Water electrolysis system

Abstract

This water electrolysis system provides a solution that can suppress the occurrence of water shortages in water electrolysis cells. [Solution] The water electrolysis system comprises a water electrolysis device, a water supply device that supplies water to the water electrolysis device, a power supply that supplies current to the water electrolysis device, and a control unit. The control unit adjusts the current density of the supplied current and the flow rate of the supplied water. The control unit includes a data set indicating the target operating state of the water electrolysis device, which is determined from the flow rate and current density. The control unit measures the flow rate and current density when the water electrolysis device is in operation. The control unit determines whether the flow rate and current density during operation of the water electrolysis device are outside the range of the respective thresholds corresponding to the target operating state, in comparison with the data set. If at least one of the flow rate and current density during operation of the water electrolysis device is outside the range of the threshold, the control unit changes the operation.

Description

[Technical Field]

[0001] This disclosure relates to a water electrolysis system. [Background technology]

[0002] Various technologies have been proposed for water electrolysis, as disclosed in Patent Documents 1 to 3. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-143346 [Patent Document 2] Japanese Patent Publication No. 2012-001745 [Patent Document 3] Japanese Patent Publication No. 2024-073918 [Overview of the project] [Problems that the invention aims to solve]

[0004] Patent Document 1 describes a technology aimed at suppressing the deterioration of the membrane electrode assembly and the decrease in efficiency of the water electrolysis device due to insufficient water in the water electrolysis cell. Specifically, it describes a water electrolysis device that estimates the amount of water in the water electrolysis cell based on the impedance of the membrane electrode assembly and adjusts the amount of water supplied to the water electrolysis cell using the estimated amount of water. However, this researcher discovered that temporary and localized water shortages, undetectable by impedance measurements, can occur within water electrolysis cells. Furthermore, it was found that these water shortages are caused by temporary and localized blockages of bubbles in the reaction water channels within the water electrolysis cell, which obstructs the flow of reaction water. Even temporary and localized water shortages, when accumulated, can cause deterioration of the electrolyte membrane, catalyst layer, mass transport layer, gas diffusion layer, and other components that make up the water electrolysis cell.

[0005] This disclosure is made in view of the above circumstances and primarily aims to provide a water electrolysis system that can suppress the occurrence of water shortages in water electrolysis cells. [Means for solving the problem]

[0006] In other words, this disclosure includes the following aspects: <1> A water electrolysis system, The aforementioned water electrolysis system is A water electrolysis apparatus that performs water electrolysis, A water supply device that supplies water to the water electrolysis device, A power supply that supplies current to the water electrolysis apparatus, It comprises a control unit and, The control unit adjusts the current density of the current supplied from the power supply to the water electrolysis device. The control unit adjusts the amount of water supplied from the water supply device to the water electrolysis device. The control unit includes a data set indicating the target operating state of the water electrolysis apparatus, which is determined from the water flow rate and the current density. The control unit measures the water flow rate and the current density during operation of the water electrolysis apparatus. The control unit determines, by comparing the water flow rate and current density during operation of the water electrolysis apparatus with the data set, whether or not they are outside the range of the respective thresholds corresponding to the target operating state. A water electrolysis system in which the control unit changes the operation when at least one of the water flow rate and the current density during operation of the water electrolysis apparatus is outside the range of the threshold.

[0007] <2> The control unit performs stop control, which involves setting the current density to zero as the operation change. <1> The water electrolysis system described above.

[0008] <3> The control unit controls at least one of the water flow rate and the current density as the operation change, so that the water electrolysis device reaches the target operating state. <1> The water electrolysis system described above.

Advantages of the Invention

[0009] According to the water electrolysis system of the present disclosure, the occurrence of a water shortage state in the water electrolysis cell can be suppressed.

Brief Description of the Drawings

[0010] [Figure 1] FIG. 1 is a graph showing the relationship between the current density, the water flow rate, and the bubble clogging occurrence rate by a bubble visualization test of a water electrolysis cell. [Figure 2] FIG. 2 is a block diagram showing an example of the water electrolysis system of the present disclosure. [Figure 3] FIG. 3 is a flowchart showing an example of the water electrolysis system of the present disclosure.

Modes for Carrying Out the Invention

[0011] Hereinafter, embodiments according to the present disclosure will be described. In addition, matters other than those specifically mentioned in this specification and necessary for the implementation of the present disclosure (for example, general configurations and manufacturing processes of water electrolysis systems that do not characterize the present disclosure) can be grasped as design matters of those skilled in the art based on the prior art in the relevant field. The present disclosure can be implemented based on the content disclosed in this specification and common technical knowledge in the relevant field. Also, the dimensional relationships (length, width, thickness, etc.) in the drawings do not reflect actual dimensional relationships.

[0012] In the present disclosure, there is provided a water electrolysis system, which includes a water electrolysis device for performing water electrolysis, a water supply device for supplying water to the water electrolysis device, a power source for supplying current to the water electrolysis device, and a control unit. The control unit adjusts the current density of the current supplied from the power source to the water electrolysis device. The control unit adjusts the water flow rate of the water supplied from the water supply device to the water electrolysis device. The control unit has a data group indicating a target operating state of the water electrolysis device determined from the water flow rate and the current density. The control unit measures the water flow rate and the current density during the operation of the water electrolysis device. The control unit determines whether the water flow rate and the current density during the operation of the water electrolysis device are outside the respective threshold ranges corresponding to the target operating state in comparison with the data group. When at least one of the water flow rate and the current density during the operation of the water electrolysis device is outside the threshold range, the control unit performs an operation change, thereby providing a water electrolysis system.

[0013] The present researcher has grasped that a phenomenon occurs in which bubbles are temporarily clogged in the flow of reaction water in the water electrolysis cell, and that this bubble clogging is related to the water flow rate and the current density of the water electrolysis cell. Such bubble clogging is considered to be caused by 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 flow path in the water electrolysis cell, resulting in the generated bubbles staying in the flow path and further growing into larger bubbles. Due to the temporary bubble clogging, a temporary local water shortage state occurs in the water electrolysis cell. Repeated bubble clogging leads to deterioration of the water electrolysis cell. Therefore, it is important to suppress the occurrence of such temporary bubble clogging, but it cannot be detected by impedance measurement or voltage monitoring.

[0014] Therefore, the inside of the water electrolysis cell (reaction water flow path) was visualized, and the current density and the water flow rate of the water electrolysis cell were changed to observe temporary bubble clogging. Specifically, a bubble visualization test was performed 450 times according to the following procedures (1) to (3). (1) Define the current density and the water flow rate of the water electrolysis cell and start water electrolysis. (2) Use a camera to take a 30-second video of the reaction water flow path of the water electrolysis cell. (3) If bubbles remain in the flow path, count it as 1.

[0015] The incidence rate of bubble clogging in the flow path was calculated from the results of 450 tests. The results are shown in Figure 1. In Figure 1, each circle represents the frequency of bubble clogging under its operating conditions; a larger circle indicates a higher frequency of bubble clogging, and a smaller circle indicates a lower frequency of bubble clogging. From Figure 1, 1 A / cm 2 ≦Current density≦5A / cm 2 Furthermore, it was confirmed that bubble clogging does not occur if the water flow rate is within the range of 60 ≤ ≤ 120 (within the rectangular frame in Figure 1). On the other hand, it can be seen that the area outside the rectangular frame in Figure 1 is a region where the frequency of bubble clogging is relatively high.

[0016] Here, the water flow rate refers to the amount of water supplied to the water electrolysis apparatus. The water flow rate is typically adjusted to obtain the desired current density. The water electrolysis reaction equation proceeding on the oxygen electrode side is (2H2O → O2 + 4H + +4e - From this, the theoretical value of the water flow rate (stoichiometric ratio = 1) is determined according to the stoichiometric ratio. However, in practice, the desired current density cannot be obtained with the theoretical value of the water flow rate, so it is common practice to supply an excess amount of water. The above 60 ≤ water flow rate ≤ 120 means that the water flow rate is within the range of 60 times or more and 120 times or less the theoretical value. Conventional water electrolysis devices typically control the current density and water flow rate to maintain a constant level during operation. However, during startup and shutdown, the balance between current and water flow is disrupted, making partial bubble clogging more likely. Furthermore, even during normal operation, the balance between current density and water flow rate can sometimes be disrupted. The water electrolysis system disclosed herein is provided based on the above findings and controls the water flow rate and current density of the water electrolysis device to a target operating state that avoids water shortage. According to the water electrolysis system disclosed herein, the occurrence of water shortage in the water electrolysis cell can be suppressed. Accordingly, according to the disclosure herein, the deterioration of components constituting the water electrolysis cell, such as the electrolyte membrane, catalyst layer, mass transport layer, and gas diffusion layer, can be suppressed.

[0017] The water electrolysis system described herein will be explained using a water electrolysis system 100, which is one embodiment. Figure 2 shows a block diagram of the water electrolysis system 100. The water electrolysis system 100 comprises a water electrolysis device 10, an oxygen electrode side piping section 20 located on the oxygen electrode side of the water electrolysis device 10, a hydrogen electrode side piping section 30 located 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.

[0018] <Water electrolysis device 10> The water electrolysis apparatus 10 is a device that performs water electrolysis. The configuration of the water electrolysis apparatus 10 is well known. An example of the water electrolysis apparatus 10 is described below.

[0019] The water electrolysis apparatus 10 includes water electrolysis cells. Typically, the water electrolysis apparatus 10 includes a water electrolysis stack in which multiple water electrolysis cells are stacked. The water electrolysis apparatus 10 also has terminals that can be connected to a power supply 40.

[0020] A water electrolysis cell can electrolyze water to produce hydrogen and oxygen. The water electrolysis cell has an oxygen electrode and a hydrogen electrode. By supplying water to the oxygen electrode and applying a voltage, oxygen is produced from the oxygen electrode and hydrogen from the hydrogen electrode. While there are no particular limitations on the type of water electrolysis cell, a PEM (Polymer Electrolyte Membrane) type water electrolysis cell can be used to improve water electrolysis efficiency. The following is a brief explanation of the configuration of a PEM-type water electrolysis cell.

[0021] A PEM-type water electrolysis cell comprises a membrane electrode assembly and a pair of separators (oxygen electrode side separator and hydrogen electrode side separator) positioned on both sides of the membrane electrode assembly. The membrane electrode assembly comprises an electrolyte membrane, an oxygen electrode positioned on one side of the electrolyte membrane, and a hydrogen electrode positioned on the other side of the electrolyte membrane. The oxygen electrode includes an oxygen electrode catalyst layer, and may optionally include an oxygen electrode side porous transport layer (oxygen electrode side PTL) between the oxygen electrode side catalyst layer and the oxygen electrode side separator. The hydrogen electrode includes a hydrogen electrode catalyst layer, and may optionally include a hydrogen electrode side porous transport layer (hydrogen electrode side PTL) between the hydrogen electrode side catalyst layer and the hydrogen electrode side separator.

[0022] Examples of electrolyte membranes include those having proton conductivity. Examples of proton-conducting membranes include those containing proton-conducting polymers. Examples of proton-conducting polymers include fluorine-based polymers having sulfonic acid groups, such as perfluoroalkyl sulfonic acid polymers.

[0023] The oxygen electrode catalyst layer contains an oxygen electrode catalyst capable of generating oxygen by water electrolysis. On the other hand, the hydrogen electrode catalyst layer contains a hydrogen electrode catalyst capable of generating hydrogen by water electrolysis. The oxygen electrode catalyst and the hydrogen electrode catalyst are not particularly limited, but examples include metal catalysts. Examples of metal catalysts include metal catalysts whose composition includes at least one of Pt, Ru, Rh, Os, Ir, Pd, and Au. The metal catalyst may also be an oxide of these metals. Furthermore, the oxygen electrode catalyst and the hydrogen electrode catalyst may be a metal-supported catalyst in which the metal catalyst is supported on an electrically conductive carrier. The oxygen electrode catalyst layer and the hydrogen electrode catalyst layer may further contain a proton-conducting electrolyte. The electrolyte used can be the same as the electrolyte constituting the electrolyte membrane.

[0024] The oxygen electrode side PTL and hydrogen electrode side PTL play a role in facilitating the supply and discharge of water from the oxygen and hydrogen electrodes, as well as the discharge of generated oxygen and hydrogen gases. The PTL is formed from, for example, porous titanium sheets such as titanium mesh.

[0025] Separators are placed on both sides of the film electrode assembly. The separators are formed from conductive materials. Examples of conductive materials include resins containing carbon materials; and metallic materials such as iron, copper, stainless steel, and titanium. A predetermined channel is formed on the catalyst layer side of the separator, and the channel serves to guide the water supplied to the water electrolysis cell and the oxygen or hydrogen produced by the water electrolysis reaction. The channel structure of the separator is not particularly limited, and for example, the shape of the channel may be a straight channel, a bent channel, a wavy channel, etc.

[0026] <Oxygen electrode side piping section 20> The oxygen electrode side piping section 20 is responsible for supplying water to the oxygen electrode of the water electrolysis device 10. As shown in Figure 2, the oxygen electrode side piping section 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.

[0027] The water supply device 21 is a device that supplies reaction water to the oxygen electrode of the water electrolysis device 10. Pressurized water may be supplied to the water electrolysis device 10 to circulate the water. The water supply device 21 may include, for example, a reaction water storage tank, a reaction water pump, an ion exchanger, etc. 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.

[0028] The water discharge channel 24 is a pipe that connects the water electrolysis device 10 and the oxygen electrode side gas-liquid separator 23, and allows the water and oxygen discharged from the oxygen electrode of the water electrolysis device 10 to flow to the oxygen electrode side gas-liquid separator 23. The oxygen electrode 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 a circulation channel 25 that connects the water supply device 21 and the oxygen electrode side gas-liquid separator 23, and is reused in the water electrolysis reaction. The circulation channel 25 is used when water is circulated in the oxygen electrode side piping section 20, and is therefore unnecessary when water is not being circulated. Meanwhile, the oxygen separated by the oxygen electrode gas-liquid separator 23 is discharged to the outside via the discharge channel 26.

[0029] <Hydrogen electrode side piping section 30> The hydrogen electrode side piping section 30 has the role of recovering the hydrogen generated at the hydrogen electrode of the water electrolysis device 10. As shown in Figure 2, the hydrogen electrode side piping section 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.

[0030] The hydrogen discharge channel 32 is a pipe that connects the water electrolysis device 10 and the hydrogen electrode-side gas-liquid separator 31, and allows the water and hydrogen discharged from the hydrogen electrode of the water electrolysis device 10 to flow to 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. Reaction water is supplied to at least the oxygen electrode of the water electrolysis device 10, but water may leak to the hydrogen electrode side through the membrane electrode assembly. For this reason, a gas-liquid separator is also provided in the hydrogen electrode-side piping section 30.

[0031] The hydrogen separated by the hydrogen electrode side gas-liquid separator 31 is sent to the hydrogen tank 33 via a hydrogen supply channel 34 connecting the hydrogen electrode side gas-liquid separator 31 and the hydrogen tank 33, and stored there. The water separated by the hydrogen electrode gas-liquid separator 31 is discharged as appropriate.

[0032] <Power supply 40> The power supply 40 is for supplying current to the water electrolysis device 10 and is connected to both the oxygen electrode and the hydrogen electrode of the water electrolysis device 10. Such a power supply 40 is well known. Water electrolysis occurs by supplying water to the water electrolysis device 10 while simultaneously supplying current through the power supply 40.

[0033] <Control Unit 50> The control unit 50 is a computer system equipped with a CPU, RAM, and input / output interfaces, etc. The control unit 50 is electrically connected to the power supply 40 and adjusts the current density of the current supplied from the power supply 40 to the water electrolysis device 10. Furthermore, the control unit 50 is electrically connected to the water supply device 21 and adjusts the amount of water supplied from the water supply device 21 to the water electrolysis device 10. Furthermore, the control unit 50 includes a data set indicating the target operating state of the water electrolysis device 10, which is determined from the water flow rate and current density. This data set is obtained in advance from evaluation tests and 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 account the required current density, etc. For example, by performing a bubble visualization test as described above in advance, a range of water flow rate and current density in which bubble clogging is less likely to occur can be identified, and the identified range can be used as the data set indicating the target operating state of the water electrolysis system. This data set also differs depending on the flow path shape (straight flow path, bent flow path, wavy flow path, etc.) and the material constituting the flow path of the reaction water channel formed by the oxygen electrode separator, so it may be obtained in advance through tests, etc. Multiple data sets may be prepared depending on the operating state.

[0034] The control unit 50 measures the water flow rate and current density when the water electrolysis device 10 is in operation. The water flow rate of the water electrolysis device 10 can be measured directly by a flow meter provided in the water electrolysis device 10 or by a flow meter provided in the water supply device 21, or it can be measured indirectly by inferring it from the operating status 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 or by a current measuring device provided in the power supply 40, etc.

[0035] Next, the control unit 50 compares the measured water flow rate and current density with the above data set to determine whether or not they are outside the range of the respective thresholds corresponding to the target operating state. If the determination shows that at least one of the water flow rate or current density of the water electrolyzer 10 is outside the threshold range, the control unit 50 changes the operation of the water electrolyzer 10. Specifically, if the water flow rate of the water electrolyzer 10 is within the threshold range for the target operating state, but the current density is outside the range, and if the current density of the water electrolyzer 10 is within the threshold range for the target operating state, but the water flow rate is outside the range, the operation of the water electrolyzer 10 is changed. The operation is also changed if both the water flow rate and current density of the water electrolyzer 10 are outside the range. On the other hand, if the determination shows that both the water flow rate and current density are within the threshold range (target operating state), the measurement of the water flow rate and current density will continue, and the water flow rate and current density will be monitored.

[0036] Operating modification refers to changing the operating conditions in order to reduce or eliminate the blockage of air bubbles in the flow path of the water electrolysis device, as described above. Examples of operating modification include changing the amount of current supplied to the water electrolysis device or changing the amount of water flowed by the water supply device 21. For example, if it is time to stop the operation of the water electrolysis device 10, the control unit 50 performs a stop control as an operation change by stopping the supply of current from the power supply 40 to the water electrolysis device 10 and reducing the current density to zero. As a result, the progress of the water electrolysis reaction stops, and the generation of bubbles also stops. On the other hand, the control unit 50 continues to supply water from the water supply device 21 to the water electrolysis device 10. This allows oxygen gas to be expelled from the flow path of the water electrolysis device, thus preventing bubble clogging when the next operation is started. The timing for attempting to stop the operation of a water electrolysis device includes, for example, when the operation of the water electrolysis device has been stopped.

[0037] Furthermore, when the water electrolyzer 10 is started up or in normal operation, the control unit 50 controls at least one of the water flow rate and current density as an operation change so that the water electrolyzer 10 reaches the target operating state. That is, in the above determination, the control unit controls the water flow rate and / or current density that were outside the threshold range corresponding to the target operating state so that the water electrolyzer 10 reaches the target operating state. After the operation change, the control unit returns to measuring the water flow rate and current density and monitors the water flow rate and current density.

[0038] Here, the control of the water flow rate and current density during operation of the water electrolysis apparatus 10 by the control unit 50 will be explained using Figure 3. Figure 3 is a flowchart showing an example of the control unit's control of the water flow rate and current density of the water electrolysis apparatus. The flowchart shown in Figure 3 controls the water flow rate and current density of the water electrolysis device based on the target operating state obtained from the bubble visualization test results shown in Figure 1. Specifically, the target operating state of the water electrolysis device is 1 A / cm². 2 ≦Current density≦5A / cm 2 Furthermore, 60 ≤ water flow rate ≤ 120.

[0039] First, monitor (measure) the water flow rate and current density during operation of the water electrolysis device (STEP 1). Next, determine whether the measured water flow rate and current density values ​​satisfy at least one of the following conditions (1) and (2) (STEP 2). Condition (1): Water flow rate < 60 or Water flow rate > 120 Condition (2): Current density <1A / cm 2 or current density>5A / cm 2

[0040] If neither condition (1) nor condition (2) is met, it means that the water flow rate and current density of the water electrolysis unit are in the target operating state. If neither condition (1) nor condition (2) is met, return to STEP 1 and continue monitoring the water flow rate and current density.

[0041] Satisfying condition (1) means that the water flow rate of the water electrolysis device is outside the range of the threshold value corresponding to the target operating state. Satisfying condition (2) means that the current density of the water electrolysis device is outside the range of the threshold value corresponding to the target operating state. When only one of conditions (1) and (2) is satisfied, and when both conditions (1) and (2) are satisfied, it is determined whether the water electrolysis device has received an end operation (operation stop operation) (STEP3). When it is determined that the end operation has been received, the current of the water electrolysis device is turned off and the current density is set to zero (STEP4). As described above, the water flow rate is not set to zero.

[0042] When it is determined that the end operation has not been received, at least one of the water flow rate and the current density of the water electrolysis device is controlled so as to satisfy the following condition (3) and the following condition (4) (STEP5). Condition (3): 60 ≤ water flow rate ≤ 120 Condition (4): 1 A / cm 2 ≤ current density ≤ 5 A / cm 2

[0043] In STEP2, when it is determined that both condition (1) and condition (2) are satisfied, in STEP5, both the water flow rate and the current density are controlled so as to satisfy conditions (3) and (4). S In STEP2, when it is determined that only condition (1) is satisfied, in STEP5, the water flow rate is controlled so as to satisfy condition (3). In STEP2, when it is determined that only condition (2) is satisfied, in STEP5, the current density is controlled so as to satisfy condition (4). After the control in STEP5, return to STEP1 and continue to monitor the water flow rate and the current density.

Explanation of Signs

[0044] 10... Water electrolysis device 20... Oxygen electrode side pipe part 21... Water supply device 22 ...Water supply channel 23 ... Oxygen electrode gas-liquid separator 24 ...Water discharge channel 25 ... Circulation channel 26 ... Discharge channel 30 ...Hydrogen electrode side piping section 31…Hydrogen electrode side gas-liquid separator 32…Hydrogen Emission Channel 33…Hydrogen tank 34…Hydrogen supply channel 40…power supply 50 ... Control Unit 100 ... Water electrolysis system

Claims

1. A water electrolysis system, The aforementioned water electrolysis system is A water electrolysis apparatus that performs water electrolysis, A water supply device that supplies water to the water electrolysis device, A power supply that supplies current to the water electrolysis apparatus, It comprises a control unit and, The control unit adjusts the current density of the current supplied from the power supply to the water electrolysis device. The control unit adjusts the amount of water supplied from the water supply device to the water electrolysis device. The control unit includes a data set indicating the target operating state of the water electrolysis apparatus, which is determined from the water flow rate and the current density. The control unit measures the water flow rate and the current density during operation of the water electrolysis apparatus. The control unit determines, by comparing the water flow rate and current density during operation of the water electrolysis apparatus with the data set, whether or not they are outside the range of the respective thresholds corresponding to the target operating state. A water electrolysis system in which the control unit changes the operation when at least one of the water flow rate and the current density during operation of the water electrolysis apparatus is outside the range of the threshold.

2. The water electrolysis system according to claim 1, wherein the control unit performs a stop control to set the current density to zero as the operation change.

3. The water electrolysis system according to claim 1, wherein the control unit controls at least one of the water flow rate and the current density as the operation change, so that the water electrolysis device reaches the target operating state.

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