Tubular cathodes provide a method to obtain high surface areas for scaling up microbial fuel cells (MFCs), but the importance of the cathode shape is not known. We therefore examined power production using cathodes in various configurations (tubes or flat). The MFC with a single internal carbon cloth tube cathode (71 W/m3) produced more power than previously obtained with an ultrafiltration membrane (8 W/m3) due to the better performance of carbon material. This power density was slightly less than that of a flat carbon cloth cathode (81 W/m3; 88 m2/m3) due to the lower total surface area of the tube (68 m2/m3) and not as a result of the tubular cathode shape. Adding a second tube increased power (83 W/m3) in proportion to specific surface area (93 m2/m3). Wrapping the cathode completely around the anode formed a fully tubular MFC (external tubular reactor) with a higher surface area that produced 128 W/m3. Volumetric power density was highly correlated with cathode specific surface area (R2 = 0.93, p = 0.008) and did not depend on the cathode shape (tubes, completely tubular, or flat). Thus, future MFC designs should focus on increasing cathode specific surface area.

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