Spectroscopic and Microscopic Characterization of Heterogenized Molecular Catalyst Systems

The carbon dioxide reduction reaction (CO2RR) is one among many electrocatalytic small molecule transformations important for a decarbonized energy economy. However, industrial CO2 electrolyzer technologies are still far from meeting benchmarks for activity, reaction selectivity, and stability needed to be economically viable, and fundamental studies are still necessary to find methods of improving these parameters. Spectroscopic and microscopic techniques are essential tools for probing the structure, reaction microenvironment, and reaction mechanism of electrocatalyst materials for the CO2RR, and provide key insights to supplement or inform electrochemical studies. My work has primarily focused on molecular catalyst-polymer composite materials, specifically cobalt phthalocyanine (CoPc) encapsulated in poly(4-vinylpyridine) (P4VP) and related polymers. Previous work by our group and others has shown this simple composite system to have high activity and selectivity for the CO2RR to CO. However, little characterization work had been done with the system and many questions remained unanswered. I first studied how the aggregation behavior of CoPc is influenced by codeposition with P4VP. I used UV-Vis to gauge the extent of CoPc monomerization in deposition solutions and in the resulting deposited layers. CoPc and other phthalocyanines readily self-associate through π-stacking interactions, but I found that axial coordination by P4VP or molecular pyridine disaggregated CoPc in deposition solutions, while polymers lacking axial coordination did not have the same effect. Upon deposition, the CoPc-P4VP composite forms a solid solution which retains the monomerization of the CoPc. Both the axial coordination and the presence of a polymeric structure are needed to achieve a solid solution, and the non-coordinating structural analogue poly(4-vinylpyridine) also created a highly monomerized solid solution with CoPc when pyridine was added to the deposition solution. The ubiquitous binding polymer Nafion was found to be ineffective at promoting or retaining CoPc disaggregation in the deposited layer. I also investigated how the composition of the electrolyte buffer solution influences the protonation of the P4VP layer. Previous investigations had revealed that proton delivery to catalytic sites in the CoPc-P4VP composite layer occurs primarily through a proton relay mechanism, which was expected to be influenced by solution pH. Through a combination of electrochemical and spectroscopic experiments, we confirmed that a lower bulk solution pH results in greater protonation of the P4VP layer, which is correlated with increased activity for HER and declining Faradaic efficiency for the CO2RR. We also discovered that layer protonation, and thus HER activity, scales with the electrolyte concentration in the buffer. We attribute this effect to increased equilibrium partitioning of ions into the P4VP layer, which increases the maximum concentration of protons allowed by local charge balancing. Eventually, our goal is to apply in situ spectroelectrochemical techniques to these questions. To that end, in a two-phase process I designed, constructed, and tested a spectroelectrochemical flow cell (SEC-FC) for polarization-modulation infrared reflection absorption spectroscopy (PM-IRRAS). The design was successful in some areas but was hindered by design challenges. Lessons learned from the PM-IRRAS SEC-FC will be inform future efforts to use in situ spectroelectrochemistry to probe the structure, microenvironment, and reaction mechanisms of electrocatalytic materials.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/174323/1/wsdean_1.pd