Impact of Ohmic Resistance on Measured Electrode Potentials and Maximum Power Production in Microbial Fuel Cells

Low solution conductivity is known to adversely impact power generation in microbial fuel cells (MFCs), but its impact on measured electrode potentials has often been neglected in the reporting of electrode potentials. While errors in the working electrode (typically the anode) are usually small, larger errors can result in reported counter electrode potentials (typically the cathode) due to large distances between the reference and working electrodes or the use of whole cell voltages to calculate counter electrode potentials. As shown here, inaccurate electrode potentials impact conclusions concerning factors limiting power production in MFCs at higher current densities. To demonstrate how the electrochemical measurements should be adjusted using the solution conductivity, electrode potentials were estimated in MFCs with brush anodes placed close to the cathode (1 cm) or with flat felt anodes placed further from the cathode (3 cm) to avoid oxygen crossover to the anodes. The errors in the cathode potential for MFCs with brush anodes reached 94 mV using acetate in a 50 mM phosphate buffer solution. With a felt anode and acetate, cathode potential errors increased to 394 mV. While brush anode MFCs produced much higher power densities than flat anode MFCs under these conditions, this better performance was shown primarily to result from electrode spacing following correction of electrode potentials. Brush anode potentials corrected for solution conductivity were the same for brushes set 1 or 3 cm from the cathode, although the range of current produced was different due to ohmic losses with the larger distance. These results demonstrate the critical importance of using corrected electrode potentials to understand factors limiting power production in MFCs.