Hierarchical Tuning of the Performance of Electrochemical Carbon Dioxide Reduction Using Conductive Two-Dimensional Metallophthalocyanine Based Metal-Organic Frameworks.

The use of reticular materials in the electrochemical reduction of carbon dioxide to value-added products has the potential to enable tunable control of the catalytic performance through the modulation of chemical and structural features of framework materials with atomic precision. However, the tunable functional performance of such systems is still largely hampered by their poor electrical conductivities. This work demonstrates the use of four systematic structural analogs of conductive two-dimensional (2D) metal-organic frameworks (MOFs) made of metallophthalocyanine (MPc) ligands linked by Cu nodes with electrical conductivities of 2.73 × 10-3 to 1.04 × 10-1 S cm-1 for the electrochemical reduction of CO2 to CO. The catalytic performance of the MOFs, including the activity and selectivity, is found to be hierarchically governed by two important structural factors: the metal within the MPc (M = Co vs Ni) catalytic subunit and the identity of the heteroatomic cross-linkers between these subunits (X = O vs NH). The activity and selectivity are dominated by the choice of metal within MPcs and are further modulated by the heteroatomic linkages. Among these MOFs, CoPc-Cu-O exhibited the highest selectivity toward CO product (Faradaic efficiency FECO = 85%) with high current densities up to -17.3 mA cm-2 as a composite with carbon black at 1:1 mass ratio) at a low overpotential of -0.63 V. Without using any conductive additives, the use of CoPc-Cu-O directly as an electrode material was able to achieve a current density of -9.5 mA cm-2 with a FECO of 79%. Mechanistic studies by comparison tests with metal-free phthalocyanine MOF analogs supported the dominant catalytic role of the central metal of the phthalocyanine over Cu nodes. Density-functional theory calculations further suggested that, compared with the NiPc-based and NH-linked analogs, CoPc-based and O-linked MOFs have lower activation energies in the formation of carboxyl intermediate, in line with their higher activities and selectivity. The results of this study indicate that the use of 2D MPc-based conductive framework materials holds great promise for achieving efficient CO2 reduction through strategic ligand engineering with multiple levels of tunability.