The MFC technology has to compete with the mature methanogenic anaerobic digestion technology that has seen wide commercial applications (Holzman, 2005; Lusk, 1998) because they can utilize the same biomass inmany cases for energy productions. MFCs are capable of converting biomass at temperatures below 20 degree celsius and with low substrate concentrations, both of which are problematic for methanogenic digesters. A major disadvantage of MFCs is their reliance on biofilms for mediator-less electron transport, while anaerobic digesters such as up-flow anaerobic sludge blanket reactors eliminate this need by efficiently reusing the microbial consortium without cell immobilization. It is likely that the MFC technology will co-exist with the methanogenic anaer-obic digestion technology in the future.
To improve the power density output, new anodo-philic microbes that vastly improve the electron transport rate from the biofilm covering an anode to the anode are much needed. Lovley claimed that an MFC's current flow could increase by four orders of magnitude if Geobacter transports electrons to the anode at the same rate as its does to its natural electron acceptor that is ferric iron . Mutagenesis and even recom-binant DNA technology can conceivably be used in the future to obtain some "super bugs" for MFCs. Microbes may be used as a pure culture or a mixed culture forming a synergistic microbial consortium to offer better performance. One type of bacterium in a consortium may provide electron mediators that are used by another type of bacterium to transport electrons more efficiently to an anode. It is possible in the future that an optimized microbial consortium can be obtained to operate an MFC without extraneous mediators or biofilms while achieving superior mass transfer and electron transfer rates.
As aforementioned, MFCs can potentially be used for different applications. When used in wastewater treatment, a large surface area is needed for biofilm to build up on the anode. A breakthrough is needed in creating inexpensive electrodes that resist fouling. It is unrealistic to expect that the power density output from anMFC to match that of conventional chemical fuel cell such as a hydrogen-powered fuel cell. The fuel in an MFC is often a rather dilute biomass (as in wastewater treatment) in the anodic chamber that has a limited energy (reflected by its BOD). Another limitation is the inherent naturally low catalytic rate of the microbes. Even at their fastest growth rate microbes are relatively slow transformers. Although Coulombic efficiency over 90% has been achieved in some cases, it has little effect on the crucial problem of low reaction rate. Although some basic knowledge has been gained in MFC research, there is still a lot to be learned in the scale-up of MFC for large-scale applications.
