124
7 Applications to Biofuel Cells and Bioreactors
redox mediators in a MET-type formate/O 2 biofuel cell realized an OCV of 1.2 V
[10]. In addition, serial assembly of biofuel cells can be employed to amplify the
output voltage. A multi-stacked sheet-type glucose biofuel cell composed of 15 multistacked biofuel cell units connected 5 in parallel and 3 in series provided an OCV
over 2.5 V, which allowed the successful operation of a radio at working voltages of
ca. 1.6 V for over 6 h [15]. The development of self-charging biosupercapacitors or
charge-storing biofuel cells has also attracted growing attention to overcome several
issues of biofuel cells [49–53].
7.3 Photobioelectrochemical Water Splitting
Water splitting is one of the important technological breakthroughs to construct a
hydrogen economy. The photosynthesis of photosynthetic organisms occurs in the
thylakoid membrane in the chloroplast and converts CO 2 in the atmosphere to organic
substances with excited electrons generated by photobiochemical splitting of H 2 O
(Eq. (7.1)):
2H 2 O + 4hν → 4e
−
+ 4H
+
+ O 2
(7.1)
The excited electrons can be transferred to an electrode in MET-type reactions.
The bioelectrode may be called photo-driven bioanodes. Biocatalysts used for photodriven bioanodes include: chloroplasts [54, 55], photosystem I (PSI) [56], PSII
[57], photosynthetic microorganisms [58–60], and thylakoid membranes [61–63].
1,2-Naphthoquinone (NQ) [64] and hexaammineruthenium (III) ([Ru(NH 3 ) 6 ]
3+ )
[65] may be utilized as mediators. Characteristics required as suitable mediators are: low barrier in electrode kinetics, high stability, suitable solubility, and
low redox potential (to minimize the overpotential), while large O 2 -tolerance and
high permeability of bio-membranes are also required. A photocurrent density
of 0.18 mA cm
−2 was reported in a photo-driven bioanode utilizing thylakoid
membranes and [Ru(NH 3 ) 6 ]
3+ [65] at a light flux density of 1.5 mmol m
−2 s
−1 ,
in which water-spread multi-walled carbon nanotubes (MWCNTs) were mounted
by π–π staking on a light-permeable ITO electrode, on which thin Au film with a
thickness of 4 nm was spattered to improve the electrode kinetics of the mediator.
Electrochemical coupling of the photo-driven bioanode with a DET-type MCObased biocathode yielded a bioelectrochemical photocell (called bio-solar cell) [65].
The cell exhibited an OCV of 0.61 V and a P max of 50 μW cm
−2 . This is a typical
and ideal example of an electrochemical device to support the hydrogen economy,
though further trial to decrease the overpotential of the photobioelectrochemical H 2 O
splitting is required in future. Thinning of cells is also very important in bio-solar
cells as well as other biofuel cells.
The reaction of Eq. (7.1) is a reverse reaction of a 4-electron reduction of O 2 , which
can be effectively catalyzed by MCO enzymes in bioelectrochemical system. Therefore, bioelectrochemical systems can realized by directional conversion between O 2
7 Applications to Biofuel Cells and Bioreactors
redox mediators in a MET-type formate/O 2 biofuel cell realized an OCV of 1.2 V
[10]. In addition, serial assembly of biofuel cells can be employed to amplify the
output voltage. A multi-stacked sheet-type glucose biofuel cell composed of 15 multistacked biofuel cell units connected 5 in parallel and 3 in series provided an OCV
over 2.5 V, which allowed the successful operation of a radio at working voltages of
ca. 1.6 V for over 6 h [15]. The development of self-charging biosupercapacitors or
charge-storing biofuel cells has also attracted growing attention to overcome several
issues of biofuel cells [49–53].
7.3 Photobioelectrochemical Water Splitting
Water splitting is one of the important technological breakthroughs to construct a
hydrogen economy. The photosynthesis of photosynthetic organisms occurs in the
thylakoid membrane in the chloroplast and converts CO 2 in the atmosphere to organic
substances with excited electrons generated by photobiochemical splitting of H 2 O
(Eq. (7.1)):
2H 2 O + 4hν → 4e
−
+ 4H
+
+ O 2
(7.1)
The excited electrons can be transferred to an electrode in MET-type reactions.
The bioelectrode may be called photo-driven bioanodes. Biocatalysts used for photodriven bioanodes include: chloroplasts [54, 55], photosystem I (PSI) [56], PSII
[57], photosynthetic microorganisms [58–60], and thylakoid membranes [61–63].
1,2-Naphthoquinone (NQ) [64] and hexaammineruthenium (III) ([Ru(NH 3 ) 6 ]
3+ )
[65] may be utilized as mediators. Characteristics required as suitable mediators are: low barrier in electrode kinetics, high stability, suitable solubility, and
low redox potential (to minimize the overpotential), while large O 2 -tolerance and
high permeability of bio-membranes are also required. A photocurrent density
of 0.18 mA cm
−2 was reported in a photo-driven bioanode utilizing thylakoid
membranes and [Ru(NH 3 ) 6 ]
3+ [65] at a light flux density of 1.5 mmol m
−2 s
−1 ,
in which water-spread multi-walled carbon nanotubes (MWCNTs) were mounted
by π–π staking on a light-permeable ITO electrode, on which thin Au film with a
thickness of 4 nm was spattered to improve the electrode kinetics of the mediator.
Electrochemical coupling of the photo-driven bioanode with a DET-type MCObased biocathode yielded a bioelectrochemical photocell (called bio-solar cell) [65].
The cell exhibited an OCV of 0.61 V and a P max of 50 μW cm
−2 . This is a typical
and ideal example of an electrochemical device to support the hydrogen economy,
though further trial to decrease the overpotential of the photobioelectrochemical H 2 O
splitting is required in future. Thinning of cells is also very important in bio-solar
cells as well as other biofuel cells.
The reaction of Eq. (7.1) is a reverse reaction of a 4-electron reduction of O 2 , which
can be effectively catalyzed by MCO enzymes in bioelectrochemical system. Therefore, bioelectrochemical systems can realized by directional conversion between O 2