B-site cation sequencing in SrMnO3 using iron for zinc–air battery electrocatalysis: a structural evaluation†

Abstract

An unprecedented approach for synthesizing strontium manganese perovskite oxides (ABO₃) and their B-site substituted variants (SrMn_1−xFe_xO₃) was employed using the molten salt synthesis route. This study aims to investigate the intrinsic property changes of perovskite oxide materials and their electrochemical response, particularly in the bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Systematic substitution at the B-site induces a phase transition from hexagonal to trigonal, and then to orthorhombic, which was confirmed through Rietveld refinement of XRD data, along with SEM, TEM and XPS analyses. The multiple oxidation states of manganese (Mn³⁺/Mn⁴⁺) and iron (Fe³⁺/Fe²⁺) at the B-site play a crucial role in redox reactions. Furthermore, the orthorhombic brownmillerite phase (Sr₂MnFeO₅) promotes the ORR even without conductive support, which is attributed to its intrinsic conductivity stemming from the specific distribution of oxygen vacancies. The favorable adsorption/desorption energies of oxygen intermediates are a result of regulated electron filling in the d orbitals. The SrMn_0.7Fe_0.3O₃ variant was evaluated as a bifunctional electrocatalyst, showing an onset potential of 0.99 V vs. RHE for the ORR, and demonstrated excellent performance in rechargeable zinc–air batteries (ZABs), with a high peak power density of 114 mW cm⁻² and a long cycle life of over 262 hours, exhibiting a specific capacity of 680 mA h g⁻¹. The unique structural properties of SrMn_0.7Fe_0.3O₃ make it a promising candidate for ZAB applications.