Electrocatalytic Nitrate Reduction on Oxide-Derived Silver with Tunable Selectivity to Nitrite and Ammonia

Abstract

Removing excess nitrate (NO 3 – ) from waste streams has become a significant environmental and health topic. However, realizing highly selective NO 3 – conversion toward N 2, primarily via electrocatalytic conversions, has proven challenging, largely because of the kinetically uncontrollable NO 3 – -to-NO 2 – pathway and unfavorable N–N coupling. Herein, we discovered unique and ultra-high electrocatalytic NO 3 – -to-NO 2 – activity on oxide-derived silver (OD-Ag). Up to 98% selectivity and 95% Faradaic efficiency (FE) of NO 2 – were observed and maintained under a wide potential window. Benefiting from the superior NO 3 – -to-NO 2 – activity, further reduction of accumulated NO 2 – to NH 4 + was well regulated by the cathodic potential and achieved an NH 4 + FE of 89%, indicating a tunable selectivity to the key nitrate reduction products (NO 2 – or NH 4 + ) on OD-Ag. Density functional theory computations provided insights into the unique NO 2 – selectivity on Ag electrodes compared with Cu, showing the critical role of a proton-assisted mechanism. Based on the ultra-high NO 3 – -to-NO 2 – activity on OD-Ag, we designed a novel electrocatalytic–catalytic combined process for denitrifying real-world NO 3 – -containing agricultural wastewater, leading to 95+% of NO 3 – conversion to N 2 with minimal NO X gases. In addition to the wastewater treatment process to N 2 and the electrochemical synthesis of NH 3, NO 2 – derived from electrocatalytic NO 3 – conversion can serve as a reactive platform for the distributed production of various nitrogen products.