Polymerized methylene blue films have been shown to be efficient electron mediators for biological redox compounds. Previously these films have been deposited on noble metal and carbon substrates. In this work, methylene blue has been polymerized on stainless steel by first applying a layer of an electronically conductive polymer to inhibit passivation or corrosion of the metal and to increase the active surface area. The redox activity of the resulting electrodes has been examined by electrochemical impedance spectroscopy and fit to an equivalent circuit model for conductive polymers. The film with methylene blue shows a large decrease in charge transfer resistance, from 130 Ω to 15 Ω, when compared to the bare polypyrrole film. The stability of the films was studied by measuring the impedance changes over time when stored in aqueous solutions. Degradation of the films was found to depend mostly on the doping ion used in the polypyrrole synthesis. NADH oxidation was also measured at different concentrations by CV.
Different configurations of anodic and cathodic half-cells were incorporated into a microbial fuel cell to determine the effectiveness of a composite electrode. This novel composite electrode consisted of poly(methylene blue) and polypyrrole electrodeposited onto a stainless steel electrode. The novel electrode/immobilized mediator was incorporated into a microbial cathodic half-cell that relied on the microalgae Chlorella vulgaris for photosynthesis, and was a net reducer of carbon dioxide. Similar microbial cathodic half-cells were also examined using electrodes fabricated from graphite and graphite deposited with methylene blue. Results from using these three different electrodes in the microbial cathodic half-cell were examined and compared with the results from others. The electrode using the novel immobilized mediator demonstrated the highest short circuit current density of 65 mA/m 2 when compared with other C. vulgaris systems. Different anodic half-cells were also incorporated into the microbial fuel cell and tested. Anodic half-cells tested included a microbial half-cell containing Saccharomyces cerevisiae and one containing no microbial material and based on purely chemical constituents.In the case of the microbial anodic half-cell, different electrodes, including the novel immobilized mediator/electrode, were tested. It was found that the anodic half-cell performed better with a soluble mediator than an immobilized mediator/electrode. In the case of a fuel cell where both the anodic and cathodic half-cells are microbial, our results demonstrate better performance than previous systems by using a soluble mediator in the anodic half-cell with an immobilized mediator in the cathodic half-cell.
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