Hierarchically devising NiFeO_xH_y catalyst with surface Fe active sites for efficient oxygen evolution reaction

Nickel iron (oxy)hydroxide (NiFeOxHy) is among the most active electrocatalysts for oxygen evolution reaction (OER) in the alkaline media. Development of new design strategy for NiFeOxHy electrode with respect to intrinsic activity and number of active species is highly attractive. Here we report a hierarchical fabrication for NiFeOxHy electrode towards efficient oxygen evolution. Three-dimensional mesoporous NiO was initially prepared and reduced to a polycrystalline Ni metal framework. Despite the poor OER activity for Ni-based catalyst, the incorporation of surface Fe species induced a notably high current density for the oxygen evolution. The results indicated that the Fe species, located at outer surface region, played a key role as the OER active sites. It is also found that the surface of metallic Ni was reconstructed to (oxy)hydroxide phase with ease compared to NiO. As a result, a heterostructure composed of an OER-active NiFe (oxy)hydroxide shell and an electron-conductive Ni core was formed. In this fabricated NiFeOxHy/Ni catalyst, the promoted intrinsic activity, number of active sites and charge-transfer ability all taken into account contribute to an efficient OER process.