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Figure 1.6 (a) Schematics of the fabrication processes of metal nitride cathode and anode materials. (b) Cycling performance of full device at 4 A g⁻¹ in 20 000 cycles with different bending situations. (c) Ragone plots of quasi‐solid‐state TiN‐Fe₂N AFSC in comparison with other PVA‐based solid electrolyte SFSCs and AFSCs. Inset: pink and white LEDs in parallel are lit up by two full devices in tandem.

      Source: Reproduced with permission [101]. © 2015, Wiley‐VCH.

      1.3.1.4 Conductive Polymer Anodes

      Conductive polymers are promising candidates as pseudocapacitive materials owing to their good conductivity and reversible redox reactions during charging/discharging, but they are mostly applied as cathode materials while rarely studied as anode materials for AFSCs. Recently, Wang et al. synthesized 150 WO₃@PPy nanowires on carbon fibers as the anode and grew Co(OH)₂ nanowires on carbon fabric as the cathode for AFSC device. The as‐fabricated AFSC device exhibited apparent pseudocapacitive behavior within a stable potential range of 0–1.6 V. The maximum volumetric capacitance of 2.8 F cm⁻³ was achieved at a scan rate of 20 mV s⁻¹. Moreover, the asymmetric supercapacitor (ASC) device delivered an energy density as high as 1.03 mWh cm⁻³.

      1.3.2 Fiber‐Type ASCs

      Despite distinct advances, the planar‐shaped SCs are still insufficient in deformability for weaving into textiles or integrating into linear‐shaped electronics. In this regard, researchers have creatively assembled electrodes with one‐dimensional geometry to fabricate fiber‐type AFSCs. Fiber‐shaped AFSCs have been developed into multiple configurations including parallel type, wrap type, coaxial‐helix type and two‐ply yarn type, in order to effectively meet the demands of different wearable energy textiles, including sensing [102–104], communication [105], and storage [106].

      1.3.2.1 Parallel‐Type Fiber AFSCs

For a parallel‐type fiber AFSC, two fiber‐shaped electrodes are assembled side‐by‐side, separated by gel/polymer electrolyte, and finally supported on a flat substrate [60, 107–109]. For instance, Yu et al. [109] reported a parallel type all‐solid‐state asymmetric micro‐SC using MnO₂‐deposited rGO/SWCNT fiber as the cathode (denoted as GCF/MnO₂‐10) and an N‐doped rGO/SWCNT fiber as the anode (denoted as GCF/N2) (Figure 1.7a). By fully utilizing the potential window of both cathode (0 ~ 0.9 V) and anode (−0.9 ~ 0 V), the device showed a high output voltage of 1.8 V (Figure 1.7b). Excellent electrochemical performances such as good cycling stability (87% capacitance retention after 10 000 cycles), high energy density (5 mW h cm⁻³) and power density (929 mW cm⁻³) were also achieved. Furthermore, such device geometry exhibited promising mechanic stability under different bending states (Figure 1.7c). This asymmetric micro‐SC device was testified as a reliable power source for a ZnO film‐based UV photodetector, suggesting its promising potential in future applications.