Biomedical Engineering Reference
In-Depth Information
E
ox
M
red
S
e¯
M
ox
E
red
P
FIGURE 13.7
Schematic illustrating the regeneration of redox species (enzymes/coenzymes
and mediators) in the bioelectrocatalysis. (S: substrate, P: product, E
ox
and
E
red
: oxidized and reduced form of the enzyme/coenzyme, M
red
and M
ox
:
reduced and oxidized form of mediator.) (Modified from Figure 1 in Ikeda, T.
and Kano, K.,
J. Biosci. Bioeng.,
92, 2001.)
a thermal mechanical pathway, it also requires an extremely complex activa-
tion and separation of the reactants to enable an ecient energy conversion
process.
Such a “balance-of-the-plant” must allow a variety of tasks to accomplish
eciently. For a biofuel cell to fulfill its promise, unlimited flow of reactants
and products to reach and remove, respectively, from the stationary cata-
lyst layer must be conducive to fuel delivery and product removal (Cooney
and Liaw 2008). If direct electron transfer is desired (the more promising
and ecient form of a biofuel cell operation) biocatalyst needs to be further
supported on a conductive surface such that electrons can transfer from the
reaction center to the catalyst support (Katz et al. 2003). Because catalysts
in an enzymatic biofuel cell are inherently unstable the desired immobiliza-
tion approach must also provide conditions that will stabilize and protect the
enzyme from gradually denaturing (Minteer et al. 2007). For microbial biofuel
cells, more recent studies on nanowired electron transfer from the microbial
species to electrode surface indeed are very intriguing (Reguera et al. 2005;
Gorby et al. 2006), which also illustrate the complexity involved in the charge
transfer processes in a biofuel cell system.
13.3 Desirable Properties and Functionalities
Conductive materials (for electrons or ions, or both) with high porosity
(i.e., high surface area and proper pore size distribution [PSD]) and suitable
chemical backbone are more likely to fulfill the requirements for ecient
energy transfer. Ideally the porosity needs to be on the tens of nanometer
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