The development of efficient and durable non-noble metal electrocatalysts for the hydrogen evolution reaction (HER) requires precise control over composition, microstructure, and electronic properties. In this work, Ni-P coatings with tunable phosphorus content were fabricated on Cu substrates via electroless deposition by adjusting the bath pH. Structural analysis revealed that increasing P content led to the formation of amorphous Ni-P alloys, while subsequent heat treatment transformed the amorphous structure into a nanocomposite architecture consisting of monodispersed nickel phosphide (NixPy, e.g., Ni2P/Ni5P4) nanocrystals embedded in a conductive Ni-P alloy matrix. Electrochemical measurements demonstrated that the as-deposited medium-P sample (pH = 4.6) exhibited the best performance among untreated coatings, delivering an overpotential of 312 mV at 10 mA cm−2 and a Tafel slope of 130 mV.dec−1. After heat treatment, the high-P sample treated at 300 °C (HP-HT300) showed the highest HER activity, with a significantly reduced overpotential of 208 mV at 10 mA cm−2 and a Tafel slope of 92 mV.dec−1, indicating accelerated reaction kinetics. The enhanced performance is attributed to the “nanocrystals-in-alloy” structure, where NixPy nanocrystals provide abundant active sites while the metallic Ni-P matrix ensures efficient charge transport. Phosphorus incorporation may induce electronic modulation in the Ni-P system (Niδ+/Pδ−), which could help optimize hydrogen adsorption and facilitate the Volmer and Heyrovsky/Tafel steps, particularly in alkaline media. Stability tests, including chronopotentiometry and cyclic voltammetry, confirmed the excellent durability of the heat-treated coatings. This work demonstrates that combining composition control via pH-regulated electroless deposition with post-deposition heat treatment provides an effective strategy for engineering high-performance, binder-free Ni-P-based HER electrocatalysts.

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