Colloidal Phase Control of Ni-P Nanocrystals Reveal a P-Site Hydrogen Evolution Reaction Mechanism Distinct from Ni-Rich Analogues

Abstract

Nickel phosphides have emerged as promising earth-abundant catalysts for the hydrogen evolution reaction (HER), yet most studies have focused on Ni-rich phases (e.g., Ni₂P, Ni₅P₄), where catalytic activity is commonly attributed to metallic Ni surface sites. In contrast, the catalytic potential of phosphorus-rich phases has remained largely unexplored due to synthetic challenges that have hindered access to phase-pure compositions. Here, we report the colloidal synthesis of phase-controlled Ni-P nanocrystals, granting access to four distinct phases (Ni₁₂P₅, Ni₂P, Ni₅P₄, NiP₂) and overcoming long-standing barriers such as phosphorus volatility and biphasic formation. This synthetic platform enables a direct and systematic comparison across the compositional gradient and reveals a fundamentally distinct HER mechanism at the P-rich end: hydrogen adsorption and evolution proceed preferentially on surface phosphorus atoms, rather than on Ni hollow or bridge sites as in conventional Nirich phosphides. Electrochemical analysis and density functional theory (DFT) calculations show that NiP₂ exhibits superior HER performance compared to its Ni-rich analogues, despite having a lower electrochemically active surface area. This P-site-driven reactivity uncovers a previously unrecognized catalytic regime and challenges the prevailing Ni-centric model in transition metal phosphide catalysis. Our findings demonstrate that tuning the stoichiometry toward phosphorus-rich compositions not only alters the surface electronic structure but also redefines the identity of the active site. This work positions NiP₂ as a prototype for anion-driven HER catalysis and introduces a new conceptual framework for designing non-precious electrocatalysts that exploit metalloid-active centers.