Patent Application
20190296381
The present invention relates to a microbial fuel cell and a waste liquid treatment system. More particularly, the present invention relates to a microbial fuel cell and a waste liquid treatment system capable of purifying waste liquid and generating electrical energy.
Microbial fuel cells that generate electricity using biomass are in increasing demand as sustainable energy. The microbial fuel cells are superior systems capable of converting organic matter into electrical energy by use of metabolism of microorganisms, so as to collect energy while treating the organic matter. Such microbial fuel cells are still required to be improved because the amount of electricity generated by microorganisms is quite small, and current density to be output is thus low.
Patent Literature 1 discloses an electrode for such a microbial fuel cell. The electrode for a microbial fuel cell disclosed in Patent Literature 1 includes an electrode base including carbon, and carbon nanowires formed across the whole or a part of the surface of the electrode base. Patent Literature 1 discloses that the electrode base has a fibrous structure or a porous structure having gaps and/or pores, and the length and/or the width of the gaps and the diameter of the pores are each set to a predetermined value. Patent Literature 1 discloses that the electrode base may have stiffness so as to keep the shape as an electrode. Patent Literature 1 further discloses that the electrode base, which may be composed of powdered microparticles such as carbon black, is required to be held on a surface of a supporting body formed of a different substance so as to keep the shape as an electrode.
Patent Literature 1: Japanese Patent No. 5494996
The electrode base disclosed in Patent Literature 1 has high stiffness and therefore has low flexibility. The electrode is thus not easy to bend into a particular shape. The electrode base formed such that the powdered microparticles are held on the supporting body, may decrease in output power of the microbial fuel cell, since contact resistance between the microparticles and the supporting body increases.
The present invention has been made in view of the above-described conventional problems. An object of the present invention is to provide a microbial fuel cell having high flexibility and capable of keeping output power high, and a waste liquid treatment system.
In order to solve the above problems, a microbial fuel cell according to a first aspect of the present invention includes a negative electrode including a sheet-like first carbon material including a graphene sheet, the negative electrode supporting microorganisms, and a positive electrode opposed to the negative electrode. The negative electrode has an arithmetical mean roughness Ra on a surface of the negative electrode set in a range of 4.0 μm to 10000 μm.
A waste liquid treatment system according to a second aspect of the present invention includes the microbial fuel cell according to the first aspect, wherein the positive electrode is arranged in contact with gas containing oxygen.
A microbial fuel cell and a waste liquid treatment system according to the present embodiment will be described in detail below. The dimensions in the drawings may be exaggerated for illustration purposes, and are not necessarily drawn to scale.
[Microbial Fuel Cell]
As shown in
As shown in
As shown in
As described below, the positive electrode 20 includes a water-repellent layer 21 having water repellency. The inside of the cassette substrate 50 is separated from the liquid 70 to be treated held in the waste liquid tank 80, so that the inner space defined by the two membrane electrode assemblies 40 and the cassette substrate 50 serves as the gas phase 2. As shown in
(Negative Electrode)
The negative electrode 10 according to the present embodiment includes a sheet-like first carbon material 11 including a graphene sheet, as shown in
The graphene sheet is obtained as follows, for example. First, natural graphite is subjected to chemical treatment with acid so as to provide inserts between graphene layers of the graphite. Subsequently, the graphene layers provided with the inserts are rapidly heated at a high temperature, so as to obtain expanded graphite in which gaps between the graphene layers are expanded by gas pressure due to thermal decomposition of the inserts between the graphene layers. Thereafter, the expanded graphite is pressed and rolled, so as to obtain the graphene sheet. When the graphene sheet thus obtained is used for the first carbon material 11, the graphene layers in the graphite are arranged in the direction Y perpendicular to the stacking direction X, as shown in
The negative electrode 10 holds anaerobic microorganisms thereon, and hydrogen ions and electrons are produced from at least one of organic matter or a nitrogen-containing compound contained in the liquid 70 to be treated due to the catalysis of the microorganisms. In particular, the negative electrode 10 has one surface 10a and the other surface 10b on the opposite side of the surface 10a, wherein the surface 10a is opposed to the positive electrode 20 with the ion transfer layer 30 interposed therebetween, and the other surface 10b holds the anaerobic microorganisms thereon. A biofilm including the anaerobic microorganisms is placed and fixed onto the other surface 10b of the negative electrode 10 so that the anaerobic microorganisms are held by the negative electrode 10. A typical biofilm has a three-dimensional structure including a microbial population and an extracellular polymeric substance (EPS) that the microbial population produces. The anaerobic microorganisms are not necessarily held by the negative electrode 10 via the biofilm. The anaerobic microorganisms may be held not only on the surface of the negative electrode 10 but also inside the negative electrode 10.
The anaerobic microorganisms held by the negative electrode 10 are preferably electricity-producing bacteria having, for example, an extracellular electron transfer mechanism. Specific examples of anaerobic microorganisms include Geobacter, Shewanella, Aeromonas, Geothrix, and Saccharomyces.
According to the present embodiment, an arithmetical mean roughness Ra on the surface of the negative electrode 10 is set in a range of 4.0 μm to 10000 μm. Preferably, the arithmetical mean roughness Ra on the other surface 10b of the negative electrode 10 on which the anaerobic microorganisms are held is in the range of 4.0 μm to 10000 μm. The arithmetical mean roughness Ra of 4.0 μm or greater on the surface of the negative electrode 10 increases the area of the corresponding surface. Since the amount of the anaerobic microorganisms supported on the negative electrode 10 is increased as the area of the surface is increased, the reaction of producing hydrogen ions and electrons from at least one of the organic matter or the nitrogen-containing compound contained in the liquid 70 to be treated is induced efficiently, so as to improve the output power of the microbial fuel cell.
The arithmetical mean roughness Ra on the surface of the negative electrode 10 can be measured in accordance with Japanese Industrial Standards JIS B0601: 2013 (Geometrical Product Specifications (GPS)-Surface texture: Profile method-Terms, definitions and surface texture parameters). In particular, the surface of the negative electrode is observed with a laser microscope, for example, so as to measure the arithmetical mean roughness Ra in an area of 500 μm×500 μm at three different points. An average of the arithmetical mean roughness Ra obtained at the three different points is calculated, so as to obtain the arithmetical mean roughness Ra on the surface of the negative electrode.
Since the amount of the anaerobic microorganisms supported can be increased as the area of the surface of the negative electrode is increased, the upper limit of the arithmetical mean roughness Ra on the surface of the negative electrode is preferably, but not necessarily, 10000 μm. The upper limit of the arithmetical mean roughness Ra may be determined as follows. When the graphene sheet is used for the first carbon material 11, the graphene sheet shown in
To ensure the mechanical strength of the negative electrode 10, the arithmetical mean roughness Ra on the surface of the negative electrode 10 is preferably set in a range of 4.0 μm to 5000 μm, more preferably in a range of 4.0 μm to 1000 μm.
As described above, according to the present embodiment, the arithmetical mean roughness Ra on the surface of the negative electrode 10 is set in the range of 4.0 μm to 10000 μm. The arithmetical mean roughness Ra may be increased by any method, and an example of the method is to support a second carbon material on the surface of the first carbon material 11. In particular, as shown in
The second carbon material 13 is preferably, but not necessarily, in a particle state. In particular, the second carbon material 13 preferably has a spherical shape or a polyhedral shape. A mean particle diameter (a median diameter: d50) of the second carbon material 13 is preferably, but not necessarily, 1 mm or smaller, more preferably 1 μm or smaller. The lower limit of the mean particle diameter of the second carbon material 13 may be, but is not limited to, 50 nm. The particle diameter of the second carbon material 13 may be obtained such that the second carbon material 13 is observed with a scanning electron microscope (SEM) or a transmission electron microscope (TEM).
The second carbon material 13 in a particle state may be at least a material selected from the group consisting of carbon black, activated carbon, and fullerene. An example of carbon black may be at least a material selected from the group consisting of Ketjenblack (registered trademark), acetylene black, channel black, lamp black, oil furnace black, and thermal black.
The shape of the second carbon material 13 is not limited to a particle state, and may be a wire state or a flake state. The second carbon material 13 in a wire state may be at least a material selected from the group consisting of carbon nanotube, carbon nanofiber, carbon nanohorn, and carbon fibril. The second carbon material 13 in a flake state may be graphene having a thickness of 0.05 μm to 1 μm and an aspect ratio of about 10 to 1000. The aspect ratio is a ratio of a mean in-plane diameter D to the thickness (mean in-plane diameter D/thickness) of graphene.
A method of supporting the second carbon material 13 on the surface 11a of the first carbon material 11 may be any method, and the second carbon material 13 may be supported as follows. First, the second carbon material 13 and a binder are mixed into a solvent to prepare carbon material slurry. The mixed amount of the second carbon material 13 and the binder is preferably adjusted as appropriate so as to provide fine gaps. The solvent used for preparing the carbon material slurry may be any solvent, and examples thereof include water, and an alcohol solvent such as methanol, ethanol, 1-propanol, 2-propanol, ethylene glycol, and propylene glycol. The slurry prepared with each solvent may be mixed with a known surfactant or thickener as appropriate.
The carbon material slurry is applied to the surface 11a of the first carbon material 11 and is then dried, so as to support the second carbon material 13 on the surface 11a of the first carbon material 11. A temperature upon drying of the carbon material slurry is preferably, but not necessarily, set in a range of 80° C. to 120° C. sufficient to remove the solvent from the slurry.
The binder used may be any compound that can bind the particles of the second carbon material 13 together. The binder preferably includes at least one of the compounds selected from the group consisting of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and an ethylene-propylene-diene copolymer (EPDM).
Another example of the method of increasing the arithmetical mean roughness Ra on the surface of the negative electrode 10 is to roughen the surface of the negative electrode 10, instead of supporting the second carbon material thereon. A specific example is to exfoliate part of graphene present on the surface layer of the first carbon material 11 with adhesive tape. Alternatively, the surface of the first carbon material 11 may be roughened by filing, for example.
The graphene sheet used for the first carbon material 11 preferably has electrical resistivity of 20 μΩ·m or lower in the direction Y perpendicular to the thickness direction X of the graphene sheet. The electrical resistivity in the thickness direction of the graphene sheet is preferably at least 100 times as high as the electrical resistivity in the direction Y perpendicular to the thickness direction X. The electrical resistivity set to the range as described above can facilitate the flow of electrons produced by a local cell reaction in the negative electrode 10 toward the external circuit 90. The lower limit of the electrical resistivity of the graphene sheet in the direction Y perpendicular to the thickness direction X may be, but is not limited to, 0.10 μΩ·m or greater. The upper limit of the electrical resistivity of the graphene sheet in the thickness direction X may be, but is not limited to, at most 1000 times as high as the electrical resistivity in the direction Y perpendicular to the thickness direction X. The electrical resistivity may be measured by a four-point probe method, for example.
The negative electrode 10 and the positive electrode 20 according to the present embodiment preferably have flexibility. The negative electrode 10 and the positive electrode 20 having flexibility can be deformed to have a cylindrical shape, for example, as described below. The resistance to water pressure can be improved when the microbial fuel cell 1 having a cylindrical shape is increased in size. Accordingly, the inner space can be defined by the membrane electrode assemblies 40 and the cassette substrate 50 stably, so as to continuously supply oxygen to each positive electrode 20.
The first carbon material 11 preferably has a sheet-like shape, as shown in
Table 1 shows the electrical resistivity when the same amount of carbon black is supported on the respective surfaces of nonwoven fabric including polyethylene (PE) and polypropylene (PP) mixed together, carbon cloth, carbon felt, stainless steel mesh, and a graphene sheet. In particular, the electrical resistivity indicated is measured when the nonwoven fabric, carbon cloth, carbon felt, stainless steel mesh, and graphene sheet having a thickness with no stiffness are used, and the carbon black is supported on the surface of each material. The results in Table 1 show that the electrical resistivity of the nonwoven fabric is high, and the carbon cloth and the carbon felt have the electrical resistivity about six times as high as the graphene sheet or over, leading to an increase in electrical resistance when the electrode is increased in area. While the stainless steel mesh has substantially the same electrical resistivity as the graphene sheet, the stainless steel mesh has lower corrosion resistance than the graphene sheet, and may be decreased in power generation performance when used for a long period of time. The first carbon material 11 according to the present embodiment thus preferably includes only the graphene sheet in view of the electrical resistance and the corrosion resistance.
There is a publication, “F. L. LaQue: Marine Corrosion Causes and Prevention, John Wiley and Sons, p. 179 (1975)”, describing corrosion potentials of various kinds of metals in stationary seawater at normal temperature. This document teaches that a potential of graphite with respect to a saturated calomel electrode is in the range of +0.3 to +0.2 (V vs. SCE), and a potential of platinum with respect to the saturated calomel electrode is in the range of +0.25 to +0.18 (V vs. SCE). Namely, graphite has higher corrosion resistance than platinum, and is therefore a superior material for use in the first carbon material 11.
As shown in
The negative electrode 10 according to the present embodiment may be modified with an electron transfer mediator molecule. Alternatively, the liquid 70 to be treated in the waste liquid tank 80 may include the electron transfer mediator molecule. The presence of the molecule can promote the transfer of the electrons from the anaerobic microorganisms to the negative electrode 10, so as to treat the liquid more efficiently.
More particularly, the electrons flow between the negative electrode 10 and cells or a final electron acceptor due to a metabolism mechanism of the anaerobic microorganisms. The mediator molecule introduced into the liquid 70 to be treated serves as a final electron acceptor and transfers the received electrons to the negative electrode 10. Accordingly, the oxidative decomposition rate of the organic matter and the like in the liquid 70 can be enhanced. An example of such an electron transfer mediator molecule may be, but is not limited to, at least a material selected from the group consisting of neutral red, anthraquinone-2,6-disulfonate (AQDS), thionine, potassium ferricyanide, and methyl viologen.
(Positive Electrode)
The positive electrode 20 according to the present embodiment has a function that causes oxygen supplied from the gas phase 2 to react with hydrogen ions and electrons produced in the negative electrode 10 so as to produce water. The positive electrode 20 according to the present embodiment may have any structure that can achieve the effect described above, but preferably includes at least the water-repellent layer 21 and a gas diffusion layer 22, as shown in
The water-repellent layer 21 has both water repellency and gas permeability. The water-repellent layer 21 has a function that allows gas to move from the gas phase 2 to the liquid 70 to be treated in the electrochemical system in the microbial fuel cell 1, while separating the gas phase 2 from the liquid 70 to be treated in a favorable state. In particular, the water-repellent layer 21 allows oxygen in the gas phase 2 to pass therethrough to reach the gas diffusion layer 22. The water-repellent layer 21 is preferably a porous layer. The porous water-repellent layer 21 can have high gas permeability.
The gas diffusion layer 22 preferably includes, for example, a porous electrical conductive material and a catalyst supported on the electrical conductive material. Alternatively, the gas diffusion layer 22 may include a porous catalyst having electrical conductivity.
In the present embodiment, the water-repellent layer 21 of the positive electrode 20 is arranged on the gas phase 2 side. The surface 20a of the water-repellent layer 21 on the opposite side of the gas diffusion layer 22 is exposed to the gas phase 2. The oxygen in the gas phase can be supplied to the gas diffusion layer 22 through the water-repellent layer 21. The gas diffusion layer 22 of the positive electrode 20 is in contact with the ion transfer layer 30 so as to be opposed to the negative electrode 10 via the ion transfer layer 30.
More particularly, the water-repellent layer 21 is preferably a porous body having water repellency. Such a water-repellent layer 21 can have high gas permeability. The water-repellent layer 21 is preferably formed of, for example, at least one of the materials selected from the group consisting of polytetrafluoroethylene (PTFE), polydimethylsiloxane (PDMS), polyethylene (PE), and polypropylene (PP).
The gas diffusion layer 22 preferably includes, for example, a porous electrical conductive material and a catalyst supported on the electrical conductive material. Alternatively, the gas diffusion layer 22 may include a porous catalyst having electrical conductivity. The electrical conductive material in the gas diffusion layer 22 may include at least one of the materials selected from the group consisting of a carbon substance, an electrical conductive polymer, a semiconductor, and metal. The carbon substance is a substance including carbon as a component. Examples of carbon substances include graphite, activated carbon, carbon powder such as carbon black, VULCAN (registered trademark) XC-72R, acetylene black, furnace black, and Denka black, carbon fiber such as graphite felt, carbon wool, and carbon woven material, a carbon plate, carbon paper, and a carbon disc. Other examples may include microstructure substances such as carbon nanotube, carbon nanohorn, and carbon nanoclusters.
The electrical conductive polymer is a general term for high molecular compounds having electrical conductivity. Examples of electrical conductive polymers include polymers each composed of a single monomer or two or more monomers, each of which is aniline, aminophenol, diaminophenol, pyrrole, thiophene, paraphenylene, fluorene, furan, acetylene, or a derivative of each compound. More specific examples of electrical conductive polymers include polyaniline, polyaminophenol, polydiaminophenol, polypyrrole, polythiophene, polyparaphenylene, polyfluorene, polyfuran, and polyacetylene. The electrical conductive material made of metal may be stainless steel mesh. The electrical conductive material is preferably a carbon substance in view of availability, costs, corrosion resistance, and durability.
The electrical conductive material is preferably in the form of powder or fiber. The electrical conductive material may be supported by a supporting body. The supporting body is a member that has stiffness and allows the gas diffusion electrode to have a fixed shape. The supporting body may be either a non-conductive body or an electrical conductive body. When the supporting body is a non-conductive body, examples thereof include glass, plastics, synthetic rubber, ceramics, waterproofing or water-repellent paper, a plant piece such as a wood piece, and an animal piece such as a bone piece or a shell. Examples of supporting bodies having a porous structure include porous ceramics, porous plastics, and a sponge. When the supporting body is an electrical conductive body, examples thereof include a carbon substance such as carbon paper, carbon fiber, and a carbon stick, metal, and an electrical conductive polymer. When the supporting body is an electrical conductive body, the supporting body may function as a current collector obtained such that an electrical conductive material supporting a carbon-based material described below is provided on the surface of the supporting body.
The catalyst in the gas diffusion layer 22 is preferably, for example, a carbon-based material doped with metal atoms. The metal atoms are preferably, but not necessarily, atoms of at least one metal selected from the group consisting of titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, silver, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, and gold. The carbon-based material doped with such metal atoms exhibits high performance as a catalyst particularly for enhancing an oxygen reduction reaction and an oxygen evolution reaction. The amount of the metal atoms included in the carbon-based material may be determined as appropriate so that the carbon-based material exhibits high catalytic performance.
The carbon-based material is preferably further doped with atoms of at least one nonmetal selected from the group consisting of nitrogen, boron, sulfur, and phosphorus. The amount of the doped nonmetal atoms may also be determined as appropriate so that the carbon-based material exhibits high catalytic performance.
The carbon-based material is obtained in a manner such that a carbon-based raw material such as graphite or amorphous carbon is used as a base, and the carbon-based raw material is doped with metal atoms and atoms of at least one nonmetal selected from the group consisting of nitrogen, boron, sulfur, and phosphorus.
Combinations of the doped metal, atoms and nonmetal atoms used for the carbon-based material are selected as appropriate. Particularly, the nonmetal atoms preferably include nitrogen, and the metal atoms preferably include iron. The carbon-based material including these atoms can achieve significantly high catalytic activity. The nonmetal atoms may only include nitrogen. The metal atoms may only include iron.
The nonmetal atoms may include nitrogen, and the metal atoms may include at least one of cobalt and manganese. The carbon-based material including such atoms can also achieve significantly high catalytic activity. The nonmetal atoms may only include nitrogen. The metal atoms may only include either cobalt or manganese, or may only include cobalt and manganese.
The carbon-based material may have any shape. For example, the carbon-based material may be in a particle state or have a sheet-like shape. The carbon-based material having a sheet-like shape may have any size, and may be a minute material. The carbon-based material having a sheet-like shape may be a porous material. The porous carbon-based material having a sheet-like shape is preferably, for example, in the form of woven or non-woven fabric. Such a carbon-based material can be used for the gas diffusion layer 22 without the electrical conductive material included.
The carbon-based material serving as a catalyst in the gas diffusion layer 22 may be produced as follows. First, a mixture of a nonmetal compound including at least one nonmetal selected from the group consisting of nitrogen, boron, sulfur, and phosphorus, a metal compound, and a carbon-based raw material is prepared. Thereafter, the mixture is heated at a temperature from 800° C. or higher to 1000° C. or lower, for from 45 seconds or longer to shorter than 600 seconds. This yields the carbon-based material serving as a catalyst.
As described above, graphite or amorphous carbon may be used for the carbon-based raw material. The metal compound may be any compound that includes metal atoms capable of being coordinated with nonmetal atoms with which the carbon-based raw material is doped. The metal compound may be at least a material selected from the group consisting of: an inorganic metal salt such as a metal chloride, nitrate, sulfate, bromide, iodide, and fluoride; an organic metal salt such as acetate; a hydrate of an inorganic metal salt; and a hydrate of an organic metal salt. When graphite is doped with iron, for example, the metal compound preferably includes iron(III) chloride. When graphite is doped with cobalt, the metal compound preferably includes cobalt chloride. When the carbon-based raw material is doped with manganese, the metal compound preferably includes manganese acetate. The amount of the metal compound used is preferably determined, for example, such that the metal atoms are present in the metal compound in an amount of from 5% to 30% by mass, more preferably from 5% to 20% by mass, with respect to the carbon-based raw material.
The nonmetal compound preferably includes at least one nonmetal selected from the group consisting of nitrogen, boron, sulfur, and phosphorus, as described above. The nonmetal compound may be at least a material selected from the group consisting of pentaethylenehexamine, ethylenediamine, tetraethylenepentamine, triethylenetetramine, ethylenedi amine, octylboronic acid, 1,2-bis(diethylphosphino)ethane, triphenyl phosphite, and benzyl disulfide. The amount of the nonmetal compound used is determined as appropriate depending on the amount of the doped nonmetal atoms applied to the carbon-based raw material. The amount of the nonmetal compound used is preferably determined such that a molar ratio of the metal atoms in the metal compound and the nonmetal atoms in the nonmetal compound is in the range from 1:1 to 1:2, more preferably in the range from 1:1.5 to 1:1.8.
When the carbon-based material serving as a catalyst is prepared, the mixture of the nonmetal compound, the metal compound, and the carbon-based raw material is obtained as follows. First, the carbon-based raw material, the metal compound, and the nonmetal compound are mixed, and a solvent such as ethanol is added thereto as necessary, so as to adjust the total mixture amount. These materials are dispersed by an ultrasonic dispersion method. The dispersed product is heated at an appropriate temperature (for example, 60° C.), and then dried so as to remove the solvent. This yields the mixture including the nonmetal compound, the metal compound, and the carbon-based raw material.
Subsequently, the mixture thus obtained is heated in a reduction atmosphere or an inert gas atmosphere. As a result, the carbon-based raw material is doped with the nonmetal atoms, and is further doped with the metal atoms when the nonmetal atoms and the metal atoms are coordinated with each other. The heating temperature is preferably in the range from 800° C. or higher to 1000° C. or lower, and the heating time is preferably in the range from 45 seconds or longer to shorter than 600 seconds. Since the heating time is short, the carbon-based material is produced efficiently, and the catalytic activity of the carbon-based material is further enhanced. The heating rate of the mixture at the beginning of heating in the heating process is preferably 50° C./second or greater. This rapid heating further increases the catalytic activity of the carbon-based material.
The carbon-based material may further be subjected to acid cleaning. For example, the carbon-based material may be dispersed in pure water with a homogenizer for 30 minutes, added in 2M sulfuric acid, and then stirred at 80° C. for three hours. This process minimizes elution of the metal components from the carbon-based material.
The production method described above allows the carbon-based material to have high electrical conductivity in which the amounts of an inert metal compound and metal crystals are significantly reduced.
(Ion Transfer Layer)
The microbial fuel cell 1 according to the present embodiment includes the ion transfer layer 30 permeable to hydrogen ions. As shown in
The ion transfer layer 30 may be an ion exchange membrane including ion exchange resin. As the ion exchange resin, for example, NAFION (registered trademark) made by DuPont Kabushiki Kaisha, and Flemion (registered trademark) and Selemion (registered trademark) made by Asahi Glass Co., Ltd. can be used.
The ion transfer layer 30 may also be a porous membrane having pores through which the hydrogen ions can be transmitted. In other words, the ion transfer layer 30 may be a sheet having spaces (voids) through which the hydrogen ions move between the negative electrode 10 and the positive electrode 20. The ion transfer layer 30 preferably includes at least one of the materials selected from the group consisting of a porous sheet, a sheet of woven fabric, and a sheet of nonwoven fabric. Alternatively, the ion transfer layer 30 may be at least a material selected from the group consisting of a glass fiber membrane, a synthetic fiber membrane, and plastic nonwoven fabric, or may be a stacked body including a plurality of these membranes stacked on one another. The porous sheet having a plurality of pores as described above allows the hydrogen ions to easily pass therethrough. The pores in the ion transfer layer 30 may have any diameter that can transmit the hydrogen ions from the negative electrode 10 to the positive electrode 20 therethrough.
As described above, the ion transfer layer 30 has a function that allows the hydrogen ions produced in the negative electrode 10 to pass therethrough so as to move toward the positive electrode 20. When the negative electrode 10 and the positive electrode 20 are not in contact with but arranged close to each other, for example, the hydrogen ions can move from the negative electrode 10 to the positive electrode 20. Therefore, the microbial fuel cell 1 according to the present embodiment does not necessarily include the ion transfer layer 30. However, since the use of the ion transfer layer 30 promotes the efficient transfer of the hydrogen ions from the negative electrode 10 to the positive electrode 20, the ion transfer layer 30 is preferably provided in view of the improvement of the output power.
The waste liquid tank 80 holds the liquid 70 to be treated inside thereof, but may have a configuration in which the liquid 70 flows through the waste liquid tank 80. For example, as shown in
The waste liquid tank 80 is preferably kept in an anaerobic condition in which molecular oxygen is not present or the concentration of molecular oxygen present is quite low. Accordingly, the liquid 70 to be treated can be held in the waste liquid tank 80 so as to hardly come into contact with oxygen.
The effects of the microbial fuel cell 1 according to the present embodiment are described below. When the microbial fuel cell 1 is in operation, the liquid 70 to be treated containing at least one of organic matter and a nitrogen-containing compound is supplied to the negative electrode 10, and gas containing air or oxygen is supplied to the positive electrode 20. The air is continuously supplied to the positive electrode 20 from the opening on the upper side of the cassette substrate 50. The liquid 70 is also preferably continuously supplied to the negative electrode 10 through the liquid supply port 81 and the liquid drain port 82.
In the positive electrode 20, the air passes through the water-repellent layer 21 and is then diffused by the gas diffusion layer 22. In the negative electrode 10, the hydrogen ions and electrons are generated, by the catalysis of the microorganisms, from at least one of the organic matter and the nitrogen-containing compound contained in the liquid 70 to be treated. The generated hydrogen ions are transferred through the ion transfer layer 30 toward the positive electrode 20. The generated electrons are transferred through the electrically conductive sheet of the negative electrode 10 to the external circuit 90, and are further transferred to the gas diffusion layer 22 of the positive electrode 20 from the external circuit 90. The hydrogen ions and electrons transferred to the gas diffusion layer 22 are bonded to the oxygen due to the effect of the catalyst, so as to turn into water to be consumed. At this time, the external circuit 90 recovers electrical energy flowing in such a closed circuit.
As described above, the microbial fuel cell 1 according to the present embodiment includes: the negative electrode 10 including the sheet-like first carbon material 11 including the graphene sheet, the negative electrode 10 supporting microorganisms; and the positive electrode 20 opposed to the negative electrode 10. The negative electrode 10 has an arithmetical mean roughness Ra on the surface of the negative electrode 10 set in a range of 4.0 μm to 10000 μm. Since the graphene sheet of the first carbon material 11 has flexibility, the negative electrode 10 can be deformed into a particular shape so as to have high resistance to water pressure, for example. In addition, since the graphene sheet itself has appropriate strength, a supporting body for supporting the graphene sheet does not need to be used.
The microbial fuel cell 1 according to the present embodiment can increase in the amount of anaerobic microorganisms supported on the negative electrode 10, since the arithmetical mean roughness Ra on the surface of the negative electrode 10 is high. Accordingly, the reaction of producing the hydrogen ions and electrons from at least one of the organic matter and the nitrogen-containing compound contained in the liquid 70 to be treated, is induced efficiently, so as to improve the output power of the microbial fuel cell 1.
When an electrode base having high electrical resistivity is used, as disclosed in Patent Literature 1, the resistivity of the electrode base greatly influences the recovering of electricity in association with an increase in size of the cell. In contrast, the negative electrode according to the present embodiment does not have stiffness, and includes the first carbon material 11 having much smaller resistivity in the plane direction YZ, so as to have the great advantage when formed into a cylindrical shape and increased in area, as described below.
The fuel cell unit 60 shown in
The fuel cell unit 60 shown in
When the microbial fuel cell according to the present embodiment is increased in size, the negative electrode, the positive electrode and the ion transfer layer need to increase in resistance to water pressure, since the water pressure acting on the negative electrode, the positive electrode and the ion transfer layer increases as the depth of the waste liquid tank 80 increases. In view of this, as shown in
The cassette substrate 50A shown in
When an electrode base having high electrical resistivity is used, as disclosed in Patent Literature 1, the resistivity of the electrode base greatly influences the recovering of electricity in association with an increase in size of the cell. In contrast, the negative electrode according to the present embodiment does not have stiffness, and includes the first carbon material 11 having much smaller resistivity in the plane direction YZ, so as to have the great advantage when formed into a cylindrical shape and increased in area.
[Waste Liquid Treatment System]
A waste liquid treatment system according to the present embodiment is described below. The waste liquid treatment system according to the present embodiment includes the microbial fuel cell described above, and the positive electrode 20 is arranged so as to be in contact with gas containing oxygen.
As described above, the microbial fuel cell 1 according to the present embodiment supplies the liquid 70 to be treated containing at least one of the organic matter and the nitrogen-containing compound to the negative electrode 10. The microbial fuel cell 1 generates carbon dioxide or nitrogen together with the hydrogen ions and electrons from at least one of the organic matter and the nitrogen-containing compound contained in the liquid 70 by the metabolism of the microorganisms supported on the negative electrode 10.
In particular, for example, when the liquid 70 to be treated includes glucose as organic matter, carbon dioxide, the hydrogen ions, and the electrons are generated by the local cell reaction shown below.
Negative electrode 10 (anode): C6H12O6+6H2O→6CO2+24H++24e−
Positive electrode 20 (cathode): 6O2+24H++24e−→12H2O
When the liquid 70 to be treated includes ammonia as a nitrogen-containing compound, nitrogen, the hydrogen ions, and the electrons are generated by the local cell reaction shown below.
Negative electrode 10 (anode): 4NH3→2N2+12H++12e−
Positive electrode 20 (cathode): 3O2+12H++12e−→6H2O
The waste liquid treatment system according to the present embodiment using the microbial fuel cell 1 causes the organic matter and the nitrogen-containing compound contained in the liquid 70 to be treated to come into contact with the negative electrode 10 to lead to oxidative decomposition, so as to purify the liquid 70. As described above, the waste liquid tank 80 may be provided with the liquid supply port 81 for supplying the liquid 70 to the waste liquid tank 80 and the liquid drain port 82 for draining the treated liquid 70 from the waste liquid tank 80, so as to continuously supply the liquid 70 to the waste liquid tank 80. Accordingly, the liquid 70 is continuously brought into contact with the negative electrode 10 so as to be treated efficiently.
Hereinafter, the present embodiment will be described in more detail with reference to Example and Comparative Example, but is not intended to be limited to these examples.
(Preparation of Positive Electrode)
First, a polytetrafluoroethylene (PTFE) dispersion liquid was applied to one surface of a carbon paper (electrode base material for fuel cell TGP-H-120, available from Toray Industries, Inc.), and was heated at 380° C. for 20 minutes. The PTFE dispersion liquid used was 60 wt % dispersion in H2O (available from Sigma-Aldrich).
Subsequently, catalyst ink was applied to the other surface of the carbon paper. The catalyst ink was prepared such that a Pt/C catalyst, Nafion, and ion exchange water were mixed. After the catalyst ink was applied to the carbon paper to result in 2 mg/cm2 of the platinum catalyst, the carbon paper was dried at 380° C. for 20 minutes, so as to prepare a positive electrode of this example. The Pt/C catalyst included in the catalyst ink was a catalyst available from Tanaka Kikinzoku Kogyo K. K., and the amount of platinum supported was 66.6%. Nafion used was a 5 wt % solution in lower aliphatic alcohols/H2O mix.
(Preparation of Negative Electrode)
First, a graphene sheet (flexible graphite material CARBOFIT (registered trademark) HGP-105, available from Hitachi Chemical Company, Ltd.) was cut out so as to have the same size as the catalyst-applied area in the positive electrode.
Subsequently, carbon black ink was applied to the surface of the graphene sheet and then dried at 380° C. for 20 minutes. The carbon black ink was prepared such that carbon black, a PTFE dispersion liquid, and ion exchange water were mixed. The carbon black used was Ketjenblack EC600JD (available from Lion Specialty Chemicals Co., Ltd.). The PTFE dispersion liquid used was 60 wt % dispersion in H2O (available from Sigma-Aldrich).
Thereafter, an electrically conductive wire was attached to an end portion of the graphene sheet supporting the carbon black with an epoxy conductive adhesive (CW2400, CircuitWorks conductive adhesive epoxy), so as to prepare a negative electrode of this example.
(Preparation of Fuel Cell Unit)
First, anaerobic microorganisms were held on the surface of the negative electrode supporting the carbon black prepared as described above. Subsequently, polyolefin nonwoven fabric was used as an ion transfer layer, and the positive electrode, the ion transfer layer, and the negative electrode were stacked in this order so as to prepare a membrane electrode assembly. Thereafter, two membrane electrode assemblies and a cassette substrate having a substantially U-shape in cross section were stacked together, so as to obtain a fuel cell unit as shown in
(Preparation of Negative Electrode)
First, a graphene sheet (flexible graphite material CARBOFIT (registered trademark) HGP-105, available from Hitachi Chemical Company, Ltd.) was cut out so as to have the same size as the negative electrode of Example. Thereafter, an electrically conductive wire was attached to an end portion of the graphene sheet with an epoxy conductive adhesive (CW2400, CircuitWorks conductive adhesive epoxy), so as to prepare a negative electrode of this example.
(Preparation of Fuel Cell Unit)
First, anaerobic microorganisms were held on one surface of the negative electrode prepared as described above. Subsequently, polyolefin nonwoven fabric was used as an ion transfer layer, and the positive electrode used in Example, the ion transfer layer, and the negative electrode were stacked in this order so as to prepare a membrane electrode assembly. Thereafter, two membrane electrode assemblies and a cassette substrate having a substantially U-shape in cross section were stacked together, so as to obtain a fuel cell unit as shown in
[Evaluation]
(Measurement of Arithmetical Mean Roughness on Surface of Negative Electrode)
The surface of the negative electrode obtained in each of Example and Comparative Example was observed with a laser microscope, so as to measure the arithmetical mean roughness Ra in accordance with JIS B0601: 2013. Table 2 shows the measurement results. As shown in Table 2, the negative electrode of Example to which the carbon black was applied has the arithmetical mean roughness on the surface supporting the anaerobic microorganisms that exceeds 4.0 μm. The arithmetical mean roughness of the negative electrode of Comparative Example without carbon black applied is less than 1.5 μm.
(Measurement of Output Power)
The fuel cell unit obtained in each of Example and Comparative Example was placed in a water tank with a volume of 300 cc, and a liquid to be treated was introduced into the water tank. The microbial fuel cell was operated therein for 20 days, and a change in output characteristics of each fuel cell was observed. The liquid to be treated was simulated sewage containing about 700 mg/L of total organic carbon and kept at 30° C. The amount of the liquid to be treated introduced to the waste liquid tank was adjusted so that a hydraulic retention time of the liquid was 24 hours. The respective evaluation results are shown in
As shown in
The steady-state output of the microbial fuel cell of Comparative Example having less than 4.0 μm of the arithmetical mean roughness Ra on the surface of the negative electrode, was much worse than that of Example, although the steady-state output was not decreased over the course of the operating days. This is presumed to be because the amount of the anaerobic microorganisms supported on the surface of the negative electrode was smaller because of the smaller arithmetical mean roughness Ra on the surface.
The entire content of Japanese Patent Application No. P2015-162728 (filed on Aug. 20, 2015) is incorporated herein by reference.
While the present embodiment has been described above, the present embodiment is not intended to be limited to the descriptions thereof, and various modifications will be made within the scope of the present embodiment.
The microbial fuel cell according to the present embodiment can be deformed into a highly water-resistant shape, for example. The microbial fuel cell can increase in the amount of microorganisms supported on the negative electrode due to the high arithmetical mean roughness Ra. Accordingly, the reaction of producing hydrogen ions and electrons from organic matter contained in a liquid to be treated can be induced efficiently, so as to enhance the output power of the microbial fuel cell. Further, since the amount of microorganisms supported is increased, the performance of treatment of the waste liquid treatment system can be improved.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2016/004395 | 9/29/2016 | WO | 00 |
