Nanostructured pseudocapacitors with pH-tunable electrolyte for electrochromic smart windows

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.