Water electrolysis device and method for controlling water electrolysis cell

Abstract

To provide a water electrolysis apparatus and a control method of a water electrolytic cell that can discharge contaminants while suppressing elution of a metal catalyst for an anode electrode.SOLUTION: A water electrolysis apparatus according to an embodiment of the present invention comprises a water electrolytic cell, a power supply unit, and a control unit. The water electrolytic cell generates hydrogen and oxygen by electrolysis of water supplied to an anode electrode using a solid-polymer electrolytic membrane, with the anode electrode disposed on one side and a cathode electrode disposed on the other side. The power supply unit supplies an electric current and an electric voltage to a gap between terminals of the water electrolytic cell from a DC power supply. The control unit includes: maintaining a connection state in which a discharge current having the reverse direction to the direction of an electric current at the time of hydrogen generation in the water electrolytic cell is generated after the hydrogen is stopped being generated; maintaining an electric potential falling speed to a prescribed value in the anode electrode when the discharge current is generated; and controlling a first mode in which an electric potential of the anode electrode is controlled to a first electric potential.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] An embodiment of the present invention relates to a water electrolysis device and a method for controlling a water electrolysis cell. [Background technology]

[0002] A water electrolysis device is a device that generates hydrogen and oxygen. During operation, an external direct current is passed through the water electrolysis cell included in the water electrolysis device, electrolyzing water to generate hydrogen from the cathode electrode and oxygen from the anode electrode. Water electrolysis cells include polymer electrolyte membrane (PEM) water electrolysis cells, which use a polymer electrolyte membrane as a diaphragm, alkaline water electrolysis cells, which have a diaphragm in an alkaline electrolyte, and solid oxide water electrolysis cells, which use a solid oxide as an electrolyte. Among these, polymer electrolyte water electrolysis cells have a low operating temperature and are capable of generating highly pure hydrogen. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-99905 Summary of the Invention [Problem to be solved by the invention]

[0004] In solid polymer water electrolysis cells, if impurities such as metal ions are mixed into the circulating water supplied to the cell, they may adhere to the cell electrodes, causing a decrease in performance. It has also been found that when operation is stopped, the residual oxygen gas in the anode electrode reacts with the hydrogen gas in the cathode electrode, generating a counter electromotive force, which allows the impurities to be released when a reverse current flows. However, the rapid voltage fluctuations and voltage drops that accompany this discharge can cause the metal catalyst in the anode electrode to dissolve, potentially degrading the water electrolysis cell.

[0005] Therefore, an object of the present invention is to provide a water electrolysis apparatus and a method for controlling a water electrolysis cell that are capable of discharging impurities while suppressing elution of the metal catalyst from the anode electrode. [Means for solving the problem]

[0006] According to this embodiment, the water electrolysis device includes a water electrolysis cell, a power supply unit, and a control unit. The water electrolysis cell uses a solid polymer electrolyte membrane having an anode electrode on one side and a cathode electrode on the other side to electrolyze water supplied to the anode electrode, generating hydrogen and oxygen. The power supply unit supplies current and voltage from a DC power supply between terminals of the water electrolysis cell. After hydrogen production is stopped, the control unit establishes a connection state in which a discharge current is generated in the water electrolysis cell in a direction opposite to the current flowing during hydrogen production, and performs control in a first mode in which the potential of the anode electrode decreases at a predetermined rate during the generation of the discharge current, and the potential of the anode electrode is controlled to a first potential. [Effects of the Invention]

[0007] According to the present invention, impurities can be discharged while suppressing the elution of the metal catalyst from the anode electrode. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a block diagram showing the configuration of a water electrolysis device. [Figure 2] FIG. 2 is a diagram showing an example of the configuration of a unit cell that constitutes a water electrolysis cell. [Figure 3A] FIG. 2 is a block diagram showing an example of the configuration of a power supply unit. [Figure 3B] FIG. 10 is a block diagram showing another example of the configuration of the power supply unit. [Figure 4] FIG. 2 is a block diagram showing an example of the configuration of a control unit. [Figure 5] FIG. 3 is a diagram schematically illustrating a reaction between an anode electrode and a cathode electrode. [Figure 6] FIG. 2 is a diagram schematically illustrating a reaction between an anode electrode and a cathode electrode when impurities are mixed in. [Figure 7]FIG. 4 is a diagram schematically illustrating the reaction between the anode electrode and the cathode electrode during a recovery operation. [Figure 8] A diagram showing the thermodynamic stability of iridium oxide. [Figure 9] FIG. 10 is a diagram showing the state of a water electrolysis cell in a comparative example. [Figure 10] FIG. 10 is a diagram showing the potential at the time of stopping in the first mode. [Figure 11] FIG. 2 is a diagram showing the state of the water electrolysis cell in the first mode. [Figure 12] FIG. 10 is a diagram showing the potential at the time of stopping in the second mode. [Figure 13] FIG. 4 is a diagram showing the state of the water electrolysis cell in the second mode. [Figure 14] 4 is a flowchart showing an example of control over a water electrolysis cell. [Figure 15] 4 is a flowchart showing an example of control when hydrogen generation is stopped. [Figure 16] FIG. 4 is a block diagram showing the configuration of a water electrolysis apparatus according to a second embodiment. [Figure 17] FIG. 10 is a block diagram showing the configuration of a control unit according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] A water electrolysis apparatus and a method for controlling a water electrolysis cell according to an embodiment of the present invention will be described in detail below with reference to the drawings. Note that the embodiment described below is merely an example of an embodiment of the present invention, and the present invention should not be construed as being limited to these embodiments. Furthermore, in the drawings referred to in this embodiment, identical parts or parts having similar functions are denoted by the same or similar reference numerals, and repeated description thereof may be omitted. Furthermore, for convenience of explanation, the dimensional proportions in the drawings may differ from the actual proportions, and some components may be omitted from the drawings.

[0010] (First embodiment) FIG. 1 is a block diagram showing the configuration of a water electrolysis device 1. As shown in FIG. 1, the water electrolysis device 1 according to this embodiment is an example of a water electrolysis device that generates hydrogen and oxygen using a solid polymer electrolyte membrane. The water electrolysis device 1 includes a water electrolysis cell 20, an oxygen gas / gas-liquid separation tank 23, a circulating water supply tank 24, a circulating water supply pump 25, a filter 26, a pure water supply unit 27, a hydrogen gas / gas-liquid separation tank 28, a power supply unit 30, a control unit 40, multiple control valves V10, V12, V14, V16, V18, and V20, supply pipes P10, P12, P14, and P16, a conductivity meter D10, and a hydrogen meter H10. FIG. 1 also shows a hydrogen gas storage tank 29, a city water supply pipe P16, and control valves V20 and V22. The water electrolysis device 1 may also be referred to as a water electrolysis unit.

[0011] FIG. 2 is a diagram showing an example of the configuration of a unit cell 100 constituting the water electrolysis cell 20. As shown in FIG. 2, the unit cell 100 includes a polymer electrolyte membrane (PEM) electrolyte membrane 101, an anode electrode 102, a cathode electrode 104, an anode current collector 106, and a cathode current collector 108. In the unit cell 100, the solid polymer electrolyte membrane 101 is sandwiched between the anode electrode 102 and the cathode electrode 104, and the anode current collector 106 and the cathode current collector 108 are disposed on both outer sides of the solid polymer electrolyte membrane 101. The water electrolysis cell 20 is formed by stacking a plurality of the unit cells 100. Note that the water electrolysis cell 20 according to this embodiment is formed by stacking a plurality of the unit cells 100 in series, but is not limited thereto. For example, the water electrolysis cell 20 may be formed of a single cell 100. Alternatively, the water electrolysis cell 20 may be formed by connecting a plurality of the unit cells 100 in parallel.

[0012] The solid polymer electrolyte membrane 101 is composed of, for example, a fluorine-based polymer having sulfonic acid groups. The anode electrode 102 is composed of, for example, a catalyst layer containing an iridium oxide catalyst (IrOx) sandwiched between the solid polymer electrolyte membrane 101 and titanium nonwoven fabric. The cathode electrode 104 is composed of, for example, a mixture of a platinum-supported carbon catalyst or a platinum alloy-supported carbon catalyst and an ionomer coated on carbon paper, carbon cloth, carbon nonwoven fabric, or titanium nonwoven fabric.

[0013] The anode current collector 106 is connected to the anode of the power supply unit 30, and the cathode current collector 108 is connected to the cathode of the power supply unit 30. This causes hydrogen ions to move within the solid polymer electrolyte membrane 101, electrolyzing the water H2O supplied from the supply unit G2. At this time, the anode electrode 102 converts 2H2O to 4H + +4e - The reaction +O2 occurs, and oxygen O2 and unreacted water H2O are discharged from outlet G4.

[0014] On the other hand, the cathode electrode 104 is + +4e - The reaction 2H2O → 2H2 + O2 occurs, and hydrogen H2 is discharged from the discharge port G6.

[0015] The opening and closing of the multiple control valves V10, V12, V14, V16, V18, V20, V20, and V22 is controlled by a controller 40. The circulating water supply pipe P10 connects the water electrolysis cell 20, the oxygen gas / gas-liquid separation tank 23, the circulating water supply tank 24, the circulating water supply pump 25, the filter 26, and the conductivity meter D10. The oxygen gas / gas-liquid separation tank 23 collects oxygen-containing water discharged from the discharge port G4 of each unit cell 100 in the water electrolysis cell 20 and separates the oxygen from the water.

[0016] The circulating water supply tank 24 stores the water separated by the oxygen gas / gas-liquid separation tank 23 and the pure water supplied from the pure water supply unit 27. The circulating water supply pump 25 supplies the oxygen-containing water discharged from the discharge unit G4 of each unit cell 100 to the oxygen gas / gas-liquid separation tank 23. The circulating water supply pump 25 also supplies the water from the circulating water supply tank 24 to the supply unit G2 of each unit cell 100 via a filter 26. The filter 26 filters out impurities in the circulating water.

[0017] The conductance meter D10 measures the conductivity (electrical conductivity) of the circulating water. The hydrogen meter H10 measures the amount of hydrogen in the discharge pipe P16.

[0018] The pure water supply unit 27 generates and supplies pure water from water supplied via the city water supply pipe P16. During normal operation, the pure water supply unit 27 supplies pure water to the circulating water supply tank 24 via the first pure water supply pipe P14. On the other hand, during the impurity removal process in the water electrolysis cell 20, pure water is supplied to each unit cell 100 via the second pure water supply pipe P12. The pure water supplied by the pure water supply unit 27 is water adjusted to a predetermined electrical conductivity or less, such as ion-exchanged water or purified water from which impurities have been removed by other methods. For example, the electrical conductivity of ion-exchanged water is 1 μS / cm or less, preferably 0.06 μS / cm or less (equivalent to an electrical resistivity of 18 MΩ·cm or more).

[0019] The discharge pipe P16 connects the hydrogen gas / gas-liquid separation tank 28 to the discharge part G6 of each unit cell 100. As a result, the hydrogen gas discharged from the discharge part G6 of each unit cell 100 and the water that has migrated with the hydrogen ions inside the solid polymer electrolyte membrane 101 and condensed are supplied to the hydrogen gas / gas-liquid separation tank 28.

[0020] The hydrogen gas / gas-liquid separation tank 28 separates the hydrogen gas from the water. The hydrogen gas / gas-liquid separation tank 28 is further connected to a hydrogen gas storage tank 29. As a result, the separated hydrogen is stored in the hydrogen gas storage tank 29 and distributed.

[0021] 3A is a block diagram showing an example configuration of the power supply unit 30. As shown in FIG. 3A, the power supply unit 30 includes a DC power supply 30a capable of controlling the supply current and voltage, a variable resistor 30b, a plurality of switches T10, T14, T16, and T20, and a voltmeter V10. The plurality of switches T10, T14, T16, and T20 are switches that are electrically connected or disconnected. Note that the electrical connection states according to this embodiment correspond to the connection states of the plurality of switches T10, T14, T16, and T20.

[0022] The anode of the DC power supply 30a is connected to one end of the switch T10, and the other end of the switch T10 is connected to the anode current collector 106. That is, when the switch T10 is in a connected state, the anode of the DC power supply 30a is connected to the anode current collector 106 via the switch T10, and the cathode is connected to the cathode current collector 108. One end of the switch T14 is connected to the other end of the switch T10, and the other end is connected to one end of the switch T16. The other end of the switch T16 is connected to the cathode of the DC power supply 30a. In this embodiment, the anode current collector 106 and the cathode current collector 108 correspond to terminals. One end of the variable resistor 30b is connected to the cathode current collector 108, and the other end is connected to one end of the switch T20. The other end of the switch T20 is connected to the other end of the switch T14.

[0023] When switches T10 and T16 are in a disconnected state and switches T14 and T20 are in a connected state, variable resistor 30b is connected to anode current collector 106 and cathode current collector 108. In other words, when switches T10 and T16 are in a disconnected state and switches T14 and T20 are in a connected state, variable resistor 30b varies the resistance between anode current collector 106 and cathode current collector 108.

[0024] On the other hand, when the switch T10 is in the disconnected state as described above, it is possible to set the DC power supply 30a in the disconnected state. The connected and disconnected states of the multiple switches T10, T14, T16, and T20 are controlled by the control unit 40.

[0025] FIG. 3B is a block diagram showing another example of the configuration of the power supply unit 30. As shown in FIG. 3B, the power supply unit 30 includes a DC power supply 30a capable of controlling the supply current and voltage, a variable resistor 30b, a plurality of switches T10, T14, T16, and T20, and a voltmeter V10. The anode of the DC power supply 30a is connected to the anode current collector 106, and the cathode is connected to one end of the switch T10. The other end of the switch T10 is connected to one end of the variable resistor 30b and one end of the switch T16. The other end of the switch T16 is connected to the cathode current collector 108. That is, when the switches T10 and T16 are connected, the anode of the DC power supply 30a is connected to the anode current collector 106, and the cathode is connected to the cathode current collector 108 via the switches T10 and T16.

[0026] The other end of variable resistor 30b is connected to one end of switch T20. The other end of switch T20 is connected to cathode current collector 108. Variable resistor 30b is connected to anode current collector 106 and cathode current collector 108 when switches T10 and T16 are in a disconnected state and switches T14 and T20 are in a connected state. In other words, variable resistor 30b varies the resistance between anode current collector 106 and cathode current collector 108 when switches T10 and T16 are in a disconnected state and switches T14 and T20 are in a connected state.

[0027] Fig. 4 is a block diagram showing an example configuration of the control unit 40. As shown in Fig. 4, the control unit 40 controls the entire water electrolysis apparatus 1 and includes, for example, a storage unit 400, an acquisition unit 402, a connection control unit 404, a current / voltage control unit 406, a resistance control unit 408, a valve control unit 410, and a determination unit 412. The control unit 40 includes, for example, a CPU (Central Processing Unit) and an MPU (Microprocessor), and configures each processing unit by executing a control program stored in the storage unit 400. Note that each processing unit may also be configured using an electronic circuit.

[0028] The specific configuration of the control unit 40 is not limited, and may be, for example, a field programmable gate array (FPGA) or other devices such as an application specific integrated circuit (ASIC). Alternatively, the control unit 40 may be configured by a general-purpose computer.

[0029] The storage unit 400 is configured, for example, with an HDD (hard disk drive), an SSD (solid state drive), etc. As described above, the storage unit 400 stores the control program, a table L10 (see FIG. 9) showing the relationship between the pH (PH) of various metals and the solvent dissolution potential, which will be described later with reference to FIG. 9, and the like.

[0030] The acquisition unit 402 acquires information related to the control of the water electrolysis apparatus 1. In this embodiment, for example, at least information related to the voltage measured by the voltmeter V10 (see FIGS. 3A and 3B) and information related to the conductivity measured by the conductance meter D10 (see FIG. 1) is acquired.

[0031] As described above, the connection control unit 404 controls the connection and disconnection states of the multiple switches T10, T14, T16, and T20. In addition, the current / voltage control unit 406 controls the voltage of the DC power supply 30a, and thereby controls the potential applied to the anode current collector 106 of the single cell 100 (see FIG. 2).

[0032] The resistance control unit 408 controls the resistance value of the variable resistor 30b. Furthermore, the valve control unit 410 controls the opening and closing of the plurality of control valves V12, V14, V16, V18, V20, and V22 as described above. The determination unit 412 determines the water quality of the circulating water based on information relating to the electrical conductivity of the conductance meter D10 (see FIG. 1), for example.

[0033] 5 is a diagram schematically illustrating the reaction between the anode electrode 102 and the cathode electrode 104 during water electrolysis. During water electrolysis, the connection control unit 404 connects the DC power supply 30a using switches T10 and T16, and disconnects the variable resistor 30b using switches T14 and T20. Furthermore, the current / voltage control unit 406 controls the DC power supply 30a based on information regarding the voltage from the voltmeter V10 (see FIGS. 3A and 3B) so that the voltage applied to the anode current collector 106 and cathode current collector 108 of the single cell 100 (see FIG. 2) becomes a predetermined value, for example, 1.8 volts.

[0034] At this time, the valve control unit 410 opens the valve of control valve V10 on the supply pipe P10 side and closes the valve on the supply pipe P12 side. The valve control unit 410 also opens the control valves V12, V20, and V22 and closes the control valves V14, V16, and V18. The open / closed states of these control valves are merely examples, and the present invention is not limited to these open / closed states.

[0035] At this time, as described above, the anode electrode 102 + +4e - The reaction +O2 occurs, and oxygen O2 and unreacted water H2O are discharged from the discharge port G4.

[0036] 6 is a diagram schematically illustrating the reaction between the anode electrode 102 and the cathode electrode 104 during water electrolysis when impurities are introduced into the circulating water over the course of operation. When control similar to that shown in FIG. 5 is performed, impurities contained in the circulating water flow into the single cell 100 and accumulate in the anode electrode 102 and the electrolyte membrane 101. As impurities accumulate, the water electrolysis cell 20 deteriorates, and in order to produce the same amount of hydrogen, it becomes necessary to control the current and voltage applied to the anode current collector 106 and the cathode current collector 108 in accordance with cell deterioration.

[0037] 7 is a diagram showing a schematic diagram of the reaction between the anode electrode 102 and the cathode electrode 104 during a recovery operation performed when impurities are mixed in. The connection control unit 404 stops the DC power supply 30a, disconnects the circuit using the switches T10 and T16, and connects the switches T14 and T20 of the variable resistor 30b.

[0038] At this time, the valve control unit 410 opens the valve on the supply pipe P12 side of the control valve V10. The valve control unit 410 also closes the valve on the supply pipe P10 side of the control valve V10 and the valve V12, or closes the valve V12 to a smaller flow rate than during water electrolysis. This causes the pure water supplied from the pure water supply unit 27 to be supplied mainly to the anode electrode 102 of each unit cell 100 (see FIG. 2). The gas remaining in each unit cell 100 immediately after water electrolysis is stopped causes a so-called fuel cell reaction, and the anode electrode 102 reacts with O2 + 4H + +4e - On the other hand, at the cathode electrode 104, the reaction 2H2 → 4H + +4e - At this time, impurities adsorbed on the catalyst by the water generation reaction in the opposite direction to water electrolysis are desorbed and removed from the anode electrode 102 and the electrolyte membrane 101. Note that the half-closed state of these control valves is an example, and the open / closed states are not limited to these.

[0039] A control example will be described as a comparative example using Figures 8 and 9. Figure 8 is a diagram showing the thermodynamic stability of iridium oxide. The vertical axis indicates the potential of the anode current collector 106 relative to hydrogen, and the horizontal axis indicates the pH (PH), which is the hydrogen ion concentration of the anode electrode 102. Line L10 indicates a numerical example, which is the dashed line L10 on a graph showing the relationship between the pH of iridium oxide and the solvent dissolution potential. In Figure 8, when the potential is high (above the dashed line L10 on the graph), the iridium oxide catalyst IrO x is stable, and at low potentials (below the dashed line L10 in the graph), the iridium oxide catalyst IrO xbecomes unstable, and the Ir metal tends to dissolve. This numerical example L10 is stored in the storage unit 400 as tabular data. In this embodiment, 0 volts relative to the hydrogen reference means the potential at which the separation and synthesis of hydrogen molecules and hydrogen ions are in equilibrium. Therefore, in this embodiment, when referring to potential relative to the hydrogen reference, it means the potential at 0 volts at which the separation and synthesis of hydrogen molecules and hydrogen ions are in equilibrium.

[0040] 9 is a diagram showing the state of the water electrolysis cell 20 in the comparative example. The horizontal axis represents time, and the vertical axis represents the water electrolysis current flowing through the water electrolysis cell 20, the voltage (potential) of the anode electrode 102 of the water electrolysis cell 20, and the electric field inside the unit cell 100 of the water electrolysis cell 20, respectively.

[0041] As shown in Fig. 8, in state St10 up to time t10 (see Fig. 9) during water electrolysis, hydrogen is being generated, and the potential of the anode electrode 102 is set to a predetermined value, for example, 1.8 V, by controlling the current. At this time, hydrogen gas is generated, as shown in Fig. 5 or 6.

[0042] As shown in FIGS. 8 and 9 , during the process of reducing the current supply to the DC power supply 30a from the operating state to zero at times t10 and t10a, hydrogen generation stops, and the potential of the anode electrode 102 continues to decrease until time t12, reaching state St12. If the circuit remains connected after time t12, the water electrolysis current becomes negative due to a reverse reaction (fuel cell reaction) caused by residual gas, generating a discharge current in the opposite direction to the current flowing during hydrogen generation. The discharge current gradually decreases to zero at time t14, and the current and potential gradually decrease to zero, entering state St14. During period T1214 (between t12 and t14) during which a discharge current in the opposite direction to the current flowing during hydrogen generation is occurring, hydrogen ions move within each cell 100 in the opposite direction to that during water electrolysis. This promotes the desorption of impurities adsorbed on the catalyst of the anode electrode 102 together with the accompanying water molecules (drug water), thereby removing the impurities as shown in FIG. 7 .

[0043] However, the peak A10 of the discharge current occurring in period T1214 changes abruptly, and the rate of potential decrease due to the decrease in the potential of the anode electrode 102 caused by the increase in the discharge current increases. In this case, the potential of the anode electrode 102 remains in the solvent dissolution potential range (the range below the dashed line of the solvent dissolution potential L10 in FIG. 8), for example, state St14, and thus solvent dissolution of iridium oxide from the anode electrode 102 occurs. This further deteriorates the catalyst of the anode electrode 102.

[0044] A control example according to this embodiment will be described with reference to Figures 10 to 14. This control example according to this embodiment has two stop modes. In the first mode, the potential of the anode electrode 102 is controlled to a predetermined first potential, for example, 0.5 volts, when the discharge current approaches zero. The first mode is primarily used when removing impurities from the anode electrode 102 and the electrolyte membrane 101.

[0045] The second mode is a mode in which the potential of the anode electrode 102 is controlled to a second potential that is higher than the first potential and does not generate a discharge current in the direction opposite to the water electrolysis reaction. For example, the second potential is 1.5 volts, and is equal to or higher than the solvent dissolution potential L10 of a predetermined catalyst, such as iridium oxide, at the anode electrode 102, and is in a stable state St12 of iridium oxide (see FIG. 12).

[0046] 8, Fig. 10 is a diagram showing the thermodynamic stability of iridium oxide and explaining the potential state at the time of shutdown in the first mode. The vertical axis represents the hydrogen-based potential of the anode current collector 106, and the horizontal axis represents the pH (pH), which is the hydrogen ion concentration of the anode electrode 102.

[0047] 11 is a diagram showing the state of the water electrolysis cell 20 in the first mode. The horizontal axis represents time, and the vertical axis represents the electrolysis current flowing through the water electrolysis cell 20, the voltage of the anode electrode 102 of the water electrolysis cell 20, and the electric field inside the unit cell 100 of the water electrolysis cell 20, respectively.

[0048] In the first mode, when hydrogen generation is stopped, the connection control unit 404 disconnects the DC power supply 30a using switches T10 and T16 of the power supply unit 30, and connects the variable resistor 30b using switches T14 and T20. The resistance control unit 408 controls the resistance value of the variable resistor 30b based on information from the voltmeter V10, and controls the rate of potential decline at T12-16 (between times t12 and t16) (see FIG. 11) to a range that suppresses catalyst degradation. The resistance control unit 408 controls the rate of potential decline to, for example, 20 mV / sec or less per 100 unit cells, preferably 15 mV / sec or less per 100 unit cells. In the first mode, as shown in FIGS. 10 and 11 , when hydrogen generation is stopped between times t10 and t10a during the process of reducing the current supply to the DC power supply 30a from the operating state to zero, the potential of the anode electrode 102 drops to state St12 until time t12. Then, after each switch is set to the first mode at time t12, the water electrolysis current becomes negative, and a discharge current occurs in the direction opposite to the current flow during hydrogen generation. By controlling the discharge current during period T1216, i.e., between times t12 and t16, the potential of the anode electrode 102 drops to state St16. At this time, the potential in state St16 is maintained at a predetermined potential, for example, 0.5 volts, which is higher than state St14 in the comparative example (see FIG. 8 ). During period T1216 during which a discharge current occurs in the direction opposite to the current flow during hydrogen generation, impurities are removed, as shown in FIG. 7 .

[0049] The peak B12 of the discharge current occurring during the period T1216 changes more gradually than the peak A10 (see FIG. 9), and the rate of potential decrease during the period T1216 is slower. At this time, the iridium oxide catalyst IrO xRapid changes in the potential of the anode electrode 102 are suppressed. Furthermore, the potential of the anode electrode 102 in state St16 is maintained higher than in the comparative example, for example, state St14 in FIG. 8, and solvent elution of iridium oxide is suppressed more than in the comparative example. This further suppresses deterioration of the catalyst in the anode electrode 102. Note that, although the variable resistor 30b suppresses changes in the potential of the anode electrode 102 in state St16, if the variable resistor 30b is kept connected, the discharge current continues to flow and gradually decreases. Therefore, the switches T14 and T20 in FIG. 3A are then disconnected to stop the discharge current. 3B, the connection control unit 404 may disconnect the switches T14 and T16 and connect the switches T10 and T20 so that the variable resistor 30b and the DC power supply 30a are connected in series within the circuit of the power supply unit 30, and the current / voltage control unit 406 may control the DC power supply 30a and the variable resistor 30b so that the potential of the anode electrode 102 in state St16 remains constant. In this case, the current / voltage control unit 406 may reduce the power supply to the DC power supply 30a from the operating state to zero, and then control the discharge current in the power supply unit 30 so that the potential of the anode electrode 102 remains equal to or higher than the solvent dissolution potential L10, allowing iridium oxide to remain in a stable state (see FIG. 10).

[0050] 8, Fig. 12 is a diagram showing the thermodynamic stability of iridium oxide, and is an explanatory diagram showing the potential state at the time of shutdown in the second mode. The vertical axis represents the hydrogen-based potential of the anode current collector 106, and the horizontal axis represents the pH (pH), which is the hydrogen ion concentration of the anode electrode 102.

[0051] 13 is a diagram showing the state of the water electrolysis cell 20 in the second mode. The horizontal axis represents time, and the vertical axis represents the electrolysis current flowing through the water electrolysis cell 20, the voltage of the anode electrode 102 of the water electrolysis cell 20, and the electric field inside the unit cell 100 of the water electrolysis cell 20, respectively.

[0052] In the second mode, during the time period from time t10 to time t10a when hydrogen generation is stopped, the connection control unit 404 connects the DC power supply 30a using the switches T10 and T16 and disconnects the variable resistor 30b using the switches T14 and T20. The current / voltage control unit 406 controls the DC power supply 30a so that the current value is a predetermined value, e.g., near zero, so that the potential of the anode electrode 102 in state St12 becomes a second potential. The second potential is a potential at which the potential of the anode electrode 102 does not generate a discharge current. For example, the second potential is 1.5 volts, which is a range that is equal to or higher than the solvent dissolution potential at which a predetermined catalyst, e.g., iridium oxide, can exist in a stable state at the anode electrode 102, as shown in FIG. 12 . By setting the potential of the anode electrode 102 equal to or higher than the solvent dissolution potential of iridium oxide, solvent dissolution of iridium oxide is suppressed. Therefore, when the amount of impurities in the circulating water is small, stopping in mode 2 can suppress the elution of iridium oxide into the solvent more than in the comparative example and mode 1. Alternatively, it is also possible to disconnect switches T10 and T16 that connect the DC power supply 30a immediately after stopping hydrogen generation, thereby preventing the generation of discharge current.

[0053] FIG. 14 is a flowchart showing an example of control of the water electrolysis cell 20 by the control unit 40 according to this embodiment. Here, an example of stop control in the first mode will be described. As shown in FIG. 14, first, the control unit 40 controls the circulating water supply pump 25 to supply water to the supply unit G2 of each unit cell 100. Next, the connection control unit 404 turns on the switches T10 and T16 and turns off the switches T14 and T20. Then, the current / voltage control unit 406 controls the voltage of the DC power supply 30a so that the potential between the anode current collector 106 and the cathode current collector 108 of each unit cell 100 is, for example, 1.8 volts, thereby starting hydrogen generation (step S100).

[0054] Next, the current / voltage control unit 406 of the control unit 40 stops the current (voltage) supply from the DC power supply 30a (step S102), and then the connection control unit 404 sets the switches T10 and T16 to a non-conductive state and the switches T14 and T20 to a conductive state (step S104), thereby starting discharge in the first mode (step S106).

[0055] Next, the resistance control unit 408 controls the resistance value of the variable resistor 30b based on the information from the voltmeter V10, thereby controlling the rate of potential decrease of T1216 (see FIG. 11) within a range that suppresses catalyst degradation (step S108). When the potential of the anode electrode 102 reaches a predetermined value (step S110), the resistance control unit 408 controls the connection control unit 404 to turn off the switches T14 and T20 (step S112). The valve control unit 410 controls the open / closed states of the various valves to predetermined states (step S114), thereby completing the stop control in the first mode. In this way, the resistance value of the variable resistor 30b is controlled to control the rate of potential decrease of T1216 (see FIG. 11) within a range that suppresses catalyst degradation. This allows the catalyst of the anode electrode 102 to be maintained at a potential that suppresses solvent dissolution, thereby suppressing degradation of the water electrolysis cell 20.

[0056] FIG. 15 is a flowchart showing an example of control of the water electrolysis cell 20 by the control unit 40 when hydrogen production is stopped. First, the acquisition unit 402 acquires the conductivity of the circulating water measured by the conductance meter D10 in step S100 during water electrolysis in FIG. 14 (step S200). Next, the determination unit 412 determines whether the conductivity of the circulating water is, for example, 1 μS / cm or less (step S202). If the determination unit 412 determines that the conductivity is, for example, 1 μS / cm or less (YES in step S202), the determination unit 412 performs a stop process in the second mode on the connection control unit 404, the current / voltage control unit 406, the resistance control unit 408, and the valve control unit 410 (step S204).

[0057] On the other hand, if the determination unit 412 determines that the conductivity of the circulating water, for example, is higher than 1 μS / cm (deterioration due to impurities in the circulating water) (NO in step S202), it performs a stop process in the first mode on the connection control unit 404, the current / voltage control unit 406, the resistance control unit 408, and the valve control unit 410 (step S205). The control unit 40 then determines whether to end the process (step S206). If it determines not to end the process (NO in step S206), it repeats the process from step S200, for example, when restarting and stopping the water electrolysis operation. On the other hand, the control unit 40 temporarily sets the system to an operating state, and when it is stopped again, it repeats the process from step S200, and if it determines to end the process (YES in step S206), it terminates the entire process.

[0058] In this way, the quality of the circulating water is determined by the electrical conductivity of the circulating water, and if the water quality is maintained within a predetermined standard range, operation is stopped in the second mode, whereby the anode electrode 102 is at a potential above the solvent dissolution potential at which the catalytic metal (iridium oxide) can exist in a stable state, i.e., a potential range where solvent dissolution is unlikely to occur, thereby suppressing deterioration. On the other hand, if the water quality does not meet the predetermined standard, for example, if the electrical conductivity of the circulating water is above a predetermined value, operation is stopped in the first mode, whereby impurity removal processing can be performed while the potential of the anode electrode 102 is controlled to a potential that suppresses solvent dissolution of the catalytic metal.

[0059] As described above, according to this embodiment, when hydrogen production in the water electrolysis cell 20 is stopped, the control unit 40 controls the potential of the anode 102 to a predetermined potential, and also controls the rate of potential decrease of the anode 102 during discharge current generation to a predetermined value using the variable resistor 30b. This reduces fluctuations in the potential of the anode 102 per unit time during discharge current generation, enables control of the potential to a level that suppresses elution of the catalytic metal, and suppresses deterioration of the water electrolysis cell 20.

[0060] (Second embodiment) The water electrolysis apparatus 1 according to the second embodiment differs from the water electrolysis apparatus 1 according to the first embodiment in that it further has multiple determination modes for determining the water quality of circulating water. The differences from the water electrolysis apparatus 1 according to the first embodiment will be described below.

[0061] Fig. 16 is a block diagram showing the configuration of a water electrolysis apparatus 1 according to a second embodiment. As shown in Fig. 16, the water electrolysis apparatus 1 according to this embodiment has a water pipe P18 that enables the oxygen gas / gas-liquid separation tank 23 to be separated from the circulating water supply pipe P10.

[0062] Fig. 17 is a block diagram showing the configuration of a control unit 40 according to the second embodiment. As shown in Fig. 17, the control unit 40 according to the present embodiment further includes a measurement unit 414. The measurement unit 414 measures the operating conditions related to hydrogen production by the water electrolysis device 1, such as at least one of the operating time, the potential of the water electrolysis cell, and the number of starts and stops.

[0063] When hydrogen production is stopped in the first mode, the control unit 40 controls the control valves V26 and V24 to recover high-purity water (water with low electrical conductivity) that has been separated from the water vapor in the oxygen gas and separated into gas and liquid from the oxygen gas / gas-liquid separation tank 23, and supply it from the control valve V26 to the control valve V24 via the pipe P18. This makes it possible to remove impurities from the water electrolysis cell 20 more efficiently depending on the system configuration.

[0064] The determination unit 412 of the control unit 40 has a 21 mode in which control is performed in the first mode when the voltage between the terminals of the water electrolysis cell 20 during hydrogen production during water electrolysis operation reaches or exceeds a predetermined value. The determination unit 412 also has a 22 mode in which control is performed in the first mode when the integrated value of the hydrogen production time of the water electrolysis cell 20 exceeds a predetermined value, and a 23 mode in which control is performed in the first mode when the number of start-ups or stoppages of the water electrolysis cell 20 exceeds a predetermined value.

[0065] More specifically, in mode 21, the determination unit 412 performs control in mode 1 when, for example, the measurement unit 414 determines that the voltage across the terminals of the water electrolysis cell 20 has increased by 10% compared to the voltage at the beginning of operation while a predetermined amount of hydrogen is being produced during water electrolysis operation. As deterioration of the water electrolysis cell 20 progresses, the cell overvoltage of the water electrolysis reaction increases, causing reaction losses and resulting in a decrease in the amount of hydrogen produced per unit of external power supply, such as from renewable energy. On the other hand, when the electrolysis cell 20 is operated to produce a predetermined amount of hydrogen, the cell overvoltage increases due to deterioration, causing the terminal voltage to become higher than before deterioration. Therefore, by measuring the terminal voltage, the degree of deterioration of the water electrolysis cell 20 can be determined with higher accuracy.

[0066] In mode 22, the determination unit 412 performs control in mode 1 when, for example, the integrated value of the hydrogen generation time exceeds 500 hours. Statistically, the water electrolysis cell 20 deteriorates when, for example, the integrated value exceeds a predetermined time. Therefore, the degree of deterioration of the water electrolysis cell 20 can be determined with higher accuracy based on the integrated value of the hydrogen generation time.

[0067] In mode 23, the determination unit 412 performs control in mode 1 when, for example, the number of start-ups or stop-ups measured by the measurement unit 414 exceeds 30. Statistically, the water electrolysis cell 20 deteriorates when the number of start-ups or stop-ups exceeds a predetermined number. Therefore, the degree of deterioration of the water electrolysis cell 20 can be determined with higher accuracy based on the number of start-ups or stop-ups.

[0068] As described above, the determining unit 412 has multiple modes for determining the degree of deterioration of the water electrolysis cell 20, enabling a more objective determination of the degree of deterioration of the water electrolysis cell 20. This allows hydrogen production to be stopped more accurately in the first mode, thereby enabling recovery of the water electrolysis cell 20 while suppressing deterioration. In other words, hydrogen production can also be stopped more accurately in the second mode, thereby suppressing solvent solutes at the anode 102.

[0069] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the inventions and their equivalents as defined in the claims. [Explanation of symbols]

[0070] 1: water electrolysis device, 20: water electrolysis cell, 30: power supply unit, 30a: DC power supply, 30b: variable resistor, 40: control unit, 100: single cell, 101: solid polymer electrolyte membrane, 102: anode electrode, 104: cathode electrode.

Claims

1. a water electrolysis cell having an anode electrode on one side and a cathode electrode on the other side, the water electrolysis cell using a solid polymer electrolyte membrane to electrolyze water supplied to the anode electrode to generate hydrogen and oxygen; a power supply unit for supplying a current and a voltage from a DC power supply between terminals of the water electrolysis cell; a control unit that performs control in a first mode in which, after the hydrogen production is stopped, a connection state is established in which a discharge current flows in a direction opposite to a direction of a current flowing through the water electrolysis cell during hydrogen production, and a potential of the anode electrode is controlled to a first potential while a potential decrease rate at the anode electrode during the generation of the discharge current is set to a predetermined value; A variable resistor connectable between the terminals; Equipped with The control unit controls the resistance value of the variable resistor so that a rate of decrease in potential of the anode electrode falls within a predetermined range.

2. The water electrolysis apparatus according to claim 1 , wherein the control unit connects the variable resistor between the terminals of the water electrolysis cell in the first mode.

3. The water electrolysis apparatus according to claim 1 , wherein in the first mode, the control unit controls the resistance value of the variable resistor so that a rate of potential decrease between the terminals falls within a predetermined value range.

4. The control unit is In the first mode when hydrogen generation is stopped, the potential of the anode electrode is controlled to the first potential; 4. The water electrolysis device according to claim 1, wherein in a second mode when hydrogen generation is stopped, the potential of the anode electrode is controlled to a second potential that is higher than the first potential and at which no reverse discharge current is generated.

5. The water electrolysis apparatus according to claim 4 , wherein the second potential is in a range equal to or higher than a solvent dissolution potential of the catalyst of the anode electrode.

6. The water electrolysis apparatus according to claim 1 , wherein the metal component of the catalyst of the anode electrode is iridium oxide.

7. The water electrolysis apparatus according to claim 1 , wherein the control unit performs control in the first mode when the electrical conductivity of the water supplied to the anode electrode becomes equal to or higher than a predetermined value.

8. 8. The water electrolysis device according to claim 1, wherein the control unit performs control in the first mode when a voltage between the terminals of the water electrolysis cell during hydrogen production reaches or exceeds a predetermined value.

9. The water electrolysis apparatus according to claim 1 , wherein the control unit performs control in the first mode to supply pure water having a predetermined conductivity or less as the water.

10. The water electrolysis apparatus according to claim 1 , wherein the control unit performs control in the first mode when an integrated value of a hydrogen production time in the water electrolysis cell exceeds a predetermined value.

11. The water electrolysis apparatus according to claim 1 , wherein the control unit performs control in the first mode when the number of times the water electrolysis cell is started or stopped exceeds a predetermined value.

12. The power supply unit includes a DC power supply and a plurality of switches that are electrically connected or disconnected; 4. The water electrolysis apparatus according to claim 1, wherein the control unit controls the switches to be in at least one of a first connection state in which the variable resistor is connected between the terminals, a second connection state in which the DC power supply is connected between the terminals, and a third connection state in which the DC power supply and the variable resistor are connected in series between the terminals, and controls the connection state to be the first connection state in the first mode.

13. A method for controlling a water electrolysis cell that uses a solid polymer electrolyte membrane having an anode electrode on one side and a cathode electrode on the other side to electrolyze water supplied to the anode electrode to generate hydrogen and oxygen, comprising the steps of: after the production of hydrogen is stopped, a connection state is established in which a discharge current flows in a direction opposite to a direction of a current flow during hydrogen production in the water electrolysis cell, and a potential of the anode electrode is controlled to a first potential while a rate of decrease in a potential of the anode electrode during the production of the discharge current is set to a predetermined value; In a second mode, the potential of the anode electrode is controlled to a second potential that is higher than the first potential and at which no reverse discharge current is generated.

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