Flexible Supercapacitors. Группа авторов
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Source: Reproduced with permission [109]. © 2014, Wiley‐VCH.
1.3.2.2 Wrap‐Type Fiber AFSCs
The design of a wrap‐type AFSC is very similar to that of a parallel‐type fiber AFSC, which encapsulates two electrodes into a protective flexible tube instead of placing them on a flexible substrate [53, 59, 62,110–112]. Recently, Lu and his co‐workers [112] successfully synthesized N and low valence‐state Mo dual‐doped MoO3 nanowires on carbon fibers, which was coupled with MnO2@TiN‐loaded carbon fiber cathode and sealed with heat‐shrinkable tube to fabricate a wrap type solid‐state ASC (denoted as MnO2@TiN//N‐MoO3‐x) (Figure 1.8a). The galvanostatic charge/discharge (GCD) curves of MnO2@TiN//N‐MoO3‐x with different current densities in Figure 1.8b indicate that the stable operating voltage of the device reaches a significantly high value of 2.0 V. The ASC device also shows superior rate capability when current density increased by 15 folds (Figure 1.8c). More importantly, the excellent flexibility and mechanic robustness enabled the fiber AFSC device to perfectly maintain its electrochemical performances in bent and even knotted conditions (Figure 1.8d). Benefiting from the ultrahigh output voltage and Faradaic electrodes with improved conductivity, the MnO2@TiN//N‐MoO3‐x device exhibited a maximum energy and power density of 2.29 mW h cm−3 and 1.64 W cm−3 respectively, outperforming many other fiber‐shaped SC devices reported (Figure 1.8e).
Figure 1.8 (a) Schematic illustration of 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
By helically wrapping a wire shape axial electrode with another wire electrode, coaxial‐helix‐type ASCs with core–shell cable‐like structures have been creatively explored [86, 113–117]. For example, the Thomas group [117] fabricated a novel cable‐like coaxial‐helix‐type AFSC as illustrated in Figure 1.9a. The AFSC device was coaxially assembled using wire‐shaped electrodes, where the MnO2@AuPd@CuO nanowiskers@copper wire core‐shell‐structured cathode was fabricated via a three‐step synthesis, and the Fe2O3@carbon fiber core‐shell‐structured anode was prepared through hydrothermal growing followed by conversion. The AFSC device could tolerate different bending states from 0° to nearly 180° (Figure 1.9b), while its promising electrochemical performances such as quasi‐rectangular‐shaped CV curves, maximum output voltage of 1.8 V and volumetric capacitance of 1.6 F cm−3 did not sacrifice (Figure 1.9c). Furthermore, the AFSC device could serve as a bi‐functional integrated electrical cable for both electricity storage and electricity transmission.
Figure 1.9 (a) Schematics illustration shows 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
Fiber AFSCs with stand‐alone two‐ply‐yarn type configuration has been proposed for efficient weaving into textiles, which are constructed by coating two fiber electrodes with polymer electrolyte and then twisting them together without outer packaging [118–120]. As shown in Figure 1.10a, Jin et al. [118] recently employed carbon‐fiber‐thread@polyaniline as cathode and functionalized carbon‐fiber thread as anode to fabricate a two‐ply‐yarn‐type AFSC device with PVA‐H3PO4 gel electrolyte. The device successfully reached a high operating voltage of 1.6 V with energy density up to 2 mW h cm−3. In order to demonstrate its potential in practical applications, the device was woven into a glove (Figure 1.10b) and exhibited excellent mechanical tolerance against bending (Figure 1.10c). The flexibility of the device was also examined via stretching to even 100% (Figure 1.10d), while the capacitance of the stretched device merely changed (Figure 1.10e).
1.4 Summary
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