Colloidal phase control of Ni–P nanocrystals reveals 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 Ni-rich 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.