The microbial species in a biofilm covering an anode are important because they determine the mode of elec­tron transfer and the mechanism of electricity genera­tion as well as what forms of organic material can be utilized in the feed stream. Theoretically, a myriad of microorganisms may be useful for MFCs, but most of them have no direct electrochemical activity and thus cannot transfer electrons directly from the cytoplasm to the anode, i. e. they are not electrogenic. However, many microorganisms with the addition of a soluble redox mediator can act as electron transfer intermediates to transfer electrons. Table 9.1 shows the microbial spe­cies and the electron transfer mechanism in the anodic chamber that can perform such processes.

In MFCs, mixed cultures usually possess higher elec­tron transfer efficiency than the pure culture because its specificity to the microbe is very strong and its growth rate is relatively slow (Hassan et al., 2012). Mixed cul­tures are often found to perform better than pure stains. This is because a synergistic biofilm consortium contains various syntrophic species, with each organism contrib­uting specific roles. A consortium can adapt to substrate variations in wastewater and harsh environmental con­ditions because generally a biofilm consortium is far more robust metabolically than a pure-culture biofilm. The consortium is able to self-select the most efficient electron transfer mechanism if several are available.

Biocathodes

Biocathodes use biofilms as catalysts to improve the cathode reaction, avoiding using precious metal cata­lysts. Another unique advantage of biocathodes is that oxidants other than oxygen can be used, including sul­fate, nitrate, carbon dioxide, H+, Fe(III), Cr(VI), U(VI), Mn(IV), tetrachloroethene, fumarate, perchlorate, and trichloroethene (Huang et al., 2011c). In addition, the sustainability of MFC may be improved with the elimi­nation of problems such as sulfur poisoning of Pt and the requirement for electron mediators in the cathodic chamber (He and Angenent, 2006).

There are two types of biocathodes: aerobic and ana­erobic. Aerobic biocathodes reduce oxygen (electron acceptor). The biofilm on the cathode surface can catalyze the oxidation of transition metal compounds, such as Fe(II) and Mn (II), releasing the electrons to oxygen. MFCs with aerobic biocathodes can produce higher power density than that of anaerobic biocathodes (Srikanth and Venkata, 2012).

The use of a biocathode also means that an MFC can potentially be used to treat an additional wastewater stream in the cathodic chamber. It may be a wastewater stream containing sulfate or nitrate that can come from agricultural runoff (Srikanth and Venkata, 2012). How­ever, the accumulation of microbial metabolites in the cathode chamber can inhibit microbial activities. In addition, metabolites which act as electron donors for bacteria can also compete against the cathode, and there­fore reduce the MFC performance (Hamid et al., 2008).

Furthermore, Zhou et al. (2013) indicated that the voltage output for the combined redox reaction involving Eqn (9.3) and the oxidation of an organic car­bon such as acetate may be too small for MFC after sub­tracting various overpotentials.