Overpotential Behavior of Stabilized Zirconia Solid Electrolyte Fuel Cells

The nature and characteristics of the overpotential in cells of the type were determined from 700° to 1100°C over a wide range of oxygen pressures. Porous Pt black electrodes were deposited on impervious tubes. Almost pure resistance polarization is observed when or mixtures are present at the anode. However, for mixtures dilute in at the cathode, diffusion overpotential readily appears and limiting currents are eventually reached. Limiting currents, proportional to , are controlled by the diffusion of gaseous oxygen in the pores of the Pt electrode. Based on the present results and adsorption data, a mechanism is proposed for the cathode reaction. For mixtures, transition overpotential associated with the reaction is observed. The transition factor is about 0.5 and exchange current densities are in the vicinity of 1 mA/cm2. Equations relating exchange current density to , , and the amount of adsorbed on the electrolyte are derived and compared with the experimental results. It is concluded that either gaseous or adsorbed and gaseous are directly involved in the electrochemical step of the reaction.