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Oxygen-reducing biocathodes previously developed for microbial fuel cells (MFCs) have required energy-intensive aeration of the catholyte. To avoid the need for aeration, the ability of biocathodes to function with passive oxygen transfer was examined here using air cathode MFCs. Two-chamber, air cathode MFCs with biocathodes produced a maximum power density of 554 ± 0 mW/m², which was comparable to that obtained with a Pt cathode (576 ± 16 mW/m²), and 38 times higher than that produced without a catalyst (14 ± 3 mW/m²). The maximum current density with biocathodes in this air-cathode MFC was 1.0 A/m², compared to 0.49 A/m² originally produced in a two-chamber MFC with an aqueous cathode (with cathode chamber aeration). Single-chamber, air-cathode MFCs with the same biocathodes initially produced higher voltages than those with Pt cathodes, but after several cycles the catalytic …