Electrocatalysis for Fuel Cells at Enzyme-Modified Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 592

Electrocatalysis by redox enzymes has now been demonstrated at the anode and/or
cathode of a variety of small scale proof-of-concept fuel cells. It remains to be seen
whether redox enzymes can be produced at low cost and can be sufficiently robust
to enable their widespread use in fuel cell technologies. Enzymes almost always
require an aqueous environment to function, and generally will not tolerate extremes
of temperature or pH, so it is unlikely that they will compete with precious metal cat-
alysts in high power output devices, for which elevated operating temperatures are
desirable. (In some cases, ambient biocatalysis of reactions that otherwise require
high temperatures can be an advantage, for example reversible interconversion of
CO and CO 2 at an electrode modified with carbon monoxide dehydrogenase
[Parkin et al., 2007].) However, the specificity of enzyme catalysis may open up
new concepts and designs for fuel cells, leading to specialized applications, such as
biologically implantable power sources [Barton et al., 2004], disposable fuel cells
where limited lifetime is not a problem, or fuel cells that function on contaminated
fuels [Vincent et al., 2005, 2006]. Whether or not enzymes find applications in real
energy cycling devices, their extremely high specificity, discussed in detail in this
chapter, provides a benchmark for desirable properties of fuel cell catalysts, and
should provide inspiration for the development of new catalysts for energy cycling.
17.1.3 Strategies for Exploiting Enzyme Electrocatalysis
17.1.3.1 Enzyme Immobilization and Electron Transfer For fuel cell appli-
cations, it is highly advantageous to immobilize the enzymes at the respective anode or
cathode in order to facilitate efficient electron transfer and to avoid washing out the
biocatalyst if the fuel is supplied in liquid form. Redox centers in the enzymes must
be in close electronic contact with an electronically conducting support. This may
be direct, or may involve co-immobilized electron transfer mediator molecules that
are able to exchange electrons with redox centers buried in the protein or centers far
from the electrode. Earlier work used soluble mediators (see, e.g., Yahiro et al.
[1964]; Davis et al. [1983]; Yue and Lowther [1986]; Palmore et al. [1999]). This
unnecessarily complicates cell design: a single compartment fuel cell is possible
with a pair of immobilized enzymes that are selective for fuel and oxidant, but if elec-
tron transfer mediators are present in the electrolyte solution, the anode and cathode
must be in separate compartments to prevent cross reaction of the mediators.
The choice of immobilization strategy obviously depends on the enzyme, electrode
surface, and fuel properties, and on whether a mediator is required, and a wide range of
strategies have been employed. Some general examples are represented in Fig. 17.4.
Key goals are to stabilize the enzyme under fuel cell operating conditions and to opti-
mize both electron transfer and the efficiency of fuel/oxidant mass transport. Here, we
highlight a few approaches that have been particularly useful in electrocatalysis
directed towards fuel cell applications.
Achieving fast electron transfer to enzyme active sites need not be complicated. As
mentioned above, many redox enzymes incorporate a relay of electron transfer centers
that facilitate fast electron transfer between the protein surface and the buried active
site. These may be iron - sulfur clusters, heme porphyrin centers, or mononuclear