Direct Current‐Voltage Characteristics of Calcia Stabilized Zirconia with Porous Platinum Electrodes

Abstract

The direct current‐voltage characteristics of symmetrical cells Pt, O2(I), , O2(II), with , were measured at 560°C under oxygen pressures from 1 to 10−20 atm. The characteristics were nonohmic, the deviation from ohmic law being mainly due to the potential drop at the cathode interface between the solid electrolyte and the gas ambient. The characteristics consist of two parts. The first is characterized by a marked oxygen pressure dependence and is observed at voltages lower than approximately 2V (weak polarization). In this range, the rate‐determining process is the diffusion of oxygen atoms (resulting from dissociation of O2 or ) through the platinum of the Pt paste electrode. The second part of the characteristic is almost independent of the oxygen pressure and is observed at voltages higher than approximately 2V (strong polarization). Here the rate‐determining step is the process in which neutral oxygen atoms, adsorbed at the cathode surface of the electrolyte, combine with effectively neutral oxygen vacancies , (oxygen ion vacancies which have trapped two electrons) to form a normal O2− lattice ion . This process utilizes the part of the electrolyte surface not in contact with the platinum, but close to points where the platinum makes contact, and involves migration of electrons from the platinum over the surface of the electrolyte.