SUMMARY

Concentrating solar technologies have the potential to mitigate anthropogenic greenhouse gas emissions and bolster energy security. However, intermittency in and localization of ideal solar irradiation conditions are significant obstacles to increased adoption. Thermochemical storage methods, namely solar fuels production and thermochemical energy storage for concentrating solar power facilities, address these obstacles with high energy storage densities, prolonged storage times, and compatibility with existing generation/transportation infrastructure. Storage material candidates must display equilibrium and kinetic behavior suitable for the heat rates, operating temperatures, and pressures of solar receivers/reactors, and optimal candidates require highly-tuned thermodynamic and kinetic properties to enhance cycle efficiency and storage capacity.

Here, the development, synthesis, and characterization of novel materials for two-step solar thermochemical cycles are investigated, with applications in solar fuels production and/or solar thermochemical energy storage. Reduction-oxidation (redox)-active binary metal oxides and mixed ion-electron conductors are studied via a combined experimental-computational approach, and a systematic characterization is performed to identify candidates with favorable thermodynamic and kinetic properties. Thermogravimetric analysis is used to extract reduction extents and reduction enthalpies/entropies for newly-synthesized materials. A novel upward flow reactor coupled to a High-Flux Solar Simulator with downstream product gas measurement is presented for kinetic evaluation of rapidly-reacting materials. The simulator is characterized experimentally and via Monte Carlo ray tracing to determine spatial heating of a material sample within the upward flow reactor. Heat inputs are coupled to a computational model of heat transfer and fluid dynamics for the reactor to determine the spatial, temporal temperature and thermal reduction of samples at high resolutions. The method is validated via measurements of well-characterized binary metal oxides. Fundamental structural properties of novel redox-active materials inform an experimental cation substitution study to enhance energy storage and tune the thermodynamics of candidate materials to intended applications. Oxidation experiments with candidate materials are performed to ensure completion of the two-step thermochemical cycle.