(305a) Solar Thermal Water Splitting By Two-Step Redox Cycle on Iron Aluminate in a Reticulated Porous Ceramic Morphology

Solar thermochemistry (STCH) uses concentrated solar power to drive highly endothermic, high temperature reactions. Splitting water by thermolysis is a route to producing green hydrogen. Water thermolysis can be carried out via a two-step reduction-oxidation cycle performed on an active material, typically a nonstoichiometric metal oxide. During reduction, a small amount (δ) of oxygen is removed from the metal oxide. During oxidation, the same amount of oxygen can be returned to the metal oxide using H₂O as the oxidant, splitting the molecule. Reduction is endothermic and occurs at the highest temperature, typically >1200°C, and under vacuum pressure or high inert sweep rates to lower p_O2 and drive the reaction forward. The oxidation temperature depends on the active material, with some materials requiring a large temperature swing to achieve a significant difference in nonstoichiometry between the oxidized and reduced states. The benchmark STCH material is ceria, which requires a temperature swing of 600°C-800°C [1]. Recently, the Weimer lab has demonstrated iron aluminate [2] as a STCH material which performs well under isothermal, pressure-swing operation, splitting more CO₂ than ceria under those conditions [3]. Elimination of the temperature swing is favorable because it eliminates thermal stresses on the system and thermal losses during repeated heating and cooling.

Active materials are often synthesized as reticulated porous ceramic (RPC) parts. This is because their porosity facilitates mass transfer by exposing a high surface area and heat transfer by being relatively optically thin, allowing radiation to transmit heat to more of the material. It also helps to have the solid phase fixed under direct irradiation to avoid thermophoresis and damage to the reactor aperture.

The iron aluminate RPCs are fabricated in-house and characterized for reaction. They are designed to fill the rooftop 4 kW high flux solar furnace (HFSF) dish at ETH Zurich. The goal of this work is to demonstrate water splitting with the iron aluminate RPCs on-sun. This work is carried out collaboratively with ETH Zurich and is expected to commence on-sun this summer. Since iron aluminate performs well under isothermal and near-isothermal operation, compared to other materials, we hypothesize that we will be able to operate with high efficiency by eliminating the temperature swing without compromising conversion. Results will be presented on the characterization and reactivity of the iron aluminate RPC and, potentially on full implementation on-sun at the ETH Zurich facility.

[1] D. Marxer, P. Furler, M. Takacs, and A. Steinfeld, "Solar thermochemical splitting of CO2 into separate streams of CO and O2 with high selectivity, stability, conversion, and efficiency," Energy & Environmental Science, vol. 10, pp. 1142-1149, May 1 2017.

[2] K. J. Warren, J. T. Tran, and A. W. Weimer, "A thermochemical study of iron aluminate-based materials: a preferred class for isothermal water splitting," Energy & Environmental Science, vol. 15, pp. 806-821, Feb 16 2022.

[3] J. T. Tran, K. J. Warren, D. Mejic, R. L. Anderson, L. Jones, D. S. Hauschulz, et al., "Pressure-enhanced performance of metal oxides for thermochemical water and carbon dioxide splitting," Joule, vol. 7, pp. 1758-1768, Aug 16 2023.