Electrocatalysis for Fuel Cells at Enzyme-Modified Electrodes - Fuel Cell Catalysis: A Surface Science Approach - page 613

advantage owing to the unique properties of biological catalysts. In particular, the
impressive substrate selectivity of enzymes leads to novel fuel cell designs and appli-
cations, and provides a benchmark for the development of catalysts for future technol-
ogies. While there are now a few examples of methanol/O 2 fuel cells that use modified
precious metal catalysts (e.g., metal chalcogenides) for O 2 -tolerant methanol oxidation
[Alonso-Vante, 2003], there are no well-understood examples of O 2 -tolerant H 2
oxidation at precious metals. In this section, we set out examples of membraneless
fuel cells operating on fuels such as sugars, ethanol, glycerol, and H 2 .
17.4.1 Sugars as Fuels
The most well-established use of enzymes in fuel cells is in generating electricity from
sugar oxidation, using enzymes such as glucose oxidase, glucose dehydrogenase, or
fructose dehydrogenase at the anode in combination with an O 2 -reducing cathode
modified with laccase or bilirubin oxidase. The availability of sugars and O 2 in
blood and other biological fluids means that implantable devices should be possible,
and thus a showcase application of biological fuel cells is likely to be in the area of
power for biomedical devices.
Heller and co-workers, in a series of papers, describe the development of a low
power sugar/O 2 biofuel cell device suitable for biomedical applications (for a
review, see Heller [2004]). The electrodes consist of carbon fibers, approximately
7 mm in diameter, on which a hydrogel is deposited that incorporates enzymes and
Os III/II redox mediator centers (as described in Sections 17.2.1 and 17.3.1)
(Fig. 17.17). The enzyme at the anode is generally glucose oxidase, and laccase
and bilirubin oxidase have been utilized at the cathode. Bilirubin oxidase is a more
suitable complement for glucose oxidase, having maximum activity close to pH 7
and remaining active in the presence of chloride. Tuning the redox potential of the
mediator in the anode and cathode polymer by modifying the ligands to Os meant
that the electrodes can operate close to the onset potential for catalysis by glucose
oxidase and bilirubin oxidase respectively, and the open circuit potential is around
0.8 V (Fig. 17.17 inset). This compares favorably with the open circuit potential of
120 mV reported for an earlier glucose/O 2 cell that utilized an enzyme with a high
overpotential for O 2 reduction at the cathode, cytochrome c oxidase [Katz et al., 1999].
Recently, there has been interest in enzyme fuel cells that utilize high surface area
forms of carbon modified with enzyme molecules that are able to undergo direct electron
transfer. Kano and co-workers achieved current densities in the range 2 - 4 mA cm 22 by
immobilizing laccase onto a mesoporous carbon aerogel and D -fructose dehydrogenase
onto nanodimensional Ketjen black particles and then coating these materials onto a
carbon paper support [Kamitaka, 2007]. A membraneless fuel cell incorporating these
electrodes and operating in O 2 -saturated aqueous solution at pH 5 containing 0.2 M fruc-
tose gave an open circuit voltage of 790 mV, and produced a current density greater than
1mAcm 22 at a cell voltage of 410 mV in an unstirred solution.
17.4.1.1 Possibilities for Enzymes in Implantable Fuel Cells There is
significant and increasing demand for power supplies for implantable medical devices,
including continuous glucose monitors for diabetic patients, thermal sensors for