Highly efficient N₂ fixation catalysts: Transition-metal carbides M₂C (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.