cell that
uses an enzyme. As shown in Fig. 8, in a biofuel cell
using glucose as the fuel, an oxidation reaction of
20 glucose progresses to extract electrons at the negative
electrode (the anode), and a reduction reaction of oxygen
(02) in the atmosphere progresses at the positive
electrode (the cathode).
In a conventional biofuel cell, an anode and a
25 cathode are positioned to face each other via an
insulating film having proton permeability and an
electrolyte layer containing a buffering substance, and
the fuel solution is not in contact with the cathode
serving as an air electrode (see Patent Documents 1
30 through 3, for example) . There has also been a suggested
biofuel cell that has the cathode in contact with a
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buffer solution saturated with dissolved oxygen (see
Patent Document 4, for example). In this biofuel cell
disclosed in Patent Document 4, the fuel solution in
contact with the anode and the buffer solution in contact
5 with the cathode are separated from each other by a salt
bridge or a polymer electrolyte film.
In a biofuel cell, the substrate specificity of the
enzyme serving as a reaction catalyst is high. Therefore,
even when a fuel such as glucose is brought into contact
10 with the air electrode (the cathode), the battery
characteristics are hardly affected, and cross-over does
not easily occur. Accordingly, not only biofuel cells of
air-exposure types as disclosed in Patent Documents 1
through 3, but also immersion-type biofuel cells each
15 having a fuel solution in contact with both the anode and
the cathode can be realized.
CITATION LIST
PATENT DOCUMENTS
20 Patent Document 1: Japanese Patent Application Laid-Open
No. 2006-93090
Patent Document 2: Japanese Patent Application Laid-Open
No. 2008-305559
Patent Document 3: Japanese Patent Application Laid-Open
25 No. 2009-245920
Patent Document 4: Japanese Patent Application Laid-Open
No. 2006-508519
SUMMARY OF THE INVENTION
30 PROBLEMS TO BE SOLVED BY THE INVENTION
To continue power generation with the fuel
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electrode (the anode) in a biofuel cell, a sufficient
fuel component such as glucose needs to exist in the fuel
solution. Further, to increase the battery capacity, a
fuel solution having a higher component concentration
5 than that of the fuel needs to be used. However, if the
concentration of a fuel component such as glucose is made
higher, the viscosity of the fuel solution becomes higher.
Therefore, in the case of an immersion-type biofuel cell,
the diffusion coefficient becomes lower, and the
10 characteristics of the air electrode (the cathode) are
degraded.
Therefore, the principal object of the present
invention is to provide a fuel cell that can increase its
battery capacity without degrading the cathode
15 characteristics.
SOLUTION TO PROBLEMS
A fuel cell according to the present invention
includes one or more battery cell units in which an
20 oxidoreductase exists on the surface of an anode and/or a
cathode, and the cathode is in contact with both a liquid
phase and a gas phase.
In this battery cell unit, a selective transmission
film that restrains permeation of at least the fuel
25 component is provided between a first solution unit
provided around the anode and a second solution unit
provided around the cathode.
Here, the surface of each of the anode and the
cathode includes the entire external surface of the
30 electrode and the entire internal surface of the space
inside the electrode, and this also applies to the cases
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described below.
In the present invention, the selective
transmission film that restrains permeation of at least
the fuel component is provided between the first solution
5 unit and the second solution unit. Accordingly,
diffusion of the fuel component into the cathode side can
be restrained.
In this fuel cell, the fuel component concentration
in the solution in contact with the anode can be made
10 higher than that in the solution in contact with the
cathode.
In that case, a fuel solution having a lower fuel
component concentration may be introduced from the first
solution unit into the second solution unit via the
15 selective transmission film.
Also, a first inlet through. which a solution is
introduced into the first solution unit and a second
inlet through which a solution is introduced into the
second solution unit may be provided, and solutions
20 having different fuel component concentrations from each
other may be stored in the first solution unit and the
second solution unit.
Further, the selective transmission film may also
restrain permeation of an enzyme and/or a mediator.
25 Further, the fuel component may be a saccharide,
for example.
Further, the selective transmission film may have a
mean pore size of 0.5 μm or smaller.
Further, the selective transmission film may be
30 formed of a cellulose film or a synthetic polymer film.
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EFFECTS OF THE INVENTION
According to the present invention, the fuel
component concentration can be made higher only on the
fuel electrode (anode) side. Accordingly, the. battery
5 capacity can be increased while a high battery power
output is maintained, without degradation of the cathode
characteristics.
BRIEF DESCRIPTION OF DRAWINGS
10 Fig. 1 is a diagram schematically showing the
structure of the battery cell unit in a fuel cell
according to a first embodiment of the present invention.
Fig. 2 is a diagram schematically showing the
structure of the battery cell unit in a fuel cell
15 according to a modification of the first embodiment of
the present invention.
Fig. 3 is a diagram schematically showing the
structure of the battery cell unit in a fuel cell
according to a second embodiment of the present invention.
20 Fig. 4(a) is a conceptual diagram showing an
example in which cellophane is provided between
respective solution units; Fig. 4(b) is a conceptual
diagram showing a comparative example in which non-woven
fabric is provided between respective solution units.
25 ' Fig. 5 is a graph showing the relationship between
the glucose concentration in the fuel solution and the
output in each of the fuel cells of the example and the
comparative example, with the abscissa axis indicating
glucose concentration and the ordinate axis indicating
30 relative current.
Fig. 6 is a diagram schematically showing a cell
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used'in Second Example of the present invention.
Figs. 7(a) and 7(b) are graphs showing temporal
changes in the cell voltages of the biofuel cells of the
example and the comparative example, with the abscissa
5 axis indicating time and the ordinate axis indicating
voltage.
Fig. 8 is a diagram showing a reaction scheme of a
biofuel cell that uses an enzyme.
10 MODE FOR CARRYING OUT THE INVENTION
The following is a detailed description of
embodiments for carrying out the present invention, with
reference to the accompanying drawings. It should be
noted that the present invention is not limited to the
15 embodiments described below. Explanation will be made in
the following order.
1. First Embodiment
(Example of a biofuel cell in which a common
solution inlet is provided, and a solution is separated
20 in the battery cell unit)
2. Modification of the First Embodiment
(Example of a biofuel cell in which a selective
transmission film also serves as a separator)
3. Second Embodiment
25' (Example of a biofuel cell in which a biofuel cell
is provided on each of the anode side and the cathode
side)
<1. First Embodiment>
[Structure of a Battery Cell Unit]
30 First, a biofuel cell according to a first
embodiment of the present invention is described. Fig. 1
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is a'diagram schematically showing the structure of the
battery cell unit in the fuel cell of this embodiment.
As shown in Fig. 1, the biofuel cell of this embodiment
is an immersion-type fuel cell in which electrolyte is in
5 contact with both an anode (a fuel electrode) 2 and a
cathode (an air electrode) 3.
In this biofuel cell, an oxidoreductase exists on
the electrode surface of one or both of the anode 2 and
the cathode 3. Here, the surface of an electrode
10 includes the entire external surface of the electrode and
the entire internal surface of the space inside the
electrode, and this also applies to the cases described
below. The cathode 3 is designed to be in contact with
both a liquid phase (a solution) and a gas phase (the
15 air). Current collectors 7 and 8 are provided in contact
with the anode 2 and the cathode 3, respectively.
An anode solution unit 4 and a cathode solution
unit 5 are provided around the anode 2 and the cathode 3,
respectively, and a selective transmission film 6 is
20 provided in between. Further, in the fuel cell of this
embodiment, a fuel solution inlet 9 for introducing a
fuel solution 10 into the battery cell unit 1 is provided,
and this fuel solution inlet 9 leads to the anode
solution unit 4.
25 [Anode 2]
The anode 2 is a fuel electrode, and may be an
anode that has an oxidoreductase immobilized onto the
surface of an electrode made of a conductive porous
material, for example. As the conductive porous material
30 used at this point, a known material can be used, but it
is particularly preferable to use a carbon-based material,
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such'as porous carbon, carbon pellet, carbon felt, carbon
paper, carbon fiber, or a stack structure formed of
carbon fine particles.
As the enzyme to be immobilized onto the surface of
5 the anode, glucose dehydrogenase (GDH), which breaks down
glucose, can be used, if the fuel component is glucose,
for example. In a case where a monosaccharide such as
glucose is used as the fuel component, a coenzyme oxidase
and an electron mediator, as well as an oxidase such as
10 GDH that facilitates oxidation of a monosaccharide and
breaks down the monosaccharide, are preferably
immobilized onto the surface of the anode.
A coenzyme oxidase oxidizes a coenzyme (such as
NAD' or NADP') that is reduced with an oxidase, and a
15 coenzyme reductant (such as NADH or NADPH). Such a
coenzyme oxidase may be diaphorase, for example. By
virtue of the action of the coenzyme oxidase, electrons
are generated when a coenzyme returns to an oxidant, and
the electrons are transferred from the coenzyme oxidase
20 to the electrode via an electron mediator.
As the electron mediator, a compound having a
quinone skeleton is preferably used, and a compound
having a naphthoquinone skeleton is more preferable.
Specifically, it is possible to use 2-amino-1,4-
25- naphthoquinone (ANQ), 2-amino-3-methyl-1,4=naphthoquinone
(AMNQ), 2-methyl-1,4-naphthoquinone (VK3), 2-amino-3-
carboxy-l,4-naphthoquinone (ACNQ), or the like. As the
compound having a quinone skeleton, it is possible to use
anthraquinone or a derivative thereof, other than a
30 compound having a naphthoquinone skeleton. Further, one
or more compounds that function as electron mediators can
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be immobilized together with a compound having a quinone
skeleton, where necessary.
In a case where a monosaccharide is used as the
fuel component, it is preferable to immobilize a
5 degrading enzyme that facilitates degradation such as
hydrolytic degradation of a polysaccharide to generate a
monosaccharide such as glucose, as well as the above
mentioned oxidase, coenzyme oxidase, coenzyme, and
electron mediator. It should be noted that a
10 "polysaccharide" used herein is in a broad sense, and
indicates all kinds of carbohydrates that generate two or
more molecules of monosaccharides through hydrolytic
degradation, including oligosaccharides such as
disaccharides, trisaccharides, and tetrasaccharides.
15 Specific examples include starch, amylose, amylopectin,
glycogen, cellulose, maltose, sucrose, and lactose. Each
of those examples is formed by two or more
monosaccharides binding together, and any of those
polysaccharides contains glucose as the monosaccharide
20 serving as the binding unit.
Amylose and amylopectin are components contained in
starch, and starch is a mixture of amylose and
amylopectin. In a case where glucoamylase is used as a
polysaccharide-degrading enzyme, and glucose
25 dehydrogenase is used as a monosaccharide-degrading
oxidase, for example, a polysaccharide that can be broken
down into glucose by glucoamylase can be used as the fuel
component. Examples of such polysaccharides include
starch, amylose, amylopectin, glycogen, and maltose.
30 Here, glucoamylase is a degrading enzyme that
hydrolytically degrades a-glucan such as starch to
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generate glucose, and glucose dehydrogenase is an oxidase
that oxides R-D-glucose into D-glucono-S-lactone.
The anode 2 is not limited to an anode having an
oxidoreductase immobilized to the surface thereof. As
5 long as an oxidoreductase exists on the electrode surface,
an electrode that has an oxidoreductase and has
microorganisms serving as a reaction catalyst adhering
to the surface can also be used.
[Cathode 3]
10 The cathode 3 is an air electrode, and is in direct
contact with. the gas phase (the air) or in contact with
the gas phase via a gas-liquid separation film. The
electrode forming the cathode 3 is not particularly
limited, and it is possible to use a cathode that has an
15 oxidoreductase and an electron mediator immobilized to
the surface of an electrode made of a conductive porous
material, for example. As the conductive porous material
forming the cathode 3, a known material can also be used,
but it is particularly preferable to use a carbon-based
20 material, such as porous carbon, carbon pellet, carbon
felt, carbon paper, carbon fiber, or a stack structure
formed of carbon fine particles.
Examples of oxygen reduction enzymes that can be
immobilized onto the cathode 3 include bilirubin oxidase,
25 laccase, and ascorbic acid oxidase. Examples of electron
mediators that can be immobilized together with those
enzymes include potassium hexacyanoferrate, potassium
ferricyanide, and potassium octacyanotungstate.
The cathode 3 is not limited to a cathode having an
30 oxidoreductase immobilized to the surface thereof either.
As long as an oxidoreductase exists on the electrode
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surface, an electrode that has an oxidoreductase and has
microorganisms serving as a reaction catalyst adhering to
the surface can also be used.
[Selective Transmission Film 6]
5 The selective transmission film 6 used in the
biofuel cell of this embodiment has permeability, and
restrain permeation of at least the fuel component
contained in the fuel solution 10. The fuel solution 10
introduced into the anode solution unit 4 is further
10 introduced into the cathode solution unit 5 via the
selective transmission film 6.
The selective transmission film 6 may restrain not
only permeation of the fuel component but also permeation
of a component contained in the fuel solution 10 other
15 than the fuel component. Particularly, the selective
transmission film 6 preferably can restrain permeation of
an enzyme or a mediator eluted in the fuel solution 10.
With this arrangement, the enzyme and the mediator
existing on each electrode can be prevented from
20 migrating toward each other electrode, and accordingly,
degradation of the battery characteristics can be
prevented.
In a case where a fuel solution having an
inhibitory effect on the cathode 3 or a commercially
25 available beverage is used as the fuel solution 10, for
example, the selective transmission film 6 may restrain
permeation of a calorie-free sweetener, a saccharide
(such as fructose or a fruit sugar) that cannot be
oxidized by the enzyme of the anode 2, or the like. As a
30 result, degradation of the battery characteristics can be
prevented, and the power generation efficiency can be
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increased.
As such a selective transmission film 6, a
cellulose film or a synthetic polymer film can be used,
for example. Specific examples of cellulose films
5 include regenerated cellulose (RC) films such as cupraammonium
rayon (CR) and saponified cellulose (SCA),
regenerated surface-modification cellulose films such as
a hemophan film and a vitamin E coating film, and
cellulose acetate (CA) films such as cellulose diacetate
10 (CDA) and cellulose triacetate (CTA).
Examples of synthetic polymer films include
polyacrylonitrile (PAN), polymethylmethacrylate (PMMA),
an ethylene vinyl alcohol (EVA) copolymer, polysulfone
(PS), polyamide (PA), and polyester polymer alloy.
15 The selective transmission film 6 can have a mean
pore size of 0.5 μm or smaller, for example, and
accordingly, can efficiently restrain permeation of the
fuel component. The mean pore size of the selective
transmission film 6 is preferably 100 nm or smaller, more
20 preferably, 20 nm or smaller, or more preferably, 10 nm
or smaller. With this, the effect to restrain permeation
of the fuel component can be increased, and permeation of
a component such as an enzyme or a mediator other than
the fuel component can also be restrained.
25- Also, the selective transmission film 6 preferably
adjusts the ion conductivity to 0.1 S/cm or higher
between the anode 2 and the cathode 3, or adjusts the
internal resistance of the battery cell unit 1 to 10 0
or lower. With this, power generation loss can be
30 reduced. The ion conductivity between the anode 2 and
the cathode 3 can be determined by carrying out impedance
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measurement, with an electrolytic solution injected.
Further, the selective transmission film 6
preferably has chemical stability even in a solution with
a pH of 3 to 12, and also has such a degree of heat
5 resistance that the selective transmission film 6 is not
modified under circumstances at 20 to 120°C. With this,
modification and breaking in the solution can be
prevented. Accordingly, power can be generated, without
any problem such as short-circuiting.
10 [Fuel Solution 10]
The fuel solution 10 is a fuel component such as
sugar, alcohol, aldehyde, lipid, or protein, or a
solution containing at least one of those fuel components.
Examples of fuel components that can be used in the
15 biofuel cell of this embodiment include saccharides such
as glucose, fructose, and sorbose, alcohols such as
methanol, ethanol, propanol, glycerin, and polynivyl
alcohol, aldehydes such as formaldehyde and acetaldehyde,
and organic acids such as acetic acid, formic acid, and
20 pyruvic acid.
Other than those, fats, proteins, and organic acids
that are intermediate products in sugar metabolism of
those fats and proteins can also be used as fuel
components. In addition to the above fuel component, the
25 fuel solution 10 may contain an electrolyte functioning
as a protonic conductor.
[Operations]
Next, operations of the biofuel cell of this
embodiment are described. In the biofuel cell of this
30 embodiment, the fuel solution 10 is first introduced into
the anode solution unit 4 through the fuel solution inlet
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9. After that, the fuel solution is also supplied to the
cathode solution unit 5 via the selective transmission
film 6. However, permeation of the fuel component in the
fuel solution 10 is restrained by the selective
5 transmission film 6, and accordingly, a solution with a
lower fuel component concentration is introduced into the
cathode solution unit 5. That is, in the biofuel cell of
this embodiment, the fuel solution in contact with the
anode 2 has a higher fuel component concentration than
10 the fuel solution in contact with the cathode 3.
At the anode 2 in this fuel cell, the fuel is
broken down by the enzyme immobilized onto the surface,
so that electrons are extracted, and proton (H') is
generated. At the cathode 3, on the other hand, water is
15 generated from proton transported from the anode 2 via
the protonic conductor, electrons transferred from the
anode 2 through an external circuit, and oxygen in the
solution (the liquid phase) stored in the cathode
solution unit 5 or in the air (the gas phase), for
20 example.
As described above, in the biofuel cell of this
embodiment, the selective transmission film 6 that
restrains permeation of the fuel component is provided
between the anode solution unit 4 and the cathode
25 solution unit S. Accordingly, the fuel component can be
restrained from diffusing to the cathode 3. With this,
even if the fuel component concentration in the fuel
solution 10 to be introduced into the anode solution unit
4 is made higher, the fuel component concentration in the
30 solution to be introduced into the cathode solution unit
5 can be restricted to a low concentration. Accordingly,
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degradation of the characteristics of the cathode 3 can
be prevented.
Further, in the biofuel cell of this embodiment,
the fuel component concentration in the solution to be in
5 contact with the anode 2 can be maintained at a high
concentration, and accordingly, the power generation
efficiency becomes higher. As a result, the power output
of the battery is the same or higher than that of a
conventional biofuel cell, and a biofuel cell with a
10 larger battery capacity than that of a conventional
biofuel cell can be realized.
This embodiment can be applied not only to "singlecell"
structures each having one battery cell unit
provided on the battery main body, but also to structures
15 each having battery cell units connected in series or in
parallel. In that case, a structure in which one fuel
solution inlet is shared by battery cell units can also
be formed.
<2. Modification of the First Embodiment>
20 Next, a biofuel cell according to a modification of
the first embodiment is described. Fig. 2 is a diagram
schematically showing the structure of the battery cell
unit in the biofuel cell of this modification. In Fig. 2,
the same components as those of the fuel cell of the
25 first embodiment illustrated in Fig. 1 are denoted by the
same reference numerals as those used in the first
embodiment, and detailed explanation thereof will not be
repeated.
In the biofuel cell of the above described first
30 embodiment, a space is left between the selective
transmission film 6 and each of the anode 2 and the
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cathode 3. However, the present invention is not limited
to that, and the. anode 2 and the cathode 3 may be in
contact with the selective transmission film 6, for
example, as in the battery cell unit 11 illustrated in
5 Fig. 2.
In that case, the selective transmission film 6 not
only restrains permeation of the fuel component, but also
functions as a separator to prevent short-circuiting of
each electrode (the anode 2, the cathode 3) and further
10 as a protonic conductor if water (ions) is contained
therein. With this, the number of components in the
battery cell unit 12 can be reduced. Accordingly, a
biofuel cell of a smaller size can be realized at lower
cost.
15 It should be noted that the aspects and effects of
this modification other than the above described ones are
the same as those of the above described first embodiment.
<3. Second Embodiment>
[Structure of a Battery Cell Unit]
20 Next, a biofuel cell according to a second
embodiment of the present invention is described. Fig. 3
is a diagram schematically showing the structure of the
battery cell unit in the fuel cell of this embodiment.
In Fig. 3, the same components as those of the fuel cell
25' of the first embodiment illustrated in Fig. 1 are denoted
by the same reference numerals as those used in the first
embodiment, and detailed explanation thereof will not be
repeated.
As shown in Fig. 3, the biofuel cell of this
30 embodiment is an immersion-type fuel cell in which a fuel
solution 10 is in contact with an anode (a fuel
16
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electrode) 2, and a solution 20 containing an electrolyte
or the like is in contact with a cathode (an air
electrode) 3. In this biofuel cell, an oxidoreductase
exists on the surface of at least one of the electrodes
5 (the anode 2 and/or the cathode 3) provided in the
battery cell unit 12.
Also, in the biofuel cell of this embodiment, the
cathode 3 is designed to be in contact not only with a
liquid phase (the solution 20) but also with a gas phase
10 (the air) . Specifically, a gas-liquid separation film 13
may be provided in contact with a surface located outside
the cathode 3, and the cathode is in contact with the gas
phase (the air) via the gas-liquid separation film 13.
Alternatively, the surface of the cathode 3 may have
15 water-repellent properties, and may be in direct contact
with the gas phase (the air). Further, current
collectors 7 and S are provided in contact with the anode
2 and the cathode 3, respectively.
An anode solution unit 4 and a cathode solution
20 unit S are provided around the anode 2 and the cathode 3,
respectively, and a selective transmission film 6 is
provided in between. In the fuel cell of this embodiment,
a fuel solution inlet 9 leading to the anode solution
unit 4 and a fuel solution inlet 19 leading to the
25 cathode solution unit 5 are provided independently of
each other. The fuel solution 10 is introduced into the
anode solution unit 4 via the fuel solution inlet 9, and
the electrolyte-containing solution 20 or the like, which
differs from the fuel solution 10, is introduced into the
30 cathode solution unit 5 via the fuel solution inlet 19.
[Solution 20]
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The solution 20 to be introduced into the cathode
solution unit 5 is not particularly limited, and it is
possible to use an aqueous solution (an electrolytic
solution) containing an electrolyte such as dihydrogen
5 phosphate or an imidazole compound, or an ionic liquid
such as a potassium chloride solution, for example. The
solution 20 functions mainly as a protonic conductor.
[Operations]
Next, operations of the biofuel cell of this
10 embodiment are described. First in the biofuel cell of
this embodiment, the fuel solution 10 is introduced into
the anode solution unit 4 through the fuel solution inlet
9, and the solution 20 such as an electrolytic solution
is introduced into the cathode solution unit 5 through
15 the fuel solution inlet 19. At this point, the fuel
component contained in the fuel solution 10 stored in the
anode solution unit 4 migrates into the solution 20 in
the cathode solution unit 5. However, permeation of the
fuel component is restrained by the selective
20 transmission film 6, and accordingly, the fuel component
concentration in the region surrounding the cathode 3 is
maintained lower than that in the region surrounding the
anode 2.
In the biofuel cell of this embodiment, the ion
25 concentration in the solution 20 to be introduced into
the cathode solution unit 5 is preferably made higher
than that in the fuel solution 10, for example, so that
the osmotic pressure of the solution 20 becomes higher
than that of the fuel solution 10. With this, the amount
30 of the fuel component migrating from the fuel solution 10
through the selective transmission film 6 can be reduced.
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At the anode 2 in this biofuel cell, the fuel is
also broken down by the enzyme immobilized onto the
surface, so that electrons are extracted, and proton (H')
is generated, as in the biofuel cell of the above
5 described first embodiment. At the cathode 3, water is
generated from proton transported from the anode 2 via
the protonic conductor, electrons transferred from the
anode 2 through an external circuit, and oxygen in the
solution 20 stored in the cathode solution unit 5 or in
10 the gas phase (the air) in contact via the gas-liquid
separation film 13, for example.
In the biofuel cell of this embodiment, the fuel
solution inlet 19 leading to the cathode solution unit 5
is provided, independently of the fuel solution inlet 9.
15 Accordingly, different solutions can be introduced into
the anode solution unit 4 and the cathode solution unit 5.
Further, in the biofuel cell of this embodiment, the
selective transmission film 6 is provided between the
anode solution unit 4 and the cathode solution unit 5.
20 Accordingly, even if the fuel component concentration in
the fuel solution 10 to be introduced into the anode
solution unit 4 is made higher, the amount of the fuel
component migrating into the solution 20 introduced into
the cathode solution unit 5 can be restricted to a small
25 amount. As a result, the fuel component concentration in
the region surrounding the cathode 3 can be maintained
low. Accordingly, degradation of the characteristics of
the cathode 3 can be prevented.
It should be noted that the aspects and effects of
30 this embodiment other than the above described ones are
the same as those of the above described first embodiment.
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Also; in the biofuel cell illustrated in Fig. 3, the
anode 2 and the cathode 3 are in contact with the
selective transmission film 6. However, the present
invention is not limited to that, and the anode, the
5 selective transmission film, and the cathode may be
positioned at predetermined intervals. Further, the
mechanism for bringing the cathode 3 into contact with
the gas phase is not limited to the structure in which
the gas-liquid separation film 13 is provided in direct
10 contact with the cathode 3 as shown in Fig. 3, and a
water-repellent electrode may be provided in direct
contact with the gas phase.
Further, this embodiment can be applied not only to
"single-cell" structures each having one battery cell
15 unit provided on the battery main body, but also to
structures each having battery cell units connected in
series or in parallel. In that case, each of the fuel
solution inlet 9 and the fuel solution inlet 19 can be
shared by two or more battery cell units.
20
EXAMPLES

In the following, the effects of the present
invention are described in detail by way of examples of
25 the present invention. First, in First Example of the
present invention, a biofuel cell according to the first
embodiment illustrated in Fig. 1 was prepared.
Cellophane 26 was provided as the selective transmission
film between the anode solution unit 4 and the cathode
30 solution unit 5, and a fuel solution 10 having a glucose
concentration varying from 0 to 1 M was used, to generate
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power at 0.25 V for 5 minutes. The current value was
then measured. In a comparative example, non-woven
fabric 106 was provided between the anode solution unit 4
and the cathode solution unit 5, and the same measurement
5 as above was carried out.
Fig. 4(a) is a conceptual diagram showing the
example in which the cellophane 26 was provided between
the respective solution units. Fig. 4(b) is a conceptual
diagram showing a comparative example in which the non-
10 woven fabric 106 was provided between the respective
solution units. As shown in Figs. 4(a) and 4(b), a
carbon fiber electrode (5 mm square, 2 mm thick) was used
as the cathode 3, and a titanium mesh material was used
as the current collector 8. Polytetrafluoroethylene
15 (PTFE) serving as the gas-liquid separation film 21 was
provided at an end of the cathode solution unit 5.
A 2.0 M imidazole/H2SO4 solution was used as the
(pH 7) protonic conductor. In the biofuel cell of the
example illustrated in Fig. 4(a), a syringe 22 was used
20 to fill the cathode solution unit 5 with a 2.0 M
imidazole/H2SO4 solution having no glucose added thereto,
and fill the anode solution unit 4 with a 2.0 M
imidazole/H2SO4 solution having a glucose concentration
of 0 M, 0.2 M, 0.4 M, 0.8 M or 1.0 M. In the biofuel
25 cell of the comparative example illustrated in Fig. 4(b),
on the other hand, all components passed through the nonwoven
fabric 106. Therefore, each of the anode solution
unit 4 and the cathode solution unit S was filled with
the same solution or a 2.0 M imidazole/H2SO4 solution
30 having a glucose concentration of 0 M, 0.2 M, 0.4 M, 0.8
M or 1.0 M.
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Fig. 5 is a graph showing the relationship between
the glucose concentration in the fuel solution and the
output in each of the fuel cells of the example and the
comparative example. In this graph, the abscissa axis
5 indicates glucose concentration, and the ordinate axis
indicates relative current. The relative current values
shown in Fig. 5 are values that were obtained where the
reference current value (1.0) was the current value
obtained when a 2.0 M imidazole/H2SO4 solution having a
10 glucose concentration of 0 M was used in the biofuel cell
of the comparative example illustrated in Fig. 4(b). As
shown in Fig. 5, in the biofuel cell of the comparative
example using non-woven fabric, the current value became
lower as the glucose concentration in the fuel solution
15 became higher. In the biofuel cell of the example using
cellophane, on the other hand, the current value hardly
decreased even when the glucose concentration was made
higher.

20 In Second Example of the present invention, the
amount of glucose as the fuel component migrating from
the anode solution unit 4 into the cathode solution unit
5 was measured in a case where the cellophane 26 was used
as the selective transmission film. Fig. 6 is a diagram
25 schematically showing a cell used in this example. In
this example, PTFE was used as the gas-liquid separation
film 21.
In this example, the cathode solution unit 5 was
filled with a 2.0 M imidazole/H2SO4 solution having no
30 glucose added thereto, and the anode solution unit 4 was
filled with a 2.0 M imidazole/H2SO4 solution containing
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0.8 M glucose. The cell was left for 2 hours. After
that, each of the solutions was collected, and was
diluted twentyfold with 2.0 M imidazole. The amount of
the glucose contained in each of the diluted solutions
5 was measured. For comparison, a 2.0 M imidazole/H2SO4
solution containing 0.8 M glucose and a 2.0 M
imidazole/H2SO4 solution containing 0.04 M glucose were
also diluted twentyfold with imidazole, and the amounts
of the glucoses therein were measured.
10 As a result, it was confirmed that glucose hardly
permeated the cathode solution unit 5. Even after the
solutions were stirred and were left for 2 hours,
migration of glucose was not observed. Further, another
cellulose film and a synthetic polymer film were used as
15 the selective transmission films 21, and the same
experiment as above was conducted. As a result, the same
effects as the effects of this example that used
cellophane were achieved.

20 In Third Example of the present invention, the
battery characteristics of the biofuel cells illustrated
in Figs. 4(a) and 4(b) were evaluated. The fuel solution
was,a 2.0 M imidazole/H2SO4-0.4 M glucose solution or a
2.0 M imidazole/H2SO4-0.8 M glucose solution, and in the
25 biofuel cell of the example illustrated in Fig. 4(a), the
cathode solution unit 5 was filled with a 2.0 M
imidazole/H2SO4 solution having no glucose added thereto.
In those biofuel cells of the example and the comparative
example, 20 mA constant current tests were conducted.
30 Figs. 7(a) and 7(b) are graphs showing temporal
changes in the cell voltages in the biofuel cells of the
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example and the comparative example. In each of the
graphs, the abscissa axis indicates time, and the
ordinate axis indicates voltage. As shown in Figs. 7(a)
and 7(b), the maximum output and the capacity of the
5 biofuel cell of this example that used the cellophane 26
were approximately twice larger than those of the biofuel
cell of the comparative example that used the non-woven
fabric 106.
The above results confirmed that the battery
10 capacity was increased without degradation of the cathode
characteristics by providing the selective transmission
film between the cathode solution unit and the anode
solution unit.
15 REFERENCE SIGNS LIST
1, 11, 12 ... Battery cell units 2 ... Anode 3 ...
Cathode 4 ... Anode solution unit 5 ... Cathode
solution unit 6 ... Selective transmission film 7, 8 ...
Current collectors 9 ... Fuel solution inlet 10 ...
20 Fuel solution 13, 21 ... Gas-liquid separation film
19 ... Solution inlet 20 ... Solution 22 ... Syringe
26 ... Cellophane 106 ... Non-woven fabric
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CLAIMS
1. A fuel cell comprising a battery cell unit or a
plurality of battery cell units each having an
5 oxidoreductase on a surface of an anode and/or a cathode,
the cathode being in contact with both a liquid phase and
a gas phase,
wherein the battery cell unit has a selective
transmission film between a first solution unit provided
10 around the anode and a second solution unit provided
around the cathode, the selective transmission film
restraining permeation of at least a fuel component.
2. The fuel cell according to claim 1, wherein a
15 solution in contact with the anode has a higher fuel
component concentration than a solution in contact with
the cathode.
3., The fuel cell according to claim 2, wherein a fuel
20 solution having a lower fuel component concentration is
introduced from the first solution unit into the second
solution unit via the selective transmission film.
4. The fuel cell according to claim 2, further
25 comprising:
a first inlet through which a solution is
introduced into the first solution unit; and
a second inlet through which a solution is
introduced into the second solution unit,
30 wherein solutions having different fuel component
concentrations from each other are stored in the first
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solution unit and the second solution unit.
5. The fuel cell according to claim 4, wherein the
selective transmission film further restrains permeation
of an enzyme and/or a mediator.
6. The fuel cell according to claim 4, wherein the
fuel component is a saccharide.
10 7. The fuel cell according to claim 4, wherein the
selective transmission film has a mean pore size of 0.5
μm or smaller.
8. The fuel cell according to claim 4, wherein the
15 selective transmission film is one of a cellulose film
and a synthetic polymer film.
9. The fuel cell according to claim 4, wherein the
cathode is in contact with the gas phase via a gas-liquid
20 separation film.