Microbial Fuel Cells for Wastewater Treatment - Biotechnologies and Biomimetics for Civil Engineering - page 413

Cathodes are made from the same materials as anodes, and catalysts are usually
contained but not necessary. Because oxygen is the terminal electron acceptor in
most cases, the high overpotential arising from oxygen reduction reaction causes
the noncatalyst cathodes to be inefficient. Thus, catalysts and/or artificial mediators
are generally required to improve performance. They are generally mounted on the
cathodes with a binder such as Nafion (perfluorosulfonic acid) or polytetrafluo-
roethylene. Pt has become the most popular catalyst (Thurston et al. 1985 ), but its
high cost and reduced activities due to formation of a PtO layer on the electrode
surface restrict its practical application. For this reason non-Pt catalysts including
nonabundant metals, e.g., Pd or Ru (Vante and Tributsch 1986 ; Fernández et al.
2005 ; Raghuveer et al. 2005 ) and nonprecious materials, e.g., Fe, Mn and Co (Park
and Zeikus 2003 ) tend to be more appealing. They could exhibit essentially equal
or slightly better performance than the more expensive Pt. Among the non-Pt
catalysts, the most promising CoTMPP and iron (II) phthalocyanine (FePc) (Zhao
et al. 2005 ) were proved to be inexpensive and efficient alternatives for MFC
applications. Integration of noncorrosive metals (titanium and nickel) and carbon
fibers can be used as cathode materials as well (Hasvold et al. 1997 ; Zhao et al.
2009 ). Additionally, catalysts are not required for catholyte cathodes, which use
the redox mediators such as ferricyanide (Oh et al. 2004 ; Venkata Mohan et al.
2008 ) or permanganate (You et al. 2006 ). Using them as terminal electron
acceptors could result in alternative cathodic reactions and further improve power
output to 258 W/m 3 (Aelterman et al. 2006 ). These catholytes seemed to be
impractical and unsustainable for practical application owning to the requirement
of regeneration of the chemicals. On the basis of the above introduction, a large
number of materials have been investigated to improve cathode performance.
However, their long-term stability on the cathode should be further evaluated for
future application.
The PEM is also an important component in the PEM-MFC configuration. It
provides a separation between the anode and cathode chambers and allows for
transport of positive charges to compensate the electron transport. Cur-
rently, the most widely used membrane material is Nafion TM (Park and Zeikus
2000 ; Bond and Lovley 2003 ), which has set the industry standard for PEM. Its
properties have been extensively reviewed (Mauritz and Moore 2004 ). Obviously,
Nafion TM was the predominant choices for current MFCs. Nevertheless, it has
been recently found that the use of Nafion TM leads to some side effects such as pH
imbalance and power reduction (Gil et al. 2003 ; Kim et al. 2004 ). In addition to
Nafion TM , polyether ether ketone (PEEK) is a promising polymer being actively
studied by the MFC researchers to overcome the drawbacks of Nafion TM (Roziere
and Jones 2003 ; Mecheri et al. 2006 ). In fact, membranes can be omitted from the
bioelectrochemical configuration. The lack of a PEM could decrease the cost of the
materials for a MFC, but substantial oxygen diffusion into the anode chamber in
the absence of the PEM could reduce the fraction of electrons recovered as current.