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 a 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)(12)P-5 nanocrystalline materials with tunable Ni/Fe ratios 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) studies reveals that enhanced charge transfer from the foreign metal (Fe) to the host (Ni) to P atoms and a strong synergistic effect exist in the composite (Ni1-xFex) P-12(5) electrocatalysts, thus rendering the as-prepared (Ni1-xFex)(12)P-5 nanocrystalline compounds (NCs) with a Ni/Fe ratio of 0.51/0.49, the highest OER activity with a small onset potential of 125 mV, a low Tafel slope of 39 mV dec(-1), and a low overpotential of 216 mV, to achieve 20 mA cm(-2) in an alkaline medium electrolyte (1.0 M KOH).