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. © 2004 The Electrochemical Society. All rights reserved

Electrochromic mechanism in nickel oxide thin films grown by pulsed laser deposition

In this paper we report further insights into the understanding of the electrochromic mechanism involved in the anodically colored nickel oxide thin films grown by Pulsed Laser Deposition (PLD) using optimized conditions, namely a 10(-1) mbar oxygen pressure and a Room Temperature substrate. The reversible color change from transparent to brownish is associated to a faradic process (Ni3+/Ni2+ redox couple) involving electrochemically oxyhydroxides phases whereas NiO acts as a reservoir. The key role played by the film morphology is demonstrated through figures of merit such as the capacity, the contrast and the cycling life of PLD NiO thin films cycled in KOH liquid electrolyte

Electrochromic degradation in nickel oxide thin film: A self-discharge and dissolution phenomenon

Nickel oxide thin films known as optical active counter electrode in electrochromic devices were grown by Pulsed Laser Deposition (PLD) at room temperature (RT) under a 10-1 mbar oxygen pressure. From intense electrochemical characterizations of as-deposited NiO electrodes in alkaline medium, a mechanism taking into account the three typical steps of their cycling life, namely the activation period, the steady state and the degradation period, is proposed. The reversible color change from transparent to brownish, generally ascribed to the Ni(II)/Ni(III) couple in the literature, was clearly identified to the electrochemically active nickel hydroxide/oxy-hydroxide phases. The reason lies in a spontaneous chemical conversion of NiO into Ni(OH)2 when the film is immersed in KOH medium. Afterwards, once cycling is performed, there is in competition with the electrochemical process, a self-discharge phenomenon associated to a partial dissolution of the thin-film oxidized phases. This second process becomes predominant on cycling leading to a progressive degradation of the electrochromic performances. © 2005 Elsevier Ltd. All rights reserved