DK201400505A1

DK201400505A1 - Pressurised Electrolysis Stack

Info

Publication number: DK201400505A1
Application number: DKPA201400505A
Authority: DK
Inventors: Jørgen Krogsgaard Jensen; Anders Rønne Rasmussen
Original assignee: Greenhydrogen Dk Aps
Priority date: 2014-09-05
Filing date: 2014-09-05
Publication date: 2016-03-14
Also published as: WO2016034183A1; DK178796B1

Abstract

An electrolysis stack (2) for an electrolyser (20) is disclosed. The electrolysis stack (2) comprises a plurality of electrolysis cells each comprises two electrodes and a (porous) gas separating membrane and a cell frame (6, 6', 6'') having a circular outer periphery. The cell frames (6, 6', 6'') are arranged adjacent to each other: The electrolysis stack (2) comprises means (18) for supplying electrolyte feed to the interior of the electrolysis cells and means (16) for removing oxygen gas and hydrogen gas from the electrolysis cells. The electrolysis stack (2) comprises electric power point members (46, 46') constituting a cathode, an anode or a cathode and an anode. The electrolysis stack (2) comprises at least one support member (12, 12', 42, 42', 42'') arranged at the outside periphery of the cell frames (6, 6', 6'').

Description

Pressurised Electrolysis Stack Field of invention

The present invention generally relates to an electrolyser for pressurised electrolysis. The present invention more particularly relates to an electrolysis stack for a pressurised electrolyser.

Prior art

The need for storing electric energy generated from solar panels or wind turbines is increasing due to the need of greener energy sources. Storing the electric energy in hydrogen by using electrolysers to convert water to hydrogen and oxygen has been known for decades. However, there is a storage problem with hydrogen, because the geometric volume needed for storing a given mass of hydrogen gas is inversely related to the storage pressure, i.e. the higher the pressure, the smaller the volume. Therefore, hydrogen needs to be stored under pressure. It is therefore desirable to conduct electrolysis at elevated pressures because this reduces or even eliminates the need for further compression of the hydrogen.

Several methods of manufacturing an electrolysis stack, which is capable of generating hydrogen at elevated pressure has been described.

The American patent US 2,881,123 describes an electrolysis stack wherein the cell frames are made of steel. Steel has a high mechanical strength and is the preferred choice of material for these types of pressure vessels; however the drawback is that the cell frames needs to be electrically insulated from each other in order to prevent short circuiting of the stack.

The American patent application US 2010/0078317 describes an electrolysis stack, which is able to tolerate a high operating pressure. The cell frames are manufactured from polymer material which is protected by a strong outer shell of steel. There is, however, no pressure difference between the inside and the outside of the stack.

The American patent US 6,554,978 describes an electrolysis stack in which the cell frames are manufactured from polymer material. However, in order for the polymer material to tolerate elevated pressure, the thickness of the cell frame is larger in order to prevent failure of the material.

For these electrolysis stacks to be able to operate at elevated pressure they have been made of pressure resistant material which is very costly. Thus, there is a need for an improved electrolysis stack which reduces or even eliminates the above mentioned disadvantages of the prior art.

Accordingly, it is an object of the present invention to provide an electrolysis stack that is capable of being operated at elevated pressure, where thickness of the cell frame may be reduced.

Summary of the invention

The object of the present invention can be achieved by an electrolysis stack as defined in claim 1 and by an electrolyser having the features as defined in claim 10. Preferred embodiments are defined in the dependent sub claims and explained in the following description and illustrated in the accompanying drawings.

The electrolysis stack according to the invention is an electrolysis stack method for an electrolyser, which electrolysis stack comprises a plurality of electrolysis cells. The stack comprises a plurality of bipolar electrodes, gas separating membranes, and cell frames, which cell frames are arranged adjacent to each other, which electrolysis stack comprises means for supplying electrolyte feed to the interior of the electrolysis cells and means for removing oxygen gas and hydrogen gas from the electrolysis cells, which electrolysis stack comprises electric power point members constituting a cathode, an anode or a cathode and an anode, where the electrolysis stack comprises at least one support member arranged at the outside periphery of the cell frames.

Hereby it is possible to provide an electrolysis stack that is capable of being operated at elevated pressure, where thickness of the cell frame can be reduced. The support member reduces deformation in the circumferential direction of the cell frames.

The electrolysis stack may comprise any suitable number of electrolysis cells (e.g. 25, 50, 100, or 400). The electrolysis cells may be arranged in the same or in several different modules.

The electrolysis stack according to the invention may be adapted to handle a strong alkali electrolyte comprising potassium hydroxide (KOH) (e.g. 30wt% KOH).

The bipolar electrodes may comprise sheet material (e.g. a metal sheet) and the separating membrane may be a porous gas separating membrane. The membrane may comprise any suitable material e.g. two layers of a polymer comprising Zr02.

It may be an advantage that each cell frames has a circular outer periphery. Hereby it is possible to provide a strong and reliable electrolysis stack.

The cell frames are arranged adjacent to each other and may be an advantage that the cell frames are sealed with O-ring gaskets made in a resilient material (e.g. EPDM rubber).

The electrolysis stack comprises means for supplying electrolyte feed to the interior of the electrolysis cells. The means for supplying electrolyte feed to the interior of the electrolysis cells may be of any suitable type and geometry. The means for supplying electrolyte feed to the interior of the electrolysis cells may comprise a channel structure constituted by the plurality of cell frames arranged side by side along the longitudinal axis of the electrolysis stack.

The electrolysis stack comprises means for removing oxygen gas and hydrogen gas from the electrolysis cells. These means may comprise a channel structure constituted by the plurality of cell frames arranged side by side along the longitudinal axis of the electrolysis stack.

It may be an advantage that each cell frame is provided with a plurality of through-bores extending through the axial length of the cell frame. These through-bores may, together with other structures, constitute a channel structure constituted by the plurality of cell frames arranged side by side along the longitudinal axis of the electrolysis stack.

The electrolysis stack comprises electric power point members constituting a cathode, an anode or a cathode and an anode. These electric power point members may have any suitable geometry. The electric power point members may e.g. be plate-shaped.

The at least one support member arranged at the outside periphery of the cell frames may have any suitable geometry and be made in any suitable material.

It may be an advantage that the inner portion of the support member is made in an electrically insulating material, such as a plastic material.

It may be beneficial that the support member is cylindrical and extends along the axial length of the cell frame.

Hereby it is possible to provide a support member having the required mechanical properties.

Moreover, a cylindrical support member will be fit to enclose cell frames having a circular outer periphery.

It may be beneficial that the cell frames are arranged between two flanges and that the flanges are mechanically attached to each other by means of a plurality of threaded rods and nuts.

Hereby it is possible to provide an electrolysis stack that is configured to resist large forces acting in the axial direction (causing expansion of the electrolysis stack along its longitudinal axis).

It may be an advantage that the cell frames are arranged between two flanges and that the flanges are mechanically attached to each other by means of a plurality of threaded rods, nuts and washers.

It may be advantageous that the support member is arranged in such a manner that it is not in mechanical contact with the flanges. Hereby it is achieved that the support member can expand in the axial direction.

It is preferred that the cell frames are mounted in an outer support member made in a high strength composite material. It may be an advantage that the outer support member extend longer in the axial direction than the cell frames contained within the outer support member. Hereby it is possible to support the cell frames when the cell frames extend axially.

It may be beneficial that the support member is constructed in such a way that the gap between the outer diameter of the cell frames and the inner diameter of the support member is as small as possible.

In one embodiment according to the invention the support member is a cylindrical tube made in a composite material (fibre-reinforced polymer) made of a polymer matrix reinforced with fibres (e.g. glass, carbon or aramid). The polymer may be any suitable polymer material, e.g. epoxy, polyphenylsulfone (PPSU) or polyether ether ketone (PEEK).

It may be beneficial that the elastic modulus of the support member is significantly larger than the elastic modulus of the cell frames.

Hereby the support member is configured to keep its geometrical shape and prevent radially expansion of the cell frames.

It may be advantageous that the coefficient of thermal expansion of the support member is smaller than the coefficient of thermal expansion of the cell frames.

Hereby the support member is configured to maintain its geometrical shape and prevent radially expansion of the cell frames during operation of the electrolysis stack.

It may be beneficial that the support member is made in an electrically insulation material e.g. a fibre reinforced plastic material.

The fibres may be glass fibre, arm id fibre or carbon fibre by way of example.

It may be an advantage that the electrolysis stack comprises a plurality of support members arranged with mutual end-to-end contact and substantially in axial extension of each other.

Hereby it is possible to provide an electrolysis stack provided with modular support members. It is thus possible to build a long electrolysis stack and apply the same support member that is used for shorter electrolysis stacks.

It may be an advantage that the electrolysis stack comprises a first support member and at least one additional support member arranged at the outside of the first support member.

Hereby it is possible to provide additional strength to the electrolysis stack so that is configured to resist the pressure within the cell stacks.

The object of the invention may be achieved by an electrolyser comprising an electrolysis stack according to the invention.

The electrolysis stack can be a single electrolysis stack or split in sections.

It is preferred that the ceil frames are made in a material that is suitable for handling high pH values (pH values above 14)

The support member may be mechanically attached outside the cell frames.

It is possible to apply a metal (steel) support structure provided with an inner isolation structure.

It may be an advantage that the electrolysis stack is divided into a plurality of electrically separated cell frame module.

Hereby, the electrical potential between the first and last cells in each of the cell frame module is reduced. Accordingly, it is possible to provide an electrolysis stack that reduces the energy losses including the stray currents.

The amount of energy loss due to stray current increases with the number of cells in the cell frame module because the electrical potential between the first cell frame and the last cell frame in a cell frame module depends of the number of cell frames in the cell frame module.

The present invention suggests a construction in which the electrolysis stack is divided into a plurality of electrically separated cell frame module. In this manner the electrical potential difference between the first cell and the last cell in a cell frame module can be significantly reduced.

The electrolysis stack may be divided into a plurality of electrically separated cell frame module by several means; however, it may be an advantage that the electrolysis stack is divided into a plurality of electrically separated cell frame modules by means of electric power point members extending along the length of the cell frames.

It may be an advantage that the electric power point members electrically separating the cell frame modules are basically plate-shaped and comprises an electrically insulating material.

It may be beneficial that the electrolysis stack is divided into a three or more electrically separated cell frame module.

It may be advantageous that each of the electrically separated cell frame module comprises 10-40, preferably 15-35, such as 20-35 cell frames. Hereby it is possible to apply standard power supplies.

It may be an advantage that each of the electrically separated cell frame modules comprises 25 cell frames. Hereby it is possible to apply a standard power supply.

It may be advantageous that each of the electrically separated cell frame modules comprises the same number of cell frames. Hereby it is possible to build an electrolysis stack by using a plurality of identical cell frame modules.

It may be beneficial that each of the electrically separated cell frame modules are electrically separated from each other by means of current terminals and/or electric power point members arranged between adjacent cell frame modules. Hereby it is possible to supply electrical current to the cell frame modules through these current terminals and/or electric power point members.

It may be an advantage that each of the electrically separated cell frame modules comprises insulation brushings configured to electrically insulate the electrolyte within the electrolysis stack from the current terminals and/or electric power point members arranged between adjacent cell frame modules during use of the electrolysis stack.

Hereby it is possible to reduce the stray currents giving rise to loss of energy in the electrolysis stack.

The brushings may preferably have a cylindrical shape.

It may be an advantage that the brushings are arranged in the channels that are provided in the cell frames to distribute electrolyte to all the cells frames in the cell frame module. Preferably the brushings extends between two adjacent cell frame modules.

Description of the Drawings

The invention will become more fully understood from the detailed description given herein below. The accompanying drawings are given by way of illustration only, and thus, they are not limitative of the present invention. In the accompanying drawings:

Fig. 1 shows two schematic view of an electrolysis stack according to the invention;

Fig. 2 shows a schematic perspective top view of an electrolyser according to the invention;

Fig. 3 illustrates schematic perspective top views of an electrolysis stack according to the invention;

Fig. 4 shows schematic cross-sectional views of an electrolysis stack according to the invention;

Fig. 5 shows two schematic perspective top views of an electrolysis stack according to the invention and Fig. 6 shows a schematic perspective top view of an electrolysis stack according to the invention.

Detailed description of the invention

Referring now in detail to the drawings for the purpose of illustrating preferred embodiments of the present invention, an electrolysis stack 2 of the present invention is illustrated in Fig. 1.

Fig. 1 illustrates two different schematic views of an electrolysis stack 2 according to the invention. Fig. 1 a) illustrates a schematic top view of an electrolysis stack 2 comprising a cylindrical support member 12 enclosing a plurality of disk-shaped cell frames 6 stacked within the support member 12.

The electrolysis stack 2 comprises a series of stacked electrolysis cells. Each of these electrolysis cells contains two bipolar electrodes (metal sheets). A gas separating porous membrane is provided between every bipolar electrode. Each electrolysis cell comprises a disk-shaped polymer cell frame 6.

In Fig. 1, however, the membrane and bipolar electrodes of the cell frames 6 have been removed for illustration purposes. It may be an advantage that the cell frames are sealed with O-ring gaskets of a resilient material (e.g. EPDM rubber).

Each cell frame 6 comprises four axially extending through bores 8, 8', 10, 10'. Each cell frame 6 comprises a centrally arranged aperture 14.

Each cell frame 6 comprises a membrane (not shown). The membrane is exposed to high temperatures (up to 100° Celsius) and pH values above 14 during operation of the electrolysis stack 2. Accordingly, the membrane must be capable of being exposed to a demanding chemical environment. The membrane may comprise any suitable material e.g. two layers of a polymer comprising Zr02. The electrolysis stack 2 according to the invention may be adapted to handle a strong alkali electrolyte comprising potassium hydroxide (KOH) (e.g. 30 wt% KOH).

Some of the through bores 8, 8', 10, 10' may be used to transport oxygen (02) and hydrogen (H2) generated by means of the electrolysis stack 2. Some of the through bores 8, 8', 10, 10' may be used to transport of the electrolyte (e.g. demineralised water with 30 wt% KOH).

Fig. 1 b) illustrates a schematic perspective top view of the electrolysis stack 2 shown in Fig. 1 a). The electrolysis stack 2 comprises a cylindrical support member 12 arranged at the outside of a stack of cell frames 6, 6', 6" stacked within the support member 12. Even though the cell frames 6, 6', 6" comprise membranes and bipolar electrodes these have been removed for illustrating that the cell frames 6, 6', 6" are stacked on the top of each other within the cylindrical support member 12.

The cell frames 6, 6', 6" may be manufactured in polymer material, e.g. polyphenylsulfone (PPSU) or polyether ether ketone (PEEK). Once the cell frames 6, 6', 6" are brought into mechanical contact with the support member 12, the support member 12 (a cylindrical tube) will significantly reduce further deformation in the circumferential direction of the cell frames 6, 6', 6". Accordingly, the use of the support member 12 makes it possible to operate the electrolysis stack 2 at high pressures (e.g. up to 3 MPa corresponding to 30 bar) without critical deformation of the cell frames 6, 6', 6".

It can be seen that the support member 12 is slightly longer (in the axial direction) than the cell frames 6, 6', 6". Hereby it is possible to support the cell frames when the cell frames extend axially.

Fig. 2 illustrates a schematic perspective top view of an electrolyser 20 according to the invention. The electrolyser 20 comprises a frame 36 having a lower frame member 38 and an upper frame member 38' interconnected by a four (only three are visible in Fig. 2) connection members 40, 40', 40" shaped as angle bars 40, 40', 40". Each angle bars 40, 40', 40" is mechanically attached to both the lower frame member 38 and an upper frame member 38'.

The electrolyser 20 comprises two electrolysis stacks 2, 2' mounted in the lower portion of the electrolyser 20. Each of the electrolysis stacks 2, 2' comprise a cylindrical support member 12 like the one shown in Fig. 1. Each of the two electrolysis stacks 2, 2' is arranged between two flanges 24, 24'. These flanges 24, 24' are mechanically attached to each other by means of a plurality of threaded rods 26, nuts 22 and washers 44. The two electrolysis stacks 2, 2' are equally constructed and extend parallel with each other.

The electrolyser 20 comprises degassing chambers, a gas purification system and a pressure control system. The two electrolysis stacks 2, 2' are electrically connected to separate power supplies.

Fig. 3 a) and Fig. 3 b) illustrate two different schematic perspective top views of an electrolysis stack 2 according to the invention. The electrolysis stack 2 is arranged between two parallel plate-shaped flanges 24, 24'. The flanges 24, 24' are mechanically attached to each other by means of a plurality of threaded rods 26 and corresponding nuts 22 and disks 44. The disks 44 are compressible disks allowing the cell frames to expand along the longitudinal axis X of the electrolysis stack 2. The stack may for example comprise 18 disks 44. This assembly prevents the flanges from being displaced from each other along the longitudinal axis X of the electrolysis stack 2.

It can be seen that the treaded rods 26 extend parallel to each other and to the longitudinal axis X of the electrolysis stack 2.

The electrolysis stack 2 comprises three cylindrical support members 11, 11', 12 arranged end to end at the periphery of a plurality of cell frames (not shown) within the interior of the electrolysis stack 2.

Four electrical connections 50, 50', 52, 52' are provided along the periphery of the support members 11, 11', 12. The electrical connections 50, 50', 52, 52' protrude radially from the periphery of the support members 11, 11', 12.

Fig. 4 illustrates two schematic cross-sectional views of an electrolysis stack 2 according to the invention. Fig. 4 a) shows a side view, while Fig. 4 b) illustrates a perspective view. The electrolysis stack 2 is arranged between two flanges 24, 24' mechanically attached to each other by means of a plurality of threaded rods 26, 26' and corresponding nuts 22, 22' and washers 44. The threaded rods 26, 26' extend along the longitudinal axis X of the electrolysis stack 2.

The electrolysis stack 2 comprises a cylindrical support member 12 arranged at periphery of a plurality of cell frames 6. The electrolysis stack 2 comprises three cylindrical support members 11, 11', 12 arranged end to end at the periphery of a plurality of cell frames 6 of the electrolysis stack 2.

Fig. 4 b) shows that a gas outlet pipe 16 and a KOH inlet pipe 18 are provided in the flange 24. Channel extending parallel to the longitudinal axis X of the electrolysis stack 2 are provided in continuation of the gas outlet pipe 16 and of the KOH inlet pipe 18. The channels extend through the plurality of cell frames 6.

The electrolysis stack 2 comprises three cell frame modules Mi, M2, M3 arranged end to end along the longitudinal axis X of the electrolysis stack 2. Fifty cell frames 6 are arranged in each of the three cell frame modules Mi, M2, M3. Accordingly, the total number of cell frames 6 in the electrolysis stack 2 is 150.

An insulating plate member 32, 32' is arranged in each end of the electrolysis stack 2. Two electric power point members (current terminals) 46, 46' are arranged next to each of the insulating plate members 32, 32'. The electric power point member 46 is a cathode, while the electric power point member 46' is an anode. Furthermore, two electric power point members formed as bipolar electrodes 48, 48' are arranged between the first cell frame module Mi and the second cell frame module M2 as well as between the second cell frame module M2 and the third cell frame module M3, respectively.

A first brushing 34, a second brushing 34' and a third brushing 34" are arranged to electrically insulate the electrolyte from the electric power point members 48, 48', 46, 46' in order to prevent unwanted currents from running through the electrolysis stack 2.

The brushings 34, 34', 34" may be made in any suitable insulating material capable of resisting the demanding working conditions (temperatures up to 100° Celsius and pH values above 14 as well as high concentration of oxygen and hydrogen gasses). The brushings 34, 34', 34" may be made in polyphenylsulfone (PPSU) or polyether ether ketone (PEEK) by way of example.

The electrolysis stack 2 is equipped with a gas outlet channels 16 (oxygen or hydrogen gasses) and a media inlets 18 (for demineralised water with KOH, e.g. demineralised water with 30wt% KOH).

The electrolysis stack 2 is enclosed by three support members 11, 11', 12 shaped cylindrical tubes. The support members 11, 11', 12 are constructed in such a way that they are configured to support the cell frames 6 in radial direction. It is possibly to apply one large support member instead of three support members 11, 11', 12.

Along the longitudinal axis X of the electrolysis stack 2, the total length of the stack of cell frames 6 will change with temperature and over time due to thermal expansion, change of elastic modulus with temperature and the compressive stress, and creep due to compressive stress.

The support members 11, 11', 12 are not subjected to any significant stress in the axial direction. Accordingly, only thermal expansion will cause changes in the length of the support members 11, 11', 12 in the direction of its longitudinal axis X.

By using support members 11, 11', 12 like, the ones illustrated in Fig. 3-4 it is possible to reduce the dimensions of the cell frames 6, 6', 6".

The electrolysis stack 2 is designed whit a modular concept in mind. The electrolysis stack 2 a number of cell frame modules Mi, M2, M3 providing a total number of cell frames of e.g. 100, 150 or 200 cells frames 6 with a volume ranging from e.g. 4 L to 10 L or more of electrolyte inside.

Depending on the customer's needs, it is possible to provide lager configurations of e.g. 50, 75, 100 or 200 cell frames 6 by putting together a number of cell frame modules Mi, M2, M3.

When put together the cell frame modules Mi, M2, M3 are separated from one another.

The cell frame modules Mi, M2, M3 each comprise 50 cells frames 6. Each cell frame module comprises electrical power point members constituting either a cathode, an anode or a cathode and an anode. A diaphragm or membrane is provided to separate the gasses generated.

When the cells frames 6 are combined into an electrolysis stack 2, three cell frame modules Mi, M2, M3 are at the end of each other. The cell frame modules Mi, M2, M3 are connected to fittings in the flanges 24, 24'.

Accordingly, a 150 cell frame electrolysis stack 2 is build up by the three cell frame modules Mi, M2, M3 with a total of 150 small chambers (anode, cathode, anode, cathode and so on) where 75 of the chambers are connected by channels to be the oxygen producing part of the stack and the remaining 75 chambers are connected to be the hydrogen producing part.

The oxygen and the hydrogen sides are completely separated from each other by membranes/diaphragms and (bipolar) electrodes. Accordingly, the electrolysis stack 2 may be considered to take form two vessels: one carrying H2 and one carrying 02.

When direct current is applied to the first and the last cell of a cell frame module Mi, M2, M3, it causes current to flow through each cell (each cell comprises two cell frames 6) in the cell frame module Mi, M2, M3, dividing the potential over each cell frame in the cell frame module Mi, M2, M3. The potential of each cell frame 6 is determined by the current passing through the each cell frame 6, the temperature, the chemical composition of the (bipolar) electrode and the thickness of the electrolyte. When producing hydrogen and oxygen, there will be a gas fraction corresponding to approximately 10% of the volume in the electrolysis stack 2.

Fig. 5 a) illustrates a schematic perspective top view of an electrolysis stack 2 according to the invention. The electrolysis stack 2 comprises only one cell frame 6 since the remaining cell frames have been removed. The cell frame 6 has a circular outer periphery and is provided with a centrally and symmetrically arranged aperture 14. The aperture 14 is defined by two circular arcs connected by two parallel straight lines.

The cell frame 6 is arranged within a cylindrical support member 12 having an inner geometry that fits the outer geometry of the cell frame 6.

Fig. 5 b) illustrates a top view of an electrolysis stack 2 according to the invention. The electrolysis stack 2 comprises a plurality of cell frames 6 (only one is visible) corresponding to the one illustrated in Fig. 5 a).

The cell frame 6 is arranged within a cylindrical support member 12 having an inner geometry that fits the outer geometry of the cell frame 6. During operation gaseous 02 and H2 is generated within the central portion of the cell frame 6 by means of two electrodes (metal sheets) and a gas separating porous membrane (these are not shown). Hereby the pressure is increased significantly (up to 3 MPa). Therefore, an outwardly directed force F is created. The force F acts in all radial directions causes the need for ensuring a rather large mechanical strength of the electrolysis stack 2.

A large mechanical strength of the electrolysis stack 2 is achieved by means of the cylindrical support member 12 enclosing the cell frames 6 of the electrolysis stack 2. The cell frame 6 bears against the inside portion of the support member 12 and hereby the mechanical strength of the support member 12 can directly be used to prevent radially expansion of the cell frames 6. Thus, the mechanical strength of the cell frames 6 may be reduced provided that the mechanical strength of the support member 12 is sufficiently large.

Fig. 6 a) illustrates a schematic perspective top view of an electrolysis stack 2 according to the invention. The electrolysis stack 2 comprises a plurality of cell frames 6, 6', 6" arranged within a cylindrical support member 12. Additional support members 42, 42', 42" are provided at the outside of the cylindrical support member 12.

Hereby it is possible to enlarge the mechanical strength of the electrolysis stack 2. The additional support members 42, 42', 42" are made as separate bands configured to fit the outer periphery of the cylindrical support member 12. However, it would be possible to apply one large additional support member 42 having the same axial extension as the cylindrical support member 12. Alternatively, it is possible to apply a larger number (e.g. four or more) of additional support members 42, 42', 42".

Fig. 6 b) illustrates a schematic top view of an electrolysis stack 2 according to the invention. The electrolysis stack 2 comprises a plurality of cell frames 6 arranged within a first cylindrical support member 12 having an inner geometry that fits the outer geometry of the cell frame 6. A second and additional support member 12' is arranged at the outside of the first cylindrical support member 12. Hereby, the mechanical strength of the construction can be increased further.

During operation of the electrolysis stack 2 gaseous 02 and H2 is generated within the central portion of the cell frames 6. The gaseous 02 and H2 can be generated through use of two electrodes (not shown) and a gas separating porous membrane (not shown).

The pressure within the central portion of the cell frames 6 is increased significantly (up to 3 MPa) due to the generated gasses and an outwardly directed force F acting in all radial directions is created.

The first cylindrical support member 12 as well as the second additional support member 12' provides the required mechanical strength of the electrolysis stack 2. Thus, radially expansion of the cell frames 6 can be prevented.

List of reference numerals 2 Electrolysis stack 6, 6', 6" Cell frame 8, 8' Bore 10, 10' Bore 11, 11' Support member 12, 12' Support member 14 Aperture 16 Pipe (gas outlet) 18 Pipe (KOH inlet) 20 Electrolyser 22, 22' Nut 24, 24' Flange 26 Threaded rod 32, 32' Plate member 34, 34', 34" Brushing (insulation) 36 Frame 38, 38' frame member 40, 40', 40" Connection member 42, 42', 42" Additional support member 44 Disk 46, 46' Current terminal 48, 48' Electric power point member 50, 50', 52, 52' Electrical connection

Mi, M2, M3 Cell frame module X Longitudinal axis F Force

Claims

1. An electrolysis stack (2) for an electrolyser (20), which electrolysis stack (2) comprises a plurality of electrolysis cells each comprising two electrodes and a gas separating membrane and a cell frame (6, 6', 6"), which cell frames (6, 6', 6") are arranged adjacent to each other, which electrolysis stack (2) comprises means (18) for supplying electrolyte feed to the interior of the electrolysis cells and means (16) for removing oxygen gas and hydrogen gas from the electrolysis cells, which electrolysis stack (2) comprises electric power point members (46, 46', 48, 48') constituting either a cathode, an anode or a cathode and an anode, characterised in that the electrolysis stack (2) comprises at least one support member (12, 12', 42, 42', 42") arranged at the outside periphery of the cell frames (6, 6', 6").

2. An electrolysis stack (2) according to claim 1, characterised in that the support member (12, 12', 42, 42', 42") is cylindrical and extends along the axial length of the cell frame (6, 6', 6").

3. An electrolysis stack (2) according to claim 1 or claim 2, characterised in that the cell frames (6, 6', 6") are arranged between two flanges (24, 24') and that the flanges (24, 24') are mechanically attached to each other by means of a plurality of threaded rods (26) and nuts (22).

4. An electrolysis stack (2) according to claim 3, characterised in that the support member (12, 12', 42, 42', 42") is arranged in such a manner that it is not in mechanical contact with the flanges (24, 24').

5. An electrolysis stack (2) according to one of the preceding claims, characterised in that the elastic modulus of the support member (12, 12', 42, 42', 42") is significantly larger than the elastic modulus of the cell frames (6, 6', 6").

6. An electrolysis stack (2) according to one of the preceding claims, characterised in that the coefficient of thermal expansion of the support member (12, 12', 42, 42', 42") is smaller than the coefficient of thermal expansion of the cell frames (6, 6', 6").

7. An electrolysis stack (2) according to one of the preceding claims, characterised in that the support member (12, 12', 42, 42', 42") is made in an electrically insulation material e.g. a fibre reinforced plastic material.

8. An electrolysis stack (2) according to one of the preceding claims, characterised in that the electrolysis stack (2) comprises a plurality of support members (12, 12', 42, 42', 42") arranged with mutual end-to-end contact and substantially in axial extension of each other.

9. An electrolysis stack (2) according to one of the preceding claims, characterised in that the electrolysis stack (2) comprises a first support member (12) and at least one additional support member (12', 42, 42', 42") arranged at the outside of the first support member (12).

10. An electrolyser (20) comprising an electrolysis stack (2) according to one of the preceding claims.