Three-step pulse strategy enhances ultradilute nitrate-to-ammonia conversion via microenvironment and mass transfer control

Abstract

Nitrate pollution in wastewater, primarily originating from industrial, agricultural, and domestic sources, typically occurs at concentrations of 10 mm or lower. Although the nitrate reduction reaction (NO3RR) has been one of the most intensively researched fields with a mature FE (Faraday efficiency) over 90% and a milligram-level yield of ammonia, it remains difficult to deal with low-nitrate environments, such as municipal wastewater. In this study, a three-step pulsed strategy is presented that attains nearly 100% ammonia FE from an ultralow 10 mm nitrate concentration electrolyte, representing a threefold enhancement over the conventional constant potentiostatic approach. Through operando characterizations, density functional theory calculations and COMSOL simulations, the mechanism is elucidated by which various potential biases concurrently modulate NO3RR intermediates, thereby enhancing reaction kinetics and effectively suppressing the competing hydrogen evolution reaction. Furthermore, practical application in a flow cell, along with techno-economic analysis, highlights the technological and economic feasibility of converting nitrate into valuable ammonia directly from wastewater without preconcentration. The research advances the understanding of pulse-driven strategies and the modulation of ionic microenvironments in electrochemical processes, paving the way for practical and environmentally friendly wastewater treatment and ammonia synthesis.