The selective electrochemical reduction of CO2 in the CO2 reduction reaction (CO2RR) is a crucial strategy for storing energy from intermittent sources in the form of chemical bonds (e.g. solar fuels). One of the focuses of my research group is studying the effects of polymer encapsulation on controlling the microenvironment surrounding molecular electrocatalysts, and how this in turn influences the catalysts’ activities and mechanisms for the CO2RR. In particular, we have shown that encapsulating cobalt phthalocyanine (CoPc) within the coordinating polymer poly-4-vinylpyridine (P4VP) results in a catalyst-polymer composite (CoPc-P4VP) that selectively reduces CO2 to CO at fast rates at low overpotential. This increased activity and selectivity for the CO2RR to CO is attributed to a combination of primary, secondary, and outer-coordination sphere effects. In this presentation, we specifically explore the role of axial coordination in the primary coordination sphere on CO2RR activity and mechanism for the CO2RR. Using a combination of experimental electrochemistry and computational studies, we show that axial coordination to CoPc by pyridyl residues and related systems increases the Co dz2 orbital energy, thus facilitating CO2 binding compared to the parent CoPc systems. We show that the magnitude of the energy increase, and thus the CO2RR activity, is directly correlated to the σ-donor strength of the axially-coordinating ligand. These studies demonstrate how computation and experimentation can be used together to understand the effect of axial coordination on catalytic activity and mechanism, and they provide design considerations for MN4-based electrocatalytic systems.

Understanding the role of axial coordination in CO2 reduction by cobalt phthalocyanine

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