Solar Energy Materials & Solar Cells 91 (2007) 1037–1050 Li + ion exchange in CeO 2 –TiO 2 sol–gel layers studied by electrochemical quartz crystal microbalance D. Sun 1 , S. Heusing, M.A. Aegerter à Leibniz-Institut fuer Neue Materialien—INM, Im Stadtwald, Gebaeude D2 2, 66123 Saarbruecken, Germany Received 26 November 2006; accepted 15 February 2007 Available online 10 April 2007 Abstract The paper reports first on the electrochemical behavior in liquid Li + electrolytes of 200 nm thick single sol–gel (CeO 2 ) 0.81 –TiO 2 electrochromic (EC) layers deposited by the dip-coating process. The electrolytes were solutions of 1 M LiClO 4 dissolved in dry propylene carbonate (PC) (containing 0.03 wt% of water) and wet PC containing up to 10 wt% of water, respectively. Then an electrochemical quartz crystal microbalance was used as a sensitive detector to analyze the mass changes occurring during the Li + ion exchange processes. These electrochemical processes were studied for 370 nm thick double layers, deposited on gold-coated quartz crystal electrodes and sintered at 450 1C in air. The electrolytes were the same solutions with water content varying from 0.03 up to 3 wt% of water. The processes have been studied in the potential range from 2.0 to +1.0 V vs. Ag/AgClO 4 during 100 voltammetry cycles. The composition of the (CeO 2 ) 0.81 –TiO 2 layers was found to change during the early cycles, mainly because of an irreversible Li + intercalation. It was found, however, that the mass change observed during cycling is not due only to a pure Li + ion exchange process but also involves the adsorption/desorption or exchange of other cations and anions contained in the electrolyte. These ions are Li + and ClO 4  in dry electrolyte and Li + , hydrated Li(H 2 O) n + and ClO 4  in wet electrolyte. The improvement of the reversibility of the intercalation and deintercalation processes as well as the faster kinetics observed in wet electrolytes are finally discussed in terms of a model in which the formation of hydrated Li + ions takes an important role. r 2007 Elsevier B.V. All rights reserved. Keywords: CeO 2 –TiO 2 coatings; Sol–gel; Electrochromism; Quartz crystal microbalance; Dry and wet electrolyte 1. Introduction Electrochromism in thin film electrodes of metal oxide is usually interpreted as an electron transfer reaction accompanying the insertion of electro active species, such as H + , Li + or OH  , into the amorphous or crystalline structure of the host materials, resulting in new absorption bands and consequently an altered layer coloration [1]. Understanding the mechanism of ion insertion in metal oxides during an electrochromic (EC) reaction is therefore a fundamental task to clarify the optical performance and also the eventual electrode failure. The detailed insertion mechanisms are usually highly complex; they may consist of several processes and have rarely been reported. Aegerter et al. [2–4] have shown already in 1989 that sol–gel CeO 2 –TiO 2 layers presented outstanding properties as H + and Li + ion storage layers and could be used as non-coloring counter electrodes in EC transparent devices of the configuration conducting glass/WO 3 /electrolyte/ CeO 2 –TiO 2 /conducting glass [5]. In the following years several papers reported on different preparations of such compounds [6–11] and one of them has been optimized to build large EC device prototypes [12,13]. Based on this recipe several large all solid state EC prototypes (up to 1m 2 ) have been developed at INM with blue coloring WO 3 [14] and Nb 2 O 5 [15–18], gray coloring Nb 2 O 5 :Mo [15–18] and brown coloring Nb 2 O 5 :Li [15–18] as EC electrodes. Although these devices presented good EC properties, they ARTICLE IN PRESS www.elsevier.com/locate/solmat 0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.02.015 à Corresponding author. Present address. Re´sidence Vert Pre´, Ch. des Placettes, 6, 1041 Bottens, Switzerland. Tel./fax: +41 21 881 5380. E-mail addresses: michel.aegerter@bluewin.ch, michel.aegerter@inm-gmbh.de (M.A. Aegerter). 1 Present address: College of Sciences, Tianjin University of Science and Technology, China.