Charge‐Balancing Redox Mediators for High Color Contrast Electrochromism on Polyoxometalates

Abstract A nonaqueous charge‐balancing electrolyte with a redox couple (I⁻/I₃⁻) is investigated to improve the electrochromic (EC) performance of polyoxometalate (POM)‐based electrochromic devices (ECDs). PW₁₂O₄₀³⁻ (PW₁₂)‐coated porous TiO₂ nanoparticle films (PWTNF) with different thicknesses are used as the working electrode. The optical contrast (ΔT) of the optimized PW₁₂‐ECDs reached 59.4% with the electrolyte containing the I⁻/I₃⁻ redox couple. PW₁₂‐ECDs also show a fast response time for bleaching (t_b,90% = 1.9 s) and coloration (t_c,90% = 3.1 s). On the contrary, without the I⁻/I₃⁻ redox couple, PW₁₂‐ECDs show a poor ΔT of 4.0% and slow response times (>9.0 s). The role of the I⁻/I₃⁻ couple is identified as a charge‐balancing redox mediator between PW₁₂ and the counter electrode. Li⁺ is the effective intercalating/deintercalating cation required for the EC reaction of PW₁₂. Thus, by simply introducing the I⁻/I₃⁻ redox couple into the electrolyte significantly boosts the EC performance of POMs.