Powering fuel cells through biocatalysis - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 389

e

substrate

A
N
O
D
E

enzyme

product

FIGURE 12.2 Schematic depicting a direct electron transfer process between an enzyme and the elec-
trode, acting as an anode in this case.

product ox Enzyme red Med ox

A
N
O
D
E

substrate red Enzyme ox Med red

E 0 (substrate/product) E 0 (enz ox /enz red ) E 0 (med ox /med red )

FIGURE 12.3 Schematic depicting the electron fl ow in an enzyme-catalyzed mediated electron transfer
oxidation of substrate. The relative magnitudes of the standard reduction potentials of each element for effi -
cient mediation are shown beneath the scheme.

and the mediator is considered to be the second substrate (co-substrate) of the catalytic
process. The use of mediators can increase the rate of electron transfer, sometimes by
several orders of magnitude.
In redox mediation, to have an effective electron exchange, the thermodynamic
redox potentials of the enzyme and the mediator have to be accurately matched. For
biocatalytic electrodes, effi cient mediators must have redox potentials downhill from
the redox potential of the enzyme: a 50 mV difference is proposed to be optimal [1,
18]. The tuning of these potentials is a compromise between the need to have a high
cell voltage and a high catalytic current. Furthermore, an obvious requirement is that
the mediator must be stable in the reduced and oxidized states. Finally, for operation
of a membraneless miniaturized biocatalytic fuel cell, the mediators for both the anode
and the cathode must be immobilized to prevent power dissipation by solution redox
reactions between them.

12.4 BIOCATALYTIC CATHODES

12.4.1 Enzymes and substrates
In this section we review research on biocatalytic cathodes for oxidant reduction. The
biocatalytic reduction of oxidants has only recently attracted renewed attention, with