Powering fuel cells through biocatalysis - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 387

compared to the microbial fuel cells. Implantable biocatalytic fuel cells have thus been
proposed, where the body's own chemicals are used to produce power in vivo.
In this chapter, progress on biocatalytic fuel cell research is reviewed. Particularly,
reported biocatalytic fuel cell prototypes are critically assessed against the long-term
goal of the design of a small, implantable, and long-lived, low power source for bio-
medical applications. In this context, the substrates of choice to power such a device
are free oxygen as the oxidant and glucose as the fuel, both present in signifi cant con-
centration in physiological media. Although other recently reported biocatalytic fuel
cells not exclusively relying on glucose or oxygen are of interest to the fi eld, we will
mainly focus on the glucose-oxygen system, with eventual implantation in mind.
The fi rst oxygen-glucose biocatalytic fuel cell working at neutral pH was reported
in 1964 [4]. The overambitious goal of the time was to power energy-demanding
devices like the artifi cial heart. The development of the fi eld occurred concomitantly
with the development of bioelectrochemistry [5]. The low current densities obtained
in these early, and to some extent current, prototypes gradually reoriented the bio-
catalytic fuel cell research towards more modest and sensible aims [1-3]. The recent
upsurge in interest in biocatalytic fuel cell research is driven in part by the convergence
of advances, on the one hand, in biosensor design and enzyme electrochemistry, par-
ticularly in terms of increasing stable current densities at modifi ed electrodes, and on
the other hand, in microelectronics, where ever smaller and lower energy consuming
devices are being manufactured. These advances now possibly make the implantable
low power biocatalytic fuel cell a realistic goal, although many issues still need to be
resolved. Several interesting reviews on aspects of biocatalytic fuel cell research have
appeared in the last decade [1-3, 6, 7]. Here, we review the development of this excit-
ing research area, concentrating initially on some basic principles, then on the cathode
compartment of the cell, then on the anode, and fi nally summarizing research on com-
bining the two electrodes to provide prototype biocatalytic fuel cells.
12.2 BIOCATALYTIC FUEL CELL DESIGN
Contrary to traditional fuel cells, biocatalytic fuel cells are in principle very simple
in design [1]. Fuel cells are usually made of two half-cell electrodes, the anode and
cathode, separated by an electrolyte and a membrane that should avoid mixing of the
fuel and oxidant at both electrodes, while allowing the diffusion of ions to/from the
electrodes. The electrodes and membrane assembly needs to be sealed and mounted
in a case from which plumbing allows the fuel and oxidant delivery to the anode and
cathode, respectively, and exhaustion of the reaction products. In contrast, the simplic-
ity of the biocatalytic fuel cell design rests on the specifi city of the catalyst brought
upon by the use of enzymes.
Provided that the required enzymes can be immobilized at, and electrically com-
municated with, the surface of an electrode, with retention of their high catalytic prop-
erties and there is no electrolysis of fuel at the cathode or oxidant at the anode, or a
solution redox reaction between fuel and oxidant, the biocatalytic fuel cell then simply