Biocathodes

Much as bacteria catalyze the anode reaction in an MFC, bacteria can also catalyze the terminal reduction reaction at the cathode (Rabaey et al., 2003). Biocathodes are both low-cost and sustainable making them an attractive research area. However, biocathodes have not made much of an impact in the MFC field yet as power densities to date have been quite low. These low power densities appear to be mainly a product of oxygen mass transfer limitations associated with diffusion through a biofilm (Clauwaert et al., 2007). Measured current densities for biocathodes were significantly lower than their Pt counterparts but may have other advantages such as being able to reduce several different oxidized contaminates, such as Cr(IV) and Cr(OH)3 (Clauwaert et al., 2007 Ter Heijne, 2010). 

Microbial fuel cells rely on bioelectrochemical reactions driven by microbial metabolism to produce electricity while degrading organic substrates. Pure culture MFC studies with bacteria such as Geobacter and Shewanella have informed researchers on the mechanisms behind extracellular electron transfer (EET), but it is mixed consortium communities that typically produce the most power and have the most metabolic flexibility to handle a variety of substrates and environmental conditions. 

MCO Surface Conditions for ORR Potential (V) Current density (mAcm ) Ref.  

Myrothe-cium ve-rrucaria BOD Adsorption to spectroscopic graphite pH = 4.0 0.3 0.03 [128]  

Myrothe-cium ve-rrucaria BOD Adsorption onto a polylysine-modified pH = 7.0, 25 °C, aeration 0.25 0.85 [121]  

Trametes versicolor laccase Glutaraldehyde crosslinking to pH = 6.0, aeration of cell 0.45 0.06 [130]  

Myrothecium verrucaria BOD derivatives, on glassy carbon electrode pH = 7.0, aeration of cell 0.3 0.03  

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W., Muhler, M., Schuhmann, W., and Stoica, L (2010) Towards a high potential biocathode based on direct bioelectrochemistry between horseradish peroxidase and hierarchically structured carbon nanombes. Physical Chemistry Chemical Physics, 12 (34), 10088-10092. 

Barton, S.C. (2005) Oxygen transport in composite biocathodes. Proton Conducting Membrane Fuel Cells III, Proceedings, 2002 (31), 324—335. 

Barton, S.C. and Hudak, N. (2004) Mediated gas diffusion biocathodes. Abstracts of Papers of the American Chemical Society, 228, U484-U484. 

An approach focused on fabrication of nanostructured three-dimensional electrodes and introduction of surface modifications for tethering/retention in an optimal orientation of the MCOs to permit DET to the Tl site from the electrode shows great promise for the production of biocathode prototypes for application to EFCs. A systematic study of such electrodes modified with each of the MCOs available, reporting on their activity for ORR, using DET, under defined conditions of pH, mass transport, and temperature is not yet available, and would be a valuable contribution to advance the technological application of EFCs. A welcome recent focus is normalization of ORR, based on DET to Trametes versico/or adsorbed on porous carbon-based electrode materials, to electrode volume and to electrode... 

Figure 5.10 [116]. Such biocathodes yielded current densities in the mAcm range. However, the use of CueO as a biocatalyst meant that steady-state currents for ORR were only observed at potentials more negative than —0.2 V vs. Ag/AgCl. It was also suggested that the current was limited by supply of protons, as the current was dependent on buffer concentration in the electrolyte. Use of a 1.0 M citrate electrolyte (pH = 5.0), resulted in a steady-state current density of 20 rriAcrrr the highest yet reported for the ORR at a biocathode.

Advances in nanostructured conducting materials for DET have resulted in impressive current densities for the ORR, and application of these three-dimensional materials to DET from MCOs other than CueO may provide biocathodes with the characteristics suitable for an implantable EFC. While a DET approach using MCOs can provide for ORR at potentials approaching the thermodynamic reduction potential for oxygen, the current density achievable in this approach still relies upon intimate contact, and correct orientation, ofthe MCO to a conducting surface. Use of a mediator, capable of close interaction with the TI site of the MCOs, and with a redox potential tailored to permit rapid electron transfer to the TI site, can eliminate the requirement for direct contact in the correct orientation between MCO and electrode, and offer the possibility of a three-dimensional biocatalytic reaction layer on electrodes for higher ORR current densities. 

Figure 5.10 Schematic representation of the air diffusion biocathode half-cell based on CueO adsorbed on carbon particle-modified carbon paper electrode. Adapted from [116] with permission from Elsevier. 

A construction of a biofuel cell is schematically illustrated in Fig. 23. A carbon felt sheet was used for both anode El and cathode E2. An anion-exchange membrane 180 /am thick was used for a separator membrane S. The contact area of S with the electrolyte in each compartment was 12.5 cm. Each compartment had an electrolyte solution (adjusted to pH 7.0 with NaH2P04 and Na2HP04) of 5 mL. The cell was used as a prototype biofuel cell to evaluate the performance of the fuel cell composed of a biocathode (ABTS / ABTS -B0D-02/H20) and a bioanode (MV /MV -Z). vulgaris (H)-2H /H2). The biofuel cell was operated with O2 and H2 gas bubbling in the cathode and anode compart-... 

Microfluidic Fuel Cells, Fig. 6 A complete microfluidic biofuel ceil featuring an upstream biocathode and a downstream bioanode integrated in a single microchannel. Magnifled views of the electrode dimensions (A ) and simulated oxygen concentration (A") in the chaimel are also provided (Reproduced with permission from Togo et al. [9]. Copyright Elsevier (2008))... 

The power of biofuel cells is directly related to the difference between the respective redox potentials of the electroenzymatic reactions occurring at each of the two electrodes the bioanode for glucose oxidation and the biocathode for the reduction of oxygen. The cell voltage and hence the power thus depends on the mode of enzyme wiring and therefore, the direct electron transfer is the most... 

Fig. 3.8 Power density of laccase-glucose oxidase biofuel cell as a function of cell potential in air-saturated 0.1 M phosphate buffer (pH 6.0) containing 5 mM glucose for biofuel cell based on a LDH-ABTS laccase biocathode and b two layers configuration (SWCNT deposit modified by a LDH-ABTS laccase coating) biocathode...

In addition to chemical cathodes, biocathodes have also been developed, and actually fit much better with the sustainable nature of BBSs than chemical cathodes. Therefore, there is increasing interest in the development of biologically driven cathode reactions, as has been demonstrated by the increasing number of biocathode studies in recent years, from two publications in 2004 to 41 in 2010 (Web of Knowledge, keyword biocathode ). 

As the scope of this chapter is focused on the biological aspects of electrochemical systems, only biocathodes and their electron-transfer mechanisms will be discussed further. 

Also in biocathodes microorganisms play a pivotal role in catalyzing reduction reactions. So far, most of the electrochemically active bacteria in biocathodes have been reported to be Gram-negative, although Gram-positive bacteria are also able to play a role in the transfer of electrons. This indicates that, similarly to anodic communities, there is a potentially wide capability of bacteria to catalyze electrode reactions [38, 78, 80, 109, 110]. 

De Schamphelaire, L, Boeckx, P., and Verstraete, W. (2010) Evaluation of biocathodes in freshwater and brackish sediment microbial fuel cells. 

C.J.N. (2010) Cathode potential and mass transfer determine performance of oxygen reducing biocathodes in microbial fuel cells. Environ. Sci. Technol,... 

C.).N. (2008) Hydrogen production with a microbial biocathode. Environ. 

Huang, LP., Regan, ).M., and Quan, X. (2011) Electron transfer mechanisms, new applications, and performance of biocathode microbial fuel cells. Bioresour. Technol., 102 (1), 316-323. 

Freguia, S., Tsujimura, S., and Kano, K. (2010) Electron transfer pathways in microbial oxygen biocathodes. Electrochim. Acta, 55 (3), 813 818. 

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