Selective nitrogen reduction over Mo2C(0001) MXene induced by fluorine adsorbates from the etching process

• Systematic screening of mixed *F/*O/*OH terminations identifies Mo 2 C(0001) – 3*F – 4*O – 2*OH as a highly selective NRR surface. • Efficient model evaluation is achieved by symmetry reduction and work function screening of configurations. • Integrating G max ( U ), DOS, and d-band center analyses uncovers the origin of NRR activity and selectivity. • Theoretical modeling predicts up to 98 % NRR selectivity for partly *F-terminated Mo 2 C surfaces at −0.15 V vs. RHE. • Transferable framework for the surface engineering of MXene-based electrocatalysts in energy conversion. Electrochemical nitrogen reduction (NRR) under ambient conditions presents a sustainable alternative to the Haber–Bosch process, but its selectivity remains limited in aqueous media due to competing hydrogen evolution reaction (HER). In this work, we model (3 × 3) Mo 2 C(0001) MXene surfaces terminated with mixed *F/*O/*OH groups—surface terminations representative of HF-etched synthesis and aqueous environments. Using symmetry reduction and work function screening, the configurational space is narrowed to 21 representative surface models, which are systematically evaluated for NRR. Using a descriptor-based analysis, we demonstrate that hydrogenation of the *NH p intermediate constitutes the thermodynamic bottleneck independent of surface configuration. Selectivity analysis identifies certain surfaces with balanced fluorine/ oxygen/ hydroxyl content (e.g., 3*F – 4*O – 2*OH) optimize NRR selectivity (∼98 % at –0.15 V vs. RHE), whereas hydroxyl- or oxygen-dominated surfaces favor HER. Complementary density of states and d-band center analyses confirm the electronic origin of these trends. Our computational work, which integrates electronic structure and descriptor-based analyses, provides a transferable framework for the rational design of MXene-based electrocatalysts.