Electrocatalysis for Fuel Cells at Enzyme-Modified Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 618

for NAD þ regeneration. An anode for the oxidation of ethanol to acetate uses two
enzymes that operate in series trapped together in the Nafion: alcohol dehydrogenase
and aldehyde dehydrogenase. Combination of this anode with a mediated bilirubin
oxidase O 2 -reducing cathode, as shown in Fig. 17.19, gives a membraneless fuel
cell that produces a small power output from oxidation of ethanol in air [Topcagic
and Minteer, 2006].
Minteer and co-workers have also exploited the broad substrate specificity of PQQ-
dependent alcohol dehydrogenase and aldehyde dehydrogenase from Gluconobacter
species trapped within Nafion to oxidize either ethanol or glycerol at a fuel cell anode
[Arechederra et al., 2007]. Although the alcohol dehydrogenase incorporates a series
of heme electron transfer centers, it is unlikely that many enzyme molecules trapped
within the mediator-free Nafion polymer are electronically engaged at the electrode.
17.4.3 H 2 / O 2 Enzyme Fuel Cells
The O 2 tolerance of a hydrogenase from the aerobic bacterium Ralstonia has been
exploited in a membraneless fuel cell that produces electricity from just 3% H 2 in
air, a level of fuel that is too dilute to burn (Fig. 17.20) (Plate 17.4) [Vincent et al.,
2006]. The design is far from optimized, and the power output is low owing to partial
inactivation of the hydrogenase by O 2 at the low level of H 2 and to short-circuiting at
the anode caused by O 2 reduction at bare regions of the graphite. However, the signifi-
cant aspects of this demonstration are the selective catalysis of H 2 oxidation in the
presence of an excess of O 2 and the simplicity of design. Both enzymes were directly
adsorbed onto PGE strip electrodes immersed in a shallow tray of electrolyte in
contact with still air containing a low level of H 2 . It should therefore be possible to
scale down such devices to provide localized micro- or nanoscale power sources.
With selective enzyme catalysts, trace H 2 in air becomes a viable fuel source, even
when contaminated with sulfides or CO [Vincent et al., 2007].
17.5 POSSIBILITIES FOR SCALING DOWN ENZYME
ELECTRODES (NANOTECHNOLOGIES)
A number of reports of enzyme electrocatalysis on carbon nanotubes are now
emerging. This approach may prove useful both for scaling up the current response
in enzyme fuel cells (by increasing the effective electrode surface area) and for scaling
down enzyme fuel cells for nanotechnology applications. The coupling between pro-
teins and nanoscopic structures has been studied extensively, and is the subject of sev-
eral reviews [Chen et al., 2007; Willner et al., 2006, 2007; Gooding, 2005; Katz et al.,
2003]. There are examples in which either Au nanoparticles or carbon nanotubes pro-
vide efficient electronic coupling between an electrode and an enzyme. Willner and
co-workers demonstrated that glucose oxidase shows unprecedented activity, albeit
at high overpotentials, when the FAD cofactor is directly wired to an Au 55 nanopar-
ticle, which in turn is wired via an aromatic diol to an Au electrode [Xiao et al.,
2003]. In a similar experiment, carbon nanotubes were used as a wire, with one end