Charge and Proton Transport in Electrocatalysis by Polymer Encapsulated Cobalt Phthalocyanine

Abstract

The electrocatalytic CO2 reduction reaction (CO2RR) is one proposed method to convert industrial waste CO2 into value-added products using renewable electricity. There are many (photo)electrocatalysts that can decrease the costly kinetic barrier to overcome for this process, but one method involves the use of molecular catalysts. Cobalt phthalocyanine (CoPc) is one catalyst that can perform CO2RR with CO and methanol as possible products and when CoPc is encapsulated in a poly(4-vinylpyridine) (P4VP) polymer, it exhibits enhanced activity and selectivity metrics for the CO2RR compared to its non-encapsulated parent complex. Specifically, the reaction activity is quadrupled, and the selectivity for CO2RR over the hydrogen evolution reaction (HER) is enhanced due to three effects of the encapsulation: a primary coordination sphere effect, a secondary coordination sphere effect, and an outer-coordination proton transport effect. Previous members of the McCrory lab have studied the primary sphere effect of pyridyl residues interacting with CoPc by using multiple tools: electrochemistry, spectroscopy, and computations. Unfortunately, the secondary coordination sphere involves the stabilization of the reactive intermediate COO- as it undergoes the reductive process at the CoPc site and has been challenging to study. Previous members of the McCrory lab also studied outer coordination sphere effects by using the electrochemical proton inventory technique to show that there was a proton relay through the polymer, controlling proton delivery from the electrolyte to CoPc active sites. With the knowledge that transport of protons is important, I focused my sties on modulating the transport of electrons and protons within the CoPc-P4VP system. First, I conducted a comprehensive study to incorporate graphite powder (GP) into the catalyst-polymer system and studied how this increased activity. Specifically, I found that the CoPc:GP ratio was important for achieving maximum activity. I also explored the best practice for preparing the CoPc-P4VP/GP slurries by showing that not including centrifugation in the preparation method of the deposition inks showed lower activity at all CoPc loadings. I also studied the impact of pH on the activity and selectivity of the CO2RR by CoPc-P4VP. The rate of transport from the electrolyte to catalytic active sites within the polymer is proportional to the bulk concentration of protons in solution. I saw that fractional protonation of the polymer changed as a function of electrolyte pH, and that increased pH resulted in an increase in CO2RR selectivity and activity. I also found that the concentration of electrolyte impacts the fractional protonation of the polymer, and these results had implications for the HER as catalyzed by CoPc-P4VP. The increase in fractional protonation of the polymer resulted in an observed kinetic isotope effect, indicating that a protonation event was likely the rate-limiting step for the HER but may not be observed unless there was sufficient protonation of the polymer. Finally, I studied the impact of incorporating styrene moieties into a polymer using copolymers with varying styrene:4-vinylpyridine molar concentrations. The study was performed with the express intent to shut down proton relays. I found that the activity decreased with small amounts of styrene within the copolymer, but kinetic isotope effect studies indicated that a possible loss of axial coordination from the pyridyl residues may have caused this diminished activity. The insights from this work may be applied to CO2 electrolyzer devices as polymer-bound molecular catalysts with carbon supports are used in flow cell reactor systems.