Understanding the anodic mechanism of a seafloor fuel cell Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/b2773z26m

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  • Anoxic sediment overlain by oxic seawater establishes a voltage gradient on the order of 0.7-0.8V across the sediment-water interface. This study follows Reimers et al. (2001) and Tender et al. (2002) who reported the development of a seafloor fuel cell to harvest electrical energy from this potential difference. Prototype fuel cells were deployed for demonstration purposes in the Yaquina Bay estuary, Newport (Oregon, USA) and in a salt marsh near Tuckerton (New Jersey, USA) and were monitored during approximately seven months at either fixed current or fixed potential. Control cells with electrodes not connected to each other (open circuit) were deployed at each site near the prototype devices. The impacts of fuel cell processes on sediment solids and porewaters were studied by taking cores of sediment down to the surface of the anode (electrode embedded in sediment) of both active and control cells. Porewater profiles showed significant increases in sulfate and iron concentrations, but also sulfide depletion approaching the active anode. Solid-phase acid volatile sulfide and pyrite decreased significantly toward the anode. Fe(III) mineral phases did not appear affected by the presence of the fuel cell. Particulate organic carbon was not depleted significantly either. Electron microprobe analyses and SEM images revealed accumulations of sulfur and iron with Fe/S ratios <1 at the electrode surface of the fuel cell anode. Sulfur deposition was also observed on electrodes simulating a marine fuel cell, under sterile conditions, using US- as sole electron acceptor. Moreover current densities and voltages displayed at both anodes and cathodes in these laboratory experiments were similar to the values measured with the fuel cell devices in the field. Collectively these results indicate that electron transfer processes at the anodes of seafloor fuel cells result in the oxidation of dissolved and solid-phase forms of reduced sulfur in sediments producing mainly S⁰ which deposits at the electrode surface. This oxidation product enhances a localized biogeochemical cycle involving biofilm bacteria that may regenerate sulfate and sulfide. This mechanism may sustain electron transfer processes or co-occur while other biofilm bacteria use the anode directly as a terminal electron acceptor.
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