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

A nonaqueous charge‐balancing electrolyte with a redox couple (I − /I 3 − ) is investigated to improve the electrochromic (EC) performance of polyoxometalate (POM)‐based electrochromic devices (ECDs). PW 12 O 40 3− (PW 12 )‐coated porous TiO 2 nanoparticle films (PWTNF) with different thicknesses are used as the working electrode. The optical contrast (Δ T ) of the optimized PW 12 ‐ECDs reached 59.4% with the electrolyte containing the I − /I 3 − redox couple. PW 12 ‐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 − /I 3 − redox couple, PW 12 ‐ECDs show a poor Δ T of 4.0% and slow response times (>9.0 s). The role of the I − /I 3 − couple is identified as a charge‐balancing redox mediator between PW 12 and the counter electrode. Li + is the effective intercalating/deintercalating cation required for the EC reaction of PW 12 . Thus, by simply introducing the I − /I 3 − redox couple into the electrolyte significantly boosts the EC performance of POMs.