Facilitating Hydrogen Evolution Reaction Activity in Alkaline Water Electrolysis via Formation of Ni‐ <scp> Co ₂ P </scp> Heterojunction

Cobalt phosphide (Co 2 P) is a promising non‐noble electrocatalyst material for hydrogen evolution reaction (HER) due to its affordability and stability in alkaline medium. Nevertheless, its HER activity still requires further improvement for practical applications in alkaline water electrolysis. In this study, we have incorporated nickel (Ni) into Co 2 P to form the junction of Ni‐Co 2 P, using co‐sputtering technique. This heterojunction, a key factor for HER activity, was quantitatively analyzed based on the ratio of Ni‐P bonds, showing that a high heterojunction fraction results in enhanced HER activity. DFT results demonstrate that water dissociation is significantly promoted at the Ni‐Co 2 P junction, while hydrogen evolution is facilitated at Ni sites, synergistically enhancing HER activity. An improper Ni concentration reduces the heterojunction fraction due to either insufficient Ni incorporation or Ni agglomeration. In contrast, the fraction is maximized at the optimal Ni concentration while maintaining high electrochemical surface area, P content, and crystallinity. Our optimized Ni‐Co 2 P exhibits remarkable HER performance, achieving a low overpotential of ~76 mV at 10 mA cm −2 and a Tafel slope of ~47 mV dec −1 . Furthermore, its current density remains stable during the on/off stability test, which simulates renewable energy‐integrated operation, indicating excellent durability under dynamic conditions.