Microbial Fuel Cells in Power Generation and Extended Applications - Biotechnology in China: Biofuels and Bioenergy - page 175

morphologies by an electrochemical deposition method. It was found that MnO x
nano-rods had the best electrochemical activity toward oxygen reduction. When
such MnO x was coated onto a carbon paper cathode, the MFC produced a power
density of 772.8 mW/m 3 , over three times higher than the plain carbon paper
control. Recently, the employment of palladium (Pd) nanoparticles as a low-cost
cathodic catalyst has also been documented [ 52 ]. The achieved catalytic efficiency
of Pd nanoparticles was high, and the cathode coated with Pd nanoparticles
showed a distinctly lower overpotential. In an effort to avoid the use of noble
metals, the employment of Pt-based alloys has also been recently investigated to
decrease cost and poisoning while maintain a high catalytic activity of cathode
[ 53 ]. Nevertheless, poisoning of these metal catalysts may be inevitable in treat-
ment of composite wastewater and their sustainability might be questionable.
In this regard, biocathodes are bringing exciting new promise.
Biocathodes
The application of biocathodes undoubtedly presents a big new step in the
development of MFCs. In a biocathode, electrochemically active microorganisms
are used as the catalyst to promote cathodic reduction reactions. It confers several
advantages over abiotic cathodes, such as low cost, self-sustainability, and the
possibility of producing a variety of useful products or removing unwanted
compounds [ 54 ]. The catalytic performances of biocathodes are directly deter-
mined by the amount and activity of the enriched microorganisms, and thus a high
surface area of the cathode electrode for bacteria adhesion is invariably needed.
You et al. [ 55 ] constructed a biocathode by using graphite fiber brush as the
cathode material. After 133 h mixed culturing, the charge transfer resistance of the
cathode decreased from 188 to 17 X, and the MFC generated a maximum power
density of 68.4 W/m 3 . To further increase the cathode area, a combination of
graphite fiber brush and graphite granules cathode was employed by Zhang et al.
[ 48 ]. This combined application of multiple materials enabled a biocathode of
higher bacteria density than when single graphite fiber brush or graphite granules
were used.
In addition to increasing the electrode area, efforts have also been devoted to
culturing bacteria of high catalytic efficiency. Mao et al. [ 56 ] developed a novel
biocathode based on the biocatalysis of ferro/manganese-oxidizing bacteria. In this
system, iron and manganese oxides were modified onto the GAC cathode to
increase the conductivity and promote bacteria activity. This improvement led to a
low internal resistance of only 14 X and an enhanced power density. In another
study, Huang et al. [ 57 ] proposed a dual strategy for biocathode enhancement.
On the one hand, a combined cathode of granular plate and granular graphite was
used to enlarge the cathode surface area to 900 m 2 /m 3 ; on the other, indigenous
microbial consortia from a hexavalent chromium (Cr 6+ ) contaminated site were
used as the inoculum to improve the overall activity of the biocathode for Cr 6+
reduction. This proved to be feasible: a power density of 2.4 W/m 3 were achieved
in the enhanced system.