Nano-cobalt oxide/viologen hybrid solid state device: Electrochromism beyond chemical cell

Abstract

An improved nanofilm of Co₃O₄ has been synthesized using controlled current electrodeposition on a conducting transparent electrode for fabricating a hybrid solid state electrochromic device by combining it with Viologen. The nanoelectrochromic electrode also acts as a counterion to support redox induced color switching of Viologen in a solid state without any liquid electrolyte. A good color contrast between its yellow and blue states under different bias conditions has been observed, which leads to overall performance enhancement as quantified using device parameters such as coloration efficiency, color contrast, stability, and cycle life. The solid state device shows an improved efficiency of as high as 360 cm²/C and a switching time of as low as 500 ms. In situ spectroelectrochemical studies reveal that the bias induced redox activity of Viologen and metal oxide leads to the color change. The two constituents are not only electrochromically active materials but also simultaneous counterions for each other, thus leading to improvement in the electrochromic performance.