Keynote: Thermodynamic Limits for Materials in Solar Thermochemical Fuel Production

Solar-thermochemical (STC) carbon dioxide and water splitting is a pathway for the direct thermal production of solar fuels—an alternative to direct thermolysis. Currently, the most widely researched implementation is the two-step redox active metal oxide cycle. Reducing the two-step cycle to practice is challenging, and there is a substantial ongoing effort to improve reactor designs, and to discover and synthesize functional materials that can exceed the STC redox capacity potential at milder operating conditions than ceria, the current state of the art redox active metal oxide. We have analyzed the underlying general thermodynamic boundaries that lead to materials performance tradeoffs between reduction enthalpy, productivity, and yield. These boundaries apply to all nonstoichiometric oxides without significant phase changes during redox cycles. We quantify the tradeoffs between the desirable decrease of the reaction enthalpy of reduction and penalties such as decreasing yields and increasing temperature difference between the two steps (reduction and re-oxidation), and show that operating parameters pose firm limitations on cycle performance and on the desirable materials properties space.