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 mol ecule at the porous platinum/yttria stabilized zirconia solid electrol yte interface has been critically analyzed. Steady state polarization experiments showed that for very thin and very porous layers and oxyge n mixtures containing 1% or more oxygen the rate of the overall proces s is apparently governed by the electrode/electrolyte interfacial diff usion of neutral and/or single ionized atoms. This process is probably fed by the formation of single ionized oxygen molecules at the electr olyte surface, particularly in the neighbours of the electrode/electro lyte/gas phase triple phase boundary. At lower oxygen partial pressure s and/or for thicker and less porous metal layers the control is proba bly 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 consis tent with the evident increase of the overall reaction rate with tempe rature. The role of platinum, however, can not be confined as a pure e lectron supply. In fact, at very negative overvoltages it enters the p rocess of morphological expansion of the triple phase boundary zone an d likely affects the activity of the electrolyte in the primary proces s of oxygen ionosorption.