Mixed Transition Metal Oxide Supported on Nitrogen Doped Carbon Nanotubes - a Simple Bifunctional Electrocatalyst Studied with Scanning Electrochemical Microscopy

A facile way to prepare a bifunctional electrocatalyst, which is active for both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) in alkaline media, is presented. A Ni0.9Co0.1Fe2O4 mixed oxide, which is synthesized using a polyol mediated synthesis route, is combined with nitrogen doped carbon nanotubes (NCNT) known as electrically conducting material with high activity for the ORR. The combined bifunctional catalyst was investigated towards its structural properties using TEM, XRD, XPS and Raman spectroscopy and studied with relevant electrochemical characterization techniques, namely rotating disk electrode measurements for ORR and linear sweep voltammetry for OER as well as with scanning electrochemical microscopy (SECM). The difference between onset potentials for OER and for ORR was similar to that of other published bifunctional catalysts, indicating that the simple synthetic approach yields materials well suited for bifunctional catalysis applications. For ORR investigations the redox competition mode of SECM indicated high selectivity of the materials towards 4-electron reduction of oxygen to water. The mixed material showed an enhanced activity for both reactions in comparison to the untreated and pristine materials. An additional heat treatment of the bifunctional catalyst led to a material with elemental Ni, Co and Fe exhibiting a decreased activity for both reactions.