Dynamic cycling of surface lattice hydrogen on OH-terminated metal borides drives efficient nitrate electroreduction...

Electrochemical nitrate reduction (NO₃RR) represents a sustainable pathway for nitrate removal and green ammonia synthesis under mild conditions. However, its efficiency is limited by insufficient active hydrogen, causing slow kinetics and possible nitrite formation. We reveal a non-traditional hydrogen supply mechanism-surface lattice hydrogen of hydroxyl-terminated 2D transition metal compounds acts as hydrogen donors and regenerates via a dynamic vacancy cycle-by employing density functional theory (DFT) calculations and experimental validation. Using 2D metal borides as models, DFT calculations show these materials primarily exist as hydroxyl-terminated forms under electrochemical conditions. The surface hydroxyl groups stabilize intermediates through hydrogen bonding and act as hydrogen donors, while generated hydrogen vacancies are rapidly replenished, creating a dynamic cycle that accelerates kinetics. OH-terminated WB exhibits an improved limiting potential (-0.10 V) compared with its bare counterpart (-0.57 V), with high selectivity and stability. The proposed mechanism enables a fundamental understanding of the experimentally observed enhancement in NO₃RR activity of OH-terminated Ti₃C₂ MXene under high applied voltage. This study highlights the critical role of surface lattice hydrogen and proposes a design strategy based on hydrogen vacancy formation energy and work function, advancing the application of NO₃RR in water denitrification and ammonia production.