Water-Induced Decoupling of Tracer and Electrochemical Oxygen Exchange Kinetics on Mixed Conducting Electrodes

Isotope exchange depth profiling and electrochemical impedance spectroscopy are usually regarded as complementary tools for measuring the surface oxygen exchange activity of mixed conducting oxides, for example used in solid oxide fuel cell (SOFC) electrodes. Only very few studies compared electrical (k(q)) and tracer (k*) exchange coefficients of solid-gas interfaces measured under identical conditions. The 1:1 correlation between k(q) and k* often made is thus more an assumption than experimentally verified. In this study it is shown that the measured rates of electrical and tracer exchange of oxygen may strongly differ. Simultaneous acquisition of k(q) and k* on La0.6Sr0.4FeO3.delta and SrTi0.3FeO3.delta thin film electrodes revealed that k* > 100 k(q) in humid oxidizing (O-16(2) + (H2O)-O-18) and humid reducing (H-2 + (H2O)-O-18) atmospheres. These results are explained by fast water adsorption and dissociation on surface oxygen vacancies, forming two surface hydroxyl groups. Hence, interpreting experimentally determined k* values in terms of electrochemically relevant oxygen exchange is not straightforward.