CONTRIBUTION TO THE INTERPRETATION OF THE MULTISTEP CATHODIC REDUCTION OF OXYGEN AT THE PT/ZIRCONIA BASE ELECTROLYTE INTERFACE AT HIGH-TEMPERATURE

The multistep cathodic reduction at high temperature of the oxygen molecule at the porous platinum/yttria stabilized zirconia solid electrolyte interface has been critically analyzed. Steady state polarization experiments showed that for very thin and very porous layers and oxygen mixtures containing 1% or more oxygen the rate of the overall process is apparently governed by the electrode/electrolyte interfacial diffusion of neutral and/or single ionized atoms. This process is probably fed by the formation of single ionized oxygen molecules at the electrolyte surface, particularly in the neighbours of the electrode/electrolyte/gas phase triple phase boundary. At lower oxygen partial pressures and/or for thicker and less porous metal layers the control is probably transferred to the interpores molecular diffusion. The adsorption of oxygen onto the metal surface, as a step for feeding the transport of reducible species to the triple phase boundary, looks hardly consistent with the evident increase of the overall reaction rate with temperature. The role of platinum, however, can not be confined as a pure electron supply. In fact, at very negative overvoltages it enters the process of morphological expansion of the triple phase boundary zone and likely affects the activity of the electrolyte in the primary process of oxygen ionosorption.