A flow field plate for use in a stack of fuel cells

TECHNICAL FIELD The present invention relates to a flow field plate, particularly for use in a stack of fuel cells, which plate comprises at least one bipolar electrode of electrically conductive material and an ion exchange membrane on each side of the bipolar electrode.

PRIORART A flow field plate (e.g. a bipolar electrode and/or a separator plate) is a disk/plate with open channels, which has to distribute the supplied reactants, gases or liquids, and which may also give mechanical strength to the fuel/reactor cell. The channels distribute die anode reactant at one side and the cathode reactant at the other side of the electrode. For instance metal, plastics and ceramics have been suggested as materials, and it has been indicated that the channels may be achieved by etching but also by removal with laser, or chip removal, embossing, pressing, or punching. The opinion has generally been that the channels at the anode side have to cross the channels at the cathode side, which implies that the bipolar electrodes have to be comparatively thick, so that it has been a large consumption of material at the manufacture. They have therefore been expensive to manufacture. Generally, a bipolar electrode may consist of graphite, for instance, where a pattern of channels has been achieved by moulding or chip removal, or consists of metal plates, where the pattern has been achieved by etching or chip removal. Also sheet metal with pressed patterns of channels ha\'e been used, wherein two plates have been connected for forming a bipolar electrode, see e.g. WO 00/3 ISl 5, but also US 6,051 ,331, where photolϋographically etched plates have beeu joined for forming a so called bipolar separator.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a flow field plate, e.g. constitutmg a bipolar electrode and/or a separator plate which may be manufactured with less consumption of material, which results in a lower weight and considerably lower costs than hitherto.

Another object is to provide a flow field plate/bipolar electrode and/or a separator plate (below only the designation electrode is used, for the sake of simplicity), which gives a reduced construction height (in a direction perpendicular to the electrode) as compared with electrodes previously known.

With an electrode and/or a separator plate of the type initially mentioned, said objects are achieved when the electrode is designed according to claim 1. In this way, manufacturing processes may be chosen which save material and costs, and the construction height and weight of the completed electrode will be lower than previously.

Additional advantages according to preferred aspects of the invention will be apparent from the following description.

BRIEF DESCRIPTION OF THE ENCLOSED DRAWINGS

Below, the invention will be described more in detail with reference to preferred embodiments and to the enclosed drawings.

Fig. 1 is a planar view of one side of a first preferred embodiment of a channel patterned plate included in an electrode according to the invention, which plate is square.

Fig. 2 is a partial view of the lower right corner of Fig. 1.

Fig. 3 is a cross sectional view according to the line HI-III of Fig. 2 and shows, partly how the flow connection between the inlet/outlet and the channel pattern has been formed, and partly how the electrode in a stack of fuel cells at both sides is surrounded by a proton exchange membrane.

Fig. 4 is a planar view of one side of a second preferred embodiment of a channel patterned plate included in an electrode according to the invention, which plεtc is square.

Fig. 5 is a cross sectional view according to the line V-V of Fig.4 and shows, partly how the flow connection between the inlet/outlet and the channel pattern has been formed, and partly how the electrode in a stack of fuel cells at both sides is surrounded by a proton exchange membrane.

Fig. 6 is a cross sectional view according to the line VI-VI of Fig. 4 and shows, partly how the flow connection between the inlet/outlet and the channel pattern has been formed through an recess in a sealing frame, and partly how the electrode

in a siack of fuel cells on both sides is surrounded by a proton exchange membrane.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figs. 1 to 6 show two different embodiments of a plate intended to represent an electrode for πse in a stock of fuel cells.

A great advantage with this invention is that it makes it possible to use only one and the same type of embossed plate 1, flow distributing zones 12, 13 and to amαige doubled inflow and outflow channels 7, 27, 8 _? 28 (see Figs. 1 to 3) by means of sealings 15A, 15B according to the flow pattern desired in a stack.

The pattern 2 of the plate 1 is surrounded by a frame region 6, 26, in which frame region apertures 7, 8, 27, 28 are arranged, partly for inflow 7, 27 (at a parallel flow) and 7, 28 (at a counter-flow), respectively, of reactants to the channels at both sides of the plate, and partly for outflow 8, 28 (at a parallel flow), and 8, 27 (at a counter-flow), respectively, of reaction products formed, from the channels 3, 23 at both sides of the plate. In the embodiments of the plate 1 shown in the figures, additional apertures are arranged in the frame region 6, 26, namely an aperture 19 for supply/discharge of a circulating medium, e.g. cooling water, and an aperture 20 for insertion of draw bars, not shown, for axially keeping a stack of fuel cells together.

The frame region 6, 26 is located in a plane P (see Fig. 3) of the bipolar electrode, which plane is located between the peaks of the ridges 5 at one side and the peaks of the ridges 25 at the other side of the electrode. The plane may consist of a median plane located halfway between the peeks of the ridges at one side and the peaks of the ridges at the other side of the plate. It is also possible, if so desired; to displace the plane in a direction towards the peaks of the ridges at one side to reduce the pressure fall at one side of the plate at the same time as an increase of the pressure fall may be achieved at the other side of the plate. This possibility may be of interest for certain types of fuel cells.

According to the embodiment shown in Fig. 1 and which will be explained more in detail below with reference to Figs. 2 and 3, where only one comer is shown, the solution is possible by, partly arranging flow distributing spaces 13a, 13B, partly arranging sealings 15 A, 15B in the two regions (here comer regions), where inlets and outlets, respectively, are arranged. The flow distributing regions 13A, 13B consist of an

essentially flat portion 13, which is located between the peaks of the ridges at one side and the peaks of the ridges at the other side of the electrode, preferably in the median plane P of the plate 1. By arranging a sealing 15 B at the upper side of the portion 13 against the edge of one outlet 28, the channel portions 3 A running from the flow distributing space 13 A are blocked towards this outlet 28 to be able to receive the gas (e.g. O ₂ )) flowing into the upper distribution space 13A. From Fig. 3 it is more evident that, in an embodiment with parallel flow, the channels 3 formed at the upper side of the "corrugated" plate, i.e. the channels 3 having valleys 4 as a bottom (seen from above according to Fig. 1), will contain the gas (here O ₂ ) flowing into the upper distribution space 13A. Because of the sealing 15A the gas flow of O2 is forced to flow oυt via the channel portions 3'B through the other outlet aperture 8. An advantage with the use of channel portions 3'A ₅ 3'B, 23'A, 23'B is that they guarantee that the membrane is hence given support to flat/tight abutment against the ridges 5 'A, 25 ^* B in the region between the distribution zones 12, 13 and me inlet3/outlets 7, 27, 8, 28.

In a corresponding manner, a sealing 15 A is arranged at the bottom side, which blocks outflow towards the second outlet 8. Instead the gas (e.g. H ₂ ), which moves at the bottom side of the plate 1 in the channels 23 (below the πdges 5, seen from above according to Fig. 1), will flow first into the distribution space at the bottom 13B and out therefrom to the first outlet 28 via the channel portions 23'A formed below the ridges 5'A between the distribution plaie 13 and the outlet 28.

The flows in connection with the inlets 27 and 7, respectively, may be arranged in a corresponding way.

In the section shown in Fig. 3 (πi-iπ of Fig. 2) the function of an imaginary example according to the invention in an embodiment of parallel flow is clearest elucidated, wherein the plate 1 is shown in a stack comprising a lower MEA (membrane electrode assembly) 39A and an upper MEA 39B. It is thus shown that in an irnaginary example, hydrogen H ₂ , in a stack, flows in through the inlet 27, is fed into the lower channels 23, and flow through these channels 23, which are formed between the lower MEA 39A and the plate 1, while tfce oxygen O ₂ flows in via the Met 7 and through the channels 3 which are formed between the plate 1 and the upper MEA 39B. Further, it is shown that a sealing frame 9, 29 seals outwards, in the outer frame region between the plate 1 and the respective MEA 39A, 39b, so that the gas (here O ₂ ), which is fed via the upper distribution space 13 A and via the sub-channels 3'B, is fed downwards therethrough, and the other gas (here Ha) is fed downwards through the second outlet 28.

According to the invention, the flow field plate consists of a single plate 1 with a very smaller material thickness than usual, i.e. a material thickness in the region 0.1 to 1 mm, preferably max. 0.5 mm, and more preferred max. 0.2 mm, which also gives weight and cost advantages. A pattern 2 of open channels 3 is arranged at one side of the plate and a pattern 22 of open channels 23 at the other side of the plate, respectively, in such way that valleys 4, 24 in the pattern at one side of the plate form ridges 25, 5 at the other side, and vice versa.

The patterned portion of the plate may have many different shapes/appearances, for instance a substantially sinusoidal cross section, or as in that embodiment ridges and valleys in the form of a substantially equally sided parallel-trapezoidal cross section. Further, the width of the valleys may differ from the width of the ridges, wherein wide valleys and narrow ridges at one side corresponds to wide ridges and narrow valleys at the other side, whereby a large flow is permitted at one side of the plate. This possibility may be of interest for certain types of fuel cells, for instance. In this way, such production processes may be chosen which save material and costs, and the construction height of the completed bipolar electrode may be lower than previously. Further, it is indicated that the channel ppttem may be given different shapes, e.g. three parallel channels arranged in a meander shape according to Fig. 1.

Different known methods for the production of the patterned plates are suitable, but according to certain aspects a particularly suitable pressing method for achieving the channel pattern 2, 22 in metal material is adiabaric pressing, e.g. according to WOOl 83132, which is incorporated herein as a reference.

In an artematrve embodiment, the plate 1 consists of a non-metallic material having good electric conductivity, which kind of material is known per se, and which material is resistant against reactants supplied to or formed in the stack of fuel cells. Alternatively, non-conductrve or less conductive material may be used in a manner known per se, which material is coated with a good conductor, e.g. gold

In an embodiment of the bipolar electrode according to Figs. 1 to 3 with counter-flow:

- the apertures for the inflow of reactants are arranged in opposite comers;

- the apertures for the outflow of reaction products formed are also arranged in opposite corners;

- a first unpattemed portion 12 is arranged adjacent to the frame region 6, 26 and in the plane P, which via intermediate channels 3'A, 3'B; 23'A, 23'B runs out to the outlet apertoe 7 and 27, respectively, and is arranged to connect the outlet aperture 7 with the channels 3 at one side of the plate and the outlet aperture 27 with the channels 23 at the other side of the plate; and

- a second unpattemed portion 13 is arranged adjacent to the frame region 6, 26 and in the plane P, which via intermediate channels 3'A, 3'B; 23'A, 23'B runs out to the second outlet aperture 8 and outlet aperture 28, respectively, and is arranged to connect the channels 3 at one side of the plate with the second inlet aperture 8, and the channels 23 at the other side of the plate with the second outlet aperture 28. hi an embodiment with parallel flow, one inlet aperture 7 or 27 and the associated outlet aperture 8 and 23, respectively, change places.

hi the embodiment shown in Fig.4, the plate is square, and said two unpattemed portions 12 and 13 are shown as being equally large and having substantially a triangular shape. However, the shape of the portions 12, 13 does not have any effect on their function but they may have an arbitrary shape, e.g. oval, rhombøidal, etc. hi contrast to Figs. 1 to 3, mere are no sub-channels, i.e. the plate 1 is not used to give any special control of the flow through parallel channel portions and/or support to the membrane in this region between the flow distributing spaces 12, 13 and the inlets/outlets 7, 8, 27, 28. The two unpattemed portions 12, 13 should, however be shaped such that they together give essentially the same effect on the flow in each individual channeL

hi an imaginary method with parallel flow, it applies that an inlet aperture 7, via the unpattemed region 12, in a straight Hne forms the inlet to the channels 3 at one side of the plate. The inlet 7 is located adjacent to a first corner, but at one side of the corner itself. An associated outlet aperture 8 is located adjacent to a diagonally opposite corner. Further, a second inlet aperture 27, for perpendicular inflow via the unpattemed region 12 to the channel 23, is arranged at the other side of the plate, and a second outlet aperture 28 for perpendicular outflow from the channels 23 is arranged at said other side of the plate, respectively. It is realized that the apertures 7, 8, 27, 28 may be positioned in a divertάfied way depending on apphcation/situation.

hi Figs. 5 and 6 it is among other shown how the bipolar electrode in a stack of fuel cells at both sides is siαrounded by a proton exchange membrane 39A, 39B. The membrane itself is designated 33 and is at both sides provided with a thin carrier 34, 35

for a catalyst, which is adapted to the reaction to be performed in the fuel celL The carrier 34, 35 is electrically conductive and may for instance consist of a carbon fibre web or of graphite paper. The unit 39A, 39B consisting of the membrane 33 itself and the catalyst carriers 34, 35 is sometimes called MEA (membrane electrode assembly). Also 39A, 39B may have a frame region 36 in the form of a sealing frame. The frame region 36 (e.g. a sealing frame) of the membrane unit sealingly abuts against the sealing frame 9, 29 of the bipolar electrode. Further, one membrane unit abuts against the peaks of the ridges 5 at one side of the bipolar electrode for delimitation of the valleys 4 between the ridges 5, and the other membrane unit abuts against the peaks of the ridges 24 at the other side of the bipolar electrode for delimitation of the valleys 24 between the ridges 25. Of course, the membrane unit is also provided with apertures corresponding to the apertures in the bipolar electrode.

hi Fig. 5 the reaction products formed run through the channels 23 and flow out onto the second unpatterned portion 13 , where they turn laterally at a right angel in order to reach the outlet aperture 28 via the indentation 31 (which ends in the aperture 28 behind the section V-V) in the sealing frame 29. In Fig. 6 the reaction products formed will in a corresponding manner run through the channels 3 and flow out over the first unpatiemed portion 13, where they continue straight forward in order to reach the outlet aperture 8 via the indentation 11 (which ends in the aperture 8 in front of the section Yl- VT) in the sealing frame 9.

If the plate 1 consists of a material which is attacked by the reactants or by the reaction products formed, both sides of the plate 1 are suitably provided with a thin protection layer of a material not being attacked

The sealing frame 9, 29 and the plate 1 have a total thickness which have to be adapted to the membrane unit 33 to 36, as the membrane υnit 33 to 36 has to abut and have a good electric contact against the peaks of the ridges 5, 25 and create good sealing against the MEA 39A, 39B.

The material of the sealing frame 9, 29 is chosen from a group which is sufficiently resistant to the reactants used and the reaction products formed, and they do not conduct electricity.

INDUSTRIAL APPLICABILITY

The bipolar electrode according to the invention described above is chiefly intended to be used in electrochemical cells such as fuel cells and electrolysers, but the man skilled in the art may, of course, simply and without any invention work modify it within the scope of the following claims, so it may be used in similar applications.

It is realized that for instance the channel pattern may be achieved in many different ways, of which adiabatic pressing is one possible method.

Further, it is realized that the plate 1 and the electrodes, respectively, may principally be given any shape (e.g. square, ovaL hexagonal, etc) and achieve the mam object of the invention. In addition, any part of the frame region, in the present case the central one, may be provided with apertures, for inflow and outflow, and that for instance the location of the cooling channels may be anywhere at the electrode.

Further, the principle with inlet and outlet channels at other places than in the frame region may be applied to plates with all shapes, as well as that a round plate may be arranged with channels only in the outer regions.