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Homogeneous Viologens for Use As Catalysts in Direct Carbohydrate Fuel Cells Brigham Young University BYU ScholarsArchive Theses and Dissertations 2012-07-12 Homogeneous Viologens for Use as Catalysts in Direct Carbohydrate Fuel Cells Dane C. Hansen Brigham Young University - Provo Follow this and additional works at: https://scholarsarchive.byu.edu/etd Part of the Chemical Engineering Commons BYU ScholarsArchive Citation Hansen, Dane C., "Homogeneous Viologens for Use as Catalysts in Direct Carbohydrate Fuel Cells" (2012). Theses and Dissertations. 3647. https://scholarsarchive.byu.edu/etd/3647 This Dissertation is brought to you for free and open access by BYU ScholarsArchive. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Homogeneous Viologens for Use as Catalysts in Direct Carbohydrate Fuel Cells Dane Cameron Hansen A dissertation submitted to the faculty of Brigham Young University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Dean R. Wheeler, Chair Gerald D. Watt William G. Pitt W. Vincent Wilding Randy S. Lewis Department of Chemical Engineering Brigham Young University August 2012 Copyright © 2012 Dane Cameron Hansen All Rights Reserved ABSTRACT Homogeneous Viologens for Use as Catalysts in Direct Carbohydrate Fuel Cells Dane Cameron Hansen Department of Chemical Engineering, BYU Doctor of Philosophy Deriving electrical energy from glucose and other carbohydrates under mild conditions is an important research objective because these biomolecules are abundant, renewable, and can provide 12 to 24 electrons per molecule, yielding substantial electrical power. It was previously observed that disubstituted viologens, salts of N,N’-disubstituted 4,4’-bipyridine, are able to oxidize glucose under alkaline conditions. Building on that initial result, the objective of this work was to understand and quantify the effectiveness and utility of viologens as catalysts for use in direct carbohydrate fuel cells. The extent that viologens oxidize carbohydrates, the conditions under which that oxidation occurs, and the mechanism for the oxidation were examined using oxygen-uptake and other methods. Viologens were found to catalytically oxidize carbohydrates extensively in alkaline solution. Viologens were also found to react with the enediol form of the carbohydrate, initiating carbohydrate oxidation with subsequent reduction of the viologen. If the viologen/carbohydrate ratio is low, electron transfer from the carbohydrate to the viologen becomes limiting and the carbohydrates undergoing oxidation rearrange into unreactive intermediates such as carboxylic acids and alcohols. At high catalyst ratios, excess viologen more rapidly oxidizes the carbohydrate and minimizes formation of unreactive intermediates. We also found that viologen polymers were more efficient than an equivalent concentration of monomers, suggesting that the higher localized concentration in polymeric viologen acts to efficiently oxidize carbohydrates and simulates high viologen/carbohydrate ratios. Monoalkyl viologens, aminoviologens, indigo carmine, and methylene blue were investigated by the method of cyclic voltammetry to inform their use as catalysts in the oxidation of carbohydrates. Redox potentials, diffusion coefficients, and heterogeneous electron-transfer rate constants were determined. Stability in alkaline solution and aqueous solubility were also examined in a semi-quantitative fashion. A comparison between the catalysts was made and viologens were found to be superior based on the examined parameters. The catalytic oxidation of carbohydrates by viologen was also examined using a fuel cell- like device. For the conditions in which a test cell was operated, oxidation efficiencies of up to 33% were observed, compared to previously reported values from about 2.5% to 80%. Anode polarization curves were obtained and used to determine the behavior of the viologen-controlled anode as a function of pH, viologen and carbohydrate concentration, and carbohydrate identity. pH was found to have a stronger effect on the performance at the anode for carbohydrates with a higher number of carbons than those with a lower number. Keywords: viologen, catalysis, carbohydrate oxidation, mechanism, fuel cell, voltammetry ACKNOWLEDGMENTS To my wife, the love of my life. To my children, my joy (most of the time). To my parents; I’m grateful for your love and support all of these years. Thank you Dr. Wheeler, for guiding me. Thank you Dr. Watt, for your generosity. I’m grateful for the many friends I’ve made as a student. Life is much better when you have friends; to teach and to learn from. TABLE OF CONTENTS LIST OF TABLES ....................................................................................................................... ix LIST OF FIGURES ..................................................................................................................... xi 1 Introduction ........................................................................................................................... 1 1.1 Motivation ....................................................................................................................... 1 1.2 Scope of work ................................................................................................................. 2 2 Background ........................................................................................................................... 7 2.1 Introduction ..................................................................................................................... 7 2.2 Fuel cell ........................................................................................................................... 7 2.2.1 Direct carbohydrate fuel cell reactions ....................................................................... 7 2.2.2 Anode .......................................................................................................................... 8 2.2.3 Cathode ....................................................................................................................... 8 2.2.4 Separator and electrolyte ............................................................................................. 9 2.2.5 Available polymer electrolyte membranes ............................................................... 11 2.2.6 Glucose-fuel-cell technology .................................................................................... 13 2.3 Viologen ........................................................................................................................ 15 2.3.1 Viologen electrochemistry ........................................................................................ 16 2.3.2 Anode reactions in a viologen-catalyzed DCFC ....................................................... 17 2.3.3 Viologen immobilization .......................................................................................... 18 2.3.4 Viologen stability ...................................................................................................... 20 2.4 The carbohydrate fuel ................................................................................................... 20 2.5 Coulombic efficiency and voltage efficiency ............................................................... 22 2.6 Experimental ................................................................................................................. 23 2.6.1 Electrochemical test apparatuses and methods ......................................................... 23 v 2.6.2 Working, reference, and counter electrodes.............................................................. 24 2.6.3 Advice for beginning experimentalists ..................................................................... 25 2.7 Summary ....................................................................................................................... 27 3 Initial investigation into the utility of viologens for use as catalysts in direct carbohydrate fuel cells ........................................................................................................ 29 3.1 Introduction ................................................................................................................... 29 3.2 Experimental ................................................................................................................. 31 3.2.1 Materials ................................................................................................................... 31 3.2.2 Synthesis of viologens .............................................................................................. 31 3.2.3 Methods ..................................................................................................................... 32 3.3 Results ........................................................................................................................... 34 3.3.1 Sealed-vial results ..................................................................................................... 34 3.3.2 Sealed-vial, viologen/carbohydrate ratio effect ........................................................ 37 3.3.3 pH dependence .......................................................................................................... 40 3.3.4 Pressure-cell results .................................................................................................
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