Exploring Ca–Ce–M–O (M = 3d Transition Metal) Oxide Perovskites for Solar Thermochemical Applications

Journal Article · 17 November 2020 · Chemistry of Materials

DOI: https://doi.org/10.1021/acs.chemmater.0c02912 · OSTI ID:1806574

Gautam, Gopalakrishnan Sai  ^[1]; Stechel, Ellen B.  ^[2]; Carter, Emily A.  ^[3]

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1. Princeton Univ., NJ (United States); Indian Inst. of Science, Karnataka (India)
2. Arizona State Univ., Tempe, AZ (United States)
3. Princeton Univ., NJ (United States); Univ. of California, Los Angeles, CA (United States)

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 (VaO) 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 VaO 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 Va_O 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. Lastly, 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.

• Research Organization: Princeton Univ., NJ (United States)
• Sponsoring Organization: USDOE Office of Energy Efficiency and Renewable Energy (EERE)
• Grant/Contract Number: EE0008090
• OSTI ID: 1806574
• Journal Information: Chemistry of Materials, Vol. 32, Issue 23; ISSN 0897-4756
• Publisher: American Chemical Society (ACS)Copyright Statement
• Country of Publication: United States
• Language: English