Unconventional molybdenum carbide phases with high electrocatalytic activity for hydrogen evolution reaction

The development of noble metal-free catalysts, which can replace noble metal ones, such as Pt, for various electrocatalytic processes in renewable energy devices is currently of huge interest. The β-phase of molybdenum carbide (i.e., β-Mo₂C) has been reported to be one of the most active noble metal-free electrocatalysts for the hydrogen evolution reaction (HER) in electrolyzers, whereas the other phases, MoC and MoC_1-x, have been widely regarded as weak electrocatalysts for HER. Herein, we report the synthesis of nanoporous substoichiometric α-MoC_1-x and η-MoC nanosheets, named np-MoC NSs, that show comparable electrocatalytic activity toward HER as β-Mo₂C does. The materials are synthesized using two-dimensional (2D) conjugated carbonitride and ammonium molybdate as precursors. Their structures contain N dopant atoms and nanopores. The pores create highly accessible catalytic sites and good mass and charge transport, and thereby excellent reaction kinetics for HER, in the materials. Theoretical calculations show that the N dopant atoms modulate the electronic properties of the catalytically active sites in the materials, leading to lower free energy of adsorption and desorption for the hydrogen species involved in the reaction and better catalytic activity for HER. This work demonstrates that, by simple structural design and electronic modulation, various phases of molybdenum carbides and related materials that have been traditionally considered inactive catalysts for HER can be made robust electrocatalysts for the reaction.