Pseudocapacitive Charge Storage in Electrochromic Transition-Metal Oxide Thin Films

Electrochromic pseudocapacitive transition-metal oxide materials, such as tungsten oxide, which combine fast response, high energy density, and optical effects, can play a significant role as energy storage materials. Here we investigate the electrochemical kinetics of thin films of tungsten oxide, which turn transparent to sky-blue color in the lithiated state due to the reduction of W ⁶⁺ to W ⁵⁺ . We investigated the charge density, charge transfer, ion diffusion, and interfacial behavior upon Li ⁺ insertion/de-insertion in WO ₃ . The electrochromic thin film’s pseudocapacitive and electrical double layer mechanism was differentiated based on the power-law. Faradaic diffusion-controlled process dominates over the surface capacitive behavior at scan rates below 40 mV s ⁻¹ . These films exhibit an areal charge density of around 100 mC cm ⁻² and a capacitance of 80 mF cm ⁻² , superior to most comparable electrochromic materials and supercapacitors. This work combines electrochromics and energy storage properties and provides a fundamental understanding of pseudocapacitive and electrochromic mechanisms in WO ₃ .