Electrochemical Analysis of New Materials for Proton-Exchange Membrane Fuel Cells: Hydrogenated N-Heterocyclic Compounds for Virtual Hydrogen Storage and “Click”-Functionalized Metallocorrole Complexes for Oxygen Reduction
- Author(s): Rubin Shen, Leah Katherine
- Advisor(s): Arnold, John
- et al.
Chapter 1: An Overview of Proton-Exchange Membrane Fuel Cell Research Needs
Proton-exchange membrane (PEM) fuel cells are a promising technology for sustainable energy, having the potential to replace vehicle combustion engines and enable grid-level storage of intermittent renewable energy sources. However, PEM fuel cells are currently fueled by hydrogen, which is difficult to store, transport, and distribute efficiently and safely. They also require precious metal catalysts for both hydrogen oxidation at the anode and oxygen reduction at the cathode. In particular, the platinum cathode catalyst reduces oxygen at a large overpotential, leading to lower overall cell voltage due to kinetic losses. Widespread adoption of PEM fuel cells is therefore limited by both the difficulties of hydrogen storage and distribution and the expense of the catalysts used.
To mitigate these difficulties, new materials for hydrogen storage and oxygen reduction catalysis are needed. This first part of this work explores the use of nitrogen heterocycles for “virtual” hydrogen storage. A virtual hydrogen storage system uses a liquid organic carrier that is electrochemically dehydrogenated to produce protons and electrons separately, allowing the compound to behave like hydrogen in the fuel cell environment. The second part of this work studies small molecule oxygen reduction catalysts using first-row transition metals. These catalysts could be an inexpensive alternative to platinum, but must be anchored within the fuel cell in order to work optimally. The catalysts studied herein were thus additionally designed to be covalently attached to an electrode surface.
Chapter 2: Electro-Dehydrogenation of Indoline: Direct Oxidation at the Electrode and Redox Catalysis
Indoline was chosen as a model for studying virtual hydrogen storage feasibility. The direct electrode reactivity of indoline favors a dimerization pathway. However, in the presence of excess base, either an electrochemical-chemical (EC) or chemical-electrochemical (CE) pathway is favored, depending on the strength of the base. The use of redox mediators in conjunction with base lowers the reaction voltage by 250 mV (ferrocene) or 750 mV (decamethylferrocene). Bulk electrolysis studies with ferrocene and imidazole demonstrate 25-35% indole formation, with no dependence on catalyst concentration. However, bulk electrolysis in the presence of decamethylferrocene and 1,1,3,3-tetramethylguanidine produces up to 50% dehydrogenation to indole at 0.13 V vs. NHE, with more indole formation at higher catalyst concentrations.
Chapter 3: Electrochemical Analysis of a Fully Hydrogenated Carbazole as a Virtual Hydrogen Storage Compound
An electrochemical study of N-ethyldodecahydrocarbazole (NEC-H12), a compound that stores 5.7% hydrogen by weight, is presented. This first study of a fully hydrogenated nitrogen heterocycle indicates that the direct electrode oxidation of NEC-H12 involves a slow electron transfer step with a rate-competitive follow-up reaction. Addition of a strong base, 1,1,3,3-tetramethylguanidine, increases the oxidative current up to five-fold. Effective redox catalysis with ferrocene is also demonstrated, lowering the voltage for NEC-H12 oxidation by nearly 0.4 V. Finally, while bulk electrolysis studies of NEC-H12 mostly result in an intractable mixture of products, use of a platinum electrode in dichloromethane results in substantial dehydrogenation, with a pathway that likely goes through N-ethyl-1,2,3,4-tetrahydrocarbazole.
Chapter 4: Oxygen Reduction Catalysis by First-Row Transition Metal Corrole Complexes
Oxygen reduction catalysis is demonstrated with copper, cobalt, and iron corrole complexes. Of these, cobalt(III) and iron(III) complexes of 5,15-bis(pentafluorophenyl)-10-(4-methoxyphenyl)corrole demonstrate good activity for oxygen reduction in solution-phase studies (acidic acetonitrile) and in surface studies using rotating ring-disk voltammetry (aqueous sulfuric acid). The cobalt(III) complex favors two-electron reduction to hydrogen peroxide while the iron(III) complex favors four-electron reduction to water, both of which are consistent with previous literature reports.
Chapter 5: “Clickable” Propargyl- and Azido-Modified Metallocorrole Complexes: Biscorrole Characterization and Covalent Attachment
Electrochemical characterization of “click”-functionalized metallocorrole complexes is presented. Voltammetric studies indicate that addition of “click” functionality to the periphery of the corrole ligand does not affect the redox processes at the metal center, validating the previous study of related complexes for oxygen reduction catalysis. Huisgen azide-alkyne cycloaddition of propargyl- and azide-functionalized metallocorroles results in copper-copper and copper-iron biscorrole complexes, which display electronic independence of the metal centers. Finally, preliminary studies of covalent attachment of copper(III) 5,15-bis(pentafluorophenyl)-10-(4-propargyloxyphenyl)corrole to an azide-functionalized electrode surface are reported.
Conclusions and Future Outlook:
This work describes studies to improve hydrogen storage technologies and oxygen reduction catalysis for PEM fuel cells. Future work recommended in the area of virtual hydrogen storage includes: further elucidation and optimization of electrochemical dehydrogenation pathways, development of catalysts capable of performing proton-electron dehydrogenation, electrochemical study of other nitrogen heterocycles and other hydrogen-carrying fuels such as alcohols, and development of new membrane technologies that can support new fuel types. In the area of “clickable” catalysts, future work includes optimization of surface attachment and comparison of catalysis for covalently-bound vs. physisorbed catalysts. More broadly, this enables the development and study of other click-functionalized metallocorrole complexes with applications in medical imaging, drug delivery, or environmental sensing.