Ni/Fe Ratio Dependence of Catalytic Activity in Monodisperse Ternary Nickel Iron Phosphide for Efficient Water Oxidation

Electrochemical water-splitting is an effective way to obtain hydrogen fuel as a clean and renewable energy source. However, a major challenge is to accelerate the sluggish oxygen evolution reaction (OER) kinetics with multistep proton-coupled electron transfer process. Transition metal Ni- and Co-based composites have been developed to replace expensive and scare noble metal-based OER catalysts. So far, layered NiFe double hydroxide represents the best OER activity among all Ni- and Co-based oxides. Herein, monodispersed ternary (Ni1-xFex)12P5 nanocrystalline with tunable Ni/Fe ratio are reported as highly efficient OER electrocatalysts, exhibiting activity and stability surpassing noble metal OER catalysts including RuO2, IrO2, and superior to nickel phosphides, layered nickel-iron double hydroxides, and other NiFe based OER catalysts. X-ray photoelectron spectroscopy (XPS) study reveals that enhanced charge transfer from the foreign metal (Fe) to host (Ni) to P atoms and strong synergistic effect do exist in the composite (Ni1-xFex)12P5 electrocatalysts, thus render the as-prepared (Ni1-xFex)12P5 nanocrystallines (NCs) with a Ni/Fe ratio of 0.51/0.49 the highest OER activity with a small onset potential of 125 mV, low Tafel slope of 39 mV dec-1, and low overpotential of 216 mV to achieve 20 mA cm-2 in an alkaline medium electrolyte (1.0 M KOH).