Ultrafine Metallic Nickel Domains and Reduced Molybdenum States Improve Oxygen Evolution Reaction of NiFeMo Electrocatalysts

An electrocatalyst for oxygen evolution reaction (OER) is essential in the realization of renewable energy conversion technologies, but its large overpotential, slow charge transfer, and degradation of surface reaction sites are yet to be overcome. Here, it is found that the metallic nickel domains and high-valence reduced molybdenum ions of NiFeMo electrocatalysts grown on a 3D conductive and porous electrode without using binders enable ultrahigh performance in OER. High resolution-transmission electron microscope and extended X-ray absorption fine structure analyses show that metallic nickel domains with Ni-Ni bonds are generated on the catalyst surface via a dry synthesis using nitrogen plasma. Also, Mo K-edge X-ray absorption near-edge spectroscopy reveals that Mo6+ ions are reduced into high-valence modulating Mo4+ ions. With the metallic nickel domains facilitating the adsorption of oxygen intermediates to low-coordinated Ni0 and the Mo4+ pulling their electrons, the catalyst exhibits about 60-fold higher activity than a Mo-free NiFe catalyst, while giving about threefold faster charge transfer along with longer stability over 100 h and repeated 100 cycles compared to a bare NiFeMo catalyst. Additionally, these metallic domains and high-valence modulating metal ions are exhibited to give high Faradaic efficiency over 95%.