Abstract

Solar thermochemical (STC) processes hold promise as efficient ways to generate renewable fuels, fuel precursors, or chemical feedstocks using concentrated sunlight. Specifically, one actively researched approach is the two-step STC cycle, which uses a redox-active, off-stoichiometric, transition-metal oxide material to split water and/or CO₂, generating H₂ and/or CO, respectively, or syngas (a combination of H₂ and CO). Identifying novel metal oxides that yield larger reduction extents (practically achievable off-stoichiometries) than the state-of-the-art CeO₂ is critical. Here, we explore the chemical space of Ca–Ce–M–O (M = 3d transition metal, except Cu and Zn) metal oxide perovskites, with Ca and/or Ce occupying the A site and M occupying the B site within an ABO₃ framework, as potential STC candidates. We use density functional theory (DFT)-based calculations and systematically evaluate the oxygen vacancy (Va_O) formation energy (≈ enthalpy of reduction in an STC cycle), electronic properties, thermodynamic stability of CaMO₃, CeMO₃, and Ca_0.5Ce_0.5MO₃ perovskites, and the Va_O formation energy within Ca_0.5Ce_0.5Ti_0.5Mg_0.5O₃ perovskite. We consider only Ca and/or Ce on the A site because of their similar size and the potential redox activity of Ce⁴⁺. If both Ce and M exhibit simultaneous reduction with Va_O formation, the resulting perovskite could exhibit a larger entropy of reduction than a single cation reduction. The increased entropy produces increased reduction for fixed temperature, partial pressure of oxygen, and reduction enthalpy, and therefore increased STC efficiency. Importantly, we identify Ca_0.5Ce_0.5MnO₃, Ca_0.5Ce_0.5FeO₃, and Ca_0.5Ce_0.5VO₃ to be promising candidates based on their VaO formation energy and thermodynamic (meta)stability. Moreover, based on our calculated on-site magnetic moments, electron density of states, and electron density differences between pristine and defective structures, we find Ca_0.5Ce_0.5MnO₃ to exhibit simultaneous reduction of both Ce⁴⁺ (A-site) and Mn³⁺ (B-site), highlighting a particularly promising candidate for STC applications with a predicted higher entropy of reduction than CeO₂. Finally, we extract metrics that govern the trends in Va_O formation energies, such as standard reduction potentials, and provide pointers for further experimental and theoretical studies, which will enable the design of improved materials for the STC cycle.