Oxygen reduction and oxidation at epitaxial model-type Pt(O2)/YSZ electrodes – On the role of PtOx formation on activation, passivation, and charge transfer

Epitaxial model-type platinum (Pt) film electrodes with a high three-phase boundary (tpb) density on single crystalline yttria-stabilized zirconia electrolyte, YSZ(1 1 1), are characterized electrochemically in air at a temperature of 673 K – which is comparatively low for the system Pt(O2)/YSZ. By combining different electrochemical measurements including impedance spectroscopy (EIS) with and without applied potential bias, cyclic voltammetry (CV), and ‘steady state’ current-overpotential characteristics (I–η curves) with different sample starting conditions, the impact of annealing time and pre-polarization on the polarization resistance and on the oxygen exchange rate are investigated in detail. A model, based on the electrochemical control of thermally grown Pt oxide, is suggested to explain the experimental results. It is assumed that Pt oxide passivates the electrode and decreases the electrocatalytic activity of Pt for the oxygen exchange reaction. Additionally, it is suggested that Pt oxide has a similar passivating effect as in aqueous electrochemistry, despite the higher temperature. Additionally, blocking impurities at the tpb and activating morphological changes are discussed. Together with the experimental results the model provides new insights in the observed electrode processes and offers routes to improve the electrode performance of Pt(O2)/YSZ at low temperature. This might be of significant interest e.g. for μ-solid oxide fuel cells (micro-SOFCs).