High-performance Pt-free oxygen reduction reaction electrocatalysts derived from carbon supported spinel cobalt ferrite and Co/Fe-nitrogen coordinated active centers for advanced energy applications

Abstract

Although the metal oxides are capable of displaying interesting catalytic activity characteristics, the intrinsic activities of these types of systems are found to be largely depending on other parameters such as particle size, morphology, and how well the catalyst particles are dispersed on a conducting substrate and additionally the surface intermediates involved in the process. Herein, we have explored the close relation of particle size, dispersion characteristics of the active particles on a conducting carbon, substitution effect of other metals and controlled interplay of the surface intermediates for the cobalt and iron metal atoms in facilitating the reactions like ORR and OER. Substituting some portion of Fe by Zn in the spinel lattice of cobalt ferrite nanocrystals can favorably influence the overpotential for ORR. However, the substitution of Zr in a cobalt ferrite matrix shows a significant level of improvement for the overpotentials of both ORR and OER, thereby positioning the composite as a potential bifunctional electrocatalyst. Basically, the electrochemical faradaic reactions are controlled by the transition states formed in between the reaction process and the adsorption energies of these states. In acidic and basic media, the stability of the transition states changes significantly with a direct impact on the ORR activity. To understand this and to gain insightful information on the mechanistic aspects, we have studied the variation in the ORR activity with the nature of the medium for cobalt and iron atom incorporated active centers. The functioning of the catalyst is demonstrated in the primary zinc-air battery in correlation of the transition states over the catalyst.