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: La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3−δ , Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ , PrBaCo 2 O 5+δ and Pr 2 NiO 4+δ . Their hydration properties were measured by TGA: Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ shows the largest water uptake. Their electrochemical performances were characterized using BaCe 0.9 Y 0.1 O 3−δ as electrolyte; polarization resistances as low as 0.5 Ω cm 2 were found at 600°C, for PrBaCo 2 O 5+δ and Pr 2 NiO 4+δ . The rate determining steps of the oxygen reduction reaction were determined on the basis of electrochemical studies performed as a function of pH 2 O, in air. Proton transfer and water release appear to be the rate determining steps for Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ , PrBaCo 2 O 5+δ and Pr 2 NiO 4+δ . No rate determining step involving proton was found for La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3−δ . On the basis of this study, one can suggest that Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ , PrBaCo 2 O 5+δ and Pr 2 NiO 4+δ show some protonic conduction as well as oxide diffusivity and can be labeled Triple Conducting (e-/O 2− /H + ) Oxides, so-called TCO.