WO2000035038A1

WO2000035038A1 - Proton exchange membrane fuel cell external manifold seal

Info

Publication number: WO2000035038A1
Application number: PCT/US1999/029345
Authority: WO
Inventors: Myron Krasij; Bryan F. Dufner
Original assignee: International Fuel Cells, Llc
Priority date: 1998-12-11
Filing date: 1999-12-09
Publication date: 2000-06-15
Also published as: AU3342300A

Abstract

A sealant system (13) for a manifold (10) of a proton exchange membrane fuel cell includes low temperature silicone rubber bridges (14) between the end plates (9) to compensate for the uneven edges of various fuel cell component layers (8), and a layer (15) of silicone rubber foam or sponge extending across the bridges and along the end plates, around the entire contact perimeter surfaces of the manifold, to seal the manifold to the fuel cell.

Description

Proton Exchange Membrane Fuel Cell External Manifold Seal

Technical Field

This invention relates to a two-part rubber/silicone foam seal system for proton exchange membrane (PEM) fuel cell reactant manifold.

Background Art

A basic fuel cell comprises an anode electrode spaced apart from a cathode electrode with an electrolyte disposed therebetween in a compartment formed between the two electrodes; each electrode also includes a catalyst layer on the electrolyte side thereof. On the non- electrolyte side of the anode electrode is a reactant gas chamber for carrying a fuel, and on the nonelectrolyte side of the cathode electrode is a reactant gas chamber for carrying an oxidant. The electrodes are constructed so that the gas diffuses therethrough and comes into contact with the electrolyte in the catalyst layer thereby causing a well- known electrochemical reaction whereby hydrogen ions and electrons produced at the anode travel from the anode electrode through, respectively, the electrolyte and the external circuit to the cathode electrode where they react with oxygen to produce heat and water. This flow of electrons is the electric current produced by the cell.

In a fuel cell power plant a number of fuel cells are connected electrically in series through plates separating adjacent cells, thereby forming a cell stack assembly (CSA). These plates in combination with the electrodes adjacent thereto, generally define the reactant gas passages or chambers. The CSA is provided with external fuel and oxidant manifolds for simultaneous supply to and exhaust of gases from the individual cells. Accordingly, it is necessary to provide manifold-to- CSA seals to prevent leakage of the gases involved in the operation of the fuel cell stack assembly. Also, during operation of the CSA, the CSA and the manifold-to-CSA seals undergo compressive creep. Thus, since the CSA is made up of a series of cells, the edges of which are not aligned, and the surface of the stack is therefore rough, and, with compressive creep of the stack and the manifold-to-CSA seals, this movement results in increased reactant leakage.

Seals for phosphoric acid fuel cells as shown in commonly owned U.S. Patent 4,774,154, have successfully employed a composition which includes a high fluorine content fluorinated hydrocarbon elastomer, a carbon black filler, azodicarbonamide blowing agent and blowing agent promotor, and an epoxy or inorganic oxide acid acceptor. However, this composition cures in the range between 176°C and 204°C (350°F and 400°F). Since the proton exchange membrane is generally damaged above 90°C (194°F), curing the prior seal compound would destroy a PEM fuel cell.

PEM fuel cells known to the prior art have utilized closed cell, neoprene rubber foam to accommodate the uneven surface of the side of the fuel cell stack. However, this material exhibits poor springback characteristics, resulting in increased leakage as a function of time and a requirement to continuously tighten the manifolds.

Disclosure of Invention

Objects of the invention include a manifold seal system for a PEM fuel cell stack assembly which can be effected at temperatures not exceeding 90°C (194°F), which is stable in the presence of hydrogen, oxygen and water, which has low creep and which is very compressible.

According to the present invention, a manifold seal system for a PEM fuel cell comprises two parts, including a low temperature curing silicone rubber applied in strips to provide a smooth bridge between one stack end plate (sometimes referred to as "pressure plate"), and the opposite end plate, there typically being one such bridge along each edge of the stack, the second part comprising a elastomer extending between the reactant manifold and the CSA, said elastomer in contact with the end plates and in contact with the silicone rubber bridges, said elastomer having a low compression set (low creep) and a low compressive modulus (being very springy). In accordance with preferred embodiments of the present invention, the cell bridges may comprise a two-part silicone rubber capable of curing at room temperature or a one- part silicone room temperature vulcanizing silicone rubber adhesive. In preferred embodiments, the elastomer may comprise closed cell silicone rubber foam of low to medium density, or closed cell silicone rubber sponge.

As used herein, "low density foam" means a foam that exerts a pressure of about 2-5 psi at a deflection of 25% while "medium density foam" exerts a pressure of about 10-20 psi at a deflection of 25%. A low density foam also has a bulk density of about 0.15-0.25 grams per cubic centimeter and a medium density foam has a bulk density of 0.25-0.35 grams per cubic centimeter. The compression set of the seal material should be less than 35% after 72 hours at 190°F. The preferred range of thickness is between 1/16 of an inch and 3/16 of an inch.

The invention provides an effective seal to the fuel cell for all pressure differentials experienced by the seal during fuel cell operation, without leaking, and does not require periodic tightening of a manifold to maintain the seal over time. The sealing system of the invention, in one form, allows easy removal of the manifold, should it be required.

Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing. Brief Description of the Drawings

Fig. 1 is a partial, partially broken away and sectioned, front elevation view of a fuel cell having a seal according to the invention.

Fig. 2 is a partially sectioned top plan view of a fuel cell having a seal according to the present invention, with the manifold of Fig. 1 removed.

Best Mode for Carrying Out the Invention

Referring to Figs. 1 and 2, the fuel cell component elements 8 of a PEM fuel cell are depicted stylistically, and, as is known include, for each cell, a membrane, an anode substrate, an anode catalyst, a cathode substrate, a cathode catalyst, an anode flow field plate and a cathode flow field plate. A cooler (not shown) is interspersed between each cell or each group of three to four cells. The positions of the edges of these elements are irregular as shown in Fig. 1 , giving rise to one aspect of the problem with providing an adequate seal system for a PEM fuel cell. The elements 8 are sandwiched between end plates 9, which are bolted together so as hold the various layers in tight contact with one another. A reactant gas manifold 10 (shown only in Fig. 1 ) is positioned above the elements 8 so as to provide flow of reactant gas (either oxygen containing oxidizing gas or hydrogen containing fuel gas) to and from the flow field plates so as to bring the reactant gases to the anode and cathode, respectively.

The seal system 13 of the invention includes bridges 14 of low temperature cured silicone rubber which provide a straight smooth surface coplanar with the adjacent surfaces of the end plates 9 and a layer 15 of silicone rubber foam or sponge beneath the entire contact perimeter surface 16 of the manifold 10. The bridges 14 may be made of two part low temperature curing silicone rubber, or one part silicone room temperature vulcanizing silicone adhesive. A preferred material is GE RTV 108 one-part silicone adhesive sealant. Another preferred material is Dow Corning 732 RTV. The layer 15 may preferably comprise Rogers Poron® BF-1000 silicone foam; or, Ja-Bar JB 0202 silicone sponge may also be used, if desired. However, other materials may be used, so long as they are compatible in the fuel cell environment, the seal system can be perfected at temperatures which will not harm the membrane (that is, below about 90°C, or 94°F), they have low creep under load, and they are very compressible. The seal system 13 of the invention may be formed by first applying low temperature curing silicone rubber directly to the edge of the stack elements 8, thereby creating the bridges 14, and allowing the bridges to cure. This provides the flat, smooth surface above the elements 8 coplanar with the adjacent surfaces of the end plates 9, in the regions where the manifold 10 crosses over from one end plate to the other as seen in Fig. 2. Then, the silicone rubber foam or sponge 15 can be applied over the bridges 14 and end plates 9, with the manifold 10 thereafter being drawn tightly toward the CSA by means of tie rods, not shown. Alternatively, the layer 15 can be adhered to the contact perimeter surface 16 of the manifold and the silicone rubber adhesive applied to the exposed surface of the adhered layer 15, with sufficient material in the cross-over regions to form the bridges 14, and the manifold then mounted to the fuel cell under low pressure as the silicone rubber of the bridges cures. However, it is deemed preferred to form the bridges 14 first, and then position layer 15 and the manifold onto the fuel cell, since this makes it easier to remove the manifold later, should it be necessary.

The aforementioned patent is incorporated herein by reference. Thus, although the invention has been shown and described with respect to exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without departing from the spirit and scope of the invention.

Claims

C-2249Claims

1 . A seal system for a reactant gas manifold in a proton exchange membrane fuel cell having a plurality of fuel cell component elements extending under compression pressure between a pair of end plates, said elements having mutually uneven surfaces at ends thereof to which said manifold is to be sealed, comprising: one or more first seal parts applied to said mutually uneven surfaces and extending from one of said end plates to the other of said end plates at one or more regions of said mutually uneven surfaces to which said manifold is to be sealed, said first seal parts each having a relatively smooth surface facing said manifold, and coplanar with surfaces of said end plates to which said manifold is to be sealed, said first seal parts comprising a low-temperature-curing, silicone rubber adhesive; and a second seal part comprising an elastomer disposed on each of said bridges and on each of said end plates, between the entire contact perimeter surface of said manifold and said bridges and end plates, said elastomer having a low compressive set and a low compressive modulus.

2. A system according to claim 1 wherein said elastomer is a silicone rubber foam closed cell sponge.

3. A system according to claim 2 wherein said elastomer is a Rogers BF-1000 Poron® silicone. C-2249

4. A system according to claim 2 wherein said elastomer is a Ja-Bar JB 0202 silicone sponge.

5. A system according to claim 1 wherein said silicone rubber adhesive is selected from GE RTV 108 and Dow 732.

6. A system according to claim 1 wherein said elastomer is a closed cell foam or sponge with a compressibility of 2-20 psi at 25 percent deflection, a compressive set of less than 35 percent at 25 percent deflection at 190°F, and a thickness of about 1/16 inch to 3/16 inch.

7. A method of sealing a perimeter surface of a reactant gas manifold to end plates and fuel cell component layers of a proton exchange membrane fuel cell, comprising: first, applying a low temperature curing silicone rubber adhesive to the regions of said fuel cell layers that are to be sealed to said manifold, to provide bridges having smooth, straight surfaces coplanar with the surfaces of said end plates to which said manifold is to be sealed; then allowing said silicone rubber adhesive to cure; thereafter applying elastomer, having a low compression set and a low compression modulus, to said coplanar surfaces of said end plates and said bridges; and installing said manifold on said fuel cell with its contact perimeter surface in contact with said elastomer.

8. A method according to claim 7 wherein said elastomer is a silicone rubber foam or closed cell sponge.

9. A method according to claim 8 wherein said elastomer is a Rogers BF-1000 Poron® silicone.

10. A method according to claim 8 wherein said elastomer is a Ja-Bar JB 0202 silicone sponge.

1 1. A method according to claim 6 wherein said silicone rubber adhesive is selected from GE RTV 108 and Dow 732.

12. A method according to claim 6 wherein said elastomer is a closed cell foam or sponge with a compressibility of 2-20 psi at 25 percent deflection, a compressive set of less than 35 percent at 25 percent deflection at 190°F, and a thickness of about 1/16 inch to 3/16 inch.

13. A method of sealing a perimeter surface of a reactant gas manifold to end plates and fuel cell component layers of a proton exchange membrane fuel cell, comprising: first, adhering an elastomer layer to the entire contact perimeter surface of said manifold; then applying the exposed surface of said adhered elastomer layer with a low temperature curing silicone rubber adhesive, said adhesive being applied sufficiently thick in regions of said elastomer layer which face said component layers so as to fill any void resulting C-2249

from ends of said fuel cell layers adjacent said elastomer layer being non-coplanar with the surfaces of said end plates facing said elastomer layer; and placing said manifold into position on said fuel cell while allowing said adhesive to cure under a low force applied to said manifold.

14. A method according to claim 13 wherein said elastomer is a silicone rubber foam or closed cell sponge.

15. A method according to claim 14 wherein said elastomer is a Rogers BF-1000 Poron® silicone.

16. A method according to claim 14 wherein said elastomer is a Ja-Bar JB 0202 silicone sponge.

17. A method according to claim 13 wherein said silicone rubber adhesive is selected from GE RTV 108 and Dow 732.

18. A method according to claim 13 wherein said elastomer is a closed cell foam or sponge with a compressibility of 2-20 psi at 25 percent deflection, a compressive set of less than 35 percent at 25 percent deflection at 190°F, and a thickness of about 1/16 inch to 3/16 inch