Materials and Components for Solar-thermochemical Fuels Production

Abstract

The ease of transportation and high power density create the basis for hydrocarbon fuels’ privileged position in our energy mix. Therefore, processes that use renewable energy sources to produce liquid hydrocarbon fuels from H2O and CO2 are of crucial importance. Concentrated solar radiation can be employed as the only energy source for the renewable production of hydrogen from water either indirectly, e.g. by supplying the electricity for electrolysis, or directly by supplying the necessary heat for thermochemically producing hydrogen and syngas. Among the various thermochemical cycles tested so far for solar-driven hydrogen production via water splitting, those based on redox-pair oxide systems, are directly adaptable to carbon dioxide splitting and/or combined CO2/H2O splitting for the production of CO or syngas, respectively. The Helmholtz Virtual Institute SolarSynGas brings together expertise from solar energy research and materials science to develop metal oxide based redox materials and to integrate them into suitable process technologies for such processes. Due to preliminary theoretical studies as well as reported in the literature, ceria-based materials are viable candidate materials for such cycles. In screening campaigns, the performance of ceria was enhanced via doping with zirconium and trivalent lanthanides. The most promising compositions were subjected to equilibrium reduction experiments with the aid of thermogravimetry analysis under varying conditions such as the temperature and the partial pressure of oxygen. Applying a basic reaction model, the thermodynamic properties of the material were determined. In particular, the reduction enthalpy and reaction conditions are crucial to assess the performance of ceria-based redox materials with the aid of basic process models, which include the major energetic relations. Further than that, kinetical parameters such as the chemical surface exchange coefficient and chemical diffusivity of oxygen were determined. Thereby, the basic understanding of interrelations between atomic structure and transport, microstructure, reactivity and life time of the materials is gained, which is crucial for the development of optimized reactor and process concepts based on ceria.