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

Renewable energy-powered co-electrolysis of CO₂ and NO₃ ^- 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 CO₂ bubbles on the catalyst surface creates numerous triple-phase boundaries that are key toward enhancing CO₂ versus NO₃ ^- 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 CO₂ bubbles at the catalyst/electrolyte interface. Notably, we demonstrate that this electrolyzer system can utilize simulated flue gas (20% CO₂ + 5% O₂) for urea production. With a Cu₉₅Ru₅ 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 CO₂. Our results introduce a simple yet effective design approach toward developing efficient electrolyzer systems for urea production.