MFC Anode

Calculations show that the effect of CL performance optimization by the gradient catalyst loading strongly depends on jo. To characterize the effect, the optimization factor kept is introduced, defined as the ratio of cell currents at optimal and uniform loadings 

FIGURE 4.38 The points the optimization factor (4.280) as a function of the dimensionless cell current density jo. The solid line the fitting function, indicated in the legend. 

Physically, this result is quite clear if the cell current is low, the reaction rate is distributed uniformly over the CL thickness, and the spatial optimization of catalyst/Nafion loading is not necessary. If, however, the dimensionless current is substantial, the reaction runs mainly in a small conversion domain close to the membrane (the section Ideal Proton Transport ). Placing more catalyst and Nation into this domain, greatly improves the performance. 

The model above ignores the relation of Nafion and catalyst loadings to the catalyst layer structure. In particular, it does not take into account that in standard CLs, higher Nafion loading usually means lower catalyst content and vice versa. An account 

FIGURE 4.39 Optimal catalyst and electrolyte loadings for DMFC anode. 

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Figure 10.2 Shapes and modes of emplacement of benthic MFC anodes, (a) Buried and (b)...

There are clear differences between chemical and microbial fuel cell anodes. The most obvious difference is that anodes of MFCs must be able to support the growth of biological organisms. MFC anodes must also be highly conductive in order to efficiently collect electrons produced by bacteria as small increases in material resistance can have a significant impact on maximum power outputs. Other considerations when selecting an anode material include the expense of the material and the ability for it to be manufactured on a large scale. 

Metals such as platinum (Schroder et al., 2003), gold (Richter et al., 2008), titanium (Ter Heijne et al., 2008), stainless steel (Dumas et al., 2007) and copper (Kargi and Eker, 2007) have been investigated as MFC anodes due to then high conductivity, but the overall performance of these materials has been underwhelming. This could be due to them having less than the optimal... 

A microbial fuel cell consists of an anode, a cathode, a proton or cation exchange membrane, and an electrical circuit. The bacteria live in the anode and convert a substrate such as glucose, acetate, as well as wastewater into CO2, protons, and electrons as shown in Figure 1.9. Under aerobic conditions, bacteria use oxygen or nitrate as a final electron acceptor to produce water. However, in the anode of an MFC, no oxygen is present and bacteria need to switch from their natural electron acceptor to an insoluble acceptor, such as the MFC anode. Because of the ability of bacteria to transfer electrons to an insoluble electron acceptor, we can use an MFC to collect the electrons originating from the microbial metabolism. The electron transfer can occur... 

Unlike the anodes of chemical fuel cells, which require catalysts to speed up the anodic reaction, the MFC anode mainly serves as current collector while providing a surface for biofilm development. In this configuration, the biofilm can function as the catalyst. Therefore, the anode should be constructed of material suitable for biofilm development as well as current collection. 

Non-carbon-based materials have also been explored for MFC anodes, including various metals, platinum [18], gold [19], titanium [11], stainless steel (SS) [20], and copper [13]. In spite of the fact that the conductivity of metals is normally higher than that of carbon materials, the performance of metal materials as MFC anodes is generally poor. Poor performance could be due to the relatively low surface area of metal electrodes and the less favorable surface properties for biofilm development compared with carbon electrodes. The high cost of Pt and Au materials impedes their large-scale applications. Attention should also be given to the corrosive nature and poisonous effects of some metals. For example, metals such as copper and stainless steel can be reactive as MFC anodes, which further limits the apphcation of metals as anode materials in MFCs. 

Enhanced performance was also reported for anode modification with conductive polymers. A commonly used conductive polymer, polyaniline, can increase the current densities of MFC anodes. But it is also susceptible to microbial attack and degradation [39]. Schroder et al. [18] reported that a platinum electrode covered with polyaniline achieved a current density up to l.SmAcm in an MFC. Modification of polyaniline can improve its performance and stability, such as fluorinated PANI [40], PANI/carbon nanotube (CNT) composite [41], and PANI/titanium dioxide composite [42]. 

The anodic electron transfer mechanism in MFC is a key issue in understanding how MFCs work. As discussed above, the redox active species at the end of the electron transfer chain links the solid electrode in MFCs anodes, completing the exo-cellular electron transfer (Figure 2.3). These linking species, for example, may be a soluble redox shuttle, an outer membrane redox protein or a pili (nanowire). For an efficient electron transfer, the linking species must fulfill the following requirements [6] ... 

Some microorganisms can directly transfer electrons to the electrode via a physical contact of the cell membrane or a membrane organelle with the anode. No diffusional redox species are involved in this electron transfer process. As illustrated in Figure 2.6a, the direct electron transfer requires the microorganisms to possess (1) membrane-bound protein relays which transfer electrons from the inside of the bacterial cell to its outside, and (2) an outer membrane (OM) redox protein which accepts the electrons and delivers them to an external, solid electron acceptor (a metal oxide or an MFC anode). The most studied OM redox proteins are c-type cytochromes, which are involved in metal-reducing microorganisms such as Geobacter, Rhodqferax and Shewanella. These bacteria often have to rely on solid terminal electron acceptors like iron(lll) oxides in their natural environments. 

Carbon paper, cloth, foams, and R VC. The use of carbon-based electrodes in paper, cloth, and foam forms for the MFC anode is very common. These materials have high conductivity and appear to be well suited for bacterial growth. Carbon paper is stiff and slightly brittle but it is easily connected to a wire (Fig. 5.1 A). It should be sealed to the wire using epoxy, with all exposed surfaces of the wire covered or sealed with epoxy as well. Copper wire can be used but it corrodes over time, either releasing copper into solution (which can be toxic to the bacteria) or causing the electrode to detach from the wire. Stainless steel or titanium wires work better in MFCs. Carbon paper is commonly... 

Fig. 5.1 Photographs of carbon materials used for MFC anodes (A) carbon paper (E-TEK) (B) carbon cloth (E-TEK) (C) three different types of reticulated vitreous carbon (RVC) with different pore sizes (10, 20, and 45 pores per inch). [Photographs A and B, B.E. Logan C, L. Angenent].

Fig. 5.2 Photographs of some graphite materials used for MFC anodes (A) graphite rod (B) thick graphite plate, (C) thinner graphite electrode, and (D) sheet shown with square electrode cut out.

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