Effect of the cathode/anode ratio and the choice of cathode catalyst on the performance of microbial fuel cell transducers for the determination of microbial activity

Microbial activity can be measured using sensors based on microbial fuel cell technology. In these sensors, microorganisms in contact with the anode generate a current proportional to their metabolic activity. Proper operation of such a device requires that activity at the anode is not impaired by the ability of the cathode to transfer current to the cathodic electron acceptor. Therefore, we have determined the minimum cathode to anode ratio required for unhindered performance of the microbial fuel cell. Our results indicate that for the same level of biological activity, the optimal cathode/anode ratios depend on the type of cathode being used. Thus, while carbon paper/ferricyanide cathodes require ratios of 4, platinum cathodes need much higher ratios of about 27. Cyclic voltammetry measurements indicate that platinum cathodes have a much slower dynamic behaviour than cathodes based on carbon paper/ferricyanide. While these results indicate that carbon paper/ferricyanide cathodes provide the most current for the same cathode area, extended experiments carried over a period of several days indicate a progressive degradation of fuel cell performance in cells using iron catalysts. Overall, our conclusion is that soluble iron-based catalysts provide much higher power output than solid phase platinum catalysts, but at the expense of a reduced life span which limits their use for applications requiring extended operation.