Electron Mediator Immobilized Carbon Electrodes And Their Applications In Microbial Fuel Cells

The microbial fuel cell (MFC) is a promising device that directly converts the chemical energy stored in biomass into electricity through the catalytic capabilities of microorganisms (bacteria). Compared with the noble metal catalysts used in conventional chemical fuel cells, microbes are not only abundant in nature, but mostly have versatile metabolic abilities to oxidize organic compounds from natural hydrocarbons to domestic and industrial wastes. Therefore, MFCs offer simultaneous electrical energy generation and waste disposal, making them a competitive solution to the worldwide crisis on energy shortage and environmental damage. Currently, the slow electron transfer rate between microbes and electrodes is among the most serious problems for MFCs to become a practically viable technology. This thesis aims to improve electron transfer rate between microbes and electrodes by immobilizing redox mediator on the anode of MFC. The main research contents and results are summarized as follows:A novel chamber-less microbial fuel cell with a continuous-flow structure was constructed to study the acclimation process of sewage strains and the optimization of anode materials. The results indicate that COD load of the culture medium, anode compositions and the discharging current can significantly affect the performance of MFC. In the initial acclimation period, it is unsuitable to use a high COD load and high current, otherwise the normal metabolism of microbes would be disturbed which results in the loss of electrical activity; the graphite/PTFE composite film with 20% PTFE showed the best anode performance for its compatible pore size with the body size of microbes and high porosity.Considering the excellent redox activity, bio-compatibility, and the comparable redox potential to NAD+/NADH (-320mV vs. NHE) which render metabolic electrons to be harvested at the top of respiration chain at negative potential, neutral red (NR) was chosen as a favorable electron-transfer mediator between MFC anode and microbial catalyst. We develop three approaches to immobilize NR on carbon electrodes to construct effective catalytic bio-electrodes: (1) Covalent immobilization of NR on carbon electrode through a stepwise amidation procedure. The -COOH groups were first produced on the surface of carbon electrode by oxidation, and then transferred into acylchlorides. Finally, acylamide bonds are formed through the reaction between the acylchlorides on carbon surface and the amine groups of NR molecules. It is shown that, the present stepwise amidation procedure can significantly increase the amounts of NR molecules immobilized on carbon surface without altering its redox reversibility. In the meantime, significantly improved MFC performance is also achieved by using NR-modified carbon anode fabricated with the present stepwise amidation procedure. For instance, the maximum current density and power density have been increased from 1300 mA m-2 and 620 mW m-2 to 1500 mA m-2 and 900 mW m-2 respectively.(2) Covalent immobilization of NR on synthetic hydrogel. A hydrogel polymer with side chains is prepared by reacting poly-4-vinylpyridine (PVP) backbone with 6-bromohexanoic acid. The resulted hexanoic-acid side chains in the hydrogel polymer are then modified with NR through a stepwise amidation procedure to form a redox hydrogel. The NR-PVP hydrogel exhibits excellent redox activity and favorable redox potential compatible with NAD+/NADH. The carbon electrodes modified with thus prepared redox hydrogel exhibit more negative anode potential (-0.460 mV) and higher fuel utilization (94%) in MFC than the ordinary carbon electrode (-0.355 mV and 65%). Accordingly, the current density and power density of MFC are also increased to 1550 mA m-2 and 950 mW m-2 respectively.(3) Embedding NR into natural hydrogel composites. The alginate-cellulose composite films embedded with NR are formed on carbon electrodes through layer-by-layer electrostatic cross-link and flocculation processes. The prepared composite films of natural hydrogels possess not only excellent biocompatibility to accommodate microbes, but good redox activity to mediate electron transfer from microbial cells to microbial cells and from microbial cells to the electrodes. The voltammetric measurements showed that NR molecules are firmly embedded in the composite hydrogel films, and exhibit excellent redox reversibility. The hydrogel modified anodes give rather long discharge time with more negative anode potential, and therefore much higher current and power densities (1700 mA m-2 and 1150 mW m-2 respectively). In addition, the composite hydrogel film modified glassy-carbon (GC) electrode also exhibits excellent electrochemical activity in E. Coli solution which shows no electrochemical activity on the ordinary GC electrode, therefore showing a broad prospect in the application of biosensors.