In Situ Characterizations of Porous Media for Applications in Biofuel Cells: Issues and Challenges - Porous Media: Applications in Biological Systems and Biotechnology - page 498

scale such that enzymes can be immobilized in a well-controlled environment
and individually accessible to the substrate which is the fuel, and on the
other hand, on the scale of hundreds of microns such that transport of the
fuel in its liquid phase will not become the limiting factor in the power pro-
ducing process. The balance between the considerations of optimal enzyme
immobilization and fuel transport will determine the ultimate pore config-
uration for the power generation. In other words, macropores in the size of
hundreds of microns with network of channels are required for facile fuel trans-
port, whereas the pore structure needs to accommodate sublayered mesopores
on the surface to enable effective enzyme immobilization. In the microbial
electrode design, the pore structure of the transport media is less stringent,
but PSD from submicron to several hundred microns remains critical in the
design. Besides pore structure, prolonging the lifetime of the biocatalytic
species in frequently strained fuel cell operating conditions to sustain the
power generation is also a critical requirement. It usually requires an appro-
priate and delicate immobilization technique to retain enzymatic catalyst's
activity or facilitate biofilm development. Furthermore, to achieve an effec-
tive immobilization of the biocatalytic species, it is also desirable to increase
its loading (density of biocatalytic species per electrode surface area) to
enhance power generation. A high loading is to create a large number of
reactive sites per unit surface area. Complicated with substrate transport to
allow effective biocatalytic reaction, it is also attractive to design electrodes
with optimal three-dimensional architecture and optimized pore structure to
allow the best combination of effective surface loading and mass transport to
increase the loading density per unit volume for power generation (Cooney and
Liaw 2008).
To achieve these objectives, an interesting possibility that has been
proposed by many is to use conductive polymer network (such as those
made of polypyrrole [PPy] and its derivatives) as an immobilization plat-
form. Such a conductive backbone remains a promising possibility to enable
favorable three-dimensional architecture for power generation in batteries
or fuel cells (Scott et al. 2008) and as proposed for its use in biosensors
(Ramanavicius et al. 1999). For instance, PPy is known for the ease in
fabrication using electrochemical deposition techniques and in manipulation
of its property through the deposition conditions and electrolyte composi-
tions (which affect growth mechanism and dopant level) (e.g., Diaz et al.
1979; Salmon et al. 1982; Yang et al. 1991; Sutton and Vaughan 1995; Li
et al. 1995; Kaynak 1997; Miles et al. 2000). In many literature reports,
the film morphology and pore structure indeed depend on a wide range of
fabrication conditions. For instance, PPy films with significant differences
in density and morphological microstructure can be developed using con-
trol of pyrrole monomer concentration and growth rate under various poten-
tiostatic and galvanostatic conditions (Sutton and Vaughan 1995; Kaynak
1997). Another interesting example is the fabrication of biocompatible chi-
tosan or its derivatives into scaffolds that can be used as immobilization media
(Cooney et al. 2008).