Porous carbon electrode and manufacturing method therefor

Abstract

[Problem] To address the problem of providing an electrode suitable for a biofuel cell capable of overcoming drawbacks of conventional electrodes, improving output density, and exhibiting biocompatibility. [Solution] An electrode comprising a base material that is a porous carbon body with a three-dimensional network structure and provided with an oxidoreductase supported thereon, wherein the porous carbon body is derived from a fired product of a foamed polymer material.

Description

Porous carbon electrode and its manufacturing method

[0001] The present invention relates to a porous carbon electrode suitable for a biofuel cell and a method for producing the same.

[0002] Conventional fuel cells have mainly used metal catalysts such as platinum to catalyze chemical reactions. Meanwhile, biofuel cells, which use biocatalysts such as enzymes and microorganisms to convert biomass fuel such as glucose into electrical energy, are being actively developed. The electrode reaction in a biofuel cell typically involves the oxidation of a fuel such as glucose by a biocatalyst, resulting in the transfer of electrons to the electrode. This electron transfer from the biocatalyst to the electrode is generally mediated by a redox compound known as a mediator. In addition to glucose, ethanol and sugars such as lactose are also being considered as fuels for biofuel cells.

[0003] A major issue with such biofuel cells is their low power density, particularly their low current density. The power density depends not only on the enzyme and mediator selected, but also on the method for immobilizing them on the electrode and the non-surface area of ​​the electrode material. To address this issue, bio(enzyme) electrodes have been proposed that improve power density by immobilizing conductive carbon nanotubes, enzymes, and metal oxides in a conductive binder, thereby increasing the efficiency of enzyme-catalyzed reactions (see, for example, Patent Document 1).

[0004] On the other hand, carbon particles and carbon films, which are conductive materials with large surface areas and are biocompatible, have the potential to be useful as electrode materials for biofuel cells. However, until now, it has been difficult to increase the amount of carbon particles attached to the electrode substrate and improve conductivity and reliability while maintaining strong adhesion that does not easily peel off.

[0005] To overcome these drawbacks, attempts have been made to use carbon nanotubes, as in Patent Document 1, or to lay carbon nanotubes in a mesh pattern on the surface of stacked carbon fiber bundles (e.g., Patent Documents 2 and 3). However, the former method suffers from the drawback that the water-repellent surface of carbon particles, such as carbon nanotubes, makes it difficult to firmly immobilize them on the electrode substrate, and also from poor affinity (e.g., hydrophilicity) with electrolytes and living organisms. The latter method, however, has a structure resembling a mesh of linear carbon fibers stretched vertically and horizontally. While this is more effective for retaining carbon particles and carbon nanotubes than flat or cylindrical structures, it suffers from the drawback that the carbon particles adhere to the mesh-like fiber surface, making it difficult for them to penetrate into the internal space, limiting the amount of particles retained. Therefore, these carbon nanotube-based methods fail to ensure the strong adhesion of carbon particles and the necessary electrical conductivity for an electrode, resulting in insufficient redox activity for use as a fuel cell electrode.

[0006] Furthermore, when metals are used as electrode substrates for biofuel cell electrodes, some metals have problems with hygiene, biocompatibility, and poor electrical conductivity. To address these issues, a method has been proposed in which a solid electrode substrate is coated with carbon powder and then coated with a biocompatible hydrogel containing a catalytic substance that controls the redox reaction. However, this hydrogel coating creates a new drawback: it inhibits contact between the catalyst and fuel at the anode and cathode electrodes, making it difficult for electrons generated by the chemical reaction to be transferred to the electrodes. This results in an insufficient redox reaction, and the problem of insufficient fuel cell output remains.

[0007] Japanese Patent Publication No. 2012-28181 Japanese Patent No. 6687285 Japanese Patent No. 6875047

[0008] Therefore, an object of the present invention is to provide an electrode suitable for a biofuel cell that can compensate for the drawbacks of conventional electrodes, can further improve the output density, and is friendly to living organisms.

[0009] The inventors conducted extensive research to solve the above-mentioned problems and discovered that by using a porous carbon substrate derived from a fired foamed polymer material and supporting an oxidative reduction enzyme on the substrate, it is possible to obtain a biofuel cell electrode that is less likely to inhibit the redox reaction of the enzyme. Furthermore, the inventors also discovered that an electrode that utilizes the redox reaction of the enzyme can be manufactured without the conventional complicated process of introducing carbon particles into the electrode substrate to improve conductivity. Based on these findings, the present invention was completed.

[0010] That is, in one aspect, the present invention relates to a novel porous carbon electrode suitable for use in a biofuel cell, and more specifically, <1> an electrode comprising a substrate that is a porous carbon material having a three-dimensional network structure and that supports an oxidative reduction enzyme, the porous carbon material being derived from a fired foamed polymer material; <2> the electrode according to <1> above, in which the porous carbon material has an average pore size in the range of 0.1 to 5 mm; <3> the electrode according to <1> above, in which the porous carbon material has a porosity in the range of 50 to 90%; <4> the electrode according to <1> above, which further comprises an electron transfer mediator; <5> the electrode according to <1> above, which has a hydrophilic coating layer on the surface of the porous carbon material, the oxidative reduction enzyme being present in the hydrophilic coating layer; <6> the electrode according to <4> above, which has a hydrophilic coating layer on the surface of the porous carbon material, the oxidative reduction enzyme and the electron transfer mediator being present in the hydrophilic coating layer; <7> the electrode according to <5> or <6> above, in which the hydrophilic coating layer contains a hydrophilic polymer; <8> The electrode according to the above item <1>, which is an electrode for a biofuel cell; and <9> A biofuel cell including the electrode according to any one of the above items <1> to <8>.

[0011] In another aspect, the present invention also relates to a method for producing the electrode, and more specifically, <10> the method for producing the electrode according to any one of the above items <1> to <8>, comprising: 1) a step of bringing a solution containing an oxidoreductase into contact with a carbon porous body having a three-dimensional network structure, and allowing the oxidoreductase to be adsorbed onto the surface of the carbon porous body; or 2) a step of coating the surface of the carbon porous body having a three-dimensional network structure with a hydrophilic polymer solution to form a hydrophilic coating layer, and a step of allowing the oxidoreductase to be adsorbed onto the hydrophilic coating layer; and <1111> the method according to the above item <10>, wherein in step 2), the hydrophilic polymer solution contains an electron transfer mediator.

[0012] According to the present invention, it is possible to provide an electrode suitable for a biofuel cell, which uses a carbon material that is relatively gentle on living organisms as a substrate and is less likely to inhibit the redox reaction of enzymes. In the electrode of the present invention, by using a carbon porous body having a three-dimensional network structure with a predetermined porosity, it is possible to support an enzyme on the skeletal surface with good adhesion and in a highly active state. In addition, the permeability and flowability of an electrolyte or fuel into the pores of the porous body is improved, which has the effect of facilitating the flow of electrons in the redox reaction catalyzed by the enzyme.

[0013] Furthermore, according to the manufacturing method of the present invention, an electrode that utilizes the oxidation-reduction reaction of an enzyme can be manufactured without the need for a complicated process such as introducing carbon particles into the electrode substrate and laboriously improving conductivity.

[0014] FIG. 1 is a schematic diagram showing the cross-sectional structure of a porous carbon body of the present invention. FIG. 2 is a schematic diagram showing an embodiment of enzyme adsorption on a porous carbon body of the present invention. FIG. 3 is a schematic diagram showing the immobilization of an enzyme and an electron transfer mediator on a porous carbon body of the present invention. FIG. 4 is a schematic diagram showing a representative example of a biofuel cell of the present invention. FIG. 5 is a current-potential curve for Example 1 and Comparative Example 1. FIG. 6 is a current-potential curve for Example 2 and Comparative Example 2. FIG. 7 is a current-potential curve for Example 3 and Comparative Example 3.

[0015] The following describes embodiments of the present invention. The scope of the present invention is not limited to these descriptions, and other than the following examples, the present invention can be implemented with appropriate modifications within the scope that does not depart from the spirit of the present invention.

[0016] 1. The electrode of the present invention has a structure in which an oxidoreductase is supported on a substrate that is a porous carbon material having a three-dimensional network structure, and the porous carbon material is a material derived from a fired foamed polymer material.

[0017] The porous carbon material used as the electrode substrate is a material composed of carbon that is relatively biocompatible. It is a porous material with a three-dimensional network structure consisting of pores and a skeleton with a high porosity, as shown in Figure 1. Conventionally, with regard to electrodes for biofuel cells or electrodes using conductive carbon microparticles for application thereto, numerous attempts have focused on the organization and microstructure of fine carbon microparticles and their use as electrode materials. However, unlike previous efforts involving carbon microparticles, the present invention focuses on the porous body having a three-dimensional network structure, as well as the surface of the skeleton that constitutes this porous body. In other words, this invention is the first to utilize the fact that the skeleton surface of such a porous carbon body is suitable for directly adsorbing and immobilizing redox substances such as enzymes.

[0018] Therefore, in one aspect, the present invention does not require a separate process, such as coating or adhering carbon fine particles to an electrode substrate, as has been done in conventional electrodes, and allows an enzyme to be directly adsorbed and immobilized on the skeletal surface of a porous carbon material. Furthermore, this skeletal structure also has favorable affinity with hydrophilic polymers (described below) and interfacial affinity with electron-transferring mediators.

[0019] As described above, the porous carbon body used as the substrate in the present invention is a material derived from a fired foamed polymer material. For example, the porous carbon body can be obtained by firing a foamed polymer material in an inert atmosphere and then metallizing it with carbon, or it can be produced by mixing carbon powder particles into a foamed polymer material and firing it under temperature and atmosphere control. For example, carbon porous bodies having such a three-dimensional mesh structure can be commercially available products manufactured by Mitsubishi Pencil Co., Ltd., Rayho Manufacturing Co., Ltd., and the like.

[0020] The inventors discovered that such porous carbon materials produced from organic materials have numerous micropores and ultramicropores on the surface of the skeleton, and in addition, hydrophilic functional groups such as hydroxyl groups and carboxyl groups are also present on the surface, so that enzyme molecules, which are organic materials, can be adsorbed and immobilized relatively smoothly and can be supported in an active state.

[0021] More specifically, an oxidoreductase can be supported on the skeletal surface of a porous carbon material, as shown in Figure 2. In this specification, "supporting" means adsorbing, coating, or immobilizing an oxidoreductase or the like on the skeletal surface (including the pore surface) of a porous carbon material. Without being bound by theory, it is believed that such support is due to the molecular attraction of the countless micropores distributed on the skeletal surface of the porous carbon material.

[0022] In the electrode of the present invention, by supporting an oxidoreductase on the surface of a porous carbon material in this manner, the permeability and flowability of electrolyte and fuel into the three-dimensional network of continuous pores of the porous carbon material, which has low resistance to fluids (i.e., high porosity), is improved, which has the effect of making it easier for electrons to flow in association with the chemical reaction of the oxidoreductase to be released.

[0023] Therefore, the lower limit of the average pore diameter of the carbon porous body in the present invention is preferably 0.1 mm or more or 0.3 mm or more, and the upper limit is preferably 5 mm or less or 3 mm or less. Typically, the carbon porous body in the present invention has an average pore diameter in the range of 0.1 to 5 mm, more preferably 0.3 to 3 mm.

[0024] The lower limit of the porosity of the carbon porous body in the present invention is preferably 50% or more or 70% or more, and the upper limit is preferably 90% or less or 85% or less. Typically, the carbon porous body in the present invention has a porosity in the range of preferably 50 to 90%, more preferably 70 to 90%.

[0025] The average pore size, porosity, and density can be adjusted by the type of foamed polymer material used to produce the carbon porous body and the amount of carbon powder particles mixed therewith.

[0026] Generally, carbon porous bodies are broadly known as carbon cloth, mesh (wire netting), particle-sintered porous bodies, porous carbon, etched porous carbon, etc., but all of these have significantly smaller porosity, specific surface area, etc. than the porous carbon bodies used in the present invention, and suffer from significantly larger losses (pressure losses) during the passage of air or liquid. Therefore, even if other substances are applied to the interior of these bodies using immersion liquids or sprays to penetrate them, the liquids or particles may not penetrate into the interior, which is a point that differentiates them from the porous carbon bodies described above.

[0027] Furthermore, although not necessarily required, a pretreatment such as heat treatment in an appropriately adjusted atmosphere can be performed on the porous carbon material before enzyme adsorption. Furthermore, in some cases, if necessary, the porous carbon material may be coated (covered and filled) with conductive carbon particles conventionally used in electrodes. Examples of such conductive carbon particles include carbon black, graphene, ketjen black, and carbon nanotubes. It is particularly preferable to use carbon nanotubes, their needle-like crystals, or carbon nanotubes whose conductivity has been improved by heat treatment in an appropriately adjusted atmosphere.

[0028] Various oxidoreductases can be used in the electrodes of the present invention depending on the cathode or anode electrode, the fuel, and the intended use. Many enzymes, including their immobilization methods, are known for use in electrodes of biofuel cells (e.g., Japanese Patent No. 6295630, Japanese Patent No. 5983392, JP-A-2005-310613, etc.), and any of these can be used as appropriate.

[0029] More specifically, when the electrode of the present invention is used as a cathode, an enzyme that catalyzes the reduction reaction of oxygen can be used. Examples of such enzymes include bilirubin oxidase, laccase, ascorbate oxidase, and pyruvate oxidase. When the electrode of the present invention is used as an anode, an enzyme that can oxidize fuel can be used. Examples of such enzymes include oxidoreductases such as glucose dehydrogenase, glucose oxidase (GOD), laccase, alcohol dehydrogenase, aldehyde oxidase, galactose dehydrogenase, xylol dehydrogenase, and nicotinamide adenine dinucleotide.

[0030] In the present invention, one or more of these enzymes are adsorbed onto a porous carbon material for use. For example, the enzymes can be added and immobilized on the three-dimensional network-like porous carbon material by immersing or applying a dispersion of the enzyme in a solution such as water, alcohol, or organic solvent to the porous carbon material, followed by drying or, in some cases, heat treatment.

[0031] In a preferred embodiment of the present invention, a hydrophilic coating layer can be formed on the surface of the porous carbon material for the addition or immobilization of an enzyme or a mediator. In this case, the above-mentioned redox enzyme and / or the electron transfer mediator described below can be present in the hydrophilic coating layer. Such a hydrophilic coating layer preferably contains a hydrophilic polymer (inorganic, organic, or composite material).

[0032] Examples of hydrophilic polymers that can be used in this case include organic polyvinyl alcohols, polyethylene glycols, polystyrene sulfonates, and water-soluble silicones. Examples include alginate, polyethylene oxide, polyethylene glycol, polyvinyl acetate, polyvinylpyrrolidone, methylcellulose, ethylcellulose, hydroxypropyl cellulose, hydroxyethyl cellulose, hydroxyethyl methylcellulose, hydroxypropyl methylcellulose, polyacrylic acid, sodium carboxymethylcellulose, polyvinyl alcohol, polyethylene oxide, ethylene oxide-propylene oxide copolymer, collagen, gelatin, albumin, polyamine, polyphosphazene, polysaccharides, chitin, chitosan, poly(meth)acrylic acid esters, poly(meth)acrylamides, ethylene vinyl alcohol (ethylene-vinyl alcohol copolymers), polyalkylene glycols, polyethyleneimines, glycosaminoglycans (e.g., hyaluronic acid and chondroitin sulfate), proteoglycans, xanthan gum, carrageenan, gellan gum, guar gum, locust bean gum, and sacran. These organic polymers can be prepared by applying a liquid prepared by dispersing uncured monomers or prepolymers of the hydrophilic resin in an appropriate solvent to the surface of the porous carbon body, followed by appropriate drying and heat treatment at a relatively low temperature.

[0033] As inorganic polymers, for example, highly concentrated aqueous sodium silicate solution (water glass), polyphosphate salts to which a small amount of water has been added, silica-titania-based and boric acid-based inorganic polymers can be used.

[0034] If necessary, the hydrophilic polymer used in the present invention may be a water-soluble photosensitive polymer. As such a photosensitive polymer, any polymer known in the art may be used, such as saponified polyvinyl acetate (polyvinyl alcohol), polyvinylpyrrolidone, poly(meth)acrylamide-diacetone(meth)acrylamide copolymer, poly(N-vinylformamide), poly(N-vinylacetamide), etc. In addition, as a compound for introducing a photosensitive group into a photosensitive polymer, for example, 3-(4-azidophenyl)-N-(4,4'-dimethoxybutyl)-2-phenylcarbonylamino-prop-2-enamide), 2-(3-(4-azidophenyl)prop-2-enoylamino)-N-(4,4-dimethoxybutyl)-3-(3-pyridyl)prop-2-enamide), 3-(4-azidophenyl)-N-(4,4'-dimethoxybutyl)-2-[(3-pyridyl)carbonylamino]-prop-2-enamide, 3-(2-dimethoxybutyl)-(4-azidobenzylidene-2-sodium sulfonate)rhodanine, 3-(2-dimethoxyethyl)-(4-azidobenzylidene-2-sodium sulfonate)rhodanine and the like photosensitive group unit can be used, but those having an azide group are preferred.

[0035] Furthermore, in a preferred embodiment of the present invention, an electron transfer mediator can be immobilized on the electrode to improve the electron transfer between the oxidoreductase and the electrode carbon. Examples of such electron transfer mediators include complexes of metals such as iron, nickel, cobalt, chromium, molybdenum, manganese, ruthenium, and osmium, heterocyclic compounds such as viologen and methylviologen, and quinones such as naphthoquinone. Typical examples include cyclopentadienyl metal complexes having an amino group and quinone compounds having an amino group. Preferably, aminoferrocene, amino-3-chloro-1,4-naphthoquinone, and the like can be used.

[0036] For example, as shown in Figure 3, a hydrophilic coating layer made of a hydrophilic polymer containing an electron-transferring mediator can be formed on the skeleton of a porous carbon body by coating, and then a hydrophilic coating layer made of a hydrophilic polymer containing an oxidative reduction enzyme can be formed on top of that. This can accelerate the overall redox reaction when used as an electrode for a biofuel cell in a living organism, and can also improve the electron transfer between the enzyme and mediator and the electron flow between the mediator and the porous carbon body.

[0037] 2. Biofuel Cell of the Present Invention In another aspect, the present invention also relates to a biofuel cell comprising the above-described electrode. As described above, the electrode of the present invention can be used as either a cathode electrode or an anode electrode. Figure 4 shows a schematic diagram of a representative example of a biofuel cell of the present invention.

[0038] The electrolyte solution in the biofuel cell of the present invention can be an aqueous electrolyte solution known in the art. For example, a wide variety of solutions of any concentration can be used, such as saturated potassium chloride solution, citrate buffer solution, organic electrolyte solution, and phosphate buffer solution. These can further contain a fuel substance depending on the application of the fuel cell.

[0039] As the fuel material for the biofuel cell of the present invention, a wide range of oxygen-containing substances can be used, such as sugars, proteins, alcohols, amines, aldehydes, and many organic substances.

[0040] In some cases, the biofuel cell of the present invention may include an electrolyte membrane between the anode and cathode electrodes, but fuel cells without such an electrolyte membrane can also be used. When an electrolyte membrane is used, a material with no or very little electronic conductivity and proton conductivity is preferred. Examples of such electrolyte membranes that can be used include perfluorocarboxylic acid polymer membranes, trifluorostyrene copolymer membranes, phosphoric acid-added polybenzimidazole resin membranes, and aromatic polyethersulfonic acid membranes. Typically, products with good proton conductivity, such as "Nafion" (manufactured by DuPont), can be used.

[0041] 3. Manufacturing Method of the Present Invention In a further aspect, the present invention also relates to a manufacturing method of the above-mentioned battery, which manufacturing method comprises at least the following steps: 1) a step of bringing a solution containing an oxidoreductase into contact with a carbon porous body having a three-dimensional network structure, and causing the oxidoreductase to be adsorbed onto the surface of the carbon porous body; or 2) a step of coating the surface of a carbon porous body having a three-dimensional network structure with a hydrophilic polymer solution to form a hydrophilic coating layer, and a step of adsorbing the oxidoreductase onto the hydrophilic coating layer.

[0042] In step 1), an oxidoreductase is directly supported on the skeletal surface (including the pore surfaces) of the porous carbon material. This step can be carried out, for example, by immersing or applying a dispersion of the enzyme in a solution such as water, alcohol, or organic solvent to the porous carbon material, as described above, followed by drying or, in some cases, heat treatment, to attach and fix the enzyme to the three-dimensional network-like porous carbon material.

[0043] Step 2) is a step of forming a hydrophilic coating layer on the skeleton surface of the porous carbon body and then supporting an oxidoreductase. Preferably, in step 2), the hydrophilic polymer solution may also contain an electron transfer mediator. For example, as described above, a hydrophilic coating layer in which an electron transfer mediator is contained in a hydrophilic polymer may be formed on the skeleton of the porous carbon body by coating, and then a hydrophilic coating layer in which an oxidoreductase is also contained in a hydrophilic polymer may be formed on top of the hydrophilic coating layer. In some cases, a hydrophilic coating layer may be formed by incorporating both the electron transfer mediator and the oxidoreductase in a hydrophilic polymer solution and coating the solution.

[0044] The details of the porous carbon material, the oxidoreductase, the hydrophilic polymer, and the electron transfer mediator used in these steps are as described above.

[0045] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0046] As the electrode substrate, a three-dimensional mesh carbon porous body was used, which was a commercially available product (trade name PFC 3A) manufactured by Mitsubishi Pencil Co., Ltd., with an average pore diameter of 0.3 mm and a porosity of 80%.

[0047] To easily evaluate the performance of the electrode of the present invention, an enzyme was reduced in an electrolyte solution and the reduction current value was measured as follows. First, bilirubin oxidase (Amano Enzyme, BO3) was adsorbed as a reducing enzyme onto the battery electrode of the present invention to prepare a sample electrode. Platinum was used as the counter electrode, PBS (phosphate buffered saline, pH 7.4) as the electrolyte, and Ag-AgCl was used as the reference electrode. The cell temperature was set to 35°C in an oxygen atmosphere.

[0048] In this battery system, the current-potential curve of the electrode was measured using an electrochemical analyzer manufactured by BAS, and the current density value at zero potential was taken as the reduction current density to evaluate the reduction ability of the electrode of the present invention. As a result, a value of -15.7 mA / cm was obtained.

[0049] As Comparative Example 1, a carbon electrode substrate was prepared by bundling and immobilizing carbon cloth manufactured by A&A Materials, which was obtained by burning and carbonizing acrylic fibers. This was used to prepare a half cell that underwent the same enzyme adsorption treatment as the electrode of the present invention. The counter electrode, electrolyte, reference electrode, oxygen atmosphere, and cell temperature were all the same, and the enzyme was reduced, and the reduction current density was measured and evaluated. A value of -3.8 mA / cm2 was obtained.

[0050] Figure 5 shows the current-potential curves for Example 1 and Comparative Example 1. It can be seen from Figure 4 that the reduction ability (magnitude of the negative current) of the enzyme electrode of the present invention is superior to that of the electrode of Comparative Example 1. This is clearly due to the fact that the three-dimensional mesh carbon electrode used as the electrode substrate is superior to electrodes made of carbon cloth substrates in terms of enzyme retention and activity (reducing ability). In other words, this is due to the excellent enzyme retention of the three-dimensional mesh carbon electrode and the excellent flow of electrons generated by the chemical reaction between the enzyme and the electrode.

[0051] The electrode substrate was a three-dimensional mesh-like porous carbon material of the same type as in Example 1. A polyethylene glycol-based hydrophilic polymer (Takiral EL-40, manufactured by Taki Chemical Industry Co., Ltd.) was prepared as a hydrophilic polymer at a concentration of 5 wt % in water to which a small amount of acetone had been added. The same bilirubin oxidase enzyme as in Example 1 was applied to the cathode electrode, and the process was repeated three times to dry, resulting in a biofuel cell electrode of the present invention. As in the example, a platinum counter electrode, PBS (phosphate buffered saline, pH 7.4) as the electrolyte, and Ag-AgCl as the reference electrode were used, and the current-potential curve was measured as a half-cell in an oxygen atmosphere at a cell temperature of 35°C.

[0052] For evaluation, the reduction ability of the electrode of the present invention was examined by taking the current density value at zero potential as the reduction current density, and the result was -17.5 mA / cm2.

[0053] As Comparative Example 2, a carbon electrode was prepared in which the carbon cloth used in Comparative Example 1 was bundled and immobilized. This was subjected to the same enzyme adsorption treatment as the electrode of the present invention to construct a half cell, and the enzyme was similarly reduced and the reduction current density was measured and evaluated. The sample electrode and the counter electrode, electrolyte, and other components were the same, and the current-potential curve of the electrode was measured at 35°C in an oxygen atmosphere. As a result, a reduction current density of -4.9 mA / cm2 was obtained at zero potential. The current-potential curves of Example 2 and Comparative Example 2 are shown in Figure 6.

[0054] As in Example 1, it was confirmed that the enzyme electrode of the present invention, when the enzyme was immobilized using a hydrophilic polymer, had a superior reduction ability to the electrode of Comparative Example 2. Here, the reduction current density values ​​of the biofuel cell electrode of the present invention and the bioelectrodes of Comparative Examples 1 and 2 used in the experiments in the Examples are shown in Table 1.

[0055] The same three-dimensional mesh carbon porous body as in Examples 1 and 2 was used as the electrode substrate to fabricate an anode electrode, and the oxidation ability of the biofuel cell electrode of the present invention was evaluated when an enzyme and an electron transfer mediator were used.

[0056] A mixture of water-soluble resin (product name: AWP) manufactured by Toyo Gosei Kogyo Co., Ltd. and polyvinyl alcohol was used as the hydrophilic polymer. The porous carbon body was immersed and coated with the polymer mixture, which first contained phenazine methosulfanate (P9625 manufactured by Sigma-Aldrich Co.) as a mediator, and the mixture was thoroughly permeated into the internal skeleton. Next, glucose oxidase was selected as the enzyme for oxidizing glucose fuel and added to a PBS solution as a buffer. The entire porous body, with the hydrophilic polymer layer containing the mediator formed on the skeleton, was thoroughly immersed in the enzyme solution and allowed to adsorb. The current-potential curve of the biofuel cell electrode of the present invention thus formed was measured under the same conditions as before in a PBS electrolyte solution containing 100 mM glucose as a fuel.

[0057] The results obtained are shown in Figure 7. From Figure 7, it was confirmed that the fuel oxidation performance of the biofuel cell electrode of the present invention is far better than when carbon cloth is used as the electrode substrate.

[0058] In the present invention, as in the process of Example 3, the skeleton of the porous carbon material can first be coated with the hydrophilic coating layer containing an electron-transferring mediator. Then, an oxidoreductase can be adsorbed onto this coating layer to form an immobilized layer. This is because it may be difficult for the enzyme to completely permeate the hydrophilic polymer layer formed on the skeleton of the porous carbon material.

[0059] Furthermore, when a hydrophilic polymer is used as a binder to bring the redox enzyme as close as possible to the mediator layer of the porous skeleton, the amount of polymer should be kept to a minimum for the reasons mentioned above. On the other hand, the mediator can easily permeate the hydrophilic polymer layer, so it is permeated and immobilized so that it is closer to and in contact with the porous skeleton than the enzyme. Therefore, as already shown in Figure 3, in the biofuel cell electrode of the present invention, it is desirable to arrange the porous carbon, mediator, and redox enzyme in roughly this order. However, this order is not strictly necessary because the electrolyte and fuel flow smoothly within the pores surrounded by the porous skeleton. This is also a major reason why it is essential to use a three-dimensional mesh-like porous carbon as the electrode substrate in the present invention.

Claims

1. An electrode comprising a substrate that is a porous carbon material having a three-dimensional network structure and that supports an oxidoreductase, wherein the porous carbon material is derived from a fired foamed polymer material.

2. The electrode according to claim 1, wherein the porous carbon body has an average pore size in the range of 0.1 to 5 mm.

3. The electrode according to claim 1, wherein the porous carbon body has a porosity in the range of 50 to 90%.

4. The electrode of claim 1, further comprising an electron transfer mediator.

5. The electrode according to claim 1, wherein the surface of said porous carbon body has a hydrophilic coating layer, and said redox enzyme is present in said hydrophilic coating layer.

6. The electrode according to claim 4, wherein the surface of said porous carbon body has a hydrophilic coating layer, and said oxidoreductase and said electron transfer mediator are present in said hydrophilic coating layer.

7. The electrode according to claim 5 or 6, wherein the hydrophilic coating layer comprises a hydrophilic polymer.

8. The electrode according to claim 1, which is an electrode for a biofuel cell.

9. A biofuel cell comprising an electrode according to any one of claims 1 to 8.

10. A method for producing an electrode as defined in any one of claims 1 to 8, comprising: 1) a step of bringing a solution containing an oxidoreductase into contact with a porous carbon material having a three-dimensional network structure, and causing the oxidoreductase to be adsorbed onto the surface of the porous carbon material; or 2) a step of coating the surface of a porous carbon material having a three-dimensional network structure with a hydrophilic polymer solution to form a hydrophilic coating layer, and a step of adsorbing the oxidoreductase onto the hydrophilic coating layer.

11. The manufacturing method according to claim 10, wherein in step 2), the hydrophilic polymer solution contains an electron-transferring mediator.

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