WO1997006571A1

WO1997006571A1 - Device provided with an opening to regulate the flow of humidified gas streams of electrochemical fuel cells

Info

Publication number: WO1997006571A1
Application number: PCT/CA1996/000523
Authority: WO
Inventors: Russell H. Barton; Henry H. Voss
Original assignee: Ballard Power Systems Inc.
Priority date: 1995-08-04
Filing date: 1996-08-02
Publication date: 1997-02-20
Also published as: CA2228383A1; AU6609296A

Abstract

In a device (15) for use with humidified gas streams in electrochemical fuel cells (10) having a reactant manifold (27) for directing a reactant stream across the fuel cell, the manifold having an inlet and an outlet, the device comprises a base layer (16) of fluid impermeable sheet material extending across the manifold outlet. The base layer has an opening formed therein. A top layer (18) of fluid impermeable sheet material superposed over a portion of the base layer extends across and beyond the base layer opening on the major surface of the base layer facing away from the manifold. An intermediate layer (17) of fluid permeable material is interposed between the base layer and the top layer. The intermediate layer extends across the opening in the base layer and laterally beyond the top layer. The intermediate and top layer have an opening formed therein for the passage of a fluid stream therethrough, said openings are circumscribed by the base layer opening.

Description

DEVICE PROVIDED WITH AN OPENING TO REGULATE THE FLOW OF HUMIDIFIED GAS STREAMS OF ELECTROCHEMICAL FUEL CELLS

Cross-Reference To Related Application

This application is related to and claims priority benefits from U.S. Provisional Patent Application Serial No. 60/001,925 filed August 4, 5 1995, entitled "Wicking Orifice For Electrochemical Fuel. Cells". The related U.S. provisional patent application is incorporated herein by reference in its entirety.

Field Of The Invention

10 This invention relates generally to electrochemical fuel cells and, more particularly, to a metering device for evenly distributing a humidified gas stream between parallel flow paths within an electrochemical fuel cell.

15 Background Of The Invention

Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly ("MEA") comprising a

20 solid polymer electrolyte or ion exchange membrane disposed between two electrodes formed of porous, electrically conductive sheet material, typically carbon fiber paper. The MEA contains a layer of catalyst, typically in the form of finely

25 comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load. At the anode, the fuel is transported through the porous electrode material and reacts at the catalyst layer to form cations, which migrate through the membrane to the cathode. At the cathode, the oxygen in the oxidant supply reacts with the cations at the catalyst layer to form a reaction product. In electrochemical fuel cells employing hydrogen as the fuel and oxygen- containing air (or substantially pure oxygen) as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of hydrogen ions from the anode to the cathode. In addition to conducting hydrogen ions, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode catalyst layer, oxygen reacts with the hydrogen ions that have crossed the membrane to form water as the reaction product. The anode and cathode reactions in hydrogen/oxygen fuel cells are shown in the following equations:

Anode reaction: H₂ -» 2H* + 2e^" Cathode reaction: 1/20-, + 2H* + 2e^" -* H₂0

Two or more fuel cells can be connected together in series or in parallel to increase overall power output. A series-connected multiple fuel cell arrangement is commonly referred to as a fuel cell stack.

Metering apertures can be used to ensure even distribution of gas flowing through parallel flow paths which are fed by a common distribution header or conduit. The pressure drop across the aperture is designed to be εubstantially greater than the pressure variation in the distribution conduit and/or the pressure variation between the parallel flow paths. The metering aperture is typically an opening of accurate and reproducible dimensions formed through a gas impermeable material, through which the gas flows (at the inlet or outlet) . If the gas passing through the aperture is carrying some vapor or entrained liquid, a droplet of liquid could occlude or partially occlude the aperture. This is especially likely to occur if, for example:

(1) there is a temperature difference across the aperture; (2) the temperature of the material through which the aperture is formed is lower than that of the gas stream passing through it; (3) the aperture is small; (4) the flow rate of the gas passing through the aperture is low; or (5) the pressure differential across the aperture is εmall and insufficient to clear away a water droplet. For fuel cells arranged in a stack, reactant gases are typically fed to individual cells in the stack via a common conduit, and then the reactant gas is distributed across the active area of each individual cell by one or more passageways branching off from the main conduit. The gases are typically exhausted from the stack via a common outlet. The distribution of gases between each of the fuel cells in the stack is determined by the pressure drop acrosε each cell. In the absence of any metering control, there maybe inherent differences in the resistance- o-flow (pressure drop) across the various cells and, during operation of the stack, conditions in individual cells (e.g. build-up of water) can cause variations between cells thereby reεulting in uneven gas distribution and lower performance of some cells.

However, in the distribution of a reactant gas stream in a solid polymer electrochemical fuel cell the use of conventional metering jets or orifices is unsuitable due to the water content of the reactant gas stream. It is desirable to place the flow restriction near or at the exit of the reactant passage or passages in each individual fuel cell, however, water vapor may condense if the gas εtream exiting the fuel cell contacts a surface which is cooler than the dew point of the exit gas stream. Because the fuel cell is the εource of heat energy, a temperature profile exists which decreases from the fuel cell to the heat sink, which is typically a coolant fluid. In conventional fuel cell designs, the reactant exhaust manifolds are typically slightly cooler than the reactant passages within the fuel cell, and water vapor may therefore condense. Liquid water which forms in the metering jet or orifice may cause occlusion of the aperture unless the pressure differential across the aperture is sufficient to overcome the surface tension effects of the occluding water droplet. In fuel cells in which one or both of the reactant streams is at low or near-ambient presεure thiε can be a particular problem.

Near-ambient air fuel cells are described in U.S. Patent Application Serial No. 08/171,732 filed December 22, 1993, entitled "Electrochemical Fuel Cell Employing Ambient Air As the Oxidant And Coolant" and U.S. Patent Application Serial No. 08/485,644 filed June 7, 1995, entitled

"Electrochemical Fuel Cell Assembly With Compliant Compression Mechanism", incorporated herein by reference in their entireties. In these fuel cells, air may be directed to the fuel cell cathodes using a low pressure fan or pump. In this case it is desirable to use metering orifices to ensure even diεtribution of oxidant across individual fuel cells in the stack (to ensure consistent performance from cells in the stack) . It is often advantageous to locate the metering aperture at the outlet of each cell where the air is vented to the outlet conduit. However, because the fuel cell reaction produces water and heat, the exiting gas stream is typically warm and at near 100% humidity. The material through which the metering aperture is formed and the environment on the far side of the aperture is typically cooler so as the air passes through the aperture water will tend to condense on the plate and occlude the aperture. Generally the pressure generated by the fan or pump will be insufficient to diεplace the droplet. When the aperture is occluded the air flow across that cell will be reduced, possibly to zero, and the performance of the cell will drop. It is therefore an object of thiε invention to provide a metering aperture which will not plug with a droplet of liquid when used to meter a gas containing vapor or entrained liquid. Simmtarv Of The Invention

A metering apparatus for an electrochemical fuel cell comprising a reactant manifold for directing a reactant stream acrosε the fuel cell, the manifold comprising an inlet and an outlet, comprises: a base layer of fluid impermeable sheet material having two major surfaces, the base layer extending across the manifold outlet, the base layer having an opening formed therein; a top layer of fluid impermeable sheet material, the top layer superposed over a portion of the major surface of the base layer facing away from the manifold and extending across and beyond the base layer opening; an intermediate layer of fluid permeable sheet material interposed between the base and top layers, the fluid permeable layer extending across the base layer opening and extending beyond the top layer, the top and intermediate layers having a metering opening formed therethrough, the metering opening circumscribed by the base layer opening. The fluid stream preferably compriseε water vapor and moεt preferably further compriεes air.

In an electrochemical fuel cell stack comprising at least two fuel cells, each with a metering device for a humidified oxidant εtream, the fuel cells each comprise: a membrane electrode aεεembly compriεing a first cathode, an anode fluidly isolated from the cathode, an ion exchange membrane interposed between the cathode and the anode; a first oxidant manifold for directing an oxidant stream across the first cathode, the first manifold having and inlet and an outlet; a first oxidant stream metering apparatus comprising: a base layer of fluid impermeable sheet material having two major surfaces, the base layer extending across the first manifold outlet, the base layer having an opening formed therein; a top layer of fluid impermeable sheet material, the top layer superposed over a portion of the major surface of the base layer facing away from the first manifold and extending acrosε and beyond the base layer opening; an intermediate layer of fluid permeable sheet material interposed between the base and top layers, the fluid permeable layer extending across the base layer opening and extending beyond the top layer, the top and intermediate layers having a metering opening formed therethrough, the metering opening circumscribed by the base layer opening; and the electrochemical fuel cell stack further compriseε: oxidant delivery means for delivering an oxidant stream to the first cathodes, the delivery means comprising an oxidant supply conduit in fluid communication with the firεt oxidant manifoldε for directing an oxidant stream to the inlets; fuel delivery means for delivering a fluid fuel stream to the anodes. The oxidant delivery means preferably compriseε a fan. In one embodiment of an electrochemical fuel cell εtack compriεing at leaεt two fuel cellε wherein each fuel cell compriεeε a bicell membrane electrode aεsembly, each fuel cell having a metering device for a humidified oxidant stream, the fuel cells each further comprise a second cathode fluidly isolated from the anode, and a second ion exchange membrane interposed between the second cathode and the anode, and wherein the first oxidant manifold is for directing an oxidant stream across both the first and second cathodes in each of the fuel cells, and wherein the oxidant delivery means is for delivering an oxidant stream to the first and second cathodes.

In an alternative embodiment of an electrochemical fuel cell stack comprising at least two fuel cells wherein each fuel cell comprises a bicell membrane electrode assembly, each fuel cell has a pair of oxidant manifolds and a pair of metering devices, one for each cathode of the bicell. In this embodiment the fuel cells each further comprise: a second cathode fluidly isolated from the anode, a second ion exchange membrane interposed between the second cathode and the anode; a second oxidant manifold for directing an oxidant stream acrosε the second cathode the second manifold having and inlet and an outlet; a second oxidant stream metering apparatus comprising: a base layer of fluid impermeable sheet material having two major surfaces, the base layer extending across the εecond manifold outlet, the base layer having an opening formed therein; a top layer of fluid impermeable sheet material, the top layer superposed over a portion of the major surface of the baεe layer facing away from the εecond manifold and extending across and beyond the base layer opening; an intermediate layer of fluid permeable sheet material interposed between the base and top layers, the fluid permeable layer extending across the base layer opening and extending beyond the top layer, the top and intermediate layers having a metering opening formed therethrough, the metering opening circumscribed by the base layer opening; and wherein the oxidant delivery means is for delivering an oxidant stream to the first and second cathodes, the delivery means comprising an oxidant supply conduit in fluid communication with each of the first and second oxidant manifolds.

In an alternative embodiment of the metering apparatus for an electrochemical fuel cell, the fuel cell comprising a reactant manifold for directing a reactant stream acrosε the fuel cell, the manifold compriεing an inlet and an outlet, the apparatus compriseε a plate having two substantially parallel major surfaces. One of the plate major surfaces extends across the manifold outlet. The plate comprises a fluid impermeable matrix having fluid permeable fibers extending through the matrix in a direction substantially parallel to the plate major surfaces. The plate has an opening formed therein for the passage of a fluid stream comprising a condensable component between the plate major surfaces. At least a portion of the fluid permeable fibers are exposed at the opening and at the plate major surface facing away from the manifold outlet. In operation, the exposed fluid permeable fibers conduct the condensable component away from the opening.

In the preferred alternative embodiment, the fluid stream compriεes water vapor and the fluid permeable fibers are hydrophilic. The fluid stream most preferably further comprises air.

Brief Description Of The Drawings

FIG. 1 is an isometric view of a bicell electrochemical fuel cell assembly with a metering device for controlling the flow of a humidified gas stream. FIG. 2 is a crosε-εectional view taken in the direction of arrows 2-2 in FIG. 1.

FIG. 3 is a side sectional view of an electrochemical fuel cell stack in which each of the fuel cell assemblies has a metering device for a humidified gas εtream asεociated therewith. FIG. 4 iε an exploded isometric view of a three-layer metering device structure.

FIG. 5 is a top view of an asεembled three- layer metering device structure.

FIG. 6 is a top view of one of the layers of the metering device structure.

FIG. 7 is a side sectional view of a unicell electrochemical fuel cell aεεembly.

FIG. 8 iε a εide sectional view of a metering device formed of a fluid impermeable matrix having fluid permeable fibers extending through the matrix.

Detailed Description Of The Preferred Embodiments

Referring first to FIGS. 1 and 2, an electrochemical fuel cell asεembly 10 comprises an electrochemical fuel cell 20 and a metering device 15 for controlling the flow of a humidified reactant stream through fuel cell 20. Fuel cell 20 includes first plate 22, second plate 24, firεt cathode 26, second cathode 28, first membrane electrolyte 30, second membrane electrolyte 32, and anode 34. Metering device 15 includes first or base layer 16, second or intermediate layer 17, and third or top layer 18. Each of layers 16 and 18 is preferably formed from substantially fluid impermeable sheet material. Layer 17 iε preferably formed from a fluid permeable hydrophilic or wicking material. Plates 22, 24 cooperate which the exposed major surfaces of cathodes 26, 28 respectively, to form a manifold 27 for directing an oxidant stream across the cathodes 26, 28. Manifold 27 has an inlet 29a and an outlet 29b, and the metering device 15 extends across the manifold outlet 29b of the fuel cell 20.

FIG. 3 showε an electrochemical fuel cell stack comprising five electrochemical fuel cell asεemblies substantially identical to assembly 10 in FIGS. 1 and 2. As shown in FIG. 3, a gas εtream (typically ambient air) iε directed by a fan 50 through a distribution header or conduit 40 to each of the fuel cell assemblies 10 having metering devices asεociated therewith. The flow of the reactant gas stream is depicted by arrows in FIG. 3.

FIG. 4 shows a three-layer metering device structure comprising base layer 16, intermediate layer 17, and top layer 18. Aε indicated above, layerε 16 and 18 are preferably formed from εubεtantially fluid impermeable εheet material. Layer 17 iε preferably formed from hydrophilic or wicking material. In a fuel cell aεsembly, base layer 16 extends across the manifold outlet 29b as shown in FIGS. 1 and 2. As shown in FIGS. 2, 4 and 6, baεe layer 16 haε a centrally diεpoεed opening 19 formed therein. Top layer 18 typically haε a smaller area than base layer 16, but extends across and beyond opening 19. Intermediate layer 17 is interposed between the overlapping portions of base layer 16 and top layer 18, and extends across opening 19 and laterally beyond top layer 18. The intermediate and top layers 17, 18 have a centrally disposed metering opening 21 formed therein for the paεsage of a reactant stream. Metering opening 21 is circumscribed by base layer opening 19 as shown in FIGS. 4 and 5. In this way a portion of one major surface of intermediate layer 17 is exposed on the base layer εide of the device 15 and prevents liquid droplets forming adjacent metering opening 21. Also, a portion of the other major surface of intermediate layer 17 is exposed on the top layer side, providing an evaporative surface from which liquid may be removed.

FIG. 7 illustrateε a unicell fuel cell aεεembly. Unicell MEA 214 includeε an ion exchange membrane 224, which iε interpoεed between anode 226 and cathode 216. A εeal 250, formed of sealant material diεposed along the exterior surfaces of the anode 226, is also shown in FIG. 7. Seal 250 forms a gas-impermeable barrier to prevent leakage of gaseous fuel supplied to the anode 226. A fuel delivery mechanism 244 delivers gaseous fuel (preferably substantially pure hydrogen) to the anode 226 of the unicell MEA 214. The fuel delivery means 244 preferably includes at least one fuel inlet 246 which extends partially into the anode 226. The fuel inlet 246 deliverε gaseouε fuel to the anode 226 at a low preεεure or at slightly greater than atmospheric pressure.

In the embodiment illustrated in FIG. 7, a clamping mechanism 218 secures the plate 262, together with itε fins 264, 266, against the cathode 216 of the unicell MEA 214. The clamping means 218 is illustrated in FIG. 7 as a pair of threaded fasteners 272, 274 and an end plate 220. FIG. 8 illuεtrates a metering device 302 formed of a fluid impermeable matrix 304 having fluid permeable fibers 306 extending through the matrix. Metering device 302 includes a plate 308 which in assembly extends acrosε a fuel cell manifold outlet (not εhown in FIG. 8) . The fluid permeable fibers 306 extend through the matrix 304 in a direction subεtantially parallel to the major surfaces 310, 312 of the plate 308. Plate 308 has a raised opening 316 formed therein for the passage of a fluid εtream in the direction of the arrow in FIG. 8, between the major εurfaces 310, 312 of plate 308. The fluid stream comprises a condensable component, typically water vapor. The fluid permeable fibers 306a at the opening, and

306b adjacent the opening, conduct the condensable component away from the opening. The condensable component can be evaporated from surface 310 where fibers 306c are exposed. In fabricating the metering device of FIGS. 4- 6, it has been noted that if the layers are first asεembled and a hole formed through the intermediate and top layerε 17 and 18 to form the metering opening in those layers, intermediate layer 17 tends to εwell and dimenεionally occlude the opening once hydrated. In addition, the prefered fluid permeable or wicking materialε employed as the intermediate layer (polyvinylalcohol or cellulose paper) tended to fray during the hole forming operation and had irregular dimensionε which could effectively restrict the opening to a greater extent than desired. A method of forming an accurate opening in the intermediate, fluid permeable layer 17 that is no smaller than the opening in the top layer 18, and that has no distorted edges that will swell over the edges of the opening in top layer 18, is to bore an undersized hole in the assembly, soak the assembly in pure water (or whatever liquid saturates the fluid permeable or wicking material) , then freeze the assembly (for example, using liquid nitrogen) , and then bore to final εize with the fluid permeable or wicking material dimensionally stabilized by being frozen. Using this fabrication method, the fluid permeable or wicking layer material, in which the opening is bored while the material is in a hydrated state, retains its bored dimensions when subsequently hydrated during operation of the fuel cell.

While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of courεe, that the invention iε not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings. It is therefore contemplated by the appended claims to cover such modifications as incorporate those features which come within the εpirit and scope of the invention.

Claims

What iε claimed iε:

1. A metering apparatuε for an electrochemical fuel cell, εaid fuel cell comprising a reactant manifold for directing a reactant stream acrosε εaid fuel cell, said manifold comprising an inlet and an outlet, said apparatus comprising: a base layer of fluid impermeable sheet material having two major surfaceε, εaid base layer extending across εaid manifold outlet, εaid baεe layer having an opening formed therein; a top layer of fluid impermeable sheet material, said top layer superposed over a portion of the major surface of said base layer facing away from said manifold and extending across and beyond said base layer opening; an intermediate layer of fluid permeable sheet material interposed between said base and top layers, said fluid permeable layer extending acrosε εaid baεe layer opening and extending beyond said top layer, said top and intermediate layers having a metering opening formed therethrough, said metering opening circumscribed by said base layer opening.

2. The metering apparatus of claim 1 wherein said reactant stream compriεes water vapor.

3. The metering apparatus of claim 2 wherein said reactant stream further compriseε air.

4. An electrochemical fuel cell εtack compriεing at leaεt two electrochemical fuel cells, each of said fuel cells comprising: a membrane electrode asεembly comprising a first cathode, an anode fluidly isolated from said cathode, an ion exchange membrane interposed between said cathode and εaid anode; a first oxidant manifold for directing an oxidant stream acrosε said first cathode, εaid first manifold having and inlet and an outlet; a firεt oxidant εtream metering apparatuε compriεing: a baεe layer of fluid impermeable sheet material having two major surfaceε, εaid base layer extending across said first manifold outlet, εaid base layer having an opening formed therein; a top layer of fluid impermeable sheet material, said top layer superposed over a portion of the major surface of said base layer facing away from said firεt manifold and extending across and beyond said base layer opening; an intermediate layer of fluid permeable sheet material interposed between said base and top layers, said fluid permeable layer extending across said base layer opening and extending beyond said top layer, said top and intermediate layers having a metering opening formed therethrough, said metering opening circumscribed by said base layer opening; εaid electrochemical fuel cell εtack further compriεing: oxidant delivery meanε for delivering an oxidant stream to said first cathodes, said delivery means comprising an oxidant supply conduit in fluid communication with said first oxidant manifolds for directing an oxidant εtream to said inlets; fuel delivery means for delivering a fluid fuel stream to said anodes.

5. The electrochemical fuel cell stack of claim 4 wherein said oxidant stream comprises water vapor.

6. The electrochemical fuel cell stack of claim 5 wherein said oxidant stream further comprises air.

7. The electrochemical fuel cell stack of claim 4 wherein said oxidant delivery means further comprises a fan.

8. The electrochemical fuel cell stack of claim 4 wherein each of εaid fuel cellε further comprises a second cathode fluidly isolated from said anode, and a εecond ion exchange membrane interposed between said second cathode and said anode, and wherein said first oxidant manifold is for directing an oxidant stream across said first and second cathodes in each of said fuel cells, and wherein said oxidant delivery means is for delivering an oxidant stream to said first and second cathodes.

9. The electrochemical fuel cell stack of claim 8 wherein said oxidant stream comprises water vapor.

10. The electrochemical fuel cell stack of claim 9 wherein said oxidant stream further compriseε air.

11. The electrochemical fuel cell εtack of claim 8 wherein εaid oxidant delivery means further compriseε a fan.

12. The electrochemical fuel cell stack of claim 4 wherein each of said fuel cells further comprises: a second cathode fluidly isolated from said anode, a εecond ion exchange membrane interpoεed between εaid εecond cathode and εaid anode; a second oxidant manifold for directing an oxidant stream acrosε said εecond cathode εaid second manifold having and inlet and an outlet; a second oxidant stream metering apparatus comprising: a base layer of fluid impermeable sheet material having two major surfaces, said base layer extending across said second manifold outlet, said base layer having an opening formed therein; a top layer of fluid impermeable sheet material, said top layer superposed over a portion of the major surface of said base layer facing away from said second manifold and extending across and beyond said base layer opening; an intermediate layer of fluid permeable sheet material interposed between said base and top layers, said fluid permeable layer extending across said base layer opening and extending beyond said top layer, said top and intermediate layers having a metering opening formed therethrough, εaid metering opening circumscribed by εaid baεe layer opening; and wherein εaid oxidant delivery means is for delivering an oxidant stream to said first and second cathodes, said delivery means comprising an oxidant εupply conduit in fluid communication with each of εaid firεt and second oxidant manifolds.

13. The electrochemical fuel cell stack of claim 12 wherein said oxidant stream compriseε water vapor.

14. The electrochemical fuel cell εtack of claim 13 wherein said oxidant stream further compriseε air.

15. The electrochemical fuel cell stack of claim 12 wherein said oxidant delivery means further compriseε a fan.

16. A metering apparatuε for an electrochemical fuel cell, εaid fuel cell compriεing a reactant manifold for directing a reactant stream acrosε εaid fuel cell, εaid manifold compriεing an inlet and an outlet, εaid apparatuε compriεing a plate having two substantially parallel major surfaces, one of said plate major surfaces extending acrosε said manifold outlet, said plate comprising a fluid impermeable matrix having fluid permeable fibers extending through εaid matrix in a direction εubstantially parallel to said plate major εurfaceε, εaid plate having an opening formed therein for the paεεage of a fluid stream comprising a condensable component between said plate major surfaceε, at least a portion of said fluid permeable fibers exposed at said opening and at said plate major surface facing away from said manifold outlet, whereby said exposed fluid permeable fibers conduct said condensable component away from εaid opening.

17. The metering apparatuε of claim 16 wherein said fluid stream compriseε water vapor and εaid fluid permeable fibers are hydrophilic.

18. The metering apparatus of claim 17 wherein said fluid stream further comprises air.