Multimetallic Catalyst Architectures for Selective Electrochemical CO2 Reduction

Abstract

The electrocatalytic CO2 reduction reaction (CO2RR) is a promising strategy of converting CO2 to value-added chemicals by renewable energy sources. Current electrocatalysts can be split into two categories: heterogeneous solid-state catalysts and homogeneous molecular catalysts. Both catalysts face challenges: the molecular catalysts usually show lower activity and stability; the solid-state catalysts are less selective. The work in this thesis focuses on the design and study of a new class of catalytic materials, multimetallic catalyst architectures, which are heterogeneous catalysts using the molecular catalysts as building units therefore they can not only maintain the product selectivity of molecular catalysts but also possess the activity and stability of solid-state catalysts. Background information for this research is discussed in Chapter 1. In Chapter 2, I discuss the design of a model system of homo- and heterobimetallic catalysts to study how intramolecular interactions influence the catalytic activity of the multimetallic catalyst assemblies for CO2RR. A homobimetallic complex [Co(PDI)-(PDI)Co] has been prepared to simulate two adjacent Co(PDI) molecular catalyst building units in the multimetallic catalysts and exhibits two main interactions: intramolecular electrostatics and electronic coupling between metal sites. The results of modulating these interactions demonstrate that intramolecular electrostatics is a crucial influence on the per-site activity of the multimetallic catalysts. Specifically, a heterobimetallic [Zn(PDI)-(PDI)Co] shows higher catalytic activity for CO2RR compared to the [Co(PDI)-(PDI)Co], which is attributed to the larger electrostatics exerted by the proximal Zn2+ in the catalytically active reduced form of the complex. In Chapter 3, I extended the bimetallic molecular catalysts to a multimetallic polymeric catalyst poly-[Co(VinylPDI)], which was prepared by radical polymerization of vinyl group attached to Co(PDI) building unit. The poly-[Co(VinylPDI)] shows higher catalytic current and more positive catalytic onset compared to monomer [Co(PDI)], and this is due to the electrostatic effects between the molecular catalyst building units in the polymer system. However, the poly-[Co(VinylPDI)] exhibits catalyst decomposition during electrolysis, resulting in lower selectivity and stability for CO2RR. XPS and FTIR studies indicate that the decomposition happens at the imine bonds of Co(PDI) units which are subsequently converted to C=O and N‒H groups. In Chapter 4, I heterogenized molecular catalysts for CO2RR, where a [Co(PDI)]-containing polymer film (poly-[Co(TTPDI)]) was directly deposited onto electrode surface by an electropolymerization method. The poly-[Co(TTPDI)] film shows enhanced catalytic performance compared to parent [Co(PDI)] complex, which is probably due to the highly conjugated polyterthiophene backbone for charge delocalization and conduction. However, the poly-[Co(TTPDI)] exhibits electrochemical instability, as evidenced by a rapid current decrease after the first CV scan, indicating a significant degradation of the polymer film. The degradation studies suggest that the nitrogen in PDI ligand is the main reason for the polymer degradation, probably because the nitrogen attacks thienyl radicals, causing irreversible structure changes. In Chapter 5, I employed density functional theory (DFT) to further explore how intramolecular electrostatics enhance catalytic performance of bimetallic [Co(PDI)-(PDI)Co] and [Zn(PDI)-(PDI)Co]. DFT calculations of CO₂ binding modes reveal that [Zn(PDI)-(PDI)Co] exhibits the strongest binding affinity to CO2, followed by [Co(PDI)-(PDI)Co] and [Co(PDI)]. These results align with the trend in the strength of electrostatics in the three catalysts and account for experimental observations. In Chapter 6, I discuss some new directions I believe this work should take based on my studies, and I include some preliminary data for some of these new directions.