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

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.