Biocathodes oxygen reduction reaction

Laccase has been commonly used in cathodes of en2yme-based biofuel cells. Only recently, it was discovered that bacteria can also catalyze the oxygen reduction reaction in MFCs. Biocathode here refers to bacteria-based cathode. 

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). 

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. 

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... 

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