Sequential electrocatalytic reactions along a membrane electrode assembly drive efficient nitrate-to-ammonia conversion
Tiange Yuan, Min Li, Siddhartha Subramanian, Jesse Kok, Mengran Li, Atsushi Urakawa, Oleksandr Voznyy, Thomas Burdyny
Abstract
Electrochemical ammonia (NH 3 ) synthesis from nitrate (NO 3 − ) 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 (NO 3 − → NH 3 ) while suppressing the two-step pathway (NO 3 − → NO 2 − → NH 3 ). 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 NO 2 − 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 NO 2 − , allowing for 91% FE NH3 in a 5 cm 2 electrolyzer with a NO 3 − to NH 3 partial current of 1.8 A. This work shows that traditional engineering principles can substantially boost the NO 3 reduction reaction, even for simple catalysts.