Phosphonium-Based Ionic Liquid: A New Phosphorus Source toward Microwave-Driven Synthesis of Nickel Phosphide for Efficient Hydrogen Evolution Reaction

Employing phosphonium-based ionic liquid, tetrabutylphosphonium chloride [P4444]Cl as novel phosphorus source and reaction medium, a facile approach for fabricating nanostructured Ni2P and Ni12P5 was developed upon microwave heating in 1–2 min or conventional heating at 350 °C for 3 h. In a microwave-driven approach, controlling counteranions of various nickel salts could conveniently tune the phase of as-synthesized nickel phosphides. Ni­(acac)2 and Ni­(OAc)2·4H2O as Ni source could yield Ni2P nanoparticles, while NiCl2·6H2O and NiSO4·7H2O offered Ni12P5 nanocrystals. The synthesized products were characterized by X-ray powder diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Their electrocatalytic behavior toward hydrogen evolution reaction in acidic medium was investigated. The as-synthesized Ni2P nanoparticles presented more excellent catalytic efficiency than Ni12P5. Ni2P nanoparticles from Ni­(acac)2 require overpotentials of only 102 mV to reach 10 mA cm–2 with a small Tafel slope of 46 mV dec–1, showing its best activity among those tested catalysts. The present novel ionic liquid-mediated strategy for the synthesis of nickel phosphide provides the remarkable advantage of operating in very short time by microwave heating, which is of particular interest from the viewpoint of energy-saving, fast synthesis, and easy operation.