JP2020087804A - Microbial fuel cell and liquid processing unit - Google Patents

Abstract

To provide a microbial fuel cell which can decrease the risk of an electrolyte solution leaking from a positive electrode and a liquid processing unit.SOLUTION: A microbial fuel cell 100 comprises: an electrolyte tank 1 for holding an electrolyte solution 4 containing an organic substance; a negative electrode 6 supporting anaerobic microorganism and immersed in the electrolyte solution 4; and a positive electrode 5 electrically connected with the negative electrode 6 and having an air electrode part 54. One face 5a of the air electrode part 54 is astride a liquid level 4a of the electrolyte solution 4 in a vertical direction Y and in contact with a gaseous phase 10 containing oxygen. The rate of a height H2 of the air electrode part 54 to a height H1 from an inner face of a bottom wall 1c of the electrolyte tank 1 to the liquid level 4a of the electrolyte solution 4 is 1/5 or more and 2/3 or less.SELECTED DRAWING: Figure 2

Description

The present invention relates to a microbial fuel cell and a liquid treatment unit using the same.

In recent years, microbial fuel cells that generate electricity using biomass have been attracting attention as sustainable energy. A microbial fuel cell is a wastewater treatment device that converts chemical energy of organic substances contained in domestic wastewater and industrial wastewater into electric energy, and oxidatively decomposes and treats the organic substances. In addition, the microbial fuel cell has features that the amount of sludge generated is small and the energy consumption is small.

The microbial fuel cell has a negative electrode supporting microorganisms, and a positive electrode in contact with a gas phase containing oxygen and an electrolytic solution (wastewater). Then, while supplying an electrolytic solution containing an organic substance and the like to the negative electrode, a gas containing oxygen is supplied to the positive electrode. The negative electrode and the positive electrode form a closed circuit by connecting to each other via a load circuit. At the negative electrode, hydrogen ions and electrons are generated from the electrolytic solution by the catalytic action of microorganisms. Then, the generated hydrogen ions move to the positive electrode via the electrolytic solution, and the electrons move to the positive electrode via the load circuit. Hydrogen ions and electrons transferred from the negative electrode combine with oxygen in the positive electrode and are consumed as water. At that time, the electric energy flowing through the closed circuit is recovered.

As the above-mentioned microbial fuel cell, the microbial fuel cell described in Patent Document 1 is known. Patent Document 1 describes a microbial fuel cell including an electrolysis cell, an anode disposed inside the electrolysis cell, and a cathode disposed in the cell body of the electrolysis cell. One surface of the cathode is in contact with the air outside the cell, and the other surface is in contact with the electrolytic solution. Further, in the cathode, the electrode base material is made of a carbon fiber-based material, and a water blocking layer containing silicone and a conductive substance is provided on one surface in contact with air.

Patent No. 6184256

The positive electrode (cathode) described in Patent Document 1 has a water stop layer on one surface that comes into contact with air, and thus has excellent water resistance. However, in such a microbial fuel cell, the cathode is pressed from the liquid phase side to the gas phase side by water pressure. Therefore, when the size of the microbial fuel cell is increased, the electrolytic solution may leak from the electrolytic cell tank due to water pressure damage or rupture of the positive electrode in a deep water location.

The present invention has been made in view of the above problems of the conventional technology. And an object of the present invention is to provide a microbial fuel cell and a liquid treatment unit capable of reducing the risk of electrolyte leakage from the positive electrode.

To solve the above problems, the microbial fuel cell according to the first aspect of the present invention is an electrolytic solution tank for holding an electrolytic solution containing an organic substance, carrying anaerobic microorganisms, and immersed in the electrolytic solution. A negative electrode and a positive electrode electrically connected to the negative electrode and having an air electrode portion are provided. One surface of the air electrode portion straddles the liquid surface of the electrolytic solution in the vertical direction and comes into contact with the gas phase containing oxygen. The ratio of the height of the air electrode portion to the height from the inner surface of the bottom wall of the electrolytic solution tank to the liquid surface of the electrolytic solution is 1/5 or more and 2/3 or less.

The liquid treatment unit according to the second aspect of the present invention includes a microbial fuel cell.

According to the present disclosure, it is possible to provide a microbial fuel cell and a liquid treatment unit that can reduce the risk of electrolyte leakage from the positive electrode.

It is a perspective view which shows roughly an example of the microbial fuel cell which concerns on 1st embodiment. It is sectional drawing which followed the AA line in FIG. It is an exploded perspective view showing an example of an electrode cassette in a microbial fuel cell. It is a perspective view which shows schematically the other example of the microbial fuel cell which concerns on 1st embodiment. It is a perspective view which shows schematically an example of the microbial fuel cell which concerns on 2nd embodiment.

Hereinafter, the microbial fuel cell and the liquid treatment unit according to the present embodiment will be described in detail. Note that the dimensional ratios in the drawings are exaggerated for convenience of explanation, and may differ from the actual ratios.

[Microbial fuel cell]
(First embodiment)
As shown in FIGS. 1 and 2, the microbial fuel cell 100 according to this embodiment includes an electrolytic solution tank 1 that holds an electrolytic solution 4 containing an organic substance. The electrolytic solution 4 held in the electrolytic solution tank 1 is waste water such as domestic waste water or factory waste water.

The electrolytic solution tank 1 is a substantially rectangular parallelepiped-shaped container, and a supply unit 2 for supplying the electrolytic solution 4 to the electrolytic solution tank 1 is provided above the front wall 1 a of the electrolytic solution tank 1. Further, on the upper part of the rear wall 1b of the electrolytic solution tank 1, there is provided a discharge part 3 for discharging the treated electrolytic solution 4 from the electrolytic solution tank 1.

In addition, in this embodiment, the supply part 2 and the discharge part 3 are not limited in the position provided as long as the electrolyte solution 4 can be supplied and discharged, and may be provided in arbitrary places. Further, in the present embodiment, an example in which the electrolytic solution tank 1 has the supply unit 2 and the discharge unit 3 has been described, but the electrolytic solution tank 1 does not have the supply unit 2 and the discharge unit 3. Alternatively, the electrolytic solution 4 may be directly supplied to and discharged from the electrolytic solution tank 1. Moreover, one common supply/discharge unit may be provided as the supply unit 2 and the discharge unit 3.

The microbial fuel cell 100 according to this embodiment includes an electrode cassette 50. As shown in FIGS. 1 to 3, the electrode cassette 50 includes an electrode assembly 20 including a positive electrode 5, a negative electrode 6, and an ion transfer layer 7. The negative electrode 6 is arranged so as to contact one surface 7a of the ion transfer layer 7, and the positive electrode 5 is arranged so as to contact a surface 7b of the ion transfer layer 7 opposite to the surface 7a. Further, as shown in FIG. 2, the positive electrode 5 and the negative electrode 6 are electrically connected to the external circuit 8, respectively.

As shown in FIG. 3, the electrode assembly 20 is laminated on the cassette base material 30. The cassette base material 30 is a U-shaped frame member, and has an open top. That is, the cassette base material 30 is a frame member in which the bottom surfaces of two first columnar members 31 are connected by the second columnar member 32.

As will be described later, the positive electrode 5 includes a water repellent layer 52 having water repellency, and the side surface 33 of the cassette substrate 30 is joined to the surface 5 a of the water repellent layer 52. The first columnar member 31 and the second columnar member 32 are arranged along the outer peripheral edge of the surface 5 a of the water repellent layer 52. The second columnar member 32 is provided at a substantially central portion of the electrode assembly 20 in the vertical direction Y that is perpendicular to the stacking direction X.

As shown in FIG. 2, the electrode cassette 50 formed by stacking two sets of the electrode assembly 20 and the cassette base material 30 has the inside of the electrolytic solution tank 1 so that the gas phase 10 communicating with the atmosphere is formed. Is located in. Since the water-repellent layer 52 has water repellency, it is separated from the electrolytic solution 4 held inside the electrolytic solution tank 1 and the inside of the cassette substrate 30, and is formed by the electrode assembly 20 and the cassette substrate 30. The formed internal space is in the vapor phase 10. That is, in the present embodiment, the cassette base material 30 is a water blocking portion that prevents the electrolytic solution 4 from entering the gas phase 10.

In the microbial fuel cell 100, the gas phase 10 is opened to the outside air, or the gas phase 10 is supplied with air from the outside by a pump, for example. On the other hand, the electrolytic solution 4 as the water to be treated is held inside the electrolytic solution tank 1, and the conductive layer 53, the negative electrode 6 and the ion transfer layer 7 of the positive electrode 5 are immersed in the electrolytic solution 4.

As shown in FIG. 2, the positive electrode 5 has an air electrode portion 54 provided on the upper end side and a non-air electrode portion 55 provided on the lower end side in the vertical direction Y.

One surface 5a of the air electrode portion 54 is in contact with the gas phase 10 containing oxygen. Specifically, in the air electrode portion 54, the water repellent layer 52 and the conductive layer 53 are laminated, and the entire surface 5 a of the air electrode portion 54 on the water repellent layer 52 side is in direct contact with the gas phase 10. Further, the other surface 5b of the air electrode portion 54 is in contact with the surface 7b of the ion transfer layer 7. Specifically, the conductive layer 53 side of the air electrode portion 54 is in contact with the entire surface 7b of the ion transfer layer 7.

One surface 5a of the air electrode portion 54 contacts the gas phase 10 across the liquid surface 4a of the electrolyte solution 4 in the vertical direction Y so that the electrolyte solution 4 does not enter the gas phase 10 beyond the air electrode portion 54. is doing. That is, the air electrode portion 54 is arranged so as to project upward from the liquid surface 4a of the electrolytic solution 4, and the surface having the same height as the liquid surface 4a of the electrolytic solution 4 in the vertical direction Y intersects with the air electrode portion 54. ing.

The height of the portion of the air electrode portion 54 protruding upward from the liquid surface 4a of the electrolytic solution 4 is such that when the liquid surface 4a of the electrolytic solution 4 becomes high, The height is preferably such that it does not enter. In the air electrode portion 54, the height of the portion of the electrolyte 4 that protrudes upward from the liquid surface 4a is preferably 5% or more, and preferably 10% or more, with respect to the height H2 of the air electrode portion 54. Is more preferable. From the viewpoint of cost reduction, the height of the portion of the air electrode portion 54 that protrudes upward from the liquid surface 4a of the electrolytic solution 4 is 30% or less of the height H2 of the air electrode portion 54. It is preferably 20% or less.

The non-air electrode portion 55 is not in direct contact with the gas phase 10. That is, the non-air electrode portion 55 is not directly exposed to the gas phase 10. As shown in FIG. 2, the non-air electrode portion 55 has a joining portion 56 provided on the upper end side and a dipping portion 57 provided on the lower end side in the vertical direction Y. The joint portion 56 is provided between the air electrode portion 54 and the immersion portion 57.

The joining portion 56 is joined to the cassette base material 30 (water blocking portion) and prevents the electrolytic solution 4 from entering the gas phase 10. Specifically, at the joint portion 56, the water repellent layer 52 and the conductive layer 53 are laminated, and the surface 5a of the water repellent layer 52 and the side surface 33 of the cassette base material 30 are joined.

The immersion part 57 has the conductive layer 53 and can utilize the dissolved oxygen in the electrolytic solution 4 for the reduction reaction. It is preferable that the surface of the conductive layer 53 is directly exposed to the electrolytic solution 4 so that the positive electrode 5 can easily utilize the dissolved oxygen in the electrolytic solution 4. Specifically, in the non-air electrode portion 55, it is preferable that at least a part of the surface of the conductive layer 53 is not covered with the water repellent layer 52, and the entire surface of the conductive layer 53 is covered with the water repellent layer 52. More preferably not.

In the microbial fuel cell 100 according to this embodiment, the ratio of the height H2 of the air electrode portion 54 to the height H1 from the inner surface of the bottom wall 1c of the electrolytic solution tank 1 to the liquid surface 4a of the electrolytic solution 4 is 1/5. It is above 2/3. The height H2 of the air electrode portion 54 corresponds to the length from the upper end to the lower end of the air electrode portion 54 in the vertical direction Y. That is, the height H2 of the air electrode portion 54 corresponds to the length from the upper end of the air electrode portion 54 to the upper end of the joining portion 56 in the vertical direction Y.

As described above, by setting the ratio of the height H2 to the height H1 to be ⅕ or more, the positive electrode 5 can take in oxygen contained in the air, so that the reaction in the positive electrode 5 can effectively proceed. be able to. In addition, by setting the ratio of the height H2 to the height H1 to 2/3 or less, it is possible to suppress the high water pressure applied to the positive electrode 5 even when the electrolytic solution tank 1 having a deep water depth is used. it can. Therefore, it is possible to prevent the positive electrode 5 from being pressed by the water pressure from the electrolytic solution tank 1 side to the gas phase 10 side and being damaged or ruptured. Therefore, the risk that the electrolytic solution 4 leaks from the positive electrode 5 can be reduced.

The height H1 from the inner surface of the bottom wall 1c of the electrolytic solution tank 1 to the liquid surface 4a of the electrolytic solution 4 is preferably 3 m or more and 5 m or less. By setting the height H1 to 3 m or more, the microbial fuel cell 100 can be upsized and the water treatment and power generation effect in the microbial fuel cell 100 can be improved. Further, by setting the height H1 to be 5 m or less, it is possible to suppress the nonuniformity of the electrolytic solution 4 and improve the water treatment and power generation effects.

The height H2 of the air electrode portion 54 is preferably 1 m or more and 2 m or less. By setting the height H2 of the air electrode portion 54 to 1 m or more, the positive electrode 5 can effectively take in oxygen contained in the air, so that the reaction in the positive electrode 5 can be effectively advanced. Further, by setting the height H2 of the air electrode portion 54 to be 2 m or less, it is possible to suppress the high water pressure applied to the positive electrode 5.

Therefore, the height H1 from the inner surface of the bottom wall 1c of the electrolytic solution tank 1 to the liquid surface 4a of the electrolytic solution 4 is 3 m or more and 5 m or less, and the height H2 of the air electrode portion 54 is 1 m or more and 2 m or less. preferable. By setting the height H1 and the height H2 within the above ranges, it is possible to further suppress the high water pressure applied to the positive electrode 5 even when the electrolytic solution tank 1 having a deep water depth is used.

(Positive electrode)
The positive electrode 5 is electrically connected to the negative electrode 6. As described above, the positive electrode 5 includes the gas diffusion electrode including the water repellent layer 52 and the conductive layer 53 laminated on the water repellent layer 52. Then, the conductive layer 53 of the positive electrode 5 is in contact with the ion transfer layer 7, and the water repellent layer 52 is exposed on the gas phase 10 side. By using such a thin plate-shaped gas diffusion electrode, it becomes possible to easily supply oxygen in the gas phase 10 to the catalyst in the positive electrode 5.

<Water repellent layer>
The water repellent layer 52 of the positive electrode 5 is a layer having both water repellency and oxygen permeability. The water repellent layer 52 is configured to allow the movement of oxygen from the gas phase 10 to the liquid phase while favorably separating the gas phase 10 and the liquid phase in the electrochemical system in the electrode cassette 50. That is, the water-repellent layer 52 allows oxygen in the gas phase 10 to permeate and move to the conductive layer 53, while suppressing movement of the electrolytic solution 4 to the gas phase 10. The term "separation" as used herein means to physically cut off.

The water-repellent layer 52 is in contact with the gas phase 10 containing oxygen and diffuses oxygen in the gas phase 10. In the structure shown in FIG. 2, the water-repellent layer 52 supplies oxygen to the conductive layer 53 substantially uniformly. Therefore, the water repellent layer 52 is preferably a porous body so that the oxygen can be diffused. Since the water-repellent layer 52 has water repellency, it is possible to prevent the pores of the porous body from being clogged due to dew condensation or the like, thereby reducing the oxygen diffusivity. Further, since the electrolytic solution 4 is less likely to soak into the water repellent layer 52, oxygen can be efficiently circulated from the surface of the water repellent layer 52 that contacts the vapor phase 10 to the surface that faces the conductive layer 53. Become.

The water-repellent layer 52 is preferably formed of a woven or non-woven fabric into a sheet shape. The material forming the water-repellent layer 52 is not particularly limited as long as it has water repellency and can diffuse oxygen in the gas phase 10. Examples of the material forming the water repellent layer 52 include polyethylene, polypropylene, polybutadiene, nylon, polytetrafluoroethylene (PTFE), ethyl cellulose, poly-4-methylpentene-1, butyl rubber and polydimethylsiloxane (PDMS). At least one selected from the group consisting of silicone can be used. Since these materials easily form a porous body and have high water repellency, it is possible to suppress clogging of pores and improve gas diffusivity. The water-repellent layer 52 preferably has a plurality of through holes penetrating in the stacking direction X of the water-repellent layer 52 and the conductive layer 53.

The water-repellent layer 52 may be subjected to water-repellent treatment using a water-repellent agent, if necessary, in order to enhance the water repellency. Specifically, water repellency may be improved by attaching a water repellent agent such as polytetrafluoroethylene to the porous body forming the water repellent layer 52.

<Conductive layer>
The conductive layer 53 in the positive electrode 5 preferably includes a porous conductive material and a catalyst carried by the conductive material. The conductive layer 53 may be made of a porous catalyst having conductivity. By using such a conductive layer 53 for the positive electrode 5, it becomes possible to conduct electrons generated by a local cell reaction described later between the catalyst and the external circuit 8. That is, as will be described later, a catalyst is supported on the conductive layer 53, and the catalyst is an oxygen reduction catalyst. Then, the electrons move from the external circuit 8 to the catalyst through the conductive layer 53, so that the oxygen reduction reaction by oxygen, hydrogen ions and electrons can be promoted by the catalyst.

In the positive electrode 5, it is preferable that oxygen permeates the water repellent layer 52 and the conductive layer 53 efficiently and is supplied to the catalyst in order to ensure stable performance. Therefore, the conductive layer 53 is preferably a porous body having a large number of pores through which oxygen permeates from the surface facing the water-repellent layer 52 to the surface opposite thereto. In addition, the shape of the conductive layer 53 is particularly preferably a three-dimensional mesh shape. With such a mesh shape, it is possible to impart high oxygen permeability and conductivity to the conductive layer 53.

In the positive electrode 5, the water-repellent layer 52 is preferably bonded to the conductive layer 53 via an adhesive in order to efficiently supply oxygen to the conductive layer 53. Thereby, the diffused oxygen is directly supplied to the conductive layer 53, and the oxygen reduction reaction can be efficiently performed. From the viewpoint of ensuring the adhesiveness between the water repellent layer 52 and the conductive layer 53, the adhesive is preferably provided on at least a part of the water repellent layer 52 and the conductive layer 53. However, from the viewpoint of enhancing the adhesiveness between the water-repellent layer 52 and the conductive layer 53 and stably supplying oxygen to the conductive layer 53 for a long period of time, the adhesive is used to bond the water-repellent layer 52 and the conductive layer 53. More preferably, it is provided over the entire surface.

The adhesive preferably has oxygen permeability and contains at least one selected from the group consisting of polymethylmethacrylate, methacrylic acid-styrene copolymer, styrene-butadiene rubber, butyl rubber, nitrile rubber, chloroprene rubber and silicone. A resin can be used.

Here, the conductive layer 53 of the positive electrode 5 in the present embodiment will be described in more detail. As described above, the conductive layer 53 can be configured to include a porous conductive material and a catalyst carried by the conductive material.

The conductive material in the conductive layer 53 can be composed of, for example, one or more materials selected from the group consisting of carbon-based substances, conductive polymers, semiconductors and metals. Here, the carbon-based substance means a substance having carbon as a constituent component. Examples of the carbon-based substance include, for example, graphite, activated carbon, carbon black, Vulcan (registered trademark) XC-72R, acetylene black, furnace black, carbon powder such as Denka black, graphite felt, carbon wool, carbon woven cloth, and the like. Examples thereof include carbon fibers, carbon plates, carbon paper, carbon disks, carbon cloth, carbon foil, and carbon-based materials obtained by compression-molding carbon particles. Further, examples of the carbon-based material also include fine-structured materials such as carbon nanotubes, carbon nanohorns, and carbon nanoclusters. Furthermore, as the conductive material in the conductive layer 53, a metal material such as mesh and foam can be used.

The conductive polymer is a general term for polymer compounds having conductivity. Examples of the conductive polymer include aniline, aminophenol, diaminophenol, pyrrole, thiophene, paraphenylene, fluorene, furan, acetylene, or a single monomer having a derivative thereof as a constituent unit or a polymer of two or more kinds of monomers. Can be mentioned. Specific examples of the conductive polymer include polyaniline, polyaminophenol, polydiaminophenol, polypyrrole, polythiophene, polyparaphenylene, polyfluorene, polyfuran, and polyacetylene. Examples of the conductive material made of metal include aluminum, copper, stainless steel, nickel and titanium. In consideration of availability, cost, corrosion resistance, durability, etc., the conductive material is preferably a carbon-based substance.

The shape of the conductive material is preferably a powder shape or a fiber shape. Further, the conductive material may be supported by the support. The support means a member which has rigidity by itself and can give a certain shape to the gas diffusion electrode. The support may be an insulator or a conductor. When the support is an insulator, examples of the support include glass, plastic, synthetic rubber, ceramics, water-resistant or water-repellent treated paper, plant pieces such as wood pieces, bone pieces, animal pieces such as shells, and the like. .. Examples of the support having a porous structure include porous ceramics, porous plastics and sponges. When the support is a conductor, examples of the support include carbon paper, carbon fiber, carbon-based substances such as carbon rods, metals, and conductive polymers. When the support is a conductor, the support can also function as a current collector by disposing a conductive material supporting a carbon-based material on the surface of the support.

The catalyst in the conductive layer 53 is a platinum-based catalyst, a carbon-based catalyst using iron or cobalt, a transition metal oxide-based catalyst such as partially oxidized tantalum carbonitride (TaCNO) and zirconium carbonitride (ZrCNO), tungsten or the like. A carbide type catalyst using molybdenum, activated carbon or the like can be used.

The catalyst in the conductive layer 53 is preferably a carbon-based material doped with metal atoms. The metal atom is not particularly limited, but titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, silver, hafnium, tantalum, tungsten, rhenium, osmium, iridium. It is preferably an atom of at least one metal selected from the group consisting of platinum and gold. In this case, the carbon-based material exhibits excellent performance as a catalyst for promoting the oxygen reduction reaction. The amount of metal atoms contained in the carbon-based material may be appropriately set so that the carbon-based material has excellent catalytic performance.

It is preferable that the carbon-based material is further doped with one or more non-metal atoms selected from nitrogen, boron, sulfur and phosphorus. The amount of non-metal atoms doped in the carbon-based material may be appropriately set so that the carbon-based material has excellent catalytic performance.

The carbon-based material is based on a carbon source material such as graphite and amorphous carbon, and the carbon source material is doped with a metal atom and one or more non-metal atoms selected from nitrogen, boron, sulfur and phosphorus. It can be obtained.

The combination of metal atoms and non-metal atoms doped in the carbon-based material is appropriately selected. In particular, it is preferable that the non-metal atom contains nitrogen and the metal atom contains iron. In this case, the carbon-based material can have a particularly excellent catalytic activity. The non-metal atom may be only nitrogen, and the metal atom may be only iron.

The non-metal atom may include nitrogen and the metal atom may include at least one of cobalt and manganese. Also in this case, the carbon-based material can have a particularly excellent catalytic activity. The non-metal atom may be nitrogen only. Further, the metal atom may be only cobalt, only manganese, or only cobalt and manganese.

The shape of the carbon-based material is not particularly limited. For example, the carbon-based material may have a particle shape or a sheet shape. The size of the carbon-based material having a sheet shape is not particularly limited, and for example, the carbon-based material may have a minute size. The carbonaceous material having a sheet shape may be porous. The sheet-shaped and porous carbon-based material preferably has, for example, a woven cloth shape, a non-woven cloth shape, or the like. Such a carbon-based material can form the conductive layer 53 without the conductive material.

The carbon-based material configured as the catalyst in the conductive layer 53 can be prepared as follows. First, a mixture containing a non-metal compound containing at least one non-metal selected from the group consisting of nitrogen, boron, sulfur and phosphorus, a metal compound and a carbon source material is prepared. Then, this mixture is heated at a temperature of 800° C. or higher and 1000° C. or lower for 45 seconds or more and less than 600 seconds. As a result, it is possible to obtain a carbon-based material configured as a catalyst.

Here, as the carbon source material, for example, graphite or amorphous carbon can be used as described above. Furthermore, the metal compound is not particularly limited as long as it is a compound containing a metal atom capable of coordinatively bonding with a non-metal atom doped in the carbon source material. Examples of the metal compound include inorganic metal salts such as metal chlorides, nitrates, sulfates, bromides, iodides, and fluorides, organic metal salts such as acetates, hydrates of inorganic metal salts, and organic metal salts. It is possible to use at least one selected from the group consisting of hydrates of For example, when graphite is doped with iron, the metal compound preferably contains iron(III) chloride. When the graphite is doped with cobalt, the metal compound preferably contains cobalt chloride. When the carbon source material is doped with manganese, the metal compound preferably contains manganese acetate. The amount of the metal compound used is preferably determined such that the ratio of metal atoms in the metal compound to the carbon source material is within the range of 5 to 30% by mass, and the ratio is 5 to 20% by mass. More preferably, it is determined to be within the range.

As described above, the non-metal compound is preferably at least one non-metal compound selected from the group consisting of nitrogen, boron, sulfur and phosphorus. Examples of the nonmetallic compound include pentaethylenehexamine, ethylenediamine, tetraethylenepentamine, triethylenetetramine, octylboronic acid, 1,2-bis(diethylphosphinoethane), triphenylphosphite, and benzyldisulfide. At least one compound selected from the group consisting of can be used. The amount of the non-metal compound used is appropriately set according to the amount of the non-metal atom doped into the carbon source material. The amount of the non-metal compound used is preferably determined such that the molar ratio of the metal atom in the metal compound to the non-metal atom in the non-metal compound is in the range of 1:1 to 1:2. , 1:1.5 to 1:1.8 is more preferably determined.

A mixture containing a non-metal compound, a metal compound, and a carbon source raw material when a carbon-based material configured as a catalyst is prepared is obtained, for example, as follows. First, a carbon source raw material, a metal compound and a non-metal compound are mixed, and a solvent such as ethanol is further added if necessary to prepare the total amount. These are further dispersed by an ultrasonic dispersion method. Then, after heating these at an appropriate temperature (for example, 60° C.), the mixture is dried to remove the solvent. As a result, a mixture containing the non-metal compound, the metal compound and the carbon source material is obtained.

Next, the obtained mixture is heated, for example, under a reducing atmosphere or an inert gas atmosphere. As a result, the carbon source material is doped with a non-metal atom, and the non-metal atom and the metal atom are coordinate-bonded to each other, thereby doping the metal atom. The heating temperature is preferably in the range of 800° C. to 1000° C., and the heating time is preferably in the range of 45 seconds to less than 600 seconds. Since the heating time is short, the carbon-based material is efficiently produced, and the catalytic activity of the carbon-based material is further increased. In the heat treatment, the temperature rising rate of the mixture at the start of heating is preferably 50°C/s or more. Such rapid heating further improves the catalytic activity of the carbon-based material.

Further, the carbonaceous material may be further acid washed. For example, the carbon-based material may be dispersed in pure water for 30 minutes with a homogenizer, and then the carbon-based material may be put into 2M sulfuric acid and stirred at 80° C. for 3 hours. In this case, elution of the metal component from the carbon-based material is suppressed.

By such a manufacturing method, a carbon-based material having a remarkably low content of an inert metal compound and a metal crystal and high conductivity can be obtained.

In the conductive layer 53, the catalyst may be bound to the conductive material using a binder. That is, the catalyst may be supported on the surface of the conductive material and inside the pores by using a binder. This can prevent the catalyst from being desorbed from the conductive material and deteriorating the oxygen reduction property. As the binder, for example, at least one selected from the group consisting of polytetrafluoroethylene, polyvinylidene fluoride (PVDF), and ethylene-propylene-diene copolymer (EPDM) is preferably used. It is also preferable to use NAFION (registered trademark) as the binder.

(Negative electrode)
The negative electrode 6 carries anaerobic microorganisms and is immersed in the electrolytic solution 4. The negative electrode 6 has a function of generating hydrogen ions and electrons from the organic substance in the electrolytic solution 4 by the catalytic action of anaerobic microorganisms. Therefore, the negative electrode 6 is not particularly limited as long as it has a configuration that produces such a function.

The negative electrode 6 has a structure in which an anaerobic microorganism is supported on a conductive sheet having conductivity. As the conductor sheet, at least one selected from the group consisting of a porous conductor sheet, a woven cloth-like conductor sheet and a non-woven cloth-like conductor sheet can be used. Further, the conductor sheet may be a laminated body in which a plurality of sheets are laminated. By using such a sheet having a plurality of pores as the conductor sheet of the negative electrode 6, hydrogen ions generated by a local cell reaction described later easily move to the direction of the ion transfer layer 7 and the oxygen reduction reaction It is possible to increase the speed. Further, from the viewpoint of improving the ion permeability, the conductor sheet of the negative electrode 6 has a continuous space (void) in the stacking direction X of the positive electrode 5, the ion transfer layer 7 and the negative electrode 6, that is, the thickness direction. Preferably.

The conductor sheet may be a metal plate having a plurality of through holes penetrating in the thickness direction. Therefore, as the material forming the conductor sheet of the negative electrode 6, for example, at least one selected from the group consisting of aluminum, copper, stainless steel, nickel and titanium, and other conductive metals, and carbon paper and carbon felt is used. be able to.

As the conductor sheet for the negative electrode 6, a graphite sheet that can be used for the positive electrode 5 may be used. Further, it is preferable that the negative electrode 6 contains graphite, and that the graphene layer in the graphite is arranged along the plane of the positive electrode 5, the ion transfer layer 7, and the negative electrode 6 in the direction YZ perpendicular to the stacking direction X. By arranging the graphene layers in this manner, the conductivity in the direction YZ perpendicular to the stacking direction X is improved rather than the conductivity in the stacking direction X. Therefore, the electrons generated by the local battery reaction of the negative electrode 6 are easily conducted to the external circuit 8, and the efficiency of the battery reaction can be further improved.

The anaerobic microorganism carried on the negative electrode 6 is not particularly limited as long as it is a current-producing bacterium that decomposes an organic substance in the electrolytic solution 4 to generate hydrogen ions and electrons. Anaerobic microorganisms do not require oxygen for oxidatively decomposing the organic substance in the electrolytic solution 4. Therefore, the electric power required to send oxygen can be significantly reduced. In addition, since the anaerobic microorganisms acquire a small amount of free energy, it is possible to reduce the amount of sludge generated.

The anaerobic microorganisms retained on the negative electrode 6 are preferably, for example, electro-producing bacteria having an extracellular electron transfer mechanism. Specifically, examples of the anaerobic microorganisms include Geobacter genus bacteria, Shewanella genus bacteria, Aeromonas genus bacteria, Geothrix genus bacteria, and Saccharomyces genus bacteria.

The anaerobic microorganisms may be retained in the negative electrode 6 by stacking and fixing the biofilm containing the anaerobic microorganisms on the negative electrode 6. In addition, a biofilm generally refers to a three-dimensional structure containing a microbial population and an extracellular polymeric substance (EPS) produced by the microbial population. However, the anaerobic microorganisms may be retained on the negative electrode 6 without depending on the biofilm. Further, the anaerobic microorganisms may be retained not only on the surface of the negative electrode 6 but also inside.

The negative electrode 6 may be modified with, for example, an electron transfer mediator molecule. Alternatively, the electrolytic solution 4 in the electrolytic solution tank 1 may include an electron transfer mediator molecule. As a result, electron transfer from the anaerobic microorganisms to the negative electrode 6 can be promoted, and more efficient liquid treatment can be realized.

Specifically, in the metabolic mechanism of anaerobic microorganisms, electrons are exchanged within cells or with the final electron acceptor. When the mediator molecule is introduced into the electrolytic solution 4, the mediator molecule acts as the final electron acceptor of metabolism, and transfers the received electron to the negative electrode 6. As a result, it becomes possible to increase the rate of oxidative decomposition of the organic substance in the electrolytic solution 4. Such an electron transfer mediator molecule is not particularly limited. As the electron transfer mediator molecule, for example, at least one selected from the group consisting of neutral red, anthraquinone-2,6-disulfonic acid (AQDS), thionine, potassium ferricyanide, and methyl viologen can be used.

(Ion transfer layer)
The electrode cassette 50 of the present embodiment further includes an ion transfer layer 7 having proton permeability, which is provided between the positive electrode 5 and the negative electrode 6. Then, as shown in FIGS. 1 and 2, the negative electrode 6 is separated from the positive electrode 5 via the ion transfer layer 7. The ion transfer layer 7 has a function of transmitting hydrogen ions generated in the negative electrode 6 and moving them to the positive electrode 5 side.

As the ion transfer layer 7, for example, an ion exchange membrane using an ion exchange resin can be used. As the ion exchange resin, for example, NAFION (registered trademark) manufactured by DuPont Co., Ltd., and Flemion (registered trademark) and Selemion (registered trademark) manufactured by Asahi Glass Co., Ltd. can be used.

Further, as the ion transfer layer 7, a porous membrane having pores through which hydrogen ions can permeate may be used. That is, the ion transfer layer 7 may be a sheet having a space (void) for hydrogen ions to move from the negative electrode 6 to the positive electrode 5. Therefore, the ion transfer layer 7 preferably includes at least one selected from the group consisting of a porous sheet, a woven sheet and a non-woven sheet. Further, the ion transfer layer 7 can use at least one selected from the group consisting of a glass fiber membrane, a synthetic fiber membrane, and a plastic non-woven fabric, and may be a laminate formed by laminating a plurality of these. Since such a porous sheet has a large number of pores inside, hydrogen ions can easily move. The pore size of the ion transfer layer 7 is not particularly limited as long as hydrogen ions can move from the negative electrode 6 to the positive electrode 5.

As described above, the ion transfer layer 7 has a function of transmitting hydrogen ions generated in the negative electrode 6 and moving them to the positive electrode 5 side. However, for example, if the negative electrode 6 and the positive electrode 5 are close to each other without coming into contact with each other, hydrogen ions can move from the negative electrode 6 to the positive electrode 5. Therefore, in the microbial fuel cell 100, the ion transfer layer 7 is not an essential component. However, by providing the ion transfer layer 7, it becomes possible to efficiently transfer hydrogen ions from the negative electrode 6 to the positive electrode 5, and therefore it is preferable to provide the ion transfer layer 7 from the viewpoint of improving the output. A space may be provided between the positive electrode 5 and the ion transfer layer 7, and a space may be provided between the negative electrode 6 and the ion transfer layer 7.

As shown in FIG. 2, the electrode cassette 50 includes an external circuit 8 electrically connected to the negative electrode 6 and the positive electrode 5. However, in the electrode cassette 50, the negative electrode 6 and the positive electrode 5 may be electrically directly connected by using a conductive member without using the external circuit 8. Further, in the electrode cassette 50, the entire upper portion of the cassette base material 30 is opened, but it may be partially opened if it is possible to introduce air (oxygen) into the inside, and it is closed. May be.

As described above, the microbial fuel cell 100 according to the present embodiment includes the electrolytic solution tank 1 that holds the electrolytic solution 4 containing an organic substance, the negative electrode 6 that carries anaerobic microorganisms, and is immersed in the electrolytic solution 4. And a positive electrode 5 electrically connected to the negative electrode 6 and having an air electrode portion 54. One surface 5a of the air electrode portion 54 straddles the liquid surface 4a of the electrolytic solution 4 in the vertical direction Y and contacts the gas phase 10 containing oxygen. The ratio of the height H2 of the air electrode portion 54 to the height H1 from the inner surface of the bottom wall 1c of the electrolytic solution tank 1 to the liquid surface 4a of the electrolytic solution 4 is 1/5 or more and 2/3 or less.

Therefore, since the positive electrode 5 can effectively take in oxygen contained in the air, the reduction reaction in the positive electrode 5 can be effectively advanced. Further, even when the electrolytic solution tank 1 having a deep water depth is used, it is possible to suppress high water pressure on the positive electrode 5. Therefore, it is possible to prevent the positive electrode 5 from being pressed by the water pressure from the four-phase side of the electrolytic solution toward the gas-phase 10 side and being damaged or ruptured. Therefore, the microbial fuel cell 100 can reduce the risk of the electrolyte solution 4 held in the electrolyte solution tank 1 leaking from the positive electrode 5.

In addition, as described above, when the positive electrode 5 has the immersed portion 57, the dissolved oxygen in the electrolytic solution 4 can be used for the reduction reaction, which is preferable from the viewpoint of power generation efficiency. However, as described above, if the ratio of the height H2 of the air electrode portion 54 to the height H1 is 1/5 or more and 2/3 or less, the risk of water leakage of the positive electrode 5 can be reduced. Therefore, as shown in FIG. 4, the positive electrode 5 may not have the immersion portion 57.

(Second embodiment)
Next, the microbial fuel cell according to the second embodiment will be described in detail with reference to the drawings. The same components as those in the first embodiment are designated by the same reference numerals, and duplicate description will be omitted.

As shown in FIG. 5, the microbial fuel cell 100A according to the present embodiment includes an electrolytic solution tank 1 that holds an electrolytic solution 4 containing an organic substance. The electrolytic solution tank 1 is a substantially rectangular parallelepiped container, and a supply unit 2 for supplying the electrolytic solution 4 to the electrolytic solution tank 1 is provided below the front wall 1 a of the electrolytic solution tank 1. Further, a discharge part 3 for discharging the treated electrolytic solution 4 from the electrolytic solution tank 1 is provided above the front wall 1 a of the electrolytic solution tank 1.

The microbial fuel cell 100A according to this embodiment includes an electrode assembly 20 including a positive electrode 5, a negative electrode 6, and an ion transfer layer 7, as in the first embodiment. The negative electrode 6 is arranged so as to contact one surface 7a of the ion transfer layer 7, and the positive electrode 5 is arranged so as to contact a surface 7b of the ion transfer layer 7 opposite to the surface 7a. Further, as shown in FIG. 5, the positive electrode 5 and the negative electrode 6 are electrically connected to the external circuit 8, respectively.

As shown in FIG. 5, the second embodiment is different from the first embodiment in that the electrode assembly 20 is joined to the rear wall 1b of the electrolytic solution tank 1, not to the cassette base material 30. Specifically, the inner surface of the rear wall 1b of the electrolytic solution tank 1 is joined to the surface 5a of the water repellent layer 52. Since the water-repellent layer 52 has water repellency, it is separated from the electrolytic solution 4 held inside the electrolytic solution tank 1 and the outside of the electrolytic solution tank 1, and is formed by the electrode assembly 20 and the rear wall 1b. The external space is a gas phase 10. That is, in the present embodiment, the rear wall 1b of the electrolytic solution tank 1 is a water blocking part that prevents the electrolytic solution 4 from entering the gas phase 10.

As shown in FIG. 5, the positive electrode 5 has an air electrode portion 54 provided on the upper end side and a joint portion 56 (non-air electrode portion 55) provided on the lower end side in the vertical direction Y. ..

One surface 5a of the air electrode portion 54 is in contact with the gas phase 10 containing oxygen. Specifically, in the air electrode portion 54, the water repellent layer 52 and the conductive layer 53 are laminated, and the entire surface 5 a of the air electrode portion 54 on the water repellent layer 52 side is in direct contact with the gas phase 10. Further, one surface 5a of the air electrode portion 54 straddles the liquid surface 4a of the electrolyte solution 4 in the vertical direction Y so that the electrolyte solution 4 does not move to the gas phase 10 beyond the air electrode portion 54. Is in contact with.

The joint portion 56 is not in direct contact with the vapor phase 10. That is, the joint portion 56 is not directly exposed to the vapor phase 10. The joint portion 56 is joined to the rear wall 1b (water blocking portion) and prevents the electrolytic solution 4 from entering the gas phase 10. Specifically, in the joint portion 56, the water repellent layer 52 and the conductive layer 53 are laminated, and the surface 5a of the water repellent layer 52 and the inner surface of the rear wall 1b are joined. Although the positive electrode 5 does not have the immersion portion 57 in the present embodiment, the immersion portion 57 may be formed on the lower end side of the joining portion 56 by removing the water-repellent layer 52 or the like.

The ratio of the height H2 of the air electrode portion 54 to the height H1 from the inner surface of the bottom wall 1c of the electrolytic solution tank 1 to the liquid surface 4a of the electrolytic solution 4 is ⅕ or more/2/ It is 3 or less. The height H2 of the air electrode portion 54 corresponds to the length from the upper end to the lower end of the air electrode portion 54 in the vertical direction Y. That is, the height H2 of the air electrode portion 54 corresponds to the length from the upper end of the air electrode portion 54 to the upper end of the joining portion 56 in the vertical direction Y.

As in the first embodiment, by setting the ratio of the height H2 to the height H1 to be ⅕ or more and ⅔ or less, the positive electrode 5 can effectively take in oxygen, and the electrolysis with a deep water depth can be performed. Even when the liquid tank 1 is used, it is possible to prevent high water pressure from being applied to the positive electrode 5. Therefore, it is possible to prevent the positive electrode 5 from being pressed by the water pressure from the electrolytic solution tank 1 side to the gas phase 10 side and being damaged or ruptured. Therefore, the risk that the electrolytic solution 4 leaks from the positive electrode 5 can be reduced.
[Liquid processing unit]
Next, the liquid processing unit according to this embodiment will be described. The liquid treatment unit of the present embodiment includes the above-mentioned microbial fuel cell.

As described above, in the microbial fuel cell of this embodiment, the electrolytic solution 4 containing the organic substance is supplied to the negative electrode 6. Then, carbon dioxide or nitrogen is generated from the organic substance in the electrolytic solution 4 along with hydrogen ions and electrons by metabolism of the microorganisms carried on the negative electrode 6.

Specifically, for example, when the electrolytic solution 4 contains glucose as an organic substance, carbon dioxide, hydrogen ions and electrons are generated by the following local cell reaction.
Negative electrode 6 (anode): C ₆ H ₁₂ O ₆ +6H ₂ O→6CO ₂ +24H ⁺ +24e ⁻
・Positive electrode 5 (cathode): 6O ₂ +24H ⁺ +24e ⁻ →12H ₂ O

As described above, by using the microbial fuel cell as the liquid treatment unit, the organic substance in the electrolytic solution 4 comes into contact with the negative electrode 6 and is oxidized and decomposed, so that the electrolytic solution 4 can be purified. Further, as described above, a supply unit 2 for supplying the electrolytic solution 4 to the electrolytic solution tank 1 and a discharging unit 3 for discharging the treated electrolytic solution 4 from the electrolytic solution tank 1 to the electrolytic solution tank 1. Is provided, and the electrolytic solution 4 can be continuously supplied. Therefore, the electrolytic solution 4 can be continuously contacted with the negative electrode 6 to efficiently process the electrolytic solution 4.

Hereinafter, the present embodiment will be described in more detail with reference to examples, but the present embodiment is not limited to these examples.

[Production of microbial fuel cell]
(Example 1)
First, a polyolefin resin water-repellent layer was coated with a silicone resin that was an adhesive, and then a graphite foil that was a conductive layer was joined to prepare a laminated sheet composed of a water-repellent layer/silicone adhesive/conductive layer. As the water-repellent layer, SERPUI (registered trademark) manufactured by Sekisui Chemical Co., Ltd. was used. As the silicone resin, one-pack type RTV rubber KE-3475-T manufactured by Shin-Etsu Chemical Co., Ltd. was used. The graphite foil used was made by Hitachi Chemical Co., Ltd.

Next, a gas diffusion electrode was produced by press-molding a catalyst layer formed by mixing an oxygen reduction catalyst and PTFE (manufactured by Aldrich) on the surface of the graphite foil opposite to the water-repellent layer. The oxygen reduction catalyst was press-molded so that the basis weight was 6 mg/cm ² .

The oxygen reduction catalyst was prepared as follows. First, a mixed solution was prepared by placing 3 g of carbon black, a 0.1 M aqueous solution of iron(III) chloride, and a 0.15 M solution of pentaethylenehexamine in ethanol in a container. As the carbon black, Ketjenblack ECP600JD manufactured by Lion Specialty Chemicals Co., Ltd. was used. The amount of the 0.1 M iron(III) chloride aqueous solution used was adjusted so that the ratio of iron atoms to carbon black was 10% by mass. The total amount was adjusted to 9 mL by further adding ethanol to this mixed solution. Then, this mixed liquid was ultrasonically dispersed and then dried at a temperature of 60° C. in a dryer. As a result, a sample containing carbon black, iron (III) chloride, and pentaethylenehexamine was obtained.

Then, this sample was packed into one end of the quartz tube, and then the inside of the quartz tube was replaced with argon. This quartz tube was put in a furnace at 900° C. and then withdrawn in 45 seconds. When inserting the quartz tube into the furnace, the quartz tube was inserted into the furnace for 3 seconds to adjust the heating rate of the sample at the start of heating to 300° C./s. Subsequently, the sample was cooled by circulating argon gas in the quartz tube. As a result, an oxygen reduction catalyst was obtained.

Further, the gas diffusion electrode obtained as described above was laminated on a cassette substrate as shown in FIGS. 1 and 2 to provide an air intake section, thereby producing a positive electrode. Then, the positive electrode and the negative electrode made of a carbon material (graphite foil) were installed in an electrolytic solution tank having a supply part and having a capacity of 300 mL. Further, a polyolefin nonwoven fabric was placed between the positive electrode and the negative electrode. Then, the positive electrode and the negative electrode were connected to a load circuit.

Next, the water repellent layer made of polyolefin was laminated on the cassette base material as shown in FIGS. 1 and 2 to provide an air intake section, thereby producing an oxygen supply section. As the water-repellent layer, SERPUI (registered trademark) manufactured by Sekisui Chemical Co., Ltd. was used. And the oxygen supply part was installed in the electrolytic solution tank which has a discharge part and has a capacity of 300 mL.

Then, as shown in FIGS. 1 and 2, the inside of the electrolytic solution tank was filled with the electrolytic solution. The electrolytic solution was prepared by dissolving starch, peptone, yeast extract, urea, potassium dihydrogen phosphate, calcium chloride, magnesium sulfide, potassium chloride and sodium hydrogen carbonate in deionized water to obtain total organic carbon (TOC). It was adjusted to 800 mg/L. Further, in order to stabilize the pH, sodium hydrogen carbonate was added as a buffer to a concentration of 20 mM. Furthermore, soil microorganisms including anaerobic microorganisms that generate electricity were planted in the electrolytic solution tank. Thus, the microbial fuel cell of this example was obtained.

(Example 2)
As shown in FIG. 4, a microbial fuel cell was obtained by the same method as in Example 1 except that the immersed portion under the positive electrode was removed.

[Evaluation]
The microbial fuel cell was operated so that the hydraulic retention time of the electrolytic solution in the electrolytic solution tank was 24 hours and the water temperature was 30° C., and the average power density from the 15th day after the start of the operation was measured.

In the microbial fuel cell according to Example 1, the average power density from the start of operation to the 15th day was 197 mW/m ² . On the other hand, in the microbial fuel cell according to Example 2, the average output density from the start of operation to the 15th day was 128 mW/m ² .

From the above results, by reducing the dissolved oxygen in the electrolytic solution even in the lower part of the immersed positive electrode, the microbial fuel cell of Example 1 has an average power density improved by 1.5 times as compared with the microbial fuel cell of Example 2. did. As a result, a higher power density can be obtained by arranging the positive electrode soaked part in the electrolytic solution while avoiding formation of the positive electrode air electrode part in the deep part of the electrolytic solution tank where high water pressure is applied. It was shown that

Although the present embodiment has been described above, the present embodiment is not limited to these, and various modifications can be made within the scope of the gist of the present embodiment.

DESCRIPTION OF SYMBOLS 1 Electrolyte tank 1c Bottom wall 4 Electrolyte 4a Liquid level 5 Positive electrode 6 Negative electrode 10 Vapor phase 52 Water repellent layer 53 Conductive layer 54 Air electrode part 100,100A Microbial fuel cell H1 Height to liquid level H2 Air electrode height Y Vertical direction

Claims (4)

An electrolytic solution tank holding an electrolytic solution containing an organic substance,
Supporting anaerobic microorganisms, the negative electrode immersed in the electrolytic solution,
A positive electrode electrically connected to the negative electrode and having an air electrode portion,
Equipped with
One surface of the air electrode portion straddles the liquid surface of the electrolytic solution in the vertical direction, and is in contact with a gas phase containing oxygen,
The microbial fuel cell, wherein the ratio of the height of the air electrode portion to the height from the inner surface of the bottom wall of the electrolytic solution tank to the liquid surface of the electrolytic solution is 1/5 or more and 2/3 or less. 2. The microorganism according to claim 1, wherein the height from the inner surface of the bottom wall of the electrolytic solution tank to the liquid surface of the electrolytic solution is 3 m or more and 5 m or less, and the height of the air electrode part is 1 m or more and 2 m or less. Fuel cell. The microbial fuel cell according to claim 1 or 2, wherein the positive electrode includes a water repellent layer and a conductive layer laminated on the water repellent layer. A liquid treatment unit comprising the microbial fuel cell according to claim 1.

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