MnO₂@TiN//EACC devices.

      Source: Reproduced with permission [90]. © 2015, Wiley‐VCH.

      1.3.1.3 Transition Metal Nitride Anodes

Owing to the high conductivity and transition metal sites with multiple valence states, transition metal nitrides are emerging as promising pseudocapacitive anode materials with fast and reversible redox reactions. Many transition metal nitrides have been exploited for AFSCs, such as titanium nitride, vanadium nitride, tungsten oxynitride, iron nitride, etc., with high performances comparable to transition metal oxide anodes [91,99–101]. For instance, Fan's group successfully fabricated an all‐metal nitrides solid‐state asymmetric SC, where the titanium nitride (TiN) cathode and iron nitride (Fe₂N) anode were grown on CC‐loaded graphene nanosheets (GNS) using atomic layered deposition followed by calcination under ammonia atmosphere (Figure 1.6a). The porous configuration of TiN and homogeneous distribution of Fe₂N nanoparticles contribute to the extraordinary cycling durability (≈98% capacity retention after 20 000 cycles) of the fabricated quasi‐solid‐state AFSC device using PVA/LiCl polymer gel as neutral electrolyte (Figure 1.6b). The AFSC device achieved a maximum energy density of 0.61 mWh cm⁻³ and a maximum power density of 422.7 mW cm⁻³, which were substantially higher than those of transition‐metal‐nitride‐based SCs and PVA‐based solid‐state SCs (Figure 1.6c). Lu's group has reported various AFSCs using CC‐loaded transition metal nitride anodes in recent years. For example, they used neutral PVA/LiCl polymer gel electrolyte to effectively stabilize porous VN NWs anode, and paired it with VO_x NWs cathode to assemble a stable and high‐performance quasi‐solid‐state AFSC device with a high output voltage of 1.8 V [91]. Furthermore, the VO_x//VN‐AFSC device was able to deliver an impressive volumetric capacitance of 1.35 F cm⁻³, a highest energy density of 0.61mWh cm⁻³ and extraordinary cycling stability with 12.5% loss of capacitance after 10 000 cycles. They also prepared holey tungsten oxynitride (WON) nanowires on CC through the annealing of WO₃ precursor nanowires in ammonia atmosphere [100]. The as‐fabricated AFSC device with WON NWs anode and MnO₂ cathode could deliver a high working voltage of 1.8 V and volumetric capacitance of 2.73 F cm⁻³. The maximum energy density of MnO₂//WON AFSC device was 1.27 mWh cm⁻³ at a power density of 0.62 W cm⁻³, which has transcended many reported AFSC devices.

Figure 1.5 (a) Schematic diagram illustrating the synthesis procedure of MnO₂ NWs and Fe₂O₃ NTs on carbon cloth. (b) Schematic sketch illustrating the designed asymmetric supercapacitor device. (c) CV curves of the assembled solid‐state AFSC device collected in different scan voltage windows. (d) Ragone plots of the solid‐state AFSC device. Inset shows a blue LED powered by the tandem AFSC devices.

      Source: Reproduced with permission [54]. © 2014, American Chemical Society.

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.

Figure 1.7 (a) Schematic illustration of the design and fabrication of the asymmetric fiber‐based micro‐SC. (b) Comparative CV curves obtained for the GCF‐N2