Electrocatalysis for Fuel Cells at Enzyme-Modified Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 591

known example of “reversible” electrocatalytic CO 2 /CO cycling [Parkin et al., 2007],
and Complex I (NADH dehydrogenase) on an electrode provides the only known
example of efficient NAD þ /NADH interconversion [Barker et al., 2007].
The laccase molecule represented in Fig. 17.3 has a diameter of about 5 nm, giving
it a footprint of about 20 nm 2 : this means that the maximum coverage supported on a
flat electrode surface would be about 8 pmol cm 22 , i.e., the surface density of active
sites is very low. A key implication is that in order to obtain high current densities
(e.g., greater than 10 mA cm 22 ) at enzyme electrodes, the effective surface area of
the electrode must be large and/or the coating of enzyme must be fairly thick.
Increasing thickness brings additional challenges, including the need to move
electrons, substrate, and product through multiple layers. Strategies for “wiring”
multiple layers of enzyme at an electrode are discussed in Sections 17.2 and 17.3.
Many of the methods discussed in this chapter are amenable to other applications
besides fuel cells. For example, the effort invested in enzymatic fuel cells is dwarfed
by efforts to develop enzymatic electrochemical sensors for species such as glucose,
methanol, and ethanol using very similar technologies. Less well developed, but
also with significant potential value, is the study of chemical conversions by
enzyme electrocatalysis, for example selective oxidation of a single enantiomer of a
secondary alcohol by an alcohol dehydrogenase [Kroutil et al., 2004]. Furthermore,
the investigation of enzyme catalysis on electrodes has provided much insight into
the potential-dependent reactions of enzymes that would be difficult to control and
study for enzymes in solution.
Two key features distinguish enzymatic fuel cells from sensors and highlight the
challenges of implementing enzymes in power-producing devices. First, it is generally
desirable to minimize current draw in a sensor, whereas in a fuel cell it is desirable to
maximize both cell current and voltage under operating conditions. The demand for
high cell voltage is not difficult to meet, because enzymes are available to catalyze
the oxidation of H 2 , sugars, or other alcohols and the reduction of O 2 at fairly low over-
potentials. Achieving high currents in enzyme fuel cells is a much more difficult prob-
lem: although enzyme turnover rates per active site can be very high, their large size
means that the turnover rate per unit volume is low compared with precious metals.
Trapping enzymes within a redox-active hydrogel or expanding a solid conducting
support into three-dimensional space are strategies that have been exploited to address
this problem.
Second, sensors are often intended for a single use, or for usage over periods of one
week or less, and enzymes are capable of excellent performance over these time scales,
provided that they are maintained in a mild environment at moderate temperature and
with minimal physical stress. Stabilization of enzymes on conducting surfaces over
longer periods of time presents a considerable challenge, since enzymes may be sub-
ject to denaturation or inactivation. In addition, the need to feed reactants to the biofuel
cell means that convection and therefore viscous shear are often present in working
fuel cells. Application of shear to a soft material such as a protein-based film can
lead to accelerated degradation due to shear stress [Binyamin and Heller, 1999].
However, enzymes on surfaces have been demonstrated to be stable for several
months (see below).