Journal of Energy Bioscience 2024, Vol.15, No.2, 85-95 http://bioscipublisher.com/index.php/jeb 91 sustainable energy production and environmental remediation. Figure 2 3D Reconstruction and Classification of Microstructure (Adopted from Liu et al., 2018b) Image caption: (a) Original XRT image of the iron carbonate binder, (b) 3D image after segmentation of unreacted iron particles; and (c) 3D image after pore and unreacted iron segmentation (three-phase; pore, unreacted iron and solid). Please refer to the online version of the paper for color images (Adopted from Liu et al., 2018b) 7 Case Study: Electron Transfer inGeobacter sulfurreducens MFCs 7.1 Description of the case study setup and conditions In this case study, we investigate the electron transfer mechanisms of Geobacter sulfurreducens in microbial fuel cells (MFCs). The setup involves cultivating G. sulfurreducens on graphite-based electrodes polarized to +400 mV versus Ag/AgCl for a period of 8 days. During this period, the biofilm formation and electron transfer processes are monitored and analyzed. The maximum current density achieved was (172±29) µA cm-2 after 7 days, indicating active electron transfer from the biofilm to the electrode (Fernandes et al., 2021). Additionally, the spatial structure of the electroactive biofilm (EAB) is tailored using iron phthalocyanine (FePc) modified carbon cloth (CC) electrodes to enhance the affinity between the anode and outer membrane c-type cytochromes (OM c-Cyts), resulting in a higher power density and biomass loading (Stöckl et al., 2019). 7.2 Analysis of electron transfer pathways inGeobacter sulfurreducens The electron transfer (ET) pathways in G. sulfurreducens involve a complex network of multiheme c-type cytochromes that facilitate the transfer of electrons from the cell interior to the exterior. Key cytochromes identified include inner-membrane associated MacA, periplasmic cytochromes (PpcA, PpcB, PpcC, PpcD, PpcE, and GSU1996), and outer membrane-associated cytochromes (OmcF, OmcS, and OmcZ) (Santos et al., 2015). The redox properties of these cytochromes are critical for efficient ET, with OmcF playing a pivotal role in the reduction of metal oxides and electricity production in MFCs (Füeg et al., 2021). The ET process is further influenced by the extracellular polymeric substances (EPS) secreted by the biofilm, which enhance the biofilm's electroactivity and stability (Fernandes et al., 2021). 7.3 Results and implications for MFC performance The results of this case study highlight several key findings regarding the ET mechanisms of G. sulfurreducens and their implications for MFC performance. Firstly, the rate-limiting steps in ET transition during biofilm growth were identified. In early to mid-stage biofilms, the rate-limiting step transitions from irreversible acetate turnover to electron transfer from inside the exoelectrogen to extracellular redox cofactors (ERCs) within the biofilm. Fully-grown biofilms exhibit a current density of more than 3.1 Am-2, with the rate-limiting step being the electron transfer from ERCs within the biofilm to ERCs at the anode (Ren et al., 2020). The introduction of FePc-modified CC anodes significantly improved the viability and activity of the EAB, resulting in a higher power density of 2 419 mW m-2 compared to 560 mW m-2 for unmodified CC anodes. This enhancement is attributed to the decreased charge transfer resistance and accelerated interfacial reaction rates, promoting direct electron transfer via OM c-Cyts (Stöckl et al., 2019). Additionally, the EPS secreted by G. sulfurreducens under electroactive conditions were found to be rich in proteins, which dominate all EPS fractions
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