Electrochromic performance of WO 3 thin films with solvent-free viscous electrolytes based on polyethylene glycol-titani
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Electrochromic performance of WO3 thin films with solvent-free viscous electrolytes based on polyethylene glycol-titanium oxide nanocomposites Narcizo Mendoza1,2, Liliana Hechavarría1,Francisco Paraguay-Delgado2 and Hailin Hu1. 1 Centro de Investigación en Energía, UNAM, Priv. Xochicalco S/N, Temixco, Morelos, 62580, México. 2 Centro de Investigación en Materiales Avanzados, Miguel de Cervantes 120, Chihuahua, Chih, 31109, México. ABSTRACT Polymeric nanocomposites of polyethylene glycol (PEG) with titanium oxide compound, PEG-Ti, have been prepared by sol-gel method from liquid PEG and titanium isopropoxide in acidic medium. Lithium salt (LiX) has been added into PEG-Ti to form PEG-Ti-LiX polymeric electrolytes. Electrochromic devices based on tungsten oxide thin films and PEG-Ti-LiX electrolyte may show excellent optical transmittance transient, however it depends on the type of lithium salt used during the sol-gel process. With LiI, the color change speed of the devices is very fast but they show a yellow color at bleaching state. The use of LiClO4 makes the devices totally transparent in visible region but the optical contrast is small. Possible molecular structure model of these polymeric electrolytes have been analyzed to explain the relation between electrochromic performance of tungsten oxide and electrolyte chemical composition. INTRODUCTION
Electrolytes work as ionic conductor -reservoir and as a separator of electrodes in electrochemical devices such as lithium batteries, dye-sensitized solar cells, electrochromic windows and sensors [1,2]. They can be classified as liquid, viscous or solid according to their viscosity. An electrolyte with volatile solvents shows fast ionic conduction, but the leak or evaporation of solvents is inevitable. The use of this type of electrolytes obligates a rigorous device sealing to reduce or prevent solvent loss. On the other hand, electrolytes without volatile solvents, or simply solvent-free electrolytes, are chemically stable and easy to be handled, which is desirable for device applications. A polymer electrolyte is usually prepared by dissolving a salt in a polymeric matrix. PEO is a commercially available polyether: HO-(CH2-CH2-O)nH. As the lithium salt is added into it, the ether oxygen atoms of the repetitive unit are responsible to dissociate such salt, interact with lithium ions and transport them from one to another.In comparison with those with solvents, the disadvantage of solvent-free electrolytes in comparison with those with solvents is their relatively low ionic conductivity, which can prejudice charge transfer process at electrode/electrolyte interfaces of electrochemical devices. Several approaches have been reported to improve the ionic conductivity of polymeric electrolytes, and one of them is the use of composite materials; those of PEO composites with ceramic nanoparticles (TiO2, SiO2, etc.) show better ionic conductivity than that of pure salt-in-polymer electrolytes [3-6]. In a recent work we reported a PEG-Ti nanocomposite made by sol-gel rou
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