Water electrolysis system

Abstract

This invention provides a water electrolysis system that can suppress the decrease in oxygen electrode potential while keeping electricity costs down. [Solution] A water electrolysis system having a hydrogen electrode and an oxygen electrode includes a voltage measuring unit that measures the voltage, which is the potential difference between the oxygen electrode and the hydrogen electrode, when the water electrolysis system is stopped, and an oxygen supply unit that supplies an oxygen-containing gas to the oxygen electrode when the measured voltage drops to a predetermined threshold.

Description

Technical Field

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

Background Art

[0002] In a water electrolysis system that electrolyzes water to generate oxygen and hydrogen, various techniques for suppressing the deterioration of the oxygen electrode have been proposed. For example, Patent Document 1 discloses a technique for suppressing a decrease in the potential of the oxygen electrode by supplying external power to the oxygen electrode when the voltage drops to a specified value during the stop of the water electrolysis system. When the potential of the oxygen electrode decreases, the catalyst of the oxygen electrode becomes in a state where it is easily reduced. The catalyst of the oxygen electrode deteriorates by being reduced.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The technique of Patent Document 1 requires a relatively large power cost because external power needs to be supplied every time the voltage drops to a specified value. There is a need for a technique that can suppress a decrease in the potential of the oxygen electrode while suppressing the power cost.

Means for Solving the Problems

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

[0006] (1) According to one embodiment of the present disclosure, a water electrolysis system having a hydrogen electrode and an oxygen electrode is provided. The water electrolysis system includes a voltage measuring unit that measures a voltage which is the potential difference of the oxygen electrode with respect to the hydrogen electrode when the water electrolysis system is stopped, and an oxygen supply unit that supplies an oxygen-containing gas to the oxygen electrode when the measured voltage falls to a predetermined threshold. In this type of water electrolysis system, the oxygen supply unit supplies oxygen-containing gas to the oxygen electrode when the voltage acquired by the voltage acquisition unit drops to a threshold level during a shutdown of the water electrolysis system, thereby preventing the voltage from falling below the threshold. Furthermore, since the decrease in the potential of the oxygen electrode is suppressed by supplying oxygen-containing gas, the power cost is lower compared to a configuration that suppresses the decrease in the potential of the oxygen electrode by supplying power to the oxygen electrode using external power. (2) The water electrolysis system of the above form may further include a hydrogen supply unit that supplies hydrogen to the hydrogen electrode when the water electrolysis system is stopped. In this type of water electrolysis system, the hydrogen supply unit supplies hydrogen to the hydrogen electrode when the water electrolysis system is stopped, so the area around the catalyst of the hydrogen electrode can be filled with hydrogen compared to a configuration in which hydrogen is not supplied to the hydrogen electrode. This suppresses fluctuations in the potential of the hydrogen electrode caused by contact with gases other than hydrogen that may be present around the catalyst of the hydrogen electrode. In other words, the potential of the hydrogen electrode as a reference becomes more stable. As a result, the voltage measuring unit can measure the potential difference of the oxygen electrode relative to the hydrogen electrode more accurately. (3) In the water electrolysis system of the above form, a water supply unit may be further provided to supply water to the oxygen electrode when the water electrolysis system is restarted. In this type of water electrolysis system, the water supply unit supplies water to the oxygen electrode when the water electrolysis system is restarted. By flushing away the oxygen-containing gas surrounding the catalyst on the oxygen electrode with water, the catalyst can be filled with water, preparing it for the start of water electrolysis. (4) In the water electrolysis system of the above form, the hydrogen supply unit may supply hydrogen to the hydrogen electrode when the water electrolysis system is restarted. In this type of water electrolysis system, the hydrogen supply unit supplies hydrogen when the system is restarted, so that any gases other than hydrogen that may be present at the hydrogen electrode can be pushed out by the hydrogen. This helps to suppress a decrease in the purity of the hydrogen produced by the water electrolysis system. (5) In the water electrolysis system of the above form, the threshold may be greater than the closed-circuit voltage in the fuel cell. In this form of water electrolysis system, the threshold voltage is higher than the closed-circuit voltage in a fuel cell, so the reduction of the oxygen electrode catalyst can be more suppressed compared to a configuration where the threshold voltage is below the closed-circuit voltage. This further suppresses the degradation of the oxygen electrode catalyst.

[0007] This disclosure can be implemented in various forms other than water electrolysis systems. For example, it can be implemented in the form of an oxygen supply method, a program for performing this method, and so on. [Brief explanation of the drawing]

[0008] [Figure 1] This diagram shows a schematic configuration of a water electrolysis system in one embodiment of the present disclosure. [Figure 2] This flowchart shows the procedure for the oxygen supply process performed by the system. [Figure 3] This is a timing chart showing the relationship between oxygen supply, the potential difference between the oxygen electrode and the hydrogen electrode, and power supply. [Figure 4] This block diagram shows the schematic configuration of the system according to the second embodiment. [Figure 5] This is a block diagram showing the schematic configuration of the system according to the third embodiment. [Modes for carrying out the invention]

[0009] A. First Embodiment: A1. System configuration: Figure 1 is a block diagram showing the schematic configuration of a water electrolysis system 1 (hereinafter also referred to as "System 1") in one embodiment of the present disclosure. System 1 is used to electrolyze water to obtain hydrogen and oxygen. Water electrolysis system 1 comprises a water electrolysis stack 100, a power supply P, a water supply unit 200, an oxygen discharge unit 300, a hydrogen discharge unit 400, a voltage measuring unit 500, an oxygen supply unit 600, and a control device 700.

[0010] <Configuration of Water Electrolysis Stack 100> The water electrolysis stack 100 includes a plurality of stacked water electrolysis cells 110. Each water electrolysis cell 110 comprises an electrolyte membrane, a hydrogen electrode, and an oxygen electrode. The electrolyte membrane is made of a polymer material having ion exchange groups. The electrolyte membrane is located between the hydrogen electrode and the oxygen electrode. The hydrogen electrode catalyzes the reaction that produces hydrogen from protons and electrons. The oxygen electrode catalyzes the reaction that produces oxygen, protons, and electrons from water. Known catalysts are used for the hydrogen electrode and the oxygen electrode. The catalyst for the oxygen electrode is preferably an oxide. For example, the catalyst for the hydrogen electrode is platinum, and for example, the catalyst for the oxygen electrode is iridium oxide.

[0011] <Power supply configuration> Power supply P supplies power to the water electrolysis stack 100. When power supply P starts supplying power, water electrolysis by the water electrolysis stack 100 begins.

[0012] <Configuration of the water supply unit 200> The water supply unit 200 supplies water to the oxygen electrode of the water electrolysis cell 110. The water supply unit 200 comprises a water tank 210, an oxygen vapor-liquid separator 220, a pump 230, and an ion separator 240. The water supply unit 200 also comprises a water supply channel 205 and a first circulation channel 215 as water flow paths. The water supply channel 205 connects the water tank 210 and the oxygen vapor-liquid separator 220. The first circulation channel 215 connects the oxygen vapor-liquid separator 220 and the water electrolysis stack 100.

[0013] The water tank 210 contains water used for electrolysis. The water in the water tank 210 is supplied to the oxygen vapor-liquid separator 220 via the water supply channel 205.

[0014] The oxygen gas-liquid separator 220 accommodates gas and water and separates the gas and water from each other. The oxygen gas-liquid separator 220 separates the water supplied from the water tank 210 from the gas and supplies it to the pump 230 via the first circulation flow path 215. The oxygen gas-liquid separator 220 is also a part of the configuration of the oxygen discharge unit 300 described later.

[0015] The pump 230 is provided in the first circulation flow path 215 and sends out the water supplied from the oxygen gas-liquid separator 220 toward the water electrolysis stack 100.

[0016] The ion separator 240 is provided downstream of the pump 230 in the first circulation flow path 215. The ion separator 240 reduces ions as impurities contained in the supplied water. The ion separator 240 supplies the water with reduced ions to the water electrolysis stack 100.

[0017] <Configuration of the oxygen discharge unit 300> The oxygen discharge unit 300 discharges the oxygen generated by the reaction at the oxygen electrode of the water electrolysis cell 110 to the outside of the system 1. Further, the oxygen discharge unit 300 discharges the cross-leaked hydrogen to the outside of the system 1. Cross-leakage means that a part of the hydrogen generated on the hydrogen electrode side moves to the oxygen electrode side across the electrolyte membrane. Furthermore, the oxygen discharge unit 300 circulates at least a part of the water not consumed at the oxygen electrode.

[0018] The oxygen discharge unit 300 includes the oxygen gas-liquid separator 220. The oxygen discharge unit 300 also includes a second circulation flow path 305 as a flow path and an oxygen discharge flow path 315 as a discharge path for oxygen and cross-leaked hydrogen. The second circulation flow path 305 connects the water electrolysis stack 100 and the oxygen gas-liquid separator 220. One end of the oxygen discharge flow path 315 is connected to the oxygen gas-liquid separator 220.

[0019] The oxygen-gas-liquid separator 220 separates the oxygen and hydrogen supplied via the second circulation channel 305 from the water. The separated oxygen and hydrogen are discharged to the outside of system 1 via the oxygen discharge channel 315.

[0020] <Configuration of hydrogen discharge unit 400> The hydrogen discharge section 400 discharges hydrogen generated at the hydrogen electrode of the water electrolysis cell 110 to the outside of the system 1. The hydrogen discharge section 400 includes a hydrogen gas-liquid separator 410 and a pressure regulating valve 420. The hydrogen discharge section 400 also includes a hydrogen discharge passage 405, a pressure regulating passage 415, and an exhaust passage 425. The hydrogen discharge passage 405 connects the water electrolysis stack 100 and the hydrogen gas-liquid separator 410. The pressure regulating passage 415 connects the hydrogen gas-liquid separator 410 and the pressure regulating valve 420. One end of the exhaust passage 425 is connected to the pressure regulating valve 420.

[0021] The hydrogen vapor-liquid separator 410 separates the hydrogen supplied from the water electrolysis stack 100 via the hydrogen discharge channel 405 from the water. The separated hydrogen is supplied to the pressure regulating valve 420 via the pressure regulating channel 415. The separated water is discharged to the outside of system 1 via a drain valve (not shown).

[0022] The pressure regulating valve 420 adjusts the pressure of the supplied hydrogen. The hydrogen produced by the water electrolysis stack 100 is adjusted to the desired hydrogen pressure by adjusting the opening of the pressure regulating valve 420. The pressure-regulated hydrogen is discharged to the outside of system 1 through the exhaust passage 425.

[0023] <Configuration of the voltage measurement unit 500> The voltage measuring unit 500 measures the voltage, which is the potential difference between the oxygen electrode and the hydrogen electrode of the water electrolysis stack 100. More specifically, the voltage measuring unit 500 measures the potential difference of the oxygen electrode relative to the hydrogen electrode when the water electrolysis system 1 is stopped. It can also be said that the voltage measuring unit 500 measures the voltage of the water electrolysis cell 110. In this disclosure, "stopping the water electrolysis system 1" means stopping the power supply for water electrolysis. Therefore, "when the water electrolysis system 1 is stopped" in this disclosure means from the time when the power supply from the power source P is stopped until the power supply from the power source P is started to restart water electrolysis. The measured voltage is transmitted to the control device 700, which will be described later. In this embodiment, the voltage measuring unit 500 is configured as a voltmeter.

[0024] <Configuration of the oxygen supply unit 600> The oxygen supply unit 600 supplies oxygen to the oxygen electrode when the water electrolysis system 1 is stopped. The oxygen supply unit 600 comprises an oxygen tank 610 and an oxygen supply valve 620. The oxygen supply unit 600 also includes an oxygen supply channel 605 as a flow path. The oxygen supply channel 605 connects the oxygen tank 610 and the water electrolysis stack 100.

[0025] The oxygen tank 610 stores oxygen. The stored oxygen may be oxygen generated by system 1, or it may be oxygen supplied from outside system 1.

[0026] The oxygen supply valve 620 is located in the oxygen supply channel 605. The oxygen supply valve 620 switches the supply of oxygen from the oxygen tank 610 to the water electrolysis stack 100 on and off. The oxygen supply valve 620 is controlled by a control device 700, which will be described later.

[0027] <Configuration of control device 700> The control device 700 is configured as a computer equipped with a CPU 710 and memory 720. The CPU 710 activates the voltage acquisition unit 711 and the oxygen supply instruction unit 712 by executing a control program stored in the memory 720.

[0028] The voltage acquisition unit 711 acquires the voltage measured by the voltage measurement unit 500. The acquired voltage is transmitted to the oxygen supply instruction unit 712.

[0029] The oxygen supply instruction unit 712 controls the opening and closing of the oxygen supply valve 620. Specifically, when the system 1 is stopped, the oxygen supply instruction unit 712 opens the oxygen supply valve 620 when the voltage acquired by the voltage acquisition unit 711 drops to a predetermined threshold. This starts the supply of oxygen to the oxygen electrode. In this embodiment, the threshold is the closed-circuit voltage in the fuel cell. More specifically, it is the closed-circuit voltage when the configuration of the water electrolysis system 1 is used as a fuel cell. The threshold is stored in the memory 720 beforehand.

[0030] Furthermore, the oxygen supply instruction unit 712 closes the oxygen supply valve 620 when System 1 is restarted. This stops the supply of oxygen to the oxygen electrode. In this disclosure, "when System 1 is restarted" means both the preparation period for restarting water electrolysis from a stopped state and the actual start of power supply by the power source P. During the preparation period, for example, the oxygen electrode is filled with water and the opening of each valve is adjusted.

[0031] A2. Oxygen supply treatment: Figure 2 is a flowchart showing the procedure for the oxygen supply process performed by System 1. The oxygen supply process is performed when System 1 is shut down.

[0032] As shown in Figure 2, the voltage measurement unit 500 measures the potential difference, i.e., the voltage, of the oxygen electrode relative to the hydrogen electrode (S100). The voltage acquisition unit 711 acquires the measured voltage (S110). The oxygen supply instruction unit 712 determines whether the acquired voltage has dropped to a predetermined threshold (S120). If the acquired voltage has dropped to the threshold (S120: Yes), the oxygen supply instruction unit 712 controls the oxygen supply valve 620 to open and supply oxygen to the oxygen electrode (S130). If the acquired voltage has not dropped to the threshold (S120: No), the oxygen supply process is terminated. The oxygen supply process is repeatedly executed until system 1 is restarted.

[0033] A3. Voltage change of water electrolysis cell 110: Figure 3 is a timing chart of oxygen supply, the potential difference (voltage) of the oxygen electrode relative to the hydrogen electrode, and power supply. In Figure 3, the top section shows the on and off timing of oxygen supply, the middle section shows the potential difference of the oxygen electrode relative to the hydrogen electrode, and the bottom section shows the on and off timing of power supply. In the middle section of Figure 3, the solid line represents the embodiment, and the dashed line represents the comparative example. In the embodiment, when system 1 is stopped, oxygen is supplied to the oxygen electrode when the potential difference of the oxygen electrode relative to the hydrogen electrode drops to the closed-circuit voltage of the fuel cell. In contrast, in the comparative example, no oxygen is supplied.

[0034] First, let's describe an example. As shown in the lower part of Figure 3, at time t1, the power supply P to the water electrolysis stack 100 is stopped. This stops the water electrolysis by the water electrolysis stack 100. At this time, as shown in the middle part of Figure 3, the voltage gradually decreases from time t1. The reason why the potential does not drop sharply to 0 when the power supply is stopped is that the oxygen produced by water electrolysis and remaining at the oxygen electrode undergoes the reverse reaction of water electrolysis.

[0035] As shown in the upper part of Figure 3, at time t2, the supply of oxygen to the oxygen electrode is started. As shown in the middle part of Figure 3, the supply of oxygen suppresses the decrease in voltage. This is because the reverse reaction of water electrolysis becomes more active when the area around the catalyst at the oxygen electrode is filled with oxygen.

[0036] In this embodiment, time t2 is the timing when the voltage drops to the closed-circuit voltage of the fuel cell. When the voltage falls below the closed-circuit voltage, the catalyst at the oxygen electrode is more easily reduced compared to when it is above the closed-circuit voltage. Since oxides are generally used as catalysts for oxygen electrodes, the catalyst deteriorates due to reduction. As in the embodiment, by supplying oxygen to the oxygen electrode and preventing the voltage from falling below the closed-circuit voltage, the reduction of the catalyst is suppressed.

[0037] As shown in the upper part of Figure 3, at time t3, the oxygen supply is stopped in preparation for restarting system 1. Since the efficiency of water electrolysis decreases when the area around the oxygen electrode is filled with oxygen, the oxygen supply is stopped as a preparatory step for water electrolysis. Once the oxygen supply is stopped, the voltage gradually decreases again, as shown in the middle part of Figure 3.

[0038] As shown in the lower part of Figure 3, at time t4, power supply P starts supplying power to the water electrolysis stack 100. This restarts system 1, and water electrolysis by the water electrolysis stack 100 is performed again. As shown in the middle part of Figure 3, the voltage rises sharply to the voltage required for water electrolysis due to the power supply.

[0039] Next, a comparative example will be described. As shown by the dashed line in the middle of Figure 3, when the water electrolysis of the water electrolysis stack 100 stops at time t1, the voltage continues to decrease until it reaches 0. This is because, unlike the example, there is no oxygen supply at time t2. The oxygen around the oxygen electrode is consumed by the reverse reaction, and the potential of the oxygen electrode continues to decrease until it becomes equal to the potential of the hydrogen electrode.

[0040] Thus, the voltage at the time of shutdown of System 1 differs significantly between the example and the comparative example. Specifically, in the example, the supply of oxygen to the oxygen electrode prevents the voltage from falling below the closed-circuit voltage. As a result, the reduction of the catalyst at the oxygen electrode is suppressed. In contrast, in the comparative example, the voltage falls below the closed-circuit voltage and continues to decrease until the potential of the oxygen electrode and the potential of the hydrogen electrode become equal. Therefore, in the comparative example, the reduction of the catalyst at the oxygen electrode proceeds more easily than in the example.

[0041] According to the system 1 of the first embodiment described above, the oxygen supply unit 600 supplies oxygen to the oxygen electrode when the voltage, which is the potential difference between the oxygen electrode and the hydrogen electrode measured by the voltage measuring unit 500 when the system 1 is stopped, drops to a predetermined threshold. This suppresses the voltage drop when the system 1 is stopped. As a result, the deterioration of the catalyst in the oxygen electrode can be suppressed.

[0042] Furthermore, according to System 1 of the first embodiment, the oxygen supply unit 600 suppresses voltage drop by supplying oxygen-containing gas to the oxygen electrode, resulting in lower power costs compared to a configuration that suppresses voltage drop by supplying power to the oxygen electrode using external power.

[0043] Furthermore, according to System 1 of the first embodiment, the threshold is the closed-circuit voltage in the fuel cell, so it is possible to suppress the voltage from falling below the closed-circuit voltage when System 1 is stopped. This suppresses the reduction of the catalyst in the oxygen electrode, and thus suppresses the degradation of the catalyst in the oxygen electrode.

[0044] B. Second Embodiment: Figure 4 is a block diagram showing the schematic configuration of system 1b of the second embodiment. System 1b of the second embodiment differs from system 1 of the first embodiment in that it further includes a hydrogen supply unit 800 and the CPU 710b further enables the hydrogen supply instruction unit 713 to function. The other configurations of system 1b of the second embodiment are the same as those of system 1 of the first embodiment, so their description is omitted.

[0045] The hydrogen supply unit 800 supplies hydrogen to the hydrogen electrode when system 1 is stopped. The hydrogen supply unit 800 includes a hydrogen tank 810 and a hydrogen supply valve 820. The hydrogen supply unit 800 also includes a hydrogen supply channel 805 as a flow path. The hydrogen supply channel 805 connects the hydrogen tank 810 and the water electrolysis stack 100.

[0046] The hydrogen tank 810 stores hydrogen. The stored hydrogen may be hydrogen produced by system 1, or it may be hydrogen supplied from outside system 1.

[0047] The hydrogen supply valve 820 is installed in the hydrogen supply channel 805. The hydrogen supply valve 820 switches the supply of hydrogen from the hydrogen tank 810 to the water electrolysis stack 100 on and off. The hydrogen supply valve 820 is controlled by the control device 700.

[0048] The hydrogen supply instruction unit 713 functions when the CPU 710b executes a control program stored in memory 720. The hydrogen supply instruction unit 713 controls the opening and closing of the hydrogen supply valve 820. Specifically, the hydrogen supply instruction unit 713 opens the hydrogen supply valve 820 when system 1 is stopped. This starts the supply of hydrogen to the hydrogen electrode, filling the area around the catalyst of the hydrogen electrode with hydrogen. The hydrogen supply instruction unit 713 also closes the hydrogen supply valve 820 when system 1 is restarted. This stops the supply of hydrogen to the hydrogen electrode.

[0049] The system 1b of the second embodiment described above further includes a hydrogen supply unit 800 that supplies hydrogen to the hydrogen electrode when system 1 is stopped, so that the area around the catalyst of the hydrogen electrode can be filled with hydrogen. This makes the voltage measured by the voltage measuring unit 500 more accurate. This is for the following reasons: When hydrogen is not supplied to the hydrogen electrode, water, hydrogen, and air are present around the catalyst of the hydrogen electrode, and the potential of the hydrogen electrode becomes unstable due to the reaction of these substances with each other. In contrast, by supplying hydrogen to the hydrogen electrode, the area around the catalyst of the hydrogen electrode can be filled with hydrogen, so the reactions that occur around the catalyst can be suppressed. This stabilizes the potential of the hydrogen electrode. Since the voltage measuring unit 500 measures the potential difference between the hydrogen electrode and the oxygen electrode, the measured potential becomes more accurate as the potential of the hydrogen electrode, which serves as the reference, becomes stable.

[0050] C. Third Embodiment: Figure 5 is a block diagram showing the schematic configuration of system 1c of the third embodiment. System 1c of the third embodiment differs from system 1b of the second embodiment in that the CPU 710c further enables the water supply instruction unit 714 to function. Other aspects of system 1c of the third embodiment are the same as those of system 1b of the second embodiment, so their explanation will be omitted.

[0051] The water supply instruction unit 714 functions when the CPU 710c executes a control program stored in memory 720. The water supply instruction unit 714 controls the operation of the pump 230. Specifically, the water supply instruction unit 714 supplies water contained in the oxygen vapor separator 220 to the oxygen electrode by operating the pump 230 when system 1 is restarted. When system 1 is stopped, oxygen is supplied to the oxygen electrode by the control of the hydrogen supply instruction unit 713, so the area around the catalyst of the oxygen electrode is filled with oxygen. Here, by supplying water to the oxygen electrode by the control of the water supply instruction unit 714, the oxygen in the catalyst of the oxygen electrode is washed away by the water. This completes the preparation of the oxygen electrode for performing water electrolysis.

[0052] According to the system 1c of the third embodiment described above, the water supply unit 200 supplies water to the oxygen electrode when the system 1 is restarted, so that the oxygen that is supplied to the oxygen electrode and covers the catalyst when the system 1 is stopped can be washed away with water. This completes the preparation for water electrolysis at the oxygen electrode.

[0053] D. Fourth Embodiment: The system of the fourth embodiment differs from system 1b of the second embodiment in that the hydrogen supply unit 800 supplies hydrogen to the hydrogen electrode when the system is restarted. That is, in system 1b of the second embodiment, the hydrogen supply unit 800 supplied hydrogen to the hydrogen electrode "when system 1b was stopped", but in the system of the fourth embodiment, in addition to this, hydrogen is also supplied to the hydrogen electrode "when the system is restarted". The supply of hydrogen is achieved by the hydrogen supply instruction unit 713 controlling the opening and closing of the hydrogen supply valve 820, as in the second embodiment. The system of the fourth embodiment may be used in combination with system 1c of the third embodiment.

[0054] According to the system of the fourth embodiment described above, the hydrogen supply unit 800 supplies hydrogen to the hydrogen electrode when the system is restarted, so that the area around the catalyst of the hydrogen electrode can be filled with hydrogen. This allows oxygen, water, and other substances that may be present around the hydrogen electrode to be flushed away, thereby suppressing a decrease in the purity of the hydrogen produced by system 1.

[0055] E. Other embodiments: (E1) In each of the above embodiments, the oxygen supply unit 600 supplied oxygen to the oxygen electrode, but the oxygen supply unit 600 may also supply oxygen-containing air to the oxygen electrode. In this configuration as well, it is possible to suppress a decrease in the potential of the oxygen electrode.

[0056] (E2) In each of the above embodiments, the threshold was the closed-circuit voltage in the fuel cell, but the disclosure is not limited thereto. The threshold may be a voltage greater than the closed-circuit voltage. In this configuration as well, it is possible to suppress the voltage from falling below the closed-circuit voltage when systems 1, 1b, and 1c are stopped, thereby suppressing the reduction and degradation of the catalyst in the oxygen electrode. The threshold may also be any voltage greater than 0V. In this configuration as well, it is possible to suppress the voltage drop compared to a configuration in which oxygen is not supplied to the oxygen electrode when systems 1, 1b, and 1c are stopped.

[0057] (E3) In each of the above embodiments, the shutdown of systems 1, 1b, and 1c may be performed by gradually reducing the amount of power supplied from the power supply P. In this case, the voltage measurement unit 500 may start measuring the voltage from the point when the power supply from the power supply P begins to decrease. Also, in each of the above embodiments, the startup and restart of systems 1, 1b, and 1c may be performed by gradually increasing the amount of power supplied from the power supply P.

[0058] (E4) In the third embodiment described above, the water supplied to the oxygen electrode was contained in the oxygen vapor-liquid separator 220, but the disclosure is not limited thereto. The water supplied to the oxygen electrode may be contained in any container other than the oxygen vapor-liquid separator 220.

[0059] (E5) In each of the above embodiments, the catalyst for the oxygen electrode was iridium oxide, but the disclosure is not limited thereto. The catalyst for the oxygen electrode may be an oxide such as ruthenium oxide.

[0060] This disclosure is not limited to the embodiments described above, and can be implemented in various configurations without departing from its spirit. For example, the technical features in the embodiments corresponding to the technical features in each form described in the summary of the invention can be replaced or combined as appropriate in order to solve some or all of the above-described problems, or to 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 deleted as appropriate. [Explanation of Symbols]

[0061] 1,1b,1c…Water electrolysis system, 100…Water electrolysis stack, 110…Water electrolysis cell, 200…Water supply unit, 205…Water supply channel, 210…Water tank, 215…First circulation channel, 220…Oxygen gas-liquid separator, 230…Pump, 240…Ion separator, 300…Oxygen discharge unit, 305…Second circulation channel, 315…Oxygen discharge channel, 400…Hydrogen discharge unit, 405…Hydrogen discharge channel, 410…Hydrogen gas-liquid separator, 415…Pressure regulating channel, 420…Pressure regulating valve B, 425...Exhaust passage, 500...Voltage measurement unit, 600...Oxygen supply unit, 605...Oxygen supply passage, 610...Oxygen tank, 620...Oxygen supply valve, 700...Control device, 710, 710b, 710c...CPU, 711...Voltage acquisition unit, 712...Oxygen supply instruction unit, 713...Hydrogen supply instruction unit, 714...Water supply instruction unit, 720...Memory, 800...Hydrogen supply unit, 805...Hydrogen supply passage, 810...Hydrogen tank, 820...Hydrogen supply valve, P...Power supply

Claims

1. A water electrolysis system having a hydrogen electrode and an oxygen electrode, When the water electrolysis system is stopped, a voltage measuring unit measures the voltage which is the potential difference between the oxygen electrode and the hydrogen electrode, When the measured voltage drops to a predetermined threshold, an oxygen supply unit supplies an oxygen-containing gas to the oxygen electrode, A water electrolysis system equipped with the following features.

2. A water electrolysis system according to claim 1, A water electrolysis system further comprising a hydrogen supply unit that supplies hydrogen to the hydrogen electrode when the water electrolysis system is stopped.

3. A water electrolysis system according to claim 2, A water electrolysis system further comprising a water supply unit that supplies water to the oxygen electrode when the water electrolysis system is restarted.

4. A water electrolysis system according to claim 2 or 3, The hydrogen supply unit is a water electrolysis system that supplies hydrogen to the hydrogen electrode when the water electrolysis system is restarted.

5. A water electrolysis system according to claim 4, The aforementioned threshold is greater than the closed-circuit voltage in a fuel cell in a water electrolysis system.

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