ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY OF CONDUCTOR-INSULATOR COMPOSITE ELECTRODES - PROPERTIES IN THE BLOCKING AND DIFFUSIVE REGIMES

The electrochemical response of graphite + high-density polyethylene c omposite electrodes as a function of the conductivity load was investi gated. Percolation theory was used in order to explain the electrochem ical behaviour of this type of composite electrode. In the blocking re gime the electrochemical impedance of this electrode material behaved as R-0 + q.(omega j)(-eta), where R-0 represents the uncompensated res istance of the cell. Its value depended on the graphite volume proport ion (v) with a power law R-0 proportional to (v-v(c))(-1) with a criti cal exponent t = 3.2 +/- 0.1 which is close to the mean field value, t =3. With potassium chloride concentrations greater than 0.7 M, the unc ompensated resistance, R-0, coincided practically with the bulk resist ivity of the solid. The electrochemical response of potassium ferricya nide was analyzed using a generalized Randles equivalent circuit where the double layer capacity and the Warburg impedance were substituted by two independent CPEs, respectively. In this diffusive regime and in the blocking regime, the values of the constants and exponents of the constant phase elements were interpreted on the basis of a surface mo del of composite electrodes and its disorder. (C) 1998 Elsevier Scienc e S.A. All rights reserved.