WO2001024295A1 - Bipolar collectors for pem-effect fuel cells - Google Patents

Abstract

The invention concerns a thermal bipolar-exchanger collector for a fuel cell, comprising both discrete collecting of electric charges and calories, a main structure (E, E') made of electronically and thermally conductive material and the use of a non-conductive open structure (Da) homogenising the flow of gas reagents. The collecting of electric charges and calories is provided by metal contacts (Bi) regularly distributed at the surface of the electrodes. Said metal needles do not completely run through the main structure, the electronic conduction relay being ensured by the main structure. Two embodiments are described: they differ in dimension of the spaces designed for the circulation of the heating medium. When the spaces are of large dimension, it is advantageous to fill them with a foam-type conductive structure enabling a good distribution of heating medium and good heat exchange.

Description

 BIPOLAR COLLECTORS FOR PEM TYPE FUEL CELL

IMPROVEMENTS ON BIPOLAR COLLECTORS FOR PEM-TYPE FUEL CELLS

The present invention relates to a bipolar collector-heat exchanger characterized in that it combines both, a main structure made of an electronically and thermally conductive material. discreet collection of charges and calories provided by metallic contacts uniformly distributed on the surface of the electrodes and penetrating into the main structure but not passing right through, and non-conductive open structures allowing the homogenization of the flow gaseous reagents.

Research relating to the development of bipolar collectors for batteries, in particular of the PEM-FC type, having interesting technical and economic characteristics, has been very active for a few years. In the first place, efforts were focused on the selection of collectors at a much lower cost than the collectors produced by machining a graphite plate. From this point of view, the most interesting results have been obtained by the development of polymer-carbon plates, the main advantage of which lies in their possible development by molding, hence very low manufacturing costs.

On the other hand, other solutions consisting of the use of fluted metal strips or metal foams prove to be more expensive since the surface of these metallic materials must necessarily be coated with a compound avoiding both corrosion and passivation.

Regarding the ohmic drop resulting from the passage of current through the collectors, it should be noted that the resistivity of the polymer-carbon composites is between 3.10 ^'2 and 1 Ω. cm while the graphite has a resistivity of the order of 1.3.10 ^"J Ω.cm. It follows that, for bipolar collectors such as those shown diagrammatically in FIG. 1, the ohmic drop caused by an apparent current density 0.5 A.cm ^'2 is less than 1 mV when the collector is made of graphite and is 20 mV for a composite collector having a resistivity of 0.05 Ω.cm.

As it is, for many uses where actually the current density does not exceed 0.5 A.cπX, it can be assumed that ^a loss of 20 mV. on a voltage of the order of 0.7 V, remains admissible without however being negligible. However, the situation is completely different if it is admitted that the progress to be hoped for in the field of PEM-CF leads to operating with current densities at least equal to 1 A. cm ^" .

Finally, this problem should be completely revised when the bipolar collectors must also constitute heat exchangers. Under these conditions, it can be shown that the dimensions of these exchanger-collectors, associated with the constraints linked to the good diffusion of heat in the materials constituting them, lead to reconsider the choices as regards the nature and structure of the collectors- exchangers.

Thus, for a collector-exchanger whose diagram is shown in Figure 2, made of a composite material of resistivity 0.05 Ω.cm. the ohmic drop reaches 50 mV for a current density of 0.5 A.cm ^'2 , a loss of 7% on the voltage of an element operating at 0.7 V.

Now, for applications such as powering electric vehicles, the nominal power required being for example 50 kW for a total voltage of 140 V, it can be seen that the power of an element is of the order of 250 W, or an energy to be evacuated per element close to 200 W. Under these conditions (and even for batteries with twice the power), it is necessary that all the bipolar collectors are also heat exchangers.

For power cells, the collector-exchanger should therefore not be considered as a special case but as the standard component.

We have already shown (patent application FR 99 04277) the advantage of discreet collection of charges by metal needles partly passing through the bipolar collectors. The device made it possible both to lighten the collectors and to minimize ohmic drops in these collectors. This concept has been taken up and necessarily adapted for the production of manifolds-exchangers. We have observed that, although the cross section of the needles is small compared to the apparent surface of the electrodes, these needles could constitute effective means for draining the calories generated at the electrodes.

The object of the present invention relates to a bipolar collector-heat exchanger for a fuel cell, characterized in that it combines both: • a main structure in an electronically and thermally conductive material,

• discreet collection of charges and calories provided by metallic contacts distributed uniformly on the surface of the electrodes and penetrating into the main structure but not passing right through.

• the use of open structures nonconductive ensuring ^{homogenization} of the flow of gaseous reactants.

According to a first embodiment, the bipolar collector-heat exchanger according to ^the invention is characterized in that the conductive main structure electronically and thermally consists of two parts one of which comprises at least on one of its faces, grooves allowing circulation of a heat transfer fluid.

In the case where the two parts of the main structure comprise grooves on one of their faces, said faces are juxtaposed during their assembly to reveal channels allowing the circulation of the heat transfer fluid.

According to another characteristic of the invention, the metal contacts are needles which do not entirely pass through each part of the main structure but penetrate into the projecting parts included between the channels, their end being situated between 0.4 and 0.9 mm from the plane separating the two parts constituting the main structure. These needles penetrate into the electrodes with a depth of between 0.1 and 0.2 mm and are distributed uniformly on the surface, in square or rectangular patterns.

The bipolar collector-heat exchanger according to the invention comprises, between each electrode and the surface of the collector-exchanger, a space defined by pins placed on the external faces of the two parts which constitute the main structure. This space, the thickness of which is between 0.8 and 2 mm, is filled by an open, non-conductive structure.

This non-conductive open structure consists of:

• or by an insulating polymeric material, the surface of which preferably has a hydrophobic character. • or by a foam, the core of which consists of a metallic material conducting heat, coated with an anticorrosion layer having a hydrophobic character

• or by a polymer mesh, open longitudinally. the surface of which preferably has a hydrophobic character.

According to another embodiment, the collector-exchanger according to the invention is characterized by the use of a main electronically and thermally conductive structure consisting of two parts, at least one of which comprises, in addition to the pins provided on one of their faces and used to define the space between the electrodes and the collector-exchanger, on their other face, cylindrical pins of height between 0.5 and 2 mm, having a diameter between 3 and 10 mm. The two parts juxtaposed by putting the pins face to face reveal a space intended for the circulation of the heat transfer fluid.

According to one of the characteristics of the invention, the cylindrical pins serving to provide a space intended for the circulation of the heat-transfer fluid are distributed uniformly over the surface of the collector-exchanger in numbers such that the total surface of their section occupies at least 40% of the apparent surface of the electrodes.

According to another characteristic of the invention, the cylindrical pins serving to provide a space intended for the circulation of the heat-transfer fluid are distributed uniformly over the surface of the collector-exchanger and in number such that the total surface of their section is less than 10% of the apparent surface of the electrodes. In this case, the space intended for the circulation of the heat-transfer fluid is filled with an open structure foam constituted by a metallic material good conductor of heat and electricity, the strands of which are in contact with the walls of the main structure. . The material of the foam is copper or nickel.

According to another characteristic of the invention, the walls in contact with the conductive foam électromquement and heat are coated with a thin electronically conductive layer such as ^a metal, a metal alloy or a conductive lacquer.

According to the second embodiment described, the metal contacts ensuring the discrete collection of charges and calories are needles which partially penetrate into the main structure, their end being located between 0.4 and 0.9 mm from the plane separating the two parts constituting the main structure. These needles penetrate into the electrodes with a depth of between 0.1 and 0.2 mm and are distributed uniformly in square, rectangular or triangular equilateral patterns.

Other characteristics and advantages of the present invention will appear more clearly on reading the description which follows, with reference to the appended figures 3 to 6 which represent respectively:

FIG. 3, a sectional view of the central part of a bipolar collector-heat exchanger according to a first embodiment, * FIG. 4, a section along the axis MM ′ of the bipolar collector-heat exchanger shown in figure 3,

FIG. 5, a sectional view of the central part of a bipolar collector-heat exchanger according to a second embodiment,

• Figure 6, a top view of the bipolar collector-heat exchanger shown in Figure 5.

The different figures are represented without taking into account the scales of the drawings.

The present invention results from a compromise between • minimization of ohmic drops, “101 lightening of the collectors,

• good heat exchange, • simplified manufacturing.

Two embodiments have been particularly studied, each giving rise to several variants.

First embodiment

FIG. 3 simply represents the central part of the collector-exchanger which is the subject of the present invention, excluding the margins where the fluid inlets and the seals are located.

The bipolar collector-heat exchanger essentially consists of two symmetrical parts E and E ^" whose juxtaposition along the plane MM 'forms the channels F. The material of these two main parts is a polymer-carbon composite whose resistivity is 0.1 Ω.cm. This is, for example, the material sold under the reference EMI 0683 by the company RTP France.

Furthermore, each of these main parts E and E 'comprises pins C and C' that ^s' based on the electrodes A and A 'leaving a free vein D or ^D'.

The veins D and D ', of thickness between 0.8 and 2 mm, are advantageously filled, as claimed in patent application FR 98 09236, by an open structure which acts as a distributor and homogenizer of the flows gaseous. In the case of a standard collector, the material constituting the open structure is, for example, a polymer whose surface is advantageously hydrophobic. In order to facilitate the transfer of calories to the central part of the manifold-exchangers, it is possible, without this being an obligation, to use for the filling of the veins D and D a metallic foam of low surface mass (20 to 30 mg / cm ") covered on all of its strands by a hydrophobic film which, moreover, prevents any corrosion of the material constituting the foam, the latter possibly being, for example, copper.

The height of the channels F is between 1 and 4 mm. These channels allow the circulation of a heat transfer fluid. In order to amplify the heat exchange between the heat transfer fluid and the collector-exchanger, a copper foam of low surface mass (20 to 50 mg / cm ² ) can be placed in the channels. By slight compression during assembly, the strands of the foam are in contact with the walls of the collector.

The thickness I of the solid part of each half-collector is between 0.5 and 1.5 mm.

The cylindrical pins C, and C, 'have a diameter of between 3 and 4 mm, their height being between 0.8 and 2 mm and their distribution is such that the total area of their section represents only a maximum of 3%. of the apparent surface of the electrode on which they bear (i.e. two to three pawns per 10 cm ² of electrode), which limits the occultation of the active surface.

Needles B, or B 'do not pass through the parts E and E' but s ^'stop at a distance between 0.4 and 0.9 mm from the plane M-M'. Their diameter is between 0.2 and 0.3 mm. They consist for example of stainless steel 316 L. The end which penetrates into ^the electrode over a length between 0.1 and 0.2 mm. is coated with a deposit preventing passivation and corrosion. Figure 4 shows the distribution of the needles B and B ': it is ^a question of a Switchgear rectangular or square patterns, the distance G between B, and B2 being generally greater than the distance H between Bi and B _α ι or B ₂ and B * ^

The width of the channels F is between 1 and 3 mm. The spacing between two channels is between 1.5 and 2.5 mm (1.5 mm when the width of the channel is 3 mm and 2.5 mm when it is only 1 mm). For a channel width of 2 mm, the spacing is 2 mm.

The position of the needles being in the middle of the spacing between two channels, it follows that the distance G between two needles (distance B, -B ₂ ) is between 3 and 4.5 mm. The distance H in the other axis (distance Bi and B _α ι) is determined as a function of the current density at the electrodes; it is between 2 and 4.5 mm.

The composite material parts are advantageously obtained by molding. As for the composite material, it is interesting to use the reference product EMI 0683 (from RTP France), which contains nickel-plated carbon fibers. In order to improve the connection at the interface between the two main parts, the latter can be surfaced or a deposit of a conductive lacquer or a metallic film deposited there.

The ^insertion of needles into the composite parts can be achieved by different means:

• insertion into holes already drilled, • hot nailing, • insertion during molding.

In the case of insertion into already drilled holes, the electrical contact between the needle and the composite can be improved by interposing a conductive powder or lacquer.

Second embodiment

FIG. 5 corresponds to the second type of embodiment of the invention and represents the central part of the bipolar-heat exchanger manifold, excluding the margins where the fluid inlets and the seals are located. Here again the collector-exchanger consists of two main parts E and E ^" . Made of the same electronic conductive composite material as that of the first embodiment, which are juxtaposed on a plane separating the pins J, and J, '.

These pins have a diameter of between 3 and 10 mm and their number is such that the surface of their section is at most equal to 8% of the apparent surface of the electrodes, their distribution being as uniform as possible. The height of the pins J, and J, 'is between 0.5 and 2 mm. Between these pins is a vein whose surface represents more than 90% of the surface of the electrodes and whose height K is between 1 and 4 mm.

As in the first exemplary embodiment described (see FIG. 3), the spaces D and D ′ where the gaseous reactants circulate are limited in their height by pawns L, and L, ^′ , the characteristics of which are identical to those given for the previous achievement.

Similarly, the spaces D and D 'are filled by an open structure which ensures the homogenization of the gas flows.

The essential difference between the two types of embodiment lies, for the second case, in the fact that the internal space allowing the heat exchanges occupies a much larger volume. Therefore, in order not only to ensure good circulation of the heat transfer fluid, good heat exchange, but also good electrical conductivity between the parts E and E ′, the central space is filled with a metallic foam whose strands are in good contact (by compression) with the surfaces of the parts E and E ', surfaces which, advantageously, can be coated with a metallic film such as copper or nickel.

As in the previous example, charge collection at the electrodes is ensured by stainless steel needles B, the tips of which, penetrating 0.1 to 0.2 mm into the electrodes A and A ', are coated with a deposit avoiding passivation and corrosion (see patent application FR 98 09236).

FIG. 6 represents a uniform distribution of the needles B "corresponding to a pattern of the equilateral triangle type. the distance between two needles, or side of the triangle, having a dimension between 2.5 and 5 mm. The counters L and L 'which ^s' based on the electrodes have a diameter such ^that they are placed between the needles.

In this configuration, the distance between the needles can therefore be shorter than in the previous example.

The distance O separating the point of the needles from the internal space is between 0.4 and 0.9 mm.

With regard to the characteristics of the materials used such as the means of implementation (insertion of the needles for example), the solutions are the same as those described for the first exemplary embodiment.

The invention is not limited to the two embodiments described above but embraces all the variants.

Claims

1 - bipolar collector-heat exchanger characterized in ^that it combines both: • a main structure in an electronically and thermally conductive material, • discreet collection of charges and calories provided by metallic contacts distributed uniformly on the surface of the electrodes and penetrating into the main structure but not passing right through, • the use of open non-conductive structures ensuring the homogenization of the flow of gaseous reactants. 2 - Bipolar collector-heat exchanger according to claim 1, characterized in that the main electronically and thermally conductive structure consists of two parts, at least one of which comprises, on one of its faces, grooves allowing the circulation of a coolant. 3 - Bipolar collector-heat exchanger according to claim 2, characterized in that, in the case where the two parts of the main structure include grooves on one of their faces, said faces are juxtaposed during their assembly to reveal channels allowing circulation of the heat transfer fluid. 4 - Bipolar collector-heat exchanger according to claim 1, characterized in that the metal contacts are metal needles which penetrate into the projecting parts included between the channels provided for the circulation of the heat transfer fluid, their end being located between 0.4 and 0.9 mm from the plane separating the two parts constituting the main structure. 5 - Bipolar collector-heat exchanger according to claim 4, characterized in that the needles penetrate into the electrodes with a depth of between 0.1 and 0.2 mm and are distributed uniformly in square or rectangular patterns. 6 - bipolar exchanger thermal collector according to claim 1, characterized in ^that it is provided between each electrode and the collector surface-exchanger, ^using pins placed on the outer faces of the two parts that constitute the main structure , a space, of which ^the thickness is between 0.8 and 2 mm and which is filled with a non-conductive open structure. 7 - Bipolar collector-heat exchanger according to claim 6. characterized in that the open non-conductive structure is a polymer foam whose surface preferably has a hydrophobic character. 8 - Bipolar collector-heat exchanger according to claim 6. characterized in that the open non-conductive structure is a foam whose core is constituted by a metallic material conductive of heat and which is coated with an anticorrosion layer having a character hydrophobic. 9 - Bipolar collector-heat exchanger according to claim 6. characterized in that the non-conductive open structure is constituted by a polymer mesh, open longitudinally and the surface of which preferably has a hydrophobic character. 10- Bipolar collector-heat exchanger according to claim 1. characterized in that the main electrically and thermally conductive structure consists of two parts, at least one of which comprises, in addition to the pins provided on one of their faces and used to define the space between the electrodes and the collector-exchanger. on their other face, cylindrical pins of height between 0.5 and 2 mm. having a diameter between 3 and 10 mm. The two juxtaposed parts reveal a space intended for the circulation of the heat transfer fluid. 11 - Bipolar collector-heat exchanger according to claim 10, characterized in that the cylindrical pins serving to provide a space intended for the circulation of the heat-transfer fluid are distributed uniformly on the surface of the collector-exchanger in number such that the total surface of their section occupies at least 40% of the apparent surface of the electrodes. 12 - Bipolar collector-heat exchanger according to claim 10, characterized in that the cylindrical pins serving to provide a space for the circulation of the heat transfer fluid are distributed uniformly on the surface of the collector-exchanger. in number such that the total area of their section is less than 10% of the apparent area of the electrodes. 13 - bipolar exchanger thermal collector according to claim 12, characterized in that ^the space for the circulation of the coolant is filled with an open structure foam consisting of a good conductive metal material of the heat and electricity which the strands are in contact with the walls of the main structure. 14 - Bipolar collector-heat exchanger according to claim 13, characterized in that the material of the foam is copper or nickel. 15 - Bipolar collector-heat exchanger according to claims 10 and 13, characterized in that the walls in contact with the foam are coated with a thin electronically conductive layer such as a metal, a metal alloy or a conductive lacquer. 16 - Bipolar collector-heat exchanger according to claims 1 and 10, characterized in that the metal contacts ensuring the discrete collection of charges and calories are metal needles which partially penetrate into the main structure, their end being located between 0.4 and 0.9 mm from the plane separating the two parts constituting the main structure. 17 - Bipolar collector-heat exchanger according to claim 12, characterized in that the needles penetrate into the electrodes with a depth of between 0.1 and 0.2 mm and are distributed uniformly in square, rectangular or triangular equilateral patterns.