Tubular membrane cathodes for scalable power generation in microbial fuel cells

One of the greatest challenges for using microbial fuel cells (MFCs) for wastewater treatment is creating a scalable architecture that provides large surface areas for oxygen reduction at the cathode and bacteria growth on the anode. We demonstrate here a scalable cathode concept by showing that a tubular ultrafiltration membrane with a conductive graphite coating and a nonprecious metal catalyst (CoTMPP) can be used to produce power in an MFC. Using a carbon paper anode (surface area A(an) = 7 cm(2), surface area per reactor volume A(an,s) = 25 m(2)/m(3)), an MFC with two 3-cm tube cathodes (A(cat) = 27 cm(2), A(cat,s) = 84 m(2)/m(3)) generated up to 8.8 W/m(3) (403 mW/m(2)) using glucose [0.8 g/L in a 50 mM phosphate buffer solution (PBS)], which was only slightly less than that produced using a carbon paper cathode with a Pt catalyst (9.9 W/m(3), 394 mW/m(2); A(cat) = 7 cm(2), A(cat,s) = 25 m(2)/m(3)). Coulombic efficiencies (CEs) with carbon paper anodes were 25-40% with tube cathodes (CoTMPP), compared to 7-19% with a carbon paper cathode. When a high-surface-area graphite brush anode was used (A(an) = 2235 cm(2), A(an,s) = 7700 m(2)/m(3)) with two tube cathodes placed inside the reactor (A(cat) = 27 cm(2), A(cat,s) = 93 m(2)/m(3)), the MFC produced 17.7 W/m(3) with a CE = 70-74% (200 mM PBS). Further increases in the surface area of the tube cathodes to 54 cm(2) (120 m(2)/m(3)) increased the total power output (from 0.51 to 0.83 mW), but the increase in volume resulted in a constant volumetric power density (similar to 18 W/m(3)). These results demonstrate that an MFC design using tubular cathodes coated with nonprecious metal catalysts, and brush anodes, is a promising architecture that is intrinsically scalable for creating larger systems. Further increases in power output will be possible through the development of cathodes with lower internal resistances.