Kinetics of electrochromic process in thin films of cathodically deposited nickel hydroxide

Ni(OH)2 films have been obtained by cathodic deposition from 1 M Ni(NO3)2 on a glass with a current-conducting SnO2 layer. The films have a porous structure and consist of chaotically joined formations of 100–150 nm in size. They contain a considerable amount of adsorbed water, because of which their refraction index was n = ~1.5, whereas for crystalline nickel hydroxide, it was n = 2.37. It has been shown that in the course of discoloration of films through a shift of their potential towards more negative values, an electric field is formed in the bulk of film, which accelerates the entry of cations into it (protons as [H3O]+) from the electrolyte and electrons from ohmic contact. In this case, the initial dependence i(t), where i ~ t−1/4, is bound up with gradual increase in proton surface concentration. It is proposed to determine separately the dependence of the effective codiffusion coefficient of charge carriers (protons and electrons) on decoloration potential by the analysis of plots of current, injected charge, and luminous transmittance of films against time. This procedure involves a series of periodic stops of potential during the recording of current–potential curves for an electrochromic electrode and allows one to monitor the ratio of the film forms NiOOH and Ni(OH)2 in the course of film reduction.