Porous metal electrode

Abstract

[Problem] To provide an electrode suitable for a biofuel cell, capable of compensating defects of a conventional electrode, having sufficient conductivity, and further improving oxidation-reduction action as an electrode catalyst. [Solution] This electrode comprises a base material, which is a porous metal body, and a conductive carbon film. The porous metal body is composed of pores and a skeleton having a three-dimensional network structure. The conductive carbon film is formed on the surface of the skeleton of the porous metal body.

Description

Metal porous electrode

[0001] The present invention relates to a porous metal electrode suitable for a biofuel cell.

[0002] Conventional fuel cells have mainly used metal catalysts such as platinum as catalysts for chemical reactions. However, in recent years, with the aim of applying fuel cells to living organisms, active development has been underway in biofuel cells that use biocatalysts such as enzymes and microorganisms, which are more biocompatible than metal catalysts, to oxidize biomass fuels such as glucose and reduce oxygen to obtain electrical energy. In addition to glucose, other sugars such as ethanol and lactose are also being considered as fuels for biofuel cells.

[0003] Since such biofuel cells have a major problem of low power density, particularly low current density, various attempts have been made to improve the power density. For example, a bio(enzyme) electrode has been proposed in which conductive carbon nanotubes are immobilized together with an enzyme and a metal oxide in a conductive binder to increase the efficiency of the enzyme-catalyzed reaction (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, on the other hand, results in a structure resembling a mesh of linear carbon fibers stretched lengthwise and widthwise. While this structure is more effective at 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 conventional 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] Japanese Patent Publication No. 2012-28181 Japanese Patent No. 6687285 Japanese Patent No. 6875047

[0007] Therefore, an object of the present invention is to provide an electrode suitable for biofuel cells that overcomes the drawbacks of conventional electrodes, has sufficient conductivity, and can further improve the oxidation-reduction action as an electrode catalyst.

[0008] As a result of intensive research conducted by the inventors to solve the above-mentioned problems, they discovered that by using a metal porous body having a three-dimensional mesh structure derived from a fired foamed polymer material as a substrate and forming a conductive carbon film on the skeletal surface (and further, within the pores) of the metal porous body, it is possible to firmly adhere conductive carbon particles to the substrate and to exhibit sufficient conductivity, which led to the completion of the present invention.

[0009] That is, in one aspect, the present invention relates to a novel porous metal electrode suitable for biofuel cells, and more specifically, <1> an electrode having a substrate that is a porous metal body and a conductive carbon film, wherein the porous metal body is composed of a skeleton and pores having a three-dimensional network structure, and the conductive carbon film is formed on the surface of the skeleton of the porous metal body; <2> the electrode according to <1> above, wherein the porous metal body has a three-dimensional network structure derived from a fired product of a foamed polymer material; <3> the electrode according to <1> above, further comprising conductive carbon particles filled in the pores of the porous metal body; <4> the electrode according to <1> above, further comprising an oxidoreductase supported on the electrode surface; <5> the electrode according to <1> above, wherein the porous metal body has an average pore diameter in the range of 0.1 to 3 mm; <6> the electrode according to <1> above, wherein the porous metal body has a porosity in the range of 40 to 98%; <7> the electrode according to <1> above, wherein the porous metal body has a porosity of 200 to 6,000 m ２ / m ３ <8> the electrode according to the above <1>, further comprising an electron transfer mediator; <9> the electrode according to the above <1>, wherein the conductive carbon film is formed from conductive carbon particles selected from the group consisting of carbon black, graphene, ketjen black, graphite, carbon nanotubes, and combinations thereof; and <10> the electrode according to the above <1>, which is an electrode for a biofuel cell.

[0010] In another aspect, the present invention also relates to a biofuel cell comprising the electrode described above. More specifically, <11> the present invention provides a biofuel cell comprising the electrode described in <1> to <10> above.

[0011] According to the present invention, a carbon film applied by coating and carbon particles adhere well to the skeletal surface of a three-dimensional mesh-like porous metal body, and optionally within the highly porous pores, and are in close contact in an active state, thereby providing an electrode suitable for biofuel cells with good conduction of generated electrons. This effect is achieved because the three-dimensional mesh-like porous metal body used as the substrate in the electrode of the present invention is made by baking a foamed polymer material in a controlled atmosphere, which provides good chemical adhesion and compatibility with the conductive carbon particles.

[0012] In a preferred embodiment of the present invention, by incorporating an oxidoreductase into the coating (application) liquid for the porous metal body, it is possible to provide the effect of further improving the current efficiency as an electrode catalyst.

[0013] Fig. 1 is an image showing the internal structure of a metal porous body of the present invention. Fig. 2 is an image showing the surface of a three-dimensional mesh-like metal porous body (pore diameter 0.8 mm) used in the examples. Fig. 3 is an image showing the surface of an electrode of the present invention produced in Example 3. Fig. 4 shows current-potential curves for Examples 1 to 3 and Comparative Examples 1 and 2. Fig. 5 shows current-potential curves for Examples 4 to 6 and Comparative Examples 3 and 4.

[0014] 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.

[0015] 1. Electrode of the Present Invention The electrode of the present invention is characterized by comprising a substrate that is a porous metal body constituted by a skeleton having a three-dimensional network structure and pores, and a conductive carbon film formed on the surface of the skeleton.

[0016] The porous metal body used as the substrate in the electrode of the present invention is a porous structure having a three-dimensional network structure with a very high porosity, which is composed of a skeleton and pores, as shown in Figure 1. Metals that can be used to compose the skeleton include nickel, nickel-chromium alloys, nickel-tin alloys, titanium, copper alloys, aluminum, and copper. Typically, porous metal bodies made of nickel-chromium alloys are preferred.

[0017] While known techniques can be used to manufacture such three-dimensional network-like porous metal bodies, they can preferably be manufactured using a fired foamed polymeric material as a mold for the three-dimensional network structure. For example, as disclosed in Japanese Patent No. 1,387,082 (JP 61-053417 B), a foamed polymeric material such as urethane foam is fired in an inert atmosphere to leave a shrunken skeleton, and the semi-fired membrane stretched between the skeletons and the burnt organic matter remaining on the skeleton are blown away with an inert gas flow. The porous structure, with only the skeleton remaining, is then subjected to metal plating, metallization, vapor deposition, metal-containing coating, etc. Next, the porous structure is fired in an inert or reducing atmosphere to remove the residual organic matter, leaving only the metal skeleton, thereby obtaining a three-dimensional network-like porous metal body. The interior of the metal skeleton can also be hollow, with the organic matter removed. Such porous metal bodies can also be commercially available products manufactured by manufacturers such as Sumitomo Electric Industries, Japan Heavy Chemical Industries, Taisei Kogyo, and Mitsubishi Pencil Co., Ltd.

[0018] The lower limit of the average pore diameter of the porous metal body of the present invention is preferably 0.1 mm or more or 0.3 mm or more, and the upper limit is preferably 3 mm or less. Typically, the porous metal body of the present invention has an average pore diameter in the range of 0.1 to 3 mm, more preferably 0.3 to 3 mm.

[0019] The lower limit of the porosity of the metal porous body in the present invention is preferably 40% or more or 50% or more, and the upper limit is preferably 98% or less or 90% or less. Typically, the metal porous body in the present invention has a porosity in the range of 40 to 98%, more preferably 50 to 90%.

[0020] The lower limit of the specific surface area of ​​the porous metal body in the present invention is preferably 200 m ２ / m ３ or more or 250m ２ / m ３ The upper limit is preferably 6000 m ２ / m ３ Less than or equal to 5000m ２ / m ３ Typically, the porous metal body of the present invention has a thickness of 200 to 6000 m２ / m ３ More preferably, in the range of 250 to 5000 m ２ / m ３ It has a specific surface area in the range of

[0021] The average pore size, porosity, and specific surface area can be easily adjusted by changing the type of foamed polymer material used to produce the metal porous body, the pressure applied during compression, and other factors.

[0022] The thickness of the porous metal body in the present invention is not particularly limited, but it can be typically a plate-like molded body in the range of 1 to 30 mm.

[0023] By using such a three-dimensional mesh-like metal porous body as a substrate, a conductive carbon film or conductive carbon particles applied by coating as described below adheres well and adheres in an active state, thereby providing good conduction of generated electrons and excellent functionality as an electrode.

[0024] Generally, metal porous bodies include mesh (wire net) metals, sintered metal particles, porous metals, and etched porous metals, but all of these have significantly smaller porosity, specific surface area, etc. than the three-dimensional mesh-like metal porous body of the present invention, and have significantly larger air resistance losses (pressure losses). Therefore, even if other substances are applied and penetrated into the interior using immersion liquid or spray, the liquid or particles may not penetrate into the interior, which is different from the above-mentioned metal porous bodies.

[0025] The conductive carbon film in the electrode of the present invention can be formed by a method such as coating (application) of conductive carbon particles. For example, a method can be used in which conductive carbon particles are dispersed in water, alcohol, or the like to which a small amount of organic solvent has been added, and the dispersion is impregnated and infiltrated into a three-dimensional network-like metal porous body, followed by repeated drying and heat treatment. Alternatively, a general application method can be used, such as a method in which the dispersion is spray-dried and then dried and baked (baked).

[0026] Furthermore, by taking advantage of the high porosity of the metal porous body, it is also possible to coat it using a thin film or thick film formation method (gas phase method) such as a sputtering method of a carbon material, a CVD (chemical vapor deposition) method, a plasma CVD method, a vapor deposition method, etc. Furthermore, conductive carbon particles can be contained or dispersed in an organic or inorganic binder, coated or filled, and then appropriately fired to coat and fix the porous body.

[0027] The conductive carbon particles used to form the conductive carbon film may be carbon black, graphene, ketjen black, graphite, carbon nanotubes, or any combination thereof. In particular, it is preferable to use carbon nanotubes, needle-like crystals thereof, or carbon nanotubes whose conductivity has been improved by heat treatment under a special atmosphere.

[0028] In a preferred embodiment, due to the large porosity of the three-dimensional network metal porous body, the skeleton thereof is coated with conductive carbon particles, and the pores can also be filled with conductive carbon particles.

[0029] In a preferred embodiment of the present invention, the electrode of the present invention can further support an oxidoreductase on the surface of the electrode on which the conductive carbon film is formed. In this specification, "supporting" means adsorbing, coating, or immobilizing an oxidoreductase or the like on the surface of the electrode (including the surface of the pores). This improves the permeability and flowability of an electrolyte or fuel into the three-dimensional network of continuous pores of the metal porous body, which has low resistance to fluids (i.e., high porosity), and thereby provides the effect of facilitating the flow of electrons involved in the chemical reaction of the oxidoreductase.

[0030] 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.

[0031] 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.

[0032] In the present invention, one or more of these enzymes can be attached and fixed to the surface of an electrode by, for example, immersing or applying a dispersion of the enzyme in a solution such as water, alcohol, or organic solvent to the electrode, and then drying or, in some cases, heat treating the electrode.

[0033] 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.

[0034] In this case, as described in, for example, Japanese Patent Publication No. 7359998, it is also effective to use a hydrogel compound and covalently bond it to an electron-transferring mediator compound, or to bond such a compound to a bioelectrode material by actively graft polymerizing it, thereby promoting the catalytic reaction accompanying electron transfer.

[0035] Alternatively, a hydrophilic coating layer made of a hydrophilic polymer containing an electron-transferring mediator can be formed on the skeleton of a porous metal 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 vivo, and can also improve the electron transfer between the enzyme and the mediator, and the electron flow between the mediator and the porous metal body.

[0036] Such a hydrophilic coating layer may preferably contain a hydrophilic polymer (inorganic, organic, or composite material), a hydrogel polymer, a zwitterionic group-containing polymer, or the like. Examples of the hydrophilic polymer used in this case include organic polyvinyl alcohol, polyethylene glycol, polystyrene sulfonic acid, and water-soluble silicone polymers. These organic polymers can be prepared by applying a dispersion of a pre-cured monomer or prepolymer of a 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. Examples of inorganic polymers that can be used include highly concentrated aqueous sodium silicate solutions (water glass), polyphosphates with a small amount of water added, silica-titania, and boric acid-based inorganic polymers. If necessary, the hydrophilic polymer used in the present invention may be a water-soluble photosensitive polymer.

[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.

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

[0042] The electrode substrate was a three-dimensional mesh-like porous metal body made of a nickel-chromium alloy (manufactured by Sumitomo Electric Industries, Ltd., under the trade name Celmet) as shown in Figure 2. The porous metal body had an average pore diameter of 0.8 mm and a porosity of 85%. The conductive carbon coating was formed by applying and penetrating a mixture of carbon black, graphene, and carbon nanotubes into a mixed dispersion (3.5 wt %) of water, ethyl alcohol, and a small amount of polyvinyl alcohol, followed by a 30-minute impregnation and drying cycle three times to coat and fill the carbon powder. The porous metal body was then heat-treated at 300°C for 2 hours in an oxygen stream. This treatment confirmed that 5.1 wt % of carbon had been applied (coated and filled) into the skeleton and pores of the porous metal body.

[0043] Next, bilirubin oxidase (manufactured by Amano Enzyme) was adsorbed and immobilized as a reducing enzyme to produce the electrode of Example 1. In this treatment method, the porous metal body coated with and filled with the conductive carbon powder was immersed for 20 minutes in a solution prepared by adding small amounts of ethanol, benzene, and ethyl acetate to a phosphate buffer solution (pH 7.4) containing bilirubin oxidase, and then dried at 35°C for 10 minutes and then at 55°C for 10 minutes.

[0044] The performance of the obtained biofuel cell electrodes was evaluated by the following method. First, an electrode onto which bilirubin oxidase was adsorbed as a reductase was used as a sample electrode. Platinum was used as the counter electrode, PBS (phosphate buffered saline, pH 7.4) as the electrolyte, and Ag-AgCl as the reference electrode, and the cell temperature was set to 35°C in an oxygen atmosphere. In this biofuel cell system, the current-potential curve of the electrode was measured using an electrochemical analyzer (manufactured by BAS Inc.), and the magnitude of the reduction current and the current density value at zero potential were used as the reduction current density to measure and evaluate the reduction ability of the electrode of the present invention. As a result, the reduction current density was found to be -4.0 mA / cm ２ The value was obtained.

[0045] As Comparative Example 1, a carbon electrode was prepared in which ordinary carbon fibers were bundled and fixed. Specifically, a large number of 10 μm diameter Toray carbon fibers were used to fabricate an electrode with a diameter of approximately 5 mm and a length of approximately 2 cm. Furthermore, as Comparative Example 2, an electrode was prepared using the same metal porous body as used in Example 1, but without treating the skeleton and pores with carbon particles. These electrodes were subjected to the same enzyme (bilirubin oxidase) immobilization treatment as the electrode in Example 1, and the enzyme was reduced to measure and evaluate the reduction current density. The measurement conditions were the same: a platinum counter electrode, a PBS electrolyte, an Ag-AgCl reference electrode, and a cell temperature of 35°C in an oxygen atmosphere. As a result, in Comparative Example 1, a current density of -0.3 mA / cm was obtained. ２ , and -0.1 mA / cm in Comparative Example 2. ２ In all cases, it was found that Example 1 had a superior reduction current density.

[0046] Using the same metal porous body as in Example 1, the same mixed dispersion (concentration: 3.5%) containing carbon black, graphene, and carbon nanotubes was used, and the carbon was impregnated and dried five times for one hour to deposit and penetrate the material. Heat treatment in an oxygen stream was then performed for the same time as above. This treatment confirmed that 7.0 wt % of carbon was deposited (coated or filled) in the skeleton and pores of the metal porous body. Next, bilirubin oxidase was adsorbed and immobilized as a reducing enzyme using the same treatment as in Example 1. That is, the treatment process was identical to Example 1 except for the amount of deposited carbon. This was used as the electrode for Example 2, and the reduction current density was measured under the same conditions as in Example 1, resulting in a value of -5.5 mA / cm. ２ The value of was obtained.

[0047] The same porous metal body as in Example 1 was used, and a mixed dispersion (3.5% concentration) containing carbon black, graphene, and carbon nanotubes was immersed in the same mixture for one hour and dried seven times to allow adhesion and penetration. The heat treatment in an oxygen stream was then performed for the same time as in Example 1. This treatment confirmed that 8.5 wt % of carbon had adhered (coated or filled) into the skeleton and pores of the porous metal body. Next, bilirubin oxidase was adsorbed as a reducing enzyme under the same treatment conditions as in Example 1. This electrode was designated Example 3. A photograph of the surface of this electrode is shown in Figure 3. The skeleton of the three-dimensional mesh-like porous metal body used as the substrate was not clearly visible, and a somewhat smooth surface coated with a carbon film containing the enzyme was observed. However, it was confirmed that the electrolyte was easily absorbed even on this surface and rapidly and sufficiently penetrated into the interior.

[0048] The electrode of Example 3 was subjected to the same enzyme treatment as in Example 1 and treatment as a battery system, and then the reduction current density was measured. As a result, it was found to be -7.2 mA / cm ２ The value of was obtained.

[0049] The measurement results of the reduction current density, which indicates the reduction ability of the electrodes obtained in Examples 1 to 3 and Comparative Examples 1 and 2, are shown in Table 1. The current-potential curves obtained by electrochemical measurement for the reduction current values ​​in Table 1 are shown in FIG.

[0050]

[0051] As shown in Table 1, in Examples 1 to 3, the skeleton and pores of the three-dimensional network metal porous body were filled with a conductive carbon film and conductive carbon particles, and as the amount of carbon attached increased, the reduction current density of the enzyme tended to increase (become larger in the negative direction). This reduction current value was significantly larger than in Comparative Examples 1 and 2, and it was found that in Examples 1 to 3, the reduction ability of the battery electrode increased as the amount of carbon attached increased.

[0052] Next, in order to further clarify the performance of the biofuel cell electrode of the present invention using oxidoreductase, the enzyme immobilization treatment similar to that in Examples 1 to 3 was repeated once more (a total of two treatments) to verify the effect of the adsorbed and immobilized enzyme.

[0053] A three-dimensional mesh-like porous metal body was treated with carbon particles, and after attaching approximately the same amount of conductive carbon as in Example 1, bilirubin oxidase was adsorbed and immobilized (for the second time) as a reducing enzyme to produce a biofuel cell electrode of Example 4. Platinum was used as the counter electrode, PBS (phosphate buffered saline, pH 7.4) as the electrolyte, and Ag-AgCl as the reference electrode, and the cell temperature was set to 35°C in an oxygen atmosphere. The enzyme was reduced in exactly the same manner as above, and the reduction current density was measured, resulting in a value of -7.3 mA / cm ２ The value of was obtained.

[0054] As Comparative Example 3, a carbon electrode (diameter: approximately 5 mm, length: approximately 2 cm) was prepared in the same manner as in Comparative Example 1, in which ordinary carbon fibers (10 μm diameter carbon fibers manufactured by Toray) were bundled and immobilized. Furthermore, as Comparative Example 4, an electrode was prepared in which the same metal porous body as used in Example 1 was used, but the skeleton and pores were not treated with carbon particles. These battery systems were subjected to the same enzyme treatment (second time) as in Example 4, and then the reduction current density was measured and evaluated under the same conditions. As a result, in Comparative Example 3, a reduction current density of -0.6 mA / cm was obtained. ２ , and -0.2 mA / cm in Comparative Example 4. ２ The value of was obtained.

[0055] In the exact same manner as in Example 2, the three-dimensional mesh-like metal porous body was subjected to the same carbon treatment, and the same amount of conductive carbon as in Example 2 was attached, followed by enzyme treatment to obtain an electrode. Subsequently, the same enzyme adsorption and immobilization treatment (second time) as in Example 4 was performed. This was used as the biofuel cell electrode of Example 5. Platinum was used as the counter electrode, PBS (phosphate buffered saline, pH 7.4) as the electrolyte, and Ag-AgCl as the reference electrode, and the cell temperature was set to 35°C in an oxygen atmosphere. The reduction current density was measured in the same manner as in Example 4, and was found to be -17.7 mA / cm ２ The value of was obtained.

[0056] In exactly the same manner as in Example 3, a three-dimensional mesh-like metal porous body was subjected to the same carbon treatment to attach substantially the same amount of conductive carbon as in Example 3, and then the enzyme treatment was carried out. This was then subjected to the same enzyme adsorption and immobilization treatment (second time) as in Example 4, and the same cell configuration as in Example 6, with the same electrolyte, counter electrode, and reference electrode, was used to produce a biofuel cell electrode. The reduction current density was measured in the same manner as in Example 4, and was found to be -25.1 mA / cm ２ The value of was obtained.

[0057] The measurement results of the reduction current density, which indicates the reduction ability of the electrodes obtained in Examples 4 to 6 and Comparative Examples 3 and 4, are shown in Table 2. The current-potential curves obtained by electrochemical measurement for the reduction current values ​​in Table 2 are shown in FIG.

[0058] The results of Examples 4 to 6 revealed that in the electrode of the present invention, smooth movement of electrons generated by chemical reactions occurs at the electrode interface between the three-dimensional mesh-like metal porous body derived from the foamed polymer material that serves as the electrode substrate, the conductive carbon, and the redox enzyme, resulting in an increase in the reduction current of the enzyme. The reduction current values ​​are significantly higher than those of Comparative Examples 3 and 4, demonstrating that the reduction ability of the electrode of the present invention is extremely excellent.

[0059] The results of the above examples (comparison of Examples 1 to 3 with Examples 4 to 6) demonstrate that the catalytic activity of enzymes is further enhanced when the electrodes of the present invention are used in biofuel cells. This is believed to be due to the excellent adsorption capacity of the three-dimensional mesh-like metal porous material used in the present invention for enzyme particles and oxidoreductase. Similarly, when a fuel oxidase is used as the anode, its oxidation activity is also improved.

[0060] In the present invention, the surface chemical bonds that contribute to the transfer of generated electrons between the conductive carbon film and carbon particles filling the internal skeleton and pores of a three-dimensional network-like porous metal body produced using a fired foamed polymer material are considered as follows. That is, it is believed that electron transfer is promoted by chemical bonds between the porous metal body surface, the conductive carbon film, and the conductive carbon particles due to hydroxyl groups (surface hydroxyl groups) derived from the production process and subsequent heat treatment, as well as surface oxidation from the air, or carboxyl groups that are presumed to remain in small amounts. That is, with regard to carbon materials, as long as they are conductive carbon particles, their production methods use organic materials as starting materials, and it is presumed that, microscopically, such functional groups as described above are likely to remain in the small amount of residue remaining after firing the organic material.

[0061] Next, the adhesive strength of the conductive carbon film formed on the surface of the three-dimensional network metal porous bodies prepared in Examples 3 and 6 was examined using electrodes of the present invention (the same as in FIG. 3 ) in which the same amount of conductive carbon was attached by performing the same carbon treatment.

[0062] Based on a typical paint film peeling test (see JIS K 5600), the test was conducted by attaching cellophane tape to the surface of the sample, rubbing the surface with a substantially constant pressure, and then quickly peeling the tape off. The same test was also conducted on the electrodes of Comparative Examples 1 and 3 for comparison. As a result, a relatively large amount of carbon powder and its broken fragments was found to be attached to the peeled tape in the two comparative samples. In contrast, in the electrodes used in Examples 3 and 6, the amount of carbon powder attached to the peeled tape was so small that it was difficult to detect with the naked eye. It was confirmed that the same results were obtained when the same test was conducted repeatedly.

[0063] These results demonstrate that in the biofuel cell electrode of the present invention, the conductive carbon particles coated and filled in the three-dimensional mesh-like metal porous body are firmly attached to the skeletal surface and inside the pores of the porous body, demonstrating that the adhesiveness of the carbon particles contributes to the improvement of the reduction current characteristics.

Claims

1. An electrode having a substrate that is a porous metal body and a conductive carbon film, wherein the porous metal body is composed of a skeleton and pores having a three-dimensional network structure, and the conductive carbon film is formed on the surface of the skeleton of the porous metal body.

2. The electrode according to claim 1, wherein the porous metal body has a three-dimensional network structure derived from a fired foamed polymer material.

3. The electrode according to claim 1, further comprising conductive carbon particles filled in the pores of the porous metal body.

4. The electrode of claim 1, further comprising an oxidoreductase supported on the electrode surface.

5. The electrode according to claim 1, wherein the porous metal body has an average pore size in the range of 0.1 to 3 mm.

6. The electrode according to claim 1, wherein the porous metal body has a porosity in the range of 40 to 98%.

7. The porous metal body has a thickness of 200 to 6000 m ２ / m ３ 10. The electrode of claim 1, having a specific surface area in the range of 8. The electrode of claim 1, further comprising an electron transfer mediator.

9. The electrode of claim 1, wherein the conductive carbon film is formed from conductive carbon particles selected from the group consisting of carbon black, graphene, ketjen black, graphite, carbon nanotubes, and combinations thereof.

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

11. A biofuel cell comprising an electrode according to any one of claims 1 to 10.

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