Design principles of perovskites for thermochemical oxygen separation

Abstract

Separation and concentration of O2 from gas mixtures is central to several sustainable energy technologies, such as solar-driven synthesis of liquid hydrocarbon fuels from CO2, H2O, and concentrated sunlight. We introduce a rationale for designing metal oxide redox materials for oxygen separation through "thermochemical pumping" of O2 against a pO2 gradient with low-grade process heat. Electronic structure calculations show that the activity of O vacancies in metal oxides pinpoints the ideal oxygen exchange capacity of perovskites. Thermogravimetric analysis and high-temperature X-ray diffraction for SrCoO3-δ, BaCoO3-δ and BaMnO3-δ perovskites and Ag2O and Cu2O references confirm the predicted performance of SrCoO3-δ, which surpasses the performance of state-of-the-art Cu2O at these conditions with an oxygen exchange capacity of 44 mmolO2molSrCoO3-δ-1 exchanged at 12.1 μmolO2min-1g-1 at 600-900 K. The redox trends are understood due to lattice expansion and electronic charge transfer. Heat and pump thermochemically: Gas-phase O2 separation often limits the solar-to-fuel energy conversion efficiency of solar-driven thermochemical H2O and CO2 splitting. A method is proposed by which O2 is separated from gas mixtures through "pumping" O2 against a partial pressure gradient using metal oxide redox materials and low-temperature solar-thermal process heat. Design principles for perovskite reactants are developed and demonstrated.