Synthetic Molecular Models for the Oxygen Reduction Active Sites in Heteroatom-Doped Graphitic Electrocatalysts: Linking Heterogeneous and Homogeneous Electrocatalysis

Abstract

The development and deployment of low-temperature fuel cell technologies require that low-cost platinum-free catalysts for the oxygen reduction reaction (ORR) that are highly active, selective, and durable be created. To this point, the advancement of potential replacement catalysts such as metal- and nitrogen-doped carbon (M-N-C) materials has been hampered by a lack of detail regarding the structures of the M-N4 active sites that facilitate the ORR coupled with a paucity of characterization with respect to the electrochemical behavior and properties of these sites. In this work, we present four studies on the structure, spectroscopy, and the electrochemical properties and reactivity of several Fe-N4 complexes as potential model complexes for the sites in iron- and nitrogen-doped carbon (Fe-N-C) catalysts and as platforms for elucidating catalyst design principles relevant to both homogeneous non-aqueous and heterogeneous aqueous ORR electrocatalysis. In Chapter 2, we synthesize a functional structural molecular platform for modeling the ORR active sites in Fe-N-C materials. We show that the complex more faithfully represents the Fe-containing sites in Fe-N-C materials compared to legacy pyrrolic and pyridinic macrocycle complexes, providing a promising platform for the construction and study of next-generation Fe-N-C materials characterized by improved active site homogeneity and increased site density without the need for high-temperature pyrolysis. In Chapter 3, we investigate the electrochemical response of the model complex synthesized in Chapter 2 to molecular poisons to bolster the previous claims made in the literature about the relative contributions of metal-centered and metal-free ORR activity in acidic and alkaline electrolytes. We show that acidic and alkaline electrolytes cause different ORR contributions from the model complexes, suggesting that improving the poison tolerance, durability, and activity of the metal-containing active sites should be the focus of future Fe-N-C material synthesis research. In Chapter 4, we examine the electrochemical ORR performance of a family of macrocycles spanning a variety of Fe-N4, Fe-C2N2, and Fe-C4 coordination environments. Our results highlight that careful control over the reaction environment is required to optimize catalysts for a specific application, and that, in general, the rate-overpotential scaling relationships for ORR allow for substantially more efficient catalysis in heterogeneous aqueous environments. In Chapter 5, we evaluate the homogeneous ORR performance of an Fe-N-C model complex derivative in non-aqueous weak acid buffers. The data indicate that structural features on the catalyst enable substantial rate enhancements as the acidity of the proton donor is reduced. Our results demonstrate that changes to the iron coordination environment, coupled with interactions with the electrolyte have the ability to radically alter the catalytic activity of Fe-N4 macrocycle complexes. The development of more sustainable and affordable fuel cell devices relies upon the evolution and elaboration of stable, highly active, and platinum-free electrocatalysts for the ORR. Through the projects outlined in this work, we have taken steps to bolster the structural identification of the nitrogen-ligated sites in Fe-N-C materials and used the resulting model complexes to investigate the role that both electrolytes and catalyst structure play in controlling ORR catalysis both in solution and on solid catalyst surfaces. In aggregate, this dissertation seeks to highlight crosstalk between molecular and heterogeneous electrochemistry, aiming to simultaneously inform Fe-N-C development for fuel cell applications and molecular catalyst design for the oxygen reduction reaction.Ph.D