Water electrolysis system

Abstract

To provide a technique capable of suppressing the production amount of oxygen and hydrogen and production efficiency thereof from decreasing.SOLUTION: The water electrolysis system generating hydrogen and oxygen by electrolysis of water, comprises: a water electrolysis cell comprising an anode, a cathode, and an electrolyte membrane interposed between the anode and the cathode; and a controller controlling electric power supplied to the water electrolysis cell. The controller carries out potential variation processing of varying a potential of the anode at least at a time of one of a startup operation and a continuous operation of the water electrolysis system, the potential variation processing including potential lowering processing of lowering the potential of the anode to a predetermined potential.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present disclosure relates to a water electrolysis system. [Background technology]

[0002] A water electrolysis system that produces oxygen and hydrogen by electrolyzing water to generate oxygen at the anode and hydrogen at the cathode is known (Patent Document 1). This water electrolysis system includes a water electrolysis stack in which multiple cells are stacked, each having a membrane electrode assembly in which an ion exchange membrane is sandwiched between an anode and a cathode. Each of the anode and cathode has a catalyst layer and a power supply. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-128165 Summary of the Invention [Problem to be solved by the invention]

[0004] During start-up and continuous operation of a water electrolysis system, the cells may be poisoned by water circulating through the system or impurities generated within the system, resulting in a decrease in catalytic activity and deterioration of the electrolyte membrane (ion exchange membrane).Poisoning of the cells may result in a decrease in the amounts and efficiency of oxygen and hydrogen production. [Means for solving the problem]

[0005] The present disclosure can be realized in the following forms.

[0006] (1) According to one embodiment of the present disclosure, there is provided a water electrolysis system. The water electrolysis system generates hydrogen and oxygen by electrolysis of water and includes a water electrolysis cell including an anode, a cathode, and an electrolyte membrane sandwiched between the anode and the cathode; and a control device that controls power supplied to the water electrolysis cell. The control device executes a potential fluctuation process to fluctuate the potential of the anode during at least one of startup and continuous operation of the water electrolysis system, the potential fluctuation process including a potential reduction process to reduce the potential of the anode to a predetermined potential. According to this embodiment, the control device executes the potential fluctuation process during at least one of startup and continuous operation of the water electrolysis system, when the water electrolysis cell may be poisoned, thereby restoring the electrolysis performance of the water electrolysis cell. This prevents a decrease in the amount and efficiency of oxygen and hydrogen produced in the water electrolysis system. (2) In the above aspect, the control device may execute the potential variation process at least once and at most 30 times. According to this aspect, the control device can restore the electrolysis performance of the water electrolysis cell by executing the potential variation process at least once and at most 30 times. (3) In the above aspect, the control device may perform the potential lowering process by causing the hydrogen generated at the cathode to permeate through the electrolyte membrane and move from the cathode to the anode after stopping the supply of power to the water electrolysis cell. According to this aspect, the control device can perform the potential lowering process by causing the hydrogen generated at the cathode to permeate through the electrolyte membrane and move from the cathode to the anode after stopping the supply of power to the water electrolysis cell. (4) In the above aspect, the control device may perform the potential variation process by controlling at least one of a current value and a voltage value when power is supplied to the water electrolysis cell. According to this aspect, the control device can perform the potential variation process by controlling at least one of a current value and a voltage value when power is supplied to the water electrolysis cell. (5) In the above aspect, the electrolyte membrane may have a thickness of 25 μm or less. According to this aspect, when the electrolyte membrane has a thickness of 25 μm or less, the control device can restore the electrolysis performance of the water electrolysis cell. The present disclosure may be realized in various forms other than the water electrolysis system described above, such as a method for manufacturing a water electrolysis system, a water electrolysis method using a water electrolysis system, a method for controlling a water electrolysis system, a computer program for implementing the control method, and a non-transitory recording medium on which the computer program is recorded. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram showing the configuration of a water electrolysis system. [Figure 2] FIG. 1 is a cross-sectional view showing a configuration of a water electrolysis cell. [Figure 3] FIG. 10 is a diagram for explaining details of a potential change process. [Figure 4] 10 is a table showing the results of examining various conditions in potential variation processing. [Figure 5] 10 is a graph showing the transition of current density for each number of executions of a potential variation process. [Figure 6] FIG. 10 is a graph showing the relationship between the thickness of the electrolyte membrane and the time required for the potential lowering process. DETAILED DESCRIPTION OF THE INVENTION

[0008] A. First embodiment: Figure 1 shows the configuration of a water electrolysis system 1. The water electrolysis system 1 generates hydrogen and oxygen through the electrolysis of water. The water electrolysis system 1 includes a cell stack 20 in which multiple water electrolysis cells 22 are stacked, a power source 30, a cell monitor 32, a water supply unit 40, an oxygen discharge unit 50, a hydrogen discharge unit 60, and a control device 70.

[0009] 2 is a cross-sectional view showing a schematic configuration of the water electrolysis cell 22. The water electrolysis cell 22 includes an electrolyte membrane 80, an anode 81, a cathode 82, an anode-side separator 85, a cathode-side separator 86, an anode-side flow path 87, and a cathode-side flow path 88.

[0010] The electrolyte membrane 80 is sandwiched between an anode 81 and a cathode 82. The electrolyte membrane 80 is a membrane made of a polymer having ion exchange groups. The ion exchange groups of the electrolyte membrane 80 may be, for example, at least one of a sulfonic acid group, a phosphate group, and a quaternary ammonium group. The electrolyte membrane 80 may be an anion exchange membrane or a cation exchange membrane. The electrolyte membrane 80 may be, for example, a membrane made of a perfluorocarbon sulfonic acid polymer, or a membrane made of a polymer containing either polyether ether ketone or polybenzimidazole as a main component. The electrolyte membrane 80 may be complexed with metals or cations such as iridium, platinum, cerium, and manganese. When the electrolyte membrane 80 is complexed with a metal, the amount of metal contained in the electrolyte membrane 80 is 5 μg / cm. 2 It may be less than 3 μg / cm 2 The metal contained in the electrolyte membrane 80 may be in the form of a metal, an oxide, or an ion. In this embodiment, the electrolyte membrane 80 is a proton (hydrogen ion) exchange membrane.

[0011] The anode 81 includes an anode catalyst layer 811 and an anode gas diffusion layer 812. The anode catalyst layer 811 is laminated on one surface of the electrolyte membrane 80. The anode gas diffusion layer 812 is laminated on the surface of the anode catalyst layer 811 opposite to the surface facing the electrolyte membrane 80 in the lamination direction D of the water electrolysis cell 22.

[0012] The anode catalyst layer 811 is a layer that functions as an anode electrode that generates oxygen. The anode catalyst layer 811 is produced, for example, by supporting an anode catalyst on a carrier using a binder.

[0013] The anode catalyst is a metal particle that catalyzes the reaction that produces oxygen. The anode catalyst contains at least one metal selected from the group consisting of platinum, ruthenium, iridium, rhodium, palladium, osnium, tungsten, lead, iron, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, and yttrium. The anode catalyst may contain two or more of the above metals. The anode catalyst preferably contains either iridium or ruthenium as a main component. The anode catalyst may be an oxide, nitride, sulfide, phosphide, or the like. The anode catalyst is preferably either an oxide or a nitride. The anode catalyst may be composed of at least one of iridium particles, iridium alloy particles, and composite particles containing iridium. Iridium alloy particles and iridium-containing composite particles contain at least one metal other than iridium, such as ruthenium, rhodium, palladium, osnium, tungsten, lead, iron, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, and yttrium. Iridium alloy particles and iridium-containing composite particles may contain two or more of the above metals. The ratio of elements other than iridium in the iridium alloy particles is not particularly limited and may be, for example, 0.11 atm% or more and 60 atm% or less. The particle size of the metal particles constituting the anode catalyst is not particularly limited and may, for example, be 1 nm or more and 5000 nm or less. In the present disclosure, the particle size of the metal particles is the average crystallite size measured by X-ray diffraction. In another embodiment, the particle diameter of the metal particles may be an average particle diameter calculated by measuring the particle diameters of a predetermined number of metal particles using an electron microscope and averaging the measured particle diameters of each metal particle. To calculate the average particle diameter, for example, the particle diameters of 100 to 1000 metal particles are measured using an electron microscope.

[0014] The anode catalyst may be supported on a carrier. The method for supporting the anode catalyst on a carrier is not particularly limited, and known methods such as impregnation can be used. The carrier supporting the anode catalyst may be primary particles or secondary particles. The particle diameter of the primary particles constituting the carrier may be, for example, 5 nm or more and 5,000 nm or less. The loading ratio of the anode catalyst supported on the carrier is not particularly limited, and may be, for example, 1% or more, 50% or more, or 100% or less. The carrier supporting the anode catalyst is composed of, for example, an oxide. The oxide constituting the carrier is, for example, at least one of titanium oxide, niobium oxide, tin oxide, tungsten oxide, and molybdenum oxide. The carrier supporting the anode catalyst may be composed of a mixture containing at least one of the above oxides.

[0015] The binder used to support the anode catalyst on the support may be composed of, for example, at least one of a polymer having an ion exchange group and an ionomer. The binder used to support the anode catalyst on the support may have, as the ion exchange group, at least one of a sulfonic acid group, a phosphate group, and a quaternary ammonium group. The binder used to support the anode catalyst on the support may be composed of an anion exchange polymer or a cation exchange polymer. The binder used to support the anode catalyst on the support may be composed of, for example, a perfluorocarbon sulfonic acid polymer, or may be composed of a polymer containing, as a main component, either polyether ether ketone or polybenzimidazole.

[0016] The anode gas diffusion layer 812 is a layer for distributing gas. The anode gas diffusion layer 812 is made of, for example, at least one of carbon paper, carbon fiber, carbon cloth, porous titanium, and titanium fiber. The anode gas diffusion layer 812 may be made of a combination of two or more of the above materials. The anode gas diffusion layer 812 may also include a microporous layer made of at least one of carbon and titanium particles.

[0017] The cathode 82 includes a cathode catalyst layer 821 and a cathode gas diffusion layer 822. The cathode catalyst layer 821 is laminated on the other surface of the electrolyte membrane 80. The cathode gas diffusion layer 822 is laminated on the surface of the cathode catalyst layer 821 opposite to the surface facing the electrolyte membrane 80 in the lamination direction D of the water electrolysis cell 22.

[0018] The cathode catalyst layer 821 is a layer that functions as a cathode electrode that generates hydrogen. The cathode catalyst layer 821 is produced, for example, by supporting a cathode catalyst on a carrier using a binder.

[0019] The cathode catalyst is a metal particle that catalyzes the reaction that produces hydrogen. The cathode catalyst contains at least one metal selected from the group consisting of platinum, ruthenium, iridium, rhodium, palladium, osnium, tungsten, lead, iron, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, and yttrium. The cathode catalyst may contain two or more of the above metals. The cathode catalyst may be an oxide, nitride, sulfide, phosphide, or the like. The cathode catalyst may be composed of at least one of platinum particles, platinum alloy particles, and platinum-containing composite particles. Platinum alloy particles and platinum-containing composite particles contain at least one metal other than platinum, such as ruthenium, iridium, rhodium, palladium, osnium, tungsten, lead, iron, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, and yttrium. Platinum alloy particles and platinum-containing composite particles may contain two or more of the above metals. The ratio of elements other than platinum in the platinum alloy particles is not particularly limited and may be, for example, 0.11 atm% or more and 60 atm% or less. The particle size of the metal particles constituting the cathode catalyst is not particularly limited and may be, for example, 1 nm or more and 100 nm or less.

[0020] The cathode catalyst may be supported on a carrier. The method for supporting the cathode catalyst on a carrier is not particularly limited, and known methods such as impregnation can be used. The carrier supporting the cathode catalyst may be primary particles or secondary particles. The particle diameter of the primary particles constituting the carrier may be, for example, 5 nm or more and 5000 nm or less. The loading ratio of the cathode catalyst supported on the carrier is not particularly limited, and may be, for example, 1% or more, 18% or more, 48% or less, or 70% or less. The carrier supporting the cathode catalyst is composed of, for example, at least one of conductive carbon, an oxide, and a mixture containing carbon and an oxide. The carbon constituting the support may be, for example, at least one of carbon blacks such as acetylene black, ketjen black, and furnace black, activated carbon, graphite, glassy carbon, graphite, graphene, carbon fiber, carbon nanotubes, carbon nitride, sulfurized carbon, carbon phosphide, channel black, roller black, disc black, oil furnace black, gas furnace black, lamp black, thermal black, and vulcanized carbon. The support supporting the cathode catalyst may be composed of a mixture containing at least one of the above carbons. The oxide constituting the support is at least one of titanium oxide, niobium oxide, tin oxide, tungsten oxide, and molybdenum oxide. The support supporting the cathode catalyst may be composed of a mixture containing at least one of the above oxides.

[0021] The binder used when supporting the cathode catalyst on the carrier may be composed of, for example, at least one of a polymer having an ion exchange group and an ionomer. The binder used when supporting the cathode catalyst on the carrier may have, as the ion exchange group, at least one of a sulfonic acid group, a phosphate group, and a quaternary ammonium group. The binder used when supporting the cathode catalyst on the carrier may be composed of an anion exchange polymer or a cation exchange polymer. The binder used when supporting the cathode catalyst on the carrier may be composed of, for example, a perfluorocarbon sulfonic acid polymer, or may be composed of a polymer mainly composed of either polyether ether ketone or polybenzimidazole.

[0022] The cathode gas diffusion layer 822 is a layer for distributing gas. The cathode gas diffusion layer 822 is made of, for example, at least one of carbon paper, carbon fiber, carbon cloth, porous titanium, and titanium fiber. The cathode gas diffusion layer 822 may be made of a combination of two or more of the above materials. The cathode gas diffusion layer 822 may include a microporous layer made of at least one of carbon and titanium particles.

[0023] The two separators 85, 86 are disposed at both ends of the water electrolysis cell 22 in the stacking direction D. The anode-side separator 85 faces the anode gas diffusion layer 812. The cathode-side separator 86 faces the cathode gas diffusion layer 822.

[0024] The anode-side flow path 87 penetrates from the anode-side separator 85 to the anode 81 along the stacking direction D of the water electrolysis cells 22. The cathode-side flow path 88 penetrates from the cathode-side separator 86 to the cathode 82 along the stacking direction D of the water electrolysis cells 22.

[0025] 1, a power supply 30 supplies power to the water electrolysis cell 22. A cell monitor 32 monitors the state of the water electrolysis cell 22.

[0026] As shown in FIGS. 1 and 2 , the water supply unit 40 supplies water to the water electrolysis cell 22. In this embodiment, the water supply unit 40 supplies water to the anode 81 of the water electrolysis cell 22. The water supply unit 40 includes a tank 41, a supply flow path 43, a circulation flow path 45, a supply pump 47, and a circulation pump 49. The tank 41 stores water to be supplied to the water electrolysis cell 22. The supply flow path 43 connects the tank 41 to an anode-side gas-liquid separator 53 (described later). The circulation flow path 45 connects the anode-side gas-liquid separator 53 to an anode-side flow path 87. The supply pump 47 is provided in the supply flow path 43 and supplies water from the tank 41 to the anode-side gas-liquid separator 53. The circulation pump 49 is provided in the circulation flow path 45 and supplies water from the anode-side gas-liquid separator 53 to the anode-side flow path 87. In other embodiments, the water supply unit 40 may supply water to the cathode 82 instead of or in addition to the anode 81.

[0027] The oxygen discharge unit 50 has an anode-side discharge channel 51, an anode-side gas-liquid separator 53, and an oxygen discharge channel 55. The anode-side discharge channel 51 connects the anode-side gas-liquid separator 53 and the anode-side flow path 87. The anode-side gas-liquid separator 53 separates the fluid discharged from the anode-side flow path 87 into oxygen and water. The oxygen discharge channel 55 discharges the oxygen separated in the anode-side gas-liquid separator 53 to the outside. The oxygen discharge channel 55 is connected, for example, to a tank (not shown) that stores oxygen.

[0028] The hydrogen discharge unit 60 has a cathode-side discharge channel 61, a cathode-side gas-liquid separator 63, and a hydrogen discharge channel 65. The cathode-side discharge channel 61 connects the cathode-side gas-liquid separator 63 to the cathode-side flow path 88. The cathode-side gas-liquid separator 63 separates the fluid discharged from the cathode-side flow path 88 into hydrogen and water. The hydrogen discharge channel 65 discharges the hydrogen separated in the cathode-side gas-liquid separator 63 to the outside. The hydrogen discharge channel 65 is connected, for example, to a tank (not shown) that stores hydrogen.

[0029] During the electrolysis process of electrolyzing water, water is supplied from the tank 41 to the anode 81, and power is supplied from the power source 30 to the water electrolysis cell 22. As a result, the water supplied to the anode 81 is electrolyzed to generate hydrogen ions and oxygen. The oxygen generated at the anode 81, together with a portion of the water remaining without being electrolyzed, is sent to the anode-side gas-liquid separator 53 through the anode-side flow path 87 and the anode-side discharge path 51. The oxygen separated at the anode-side gas-liquid separator 53 is discharged to the outside via the oxygen discharge path 55. The water separated at the anode-side gas-liquid separator 53, together with the water supplied from the tank 41, passes through the circulation flow path 45 and is supplied again to the anode 81. The hydrogen ions generated at the anode 81, together with a portion of the water remaining without being electrolyzed, permeate the electrolyte membrane 80 and move to the cathode 82. At the cathode 82, the hydrogen ions combine with electrons to generate hydrogen. The hydrogen produced at the cathode 82, together with water that has moved from the anode 81 to the cathode 82 by permeating the electrolyte membrane 80 along with the hydrogen ions, is sent to the cathode-side gas-liquid separator 63 via the cathode-side flow path 88 and the cathode-side discharge channel 61. The hydrogen separated in the cathode-side gas-liquid separator 63 is discharged to the outside via the hydrogen discharge channel 65.

[0030] The control device 70 controls the water electrolysis system 1. During the electrolysis process, the control device 70 controls the operation of the supply pump 47 and the circulating pump 49. As a result, the control device 70 supplies water to the anode 81 at a desired flow rate. During the electrolysis process, the control device 70 sets the current and voltage values ​​and operates the power supply 30 so that the anode 81 and the cathode 82 each have a predetermined electrolysis potential. As a result, the control device 70 controls the power supplied to the water electrolysis cell 22. The electrolysis potential is an arbitrary potential. The electrolysis potential of the anode 81 is, for example, 1.5 V or higher. During at least one of the start-up and continuous operation of the water electrolysis system 1, the control device 70 performs a potential variation process at least once to restore the electrolysis performance of the water electrolysis cell 22.

[0031] FIG. 3 is a diagram illustrating the details of the potential change process PF. The vertical axis of each diagram in FIG. 3 represents the potential of the anode 81. The horizontal axis of each diagram in FIG. 3 represents time. The first potential P1 may be equal to the electrolytic potential P3, may be higher than the electrolytic potential P3, or may be lower than the electrolytic potential P3. The first potential P1 is preferably 1.0 V (vs. RHE) or higher, and more preferably 1.4 V (vs. RHE) or higher. The first potential P1 is preferably 3.0 V (vs. RHE) or lower, and more preferably 2.0 V (vs. RHE) or lower. The second potential P2 is lower than the first potential P1 and the electrolytic potential P3. The second potential P2 may be equal to the natural potential P4 or may be higher than the natural potential P4. The second potential P2 is preferably −0.5 V (vs. RHE) or higher, and more preferably 0.0 V (vs. RHE) or higher. The second potential P2 is preferably 1.0 V (vs. RHE) or less, more preferably 0.7 V (vs. RHE) or less, and even more preferably 0.2 V (vs. RHE) or less.

[0032] The potential change process PF is a process for changing the potential of the anode 81. The potential change process PF includes a potential decrease process PD for decreasing the potential of the anode 81 to a predetermined second potential P2. The potential change process PF may further include a potential increase process PU for increasing the potential of the anode 81 to a predetermined first potential P1.

[0033] At the start-up of the water electrolysis system 1, the control device 70 executes a potential change process PF before starting the electrolysis process PE. The start-up of the water electrolysis system 1 refers to the time when the power supply 30 is turned on and the control device 70 is started. At the start-up of the water electrolysis system 1, the potential of the anode 81 is the natural potential P4. Therefore, when the potential change process PF is executed once at the start-up of the water electrolysis system 1, the control device 70 executes the following processes. In this case, the control device 70 executes a potential change process PU to increase the potential of the anode 81 from the natural potential P4 to the first potential P1, and a potential change process PD to decrease the potential of the anode 81 from the first potential P1 to the second potential P2, in this order. When the potential change process PF is executed N times (N is an integer greater than or equal to 2) at the start-up of the water electrolysis system 1, the control device 70 executes the following processes. In this case, in the first potential change processing PF, the control device 70 executes a potential increase processing PU that increases the potential of the anode 81 from the natural potential P4 to the first potential P1, and a potential decrease processing PD that decreases the potential of the anode 81 from the first potential P1 to the second potential P2, in that order. In the second and subsequent potential change processing PF, the control device 70 executes a potential increase processing PU that increases the potential of the anode 81 from the second potential P2 to the first potential P1, and a potential decrease processing PD that decreases the potential of the anode 81 from the first potential P1 to the second potential P2, in this order, N-1 times. After the potential change processing PF is completed, the control device 70 starts the electrolysis processing PE by changing the potential of the anode 81 from the second potential P2 to the electrolysis potential P3.

[0034] During continuous operation of the water electrolysis system 1, the control device 70 executes a potential change process PF before restarting the electrolysis process PE. Continuous operation of the water electrolysis system 1 refers to a period of time during which the electrolysis process PE is continuously executed for a predetermined period of time or more. The predetermined period of time is calculated in advance, for example, based on the time required from the start of the electrolysis process PE until the current density, calculated using data output from the cell monitor 32, falls below a predetermined threshold. During continuous operation of the water electrolysis system 1, the potential of the anode 81 is the electrolysis potential P3. Therefore, when the potential change process PF is executed once during continuous operation of the water electrolysis system 1, the control device 70 executes the following process. In this case, the control device 70 executes a potential change process PD to decrease the potential of the anode 81 from the electrolysis potential P3 to the second potential P2 without executing a potential increase process PU. When the potential change process PF is executed N times during continuous operation of the water electrolysis system 1, the control device 70 executes the following process. In this case, in the first potential change processing PF, the control device 70 performs a potential change processing PD to decrease the potential of the anode 81 from the electrolysis potential P3 to the second potential P2 without performing a potential change processing PU. In the second and subsequent potential change processing PF, the control device 70 repeats the potential change processing PU to increase the potential of the anode 81 from the second potential P2 to the first potential P1 and the potential change processing PD to decrease the potential of the anode 81 from the first potential P1 to the second potential P2 N−1 times. After the potential change processing PF is completed, the control device 70 resumes the electrolysis processing PE by changing the potential of the anode 81 from the second potential P2 to the electrolysis potential P3. Note that after the potential change processing PF performed during continuous operation of the water electrolysis system 1 is completed, the control device 70 may stop the water electrolysis system 1 and terminate the electrolysis processing PE.

[0035] 2, the control device 70 performs the potential lowering process PD by, for example, stopping the supply of power to the water electrolysis cell 22 and then causing hydrogen generated at the cathode 82 to permeate through the electrolyte membrane 80 and move from the cathode 82 to the anode 81. In other words, the control device 70 uses the hydrogen permeability of the electrolyte membrane 80, which allows hydrogen generated at the cathode 82 to permeate, to lower the potential of the anode 81 to a second potential P2, without supplying power from the power source 30 to the water electrolysis cell 22. The hydrogen permeability of the electrolyte membrane 80 is also known as hydrogen cross-leak (crossover).

[0036] The control device 70 may perform the potential lowering process PD by supplying power from the power supply 30 to the water electrolysis cell 22 by controlling at least one of the current value and voltage value when power is supplied to the water electrolysis cell 22. When performing the potential lowering process PD by controlling the current value when power is supplied to the water electrolysis cell 22, the control device 70 performs the following process. In this case, the control device 70 operates the power supply 30, for example, by setting the current value to a value lower than the current value that is set when the potential of the anode 81 is set to either the first potential P1 or the electrolysis potential P3. When performing the potential lowering process PD by controlling the voltage value when power is supplied to the water electrolysis cell 22, the control device 70 performs the following process. In this case, the control device 70 operates the power supply 30, for example, by setting the voltage value that is lower than the voltage value that is set when the potential of the anode 81 is set to either the first potential P1 or the electrolysis potential P3.

[0037] The control device 70 performs the potential-raising process PU by supplying power from the power supply 30 to the water electrolysis cell 22, for example, by controlling at least one of the current value and voltage value when power is supplied to the water electrolysis cell 22. When performing the potential-raising process PU by controlling the current value when power is supplied to the water electrolysis cell 22, the control device 70 performs the following process. In this case, the control device 70 operates the power supply 30, for example, by setting a current value greater than the current value set when the potential of the anode 81 is set to the second potential P2. When performing the potential-raising process PU by controlling the voltage value when power is supplied to the water electrolysis cell 22, the control device 70 performs the following process. In this case, the control device 70 operates the power supply 30, for example, by setting a voltage value greater than the voltage value set when the potential of the anode 81 is set to the second potential P2. Note that power is supplied to the water electrolysis cell 22 from the power supply 30 during the period when the potential-raising process PU is being performed. Therefore, during the period in which the potential increase process PU is being performed, the amount of change in the potential of the anode 81 due to the supply of power is greater than the amount of change in the potential of the anode 81 due to hydrogen cross-leakage. Therefore, during the period in which the potential increase process PU is being performed, the change in the potential of the anode 81 due to hydrogen cross-leakage can be substantially ignored.

[0038] In the potential change process PF, the potential of the anode 81 can be detected using, for example, at least one of a cell having a reference electrode and a two-electrode cell. Furthermore, if the potential of the anode 81 decreases due to hydrogen cross-leakage, the potential difference between the anode 81 and the cathode 82 decreases. If the potential of the anode 81 increases due to the supply of power, the potential difference between the anode 81 and the cathode 82 increases. The potential difference is equal to the voltage. Therefore, in the potential change process PF, the control device 70 can check the progress of the potential change process PF by checking the transition of the voltage value of each of the multiple water electrolysis cells 22 that make up the cell stack 20 using the output result of the cell monitor 32.

[0039] Fig. 4 is a table showing the results of examining various conditions for the potential variation process PF. Fig. 4 shows the various conditions and the evaluation results for the water electrolysis cells 22 when the potential variation process PF was performed different times for each of the water electrolysis cells 22 having electrolyte membranes 80 with different thicknesses and cerium contents at the start-up of the water electrolysis system 1.

[0040] The water electrolysis cells 22 in Examples 1 to 11 and the Reference Example were fabricated as follows. An anode catalyst layer 811 was fabricated by dispersing, mixing, and applying an anode catalyst to a carrier. Elyst Ir75 manufactured by Umicore was used as the anode catalyst. A cathode catalyst layer 821 was fabricated by dispersing, mixing, and applying a cathode catalyst to a carrier. Platinum-supported carbon was used as the cathode catalyst. The platinum-supported carbon used as the cathode catalyst had a platinum loading of 10 wt% or more and 50 wt% or less. A Nafion (registered trademark)-based perfluorocarbon sulfonic acid polymer manufactured by Chemours was used as the binder for each catalyst layer. The perfluorocarbon sulfonic acid polymer used as the binder had an equivalent weight (EW) of 1,100. The electrolyte membrane 80 was sandwiched between the anode catalyst layer 811 and the cathode catalyst layer 821, and a pressure of 3 MPa was applied at a temperature of 145°C to thermocompress the anode catalyst layer 811, the electrolyte membrane 80, and the cathode catalyst layer 821 together, thereby producing a membrane electrode assembly Q. Perfluorocarbon sulfonic acid polymers with different thicknesses and cerium contents were used as the electrolyte membrane 80. A membrane electrode gas diffusion layer assembly R was produced by joining one anode gas diffusion layer 812 made of a porous metal to the anode 81 side of the membrane electrode assembly Q, and joining one cathode gas diffusion layer 822 made of carbon fiber to the cathode 82 side of the membrane electrode assembly Q. GDL 22BB, manufactured by SGL, was used as the carbon fiber. The size of the produced membrane electrode gas diffusion layer assembly R was 1 cm. 2The water electrolysis cell 22 can use the electrolyte membrane 80, anode catalyst layer 811, anode gas diffusion layer 812, cathode catalyst layer 821, and cathode gas diffusion layer 822 made of any material and composition. The temperature and pressure used when thermocompression bonding the anode catalyst layer 811, electrolyte membrane 80, and cathode catalyst layer 821 together are not particularly limited, and any temperature and pressure can be set as appropriate.

[0041] The various conditions for the potential variation process PF were set as follows: The temperature of the water electrolysis cell 22 was set to 50°C. The second potential P2 was set to the natural potential P4 of the anode 81. The natural potential P4 of the anode 81 was 0.1 V or less. The electrolysis potential P3 was set to 1.8 V. The potential increase process PU was performed by controlling at least one of the current value and voltage value when power was supplied to the water electrolysis cell 22. The required time for the potential increase process PU was set to 3 minutes. The potential decrease process PD was performed by utilizing the hydrogen permeability characteristics of the electrolyte membrane 80 without supplying power to the water electrolysis cell 22.

[0042] The various conditions for the potential fluctuation process PF were evaluated as follows. The hydrogen permeation characteristics of the electrolyte membrane 80, the time required for the potential drop process PD, and the current-voltage characteristics were evaluated while ultrapure water was flowing through the anode 81. The total time required for the potential fluctuation process PF was calculated by multiplying the sum of the time required for the potential increase process PU and the time required for the potential drop process PD by the number of times the potential fluctuation process PF was performed. The longer the total time required for the potential fluctuation process PF, the later the timing at which the electrolysis process PE can be started. Therefore, it is preferable that the total time required for the potential fluctuation process PF is as short as possible, within the range that allows the electrolysis performance of the water electrolysis cell 22 to be restored. Therefore, the total time required for the potential fluctuation process PF is evaluated as "A," which indicates a shorter time, with the highest rating, and as "C," which indicates a longer time, with the lowest rating.

[0043] The water electrolysis cell 22 was evaluated as follows. The performance of the water electrolysis cell 22 was evaluated under atmospheric pressure without applying gas pressure to either the anode 81 or the cathode 82. In Examples 1 to 11, the water electrolysis cell 22 was evaluated based on the current density when the electrolysis process PE was performed with the electrolysis potential P3 controlled at 1.8 V after the potential fluctuation process PF was completed. In the Reference Example, the water electrolysis cell 22 was evaluated based on the current density when the electrolysis process PE was performed with the electrolysis potential P3 controlled at 1.8 V after the water electrolysis system 1 was started without performing the potential fluctuation process PF. The higher the electrolysis performance of the water electrolysis cell 22, the higher the current density when the electrolysis process PE was performed. The lower the electrolysis performance of the water electrolysis cell 22, the lower the current density when the electrolysis process PE was performed. Therefore, the effect of the potential fluctuation process PF was evaluated best as "A" indicating a higher current density when the electrolysis process PE was performed, and worst as "C" indicating a lower current density when the electrolysis process PE was performed. In each of the water electrolysis cells 22 in Examples 1 to 11, in which the electrolysis process PE was performed after the potential fluctuation process PF, the maximum value of the saturated current density when the electrolysis process PE was performed with the electrolysis potential P3 controlled at 1.8 V was 3.0 A / cm 2 In the water electrolysis cell 22 of the reference example in which the electrolysis process PE was performed without performing the potential variation process PF, the current density when the electrolysis process PE was performed with the electrolysis potential P3 controlled to 1.8 V was 2.5 A / cm 2 Therefore, the effect of the potential variation treatment PF was evaluated based on the saturation current density. When the value of the current supplied to the water electrolysis cell 22 is controlled during the electrolysis treatment PE, the water electrolysis cell 22 may be evaluated using, for example, the saturation voltage when the electrolysis treatment PE is performed.

[0044] In Examples 1 to 11, in which the electrolysis process PE was performed after the potential fluctuation process PF, the current density when the electrolysis process PE was performed was higher than in the reference example in which the electrolysis process PE was performed without performing the potential fluctuation process PF. Therefore, the control device 70 can restore the electrolysis performance of the water electrolysis cell 22 by performing the potential fluctuation process PF at least once.

[0045] FIG. 5 is a graph showing the change in current density over time for each execution count of the potential change process PF. The vertical axis of FIG. 5 represents the current density when the electrolysis process PE was performed with the electrolysis potential P3 controlled at 1.8 V. The horizontal axis of FIG. 5 represents the elapsed time from the start of the electrolysis process PE. The current density when the electrolysis process PE was performed six, eighteen, and thirty times was equivalent. Furthermore, as shown in Examples 1 to 3, 9, and 10 of FIG. 4, the current density when the electrolysis process PE was performed once, twice, six, eighteen, and thirty times was equivalent. Therefore, the control device 70 executes the potential change process PF, for example, one to thirty times. Here, the total time required for the potential change process PF is preferably shorter if it can achieve the same effect of restoring the electrolysis performance of the water electrolysis cell 22. As shown in Examples 1 to 3 in Figure 4, when the thickness and cerium content of the electrolyte membrane 80 are the same, the fewer the number of times the potential change process PF is performed, the shorter the total time required for the potential change process PF. Therefore, the control device 70 preferably performs the potential change process PF from 1 to 18 times, and more preferably from 1 to 6 times. Note that the number of times the potential change process PF is performed is not limited to the above. The control device 70 may also perform the potential change process PF 31 or more times.

[0046] As shown in Examples 1 to 3 in Fig. 4, the current density when the electrolysis process PE is performed is equivalent when the total time required for the potential change process PF is 30 minutes, 90 minutes, or 150 minutes. Therefore, the control device 70 performs the potential change process PF, for example, so that the total time required for the potential change process PF is within 150 minutes. Here, if the same effect of restoring the electrolysis performance of the water electrolysis cell 22 can be obtained, it is preferable that the total time required for the potential change process PF is shorter. Therefore, the control device 70 preferably performs the potential change process PF so that the total time required for the potential change process PF is within 90 minutes, and more preferably performs the potential change process PF so that the total time required for the potential change process PF is within 30 minutes.

[0047] FIG. 6 shows the relationship between the thickness of the electrolyte membrane 80 and the time required for the potential-dropping process PD. The thicker the electrolyte membrane 80, the less likely it is that hydrogen generated at the cathode 82 will permeate the electrolyte membrane 80, potentially resulting in a smaller amount of hydrogen moving from the cathode 82 to the anode 81 per unit time. The smaller the amount of hydrogen moving from the cathode 82 to the anode 81 per unit time, the longer the time required for the potential-dropping process PD. In fact, in Example 11, where the electrolyte membrane 80 is 50 μm thick, the time required for the potential-dropping process PD is longer than in Examples 1 to 10, where the electrolyte membrane 80 is 25 μm or less thick. The longer the time required for the potential-dropping process PD, the longer the total time required for the potential-fluctuation process PF. Furthermore, as shown in FIG. 4, in Example 11, where the electrolyte membrane 80 is 50 μm thick, the current density when the electrolysis process PE is performed is smaller than in Examples 1 to 10, where the electrolyte membrane 80 is 25 μm or less thick. Therefore, when the control device 70 performs the potential-lowering process PD by utilizing the hydrogen permeability characteristics of the electrolyte membrane 80, the thickness of the electrolyte membrane 80 is preferably 25 μm or less. Here, the current density when the electrolysis process PE is performed is equivalent between Example 2, in which the electrolyte membrane 80 is 15 μm thick, and Example 8, in which the electrolyte membrane 80 is 25 μm thick, and the electrolysis performance of the water electrolysis cell 22 can be restored to the same extent. Therefore, when the control device 70 performs the potential-lowering process PD by utilizing the hydrogen permeability characteristics of the electrolyte membrane 80, the thickness of the electrolyte membrane 80 is more preferably 15 μm or less. Furthermore, in Example 4, in which the electrolyte membrane 80 is 8 μm thick, the time required for the potential-lowering process PD is shorter than in Example 1, in which the electrolyte membrane 80 is 15 μm thick. The shorter the time required for the potential-lowering process PD, the shorter the total time required for the potential-fluctuation process PF. Therefore, when the control device 70 performs the potential-lowering process PD by utilizing the hydrogen permeability characteristics of the electrolyte membrane 80, the thickness of the electrolyte membrane 80 is even more preferably 8 μm or less. The thickness of the electrolyte membrane 80 is not limited to the above. The thickness of the electrolyte membrane 80 may be greater than 25 μm or greater than 50 μm. The thickness of the electrolyte membrane 80 can be measured, for example, by at least one of SEM (Scanning Electron Microscope) measurement and microscope measurement.

[0048] By incorporating cerium into the electrolyte membrane 80, the durability of the electrolyte membrane 80 can be improved. However, because cerium is a cation, the greater the cerium content of the electrolyte membrane 80, the more difficult it becomes for hydrogen generated at the cathode 82 to permeate the electrolyte membrane 80, and the less hydrogen moves from the cathode 82 to the anode 81 per unit time. In fact, when the cerium content of the electrolyte membrane 80 is 5 μg / cm 2 In Example 2, the cerium content of the electrolyte membrane 80 was 0 μg / cm 2 The time required for the potential reduction treatment PD is longer than that of Example 7. 2 In Example 5, the cerium content of the electrolyte membrane 80 was 0 μg / cm 2 The time required for the potential drop process PD is longer than in Example 4. In Examples 4 and 5, in which the thickness of the electrolyte membrane 80 is 8 μm, the increase in the time required for the potential drop process PD due to the inclusion of cerium in the electrolyte membrane 80 is greater than in Examples 2 and 7, in which the thickness of the electrolyte membrane 80 is 15 μm. Therefore, the cerium content of the electrolyte membrane 80 may be determined according to the thickness of the electrolyte membrane 80 so that the time required for the potential drop process PD is within a desired time. However, the cerium content of the electrolyte membrane 80 is not limited to the above. The cerium content of the electrolyte membrane 80 is 5 μg / cm 2 The amount of metals such as cerium, platinum, and manganese contained in the electrolyte membrane 80 can be detected by, for example, ICP (Inductively Coupled Plasma) measurement.

[0049] According to the above embodiment, during at least one of start-up and continuous operation of the water electrolysis system 1, when there is a risk of poisoning of the water electrolysis cell 22, the control device 70 executes the potential variation process PF to restore the electrolysis performance of the water electrolysis cell 22. This makes it possible to suppress a decrease in the amounts and efficiency of oxygen and hydrogen produced in the water electrolysis system 1.

[0050] Furthermore, according to the above embodiment, the control device 70 can restore the electrolysis performance of the water electrolysis cell 22 by performing the potential fluctuation process PF from 1 to 30 times. Furthermore, the control device 70 can restore the electrolysis performance of the water electrolysis cell 22 by performing the potential fluctuation process PF from 1 to 18 times. In this manner, the control device 70 can restore the electrolysis performance of the water electrolysis cell 22 in a shorter time. Furthermore, the control device 70 can restore the electrolysis performance of the water electrolysis cell 22 by performing the potential fluctuation process PF from 1 to 6 times. In this manner, the control device 70 can restore the electrolysis performance of the water electrolysis cell 22 in an even shorter time.

[0051] Furthermore, according to the above embodiment, the control device 70 can restore the electrolysis performance of the water electrolysis cell 22 by performing the potential change treatment PF so that the total time of the potential change treatment PF is within 150 minutes. Furthermore, the control device 70 can restore the electrolysis performance of the water electrolysis cell 22 by performing the potential change treatment PF so that the total time of the potential change treatment PF is within 90 minutes. In this manner, the control device 70 can restore the electrolysis performance of the water electrolysis cell 22 in a shorter time. Furthermore, the control device 70 can restore the electrolysis performance of the water electrolysis cell 22 by performing the potential change treatment PF so that the total time of the potential change treatment PF is within 30 minutes. In this manner, the control device 70 can restore the electrolysis performance of the water electrolysis cell 22 in an even shorter time.

[0052] Furthermore, according to the above embodiment, after stopping the supply of power to the water electrolysis cell 22, the control device 70 can perform the potential reduction process PD by causing hydrogen generated at the cathode 82 to permeate through the electrolyte membrane 80 and move from the cathode 82 to the anode 81. In this case, the electrolyte membrane 80 constituting the water electrolysis cell 22 may have a thickness of 25 μm or less. In this way, the control device 70 can restore the electrolysis performance of the water electrolysis cell 22 in a shorter time.

[0053] Furthermore, according to the above embodiment, the control device 70 can execute the potential variation process PF by controlling at least one of the current value and the voltage value when power is supplied to the water electrolysis cell 22.

[0054] B. Other Embodiments: The potential change process PF may further include a first holding process after the potential increase process PU in which the potential of the anode 81 is held at the first potential P1 for a predetermined period. The potential change process PF may further include a second holding process after the potential decrease process PD in which the potential of the anode 81 is held at the second potential P2 for a predetermined period.

[0055] The present disclosure is not limited to the above-described embodiments and can be realized in various configurations without departing from the spirit thereof. For example, the technical features of the embodiments corresponding to the technical features in each aspect described in the Summary of the Invention section can be appropriately replaced or combined to solve some or all of the above-described problems or achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted. [Explanation of symbols]

[0056] 1...water electrolysis system, 20...cell stack, 22...water electrolysis cell, 30...power supply, 32...cell monitor, 40...water supply unit, 41...tank, 43...supply flow path, 45...circulation flow path, 47...supply pump, 49...circulation pump, 50...oxygen discharge unit, 51...anode side discharge path, 53...anode side gas-liquid separator, 55...oxygen discharge path, 60...hydrogen discharge unit, 61...cathode side discharge path, 63...cathode side gas-liquid separator, 65...hydrogen discharge path, 70...control device, 80...electrolyte membrane, 81...anode, 82 ...cathode, 85...anode side separator, 86...cathode side separator, 87...anode side flow path, 88...cathode side flow path, 811...anode catalyst layer, 812...anode gas diffusion layer, 821...cathode catalyst layer, 822...cathode gas diffusion layer, D...stacking direction, P1...first potential, P2...second potential, P3...electrolysis potential, P4...natural potential, PD...potential decrease treatment, PE...electrolysis treatment, PF...potential fluctuation treatment, PU...potential increase treatment, Q...membrane electrode assembly, R...membrane electrode gas diffusion layer assembly

Claims

1. A water electrolysis system that generates hydrogen and oxygen by electrolysis of water, a water electrolysis cell including an anode, a cathode, and an electrolyte membrane sandwiched between the anode and the cathode; a control device that controls the power supplied to the water electrolysis cell, the control device performs a potential variation process to vary the potential of the anode during at least one of startup and continuous operation of the water electrolysis system; the potential variation process includes a potential reduction process of reducing the potential of the anode to a predetermined potential.

2. The water electrolysis system according to claim 1, The control device executes the potential variation process at least once but not more than 30 times.

3. The water electrolysis system according to claim 1, and after stopping the supply of power to the water electrolysis cell, the control device causes the hydrogen generated at the cathode to permeate through the electrolyte membrane and move from the cathode to the anode, thereby performing the potential lowering process.

4. The water electrolysis system according to claim 1, The control device performs the potential variation process by controlling at least one of a current value and a voltage value when the power is supplied to the water electrolysis cell.

5. The water electrolysis system according to claim 1, a thickness of the electrolyte membrane is 25 μm or less.

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