To continue to meet global energy demands, efficient methods of utilizing renewable energy must be developed. Converting solar energy into chemical fuels is a promising approach, but efficient and cost effective methods for producing solar fuels have not yet been developed. Solar thermal water splitting (STWS) is a promising possibility because it has a high theoretical hydrogen production efficiency. In two-step STWS, a metal oxide is heated under low O2 partial pressures by concentrated sunlight to a high temperature to undergo reduction and generate O2. In the second step, the reduced metal oxide is exposed to steam which re-oxidizes the material and forms H2. However, achieving the promised high solar to H2 efficiency requires finding the proper redox material. Currently, the most promising materials include spinels and perovskites. These materials split water by forming oxygen vacancies in the high temperature step, and then stripping oxygen from water to fill the vacancies in the next. An efficient STWS process is extremely demanding on materials and requires that they be thermodynamically capable of being reduced, withstand the high temperatures of solar thermal water splitting, and form oxygen vacancies with enough energy to reduce water. Furthermore, materials must also be kinetically viable, in other words they must be able to complete the water splitting cycle quickly enough to allow for large scale production of hydrogen. The thermodynamic and kinetics of STWS chemistry can be directly modeled using quantum chemical methods, but these tools are too time intensive to use over a class of materials to identify promising candidates.
We have developed computational methods to identify STWS materials with desirable thermodynamic and kinetic properties. We have used high level calculations on a subset of the materials to identify descriptors for the activation barriers for the rate limiting step for water splitting. These descriptors can be determined from simpler calculations and will allow for rapid calculation and determination of water splitting materials. This work focuses on the use of density functional theory to develop general descriptors of the water splitting reaction and in turn a more fundamental understanding of the reaction mechanism.
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