Structure- and Morphology-Controlled Synthesis of Hexagonal Ni2-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 Ni2P 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 Ni2P 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 (Delta G(H)) closer to a thermoneutral value. Consequently, phase pure hexagonal Ni2-xZnxP 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 (eta(-10)) of 188.67, 170.01, and 135.35 mV, respectively at a current density of -10 mA/cm(2), in comparison to Ni2P NCs (216.2 +/- 4.4 mV). Conversely, Ni2-xZnxP NCs with x = 0.01, 0.38, 0.44, and 0.50 compositions showed a notable decrease in HER activity, with corresponding eta(-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 Ni1.85Zn0.15P NCs, featuring a Zn concentration of 5.24%, consistent with composition-dependent Delta G(H) calculations. The highest performing Ni1.85Zn0.15P NCs displayed a Heyrovsky HER mechanism, enhanced kinetics and electrochemically active surface area (ECSA), and superior corrosion tolerance with a negligible increase of eta(-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.