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

Electrochemical ammonia (NH3) synthesis from nitrate (NO3-) (NO3RR) offers a promising green alternative to the fossil fuel-based Haber-Bosch approach to support the increasing global demand for nitrogen fertilizers, while removing environmental waste. Previous efforts have centered on designing catalysts to promote the direct conversion from NO3- àNH3 while suppressing the two-step reaction pathway: NO3- àNO2-àNH3. We took the view that efficient NO3RR is possible on simple catalysts by instead promoting the two-step reaction process, and using chemical reactor principles in a membrane electrode assembly (MEA) to drive high ammonia current densities and Faradaic efficiency, despite nitrite intermediates. Here we use an unmodified copper catalyst in an MEA and control reactivity through current density, flow rate, and electrolyte recycling. We show that balancing the electrolyte flow rates with current density results in ideal residence times of NO2- by-products, allowing for 91% FENH3 in a 5 cm2 electrolyzer with a NO3- to NH3 partial current 1.8 A. Full NO3- conversion was also achieved via recycling with FENH3 maintained > 60% with H2 as a by-product. Importantly this work shows how traditional engineering principles can substantially boost ammonia production, even for simple catalysts.