Highly Efficient and Selective Nitrogen Reduction Reaction Catalysis of Cluster-Modified MXene Nanosheets

The electrocatalytic synthesis of NH₃ holds immense significance for energy conservation in industrial and agricultural production. Herein, an efficient solution is proposed for MXene-based high-activity nitrogen reduction reaction (NRR) catalysts that are modified using tetranuclear non-noble 3d transition metal clusters (M₄). The thorough exploration of M₄/Ti₂CO₂ candidates reveals that the thermodynamically and kinetically stable Cr₄/Ti₂CO₂ possesses the lowest overpotential (0.35 V) and high selectivity, comparable to those of well-known NRR catalysts such as Ru(0001) (0.43 V) and Au(310) (1.91 V). In addition, the doping of Fe into Cr₄ clusters can further reduce the overpotential and kinetic barriers by 31 and 46%, respectively. The analysis of the complicated bonding nature reveals the mechanism of the catalytic activity, which demonstrates the role of clusters pulling π/σ electrons from N₂ while simultaneously back-donating d orbital electrons to the π* orbital. A descriptor (φ), related to intrinsic transferred charges (Δe) of the cluster, is proposed to accurately determine the NRR catalytic activity using simple calculations, and the linear correlation between them can reach 0.98. This work provides guidance for designing promising cluster-modified MXene catalysts for NRR and an elucidation of the electronic factors governing catalytic activity.