Bifunctional Oxygen Reduction and Evolution Activity\nin Brownmillerites Ca₂Fe_{(1–<i>x</i>)}Co_<i>x</i>O₅

Abstract

State-of-the-art\ncatalysts for oxygen reduction and evolution reactions\n(ORR and OER), which form the basis of advanced fuel cell applications,\nare based on noble metals such as Pt and Ir. However, high cost and\nscarcity of noble metals have led to an increased demand of earth-abundant\nmetal oxide catalysts, especially for bifunctional activity in ORR\nand OER. The fact that Pt and Ir or C, the cost-effective alternatives\nsuggested, do not display satisfactory bifunctional activity has also\nhelped in turning the interest to metal oxides which are stable under\nboth ORR and OER conditions. Brownmillerite A₂B₂O₅ type oxides are promising as bifunctional oxygen electrocatalysts\nbecause of intrinsic structural features, viz., oxygen vacancy and\ncatalytic activity of the B-site transition metal. In this study,\nCo-doped Ca₂Fe₂O₅ compounds are synthesized\nby the solid state method and structurally analyzed by Rietveld refinement\nof powder X-ray diffraction data. The compound Ca₂Fe₂O₅, crystallizing in the <i>Pcmn</i> space\ngroup has alternative FeO₄ tetrahedral and FeO₆ octahedral layers. Its Co-doped analogue, Ca₂Fe_1.75Co_0.25O₅, also crystallizes in the same space\ngroup with both tetrahedral and octahedral Fe positions substituted\nwith Co. However, Ca₂FeCoO₅ in the <i>Pbcm</i> space group shows interlayer ordering with Co-rich octahedra connected\nto Fe-rich tetrahedra and vice versa. Oxygen bifunctional activities\nof these catalysts are monitored by rotating disc electrode and rotating\nring disc electrode techniques in alkaline media. A close analysis\nof the ORR and OER was conducted through comparison of various parameters\nsuch as onset potential, current density, halfwave potential, and\nother kinetic parameters, which suggests that the presence of Co in\nthe B site aids in achieving better bifunctional activity and bulk\nconductivity. In addition, Co­(II)/Co­(III) redox systems and their\ncomparative concentrations also play a decisive role in enhancing\nthe activity.