An in situ colorimetric measurement study of electrochromism in the thin-film nickel hydroxide/oxyhydroxide system

Abstract

Electrochromic thin-film ‘hydrous nickel oxide’ (nickel hydroxide) was deposited on fluorine-doped tin oxide (FTO)/glass substrates by galvanostatic electrochemical reduction at −0.1 mA cm−2 for 100, 400 and 800 s from aqueous 0.01 mol dm−3 nickel nitrate solution. Following transfer to aqueous 0.1 mol dm−3 potassium hydroxide, the transmittance and absorbance changes, colouration efficiencies and stability to multiple potential cycling were measured on reversibly electrochemically switching the transmissive light green (essentially ‘bleached’) ‘hydrous nickel oxide’ to the deep brown (‘coloured’) nickel oxyhydroxide redox state. The ‘hydrous nickel oxide’ (as deposited) films showed excellent transmittance modulation, achieving a maximum Δ%T of 83.2 at 432 nm, for the highest deposition time (800 s) studied. Colouration efficiency (CE) values were ∼30 cm2 C−1, with response times of 12.0 s (colouration) and 9.5 s (‘bleaching’). Using the Commission Internationale de l’Eclairage (CIE) system of colorimetry, the colour stimuli at rest, and on dynamically reversibly switching between the electrochromic redox states were calculated from in situ visible spectra recorded under electrochemical control. Reversible changes in hue and saturation occur, as shown by the track of the CIE 1931 xy chromaticity coordinates, together with a sharp decrease in luminance on oxidation. CIELAB L*a*b* coordinates were also used to quantify the colour of the electrochromic films. A combination of a low L*, and positive a* and b* values, quantified the perceived deep brown ‘coloured’ state.