Fundamentals of Surface Kinetics in High Temperature Electrochemistry (FUSKE)

The aim of the project FUndamentals of Surface Kinetics in high temperature Electrochemistry (FUSKE) is to generate groundbreaking insight to the surface processes involved in high temperature electrochemical transport, addressing scientific questions critical for further progress and new discovery within this field of science. The technological relevance of the fundamental investigations of the project is topical; electrochemistry will play a vital role in sustainable technologies for interconversion of electrical and chemical energy. Examples of these embrace batteries, fuel cells, electrolysers, gas separation membranes. The electrochemical devices rely on the surface reactions (electrode reactions) at each side of a membrane through which the conductivity of ions is also essential. All the processes involved are, in principle, equally important as they may all determine the performance of the energy conversion. In FUSKE the surface reactions are the main objectives. Focus in the present reporting period has been the elementary reactions governing the processes involved from a gas molecule adsorbs on the surface of an oxide, dissociates and is incorporated into the oxide, typically either an electrode or an electrolyte material. Acceptor-substituted ceria is one of the materials that has been studied rather intensively with an array of methodologies. Effects of water vapor on surface processes were, based on computational approaches, anticipated to be rather important. However, the effect was not pronounced at all. To further elucidate the effects of water vapor on the oxygen surface exchange of fluorite based oxide ion conductors, we substituted 50% of Ce with La, yielding the compound lanthanum cerate, La2Ce2O7. Since La is less acidic than Ce, it was expected that hydroxide defects would influence the surface exchange more for lanthanum cerate than for ceria. The presence of water vapor decelerated the oxygen exchange. This was interpreted to reflect that hydroxide defects adsorb to the active sites for oxygen adsorption, as such partly blocking oxygen exchange. To perform these measurements of surface kinetics there has been major development in the experimental approaches, designing experimental setups enabling the study of the delicate reaction sequences of the overall surface reaction. In the present reporting period an experimental set-up has been constructed that enables electrochemical measurements in parallel to gas-phase analysis with isotopolouges. As far as we know, this is the first set-up of its kind in the world. The approach will generated data that may lead to new in-depth understanding of the surface processes that rate limits electrochemical energy conversion reaction determining the performance of these, e.g. solid oxide fuel cells and electrolyzer cells, in applied as part of a sustainable energy technology.

The research performed in the project has contributed to increase the detailed understanding of surface processes for materials relevant as oxygen gas separation membranes and for electrodes in solid-state electrochemical energy interconversion devices and importance of the presence of water vapor. Also the increased understanding of the surface exchange mechanism for complex mixed proton-oxide ion-electron hole double perovskite will be essential input to develop further positrodes for proton ceramic fuel- and electrolyzer cells. Model materials for mixed oxide ion-electron and mixed proton-electron conductors have been investigated and the mechanisms for oxygen reduction and incorporation of protons have been determined increasing the overall understanding of the heterogenous gas-sold surface processes. An important outcome of the project, which will benefit the solid-state ionic research community in the years to come, is the significant development and implementation of gas-phase isotope based methodologies to investigate oxide kinetics in general, but most importantly the surface kinetics. As outlined previously in this report, there are already numerous projects that will benefit the methodology developed addressing both fundamental and more applied surface science by means of the IE-GPA and the PIE systems.

High temperature electrochemistry lacks fundamental understanding of the surface processes at play, processes of which kinetics in many cases rate-limit electrochemical transport. The present project addresses high temperature gas-solid reaction with the aim to develop the fundamental mechanistic insight providing the answers that enable development of a future clean and sustainable energy production. The present project, FUndamentals of Surface Kinetics in high temperature Electrochemistry (FUSKE), sets out with the hypothesis that water vapor is one of the keys to the understanding of surface mechanisms. Moreover, that effects of space charge zones at, or in the vicinity of, the surface strongly influence electrocatalytic properties. An array of novel experimental and computational based approaches will be utilized to address these two fundamental, scientific statements and to generate ground breaking, detailed mechanistic comprehension. This includes extensive use of isotopes in combination with transient approaches and isotope exchange annealing followed by ToF-SIMS. In particular novel approaches based on "super-heavy" water, D218O will be utilized and further developed. Instrumentation including both electron microscopy, ex-situ and in-situ, and spectroscopy based techniques, will be applied to characterize the surfaces structure, defect structure, electronic "states", microstructure and composition. Computational based approaches are included as the third important pillar in the projects overall methodology to develop and derive models for the aforementioned properties of the surfaces, and for comparison to the findings of the experiments. The project will be complementary to another projects focusing on methodology development and these two together will contribute important leaps in the understanding of surface processes in high temperature electrochemistry. The project will educate one PhD and train one researcher/post doc.