Unconventional direct synthesis of Ni3N/Ni with N-vacancies for efficient and stable hydrogen evolution

Transition metal nitrides are a fascinating class of catalyst materials due to their superior catalytic activity, low electrical resistance, good corrosion resistance and earth-abundance; however, their conventional synthesis relies on high temperature nitridation process in hazardous environments. Here, we report direct synthesis of Ni3N/Ni enriched with N-vacancies using one-step magnetron sputtering. The surface state of Ni3N (001) with 75% N-vacancies as hydrogen terminated exhibit four inequivalent Ni3-hollow sites. This leads to a lowest surface free energy (γ), strongest H* binding strength compared to Ni (111), and is affirmed as the most stable surface terminations under electrochemical working conditions (pH≈13.8 and E = -0.1 eV) by a Pourbaix diagram. The Ni3N/Ni catalyst shows low crystallinity and good wettability, and exhibits a low overpotential of 89 mV vs RHE at 10 mA cm-2 in 1.0 M KOH with an excellent stability of over 3 days. This performance closely matches with that of the Pt catalyst synthesized under the same conditions, and surpasses other reported earth-abundant catalysts on planar substrates. The application of Ni3N/Ni as a cocatalyst on Si photocathodes produces an excellent ABPE of 9.3% and over 50 h stability. Moreover, its feasibility for practical application is confirmed with excellent performance on porous substrates and robustness at high operating currents in zero-gap alkaline electrolysis cells. Our work demonstrates a general approach for feasible synthesis of other transition metal nitride catalysts for electrochemical and photoelectrochemical energy conversion applications.