Page 300 - Biofuels Refining and Performance

Fuel Cells 279

Some designs in which a thin, dense layer of the electrolyte is physically
supported on one of the electrodes (electrode-supported design) are sug-
gested. This structure of a very porous support is difficult to manufac-
ture, and an expensive thin-film deposition technique such as chemical
vapor deposition (CVD) is needed to manufacture these systems. Even
then, the mechanical strength of the structure (defined by the porous
electrode) is often poor, and the handling of the structure through sub-
sequent processing and assembly is difficult. Another approach to
improve SOFC performance at low operating temperatures is to use
different materials for the electrolyte and the electrode. Several mate-
rials options are being investigated [2, 6, 26, 27].

9.3.7 Biofuel cells
A biofuel cell operation is very similar to a conventional fuel cell, except
that it uses biocatalysts such as enzymes, or even whole organisms
instead of inorganic catalysts like platinum, to catalyze the conversion
of chemical energy into electricity. They can use available substrates
from renewable sources and convert them into benign by-products with
the generation of electricity. As mentioned earlier, in recent years, med-
ical science is increasingly relying on implantable electronic devices for
treating a number of conditions. These devices demand a very reliable
and maintenance-free (any maintenance that might require surgery)
power source. Biofuel cells can provide solutions to most of these prob-
lems. A biofuel cell can use fuel that is readily available in the body, for
example, glucose in the bloodstream, and it would ideally draw on this
power for as long as the patient lives. Since they use concentrated
sources of chemical energy, they can be small and light.
A biofuel cell can operate in two ways: It can utilize the chemical
pathways of living cells (microbial fuel cells), or, alternatively, it can use
isolated enzymes [7, 28]. Microbial fuel cells have high efficiency in
terms of conversion of chemical energy into electrical energy; however,
they suffer from the low volumetric catalytic activity of the whole organ-
ism and low power densities due to slow mass transport of the fuel
across the cell wall. Isolated enzymes extracted from biological systems
can be used as catalysts to oxidize fuel molecules at the anode and to
enhance oxygen reduction at the cathode of the biofuel cell. Isolated
enzymes are attractive catalysts for biofuel cells due to their high cat-
alytic activity and selectivity. The theoretical value of the current that
3
can be generated by an enzymatic catalyst with an activity of 10 U/mg
is 1.6 A, a catalytic rate greater than platinum! However, practical
observed currents are much lower due to the loss of catalytic activity
from immobilization of the enzymes at the electrode surface and energy
losses of the overall system. A major challenge in the biofuel cell design