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 cm², surface area per reactor volume A_an,s = 25 m²/m³), an MFC with two 3-cm tube cathodes (A_cat = 27 cm², A_cat,s = 84 m²/m³) generated up to 8.8 W/m³ (403 mW/m²) 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/m³, 394 mW/m²; A _cat = 7 cm², A_cat,s = 25 m²/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 cm², A_an,s = 7700 m²/m³) with two tube cathodes placed inside the reactor (A_cat = 27 cm², A_cat,s = 93 m²/m³), the MFC produced 17.7 W/m³ with a CE = 70-74% (200 mM PBS). Further increases in the surface area of the tube cathodes to 54 cm² (120 m²/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 (≃18 W/m³). 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.