Polarization reduction by a highly resistant electrolyte film in SOFC oxygen electrodes

Abstract

It is known that the polarization resistance ( R p) of oxygen electrode increases with increasing electrolyte resistivity ( ρ). If this relationship is attributed to the interfacial electrolyte, the R p will be increased by a high resistance film, which is coated on the substrate electrolyte. This study had started to prove this prediction. An 8YSZ electrolyte (low resistance substrate) was coated with a (ZrO 2) 0.9(Nd 2O 3) 0.1 film (high resistance film). The electrodes used were Pt, Ag and Au electrodes. The polarization resistance was measured using the current interruption method to eliminate the ohmic drop. On the contrary to the prediction, it was found that the R p is lowered by the high resistance film in common to all the electrodes studied. The effective reaction area of the Pt electrode was measured from the peak area of the cyclic voltammetry. It was found that the enhancement of the electrode by the high resistance film is accompanied by a decrease in the reaction area. This uncommon experimental fact is explained by the non-uniform electrode model as follows. If the current distribution is completely uniform over the entire electrode area, the reaction area is a maximum, but the R p is the highest because it is determined by the local area of the lowest performance, which is, for this case, most polarized. If the current density is concentrated on the high performance local area, the reaction area is effectively smaller, but the local area of the lower performance is equally polarized with that of the higher performance, thus the R p being lower than that for the first case. If one can assume that the interfacial resistance is decreased by the high resistance film, this model assigns the former case to the uncoated electrolyte and the latter to the coated electrolyte.