Proton Coupled Electron Transfer Reactions at the Surface of Metal Oxide Nanomaterials

Thesis (Ph.D.)--University of Washington, 2015-10Nanostructured metal oxide materials are found in many products and processes in our society today, but they play a particularly important role in the conversion and storage of energy. The materials are used as catalysts and redox active supports in devices such as dye sensitized solar cells, solid oxide fuel cells, and flow batteries, where they transfer and store electrons and charge balancing cations. Oftentimes electron transfer is modulated by the cations and when the cation is a proton, these redox reactions are known as proton coupled electron transfer (PCET) reactions. The work described in this dissertation focuses on understanding the PCET reactivity of nanocrystalline metal oxide materials. Chapter 1 introduces the concept of PCET and provides background information on the zinc oxide (ZnO) nanocrystals (NCs) which the majority of the research is focused on. Chapter 2 examines the chemistry that occurs during the photoreduction of ZnO NCs. Chapter 3 describes experiments probing how ZnO NC capping ligand concentration and NC size modulate PCET reaction rates. Chapter 4 describes experiments that compare the PCET reactivity of ZnO NCs with different numbers of electrons and protons stored on them. Chapter 5 describes attempts to observe the electrochemical reduction of ZnO NCs attached to gold electrodes. Finally, Chapter 6 contains attempts to identify a nanostructured metal oxide alkane oxidation catalyst for use in fuel cell

Effect of Protons on the Redox Chemistry of Colloidal Zinc Oxide Nanocrystals

Electron transfer (ET) reactions of colloidal 3–5 nm diameter ZnO nanocrystals (NCs) with molecular reagents are explored in aprotic solvents. Addition of an excess of the one-electron reductant Cp*₂Co (Cp* = pentamethylcyclopentadienyl) gives NCs that are reduced by up to 1–3 electrons per NC. Protons can be added stoichiometrically to the NCs by either a photoreduction/oxidation sequence or by addition of acid. The added protons facilitate the reduction of the ZnO NCs. In the presence of acid, NC reduction by Cp*₂Co can be increased to over 15 electrons per NC. The weaker reductant Cp*₂Cr transfers electrons only to ZnO NCs in the presence of protons. Cp*₂M⁺ counterions are much less effective than protons at stabilizing reduced NCs. With excess Cp*₂Co or Cp*₂Cr, the extent of reduction increases roughly linearly with the number of protons added. Some of the challenges in understanding these results are discussed

High-Performance Oligomeric Catholytes for Effective Macromolecular Separation in Nonaqueous Redox Flow Batteries.

Nonaqueous redox flow batteries (NRFBs) represent an attractive technology for energy storage from intermittent renewable sources. In these batteries, electrical energy is stored in and extracted from electrolyte solutions of redox-active molecules (termed catholytes and anolytes) that are passed through an electrochemical flow cell. To avoid battery self-discharge, the anolyte and catholyte solutions must be separated by a membrane in the flow cell. This membrane prevents crossover of the redox active molecules, while simultaneously allowing facile transport of charge-balancing ions. A key unmet challenge for the field is the design of redox-active molecule/membrane pairs that enable effective electrolyte separation while maintaining optimal battery properties. Herein, we demonstrate the development of oligomeric catholytes based on tris(dialkylamino)cyclopropenium (CP) salts that are specifically tailored for pairing with size-exclusion membranes composed of polymers of intrinsic microporosity (PIMs). Systematic studies were conducted to evaluate the impact of oligomer size/structure on properties that are crucial for flow battery performance, including cycling stability, charge capacity, solubility, electron transfer kinetics, and crossover rates. These studies have led to the identification of a CP-derived tetramer in which these properties are all comparable, or significantly improved, relative to the monomeric counterpart. Finally, a proof-of-concept flow battery is demonstrated by pairing this tetrameric catholyte with a PIM membrane. After 6 days of cycling, no crossover is detected, demonstrating the promise of this approach. These studies provide a template for the future design of other redox-active oligomers for this application

Effect of the Backbone Tether on the Electrochemical Properties of Soluble Cyclopropenium Redox-Active Polymers

Few reports to date have focused on the chemical and electrochemical reversibility of redox pendants assembled into soluble redox-active polymers (RAPs). Here we report a series of soluble RAPs for flow battery applications designed with cyclopropenium (CP) pendants. The tether length between CP and a polystyrene backbone was varied and found to influence electrochemical activity and stability. Different tether lengths of <i>x</i> methylene groups (<i>x</i> = 1–7) were simulated, and <i>x</i> = 1, 5, and 7 were synthesized to evaluate experimentally. This study illustrates that polymers with extended tether groups display an improved reversibility in cyclic voltammetry. The behavior is mirrored in the stability of the charged state tested in galvanostatic half-cells. When paired with a viologen polymer, these CP-based polymers produce a 1.55 V nonaqueous flow battery. The capacity decays for the polymers were structure-dependent, which provides empirical insight into materials design for high potential catholyte polymers

Designing Redox-Active Oligomers for Crossover-Free, Nonaqueous Redox-Flow Batteries with High Volumetric Energy Density

Here we show how to design organic redox-active solutions for use in redox-flow batteries, with an emphasis on attaining high volumetric capacity electrodes that minimize active-material crossover through the flow cell’s membrane. Specifically, we advance oligoethylene oxides as versatile core motifs that grant access to liquid redox-active oligomers having infinite miscibility with organic electrolytes. The resulting solutions exhibit order-of-magnitude increases in volumetric capacity and obviate deleterious effects on redox stability. The design is broadly applicable, allowing both low potential and high potential redox centers to be appended to these core motifs, as demonstrated by benzofurazan, nitrobenzene, 2,2,6,6-tetramethylpiperidin-1-yl)­oxyl, and 2,5-di-<i>tert</i>-butyl-1-methoxy-4-(2′-methoxy)­benzene pendants, whose reduction potentials range from −1.87 to 0.76 V vs Ag/Ag⁺ in acetonitrile. Notably, the oligoethylene oxide scaffold minimizes membrane crossover relative to redox-active small molecules, while also providing mass- and electron-transfer kinetic advantages over other macromolecular architectures. These characteristics collectively point toward new opportunities in grid-scale energy storage using all-organic redox-flow batteries

Lectin Chromatography/Mass Spectrometry Discovery Workflow Identifies Putative Biomarkers of Aggressive Breast Cancers

We used a lectin chromatography/MS-based approach to screen conditioned medium from a panel of luminal (less aggressive) and triple negative (more aggressive) breast cancer cell lines (<i>n</i> = 5/subtype). The samples were fractionated using the lectins <i>Aleuria aurantia</i> (AAL) and <i>Sambucus nigra</i> agglutinin (SNA), which recognize fucose and sialic acid, respectively. The bound fractions were enzymatically <i>N</i>-deglycosylated and analyzed by LC–MS/MS. In total, we identified 533 glycoproteins, ∼90% of which were components of the cell surface or extracellular matrix. We observed 1011 glycosites, 100 of which were solely detected in ≥3 triple negative lines. Statistical analyses suggested that a number of these glycosites were triple negative-specific and thus potential biomarkers for this tumor subtype. An analysis of RNaseq data revealed that approximately half of the mRNAs encoding the protein scaffolds that carried potential biomarker glycosites were up-regulated in triple negative vs luminal cell lines, and that a number of genes encoding fucosyl- or sialyltransferases were differentially expressed between the two subtypes, suggesting that alterations in glycosylation may also drive candidate identification. Notably, the glycoproteins from which these putative biomarker candidates were derived are involved in cancer-related processes. Thus, they may represent novel therapeutic targets for this aggressive tumor subtype

Lectin Chromatography/Mass Spectrometry Discovery Workflow Identifies Putative Biomarkers of Aggressive Breast Cancers

We used a lectin chromatography/MS-based approach to screen conditioned medium from a panel of luminal (less aggressive) and triple negative (more aggressive) breast cancer cell lines (<i>n</i> = 5/subtype). The samples were fractionated using the lectins <i>Aleuria aurantia</i> (AAL) and <i>Sambucus nigra</i> agglutinin (SNA), which recognize fucose and sialic acid, respectively. The bound fractions were enzymatically <i>N</i>-deglycosylated and analyzed by LC–MS/MS. In total, we identified 533 glycoproteins, ∼90% of which were components of the cell surface or extracellular matrix. We observed 1011 glycosites, 100 of which were solely detected in ≥3 triple negative lines. Statistical analyses suggested that a number of these glycosites were triple negative-specific and thus potential biomarkers for this tumor subtype. An analysis of RNaseq data revealed that approximately half of the mRNAs encoding the protein scaffolds that carried potential biomarker glycosites were up-regulated in triple negative vs luminal cell lines, and that a number of genes encoding fucosyl- or sialyltransferases were differentially expressed between the two subtypes, suggesting that alterations in glycosylation may also drive candidate identification. Notably, the glycoproteins from which these putative biomarker candidates were derived are involved in cancer-related processes. Thus, they may represent novel therapeutic targets for this aggressive tumor subtype

Lectin Chromatography/Mass Spectrometry Discovery Workflow Identifies Putative Biomarkers of Aggressive Breast Cancers

We used a lectin chromatography/MS-based approach to screen conditioned medium from a panel of luminal (less aggressive) and triple negative (more aggressive) breast cancer cell lines (<i>n</i> = 5/subtype). The samples were fractionated using the lectins <i>Aleuria aurantia</i> (AAL) and <i>Sambucus nigra</i> agglutinin (SNA), which recognize fucose and sialic acid, respectively. The bound fractions were enzymatically <i>N</i>-deglycosylated and analyzed by LC–MS/MS. In total, we identified 533 glycoproteins, ∼90% of which were components of the cell surface or extracellular matrix. We observed 1011 glycosites, 100 of which were solely detected in ≥3 triple negative lines. Statistical analyses suggested that a number of these glycosites were triple negative-specific and thus potential biomarkers for this tumor subtype. An analysis of RNaseq data revealed that approximately half of the mRNAs encoding the protein scaffolds that carried potential biomarker glycosites were up-regulated in triple negative vs luminal cell lines, and that a number of genes encoding fucosyl- or sialyltransferases were differentially expressed between the two subtypes, suggesting that alterations in glycosylation may also drive candidate identification. Notably, the glycoproteins from which these putative biomarker candidates were derived are involved in cancer-related processes. Thus, they may represent novel therapeutic targets for this aggressive tumor subtype