Method for producing a proton conductor

Abstract

[Problem] To provide: a proton conductor suitable for use at a temperature range between 200°C and 600°C; and a method for producing the same. [Solution] This proton conductor is configured such that a portion of lithium ions of Li14-2xZn1+x(GeO4)4 is replaced with protons. The proton conductor has an electroconductivity of at least 0.01 S / cm at 300°C. Here, x is a number equal to or more than zero. Of movable lithium ions included in Li14-2xZn1+x(GeO4)4, 40-70% of the lithium ions may be replaced with protons. Of movable lithium ions included in Li14-2xZn1+x(GeO4)4, 50-60% of the lithium ions may be replaced with protons. The method for producing a proton conductor includes a step for immersing Li14-2xZn1+x(GeO4)4 in a non-aqueous organic solution containing an acid to cause a portion of lithium ions to be replaced with protons.

Description

Technical Field

[0001] The present invention relates to a proton conductor, and more particularly to a proton conductor having a sufficient proton conductivity in a medium temperature range of 200°C or higher, more preferably 300 to 600°C.

Background Art

[0002] Solid polymer fuel cells (PEFCs) used in automobiles, phosphoric acid fuel cells (PAFCs) used as stationary fuel cells, molten carbonate fuel cells (MCFCs), and solid oxide fuel cells (SOFCs) have been put into practical use. The operating temperatures are as follows: for solid polymer fuel cells, room temperature to 100°C; for phosphoric acid fuel cells, 180 to 200°C; for molten carbonate fuel cells, 600 to 700°C; and for solid oxide fuel cells, 600 to 900°C. However, there is no fuel cell that operates in the medium temperature range of 200 to 600°C.

[0003] Fuel cells operating in the medium temperature range of 200 to 600°C are suitable not only for hydrogen-oxygen fuel cells but also for direct-type fuel cells that generate hydrogen from various fuels in the fuel electrode chamber of the fuel cell and generate electricity through a fuel cell reaction using the generated hydrogen. In addition, fuel cells operating in the medium temperature range of 200 to 600°C can promote the fuel cell reaction compared to fuel cells operating in the low temperature range of 200°C or lower, and thus can improve the efficiency.

[0004] The reason why there is no fuel cell operating in the medium temperature range of 200 to 600°C is that there is no ion conductor having a sufficient ion conductivity in this temperature range. So far, cesium dihydrogen phosphate (CsH2PO4) discovered in 1997 has been attracting attention as the proton conductor with the highest conductivity. However, since cesium dihydrogen phosphate undergoes a phase transition at 270°C or higher, its maximum operating temperature is set at 270°C. Cesium dihydrogen phosphate usually has an operating temperature of 250°C, and the conductivity σ (S / cm) at this time is about 0.008. Therefore, the realization of a fuel cell operating in the medium temperature range has been an important research topic since the 1990s.

[0005] Patent Document 1 discloses a direct-type fuel cell. In a direct-type fuel cell, an organic hydride such as methylcyclohexane and decalin is supplied as fuel to a fuel cell stack, and dehydrogenation reaction is carried out by bringing it into contact with a noble metal catalyst fixed to the electrode of the fuel electrode. Hydrogen generated at the fuel electrode becomes protons by passing electrons to the fuel electrode. Protons move through the electrolyte membrane and receive electrons from the electrode together with oxygen atoms activated at the air electrode of the counter electrode to advance the fuel cell reaction. The electrolyte membrane is a membrane composed of a mixture of microcrystals of cesium dihydrogen phosphate (CsH2PO4) and polytetrafluoroethylene. The direct-type fuel cell of Patent Document 1 has an output of 40 mW / cm at an operating temperature of 170 to 220 °C. 2 and becomes.

[0006] However, the operating temperature when using a solid electrolyte that is an organic membrane is generally 100 °C or lower, and the heat resistance of the organic membrane is not sufficient at 200 °C or higher. Cesium dihydrogen phosphate is known as a solid electrolyte that can be used at 200 °C or higher. However, since the maximum usable temperature of cesium dihydrogen phosphate is 270 °C, a new proton conductor that can be used at higher temperatures is desired.

[0007] In response to the above demands, Non-Patent Document 1 discloses Li 14 Zn(GeO4)4 in which part of the Li of a kind of LISICON which is a solid electrolyte is replaced with Sr. 13.9 Sr 0.1 Zn(GeO4)4. Li 13.9 Sr 0.1 Zn(GeO4)4 shows a conductivity of 0.039 S / cm at 600 °C and has a higher conductivity than conventional solid electrolytes of zirconia-based materials or ceria-based materials. Further, a fuel cell to which Li 13.9 Sr 0.1 Zn(GeO4)4 is applied has an output of about 0.4 W / cm at an operating temperature of 600 °C. Further, Li 2 Sr 13.9 Sr 0.1When lithium ions that can move in Zn(GeO4)4 are completely replaced by protons, the conductivity improves to 0.048 S / cm at an operating temperature of 600 °C. The exchange of lithium ions and protons is carried out in water or dilute acetic acid. As an example, Li 13.9 Sr 0.1 Ion exchange is performed by stirring Zn(GeO4)4 in a 5 mM aqueous acetic acid solution for 24 hours.

[0008] Non-Patent Document 2 discloses a proton conductor in which Li 14-2x Zn 1+x (GeO4)4 is ion-exchanged in a 5 mM aqueous acetic acid solution to exchange lithium ions and protons. In Non-Patent Document 2, by changing the Li + / Zn 2+ ratio and performing ion exchange on Li 14 Zn(GeO4)4, Li 12 Zn2(GeO4)4, Li 10 Zn3(GeO4)4, and identifying each sample and measuring the weight change during heating, it is confirmed that the higher the lithium content in the sample, the higher the ion exchange amount to protons. Also, Non-Patent Document 2 finds the possibility of obtaining the same conductivity as the proton conductor disclosed in Non-Patent Document 1 from the results of measuring the electromotive force of a hydrogen concentration cell using the proton conductor.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0010] However, in the temperature range of 200 to 600 °C, a novel proton conductor having a higher conductivity is desired.

[0011] In view of the above background, an object of the present invention is to provide a proton conductor suitable for use in the temperature range of 200 to 600 °C. Another object of the present invention is to provide a method for producing a proton conductor suitable for use in the temperature range of 200 to 600 °C.

Means for Solving the Problems

[0012] In order to solve the above problems, an aspect of the present invention provides a proton conductor in which a part of the lithium ions in Li 14-2x Zn 1+x (GeO4)4 is replaced by protons and which has a conductivity of 0.01 S / cm or more at 300 °C. Here, x is a number of 0 or more. x may include a decimal. Also, Li 14-2x Zn 1+x (GeO4)4 can be expressed as Li 2+2y Zn 1-y GeO4. Here, x = 3 - 4y. Also, the structure in which a part of the lithium ions in Li 14-2x Zn 1+x (GeO4)4 is replaced by protons can be expressed as (Li,H) 14-2x Zn 1+x (GeO4)4 or (Li,H) 2+2y Zn 1-y GeO4. In this aspect, the x may be 0.

[0013] According to this aspect, a proton conductor that can be used in the temperature range of 200 to 600 °C can be provided.

[0014] In the above aspect, 40% or more and 70% or less of the mobile lithium ions contained in Li 14-2x Zn 1+x (GeO4)4 may be replaced by protons. Also, Li 14-2x Zn 1+x50% or more and 60% or less of the mobile lithium ions contained in (GeO4)4 may be replaced with protons. The mobile lithium ions are Li 14-2x Zn 1+x Among all the lithium ions contained in (GeO4)4, Li 14-2x Zn 1+x refers to the lithium ions that can move within (GeO4)4. Li 14-2x Zn 1+x The ratio of the mobile lithium ions to all the lithium ions in (GeO4)4 is (3 - x) / (14 - 2x).

[0015] Another aspect of the present invention is a method for manufacturing a proton conductor, which includes a step of immersing Li 14-2x Zn 1+x (GeO4)4 in a non-aqueous organic solution containing an acid to replace a part of the lithium ions with protons. Here, x is a number of 0 or more. x may include a decimal. In this aspect, the x may be 0.

[0016] According to this aspect, a proton conductor that can be used in the temperature range of 200 to 600 °C can be manufactured. Li 14-2x Zn 1+x (GeO4)4 is immersed in a non-aqueous organic solution containing an acid, whereby Li 14-2x Zn 1+x The ion exchange rate of the mobile lithium ions contained in (GeO4)4 to protons can be made 40% or more and 70% or less. When the ion exchange rate of the mobile lithium ions to protons is 40% or more and 70% or less, the structural stability of the proton conductor is improved and the conductivity becomes relatively high.

[0017] In the above aspect, the acid may preferably include at least one selected from the group consisting of benzoic acid, m-nitrophenol, acetic acid, p-toluenesulfonic acid, oxalic acid, and methanesulfonic acid. Also, the non-aqueous solvent may preferably include at least one selected from the group consisting of toluene, dimethyl sulfoxide, tetrahydrofuran, and N,N-dimethylformamide. [Advantages of the Invention]

[0018] According to the above configuration, a proton conductor suitable for use in the temperature range of 200 to 600 °C can be provided. Further, a method for manufacturing a proton conductor suitable for use in the temperature range of 200 to 600 °C can be provided.

Brief Description of the Drawings

[0019]

Figure 1

Embodiments for Carrying Out the Invention

[0020] Embodiments of the proton conductor of the present invention will be described below. The proton conductor has a structure in which a part of Li in Li 14-2x Zn 1+x (GeO4)4 is substituted with protons. Here, x is a number of 0 or more, and may include a decimal. Li 14-2x Zn 1+x (GeO4)4 can be expressed as Li 2+2y Zn 1-y GeO4. Here, x = 3 - 4y. Further, the structure in which a part of the lithium ions in Li 14-2x Zn 1+x (GeO4)4 is substituted with protons can be expressed as (Li,H) 14-2x Zn 1+x (GeO4)4 or (Li,H) 2+2y Zn 1-y GeO4. Li 14-2x Zn 1+x (GeO4)4 is a kind of LISICON (lithium superionic conductor) which is a solid electrolyte. x may be, for example, 0, 1, or 2.

[0021] LISICON (Lithium super ionic conductor) has a framework structure formed by tetrahedrons of γ-Li3PO4 type LiO4, GeO4, SiO4, PO4, ZnO4, VO4 and octahedrons of LiO6. Li 14Zn(GeO4)4 is a solid solution of Zn in the parent structure of Li4GeO4 and has high conductivity.

[0022] The proton conductor has a conductivity of 0.01 S / cm or more at 300 °C. The proton conductor is Li 14-2x Zn 1+x In (GeO4)4, 40% or more and 70% or less of the mobile lithium ions are replaced by protons. Also, for the proton conductor, it is preferable that 50% or more and 60% or less of the Li of the mobile lithium ions contained in Li 14-2x Zn 1+x (GeO4)4 are replaced by protons.

[0023] The manufacturing method of the proton conductor will be described below. First, the preparation method of Li 14-2x Zn 1+x (GeO4)4 before ion exchange will be described. The preparation method of Li 14-2x Zn 1+x (GeO4)4 is also disclosed in Non-Patent Document 2 above. Li 14-2x Zn 1+x (GeO4)4 can be prepared by a solid-phase method. Powders of reagents of a Li source, a Zn source, and a Ge source are mixed overnight in an organic solvent and pulverized, and then the organic solvent is evaporated to obtain a mixture. The Li source preferably contains at least one selected from the group consisting of LiOH, Li2O, and LiNO3. The Zn source preferably contains at least one selected from the group consisting of Zn(OH)2, ZnCO3, and Zn(NO3)2. The Ge source preferably contains at least one selected from the group consisting of GeO and GeCl2. The combination of the Li source, the Zn source, and the Ge source is, for example, preferably Li2CO3, ZnO, GeO2. The organic solvent is preferably at least one selected from the group consisting of ethanol, methanol, 1-propanol, 2-propanol, and 1-butanol. After that, the mixture is formed into pellets using a molding machine, the molded product is fired in air, and then pulverized to be powdered to obtain Li 14-2x Zn 1+x (GeO4)4.

[0024] The air firing temperature of the formed product is preferably 1000 to 1200 °C, more preferably 1100 to 1150 °C. When the firing temperature is lower than 1000 °C, there is a problem that the solid-phase reaction does not proceed, and when the firing temperature is higher than 1200 °C, there is a problem that the formed product melts. The firing time of the formed product is preferably 3 to 7 hours, more preferably 4 to 6 hours. The formed product may be fired, for example, at 1150 °C for 5 hours in air.

[0025] Li 14-2x Zn 1+x (GeO4)4 may be, for example, Li 14 Zn(GeO4)4, Li 12 Zn2(GeO4)4, Li 10 Zn3(GeO4)4. Li 14-2x Zn 1+x (GeO4)4, the ratio of Li to Zn can be varied depending on the ratio of the Li source, Zn source, and Ge source to be mixed.

[0026] Next, a method for replacing a part of the lithium in Li 14-2x Zn 1+x (GeO4)4 with protons will be described. Li 14-2x Zn 1+x (GeO4)4 powder sample is stirred in a non-aqueous solvent containing an acid to replace a part of the mobile lithium ions contained in Li 14-2x Zn 1+x (GeO4)4 with protons. The non-aqueous solvent is preferably an aprotic solvent. The non-aqueous solvent may contain one selected from the group consisting of toluene, dimethyl sulfoxide, tetrahydrofuran, and N,N-dimethylformamide. The acid may contain at least one selected from the group consisting of benzoic acid, m-nitrophenol, acetic acid, p-toluenesulfonic acid, oxalic acid, and methanesulfonic acid. For example, using toluene from which water has been removed with a dehydrating agent as the non-aqueous solvent, and dissolving benzoic acid as a proton source at a concentration of 5 mM in 100 mL of a non-aqueous organic solution, Li 14-2x Zn 1+x (GeO4)4 may be stirred for 24 hours to perform ion exchange.

[0027] Li 14-2x Zn 1+x (GeO4)4 contains mobile lithium ions. The ion exchange rate of these lithium ions to protons can be adjusted by changing the concentration of Li 14-2x Zn 1+x (GeO4)4 and the type of acid. When the solvent is aqueous and the acid is acetic acid, it has been confirmed that the ion exchange rate of the mobile lithium ions contained in Li 14-2x Zn 1+x (GeO4)4 to protons reaches 100%.

[0028] The powder of the proton conductor after ion exchange is obtained by removing the solvent. At this time, the drying temperature is preferably not lower than the boiling point of the solvent used and not higher than 300 °C. If the temperature is lower than the boiling point, there will be a problem of solvent residue. If the temperature is higher than 300 °C, there will be a problem of proton desorption in the sample. Thus, a powdery proton conductor is obtained.

[0029] The powdery proton conductor prepared as described above can be formed into a thin film and used as an electrolyte membrane for fuel cells, electrolytic cells, solid batteries, etc. A direct fuel cell, which is a type of fuel cell, supplies a substance different from hydrogen as fuel to the fuel electrode of the fuel cell. At the fuel electrode, hydrogen is generated from the fuel by a hydrogen generation reaction, and the generated hydrogen is used to operate a hydrogen-oxygen fuel cell. Here, since the chemical energy of the hydrogen gas generated is mostly higher than that of the fuel used, the hydrogen generation reaction from the fuel is an endothermic reaction. On the other hand, on the side of the counter electrode where water is generated, a hydrogen combustion reaction proceeds and is converted into electrical energy and thermal energy. Since this thermal energy can be used for the endothermic reaction of the fuel electrode in a direct fuel cell, it is possible to efficiently supply the energy required for hydrogen generation within the cell, and there is an advantage that the energy efficiency up to power generation including the hydrogen production process can be significantly improved.

[0030] From the above, by using the proton conductor according to this embodiment as an ion exchange membrane, a fuel cell that operates in a temperature range of 200°C or more and uses hydrogen, methylcyclohexane, ammonia, methanol, dimethyl ether, formic acid, or the like as fuel can be configured. The reaction temperature when generating hydrogen from these fuels is in the temperature range of 300 to 500°C, and a hydrogen generation catalyst suitable for each fuel may be used. The hydrogen generation catalyst may be a known catalyst, and may be an ammonia decomposition catalyst or a reforming catalyst for methanol, dimethyl ether, formic acid, or the like.

[0031] Cyclohexane, which is used as a fuel in direct fuel cells, is one of the organic chemical hydride compounds that are expected to be hydrogen energy carriers. The organic chemical hydride method is a method of storing and transporting hydrogen as an organic chemical hydride compound (hydrogenated organic compound) that incorporates hydrogen into the molecular structure of a chemical product through a chemical reaction. In this method, hydrogen gas is reacted with toluene in a hydrogenation reactor to produce methylcyclohexane (MCH) as a hydrogen storage process. Toluene is a liquid chemical at room temperature and pressure, and is a general-purpose chemical that is widely used in large quantities as a solvent for paints and other products as a general-purpose solvent with low toxicity. In this hydrogenation process, hydrogen atoms are incorporated into the molecules of MCH. MCH is in a liquid state at room temperature and pressure like toluene, and is used in large-scale transportation of chemical products in existing chemical tankers, as well as toluene. MCH is an industrial agent that is used as a solvent for correction ink and is a general-purpose chemical product with low toxicity that is also used in household office supplies. This MCH can be transported on a large scale by sea using large ships such as chemical tankers.

[0032] When MCH is transported by sea, it is unloaded into large tanks in coastal areas for use in power generation or as a chemical raw material, but because land transportation by chemical tanker trucks and rail freight has also become practical, it can be transported to hydrogen stations, regional bases, and remote islands in the same way as existing kerosene and gasoline.

[0033] In this way, the MCH transported to the hydrogen utilization site generates hydrogen in a dehydrogenation device, and the generated hydrogen is supplied as power generation or chemical raw material. After hydrogen is generated by this dehydrogenation reaction, the MCH returns to toluene. Therefore, toluene is transported back to the hydrogen production site and reused as a raw material for the hydrogenation reaction again and again. The characteristics of the organic chemical hydride method and the process of completing the international hydrogen supply chain demonstration between Southeast Asia and Japan in 2020 and transitioning to the commercialization stage are introduced in the literature (see Journal of the Gas Turbine Society, Vol. 49, No. 2, p. 1-6 (2021)).

[0034] The organic chemical hydride method has been proposed since the 1980s. However, the dehydrogenation catalyst for generating hydrogen from MCH incorporating hydrogen has an extremely short lifespan, making industrial implementation difficult, so it has not been put into practical use. The key to technology development was the development of a new dehydrogenation catalyst with sufficient performance such as an industrially applicable catalyst lifespan. Currently, the development of a platinum-supported alumina catalyst with high performance has been completed, and technical improvements contributing to cost reduction in each process of the above scheme are being implemented. The hydrogen energy carrier system based on the organic chemical hydride method is the only system that has completed the demonstration of all processes and established the technology, and can be put into practical use at an early stage.

[0035] On the other hand, Japan has incorporated the promotion of the practical application and popularization of hydrogen energy into its national policy since the 4th Energy Basic Plan after the earthquake, and the Cabinet has decided on the Hydrogen Basic Strategy in 2017 following the formulation of the Hydrogen and Fuel Cell Technology Roadmap. The above-mentioned organic chemical hydride method is included in the Hydrogen Basic Strategy as a hydrogen energy carrier for "storing" and "transporting" hydrogen on a large scale, and the hydrogen supply price target is set at ¥30 / Nm 3 by 2030 and 3 ¥20 / Nm by 2050. Therefore, cost reduction through continuous improvement technology development is required.

[0036] In contrast, for the ¥30 / Nm in 20303 Regarding the target, it is planned to achieve it through measures such as technological improvements and reuse of existing facilities. However, to achieve the 2050 target of ¥20 / Nm 3 it is recognized that further technological innovation is necessary. We are in the stage of planning various improvements and developments, such as on the MCH manufacturing side, increasing the size of the tanker, and the utilization method after MCH transportation. Since the proton conductor of the present invention can be used in a direct MCH fuel cell, it is recognized as a technology that can expect an extremely high cost reduction effect in power generation applications.

[0037] The dehydrogenation reaction of MCH is an endothermic reaction, similar to the cases of other hydrogen generation raw materials described above. Heat equivalent to 30% of the energy of the hydrogen transported as MCH is required for the dehydrogenation reaction. Therefore, when generating hydrogen with the current dehydrogenation device and using it as a power generation fuel for turbines or SOFCs, etc., there is a problem that the heat generated in these high-temperature power generation facilities needs to be supplied to the dehydrogenation reaction, resulting in a decrease in power generation efficiency. In addition, when using fossil fuels as the heat source, CO2 is generated, which also causes a problem of an increase in LCA CO2 when using hydrogen.

[0038] As described above, since MCH can be used as a fuel in a direct fuel cell and the heat generated at the counter electrode of the fuel electrode can be used for the dehydrogenation reaction of the fuel electrode, not only the cost of the heat source is reduced, but also the problem of an increase in LCA CO2 is solved. When purchasing and using natural gas as the heat source, when the hydrogen supply cost is set at ¥30 / Nm in 2030 3 the purchase cost of natural gas required for the dehydrogenation heat source is ¥5 / Nm 3 or more. From this, when the heat source is not required in the direct MCH fuel cell, there is a cost reduction effect of ¥5 / Nm 3 or more.

[0039] In a MCH direct fuel cell, it is necessary to generate hydrogen by dehydrogenation reaction in the vicinity of the fuel electrode. At this time, a dehydrogenation catalyst is required, and the dehydrogenation catalyst used in the above-mentioned current technology can be used. This catalyst is a platinum-supported alumina catalyst in which fine particles of active metal platinum are supported on a γ-alumina carrier, and it is characterized in that platinum particles with extremely small sizes are supported compared to conventional platinum-alumina catalysts.

[0040] Moreover, the proton conductor according to this embodiment can be used as a proton exchange membrane of an electrolytic cell. Thereby, an electrolytic cell operating in the intermediate temperature range of 200 to 600 °C can be provided. Currently, a chlor-alkali type electrolytic cell operating at about 90 °C and a PEM type electrolytic cell in which both electrodes are provided on both sides of a polymer electrolyte membrane for electrolysis of water have been put into practical use. In addition, a solid electrolyte electrolytic cell (SOEC) that uses the cells of an SOFC fuel cell for high-temperature electrolysis has been researched and developed. However, for an electrolytic cell operating in the intermediate temperature range of 200 to 600 °C, there is no conductive material having sufficient conductivity in this temperature range, so the development has not made progress. Since the proton conductor according to this embodiment has high conductivity at 200 to 600 °C, by using the proton conductor as an ion exchange membrane, various electrolytic cells operating in a temperature range of 200 °C or higher can be configured. In recent years, technological development for performing the production of various substances by electrolysis reaction has been actively carried out. The proton conductor according to this embodiment can increase the temperature of these electrolytic cells and improve the efficiency. The proton conductor according to this embodiment has a wide application range and a very large ripple effect.

[0041] The effects of the proton conductor according to this embodiment will be described below. FIG. 1 is a graph showing the conductivity of various solid electrolytes. The star mark in the figure indicates the conductivity σ (0.008 S / cm) (log σ = -2.1) of cesium dihydrogen phosphate (CsH2PO4) at 250 °C. The conductivity plotted with round marks is the proton conductor ((Li,H) according to this embodiment 14It shows the conductivity of Zn(GeO4)4). The conductivity of the proton conductor according to this embodiment at 300 °C is higher than that of cesium dihydrogen phosphate (CsH2PO4) at 250 °C. Also, the conductivity of the proton conductor according to this embodiment at 600 °C is higher than that of various solid electrolytes used in SOFCs.

[0042] Also, in the upper right of the figure, the conductivity of Nafion ion exchange membrane (Nafion117), which is a type of ion exchange membrane of the organic polymer used in automotive fuel cells, is shown. The conductivity of the proton conductor according to this embodiment at 500 to 600 °C is equivalent to the conductivity of the Nafion membrane at an operating temperature of about 90 °C. The PEFC using the Nafion ion exchange membrane is a 100 kW-class fuel cell and has a small size. SOFCs are significantly larger in size than PEFCs. When the proton conductor according to this embodiment is used as a solid electrolyte, the operating temperature of current SOFCs can be lowered and the size can be reduced.

[0043] The proton conductor according to this embodiment, as a solid electrolyte operating in the temperature range of 200 °C to 250 °C, has a higher conductivity than that of cesium dihydrogen phosphate. Also, the proton conductor according to this embodiment has a high conductivity even in the temperature range of 300 to 600 °C. The proton conductor according to this embodiment has a conductivity equivalent to that of the Nafion ion exchange membrane used in automotive fuel cells in the temperature range of 500 °C or higher. Also, the proton conductor according to this embodiment can be used even at a high temperature of 600 °C or higher, and the conductivity at 600 °C is equivalent to that of the solid electrolytes used in existing SOFCs. Since the proton conductor according to this embodiment moves protons instead of oxide ions, its operating temperature can be lowered to about 600 °C. As a result, the proton conductor according to this embodiment can provide a fuel cell that is more efficient and easier to handle than existing SOFCs.

[0044] In the manufacturing method of the proton conductor according to this embodiment, Li 14-2x Zn 1+xImmerse (GeO4)4 in a non-aqueous organic solution containing an acid and stir it to obtain Li 14-2x Zn 1+x The ion exchange rate of the mobile lithium ions contained in (GeO4)4 to protons can be set to 40% or more and 70% or less. When the ion exchange rate of the mobile lithium ions to protons is 40% or more and 70% or less, the structural stability of the proton conductor is improved and the conductivity becomes relatively high. In contrast, when Li 14-2x Zn 1+x (GeO4)4 is dissolved in an aqueous acetic acid solution, it has been confirmed that the ion exchange rate of the mobile lithium ions contained in Li 14-2x Zn 1+x (GeO4)4 to protons becomes 100%. However, in this case, the proton conductor has low structural stability, and by-products are generated when the powder is molded, and it has been confirmed that the conductivity decreases as a result. When a non-aqueous solvent is used during the production of the proton conductor, the ion exchange rate decreases compared to the case where an aqueous solvent is used, but the conductivity increases because the structural stability is improved.

Example

[0045] (Preparation method of Li 14 Zn(GeO4)4) Lithium carbonate was used as the Li source, zinc oxide was used as the Zn source, and germanium oxide was used as the Ge source. Lithium carbonate, zinc oxide, and germanium oxide were added in a ratio of 25:4:21 by weight, and the slurry obtained by finely mixing with ethanol and zirconia balls in a sealed container for 24 hours was dried at 130 °C. The resulting powder was molded into pellets using a press. After firing this pellet in an alumina crucible in air at 1150 °C for 5 hours, it was pulverized in a magnetic mortar for 2 hours, molded into pellets again, and fired in an alumina crucible in air at 1150 °C for 5 hours. The fired pellet was pulverized again in a magnetic mortar for 2 hours to obtain Li 14 Zn(GeO4)4 powder before ion exchange.

[0046] (Example 1) Li before ion exchange 142.5 g of a sample of Zn(GeO4)4 powder was stirred in 100 ml of a non-aqueous organic solution prepared by dissolving benzoic acid as a proton source to a concentration of 5 mM in toluene from which water had been removed with a dehydrating agent as a non-aqueous solvent for 24 hours to carry out ion exchange. After ion exchange, the powder was recovered by filtration, washed with toluene, and then vacuum-dried at 130 °C overnight to obtain the ion-exchanged powder of Example 1. The ion exchange rate of the mobile lithium ions of the proton conductor of Example 1 to protons was 52%.

[0047] (Comparative Example 1) Li before ion exchange 14 Zn(GeO4)4 powder was stirred in a 5 mM aqueous acetic acid solution 40 times its weight at room temperature for 24 hours to carry out ion exchange. After filtration and washing, it was dried in a vacuum dryer at 130 °C to obtain the ion-exchanged product of Comparative Example 1. The ion exchange rate of the mobile lithium ions of the proton conductor of Comparative Example 1 to protons was 100%.

[0048] (Comparison between Example 1 and Comparative Example 1) The conductivities of the ion-exchanged products of Example 1 and Comparative Example 1 were measured. The measurement was carried out by the direct current four-terminal method and the alternating current two-terminal method using an electrochemical evaluation apparatus (ModuLab, manufactured by Solartron analytical) in a 10% humidified nitrogen atmosphere. The measurement results are shown in Table 1 below.

Table 1

[0049] As shown in Table 1, it was confirmed that Example 1 had a higher conductivity than Comparative Example 1.

[0050] (Example 2) Using dimethyl sulfoxide as a non-aqueous solvent and m-nitrophenol, acetic acid, benzoic acid, p-toluenesulfonic acid, oxalic acid, and methanesulfonic acid as proton sources, the concentration was changed in the range of 5 to 100 mM, and the ion exchange operation was carried out in the same manner as in Example 1. Then, the Li on which the ion exchange operation was carried out 14The ion exchange amount was confirmed by thermogravimetric analysis of Zn(GeO4)4 powder. As a result, the ion exchange rate was 45 - 65% for each proton source.

[0051] (Comparative Example 2) Li before ion exchange with only a non-aqueous solvent 14 The effect of ion-exchanging Zn(GeO4)4 powder was confirmed. Toluene, tetrahydrofuran, ethanol, N,N-dimethylformamide, dimethyl sulfoxide, and propylene carbonate as non-aqueous solvents were treated with a dehydrating agent and used. The ion exchange operation was performed in the same manner as in Comparative Example 1 without using a proton source. After that, the ion exchange amount of the Li 14 The ion exchange amount was confirmed by thermogravimetric analysis of Zn(GeO4)4 powder. As a result, it was confirmed that the ion exchange hardly proceeded.

Industrial Applicability

[0052] The proton conductive material of the present invention can be suitably used as an ion exchange membrane for various fuel cells and various electrolytic cells operating in the intermediate temperature range of 200 - 600 °C, which has not existed so far. Specifically, various fuels capable of generating hydrogen in the intermediate temperature range are directly supplied as fuels for fuel cells to the fuel cells, and hydrogen is generated by a catalytic reaction in the cell to perform fuel cell power generation. At the same time, the reaction heat required for the reaction of generating hydrogen from the fuel can be covered by the fuel cell reaction, eliminating the need for a heat source and contributing to cost reduction of fuel cell power generation. In addition, in existing fuel cells that supply hydrogen fuel, it is possible to provide a highly efficient fuel cell that can operate in the intermediate temperature range compared to low-temperature types, and it is possible to achieve a lower temperature in the intermediate temperature range that high-temperature SOFC fuel cells aim for. Furthermore, it can be applied to intermediate-temperature electrolytic cells. Thus, since the present invention is a basic technology regarding proton conductors used in cells such as fuel cells and electrolysis, it is an invention with extremely high industrial applicability.

[0053] The description of the specific embodiments ends here, but the present invention can be widely modified and implemented without being limited to the above embodiments.

Claims

1. A method for producing a proton conductor, Li １４－２ｘ Zn １＋ｘ (GeO ４ ) ４ A method for producing a proton conductor, which includes a step of substituting a part of lithium ions with protons by immersing where x is an integer of 0 or more.

2. The method for producing a proton conductor according to Claim 1, wherein x is 0.

3. The method for producing a proton conductor according to Claim 1 or 2, wherein the acid contains at least one selected from the group consisting of benzoic acid, m-nitrophenol, acetic acid, p-toluenesulfonic acid, oxalic acid, and methanesulfonic acid.

4. The method for producing a proton conductor according to any one of Claims 1 to 3, wherein the non-aqueous solvent contains at least one selected from the group consisting of toluene, dimethyl sulfoxide, tetrahydrofuran, and N,N-dimethylformamide.

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