Hydration Properties and Rate Determining Steps of the Oxygen Reduction Reaction of Perovskite-Related Oxides as H⁺-SOFC Cathodes

Four mixed ionic-electronic conducting (MIEC) perovskite-related oxides were studied as potential H+-SOFC cathode materials: La0.6Sr0.4Fe0.8Co0.2O3−, Ba0.5Sr0.5Co0.8Fe0.2O3−, PrBaCo2O5+ and Pr2NiO4+. Their hydration properties were measured by TGA: Ba0.5Sr0.5Co0.8Fe0.2O3− shows the largest water uptake. Their electrochemical performances were characterized using BaCe0.9Y0.1O3− as electrolyte; polarization resistances as low as 0.5 cm2 were found at 600°C, for PrBaCo2O5+ and Pr2NiO4+. The rate determining steps of the oxygen reduction reaction were determined on the basis of electrochemical studies performed as a function of pH2O, in air. Proton transfer and water release appear to be the rate determining steps for Ba0.5Sr0.5Co0.8Fe0.2O3−, PrBaCo2O5+ and Pr2NiO4+. No rate determining step involving proton was found for La0.6Sr0.4Fe0.8Co0.2O3−. On the basis of this study, one can suggest that Ba0.5Sr0.5Co0.8Fe0.2O3−, PrBaCo2O5+ and Pr2NiO4+ show some protonic conduction as well as oxide diffusivity and can be labeled Triple Conducting (e-/O2−/H+) Oxides, so-called TCO.