(707b) Electrocatalytic Nitrate Reduction with Tunable Selectivity to Nitrite and Ammonia over Silver Catalysts

Hengzhou Liu, Jaeryl Park, Yifu Chen, Luke T. Roling, Wenzhen Li*

The removal of excess nitrate (NO₃^-) from waste streams has become an important environmental and health issue. However, converting NO₃^- into N₂ with high selectivity, primarily through electrocatalytic methods, has been challenging due to the uncontrollable NO₃^- to NO₂^- pathway and unfavorable N-N coupling. In this study, we first screened over 18 metals and found that only Ag and Cu have less negative onset potentials of NO₃^- to NO₂^- than that of NO₂^- to NH₄⁺. Then we prepared high-surface area oxide-derived silver (OD-Ag) electrodes for highly efficient electrocatalytic NO₃^- reduction to NO₂^-. Up to 98% selectivity and 95% Faradaic efficiency (FE) of NO₂^- was maintained in a wide range of potentials, and deep reduction of accumulated NO₂^- to NH₄⁺ was achieved with an NH₄⁺ FE of 89% only by applying a more negative cathodic potential (gating potential). This indicates a tunable selectivity to the key NO₃^- reduction products (NO₂^- or NH₄⁺) on OD-Ag. We also used density functional theory (DFT) computations to gain insights into the unique NO₂^- selectivity on Ag electrodes as compared to Cu, highlighting the critical role of a proton-assisted mechanism. Finally, we designed a novel electrocatalytic-catalytic combined process to denitrify real-world NO₃^--containing agricultural wastewater with minimal NO_x gas emission. In addition to the wastewater treatment process to N₂ and the electrochemical synthesis of NH₃, NO₂^– derived from electrocatalytic NO₃^– conversion can serve as a reactive platform for the distributed production of various nitrogen products.