Nanostructured pseudocapacitors with pH-tunable electrolyte for electrochromic smart windows

Highlights: • The work reversibly tuned pH value of electrolyte, inducing the color change of pH indicators dissolved in the electrolyte. • This is the first time to link the electrochemical way of controlling solution pH value with their color changes. • Electrochromic windows are successfully built with both tunable color saturation and multiple color combinations. -- Abstract: Electrochromic windows have attracted increasing attention, particularly for smart and energy-saving buildings. While current electrochromic devices mostly utilize the electrode phase changes during the ion implantation/extraction, the possibility of color changes in electrolyte is also a promising method to be investigated. Here we show a new family of colorful electrochromic windows by electrochemical switching of the OH⁻ concentration in the electrolyte of pseudocapacitive devices with pH indicators to create color changes. The electrodes in pseudocapacitors produce and consume OH⁻ ions in electrolyte associated with the chare/discharge cycles. By tuning alkalinity of the electrolyte using pseudocapacitive reactions of nanoflake cobaltosic oxide (Co₃O₄) electrode, we demonstrated reversible electrochemical coloring in red, blue, yellow, reddish orange, violet, green and dark blue, with the combination of cells containing phenolphthalein, thymolphthalein and 3-nitrophenol in electrolyte. This work introduces a new electrochromic mechanism based on electrolyte color changes controlled by nanostructured pseudocapacitors, which can fill the color palette.