Electrocatalytic Activity of Bimetallic Ni–Mo–P Nanocrystals for Hydrogen Evolution Reaction

Electrochemical water splitting represents a sustainable method to produce molecular hydrogen, a foreseeable clean energy alternative to exhaustible fossil fuels. Transition-metal phosphides (TMPs) are emerging as earth-abundant catalysts for water splitting, and their activity can be further improved by incorporation of synergetic metals to produce bimetallic TMP catalysts. Herein, two distinct colloidal chemistry methods were developed to produce discrete nickel molybdenum phosphide (Ni–Mo–P) nanoparticles (NPs) that show varying crystal structures, morphologies, and compositions as alkaline hydrogen evolution reaction (HER) catalysts. The one-pot route produced smaller homogeneous NPs, ranging from 4 to 11 nm, with a near-spherical morphology. The two-pot synthesis resulted in larger heterogeneous NPs, ranging from ∼50 to 80 nm, with a polygonal morphology. Both nanostructures show either a hexagonal Ni2P or tetragonal Ni12P5 crystal structure and a shift in X-ray diffraction patterns to lower 2θ angles, consistent with the formation of bimetallic TMPs. The X-ray photoelectron spectra indicate the presence of partially charged core species (Niδ+, Moδ+, and Pδ−) as well as minor higher valent (Nin+, Mon+, and PO43–, n ≥ 2) surface species, presumably bound to surfactant ligands and/or oxides. Among heterogeneous and homogeneous NPs investigated, the hexagonal Ni2–xMoxP NPs show lower overpotentials (i.e., high HER activity) in comparison to tetragonal Ni12–xMoxP5 NPs. The HER activity of both nanostructures follows a mixed Volmer–Heyrovsky reaction mechanism consistent with Tafel slopes of 49.5–100.6 mV/dec. The homogeneous and heterogeneous Ni1.87Mo0.13P NPs showed the lowest overpotentials of 101 and 96 mV, respectively, and outperformed both hexagonal Ni2P (156 mV) and tetragonal Ni12–xMoxP (198 mV) NPs at a current density of −10 mA/cm2. This work provides insights into the design and synthesis of high-efficiency TMP nanostructures for alkaline HER studies.