Termination-acidity tailoring of molybdenum carbides for alkaline hydrogen evolution reaction

Transition-metal carbides have been advocated as the promising alternatives to noble-metal platinum-based catalysts in electrocatalytic hydrogen evolution reaction over half a century. However, the effectiveness of transition-metal carbides catalyzing hydrogen evolution in high-pH electrolyte is severely compromised due to the lowered proton activity and intractable alkaline-leaching issue of transition-metal centers. Herein, on the basis of validation of molybdenum-carbide model-catalyst system by taking advantage of surface science techniques, we synthesize Mo2C nanospheres with spontaneously formed MoO2 terminations through a feasible carbonization method. By employing a straightforward termination-acidity tailoring engineering using unsaturated penta-coordination Al3+ sites, the as-obtained Al3+-tailoring molybdenum-carbide catalyst demonstrates a superior performance of alkaline hydrogen evolution with a near-zero onset-potential, a low overpotential (40 mV) at a typical current density of 10 mA/cm2, and a small Tafel slope (45 mV/dec), as well as a long-term stability for continuous hydrogen production over 200 h. Comprehensive advanced morphology/spectroscopy characterizations and density functional theory calculations reveal that penta-coordination Al3+ sites play a crucial role in the enhancements of acidity and structural robustness for MoO2 termination, which are identified as responsible for the superior performance of alkaline hydrogen evolution on molybdenum-carbide catalyst. Notably, the abundant bridge-type -Al-OH-Mo- structures serve as strong Brønsted acid sites that accelerate the deprotonation kinetics in alkaline HER process. As all transition-metal compounds are inevitably terminated by oxide covering during synthesis, storage, and catalysis, our termination-acidity tailoring strategy is believed to offer a versatile approach for exploring other forms of low-cost but highly-active catalysts for replacing benchmark Pt catalyst in high-pH electrocatalytic hydrogen production and beyond.