U.S. patent application number 13/073477 was filed with the patent office on 2011-07-14 for fuel cell, electronic device, movable body, power generation system, and cogeneration system.
This patent application is currently assigned to SONY CORPORATION. Invention is credited to Hideki Sakai, Atsushi Sato, Yuichi Tokita, Takashi Tomita.
Application Number | 20110171542 13/073477 |
Document ID | / |
Family ID | 35503402 |
Filed Date | 2011-07-14 |
United States Patent
Application |
20110171542 |
Kind Code |
A1 |
Sakai; Hideki ; et
al. |
July 14, 2011 |
FUEL CELL, ELECTRONIC DEVICE, MOVABLE BODY, POWER GENERATION
SYSTEM, AND COGENERATION SYSTEM
Abstract
A fuel cell which can directly extract electric power from a
polysaccharide, such as starch, is provided. A fuel electrode is
formed by immobilizing with an immobilizer, on an electrode
comprised of, e.g., carbon, an enzyme responsible for decomposing a
polysaccharide into monosaccharides, an enzyme responsible for
decomposing the monosaccharide formed, a coenzyme (e.g., NAD.sup.-
or NADP.sup.+) which forms a reductant due to the oxidation
reaction in the monosaccharide decomposition process, a coenzyme
oxidase (e.g., diaphorase) for oxidizing the reductant of the
coenzyme (e.g., NADH or NADPH), and an electron mediator (e.g.,
ACNQ or vitamin K3) for receiving electrons generated due to the
oxidation of the coenzyme from the coenzyme oxidase and delivering
the electrons to the electrode. The fuel cell comprises the fuel
electrode and the air electrode that sandwich an electrolyte
layer.
Inventors: |
Sakai; Hideki; (Kanagawa,
JP) ; Tomita; Takashi; (Kanagawa, JP) ; Sato;
Atsushi; (Kanagawa, JP) ; Tokita; Yuichi;
(Kanagawa, JP) |
Assignee: |
SONY CORPORATION
Tokyo
JP
|
Family ID: |
35503402 |
Appl. No.: |
13/073477 |
Filed: |
March 28, 2011 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11570113 |
Mar 13, 2007 |
|
|
|
PCT/JP05/10415 |
Jun 7, 2005 |
|
|
|
13073477 |
|
|
|
|
Current U.S.
Class: |
429/401 |
Current CPC
Class: |
H01M 4/9008 20130101;
H01M 4/9083 20130101; Y02E 60/527 20130101; H01M 4/8846 20130101;
Y02E 60/50 20130101; H01M 8/16 20130101 |
Class at
Publication: |
429/401 |
International
Class: |
H01M 8/16 20060101
H01M008/16 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 7, 2004 |
JP |
P2004-168430 |
May 31, 2005 |
JP |
P2005-159092 |
Claims
1. An apparatus comprising a fuel cell for generating electric
power by decomposing a fuel using an enzyme, wherein the fuel is a
gelatinized starch.
2. The apparatus according to claim 1, wherein the enzyme comprises
a decomposing enzyme for promoting decomposition of the gelatinized
starch to form a monosaccharide, and an oxidase for promoting
oxidation of the monosaccharide formed to decompose the
monosaccharide.
3. The apparatus according to claim 2, wherein the enzyme comprises
a coenzyme oxidase for changing a coenzyme reduced due to the
oxidation of the monosaccharide to an oxidant and giving electrons
to a negative electrode through an electron mediator.
4. The apparatus according to claim 3, wherein the oxidant of the
coenzyme is NAD.sup.+ and the coenzyme oxidase is diaphorase.
5. The apparatus according to claim 3, wherein in that the coenzyme
oxidase, the coenzyme, and the electron mediator are immobilized on
the negative electrode.
6. The apparatus according to claim 5, wherein the oxidase is
immobilized on the negative electrode.
7. The apparatus according to claim 5, wherein the oxidase and the
decomposing enzyme are immobilized on the negative electrode.
8. The apparatus according to claim 7, wherein the gelatinized
starch is immobilized on the negative electrode.
9. The apparatus according to claim 5, wherein the immobilization
uses an immobilizer comprising glutaraldehyde and
poly-L-lysine.
10. An electronic device comprising a fuel cell, wherein the fuel
cell generates electric power by decomposing a fuel using an
enzyme, wherein the fuel is a gelatinized starch.
11. A movable body comprising a fuel cell, wherein the fuel cell
generates electric power by decomposing a fuel using an enzyme,
wherein the fuel is a gelatinized starch.
12. A power generation system comprising a fuel cell, wherein the
fuel the cell generates electric power by decomposing a fuel using
an enzyme, wherein the fuel is a gelatinized starch.
13. A cogeneration system comprising a fuel cell, wherein the fuel
cell generates electric power by decomposing a fuel using an
enzyme, wherein the fuel is a gelatinized starch.
Description
CROSS REFERENCES TO RELATED APPLICATIONS
[0001] The present application is a Continuation of U.S.
application Ser. No. 11/570,113 filed on Mar. 13, 2007, which is a
National Stage of International Application No. PCT/JP05/10415
filed on Jun. 7, 2005 and which claims priority to Japanese Patent
Application Nos. JP 2004-168430 filed on Jun. 7, 2004 and JP
2005-159092 filed on May 31, 2005, the entire contents of which are
being incorporated herein by reference.
BACKGROUND
[0002] A fuel cell basically comprises a fuel electrode (negative
electrode), an oxidizer electrode or air electrode (positive
electrode), and an electrolyte (proton conductor), and has an
operational principle, in accordance with a reverse reaction of the
electrolysis of water, such that hydrogen and oxygen are reacted to
form water (H.sub.2O) and generate electricity. Specifically, a
fuel (hydrogen) supplied to the fuel electrode is oxidized and
divided into electrons and protons (H.sup.+), and the electrons go
to the fuel electrode and the protons H.sup.+ travel through the
electrolyte to the oxidizer electrode. At the oxidizer electrode,
the protons H.sup.+ are reacted with oxygen supplied from the
outside and electrons fed from the fuel electrode through an
external circuit to form H.sub.2O.
[0003] The fuel cell is a high-efficient power generator which
directly converts the chemical energy of a fuel to electrical
energy, and can extract electrical energy from the chemical energy
of fossil energy, such as natural gas, petroleum, or coal, with
high conversion efficiency, irrespective of where or when the fuel
cell is used. For this reason, conventionally, fuel cells in
large-scale power generation applications and others have been
extensively researched and developed. For example, fuel cells are
mounted on a space shuttle, which has demonstrated that the fuel
cells can supply not only electric power but also water for a crew,
and that the fuel cells are clean power generators.
[0004] Further, in recent years, fuel cells operating in a range of
relatively low temperatures of from room temperature to about
90.degree. C., such as solid polymer fuel cells, are developed and
have attracted attention. Therefore, attempts are being made to
apply the fuel cell not only to the large-scale power generation
but also to small-size systems, such as a power source for driving
an automobile, and a portable power source for personal computer or
mobile device.
[0005] As mentioned above, the fuel cell is possibly applied to a
wide range of uses from the large-scale power generation to the
small-scale power generation, and has attracted considerable
attention as a high-efficient power generator. However, the fuel
cell has various problems in that the fuel cell generally uses, as
a fuel, hydrogen gas converted by means of a reformer from natural
gas, petroleum, or coal, and hence consumes limited resources and
requires high-temperature heating, and that the fuel cell needs a
catalyst comprised of an expensive noble metal, such as platinum
(Pt). In addition, when hydrogen gas or methanol itself is directly
used as a fuel, it must be carefully handled.
[0006] For solving the problems, the application of biological
metabolism proceeding in a living body, which is a high-efficient
energy conversion mechanism, to a fuel cell has been proposed. The
biological metabolism used here includes respiration,
photosynthesis, and the like conducted in microorganism somatic
cells. The biological metabolism has advantageous features not only
in that the power generation efficiency is extremely high, but also
in that the reaction proceeds under mild conditions at about room
temperature.
[0007] For example, respiration is a mechanism such that
microorganisms or cells take in nutrients, such as saccharides,
fat, and protein, and, during the formation of carbon dioxide
(CO.sub.2) through the glycolytic pathway and tricarboxylic acid
(TCA) cycle having a number of enzyme reaction steps, nicotinamide
adenine dinucleotide (NAD.sup.+) is reduced to form reduced
nicotinamide adenine dinucleotide (NADH), thus converting the
chemical energy of the nutrients to redox energy, i.e., electrical
energy, and further, in the electron transport system, the
electrical energy of NADH is directly converted to proton-gradient
electrical energy and oxygen is reduced to form water. The
resultant electrical energy forms ATP from adenosine diphosphate
(ADP) through adenosine triphosphate (ATP) synthase, and ATP is
utilized in the reactions required for living of microorganisms or
cells. This energy conversion is carried out in cytosol and
mitochondria.
[0008] Photosynthesis is a mechanism such that, during the
conversion of the optical energy taken in to electrical energy by
reducing nicotinamide adenine dinucleotide phosphate (NADP +)
through the electron transport system to reduced nicotinamide
adenine dinucleotide phosphate (NADPH), water is oxidized to form
oxygen. The resultant electrical energy is utilized in a carbon
fixing reaction by taking in CO.sub.2 and a synthesis of
carbohydrates.
[0009] As a technique for utilizing the above-mention biological
metabolism in a fuel cell, a microorganism battery in which
electrical energy generated in microorganisms is removed from the
microorganisms through an electron mediator and the resultant
electrons are delivered to an electrode to obtain an electric
current has been reported (see, for example, Unexamined Japanese
Patent Application Laid-Open Specification No. 2000-133297).
[0010] However, microorganisms and cells have many functions other
than the desired reactions including the conversion of chemical
energy to electrical energy. Therefore, in the above method, an
undesired reaction consumes the electrical energy, making it
difficult to achieve a satisfactory energy conversion
efficiency.
[0011] For solving the problem, a fuel cell in which only a desired
reaction is advanced using an enzyme and an electron mediator has
been proposed (see, for example, Japanese Patent Application
Publication Nos. 2003-282124 and 2004-71559). In this fuel cell, a
fuel is decomposed into protons and electrons by an enzyme, and
fuel cells using as a fuel an alcohol, such as methanol or ethanol,
or a monosaccharide, such as glucose, have been developed.
[0012] However, the above-mentioned conventional fuel cell using
alcohol or glucose as a fuel is unsatisfactory in power generation
efficiency, and hence is difficult to put into practical use.
[0013] Accordingly, a task to be achieved by the present invention
is to provide a fuel cell which is advantageous not only in that it
can directly extract electric power from a polysaccharide to
achieve high-efficient power generation, but also in that it does
not require limited fossil fuel and contributes to the realization
of resource circulation society.
[0014] Another task to be achieved by the present invention is to
provide an electronic device, a movable body, a power generation
system, and a cogeneration system using the above excellent fuel
cell.
SUMMARY
[0015] The present inventors have conducted extensive and intensive
studies with a view toward solving the above-mentioned problems
accompanying the prior art technique. The studies are briefly
described below.
[0016] Glucose used as a fuel in the above-mentioned conventional
fuel cell is produced by industrially decomposing a variety of
polysaccharides. On the other hand, in the natural world,
substances in the form of monosaccharides including glucose are not
present, and many substances are present in the form of
polysaccharides. Generally, organisms do not obtain energy from
glucose, and actually obtain energy by taking in polysaccharides
and decomposing them by an enzyme. Such extraction of energy from
polysaccharides has been realized in a biomass system and the like
using, e.g., garbage as a fuel. This system produces a chemical
substance, such as hydrogen gas or methanol, by biomass, and many
of the chemical substances produced provide heat energy by burning
them. Further, heat generated by biological actions is utilized to
obtain heat energy. The heat energy is converted to kinetic energy
using a turbine or the like, and further converted to electrical
energy by a power generator. On the stages of energy conversion,
energy loss occurs and the energy of the fuel is considerably
wasted.
[0017] Therefore, when a fuel cell that uses polysaccharides
present in the natural world as a fuel to generate electric power
can be realized, electrical energy can be directly extracted from
not only garbage but also chemical substances produced in the
natural world (e.g., starch and cellulose), thus making it possible
to obtain electric power without using limited fossil fuel.
Further, garbage or waste paper can be effectively utilized and
hence the amount of waste is reduced, which contributes to the
realization of resource circulation society. Furthermore, plants
fix CO.sub.2 in air in the synthesis of polysaccharides by
photosynthesis, which possibly contributes to the reduction of
CO.sub.2 in air which is a current problem.
[0018] The present inventors have made extensive and intensive
studies. As a result, it has been found that the use of a
polysaccharide, such as starch, as a fuel in the fuel cell can
solve all the above problems, and the present invention has been
completed.
[0019] Specifically, for solving the above problems, the first
invention in an embodiment is directed to a fuel cell for
generating electric power by decomposing a fuel using an enzyme,
characterized in that the fuel contains a polysaccharide.
[0020] The second invention in an embodiment is directed to an
electronic device using a fuel cell, characterized in that the fuel
cell generates electric power by decomposing a fuel using an
enzyme, wherein the fuel contains a polysaccharide.
[0021] The third invention in an embodiment is directed to a
movable body using a fuel cell, characterized in that the fuel cell
generates electric power by decomposing a fuel using an enzyme,
wherein the fuel contains a polysaccharide.
[0022] The fourth invention in an embodiment is directed to a power
generation system using a fuel cell, characterized in that the fuel
cell generates electric power by decomposing a fuel using an
enzyme, wherein the fuel contains a polysaccharide.
[0023] The fifth invention in an embodiment is directed to a
cogeneration system using a fuel cell, characterized in that the
fuel cell generates electric power by decomposing a fuel using an
enzyme, wherein the fuel contains a polysaccharide.
[0024] The fuel cell in each of the first to fifth inventions
generally has a structure comprising a positive electrode and a
negative electrode that sandwich a proton conductor.
[0025] In the fuel cell, by decomposing a polysaccharide using an
enzyme, electrical energy can be directly extracted from the
polysaccharide.
[0026] Examples of polysaccharides usable as a fuel in the fuel
cell (polysaccharides in a broad sense, meaning any carbohydrates
that form two molecules or more of monosaccharides by hydrolysis,
and including oligosaccharides, such as disaccharides,
trisaccharides, and tetrasaccharides) include starch, amylose,
amylopectin, glycogen, cellulose, maltose, sucrose, and lactose.
These comprise two or more monosaccharides bonded together, and
each polysaccharide contains glucose as a monosaccharide which is a
bonding unit. Amylose and amylopectin are components of starch, and
starch is a mixture of amylose and amylopectin. As the fuel, any
fuel can be used as long as it contains a decomposable
polysaccharide, and the fuel may contain glucose which is a
decomposition product of a polysaccharide. Therefore, garbage or
the like can be used as a fuel.
[0027] In the fuel cell, as the enzyme, at least a decomposing
enzyme for promoting decomposition, e.g., hydrolysis of the
polysaccharide to form a monosaccharide, e.g., glucose and an
oxidase for promoting oxidation of the monosaccharide formed to
decompose it are used. Further, a coenzyme oxidase for changing a
coenzyme reduced by the oxidase to an oxidant is also used.
[0028] When the reduced coenzyme is changed to an oxidant due to
the action of the coenzyme oxidase, electrons are generated, and
the electrons are delivered from the coenzyme oxidase to an
electrode (negative electrode) through an electron mediator. As the
coenzyme, for example, NAD.sup.+ is used, and, as the coenzyme
oxidase, for example, diaphorase is used.
[0029] In the fuel cell using glucoamylase as the decomposing
enzyme for decomposing a polysaccharide and using glucose
dehydrogenase as the oxidase for decomposing a monosaccharide, a
polysaccharide decomposable into glucose by glucoamylase, for
example, a substance comprising any one of starch, amylose,
amylopectin, glycogen, and maltose can be used as a fuel to
generate electric power. Glucoamylase is a decomposing enzyme that
hydrolyzes an .alpha.-glucan, such as starch, to form glucose, and
glucose dehydrogenase is an oxidase that oxidizes .beta.-D-glucose
to D-glucono-.delta.-lactone.
[0030] In the fuel cell using cellulase as the decomposing enzyme
and using glucose dehydrogenase as the oxidase, cellulose
decomposable into glucose by cellulase can be used as a fuel.
Cellulase is, more specifically, at least one member selected from
cellulase (EC 3.2.1.4), exocellobiohydrase (EC 3.2.1.91), and
.beta.-glucosidase (EC 3.2.1.21). As the decomposing enzyme, a
mixture of glucoamylase and cellulase may be used, and, in this
case, the decomposing enzyme can decompose almost all the naturally
occurring polysaccharides, and therefore a power generation system
using as a fuel a material containing a large amount of
polysaccharides, for example, garbage can be achieved.
[0031] In the fuel cell using .alpha.-glucosidase as the
decomposing enzyme and using glucose dehydrogenase as the oxidase,
maltose decomposable into glucose by .alpha.-glucosidase can be
used as a fuel.
[0032] In the fuel cell using sucrase as the decomposing enzyme and
using glucose dehydrogenase as the oxidase, sucrose decomposable
into glucose and fructose by sucrase can be used as a fuel. Sucrase
is, more specifically, at least one member selected from
.alpha.-glucosidase (EC 3.2.1.20), sucrose-.alpha.-glucosidase (EC
3.2.1.48), and .beta.-fructofuranosidase (EC 3.2.1.26).
[0033] In the fuel cell using .beta.-galactosidase as the
decomposing enzyme and using glucose dehydrogenase as the oxidase,
lactose decomposable into glucose and galactose by
.beta.-galactosidase can be used as a fuel.
[0034] For efficiently capturing the enzyme reaction phenomenon
occurring near the negative electrode as an electric signal, it is
preferred that the coenzyme oxidase, coenzyme, and electron
mediator are immobilized on the negative electrode using an
immobilizer. It is preferred that the oxidase is also immobilized
on the negative electrode. Further, the decomposing enzyme for
decomposing a polysaccharide may be immobilized on the negative
electrode, and the polysaccharide finally used as a fuel may also
be immobilized on the negative electrode.
[0035] In the fuel cell using starch as a fuel, a gelatinized solid
fuel obtained by gelatinizing starch can be used. In this case,
there can be employed a method in which gelatinized starch is
brought into contact with the negative electrode having immobilized
thereon an enzyme and others, or a method in which gelatinized
starch is immobilized on the negative electrode, together with an
enzyme and others. When using such an electrode, the starch
concentration on the surface of the negative electrode can be kept
high, as compared to that achieved when using starch in the form of
a solution. Therefore, the decomposition reaction by the enzyme is
faster and hence the output is improved, and further the solid fuel
is easier to handle than a solution fuel, and can simplify the fuel
supplying system, and, in addition, the fuel cell can be moved
upside down and hence is very advantageously used in mobile
devices.
[0036] The fuel cell of the first invention can be used in any
devices which need electric power and have any sizes, and can be
used in, for example, electronic devices, movable bodies, power
devices, construction machines, machine tools, power generation
systems, and cogeneration systems, and the application of the fuel
cell determines the output, size, or form of the fuel cell or the
type of the fuel.
[0037] The electronic device of the second invention may be
basically any electronic device, and involves both an electronic
device of a portable type and an electronic device of a fixed type,
and, as specific examples, there can be mentioned cellular phones,
mobile devices, robots, personal computers, game machines, devices
mounted on car, household appliances, and industrial products.
[0038] The movable body of the third invention may be basically any
movable body, and specific examples include automobiles,
two-wheeled vehicles, aircrafts, rockets, and spacecrafts.
[0039] The power generation system of the fourth invention may be
basically any power generation system and may be on either a large
scale or a small scale, and, as a fuel, a polysaccharide, garbage
comprising polysaccharides, or the like can be used.
[0040] The cogeneration system of the fifth invention may be
basically any cogeneration system and may be on either a large
scale or a small scale, and, as a fuel, a polysaccharide, garbage
comprising polysaccharides, or the like can be used.
[0041] In the present invention having the above-mentioned
construction, a polysaccharide contained in the fuel is decomposed
by an enzyme into monosaccharides, and electrical energy can be
efficiently extracted upon oxidizing the monosaccharides by an
enzyme.
[0042] In the present invention, an enzyme is used as a catalyst
and a material comprising a polysaccharide is used as a fuel, and
thus a fuel cell capable of generating electric power with high
efficiency using a polysaccharide as a fuel can be obtained. The
fuel cell makes it possible to directly extract electrical energy
from, for example, garbage containing a large amount of
polysaccharides, thus enabling effective utilization of garbage.
Further, the fuel cell does not require limited fossil fuel, and
contributes to the realization of resource circulation society. In
addition, a polysaccharide which is safe when it is eaten can be
used as a fuel, and therefore a fuel cell advantageously used as a
mobile product can be obtained. Furthermore, for example, when
using gelatinized starch as a fuel, the fuel is easier to handle
than a solution fuel, and can simplify the fuel supplying system,
thus obtaining a fuel cell advantageously used as a mobile product.
The use of this excellent fuel cell can realize a high-performance
electronic device, movable body, power generation system, or
cogeneration system.
[0043] Additional features and advantages are described herein, and
will be apparent from the following Detailed Description and the
figures.
BRIEF DESCRIPTION OF THE FIGURES
[0044] FIG. 1 is a schematic diagram showing the construction of a
fuel cell according to one embodiment of the present invention.
[0045] FIG. 2 is a schematic diagram illustrating the decomposition
of starch and cellulose into glucose by an enzyme.
[0046] FIG. 3 is a schematic diagram showing the construction of a
fuel cell according to one embodiment of the present invention
using starch glue.
[0047] FIG. 4 are diagrams showing the glucose concentration
distribution in the direction perpendicular to the surface of the
fuel electrode in the fuel cell according to one embodiment of the
present invention.
[0048] FIG. 5 are diagrammatic views showing a fuel cartridge used
in the fuel cell according to one embodiment of the present
invention.
[0049] FIG. 6 is a diagrammatic view for explaining a method for
supplying a fuel to the fuel cell according to one embodiment of
the present invention.
[0050] FIG. 7 is a diagrammatic view for explaining a method for
supplying a fuel to the fuel cell according to one embodiment of
the present invention.
[0051] FIG. 8 is a diagrammatic view for explaining an example of
the method for supplying a fuel to the fuel cell according to one
embodiment of the present invention.
[0052] FIG. 9 is a diagrammatic view for explaining another example
of the method for supplying a fuel to the fuel cell according to
one embodiment of the present invention.
[0053] FIG. 10 is a diagram showing the results of the CV
measurement in Example 1 of the present invention.
[0054] FIG. 11 is a diagram showing a change of the current density
with time at a constant potential 0 V of the working electrode
relative to the reference electrode in the electrochemical
measurements in Examples 1 and 2 of the present invention.
[0055] FIG. 12 is a diagram showing the results of the CV
measurement in Example 3 of the present invention.
[0056] FIG. 13 are diagrammatic views of a fuel cell used in the
evaluation in Example 4 of the present invention.
[0057] FIG. 14 is a diagram showing the results of the measurement
of current-voltage characteristics of the fuel cell used in the
evaluation in Example 4 of the present invention.
DETAILED DESCRIPTION
[0058] Hereinbelow, an embodiment of the present invention will be
described with reference to the accompanying drawings.
[0059] FIG. 1 diagrammatically shows a fuel cell according to one
embodiment of the present invention. As shown in FIG. 1, the fuel
cell comprises a fuel electrode (negative electrode) 1 which
decomposes a polysaccharide supplied as a fuel using an enzyme to
get electrons and generate protons (H.sup.+), an electrolyte layer
3 which conducts only protons, and an air electrode (positive
electrode) 5 which is separated from the fuel electrode 1 by the
electrolyte layer 3, and which forms water from protons transported
from the fuel electrode 1 through the electrolyte layer 3,
electrons fed from the fuel electrode 1 through an external
circuit, and oxygen in air.
[0060] The fuel electrode 1 comprises an electrode 11 comprised of,
e.g., carbon having immobilized thereon, by an immobilizer (e.g., a
polymer), an enzyme responsible for decomposing a polysaccharide
into monosaccharides, an enzyme responsible for decomposing the
monosaccharide formed, a coenzyme (e.g., NAD.sup.+ or NADP.sup.+)
which forms a reductant due to the oxidation reaction in the
monosaccharide decomposition process, a coenzyme oxidase (e.g.,
diaphorase) for oxidizing the reductant (e.g., NADH or NADPH) of
the coenzyme, and an electron mediator (e.g.,
2-amino-3-carboxy-1,4-naphthoquinone; ACNQ, or vitamin K3) for
receiving electrons generated due to the oxidation of the coenzyme
from the coenzyme oxidase and delivering the electrons to the
electrode 11.
[0061] The polysaccharide usable as a fuel comprises two or more
monosaccharides bonded together. Examples of the polysaccharides
include disaccharides, such as maltose, sucrose, and lactose,
starch comprising amylose having a linear molecule and amylopectin
having a branched molecule, high-molecular glycogen having an
amylopectin-like branched molecule, cellulose having a linear
molecule, and saccharides as intermediates thereof.
[0062] As an enzyme responsible for decomposing a polysaccharide, a
decomposing enzyme capable of cutting a glycoside linkage by
hydrolysis or the like is used. When the polysaccharide is starch,
examples of the decomposing enzymes include hydrolases, such as
glucoamylase (EC 3.2.1.3), .alpha.-amylase (EC 3.2.1.1), and
.beta.-amylase (EC 3.2.1.2). Among these, glucoamylase (GAL)
decomposes starch into glucose. EC indicates an enzyme number.
[0063] Other polysaccharides can be decomposed by, for example, the
following decomposing enzymes.
<Glycogen>
[0064] Glucoamylase (EC 3.2.1.3) [0065] .alpha.-Amylase (EC
3.2.1.1)
<Dextrin>
[0065] [0066] Glucoamylase (EC 3.2.1.3)
<Cellulose>
[0066] [0067] Cellulase (EC 3.2.1.4) [0068] Exocellobiohydrase (EC
3.2.1.91) [0069] .beta.-Glucosidase (EC 3.2.1.21)
[0070] Here, enzymes capable of hydrolyzing cellulose are
collectively referred to as "cellulase". As examples of cellulases,
there can be mentioned the above three types of enzymes, and
cellulose can be decomposed into glucose in the presence of at
least one member selected from the three enzymes.
<Maltose>
[0071] .alpha.-Glucosidase (EC 3.2.1.20)
[0072] This enzyme is also called maltase, but it acts on sucrose.
Maltose can be decomposed also by glucoamylase.
<Sucrose>
[0073] .alpha.-Glucosidase (EC 3.2.1.20) [0074]
Sucrose-.alpha.-glucosidase (EC 3.2.1.48) [0075]
.beta.-Fructofuranosidase (EC 3.2.1.26)
[0076] Here, enzymes capable of hydrolyzing sucrose are
collectively referred to as "sucrase". As examples of sucrases,
there can be mentioned the above three types of enzymes. Glucose
can be formed from sucrose in the presence of at least one member
selected from the three enzymes.
<Lactose>
[0077] .beta.-Galactosidase (EC 3.2.1.23)
<1,3-.beta.-Glucan>
[0077] [0078] Glucanendo-1,3-.beta.-D-glucosidase (EC 3.2.1.39)
<.alpha.,.alpha.-Trehalose>
[0078] [0079] .alpha.,.alpha.-Trehalase (EC 3.2.1.28) [0080]
.alpha.,.alpha.-Trehalose phosphorylase (EC 2.4.1.64)
<Stachyose>
[0080] [0081] .alpha.-Galactosidase (EC 3.2.1.22) [0082]
.alpha.-Glucosidase (EC 3.2.1.20)
<Glucosides>
[0082] [0083] .beta.-Glucosidase (EC 3.2.1.21)
[0084] FIG. 2 shows an example of the decomposition of starch and
cellulose into glucose by an enzyme.
[0085] The enzyme responsible for decomposing a monosaccharide
includes an oxidase that participates in the redox reaction in the
decomposition process. When the polysaccharide is starch, glycogen,
cellulose, or maltose, the monosaccharide formed by hydrolysis of
the polysaccharide is glucose, and, when the polysaccharide is
sucrose or lactose, glucose constitutes half of the monosaccharide
formed. As an example of an enzyme responsible for decomposing
glucose, there can be mentioned glucose dehydrogenase (GDH). The
use of this oxidase can oxidize .beta.-D-glucose to
D-glucono-.delta.-lactone.
[0086] Further, the resultant D-glucono-.delta.-lactone can be
decomposed into 2-keto-6-phospho-D-gluconate in the presence of two
enzymes, i.e., gluconokinase and phosphogluconate dehydrogenase
(PhGDH). Specifically, D-glucono-.delta.-lactone is hydrolyzed into
D-gluconate, and D-gluconate is phosphorylated by hydrolyzing
adenosine triphosphate (ATP) in the presence of gluconokinase into
adenosine diphosphate (ADP) and phosphoric acid, thus forming
6-phospho-D-gluconate. The resultant 6-phospho-D-gluconate is
oxidized to 2-keto-6-phospho-D-gluconate due to the action of
oxidase PhGDH.
[0087] In addition to the above decomposition process, glucose can
be decomposed into CO.sub.2 utilizing carbohydrate metabolism. The
decomposition process utilizing carbohydrate metabolism is roughly
classified into decomposition of glucose and formation of pyruvic
acid through a glycolytic pathway and a TCA cycle, and these are
widely known reaction systems.
[0088] The oxidation reaction in the monosaccharide decomposition
process accompanies a reduction reaction of a coenzyme. The
coenzyme is determined almost always depending on the enzyme used,
and, when the enzyme is GDH, NAD.sup.+ is used as a coenzyme.
Specifically, when GDH causes .beta.-D-glucose to be oxidized to
D-glucono-.delta.-lactone, NAD.sup.+ is reduced to NADH to generate
H.sup.+.
[0089] The resultant NADH is immediately oxidized to NAD.sup.+ in
the presence of diaphorase (DI) to form two electrons and two
protons H.sup.+. In other words, in the oxidation reaction on the
first stage, two electrons and two protons H.sup.+ are formed per
one molecule of glucose. In the oxidation reaction on the second
stage, four electrons and four protons H.sup.+ in total are
formed.
[0090] The electrons generated in the above process are delivered
from diaphorase to the electrode 11 through an electron mediator,
and the protons H.sup.+ are transported to the air electrode 5
through the electrolyte layer 3.
[0091] The electron mediator delivers electrons to the electrode,
and the voltage of the fuel cell depends on the oxidation-reduction
potential of the electron mediator. In other words, for obtaining a
higher voltage, an electron mediator having a more negative
potential on the fuel electrode 1 side is preferably selected, but
the reaction affinity of the electron mediator with an enzyme, the
electron exchange rate for the electrode 11, the structure
stability of the electron mediator to an inhibition factor (such as
light or oxygen), and the like must be taken into consideration.
From this point of view, as an electron mediator for the fuel
electrode 1, for example, ACNQ or vitamin K3 is preferred.
Alternatively, a compound having, e.g., a quinone skeleton, a metal
complex of Os, Ru, Fe, or Co, a violegen compound, such as benzyl
violegen, a compound having a nicotinamide structure, a compound
having a riboflavin structure, or a compound having a
nucleotide-phosphate structure can be used as an electron
mediator.
[0092] For achieving an efficient and steady electrode reaction, it
is preferred that the enzyme, coenzyme, and electron mediator are
maintained at pH optimum for the enzyme, for example, pH about 7,
using a buffer, such as a Tris buffer or a phosphate buffer.
Further, too large or too small ion strength (I.S.) adversely
affects the enzyme activity, and, from the viewpoint of achieving
excellent electrochemical response, it is preferred that the ion
strength is an appropriate value, for example, about 0.3. The
enzymes used individually have the optimum pH and the optimum ion
strength, and hence the pH and ion strength are not limited to the
above values.
[0093] The enzyme, coenzyme, and electron mediator may be used in
the form of a solution in a buffer, but, for efficiently capturing
the enzyme reaction phenomenon occurring near the electrode as an
electric signal, it is preferred that at least the coenzyme oxidase
and the electron mediator are immobilized on the electrode 11 using
an immobilizer. When the enzyme for decomposing a fuel and the
coenzyme are further immobilized on the electrode 11, the enzyme
reaction system at the fuel electrode 1 can be stabilized. As the
immobilizer, for example, glutaraldehyde (GA) and poly-L-lysine
(PLL) can be used in combination. They may be used individually, or
other polymers may be used. By using an immobilizer comprising a
combination of glutaraldehyde and poly-L-lysine, the enzyme
immobilizing abilities of the individual components can be fully
utilized, so that the immobilizer exhibits collectively excellent
enzyme immobilizing ability. In this case, an optimum ratio of
glutaraldehyde to poly-L-lysine varies depending on the enzyme to
be immobilized and a substrate of the enzyme, but, generally, the
ratio may be arbitrary. Specifically, a ratio of an aqueous
glutaraldehyde solution (0.125%) to an aqueous poly-L-lysine
solution (1%) is 1:1, 1:2, or 2:1.
[0094] FIG. 1 shows an example in which the polysaccharide is
starch, the enzyme responsible for decomposing the polysaccharide
into monosaccharides is glucoamylase (GAL) which decomposes starch
into glucose, the enzyme responsible for decomposing the
monosaccharide formed (.beta.-D-glucose) is glucose dehydrogenase
(GDH), the coenzyme which forms a reductant due to the oxidation
reaction in the monosaccharide decomposition process is NAD.sup.+,
the coenzyme oxidase for oxidizing NADH which is the reductant of
the coenzyme is diaphorase (DI), and the electron mediator for
receiving from the coenzyme oxidase electrons generated due to the
oxidation of the coenzyme and delivering the electrons to the
electrode 11 is ACNQ.
[0095] The electrolyte layer 3 is comprised of a material which is
a proton-conductive membrane for transporting protons H.sup.+
generated at the fuel electrode 1 to the air electrode 5 and which
has no electron conduction properties and can transport protons
H.sup.+. Examples of the electrolyte layers 3 include layers
comprised of a perfluorocarbonsulfonic acid (PFS) resin membrane, a
trifluorostyrene derivative copolymer membrane, a polybenzimidazole
membrane impregnated with phosphoric acid, an aromatic polyether
ketone sulfonic acid membrane, PSSA-PVA (polystyrenesulfonic
acid-polyvinyl alcohol copolymer), or PSSA-EVOH
(polystyrenesulfonic acid-ethylenevinyl alcohol copolymer). Of
these, preferred is a layer comprised of an ion-exchange resin
having a fluorine-containing carbon sulfonic acid group, and,
specifically, Nafion (trade name; manufactured and sold by Du Pont
Co., U.S.A.) is used.
[0096] The air electrode 5 is comprised of carbon powder having a
catalyst carried thereon or catalyst particles which are not
supported on carbon. In the catalyst, for example, fine particles
of platinum (Pt), or fine particles of an alloy of a transition
metal, such as iron (Fe), nickel (Ni), cobalt (Co), or ruthenium
(Ru), and platinum or an oxide are used. The air electrode 5 has a
structure such that, for example, a catalyst or a catalyst layer
comprised of carbon powder comprising a catalyst and a gas
diffusion layer comprised of a porous carbon material are stacked
in this order from the electrolyte layer 3 side. The structure of
the air electrode 5 is not limited to this, and an oxidoreductase
can be used as a catalyst. In this case, the oxidoreductase and an
electron mediator for delivering electrons to the electrode are
used in combination.
[0097] At the air electrode 5, protons H.sup.+ from the electrolyte
layer 3 and electrons from the fuel electrode 1 reduce oxygen in
air in the presence of a catalyst to form water.
[0098] In the fuel cell having the above-described construction,
when a polysaccharide, such as starch, is supplied to the fuel
electrode 1, the polysaccharide is hydrolyzed by an enzyme into
monosaccharides, such as glucose, and further the monosaccharide is
decomposed by a decomposing enzyme comprising an oxidase. The
oxidase participates in the monosaccharide decomposition process to
form both electrons and protons H.sup.+ on the fuel electrode 1
side, making it possible to generate an electric current between
the fuel electrode 1 and the air electrode 5.
[0099] In the fuel cell, the type of the decomposable
polysaccharide is determined depending on the type of the
decomposing enzyme held or immobilized on the fuel electrode 1.
When a mixture containing a plurality of polysaccharides is used as
a fuel, enzymes for respectively decomposing the polysaccharides
are held or immobilized on the fuel electrode 1, thus improving the
fuel efficiency. In addition, garbage or the like can be used as a
fuel to generate electric power, enabling effective utilization of
garbage and others.
[0100] Furthermore, the above fuel cell can use as a fuel a
polysaccharide having a high energy density, which is safe when it
is eaten, and further can work at room temperature, and therefore
is advantageously used in mobile products. With respect to the
energy density obtained when using a polysaccharide as a fuel,
taking cooked white rice as an example, the energy of starch
contained in about 100 g of cooked rice (corresponding to one rice
bowl and about 160 kcal) corresponds to the energy of 64 AA-size
alkali dry cells (about 3 Wh per cell), which is a high fuel energy
density equal to or more than that obtained from a glucose
solution. Polysaccharide as a fuel can be used in the form of an
aqueous solution, but there can be employed a method in which the
polysaccharide is gelatinized like starch glue and brought into
contact with the fuel electrode 1 or a method in which the
polysaccharide is disposed within the fuel electrode 1, and thus a
solidified fuel can be used in the fuel cell, which is further
advantageous to mobile products. Glucose has a problem in that
glucose having a small diffusion coefficient, as compared to
methanol or ethanol, is disadvantageous to a fuel-molecule
diffusion controlled reaction which proceeds when the glucose is
used as a fuel in the form of a solution. However, when a method in
which starch is gelatinized and brought into contact with the fuel
electrode 1 or a method in which starch is disposed within the fuel
electrode 1 is employed, the starch concentration on the surface of
the fuel electrode 1 or in the fuel electrode 1, namely, the
glucose concentration can be kept high, so that the output is
improved, as compared to that achieved when using starch in the
form of a solution. In addition, a solidified fuel, such as starch
glue, is easier to handle than a liquid fuel, and can simplify the
fuel supplying system, and thus it is very effective when the fuel
cell is applied to mobile products. FIG. 3 shows an example in
which starch glue 6 as a fuel is immobilized on the fuel electrode
1.
[0101] When starch which is a polysaccharide is used as a fuel, the
glucose concentration on the surface of the fuel electrode 1 or in
the fuel electrode 1 can be kept high, as compared to that obtained
when using glucose which is a monosaccharide as a fuel.
Specifically, for example, amylose contained in starch comprises
several hundred to several thousand molecules of glucose bonded
together, and, when one molecule of amylose as a fuel molecule
arrives at the surface of the fuel electrode 1 by diffusion, the
transport speed of glucose to the surface of the fuel electrode 1
is several hundred to several thousand times that achieved when
using glucose as a fuel. In other words, the use of starch as a
fuel makes it possible to transport glucose to the surface of the
fuel electrode 1 at a higher speed.
[0102] FIG. 4A shows a state such that a CA measurement (in the
measurement of a change of the current with time at a constant
potential, a state in which a steady current flows is a
diffusion-controlled state) was conducted in a solution containing
starch and glucoamylase (GAL) using an enzyme-immobilized electrode
having ACNQ, diaphorase (DI), and glucose dehydrogenase (GDH)
immobilized on the electrode 11 by an immobilizer and a
satisfactorily long period of time had lapsed (diffusion-controlled
state). Similarly, FIG. 4B shows a state such that a CA measurement
was conducted in a glucose solution using the same
enzyme-immobilized electrode and a satisfactorily long period of
time had lapsed (diffusion-controlled state). The enzyme reaction
in the enzyme-immobilized electrode is satisfactorily fast, that
is, glucose which has reached the surface of the electrode by
diffusion can be very rapidly decomposed to deliver electrons to
the electrode. In the case of FIG. 4B, on the surface of the
electrode, the consumption of glucose by the enzyme-immobilized
electrode balances with the supply of glucose by diffusion from the
glucose solution far away from the enzyme-immobilized electrode to
exhibit a constant glucose concentration gradient. This glucose
concentration gradient determines a current, and, the larger the
glucose concentration gradient, the larger the current. That is,
the current can be increased by increasing the glucose
concentration. On the other hand, in the case of FIG. 4A, no
glucose is present in the solution at the beginning of the
measurement, but the solution contains glucoamylase as well as
starch, and hence glucoamylase hydrolyzes starch to form glucose.
In this case, on the surface of the electrode, the consumption of
glucose by the enzyme-immobilized electrode balances with the
supply by diffusion of glucose formed in the solution containing
starch and glucoamylase and the supply of glucose formed due to
glucoamylase present on the surface of the electrode, determining a
current. The glucose formed on the surface of the electrode
increases the glucose concentration on the surface of the
electrode, as compared to that obtained when using a glucose
solution (compared in terms of the ultimate glucose saturated
solution). When glucoamylase and starch are further immobilized on
the enzyme-immobilized electrode, that is the construction shown in
FIG. 3 is employed, the above effect can be further improved.
[0103] Next, a method for supplying a fuel to the fuel cell is
described. Here, the case where starch is used as a fuel is
mentioned.
[0104] FIG. 5A shows an unused, card-shaped fuel cartridge 32
filled with a fuel 31 comprising a starch solution (amylose,
amylopectin), starch glue, or the like. The fuel 31 may contain
glucose, NADH, or the like, and, in such a case, the current at the
beginning of the operation can be large, as compared to the current
obtained when using a fuel 31 comprising only starch. FIG. 5B shows
the used fuel cartridge 32 which has used all the fuel 31. In FIGS.
5A and 5B, reference numerals 33a, 33b designate fuel pushers.
Reference numeral 33c designates a spring for pushing the fuel,
having both ends fixed to the fuel pushers 33a, 33b. The fuel
pusher 33a is fixed to the fuel cartridge 32, and the spring 33c
pushes the fuel pusher 33b against the fuel 31.
[0105] FIG. 6 shows the fuel cell in which the fuel cartridge 32
has used all the fuel 31. The fuel cartridge 32 is contained in a
fuel cartridge container 34. The fuel cartridge container 34 has a
cartridge inlet 34a through which the fuel cartridge 32 is inserted
into the fuel cartridge container and a cartridge outlet 34b
through which the fuel cartridge 32 is removed from the container.
The fuel cell has a construction such that an air electrode 5
comprised of an enzyme-immobilized carbon electrode having an
enzyme immobilized on porous carbon and a fuel electrode 1
comprised of an enzyme-immobilized carbon electrode having an
enzyme and an electron mediator immobilized by an immobilizer on
porous carbon like in Example 1 are disposed so that they face each
other through a separator 35 as a proton conductor (corresponding
to the electrolyte layer 3). In FIG. 6, as an example of a load of
an external circuit, an electric bulb 36 is connected to the air
electrode 5 and the fuel electrode 1. The fuel cartridge 32 has
used all the fuel 31, and therefore the electric bulb 36 is not
lightening. The fuel cartridge 32 generally has a size larger than
that of the air electrode 5 or fuel electrode 1.
[0106] The used fuel cartridge 32 is changed to an unused fuel
cartridge 32 as follows. As shown in FIG. 7, the cartridge inlet
34a is opened, and the unused fuel cartridge 32 is inserted into
the fuel cartridge container 34 to push the used fuel cartridge 32,
letting it go out of the container through the cartridge outlet
34b. At a point in time when the used fuel cartridge 32 is
completely removed from the cartridge outlet 34b, the unused fuel
cartridge 32 is set in a predetermined position. This state is
shown in FIG. 8. As shown in FIG. 8, when the unused fuel cartridge
32 is set in the position, a feed passage for the fuel 31 is formed
between the fuel cartridge 32 and the fuel electrode 1, so that the
fuel 31 is supplied to the fuel electrode 1 through the feed
passage. This can be easily realized in an electromechanical
system. In this instance, in the fuel cartridge 32, the fuel pusher
33b pushes the fuel 31, and therefore the fuel 31 can be fed to the
inside of the fuel electrode 1 comprised of an enzyme-immobilized
carbon electrode having an enzyme and an electron mediator
immobilized by an immobilizer on porous carbon. This is effective
when, for example, liquid having a high viscosity is used as the
fuel 31. When the fuel 31 can reach the inside of the fuel
electrode 1 merely by diffusion, the fuel pushers 33a, 33b and
spring 33c can be omitted, but, when using the fuel pushers 33a,
33b and spring 33c, the fuel 31 can be more surely supplied to the
inside of the fuel electrode 1. Thus, the fuel 31 is supplied to
the fuel electrode 5 to start power generation, so that the
electric bulb 36 lightens.
[0107] It is preferred that CO.sub.2 or H.sub.2O, or both generated
in the power generation are stored in a vacant space of the fuel
cartridge 32 left after all the fuel 31 has been used.
Specifically, CO.sub.2 or H.sub.2O may be discharged from the fuel
cell, but, from an environmental point of view, more specifically,
discharging CO.sub.2 from the fuel cell is not preferred from the
viewpoint of preventing global warming, and further, discharging
H.sub.2O from the fuel cell has a problem in that, when, e.g., a
cellular phone having the fuel cell mounted is put in a pocket or
bag, the pocket or bag may be wetted with water discharged, and
therefore it is preferred to store CO.sub.2 or H.sub.2O, or both in
the fuel cartridge 32. CO.sub.2 or H.sub.2O can be efficiently
stored in a vacant space of the fuel cartridge 32 left after all
the fuel 31 has been used.
[0108] Amylase may be put in the portion of the fuel cartridge 32
containing the fuel 31. In this case, the concentration of glucose
supplied to the surface of the fuel electrode 1 is increased, thus
making it possible to obtain a large current and a large initial
current.
[0109] As the fuel cartridge 32, a fuel cartridge preliminarily
filled with the fuel 31 may be used, or, for emergency, the fuel
cartridge 32 into which easily available cooked rice, pasta,
potato, or the like, which is appropriately treated, is charged may
be used. A method for charging the fuel 31 into the fuel cartridge
32 may be in which, for example, a fuel reservoir container is
prepared and a fuel inlet formed in the fuel cartridge 32 is
connected to the container to charge the fuel 31. In this instance,
the fuel cartridge 32 may be either removed from the fuel cell or
not.
[0110] With respect to the treatment of starch, starch in raw rice
or potato is comprised mainly of .beta.-starch crystallites and
amylase exhibits almost no activity, but, when starch is heated,
.beta.-starch is changed to gelatinized .alpha.-starch, so that
amylase exhibits an activity. For this reason, it is preferred to
supply .alpha.-starch as the fuel 31 to the fuel electrode 1, but
.alpha.-starch is changed to .beta.-starch with the passage of time
(aging).
[0111] The fuel 31 having moisture extremely reduced or having
almost no moisture can be used. Starch can be solidified by
pressing it. Glucose can be solidified by this method, but it has
poor formability. In the solidified fuel 31, molecules are unlikely
to diffuse, and therefore the solidified fuel cannot be used as it
is. In this case, while contacting the fuel 31 and the fuel
electrode 1 with each other, water may be supplied from the outside
or from the inside of the fuel cartridge 32 (in which the starch
solid material and water are separated from each other), so that
the fuel cell starts power generation. As the water, water formed
at the air electrode 5 may be utilized based on the principle
similar to that of a direct methanol fuel cell (DMFC) using a 100%
methanol fuel. This is a system in which the fuel electrode 1 and
the air electrode 5 collectively form water in principle. The
reaction in this system is represented by the following
formula:
C.sub.6H.sub.12O.sub.6+6O.sub.2.fwdarw.6CO.sub.2+6H.sub.2O
.DELTA.G.degree.=-4,928 kJ/mol.
[0112] Next, a method for supplying a fuel to the fuel cell used as
a throwaway primary battery, such as a dry cell, is described.
[0113] In this case, a mechanism for removing and inserting the
fuel cartridge 32 to the fuel cell is not needed, and, as shown in
FIG. 9, the fuel cartridge 32 is preliminarily unified with the
fuel electrode 1. In this case, the method for supplying the fuel
31 from the fuel cartridge 32 to the fuel electrode 1 is similar to
the method described above.
[0114] Hereinbelow, the present invention will be described with
reference to the following Examples.
Example 1
[0115] Onto a glassy carbon (GC) electrode (BAS, .phi.=3.0 mm) were
applied dropwise 3 .mu.l of a phosphate buffer solution (83 .mu.M)
of diaphorase (DI)(UNITIKA LTD., from Bacillus stearothermophilus),
6 .mu.l of a phosphate buffer solution (60 .mu.M) of glucose
dehydrogenase (GDH)(TOYOBO LTD.), 3 .mu.l of a phosphate buffer
solution (1.4 mM) of glucoamylase (GAL)(Oriental Yeast Co., Ltd.),
3 .mu.l of an aqueous solution (1%) of poly-L-lysine (PLL), 2 .mu.l
of a phosphate buffer solution (0.4 M) of NADH, 2 .mu.l of an
ethanol solution (28 mM) of ACNQ, and 3 .mu.l of an aqueous
solution (0.125%) of glutaraldehyde (GA), and they were mixed with
each other and air-dried at room temperature, followed by washing
with distilled water, preparing a GAL/GDH/NADH/DI/ACNQ-immobilized
electrode (see FIG. 1).
[0116] The thus prepared immobilized electrode was used as a
working electrode, an Ag/AgCl electrode was used as a reference
electrode, a Pt electrode was used as a counter electrode, an
electrolytic cell made of polytetrafluoroethylene having a capacity
of 1 ml was used as a reaction bath, and 1 ml of a 0.1 M phosphate
buffer solution (pH: 7; I.S.=0.3) containing water-soluble starch
in a concentration of 1% was used as a reaction solution, and they
were subjected to deoxygenation using Ar gas, and then an
electrochemical measurement was conducted at room temperature
(25.degree. C.).
Example 2
[0117] An electrochemical measurement was conducted in
substantially the same manner as in Example 1 except that 5 mg of a
material obtained by gelatinizing a 50% phosphate buffer solution
of starch at 70.degree. C. was applied to the
GAL/GDH/NADH/DI/ACNQ-immobilized electrode prepared in Example 1,
and that the reaction solution was changed to 1 ml of a 0.1 M
phosphate buffer solution (pH 7; I.S.=0.3).
Comparative Example 1
[0118] An electrochemical measurement was conducted in
substantially the same manner as in Example 1 except that
glucoamylase (GAL) was omitted in the process of preparing the
immobilized electrode in Example 1 to prepare a
GDH/NADH/DI/ACNQ-immobilized electrode.
[0119] With respect to Example 1, potential scan was performed in
accordance with a potential sweep method (CV) at a scanning speed
of 20 mV/sec. The result of the CV measurement is shown in FIG. 10
(solid line a in the graph). In FIG. 10, for reference, the result
of the CV measurement in the case where a 0.1 M phosphate buffer
solution containing no starch was used as a reaction solution is
shown by a broken line b, and the result of the CV measurement in
the case where a 0.1 M phosphate buffer solution containing starch
in a concentration of 0.1% was used as a reaction solution is shown
by a dot-dash line c. In the measurement in the case where a
solution containing starch in a concentration of 0.5% was used, a
result substantially the same as the solid line a was obtained.
[0120] As seen from FIG. 10, when the reaction solution contains
starch, an oxidation current is observed, which indicates that
starch in the membrane immobilized on the electrode is decomposed
by GAL into glucose and the glucose is decomposed by GDH, and the
reactions successively proceed, so that the electrode receives
electrons.
[0121] In each of the electrochemical measurements in Examples 1
and 2 and Comparative Example 1, a change of the current density
with time at a constant potential 0 V of the working electrode
relative to the reference electrode is shown in FIG. 11. Further,
steady currents obtained after a satisfactorily long period of time
has lapsed in FIG. 11 are shown in Table 1.
TABLE-US-00001 TABLE 1 Steady current (.mu.A/cm.sup.2 @0 V vs
Ag|AgCl) Example 1 58 Example 2 395 Comparative Example 1 2
[0122] As can be seen from Table 1, in Examples 1 and 2 in which
glucoamylase which is an enzyme decomposing starch was present, a
current could be generated using starch as a fuel. Especially in
Example 2 in which starch was gelatinized and immobilized on the
surface of the electrode, there could be obtained a current larger
than that obtained in Example 1 in which starch in the form of a
solution was in contact with the electrode. The reason for this is
that the starch concentration, namely, glucose concentration on the
surface of the electrode can be kept high as mentioned above,
making it possible to promote the decomposition reaction of the
fuel.
[0123] Glucose has a problem in that glucose having a small
diffusion coefficient, as compared to methanol or ethanol, is
disadvantageous to a diffusion controlled reaction which is likely
to proceed when the glucose is used as a fuel in the form of a
solution, but, as apparent from the above results, the problem can
be solved by using starch as a fuel or immobilizing gelatinized
starch on the surface of the electrode. Further, the use of a
gelatinized solid fuel makes easy handling of the fuel and can
simplify the fuel supplying system, and hence the resultant fuel
cell is very useful as a fuel cell mounted on mobile devices, such
as cellular phones.
Example 3
[0124] With respect to the fuel electrode 1 having immobilized
thereon starch glue 6 as a fuel and glucoamylase (GAL) as an enzyme
decomposing starch into glucose, a CV measurement was conducted
under the same conditions as those used in Example 1. The results
are shown in FIG. 12 (curved line a). In FIG. 12, for reference,
the result of the CV measurement in the case where a glucose
solution was used as a fuel is also shown (curved line b). As can
be seen from FIG. 12, when the starch glue 6 is used as a fuel, a
considerably large current can be obtained, as compared to the
maximum current obtained when the glucose solution (glucose
concentration: 200 mM) is used as a fuel. This result reflects the
extremely high glucose concentration on the surface of the fuel
electrode 1 as mentioned above. Further, the reason why the current
increases with the passage of time resides in that the starch is
gradually hydrolyzed by glucoamylase (GAL) and, consequently, the
glucose concentration on the surface of the electrode increases
with the passage of time. The curved line b has a shape unique to a
diffusion controlled reaction.
Example 4
[0125] A fuel cell shown in FIGS. 13A and 13B was assembled and an
evaluation was made. As shown in FIGS. 13A and 13B, the fuel cell
has a construction such that an air electrode 5 comprised of an
enzyme-immobilized carbon electrode having an enzyme immobilized on
0.25 cm.sup.2 carbon felt and a fuel electrode 1 comprised of an
enzyme-immobilized carbon electrode having an enzyme and an
electron mediator immobilized by an immobilizer on 0.25 cm.sup.2
carbon felt like in Example 1 are disposed so that they face each
other through a separator 35 as a proton conductor. In this case,
the separator 35 is comprised of a predetermined film having proton
conduction properties, e.g., cellophane. Ti current collectors 41,
42 are disposed, respectively, under the air electrode 5 and on the
fuel electrode 1, thus facilitating current collection. Reference
numerals 43, 44 designate clamp plates. The clamp plates 43, 44 are
fastened to each other with screws 45, and between them are
sandwiched whole of the air electrode 5, fuel electrode 1,
separator 35, and Ti current collectors 41, 42. A circular recessed
portion 43a for drawing air is formed in one side (outer side) of
the clamp plate 43, and a number of holes 43b are formed at the
bottom of the recessed portion 43a so that they penetrate the clamp
plate 43 to another side. The holes 43b serve as air feed passages
to the air electrode 5. On the other hand, a circular recessed
portion 44a for charging a fuel is formed in one side (outer side)
of the clamp plate 44, and a number of holes 44b are formed at the
bottom of the recessed portion 44a so that they penetrate the clamp
plate 44 to another side. The holes 44b serve as fuel feed passages
to the fuel electrode 1. A spacer 46 is provided at the periphery
portion of another side of the clamp plate 44 so that the clamp
plates 43, 44 fastened with the screws 45 have a predetermined
space between them.
[0126] As shown in FIG. 13B, a load 47 was connected to the Ti
current collectors 41, 42, and a starch/buffer solution was charged
as a fuel into the recessed portion 44a in the clamp plate 44 to
perform power generation. The working temperature was 25.degree. C.
FIG. 14 shows the current-voltage characteristics of this fuel
cell. The open circuit voltage is about 0.86 V. In FIG. 14, a
curved line a indicates a current density, and a curved line b
indicates a power density. As shown in FIG. 14, the current density
is about 1 mA/cm.sup.2 at the maximum and the power density is
about 0.4 mW/cm.sup.2 at the maximum, which indicates that both
values are high.
[0127] Hereinabove, one embodiment and Examples of the present
invention are described in detail, but the present invention is not
limited to the above embodiment or Examples and can be changed or
modified based on the technical concept of the present
invention.
[0128] For example, values, structures, constructions, forms, and
materials mentioned in the above embodiment and Examples are merely
examples, and, if necessary, a value, structure, construction,
form, or material different from the above may be used.
[0129] Specifically, for example, the form of the fuel cell or fuel
cartridge 32 may be a form different from that mentioned in the
above embodiment or Examples.
[0130] It should be understood that various changes and
modifications to the presently preferred embodiments described
herein will be apparent to those skilled in the art. Such changes
and modifications can be made without departing from the spirit and
scope of the present subject matter and without diminishing its
intended advantages. It is therefore intended that such changes and
modifications be covered by the appended claims.
* * * * *