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

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.