Highly efficient N 2 fixation catalysts: transition-metal carbides M 2 C (MXenes)

The development of highly efficient metal or metal compound electrocatalysts under mild conditions has always been a challenging task for N2 reduction. Herein, we show that pristine two-dimensional (2D) MXenes are promising N2 electroreduction catalysts due in part to the availability of multiple active sites per unit area. We systematically explore a series of 3d, 4d and 5d-transition metal M2C (M = Sc, Ti, V, Cr, Mn, Fe, Zr, Nb, Mo, Ta and Hf) MXenes and compute their limiting potentials for the N2 reduction reaction (NRR). We find that 4d4-Mo2C gives rise to the lowest free-energy barrier (ΔG) of 0.46 eV, among the synthesized M2C MXenes as of today. More importantly, we find that two hypothetical MXenes, 3d5-Mn2C and 3d6-Fe2C, possess even lower ΔG of 0.28 and 0.23 eV, respectively, compared to the state-of-the-art 4d4-Mo2C, thereby likely being more efficient NRR catalysts. The N2 capture strength, a key parameter of the potential-limiting step, is found to be closely related to the d-electron arrangement on the occupied and empty spin-split d-orbitals. Hence, the excellent NRR performance of Mn2C and Fe2C can be attributed to the desirable half-filled 3d5 or 3d6 electron arrangements. The adsorption of N2 on Mn2C results in the donation of 1σ electrons to the empty spin-down 3d orbitals of Mn. The donated electrons weaken the N2 adsorption strength and lower the energy barrier of the potential-limiting step of hydrogenation. The insights obtained from this comprehensive study offer guidance to design new and efficient electrocatalysts for N2 fixation.