A copper-based reversible electrochemical mirror device with switchability between transparent, blue, and mirror states

Herein, a reversible electrochemical mirror (REM) device was electrochemically tuned to achieve dual transmittance and reflectance modulations in a single device. Conventional REM devices reversibly switch between transparent and mirror states. However, it is a significant challenge to maintain the mirror state of the REM devices due to the diffusion of anions into the metal film at the open-circuit state. In this study, we report a Cu-based REM device that offers reversible switching between transparent, blue, and mirror states via a judicious selection of electrolyte and controllable electrodeposition. The blue state can be obtained through the formation of copper(I) chloride (CuCl) when copper(II) chloride (CuCl2) undergoes electrochemical reduction. Moreover, the polymer host PVA (polyvinyl alcohol) plays an important role in reducing the surface roughness of the electrodeposited mirror film, improving film uniformity, and maintaining the mirror state of the device during the voltage-off state.