WO2008089553A1

WO2008089553A1 - Fuel cell stack compression system

Info

Publication number: WO2008089553A1
Application number: PCT/CA2008/000127
Authority: WO
Inventors: Hao Tang; Dingrong Bai; David ELKAÏM; Jean-Guy Chouinard
Original assignee: Hyteon Inc.
Priority date: 2007-01-22
Filing date: 2008-01-22
Publication date: 2008-07-31

Abstract

A fuel cell stack comprising: a top endplate and a bottom endplate; a plurality of fuel cells arranged on top of one another between the top endplate and the bottom endplate, each cell having a membrane electrode assembly between two electrically conductive flow field plates; and a compression system embedded within the fuel cell stack comprising at least one compressive plate, for maintaining a substantially constant pressure between the fuel cells during stack operation.

Description

FUEL CELL STACK COMPRESSION SYSTEM

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority of US Provisional Patent Application filed on January 22, 2007 and bearing serial number 60/881,523.

TECHNICAL FIELD

The present invention relates to fuel cells, and more particularly, to a method and design for stack assembly to reduce stack volume and increase stack pressure uniformity and durability.

BACKGROUND OF THE INVENTION

Fuel cell stack performance and lifetime is one of the key control factors for fuel cell system reliability, efficiency and cost. Generally, stack performance degrades with system operation time due to a variety of reasons. Among these reasons, stack pressure relaxation or decrease is a big issue. The lower stack pressure resulting from gaskets and/or fuel cell plate materials aging or relaxations leads to stack performance losses due to Membrane Electrode Assembly (MEA) resistance increase and/or gas diffusion layer structure changes .

To maximally maintain stack initial pressure and its uniformity during fuel cell operation, elastic springs are commonly used in fuel cell stacks to compensate stack compression variations. The stack volume, however, increases dramatically with the springs' installation in the fuel cell system. This is because the springs are placed at the top of the stack endplate and act as a stack compression system. The length of the springs can be as long as 30% of the stack height, thereby increasing the total stack volume up to 30% or 40%.

5 US patent 4,430,390 discloses a method to control or adjust stack compression using a spring resilience system. There is described a complex mechanically-designed resilience system to control and/or adjust stack pressures locally. However, the complex mechanical design is very costly, for both0 manufacturing and assembling, and hence reduces stack reliability.

Therefore, there is a need for a system that will provide the required compression without increasing stack volume.

SUMMARY OF THE INVENTION 5 There is described herein a system that improves stack pressure uniformity with a new stack design concept for the compression system. The compression system is embedded inside the stack to reduce stack volume and to improve stack pressure uniformity and durability.

0 In accordance with a first broad aspect of the present invention, there is provided a fuel cell stack comprising: a top endplate and a bottom endplate,- a plurality of fuel cells arranged on top of one another between the top endplate and the bottom endplate, each cell having a membrane electrode5 assembly between two electrically conductive flow field plates; and a compression system embedded within the fuel cell stack comprising at least one compressive plate, for maintaining a substantially constant pressure between the fuel cells during stack operation.

In accordance with a second broad aspect of the present invention, there is provided a method of maintaining pressure 5 between a plurality of fuel cells substantially constant during stack operation of a fuel cell stack, the method comprising: providing a top endplate and a bottom endplate; providing the plurality of the fuel cells arranged on top of one another between the top endplate and the bottom endplate,0 each cell having a membrane electrode assembly between two electrically conductive flow field plates; embedding a compression system within the stack by providing at least one compressive plate therein for maintaining the substantially constant pressure between the plurality of fuel cells during5 stack operation.

In the specification, the term "compressive plate" should be understood as a plate that is compressible without being permanently deformable and having expansion capabilities when compressed. The compression system produces a certain amount 0 of pressure at a given range of strain. Various types of elastic materials, springs, or specially designed metal foams can be used as compressive plates.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will5 become apparent from the following detailed description, taken in combination with the appended drawings, in which:

Fig. 1 is a front cross- sectional view of a fuel cell stack with compression springs in accordance with the prior art;

_ "2 _ Fig. 2 is a front cross-sectional view of a fuel cell stack with a compressive plate embedded within a top endplate, in accordance with an embodiment of the invention;

Fig. 3 is a front cross-sectional view of a fuel cell stack 5 with a central compressive plate embedded within a top endplate, in accordance with an embodiment of the invention;

Fig. 4 is a front cross-sectional view of a fuel cell stack with two lateral compressive plates embedded within a top endplate, in accordance with an embodiment of the invention; 0 Fig. 5 is a front cross-sectional view of a fuel cell stack with a compressive plate placed between an end plate and an insulation plate, in accordance with an embodiment of the invention;

Fig. 6 is a front cross-sectional view of a fuel cell stack5 with compressive plates placed inside recesses formed on an endplate of the stack, in accordance with an embodiment of the invention;

Fig. 7 is a front cross-sectional view of a fuel cell stack with two compressive plate embedded between two fuel cells, 0 in accordance with an embodiment of the invention;

Fig. 8 .illustrates the stress-strain curve of a metal foam material compressive plate, in accordance with an embodiment of the present invention;

Fig. 9 is a flow field plate of which parts are made of an5 elastic conductive material, in accordance with an embodiment of the present invention; Fig. 10 is an MEA gasket comprising a compressive plate, in accordance with an embodiment of the present invention; and

Fig. 11 is an MEA gasket comprising a discontinuous compressive plate, in accordance with an embodiment of the 5 present invention.

It will be noted that throughout the appended drawings, like features are identified by like reference numerals.

DETAILED DESCRIPTION

Figure 1 illustrates a traditional stack 50 design according0 to the prior art. The fuel cell stack 50 comprises end plates 52, 54, insulator plates 56, 58, current collector plates 60, 62, and a plurality of fuel cells 64 therebetween. Rods 66 maintain the assembly in position. For example, a secure plate or screw 68 is fixedly attached to rods 66 on the top5 of the fuel cell stack 50 so that no relative movement between them occurs. Bottom endplate 52 is also fixedly attached to rods 66 in order to prevent any relative movement. Springs 70 are placed between the top endplate 54 and the secure plate or screw 68. Springs 70 are in 0 compression to work as a stack compression system. The compression of springs 70 results in a force exerted on the top endplate 54 following arrow 72 and maintains the required fuel cells' 64 compressive loading to the fuel cell stack 50. As a result, the clamping pressure exerted upon fuel cells 645 is kept constant during system operation.

However, the length of springs 70 can be as long as 30% of the fuel cell stack 50 height. This presents the disadvantage of increasing the size of the fuel cell stack 50 since the total volume of the stack 50 is increased by about 30%.

Figure 2 illustrates one embodiment of a fuel cell stack 100 with a compression system embedded between top endplate 102 5 of the fuel cell stack 100. A compression system is an elastic system which maintains constant the pressure between fuel cells 64. Endplates 52 and 102 are fixedly attached to rods 108 by securing locks or screws to prevent any relative movement. Alternatively, this can be done by welding0 endplates 52 and 102 to rods 108. Alternatively, any means securing endplates 52 and 102 to rods 108, such as a locking mechanism, and preventing any relative movement can be used.

The compression system includes a compressive plate 106 made of an elastic material. The compressive plate 106 is placed5 in compression within endplate 102 so that a force following arrow 72 is exerted on the insulator plate 58. This force results in a loading charge on fuel cells 64 and maintains substantially constant the pressure between fuel cells 64 during fuel cell stack 100 operation. As a result, the0 pressure between fuel cells 64 at high temperature during stack 100 operation is substantially the same as that at room temperature during stack 100 inactivity. The pressure between the flow field plates and the MEA gasket inside a fuel cell 64 also remains substantially constant. 5 The use of compressive plate 106 and its embedment into the fuel cell stack 100 reduces the total length of the stack in comparison to stack 50.

While the compression system described in figure 2 only comprises a single compressive plate 106, it should be understood that several compressive plates can be embedded within the fuel cell stack. For example, the fuel cell stack could further include a compressive plate embedded within endplate 52.

5 While, in figure 2, the compressive plate 106 has the same length as endplate 102, the length of the compressive plate 106 can be less than that of the endplate 102. Figure 3 illustrates a fuel cell stack 150 with a central compressive plate 152. The width of the compressive plate 152 is less0 than that of endplate 102 and the compressive plate 152 is located substantially at the center of the fuel cell stack 150. Figure 4 illustrates a fuel cell stack 200 comprising two compressive plates 202 embedded within endplate 102 according to another embodiment. The compressive plates 2025 comprise apertures to receive rods 108. During the assembly of the fuel cell stack 150, compressive plate 152 is compressed to a certain amount which corresponds to a predetermined loading charge to the fuel cells 64.

Figure 5 illustrates a fuel cell stack 250 comprising a0 compressive plate 254 which is placed in compression between top endplate 252 and insulator plate 58. Alternatively, the compressive plate can be located between insulator plate 58 and collector plate 62. Fuel cell stack 250 could also comprise several compressive plates. For example, one5 compressive plate could be located between top endplate 252 and insulator plate 58 and another compressive plate could be placed between insulator plate 58 and collector plate 62.

Figure 6 illustrates a fuel cell stack 300 having a compression system embedded in a top endplate 302 according to one embodiment. Top endplate 302 is provided with recesses 304 in which compressive plates 306 are embedded. A secure plate or screw 308 is fixedly attached to rods 108 forming a fulcrum for the compressive plates 306. The compression of 5 compressive plates 306 results in a force applied on the rest of the fuel cell stack 300 and having the direction of arrow 72.

While recesses 304 are located on a side that is towards the outer side of the stack, they can be located on an inner side0 of the stack. In this case, there is no need for the secure plate 308, and endplate 302 is fixedly attached to rods 108 so that no relative movement occurs. Alternatively, the recesses and compressive plates can be located on the top side and/or bottom side of the insulator plate 58. The5 recesses and the compressive plates could also be placed on the top and/or bottom side of the current collector plate 62.

It should be understood that the number and the location of the recesses may vary. For example, endplate 302 and/or insulator plate 58 and/or current collector plate can be 0 provided with a recess located substantially at the center of the plates .

In one embodiment, the resilient material of the compressive plate can be any type of elastic materials which are compressible but non-permanently deformable. When a force is5 applied to the material, the material is capable of stretching or compression and when the force is relaxed, the material comes back to its initial shape. Examples of material that can be used for the compressive plates are vinyl material, plastics, rubbers, polymer composites or plastics with in-organic additions. Alternatively, non- elastic materials embodied with elastic materials can also be used. For example, the compressive plate can be made of any- elastic materials, elastic materials embodied with springs, 5 or non-elastic materials embodied with springs. The springs embodied inside the materials can be one or more, of any shape .

In all of the embodiments described above, the plates may be single or multiple and can be of any shape. The shape of the0 compression system can be, without being limited to, rectangular, cylindrical, or cubic. These embodiments may be employed alone or in combination.

While the compressible systems described in figures 2-6 are located at the top of the fuel cell stack, it should be5 understood that the compression system might be located at the bottom of the fuel cell stack. Several compression systems located at the top and/or the bottom of the fuel cell stack can be provided in the stack.

Figure 7 illustrates a fuel cell stack 350 with compressive0 plates 352 located between two fuel cells 64 according to another embodiment. The compressive plates 352 are made of an elastic conductive material such as metal foam and are placed in compression between fuel cell units 64. Metal open-cell foam or close-cell foam are examples of metal foam which can5 be used. Figure 8 illustrates an example of the stress-strain curve of a metal foam material compressive plate. The stress first increases as a function of the applied strain before reaching a maximum value and being substantially independent of the strain. In the illustrated example, the range of strain between 0.09 and 0.16 in/in represents a desired range. The stress resulting from the applied strain within this range is substantially constant and equal to 350 psi.

The number and the location of compressive plates 352 may 5 vary. For example, the fuel cell stack 350 may comprise a single compressive plate 352. Alternatively, a compressive plate 352 may be inserted between each fuel cell 64. A compressive plate 352 can also be located between current collector plate 60, 62 and the next fuel cell 64. 0 In one embodiment, the compressive plate is integrated into a fuel cell. At least one of the flow field plates and/or the MEA gasket constituting the fuel cell can be the compression system of the fuel cell stack. It should be noted that the flow field plates can be bi-polar plates. In this case, the5 flow field plates are partially or entirely made of conductive elastic material while the MEA gaskets are partially or entirely made of non-conductive elastic material. Alternatively conductive elastic material can be added on at least one side of the flow field plate while non-0 conductive elastic material can be added on at least one side of the MEA gasket. Figure 9 illustrates a flow field plate 370 in which areas 372 and 374 are made of an elastic conductive material. Areas 372 and 374 represent the compression system which maintains constant the pressure of5 the stack. Alternatively, the entire flow field plate 370 can be made of elastic conductive material or only the outer perimeter of the flow field plate can made of elastic conductive material. When the fuel cell is assembled, the flow field plate is placed in compression to ensure a substantially constant pressure between the elements of the fuel cell and also between the fuel cells.

The compression system can also be integrated in the MEA, such as in the gasket. Figure 10 illustrates an MEA gasket 5 380 comprising a membrane 382. A rectangular compressive plate 384 having the shape of a frame is added to MEA gasket 380. Compressive plate 384 is made of an elastic non- conductive material. Figure 11 illustrates an MEA gasket 390 comprising a discontinuous compressive plate 392 in the shape0 of a frame. Compressive plate 392 is made of elastic non- conductive material with portion 394 of regular gasket materials. Compressive plates 384 and 392 can be attached to or part of MEA gasket 380 and 390, respectively. Compressive plates 384 and 392 can also be attached to bi-polar plates. 5 Compressive plates 384 and 392 can be of any shape. Several compressive plates can be provided in the MEA gasket. An MEA gasket can also be provided with a compressive plate on each side. Alternatively, the entire MEA gasket can be made of an elastic non-conductive material. 0 The number and the location of fuel cells comprising a compression system may vary in the fuel cell stack. For example, each fuel cell can comprise at least one compression system. It should be understood that the different embodiments of the compression system may be employed either5 alone or in combination. The embodiments of the invention described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.

Claims

I/WE CLAIM:

1. A fuel cell stack comprising: a top endplate and a bottom endplate; a plurality of fuel cells arranged on top of one another between said top endplate and said bottom endplate, each cell having a membrane electrode assembly between two electrically conductive flow field plates; and a compression system embedded within said fuel cell stack comprising at least one compressive plate, for maintaining a substantially constant pressure between said fuel cells during stack operation.

2. A fuel cell stack as claimed in claim 1, wherein said at least one compressive plate is embedded within at least one of said top endplate and said bottom end plate .

3. A fuel cell stack as claimed in claim 1, wherein said at least one compressive plate is between at least one of said top endplate and said bottom endplate and an insulation plate .

4. A fuel cell stack as claimed in claim 1, wherein said at least one compressive plate is between at least an insulation plate and a current collector plate.

5. A fuel cell stack as claimed in claim 1, wherein said at least one compressive plate is between at least one of said plurality of fuel cells and a current collector plate.

6. A fuel cell stack as claimed in claim 1, wherein at least one of said top endplate, said bottom endplate, an insulation plate and a current collector plate has a recessed portion, and said at least one compressible plate is placed within said recessed portion.

7. A fuel cell stack as claimed in claim 1, wherein said at least one compressive plate is between two of said plurality of fuel cells.

8. A fuel cell stack as claimed in claim 1, wherein said at least one compressive plate is embedded within at least one of said plurality of fuel cells.

9. A fuel cell stack as claimed in claim 8, wherein said at least one compressive plate is embedded in one of said two electrically conductive flow field plates.

10. A fuel cell stack as claimed in claim 8, wherein said at least one compressive plate is part of said membrane electrode assembly.

11. A fuel cell stack as claimed in any one of claims 7 to 9 , wherein said at least one compressive plate is made of a conductive material.

12. A fuel cell stack as claimed in any one of claims 1 to 5 , wherein said compressive plate is of a same length and width as said top endplate and said bottom endplate .

13. A fuel cell stack as claimed in any one of claims 7 to 10, wherein said compressive plate is of a same length and width as said flow field plates.

14. A method of maintaining pressure between a plurality of fuel cells substantially constant during stack operation of a fuel cell stack, the method comprising: providing a top endplate and a bottom endplate; providing said plurality of said fuel cells arranged on top of one another between said top endplate and said bottom endplate, each cell having a membrane electrode assembly between two electrically conductive flow field plates; and embedding a compression system within said stack by providing at least one compressive plate therein for maintaining said substantially constant pressure between said plurality of fuel cells during stack operation.

15. A method as claimed in claim 14, wherein said embedding comprises embedding said at least one compressible plate within at least one of said top endplate and said bottom endplate .

16. A method as claimed in claim 14, wherein said embedding comprises embedding said at least one compressive plate between at least one of said top end plate and said bottom end plate and an insulation plate.

17. A method as claimed in claim 14, wherein said embedding comprises embedding said at least one compressive plate between at least an insulation plate and a current collector plate .

18. A method as claimed in claim 14, wherein said embedding comprises embedding said at least one compressive plate between at least one of said plurality of fuel cells and a current collector plate.

19. A method as claimed in claim 14, further comprising providing a recessed portion in at least one of said top endplate, said bottom endplate, an insulation plate and a current collector plate, and said embedding comprises embedding said at least one compressible plate within said recessed portion.

20. A method as claimed in claim 14, wherein said embedding comprises embedding said at least one compressive plate between two of said plurality of fuel cells.

21. A method as claimed in claim 14, wherein said embedding comprises embedding said at least one compressive plate within at least one of said plurality of fuel cells.

22. A method as claimed in claim 21, wherein said embedding comprises embedding said at least one compressive plate within one of said two electrically conductive flow field plates .

23. A method as claimed in claim 21, wherein said embedding comprises embedding said at least one compressive plate within said membrane electrode assembly.