Fuel cell direct water injection

A fuel cell assembly provides for direct water delivery to water injection points in the active area of fluid flow field plates of the assembly. The fluid flow field plate has a plurality of channels formed in the surface thereof which extend across the surface of the plate in a predetermined pattern, defining the active areas of the plate. A distribution foil has a plurality of channels formed in a surface thereof which channels extend from a first edge of the distribution foil to a second edge of the distribution foil. The channels terminate at the second edge at positions substantially coincident with respective ones of the field plate channels at water injection points. A cover foil extends over the distribution foil to enclosed the distribution foil channels and thereby form conduits for the water between the two foils.

This patent application is a national stage application of PCT application no. PCT/GB03/02973, which claims priority to GB02157907. This patent application claims priority to both PCT/GB03/02973 and to GB02157907.

TECHNICAL FIELD

The present invention relates to fuel cells, and in particular to flow field plates suitable for use in solid polymer electrolyte fuel cells, which flow field plates act as fluid delivery conduits to electrode surfaces of the fuel cell.

BACKGROUND

Conventional electrochemical fuel cells convert fuel and oxidant into electrical energy and a reaction product. A typical layout of a conventional fuel cell10is shown inFIG. 1which, for clarity, illustrates the various layers in exploded form. A solid polymer ion transfer membrane11is sandwiched between an anode12and a cathode13. Typically, the anode12and the cathode13are both formed from an electrically conductive, porous material such as porous carbon, to which small particles of platinum and/or other precious metal catalyst are bonded. The anode12and cathode13are often bonded directly to the respective adjacent surfaces of the membrane11. This combination is commonly referred to as the membrane-electrode assembly, or MEA.

Sandwiching the polymer membrane and porous electrode layers is an anode fluid flow field plate14and a cathode fluid flow field plate15. Intermediate backing layers12aand13amay also be employed between the anode fluid flow field plate14and the anode12and similarly between the cathode fluid flow field plate15and the cathode13. The backing layers are of a porous nature and fabricated so as to ensure effective diffusion of gas to and from the anode and cathode surfaces as well as assisting in the management of water vapour and liquid water. Throughout the present specification, references to the electrodes (anode and/or cathode) are intended to include electrodes with or without such a backing layer.

The fluid flow field plates14,15are formed from an electrically conductive, non-porous material by which electrical contact can be made to the respective anode electrode12or cathode electrode13. At the same time, the fluid flow field plates must facilitate the delivery and/or exhaust of fluid fuel, oxidant and/or reaction product to or from the porous electrodes. This is conventionally effected by forming fluid flow passages in a surface of the fluid flow field plates, such as grooves or channels16in the surface presented to the porous electrodes12,13.

With reference also toFIG. 2(a), one conventional configuration of fluid flow channel provides a serpentine structure20in a face of the anode14(or cathode15) having an inlet manifold21and an outlet manifold22as shown inFIG. 2(a). According to conventional design, it will be understood that the serpentine structure20comprises a channel16in the surface of the plate14(or15), while the manifolds21and22each comprise an aperture through the plate so that fluid for delivery to, or exhaust from, the channel20can be communicated throughout the depth of a stack of plates in a direction orthogonal to the plate as particularly indicated by the arrow in the cross-section on A-A shown in theFIG. 2(b).

Other manifold apertures23,25may be provided for fuel, oxidant, other fluids or exhaust communication to other channels in the plates, not shown.

The channels16in the fluid flow field plates14,15may be open ended at both ends, ie. the channels extending between an inlet manifold21and an outlet manifold22as shown, allowing a continuous throughput of fluid, typically used for a combined oxidant supply and reactant exhaust. Alternatively, the channels16may be closed at one end, ie. each channel has communication with only an input manifold21to supply fluid, relying entirely on 100% transfer of gaseous material into and out of the porous electrodes of the MEA. The closed channel may typically be used to deliver hydrogen fuel to the MEA11-13in a comb type structure.

With reference toFIG. 3, a cross-sectional view of art of a stack of plates forming a conventional fuel cell assembly30is shown. In this arrangement, adjacent anode and cathode fluid flow field plates are combined in conventional manner to form a single bipolar plate31having anode channels32on one face and cathode channels33on the opposite face, each adjacent to a respective membrane-electrode assembly (MEA)34. The inlet manifold apertures21and outlet manifold apertures22are all overlaid to provide the inlet and outlet manifolds to the entire stack. The various elements of the stack are shown slightly separated for clarity, although it will be understood that they will be compressed together using sealing gaskets if required.

In order to obtain high and sustained power delivery capability from a fuel cell, it is generally necessary to maintain a high water content within the membrane-electrode assembly, and in particular within the membrane.

In the prior art, this is conventionally achieved by humidifying the feed gases, either fuel, air or both, fed via manifolds21,22or23and channels16. A disadvantage with this technique is that in order to maintain sufficient humidification levels, the inlet gas streams often require heating and supplementary apparatus to introduce water vapour into the flowing gas streams.

In the prior art, the supplementary apparatus has been implemented in a number of ways. Bubbling the fuel or oxidant gases through heated water columns prior to introduction into the fuel cell has been applied. Alternatively, permeable membranes have been utilised as water transfer media such that water is carried into a gas stream from an adjacent plenum containing liquid water. Wicks have similarly been adopted to act as water transport media, liquid to vapour phase.

The additional apparatus may be separate from, or form an integral part of, the fuel cell stack. In either case, there is an associated increase in size and complexity of the assembly as a whole.

An alternative method is to deliver water directly to the membrane11,34, eg. directly to the electrode surfaces or into the channels16of the bipolar plates31. This technique has the advantage of not only supplying the water to maintain a high membrane water content but also can act to cool the fuel cell through evaporation and extraction of latent heat of vaporisation.

This direct heat removal process that provides for the extraction of energy via the exit gas stream has distinct advantages associated with the elimination of intermediate cooling plates within the fuel cell stack assembly.

In the prior art, it is common to adopt a cooling regime which intersperses heat exchange plates between the electrochemically active plates so as to extract the thermal energy resulting from resistive and thermodynamic inefficiency of the fuel cell. These heat exchange, or cooling, plates utilise a recirculating or, less commonly, once-through fluid flow which carries heat away from the fuel cell stack. The cooling plates are in general of a different design to the active plates thereby adding to the complexity, size and cost of the fuel cell assembly.

A difficulty that can be encountered in the direct introduction of water is to deliver precise quantities of water to the many fluid flow fuel plate channels16within a fuel cell stack30. Typically, this requires the delivery of precise quantities of water to many thousands of individual locations. To achieve this, a complex design of fluid flow field plate14,15or31is required, which is more difficult to achieve and which increases costs of production.

If the water delivery process is uneven then the cooling effect can be poorly distributed, resulting in localised hot spots where overheating may result in physical stress and a deterioration of the membrane11mechanical properties and ultimately rupture. This effect applies with both poor (uneven) delivery across a plate surface and uneven delivery to each of the individual cells that make up the stack. In other words, temperature variations mail occur within a cell, or from cell to cell.

SUMMARY

It is an object of the present invention to provide an improved method and apparatus for controlled delivery of water to individual channels in the fluid flow plates. It is a further object of the invention to provide such a method and apparatus which is easy to manufacture and assemble.

According to one aspect, the present invention provides a fuel cell assembly comprising:a fluid flow field plate having a plurality of channels formed in the surface thereof and extending across the surface of the plate in a predetermined pattern;a distribution foil having a plurality of channels formed in a surface thereof and extending from a first edge of the distribution foil to a second edge of the distribution foil, the channels terminating at the second edge at positions substantially coincident with respective ones of the field plate channels; anda cover foil extending over the distribution foil to enclose the distribution foil channels and thereby form conduits for water between the two foils.

According to another aspect, the present invention provides a fuel cell assembly comprising:a fluid flow field plate hating a plurality of channels formed in the surface thereof and extending across the surface of the plate in a predetermined pattern;a distribution foil having a plurality of channels formed in a surface thereof, the channels each extending from first positions proximal to or at a first edge of the distribution foil to second positions proximal to or at a second edge of the distribution foil, the channels terminating at the second positions substantially coincident with respective ones of the underlying plate channels; anda cover foil co-extensive with a substantial part of the distribution foil to enclose the distribution foil channels over at least part of their length between the first and second positions and thereby form conduits for water between the two foils.

According to a further aspect, the present invention provides a fuel cell assembly comprising:a fluid flow field plate having a plurality of channels formed in the surface thereof and extending across the surface of the field plate in a predetermined pattern;an adjacent membrane-electrode assembly (MEA) in contact with the fluid flow field plate over an active area of the MEA and;a distribution membrane interposed between the fluid flow field plate and the MEA, the membrane having a plurality of water conduits extending therethrough between first positions proximal to or at a first edge of the membrane to second positions proximal to or at a second edge of the membrane, the conduits terminating at the second positions substantially coincident with respective ones of the plate channels.

DETAILED DESCRIPTION

With reference toFIGS. 4(a) and4(b), the present invention provides a series of water injection conduits extending between a water inlet manifold25and the individual channels16of a fluid flow field plate40aor40b. Generally speaking, the water injection conduits are provided by way of a membrane or laminated structure which lies on the surface of the fluid flow field plate40. The water injection conduits are provided with inlets communicating with the water inlet manifold25and outlets which define predetermined water injection points over the channels16in the fluid flow field plate.

In a preferred arrangement, the laminated structure is provided in the form of two foil layers41,42overlying the plate40, the position of which foils are shown in dashed outline inFIGS. 4(a) and4(b).

FIG. 4(a) illustrates a plan view of a fluid flow field plate40awith serpentine channel16, with foils41a,42ahaving first edges43a,44acoincident with the water inlet manifold25, and second edges45a,46alocated at or adjacent to predetermined water injection points49of the channels16.

FIG. 4(b) illustrates a plan view of a fluid flow field plate40bwith two interdigitated comb channels47,48each communicating with a respective manifold21,22, and foils41b,42bhaving first edges43b,44bcoincident with the water inlet manifold25, and second edges45b,46blocated at or adjacent to predetermined water injection points of the channel47. It will be noted that the foils may be repeated on the opposite edge of the plate40bbetween a second water inlet manifold25and predetermined water injection points on the channel48.

FIG. 5shows a detailed plan view of the water distribution foil41layout, illustrating the preferred paths of the water injection conduits50. The conduits50are formed by a first series of channels51which extend from the first edge43of the foil41located at the water inlet manifold25, to a pressure distribution gallery or plenum52that extends along the length of the water injection foil41. The pressure distribution gallery52communicates with a second series of channels53which extend to the second edge45of the foil for communication with the channels16in the fluid flow field plate. For this purpose, the second series of channels53are grouped to terminate at respective convergence structures54at the second edge45of the water injection foil41.

In the preferred embodiment as illustrated, the convergence structures54comprise arcuate recesses55cut into the second edge45of the foil41at water injection points49adapted to be coincident with predetermined positions over channels16, shown in outline on the figure.

The pressure distribution gallery52preferably comprises an array of intercommunicating channels56which baffle the incoming water from the first series of channels51and effectively distribute it along the entire length of the foil41so that each group of the second series of channels53receives water at a substantially similar pressure.

Referring back toFIGS. 4(a) and4(b), the cover foil42comprises an unpatterned foil (ie. without channels) of substantially similar peripheral shape to the lower foil. The cover foil42extends beyond the edge of the distribution foil41at least at the ends of the second series of channels to ensure that water is directed downwards into the desired flow field plate channel16. Most conveniently, this overlap is achieved by the recesses55being formed in the distribution foil41, but not in the cover foil42. Thus, as best seen in the cross-sectional diagram ofFIG. 6, in exaggerated form, the cover foil42forms a top closure to the channels51,52and53to form the water injection conduits50, leaving open the ends of the channels51and53. In the embodiment shown, the cover foil42may be formed slightly larger than the distribution foil41such that it overlaps the second edge45(and possibly the first edge43) to achieve a similar effect.

It is noted that the foil layers are very thin compared with the plate40thickness, the thickness of the foil layers being easily absorbed by the MEA34and any gaskets interposed between the plates. The components in theFIG. 6are shown slightly separated for clarity, although they will, of course, be compressed together.

FIG. 7shows a perspective diagram of the water distribution foil41in position over the flow field plate40showing alignment of the various channels and manifolds.

It will be recognised that the water distribution channels51,52,53need not be formed in the lower foil41. In another embodiment, shown inFIG. 8, the water distribution channels80are formed in the lower surface of upper foil82, while the lower foil81serves to form the closure of the channels80to form the water injection conduits. In other words, the distribution foil82and cover foil81are inverted compared with the arrangement ofFIG. 6.

In theFIG. 8arrangement, at least the second series of channels (compare channels53inFIG. 5) will not extend right to the second edge83of the upper foil, but will terminate at positions proximal to the second edge. The lower (cover) foil81will extend almost to the end of the channels80, but will preferably stop slightly short thereof in order that there is fluid communication from the end of the channel80into the plate channel16at the water injection points49.

As indicated above, the lower (cover) foil81provides a closure to the channels80forming a barrier preventing water from escaping into underlying channels16in the fluid flow plate40in the wrong places, eg. where the water injection conduits traverse the fuel and/or oxidant channels16(eg. at location85).

Preferably, the foils as described above are formed from a metal, such as stainless steel. However, any suitable material having appropriate pressurised water containment properties could be used, and the expression “foil” used throughout the present specification is to be construed accordingly. Preferably, the foils are electrically conductive but they need not be so, since they do not impinge on the active area of the MEA.

In a preferred embodiment, the fluid flow channels16in the anode or cathode plates40are tropically between 0.4 mm and 1.2 mm in width and depth. It is found that a channel width and depth of 10 μm, chemically etched into the water distribution foil, selves to provide the necessary degree of water injection.

In use, the pressure of water being delivered via manifold25is controlled to ensure a significant pressure difference between the water supply and the gas pressure in the fluid flow channels16, achieving an equal distribution of water between the thousands of flow paths. In the preferred embodiment, water is delivered to the manifold at a pressure in the range 0.5-3 bar H2O.

An advantage of this approach is that the water distribution membrane is extremely thin and can easily be located within the available space within bipolar plates or in the gasket area.

The volumetric water dispensing accuracy can also be very precisely controlled by suitable design of the water injection conduit pattern and channel dimensions.

The water can be dispensed to either the fuel stream (anode) or the oxidant (cathode) side of the bipolar plate34, or both. Preferably, the water is injected into the cathode side.

As illustrated inFIG. 9, water that is dispensed into interdigitated channels90in the flow field plate40can be introduced at either the entry point91to the channel, after the feeder channel92, or alternatively into the exit track93at an injection point94at the same end of the bipolar plate as the feed manifold.

An advantage of water injection into the exit tracks is a reduction of pressure drop in reactant gas flows. This is because the water does not pass through the diffusion medium causing masking of void space for the gas passage. Similarly the elimination of water flow through the diffusion medium will also reduce the attrition of the medium and its gradual fragmentation and structural deterioration.

The evaporative cooling process is effective in the exit tracks and water content of the membrane is maintained due to saturation of the air with water vapour.

Although embodiments of the present invention have been described in the context of water injection into a proton exchange membrane fuel cell, it will be understood that the same structures may be used to inject any fluid material to injection points on a field plate.

Other embodiments are within the accompanying claims.