Overpotential Behavior of Stabilized Zirconia Solid Electrolyte Fuel Cells

The nature and characteristics of the overpotential in cells of the type Ar ‐ O 2 , H 2 ‐ H 2 O , or CO ‐ CO 2 / ZrO 2 ‐ CaO / O 2 were determined from 700° to 1100°C over a wide range of oxygen pressures. Porous Pt black electrodes were deposited on impervious ZrO 2 + 10 m / o ( mole per cent ) CaO tubes. Almost pure resistance polarization is observed when Ar ‐ O 2 or H 2 ‐ H 2 O mixtures are present at the anode. However, for Ar ‐ O 2 mixtures dilute in O 2 at the cathode, diffusion overpotential readily appears and limiting currents are eventually reached. Limiting currents, proportional to P O 2 , 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 CO ‐ CO 2 mixtures, transition overpotential associated with the reaction CO 2 + 2 e ⇄ CO + O − − is observed. The transition factor is about 0.5 and exchange current densities are in the vicinity of 1 mA/cm 2 . Equations relating exchange current density to P CO , P CO 2 , and the amount of CO adsorbed on the electrolyte are derived and compared with the experimental results. It is concluded that either gaseous or adsorbed CO and gaseous CO 2 are directly involved in the electrochemical step of the reaction.