Bioelectrochemical films on electrodes for application to biofuel cells
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This thesis focuses on studies of both enzymatic (EFC) and microbial (MFC) fuel cells. In the enzymatic fuel cell studies, interactions between enzymes, osmium based complexes and redox polymers are characterized electrochemically. These enzymes and redox polymer films are immobilizedby either physisorption of covalent anchoring at electrodesandtheir response investigated for operation as enzymatic fuel cells with a view to enhancing the power output and stability of such assemblies. Microbial fuel cell studies deal with the electrochemical characterization of model organism,Geobacter sulfurreducens and Rhodoferax ferrireducens,biofilms on electrodes that are induced to grow under fixed applied potential. Chapter 1 introduces the enzymes, osmium based complexes and redox polymers, immobilization strategies for EFC operation and microbial electrocatalysis of electroactive bacteria. Chapter 2 uses cyclic voltammetry to extract parameters for solution phase mediators, and their interaction with glucose oxidase,to permit screening of redox mediators for their suitabilityfor glucose oxidation by glucose oxidase in anodes of an EFC. Osmium complexes with polpyridyl ligands exhibiting a wide range of redox potentials from -0.24 V to 0.43 V vs. Ag/AgCl were examined as mediators, and CV was used to estimate pseudo-first and second order rate constants for mediation of glucose oxidase oxidation of glucose. The rate constants increased with increase indifference between the redox potential of glucose oxidase and the mediators.Although osmium complexeswith Eo¿above 0.2 V vs.Ag/AgCl showed high rate constantsfor reactionwith glucose oxidase, complexes with lower rate constants and redox potentials, such as Os[(4,4¿-dimethoxy-2,2¿-bipyridine)(4-aminoethyl pyridine)Cl)]PF6 with Eo¿= +0.025 V vs. Ag/AgCl were selectedfor further study in anodes of an EFC. This was to provide a compromise between current density and anode potential. Operation of a solution phase, Nafion separated, EFCusing a laccase-based cathode, revealed that selection of appropriate mediators can decrease anode overpotential. Chapter 3 investigates an immobilization strategy for enzymes in redox hydrogels as carbon electrodes, asimmobilization can eliminate the need for membrane separators. Glassy carbon and graphite electrodes were modified with films of enzyme and osmium redox polymer, cross linked with poly(ethylene glycol)diglycidyl ether, and used for elaboration of a glucose/O2 enzymatic fuel cell (EFC). The redox polymers [Os(4,4¿-dimethoxy-2,2¿-bipyridine)2(polyvinylimidazole)10Cl]+ and [Os(4,4¿-dichloro-2,2¿-bipyridine)2(polyvinylimidazole)10Cl]+ were selected to facilitate transfer of electrons from the glucose oxidase active site to the T1 Cu site of Trametes hirsuta laccase and Myrothecium verrucaria bilirubin oxidase. Using graphite resulted in an increased redox polymer loading, and as a consequence increased current densities, leading to a maximum power output of 43 µW cm-2 at 0.25 V under physiological conditions for assembled EFCs. Improved stabilization of biofilms was achieved through covalent anchoring of enzyme and redox polymer on graphite electrodes, derivatized via electrochemical reduction of the diazonium cation generated in situ from p-phenylenediamine, using the di-epoxide cross linker. In order to investigate means to improve current and power densities, chapter 4 focused onLayer-by-layer (LBL) assembly of alternate osmium redox polymer and enzymes at graphite electrode to attempt to increase film component loading, and thus current densities. The same redox polymers used in chapter 3 were selected for anode and cathode. A bilayer assembly (two layers each) of redox polymer and enzymes, glucose oxidase for anode and Trametes hirsuta laccase for cathode, produced an EFC with a maximum power density of 103 µWcm-2 at pH 5.5 and 40 µWcm-2 at pH 7.4, with power limited by the acidophilic laccase at pH 7.4. In chapter 5, an alternate immobilization strategy, based on anchoring of apoly (L-Lysine) layer on oxidized pyrolytic graphite electrodes to provide support for cross linking of redox polymer and enzyme, was investigated for application to membrane-less EFC assembly. The modified PG electrodes showed improved catalytic currents for both glucose oxidation and oxygen reduction, using redox polymers and glucose oxidase or bilirubin oxidase, compared to PG electrodes with physisorbed films of cross linked enzyme and redox polymer. Operational stability for glucose oxidation with [Os(4,4¿-dimethoxy-2,2¿-bipyridine)2(polyvinylimidazole)10Cl]+(Med1)/glucose oxidase films and [Os(2,2¿-bipyridine)2(polyvinylimidazole)10Cl]+(Med3)/bilirubin oxidasefilms for oxygen reduction at physiological buffer conditions and37 °C were evaluated independentlyfor modified and control PG electrodes. Enzyme electrodes were cured for 24 h and 48h, in order to check this influence over catalytic current and stability. Assembled EFCs produced a maximum power density of 96 µW cm-2 at 0.33 V under these conditions, withoperational stability studies revealing that the limiting factor for power loss being the decrease in anode redox polymer signal. There are only a few bacteria reported to be capable of transferring electrons directly to electrodes while oxidizing substrate. Chapter 6focused on the use of cyclic voltammetry and chronoamperometryto probe the response ofGeobacter sulfurreducens and Rhodoferax ferrireducens to applied potentials. Growth of biofilms over carbon electrodesat fixed applied potentials yielded a bioelectrocatalytic response to acetate oxidation. Electrodes were held at different applied potentialsto induce for Geobacter sulfurreducens orRhodoferax ferrireducens biofilm growth from culture media that did not contain a natural electron acceptor for the bacteria. This study is the first such study on the Rhodoferax ferrireducens electroactive bacteria. The difference in cyclic voltammograms and electron micrographs recorded for biofilms of the two bacteria highlighted differences in their rates of growth under applied potentials. Chapter 7 summarizes the main research findings and possible future research directions are presented.
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