Electrochemical nitrate reduction to ammonia in a BPM-MEA system

Electrochemical nitrate reduction reaction (NO3RR) has garnered increasing attention as a pathway for converting a harmful pollutant (nitrate) into a value-added product (ammonia). Technologies that take advantage of this reaction also allow for a reduction in nitrogen waste accumulation enabling more balance in the global nitrogen cycle. However, high selectivity toward ammonia (NH3) is imperative for process viability. Recent studies suggest that proton plays a role in achieving high NH3 production; yet, systematic examination of the impact of proton on NO3RR selectivity is not widely conducted due to the lack of effective control over proton availability during electrolysis. Here, we employed a bipolar membrane (BPM)-based membrane electrode assembly (MEA) system to investigate the influence of protons on the selectivity of NO3RR. The BPM generates a robust proton flux during electrocatalytic reactions, making it a suitable candidate for investigating the proton effects on NO3RR. We employed the interposer layer (mixed cellulose ester membrane filter) and proton scavenger (CO32-) as an approach to control the proton flux. Furthermore, we redesigned the configuration of BPM-based MEA cells, allowing us to regulate the mass transfer of the reactants (e.g. proton and nitrate), creating local environments that favor ammonia formation. We find that a moderate proton supply allowed for an increase in ammonia yield by 576% when compared to the standard membrane electrode assembly, and a high selectivity of 26 (NH3 over NO2-) was achieved at an applied current density of 200 mA cm-2 with electrolyte mirrors that found in wastewater. Our work introduces a novel approach to address the selectivity of NO3RR from the perspective of cell design and the findings lay the groundwork for a deeper understanding of the influence of proton on selective NH3 synthesis from nitrate.