Powering fuel cells through biocatalysis - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 386

Fuel cells generate electricity through an electrochemical process in which the
energy stored in a fuel is converted directly into electricity. Fuel cells chemically com-
bine the molecules of a fuel and oxidant, without burning, dispensing with the ineffi -
ciencies and pollution of traditional combustion.
In principle, a fuel cell operates like a battery. Unlike a battery, a fuel cell does not
run down or require recharging: it will produce electricity as long as fuel and oxidant
are supplied. The electrochemical reactions of a fuel cell consist of two separate reac-
tions: an oxidation half-reaction occurring at the anode and a reduction half-reaction
occurring at the cathode. The anode and the cathode are separated from each other
by the electrolyte and an ion-exchange membrane. In the oxidation half-reaction, the
input fuel passes over the anode and is catalytically split, producing ions, which travel
through the electrolyte to the cathode, and electrons, which travel through an external
circuit to serve an electric load, which consumes the power generated, to the cathode.
In the reduction half-reaction, an oxidant, supplied from air or fl uid fl owing past the
cathode, combines with the ions and electrons to complete the circuit. The power out-
put of the fuel cell is the product of the cell voltage and the cell current. The theoreti-
cal thermodynamic cell voltage is the difference in standard reduction potentials of the
oxidant and fuel. System losses, however, in the form of kinetic and mass transport
limitations and IR drop can severely lower the power output of fuel cells. For example,
in commercial fuel cells, catalysts are used on both the anode and cathode to increase
the kinetics of each half-reaction. The catalyst that works the best on each electrode is
platinum, a non-selective catalyst, and a very expensive material.
Biocatalytic fuel cells are fuel cells which rely upon biocatalytic reactions at the
electrodes to convert chemical fuels and oxidants into electrical power. The biocata-
lytic reactions used to produce power range from reactions of fermentation broths con-
taining whole cells, to isolated enzyme biocatalysts. Fermentation broths containing
microbial cells can be used to produce chemical fuels, such as sugars or hydrogen,
in the anodic compartment of a fuel cell [2]. The production of the fuels from micro-
bial cells by fermentation may alternatively be decoupled from the fuel cell, with the
fuel being fed into a conventional fuel cell [2]. Extraction of electrical power from
microbial fermentation processes can also be by addition of small redox molecules
that can mediate electron transfer from the microbial electron transport pathway to
the electrode surface [2]. An advantage of using whole cells to produce power is that
the biocatalysts and micro-organisms can be maintained in their natural environment,
while effi ciently producing power over long periods. The power densities that can be
extracted from these fuel cells are, however, typically low, and they are thus expected
to fi nd limited application in implanted electronic devices. Renewed research in devel-
opment of these types of fuel cells has instead been driven by the goal of large-scale
clean power production. Given the low power densities of these cells, however, it is
doubtful if this technology will ever compete with conventional fuel cells [3].
Biocatalytic fuel cells using isolated redox enzymes were fi rst investigated in 1964
[4]. These fuel cells represent a more realistic opportunity for provision of implant-
able power, given the exquisite selectivity of enzyme catalysts, their activity under
physiological conditions, and the relative ease of immobilization of isolated enzymes,