Perovskite Materials Design for Two-Step SolarThermochemical Redox Cycles

Kurzfassung

Solar-thermochemical redox cycles are a promising technological option in the framework of utilization and conversion of renewable energy. By reducing metal oxides at high temperature and/or low oxygen partial pressure, one can generate a material in a state which can be used to capture oxygen from a gas stream or split water or carbon dioxide. By this means, air can be separated, oxygen can be pumped, or so-called solar fuels can be generated. One especially attractive materials class for application in such redox cycles is constituted by perovskites. These materials form stable phases over a large compositional range. Within this work, we show how these perovskite oxides can be applied in thermochemical redox cycles and study the mechanisms behind these redox reactions using in-situ X-Ray techniques. We also show that the kinetic properties of the oxidation reaction are very appealing. It is furthermore presented how perovskite solid solutions can be formed over a large compositional range and how phase formation and stability are affected by the perovskite composition. Based on this knowledge, the focus of this work is set on the materials thermodynamics. A new method of rational perovskite materials design is developed by adjusting the tolerance factor of the perovskites and their thermodynamics. Both experimental and theoretical materials development are conducted, the latter based on density functional theory (DFT) within the framework of the online resource “Materials Project”. Over 240 perovsite-brownmillerite pairs are included in the search. Detailed models describing the thermodynamics of such perovskite solid solutions are established which allow describing the perovskite redox properties as a function of the temperature, oxygen partial pressure, and oxygen non-stoichiometry