Scale-up of sediment microbial fuel cells (SMFCs) is important to generating practical levels of power for undersea devices. Sustained operation of many sensors and communications systems require power in the range of 0.6 mW to 20 W. Small scale SMFC systems evaluated primarily in the laboratory indicate power densities for typical graphite plate anodes on the order of 10-50 mW m 22 . However, previous work also suggests that SMFC power production may not scale directly with size. Here, we describe a combination of lab and field studies to evaluate scale up for carbon fabric anodes with a projected surface area ranging from 25 cm 2 to 12 m 2 . The results indicate that power generation scales almost linearly with anode size up to about 1-2 m 2 of projected surface area. Our model suggests that anodes larger than this can experience significant reduction in power density, confirming laboratory observations. These results suggest that the majority of losses along the anode surface occur closest to the electronics, where the amount of current passing along an anode is the greatest. A multi-anode approach is discussed for SMFCs, suggesting that scale-up can be achieved using segmented anode arrays.

In this study, multiple reaction mechanisms in cathodes of sediment microbial fuel cells (SMFCs) were characterized by using cyclic voltammetry and microelectrode measurements of dissolved oxygen and pH. The cathodes were acclimated in SMFCs with sediment and seawater from San Diego Bay. Two limiting current regions were observed with onset potentials of approximately +400 mVAg/AgCl for limiting current I and -120 mVAg/AgCl for limiting current II. The appearance of two catalytic waves suggests that multiple cathodic reaction mechanisms influence cathodic performance. Microscale oxygen concentration measurements showed a zero surface concentration at the electrode surface for limiting current II but not for limiting current I, which allowed us to distinguish limiting current II as the conventional oxygen reduction reaction and limiting current I as a currently unidentified cathodic reaction mechanism. Microscale pH measurements further confirmed these results.

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Benthic microbial fuel cells (BMFCs) are a class of bioelectrochemical systems (BESs) that are an emerging technology under development for applications of persistent ocean monitoring. BMFCs function via anaerobic bacteria metabolizing organic matter in sediment and providing electrons to buried anodes. These anodes are electrically connected via an external circuit to an aerobic cathode positioned in the water column. Previous projects by this same group involved largescale deployments that demonstrated deployment of BMFC systems via a boat-towed sled. These long carpet like anodes (termed "Magic Carpet") have been shown to generate power densities of 5-10 mW m-2 in systems ranging from 10-40 m 2 systems. A major challenge in the field remains scaling up these systems to generate practical levels of power. This paper evaluates two iterations of segmented 20 m 2 Magic Carpet BMFC systems and compares their long-term performance.

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