Unraveling Ligand-Mediated Electron and Proton Transfer in Nitrate Electroreduction by In Situ Bipolar Electrode Mass Spectrometry

The electrocatalytic nitrate reduction reaction (NO 3 RR) serves as a dual-functional strategy for simultaneous ammonia production and nitrate pollutant removal. However, the rational design of high-performance catalysts is often hindered by a limited understanding of catalyst structure–activity relationships at the molecular level. Here, this study systematically investigated the reaction mechanisms of three cobalt-based molecular catalysts: Co(DIM), Co(cyclam), and Co(TIM) with similar ligand structures for NO 3 RR, combining in situ bipolar electrode electrochemical mass spectrometry (BPE-EC-MS) and theoretical calculations. This approach elucidates the influence of ligand structure on reaction pathways and catalytic performance. Using high-temporal-resolution BPE-EC-MS, key intermediates such as [Co(DIM) + H 2 NO + OH─H] − were directly observed for the first time. Integrated mass spectrometry and theoretical simulations reveal that the electronic effects of ligands are the core factors governing catalytic performance. Co(DIM) achieves optimal catalytic performance by redistributing electron density to ligands through molecular symmetry breaking and electron storage via double bonds; simultaneously, its amino protons thermodynamically and kinetically synergize with reaction progression through intramolecular hydrogen bonding and proton transfer. This study provides both methodological tools and theoretical foundations for the rational design of high-efficiency molecular NO 3 RR catalysts.