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

The economic feasibility of electrocatalytic carbon dioxide reduction reaction (CO 2 RR) 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 CO 2 RR. 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 CO 2 RR in an alkaline flow cell electrolyzer. The highest CO partial current density of 53.2 mA/cm 2 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/cm 2 . Experimental and density functional theory (DFT) results indicate that the sp 2 carbon atoms in azole ligands coordinated with the metal center in the MOFs are the active sites for CO 2 RR, due to the fully occupied 3 d 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− sp 2 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 CO 2 RR catalyst.