Journal of Energy Bioscience 2024, Vol.15, No.2, 85-95 http://bioscipublisher.com/index.php/jeb 90 Another study investigated the effect of electrode position on the performance of MFCs designed for simultaneous Cr(VI) reduction and bioelectricity production. The results indicated that optimal electrode spacing could enhance electron transfer and electrochemical activity, leading to higher power densities and more efficient contaminant removal. This study underscores the importance of system configuration in maximizing the benefits of MET. Furthermore, research on the application of specific potentials at the bioanode has shown that different potentials can stimulate the formation of electroactive biofilms and influence the mode of electron transfer (Pinto et al., 2018). For instance, a negative applied potential favored mediated electron transfer and biofilm formation, while a positive potential promoted direct electron transfer. These findings provide valuable insights into the complex interactions between bacteria and electrodes in MFCs and the role of redox mediators in these processes. In summary, the use of redox mediators, both natural and synthetic, plays a critical role in enhancing electron transfer in MFCs. By understanding and optimizing the use of these mediators, researchers can improve the efficiency and performance of MFCs for various applications, including wastewater treatment, bioenergy production, and environmental remediation. 6 Factors Influencing Electron Transfer Efficiency 6.1 Impact of electrode materials and surface properties The choice of electrode materials and their surface properties significantly influence the efficiency of electron transfer in microbial fuel cells (MFCs). For instance, the modification of carbon cloth (CC) electrodes with iron phthalocyanine (FePc) has been shown to enhance the affinity between the anode and outer membrane c-type cytochromes (OM c-Cyts), leading to a highly active electroactive biofilm (EAB). This modification resulted in a substantial increase in power density and biomass loading, as well as a significant reduction in charge transfer resistance, thereby promoting direct electron transfer (Li et al., 2021). Additionally, the use of electron-conducting polymers as mediators has been suggested as a promising strategy to enhance electron transfer efficiency, particularly in Gram-positive bacteria, which typically exhibit weak extracellular electron shuttling activity due to their thick, non-conductive cell walls (Yang et al., 2019). 6.2 Environmental Conditions (pH, temperature, substrate availability) Environmental conditions such as pH, temperature, and substrate availability play crucial roles in the efficiency of electron transfer in MFCs. The extracellular electron transfer (EET) mechanisms are highly dependent on the redox potentials of the species involved and the microbial oxidative metabolism that liberates electrons (Aiyer, 2019). For example, the presence of specific substrates can influence the metabolic pathways of electroactive bacteria, thereby affecting their electron transfer capabilities. In a study where an electroactive biofilm was switched from acetate oxidation to nitrate reduction conditions, it was observed that the same bacterial consortium could catalyze both anodic and cathodic reactions using the same electron conduit, highlighting the adaptability of electroactive bacteria to different environmental conditions (Pous et al., 2016). 6.3 Biofilm Formation and Its Influence on Electron Transfer Biofilm formation is a critical factor that influences electron transfer in MFCs. The spatial structure and composition of the biofilm can significantly impact its electrochemical activity. For instance, the introduction of magnetite into the biofilm, facilitated by a magnetic field, has been shown to improve electron delivery within the biofilm, leading to increased power density and enhanced extracellular electron transfer (Figure 2) (Liu et al., 2018b). Moreover, the regulation of biofilm formation through engineering strategies, such as enhancing transmembrane electron transport via cytochrome protein channels and promoting microbe-electrode interface reactions, can further improve the EET capabilities of electroactive microorganisms (Zhao et al., 2020). The distribution and penetration of conductive materials within the biofilm are essential for facilitating efficient electron transfer across the biofilm, thereby improving the overall performance of MFCs. In summary, the efficiency of electron transfer in MFCs is influenced by a combination of factors, including the properties of electrode materials, environmental conditions, and the formation and structure of biofilms. By optimizing these factors, it is possible to enhance the performance and practical applications of MFCs in
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