Room-temperature spin-coatable polyoxometalate composites for high-contrast, large-area electrochromic capacitive films†

Room-temperature spin-coatable polyoxometalate-based composites are explored for low cost electrochromic capacitive films. A transparent stable K₆P₂W₁₈O₆₄–polyvinyl alcohol (P₂W₁₈–PVA) solution suitable for both rigid and flexible substrates was developed. Electrodeposition of HKUST-1 particles was employed to create pore structures in the film leading to facile ion transportation. The excellent EC performance of P₂W₁₈ was fully expressed in this thin film with a thickness of 500 nm. The state-of-the-art record of 95% optical contrast and 1600 stable EC cycles were achieved. An electrochromic energy storage device with a capacity of 7.89 mF cm⁻² was realized with MnO₂ as the counter electrode. This strategy for the preparation of highly stable and transparent POM coatings at room temperature is a general and simple solution that is not limited to the preparation of electrochromic films.