Reversible Fuel Cell

Abstract

To provide a reversible fuel cell that does not require an external hydrogen supply source.SOLUTION: The reversible fuel cell includes an anode, an electrolyte membrane, and a cathode. A proton storage layer including organic redox molecules is provided on the anode side of the electrolyte membrane. The organic redox molecule is an organic molecule capable of reversibly performing protonation and deprotonation by a redox reaction without the intervention of hydrogen gas.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a fuel cell, and more particularly to a reversible (also called "rechargeable") fuel cell that can be charged by applying a voltage between the anode and cathode. [Background technology]

[0002] A fuel cell can generate electricity by an electrochemical reaction by supplying fuel (hydrogen) to the anode and air (oxygen) to the cathode. Patent Document 1 assumes that electricity is generated by supplying hydrogen to the fuel cell from a hydrogen supply source provided outside the fuel cell. In this specification, hydrogen and oxygen refer to hydrogen gas and oxygen gas, respectively, unless otherwise specified. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2017-16942 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the configuration of Patent Document 1, an external hydrogen supply source such as a hydrogen tank is required to operate the fuel cell.

[0005] The present invention has been made in view of the above circumstances, and provides a reversible fuel cell that does not require an external hydrogen source. [Means for solving the problem]

[0006] According to the present invention, there is provided a reversible fuel cell comprising an anode, an electrolyte membrane, and a cathode, wherein a proton storage layer containing an organic redox molecule is provided on the anode side of the electrolyte membrane, and the organic redox molecule is an organic molecule that can be reversibly protonated and deprotonated by a redox reaction without the intervention of hydrogen gas.

[0007] The fuel cell of the present invention can generate power using protons released by deprotonation of organic redox molecules, and therefore does not require hydrogen supply from an external hydrogen source, making it possible to reduce the size and weight, and does not require piping, making the device configuration simple. Also, the fuel cell of the present invention can be charged (i.e., protonation of organic redox molecules) by applying a voltage between the anode and cathode, making it possible to reduce the size and weight, and does not require an external hydrogen source, making the device configuration simple.

[0008] Various embodiments of the present invention will be described below. The embodiments described below can be combined with each other. Preferably, in the reversible fuel cell described above, the proton storage layer comprises conductive particles and a polymer electrolyte. Preferably, in the above-described reversible fuel cell, the mass ratio of (the organic redox molecules / the conductive particles) is 10 or less. Preferably, in the above-described reversible fuel cell, the organic redox molecule has a weight average molecular weight of 5000 or more. Preferably, in the reversible fuel cell described above, the proton storage layer does not contain a precious metal catalyst. Preferably, in the reversible fuel cell described above, the proton storage layer is applied to the electrolyte membrane. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a perspective view of a reversible fuel cell 1 according to one embodiment of the present invention. [Diagram 2] 2A is a cross-sectional view of catalyst-coated membrane 2 and support frame 3 in FIG. 1, and FIG. 2B is an enlarged view of region B in FIG. 2A. [Diagram 3] FIG. 2 is an explanatory diagram of the power generation operation of the reversible fuel cell 1. [Figure 4] FIG. 2 is an explanatory diagram of the charging operation of the reversible fuel cell 1. [Diagram 5] 2 is a graph showing the results of a power generation test of the reversible fuel cell 1 of Example 1. [Figure 6] 4 is a graph showing the results of a power generation test of the reversible fuel cell 1 of Example 2. [Figure 7] 11 is a graph showing the results of a power generation test of the reversible fuel cell 1 of Example 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Various characteristic features shown in the following embodiment can be combined with each other. Also, each characteristic feature can be an invention independently.

[0011] 1. Configuration of reversible fuel cell 1 As shown in Fig. 1, a reversible fuel cell 1 according to one embodiment of the present invention comprises a catalyst-coated membrane 2, a support frame 3, an anode unit 10, and a cathode unit 20. In this specification, the electrode from which electrons flow out to an external circuit during power generation operation of the reversible fuel cell 1 is referred to as the anode, and the electrode into which electrons flow from the external circuit is referred to as the cathode. During charging operation of the reversible fuel cell 1, the direction in which electrons flow is reversed, but the names of the components are set based on the direction in which electrons flow during power generation.

[0012] 1-1. Catalyst coated membrane 2 and support frame 3 2, the catalyst-coated membrane 2 is configured by applying a proton storage layer 2b to the anode side of an electrolyte membrane 2a, and applying a cathode catalyst layer 2c to the cathode side of the electrolyte membrane 2a. A peripheral portion 2d of the catalyst-coated membrane 2 is supported by a support frame 3. The electrolyte membrane 2a is a polymer membrane having ion conductivity, and protons can move from the proton storage layer 2b side to the cathode catalyst layer 2c side through the electrolyte membrane 2a.

[0013] The proton storage layer 2b is a layer capable of storing protons, and its configuration will be described later. The cathode catalyst layer 2c contains a catalyst that promotes the reaction of oxidizing protons with oxygen to produce water and the reaction of electrolyzing water to produce oxygen and protons. Examples of such catalysts include precious metals such as platinum.

[0014] When the fuel cell 1 is in a power generating operation, the proton storage layer 2b serves as the anode, and the cathode catalyst layer 2c serves as the cathode.

[0015] 1-2. Anode unit 10 As shown in FIG. 1, the anode unit 10 is disposed on the anode side of the electrolyte membrane 2 a , and includes an anode diffusion member 12 , an anode gasket 13 , an anode separator 14 , and an anode current collector plate 15 .

[0016] The anode diffusion member 12 is disposed adjacent to the proton storage layer 2b. The anode diffusion member 12 is a member that diffuses gas, and is made of, for example, a porous material. By providing the anode diffusion member 12, it is possible to promote humidification of the proton storage layer 2b, but the anode diffusion member 12 is not essential and may be omitted.

[0017] The anode gasket 13 is disposed so as to surround the anode diffusion member 12. The anode gasket 13 is made of, for example, a rubber sheet, and has a function of preventing the proton storage layer 2b from coming into contact with the outside air.

[0018] The anode separator 14 is disposed adjacent to the anode diffusion member 12 and the anode gasket 13. The anode separator 14 is made of a conductive material such as metal or carbon.

[0019] The anode current collector 15 is disposed adjacent to the anode separator 14 and is made of a conductive material such as metal or carbon. External terminals are connected to the anode current collector 15 and the cathode current collector 25, making it possible to charge the reversible fuel cell 1 and to extract the current discharged by the reversible fuel cell 1.

[0020] 1-3. Cathode unit 20 As shown in FIG. 4, the cathode unit 20 is disposed on the cathode side of the electrolyte membrane 2a, and includes a cathode diffusion member 22, a cathode gasket 23, a cathode separator 24, and a cathode current collector plate 25.

[0021] The cathode diffusion member 22 is disposed adjacent to the cathode catalyst layer 2c. The cathode diffusion member 22 is a member that diffuses the cathode gas containing oxygen, and is made of, for example, a porous material. By providing the cathode diffusion member 22, efficient diffusion of the cathode gas becomes possible.

[0022] The cathode gasket 23 is disposed so as to surround the cathode diffusion member 22. The cathode gasket 23 is made of, for example, a rubber sheet, and has the function of preventing leakage of the cathode gas.

[0023] The cathode separator 24 is disposed adjacent to the cathode diffusion member 22 and the cathode gasket 23. The cathode separator 24 is formed of a conductive material. If the cathode separator 24 is formed of carbon, there is a risk of it being corroded during electrolysis for charging the reversible fuel cell 1. Therefore, the cathode separator 24 is preferably formed of a metal with excellent corrosion resistance such as titanium, and further, it is preferable that the surface of the cathode separator 24 is plated with platinum.

[0024] The cathode current collector 25 is made of a conductive material such as metal or carbon. The cathode current collector 25 is provided with a refill port 25a. Although the reversible fuel cell 1 of this embodiment can operate in a closed system, it may be preferable to replenish substances necessary for the redox reaction (at least one of water, oxygen, and hydrogen) in order to suppress performance degradation due to gas leakage. In such cases, such substances can be replenished through the refill port 25a.

[0025] 2. Configuration of proton storage layer 2b The proton storage layer 2b contains organic redox molecules, and serves as a source of protons for the power generation operation of the reversible fuel cell 1. In this embodiment, the proton storage layer 2b is applied to the electrolyte membrane 2a, but the proton storage layer 2b may be present between the electrolyte membrane 2a and the anode current collector 15 so as to reduce the resistance to the movement of protons and electrons. For example, the proton storage layer 2b may be applied to the anode diffusion member 12, the anode separator 14, or the anode current collector 15, or a film constituting the proton storage layer 2b may be formed separately and this film may be disposed between the electrolyte membrane 2a and the anode current collector 15.

[0026] The organic redox molecule is an organic molecule that can reversibly undergo protonation and deprotonation without the intervention of hydrogen gas by a direct redox reaction (a redox reaction that does not involve hydrogen gas). The organic redox molecule is preferably one in which the redox reaction proceeds without the catalytic action of a precious metal such as platinum or iridium. In this case, it is preferable that the proton storage layer 2b does not contain a precious metal. This makes it possible to reduce the manufacturing cost of the reversible fuel cell 1. A catalyst that promotes the redox reaction may be added.

[0027] The anode potential of the reversible fuel cell 1 depends on the redox potential of the organic redox molecule, and the cell voltage of the reversible fuel cell 1 (the difference between the anode potential and the cathode potential) depends on the anode potential. Therefore, the output voltage of the reversible fuel cell 1 can be changed by selecting the organic redox molecule.

[0028] Organic redox molecules can be small molecules or polymers (molecules with repeating units). Small organic redox molecules store and release, for example, two protons per molecule. Polymeric organic redox molecules store and release, for example, two protons per repeating unit.

[0029] The organic redox molecule is preferably a polymer. In this case, it is easy to exist stably in the proton storage layer 2b. The weight-average molecular weight of the organic redox molecule is preferably 5000 or more. In this case, it is easy to exist stably in the proton storage layer 2b. This weight-average molecular weight is preferably 300,000 or less. The organic redox molecule is preferably mixed with conductive particles such as carbon in a state dissolved in a solvent, but if the weight-average molecular weight of the organic redox molecule is too large, the organic redox molecule may be difficult to dissolve in the solvent or difficult to mix. The weight-average molecular weight is specifically, for example, 5000, 10,000, 20,000, 30,000, 40,000, 50,000, 60,000, 70,000, 80,000, 90,000, 100,000, 150,000, 200,000, 250,000, 300,000, and may be within a range between any two of the numerical values ​​exemplified here. The weight-average molecular weight can be measured by gel permeation chromatography (GPC method).

[0030] An example of a low molecular weight organic redox molecule is diphenylquinoxaline, and an example of a high molecular weight organic redox molecule is polyvinylfluorenone. These organic redox molecules are capable of reversible protonation and deprotonation without the use of hydrogen gas through the redox reactions shown in the following formulas (1) and (2).

[0031] [ka]

[0032] [ka]

[0033] Other examples of small organic redox molecules are as follows: [ka]

[0034] Examples of other polymeric organic redox molecules are as follows: [ka]

[0035] The proton storage layer 2b preferably contains conductive particles and a polymer electrolyte. The conductive particles increase the electronic conductivity, and the polymer electrolyte increases the proton conductivity, so that the proton storage layer 2b contains the conductive particles and the polymer electrolyte, and thus the redox reaction is promoted. An example of the conductive particles is carbon. An example of the polymer electrolyte is Nafion.

[0036] When the conductive particles are made of a hydrophobic material such as carbon, it is preferable that the organic redox molecules are also hydrophobic, since in this case the conductive particles and the organic redox molecules tend to mix uniformly.

[0037] The mass ratio of (organic redox molecules / conductive particles) is, for example, 15 or less, preferably 10 or less, and more preferably 5 or less. If this mass ratio is too large, the electronic conductivity becomes insufficient, and the redox reaction does not easily proceed. This value is, for example, 0.1 to 15, preferably 0.5 to 5, specifically, for example, 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15, and may be within a range between any two of the numerical values ​​exemplified here.

[0038] The mass ratio of (polymer electrolyte / conductive particles) is, for example, 0.1 to 10, specifically, for example, 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10, and may be within a range between any two of the numerical values ​​exemplified here. In this case, both electronic conductivity and proton conductivity tend to be good.

[0039] The proton storage layer 2b has an organic redox molecule loading of 0.1 to 50 mg / cm 2 The amount of the organic redox molecule supported is preferably 0.1 to 30 mg / cm. 2 is preferable, and specifically, for example, 0.1, 0.5, 1, 1.5, 2, 5, 10, 15, 20, 30, 40, 50 mg / cm 2 and may be in the range between any two of the values ​​given here.

[0040] 3. Operation of reversible fuel cell 1 3-1.Power generation operation As shown in FIG. 3, the reversible fuel cell 1 can generate electricity by connecting a load 4 between the anode and the cathode (more specifically, between the anode current collector 15 and the cathode current collector 25).

[0041] At this time, an oxidation reaction (deprotonation reaction) of the organic redox molecule occurs on the anode current collector 15 side, and a reduction reaction of oxygen occurs on the cathode current collector 25 side. The reaction formula of this electrochemical redox reaction is as follows. The organic redox molecule in the reduced state and the oxidized state are represented as the organic redox molecule (Re) and the organic redox molecule (Ox), respectively. In the following, an example is given in which one organic redox molecule or repeating unit is oxidized or reduced by two protons and two electrons, but the number of protons and electrons is not limited to this.

[0042] (Reaction at the anode side) Organic redox molecule (Re) → Organic redox molecule (Ox) + 2H + +2e - (Cathode reaction) 1 / 2O 2 +2H + +2e - →H 2 O

[0043] As a result of this reaction, electrons released from the organic redox molecules in the proton storage layer 2b reach the cathode catalyst layer 2c via the anode current collector 15, the load 4, and the cathode current collector 25 in this order. Meanwhile, protons released from the organic redox molecules reach the cathode catalyst layer 2c through the electrolyte membrane 2a. Oxygen is present in the cathode catalyst layer 2c, and the oxygen is reduced by the cathode reaction to produce water.

[0044] In addition, in the reaction on the anode side, protons are generated directly from the organic redox molecules, not via hydrogen gas, so no hydrogen gas is generated in the reaction on the anode side.

[0045] 3-2.Charging operation As shown in FIG. 4, the reversible fuel cell 1 can be charged by connecting a power source 5 and applying a voltage between the anode and cathode (more specifically, between the anode current collector 15 and the cathode current collector 25) so that the potential on the cathode side (i.e., the cathode current collector 25 side) is higher.

[0046] At this time, a reduction reaction (protonation reaction) occurs on the anode current collector 15 side, and water electrolysis (water oxidation reaction) occurs on the cathode current collector 25 side. The reaction formula of this electrochemical redox reaction is as follows.

[0047] (Reaction at the anode side) Organic redox molecule (Ox) + 2H + +2e - →Organic redox molecules (Re) (Cathode reaction)H 2 O → 1 / 2O 2 +2H + +2e -

[0048] The electrons released from the water by this reaction reach the proton storage layer 2b via the cathode current collector 25, the power source 5, and the anode current collector 15 in this order. Meanwhile, the protons released from the water reach the proton storage layer 2b through the electrolyte membrane 2a. The organic redox molecules (Ox) present in the proton storage layer 2b are reduced by the arriving protons and electrons to become organic redox molecules (Re). In this way, protons are stored in the proton storage layer 2b by charging.

[0049] 4. Application Examples of the Present Invention As described above, the reversible fuel cell of the present invention does not require an external hydrogen supply source such as a hydrogen tank, so it is easy to reduce the size and weight, and can be used, for example, in drones to extend their flight time. When applied to stationary power generation equipment, since no external hydrogen supply source is required, no piping is required, so the equipment is simple and the freedom of installation is increased. When applied to electric vehicles such as EVs, it is lighter than conventional batteries, so it is expected to improve fuel efficiency. EXAMPLES

[0050] A power generation experiment of the reversible fuel cell 1 was carried out in the following manner.

[0051] 1. Fabrication of Experimental Fuel Cell The reversible fuel cells 1 of Examples 1 to 3 having the configuration shown in Figs. 1 and 2 were produced in the following procedure.

[0052] <Formation of proton storage layer> An anode paste was prepared by mixing 0.25 g of the organic redox molecule shown in Table 1, 4.13 g of water, 8.09 g of ethanol, 5 wt% Nafion solution, and carbon black (CB). The 5 wt% Nafion solution and carbon black (CB) were mixed so that the mass ratio of Nafion / CB and the mass ratio of organic redox molecule / CB were the values ​​shown in Table 1. The obtained anode paste was applied to one side of a 50 μm thick solid polymer electrolyte membrane to form a proton storage layer. The anode paste was applied so that the amount of organic redox molecule supported was the value shown in Table 1.

[0053] [Table 1]

[0054] <Formation of cathode catalyst layer> A cathode catalyst paste was prepared by mixing 0.5 g of carbon-supported platinum catalyst, 4.13 g of water, 8.09 g of ethanol, and 2.50 g of 5 wt% Nafion solution, and the cathode catalyst layer was formed by applying the cathode catalyst paste to the other side of the solid polymer electrolyte membrane. The cathode catalyst paste had a platinum loading of 0.5 mg. Pt / cm 2 The coating was applied so that the result was as follows.

[0055] <Hot press> The above steps yield a catalyst-coated membrane in which the solid polymer electrolyte membrane is sandwiched between the cathode catalyst layer and the proton storage layer. This catalyst-coated membrane is dried at 60°C for 12 hours, and then subjected to a 140°C, 10 kgf / cm 2 The cathode catalyst layer and the proton storage layer were applied to the substrate and hot pressed at 4.41 cm2 for 3 min. 2 It was.

[0056] <Manufacture of reversible fuel cell 1> The above catalyst-coated membrane was incorporated into a JARI standard cell to produce a reversible fuel cell 1. On the cathode side and the anode side, graphite fiber nonwoven fabrics with a thickness of about 200 μm were placed as gas diffusion layers.

[0057] The description of the above manufacturing method and details of the various components in Table 1 are as follows. Carbon-supported platinum catalyst: TEC10E50E, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. ·5wt%Nafion solution: DE521, EI Du Pont de Nemours & Co., Inc. Solid polymer electrolyte membrane: Nafion NRE-212, manufactured by Sigma-Aldrich Japan LLC Carbon black: High specific surface area carbon, manufactured by Tanaka Kikinzoku Co., Ltd. DPQ: Diphenylquinoxaline ·PVF: Polyvinyl fluorenone Graphite fiber nonwoven fabric: 29BC, manufactured by SGL Carbon

[0058] 2. Measurement of anode powder conductivity The anode powder electrical conductivity of Examples 1 to 3 was measured by drying the anode paste, compacting the powder obtained, and using a Panasonic powder resistance measuring device by a four-probe method.

[0059] 3. Power generation test of reversible fuel cell 1 First, while flowing nitrogen gas at 0.1 L / min on the anode and cathode sides at 40° C., a power source 5 was connected between the anode current collector 15 and the cathode current collector 25 so that the potential on the cathode current collector 25 side was higher, and a current of 1 mA / cm was applied. 2 The reversible fuel cell 1 was charged by passing a current of 1000 V until the cell voltage reached 2 V.

[0060] Next, the load 4 was connected between the anode current collector 15 and the cathode current collector 25 at 40° C. to start power generation in the reversible fuel cell 1. The resistance value of the load 4 was 1 mA (=0.227 mA / cm 2 ) was set.

[0061] The cathode potential and anode potential relative to a standard hydrogen electrode, and the cell resistance were measured during power generation by the reversible fuel cell 1. The cell resistance value at the start of the measurement is shown in Table 1. The graphs in Figs. 5 to 7 show the changes over time in the cathode potential, anode potential, cell voltage (=cathode potential-anode potential), and cell resistance when the reversible fuel cells 1 of Examples 1 to 3 were allowed to generate power. In Figs. 5 to 7, the horizontal axis shows the number of seconds per unit mass (1 mg) of organic redox molecules. The current efficiency in Table 1 shows the ratio (%) of the actual power generation time to the power generation time expected when it is assumed that protons are released from all the organic redox molecules.

[0062] As shown in FIGS. 5 to 7, in all of the reversible fuel cells 1 of Examples 1 to 3, it is clear that power generation was successful using protons from organic redox molecules without the supply of hydrogen from the outside.

[0063] Comparing Examples 1 and 2 shown in Table 1 and Figures 5 and 6, it can be seen that in Example 2, the cell resistance was 1 / 30, the power generation time was 8 times, and the current efficiency was approximately 8.2 times, compared to Example 1. Example 2 differs from Example 1 in that the mass ratio of organic redox molecule / CB was lower than that of Example 1 (i.e., the blended amount of CB was higher), and therefore the difference in the test results is believed to be due to the difference in the mass ratio of organic redox molecule / CB.

[0064] Comparing Examples 2 and 3 shown in Table 1 and Figures 6 to 7, it can be seen that in Example 3, the cell resistance was 2 / 5, the power generation time was 2.4 times, and the current efficiency was 3.9 times, compared to Example 2. Example 3 differs from Example 2 in that the organic redox molecule is a polymer (Example 2 is a low molecular weight), and the difference in the test results is thought to be due to the organic redox molecule in Example 3 being a polymer. [Explanation of symbols]

[0065] 1:Fuel cell 2: Catalyst coating film 2a: Electrolyte membrane 2b: Proton storage layer 2c: Cathode catalyst layer 2d: Periphery 3: Support frame 4: Load 5: Power supply 10: Anode unit 12: Anode diffusion member 13: Anode gasket 14: Anode separator 15: Anode current collector 20: Cathode unit 22: Cathode diffusion member 23: Cathode gasket 24: Cathode separator 25: Cathode current collector 25a: Refill port

Claims

1. A fuel cell comprising an anode, an electrolyte membrane, and a cathode, a proton storage layer including an organic redox molecule is provided on the anode side of the electrolyte membrane; The organic redox molecule is an organic molecule that can be reversibly protonated and deprotonated by a redox reaction without the intervention of hydrogen gas. A reversible fuel cell (excluding those in which a reducing agent is supplied to the anode in the form of a gas).

2. 2. The reversible fuel cell of claim 1, The proton storage layer comprises conductive particles and a polymer electrolyte.

3. 3. A reversible fuel cell according to claim 2, A reversible fuel cell, wherein the mass ratio of (the organic redox molecule / the conductive particle) is 10 or less.

4. A reversible fuel cell according to any one of claims 1 to 3, A reversible fuel cell, wherein the organic redox molecule has a weight average molecular weight of 5,000 or more.

5. A reversible fuel cell according to any one of claims 1 to 4, A reversible fuel cell, wherein the proton storage layer does not contain a precious metal catalyst.

6. A reversible fuel cell according to any one of claims 1 to 5, A reversible fuel cell, wherein the proton storage layer is applied to the electrolyte membrane.

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