Room-temperature synthesis of self-supported NiS-Ni₂P heterojunctions via a mild plasma strategy for efficient bifunctional water splitting

Developing efficient and stable bifunctional electrocatalysts is critical for alkaline overall water splitting. To address the phosphorus leaching and structural degradation of Ni₂P during the oxygen evolution reaction (OER), we propose a mild two-step strategy to precisely construct a crystalline Ni₂P core/low-crystallinity NiS shell heterostructure. Argon plasma etching introduces phosphorus vacancies on Ni₂P, enabling directed anchoring of sulfur ions and controlled growth of a NiS shell via room-temperature chemical bath deposition. During OER, the NiS shell rapidly reconstructs into highly active NiOOH, forming a stable NiOOH–Ni₂P heterojunction with the conductive Ni₂P core. Interfacial electron transfer optimizes the Ni d-band and P p-band centers, intrinsically enhancing catalytic activity. The resulting catalyst (Ar-Ni₂P|S-5) achieves overpotentials of only 90 mV for HER and 260 mV for OER at 10 mA cm⁻² in 1.0 M KOH, with stability exceeding 150 h. An overall water splitting cell voltage of 1.61 V is attained at 10 mA cm⁻². This work provides a new route to efficient and durable non‑noble metal bifunctional electrocatalysts.