JP2022035428A - Particulate electrodes and microbial fuel cells - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode which is inexpensive, durable and has high power generation characteristics. The problem can be solved by a particulate electrode having core particles and a conductive layer containing a conductive material formed on the surface of the core particles. Further, the conductive layer can be solved by the particulate electrode which is a coating film of the ink composition. Further, the problem can be solved by the particulate electrode in which the core particles are non-conductive particles. Further, it can be solved by a microbial fuel cell using the particulate electrode. [Selection diagram] Fig. 1

Description

The present invention relates to particulate electrodes and microbial fuel cells.

Wastewater treatment is an indispensable technology for preserving the water quality environment, but the activated sludge method currently widely used in wastewater treatment is a method that utilizes the aerobic metabolism of microorganisms, so aeration and sludge in the treatment tank. There is a problem that it consumes a lot of energy for processing.
In recent years, there has been a demand for energy saving in wastewater treatment due to increasing interest in energy problems, and microbial fuel cells are attracting attention as a new method.
A microbial fuel cell is a power generation method that recovers electrons generated when a microorganism called a power generation bacterium decomposes an organic substance by anaerobic metabolism and uses it as electric energy. By utilizing this property, for example, decomposition treatment of organic substances contained in domestic wastewater and power generation can be performed in parallel, so that it is expected as a water treatment method capable of reducing energy consumption (Non-Patent Document 1).
Two-tank type and one-tank type are known as configurations of microbial fuel cells (Patent Document 1, Non-Patent Document 2). It is common that electrons generated from the decomposition reaction by microorganisms at the anode pass through an external circuit and are consumed in the reduction reaction at the cathode. In the two-tank type, both poles are separated by a partition wall such as an ion exchange membrane, and an oxidizing agent such as oxygen or potassium ferricyanide is reacted at the cathode. On the other hand, the one-tank type has no partition wall, and the cathode and anode are arranged in one tank, and oxygen in the atmosphere is used as an oxidizing agent to react with the cathode.
In any form, microorganisms metabolize at the anode, so it is important to devise the form of the anode to accumulate microorganisms and bring them into contact with organic substances efficiently. Various studies have been conducted so far on the configuration of this anode.
In Patent Document 2, a group of expanded graphite particles hardened by pressure molding is used as an anode which is cheaper than the case where carbon felt or the like is used. It has been reported that this expanded graphite particle group has a larger specific surface area than the conventional expanded graphite particles, and can obtain an anode having a high degree of freedom in design. Further, in Patent Document 3, it is proposed to use carbon black on the surface of the anode base material to increase the contact with microorganisms.
However, in any of the forms, the durability is low when the particles are immersed in water and used, and the gaps between the particles are small and dirt is easily clogged, so that the deterioration of power generation characteristics becomes a problem. In addition, there is a problem that it is difficult to remove sludge and biofilm generated when microorganisms become dead bodies.

Japanese Unexamined Patent Publication No. 2004-342212 Japanese Unexamined Patent Publication No. 2019-076833 Japanese Unexamined Patent Publication No. 2017-224383

Forefront of wastewater treatment system using microbial fuel cell, NTS Co., Ltd. Environmental Science & Technology, 2004,38,4040-4046

An object of the present invention is to provide an electrode for a microbial fuel cell, which is inexpensive, durable, and has excellent power generation characteristics, and a microbial fuel cell using the electrode.

The present inventors have reached the present invention as a result of repeated diligent studies to solve the above-mentioned problems.
That is, the present invention relates to a particulate electrode having core particles and a conductive layer containing a conductive material formed on the surface of the core particles.

The present invention also relates to the particulate electrode in which the conductive layer is a coating film of an ink composition.

The present invention also relates to the particulate electrode in which the core particles are non-conductive particles.

The present invention also relates to a microbial fuel cell using the particulate electrode.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide an electrode for a microbial fuel cell, which is inexpensive, durable, and has excellent power generation characteristics, and a microbial fuel cell using the electrode.

It is sectional drawing for demonstrating the particulate electrode. It is a schematic diagram for demonstrating an electrode. It is a schematic diagram for demonstrating the microbial fuel cell produced using the particulate electrode of this invention.

<Particulate electrode>
First, the electrodes used in the present invention will be described with reference to the cross-sectional view shown in FIG. As shown in FIG. 1, the particulate electrode 3 has a conductive layer 2 formed from a composition for forming a conductive layer containing a conductive material on the surface of the core particles 1.
Further, FIG. 2 is a schematic diagram showing an electrode 4 that can be formed by gathering a plurality of particulate electrodes 3. In the electrode 4, the particulate electrode 3 may be brought into close contact with each other, or may be carried out by a conductive material such as metal or carbon at a distant distance.

<Core particles>
The core particles can be formed by using a non-conductive material such as glass, sponge, non-woven fabric, or resin. Further, it may be constructed by using a conductive material such as an iron ball, carbon particles, or carbon felt. From the viewpoint of cost, it is preferable that the core particles 1 are made of a non-conductive material. In particular, a non-porous material such as glass is particularly preferable from the viewpoint of durability because it can maintain good conductivity even in water. The diameter of the core particles is preferably 0.1 mm to 100 cm, and particularly preferably 1 mm to 10 cm. When the diameter of the core particles is 0.1 mm or more, it is preferable that the gaps between the particles are less likely to be clogged with dirt and the power generation characteristics can be maintained. The diameter of the core particles was calculated by the ID-C1012X Digimatic Indicator manufactured by Mitutoyo Co., Ltd., and the average value was used.

<Conductive layer>
The conductive layer is not particularly limited, but can be configured by using a composition for forming a conductive layer containing a conductive material. The composition for forming the conductive layer is preferably an ink composition, and the conductive layer can be obtained as a coating film of the ink composition.
The composition for forming a conductive layer contains at least a conductive material and a binder. In the present invention, the combined use of the conductive material and the binder can improve the conductivity of the electrode and optimize the surface morphology to improve the material diffusivity. In addition, the bond between the conductive materials and the core particles (base material) is strengthened, and the durability is improved. Moreover, among the conductive materials, the carbon material has a high affinity for microorganisms, so that many microorganisms can be supported. Due to these effects, according to the present invention, the durability can be improved in addition to the power generation characteristics of the microbial fuel cell.
The thickness of the conductive layer may be appropriately adjusted in consideration of the size of the core particles, the particle diameter of the conductive material, and the like, and is preferably 0.1 to 2000 μm, more preferably 10 to 1000 μm. The thickness of the conductive layer can be measured using, for example, Mitutoyo's Digimatic Indicator ID-C1012X.

<Conductive material>
The conductive material is not particularly limited as long as it is a material capable of transmitting electrons. Examples of the conductive material include metals, metal oxides, carbon materials, silicon carbide, conductive polymers and the like, and these may be used alone or in combination of two or more.

(Carbon material)
The carbon material increases the surface area of the electrode and promotes the transfer of electrons in the redox reaction. Further, when used as an anode, the adsorptivity of microorganisms can be enhanced due to the morphology of the surface and the biocompatibility. Examples of the carbon material include artificial graphite, natural graphite, carbon black, activated carbon, conductive carbon fiber (carbon nanotube, carbon nanofiber, carbon fiber), graphene, fullerene, carbon catalyst and the like. These can be used alone or in combination of two or more. In particular, when graphite and a carbon material other than graphite are used in combination, the electron paths inside the electrodes become dense, and the material diffusivity is enhanced by the suitable surface morphology, so that high power generation characteristics can be obtained.

The content of graphite contained in the carbon material ((graphite / carbon material) × 100) is preferably 5 to 99% by mass, more preferably 10 to 90% by mass, and further preferably 15 to 85% by mass. Is.

As the graphite, for example, artificial graphite, natural graphite, or the like can be used. Artificial graphite is made by artificially orienting irregularly arranged micrographite crystals by heat treatment of amorphous carbon, and is generally manufactured using petroleum coke or coal-based pitch coke as the main raw material. To. As the natural graphite, scaly graphite, lump graphite, earth-like graphite, spheroidal graphite obtained by spheroidizing scaly graphite by physical or chemical treatment can be used. It is also possible to use expanded graphite (also referred to as expandable graphite) obtained by chemically treating scaly graphite, or expanded graphite obtained by heat treatment and expansion of expanded graphite and then miniaturization or pressing. You can. Among these graphites, natural graphite is preferable from the viewpoint of conductivity, and flaky graphite such as spheroidal graphite, scaly graphite, expanded graphite, and flaky graphite is preferable.

The aspect ratio of graphite is preferably 1 to 15, and more preferably 1 to 5. The aspect ratio is a value represented by x / y when the longest diameter of the particle is x and the shortest diameter is y. Particles having a large aspect ratio are generally easy to be oriented, but when the aspect ratio is smaller than 15, the graphite particles on the electrode surface are difficult to be oriented and the surface unevenness becomes large, so that the surface area of the electrode increases. Especially when used as an anode, the amount of microorganisms attached increases due to the optimum surface morphology. From the viewpoint of aspect ratio, artificial graphite or spheroidal graphite is preferable as graphite.

The aspect ratio can be measured, for example, as follows. The particles are photographed with a scanning electron microscope, and x / y is obtained for 10 particles in an arbitrarily selected region, where x is the longest diameter and y is the shortest diameter of each particle. The average value of 10 x / y is used as the aspect ratio of the sample.

The average particle size of the graphite used is preferably 0.5 to 500 μm, and particularly preferably 2 to 100 μm.

The average particle size is the particle size (D50) at which the volume ratio of the particles is 50% when the volume ratio of the particles is integrated from the fine particle size distribution in the volume particle size distribution, and is a general particle size distribution. It is measured with a meter, for example, a dynamic light scattering type particle size distribution meter (“Microtrack UPA” manufactured by Nikkiso Co., Ltd.) or the like.

As commercially available graphite, for example, as scaly graphite, CMX, UP-5, UP-10, UP-20, UP-35N, CSSP, CSPE, CSP, CP, CB-150, CB manufactured by Nippon Graphite Industry Co., Ltd. -100, ACP, ACP-100, ACB-50, ACB-100, ACB-150, SP-10, SP-20, J-SP, SP-270, HOP, GR-60, LEP, F # 1, F # 2, F # 3, CX-3000, FBF, BF, CBR, SSC-3000, SSC-600, SSC-3, SSC, CX-600, CPF-8, CPF-3, CPB- made by Chuetsu Graphite Co., Ltd. 6S, CPB, 96E, 96L, 96L-3, 90L-3, CPC, S-87, K-3, CF-80, CF-48, CF-32, CP-150, CP-100, CP, HF- 80, HF-48, HF-32, SC-120, SC-80, SC-60, SC-32, EC1500, EC1000, EC500, EC300, EC100, EC50 manufactured by Ito Graphite Industry Co., Ltd. 10099M manufactured by Nishimura Graphite Co., Ltd. , PB-99 and the like. Examples of the spherical natural graphite include CGC-20, CGC-50, CGB-20, and CGB-50 manufactured by Nippon Graphite Industry Co., Ltd. Examples of earth-like graphite include blue P, AP, AOP, P # 1 manufactured by Nippon Graphite Industry Co., Ltd., and APR, S-3, AP-6, 300F manufactured by Chuetsu Graphite Co., Ltd. As artificial graphite, PAG-60, PAG-80, PAG-120, PAG-5, HAG-10W, HAG-150 manufactured by Nippon Graphite Industry Co., Ltd., RA-3000, RA-15, RA- of Chuetsu Graphite Co., Ltd. 44, GX-600, G-6S, G-3, G-150, G-100, G-48, G-30, G-50, SGP-100, SGP-50, SGP-25 manufactured by SEC Carbon Co., Ltd. , SGP-15, SGP-5, SGP-1, SGO-100, SGO-50, SGO-25, SGO-15, SGO-5, SGO-1, SGX-100, SGX-50, SGX-25, SGX -15, SGX-5, SGX-1 can be mentioned.

The carbon material other than graphite is not particularly limited, but a conductive carbon material is preferable, and carbon black or conductive carbon fiber is preferably used from the viewpoint of cost and conductivity. When the electrode is used as a cathode, it is preferable to use a carbon catalyst in order to enhance the oxygen reduction catalytic activity.

Carbon black includes furnace black, which is produced by continuously pyrolyzing gas or liquid raw materials in a reactor, especially acetylene black made from ethylene heavy oil, and the flame of the raw material is burned to the bottom of the channel steel. Various types such as channel black that has been rapidly cooled and precipitated, thermal black that is obtained by periodically repeating combustion and pyrolysis using gas as a raw material, and acetylene black that uses acetylene gas as a raw material, alone or 2 It can be used in combination with more than one type. In addition, normally oxidized carbon black, hollow carbon, and the like can also be used. The oxidation treatment of carbon black is carried out by treating the carbon black at a high temperature in the air or secondarily treating it with nitric acid, nitrogen dioxide, ozone, etc., for example, for a phenol group, a quinone group, a carboxyl group, or a carbonyl group. It is a process of directly introducing (covalently bonding) such an oxygen-containing polar functional group onto the surface of carbon black, and is generally performed in order to improve the dispersibility of carbon black. However, since the conductivity of carbon black generally decreases as the amount of the functional group introduced increases, it is preferable to use carbon black that has not been oxidized.

As the specific surface area of the carbon black used increases, the contact points between the carbon black particles increase, which is advantageous for lowering the internal resistance of the electrode, but the dispersibility of the carbon black decreases. Therefore, as a preferable range of the specific surface area, specifically, the specific surface area (BET) obtained from the amount of adsorbed nitrogen is 10 to 3000 m ² / g, more preferably 20 to 1500 m ² / g. The specific surface area can be measured by the BET method (JIS Z 8803: 2013) using nitrogen gas as an adsorbent.

The particle size of the carbon black used is preferably 0.005 to 1 μm in primary particle size, and particularly preferably 0.01 to 0.2 μm. However, the primary particle diameter referred to here is an average of the particle diameters measured by an electron microscope or the like.

Examples of commercially available carbon black include Talker Black # 4300, # 4400, # 4500, # 5500 manufactured by Tokai Carbon, Printex L manufactured by Degusa, Raven 7000, 5750, 5250, 5000 ULTRAIII and 5000 ULTRA manufactured by Colombian. Conductex SC ULTRA, Conductex 975 ULTRA, PUER BLACK100, 115, 205, # 2350, # 2400B, # 2600B, # 3050B, # 3030B, # 3230B, # 3350B, # 3400B, # 5400B, manufactured by Mitsubishi Chemical Corporation. MONARCH1400, 1300, 900, VulcanXC-72R, BlackPearls2000, Ensaco250G, Ensaco260G, Ensaco350G, SuperP-Li and other furnace blacks manufactured by TIMCAL), EC-200L, EC-300J manufactured by Lion Specialty Chemicals. Examples thereof include Ketjen black such as 600JD, Denka black manufactured by Denka, and acetylene black such as Denka Black HS-100 and FX-35, but the present invention is not limited to these, and even if two or more types are used in combination. good.

Specific examples of the activated carbon include activated carbons activated by phenol-based, coconut shell-based, rayon-based, acrylic-based, coal-petroleum-based pitch coke, mesocarbon microbeads (MCMB) and the like. Those having a large specific surface area, which can form an interface having a larger area even with the same mass, are preferable. Specifically, the specific surface area is preferably 30 m ² / g or more, more preferably 500 to 5000 m ² / g, and further preferably 1000 to 3000 m ² / g.

As the conductive carbon fiber, one obtained by firing from a raw material derived from petroleum is preferable, but one obtained by firing from a raw material derived from a plant can also be used. Further, the carbon nanotubes include a single-walled carbon nanotube in which a graphene sheet forms a tube having a diameter in a nanometer region, and a multi-walled carbon nanotube in which the graphene sheet is a multi-walled layer. The diameter of the single-walled carbon nanotubes is preferably 0.5 to 3.0 nm, and the diameter of the multi-walled carbon nanotubes is preferably 5 to 150 nm.

Examples of commercially available conductive carbon fibers and carbon nanotubes include vapor-phase carbon fibers such as VGCF manufactured by Showa Denko Co., Ltd., EC1.0, EC1.5, EC2.0, EC1.5-P manufactured by Meijo Nanocarbon Co., Ltd., etc. Examples thereof include single-walled carbon nanotubes manufactured by CNano, FloTube9000, FloTube9100, FloTube9110, FloTube9200, NC7000 manufactured by Nanocyl, and 100T manufactured by Knano.

The carbon catalyst is a carbon catalyst produced by mixing one or more kinds of carbon materials and a compound containing a nitrogen element and a base metal element and heat-treating them, and conventionally known carbon catalysts can be used. The carbon material used for the carbon catalyst is not particularly limited as long as it is a carbon particle derived from an inorganic material and / or a carbon particle obtained by heat-treating an organic material. Generally, the active points of the carbon catalyst include the base metal element contained in the base metal-N4 structure (a structure in which four nitrogen elements are arranged on a plane centering on the base metal element) on the surface of the carbon particles, and the surface of the carbon particles. Examples include carbon elements in the vicinity of nitrogen elements introduced into the edge portion. Therefore, it is important for the carbon catalyst to contain the nitrogen element and the base metal element constituting the active site in order to have the oxygen reduction activity. Further, the carbon catalyst preferably has a BET specific surface area of 20 to 2000 m ² / g, more preferably 40 to 1000 m ² / g, and even more preferably 60 to 600 m ² / g.

(Metal material)
The metal material is not particularly limited as long as it is a conductive metal material, but base metals such as aluminum, titanium, nickel, iron, tin and copper, precious metals such as silver, gold and platinum, stainless steel, etc. Alloys such as brass and duralumin can be used alone or in combination of two or more. The metal material used is preferably in the form of powder. The particle size of the metal material used is preferably 0.005 μm to 1 mm in primary particle size, and particularly preferably 0.01 to 100 μm.

The average specific surface area of the conductive material is preferably 5 to 300 m ² / g, more preferably 20 to 200 m ² / g. Here, the average specific surface area of the conductive material means the arithmetic mean of the specific surface areas of the conductive material contained in the composition for forming an anode. As an example, a case where two kinds of conductive materials C1 and C2 are used as the conductive materials is shown. The specific surface area of the conductive material C1 is A1 (m ² / g), the amount added to the composition of the conductive material C1 is B1 (g), the specific surface area of the conductive material C2 is A2 (m ² / g), and the amount added is B2. In the case of (g), the average specific surface area of the conductive material is represented by the following formula (1).
(A1 x B1 + A2 x B2) / (B1 + B2) ... Equation (1)

When the average specific surface area of the conductive material is 5 m ² / g or more, the surface area of the anode becomes large and the adsorptivity of microorganisms is improved. Further, when the average specific surface area of the conductive material is 300 m ² / g or less, the surface roughness of the anode becomes large and the adsorptivity of microorganisms is improved.

<Binder>
The type of the binder is not particularly limited as long as it can impart dispersibility of the conductive material, stability of the composition for forming the conductive layer, adhesion to the core particles, and flexibility of the electrode, and is not particularly limited. Can be mentioned.

As the binder resin, polyurethane resin, polyamide resin, acrylonitrile resin, acrylic resin, butadiene resin, polyvinyl resin, polyvinyl butyral resin, polyolefin resin, polyester resin, polystyrene resin, EVA resin, etc. It can contain one or more kinds selected from the group consisting of a polyvinylidene-based resin, a polytetrafluoroethylene-based resin, a silicone-based resin, a polyether-based resin, a cellulose-based resin such as carboxymethyl cellulose, and the like. However, it is not limited to these resins. The binder resin may be used alone or in combination of two or more. As the binder resin, a resin that is not fluoride is preferable from the viewpoint of price and adhesion.

As the binder resin, a curable resin that undergoes a curing (crosslinking) reaction can also be used. As the binder resin, a soluble resin or dispersed resin fine particles that are soluble in an aqueous or non-aqueous solvent can also be used. Dispersed resin fine particles exist in the state of fine particles without dissolving the resin fine particles in an aqueous or non-aqueous dispersion medium, and the dispersion is also generally called an emulsion. These may be used alone or in combination of two or more.

The particle structure of the dispersed resin fine particles may be a multilayer structure, so-called core-shell particles. For example, by localizing a resin in which a monomer having a functional group is mainly polymerized in the core portion or the shell portion, or by providing a difference in Tg and composition between the core and the shell, curability and drying are performed. It is possible to improve the property, film forming property, and mechanical strength of the binder. The average particle size of the resin fine particles is preferably 10 to 1000 nm, preferably 10 to 300 nm, from the viewpoint of binding properties and particle stability. Further, if a large amount of coarse particles exceeding 1 μm is contained, the stability of the particles is impaired, so that the amount of coarse particles exceeding 1 μm is preferably 5% or less at most. The average particle size of the dispersed resin fine particles can be measured by, for example, a dynamic light scattering method. The average particle size can be measured by the dynamic light scattering method as follows. Dilute 200 to 1000 times with the same dispersion as the dispersion medium, depending on the solid content of the resin fine particles. Approximately 5 ml of the diluted dispersion is injected into a cell of a measuring device (Nanotrack manufactured by Nikkiso Co., Ltd.), and the dispersion medium and the refractive index conditions of the resin corresponding to the sample are input, and then the measurement is performed. It can be measured by the peak of the volume particle size distribution data (histogram) obtained at this time.

The dispersed resin fine particles preferably include crosslinked resin fine particles. The crosslinked resin fine particles indicate resin fine particles having an internal crosslinked structure (three-dimensional crosslinked structure), and it is important that the particles are crosslinked inside the particles. Further, since the crosslinked resin fine particles contain a specific functional group, it is possible to contribute to the adhesion to other electrode constituent materials and base materials. Furthermore, by adjusting the crosslinked structure and the amount of functional groups, excellent durability of the battery can be obtained.

From the viewpoint of wettability and permeability to the electrolytic solution, as the binder, a water-soluble resin that is soluble in an aqueous solvent or an aqueous resin fine particle that does not dissolve in an aqueous dispersion medium and exists in the form of fine particles is used. It is preferable to do so. Further, from the viewpoints of slurry stability and coatability of the composition for forming a conductive layer, water resistance and flexibility of the electrode, it is more preferable to use the water-soluble resin and the water-based resin fine particles in combination.

(Water-soluble resin)
The water-soluble resin is a resin that can be completely dissolved in water without separation and precipitation after 1 g of the resin is put in 99 g of water at 25 ° C., stirred, and left at 25 ° C. for 24 hours.

Since the water-soluble resin has the effect of increasing the dispersibility of the conductive material, a stable composition can be obtained with a small amount of resin, and the conductivity of the electrode is improved. Further, when the electrode containing the water-soluble resin is immersed in water, the wettability of the electrode to the electrolytic solution is improved due to the high affinity of the water-soluble resin with water, and the flow path due to the swelling or elution of the water-soluble resin. Therefore, it is possible to provide an electrode having excellent diffusion of protons required for power generation of a microbial fuel cell. In addition, the high affinity of the water-soluble resin with microorganisms promotes the adhesion and growth of microorganisms on the anode, so that many microorganisms can be supported and the binding between microorganisms and carbon materials and base materials is strengthened. ..

Water-soluble resins are roughly classified into anionic resins, cationic resins, amphoteric resins having both anionic and cationic properties, and other nonionic resins, and the resins are further composed of a plurality of monomers. May be good. Further, the water-soluble resin may be used alone or in combination of two or more.

Examples of the anionic resin include a resin containing a carboxyl group, a sulfo group, a phosphoric acid group, and a skeleton in which some or all of them are neutralized. For example, homopolymers of polymerizable monomers such as (meth) acrylic acid, itaconic acid, fumaric acid, maleic acid, 2-sulfoethylmethacrylate, 2-methacryloyloxyethyl acid phosphate, or other polymerizable single amounts. Examples thereof include copolymers with the body, carboxymethyl cellulose, and alkali-neutralized products thereof.

Examples of the cationic resin include an amino group containing a cyclic substance, a skeleton in which a part or all of the amino group is neutralized, a resin containing a quaternary ammonium salt, and the like. For example, homopolymers of polymerizable monomers such as N, N-dimethylaminoethyl (meth) acrylate, N, N-diethyl (meth) acrylate, vinylpyridine, or co-polymerization with other polymerizable monomers. Polymers and their acid-neutralized products are included.

Examples of the amphoteric resin include a resin containing both the anionic skeleton and the cationic skeleton. Examples thereof include a copolymer of styrene-maleic acid-N, N-dimethylaminoethyl (meth) acrylate.

The nonionic resin is a resin other than the anionic, cationic and amphoteric resins. Examples thereof include polyvinylpyrrolidone, polyvinyl alcohol, polyvinyl butyral, polyacrylamide, poly-N-vinylacetamide, polyalkylene glycol and the like.

The molecular weight of the water-soluble resin is not particularly limited, but the mass average molecular weight is preferably 5,000 to 2,500,000. The mass average molecular weight (Mw) indicates a polyethylene oxide-equivalent molecular weight in gel permeation chromatography (GPC).

(Aqueous resin fine particles)
Aqueous resin fine particles are dispersed resin fine particles in which the resin does not dissolve in water and exists in the state of fine particles, and are also called water-dispersed resin fine particles. The aqueous dispersion is also commonly referred to as an aqueous emulsion.

Examples of the aqueous resin fine particles include (meth) acrylic emulsions, nitrile emulsions, urethane emulsions, polyolefin emulsions, fluorine emulsions (polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), etc.), and diene emulsions (diene emulsions). Styrene-butadiene rubber (SBR), etc.) and the like. In addition, (meth) acrylic means methacrylic or acrylic.

When a coating film is formed, the composition for forming a conductive layer containing the aqueous resin fine particles has excellent adhesion between the particles and the substrate, and can provide an electrode having high strength. Further, since the binding between the aqueous resin fine particles and the carbon material is due to point contact, it is difficult to inhibit the reaction using the interface as the reaction field. In addition, it is possible to form a surface morphology to which microorganisms easily adhere. Further, since the adhesiveness is excellent, a small amount of water-based resin fine particles is required, and as a result, the conductivity of the electrode is improved. Further, since the aqueous resin fine particles can be manufactured at low cost depending on the constituent monomers, the manufacturing cost of the electrode can be reduced. In order to obtain the above-mentioned effects, as the aqueous resin fine particles, a (meth) acrylic emulsion or a urethane emulsion having excellent binding properties and flexibility (flexibility of the film) between the particles is preferable.

The (meth) acrylic emulsion is an emulsion polymer containing 10 parts by mass or more of a monomer having a (meth) acryloyl group, preferably 20 parts by mass or more, and more preferably 30 parts by mass or more. It is good. Since the monomer having an acryloyl group has excellent reactivity, resin fine particles can be produced relatively easily. Therefore, the (meth) acrylic emulsion is particularly preferable as the aqueous resin fine particles.

<Solvent (dispersion medium)>
When the conductive material and the binder are uniformly mixed, a solvent can be appropriately used. The solvent is not particularly limited as long as it can dissolve the resin or stably disperse the resin fine particle emulsion, and examples thereof include water and organic solvents.

Organic solvents include alcohols such as methanol, ethanol, propanol, butanol, ethylene glycol methyl ether and diethylene glycol methyl ether, ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone, tetrahydrofuran, dioxane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether and the like. Suitable for the composition of the conductive composition from among ethers, hydrocarbons such as hexane, heptane and octane, aromatics such as benzene, toluene, xylene and cumene, and esters such as ethyl acetate and butyl acetate. Can be used.
Further, as the solvent, water and an organic solvent, or two or more kinds of organic solvents may be used.

When a water-soluble resin or water-based resin fine particles are used, it is preferable to use water as a solvent from the viewpoint of solubility and dispersibility, and if necessary, a liquid medium compatible with water may be added. As the liquid medium compatible with water, an alcohol solvent having 4 or less carbon atoms is preferable.

Further, the composition for forming a conductive layer of the present invention contains, if necessary, an ultraviolet absorber, an ultraviolet stabilizer, a radical catching agent, a filler, a thixotropic agent, and an antistatic agent, as long as the effects of the present invention are not impaired. Agents, Antistatics, Antistatics, Flame Retardants, Thermal Conductivity Improvers, Plasticizers, Anti-Dripping Agents, Antifouling Agents, Preservatives, Bactericides, Defoamers, Leveling Agents, Anti-blocking Agents, Hardeners, Various additives such as thickeners, dispersants, and silane coupling agents may be added.

<Composition of composition for forming a conductive layer>
The content of the conductive material is preferably 50% by mass or more, more preferably 60% by mass, based on 100% by mass of the total solid content of the composition for forming the conductive layer, from the viewpoint of conductivity, adhesion to the substrate, and the like. It is preferably ~ 99% by mass, and particularly preferably 85 to 99% by mass. When the content of the conductive material is 50% by mass or more, the electron path inside the anode is developed and sufficient conductivity is obtained. When the content of the conductive material is 60% by mass or more, the exposure of the conductive material on the surface increases the electron path between the conductive material and the microorganism, and promotes the anodic reaction, so that the power generation characteristics of the battery are improved. do.
The content of the binder is preferably 0.1 to 50% by mass, more preferably 0.5 to 40% by mass, and particularly preferably 1 with respect to 100% by mass of the total solid content of the composition for forming the conductive layer. It is ~ 15% by mass.

The composition for forming a conductive layer containing a water-soluble resin exhibits high affinity with microorganisms after forming the conductive layer. Therefore, the adhesion and growth of microorganisms to the anode are promoted, and many microorganisms can be supported. In addition, the binding between the microorganism and the conductive material or the base material is strengthened. Further, for example, when immersed in water, the wettability of the electrode to the electrolytic solution is improved due to the high affinity of the water-soluble resin with water, and the swelling of the water-soluble resin or the formation of a flow path due to the elution of the water-soluble resin. , It is possible to provide an electrode excellent in diffusion of protons required for power generation of a microbial fuel cell. Due to these effects, according to the present invention, the durability can be improved in addition to the power generation characteristics of the microbial fuel cell.
When the water-soluble resin is contained, the content thereof is preferably 0.1 to 20% by mass, more preferably 1 to 15% by mass, based on 100% by mass of the total solid content of the composition for forming the conductive layer. Is. When the content of the water-soluble resin is 0.5% by mass or more, a larger amount of microorganisms can be supported and an anode having good proton diffusivity can be obtained. Further, when the content of the water-soluble resin is 20% by mass or less, an anode having good durability in the electrolytic solution can be obtained.

When the aqueous resin fine particles are contained, the content thereof is preferably 0.1 to 40% by mass, more preferably 1 to 30% by mass, based on 100% by mass of the total solid content of the composition for forming the conductive layer. Particularly preferably, it is 1 to 10% by mass.
When the content of the aqueous resin fine particles is 1% by mass or more, the strength of the anode is improved. Further, when the content of the aqueous resin fine particles is 40% by mass or less, an anode having good conductivity can be obtained. Further, when the content of the water-based resin fine particles is 30% by mass or less, the surface morphology suitable for the adsorption of microorganisms is formed by the point binding between the water-based resin fine particles and the conductive material. Further, when the content of the aqueous resin fine particles is 10% by mass or less, the conductive material is more exposed on the surface of the coating film, so that the microorganisms and the conductive material are easily in contact with each other, and the anode reaction is promoted.
Further, when both the water-soluble resin and the water-based resin fine particles are contained, it is preferable to contain 50% by mass or more of the water-based resin fine particles with respect to the total solid content of 100% by mass, because the substance diffusivity is improved. More preferably, it is 50% by mass to 80% by mass.

The composition for forming a conductive layer may contain any component other than the conductive material and the binder, but from the viewpoint of conductivity and adhesion, the conductive material and the binder for the total solid content of the composition for forming the conductive layer. The total ratio of the above is preferably 50% by mass or more, and more preferably 80% by mass or more.

The appropriate viscosity of the electrode composition depends on the coating method of the composition, but is generally preferably 10 mPa · s or more and 30,000 mPa · s or less.

<Method for preparing a composition for forming a conductive layer>
There is no particular limitation on the method for preparing the composition for forming the conductive layer. The preparation method is
(1) Each component may be dispersed at the same time.
(2) After dispersing the conductive material in the solvent, another material may be added.
(3) After dispersing the conductive material and the binder in the solvent, another material may be added.
It can be optimized depending on the conductive material and binder used.
When using water-soluble resin or water-based resin fine particles, adding the water-based resin fine particles after dispersing the conductive material and the water-soluble resin in the water-based liquid medium can greatly contribute to cost reduction such as shortening the dispersion time. can.

<Disperser / Mixer>
As an apparatus used for obtaining the composition for forming a conductive layer, a disperser or a mixer usually used for pigment dispersion or the like can be used.

For example, mixers such as disper, homomixer, or planetary mixer; homogenizers such as "Clairemix" manufactured by M-Technique or "Philmix" manufactured by PRIMIX; paint conditioner (manufactured by Red Devil), ball mill, sand mill. Media type disperser such as (Symmal Enterprises "Dyno Mill" etc.), Atreiter, Pearl Mill (Eirich "DCP Mill" etc.) or Coball Mill; Wet Jet Mill (Genus PY "Genus PY", Sugino Medialess dispersers such as Machine's "Starburst", Nanomizer's "Nanomizer", etc.), M-Technique's "Claire SS-5", or Nara Machinery's "MICROS"; or other roll mills, etc. However, it is not limited to these.

For example, when using a media-type disperser, a method in which the agitator and vessel use a disperser made of ceramic or resin, or a disperser in which the surface of the metal agitator and vessel is treated with tungsten carbide spraying or resin coating. It is preferable to use. As the medium, it is preferable to use glass beads, zirconia beads, or ceramic beads such as alumina beads. Also, when using a roll mill, it is preferable to use a ceramic roll. As the dispersive device, only one type may be used, or a plurality of types of devices may be used in combination. Further, when the particles are easily broken or crushed by a strong impact, a medialess disperser such as a roll mill or a homogenizer is preferable to the media type disperser.

<Method of forming the conductive layer>
The method for forming the conductive layer on the surface of the core particles is not particularly limited, and a known method can be used. For example, a method of applying an adhesive to the surface of core particles to attach a metal powder such as silver powder to the surface, a method of forming a coating film of an ink composition, and the like can be mentioned. Examples of the method for forming the coating film of the ink composition include a spray coating method, a dip coating method, and the like, and examples of the drying method include leaving drying, blower dryer, warm air dryer, infrared heater, and the like. A far-infrared heater or the like can be used, but the present invention is not particularly limited to these.
The amount of ink applied is preferably 0.1 μm to 2000 μm, more preferably 10 μm to 1000 μm. If the thickness is 0.1 μm or less, it is difficult to secure conductivity, and if the thickness exceeds 2000 μm, there is a high possibility that defects such as cracks occur in the conductive layer.

<Microbial fuel cell>
The microbial fuel cell used in the present invention will be described. The microbial fuel cell of the present invention includes a cathode (positive electrode) and an anode (negative electrode). The cathode is in contact with the anode via a medium having ionic conductivity and is arranged at a position where it can come into contact with the oxidant. As the oxidizing agent, it is preferable to use oxygen existing in the atmosphere. The anode is partially or wholly installed inside the wastewater containing organic matter and is in contact with the cathode via a medium having ionic conductivity. The cathode and anode are electrically connected via a resistor. Here, the resistance indicates an electric resistance in an electric circuit.
The particulate electrode of the present invention may be used for both the cathode and the anode, or may be used for only one of them. When used for only one, it is preferable to use it for the anode. Further, it may be used in combination with an electrode other than the present invention, a plurality of types of anodes may be used for one cathode, a plurality of types of cathodes may be used for one cathode, and both the cathode and the anode may be used. It may be configured by using a plurality of types.
It is preferable to use the particulate electrode of the present invention in a state where a plurality of the particulate electrodes of the present invention are put in a container or the like and assembled. The container is preferably cup-shaped with a lid, and examples thereof include a 300 mL container. It is preferable that the particulate electrodes are densely packed in the container. Further, it is preferable to make a hole in the container through which the electrolytic solution can pass and use the entire container as an anode. Since the liquid can pass through, it is preferable to form a water flow path by, for example, a pump or the like and arrange the water flow through the electrodes. Further, it is preferable to use it together with a material generally used as a filter material, for example, activated carbon, zeolite, etc., because further purification of water and power generation can be achieved at the same time.

Electrodes other than the present invention can be configured by using a carbon material such as carbon felt. Further, a base material (carbon material such as carbon felt, metal material such as stainless steel, polymer material) coated with conductive carbon or an oxygen reduction catalyst may be used. The base material used at this time is preferably a porous material such as a mesh, a punching metal, or a non-woven fabric. From the viewpoint of promoting the reaction, the cathode preferably contains an oxygen reduction catalyst. As the oxygen reduction catalyst, a carbon catalyst is preferable from the viewpoint of durability and cost. The carbon catalyst means a catalyst containing carbon as a main component, and a conventionally known carbon catalyst can be used. In one embodiment, the carbon catalyst may be a carbon-based material doped with metallic and / or non-metallic elements. Such carbon-based materials are obtained, for example, by heat-treating a mixture of carbon particles and other components such as a metal element-containing compound.

In addition, the organic matter generally refers to what is called an organic compound, and examples thereof include organic matter in water, sediment or soil, feed and decomposition products thereof. Organic matter in water, sediment or soil includes organic matter (carbohydrates, proteins, amino acids, lignins, sugars, lipids, etc.) consisting of organisms such as plants, animals and microorganisms, biological remains such as animals and plants, and animal excreta. , Corrosive substances (corrosive acid, fluboic acid, fumin, etc.), non-biological organic substances consisting of non-corrosive substances, glucose produced by photosynthesis of plants, etc., lactic acid and acetic acid produced by the metabolism of microorganisms, etc. Organic acids, alcohols, etc. can also be mentioned. In addition, as feeds, vegetable feeds (grains such as corn and barley, oil cakes such as soybeans and peanuts, bran such as brown rice and barley, starch cakes, production meals such as beer meals, etc.) and animal feeds. (Fish flour, meat-and-bone meal, etc.), compound feeds made by mixing multiple types of feeds, etc., include carbohydrates, proteins, amino acids, lignins, sugars, lipids, etc. For example, when the number of microorganisms in the wastewater is small, the microorganisms may be planted in the anode in advance.

An example of the reaction of a microbial fuel cell when the wastewater is in an acidic state is shown. At the anode, organic matter is decomposed by the metabolism of microorganisms. At this time, the reaction represented by the following formula 1 occurs.

Organic matter + 2H ₂ O → CO ₂ + 4H ⁺ + 4e ^-... Equation 1

The electrons (e ⁻ ) generated in the reaction of Equation 1 are taken into the anode and move to the cathode side via the resistor. Further, the proton (H ⁺ ) generated in the reaction of the formula 1 passes through the wastewater and moves to the cathode side.

Further, at the cathode, electrons and protons transferred from the anode side and oxygen near the cathode undergo the reaction shown in the following formula 2.

O ₂ + 4H ⁺ + ^4e- → 2H ₂ O ・ ・ ・ Equation 2

Then, by repeating the reactions shown in the above formulas 1 and 2, an electromotive force is generated between the cathode and the anode. By such an operation, when an electron flows through a resistance, a voltage is generated at both ends according to Ohm's law, and electric energy is consumed. That is, in a microbial fuel cell, electric energy can be recovered by resistance. Therefore, as the resistance, a device that consumes or stores electric power or the like is used. Since electric power is consumed by being converted into heat, light, kinetic energy, and the like, examples of devices that consume electric power include resistors, heaters, lights, motors, and pumps. Further, examples of the device for storing electric power include a secondary battery, a capacitor, a capacitor and the like.

Hereinafter, the present invention will be described in more detail by way of examples. However, the following examples do not limit the scope of rights of the present invention at all. In the examples and comparative examples, "parts" means "parts by mass" and "%" means "% by mass".

<Cathode for microbial fuel cell>
Weigh Ketjen Black (manufactured by Lion Specialty Chemicals Co., Ltd.) and Iron Phthalocyanine P-26 (manufactured by Sanyo Pigment Co., Ltd.) so that the mass ratio is 1/0.5 (Ketjen Black / Iron Phthalocyanine), and dry. Mixing was performed to obtain a mixture. The above mixture was filled in an alumina crucible and heat-treated in an electric furnace at 600 ° C. for 2 hours in a nitrogen atmosphere to obtain a carbon catalyst.

14 parts of the above carbon catalyst, 6 parts of PVDF # 7200 (polyvinylidene fluoride, manufactured by Kureha Corporation), and 80 parts of N-methylpyrrolidone were mixed to obtain an electrode forming composition. The above electrode forming composition was applied onto carbon felt (size 10 cm × 10 cm, manufactured by Nippon Carbon Co., Ltd.) so that the basis weight was 3 mg / cm ² , and then dried in an oven to prepare a cathode. ..

<Anode for microbial fuel cell>
(Preparation of composition for forming a conductive layer)
[Manufacturing Example 1]
Mixer 17% by mass of copper powder (manufactured by Wako Pure Chemical Industries, Ltd.) as a conductive material, 3% by mass of KF polymer # 9200 (manufactured by Kureha) which is polyvinylidene fluoride as a binder, and 80% by mass of N-methylpyrrolidone as a solvent. The composition (1) for forming a conductive layer having a solid content of 20% by mass was obtained.

[Manufacturing Examples 2 to 10]
Using the conductive material, the binder, and the solvent shown in Table 1, the compositions (2) to (10) for forming the conductive layer were prepared in the same manner as the composition for forming the conductive layer (1).

(Preparation of electrodes)
The materials used as core particles are shown below.
・ Glass beads: 3 mm in diameter
・ Zirconia beads: 5 mm in diameter

[Example 1]
30 g of glass beads having a diameter of 3 mm and 3 g of the electrode composition (1) were mixed by a stirring defoaming machine. Then, it dried at 80 degreeC, and the particulate electrode (1) was obtained. The film thickness of the conductive layer of the particulate electrode (1) was 40 μm.

[Examples 2 to 11]
Using the base material shown in Table 2 and the composition for forming a conductive layer, particulate electrodes (2) to (11) were obtained in the same manner as in the particulate electrode (1). The film thickness of the conductive layer of the particulate electrodes (2) to (10) was the same as that of the particulate electrode (1). The film thickness of the conductive layer of the particulate electrode (11) was 150 μm.

[Comparative Example 1]
Spheroidal graphite CGB-50 (manufactured by Nippon Graphite Co., Ltd., average particle size 50 μm) was used as the conductive material as the particulate electrode.

(Evaluation of electrode conductivity)
30 g of the produced particulate electrode is sandwiched between carbon felts (size 5 cm x 5 cm, manufactured by Nippon Carbon Co., Ltd.), and the conductivity between the carbon felts when a load of 1 kg is applied is measured by a tester (Digidal Multitester TDE-200A, TRUSCO). Made).
The criteria for determining the resistance value are shown below.
: Less than 1.0 x 10Ω (extremely good)
〇: 1.0 x 10 Ω or more, less than 1.0 x 10 ² Ω (good)
〇 △: 1.0 × 10 ² Ω or more, less than 1.0 × 10 ³ Ω (no problem in practical use)
Δ: 1.0 × 10 ³ Ω or more, less than 1.0 × 10 ⁴ Ω (practical use is limited)
×: 1.0 × 10 ⁴ Ω or more (not practical)

Since the conductive material of the particulate electrode of the present invention is firmly adhered to the base material, an electrode having a low resistance value can be manufactured at low cost.

(Evaluation of power generation characteristics by microbial fuel cell)
After anaerobically culturing a mixed solution of Shewanella oneidenis MR-1 ⁽ single culture, 105 cells / mL) and paddy soil at 30 ° C. for 3 days as an electron donating microorganism in an electrolytic cell having a capacity of 1000 mL. A buffer solution of K ₂ HPO ₄ / KH ₂ PO ₄ (pH 7.0) was used as the electrolyte solution, and 2.0 g COD / L / day (COD: chemical oxygen demand) of domestic wastewater containing glucose as a nutrient substrate. Inflowed continuously. The cathode described in the example and the anode using the particulate electrode of the example were inserted into the electrolytic cell, respectively. The anode was used by putting up to 150 mL of particulate electrodes in a 300 mL container and applying a weight from above. Wiring was attached to the cathode and anode and connected to an external resistance (1 kΩ) to continuously operate the microbial fuel cell.

(Evaluation of power generation characteristics)
The voltage and current of the microbial fuel cell after one week were measured with a tester to calculate the output. The judgment criteria are shown below, and the results are shown in Table 2.
⊚: Power generation characteristics 25 μW or more (extremely good)
〇: Power generation characteristics 20 μW or more and less than 25 μW (good)
〇 △: Power generation characteristics 15 μW or more and less than 20 μW (no problem in practical use)
Δ: Power generation characteristics 5 μW or more and less than 15 μW (defective)
×: Power generation characteristics less than 5 μW (extremely defective)

(Durability evaluation)
The power generation characteristics at the time when the power generation characteristics were evaluated were set to 100%, and the power generation characteristics after 3 weeks were measured to calculate and evaluate the maintenance rate of the power generation characteristics. The judgment criteria are shown below, and the results are shown in Table 2.
◎: Power generation characteristic maintenance rate 80% or more (extremely good)
〇: Power generation characteristic maintenance rate 70% or more and less than 80% (good)
〇 △: Power generation characteristic maintenance rate 60% or more and less than 70% (no problem in practical use)
Δ: Power generation characteristic maintenance rate 40% or more and less than 60% (defective)
×: Power generation characteristic maintenance rate less than 40% (extremely poor)

From Table 2, it was shown that the durability was significantly reduced in the comparative example as compared with the example. In Comparative Example 1, the resistance value was low by using the spheroidal graphite itself, but it seems that it was difficult to maintain the binding between the graphites when flooded. Further, since the spheroidal graphite having an average particle diameter of less than 100 μm, the gaps between the particles are small, so that dirt is easily clogged, and it is considered that the durability and power generation characteristics are significantly deteriorated.
In addition, the combination of the conductive material and the water-soluble resin increased the power generation characteristics and durability. It is considered that this is because the presence of the water-soluble resin near the surface of the electrode enhances the affinity for microorganisms and increases the amount of microorganisms supported, so that the power generation characteristics and durability are improved. In addition, the high affinity of the water-soluble resin with water improves the wettability of the electrode with respect to the electrolytic solution, and the swelling of the water-soluble resin or the formation of a flow path due to the elution of the water-soluble resin improves the diffusion of protons. It is considered that the power generation characteristics have improved. Further, when the content of the conductive material in the composition was 85% by mass or more with respect to the total solid content of 100% by mass, higher power generation characteristics were exhibited. Since there are sufficient electron paths between the conductive materials, it is considered that the electron paths between the conductive materials and the microorganisms increased due to the exposure of more conductive materials on the surface.

Further, it was considered that the combination of graphite and other carbon materials as the conductive material improved the battery performance, and thus the surface morphology suitable for the adsorption of microorganisms was formed. Further, when the average specific surface area of the conductive material is 5 m ² / g or more, the power generation characteristics are improved. It is considered that this is because the surface area of the anode has increased and the adsorptivity of microorganisms has improved. Further, when the average specific surface area was 300 m ² / g or less, the durability was improved. It is considered that the surface roughness of the anode has increased and the adsorptivity of microorganisms has improved.

In addition, a biofilm formed by power generation bacteria or the like was formed on the surface of each electrode, but it was difficult to remove it in the form of Comparative Example 1. On the other hand, the conductive beads of the present invention could be used as an electrode without any problem even after the biofilm could be washed away with water. By using the particulate electrode of the present invention, it is possible to provide a microbial fuel cell having excellent recyclability.

1 Core particle 2 Conductive layer 3 Particle-like electrode 4 Electrode 5 Cathode electrode 6 External resistance 7 Electrolyte solution 8 Container

Claims (4)

A particulate electrode having core particles and a conductive layer containing a conductive material formed on the surface of the core particles. The particulate electrode according to claim 1, wherein the conductive layer is a coating film of an ink composition. The particulate electrode according to claim 1 or 2, wherein the core particles are non-conductive particles. A microbial fuel cell using the particulate electrode according to any one of claims 1 to 3.

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