Triple‐Phase Boundaries Enable Selective Urea Production From Simulated Flue Gas in a Zero‐Gap Electrolyzer

Abstract

Abstract Renewable energy‐powered co‐electrolysis of CO 2 and NO 3 − offers a promising pathway toward sustainable urea production. However, achieving high urea selectivity is challenging due to substantial competing side reactions. Here, we show that engendering a high density of CO 2 bubbles on the catalyst surface creates numerous triple‐phase boundaries that are key toward enhancing CO 2 versus NO 3 − availability for selective urea production. We implemented this strategy using a bipolar membrane in a zero‐gap electrolyzer, which allows for the in situ conversion of (bi)carbonate to CO 2 bubbles at the catalyst/electrolyte interface. Notably, we demonstrate that this electrolyzer system can utilize simulated flue gas (20% CO 2  + 5% O 2 ) for urea production. With a Cu 95 Ru 5 catalyst, we achieve a urea Faradaic efficiency of 58% at 2 V full‐cell voltage over an extended 30‐h period and a peak production rate of 35.46 mmol h −1  g cat −1 . Under these conditions, the Faradaic efficiency to hydrogen evolution and nitrate reduction are 14.7% and 21.6%, respectively. Strikingly, these results with simulated flue gas are comparable to previously reported systems that employ pure CO 2 . Our results introduce a simple yet effective design approach toward developing efficient electrolyzer systems for urea production.