Composition-Dependent Electrocatalytic Activity of Zn-Doped Ni₅P₄ Nanocrystals for the Hydrogen Evolution Reaction

Abstract

Electrocatalytic water splitting presents an exciting opportunity to produce environmentally benign hydrogen fuel to power human activities. Earth-abundant Ni 5 P 4 has emerged as an efficient catalyst for the hydrogen evolution reaction (HER), and its activity can be enhanced by admixing synergistic metals to modify the surface affinity and consequently the kinetics of HER. Computational studies suggest that the HER activity of Ni 5 P 4 can be improved by Zn doping, causing a chemical pressure-like effect on Ni 3 hollow sites. Herein, we report a facile colloidal route to produce Ni 5−x Zn x P 4 nanocrystals (NCs) with control over structure, morphology, and composition and investigate their composition-dependent HER activity in alkaline solutions. Ni 5−x Zn x P 4 NCs retain the hexagonal structure and solid spherical morphology of binary Ni 5 P 4 NCs, with a notable size increase from 9.2−28.5 nm for x = 0.00−1.27 compositions. Elemental maps affirm the homogeneous ternary alloy formation with no evidence of Zn segregation. Surface analysis of Ni 5−x Zn x P 4 NCs indicates significant modulation of the surface polarization upon Zn incorporation, resulting in a decrease in Ni δ+ and an increase in P δ− charges. Although all compositions followed a Volmer−Heyrovsky HER mechanism, the modulated surface polarization enhances the reaction kinetics, producing lower Tafel slopes for Ni 5−x Zn x P 4 NCs (82.5−101.9 mV/dec for x = 0.10−0.84) compared to binary Ni 5 P 4 NCs (109.9 mV/dec). Ni 5−x Zn x P 4 NCs showed higher HER activity with overpotentials of 131.6−193.8 mV for x = 0.02− 0.84 in comparison to Ni 5 P 4 NCs (218.1 mV) at a current density of −10 mA/cm 2 . Alloying with Zn increases the material's stability with only a ∼10% increase in overpotential for Ni 4.49 Zn 0.51 P 4 NCs at −50 mA/cm 2 , whereas a ∼33% increase was observed for Ni 5 P 4 NCs. At current densities above −40 mA/cm 2 , bimetallic NCs with x = 0.10, 0.29, and 0.51 compositions outperformed the benchmark Pt/C catalyst, suggesting that hexagonal alloyed Ni 5−x Zn x P 4 NCs are excellent candidates for practical applications that necessitate lower HER overpotentials at higher current densities.

Structure- and Morphology-Controlled Synthesis of Hexagonal Ni2–xZnxP Nanocrystals and Their Composition-Dependent Electrocatalytic Activity for Hydrogen Evolution Reaction

“…Although no apparent shifts in the diffraction patterns were observed, slight broadening of Bragg reflections was noted and confirmed with increased full width at half maxima (fwhm) of Zn-doped samples. This observation is consistent with an earlier report, where no shifts in Bragg reflections were noted for Ni 5– x Zn x P 4 NCs because of the similar size of Ni and Zn . This combined with broader diffraction peaks obtained for Ni 2– x Zn x P NCs make it more challenging to distinguish minor peak shifts.…”

“…This observation is consistent with an earlier report, where no shifts in Bragg reflections were noted for Ni 5– x Zn x P 4 NCs because of the similar size of Ni and Zn. 50 This combined with broader diffraction peaks obtained for Ni 2– x Zn x P NCs make it more challenging to distinguish minor peak shifts. Nevertheless, the effect of Zn doping was evident in the crystallite size estimated by applying the Scherrer formula to the (111) reflection.…”