DESCRIPTION
BIOFUEL CELL
TECHNICAL FIELD
5 The present invention relates to a biofuel cell
using a redox enzyme. More specifically, the present
invention relates to a technique to improve an electrode
performance.
10 BACKGROUND ART
A biofuel cell using a redox enzyme as a reaction
catalyst can take out electrons effectively from fuels
that cannot be utilized in general industrial catalysts
such as glucose and ethanol, and thus gains attentions as
15 a next-generation fuel cell that has a high capacity and
high safety. Fig. 11A is a drawing showing a reaction
scheme of a negative electrode of a biofuel cell using an
enzyme. Fig. 11B is a drawing showing a reaction scheme
of a positive electrode of a biofuel cell using an enzyme:
20 For example, as shown in Figs. 11A and 11B, in the case
of a biofuel cell using glucose as a fuel, an oxidation
reaction of glucose proceeds on a negative electrode
(anode) and electrons are taken out, whereas a reduction
reaction of oxygen (O2) in the. air proceeds on a positive
25 electrode (cathode).
On the other hand, output differs depending on the
materials of electrodes in biofuel cells. Therefore, it
is necessary to select a suitable electrode material so
as to attain a high-output cell. Therefore, electrodes
30 mainly using carbon materials are used in conventional
biofuel cells (for example, see Patent Documents 1 to 5)
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For example, electrodes formed of porous carbon such as
carbon felt are used in the biofuel cell described in
Patent Document 1.
Furthermore, Patent Document 2 discloses a process
5 for forming a porous carbon electrode by applying a
carbon ink containing porous carbon and a void-forming
material powder to a carbon substrate such as carbon felt
and carbon cloth, drying, and removing the void-forming
material powder. In addition, in the enzyme electrodes
10 for biofuel cells described in Patent Documents 3 to 5,
carbon paper, carbon cloth and the like are used as
sheet-like carbon substrates for fixing an enzyme.
CITATION LIST
15 PATENT DOCUMENTS
Patent Document 1: JP 2009-158466 A
Patent Document 2: JP 2009-181889 A
Patent Document 3: JP 2010-43978 A
Patent Document 4: JP 2006-508519 W
20 Patent Document 5: JP 2007-324005 A
SUMMARY OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
However, the conventional biofuel cells mentioned
25. above have problems shown below: Namely, electrodes
formed of porous carbon which are generally used in
biofuel cells at present have -a problem in that it is
difficult to uniformly form enzyme-immobilized membranes
on the surfaces thereof and thus the reactivity as
30 electrodes is not sufficient. Furthermore, porous carbon
electrodes also have a problem in that the physical
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stability is decreased when the porosity thereof is
increased.
On the other hand, the electrodes using a sheetlike
carbon material such as carbon cloth and carbon
5 paper described in Patent Documents 3 to 5 were selected
from the viewpoints of electroconductive materials having
a large surface area, and the physical stability and
liquid permeability were not considered. Therefore, in
the case where such sheet-like carbon material is used in
10 a biofuel cell, there is a problem in that a high current
density is difficult to obtain.
Therefore, a main object of the present invention
is to improve the electrode performance without
decreasing the surface areas, physical stability and
15 liquid permeability of electrodes, to thereby provide a
biofuel cell having high output.
SOLUTION TO PROBLEM
The biofuel cell according to the present invention
20 comprises an electrode that is formed of a carbon fiber
fabric having a network structure constituted by a
monofilament strand of a carbon fiber and comprises a
redox enzyme on the surface thereof.
As used herein, the surface of the electrode
25 includes the entirety of the outer surface of the
electrode and the inner surface of a void inside the
electrode, and the same also applies to the following
descriptions.
In the present invention, since the electrodes are
30 formed of a carbon fiber fabric; thin electrodes having
high strength and flexibility can be attained.
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Furthermore, since the electrodes of the carbon fiber
fabric have large surface areas, an excellent electrode
performance can be obtained as compared to conventional
biofuel cells.
5 In this biofuel cell, it is desirable that at least
a negative electrode be formed of the carbon fiber fabric.
Alternatively, both of the negative electrode and a
positive electrode may be formed of the carbon fiber
fabric.
10 Furthermore, current collectors that are crimpcontacted
to the electrodes may also be formed of the
carbon fiber fabric.
In addition, a carbon fiber fabric having a
monofilament diameter of, for example, 15 μm or less can
15 be used as the carbon fiber fabric.
EFFECTS OF THE INVENTION
According to the present invention, since the
electrodes are formed of a carbon fiber fabric, the
20 electrode performance can be improved without decreasing
the surface areas, physical stability and liquid
permeability of the electrodes, and thus a biofuel cell
having high output can be attained.
25-' BRIEF DESCRIPTION OF DRAWINGS
Fig. 1 is an exploded perspective view showing the
constitution of a biofuel cell of an embodiment of the
present invention.
Fig. 2 is a drawing schematically showing the
30 principle of power generation of the biofuel cell
according to an embodiment of the present invention.
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Figs. 31k to 30 are SEM photographs of a carbon
fiber fabric ofExample 1.
Figs. 41k to 40 are SEM photographs of a carbon
fiber fabric of Example 2.
5 Figs. 51k to SC are SEM photographs of a carbon
fiber fabric of Example 3.
Figs. 6A to 6C are SEM photographs of a carbon
fiber fabric of Comparative Example 1.
Figs. 71k to 70 are SEM photographs of a carbon
10 fiber fabric of Comparative Example 2.
Figs. 8A and 8B are graphs showing the results of
potentiostatic measurements.
Fig. 9 is a graph showing the relationship between
the monofilament diameter and current density, wherein
15 the horizontal axis shows the monofilament diameter of
the carbon fiber fabric and the vertical axis shows the
current density at after 300 seconds.
Fig. 101k is an SEM photograph showing the state in
which the enzyme is fixed on the carbon fiber fabric
20 electrode of Example 1. Fig. IOB is an SEM photograph
showing the state in which the enzyme is fixed on the
porous carbon electrode of Comparative Example 3.
Fig. 111k is a drawing showing a reaction scheme of
a negative electrode of a biofuel cell using an enzyme.
25 Fig. 11B is a drawing showing a reaction scheme of a
positive electrode of a biofuel cell using an enzyme.
MODE FOR CARRYING OUT THE INVENTION
Hereinafter the embodiments for carrying out the
30 present invention will be explained in detail with
reference to the accompanied drawings. The present
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invention is hot construed to be limited to the
respective embodiments shown below.
[Entire structure]
First, the biofuel cell according to the embodiment
5 of the present invention will be explained. Fig. 1 is an
exploded perspective view showing the constitution of the
biofuel cell of the embodiment of the present invention.
As shown in Fig. 1, in the biofuel cell 10 of the present
embodiment, an anode (negative electrode) 1 to be a fuel
10 electrode, an anode current collector 4, a separator 3, a
cathode current collector 5, a cathode (positive
electrode) 2 to be an air electrode and a gas-liquid
separation membrane 6 are disposed in this order between
a fuel tank 7 and a positive electrode cover 8.
15 [Anode 1 and cathode 2]
In the biofuel cell 10 of the present embodiment,
the anode 1 and/or the cathode 2 is/are formed of a
carbon fiber fabric having a network structure
constituted by a monofilament strand of a carbon fiber,
20 and have/has a redox enzyme that functions as a reaction
catalyst on the surface(s) thereof. An electrode formed
of this carbon fiber fabric is desirably applied to at
least the anode 1, more desirably applied to both the
anode 1 and the cathode 2. -
25, Since the anode 1 is a fuel electrode with which a
fuel solution contacts, in order to improve the
performance thereof, it is necessary not only to increase
the surface area but also to enhance the physical
stability and liquid permeability. Specifically, since a
30 liquid fuel has a lower permeation velocity (diffusion
velocity) than that of a gas fuel, the quality of the
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permeability' in an electrode significantly affects the
electrode performance. Therefore, when a carbon fiber
fabric having a network structure constituted by a
monofilament strand of a carbon fiber, which has a large
5 surface area and is excellent in physical stability and
liquid permeability, is used in the anode 1, the
electrode performance is significantly improved, and the
output of the entirety of the cell can be effectively
enhanced.
10 Furthermore, as a matter of course, also in the
case where the carbon fiber fabric having a network
structure constituted by a monofilament strand of a
carbon fiber is used in the cathode 2, the electrode
performance is improved and the output of the entirety of
15 the cell can be enhanced as compared to the cases where
an electrode formed of a conventional electrode material
is used. Furthermore, it is preferable to use a?carbon
fiber fabric having a monofilament diameter of 15 μm or
less as the carbon fiber fabric for forming each
20 electrode. By doing so, it becomes possible to increase
the surface area of the cell without decreasing the
porosity that affects the liquid permeability, and thus
the electrode performance, specifically the performance
of the anode 1 can be improved more.
25 5 In addition, since the perviousness of a fuel
affects the electrode property of the anode 1 and the
perviousness of oxygen affects the electrode property of
the cathode 2, the network structure, monofilament
diameter and the likecan be selected in accordance with
30 the requirements thereof. Accordingly, in the biofuel
cell 10 of the present embodiment, carbon fiber fabrics
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having network structures and monofilament diameters that
differ from each other may be used in the anode 1 and the
cathode 2.
On the other hand, as the enzyme that is fixed on
5 the surface of the anode 1, for example, in the case
where the fuel component is glucose, glucose
dehydrogenase (GDH) that decomposes glucose can be used.
Furthermore, in the case where a monosaccharide such as
glucose is used as a fuel component, it is desirable that
10 a coenzyme oxidase and an electron mediator be fixed
together with an oxidase that promotes the oxidation of a
monosaccharide and decompose the monosaccharide such as
GDH on the surface of the anode.
The coenzyme oxidase oxidizes a coenzyme that is
15 reduced by an oxidase (for example, NAD*, NADP and the
like) and a reduced form of a coenzyme (for example, NADH,
NADPH and the like), and examples include diapho'lase and
the like. By the action of this coenzyme oxidase,
electrons are generated when the coenzyme returns to an
20 oxidized form, and the electrons are transmitted from the
coenzyme oxidase to the electrode through the electron
mediator.
Furthermore, it is preferable to use a compound
having a quinone backbone as the electron mediator, and a
25 compound having a naphthoquinone backbone is specifically
preferable. Specifically, 2-amino-l,4-naphthoquinone
(ANQ), 2-amino-3-methyl-1,4-naphthoquinone (AMNQ), 2-
methyl-l,4-naphthoquinone (VK3), 2-amino-3-carboxy-1,4-
naphthoquinone (ACNQ) and the like can be used.
30 Furthermore, as the compound having a quinone backbone,
for example, anthraquinone and derivatives thereof can
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also be used besides the compounds having a
naphthoquinone backbone. Furthermore, where necessary,
one kind or two or more kinds of other compound that acts
as an electron mediator may be fixed together with the
5 compound having a quinone backbone.
On the other hand, in the case where a
polysaccharide is used as a fuel component, it is
desirable to fix a degradative enzyme that promotes the
degradation such as hydrolysis of the polysaccharide to
10 generate a monosaccharide such as glucose besides the
above-mentioned oxidase, coenzyme oxidase, coenzyme and
electron mediator. The "polysaccharide" as used herein
is a polysaccharide in the broad sense of the term,
refers to all carbohydrates that generate two or more
15 molecules of a monosaccharide by hydrolysis, and
comprises oligosaccharides such as disaccharides,
trisaccharides and tetrasaccharides. Specific examples
may include starch, amylose, amylopectin, glycogen,
cellulose, maltose, sucrose and lactose, and the like.
20 These are formed by binding of two or more
monosaccharides, and glucose is included as a
monosaccharide of a binding unit in all of the
polysaccharides.
Amylose and amylopectin are components included in
25. starch, and starch is a mixture of amylose and
amylopectin. For example, in the case where glucoamylase
is used as a degradative enzyme for a polysaccharide and
glucose dehydrogenase is used as an oxidase for degrading
a monosaccharide, a polysaccharide that can be degraded
30 to glucose by glucoamylase can be used as a fuel
component. Examples of such polysaccharide may include
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starch, amylase, amylopectin, glycogen and maltose, and
the like. Glucoamylase is a degradative enzyme that
hydrolyzes a-glucan such as starch to generate glucose,
and glucose dehydrogenase is an oxidase that oxidizes 3-
5 D-glucose to form D-glucono-S-lactone.
On the other hand, in the case where a redox enzyme
is also fixed on the cathode 2, for example, bilirubin
oxydase, laccase and ascorbate oxydase, and the like can
be used. Furthermore, examples of the electron mediator
10 that is fixed together with these enzymes may include
potassium hexacyanoferrate, potassium ferricyanide and
potassium octacyanotungstate, and the like.
The anode 1 and cathode 2 are not limited to those
in which a redox enzyme is fixed on the surface, and it
15 is sufficient that the redox enzyme is present on the
surfaces of the electrodes; for example, it is also
possible to use electrodes to which a microorganism that
has a redox enzyme and acts as a reaction catalyst is
attached, and the like.
20 [Separator 3]
The separator 3 prevents short-circuiting of the
respective electrodes (anode 1 and cathode 2), and is
formed of a material that allows the permeation of
protons such as a nonwoven fabric, cellophane and PTFE
25. (polytetrafluoroethylene). Furthermore, the thickness
and physical properties thereof are not specifically
limited.
[Current collectors 4 and 5]
The material for the current collectors 4 and 5 is
30 not specifically limited, and may be a material that can
be electrically connected to outside and does not cause
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an electrochemical reaction in the biofuel cell.
Specific examples may include metal materials such as Pt,
Ag, Au, Ru, Rh, Os, Nb, Mo, In, Ir, Zn, Mn, Fe, Co, Ti, V,
Cr, Pd, Re, Ta, W, Zr, Ge and Hf, alloys such as alumel,
5 brass, duralumin, bronze, nickelin, platinum rhodium,
permalloy, permendur, German silver and phosphor bronze,
electroconductive polymers such as polyacetylenes,
carbon-based materials such as carbon felt, carbon paper,
carbon fibers or laminates of carbon microparticles,
10 borides such as HfB2, NbB, CrB2 and B4C, nitrides such as
TiN and ZrN, silicides such as VSi2, NbSi2i MoSi2 and
TaSi2, and composite materials thereof, and the like.
Alternatively, as the current collectors 4 and 5, a
carbon fiber fabric can also be used as in the above-
15 mentioned anode 1 and cathode 2. By this way, the
electrodes adhere more tightly to the current collectors
4 and 5, thereby the connection state becomes fiie. As a
result, the current collecting efficiency is improved,
and therefore the output of the entirety of the cell can
20 also be increased.
[Gas-liquid separation membrane 6]
The gas-liquid separation membrane 6 does not allow
the permeation of a liquid but allows only the permeation,
of a gas, and for example, a PTFE membrane or the like
25, can be used. Furthermore, the thickness and physical
properties thereof are not specifically limited, and a
gas-liquid separation membrane that prevents the leakage
of a fuel solution and can feed oxygen that is required
for a reaction to the cathode 2 may be used.
30 [Positive electrode cover 8]
The positive electrode cover 8 presses the
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respective elements, and the material thereof and the
like are not specifically limited, and it is sufficient
that the cathode cover comprises air pores for taking the
air outside into the cell.
5 [Fuel solution]
The fuel solution is a solution that contains a
fuel component such as a sugar, an alcohol, an aldehyde,
a lipid and a protein, or at least one kind of these fuel
components. Examples of the fuel component used in the
10 biofuel cell 10 of the present embodiment may include
sugars such as glucose, fluctose and sorbose, alcohols
such as methanol, ethanol, propanol, glycerin and
polyvinyl alcohol, aldehydes such as formaldehyde and
acetaldehyde, organic acids such as acetic acid, formic
15 acid and pyruvic acid, and the like. It is also possible
to use fats and proteins, and organic acids and the like
which are intermediate products of the sugar metabolism
of these, and the like as the fuel component.
[Behavior]
20 Next, the behavior of the biofuel cell 10 of the
present embodiment will be explained. Fig. 2 is a
drawing that schematically shows the principle of power
generation of the biofuel cell 10 according to the
present embodiment. In the biofuel cell 10 of the
25.' present embodiment, in the anode 1, the fuel is
decomposed by the enzyme fixed on the surface, thereby an
electron is taken out and a proton (H+) is generated. On
the other hand, in the cathode 2, water (H2O) is
generated by the proton (H+) transported from the anode 1
30 through the proton conductor and the electron (e-) sent
from the anode 1 through an outer circuit, and for
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example, the oxygen (02) that is present around the
cathode 2.
In the biofuel cell 10 of the present embodiment,
since the electrodes are formed of the carbon fiber
5 fabric having a network structure constituted by a
monofilament strand of a carbon fiber, the surface areas
of the electrodes can be increased. Furthermore, since
this carbon fiber fabric has a network structure and thus
has regularity and directional property, it can allow
10 quick permeation of the solution into the inside of the
electrodes during the preparation of the electrodes and
power generation.
Therefore, for example, in the preparation of the
electrodes, the enzyme, mediator and the like can be
15 applied not only to the surfaces of the electrodes but
also to the inside of the electrodes, and during power
generation, the fuel solution can be quickly spread to
the inside of the electrodes. Furthermore, by subjecting
the surfaces of the electrodes to a water repellent
20 treatment, it is also possible to quickly spread oxygen
that is a gas fuel to the inside of the electrodes.
Since the reaction amount in the electrodes is increased
by this way, the electrode performance can be improved.
Furthermore, when an electrode and a current
25.` collector are energized, these are pressurized by a
strong pressure so as to decrease contact resistance;
since the carbon fiber fabric having a network structure
constituted by a monofilament strand of a carbon fiber
has a high physical.strength despite its high porosity,
30 it can tolerate such a high pressure of pressurization.
Furthermore, the carbon fiber fabric is also excellent in
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physical stability.
In addition, since the carbon fiber fabric having a
network structure constituted by a monofilament strand of
a carbon fiber is thin and also excellent in flexibility,
5 the degree of freedom of designing in the preparation of
a cell is increased more than in conventional porous
carbon electrodes. Specifically, it is possible to form
a thin film-type electrode and adjust the thickness of
the electrode by lamination, and further cut the
10 electrode into an arbitrary shape. By utilizing the
flexibility of the carbon fiber nonwoven fabric, it also
becomes possible to eject the fuel solution remaining in
the electrode by applying a pressure during the exchange
of the fuel solution.
15 As mentioned above, the biofuel cell of the present
embodiment can improve the electrode performance without
decreasing the surface areas, physical stability and
liquid permeability of the electrodes, and thus can
enhance the output of the entirety of the cell as
20 compared to conventional biofuel cells.
In addition, the constitution of the present
embodiment can be applied not only to a "single cell"
structure in which one cell unit is disposed in the main
body of a cell but also to a structure in which plural
25 cell units are connected in series or parallel.
Furthermore, electrodes formed of the above-mentioned
carbon fiber fabric can be applied to either a biofuel
cell of an air-exposed system in which a fuel solution
contacts with only an anode, or a biofuel of a water-
30 immersion system having a constitution in which a fuel
solution contacts with both an anode and a cathode.
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Examples
Hereinafter the effect of the present invention
will be specifically explained by Examples of the present
5 invention. In the present Examples, negative electrodes
(anodes) were prepared by using carbon fiber fabrics
having a network structure constituted by a monofilament
strand of a carbon fiber (Examples 1 to 3), carbon fiber
fabrics in which a monofilament yarn is randomly
10 accumulated (Comparative Examples 1 and 2) and porous
carbon (Comparative Example 3), and the properties
thereof were evaluated.
The monofilament diameters of the respective carbon
fiber fabrics of Examples 1 to 3 and Comparative Examples
15 1 and 2 are shown in the following Table 1. The
monofilament diameters shown in the following Table 1 are
values (average values) obtained by observations by a
scanning electron microscope (SEM). Furthermore, Figs. 3
to 7 are the SEM photographs of the carbon fiber fabrics
20 of Examples 1 to 3 and Comparative Examples 1 and 2.
[Table 1]
Carbon fiber fabric Average monofilament
diameter (μm)
Example 1 6.7
Example 2 10.6
Example 3 10.0
Comparative Example 1 15.8
Comparative Example 2 19.7
The electrodes for evaluation of Examples 1 to 3
and Comparative Examples 1 and 2 were each prepared by
25 fixing an enzyme (GDH) and a mediator (NADH) on the
carbon fiber fabric of 1 cm square. Furthermore, the
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porous carbon electrode of Comparative Example 3 was
prepared by a. similar process. The porosities of the
respective electrodes of Examples 1 to 3 and Comparative
Examples 1 to 3 are shown in the following Table 2. The
5 values shown in the following Table 2 were each obtained
from the mass and density, and all of the values were
measured in the state before fixing the enzyme and the
like.
[Table 2]
Carbon fiber fabric Porosity (o)
Example 1 84.4
Example 2 81.6
Example 3 81.1
Comparative Example 1 90.8
Comparative Example 2 90.0
Comparative Example 3 83.4
10
Furthermore, potentiostatic measurements (-0.35 V
vs. Ag/AgCI) were conducted on the respective electrodes,
of Examples and Comparative Examples in an electrolyte
solution (a 2.0 M imidazole-HZSO4 solution containing 0.6
15 M glucose: pH 7.0), and the current densities were
compared. Figs. BA and BE are graphs showing the results
of the potentiostatic measurements, and Fig. 9 is a graph
showing the relationship between the monofilament
diameter and current density, wherein the horizontal axis
20 ,'shows the monofilament diameter of the carbon fiber
fabric and the vertical axis shows the current density at
after 300 seconds.
As shown in Figs. BA, BB and 9, the electrodes of
Examples 1 to 3 using the carbon fiber fabrics having a
25 network structure constituted by a monofilament strand of
a carbon fiber had a higher current density and more
16
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excellent electrode performance than those of the
electrodes of Comparative Examples 1 and 2 using the
carbon fiber fabrics in which, a monofilament yarn is
randomly accumulated and the porous carbon electrode of
5 Comparative Example 3. Furthermore, it was confirmed
that the current density was higher and the electrode
performance was improved more as the monofilament
diameter of the carbon fiber fabric decreased.
Furthermore, the state of fixing of the enzyme was
10 observed by an SEM on the electrode using the carbon
fiber fabric of Example 1 and the porous carbon electrode
of Comparative Example 3. Fig. 10A is an SEM photograph
showing the state in which the enzyme is fixed on the
carbon fiber fabric electrode of Example 1, and Fig. 10B
15 is an SEM photograph showing the state in which the
enzyme is fixed on the porous carbon electrode of
Comparative Example 3. As shown in Fig. 10B, in the
porous carbon electrode of Comparative Example 3, the
voids in the electrode were filled with the enzyme-
20 immobilized membrane.
On the other hand, as shown in Fig. 10A, the
enzyme-immobilized membrane adhered to the carbon fiber,
but the voids were maintained to some extent in the
carbon fiber fabric electrodeof Example 1. Therefore,
25 '''it was confirmed that the enzyme-immobilized membrane can
be formed more evenly on the surface of the electrode by
using the carbon fiber fabric having a network structure
constituted by a monofilament strand of a carbon fiber as
an electrode material.
30
REFERENCE SIGNS LIST
17
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1 Anode (negative electrode)
2 Cathode (positive electrode)
3 Separator
4, 5 Current collector
6 Gas-liquid separation membrane
7 Fuel tank
8 Cover
10 Biofuel cell
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carbon fiber and comprises
1. A biofuel cell comprising an electrode that is
formed of a carbon fiber fabric having a network
structure constituted by a monofilament strand of a
thereof.
CLAIMS
a redox enzyme on the surface
2. The biofuel cell according to claim 1, comprising
10 at least a negative electrode formed of the carbon fiber
fabric.
3. The biofuel cell according to claim 2, comprising
the negative electrode and a positive electrode both of
15 which are formed of the carbon fiber fabric.
4. The biofuel cell according to claim 3, further
comprising current collectors that are crimp-contacted to
the electrodes and also formed of the carbon fiber fabric'.
20
5. The biofuel cell according to any one, of claims 1
to 4, wherein the monofilament that constitutes the
carbon fiber fabric has a diameter of 15 pm or less.