Nitrogen Doping in Oxygen-Deficient Ca₂Fe₂O₅: A Strategy for Efficient Oxygen Reduction Oxide Catalysts

Oxygen reduction reaction (ORR) is increasingly being studied in oxide systems due to advantages ranging from cost effectiveness to desirable kinetics. Oxygen-deficient oxides like brownmillerites are known to enhance ORR activity by providing oxygen adsorption sites. In parallel, nitrogen and iron doping in carbon materials, and consequent presence of catalytically active complex species like C-Fe-N, is also suggested to be good strategies for designing ORR-active catalysts. A combination of these features in N-doped Fe containing brownmillerite can be envisaged to present synergistic effects to improve the activity. This is conceptualized in this report through enhanced activity of N-doped Ca₂Fe₂O₅ brownmillerite when compared to its oxide parents. N doping is demonstrated by neutron diffraction, UV-vis spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. Electrical conductivity is also found to be enhanced by N doping, which influences the activity. Electrochemical characterization by cyclic voltammetry, rotating disc electrode, and rotating ring disk electrode (RRDE) indicates an improved oxygen reduction activity in N-doped brownmillerite, with a 10 mV positive shift in the onset potential. RRDE measurements show that the compound exhibits 4-electron reduction pathways with lower H₂O₂ production in the N-doped system; also, the N-doped sample exhibited better stability. The observations will enable better design of ORR catalysts that are stable and cost-effective.