Tailoring Oxygen Vacancies in CuO via Ion Irradiation for Efficient Conversion of Nitrate to Ammonia

Electrocatalytic nitrate reduction to ammonia (NO3–RR) offers a sustainable route for simultaneously removing nitrate pollutant and recovering value-added ammonia product under ambient conditions. However, the competitive hydrogen evolution reaction (HER) diverts active hydrogen species away from nitrate hydrogenation, thereby limiting the Faradaic efficiency and hydrogenation kinetics. In this work, we construct CuO nanoneedle arrays and employ Ar–ion irradiation to regulate oxygen vacancy concentration. Spectroscopic characterization reveals a progressive increase in the oxygen-vacancy concentration with increasing irradiation fluence. Among the investigated electrodes, the CuO electrode irradiated at 1 × 1015 ions cm–2 exhibits the best NO3–RR performance, delivering an NH3 Faradaic efficiency of 94.15% and a yield rate of 1.28 mg·N·h–1·cm–2 at a low potential of –0.1 V vs the reversible hydrogen electrode. Combining electron paramagnetic resonance measurements and *H scavenging experiment, we reveal that the enhanced performance originates from irradiation-induced oxygen vacancies, which not only facilitate the generation of active hydrogen species derived from water dissociation but also effectively utilize the active hydrogen species toward nitrate hydrogenation rather than H–H coupling. A preliminary techno-economic analysis estimates a levelized cost of ammonia of $0.935 kg–1 under the assumed conditions. We further extended this strategy to a coupled NO3–RR//EGOR system, in which replacing OER with ethylene glycol oxidation lowered the anodic overpotential. This work demonstrates that ion irradiation provides a controllable defect-engineering strategy for regulating active-hydrogen-mediated nitrate electroreduction on metal-oxide catalysts.