Effect of water on the electrochromic properties of CeO2-TiO2, WO3 and Nb2O5:Mo sol-gel layers and devices prepared with them

Abstract

Electrochromic (EC) materials change their optical properties (transmittance or reflection) in a reversible manner when a current flows through them. Large EC glazings can be used for architectural and automotive applications in order to control the solar radiation entrance. The typical configuration of EC devices made with the sol-gel process (and used in our lab) is glass/fluorine doped tin oxide (FTO)/EC layer/inorganic-organic composite electrolyte/ion-storage (IS) layer/FTO/glass. The ion storage capacity of the IS layer plays an important role in the transmittance change of EC devices. WO3 layers are the best studied EC layers and their Li+ intercalation ability is larger than 30 mC/cm². The charge capacity of the IS layer should be similar to obtain a high coloration change. My work was dedicated to improve the ion storage capacity of (CeO2)x(TiO2)1 as IS layer. First the composition of the sol and the dip coating and annealing parameters have been optimized. Then the influence of water added to the electrolyte on the properties of the EC and IS layers and the EC devices has been studied. The ion storage capacity was improved from 7 mC/cm² to 14.5mC/cm² by improving the sintering temperature to 550°C. It can be even increased to 26mC/cm² by increasing the thickness of the layer and with 3 wt.% water added into the liquid electrolyte. When a small amount of water is added to the liquid electrolyte, 1 M LiClO4 in propylene carbonate (PC), the ion storage capacity of the (CeO2)x(TiO2)1 layer is greatly increased by a factor up to 3 and the ion intercalation kinetics is faster than that in the dry electrolyte. The ion intercalation reversibility is also improved. The nature of the intercalated ion was studied using an Electrochemical Quartz Crystal Microbalance (EQCM). It was foun that the charge balance of the (CeO2)0.81(TiO2)1 layer during reduction and oxidation is not only due to Li+ intercalation and deintercalation, but also to ClO4 desorption and adsorption. [...]