Electrochromic properties of MnO2/WO3 bilayered electrodes for enhanced charge storage and superior stability†

Electrochromic devices (ECDs), which combine optical modulation and energy storage, have sparked widespread interest in window/façade applications and are becoming increasingly popular for energy conservation. It is established that by adjusting the electrochromic (EC) layer surface, the charge–discharge profile and the optical output can be individually regulated. Here, EC bilayers were created by overcoating MnO₂ on both amorphous and crystalline WO₃ thin films. The heterojunction considerably improved the cyclic stability and charge storage capacity of the WO₃ electrode, without affecting the EC functions. The presence of the MnO₂ layer has significantly enhanced both the areal capacitance and volumetric capacitance of the electrodes. The crystalline WO₃ electrodes have a peak volumetric capacitance of 341 F cm⁻³ at 0.1 mA cm⁻² discharge current, associated with an impressive retention rate of 50% even when charging at a higher rate of 1.0 mA cm⁻². The ECDs exhibited outstanding visible and IR blocking capability of around 98% beyond 600 nm. A comprehensive study employing spectroscopy and electrochemistry was performed to examine the chemical and electrochemical effects of MnO₂ overcoating. The results showed that these bilayers may be effectively employed to create EC energy storage devices (EESD) that are both highly stable and superior in performance.