Electrocatalytic performance of ultrasmall Mo2C affected by different transition metal dopants in hydrogen evolution reaction

Abstract

Molybdenum carbides are considered as one type of privileged noble-metal-free electrocatalysts for hydrogen evolution reactions (HER) due to their d-band electron structure, which is similar to Pt. Especially, the electronic structure of such materials can be further adjusted by elemental doping to improve their electrocatalytic activity. Herein, we selected the Anderson-type polyoxometalates (POMs) (NH₄)_n[TMMo₆O₂₄H₆]·5H₂O (TM = Ni²⁺, Co²⁺, n = 4; TM = Fe³⁺, Cr³⁺, n = 3) as precursors to prepare new transition-metal-doped Mo₂C materials. When these POMs were mixed with dicyandiamide (DCA) by solid grinding, and carbonized at a high temperature, a series of Ni-, Co-, Fe-, and Cr-doped Mo₂C composite nanoparticles covered by few-layer graphitic carbon shells (abbr. TM-Mo₂C@C) were obtained. All these nanoparticles possess a similar size, morphology, and TM/Mo component ratio, and thus it is feasible to systematically investigate the influence of different TM dopants on the electrocatalytic activity of Mo₂C for HER. Both electrocatalytic experiments and DFT calculations reveal that TM dopants have a significant effect on the hydrogen binding energy (ΔG_H*) and the catalytic activity of Mo₂C. The sequence of HER electrocatalytic activity is as follows: Ni-Mo₂C > Co-Mo₂C > Fe-Mo₂C > Cr-Mo₂C. As a result, Ni-Mo₂C@C possesses the best HER performance, which required an overpotential of 72 mV at a current density of 10 mA cm⁻² and the Tafel slope is 65.8 mV dec⁻¹. This work suggests a shortcut to reasonably investigate the effects of elemental doping on molybdenum carbides and explore new high-efficient and low-cost electrocatalysts for HER.