Water electrolysis device and method for controlling same

Abstract

A water electrolysis device (10) is provided with a bypass flow path (56) that connects a water discharge flow path (60) and a water supply flow path (52). When water electrolysis of a water electrolysis stack (14) in which supply of current from a power supply (90) has been stopped is stopped, the water electrolysis device (10) circulates a portion of a bubble-containing alkaline water (AWO) containing oxygen bubbles flowing through the water discharge flow path (60) by returning said portion, via the bypass flow path (56) and the water supply flow path (52), to a gas-liquid diffusion layer (22) in the water electrolysis stack (14).

Description

Water electrolysis apparatus and its control method

[0001] This disclosure relates to a water electrolysis apparatus that performs water electrolysis using an anion exchange membrane as an electrolyte membrane, and a control method thereof.

[0002] Japanese Patent Publication No. 2023-58306 discloses a PEM (Proton Exchange Membrane) water electrolysis apparatus (paragraphs

[0013] and

[0014] of the said publication).

[0003] Because PEM water electrolysis systems use a large amount of precious metal catalysts, AEM (Anion Exchange Membrane) water electrolysis systems, which use fewer precious metal catalysts, are attracting attention.

[0004] This AEM water electrolysis device includes a water electrolysis stack (AEM water electrolysis stack) which is made up of multiple stacked water electrolysis cells, each in which an anion exchange membrane is sandwiched between an anode electrode and a cathode electrode.

[0005] In this AEM water electrolysis stack, water (alkaline water) supplied to the anode electrode side through the gas-liquid diffusion layer of each water electrolysis cell via water supply communication holes inside the stack permeates the anion exchange membrane and reaches the cathode electrode, where hydrogen and hydroxide ions are generated from the water by the power supply current at the cathode electrode.

[0006] When the hydroxide ions permeate the anion exchange membrane and reach the anode electrode, oxygen is generated from the hydroxide ions at the anode electrode.

[0007] The hydrogen and oxygen produced in the AEM water electrolysis stack in this manner are used for industrial purposes, such as fuel for fuel cells.

[0008] Incidentally, in an AEM water electrolysis stack in which AEM water electrolysis cells are stacked, immediately after stopping water electrolysis, the water electrolysis cells behave as batteries due to their own capacitor components. In an AEM water electrolysis stack in which these water electrolysis cells behaving as batteries are stacked, one stacked adjacent water electrolysis cell and another water electrolysis cell are electrically connected through the alkaline water flowing through a passage (water supply communication hole) that supplies conductive alkaline water to each water electrolysis cell. As a result, a current may flow to bridge the potential difference between the water electrolysis cells. In this case, depending on the potential difference between the water electrolysis cells, a current may flow in the opposite direction to the normal direction (called a reverse current).

[0009] This reverse current can cause deterioration of the anode electrode, cathode electrode, and separator of the AEM water electrolysis cell, which presents a problem.

[0010] This disclosure aims to solve the problems described above.

[0011] One aspect of the present disclosure is a water electrolysis stack comprising a plurality of stacked water electrolysis cells, each stacked in the order of an anode separator, a gas-liquid diffusion layer, an anode electrode, an anion exchange membrane, a cathode electrode, a gas diffusion layer, and a cathode separator, the stack comprising: a water inlet for which alkaline water is supplied; water supply communication holes communicating with each other in the stacking direction and supplying the alkaline water supplied from the water inlet to one end of each gas-liquid diffusion layer; water discharge communication holes communicating with each other in the stacking direction and supplying alkaline water containing oxygen bubbles produced by water electrolysis discharged from the other end of each gas-liquid diffusion layer; and a water outlet communicating with the water discharge communication holes and supplied with alkaline water containing oxygen bubbles from the water discharge communication holes, the stack comprising a water electrolysis stack communicating with the water outlet on the outside of the water electrolysis stack, A water electrolysis apparatus comprising: a water discharge channel for discharging alkaline water containing oxygen bubbles; a gas-liquid separator to which the alkaline water containing oxygen bubbles discharged from the water discharge channel is supplied and separated into oxygen and the alkaline water; a pump; a water supply channel for circulating the alkaline water separated by the gas-liquid separator to the water inlet by the pump; and a power supply for powering the water electrolysis stack, wherein a bypass channel is provided connecting the water discharge channel and the water supply channel, and when water electrolysis by the water electrolysis stack is stopped by stopping the supply of current from the power supply, a portion of the alkaline water containing oxygen bubbles flowing in the water discharge channel is returned to the water supply channel through the bypass channel by the pump.

[0012] Other aspects of the present disclosure include a water electrolysis stack comprising a plurality of stacked water electrolysis cells, each stacked in the order of an anode separator, a gas-liquid diffusion layer, an anode electrode, an anion exchange membrane, a cathode electrode, a gas diffusion layer, and a cathode separator, the water electrolysis stack comprising: a water inlet for supplying alkaline water; water supply communication holes communicating with each other in the stacking direction and supplying the alkaline water supplied from the water inlet to one end of each gas-liquid diffusion layer; water discharge communication holes communicating with each other in the stacking direction and supplying alkaline water containing oxygen bubbles produced by water electrolysis discharged from the other end of each gas-liquid diffusion layer; a water outlet communicating with the water discharge communication holes and supplied with alkaline water containing oxygen bubbles from the water discharge communication holes; a water discharge channel communicating with the water outlet outside the water electrolysis stack and discharging the alkaline water containing oxygen bubbles; and the water discharge channel A control method for a water electrolysis apparatus comprising: a gas-liquid separator that receives the alkaline water containing oxygen bubbles discharged from and separates it into oxygen and the alkaline water; a pump; a water supply channel that circulates the alkaline water separated by the gas-liquid separator to the water inlet using the pump; and a power supply that supplies current to the water electrolysis stack, further comprising: a bypass channel connecting the water discharge channel and the water supply channel; a current supply stop monitoring step that monitors whether or not the supply of current from the power supply is stopped; and an oxygen-bubble alkaline water return step that, when it is confirmed that the supply of current has been stopped, returns a portion of the alkaline water containing oxygen bubbles circulating in the water discharge channel to the water supply channel through the bypass channel using the pump.

[0013] According to this disclosure, it is possible to suppress the deterioration of the anode electrode and cathode electrode of the electrolytic cell when the water electrolysis device is shut down.

[0014] Figure 1 is a schematic diagram showing the configuration of an example of a water electrolysis apparatus according to an embodiment that implements the control method of the water electrolysis apparatus according to the embodiment. Figure 2 is a process diagram used to explain the operation of the water electrolysis apparatus of the example in Figure 1. Figure 3A is a schematic diagram used to explain the state in which only alkaline water is supplied to the water electrolysis stack. Figure 3B is an internal schematic diagram used to explain the leakage current flowing through the water electrolysis stack. Figure 3C is an electrical circuit schematic diagram used to explain the leakage current flowing through the water electrolysis stack. Figure 4A is a schematic diagram used to explain the state in which alkaline water containing oxygen bubbles is circulating in the part of the water electrolysis apparatus including the water electrolysis stack. Figure 4B is an internal schematic diagram used to explain the state in which alkaline water containing oxygen bubbles is flowing through the water supply communication hole. Figure 4C is an electrical circuit schematic diagram used to explain the state in which the leakage current flowing through the water electrolysis stack is interrupted. Figure 5A is a time chart used to explain the time change of the cell voltage in Example 1 of the stopping process and the time change of the cell voltage in Comparative Example 1. Figure 5B is a time chart used to explain the time change of cell voltage in Example 2 of the stopping process and in Comparative Example 2. Figure 6A is a schematic circuit diagram used to explain the phenomenon in Comparative Example 1 in which a reverse current flows immediately after stopping. Figure 6B is an internal schematic diagram used to explain the phenomenon in Comparative Example 1 in which a reverse current flows immediately after stopping. Figure 7 is a time chart used to explain the time change of cell voltage in Example 3 of the stopping process and in Comparative Example 3.

[0015] [Embodiment] [Configuration] Figure 1 is a schematic configuration diagram showing an example of a water electrolysis apparatus 10 according to an embodiment that implements the control method of the water electrolysis apparatus according to the embodiment. In the drawings referred to below, common or corresponding parts are denoted by the same reference numerals, and redundant explanations may be omitted.

[0016] As shown in Figure 1, the water electrolysis apparatus 10 comprises a water electrolysis stack 14 in which a plurality of schematically drawn water electrolysis cells 12 are stacked vertically (arrow A direction) or horizontally (arrow B direction). For the sake of understanding, in the following explanation, it will be assumed that the plurality of water electrolysis cells 12 are stacked vertically.

[0017] The water electrolysis stack 14 has a cylindrical appearance, and each water electrolysis cell 12 is constructed by stacking the following components in the order of bottom to top, in the direction of arrow A: a disc-shaped end plate (not shown), a ring-shaped anode separator 21, a ring-shaped gas-liquid diffusion layer 22, a ring-shaped anode electrode (anode catalyst layer) 23, a ring-shaped anion exchange membrane 24, a ring-shaped cathode electrode (cathode catalyst layer) 25, a ring-shaped gas diffusion layer 26, a ring-shaped cathode separator 27, and a disc-shaped end plate (not shown).

[0018] The gas-liquid diffusion layer 22 is a porous layer through which gas (oxygen in this embodiment) and liquid (alkaline water in this embodiment) can diffuse and flow, but it may also be a water channel consisting of a groove formed on one side (upper side) of the anode separator 21.

[0019] The anode electrode 23, anion exchange membrane 24, cathode electrode 25, and gas diffusion layer 26 constitute a so-called membrane electrode assembly (MEA). The membrane electrode assembly is sandwiched between an anode separator 21 and a cathode separator 27. The circumferential surface of the membrane electrode assembly is airtightly sealed with a gasket (not shown) or the like.

[0020] A water electrolysis stack 14, in which multiple water electrolysis cells 12 are stacked, is held together by tie rods (not shown) between the disc-shaped end plates, and is fastened in the stacking direction.

[0021] At one radial end of the water electrolysis stack 14, there is an alkaline water inlet 30 that communicates with the inlet side of the gas-liquid diffusion layer 22, and at the other radially opposite end, there is an alkaline water outlet 32 ​​that communicates with the outlet side of the gas-liquid diffusion layer 22.

[0022] Near the water inlet 30 in the water electrolysis cell 12, one radial end is provided with a water supply communication hole 34 that communicates with each other in the stacking direction to supply alkaline water to each water electrolysis cell 12.

[0023] Near the water outlet 32 ​​in the water electrolysis cell 12, at the other radial end, a water discharge communication hole 36 is provided that communicates with each other in the stacking direction and discharges alkaline water from each water electrolysis cell 12.

[0024] In other words, the alkaline water supplied from the water inlet 30 of the water electrolysis stack 14 flows through the water supply communication hole 34, through the gas-liquid diffusion layer 22 of each water electrolysis cell 12, and is discharged from the water electrolysis stack 14 through the water discharge communication hole 36 and the water outlet 32.

[0025] The water electrolysis cell 12 is provided with a hydrogen communication hole 37 that penetrates the radial center along the stacking direction.

[0026] The hydrogen communication hole 37 is connected to a hydrogen outlet (hydrogen outlet) 38, and high-pressure hydrogen generated by the electrolysis of water by the water electrolysis cell 12 of the water electrolysis stack 14 is discharged from the hydrogen outlet 38.

[0027] At the hydrogen outlet 38, the hydrogen discharge channel 40, the hydrogen inlet 42a of the gas-liquid separator (first gas-liquid separator: hydrogen gas-liquid separator) 42, the gas chamber of the gas-liquid separator 42, the hydrogen outlet 42b, the hydrogen discharge channel 40, the check valve 46, and the hydrogen discharge channel 48 are provided in this order.

[0028] A back pressure valve (not shown) is provided in the hydrogen discharge channel 40, which is connected to the gas-liquid separator 42. Hydrogen pressurized to 1 MPa to 80 MPa by this back pressure valve is supplied to a hydrogen tank (not shown) via a dryer (not shown) through the hydrogen inlet 42a of the gas-liquid separator 42, the gas chamber, the hydrogen outlet 42b, the hydrogen discharge channel 40, the check valve 46, and the hydrogen discharge channel 48 of the gas-liquid separator 42, and stored in the hydrogen tank (not shown). Compared to a PEM water electrolysis device, the dryer (not shown) discharges less water from the hydrogen outlet 38, allowing for a simpler and less expensive configuration.

[0029] The water stored in the liquid chamber of the gas-liquid separator 42 is discharged through the water outlet 42c and the drainage channel 50.

[0030] The water electrolysis apparatus 10 is equipped with a gas-liquid separator (second gas-liquid separator: oxygen gas-liquid separator) 70. The gas-liquid separator 70 stores, for example, an aqueous solution of potassium hydroxide KOH with a concentration of 0.5 molars (the alkaline water, hereinafter also referred to as alkaline water AW) in its liquid chamber. Note that the hydroxide ions OH in alkaline water AW － The molar concentration should be such that the pH is 10 or higher.

[0031] A water supply channel 84, which is equipped with an electric pump 80 and a water purification filter 82, is connected to the water inlet 70b of the gas-liquid separator 70, and purified water is supplied to the water inlet 70b.

[0032] The oxygen outlet 70c located in the gas chamber of the gas-liquid separator 70 is connected to the oxygen discharge channel 94, which is equipped with a check valve 96.

[0033] The oxygen generated by water electrolysis in the water electrolysis cell 12 can be stored in an oxygen tank (not shown) or released into the atmosphere (not shown) via the oxygen discharge channel 94, passing through the water outlet 32, the water discharge channel 60, the fluid inlet 70a of the gas-liquid separator 70, the gas chamber of the gas-liquid separator 70, and the oxygen outlet 70c of the gas-liquid separator 70.

[0034] A back pressure valve (not shown) is provided in the oxygen discharge channel 94. Oxygen pressurized to 1 MPa to 80 MPa by this back pressure valve is supplied to and stored in an oxygen tank (not shown).

[0035] Furthermore, when releasing oxygen into the atmosphere, a check valve 96 capable of withstanding a pressure (approximately 0.1 to 0.5 MPa) sufficient to prevent backflow is provided.

[0036] A water supply channel 52 is provided between the water outlet 70d of the gas-liquid separator 70 and the water inlet 30 of the water electrolysis stack 14.

[0037] The alkaline water AW stored in the liquid chamber of the gas-liquid separator 70 is supplied to the gas-liquid diffusion layer 22 of the water electrolysis stack 14 through the water supply channel 52, which is connected to the water outlet 70d of the liquid chamber of the gas-liquid separator 70, via the water inlet 30 and the water supply communication hole 34.

[0038] An electric pump 72 for circulating alkaline water AW is provided in the water supply channel 52. The reason for circulating alkaline water AW is that in AEM water electrolysis, the water used for electrolysis must be alkaline water AW, and the alkaline water AW that is not used in the reaction cannot be discharged as is.

[0039] The electric pump 72 imparts mechanical energy to the fluid (alkaline water AW or alkaline water with oxygen bubbles described later (hereinafter referred to as oxygen-bubbled alkaline water AWO)) flowing through the water supply channel 52 to cause it to flow in the water supply channel 52, and supplies the fluid to the water electrolysis stack 14.

[0040] On the outlet side of the gas-liquid diffusion layer 22, a mixed fluid (the above-mentioned oxygen-bubbled alkaline water AWO) of oxygen generated at the anode electrode 23 by electrolysis of water by the membrane electrode assembly of the water electrolysis cell 12 and the alkaline water AW flowing through the gas-liquid diffusion layer 22 is discharged. The oxygen-bubbled alkaline water AWO is discharged from the water outlet 32 to the water discharge channel 60 of the water electrolysis stack 14.

[0041] The water discharge channel 60 communicates the water outlet 32 and the gas chamber of the gas-liquid separator 70 through the fluid inlet 70a of the gas-liquid separator 70. An electromagnetic three-way valve 71 is provided in the water discharge channel 60.

[0042] The three-way valve 71 includes a first valve 7I a, a second valve (also referred to as a passing valve) 71b, and a third valve (also referred to as a return valve or a bypass valve) 71c. The first valve 71a communicates with the water outlet 32 through the water discharge channel 60. The second valve 71b communicates with the fluid inlet 70a of the gas-liquid separator 70 through the water discharge channel 60. The third valve 71c communicates with a bypass channel 56 provided with a check valve 74.

[0043] The three-way valve 71 is a fluid-dividing type three-way valve that can divide the oxygen-bubbled alkaline water AWO discharged from the water outlet 32 into the water discharge channel 60 communicating with the gas-liquid separator 70 at a set ratio and the bypass channel 56 bypassing the gas-liquid separator 70.

[0044] The water electrolysis device 10 includes a power source 90. The power source 90 is supplied with, for example, surplus power Ps among the power generated by solar power generation, wind power generation, and the like.

[0045] The power supply 90 generates a current I for water electrolysis from the power Ps. This current I is supplied to the water electrolysis stack 14. The current I flows through the anode separator 21 and anode electrode 23 of the water electrolysis cell 12 at the lower end of the water electrolysis stack 14 in the stacking direction, through each water electrolysis cell 12 that is electrically connected in series, through the cathode electrode 25 and cathode separator 27 at the upper end in the stacking direction, and returns to the power supply 90. In other words, the power from the power supply 90 is supplied to the water electrolysis stack 14.

[0046] The power supply 90 generates a predetermined potential Vp when water electrolysis is stopped and no current I is supplied to the water electrolysis stack 14. By applying the potential Vp between the anode separator 21 at the lower end in the stacking direction of the water electrolysis stack 14 and the cathode separator 27 at the upper end in the stacking direction, a potential Vpd, which is a division of the potential Vp, can be applied between the anode electrode 23 and cathode electrode 25 of each water electrolysis cell 12.

[0047] The current I supplied from the power supply 90 (supply current) is detected by the current sensor 98. The voltage V (Vp) generated by the power supply 90 is detected by the voltage sensor 99.

[0048] The water electrolysis apparatus 10 includes a control device 100 that controls the water electrolysis apparatus 10.

[0049] The control device 100 is a computer installed in the water electrolysis apparatus 10.

[0050] The control device 100 includes an arithmetic unit (processing unit) 102, a storage unit 104, a display unit (not shown), and an operation unit (not shown).

[0051] The arithmetic unit 102 may be composed of a processor such as a CPU or GPU. In other words, the arithmetic unit 102 may be composed of processing circuits.

[0052] The arithmetic unit 102 includes a determination unit 106 and a control unit 108. The determination unit 106 and the control unit 108 can be realized by the execution of a program stored in the storage unit 104 by the arithmetic unit 102.

[0053] Furthermore, at least a portion of the determination unit 106 and the control unit 108 may be implemented using integrated circuits such as ASICs and FPGAs. Also, at least a portion of the determination unit 106 and the control unit 108 may be composed of electronic circuits including discrete devices.

[0054] The storage unit 104 may consist of volatile memory and non-volatile memory. Examples of volatile memory include RAM. This volatile memory is used as the working memory of the processor and temporarily stores data necessary for processing or calculation. Examples of non-volatile memory include ROM and flash memory. This non-volatile memory is used as storage memory and stores programs, tables, maps, etc. At least a part of the storage unit 104 may be provided in the processor, integrated circuit, etc. as described above.

[0055] [Operation] Next, the operation of the water electrolysis apparatus 10, which is basically configured as described above, will be explained in detail based on the process diagram (processes P1, P2, P3 (P3'), P4, P5) shown in Figure 2.

[0056] Furthermore, the three-way valve 71 is configured such that, when not driven by the control unit 108, in other words, when no drive current is supplied to the electromagnetic coil, the first valve 71a and the second valve 71b are fully open (opening rate 100%), and the third valve 71c is fully closed (opening rate 0%), due to the built-in spring.

[0057] In step P1 of the process diagram (current supply start step), if the determination unit 106 detects that the start (activation) of water electrolysis by the water electrolysis device 10 was performed manually by the user or automatically by timer activation by the control unit 108, the control unit 108 starts supplying (powering) current I (I>0) from the power supply 90 to the water electrolysis stack 14 in step P1 and proceeds to step P2.

[0058] In process (startup process) P2, the electric pump 72 is driven.

[0059] As a result, the alkaline water AW, which is supplied with mechanical energy and flows, flows from the gas-liquid separator 70 through the water supply channel 52 into the water electrolysis stack 14 from the water inlet 30. The alkaline water AW that flows into the water electrolysis stack 14 flows through the water supply communication hole 34 and the gas-liquid diffusion layer 22 of the water electrolysis cell 12, and then reaches the water outlet 32 ​​through the water discharge communication hole 36. The alkaline water AW discharged from the water outlet 32 ​​returns to the gas-liquid separator 70 through the water discharge channel 60.

[0060] In this startup process P2, the opening degree of the third valve 71c is controlled by the control unit 108 to a fully closed state of 0%. Therefore, alkaline water AW is circulated and supplied from the liquid chamber of the gas-liquid separator 70 through the water supply channel 52, the gas-liquid diffusion layer 22 in the water electrolysis stack 14, and the water discharge channel 60 back to the gas-liquid separator 70.

[0061] When alkaline water AW flows through the gas-liquid diffusion layer 22 of the water electrolysis cell 12, the alkaline water AW passes through the anode electrode 23 and the anion exchange membrane (electrolyte membrane) 24 and reaches the cathode electrode 25.

[0062] In this case, at the cathode electrode 25, a chemical reaction (called the cathode reaction) shown in equation (1) occurs due to water and electric current (electrons), producing hydrogen and hydroxide ions. ２ O + 2e － →H ２ +2OH － …(1)

[0063] The hydrogen generated by the cathode reaction flows through the gas diffusion layer 26 and is supplied from the hydrogen outlet 38 through a hydrogen communication hole 37 that penetrates the radial center of the water electrolysis cell 12 along the stacking direction to a hydrogen tank (not shown) via a hydrogen discharge channel 40, a gas-liquid separator 42, and a check valve 46.

[0064] The hydroxide ions generated in the cathode reaction pass through the anion exchange membrane 24, which is moistened with alkaline water, and return to the anode electrode 23.

[0065] In this case, the chemical reaction shown in equation (2) (called the anodic reaction) occurs at the anode electrode 23, producing oxygen, water, and electrons. 2OH － → 1 / 2O２ +H ２ O + 2e － …(2)

[0066] The oxygen generated by the anodic reaction flows through the gas-liquid diffusion layer (also referred to as the water supply flow path) 22 as oxygen-bubbled alkaline water AWO and is discharged from the water outlet 32.

[0067] The oxygen-bubbled alkaline water AWO discharged from the water outlet 32 flows through the water discharge flow path 60 and is supplied to the gas-liquid separator 70. The gas-liquid separator 70 separates the oxygen-bubbled alkaline water AWO into oxygen as a gas component and alkaline water AW as a liquid component.

[0068] The separated oxygen is stored in an oxygen tank (not shown) from the gas chamber of the gas-liquid separator 70 via the oxygen discharge flow path 94 and the check valve 96.

[0069] The separated alkaline water AW is stored in the liquid chamber of the gas-liquid separator 70, discharged from the water outlet 70d to the water supply flow path 52, and circulated as described above.

[0070] In the startup process P2, the determination unit 106 determines whether the cell voltage Vs rising from 0 [V] or from a forced potential (described later) reaches a predetermined voltage Vth of about 2 [V], or whether a predetermined time has elapsed since the predetermined voltage Vth was reached.

[0071] When the determination unit 106 makes an affirmative determination (Vs = Vth or a predetermined time has elapsed since Vs = Vth), the control unit 108 determines that the water electrolysis stack 14 has reached a stable water electrolysis operating state, ends the startup process P2, and advances the process to the next process (water electrolysis operation process) P3.

[0072] The control (processing) of the water electrolysis operation process P3 will be described with reference to the schematic diagrams of FIGS. 3A to 3C (diagrams for explaining the problems during the operation of the water electrolysis device 10) and the schematic diagrams of FIGS. 4A to 4C (diagrams for explaining the solutions to the problems during the operation of the water electrolysis device 10).

[0073] As shown in Figure 3A, when all of the alkaline water AWO containing oxygen bubbles discharged from the water electrolysis stack 14 flows into the gas-liquid separator 70, oxygen is separated from the alkaline water AWO containing oxygen bubbles in the gas-liquid separator 70. As a result, all of the fluid supplied from the gas-liquid separator 70 to the water electrolysis stack 14 becomes alkaline water AW.

[0074] As shown in Figure 3B, the alkaline water AW that flows into the water electrolysis stack 14 flows into the gas-liquid diffusion layer 22 of each water electrolysis cell 12 through the water supply communication holes 34 of the water electrolysis cell 12. Note that the alkaline water AW is electrically conductive.

[0075] As a result, as shown in Figure 3B, the current I supplied from the power supply 90 flows through the water electrolysis cell 12 as the current Ii for water electrolysis, and at the same time, it can be divided and flow as the current Iex shown by the dashed line in the alkaline water AW flowing through the water supply communication hole 34.

[0076] Figure 3C schematically shows the state in which the current I flowing out from the power supply 90 is divided into a current Ii for water electrolysis and a current Iex (called leakage current) flowing through the water supply communication hole 34.

[0077] This leakage current Iex flows around the water electrolysis cell 12 and therefore does not contribute to water electrolysis, causing the power supply 90 to consume extra power. This reduces the power efficiency of the water electrolysis device 10.

[0078] To suppress this decrease in power efficiency, as shown in Figure 4A, in the water electrolysis operation process P3, the control unit 108 operates the valve of the three-way valve 71 to adjust so that a portion of the alkaline water AWO containing oxygen bubbles flowing into the first valve 71a of the three-way valve 71 is diverted to the bypass flow path 56 via the third valve 71c.

[0079] In other words, the control unit 108 can adjust the flow rate ratio of alkaline water AWO containing oxygen bubbles flowing through the second valve 71b and the third valve 71c by proportionally controlling the valve openings of the second valve 71b and the third valve 71c.

[0080] As a result, as shown in Figure 4A, the alkaline water AW supplied to the water electrolysis stack 14 from the water outlet 70d of the gas-liquid separator 70 through the water supply channel 52 is mixed with the alkaline water AWO containing oxygen bubbles flowing from the bypass channel 56.

[0081] In this way, alkaline water AWO containing oxygen bubbles is supplied to the water electrolysis stack 14.

[0082] At this time, as schematically shown by the dashed line with arrow in Figure 4A, the alkaline water AWO containing oxygen bubbles circulates through the water supply communication hole 34 in the water electrolysis stack 14, the gas-liquid diffusion layer 22, the water discharge communication hole 36, the water discharge channel 60, the three-way valve 71, the bypass channel 56, the water supply channel 52, and the water supply communication hole 34 in the water electrolysis stack 14 (referred to as the circulation path CR).

[0083] In this state, as shown in Figure 4B, alkaline water AWO containing oxygen bubbles also flows through the water supply communication hole 34 of the water electrolysis stack 14.

[0084] Alkaline water AW is conductive, but oxygen O ２ It is an insulator and has no conductivity.

[0085] Therefore, as shown in Figure 4C, during the water electrolysis operation process P3, the resistance value of the water supply communication hole 34 increases (shown by breaking a part of the flow path and resistor), preventing leakage current Iex from the power supply 90 from flowing through the water supply communication hole 34. As a result, the entire current I from the power supply 90 can be used for water electrolysis, thereby increasing power efficiency. Furthermore, when the cell voltage is high during startup, the leakage current Iex can be reduced by adjusting the opening degree of the third valve 71c to be greater than the opening degree of the second valve 71b compared to when the voltage is low.

[0086] Here, we will explain how to adjust (set) the opening degree of the third valve (return valve) 71c (the ratio of the amount of alkaline water AWO containing oxygen bubbles returned to the bypass channel 56) in the water electrolysis operation process P3.

[0087] During normal operation, if the power Ps supplied to the power supply 90 from an external source is small, or if the storage capacity of the hydrogen tank (not shown) and oxygen tank (not shown) is large, etc., and the water electrolysis device 10 is required to operate in a so-called "energy-saving, low-current-density manner," the leakage current Iex is suppressed as much as possible.

[0088] In this "energy-saving, low-current-density" operating state, the amount of alkaline water AW supplied to the water electrolysis stack 14 can be small, so the amount of alkaline water AWO containing oxygen bubbles supplied to the water electrolysis device 10 is increased. That is, the control unit 108 increases the opening ratio of the third valve (return valve) 71c of the three-way valve 71. The opening ratio of the second valve (return valve) 71b is decreased by the amount that the opening ratio is increased. The opening ratio of the return amount of the third valve 71c can be adjusted from 0 to 99%, and the opening ratio of the passage amount of the second valve 71b can be adjusted from 1 to 100%.

[0089] In contrast, when "high current density operation" is required during normal operation, a larger supply of alkaline water AW is needed. To prevent the cell voltage from overvoltage due to insufficient water supply, which would actually worsen power efficiency, the amount of alkaline water AWO containing oxygen bubbles is reduced, although a leakage current Iex is generated. Specifically, the control unit 108 reduces the opening ratio of the third valve (return valve) 71c of the three-way valve 71. As a result of reducing the opening ratio, the opening ratio of the second valve 71b increases.

[0090] During the water electrolysis operation process P3, the cell voltage of the water electrolysis cell 12 is constantly monitored by the determination unit 106. If, during constant monitoring, oxygen bubbles (air bubbles) adhere to the anode electrode 23, causing the cell voltage to drop and the potential difference between the water electrolysis cells 12 to change significantly beyond a predetermined level (the cell voltage of the water electrolysis cell 12 becomes greater than a threshold), or if the determination unit 106 determines that there is a water electrolysis cell 12 whose performance has deteriorated due to a lower cell voltage than the initial level, a recovery process may be performed.

[0091] In this recovery process, by intermittently opening the second valve 71b of the three-way valve 71, air bubbles adhering to the anode electrode 23, etc., can be removed by a stream of alkaline water AW or by bubbles (bubble stream) of alkaline water AWO containing oxygen bubbles. In this process, the location of bubble generation at a specific point (gas diffusion layer) on the anode electrode 23 may change, and in some cases, the performance degradation can be restored.

[0092] During the water electrolysis operation process P3, if the determination unit 106 determines that the amount of water stored in the hydrogen tank (not shown) and the oxygen tank (not shown) has reached a desired amount (a predetermined threshold), the control unit 108 proceeds to the current supply stop process P4.

[0093] In other words, the determination unit 106 continuously performs a current supply stop monitoring process P3' in parallel, which monitors whether or not the power supply from the power source 90 is stopped during the water electrolysis operation process P3.

[0094] When the determination unit 106 confirms that the power supply from the power source 90 has been stopped during the water electrolysis operation process P3 (current supply stop monitoring process P3'), the control unit 108 proceeds to process P4 (current supply stop process).

[0095] In the current supply stop step P4, the control unit 108 transitions the power supply 90 from the ON state to the OFF state, stops the supply of current I from the power supply 90 (stops power supply) (I=0), and proceeds to the stop step P5.

[0096] The following describes [Example 1] to [Example 3] of the stopping process P5. At the start of this stopping process P5, in other words, when the determination unit 106 detects that the power supply of the power supply 90 has been stopped, in [Example 1] to [Example 3], the control unit 108 executes a foam-containing alkaline water return process (process) in which a portion of the oxygen-containing alkaline water AWO flowing through the water discharge channel 60 is returned to the water supply channel 52 through the bypass channel 56.

[0097] [Example 1] Figure 5A is a time chart showing the time change of the cell voltage in Example 1 of the stopping process P5. Example 1 and Comparative Example 1 of the stopping process P5 will be explained in comparison with Figure 5A.

[0098] In Figure 5A, the solid line shows the time change of the cell voltage 201 in Example 1 of the stopping process P5, and the dashed line shows the time change of the cell voltage 301 in Comparative Example 1.

[0099] At time t0, the control unit 108 stops supplying current I from the power supply 90 to the water electrolysis stack 14 (I = 0) in order to stop the water electrolysis operation by the water electrolysis stack 14. At this time t0, since the current I becomes zero, the cell voltage 201 quickly drops to the equilibrium potential of the electrolysis reaction. After that, the hydrogen and oxygen generated by water electrolysis remain in the water electrolysis cell 12, so the cell voltage 201 quickly drops slightly, as it becomes similar to the open-circuit voltage of a fuel cell.

[0100] At time t0, in the water electrolysis apparatus according to Comparative Example 1, the electric pump 72 is stopped, and the supply of alkaline water AW to the water electrolysis stack 14 is stopped.

[0101] Figure 6A is a schematic circuit diagram relating to Comparative Example 1, illustrating the phenomenon in which a current flows in the reverse direction (referred to as reverse current I2) immediately after stopping, as indicated by the dashed line. The power supply 90 is in the off state.

[0102] At this time, as shown in Figure 6A, the water electrolysis cell 12 behaves as a battery (referred to as a deemed battery) due to its own capacitor component. The current I2 from this deemed battery circulates from the water electrolysis cell 12 to the water supply communication hole 34, so a current (reverse current I2) flows in the water electrolysis cell 12 in the opposite direction to that during water electrolysis.

[0103] Figure 6B shows an example of a situation where, if there is a difference in cell voltage due to differences in the state (voltage generation state) of the water electrolysis cells 12 immediately after stopping, a relatively large reverse current I2 may flow in an attempt to bridge the potential difference between each water electrolysis cell 12. In Figure 6B, the cell voltages are shown as 0.8 [V], 0.6 [V], and 1.0 [V] from top to bottom.

[0104] When a reverse current I2 flows, it can cause the catalyst to deteriorate, potentially leading to degradation of the cathode electrode 25 and the anode electrode 23.

[0105] Returning to Figure 5A, in the stopping process P5 according to Embodiment 1 shown by the solid line, in order to reduce or suppress the amount of deterioration caused by the generation of reverse current I2 due to the rapid decrease in cell potential, the driving state of the electric pump 72 is maintained and the opening ratio of the third valve 71c is increased during the water electrolysis stopping period from time (stopping time) t0 to time (starting time) t3.

[0106] As a result, when water electrolysis in the water electrolysis stack 14 stops, the alkaline water AWO containing oxygen bubbles circulates through the water electrolysis stack 14, maintaining a state in which a large amount of alkaline water AWO containing oxygen bubbles is present within the water electrolysis stack 14. The oxygen bubbles reduce the conductivity of the water supply communication holes 34, creating a state of interruption of the reverse current I2 caused by the flow of water through the water supply communication holes 34. In this case, since a large amount of oxygen remains in the gas-liquid diffusion layer 22, the time that oxygen remains on the anode electrode 23 in contact with the gas-liquid diffusion layer 22 is extended, slowing down the rate of potential decrease in the cell voltage, and consequently reducing the reverse current I2. Furthermore, the more oxygen, which is the energy source that generates the cell voltage, remains, the higher the cell potential is maintained.

[0107] At time t3, the control unit 108 starts supplying a current I (I>0) from the power supply 90 to the water electrolysis stack 14 in order to restart the water electrolysis operation by the water electrolysis stack 14.

[0108] Thus, in Example 1, while water electrolysis is stopped (water electrolysis of the water electrolysis stack 14 is stopped), the circulation of the oxygen-bubble-containing alkaline water AWO in the circulation path CR (see Figure 4A), which had been continuing until time t0, is continued from time t0 until time t3, when the water electrolysis operation is restarted, by driving the electric pump 72.

[0109] Therefore, even after the power supply from the power source 90 is stopped from time t0, oxygen bubbles remain in the water supply communication hole 34, the gas-liquid diffusion layer 22, and the water discharge communication hole 36, increasing the electrical resistance, which can suppress the decrease in the cell voltage 201 of the water electrolysis cell 12.

[0110] By starting water electrolysis at point t3 before the cell voltage 201 drops to a threshold voltage, for example, 0.5 [V] which does not cause a phase change in the catalyst, the deterioration of the anode electrode 23, cathode electrode 25, and anode separator 21 can be suppressed or prevented.

[0111] In contrast, in Comparative Example 1, as explained with reference to Figure 6A, during the current supply stop period (times t0 to t3 in Figure 5A), there is no circulation of the oxygen-bubble-containing alkaline water AWO in the circulation path CR, and a reverse current I2 flows until the cell voltage decreases. Therefore, for example, deterioration of electrodes and the like may occur after time t2 when the cell voltage drops to 0.5 [V].

[0112] [Example 2] Figure 5B is a time chart showing the time change of the cell voltage 202 in Example 2 of the stopping process P5. Example 2 and Comparative Example 2 of the stopping process P5 will be explained in comparison with Figure 5B. Note that explanations of operations that overlap with the explanations of operations in Example 1 and Comparative Example 1 will be omitted. The same applies below.

[0113] In Figure 5B, the solid line shows the time change of the cell voltage 202 in Example 2 of the stopping process P5, and the dashed line shows the time change of the cell voltage 302 in Comparative Example 2.

[0114] Figure 5B represents a case where water electrolysis by the water electrolysis stack 14 is stopped for a longer period of time compared to Figure 5A.

[0115] In the second embodiment of the stopping process P5 shown by the solid line in Figure 5B, in order to reduce or suppress the amount of degradation caused by the generation of reverse current I2 due to the rapid decrease in cell potential, the driving state of the electric pump 72 is maintained and the opening ratio of the third valve 71c is increased between time point t0 and time point ta. As a result, a large amount of alkaline water AWO containing oxygen bubbles is present in the water electrolysis stack 14. That is, the oxygen bubbles reduce the conductivity of the water supply communication hole 34, and a state of interruption of the reverse current I2 caused by the flow of reverse current I2 through the water supply communication hole 34 can be formed.

[0116] In this case, since a large amount of oxygen remains in the gas-liquid diffusion layer 22, the time that oxygen remains on the anode electrode 23 in contact with the gas-liquid diffusion layer 22 increases, the rate at which the cell voltage 202 decreases slows down, and consequently, the reverse current I2 becomes smaller. Furthermore, the more oxygen that is the energy source for generating the cell voltage remains, the higher the cell potential is maintained.

[0117] At time ta, when the start of re-electrolysis of water at time t3' (restart) is predicted, processing is performed to prepare for the next startup.

[0118] As a process to prepare for restarting, at time ta, the three-way valve 71 is operated to prevent the oxygen-bubble-containing alkaline water AWO from flowing into the bypass channel 56.

[0119] Between time points ta and tb, alkaline water AW is supplied from the gas-liquid separator 70 to the water electrolysis stack 14, and the alkaline water AW is circulated to sufficiently wet the anode electrode 23 and the anion exchange membrane 24 with alkaline water.

[0120] At time tb, the electric pump 72 is stopped to prepare for the next start at time t3'.

[0121] In the above-described embodiment 2, while suppressing a rapid drop in the cell voltage 202, preparations for starting up at the next time point t3' can be made from time point ta before the circulation stop time point tb.

[0122] [Example 3] Figure 7 is a time chart showing the time change of the cell voltage 203 in Example 3 of the stopping process P5. Example 3 and Comparative Example 3 of the stopping process P5 will be explained in comparison with Figure 7.

[0123] In the cell voltage 202 according to the above-described embodiment 2, some dissolution of the metallic material may occur when the cell voltage falls below 0.5 [V] during the period from time tb to time t3' (Figure 5B).

[0124] To prevent the dissolution of metal materials, the circulation of alkaline water AWO containing oxygen bubbles, which has been ongoing since before time t0 shown in Figure 7, is continued until time ta, when the potential drops to a predetermined level of 0.5 [V] or higher without a phase change. The predetermined potential is set by pre-determining a potential at which the cell voltage 203 does not exceed (fluctuate above) 0.5 [V] as much as possible.

[0125] During the predetermined time from time ta to time tb', the power supply 90 switches from supplying current to applying a voltage of the predetermined potential (corresponding to the forced potential described above). After the switch, the power supply 90 applies a predetermined voltage Vb to the water electrolysis stack 14, for example, by a voltage follower (buffer circuit) using an operational amplifier, such that the cell voltage 203 becomes 0.5 [V] or higher as a threshold voltage.

[0126] During the predetermined time period from time ta to time tb', the circulation of the oxygen-bubble-containing alkaline water AWO may be stopped or continued.

[0127] From time tb' onward, the cycle is stopped to prepare for the next startup at time t3''.

[0128] In this case, from time tb' onward, oxygen bubbles remain in the water supply communication holes 34, the gas-liquid diffusion layer 22, and the water discharge communication holes 36 within the water electrolysis stack 14. Therefore, theoretically, once a voltage is applied, the potential can be maintained, preventing a decrease in the cell voltage 203.

[0129] [Addendum] In addition to the above disclosure, the following addendum is disclosed.

[0130] (Note 1) The water electrolysis device (10) is a water electrolysis stack (14) comprising a plurality of stacked water electrolysis cells (12) in which an anode separator (21), a gas-liquid diffusion layer (22), an anode electrode (23), anion exchange membrane (24), a cathode electrode (25), a gas diffusion layer (26), and a cathode separator (27) are stacked in this order, and a water inlet (30) into which alkaline water (AW) is supplied, water supply communication holes (34) communicating with each other in the stacking direction and supplying the alkaline water supplied from the water inlet to one end of each gas-liquid diffusion layer, water discharge communication holes (36) communicating with each other in the stacking direction and supplying alkaline water with oxygen bubbles (AWO) produced by water electrolysis discharged from the other end of each gas-liquid diffusion layer, and a water outlet (32) communicating with the water discharge communication holes and supplied with alkaline water with oxygen bubbles from the water discharge communication holes, and the water A water electrolysis apparatus (10) comprising: a water discharge channel (60) communicating with the outside of the water electrolysis stack at the outlet and discharging the alkaline water containing oxygen bubbles; a gas-liquid separator (70) to which the alkaline water containing oxygen bubbles discharged from the water discharge channel is supplied and separated into oxygen and the alkaline water; a pump (72); a water supply channel (52) to which the alkaline water separated by the gas-liquid separator is circulated and supplied to the water inlet by the pump; and a power supply (90) to which current is supplied to the water electrolysis stack, wherein a bypass channel (56) is provided connecting the water discharge channel and the water supply channel, and when water electrolysis by the water electrolysis stack is stopped by stopping the supply of current from the power supply, a portion of the alkaline water containing oxygen bubbles flowing in the water discharge channel is returned to the water supply channel through the bypass channel by the pump.

[0131] This configuration makes it possible to suppress the deterioration of the anode and cathode electrodes of the water electrolysis cell when the water electrolysis device is shut down.

[0132] (Note 2) In the water electrolysis apparatus described in Note 1, a three-way valve (71) may be provided in the water discharge channel, wherein the first valve (71a) communicates with the water discharge channel, the second valve (71b) communicates with the gas-liquid separator, and the third valve (71c) communicates with the bypass channel.

[0133] With this configuration, when the water electrolysis of the water electrolysis stack is stopped, at least a portion of the alkaline water containing oxygen bubbles flowing through the water discharge channel can be easily returned to the water supply channel through the bypass channel.

[0134] (Note 3) In the water electrolysis apparatus described in Note 2, immediately after the supply of current is stopped, the flow rate of the alkaline water containing oxygen bubbles flowing out from the third valve may be made greater than the flow rate of the alkaline water containing oxygen bubbles flowing out through the second valve.

[0135] With this configuration, it is possible to suppress or prevent the flow of current (so-called reverse current) that would otherwise flow through the water supply communication hole that supplies alkaline water to the water electrolysis cell.

[0136] (Note 4) In the water electrolysis apparatus described in Note 2 or 3, when the supply of current is stopped and the cell voltage of the water electrolysis cell drops to a threshold voltage while the third valve is open, the third valve is closed, and with the third valve closed, the alkaline water supplied from the gas-liquid separator is circulated for a predetermined time (from time ta to time tb in Figure 5B) through the gas-liquid separator, the water supply channel, the water supply communication hole, the gas-liquid diffusion layer, and the water discharge channel, and then the circulation of the alkaline water is stopped (from time tb to time t3' in Figure 5B).

[0137] In this way, in order to ensure good water electrolysis in the next step (from time t3' in Figure 5B), alkaline water (alkaline water without oxygen bubbles) can be circulated to wet the anode electrode, anion exchange membrane, and cathode electrode, and to drive bubbles away from the anode electrode (from time ta to time tb in Figure 5B).

[0138] (Note 5) In the water electrolysis apparatus described in Note 2 or 3, when the supply of current is stopped and the cell voltage of the water electrolysis cell drops to a threshold voltage while the third valve is open (time ta in Figure 7), the threshold voltage may be forcibly applied from the power supply to the water electrolysis cell during the period until the next water electrolysis starts (time ta to time t3'' in Figure 7).

[0139] With this configuration, oxygen bubbles remain in the water supply communication holes, gas-liquid diffusion layer, and water discharge communication holes within the water electrolysis stack. Theoretically, once a voltage is applied, the potential can be maintained, preventing a decrease in cell voltage.

[0140] (Note 6) The control method for the water electrolysis apparatus is a water electrolysis stack (14) comprising a plurality of stacked water electrolysis cells (12) in which an anode separator (21), a gas-liquid diffusion layer (22), an anode electrode (23), anion exchange membrane (24), a cathode electrode (25), a gas diffusion layer (26), and a cathode separator (27) are stacked in this order, a water inlet (30) into which alkaline water (AW) is supplied, water supply communication holes (34) communicating with each other in the stacking direction and supplying the alkaline water supplied from the water inlet to one end of each gas-liquid diffusion layer, water discharge communication holes (36) communicating with each other in the stacking direction and supplying alkaline water with oxygen bubbles (AWO) produced by water electrolysis discharged from the other end of each gas-liquid diffusion layer, and a water outlet (32) communicating with the water discharge communication holes and supplied with alkaline water with oxygen bubbles from the water discharge communication holes, and a water outlet (32) communicating with the water outlet on the outside of the water electrolysis stack, which supplies the alkaline water with oxygen bubbles A control method for a water electrolysis apparatus (10) comprising: a water discharge channel (60) for discharging potassium water; a gas-liquid separator (70) to which the alkaline water containing oxygen bubbles discharged from the water discharge channel is supplied and separated into oxygen and the alkaline water; a pump (72); a water supply channel (52) for which the alkaline water separated by the gas-liquid separator is circulated and supplied to the water inlet by the pump; and a power supply (90) for supplying current to the water electrolysis stack, further comprising: a bypass channel (56) connecting the water discharge channel and the water supply channel; a current supply stop monitoring step (P3') for monitoring whether or not the supply of current from the power supply is stopped; and, when it is confirmed that the supply of current has been stopped, an oxygen-bubble alkaline water return step for which a portion of the alkaline water containing oxygen bubbles circulating in the water discharge channel is returned to the water supply channel through the bypass channel by the pump.

[0141] This process makes it possible to suppress the deterioration of the anode and cathode electrodes of the water electrolysis cell when the water electrolysis device is shut down.

[0142] This disclosure is not limited to the individual embodiments described above. These embodiments can be added, replaced, modified, partially deleted, etc., in any way that does not depart from the gist of this disclosure or from the spirit of this disclosure derived from the claims and their equivalents. These embodiments can also be implemented in combination. For example, the order of operations and processes in the embodiments described above are given as examples only and are not limited thereto. The same applies when numerical values ​​or mathematical formulas are used in the description of the embodiments described above.

[0143] 10...Water electrolysis device 12...Water electrolysis cell 14...Water electrolysis stack 21...Anode separator 22...Gas-liquid diffusion layer 23...Anode electrode 24...Anion exchange membrane 25...Cathode electrode 26...Gas diffusion layer 27...Cathode separator 30...Water inlet 32...Water outlet 34...Water supply communication hole 36...Water discharge communication hole 37...Hydrogen communication hole 38...Hydrogen outlet 42, 70...Gas-liquid separator 70a...Fluid inlet 70b...Water inlet 70c...Oxygen outlet 70d...Water outlet 71...Three-way valve 71a...First valve 71b...Second valve 71c...Third valve 72...Electric pump 74...Check valve 90...Power supply 100...Control device 102...Calculation unit 104...Storage unit 105, 106...Determination unit 108... Control Unit

Claims

1. A water electrolysis stack (14) comprising multiple stacks of water electrolysis cells (12) in which an anode separator (21), a gas-liquid diffusion layer (22), an anode electrode (23), anion exchange membrane (24), a cathode electrode (25), a gas diffusion layer (26), and a cathode separator (27) are stacked in this order, and comprising: a water inlet (30) into which alkaline water (AW) is supplied; water supply communication holes (34) communicating with each other in the stacking direction and supplying the alkaline water supplied from the water inlet to one end of each gas-liquid diffusion layer; water discharge communication holes (36) communicating with each other in the stacking direction and supplying alkaline water with oxygen bubbles (AWO) produced by water electrolysis discharged from the other end of each gas-liquid diffusion layer; and a water outlet (32) communicating with the water discharge communication holes and supplied with alkaline water with oxygen bubbles from the water discharge communication holes, A water electrolysis apparatus (10) comprising: a water discharge channel (60) communicating with the water outlet on the outside of the water electrolysis stack for discharging the alkaline water containing oxygen bubbles; a gas-liquid separator (70) to which the alkaline water containing oxygen bubbles discharged from the water discharge channel is supplied and separated into oxygen and the alkaline water; a pump (72); a water supply channel (52) for circulating the alkaline water separated by the gas-liquid separator to the water inlet by the pump; and a power supply (90) for supplying current to the water electrolysis stack, wherein a bypass channel (56) is provided connecting the water discharge channel and the water supply channel, and when water electrolysis by the water electrolysis stack is stopped by stopping the supply of current from the power supply, a portion of the alkaline water containing oxygen bubbles flowing in the water discharge channel is returned to the water supply channel through the bypass channel by the pump.

2. A water electrolysis apparatus according to claim 1, wherein a three-way valve (71) is provided in the water discharge channel, the first valve (71a) communicating with the water discharge channel, the second valve (71b) communicating with the gas-liquid separator, and the third valve (71c) communicating with the bypass channel.

3. A water electrolysis apparatus according to claim 2, wherein immediately after the supply of current is stopped, the ratio of the flow rate of the alkaline water containing oxygen bubbles flowing out from the third valve is made greater than the ratio of the flow rate of the alkaline water containing oxygen bubbles flowing out from the second valve.

4. A water electrolysis apparatus according to claim 2 or 3, wherein when the supply of current is stopped and the cell voltage of the water electrolysis cell drops to a threshold voltage while the third valve is open, the third valve is closed, and with the third valve closed, the alkaline water supplied from the gas-liquid separator is circulated for a predetermined time through the gas-liquid separator, the water supply channel, the water supply communication hole, the gas-liquid diffusion layer and the water discharge channel, and then the circulation of the alkaline water is stopped.

5. A water electrolysis apparatus according to claim 2 or 3, wherein when the supply of current is stopped and the cell voltage of the water electrolysis cell falls to a threshold voltage while the third valve is open, the threshold voltage is forcibly applied from the power supply to the water electrolysis cell for a period until the next water electrolysis is started.

6. A water electrolysis stack (14) comprising multiple stacks of water electrolysis cells (12) in which an anode separator (21), a gas-liquid diffusion layer (22), an anode electrode (23), anion exchange membrane (24), a cathode electrode (25), a gas diffusion layer (26), and a cathode separator (27) are stacked in this order, and comprising: a water inlet (30) into which alkaline water (AW) is supplied; water supply communication holes (34) communicating with each other in the stacking direction and supplying the alkaline water supplied from the water inlet to one end of each gas-liquid diffusion layer; water discharge communication holes (36) communicating with each other in the stacking direction and supplying alkaline water with oxygen bubbles (AWO) produced by water electrolysis discharged from the other end of each gas-liquid diffusion layer; and a water outlet (32) communicating with the water discharge communication holes and supplied with alkaline water with oxygen bubbles from the water discharge communication holes, A control method for a water electrolysis apparatus (10) comprising: a water discharge channel (60) communicating with the water outlet on the outside of the water electrolysis stack for discharging the alkaline water containing oxygen bubbles; a gas-liquid separator (70) to which the alkaline water containing oxygen bubbles discharged from the water discharge channel is supplied and separated into oxygen and the alkaline water; a pump (72); a water supply channel (52) for circulating the alkaline water separated by the gas-liquid separator to the water inlet by the pump; and a power supply (90) for supplying current to the water electrolysis stack, further comprising: a bypass channel (56) connecting the water discharge channel and the water supply channel; a current supply stop monitoring step (P3') for monitoring whether or not the supply of current from the power supply is stopped; and an oxygen-bubble alkaline water return step for returning a portion of the alkaline water containing oxygen bubbles circulating in the water discharge channel to the water supply channel through the bypass channel by the pump when it is confirmed that the supply of current has been stopped.

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