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Figure 1.3 (a) Schematic diagram illustrates the growth process for preparing H‐MnO₂ NRs on carbon cloth substrate. (b) CV curves obtained at different bent conditions at 200 mV s⁻¹. Insets are the photos of ASCs device on finger. (c) Schematic diagram and photo images of wearable ACS in real applications (sewing on the clothes model and powering electronic watch).

      Source: Reproduced with permission [98]. © 2014, Elsevier Publishing.

Unfortunately, most carbon‐based anodes are relatively low in capacitance due to the electrochemical double layer energy storage mechanism. To this end, effective strategies of achieving high‐energy‐dense AFSCs has been extensively developed by employing pseudocapacitive anodes such as functionalized carbon, transition metal oxides, transition metal nitrides, conductive polymers, etc. [90] Recently, Wang et al. [90] creatively applied electrochemical activation to CC (Figure 1.4a), which were rarely employed as SC electrode materials because of its intrinsic low capacitance as a result of the small surface area and poor electrochemical activity [32]. The obtained electrochemically activated carbon cloth (EACC) anode was coupled with MnO₂@TiN loaded on CC as cathode to fabricate a novel sandwich‐type AFSC (denoted as MnO₂@TiN//EACC) with an extended operation voltage window of 2 V (Figure 1.4b). Besides the broadened voltage window, the impressively boosted capacitance of EACC due to the roughened surface and the introduction of oxygen‐containing groups on the surface for redox reactions also contribute to an excellent energy density as high as 1.5 mWh cm⁻³, which enables its successful application in powering light emitting diode (LED) indicator even under bent condition (Figure 1.4c).

      1.3.1.2 Transition Metal Oxide Anodes

Transition metal oxides can generate reversible redox reactions on the surface or even in the bulk during charging/discharging, which results in much higher capacitances compared to carbon‐based anodes. By integrating transition metal oxides with flexible current collectors as pseudocapacitive anodes, the energy density of AFSC can be drastically enhanced. For example, Yang et al. [54] grew α‐MnO₂ nanowires (NWs) and amorphous Fe₂O₃ nanotubes (NTs) on flexible carbon textile as the pseudocapacitive cathode and anode respectively (Figure 1.5a) to fabricate a sandwich‐type flexible asymmetric pseudocapacitor (Figure 1.5b). The as‐fabricated sandwich‐type AFSC operates at a maximum cell voltage of 1.6 V (Figure 1.5c) and delivers high energy density of 0.55 mWh cm⁻³ (Figure 1.5d). Two devices connected in series can readily operate a blue LED after charging (Figure 1.5d inset), indicating the potential of the AFSC in future applications. Similarly, a novel flexible all‐solid‐state asymmetric SC fabricated with a carbon‐fabric‐loaded WO_3–x/MoO_3–x core/shell nanowires anode and a polyaniline cathode was reported by Xiao and his co‐workers. The device showed satisfactory energy density (1.9 mWh cm⁻³), impressive cyclic stability, as well as good mechanical flexibility.