Ligand Engineered Metal−Organic Frameworks for Electrochemical Reduction of Carbon Dioxide to Carbon Monoxide

Abstract

The economic feasibility of electrocatalytic carbon dioxide reduction reaction (CO2RR) relies on the development of highly selective and efficient catalysts operating at a high current density. Herein, we explore a ligand engineering strategy involving the use of metal-organic frameworks (MOFs) and combining the desirable features of homogeneous and heterogeneous catalysts for boosting the activity of CO2RR. Zn-based MOFs involving two different azolate functional ligands i.e., 1,2,4-triazole (Calgary Framework-20, CALF-20) and 2-methylimidazole (zeolitic imidazolate framework-8, ZIF-8) were investigated for CO2RR in an alkaline flow cell electrolyzer. The highest CO partial current density of 53.2 mA/cm2 was observed for a Zn-based MOF (CALF-20). CALF-20 showed the highest reported Faradaic efficiency of Zn-based MOFs for CO production (ca. 94% at -0.969 V vs. reversible hydrogen electrode, RHE), with a TOF of 1360.8 h-1 and a partial current density -32.8 mA/cm2. Experimental and density functional theory (DFT) results indicate that the sp2 carbon atoms in azole ligands coordinated with the metal center in the MOFs are the active sites for CO2RR, due to the fully occupied 3d orbital of Zn(II) centers. Ab initio investigation shows that both azolate frameworks in CALF-20 and ZIF-8 have most favorable adsorption sites at the N−sp2 C. Adopting the triazole ligand in CALF-20 enhances the charge transfer (as compared with diazole group in ZIF-8), which induces more electrons in the adjacent active sites at the azole ligand and facilitates *COOH formation, boosting the current density and Faradaic efficiency towards CO production. This study suggests that ligand engineering in MOFs could be a viable approach to design highly efficient CO2RR catalyst.