Water electrolysis system

Abstract

The present invention addresses the problem of providing a water electrolysis system capable of suppressing reduction in the potential of an oxygen electrode while suppressing power cost. A water electrolysis system having a hydrogen electrode and an oxygen electrode, the water electrolysis system being provided with: a voltage measurement unit that measures a voltage, which is a potential difference between the oxygen electrode and the hydrogen electrode, when the water electrolysis system is stopped; and an oxygen supply unit that supplies a gas containing oxygen to the oxygen electrode when the measured voltage drops to a preset threshold value.

Description

Technical Field

[0001] This invention relates to a water electrolysis system. Background Technology

[0002] In water electrolysis systems that electrolyze water to produce oxygen and hydrogen, various techniques have been proposed to suppress the degradation of the oxygen electrode. For example, Patent Document 1 discloses a technique in which, when the water electrolysis system stops and the voltage drops to a predetermined value, external power is supplied to the oxygen electrode to suppress the potential decrease of the oxygen electrode. If the potential of the oxygen electrode decreases, the catalyst of the oxygen electrode becomes easily reduced. The catalyst of the oxygen electrode degrades due to reduction.

[0003] Patent Document 1: Japanese Patent Application Publication No. 2021-105194 Summary of the Invention

[0004] The technology in Patent Document 1 requires external power every time the voltage drops to a specified value, resulting in relatively high power costs. There is a need for a technology that can suppress power costs while also suppressing the potential drop of the oxygen electrode.

[0005] The present invention can be implemented in the following ways.

[0006] (1) According to one aspect of the present invention, 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 as a potential difference between the oxygen electrode and the hydrogen electrode when the water electrolysis system is stopped; and an oxygen supply unit that supplies oxygen-containing gas to the oxygen electrode when the measured voltage drops to a preset threshold.

[0007] According to this water electrolysis system, when the voltage obtained by the voltage acquisition unit drops to a threshold value when the water electrolysis system stops, the oxygen supply unit supplies oxygen-containing gas to the oxygen electrode, thus suppressing the voltage from falling below the threshold. Furthermore, since the potential drop of the oxygen electrode is suppressed by supplying oxygen-containing gas, the electricity cost is lower compared to a structure that suppresses the potential drop of the oxygen electrode by supplying electricity to it from an external power source.

[0008] (2) In the water electrolysis system described above, a hydrogen supply unit may be further provided, which supplies hydrogen to the hydrogen electrode when the water electrolysis system is stopped.

[0009] In this water electrolysis system, because the hydrogen supply unit supplies hydrogen to the hydrogen electrode when the water electrolysis system is stopped, the catalyst of the hydrogen electrode can be surrounded by hydrogen, compared to a structure where no hydrogen is supplied to the hydrogen electrode. This suppresses potential fluctuations in the hydrogen electrode caused by contact with gases other than hydrogen that may be present around the catalyst. In other words, the potential of the hydrogen electrode, serving as a reference, is more stable. Consequently, the voltage measuring unit can more accurately measure the potential difference between the oxygen electrode and the hydrogen electrode.

[0010] (3) In the water electrolysis system described above, a water supply unit may be further provided, which supplies water to the oxygen electrode when the water electrolysis system is restarted.

[0011] According to this water electrolysis system, since the water supply unit supplies water to the oxygen electrode when the water electrolysis system is restarted, the oxygen-containing gas covering the catalyst covering the oxygen electrode can be flushed away with water, and the catalyst can be filled with water, thus preparing to start water electrolysis.

[0012] (4) In the water electrolysis system described above, the hydrogen supply unit can supply hydrogen to the hydrogen electrode when the water electrolysis system is restarted.

[0013] In this water electrolysis system, since the hydrogen supply unit supplies hydrogen when the system restarts, it is possible to displace gases other than hydrogen that may be present at the hydrogen electrode. This helps to suppress the decrease in the purity of the hydrogen produced by the water electrolysis system.

[0014] (5) In the water electrolysis system described above, the threshold can be greater than the closed-circuit voltage in the fuel cell.

[0015] According to this water electrolysis system, since the threshold voltage is higher than the closed-circuit voltage in the fuel cell, the reduction of the oxygen electrode catalyst can be further suppressed compared to structures where the threshold voltage is lower than the closed-circuit voltage. Therefore, catalyst degradation of the oxygen electrode can be further suppressed.

[0016] This invention can be implemented in various ways other than water electrolysis systems. For example, it can be implemented in the form of oxygen supply methods, programs for executing those methods, etc. Attached Figure Description

[0017] Figure 1 This is a block diagram illustrating the schematic structure of a water electrolysis system according to one embodiment of the present invention.

[0018] Figure 2 This is a flowchart representing the steps of the oxygen supply process performed by the system.

[0019] Figure 3 It is a timing diagram of oxygen supply, the potential difference between the oxygen electrode and the hydrogen electrode, and the power supply.

[0020] Figure 4 This is a block diagram showing the general structure of the system according to the second embodiment.

[0021] Figure 5 This is a block diagram showing the general structure of the system according to the third embodiment. Detailed Implementation

[0022] A. Implementation Method 1:

[0023] A1. System Structure:

[0024] Figure 1 This is a block diagram showing the schematic structure of a water electrolysis system 1 (hereinafter also referred to as "System 1") according to one embodiment of the present invention. System 1 is used to electrolyze water to obtain hydrogen and oxygen. The water electrolysis system 1 includes a water electrolysis stack 100, a power supply P, a water supply unit 200, an oxygen removal unit 300, a hydrogen removal unit 400, a voltage measuring unit 500, an oxygen supply unit 600, and a control device 700.

[0025] <Structure of Water Electrolysis Reactor 100>

[0026] The water electrolysis reactor 100 includes multiple stacked water electrolysis units 110. Each water electrolysis unit 110 has an electrolyte membrane, a hydrogen electrode, and an oxygen electrode. The electrolyte membrane is made of a polymer material with ion-exchange groups. The electrolyte membrane is located between the hydrogen electrode and the oxygen electrode. The hydrogen electrode catalyzes the reaction that generates hydrogen from protons and electrons. The oxygen electrode catalyzes the reaction that generates oxygen, protons, and electrons from water. Known catalysts are used for both the hydrogen and oxygen electrodes. The catalyst for the oxygen electrode is preferably an oxide. For example, platinum is used as the catalyst for the hydrogen electrode, and iridium oxide is used as the catalyst for the oxygen electrode.

[0027] <Structure of power supply P>

[0028] Power source P supplies electricity to water electrolyzer 100. With the start of power supply from power source P, water electrolysis begins based on water electrolyzer 100.

[0029] <Structure of Water Supply Unit 200>

[0030] The water supply unit 200 supplies water to the oxygen electrode of the water electrolysis unit 110. The water supply unit 200 includes a water storage tank 210, an oxygen-liquid separator 220, a pump 230, and an ion separator 240. Furthermore, the water supply unit 200 has a water supply path 205 and a first circulation path 215 as water flow paths. The water supply path 205 connects the water storage tank 210 and the oxygen-liquid separator 220. The first circulation path 215 connects the oxygen-liquid separator 220 and the water electrolysis reactor 100.

[0031] Water storage tank 210 contains water for electrolysis. The water in water storage tank 210 is supplied to oxygen-liquid separator 220 via water supply path 205.

[0032] The oxygen-liquid separator 220 contains gas and water and separates the gas and water from each other. The oxygen-liquid separator 220 separates the water and gas supplied from the water storage tank 210 and supplies them to the pump 230 via the first circulation path 215. The oxygen-liquid separator 220 is also part of the structure of the oxygen exhaust section 300, which will be described later.

[0033] Pump 230 is installed on the first circulation path 215 and pumps water supplied from oxygen liquid separator 220 toward water electrolysis reactor 100.

[0034] Ion separator 240 is located downstream of pump 230 in the first circulation path 215. Ion separator 240 reduces the ions of impurities contained in the supplied water. Ion separator 240 supplies the deionized water to water electrolysis reactor 100.

[0035] <Structure of Oxygen Exhaust Section 300>

[0036] The oxygen removal unit 300 discharges oxygen generated by the reaction in the oxygen electrode of the water electrolysis unit 110 to the outside of system 1. Furthermore, the oxygen removal unit 300 discharges cross-leaked hydrogen to the outside of system 1. Cross-leaking refers to a portion of the hydrogen generated on the hydrogen electrode side moving to the oxygen electrode side through the electrolyte membrane. In addition, the oxygen removal unit 300 circulates at least a portion of the water not consumed in the oxygen electrode.

[0037] The oxygen removal section 300 includes an oxygen-liquid separator 220. Furthermore, the oxygen removal section 300 includes a second circulation flow path 305 as a flow path and an oxygen removal flow path 315 as a discharge path for oxygen and cross-leaked hydrogen. The second circulation flow path 305 connects the water electrolysis reactor 100 and the oxygen-liquid separator 220. One end of the oxygen removal flow path 315 is connected to the oxygen-liquid separator 220.

[0038] The oxygen-liquid separator 220 separates the oxygen and hydrogen supplied via the second circulation path 305 from the water. The separated oxygen and hydrogen are discharged to the outside of system 1 via the oxygen exhaust path 315.

[0039] <Structure of Hydrogen Emission Section 400>

[0040] The hydrogen discharge unit 400 discharges the hydrogen generated in the hydrogen electrode of the water electrolysis unit 110 to the outside of system 1. The hydrogen discharge unit 400 includes a hydrogen-liquid separator 410 and a pressure regulating valve 420. Furthermore, the hydrogen discharge unit 400 includes a hydrogen discharge path 405, a pressure regulating path 415, and an exhaust path 425 as flow paths. The hydrogen discharge path 405 connects the water electrolysis stack 100 to the hydrogen-liquid separator 410. The pressure regulating path 415 connects the hydrogen-liquid separator 410 to the pressure regulating valve 420. One end of the exhaust path 425 is connected to the pressure regulating valve 420.

[0041] Hydrogen-liquid separator 410 separates hydrogen and water supplied from water electrolyzer 100 via hydrogen discharge path 405. The separated hydrogen is supplied to pressure regulating valve 420 via pressure regulating path 415. The separated water is discharged to the outside of system 1 via drain valve (not shown).

[0042] The pressure regulating valve 420 regulates the pressure of the supplied hydrogen. By adjusting the opening of the pressure regulating valve 420, the hydrogen generated by the water electrolysis reactor 100 is adjusted to the desired hydrogen pressure. The pressure-regulated hydrogen is discharged to the outside of system 1 through the exhaust passage 425.

[0043] <Structure of Voltage Measuring Unit 500>

[0044] The voltage measuring unit 500 measures the voltage of the potential difference between the oxygen electrode and the hydrogen electrode, which constitutes the water electrolysis stack 100. More specifically, the voltage measuring unit 500 measures the potential difference between the oxygen electrode and the hydrogen electrode when the water electrolysis system 1 is stopped. Alternatively, the voltage measuring unit 500 measures the voltage of the water electrolysis unit 110. Furthermore, in this invention, "stopping the water electrolysis system 1" refers to stopping the power supply used for water electrolysis. Therefore, "when the water electrolysis system 1 is stopped" in this invention refers to the period from when the power supply based on power source P stops until when the power supply based on power source P is started again 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.

[0045] <Structure of Oxygen Supply Unit 600>

[0046] The oxygen supply unit 600 supplies oxygen to the oxygen electrode when the water electrolysis system 1 is stopped. The oxygen supply unit 600 includes an oxygen storage tank 610 and an oxygen supply valve 620. Furthermore, the oxygen supply unit 600 includes an oxygen supply flow path 605 as a flow path. The oxygen supply flow path 605 connects the oxygen storage tank 610 and the water electrolysis reactor 100.

[0047] Oxygen storage tank 610 stores oxygen. The stored oxygen can be generated by system 1 or supplied from outside system 1.

[0048] An oxygen supply valve 620 is installed on the oxygen supply path 605. The oxygen supply valve 620 switches the oxygen supply from the oxygen storage tank 610 to the water electrolysis reactor 100. The oxygen supply valve 620 is controlled by the control device 700 described later.

[0049] <Structure of Control Device 700>

[0050] The control device 700 is configured as a computer with a CPU 710 and a memory 720. The CPU 710 executes the control program stored in the memory 720 to enable the voltage acquisition unit 711 and the oxygen supply indicator unit 712 to function.

[0051] The voltage acquisition unit 711 acquires the voltage measured by the voltage measuring unit 500. The acquired voltage is transmitted to the oxygen supply indicator unit 712.

[0052] The oxygen supply indicator 712 controls the opening and closing of the oxygen supply valve 620. Specifically, when system 1 stops, the oxygen supply indicator 712 opens the oxygen supply valve 620 when the voltage acquired by the voltage acquisition unit 711 drops to a preset threshold. This initiates the supply of oxygen to the oxygen electrode. Furthermore, in this embodiment, the threshold is the closed-circuit voltage in the fuel cell. More specifically, it is the closed-circuit voltage when the structure of the water electrolysis system 1 is used as a fuel cell. The threshold is pre-stored in the memory 720.

[0053] Furthermore, the oxygen supply indicator 712 closes the oxygen supply valve 620 when system 1 is restarted. This stops the supply of oxygen to the oxygen electrode. In addition, "when system 1 is restarted" in this invention refers to both the preparation period for restarting water electrolysis from a stopped state and the actual start of power supply based on power source P. During the preparation period, for example, actions are taken such as filling the oxygen electrode with water and adjusting the opening of each valve.

[0054] A2. Oxygen supply treatment:

[0055] Figure 2 This is a flowchart illustrating the steps of the oxygen supply process performed by System 1. The oxygen supply process occurs when System 1 is stopped.

[0056] like Figure 2 As shown, the voltage measuring unit 500 measures the potential difference, i.e., the voltage, between the oxygen electrode and the hydrogen electrode (S100). The voltage acquisition unit 711 acquires the measured voltage (S110). The oxygen supply indicator unit 712 determines whether the acquired voltage has decreased to a preset threshold (S120). If the acquired voltage has decreased to the threshold (S120: Yes), the oxygen supply indicator unit 712 controls the opening of the oxygen supply valve 620 to supply oxygen to the oxygen electrode (S130). If the acquired voltage has not decreased to the threshold (S120: No), the oxygen supply process ends. The oxygen supply process is repeated until the system 1 is restarted.

[0057] A3. Voltage change in water electrolysis unit 110:

[0058] Figure 3 This is a timing diagram of oxygen supply, the potential difference (voltage) between the oxygen electrode and the hydrogen electrode, and the power supply. Figure 3 The upper section shows the timing of the oxygen supply being turned on and off, the middle section shows the potential difference between the oxygen electrode and the hydrogen electrode, and the lower section shows the timing of the power supply being turned on and off. Figure 3The solid lines in the middle section represent the embodiments, and the dashed lines represent the comparative examples. In the embodiments, when system 1 stops, oxygen is supplied to the oxygen electrode when the potential difference between the oxygen electrode and the hydrogen electrode decreases to the closed-circuit voltage in the fuel cell. In contrast, in the comparative examples, no oxygen is supplied.

[0059] First, the embodiments will be described. Figure 3 As shown in the next paragraph, at time t1, the power supply from power source P to the water electrolysis reactor 100 is stopped. Thus, water electrolysis based on the water electrolysis reactor 100 ceases. At this time, as... Figure 3 As shown in the middle section, the voltage gradually decreases from time t1. The reason why the potential does not drop sharply to 0 when the power supply stops is that the oxygen produced by water electrolysis and remaining at the oxygen electrode carries out the reverse reaction of water electrolysis.

[0060] like Figure 3 As shown in the upper paragraph, oxygen is supplied to the oxygen electrode at time t2. Figure 3 As shown in the middle section, the voltage drop is suppressed by supplying oxygen. This is because by filling the catalyst of the oxygen electrode with oxygen, the reverse reaction of water electrolysis becomes more active.

[0061] Furthermore, in this embodiment, time t2 is the moment when the voltage drops to the closed-circuit voltage of the fuel cell. If the voltage is lower than the closed-circuit voltage, the catalyst serving as the oxygen electrode is more easily reduced compared to the case where the voltage is higher than the closed-circuit voltage. Typically, oxides are used as catalysts for the oxygen electrode, thus the catalyst deteriorates due to reduction. As shown in the example, by supplying oxygen to the oxygen electrode to ensure that the voltage does not fall below the closed-circuit voltage, the reduction of the catalyst is suppressed.

[0062] like Figure 3 As shown in the previous paragraph, at time t3, in preparation for the restart of system 1, the oxygen supply is stopped. If the area around the oxygen electrode is filled with oxygen, the efficiency of water electrolysis will decrease; therefore, as a preparation stage for water electrolysis, the oxygen supply is stopped. Figure 3 As shown in the middle section, if the oxygen supply is stopped, the voltage gradually decreases again.

[0063] like Figure 3 As shown in the next paragraph, at time t4, power supply begins from power source P to the water electrolyzer 100. Consequently, system 1 restarts and resumes water electrolysis based on the water electrolyzer 100. Figure 3 As shown in the middle section, the voltage rises sharply due to the power supply to the voltage required for water electrolysis.

[0064] Next, comparative examples will be explained. For example... Figure 3As shown by the dashed line in the middle section, if water electrolysis in the water electrolysis reactor 100 stops at time t1, the voltage continues to decrease to 0. This is because, unlike the embodiment, no oxygen is supplied at time t2. The oxygen around the oxygen electrode will be depleted due to the reverse reaction, and the potential of the oxygen electrode will continue to decrease until it is equal to the potential of the hydrogen electrode.

[0065] Thus, there is a significant difference in the voltage at which system 1 stops between the examples and the comparative examples. Specifically, in the examples, by supplying oxygen to the oxygen electrode, the voltage is prevented from falling below the closed-circuit voltage. Therefore, the reduction of the catalyst at the oxygen electrode is suppressed. In contrast, in the comparative examples, the voltage is below the closed-circuit voltage and continues to decrease until the potential of the oxygen electrode is equal to that of the hydrogen electrode. Therefore, in the comparative examples, the reduction of the catalyst at the oxygen electrode is easier to occur compared to the examples.

[0066] According to the first embodiment of system 1 described above, when the voltage, which is the potential difference between the oxygen electrode and the hydrogen electrode, measured by the voltage measuring unit 500 when system 1 is stopped, decreases to a preset threshold, the oxygen supply unit 600 supplies oxygen to the oxygen electrode, thereby suppressing the voltage drop when system 1 is stopped. This, in turn, suppresses the degradation of the catalyst in the oxygen electrode.

[0067] Furthermore, according to System 1 of the first embodiment, the oxygen supply unit 600 suppresses voltage drop by sending oxygen-containing gas into the oxygen electrode, thus reducing power costs compared to a structure that suppresses voltage drop by supplying power to the oxygen electrode using external power.

[0068] Furthermore, according to System 1 of the first embodiment, the threshold is the closed-circuit voltage in the fuel cell, thus preventing the voltage from falling below the closed-circuit voltage when System 1 stops. Therefore, the reduction of the oxygen electrode catalyst can be suppressed, thereby preventing the degradation of the oxygen electrode catalyst.

[0069] B. Second Implementation Method:

[0070] Figure 4 This is a block diagram showing the schematic structure of system 1b according to the second embodiment. The difference between system 1b of the second embodiment and system 1 of the first embodiment is that system 1b of the second embodiment further includes a hydrogen supply unit 800, and the CPU 710b further functions the hydrogen supply indicator unit 713. Other structures of system 1b of the second embodiment are the same as those of system 1 of the first embodiment, and therefore their description is omitted.

[0071] The hydrogen supply unit 800 supplies hydrogen to the hydrogen electrode when system 1 is stopped. The hydrogen supply unit 800 includes a hydrogen storage tank 810 and a hydrogen supply valve 820. Furthermore, the hydrogen supply unit 800 includes a hydrogen supply flow path 805 as a flow path. The hydrogen supply flow path 805 connects the hydrogen storage tank 810 and the water electrolyzer 100.

[0072] Hydrogen storage tank 810 stores hydrogen. The stored hydrogen can be generated by system 1 or supplied from outside system 1.

[0073] Hydrogen supply valve 820 is installed on hydrogen supply path 805. Hydrogen supply valve 820 switches the supply of hydrogen from hydrogen storage tank 810 to water electrolysis reactor 100 and stops it. Hydrogen supply valve 820 is controlled by control device 700.

[0074] The hydrogen supply indicator 713 functions by executing a control program stored in the memory 720 via the CPU 710b. The hydrogen supply indicator 713 controls the opening and closing of the hydrogen supply valve 820. Specifically, the hydrogen supply indicator 713 opens the hydrogen supply valve 820 when system 1 stops. This begins supplying hydrogen to the hydrogen electrode, filling the catalyst surrounding the hydrogen electrode with hydrogen. Furthermore, the hydrogen supply indicator 713 closes the hydrogen supply valve 820 when system 1 restarts. This stops the supply of hydrogen to the hydrogen electrode.

[0075] Regarding the system 1b of the second embodiment described above, it further includes a hydrogen supply unit 800 that supplies hydrogen to the hydrogen electrode when system 1 is stopped, thus enabling the area around the catalyst of the hydrogen electrode to be filled with hydrogen. This allows for more accurate measurement of the voltage measured by the voltage measuring unit 500. This is because, when hydrogen is not supplied to the hydrogen electrode, water, hydrogen, and air are present around the catalyst of the hydrogen electrode, and these substances react with each other, causing the potential of the hydrogen electrode to become unstable. In contrast, by supplying hydrogen to the hydrogen electrode, the area around the catalyst of the hydrogen electrode is filled with hydrogen, thus suppressing reactions occurring around the catalyst. Consequently, the potential of the hydrogen electrode becomes stable. Since the voltage measuring unit 500 measures the potential difference between the hydrogen electrode and the oxygen electrode, the potential of the hydrogen electrode, serving as a reference, becomes stable, resulting in a more accurate measured potential.

[0076] C. Third implementation method:

[0077] Figure 5 This is a block diagram illustrating the general structure of system 1c according to the third embodiment. The difference between system 1c of the third embodiment and system 1b of the second embodiment is that the CPU 710c further enhances the functionality of the water supply indicator 714. Other aspects of system 1c of the third embodiment are the same as those of system 1b of the second embodiment, and therefore their description is omitted.

[0078] The water supply indicator 714 functions by executing a control program stored in the memory 720 via the CPU 710c. The water supply indicator 714 controls the operation of the pump 230. Specifically, when system 1 restarts, the water supply indicator 714 activates the pump 230 to supply water contained in the oxygen-liquid separator 220 to the oxygen electrode. When system 1 stops, oxygen is supplied to the oxygen electrode under the control of the hydrogen supply indicator 713, thus filling the catalyst of the oxygen electrode with oxygen. Here, water is supplied to the oxygen electrode under the control of the water supply indicator 714, flushing away the oxygen from the catalyst. This completes the preparation of the oxygen electrode for performing water electrolysis.

[0079] According to the third embodiment of system 1c described above, since the water supply unit 200 supplies water to the oxygen electrode when system 1 is restarted, the oxygen supplied to the oxygen electrode and covering the catalyst when system 1 is stopped can be flushed away with water. Thus, preparation for water electrolysis in the oxygen electrode can be completed.

[0080] D. Fourth implementation method:

[0081] The system of the fourth embodiment differs from the system 1b of the second embodiment in that the hydrogen supply unit 800 supplies hydrogen to the hydrogen electrode when the system restarts. That is, in the system 1b of the second embodiment, the hydrogen supply unit 800 supplies hydrogen to the hydrogen electrode when the system 1b is "stopped," but in the system of the fourth embodiment, hydrogen is supplied to the hydrogen electrode not only when the system is "restarted," but also when the system restarts. Similar to the second embodiment, the hydrogen supply is achieved by controlling the opening and closing of the hydrogen supply valve 820 via the hydrogen supply indicator unit 713. Furthermore, the system of the fourth embodiment can be used in combination with the system 1c of the third embodiment.

[0082] According to the system of the fourth embodiment described above, since the hydrogen supply unit 800 supplies hydrogen to the hydrogen electrode when the system is restarted, the area around the catalyst of the hydrogen electrode can be filled with hydrogen. As a result, oxygen or water that may be present around the hydrogen electrode can be flushed away, thus suppressing the decrease in the purity of the hydrogen generated by system 1.

[0083] E. Other implementation methods:

[0084] (E1) In the above embodiments, the oxygen supply unit 600 supplies oxygen to the oxygen electrode, but the oxygen supply unit 600 may also supply oxygen-containing air to the oxygen electrode. In this way, the potential drop of the oxygen electrode can also be suppressed.

[0085] (E2) In the above embodiments, the threshold voltage is the closed-circuit voltage in the fuel cell, but the present invention is not limited to this. The threshold voltage can be a voltage greater than the closed-circuit voltage. In this way, it is also possible to suppress the voltage from falling below the closed-circuit voltage when systems 1, 1b, and 1c stop, thereby suppressing the reduction and degradation of the catalyst in the oxygen electrode. Furthermore, the threshold voltage can be any voltage greater than 0V. In this way, compared to a structure in which no oxygen is supplied to the oxygen electrode when systems 1, 1b, and 1c stop, voltage reduction can also be suppressed.

[0086] (E3) In the above embodiments, the shutdown of systems 1, 1b, and 1c can be achieved by gradually reducing the power supply from power source P. In this case, the voltage measuring unit 500 can measure the voltage starting from the moment the power supply from power source P begins to decrease. Furthermore, in the above embodiments, the startup and restart of systems 1, 1b, and 1c can be achieved by gradually increasing the power supply from power source P.

[0087] (E4) In the third embodiment described above, the water supplied to the oxygen electrode is contained in the oxygen-liquid separator 220, but the present invention is not limited thereto. The water supplied to the oxygen electrode can be contained in any container other than the oxygen-liquid separator 220.

[0088] (E5) In the above embodiments, the catalyst for the oxygen electrode is iridium oxide, but the present invention is not limited thereto. The catalyst for the oxygen electrode may be an oxide such as ruthenium oxide.

[0089] This invention is not limited to the embodiments described above, and can be implemented in various structures without departing from its spirit. For example, in order to solve some or all of the above-described problems, or to achieve some or all of the above-described effects, the technical features in the embodiments corresponding to the technical features in the various methods described in the Summary of the Invention section can be appropriately replaced or combined. Furthermore, if a technical feature is not described as an essential feature in this specification, it can be appropriately deleted.

[0090] Symbol Explanation

[0091] 1, 1b, 1c - Water electrolysis system; 100 - Water electrolysis reactor; 110 - Water electrolysis unit; 200 - Water supply section; 205 - Water supply path; 210 - Water storage tank; 215 - First circulation path; 220 - Oxygen-liquid separator; 230 - Pump; 240 - Ion separator; 300 - Oxygen removal section; 305 - Second circulation path; 315 - Oxygen removal path; 400 - Hydrogen removal section; 405 - Hydrogen removal path; 410 - Hydrogen-liquid separator; 415 - Pressure regulating path; 420 - Pressure regulating... Valve, 425 - Exhaust flow path, 500 - Voltage measuring unit, 600 - Oxygen supply unit, 605 - Oxygen supply flow path, 610 - Oxygen storage tank, 620 - Oxygen supply valve, 700 - Control device, 710, 710b, 710c - CPU, 711 - Voltage acquisition unit, 712 - Oxygen supply indicator, 713 - Hydrogen supply indicator, 714 - Water supply indicator, 720 - Memory, 800 - Hydrogen supply unit, 805 - Hydrogen supply flow path, 810 - Hydrogen storage tank, 820 - Hydrogen supply valve, P - Power supply.

Claims

1. A water electrolysis system, characterized by, A water electrolysis system having a hydrogen electrode and an oxygen electrode, the water electrolysis system comprising: a voltage measurement unit that measures a voltage, which is a 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 a gas containing oxygen to the oxygen electrode when the measured voltage decreases to a predetermined threshold value.

2. The water electrolysis system of claim 1, wherein, Further comprising: a hydrogen supply unit that supplies hydrogen to the hydrogen electrode when the water electrolysis system is stopped.

3. The water electrolysis system of claim 2, wherein, Further comprising: a water supply unit that supplies water to the oxygen electrode when the water electrolysis system is restarted.

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

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

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