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

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

Shallow Rate-Overpotential Scaling in Aqueous Molecular Oxygen Reduction Electrocatalysis Across a Family of Iron Macrocycles

Rate-overpotential scaling relationships have been employed widely to understand trends in oxygen reduction reaction (ORR) electrocatalysis by dissolved metal macrocycles in organic electrolytes. Similar scaling relationships remain unknown for surface-adsorbed ORR electrocatalysts in the acidic aqueous environments germane to proton-exchange membrane (PEM) fuel cells. Herein, we examine ORR catalysis in aqueous perchloric acid media for a structurally diverse array of iron macrocycle complexes adsorbed on Vulcan carbon black. The macrocycles encompass Fe–N4, Fe–N2N′2 and Fe–NxC4−x motifs bearing pyrrolic, pyridinic, and N-heterocyclic carbene (NHC) moieties in the primary ligation sphere, giving rise to a 530 mV range in Fe(III/II) redox potentials, EFe(III/II). Experimental Tafel data in the micropolarization regime were extrapolated to the EFe(III/II) to furnish estimated TOF values that span ~3 orders of magnitude across the family of compounds. Despite the structural diversity of this family of compounds, extrapolated TOF values correlate with Fe(III/II) redox potentials in a roughly log-linear fashion with a shallow scaling factor of approximately 180 mV/decade. These findings highlight that negative shifts in EFe(III/II) lead to diminishing returns in catalytic rate promotion and suggest that changes to the primary ligating environment in a macrocycle are insufficient to break fundamental rate-overpotential scaling relationships in aqueous ORR catalysis. Together these studies motivate the development of new higher-potential iron complexes that employ motifs beyond the equatorial ligation plane to enhance ORR catalysis

A Pyridinic Fe-N4 Macrocycle Effectively Models the Active Sites in Fe/N-Doped Carbon Electrocatalysts

Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum in fuel cells, but their active site structures are poorly understood. A leading postulate is that iron active sites in this class of materials exist in an Fe-N4 pyridinic ligation environment. Yet, molecular Fe-based catalysts for the oxygen reduction reaction (ORR) generally feature pyrrolic coordination and pyridinic Fe-N4 catalysts are, to the best of our knowledge, non-existent. We report the synthesis and characterization of a molecular pyridinic hexaazacyclophane macrocycle, (phen2N2)Fe, and compare its spectroscopic, electrochemical, and catalytic properties for oxygen reduction to a prototypical Fe-N-C material, as well as iron phthalocyanine, (Pc)Fe, and iron octaethylporphyrin, (OEP)Fe, prototypical pyrrolic iron macrocycles. N 1s XPS signatures for coordinated N atoms in (phen2N2)Fe are positively shifted relative to (Pc)Fe and (OEP)Fe, and overlay with those of Fe-N-C. Likewise, spectroscopic XAS signatures of (phen2N2)Fe are distinct from those of both (Pc)Fe and (OEP)Fe, and are remarkably similar to those of Fe-N-C with compressed Fe–N bond lengths of 1.97 Å in (phen2N2)Fe that are close to the average 1.94 Å length in Fe-N-C. Electrochemical studies establish that both (Pc)Fe and (phen2N2)Fe have relatively high Fe(III/II) potentials at ~0.6 V, ~300 mV positive of (OEP)Fe. The ORR onset potential is found to directly correlate with the Fe(III/II) potential leading to a ~300 mV positive shift in the onset of ORR for (Pc)Fe and (phen2N2)Fe relative to (OEP)Fe. Consequently, the ORR onset for (phen2N2)Fe and (Pc)Fe is within 150 mV of Fe-N-C. Unlike (OEP)Fe and (Pc)Fe, (phen2N2)Fe displays excellent selectivity for 4-electron ORR with 2O2 production, comparable to Fe-N-C materials. The aggregate spectroscopic and electrochemical data establish (phen2N2)Fe as a pyridinic iron macrocycle that effectively models Fe-N-C active sites, thereby providing a rich molecular platform for understanding this important class of catalytic materials.</p

A pyridinic Fe-N4 macrocycle models the active sites in Fe/N-doped carbon electrocatalysts

© 2020, The Author(s). Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum catalysts for the oxygen reduction reaction (ORR) in fuel cells; however, their active site structures remain poorly understood. A leading postulate is that the iron-containing active sites exist primarily in a pyridinic Fe-N4 ligation environment, yet, molecular model catalysts generally feature pyrrolic coordination. Herein, we report a molecular pyridinic hexaazacyclophane macrocycle, (phen2N2)Fe, and compare its spectroscopic, electrochemical, and catalytic properties for ORR to a typical Fe-N-C material and prototypical pyrrolic iron macrocycles. N 1s XPS and XAS signatures for (phen2N2)Fe are remarkably similar to those of Fe-N-C. Electrochemical studies reveal that (phen2N2)Fe has a relatively high Fe(III/II) potential with a correlated ORR onset potential within 150 mV of Fe-N-C. Unlike the pyrrolic macrocycles, (phen2N2)Fe displays excellent selectivity for four-electron ORR, comparable to Fe-N-C materials. The aggregate spectroscopic and electrochemical data demonstrate that (phen2N2)Fe is a more effective model of Fe-N-C active sites relative to the pyrrolic iron macrocycles, thereby establishing a new molecular platform that can aid understanding of this important class of catalytic materials

A pyridinic Fe-N4 macrocycle models the active sites in Fe/N-doped carbon electrocatalysts

Iron- and nitrogen-doped carbon materials are effective catalysts for the oxygen reduction reaction whose active sites are poorly understood. Here, the authors establish a new pyridinic iron macrocycle complex as a more effective active site model relative to legacy pyrrolic model complexes