Effects of electrolyte pH on the formation of nickel oxide films and the corresponding electrochromic properties

The electrolyte pH has been determined to be an important factor that has a direct influence on the formation of film morphologies. Herein, we investigate the effect of electrolyte pH on the formation of nickel oxide (NiO_x) films in borate solutions and examine their corresponding electrochromic characteristics. The result shows that the formation of hexa-aquo Ni complexes due to an appropriate electrolyte pH can aid the electrodeposition of NiO_x films and produce a film of small grains. Raman and in situ X-ray absorption fine structure (XAFS) measurements indicate that the structure of the NiO_x films in their oxidized state is similar to γ-NiOOH, regardless of the electrolyte pH, although the grain size of the films is dependent on the electrolyte pH. The different grain size results in significant differences in electrochromic performance. The film made of small grain size exhibits the best electrochromic performance. The optical contrast of the NiO_x film achieved 87%. The coloration efficiency increased by about 50% to 51 cm²/C and the switching time reduced to 5.4 s for bleaching. These results demonstrate the imperative for an appropriate electrolyte pH to enable sustained electrochemical activity, reactive site formation, and functional stability.