Electrochromic nickel oxide films were deposited by radio-frequency magnetron sputtering of a Ni target in gas mixtures of Ar and O2. When electrochemically cycled in 0.1 N KOH solution, large reversible changes in visible absorption were obtained within the first 10 bleach/color cycles. Films which showed the greatest changes in optical properties and charge capacity were produced within a narrow range of oxygen flow rates. The coloration efficiency was found to be insensitive to the sputtering conditions and stoichiometry, with a value of 34±4 cm2C−1 at 550 nm. This indicates that microstructure and charge capacity determine the absorption coefficient. The reaction rate was found to depend on the concentration of OH− in the electrolyte, indicating that OH− diffusion in the electrolyte limits the rate of reaction.
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Wruck et al. (1991) studied this question.