Electrochromic properties of NiOOH thin films prepared by reactive sputtering in an H2O atmosphere in various aqueous electrolytes

Nickel oxyhydroxide (NiOOH) thin films were prepared by reactive sputtering using H2O as a reactive gas, and the electrochromic (EC) properties of these thin films were investigated in alkaline, neutral, and acidic aqueous electrolyte solutions. A stable EC cycling of up to 100 cycles was obtained for the NiOOH thin films in electrolyte solutions with a wide pH range of 4.1–13.5. However, the EC cycling degraded drastically after several tens of cycles in an electrolyte with pH=2.0. In the stable pH region, the NiOOH films exhibited a large transmittance change from approximately 30–90%, although the response speed slowed with decreasing pH of the electrolytes.