Oil-Dispersible Nickel Hexacyanoferrate Nanoparticles as Counter Electrode Materials for Polymer Electrochromic Devices

Counter electrodes are one of the crucial parts for obtaining high-performance electrochromic devices, though their research is still scarce. In this work, a Prussian blue analog, nickel hexacyanoferrate (NiHCF), was synthesized via a coprecipitation method, and the dispersion of the derived NiHCF nanoparticles was improved by surface treatment with oleylamine to induce good film-forming properties. The spray-coated NiHCF films demonstrate good redox behavior and high transmittance in the visible and near-infrared bands, which can meet the requirements of electrochromic devices (ECDs) as the counter electrode layer. Consequently, several ECDs based on the reported electrochromic polymers (orange, red, blue, and black-to-transmissive polymers) are successfully assembled and showcase excellent electrochromic properties, such as high optical contrast, swift switching response, high coloration efficiency, and good switching stability, confirming that the synthesized NiHCF nanoparticles can be a good counter electrode for constructing high-performance polymer ECDs.