Exoelectrogenic

One exoelectrogenic microorganism usually has more than one electron transfer route to exchange electrons between the cells and the electrode. For example, S. oneidensis MR-1 contains all three pathways for electron transfer. It can synthesize flavins and secret into the medium, which serve as... 

Exoelectrogenic strains Chemical electron shuttles (self-generated) Chemical electron shuttles (exogenously added) Ref. ... 

Exoelectrogen is not limited to be Geobacter species Geobacter sulfurreducens, Geobacter metallireducens) and Shewanella species Shewanella oneidensis MR-1, Shewanella putrefaciens IR-1, Shewanella oneidensis DSPIO). Pseudomonas species Pseudomonas aeruginosa KRPl), Rhodopseudomonas palustris DX-1, Saccharomyces cerevisiae, Escherichia coli, etc., have been also reported as exoelectrogen, having the capability of EET [6]. 

Columbic efficiency is the ratio of coulombs harvested by the anode of an MFC via decomposing organic substrate by exoelectrogen to the theoretical maximum coulombs cmiverted from all organic substrates ... 

Optical imaging is often used to characterize exoelectrogen on the anode, including the morphology and thickness of biofilm. Two types of optical imaging techniques are popularly used scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). 

Microscale MFCs has been deployed for various exoelectrogen identification and characterization. The small size and capability of forming array allow microscale MFCs as attractive platforms to identify and characterize exoelectrogen. 

The anode resistivity is attributed from electrode resistivity and the resistivity associated with the electron transfer from exoelectrogen to the anode (it can also be seen as the contact resistance. Resistance of biofilm is also incorporated into the electron transfer resistivity). For conductive anode, such as gold, CNT, carbon, etc., the electrode resistivity is negligible. Consequently the main source for the anode resistivity comes from the electron transfer from exoelectrogen to the anode. 

Materials with higher SAV, lower contact resistance, and better biocompatibility, which results in a larger exoelectrogen population, help to reduce the high areal resistivity. 3D structured materials with a high surface-area-to-volume ratio, such as CNT, have been adopted in MFCs, and a significant power density enhancement has been achieved. Figure 7 shows the CNT-based anodes presented by Inoue et al. and Mink et al. 

Parra E, Lin L (2009) Microbial fuel cell based on electrode-exoelectrogenic bacteria interface. Proc. of 19th IEEE Int. Conf. on Micro Electro Mechanical Systems (MEMS) 31-34... 

Logan, B.E. (2009) Exoelectrogenic bacteria that power microbial fuel cells. Nat. Rev. Microbiol, 7 (5), 375-381. 

Kimura Z, Okabe S. Acetate oxidation by syntrophic association between Geobacter sulfurreducens and a hydrogen-utilizing exoelectrogen. ISME J 2013 7 1472-1482. 

To date, two EET mechanisms have been found. One is indirect EET, relying on electron shuttles, which can be either self-secreted or externally added. Electron shuttles are redox-driven and have two states oxidized and reduced states. Driven by redox, the oxidized shuttles become reduced via the acquisition of electrons at the exoelectrogen outer membrane. The reduced shuttles are then driven to the anode and release electrons to the anode and become oxidized. This process repeats to transfer electrons from the exoelectrogens to the anode. For instance, Shewanella oneidensis MR-1 is reported to mainly rely on electron shuttles named flavins (Marsili et al, 2008). 

Figure 8.1. Illustration of a typical two-chamber microbial fuel cell (MFC) specific bacteria species in the anode chamber, named exoelectrogenic or anode-respiring bacteria (ARB), break down organic substrates, i.e., acetate, to produce electrons, protons, and CO2. The electrons pass through an external resistor to be reduced at the cathode while protons pass through the proton exchange membrane (PEM) from the anode to the cathode chamber.

By improving the mass transfer of substrate into exoelectrogens, a 100 xL MFC was presented by Ren and Chae (Ren and Chae, 2012). A power density of 0.83 Wm and 3320 Wm was reported. Mass transfer was further improved by implementing an ultramicroelectrode (UME), and a record high power density of 7.72 W m , regardless of the size of the MFCs, was achieved (Ren and Chae 2012 Ren et al. 2014). Furthermore, by successfully mitigating the oxygen intrusion... 

Figure 8.5. Miniaturized MFCs (a) the first miniaturized MFC, using Saccharomyces cerevisiae, by Chiao et al (2002) (b) a miniaturized MFC presented by Siu and Chiao (2008), using Saccharomyces cerevisiae (c) a miniaturized MFC presented by Choi et al. (2011) using Geobacter sulfurreducens mixed culture (d) a miniaturized MFC with an ultramicroelectrode anode, using Geobacter sulfurreducens mixed culture, presented by Ren et al. 2013 (e) a miniaturized MFC presented by Qian et al., which used Shewanella as the exoelectrogen ( a miniaturized MFC presented by Inoue et al. (2012), using Geobacter sulfurreducens.

The very first outcome of miniaturization is the small feature size, which allows for less building materials to be used, less space to be taken, and less electrol5des required for the operation, etc. This results in substantially less expense for MFC research than for macro-scale research a practical issue that most researchers may consider. Along with the small feature size, batch fabrication allows ease of forming arrays. An MFC reactor array is useful for screening exoelectrogens and can be connected in a series or parallel to boost the voltage of the current. 

The first step in the generation of electricity from MFCs is the exoelectrogen oxidizing an organic substrate in the anode chamber (Fig. 9.1). This reaction produces electrons as well as protons and carbon dioxide. While the electrons travel to the anode and through the circuit to the cathode, the protons diffuse through a chamber separator (optional) to the cathode. At the... 

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