Role of crystal structure and electrical polarization of an electrocatalyst in enhancing oxygen evolution performance: Bi-Fe-O system as a case study

This work aims to give an insight into the influence of crystal structure (for a system containing same elements but crystallizing in different structures) and the effect of electrical polarization on these oxides on the performance of oxygen evolution reaction (OER). We have tried to highlight this influence by taking the Bi-Fe-O system for the study. Herein, we have synthesized three structures of the Bi-Fe-O system viz. BiFeO₃ (perovskite structure), Bi₂Fe₄O₉ (mullite structure), and Bi₂₅FeO₄₀ (sillenite structure) as an example to establish the relation. These oxides were characterized by Rietveld refinement for structure and scanning electron microscopy (SEM) for morphology. Their optical and magnetic properties were also investigated. Systematic studies were carried out with both as-synthesized and electrically polarized oxides for their performance towards OER. We observed that the order for OER activity (using non-polarized catalyst) of the three stable structures synthesized was Bi₂Fe₄O₉ > BiFeO₃> Bi₂₅FeO₄₀, which was attributed to the presence of Fe_(oct)-O-Fe_(td) linkages in Bi₂Fe₄O₉. While the current density of Bi₂Fe₄O₉ and BiFeO₃ remained unchanged after poling, that of Bi₂₅FeO₄₀ increased by four-fold. From the study, we have demonstrated that proper choice of the crystal structure and utilization of electrical polarization can be effective strategies to manipulate the surfaces of an electrocatalytic material.