Towards an Improved Understanding of Electrochemical Oxygen Exchange Reactions on Mixed Conducting Oxides

In solid oxide fuel cells or electrolysis cells with ionic and electronic conducting electrodes, the electrochemical reactions take place at the gas/solid interface of the mixed conducting electrode. This is in contrast to the situation met in aqueous electrolytes, where the reactions occur at the electrode/electrolyte interface. Thus, modified concepts are required to describe the reaction rates of such reactions on mixed conducting oxides. Here, it is discussed how overpotential and gas partial pressure affect reaction rates. A rate equation is suggested that also includes defects and the meaning of the empirical reaction orders is illustrated. Three experiments are presented that emphasize different aspects required for a mechanistic understanding of such reactions: A simultaneous variation of overpotential and partial pressure reveals the role of defects in oxygen evolution on (La,Sr)FeO3-δ electrodes. Bias dependent impedance measurements with an analysis of the chemical capacitance gives information on the concentration of minority defects in Sr(Ti,Fe)O3-δ. Modification of (La,Sr)CoO3-δ thin film surfaces by Sr or Co and impedance measurements in the pulsed laser deposition chamber help understanding catalytically active sites.