First-principles evaluation of Ca-Ce-M-O (M = 3<i>d</i> transition metal) oxide perovskites for solar thermochemical applications

Solar thermochemical (STC) processes that use redox-active, transition-metal oxide substrates to split water and/or CO₂ have potential to be an efficient way to generate renewable fuels or fuel precursors. STC processes require oxides that are thermally stable over a wide range of temperatures, tolerate high degrees of oxygen off-stoichiometry and resistant to adverse phase transformations, to yield better efficiencies than state-of-the-art CeO₂. In this work, we explore the chemical space of Ca-Ce-M-O (M=3d transition metal) perovskites as potential STC candidates using first principles calculations. Specifically, we use a Hubbard U corrected strongly constrained and appropriately normed exchange-correlation functional to treat the electronic exchange and correlation. While we consider Ca and Ce occupation of the A site (in an ABO₃ perovskite) because of their similar ionic radii and the potential redox-activity of Ce, we consider all 3d transition metals except Zn on the B-site. Subsequently, we evaluate the oxygen vacancy formation energy (~enthalpy of reduction in an STC process), electronic properties, and thermodynamic stability of ternary Ca-M-O, Ce-M-O, and quaternary Ca-Ce-M-O perovskites and identify promising candidates that might optimize STC water and CO₂ splitting.