Powering fuel cells through biocatalysis - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 390

most research on biocatalytic fuels cells previously devoted to biocatalytic oxidation
of fuels. However, in order to construct a biocatalytic fuel cell, it is essential to design
a functional cathode for the reduction of the oxidant that is coupled to the anode and
allows the electrically balanced current fl ow. Conventional platinum metal cathode cat-
alysts used in fuel cells for reduction of oxygen are usually not compatible with oxida-
tion of biocatalytic fuels since they can be poisoned and passivated by components in
the electrolyte. In addition, in the absence of a membrane assembly in miniaturized
membraneless fuel cells, oxidation of the fuel can occur at the cathode catalyst [1-3].
Biocatalytic processes at the cathode offer the advantages of selectivity for the oxidant
over fuel, allowing removal of the membrane assembly, and the possibility of decreas-
ing poisoning and passivation, over traditional fuel cell catalytic processes. Inhibition
and modulation of the biocatalytic processes remains, however, a major, unresolved,
problem for technological advances in the adoption of prototype biocatalytic cathodes
on an industrial scale.
We focus here on the use of oxygenases, particularly the “blue” copper oxygen-
ases, such as laccase and bilirubin oxidase, which can biocatalytically reduce oxygen
directly to water at relatively high reduction potentials under mild conditions. First,
however, we will briefl y consider reports on the use of hydrogen peroxide as an oxi-
dant in biocatalytic fuel cells.

12.4.2 Peroxidases
The use of hydrogen peroxide as an oxidant is not compatible with the operation of
a biocatalytic fuel cell in vivo , because of low levels of peroxide available, and the
toxicity associated with this reactive oxygen species. In addition peroxide reduction
cannot be used in a membraneless system as it could well be oxidized at the anode.
Nevertheless, some elegant approaches to biocatalytic fuel cell electrode confi gura-
tion have been demonstrated using peroxidases as the biocatalyst and will be briefl y
reviewed here.
Peroxide is a strong oxidant, with a standard reduction potential of

1.78 V vs
NHE, and is thus a good candidate for an oxidant [2b]. The direct reduction of per-
oxide at electrodes, however, has a high overpotential, thus necessitating the use of
catalysts. A recent interesting development for the design of peroxide-reducing cath-
odes is the use of ferrocene-mediated peroxide reduction by the enzyme horseradish
peroxidase at electrodes prepared by spray-painting of graphite, enzyme, mediator, and
binder onto a polymeric support [19]. These electrodes demonstrated peroxide reduc-
tion occurring close to the ferrocene/ferricenium redox potential of 0.25 V vs SCE

( 0.50 V vs NHE). Willner and co-authors have developed an impressive protocol for

the tethering and immobilization of microperoxidase-11 at gold electrodes, to yield elec-
trodes that reduce peroxide by direct electron transfer to the microperoxidase [15, 20,
21]. Microperoxidases are produced via proteolytic digestion of cytochrome c (Cyt. c )
to yield a heme-bound peptide of six, eight, nine or 11 amino acids. Microperoxidase-11
thus consists of 11 amino acids and a covalently linked heme site. Microperoxidase-11
was covalently linked via carbodiimide coupling chemistry of carboxylic acid functions