Proton diffusion and electrochromism in hydrated NiOy and Ni1-xVxOy thin films

Electrochromic hydrated nickel oxide and nickel vanadium oxide thin films were deposited by reactive dc magnetron sputtering. Optical modulation was effected by insertion and extraction of protons. The proton diffusion coefficient D during the insertion/extraction process was determined by galvanostatic intermittent titration. We analyzed D as a function of changes in the stoichiometry and found deep minima at certain H/Ni ratios. These minima were interpreted, within the bleached state, as due to a structural phase transition from beta-nickel hydroxide to alpha-nickel hydroxide. The optical absorption increased rapidly at the H/Ni ratio corresponding to the second phase transition. At the transition to the colored state, our data were consistent with a structural phase transition from alpha-nickel hydroxide to gamma-nickel oxy-hydroxide, in accordance with the Bode reaction scheme. Those structural changes were corroborated by infrared reflection-absorption spectroscopy.