The reaction rates of mixed conducting solid oxide fuel cell electrodes are often limited by the kinetics of oxygen exchange at the surface, e.g. either O2 incorporation and release, or H2 oxidation or H2O splitting. Furthermore, the kinetics may exhibit remarkable variability even for the same materials, depending on prior sample treatment or fabrication techniques and impurities in the used materials or measurement setup. All these aspects have a strong impact on the actual composition and structure of the surface of the electrode materials, which usually drastically differs from the bulk stoichiometry. Depending on the surface chemical state, even the reaction mechanism may change. By the coupling of in-situ surface characterization and modification methods with electrochemical testing, a correlation between surface chemistry and reactivity of electrode materials can be drawn.
We are therefore very active in optimizing in-situ methods, partly by constructing novel measurement setups and partly by using synchrotron-based central facilities. In these experiments, we can for example control the oxygen stoichiometry and defect chemistry of the electrode material or trigger the exsolution of metallic nanoparticles by in-situ heating and application of a sample bias. Simultaneously, we can investigate the surface chemistry by ambient pressure XPS, or surface sensitive XRD. With these in-situ methods we are therefore able to identify the electrochemically active sites for oxygen exchange reactions on mixed conducting electrodes in various atmospheres.