at a wider potential window of 4 V and a superior volumetric energy density of 1.36 mW h cm 3. Given the simplicity, costeffectiveness, scalable synthesis, high energy storage performance and flexibility, these LSG supercapacitors are expected to be a promising candidate for flexible energy storage devices. In summary, freestanding graphene-based electrodes can exhibit exciting characteristics, such as high electrical conductivity, enhanced specific surface area, high capacitance, excellent rate capability and stability. The graphene films can serve as mechanically flexible scaffolds for constructing high performance


flexible electrodes. The interconnected graphene networks in 3D porous graphene films suppress agglomeration/re-stacking of the graphene sheets and ensure good electrical conductivity of the electrodes. Although many remarkable achievements have already been made in this field, there is still a lot work needed. With an aim towards practical applications, the mechanical performance of the flexible supercapacitors need to be given more attention. For this reason many research efforts are focused on coating electrochemically active materials on substrates with high mechanical flexibility and stability. 2.2

Substrate supported graphene-based electrodes

As flexible supercapacitors aim to serve as practical energy storage devices, the mechanical properties are important for assessing their potential in flexible/foldable applications. Generally, the thickness of the electrode active materials is less than 50 micrometers, often just several hundred nanometers. Thus, the freestanding active material is relatively fragile and can flake off the electrode after prolonged bending. Even combined with commonly used metallic planar current collectors (such as Al or Cu foils), the electrodes cannot readily readjust their structures after repeated deformations. To solve this problem, many attempts have been made to coat electrode active materials onto substrates with excellent mechanical properties. 2.2.1 Graphene-based electrodes coated onto nonconductive substrates. Based on their mechanical flexibility, thinner, and light-weight plastics such as polyethylene terephthalate (PET) have been used as substrates for flexible supercapacitors. For example, an ultrathin graphene film coated on a PET substrate was fabricated by Yu et al.70 This ultrathin graphene film

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Fig. 11 (A) Photographs of transparent thin-films of varying thickness on glass slides. (B) A TEM image of graphene collected from a dispersion before filtration. (C) An SEM image of a 100 nm graphene film. Reproduced with permission.70 Rights managed by AIP Publishing LLC.

exhibited controllable transparency by altering the thickness, as shown in Fig. 11A. The surface morphology, observed by TEM and SEM, revealed a homogeneous surface with intimate contact between graphene sheets. The ultrathin graphene film gave a specific capacitance of 135 F g 1 at a current density of 0.75 A g 1, with a transmittance of 70%. However, the thickness of the graphene film was only 25 to 100 nm. This means the amount of energy stored is limited. Niu et al. reported an electrophoretic layer-by-layer assembly for constructing a multilayered RGO–gold nanoparticle film.169 The gold nanoparticle layers in the hybrid structure provide diffusion channels for ion transport and form conductive bridges between the graphene layers. The conductivity of the hybrid film was as high as 8375 S m 1. The specific capacitance of the supercapacitor was B61 F g 1 at a scan rate of 5 mV s 1, with a calculated maximum energy density of 36 W h kg 1 and power density of 49 kW kg 1. In order to further increase the capacitance, Dong et al. introduced Co Al layered double hydroxide nanosheets to fabricate hybrid graphene films through a layer-by-layer assembly method.170 These multilayer films exhibited a high specific capacitance of 1204 F g 1 and 9 mF cm 2. Although plastics can be used as flexible substrates, they suffer from limited active material loading and low effective surface area for infiltration of the electrolyte. According to the energy storage mechanism for supercapacitors, the specific area, pore size distribution, pore shape and structure are key factors influencing the electrochemical performance.49,171 It has been suggested that the presence of macropores, mesopores, and micropores are all crucial to ensure good performance of supercapacitors in terms of both power delivery rate and energy storage capacity. Macropores (450 nm) can serve as ionbuffering reservoirs, whereas mesopores (2–50 nm) can accelerate the kinetic processes of ion diffusion and improve the power performance at high current densities. In addition, micropores

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(o2 nm) that are accessible to the electrolyte ions are essential for high energy storage. Therefore, textile, sponge,172,173 and cellulose paper67,174 based flexible substrates have shown enhanced electrochemical performance due to their large surface area and porous structures, when compared to planar flexible structures. Due to their high flexibility, light-weight, and excellent mechanical strength, these porous substrates have also been frequently employed in flexible supercapacitors. For instance, Cheng et al. developed a flexible graphene– cellulose paper membrane by simply filtering a graphene nanosheets solution through cellulose paper.67 Here the graphene nanosheets strongly bind to the cellulose fibers and fill the pores. The numerous functional groups on the cellulose fibers provide abundant interactive sites to bind the graphene nanosheets (Fig. 12). The macroporous texture of the cellulose paper coated by the graphene nanosheets forms a conductive interconnected network. The resulting film combines the macroporous structure of cellulose paper and the excellent conductivity of graphene. Such a flexible electrode exhibited an areal capacitance of 81 mF cm 2 and a gravimetric capacitance of 120 F g 1 based just on the mass of graphene. The graphene–cellulose paper was also fabricated into an all-solid-state supercapacitor by using H2SO4–PVA gel as the electrolyte. Based on the geometric area of the whole device, its capacitance exhibited a value as high as 46 mF cm 2. Recently, Liu et al. demonstrated a simple ‘‘dipping and drying’’ process for building flexible supercapacitors via the assembly of PANi–RGO nanocomposites on a cellulose fiber paper. The process starts with coating a GO layer onto the surface of cellulose fiber paper and then changing the GO layer into a porous RGO by using a hydrothermal reduction process.174 This is followed by an in situ polymerization process to deposit a layer of PANi onto the surface of the RGO. Thanks to the good mechanical properties of the cellulose paper, the PANi– RGO hybrid cellulose paper exhibited high tensile strength and Young’s modulus of 1.9 and 27.2 M Pa, which are even larger than pure cellulose paper (0.73 and 18.3 M Pa). Moreover, the specific capacitance of PANi–RGO hybrid paper can reach up to 464 F g 1 (based on the total mass of PANi and RGO). Analogously, sponges are composed of an interconnected hierarchical macroporous network and high surface area. Thus, Ge et al. demonstrated a dip-coating method to deposit graphene–MnO2 onto a sponge framework to fabricate nanostructured hybrid electrodes.172 The highest specific capacitance achieved was 450 F g 1 at a scan rate of 2 mV s 1. This device had a maximum energy density of 8.34 W h kg 1, with a power density up to 47 kW kg 1. Among the possible porous flexible substrates, textiles are one of the most commonly used and low-cost materials. These textiles have hierarchical network structures with a complex surface morphology, functional groups such as hydroxyls, and high porosity, which is important for the transport of electrolyte ions and integration with the active material when serving as the electrode substrate. An ideal wearable energy device should incorporate a textile as a component. Liu et al. reported a simple ‘‘brush-coating and drying’’ method to prepare flexible supercapacitor electrodes by coating a GO suspension onto a


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Fig. 12 SEM images of the filter paper or graphene–cellulose paper surfaces with different graphene nanosheet loading amounts. (A) 0 wt% (pristine filter paper), (B) 2.3 wt%, and (C) 7.5 wt%. (D) Illustration of the structural evolution of graphene–cellulose paper as the graphene nanosheets loading increases. Reproduced with permission.67 Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

commonly used cotton textile followed by thermal reduction.175 A simple symmetric supercapacitor was fabricated by using composite fabrics as the electrodes, and a pure cotton textile as a separator. The cell exhibited a specific capacitance of 81.7 F g 1 at a scan rate of 10 mV s 1, based on the active material mass of the two electrodes. Yu et al. demonstrated that a graphene– MnO2 hybrid textile can be fabricated by coating solutionexfoliated graphene sheets onto a 3D framework and further controlled the electrodeposition of the pseudocapacitive MnO2 nanomaterial (Fig. 13A).176 Such 3D porous networks not only permit large loading of active electrode materials but also facilitate easy transport of electrolyte ions. Representative SEM images (Fig. 13B and C) show the 3D porous structures of graphene-wrapped textiles and nanoflower shaped hierarchical architectures of the MnO2 particles with dimensions of 300– 800 nm and 5–30 nm mesoporous branches. The unique hierarchical structure of this graphene–MnO2 nanostructure textile provides several interesting features for enhancing the electrochemical performance. First, the 3D porous macrostructure of the polyester textile facilitates transport of electrolyte ions. Second, the graphene nanosheet coating provides electronic conductivity and interfacial contact. Third, the electrodeposited MnO2 nanoflower architecture with mesopores offers an electrochemically active surface for energy storage. Thus, the graphene–MnO2 composite nanostructured based energy textiles yielded a specific capacitance of 315 F g 1, which is 4 times higher than that of a pure graphene nanosheet coated textile. Furthermore, an asymmetric supercapacitor was fabricated with a graphene–MnO2–textile as the positive electrode and a single walled CNTs (SWCNTs)–textile as the negative electrode, which exhibited a maximum power density of 110 kW kg 1 and a maximum energy density of 12.5 W h kg 1. However, due to


Fig. 13 (A) Schematic illustration of two key steps for preparing hybrid graphene–MnO2 textiles as high-performance supercapacitor electrodes. (B, C) SEM images of hybrid graphene–MnO2 textiles under different magnifications. Reproduced with permission.176 Copyright 2011, American Chemical Society.

the poor electronic and ionic conductivities of MnO2, the graphene–MnO2–textile electrodes still resulted in very limited specific capacitance. To further improve the electrochemical performance, Yu et al. developed a ‘‘conductive wrapping’’ method to promote the supercapacitor performance of graphene–MnO2 nanostructured textile electrodes (Fig. 14A and B).76 They wrapped CNTs with the conducting polymer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) to form a conductive paint

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Fig. 14 (A, B) Schematic illustration showing conductive wrapping of graphene–MnO2 with CNTs and conducting polymers, (C, D) with their corresponding SEM images of GMC and GMP. (E) Plots of specific capacitance values at various current densities. Reproduced with permission.76 Copyright 2011, American Chemical Society.

and designed ternary systems, such as graphene–MnO2–CNT and graphene–MnO2–PEDOT:PSS composites for high performance electrodes. The 3D conductive wrapping not only provides an additional electron transport pathway besides the graphene layer underneath the MnO2, but actively participates in the charge storage process as an electric double-layer capacitor or pseudocapacitor material. Representative SEM images (Fig. 14C and D) of the surface of the composites, demonstrate that an ultrathin film of interconnected CNT and PEDOT is uniformly coated on the nanoflower-shaped MnO2 surfaces. The equivalent series resistances (ESR) of these composites were estimated to be 41 O and 27 O respectively, in contrast to 87 O for a modified graphene-based electrode. The hybrid textile electrodes exhibited significantly enhanced performance with specific capacitances as high as B380 F g 1, as shown in the summary plot of specific capacitance of different electrodes (Fig. 14E). This 3D conductive wrapping method represents an effective technique to promote the electrochemical performance of the insulating pseudo-capacitor materials. Sponges, cellulose papers and textiles used in the above flexible supercapacitors are abundant materials that are lightweight and robust, promoting electrode flexibility and enhanced mechanical stability. The sustainability and recyclability of these substrates are promising for energy-saving and environmentally friendly devices. Moreover, the 3D porous microstructures provide abundant space for transport and storage of electrolyte ions. The functional groups (e.g. hydroxyl) allow the substrates to absorb large amounts of electrolyte or other polar molecules, which promotes infiltration of electrolyte ions improving connections between the substrate and the graphene sheets. Therefore, coating electrode active materials on sponges, textiles and cellulose paper appears to be an effective strategy to create mechanical flexibility for supercapacitors. 2.2.2 Graphene-based electrodes coated on conductive substrates. Although the substrates discussed in the previous section promote mechanical performance, they are electrically

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insulating and could not contribute any capacitance, resulting in a decrease in gravimetric and volumetric performance. Highly conductive films or networks with good flexibility and mechanical strength can be also used as flexible supports for active materials. In addition, nitrogen doping has been demonstrated to be a simple, yet effective method to improve performance of graphene in device applications because it changes the local electronic structure, thus leading to enhanced binding with ions in the electrolyte, which improves performance.177–179 Based on this concept, an interesting protocol was proposed to dope nitrogen atoms into graphene sheets via plasma-enhanced CVD (PECVD).180 Through plasma doping, nitrogen atoms are expected to replace carbon atoms in the original graphene sheets and form three types of nitrogen-configurations: pyridine-like (N-6), pyrrole-like (N-5), and graphite-like (N-Q) (Fig. 15). The fabricated nitrogendoped graphene was coated on Ni foil to prepare electrodes for electrochemical measurements. The highest specific capacitance reported was 282 F g 1, compared with 69 F g 1 for pristine graphene. The specific capacitance of nitrogen-doped graphene is even comparable to those of some pseudo-capacitor materials and graphene composites. Note that the PECVD treatment modifies the atomic configuration of the carbon bonds in graphene rather than changing the surface area or creating composites with other materials. Thus, these modified electrodes also exhibit good rate stability (165 F g 1 at a current density of 33 A g 1) and cycling stability (retaining 95.3% of initial capacitance after 10 000 cycles). Flexible, nitrogen-doped graphene supercapacitor devices were developed on commercial conductive carbon textiles. A wearable supercapacitor was tested by wrapping it around an arm; the device easily lit up a light emitting diode (LED) even with arm movements (Fig. 16). Vanadyl phosphate (VOPO4) can also be used as a high performance electrochemical layered material. Wu et al. demonstrated a layer-by-layer assembly method to fabricate a VOPO4– graphene hybrid film.181 This VOPO4 nanosheet is a new kind of ultrathin inorganic nanosheet with no more than 6 atomic layers.


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Fig. 15 (A) Schematic illustration of the plasma doping process. (B) Gravimetric capacitances of supercapacitors built on nickel foil and paper substrates measured at a series of different current densities. (inset) A photograph shows that a wearable supercapacitor wrapped around an arm can store enough electrical energy to power a LED. Reproduced with permission.180 Copyright 2011, American Chemical Society.

It was integrated on the graphene sheets and assembled into stacks to form the final VOPO4–graphene hybrid films. With PVA–LiCl as a gel electrolyte, the VOPO4–graphene hybrid films were fabricated to form a flexible all-solid-state supercapacitor (Fig. 16). The device showed a high areal specific capacitance of 8.36 mF cm 2 and an energy density of 1.7 mW h cm 2 with a power density of 5.2 mW cm 2. As previously discussed, typical graphene sheets in supercapacitor electrodes are randomly oriented with respect to the current collectors in a stacked microstructure. In such cases, electrolyte ions could not completely penetrate inside the graphene sheets, leading to incomplete utilization of the electrochemical surface area which consequently limits the performance. Ajayan et al. designed an ‘‘in-plane’’ fabrication approach for flexible supercapacitors, in which a large amount of conductive planar graphene sheets are oriented in a direction perpendicular to the current collectors.68 The 2D in-plane design takes advantage of the atomic layer thickness and flat morphology of graphene and offers opportunities for the electrolyte ions to interact with all the graphene layers. An all-solid-state supercapacitor constructed with this electrode and gel electrolyte gave a gravimetric capacitance for the active material of up to 250 F g 1. However, based on the low mass loading of graphene sheets (0.283 mg), the areal specific capacitance was only 394 mF cm 2.

Similarly, Sheng et al. reported interpenetrating graphene electrodes fabricated by electrochemical deposition of vertical GO on gold foil.182 The double-layer capacitor with these graphene electrodes exhibited a specific capacitance of 283 mF cm 1, and a short resistor-capacitor time constant of 1.35 ms. The conductive substrates used in graphene-based flexible supercapacitor electrodes (Cu and Al foils of about 13.0 and 5.0 mg cm 2) are usually much thicker and heavier than the coated graphene sheets. Therefore, even if the gravimetric specific capacitance based only on the active materials is increased, the low mass loading of graphene sheets will consequently lead to only a marginal increase in capacitance for the whole device. Furthermore, the double layer charges in the thick electrode materials cannot be conveniently transferred to the surface of the conductive substrates, which would greatly decrease the rate capability and capacitance of supercapacitors with thick electrodes. Thus, the loading quantity of active materials is always limited. In order to solve this problem, using a 3D conductive structure as substrate could be an effective way to increase the loading mass of an electrode. Therefore, Chen et al. reported a method for depositing large amounts of a RGO hydrogel in a 3D porous nickel foam to form graphene hydrogel–nickel foam (G-Gel–NF) composite electrodes.154 A cross-sectional SEM image of a freeze-dried G-Gel–NF electrode clearly shows that the G-Gel

Fig. 16 (A) Schematic illustration of an as-fabricated flexible graphene–VOPO4 ultrathin-film supercapacitor. (B) Cycling stability of a flexible all-solidstate supercapacitor under repeated bending/extending deformations. Reproduced with permission.181 Copyright 2013, Rights Managed by Nature Publishing Group.


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has been successfully deposited and filled into the macropores of the entire nickel foam. The framework exhibits a threedimensional interpenetrating porous structure with pore sizes in the range from sub-micrometer to several micrometers. The areal specific capacitance was calculated to be 45.6 mF cm 2 at a current density of 0.6 mA cm 2, and only decreased slightly to 41.1 mF cm 2 at a current density of 50 mA cm 2. Due to the excellent mechanical strength and good conductivity, carbon fiber is commonly employed as substrates in flexible supercapacitors. Moreover, carbon fibers can be woven into various 3D porous forms such as cloths or textiles. These carbon fiber fabrics are often combined with other kinds of active materials to serve as flexible electrodes. For example, Xu et al. demonstrated a screen printable strategy to coat graphene–PANi ink onto carbon fabric substrates to fabricate graphene–PANi flexible electrodes.183 In order to produce thin film electrodes quickly and scalably, they used a ball milling method to prepare the graphene–PANi ink. By employing carbon fabric as a conductive substrate, the macroporous fiber woven structure significantly improved film adhesion and simplified the coating process. Due to the high conductivity of carbon fabric, the graphene–PANi hybrid films can be directly used as electrodes. Electrochemical measurements showed a maximum specific capacitance of 269 F g 1 at a scan rate of 20 mV s 1. In summary, conductive carbon fibers, metal coated-PETs, metal foils and foams can serve as platforms to construct flexible supercapacitors with improved properties and functionalities because of their excellent structural stability and high electrical conductivity. Table 1 summarizes the preparation methods, capacitances and, energy density, power density and retention (after cycling) for the aforementioned flexible planar supercapacitor electrodes. Directly growing or coating graphene sheets on these conductive substrates has been demonstrated to be an effective method for promoting the electrochemical and mechanical performance of flexible electrodes. Tight adhesion of active materials to the conductive substrates can provide extremely low electrical contact resistance and excellent structural stability for the electrodes. More efforts should be focused on the interactions between conductive substrates and graphene sheets, as a deeper understanding of the interaction mechanisms is crucial for achieving stable interfaces to provide long cycle life.

3. Different prototypes for flexible supercapacitors The future development of multifunctional flexible electronics such as roll-up displays, photovoltaic cells, and even wearable devices presents new challenges for designing and fabricating light-weight, flexible energy storage devices. Commercially available supercapacitors always consist of a separator sandwiched between two electrodes with liquid electrolyte. The contents are either spirally wound and packaged into a cylindrical container or stacked into a button cell. Unfortunately, these device architectures not only suffer from the possible harmful leakage

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of electrolytes, but their packaging makes it difficult to use them for practical flexible electronics. Therefore, as mentioned earlier, many remarkable achievements have been made in developing planar sandwiched flexible supercapacitors. In addition, the recent emergence of flexible integrated electronic microchips and 1D fiber electronics has inspired great interest in developing high performance micro-supercapacitors and fiber-type supercapacitors. 3.1

Micro-supercapacitors

The rapid development of miniaturized portable electronic equipment has increased the demand for flexible energy storage devices that are sufficiently compact so they can be directly integrated on chips with other electronic components. However, due to the limits of size and geometry, miniaturizing traditional energy storage devices (sandwiched supercapacitors or batteries) onto electronic circuits is still a challenge. Recently, microsupercapacitors with dimensions of tens to hundreds of micrometers based on nanomaterials such as VS2,184 MnO2,185–187 carbide-derived carbon,188 carbon onions,189 CNTs,190,191 and graphene192–195 produced by common micro-fabrication techniques provide a new path to design devices for driving miniaturized electronics.196 The micro-supercapacitors can be directly integrated into other miniaturized electronic devices, such as micro energy-harvesting systems, thereby providing microscale power. From a historical viewpoint, the first example of a flexible micro-supercapacitor was fabricated in 2003 by photolithography and an electrochemical polymerization technique with the conductive polymer polypyrrole, as the active electrode material.197 But the device only showed moderate performance due to the high resistance and limited capacitance of the electrode. Afterwards, many research efforts have been dedicated to developing flexible micro-supercapacitors by focusing on graphene and its composites. For instance, Cao et al. demonstrated a direct laser writing of a GO film to fabricate a RGO–GO micro-supercapacitor, by using GO as a solid electrolyte.193 The as-prepared laser patterned micro-supercapacitor exhibited good electrochemical performance without the use of an external electrolyte. Based on the whole device, the micro-supercapacitor gave an area specific capacitance of 0.51 mF cm 2, which was nearly twice that of a sandwich structured supercapacitor. In comparison with conventional planar sandwich-structured supercapacitors, one of the key features of micro-supercapacitors is to make two small size interdigital electrodes on a 2D flexible plane, in which electrolyte ions in the narrow interspaces between electrode fingers can be readily and rapidly transported to offer higher energy density and power capability. In order to prove the advantages of micro-supercapacitors, Niu et al. combined photolithography with electrophoretic deposition to prepare graphene microelectrodes, then fabricated these into flexible all-solid-state micro-supercapacitors and compared the performance with a conventional graphene supercapacitor.198 The open edges of the graphene microelectrodes enhanced the infiltration of the electrolyte ions into the micro-patterned graphene electrodes, due to the shortened diffusion pathways. As a result, electrolyte ions could access more surface area of



(2 mV s 1, 1 M H2SO4) (0.4 A g 1, 1 M H2SO4) (0.3 A g 1, 1 M H2SO4)

B816 F g 1 (5 mV s 1, 3 M KOH) 372 F g 1 (1 M Na2SO4)

Brush-coating and drying

RGO–gold particles hybrid films

81.7 F g 1 (10 mV s 1, 6 M KOH, based on two electrode) 1240 F g 1 or 90 mF cm 2 (5 mV s 1, 1 M KOH) 61 F g 1 (5 mV s 1, 1 M LiClO4, based on device)

Laser scribe Vacuum filtration Vacuum filtration Dip-drying Dip-drying and electrochemical deposition

Laser-scribed graphene Transparent graphene film Graphene–cellulose paper Graphene–MnO2–PEDOT:PSS Graphene–MnO2 nanostructured textiles Graphene sheets–cotton cloth composite fabric Co-Al LDH/RGO films Layer-by-layer deposition

1

1

1

1

1

1

1

36 W h kg

1

7.13 W h kg

12.5 W h kg

1

1

1.36 mW h cm

35.1 mW h cm 6.8 W h kg 1

19.7 W h kg

3

2

2

1

8.7 W h kg (all solid state)

44 W h kg

32.7 W h kg

2.2 W h kg

0.61 W h kg 1 (total mass of device)

59.9 W h L

4.66 mW h cm

26 W h kg

1

150.9 W h kg

26 W h kg

Energy density

1

1

1

1

1

1

1

1

1

49 kW kg

1.5 kW kg

1 1

1

3

110 kW kg

20 W cm

37.5 mW cm 2 2.5 W h kg 1

6.8 kW kg

1.65 kW kg (all solid state)

25 kW kg

42 kW kg

33.9 kW kg

0.67 kW kg 1 (total mass of device)

75 kW L

369.8 mW cm

414 kW kg 776.8 kW kg

5.1 kW kg

Power density

2

Ref.

137 139 130

64

66 159 55 41 164 65 106

105

102 101

148 145

140 110 131 141 144 163

B99% (2000)

169

170

B93.8% (1500) 175

B97% (10 000) 22 70 B99% (5000) 67 B96% (3000) 76 B95% (5000) 176

93% (10 000) 79.4% (2000) 90% (5000) 82% (1000) 93% (1350) 81.2% (5000)

95% (1000) 156 86% (10 000) 168 B100% (1000) 153

96% (700) 86% (10 000)

B100% (2000) 161 B95% (1000) 146

96% (500) 79% (800)

91.6% (10 000)

B95% (2000) B99% (5000) 97.8% (4000)

B90% (1000)

96.15% (2000) 95.65% (2000) 97% (10 000)

Retention (cycles)

Chem Soc Rev

Layer-by-layer assembly

574 F g 1 233 F g 1 (1 M H2SO4) 385 F g 1 (0.5 A g 1, 1 M H2SO4) 763 F g 1 (1 A g 1, 1 M H2SO4) 243 F g 1 (0.05 A g 1, 1 M Na2SO4) 465 F g 1 or 1.42 F cm 2 (2 mV s 1, 0.5 M Na2SO4) 276 F g 1 or 5.02 mF cm 2 (EMIMBF4) 135 F g 1(10 mV s 1, 2 M KCl) 120 F g 1 (1 mV s 1, 1 M H2SO4) B380 F g 1 (0.1 mA cm 2, 0.5 M Na2SO4) B315 F g 1 (2 mV s 1, 0.5 M Na2SO4)

Vacuum filtration and in situ polymerization Vacuum filtration Template filtration and polymerization Coating and electropolymerization Vacuum filtration CVD and electrochemical deposition

Vacuum filtration and hydrothermal treatment 378 F g 1 (2 A g 1, 3 M KOH) Vacuum filtration 72.6 F g 1 (0.5 A g 1, PAAK–KCl, asymmetric device) Vacuum filtration and sacrificial template 389 F g 1 (1 A g 1, 1 M Na2SO4) Hydrothermal treatment 441 F g 1 (1 A g 1, 1 M H2SO4) Hydrothermal treatment 239 F g 1 (1 mV s 1, 1 M H2SO4)

1

1

CVD and sacrificial template Co-precipitation and vacuum filtration

1

233 F g 489 F g 210 F g

Vacuum filtration and electropolymerization Vacuum filtration and polymerization Vacuum filtration

Modified hydrothermal reduction method

Vacuum filtration Hard template and vacuum filtration Vacuum filtration Mechanical pressing Interfacial gelation Vacuum filtration and leavening strategy Vacuum filtration

B140 F g 1 (0.1 A g 1, 1 M H2SO4) 138 F g 1 (10 mV s 1, 6 M KOH) 83.2 F g 1 (10 mV s 1, 1 M LiPF6) 215 F g 1 (0.1 A g 1, 1 M H2SO4) 273 F g 1 (0.1 A g 1, EMIMPF6) 118.5 F g 1 (1 A g 1, Nafion) 92.7 F g 1 (3 mV s 1) 120 F g 1 (10 A g 1, TEABF4–AN) 172 F g 1 (1 A g 1, 1 M H2SO4) 33.8 mF cm 2 (1 mA cm 2, 1 M H2SO4) 110 F g 1 255.5 F cm 3 (0.1 A g 1, 1 M H2SO4), 261.3 F cm 3 (0.1 A g 1, EMIMBF4 /AN) 372 mF cm 2 (PVA–H2SO4)

Capacitance (rate, electrolyte)

PANi–oriented graphene hybrid films RuO2–liquid functionalized-RGO 3D RGO–PANi film Graphene–PANi composite paper RGO–MnO2 paper 3D graphene–MnO2 composite networks

Graphene–PANi composite paper Graphene–PANi hybrid paper Graphene–PANi nanofiber composite films NiO–graphene three-dimensional networks Graphene–MnO2–CNTs nanocomposite films Co3O4–RGO–CNTs hybrid paper Mn3O4 nanoparticles–graphene paper and CNTs–graphene paper Embossed RGO–MnO2 hybrid films Functionalized graphene hydrogels 3D N and B co-doped graphene hydrogels

Functionalized RGO film Macroporous graphene film Activated RGO film Folded graphene paper RGO gelation RGO foam Liquid electrolyte mediated chemically RGO films Graphene hydrogel films

Vacuum filtration Vacuum filtration

CNTs–graphene films Carbon black pillared graphene paper Graphene film Vacuum filtration

Preparation methods

Comparison of preparation methods, capacitances and, energy density, power density and retention (after cycling) for the reported flexible planar supercapacitors electrodes

Electrode materials

Table 1


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B100% (10 000) 182 96% (7000) 181 183 2

B94% (1500) 68 99.8% (10 000) 180 B90% (10 000) 154

Chem. Soc. Rev.

247.3 F g 1 (176 mA g 1, PVA–H3PO4) B282 F g 1 (1 A g 1) 45.6 mF cm 2 (0.67 mA cm 2, 5 M KOH) 283 mF cm 2 8.36 mF cm 2 (0.02 mA cm 2, PVA–LiCl) 1.7 mW h cm 9.3 W h kg 1 269 F g 1 (20 mV s 1, 1 M H2SO4) CVD Plasma-enhanced CVD Chemical reduction

Electrochemical reduction Vacuum filtration Blade coating Electrochemical RGO films VOPO4–graphene hybrid film Graphene–PANi thin film

1

1

(2 mV s , 1 M Na2SO4)

8.34 W h kg B450 F g Dip-drying

Graphene–MnO2 nanostructured sponges Ultrathin planar graphene N-doped graphene films Graphene hydrogel–Ni foam

1

Capacitance (rate, electrolyte) Preparation methods Electrode materials

Table 1

(continued)

Energy density

2

94 kW kg

5.2 mW cm

1

Retention (cycles) Power density


B90% (10 000) 172

Chem Soc Rev Ref.

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the graphene sheets compared with traditional large-area graphene film electrodes. Furthermore, the graphene microelectrodes allowed for uptake of the electrolyte ions into or removal from the graphene layers with short diffusion pathways. Therefore, the graphene micro-supercapacitors exhibited a much better electrochemical performance than the conventional supercapacitor configuration. The specific capacitance of the graphene microsupercapacitor was 285 F g 1, which is about three times higher than that of a conventional graphene supercapacitor (86 F g 1). In addition, Wu et al. recently developed a flexible all-solidstate graphene based micro-supercapacitor through micropatterning of high temperature RGO films. According to the relatively high electrical conductivity (345 S cm 1) of the fabricated RGO films and the in-plane interdigitated geometry, the resulting micro-supercapacitor delivered a superior electrochemical performance.199 The device exhibited a maximum areal capacitance of 80.7 mF cm 2 and a volumetric capacitance of 17.9 F cm 3. More remarkably, the device showed a power density of 495 W cm 3, and an energy density of 2.5 mW h cm 3, which is comparable to that of lithium thin-film batteries. This device retained 98.3% of its initial capacitance after 100 000 cycles. Operation at high scan rates up to 1000 V s 1 is possible, which illustrates the ultrafast charge and discharge capability and ultrahigh power density of the micro-supercapacitor. Recently, our group introduced a scalable fabrication method for graphene micro-supercapacitors over large areas by using a commercial LightScribe DVD burner (Fig. 17).192 With this method, more than 100 micro-supercapacitors can be produced on a single disc in less than 30 min. The flexible all-solid state microsupercapacitors exhibited an areal specific capacitance of 3.05 F cm 3 at 16.8 mA cm 3, and maintained 60% of this value when operated at a current density of 1.84 104 mA cm 3. This is equivalent to the operation of the device at 1100 A g 1, which is about 3 orders of magnitude higher than the normal current density used for traditional supercapacitors. Furthermore, we introduced an experimental demonstration of an all-solid-state micro-supercapacitor by using an ionogel electrolyte that allows for an operational window of 2.5 V. The micro-supercapacitor also showed exceptional electrochemical stability under different bending and twisting conditions. These micro-supercapacitors exhibited an ultrahigh volumetric power density of 200 W cm 3 and excellent frequency response with an RC time constant of only 19 ms. In order to prevent the aggregation and restacking of graphene sheets and increase the ion accessible surface area for energy storage, adding capacitive spacers such as CNTs,190,200 metal oxides (e.g. MnO2),201 or conductive polymers (e.g. PANi)195,202 proves to be a useful way to improve the overall performance of devices. For instance, Beidaghi et al. developed a RGO–CNTs composite micro-supercapacitor with interdigital microelectrodes (100 mm width and 50 mm spacing) by combining electrostatic spray deposition and photolithography lift-off techniques.200 Electrochemical measurements demonstrated that the addition of CNTs between graphene sheets and the planar interdigital design of microelectrodes sufficiently increase the accessibility of electrolyte ions between stacked


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Fig. 17 (A–C) Schematic diagram showing the fabrication process for an LSG micro-supercapacitor. (D and E) The digital image of the direct writing of micro-devices with high areal density. Reproduced with permission.192 Copyright 2013, Rights Managed by Nature Publishing Group.

RGO sheets and improves the energy and power densities. The RGO–CNTs micro-supercapacitors showed a areal specific capacitance of B6.1 mF cm 2 at a low scan rate of 0.01 V s 1 and retained B2.8 mF cm 2 at a 5000 times higher scan rate of 50 V s 1. To increase the specific capacitance of the microsupercapacitor, Peng et al. fabricated a high performance micro-supercapacitor by forming a hybrid of the pseudocapacitor material MnO2 with graphene films.201 The planar structure based on the MnO2 integrated on graphene sheets can introduce more electrochemically active surfaces for electrolyte ions and additional channels to facilitate charge transport and increase the capacitance by adding MnO2. The all-solid-state micro-supercapacitor was fabricated by using gold-coated PET as a flexible substrate and PVA–H3PO4 as a gel electrolyte. The device exhibited high specific capacitance: 267 F g 1 at a current density of 0.2 A g 1 and 208 F g 1 at 10 A g 1, indicating good rate capability. We have summarized preparation methods, capacitances and, energy density, power density and retention for the recent development of graphene-based interdigital micro-supercapacitors in Table 2. Due to the excellent characteristics of graphene and promising interdigital structure, graphene-based microsupercapacitors are expected to play an important role in serving as energy storage devices for flexible electronics. Although many research efforts have been dedicated to the field of microsupercapacitors, further development, such as designing high performance 3D electrode architectures, and integration with other microelectronic devices, still need to be explored. 3.2

Fiber type supercapacitors

Recently, a fiber type design was introduced to provide supercapacitors for maximum flexibility, far beyond that of conventional 2D planar supercapacitors.35,61,203–211 These fiber supercapacitors are being extensively studied to meet aesthetic demands and the needs of specific operating conditions. They provide


advantages for the direct use as energy storage devices in wearable and embedded electronics and multi-functional ‘‘smart fabrics’’. According to their omni-directional flexibility, the operational space of these fiber supercapacitors are free from restrictions. These devices could stimulate the rapid development of portable and flexible electronics. Graphene-based electrode materials are promising in fiber type flexible supercapacitors. For example, a flexible prototype fiber-shaped solid supercapacitor has been fabricated by Li et al. with electrochemical reduction of GO on gold wires as the electrodes.212 A typical capacitance per unit length of the fiber type flexible supercapacitor was calculated to be 11.4 mF cm 1, corresponding to an areal capacitance of 0.73 mF cm 2. A CV measurement showed a negligible capacitance decrease (o1%) when the supercapacitor was bent to 1201 or even twisted into an S-shaped structure. Meanwhile, Meng et al. fabricated an all-graphene core-sheath fiber to serve as electrodes (GF@3D-G), in which a core of graphene fiber is covered with a sheath of an electrochemically deposited graphene framework (Fig. 18A).213 When driven by an electric field, the graphene sheets were quickly deposited on the surface of the graphene fiber and assembled into 3D interpenetrating porous networks. Most of the graphene sheets were deposited nearly vertically to the surface of the graphene fibers (Fig. 18B and C). Therefore, micro-pores in the electrode are fully exposed to the electrolyte for access of ions to form electrochemical double-layers. Flexible all-solid-state fiber supercapacitors were built by intertwining two GF@3D-G electrodes with H2SO4–PVA gel electrolyte (Fig. 18D). Owing to the high conductivity and highly exposed surfaces of the graphene sheets, the GF@3D-G fiber supercapacitor showed an areal capacitance of 1.2–1.7 mF cm 2 (ca. 25–40 F g 1). The spring-like supercapacitor was fabricated by annealing at room temperature for several hours, making it compressible and stretchable (Fig. 18E and F). To demonstrate applications in portable devices, the fiber supercapacitor was incorporated into

Chem. Soc. Rev.

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200

202 195

201

90 (1700) 97.3% (1500)

B92% (7000)

970 F g 1 (2.5 A g 1, PVA–H3PO4) 667.5 mF cm 2 (15.0 mA cm 2, 0.5 M Na2SO4)

Electrostatic spray deposition and photolithography lift-off Electrodeposition Electrodeposition

3D graphene–CNTs carpets

RGO–CNTs

1

18.64 W h kg (0.2 A g 1) 1

267 F g

RGO–PANi Graphene quantum dots PANi asymmetric device Ultrathin MnO2–graphene films

CVD

RGO Graphene quantum dots Laser-scribed graphene

Vacuum filtration

2.42 mW h cm

0.093 mW h cm

3

2

12.6 kW kg

7.52 mW cm

3

46 W cm

1

2

96% (10 000)

199 194 192 98% (100 000)

495 W cm 3 56.7 mW cm 200 W cm 3 2

9 mW cm 324 W cm 3

14 nW h cm 31.9 mW h cm

Laser scribe Photolithography and electrophoretic deposition Thermal reduction and lithography Electrodeposition Laser scribe RGO RGO

2

3

2

2.5 mW h cm 3 0.474 mW h cm

3

2

B70 (10 000)

193 198

0.51 mF cm and 3.1 F cm (hydrated GO) 86 F g 1, 462 mF cm 2, 359 F cm 3 (1 A g 1, H3PO4–PVA) 80.7 mF cm 2 and 17.9 F cm 3 (H2SO4–PVA) 468.1 mF cm 2 (15 mA cm 2, EMIMBF4–AN) 2.32 mF cm 3 (16.8 mA cm 3, PVA–H2SO4) 2.35 mF cm 3 (16.8 mA cm 3, ionogel) 2.16 mF cm 2 (1 M Na2SO4) 4.82 mF cm 2 (BMIM-BF4) 6.1 mF cm 2 (10 mV s 1, 3 M KCl)

2

Retention (cycles) Power density Energy density Capacitance (rate, electrolyte) Preparation methods Electrode materials


Comparison of preparation methods, capacitances and, energy density, power density and retention for the reported interdigital micro-supercapacitor electrodes Table 2

Chem. Soc. Rev.

190

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a textile by conventional weaving technology. Moreover, in order to make full use of the benefits of CNTs and graphene, Cheng et al. fabricated CNT–graphene hybrid fibers by directly growing CNTs along graphene fibers with a CVD method.214 The prepared CNT–graphene hybrid fibers exhibited an areal capacitance of 1.2–1.3 mF cm 2. Instead of fabricating various structural fiber electrodes, Kou et al. designed a coaxial wetspinning assembly method to fabricate continuous polyelectrolytewrapped coaxial fibers with different cores of pure RGO, pure CNTs and mixtures of RGO and CNTs.215 Given the attributes of the shell of the coaxial fibers, the fabricated fiber supercapacitor can effectively decrease the distance of the two electrodes and avoid the risk of short circuit, which is one of the main safety concerns for fiber supercapacitors. By using H3PO4–PVA as gel electrolyte, the RGO–CNTs hybrid fiber supercapacitor exhibited the highest capacitance among the three types (CL B 5.3 mF cm 1, CA B 177 mF cm 2 and CV B 158 F cm 3). The significant improvement of capacitance could be attributed to the large surface area of the materials and the fast ion transportation between the two electrodes. Furthermore, Yu et al. developed a scalable method to continuously produce hierarchically structured fiber made of nitrogen-doped RGO hybrids with SWCNTs.216 The nanocomposite fibers have mesoporous structures of large specific surface area of 396 m2 g 1 and high conductivity of 102 S cm 1. The resultant fiber type supercapacitor shows a high volumetric capacitance of 300 F cm 3 in PVA–H3PO4 gel electrolyte. The all solid state fiber supercapacitor can be easily integrated into miniaturized flexible devices and power an ultraviolet photodetector and a LED. To further increase the electrochemical performance of fiber supercapacitors, an asymmetric fiber flexible all-solid-state supercapacitor was fabricated by using Co3O4 coated titanium wire as the positive electrode and carbon fiber–graphene as the negative electrode with a PVA–KOH gel electrolyte.217 The asymmetric fiber supercapacitor can be operated up to 1.5 V. The extended potential window and porous microstructures simultaneously enhanced the electrochemical performance. The fiber supercapacitor exhibited a high volumetric capacitance at a current density of 20 mA cm 3, and a corresponding gravimetric specific capacitance of B250 F g 1, achieving a maximum volumetric energy density of 0.62 mW h cm 3 and power density of 1.47 W cm 3. Similarly, Yu et al. constructed a solid state asymmetric fiber supercapacitor by using MnO2 coated RGO–SWCNTs hybrid fiber as a positive electrode and N-doped RGO–SWCNTs fiber as a negative electrode.218 The optimized fiber supercapacitor can offer a volumetric energy density of 5 mW h cm 3 and a significantly high power density of 929 mW cm 3 and operate at a high voltage window of 1.8 V. According to the distinctive features of omni-directional flexibility, the fiber type supercapacitors can be interconnected into electronic devices with exceptional mechanical flexibility. For example, fiber type supercapacitors could be directly mounted onto the wrist, neck, or waist of a user’s body. As summarized in Table 3, although great efforts have been paid to the field of graphene-based materials for fiber type supercapacitors, the electrochemical performance are also far from satisfied, when


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Fig. 18 (A) A photo of a distorted GF@3D-G. (B, C) Different magnification SEM images of a GF@3D-G. (D) Schematic illustration of a fiber-shaped supercapacitor fabricated from two intertwined GF@3D-Gs with polyelectrolyte. (E, F) A photo of a spring-like supercapacitor. Reproduced with permission.213 Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

compared with some other kinds of materials, such as CNT fibers219,220 and composites.210,221–224 Therefore, several technical challenges remain including interface engineering of the electrodes and electrolyte, the prevention of graphene sheet restacking, and their large-scale assembly, before the promise of fiber type supercapacitors can be realized in commercial applications.

4. Summary Flexible supercapacitors are receiving tremendous attention and research interest due to the increasing need for flexible/ wearable energy storage devices. In this review, we have summarized the recent progress made on graphene-based materials for flexible supercapacitor electrodes, with a specific focus on the fabrication methods, different kinds of substrates, the conductivity and macro/micro-structures needed for graphenebased electrodes, and their influences on the electrochemical performance. In addition, recent efforts on innovative flexible supercapacitors, such as micro-supercapacitors and fiber-type supercapacitors have also been presented. In order to apply these supercapacitors for practical flexible/wearable applications, each component in the flexible supercapacitor must withstand a certain amount of deformation. Thus, current research has mainly focused on fabricating flexible graphenebased materials, and combining them with shape-conformable solid electrolytes, and soft substrates. Although recent developments in graphene based materials for flexible supercapacitors appear extremely promising, there remain challenges for different types of electrode materials for practical applications. For example, while freestanding 2D graphene-based electrodes exhibit high electrical conductivity, and excellent mechanical flexibility, they have only moderate rate stability and power density due to limited porous structures


that generally confine the effective transport and diffusion of electrolyte ions. In contrast, interconnected graphene networks and multi-stage porous structures give the freestanding 3D graphene based electrodes excellent rate and power performance, but the 3D porous electrodes have finite mechanical strength and the preparation methods are often complicated. Graphene-based electrodes supported on 3D substrates generally show better mechanical and electrochemical performance due to their robust, large surface area and porous substrates; however, the low capacitance contribution of the inactive substrates results in a decrease of the actual gravimetric and volumetric capacitance of the whole device. Advantages always seem to be accompanied by trade-offs in these different kinds of graphene-based electrodes. Thus, there exists substantial room for stimulating research and development of high-performance flexible supercapacitors for flexible/wearable electronic applications.

5. Future prospects (1) Despite the above mentioned impressive achievements, further developments are needed for high-performance flexible supercapacitor electrodes that can simultaneously ensure high capacitance and excellent rate and cycling stability. Graphenebased materials are promising candidates due to their large surface area, high conductivity, low weight and superior mechanical flexibility. Fabrication of composites that combine the advantages of graphene and pseudo-capacitor materials is an effective way to enhance performance. However, to further improve the flexible supercapacitors, the following aspects should be considered: (i) develop a deeper understanding of the energy storage mechanisms of supercapacitors, especially for the interfacial reactions between the electrode and electrolyte; (ii) rationally design the porous structures inside the supercapacitor electrodes to form hierarchical interconnected porous

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217

216 210 218 93% (10 000)

84% (1000)

225 215 95% (1000) B100% (2000)

87% (10 000)

214 40.8% (1000)

212 213 B94% (10 000)

3

1.47 W cm

929 mW cm

3

W cm Hydrothermal treatment and CVD

CVD Coaxial wet-spinning assembly

Capillary hydrothermal Wet-spinning Hydrothermal synthesis and redox deposition Solvothermal method

Graphene–carbon fabric Graphene–CNTs

CNTs–N-doped graphene Graphene–PPy composite fiber MnO2–RGO–CNTs and N-doped RGO–CNTs asymmetric device Co3O4 and carbon fiber–graphene asymmetric device

(20 mA cm 3, 6 M KOH) 3

2.1 F cm

7

3

3.84 mW h cm 2 5.91 mW h cm 2 6.3 mW h cm 3 9.7 mW h cm 2 5 mW h cm 3 0.98 mF cm 2 and 200.4 F g 1 (20 mA cm 2, 1 M Na2SO4) 173 F g 1 and 44.7 mF cm 2 (1 M Na2SO4) 177 mF cm 2 and 158 F cm 3 (H3PO4–PVA) 269 mF cm 2 (239 F cm 3) (1 M H2SO4) 300 F cm 3 (26.7 mA cm 3, PVA–H3PO4) 115 mF cm 2 (0.2 mA cm 2, 1 M NaClO4) B11.1 F cm 3 (25 mA cm 3, PVP–Na2SO4)

Electrochemically reduction Electrochemically reduction

Electrochemical RGO Graphene fiber/3D graphene framework CNT–graphene hybrid fibers

1 2

Capacitance (rate, electrolyte)

0.62 mW h cm

1.7 10 78.3 mF cm and 3.12 mF cm (0.1 A g 1, H3PO4–PVA) 10 mF cm 1 (50 mV s 1, H3PO4–PVA) 1.7 mF cm 2 (H2SO4–PVA) Rolling assembly and freeze dry Porous graphene ribbons

W h cm

2

1 10

4

Power density Energy density Preparation methods Electrode materials

Comparison of preparation methods, capacitances and, energy density, power density and retention for the reported fiber type supercapacitor electrodes Table 3


2

209

Retention (cycles)

99% (5000)

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microstructures and avoid the formation of dead volume or the collapse of the porous microstructures; and (iii) control the interfacial interactions between graphene nanosheets and pseudo-capacitive materials to achieve a well-designed flexible structure with high electrochemical performance. In addition to fabricating high performance electrode materials, a unified system to evaluate the electrochemical performance of flexible supercapacitors is needed. In addition, as can be seen in Tables 1–3 of this review, it is difficult to compare the reported performances, due to the inconsistent performance evaluation methods of different flexible supercapacitors. Therefore, uniform standards for calculating performance of flexible supercapacitors are needed. (2) As flexible supercapacitors are being developed for future practical application, the same effort needs to go into investigating mechanical flexibility. To date, there is no unified evaluation standard to characterize flexibility. Therefore, a comprehensive standardized metrology that accurately evaluates the mechanical flexibility should be established. (3) In general, the trend for the development of flexible supercapacitors is to integrate or embed them into flexible electronic devices to create multifunctional or self-powered hybrid systems. Some efforts have been paid to combine flexible supercapacitors with different electronics, such as solar cells,226–229 Li-ion batteries,34,230 electrochromic devices,231 and nanogenerators.232–235 However, integration of these devices into practical applications is still a challenge. In conclusion, graphene-based materials for flexible supercapacitors have been extensively explored. The research results indicate that graphene based materials will play an important and perhaps irreplaceable role in flexible supercapacitors due to their intriguing features. Although many challenges remain, we believe that flexible graphene-based supercapacitors will emerge as a widely used technology in the near future.

Acknowledgements The authors would like to thank all the students and collaborators who have worked diligently to carry out the original experiments described here. Prof. Hongzhi Wang gratefully acknowledges financial support from the Fundamental Research Funds for the Central Universities (2232014A3-06), and Innovative Research Team in University (IRT 1221). Yuanlong Shao would like to thank the China Scholarship Council for financial support for studying abroad at UCLA. Mir Mousavi would like to thank the Tarbiat Modares University Research Council and Maher El-Kady and Richard Kaner thank Nanotech Energy for financial support.

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