Scalable Synthesis of Nanoscale Ni5P4 As a Novel HER Catalyst: Investigating the Correlation Between Particle Size and Electrochemical Activity

Renewable H₂ can be produced from water by electrolysis with either low-efficiency alkaline electrolyzers, or by more efficient acidic electrolyzers using expensive noble metal catalysts, most notably Pt. Replacement of Pt with inexpensive, earth-abundant metals is a key step to the global scalability of clean energy systems. Nanocrystalline microparticles of Ni₅P₄ have been shown to be highly catalytically active for the hydrogen evolution reaction (HER) by the electrolysis of water, achieving comparable efficiency as Pt in acidic and basic electrolyte, with no detectable degradation at current densities of ‐10 mA/cm², corresponding to ~10% photoelectric conversion. In order to apply this catalyst to commercial-scale electrolyzers, it is necessary to optimize both the method of producing the material and the crystalline properties of the material itself to achieve the highest possible affordability and efficiency.

 The work presented here aims to:

1. Devise a scalable synthetic route to produce <100 nm nanoparticles of the Ni₅P₄catalyst using inexpensive Ni and P precursors

2. Elucidate the dependency of its electrochemical activity on particle/crystallite size and morphology

Synthesis of the Ni₅P₄ nanoparticles was carried out via low-temperature solid-state reaction, hydrothermal synthesis, and by thermal hypophosphite reduction. The composition of the synthesized products was characterized by powder X-ray diffraction (PXRD) and the morphology of the products was determined by scanning electron microscopy (SEM). Through optimization of reaction conditions and precursor quantities to account for P lost during reaction near 300^oC, the three synthesis methods yielded particles with Scherrer crystallite size less than 100 nm.

In order to elucidate the correlation between particle size and electrochemical activity, three samples of phase-pure Ni₅P₄ were prepared by solid-state reaction using three different Ni precursors: 150 μm bulk Ni particles, Raney Ni, and commercially-purchased, passivated, spray-pyrolyzed 20 nm Ni nanoparticles. In addition, samples of Ni₅P₄ produced from the 20 nm nanoparticles were sintered at 550^oC and 650^oC, respectively, to yield intermediate particle sizes. Electrochemical activity was measured by cyclic voltammetry, with currents normalized to both geometric (GSA) and electrochemical surface area (ECSA). Electrodes were fabricated from pressed pellets of each catalyst sample with and without neutralized Nafion ionomer to compare differences in activity related to proton transfer capability within the catalyst framework.  While ECSA-normalized current densities demonstrated a clear trend with particle size distribution, GSA-normalized current densities varied by nanoscale morphology.