FR2764122A1 - BIPOLAR PLATE FOR FUEL CELL ASSEMBLY - Google Patents

Abstract

A fuel cell assembly has bipolar thermoplastic plates sandwiched between a number of electrode assemblies (30, 30 ') to form a stack. A thermoplastic bipolar plate (35) is placed alternately with a set of electrodes (30) so that one side (31) of the plate is adjacent and bonded by adhesion to the corresponding surface of an anode of the assembly of electrodes (34), and the other side (39) is adjacent to the corresponding surface of a cathode (32) in the neighboring electrode assembly (30 '). The adhesion bond also serves to seal the fuel and oxidant grooves (37).

Description

Priority: no 08/868 330 of 03.06.1997 Motorola, Inc. Title Plaque

  Technical Field The present invention generally relates to fuel cells and more particularly to a bipolar plate for fuel cells. Background of the invention Fuel cells are electrochemical cells in which a change in free energy resulting from the oxidation reaction of the fuel is converted into electrical energy. As shown in FIG. 1, a typical fuel cell 10 is composed of a fuel electrode (anode) 12 and an oxidant electrode (cathode) 14, separated by an ion-conducting electrolyte 16. The electrodes are electrically connected via a load (such as an electronic circuit) 19 by an external circuit conductor. In the circuit conductor, the electric current is transported by a flow of electrons, while in the electrolyte it is transported by the flow of ions, for example hydrogen ion (H +) in acid electrolytes, or the hydroxyl ion (OH-) in alkaline electrolytes. In theory, any substance capable of chemical oxidation which can be supplied continuously (such as a gas or a fluid) can be galvanically oxidized, like fuel 11 at the anode 12 of a fuel cell. Likewise, the oxidant 13 can be any material which can be reduced at a sufficient speed. In specialized systems, the two reactants can be liquids, such as hydrazine for fuel and peroxide

  hydrogen or nitric acid for the oxidant.

  Hydrogen gas has become the preferred fuel for most applications because of its high reactivity in the presence of suitable catalysts and because of its high energy density when stored as a cryogenic liquid, for example for use in space. Similarly, at the cathode of fuel cell 14, the most common oxidant is gaseous oxygen, which is easily and inexpensively found in air for fuel cells used in terrestrial applications. When hydrogen and gaseous oxygen are used as fuel and oxidizer, the electrodes are porous and allow the gas-electrolyte junction to be optimal. The electrodes must be electronic conductors, and have the appropriate reactivity to give high reaction rates. The batteries

  the most common fuels are hydrogen-

  oxygen and use an acid electrolyte. At the anode 12, the incoming hydrogen gas 11 ionizes to produce electrons and hydrogen ions. Since the electrolyte is a non-electronic conductor, the electrons exit the anode via the metallic external circuit. At the cathode 14, the oxygen gas 13 reacts with the hydrogen ions which migrate in the electrolyte 16 and the incoming electrons coming from the external circuit to produce water as a derived product. Depending on the operating temperature of the cell, the water produced can enter the electrolyte, which dilutes it and increases its volume, or can be extracted through the cathode as vapor. The overall reaction which takes place in the fuel cell corresponds to the sum of the reactions at the anode and at the cathode; in this case, the combination of hydrogen and oxygen produces water, part of the free energy of the reaction being released directly in the form of electrical energy. The difference between this available free energy and the heat of the reaction is produced as heat at the temperature of the fuel cell. In all cases, it can be seen that as long as hydrogen and oxygen are supplied to the fuel cell, the flow of electric current is supported by the electronic flow in the circuit.

  external and ionic flux in the electrolyte.

  In practice, a number of fuel cells are normally stacked or grouped together to form a set of fuel cells. Referring now to Figure 2, the anode / electrolyte / cathode subunit is generally referred to as an "electrode assembly" (EE). The cathode 24 of a first EE 20 is generally placed near the anode 22 of a next EE 20 ', but separated from it by a bipolar plate 25. In the prior art, the bipolar plate is generally made of carbon. , chosen for its unique combination of properties: chemical inertness, electrical conductivity, rigidity and ability to be machined. A network of grooves 27 is generally formed in the bipolar plate by mechanical machining. These grooves or grooves ensure the distribution of the oxidant or of the liquid or gaseous fuel to the cathode and to the anode, respectively. The bipolar plate provides an electrical connection between one EE and the next, and also serves to isolate the anode fuel from the oxidant of the cathode in the adjacent EEs. In order to further contain the fuel and keep it separate from the oxidant, a sealing means 28, such as an O-ring or other external seal, must be provided. As can be easily understood, the cost of manufacturing the carbon bipolar plate and then assembling it into a set of fuel cells is considerable, due to the materials and work involved. As a result, this is one of the factors preventing fuel cell technology from spreading. A low cost bipolar plate would be a

more in the field.

Brief description of the drawings

  Figure 1 is a schematic representation of a conventional fuel cell, as put in

  practice in the prior art.

  Figure 2 is a schematic sectional view of a set of fuel cells, as set

  practice in the prior art.

  Figure 3 is a schematic sectional view of a

  set of fuel cells according to the invention.

  Figure 4 is a schematic sectional view of another embodiment of a set of batteries to

fuel according to the invention.

  Figure 5 is a schematic sectional view of another embodiment of a set of batteries to

fuel according to the invention.

  Detailed description of the preferred embodiment

  A fuel cell assembly has thermoplastic bipolar plates sandwiched between a number of electrode assemblies to form a stack. A thermoplastic bipolar plate is alternately placed with an electrode assembly so that one side of the plate is adjacent to the corresponding surface of an anode of the electrode assembly, and the other side is adjacent to the corresponding surface of a cathode in the neighboring electrode assembly. The thermoplastic bipolar plate is bonded and adheres to each of the adjacent electrode assemblies, sealing the grooves containing the oxidant and the fuel while eliminating

  the need for additional seals.

  Referring now to Figure 3, a fuel cell assembly is composed of a fuel cell stack containing more than one electrode assembly 30, 30 ', 30 ". As used in the

  context of this description and in the literature in

  In general, a set of electrodes (EE) or set of membrane electrodes (EEM) is a unit cell composed of an anode 32, a cathode 34 and an electrolyte 36. When a plurality of these cells units are interconnected, they are referred to as fuel cell stacking or a set of fuel cells. In the preferred embodiment, the electrolyte is a polymer electrolyte membrane (PEM), such as that generally used in a hydrogen fuel cell, a direct methanol PEM cell or a PEM fuel cell using a organic fuel such as ethanol or formaldehyde. PEMs are ionic polymers with high ionic conductivity. The polymeric nature of PEMs makes them easier to handle than liquid electrolytes. The physical construction of the electrochemical cell is considerably simplified since finished joints and containment systems do not necessarily have to contain corrosive liquid electrolytes. A PEM must have the following properties: (1) high ionic conductivity, (2) zero electronic conductivity, (3) very low gas permeability, (4) chemical stability at operating temperature, (5) mechanical resistance, (6 ) low sensitivity to humidity, and (7) compatibility with the catalyst. Fuel cells using PEMs are described and known in the literature, for example in U.S. Patent No. 5,403,675 and since a skilled person is expected to be familiar with PEM cells, PEMs will not be detailed by the following. Between each of the unit cells of the stack is a bipolar thermoplastic and electrically conductive plate providing electrical conductivity between the cathode 34 of a fuel cell 30 and the anode 32 of the neighboring fuel cell 30 '. Each cathode of the stack is isolated from the previous anode by the bipolar plate. A fuel cell stack can be created by placing the unit cells alternately with the bipolar plates so that n (n being an integer greater than 2) sets of electrodes are combined with nl bipolar plates to create the stack fuel cells. Obviously, a person skilled in the art will understand that the stack of fuel cells must also contain end caps above the outermost electrodes, which have not been shown on the

  Figures 2 to 4 for clarity.

  The thermoplastic bipolar plate is made electrically conductive by any number

  methods known to those skilled in the art.

  For example, an electrically conductive filler such as carbon powder, carbon fiber or metallic particles in the form of powder or flakes (for example titanium, aluminum, stainless steel, l (silver, gold, etc.), can be added to the thermoplastic. In addition, the thermoplastic bipolar plate can be made electrically conductive by coating the surface with a thin film of a conductive material such as carbon, gold, nickel, titanium, silver, platinum, palladium. , chromium or rhodium, as is well known in the techniques of vacuum deposition of thin films and of electroplating. In this case, the edges of the bipolar plate must also be treated to ensure that the plate is capable of conducting the electrical charge between the cathodes and the neighboring anodes. If the outside of the plastic is completely metallized, the conductive path goes from the anode side to the cathode side, turning around the plate, and does not pass through the plastic. In other words, the two sides or faces of the bipolar plate are electrically connected via the

metallization of the edges.

  To return to the choice of materials, the thermoplastic bipolar plate can be any number of plastics such as basic thermoplastics such as polyethylene, polypropylene or polyacrylate or else can be a thermoplastic

  processed such as polycarbonate, acrylonitrile-

  butadiene-styrene (ABS), polyetherimide, polyimide or polyamide. In general, lower cost base thermoplastics are preferred since they will give a lower cost set of fuel cells, however, other performance criteria such as temperature or chemical resistance may prompt the need for a more efficient material. Processing these materials to make them electrically conductive can be easily accomplished by filling or

  laying a coating, as explained above.

  A plurality of grooves or grooves 37 is formed on the surface of the thermoplastic bipolar plate to ensure the distribution of gas to the anode and to the cathode. Although these grooves are generally formed in the carbon plates by mechanical machining, it has been discovered that, when thermoplastics are used, they can be formed in the surface by various more efficient and less costly methods, for example by stamping. , molding, thermoforming, photolithography using an active photographic layer, micro-machining or incorporation

  a stainless steel screen in the surface.

  Shimshon Gottesfeld et al., Of Los Alamos National Laboratories have demonstrated the effectiveness of a process of embedding a mesh matrix in a

  material for forming grooves or pockets.

  When a thermoplastic material with a relatively low melt index, such as polyethylene or polypropylene, is used as a bipolar plate, the plate can then be easily laminated directly on the EEM by applying heat and pressure to melt it. on cathode 34 or anode 32. This fusion process eliminates the need for external seals and serves to seal the gas distribution grooves 37 to each other, which provides integrity and mechanical rigidity to the stacking of fuel cells and allows to seal it in a single step. This reduces the cost and size of fuel cell stacking by eliminating seals, the cost and size of neighboring elements, and eliminates the need for expensive machined carbon bricks. According to another embodiment of the invention, as shown in FIG. 4, the bipolar plate is a thermoplastic with a higher melting point, and a thin layer of a thermoplastic material 48 which has a relatively low melt index. , such as polyethylene or polypropylene, is applied to the plate. The polyethylene or polypropylene is then used to laminate the EEM on the bipolar plate by applying heat and pressure to make it melt on the cathode 34 or the anode 32. Care must be taken not to block the distribution grooves gas with the thin layer of bonding thermoplastic. In addition, the electrical conductivity must be maintained between an electrode and the bipolar plate, so that the thin layer of thermoplastic material is preserved. In addition, the thin layer of thermoplastic material 48 can, if desired, be made electrically conductive by filling it with conductive particles in a manner similar to that used to make the conductive.

base plate.

  According to another embodiment of the invention, an adhesive can be used in place of the thin layer of thermoplastic material to ensure the necessary mechanical connection between the bipolar plate and the EEM. For example, grade B epoxy resin can be placed on the surface of the bipolar plate, and the plate can be adhesively placed on the EEM using heat and pressure to cure the epoxy resin. Hot melt adhesives are also useful as adhesive agents and can be placed on the surface in different ways and then assembled to form the stack. The reference number 48 in Figure 4, although previously described as a thin layer of thermoplastic material, is also used to show how the hot melt epoxy resin or adhesive is applied to the bipolar plate / EEM stacking. If desired, if a grade B epoxy resin is used, the gas distribution grooves can be formed in the adhesive layer using mechanical techniques

or photolithography.

  Referring now to Figure 5, the bipolar thermoplastic plate can also be manufactured in a form other than planar. The very nature of the thermoplastic material makes it possible to create bipolar plates of different and varied shapes. For example, the plate 55 can be constructed so as to

  present a cavity or recess 52 on one side.

  The interior of the recess 52 is made conductive, for example by coating the bottom surface of the recess with a conductive metal (electrodeposition or deposition under vacuum). The opposite side is also made conductive. In order to assemble the fuel cell stack, the EEMs 50 are placed inside the recesses 52 so that one electrode of each EEM

  touches the conductive coating in the recess.

  The electrode is preferably linked to the EEM in a manner described above. These sub-assemblies are then stacked on top of each other to create a stack shown in FIG. 5, a thermoplastic bipolar plate being linked directly to another bipolar plate by melting a non-metallic part 51 near the edges of the bipolar plate . This seals the gas tightly and keeps the assembly assembled without the need for external seals or fasteners. Thus, fuel cell stacking is achieved by combining a plurality of EEMs (50, 50 ', 50 ") with a plurality of plates

  shaped thermoplastic bipolar (55, 55 ', 55 ").

  In summary, a set of PEM fuel cells has been described and uses a bipolar thermoplastic plate to connect the individual EEMs. The bipolar plate uses low cost materials and is bonded by adhesion to EEM, which eliminates the need for gaskets and other means of sealing. The overall size is reduced as well

than its cost.

  Although the preferred embodiments of the invention have been illustrated and described, it will be readily understood that the invention is not limited thereto and that variants will come to mind of those skilled in the art, these should remain in the spirit and in the scope of this

  invention as described in the claims

attached.

Claims (5)

  1. A fuel cell assembly, comprising: two sets of electrodes having an anode and a cathode separated by an electrolyte; a bipolar plate, comprising an electroconductive thermoplastic polymer substrate having two opposite main faces, a plurality of grooves being formed on each of the faces; and the bipolar plate disposed between the two sets of electrodes so that a main face is adjacent and linked by adhesion to a main surface of the anode of the first set of electrodes and the other main face is adjacent and linked by adhesion to a main surface of the cathode of second set of electrodes.   2. Fuel cell assembly according to claim 1, further comprising an additional layer of thermoplastic located between the main face and the cathode, said layer being melted   by heating on the bipolar plate and the cathode.   3. Fuel cell assembly according to claim 1, further comprising a layer of adhesive situated between the main face and the cathode, said layer of adhesive bonding the plate. bipolar at the cathode.   4. The fuel cell assembly according to claim 1, wherein the electrolyte is a polymer electrolyte membrane.   5. A fuel cell assembly, comprising: a plurality of n sets of membrane electrodes each having an anode and a cathode separated by a polymer electrolyte membrane; a plurality of n-1 bipolar plates, each plate comprising: a thermoplastic polymer substrate selected from the group consisting of polyethylene, polypropylene, polycarbonate, acrylonitrile-butadiene-styrene, polyetherimide, polyimide, polyamide and polyacrylate; the substrate having two opposite main faces having a plurality of fuel grooves formed on one side and a plurality of oxidant grooves formed on the other side; and the thermoplastic polymer substrate made electroconductive by filling it with a metal or a carbon filling material; the plurality of n-1 bipolar plates and the plurality of sets of membrane electrodes assembled in a stack so that a bipolar plate is placed alternately with a set of membrane electrodes, each bipolar plate being linked by adhesion to two sets of corresponding membrane electrodes, said adhesive bond also serving to seal the plurality of grooves   fuel and the plurality of oxidant grooves.