Influence of soluble microbial products on the long-term stability of air cathodes in microbial fuel cells

The practical application of microbial fuel cells (MFCs) is currently restricted by the poor long-term stability of air cathodes, which has proven closely related to the inevitable biofilm growth on catalyst layer. However, the mechanisms how the biofilm inhibits the cathodic oxygen reduction reaction (ORR) remain controversial. This work examined the possible role of soluble microbial products (SMP) of cathode biofilm in fouling of air cathodes. The cathode biofilm SMP were found to primarily consist of polysaccharides, proteins and some humic-like substances. Excitation-emission matrix (EEM), scanning electron microscopy (SEM) and molecular size analyses revealed that the SMP could diffuse into the catalyst layer during MFC operation. Removing SMP from the catalyst layer by chemical cleaning reduced the ohmic resistance (Rohm), charge transfer resistance (Rct) and diffusion resistance (Rd) by 5.3%, 33.3% and 27.7% respectively after 6 months of operation. As a result, the maximum power density of MFCs was increased by 10.2% from 965.1 ± 45.1 mW m−2 to 1063.1 ± 29.5 mW m−2. Impregnating simulated SMP into the catalyst layer increased Rohm, Rct and Rd by 16.8%, 63.6% and 76.2% respectively, resulting in an obvious reduction in the ORR current. These results demonstrated that cathode biofilm SMP are an important factor reducing the long-term stability of air cathodes and thus MFCs, which will be valuable for promoting the practicability of MFCs.