Method for Producing Separator Plates for a Fuel Cell

Abstract

A method for producing separator plates, in particular bipolar plates, for a fuel cell. The method comprises use of a sacrificial binder.

Claims

1. A method of producing a separator plate for a fuel cell by providing a powder containing at least 70% graphite or carbon black or both and 10-30% thermoplastic polymer different from PTFE, all percentages by weight of the powder, the method comprises providing a liquid solution of a sacrificial binder and mixing the liquid solution with the powder and sedimenting the sacrificial binder and the powder as a slurry from the liquid solution, drying the slurry to form a mat of powder and sacrificial binder, and hot-press moulding the mat in a press mold into a shape of a separator plate at a molding temperature that causes evaporation of at least part of the sacrificial binder, wherein the sacrificial binder is chosen from a polycarbonate polymer, a polysaccharide or a mix of polysaccharides.

2. A method according to claim 1, wherein the thermoplastic polymer is polyphenylene sulfide, PPS.

3. A method according to claim 1, wherein the method comprises adding coagulation agent to the solution at a concentration that causes the sedimentation of the sacrificial binder from the solution.

4. A method according to claim 3, wherein the coagulation agent is iso-propanol.

5. A method according to claim 1, wherein the method comprises, prior to the hot-press moulding, drying the slurry into a mat by heating the solution with the mixed powder to a temperature that does not exceed the boiling point of the solvent and causing evaporation of the solvent.

6. A method according to claim 1, wherein the method comprises hot-press molding the mat into a separator plate at a pressure in the range of 10 to 100 MPa and a temperature that is at least 25% higher than the decomposition temperature of the sacrificial binder and in the range of 280 to 480° C.

7. A method according to claim 1, wherein the sacrificial binder is polycarbonate polymer, and the method comprises dissolving the sacrificial binder in an organic solvent for providing the liquid solution of the sacrificial binder.

8. A method according to claim 7, wherein the sacrificial binder is a copolymer of carbon dioxide and epoxide.

9. A method according to claim 8, wherein the sacrificial binder is at least one of polyethylene carbonate, polypropylene carbonate, or polycyclohexene carbonate, and the method comprises decomposing at least 80% of the sacrificial binder.

10. A method according to claim 7, wherein the solvent comprises at least 50% of its weight as acetone.

11. A method according to claim 1, wherein the sacrificial binder is a polysaccharide or a mix of polysaccharides, and the solvent is aqueous, and the method comprises dissolving the polysaccharide in the aqueous solvent for providing the liquid solution of the sacrificial binder.

12. A method according to claim 11, wherein the polysaccharide is at least one of agarose, gluten or starch, and the method comprises decomposing at least 20% of the sacrificial binder.

13. A method according to claim 12, wherein the method comprises adding a non-ionic surfactant to the aqueous solution.

14. A method according to claim 13, wherein the method comprises adding octyl phenol ethoxylate as the non-ionic surfactant.

Description

SHORT DESCRIPTION OF THE DRAWINGS

[0024] The invention will be explained in more detail with reference to the drawing, where

[0025] FIG. 1 is a perspective exploded view of a fuel sell stack assembly according to the present invention showing bipolar plates, membranes, sealants and endplates;

[0026] FIG. 2 is a perspective view of the cathode side of one “sandwich element” comprising (from left to right): a sealant for sealing off the cathode side of a PEM bipolar plate; a PEM bipolar plate; a sealant for sealing off the anode side of a PEM bipolar plate; and finally a membrane;

[0027] FIG. 3 is a perspective view of the anode side of one “sandwich element” comprising (from left to right): a membrane; a sealant for sealing off the anode side of a PEM bipolar plate; a PEM bipolar plate; and finally a sealant for sealing off the cathode side of a PEM bipolar plate;

[0028] FIG. 4 illustrates a fuel cell stack principle, where a bipolar plate is used between electrolytic membranes;

[0029] FIG. 5 illustrates alternative fuel cell stack principles, where an anode plate and a cathode plate are oriented back-to-back with a cooling section between the anode plate and the cathode plate;

[0030] FIG. 6 illustrates a further alternative fuel cell stack principle, where a cooling plate is sandwiched between a cathode plate and an anode plate and cooling is provided in the volume between the cooling plate and the anode plate and in the volume between the cooling plate and the cathode plate;

[0031] FIG. 7 illustrates stages of the production method of a separator plate.

DETAILED DESCRIPTION/PREFERRED EMBODIMENT

[0032] FIG. 1 illustrates a PEM fuel cell stack 90 comprising a plurality of bipolar plates 1 assembled between endplates 92. Proton exchange membranes (PEM) 40 between adjacent bipolar plates 1 are sealed against the environment by sealants 70 and 50. FIG. 2 is a perspective view onto the cathode side of the bipolar plate 1 assembly comprising the membrane 40 and a sealant 70 for sealing off the cathode side of a PEM bipolar plate and a sealant 50 for sealing off the anode side of a PEM bipolar plate. Correspondingly, FIG. 3 is a perspective view onto the anode side of the bipolar plate 1 assembly. The cathode side comprises a serpentine channel pattern for flow of oxygen gas along the membrane 40 and efficient cooling by the oxygen gas, typically air. The anode side comprises straight channels for transport of hydrogen along the membrane 40.

[0033] FIG. 4 illustrates such configuration with a bipolar plate 10, on the anode side 28 of which a hydrogen flow is provided for donating protons to the electrolytic membrane 30 and with a cathode side 26 on which oxygen or air or other fluid flows for accepting protons from the membrane 30. The cathode fluid, for example oxygen or air is used as a cooling medium for cooling the bipolar plate. The cathode side 26 of the bipolar plate 1 is provided with a serpentine channel pattern as described above. Exemplary details of the channel patterns and other details of the bipolar plate are explained in WO2009/010066 and WO2009/010067.

[0034] The production method for separator plates as described herein is not only suitable for bipolar plates. It applies equally well to other separator plates, such as cathode plates, anode plates and cooling plates. Such examples are illustrated in FIGS. 5 and 6.

[0035] FIG. 5 illustrates an embodiment, where a cathode plate 34 with a cathode side 26 is combined with an anode plate 36 with anode side 28 and with cooling fluid 32, for example gas or liquid in a space 32 between the two plates. In the space 32, the cathode plate 34 or the anode plate 36 are provided with a channel pattern for example serpentine channel pattern, as described above for efficient cooling by the cooling fluid.

[0036] FIG. 6 illustrates a further alternative, where a cathode plate 34 and an anode plate 36 are sandwiching a cooling plate 38 such that two cooling spaces 32 are provided, one cooling volume between the cooling plate 38 and the cathode plate 34 and another cooling volume between the cooling plate 38 and the anode plate 36. The cooling plate 38 is provided with a channel pattern on both of its sides, for example a serpentine channel pattern as described above.

[0037] The production of the separator plates, for example BPP, is based on use of sacrificial binders, such as polymers which decompose to gaseous substances for removal from the composites during the molding process.

[0038] Data of temperatures T.sub.d when rapid decomposition starts and residual contents C.sub.r for the mentioned polymers at 360° C. are collected in Table 1 below. It should be mentioned that 360° C. is a useful reference point because the highest crystallinity index is achieved for molded PPS at that temperature.

TABLE-US-00001 TABLE 1 Decomposition temperatures of some polymers determined via thermogravimetric analysis Sacrificial polymer name T.sub.d (° C.) C.sub.R (%) polyethylene carbonate ca. 220 ca. 0 polypropylene carbonate ca. 250 ca. 0 polycyclohexene carbonate ca. 250 ca. 0 agarose ca. 280 ca. 25 gluten ca. 240 ca. 30 starch ca. 300 ca. 25

[0039] With reference to the Table 1 given above, polycarbonates are more preferred due to their complete decomposition at specified temperature. However, despite incomplete decomposition, polysaccharides are interesting for this purpose, as well.

[0040] In more detail, the following production method has been found useful, in which separator plates (anode plates, cathode plates, or bipolar plates) were manufactured as follows, with reference to FIG. 7 as an exemplary embodiment thereof.

[0041] A powder is provided which contains at least 70%, for example 70-90% or 80-90%, graphite and/or carbon black, as well as 10-20% of thermoplastic polymer. For example, the powder is a ground powder made from a composite of these ingredients. Alternatively, the powder is a mix of graphite powder and/or carbon black with an average grain size in the range of 0.25 to 5 microns and 10-20% of thermoplastic polymer powder. The combination of carbon and thermoplastic polymer containing 80 to 90 wt. % carbon material and 10 to 20 wt. % thermoplastic polymer, the latter adding up to 100% relative to the carbon. The percentage by weight and calculated relatively to the weight of the mix of carbon and thermoplastic polymer.

[0042] A useful example of a thermoplastic polymer is PPS, which is advantageous due to its high chemical stability. In the following, the method is exemplified with PPS, although also other thermoplastic polymers or blends of thermoplastic polymers can be used. If another thermoplastic polymer is used, the PPS in the method below is substituted by the other thermoplastic polymer or blend of thermoplastic polymers. This mix of carbon and thermoplastic polymer mix was added to a liquid binder solution.

[0043] One option for a liquid binder material solution is a solution that contains sacrificial polycarbonate polymers. In this case, the polymer was dissolved in organic solvents, for example acetone-based, such as acetone. Optionally, the concentrations of the polymer is ranges from 0.5 to 30 wt. % of the solution.

[0044] Another option for binder material are polysaccharides. In this case, the solvent is aqueous, for example water. Optionally, the concentrations of the polysaccharides is ranges from 0.5 to 30 wt. % of the aqueous solution. Optionally, in order to improve wettability of the carbon-based composites, a non-ionic surfactant is added, for example at a concentration of 1-2 vol. %. A useful example of a non-ionic surfactant is octyl phenol ethoxylate, for example commercially available under the trade name Triton™ X-100 from Dow Chemical Company®.

[0045] For example, the solutions are prepared by equal weight amounts of carbon/PPS composite and binder solution. The combination of the composite and binder solution is advantageously made while stirring.

[0046] For example, the amount of sacrificial polymer solid in the final composition is in the range of 1 to 10 wt. %.

[0047] Advantageously, further solvent is added to the combined mix of composite and binder solution, where the solvent is of the type that easy mixes with the solvent and provokes sedimentation of the sacrificial binder from the solution. A useful example in the aqueous case is water or iso-propanol, which is also a useful example for the acetone based binder solution. Other useful organic solvents include polar solvents that have low surface tension and good wetting capabilities for the components. A useful candidate is metoxybenzen.

[0048] The sedimentation is typically achieved during stirring.

[0049] The sedimentation of the binder from the solution leads to a highly viscous material, which is used for the hot-pressing step in the pressing tool. However, before hot-pressing, the liquid from the binder, for example containing a mixture of iso-propanol with water or acetone, is subjected to evaporation at temperatures that do not exceed their boiling points. For example, the evaporation stage is done at a temperature in the range of 70-80° C., optionally at 80° C., for a water/iso-propanol azeotropic mixture and at a temperature in the range of 50-56° C., optionally at 56° C., for acetone/iso-propanol, the latter temperature being determined by the boiling point of acetone.

[0050] Due to the evaporation, the viscous material dries into mechanically stable mats. Typically, the drying time is at least 1 h. Optionally, this drying step is made while the mix is already in the pressing tool.

[0051] As a second heating step, the pressing tool is used for hot-pressing the mats located in the pressing tool at temperatures in the range between 280 and 480° C., depending on the type of sacrificial binder. This range is limited by the melting point and decomposition temperature of the PPS, which is not desired to decompose.

[0052] During the hot-pressing, pressure is applied, typically in the range 10 to 100 MPa, to form a separator plate, for example bipolar plate, with specified desired parameters, such as thickness and density.

[0053] Key characteristics for separator plates for the fuel cell's stack are their electrical conductivity, especially through-plane conductivity. Experimentally, measurements were carried out in this respect. According to these measurements, through-plane conductivity for BPPs with 2 wt. % sacrificial binder produced by the above-described method reached 30 S/cm. In comparison, similar BPPs with 2 wt. % PTFE had about 20 S/cm. The latter were produced by the method as disclosed in WO2018/072803.

[0054] In summary, a number of advantages were achieved as compared to the method as disclosed in WO2018/072803: [0055] better electrical properties due to thermal decomposition of binders forming the carbon-based mats during molding process; [0056] less toxic molding process due to use of biodegradable polymers instead of PTFE, which can be the source of toxic fluorine-contained substances at elevated temperatures; [0057] no need to apply heat to coagulate (sediment) the binder as it is performed in WO2018/072803; [0058] the mat forming process moves faster, because coagulation is rapid.

[0059] As it appears from the above, a useful scalable production method has been found in use of a sacrificial binder and the two-step heating process for first evaporating the solvent and then the binder. Also, a useful part of the method is the precipitation process.