Analysis of cyclic voltammograms of electrochromic a-WO3 films from voltage-dependent equilibrium capacitance measurements

The main features of cyclic voltammograms (CVs) of thin electrochromic films based on a-WO3 were investigated. First, the chemical capacitance is defined in terms of the electrochemical potential variation with the insertion level, x, and is measured under galvanostatic (chronopotentiometry) quasi-equilibrium conditions. An equilibrium capacitance increasing rapidly with respect to insertion level or negative potential is observed, respectively C-ch proportional to x(a) at x > 10(-3)(a approximate to 0.74) and C-ch proportional to V-alpha(alpha approximate to 3). Simulation methods used to generate the observed CVs are described in detail. Major CV peaks can be simply understood as charging and discharging of the variable capacitor, in conjunction with a distortion of the voltage scale due to a series transport process. Therefore a simple RC equivalent circuit allows us to explain the principal CVs characteristies of lithium intercalation and deintercalation in amorphous films. (C) 2003 Elsevier B.V. All rights reserved.