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Introduction
Microbial fuel cells (MFCs) employ microorganisms as biocatalysts to convert chemical energy comprised in electron donors to electrical energy (Allen and Bennetto, 1993; Logan et al., 2006). Due to their unique characteristics, MFCs are considered to have potential applications in the domains of power supply (Wilkinson, 2000; Aelterman et al., 2006; Biffinger et al., 2007), biological oxygen demand sensors (Chang et al., 2006) and especially in sustainable wastewater treatment (Rabaey and Verstraete, 2005). However, to make these applications of MFCs feasible, a number of limitations need to be overcome (Aelterman et al., 2006; Pham et al., 2006). Most researchers thus far have been focusing on the activity of the biocatalysts, electron transfer losses both at the anodes and the cathodes, and the internal resistance (Logan et al., 2006). The electron transfer between the bacterial catalysts and the anodic electrode poses a serious challenge.
Attempts have been made to understand the mechanisms of the anodic electron transfer of an MFC. To date, the proposed mechanisms include: (i) direct electron transfer either by membrane‐associated cytochromes (Kim et al., 1999; Bond and Lovley, 2003) or conductive pili or pilus‐like appendages (Reguera et al., 2005; Gorby et al., 2006) and (ii) the mobile redox mediator‐associated electron transfer (Rabaey and Verstraete, 2005; Marsili et al., 2008). In addition, recently, the interactions between bacteria have been shown to play an important role in the anodic electron transfer of, e.g. a Gram‐positive bacterium dominant in the microbial community of an acetate‐fed MFC (Rabaey et al., 2007; Pham et al., 2008). Nevertheless, from the existing concepts, a strategy to improve the electron transfer with an order of magnitude is still needed.
The anodic microbial consortium of an MFC grows in biofilm structures (Kim et al., 2004; Rabaey and Verstraete, 2005; Reguera et al., 2006). Studies on the composition of such microbial communities have been carried out but no work has been related to the engineering of the MFC biofilm structure in order to improve the performance of the MFC. It has been reported that high shear rates (below the level...