Favorable Redox Thermodynamics of SrTi_0.5Mn_0.5O_3−δ in Solar Thermochemical Water Splitting

Two-step, solar thermochemical splitting of water using nonstoichiometric redox-active metal oxides has emerged as an intriguing approach for large-scale hydrogen production. Perovskites have been proposed as alternatives to state-of-the-art fluorite CeO2−δ because of their potential for lowering reduction temperature while maintaining high fuel productivity. Guided by computational insights, we explore the thermodynamic properties and water splitting efficacy of the cubic perovskite SrTi0.5Mn0.5O3−δ (STM55). Thermogravimetric analysis is performed under controlled oxygen partial pressures (pO2) and temperatures up to 1500 °C, from which both the enthalpy and entropy of reduction as a function of oxygen nonstoichiometry are determined. STM55 provides an attractive combination of moderate enthalpy, 200–250 kJ (mol-O)−1, and high entropy, with unusual δ dependence. Using a water splitting cycle in which the material is thermally reduced at 1350 °C (pO2, ∼10–5 atm) and subsequently exposed to steam at 1100 °C (steam partial pressure of pH2O = 0.4 atm), we demonstrate a hydrogen yield of 7.4 mL g–1. Through both half-cycles, the material remains largely in quasi-equilibrium with the gas phase, as reflected in the agreement of the measured data with predicted profiles based on the thermodynamic data. This behavior indicates rapid surface and bulk diffusion kinetics. Cyclic operation showed the material to be free of degradation and always resulted in a 2:1 yield of H2/O2. Overall, STM55 provides outstanding performance characteristics for thermochemical hydrogen production.

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