Scale-up of membrane-free single-chamber microbial fuel cells

Scale-up of microbial fuel cells (MFCs) will require a better understanding of the effects of reactor architecture and operation mode on volumetric power densities. We compared the performance of a smaller MFC (SMFC, 28 mL) with a larger MFC (LMFC, 520 mL) in fed-batch mode. The SMFC produced 14 W m−3, consistent with previous reports for this reactor with an electrode spacing of 4 cm. The LMFC produced 16 W m−3, resulting from the lower average electrode spacing (2.6 cm) and the higher anode surface area per volume (150 m2 m−3 vs. 25 m2 m−3 for the SMFC). The effect of the larger anode surface area on power was shown to be relatively insignificant by adding graphite granules or using graphite fiber brushes in the LMFC anode chamber. Although the granules and graphite brushes increased the surface area by factors of 6 and 56, respectively, the maximum power density in the LMFC was only increased by 8% and 4%. In contrast, increasing the ionic strength of the LMFC from 100 to 300 mM using NaCl increased the power density by 25% to 20 W m−3. When the LMFC was operated in continuous flow mode, a maximum power density of 22 W m−3 was generated at a hydraulic retention time of 11.3 h. Although a thick biofilm was developed on the cathode surface in this reactor, the cathode potentials were not significantly affected at current densities <1.0 mA cm−2. These results demonstrate that power output can be maintained during reactor scale-up; increasing the anode surface area and biofilm formation on the cathode do not greatly affect reactor performance, and that electrode spacing is a key design factor in maximizing power generation.