Nickel phosphides are a prominent class of 3d metal phosphides that are widely explored as potentially useful hydrogen evolution reaction (HER) electrocatalysts. Metal-rich (x &gt; y, NixPy) to phosphorus-rich nickel phosphides (x &lt; y, NixPy) have distinct structural and electronic properties which offer a variety of advantages in HER activity and stability. Facile and flexible synthetic routes to metal-rich and phosphorus-rich metal phosphides are highly desired for comparable electrocatalytic HER studies on similarly synthesized materials. The present study describes the successful synthetic tunability of solvent-free, direct and Sn-flux assisted NiCl2/P reactions at moderate temperatures (500 °C) to access several metal-rich (Ni2P, Ni5P4) and phosphorus-rich structures (cubic c-NiP2, monoclinic m-NiP2, NiP3) through simple modifications in reaction stoichiometry. The stoichiometric direct synthesis produced Ni2P, Ni5P4 and c-NiP2 while Sn-flux assisted synthesis resulted in m-NiP2 and NiP3 at 500 °C. All nickel phosphide products were crystalline, micrometer-sized, and have bulk compositions consistent with its crystalline phase. Different nickel phosphides (Ni2P, Ni5P4, c-NiP2) from similar direct synthesis were investigated for their HER electrocatalytic activity and stability in acidic conditions. As phosphorus is introduced in the structure, higher levels of H2 generation are observed at smaller overpotentials (c-NiP2 ≥ Ni5P4 &gt; Ni2P). m-NiP2 and NiP3 synthesized from molten Sn-fluxes exhibit moderate HER activity that is greater than Ni2P. Based on overpotentials to achieve 10 mA/cm2 current density (-260 mV to -160 mV range), the HER activity follows the trend of c-NiP2 ≥ Ni5P4 &gt; m-NiP2 &gt; NiP3 &gt; Ni2P. In contrast to metal-rich Ni2P, several phosphorus-rich nickel phosphides show notably improved H2 generation at lower overpotentials and are more stable in acidic conditions. The observed electrocatalytic HER behaviors of different nickel phosphides may be attributed to differences in particle size, surface area, phosphorus-rich surface structures, M-P bonding, and electronic properties.

Synthesis of metal-rich and phosphorus-rich nickel phosphides for hydrogen evolution reaction (HER) electrocatalysis

ACS Spring 2022

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