Fuel cell separator plate, precursor thereof and manufacturing method thereof

Abstract

For the manufacture of fuel cell separator plates, a malleable precursor sheet is made by mixing a thermoplastic polymer, carbon fibers, and conductive carbon particles, which is then hot compression molded into a single layer or multi-layer structure, where the thickness of the layer is less than the length of the carbon fibers.

Description

[Technical Field]

[0001] The present invention relates to separator plates, such as bipolar plates, for fuel cells. The present invention also relates to hot compression molding of such separator plates and precursors for their production. [Background technology]

[0002] Bipolar plates (BPPs) are a critical component of fuel cell stacks because they separate the individual membrane-electrode assemblies and deliver fuel to the membranes [Ref. 1]. High-temperature proton exchange membrane (PEM) fuel cells offer significant advantages over low-temperature PEM fuel cells due to their high tolerance to impurities in the input gas, particularly residual carbon monoxide in hydrogen gas [Ref. 2]. However, the relatively high operating temperatures of 160–200 °C and acidic medium place severe limitations on the materials suitable for use in bipolar plates [Ref. 3]. Graphite appears to be a very attractive material due to its high electrical and thermal conductivity and good chemical stability [Ref. 4]. Other carbon materials, such as carbon black (CB), carbon fiber (CF), carbon nanotubes (CNTs), and graphene, can be used as additives in BPP composites to improve their mechanical and electrical properties [Refs. 5, 6]. However, binders, such as thermoplastic polymers, are required to bind the carbon particles together. Polyphenylene sulfide (PPS) is a good candidate because it meets all the stringent requirements of high-temperature PEM fuel cells, including being chemically inert, having a high operating temperature above 200°C, and having excellent mechanical properties [7].

[0003] Alternative BPP structures are disclosed in CN111048800A [Reference 58] and US7887927 [Reference 59], where multi-layer BPPs are fabricated by stacking carbon fiber or graphite sheets with conductive polymer fillers sandwiched between them.

[0004] The U.S. Department of Energy (DoE) has declared ambitious goals for 2020 for BPP used in transportation applications, including a surface-specific resistivity of 10 mΩ·cm. 2 The requirements are that the thickness should be less than 1.5 mm and the bending strength should be greater than 25 MPa [Reference 8]. It is noteworthy that graphite-based BPPs typically have a thickness of approximately 1.5–3 mm [References 9–12]. However, recent studies have demonstrated that graphite compound-based graphite BPP samples with thicknesses of less than 1 mm, e.g., 0.85 mm [Reference 13], 0.6 mm [Reference 14], or even 0.4 mm [Reference 15], offer several advantages, including more compact fuel cell designs and cost savings due to reduced material consumption. A multi-step process has been disclosed for producing thin BPPs from powdered compounds [Reference 16]. It would be desirable to provide a simple yet efficient process for mass production of BPPs.

[0005] Only a few patents and research papers describe the preparation process of carbon / PPS compounds and the corresponding BPPs [17-20]. For example, in International Patent Application WO2014 / 100082

[17] , PPS is mixed with carbon nanotubes (CNTs) in a high-temperature extruder, where the polymer is melted. High polymer loadings, typically greater than 50% by weight, are usually required for the extrusion process, which unfortunately negatively impacts the electrical properties of such compounds. After extrusion, PPS-based compounds can be formed into sheets, as proposed, for example, in U.S. Patent No. 7,736,786

[18] .

[0006] Chinese Patent Application No. CN101174695A

[63] discloses a dry process for producing BPP made from a material containing 45-55% graphite, 5-10% carbon fiber, 30-40% PPS, and 5-10% fluororesin, such as PTFE. The flow field structure is 0.5-1.5 mm deep within a sheet with a thickness of the order of 5 mm. The carbon fiber length ranges from 10 to 200 micrometers. The powder mixture is first cold-pressed at 3-5 MPa, then slowly heated to 310-330°C and pressed at 15-25 MPa for 30 minutes to consolidate, followed by quenching in water to prevent crystallization and improve toughness.

[0007] US2014 / 087287 [Reference 60] discloses a dry manufacturing method for separator plates for fuel cells, in which the separator plate is made from two sheets, A and B. Sheet A contains 100 parts thermoplastic resin and 130-3200 parts carbonaceous material, while Sheet B contains 100 parts thermoplastic resin, e.g., polypropylene, and 3-280 parts carbonaceous material. Half of the carbonaceous material is fibrous carbon. The fiber length is in the range of 0.001-20 mm, more preferably 1-10 mm, e.g., 6 mm. The sheets are formed by compressing a powder mixture at a temperature 60°C higher than the higher melting point of each binder component. The groove depths present in the final plate are 0.3 and 0.5 mm for a 1 mm thick separator.

[0008] US2019 / 0341630 [Reference 61] discloses a method for producing a fuel cell separator, e.g., 1 mm thick. In this method, carbon fibers are dispersed in an aqueous dispersion together with a fibrous resin, and the resulting slurry is then dried to obtain a paper-like composite sheet, into which carbon particles are then pressed at high temperature. The length of the carbon fibers is not particularly limited, but is, for example, in the range of 20 μm to 6 mm, and is exemplified as 3 mm for a 2 mm thick composite sheet.

[0009] US2009 / 0152105 [Reference 62] discloses a method for manufacturing compressed pads in fuel cells, in which 1-inch-long carbon fibers are blended with molten PTFE, cured, and chopped in a coffee grinder. This blend is then mixed with PVDF and heated to 200°C while being compressed. Alternatively, the carbon fibers are melted into the PVDF. The carbon fibers are preferably 3 mm (1 / 8 inch) long. Typical pad thicknesses are 10-15 mils, which roughly equates to 0.25-0.40 mm.

[0010] WO2008 / 075812 [Reference 64] discloses a hydrophilic polymer composite for bipolar plates comprising carbon black aggregates with hybrid particles embedded on the surface of the carbon black particles.

[0011] US2019 / 0341630 [Reference 61] discloses a method for producing a fuel cell separator, e.g., 1 mm thick. In this method, carbon fibers are dispersed in an aqueous dispersion together with a fibrous resin, and the resulting slurry is then dried to obtain a paper-like composite sheet, into which carbon particles are then pressed at high temperature. The length of the carbon fibers is not particularly limited, but is, for example, in the range of 20 μm to 6 mm, and is exemplified as 3 mm for a 2 mm thick composite sheet.

[0012] US2009 / 0152105 [Reference 62] discloses a method for manufacturing compressed pads in fuel cells, in which 1-inch-long carbon fibers are blended with molten PTFE, cured, and chopped in a coffee grinder. This blend is then mixed with PVDF and heated to 200°C while being compressed. Alternatively, the carbon fibers are melted into the PVDF. The carbon fibers are preferably 3 mm (1 / 8 inch) long. Typical pad thicknesses are 10-15 mils, which roughly equates to 0.25-0.40 mm.

[0013] US2018 / 0358630 [Reference 65] discloses a method for manufacturing bipolar plates using a mixture of two types of carbon powder with different powder particle sizes and compression molding. US2004 / 033413 [Reference 66] discloses a polymer electrolyte membrane for fuel cells, in which the membrane has a conductive coating. US2017 / 298200 [Reference 67] discloses a thermoplastic prepreg intermediate material for fuel cell separator plates and a method for manufacturing the thermoplastic prepreg, in which it is compressed at a temperature above the melting point of the polymer.

[0014] The wet process, as described, for example, in [Reference 19] and US 2019 / 0341630 [Reference 20], suggests mixing graphite, carbon fiber, and PPS binder in a liquid phase, specifically water, followed by filtering and forming the solid mixture into sheets, which are then dried and molded into BPP. Unfortunately, PPS exhibits very poor water wettability, picking up only about 0.1% water after 500 hours at 23°C [Reference 21]. Therefore, dispersing PPS in aqueous media without particle agglomeration is difficult. Perhaps for this reason, [References 19, 20] employ highly diluted dispersions with total solids content not exceeding 10 wt%, preferably in the range of 1-3 wt%.

[0015] A process for making sheets from a powdered compound prepared by dry-blending graphite and PPS has also been proposed

[22] . The compound is dispersed in isopropanol, a surfactant that is good for wetting hydrophobic surfaces, along with another thermoplastic polymer, polytetrafluoroethylene (PTFE)

[23] . Upon raising the temperature to the boiling point of the alcohol, the PTFE rapidly solidifies, forming a flexible, malleable material resembling a dough-like structure (see US 2019 / 0260037

[22] ). PTFE has a high elongation capacity compared to other polymers, which is estimated to reach 550%

[24] .

[0016] In the patent literature, various processes have been disclosed relating to the handling of PPS (e.g., US8563681 [Reference 30]) and the formation of articles therefrom (e.g., US5043112 [Reference 27]), as well as the manufacture of BPP (e.g., US2008 / 0318110 [Reference 31] and US2006 / 0084750 [Reference 50]).

[0017] US6544680 [Reference 54] discloses molded separator plates made from carbon and PPS with a thermosetting resin. US6803139 [Reference 55] discloses molded separator plates made from carbon and a thermoplastic resin, such as polyphenylene sulfide (PPS), with a carbodiimide additive. EP1758185 [Reference 56] discloses a separator plate containing 84% carbon, 2% PTFE, and 14% epoxy, cured by hot pressing. Polyphenylene sulfide (PPS) or polytetrafluoroethylene (PTFE) is mentioned as a thermoplastic resin, but no examples are given. US2005 / 0042496 [Reference 57] discloses a continuous process in which a polymer is blended with a filler, such as graphite, kneaded, extruded, and then transferred into a compacted form, for example, into a separator plate.

[0018] International Patent Application WO2018 / 072803 [Reference 53] discloses a method for manufacturing separator plates by suspending a powder mixture of carbon and a thermoplastic polymer in alcohol and mixing it with an aqueous suspension of PTFE, then evaporating the liquid, and rolling the resulting malleable slurry into a sheet and pressing it into a separator plate. Examples of carbon powders include graphite, carbon black, graphene, carbon nanotubes, and amorphous carbon.

[0019] Compared to pressed powder compounds for BPP, the use of malleable sheets or slabs for BPP formulation appears to offer advantages in terms of ease of storage, handling, and administration. However, further improvements are needed to optimize this method and the resulting BPP, especially with regard to the goals set by the U.S. DOE's 2020 program. Ease, cost, and speed of production are also important factors that must be considered for a successful production method. Summary of the Invention

[0020] It is an object of the present invention to provide improvements in the art. In particular, it is an object to provide improved separator plates, such as BPP, and improved fuel cells having such separator plates. A further object is to provide improved methods for providing separator plates, such as BPP, and improved material mixtures of carbon and polymer, as well as corresponding precursors for hot compression molding of separator plates. This is achieved by the manufacturing methods and precursors for hot compression molding of separator plates, such as BPP, as claimed and described below.

[0021] This objective is achieved by a manufacturing process of a graphite-based compound with a thermoplastic polymer and its formable, malleable precursor sheets for hot compression molding into conductive, rigid separator plates for fuel cells, such as BPP.

[0022] While the present invention is directed to the manufacture of BPPs for use in fuel cell stacks, its processes and materials are equally applicable and therefore equally useful for the manufacture of separator plates in general. In particular, the method is equally useful for the manufacture of single electrode plates for fuel cells, as well as end plates for fuel cell stacks. An example configuration is described in WO 2018 / 072803 [Reference 53]. For example, the separator plates are bipolar plates with opposing fluid flow fields, specifically, an oxygen flow field on one side and a hydrogen flow field on the other. Alternatively, a separator plate for a fuel cell stack may include a flow field for oxygen gas, such as air, on one side and be attached, e.g., back-to-back, to a second separator plate that includes a flow field for hydrogen gas on the opposite side. Optionally, a cooling flow field for a coolant is provided between two separator plates for fuel gas, e.g., by inserting a corresponding separator plate with a coolant flow field on one or both sides. Optionally, a separator plate may have an oxygen flow field on one side and a coolant flow field on the other side. Depending on the need, the methods as described herein can be used to manufacture a variety of different separator plates, whether they have fluid flow fields on only one side or both sides, for oxygen, hydrogen, or coolant in each flow field.

[0023] Briefly, in a general aspect, for the manufacture of such separator plates for fuel cells, a malleable precursor sheet is made by mixing a thermoplastic polymer, carbon fiber, and conductive carbon particles, and the precursor is then hot-compressed into a single-layer or multi-layer structure that constitutes the separator plate. Advantageously, the thickness of the layers is less than the average length of the carbon fiber, as described below.

[0024] The compound is provided as a polymer matrix comprising a thermoplastic polymer blend having dispersed therein carbon fibers and conductive carbon particles, the thermoplastic polymer blend including PTFE and a thermoplastic polymer different from PTFE, such as polyphenylene sulfide (PPS).

[0025] Alternatives to PPS include ethylene tetrafluoroethylene (ETFE), fluorinated ethylene propylene (FEP), perfluoroalkoxyalkane (PFA), polyamideimide (PAI), polychlorotrifluoroethylene (PCTFE), polyetheretherketone (PEEK), polyetherketone (PEK), polyetherimide (PEI), polyethersulfone (PES), polyphenylsulfone (PPSU), polysulfone (PSU), or polyvinylidene fluoride (PVDF).

[0026] In this specification and below, all weight percentages given are given relative to the total weight of the polymer blend, carbon fiber, and carbon particles, and therefore do not take into account liquids such as water and organic solvents, as well as liquid additives, e.g., surfactants, used in the manufacturing process, as these evaporate before the final manufacturing step.

[0027] For example, the precursor and final separator plates may include one or more of the following parameters, and two, three or more of the parameters may be combined: PTFE: at least 0.05% by weight PTFE: Less than 0.5% by weight Thermoplastic polymers other than PTFE: minimum 5-30% by weight Relative weight of carbon fiber: 2-20% by weight, e.g., 5-20% Average length of carbon fiber L: 0.1 to 1 mm Relative weight of carbon particles (different from carbon fiber): 25-90% by weight Average particle size of carbon particles (different from carbon fiber): 0.1 to 100 μm.

[0028] The particle sizes given herein for polymer particles and carbon particles, including graphite and carbon black, are average particle sizes, which means the average over the three dimensions of the particle as well as the average over the number of particles of that particular group or type.

[0029] Typically, the particles have a statistical distribution associated with the dimensionally averaged particle size, including the length of the carbon fibers, with a FWHM of less than or on the order of ±20%.

[0030] The compound is formed into a moldable, malleable precursor sheet at a forming station, such as a stationary forming station, but preferably the forming station is a calendering station and forming includes press-rolling the sheet through a calender roller calendering station.

[0031] The precursor sheet is optionally formed as a single layer slab having a thickness X1 and then formed into a single layer separator plate. Alternatively, the precursor sheet is formed as a multi-layer slab structure with multiple layers having a thickness X1 stacked together. Typically, the precursor is cut to shape before forming, and the resulting slab is hot-compressed into a separator plate.

[0032] In the case of a multi-layer separator plate, the precursor multi-layer slab is composed of multiple layers of thickness X2, although this is not required and there may be additional layers of different thicknesses in addition to the multiple layers having thickness X2, for example there may be multiple layers having thickness X2, typically thicker top and bottom layers with identical layers provided between them.

[0033] The terms precursor slab thickness X1 and corresponding separator plate thickness X2 are used to distinguish between the two because the compression stage increases the slab's density through high-pressure consolidation. While slab thickness X1 applies to the precursor slab before molding, thickness X2 applies to the separator plate after molding. For example, the density of the slab to the final separator plate, such as a BPP, can increase by a factor of two. However, it should be noted that the formation of flow fields during compression molding can compensate for some of the thickness reduction.

[0034] Optionally, the number of the plurality of layers is in the range of 2 to 10, for example, 4 to 8. However, more layers are possible.

[0035] Advantageously, as discussed in more detail below, the thickness X1 is less than the average length L of the carbon fibers. This is advantageous because it allows the carbon fibers to be at least partially parallel to the slab, particularly during the rolling process, improving the mechanical strength of the final separator plate. For example, the average length L is at least twice the thickness X1 of the precursor sheet.

[0036] When comparing the length L of the carbon fibers with the thickness X2 of the separator plate, it is useful to consider that this thickness X2 should be smaller than the average length L of the carbon fibers, but rather at least two or even four times smaller than L, due to the compression of the precursor during compression molding.

[0037] The choice of carbon fiber length is a balance between the positive effect on the strength of the final separator plate and the ability to disperse the carbon fibers, which is easiest with shorter carbon fibers. From these perspectives, it has been found useful for the average length of the carbon fibers to be in the range of 0.05 to 1 mm, e.g., 0.1 to 1 mm.

[0038] For example, if the carbon fiber length is 1 mm and the requirement is that the carbon fiber length be at least twice the thickness of the precursor sheet, then the maximum sheet thickness should be selected to be 0.5 mm.

[0039] The fiber lengths are average values, provided that the lengths do not deviate significantly from the average value, e.g., less than 20% of the fibers have lengths that deviate more than 20% from the average value.

[0040] The choice of fiber length depends on the thickness of the final separator plate.

[0041] That is, if the length L is selected to be twice the precursor thickness X1, compaction during molding will typically result in the length L being on the order of four times the final separator plate thickness X2.

[0042] When mixing carbon fibers into the initial dispersion, it is desirable that the fibers are not too long, making mixing difficult. For example, fibers 6 or 10 mm long, as disclosed in prior art US2014 / 087287, US2019 / 0341630, and US2009 / 0152105, would be disadvantageous in producing separator plates with a thickness X2 of 0.05 to 0.3 mm. On the other hand, fiber lengths of 0.01 mm or 0.02 mm, as disclosed in the same prior art, would be too short. Therefore, the ranges proposed by the prior art are quite arbitrary and do not seem to balance adequate mixing with increased strength.

[0043] The amount of carbon fiber in the compound should also not be chosen arbitrarily. Experiments have shown that the strength of the final separator plate increases with increasing amounts of carbon fiber up to 10% by weight, but that the increase is more gradual within the range of 7-10% by weight. It has been found that amounts greater than 10% by weight result in a decrease in flexural strength. For this reason, amounts between 5-20% by weight, e.g., 5-15% by weight, are considered most useful, with an optimal range being 6-12% by weight, e.g., 6-10%.

[0044] As discussed in the introduction, thin separator plates are desirable because they affect the size, weight, performance, and cost of the final fuel cell stack. For example, separator plate thickness X2 may be in the range of 0.05-1 mm, e.g., 0.05-0.5 mm, or 0.05-0.6 mm, or even 0.05-0.3 mm, potentially fabricated in multiple layers.

[0045] For example, the thickness X1 of the precursor sheet is in the range of 0.05 to 1 mm, e.g., 0.1 to 1 mm, or 0.05 to 0.5 mm, 0.05 to 0.6 mm, optionally 0.1 to 0.6 mm, or 0.05 to 0.3 mm. For multi-layer separators, the thickness X1 per layer is typically at the lower end thereof, e.g., in the range of 0.05 to 0.3 mm, optionally 0.05 to 0.2 mm.

[0046] The following method for producing a precursor sheet has proven advantageous: an aqueous dispersion and a solvent dispersion are provided and mixed, the aqueous dispersion comprising PTFE particles and carbon fibers, and optionally a first portion of graphite particles, for example having an average particle size in the range of 0.1 to 10 μm.

[0047] The solvent dispersion includes a second thermoplastic particulate polymer different from PTFE, such as PPS, and carbon black particles in the mixture, which are more hydrophobic than the graphite particles.

[0048] The organic solvent is optionally N-methylpyrrolidone, NMP. Alternative solvents can also be used, such as N,N-dimethylacetamide, N,N-dimethylformamide, dimethylsulfoxide.

[0049] Both dispersions are stirred to prevent settling of the particles, then combined and mixed.

[0050] Furthermore, graphite particles having an average particle size in the range of, for example, 10 to 100 μm are mixed into the mixed dispersion. Typically, the portion of these graphite particles has a weight that is 5 to 20 times the weight of the carbon black.

[0051] It has been found advantageous to use two portions of graphite in a mixture, the first portion having a smaller particle size than the second portion. As already mentioned, the first portion is advantageously mixed into a first dispersion. The second portion of graphite particles is advantageously mixed with the two mixed dispersions. For example, the graphite particles of the second portion have an average particle size in the range of 10 to 100 μm. This second portion is relatively large compared to the amount of the first portion and the carbon black. Typically, the second portion has a weight that exceeds the sum of the weight of the first portion and the weight of the carbon black by 5 to 20 times.

[0052] This large amount of graphite makes further mixing difficult, so proper mixing is done in a kneader.

[0053] During kneading, the temperature is raised to a level high enough to evaporate the organic solvent and water from the mixture. Furthermore, by raising the temperature to a level above the glass transition temperature of PTFE but below its melting point, kneading results in fiberization of the PTFE, which is beneficial to the strength of the final separator plate.

[0054] In a preferred process, the elevated temperature during kneading in the kneader is below the melting temperature of the PTFE and the second thermoplastic polymer. While it is possible to raise the temperature of the mixture in the kneader above the melting temperature of the second thermoplastic polymer, it has been found to be more advantageous to melt the second thermoplastic polymer after removing the mixture from the kneader. Thus, only after removal is the temperature of the mixture raised to a level high enough to melt the second thermoplastic polymer before forming the sheet into a slab of thickness X1 in the forming station.

[0055] For example, a conveyor may be provided between the kneader and the rolling station, in which the compound for melting the second thermoplastic polymer is heated so that the second thermoplastic polymer is in a molten state when the mixture is formed into a sheet in a forming station, such as a calender rolling station.

[0056] For example, after the forming station, the sheet is cut into slabs having appropriate dimensions for hot compression molding in an in-line process including a kneader and forming station. Optionally, multiple slabs are stacked into multi-layer slabs to produce multi-layer separator plates.

[0057] For very thin precursor sheets, a rolling station can be equipped with multiple rolling stages. Experiments have shown that a rolling station with six rolling stages successfully produced a sheet with a thickness X1 of 0.05 mm. In these experiments, the average particle size of the carbon particles was 20 μm, and the thickness X1 of the sheet of 0.05 mm was the theoretical lower limit for a sheet with such particles.

[0058] In a successful multilayer separator plate experiment, six thin precursor sheets with a thickness of 0.1 mm were cut and stacked into a 0.6 mm thick slab, which was then used to hot compression mold a separator sheet, specifically BPP, with a final thickness of 0.3 mm.

[0059] It should be noted that such thin precursor sheets with a thickness X1 of 0.05-0.1 mm cannot be produced by pressing a powder mixture of carbon particles and polymer particles. One of the factors for successfully producing thin plates is the fiberization of PTFE during the kneading process, so kneading is a critical step for production.

[0060] Alternatively, the sheet is provided as a semi-endless sheet having a thickness X1, which is cooled to solidify and then wound into a roll for storage until final shaping, e.g., by cutting, prior to hot compression molding. Optionally, after storage, the semi-endless sheet is cut into slabs that are laminated into multi-layer slabs to produce a multi-layer separator plate.

[0061] Calendering has been found to be advantageous for aligning the carbon fibers at least partially in a direction parallel to the surface.

[0062] Furthermore, it was found that calendering the slab in different directions tends to align the fibers in different directions. This is surprising, since the fiber orientation is determined not only by the final calendering step, but also by the previous steps. Therefore, it is believed that subsequent calendering steps tend to align most of the outermost carbon fibers, so that the carbon fiber orientation from the previous calendering steps is maintained within the bulk of the sheet. Therefore, when a precursor slab is calendered in one direction and then calendered in another direction, such as the transverse direction, an improvement in strength is observed in this calendering direction as well.

[0063] For example, the endless sheet may be first calendered in a direction parallel to the endless sheet, then cut into slabs, which are then further calendered in a different direction, for example the transverse direction.

[0064] Subsequent hot compression molding converts the slabs into conductive rigid separator plates, such as BPP, for fuel cells.

[0065] Layer-by-layer fabrication results in a self-assembled laminate structure of multilayer slabs, making the precursor particularly suitable for hot compression molding of ultra-thin separator bipolar plates. The resulting multilayer bipolar plates have been experimentally confirmed to have excellent mechanical and electrical properties.

[0066] For example, separator plates have a maximum resistance of 2 mΩ·cm per 0.3 mm thickness unit. 2 It has an areal-specific resistance of

[0067] Optionally, the separator plate has a flexural strength greater than 180 MPa per 0.3 mm thickness unit.

[0068] Hot compression molding modifies the shape of the separator plate, for example, imprinting flow field structures into the material for fluid flow on at least one side of the separator plate, but typically both sides, particularly if the separator plate is BPP.

[0069] Such separator plates, e.g., BPP, are useful in high-temperature polymer electrolyte membrane fuel cells (HT-PEM), which operate above 120°C, and can distinguish HT-PEM fuel cells from low-temperature PEM fuel cells, which operate at temperatures below 100°C, e.g., 70°C. Typical operating temperatures for HT-PEM fuel cells are in the range of 120-200°C, e.g., 160-170°C. Such HT-PEM fuel cells are advantageous for compact fuel cell systems, e.g., for the automotive industry.

[0070] The following describes various interrelated aspects that also fulfill the same objective.

[0071] Aspect 1. A moldable, malleable precursor sheet for hot compression molding into an electrically conductive, rigid separator plate for a fuel cell, the precursor sheet being formed as a multilayer structure with multiple layers stacked together or as a single layer, each layer of the multiple layers of the single layer or multilayer structure having a thickness X, and provided as a polymer matrix including a thermoplastic polymer blend having dispersed therein carbon fibers and electrically conductive carbon particles, the thermoplastic polymer blend including PTFE and a thermoplastic polymer different from PTFE, such as PPS, the carbon fibers having an average length L greater than the layer thickness X1.

[0072] Embodiment 2. The precursor sheet of embodiment 1, wherein the carbon fibers have an average length L that is at least twice as large as the layer thickness X1.

[0073] Embodiment 3. The precursor sheet of any preceding embodiment, wherein the thickness X1 is in the range of 0.05 to 1 mm.

[0074] Embodiment 4. The precursor sheet of any preceding embodiment, wherein the weight concentration of the carbon fibers is in the range of 5 to 20 wt %, based on the combined weight of the polymer blend, the carbon fibers, and the conductive carbon particles.

[0075] Embodiment 5. The precursor sheet of any one of the preceding embodiments, wherein the weight concentration of PTFE is at least 0.05 wt.% but less than 0.5 wt.%, and the weight concentration of the thermoplastic polymer different from PTFE, e.g., PPS, is 5 to 30 wt.%, based on the combined weight of the polymer blend, the carbon fibers, and the conductive carbon particles.

[0076] Embodiment 6. The precursor sheet of any one of the preceding embodiments, wherein the conductive carbon particles in the thermoplastic polymer blend comprise at least a first portion and a second portion, wherein the carbon particles of the first portion are graphite particles having an average particle size in the range of 10-100 μm, and the carbon particles of the second portion have a particle size in the range of 0.1-10 μm, wherein a weight concentration of the first portion is in the range of 50-90 wt %, based on a total weight of the polymer blend, the carbon fibers, and the conductive carbon particles, and wherein a weight ratio between the first portion and the second portion of the conductive carbon particles is in the range of 5-20.

[0077] Embodiment 7. The precursor sheet of any one of the preceding embodiments, wherein the thermoplastic polymer is PPS.

[0078] Embodiment 8. The precursor sheet of any one of the preceding embodiments, wherein the sheet is multilayered, and wherein multiple layers are identical.

[0079] Embodiment 9. The precursor sheet of any one of the preceding embodiments, wherein the sheet has a multilayer structure, each of the plurality of layers has a thickness X1 in the range of 0.05 to 0.3 mm, and the number of the plurality of layers is in the range of 2 to 10, e.g., 4 to 8.

[0080] Embodiment 10. The precursor composition of any preceding embodiment, wherein the carbon fibers have an average length in the range of 0.1 to 1 mm.

[0081] Embodiment 11. A method of making a precursor sheet according to any preceding embodiment, comprising: providing an aqueous dispersion comprising PTFE particles, carbon fibers, and a first portion of graphite particles, wherein the graphite particles of the first portion have an average particle size in the range of 0.1 to 10 μm, and the carbon fibers have an average length L; providing a solvent dispersion comprising carbon black particles and particles of a second thermoplastic polymer different from PTFE, such as PPS, dispersed in an organic solvent; - stirring both dispersions to prevent settling of the particles; - combining and mixing two dispersions; - mixing a second portion of the graphite particles with the two dispersions, wherein the graphite particles of the second portion have an average particle size in the range of 10 to 100 μm, and the second portion has a weight that is 5 to 20 times greater than the sum of the weight of the first portion and the weight of the carbon black; - kneading the mixture in a kneader; - during kneading in the kneader, increasing the temperature to a high enough level to evaporate the organic solvent and water from the mixture, the high temperature level being above the glass transition temperature of PTFE; - forming the mixture into a sheet in a forming station after evaporating the organic solvent and water and while the second thermoplastic polymer is in a molten state; A method comprising:

[0082] Embodiment 12. The method of embodiment 11, comprising forming the sheet at a forming station to a thickness X1 that is less than the average length L of the carbon fibers.

[0083] Embodiment 13. The method of embodiment 12, further comprising preparing a precursor sheet ready for hot compression molding, the preparing comprising cutting the molten sheet into slabs and laminating a plurality of such slabs of thickness X1 together in a molten state to provide a multilayer precursor for compression molding into a separator plate.

[0084] Embodiment 14. The method of embodiment 12 or 13, wherein the forming station is a calender rolling station, and forming comprises press-rolling the sheet by the calender rolling station, and wherein the method further comprises cooling the sheet after press-rolling to solidify the sheet, providing the sheet as a semi-endless slab of sheet thickness X1, and winding the semi-endless slab onto a roll after cooling for storage as a solidified, wound semi-endless slab and for subsequent cutting and hot compression forming.

[0085] Embodiment 15. The method of any one of embodiments 11-14, wherein the elevated temperature level during kneading in the kneader is below the melting temperature of the PTFE and the second thermoplastic polymer, and the method includes removing the mixture from the kneader and then increasing the temperature of the mixture to a level high enough to melt the second thermoplastic polymer before forming the sheet into a slab of thickness X1 at a forming station.

[0086] 16. The method of embodiment 15, wherein providing the aqueous dispersion comprises adding a surfactant to the aqueous dispersion, the surfactant having a boiling point greater than the boiling point of water and greater than the boiling point of the organic solvent, and the method comprises removing the mixture from the kneader while the mixture comprises the surfactant but not the solvent or water, and then increasing the temperature of the mixture to a level high enough to evaporate the surfactant before forming the sheet into a slab of thickness X1 at a forming station.

[0087] Embodiment 17. The method of any one of embodiments 11-16, wherein the weight concentration of PTFE in the mixture is at least 0.05 wt% but less than 0.5 wt% PTFE, and the weight concentration of the thermoplastic polymer different from PTFE is in the range of 5-30 wt%, the weight percentages being relative to the combined weight of the carbon fibers and carbon particles, PTFE, and thermoplastic polymer.

[0088] Aspect 18. A rigid press-formed separator plate for a fuel cell, the separator plate being formed as a multilayer structure with multiple layers stacked together, or as a single layer, each layer of the multiple layers of the single layer or multilayer structure having a thickness X2, and provided as a polymer matrix including a thermoplastic polymer blend having dispersed therein carbon fibers and conductive carbon particles, the thermoplastic polymer blend including PTFE and a thermoplastic polymer different from PTFE, e.g., PPS, the carbon fibers having an average length L greater than the layer thickness X2.

[0089] Embodiment 19. The separator plate of embodiment 18, wherein the carbon fibers have an average length L that is at least twice as large as the layer thickness X2.

[0090] Embodiment 20. The separator plate of embodiment 19, wherein the separator is provided as a multilayer structure including a plurality of layers in a stacked state, and each of the plurality of layers has a thickness X2 in the range of 0.05 to 0.2 mm.

[0091] Aspect 21. The separator plate has a maximum resistance of 2 mΩ·cm per 0.3 mm thickness unit. 2 21. The separator plate of embodiment 20, having an areal resistivity of

[0092] Aspect 22. The separator plate of aspect 20 or 21, wherein the separator plate has a flexural strength of greater than 180 MPa per 0.3 mm thickness unit.

[0093] Embodiment 23. The separator plate of any one of embodiments 18-22, wherein the separator plate has flow field structures for fluid flow engraved into the material on at least one side of the separator plate.

[0094] Embodiment 24. The separator plate of embodiment 23, wherein the separator plate is a bipolar plate having fluid flow structures on both sides of the bipolar plate.

[0095] Embodiment 25. A fuel cell comprising the separator plate according to any one of embodiments 18 to 24, or a separator plate provided as a plate hot-pressed from the precursor sheet according to any one of embodiments 1 to 10.

[0096] Embodiment 26. A moldable, malleable precursor sheet for hot compression molding into an electrically conductive, rigid separator plate for a fuel cell, the precursor sheet being formed as a single layer, the single layer having a thickness X1 in the range of 0.05 to 1 mm, and provided as a polymer matrix comprising a thermoplastic polymer blend having dispersed therein carbon fibers and electrically conductive carbon particles, the thermoplastic polymer blend comprising polytetrafluoroethylene (PTFE) and a thermoplastic polymer different from PTFE, e.g., PPS, and the carbon fibers having an average length L in the range of 0.1 to 1 mm.

[0097] Embodiment 27. The precursor sheet of embodiment 26, optionally wherein the carbon fibers have an average length L that is at least twice as large as the layer thickness X1.

[0098] Aspect 28. A precursor sheet according to aspect 26 or 27, wherein the thermoplastic polymer different from PTFE is polyphenylene sulfide (PPS).

[0099] Embodiment 29. The precursor sheet of any one of embodiments 26 to 28, wherein the weight concentration of the carbon fibers is in the range of 5 to 20 wt % based on the total weight of the polymer blend, the carbon fibers, and the conductive carbon particles.

[0100] Embodiment 30. A precursor sheet according to any one of embodiments 26 to 29, wherein the weight concentration of PTFE is at least 0.05 wt% but less than 0.5 wt% PTFE, and the weight concentration of the thermoplastic polymer different from PTFE, e.g., PPS, is 5 to 30 wt% based on the total weight of the polymer blend, the carbon fibers, and the conductive carbon particles.

[0101] Embodiment 31. The precursor sheet of any one of embodiments 26 to 30, wherein the conductive carbon particles in the thermoplastic polymer blend include at least a first portion and a second portion, wherein the carbon particles of the first portion are graphite particles having an average particle size in the range of 10 to 100 μm, and the carbon particles of the second portion have a particle size in the range of 0.1 to 10 μm, wherein the weight concentration of the first portion is in the range of 50 to 90 wt %, based on the total weight of the polymer blend, the carbon fibers, and the conductive carbon particles, and wherein the weight ratio between the first portion and the second portion of the conductive carbon particles is in the range of 5 to 20.

[0102] Embodiment 32. A method of manufacturing a separator plate, comprising: - mixing a thermoplastic polymer, carbon fibers, and conductive carbon particles in a dispersion, wherein the carbon fibers have an average length L in the range of 0.1 to 1 mm; - forming a formable, malleable, single-layer precursor sheet of thickness X1 from the mixture by calender rolling in a forming station; - hot compression molding the precursor into a monolayer separator plate to a thickness X2 in the range of 0.05 to 0.6, for example 0.05 to 0.3 mm, where X2 is smaller than the average length L of the carbon fibers; Manufacturing method.

[0103] Embodiment 33. The method of embodiment 32, comprising providing carbon fibers having an average length L at least twice as large as the layer thickness X2.

[0104] Aspect 34. The method of aspect 32 or 33, wherein the thermoplastic polymer is a blend comprising PTFE and a thermoplastic polymer different from PTFE.

[0105] Embodiment 35. The method of embodiment 34, wherein the thermoplastic polymer different from PTFE is polyphenylene sulfide (PPS).

[0106] Embodiment 36. The method according to any one of embodiments 32 to 35, comprising: providing an aqueous dispersion comprising PTFE particles and carbon fibers, the carbon fibers having an average length L; providing a solvent dispersion comprising carbon black particles and particles of a second thermoplastic polymer different from PTFE, such as PPS, dispersed in an organic solvent; - stirring both dispersions to prevent settling of the particles; - combining and mixing two dispersions; - mixing a portion of the graphite particles with two dispersions, the graphite particles of said portion having an average particle size in the range of 10 to 100 μm, and said portion having a weight that exceeds the weight of the carbon black by 5 to 20 times; - kneading the mixture in a kneader; - during kneading in the kneader, increasing the temperature to a high enough level to evaporate the organic solvent and water from the mixture, the high temperature level being above the glass transition temperature of PTFE; - forming the mixture into a precursor sheet at a forming station after evaporating the organic solvent and water and while the second thermoplastic polymer is in a molten state; A method comprising:

[0107] Embodiment 37. The method of embodiment 36, wherein the portion of graphite particles is a second portion of graphite particles, and the method comprises providing an aqueous dispersion comprising the first portion of graphite particles in addition to PTFE and carbon fibers, wherein the graphite particles of the first portion have an average particle size in the range of 0.1 to 10 μm.

[0108] Embodiment 38. The method of any one of embodiments 32 to 37, wherein the elevated temperature level during kneading in the kneader is below the melting temperature of the PTFE and the second thermoplastic polymer, and the method includes removing the mixture from the kneader and then increasing the temperature of the mixture to a level high enough to melt the second thermoplastic polymer before forming the sheet into a precursor sheet of thickness X1 at a forming station.

[0109] Embodiment 39. The method of embodiment 38, wherein providing the aqueous dispersion comprises adding a surfactant to the aqueous dispersion, the surfactant having a boiling point greater than the boiling point of water and greater than the boiling point of the organic solvent, and wherein the method comprises removing the mixture from the kneader while the mixture comprises the surfactant but not the solvent or water, and then increasing the temperature of the mixture to a level high enough to evaporate the surfactant before forming the sheet into a slab of thickness X1 at a forming station.

[0110] Embodiment 40. The method of any one of embodiments 32 to 39, wherein the weight concentration of PTFE in the mixture is at least 0.05 wt% but less than 0.5 wt% PTFE, the weight concentration of the thermoplastic polymer different from PTFE is in the range of 5 to 30 wt%, and the weight concentration of the carbon fibers is 2 to 20 wt%, the weight percentages being relative to the combined weight of the carbon fibers and carbon particles, PTFE, and thermoplastic polymer.

[0111] Embodiment 41. The method of any one of embodiments 32-40, comprising calendering the precursor sheet in at least two different directions to align the carbon fibers in different directions.

[0112] Embodiment 42. A rigid rolled and press-formed separator plate for a fuel cell, the separator plate being formed as a single layer from a monolayer precursor sheet, the single layer having a thickness X2 in the range of 0.05-0.6, e.g., 0.05-0.3 mm, and provided as a polymer matrix comprising a thermoplastic polymer blend having dispersed therein carbon fibers and conductive carbon particles, the thermoplastic polymer blend comprising PTFE and a thermoplastic polymer different from PTFE, e.g., PPS, the carbon fibers having an average length L in the range of 0.1-1 mm and greater than the layer thickness X2.

[0113] Embodiment 43. The separator plate of embodiment 42, wherein the carbon fibers have an average length L that is at least twice as large as the layer thickness X2.

[0114] Aspect 44. The separator plate of aspect 42 or 43, wherein the weight concentration of the carbon fiber is in the range of 5 to 20 wt %, based on the total weight of the polymer blend, the carbon fiber, and the conductive carbon particles; the weight concentration of the PTFE is at least 0.05 wt % but less than 0.5 wt % PTFE; and the weight concentration of the thermoplastic polymer other than PTFE, e.g., PPS, is 5 to 30 wt %, based on the total weight of the polymer blend, the carbon fiber, and the conductive carbon particles.

[0115] Embodiment 45. The separator plate of embodiment 42, 43, or 44, wherein the conductive carbon particles in the thermoplastic polymer blend include at least a first portion and a second portion, wherein the carbon particles of the first portion are graphite particles having an average particle size in the range of 10 to 100 μm, and the carbon particles of the second portion have a particle size in the range of 0.1 to 10 μm, wherein the weight concentration of the first portion is in the range of 50 to 90 wt %, based on the total weight of the polymer blend, the carbon fibers, and the conductive carbon particles, and wherein the weight ratio between the first portion and the second portion of the conductive carbon particles is in the range of 5 to 20.

[0116] The invention will now be explained in more detail with reference to the drawings. [Brief explanation of the drawings]

[0117] [Figure 1] 1 is a continuous process scheme for making a graphite-based compound, preforming the compound into slabs, and then forming bipolar plates. [Figure 2] (a) Flexural strength and (b) areal resistivity as a function of thickness for MFG / SFG / CF / CB / PPS / PTFE-based BPPs. [Figure 3] A micrograph of the cross section of the slab is shown. [Figure 4] Figure 1 shows the thickness-dependent distributed load at break for MFG / SFG / CF / CB / PPS / PTFE-based BPPs (oval symbols indicate single-layer BPPs, cross symbols indicate multilayer BPPs). [Figure 5]A simplified diagram of a composite with conductive carbon particles of different particle sizes when an electric current is passed through it. a) The composite has only one particle size, while b) the composite has different particle sizes. [Figure 6] 1 shows the formation of a sheet in a rolling station. DETAILED DESCRIPTION OF THE INVENTION

[0118] The manufacturing method described herein combines several partial processes, namely, blending raw material powders, followed by their kneading and calendering into preform shapes such as thin slabs of a predetermined density, and then compression molding of such slabs to provide separator plates for fuel cells, and optionally electrode plates, end plates, or BPPs. Hereinafter, the method will be described with respect to BPPs, but the method equally applies to such various plates of a fuel cell or fuel cell stack. Therefore, all partial methods described below should also be read with respect to such other types of separator plates, although it is believed that the greatest advantages are achieved by this method with respect to BPPs.

[0119] In Figure 1, a scheme showing the production for producing BPP based on graphite, carbon fiber (CF), and carbon black (CB) and their combination in a polymer matrix with the polymers PPS and PTFE is shown.

[0120] It should be mentioned here that other thermoplastic polymers can also be used in the fabrication of BPP for high-temperature PEM fuel cells. Candidates include, among others, ethylene tetrafluoroethylene (ETFE), fluorinated ethylene propylene (FEP), perfluoroalkoxyalkane (PFA), polyamide-imide (PAI), polychlorotrifluoroethylene (PCTFE), polyetheretherketone (PEEK), polyetherketone (PEK), polyetherimide (PEI), polyethersulfone (PES), polyphenylsulfone (PPSU), polysulfone (PSU), and polyvinylidene fluoride (PVDF) (see also [Reference 25]).

[0121] The PTFE is provided, for example, in an aqueous dispersion at a relative concentration of 10-80% by weight, optionally 50-70% by weight, PTFE in water, for example, a 60% by weight aqueous dispersion. Such latter dispersions are commercially available, for example, from Merck®. Once purchased, the dispersion can optionally be further diluted to the appropriate concentration by mixing with deionized water. Additionally, a surfactant may be added.

[0122] As shown in Figure 1, PTFE, CF, and graphite, as well as other potential ingredients such as surfactants, are provided in a first container 1. The surfactant content does not exceed 10% by volume of the total liquid composition, typically in the range of 0.2-2% by volume. It should be noted here that PTFE dispersions from suppliers typically already contain a small amount of surfactant to prevent agglomeration of the polymer particles [Reference 26].

[0123] Non-limiting examples of surfactants include the Tergitol™ 15-S series from Dow Chemicals®, the Triton® X series from Union Carbide Corporation®, or the Tween® series from Croda International®. For example, Triton X-100™ from the Triton X series has a hydrophilic polyethylene oxide chain and an aromatic hydrocarbon lipophilic or hydrophobic group. The hydrocarbon group is a 4-phenyl group. The formula is: 14 H 22 O(C2H4O) n where n is 9 to 10.

[0124] For example, a useful mass ratio between the solid and liquid phases in the first vessel 1 is in the range of 1:4 to 1:6, e.g., about 1:5. On the one hand, a low amount of water is desired in the process to minimize energy and resource consumption in the process, and on the other hand, the process requires a sufficient amount of liquid to maintain adequate dispersion.

[0125] Optionally, the CFs are provided with an average length in the range of 0.1-1 mm, e.g., a length in the range of 0.2-0.4 mm, e.g., about 0.3 mm. The lengths given here are average lengths. For example, the statistical distribution associated with the averaged lengths has a FWHM of less than or on the order of ±20%. As will become more apparent below, the average length of the CFs is selected relative to the layer thickness X1.

[0126] Advantageously, a small proportion of graphite (SFG) is added. The term "small proportion" refers to a percentage in the range of 2-10% by weight relative to the final product, i.e., total dry weight without liquid. Optionally, the SFG graphite particles have an average particle size in the range of 0.1-10 μm, for example in the range of 0.5-2 μm. In experiments, the average particle size of the SFG graphite particles was 1 μm.

[0127] As previously mentioned, the particle sizes given herein for polymer particles and carbon particles, including graphite and carbon black, are average particle sizes, meaning the average over three dimensions of the particle and the average over the number of particles of that particular group or type. Typically, for particles, the statistical distribution associated with the dimensionally averaged particle size has a FWHM of less than or on the order of ±20%.

[0128] In this first container 1, all ingredients are intensively mixed.

[0129] An example of the weight ratio of PTFE, CF, and graphite (SFG) is in the range of (0.05-0.5):(0.05-15):(0.05-15). In our experiments, this ratio was 0.25:10:5.

[0130] In parallel, a second powder mixture is provided in a second container 2. This second powder mixture optionally comprises PPS powder having an average particle size in the range of 10-100 μm, such as in the range of 20-30 μm, for example about 25 μm.

[0131] Furthermore, this second container 2 is optionally provided with CB particles having an average particle size in the range of 10-100 nm, such as in the range of 30-50 nm, for example about 40 nm.

[0132] The particles in the second vessel 2 are mixed with N-methyl-2-pyrrolidone (NMP) to form a viscous slurry. NMP, with its polarity and low surface tension, has excellent wetting properties and is therefore provided in vessel 2 to wet the hydrophobic PPS and CB particles before dispersing them from the first vessel 1 into the aqueous medium [Reference 27]. It has been noted that NMP can dissolve approximately 10% by weight of PPS at 203°C [Reference 28]. At temperatures below this, NMP likely dissolves only a very thin layer near the surface of the polymer particles [Reference 29]. Because NMP is miscible with water at all temperatures [Reference 30], it acts as a "bridge" for water molecules, delivering them directly to the surface of the hydrophobic particles.

[0133] For this purpose, several other solvents can be used instead of NMP, such as N,N-dimethylacetamide, N,N-dimethylformamide, and dimethyl sulfoxide. These solvents, when used in combination with surfactants, can produce long-term stable PPS dispersions, but the process occurs at temperatures in the range of 220–320 °C

[31] , which is not optimal.

[0134] The distribution of some of the carbon particles to the first container 1 and the other to the second container 2 is based on the consideration that the overall volume of liquid should be minimized to avoid unnecessary energy consumption for subsequent liquid evaporation. In principle, it would be possible to add all of the particles to the second container 2 along with the NMP, but in that case, the solids content in container 2 would require an unnecessarily large amount of NMP to achieve an acceptable particle concentration for efficient mixing. Furthermore, this manufacturing method allows for the minimization of organic solvent usage, making the method environmentally friendly, especially if the solvent is recycled. CB and PPS were chosen to be dispersed in the second container 2 because these carbon particles are less wettable with water than NMP. SFG and CF, on the other hand, are less hydrophobic and therefore more suitable for addition to the aqueous dispersion in the first container 1.

[0135] In this case, settling or agglomeration of the PPS microparticles in the second vessel 2 is avoided because the dispersion is continuously stirred until the main filler, i.e., graphite, is added. After adding significant concentrations of graphite, the viscosity of the system becomes so high that settling becomes almost impossible, even if stirring is stopped for a long period of time. As stirring is no longer feasible, a kneader is used in the next stage, which will be described in more detail below.

[0136] After preparing these two dispersions separately in the first container 1 and the second container 2, they are mixed in a third container 3, for example as shown in FIG. 1, for uniform distribution of the polymer and carbon particles. Usually, the content of NMP in water after mixing them in the third container 3 does not exceed 25% by volume relative to the total amount of the mixture in the container 3. For example, the concentration of NMP in the mixture in the third container 3 is in the range of 10 to 25% by volume.

[0137] Once this mixing process step is complete, the SFG / CF / CB / PPS / PTFE suspension is transferred from the third vessel 3 to a kneader 5 along with a quantity of graphite from the fourth vessel 4. Because the quantity of graphite from the fourth vessel 4 is relatively large and contributes a relatively large proportion in the final mixture, it is referred to as the "major proportion of graphite" (MFG). The ratio between MFG:SFG is at least 2, e.g., at least 3, but typically at least 5, and typically up to 20, e.g., in the range of 5-20. In experiments, an MFG:SFG ratio of 13 has been used.

[0138] For example, the relative content of MFG in wt% relative to the dry mass of the mixture in the kneader 5 is in the range of 40-80 wt%. In the experiment, the concentration of MFG was 66.25 wt% relative to the dry mass.

[0139] Optionally, the graphite for this MFG has an average particle size in the range of 10-100 μm, e.g., 10-30 μm. In experiments, the average particle size of the graphite particles in the MFG was 20 μm. Note that the particle size of the MFG is an order of magnitude larger than that of the SFG.

[0140] MFG is the primary carbon component of the composite. Figure 5 provides a simplified illustration of the benefits of different particle sizes. While Figure 5a presents only one carbon particle size, Figure 5b clearly shows how smaller particles act as conductive bridges between larger particles, lowering the overall resistivity of the final composite. When constructing such composite matrices, realistic and competitive production costs for commercial products must also be considered. Therefore, the amount of relatively expensive nano-sized graphite must be balanced against the benefits of its concentration. The addition of CB represents a compromise between high conductivity and competitive production costs.

[0141] Due to the addition of MFG, the solids content in the mixture in the kneader 5 increases, reaching a solids content in the range of 30-70 wt.%, for example in the range of 40-60 wt.%, for example about 50 wt.%. The mixing procedure in vessels 1, 2, 3 and 4 is carried out at a first temperature level T1, typically room temperature, for example in the range of 20-25°C, which is sufficient for a uniform distribution of the powdered materials.

[0142] This compares favorably with the process described in [Reference 32], in which the polymer used to bind the carbon particles must be completely dissolved in a suitable solvent, which requires temperatures in excess of 200 °C, which is not so easy for highly engineered plastics like PPS. Worse yet, temperatures in excess of 300 °C are required to achieve a solubility of PPS greater than 50 wt% [Reference 28]. Furthermore, suitable solvents typically have high boiling points, making their further removal from the compound prior to molding problematic.

[0143] The kneading of the MFG / SFG / CF / CB / PPS / PTFE mixture in the kneader 5 begins at a first temperature level T1, e.g., room temperature, and continues while the mixture is heated to an elevated temperature above the boiling point of the liquid used in order to remove the liquid from the mixture by evaporation. Kneading during evaporation prevents or at least minimizes the risk of bubble formation.

[0144] However, the kneading process typically takes 10 to 30 minutes, depending on the rate of temperature increase and evaporation of the liquid.

[0145] The mixture is heated from a first temperature level T1, for example, in the range of 20-25°C, and the temperature is gradually increased from the first temperature level T1 to a second temperature level T2, which is the boiling point of water, to remove water by evaporation.

[0146] To ensure that the kneading step continues without the formation of bubbles due to residual water in the mixture, the mixture is heated to a third temperature level T3 that is sufficiently higher than the boiling point of water, for example, in the range of 102 to 120°C.

[0147] In the experiment, kneading continues at a third temperature level T3, which is 116°C, well above the boiling point of water, thus ensuring that all water is removed from the mixture.

[0148] PTFE undergoes a phase change at its glass transition temperature, determined to be 116 °C in this case, where the polymer becomes rigid and amorphous

[33] , and has been shown to have an increased tendency to form fibers from nanoparticles. This makes the compound softer and easier to knead. Fiberization is ultimately beneficial in that it increases the cohesive forces between the components in the mixture.

[0149] After the water is removed by evaporation, the temperature is further increased to a fourth temperature level T4 to remove the solvent by evaporation. In the experimental examples herein where NMP was used as the solvent, the fourth temperature T4 was adjusted to 204°C, the boiling point of NMP, to remove the NMP by evaporation [Reference 30].

[0150] Optionally, for recycling purposes, all evaporated material can be concentrated back into the liquid phase in a further vessel 6 and separated into pure solvents. Useful separation methods include distillation and / or membrane separation [References 34, 35]. Water and NMP are collected in separate vessels 7 and 8, respectively, and returned to the production process. It should be noted that small amounts of dissolved surfactants are acceptable.

[0151] After kneading, the soft compound is removed from the kneader 5 as a dough-like malleable mass and extruded onto a heated conveyor 9 .

[0152] To remove further high-boiling liquids, such as nonionic surfactants, from the compounded MFG / SFG / CF / CB / PPS / PTFE mixture [Reference 26], the temperature of the mixture is further increased to a fifth temperature level, T5, which causes evaporation of the surfactant. For example, the fifth temperature level, T5, is below the melting point of PPS, which is in the range of 271–292 °C. In experiments, the mixture was heated to the fifth temperature level, T5, of 270 °C, which removed the surfactant but did not melt the PPS.

[0153] This temperature increase can occur while the mixture is still in the kneader. However, for smooth extrusion of compounds with a relatively low polymer content, it has been found to be advantageous if the compound contains some surfactant, which does not evaporate. Therefore, the temperature is increased to level T5 after extrusion from the kneader to evaporate the surfactant, since the surfactant is no longer needed after extrusion. For example, the boiling point of the surfactant Triton® X-100 is 270°C [Reference 36]. This temperature increase can also occur on the conveyor. While in the conveyor, the mixture's temperature is further increased to a sixth temperature level T6, which is higher than the melting point of the second thermoplastic polymer. In this experiment, this was 347°C to melt the PPS and reduce its viscosity. However, the final temperature depends on the molding parameters selected.

[0154] A conveyor 9 advances the dough-like structure through at least one rolling station 10, with the gap between the calender rollers adjusted as needed, to calender and press the dough-like structure into a sheet of a predetermined thickness, typically in the range of 0.05 to 10 mm. This process is shown in more detail in FIG. 6. However, as will become apparent from the discussion below, for BPP, a thickness of less than 1 mm is advantageous to save material and weight and achieve good performance. For example, the thickness may be adjusted to a value in the range of 0.05 to 1 mm.

[0155] It should be noted that multiple rolling stations are typically required to produce films at the lower end of the thickness range. After the film is formed from the malleable structure, a cutting tool 11 cuts it into slabs.

[0156] In some experiments, thicknesses of 0.1 mm and 0.6 mm were used for comparison; 0.1 mm slabs were used for stacks of six precursor slabs, while 0.6 mm slabs were used for single-layer precursors and BPPs. The thickness of the BPP obtained after compression molding was 0.3 mm, which is half the thickness of the precursor and is due to the thickness reduction caused by compression. In this regard, the following should be noted: the slabs, as used in the experiments, had a density of approximately 1 g / cm. 3 etc., usually 0.5 to 1.5 g / cm 3 However, compression molded BPP has a density in the range of 2 g / cm 3 Therefore, a thicker precursor slab, up to about twice as thick, must be provided. However, it has been suggested that the reduction in thickness can be partially compensated for by the formation of a flow field during compression molding.

[0157] The experimental dimensions of the slabs are 400 x 100 mm, excluding thickness, but it is possible to produce slabs of any size by this method.

[0158] Finally, the preheated graphite-based slab is formed into BPP in a hot press machine 12 by compression molding. Compression molding is advantageously carried out at a temperature between the melting points of the thermoplastic polymers used. For example, in the case of PPS-PTFE, the molding temperature is advantageously in the range between the melting point of PPS (271-292°C) and the melting point of PTFE (320-347°C). Optionally, the temperature is in the range of 300-320°C.

[0159] Alternatively, the temperature should be slightly higher than the melting point of PTFE, but below the decomposition temperature of PPS (around 475°C) [Reference 37].

[0160] The molding temperature generally depends on the molecular weight of the polymers and their melting points, as well as their behavior during thermal processing [References 38-40].

[0161] This temperature range helps prevent the slab from sticking to the mold because not all of the polymer melts. Additionally, PTFE can act as an anti-stick component in MFG / SFG / CF / CB / PPS / PTFE compounds.

[0162] During molding, the applied pressure is typically in the range of 25-225 MPa, for example in the range of 75-175 MPa. In the experiments, a pressure of 125 MPa was used.

[0163] The processing time is determined by the cooling rate of the BPP in the mold. The pressure is released when the mold temperature drops below the glass transition temperature of PPS, i.e., below 93°C

[38] .

[0164] It should be noted that BPPs can be fabricated either as a single layer using one slab or as a stack of multiple slabs, e.g., 4 to 8 slabs. In this experiment, a multi-layer slab was obtained using six slabs, which were then pressed into a multi-layer BPP.

[0165] Each component of the compound thus produced has a specific purpose: MFG is the primary filler for PPS, while the other additives improve both the mechanical properties, particularly influenced by CF, and the electrical properties, particularly influenced by SFG and CB, as well as the ability to bind finely powdered materials together in a malleable, dough-like structure, achieved in particular by PTFE.

[0166] The percentage ratios between all these components in the final mixture can vary within certain constraints. For example, all percentages are by weight: The MFG content is in the range of 25-90% by weight, for example 50-90% by weight. In the experiments, a proportion slightly less than 70% was used. The minimum amount of PPS is 5% by weight, typically less than 30%. In the experiments, 20% by weight was used. - The total content of additives, SFG, CF, CB, PTFE in the final compound is usually less than 45% by weight. The CF content is in the range of 2-20% by weight, but advantageously in the range of 3-15% by weight, for example in the range of 5-15% by weight. - Example ranges of ingredients are 25-90 wt% MFG, 5-30 wt% PPS, 2-20 wt% CF, 0.05-15 wt% SFG, 0.05-10 wt% CB, 0.05-5 wt% PTFE, for example at least 0.05 wt% but less than 0.5 wt% PTFE.

[0167] All weight percentages are given relative to the polymer blend with particles and fibers, and therefore the polymer blend after liquid removal.

[0168] Experimentally, optimal electromechanical properties were demonstrated with a liquid-free final compound containing the following amounts of the individual components: MFG (66.25 wt%), PPS (17.50 wt%), CF (10.00 wt%), SFG (5.00 wt%), CB (1.00 wt%), and PTFE (0.25 wt%).

[0169] Figure 2 shows the thickness dependence of the bending strength and area specific resistance of BPP made from compounds with such component ratios.

[0170] As can be seen from the figures presented above, these two dependencies are nonlinear. Decreasing the BPP thickness leads to a decrease in areal resistivity, as seen in Figure 2b, and a significant increase in bending strength, as seen in Figure 2a. In particular, the thickness-dependent bending strength increases with decreasing thickness, deviating from a quasi-linear shape for thicknesses below 1 mm, particularly for BPP thicknesses below 0.5 mm. The increase in bending strength at smaller thicknesses is likely due to the planar orientation of CF in the near-surface layer. This near-surface layer, formed during rolling of the slab, likely provides improved mechanical properties compared to the remaining volume below the near-surface layer, where the nearly perpendicular orientation of CF is maintained. This explanation suggests the emergence of a self-organized layer structure, which is experimentally verified by microtomography, the image of which is reproduced in Figure 3, showing the existence of two main zones in the slab with different CF orientations. This is an important finding, and we emphasize that it can be exploited to great advantage, as we will explain below.

[0171] In ultrathin slabs and the resulting BPPs, there is insufficient space for the CFs to be oriented perpendicular to the slab, which is mechanically weaker. This requires the CFs to be oriented at least partially parallel to the slab, resulting in a superior mechanical structure, as demonstrated by the exponential increase in flexural strength with decreasing thickness, as reflected in Figure 2a. It should be noted here that the flexural strength of PPS is in the range of 125–135 MPa

[41] , and therefore this behavior appears to be related to the anisotropy of the carbon materials used, primarily the CFs. Similar results were shown in

[42] , where good correlation with mathematical models also exists

[43] .

[0172] To determine the minimum practical thickness of a BPP that can be used in a PEM fuel cell stack, the criterion of needing to withstand a pressure of 1 MPa is used, which is also recommended by one gas diffusion layer supplier [Reference 44]. Therefore, strength should also be considered in terms of the distributed load, which is shown for various BPP thicknesses in Figure 4. Figure 4 demonstrates that for a single-layer, i.e., single-slab-based BPP, the minimum thickness to meet the 1 MPa criterion is 0.38 mm.

[0173] However, for multilayer BPPs, the minimum thickness required to meet this criterion is significantly smaller—only 0.29 mm. This reflects the fact that the thinner the slabs used to fabricate the multilayer BPP, the higher its relative strength. This 0.29 mm minimum thickness is consistent with the simulation shown by the solid curve on the left side of Figure 2. However, this curve is only valid for small multilayer slab thicknesses and appears to reproduce the correct minimum thickness. As the thickness increases, the theoretical simulation curve in Figure 4 clearly shows a significant deviation from the experimental data obtained for multilayer BPPs. This can be explained by the effect of carbon fibers reverting from a planar orientation to a nearly perpendicular orientation during the molding process. This can occur when the thickness of the BPP exceeds the length of the carbon fibers used in the material, which in our experiments was 300 μm. In other words, improved strength is achieved when the layer thickness is less than the average length of the embedded carbon fibers.

[0174] However, it is noted that this effect is particularly pronounced in sub-millimeter thick layers, hence the advantage of multilayers in thicker slabs and correspondingly thicker separator plates such as BPP. If each layer of the multilayer stack is rolled as a separate slab before laminating the rolled layers into a multilayer slab, the resulting strength improvement due to realignment of CFs in the near-surface layers is correspondingly multiplied.

[0175] It is noteworthy that our experimental six-layer BPP, with a total thickness of 0.3 mm, produced in this way is not only the thinnest graphite-based BPP in the world at the time of writing this patent application, but also the strongest BPP to date.

[0176] Table 1 shows data collected on thickness, flexural strength, areal resistivity, and in-plane electrical conductivity for a six-layer MFG / SFG / CF / CB / PPS / PTFE-based BPP (referred to as "BWT" in the table) experimentally produced by the method outlined herein, compared to BPPs made from graphite-based compounds with PPS binders obtained from commercial suppliers for testing, and BPPs for which corresponding test data was obtained from the literature [References 15, 19, 45-48]. [Table 1]

[0177] It is pertinent to point out that for ultra-thin BWT BPPs, better results have been obtained than required by the DOE's 2020 targets [Reference 8]. Furthermore, compared to other graphite-based BPPs, many advantages have been obtained, namely higher strength and electrical conductivity, in addition to providing higher power density due to the reduced overall stack volume.

[0178] In comparison with the method of WO2018 / 072803 [Reference 53], it should be pointed out that the carbon powder in the present disclosure is mixed with a polymer and then ground into a carbon-polymer powder. If carbon fibers were part of such a carbon mixture, the grinding process would destroy many of the carbon fibers, and the beneficial results described above would not be obtained.

[0179] The main features achieved by this manufacturing method are shown in the following list: 1. An integrated manufacturing process, i.e., material blending, slab rolling, and BPP forming, is carried out continuously in one production line with a high utilization rate of raw materials, which is different from the method applied in [Reference 22]. 2. The use of a combination of different solvents, including water and organic solvents, facilitates the distribution and blending of various powder materials even at room temperature, which is advantageous over processes that require high temperatures, such as [References 17, 30]. 3. The solid content in the suspension is high compared to other prior art methods, especially [Reference 20], so evaporation and drying occur very quickly. 4. The dough-like structure of the slabs is obtained by adding much less PTFE, i.e., 0.25 vs. 2 wt. %, compared to other prior art, especially [Reference 22], thus reducing the negative effect of the polymer on the electrical properties of the BPP. 5. The flexibility of the slabs allows them to be rolled and subsequently formed in a wide range of thicknesses, the lower limit of which reaches 0.05 mm, which is very close to the theoretical value set by the dimensions of the largest components of the compound, i.e. 20 μm (MFG) + 25 μm (PPS). 6. The slab manufacturing process results in the formation of self-assembled laminated structures with enhanced mechanical properties, which is advantageous compared to prior art, particularly [Reference 52], where similar structures could only be achieved by additional coatings on the core plate. 7. By applying the multilayer design, the bending strength can be improved by 40%, resulting in a 25% reduction in the minimum required thickness of BPP acceptable for assembling PEM fuel cell stacks, taking into account the strength criteria. 8. Experimentally prepared 0.3 mm thick multilayer BPP demonstrated very high flexural strength at relatively low polymer content, i.e., 186 MPa at less than 18 wt. % PPS. 9. The small amount of polymer binder, combined with a low-profile design, ensures that the resistance of the BPP is a small percentage of the total resistance of the fuel cell.

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Claims

1. 1. A moldable and malleable precursor sheet for hot compression molding into an electrically conductive, rigid separator plate for a fuel cell, the precursor sheet being formed as a multilayer structure with multiple layers stacked together, or as a single layer, each layer of the single layer or multiple layers of the multilayer structure having a thickness X1, the precursor sheet being provided as a polymer matrix including a thermoplastic polymer blend having dispersed therein carbon fibers and electrically conductive carbon particles, the thermoplastic polymer blend including polytetrafluoroethylene (PTFE) and a thermoplastic polymer different from PTFE, such as PPS, the carbon fibers having an average length greater than the thickness X1 of the layer.

2. 2. The precursor sheet of claim 1, wherein the carbon fibers have an average length L that is at least twice as large as the layer thickness X1.

3. The precursor sheet of claim 2, wherein the precursor sheet is formed as a single layer having a thickness X1 in the range of 0.05 to 1 mm.

4. 3. The precursor sheet of claim 1, wherein the precursor sheet is formed as a multi-layer structure, and the thickness X1 is in the range of 0.05 to 0.5 mm, optionally 0.05 to 0.3 mm.

5. 5. The precursor sheet according to claim 1, wherein the weight concentration of PTFE is at least 0.05 wt % but less than 0.5 wt %, and the weight concentration of the thermoplastic polymer different from PTFE, for example PPS, is 5 to 30 wt %, relative to the total weight of the polymer blend, carbon fibers, and conductive carbon particles.

6. 6. The precursor sheet according to claim 1, wherein the conductive carbon particles in the thermoplastic polymer blend include at least a first portion and a second portion, the carbon particles in the first portion are graphite particles having an average particle size in a range of 10 to 100 μm, the carbon particles in the second portion have a particle size in a range of 0.1 to 10 μm, a weight concentration of the first portion is in a range of 50 to 90 wt % with respect to a total weight of the polymer blend, carbon fibers, and conductive carbon particles, and a weight ratio between the first portion and the second portion of the conductive carbon particles is in a range of 5 to 20.

7. The precursor sheet according to any one of claims 1 to 6, wherein the thermoplastic polymer is PPS.

8. The precursor sheet according to any one of claims 1 to 7, wherein the carbon fibers have an average length in the range of 0.1 to 1 mm.

9. A method for producing a separator plate by using a precursor sheet according to any one of claims 1 to 8, said method comprising: - mixing a thermoplastic polymer, carbon fibers and conductive carbon particles in a dispersion, said carbon fibers having an average length L in the range of 0.1 to 1 mm; forming from said mixture a formable, malleable precursor sheet of thickness X1 by calendering in a forming station; - hot pressing said precursor into a monolayer separator plate to a thickness X2 in the range of 0.05 to 0.6, for example 0.05 to 0.3 mm, or cutting said sheet into slabs and laminating a plurality of such slabs of thickness X1 together in the molten state into a stack and hot pressing said stack, wherein each layer of the plurality of layers after shaping has a thickness X2 in the range of 0.05 to 0.2 mm, X2 is smaller than the average length L of the carbon fibers; method.

10. 10. The method of claim 9, comprising providing the carbon fibers having an average length L at least twice as large as the layer thickness X2.

11. 11. The method of claim 9 or 10, wherein the thermoplastic polymer is a blend comprising PTFE and a thermoplastic polymer different from PTFE.

12. 12. The method of claim 11, wherein the thermoplastic polymer different from PTFE is polyphenylene sulfide (PPS).

13. The method according to any one of claims 9 to 12, providing an aqueous dispersion, the aqueous dispersion comprising PTFE particles and carbon fibers; providing a solvent dispersion comprising carbon black particles and particles of a second thermoplastic polymer different from PTFE, for example PPS, dispersed in an organic solvent; - stirring both dispersions to prevent settling of the particles; - combining and mixing the two dispersions; - mixing a portion of graphite particles with the two dispersions, the graphite particles of said portion having an average particle size in the range of 10 to 100 μm, said portion having a weight that exceeds the weight of the carbon black by 5 to 20 times; - kneading the mixture in a kneader; - during mixing in the kneader, increasing the temperature to an elevated temperature level high enough to evaporate the organic solvent and water from the mixture, said elevated temperature level being above the glass transition temperature of PTFE; forming the mixture into a precursor sheet at the forming station after evaporating the organic solvent and water and while the second thermoplastic polymer is in a molten state; A method comprising:

14. 14. The method of claim 13, wherein the portion of graphite particles is a second portion of graphite particles, the method comprising providing an aqueous dispersion comprising a first portion of graphite particles in addition to PTFE and carbon fiber, the first portion of graphite particles having an average particle size in the range of 0.1 to 10 μm.

15. 15. The method of claim 13 or 14, wherein the elevated temperature level during kneading in the kneader is below the melting temperature of the PTFE and the second thermoplastic polymer, the method comprising removing the mixture from the kneader and then increasing the temperature of the mixture to a level high enough to melt the second thermoplastic polymer before forming the sheet into a precursor sheet of thickness X1 at the forming station.

16. 16. The method of claim 15, wherein providing the aqueous dispersion comprises adding a surfactant to the aqueous dispersion, the surfactant having a boiling point above the boiling point of water and above the boiling point of the organic solvent, and the method comprises removing the mixture from the kneader while the mixture contains the surfactant but not the solvent or water, and then increasing the temperature of the mixture to a level high enough to evaporate the surfactant before forming the sheet into a slab of thickness X1 at the forming station.

17. 17. The method of any one of claims 13 to 16, wherein the weight concentration of PTFE in the mixture is at least 0.05 wt% but less than 0.5 wt% PTFE, the weight concentration of the thermoplastic polymer different from PTFE is in the range of 5 to 30 wt%, and the weight concentration of the carbon fibers is 2 to 20 wt%, said weight percentages being relative to the combined weight of the carbon fibers and carbon particles, PTFE, and the thermoplastic polymer.

18. 18. The method of any one of claims 9 to 17, comprising calendering the precursor sheet in at least two different directions to align the carbon fibers in different directions.

19. 10. A rigid rolled and press-formed separator plate for a fuel cell, comprising one or more precursor sheets according to any one of claims 1 to 8, the separator plate being formed as a multi-layer structure in which a plurality of precursor sheets are stacked in layers, or as a single layer of a single precursor sheet, the precursor sheet in the single layer or in each of the plurality of layers of the multi-layer structure having a thickness X2 in the range of 0.05 to 0.6, for example 0.05 to 0.3 mm, and provided as a polymer matrix comprising a thermoplastic polymer blend having dispersed therein carbon fibers and conductive carbon particles, the thermoplastic polymer blend comprising PTFE and a thermoplastic polymer different from PTFE, for example PPS, the carbon fibers having an average length L in the range of 0.1 to 1 mm and greater than the thickness X2 of the layer.

20. 20. The separator plate of claim 19, wherein the carbon fibers have an average length L that is at least twice as large as the layer thickness X2.

21. 21. The separator plate of claim 19 or 20, wherein the weight concentration of the carbon fibers is in the range of 5 to 20 wt %, based on the total weight of the polymer blend, the carbon fibers, and the conductive carbon particles; the weight concentration of PTFE is at least 0.05 wt % but less than 0.5 wt % PTFE; and the weight concentration of the thermoplastic polymer different from PTFE, e.g., PPS, is 5 to 30 wt %, based on the total weight of the polymer blend, the carbon fibers, and the conductive carbon particles.

22. 22. The separator plate of claim 19, 20, or 21, wherein the conductive carbon particles in the thermoplastic polymer blend include at least a first portion and a second portion, the carbon particles of the first portion are graphite particles having an average particle size in a range of 10 to 100 μm, and the carbon particles of the second portion have a particle size in a range of 0.1 to 10 μm, the weight concentration of the first portion is in a range of 50 to 90 wt % with respect to the total weight of the polymer blend, carbon fibers, and conductive carbon particles, and the weight ratio between the first portion and the second portion of conductive carbon particles is in a range of 5 to 20.

23. 23. The separator plate of claim 19, wherein the separator is provided as a multilayer structure including a plurality of layers in a stacked state, each layer of the plurality of layers having a thickness X2 in the range of 0.05 to 0.2 mm.

24. The separator plate has a resistance of up to 2 mΩ·cm per 0.3 mm thickness unit. ２ 24. The separator plate of claim 23 having an areal resistivity of

25. 25. The separator plate of claim 23 or 24, wherein the separator plate has a flexural strength of greater than 180 MPa per 0.3 mm thickness unit.

26. A fuel cell comprising the separator plate according to any one of claims 19 to 25 or a separator plate provided as a hot-press-formed plate from the precursor sheet according to any one of claims 1 to 8.

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