Reference electrode placement in thin solid electrolytes

To separate anode and cathode contributions to the impedance of a thin solid electrolyte cell, many workers place a reference electrode on the surface of the inactive electrolyte, coplanar with the working electrode. This practice is evaluated theoretically using finite-element calculations of the real and imaginary potential distributions in the electrolyte under the conditions of ac impedance. The results of this analysis show that minor errors in the alignment of the anode and cathode can create significant errors in the measured half-cell overpotential. These errors involve not only quantitative scaling factors, but also cross-contamination of anode and cathode frequency response in the measured half-cell impedances. Even if electrodes are perfectly aligned, differences between the anode and cathode kinetics and/or frequency response may cause inherent distortion of the impedance, including frequency dispersion and inductive artifacts. We evaluate two approaches (pellets and microelectrode arrays) that can be used to avoid ambiguities. (C) 2002 The Electrochemical Society.