Optimal Rule-of-Thumb Design of Nickel–Vanadium Oxides as an Electrochromic Electrode with Ultrahigh Capacity and Ultrafast Color Tunability

The use of electrodes capable of functioning as both electrochromic windows and energy storage devices has been extended from green building development to various electronics and displays to promote more efficient energy consumption. Herein, we report the electrochromic energy storage of bimetallic NiV oxide (NiVO) thin films fabricated using chemical bath deposition. The best optimized NiVO electrode with a Ni/V ratio of 3 exhibits superior electronic conductivity and a large electrochemical surface area, which are beneficial for enhancing electrochemical performance. The color switches between semitransparent (a discharged state) and dark brown (a charged state) with excellent reproducibility because of the intercalation and deintercalation of OH–ions in an alkaline KOH electrolyte. A specific capacity of 2403 F g–1, a coloration efficiency of 63.18 cm2C–1, and an outstanding optical modulation of 68% are achieved. The NiVO electrode also demonstrates ultrafast coloration and bleaching behavior (1.52 and 4.79 s, respectively), which are considerably faster than those demonstrated by the NiO electrode (9.03 and 38.87 s). It retains 91.95% capacity after 2000 charge–discharge cycles, much higher than that of the NiO electrode (83.47%), indicating that it has significant potential for use in smart energy storage applications. The superior electrochemical performance of the best NiVO compound electrode with an optimum Ni/V compositional ratio is due to the synergetic effect between the high electrochemically active surface area induced by V-doping-improved redox kinetics (low charge-transfer resistance) and fast ion diffusion, which provides a facile charge transport pathway at the electrolyte/electrode interface.