Bifunctional α-MnO2 and Co3O4 Catalyst for Oxygen Electrocatalysis in Alkaline Solution

Abstract

Low cost and abundant catalysts demonstrating high activity and stability towards the oxygen reactions, i. e., the oxygen reduction (ORR) and oxygen evolution reaction (OER), are crucial for the development of electrically rechargeable zinc-air batteries. Herein, the facile synthesis and systematic characterisation of two highly active and stable oxygen electrocatalysts, i. e., high surface area α-MnO2 microspheres and nanoparticulate Co3O4, are reported. α-MnO2 exhibits low half-wave potential and potential of −0.197 and −0.226 V (vs. Ag/AgCl) at −3 mA cm−2, respectively, that are only marginally higher compared to commercial Pt/C (E1/2=−0.161 V, Ej=-3=−0.171 V) for ORR. Meanwhile, Co3O4 needs a potential of 0.601 V (vs. Ag/AgCl) to drive 10 mA cm−2 being competitive to commercial Ir/C (Ej=10=0.60 V) for OER. In order to create a bifunctional catalyst, two approaches were pursued: i) Co3O4 nanoparticles were homogeneously grown on the surface of α-MnO2 microspheres yielding a radial hybrid composite catalyst material in the form of a core (α-MnO2) shell (Co3O4) structure and ii), much simpler, individual α-MnO2 microspheres and Co3O4 nanoparticles were physically mixed in a powder blend. The powder blend demonstrates superior overall bifunctional catalytic properties such that the individual catalysts still dominate their respective oxygen reaction and, due to synergistic interactions between both catalysts, an improved ORR activity could be achieved.