Improved Optical and Electrochromic Properties of NiO_x Films by Low-Temperature Spin-Coating Method Based on NiO_x...

Solution approaches to NiO_x films for electrochromic applications are problematic due to the need of an additional high-temperature annealing treatment step in inert gas. In this study, nanostructured NiO_x powder with grain size of about 10.1 nm was synthesized for fabrication of NiO_x films for electrochromic application. Non-toxic dispersants of isopropanol and deionized water were used and the whole process was carried out in air. The effects of the number of spin-coating layers, annealing temperature, and the volume ratios of isopropanol to deionized water were systematically investigated. Large transmittance change of 62.3% at 550 nm, high coloration efficiency (42.8 cm²/C), rapid switching time (coloring time is 4 s, bleaching time is 3 s), and good stability were achieved in the optimized NiO_x film. The optimized process only required a low processing temperature of 150 °C in air with spin-coating three times and 1:2 volume ratio of isopropanol to deionized water. Finally, good cycle durability of up to 2000 cycles without obvious degradation was demonstrated by cyclic voltammetry tests in a LiClO₄/propylene carbonate electrolyte. This study provides a simple and effective approach for fabrication of NiO_x films at low temperature in air, which is attractive for further commercialization of electrochromic devices.