Technologies such as solar thermochemical hydrogen (STCH) production, which leverage the oxygen-exchange capacity of metal oxides to split water, are thermodynamically attractive, as the entire solar spectrum can be exploited to supply process heat. Here, a metal oxide is subjected to cyclic reduction-oxidation reactions that facilitate either oxygen or hydrogen production, depending on the operating conditions. Chemical thermodynamics suggest that the forward direction of each redox half-reaction is favorable in different temperature and/or oxygen partial pressure regions, and as a result, in practice, large irreversibilities generally arise from sensible heating between redox regimes. To address this issue, recent research has primarily focused on: (1) investigating novel perovskite materials with tunable thermodynamic properties that enable greater reduction extents at more moderate temperatures; and (2) integrating auxiliary techniques, such as mechanical or thermochemical pumping, that reduce the operating oxygen partial pressure (and thus temperature) in the endothermic reduction step. However, until highly effective and reliable solid-solid heat recovery methods are devised, inefficiencies inherent to temperature-swing operation will continue to negatively impact performance. Alternatively, at the expense of lower hydrogen productivity, the water-splitting (oxidation) step can be operated at temperatures where reduction is favorable, thereby minimizing thermal stresses and avoiding complexities associated with solid-solid heat recuperation. Herein, motivated by the immediate practicality of isothermal operation, the behavior of low-cost iron-based aluminate spinels (e.g., hercynite) was characterized under conditions relevant to isothermal STCH. Using thermogravimetry, equilibrated changes in oxygen content were quantified at different oxygen partial pressures and temperatures to extract thermodynamic properties, namely partial molar enthalpy and entropy, and thus enable performance predictions. Results indicate that this class of redox intermediates is capable of higher hydrogen productivities under isothermal operation than previously established benchmark materials (e.g., ceria and lanthanum manganite-based perovskites) evaluated under temperature-swing. The favorable thermodynamic properties of spinels at conditions better suited for industrialization present a step forward towards sustainable hydrogen production with solar thermal energy.
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