Structural and Thermodynamic Assessment of Ba and Ba/Mg Substituted SrFeO3−δ for “Low-Temperature” Chemical Looping Air Separation

The increasing demand for high-purity oxygen has prompted interest in chemical looping air separation (CLAS), a promising approach to reduce cost and energy consumption. The current study presents a structural and thermodynamic analysis of Sr0.75Ba0.25FeO3−δ (SBF628), Sr0.75Ba0.25Fe0.875Mg0.125O3−δ (SBFM6271), and Sr0.25Ba0.75FeO3−δ (SBF268) for CLAS applications. Our results confirm that through partial substitution of A- and B-site cations, we can tailor the thermochemical properties of SrFeO3−δ under practical operating conditions, e.g. 400–700 °C and 0.2–0.01 atm O2. Based on X-ray diffraction (XRD) and Rietveld refinement, 25% Ba can be fully substituted into a SrxBa1–xFeO3−δ structure (SBF628), whereas SBF268 and SBFM6271 form minor secondary phases. An increased Ba:Sr ratio in the A-site favors facile oxygen exchange by reducing partial molar enthalpy and improving redox capacity at low temperatures (400–500 °C) while leading to an increase in Fe–O bond length in the B-site. Our findings support that the Fe–O bond distance can be a useful descriptor in the optimization of redox performance in SrFeO3−δ (SF) based oxygen carrier candidates for CLAS. All the substituted candidates exhibit superior redox performance to unsubstituted SF at temperatures below 600 °C. SBF628 achieves the redox (oxygen storage) capacity of 1.24 wt % under a combined temperature (400–600 °C) and pressure swing (0.2–0.01 atm O2) mode. The introduction of 12.5% Mg in the B-site improves the oxidation kinetics, but it does not have a substantial impact on the oxygen storage capacity. SBFM6271 and SBF628 exhibited excellent recyclability and robustness while a minimal decrease (3%, on a relative basis) in the capacity of SBF268 was observed during 100 redox cycles under pressure swing between 0.2 and 0.01 atm O2 at 600 °C.