Solar thermochemical (STC) processes are an efficient way to generate renewable fuels or fuel precursors using concentrated sunlight. Specifically, an STC process uses redox-active, off-stoichiometric transition- metal oxide substrates to split water and CO2, generating syngas, H2 and CO. Importantly, STC processes require oxide substrates that can i) withstand large temperature swings, ii) sustain high degrees of oxygen off-stoichiometry and iii) be resistant to adverse phase transformations. Currently, the state-of-the-art material used in STC applications is pure or doped CeO2; identifying better materials is an active area of research. In this work, we explore the chemical space of Ca-Ce-M-O (M=3d transition 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, using density functional theory (DFT) based calculations. To account properly for electronic exchange and correlation, we use the strongly constrained and appropriately normed (SCAN) functional, with an appropriately determined Hubbard U correction to reduce the inherently large self-interaction error DFT engenders when describing 3d (M) and 4f (Ce) electrons. While we consider only Ca and/or Ce on the A-site because of their similar size and the potential redox-activity of Ce, we evaluate all 3d transition metals except Zn on the B-site since Zn+3 is an uncommon oxidation state in solids. We evaluate the oxygen vacancy formation energy (enthalpy of reduction in a STC process), electronic properties, and thermodynamic stability of ternary Ca-M-O, Ce-M-O, and quaternary Ca-Ce-M-O perovskites, identify promising candidates, and extract metrics that govern the enthalpy and entropy of reduction, which is key to improving STC efficiencies.

Exploring Ca-Ce-M-O (M = 3d transition metal) oxide perovskites for solar thermochemical applications

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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