Impedance analysis of amorphous WO3 thin films in hydrated LiClO4-propylene carbonate electrolytes

An alternating current (ac) method has been applied to study the electrical properties of electrochromic electrodes in liquid anhydrous and hydrated organic electrolytes. The influence of the three following parameters, on the shape of the complex impedance diagrams, has been studied: the insertion coefficient into WO3 the water content in the electrolyte and the electronic conductivity of the substrate onto which WO3 is deposited. From an equivalent electrical circuit, we determined the values of the different components of the cell and we associated them with electrical processes occuring in the cell, i.e. the low-frequency capacitance, the charge-transfer resistance, the diffusion coefficient of the inserted species and the high-frequency resistance. Their variations with the above-mentioned paramaters are discussed. From the diffision coefficient values obrained in hydrated organic electrolytes, a Li+/H+ exchange machanism in WO3 may be confirmed. Moreover, the roughness of the electrode surface can be observed on the diagrams at low frequency when using a low conducting electronic electrode as substrate for WO3. A specific constant phase element (CPE) behavior is observed for frequencies lower than 0.1 Hz. The measurement of the electrode roughness is very important since this paramater plays an important role in the electrochromic process (author). 23 refs.; 12 figs.; 6 tabs