Page 302 - Biofuels Refining and Performance

Fuel Cells 281

a potassium phosphate buffer (pH 7.0), and a woven graphite felt elec-
trode. Potassium ferricyanide reaction helps in rapid electron uptake.

Hydrogen ions (H ) migrate across the PEM and combine with oxygen
from air and the electrons to produce water at the cathode. The cath-
ode compartment has to be oxygenated by constant bubbling with air
to promote the cathode reactions. It may be worth mentioning that the
electron transport chain occurs in the cell membrane of prokaryotes
(a unicellular organism having cells lacking membrane-bound nuclei,
such as bacteria), while this process occurs in the mitochondrial mem-
brane of eukaryotes (animal cells). Therefore, attempts to substitute
eukaryotic cells for bacterial cells in a biofuel cell may present a sig-
nificant challenge.

Electrochemistry of microbial fuel cells. In a microbial fuel cell, two redox
couples are required in order to generate a current: (a) coupling of the
reduction of an electron mediator to a bacterial oxidative metabolism and
(b) coupling of the oxidation of the electron mediator to the reduction of
the electron acceptor on the cathode surface. The electron acceptor is
subsequently regenerated by the presence of O at the cathode surface.
2
The electrochemical reactions in a biofuel cell using glucose as a fuel are
At the anode:

2
C H O 1 6H O S 6CO 1 24e 1 24H 1
6
6
2
2
12
At the cathode:
32 2 42
4FesCNd 6 1 4e S 4FesCNd 6
42 1 32
2
4FesCNd 6 1 4H 1 O S 4FesCNd 6 1 2H O
2
Complete oxidation of glucose does not always occur. One might often
get additional products besides CO and water. For example, E. coli forms
2
acetate, being unable to completely breakdown glucose, thereby limit-
ing electricity production. Recently, an elegant approach to address this
long-standing problem of limited enzyme stability has been reported
[30]. It is suggested that the immobilization of enzymes in Nafion layers
to create a bio-anode results in stable performance over months.
Another way of using a microorganism’s ability to produce electro-
chemically active substances for energy generation is to combine a biore-
actor with a biofuel cell or a hydrogen fuel cell. The fuel can be produced
in a bioreactor at one place and transported to a (H or bio-) fuel cell to
2
be used as a fuel. In this case, the biocatalytic microbial reactor produces
the fuel, and the biological part of the device is not directly integrated
with the electrochemical part (see Fig. 9.12).