Electrocatalysis for Fuel Cells at Enzyme-Modified Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 586

Figure 17.1 Active sites of some enzymes that carry out reactions relevant to fuel cell

catalysis.

of bacteria that live on trace H 2 in aerobic conditions expresses hydrogenases that are
able to oxidize low levels of H 2 in air (even at high levels of carbon monoxide), and the
hydrogenases from sulfate-reducing bacteria are tolerant to sulfide over a wide poten-
tial window [Vincent et al., 2007]. The remarkable substrate specificity of many
enzymes forms the basis for their application in novel fuel cell devices that would
not be possible with conventional metal catalysts. Other interesting examples of
enzyme catalysis do indeed arise from organisms found in unusual environments,
such as the reversible interconversion of CO and CO 2 at an iron- and nickel-containing
cluster in carbon monoxide dehydrogenase [Parkin et al., 2007], or H 2 oxidation at
100 8C by hydrogenases from thermophilic microorganisms.
Enzymes that catalyze redox reactions are usually large molecules (molecular mass
typically in the range 30 - 300 kDa), and the effects of the protein environment distant
from the active site are not always well understood. However, the structures and
reactions occurring at their active sites can be characterized by a combination of
spectroscopic methods, X-ray crystallography, transient and steady-state solution
kinetics, and electrochemistry. Catalytic states of enzyme active sites are usually
better defined than active sites on metal surfaces.
The large size of redox enzymes means that diffusion to an electrode surface will be
prohibitively slow, and, for enzyme in solution, an electrochemical response is usually
only observed if small, soluble electron transfer mediator molecules are added. In this
chapter, discussion is limited to examples in which the enzyme of interest is attached
to the electrode surface. Electrochemical experiments on enzymes can be very simple,
involving direct adsorption of the protein onto a carbon or modified metal surface from
dilute solution. Protein film voltammetry, a method in which a film of enzyme in direct