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In general, either enhanced capacitance (C) or enlarged operating voltage (V) of the device should make sense. Of which, the C of a FSC device can be equivalent to the negative electrode capacitance (C n) and positive electrode capacitance (C p) connected in series (Figure 1.2a), which can be calculated using Eq. (1.2)
Figure 1.1 (a, b) Scheme and optical image of a flexible acoustic device.
Source: Reproduced with permission from Ref. [121], © 2017, Springer Nature.
Optical image of (c) a flexible circuit
Source: Reproduced with permission from Ref. [2], © 2018, NPG
, (d) multiplexed fingerprint sensor. Scale bar, 1 cm.
Source: Reproduced with permission from Ref. [5], © 2018, NPG
and (e) artificial skin electronics
Source: Reproduced with permission from Ref. [8], © 2018, NPG.
(f) 3 × 3 honeycomb‐like supercapacitor array powering LED panel.
Source: Reproduced with permission from Ref. [13], © 2017, Wiley.
(g) Image of an array of field‐effect heterojunctions on textile.
Source: Reproduced with permission from Ref. [14], © 2017, NPG.
(h, i) Fabrication and optical image of the fiber‐shaped Al‐air battery.
Source: Reproduced with permission from Ref. [15], © 2016, Wiley.
Figure 1.2 (a) The equivalent circuit of an AFSC. (b) Schematic illustration of the typical configuration of AFSCs and (c) Cyclic voltammograms (CV) curves as schematic illustrations of typical AFSCs.
Source: Reproduced with permission [30]. © 2016, Royal Society of Chemistry.
The maximum C value of the FSCs can be reached when C n is equal to C p . Thus, early investigations focused on the symmetric flexible supercapacitors (SFSCs) with cathodes and anodes being identical for achieving higher device capacitance [31–34]. However, due to their limited potential voltage (<1 V in aqueous electrolyte), the energy density of SFSCs is still unsatisfactory. Notably, the V of a FSC device related to the capacitive potential range of electrodes. Thus, asymmetric flexible supercapacitors (AFSCs), also called hybrid SCs or battery‐capacitor SCs, are designed with different electrodes configured together (Figure 1.2b) [24–28]. By making use of the distinct capacitive potential range, AFSCs have been widely proven to effectively achieve high operating voltages (even >2 V in aqueous electrolyte) as well as optimized capacitance after balancing the charge between the specific positive and negative electrodes (Figure 1.2c) [30, 35]. In addition, they have several important advantages including small size, low weight, ease of handling, excellent reliability, and a wider range of operating temperatures. Therefore, AFSCs have become one of the most promising energy storage devices for flexible and wearable electronics.
In this context, to achieve high electrochemical performance while maintaining good mechanical stability, FSCs with asymmetric structure could realize further gains, and thus arouse global efforts in relative research. This chapter enumerates some typical newly developed AFSCs in terms of structure design of electrode materials and device's configuration engineering. We first focus on the guidelines on the material design and charge balance of a typical AFSC device. Furthermore, different types of various newly developed AFSCs, including sandwich‐type, fiber‐type, and the other type of AFSCs devices, are illustrated based on various electrode materials. Finally, the future developing trends and challenges are discussed to provide certain reference to readers on how to contrive this device.
1.2 Configurations of AFSCs Device
Specifically, AFSCs device can be fabricated by constructing two flexible dissimilar electrodes (a Faradaic positive electrode and a capacitor‐type negative electrode), a separator and, in most cases, quasi‐solid‐state electrolyte in a soft package. Among various types of quasi‐solid‐state electrolytes, gel polymer electrolytes have been extensively used in FSCs due to its relatively high ionic conductivity [36–40]. Soft and bendable plastics including polyethylene terephthalate (PET) [41–44], polydimethylsiloxane (PDMS) [45], and ethylene/vinyl acetate copolymer (EVA) film [46] are typically used as packaging materials for FSC devices.
Considering that the fundamental limit of energy storage capability is largely determined by the electrode material, either the material choice or structure design of electrode materials are of vital importance. Apart from directly fabricated freestanding films like carbon nanotube (CNT) films [47, 48] and graphene films [49, 50], previous reports for FSCs indicate that the flexible electrodes can also rely on a flexible substrate such as thin metal foils [51, 52], polymer substrates [53], textiles [54], and papers [55], to provide flexibility. The main differences between the AFSCs and SFSCs are that the AFSCs require that the positive and negative electrodes are not the same, but they need to be matched well. Electrode materials that are dominated by Faradaic reactions such as metal oxides (RuO2 [56, 57], MnO2 [58–64], CoO [60,65–67] NiO [68–70], V2O5 [71, 72], etc.), metal sulfides (NiCo2S4 [73–75], MoS2 [76], CoS2 [77, 78], NiS [58, 63, 79, 80], etc.) and conductive polymers (polyaniline (PANI) [32, 81], polypyrrole (PPy) [82, 83], poly (3,4‐ethylenedioxythiophene) (PEDOT) [84, 85] etc.) are normally applied as positive electrodes in AFSCs due to their high specific capacitance and relatively higher potential window. Notably, carbon‐based materials (activated carbon [60, 66, 67], graphene [59, 86], CNTs [87], carbon fibers [88–90] etc.), metal nitrides (TiN [20], VN [91], MoN [92], etc.), and some metal oxides (FeOx [93], MoOx [94] etc.) are usually employed as negative electrodes because of their fast charging/discharging rate and suitable working window at negative potential.
However, before they are assembled in an AFSC, the matching problems of the two electrodes with different theoretical capacitance need to be solved [91]. As for an AFSC, the charge balance will follow the relationship q+ = q−. The charge stored by each electrode depends on the specific capacitance (C), the potential range for the charge/discharge process (E) and the mass of the active electrode material (m), following the Eq. (1.3):
(1.3)