Controlling the 3-D morphology of Ni–Fe-based nanocatalysts for the oxygen evolution reaction

Abstract

Controlling the 3-D morphology of nanocatalysts is one of the underexplored but important approaches for improving the sluggish kinetics of the oxygen evolution reaction (OER) in water electrolysis. This work reports a scalable, oil-based method based on thermal decomposition of organometallic complexes to yield highly uniform Ni–Fe-based nanocatalysts with a well-defined morphology (i.e. Ni–Fe core–shell, Ni/Fe alloy, and Fe–Ni core–shell). Transmission electron microscopy reveals their morphology and composition to be NiO_x–FeO_x/NiO_x core-mixed shell, NiO_x/FeO_x alloy, and FeO_x–NiO_x core–shell. X-ray techniques resolve the electronic structures of the bulk and are supported by electron energy loss spectroscopy analysis of individual nanoparticles. These results suggest that the crystal structure of Ni is most likely to contain α-Ni(OH)₂ and that the chemical environment of Fe is variable, depending on the morphology of the nanoparticle. The Ni diffusion from the amorphous Ni-based core to the iron oxide shell makes the NiO_x–NiO_x/FeO_x core-mixed shell structure the most active and the most stable nanocatalyst, which outperforms the comparison NiO_x/FeO_x alloy nanoparticles expected to be active for the OER. This study suggests that the chemical environment of the mixed NiO_x/FeO_x alloy composition is important to achieve high electrocatalytic activity for the OER and that the 3-D morphology plays a key role in the optimization of the electrocatalytic activity and stability of the nanocatalyst for the OER.