Document Type

Dissertation

Date of Degree

Spring 2015

Degree Name

PhD (Doctor of Philosophy)

Degree In

Chemistry

First Advisor

Johna Leddy

Abstract

Electroanalytical techniques are used to investigate mass transport through density gradient films; lanthanide triflate reduction and oxidation in a Nafion/acetonitrile matrix; and magnetic field effects on hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and lanthanide electrochemistry.

Graded density films are more dense at the electrode surface and become less dense out into solution due to a brush polymer structure. Fick's second law expands to account for a diffusion coefficient that varies with distance x normal to the electrode surface. Confocal microscopy, cyclic voltammetry, and computer simulations are used to investigate density graded Ficoll® films. Mass transport approaches steady state (scan rate independence) at slow scan rates where the diffusion length samples the entire film. The use of Ficoll to template an ion exchange polymer is explored by casting Nafion® Ficoll composites.

Lanthanide electrochemistry is enabled in acetonitrile at a Nafion modified platinum electrode in the presence of triflate ligands. Formal potentials are shifted into the voltage window of acetonitrile accessible due to triflate complexation. The Nafion further solubilizes the compounds. The mechanism (ECEC) is studied with cyclic voltammetry and x-ray photoelectron spectroscopy.

Magnetic field effects on electrochemical systems have been of interest to researchers for the past 65 years. Mass transport effects, such as magnetohydrodynamics and magnetic field gradient effects have been reported, but the Leddy group focuses on electron transfer effects. Electrode surfaces are modified with composite films of magnetic microparticles suspended in ion exchange polymer Nafion. Effects are verified to be electron transfer related and due to the magnetization of chemically inert microparticles. The magnets catalyze the rates of important electron transfer reactions such as hydrogen evolution and oxygen reduction.

Magnetic field effects on HER at various noncatalytic metal electrodes are explored with linear scan voltammetry. There is a correlation between the magnetic susceptibility of the electrode metals and the HER exchange currents (reaction rates). Exchange currents are 103× larger for a paramagnetic metal electrode than a diamagnetic one with the same work function. The overpotential at diamagnetic electrodes is decreased by modification with a Nafion + magnetic microparticle composite film. A decrease in overpotential of ∼70 % for all electrodes except platinum is observed. The overpotential decrease correlates with the magnetic susceptibility of the particles.

Magnets can enhance differences between lanthanide cyclic voltammograms by shifting current densities at a given potential and enhancing current based on the number of 4f electrons and magnetic moment of each lanthanide ion.

Magnetic field effects on ORR in acetonitrile are investigated with cyclic voltammetry. In aprotic solvents, ORR proceeds by a one electron transfer reaction to paramagnetic O2.. Enhanced reversibility and electron transfer kinetics are observed as well as a decrease in overpotential of ∼100 mV. Magnetic field effects on ORR in a lanthanide triflate solution are also examined. Electron transfer kinetics and reversibility are further enhanced in the presence of lanthanide triflate.

Public Abstract

Electrochemical reactions are processes that involve the transfer of an electron. These reactions are critical to the operation of many common devices such as batteries and fuel cells. Electrochemical techniques such as cyclic voltammetry evaluate how fast the electrons and molecules move (electron transfer and mass transport, respectively). Here, several systems are electrochemically investigated.

Lanthanides are heavy elements generated as nuclear waste products decay. Lanthanides are nonradioactive and can be recycled for applications in lasers, medical imaging, and high power magnets. When magnets are added to the electrode, enhanced electron transfer rates are observed as increases in current and decreases in the energy required to drive the reaction.

In energy technologies, hydrogen evolution reaction (HER) generates hydrogen as a fuel and the oxygen reduction reaction (ORR) drives metal air batteries and fuel cells. Electrochemical systems generate H2 and O2 by splitting water and consume H2 and O2 as fuels that provide clean (no pollutants) sources of renewable energy. Metal air batteries have significantly more inherent energy than lithium ion batteries. Addition of magnetic microparticles to the electrodes increases current (rates) of HER and ORR.

Ficoll® is a polymer that forms a graded density …lm, where the …lm is more viscous at one edge, then gradually becomes less viscous. When molecules move through the …lm along this gradient, they can be delivered to the most dense side at a steady, fixed rate. This has applications for coatings and stabilizers and steady delivery of pharmaceuticals such as time release hormones, insulin, nicotine, and mood enhancers. Microscopy, cyclic voltammetry, mathematical equations, and computer simulations are used to explore the properties of these films. Methods to make films with density gradients out of other materials are also explored.

This work contributes to important technologies in energy generation and storage, nuclear waster remediation, and the time release of pharmaceuticals.

Keywords

publicabstract, Catalysis, Density Gradient, Electrochemistry, Films, Lanthanide, Magnets

Pages

xxiv, 205 pages

Bibliography

Includes bibliographical references (pages 201-205).

Copyright

Copyright 2015 Krysti Lynn Knoche

Included in

Chemistry Commons

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