Date of Award
Doctor of Philosophy (PhD)
Energy storage and conversion are key facets of sustainable energy science. Fuel cells, for instance, allow us to convert chemical and electrical energies while minimizing energy loss in the form of heat. The oxygen reduction reaction—which involves combining O2, protons, and electrons to make water—is one such reaction common to many types of fuel cells but is hard to perform both rapidly and efficiently. Typically there are trade-offs, and faster catalysis is only achieved at lower efficiencies. Overcoming these tradeoffs is a long withstanding goal in electrocatalysis and would be broadly impactful for the optimization of energy-relevant reactions. The atomistic insight obtained from homogeneous (electro)catalysts can help realize this goal, for instance by identifying kinetic/thermodynamic relationships or key structure-function properties in catalyst design. This thesis describes a series of related stories in which homogeneous, molecular iron porphyrins are used as i) (electro)catalysts for the oxygen reduction reaction (ORR) and ii) electrostatic models for small molecule activation. Chapters 1-3 begin this thesis with the derivation, utility, and application of molecular “scaling relationships,” the tradeoffs described above, for iron porphyrin catalyzed ORR. Chapter 1 starts this narrative with a summary of the kinetic and thermodynamic studies required to derive and apply the scaling relationships. Chapter 2 details a complete mechanistic study of O2 reduction by iron tetraphenylporphyrin in nonaqueous solvents and identifies key intermediates, energies, and steps involved during turnover. Using an iron porphyrin bearing four ortho-trimethylanilinium groups and buffered carboxylic acids in acetonitrile, Chapter 3 demonstrates that molecular scaling relationships can be additive and that a cooperative scaling approach is a powerful way to improve multistep electrocatalytic processes. Together, these studies demonstrate the power of kinetic/thermodynamic scaling relationships and show that cooperative, or “tandem” scaling is an untapped method for optimization of energy-relevant reactions. Chapter 4 is a turning point in this thesis, wherein the focus pivots from molecular scaling relationships to intramolecular electrostatic effects. Using iron(III) abab-tetra(o-N,N,N-trimethylanilinium)porphyrin pentatriflate—an atropisomerically-pure form of the catalyst used in Chapter 3—this Chapter details how the charged o-[N(CH3)3]+ groups on the iron porphyrin affect acetate, O2, and CO2 binding, various pre-equilibria that are invoked in the molecular electrocatalysis literature. The results of this study highlight how electrostatic, secondary sphere motifs affect specific small molecule binding in nonaqueous media and underscore their utility in catalyst design. Chapters 5 details the synthesis and characterization of all four atropisomers (abab, aabb, aaab, and aaaa) of the polycationic catalyst used in Chapters 3 and 4 and identifies the role of electrostatic effects in both solution and solid-state data. Chapter 6 takes advantage of the molecules prepared in Chapter 5 and reports both O2 reduction and CO2 reduction using the set of charged isomers. Together, Chapters 5-6 offer a first-of-a-kind look at the role of charge orientation on the behavior of molecular electrocatalysts and small molecule activation. Chapter 7 closes this thesis and reports the synthesis and prior misidentification of 4-tert-butyl-2,6-dinitrobenzaldehyde. This small molecule is a common precursor for bis-picket fence porphyrins and is potential starting point for the synthesis of new, highly charged iron porphyrin complexes. Taken together, the results and conclusions presented in this thesis have direct, immediate ramifications for ORR and CO2RR electrocatalysis and are broadly relevant for many other chemical-to-electrical energy processes. I hope that readers are inspired by some of the ideas presented herein and can find takeaways that aid in their own scientific pursuits.
Martin, Daniel James, "Electrocatalytic Oxygen Reduction using Molecular Iron Porphyrins: Detailing the Role of Electrostatics in Small Molecule Activation" (2021). Yale Graduate School of Arts and Sciences Dissertations. 262.