In Situ Characterizations of Porous Media for Applications in Biofuel Cells: Issues and Challenges - Porous Media: Applications in Biological Systems and Biotechnology - page 508

practical applications. The recent rapid expansion of interests on nanomate-
rials and fabrications demands better temporal and spatial characterization
techniques to facilitate research and development. However, only few nano-
or meso-scale characterization techniques (such as atomic force microscope,
or AFM; e.g., Picart et al. 2007; Xu and Siedlecki 2007; Zhitomirsky and
Hashambhoy 2007) are available for in situ observations, particularly, in solu-
tion. In addition, few can easily allow any moderate to large area observation
with respect to experimental conditions in transient. Thus, any in situ , non-
intrusive technique to offer such utility appears very attractive, especially for
surface or interface characterizations in liquid environments. Here we present
some simple illustrations to show that imaging ellipsometric measurements
could offer useful utility for studying conductive polymer film depositions.
The first example is the deposition of conductive PPy films on Pt or glassy
carbon electrode surface. Depending on the surface condition, the PPy film could
have different morphological appearances. It also depends on the deposition con-
ditions, such as monomer concentration, electrolyte species and its concentra-
tion, current density or working potential. In Figure 13.10, an example is shown.
By varying the current density (thus, with different deposition rates), films of
different density and morphology can be developed (Liaw et al. 2005).
In another example (Figure 13.11), as shown by the deposition of poly-
meric methylene green, using the imaging ellipsometric capability, Svoboda
et al. (Svoboda et al. 2007) show that a dense film of the polymer can be
obtained, judging from the retention of the microstructure and roughness
of the underlying Pt surface. Methylene green is an important mediator for
NADH (the reduced form of nicotinamide adenine dinucleotide or NAD + )
reoxidation in biofuel cell operation, and the polymeric form of this azine
has been suggested (Zhou et al. 1996; Akkermans et al 1999; Karyakin et al.
1999) more effective in performing such a function. Without the shuttling of
electrons by the coenzyme NAD + -NADH redox reaction and the poly-azine to
transfer the electrons to the electrode surface, the energy harnessing from the

2 mA/cm 2

0.5 mA/cm 2 1 mA/cm 2

FIGURE 13.10
Polypyrrole deposition using galvanostatic technique at various current den-
sities was shown to produce different morphologies and densities in the films.