Quantitative Analysis of Charge Storage Process of Tungsten Oxide that Combines Pseudocapacitive and Electrochromic Properties

Tungsten oxide has been recently demonstrated interesting and promising bifunctionalities that combine electrochromism and pseudocapacitance. However, understanding about the charge storage process of pseudocapacitive tungsten oxide film is very limited. Our quantitative investigation clearly reveals that the capacity performance of tungsten oxide film is thickness-dependent. In particular, the 100 nm-thick tungsten oxide film exhibits highest charge capacity density at high rates, with nearly 242.1 C g(-1) stored reversibly in 6 s. The 100 nm-thick tungsten oxide film stores charges mainly by capacitive effects (including both electric double layer capacitance and pseudocapacitance). For example, at a scan rate of 5 mV s(-1), more than 78% stored charges is attributed to capacitive effects, according to the cyclic voltammetry analysis. Furthermore, pseudocapacitance is responsible for around 70% of the capacitive charge storage based on the electrochemical impedance spectroscopy analysis. The contributions of diffusion-controlled process, electric double layer capacitive process, and pseudocapacitive process have been discussed in detailed and successfully identified. Overall, this work provides insight into the charge storage process of tungsten oxide, and our new findings can shed light on other transition metal oxide-based electrochemical energy storage systems.