Page 304 - Biofuels Refining and Performance

Fuel Cells 283

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extended periods. Current density of 31 mA/m over a period of more than
600 h has been reported [31]. MFCs using R. ferrireducens have the abil-
ity to be recharged, and have a reasonable cycle life and low capacity
loss under open-circuit conditions. They allow the harvest of electricity
from many types of organic waste matter or renewable biomass. This is
an advantage over other microorganisms in the family Geobacteraceae,
which cannot metabolize sugars.
Another recent development has been the use of microfibers rather
than flat electrodes and the enzyme-based electroactive coatings. The
anode coating used is glucose oxidase, which is covalently bound to a
reducing-potential copolymer and has osmium complexes attached to its
backbone. The cathode coating contains the enzyme laccase and an
oxidizing-potential copolymer. The osmium redox centers in the coatings
electrically “wire” the reaction centers of the enzymes to the carbon
fibers. This electrode design avoids glucose oxidation at the cathode
and O reduction at the anode, eliminating the need for an electrode-
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separating membrane. This has led to miniature “one-compartment bio-
fuel cells” for implantable devices within humans, such as pacemakers,
insulin pumps, sensors, and prosthetic units. Biofuel cells with two
7-
m-diameter, electrocatalyst-coated carbon fiber electrodes placed in
1-mm grooves machined into a polycarbonate support with a power
output of 600 nW at 37 C (enough to power small silicon-based micro-
electronics) have been reported [32].
Microbial fuel cells have a long way to go before they compete with
more established hydrogen fuel cells or electrical batteries. However, a
number of factors provide motivation for research into microbial fuel
cells for electricity production.

1. Bacteria are adapted to feeding on virtually all available carbon
sources (carbohydrates or more complex organic matter present in
sewage, sludge, or even marine sediments). This makes them poten-
tial catalysts for electricity generation from organic waste.
2. Bacteria are omnipresent in the environment and are self-
reproducing, self-renewing catalysts; thus a simple initial inoculation
of a suitable strain could be cultured continuously in an MFC for long-
term operation.
3. The catalytic core of conventional fuel cells uses very expensive pre-
cious metals such as platinum, and biocatalysts like bacteria may
become a serious cost-reducing alternative.
Although biofuel cells are still in an early stage of development and
work toward optimizing the performance of a biofuel cell system is needed,
the utilization of white blood cells as a source of electrons for a biofuel
cell could mark an important step in developing a perpetual power