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      Source: Reproduced with permission [109]. © 2014, Wiley‐VCH.

      1.3.2.2 Wrap‐Type Fiber AFSCs

Schematic illustrations of (a) the as-assembled fiber-shaped MnO2@TiN//N-MoO3-x-ASC device. (b) GCD curves of our fiber-shaped AFSC device. (c) Linear capacitances and volumetric capacitances of the fiber-shaped AFSC device as a function of current density. (d) CV curves collected at 100 mV s-1 for the fiber-shaped AFSC device under different conditions (left) and corresponding device pictures (right). (e) Ragone plots for the fiber-shaped AFSC device and other recently reported fiber-shaped FSCs.

      Source: Reproduced with permission [112]. © 2016, Wiley‐VCH.

      1.3.2.3 Coaxial‐Helix‐Type Fiber AFSCs

Schematic illustrations of (a) the fabrication process of an anode and a cathode, respectively, and the structure of a coil-type asymmetric supercapacitor electrical cable. (b) Optical images of a coil-type asymmetric supercapacitor electrical cable at different bending states. (c) CV curves obtained at different bending states at 200 mV s-1.

      Source: Reproduced with permission [117]. © 2015, Wiley‐VCH.

      1.3.2.4 Two‐Ply‐Yarn‐Type AFSCs

      To conclude, AFSCs have been universally accepted as one of the most promising energy storage devices, which effectively utilize the different potential windows of the pseudocapacitive cathodes and electric double‐layer capacitive anodes to increase the operating voltage of the device, thus contributing to the significant boosting of their energy density. Furthermore, researchers have developed AFSCs with impressive lightweight, small size, and high flexibility to satisfy the growing demand of portable/wearable electronics. Many novel and efficient configurations have been designed for easy integration with textiles and miniature electronics.

      Still, enormous effort should be paid to the future improvement and popularization of AFSCs. Firstly, further optimizing the overall electrochemical performances of the flexible electrodes and the devices remains as the biggest obstacle for AFSCs. Although the employment of pseudocapacitive anodes could well improve the limited energy density of AFSCs due to the relatively low capacitance of carbon‐based anodes, their poor conductivity results in unsatisfactory power density, which is worth more effective solutions. Secondly, the fabrication of a high‐mass‐loading electrode with good electrochemical performances and simultaneously good mechanical properties is rather challenging. Thirdly, most of the AFSCs are arduously handmade, which is difficult

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