Hydroxyl-Boosted Nitrogen Reduction Reaction: The Essential Role of Surface Hydrogen in Functionalized MXenes

MXenes, an emerging family of two-dimensional (2D) metal carbides and nitrides, have been demonstrated to be effective nitrogen reduction reaction (NRR) catalysts. So far, most of the theoretical studies toward NRR are based on bare MXenes; however, the structural stabilities are questionable. In this work, we studied the NRR process on several synthesized MXenes (Ti₂C, V₂C, Cr₂C, Zr₂C, Nb₂C, Mo₂C, Hf₂C, and Ta₂C) with hydroxyl (OH) termination since the structures are preferred under NRR operating conditions as per Pourbaix stability diagrams. It is found that OH plays an essential role in tuning the NRR chemistry, as a new surface-hydroxylation mechanism. Different from the widely accepted NRR mechanism where only protons are involved in the reaction, hydrogen (H) atoms from surface hydroxyl could be captured by the intermediate and participate into the NRR, while the remaining H vacancy can subsequently be self-repaired by the protons under the applied potential. The cooperative effect of surface hydroxylation can effectively boost the NRR, while Mo₂C(OH)₂ stands out with the most favorable limiting potential of -0.62 V and highest selectivity. Moreover, new scaling relationships based on the H vacancy energy are established, elucidating the possibility for structure-activity tuning. This study not only elaborates the essential role of surface OH functionalization in evaluating NRR performance but also affords new insights into advance sustainable NH₃ production.