Synthesis of Nickel Phosphide Electrocatalysts from Hybrid Metal Phosphonates

Transition-metal phosphides (TMPs) have recently emerged as efficient and inexpensive electrocatalysts for electrochemical water splitting. The synthesis of nanostructured phosphides often involves highly reactive and hazardous phosphorous-containing compounds. Herein, we report the synthesis of nickel phosphides through thermal treatment under H₂(5%)/Ar of layered nickel phenylphosphonate (NiPh) or methylphosphonate (NiMe) that act as single-source precursors. Ni₁₂P₅, Ni₁₂P₅-Ni₂P, and Ni₂P nanoparticles (NPs) with sizes of ca. 15-45 nm coated with a thin shell of carbonaceous material were produced. Thermogravimetric analysis coupled with mass spectrometry (TG-MS) showed that H₂, H₂O, P₂, and -C₆H₅ are the main compounds formed during the transformation of the precursor under argon and no hazard phosphorous-containing compounds are created, making this a simple and relatively safe route for fabricating nanostructured TMPs. The H₂ most likely reacts with the -PO₃ groups of the precursor to form H₂O and P₂, and the latter subsequently reacts with the metal to produce the phosphide. The Ni₁₂P₅-Ni₂P and Ni₂P NPs efficiently catalyze the hydrogen evolution reaction (HER), with Ni₂P showing the best performance and generating a current density of 10 mA cm^-2 at an overpotential of 87 mV and exhibiting long-term stability. Co₂P and CoP NPs were also synthesized following this method. This approach may be utilized to explore the rich metal phosphonate chemistry for fabricating phosphide-based materials for electrochemical energy conversion and storage applications.