Electrochemically inactive nickel oxide as electrochromic material

Nickel oxide thin films, known as optically active counter electrodes in electrochromic devices, were grown at room temperature under an oxygen pressure of 10(-1) mbar, using pulsed laser deposition (PLD). The influence of the film thickness, potential window, and extended cycling on the electrochromic properties was studied by means of complementary electrochemical, X-ray diffraction, and electron microscopy techniques. Independently of the substrate nature, transparent as-deposited films were crystallized with a [1 1 1] preferred orientation. The reversible color change from transparent to brownish is ascribed to a faradic process, involving the Ni3+/Ni2+ redox couple, the intensity of which is enhanced with decreasing film thickness. Finally, taking into account a first chemical reaction when NiO thin films are immersed in KOH, a mechanism based on hydroxide electrochemically active phases is proposed for the electrochromic properties of PLD NiO thin films. (C) 2004 The Electrochemical Society.