Sequential electrocatalytic reactions along a membrane electrode assembly drive efficient nitrate-to-ammonia conversion

Abstract

Electrochemical ammonia (NH3) synthesis from nitrate (NO3−) offers a promising greener alternative to the fossil-fuel-based Haber-Bosch process to support the increasing demand for nitrogen fertilizers while removing environmental waste. Previous studies have mainly focused on designing catalysts to promote the direct conversion (NO3− → NH3) while suppressing the two-step pathway (NO3− → NO2− → NH3). We hypothesize that efficient nitrate reduction is possible on simple catalysts by instead promoting the two-step reaction and using chemical reactor principles in a membrane electrode assembly, despite NO2− intermediates. Here, we use an unmodified copper catalyst and control reactivity through current density, flow rate, and electrolyte recycling. Balancing the electrolyte flow rate with current density results in ideal residence times for NO2−, allowing for 91% FENH3 in a 5 cm2 electrolyzer with a NO3− to NH3 partial current of 1.8 A. This work shows that traditional engineering principles can substantially boost the NO3 reduction reaction, even for simple catalysts.