Highly Efficient N2 Fixation Catalysts: Transition-metal Carbides M2C (MXenes)
Development of highly efficient metal or metal compound electrocatalysts under mild conditions have always been a challenging task for N2 reduction. Here, we show that pristine two-dimensional (2D) MXenes are promising N2 electroreduction catalysts due in part to 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 N2 reduction reaction (NRR). We find that 4d4-Mo2C gives rise to the lowest free-energy barrier (G) of 0.46 eV among the synthesized M2C MXenes as of today. More importantly, we find two hypothetical MXenes, 3d5-Mn2C and 3d6-Fe2C, possess even lower G of 0.28 and 0.23 eV, respectively, than the state-of-the-art 4d4-Mo2C, thereby likely being more efficient for NRR. The N2-capture strength, a key parameter of the limiting-step potential, is found to be closely related to the d-electrons arrangement on the occupied and empty spin-split d orbitals. Hence, the excellent NRR performance of Mn2C and Fe2C can be attributed to desirable half-filled 3d5 or 3d6 electron arrangements. The adsorption of N2 on Mn2C results in 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 to the potential-limiting step of hydrogenation. The insights obtained from this comprehensive study offer guidance to design new and efficient electrocatalysts for N2 fixation.