The selective and efficient electrochemical reduction of CO 2 to single products is crucial for solar fuels development. Encapsulating molecular catalysts such as cobalt phthalocyanine within coordination polymers such as poly-4-vinylpyridine leads to dramatically increased activity and selectivity for CO 2 reduction. In this study, we use a combination of kinetic isotope effect and proton inventory studies to explain the observed increase in activity and selectivity upon polymer encapsulation. We provide evidence that axial-coordination from the pyridyl moieties in poly-4-vinylpyridine to the cobalt phthalocyanine complex changes the rate-determining step in the CO 2 reduction mechanism accounting for the increased activity in the catalyst-polymer composite. Moreover, we show that proton delivery to cobalt centers within the polymer is controlled by a proton relay mechanism that inhibits competitive hydrogen evolution. These mechanistic findings provide design strategies for selective CO 2 reduction electrocatalysts and serve as a model for understanding the catalytic mechanism of related heterogeneous systems.

In situ X-ray absorbance spectroscopy measurements show that encapsulating cobalt phthalocyanine within coordinating polymers leads to axial ligation of Co and results in enhanced rates for electrocatalytic CO2 reduction.

Axial coordination of pyridyl moieties to CoPc (either exogenous or within poly-4-vinylpyridine polymer) dramatically increases the complex’s activity for the CO2 reduction reaction (CO2RR). It has been hypothesized that axial coordination to the Co active site leads to an increase in the Co d z 2 orbital energy, which increases the complex’s nucleophilicity and facilitates CO2 coordination compared to the parent CoPc complex. The magnitude of the energy increase in the Co d z 2 orbital should depend on the σ-donor strength of the axial liganda stronger σ-donating ligand (L) will increase the overall CO2RR activity of axially coordinated CoPc­(L) and vice versa. To test this, we have studied a series of CoPc­(L) complexes where the σ-donor strength of L is varied. We show that CoPc­(L) reduces CO2 with an increased activity as the σ-donor ability of L is increased. These observed electrochemical activity trends are correlated with computationally derived CO2 binding energy and charge transfer terms as a function of σ-donor strength. The findings of this study support our hypothesis that the increased CO2RR activity observed upon axial coordination to CoPc is due to the increased energy of the d z 2 orbital, and highlight an important design consideration for macrocyclic MN4-based electrocatalysts.

Cobalt phthalocyanine (CoPc) has been extensively studied as a catalyst for the electrochemical reduction of CO2 to value-added products. Previous studies have shown that CoPc is a competent and efficient catalyst when immobilized onto carbon-based electrodes using a polymer binder, especially when immobilized with a graphitic carbon powder support to increase charge transport. In this study, we systematically explore the influence of incorporating graphite powder (GP) into a polymer-encapsulated CoPc on the system’s activity for the electrochemical reduction of CO2. We report a protocol for incorporating GP into CoPc/polymer/GP catalyst films that facilitates physisorption of CoPc to GP, leading to increased activity for CO2 reduction. We show that the activity for CO2 reduction increases with GP loading at low GP loadings, but at sufficiently high GP loadings the activity plateaus as charge transfer is sufficiently fast to no longer be rate limiting. We also demonstrate that axial coordination is still important even in the presence of GP, suggesting that CoPc does not fully coordinate to heteroatoms on the GP surface. We develop a set of optimized conditions under which the CoPc/polymer/GP catalyst systems reduce CO2 with higher activity and similar selectivity to previously reported CoPc/polymer films on edge-plane graphite electrodes. The procedures outlined in this study will be used in future studies to optimize catalyst, polymer, and carbon support loadings for other polymer–catalyst composite systems for electrocatalytic transformations.

No abstract