Electrocatalysis for Fuel Cells at Enzyme-Modified Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 620

[NiFe]-hydrogenase will adsorb onto nanotubes pretreated with polymyxin as shown in
Fig. 17.21a, and undergoes direct electrocatalysis of H 2 oxidation and production as
shown in Fig. 17.21b [Hoeben et al., 2008]. An enzyme fuel cell has been constructed
with glucose dehydrogenase at the anode, relying upon NAD þ regeneration via methyl-
ene blue-modified nanotubes, combined with a cathode based on laccase adsorbed on
carbon nanotubes [Yan et al., 2006]. These and other examples demonstrate that the
graphite-like properties, large aspect ratio (a few nanometers in diameter, micrometers
long), high surface area, and high conductivity of carbon nanotubes makes them
an excellent material for wiring enzymes [Gooding, 2005; Heller et al., 2005; Tasis
et al., 2006].
Microscopic or nanoscopic surface patterning can be applied using photo-
lithography or electron-beam lithography (top-down chip technology), but can also
be achieved, possibly more cheaply, by bottom-up assembly of nanoparticles or nano-
tubes on surfaces. The use of microscopic biofuel cells as localized power sources for
autonomous “lab-on-a-chip” applications can be envisioned, for example powering
microscopic sensors on an analytical microchip. Nanoscopic patterning can also
enhance mass transport, reducing the need to stir or to create forced flow. An assembly
of nanoscopic features may function as an array of nanoelectrodes, which promotes the
even distribution of the adsorbed enzyme. The diffusion profile of the fuel to each of
these small electrodes is radial. Because the flux is inversely proportional to the elec-
trode radius and is time-independent, a nanoscopic electrode will yield an extremely
high mass transport rate [Bard and Faulkner, 2001; Armstrong et al., 1993].
17.6 FUTURE PERSPECTIVES
Enzymes are efficient catalysts for cathodic and anodic reactions relevant to fuel cell
electrocatalysis in terms of overpotential, active site activity, and substrate/reaction
specificity. This means that design constraints (e.g., fuel containment and anode -
cathode separation) are relaxed, and very simple devices that may take up ambient
fuel or oxidant from their environment are possible. While operation is generally confined
to conditions close to ambient temperature, pressure, and pH, and power densities over
about 10 mW cm 22 are rarely achieved, enzyme fuel cells may be particularly useful in
niche environments, for example scavenging trace H 2 released into air, or sugar and
O 2 from blood. Thus, trace or unusual fuels become viable for energy production.
Vast numbers of different redox enzymes are expressed by microorganisms, and
new variants on known enzymes are still being discovered. Organisms have adapted
to life in a range of environments, and have therefore evolved to express enzymes
with specific properties. Isolation of new enzymes is likely to lead to biological
catalysts with novel properties, and the range can be extended further by targeted
genetic modifications. Of particular interest will be enzymes with improved electron
transfer properties (efficient electron relay chains) and even greater substrate and
reaction specificity.
Enzymes are not as unstable as is commonly believed. This is demonstrated by a
number of examples discussed in this Chapter: electrocatalysis of H 2 oxidation up