Effect of Annealing Temperature on the Electrochromic Properties of Electrochemically Oxidized Nickel

Electrochromic (EC) films are widely used in smart windows, automobile mirrors, and low-power displays due to their capability of reversibly changing color under an external potential. Transition metal oxides, due to their ability to switch between multiple oxidation states, are primarily used as EC films. They require low operating power and provide high coloration efficiency in EC applications. However, their thin-film fabrication through a low-cost and scalable process is still challenging. Here, nickel oxide (NiO) EC films have been synthesized by room-temperature electrochemical oxidation of Ni thin films. The Ni films were grown by electroless deposition and annealed at three different temperatures of 100°C, 200°C, and 300°C. The morphological, optoelectronic, and electrochemical properties of the films have been comparatively analyzed to obtain an optimized EC film. The NiO film annealed at 100°C exhibited superior properties with coloration and bleaching times of 1.8 s and 2.9 s, respectively, and a coloration efficiency of 20 cm 2 /C. This annealed film is further used to fabricate a EC device with NiO and tungsten oxide (WO 3 ) as the electrode layers and a phosphoric acid-based gel as the electrolyte. The low-temperature deposition process used here can also be extended to flexible substrates.