The Ion-Storage Capacity and Surface Characterization of Ce/Cu Thin Films

Irena Kozjek Škofic,^a Janez Kovač^b and Nataša Bukoveca*

^a Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5, SI-1000 Ljubljana, Slovenia
^b Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia
* Corresponding author: E-mail: natasa.bukovec@fkkt.uni-lj.si

Abstract
Ce/Cu mixed oxide thin films with molar ratio 1 were prepared by sol-gel method and dip-coated on SnO₂/F covered glass. The cyclic voltammetry and chronocoulometry were carried out to determine their suitability for application as a counter-electrode in electrochromic devices. The charge capacity is up to 20 mC cm^–2, however, the reversibility of the redox process decreases with the number of cycles. Surface composition and chemical bonding of elements on the surface of the cycled and non-cycled part of the Ce/Cu mixed oxide thin film were investigated by X-Ray photoemission spectroscopy (XPS) and surface morphology was analyzed by atomic force microscopy (AFM). The Cu atoms on the non-cycled surface are bonded as Cu(2+), whereas after cycling the Cu atoms are reduced to Cu(1+). The Ce atoms in both areas are in the Ce(4+) state. The Ce/Cu ratio measured by XPS on the surface of the non-cycled area is higher than on the surface of the cycled one. In the cycled area the Li is present, bonded in thin film as Li₂CO₃.

Keywords: Ce/Cu mixed oxide thin film, counter-electrode, sol-gel process, XPS, AFM