JP5011691B2 - Fuel cells, electronic devices, mobile objects, power generation systems, and cogeneration systems - Google Patents

Description

  The present invention relates to a fuel cell in which an enzyme is immobilized as a catalyst on at least one of a positive electrode and a negative electrode, and an electronic device, a moving body, a power generation system, and a cogeneration system using the fuel cell.

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 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 to carbon dioxide (glycolytic and tricarboxylic acid (TCA) cycles through a number of enzymatic reaction steps. 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 biological metabolism in 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 functions in addition to the intended reaction such as conversion from chemical energy to electrical energy, the above-described method consumes electrical energy for unwanted reactions, and sufficient energy conversion efficiency is achieved. Is not demonstrated.
Therefore, a fuel cell (biofuel cell) that performs only a desired reaction using an enzyme has been proposed (see, for example, Patent Documents 2, 3, and 4). 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, a buffer substance (buffer solution) is generally contained in the electrolyte. This is because the enzyme used as the catalyst is very sensitive to the pH of the solution, so that the buffer is controlled near the pH at which the enzyme easily functions. Conventionally, the concentration of the buffer substance contained in the electrolyte is generally 0.1 M or less. This is because the concentration of the buffer substance is usually made as dilute as possible to keep the pH constant, and it is usual to add appropriate inorganic ions and organic ions to approximate physiological conditions.

JP 2000-133297 A JP 2003-282124 A JP 2004-71559 A JP 2005-13210 A

  However, according to the study by the present inventors, when the current value obtained by the biofuel cell is very small, sufficient buffering is possible even if the concentration of the buffer substance contained in the electrolyte is 0.1 M or less. Can be obtained by using an enzyme immobilized on a high surface area electrode such as a carbon porous body or increasing the concentration of the enzyme to be immobilized on at least one of the positive electrode and the negative electrode. In the output biofuel cell, the buffer capacity is insufficient, the pH of the electrolyte around the enzyme deviates from the optimum pH, and the ability inherent to the enzyme cannot be fully exhibited.

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, a sufficient buffer capacity can be obtained even during high output operation, and the enzyme originally has It is an object of the present invention to provide a fuel cell that can fully exhibit its ability and has excellent performance.
Another problem to be solved by the present invention is to provide an electronic device, a moving body, a power generation system, and a cogeneration system using the excellent fuel cell as described above.

As a result of intensive studies to solve the above-described problems of the prior art, the present inventors have a structure in which the positive electrode and the negative electrode face each other with an electrolyte containing a buffer substance, and at least the positive electrode and the negative electrode On the other hand, in a fuel cell (biofuel cell) in which an enzyme is immobilized, a sufficient buffer capacity cannot be obtained during high output operation because the increase or decrease in protons occurs in the electrode or in the enzyme due to the proton-mediated enzyme reaction. This is because the buffering action when it occurs in the immobilized membrane is too weak when the concentration of the buffer substance contained in the electrolyte is 0.1 M or less, and in order to prevent this, the concentration of the buffer substance is conventionally reduced. It came to the conclusion that it needs to be much higher. Specifically, as a result of various studies, it is effective to increase the concentration of the buffer substance to 0.2 M or more, which is twice or more that of the conventional one, and this increases the output current value several times or more. I found out. On the other hand, it has also been found that when the concentration of the buffer substance exceeds 2.5M, the output current value decreases. That is, in a biofuel cell, in order to be able to obtain a sufficient buffer capacity even during high-power operation and to fully exhibit the inherent ability of the enzyme, a buffer substance contained in the electrolyte It is effective to set the concentration of 0.2M to 2.5M.
The present invention has been devised based on the above-mentioned knowledge obtained independently by the present inventors.

That is, in order to solve the above problem,
The first invention is
In the fuel cell in which the positive electrode and the negative electrode are opposed to each other via an electrolyte containing a buffer substance, and an enzyme is immobilized as a catalyst on at least one of the positive electrode and the negative electrode.
The concentration of the buffer substance contained in at least the electrolyte around the enzyme is 0.2 M or more and 2.5 M or less.

Here, the concentration of the buffer substance is preferably from 0.2 M to 2 M, more preferably from 0.4 M to 2 M, and even more preferably from 0.8 M to 1.M from the viewpoint of obtaining a sufficiently high buffer capacity. 2M or less. Buffer substances, in general, as long as _a pK _a of 6 to 9, may it be used What Specific examples and, dihydrogen phosphate ion (H ₂ PO ₄ ^- ), 2-amino-2-hydroxymethyl-1,3-propanediol (abbreviated to Tris), 2- (N-morpholino) ethanesulfonic acid (MES), cacodylic acid, carbonic acid (H ₂ CO ₃ ), hydrogen citrate Ions, 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-hydroxyethylpiperazine- '-3-propanesulfonic acid (HEPPS), N- [tris (hydroxymethyl) methyl] glycine (abbreviation tricine), glycylglycine, N, N-bis (2-hydroxyethyl) glycine (abbreviation bicine), etc. . 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 ). The pH of the electrolyte containing the buffer substance is preferably around 7, but may generally be any of 1-14.

The enzyme immobilized on at least one of the positive electrode and the negative electrode may be various, and is selected as necessary. In addition to the enzyme, an electron mediator is preferably immobilized on at least one of the positive electrode and the negative electrode.
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 that degrades monosaccharides, polysaccharides that can be degraded 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. The decomposing enzyme that decomposes the polysaccharide may also be fixed on the negative electrode, and the polysaccharide that will eventually become the fuel may also be fixed 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, the gelatinized starch can be brought into contact with the negative electrode on which the enzyme or the like is immobilized, or the enzyme and the like can be immobilized on the negative electrode. 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 can be made useless, so it is very advantageous when used for mobile devices, for example. .
On the other hand, the enzyme immobilized on the positive electrode includes an oxidase. As this oxidase, for example, bilirubin oxidase (BOD) can be used.

  This fuel cell can be used for anything that requires electric power, and can be of any size. For example, it can be used for electronic devices, moving objects, power units, construction machines, machine tools, power generation systems, cogeneration systems, etc. The output, size, shape, fuel type, etc. are determined depending on the application.

The second invention is
In an electronic device using one or more fuel cells,
At least one of the fuel cells is
In the fuel cell in which the positive electrode and the negative electrode are opposed to each other via an electrolyte containing a buffer substance, and an enzyme is immobilized as a catalyst on at least one of the positive electrode and the negative electrode.
The concentration of the buffer substance contained in at least the electrolyte around the enzyme is 0.2 M or more and 2.5 M or less.
This electronic device may basically be any type, and includes both portable and stationary types. Specific examples include cell phones, mobile devices, robots, personal computers, and the like. Computers, game devices, in-vehicle devices, home appliances, industrial products, etc.

The third invention is
In a mobile body using one or more fuel cells,
At least one of the fuel cells is
In the fuel cell in which the positive electrode and the negative electrode are opposed to each other via an electrolyte containing a buffer substance, and an enzyme is immobilized as a catalyst on at least one of the positive electrode and the negative electrode.
The concentration of the buffer substance contained in at least the electrolyte around the enzyme is 0.2 M or more and 2.5 M or less.
This moving body may be basically any type, and specific examples include automobiles, two-wheeled vehicles, aircraft, rockets, and space ships.

The fourth invention is:
In a power generation system using one or more fuel cells,
At least one of the fuel cells is
In the fuel cell in which the positive electrode and the negative electrode are opposed to each other via an electrolyte containing a buffer substance, and an enzyme is immobilized as a catalyst on at least one of the positive electrode and the negative electrode.
The concentration of the buffer substance contained in at least the electrolyte around the enzyme is 0.2 M or more and 2.5 M or less.
This power generation system may be basically any type, and the scale may be any fuel. In addition to monosaccharides and polysaccharides themselves, garbage containing polysaccharides may be used. .

The fifth invention is:
In a cogeneration system using one or more fuel cells,
At least one of the fuel cells is
In the fuel cell in which the positive electrode and the negative electrode are opposed to each other via an electrolyte containing a buffer substance, and an enzyme is immobilized as a catalyst on at least one of the positive electrode and the negative electrode.
The concentration of the buffer substance contained in at least the electrolyte around the enzyme is 0.2 M or more and 2.5 M or less.
This cogeneration system may be basically any type, and the scale may be any fuel. In addition to monosaccharides and polysaccharides themselves, garbage containing polysaccharides may be used as fuel. .
In the second to fifth inventions, what has been described in relation to the first invention holds true for matters other than those described above, unless they are contrary to the nature thereof.

  In the fuel cell configured as described above, a sufficient buffer capacity can be obtained when the concentration of the buffer substance contained in at least the electrolyte around the enzyme is 0.2 M or more and 2.5 M or less. In high output operation, even if protons increase or decrease in the electrode or in the immobilized membrane of the enzyme due to the enzyme reaction via protons, a sufficient buffering action can be obtained, and the pH deviation from the optimum pH can be obtained. It can be kept small enough.

  According to the present invention, a sufficient buffer capacity can be obtained even during a high output operation, and a fuel cell having excellent performance can be obtained by fully exhibiting the ability of the enzyme. . By using such an excellent fuel cell, it is possible to realize a high-performance electronic device, a moving body, a power generation system, a cogeneration system, and the like.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 schematically shows a biofuel cell according to this embodiment. 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 the coenzyme (eg, NADH, NADPH, etc.), and coenzyme oxidase is produced along with the oxidation of the coenzyme. An electron mediator that receives 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 to adenosine triphosphate (ATP) and adenosine diphosphate (ADP) 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 above enzyme, coenzyme and electron mediator are optimal for the enzyme by using a buffer solution such as a phosphate buffer solution or a Tris buffer solution contained in the electrolyte layer 3 in order to perform the electrode reaction efficiently and constantly. It is preferably maintained at a low pH, for example, around pH 7. For this reason, in this embodiment, at least the electrolyte layer 3 around the enzyme immobilized on the negative electrode 1 and the positive electrode 2 has a buffer solution of 0.2 M or more and 2.5 M or less, preferably 0. The concentration is 2M or more and 2M or less, more preferably 0.4M or more and 2M or less, and further preferably 0.8M or more and 1.2M or less. 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. As the phosphate buffer, for example, NaH ₂ PO ₄ or KH ₂ PO ₄ is used. Further, the ionic strength (IS) is too large or too small to adversely affect the enzyme activity, but considering the electrochemical response, it should be an appropriate ionic strength, for example, about 0.3. Is preferred. However, pH and ionic strength have optimum values for each enzyme used, and are not limited to the values described above.

  The enzyme, coenzyme and electron mediator are preferably immobilized on the electrode 11 using an immobilizing material in order to efficiently capture the enzyme reaction phenomenon occurring in the vicinity of the electrode as an electrical 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.

The positive electrode 2 is composed of carbon powder on which a catalyst is supported or catalyst particles that are not held by carbon. As the catalyst, for example, platinum (Pt) fine particles, or fine particles of transition metal such as iron (Fe), nickel (Ni), cobalt (Co) or ruthenium (Ru) and platinum, or oxide fine particles are used. It is done. For example, the positive electrode 2 is formed in a structure in which a catalyst layer made of a catalyst or a carbon powder containing a catalyst and a gas diffusion layer made of a porous carbon material are laminated in order from the electrolyte layer 3 side. The positive electrode 2 is not limited to this configuration, and oxygen reductase can also be used as a catalyst. In this case, it is used in combination with an electron mediator that transfers electrons to and from the electrode.
In the positive electrode 2, in the presence of a catalyst, oxygen in the air is reduced by H ⁺ from the electrolyte layer 3 and electrons from the negative electrode 1 to generate water.

The electrolyte layer 3 is a proton conductive membrane that transports H ⁺ generated in the negative electrode 1 to the positive electrode 2 and is made of a material that does not have electron conductivity and can transport H ⁺ . Specific examples of the electrolyte layer 3 include a perfluorocarbon sulfonic acid (PFS) resin film, a copolymer film of a trifluorostyrene derivative, a polybenzimidazole film impregnated with phosphoric acid, and an aromatic polyether ketone. Examples thereof include a sulfonic acid film, PSSA-PVA (polystyrene sulfonate polyvinyl alcohol copolymer), PSSA-EVOH (polystyrene sulfonate ethylene vinyl alcohol copolymer), and the like. Among these, those made of an ion exchange resin having a fluorine-containing carbon sulfonic acid group are preferable, and specifically, Nafion (trade name, DuPont, USA) is used.

In the fuel cell configured as described above, when glucose is supplied to the negative electrode 1, 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 result of electrochemical measurement 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, EC1.1.1.17, manufactured by Toyobo Co., Ltd., GLD-311) was weighed in an amount of 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

  Using the measurement solution shown in Table 1, the enzyme / electron mediator-immobilized electrode thus prepared was set to 0.1 V with respect to the reference electrode Ag | AgCl, which is sufficiently higher than the redox potential of the electron mediator, and the chronoampero Measurement was performed. The glucose concentration of the fuel was 0.4M. FIG. 3 shows the results of chronoamperometry measured by changing the concentration of the buffer solution contained in the electrolyte layer 3. Moreover, the result of the chronoamperometry measured by changing the ionic strength of the electrolyte layer 3 is shown in FIG.

As can be seen from FIG. 3 and Table 1, in Comparative Example 1 where the NaH ₂ PO ₄ concentration was 0, that is, no buffer solution was added, the initial current was higher than that in Comparative Example 2 where the NaH ₂ PO ₄ concentration was 0.1M. The current is 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.

Further, as can be seen from FIG. 4 and Table 1, in Comparative Examples 2, 4, and 5, when the NaH ₂ PO ₄ concentration was kept constant at 0.1 M and only the ionic strength was increased, However, the current after 1800 seconds tended to decrease.
Furthermore, enzyme activity was measured by ultraviolet-visible spectroscopy (UV-vis) method. The results are shown in FIGS. Here, FIGS. 5 and 6 show changes in the reaction rate (v ₀ ) of the reaction by diaphorase (DI) with respect to the NaH ₂ PO ₄ concentration and ionic strength of the electrolyte layer 3, respectively. However, the concentration of DI is 2.5 nM, the concentration of ANQ is 2.2 mM, and the concentration of NADH is 40 mM. 7 and 8 show changes in the reaction rate (v ₀ ) of the reaction by glucose dehydrogenase (GDH) with respect to the NaH ₂ PO ₄ concentration and ionic strength of the electrolyte layer 3, respectively. However, the concentration of GDH is 1.59 nM, the concentration of glucose is 133 mM, and the concentration of NAD is 1.50 mM. According to the measurement results shown in FIGS. 5 to 8, there was no significant increase in enzyme activity accompanying the increase in NaH ₂ PO ₄ concentration, but there was a tendency to decrease slightly. From this, it can be considered that the increase in the current value accompanying the increase in the NaH ₂ PO ₄ concentration is due to the effect of suppressing the pH change inside the electrode or the immobilized film.

On the other hand, even when bilirubin oxidase (BOD) was immobilized as the oxidase on the positive electrode 2, the current value was maintained and improved as the NaH ₂ PO ₄ concentration increased. FIG. 9 and FIG. 10 show the results of measuring enzyme activity by UV-vis method in this case. Here, FIGS. 9 and 10 show changes in the reaction rate (v ₀ ) of the reaction by BOD with respect to the NaH ₂ PO ₄ concentration and ionic strength of the electrolyte layer 3, respectively. However, the concentration of BOD is 2.4 nM and the concentration of bilirubin is 1.8 mg / ml. According to the measurement results shown in FIG. 9 and FIG. 10, in the positive electrode 2 as well, there was no significant increase in enzyme activity accompanying the increase in NaH ₂ PO ₄ concentration, but there was a tendency to decrease slightly.
As a cause of obtaining the above result, it is conceivable that proton movement in the electrode or in the immobilized membrane is slow.

Next, the results of assembling and evaluating a biofuel cell as shown in FIGS. 11A and 11B will be described.
As shown in FIGS. 11A and 11B, this biofuel cell includes an anode 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. The 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 43 and 44 are sandwiched therebetween. 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 fuel supply paths 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 was connected between the Ti current collectors 41 and 42, and a glucose / buffer solution was added as fuel to the recess 44a of the fixed plate 44 to generate power. The operating temperature was 25 ° C. FIG. 12 shows a change in output (power density) with respect to the NaH ₂ PO ₄ concentration of the electrolyte layer 3. As shown in FIG. 12, when the NaH ₂ PO ₄ concentration is 0.2 M or more, the output is twice or more that of Comparative Example 2 where the concentration is 0.1 M, and about 6 times in Example 1 where the concentration is 0.4 M, In Example 2 where the concentration is 1.0 M, it is about 9 times, and it can be seen that both values are extremely high.

  As described above, according to this embodiment, a sufficient buffer capacity can be obtained when the concentration of the buffer substance (buffer solution) contained in the electrolyte layer 3 is 0.2 M or more and 2.5 M or less. Therefore, even when the fuel cell operates at a high output, the pH of the electrolyte layer 3 around the enzyme immobilized on the positive electrode 2 and the negative electrode 1 can be maintained in the vicinity of the optimum pH. The ability can be fully demonstrated. 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.

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 measurement result of the enzyme activity performed for evaluation of the biofuel cell by one Embodiment of this invention. It is a basic diagram which shows the measurement result of the enzyme activity performed for evaluation of the biofuel cell by one Embodiment of this invention. It is a basic diagram which shows the measurement result of the enzyme activity performed for evaluation of the biofuel cell by one Embodiment of this invention. It is a basic diagram which shows the measurement result of the enzyme activity performed for evaluation of the biofuel cell by one Embodiment of this invention. It is a basic diagram which shows the measurement result of the enzyme activity performed for evaluation of the biofuel cell by one Embodiment of this invention. It is a basic diagram which shows the measurement result of the enzyme activity performed for evaluation of the biofuel cell by one Embodiment of this invention. It is a basic diagram which shows the biofuel cell used for evaluation in one Embodiment of this invention. It is a basic diagram which shows the measurement result of the output of the biofuel cell used for evaluation in one Embodiment of this invention.

Explanation of symbols

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

Claims (18)

Oxygen reduction in which a positive electrode and a negative electrode are opposed to each other through an electrolyte containing a buffer substance, an enzyme is immobilized as a catalyst on at least one of the positive electrode and the negative electrode, and the enzyme is immobilized on the positive electrode Contains enzymes,
At least the buffer concentration fuel cells Ru der least 2.5M or less 0.2M substances contained in the electrolyte around the enzyme. Fuel cell concentrations 請 Motomeko 1 wherein Ru der least 2M or less 0.4M in the buffer substance. The fuel cell of the buffer concentration of the substance is 請 Motomeko 1 wherein Ru der least 1.2M or less 0.8 M. The buffer pK _a of 6 to 9 fuel cell Der Ru請 Motomeko 1, wherein the following materials. The fuel cell of Der Ru請 Motomeko 1, wherein ^- the buffer substance is H ₂ PO _4. Fuel cell Oh Ru請 Motomeko 1, wherein said buffering agent is 2-amino-2-hydroxymethyl-1,3-propanediol. The positive electrode and the fuel cell 請 Motomeko 1, wherein that an electron mediator in addition to the enzyme is immobilized to at least one of the negative electrode. The enzyme is, the negative electrode is fixed, the fuel cell of the oxidase degrades to promote the oxidation of monosaccharides including請 Motomeko 1 wherein. The enzyme is, the fuel cell including請 Motomeko 8 wherein the coenzyme oxidase to pass electrons to the negative electrode through the electron mediator with returning the coenzyme is reduced along with oxidation of the monosaccharide to an oxidized form. The oxidized form of the coenzyme is NAD ^+, the fuel cell of Ah Ru請 Motomeko 9, wherein the coenzyme oxidase with diaphorase. Fuel cell Oh Ru請 Motomeko 8, wherein said oxidase by NAD ⁺ dependent glucose dehydrogenase. The enzyme is, the negative electrode is fixed, polysaccharide-degrading enzymes and fuel generated monosaccharides oxidase degrades to promote the oxidation of including請 Motomeko 1 wherein generating a promoting monosaccharides decomposition of battery. The enzyme is glucoamylase, fuel cell Oh Ru請 Motomeko 12, wherein said oxidizing enzyme in NAD ⁺ dependent glucose dehydrogenase. The fuel cell according to claim 1, wherein the oxygen reductase is bilirubin oxidase. Using one or more fuel cells,
At least one of the fuel cells is
Oxygen reduction in which a positive electrode and a negative electrode are opposed to each other through an electrolyte containing a buffer substance, an enzyme is immobilized as a catalyst on at least one of the positive electrode and the negative electrode, and the enzyme is immobilized on the positive electrode Contains enzymes,
An electronic device in which the concentration of the buffer substance contained in at least the electrolyte around the enzyme is 0.2 M or more and 2.5 M or less. Using one or more fuel cells,
At least one of the fuel cells is
Oxygen reduction in which a positive electrode and a negative electrode are opposed to each other through an electrolyte containing a buffer substance, an enzyme is immobilized as a catalyst on at least one of the positive electrode and the negative electrode, and the enzyme is immobilized on the positive electrode Contains enzymes,
A moving body in which the concentration of the buffer substance contained in at least the electrolyte around the enzyme is 0.2 M or more and 2.5 M or less. Using one or more fuel cells,
At least one of the fuel cells is
Oxygen reduction in which a positive electrode and a negative electrode are opposed to each other through an electrolyte containing a buffer substance, an enzyme is immobilized as a catalyst on at least one of the positive electrode and the negative electrode, and the enzyme is immobilized on the positive electrode Contains enzymes,
A power generation system in which the concentration of the buffer substance contained in at least the electrolyte around the enzyme is 0.2 M or more and 2.5 M or less. Using one or more fuel cells,
At least one of the fuel cells is
Oxygen reduction in which a positive electrode and a negative electrode are opposed to each other through an electrolyte containing a buffer substance, an enzyme is immobilized as a catalyst on at least one of the positive electrode and the negative electrode, and the enzyme is immobilized on the positive electrode Contains enzymes,
A cogeneration system in which the concentration of the buffer substance contained in at least the electrolyte around the enzyme is 0.2 M or more and 2.5 M or less.

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