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The most reliable anode materials for sandwich‐type AFSCs are carbon‐based materials with significant excellence in conductivity and mechanical stability, such as graphene, CNTs, carbon fibers, etc. [24, 25, 27, 56,95–97] For example, Zhai et al. [98] successfully synthesized hydrogenated MnO₂ nanorods (H‐MnO₂) on carbon cloth (CC) via electrodeposition followed by annealing in hydrogen atmosphere (Figure 1.3a), and loaded reduced graphene oxide (RGO) on CC using vacuum process. The obtained H‐MnO₂ cathode and RGO anode were assembled as flexible solid‐state AFSC with LiCl/PVA gel electrolyte and a separator sandwiched in between. The as‐fabricated sandwich‐type AFSC (denoted as H‐MnO₂//RGO) exhibited a reliable operating voltage window as wide as 1.8 V and extraordinary mechanical tolerance to bending (Figure 1.3b). Owing to the significantly wide potential window, the device achieved a high energy density of 0.25 mWh cm⁻³ at power density of 1.01 W cm⁻³, which has surpassed many SFSCs and some AFSCs previously reported. To verify the feasibility of the AFSCs device as energy storage device for wearable electronics, two H‐MnO₂//RGO devices were tailored on a laboratory coat in series and able to power an electronic watch (Figure 1.3c). Recently, Yu and his co‐workers [25] reported a sandwich‐type AFSC with CNT‐textile anode and MnO₂/graphene‐textile cathode, which achieved an operating potential window of 1.5 V and a maximum energy density of 12.5 W h kg⁻¹. Choi et al. [56] developed a solid‐state AFSC based on an ionic liquid functionalized chemically modified graphene (IL‐CMG) film as anode and a hydrous RuO₂‐ILCMG composite film as cathode, which reached a high output voltage of 1.8 V and thus delivered a maximum energy density of 19.7 W h kg⁻¹ and maximum power density of 6.8 kW kg⁻¹. Moreover, the as‐fabricated device exhibited superior cyclic stability even when bent or twisted.

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).