JP2009140814A - Fuel cell and electronic apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell with an excellent function, which has an original capability of an enzyme sufficiently demonstrated when the enzyme is fixed to at least one of a positive electrode or a negative electrode; and to provide an electronic apparatus using this excellent fuel cell. <P>SOLUTION: The fuel cell is configured such that a positive electrode 2 and a negative electrode 1 are faced through an electrolyte layer 3 and the enzyme is fixed to at least one of the positive electrode 2 or the negative electrode 1. Furthermore, electric conductivity of the electrolyte layer 3 is ≥10 mS/cm and preferably ≥50 mS/cm. NaCl, for example, is added to the electrolyte layer 3 in addition to cushioning substance such as imidazole. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a fuel cell and an electronic device, and is particularly suitable when applied to a biofuel cell using an enzyme and various electronic devices using the biofuel cell as a power source.

The fuel cell has a structure in which a positive electrode (oxidant electrode) and a negative electrode (fuel electrode) face each other with an electrolyte (proton conductor) interposed therebetween. In the conventional fuel cell, the fuel (hydrogen) supplied to the negative electrode is oxidized and separated into electrons and protons (H ⁺ ), the electrons are transferred to the negative electrode, and H ⁺ moves through the electrolyte to the positive electrode. In the positive electrode, this H ⁺ reacts with oxygen supplied from the outside and electrons sent from the negative electrode through the external circuit to generate water (H ₂ O).

  In this way, fuel cells are highly efficient power generators that directly convert the chemical energy of fuel into electrical energy, and the chemical energy of fossil energy such as natural gas, oil, and coal can be used regardless of where and when it is used. Moreover, it can be extracted as electric energy with high conversion efficiency. For this reason, research and development of fuel cells for large-scale power generation has been actively conducted. For example, a fuel cell is mounted on the space shuttle, and it has proven that it can supply water for crew members at the same time as electric power, and that it is a clean power generator.

Furthermore, in recent years, fuel cells having a relatively low operating temperature range from room temperature to about 90 ° C., such as solid polymer fuel cells, have been developed and attracting attention. For this reason, not only large-scale power generation applications but also applications to small systems such as automobile power supplies and portable power supplies such as personal computers and mobile devices are being sought.
Thus, the fuel cell can be used in a wide range of applications from large-scale power generation to small-scale power generation, and has attracted much attention as a highly efficient power generation device. However, in fuel cells, natural gas, petroleum, coal, etc. are usually used as fuel by converting them into hydrogen gas using a reformer, which consumes limited resources and needs to be heated to a high temperature. There are various problems such as the need for an expensive noble metal catalyst such as (Pt). Even when hydrogen gas or methanol is used directly as a fuel, care must be taken when handling it.

Accordingly, attention has been paid to the fact that biological metabolism performed in living organisms is a highly efficient energy conversion mechanism, and proposals have been made to apply this to fuel cells. The biological metabolism here includes respiration, photosynthesis and the like performed in microbial somatic cells. Biological metabolism has the characteristics that the power generation efficiency is extremely high and the reaction proceeds under mild conditions of about room temperature.
For example, respiration takes nutrients such as sugars, fats, and proteins into microorganisms or cells, and these chemical energies are converted into carbon dioxide (glycolytic and citrate (TCA) circuits through multiple enzymatic reaction steps and carbon dioxide (TCA) circuit. In the process of generating CO ₂ ), nicotinamide adenine dinucleotide (NAD ⁺ ) is reduced to reduced nicotinamide adenine dinucleotide (NADH) to convert it into redox energy, that is, electric energy, and further, an electron transfer system In this mechanism, the electric energy of NADH is directly converted into electric energy of proton gradient, and oxygen is reduced to generate water. The electrical energy obtained here produces ATP from adenosine diphosphate (ADP) via adenosine triphosphate (ATP) synthase, and this ATP is used for reactions necessary for the growth of microorganisms and cells. Used. Such energy conversion occurs in the cytosol and mitochondria.

In addition, photosynthesis captures light energy and reduces nicotinamide adenine dinucleotide phosphate (NADP ⁺ ) via an electron transfer system to reduce nicotinamide adenine dinucleotide phosphate (NADPH), thereby converting it into electrical energy. In the process of conversion, it is a mechanism that oxidizes water to produce oxygen. This electric energy takes in CO ₂ and is used for carbon fixation reaction, and is used for carbohydrate synthesis.
As a technique for utilizing the above-described biometabolism for a fuel cell, a microbial cell that obtains an electric current by taking out the electric energy generated in the microorganism through the electron mediator and passing the electrons to the electrode has been reported. (For example, refer to Patent Document 1).

However, since microorganisms and cells have many unnecessary reactions in addition to the intended reaction such as conversion from chemical energy to electrical energy, the above method consumes electrical energy for unwanted reactions and has sufficient energy conversion efficiency. Is not demonstrated.
Then, the fuel cell (biofuel cell) which performs only a desired reaction using an enzyme is proposed (for example, refer patent documents 2-13). In this biofuel cell, a fuel is decomposed by an enzyme and separated into protons and electrons, and those using alcohols such as methanol and ethanol or monosaccharides such as glucose have been developed as fuel.
In this biofuel cell, it was generally considered that a smaller ion concentration of the electrolyte was preferable. This is because electrolyte ions were thought to lower the activity of the enzyme used as a catalyst.

JP 2000-133297 A JP 2003-282124 A JP 2004-71559 A JP 2005-13210 A JP 2005-310613 A JP 2006-24555 A JP 2006-49215 A JP 2006-93090 A JP 2006-127957 A JP 2006-156354 A JP 2007-12281 A JP 2007-35437 A JP 2007-87627 A

However, according to the study by the present inventors, when the ion concentration of the electrolyte is decreased, the output of the fuel cell is decreased, which is not appropriate. Rather, it has been found that increasing the ion concentration of the electrolyte increases the output of the fuel cell.
Therefore, the problem to be solved by the present invention is that when the enzyme is immobilized on at least one of the positive electrode and the negative electrode, the ability inherently possessed by the enzyme can be sufficiently exerted, and excellent performance is achieved. And an electronic device using this excellent fuel cell.

  As a result of intensive studies to solve the above-described problems of the prior art, the present inventors have a positive electrode, a negative electrode, and an electrolyte. Typically, the positive electrode and the negative electrode face each other through the electrolyte. In a fuel cell (biofuel cell) in which an enzyme is immobilized on at least one of a positive electrode and a negative electrode, when the ion concentration of the electrolyte is large, the electric conductivity of the electrolyte is It has been found that the output of the fuel cell is larger at 10 mS / cm or more, and the present invention has been devised based on this finding.

That is, in order to solve the above problem, the first invention
In a fuel cell having a positive electrode, a negative electrode, and an electrolyte, and an enzyme is immobilized on at least one of the positive electrode and the negative electrode,
The electrical conductivity of the electrolyte is 10 mS / cm or more.

The second invention is
In an electronic device using one or more fuel cells,
At least one of the fuel cells is
In a fuel cell having a positive electrode, a negative electrode, and an electrolyte, and an enzyme is immobilized on at least one of the positive electrode and the negative electrode,
The electrical conductivity of the electrolyte is 10 mS / cm or more.

In the first and second inventions, the fuel cell typically has a structure in which a positive electrode and a negative electrode face each other with an electrolyte interposed therebetween. The electric conductivity of the electrolyte is preferably 50 mS / cm or more, but is not limited thereto. The ion concentration of the electrolyte is generally 0.2M or more and 3M or less, preferably 0.2M or more and 2M or less, more preferably 0.4M or more and 2M or less, and more preferably, from the viewpoint of obtaining a sufficiently high buffer capacity. Is 0.8M or more and 1.2M or less. The electrolyte may include a buffer substance (buffer solution). The buffer substances are generally as long as _a pK _a 9 below 4 to 10 or 6 or more, or it be used What, Specific examples, dihydrogen phosphate ions (H ₂ PO ₄ ⁻ ), 2-amino-2-hydroxymethyl-1,3-propanediol (abbreviated as Tris), 2- (N-morpholino) ethanesulfonic acid (MES), cacodylic acid, carbonic acid (H ₂ CO ₃ ), hydrogen citrate ion, N- (2-acetamido) iminodiacetic acid (ADA), piperazine-N, N′-bis (2-ethanesulfonic acid) (PIPES), N- (2-acetamido) -2 -Aminoethanesulfonic acid (ACES), 3- (N-morpholino) propanesulfonic acid (MOPS), N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES), N-2-H Roxyethylpiperazine-N′-3-propanesulfonic acid (HEPPS), N- [tris (hydroxymethyl) methyl] glycine (abbreviation tricine), glycylglycine, N, N-bis (2-hydroxyethyl) glycine (abbreviation) Bicine). Examples of the substance that generates dihydrogen phosphate ions (H ₂ PO ₄ ⁻ ) include sodium dihydrogen phosphate (NaH ₂ PO ₄ ) and potassium dihydrogen phosphate (KH ₂ PO ₄ ). As the buffer substance, a compound containing an imidazole ring is also preferable. Specific examples of the compound containing an imidazole ring include imidazole, triazole, pyridine derivatives, bipyridine derivatives, imidazole derivatives (histidine, 1-methylimidazole, 2-methylimidazole, 4-methylimidazole, 2-ethylimidazole, Ethyl imidazole-2-carboxylate, imidazole-2-carboxaldehyde, imidazole-4-carboxylic acid, imidazole-4,5-dicarboxylic acid, imidazol-1-yl-acetic acid, 2-acetylbenzimidazole, 1-acetylimidazole, N-acetylimidazole, 2-aminobenzimidazole, N- (3-aminopropyl) imidazole, 5-amino-2- (trifluoromethyl) benzimidazole, 4-azabenzimidazole, 4-aza-2 -Mercaptobenzimidazole, benzimidazole, 1-benzylimidazole, 1-butylimidazole) and the like. As buffer substances, 2-aminoethanol, triethanolamine, TES (N-Tris (hydroxymethyl) methyl-2-aminoethanesulfonic acid), BES (N, N-Bis (2-hydroxyethyl) -2-aminoethanesulfonic acid), etc. It may be used. The pH of the electrolyte containing the buffer substance is preferably around 7, but may generally be any of 1-14. The electrolyte may include a cation or an anion. For example, the electrolyte contains at least one kind of ions selected from the group consisting of Na ions, K ions, Mg ions, and Ca ions as cations. Or an electrolyte contains at least 1 or more types of ion chosen from the group which consists of F ion, Cl ion, Br ion, and I ion as an anion. Such an electrolyte can be produced, for example, by adding NaCl, KCl, or the like. The electrolyte can be prepared by adding an acid such as HCl. The electrolyte has a ratio of cation to anion, that is, cation / anion is 0.01 or more, preferably 1/3 or more, but is not limited thereto.

The enzyme immobilized on the positive electrode and the negative electrode may be various, and is selected as necessary. In addition, when an enzyme is immobilized on the positive electrode and the negative electrode, an electron mediator is preferably immobilized in addition to the enzyme.
The enzyme immobilized on the positive electrode typically includes an oxygen reductase. As this oxygen reductase, for example, bilirubin oxidase, laccase, ascorbate oxidase and the like can be used. In this case, an electron mediator is preferably immobilized on the positive electrode in addition to the enzyme. As the electron mediator, for example, potassium hexacyanoferrate, potassium ferricyanide, potassium octacyanotungstate and the like are used. The electron mediator is preferably immobilized at a sufficiently high concentration, for example, 0.64 × 10 ⁻⁶ mol / mm ² or more on average.

For example, when a monosaccharide such as glucose is used as the fuel, the enzyme immobilized on the negative electrode includes an oxidase that promotes and decomposes the monosaccharide and is usually reduced by the oxidase. A coenzyme oxidase that returns the coenzyme to an oxidant. By the action of the coenzyme oxidase, electrons are generated when the coenzyme returns to the oxidized form, and the electrons are transferred from the coenzyme oxidase to the electrode via the electron mediator. For example, NAD ⁺ -dependent glucose dehydrogenase (GDH) is used as the oxidase, nicotinamide adenine dinucleotide (NAD ⁺ ) is used as the coenzyme, and diaphorase is used as the coenzyme oxidase.

  When using polysaccharides (a broadly defined polysaccharide, which refers to all carbohydrates that produce two or more monosaccharides by hydrolysis, including oligosaccharides such as disaccharides, trisaccharides, and tetrasaccharides) as fuel, Preferably, in addition to the above-mentioned oxidase, coenzyme oxidase, coenzyme and electron mediator, a degradation enzyme that promotes degradation such as hydrolysis of polysaccharides and produces monosaccharides such as glucose is also immobilized. . Specific examples of the polysaccharide include starch, amylose, amylopectin, glycogen, cellulose, maltose, sucrose, and lactose. These are a combination of two or more monosaccharides, and any polysaccharide contains glucose as a monosaccharide of the binding unit. Note that amylose and amylopectin are components contained in starch, and starch is a mixture of amylose and amylopectin. When glucoamylase is used as a polysaccharide degrading enzyme and glucose dehydrogenase is used as an oxidase degrading monosaccharides, polysaccharides that can be decomposed to glucose by glucoamylase, such as starch, amylose, amylopectin, glycogen As long as it contains any one of maltose, it is possible to generate electricity using this as fuel. Glucoamylase is a degrading enzyme that hydrolyzes α-glucan such as starch to produce glucose, and glucose dehydrogenase is an oxidase that oxidizes β-D-glucose to D-glucono-δ-lactone. Preferably, the degradation enzyme that decomposes the polysaccharide is also immobilized on the negative electrode, and the polysaccharide that will eventually become the fuel is also immobilized on the negative electrode.

  Further, when starch is used as fuel, it is possible to use a gelatinized fuel obtained by gelatinizing starch. In this case, it is preferable to use a method in which gelatinized starch is brought into contact with a negative electrode on which an enzyme or the like is immobilized, or is immobilized on the negative electrode together with the enzyme or the like. When such an electrode is used, the starch concentration on the negative electrode surface can be maintained at a higher level than when starch dissolved in the solution is used, the enzymatic degradation reaction becomes faster, and the output is improved. The fuel handling is easier than in the case of a solution, the fuel supply system can be simplified, and the fuel cell does not need to be used upside down, which is very advantageous when used for mobile devices, for example. .

  Basically, any electron mediator may be used, but a compound having a quinone skeleton, particularly, a compound having a naphthoquinone skeleton is preferably used. As the compound having a naphthoquinone skeleton, various naphthoquinone derivatives can be used. Specifically, for example, 2-amino-1,4-naphthoquinone (ANQ), 2-amino-3-methyl-1 2, 4-naphthoquinone (AMNQ), 2-methyl-1,4-naphthoquinone (VK3), 2-amino-3-carboxy-1,4-naphthoquinone (ACNQ), and the like are used. As a compound having a quinone skeleton, for example, anthraquinone or a derivative thereof can be used in addition to a compound having a naphthoquinone skeleton. In addition to the compound having a quinone skeleton, the electron mediator may contain one or two or more other compounds that function as an electron mediator, if necessary. As a solvent used when a compound having a quinone skeleton, particularly a compound having a naphthoquinone skeleton, is immobilized on the negative electrode, acetone is preferably used. Thus, by using acetone as a solvent, the solubility of the compound having a quinone skeleton can be increased, and the compound having a quinone skeleton can be efficiently immobilized on the negative electrode. If necessary, the solvent may contain one or two or more other solvents other than acetone.

  Various materials can be used as an immobilizing material for immobilizing an enzyme, a coenzyme, an electron mediator, etc. on the negative electrode and the positive electrode, preferably poly-L-lysine (PLL). A polyion complex formed using a polycation or a salt thereof and a polyanion or a salt thereof such as polyacrylic acid (for example, sodium polyacrylate (PAAcNa)) can be used, and an enzyme is contained inside the polyion complex. , Coenzymes, electron mediators, and the like can be included.

By the way, when an electron mediator is immobilized on the positive electrode and the negative electrode of this fuel cell, since the electron mediator is generally a low molecule, elution is completely suppressed, and the state in which the electron mediator is immobilized on the positive electrode and the negative electrode is long. Maintaining time is not always easy. For this reason, the electron mediator used for the positive electrode may move to the negative electrode side, and conversely, the electron mediator used for the negative electrode may move to the positive electrode side. There is a risk of lowering. In order to solve this problem, it is effective to use an electrolyte having the same sign as the charge of the oxidized or reduced form of the electron mediator. By doing so, a repulsive force acts between the charge of the electrolyte and the charge of the oxidized or reduced form of the electron mediator. For this reason, it becomes difficult for the electron mediator to move to the electrolyte side, and it is possible to effectively suppress the electron mediator from passing through the electrolyte and moving to the opposite side. Typically, the electrolyte includes a polymer having a charge of the same sign as the charge of the oxidized or reduced form of the electron mediator, such as a polyanion or polycation, so that the electrolyte is charged in the oxidized or reduced form of the electron mediator. However, the present invention is not limited to this, and the electrolyte may have the same sign as the charge of the oxidized or reduced form of the electron mediator by other methods. Specifically, when the oxidized or reduced form of the electron mediator used for at least one of the positive electrode and the negative electrode has a negative charge, a polymer having a negative charge, such as a polyanion, is included in the electrolyte, and the electron mediator When the oxidant or reductant has a positive charge, a polymer having a positive charge, such as a polycation, is included in the electrolyte. Examples of the polyanion include Nafion (trade name, DuPont, USA), which is an ion exchange resin having a fluorine-containing carbon sulfonic acid group, dichromate ion (Cr ₂ O ₇ ²⁻ ), paramolybdate ion ([ Mo ₇ O ₂₄ ] ⁶⁻ ), polyacrylic acid (for example, sodium polyacrylate (PAAcNa)), or the like can be used. For example, poly-L-lysine (PLL) can be used as the polycation.

  Various proton conductors can be used and are selected as necessary. Specifically, for example, cellophane, non-woven fabric, perfluorocarbon sulfonic acid (PFS) resin film, trifluorostyrene derivative Copolymer film, polybenzimidazole film impregnated with phosphoric acid, aromatic polyether ketone sulfonic acid film, PSSA-PVA (polystyrene sulfonate polyvinyl alcohol copolymer), PSSA-EVOH (polystyrene sulfonate ethylene vinyl alcohol) Copolymer), an ion exchange resin having a fluorine-containing carbon sulfonic acid group (Nafion (trade name, DuPont, USA), etc.) and the like.

This fuel cell can be used for almost anything that requires electric power, and can be of any size. For example, electronic devices, mobile objects (automobiles, motorcycles, aircraft, rockets, spacecrafts, etc.), power devices, construction machinery It can be used for machine tools, power generation systems, cogeneration systems, etc. The output, size, shape, type of fuel, etc. are determined depending on the application.
Electronic devices may be basically any type, including both portable and stationary types, but specific examples include mobile phones, mobile devices, robots, personal computers. , Game equipment, in-vehicle equipment, home appliances, industrial products, etc.

  In the present invention configured as described above, when the electrical conductivity of the electrolyte is 10 mS / cm or more, ion conduction is performed without deactivating the enzyme immobilized on at least one of the positive electrode and the negative electrode. The degree can be increased.

  According to the present invention, when the enzyme is immobilized on at least one of the positive electrode and the negative electrode, the ability inherent in the enzyme can be fully exhibited, and a fuel cell having excellent performance can be obtained. be able to. By using this excellent fuel cell, a high-performance electronic device can be realized.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 schematically shows a biofuel cell according to an embodiment of the present invention. In this biofuel cell, glucose is used as the fuel. FIG. 2 schematically shows details of the configuration of the negative electrode of the biofuel cell, an example of an enzyme group immobilized on the negative electrode, and an electron transfer reaction by the enzyme group.
As shown in FIG. 1, this biofuel cell has a structure in which a negative electrode 1 and a positive electrode 2 face each other with an electrolyte layer 3 conducting only protons. The negative electrode 1 decomposes glucose supplied as fuel with an enzyme to extract electrons and generate protons (H ⁺ ). The positive electrode 2 generates water by protons transported from the negative electrode 1 through the electrolyte layer 3, electrons transmitted from the negative electrode 1 through an external circuit, and oxygen in the air, for example.

The negative electrode 1 is composed of, for example, an electrode 11 (see FIG. 2) made of porous carbon or the like, an enzyme involved in glucose decomposition, and a coenzyme in which a reductant is generated in an oxidation reaction in the glucose decomposition process (for example, , NAD ⁺ , NADP ^{+ and the} like), a coenzyme oxidase (eg, diaphorase) that oxidizes a reduced form of this coenzyme (eg, NADH, NADPH, etc.), and the coenzyme oxidase accompanying oxidation of the coenzyme An electron mediator that receives the generated electrons and passes them to the electrode 11 is configured to be fixed by a fixing material made of, for example, a polymer.

As an enzyme involved in the degradation of glucose, for example, glucose dehydrogenase (GDH) can be used. In the presence of this oxidase, for example, β-D-glucose can be oxidized to D-glucono-δ-lactone.
Furthermore, this D-glucono-δ-lactone can be decomposed into 2-keto-6-phospho-D-gluconate by the presence of two enzymes, gluconokinase and phosphogluconate dehydrogenase (PhGDH). Can do. That is, D-glucono-δ-lactone is converted to D-gluconate by hydrolysis, and D-gluconate is converted from adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and phosphate in the presence of gluconokinase. And then phosphorylated to 6-phospho-D-gluconate. This 6-phospho-D-gluconate is oxidized to 2-keto-6-phospho-D-gluconate by the action of the oxidase PhGDH.

In addition to the above decomposition process, glucose can also be decomposed to CO ₂ by utilizing sugar metabolism. The decomposition process utilizing sugar metabolism is roughly divided into glucose decomposition and pyruvic acid generation by a glycolysis system and a TCA cycle, which are widely known reaction systems.
The oxidation reaction in the monosaccharide decomposition process is accompanied by a coenzyme reduction reaction. This coenzyme is almost determined by the acting enzyme. In the case of GDH, NAD ⁺ is used as the coenzyme. That is, when β-D-glucose is oxidized to D-glucono-δ-lactone by the action of GDH, NAD ⁺ is reduced to NADH to generate H ⁺ .

The produced NADH is immediately oxidized to NAD ⁺ in the presence of diaphorase (DI), generating two electrons and H ⁺ . Therefore, two electrons and two H ⁺ are generated by one-step oxidation reaction per glucose molecule. In the two-stage oxidation reaction, a total of four electrons and four H ⁺ are generated.
The electrons generated in the above process are transferred from diaphorase to the electrode 11 through the electron mediator, and H ⁺ is transported to the positive electrode 2 through the electrolyte layer 3.

  The electron mediator transfers electrons to and from the electrode 11, and the output voltage of the fuel cell depends on the redox potential of the electron mediator. That is, in order to obtain a higher output voltage, it is preferable to select an electron mediator having a more negative potential on the negative electrode 1 side. However, the reaction affinity of the electron mediator to the enzyme, the rate of electron exchange with the electrode 11, an inhibitor (light, Structural stability against oxygen) must also be considered. From such a viewpoint, 2-amino-3-carboxy-1,4-naphthoquinone (ACNQ), vitamin K3, and the like are preferable as the electron mediator acting on the negative electrode 1. In addition, for example, compounds having a quinone skeleton, metal complexes such as osmium (Os), ruthenium (Ru), iron (Fe), cobalt (Co), viologen compounds such as benzyl viologen, compounds having a nicotinamide structure, riboflavin structure A compound having a nucleotide, a compound having a nucleotide-phosphate structure, or the like can also be used as an electron mediator.

The electrolyte layer 3 is a proton conductor that transports H ⁺ generated in the negative electrode 1 to the positive electrode 2, and is made of a material that does not have electronic conductivity and can transport H ⁺ . Here, the electric conductivity of the electrolyte layer 3 is selected to be 10 mS / cm or more, preferably 50 mS / cm or more, and more preferably 100 mS / cm or more. As the electrolyte layer 3, for example, one appropriately selected from those already mentioned can be used. In this case, the electrolyte layer 3 contains, for example, a buffer substance appropriately selected from those already mentioned. The concentration of the buffer substance is selected as necessary, but preferably, for example, a concentration of 0.2 M or more and 2 M or less is included. By doing so, a high buffering capacity can be obtained, and the original ability of the enzyme can be fully exhibited even during high power operation of the fuel cell. In addition to the buffer substance, the electrolyte layer 3 contains at least one ion selected from the group consisting of Na ion, K ion, Mg ion and Ca ion as a cation, or The anion includes at least one ion selected from the group consisting of F ion, Cl ion, Br ion and I ion.

  The enzyme, coenzyme, and electron mediator are preferably immobilized on the electrode 11 using an immobilizing material in order to efficiently capture an enzyme reaction phenomenon occurring in the vicinity of the electrode as an electric signal. Furthermore, the enzyme reaction system of the negative electrode 1 can be stabilized by immobilizing the enzyme and the coenzyme for decomposing the fuel on the electrode 11. As such an immobilizing material, for example, a combination of glutaraldehyde (GA) and poly-L-lysine (PLL) or a combination of sodium polyacrylate (PAAcNa) and poly-L-lysine (PLL). These may be used, these may be used alone, or other polymers may be used. By using an immobilization material combining glutaraldehyde and poly-L-lysine, it becomes possible to greatly improve the enzyme immobilization ability of each, and to obtain an excellent enzyme immobilization ability as a whole immobilization material. Can do. In this case, the optimum composition ratio between glutaraldehyde and poly-L-lysine varies depending on the enzyme to be immobilized and the substrate of the enzyme, but generally an arbitrary composition ratio may be used. As a specific example, a glutaraldehyde aqueous solution (0.125%) and a poly-L-lysine aqueous solution (1%) are used, and the ratio thereof is 1: 1, 1: 2, 2: 1, or the like.

In FIG. 2, for example, glucose dehydrogenase (GDH) is an enzyme involved in the degradation of glucose, NAD ⁺ is a coenzyme that produces a reductant in the oxidation reaction in the glucose degradation process, and a reductant of coenzyme. The case where the coenzyme oxidase that oxidizes a certain NADH is diaphorase (DI), and the electron mediator that receives electrons from the coenzyme oxidase accompanying the oxidation of the coenzyme and passes it to the electrode 11 is ACNQ is shown.

In the positive electrode 2, an oxygen reductase and an electron mediator that transfers electrons to and from the electrode are fixed on an electrode made of, for example, porous carbon. As the oxygen reductase, for example, bilirubin oxidase (BOD), laccase, ascorbate oxidase and the like can be used. As the electron mediator, for example, hexacyanoferrate ions generated by ionization of potassium hexacyanoferrate can be used. This electron mediator is preferably immobilized at a sufficiently high concentration, for example, 0.64 × 10 ⁻⁶ mol / mm ² or more on average.

In the positive electrode 2, in the presence of oxygen reductase, oxygen in the air is reduced by H ⁺ from the electrolyte layer 3 and electrons from the negative electrode 1 to generate water.
When glucose is supplied to the negative electrode 1 side during operation (in use) of the fuel cell configured as described above, the glucose is decomposed by a decomposing enzyme including an oxidase. Since oxidase is involved in the monosaccharide decomposition process, electrons and H ⁺ can be generated on the negative electrode 1 side, and a current can be generated between the negative electrode 1 and the positive electrode 2.

Next, the range of the concentration of the buffer substance contained in the electrolyte layer 3 will be described. For this purpose, first, the results of electrochemical measurements at a single electrode of the enzyme / electron mediator immobilized electrode used as the negative electrode 1 will be described.
The enzyme / electron mediator immobilized electrode was prepared as follows.
First, various solutions were prepared as follows. As a buffer solution, a 100 mM sodium dihydrogen phosphate (NaH ₂ PO ₄ ) buffer solution (IS = 0.3, pH = 7.0) was used.
Diaphorase (DI) (EC1.6.99.-, manufactured by Unitika, B1D111) was weighed 5 to 10 mg and dissolved in 1.0 ml of a buffer solution to obtain a DI enzyme buffer solution ((1)).
Glucose dehydrogenase (GDH) (NAD-dependent type, EC 1.1.1.17, manufactured by Toyobo, GLD-311) was weighed 10 to 15 mg, dissolved in 1.0 ml of buffer solution, and GDH enzyme buffer solution ((2)) It was.
The buffer solution for dissolving the enzyme is preferably refrigerated until just before, and the enzyme buffer solution is preferably stored as refrigerated as much as possible.
3DH to 60.0 mg of NADH (manufactured by Sigma Aldrich, N-8129) was weighed and dissolved in 0.1 ml of buffer solution to obtain NADH buffer solution ((3)).
An appropriate amount of poly-L-lysine hydrobromide (PLL) (manufactured by Wako, 164-16961) was weighed and dissolved in ion-exchanged water so as to be 1 to 2 wt% to obtain a PLL aqueous solution ((4)). .
10-50 mg of 2-amino-1,4-naphthoquinone (ANQ) (synthetic product) was weighed and dissolved in 1 ml of an acetone solution to obtain an ANQ acetone solution ((5)).
Sodium polyacrylate (PAAcNa) (manufactured by Aldrich, 041-00595) was weighed in an appropriate amount and dissolved in ion-exchanged water so as to be 0.01 to 0.1 wt% to obtain a PAAcNa aqueous solution ((6)).

The various solutions prepared as described above were porous using a microsyringe in the amounts shown below in the order of (5), (1), (3), (2), (4), (6). After coating on quality carbon (made by Tokai Carbon, 0.5 × 0.5 cm ² , thickness 2 mm), it was appropriately dried to prepare an enzyme / electron mediator fixed electrode.
DI enzyme buffer solution (1): 10 μl
GDH enzyme buffer solution (2): 10 μl
NADH buffer solution (3): 10 μl
PLL aqueous solution (4): 10 μl
ANQ acetone solution (5): 7 μl
PAAcNa aqueous solution (6): 4 μl

The thus prepared enzyme / electron mediator-immobilized electrode was measured using a measurement solution containing glucose as a fuel and NaH ₂ PO ₄ buffer, and 0.1 V relative to the reference electrode Ag | AgCl, and the redox potential of the electron mediator. Chronoamperometry was performed at a sufficiently higher potential. The glucose concentration of the fuel was 0.4M. The results of chronoamperometry measured by changing the concentration of NaH ₂ PO ₄ buffer are shown in FIG. In FIG. 3, the NaH ₂ PO ₄ concentrations of Examples 1, _{2, and} 3 are 0.4 M, 1.0 M, and 2.0 M, respectively, and the NaH ₂ PO ₄ concentrations of Comparative Examples 1, _{2, and} 3 are 0.0 M, respectively. 0.1M and 3.0M.

As can be seen from FIG. 3, in Comparative Example 1 in which the NaH ₂ PO ₄ concentration is 0, that is, no buffer solution is added, the initial current is generated as compared with Comparative Example 2 in which the NaH ₂ PO ₄ concentration is 0.1 M. The current becomes almost zero after 1800 seconds. Further, when Comparative Example 2 is compared with Examples 1, 2, and 3, the NaH ₂ PO ₄ concentration is 0.2 to 2.5 M, and in particular 0.4 to 2.0 M, so that the current value is Comparative Example 2. It can be seen that it increases several times or more, especially at 1M.
On the other hand, when the NaH ₂ PO ₄ concentration is 3M as in Comparative Example 3, the obtained current value decreases to the same extent as in Comparative Example 1.

Next, when BOD is immobilized on the positive electrode 2 as an oxygen reductase and imidazole and hydrochloric acid are mixed and adjusted to pH 7 as a buffer solution, the current value is maintained and improved as the buffer solution concentration increases. Was noticeable. Table 1 and FIG. 4 show the results of chronoamperometry measured by changing the concentration of imidazole in this case. FIG. 5 shows the buffer concentration dependence of the current value (the value after 3600 seconds in Table 1 and FIG. 4). Table 1, FIG. 4 and FIG. 5 also show the results when NaH ₂ PO ₄ buffer is used as the buffer. As shown in FIG. 6, this measurement was performed with a film-like cellophane 21 placed on the positive electrode 2 and a buffer solution 22 in contact with the cellophane 21. As the positive electrode 2, an enzyme / electron mediator fixed electrode prepared as follows was used. First, commercially available carbon felt (BORAY made by TORAY) was used as porous carbon, and this carbon felt was cut into 1 cm square. Next, 80 μl of hexacyanoferrate ion (100 mM), 80 μl of poly-L-lysine (1 wt%), 80 μl of BOD solution (50 mg / ml) are sequentially infiltrated into the above carbon felt and dried to dry the enzyme / An electron mediator fixed electrode was obtained. Two sheets of the enzyme / electron mediator-immobilized electrode produced in this way were stacked to make a positive electrode 2.

As can be seen from Table 1 and FIG. 4, when the NaH ₂ PO ₄ concentration is 1.0 M, an initial current is generated, but after 3600 seconds, the current is greatly reduced. On the other hand, when the imidazole concentration is 0.4M, 1.0M and 2.0M, almost no decrease in current is observed even after 3600 seconds. In the case of 10% acetate buffer (pH = 2.8), the initial current is low, but the current at the same level as in the case where the imidazole concentration is 2.0 M is maintained after 3600 seconds. As can be seen from FIG. 5, when the imidazole concentration is in the range of 0.2 to 2.5 M, the current value increases linearly with respect to the concentration. Further, the NaH ₂ PO ₄ buffer and imidazole buffer, near both pK _a of 7, even though almost be oxygen solubility same, if the imidazole is present in a buffer of the same concentration, a large oxygen reduction current obtained It was.

Chronoamperometry was performed for 3600 seconds as described above, and then cyclic voltammetry (CV) between −0.3 and +0.6 V was performed. The result is shown in FIG. However, in this measurement, as shown in FIG. 8, a positive electrode 2 composed of an enzyme / electron mediator-immobilized electrode similar to the above was used as a working electrode, and this was placed on a PTFE (polytetrafluoroethylene) membrane 23. The test was performed in a state where the buffer solution 22 was brought into contact with 2. The counter electrode 24 and the reference electrode 25 were immersed in the buffer solution 22, and an electrochemical measurement device (not shown) was connected to the positive electrode 2, the counter electrode 24, and the reference electrode 25 as working electrodes. Pt line was used as the counter electrode 24 and Ag | AgCl was used as the reference electrode 25. The measurement was performed at atmospheric pressure, and the measurement temperature was 25 ° C. As the buffer solution 22, two types of imidazole / hydrochloric acid buffer solution (pH 7, 1.0 M) and NaH ₂ PO ₄ / NaOH buffer solution (pH 7, 1.0 M) were used.
From FIG. 7, it can be seen that when an imidazole / hydrochloric acid buffer (pH 7, 1.0 M) is used as the buffer 22, extremely good CV characteristics are obtained.
From the above, it was confirmed that the imidazole buffer solution has an advantage even if the measurement system is changed.

  FIG. 9 shows the results of cyclic voltammetry of the negative electrode 1 with and without the addition of 1.0 M NaCl in an imidazole / hydrochloric acid buffer (pH 7, 2.0 M). As the negative electrode 1, the above-described enzyme / electron mediator fixed electrode was used. FIG. 10 shows the results of chronoamperometry at −0.3V. As shown in FIGS. 9 and 10, when 1.0 M NaCl is added to the imidazole / hydrochloric acid buffer, a higher current value is obtained than when no NaCl is added. The reason why such a high current value is obtained is that the resistance of the biofuel cell is reduced because the electric conductivity of the electrolyte layer 3 is as high as 10 mS / cm or more. Here, when 1.0 M NaCl is added to the imidazole / hydrochloric acid buffer, the electric conductivity of the electrolyte layer 3 is 120 mS / cm, and when not added, the electric conductivity of the electrolyte layer 3 is 60 mS / cm.

Next, a specific configuration example of this biofuel cell is shown in FIGS. 11A and 11B.
As shown in FIGS. 11A and 11B, this biofuel cell includes a negative electrode 1 composed of an enzyme / electron mediator-immobilized carbon electrode in which an enzyme or an electron mediator is immobilized on a carbon felt of 0.25 cm ² with an immobilizing material; A structure in which a positive electrode 2 composed of an enzyme / electron mediator-immobilized carbon electrode in which an enzyme or an electron mediator is immobilized on a carbon felt of 0.25 cm ² with an immobilizing material is opposed to an electrolyte layer 3 containing a buffer substance. Have. In this case, Ti current collectors 41 and 42 are placed under the positive electrode 2 and the negative electrode 1, respectively, so that current can be easily collected. Reference numerals 43 and 44 denote fixed plates. These fixing plates 43 and 44 are fastened to each other by screws 45, and the positive electrode 2, the negative electrode 1, the electrolyte layer 3, and the Ti current collectors 41 and 42 are sandwiched between them. One surface (outer surface) of the fixing plate 43 is provided with a circular recess 43a for taking in air, and a plurality of holes 43b penetrating to the other surface are provided in the bottom surface of the recess 43a. These holes 43 b serve as air supply paths to the positive electrode 2. On the other hand, a circular recess 44a for fuel loading is provided on one surface (outer surface) of the fixing plate 44, and a number of holes 44b penetrating to the other surface are provided on the bottom surface of the recess 44a. These holes 44 b serve as a fuel supply path to the negative electrode 1. A spacer 46 is provided on the periphery of the other surface of the fixing plate 44, and when the fixing plates 43 and 44 are fastened to each other with screws 45, the interval between them becomes a predetermined interval. .
As shown in FIG. 11B, a load 47 is connected between the Ti current collectors 41 and 42, and a glucose / buffer solution is put as fuel in the recess 44a of the fixed plate 44 to generate power.

  As described above, according to this embodiment, the electrical conductivity of the electrolyte layer 3 is 10 mS / cm or more, so that the resistance of the biofuel cell can be reduced and the ability of the enzyme is inherent. Can be fully exhibited. As a result, a high-performance biofuel cell capable of high output operation can be realized. This biofuel cell is suitable for application to power sources of various electronic devices, mobile objects, power generation systems, and the like.

Although one embodiment of the present invention has been specifically described above, the present invention is not limited to the above-described embodiment, and various modifications based on the technical idea of the present invention are possible.
For example, the numerical values, structures, configurations, shapes, materials, and the like given in the above-described embodiments are merely examples, and different numerical values, structures, configurations, shapes, materials, and the like may be used as necessary.
When carbon felt is used as the electrode material for the enzyme-immobilized electrode, the effective surface area of the electrode can be increased by activating the carbon felt, and the current value of the biofuel cell using this enzyme-immobilized electrode Can be improved. In addition, by using the electrode thus activated, when the enzyme is immobilized, the enzyme is adsorbed to the pores formed by activation, so that the enzyme can be easily immobilized on the electrode. You can also get Furthermore, when an enzyme-immobilized electrode in which an oxygen reductase is immobilized in this way is used, the initial oxygen reduction current value increases. The activation electrode can be produced, for example, as follows. Iron (II) acetate 95% was dissolved in ethanol to obtain a 0.01 w / v% solution. This solution is applied to one piece of carbon felt, 40 μl per side, and then dried in air for 1 hour. Then, the target activated carbon felt electrode was obtained by heating in vacuum at 600 ° C. for 1 hour. When this activated carbon felt electrode was observed with a scanning electron microscope, it was confirmed that many pores were formed. When the effective surface area of this activated carbon felt electrode was measured, it was confirmed that it increased compared to the electrode before the activation treatment. Next, an oxygen reductase was immobilized on the activated carbon felt electrode, and an oxygen reduction current value was measured. As a result, it was found that the current value after 100 seconds significantly increased.

It is a basic diagram which shows the biofuel cell by one Embodiment of this invention. It is a basic diagram which shows typically the detail of a structure of the negative electrode of the biofuel cell by one Embodiment of this invention, an example of the enzyme group fix | immobilized by this negative electrode, and the electron transfer reaction by this enzyme group. It is a basic diagram which shows the result of the chronoamperometry performed for evaluation of the biofuel cell by one Embodiment of this invention. It is a basic diagram which shows the result of the chronoamperometry performed for evaluation of the biofuel cell by one Embodiment of this invention. It is a basic diagram which shows the relationship between the buffer solution density | concentration obtained from the result of the chronoamperometry performed for evaluation of the biofuel cell by one Embodiment of this invention, and the current density obtained. It is a basic diagram which shows the measuring system used for the measurement of the chronoamperometry shown in FIG. It is a basic diagram which shows the result of the cyclic voltammetry performed for evaluation of the biofuel cell by one Embodiment of this invention. It is a basic diagram which shows the measuring system used for the measurement of cyclic voltammetry shown in FIG. It is a basic diagram which shows the result of the cyclic voltammetry performed for evaluation of the biofuel cell by one Embodiment of this invention. It is a basic diagram which shows the result of the chronoamperometry performed for evaluation of the biofuel cell by one Embodiment of this invention. It is a basic diagram which shows the specific structural example of the biofuel cell by one Embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Negative electrode, 2 ... Positive electrode, 3 ... Electrolyte layer, 11 ... Electrode

Claims (10)

In a fuel cell having a positive electrode, a negative electrode, and an electrolyte, and an enzyme is immobilized on at least one of the positive electrode and the negative electrode,
The fuel cell, wherein the electrolyte has an electric conductivity of 10 mS / cm or more.   2. The fuel cell according to claim 1, wherein the electrolyte contains at least one kind of ions selected from the group consisting of Na ions, K ions, Mg ions and Ca ions as cations.   2. The fuel cell according to claim 1, wherein the electrolyte contains at least one kind of ions selected from the group consisting of F ions, Cl ions, Br ions and I ions as anions.   2. The fuel cell according to claim 1, wherein the electrolyte has a cation to anion ratio of 0.01 or more.   The fuel cell according to claim 1, wherein the electrolyte includes a buffer substance. The fuel cell according to claim 4, wherein the pK _a of the buffer substance is 4 to 10.   2. The fuel cell according to claim 1, wherein the buffer substance contains a compound containing an imidazole ring.   The fuel cell according to claim 1, wherein an enzyme is immobilized on the negative electrode, and the enzyme contains an oxidase immobilized on the negative electrode that promotes and decomposes monosaccharides.   The enzyme immobilized on the negative electrode contains a coenzyme oxidase that returns the coenzyme reduced by oxidation of the monosaccharide to an oxidized form and passes electrons to the negative electrode through an electron mediator. The fuel cell according to claim 8. In an electronic device using one or more fuel cells,
At least one of the fuel cells is
In a fuel cell having a positive electrode, a negative electrode, and an electrolyte, and an enzyme is immobilized on at least one of the positive electrode and the negative electrode,
An electronic device, wherein the electrolyte has an electrical conductivity of 10 mS / cm or more.

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