Triggering the Intrinsic Catalytic Activity of Ni-Doped Molybdenum Oxides via Phase Engineering for Hydrogen Evolution and Application in Mg/Seawater Batteries

Molybdenum oxides have been regarded as promising non-noble metal electrocatalysts for hydrogen evolution reaction (HER) due to their low cost, nontoxicity, and chemical stability. However, promoting the intrinsic catalytic activity of molybdenum oxides is crucial for achieving high HER performance. Herein, we demonstrate that the intrinsic HER activity of Ni-doped molybdenum oxides is triggered via a thermal treatment induced phase engineering strategy. The HER overpotential at 10 mA cm^–2 decreases from 493 mV (1 M KOH) and 818 mV (seawater) over Ni-doped molybdenum trioxide (Ni-MoO₃) to only 234 and 412 mV over Ni-doped molybdenum dioxide (Ni-MoO₂), respectively. Moreover, the electrochemical surface areas (ECSAs)-normalized current density over Ni-MoO₂, as compared to Ni-MoO₃, is at least a 35-fold increase in alkaline (at −0.2 V vs reversible hydrogen electrode (RHE)) and a 59-fold increase in seawater (at −0.4 V vs RHE), confirming the significantly triggered intrinsic HER activity via engineering orthorhombic MoO₃ to monoclinic MoO₂. Finally, an assembled Mg/seawater battery with the Ni-MoO₂ cathode reveals a peak power density of 6.54 mW cm^–2 and a continuous stable discharge for over 24 h. This study offers a facile strategy for promoting the intrinsic HER activity of non-noble metal electrocatalysts.