Fluoridated Iron-Nickel Layered Double Hydroxide for Enhanced Performance in the Oxygen Evolution Reaction

Layered double hydroxides (LDHs) are very promising but still far from satisfactory for catalyzing the electrochemical oxygen evolution reaction (OER) in water electrolysis. Herein, it was found that the catalytic performance of iron-nickel LDHs for OER can be largely boosted by a facile and controllable fluoridation approach at low temperatures. Temperature dependence of the crystal structure and surface chemical state was observed for the simple fluoridation of the iron-nickel LDH. However, no significant surface roughness and electrochemical active surface area increases were found, which was probably owing to the structure change from nanosheets to nanorods. Significant improvements in the performance, including the catalytic activity, stability, efficiency, and kinetics, were found compared with the pristine iron-nickel LDH. Specifically, iron-nickel fluoride obtained at 250 °C afforded the lowest overpotential of 225 mV (no iR correction) to drive 10 mA cm^-2 loaded on an inert glassy carbon electrode with a small Tafel slope of 79 mV dec^-1 , outperforming the noble-metal IrO₂ catalyst and most of the similar Fe-Ni based catalysts. The performance improvement could be mainly attributed to the phase-structure transfer from metal-O bonding in the FeNi-LDHs to metal-F bonding after fluoridation, which means it is easier to form the real active sites of Fe-doped high-valence Ni-(oxy)hydroxide over the iron-nickel fluoride surface.