Ultrathin Polyoxometalate Coating as the Redox Shuttle for Acid‐Free Electrochromic Polymer Capacitive Windows

Abstract Polyoxometalate is explored as a redox shuttle for non‐electrochromic charge‐balancing material of the conjugated polymer (CP)‐based electrochromic window (ECW). H 3 PW 12 O 40 is electrochemically deposited on a TiO 2 nanoparticle film that is coated on a fluorine doped tin oxide glass. The content of PW 12 is increased as the TiO 2 film is thicker. The PW 12 ‐coated TiO 2 film (PWTNF) is applied as a counter electrode (CE) of a CP‐based ECW in an ionic liquid electrolyte, to increase the potential distribution at the CP‐coated working electrode, thereby decreasing the operation voltage of the ECW. This could be ascribed to the function of PW 12 as an electron acceptor and redox shuttle that allows effective charge transport from/to the CE and nearby TiO 2 nanoparticles. For the optimized ECW with PWTNF as a CE, the maximum optical contrast is obtained at the applied voltage of 1.5 to −2.0 V (68%), along with long cyclability (2500 cycles), bistability (> 2.35 h) and UV stability (> 60 h). The ECW with PWTNF exhibits reproducible capacitive properties, along with high figure of merit for color change with a high power output, in acid‐free electrolyte for the first time.