Electrochromism in oxide films based on lanthanides

This paper reports on studies of mixed Me : lanthanide (Me: W, Ti, Zr; lanthanide: La, Ce, Pr) oxide and Ni-Ce hydroxide films as possible candidates for optically passive counter electrodes in transparent electrochromic devices. Materials for mixing with lanthanides were chosen to represent different types of electrochromic oxides/hydroxides. W and Ti oxides are cathodic materials (color on charge insertion), Ni hydroxide is anodic (colors on charge extraction), and Zr oxide has virtually zero charge capacity in a potential window typical for electrochromic device operation. Absence of coloration upon charge insertion/extraction for Ti-Ce, W-Ce and Zr-Ce films with certain Me/Ce atom ratios is discussed in terms of localized Ce 4f electron states. Oxides containing La and Pr, having different electron configurations from that of Ce, were studied to elucidate the role of the 4f electrons. From a practical point of view, the oxides of Ti-Ce and Zr-Ce seem to be the prime candidates for use in transparent electrochromic devices. (C) 1999 Elsevier Science B.V. All rights reserved.