JP7559490B2 - Composition for forming an electrode for a microbial fuel cell, an electrode for a microbial fuel cell, and a microbial fuel cell - Google Patents

Description

The present invention relates to a composition for forming an electrode for a microbial fuel cell, an electrode for a microbial fuel cell, and a microbial fuel cell.

In recent years, the development of biofuel cells, such as biofuel cells using enzymes as biocatalysts and microbial fuel cells using microorganisms, has been progressing. Microbial fuel cells are a power generation method that uses electrons generated when organic matter is decomposed by microorganisms called power-generating bacteria to recover and use them as electrical energy. For example, when used to purify domestic or industrial wastewater, the decomposition process of organic matter in the wastewater and power generation can be carried out in parallel, and it is expected to be used as a water treatment method that can reduce energy consumption (Non-Patent Document 1). In addition, since microbial fuel cells can generate power using organic matter contained in water, bottom mud, soil, etc., they can also be expected to be used as a power source in wastewater or soil (Patent Document 1, Non-Patent Document 2).

In a microbial fuel cell, electrons are generated from an oxidation reaction by electricity-generating bacteria that breaks down organic matter at the anode to extract electrons, and the generated electrons are then consumed in a reduction reaction at the cathode via an external circuit from the electrode. At the cathode, oxygen and oxidants such as potassium ferricyanide are reacted.

The cathode and anode materials used in microbial fuel cells need to be conductive in the electrode reaction, and conductive materials are generally used (Patent Documents 1 to 3). Electrodes are made of carbon materials such as carbon paper, carbon felt, carbon cloth, or activated carbon sheets, or metal materials such as aluminum, copper, stainless steel, nickel, or titanium. In the anode, electricity-generating bacteria are used by contacting these materials (Patent Documents 2 and 3). In the cathode, it is also known to use these materials with oxygen reduction catalysts such as platinum supported on them. The efficiency of these electrode reactions is still insufficient, and improving the power generation characteristics is an issue. There are also problems with deterioration due to metal oxidation and high costs.

To solve these problems, it is necessary to improve the electrode reaction efficiency, and there is a demand for improving electrical conductivity and material diffusivity. Furthermore, it is desirable to increase the amount and activity of microorganisms supported on the anode. For example, Patent Document 4 discloses a laminate in which a non-conductive substrate is coated with a conductive portion containing a conductive carbon material and a resin. However, further improvement of power generation efficiency remains an issue, and there is a demand for technology that can achieve both durability and the ability to maintain stable power generation efficiency.

JP 2004-342412 A JP 2010-102953 A JP 2015-002070 A International Publication No. 2016/129678

The cutting edge of wastewater treatment systems using microbial fuel cells, NTS Corporation Environmental Science & Technology, 2004, 38, 4040-4046

The object of the present invention is to provide an electrode-forming composition, an electrode, and a microbial fuel cell that have excellent power generation characteristics and durability at low cost.

The inventors conducted extensive research to solve the above problems and came up with the present invention.

That is, the present invention relates to a composition for forming an electrode for a microbial fuel cell, which comprises a carbon material (A) and a binder (B), and the carbon material (A) comprises graphite (A-a) and a carbon material other than graphite (A-b).

The present invention also relates to the composition for forming an electrode for a microbial fuel cell, in which the content of graphite (A-a) in a total of 100% by mass of the carbon material (A) is 25 to 99% by mass.

The present invention also relates to the composition for forming an electrode for a microbial fuel cell, in which the content of the carbon material (A) is 80 to 99 mass% based on 100 mass% of the total solid content of the composition.

The present invention also relates to the composition for forming a microbial fuel cell electrode, in which the aspect ratio of the graphite (A-a) is 1 to 10.

The present invention also relates to an electrode for a microbial fuel cell having a coating film of the composition for forming an electrode for a microbial fuel cell on a substrate.

The present invention also relates to a microbial fuel cell using the above-mentioned microbial fuel cell electrode.

The present invention provides an electrode-forming composition and electrodes with excellent power generation and durability at low cost, and can provide a microbial battery with excellent power generation characteristics and durability.

FIG. 1 is a diagram showing the structure of a microbial fuel cell used in the present invention.

The following describes an embodiment of the present invention in detail.

<Composition for forming microbial fuel cell electrode>
The composition for forming a microbial fuel electrode (hereinafter, referred to as an electrode composition) of the present invention contains at least graphite (Aa), a carbon material other than graphite (Ab), and a binder (B). By using graphite (A-a), a carbon material other than graphite (A-b) and a binder (B) in combination, the electrical conductivity of the electrode can be improved, and the surface morphology can be optimized to improve the material diffusion. Therefore, the adhesion between the carbon materials (A) and the substrate is strengthened, and durability is improved. In addition, since the affinity between the carbon materials (A) in the electrode and microorganisms is high, a large number of microorganisms can be supported. Due to these effects, according to the present invention, it is possible to improve not only the power generation characteristics but also the durability of the microbial fuel cell.

<Carbon material (A)>
The carbon material (A) increases the surface area of the electrode and promotes the transfer of electrons in the redox reaction. In addition, when used as an anode, the surface morphology and biocompatibility can enhance the adsorption of microorganisms. The carbon material (A) contains graphite (A-a) and a carbon material other than graphite (A-b). Examples of the graphite (A-a) include artificial graphite and natural graphite, and examples of the carbon material other than graphite (A-b) include carbon black, activated carbon, conductive carbon fibers (carbon nanotubes, carbon nanofibers, carbon fibers), graphene, fullerene, and carbon catalysts. These can be used alone or in combination of two or more types. By using the graphite (A-a) and the carbon material other than graphite (A-b) in combination, the electron path inside the electrode becomes dense, and the material diffusibility is also increased due to the favorable surface morphology, so that high power generation characteristics can be obtained.

The content of graphite (A-a) in the carbon material (A) ((A-a)/(A) x 100) is preferably 25 to 99% by mass, more preferably 50 to 90% by mass, and even more preferably 60 to 85% by mass.

As graphite (A-a), for example, artificial graphite and natural graphite can be used. Artificial graphite is made by artificially aligning irregularly arranged micrographite crystals by heat treatment of amorphous carbon, and is generally produced using petroleum coke or coal-based pitch coke as the main raw material. Natural graphite is classified into scaly graphite, semi-scaly graphite, and earthy graphite, and scaly graphite includes scaly graphite and lump graphite. In addition, spherical graphite obtained by scaly graphite being scaly graphite scaly graphite being scaly graphite by physical and chemical treatment, expanded graphite (also called expandable graphite) obtained by chemically treating scaly graphite, and expanded graphite obtained by heat treating expanded graphite to expand it and then refining or pressing it can also be used. Among these graphites, natural graphite is preferred from the viewpoint of electrical conductivity, and flaky graphite such as spherical graphite, scaly graphite, and expanded graphite are preferred.

The aspect ratio of graphite (A-a) is preferably 1 to 10, and more preferably 1 to 5. The aspect ratio is a value expressed as x/y, where x is the longest diameter of the particle and y is the shortest diameter. Particles with a large aspect ratio are generally more easily oriented, but when the aspect ratio is less than 10, the graphite particles on the electrode surface are less likely to be oriented and the surface becomes more uneven, which increases the surface area of the electrode and the number of reaction sites. In particular, when used as an anode, the amount of attached microorganisms increases due to the optimal surface morphology. From the viewpoint of aspect ratio, artificial graphite and spherical graphite are preferred as graphite (A-a).

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

The average particle size of the graphite (A-a) used is preferably 0.5 to 500 μm, and more preferably 2 to 100 μm.

The average particle size is the particle size (D50) at which the volumetric percentage of particles is 50% when the volumetric percentage of particles is accumulated starting from the finest particle size in the volumetric particle size distribution, and is measured using a general particle size distribution analyzer, such as a dynamic light scattering particle size distribution analyzer (Microtrac UPA, manufactured by Nikkiso Co., Ltd.).

Examples of commercially available graphite include flake graphite such as CMX, UP-5, UP-10, UP-20, UP-35N, CSSP, CSPE, CSP, CP, CB-150, CB-100, ACP, ACP-1000, ACB-50, ACB-100, ACB-150, SP-10, SP-20, J-SP, SP-270, HOP, GR-60, LEP, F#1, F#2, F#3, and FB-150 manufactured by Nippon Graphite Industries Co., Ltd., and CX-3000, FBF, BF, CBR, SSC-3000, and SSC-600 manufactured by Chuetsu Graphite Co., Ltd. , SSC-3, SSC, CX-600, CPF-8, CPF-3, CPB-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 Industries Co., Ltd., 10099M, PB-99 manufactured by Nishimura Graphite Co., Ltd., and the like. Examples of spherical graphite include CGC-20, CGC-50, CGB-20, and CGB-50 manufactured by Nippon Graphite Industries Co., Ltd. Examples of amorphous graphite include Blue P, AP, AOP, and P#1 manufactured by Nippon Graphite Industries Co., Ltd., and APR, S-3, AP-6, and 300F manufactured by Chuetsu Graphite Co., Ltd. Examples of artificial graphite include PAG-60, PAG-80, PAG-120, PAG-5, HAG-10W, and HAG-150 manufactured by Nippon Graphite Industries Co., Ltd., and RA-3000, RA-15, RA-44, GX-600, G-6S, G-3, G-150, G-100, G-48, G-30, G-50, and SEC-1 manufactured by Chuetsu Graphite Co., Ltd. Examples include SGP-100, SGP-50, SGP-25, 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, and SGX-1 manufactured by Carbon.

The carbon material (A-b) other than graphite is not particularly limited, but a conductive carbon material is preferred, and from the standpoint of cost, conductivity, etc., it is preferable to use carbon black or conductive carbon fiber. In addition, when the electrode is used as a cathode, it is preferable to use a carbon catalyst in order to increase the oxygen reduction catalytic activity.

Carbon black can be produced by continuously pyrolyzing a gaseous or liquid raw material in a reactor, such as furnace black, which is produced by pyrolyzing the gaseous or liquid raw material continuously in a reactor, particularly ketjen black, which is produced from ethylene heavy oil, channel black, which is produced by burning the raw material gas and applying the flame to the bottom surface of the channel steel to rapidly cool and precipitate, and thermal black, which is produced by periodically repeating combustion and pyrolysis of a gas raw material, particularly acetylene black, which is produced from acetylene gas. Carbon black that has been subjected to the usual oxidation treatment and hollow carbon can also be used. The oxidation treatment of carbon black is a treatment in which oxygen-containing polar functional groups such as phenol groups, quinone groups, carboxyl groups, and carbonyl groups are directly introduced (covalently bonded) to the carbon black surface by treating the carbon black at high temperatures in air or by treating it secondarily with nitric acid, nitrogen dioxide, ozone, etc., and is generally performed to improve the dispersibility of carbon black. However, it is generally the case that the conductivity of carbon black decreases as the amount of functional groups introduced increases, so it is preferable to use carbon black that has not been subjected to the oxidation treatment.

The larger the specific surface area of the carbon black used, the more contact points between the carbon material particles there are, which is advantageous for lowering the internal resistance of the electrode, but the dispersibility of the carbon black decreases. Therefore, the preferred range of the specific surface area is specifically 10 to 3000 m ² /g, more preferably 20 to 1500 m ² /g, in terms of the specific surface area (BET) calculated from the amount of nitrogen adsorption. The specific surface area can be measured by the BET method (JIS Z 8803:2013) using nitrogen gas as the adsorbate.

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

Examples of commercially available carbon black include Toka Black #4300, #4400, #4500, and #5500 manufactured by Tokai Carbon Co., Ltd., Printex L manufactured by Degussa Co., Ltd., and Raven 7000, 5750, 5250, 5000 ULTRA III, and 500 manufactured by Columbian Co., Ltd.
0ULTRA, Conductex SC ULTRA, Conductex 975 ULTRA, PUER BLACK 100, 115, 205, Mitsubishi Chemical Corporation's #2350, #2400B, #2600B, #3050B, #3030B, #3230B, #3350B, #3400B, #5400B, Cabot Corporation's MONARCH 1400, 1300, 900, Vulcan XC-72R, Black Pearls 2000, TIMCAL Corporation's Ensaco 250G, Ensaco 260G, Examples of the black include furnace blacks such as Ensaco 350G and Super P-Li manufactured by Kuraray Co., Ltd.; ketjen blacks such as EC-200L, EC-300J and EC-600JD manufactured by Lion Specialty Chemicals; and acetylene blacks such as Denka Black, Denka Black HS-100 and FX-35 manufactured by Denka Co., Ltd., but are not limited to these. Two or more of these may be used in combination.

Specific examples of activated carbon include activated carbons obtained by activating phenol-based, coconut shell-based, rayon-based, acrylic-based, coal-petroleum pitch coke, mesocarbon microbeads (MCMB), etc. Activated carbons with a large specific surface area are preferred, since they are capable of forming a larger interface area with the same mass. Specifically, a specific surface area of 30 ^m2 /g or more is preferred.
The surface area is more preferably from 500 to 5,000 ^{m 2} /g, and further preferably from 1,000 to 3,000 m ² /g.

Conductive carbon fibers obtained by burning petroleum-derived raw materials are preferable, but those obtained by burning plant-derived raw materials can also be used. Carbon nanotubes are classified into single-walled carbon nanotubes, which form tubes with a diameter in the nanometer range from a single graphene sheet, and multi-walled carbon nanotubes, which have multiple graphene sheets. Therefore, the diameter of multi-walled carbon nanotubes is large, at 30 nm, compared to the 0.7 to 2.0 nm of typical single-walled carbon nanotubes.

Commercially available conductive carbon fibers and carbon nanotubes include vapor grown carbon fibers such as VGCF manufactured by Showa Denko K.K., single-walled carbon nanotubes such as EC1.0, EC1.5, EC2.0, and EC1.5-P manufactured by Meijo Nano Carbon Co., Ltd., FloTube 9000, FloTube 9100, FloTube 9110, and FloTube 9200 manufactured by CNano Corporation, NC7000 manufactured by Nanocyl Corporation, and 100T manufactured by Knano Corporation.

The carbon catalyst is a carbon catalyst produced by mixing one or more kinds of carbon materials with a compound containing a nitrogen element and a base metal element and subjecting the mixture to a heat treatment, and a conventionally known carbon catalyst can be used. The carbon material used for the carbon catalyst is not particularly limited as long as it is a carbon particle obtained by subjecting an inorganic material-derived carbon particle and/or an organic material to a heat treatment. In general, examples of the active site of the carbon catalyst include a base metal element contained in a base metal-N4 structure (a structure in which four nitrogen elements are arranged on a plane with a base metal element at the center) on the surface of the carbon particle, and a carbon element in the vicinity of a nitrogen element introduced to an edge portion of the carbon particle surface. Therefore, it is important for the carbon catalyst to contain the nitrogen element and the base metal element that constitute the active site in order to have oxygen reduction activity. Furthermore, 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.

The average specific surface area of the carbon material (A) is preferably 5 to 300 m ² /g, and more preferably 5 to 100 m ² /g. Here, the average specific surface area of the carbon material (A) refers to the arithmetic mean of the specific surface areas of two or more types of carbon materials (A) contained in the electrode composition. As an example, a case will be shown in which C1 and C2 are used as the carbon materials (A). When the specific surface area of C1 is A1 m ² /g, the amount of C1 added in the composition is B1 g, and the specific surface area of C2 is A2 m ² /g, and the amount added is B2 g, the average specific surface area of the carbon material (A) is expressed by the following formula 1.
(A1×B1+A2×B2)/(B1+B2) …Formula 1

When the average specific surface area of the carbon material (A) is 5 m ² /g or more, the surface area of the electrode is increased, and the redox reaction is promoted. When the average specific surface area of the carbon material (A) is 300 m ² /g or less, the strength of the electrode is improved. When the average specific surface area of the carbon material (A) is 100 m ² /g or less, the surface morphology of the electrode becomes rough, and the adsorption of microorganisms is improved.

<Binder (B)>
The type of binder (B) is not particularly limited as long as it can impart dispersibility of the carbon material (A), stability of the electrode composition, adhesion to the substrate, and flexibility to the electrode, and examples of the binder include binder resins.

The binder resin may include one or more selected from the group consisting of polyurethane resins, polyamide resins, acrylonitrile resins, acrylic resins, butadiene resins, polyvinyl resins, polyvinyl butyral resins, polyolefin resins, polyester resins, polystyrene resins, EVA resins, polyvinylidene fluoride resins, polytetrafluoroethylene resins, silicone resins, polyether resins, cellulose resins such as carboxymethyl cellulose, etc. However, the binder resin is not limited to these resins. One type of binder resin may be used alone, or two or more types may be used in combination. As the binder resin, a non-fluoride resin is preferable from the viewpoint of cost and adhesion.

The binder resin may be a curable resin that undergoes a curing (crosslinking) reaction after the binder resin is applied to the substrate. The binder resin may be a soluble resin that dissolves in an aqueous or non-aqueous solvent, or a dispersion-type resin microparticle. Dispersion-type resin microparticles are resin microparticles that do not dissolve in an aqueous or non-aqueous dispersion medium and exist in the form of fine particles, and such dispersions are generally also called emulsions. These may be used alone or in combination of two or more types.

The particle structure of the dispersed resin microparticles can be a multi-layer 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 or shell, or by providing a difference in Tg or composition between the core and shell, the curability, drying property, film-forming property, and mechanical strength of the binder can be improved. From the viewpoint of binding property and particle stability, the average particle diameter of the resin microparticles is preferably 10 to 1000 nm, and more preferably 10 to 300 nm. In addition, since the stability of the particles is impaired when a large amount of coarse particles exceeding 1 μm are contained, it is preferable that the amount of coarse particles exceeding 1 μm is at most 5% or less. The average particle diameter can be measured as follows. Depending on the solid content of the resin microparticles, the resin is diluted 200 to 1000 times with the same dispersion liquid as the dispersion medium. Approximately 5 ml of the diluted dispersion liquid is poured into the cell of a measuring device (Nanotrack manufactured by Nikkiso Co., Ltd.), and the dispersion medium and the refractive index conditions of the resin according to the sample are input, and then the measurement is performed. This can be measured based on the peak of the volume particle size distribution data (histogram) obtained at this time.

The dispersed resin particles preferably contain crosslinked resin particles. Crosslinked resin particles refer to resin particles having an internal crosslinked structure (three-dimensional crosslinked structure), and it is important that the particles are crosslinked inside. In addition, the crosslinked resin particles contain specific functional groups, which can contribute to adhesion with other electrode constituent materials and substrates. Furthermore, by adjusting the amount of the crosslinked structure and functional groups, excellent durability of the battery can be obtained.

From the viewpoint of wettability and permeability to the electrolyte, it is preferable to use, as the binder (B), a water-soluble resin (B-a) that can be dissolved in an aqueous solvent, or aqueous resin microparticles (B-b) that do not dissolve in an aqueous dispersion medium and exist in the form of fine particles. Furthermore, from the viewpoint of the slurry stability and coatability of the electrode composition, and the water resistance and flexibility of the electrode, it is even more preferable to use a combination of the water-soluble resin (B-a) and the aqueous resin microparticles (B-b).

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

The water-soluble resin (B-a) has the effect of increasing the dispersibility of the carbon material (A), so a stable composition can be obtained with a small amount of resin, and the conductivity of the electrode is improved. In addition, when an electrode containing the water-soluble resin (B-a) is immersed in water, the high affinity of the water-soluble resin for water improves the wettability of the electrode to the electrolyte, and a flow path is formed due to the swelling and elution of the water-soluble resin, so that an electrode with excellent diffusion of protons necessary for power generation in a microbial fuel cell can be provided. In addition, the high affinity of the water-soluble resin (B-a) for microorganisms promotes the attachment and growth of microorganisms to the anode, so that a large number of microorganisms can be supported, and the binding between the microorganisms and the carbon material (A) and the substrate is also strengthened.

The water-soluble resin (B-a) is broadly classified into anionic resins, cationic resins, amphoteric resins that have both anionic and cationic properties, and other nonionic resins, and the resin may be composed of multiple monomers. Furthermore, the water-soluble resin (B-a) may be used alone or in combination of two or more types.

Examples of anionic resins include resins containing a skeleton containing a carboxyl group, a sulfo group, a phosphate group, or a partially or fully neutralized skeleton of these groups. Examples include homopolymers of polymerizable monomers such as (meth)acrylic acid, itaconic acid, fumaric acid, maleic acid, 2-sulfoethyl methacrylate, and 2-methacryloyloxyethyl acid phosphate, or copolymers with other polymerizable monomers, carboxymethyl cellulose, and alkali-neutralized products thereof.

Examples of cationic resins include resins containing cyclic amino groups, skeletons in which some or all of the amino groups have been neutralized, and quaternary ammonium salts. Examples include homopolymers of polymerizable monomers such as N,N-dimethylaminoethyl (meth)acrylate, N,N-diethyl (meth)acrylate, and vinylpyridine, or copolymers with other polymerizable monomers, and acid-neutralized products thereof.

Examples of amphoteric resins include resins that contain both the anionic and cationic skeletons. Examples include copolymers of styrene, maleic acid, and N,N-dimethylaminoethyl (meth)acrylate.

Nonionic resins are resins other than the anionic, cationic and amphoteric resins mentioned above. Examples include polyvinylpyrrolidone, polyvinyl alcohol, polyvinyl butyral, polyacrylamide, poly-N-vinyl acetamide, polyalkylene glycol, etc.

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

(Aqueous resin fine particles (B-b))
The aqueous resin particles are dispersion-type resin particles in which the resin does not dissolve in water but exists in the form of fine particles, and are also called water-dispersed resin particles. The aqueous dispersion is generally called an aqueous emulsion.

Examples of aqueous resin microparticles (B-b) include (meth)acrylic emulsions, nitrile emulsions, urethane emulsions, polyolefin emulsions, fluorine-based emulsions (such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE)), diene emulsions (such as styrene-butadiene rubber (SBR)), etc. Note that (meth)acrylic means methacrylic or acrylic.

When a coating film is formed, the electrode composition containing the aqueous resin fine particles (B-b) has excellent adhesion between particles and with the substrate, and can provide a strong electrode. In addition, since the aqueous resin fine particles (B-b) and the carbon material (A) are bonded by point contact, it is difficult to inhibit the reaction in which the interface is the reaction field. In addition, it is possible to form a surface morphology to which microorganisms can easily attach. Furthermore, since the aqueous resin fine particles (B-b) have excellent adhesion, only a small amount of the aqueous resin fine particles (B-b) is required, and as a result, the conductivity of the electrode is improved. Furthermore, since the aqueous resin fine particles can be produced inexpensively depending on the monomer that constitutes them, the manufacturing cost of the electrode can be reduced. In order to obtain the above-mentioned effects, it is preferable to use a (meth)acrylic emulsion or a urethane emulsion as the aqueous resin fine particles (B-b), which have excellent adhesion between particles and flexibility (flexibility of the film).

A (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. A monomer having an acryloyl group has excellent reactivity, so resin microparticles can be produced relatively easily. Therefore, a (meth)acrylic emulsion is particularly preferable as the aqueous resin microparticles (B-b).

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

The organic solvent can be appropriately selected from among 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; ethers such as tetrahydrofuran, dioxane, ethylene glycol dimethyl ether, and diethylene glycol dimethyl ether; hydrocarbons such as hexane, heptane, and octane; aromatics such as benzene, toluene, xylene, and cumene; and esters such as ethyl acetate and butyl acetate, depending on the composition of the conductive composition.
The solvent may be water and an organic solvent, or two or more organic solvents may be used.

When using a water-soluble resin (B-a) or aqueous resin microparticles (B-b), it is preferable to use water as the solvent from the viewpoint of solubility and dispersibility, and a liquid medium compatible with water may be added as necessary. As a liquid medium compatible with water, an alcohol-based solvent having a carbon number of 4 or less is preferable.

In addition, various additives such as ultraviolet absorbers, ultraviolet stabilizers, radical scavengers, fillers, thixotropy-imparting agents, antiaging agents, antioxidants, antistatic agents, flame retardants, thermal conductivity improvers, plasticizers, antisagging agents, antifouling agents, preservatives, bactericides, defoamers, leveling agents, antiblocking agents, hardeners, thickeners, dispersants, and silane coupling agents may be added to the electrode composition of the present invention, if necessary, within the scope of not impairing the effects of the present invention.

<Composition of electrode composition>
The content of the carbon material (A) is preferably 50% by mass or more, more preferably 80 to 99% by mass, and particularly preferably 85 to 99% by mass, based on the total of 100% by mass of the total solid content of the electrode composition, in terms of electrical conductivity and adhesion to the substrate. When the content of the carbon material (A) is 50% by mass or more, the electron path inside the electrode is developed, and sufficient electrical conductivity is obtained. When the content of the carbon material (A) is 80% by mass or more, the carbon material (A) is exposed on the surface, increasing the electron path between the carbon material (A) and the microorganisms, and promoting the anode reaction, thereby improving the power generation characteristics of the battery.
From the viewpoint of electrical conductivity, the content of graphite (A-a) is preferably 25 to 99 mass%, more preferably 50 to 90 mass%, and particularly preferably 60 to 85 mass% relative to 100 mass% of the total carbon material (A). By using graphite (A-a) and a carbon material other than graphite (A-b) in combination within a suitable range, packing inside the electrode is improved and electrical conductivity is improved. Furthermore, since a suitable surface morphology is formed, substance diffusibility and microbial adsorption are enhanced, reactivity is improved, and high power generation characteristics are obtained.
The content of the binder (B) is preferably 0.1 to 50 mass%, more preferably 0.5 to 20 mass%, and particularly preferably 1 to 15 mass%, relative to 100 mass% of the total solid content of the electrode composition in terms of electrical conductivity, adhesion to a substrate, and the like.

When the water-soluble resin (Ba) is contained, the content thereof is preferably 0.1 to 30 mass%, more preferably 0.5 to 20 mass%, and particularly preferably 1 to 15 mass%, based on 100 mass% of the total solid content of the electrode composition. When the content of the water-soluble resin is 0.1 mass% or more, as described above, a larger amount of microorganisms can be supported, and an anode with good proton diffusivity can be obtained. Furthermore, when the content of the water-soluble resin is 30 mass% or less, an anode with good durability in the electrolyte can be obtained.
When the aqueous resin fine particles (B-b) are contained, the content thereof is preferably 0.5 to 40 mass%, more preferably 1 to 30 mass%, and particularly preferably 2 to 25 mass%, based on 100 mass% of the total solid content of the electrode composition. When the content of the aqueous resin fine particles is 0.5 mass% or more, the strength of the anode is improved. When the content of the aqueous resin fine particles is 40 mass% or less, good electrical conductivity is obtained. Furthermore, when the content of the aqueous resin fine particles is 30 mass% or less, a surface morphology suitable for adsorption of microorganisms is formed by point-bonding between the aqueous resin fine particles and the carbon material (A). Furthermore, when the content of the aqueous resin fine particles is 25 mass% or less, a larger amount of the carbon material (A) is exposed on the coating surface, and thus the oxidation-reduction reaction is promoted.
In addition, when both the water-soluble resin (Ba) and the aqueous resin fine particles (Bb) are contained, it is preferable to contain more aqueous resin fine particles (Bb) than the water-soluble resin (Ba), since this improves the substance diffusibility.

The electrode composition may contain any components other than the carbon material (A) and the binder (B), but from the viewpoint of electrical conductivity, adhesion, etc., the total ratio of the carbon material (A) and the binder (B) to the total solid content of the electrode composition is preferably 50 mass% or more, and more preferably 80 mass% or more.

The appropriate viscosity of the electrode composition depends on the coating method of the composition, but in general, it is preferable to set it to 10 mPa·s or more and 30,000 mPa·s or less.

<Method of preparing electrode composition>
There is no particular limitation on the method for preparing the electrode composition.
(1) Each component may be dispersed simultaneously;
(2) After dispersing the carbon material (A) in the solvent, other materials may be added.
(3) After dispersing the carbon material (A) and the binder (B) in a solvent, other materials may be added.
This can be optimized by the carbon material (A) and binder (B) used.
In the case of using the water-soluble resin (Ba) or the aqueous resin fine particles (B-b), adding the aqueous resin fine particles (B-b) after dispersing the carbon material (A) and the water-soluble resin (Ba) in the aqueous liquid medium can contribute greatly to cost reduction, for example by shortening the dispersion time.

<Dispersing machine/mixing machine>
As the apparatus used in obtaining the electrode composition, a dispersing machine or a mixer that is generally used for dispersing pigments or the like can be used.

For example, mixers such as a Disper, Homomixer, or Planetary Mixer; homogenizers such as M Technique's "Clearmix" or PRIMIX's "Filmix"; media-type dispersers such as a paint conditioner (Red Devil), ball mill, sand mill (Shinmaru Enterprises' "Dynomill" etc.), attritor, pearl mill (Eirich's "DCP Mill" etc.), or Coball mill; media-less dispersers such as wet jet mills (Genus PY, Sugino Machine's "Starburst", Nanomizer Inc.'s "Nanomizer", etc.), M Technique's "Clear SS-5", or Nara Machine's "MICROS"; or other roll mills, but are not limited to these.

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

<Electrode manufacturing method>
Next, a method for producing an electrode from the electrode composition will be described. The electrode can be produced using a conventionally known method, and is not particularly limited. For example,
(1) An electrode prepared by applying an electrode composition to a substrate,
(2) An electrode obtained by forming the electrode composition into a pellet shape,
(3) An electrode prepared by wrapping a pellet-shaped electrode composition in a conductive substrate; (4) An electrode prepared by encapsulating a pellet-shaped electrode composition in a conductive substrate having a frame or basket shape;
etc.
In particular, for anodes involving microorganisms, for example, the anode may be prepared after mixing the microorganisms into the electrode composition, or the electrode may be prepared in advance as described above, and then the microorganisms may be added or inoculated. Also, the electrode may be placed in soil and then function as an anode by adsorption of the microorganisms in the soil.

<Substrate>
The substrate used in the electrode may be a conductive substrate or a non-conductive substrate.

The conductive substrate functions as a current collector. The conductive substrate is preferably one that has excellent corrosion resistance and electrical conductivity, a large surface area, and excellent diffusion of reactants and products, and there are no particular limitations on the material or shape. For example, carbon materials such as graphite paper (carbon paper), graphite cloth (carbon cloth), and graphite felt (carbon felt) can be used, as well as metal materials such as stainless steel mesh, copper mesh, and platinum mesh, but this is not limited thereto. The conductive substrate used for the electrode may be previously treated to be water-repellent. For example, the cathode is impregnated with a dispersion of PTFE, and the water-repellent property is expressed by heating the PTFE dispersion at about 400°C after drying. A conductive material may also be dispersed in the PTFE dispersion. Note that the water-repellent treatment is not limited to these.

In addition, a composition capable of forming a conductive film or a conductive polymer such as polyaniline, polyacetylene, polypyrrole, or polythiophene may be applied to a non-conductive support such as paper, cloth, or resin film, and then dried, or a combination of these may be used.

The non-conductive substrate can be the same as the non-conductive support described above.

As the substrate, conductive substrates with a non-conductive support and non-conductive substrates are preferred from the standpoint of cost and corrosion resistance.

<Coating method>
The method of applying (coating) the electrode composition of the present invention to a substrate is not particularly limited, and any known method can be used. Examples include gravure coating, spray coating, screen printing, dip coating, die coating, roll coating, doctor coating, knife coating, screen printing, and electrostatic painting, and drying methods include, but are not limited to, standing drying, air blowing dryer, hot air dryer, infrared heater, far infrared heater, and the like.

The volume resistivity of the electrode for a microbial fuel cell of the present invention is preferably less than 5×10 ¹ Ω·cm, more preferably less than 5×10 ⁰ Ω·cm, and even more preferably less than 1×10 ⁻¹ Ω·cm. The better the conductivity of the electrode, the more effectively it is possible to extract electric power.

<Microbial fuel cell>
As shown in FIG. 1, the microbial fuel cell of the present invention uses an anode, a cathode, an electrolyte, an external resistor, and the like. The anode contains a microorganism, and the cathode contains a reduction catalyst. The anode and the cathode are in contact with each other via an electrolyte, and the cathode is disposed in a position where it can come into contact with an oxidizing agent. As the oxidizing agent, oxygen present in the atmosphere is preferably used. In addition, the anode and the cathode are electrically connected via an external resistor. Here, the external resistor refers to the electrical resistance in an electric circuit.

At the anode, organic matter is oxidized and decomposed by the metabolism of microorganisms, and electrons and protons are generated at the same time. The generated electrons are taken up by the anode and move to the cathode via an external resistor. The generated protons pass through the electrolyte and move to the cathode.
At the cathode, the electrons and protons transferred from the anode side and the oxidant near the cathode react with each other due to the action of the reduction catalyst.
As the above reaction is repeated, an electromotive force is generated between the cathode and the anode.

The electrolyte used in the microbial fuel cell contains a substrate that is proton conductive and is oxidatively decomposed by the microorganism. The substrate is not particularly limited as long as it is a substance that can be metabolized by the microorganism that donates electrons. Examples of the substrate include organic substances such as sugars, proteins, and lipids, as well as inorganic substances such as ammonia, and the electrolyte may contain one or more of these substances. Therefore, the electrolyte may be made of domestic wastewater, industrial wastewater, environmental wastewater (ponds, lakes, rivers, seas), soil, sludge, etc. that satisfy the above conditions. In addition, a mediator that is responsible for electron transfer between the microorganism and the anode may be introduced, and examples of such mediators include methylene blue and neutral red.

The microorganisms that donate electrons at the anode may be of a single species or multiple species, so long as they cause an anode reaction that oxidizes and decomposes the substrate to generate electrons. The microorganisms are retained in the electrolytic cell by floating in the electrolytic cell or by being immobilized on the anode. There are no particular limitations on the type of microorganism, but examples include those belonging to the genera Shewanella and Geobacter.

The catalyst that can be used in the cathode is not particularly limited as long as it functions as a microbial fuel cell, but we will use an oxygen reduction catalyst as an example. Examples of oxygen reduction catalysts include precious metal catalysts, base metal oxide catalysts, carbon catalysts, activated carbon, and reducing enzymes. These may be used alone or in combination of two or more types.

The microbial fuel cell may be configured as a single tank type in which an anode holding electron donor microorganisms and a cathode containing a catalyst material are inserted into an electrolytic cell containing organic wastewater or the like as an electrolyte without a partition, or as a two tank type in which an anode tank and a cathode tank are separated by a solid polymer membrane, as in a solid polymer fuel cell. Furthermore, it may be configured not only in an electrolytic cell surrounded by a surface, but also in an unenclosed environment such as a rice field, a lake, a river, or the sea. For example, a cathode may be installed on the liquid surface, side, or bottom surface that separates the inside of the electrolytic cell that holds the electrolyte from the outside of the electrolytic cell, such as the atmosphere containing oxygen, or a cassette-type electrode that can take in outside air may be immersed in the liquid. In this case, the catalyst layer of the cathode is installed so that it is in contact with the electrolyte, and its back surface is in contact with the atmosphere containing oxygen. As a result, the atmosphere containing oxygen flows directly into the cathode from outside the electrolytic cell, and a reaction occurs on the catalyst in the cathode that is in contact with the electrolyte.

The present invention will be explained in more detail below with reference to examples, but the following examples are not intended to limit the scope of the invention in any way.

<Carbon materials for electrodes>
[Production Example A1]
(Preparation of Carbon Material (1))
Graphene nanoplatelets xGnP-C-750 (manufactured by XGscience) and iron phthalocyanine P-26 (manufactured by Sanyo Pigment Co., Ltd.) were weighed out to a mass ratio of 1/0.5 (graphene nanoplatelets/iron phthalocyanine) and dry-mixed to obtain a mixture. The mixture was filled into an alumina crucible and heat-treated in an electric furnace under a nitrogen atmosphere at 800°C for 2 hours to obtain a carbon material (1). The specific surface area of the carbon material (1) was 220 m ² /g.

<Preparation of electrode composition>
[Example 1]
12% by mass of spherical graphite CGB-50 (manufactured by Nippon Graphite Co., Ltd.) as graphite (A-a), 3% by mass of acetylene black HS-100 (manufactured by Denka Co., Ltd.) as a carbon material other than graphite (A-b), 5% by mass of KF polymer #9200 (manufactured by Kureha Co., Ltd.) which is polyvinylidene fluoride as binder (B), and 80% by mass of N-methylpyrrolidone as a solvent were mixed in a mixer to obtain an electrode composition (1) having a solid content of 20% by mass.

[Examples 2 to 27, Comparative Examples 1 and 2]
Using the graphite (A-a), the carbon material other than graphite (A-b), the binder (B), and the solvent shown in Table 1, electrode compositions (2) to (29) were prepared in the same manner as for the electrode composition (1).

<Preparation of electrodes for microbial fuel cells>

[Examples 28 to 45, 48 to 54, Comparative Examples 3 and 4]
The electrode compositions (1) to (18), (21) to (29) of Examples 1 to 18, 21 to 27 and Comparative Examples 1 and 2 were added to a non-conductive substrate of a 100 μm thick PET film (size 10 cm × ¹⁰ cm) so as to give a basis weight of 2 mg/cm2, and then heated and dried at 110°C for 10 minutes to produce microbial fuel cell electrodes (1) to (18), (21) to (29).

[Examples 46 and 47]
The electrode compositions (19) and (20) of Examples 19 and 20 were added to a conductive substrate, carbon paper (size 10 cm x 10 cm, manufactured by Toray Industries, Inc.), so that the basis weight ^was 2 mg/cm2, and then the composition was dried by heating at 110°C for 10 minutes to prepare electrodes (19) and (20) for microbial fuel cells.

Since the coating on the microbial fuel cell electrode (28) had peeled off, further evaluation was not performed.

[Production Example B1]
12% by mass of carbon material (1), 40% by mass of Nafion (registered trademark) (20% by mass solution), 24% by mass of water, and 24% by mass of 1-propyl alcohol were mixed in a mixer to obtain a cathode composition (1). The cathode composition (1) was added onto carbon felt LFP-210 (size 10 cm x 10 cm, manufactured by Osaka Gas Chemicals Co., Ltd.), and then heated and dried at 110°C for 10 minutes to prepare a cathode (1) for a microbial fuel cell (basis weight 10 mg/cm ² ).

<Microbial fuel cell>
[Example 55]
The evaluation was carried out using the microbial fuel cell shown in Figure 1. 400 g of soil (Flower and Vegetable Soil: Akagi Farm Co., Ltd.), a cathode, and an anode were placed in a container with a capacity of 500 mL, and 250 g of tap water as an electrolyte was added to the soil to create a microbial fuel cell. The anode used was a microbial fuel cell electrode (1), and the cathode used was a microbial fuel cell cathode (1). Next, wiring was attached to the cathode and anode and connected to an external resistor (1 kΩ), and the microbial fuel cell was operated continuously.

[Examples 56 to 72, 75 to 81, Comparative Example 5]
As shown in Table 2, the microbial fuel cell was operated continuously in the same manner as in Example 55, except that the microbial fuel cell electrodes (2) to (18), (21) to (27), and (29) were used instead of the microbial fuel cell electrode (1) as the anode.

[Examples 73 and 74]
As shown in Table 2, the microbial fuel cell was operated continuously in the same manner as in Example 55, except that the microbial fuel cell electrodes (19) and (20) were used instead of the microbial fuel cell cathode (1) as the cathode.

(Evaluation of power generation characteristics)
After three weeks, the voltage and current of the microbial fuel cell were measured with a tester and the output was calculated. The evaluation criteria are shown below, and the results are shown in Table 2.
◎: Power generation characteristics 30μW or more (very good)
〇: Power generation characteristics: 20μW or more and less than 30μW (good)
〇△: Power generation characteristics: 10μW or more and less than 20μW (no practical problems)
△: Power generation characteristics: 5 μW or more and less than 10 μW (bad)
×: Power generation characteristic: Less than 5 μW (extremely poor)

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

As shown in Table 2, by using the electrode composition of the present invention, good power generation characteristics and durability were obtained in the microbial fuel cell. It is believed that the reaction rate was high due to the high conductivity of the electrode and the high diffusivity of the substance. In addition, it is believed that the surface morphology of the anode was optimized, which improved the adsorption of microorganisms.
Comparing Examples 8 to 10, Example 8, which used a combination of the water-soluble resin (Ba) and the aqueous resin particles (Bb), was superior in power generation characteristics and durability.
Compared to Example 27 in which the content of the water-soluble resin (Ba) in 100% by mass of the total solid content of the composition was 20%, Examples 15 and 25 in which the content was 1 to 15% had excellent power generation characteristics and durability.
Compared to Example 12, which used flaky graphite, Examples 3 and 11, which used spherical graphite or artificial graphite with an aspect ratio of 1 to 10, had excellent power generation characteristics and durability.
Compared with Example 15 and Example 18, in which the content of the carbon material (A) in the total solid content of the composition (100 mass%) is 75%, Examples 25 and 26, in which the content of the carbon material (A) is 80 to 99 mass%, had excellent power generation characteristics and durability. Also, compared with Example 24, in which the content of the carbon material (A) is 80%, Examples 4, 5, and 23, in which the content of the carbon material (A) is 85 to 99%, had excellent power generation characteristics.
Example 15, which uses Ketjen black with a smaller specific surface area, had better durability than Example 16, which uses activated carbon with a larger specific surface area. Also, Examples 6 and 26, which use acetylene black with an even smaller specific surface area, had better power generation characteristics and durability than Example 14, which uses furnace black, and Example 25, which uses Ketjen black.
Compared with Example 17 in which the content of graphite (A-a) in the carbon material (A) is 60%, Example 26 in which the content of graphite (A-a) is 80% had superior power generation characteristics. Also, compared with Example 22 in which the content of graphite (A-a) is 40%, Example 23 in which the content of graphite (A-a) is 80% had superior power generation characteristics.
Example 15, in which the average specific surface area of the carbon material (A) was about 150 ^m2 /g, had superior power generation characteristics and durability compared to Example 21, in which the average specific surface area of the carbon material (A) exceeded ^{300 m2} /g. Also, Example 26, in which the average specific surface area of the carbon material (A) was about 8 ^m2 /g, had superior power generation characteristics compared to Example 25, in which the average specific surface area of the carbon material (A) was about 150 m2/g.

1 Container 2 Cathode 3 Anode 4 Electrolyte 5 External resistance

Claims (6)

A composition for forming an electrode for a microbial fuel cell comprising a carbon material (A) and a binder (B), wherein the carbon material (A) comprises graphite (A-a) and a carbon material other than graphite (A-b);
the binder (B) contains a water-soluble resin (Ba) and aqueous resin particles (B-b);
A composition for forming an electrode for a microbial fuel cell, wherein the content of graphite (A-a) in a total of 100% by mass of the carbon material (A) is 20.0% by mass or more, and the content of the carbon material (A) in a total of 100% by mass of the solid content of the composition is 50% by mass or more . The composition for forming a microbial fuel cell electrode according to claim 1, wherein the content of graphite (A-a) in a total of 100% by mass of the carbon material (A) is 25 to 99% by mass. The composition for forming a microbial fuel cell electrode according to claim 1 or 2, wherein the content of the carbon material (A) is 80 to 99 mass% out of a total solid content of the composition of 100 mass%. A composition for forming an electrode for a microbial fuel cell according to any one of claims 1 to 3, in which the aspect ratio of the graphite (A-a) is 1 to 10. An electrode for a microbial fuel cell having a coating film of a composition for forming a microbial fuel cell electrode according to any one of claims 1 to 4 on a substrate. A microbial fuel cell using the electrode for a microbial fuel cell according to claim 5.