JP7784802B2 - Multipolar electrolyzer for water electrolysis - Google Patents

Description

The present invention relates to a bipolar electrolytic cell for water electrolysis.

In recent years, renewable energy technologies such as wind power and solar power generation have been attracting attention as a way to solve problems such as global warming caused by greenhouse gases such as carbon dioxide and dwindling fossil fuel reserves.

Renewable energy output is subject to extreme fluctuations due to its dependence on weather conditions. As a result, it is not always possible to transport electricity generated by renewable energy sources to the general power grid, raising concerns about social impacts such as imbalances in power supply and demand and destabilization of the power grid.

Research is therefore being conducted to convert electricity generated from renewable energy sources into a form that can be stored and transported, and to utilize this energy. Specifically, research is being conducted into the use of electricity generated from renewable energy sources to electrolyze water, generating hydrogen that can be stored and transported, and using this hydrogen as an energy source or raw material.

Hydrogen is widely used industrially in processes such as petroleum refining, chemical synthesis, and metal refining, and in recent years its potential use has expanded in hydrogen stations for fuel cell vehicles (FCVs), smart communities, and hydrogen power plants. For this reason, there are high expectations for the development of technologies to obtain hydrogen, especially from renewable energy sources.

Water electrolysis methods include solid polymer water electrolysis, high-temperature steam electrolysis, and alkaline water electrolysis, but alkaline water electrolysis is considered to be one of the most promising methods because it has been industrialized for over several decades, can be carried out on a large scale, and is inexpensive compared to other water electrolysis devices.

However, in order to adapt water electrolysis as a means of storing and transporting energy in the future, it will be necessary to make it possible to carry out water electrolysis using electric power, which has large output fluctuations as mentioned above, efficiently and stably, and there is a need to resolve various issues with electrolytic cells and equipment for water electrolysis.

For example, with regard to extending the life of the device, Patent Document 1 reports that, in order to obtain an electrochemical system (fuel cell, electrolyzer, etc.) with a sealing device that exhibits a long life even under conditions of significant temperature changes, an electrochemical system is provided with a terminal bipolar plate and an end plate having different thermal expansion coefficients and a sealing device disposed between the two plates, and the sealing function of the sealing device is designed to be provided by sliding of the end plate and/or the terminal bipolar plate along the sealing device.
Furthermore, Patent Document 2 reports that in order to obtain a high-temperature steam electrolyzer or high-temperature fuel cell in which the seal has sufficient leak resistance even at high temperatures of 500°C or higher, an assembly is placed between two components having different average thermal expansion coefficients to seal the components, and the assembly is designed to be sealed by a combination of orthogonal compression perpendicular to each component, which is achieved by initial clamping, and radial compression, which is achieved by sliding the seal due to the thermal expansion difference.

Special Publication No. 2017-509123 Special Publication No. 2011-526329

Furthermore, particularly in an electrolytic cell in which a plurality of electrolytic cells are connected in series (stacked) by tie rods, when the cell temperature becomes high due to electrolysis, it becomes necessary to retighten the tie rods as described below, and this retightening of the tie rods is a factor that shortens the life of the electrolytic cell.
(1) When the cell temperature rises, the elastic modulus of the gasket that seals between the electrode elements that make up the electrolytic cell decreases, thereby reducing the surface pressure (sealing surface pressure) between the gasket and each element. In addition, the temperature of the tie rod also rises due to heat conduction from the cell, causing the cell and tie rod to expand, and the length of the entire electrolytic cell in the stacking direction of the cells increases. If the thermal expansion amount (elongation amount) of the tie rod in the stacking direction is greater than the thermal expansion amount (elongation amount) of the stacked cells, the tightening force of the tie rod weakens, further reducing the sealing surface pressure.
(2) When the seal surface pressure is reduced as in (1) above, it is necessary to secure the seal surface pressure by retightening the tie rod (raising the surface pressure to adjust it back to its original value) in order to prevent leakage of the electrolyte and generated gas. When the tie rod is retightened, the gasket is further compressed in the stack direction, causing the gasket to escape (protrude) outside the tightened surface.
(3) When electrolysis is completed and the cell temperature returns to the temperature before electrolysis, the elastic modulus of the gasket recovers, but because it is in a retightened state, the gasket is in an over-compressed state. At this time, even if the tie rod is loosened by the amount of retightening, the permanent strain remains in the gasket, so the sealing surface pressure does not return to its original value before electrolysis.
(4) As in (3) above, the gasket and seal surface pressure do not return to the state they were in before electrolysis, so when electrolysis is performed again and the cell temperature becomes high, further retightening is required.
(5) Repeated tightening as in (1) to (4) causes the length of the electrolytic cell in the stack direction to gradually shorten compared to the initial state, and the compressive strain of the gasket gradually increases. The over-compressed gasket eventually breaks, causing leakage of the electrolyte and generated gas.

As mentioned above, retightening the tie rods increases the gasket's protrusion and compressive strain, leading to leakage of electrolyte and generated gas, and the gasket's tearing and damage. Therefore, reducing the number of times the tie rods are retightened will extend the life of the water electrolyzer and water electrolyzer equipment and reduce the frequency of maintenance.

In view of the above-mentioned problems, the present invention aims to provide a bipolar electrolytic cell for water electrolysis that has a long life even when used with a variable power supply or under conditions where operation is frequently stopped.

That is, the present invention is as follows.
[1]
an anode terminal element comprising an anode;
a cathode terminal element comprising a cathode;
a plurality of bipolar elements positioned between the anode terminal element and the cathode terminal element, each element including an anode, a cathode, a partition wall separating the anode and the cathode, and an outer frame that frames the partition wall;
A cell stack (a) in which adjacent elements are stacked with a diaphragm disposed between them via a gasket;
press plates (b) disposed at both ends of the cell stack (a) and applying surface pressure between the gasket and the diaphragm, and between the gasket and each of the elements;
a tie rod (c) fastening the cell stack (a) and the press plate (b),
a heat insulating member (d) is provided between the press plate (b) and the cell stack (a);
a mean thermal expansion coefficient α of the cell stack (a), the press plate (b), and the heat insulating member (d) in the stacking direction at electrolysis temperature is greater than a mean thermal expansion coefficient β of the tie rod (c) in the stacking direction at electrolysis temperature , and an amount of thermal expansion of the entire stack of the cell stack (a), the press plate (b), and the heat insulating member (d) at electrolysis temperature (elongation in the stacking direction) is greater than an amount of thermal expansion of the tie rod (c) at electrolysis temperature (elongation in the stacking direction).
[2]
The bipolar electrolytic cell for water electrolysis according to [1], wherein a thermal conductivity A of the heat insulating member (d) at an electrolysis temperature is smaller than a thermal conductivity B of the press plate (b) at an electrolysis temperature.
[3]
The bipolar electrolytic cell for water electrolysis according to [1] or [2], wherein the thermal conductivity A of the heat insulating member (d) at 100°C is 1 W/m K or less.
[4]
The bipolar electrolytic cell for water electrolysis according to any one of [1] to [3], wherein the heat insulating member (d) has a volume resistivity of 1 kΩ cm or more at an electrolysis temperature.
[5]
The bipolar electrolytic cell for water electrolysis according to any one of [1] to [4], wherein the thickness of the gasket is 5 to 20% of the thickness of the bipolar element.
[6]
a spring member (e) arranged on an outer periphery of the axis of the tie rod (c) and elastically deformable in the stacking direction of the cell stack (a),
The bipolar electrolytic cell for water electrolysis according to any one of [1] to [5], wherein an expansion length ΔL of the cell stack (a) in the stack direction at an electrolysis temperature is 75% or less of a total deflection T of the spring member (e) in the stack direction.
[7]
[6] The bipolar electrolytic cell for water electrolysis according to [6], wherein the number of the tie rods (c) is n, and the spring members (e) arranged on the n tie rods (c) have a spring constant of 5/n to 20/n kN/mm.
[8]
The bipolar electrolytic cell for water electrolysis according to any one of [1] to [7], wherein the surface pressure between the gasket and each of the elements during electrolysis is at least twice the maximum internal pressure of the electrolytic cell, and the compressibility of the gasket in the stacking direction of the cell stack (a) is 10 to 25%.

The present invention makes it possible to provide a bipolar electrolytic cell for water electrolysis that has a long life even when used with a variable power supply or under conditions where operation is frequently stopped.

FIG. 1 is a side view showing an example of an entire bipolar electrolytic cell for water electrolysis according to an embodiment of the present invention. 2 is a diagram showing a cross section of a zero-gap structure portion inside the electrolysis cell in the dashed rectangular frame portion of FIG. 1. FIG. 1 is a diagram showing an outline of an electrolysis apparatus for water electrolysis according to an embodiment of the present invention.

The following describes in detail the mode for carrying out the present invention (hereinafter referred to as the "present embodiment"). Note that the present invention is not limited to the following embodiment, and various modifications can be made within the scope of its gist.

<Bipolar electrolyzer for water electrolysis>
The bipolar electrolytic cell for water electrolysis of this embodiment is an electrolytic cell including a cell stack (a), press plates (b) located at both ends of the cell stack (a), and tie rods (c) that fasten the cell stack (a) and the press plates (b), and is further characterized by including a heat insulating member (d) between the cell stack (a) and the press plates (b).
The cell stack (a) is formed by stacking (layering) an anode terminal element having an anode, a cathode terminal element having a cathode, a plurality of bipolar elements positioned between the anode terminal element and the cathode terminal element and having an anode, a cathode, a partition wall separating the anode and the cathode, and an outer frame bordering the partition wall, with a diaphragm disposed between each adjacent element via a gasket. In this disclosure, the stacking (layering) direction of the cell stack (a) is also simply referred to as the "stack direction."
The press plate (b) is fastened to the cell stack (a) by tie rods (c), and applies surface pressure (sealing surface pressure) between the gasket and the diaphragm, and between the gasket and each element.
The bipolar electrolytic cell for water electrolysis of this embodiment includes a heat insulating member (d) between the cell stack (a) and the press plate (b), thereby reducing heat conduction from the cell stack (a) to the tie rods (c) when the temperature of the cell stack (a) rises during electrolysis. This prevents the tie rods (c) from expanding and increasing in length in the stack direction, which would otherwise cause a decrease in seal surface pressure. This reduces the number of times the tie rods (c) need to be retightened, thereby achieving a bipolar electrolytic cell for water electrolysis with a long life.
Hereinafter, the configuration of an example of a bipolar electrolytic cell for water electrolysis according to this embodiment will be described with reference to the drawings.

FIG. 1 shows a side view of an example of an entire bipolar electrolytic cell for water electrolysis according to this embodiment.
FIG. 2 is a side view of an example of a zero-gap structure of a bipolar electrolytic cell for water electrolysis according to this embodiment, showing the area enclosed by a dashed square frame in FIG.
In the example of the bipolar electrolytic cell 50 for water electrolysis according to this embodiment shown in FIGS. 1 and 2 , a press plate (b), a heat insulating member (d), and an anode terminal element 51a are arranged in this order from one end, and further an anode side gasket 7, a diaphragm 4, a cathode side gasket 7, and a bipolar element 60 are arranged in this order. The bipolar element 60 is arranged so that the cathode 2c faces the anode terminal element 51a. The anode side gasket 7 to the bipolar element 60 are arranged repeatedly as many times as necessary for the designed production volume. After the anode side gasket 7 to the bipolar element 60 are arranged repeatedly as necessary, the anode side gasket 7, the diaphragm 4, and the cathode side gasket 7 are again arranged in this order, and finally the cathode terminal element 51c, the heat insulating member (d), and the press plate (b) are arranged in this order. The bipolar electrolytic cell 50 for water electrolysis is integrated by fastening the entire assembly with tie rods (c) to form the bipolar electrolytic cell 50. The elements from the anode terminal element 51a to the cathode terminal element 51c are referred to as a cell stack (a).
The arrangement of the bipolar electrolytic cell 50 for water electrolysis can be arbitrarily selected from either the anode 2a side or the cathode 2c side, and is not limited to the above order.
Furthermore, although not particularly limited, the bipolar electrolytic cell 50 for water electrolysis according to this embodiment preferably has a zero-gap structure Z in which the diaphragm 4 is in contact with the anode 2 a and the cathode 2 c, as shown in FIG. 2 .

The bipolar type is one method of connecting a large number of cells to a power source, in which multiple bipolar elements 60, one side of which is an anode 2a and the other side of which is a cathode 2c, are arranged in the same direction and connected in series, and only both ends are connected to the power source.
The bipolar electrolytic cell 50 for water electrolysis has the advantage of being able to reduce the current of the power supply, and is capable of producing large amounts of compounds, predetermined substances, etc. in a short period of time through electrolysis. For power supply equipment with the same output, a constant current, high voltage type is cheaper and more compact, so from an industrial perspective, a bipolar type is preferable to a monopolar type.

<<Cell stack (a)>>
As shown in FIG. 1 , the cell stack (a) included in the bipolar electrolytic cell 50 for water electrolysis of this embodiment is configured by stacking a required number of bipolar elements 60 between an anode terminal element 51 a and a cathode terminal element 51 c.
In the cell stack (a), the diaphragms 4 are arranged between the anode terminal element 51a and the bipolar element 60, between adjacent bipolar elements 60, and between the bipolar element 60 and the cathode terminal element 51c.
As shown in FIG. 1, adjacent elements 51 a , 60 , 51 c and adjacent elements 51 a , 60 , 51 c and diaphragms 4 are stacked with gaskets 7 interposed therebetween.

1 and 2, the bipolar element 60 includes a partition wall 1 that separates the anode 2a and the cathode 2c, and an outer frame 3 that frames the partition wall 1. More specifically, the partition wall 1 is conductive, and the outer frame 3 is provided along the outer edge of the partition wall 1 so as to surround the partition wall 1.
In this embodiment, the bipolar element 60 may be used so that the given direction D1 along the partition wall 1 is normally the vertical direction. Specifically, when the partition wall 1 has a rectangular shape in a plan view as shown in Fig. 2, the given direction D1 along the partition wall 1 may be the same direction as the direction of one of two pairs of opposing sides. In this specification, the vertical direction is also referred to as the electrolyte passage direction.

In the bipolar electrolytic cell 50 for water electrolysis in this embodiment, as shown in FIG. 2, an electrode chamber 5 through which the electrolyte passes is defined by a partition wall 1, an outer frame 3, and a diaphragm 4.
In this embodiment, in particular, the portion between the partition walls 1 between two adjacent bipolar elements 60 and the portion between the partition walls 1 between an adjacent bipolar element 60 and a terminal element in the bipolar electrolytic cell 50 for water electrolysis are referred to as the electrolytic cell 65. The electrolytic cell 65 includes the partition wall 1, anode chamber 5a, anode 2a, and diaphragm 4 of one element, and the cathode 2c, cathode chamber 5c, and partition wall 1 of the other element.
In the examples shown in FIGS. 1 and 2 , the partition wall 1, the anode 2 a, and the cathode 2 c all have a plate-like shape with a predetermined thickness; however, the present invention is not limited thereto, and they may all or partly have a zigzag or wavy shape in cross section, or may have a shape with rounded edges.

Specifically, the electrode chamber 5 has, at the boundary with the outer frame 3, an electrolyte inlet for introducing electrolyte into the electrode chamber 5 and an electrolyte outlet for discharging electrolyte from the electrode chamber 5. More specifically, the anode chamber 5a is provided with an anolyte inlet for introducing electrolyte into the anode chamber 5a and an anolyte outlet for discharging electrolyte from the anode chamber 5a. Similarly, the cathode chamber 5c is provided with a catholyte inlet for introducing electrolyte into the cathode chamber 5c and a catholyte outlet for discharging electrolyte from the cathode chamber 5c.

In this embodiment, the anode chamber 5a and the cathode chamber 5c may be provided with internal distributors for uniformly distributing the electrolyte solution across the electrode surfaces inside the bipolar electrolytic cell 50 for water electrolysis. The electrode chambers 5 may also be provided with baffle plates that function to restrict the flow of the solution inside the bipolar electrolytic cell 50 for water electrolysis. Furthermore, the anode chamber 5a and the cathode chamber 5c may be provided with protrusions for creating Karman vortices to uniform the concentration and temperature of the electrolyte solution inside the bipolar electrolytic cell 50 for water electrolysis and to promote degassing of gases adhering to the electrodes 2 and diaphragm 4.

In the example shown in Figures 1 and 2, the rectangular partition wall 1 and rectangular diaphragm 4 are arranged parallel to each other, and the inner surface of the rectangular outer frame 3 attached to the edge of the partition wall 1 on the partition wall 1 side is perpendicular to the partition wall 1, resulting in the electrode chamber 5 having a rectangular parallelepiped shape. The shape of the electrode chamber 5 is not limited to this, and may be modified as appropriate depending on the planar shapes of the partition wall 1 and diaphragm 4, the angle between the inner surface of the outer frame 3 on the partition wall 2 side and the partition wall 2, etc. Any shape is acceptable as long as the effects of the present invention are achieved.

The bipolar electrolytic cell 50 for water electrolysis is usually fitted with headers, which are pipes for distributing or collecting the electrolyte, and is provided with an anode inlet header for introducing the electrolyte into the anode chamber 5a and a cathode inlet header for introducing the electrolyte into the cathode chamber 5c, both located at the lower part of the outer frame 3 at the edge of the partition wall 1. Similarly, an anode outlet header for discharging the electrode solution from the anode chamber 5a and a cathode outlet header for discharging the electrolyte from the cathode chamber 5c are provided at the upper part of the outer frame 3 at the edge of the partition wall 1.
The headers attached to the bipolar electrolytic cell 50 for water electrolysis are typically of an internal header type and an external header type, but either type may be employed in the present invention, and there is no particular limitation.

In the bipolar electrolytic cell 50 for water electrolysis of this embodiment, the electrolyte dispensed in the anode inlet header is introduced into the anode chamber 5a through the anode electrolyte inlet, passes through the anode chamber 5a, and is discharged from the anode chamber 5a through the anode electrolyte outlet, and is collected in the anode outlet header.

The bipolar electrolytic cell 50 for water electrolysis of this embodiment may be provided with multiple straightening plates 6 arranged parallel to a given direction D1 along the partition wall 1 to reduce convection that occurs in the electrolysis chamber 5 due to turbulence in the gas-liquid flow within the electrolysis chamber 5 and suppress local increases in the temperature of the electrolyte (see Figure 2).

In the cell stack (a), the thickness of the gasket 7 is preferably 5 to 20% of the thickness of the bipolar element, more preferably 5 to 15%, and even more preferably 5 to 10%.
As will be described later, the outer frame, partition walls, etc. of the bipolar element 60 are made of materials such as stainless steel (SUS), steel, nickel, etc., and the gasket 7 is made of material such as rubber, so the thermal expansion coefficient of the gasket 7 is higher than that of the bipolar element 60, and the thicker the gasket 7 (the longer its length in the stack direction), the higher the average thermal expansion coefficient of the entire cell stack (a). Therefore, by setting the thickness of the gasket 7 within the above range, the average thermal expansion coefficient of the cell stack (a) can be reduced, and a decrease in seal surface pressure can be prevented.
The specific thickness of the gasket 7 is determined depending on the size and shape of the bipolar electrolytic cell 50 for water electrolysis, but is, for example, preferably 0.5 to 10 mm, more preferably 2 to 7 mm, and even more preferably 3 to 6 mm.

Furthermore, it is preferable that the surface pressure (sealing surface pressure) between the gasket 7 and each element 51a, 60, 51c during electrolysis is at least twice the maximum internal pressure of the bipolar electrolytic cell for water electrolysis 50, and that the compression rate of the gasket 7 in the stacking direction of the cell stack (a) is 10 to 30%. When the sealing surface pressure and the compression rate of the gasket 7 are within the above ranges, there is no leakage of the electrolyte or generated gas, and the gasket is less likely to tear or break.
The seal surface pressure is more preferably 5 times or more, and even more preferably 10 times or more, the maximum internal pressure of the bipolar electrolytic cell 50 for water electrolysis.
The maximum internal pressure of the bipolar electrolytic cell 50 for water electrolysis can be measured by a pressure gauge provided in the outlet flow path for the generated gas. The seal surface pressure can be determined by inserting a load cell between the press plate (b) and the heat insulating member (d) to measure the load in the stack direction.
The compression ratio of the gasket 7 in the stacking direction is more preferably 13 to 25%, and even more preferably 15 to 20%.
The compression ratio of the gasket 7 refers to the shrinkage ratio of the length of the gasket 7 in the stack direction compared before and after electrolysis.

<<Press plate (b)>>
In the bipolar electrolytic cell 50 for water electrolysis of this embodiment, press plates (b) are arranged on both ends of the cell stack (a). The press plates (b) are fastened to the cell stack (a) by tie rods (c), and the fastening load of the tie rods (c) applies surface pressure (sealing surface pressure) between the gasket 7 and the diaphragm 4, and between the gasket 7 and each of the elements 51 a, 60, 51 c. The gasket 7 to which surface pressure is applied by the press plates (b) seals the spaces between each bipolar element and the diaphragm 4, and between adjacent bipolar elements, against the electrolytic solution and the generated gas, thereby preventing leakage of the electrolytic solution or the generated gas to the outside of the bipolar electrolytic cell 50 for water electrolysis and mixing of the generated gas between the two electrode chambers.

The material of the press plate (b) is not particularly limited, and conventionally known materials can be used. Specific examples include stainless steel, carbon steel for mechanical structures, nickel, etc.

<<Tie rod (c)>>
In the bipolar electrolytic cell 50 for water electrolysis of this embodiment, the tie rods (c) are attached to the press plates (b) and fasten the cell stack (a) and the press plates (b) together, thereby integrating the bipolar electrolytic cell 50 for water electrolysis.

The material for the tie rod (c) is preferably one with a low thermal expansion coefficient, such as titanium.

The average thermal expansion coefficient β of the tie rods (c) in the stack direction at the electrolysis temperature is preferably 12×10 ⁻⁶ K ⁻¹ or less, more preferably 8×10 ⁻⁶ K ⁻¹ or less, and even more preferably 3×10 ⁻⁶ K ⁻¹ or less.
In the present disclosure, the electrolysis temperature refers to the temperature of the electrolyte during electrolysis, and may be 80±30°C.

<<Thermal insulation member (d)>>
In the bipolar electrolytic cell 50 for water electrolysis of this embodiment, the heat insulating member (d) plate is disposed between the cell stack (a) and the press plate (b) to reduce heat conduction from the cell stack (a) to the press plate (b) during electrolysis. This prevents the tie rods (c) in contact with the press plate (b) from expanding and increasing in length in the stack direction, which would result in a decrease in seal surface pressure, and reduces the number of times the tie rods (c) need to be retightened, thereby achieving a long-life bipolar electrolytic cell for water electrolysis.

The material for the insulating member (d) is preferably one with low thermal conductivity, and specific examples include polytetrafluoroethylene (PTFE), polyvinyl chloride, polyether ether ketone (PEEK), polyoxymethylene (POM), etc.

The thermal conductivity A of the heat insulating member (d) at 100°C is preferably 10 W/m·K or less, more preferably 1 W/m·K or less, even more preferably 0.5 W/m·K or less, still more preferably 0.3 W/m·K or less, and particularly preferably 0.1 W/m·K or less.
Furthermore, in a preferred example of the bipolar electrolytic cell 50 for water electrolysis according to this embodiment, the thermal conductivity A of the heat insulating member (d) at the electrolysis temperature is smaller than the thermal conductivity B of the press plate (b) at the electrolysis temperature. Since the thermal conductivity A is smaller than the thermal conductivity B, heat conduction to the tie rod (c) in contact with the press plate (b) is reduced, and the temperature rise of the tie rod (c) can be reduced, thereby reducing thermal expansion of the tie rod (c) and preventing a decrease in seal surface pressure.
The thermal conductivity can be measured specifically by the method described in the examples below.

The insulating member (d) is preferably electrically insulating. Therefore, the volume resistivity of the insulating member (d) at the electrolysis temperature is preferably 1 kΩ·cm or more, more preferably 1 MΩ·cm or more, and even more preferably 1 GΩ·cm or more.

<<Spring member (e)>>
The bipolar electrolytic cell 50 for water electrolysis of this embodiment may further include a spring member (e) elastically deformable in the stacking direction of the cell stack (a) and attached to the outer periphery of the axis of the tie rod (c). When the spring member (e) is provided, when the cell stack (a) thermally expands (i.e., its length in the stacking direction) the spring member (e) compresses, thereby absorbing the amount of thermal expansion of the cell stack (a), thereby preventing a decrease in seal surface pressure.

The spring member (e) is not particularly limited and can be any conventionally known member, with a preferred example being a disc spring. The material of the spring member (e) is also not particularly limited and examples include stainless steel, spring steel, and carbon steel for mechanical structures.

Furthermore, it is preferable to use a spring member (e) such that the expansion length ΔL in the stack direction of the cell stack (a) at the electrolysis temperature of the cell stack (a) is 75% or less of the total deflection T, where T is the total deflection of the spring member (e). More preferably, the spring member has ΔL of 50% or less of the total deflection T, and even more preferably 25% or less. When the relationship between the expansion length ΔL and the total deflection T is within the above range, the spring member (e) can effectively absorb the amount of thermal expansion of the cell stack (a).
The total deflection T refers to the value obtained by subtracting the length of the spring when in close contact from the free length of the spring.

The spring member (e) preferably has a spring constant of 5/n to 20/n kN/mm, more preferably 5/n to 15/n kN/mm, and even more preferably 5/n to 10/n kN/mm. Here, n is the number of tie rods (c) to which the spring member (e) is attached. A spring constant within the above range allows the spring member (e) to effectively absorb the thermal expansion of the cell stack (a).

In the bipolar electrolytic cell 50 for water electrolysis of this embodiment, the average thermal expansion coefficient α in the stacking direction of the entire stack of the cell stack (a), the press plate (b), and the heat insulating member (d) at the electrolysis temperature is preferably greater than the average thermal expansion coefficient β in the stacking direction of the tie rod (c) at the electrolysis temperature. In other words, the amount of thermal expansion (length extension in the stacking direction) of the entire stack of the cell stack (a), the press plate (b), and the heat insulating member (d) is preferably greater than the amount of thermal expansion (length extension in the stacking direction) of the tie rod (c). This prevents a decrease in the tightening force of the tie rod (c) when the cell stack (a), the press plate (b), the heat insulating member (d), and the tie rod (c) thermally expand during electrolysis.

The average thermal expansion coefficient α in the stack direction at the electrolysis temperature of the cell stack (a), press plate (b), and heat insulating member (d) is preferably 10×10 ⁻⁶ K ⁻¹ or more, more preferably 30×10 ⁻⁶ K ⁻¹ or more, and even more preferably 50×10 ⁻⁶ K ⁻¹ or more.
The average thermal expansion coefficient α can be determined by the method described in the examples below.

Hereinafter, each of the components of the bipolar electrolytic cell 50 for water electrolysis of this embodiment other than the press plate (b), the tie rod (c), the heat insulating member (d), and the spring member (e) will be described in detail.
In addition, preferred embodiments for enhancing the effects of the present invention will be described in detail below.

-Bulkhead-
The shape of the partition wall 1 in this embodiment may be a plate shape having a predetermined thickness, but is not particularly limited to this.
The shape of the partition wall 1 in plan view is not particularly limited, and may be rectangular (square, oblong, etc.) or circular (circle, ellipse, etc.), and the rectangle may have rounded corners.
In an embodiment, the partition wall 1 and the outer frame 3 may be integrated by joining them by welding or a other method. For example, the partition wall 1 may be provided with a flange portion protruding in a direction perpendicular to the plane of the partition wall 1 (an anode flange portion protruding toward the anode 2a side and a cathode flange portion protruding toward the cathode 2c side), and the flange portions may be part of the outer frame 3.

The partition wall 1 may typically be used so that the given direction D1 along the partition wall 1 is the vertical direction. Specifically, when the partition wall 1 has a rectangular shape in plan view as shown in Figure 2, the given direction D1 along the partition wall 1 may be used so that it is the same direction as the direction of one of two pairs of opposing sides. In this specification, the vertical direction is also referred to as the electrolyte passage direction.

The material for the partition wall 1 is preferably a conductive material from the perspective of realizing a uniform supply of power, and from the perspective of durability and heat resistance, stainless steel, steel, nickel, nickel alloy, mild steel, or nickel alloy plated with nickel is preferred.

-electrode-
In hydrogen production by water electrolysis according to this embodiment, reducing energy consumption, specifically reducing the electrolysis voltage, is a major issue. Since this electrolysis voltage is highly dependent on the electrodes 2, the performance of both electrodes 2 is important.

The electrolysis voltage for water electrolysis is divided into the theoretically required voltage for water electrolysis, the overvoltage for the anodic reaction (oxygen generation), the overvoltage for the cathodic reaction (hydrogen generation), and the voltage due to the distance between the electrodes 2, the anode 2a and the cathode 2c. Here, overvoltage refers to the voltage that must be applied in excess of the theoretical decomposition potential when a certain current is passed, and its value depends on the current value. When passing the same current, power consumption can be reduced by using an electrode 2 with a lower overvoltage.

Requirements for electrode 2 to achieve low overvoltage include high electrical conductivity, high oxygen generation capacity (or hydrogen generation capacity), and high wettability of the electrolyte on the surface of electrode 2.

As an electrode 2 for water electrolysis, in addition to having a low overvoltage, it is also less susceptible to corrosion of the electrode 2 substrate and catalytic layer, detachment of the catalytic layer, dissolution in the electrolyte, and adhesion of inclusions to the diaphragm 4, even when using an unstable current such as that from renewable energy.

In this embodiment, the electrode 2 is preferably porous in order to increase the surface area available for electrolysis and to efficiently remove gas generated by electrolysis from the surface of the electrode 2. In particular, in the case of a zero-gap electrolytic cell, it is necessary to degas the gas generated from the back side of the surface that comes into contact with the diaphragm 4, so it is preferable that the surface of the electrode 2 opposite the surface that comes into contact with the membrane be perforated.

Examples of porous materials include plain woven mesh, punched metal, expanded metal, and metal foam.

In this embodiment, the electrode 2 may be the substrate itself, or may have a highly reactive catalyst layer on the surface of the substrate, but it is preferable that the electrode 2 has a highly reactive catalyst layer on the surface of the substrate.

There are no particular restrictions on the material of the substrate, but mild steel, stainless steel, nickel, and nickel-based alloys are preferred due to their resistance to the usage environment.

The catalytic layer of the anode 2a preferably has high oxygen generation capacity, and nickel, cobalt, iron, or platinum group elements can be used. To achieve the desired activity and durability, the catalytic layer can be formed as a single metal, a compound such as an oxide, a composite oxide or alloy made of multiple metal elements, or a mixture of these. Organic substances such as polymers may also be included to improve durability and adhesion to the substrate.

The catalytic layer of the cathode 2c preferably has high hydrogen generation capacity, and nickel, cobalt, iron, platinum group elements, etc. can be used. To achieve the desired activity and durability, the catalytic layer can be formed as a single metal, a compound such as an oxide, a composite oxide or alloy made of multiple metal elements, or a mixture of these. Organic substances such as polymeric materials may also be included to improve durability and adhesion to the substrate.

Methods for forming a catalyst layer on a substrate include plating, thermal spraying methods such as plasma spraying, thermal decomposition methods in which a precursor layer solution is applied to the substrate and then heated, methods in which a catalyst substance is mixed with a binder component and immobilized on the substrate, and vacuum film formation methods such as sputtering.

-Outer frame-
The shape of the outer frame 3 in this embodiment is not particularly limited as long as it can frame the partition wall 1, but it may be a shape that has an inner surface that is perpendicular to the plane of the partition wall 1 and extends along the outer periphery of the partition wall 1.
The shape of the outer frame 3 is not particularly limited and may be determined appropriately in accordance with the shape of the partition wall 1 in a plan view.

The material for the outer frame 3 is preferably a conductive material, and from the standpoint of alkali resistance and heat resistance, stainless steel, steel, nickel, nickel alloy, mild steel, or nickel alloy plated with nickel is preferred.

-diaphragm-
The diaphragm 4 used in the bipolar electrolytic cell 50 for water electrolysis of this embodiment is an ion-permeable diaphragm 4 that separates the generated hydrogen gas and oxygen gas while conducting ions. The ion-permeable diaphragm 4 can be an ion exchange membrane having ion exchange capacity or a porous membrane that is permeable to the electrolytic solution. The ion-permeable diaphragm 4 preferably has low gas permeability, high ionic conductivity, low electronic conductivity, and high strength.

--Porous membrane--
The porous membrane has a structure that has a plurality of fine through-holes and allows the electrolyte to pass through the diaphragm 4. Since the electrolyte permeates the porous membrane to exhibit ionic conduction, it is extremely important to control the porous structure, such as the pore size, porosity, and hydrophilicity. On the other hand, it is required that not only the electrolyte but also the generated gas does not pass through, i.e., the membrane has gas barrier properties. From this viewpoint as well, control of the porous structure is important.

The porous membrane has a plurality of fine through-holes, and examples thereof include polymer porous membranes, inorganic porous membranes, woven fabrics, nonwoven fabrics, etc. These can be produced by known techniques.
Examples of methods for producing porous polymer membranes include a phase inversion method (microphase separation method), an extraction method, a stretching method, and a wet gel stretching method.

The porous membrane preferably contains a polymer material and hydrophilic inorganic particles, and the presence of the hydrophilic inorganic particles can impart hydrophilic properties to the porous membrane.

---Polymer materials---
Examples of polymeric materials include polysulfone, polyethersulfone, polyphenylsulfone, polyvinylidene fluoride, polycarbonate, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-ethylene copolymer, polyvinylidene fluoride, polytetrafluoroethylene, perfluorosulfonic acid, perfluorocarboxylic acid, polyethylene, polypropylene, polyphenylene sulfide, polyparaphenylene benzobisoxazole, polyketone, polyimide, polyetherimide, etc. Among these, polysulfone, polyethersulfone, polyphenylsulfone, polyphenylene sulfide, and polytetrafluoroethylene are preferred, and polysulfone is more preferred. These may be used alone or in combination of two or more.

The pore size of the porous membrane is preferably controlled in order to obtain appropriate membrane properties such as separation ability, strength, etc. When the porous membrane is used for water electrolysis, it is preferable to control the pore size of the porous membrane from the viewpoints of preventing mixing of oxygen gas generated from the anode 2a and hydrogen gas generated from the cathode 2c and reducing voltage loss during electrolysis.
The larger the average pore size of the porous membrane, the larger the amount of ion passing through the porous membrane per unit area, and particularly in electrolysis, the better the ion permeability of the porous membrane becomes, which tends to facilitate reduction in voltage loss. Also, the larger the average pore size of the porous membrane, the smaller the contact surface area with alkaline water becomes, which tends to suppress degradation of the polymer.
On the other hand, the smaller the average pore size of the porous membrane, the higher the separation accuracy of the porous membrane, and the better the gas barrier property of the porous membrane in electrolysis tends to be. Furthermore, when hydrophilic inorganic particles having a small particle size, as described later, are supported on the porous membrane, they can be firmly held without falling off. This allows the high holding ability of the hydrophilic inorganic particles to be imparted, and the effect can be maintained for a long period of time.

From this perspective, the porous membrane of this embodiment preferably has an average pore size in the range of 0.1 to 1.0 μm. A porous membrane with a pore size in this range can achieve both excellent gas barrier properties and high ion permeability. The pore size of the porous membrane is preferably controlled within the temperature range in which it will actually be used. Therefore, for example, when used as an electrolysis diaphragm 4 in an environment of 90°C, it is preferable that the pore size satisfy the above-mentioned range at 90°C. Furthermore, the porous membrane more preferably has an average pore size of 0.1 to 0.5 μm, which is the range in which the porous membrane can achieve even better gas barrier properties and high ion permeability as a water electrolysis diaphragm 4.

The average pore size of the porous membrane can be measured by the following method.
The average pore size of the porous membrane refers to the average water permeation pore size measured using an integrity tester (manufactured by Sartorius Stedim Japan, "Sartocheck Junior BP-Plus") using the following method. First, the porous membrane, including the core material, is cut to a predetermined size and used as a sample. This sample is placed in an arbitrary pressure-resistant container, and the container is filled with pure water. Next, the pressure-resistant container is held in a thermostatic chamber set to a predetermined temperature, and measurement is started once the inside of the pressure-resistant container has reached the predetermined temperature. When measurement begins, the upper surface of the sample is pressurized with nitrogen, and the pressure and permeation flow rate when pure water permeates from the lower surface of the sample are recorded. The average water permeation pore size can be determined from the following Hagen-Poiseuille equation using the gradient between pressure and water permeation flow rate at pressures of 10 kPa to 30 kPa.
Average permeable pore diameter (m) = {32ηLμ ₀ /(εP)} ^0.5
where η is the viscosity of water (Pa·s), L is the thickness of the porous membrane (m), μ ₀ is the apparent flow velocity, μ ₀ (m/s) = flow rate (m ³ /s) / flow path area (m ² ), ε is the porosity, and P is the pressure (Pa).

It is preferable to control the porosity of the diaphragm 4 from the viewpoints of gas barrier properties, maintaining hydrophilicity, preventing a decrease in ion permeability due to adhesion of gas bubbles, and further obtaining stable electrolytic performance (low voltage loss, etc.) for a long period of time.
From the viewpoint of achieving high levels of gas barrier properties and low voltage loss, the lower limit of the porosity of the porous membrane is preferably 30% or more, more preferably 35% or more, and even more preferably 40% or more. The upper limit of the porosity is preferably 70% or less, more preferably 65% or less, and even more preferably 55% or less. If the porosity of the porous membrane is the above upper limit or less, ions can easily permeate the membrane, and the voltage loss of the membrane can be suppressed.

The porosity of the porous film refers to the open porosity determined by Archimedes' method, and can be calculated by the following formula.
Porosity P (%) = ρ / (1 + ρ) × 100
Here, ρ = (W3 - W1) / (W3 - W2), W1 is the dry mass (g) of the porous membrane, W2 is the mass (g) of the porous membrane in water, and W3 is the saturated mass (g) of the porous membrane.

To measure porosity, a porous membrane washed with pure water is cut into three pieces measuring 3 cm x 3 cm to serve as measurement samples. First, W2 and W3 of the sample are measured. The porous membrane is then left to dry for at least 12 hours in a dryer set at 50°C, and W1 is measured. The porosity is then calculated from the values of W1, W2, and W3. The porosity of the three samples is calculated, and their arithmetic average is taken as the porosity P.

The thickness of the porous membrane is not particularly limited, but is preferably 100 to 700 μm, more preferably 100 to 600 μm, and even more preferably 200 to 600 μm.
When the thickness of the porous membrane is equal to or greater than the lower limit, it is less likely to be broken by piercing or the like, and it is less likely to short-circuit between electrodes. Furthermore, the gas barrier property is improved. When the thickness is equal to or less than the upper limit, the voltage loss is less likely to increase. Furthermore, the influence of variations in the thickness of the porous membrane is reduced.
Furthermore, when the thickness of the diaphragm is 100 μm or more, it is difficult to break by piercing or the like, and short circuits between electrodes are unlikely to occur. Furthermore, gas barrier properties are good. When the thickness is 600 μm or less, voltage loss is unlikely to increase. Furthermore, the influence of variations in the thickness of the porous membrane is reduced.
If the thickness of the porous membrane is 250 μm or more, better gas barrier properties can be obtained, and the strength of the porous membrane against impact can be further improved. From this viewpoint, the lower limit of the thickness of the porous membrane is more preferably 300 μm or more, even more preferably 350 μm or more, and even more preferably 400 μm or more. On the other hand, if the thickness of the porous membrane is 700 μm or less, the ion permeability is less likely to be hindered by the resistance of the electrolyte contained in the pores during operation, and better ion permeability can be maintained. From this viewpoint, the upper limit of the thickness of the porous membrane is more preferably 600 μm or less, even more preferably 550 μm or less, and even more preferably 500 μm or less.

--- Hydrophilic inorganic particles ---
The porous membrane preferably contains hydrophilic inorganic particles to exhibit high ion permeability and high gas barrier properties. The hydrophilic inorganic particles may be attached to the surface of the porous membrane, or may be partially embedded in the polymer material constituting the porous membrane. Furthermore, when the hydrophilic inorganic particles are encapsulated in the voids of the porous membrane, they are less likely to be detached from the porous membrane, and the performance of the porous membrane can be maintained for a long period of time.

Hydrophilic inorganic particles include, for example, at least one inorganic material selected from the group consisting of oxides or hydroxides of zirconium, bismuth, and cerium; oxides of Group IV elements of the periodic table; nitrides of Group IV elements of the periodic table; and carbides of Group IV elements of the periodic table. Among these, from the standpoint of chemical stability, oxides of zirconium, bismuth, and cerium and oxides of Group IV elements of the periodic table are more preferred, oxides of zirconium, bismuth, and cerium are even more preferred, and zirconium oxide is even more preferred.

The hydrophilic inorganic particles are preferably in the form of fine particles.

--Porous support--
When a porous membrane is used as the diaphragm 4, the porous membrane may be used together with a porous support. Preferably, the porous membrane has a structure in which the porous support is embedded, and more preferably, the porous membrane is laminated on both sides of the porous support. Alternatively, the porous membrane may be laminated symmetrically on both sides of the porous support.

Examples of porous supports include meshes, porous membranes, nonwoven fabrics, woven fabrics, and composite fabrics containing nonwoven fabrics and woven fabrics embedded within the nonwoven fabrics. These may be used alone or in combination of two or more. More suitable embodiments of the porous support include, for example, mesh substrates made of polyphenylene sulfide monofilaments, or composite fabrics containing nonwoven fabrics and woven fabrics embedded within the nonwoven fabrics.

--Ion exchange membrane--
Ion exchange membranes include cation exchange membranes that selectively allow cations to pass through and anion exchange membranes that selectively allow anions to pass through, and either type of exchange membrane can be used.
The material of the ion exchange membrane is not particularly limited, and known materials can be used. For example, fluorine-containing resins and modified resins of polystyrene-divinylbenzene copolymers can be suitably used. Fluorine-containing ion exchange membranes are particularly preferred because of their excellent heat resistance and chemical resistance.

Examples of fluorine-containing ion exchange membranes include those that selectively transmit ions generated during electrolysis and contain fluorine-containing polymers that have ion exchange groups. Here, a fluorine-containing polymer with ion exchange groups refers to a fluorine-containing polymer that has ion exchange groups or ion exchange group precursors that can become ion exchange groups through hydrolysis. Examples include polymers that have a fluorinated hydrocarbon main chain, pendant side chains with functional groups that can be converted to ion exchange groups through hydrolysis or other processes, and are melt-processable.

The molecular weight of the fluorine-containing copolymer is not particularly limited, but the melt flow index (MFI) of the precursor measured in accordance with ASTM D1238 (measurement conditions: temperature 270°C, load 2160 g) is preferably 0.05 to 50 (g/10 min), and more preferably 0.1 to 30 (g/10 min).

Examples of ion exchange groups possessed by ion exchange membranes include cation exchange groups such as sulfonic acid groups, carboxylic acid groups, and phosphate groups, and anion exchange groups such as quaternary ammonium groups.

By adjusting the equivalent mass (EW) of the ion exchange groups, the ion exchange membrane can be endowed with excellent ion exchange capacity and hydrophilicity. In addition, it can be controlled to have many smaller clusters (microscopic portions where the ion exchange groups coordinate and/or adsorb water molecules), which tends to improve alkali resistance and ion selective permeability.
The equivalent weight EW can be measured by salt-substituting the ion exchange membrane and back-titrating the resulting solution with an alkaline or acid solution. The equivalent weight EW can be adjusted by the copolymerization ratio of the raw material monomers, the selection of the monomer species, etc.
The equivalent mass EW of the ion exchange membrane is preferably 300 or more from the viewpoint of hydrophilicity and water resistance of the membrane, and is preferably 1300 or less from the viewpoint of hydrophilicity and ion exchange capacity.

The thickness of the ion exchange membrane is not particularly limited, but from the standpoint of ion permeability and strength, a range of 5 to 300 μm is preferred.

The ion exchange membrane may be subjected to a surface treatment to improve its hydrophilicity. Specific examples include coating the surface with hydrophilic inorganic particles such as zirconium oxide, or imparting fine irregularities to the surface.

From the standpoint of membrane strength, it is preferable to use the ion exchange membrane together with a reinforcing material. Reinforcing materials are not particularly limited, and include common nonwoven fabrics, woven fabrics, and porous membranes made from various materials. In this case, the porous membrane is not particularly limited, but is preferably a PTFE-based membrane that has been stretched to make it porous.

((Zero gap structure))
In the bipolar element 60 of the zero-gap cell, a means for reducing the inter-electrode distance is preferably provided by disposing an elastic spring between the electrode 2 and the partition wall 1 and supporting the electrode 2 with this spring. For example, in a first example, a spring made of a conductive material may be attached to the partition wall 1, and the electrode 2 may be attached to this spring. In a second example, a spring may be attached to an electrode rib attached to the partition wall 1, and the electrode 2 may be attached to this spring. When adopting such a configuration using an elastic body, the strength, number, shape, etc. of the spring must be appropriately adjusted as necessary to prevent uneven contact pressure between the electrode 2 and the diaphragm 4.

-Electrode chamber-
In the bipolar electrolytic cell 50 for water electrolysis in this embodiment, as shown in FIG. 2, an electrode chamber 5 through which the electrolyte passes is defined by a partition wall 1, an outer frame 3, and a diaphragm 4.

In this embodiment, the header 10 of the bipolar electrolytic cell can be arranged in either an internal header type or an external header type. For example, in the example shown, the space occupied by the anode 2a and cathode 2c themselves may also be considered to be part of the space inside the electrode chamber 5. Furthermore, particularly when a gas-liquid separation box is provided, the space occupied by the gas-liquid separation box may also be considered to be part of the space inside the electrode chamber 5.

- Rectifier plate -
In the bipolar electrolytic cell 50 for water electrolysis of this embodiment, it is preferable that the current rectifying plates 6 (anode current rectifying plates, cathode current rectifying plates) are attached to the partition wall 1, and the current rectifying plates 6 are physically connected to the electrodes 2. With such a configuration, the current rectifying plates 6 serve as supports for the electrodes 2, making it easy to maintain the zero gap structure Z.
Here, the electrode 2 may be provided on the rectifying plate 6, or the current collector 2r, the conductive elastic body 2e, and the electrode 2 may be provided on the rectifying plate 6 in this order.
In the bipolar electrolytic cell 50 for water electrolysis described above as an example, a structure is adopted in the cathode chamber 5c in which the current rectifying plate 6, the current collector 2r, the conductive elastic body 2e, and the electrode 2 are stacked in this order, and a structure is adopted in the anode chamber 5a in which the current rectifying plate 6 and the electrode 2 are stacked in this order.

In the example bipolar electrolytic cell 50 for water electrolysis described above, the cathode chamber 5c employs the above-described "rectifying plate 6 - current collector 2r - conductive elastic body 2e - electrode 2" structure, and the anode chamber 5a employs the above-described "rectifying plate 6 - electrode 2" structure. However, the present invention is not limited to this, and the anode chamber 5a may also employ the "rectifying plate 6 - current collector 2r - conductive elastic body 2e - electrode 2" structure.

It is preferable that the current rectifier plate 6 (anode current rectifier plate, cathode current rectifier plate) not only supports the anode 2a or cathode 2c, but also transmits current from the partition wall 1 to the anode 2a or cathode 2c.

In the bipolar electrolytic cell 50 for water electrolysis of this embodiment, it is preferable that at least a portion of the rectifying plate 6 is conductive, and it is even more preferable that the entire rectifying plate 6 is conductive. With this configuration, it is possible to suppress an increase in cell voltage due to electrode deflection.

The current plate 6 is generally made of a conductive metal. For example, nickel-plated mild steel, stainless steel, nickel, etc. can be used.

The distance between adjacent anode rectifier plates or between adjacent cathode rectifier plates is determined taking into account factors such as the electrolysis pressure and the pressure difference between the anode chamber 5a and the cathode chamber 5c.

The length of the current plates 6 (anode current plates, cathode current plates) may be determined appropriately depending on the size of the partition wall 1 .
The height of the rectifying plate 6 may be determined appropriately depending on the distance from the partition wall 1 to each flange portion, the thickness of the gasket 7, the thickness of the electrode 2 (anode 2 a, cathode 2 c), the distance between the anode 2 a and the cathode 2 c, etc.
The thickness of the current plate 6 may be 0.5 to 5 mm, taking into consideration cost, manufacturability, strength, etc., and a thickness of 1 to 2 mm is easy to use, but is not particularly limited.

-gasket-
In the bipolar electrolytic cell 50 for water electrolysis of this embodiment, it is preferable that a gasket 7 having a diaphragm 4 is sandwiched between the outer frames 3 that frame the partition walls 1 .
The gasket 7 is used to seal the space between the bipolar element 60 and the diaphragm 4 and between the bipolar elements 60 against the electrolytic solution and the generated gas, and can prevent leakage of the electrolytic solution or the generated gas to the outside of the electrolytic cell and mixing of gases between the two electrode chambers.

The gasket 7 generally has a rectangular or annular structure with the electrode surface hollowed out to match the surface that contacts the element frame. The diaphragm 4 can be stacked between elements by sandwiching it between two such gaskets. Furthermore, the gasket 7 preferably has a slit that can accommodate the diaphragm 4 so that it can hold the diaphragm 4, and also has openings that allow the accommodated diaphragm 4 to be exposed on both surfaces of the gasket 7. This allows the gasket 7 to accommodate the edge of the diaphragm 4 within the slit, covering the edge faces of the diaphragm 4. This more reliably prevents electrolyte and gas from leaking from the edge faces of the diaphragm 4.

The material of the gasket 7 is not particularly limited, and known insulating rubber materials, resin materials, etc. can be selected.
Specific examples of rubber and resin materials that can be used include rubber materials such as natural rubber (NR), styrene butadiene rubber (SBR), chloroprene rubber (CR), butadiene rubber (BR), acrylonitrile-butadiene rubber (NBR), silicone rubber (SR), ethylene-propylene rubber (EPT), ethylene-propylene-diene rubber (EPDM), fluororubber (FR), isobutylene-isoprene rubber (IIR), urethane rubber (UR), and chlorosulfonated polyethylene rubber (CSM); fluororesin materials such as polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-ethylene copolymer (ETFE), and chlorotrifluoroethylene-ethylene copolymer (ECTFE); and resin materials such as polyphenylene sulfide (PPS), polyethylene, polyimide, and polyacetal. Among these, ethylene-propylene-diene rubber (EPDM) and fluororubber (FR) are particularly suitable from the viewpoint of elastic modulus and alkali resistance.

A reinforcing material may be embedded in the gasket 7. This makes it possible to prevent the gasket 7 from being crushed when it is sandwiched and pressed between the frames during stacking, and makes it easier to prevent damage.
Such reinforcing materials can be made of known metal materials, resin materials, carbon materials, etc., and specific examples include metals such as nickel and stainless steel, resins such as nylon, polypropylene, PVDF, PTFE, and PPS, and carbon materials such as carbon particles and carbon fibers.

The size of the gasket 7 is not particularly limited and may be designed to match the dimensions of the electrode chamber 5 and the membrane, but it is preferable that the width be 10 to 40 mm.
In this case, when the gasket 7 has a slit portion, the size of the slit portion is preferably set so that the inner dimensions of the slit are 0.5 to 5 mm larger in length and width than the size of the membrane.

The thickness of the gasket 7 is as described above.
Furthermore, when the gasket 7 has a slit portion, the opening width of the slit portion may be 0.5 to 1.0 times the thickness of the film.

The elastic modulus of the gasket 7 is not particularly limited and is designed depending on the material and cell area of the electrode 2. A preferred range of the elastic modulus is a tensile stress at 100% deformation of 0.20 to 20 MPa, and from the viewpoint of sealing properties and cell strength when stacked, a range of 1.0 to 10 MPa is more preferred.
The tensile stress can be measured in accordance with JIS K6251. For example, an Autograph AG manufactured by Shimadzu Corporation may be used.

In particular, in this embodiment, it is preferable that the thickness of the gasket 7 is 3.0 to 10 mm and that the tensile stress at 100% deformation is 1.0 to 10 MPa, from the viewpoints of suppressing an increase in cell voltage due to electrode deflection, as well as sealing properties and cell strength when stacked.

In this embodiment, it is preferable to cover the surface of the gasket 7 with an insulating resin sheet (e.g., a fluororesin such as polytetrafluoroethylene). This ensures that the multiple elements 60 are insulated from one another, preventing the charge accumulated in each element 60 during the energization process (the process in which electrolysis of the electrolyte is carried out) from affecting other elements 60 during the stop process (the process in which electrolysis of the electrolyte is stopped).

-Header-
The bipolar electrolytic cell 50 for water electrolysis has a cathode chamber 5c and an anode chamber 5a for each electrolytic cell 65. In order to continuously perform the electrolytic reaction in the bipolar electrolytic cell 50 for water electrolysis, it is necessary to continuously supply an electrolyte solution containing a sufficient amount of raw materials to be consumed by electrolysis to the cathode chamber 5c and the anode chamber 5a of each electrolytic cell 65.

The electrolytic cells 65 are connected to electrolyte supply and discharge pipes called headers that are common to multiple electrolytic cells 65. Generally, the anode distribution pipe is called the anode inlet header, the cathode distribution pipe is called the cathode inlet header, the anode collection pipe is called the anode outlet header, and the cathode collection pipe is called the cathode outlet header. The electrolytic cells 65 are connected to each electrode distribution pipe and each electrode collection pipe via hoses, etc.

There are no particular restrictions on the material of the header, but it must be able to withstand the corrosiveness of the electrolyte used and operating conditions such as pressure and temperature. Materials that may be used for the header include iron, nickel, cobalt, PTFE, ETFE, PFA, polyvinyl chloride, polyethylene, etc.

In this embodiment, the extent of the electrode chambers 5 varies depending on the detailed structure of the outer frame 3 provided at the outer end of the partition wall 1, and the detailed structure of the outer frame 3 may differ depending on the arrangement of the headers (pipes for distributing or collecting the electrolyte) attached to the outer frame 3. Typical header arrangements for a bipolar electrolytic cell 50 for water electrolysis are internal header and external header types.

--Internal Header--
The internal header type refers to a type in which the bipolar electrolytic cell 50 for water electrolysis and a header (a pipe for distributing or collecting the electrolyte) are integrated.

More specifically, in an internal header type bipolar electrolytic cell, the anode inlet header and cathode inlet header are provided at the lower part of the partition wall 1 and/or the outer frame 3, and extend perpendicular to the partition wall 1, while the anode outlet header and cathode outlet header are provided at the upper part of the partition wall 1 and/or the outer frame 3, and extend perpendicular to the partition wall 1.

The anode inlet header, cathode inlet header, anode outlet header, and cathode outlet header inherent in an internal header type bipolar electrolyzer are collectively referred to as the internal headers.

In an example of the internal header type, an anode inlet header and a cathode inlet header are provided in a portion of the lower part of the outer frame 3 at the edge of the partition wall 1, and similarly, an anode outlet header and a cathode outlet header are provided in a portion of the upper part of the outer frame 3 at the edge of the partition wall 1.

--External headers--
The external header type refers to a type in which the bipolar electrolytic cell 50 for water electrolysis and the header (a pipe for distributing or collecting the electrolyte) are independent.

In an external header type bipolar electrolytic cell, the anode inlet header and cathode inlet header are installed independently, running parallel to the bipolar electrolytic cell 50 for water electrolysis, in a direction perpendicular to the current-carrying surface of the electrolytic cell 65. The anode inlet header and cathode inlet header are connected to each electrolytic cell 65 with hoses.

The anode inlet header, cathode inlet header, anode outlet header, and cathode outlet header, which are externally connected to an external header type bipolar electrolyzer, are collectively called external headers.
In the external header type example, a tubular member is installed in a header through hole provided in a lower portion of the outer frame 3 at the edge of the partition wall 1, and the tubular member is connected to an anode inlet header and a cathode inlet header. Similarly, a tubular member (e.g., a hose or a tube) is installed in a header through hole provided in an upper portion of the outer frame 3 at the edge of the partition wall 1, and the tubular member is connected to an anode outlet header and a cathode outlet header.

In addition, both internal header and external header bipolar electrolytic cells 50 for water electrolysis may have a gas-liquid separation box inside them that separates the gas generated by electrolysis from the electrolyte. The installation position of the gas-liquid separation box is not particularly limited, but it may be installed between the anode chamber 5a and the anode outlet header, or between the cathode chamber 5c and the cathode outlet header.

The surface of the gas-liquid separation box may be coated with a coating material that is sufficiently resistant to the corrosiveness of the electrolyte and operating conditions such as pressure and temperature. The coating material may be insulating in order to increase the electrical resistance of the leakage current circuit inside the electrolytic cell. EPDM, PTFE, ETFE, PFA, polyvinyl chloride, polyethylene, etc. may be used as the coating material.

<Electrolyzer for water electrolysis>
FIG. 3 shows an outline of an electrolysis device 70 for water electrolysis using the bipolar electrolysis cell 50 for water electrolysis of this embodiment.
The electrolysis device 70 for water electrolysis of this embodiment includes the bipolar electrolytic cell 50 for water electrolysis of this embodiment, a liquid feed pump 71 for circulating the electrolytic solution, a gas-liquid separation tank 72 for separating the electrolytic solution from hydrogen and/or oxygen, and a water supply device 73 for supplying water consumed by electrolysis.

According to the electrolysis device 70 for water electrolysis of this embodiment, the effects of the bipolar electrolytic cell 50 for water electrolysis of this embodiment can be obtained.
That is, according to this embodiment, a bipolar electrolysis device for water electrolysis having a long life even when used under conditions of variable power supply or frequent operation stoppages can be obtained.

The components of the water electrolysis device 70 of this embodiment are described below.

- Liquid delivery pump -
The liquid feed pump 71 used in this embodiment is not particularly limited and may be determined as appropriate.

-Gas-liquid separation tank-
The gas-liquid separation tank 72 used in this embodiment includes a hydrogen separation tank 72h that separates the hydrogen gas generated in the cathode chamber 5c from the electrolytic solution, and an oxygen separation tank 72o that separates the oxygen gas generated in the anode chamber 5a from the electrolytic solution.
The hydrogen separation tank 72h is connected to the cathode chamber 5c, and the oxygen separation tank 72o is connected to the anode chamber 5a.

The mixture of electrolyte and generated gas discharged from the electrolytic cell 65 is allowed to flow into the gas-liquid separation tank 72. If gas-liquid separation is not performed properly, oxygen gas and hydrogen gas will be mixed when the electrolytes in the cathode chamber 5c and anode chamber 5a mix, reducing the purity of the gas. In the worst case scenario, there is a risk of explosive gas formation.

When the gas and electrolyte flow into the gas-liquid separation tank 72, the gas separates into the gas phase in the upper layer of the tank, and the electrolyte separates into the liquid phase in the lower layer of the tank. The degree of gas-liquid separation is determined by the flux of the electrolyte in the gas-liquid separation tank 72, the floating speed of the generated gas bubbles, and the residence time in the gas-liquid separation tank 72.

After the gas is separated, the electrolyte flows out of the outlet at the bottom of the tank and flows back into the electrolytic cell 65, forming a circulation path. Because the oxygen and hydrogen gas discharged from the outlet at the top of the tank both contain alkaline mist, it is preferable to install a device downstream of the outlet that can liquefy the excess mist and return it to the gas-liquid separation tank 72, such as a mist separator or cooler.

The gas-liquid separation tank 72 may also be equipped with a level gauge to monitor the liquid level of the electrolyte stored inside.

It is also preferable that the gas-liquid separation tank 72 be equipped with a pressure relief valve. This allows the pressure to be safely reduced if it exceeds the design pressure, even if it is increased by gas generated during electrolysis.

The inlet to the gas-liquid separation tank 72 is preferably located above the electrolyte surface in order to improve gas-liquid separation, but is not limited to this.
In order to prevent the liquid level in the electrolytic cell from dropping when circulation is stopped, it is preferable that the electrolyte level in the gas-liquid separation tank 72 be higher than the upper surface of the electrolytic cell, but this is not limitative.
It is preferable to provide a shutoff valve between the electrolytic cell 65 and the gas-liquid separation tank 72, but this is not a limitation.

The gas-liquid separation tank 72 is made of an alkali-resistant metal such as nickel. On the other hand, when a general-purpose metal such as iron is used as the tank housing material, the electrolyte-contacting surface inside the tank may be coated with a fluorine-based resin or the like, but this does not limit the material of the gas-liquid separation tank 72 in the present invention.

Considering the installation volume, it is preferable that the capacity of the gas-liquid separation tank 72 is small. However, if the volume is too small, the liquid level in the tank will fluctuate if the pressure difference between the cathode 2 c and the anode 2 a increases or if there is a fluctuation in the electrolysis current value, and therefore it is necessary to take this fluctuation into consideration.
Similarly, if the tank height is low, it is susceptible to the influence of the above fluctuations, so it is preferable to make it high.

-Water supply device-
The water supply device 73 used in this embodiment is not particularly limited and may be determined as appropriate.
As the water, ordinary tap water may be used, but in consideration of long-term operation, it is preferable to use ion-exchanged water, RO water, ultrapure water, etc.

-others-
The electrolysis apparatus 70 for water electrolysis of this embodiment may include a rectifier 74, an oxygen concentration meter 75, a hydrogen concentration meter 76, a flow meter 77, a pressure meter 78, a heat exchanger 79, and a pressure control valve 80, in addition to the bipolar electrolytic cell 50 for water electrolysis, a liquid feed pump 71, a gas-liquid separation tank 72, and a water supply device 73.

Furthermore, the water electrolysis device 70 of this embodiment preferably further includes a detector that detects the interruption of power supply when the power supply is stopped, and a controller that automatically stops the liquid feed pump. By including a detector and a controller, it becomes possible to efficiently reduce the effects of self-discharge without manual operation, even with a power source that fluctuates greatly, such as renewable energy.

The bipolar electrolytic cell for water electrolysis and the electrolysis device for water electrolysis according to the embodiments of the present invention have been described above with reference to the drawings. However, the bipolar electrolytic cell for water electrolysis and the electrolysis device for water electrolysis according to the present invention are not limited to the above examples, and appropriate modifications can be made to the above embodiments.

The present invention will be explained in detail below using examples and comparative examples, but it goes without saying that the present invention is not limited to these examples and can be practiced in various modifications within the scope of the present invention.

Example 1
A bipolar electrolysis device for water electrolysis with the configuration shown in FIG. 3 was used. Specifically, the bipolar electrolysis cell for water electrolysis used a cell stack (a) (five electrolysis cells) in which one anode terminal element, four bipolar elements, one cathode terminal element, and five diaphragms were stacked via five rubber gaskets (compression rate before electrolysis: 20%) as shown in FIG. 1 . In all elements, nickel was used for all components that come into contact with the electrolyte, such as the partition wall and outer frame. Nickel (with a cobalt catalytic layer) was used for the anode, and nickel (with platinum and palladium catalytic layers) was used for the cathode. Other components included two SUS press plates (b), six titanium tie rods (c) (three for each press plate (b)), two PTFE heat insulating members (d), and six SUS spring members (e) (disc springs) (one for each tie rod (c)). The electrolyte used was an aqueous potassium hydroxide solution.
The electrolysis was carried out for 7 hours at a current density of 6000 A/m ^2. The atmospheric temperature was 10° C., the temperature of the electrolytic cell during electrolysis was 100° C., and the internal pressure of the electrolytic cell during electrolysis was adjusted to 0.01 MPa.
The configuration of the bipolar electrolytic cell for water electrolysis and the measurement results are shown in Table 1.
The average thermal expansion coefficient α of the cell stack (a), the press plate (b), and the heat insulating member (d) was calculated using the following formula.
α=(t _(a) α _(a) +t _(b) α _(b) +t _(d) α _(d) )/(t _(a) +t _(b) +t _(d) )
(In the formula, t _(a) , t _(b) , and t _(d) represent the lengths (thicknesses) of the cell stack (a), press plate (b), and heat insulating member (d) in the stacking direction, respectively, and α _{(a )} , α _(b) , and α _(d) represent the thermal expansion coefficients of the cell stack (a), press plate (b), and heat insulating member (d), respectively.)
The internal pressure and seal surface pressure of the electrolytic cell were measured by a pressure gauge provided at the outlet header of the electrolytic cell and a load cell installed between the press plate (b) and the heat insulating member (d), respectively.

Example 2
The same procedure as in Example 1 was carried out except that the material of the tie rod (c) was SUS.
The configuration of the bipolar electrolytic cell for water electrolysis and the measurement results are shown in Table 1.

(Comparative Example 1)
The same procedure as in Example 1 was carried out except that a SUS member (d') was used instead of the heat insulating member (d) and the tie rod (c) was made of SUS.
The configuration of the bipolar electrolytic cell for water electrolysis and the measurement results are shown in Table 1.

In Comparative Example 1, the seal surface pressure (the surface pressure between the gasket and each element) decreased from 2 MPa before electrolysis to 1.9 MPa during electrolysis, making it necessary to retighten the tie rods. In contrast, in Examples 1 and 2, the seal surface pressure increased from 2 MPa before electrolysis to 2.3 MPa and 2.1 MPa, respectively, during electrolysis, making it unnecessary to retighten the tie rods.

The bipolar electrolytic cell for water electrolysis of the present invention has a long life even when used under conditions of variable power supplies or frequent shutdowns, and can therefore operate stably for long periods of time even when operated under variable power supplies such as renewable energy sources.

(a) cell stack (b) press plate (c) tie rod (d) heat insulating member (e) spring member 1 partition wall 2 electrode 2a anode 2c cathode 2e conductive elastic body 2r current collector 3 outer frame 4 diaphragm 5 electrode chamber 5a anode chamber 5c cathode chamber 6 rectifier plate 7 gasket 50 bipolar electrolytic cell for water electrolysis 51a anode terminal element 51c cathode terminal element 60 bipolar element 65 electrolytic cell 70 electrolysis device 71 liquid feed pump 72 gas-liquid separation tank 72h hydrogen separation tank 72o oxygen separation tank 73 water supply device 74 rectifier 75 oxygen concentration meter 76 hydrogen concentration meter 77 flow meter 78 pressure gauge 79 heat exchanger 80 pressure control valve D1 A given direction along the partition (electrolyte flow direction)
Z Zero gap structure

Claims (8)

an anode terminal element comprising an anode;
a cathode terminal element comprising a cathode;
a plurality of bipolar elements positioned between the anode terminal element and the cathode terminal element, each element including an anode, a cathode, a partition wall separating the anode and the cathode, and an outer frame that frames the partition wall;
A cell stack (a) in which adjacent elements are stacked with a diaphragm disposed between them via a gasket;
press plates (b) disposed at both ends of the cell stack (a) and applying surface pressure between the gasket and the diaphragm, and between the gasket and each of the elements;
a tie rod (c) fastening the cell stack (a) and the press plate (b),
a heat insulating member (d) is provided between the press plate (b) and the cell stack (a);
a mean thermal expansion coefficient α of the cell stack (a), the press plate (b), and the heat insulating member (d) in the stacking direction at electrolysis temperature is greater than a mean thermal expansion coefficient β of the tie rod (c) in the stacking direction at electrolysis temperature , and an amount of thermal expansion of the entire stack of the cell stack (a), the press plate (b), and the heat insulating member (d) at electrolysis temperature (elongation in the stacking direction) is greater than an amount of thermal expansion of the tie rod (c) at electrolysis temperature (elongation in the stacking direction). The bipolar electrolytic cell for water electrolysis according to claim 1, wherein the thermal conductivity A of the insulating member (d) at the electrolysis temperature is smaller than the thermal conductivity B of the press plate (b) at the electrolysis temperature. The bipolar electrolytic cell for water electrolysis according to claim 1 or 2, wherein the thermal conductivity A of the heat insulating member (d) at 100°C is 1 W/m·K or less. The bipolar electrolytic cell for water electrolysis according to any one of claims 1 to 3, wherein the volume resistivity of the heat insulating member (d) at the electrolysis temperature is 1 kΩ·cm or more. A bipolar electrolytic cell for water electrolysis according to any one of claims 1 to 4, wherein the thickness of the gasket is 5 to 20% of the thickness of the bipolar element. a spring member (e) arranged on an outer periphery of the axis of the tie rod (c) and elastically deformable in the stacking direction of the cell stack (a),
6. The bipolar electrolytic cell for water electrolysis according to any one of claims 1 to 5, wherein an expansion length ΔL of the cell stack (a) in the stack direction at an electrolysis temperature is 75% or less of a total deflection T of the spring member (e) in the stack direction. The bipolar electrolytic cell for water electrolysis according to claim 6, wherein the number of tie rods (c) is n, and the spring constant of the spring members (e) arranged on the n tie rods (c) is 5/n to 20/n kN/mm. 8. The bipolar electrolytic cell for water electrolysis according to any one of claims 1 to 7, wherein a surface pressure between the gasket and each of the elements during electrolysis is at least twice the maximum internal pressure of the electrolytic cell, and a compressibility of the gasket in a stacking direction of the cell stack (a) is 10 to 25%.