7.2 Biofuel Cell
121
combined a glucose bioanode, in which GOD or GDH is usually employed as redox
enzymes, and an O 2 biocathode, in which MCOs, such as BOD, Lac, and CueO are
often utilized. For an example, a glucose/O 2 biofuel cell was reported by combination of an NAD-dependent GDH/diaphorase-based bioanode and a BOD-based
O 2 biocathode; the cell provided a maximum power density (P max ) of 5 mW cm
−2
at an operational cell voltage (E cell ) of 0.5 V [30] At the bioanode in the system,
glucose is 2-electron-oxidized with NAD
+ by NAD-dependent GDH, and FMN in
Dp accepts the hydride ion from NADH and provides two electrons to 2-amino1,4-naphthoquinone as a mediator, which transfer the electrons to the bioanode in a
MET-type bioelectrocatalytic mode. At the biocathode, O 2 acts as the final electronacceptor in a mode of a MET-type bioelectrocatalysis of BOD using K 3 [Fe(CN) 6 ]
as a mediator [30].
Fructose also is a common sugar utilized for biofuel cells. Due to the DET capability of FDH, a DET-type fructose/O 2 biofuel cell was reported with a P max of 0.85
mW cm
−2 at a E cell of 0.41 V, in which fructose was 2-electron-oxidized by FDH in a
DET-type bioelectrocatalytic mode at the bioanode, while O 2 was 4-electron-reduced
in a DET-type bioelectrocatalysis of Lac at the biocathode [26]. Furthermore, a P max
density of 2.6 mW cm
−2 at an E cell of 0.46 V was achieved under quiescent conditions by combining an FDH-based fructose bioanode with a BOD-based air-breathing
O 2 biocathode [22]. In addition, many other sugars including xylose, sucrose, and
polysaccharides such as maltodextrin have been also used as fuels in biofuel cells.
H 2 /O 2 Biofuel Cells
Dihydrogen (H 2 ) is also a common fuel for biofuel cells because it has the highest
energy density value per mass and has been widely used in conventional fuel cells.
H 2 /O 2 fuel cells are known as clean and highly efficient energy conversion devices
and have attracted increasing attentions for establishing the “hydrogen economy”.
In H 2 /O 2 biofuel cells, H 2 is oxidized by H 2 ase at the bioanode and O 2 is typically
reduced by MCOs at the biocathode, the product being just H 2 O. The 2-electron
oxidation of H 2 is a complete oxidation, unlike that of sugars and alcohol. Although
H 2 ase shows high catalytic efficiency, two major issues have to be considered in
the utilization of H 2 ase as an electrocatalyst: their O 2 sensitivity and the oxidative
inactivation that occurs at positive electrode potentials [31, 32]. A useful approach
to protect H 2 ase from O 2 damage and high-potential deactivation was to entrap
the enzyme into a specifically designed viologen-based redox polymer [33]; almost
whole parts of the polymer were reduced in a MET-type reaction of H 2 ase, and
the reduced polymer can scavenge (or reduce) O 2 diffusing from the solution phase
and sometimes reactivate (or reduce) H 2 ase that is oxidatively deactivated at electrodes at positive potentials. An H 2 /O 2 biofuel cell combining the viologen polymerfunctionalized H2ase bioanode and BOD biocathode provided a P max of 0.178 mW
cm
−2 and an open circuit voltage (OCV ) of 0.947 V [34].
In contrast to the viologen-based redox hydrogel, gas-diffusion systems were
also proposed to prevent the H 2 ase inactivation for DET-type bioelectrocatalysis by
providing high-speed H 2 -supply around the bioelectrodes [35] (Fig. 7.2). Since the
oxidation of the ready-form enzyme with O 2 or at electrodes in the deactivation
121
combined a glucose bioanode, in which GOD or GDH is usually employed as redox
enzymes, and an O 2 biocathode, in which MCOs, such as BOD, Lac, and CueO are
often utilized. For an example, a glucose/O 2 biofuel cell was reported by combination of an NAD-dependent GDH/diaphorase-based bioanode and a BOD-based
O 2 biocathode; the cell provided a maximum power density (P max ) of 5 mW cm
−2
at an operational cell voltage (E cell ) of 0.5 V [30] At the bioanode in the system,
glucose is 2-electron-oxidized with NAD
+ by NAD-dependent GDH, and FMN in
Dp accepts the hydride ion from NADH and provides two electrons to 2-amino1,4-naphthoquinone as a mediator, which transfer the electrons to the bioanode in a
MET-type bioelectrocatalytic mode. At the biocathode, O 2 acts as the final electronacceptor in a mode of a MET-type bioelectrocatalysis of BOD using K 3 [Fe(CN) 6 ]
as a mediator [30].
Fructose also is a common sugar utilized for biofuel cells. Due to the DET capability of FDH, a DET-type fructose/O 2 biofuel cell was reported with a P max of 0.85
mW cm
−2 at a E cell of 0.41 V, in which fructose was 2-electron-oxidized by FDH in a
DET-type bioelectrocatalytic mode at the bioanode, while O 2 was 4-electron-reduced
in a DET-type bioelectrocatalysis of Lac at the biocathode [26]. Furthermore, a P max
density of 2.6 mW cm
−2 at an E cell of 0.46 V was achieved under quiescent conditions by combining an FDH-based fructose bioanode with a BOD-based air-breathing
O 2 biocathode [22]. In addition, many other sugars including xylose, sucrose, and
polysaccharides such as maltodextrin have been also used as fuels in biofuel cells.
H 2 /O 2 Biofuel Cells
Dihydrogen (H 2 ) is also a common fuel for biofuel cells because it has the highest
energy density value per mass and has been widely used in conventional fuel cells.
H 2 /O 2 fuel cells are known as clean and highly efficient energy conversion devices
and have attracted increasing attentions for establishing the “hydrogen economy”.
In H 2 /O 2 biofuel cells, H 2 is oxidized by H 2 ase at the bioanode and O 2 is typically
reduced by MCOs at the biocathode, the product being just H 2 O. The 2-electron
oxidation of H 2 is a complete oxidation, unlike that of sugars and alcohol. Although
H 2 ase shows high catalytic efficiency, two major issues have to be considered in
the utilization of H 2 ase as an electrocatalyst: their O 2 sensitivity and the oxidative
inactivation that occurs at positive electrode potentials [31, 32]. A useful approach
to protect H 2 ase from O 2 damage and high-potential deactivation was to entrap
the enzyme into a specifically designed viologen-based redox polymer [33]; almost
whole parts of the polymer were reduced in a MET-type reaction of H 2 ase, and
the reduced polymer can scavenge (or reduce) O 2 diffusing from the solution phase
and sometimes reactivate (or reduce) H 2 ase that is oxidatively deactivated at electrodes at positive potentials. An H 2 /O 2 biofuel cell combining the viologen polymerfunctionalized H2ase bioanode and BOD biocathode provided a P max of 0.178 mW
cm
−2 and an open circuit voltage (OCV ) of 0.947 V [34].
In contrast to the viologen-based redox hydrogel, gas-diffusion systems were
also proposed to prevent the H 2 ase inactivation for DET-type bioelectrocatalysis by
providing high-speed H 2 -supply around the bioelectrodes [35] (Fig. 7.2). Since the
oxidation of the ready-form enzyme with O 2 or at electrodes in the deactivation