Three weave styles of carbon fabrics. Adopted from Hexcel Corporation. 

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... an issue of national security for every country on earth. Meeting energy needs in an environmentally sustainable manner is currently the most important technologi- cal challenge facing society. Energy storage devices can efficiently store electricity generated from renewable sources, such as solar, water, wind, thermoelectric, fuel cells, for reuse at many different scales. Therefore, energy storage devices, such as batteries and supercapacitors, have become key to society (Guerrero, Romero, Barrero, Milanes, & Gonzalez, 2009; Liu, Li, Ma, & Cheng, 2010; Wang, Wei, & Qi, 2007). Electrochemical capacitors (supercapacitors) have been progressing rapidly in recent years. Because they have a particularly high power density, long life cycle, rapid charge/discharge, a wide operating temperature range, environmentally benign and safe (Wang et al., 2007; Zhang et al., 2009), supercapacitors show a great potential in both consumer electronics and large-sized energy storage applications, such as in the communications, transportation, aviation and power indus- tries. Supercapacitors have been produced at an industry scale and commercialized for decades. Typically, a supercapacitor includes a plastic outer package, positive electrode, negative electrode, and separators. The electrodes, the key to a supercapacitor, are generally made from active powder materials, conductive additives and binders. The active material and conductive additive powders are brought together on a metal current collector by binders at several weight percents. Then, the positive and negative electrodes together with the separators are folded into several layers, either rectangular shaped or wired to be cylindrically shaped (Burke, 2000). Because the concept of wearable, miniaturized, portable, flexible electronic products is being put forward, there is currently a strong need for the development of new, flexible, lightweight, low-cost and environmentally friendly energy storage devices. However, the state-of-the-art supercapacitors are far from their potential. Therefore, particular attention is now being given to transparent and/or flexible supercapacitors. Large- scale transparent, flexible electronic devices have been pursued for their potential applications in touch sensors and display technologies (Hu, Zhu, Wang, & Chen, 2011; Ju et al., 2008; Nyholm, Nystrom, Mihranyan, & Stromme, 2011; Pushparaj et al., 2007; Wei et al., 2009). In this paper, the recent progress on flexible supercapacitors, flexible electrodes and electrolytes is reviewed. In addition, the future challenges and opportunities are discussed. Two types of supercapacitors are based on the charge storage mechanism: the electric double-layer capacitor (EDLC) and pseudocapacitor. Carbon materials are typically used as electrode materials of an EDLC, whereas pseudocapacitor materials include metal oxides and conducting polymers (Xu, Kang, Li, & Du, 2010; Xu, Wei, Li, Kang, & Guan, 2011). Flexible electrodes are primarily based on carbon materials (Pushparaj et al., 2007; Wang et al., 2009). Carbon materials have a variety of geometric shapes from a macroscopic point of view, such as zero-dimensional (0D) fullerene or carbon particles, one- dimensional (1D) carbon nanotubes (CNTs) or carbon fibers (CFs), and two-dimensional (2D) graphene or graphite sheets (Fig. 1(a)). Among them, 1D and 2D carbon materials have been widely used in flexible electrodes because they can readily form highly conductive and flexible carbon networks (such as carbon fabric, carbon film, carbon paper and carbon textile, as shown in Fig. 1(b)) with outstanding electrochemical performances. Based on these carbon networks, we can roughly divide flexible electrodes into two categories: single carbon electrodes and carbon composite (CC) electrodes. Single carbon electrodes are composed of only carbon networks made from one or more carbon shapes, whereas carbon composite electrodes are composites of versatile carbon networks with various pseudocapacitor materials. Carbon networks consisting of carbon fabric, cloth, film, coating, paper or textiles are important in the development of flexible electrodes. These carbon architectures generally come from single 1D and/or 2D carbon particles, which form aggregates by hydrogen bonds or van der Waals forces (Hu & Cui, 2012; Hu, Pasta, et al., 2010; Hu, Wu, La Mantia, Yang, & Cui, 2010). Various methods have been used to create carbon networks from 1D or 2D carbon particles, such as weaving, chemical vapor deposition (CVD), printing, filtration, evaporation and dipping-drying (Fig. 1(b)). In this section, we explain the preparation procedures for carbon cloth, film, paper, and textiles. The best-known network used for flexible electrodes is carbon fabric (CF), which can be fabricated by carbon fibers through a commercial weaving method (Fig. 1(b), F1). Carbon fabrics manufactured with a loom commonly use the three main weave styles: plain, satin and twill weaves (Fig. 2). In addition, fibers can be mixed to obtain hybrid fabrics. Woven fabric exhibits good strength, stiff- ness, and excellent flexibility; however, its low capacity severely limits its application for electrodes (Masarapu, Wang, Li, & Wei, 2012). Therefore, woven fabric combined with pseudocapacitor materials are important in the following carbon composite electrodes (Reddy, Amitha, Jafri, & Ramaprabhu, 2008). The other three carbon networks, which include carbon film, carbon paper, and carbon textile, are achieved by using 1D carbon nanotubes or 2D graphene sheets as the starting materials via a variety of methods. As described in Fig. 1, the procedure to create the three carbon networks all involve a similar starting procedure, which is the dispersion of carbon materials in a suitable solvent to produce a stable solution (also known as conductive ink in printing process and dye solution in dipping-drying technology). Impor- tantly, surfactants, such as sodium dodecylbenzene sulfonate, are also necessary for good dispersion. Carbon films are usually fabricated by a CVD method and a printing process (Fig. 1(b), C1 and C2). A buckled single-walled carbon nanotube (SWNT) film has been prepared with a simple CVD method on an elastomeric polydimethylsiloxane (PDMS) substrate followed by the relaxation of the prestrained substrate (Yu, Masarapu, Rong, Wei, & Jiang, 2009). The electrochemical performance of the supercapacitor assembled by SWNTs macrofilms is not influenced by the change in bending degree as shown in experiments performed at different levels of prestrain load on the substrate. We note that the printing process refers to printing of specially made inks on a substrate with Meyer rods, brushes (Fig. 1(b), C2), ink-jet printers or spin-coating tools (Chen, Chen, Qiu, & Zhou, 2010; Grande et al., 2012; Hu & Cui, 2012). The substrate can be flexible plastic or paper; paper has a relatively high conductivity due to its porous structure, which leads to a large contact area and good adhesion with inks. It has been found that the sheet resistance of printed paper by a scalable Meyer rod coating method is as low as 10 /sq (Hu et al., 2009; Hu & Cui, 2012). However, occasionally, certain pores in the paper can be too large, which results in nanotubes penetrating into the pores, leading to the supercapacitor being short-circuited. To avoid this outcome, a surface pretreatment that applies coatings onto both faces with polyvinylidene fluoride (PVDF) has been applied (Kaempgen, Chan, Ma, Cui, & Gruner, 2009). The preparation of carbon papers is represented in Fig. 1(b) (P1 and P2). Suspensions of 1D carbon materials can become freestanding paper after a filtration method (Kang, Li, Hou, Wen, & Su, 2012; Yu, Roes, Davies, & Chen, 2010) or an evaporation procedure (Izadi-Najafabadi et al., 2011; Torop et al., 2011). In the filtration process, special paper is fabricated by carbon materials through hydrogen bonds or van der Waals forces in the filter membrane. The carbon paper can be used as an electrode directly (Kang et al., 2012) or after a further treatment (Yu et al., 2010) to remove the filter membrane by transferring it onto a plastic substrate. For example, graphene/cellulose paper can be fabricated by filtrating the mixed solution of graphene sheets with cellulose pulp. Moreover, it has been shown that a supercapacitor assembled by two flakes of graphene/cellulose papers in an organic electrolyte of 1 M LiPF 6 in ethylene carbonate/propylene carbonate/dimethyl carbonate (EC/PC/DMC = 1:1:1) exhibited a large specific capacitance, approximately 252 F/g at a current density of 1 A/g (Kang et al., 2012). In addition, self-standing carbon paper can be created by evaporating solvent on a Petri dish (Izadi-Najafabadi et al., 2011) or a special mold (Pushparaj et al., 2007; Torop et al., 2011). These paper-making methods are also extensively used in fabricat- ing carbon composite electrodes, which will be further described subsequently. Carbon textiles are fabricated by dipping-drying technology, which is similar to the cloth dyeing process in the cloth industry. As the T1 procedure is described in Fig. 1(b), a piece of fabric is dipped into a pre-made solution (so-called dye solution), and then, the solvent is removed after drying (Hu, Pasta, et al., 2010; Pasta, La Mantia, Hu, Deshazer, & Cui, 2010). Used as electrodes, these highly conductive carbon textiles have outstanding flexibility and exhibit outstanding electrochemical performance. Mauro Pasta et al. (2010) fabricated a SWNTs textile by this method with a sheet resistance as low as 1 /sq and demonstrated that aqueous supercapacitors made from these textiles in a Li 2 SO 4 electrolyte displayed a specific capacitance of 70–80 F/g at a current density of 0.1 A/g, which rarely decreased after 35,000 cycles. Hu and Cui (2012) concluded that coating pseudocapacitor materials (such as MnO 2 ) onto these textiles can greatly increase their specific ...

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... best-known network used for flexible electrodes is carbon fabric (CF), which can be fabricated by carbon fibers through a commercial weaving method ( Fig. 1(b), F1). Carbon fabrics man- ufactured with a loom commonly use the three main weave styles: plain, satin and twill weaves (Fig. 2). In addition, fibers can be mixed to obtain hybrid fabrics. Woven fabric exhibits good strength, stiff- ness, and excellent flexibility; however, its low capacity severely Fabrication of carbon networks with 1D carbon fibers (CF), 1D carbon nanotubes (CNTs) or 2D graphene as the starting materials using a variety of methods. With ...