The orientation of interfacial water molecules governs the electrochemical nitrogen reduction reaction on the Mo–N4–C surface

Abstract

The nitrogen reduction reaction (NRR) is a sustainable method for NH₃ synthesis but remains challenging due to the competitive hydrogen evolution reaction (HER). While Fe/Ru single atoms on N-doped graphene (MN₄) are well studied, MoN₄ remains underexplored, despite its predicted strong affinity for N₂. We employed constant-potential ab initio molecular dynamics simulations to investigate how the interfacial microenvironment is reorganized on a Mo single atom anchored N-doped graphene surface under acidic conditions (pH = 1) at an applied potential of 0, −0.2, and −0.4 V vs. RHE. The enhanced electrostatic interaction leads to more ordered and dense layering, reinforced by a hydrogen-bond network, at −0.4 V, facilitating rapid proton transfer compared to 0 and −0.2 V. The NN bond is effectively activated at a more negative potential by enhancing charge transfer from d → 2π*, shifting the orbital to a lower energy level. The *NH₂ protonation barrier is significantly reduced at −0.4 V, due to a prealigned proton-donating water and a dense hydrogen-bond network at the interface. Our findings reveal that reaction barriers are not solely dictated by interfacial water rearrangements but also by the reorganization and orientation of the proton-donating water molecules in the electric double layer, which indirectly favors catalytic activity. The oxyphilic nature of Mo promotes hydrogen bonding between surface-bound oxygenated intermediates (*OH) and interfacial water, which plays a crucial role in modulating proton-coupled electron transfer steps that influence HER kinetics. These insights highlight the potential-dependent interfacial dynamics as a key to NRR-HER selectivity and rationalize the limited exploration of the MoN₄ system.