Understanding the Anodic Mechanism of a Seafloor Fuel Cell: Interactions Between Geochemistry and Microbial Activity |
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Authors: | Natacha?Ryckelynck III" target="_blank">Hilmar?A?StecherIII Email author" target="_blank">Clare?E?ReimersEmail author |
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Institution: | (1) College of Oceanic and Atmospheric Sciences, Oregon State University, 104 Ocean Admin. Bldg., 97331-5503 Corvallis, OR, USA;(2) College of Oceanic and Atmospheric Sciences, Oregon State University, Hatfield Marine Science Center, Newport, OR 97365, USA |
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Abstract: | Seafloor fuel cells made with graphite electrodes generate electricity by promoting electron transfer in response to a natural
voltage difference (−0.7 to −0.8 V) between anoxic sediments and overlying oxic seawater. Geochemical impacts of a seafloor
fuel cell on sediment solids and porewaters were examined to identify the anodic mechanisms and substrates available for current
production. In an estuarine environment with little dissolved sulfide, solid-phase acid volatile sulfide and Cr2+-reducible sulfur minerals decreased significantly toward the anode after 7 months of nearly continuous energy harvesting.
Porewater iron and sulfate increased by millimolar amounts. Scanning electron microscope images showed a biofilm overcoating
the anode, and electron microprobe analyses revealed accumulations of sulfur, iron, silicon and phosphorus at the electrode
surface. Sulfur deposition was also observed on a laboratory fuel cell anode used to generate electricity with only dissolved
sulfide as an electron donor. Moreover, current densities and voltages displayed by these purely chemical cells were similar
to the values measured with field devices. These results indicate that electron transfer to seafloor fuel cells can readily
result in the oxidation of dissolved and solid-phase forms of reduced sulfur producing mainly S0 which deposits at the electrode surface. This oxidation product is consistent with the observed enrichment of bacteria most
closely related to Desulfobulbus/Desulfocapsa genera within the anode biofilm, and its presence is proposed to promote a localized biogeochemical cycle whereby biofilm
bacteria regenerate sulfate and sulfide. This electron-shuttling mechanism may co-occur while these or other bacteria use
the anode directly as a terminal electron acceptor. |
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Keywords: | Anoxic sediments Energy production Fuel cell Microbial activity Pyrite Sulfur |
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