Insights into Reactive Carbon Capture through Heterogeneously Catalyzed Electrochemical Reduction of an Imidazolium...

In reactive carbon capture (RCC) approaches, sorbents capture CO₂ from a dilute source as opposed to pure CO₂ and electrochemical conversion proceeds through the bound-CO₂ form, bypassing the energy losses and expense of sequential CO₂ capture, purification, and compression. N-heterocyclic carbenes (NHCs) have shown promise as capture agents, with reports of RCC conversion of CO₂ to CO or CH₄ via homogeneous electrocatalysis. However, RCC using NHCs is underexplored with heterogeneous electrocatalysts. Herein, the electroreduction of 1,3-bis(2,6-diisopropylphenyl)imidazolium-2-carboxylate (IPrCO₂) was investigated as the CO₂-adduct form of the corresponding imidazole carbene on Ag and other cathodes. In acetonitrile with dilute water, IPrCO₂ reduction led to formate product at up to 88% faradaic efficiency (FE). Carboxylation of the NHC with ¹³CO₂ and subsequent electroreduction led to ¹³C-labeled formate, supporting the interpretation of RCC conversion of the carboxylate. Compared to reduction of IPrCO₂, reduction of the smaller -R group 1,3-dimethylimidazolium-2-carboxylate (IMeCO₂) led to much lower formate selectivity (∼8% FE), in part due to rapid degradation of the IMeCO₂ molecule. Ab initio calculations further supported favorable energetics of IPrCO₂ conversion compared to IMeCO₂, while indicating that a mechanism through carboxylate adsorption is unfavorable and more likely to proceed through a surface hydride. In situ surface-enhanced Raman spectroscopy (SERS) supported a weak surface adsorption of IPrCO₂ during the reaction. Despite relatively high selectivity conversion of IPrCO₂ to formate, CO₂ sorption and reduction cycling studies showed limited durability of the system due to decomposition of the IPrCO₂, indicating a need for more reductively stable NHCs.