There are numerous investigations on the mecha­nisms of electron transfer to the anode by the microbes, while the reports about electron transfer to a biocathode are rather limited (Lovley, 2008). The two electron trans­fer directions are opposite. The biocathode is an electron donor while the anode is an electron acceptor. Despite this difference, biocathodes use the same electron trans­fer mechanisms, DET and MET (Rosenbaum et al., 2011), because biofilm electron transfer can be bidirectional.

DET for Biocathodes

Similar to DET for anodes, DET for biocathodes also requires physical contact of the microbial cell wall with the electrode surface. At the site of direct contact, the electrons directly transfer to the outer cell membrane-bound redox macromolecules (such as c-type cytochromes) from the electrode (Figure 9.4(a); Huang et al., 2011c). However, this kind of DET can only utilize a monolayer of sessile cells on the cathode, thus limiting the biocathode performance. With an in­crease in biofilm thickness, the power generation decreased due to mass transfer resistance to oxidant diffusion from the bulk fluid to the cathode surface (Behera et al., 2010). Geobacter species and mixed cultures that use nitrate, fumarate, tetrachloroethene, O2, CO2, U(VI)/U(IV), and so on as an electron acceptor generally transfer the electrons via DET (Table 9.2). On biocathodes, most of the microbes are found to be Gram negative although some Gram-positive microbes exhibit the DET mechanism in cyclic voltammetry (Huang et al., 2011c). Compared with the pure culture

systems, the mixed culture biocathodes also can transfer electrons via DET. When nitrate, carbon dioxide or tri — chloroethene is used as the electron acceptor, the DET in the mixed culture biocathode improves the power generation (Aulenta et al., 2010; Cao et al., 2009; Clau — waert et al., 2007a).