Water electrolysis system and method for operating water electrolysis system

Abstract

This water electrolysis system uses an alkaline aqueous solution as an electrolytic solution, and is provided with: a cell stack to which the electrolytic solution is supplied; a storage unit in which the electrolytic solution is stored; an annular flow path that connects the storage unit and the cell stack; a pump unit that is provided on the annular flow path; a scale removal unit that is provided on the annular flow path and is capable of removing a scale contained in the electrolytic solution; and a scale component removal unit that is capable of removing scale components dissolved in the electrolytic solution at a saturation concentration or less.

Description

Water electrolysis system and method for operating the water electrolysis system

[0001] This application claims priority to Japanese Patent Application No. 2024-068450, filed on April 19, 2024, the contents of which are incorporated herein by reference.

[0002] Water electrolysis systems that produce hydrogen by electrolyzing an aqueous solution are being put into practical use. This type of device mainly comprises a cell stack consisting of multiple electrolysis cells, a tank for supplying the aqueous solution, and a power supply unit for applying voltage to the electrolysis cells. An ion exchange membrane and electrodes are arranged in the electrolysis cell, and when a voltage is applied between the electrodes, the aqueous solution is electrolyzed, ultimately producing hydrogen.

[0003] In water electrolysis systems, an aqueous potassium hydroxide solution is sometimes used to increase the solubility of potassium hydroxide in water. In this case, impurities generated during the production of the aqueous potassium hydroxide solution include trace amounts of scale components (dissolved in the electrolyte at a concentration below saturation), which may precipitate as scale in the aqueous solution. Meanwhile, Patent Document 1 listed below discloses a technology for removing ions contained in cathode water using an ion exchange membrane in an ozone water generator.

[0004] JP 2010-155227 A

[0005] It has been confirmed that water electrolysis systems have a problem in which membrane resistance increases due to scale buildup inside the cells and scale components accumulated on the ion exchange membrane, resulting in a rise in electrolysis voltage. However, no effective measures have been proposed to remove the scale and scale components in this system.

[0006] The present disclosure has been made to solve the above-mentioned problems, and has an object to provide a water electrolysis system and an operation method for a water electrolysis system that can further reduce the electrolysis voltage.

[0007] In order to solve the above problems, the water electrolysis system according to the present disclosure is a water electrolysis system that uses an alkaline aqueous solution as an electrolyte, and includes: a cell stack to which the electrolyte is supplied; a storage section in which the electrolyte is stored; an annular flow path connecting the storage section and the cell stack; a pump section provided on the annular flow path; a scale removal section provided on the annular flow path that is capable of removing scale contained in the electrolyte; and a scale component removal section that is capable of removing scale components dissolved in the electrolyte at a concentration equal to or lower than a saturation concentration.

[0008] A method for operating a water electrolysis system according to the present disclosure is a method for operating the above-described water electrolysis system, including the steps of circulating the electrolytic solution within the annular flow path and, after the circulating step, applying a voltage to the cell stack.

[0009] According to the present disclosure, it is possible to provide a water electrolysis system and a method for operating a water electrolysis system that can further reduce the electrolysis voltage.

[0010] 1 is a schematic diagram illustrating a configuration of a water electrolysis system according to an embodiment of the present disclosure; 2 is a flowchart illustrating an operation method of a water electrolysis system according to an embodiment of the present disclosure; 3 is a graph illustrating the relationship between elapsed time and electrolysis voltage in a water electrolysis system according to an embodiment of the present disclosure;

[0011] (Configuration of Water Electrolysis System 1) Hereinafter, a water electrolysis system 1 and an operation method thereof according to an embodiment of the present disclosure will be described with reference to Figs. 1 to 3 .

[0012] As shown in FIG. 1 , the water electrolysis system 1 includes a cell stack 10 , a storage unit 11 , an annular flow path 12 , a pump 13 , a filter unit 14 , a scale removal unit 15 , and a scale component removal unit 16 .

[0013] The cell stack 10 includes multiple electrolysis cells. Each electrolysis cell mainly includes an ion exchange membrane, a cathode, and an anode. The ion exchange membrane is sandwiched between the anode and the cathode. The ion exchange membrane is, for example, a solid polymer electrolyte membrane. The ion exchange membrane is, for example, an anion exchange membrane (AEM) that is hydroxide ion conductive. However, the ion exchange membrane is not limited to the above example, and may be a proton exchange membrane (PEM: Polymer Electrolyte Membrane) of a type different from the above example that is proton conductive.

[0014] The surroundings of these electrolytic cells are filled with an electrolyte. An alkaline aqueous solution is used as the electrolyte. Specifically, an aqueous solution of potassium hydroxide is preferably used. When a voltage is applied between the anode and cathode, water and oxygen are produced at the anode, and hydroxide ions and hydrogen are produced at the cathode. Specifically, the following chemical reaction occurs on the cathode side: 2H ２ O + 2e － →H ２ +2OH － ...(Chemical 1)

[0015] The following chemical reaction occurs on the anode side: － →1 / 2O ２ +H ２ O + 2e － ...(Case 2)

[0016] As a result, when viewed as a whole electrolytic cell, the chemical reaction shown in Chemical Formula 3 below occurs: ２ O → H ２ +1 / 2O ２ ...(Chem.3)

[0017] The hydroxide ions pass through the ion exchange membrane and are supplied to the anode side. At the anode, electrons are removed from these hydroxide ions, producing water and oxygen as shown above. This cycle occurs continuously, producing hydrogen gas as the final product.

[0018] The reservoir 11 is a container that stores the above-mentioned electrolyte. An annular flow path 12 is provided between the reservoir 11 and the cell stack 10. The annular flow path 12 connects the reservoir 11 and the cell stack 10 in a ring shape. A pump 13, a filter unit 14, a scale removal unit 15, and a scale component removal unit 16 are arranged on the annular flow path 12 in this order. The pump 13 is provided to pump the electrolyte.

[0019] The filter unit 14 removes coarse suspended particles contained in the electrolyte. Specifically, a porous material or a mesh member is used as the filter unit 14. It is also desirable that the filter unit 14 be alkali-resistant. An example of the filter unit 14 is MCS-020-D10ER, manufactured by ADVANTEC, which is made of polyethersulfone.

[0020] The scale removal unit 15 removes scale from the electrolyte. The scale refers to a substance in a state where scale components described later are precipitated in the electrolyte.

[0021] The scale component removal unit 16 selectively removes specific metal ions (scale components) contained in the electrolyte. Here, the specific metal ions refer to calcium ions, magnesium ions, iron ions, nickel ions, chromium ions, manganese ions, and molybdenum ions. These metal ions are impurities contained in trace amounts in potassium hydroxide, which is used as a reagent when producing an aqueous potassium hydroxide solution as the electrolyte. The inventors of the present application have determined that, when precipitated, these metal ions adhere to the cell stack 10 as scale, increasing the membrane resistance and ultimately causing an increase in the electrolysis voltage of the cell stack 10.

[0022] The scale component removal unit 16 includes at least one of a first removal unit 21, a second removal unit 22, and a third removal unit 23. The first removal unit 21 is filled with a chelating resin. The second removal unit 22 is filled with a cation exchange resin. The third removal unit 23 is filled with an anion exchange resin. More specifically, the scale component removal unit 16 includes a resin material having at least one of iminodiacetic acid, aminophosphoric acid, and type I quaternary ammonium as a functional group. FIG. 1 illustrates an example using the first removal unit 21 and the second removal unit 22. The first removal unit 21 and the second removal unit 22 are arranged in series on the annular flow path 12. When arranging the third removal unit 23, it is desirable to further arrange the first removal unit 21 and the second removal unit 22 in series. Alternatively, a mixed bed system may be employed in which the second removal unit 22 and the third removal unit 23 are housed in the same housing as the first removal unit 21.

[0023] (Operation Method of Water Electrolysis System 1) Next, an operation method of the water electrolysis system 1 will be described with reference to FIG. 2 . As shown in the figure, in this operation method, in step S1, the electrolyte is first circulated through the annular flow path 12. During this circulation, the electrolyte passes through the filter unit 14, the scale removal unit 15, and the scale component removal unit 16. By repeating the circulation multiple times, the suspended matter, scale, and scale components are removed from the electrolyte, resulting in a clean state. In this state, in step S2, application of voltage to the cell stack 10 is initiated. Thereafter, the electrolysis reaction described above occurs, and hydrogen production begins (step S3). These steps S1 to S3 are used not only during initial operation of the system, but also when starting operation under normal conditions. Furthermore, it is possible to continue hydrogen production while the electrolyte is normally circulated, as necessary.

[0024] (Effects) In the water electrolysis system 1, a potassium hydroxide aqueous solution may be used to increase the ionic conductivity of water. In this case, trace amounts of various scale components may dissolve in the aqueous solution as impurities generated during the production of the potassium hydroxide aqueous solution. Furthermore, trace amounts of various metal ions may leach into the potassium hydroxide solution (electrolyte) from the metal surfaces of the cell stack components and system piping that come into contact with the potassium hydroxide solution. The inventors have now identified a problem in which scale and scale components are liberated in the electrolyte due to these trace amounts of metal ions, increasing the membrane resistance and resulting in an increase in electrolysis voltage. However, in the field of such systems, no effective measures for removing scale and scale components have been proposed. To solve this problem, the present embodiment employs the above-described configurations.

[0025] According to the above configuration, the electrolytic solution passes through the scale removal unit 15 and the scale component removal unit 16 during circulation through the annular flow path 12. This allows selective removal of specific scale and scale components contained in the electrolytic solution. Specifically, calcium ions, magnesium ions, iron ions, nickel ions, chromium ions, manganese ions, and molybdenum ions can be removed. The present inventors have identified these scale components as substances that cause an increase in the electrolysis voltage of water. Therefore, removing these components can significantly reduce the electrolysis voltage (see FIG. 3 ). Note that the dashed line in FIG. 3 is a reference example showing the change in electrolysis voltage without the filter unit 14, the scale removal unit 15, and the scale component removal unit 16, while the solid line shows the change in electric field voltage with the configuration according to this embodiment. This significantly reduces the operating costs of the water electrolysis system 1. As a result, the production cost of hydrogen, the final product, can be reduced, contributing to the further spread of hydrogen energy.

[0026] According to the above configuration, the electrolyte passes through the first removal section 21, the second removal section 22, and the third removal section 23, whereby selective ion removal by the respective resin materials, i.e., removal of scale and scale components, can be realized. For example, in the mesh filter, magnesium oxide (MnO ２ ), calcium oxide (CaO), calcium carbonate (CaCO ３ ) and other metal scales. Chelate resins have high selectivity for specific metal ions, forming chelates (complexes) to remove calcium (Ca ２＋ ), magnesium (Mg ２＋ ), barium (Ba ２＋ ), manganese (Mn ２＋ ), nickel (Ni ２＋ ), iron (Fe ２＋ , Fe ３＋ ), chromium (Cr ２＋ , Cr ３＋ ) and other metal ions. Cation exchange resins also have the ability to remove or recover fixed ions -SO ３ － and counter ion H ＋ It dissociates into hydrogen ions (H-type). ＋ ), sodium (Na ＋ ) and are replaced by hydrogen ions H ＋ The anion exchange resin dissociates into fixed ions and counter ions in the solution, becoming hydroxide ion form (OH form). Then, chloride (Cl) in the water is released. － ) and sulfuric acid (SO ４ ２－ ), nitric acid (NO ３ － ) and replaced with hydroxide ions (OH) － is released into the electrolyte.

[0027] Here, biofilms and other suspended matter may adhere to the ion exchange membrane contained in the cell stack 10. If such suspended matter becomes free in the electrolyte, it can cause a decrease in the performance of the ion exchange membrane itself. With the above configuration, by removing coarse suspended matter in advance using the filter unit 14, it becomes possible to efficiently remove scale from the electrolyte using the scale removal unit 15. As a result, the life of the cell stack 10 is extended, which can reduce the operating costs of the system and the production costs of hydrogen, the final product.

[0028] According to the above configuration, a resin material having at least one of iminodiacetic acid, aminophosphate, and type I quaternary ammonium as a functional group selectively binds to metal ions that are reactive with each other, thereby enabling the efficient removal of these metal ions.

[0029] According to the above method, the electrolyte can be circulated in the annular flow path 12 before a voltage is applied to the cell stack 10, thereby supplying the electrolyte in a clean state to the cell stack 10. This significantly reduces the possibility that the electrolysis voltage of the cell stack 10 will increase.

[0030] (Other Embodiments) Although the embodiments of the present disclosure have been described in detail above with reference to the drawings, the specific configuration is not limited to this embodiment, and design changes and the like within the scope that does not deviate from the gist of the present disclosure are also included.

[0031] For example, the water electrolysis system 1 described above may be configured without the filter unit 14. In this case, the effects of removing metal ions and scale and thereby reducing the electrolysis voltage can be obtained.

[0032] <Additional Notes> The water electrolysis system 1 and the method of operating the water electrolysis system 1 described in each embodiment can be understood, for example, as follows.

[0033] (1) A water electrolysis system 1 according to a first aspect uses an alkaline aqueous solution as an electrolyte, and includes a cell stack 10 to which the electrolyte is supplied, a storage unit 11 in which the electrolyte is stored, an annular flow path 12 connecting the storage unit 11 and the cell stack 10, a pump 13 provided on the annular flow path 12, a scale removal unit 15 provided on the annular flow path 12 and capable of removing scale contained in the electrolyte, and a scale component removal unit 16 capable of removing scale components dissolved in the electrolyte at a concentration equal to or lower than a saturation concentration.

[0034] According to the above configuration, the electrolytic solution passes through the scale removal section 15 and the scale component removal section 16 while circulating through the annular flow path 12. At this time, scale and scale components contained in the electrolytic solution can be selectively removed.

[0035] (2) A water electrolysis system 1 according to a second aspect is the water electrolysis system 1 according to (1), wherein the scale component removal unit 16 has at least one of a first removal unit 21 filled with a chelating resin, a second removal unit 22 filled with a cation exchange resin, and a third removal unit 23 filled with an anion exchange resin.

[0036] According to the above configuration, by passing the electrolyte through the first removal section 21, the second removal section 22, and the third removal section 23, selective ion removal by each resin material, i.e., removal of scale components, can be achieved.

[0037] (3) A water electrolysis system 1 according to a third aspect is the water electrolysis system 1 according to (1) or (2), further comprising a filter unit 14 provided upstream of the scale removal unit 15 and capable of removing suspended solids in the electrolytic solution.

[0038] According to the above configuration, the suspended matter can be efficiently removed by the filter portion 14.

[0039] (4) A water electrolysis system 1 according to a fourth aspect is the water electrolysis system 1 according to any one of the aspects (1) to (3), wherein the scale component removal unit 16 includes a resin material having at least one of iminodiacetic acid, aminophosphoric acid, and type I quaternary ammonium as a functional group.

[0040] According to the above-mentioned configuration, each of the functional groups selectively binds to reactive metal ions, and these metal ions can be efficiently removed.

[0041] (5) A fifth aspect of the water electrolysis system 1 is the water electrolysis system 1 according to (1) or (2), wherein the scale removal unit 15 includes a mesh filter capable of removing scale deposited in the electrolytic solution.

[0042] According to the above configuration, scale deposited in the electrolytic solution can be removed.

[0043] (6) A water electrolysis system 1 according to a sixth aspect is the water electrolysis system 1 according to (1) or (2), including an anion exchange membrane electrolysis cell.

[0044] According to the above configuration, water electrolysis can be carried out efficiently.

[0045] (7) A method for operating a water electrolysis system 1 according to a seventh aspect is a method for operating a water electrolysis system 1 according to any one of the aspects (1) to (4), including the steps of circulating the electrolytic solution within the annular flow path 12 and applying a voltage to the cell stack 10 after the circulating step.

[0046] According to the above method, the electrolyte can be circulated in the annular flow path 12 before a voltage is applied to the cell stack 10, thereby supplying the electrolyte in a clean state to the cell stack 10. This significantly reduces the possibility that the electrolysis voltage of the cell stack 10 will increase.

[0047] According to the present disclosure, it is possible to provide a water electrolysis system and a method for operating a water electrolysis system that can further reduce the electrolysis voltage.

[0048] REFERENCE SIGNS LIST 1... Water electrolysis system 10... Cell stack 11... Storage section 12... Annular flow path 13... Pump 14... Filter section 15... Scale removal section 16... Scale component removal section 21... First removal section 22... Second removal section 23... Third removal section

Claims

1. A water electrolysis system using an alkaline aqueous solution as an electrolyte, comprising: a cell stack to which the electrolyte is supplied; a storage unit in which the electrolyte is stored; an annular flow path connecting the storage unit and the cell stack; a pump unit provided on the annular flow path; a scale removal unit provided on the annular flow path and capable of removing scale contained in the electrolyte; and a scale component removal unit capable of removing scale components dissolved in the electrolyte at a concentration equal to or lower than saturation.

2. The water electrolysis system according to claim 1, wherein the scale component removal section has at least one of a first removal section filled with a chelating resin, a second removal section filled with a cation exchange resin, and a third removal section filled with an anion exchange resin.

3. The water electrolysis system according to claim 1, further comprising a filter unit provided upstream of the scale removal unit and capable of removing suspended solids from the electrolyte.

4. A water electrolysis system according to any one of claims 1 to 3, wherein the scale component removal section includes a resin material having at least one of iminodiacetic acid, aminophosphoric acid, and type I quaternary ammonium as a functional group.

5. The water electrolysis system according to claim 1 or 2, wherein the scale removal unit includes a mesh filter capable of removing scale deposited in the electrolytic solution.

6. The water electrolysis system according to claim 1 or 2, comprising an anion exchange membrane electrolysis cell.

7. A method for operating a water electrolysis system according to any one of claims 1 to 3, comprising: a step of circulating the electrolyte solution within the annular flow path; and a step of applying a voltage to the cell stack after the circulating step.

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