WATER ELECTROLYSIS SYSTEM

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2024-166891 filed on Sep. 26, 2024. The disclosure of the above-identified application, including the specification, drawings, and claims, is incorporated by reference herein in its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates to a water electrolysis system.

2. Description of Related Art

Various techniques related to water electrolysis have been proposed, such as those disclosed in Japanese Unexamined Patent Application Publication No. 2020-143346 (JP 2020-143346 A), Japanese Unexamined Patent Application Publication No. 2012-001745 (JP 2012-001745 A), and Japanese Unexamined Patent Application Publication No. 2024-073918 (JP 2024-073918 A).

SUMMARY

JP 2020-143346 A describes a technique aimed at reducing deterioration of a membrane electrode assembly and a decrease in the efficiency of a water electrolysis device due to insufficient water in a water electrolysis cell. Specifically, a water electrolysis device is described in which the amount of water in a water electrolysis cell is estimated from the impedance of a membrane electrode assembly, and the amount of water to be supplied to the water electrolysis cell is adjusted using the estimated water amount.

However, the researchers of the present disclosure found that a state of temporarily and locally insufficient water that is not detectable in the impedance measurements occurs in the water electrolysis cell. Moreover, the state of insufficient water was found to occur due to bubble clogging that occurs temporarily and locally in a reaction water channel in the water electrolysis cell and hinders the flow of reaction water. Even though the insufficient water is temporary and local, accumulation of such insufficient water causes deterioration of the electrolyte membrane, catalyst layer, mass transport layer, gas diffusion layer, and the like of the water electrolysis cell.

The present disclosure has been made in consideration of the above circumstances, and a main object of the present disclosure is to provide a water electrolysis system capable of reducing the occurrence of the state of insufficient water in a water electrolysis cell.

That is, the present disclosure includes the following aspects.

• • (1) A water electrolysis system includes:
    • a water electrolysis device configured to perform water electrolysis;
    • a water supply device configured to supply water to the water electrolysis device;
    • a power supply configured to supply current to the water electrolysis device; and
    • a control unit.

The control unit is configured to adjust a current density of the current supplied from the power supply to the water electrolysis device, and

• • adjust a water flow rate of the water supplied from the water supply device to the water electrolysis device.

The control unit includes a data group indicating a target operating state of the water electrolysis device that is determined from the water flow rate and the current density.

The control unit is configured to: measure the water flow rate and the current density during operation of the water electrolysis device;

• • check the water flow rate and the current density during the operation of the water electrolysis device against the data group, and determine whether the water flow rate and the current density are outside respective threshold ranges corresponding to the target operating state; and
  • perform an operation change when at least one of the water flow rate and the current density during the operation of the water electrolysis device is outside a corresponding one of the threshold ranges.
  • (2) In the water electrolysis system according to (1), the control unit is configured to perform a stop control, as the operation change, that sets the current density to zero.
  • (3) In the water electrolysis system according to (1), the control unit is configured to control, as the operation change, the at least one of the water flow rate and the current density such that the water electrolysis device is in the target operating state.

According to the water electrolysis system of the present disclosure, it is possible to reduce the occurrence of the state of insufficient water in the water electrolysis cell.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like signs denote like elements, and wherein:

FIG. 1 is a graph illustrating the relationship among current density, the water flow rate, and the occurrence rate of bubble clogging obtained by a bubble visualization test on a water electrolysis cell;

FIG. 2 is a block diagram illustrating an example of a water electrolysis system of the present disclosure; and

FIG. 3 is a flowchart illustrating an example of the water electrolysis system according to the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

An embodiment according to the present disclosure will be described below. It should be noted that matters other than those specifically mentioned in the present specification and necessary to carry out the present disclosure (for example, general configurations and manufacturing processes of a water electrolysis system that do not characterize the present disclosure) may be regarded as design matters for those skilled in the art based on the related art in the field. The present disclosure may be carried out based on the content disclosed in the present specification and the common general technical knowledge in the field.

The dimensional relationships (length, width, thickness, etc.) in the drawings do not reflect the actual dimensional relationships.

The present disclosure provides a water electrolysis system including: a water electrolysis device configured to perform water electrolysis; a water supply device configured to supply water to the water electrolysis device; a power supply configured to supply current to the water electrolysis device; and a control unit. The control unit is configured to adjust a current density of the current supplied from the power supply to the water electrolysis device, and adjust a water flow rate of the water supplied from the water supply device to the water electrolysis device. The control unit includes a data group indicating a target operating state of the water electrolysis device that is determined from the water flow rate and the current density. The control unit is configured to: measure the water flow rate and the current density during operation of the water electrolysis device; check the water flow rate and the current density during the operation of the water electrolysis device against the data group, and determine whether the water flow rate and the current density are outside respective threshold ranges corresponding to the target operating state; and perform an operation change when at least one of the water flow rate and the current density during the operation of the water electrolysis device is outside a corresponding one of the threshold ranges.

The researchers discovered that a phenomenon occurs in which the flow of reaction water inside the water electrolysis cell becomes temporarily clogged with bubbles, and that the bubble clogging is related to the water flow rate and the current density of the water electrolysis cell. It is believed that such bubble clogging occurs because the balance between the amount of bubbles generated by the water electrolysis reaction and the flow rate and water pressure of the reaction water (liquid water) flowing through the channels in the water electrolysis cell causes the generated bubbles to remain in the channels and further grow into larger bubbles. Temporary bubble clogging causes a state of temporarily and locally insufficient water in the water electrolysis cell. Repeated bubble clogging leads to deterioration of the water electrolysis cell. Although it is important to reduce the occurrence of the temporary bubble clogging, such clogging is not detectable by impedance measurement or voltage monitoring.

Therefore, the inside of the water electrolysis cell (reaction water channel) was visualized, and temporary bubble clogging was observed by changing the current density and the water flow rate of the water electrolysis cell. Specifically, the bubble visualization test was performed 450 times in the following steps (1) to (3).

• • (1) The current density and the water flow rate of the water electrolysis cell are specified and water electrolysis is started.
  • (2) The reaction water channel of the water electrolysis cell was recorded using a camera for 30 seconds.
  • (3) When bubbles remain in the channel, count the clogging as one.

The occurrence rate of bubble clogging in the channel was calculated from the results of the test performed 450 times. The results are shown in FIG. 1. In FIG. 1, each circle represents the frequency of bubble clogging under a corresponding operating condition. A larger circle represents a higher frequency of bubble clogging and a smaller circle represents a lower frequency of bubble clogging.

It was confirmed from FIG. 1 that no bubble clogging occurs within the ranges of 1 A/cm²≤current density≤5 A/cm² and 60≤water flow rate≤120 (within the rectangular frame in FIG. 1). On the other hand, it can be seen that the range outside the rectangular frame in FIG. 1 is a region where the frequency of bubble clogging is relatively high.

Here, the water flow rate refers to the amount of water supplied to the water electrolysis device. The water flow rate is typically adjusted to obtain a desired current density. The theoretical value of the water flow rate (stoichiometric ratio=1) is determined according to the stoichiometric ratio from the water electrolysis reaction (2H₂O→O₂+4H⁺+4e⁻) that proceeds on an oxygen electrode side. In practice, however, the theoretical value of the water flow rate is not enough to obtain the desired current density, and therefore an excess amount of water is generally supplied. The above range of 60≤water flow rate≤120 means that the water flow rate is within the range of 60 to 120 times the theoretical value. In conventional water electrolysis devices, the current is usually controlled so that the current density is constant during operation, and the water flow rate is also controlled to be constant. However, when the water electrolysis device is started up or stopped, the balance between the current and the water flow rate is lost, making it easy for partial bubble clogging to occur. Even during normal operation, the balance between the current density and the water flow rate may be lost.

The water electrolysis system disclosed in the present disclosure is provided based on the above findings, and controls the water flow rate and current density of the water electrolysis device so as to achieve target operating states that avoid a state of insufficient water. According to the water electrolysis system of the present disclosure, it is possible to reduce the occurrence of the state of insufficient water in the water electrolysis cell. Therefore, according to the present disclosure, it is possible to reduce deterioration of members included in the water electrolysis cell, such as an electrolyte membrane, a catalyst layer, a mass transport layer, and a gas diffusion layer.

The water electrolysis system according to the present disclosure will be described using a water electrolysis system 100 that is one embodiment. FIG. 2 illustrates a block diagram of the water electrolysis system 100.

The water electrolysis system 100 includes a water electrolysis device 10, an oxygen electrode side piping portion 20 disposed on the oxygen electrode side of the water electrolysis device 10, a hydrogen electrode side piping portion 30 disposed on the hydrogen electrode side of the water electrolysis device 10, a power supply 40 that supplies current to the water electrolysis device 10, and a control unit 50.

Water Electrolysis Device 10

The water electrolysis device 10 is a device that performs water electrolysis. The configuration of the water electrolysis device 10 is known. An example of the water electrolysis device 10 will be described below.

The water electrolysis device 10 includes a water electrolysis cell. Typically, the water electrolysis device 10 includes a water electrolysis stack with a plurality of stacked water electrolysis cells. The water electrolysis device 10 also includes a terminal that is connectable to the power supply 40.

The water electrolysis cell can electrolyze water to produce hydrogen and oxygen. The water electrolysis cell includes an oxygen electrode and a hydrogen electrode. Oxygen is generated from the oxygen electrode and hydrogen is generated from the hydrogen electrode when water is supplied and voltage is applied to the oxygen electrode of the water electrolysis cell. The type of water electrolysis cell is not particularly limited, but from the viewpoint of improving water electrolysis efficiency, a PEM (polymer electrolyte membrane) water electrolysis cell can be adopted.

Hereinafter, a configuration of the PEM water electrolysis cell will be briefly described.

The PEM water electrolysis cell includes a membrane electrode assembly and a pair of separators (an oxygen electrode side separator and a hydrogen electrode side separator). The respective separators are disposed on both sides of the membrane electrode assembly.

The membrane electrode assembly has an electrolyte membrane, an oxygen electrode disposed on one side of the electrolyte membrane, and a hydrogen electrode disposed on the other side of the electrolyte membrane.

The oxygen electrode includes an oxygen electrode catalyst layer, and may include an oxygen electrode side porous transport layer (oxygen electrode side PTL) between the oxygen electrode catalyst layer and the oxygen electrode side separator.

The hydrogen electrode includes a hydrogen electrode catalyst layer, and may include a hydrogen electrode side porous transport layer (hydrogen electrode side PTL) between the hydrogen electrode catalyst layer and the hydrogen electrode side separator.

The electrolyte membrane may be one having proton conductivity. Proton conducting membranes include, for example, those containing proton conducting polymers. Examples of the proton conducting polymers include fluorine-based polymers having sulfonic acid groups, such as perfluoroalkylsulfonic acid polymers.

The oxygen electrode catalyst layer contains an oxygen electrode catalyst capable of producing oxygen by water electrolysis. The hydrogen electrode catalyst layer contains a hydrogen electrode catalyst capable of producing hydrogen by water electrolysis. The oxygen electrode catalyst and the hydrogen electrode catalyst are not particularly limited, but examples thereof include metal catalysts. Examples of the metal catalysts include metal catalysts that contain at least one of Pt, Ru, Rh, Os, Ir, Pd, and Au in their composition. The metal catalysts may be oxides of the metals above. The oxygen electrode catalyst and the hydrogen electrode catalyst may be a metal-supported catalyst in which a metal catalyst is supported on an electrically conductive support.

The oxygen electrode catalyst layer and the hydrogen electrode catalyst layer may further contain an electrolyte having proton conductivity. As the electrolyte, the same electrolyte as that constituting the electrolyte membrane can be used.

The oxygen electrode side PTL and the hydrogen electrode side PTL serve to facilitate the supply and discharge of water to and from the oxygen electrode side and the hydrogen electrode side, as well as the discharge of generated oxygen gas and hydrogen gas. The PTL is formed, for example, from a porous titanium sheet, such as titanium mesh.

The separators are respectively disposed on both sides of the membrane electrode assembly. The separator is formed from a conductive member. Examples of the conductive member include resins containing carbon materials; and metal materials, such as iron, copper, stainless steel, and titanium.

A predetermined channel is provided on the surface of the separator facing the catalyst layer, and the channel serves to guide water supplied to the water electrolysis cell, and oxygen or hydrogen produced by the water electrolysis reaction. The channel structure of the separator is not particularly limited, and the shape of the channel may be, for example, a straight channel, a curved channel, and a wavy channel.

Oxygen Electrode Side Piping Portion 20

The oxygen electrode side piping portion 20 serves to supply water to the oxygen electrode of the water electrolysis device 10. As shown in FIG. 2, the oxygen electrode side piping portion 20 includes a water supply device 21, a water supply channel 22, an oxygen electrode side gas-liquid separator 23, a water discharge channel 24, a circulation channel 25, and a discharge channel 26.

The water supply device 21 is a device that supplies reaction water to the oxygen electrode of the water electrolysis device 10. The water may be supplied to the water electrolysis device 10 under pressure so as to circulate the water. The water supply device 21 may include, for example, a reaction water storage tank, a reaction water pump, and an ion exchanger.

The water supply channel 22 is a pipe that connects the water electrolysis device 10 and the water supply device 21 and supplies water from the water supply device 21 to the water electrolysis device 10.

The water discharge channel 24 connects the water electrolysis device 10 and the oxygen electrode side gas-liquid separator 23, and is a pipe that allows water and oxygen discharged from the oxygen electrode of the water electrolysis device 10 to flow into the oxygen electrode side gas-liquid separator 23.

The oxygen electrode side gas-liquid separator 23 is a device that separates water and oxygen discharged from the oxygen electrode of the water electrolysis device 10.

The water separated by the oxygen electrode side gas-liquid separator 23 is sent to the water supply device 21 via the circulation channel 25 connecting the water supply device 21 and the oxygen electrode side gas-liquid separator 23, and is reused for the water electrolysis reaction. The circulation channel 25 is used when water is circulated in the oxygen electrode side piping portion 20. Therefore, the circulation channel 25 is unnecessary when water is not circulated.

On the other hand, the oxygen separated by the oxygen electrode side gas-liquid separator 23 is discharged to the outside via the discharge channel 26.

Hydrogen Electrode Side Piping Portion 30

The hydrogen electrode side piping portion 30 serves to recover hydrogen generated at the hydrogen electrode of the water electrolysis device 10. As shown in FIG. 2, the hydrogen electrode side piping portion 30 includes a hydrogen electrode side gas-liquid separator 31, a hydrogen discharge channel 32, a hydrogen tank 33, and a hydrogen supply channel 34.

The hydrogen discharge channel 32 connects the water electrolysis device 10 and the hydrogen electrode side gas-liquid separator 31, and is a pipe that allows water and hydrogen discharged from the hydrogen electrode of the water electrolysis device 10 to flow into the hydrogen electrode side gas-liquid separator 31.

The hydrogen electrode side gas-liquid separator 31 is a device that separates water and hydrogen discharged from the hydrogen electrode of the water electrolysis device 10. The reaction water is supplied to at least the oxygen electrode of the water electrolysis device 10, but the water may permeate the membrane electrode assembly and leak to the hydrogen electrode side. For this reason, the gas-liquid separator is also provided in the hydrogen electrode side piping portion 30.

The hydrogen separated by the hydrogen electrode side gas-liquid separator 31 is sent to the hydrogen tank 33 via the hydrogen supply channel 34 connecting the hydrogen electrode side gas-liquid separator 31 and the hydrogen tank 33, and is stored in the hydrogen tank 33.

The water separated by the hydrogen electrode side gas-liquid separator 31 is discharged as appropriate.

Power Supply 40

The power supply 40 is for supplying current to the water electrolysis device 10, and is connected to both of the oxygen electrode and the hydrogen electrode of the water electrolysis device 10. The power supply 40 as described above is a known power supply. Water electrolysis occurs by supplying water to the water electrolysis device 10 and passing current from the power supply 40.

Control Unit 50

The control unit 50 is a computer system including a CPU, a RAM, an input/output interface, and the like.

The control unit 50 is electrically connected to the power supply 40 and adjusts the current density of current supplied from the power supply 40 to the water electrolysis device 10.

The control unit 50 is also electrically connected to the water supply device 21 and adjusts the water flow rate of water supplied from the water supply device 21 to the water electrolysis device 10.

Furthermore, the control unit 50 includes a data group indicating a target operating state of the water electrolysis device 10 that is determined from the water flow rate and the current density. This data group is obtained in advance from evaluation tests or evaluation simulations of the water electrolysis device based on the configuration of the water electrolysis device 10 and the water electrolysis system 100 and taking into consideration the required current density, etc. For example, by performing the above bubble visualization test in advance, it is possible to identify ranges of the water flow rate and current density in which bubble clogging is unlikely to occur, and to use the identified ranges as a data group indicating a target operating state of the water electrolysis system 100. The data group may be obtained in advance by testing, etc., since it differs depending on the channel shape (straight channel, curved channel, wavy channel, etc.) of the reaction water channel provided in the separator of the oxygen electrode and the material forming the channel. A plurality of data groups may be prepared according to the operating conditions.

The control unit 50 measures the water flow rate and the current density during operation of the water electrolysis device 10. The water flow rate of the water electrolysis device 10 can be directly measured by a flow meter provided in the water electrolysis device 10 or a flow meter provided in the water supply device 21, or can be indirectly measured by inference from the operating state of the reaction water pump. The current density of the water electrolysis device 10 can be measured by a current measuring device provided in the water electrolysis device 10, a current measuring device provided in the power supply 40, or the like.

Next, the control unit 50 checks the measured water flow rate and the measured current density against the above data group, and determines whether they are outside respective threshold ranges corresponding to the target operating state.

When the determination result indicates that at least one of the water flow rate and the current density of the water electrolysis device 10 is outside a corresponding one of the threshold ranges, the control unit 50 performs an operation change of the water electrolysis device 10. In other words, the operation change of the water electrolysis device 10 is performed when the current density of the water electrolysis device 10 is outside the threshold range of the target operating state even though the water flow rate of the water electrolysis device 10 is within the threshold range. The operation change of the water electrolysis device 10 is performed when the water flow rate of the water electrolysis device 10 is outside the threshold range of the target operating state even though the current density of the water electrolysis device 10 is within the threshold range. The operation change is also performed when both of the water flow rate and the current density of the water electrolysis device 10 are outside the respective threshold ranges.

On the other hand, when the determination result indicates that both of the water flow rate and the current density of the water electrolysis device 10 are within the respective threshold ranges (target operating state), the measurement of the water flow rate and the current density is continued and the water flow rate and the current density are monitored.

The operation change means changing operating conditions to achieve an operating state in which bubble clogging in the channels of the water electrolysis device 10 as described above is reduced or does not occur. Examples of the operation change include a change in the amount of current supplied to the water electrolysis device 10, a change in the water flow rate of the water supply device 21, and the like.

For example, at a timing when the operation of the water electrolysis device 10 is about to be stopped, the control unit 50 performs stop control, as the operation change, that stops the supply of current from the power supply 40 to the water electrolysis device 10 and sets the current density to zero. This stops the progress of water electrolysis reaction, and therefore the generation of bubbles also stops. On the other hand, the control unit 50 continues the supply of water from the water supply device 21 to the water electrolysis device 10. This allows oxygen gas to be discharged from the channel of the water electrolysis device 10, and allows avoiding the occurrence of bubble clogging when the device is started up next time. An example of the timing at which the operation of the water electrolysis device 10 is about to be stopped is when an operation to stop the operation of the water electrolysis device 10 is being performed.

During the start-up or normal operation of the water electrolysis device 10, the control unit 50 controls, as the operation change, at least one of the water flow rate and the current density such that the water electrolysis device 10 achieves the target operating state. That is, the water flow rate and/or the current density that are determined to be outside the threshold ranges corresponding to the target operating state are controlled such that the water electrolysis device 10 achieves the target operating state. After the operation change, the water flow rate and the current density are again measured and monitored.

Here, the control of the water flow rate and the current density by the control unit 50 during operation of the water electrolysis device 10 will be described with reference to FIG. 3. FIG. 3 is a flowchart illustrating an example of the control over the water flow rate and the current density of the water electrolysis device 10 by the control unit 50.

The flowchart in FIG. 3 illustrates control over the water flow rate and the current density of the water electrolysis device 10 based on the target operating state obtained from the results of the bubble visualization test illustrated in FIG. 1. That is, the target operating state of the water electrolysis device 10 is 1 A/cm²≤current density≤5 A/cm² and 60≤water flow rate≤120.

First, the water flow rate and the current density of the water electrolysis device 10 during operation is monitored (measured) (STEP 1).

Next, for the measured water flow rate and the measured current density, it is determined whether at least one of the following conditions (1) and (2) is satisfied (STEP 2).

• • Condition (1): water flow rate<60 or water flow rate>120
  • Condition (2): current density<1 A/cm² or current density>5 A/cm²

When none of the conditions (1) and (2) is satisfied, this means that the water flow rate and the current density of the water electrolysis device 10 are in the target operating state. When none of the conditions (1) and (2) is satisfied, the process returns to STEP 1, and the monitoring of the water flow rate and the current density is continued.

Satisfying the condition (1) means that the water flow rate of the water electrolysis device 10 is outside the threshold range corresponding to the target operating state.

Satisfying the condition (2) means that the current density of the water electrolysis device 10 is outside the threshold range corresponding to the target operating state.

When either one of the conditions (1) and (2) is satisfied, or when both of the conditions (1) and (2) are satisfied, it is determined whether the water electrolysis device 10 has received a termination operation (operation to stop the operation) (STEP 3).

When it is determined that a termination operation has been received, the current to the water electrolysis device 10 is turned off and the current density is set to zero (STEP 4). As mentioned above, the water flow rate is not set to zero.

When it is determined that a termination operation has not been received, at least one of the water flow rate and the current density of the water electrolysis device 10 is controlled such that a corresponding one of the following conditions (3) and (4) is satisfied (STEP 5).

• • Condition (3): 60≤water flow rate≤120
  • Condition (4): 1 A/cm²≤current density≤5 A/cm²

When it is determined in STEP 2 that both of the conditions (1) and (2) are satisfied, then in STEP 5, both of the water flow rate and the current density are controlled such that the conditions (3) and (4) are satisfied.

When it is determined in STEP 2 that only the condition (1) is satisfied, then in STEP 5, the water flow rate is controlled such that the condition (3) is satisfied.

When it is determined in STEP 2 that only the condition (2) is satisfied, then in STEP 5, the current density is controlled such that the condition (4) is satisfied.

After the control in STEP 5, the process returns to STEP 1 to continue monitoring the water flow rate and the current density.