Abstract
The selective and efficient electrochemical reduction of CO2 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 CO2 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 CO2 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 CO2 reduction electrocatalysts and serve as a model for understanding the catalytic mechanism of related heterogeneous systems.
Highlights
The selective and efficient electrochemical reduction of CO2 to single products is crucial for solar fuels development
Using proton inventory studies—a technique that is used in enzymology to study the kinetics of proton delivery to enzymatic active centers based on the attenuation of kinetic rates as a function of fractional solvent deuteration32–35—we show that proton-transport to the Co active site in CoPc-P4VP and related systems is controlled by proton relays in the polymer rather than diffusion through the film
To explore whether aggregation influences the results of our mechanistic studies, we explored the loading dependence on CO2 reduction reaction (CO2RR) activity by CoPc both physisorbed onto edge plane graphite (EPG) and within the P4VP films over four orders of magnitude of loading between 2.19 × 10−11 mol cm−2 and 2.19 × 10−7 mol cm−2
Summary
The selective and efficient electrochemical reduction of CO2 to single products is crucial for solar fuels development. The results of the CoPc-P4VP proton inventory studies support our hypothesis that proton delivery to the active site is controlled by a polymer-based proton relay mechanism.
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