Coupling nitrate capture with ammonia production through bifunctional redox-electrodes

Coupling nitrate capture with conversion to ammonium is attractive because it combines pollutant remediation and chemical production. Here, the authors design a bifunctional redox-electrode to achieve the reactive separation of nitrate to ammonium in a single electrochemical cell. Nitrate is a ubiquitous aqueous pollutant from agricultural and industrial activities. At the same time, conversion of nitrate to ammonia provides an attractive solution for the coupled environmental and energy challenge underlying the nitrogen cycle, by valorizing a pollutant to a carbon-free energy carrier and essential chemical feedstock. Mass transport limitations are a key obstacle to the efficient conversion of nitrate to ammonia from water streams, due to the dilute concentration of nitrate. Here, we develop bifunctional electrodes that couple a nitrate-selective redox-electrosorbent (polyaniline) with an electrocatalyst (cobalt oxide) for nitrate to ammonium conversion. We demonstrate the synergistic reactive separation of nitrate through solely electrochemical control. Electrochemically-reversible nitrate uptake greater than 70 mg/g can be achieved, with electronic structure calculations and spectroscopic measurements providing insight into the underlying role of hydrogen bonding for nitrate selectivity. Using agricultural tile drainage water containing dilute nitrate (0.27 mM), we demonstrate that the bifunctional electrode can achieve a 8-fold up-concentration of nitrate, a 24-fold enhancement of ammonium production rate (108.1 ug h(-1) cm(-2)), and a >10-fold enhancement in energy efficiency when compared to direct electrocatalysis in the dilute stream. Our study provides a generalized strategy for a fully electrified reaction-separation pathway for modular nitrate remediation and ammonia production.

Automated summary

In this study, a bifunctional redox-electrode was designed to achieve the reactive separation of nitrate to ammonium in a single electrochemical cell, providing an attractive solution for pollutant remediation and ammonia production. The results show that electrochemical control can achieve reversible nitrate uptake greater than 70 mg/g and greatly enhance energy efficiency. This study provides a generalized strategy for a fully electrified reaction-separation pathway for modular nitrate remediation and ammonia production.