Polyoxotungstate based nanocomposite film with multi-color change and high volumetric capacitance toward electrochromic energy-storage applications

2021 
Electrochromic energy storage devices (EESD) can directly reflect the level of energy storage through color changes, and exploring high-performance electrode materials is an important means to develop these devices. In this study, we report a nanocomposite film NW/P2W17/Fe(phen)3 based on Dawson-type monolacunary polyoxotungstate K10P2W17O61 (abbreviated as P2W17) and transition metal complex [FeII(phen)3](ClO4)2 [abbreviated as Fe(phen)3] formed on TiO2 nanowire (abbreviated as NW) array substrate by combining facile hydrothermal process and layer-by-layer self-assembly method. The existence of TiO2 nanowires substrate significantly increased the active area of nanocomposite film and facilitatd the diffusion of ions during the electrochemical process. As bi-functional electrodes, the NW/P2W17/Fe(phen)3 films successfully achieved bridging electrochromic behavior with energy storage, realized the multi-color transition from orange-red to light blue-violet and then to dark blue-violet during the charging and discharging process, with superior electrochemical properties of large optical modulation (34.0 % at 600 nm), fast switching time (tc = 2.3, tb = 6.2 s), high coloring efficiency (194.5 cm2/C at 600 nm) and volumetric capacitance (135.8 F·cm-3 at 0.2 mA∙cm-2). The charged EESD constructed based on NW/P2W17/Fe(phen)3 can light up an LED, indicating the feasibility of practical application of polyoxometalates for electrochromic and energy storage.
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