Anodic Electron Transfer Mechanism in Bioelectrochemical Systems

Abstract

The reality of bacteria in transporting electron beyond their cell wall and ability to electrically interact with electrode has been nearly over a century (Potter 1911). Microbial fuel cells are growing bioelectrochemical systems that use bacteria as a catalyst and generate bioelectricity using organic matter. The bacteria act as powerhouse at the anode of MFC and oxidize organic matter to CO2 by generating electrons and protons (Kondaveeti 2014). These electrons move from anode to cathode and get reduced as water by using oxygen as an electron acceptor. The generated electrons from bacteria can be transferred to anode by direct contact with biofilm or by using mediators, which can be either exogenic or endogenic (Kondaveeti and Min 2015). The natural mediators such as flavins which are secreted by bacteria or other active complexes such as c-type chromosomes present on outer cell membranes can shuttle electrons. Up to date the metal reducing bacterial species such as Geobacter and Shewanella have been widely noticed in MFC technology, due to their external electron transfer mechanism and for synthesis of natural mediators (riboflavins), which can be a rival for other exoelectrogens (Logan 2008). The external insoluble shuttles such as neutral red, and methyl viologen etc. were used in microbial fuel cells for electron transfer from the bacterial cell wall to electrodes. The initial studies in addition of exogenous mediators to MFC were pursued (Cohen 1931; Schroder 2007). In this study low current generation in MFC might be due to lack of electromotive oxidation and reductive force. These were resurfaced in 1980 by Bennetto and coworkers and it was further investigated by many other researchers. In the present chapter, the electron transfer mechanisms such as direct electron transfer, mediated electron transfer and interspecies electron transfer mechanisms with electroactive anode bacteria are discussed.