Cation Defect Chemistry and Segregation in Perovskites

Efficient and economical large-scale storage of electrical energy is a still missing key technology on the way towards fully renewable electricity production. The most promising pathway for this is electrolysis, which uses electrical energy to produce stable chemical energy carriers (e.g. H2). These can be converted back into electricity or used in the chemical industry. High-temperature cells based on solid oxides (solid oxide fuel and electrolysis cells) operate at temperatures of 600-800C, where the reactions run fast and efficiently. Nonetheless, further optimization - especially regarding activity and stability of the electrodes - is required. Perovskite-type ceramics with ABO3 structure are particularly well suited as electrode materials. These are not only active catalysts but also exhibit good electrical conductivity and mobility of oxygen ions which are prerequisites for usage as electrodes in electrochemical cells. Although perovskite-type materials with complex stoichiometry such as La0.6Sr0.4Fe0.8CoO3 are already used commercially in electrochemical cells, neither the electrochemical reaction mechanism nor the chemical composition of the catalyst surface on which the reaction takes place have been clarified in detail. A major hindrance for atomistic understanding is that the well-understood bulk crystal structure is interrupted at the surface, which triggers cation segregation. This means that on the surface, a very thin (2-5 atomic layers) but thermodynamically stable termination layer forms, which is very different from the underlying crystal in structure stoichiometry. As this surface layer is very thin, and its exact structure and composition may change with the environmental conditions, it is very hard to resolve on an atomistic level. We plan to tackle this task and find ways to optimise the activity of this surface layer within this research project. At the Vienna University of Technology, as well as at the partner organisations "CEST" and the MU Leoben, several powerful analytical instruments have recently been installed, with which the composition of the top atomic layers of the catalyst can be measured in conditions that resemble real operation. Over the next 3 years, project leader Andreas Nenning, as well as a PhD student will test various catalytically active perovskite materials for their surface composition and catalytic activity under operating conditions. The final goal of the research work is to gain insight into the relationship between composition, operating conditions and surface properties, which will enable the targeted development of catalysts with higher activity and long-term stability. Such catalysts are urgently needed to make high-temperature electrolysis more efficient, more long-term stable and thus economically successful.