Journal of Energy Bioscience 2024, Vol.15, No.2, 85-95 http://bioscipublisher.com/index.php/jeb 86 2 Fundamentals of Electron Transfer in MFCs 2.1 Description of electron transfer processes in MFCs Microbial fuel cells (MFCs) are innovative bioelectrochemical systems that convert chemical energy from organic substrates directly into electrical energy through the metabolic activities of microorganisms. The core process in MFCs involves the transfer of electrons from microbial cells to the anode, a process known as extracellular electron transfer (EET). This electron transfer is crucial for the generation of electricity in MFCs and is facilitated by electroactive bacteria (EAB) that can transfer electrons to solid-phase electron acceptors such as electrodes (Logan et al., 2019; Pankratova et al., 2019; Aiyer, 2020). The EET process begins with the microbial oxidation of organic matter, which releases electrons and protons. The electrons are then transferred to the anode, while the protons migrate through a proton exchange membrane to the cathode, where they combine with oxygen to form water. The efficiency of this process is influenced by several factors, including the type of microorganisms, the nature of the electrode material, and the presence of electron mediators (Lan et al., 2018; Aiyer, 2020; Li et al., 2021). 2.2 Role of electroactive bacteria in electron transfer Electroactive bacteria (EAB) play a pivotal role in the electron transfer processes within MFCs. These microorganisms possess unique metabolic pathways that enable them to transfer electrons to external electron acceptors. Among the most studied EAB are species from the genera Geobacter and Shewanella, which have been shown to produce high power densities in MFCs (Logan et al., 2019; Zhao et al., 2020; Zhou et al., 2020). Geobacter sulfurreducens, for instance, is known for its ability to form conductive biofilms and transfer electrons directly to the anode via conductive pili or nanowires. These biofilms enhance the surface area for electron transfer and improve the overall efficiency of the MFC (Logan et al., 2019; Li et al., 2021). Similarly, Shewanella oneidensis can transfer electrons through outer membrane cytochromes and soluble redox mediators, facilitating both direct and mediated electron transfer mechanisms (Pinto et al., 2018; Zhao et al., 2020). The diversity of EAB extends beyond these model organisms, with many other bacteria, fungi, and archaea also capable of EET. This diversity offers opportunities for optimizing MFC performance by selecting or engineering microorganisms with enhanced electron transfer capabilities (Logan et al., 2019; Zhao et al., 2020). 2.3 Types of electron transfer mechanisms The electron transfer mechanisms in MFCs can be broadly categorized into direct electron transfer (DET) and mediated electron transfer (MET). Direct Electron Transfer (DET): In DET, electrons are transferred directly from the microbial cells to the anode without the involvement of soluble mediators. This process is facilitated by conductive structures such as pili, nanowires, and outer membrane cytochromes. For example, Geobacter species utilize conductive pili and cytochromes to transfer electrons directly to the anode, forming a highly efficient electron transfer pathway (Lan et al., 2018; Aiyer, 2020; Li et al., 2021). The spatial structure of the biofilm and the interaction between the microbial cells and the electrode surface are critical factors influencing DET efficiency (Pinto et al., 2018; Li et al., 2021). Mediated Electron Transfer (MET): In MET, soluble redox mediators facilitate the transfer of electrons from the microbial cells to the anode. These mediators can be naturally produced by the microorganisms, such as flavins and pyocyanins, or artificially added to the system, such as methylene blue and neutral red (Kalathil et al., 2016; Liu et al., 2018). The mediators shuttle electrons between the microbial cells and the anode, enabling electron transfer even when direct contact is not possible. The efficiency of MET depends on the redox potential of the mediators and their ability to diffuse between the cells and the electrode (Kalathil et al., 2016; Liu et al., 2018). Both DET and MET are essential for the operation of MFCs, and understanding these mechanisms is crucial for optimizing the design and performance of these systems. Advances in nanotechnology and bioengineering hold promise for enhancing both DET and MET, thereby improving the overall efficiency and applicability of MFCs in sustainable energy production and environmental remediation (Kalathil et al., 2016; Li et al., 2021).
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