Reduction enthalpy and charge distribution of substituted ferrites and doped ceria for thermochemical water and carbon dioxide splitting with DFT+U

Abstract

The thermal reduction step of substituted ferrites (MFe₂O₄ where M = Fe, Ni, Co, Gd) and doped ceria (M_xCe_1−xO₂, where M = Ce, Zr, Hf and x = 0.25) in two-step thermochemical cycles for H₂O and CO₂ splitting is investigated within the DFT+U framework. This thermal reduction step is described as the oxygen vacancy formation energy (reduction enthalpy), i.e. the energy required to create an oxygen vacancy in the crystal lattice. Oxides with a lower oxygen vacancy creation energy are easier to reduce. A Bader charge analysis of the reduction mechanism is carried out providing the charge distribution of the bulk and reduced ions, enabling interrelations of the substitute ions and the resulting reduction energies. Based on the approach presented here, interesting solar fuels producing materials are CoFe₂O₄, NiFe₂O₄ and Hf_0.25Ce_0.75O₂.