DE69804019T2 - CONTINUOUS METHOD FOR PRODUCING A LAMINATED ELECTROLYTE ELECTRODE ARRANGEMENT - Google Patents

Description

Field of the invention

The present invention relates to an improved method for producing a continuous, laminated multilayer electrolyte and an electrode structure ("laminate structure") for an electrochemical fuel cell.

Background of the invention

Electrochemical fuel cells convert fuel and oxidizer into electricity and reaction product. Solid polymer electrochemical fuel cells generally use a membrane electrode assembly ("MEA") in which an electrolyte in the form of an ion exchange membrane is sandwiched between two electrode layers. The electrode layers are made of porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. In a typical MEA, the electrode layers provide structural support for the membrane, which is typically thin and flexible.

The MEA contains an electrocatalyst, typically comprising finely crushed platinum particles arranged in a layer at each membrane/electrode layer junction to induce the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes via an external load.

During operation of the fuel cell, the fuel at the anode penetrates the porous electrode layer and reacts at the electrocatalyst layer to generate protons and electrons. The protons migrate through the ion exchange membrane to the cathode. At the cathode, the oxygen-containing gas supply penetrates the porous electrode material and reacts with the protons at the cathode electrocatalyst layer to form water as a reaction product.

In conventional fuel cells, the MEA is disposed between two electrically conductive plates, each typically having at least one flow channel formed therein. The flow channels direct the fuel and oxidant to the respective electrodes, namely the anode on the fuel side and the cathode on the oxidant side. In a single cell assembly, fluid flow field plates are provided on each of the anode and cathode sides. The fluid flow field plates act as current collectors, provide fuel and oxidant to the respective anode and cathode catalytic surfaces, and provide channels for the removal of exhaust fluid streams.

One known method for manufacturing an MEA for use in an electrochemical fuel cell is to use heat pressing to bond the MEA components together. A disadvantage of this method is that heat pressing the entire structure, which includes the porous electrode layers, the catalyst material, and the solid polymer electrolyte, subjects each of these components to undesirable mechanical and thermal stresses. Such mechanical and thermal stresses can reduce the performance and lifetime of an MEA in an operating fuel cell. Another disadvantage relates to the applicability of this known method for mass production. The heat pressing process is typically a discontinuous or "burst" process. While the press is heated, a layered structure, which includes electrodes, catalyst layers, and a solid ion exchange membrane, is typically inserted into a press, pressed therein, and subsequently removed from the press. This traditional intermittent procedure involves inefficient, costly and time-consuming process control.

The German patent DE 195 09 748 C2 discloses a generic process for producing a composite laminate comprising an electrode material, a catalyst material and a solid electrolyte material. The Component materials are placed on an electrostatically charged surface, and an external heater heats the solid electrolyte material until the top side of the electrolyte material softens. While the top side of the electrolyte material is still soft, it is applied to the catalyst material under pressure to bond the catalyst material to the polymer electrolyte. After bonding is effected, the composite laminate is removed from the surface. One problem with this process is that the dimensions of the electrostatically charged surface limit the size of the MEA produced. This process cannot produce a continuous sheet of surface.

Accordingly, there is a need for a continuous process for manufacturing a continuous laminated electrolyte electrode assembly. A continuous process is desirable to improve efficiency by increasing productivity and speed of the manufacturing process, thereby reducing manufacturing costs.

Summary of the invention

DE 195 09 749 A1 discloses a continuous process for producing a continuous multilayer laminate structure for an electrochemical cell, the process comprising applying a catalyst powder by rolling it onto an electrode substrate layer and subsequently laminating a membrane layer thereon. The catalyst layer comprises a mixture of electrode material, catalyst material and solid electrolyte material, and in the lamination step the surface of the catalyst layer is softened by heating before the membrane layer is applied thereon and the structure is passed through a gap defined by two laminating rollers.

WO 97/23919 A1 discloses another method of manufacturing a continuous multilayer laminate structure for an electrochemical cell, the method comprising coating a preformed membrane layer and/or a preformed electrode substrate layer with a catalyst layer by spraying, dipping or wetting and then laminating such a preformed electrode substrate layer on one or both sides of the membrane layer with the applied catalyst coating in between.

The problem underlying the present invention is to provide an improved continuous manufacturing process for producing a continuous multilayer laminate structure for use in electrochemical energy converters, in particular fuel cells, but also electrolyzers and storage media (for example double-layer catalysts).

This problem is solved by providing a continuous process for producing a continuous multilayer laminate structure for an electrochemical cell according to claim 1.

The laminated structure thus produced includes first and second preformed electrode layers, an electrolyte layer interposed between the first and second electrode layers, a first catalyst layer interposed between the first electrode layer and a first major surface of the electrolyte layer, and a second catalyst interposed between the second electrode layer and a second major surface of the electrolyte layer. The electrolyte layer includes an ionic electrolyte or an electrolyte precursor that can be converted to an ionic form suitable as an electrolyte. The method includes forming at least one of the catalyst and electrolyte layers as an extrusion layer and using the layer as a laminating medium to bond the same to the two immediately adjacent layers of the laminate structure provided as preformed layers, thereby bonding these two preformed layers of the laminate structure together. The in-situ fabricated layer preferably acts as the laminating medium while still being soft or in a malleable state when the layers of the laminate structure are pressed together.

In one embodiment of the method, the first and second catalyst layers as well as the electrolyte layer are all created in-situ by a three-layer extrusion. The opposing major surfaces of the three-layer extrusion act as the laminating media for bonding the three-layer extrusion to the immediately adjacent, prefabricated electrode layers. The three-layer extrusion can be produced by a multi-channel single-slot extrusion die. Suitable catalyst mixtures and electrolyte materials (or electrolyte precursor materials) can be continuously fed into separate inlet chambers of an extrusion setup. These materials are preferably in the form of granules or a paste. Each of the inlet chambers is fluidly connected to one of the channels of the extrusion die.

The material selected for the electrolyte layer is suitable as an ion exchange layer in an electrochemical cell or is a precursor electrolyte material that is transformable into an ion exchange layer. For example, a precursor material can be converted from a non-ionic to an ionic form after extrusion using known methods such as hydrolysis.

The manufactured electrolyte layer preferably has the following properties:

- essentially impermeable to reactant fluids;

- ionic conductors;

- electrical insulator; and

- essentially inert in an electrochemical fuel cell environment.

In the laminate structure, the electrolyte layer may take a solid form or remain in the form of a gel or paste. The electrolyte material is preferably polymeric, but it may also be non-polymeric as long as it has the properties identified above. Some examples of some preferred electrolyte materials are perfluorosulfonic acid based materials such as Nafion™, polyetheretherketoneketone (poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene) or PEEKK) based ion-conducting polymers, sulfonated polytrifluorostyrene based ion-conducting polymers such as BAM™ polymer.

The catalyst mixture may include catalyst particles and a binding material. The binding material is preferably compatible with bonding with the electrolyte layer and may be the same material as that used in the electrolyte layer. The catalyst mixture may also contain additives which improve the desired properties of the catalyst layer, such as fluid transport properties, permeability, ionic conductivity and electrical conductivity. Using the described method, it is possible to use different catalyst mixture compositions for the two catalyst layers by introducing different catalyst mixtures into the separate extrusion buildup chambers.

After the co-extrusion step, the three-layer extrusion is continuously applied between two pre-fabricated electrode layers. The electrode layers may be continuous strips of material fed from rolls. The electrode layers preferably comprise flexible carbon fabric or carbon paper sheet material with a longitudinal tensile strength of not less than 30 N/50 mm. The three-layer extrusion is thus inserted between and bonded to the pre-fabricated electrode layers. The laminate structure is preferably passed between opposing calender rolls which press the layers together. The product of the process is a continuous laminate structure which can be wound onto a roll for storage or fed to further manufacturing processes such as a cutting machine to cut the laminate structure into the desired size and shape.

In a second embodiment of the process, only the catalyst layers are generated in-situ and these serve as the laminating medium. In this embodiment, the electrolyte and electrode layers preferably consist of prefabricated layers that are dispensed from rolls. The anode catalyst and cathode catalyst mixtures are continuously extruded and introduced between the electrolyte layer and the respective porous electrode layers.

The laminate structure is preferably passed between at least one pair of calender rolls to press the layers together. The insertion of the catalyst layers can be carried out sequentially or simultaneously. If the insertion is carried out sequentially, After inserting each catalyst layer, the laminate structure can be passed through a laminating machine with a pair of calender rolls.

In the second embodiment, the process preferably uses a tandem extrusion assembly having separate chambers and extrusion nozzles for the first and second catalyst mixtures. The first and second catalyst mixtures may be different in composition and are preferably fed into the extrusion assembly in granular or paste form.

In a third embodiment, the electrolyte layer serves as the laminating medium. The anode and cathode catalyst mixture are first applied to respective anode and cathode layers. Alternatively, the prefabricated electrodes may be precoated with a catalyst layer. There are several known methods for applying the catalyst layers to the electrode layers. For example, there are known methods for spraying, screen printing or rolling the catalyst onto the electrode layers.

The electrolyte layer is extruded and created in-situ between the coated anode and cathode layers. The laminate assembly is then passed through a pair of calender rolls to press the layers together, thus completing the lamination procedure. The method may further include the step of treating the laminate assembly to convert the precursor electrolyte material to an ionic form, in the same manner as disclosed with respect to the first embodiment of the method.

There are several advantages to using the described method. For example, one advantage of using at least one of the layers of the laminate structure as the laminating medium is that there is no need for additional steps to bond the individual layers together (i.e. no need for a heat-pressing process or the application and later removal of solvents or the use of adhesives).

An advantage of using an extrusion process is that an extrusion layer thickness between 2 µm and 250 µm is possible. Accordingly, a catalyst layer thickness of ≤ 10 µm can be chosen to reduce the amount of expensive precious metal used. Furthermore, the co-extrusion step can be designed to prevent any significant mixing of the electrolyte and catalyst layers of the laminate structure.

Yet another advantage of the present process is the production speeds that can be achieved. For similar processes, production speeds of up to 400 m/min are achievable. Although the three embodiments described involve extruding the laminating medium, other methods for producing coating layers are also contemplated as methods for producing the laminating medium in-situ, such as spray coating, brush coating and rolling.

Short description of the drawings

The advantages, nature and additional features of the invention will become more apparent from the following description taken in conjunction with the accompanying drawings in which:

Fig. 1 schematically illustrates the manufacture of a laminate structure according to a first embodiment of an improved continuous manufacturing process;

Fig. 2 schematically illustrates the manufacture of a laminate structure according to a second embodiment of an improved continuous manufacturing process; and

Fig. 3 schematically illustrates the manufacture of a laminate structure according to a third embodiment of an improved continuous manufacturing process.

Detailed description of preferred embodiments

The invention relates to improved methods for producing a laminate structure for use in electrochemical fuel cells. The method involves using at least one of the electrolyte or catalyst layers as a laminating medium for the laminate structure. The method preferably uses a calendering process combined with an extrusion process to produce a laminate structure in a continuous layer.

In a first embodiment shown in Figure 1, a co-extrusion assembly 10 provides a three-layer electrolyte catalyst extrusion sandwiched between two electrode layers 15, 16 that are drawn off respective rollers 15a, 16a.

The co-extrusion structure 10 is equipped with three inlets 11, 12, 13 for receiving catalyst and electrolyte materials, which may be in granular or paste form. In practice, the central inlet 13 is preferably loaded with a precursor of an ionic electrolyte (for example, non-ionomeric polymer granules). The outer inlets 11, 12 are loaded with suitable granules containing the anode catalyst and the cathode catalyst, respectively. The anode catalyst may differ in composition from the cathode catalyst. Instead of granules, the outer inlets 11, 12 may be loaded with a paste mixture of catalyst and, for example, polymer electrolyte solution, which is extrudable and can serve as the laminating medium.

Inside the co-extrusion assembly 10 there are three screw conveyors (not shown) and a multi-channel single slot extrusion die 14 from which the co-extruded three-layer extrusion exits, which includes the anode and cathode catalyst layers with a polymer electrolyte layer disposed therebetween.

The three-layer extrusion exiting the extrusion nozzle 14 is introduced between two electrode layers 15, 16, while the anode and cathode catalyst layer from the extrusion process continues are soft. The electrode layers 15, 16 may consist of flexible, non-woven carbon fiber paper, carbon fiber fabric or any other porous, electrically conductive material.

The three-layer extrusion sandwiched between the electrode layers 15, 16 is fed to a laminating station 17 including at least one pair of calender rolls. The laminating station 17 laminates the three-layer extrusion to the electrode layers 15, 16, producing a continuous laminate build-up layer 18 that can be wound onto a storage roll 19.

In a second embodiment, illustrated by Figure 2, the process uses a tandem extrusion setup 20 that introduces catalyst layers between a prefabricated membrane electrolyte layer 23 and prefabricated electrode layers in two separate lamination stages.

The tandem extrusion structure 20 is equipped with two inlets 21, 22. Each inlet 21, 22 supplies one extruder 28, 29 with anode catalyst and cathode catalyst granules.

In this second embodiment, the anode catalyst layer is extruded in the extruder 28 between the electrode layer 24 and the membrane electrolyte 23, which is drawn off from a storage roll. The anode catalyst layer inserted between the electrode layer 24 and the membrane electrolyte 23 is fed to a laminating station 25 and laminated therein.

The laminated layer produced by the laminating station 25 is fed to a second laminating station 27 together with a second electrode layer and the cathode catalyst layer interposed by the extruder 29. The laminate leaving the laminating station 27 is a continuous laminate build-up layer made according to the invention and can be fed to a storage roll (not shown).

Each of the laminating stations 25 and 27 includes at least one pair of calender rolls 26 for pressing the component layers together with the laminating medium interposed therebetween.

In this second embodiment, the two catalyst layers serve as a laminating medium between the prefabricated electrode layers and the centrally arranged, prefabricated membrane electrolyte 23.

The pellets fed into the inlets 21 and 22 of the extrusion assembly preferably contain a mixture of catalyst and polymer materials. For example, if the catalyst is platinum (Pt), the pellet mixture should contain between 1% Pt and 40% Pt, with the desired catalyst loading being 0.1 mg/cm2 Pt. The remainder of the pellet mixture may contain precursors for a polymer electrolyte, electrically conductive particles and other additives such as pore-forming agents. For different catalytic materials, different proportions may be preferred depending on the desired loading. In a preferred embodiment, when using Pt, this percentage is approximately 10% Pt to obtain a loading between 0.1 mg/cm² Pt to 4.0 mg/cm² Pt for the catalyst layers, preferably 1.0 mg/cm².

The extrusion process may involve heating and melting the granular materials. Therefore, to practice the process of heat extruding the precursor catalyst and/or electrolyte layer, the granular materials preferably do not contain any volatile components to avoid hazardous conditions. For polymer electrolytes and their precursors, suitable process temperatures typically range between 150ºC and 350ºC. More commonly, the preferred process temperature range to avoid degradation of typical polymer electrolyte materials is between about 180ºC and 185ºC.

Alternatively, a cold extrusion process can be used in which the extruded material is in the form of a paste or an emulsion mixture.

In a third embodiment, illustrated by Fig. 3, the method of the invention involves first applying the catalyst layers to electrode layers 33, 34. The catalyst coated electrode layers 33, 34 are then laminated to the electrolyte layer, with the electrolyte layer being generated in-situ and acting as the laminating medium.

As shown by Fig. 3, two prefabricated electrode layers 33 and 34 are pulled from respective stock rolls. The electrode layers 33 and 34 then pass through respective coating or priming stations 30 and 31 in which a respective anode or cathode catalyst layer is applied to the electrode layers 33 and 34. The coating stations 30 and 31 are known to those skilled in the art and are each indicated only by three arrows to simplify the illustration of this known procedure. The coating stations 30 and 31 may include, for example, roller application, spray application, screen printing or other known methods. The coating stations 30 and 31 may also include a secondary drying step.

After coating the electrode layers 33 and 34 with the respective catalyst layer, an electrolyte material or a precursor material for the electrolyte is extruded between the catalyst coated electrode layers 33 and 34. The laminate assembly is then fed to a laminating station 35 where the assembly is laminated together. The laminating station 35 includes at least one pair of calender rolls which press the component layers together with the laminating medium interposed therebetween. In this embodiment, the electrolyte layer 32, typically an electrolyte precursor material, acts as the laminating medium. The laminate assembly can then be fed to a storage roll 37 or further manufacturing processes.

In any of the embodiments of the method, one of the further manufacturing processes may include the step of converting a non-ionic electrolyte precursor material into an ionic electrolyte form using known methods, such as hydrolysis.

Furthermore, in some cases it is desirable to produce the anode catalyst with a different composition than the cathode catalyst in order to improve the efficiency of the respective catalyst, for example for use with different fuels or oxidants in a fuel cell.

Since the electrode layers are dispensed from and stored on rolls and kept tensioned by the lamination process, in preferred embodiments it is desirable that the electrode layers are formed from flexible, tear-resistant carbon material, such as carbon fiber paper or fabric. The desired tear force for the electrode layer material depends on the tension and the winding speed of the lamination machine. For lamination machines operating at production speeds of up to 400 m/min, the longitudinal tear force of flexible carbon fiber paper or fabric used as electrode layers is preferably not less than 30 N/50 mm.

It will be understood by those skilled in the art that the term "continuous" in reference to the process describes a process, such as a roll-to-roll process, that produces a continuous layer that can be wound onto a roll. The laminate structure is later cut into the desired shape and size for use in electrochemical cells. Of course, when materials for some of the layers are prefabricated and fed from rolls, or when the finished laminate structure is wound onto a roll, it may be necessary to interrupt such a continuous process periodically to switch rolls.

The calender rolls used in the lamination step may also be provided with means for forming apertures in the laminate structure. For example, one of the calender rolls may have cutting blades for forming an aperture to receive an internal manifold and/or an internal pull rod in a fuel cell stack.

Claims (18)

1. A continuous process for producing a continuous multilayer laminate structure for an electrochemical cell, the laminate structure comprising: a first and a second prefabricated electrode substrate layer; an electrolyte layer interposed between the first and second electrode substrate layers, the electrolyte layer including an electrolyte or an electrolyte precursor; a first catalyst layer interposed between the first electrode substrate layer and a first main surface of the electrolyte layer; and a second catalyst layer interposed between the second electrode substrate layer and a second major surface of the electrolyte layer; the method comprising: Forming at least one of the electrolyte layer and the catalyst layers as an extrusion layer by extruding a flowable material and laminating two preformed layers with the extruded layer therebetween as a laminating medium, wherein the preformed layers are selected from preformed ones of the electrode substrate layers and the electrolyte layer. 2. The method of claim 1, wherein the first catalyst layer and the second catalyst layer contain different catalyst compositions. 3. The method of claim 1, wherein the first and second prefabricated electrode substrate layers comprise a flexible carbon cloth or carbon paper. 4. The method according to claim 1, wherein the electrode substrate layers have a longitudinal fracture load of not less than 30 N/50 mm. 5. The method of claim 1, further comprising the step of winding the laminated structure onto a roll. 6. The method of claim 1, wherein the step of forming at least one of the electrolyte layer and the catalyst layers includes co-extruding a three-layer extrusion including the first and second catalyst layers and the electrolyte layer interposed therebetween, the three-layer extrusion being the laminating medium between the pre-formed first and second electrode substrate layers. 7. The method of claim 6, further comprising passing the multilayer laminate structure through opposing calender rolls which compress the layers together. 8. The method of claim 6, wherein at least one of the first catalyst layer, the electrolyte layer and the second catalyst layer is formed from a granular or paste material that is fed into a co-extrusion assembly. 9. The method of claim 8, wherein the co-extrusion assembly includes a multi-channel single slot extrusion die for producing the three-layer extrusion. 10. The method of claim 8, wherein each of the first catalyst layer, the electrolyte layer and the second catalyst layer are made from respective granular materials which are each fed into corresponding inlets of the co-extrusion structure. 11. The method of claim 1, wherein the electrolyte layer is prefabricated, the first catalyst layer is extruded between a first major surface of the prefabricated electrolyte layer and the first electrode substrate layer, and the second catalyst layer is extruded between a second major surface of the prefabricated electrolyte layer and the second electrode substrate layer. 12. The method of claim 11, wherein the method further comprises the steps of: Extruding a first catalyst mixture to form a first catalyst layer between the first electrode substrate layer and the first major surface of the electrolyte layer; passing the first electrode substrate layer, the first catalyst layer and the electrolyte layer between a pair of calender rolls; Extruding a second catalyst mixture to form a second catalyst layer between the first electrode substrate layer and the first major surface of the electrolyte layer; and Passing the second electrode substrate layer, the second catalyst layer and the electrolyte layer between a pair of calender rolls. 13. The method of claim 11, further comprising the step of introducing the first catalyst mixture and/or the second catalyst mixture into an extrusion assembly as a granular material. 14. The method of claim 11, wherein the first catalyst mixture and the second catalyst mixture are fed as granules into separate inlets of a tandem extrusion setup. 15. The method of claim 1, wherein the step of forming at least one of the electrolyte layer and the catalyst layers includes extruding the electrolyte layer, and the electrolyte layer is the laminating medium between immediately adjacent prefabricated layers of the laminate structure. 16. The method of claim 15, further comprising the continuous steps: coating a first major surface of the first electrode substrate layer with a first catalyst mixture to form the first catalyst layer; coating a first major surface of the second electrode substrate layer with a second catalyst mixture to form the second catalyst layer; and subsequently extruding the electrolyte layer between the catalyst-coated main surfaces of the first and second electrode substrate layers. 17. The method of claim 6 or 16, further comprising the step of converting an electrolyte precursor to an ionic electrolyte after the layers of the laminated structure have been bonded together. 18. The method of claim 17, wherein the ionic electrolyte is selected from the group consisting of perfluorosulfonic acid based materials, polyetheretherketoneketone based ion-conducting polymers, and sulfonated polytrifluorostyrene based ion-conducting polymers.