Microbial fuel cells

Abstract

The objective is to provide a microbial fuel cell that can efficiently extract electrical energy from organic fertilizers given to plants, thereby improving power production capacity. [Solution] A microbial fuel cell characterized by comprising an anode electrode which is a metal with a negative standard electrode potential, a cathode electrode which is a carbon material or a metal with a higher standard electrode potential than the anode electrode, and an electrolyte which is an organic fertilizer containing a microorganism that promotes the growth of current-generating bacteria that generate electrons in an anaerobic environment when a voltage is applied.

Description

Technical Field

[0001] The present invention relates to a microbial fuel cell that extracts electrical energy from organic fertilizers.

Background Art

[0002] A microbial fuel cell is a system that produces electricity from organic substances by utilizing the dissimilatory metabolism ability of microorganisms. Sewage sludge contains organic substances included in industrial wastewater and domestic wastewater, etc., and accumulates on the river bottom and the sea bottom together with mud. When biomass treatment is performed to decompose the organic substances in the sludge by microorganisms, protons and electrons are generated and electrical energy can be directly recovered in the form of electricity. In a microbial fuel cell, electrons released from microorganisms are transferred to an anode electrode. The electrons move from the anode electrode through an external load to a cathode electrode, where they react with a compound serving as an oxidant (electron acceptor) and protons (H + ) diffused from the anode side. Therefore, by connecting a load between the anode electrode and the cathode electrode, it functions as an electric circuit.

[0003] As the material of the anode electrode, graphite felt having conductivity and a large surface area is often used. The electrons move from the anode through an external load to the cathode, where they react with a compound serving as an oxidant (electron acceptor) and protons diffused from the anode side. As the oxidant, potassium ferricyanide or manganese oxide may be used, but in many microbial fuel cells, oxygen in the atmosphere that does not cost much is used. As the material of the cathode electrode, graphite felt having conductivity and a large surface area, similar to the anode electrode, is often used. Since oxygen in the atmosphere is used, a breathable material is preferable.

[0004] Platinum is often used as a catalyst for the cathode reaction. In essence, a microbial fuel cell can extract electrical energy if protons and electrons are generated. Plants growing in soil produce sugar and oxygen using carbon dioxide and water through photosynthesis. The sugar produced by photosynthesis does not remain in the plant but is released, with some of the sugar being released from the plant's roots. The sugar released from the roots is broken down by bacteria present in the soil, producing protons and electrons, and these bacteria are called current-generating bacteria. The electrons move from the anode electrode through an external load to the cathode electrode, where they are converted into electron acceptors and protons (H + ) will be the reaction.

[0005] There are two main types of microbial fuel cells: double-chamber and single-chamber. In a double-chamber microbial fuel cell, the anode and cathode are separated by a proton exchange membrane, allowing protons to permeate from the anode to the cathode. This type has the advantage of high airtightness. On the other hand, single-chamber microbial fuel cells use a membrane-type cathode called an air cathode (oxygen positive electrode). The air cathode is permeable to oxygen, and oxygen that permeates from the atmosphere reacts with protons via a platinum catalyst coated on the inside, converting it to water. While the energy recovery efficiency from organic matter is lower in this single-chamber type, and its structure is more complex and difficult to install, it offers lower costs, lower internal resistance, and higher output compared to the double-chamber type because it does not use a proton exchange membrane.

[0006] Patent Document 1 discloses a method for increasing electric current by adding fertilizer. Fertilizer, as defined in the Fertilizer Law, is a substance applied to soil for the purpose of providing nutrients to plants or causing chemical changes in the soil to facilitate plant cultivation.

[0007] The device comprises a cathode electrode made of a metal or carbon with a positive standard oxidation-reduction potential, which serves as an indicator of ionization tendency; an anode electrode made of a metal or Prussian blue with a negative standard oxidation-reduction potential, which serves as an indicator of ionization tendency; and an electrolyte composed of at least water or soil. By adding fertilizer to the electrolyte, power is increased by applying a voltage that utilizes the difference in oxidation-reduction potential between the cathode and anode electrodes.

[0008] Patent Document 2 discloses a two-compartment microbial fuel cell utilizing plants. The container comprises an anode compartment and a cathode compartment. An anode electrode is placed in the anode compartment, and a cathode electrode is placed in the cathode compartment. The cathode compartment and the anode compartment are separated from each other by an ion exchange membrane. The plant is housed with its roots located in the anode compartment. When light energy such as sunlight is irradiated onto the plant, sugars are released from the plant roots into the anode compartment through photosynthesis.

[0009] Graphite felt is used for the cathode and anode electrodes. The cathode compartment is filled with 50 mM K3Fe(CN)6 and 100 mM KH2PO4, neutralized to a pH of approximately 7.

[0010] Patent Document 3 discloses an electrode for a microbial fuel cell capable of generating a high-power current in a microbial fuel cell, and a microbial fuel cell using the same. As the anode electrode of the microbial fuel cell, a nanowire structure is formed on the surface of the electrode substrate using a conductive polymer to increase the electrode surface area. It has been found that this increases the charge transfer efficiency from microorganisms to the electrode by 10 to 100 times compared to conventional electrodes for microbial fuel cells.

[0011] Patent Document 4 discloses a microbial fuel cell capable of improving power production capacity and suppressing power generation costs, an electrode for a microbial fuel cell and a method for manufacturing the same, a method for producing electricity using microorganisms, and a method for selectively culturing microorganisms used in the electricity production method. The fuel cell has a liquid containing an organic substance and an anode electrode, and the anode electrode, which biodegrades the organic substance by microorganisms in an anaerobic atmosphere, a cathode electrode, and an external circuit that electrically connects the anode electrode and the cathode electrode, and the anode electrode is made of graphene. The graphene provided in the anode electrode is an excellent electron conductive material, and therefore, the graphene can facilitate electron transfer from the microorganisms to the negative electrode, thereby improving power production.

[0012] Patent Document 5 discloses a microbial fuel cell that can increase current density without using an electron-transferring mediator. The current density is improved by adding nano-sized iron oxide nanoparticles (Fe2O3) to Shewanella bacteria, which are current-generating bacteria, and forming aggregates with a three-dimensional structure. [Prior art documents] [Patent Documents]

[0013] [Patent Document 1] Japanese Patent Publication No. 2022-125379 [Patent Document 2] U.S. Patent Application Publication No. 2010 / 0190039 [Patent Document 3] International Publication No. 2011 / 025021 [Patent Document 4] International Publication No. 2013 / 073284 [Patent Document 5] International Publication No. 2009 / 119846 [Overview of the Initiative] [Problems that the invention aims to solve]

[0014] In conventional microbial fuel cells, the extractable electric power is extremely low, and there are problems such as further increasing the voltage and improving the power production amount in practical applications.

[0015] Conventionally, in order to improve the power production amount, it has been proposed to perform improvements on the electrodes and add fertilizers to the soil. However, the power production amount was still low and improvement was necessary.

[0016] If a nanocarbon material is used, the battery characteristics can be improved. However, the production of nanocarbon materials requires advanced technology, and it is difficult to mass-produce those with excellent electrical conductivity at low cost. In addition, since it is a mechanism that collects electricity from only a small number of microorganisms that come into contact with the anode electrode among the microorganisms present throughout the container, even if the battery performance is improved by improving the anode electrode, there is a problem that it is difficult to significantly improve the power production.

[0017] The technology of using graphene for the anode electrode is limited to about twice that without graphene input because it is difficult for graphene to disperse in an aqueous solution and efficiently contact the enzyme, which is a catalyst.

[0018] Furthermore, the soil has a lot of variation, and the presence of current-generating bacteria such as required Shewanella bacteria and Geobacter bacteria also has a high dependence on the soil.

[0019] The present invention has been made to solve the above problems, and an object thereof is to provide a microbial fuel cell that can efficiently extract electrical energy using organic fertilizer as an electrolyte and improve the power productivity.

Means for Solving the Problems

[0020] The present invention is a microbial fuel cell that can efficiently extract electrical energy using organic fertilizer as an electrolyte.

[0021] Note that "microorganisms" means minute organisms, typically cells or multicellular organisms in which little cell function or tissue differentiation is observed, and is used in a broad sense to include single-celled bacteria (bacteroids) with prokaryotic cells and fungi, which are eukaryotes that perform absorptive heterotrophy. Also, it is assumed that the soil naturally contains moisture.

[0022] (1) A microbial fuel cell characterized by comprising an anode electrode made of a metal with a negative standard electrode potential, a cathode electrode made of a carbon material or a metal with a standard electrode potential higher than that of the anode electrode, and an electrolyte composed of an organic fertilizer containing a microbial flora that generates electrons in an anaerobic environment by applying a voltage.

[0023] (2) A microbial fuel cell characterized in that the microbial flora is Deltaproteobacteria.

[0024] (3) A microbial fuel cell characterized in that the microbial flora is Gammaproteobacteria.

[0025] (4) A microbial fuel cell characterized in that the microbial flora is the phylum Bacteroidetes.

[0026] (5) A microbial fuel cell characterized in that the metal of the anode electrode is magnesium or aluminum.

[0027] (6) A microbial fuel cell characterized in that a conductive film is formed on the magnesium or the aluminum.

[0028] (7) A microbial fuel cell characterized in that the carbon material of the cathode electrode is Binchotan charcoal, or carbon graphite, hard carbon, or graphite.

[0029] (8) A microbial fuel cell characterized in that the metal of the cathode electrode is stainless steel, copper, silver, or gold.

[0030] (9) The microbial fuel cell is characterized in that the cathode electrode is coated with a photocatalyst.

[0031] (10) A microbial fuel cell characterized in that a microbial fuel cell installation area is provided in a part of the soil, containing only the organic fertilizer or a mixture of the organic fertilizer and a conductive material as the electrolyte, and the cathode electrode and the anode electrode are embedded in it.

[0032] (11) An energy storage system comprising: a number of microbial fuel cells according to claim 1 buried in the soil; a plurality of solar panels installed on the surface of the soil; a battery for storing electrical energy from the microbial fuel cells and the solar panels; and a control unit having a DC-DC converter for controlling the storage of electrical energy from the microbial fuel cells and the solar panels into the battery.

[0033] (12) The control unit is equipped with an on / off function that depends on the amount of electrical energy from the solar panel and controls the supply of electrical energy to a load connected to the battery, and is a power storage system characterized in that. [Effects of the Invention]

[0034] (1) A microbial fuel cell consists of a cathode electrode, an anode electrode, and an electrolyte. Microorganisms oxidize and decompose organic matter, and an electric current is generated when electrons produced in this process move from inside the microbial cells to the electrode (anode electrode). The present invention is characterized by an electrode structure in which the anode electrode is made of a metal with a negative standard electrode potential and the cathode electrode is made of a carbon material or a metal with a higher standard electrode potential than the anode electrode, and a voltage can be applied to the electrolyte due to the difference in standard electrode potentials.

[0035] The electrolyte was an organic fertilizer containing a microorganism that generates electrons in an anaerobic environment when voltage is applied. Bacteria whose electron generation and electron transfer are promoted by the application of voltage possess an external electron transport mechanism and can release electrons into the external environment by using the electrode as an electron acceptor.

[0036] (2) The microbiome, which consists of bacteria whose electron generation and electron transfer are promoted by the application of voltage, is preferably deltaproteobacteria. It has been experimentally confirmed that applying a constant voltage improves the electron supply efficiency of the bacteria, resulting in an increase in power generation.

[0037] (3) The microbiome, which consists of bacteria whose electron generation and electron transfer are promoted by the application of voltage, is preferably gammaproteobacteria. Similar to deltaproteobacteria, it has been experimentally confirmed that applying a constant voltage improves the electron supply efficiency of the bacteria, resulting in an increase in power generation.

[0038] (4) The microbiome, which consists of bacteria whose electron generation and electron transfer are promoted by the application of voltage, is preferably of the Bacteroidetes phylum. Similar to Deltaproteobacteria, it has been experimentally confirmed that applying a constant voltage improves the electron supply efficiency of bacteria, resulting in an increase in power generation.

[0039] (5) The metal of the anode electrode is preferably magnesium or aluminum. There are various metals with a negative standard electrode potential, and the lower the potential, the greater the voltage that can be obtained, but lithium, for example, is expensive and an unstable metal. For this reason, magnesium and aluminum are suitable as they are abundant and readily available resources.

[0040] (6) It is preferable that a conductive coating is formed on the magnesium or aluminum. By forming a conductive coating, the metal electrode does not come into direct contact with the electrolyte, which prevents chemical reactions and corrosion. This results in a microbial fuel cell with stable characteristics over the long term.

[0041] (7) The carbon material of the cathode electrode is preferably Binchotan charcoal, carbon graphite, hard carbon, or graphite. These carbon materials are electrochemically very stable, and the standard electrode potential can be considered to be approximately 0V.

[0042] (8) The metal of the cathode electrode may be stainless steel, copper, silver, or gold. These metal materials have a positive standard electrode potential, which is more positive than that of carbon materials, and thus a higher voltage can be obtained as a microbial fuel cell.

[0043] (9) The cathode electrode may be coated with a photocatalyst. Generally, photocatalysts excite electrons from the valence band to the conduction band when irradiated with light, and a strong oxidation-reduction reaction functions. For example, they can be used for water purification and the decomposition of harmful substances, and can decompose organic pollutants and heavy metals using sunlight as an energy source.

[0044] However, in this invention, an anode electrode made of a metal with a negative standard electrode potential and a cathode electrode made of carbon material or a metal with a higher standard electrode potential than the anode electrode are used, and a voltage is applied to the electrolyte due to the difference in standard electrode potentials. Therefore, even if binchotan charcoal (cathode electrode) is placed in an anaerobic atmosphere where no light is present, the photocatalyst coated on its surface performs oxidation-reduction, decomposing organic matter and generating electrons. This phenomenon has been confirmed experimentally. As a result, high performance can be obtained as a microbial fuel cell.

[0045] (10) A microbial fuel cell can be created by providing a microbial fuel cell installation area in a part of the soil, using only organic fertilizer or a mixture of organic fertilizer and conductive material as the electrolyte, and by providing a microbial fuel cell installation area and burying the cathode electrode and anode electrode. By providing a microbial fuel cell installation area, the soil can be fertilized with fertilizer suitable for plant growth, and the electrolyte of the microbial fuel cell can be made suitable for the microbial fuel cell, using only organic fertilizer or a mixture of organic fertilizer and conductive material as the electrolyte. The conductive material is, for example, iron oxide, and the acquisition current increases by reducing the internal resistance.

[0046] (11) By providing multiple microbial fuel cells buried in the soil, multiple solar panels installed on the soil surface, a battery for storing electrical energy from the microbial fuel cells and solar panels, and a control unit that has a DC-DC converter and controls the storage of electrical energy from the microbial fuel cells and solar panels into the battery, it can be used as an energy storage system. Since solar panels are installed above ground and electrodes for the microbial fuel cells are set underground, effective land use is possible.

[0047] Furthermore, since obtaining power from microbial fuel cells is effective on rainy or cloudy days and at night, and obtaining power from solar panels is effective on sunny days, this energy storage system is less dependent on weather conditions.

[0048] (12) By providing a control unit with an on / off function that depends on the amount of electrical energy from the solar panels, the supply of electrical energy to the system can be automatically controlled to be limited to nighttime or daytime only. Examples of nighttime-only electrical energy supply include streetlights. In addition, LED lighting in plant cultivation is mainly done during the daytime. [Brief explanation of the drawing]

[0049] [Figure 1] This is a block diagram illustrating the microbial fuel cell according to the present invention. [Figure 2] This is a cross-sectional view of the anode electrode 10. [Figure 3] This is a cross-sectional view of the cathode electrode 12. [Figure 4] This shows the experimental plot where organic fertilizer was applied. [Figure 5] This shows the changes in pH and EC values ​​between the control group (no organic fertilizer applied) and the control group (organic fertilizer applied). [Figure 6] This shows data measuring the relationship between voltage (open-circuit voltage) and current (short-circuit current), as well as the relationship between the number of microorganisms and the number of days elapsed, in a control group (no organic fertilizer applied) and an experimental group (organic fertilizer applied). [Figure 7]This data shows the types and proportions of soil microbiota in the control and experimental plots, with the number of days elapsed on the horizontal axis. [Figure 8] This graph focuses on Bacteroidetes, Gammaproteobacteria, and Deltaproteibacteria, which are microorganisms that contribute to electric current generation in the soil microbial community when organic fertilizers are applied. [Figure 9] This data shows the proportion of microbial flora in organic fertilizer compost. [Figure 10] This data shows how the number of power-generating bacteria (iron-reducing bacteria) and microbial corrosion-related microorganisms changes over time. [Figure 11] This figure shows the effect of applying organic fertilizer on power generation and the microbial flora. [Figure 12] This diagram illustrates an example of installing a microbial fuel cell in soil. [Figure 13] This diagram illustrates an energy storage system that utilizes efficient land use to enable the coexistence of solar panels and microbial fuel cells. [Modes for carrying out the invention]

[0050] First, the principle of microbial fuel cells will be explained, and then the differences between this invention and conventional microbial fuel cells will be clarified.

[0051] Living organisms survive by taking in the energy necessary to maintain their homeostasis and reproduce from the outside world. Electrons are generated during this energy acquisition process, and by extracting these generated electrons, microbial fuel cells are created. The mechanisms by which living organisms acquire energy can be broadly classified into three types: respiration, fermentation, and photosynthesis. Most living organisms obtain the energy necessary for life activities by using oxygen for respiration (aerobic respiration). The essential meaning of respiration is to synthesize ATP based on the energy (difference in oxidation-reduction potential between the two reactions) generated when the oxidation reaction of an electron-donating substance (organic matter, etc.) and the reduction reaction of an electron-accepting substance (oxygen, etc.) are coupled.

[0052] ATP (adenosine triphosphate) is involved in the transport and supply of energy within living organisms and is produced through metabolic processes such as cellular respiration (aerobic respiration) and fermentation (anaerobic respiration). In particular, oxidative phosphorylation in mitochondria is the major ATP production pathway. ATP is an essential molecule for organisms to maintain life activities, and its production and utilization play a crucial role in the basic life processes of living organisms.

[0053] Respiration in eukaryotes takes place in the inner membrane of mitochondria, an organelle within the cell, while in prokaryotes such as bacteria, it occurs in the intracellular membrane. When organisms oxidize and decompose organic matter, the electrons that were present in the organic matter are transferred to electron transport molecules (NAD) within the cell. + ) is transferred to ), and reduced NAD (NADH) is synthesized. In aerobic respiration, NADH is first oxidized by enzymes present in the inner membrane, and the electrons released as a result move through the respiratory chain electron transport system, which is composed of quinones and cytochrome proteins, and are finally transferred to oxygen by the action of terminal oxidases.

[0054] During this process, energy equivalent to the difference in redox potential between the electron donor (NADH) and the final electron acceptor (oxygen) is released. This energy is used by the proton pump to utilize the electrochemical potential difference between protons across the membrane as electrons pass through the respiratory electron transport chain, enabling organisms to synthesize ATP. The respiratory electron transport chain functions as an internal electrical network within the cell, and organisms extract energy by passing an electric current through this network.

[0055] Some microorganisms can transfer electrons to substances other than oxygen. Such microorganisms can perform respiration (anaerobic respiration) and obtain energy even under anaerobic conditions where oxygen is absent. Electrical microorganisms are anaerobic respiration microorganisms that obtain the energy necessary for cell maintenance and proliferation through respiration (electrode respiration) in which electrodes act as the final electron acceptors. Microorganisms that perform electrode respiration possess a respiratory chain transmission pathway (extracellular electron transport pathway) that spans the cell membrane, allowing them to transfer electrons to electrodes located outside the cell.

[0056] The amount of energy an organism can obtain through respiration is determined by the difference in redox potential between the electron donor and the electron acceptor. In electrode respiration, the redox potential of the electrode is generally lower than that of oxygen, so the amount of energy that microorganisms can obtain is less than in oxygen respiration. However, in MFCs, it can be thought that the difference in energy is recovered as electrical power. When oxygen is used as the oxidizing agent for the cathode electrode, the difference between the redox potential of NADH and the electrode potential of the anode becomes the maximum amount of energy that the microorganism can obtain, and the potential difference between the anode potential and oxygen is recovered as electrical power.

[0057] Bacteria that perform electrode respiration are thought to respire using solid metal oxides (such as iron oxide) as electron acceptors in their natural environment (metallic respiration). Bacteria that perform metallic respiration are called metal-reducing bacteria, and Shewanella and Geobacter have been discovered. Shewanella is a Gram-negative facultative anaerobic bacterium and is attracting attention as an "electric current-generating bacterium." This bacterium often lives in water and soil, especially in oxygen-poor environments, and has the ability to generate electric current by using specific substances as electron acceptors even in the absence of oxygen.

[0058] Shewanella bacteria use metal ions such as iron and manganese, or sulfates and nitrates, as electron acceptors instead of oxygen, and expel electrons from the cell through metabolism, generating an electric current in this process. Proteins such as cytochrome present in the outer membrane play a crucial role, acting as an "electron shuttle" to move electrons out of the cell. Shewanella bacteria are used in microbial fuel cells because they form bacterial aggregates called biofilms, increasing the contact area with electrodes and efficiently generating electric current. Furthermore, Shewanella bacteria also have the ability to reduce heavy metals and harmful substances; for example, they can reduce and insolubilize metals such as uranium and chromium, playing a role in soil and water purification.

[0059] While Geobacter bacteria share similar properties with Shewanella bacteria, there are also differences. In terms of electron transport mechanisms, Geobacter bacteria utilize pyripyri proteins, while Shewanella bacteria utilize cytochromes. The pyripyri proteins of Geobacter bacteria are particularly excellent for long-distance electron transfer, enabling more efficient electric current generation. Regarding habitat, Geobacter bacteria are commonly found in soil and water, while Shewanella bacteria are more commonly found in aquatic environments such as oceans and lakes.

[0060] In the principle of microbial fuel cells, electrons released from microorganisms are first transferred to the anode electrode. Graphite felt, which is stable in microbial culture tanks and has a large surface area, is often used as the anode electrode material. The electrons then move from the anode to the cathode via an external load, where they react with an oxidizing agent (electron acceptor) compound and protons diffused from the anode, completing the circuit of the microbial fuel cell. While compounds such as potassium ferricyanide and manganese oxide are sometimes used as oxidizing agents, in most cases, cost-effective atmospheric oxygen is utilized.

[0061] However, since the reduction reaction of oxygen involves a large activation energy, a catalyst that reduces this energy may be used. Chemical catalysts (mainly platinum) are often used as catalysts for the cathode reaction. Research is also underway to find alternative catalysts to expensive platinum, and there have been reports that catalysts in which inexpensive transition metals (iron or cobalt) are coordinated to a nitrogen-containing carbon skeleton are useful as cathode catalysts for MFCs.

[0062] Microbial fuel cells are devices that generate electricity using the metabolism of microorganisms, and their structure is broadly classified into two types: "single-tank type" and "double-tank type." In a double-tank microbial fuel cell, the anode tank and cathode tank are separated by a proton exchange membrane, and oxidizing agents such as oxygen are supplied to the cathode tank. This type has the advantage of being highly airtight, but because oxygen has low solubility in water, aeration is necessary when using oxygen as an oxidizing agent.

[0063] On the other hand, single-tank microbial fuel cells use a membrane-type cathode called an air cathode (oxygen diffusion positive electrode) that is in contact with air. The air cathode is permeable to oxygen, and the oxygen that permeates from the atmosphere reacts with protons by a cathode catalyst coated on the inside to form water. In this system, the energy recovery efficiency from organic matter is low because the excess oxygen that permeates into the tank is consumed by microorganisms, but the running costs are lower compared to the two-tank type because aeration is not required. In addition, because the distance between the anode and cathode can be shortened, the internal resistance due to proton diffusion transfer between electrodes is reduced, and a higher output can be obtained.

[0064] Recently, attempts have been made to efficiently obtain electricity by integrating two-layer and single-tank types. Specifically, the cathode electrode of the cathode tank, which is separated by a proton exchange membrane, is made into an air cathode structure, allowing for efficient acquisition of electrical energy without aeration treatment. However, this structure becomes more complex and costly.

[0065] Microorganisms in the soil naturally play various roles and influence the characteristics of microbial fuel cells. Because microbial fuel cells can use biomass such as sludge and food waste as fuel, they are also a sustainable power generation system. Furthermore, since the microorganisms themselves function as biocatalysts that extract electrons from organic matter, they also have the advantage of being low-cost.

[0066] Among the microorganisms present in soil are highly thermophilic sulfur-dependent archaea that use hydrogen as an electron emitter and sulfur as an electron acceptor, producing hydrogen sulfide. Furthermore, there are methanogenic bacteria that use carbon dioxide as an electron acceptor, sulfate-reducing bacteria that use sulfates as electron acceptors, acetate-producing bacteria that use carbonates as electron acceptors, and catabolic iron-reducing bacteria that use iron as an electron acceptor.

[0067] Among microorganisms, there are bacteria called electrogenic bacteria that have the property of releasing electrons into the environment during the process of decomposing organic matter. Electrogenic bacteria are catabolic iron-reducing bacteria, which, by directly contacting amorphous iron oxides, reduce trivalent iron to divalent iron, producing nano-sized magnetic particles.

[0068] Examples of catabolic iron-reducing bacteria include Geobacter and Shewanella. These bacteria dislike oxygen in the air and live in environments with little to no oxygen, such as underground, the seabed, and swamp bottoms. When these current-generating bacteria are given water along with organic matter, they decompose the organic matter, releasing protons and electrons. If the released electrons are passed to the anode electrode and flow to the cathode electrode, an electric current flows, and at the cathode electrode, the protons react with oxygen to form water. Therefore, if organic matter and water are continuously supplied to the microorganisms, sustained power generation becomes possible.

[0069] Plant photosynthesis can be used to continuously supply organic matter to the soil. Plants perform photosynthesis in chloroplasts using sunlight, and from water absorbed through xylem vessels in the roots and carbon dioxide from the air, they produce organic matter, such as sucrose (C6H2). 12 Photosynthesis synthesizes oxygen (O6) and starch. The oxygen produced during the decomposition of water is released into the air. Some of the organic matter produced by photosynthesis is released into the soil from the plant roots. This allows for a continuous supply of organic matter as an energy source.

[0070] The technical background of microbial fuel cells has been explained above. Conventional microbial fuel cells use conductors containing carbon as an electrode component, and because the cathode electrode and anode electrode are made of the same material, the amount of electrical energy that can be extracted is small, for example, the open-circuit voltage is only a few hundred mV. In addition, electron generation relies on respiration and current-generating bacteria, so the amount obtained is small, and further improvement in power output is desired for practical application.

[0071] In this invention, the electrodes and electrolytes are redesigned to improve the performance of the microbial fuel cell.

[0072] Figure 1 is a block diagram illustrating a microbial fuel cell according to the present invention. The microbial fuel cell 1 comprises an anode electrode 10, an electrolyte 14, and a cathode electrode 12. The electrolyte 14 is an organic fertilizer, and the anode electrode 10 and the cathode electrode 12 are connected by a wire 18 via a load 16 to form an electrical circuit, becoming a microbial fuel cell. The anode electrode 10 and the cathode electrode 12 use materials with different standard electrode potentials, and the voltage of the microbial fuel cell is determined depending on this difference in standard electrode potential.

[0073] Figure 2 is a cross-sectional view of the anode electrode 10. A conductive coating 22 is formed on the anode electrode metal 20. The standard electrode potential of the anode electrode metal 20 should be small. Examples of metals with a negative standard electrode potential include lithium (standard electrode potential: -3.05V), potassium (standard oxidation-reduction potential: -2.93V), magnesium (standard electrode potential: -2.36V), aluminum (standard electrode potential: -1.68V), titanium (standard oxidation-reduction potential: -1.63V), zinc (standard electrode potential: -0.76V), iron (standard oxidation-reduction potential: -0.44V), and nickel (standard electrode potential: -0.26V).

[0074] Of these metals with negative standard electrode potentials, magnesium and aluminum are preferred because they are abundant, readily available, and have as low a standard electrode potential as possible. In particular, magnesium is a substance that is abundant in seawater, and its standard electrode potential is low at -2.36V, making it a preferred material for the anode electrode metal 20.

[0075] The anode electrode 10 has a conductive coating 22 formed on the surface of the anode electrode metal 20. This is to prevent chemical reactions of the anode electrode metal. Generally, chemical batteries are based on the voltaic cell principle, in which the metal on the anode side is electrolyzed by a chemical reaction, and the electrons generated at that time are extracted at the anode electrode and flowed to the cathode electrode side, thereby operating as a battery.

[0076] However, when used as a microbial fuel cell 1, electrons are generated by microorganisms, so there is no need to chemically react the anode metal. In fact, chemical reactions can shorten the lifespan or cause corrosion that makes the metal an insulator, hindering operation and posing a barrier to practical application. Therefore, in order to prevent chemical reactions while maintaining voltage, it is necessary to form a coating so that the electrolyte 14 and the anode electrode metal 20 do not come into direct contact. Even when the electrolyte 14 and the anode electrode metal 20 do not come into direct contact, a voltage is still generated between the anode electrode 10 and the cathode electrode 12.

[0077] When the coating is insulating, the open-circuit voltage also increases. For example, when magnesium is used as the anode electrode metal 20 and binchotan charcoal is used as the cathode electrode 12, the open-circuit voltage was approximately 2V. Naturally, because the coating is insulating, no short-circuit current flows. By making the coating conductive, current flows, and the higher the conductivity, the easier it is for current to flow.

[0078] The conductive coating 22 can be realized by imparting conductivity to a resin (paint) by mixing a conductive material. Urethane resin, silicone resin, and fluororesin can be used as the resin. The conductive material can be broadly classified into carbon-based and metal filler-based types.

[0079] One example of a carbon-based material is graphite. Graphite, also known as carbonite, is an elemental mineral composed of carbon, with a hexagonal crystal system and hexagonal plate-like crystal structure. Its structure is a layered material resembling a tortoise shell, where carbon atoms are connected by strong covalent bonds within each layer's plane, but the spaces between layers (interplanes) are bonded by weak van der Waals forces. Furthermore, carbon-based materials include amorphous carbon, which does not have a distinct crystalline structure, such as carbon fiber, charcoal, and activated carbon, and this amorphous carbon can also be used. Because it has a tubular hollow space inside, it can encapsulate various molecules.

[0080] Carbon nanotubes can also be used. Carbon nanotubes have a structure like a uniform, flat graphite rolled into a cylinder. When closed, both ends are closed with a hemispherical structure similar to that of fullerenes, and each end always has six five-membered rings. Because of the small number of five-membered rings, they are not easily soluble in organic solvents. Because the tube has a cylindrical structure, various substances can be incorporated inside by burning off the cap, etc.

[0081] Furthermore, Ketjenblack can also be used. Compositionally, Ketjenblack consists of carbon, similar to carbon black for rubber, carbon fibers, and graphite, and is composed of crystallites called pseudo-graphite structures. The crystallites consist of condensed benzene rings with π electrons, and these π electrons can move freely on the carbon black. Because the π electrons move on conductive circuits formed by aggregates and agglomerates, materials to which Ketjenblack is added exhibit conductivity. Ketjenblack is characterized by its high specific surface area and porosity, and the formation of conductive circuits due to increased particle density is dominant, resulting in high conductivity.

[0082] Commonly used materials as conductive fillers (metal fillers) are gold, silver, and copper. Gold and silver, in particular, have stable properties and are excellent conductive materials, but they are expensive. Copper has high conductivity second only to silver and is relatively inexpensive, but it oxidizes easily, so oxidation prevention treatment is necessary. Nickel, like copper, oxidizes easily, but it has relatively good corrosion resistance. It is used as a filler in nickel powder or nickel coatings. Aluminum is lightweight and conductive, but it easily forms oxide films, and its conductivity may not be stable. Therefore, oxidation prevention treatment is necessary, similar to copper.

[0083] Other materials include conductive titanium oxide and antimony-doped tin oxide. Antimony doping improves the electrical conductivity of tin oxide. Antimony acts as a dopant, supplying electrons to the band structure of tin oxide, thereby increasing its conductivity as an n-type semiconductor. Furthermore, when combined with titanium oxide, its heat resistance and chemical resistance are improved, resulting in stability in various environments.

[0084] Conductive materials include carbon-based and metal filler-based materials. However, with carbon-based materials, the standard electrode potential of carbon itself is considered to be around 0V, so when a conductive film is formed, the open-circuit voltage of the microbial fuel cell tends to be low. Similarly, with metal-based materials, gold, silver, and copper have positive standard electrode potentials, which tends to lower the open-circuit voltage. Titanium oxide and tin oxide also tend to lower the open-circuit voltage. When binchotan charcoal was used as the cathode electrode 12, the open-circuit voltage for all of these materials was around 1.2 to 1.4V.

[0085] On the other hand, when aluminum and nickel powders treated to prevent oxidation were used as conductive materials, an open-circuit voltage of 1.5 to 1.8 V was obtained when Binchotan charcoal was used as the cathode electrode 12. In particular, since aluminum has a lower standard electrode potential than nickel, at -1.68 V, the open-circuit voltage was higher for aluminum.

[0086] Figure 3 is a cross-sectional view of the cathode electrode 12. The cathode electrode 12 is made of a carbon material or a metal 24 for cathode electrodes, with a photocatalytic coating 26 formed on it. Since the open-circuit voltage of the cathode electrode 12 depends on the difference in standard electrode potential between it and the anode electrode 10, a higher standard electrode potential is preferable. Carbon material is an extremely stable material, and its standard electrode potential can be considered to be 0V, making it a suitable material for cathode electrodes. Examples of carbon materials include graphite, activated carbon, carbon graphite, hard carbon, and binchotan charcoal, all of which can be used.

[0087] Metals with a positive standard electrode potential include, for example, copper (standard oxidation-reduction potential: 0.34V), silver (standard oxidation-reduction potential: 0.80V), platinum (standard oxidation-reduction potential: 1.19V), and gold (standard oxidation-reduction potential: 1.52V). Furthermore, stainless steel (especially austenitic stainless steel such as SUS304 and SUS316) has a relatively positive potential due to the formation of an oxide film, which gives it high corrosion resistance. Generally, the standard electrode potential is considered to be around +0.1V to +0.3V (SHE reference). Flame-oxidized stainless steel, which is produced by oxidizing stainless steel to create a thicker oxide layer, has improved surface corrosion resistance, and the electrode potential may rise slightly, sometimes reaching around +0.3V to +0.5V (SHE reference). However, the potential changes depending on the thickness and uniformity of the oxide layer, so it is easily affected by measurement conditions and environment.

[0088] The cathode electrode 12 has a photocatalytic coating 26 formed on the surface of a carbon material or a metal 24 for the cathode electrode. The photocatalysts include those primarily composed of anatase-type titanium dioxide and those primarily composed of tungsten oxide. In both cases, coating the carbon material with the photocatalyst increased the open-circuit voltage and short-circuit current. The open-circuit voltage was approximately 0.1-0.2V higher than without the photocatalytic coating. Furthermore, the short-circuit current increased by 10-20% with the photocatalytic coating.

[0089] This result was obtained under conditions where the photocatalyst was not exposed to light (in soil), and the phenomenological explanation remains unclear. Photocatalysts work by exciting electrons in the valence band into the conduction band upon irradiation with light, thereby causing a strong oxidation-reduction reaction and resulting in phenomena such as the decomposition of organic matter. However, in this invention, by coating the cathode electrode 12 with a photocatalyst to form a photocatalytic film 26, it is presumed that a similar phenomenon to that occurring when light is irradiated occurs even in soil where light is not irradiated.

[0090] Currently, the reason is under investigation, but it is presumed that an activation effect is occurring in the soil, suggesting that electrons are excited from the valence band to the conduction band due to some cause. The most likely cause is the application of a voltage due to the difference in standard electrode potentials between the anode electrode 10 and the cathode electrode 12. It seems natural to think that the electrons were excited by the energy from this applied voltage. However, this has not been definitively verified and remains merely a hypothesis. We plan to continue investigating this further.

[0091] In the microbial fuel cell 1 of the present invention, the electrolyte is organic fertilizer. The voltage can be increased by utilizing the difference in standard electrode potentials of the electrodes, but the current is generated by electrons produced by the decomposition of organic matter. Therefore, in order to increase the current, it is necessary to increase the number of Shewanella and Geobacter bacteria that decompose organic matter. Normally, Shewanella and Geobacter bacteria exist in the soil, and it is difficult to increase them by adding fertilizer from outside. Therefore, it is necessary to analyze the microbial community of organic fertilizer and experimentally find a microbial community that promotes the growth of Shewanella and Geobacter bacteria. By doing so, the microbial community that promotes the growth of Shewanella and Geobacter bacteria can be identified, and by using organic fertilizer that contains a large amount of this microbial community, the current can be increased.

[0092] Next, we will install a microbial fuel cell and clarify the microbial community when organic fertilizer is used as the electrolyte. Example 1

[0093] Figure 4 shows the experimental plot where organic fertilizer was applied. The control plot where organic fertilizer was not applied was similar. Two plants were planted, and a microbial fuel cell area was set up between them (Figure 4, top). First, the experimental plot was fertilized with the chemical fertilizer MagAmp K (nitrogen N, phosphorus P, potassium K, magnesium Mg) (April 18, 2023). At this time, BL (Botanical Light: microbial fuel cell) was also installed. Approximately one month later (May 26, 2023), the experimental plot was fertilized with organic fertilizers, cow manure compost and bark compost (Figure 4, bottom). Measurement and analysis of soil samples are described below.

[0094] For soil samples, pH and EC were measured using measuring instruments. The open-circuit voltage and short-circuit current of the microbial fuel cell were measured using a digital multimeter. Analysis included bacterial count analysis and microbiome analysis. Bacterial count analysis (qPCR, copies / g-soil) was a total bacterial count analysis using the QP-PCR method targeting 16S rRNA. Microbiome analysis (relative abundance: %) was an amplicon sequencing analysis targeting the 16S rRNA V4 region.

[0095] Microbial DNA was extracted from a 0.5g soil sample using the Extra Soil DNA Kit Plus Ver.2 (Biodynamics), and the extracted DNA was analyzed via PCR amplification. The number of eubacteria was estimated by quantitative analysis of the 16S rRNA gene count (copy number). Genetic data was obtained by amplicon sequencing analysis targeting the 16S rRNA V4 region, identifying microbial species present in the soil and calculating their relative abundances.

[0096] Figure 5 shows the changes in pH and EC values ​​in the control group (no organic fertilizer applied) and the control group (organic fertilizer applied). The horizontal axis represents the number of days elapsed, and the vertical axis represents the EC value (● marks) and pH (◆ marks). A BL (Blue Line) was installed in both the control group and the experimental group. MagAmp K was applied to the experimental group at the time of BL installation, and then organic fertilizer (cow manure compost and bark compost) was applied approximately one month later.

[0097] In the control group where neither chemical nor organic fertilizers were applied, the EC value remained largely unchanged over time. However, in the experimental group where chemical and organic fertilizers were applied, the EC value increased from approximately 0.02 ms / cm to approximately 0.10 ms / cm. This is thought to be because the components of the chemical and organic fertilizers dissolve into the soil, improving its conductivity.

[0098] The pH changed from 5 to 7.5 in both the control group and the experimental group. In the experimental group, the pH changed from 5.5 to 7.5 with the application of chemical fertilizer, and remained unchanged even with the addition of organic fertilizer. It is likely that the pH would also change from 5 to 7.5 with the application of organic fertilizer.

[0099] Figure 6 shows data on the relationship between voltage (open-circuit voltage) and current (short-circuit current), as well as the number of microorganisms and the number of days elapsed, in the control group (no organic fertilizer) and the experimental group (organic fertilizer). The vertical axis shows voltage (■), current (▲), and number of microorganisms (◆). For the analysis of the number of microorganisms, quantitative PCR analysis targeting the 16S rRNA V7-V8 region (eubacteria) was performed using QProbe. Soil samples collected in 25 ml centrifuge tubes (sterilized) were used as samples, and microbial DNA was extracted from 0.5 g of each sample using a DNA extraction reagent (Extrap Soil DNA Kit Plus Ver.2, Biodynamics). Then, QP-PCR was performed using the extracted DNA as a template.

[0100] Regarding voltage (open-circuit voltage), there was no change in either the control group or the experimental group, and no dependence on voltage due to chemical or organic fertilizers was observed. Regarding current (short-circuit current), it remained almost constant at approximately 7 mA in the group without organic fertilizer (control group), and the number of microorganisms was also approximately 2 × 10⁻⁶. 8 The current remained constant. In the organic fertilizer application plot (experimental plot), the application of organic fertilizer doubled the current to approximately 15 mA. Consequently, the number of microorganisms also increased to approximately 8 × 10⁻⁶. 8 And it became four times that amount.

[0101] Next, the soil microbiome was identified, and its relative proportion (%) was analyzed. The analysis was performed using a high-throughput DNA sequencer, Miseq, by targeting the 16S rRNA V4 region (prokaryotes: bacteria and archaea) with targeted amplicon analysis. Furthermore, soil samples collected in 25 ml centrifuge tubes (sterilized) were used as sample materials, and microbial DNA was extracted from 0.5 g of each sample using a DNA extraction reagent (Extrap Soil DNA Kit Plus Ver.2, Biodynamics). PCR amplification was then performed using the extracted DNA as a template, and the amplified PCR fragments (amplicons) were analyzed with Miseq to obtain DNA sequencing data. In addition, for microbiome analysis, the acquired data was queried against the DNA database: SILVA132, and homology search (BLAST+ (ver 2.13.0+)) was performed to obtain microbiome data for each sample.

[0102] Figure 7 shows data on the types and proportions of soil microbiota in the control and experimental plots, with the number of days elapsed on the horizontal axis. On the horizontal axis, the first letter indicates the measurement location, and the following four-digit number indicates the month (first two digits) and the day (next two digits). For example, P0322 indicates data measured at location P on March 22nd. The extracted microbiota are as follows:

[0103] Acidobacteria is a phylum of bacteria widely distributed in soil and water. These bacteria are found in a variety of environments around the world, particularly in forest soils and agricultural lands, and are one of the third most common groups in soil microbiota. Acidobacteria generally contribute to the carbon and nitrogen cycle through the decomposition of organic matter, and some species have the ability to reduce nitrates and iron, making them useful in removing nutrients in water treatment facilities and other applications.

[0104] Chloroflexi (the phylum Chloroflexi) is a group of bacteria widely distributed in various ecosystems, and is particularly common in water treatment plants and soil environments. Because they are especially abundant in facilities with long sludge lifespans and those removing nitrogen and phosphorus, they play an important role in the decomposition and recycling of harmful substances in the environment. Furthermore, since Chloroflexi can survive in anaerobic environments, they play a vital role in the sludge decomposition process, contributing to the ecosystem's nutrient cycle through the decomposition of plant and animal remains and organic residues.

[0105] Bacteroidetes are a group of Gram-negative bacteria primarily found in the gut and soil, involved in decomposition and nutrient cycling. This phylum includes many bacteria capable of efficiently breaking down carbohydrates and complex organic compounds. They are involved in nutrient breakdown and the production of short-chain fatty acids. They are also distributed in soil and water, playing a role in supporting the carbon cycle through the decomposition of complex organic matter. Derived from cow manure compost, they produce organic acids such as succinic acid and acetic acid using sugars as substrates, supplying substrates (electron donors) for power-generating bacteria. It has been suggested that microbial communities including Bacteroidetes play a role in energy generation in microbial fuel cells, as Bacteroidetes may decompose organic matter, releasing electrons that can be converted into electrical energy.

[0106] The metabolites produced by Bacteroidetes (such as SCFAs and organic acids) are often utilized by electrogenic bacteria (e.g., Shewanella and Geobacter), which serve as substrates for these bacteria when generating electricity in microbial fuel cells or in the natural environment. Therefore, Bacteroidetes are important as "support" for other electrogenic bacteria and contribute as part of the overall electrogenic process. Some studies have shown that complex microbial communities containing Bacteroidetes improve the efficiency of organic matter decomposition and power generation in microbial fuel cells (MFCs), because the metabolites produced by Bacteroidetes are utilized by other electrogenic bacteria.

[0107] Alphaproteobacteria are a highly diverse group of bacteria that have adapted to specific environments and developed different lifestyles. Common examples of photosynthetic alphaproteobacteria include bacteria of the genera Rhodobacter and Rosebacter, which convert light energy into chemical energy in marine and freshwater environments. They can obtain energy in diverse environments through photosynthesis, symbiosis, and chemosynthesis.

[0108] Gammaproteobacteria are the most diverse group of Gram-negative bacteria, found in a wide variety of environments and ecosystems. Gammaproteobacteria include not only species that freely inhabit the environment, but also many pathogenic bacteria that live in symbiosis with hosts and cause infectious diseases. Therefore, they exhibit extremely high ecological diversity and are attracting attention in various fields. Among Gammaproteobacteria, the Beta group in particular is involved in interactions with plants and animals, as well as the decomposition of environmental pollutants. For example, some species, such as Xanthomonadales, are known as plant pathogens and can infect crops, affecting yields.

[0109] Some Gammaproteobacteria (e.g., bacteria of the genera Geobacter and Shewanella) possess the ability to perform electron transport and generate electricity in microbial fuel cells (MFCs). These bacteria generate electric current by transferring electrons to external electron acceptors during the metabolism of organic matter. Microbial fuel cells utilizing Gammaproteobacteria are being studied as a means of wastewater treatment and sustainable energy generation, as these systems can recover electrical energy during the decomposition of organic matter.

[0110] Thaumarchaeota is a group of archaea that have the ability to oxidize ammonia in the nitrification process and play an important role in the nitrogen cycle. They mainly inhabit nutrient-poor environments such as water and soil, and oxidize ammonia at lower concentrations than other nitrifying bacteria. They are also adapted to low-oxygen environments and can be found in sediments and hot springs. They also function as a source of vitamin B12 as part of carbon sequestration, contributing to the health of other ecosystems.

[0111] Actinobacteria are a phylum of Gram-positive bacteria widely distributed in various natural environments, including soil, water, and within the bodies of plants and animals. In soil, in particular, they play a crucial role in the decomposition of organic matter, contributing to nutrient cycling and the maintenance of ecosystems. Many Actinobacteria species decompose difficult-to-decompose organic substances such as cellulose and lignin, thereby maintaining soil health and fertility.

[0112] Deltaproteobacteria are a group of bacteria belonging to the phylum Proteobacteria, possessing a wide variety of properties. They inhabit natural environments such as soil and water, and some have interesting ecological and metabolic characteristics. Some Deltaproteobacteria (e.g., the genus Desulfovibrio) are "sulfate-reducing bacteria" that reduce sulfates to produce hydrogen sulfide (H₂S).

[0113] Deltaproteobacteria often have the ability to form biofilms in cooperation with other microorganisms. Metabolically, deltaproteobacteria are highly diverse; while they do not perform photosynthesis, they have the ability to metabolize a variety of organic and inorganic substances. Some are known as "iron-reducing bacteria" or "nitrate-reducing bacteria," contributing to the cycling of metals and nitrogen compounds in the environment. Some deltaproteobacteria perform metabolic activities involving electron generation and transfer, particularly in groups such as iron-reducing and sulfate-reducing bacteria. These bacteria release electrons during metabolic processes and obtain energy by transferring those electrons to surrounding inorganic and organic matter.

[0114] Some deltaproteobacteria "respire" compounds such as iron and sulfates, transferring electrons to the outside during the reduction reaction. For example, the genera Geobacter and Shewanella (strictly speaking, they are included in gammaproteobacteria, but have similar functions) are known to transfer electrons directly to electrodes and minerals.

[0115] These bacteria are anaerobic bacteria that can grow in oxygen-free environments and can generate electric current, making them useful in microbial fuel cells. Delta proteobacteria such as Desulfovibrio and Geobacter are used in microbial fuel cells, generating electric current by transferring electrons obtained from organic matter decomposed by the bacteria to electrodes. Some delta proteobacteria possess an EET (External Electron Transfer) mechanism, which allows them to transfer electrons to the outside of the cell, enabling them to transmit electrons to electrodes or other mineral surfaces.

[0116] Verrucomicrobia is a phylum of Gram-negative bacteria that is widely distributed in the natural environment. It can be found in various environments, including soil, freshwater, marine environments, and the intestines of animals. Verrucomicrobia possesses heterogeneous metabolic capabilities; for example, some species live in symbiosis with methanogenic bacteria, consuming methane, while others break down sugars, and some even perform photosynthesis.

[0117] Gemmatimonadetes are Gram-negative bacteria widely found in various natural environments such as soil, aquatic environments, and sediments. Bacteria in this phylum play a crucial role in the Earth's carbon and nitrogen cycles. Some of these bacteria have been shown to possess photosynthetic capabilities, allowing them to utilize light energy for metabolism.

[0118] Planctomycetes is a group of bacteria with unique characteristics, possessing an evolutionary lineage and morphology distinct from other bacteria. Planctomycetes inhabit a wide variety of environments, including aquatic environments, soil, wetlands, and oceans, and are particularly abundant in nutrient-poor areas such as the seabed and wetlands. Because Planctomycetes have a cellular structure similar to that of eukaryotes, they are also studied for their role in exploring the evolutionary processes of eukaryotic cells. In particular, nitrogen removal using Anammox is seeing increasing applications in water treatment and environmental remediation technologies. Furthermore, their evolutionary characteristics contribute to research on the evolution of life and the formation of cellular structures.

[0119] The Nitrospirae phylum is a group of microorganisms primarily involved in nitrification (the process of oxidizing ammonia and nitrite to nitrate), playing a crucial role in the nitrogen cycle in the environment. Nitrospirae are distributed in various environments, including freshwater, oceans, and soils, and are particularly common in environments where ammonia and nitrite are present. They can also function under anaerobic conditions where oxygen is present. The Nitrospirae phylum is a group of microorganisms that plays an extremely important role in the nitrogen cycle in the environment and has the potential to make significant contributions to water treatment technologies and nitrogen management in ecosystems.

[0120] Cyanobacteria (also known as blue-green algae) are a group of Gram-negative bacteria that perform photosynthesis and play an important role in Earth's ecosystems. Like plants, cyanobacteria absorb carbon dioxide and produce oxygen and organic matter through photosynthesis. In addition to chlorophyll a, they possess light-harvesting pigments such as phycocyanin and phycoerythrin, and these pigments give them their blue-green color, hence they are also called blue-green algae.

[0121] The above describes the microbial community analyzed experimentally. The power generation mechanism of microbial fuel cells, inferred from the microbial community data, is that sugars released from the roots of organisms (plants) through photosynthesis are fermented by Bacteroidetes and other microorganisms into organic acids, which then proliferate to generate electrons and produce electricity. In soil microbial communities, Bacteroidetes, Gammaproteobacteria, and Deltaproteibacteria play a significant role in power generation, and their proportion is large with the application of organic fertilizers.

[0122] Figure 8 is a graph that focuses on Bacteroidetes, Gammaproteobacteria, and Deltaproteibacteria, which are microorganisms that cause electric current generation in the soil microbial community when organic fertilizers are applied.

[0123] Bacteroidetes is a large phylum of Gram-negative bacteria, mainly classified into three classes: Bacteroidia, Flavobacteriia, and Sphingobacteriia. Bacteria of this phylum are adapted to a wide range of habitats, from natural environments to the intestines of humans and animals, and play a very diverse role. Bacteroidetes bacteria perform anaerobic metabolism, breaking down organic matter and complex carbohydrates without normal oxygen respiration. In addition, bacteria of the Bacteroidetes phylum readily form biofilms, and this property allows them to secure a stable survival environment in the environment.

[0124] Bacteroidetes bacteria generally do not directly generate electrons but rather obtain energy mainly through fermentation and anaerobic metabolism, and are therefore thought to have a low capacity to directly supply electrons to electrodes. However, some Bacteroides bacteria produce organic substances such as short-chain fatty acids (acetic acid, propionic acid, etc.) during their metabolic processes, which can be further broken down by other electron-transferring bacteria within a microbial fuel cell.

[0125] Bacteroidetes bacteria have few mechanisms to directly release electrons, but they contribute indirectly to electron generation by living in symbiosis with other microorganisms (e.g., deltaproteobacteria such as Geobacter and Shewanella) and providing metabolites produced from the decomposition of organic matter. For example, metabolites of Bacteroides are used as electron donors by other bacteria, generating electric current in microbial fuel cells.

[0126] Bacteroidetes bacteria often function as "initial decomposers" within the overall microbial community, breaking down organic matter and producing byproducts that are then used by other bacteria for electron generation and transfer. Some studies suggest that Bacteroidetes bacteria, when combined with other electron-producing bacteria in certain environments, can indirectly improve the efficiency of microbial fuel cells. Therefore, although Bacteroidetes bacteria do not independently produce electrons, they can indirectly contribute to electron transport and generation processes through metabolic flows within the microbial community.

[0127] Bacteroidetes bacteria are not typically the type of bacteria whose electron production is directly stimulated by the application of voltage. These bacteria primarily obtain energy through anaerobic fermentation in their natural environment and do not have a high capacity to transfer electrons through electrodes. However, the application of voltage may indirectly affect electron production and metabolic activity, and it is thought that the application of voltage may affect microbial metabolism for the following reasons.

[0128] When bacteria of the phylum Bacteroidetes are co-cultured with other electron-generating bacteria, applying an external voltage can increase the number of electron acceptors within the microbial fuel cell, thereby promoting symbiotic metabolic activity. In such an environment, the organic matter decomposition products provided by the Bacteroidetes bacteria become an energy source for the electron-generating bacteria, and the applied voltage enhances metabolic efficiency.

[0129] Some microorganisms are known to form biofilms on electrode surfaces, thereby increasing their metabolic activity. Bacteroidetes bacteria also possess biofilm-forming capabilities, and voltage application can stimulate this, potentially indirectly increasing metabolic activity and the release of degradation products. In particular, under low voltage conditions, the biofilm metabolism of the entire microorganism is activated, resulting in an indirect promotion of electron transport.

[0130] Applying voltage alters the amount of by-products, such as organic acids and short-chain fatty acids, produced by bacteria of the Bacteroidetes phylum. These by-products are then used as electron donors by surrounding electron-producing bacteria, influencing their current generation.

[0131] While Bacteroidetes bacteria themselves do not directly improve their electron-generating ability through voltage application, it is expected that their electron transport and power generation efficiency will be indirectly promoted through interactions with other microorganisms and changes in the environment. Therefore, it is effective to apply an external voltage to complex microbial systems containing Bacteroidetes bacteria, such as when used in microbial fuel cells.

[0132] In this invention, a voltage application state is realized by utilizing the difference in standard electrode potential between the anode electrode 10 and the cathode electrode 12. This indirectly promotes electron transfer and power generation efficiency through interactions with other microorganisms and changes in the environment, contributing to improved performance as a microbial fuel cell.

[0133] Gammaproteobacteria are the most diverse and generally abundant group of microorganisms within the phylum Proteobacteria. Gammaproteobacteria include a wide variety of species, from pathogenic bacteria to those widely distributed in the environment.

[0134] Some Gammaproteobacteria species possess the ability to generate electric current. Generally, these current-generating bacteria have the ability to release electrons onto electrodes or metal surfaces, and this ability can be utilized in the fields of microbial fuel cells and bioremediation.

[0135] Among Gammaproteobacteria, representative bacteria capable of generating electric current include the genera Pseudomonas and Shewanella. Pseudomonas aeruginosa (Pseudomonas aeruginosa) of the Pseudomonas genus has the ability to transfer electrons to electrodes and is used in microbial fuel cells. It attaches to and grows on metal surfaces and generates electric current under specific conditions. Shewanella oneidensis of the Shewanella genus, for example, uses metal as another electron acceptor and has the ability to directly transfer electrons to metal. This function makes it possible to corrode metals or generate electric current. These bacteria generate electrons through oxidation-reduction reactions and transfer them to electrodes to generate electric current.

[0136] Gammaproteobacteria can accelerate redox reactions by applying voltage. This process is also called "electrochemical acceleration" or "electrocatalysis," and because the activation energy required for the reaction is reduced by the applied voltage, the redox reaction proceeds more rapidly. In redox reactions, electrons are transferred. By applying voltage, electrodes that supply electrons (reduction) or remove electrons (oxidation) act according to their polarity, allowing the reaction to proceed.

[0137] In the reduction reaction, electrons are supplied at the anode electrode 10 by applying a voltage, and the oxidized substance is reduced. In the oxidation reaction, electrons are removed at the cathode electrode, and the reduced substance is oxidized. In a microbial fuel cell, electroactive bacteria generate an electric current by causing oxidation-reduction reactions on the electrodes. By applying an appropriate voltage from the outside, the bacteria can release electrons more efficiently, thus promoting the oxidation-reduction reaction. This manipulation of external voltage is precisely controlled by a "potentiostat," and is sometimes used to enhance specific oxidation-reduction reactions.

[0138] In this invention, a voltage application state is achieved by utilizing the difference in standard electrode potentials between the anode electrode 10 and the cathode electrode 12, enabling bacteria to release electrons more efficiently and promote oxidation-reduction reactions. This contributes to improving the performance of the microbial fuel cell.

[0139] Deltaproteobacteria are a group of bacteria belonging to the phylum Proteobacteria, possessing a wide variety of properties and primarily inhabiting natural environments such as soil and water. Many species of Deltaproteobacteria are parasitic or symbiotic with other microorganisms, including so-called "predators" and "phagocytic bacteria."

[0140] Some deltaproteobacteria (e.g., the genus Desulfovibrio) are sulfate-reducing bacteria that reduce sulfates to produce hydrogen sulfide (H2S), and they play an important role in anaerobic environments where oxygen is absent. This sulfate reduction has a significant impact on water quality and the cycling of chemical substances in the environment, and is widely observed, for example, in wetland, lakebed, and seabed sediment layers.

[0141] Delta proteobacteria have the ability to form biofilms in cooperation with other microorganisms, which makes them more likely to survive in more complex and protected environments. Myxobacteria also have the characteristic of forming multicellular structures called "fruiting bodies" as a collective behavior, and producing spores within them. Delta proteobacteria are metabolically very diverse; while they do not perform photosynthesis, they have the ability to metabolize a variety of organic and inorganic substances. Some are known as "iron-reducing bacteria" or "nitrate-reducing bacteria," and contribute to the cycling of metals and nitrogen compounds in the environment.

[0142] Some deltaproteobacteria perform metabolic activities that involve the generation and transfer of electrons. This ability is particularly pronounced in groups such as iron-reducing and sulfate-reducing bacteria. These bacteria release electrons during metabolic processes and obtain energy by transferring those electrons to surrounding inorganic and organic matter. Some deltaproteobacteria "respire" compounds such as iron and sulfates, transferring electrons to the outside during the reduction reaction. For example, the genera Geobacter and Shewanella are known to transfer electrons directly to electrodes and minerals.

[0143] These bacteria are anaerobic bacteria that can grow even in oxygen-free environments and can generate electric current, making them suitable for use in microbial fuel cells. Delta proteobacteria such as Desulfovibrio and Geobacter are used in microbial fuel cells, generating electric current by transferring electrons obtained from organic matter decomposed by the bacteria to electrodes. In microbial fuel cells, bacteria play a role in generating electricity and are used as a sustainable power source, and can be applied, for example, to devices that generate electricity simultaneously with wastewater treatment.

[0144] Some deltaproteobacteria possess an EET (Extracellular Electron Transfer) mechanism, allowing them to transfer electrons to the outside of the cell. This mechanism enables them to deliver electrons to electrodes and other mineral surfaces. Geobacter sulfurreducens, for example, is known to transfer electrons to the outside via a protein called philipin. Due to these characteristics, deltaproteobacteria are highly promising microorganisms in fields such as energy production, environmental remediation, and even bioelectronics.

[0145] Some species of Deltaproteobacteria can generate electric current when an external voltage is applied. In particular, many Deltaproteobacteria possess electric current-generating capabilities and are utilized in applications such as metal corrosion in soil and the environment, and in microbial fuel cells (MFCs). The Geobacter genus (e.g., Geobacter sulfurreducens), a representative electric current-generating bacterium, is known for its particularly high electrical activity among Deltaproteobacteria and its ability to transfer electrons to metals and electrodes. Geobacter attaches to metal surfaces and electrodes, releasing electrons and playing a role in energy metabolism through metal reduction in nature.

[0146] When a voltage is applied while delta-proteobacteria are attached to an electrode, processes such as electron transport and increased metabolic reactions are accelerated. Applying voltage further activates the electron transport system of the bacteria, transferring more electrons to the electrode. The applied voltage promotes the metabolic reactions of the bacteria, and the increased amount of electron transport increases the current. In this way, delta-proteobacteria can enhance their current generation capacity by applying an appropriate voltage, thereby improving the power generation efficiency of microbial fuel cells. The binding to electrodes and electron transport efficiency of Geobacter species, in particular, are attracting attention. Some delta-proteobacteria can generate current when an external voltage is applied, and electroactive bacteria such as Geobacter species are being applied not only to microbial fuel cells but also to various fields such as environmental remediation.

[0147] Anaeromyxobacter is also known as an electrogenic bacterium that generates electricity. Anaeromyxobacter inhabits anaerobic environments and is mainly found in soil, compost, and moist environments. It has the ability to release electrons in the process of decomposing organic matter, which contributes to the generation of electric current. Anaeromyxobacter can use organic acids to transfer electrons to external electron acceptors, so it can generate electricity in systems such as microbial fuel cells, making it possible to use it as a sustainable energy source. Because Anaeromyxobacter has the characteristics of an electrogenic bacterium and the ability to generate electricity through the decomposition of organic matter in the environment, it is effective in the development of microbial fuel cells.

[0148] In this invention, a voltage application state is realized by utilizing the difference in standard electrode potentials between the anode electrode 10 and the cathode electrode 12. Bacteria capable of generating current by voltage application exist, and electroactive bacteria such as Geobacter and Anaeromyxobacter contribute to improving the performance of the microbial fuel cell. Anaeromyxobacter has the ability to send electrons to external electron acceptors (such as redox substances) via its electron transport chain. Therefore, by applying voltage, the potential difference between the inside and outside of the cell increases, promoting electron movement and enabling efficient transfer of electrons to the external electrode, leading to an improvement in power generation. When Anaeromyxobacter coexists with other power-generating bacteria, voltage application also affects these microbial communities, enhancing the overall power generation effect.

[0149] As described above, the microbial fuel cell of the present invention has electrodes composed of an anode electrode, which is a metal with a negative standard electrode potential, and a cathode electrode, which is a carbon material or a metal with a higher standard electrode potential than the anode electrode. A voltage is generated by the difference in standard electrode potentials, making it possible to apply a voltage to the electrolyte. The electrolyte is an organic fertilizer containing electrolytic bacteria that generate electrons in an anaerobic environment when a voltage is applied. The application of voltage affects the microbial flora of the organic fertilizer, and the power generation function of the microbial fuel cell as a whole is enhanced.

[0150] The preferred microbial community in organic fertilizers consists of Bacteroidetes, Gammaproteobacteria, and Deltaproteibacteria, as these microorganisms play a significant role in power generation. Therefore, organic fertilizers containing large amounts of these three microorganisms are preferred, and the components of cow manure compost, chicken manure compost, bark compost, leaf mold, and bamboo crab compost were investigated.

[0151] Figure 9 shows data indicating the proportion of microbial flora in organic fertilizer compost. The organic fertilizers containing the most Bacteroidetes were cow manure compost, bark compost, and leaf mold. The proportion of Bacteroidetes was highest in cow manure compost at approximately 55%, followed by bark compost at approximately 45%, and leaf mold at approximately 35%. Furthermore, when Gammaproteobacteria and Deltaproteibacteria are included, the total amount exceeds approximately 50% in cow manure compost, bark compost, and leaf mold. Therefore, it can be seen that cow manure compost, bark compost, and leaf mold are suitable among organic fertilizers.

[0152] Figure 10 shows data illustrating how the number of power-generating bacteria (iron-reducing bacteria) and microbial corrosion-related microorganisms changes over time. On the horizontal axis, the first letter indicates the measurement location, and the following four-digit number indicates the month (first two digits) and the day (next two digits). For example, P0322 indicates data measured at location P on March 22nd. Iron-reducing bacteria obtain energy by reducing iron ions such as iron oxide (Fe³⁺) and releasing electrons. Power-generating bacteria utilize metal ions such as iron oxide as electron acceptors and carry out a similar process to iron-reducing bacteria.

[0153] Sulfur-oxidizing bacteria are a group of bacteria that obtain energy by oxidizing sulfur compounds such as sulfur and sulfides, but the hydrogen sulfide produced causes metal corrosion. Sulfate-reducing bacteria are corrosive organisms that directly promote corrosion on metal surfaces through the hydrogen sulfide produced by their metabolism. Methanogenic bacteria are involved in the activity of other corrosive microorganisms (especially sulfate-reducing bacteria) and indirectly promote the progression of corrosion.

[0154] In the plots without organic fertilizer, the electrogenic bacteria remained largely unchanged, and very few microbial decay-related microorganisms were detected. In the plots with organic fertilizer, the application of organic fertilizer (cow manure compost and bark compost) slightly increased microbial decay-related microorganisms, but the increase in electrogenic bacteria was overwhelmingly greater. The activity of electrogenic bacteria (iron-reducing bacteria) creates an environment where metal (microbial) corrosion and Mn oxidation are less likely to occur.

[0155] Figure 11 shows the effects of applying organic fertilizers on power generation and the microbial flora. Bark compost and cow manure compost were applied as organic fertilizers, and their effects were investigated. The current value was 1.2 times that of the control group before application of bark compost and cow manure compost, but increased to 1.8 times after application. The number of soil microorganisms remained unchanged from the control group before application of bark compost and cow manure compost, but increased to 3.6 times after application. Along with the increase in soil microorganisms, the number of power-generating bacteria increased to 6.1 times. This clearly demonstrates that the application of bark compost and cow manure compost contributed to the increase in current value.

[0156] Figure 12 illustrates an example of installing a microbial fuel cell in soil. Given the relationship between electrodes and electrolytes in this invention, it is preferable to maintain this relationship when installing a microbial fuel cell. For example, when installing in soil, a microbial fuel cell can be created by providing a microbial fuel cell installation area in a portion of the soil, using only organic fertilizer or a mixture of organic fertilizer and conductive material as the electrolyte, and then burying the cathode electrode and anode electrode in this area. By providing a microbial fuel cell installation area, the soil can be fertilized with fertilizers suitable for plant growth, and the electrolyte in the microbial fuel cell can be adapted to the microbial fuel cell, using only organic fertilizer or a mixture of organic fertilizer and conductive material as the electrolyte. The conductive material is, for example, iron oxide, and the acquisition current increases due to the reduction of internal resistance.

[0157] Figure 13 illustrates an energy storage system that utilizes land effectively to allow solar panels and microbial fuel cells to coexist. Multiple microbial fuel cells 1 are installed in a microbial fuel cell installation area 34 provided in the soil 32. The system can be used as an energy storage system 36 by providing multiple solar panels 46 fixed to the surface of the soil 32 by support pillars 48, a battery 44 for storing electrical energy from the multiple microbial fuel cells 1, and a control unit 38 that has a DC-DC converter 40 and controls the storage of electrical energy from the microbial fuel cells 1 and solar panels 46 into the battery 44. Since solar panels 46 are installed on the ground and electrodes for the microbial fuel cells 1 are set underground, effective land use is possible.

[0158] Furthermore, since obtaining power from the microbial fuel cell 1 is effective on rainy or cloudy days and at night, and obtaining power from the solar panels 46 is effective on sunny days, the energy storage system 36 is less dependent on weather conditions.

[0159] By providing a control unit 38 with an on / off function 42 that depends on the amount of electrical energy from the solar panel 46, the system can be automatically supplied with electrical energy only at night or only during the day. Examples of electrical energy supplied only at night include streetlights. In addition, LED lighting in plant cultivation is mainly done during the day.

[0160] Although embodiments of the present invention have been described above, the present invention includes appropriate modifications that do not impair its purpose and advantages, and is not limited by the above embodiments. [Explanation of symbols]

[0161] 1 Microbial fuel cell 10 Anode electrodes 12 Cathode electrodes 14 Electrolytes 16 load 18 Conductor 20 Metals for anode electrodes 22 Conductive coating 24 carbon material 26 Metals for Cathode Electrodes 28 Photocatalytic coating 30 plants 32 Soil 34 Microbial fuel cell installation area 36 Energy Storage Systems 38 Control Unit 40 DC-DC converters 42 On / Off Switch 44 Storage batteries 46 Solar panels 48 Pillar

Claims

1. The anode electrode is a metal with a negative standard electrode potential, A cathode electrode made of carbon material, or a metal whose standard electrode potential is higher than that of the anode electrode, An electrolyte consisting of an organic fertilizer containing a microbial community that promotes the growth of current-generating bacteria that generate electrons in an anaerobic environment when voltage is applied, A microbial fuel cell characterized by being composed of the following.

2. The aforementioned organic fertilizer contains Delta Proteobacteria. A microbial fuel cell according to claim 1, characterized by the above.

3. The aforementioned organic fertilizer contains gamma proteobacteria. A microbial fuel cell according to claim 1, characterized by the above.

4. The aforementioned organic fertilizer contains Bacteroidetes, A microbial fuel cell according to claim 1, characterized by the above.

5. The metal of the anode electrode is magnesium or aluminum. A microbial fuel cell according to claim 1, characterized by the above.

6. The magnesium or aluminum has a conductive coating formed on it. A microbial fuel cell according to claim 4, characterized by the above.

7. The carbon material of the cathode electrode is Binchotan charcoal, carbon graphite, hard carbon, or graphite. A microbial fuel cell according to claim 1, characterized by the above.

8. The metal of the cathode electrode is stainless steel, copper, silver, or gold. A microbial fuel cell according to claim 1, characterized by the above.

9. The cathode electrode is coated with a photocatalyst. A microbial fuel cell according to claim 1, characterized by the above.

10. In a portion of the soil mixed with fertilizer, a microbial fuel cell installation area was provided containing only the organic fertilizer or a mixture of the organic fertilizer and a conductive material as the electrolyte, and the cathode electrode and anode electrode were embedded. A microbial fuel cell according to claim 1, characterized by the above.

11. A microbial fuel cell according to claim 1, which is buried in the soil, Multiple solar panels installed on the soil surface, The microbial fuel cell and the battery that stores electrical energy from the solar panel, A DC-DC converter is provided, along with a control unit that controls the storage of electrical energy from the microbial fuel cell and the solar panel into a battery, A well-equipped energy storage system.

12. The control unit has an on / off function that depends on the amount of electrical energy from the solar panel, and controls the supply of electrical energy to the load connected to the storage battery. The energy storage system according to claim 11, characterized by the following:

Images