Electrochemical Cell Stack

Abstract

The present invention relates to a cell stack having at least one electrochemical cell arranged between a first end plate connected to an electrical bolt and a second end plate connected to another electrical bolt, said stack having a housing, means for providing fixed support of said stack to said housing and means for maintaining a constant load over said cell stack. Said means for maintaining a constant load comprise an elastic pad inserted into the space between said cell stack and said housing wall. The cell stack is applicable in the area of PEM fuel cells and PEM water electrolyser cells.

Description

The present invention relates to an electrochemical cell stack having means for maintaining constant mechanical load over the components in said cell stack.

Electrochemical cells using a polymer membrane as electrolyte, e.g. a Proton Exchange Membrane (PEM), are interesting in the area of PEM fuel cells and PEM water electrolyser cells where protons constitute the carrier of ionic charge.

Each cell consists of an assembly of a membrane and two catalytic layers on each side, i.e. a Membrane Electrode Assembly (MEA) where the catalytic layers constitute the electrodes and are in intimate contact with the membrane. The MEA is further supported by electrode backings on each side, typically a porous plate, a mesh or a porous sheet with good electrically conductivity and with high electrochemical/chemical stability. The electrode backing has a certain integrity which matches the electrode layer and the operation regime. The water electrolysis process in a PEM cell is shown in FIG. 1.

PEM cells, and similar cells, are in most practical applications arranged in a so-called bipolar design, i.e. the cells are connected in series, in a cell stack, where the current passes from one cell to another. Each cell is enclosed within so-called bipolar plates or end plates, which separate the individual PEM cells from each other and where electronically current passes from cell to cell. Between the bipolar plate and the electrode backing there must be some void space for transport of fluid in and out of the cell and at the same time there must be electrical contact between the bipolar plate (or end plate) and the electrode backing (as shown in FIGS. 2a and 2b).

Large stack units, with a considerable number of cells in series, provide many interfaces between all the layers, where every interface possesses an interfacial contact resistance to passage of electrical current. Different materials possess different interfacial contact resistance. It is therefore of great importance to secure high electronically contact between all the different interfaces within a cell assembly to minimize the electrical contact resistance.

The electrical contact resistance represents a direct energy loss given by:

P=I²·R

where P is the energy loss in watt, I is the electronic current in ampere and R is the contact resistance in ohm. The energy loss is transformed to heat and, in the case where metallic components are used within the cells, local heating further accelerates corrosion and passivation of the metallic interfaces, which further increases the ohmic resistance and energy loss.

The contact resistance is a function of the mechanical load that compresses the cell together, i.e. the compression force or the clamping force. According to classical theory the contact resistance as a function of the compression force (F) is given by:

R=k·F^−x

where k is a constant, F is the compression force and x is a constant depending on the type of deformation at the contact point and is near 0.5 for most contacts. From this equation it is clear that R decreases substantially until a certain load F where R decreases only infinitesimal for higher F.

During the operation of a PEM cell, i.e. an electrolyser or a fuel cell, heat is generated. At a given temperature the water content of the membranes is in equilibrium with the surrounding environment. Increased water content causes swelling of the membrane whereas decreased water content causes shrinking of the membrane. The dimensions of the other cell parts will also vary with temperature. During operation, each cell therefore expands and contracts according to the temperature caused by the heat generated during operation. Under repeated expansion and contraction of the cell, the cell parts that support the electrodes, e.g. gas diffusion layers in fuel cells, current/water/gas distribution layers and electrode supports, will undergo mainly elastic deformation. However, in the process of expansion and contraction some constituents of a cell stack will be deformed plastically to a certain degree.

In order to ensure gas tightness of the cell, when operating the cell/cell stack under pressure, it is often beneficial to insert the cell body into a pressurised housing (a pressure vessel), where the electrochemical cell is in open communication, or at least partly, with the interior of the housing. Still a compression force over the end plates of the cell body is necessary.

Either the anode side or the cathode side must be in open communication with the vessel environment surrounding the cell stack, and the pressure difference between anode and cathode side must then be controlled externally. Depending on the specific cell design the MEA can withstand several bars pressure difference.

For an electrochemical cell stack the clamping pressure over the cell stack is important for maintaining electronic contact between the stacked components. Due to expansion and contraction of the cell stack during operation, and thereby relaxation of the clamping force, a means is necessary to maintain almost constant mechanical load over the cell stack.

Means for maintaining constant load is well known in the art of PEM technology and is absolutely necessary to obtain long operation lifetime of the electrochemical cell. Different concepts have been used in different designs to provide some beneficial features. Low cost components are the most important but also more advance components with extra functionality or advantages can be provided.

The conventional means clamp the cell stack between two end plates by several bolts and using springs to bear the load as shown in FIG. 3.

For high-pressure systems, where the cell stack is inserted into a pressurised housing, the conventional spring system is not suitable as the springs are usually made from materials that will lead to poisoning of the environment within the housing.

GB 497956 describes a very simple system where springs are inserted between a press plate and a first cell body end plate. Tie rods are pressing the second cell body end plate and the press plate together.

EP 1231298-A1 describes another simple and a very common method. The tie rods, or bolts, compress the cell body end plates together using springs between the bolt nuts and one cell body end plate. A similar design is also shown in WO 0209208-A2.

JP 2003-160891-A describes a more advanced system using a hydraulic cylinder between a press plate and one cell body end plate to maintain constant compression over the cell body when the cell is operating under varying pressure.

U.S. Pat. No. 3,507,704 describes a so-called regenerative alkaline fuel cell system, i.e. an electrochemical device that can operate both as an electrolyser and a fuel cell, where the cell body is inserted into a tank. The cell body is compressed by tie rods and springs between the bolt nut and one cell body end plate. Material compatibility of tie rods and springs with the chemical environment within the interior of the vessel and within the cell stack is very important.

JP 2003-160891-A describes a system where the oxygen pressure is used to compress the cell stack by pressing a moveable electrical end plate towards the opposite electrical end plate.

Several patents, e.g. U.S. Pat. No. 5,547,777 and US 20040115511 A1, describe a thin film compression layer located at the end plate of the cell body, or within the compartment of each cell, for evening out locally pressure differences within the cells. These may be caused for example by an uneven thickness of different electrically conducting cell parts or by bending of the cell end plates during compression. The main purpose of these thin film compression layers is to secure a high inter-layer contact area, i.e. of the area of the electrically conductive layers. Also, the thin film compression layer is located within one or more cell compartments and compensates only within the given cell.

EP 1304757-A2 describes an electrochemical cell body compressed between two plates. Since the plates become bent under compression of the cell stack, two electrical conductive elements, that undergo temporary plastic deformation, are inserted between each of the compression plate and each of the two cell body plates. Upon compressing the cell stack, the plastic deformation of the two elements secures high contact surface between the bent compression plates and the cell body end plates.

The main objective of the present invention was to arrive at an electrochemical cell stack which is designed to maintain an almost constant load over said cell stack in order to secure long term operability of said cell stack.

Another objective of the present invention was to arrive at an electrochemical cell stack which is designed to compensate thermal expansion or contraction of components of said cell stack during operation.

A further objective of the present invention was to arrive at an electrochemical cell stack which is designed to secure proper sealing around electrical bolts and insulating the electrical end plate(s) from its environment.

Still another objective of the present invention was to arrive at an electrochemical cell stack which is designed to avoid poisoning of its environment.

The environment is the void space inside a housing where the cell stack is inserted.

In accordance with the present invention, these objectives are accomplished by a cell stack having at least one electrochemical cell arranged between a first end plate connected to an electrical bolt and a second end plate connected to another electrical bolt, said stack having a housing, means for providing fixed support of said cell stack to said housing and means for maintaining a constant mechanical load over said cell stack. Said means for maintaining a constant load comprise at least one elastic pad inserted into the space between said cell stack and said housing wall.

Preferably, said pad is inserted into the space between one of said end plates and the adjacent end plate of said housing.

The cell stack can also have an elastic pad in each end. The pad is then inserted into the space between each of said end plates and each of its adjacent end plate of said housing.

An elastic pad means that the pad has the ability to recover its original shape partially or completely after the deforming force (thermal expansion or contraction) has been removed.

Preferably, said pad is made of silicon or another elastic polymeric material. Its shape can be spherical or it can be a pad with indentation on the side, i.e. like a bellow.

Alternatively, the elastic pad can be in the form of two or more individual pads.

The pad is placed on at least one of the end plates of the cell stack.

When in operation, the cell stack expands or contracts. This movement is countered by the compression and expansion of the pad thereby providing the necessary pressure on the cell stack at any time.

Thus, the pressure acting on the several parts of the cell stack will be substantially constant and always so high that perfect tightness is secured. This will secure a long-term operability of said cell stack.

The thickness of the pad is chosen such that its elastic properties are equivalent to those of the cell stack. Thereby any thermal elongation or contraction of the cell stack under operation will be compensated. The elastic pad secures a high contact surface throughout operation of the cell. Furthermore, the elastomeric compression pad is made of a material that is viscoelastic, and will undergo plastic deformation over time. Therefore the thickness of the pad must be much larger than the expected plastic deformation, in order to maintain a minimum load over the cell stack.

The use of an elastic pad has the advantage of bringing a compression pad into the housing without compromising the materials compatibility. Furthermore, said pad is electrical insulating and serves the purpose of isolating the bolt for electrical connection. Furthermore, the sealing properties of the elastic pad offer the option of bringing current-bolts of high electronic conductivity but with a poisoning-effect into the housing.

The pad functions both as a spring and as a sealing. Hence, the conventional used springs, nuts and bolts are replaced with said pad.

In one embodiment of the present invention the housing can be a pressure vessel.

The present invention will now be described with reference to the accompanying drawings in which:

FIG. 1 shows a membrane electrode assembly (MEA),

FIGS. 2a and 2b show possible concepts of bipolar plates and the integration of flow fields in a cell,

FIG. 3 shows a cell stack clamped between two end plates using conventional dish-springs to maintain a constant load,

FIG. 4 shows a cell stack inside a housing according to the present invention.

FIG. 1 illustrates a water electrolysis process in a PEM cell where water is fed to the anode side 1 and, under applied potential field, becomes split to oxygen gas, protons and electrons. Protons migrate to the cathode side 3 where it recombines with an electron to form hydrogen gas.

FIGS. 2a and 2b show two typical concepts where a) the flow pattern is integrated into the bipolar plate or b) a porous layer is inserted between the bipolar plate and the electrode backing. This porous layer must then provide electronic contact between the bipolar plate and the electrode backing and it must provide flow of fluid in and out of the cell compartment. Gaskets are placed around the electrode backing area between the membrane and the bipolar plate (not shown in FIG. 2). Flow of fluid in and out of the cells is typically arranged through channels within the gaskets and the bipolar plates.

FIG. 3 shows a cell stack 4 of 10 cells connected in a bipolar arrangement, where the cells are clamped between a first end plate 1 and a second end plate 8. Number of cells can be from one to several hundred. The end plates are compressed together by a certain number of bolts 2, usually more than four bolts. Springs 5 are inserted on the bolts between the second end plate 8 and the nut 7 to maintain an almost constant clamping pressure.

FIG. 4 shows a housing comprising a wall 5 and a first end plate 2 and a second end plate 8. An electrochemical cell stack is compressed between end plates 2 and 8. The end plate 2 functions also as an electrical end plate and is connected to an electrical end bolt 1. The electrochemical cell stack 3 is arranged between said electrical end plate 2 and a second electrical end plate 6. The electrical end plate 6 is connected to an electrical bolt 10 and inserted into the housing and electrically insulated from the housing. The insulation is achieved by an insulation ring 9 between bolt 10 and end plate 8 and an electrical insulating elastic pad 7. The elastic pad 7 is placed between the electrical end plate 6 and end plate 8.

The electrochemical cell stack is stacked on the top of the end plate 6, the end plate 6 is placed on the pad 7 and the end plate 8. After stacking, the wall 5 is placed on and fixed to the plate 8. End plate 2 is then positioned on the top of the cell stack. By exerting a force normal to end plate 2 the stack is compressed until sealing is obtained between the end plate 2 and the wall 5 and the end plate 8. End plate 2 and end plate 8 are fixed to the wall 5.

Said elastic pad has the following distinct functions:

• • to insulate the electrical end plate (6) from the housing.
  • to assure sealing around electrical bolt (9).
  • to exert the correct compression force on the end plate of the cell stack.
  • to compensate thermal expansion or contraction of the cell stack under operation while maintaining almost constant compression force.

Having described a preferred embodiment of the present invention it will be apparent to those skilled in the art that other embodiments incorporating the concepts may be used. The embodiments illustrated above are intended by way of example only and the actual scope of the present invention is to be determined from the following claims.

Claims

1. A cell stack having at least one electrochemical cell arranged between a first end plate connected to an electrical bolt and a second end plate connected to another electrical bolt, where said stack having a housing, means for providing fixed support of said cell stack to said housing and means for maintaining a constant mechanical load over said cell stack,

characterised in that

said means for maintaining a constant load comprise at least one elastic pad inserted into the space between said cell stack and said housing wall.

2. A cell stack according to claim 1,

characterised in that

said pad is inserted into the space between one of said end plates and the adjacent end plate of said housing.

3. A cell stack according to claim 1,

characterised in that

said pad is inserted into the space between each of said end plates and each of its adjacent end plate of said housing.

4. A cell stack according to claim 1,

characterised in that

said means for maintaining a constant load comprise two or more individual pads.

5. A cell stack according to claim 1,

characterised in that

said elastic pad is a silicon pad.

6. A cell stack according to claim 1,

characterised in that

said pad is spherical shaped.

7. A cell stack according to claim 1,

characterised in that

said pad has indentation.

8. A cell stack according to claim 1,

characterised in that

said housing is a pressure vessel.