| Abstract
| - Microbial fuel cells (MFCs) are typically designed as atwo-chamber system with the bacteria in the anodechamber separated from the cathode chamber by apolymeric proton exchange membrane (PEM). Most MFCsuse aqueous cathodes where water is bubbled with airto provide dissolved oxygen to electrode. To increase energyoutput and reduce the cost of MFCs, we examinedpower generation in an air-cathode MFC containing carbonelectrodes in the presence and absence of a polymericproton exchange membrane (PEM). Bacteria presentin domestic wastewater were used as the biocatalyst,and glucose and wastewater were tested as substrates.Power density was found to be much greater than typicallyreported for aqueous-cathode MFCs, reaching a maximumof 262 ± 10 mW/m2 (6.6 ± 0.3 mW/L; liquid volume)using glucose. Removing the PEM increased the maximumpower density to 494 ± 21 mW/m2 (12.5 ± 0.5 mW/L).Coulombic efficiency was 40−55% with the PEM and 9−12%with the PEM removed, indicating substantial oxygendiffusion into the anode chamber in the absence of thePEM. Power output increased with glucose concentrationaccording to saturation-type kinetics, with a half saturationconstant of 79 mg/L with the PEM-MFC and 103 mg/L in theMFC without a PEM (1000 Ω resistor). Similar results onthe effect of the PEM on power density were found usingwastewater, where 28 ± 3 mW/m2 (0.7 ± 0.1 mW/L)(28% Coulombic efficiency) was produced with the PEM,and 146 ± 8 mW/m2 (3.7 ± 0.2 mW/L) (20% Coulombicefficiency) was produced when the PEM was removed. Theincrease in power output when a PEM was removedwas attributed to a higher cathode potential as shown byan increase in the open circuit potential. An analysisbased on available anode surface area and maximumbacterial growth rates suggests that mediatorless MFCsmay have an upper order-of-magnitude limit in power densityof 103 mW/m2. A cost-effective approach to achievingpower densities in this range will likely require systemsthat do not contain a polymeric PEM in the MFC and systemsbased on direct oxygen transfer to a carbon cathode.
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