WATER ELECTROLYSIS SYSTEM

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2024-162115 filed on Sep. 19, 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 technology disclosed in this specification relates to a water electrolysis system.

2. Description of Related Art

Japanese Unexamined Patent Application Publication No. 2024-62492 (JP 2024-62492A) discloses a water electrolysis cell. This water electrolysis cell has a structure in which a membrane-electrode assembly is sandwiched between two separators. A hydrogen flow passage is provided between one of the separators and a hydrogen electrode, and an oxygen flow passage is provided between the other separator and an oxygen electrode. When the water electrolysis cell operates, hydrogen is generated in the hydrogen flow passage while oxygen is generated in the oxygen flow passage.

SUMMARY

During water electrolysis, members constituting the oxygen flow passage become oxidized by the oxygen in the oxygen flow passage, resulting in deterioration of these members. This specification proposes a technology that mitigates oxidation of members constituting an oxygen flow passage in a water electrolysis cell.

A water electrolysis system disclosed in this specification has: a membrane-electrode assembly; a first separator in contact with a hydrogen electrode of the membrane-electrode assembly; a hydrogen flow passage provided between the first separator and the hydrogen electrode; a second separator in contact with an oxygen electrode of the membrane-electrode assembly; an oxygen flow passage provided between the second separator and the oxygen electrode; and a cooling device that cools the hydrogen electrode such that a temperature of the hydrogen electrode becomes lower than a temperature of the oxygen electrode.

In this water electrolysis system, the cooling device cools the hydrogen electrode such that the temperature of the hydrogen electrode becomes lower than the temperature of the oxygen electrode. As the hydrogen electrode is cooled, constituent members of the oxygen flow passage are cooled, which mitigates oxidation of the constituent members of the oxygen flow passage. In addition, the temperature of the oxygen electrode becomes higher than the temperature of the hydrogen electrode, so that a decrease in the efficiency of water electrolysis can be avoided.

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 an exploded perspective view of a water electrolysis cell 1;

FIG. 2 is a partial sectional view along line II-II of FIG. 1; and

FIG. 3 is a configuration diagram of a water electrolysis system 70.

DETAILED DESCRIPTION OF EMBODIMENTS

The water electrolysis system may further have a water supply device that supplies water to the oxygen flow passage. The cooling device may supply a cooling liquid at a lower temperature than the water in the oxygen flow passage to the hydrogen flow passage.

With this configuration, the hydrogen electrode can be cooled such that the temperature of the hydrogen electrode becomes lower than the temperature of the oxygen electrode.

The water electrolysis system may further have a frame body made of resin that is sandwiched between the first separator and the second separator and provided along a peripheral edge of the membrane-electrode assembly. The cooling device may cool a joint portion between the membrane-electrode assembly and the frame body.

With this configuration, oxidation of the joint portion between the membrane-electrode assembly and the frame body can be mitigated.

General Configuration of Water Electrolysis Cell 1

FIG. 1 shows an exploded perspective view of a water electrolysis cell 1. The water electrolysis cell 1 mainly includes a first separator 10, a second separator 20, a membrane-electrode assembly 40, and a frame body 50. The membrane-electrode assembly 40 electrolyzes water to generate hydrogen and oxygen. The structure of the membrane-electrode assembly 40 will be described later.

The frame body 50 is composed of a resin having insulation properties. As shown in FIG. 1, a housing hole 54 that penetrates the frame body 50 is provided at a center of the frame body 50. The membrane-electrode assembly 40 is disposed inside the housing hole 54. That is, the frame body 50 surrounds the membrane-electrode assembly 40.

The first separator 10 and the second separator 20 are composed of a gas-impermeable conductive material. Examples of the material of the separators include metal materials, such as stainless steel, and carbon materials. The first separator 10 and the second separator 20 face each other through the membrane-electrode assembly 40 and the frame body 50.

In the frame body 50, a plurality of through-holes 56 is provided around the housing hole 54. In the first separator 10, a plurality of through-holes 16 is provided. In the second separator 20, a plurality of through-holes 26 is provided. The through-holes 16, 26 are located at positions coinciding respectively with the through-holes 56. As the through-holes 16, 56, 26 connect to one another, each of a first supply passage 61, a first discharge passage 62, a second supply passage 63, a second discharge passage 64, a third supply passage 65, and a drain passage 66 is formed. These flow passages penetrate the water electrolysis cell 1 in a thickness direction.

Specific Configuration of Water Electrolysis Cell 1

FIG. 2 shows a partial sectional view along line II-II of FIG. 1. The membrane-electrode assembly 40 includes a hydrogen electrode 41, an oxygen electrode 42, and an electrolyte membrane 43. The electrolyte membrane 43 is an ion-exchange membrane having proton conductivity that is formed by a solid polymer material. The hydrogen electrode 41 includes a first catalyst layer 44 and a first gas diffusion layer 45. The oxygen electrode 42 includes a second catalyst layer 46 and a second gas diffusion layer 47. The first catalyst layer 44 and the second catalyst layer 46 are porous layers in which carbon particles or metal oxide particles supporting a catalyst are coupled together by resin. Examples of catalysts that can be used include iridium (Ir), ruthenium (Ru), platinum (Pt), and alloys composed of Pt and other metals (e.g., Pt alloys in which cobalt, nickel, etc. are mixed). The first gas diffusion layer 45 and the second gas diffusion layer 47 are conductive members having water permeability and gas permeability.

The electrolyte membrane 43, the hydrogen electrode 41, and the oxygen electrode 42 have a rectangular shape. The hydrogen electrode 41 is equal in size to the electrolyte membrane 43, and the oxygen electrode 42 is smaller than the electrolyte membrane 43. At an outer peripheral portion of an upper surface 43u of the electrolyte membrane 43, an outer peripheral region PA where the second catalyst layer 46 is not present is formed. On the upper surface 43u in the outer peripheral region PA, an adhesive layer 49 is disposed. The adhesive layer 49 is a layer formed by an adhesive applied thereto. One example of adhesives is an adhesive that contains an organic solvent and has ultraviolet-curable properties.

The frame body 50 includes a three-layer structure in which a first resin layer 51, a core layer 53, and a second resin layer 52 are laminated in the thickness direction. The core layer 53 is a structural member having gas sealing properties and insulation properties. The first resin layer 51 is a layer that is bonded to the first separator 10. The second resin layer 52 is a layer that is bonded to the second separator 20. A lower surface of the first resin layer 51 constitutes a lower surface 51b of the frame body 50. An upper surface of the second resin layer 52 constitutes an upper surface 52u of the frame body 50.

The first resin layer 51 and the second resin layer 52 may have properties with lower viscosity and a lower melting point than the core layer 53. Specifically, the first resin layer 51 and the second resin layer 52 may be a thermoplastic resin, such as an acid-modified olefin-based resin or a polyester-based resin. The frame body 50 of a multi-layer structure can be formed by various methods. For example, it may be formed by coextrusion molding.

When seen in a direction perpendicular to the membrane-electrode assembly 40 (z-direction), an outer periphery of the membrane-electrode assembly 40 and an inner periphery of the frame body 50 overlap each other. An overlap region between the membrane-electrode assembly 40 and the frame body 50 is a joint portion OA where the membrane-electrode assembly 40 is joined to the frame body 50. In the joint portion OA, the lower surface 51b of the frame body 50 is bonded to the upper surface 43u of the electrolyte membrane 43 through the adhesive layer 49. Thus, a structure is created in which an outer peripheral portion 40e of the membrane-electrode assembly 40 extends to between the lower surface 51b of the frame body 50 and the first separator 10.

In the first separator 10, ribs 10r are provided. The first separator 10 is in contact with the hydrogen electrode 41 at portions other than the ribs 10r, and a space is provided between each rib 10r and the hydrogen electrode 41. The space between each rib 10r and the hydrogen electrode 41 constitutes a hydrogen flow passage 14. Some of the hydrogen flow passages 14 are provided at positions coinciding with the joint portion OA when seen along the z-direction. In the second separator 20, ribs 20r are provided. The second separator 20 is in contact with the oxygen electrode 42 at portions other than the ribs 20r, and a space is provided between each rib 20r and the oxygen electrode 42. The space between each rib 20r and the oxygen electrode 42 constitutes an oxygen flow passage 24. Some of the oxygen flow passages 24 are provided at positions coinciding with the joint portion OA when seen along the z-direction. Portions of surfaces of the adhesive layer 49 and the first resin layer 51 constituting the joint portion OA are exposed to an inside of the oxygen flow passage 24.

Configuration of Water Electrolysis System

A water electrolysis system 70 shown in FIG. 3 has a water electrolysis stack 72. The water electrolysis stack 72 is composed of a stack in which a plurality of water electrolysis cells 1 is stacked. The water electrolysis system 70 has a pure water supply system 80 that supplies pure water to the water electrolysis stack 72 and a cooling liquid supply system 90 that supplies a cooling liquid to the water electrolysis stack 72.

The pure water supply system 80 has a pure water supply passage 82, a pure water discharge passage 84, a pure water pump 86, and a pure water temperature sensor 88. The pure water supply passage 82 is connected to the oxygen flow passages 24 of each water electrolysis cell 1 through the second supply passage 63 (see FIG. 1). The oxygen flow passages 24 of each water electrolysis cell 1 are connected to the pure water discharge passage 84 through the second discharge passage 64 (see FIG. 1). The pure water pump 86 supplies the pure water to the water electrolysis stack 72 through the pure water supply passage 82. When the pure water pump 86 operates, the pure water flows into the oxygen flow passages 24 of each water electrolysis cells 1 through the pure water supply passage 82. The pure water having passed through the oxygen flow passages 24 of each water electrolysis cell 1 is discharged to the pure water discharge passage 84. The pure water temperature sensor 88 detects a temperature of the pure water discharged from the water electrolysis stack 72 to the pure water discharge passage 84.

The cooling liquid supply system 90 has a cooling liquid supply passage 92, a cooling liquid discharge passage 94, a cooling liquid pump 96, a cooling liquid temperature sensor 98, and a radiator 99. The cooling liquid supply passage 92 is connected to the hydrogen flow passages 14 of each water electrolysis cell 1 through the first supply passage 61 (see FIG. 1). The hydrogen flow passages 14 of each water electrolysis cell 1 are connected to the cooling liquid discharge passage 94 through the first discharge passage 62 (see FIG. 1). The cooling liquid pump 96 supplies the cooling liquid to the water electrolysis stack 72 through the cooling liquid supply passage 92. A supply pressure of the cooling liquid pump 96 is equal to or higher than a pressure of the hydrogen in each water electrolysis cell 1. When the cooling liquid pump 96 operates, the cooling liquid flows into the hydrogen flow passages 14 of each water electrolysis cell 1 through the cooling liquid supply passage 92. The cooling liquid having passed through the hydrogen flow passages 14 of each water electrolysis cell 1 is discharged to the cooling liquid discharge passage 94. The cooling liquid temperature sensor 98 detects a temperature of the cooling liquid discharged from the water electrolysis stack 72 to the cooling liquid discharge passage 94. The radiator 99 cools the cooling liquid in the cooling liquid supply passage 92 by heat exchange with outside air.

The water electrolysis system 70 has a control device 100. The control device 100 is electrically connected to the pure water pump 86, the pure water temperature sensor 88, the cooling liquid pump 96, the cooling liquid temperature sensor 98, and the radiator 99. The control device 100 controls the cooling liquid pump 96 and the radiator 99 according to detection values of the pure water temperature sensor 88 and the cooling liquid temperature sensor 98.

Operation of Water Electrolysis System

During operation of the water electrolysis system 70, the control device 100 operates the pure water pump 86 to thereby supply the pure water to the oxygen flow passages 24 of each water electrolysis cell 1. The control device 100 operates the cooling liquid pump 96 to thereby supply the cooling liquid to the hydrogen flow passages 14 of each water electrolysis cell 1. The control device 100 applies a voltage to each water electrolysis cell 1 by a power source (not shown). The voltage is applied in such a direction that, in each water electrolysis cell 1, the second separator 20 has a higher potential than the first separator 10. When the voltage is applied, the pure water in the oxygen flow passages 24 is electrolyzed in the membrane-electrode assembly 40. As a result, oxygen is generated in the oxygen flow passages 24 while hydrogen is generated in the hydrogen flow passages 14. The oxygen generated in the oxygen flow passages 24 is discharged along with the pure water to the pure water discharge passage 84. The oxygen discharged to the pure water discharge passage 84 is separated from the pure water by a gas-liquid separator or the like (not shown) and utilized. The hydrogen generated in the hydrogen flow passages 14 is discharged along with the cooling liquid to the cooling liquid discharge passage 94. The hydrogen discharged to the cooling liquid discharge passage 94 is separated from the cooling liquid by a gas-liquid separator or the like (not shown) and utilized.

As described above, oxygen is generated in the oxygen flow passages 24 during water electrolysis. During water electrolysis, the water electrolysis cell 1 generates heat. Thus, members constituting the oxygen flow passages 24 undergo oxidative deterioration, which may degrade the durability of the water electrolysis cell 1. In particular, when the adhesive layer 49 and the first resin layer 51 undergo oxidative deterioration at the joint portion OA, sealing properties between the frame body 50 and the membrane-electrode assembly 40 cannot be maintained and the durability of the water electrolysis cell 1 degrades.

In the technology in this example of implementation, as described above, the cooling liquid is supplied into the hydrogen flow passages 14 during operation of the water electrolysis system 70. The control device 100 mitigates a temperature rise of the water electrolysis cell 1 by controlling the cooling liquid pump 96 such that a flow rate of the cooling liquid becomes higher as a current load on the water electrolysis cell 1 becomes higher. The control device 100 controls the cooling liquid pump 96 and the radiator 99 such that the temperature of the cooling liquid detected by the cooling liquid temperature sensor 98 becomes lower than the temperature of the pure water detected by the pure water temperature sensor 88. Thus, in each water electrolysis cell 1, the hydrogen electrode 41 is cooled such that the temperature of the hydrogen electrode 41 becomes lower than the temperature of the oxygen electrode 42. As the hydrogen electrode 41 is cooled, each of the members constituting the oxygen flow passages 24 is cooled, which mitigates oxidative deterioration of each of the members constituting the oxygen flow passages 24. In particular, since some hydrogen flow passages 14 are disposed at positions coinciding with the joint portion OA, the adhesive layer 49 and the first resin layer 51 constituting the joint portion OA are cooled by the cooling liquid in these hydrogen flow passages 14. Thus, oxidative deterioration of the adhesive layer 49 and the first resin layer 51 constituting the joint portion OA is mitigated and the sealing properties are maintained.

When the oxygen electrode 42 is excessively cooled, water electrolysis occurs less in the oxygen electrode 42, resulting in reduced efficiency of generating oxygen and hydrogen. As a countermeasure, in the technology in this example of implementation, the cooling liquid is passed through the inside of the hydrogen flow passages 14 located on the opposite side from the oxygen electrode 42, so that excessive cooling of the oxygen electrode 42 is prevented and the temperature of the oxygen electrode 42 becomes higher than the temperature of the hydrogen electrode 41. This allows water electrolysis to occur efficiently in the oxygen electrode 42. For example, setting the temperature of the pure water 5 to 10° C. higher than the temperature of the cooling liquid can mitigate oxidative deterioration of the joint portion OA while maintaining high water electrolysis efficiency.

The technology of this specification can be applied to various structures. For example, the frame body 50 is not limited to a three-layer structure. The technology of this specification can be applied also to frame bodies having a single-layer structure, a two-layer structure, or a structure with four or more layers.

The cooling liquid supply system 90 in the example of implementation is one example of the cooling device that cools the hydrogen electrode such that the temperature of the hydrogen electrode becomes lower than the temperature of the oxygen electrode.

While embodiments have been described in detail above, these are merely illustrative and not intended to restrict the claims. The technology described in the claims includes the above-illustrated specific examples to which various changes and modifications have been made. The technical elements described in this specification or the drawings exhibit technical utility independently or in various combinations, and are not limited to the combinations described in the claims as filed. The technology illustrated in this specification or the drawings achieves a plurality of objects at the same time, and its achieving one of these objects in itself means that it has technical utility.