Conspectus The electrochemical CO2 reduction reaction (CO2RR) is an attractive method for capturing intermittent renewable energy sources in chemical bonds, and converting waste CO2 into value-added products with a goal of carbon neutrality. Our group has focused on developing polymer-encapsulated molecular catalysts, specifically cobalt phthalocyanine (CoPc), as active and selective electrocatalysts for the CO2RR. When CoPc is adsorbed onto a carbon electrode and encapsulated in poly­(4-vinylpyridine) (P4VP), its activity and reaction selectivity over the competitive hydrogen evolution reaction (HER) are enhanced by three synergistic effects: a primary axial coordination effect, a secondary reaction intermediate stabilization effect, and an outer-coordination proton transport effect. We have studied multiple aspects of this system using electrochemical, spectroscopic, and computational tools. Specifically, we have used X-ray absorption spectroscopy measurements to confirm that the pyridyl residues from the polymer are axially coordinated to the CoPc metal center, and we have shown that increasing the σ-donor ability of nitrogen-containing axial ligands results in increased activity for the CO2RR. Using proton inventory studies, we showed that proton delivery in the CoPc–P4VP system is controlled via a proton relay through the polymer matrix. Additionally, we studied the effect of catalyst, polymer, and graphite powder loading on CO2RR activity and determined best practices for incorporating carbon supports into catalyst–polymer composite films. In this Account, we describe these studies in detail, organizing our discussion by three types of microenvironmental interactions that affect the catalyst performance: ligand effects of the primary and secondary sphere, substrate transport of protons and CO2, and charge transport from the electrode surface to the catalyst sites. Our work demonstrates that careful electroanalytical study and interpretation can be valuable in developing a robust and comprehensive understanding of catalyst performance. In addition to our work with polymer encapsulated CoPc, we provide examples of similar surface-adsorbed molecular and solid-state systems that benefit from interactions between active catalytic sites and a polymer system. We also compare the activity results from our systems to other results in the CoPc literature, and other examples of molecular CO2RR catalysts on modified electrode surfaces. Finally, we speculate how the insights gained from studying CoPc could guide the field in designing other polymer–electrocatalyst systems. As CO2RR technologies become commercially viable and expand into the space of flow cells and gas-diffusion electrodes, we propose that overall device efficiency may benefit from understanding and promoting synergistic polymer-encapsulation effects in the microenvironment of these catalyst systems.

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.

The electrochemical reduction of CO 2 and H 2 O to solar fuels remains a promising strategy for storing intermittent energy sources in the form of chemical bonds. These electrochemical reductions occurring at the cathode typically are coupled to the oxygen evolution reaction at the anode. The electrochemical oxidation of organic alcohols in the alcohol oxidation reaction is a promising alternative anode reaction that occurs at decreased operating potentials compared to the oxygen evolution reaction and that produces more valuable products than O 2 . Co 2 NiO 4 is a particularly promising catalyst for the oxidation of alcohols, able to promote alcohol oxidation at current densities of 10 mA cm −2 at potentials of only 1.42 V vs reversible hydrogen electrode (RHE) in alkaline aqueous conditions, significantly less positive than typical potentials required for the oxygen evolution reaction. In this work, we study the alcohol oxidation reaction by Co 2 NiO 4 for a series of straight-chain primary alcohols of increasing chain length from ethanol to n-pentanol. We show that the product distribution for alcohol oxidation depends on the alcohol chain length, changing from primarily aldehyde products for shorter-chain alcohols to primarily carboxylic acid products for longer-chain alcohols. These results suggest that alcohols are oxidized sequentially to first aldehydes and then carboxylic acids at Co 2 NiO 4 . During the oxidation of longer-chain alcohols, the aldehyde intermediates are retained at the catalyst surface for longer times, facilitating further oxidation to terminal carboxylic acid products. We also explored the potential-dependent activities and product distributions for nbutanol oxidation at Co 2 NiO 4 , and showed that alcohol oxidation is able to outcompete chloride oxidation in aqueous solutions containing Cl − at seawater concentrations. These studies provide further insight into the alcohol oxidation reaction at Co 2 NiO 4 and highlight its promise as an alternative anode reaction for the production solar fuels.

<p>Axial coordination of a pyridyl moieties to CoPc (either exogenous or within poly-4-vinylpyridine polymer) dramatically increases the complex’s activity for CO₂RR. It has been hypothesized that axial coordination to the Co active site leads to an increase in the Co dz² 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 dz² 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 supports our hypothesis that the increased CO₂RR activity observed upon axial coordination to CoPc is due to the increased energy of the dz² orbital, and highlight an important design consideration for macrocyclic MN₄-based electrocatalysts.</p><p> </p><p> </p>

Polymer-encapsulated cobalt phthalocyanine (CoPc) is a model system for studying how polymer–catalyst interactions in electrocatalytic systems influence performance for the CO2 reduction reaction. In particular, understanding how bulk electrolyte and proton concentration influence polymer protonation and in turn how the extent of polymer protonation influences catalytic activity and selectivity is crucial to understanding polymer–catalyst composite materials. We report a study of the dependence of bulk pH and electrolyte concentration on the fractional protonation of poly­(4-vinylpyridine) and related polymers with both electrochemical and spectroscopic evidence. In addition, we show that the fractional protonation of the polymer is directly related to both the activity of the catalyst and the reaction selectivity for the CO2 reduction reaction over the competitive hydrogen evolution reaction. Of particular note is that the fractional protonation of the film is related to electrolyte concentration, which suggests that the transport of counterions plays an important role in regulating proton transport within the polymer film. These insights suggest that electrolyte concentration and pH play an important role in the electrocatalytic performance for polymer–catalyst composite systems, and these influences should be considered in both experimental preparation and analysis.