Copper complex/polyoxometalate-based tunable multi-color film for energy storage

Polyoxometalates (POMs) are considered as an attractive option for electrochromic energy storage (EES) system due to their unique electrochemical feature. Herein, a composite film NW/P 2 W 17 /Cu (phen) 2 based on Dawson-type monolacunary POMs K 10 P 2 W 17 O 61 (abbreviated as P 2 W 17 ) and copper complex [Cu II (phen) 2 ](NO 3 ) 2 (abbreviated as Cu (phen) 2 ) formed on TiO 2 nanowire (abbreviated as NW) array substrate was fabricated by hydrothermal process and layer-by-layer self-assembly combination method. This rational designed film NW/P 2 W 17 /Cu (phen) 2 exhibits excellent tunable multicolor electrochromic behavior and improved pseudocapacitive performance with large optical modulation (43.7% at 600 nm), fast switching time (t c = 2.9 s, t b = 9.5 s), and high volumetric capacitance (228.0 F cm −3 at 0.3 mA cm −2 ). In the process of charge and discharge, the composite film can realize the transformation from light green to orange, green, and finally to blue-green color, successfully achieve the bridge between electrochromic behavior and energy storage, and monitor the level of stored energy by color change. This work showed great potential of POMs-based electrode materials for integrated electrochromism and energy storage applications.