WO2000002267A2

WO2000002267A2 - Internal cooling arrangement for undulate mea fuel cell stack

Info

Publication number: WO2000002267A2
Application number: PCT/GB1999/002056
Authority: WO
Inventors: Gerard Francis Mclean
Original assignee: Ballard Power Systems Inc.
Priority date: 1998-07-01
Filing date: 1999-07-01
Publication date: 2000-01-13
Also published as: AU4525399A; WO2000002281A3; WO2000002271A2; AU4526299A; GB9915293D0; WO2000002271A3; GB2339066A; WO2000002267A3; GB9814120D0; WO2000002281A2; AU4526499A; CA2336314A1; GB2339068A; GB9915294D0; GB9915295D0; GB2339067A

Abstract

In a PEM-type fuel cell, coolant conduits (79) are provided that follow the reactant gas flowpaths on either side of the MEA (70) and are formed in the interface between adjacent contacting separator layers (72) of consecutive fuel cells in the stack. These coolant conduits can eliminate the need for coolant conduits that pass in the stack dimension through the MEA layers.

Description

Internal Cooling Arrangement for Undulate MEA Fuel

Cell Stack Field of Invention

This invention relates to the cooling of fuel cell stacks, particularly proton exchange membrane (PEM)-type fuel cell stacks employing an undulate membrane electrode assembly (MEA) .

Background

In PEM-type fuel cell stacks of the type heretofore manufactured, typically heat-conductive cooling plates are interspersed with fuel cell strata in order to conduct heat from the stack to a coolant, such as water, flowing therethrough. Typically, the coolant plates have a coolant flow field extending over at least one of their surfaces; the flow field entrance and exit ports are connected to supply and sink plena for the coolant. Typically, the fuel cell strata in the stack are provided with aligned apertures that, in the stack dimension, constitute plena for the supply of coolant fluid to the stack and exhaust of heated effluent coolant from the stack. Heat is removed from the fuel cells to the cooling plates and thence to the coolant as it flows past the cooling plates, thereby cooling the stack. Ballard U.S. Patent No. 5,230,966 (Voss) issued 27 July 1993 illustrates a typical cooling arrangement for a PEM-type fuel cell stack.

Typically in conventional PEM-type fuel cell stacks, the cooling plates are interspersed in the stack in the ratio of approximately one cooling plate for every three to five fuel cells in the stack. This inclusion of a cooling plate in the stack of course contributes appreciably to the stack dimensions, increasing the overall stack volume and weight by an amount depending upon the size and mass of the cooling plate. As fuel cell stacks of this type are intended for use in vehicles and other mobile applications, it is desirable to keep bulk and weight of fuel cell stacks to a minimum. Since the use of such coolant plates contributes to both bulk and weight of the fuel cell stack in an undesirable manner, it would be desirable to eliminate the coolant plates.

Further, since heat is generated approximately equally in each of the fuel cells within the stack, yet the proximity of any given fuel cell to the nearest cooling plate may vary due to the overall configuration, it follows that some fuel cells nearer the cooling plates can operate at a lower temperature than those fuel cells that are more remote from the nearest cooling plate, creating temperature gradients within the fuel cell stack. Since there is normally an optimum operating temperature for each fuel cell, it follows that at least some fuel cells in the stack will not operate at optimum temperature, with the consequence that overall performance of the fuel cell stack is reduced relative to the theoretical maximum available from the fuel cells in the stack.

As mentioned briefly above, when cooling plates are interspersed with the fuel cell layers in the stack, typically the coolant is manifolded in a manner similar to the manner of manifolding reactant gases to the stack. Typically, this implies that a common plenum configuration is required for the coolant flow, as would also be the case for supply and exhaust of the reactant gases. The requirement for coolant flow in plena extending in the stack dimension through the stack necessitates that the fuel cell membrane electrode assembly (MEA) layers in the stack be punctured to provide the necessary aperture for the flow of coolant therethrough. Puncturing of the MEA of the stack is inherently undesirable; for example, it complicates the manufacturing process and risks damage to the MEA layers .

A number of prior proposals for alternative cooling arrangements have been made. Breault, U.S. Patent 4,233,369, issued 11 November 1980, describes a cooler assembly for use between consecutive cells in a fuel cell stack that comprises a fibrous gas porous holder layer sandwiched between and bonded to a pair of gas impervious graphite plates. The fibrous gas porous holder layer has coolant tubes passing therethrough within channels in the layer. The channels have substantially the same depth as the holder layer thickness and as the outer diameter of the tubes. Breault 's design does not appreciably reduce the volume required by the cooling components of the stack, and thus does not appreciably contribute to the efficient use of space within the fuel cell stack.

Grevstad, U.S. Patent 3,969,145, issued 13 July 1976, describes a cooler comprising a number of cooler tubes located in the separator plates between fuel cells in a stack. In the Grevstad configuration, a separate sealed piping structure is inserted into grooves cut into separator plates. The Grevstad design is similar to that of Voss, mentioned previously, except that instead of forming coolant flow channels directly within separator plates as Voss discloses, Grevstad instead inserts a separately formed coolant conduit into the separator plate grooves. As best seen in Figure 2 of the Grevstad patent, this prior design does not alter the - manner in which the reactant gas flow channel walls contact the electrode layers relative to conventional structures. Indeed, Grevstad pays little or no attention to the resulting coolant flow field deriving from his cooling arrangement.

McElroy, U.S. Patent 4,678,724, issued 7 July 1987, discloses internally cooled bipolar separator plates within a fuel cell stack. The cooling structure proposed by McElroy is in addition to the reactant gas flow path arrangements and does not attempt, in conjunction with the flow paths, to minimize the overall volume required.

Reiser, U.S. Patent 3,964,930, issued 22 June 1976, discloses tubes for carrying coolant through a fuel cell stack. The tubes are located in tubes or passageways formed in the separator plates. Reiser's coolant conduit structure does not occupy its own plane within the fuel cell stack, but is coplanar with one of the reactant gas flow fields. As illustrated in Figure 3 of the Reiser patent, the cooling conduits lie in direct contact with the adjacent electrode layer. Reiser's coolant tubes are electrically insulated from the rest of the fuel cell structure, and thus are not useful to carry electric current between adjacent fuel cells. Reiser does not mention the potential decrease in fuel cell performance that may result from insertion of non- conducting cooling components immediately next to the electrolyte layer -there will be significant portions of Reiser's electrolyte layer that are in contact with gas impermeable electrically insulating components, and thus fuel cell efficiency necessarily must decrease. Reiser does not disclose the utility of this design for PEM- type fuel cells.

Chan, U.S. Patent 5,804,326 discloses a structure in which coolant and reactant gases flow in the same plane within a separator plate. The coolant flow is segmented into a different region than the reactant gas flow, however, and is isolated to the perimeter of the cell. This is done to provide cooling of the seals used in the stack. Thus, while the cooling facility is integrated into the bipolar separator, it consumes space that is not used (and is not disclosed by the patentee to be available to be used) for any fuel cell function other than cooling.

Katsunori, Japanese published patent specification 3-89467 filed 31 August 1989 discloses a design for a molten carbonate fuel cell in which a connected set of coolant-carrying jackets is inserted between an eletrode layer and a current collecting layer, interspersed with the reactant gas flowfield. The coolant jackets consume the full height of the flow region and thus displace the functions of either current collection or gas distribution. Thus the insertion of the cooling jackets next to the electrode layers renders certain portions of the electrode surface ineffective. Overall, the design does not offer significant volume reduction.

Horioka, Japanese published patent specification 6- 260190 filed 1 March, 1993 presents a design for concurrent cooling and humidification of a PEM fuel cell by inserting porous coolant carrying conduits within the reactant gas distribution channels. No means of manufacturing the proposed cell are disclosed, nor is there any discussion of the effect of the coolant/humidification channel on the overall performance of the cell. Improving the volumetric power density of the stack through the improved utilization of space within the stack does not appear to be an objective of the invention.

Externally manifolded coolant layers have also been described in the prior literature. Kamoshita, U.S. Patent 5,041,344, issued 20 August 1991, is representative. Kamoshita requires separate cooling plates to achieve cooling of the stack.

Summary of the Invention

A fuel cell stack to which the present invention relates comprises a series of fuel cells in which reactant flowfields are provided between an undulate membrane electrode assembly (MEA) layer and separator layers of each fuel cell. The flowfield comprises, for each reactant gas, a number of parallel flow channels formed implicitly by the association of the undulate MEA layer with a separator layer, which flow channels are interconnected via an internal or external manifolding arrangement to form a flowpath for each reactant gas that involves more than one pass along the length of the cell. Preferably the flowpath is serpentine, comprising an array of longitudinally extending parallel flow channels suitably interconnected to form a serpentine path. (In this specification, "explicit" formation of a component element occurs when special steps are taken to form and fabricate that component, as a result of which the component occupies space in the fuel cell stack that must be separately made available for it. By contrast, "implicit" formation occurs when the component is formed inherently when two or more other structural components of the fuel cell stack are assembled; these other components are deliberately designed to generate the implicit component and to provide within themselves a suitable space to accommodate the implicit component.)

According to the invention, internal coolant conduits are provided that lie next to and in the plane of or nearly in the plane of at least some of the separator layers (enough conduits are provided for cooling of the stack) . The coolant conduits extend adjacent and parallel to the reactant gas flow channels. Cooling of the stack is obtained by flow of coolant through the internal coolant conduits thus provided, and without the need of any special cooling plates. Such coolant conduits may be provided in association with one or both of the reactant gas flow fields for each fuel cell, depending upon the configuration and the cooling needs of the stack. It is contemplated that at least one set of such coolant conduits will be provided for each fuel cell within a stack, although depending upon stack geometry, some stacks may be adequately cooled with fewer sets of coolant conduits.

The inventor has recognized that in conventional designs, the reactant gas flow channels formed in fuel cell strata within a fuel cell stack are not optimally utilized throughout the cross-section of the flow channels for their primary purpose, viz to supply reactant gas to the adjacent porous electrode layers of the MEA strata in the stack. Those portions of the flow-channel cross-section that are nearest to the porous electrode layer are most effective in enabling reactant gas to reach and penetrate the porous electrode _ layer. On the other hand, those portions of the flow channel more remote from the MEA layer are relatively inefficient for transferring reactant gas to the porous electrode layer. From this insight, the inventor has recognized that such inefficient portion of the flow channel could instead be utilized for the provision of a coolant conduit that would follow at least a portion of such flow channel, and would provide individual cooling for the fuel cell in which it is located.

The inventor has recognized that coolant conduits may be more favourably located within a fuel cell stack than are conventional coolant conduits, and may be conveniently and economically located within or between other layers in a fuel cell stack so as to eliminate the need for separate cooling plates. This invention achieves the function of providing cooling without the disadvantage of thereby increasing the volume of the stack. An increase in stack volume is avoided by using space within the stack for cooling that was previously under-utilized for its intended function of delivering reactant gases.

In one embodiment of the invention, the coolant conduits may be formed within a composite separator stratum in the fuel cell stack.

In another embodiment of the invention, the coolant conduits may be formed between adjacent separator strata in the stack. In this latter instance, sequential separator strata are configured for mating engagement that creates a cooling conduit whose boundaries are partly on one of the strata and partly on the other of the strata.

The inventor has recognized that given the possibility of altering flow-channel cross-section (given flow channels of conventional size) without unduly impeding reactant transfer to the porous one of the electrode layer adjacent, it is possible to reconfigure the internal structure of fuel cells so as to provide suitable coolant conduits in convenient spaces within or between fuel cell strata. Further, because the fuel cells may each provide (or share in the provision of) integrated internal cooling arrangements according to the invention, the provision in the stack of separate special-purpose cooling plates can be completely eliminated, and fuel cell stack dimensions and weight can be correspondingly reduced, thereby improving the overall power density of the fuel cell stack.

According to the invention, the separator strata in the fuel cell stack may advantageously be designed to perform not only their primary conventional separation function but also an auxiliary cooling function. The auxiliary cooling function may be implemented within or between the separator strata while maintaining the overall thin dimensions of the separator for stacking efficiency.

The design of a fuel cell stack in accordance with the principles of the invention is intended primarily for use with PEM-type fuel cells, but the principles may be applied to other types of fuel cells in which internal coolant conduits may conveniently follow reactant gas flow channels (or may be positioned in close proximity thereto, preferably with end-to-end - orientation generally following the flow-path pattern) .

The principles of the present invention can be advantageously applied to undulate tube cell stack configurations of the sort described and claimed in the applicant's British Patent Application Serial No. 9814123.7 (McLean) filed on 1 July 1998 and derivatives and divisionals thereof. Such undulate MEA fuel cell stack configurations comprise extended parallel reactant gas conduits that are space-efficient and economical to manufacture. Such configurations lend themselves to end connection or coupling to supply and exhaust plena for the reactant gases. Coolant conduits according to the present invention could be coupled to inlet and outlet coolant plena respectively located at the ends of the coolant conduits. With such configurations, the risk of puncturing MEA layers is low, thus lowering the risk of damage or contamination and also obviating the special sealing problems associated with conventional PEM-type fuel cell stacks.

Because the cooling conduits of the present invention are easily isolated from the electrochemical activity of the fuel cell stack (and indeed may be made from electrically insulating material of adequately high thermal conductivity) , and consequently may carry readily ionizable cooling fluids such as antifreeze, technically and economically superior cooling is possible without undue risk of contamination of, or electrochemical interaction with, the vital active components of the fuel cell stack. By contrast, present conventional designs frequently require the use of such fluids as de-ionized water for use as the coolant in order to avoid creating unwanted conductive paths within - the stack, and so as to avoid damage to membranes and catalysts.

It is also an aspect of the invention to use hydrogen in a fuel cell stack made up of fuel cells having separator layers as heretofore described and that is connectable via an anode terminal and a cathode terminal to an external load. Each fuel cell has an MEA layer and two discrete associated reactant-gas impermeable separator layers. The MEA layer has a porous anode electrode, a porous cathode electrode, an electrolytic membrane layer disposed between the two electrodes, an anode electro-catalyst layer disposed between the electrolytic membrane layer and the anode electrode, and a cathode electro-catalyst layer disposed between the electrolytic membrane layer and the cathode electrode. One side of one separator layer in conjunction with the MEA layer provides at least one flowpath of a flow field for hydrogen and one side of the other separator layer in conjunction with the MEA layer provides at least one flowpath of a flow field for a selected oxidant. The flowpaths are constituted over their greater length by parallel transversely spaced and longitudinally extending flow channels interconnected in the vicinity of their ends to form the flowpaths. The MEA layer is installed in the stack between the associated separator layers so that the side of the separator layer that in conjunction with the MEA layer provides flow channels of a flow field for hydrogen faces and is in contact with the anode side of the MEA layer, whilst the side of the separator layer providing flow channels of a flow field for oxidant faces and is in contact with the cathode side of the MEA layer, so - that the hydrogen flow channels are closed to form a conduit for supplying hydrogen to the MEA layer and the oxidant flow channels are closed to form a conduit for supplying oxidant to the MEA layer. The fuel cells are stacked in sequence, the anode electrode of the fuel cell at one extremity of the stack being electrically connected to the anode terminal, the cathode electrode of the fuel cell at the other extremity of the stack being electrically connected to the cathode terminal, and the anode electrode of each of the other fuel cells in the stack being electrically connected to the cathode electrode of the next adjacent fuel cell. When the anode terminal and cathode terminal are electrically connected through an external load and for each fuel cell hydrogen is supplied to the hydrogen conduit and oxygen is supplied to the oxidant conduit, then in each fuel cell hydrogen moves from the hydrogen flow field through the anode electrode and is ionized at the anode electro-catalyst layer to yield electrons and hydrogen ions, the hydrogen ions migrate through the electrolytic membrane layer to react with oxygen that has moved from the oxidant flow field through the cathode to the cathode electro-catalyst layer and with electrons that have moved from the anode electrode electrically connected to the cathode electrode, thereby to form water as a reaction product, and a useful current of electrons is thereby produced through the load. Summary of the Drawings

Figure 1 (prior art) is a schematic isometric view of a portion of a fuel cell stack of conventional design illustrating coolant plates interspersed with fuel cells in the stack.

Figure 2 (prior art) is a schematic fragmentary section view of a portion of a conventional fuel cell arrangement showing a membrane electrode assembly (MEA) layer sandwiched between fuel and oxidant distribution/separator plates.

Figure 3 is a fragmentary section view of an undulate MEA layer sandwiched between consecutive separator plates in a stackable fuel cell arrangement and including cooling conduits within the troughs of the undulate MEA layer, constructed in accordance with an explicit exemplification of a coolant conduit formed in a "waste space" portion of the fuel cell.

Figure 4 is a schematic fragmentary section view of an undulate MEA layer positioned in contact with an adjacent separator layer, and showing the relatively effectively utilized portion of the reactant flow channel and the relatively poorly utilized portion of the reactant flow channel.

Figure 5 is a schematic fragmentary section view of an undulate MEA layer sandwiched between separator plates in a stacked series, in which the cooling conduits are formed by mating planar and oblique portions of contacting consecutive separator layers in the stack, constructed in accordance with the present invention .

Figure 6 is a schematic fragmentary exploded section view of the structure of Figure 5.

Figure 7 is a schematic fragmentary section view - of a portion of an alternative undulate MEA layer/separator layer configuration of the general type illustrated in Figure 5 and including cooling conduits in accordance with the invention, but in which the cooling conduits are smaller and the spacing between consecutive layers of the fuel cell stack is smaller than what appears in Figure 5, permitting a higher power density than is available from the Figure 5 arrangement.

Figure 8 is a schematic exploded section view of the structure of Figure 7.

Detailed Description

In this specification, three dimensional referents are used. The stack dimension is the dimension in which fuel cell layers and other layers are stacked; it is generally perpendicular to the broad surfaces of the layers within the stack. The flow dimension is the dimension parallel to the major flow channels comprising the reactant gas flow fields. (In a stack, depending upon configuration, it is conceivable that the fuel gas flow channels might be designed to lie perpendicular to the oxidant gas flow channels. However, relative to any one flow field, the flow dimension is free from ambiguity. ) The transverse dimension is that dimension that, for any given flow field, is perpendicular to both the flow and stack dimensions. Note that the flow dimension presupposes a generally parallel arrangement of straight flow channels suitably interconnected at their ends to form the complete flowfield. This configuration lends itself to relatively inexpensive manufacture and is highly suitable as a foundation for the implementation of the present invention. However, it is possible to consider the flow dimension as parallel to reactant gas flow at any selected portion of a flowfield, and to adjust the concept of the transverse dimension accordingly.

Referring to Figure 1, in a conventional fuel cell stack 10, interspersed among fuel cell layers 12 are cooling plates 14 made of a suitable material that functions as an effective heat conductor while avoiding contamination of the fuel cells or reactant gases therein. The fuel cell stack 10 illustrated in Figure 1 is partial only; one might expect as many as 100 or more fuel cells to be mounted in a stack. In the case of the exemplary schematic arrangement of Figure 1, the lowermost portion of the stack 10 is illustrated, resting upon terminal base plate 15. Cooling plates 14 are illustrated in Figure 1 as constituting every fourth layer of the fuel cell stack, three fuel cells 12 being interposed between each consecutive pair of cooling plates 14. It follows that fuel cell 12b, relatively remote from the nearest cooling plate 14, will operate at a higher temperature than fuel cells 12a and 12c that are in contact with an adjacent cooling plate 14.

The cooling plates 14 and the fuel cells 12 are provided with apertures 16 so as to provide, in the stack dimension of stack 10, plenum chambers for incoming and effluent fuel and oxidant gases. Coolant plenum apertures 18 are provided for inlet and effluent coolant flow within the stack. Heat flow is predominantly from the coolant plates 14 (which receive heat from the fuel cells in the stack) into the conductive coolant that flows therethrough from and into respective supply and exhaust plenum chambers formed by - the sequence of aligned apertures 18 throughout the stack 10.

Figure 2 illustrates a fragment of an individual fuel cell layer within the stack 10. A membrane electrode assembly (MEA) layer 22 is shown sandwiched between a fuel gas distribution and separator plate 24 and an oxidant gas distribution and separator plate 26. Each of the plates 24, 26 is provided with a serpentine flowpath (not apparent from Figure 2, but conventional) which, in cross-section, appears as a series of spaced flow channels 28. Electrical conductivity through the stack is maintained by physical and electrical contact between the solid abutting portions 27 of the distribution plates 24, 26, and the sandwiched MEA layer 22 kept under compression between the abutting solid portions 27 of the distribution plates 24, 26.

It is to be observed that although fuel and oxidant gases flow through the entirety of the flow channels 28 forming the respective fuel and oxidant flowpaths in the distribution plates 24, 26, nevertheless, primarily that portion of the gas flow immediately adjacent the exposed MEA layer 22 is able effectively to reach the fuel cell electrodes and contribute to the electrochemical performance of the fuel cell.

Similar observations can be made as to the flow of reactant gases in unconventional MEA fuel cells such as those illustrated in Figures 3 and 5. In Figure 3 a waved, or undulate, MEA layer 70 is sandwiched between separator layers 72 that make electrical connection with the apices 74 of the MEA layer 70. The separator layers 72 together with the sandwiched undulate MEA layer 70 - form flow channels 76, 78 for the reactant gases. Such an arrangement is described in British patent application 9814123.7 (McLean) filed on 1 July 1998 and in derivative and divisional applications thereof. The uppermost "troughs" of the undulate MEA layer 70 constitute fuel flow channels 76 and the lower complementary channels 78 serve to carry the oxidant gas, or vice versa - the selection is arbitrary, as is the orientation shown in the illustration.

Because the reactant flow channels 76, 78 operate with maximum electrochemical efficiency immediately adjacent the MEA layer 70, coolant flow conduits 79 may be provided immediately adjacent the separator layers 72 and spaced away from the MEA layer 70 so as to leave ample flow within channels 76, 78 sufficient to feed the exposed surfaces of MEA layer 70. In Figure 3, such coolant conduits 79 are shown both in the oxidant flow channels 78 and in the fuel flow channels 76. Depending upon the cooling requirements of the fuel cell, the dimensions of the structures, the materials used, and other parameters, it may not be necessary to provide such coolant conduits in all of the flow path channels. Each of the coolant conduits 79 can be connected between inlet and outlet coolant plenum chambers (not shown) to complete the coolant cycling system. While the Figure 3 arrangement embodies the "waste space utilization" principle of the invention, it does not embody the coolant channel implicit formation principle of the invention, and for this latter reason, is expected to be less economical to manufacture than various implicitly formed coolant conduit arrangements.

To elaborate on the foregoing, it will be noted from an inspection of Figure 3 that each coolant conduit - 79 is located so as to lie in a region that is relatively remote from the active surfaces of the nearby MEA layer 70. The reason for this can be better understood by referring to Figure 4, which shows the layout of a single undulation in an undulate MEA layer 70, a planar separator layer 72 and an interior space relatively remote from the MEA layer 70 that can be used to locate cooling conduit 79. It is logical to excise from flow channel 78, shown bounded by separator layer 72 and MEA layer 70 in that illustration, the inefficient portion of the flow channel and to make the inefficient portion available to accommodate the coolant conduit 79. By installing a coolant conduit 79 in an inefficient portion of the previously available flow channel space, the residual space 78 left for reactant flow gas is considerably diminished, yet, because the region 78 is the region that is immediately adjacent the MEA surface 70, the overall efficiency of the fuel cell can be maintained (with such adjustments to pressures and flow rates as are needed to maintain operating efficiency of each fuel cell) . The cooling conduit 79 is placed adjacent and preferably fixed to the separator plate 72 so that it remains well out of contact with the MEA layer 70.

While Figure 4 has taken, by way of example, an undulate MEA layer and planar separator arrangement such as that illustrated in Figure 3, the principles derived from a review of Figure 4 can be applied to most, if not all, other reactant channel configurations, such as reactant channels formed between an undulate MEA and undulate separators.

Some trade-offs may have to be made. For example, let us suppose that despite adjustment of other parameters, the inclusion of a coolant channel 79 within what would otherwise be available reactant gas flow path space 78 in Figure 4 diminishes the overall performance of the fuel cell to some extent. Notwithstanding this hypothetical diminution in operational performance, we have, by reason by the present invention to be described more fully below, eliminated cooling plates 14 (Figure 1) from the fuel cell stack. The result is expected to be an overall improvement in power density for the fuel cell stack, even if, contrary to expectations, there is some diminution in individual fuel cell performance that is more than trivial.

Preferably the coolant conduits 79 are bonded to the adjacent portions of separator layers 72. If the separator layers 72 and the coolant conduits 79 are made of metal, then such bonding may be by welding or soldering. If they are not made of metal or are provided with non-metallic surfaces, the use of a cement such as an high-temperature-resistant epoxy resin may be suitable. Materials other than metals can be used for constructing the conduits 79; suitable materials that have adequate heat transfer capability and yet are sufficiently inert that they can be used in direct contact with the reactant gases include solid and woven graphite materials. An alternative inert material with satisfactory heat conductivity that is expected to be useful is polytetrafluoroethylene (PTFE) such as that sold under the trademark TEFLON. Of course, the shape and dimensions of the cooling conduits 79, and its orientation within flow channels 76, 78, are in the discretion of the designer.

Boundary walls 80 of cooling conduits 79 (and boundary walls of other cooling conduit designs and arrangements to be described below) may be made of either conductive material or non-conductive material in the designer's preference. Non-conducting walls for the cooling conduits would have the advantage of allowing conductive coolant fluids to be used without disturbing the internal electrical balance within the fuel cell, since the conductive coolant fluids would be electrically isolated from the electrochemically active portions of the fuel cell by the non-conductive walls of the cooling conduit. On the other hand, if the walls 80 are made of conductive material, then the conduit walls themselves may, depending upon the configuration chosen, serve a useful function in current collection and transmission. The specific configuration of Figure 3 does not lend itself to such electrically conductive function of the conduit walls 80, but other configurations can easily be imagined in which the coolant conduit is shaped, dimensioned and located in such a position that electrical conductivity of the walls might be advantageous for collection and transmission of fuel cell current. Whether or not the walls 80 or other coolant conduit walls to be described below are selected to be conductive or non-conductive, it is of course important that the coolant within the coolant conduits 79 be isolated physically from the reactant gases and MEA layers; it is important to prevent migration of the coolant materials into the electrolytic membrane layer.

In a preferred embodiment, the separator plates are configured to provide coolant channels without requiring separate manufacturing operations to form and attach the coolant channels. Figure 5 illustrates this possibility - in the case of a fuel cell stack a portion of which is schematically illustrated, each stratum in the stack including an undulate MEA layer 70, essentially similar to the layer 70 illustrated in Figure 3. In the case of Figure 5, however, the separator layers 72 are not formed as simple planar or undulate structures, but rather are constructed as the composite of at least two layers 82, 84 each of which is formed by suitable bending of sheet material to form generally triangular cross-sectioned peaks or troughs (depending upon orientation). The layers 82, 84 are then oriented in opposite directions, transversely offset from one another so that the triangular "waves" are out of phase, and are then bonded together to form the overall non- planar separator containing interior flow channels for coolant 79.

More specifically, each component layer 82, 84 of the composite separator layer 72 is made by bending planar sheet material to form transversely spaced triangular troughs 86 alternating transversely with planar web portions 88. In assembly of the composite layer 72, separator sheets 82, 84 are transversely offset from one another through approximately a 180° phase shift so that the triangular trough 86 of any given component layer 82 lies immediately adjacent a planar web portion 88 of the next adjacent separator component layer 84. This structural arrangement is more obviously apparent from an inspection of the schematic exploded view of Figure 6. When consecutive separator component layers 82, 84 come into contact with one another, the triangular trough portions 86 combine with facing and mating planar portions 88 to form conduits 79 - of triangular cross-section that are suitable for coolant flow. Note that it is not essential that there be a sealing contact between consecutive separator component layers 82, 84; no harm is done if there are small leaks between neighbouring triangular conduits 79. Note also that triangular configuration of the coolant conduit cross-sections is not essential; trapezoidal or continuously curved shapes would work; triangular configuration however is simple and likely to be relatively inexpensive to manufacture. In the choice of cross-section design, the designer will take into account not only manufacturing cost but other pertinent factors such as heat transfer requirements, flow characteristics (especially the desideratum of inducing turbulent flow within the cooling conduits) , structural strength and integrity, etc. The configuration of Figure 5 accordingly provides a relatively inexpensive solution to provision of coolant conduits within a fuel cell stack that eliminates the need for explicit (separately formed) cooling strata and provides relatively uniform cooling. The coolant conduits 79 are effective and cheaply manufactured, yet the Figure 5 design avoids the creation of any undue impediment to gas flow within reactant flow channels 76, 78 formed between the MEA layers 70 and adjacent separator layers 72.

The layout shown in Figures 5 and 6 illustrates a design in which the separating layer remains roughly planar, with cooling conduits formed in the separator 72 and protruding into the reactant gas flow channels 76,78. The shape of the protruding conduits can be altered, as can their frequency relative to the reactant gas flow channels, i.e. it may not always be necessary to design one coolant conduit for every reactant gas _ flow channel.

In another embodiment of this invention, it is possible to increase the power density of the fuel cell stack relative to the power density available from the Figure 5 arrangement by reducing the cross-sectional area of the coolant conduits 79, thereby permitting a closer stacking of the MEA layers 70. Such higher power-density arrangement is illustrated in Figure 7. In this case, the separator layers 72 are provided with wider and deeper triangular troughs 86 and narrower planar web portions 88 than are respectively found in the counterpart separator component layers 72 of Figure 5. Figure 8 is an exploded view of the schematic design shown in Figure 7, showing in particular how the undulate composite separator 72 with integral cooling conduits 79 is formed from two separator components 82, 84. It is critical to understand that through the interaction of the shapes of components 82, 84, the overall composite separator 72 of Figure 7 takes on an overall undulate rather than planar configuration. This allows the neighbouring MEA layers 70 to overlap each other in the stack dimension without interfering with one another, thus facilitating an overall volumetric power density improvement relative to the design shown in Figure 5.

It is understood that the design of separator components 82, 84 need not be limited to triangular coolant flow conduits. Any shape may be selected so long at separator components 82, 84 mate together to provide a means of current conduction through the fuel cell and to permit the other usual fuel cell component functions to be implemented. Accordingly, when consecutive separator component layers 82, 84 are offset transversely from one another and placed in contact with one another, the change in relative dimensions of the troughs 86 and planar portions 88 results in the creation of smaller coolant conduits 79 than is obtained in the Figure 5 construction. Note also that there is a narrowing of the available space for reactant gas to reach those portions of MEA layer 70 that lie closest to the separator layers 72 as compared with the counterpart configuration of Figure 5. The degree to which the neighbouring MEA layers 70 can be compressed together is limited by the considerations for supply of reactant gases to the MEA layers 70, as previously mentioned. Care must be taken to ensure efficient transport of reaction gases to the MEA layer. Considerations such as the foregoing will influence the specific design in terms of coolant conduit shape, overall dimensions etc. and will be understood by persons skilled in the art of fuel cell design.

The key value of the Figure 7 embodiment is that a composite separator, suitable for use with undulate MEA layers, including internal cooling conduits and permitting efficient MEA layer stacking by virtue of the overall non-planar structure, can be created through the interaction of two separator component layers each formed from thin substantially planar sheet material. It is important to note that the inclusion of coolant conduits within the separator layer, as described herein, does not limit the options available to the designer for manifolding the cell in order to create the desired flowpaths. Such options may include internal manifolding arrangements. Since the present - invention is based on the interaction of interconnected but separate component layers, it remains possible to employ an internal manifolding technique by forming interconnecting reactant flowpaths within the component layers .

Some empirical testing and selection of preferred positioning of coolant conduits within individual fuel cell layers may well be appropriate, because a number of factors have to be taken into account in deciding how to locate, shape and size such coolant conduits, and how many to have in a fuel cell stack. Persons skilled in the technology know that it is desirable, within a reactant gas flow channel, to have a pressure drop from the inlet to the outlet side of the channel, and to have a channel shape that facilitates turbulence within the channel, thereby improving the ability of the gas to reach the exposed surface of the MEA layer and to penetrate same. The fuel cell reaction product (water) must also be satisfactorily removed, and the flow channel design must meet this objective. The present description does not elaborate such empirical approaches as may be desirable to optimize reaction product removal, pressure drop, turbulent flow, and other factors of significance in the design of the flow channel and coolant conduit configuration for any given fuel cell stack design. Manufacturing cost may be expected to play a significant role; that parameter, together with the power density parameter previously discussed, may influence a number of design choices. However, individual designers will have their own respective preferences and priorities, and will be able to choose - appropriate designs while relying upon the general principles apparent from the foregoing description.

For simplicity, the accompanying drawings are schematic; engineering details of any given design implementation of the invention may vary according to designers' preferences that meet objectives that may or may not directly relate to cooling. For example, the designer may wish to increase reactant gas turbulence to improve transfer of reactant gas to the electrochemically active portions of the fuel cell. To this end, the designer may choose rough or irregular surfaces for the walls enclosing reactant gas flowpaths. It may be convenient for the designer to select rough or irregular coolant conduit external walls (that will be in contact with reactant gases) to help achieve such objective. Further, the cross-sectional shape and dimensions of the coolant conduits can be selected not only with cooling in mind but also for optimization of the reactant gas flowpath cross-section. An integrated design approach should facilitate the achievement of such technical objectives while permitting the designer to aim for manufacturing simplicity and relatively low cost of manufacture.

Hydrogen may be used as a fuel gas in a fuel cell stack incorporating the separators described above. In Figures 5 and 7 portions of exemplary fuel cell stacks are shown as made up of fuel cells and include composite separator layers 72 made up of component layers 82, 84.

Each fuel cell stack is connectable via a cathode terminal (not shown) and an anode terminal (not shown) to an external load (not shown) . Each fuel cell has a discrete MEA layer 70 and is associated with two of the reactant-gas impermeable composite separator layers 72.

Each MEA layer 70 has a porous anode electrode (not shown) , a porous cathode electrode (not shown) , an electrolytic membrane layer (not shown) disposed between the two electrodes, an anode electro-catalyst layer (not shown) disposed between the electrolytic membrane layer and the anode electrode, and a cathode electro-catalyst layer (not shown) disposed between the electrolytic membrane layer and the cathode electrode. One side of one associated separator layer in conjunction with the MEA layer 70 provides at least one flowpath of a flow field for hydrogen and one side of the other associated separator layer in conjunction with the MEA layer 70 provides at least one flowpath of a flow field for a selected oxidant. The flowpaths are constituted over their greater length by parallel transversely spaced and longitudinally extending flow channels interconnected in the vicinity of their ends to form the flowpaths. Each MEA layer 70 is installed in the stack between the associated composite separator layers 72 so that the side of the composite separator layer 72 that in conjunction with the MEA layer provides flow channels of a flow field for hydrogen faces and is in contact with the anode side of the MEA layer, whilst the side of the separator layer providing flow channels of a flow field for oxidant faces and is in contact with the cathode side of the MEA layer 70, so that the hydrogen flow channels are closed to form a conduit for supplying hydrogen to the MEA layer 70 and the oxidant flow channels are interconnected in the manner described above to form a conduit for supplying oxidant to the MEA layer 70. For example, in the Figures 5 and 7, the reactant gas flow channels are indicated by reference _ numerals 76 and 78. The fuel cells are stacked in sequence and the anode electrode of the fuel cell, say, at one extremity of the stack electrically connected to the anode terminal, the cathode electrode of the fuel cell at the other extremity of the stack electrically connected to the cathode terminal, and the anode electrode of each of the other fuel cells in the stack electrically connected to the cathode electrode of the next adjacent fuel cell. When the anode terminal and cathode terminal are electrically connected through an external load and for each fuel cell hydrogen is supplied to the hydrogen conduit and oxygen is supplied to the oxidant conduit, then in each fuel cell hydrogen moves from the hydrogen flow field through the anode electrode and is ionized at the anode electro-catalyst layer to yield electrons and hydrogen ions, the hydrogen ions migrate through the electrolytic membrane layer to react with oxygen that has moved from the oxidant flow field through the cathode to the cathode electro- catalyst layer and with electrons that have moved from the anode electrode electrically connected to the cathode electrode, thereby to form water as a reaction product, and a useful current of electrons is thereby produced through the load.

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 applicable technologies, particularly in light of the foregoing description. The appended claims include within their ambit such modifications and variants of the exemplary embodiments of the invention described herein as would be apparent to those skilled in the applicable technologies.

Claims

What is claimed is :

1. A PEM-type fuel cell for incorporation into a fuel cell stack, having an undulate MEA layer and reactant gas flow channels forming at least one flowpath on either side of the MEA layer and separator layers for segregating supplied reactant gases from one another, characterized in that at least one coolant conduit is provided for flow of cooling fluid so as to remove heat from the fuel cell, each said coolant conduit being formed between contacting surfaces of separator layers of consecutive fuel cells in the fuel cell stack.

2. A fuel cell as defined in claim 1, wherein each said coolant conduit is formed implicitly between contacting surfaces of separator layers of consecutive fuel cells in the fuel cell stack.

3. A fuel cell as defined in claim 1 or 2, wherein the fuel cell is bounded by separator layers that mate with separator layers of adjacent consecutive fuel cells in the stack to form the coolant conduits.

4. A fuel cell as defined in claim 3, wherein the outer surfaces of said separator layers are formed as a regular alternating sequence of surface portions parallel to the MEA layer and surface portions oblique to the MEA layer, the surface pattern of one of said surfaces being offset from the surface pattern of the other of said surfaces, such that adjacent mating offset surfaces of consecutive fuel cell separator layers form the coolant conduits.

5. A fuel cell as defined in claim 4, wherein the coolant conduits are of triangular cross-section.

6. A fuel cell as defined in claim 1 or 2, wherein the flow channels extend substantially parallel to one another, and those lying on either side of the MEA layer are coupled to selected ones of one another in the - vicinity of their respective ends to form at least one said reactant gas flowpath.

7. A fuel cell as defined in claim 1, 2 or 6, wherein a plurality of said coolant conduits are provided to follow substantially the entirety of the flowpaths on the surface of the side of the MEA layer on which said selected one of the reactant gas flowpaths lies, thereby to provide substantially uniform cooling over said last- mentioned surface of the fuel cell.

8. A fuel cell as defined in claim 1, 2 or 6, wherein at least two said coolant conduits are provided within the fuel cell, one following one of the two reactant gas flowpaths, and the other following the other of the two reactant gas flowpaths.

9. A fuel cell as defined in claim 8, wherein a plurality of said coolant conduits are provided to follow substantially the entirety of the flowpaths, thereby to provide substantially uniform cooling over the surfaces of the fuel cell.

10. A fuel cell stack having fuel cells in which a reactant gas flow field is provided between an MEA layer and a separator layer, the flow field comprising one or more reactant gas flow channels, characterized in that coolant conduits interspersed throughout the stack are provided to lie next to at least some of the separator layers and to extend adjacent and parallel to neighbouring flow channels, whereby cooling of the stack is obtained by flow of coolant through the coolant conduits without benefit of special-purpose cooling plates, the coolant conduits formed in the interface - between adjacent contacting separator layers of consecutive fuel cells in the stack.

11. A fuel cell as defined in any of claims 1 to 10, wherein:

the reactant gas flow channels are formed between first and second separator layers and the MEA layer, the channels on a selected side of the first separator layer constituting flow channels of a hydrogen flow field and the channels on a selected side of the second separator layer constituting flow channels of an oxidant flow field;

the MEA layer has an anode side comprising porous anode material, a cathode side comprising porous cathode material, an electrolyte membrane disposed between the two sides, an anode electro-catalyst layer disposed between the electrolyte membrane and the anode material, and a cathode electro-catalyst layer disposed between the electrolyte membrane and the cathode material; and

the MEA layer is installed in the fuel cell between the selected side of the first separator layer and the selected side of the second separator layer, the selected side of the first separator layer facing and in contact with the anode side of the MEA layer and the selected side of second separator layer facing and in contact with the cathode side of the MEA layer so that the hydrogen flow channels are closed to form a conduit for supplying hydrogen to the MEA layer and a conduit for supplying oxidant to the MEA layer.

12. The use of hydrogen as a fuel gas in a fuel cell as - defined in claim 11 whereby,

hydrogen is supplied to the hydrogen conduit, moves from the hydrogen flow field through the porous anode electrode material of the MEA layer, is oxidised at the anode electro-catalyst of the MEA layer to yield electrons and hydrogen ions which migrate through the electrolyte membrane of the membrane electrode, oxygen is supplied to the oxidant conduit, moves from the oxidant flow field through the porous cathode electrode material of the MEA layer and is reduced at the cathode electro-catalyst of the MEA layer with electrons to yield oxygen ions which migrate through the electrolyte membrane of the membrane electrode, the electrons being supplied to the cathode electro-catalyst by way of an external circuit which includes a load and connects the anode electrode to the cathode electrode, the hydrogen ions and the oxygen ions combining to form water thereby producing a useful current of the electrons travelling from the anode electrode through the external circuit to the cathode electrode.

13. The use of hydrogen as a fuel gas in fuel cells in a fuel cell stack connectable via an anode terminal and a cathode terminal to an external load, each said fuel cell having:

(i) an MEA layer having a porous anode electrode, a porous cathode electrode, an electrolytic membrane layer disposed between the two electrodes, an anode electro- catalyst layer disposed between the electrolytic membrane layer and the anode electrode, and a cathode electro-catalyst layer disposed between the electrolytic membrane layer and the cathode electrode; and

(ii) two discrete associated reactant-gas impermeable separator layers, one side of one layer in conjunction with the MEA layer providing flow channels of a flow field for hydrogen and one side of the other layer in conjunction with the MEA layer providing flow channels of a flow field for a selected oxidant;

the MEA layer being installed in the stack between the associated separator layers so that the side of the separator layer that in conjunction with the MEA layer provides flow channels of a flow field for hydrogen faces and is in contact with the anode side of the MEA layer, whilst the side of the separator layer providing flow channels of a flow field for oxidant faces and is in contact with the cathode side of the MEA layer, so that the hydrogen flow channels are closed to form a conduit for supplying hydrogen to the MEA layer and the oxidant flow channels are closed to form a conduit for supplying oxidant to the MEA layer; and

the fuel cells being stacked in sequence, the anode electrode of the fuel cell at one extremity of the stack being electrically connected to the anode terminal, the cathode electrode of the fuel cell at the other extremity of the stack being electrically connected to the cathode terminal, and the anode electrode of each of the other fuel cells in the stack being electrically connected to the cathode electrode of the next adjacent fuel cell,

so that when the anode terminal and cathode terminal are electrically connected through an external load and for each fuel cell hydrogen is supplied to the hydrogen . conduit and oxygen is supplied to the oxidant conduit, then in each fuel cell hydrogen moves from the hydrogen flow field through the anode electrode and is ionized at the anode electro-catalyst layer to yield electrons and hydrogen ions, the hydrogen ions migrate through the electrolytic membrane layer to react with oxygen that has moved from the oxidant flow field through the cathode to the cathode electro-catalyst layer and with electrons that have moved from the anode electrode electrically connected to the cathode electrode, thereby to form water as a reaction product, and a useful current of electrons is thereby produced through the load

characterized in that

at least one coolant conduit is provided for flow of cooling fluid so as to remove heat from the fuel cell, each coolant conduit formed between contacting surfaces of separator layers of consecutive fuel cells in the fuel cell stack.

14. The use of hydrogen as defined in claim 13, wherein each said coolant conduit is formed implicitly between contacting surfaces of separator layers of consecutive fuel cells in the fuel cell stack.

15. The use of hydrogen as defined in claim 13 or 14, wherein the fuel cell is bounded by separator layers that mate with separator layers of adjacent consecutive fuel cells in the stack to form the coolant conduits.

16. The use of hydrogen as defined in claim 15, wherein the outer surfaces of said separator layers are formed as a regular alternating sequence of surface portions parallel to the MEA layer and surface portions oblique to the MEA layer, the surface pattern of one of said surfaces being offset from the surface pattern of the other of said surfaces, such that adjacent mating offset surfaces of consecutive fuel cell separator layers form the coolant conduits.

17. The use of hydrogen as defined in claim 16, wherein the coolant conduits are of triangular cross-section.

18. The use of hydrogen as defined in claim 13 or 14, wherein the flow channels extend substantially parallel to one another, and those lying on either side of the MEA layer are coupled to selected ones of one another in the vicinity of their respective ends to form at least one said reactant gas flowpath.

19. The use of hydrogen as defined in claim 13, 14 or 18, wherein a plurality of said coolant conduits are provided to follow substantially the entirety of the flowpaths on the surface of the side of the MEA layer on which said selected one of the reactant gas flowpaths lies, thereby to provide substantially uniform cooling over said last-mentioned surface of the fuel cell.

20. The use of hydrogen as defined in claim 13, 14 or 18, wherein at least two said coolant conduits are provided within the fuel cell, one following one of the two reactant gas flowpaths, and the other following the other of the two reactant gas flowpaths.

21. The use of hydrogen as defined in claim 20, wherein a plurality of said coolant conduits are provided to _ follow substantially the entirety of the flowpaths, thereby to provide substantially uniform cooling over the surfaces of the fuel cell.

22. The use of hydrogen as a fuel gas in fuel cells in a fuel cell stack connectable via an anode terminal and a cathode terminal to an external load, each said fuel cell having:

the MEA layer being installed in the stack between the associated separator layers so that the side of the separator layer that in conjunction with the MEA layer provides flow channels of a flow field for hydrogen faces and is in contact with the anode side of the MEA layer, whilst the side of the separator layer providing flow channels of a flow field for oxidant faces and is in contact with the cathode side of the MEA layer, so that the hydrogen flow channels are closed to form a conduit for supplying hydrogen to the MEA layer and the - oxidant flow channels are closed to form a conduit for supplying oxidant to the MEA layer; and

so that when the anode terminal and cathode terminal are electrically connected through an external load and for each fuel cell hydrogen is supplied to the hydrogen conduit and oxygen is supplied to the oxidant conduit, then in each fuel cell hydrogen moves from the hydrogen flow field through the anode electrode and is ionized at the anode electro-catalyst layer to yield electrons and hydrogen ions, the hydrogen ions migrate through the electrolytic membrane layer to react with oxygen that has moved from the oxidant flow field through the cathode to the cathode electro-catalyst layer and with electrons that have moved from the anode electrode electrically connected to the cathode electrode, thereby to form water as a reaction product, and a useful current of electrons is thereby produced through the load

characterized in that

coolant conduits interspersed throughout the stack are provided to lie next to at least some of the separator layers and to extend adjacent and parallel to neighbouring flow channels, whereby cooling of the stack is obtained by flow of coolant through the coolant conduits without benefit of special-purpose cooling plates, the coolant conduits formed in the interface between adjacent contacting separator layers of consecutive fuel cells in the stack.