Electrosynthesis of pure urea from pretreated flue gas in a proton-limited environment established in a porous solid-state electrolyte electrolyser

Abstract

The electrosynthesis of pure urea via the co-reduction of CO₂ and N₂ remains challenging. Here we show that a proton-limited environment established in an electrolyser equipped with porous solid-state electrolyte, devoid of an aqueous electrolyte, can suppress the hydrogen evolution reaction and excessive hydrogenation of N₂ to ammonia. This can instead be conducive to the C–N coupling of *CO₂ with *NHNH (the intermediate from the semi-hydrogenation of N₂), thereby facilitating the production of urea. By using nanosheets of an ultrathin two-dimensional metal–azolate framework with cyclic heterotrimetal clusters as catalyst, the Faradaic efficiency of urea production from pretreated flue gas (which contains mainly 85% N₂ and 15% CO₂) is as high as 65.5%, and no ammonia and other liquid products were generated. At a low cell voltage of 2.0 V, the current can reach 100 mA, and the urea production rate is as high as 5.07 g g_cat⁻¹ h⁻¹ or 84.4 mmol g_cat⁻¹ h⁻¹. Notably, it can continuously produce 6.2 wt% pure urea aqueous solution for at least 30 h, and about 1.24 g pure urea solid was obtained. The use of pretreated flue gas as a direct feedstock significantly reduces input costs, and the high reaction rate and selectivity contribute to a reduction in system scale and operational costs.