Structure- and Morphology-Controlled Synthesis of Hexagonal Ni 2- x Zn x P Nanocrystals and Their Composition-Dependent Electrocatalytic Activity for Hydrogen Evolution Reaction.

Nickel phosphides are an emerging class of earth-abundant catalysts for hydrogen generation through water electrolysis. However, the hydrogen evolution reaction (HER) activity of Ni 2 P is lower than that of benchmark Pt group catalysts. To address this limitation, an integrated theoretical and experimental study was performed to enhance the HER activity and stability of hexagonal Ni 2 P through doping with synergistic transition metals. Among the nine dopants computationally studied, zinc emerged as an ideal candidate due to its ability to modulate the hydrogen binding free energy (Δ G H ) closer to a thermoneutral value. Consequently, phase pure hexagonal Ni 2- x Zn x P nanocrystals (NCs) with a solid spherical morphology, variable compositions ( x = 0-17.14%), and size in the range of 6.8 ± 1.1-9.1 ± 1.1 nm were colloidally synthesized to investigate the HER activity and stability in alkaline electrolytes. As predicted, the HER performance was observed to be composition-dependent with Zn compositions ( x ) of 0.03, 0.07, and 0.15 demonstrating superior activity with overpotentials (η -10 ) of 188.67, 170.01, and 135.35 mV, respectively at a current density of -10 mA/cm 2 , in comparison to Ni 2 P NCs (216.2 ± 4.4 mV). Conversely, Ni 2- x Zn x P NCs with x = 0.01, 0.38, 0.44, and 0.50 compositions showed a notable decrease in HER activity, with corresponding η -10 of 225.3 ± 3.2, 269.9 ± 4.3, 276.4 ± 3.7 and 263.9 ± 4.9 mV, respectively. The highest HER active catalyst was determined to be Ni 1.85 Zn 0.15 P NCs, featuring a Zn concentration of 5.24%, consistent with composition-dependent Δ G H calculations. The highest performing Ni 1.85 Zn 0.15 P NCs displayed a Heyrovsky HER mechanism, enhanced kinetics and electrochemically active surface area (ECSA), and superior corrosion tolerance with a negligible increase of η -10 after 10 h of continuous HER. This study provides critical insights into enhancing the performance of metal phosphides through doping-induced electronic structure variation, paving the way for the design of high-efficiency and durable nanostructures for heterogeneous catalytic studies.