WO2017216621A2

WO2017216621A2 - Fuel cell stacks with bent perimeter flow field plates

Info

Publication number: WO2017216621A2
Application number: PCT/IB2017/000659
Authority: WO
Inventors: Simon Farrington
Original assignee: Daimler Ag; Ford Motor Company
Priority date: 2016-06-14
Filing date: 2017-05-31
Publication date: 2017-12-21
Also published as: WO2017216621A3

Abstract

By introducing appropriate bends at the perimeters in the flow field plates used in solid polymer electrolyte fuel cell stacks, certain improvements can be obtained. Bends introduced at the outer perimeter of either or both of the oxidant and fuel flow field plates can serve to mechanically protect and electrically isolate the cell edges from each other and from external support structures. Bends introduced at the perimeters of reactant ports in the flow field plates can improve the flow of reactants through the manifolds created from the stacking of the ports in the stack.

Description

FUEL CELL STACKS WITH BENT PERIMETER FLOW FIELD PLATES

BACKGROUND Field of the Invention

This invention relates to flow field plates for solid polymer electrolyte fuel cell stacks. In particular, it relates to designs and methods for obtaining certain improvements via modifications at the edges of the flow field plates.

Description of the Related Art

Fuel cells electrochemically convert fuel and oxidant reactants, (e.g. hydrogen and oxygen or air respectively), to generate electric power. One type of fuel cell is a solid polymer electrolyte fuel cell which generally employs a proton conducting polymer membrane electrolyte between cathode and anode electrodes. The electrodes contain appropriate catalysts and typically also comprise conductive particles, binder, and material to modify wettability. A structure comprising a proton conducting polymer membrane sandwiched between two electrodes is known as a membrane electrode assembly (MEA). Such assemblies can be prepared in an efficient manner by appropriately coating catalyst mixtures onto the polymer membrane, and thus are commonly known as catalyst coated membranes (CCMs). For purposes of handling, assembly, and electrical insulation, CCMs are often framed with suitable electrically insulating plastic frames.

Anode and cathode gas diffusion layers are usually employed adjacent their respective electrodes on either side of a catalyst coated membrane. The gas diffusion layers serve to uniformly distribute reactants to and remove by-products from the catalyst electrodes. Fuel and oxidant flow field plates are then typically provided adjacent their respective gas diffusion layers and the combination of all these components represents a typical individual fuel cell assembly. The flow field plates comprise flow fields that usually contain numerous fluid distribution channels. The flow field plates serve multiple functions including: distribution of reactants to the gas diffusion layers, removal of by-products therefrom, structural support and containment, and current collection. Often, the fuel and oxidant flow field plates are assembled into a unitary bipolar plate in order to incorporate a coolant flow field therebetween and/or for other assembly purposes. Because the output voltage of a single cell is of order of IV, a plurality of fuel cell assemblies is usually stacked together in series for commercial applications. Fuel cell stacks can be further connected in arrays of interconnected stacks in series and/or parallel for use in automotive applications and the like. Along with water, heat is a significant by-product from the electrochemical reactions taking place within the fuel cell. Means for cooling a fuel cell stack is thus generally required. Stacks designed to achieve high power density (e.g. automotive stacks) typically circulate liquid coolant throughout the stack in order to remove heat quickly and efficiently. To accomplish this, coolant flow fields comprising numerous coolant channels are also typically incorporated in the flow field plates of the cells in the stacks. The coolant flow fields are typically formed on the electrochemically inactive surfaces of both the anode side and cathode side flow field plates and, by appropriate design, a sealed coolant flow field is created when both anode and cathode side plates are mated together into a bipolar plate assembly.

Numerous seals are required in typical fuel cell stack construction, and achieving adequate, reliable seals in a manner suitable for commercial, high volume manufacture is challenging. Around the periphery of the MEAs, pressurized fuel and oxidant gases must be separated from each other (i.e. gas shorting around the edges of the membrane must be prevented) and also prevented from leaking to the external environment. Frames are commonly used to seal working fluids at the edges of MEAs. To provide both reactants and the coolant to and from the individual cells in the stack, a series of ports are generally provided at opposing ends of the individual cells such that when the cells are stacked together they form manifolds for these fluids. Further required design features then include passageways in the plates to distribute the bulk fluids in these formed manifolds to and from the various channels in the reactant and coolant flow fields in the plates. These passageway regions are referred to as transition regions. The transition regions can themselves comprise numerous fluid distribution channels, e.g. oxidant and/or fuel transition channels.

A typical solid polymer electrolyte fuel cell stack is thus quite a complex article structurally. It comprises numerous, thin stacked components and the component flow field plates comprise numerous finely detailed features.

In fuel cell stacks comprising frames around the CCMs of the component cells, because the frames are typically relatively flimsy, they can undesirably be deformed via interaction with adjacent external support structures (such as stiffener bars or external compression members for compressing the stack together). The nature of such deformation is unpredictable and, as illustrated below, it can lead to electrical shorting between cells or between cells and external compression members. The possibility of such electrical shorting is a significant concern and thus there is a requirement to ensure it does not occur. Another requirement in fuel cell stacks generally is to provide for substantial fluid flow through the various manifolds formed by the stacked inlet and outlet ports in the plates. This is required in order that substantial amounts of fluid can be uniformly provided to and removed from the numerous flow fields in the fuel cell stack. Preferably then, the flow resistance to fluids in these manifolds is minimized. While flow resistance can be decreased if desired by increasing the size of the ports, of course this undesirably increases the size of all the associated components and the fuel cell stack itself.

There remains a continuing need for new designs and methods to protect against such problems as edge deformation of component flow field plates and to improve the flow of fluids in solid polymer electrolyte fuel cell stacks. This invention fulfills these needs and provides further related advantages.

SUMMARY

The present invention provides for certain improvements in solid polymer electrolyte fuel cell stacks by introducing appropriate bends at the perimeters in the flow fields plates therein. Mechanical protection and electrical isolation at the cell edges can be obtained by introduction of appropriate bends at the outer perimeter of either or both of the oxidant and fuel flow field plates. Further, improved flow of reactants or their by-products can be obtained by introduction of appropriate bends at the perimeter of reactant ports in the flow field plates. A solid polymer electrolyte fuel cell stack comprises a series stack of a plurality of solid polymer electrolyte fuel cells. In a relevant stack, each fuel cell comprises a membrane electrode assembly comprising a solid polymer electrolyte membrane electrolyte, a cathode on one side of the membrane electrolyte, and an anode on the other side of the membrane electrolyte. Further, each fuel cell comprises essentially planar oxidant and fuel flow field plates on the side of the cathode opposite the membrane electrolyte and the side of the anode opposite the membrane electrolyte respectively. Each of the oxidant and fuel flow field plates comprise an outer perimeter, a plurality of formed non-planar features, and at least one reactant port comprising a port perimeter for a reactant fluid. Further still, each fuel cell comprises a frame attached to the periphery of the membrane electrode assembly, and a seal that seals the frame to the oxidant and fuel flow field plates. Generally, in the present invention, a portion selected from the group consisting of the outer perimeter and the port perimeter of at least one of the oxidant and fuel flow field plates in each fuel cell is bent in the same out-of-plane direction. The portion does not necessarily include the entire perimeter or perimeters involved since it may be desirable to leave certain sections unbent. Further, the bent portion may only be in the oxidant flow field plates and not the fuel flow field plates, or vice versa. (This is possible at the outer perimeter for instance if one plate is smaller than the other and is possible at a port perimeter if one port is larger than the other.)

In an exemplary embodiment, the bent portions are in both the oxidant and the fuel flow field plates. That is, a portion selected from the group consisting of the outer perimeter and the port perimeter of the oxidant flow field plate in each fuel cell and a corresponding portion of the fuel flow field plate in each fuel cell are bent in the same out-of-plane direction.

To obtain electrical isolation at the cell edges, essentially the entire outer perimeters of the oxidant flow field plate and the fuel flow field plate may be bent in the same out-of-plane direction. In such an embodiment, the frame of each fuel cell desirably extends beyond the outer perimeters of the oxidant and fuel flow field plates. Further, in such an embodiment, the bent outer perimeters of the oxidant and fuel flow field plates of a given fuel cell in the stack can overlap the bent outer perimeters of the oxidant and fuel flow field plates of an adjacent fuel cell in the stack. This results in a fuel cell stack with "armadillolike" protection at the edges of the cells. And for instance, in a fuel cell stack comprising external compression members, the extended frames of the fuel cells electrically insulate the fuel cells from the external compression members.

To obtain an improved flow of a reactant fluid through a reactant port in the flow field plates, the port perimeter portion of the reactant port is bent in the out-of-plane direction that is towards the direction of flow of the reactant fluid through the reactant port. In an exemplary embodiment, both the oxidant and fuel flow field plates are involved. That is, the portion of the oxidant flow field plate comprises a port perimeter portion and the corresponding portion of the fuel flow field plate comprises a corresponding port perimeter portion in which the out-of-plane direction is towards the direction of flow of the reactant fluid through the reactant port. Further, in an exemplary embodiment, essentially the entire port perimeters of the oxidant flow field plate and the fuel flow field plate may be bent in the same out-of- plane direction. As suggested in the example calculations below, a significant improvement in flow can be obtained when the port perimeter portion of the oxidant flow field plate and the corresponding port perimeter portion of the fuel flow field plate are bent at an angle in the range from about 16 to 30 degrees from the normal direction to the plane of the plates.

To obtain both electrical isolation at the cell edges and to obtain an improved flow of a reactant fluid through a reactant port in the flow field plates, both the outer perimeters and the port perimeters in the flow field plates may be bent appropriately. That is, a first portion of the outer perimeter of the oxidant flow field plate in each fuel cell and a corresponding first portion of the fuel flow field plate in each fuel cell may be bent in the same first out-of-plane direction, and a second portion of the port perimeter of the oxidant flow field plate in each fuel cell and a corresponding second portion of the fuel flow field plate in each fuel cell may be bent in the same second out-of-plane direction. In such an embodiment, the first out-of-plane direction can be the same direction as the second out-of-plane direction.

Of course in an embodiment of the invention, improved flow of either or both reactant fluids can be obtained at either or both of their respective inlet and outlet ports. In an embodiment where at least two such improved flows are involved, the oxidant and fuel flow field plates comprise first and second reactant ports comprising first and second port perimeters for first and second reactant fluids respectively, in which a first portion of the first port perimeter of the oxidant flow field plate in each fuel cell and a corresponding first portion of the fuel flow field plate in each fuel cell are bent in the same first out-of- plane direction, and a second portion of the second port perimeter of the oxidant flow field plate in each fuel cell and a corresponding second portion of the fuel flow field plate in each fuel cell are bent in the same second out-of-plane direction. As before, the first out-of-plane direction here can be the same direction as the second out-of-plane direction.

The invention is suitable for fuel cell stacks employing metal flow field plates but can also be applied in stacks employing flow field plates made of other materials, such as conductive plastics, flexible graphite, molded carbon, certain polymeric materials, and the like. Further, the invention is suitable for stacks in which the membrane electrode assembly of each fuel cell comprises a cathode gas diffusion layer adjacent to the cathode on the side opposite the membrane electrolyte and an anode gas diffusion layer adjacent to the anode on the side opposite the membrane electrolyte.

Generally, the method of making fuel cell stacks of the invention involves preparing the special flow field plates which are essentially planar but are characterized by the aforementioned bent portions, and otherwise assembling the stack in a conventional manner. That is, the general method comprises the steps of: preparing a plurality of the essentially planar oxidant flow field plates and a plurality of the essentially planar fuel flow field plates; obtaining a plurality of the membrane electrode assemblies, a plurality of the frames, and a plurality of the seals; attaching a frame from the plurality of frames to the periphery of each of the membrane electrode assemblies; and assembling the oxidant flow field plates, the fuel flow field plates, the framed membrane electrode assemblies, and the seals to form the series stack of a plurality of solid polymer electrolyte fuel cells.

In embodiments in which a portion of the outer perimeter of the oxidant flow field plate and a corresponding portion of the outer perimeter of the fuel flow field plate are bent in the same out-of-plane direction, and in which both the oxidant and fuel flow field plates are made of metal, the preparing of the oxidant and fuel flow field plates can comprise the steps of: obtaining flat metal sheet workpieces larger than the oxidant and fuel flow field plates; forming the plurality of non-planar features into the workpieces; forming precursor features for the bent outer perimeters into the workpieces; cutting the workpieces at the reactant port perimeters; and cutting the workpieces at the precursor features for the bent outer perimeters, thereby preparing flow field plates comprising reactant ports and outer perimeters bent in an out-of-plane direction.

In embodiments in which a portion of the port perimeter of the oxidant flow field plate and a corresponding portion of the port perimeter of the fuel flow field plate are bent in the same out-of-plane direction, and in which both the oxidant and fuel flow field plates are made of metal, the preparing of the oxidant and fuel flow field plates can comprise the steps of: obtaining flat metal sheet workpieces larger than the oxidant and fuel flow field plates; forming the plurality of non-planar features into the workpieces; forming precursor features for the bent port perimeters into the workpieces; cutting the workpieces at the outer perimeters; and cutting the workpieces at the precursor features for the bent port perimeters, thereby preparing flow field plates comprising outer perimeters and reactant ports with port perimeters bent in an out-of-plane direction.

These and other aspects of the invention are evident upon reference to the attached Figures and following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a top view of the oxidant flow field side of a prior art bipolar plate assembly for a solid polymer electrolyte fuel cell. Figure 2 shows an enlarged cross-sectional view of a few fuel cells in a prior art fuel cell stack in the region near the outer perimeters of the flow field plates.

Figure 3a shows an enlarged cross-sectional view of workpieces partway through the preparation process of making flow field plates whose outer perimeters are bent in accordance with the invention. The view is in the region near the outer perimeters and shows how the workpieces appear after forming suitable non-planar features and precursor features for the bent outer perimeters, but before the cutting operations.

Figure 3b shows a similar enlarged cross-sectional view as that shown in Figure 2 except here the outer perimeters of the flow field plates are bent in accordance with the invention. Further, the flow field plates shown here have been prepared from the workpieces depicted in Figure 3 a.

Figure 4a shows an enlarged cross-sectional view of a few fuel cells in a prior art fuel cell stack near the perimeters of the oxidant outlet ports in the flow field plates. The cross-sectional view shown here is taken in the areas of the vias connecting the oxidant outlet ports to the oxidant flow fields.

Figure 4b shows a similar enlarged cross-sectional view as that shown in Figure 4a except here the oxidant outlet port perimeters are bent in accordance with the invention.

Figure 4c shows an enlarged cross-sectional view of the inventive embodiment of Figure 4b except that the view show here is taken in areas away from the via areas.

DETAILED DESCRIPTION Herein, the following definitions have been used. In a quantitative context, the term "about" should be construed as being in the range up to plus 10% and down to minus 10%.

The term "essentially planar" is used herein with reference to the flow field plates in fuel cell stacks. It is intended to refer to plates that are flat or planar on average or on a macroscopic scale, but to recognize that such plates also comprise numerous out-of-plane features that are relatively modest in size (e.g. channels, landings, sealing beads, and the like). The term "frame" has been used herein to refer to an element attached to the periphery of a membrane electrode assembly in fuel cells. While such elements are typically distinct from the membrane electrolyte and made of different plastic materials having certain desirable mechanical and chemical properties, a "frame" may also include an extension of the membrane electrolyte itself (e.g. if laminated out to edges of the cell) and/or be made of any functionally suitable material.

In the present application, the terms "periphery" and "perimeter" appear frequently and are used in a similar sense, namely to reference the outer limits of the element under consideration. Although the terms have similar meanings in general, to assist in comprehension, the term "perimeter" has been used in reference to features of significant importance to the invention (e.g. the outer limits of the flow field plates and the outer limits of the reactant ports) while "periphery" has been used in reference to other features.

A top view of the oxidant flow field side of an exemplary prior art bipolar plate assembly for a solid polymer electrolyte fuel cell is shown in Figure 1 and serves to identify those features commonly found in such bipolar plate assemblies. On the visible side of the assembly is oxidant flow field plate 1 which comprises oxidant flow field 2 which in turn comprises a plurality of oxidant channels separated by landings (these features are so small in Figure 1 that they have not been identified with reference numerals). On the opposite side of oxidant flow field plate 1 is a coolant flow field (not visible in Figure 1).

Oxidant flow field plate 1 comprises oxidant inlet port 3, fuel inlet port 4, coolant inlet port 5, oxidant outlet port 6, fuel outlet port 7, and coolant outlet port 8. Seals 9 surround the various ports and seal 10 runs around the perimeter of plate 1 and serves several sealing functions including sealing the frame of a framed MEA (not present in Figure 1) to plate 1. Oxidant inlet transition region 11 occupies the space between oxidant inlet port 3 and oxidant flow field 2. An oxidant backfeed port and associated vias (not visible in Figure 1) provide a fluid connection for supplied oxidant between oxidant inlet port 3 and transition region 11. In a like manner, oxidant outlet transition region 12 occupies the space between oxidant outlet port 6 and oxidant flow field 2. Another oxidant backfeed port and associated vias (again not visible in Figure 1) provide a fluid connection for exhausted oxidant between transition region 12 and oxidant outlet port 6. Transition regions 11 and 12 provide for fluid flow, transitions from a single, large body of flow into a multiplicity of small channel flow streams and vice versa. Figure 2 shows an enlarged cross-sectional view of a few fuel cells in an exemplary prior art fuel cell stack in the region near the outer perimeters of the flow field plates. In Figure 2, three cell's worth of components are shown. Specifically, three bipolar plate assemblies 20 and three framed MEAs 21 are shown stacked together. Each bipolar plate assembly 20 comprises oxidant flow field plate 1 and fuel flow field plate 22 which are welded together at numerous weld locations 23. Each framed MEA 21 comprises catalyst coated membrane (CCM) 24, anode gas diffusion layer 25, cathode gas diffusion layer 26, and plastic frame 27. As shown in Figure 2, frame 27 is captured and attached between the edges of CCM 24 and anode gas diffusion layer 25. Frame 27 extends beyond seal 10 and beyond the outer perimeters (indicated by arrow 28) of oxidant and fuel flow field plates 1 , 22 in bipolar plate assemblies 20. In the exemplary embodiment of Figure 2, seals 10 actually comprise portions on both sides of bipolar plate assemblies 20 and are formed and remain connected together via appropriate holes in bipolar plate assemblies 20 (not shown). Frames 10 are sealed via seals 10 such that the anode and cathode sides of MEAs 21 are isolated not only from each other but from the environment as well. Figure 2 further illustrates a problem with such prior art fuel cell stacks. External support structures such as inner surface of stack enclosure 29 are often employed immediately adjacent to the fuel cell stack. While it is intended that stack enclosure 29 does not contact any of adjacent bipolar plate assemblies 20, contact may nonetheless occur as a result of unintentional events (e.g. shifting arising from a crash event, other mechanical contact, tolerance issues, etc.). Electrical shorting between cells can occur however if two or more bipolar plate assemblies 20 come into contact with an electrically conductive stack enclosure 29. Further, even use of non-electrically conductive stack enclosure walls can result in electrical shorting if there is sufficient contact between it and adjacent bipolar plate assemblies so as to deform unprotected outer perimeters of the plates into electrical contact. However, because frames 27 are relatively flimsy, they can undesirably and unpredictably be deformed via interaction with stack enclosure 29. For instance, as illustrated in Figure 2, frame edges 27a may be deformed upwards, downwards, or even into the space between adjacent bipolar plate assemblies 20. As a consequence, such a fuel cell stack can be vulnerable to electrical shorting at the cell edges for the aforementioned reasons.

The potential electrical shorting problem can be addressed by employing flow field plates in the bipolar plate assemblies that have been bent at their outer perimeters in accordance with the invention. Figure 3a shows an enlarged cross-sectional view of workpieces partway through the preparation process of making such flow field plates. The view is of a region near the outer perimeters of the plates and similar to the region shown in Figure 2. Here, workpiece 30 comprises precursor oxidant flow field plate 31 and precursor fuel flow field plate 32 which are welded together at numerous weld locations 33. At this stage of the process, suitable non-planar features 34 (e.g. flow field channels, landings, etc.) have already been formed from flat metal sheet workpieces using conventional embossing techniques. In addition though, precursor features 35 have also been formed for the bent outer perimeters. The preparation process is then completed by cutting off the edge of workpiece 30 at an appropriate point (indicated by dashed line 36) in the precursor features 35. (In principle the embossing and cutting operations may be reversed and suitable flow field plates formed directly via embossing appropriate pre-cut blank sheets. But such an approach is not generally preferred tooling practice because it is more difficult to control and to prevent distortion.)

Figure 3b shows an enlarged cross-sectional view of a few fuel cells in an exemplary fuel cell stack of the invention. The flow field plates shown here have been prepared from the workpieces depicted in Figure 3a. The view here is in the region near the outer perimeters of the flow field plates and is similar to that shown in Figure 2 except here the outer perimeters of the flow field plates are now bent in accordance with the invention. (In Figure 3b, elements common to those in Figures 2 and 3a have been identified with like numerals.)

In Figure 3b, portions 31b at the outer perimeters of oxidant flow field plates 31 and portions 32b at the outer perimeters of fuel flow field plates 32 are bent in the same out-of-plane direction (downward as shown). As a result, when the fuel cell stack is assembled and the various components are compressed together in the stacking direction, frames 27 (which extend beyond the outer perimeters of the plates) are necessarily directed in the same out-of-plane direction. Edges 27b of frames 27 overlap and nest together as shown in a manner analogous to the armour plates of an armadillo. And in a like manner to the protection offered to an armadillo, the overlapping edges 27b insulate and protect plates 31, 32 from contacting stack enclosure 29 or each other, and thus prevent electrical shorting. Figure 4a shows an enlarged cross-sectional view of a few fuel cells in the same prior art fuel cell stack as that shown in Figure 2. Here however, the view is near the perimeters of the oxidant outlet ports 6 in flow field plates 1, 22, and specifically it is taken in the areas of vias 40 which connect oxidant outlet ports 6 to the oxidant flow fields. In Figure 4a, the stacked oxidant outlet ports 6 of the numerous cells in the stack form a manifold for the removal of the oxidant exhaust. Arrow 41 indicates the direction (i.e. the stack direction) of the bulk oxidant exhaust flowing through manifold 6 which is formed by numerous stacked oxidant outlet ports 6. Arrows 42 indicate the direction of the oxidant exhaust (originating from the oxidant flow fields in the cells) as it exits vias 40 and enters manifold 6. In this region, when under compression and assembled in the stack, the edges of frames 27 are deformed only slightly from the plane of the plates. A representative value may be just less than about 90° (as illustrated in Figure 4a as the angle between frames 27 and oxidant exhaust direction (also stack direction) 41.

In the prior art embodiment of Figure 4a, the oxidant exhaust flow from vias 40 (indicated by arrows 42) enters manifold 6 at relatively high velocity and essentially perpendicular to the direction 41 of the bulk oxidant exhaust flow through manifold 6. This high velocity, perpendicular flow 42 interferes with the flow of the bulk fluid in manifold 6 and from a mathematical perspective, this results in an apparent reduction in the effective flow area through manifold 6 (i.e. as a consequence of the high velocity flow, the stacked oxidant outlet ports act as if they are smaller in size than they actually are with regards to fluid flowing therethrough). As discussed in the Example below, the reduction in effective flow area can be numerically determined by modeling.

Figure 4b shows an enlarged cross-sectional view of a few fuel cells in the same exemplary fuel cell stack of the invention shown in Figure 3 b. Here however, the view is near the perimeters of the oxidant outlet ports 6 and specifically in the areas of vias 40 like that of Figure 4a. Here however, the oxidant outlet port perimeters are bent in accordance with the invention. Note that the preparation of plates with such bent port perimeters can be accomplished in a similar manner to the preparation of plates with bent outer perimeters. Further, preparation of plates with both bent port perimeters and bent outer perimeters can be accomplished at the same time.

In Figure 4b, portions 31c at the oxidant port perimeters of oxidant flow field plates 31 and portions 32c at the oxidant port perimeters of fuel flow field plates 32 are bent in the same out-of-plane direction (downward as shown). The edges of frames 27 are now also bent more significantly in the same out-of- plane direction and form a much shallower angle with the oxidant exhaust and stack directions 41 (shown as 30°). In a like manner then, oxidant exhaust flow 42 is also directed at a shallower angle into manifold 6 and consequently it interferes less with the bulk flow in manifold 6. The effective flow area through manifold is thus not reduced as much as in Figure 4a and thus flow in the manifold is improved.

The extent of the bend achieved in the oxidant port perimeters can vary at different locations around the perimeters due to the asymmetric shape of oxidant port 6 (see Figure 1) and/or due to deliberate efforts to do so. Figure 4c shows an enlarged cross-sectional view of the inventive embodiment of Figure 4b taken in an area away from the area of vias 40 where the extent of the bend is different and even shallower yet (shown as 16°). The preceding figures show certain exemplary embodiments of the invention in which both oxidant and fuel flow field plates are bent at outer perimeters and/or at port perimeters therein. Alternative embodiments are also possible in which only one of the oxidant and fuel flow field plates are bent to obtain a desired result. For instance, in relatively thick assemblies, a combination where the bent plates are larger and the other unbent plates are appropriately smaller might allow for the bend in the larger plates to sufficiently curve around the edges of the other smaller plates to obtain the desired results. Further, while it may generally be advantageous to incorporate bends around most or all of the outer perimeters and/or port perimeters, it may instead be desirable to leave certain areas unbent or open for specific purposes (e.g. to allow for contact with alignment rods during assembly of the stack). Further still, it may only be required to incorporate bends in specific locations to address certain problems (e.g. only in locations which will be adjacent the stack enclosure walls). Thus, numerous variations of the invention are possible.

The following Example has been included to illustrate certain aspects of the invention but should not be construed as limiting in any way.

EXAMPLE

CFD modeling was carried out using STAR CCM software on representative automotive fuel cell stack designs in order to estimate the extent of improvement that might be expected in flow through the reactant manifolds via use of the invention. In the following, the flow through the oxidant outlet ports of a comparative stack was modeled and compared to the flow through the oxidant outlet ports of a stack of the invention.

The comparative stack was assumed to be a typical prior art automotive solid polymer fuel cell stack in which the oxidant port perimeters in the flow field plates were unbent. The comparative stack was further assumed to be operating under typical operating conditions. The inventive stack was assumed to be similar to the comparative stack and operating under the same operating conditions except that the oxidant outlet port perimeters in the inventive stack were bent at a 30° angle in the direction of flow in the oxidant manifold. As discussed above with regards to Figure 4a, the high velocity, perpendicular flow 42 from the flow fields interferes with the flow of the bulk fluid through the oxidant outlet ports (i.e. manifold 6) and result in an apparent reduction in the effective flow area therethrough. In other words, as a result of interference from this perpendicular flow, the cross-sectional area of the oxidant outlet ports (manifold 6) appears to be is smaller than it actually is. From the CFD modeling done here, an "effective flow area" was calculated for the oxidant outlet ports in each of these stack designs and operating conditions. In the comparative stack, the effective flow area of the oxidant outlet ports was determined to be about 30% smaller than the actual area. The comparative stack thus experiences a significant reduction in the effective flow area as a result of the perpendicular flow from the flow fields. In the inventive stack however, a marked improvement was seen over the comparative stack. Here, an approximate 30% improvement in effective flow area was determined over that of the comparative stack.

All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification, are incorporated herein by reference in their entirety.

While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the foregoing teachings. Such modifications are to be considered within the purview and scope of the claims appended hereto.

Claims

What is claimed is:

1. A solid polymer electrolyte fuel cell stack comprising a series stack of a plurality of solid polymer electrolyte fuel cells, each fuel cell comprising:

a membrane electrode assembly comprising a solid polymer electrolyte membrane electrolyte, a cathode on one side of the membrane electrolyte, and an anode on the other side of the membrane electrolyte;

an essentially planar oxidant flow field plate on the side of the cathode opposite the membrane electrolyte, the oxidant flow field plate comprising an outer perimeter, a plurality of formed non-planar features, and at least one reactant port comprising a port perimeter for a reactant fluid;

an essentially planar fuel flow field plate on the side of the anode opposite the membrane electrolyte, the fuel flow field plate comprising an outer perimeter, a plurality of formed non-planar features, and at least one reactant port comprising a port perimeter for the reactant fluid;

a frame attached to the periphery of the membrane electrode assembly; and a seal that seals the frame to the oxidant and fuel flow field plates;

characterized in that a portion selected from the group consisting of the outer perimeter and the port perimeter of at least one of the oxidant and fuel flow field plates in each fuel cell is bent in the same out-of-plane direction.

2. The fuel cell stack of claim 1 characterized in that a portion selected from the group consisting of the outer perimeter and the port perimeter of the oxidant flow field plate in each fuel cell and a corresponding portion of the fuel flow field plate in each fuel cell are bent in the same out-of-plane direction.

3. The fuel cell stack of claim 2 wherein essentially the entire outer perimeters of the oxidant flow field plate and the fuel flow field plate are bent in the same out-of-plane direction.

4. The fuel cell stack of claim 3 wherein the frame of each fuel cell extends beyond the outer perimeters of the oxidant and fuel flow field plates.

5. The fuel cell stack of claim 4 wherein the bent outer perimeters of the oxidant and fuel flow field plates of a fuel cell in the stack overlap the bent outer perimeters of the oxidant and fuel flow field plates of an adjacent fuel cell in the stack.

6. The fuel cell stack of claim 4 comprising external compression members wherein the extended frames of the fuel cells electrically insulate the fuel cells from the external compression members.

7. The fuel cell stack of claim 2 wherein essentially the entire port perimeters of the oxidant flow field plate and the fuel flow field plate are bent in the same out-of-plane direction.

8. The fuel cell stack of claim 2 wherein the portion of the oxidant flow field plate comprises a port perimeter portion and the corresponding portion of the fuel flow field plate comprises a corresponding port perimeter portion and wherein the out-of-plane direction is towards the direction of flow of the reactant fluid through the reactant port.

9. The fuel cell stack of claim 8 wherein the port perimeter portion of the oxidant flow field plate and the corresponding port perimeter portion of the fuel flow field plate are bent at an angle in the range from about 16 to 30 degrees from the normal direction to the plane of the plates.

10. The fuel cell stack of claim 2 wherein:

a first portion of the outer perimeter of the oxidant flow field plate in each fuel cell and a corresponding first portion of the fuel flow field plate in each fuel cell are bent in the same first out-of-plane direction; and

a second portion of the port perimeter of the oxidant flow field plate in each fuel cell and a corresponding second portion of the fuel flow field plate in each fuel cell are bent in the same second out-of-plane direction.

11. The fuel cell stack of claim 10 wherein the first out-of-plane direction is the same direction as the second out-of-plane direction.

12. The fuel cell stack of claim 2 wherein the oxidant and fuel flow field plates comprise first and second reactant ports comprising first and second port perimeters for first and second reactant fluids respectively, and wherein: a first portion of the first port perimeter of the oxidant flow field plate in each fuel cell and a corresponding first portion of the fuel flow field plate in each fuel cell are bent in the same first out-of-plane direction; and

a second portion of the second port perimeter of the oxidant flow field plate in each fuel cell and a corresponding second portion of the fuel flow field plate in each fuel cell are bent in the same second out-of-plane direction.

13. The fuel cell stack of claim 12 wherein the first out-of-plane direction is the same direction as the second out-of-plane direction.

14. The fuel cell stack of claim 1 wherein the oxidant and fuel flow field plates are made of metal.

15. The fuel cell stack of claim 1 wherein the membrane electrode assembly of each fuel cell comprises a cathode gas diffusion layer adjacent to the cathode on the side opposite the membrane electrolyte and an anode gas diffusion layer adjacent to the anode on the side opposite the membrane electrolyte.

16. A method of making the fuel cell stack of claim 1 comprising:

preparing a plurality of the essentially planar oxidant flow field plates and a plurality of the essentially planar fuel flow field plates;

obtaining a plurality of the membrane electrode assemblies, a plurality of the frames, and a plurality of the seals;

attaching a frame from the plurality of frames to the periphery of each of the membrane electrode assemblies; and

assembling the oxidant flow field plates, the fuel flow field plates, the framed membrane electrode assemblies, and the seals to form the series stack of a plurality of solid polymer electrolyte fuel cells.

17. The method of claim 16 wherein a portion of the outer perimeter of the oxidant flow field plate and a corresponding portion of the outer perimeter of the fuel flow field plate are bent in the same out-of- plane direction, both the oxidant and fuel flow field plates are made of metal, and the preparing of the oxidant and fuel flow field plates comprises:

obtaining flat metal sheet workpieces larger than the oxidant and fuel flow field plates;

forming the plurality of non-planar features into the workpieces; forming precursor features for the bent outer perimeters into the workpieces;

cutting the workpieces at the reactant port perimeters; and

cutting the workpieces at the precursor features for the bent outer perimeters, thereby preparing flow field plates comprising reactant ports and outer perimeters bent in an out-of-plane direction.

18. The method of claim 16 wherein a portion of the port perimeter of the oxidant flow field plate and a corresponding portion of the port perimeter of the fuel flow field plate are bent in the same out-of-plane direction, both the oxidant and fuel flow field plates are made of metal, and the preparing of the oxidant and fuel flow field plates comprises:

forming the plurality of non-planar features into the workpieces;

forming precursor features for the bent port perimeters into the workpieces;

cutting the workpieces at the outer perimeters; and

cutting the workpieces at the precursor features for the bent port perimeters, thereby preparing flow field plates comprising outer perimeters and reactant ports with port perimeters bent in an out-of-plane direction.