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

Electrochromic energy storage devices (EESDs) can directly reflect the level of energy storage via color changes, and the exploration of 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 a transition metal complex, [FeII(phen)3](ClO4)2 [abbreviated as Fe(phen)3], formed on a TiO2 nanowire (abbreviated as NW) array substrate combining a facile hydrothermal process and a layer-by-layer self-assembly method. The existence of TiO2 nanowire substrates significantly increased the active area of the nanocomposite film and facilitated 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, realizing 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 (34.3% at 600 nm), fast switching time (tc = 2.8, tb = 6.2 s), high coloring efficiency (194.5 cm2 C−1 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.