Electricity generation from wastewater using microbial fuel cells :a study of electrode and membrane materials

Abstract

The environmental, social and economic challenges facing society make sustainability and resource efficiency a necessity. To overcome these challenges in wastewater treatment, energy conservation and recovery are two central requirements. Microbial fuel cells (MFCs) as an energy producer could provide a sustainable solution to fulfil both objectives.

A challenge for reliable and efficient use of microbial fuel cells is achieving low material costs due to the low power output especially when using wastewater as substrate. Low cost materials for the anode, cathode and membrane in MFCs were studied to increase the knowledge of reactions and interactions between microbiology and materials in a MFC and to try to increase the power performance and coulombic efficiencies when using complex wastewaters as substrate.

Activated carbon cloth, as one of the anode materials tested, showed the greatest potential for high power performances from wastewater. It reached power densities of 67 mW m -2 during polarisation in a membrane-less reactor and 29±3.4 mW m under 1000 Ω external load using a radiation grafted ion exchange membrane based on ethylene tetrafluoroethylene (ETFE). Coulombic efficiencies (CE) observed reached 92±6% CE for reactors using ETFE radiation grafted membranes and 68±11% CE for membranes-less reactors with an interior biocathode opposite to the anode in the anode chamber. Both reactors used activated carbon cloth as anode material and carbon black as cathode catalyst.

The low conductivity of the wastewater (1-2 mS cm -1 ) limited the power density achieved as it created high ohmic losses because of a high internal resistance in the reactors using 4 cm electrode spacing. Reducing the electrode distance to 2 mm decreased the internal resistance by a factor of ten. A simultaneous reduction in anode potential of 100 mV lead to lower power densities presumably due to oxygen diffusing into the anode chamber. With the exception of carbon cloth, the anode materials investigated showed low overpotential losses and charge transfer resistivities. The cathode was the more limiting influence in the system when a low costs separator (Rhinohide) was used as membrane. However when an ion exchange membrane (Nafion or a radiation grafted membrane based on ETFE) or membrane-less (carbon paper with an internal cathode) was used lower overpotential losses were observed on the carbon black cathode than the anode. The different membranes and separators tested showed a greater influence on the system than previously anticipated.

Although durability studies of the activated carbon cloth as anode material and the different cathode materials over three and two month respectively showed a concerning decline in power performance, the coulombic efficiencies increased over time. So the high coulombic efficiencies achieved and a capability for high power densities using inexpensive materials give hope for the use of microbial fuel cell systems for economical energy generation from wastewater.