Phase-transition engineering induced lattice contraction of the molybdenum carbide surface for highly efficient hydrogen evolution reaction

The electrochemical hydrogen evolution reaction (HER) has been considered as an efficient way of producing hydrogen energy. Molybdenum carbide has received a lot of attention as an important catalyst because of its noble metal-like surface electronic properties. However, the effect of surface reconstruction of molybdenum carbide during phase transition on electrocatalysis has attracted little attention. Herein, different degrees of the lattice contraction effect on the alpha-MoC surface are discovered during the phase-transition from cubic alpha-MoC to hexagonal beta-Mo2C by CH4/H-2 pretreatment. Density functional theory (DFT) calculations reveal that the lattice contraction imposed on the molybdenum carbide surface results in a downshift of the d-band center and then intensively regulates the Mo-H binding energy and Delta G(H*) value close to thermodynamic neutral, which greatly enhances the HER kinetics. Benefiting from these, the optimized alpha-MoC-5 h (alpha-MoC pretreated in a CH4/H-2 atmosphere for 5 h) with a lattice contraction degree of 2.86% manifests a much lower overpotential (eta(10) = 126 mV in 1 M HClO4; eta(10) = 122 mV in 1 M KOH), which is 2-fold lower than that of untreated alpha-MoC, and exhibits good stability over 120 hours. This work unravels the underlying lattice contraction effect of alpha-MoC on the phase-transition process during thermal pretreatment, which deepens the understanding of molybdenum carbide and provides inspiration for the design of other metal carbides.