Separator for fuel cells, precursor thereof and method for producing the same

Abstract

To produce separators in fuel cells, a ductile precursor sheet is made by mixing a thermoplastic polymer, carbon fibers and electrically conductive carbon particles, and then hot-pressing the precursor sheet into a single layer or a multi-layer structure, or a multi-layer structure where the layer thickness is less than the length of the carbon fibers.

Description

TECHNICAL FIELD

[0001] The present invention relates to separators for fuel cells, such as bipolar plates. The invention also relates to a hot-press mould for moulding such separators and to a precursor for the production thereof. BACKGROUND

[0002] Bipolar plates (BPPs) are key components of fuel cell stacks as they separate individual membrane electrode assemblies and deliver fuel to the membrane [Ref. 1]. High temperature proton exchange membrane (PEM) fuel cells have a great advantage over low temperature PEM fuel cells as they have a high tolerance to impurities in the input gas, especially to carbon monoxide residues in hydrogen gas [Ref. 2]. However, the relatively high operating temperature in the range of 160-200 °C and the acidic medium set strict limitations on those materials suitable for bipolar plates [Ref. 3]. Graphite as a material seems very attractive as it has high electrical and thermal conductivity and good chemical stability [Ref. 4]. Other carbon materials such as carbon black (CB), carbon fibres (CF), carbon nanotubes (CNT) and graphene can be used as additives for composites of BPPs in order to improve mechanical and electrical properties [Refs. 5, 6]. However, in order to keep the carbon particles together, the use of a binding material, such as a thermoplastic polymer, is required. Polyphenylene sulfide (PPS) is a good candidate material as it meets all the strict requirements for high temperature PEM fuel cells, including chemical inertness, having a high use temperature above 200 °C and having good mechanical properties [Ref. 7].

[0003] Alternative BPP structures are disclosed in CN111048800A [Ref. 58] and US7887927 [Ref. 59], in which a multi-layer BPP is produced from stacked carbon fibre or graphite sheet materials with an electrically conductive polymer filler in between.

[0004] The US Department of Energy (DoE) announced high targets for BPPs for transportation applications in 2020, in which the area specific resistance should be less than 10 mW-cm 2and the flexural strength must exceed 25 MPa [Ref. 8]. It should be noted here that typically, for graphite-based BPPs, the thickness is in the range of about 1.5 mm - 3 mm [Refs. 9-12]. However, recent research work has demonstrated graphite BPP samples based on graphitized compounds with thicknesses less than 1 mm, for example 0.85 mm in [Ref. 13], 0.6 mm in [Ref. 14], or even 0.4 mm in [Ref. 15], which brings some benefits such as more compact fuel cell design and cost reduction due to lower material consumption. To produce thin BPPs from powdered compounds, a multi-step process is disclosed in [Ref. 16]. It is desirable to provide a simple but effective process for mass production of BPPs.

[0005] Few patents and research papers describe processes for making carbon / PPS compounds and corresponding BPPs [Refs. 17-20]. For example, in international patent application WO2014 / 100082 [Ref. 17], PPS is mixed with carbon nanotubes (CNTs) in a hot extruder where the polymer is melted. Typically, the extrusion process requires high polymer loading, usually exceeding 50 wt.%, which unfortunately has a negative impact on the electrical properties of such compounds. After extrusion, the PPS-based compound can be shaped into a sheet, for example as presented in US patent No. 7,736,786 [Ref. 18].

[0006] Chinese patent application CN101174695A

[63] discloses a dry method for producing BPPs made of a material containing 45% - 55% graphite, 5% - 10% carbon fibers, 30% - 40% PPS, and 5% - 10% fluorine-containing polymer resin (e.g. PTFE). The depth of the flow field structure is 0.5 mm - 1.5 mm within a sheet of about 5 mm thickness. The carbon fiber length is in the range of 10 microns - 200 microns. The powder mixture is first cold-pressed at 3 MPa - 5 MPa, then slowly heated to 310°C - 330°C and pressed at 15 MPa - 25 MPa for 30 minutes to compact, after which it is rapidly cooled in water to prevent crystallization and increase toughness.

[0007] US2014 / 087287 [Reference 60] discloses a dry method for making separators for fuel cells, where the separator is made from two sheets A and B. Sheet A comprises 100 parts of a thermoplastic resin and 130-3200 parts of a carbonaceous material, and sheet B comprises 100 parts of a thermoplastic resin (e.g. polypropylene) and 3-280 parts of a carbonaceous material. Half of the carbonaceous material is fibrous carbon. The fiber length is in the range of 0.001 mm-20 mm, more preferably 1 mm-10 mm, and 6 mm is exemplified. The sheets are molded by compression molding the powder mixture at a temperature 60 degrees higher than the higher of the melting points of its respective binder components. The groove depth in the final plate is 0.3 mm and 0.5 mm in a 1 mm thick separator.

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

[0009] US2009 / 0152105 [Reference 62] discloses a method of producing a compression molded mat in a fuel cell, where 1 inch long carbon fibers are blended into molten PTFE, which is then hardened and shredded in a coffee grinder. The blend is then mixed with PVDF, and heated to 200°C while compression molded. Alternatively, the carbon fibers are melted into PVDF. The carbon fibers are preferably 3 mm long (1 / 8 inch). The mat is typically 10-15 mils thick, corresponding to approximately 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 of producing a fuel cell separator plate (e.g. 1 mm thick). In the method, carbon fibers are dispersed with a fibrous resin in an aqueous dispersion, after which the slurry is dried to obtain a paper-like composite sheet, as a next step, carbon particles are pressed into the sheet at high temperature. The length of the carbon fibers is not limited, but is for example in the range of 20 microns to 6 mm, and 3 mm of a 2 mm thick composite sheet is exemplified.

[0012] US2009 / 0152105 [Ref. 62] discloses a method of production of compression molded pads in fuel cells, where 1 inch long carbon fibers are blended into molten PTFE, then the PTFE is hardened and shredded in a coffee grinder. The blend is then mixed with PVDF and heated to 200°C while compression molded. Alternatively, the carbon fibers are melted into PVDF. The carbon fibers are preferably 3 mm long (1 / 8 inch). The typical thickness of the pads is 10-15 mils, roughly corresponding to 0.25-0.40 mm.

[0013] US2018 / 0358630 [Ref. 65] discloses a method of manufacturing bipolar plates, where two carbon powders with different powder sizes are mixed and compression molded is used. US2004 / 033413 [Ref. 66] discloses a polymer electrolyte membrane for a fuel cell, where the membrane has an electrically conductive coating. US2017 / 298200 [Ref. 67] discloses a thermoplastic prepreg intermediate material for a fuel cell separator and a method for manufacturing the thermoplastic prepreg, where the thermoplastic prepreg is compression molded at a temperature above the polymer melting point.

[0014] A wet process, such as the one described in [Ref. 19] and US2019 / 0341630 [Ref. 20], means that graphite, carbon fibers and PPS binder are mixed in a liquid phase, in particular in water, after which the solid mixture is filtered and shaped into a sheet, to be further dried and molded into a BPP. However, disadvantageously, PPS is very difficult to wet with water, as it can only absorb about 0.1% of water after 500 hours at 23°C [Ref. 21]. Therefore, dispersing PPS in an aqueous medium without causing the particles to agglomerate is a challenge. It is likely that this is why in [Ref. 19, 20] highly diluted dispersions are applied, where the total solids content does not exceed 10 wt%, and preferably is kept in the range of 1-3 wt%.

[0015] In [Ref. 22] it is also proposed a process of making a sheet from a powdered compound (prepared by dry mixing of graphite and PPS), where the compound is dispersed together with another thermoplastic polymer, namely polytetrafluoroethylene (PTFE), in isopropyl alcohol, which is a good surfactant for wetting hydrophobic surfaces [Ref. 23]. The temperature is raised to the boiling point of the alcohol, which causes the PTFE to coagulate rapidly, forming a pliable and ductile material, similar to a dough-like structure, see US2019 / 0260037 [Ref. 22], since PTFE has the ability to stretch high, compared to other polymers, as it can reach 550% [Ref. 24].

[0016] In the patent literature various processes are disclosed related to the handling of PPS (e.g. US 8563681 [Ref. 30]) and various processes for forming articles from PPS (e.g. US 5043112 [Ref. 27]) as well as the production of BPP (e.g. US 2008 / 0318110 [Ref. 31] and US 2006 / 0084750 [Ref. 50]).

[0017] US 6544680 [Ref. 54] discloses a molded separator with carbon and PPS, added with a thermosetting resin. US 6803139 [Ref. 55] discloses a molded separator with carbon and a thermoplastic, such as polyphenylene sulfide (PPS), and added with a carbodiimide. EP 1758185 [Ref. 56] discloses a molded separator with 84% carbon, 2% PTFE, 14% epoxy cured in hot press. Polyphenylene sulfide (PPS) or polytetrafluoroethylene (PTFE) are mentioned as thermoplastic resins, but not exemplified. US 2005 / 0042496 [Ref. 57] discloses a continuous process in which a polymer is blended, kneaded and extruded with a filler, such as graphite, after which it is converted into a compacted form, such as into a separator.

[0018] International patent application WO 2018 / 072803 [Ref. 53] discloses a production method of a separator in which a powder mixture of carbon and a thermoplastic polymer is suspended in alcohol and mixed with a water suspension of PTFE, after which the liquid is evaporated and the final ductile slurry is rolled into a sheet and compression molded into a separator. Examples of carbon powder include graphite, carbon black, graphene, carbon nanotubes or amorphous carbon.

[0019] The use of a ductile sheet or slab to mold a BPP seems to be also beneficial in terms of easier storage, handling and metering compared to compression powder compounds for BPP. However, in order to optimize such a process and the resulting BPP, further improvements are desired, especially in view of the targets set in the US Department of Energy 2020 plan. Since the production method is also easy, in order to be successful, cost and production speed are important factors to consider. SUMMARY

[0020] It is an object of the present invention to provide an improvement in the art. In particular, it is an object to provide an improved separator, such as a BPP, and an improved fuel cell with such a separator. It is a further object to provide an improved method for providing a separator, such as a BPP, as well as an improved material mixture of carbon and polymer, and a corresponding precursor for compression molding of a separator. This is achieved using a production method as described in the claims and below and using a precursor for hot press plastic molding of a separator, such as a BPP.

[0021] This object is achieved by the preparation of a graphite-based compound with a thermoplastic polymer therein and a moldable, ductile precursor sheet thereof for hot-press plastic molding into an electrically conductive rigid separator for a fuel cell, such as a BPP.

[0022] Although the object of the invention is to produce a BPP for a fuel cell stack, it is equally valid for the production of separators in general, as the process and materials are equally well applicable. In particular, the method is equally applicable for the production of single electrode plates for a fuel cell as well as end plates for a fuel cell stack. Examples of configurations are given in WO2018 / 072803 [Ref. 53]. For example, a separator is a bipolar plate with fluid flow fields on opposite sides, in particular with an oxygen flow field on one side and a hydrogen flow field on the opposite side. Alternatively, a separator for a fuel cell stack comprises an oxygen (such as air) flow field on one side and is attached (e.g. back-to-back attached) to a second separator containing a hydrogen flow field on its opposite side. Optionally, a cooling flow field of a coolant is provided between the two separators of fuel gas, for example by interposing a corresponding separator with a coolant flow field on one or both of its sides. Optionally, a separator has an oxygen flow field on one side and a coolant flow field on its opposite side. As explained herein, the method can be used to produce various different separators as required, whether in terms of having fluid flow fields on only one side or on both sides, or in terms of oxygen, hydrogen or coolant in the respective flow fields.

[0023] In brief, as a general aspect, for the production of such separators in a fuel cell, a ductile precursor sheet is made by mixing a thermoplastic polymer, carbon fibers and electrically conductive carbon particles, where the precursor is then hot-press plastic molded into a single layer or a multi-layer structure constituting the separator. Advantageously, as will be discussed below, the layer thickness is less than the average length of the carbon fibers. Details are as described below.

[0024] The compound is provided as a polymer matrix comprising a thermoplastic polymer blend with carbon fibers and electrically conductive carbon particles dispersed therein. The thermoplastic polymer blend comprises 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), perfluoroalkoxy alkane (PFA), polyamide-imide (PAI), polychlorotrifluoroethylene (PCTFE), polyether ether ketone (PEEK), polyether ketone (PEK), polyetherimide (PEI), polyether sulfone (PES), polyphenylsulfone (PPSU), polysulfone (PSU) or polyvinylidene fluoride (PVDF).

[0026] In this and the following, all the weight percentages are given relative to the total weight of the polymer blend, carbon fibers and carbon particles. Thus, liquids used in the production process, such as water and organic solvents, as well as liquid additives, e.g. surfactants, are not taken into account, as they are evaporated before the final production step.

[0027] For example, the precursor and the final separator comprise one or more of the following parameters, two, three or more of which can be successfully combined:

[0028] PTFE: at least 0.05 wt.%

[0029] PTFE: less than 0.5 wt.%

[0030] Thermoplastic polymer different from PTFE: minimum 5 wt.% - 30 wt.%

[0031] Relative weight of carbon fibers: 2 wt.% - 20 wt.%, e.g. 5% - 20%

[0032] Average length L of carbon fibers: 0.1 mm - 1 mm

[0033] Relative weight of carbon particles (different from carbon fibers): 25 wt.% - 90 wt.%

[0034] Average size of carbon particles (different from carbon fibers): 0.1 pm - 100 pm.

[0035] The size of the polymer particles and carbon particles (including graphite and carbon black) given herein is the average size, which means averaged over the three dimensions of the particles and over the number of particles of the herein specified group or type.

[0036] Generally, for the particles, the statistical distribution associated with the average size of the dimension, including the length of the carbon fibers, has a FWHM lower than or about ± 20%.

[0037] The compounds are formed into moldable, ductile precursor sheets in a forming station, e.g. a stationary forming station. Advantageously, however, the forming station is a calendering station and the forming comprises calendering the sheet through calendering rolls in the calendering station.

[0038] The precursor sheet is optionally formed into a single layer blank of thickness X1, which is then molded into a single layer separator. Alternatively, the precursor sheet is formed into a multi-layer blank structure having a plurality of layers of thickness X1 in a stacked state. Typically, the precursor is cut to shape before molding and the resulting blank is then hot press molded into a separator.

[0039] For a multi-layer separator, the precursor's multi-layer slab is composed of a number of layers of thickness X2, but this is not mandatory, as additional layers of different thicknesses, for example thicker top and bottom layers, can be added to the number of layers of thickness X2, providing between them a number of layers of thickness X2, typically identical layers.

[0040] The terms thickness X1 of the precursor slab and thickness X2 of the corresponding separator are used to distinguish, as the compression molding stage increases the density of the slab due to the high pressure compaction. The thickness X1 of the slab applies to the precursor slab before molding, while the thickness X2 applies to the separator after the slab has been molded. For example, the density can increase 2 times from the slab to the final separator, such as BPP. However, it is also pointed out that the formation of the flow field during compression molding can compensate to some extent for the reduction in thickness.

[0041] Optionally, the number of layers is in the range of 2-10, for example 4-8. However, more layers are possible.

[0042] Advantageously, and as will be discussed in more detail below, the thickness X1 is smaller than the average length L of the carbon fibers. This is advantageous as it makes the carbon fibers at least partially parallel to the slab, especially during the rolling process, which increases the mechanical strength of the final separator. For example, the average length L is at least twice the thickness X1 of the precursor sheet.

[0043] When comparing the length L of the carbon fibers to the thickness X2 of the separator, it is also valid to assume that this thickness X2 should be smaller than the average length L of the carbon fibers, more precisely at least half of L or even at least a quarter of L, due to the compaction of the precursor during compression molding.

[0044] The choice of the length of the carbon fibers is a balance between the positive effect on the strength of the final separator and the ability to disperse the carbon fibers, which is easiest for short carbon fibers. From these perspectives, the average length of the carbon fibers has been found to be suitable if it is in the range of 0.05 mm - 1 mm, for example 0.1 mm - 1 mm.

[0045] For example, if the length of the carbon fibers is 1 mm, and it is required that the length of the carbon fibers is at least twice the thickness of the precursor sheet, the maximum sheet thickness must be chosen to be 0.5 mm.

[0046] The length of the fibers is the average length, but it is assumed that the length does not deviate substantially from the average. For example, less than 20% of the fibers deviate from the average length more than 20%.

[0047] The choice of the length of the fibers depends on the thickness of the final separator.

[0048] In other words, when the length L is chosen to be twice the thickness X1 of the precursor, it will typically be about four times the thickness X2 of the final separator due to compaction during moulding.

[0049] When mixing the carbon fibres in the initial dispersion, the fibres should not be too long, otherwise mixing becomes difficult. For example, when producing separators with a thickness X2 of 0.05-0.3 mm, a length of 6 mm or 10 mm of fibres as disclosed in prior art US2014 / 087287, US2019 / 0341630 and US2009 / 0152105 would be disadvantageous. On the other hand, a fibre length of 0.01 mm or 0.02 mm as disclosed in the same prior art would be too short. The range suggested by the prior art thus appears rather arbitrary and does not appear to take into account a balance between proper mixing and increased strength.

[0050] In addition, the amount of carbon fibres in the compound should not be chosen arbitrarily. It has experimentally been confirmed that the strength of the final separator increases with increasing amount of carbon fibres up to 10 wt%, but increases more slowly in the range of 7-10 wt%. For amounts above 10 wt%, the bending strength was found to decrease. For this reason, an amount of 5-20 wt%, for example 5-15 wt%, appears most useful, with an optimal range within 6-12 wt%, for example 6-10%.

[0051] As discussed in the introduction, thin separators are desirable as they affect the size, weight, performance and cost of the final fuel cell stack. For example, the thickness X2 of the separator is in the range of 0.05-1 mm, for example in the range of 0.05-0.5 mm or 0.05-0.6 mm or even 0.05-0.3 mm, however, it is possible to make it multi-layered.

[0052] For example, the thickness X1 of the precursor sheet is in the range of 0.05-1 mm, for example 0.1-1 mm or 0.05-0.5 mm, 0.05-0.6 mm, optionally 0.1-0.6 mm or even 0.05-0.3 mm. For a multi-layered separator, the thickness X1 of each layer is typically at the lower end thereof, for example in the range of 0.05-0.3 mm, optionally 0.05-0.2 mm.

[0053] It has been found that the following method of producing a precursor sheet is advantageous. In this method, an aqueous dispersion and a solvent dispersion are provided and mixed. The aqueous dispersion comprises PTFE particles and carbon fibres, and optionally a first portion of graphite particles, for example with an average particle size in the range of 0.1-10 pm.

[0054] The solvent dispersion comprises second thermoplastic particulate polymer (e.g. PPS) different from PTFE, and carbon black particles that are more hydrophobic than the graphite particles in the mixture.

[0055] The organic solvent is optionally N-methyl pyrrolidone NMP. Alternative solvents can also be used, such as N,N-dimethylacetamide, N,N-dimethylformamide and dimethyl sulfoxide.

[0056] The two dispersions are stirred to prevent settling of the particles, and then the two dispersions are combined and mixed.

[0057] Furthermore, the graphite particles are mixed into the mixed dispersion, for example with an average size in the range of 10-100 pm. Typically, the weight of these graphite particle fractions is 5-20 times the weight of the carbon black.

[0058] It has been found advantageous to use two portions of graphite in the mixture, wherein the particle size of the first portion is smaller than the second portion. As already mentioned, the first portion is advantageously mixed into the first dispersion. The second portion of graphite particles is advantageously mixed with the mixed two dispersions. For example, the average size of the second portion of graphite particles is in the range of 10-100 pm. This second portion is relatively large compared to the first portion and the amount of carbon black. Typically, the weight of the second portion is 5-20 times the sum of the weight of the first portion and the weight of the carbon black.

[0059] These large amounts of graphite make further stirring difficult, which is why proper blending is done in a kneader.

[0060] During kneading in the kneader, the temperature is raised to a high level that is high enough to evaporate the organic solvent and the water from the mixture. Furthermore, by raising the temperature to a level above the glass transition temperature of PTFE but below the melting temperature of PTFE, the kneading results in fiberization of the PTFE that is advantageous for the strength of the final separator.

[0061] In an advantageous process, the high temperature level during kneading in the kneader is below the melting temperature of the PTFE and the second thermoplastic polymer. Although 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 more advantageous to melt the second thermoplastic polymer after extraction of the mixture from the kneader. Thus, only after extraction, the temperature of the mixture is raised to a level that is high enough to melt the second thermoplastic polymer, after which the sheet is shaped into a slab of thickness X1 in a shaping station.

[0062] For example, a conveyor belt is provided between the kneader and the rolling station, in which conveyor belt the compound is heated to melt the second thermoplastic polymer, so that the second thermoplastic polymer is in a molten state when the mixture is shaped into a sheet in a shaping station, for example a calender rolling station.

[0063] For example, after the forming station, the sheet is cut into slabs having the correct dimensions for hot compression moulding in an in-line process comprising a kneader and a forming station. Optionally, a plurality of slabs is stacked into a multi-layer slab for producing a multi-layer separator.

[0064] For very thin precursor sheets, several calendering stages are provided in the calendering station. In experiments, the successful production of a sheet having a thickness X1 of 0.05 mm was achieved using a calendering station having 6 calendering stages. In the experiments performed like this, the average size of the carbon particles was 20 pm, so that the 0.05 mm thickness X1 of the sheet was at the theoretical lower limit for a sheet having such particles.

[0065] In experiments with successful multi-layer separators, six of the thin precursor sheets having a thickness 1 of 0.1 mm were cut and stacked into a 0.6 mm thick slab, which was used for hot compression moulding into a separator having a final thickness X2 of 0.3 mm, in particular a BPP.

[0066] Note that such thin precursor sheets having a thickness X1 of 0.05 mm - 0.1 mm cannot be produced by pressing a powder mixture of carbon particles and polymer particles. One of the factors for the successful production of thin sheets is the fibrillation of the PTFE during the kneading process, which is why kneading is an important stage of the production.

[0067] Alternatively, the sheet is provided as a quasi-annular sheet having a thickness X1, which is cooled to solidify and then wound onto a roll for storage until final shaping (e.g. by cutting), after which hot compression moulding takes place. Optionally, the quasi-annular sheet after storage is cut into slabs, which are stacked into a multi-layer slab for producing a multi-layer separator.

[0068] It has been proven that calendering is advantageous for at least partially aligning the carbon fibres in a direction parallel to the surface.

[0069] Furthermore, it has been proven that calendering a slab in a different direction tends to align the fibres in different directions. This is surprising, because not only the last calendering step determines the direction of the fibres, but also the previous steps. It is believed that the subsequent calendering steps tend to align the carbon fibres of the outermost layers mostly, so that the direction of the carbon fibres from the previous calendering steps is maintained inside the bulk of the sheet. Therefore, when a precursor slab that was calendered in one direction is calendered in a different direction (e.g. transversely), an increase in strength is also observed in this calendering direction.

[0070] For example, an annular sheet is first calendered in a direction parallel to the annular sheet, then cut into slabs, and then further calendered in a different direction (e.g. transversely).

[0071] Subsequent hot compression molding transforms the slab into an electrically conductive rigid separator for fuel cells, for example BPP.

[0072] The use of the stack production results in a self-organizing lamination structure of the multi-layer slab, which makes the precursor particularly suitable for hot compression molding of the precursor into an ultra-thin bipolar separator. It has been experimentally confirmed that the resulting multi-layer bipolar plate has excellent mechanical and electrical properties.

[0073] For example, the area specific resistance of the separator is at most 2 mW-cm per 0.3 mm thickness unit. 2 .

[0074] Optionally, the flexural strength of the separator is greater than 180 MPa per 0.3 mm thickness unit.

[0075] The hot compression molding changes the shape of the separator, for example with a flow field structure for pressing fluid flow into the material on at least one side of the separator, although typically on both sides, especially in case the separator is a BPP.

[0076] Such separators, for example BPP, can be used in high temperature polymer electrolyte membrane fuel cells (HT-PEM), which operate at more than 120 degrees Celsius, which distinguishes the HT-PEM fuel cells from low temperature PEM fuel cells, which operate at temperatures below 100 degrees, for example at 70 degrees. The normal operating temperature of HT-PEM fuel cells is in the range of 120 degrees Celsius to 200 degrees Celsius, for example in the range of 160 degrees Celsius to 170 degrees Celsius. Such HT-PEM fuel cells are advantageous for compact fuel cell systems, for example for use in the automotive industry.

[0077] Aspects

[0078] In the following, various aspects are described which are also implemented to achieve the object described.

[0079] Aspect 1. A moldable, ductile precursor sheet for hot compression molding into an electrically conductive rigid separator for fuel cells, wherein the precursor sheet is shaped into a multi-layer structure having a plurality of layers in a stacked state or into a single layer, wherein the single layer, or each layer of the plurality of layers of the multi-layer structure, has a thickness X1 and is provided as a polymer matrix comprising a thermoplastic polymer blend having dispersed therein carbon fibers and electrically conductive carbon particles; wherein the thermoplastic polymer blend comprises PTFE and a thermoplastic polymer different from PTFE, for example PPS; wherein the average length L of the carbon fibers is greater than the thickness X1 of the layer.

[0080] Aspect 2. The precursor sheet according to aspect 1, wherein the average length L of the carbon fibers is at least twice the thickness X1 of the layer.

[0081] Aspect 3. The precursor sheet according to any preceding aspect, wherein the thickness X1 is in the range of 0.05 mm - 1 mm.

[0082] Aspect 4. The precursor sheet according to any preceding aspect, wherein the weight concentration of the carbon fibers is in the range of 5 wt% - 20 wt% relative to the total weight of the polymer blend, carbon fibers and electrically conductive carbon particles.

[0083] Aspect 5. The precursor sheet according to any of the preceding aspects, wherein the weight concentration of PTFE is at least 0.05 wt% but less than 0.5 wt% PTFE relative to the total weight of the polymer blend, carbon fibers and electrically conductive carbon particles, and wherein the weight concentration of the thermoplastic polymer different from PTFE (e.g. PPS) is in the range of 5 wt% - 30 wt%.

[0084] Aspect 6. The precursor sheet according to any of the preceding aspects, wherein the electrically 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 size in the range of 10 pm - 100 pm, and wherein the carbon particles of the second portion have a size in the range of 0.1 pm - 10 pm, wherein the weight concentration of the first portion is in the range of 50 wt% - 90 wt% relative to the total weight of the polymer blend, carbon fibers and electrically conductive carbon particles, and wherein the weight ratio between the first portion and the second portion of electrically conductive carbon particles is in the range of 5 - 20.

[0085] Aspect 7. The precursor sheet according to any of the preceding aspects, wherein the thermoplastic polymer is PPS.

[0086] Aspect 8. The precursor sheet according to any of the preceding aspects, wherein the sheet is a multi-layer structure and the plurality of layers are identical.

[0087] Aspect 9. The precursor sheet according to any of the preceding aspects, wherein the sheet is a multi-layer structure and the thickness X1 of each layer of the plurality of layers is in the range of 0.05 mm - 0.3 mm, wherein the number of the plurality of layers is in the range of 2 - 10, e.g. 4 - 8.

[0088] Aspect 10. The precursor composition according to any preceding aspect, wherein the average length of the carbon fibers is in the range of 0.1 mm - 1 mm.

[0089] Aspect 11. A method of producing a precursor sheet according to any preceding aspect, the method comprising:

[0090] - providing an aqueous dispersion comprising a first portion of PTFE particles, carbon fibers, and graphite particles; wherein the average particle size of the graphite particles of the first portion is in the range of 0.1 pm - 10 pm; wherein the carbon fibers have an average length L;

[0091] - providing a solvent dispersion comprising particles of carbon black and a second thermoplastic polymer different from PTFE (e.g. PPS) dispersed in an organic solvent;

[0092] - stirring both dispersions to prevent the particles from settling;

[0093] - combining and mixing the two dispersions;

[0094] - mixing a second portion of graphite particles with the two dispersions, the average size of the graphite particles of the second portion being in the range of 10 pm - 100 pm; wherein the weight of the second portion is 5 times - 20 times the sum of the weight of the first portion and the weight of the carbon black;

[0095] - kneading the mixture in a kneader;

[0096] - during kneading in the kneader, increasing the temperature to a high temperature level high enough to evaporate the organic solvent and water from the mixture, wherein the high temperature level is higher than the glass transition temperature of PTFE,

[0097] - after evaporation of the organic solvent and water and while the second thermoplastic polymer is in a molten state, shaping the mixture into a sheet in a shaping station.

[0098] Aspect 12. The method according to aspect 11, wherein the method comprises shaping the sheet into a thickness X1 smaller than the average length L of the carbon fibers in the shaping station.

[0099] Aspect 13. The method according to aspect 12, further comprising preparing the precursor sheet ready for hot compression moulding, the preparing comprising cutting the molten sheet into slugs and stacking a plurality of such slugs of thickness X1 on top of each other in a molten state to provide a multi-layer precursor for compression moulding into a separator.

[0100] Aspect 14. The method according to aspect 12 or 13, wherein the shaping station is a calendering station and the shaping comprises calendering the sheet through the calendering station; wherein the method further comprises cooling the sheet after calendering to solidify the sheet, and providing the sheet as a quasi-annular slug having a sheet thickness X1, and after cooling, winding the quasi-annular slug onto a drum for storage as a wound quasi-annular slug in a solidified state and for later cutting and hot compression moulding.

[0101] Aspect 15. The method according to any one of aspects 11 to 14, wherein the high temperature level during the kneading in the kneader is lower than the melting temperature of the PTFE and the second thermoplastic polymer; wherein the method comprises extracting 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 shaping the sheet into a mat of thickness X1 in the shaping station.

[0102] 16. The method according to aspect 15, wherein the providing of the aqueous dispersion comprises adding a surfactant to the aqueous dispersion, wherein the boiling temperature of the surfactant is higher than the boiling temperature of water and higher than the boiling temperature of the organic solvent; wherein the method comprises extracting the mixture from the kneader when the mixture contains the surfactant but not both the solvent and water, and then increasing the temperature of the mixture to a level high enough to evaporate the surfactant, before shaping the sheet into a mat of thickness X1 in the shaping station.

[0103] Aspect 17. The method according to any one of aspects 11 to 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 wt% - 30 wt%, the weight percentages being relative to the total weight of the carbon fibers and carbon particles, the PTFE and the thermoplastic polymer.

[0104] Aspect 18. A rigid, compression-molded separator for a fuel cell, wherein the separator

[0105] is shaped into a multi-layer structure having a plurality of layers in a stacked state or into a single layer, wherein the single layer, or each layer of the plurality of layers of the multi-layer structure, has a thickness X2 and is provided as a polymer matrix comprising a thermoplastic polymer blend having dispersed therein carbon fibers and electrically conductive carbon particles; wherein the thermoplastic polymer blend comprises PTFE and a thermoplastic polymer different from PTFE, such as PPS; wherein the average length L of the carbon fibers is larger than the thickness X2 of the layer.

[0106] Aspect 19. The separator according to aspect 18, wherein the average length L of the carbon fibers is at least twice the thickness X2 of the layer.

[0107] Aspect 20. The separator according to aspect 19, wherein the separator is provided as a multi-layer structure comprising a plurality of layers in a stacked state, wherein the thickness X2 of each layer of the plurality of layers is in the range of 0.05 mm - 0.2 mm.

[0108] Aspect 21. The separator according to aspect 20, wherein the area specific resistance of the separator is at most 2 mW-cm2per 0.3 mm thickness unit.

[0109] Aspect 22. The separator according to aspect 20 or 21, wherein the bending strength of the separator is greater than 180 MPa per 0.3 mm thickness unit.

[0110] Aspect 23. The separator according to any one of aspects 18 to 22, wherein the separator has a flow field structure for pressing a fluid flow into the material on at least one side of the separator.

[0111] Aspect 24. The separator according to aspect 23, wherein the separator is a bipolar plate having a fluid flow structure on both sides of the bipolar plate.

[0112] Aspect 25. A fuel cell having a separator according to any one of aspects 18 to 24 or having a separator provided as a hot press molded separator plate from a moldable, ductile precursor sheet according to any one of aspects 1 to 10.

[0113] Aspect 26. A moldable, ductile precursor sheet for hot press molding into an electrically conductive rigid separator plate for a fuel cell, wherein the precursor sheet is shaped as a single layer, wherein the single layer has a thickness X1 in the range of 0.05 mm - 1 mm and is provided as a polymeric matrix comprising a thermoplastic polymer blend having dispersed therein carbon fibers and electrically conductive carbon particles; wherein the thermoplastic polymer blend comprises polytetrafluoroethylene PTFE and a thermoplastic polymer different from PTFE, such as PPS; wherein the average length L of the carbon fibers is in the range of 0.1 - 1 mm.

[0114] Aspect 27. The precursor sheet according to aspect 26, wherein optionally the average length L of the carbon fibers is at least twice the thickness X1 of the layer.

[0115] Aspect 28. The precursor sheet according to aspect 26 or 27, wherein the thermoplastic polymer different from PTFE is polyphenylene sulfide PPS.

[0116] Aspect 29. The precursor sheet according to any one of aspects 26 to 28, wherein the weight concentration of the carbon fibers is in the range of 5 wt% - 20 wt% relative to the total weight of the polymer blend, carbon fibers and electrically conductive carbon particles.

[0117] Aspect 30. The precursor sheet according to any one of aspects 26 to 29, wherein the weight concentration of PTFE is at least 0.05 wt% but less than 0.5 wt% PTFE, relative to the total weight of the polymer blend, carbon fibers and electrically conductive carbon particles, and wherein the weight concentration of the thermoplastic polymer different from PTFE (e.g. PPS) is 5 wt% - 30 wt%.

[0118] Aspect 31. The precursor sheet according to any one of aspects 26 to 30, wherein the electrically 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 size in the range of 10 pm - 100 pm, and wherein the carbon particles of the second portion have a size in the range of 0.1 pm - 10 pm, wherein the weight concentration of the first portion is in the range of 50 wt% - 90 wt% relative to the total weight of the polymer blend, carbon fibers and electrically conductive carbon particles, and wherein the weight ratio between the first portion and the second portion of electrically conductive carbon particles is in the range of 5 - 20.

[0119] Aspect 32. A method of producing a separator by

[0120] - mixing a thermoplastic polymer, carbon fibers and electrically conductive carbon particles in a dispersion, wherein the carbon fibers have an average length L in the range of 0.1 mm - 1 mm;

[0121] - forming from the mixture a moldable, ductile single layer precursor sheet having a thickness X1 by calendering in a forming station;

[0122] - hot press molding the precursor into a single layer separator having a thickness X2 in the range of 0.05 mm - 0.6 mm, e.g. 0.05 mm - 0.3 mm, wherein X2 is less than the average length L of the carbon fibers.

[0123] Aspect 33. The method according to aspect 32, wherein the method comprises providing the carbon fibers having an average length L which is at least twice the thickness X2 of the layer.

[0124] Aspect 34. The method according to aspect 32 or 33, wherein the thermoplastic polymer is a blend comprising PTFE and a thermoplastic polymer different from PTFE.

[0125] Aspect 35. The method according to aspect 34, wherein the thermoplastic polymer different from PTFE is polyphenylene sulfide PPS.

[0126] Aspect 36. A method according to any one of aspects 32 to 35, the method comprising:

[0127] - providing an aqueous dispersion comprising PTFE particles, carbon fibers; wherein the carbon fibers have an average length L;

[0128] - 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;

[0129] - stirring both dispersions to prevent the particles from settling;

[0130] - combining and mixing the two dispersions;

[0131] - mixing a portion of graphite particles with the two dispersions, the average size of the graphite particles of the portion being in the range 10-100 pm; wherein the weight of the portion is 5-20 times the weight of the carbon black;

[0132] - kneading the mixture in a kneader;

[0133] - during the kneading in the kneader, increasing the temperature to a high temperature level high enough to evaporate the organic solvent and water from the mixture, wherein the high temperature level is higher than the glass transition temperature of PTFE,

[0134] - after evaporation of the organic solvent and water and while the second thermoplastic polymer is in a molten state, shaping the mixture into a precursor sheet in a shaping station.

[0135] Aspect 37. The method according to aspect 36, wherein the portion of graphite particles is a second portion of graphite particles, and wherein the method comprises providing the aqueous dispersion containing a first portion of graphite particles in addition to the PTFE and carbon fibers; wherein the average particle size of the graphite particles of the first portion is in the range 0.1-10 pm.

[0136] Aspect 38. The method according to any one of aspects 32-37, wherein the high temperature level during the kneading in the kneader is lower than the melting temperature of the PTFE and the second thermoplastic polymer; wherein the method comprises extracting 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 shaping the sheet into a precursor sheet of thickness X1 in the shaping station.

[0137] Aspect 39. The method according to aspect 38, wherein said providing of said aqueous dispersion comprises adding a surfactant to said aqueous dispersion, wherein said surfactant has a boiling temperature higher than the boiling temperature of water and higher than the boiling temperature of said organic solvent; wherein said method comprises extracting said mixture from said kneader when said mixture contains said surfactant but not both said solvent and water, then raising the temperature of said mixture to a level high enough to evaporate said surfactant, before forming said sheet into a mat of thickness X1 in said forming station.

[0138] Aspect 40. The method according to any one of aspects 32-39, wherein the weight concentration of PTFE in said mixture is at least 0.05 wt% but less than 0.5 wt% PTFE, and the weight concentration of said thermoplastic polymer different from PTFE is in the range of 5-30 wt%, wherein the weight concentration of said carbon fibers is 2-20 wt%, the weight percentages being relative to the total weight of said carbon fibers and carbon particles, said PTFE and said thermoplastic polymer.

[0139] Aspect 41. The method according to any one of aspects 32-40, wherein said method comprises calendering the precursor sheet in at least two different directions to align said carbon fibers in different directions.

[0140] Aspect 42. A rigid, calendered and pressed molded separator for a fuel cell, wherein said separator is formed from a single layer precursor sheet into a single layer, wherein said single layer has a thickness X2 in the range of 0.05-0.6 mm, for example 0.05-0.3 mm, and is provided as a polymer matrix comprising a thermoplastic polymer blend having dispersed therein carbon fibers and electrically conductive carbon particles; wherein said thermoplastic polymer blend comprises PTFE and a thermoplastic polymer different from PTFE, for example PPS; wherein said carbon fibers have an average length L in the range of 0.1-1 mm and are larger than said thickness X2 of said layer.

[0141] Aspect 43. The separator according to aspect 42, wherein said carbon fibers have an average length L at least twice said thickness X2 of said layer.

[0142] Aspect 44. The separator of Aspect 42 or 43, wherein the weight concentration of the carbon fibers is in the range of 5-20 wt.%, with respect to the total weight of the polymer blend, carbon fibers and conductive carbon particles, wherein the weight concentration of PTFE is at least 0.05 wt.% but less than 0.5 wt.% PTFE, with respect to the total weight of the polymer blend, carbon fibers and conductive carbon particles, and wherein the weight concentration of the thermoplastic polymer different from PTFE (e.g. PPS) is in the range of 5-30 wt.%.

[0143] Aspect 45. The separator of Aspect 42, 43 or 44, 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 size in the range of 10-100 pm, and wherein the carbon particles of the second portion have a size in the range of 0.1-10 pm, wherein the weight concentration of the first portion is in the range of 50-90 wt.%, with respect to the total weight of the polymer blend, carbon fibers and conductive carbon particles, and wherein the weight ratio between the first portion and the second portion of conductive carbon particles is in the range of 5-20. BRIEF DESCRIPTION OF DRAWINGS

[0144] The application is explained in greater detail with reference to the accompanying drawings, in which:

[0145] Figure 1 is a schematic representation of a continuous process for the preparation of a graphite-based compound, pre-forming the compound into a slab, and molding a bipolar plate therefrom;

[0146] Figure 2 illustrates (a) the bending strength and (b) the area specific resistance of MFG / SFG / CF / CB / PPS / PTFE based BPPs as a function of thickness;

[0147] Figure 3 is a micrograph of a cross-section of a slab;

[0148] Figure 4 illustrates the fracture distribution load as a function of thickness for MFG / SFG / CF / CB / PPS / PTFE based BPPs (elliptical symbols refer to single layer BPPs, and cross symbols refer to multi-layer BPPs);

[0149] Figure 5 is a simplified illustration of current flow through a composite material having conductive carbon particles of different sizes, where a) there is only one size of particles, and b) there are different sizes of particles;

[0150] Figure 6 illustrates sheet forming in a rolling station. DETAILED DESCRIPTION

[0151] In the production process described herein, several partial processes are combined, i.e. the mixing of the powdered raw materials, after which they are kneaded and calendered into preformed shapes, such as a thin sheet of a specified density, and further compression-molded such a sheet to provide a separator plate for a fuel cell, optionally an electrode plate, end plate or BPP. In the following, the process will be explained for a BPP, however, the process is equally well applicable to the various plates in a fuel cell or fuel cell stack. Therefore, all partial process steps described below should also be read on the basis of such other types of separator plates, although this method is considered to be most advantageous for a BPP.

[0152] Figure 1 A schematic diagram is shown in Fig. 1, which illustrates the manufacturing for the production of a graphite, carbon fiber (CF) and carbon black (CB) based BPP, and its combination in a polymer matrix with the polymers PPS and PTFE.

[0153] It is mentioned here that other thermoplastic polymers can also be used for the manufacture of a BPP for high temperature PEM fuel cells. Candidate materials are inter alia ethylene tetrafluoroethylene (ETFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy alkane (PFA), polyamide-imide (PAI), polychlorotrifluoroethylene (PCTFE), polyether ether ketone (PEEK), polyether ketone (PEK), polyether imide (PEI), polyether sulfone (PES), polyphenylsulfone (PPSU), polysulfone (PSU), polyvinylidene fluoride (PVDF), see also [Ref. 25].

[0154] PTFE is provided in the form of an aqueous dispersion, for example with a relative concentration in the range of 10-80 wt.%, optionally 50-70 wt.% PTFE in water, for example 60 wt.% of an aqueous dispersion. Such latter dispersion is commercially available, for example from the company Solvay®. The purchased dispersion can optionally be further diluted to the appropriate concentration by mixing with deionized water. In addition, a surfactant can be added.

[0155] As Figure 1 explained in [Ref. 1], PTFE, CF and graphite, and other possible ingredients, such as a surfactant, are provided in a first container 1. The content of surfactant does not exceed 10 vol.% of the total liquid composition, but is typically in the range of 0.2-2 vol.%. It is noted here that the PTFE dispersion from the supplier usually already contains a small amount of surfactant to avoid aggregation of the polymer particles [Ref. 26].

[0156] Non-limiting examples of surfactants are Tergitol® from Dow Chemical Company. TM15-S series, from Union Carbide X series or from Croda Triton X-100 from the Triton X series TM have a hydrophilic polyethylene oxide chain and an aromatic hydrocarbon lipophilic or hydrophobic group. The hydrocarbon group is a 4-phenyl group. The formula is C 14 H 22 O(C2H4O) n where n is 9-10.

[0157] For example, the available mass ratio between the solid phase and the liquid phase in the first container 1 is in the range of 1 :4 to 1 :6, such as about 1 :5. In one aspect, it is desirable to have less volume of water in the process in order to minimize energy and resource consumption in the process, in another aspect, it is required to have sufficient amount of liquid to maintain proper dispersion.

[0158] Optionally, the CF is provided with an average length in the range of 0.1 mm - 1 mm, for example a length in the range of 0.2 mm - 0.4 mm, such as about 0.3 mm. The length given here is an average length. For example, the statistical distribution associated with the average length has a FWHM of less than or about ± 20%. As will become more apparent in the following, the average length of the CF is selected with respect to the layer thickness X1.

[0159] Advantageously, small fraction graphite (SFG) is added. The term "small fraction" refers to a relative percentage range of 2 wt% - 10 wt% with respect to the total dry weight (i.e. without liquid) of the final product. Optionally, the SFG graphite particles are provided with an average particle size in the range of 0.1 pm - 10 pm, for example in the range of 0.5 pm - 2 pm. In the experiment, the average size of the graphite particles of the SFG was 1 pm.

[0160] As already mentioned, the size of the polymer particles and carbon particles (including graphite and carbon black) given here is an average size, which means averaged over the three dimensions of the particles and over the number of particles of the here specified group or type. Typically, for the particles, the statistical distribution associated with the average size of the dimension has a FWHM of less than or about ± 20%.

[0161] In the here first container 1, all ingredients are intensively stirred.

[0162] Examples of the weight ratio between PTFE, CF and graphite (SFG) are in the range of (0.05 - 0.5) : (0.05 - 15) : (0.05 - 15). In the experiment, the ratio was 0.25 : 10 : 5.

[0163] ​​In parallel, a second powder mixture is provided in a second container 2. This second powder mixture contains PPS powder, the average particle size of which is optionally in the range of 10-100 pm, for example in the range of 20-30 pm, such as approximately 25 pm.

[0164] Furthermore, in this second container 2, CB particles are provided, the average size of which is optionally in the range of 10-100 nm, for example in the range of 30-50 nm, such as approximately 40 nm.

[0165] The particles in the second container 2 are mixed with N-methyl-2-pyrrolidone (NMP) in order to provide a viscous slurry. NMP is provided in the container 2 to achieve wetting of the hydrophobic PPS and CB particles before their dispersion into the aqueous medium from the first container 1, since NMP has outstanding wetting properties due to polarity and low surface tension [reference 27]. It is indicated that NMP can dissolve approximately 10 wt% of PPS at 203 °C [reference 28]. At lower temperatures, NMP can only dissolve a very thin near-surface layer of the polymer particles [reference 29]. Water-miscible at all temperatures [reference 30], NMP acts as a “bridge” for water molecules, delivering them directly to the surface of the hydrophobic particles.

[0166] Some other solvents can be used as an alternative to NMP for this purpose, such as N,N-dimethylacetamide, N,N-dimethylformamide and dimethyl sulfoxide. The use of these solvents in combination with surfactants allows the production of long-term stable PPS dispersions, but the process takes place in the temperature range of 220-320 °C [reference 31], which is not optimal.

[0167] The distribution of a part of the carbon particles into the first container 1 and the other part of the carbon particles into the second container 2 is based on the consideration that the total amount of liquid should be minimized in order to avoid unnecessary energy consumption for the subsequent evaporation of the liquid. In principle, all particles can be added to the second container 2 together with NMP, but in this case the solids content in the container 2 would require an unnecessarily high amount of NMP in order to bring the particle concentration to an acceptable level for efficient mixing. Furthermore, the production method takes into account that the amount of organic solvent used is minimized, which adds environmental friendliness to the method, especially when the solvent is recycled. The choice to disperse CB and PPS into the second container 2 is because these carbon particles are more difficult to wet with water than NMP. In another aspect, SFG and CF are less hydrophobic, which is why they are more suitable for addition to the aqueous dispersion in the first container 1.

[0168] In our case, since the dispersion is continuously stirred until the main filler (i.e. graphite) is added, sedimentation and agglomeration of the fine PPS particles in the second vessel 2 is avoided. After the addition of an appreciable concentration of graphite, the viscosity of the system increases so much that sedimentation becomes almost impossible even if stirring is stopped for a long time. Since stirring is no longer feasible, a kneader is used for the next stage, as described in more detail below.

[0169] After the two dispersions are separately prepared in the first vessel 1 and the second vessel 2, they are mixed (e.g. in a third vessel 3), as shown in Figure 1 to obtain a uniform distribution of the polymer and the carbon particles. Typically, after mixing in the third vessel 3, the NMP content in water is no more than 25 vol% relative to the total mixture volume in vessel 3. For example, in the third vessel 3, the concentration of NMP in the mixture is in the range of 10 vol% - 25 vol%.

[0170] When this mixing process step is completed, the SFG / CF / CB / PPS / PTFE suspension is transferred from the third vessel 3 to a kneader 5 together with an amount of graphite from a fourth vessel 4. Since the amount of graphite from the fourth vessel 4 is relatively large and results in a relatively large fraction in the final mixture, the graphite from the fourth vessel 4 is referred to as “main fraction graphite” (MFG). The ratio between MFG : SFG is at least 2, for example at least 3, but typically at least 5, and typically at most 20, for example in the range of 5 - 20. In the experiments, a MFG : SFG ratio of 13 has been used.

[0171] For example, the relative content of MFG is in the range of 40 wt% - 80 wt% relative to the dry mass of the mixture in the kneader 5. In the experiments, the concentration of MFG was 66.25 wt% of the dry mass.

[0172] Optionally, the average particle size of this graphite of the MFG is in the range of 10 pm - 100 pm, for example in the range of 10 pm - 30 pm. In the experiments, the average size of the graphite particles of the MFG was 20 pm. Please note that the particle size of the MFG is one order of magnitude larger than the particle size of the SFG.

[0173] The MFG is the main carbon component in the composite material. Figure 5 illustrates in a simplified way the advantage of different particle sizes. In Figure 5a only one size of carbon particles is provided, while in Figure 5bIn this context, it is clear that the smaller particles act as conductive bridges between the larger particles, which reduces the overall resistivity of the final composite material. In building such a composite matrix, also the consideration of realistic and competitive production costs of commercial products comes into play, which is why the amount of relatively expensive nanoscale graphite has to be balanced against the advantages it provides depending on its concentration. The addition of CB is based on a compromise between obtaining high conductivity and competitive production costs.

[0174] By adding MFG, the solids content in the mixture within the kneader 5 is increased to a solids content in the range of 30-70 wt.-%, for example in the range of 40-60 wt.-%, such as approximately 50 wt.-%. The mixing procedure in the containers 1, 2, 3 and 4 is carried out at a first temperature level T1, typically at room temperature, for example in the range of 20-25 °C, which is sufficient to homogeneously distribute the powder materials.

[0175] This is an advantage compared to the procedure described in [Reference 32], where the polymer used to bind the carbon particles has to be completely dissolved in a suitable solvent, which is not easy for a high engineering plastic like PPS, since temperatures above 200 °C are required. And even worse, to obtain a solubility of more than 50 wt.-% of PPS, the temperature has to be above 300 °C [Reference 28]. In addition, solvents suitable for this usually have a high boiling point, which makes it problematic to remove them further from the compound before molding.

[0176] The kneading of the MFG / SFG / CF / CB / PPS / PTFE mixture in the kneader 5 is started at a first temperature level T1, for example at room temperature, and the kneading is continued during heating of the mixture to a high temperature above the boiling point of the liquid used in order to remove the liquid from the mixture by evaporation. The kneading during evaporation prevents bubble formation or at least minimizes the risk of bubble formation.

[0177] However, the kneading process usually takes 10-30 minutes, depending on the speed of the temperature increase and the speed of the liquid evaporation.

[0178] The mixture is heated from the first temperature level T1, for example in the range of 20-25 °C, to gradually increase the temperature from the first temperature level T1 to a second temperature level T2, wherein T2 is the boiling point of water in order to remove the water by evaporation.

[0179] In order to ensure that the kneading step is continued without bubble formation due to residual water in the mixture, the mixture is heated to a third temperature level T3, which is significantly above the boiling point of water. For example, the third temperature level T3 is in the range of 102-120 °C.

[0180] In the experiment, kneading continues at a third temperature level T3, which is 116°C, which is well above the boiling point of water and thus ensures removal of all water from the mixture.

[0181] It is proposed that PTFE undergoes a phase transition at the glass transition temperature, which in our case was determined to be 116°C, at which temperature the polymer is in a rigid amorphous state [reference 33] and its tendency to fibrillate from nanoparticles increases. For this reason, the compound becomes softer and ductile for kneading, i.e. fibrillation is useful because it ultimately leads to an increase in cohesion between the components in the mixture.

[0182] After the water has been removed by evaporation, the temperature is further increased to a fourth temperature level T4 in order to remove the solvent by evaporation. In the experimental case described herein, where NMP is used as solvent, the fourth temperature T4 is adjusted to 204°C (boiling point of NMP [reference 30]) in order to remove the NMP by evaporation.

[0183] Optionally, for recycling purposes, all evaporated substances can be condensed again into a liquid phase in another container 6 and separated into pure solvents. Available separation methods include distillation and / or membrane separation [references 34, 35]. Water and NMP are collected in other containers 7 and 8, respectively, to be returned into the manufacturing process. It is pointed out that the possible remaining small amount of surfactant dissolved therein does not interfere with this.

[0184] The soft compound is extracted from the kneader 5 after kneading as a dough-like ductile material. It is extruded onto a heated conveyor belt 9.

[0185] In order to further remove liquids with a higher boiling point from the kneaded MFG / SFG / CF / CB / PPS / PTFE compound, for example non-ionic surfactants [reference 26], the temperature of the mixture is even further increased to a fifth temperature level T5 which leads to evaporation of the surfactants. For example, the fifth temperature level T5 is below the melting point in the range of 271-292°C of PPS. In the experiment, the mixture is heated to a fifth temperature level T5 of 270°C, which removes the surfactants used but does not melt the PPS.

[0186] This temperature increase can be done while the mixture is in the kneader. However, for a smooth extrusion of the compound with a relatively low content in the polymer, it has been found to be advantageous if the compound still contains some non-evaporated surfactant. Therefore, after extrusion from the kneader, the temperature is increased to level T5 to evaporate the surfactant, because the surfactant is no longer needed after extrusion. For example, the surfactant The boiling point of X-100 is 270°C [Ref. 36]. This temperature increase can be performed on a conveyor belt. The temperature of the mixture (e.g. while in the conveyor belt) is even further increased to a sixth temperature level T6 (which in the experiments was a level of 347°C) above the melting level of the second thermoplastic polymer, in order to melt the PPS and reduce the viscosity, as was performed in the experiments. The final temperature, however, depends on the chosen molding parameters.

[0187] The conveyor belt 9 advances the dough-like structure through at least one rolling station 10, the gap between the calender rolls being adjusted as needed in order to calender press the structure into a sheet having a specified thickness, typically in the range of 0.05 mm to 10 mm. The process is performed at a temperature above the melting point of the first thermoplastic polymer, but below the melting point of the second thermoplastic polymer. Figure 6 More details will be explained. However, as will be apparent from the discussion later, thicknesses of less than 1 mm are advantageous for BPP in order to save material and weight and to obtain good properties. For example, the thickness is adjusted to a value in the range of 0.05 mm to 1 mm.

[0188] It is noted here that the production of thin films, typically at the lower end of the thickness range, requires more than one rolling station. After the formation of the thin film from the ductile structure, a cutting tool 11 cuts it into slabs.

[0189] In some of the experiments performed, a comparison was made using thicknesses of 0.1 mm and 0.6 mm, wherein the 0.1 mm thick slabs were used in a stack of 6 slabs precursors, while the 0.6 mm slabs were used in a single layer precursor and BPP. The thickness of the resulting BPP after compression molding was 0.3 mm, which is half the thickness of the precursor, due to the thickness reduction caused by compression molding. In this regard, the following is noted. The slabs have a specified density, typically in the range of 0.5 to 1.5 g / cm 3 , such as approximately 1 g / cm 3 , as used in the experiments. However, the density of the compression molded BPP is slightly higher than 2 g / cm 3 . Therefore, thicker precursor slabs should be provided, in some cases up to approximately 2 times thicker. However, it is also noted that the flow field that is formed during compression molding can compensate for the thickness reduction to some extent.

[0190] In addition to the thickness, the experimental size of the slabs was 400 x 100 mm, but slabs of any size can be produced by this method.

[0191] Finally, the pre-heated, graphite-based slabs are shaped into BPPs in a hot press 12 by compression molding. Compression molding is advantageously performed at a temperature between the melting points of the various 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.

[0192] Alternatively, the temperature is slightly higher than the melting point of PTFE. However, it should be lower than the decomposition temperature of PPS (approximately 475 °C) [Ref. 37].

[0193] The temperature for molding generally depends on the molecular weight of the polymers and their melting temperature, as well as the behavior during heat treatment [Refs. 38-40].

[0194] This temperature range (not all polymers melt) helps to avoid sticking of the slabs to the mold. In addition, PTFE can act as an anti-sticking component of the MFG / SFG / CF / CB / PPS / PTFE compound.

[0195] During molding, the pressure applied is generally 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.

[0196] The process time is defined by the cooling speed of the BPP inside the mold. When the mold temperature is below the glass transition temperature of PPS (i.e., below 93 °C), pressure release occurs [Ref. 38].

[0197] It should be mentioned here that the BPP can be manufactured by utilizing a single slab, thereby forming a single-layer BPP, or by using multiple slabs in a stacked state (for example, 4-8 slabs stacked on each other). In the experiments, 6 slabs were used to obtain a multi-layer slab and to press it into a multi-layer BPP.

[0198] Each component in the compound produced in this way has a specific purpose. MFG is the main filler of PPS, while the other additives improve the mechanical properties (especially influenced by CF) as well as the electrical properties (especially influenced by SFG and CB), in addition to improving the ability to bind the fine powdery material into an extensible dough-like structure, which is achieved especially by PTFE.

[0199] The percentage ratio between all these components in the final mixture can vary within some limits. For example, all percentages are calculated by weight:

[0200] - the content of MFG is in the range of 25-90 wt%, for example 50-90 wt%. In the experiments slightly less than 70% was used.

[0201] - the minimum amount of PPS is 5 wt% and will generally be lower than 30%. In the experiments 20 wt% was used.

[0202] - the total content of additives SFG, CF, CB, PTFE in the final compound is generally lower than 45 wt%.

[0203] - the content of CF is in the range of 2-20 wt%, however, advantageously in the range of 3-15 wt%, such as 5-15 wt%.

[0204] - a range example of components is 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.

[0205] Thus, after removal of the liquid, all weight percentages are given with respect to the polymer blend with particles and fibers.

[0206] In the experiments the final compound without liquid containing the following amounts of the individual components showed the best electro-mechanical properties: MFG (66.25 wt%), PPS (17.50 wt%), CFs (10.00 wt%), SFG (5.00 wt%), CB (1.00 wt%), PTFE (0.25 wt%).

[0207] Figure 2 shows the correlation of the bending strength and the area specific resistance versus thickness of BPP produced from a compound with the percentage ratios between the components as given in the figure.

[0208] As can be seen from the above given figures, both correlations are not linear. A decrease of the thickness of the BPP leads to a decrease of the area specific resistance, as can be seen in Figure 2b and a significant increase of the bending strength, as can be seen in Figure 2athickness and deviates from the quasi-linear shape for thicknesses below 1 mm and is particularly pronounced for BPPs with thicknesses below 0.5 mm. The increase in the bending strength for small thicknesses is believed to be due to the planar orientation of the CFs in a near-surface layer. This near-surface layer is believed to be formed upon rolling the slab and is believed to have improved mechanical properties compared to the remaining volume below the near-surface layer where the almost perpendicular orientation of the CFs is maintained. This explanation implies that a self-organizing laminated structure occurs, which is experimentally confirmed by microtomography studies, images of which are reproduced in Figure 3 and which show that there are two main regions in the slab where the orientation of the CFs is different. It is emphasized that this is an important finding that can be exploited to great advantage as explained below.

[0209] For ultra-thin slabs and BPPs based thereon, there does not seem to be enough space for the perpendicular and mechanically weak orientation of the CFs, which is why the CFs must be oriented at least partially parallel to the slab, which leads to an excellent mechanical structure, exhibiting an exponential increase in the bending strength towards small thicknesses, as reflected in Figure 2a It is noted here that the bending strength of PPS is in the range of 125-135 MPa [Ref. 41], which is why this behavior seems to be related to the anisotropic properties of the carbon material used, mainly CFs. [Ref. 42] shows something similar and there is also a good correlation with a mathematical model [Ref. 43].

[0210] To delimit the actual minimum thickness of a BPP that is acceptable for use in a PEM fuel cell stack, the following criterion was used, namely that it needs to withstand a pressure of 1 MPa, which is also recommended by one of the gas diffusion layer suppliers [Ref. 44]. Therefore, the strength should also be considered according to the distributed load, which is illustrated in Figure 4 for various BPP thicknesses. Figure 4 It is shown that for the criterion of 1 Mpa to be met, the minimum thickness of a single-layer (i.e. based on 1 slab) BPP is 0.38 mm.

[0211] However, for multi-layer BPPs, the necessary minimum thickness of this criterion is significantly lower, namely only 0.29 mm. This smaller necessary minimum thickness reflects the higher relative strength of the thinner slabs used to make multi-layer BPPs. This value of 0.29 mm for the necessary minimum thickness is in agreement with the simulation represented by the left-hand solid curve in Fig. 2. However, said curve seems to be valid and reproduces the correct minimum thickness only for small thicknesses of the multi-layer slab. For larger thicknesses, Figure 4Theoretical simulation curves in Figure 6 clearly show a significant deviation from the experimental data obtained for the multilayer BPP. This can be explained by the influence of the reorientation of the CF from planar orientation to almost perpendicular orientation during the molding process. This becomes possible when the thickness of the BPP exceeds the length of the CF used in the material. In our experimental case, this length is 300 pm. In other words, if the thickness of the layer is less than the average length of the embedded carbon fibers, an increase in strength is achieved.

[0212] However, it is pointed out that the effect is particularly pronounced in layers of sub-millimeter thickness, which is why multilayer is advantageous for thicker slabs and corresponding thicker separators, such as BPP. When each layer in the multilayer stack is first rolled into an individual slab, and then the rolled layers are stacked into a multilayer slab, the strength increase resulting from the realignment of the CF in the near-surface layers is correspondingly multiplied.

[0213] It is worth noting that the experimental 6-layer BPP of total thickness 0.3 mm produced by the method described herein is not only the thinnest graphite-based BPP in the world at the time of writing this patent application, but also the strongest to date.

[0214] Table 1 shows the collected data of thickness, flexural strength, area specific resistance, and in-plane conductivity of the 6-layer MFG / SFG / CF / CB / PPS / PTFE-based BPP produced experimentally by the method outlined herein (referred to as “BWT” in the table) compared to the BPP made from graphite-based compounds with PPS binder obtained from commercial suppliers for testing, and corresponding test data of BPP obtained from literature sources [references 15, 19, 45-48].

[0215] Table 1. Mechanical and electrical properties of 6-layer BWT BPP, tested comparison BPP, and BPP from literature sources

[0216]

[0217] * ASTM D790-17 standard [reference 49]

[0218] ** DOE test protocol [reference 50]

[0219] *** Four-probe method [reference 51]

[0220] It is important to note that better results have been achieved than required by the DOE's 2020 target for ultra-thin BWT BPP [Ref. 8]. Moreover, many advantages have been achieved compared to other graphite-based BPPs, namely higher strength and electrical conductivity, and due to the volume reduction of the entire stack, the BPP offers a higher level of power density.

[0221] It should be noted that, in contrast to the method in WO2018 / 072803 [Ref. 53], the carbon powder in the present disclosure is mixed with the polymer and then ground into a carbon-polymer powder. If carbon fibers are part of this carbon mixture, the grinding process will destroy a significant amount of carbon fibers, making it impossible to achieve the beneficial results discussed above.

[0222] The list given below presents highlights of the features achieved by this manufacturing process.

[0223] 1. An integrated production process, i.e. material compounding, slab rolling, and BPP molding, occurs in one manufacturing line in succession with high raw component utilization, which is different from the method applied in [Ref. 22].

[0224] 2. The use of a combination of different solvents, including water and organic solvents, facilitates the dispersion and compounding of various powder materials, even at room temperature, which is advantageous over processes that require high temperatures, such as in [Refs. 17, 30].

[0225] 3. In contrast to other prior art, in particular [Ref. 20], the high solids content in the suspension results in much faster evaporation and drying.

[0226] 4. In contrast to other prior art, in particular [Ref. 22], the dough-like structure of the slab is achieved by adding a much smaller amount of PTFE, i.e. 0.25 wt% vs. 2 wt%, i.e. the negative impact of the polymer on the electrical properties of the BPP is reduced.

[0227] 5. The softness of the slab allows it to be rolled and subsequently molded in a very wide thickness range, with the lower limit reaching 0.05 mm, very close to the theoretical value set by the largest component size in the compound, i.e. 20 pm (MFG) + 25 pm (PPS).

[0228] 6. The process of manufacturing the slab results in the formation of a self-organized laminated structure with enhanced mechanical properties, which is beneficial compared to the prior art, in particular [Ref. 52], where similar structures can only be achieved by means of additional coatings on the core slab.

[0229] 7. The application of a multi-layer design can increase the bending strength by 40% when considering the strength criteria, and thus reduce the necessary minimum thickness of BPP acceptable for assembly in a PEM fuel cell stack by 25%.

[0230] 8. Experimentally produced 0.3 mm thick multi-layer BPPs exhibit very high bending strength levels with relatively low polymer content, i.e. 186 MPa at less than 18% PPS.

[0231] 9. The combination of low amounts of polymer binder and thin design results in a negligible contribution of BPP resistance to the total fuel cell resistance.

[0232] REFERENCES

[0233] [1] Lin X, Sabir I. Review of bipolar plates in PEM fuel cells: Flowfield designs. Journal of Power Sources 30 (2005) 359

[0234] [2] Devrim Y, Albostan A, Devrim H. Experimental investigation of CO tolerance in high temperature PEM fuel cells. International Journal of Hydrogen Energy 43 (2018) 18672

[0235] [3] Yan WM, Chen CY, Liang CH. Comparison of performance degradation of high-temperature PEM fuel cells with different bipolar plates. Energy 186 (2019) 115836

[0236] [4] Simaafrookhteh S, Khorshidian M, Momenifar M. Fabrication of multi-filler thermoset-based composite bipolar plates for PEMFCs: Molding defects and properties characterizations. International Journal of Hydrogen Energy 45 (2020) 14119

[0237] [5] Guo N, Leu MC. Effect of different graphite materials on the electrical conductiv-ity and flexural strength of bipolar plates fabricating using selective laser sintering. International Journal of Hydrogen Energy 37 (2012) 3558

[0238] [6] Singh RS, Gautam A, Rai V. Graphene-based bipolar plates for polymer electrolyte membrane fuel cells. Frontiers of Materials Science 13 (2019) 217

[0239] [7] The advanced properties of PPS materials: https: / / readingplastic.com / pps-materi-als /

[0240] [8] Technical targets: bipolar plates for transportation applications: https: / / www.energy.gov / eere / fuel-cells / doe-technical-targets-polymer-electrolyte-membrane-fuel-cell-components

[0241] [9] Yen CY, Liao SH, Lin YF, Hung CH, Lin YY, Ma MCC. Preparation and properties of high performance nanocomposite bipolar plate for fuel cell. Journal of Power Sources 162 (2006) 309

[0242]

[10] Rzeczkowski P, Lucia M, Muller A, Facklam M, Cohnen A, P, Hopmann C, Hickmann T, P, Krause B. Development of joining methods for highly filled graphite / PP composite based bipolar plates for fuel cells: Adhesive joining and welding. Proceedings of the 33 rd International Conference of the Polymer Processing Society. Cancun, Mexico, December 10-14 th , 2017

[0243]

[11] Cunningham B, Baird DG. The development of economical bipolar plates for fuel cells. Journal of Materials Chemistry 16 (2006) 4385

[0244]

[12] Din R, Arshad M, Saleem A, Shahzad M, Subhani T, Hussain S. Fabrication and characterization of bipolar plates of vinyl ester resin / graphite-based composite for polymer electrolyte membrane fuel cells. Journal of Thermoplastic Composite Materials 29 (2014) 1315

[0245]

[13] Horizon announces a breakthrough in ultra-thin high-performance graphite bipolar plate technology for fuel cells: https: / / fuelcellsworks.com / news / horizon-announce-a-breakthrough-in-ultra-thin-high-performance-graphite-bipolar-plates-technology-for-fuel-cells /

[0246]

[14] bipolar plate: https: / / www.sglcarbon.com / pdf / SGL-Datasheet-SIGRACELL-Bipolar-Plates-EN.pdf

[0247]

[15] Highly flexible bipolar plates for redox-flow batteries: https: / / www.umsicht.fraunhofer.de / content / dam / umsicht / en / documents / research-for-the-market / electrically-conductive-polymers / highly-flexible-bipolar-plates-redox-flow-batteries.pdf

[0248]

[16] US7910501B2 Jiang J, Harada T. Sheet molding material for fuel cell bipolar plate, method for producing same and bipolar plate of fuel cell. US 7,910,501 B2

[0249]

[17] WO2014 / 100082A1 Golba JC, Spikowski J, Davison BJ, Geng K. Electrically conductive polyphenylene sulfide compounds. WO 2014 / 100082 A1

[0250]

[18] US7,736,786B2 Hong CM, Park SH, Lee YJ. Composition for fuel cell bipolar plate.

[0251]

[19] Cunningham BD, Baird DG. Development of bipolar plates for fuel cells from graphite-filled wet-lay material and a compatible thermoplastic laminate skin layer. Journal of Power Sources 168 (2007) 418

[0252]

[20] US2019 / 0341630A1 Ando H, Suzuki T, Okada A, Koizumi A. Separator for fuel cells and method for producing same.

[0253]

[21] Torelina TM PPS resin:

[0254] https: / / www.toray.jp / plastics / en / torelina / technical / tec_023.html

[0255]

[22] Gromadskyi D. A method for producing a separator plate for a fuel cell and a method for producing a fuel cell stack with such separator. US2019 / 0260037 Al

[0256]

[23] Gromadskyi DG, Chae JH, Norman SA, Chen GZ. Correlation of energy storage performance of supercapacitor with iso-propanol improved wettability of aqueous electrolyte on activated carbon electrodes of various apparent densities. Applied En-ergy 159 (2015) 39

[0257]

[24] Tensile testing of PTFE: https: / / polyfluoroltd.com / blog / tensile-testing-of-ptfe /

[0258]

[25] Highly engineered thermoplastic materials: https: / / www.performanceplastics.com / materials /

[0259]

[26] Dupont™ PTFE DISP 30 fluoropolymer resin: http: / / download.ce-ris.purdue.edu / file / 3187

[0260]

[27] SIA comments on the preliminary information on manufacturing, processing, distribution, use, and disposal: N-methylpyrrolidone (NMP): https: / / www.semiconductors.org / wp-content / uploads / 2018 / 06 / SIA-Comments-to-EPA-on-N-Methylpyrrolidone-NMP-March-15-2017.pdf

[0261]

[28] US5043112 Beck HN. Process for forming articles comprising poly(phenylene sulfide) (PPS).

[0262]

[29] Lequeux F, Talini L, Verneuil E, Delannoy G, Valois P. Wetting of polymers by their solvents. The European Physical Journal E 39 (2016) 12

[0263]

[30] NMP: https: / / www.eastman.com / Pages / ProductHome.aspx?product=71103627

[0264]

[31] US8563681B2 Makita K, Ono T, Takahashi T, Akasaka H, Sakane T. Process for producing fine polyphenylene sulfide resin particles fine polyphenylene sulfide particles, and dispersion thereof.

[0265]

[32] US2008 / 0318110A1 Budinski MK, Fuller TJ, Dobulis BT. Thermoplastic bipolar plate.

[0266]

[33] Calleja G, Jourdan A, Ameduri B, Habas JP. Where is the glass transition temper-ature of poly(tetrafluoroethylene)? A new approach by dynamic rheometry and me-chanical tests. European Polymer Journal 49 (2013) 2214

[0267]

[34] US2015 / 0367249A1 Miyata K, Kawada T, Katou K. N-methyl-pyrrolidone distil-ling apparatus.

[0268]

[35] Sunitha K, Rani KY, Moulik S, Satyanarayana SV, Sridhar S. Separation of NMP / water mixtures by nanocomposite PEBA membrane: Part I. Membrane synthesis, characterization and pervaporation performance. Desalination 330 (213) 1

[0269]

[36] Triton™ X-100, ACROS Organics TM : https: / / www.fishersci.com / shop / products / triton-x-100-acros-organics-4 / AC215682500

[0270]

[37] Li XG, Huan MR, Bai H, Yang YL. High-resolution thermogravimetry of poly- phenylene sulfide film under four atmospheres. Journal of Applied Polymer Science 83 (2002) 2053

[0271]

[38] Cebe P, Chung S. Melting behaviour of high performance composite matrix poly-mers: Poly(phenylene sulfide) Polymer Composites 11 (1990) 265

[0272]

[39] Wang H, Ding S, Zhu H, Wang F, Guo Y, Zhang H, Chen J. Effect of scretching ratio and heating temperature on structure and performance of PTFE hollow fiber membrane in VMD for RO brine. Separation and Purification Technology 126 (2014) 82

[0273]

[40] Menczel JD, Collins GL. Thermal analysis of poly(phenylene sulfide) polymers I: Thermal characterization of PPS polymers of different molecular weights. Polymer Engineering and Science 32 (1992) 1264

[0274]

[41] Polyphenylene sulfide (PPS): http: / / polymerdatabase.com / Commercial%20Poly-mers / PPS.html

[0275]

[42] Marinho B, Ghislandi M, Tkalya E, Coning CE, With GD. Electrical conductivity of compacts of graphene, multi-wall carbon nanotubes, carbon black and graphite powder. Powder Technology 221 (2012) 351

[0276]

[43] Mijajlovikj M, Ristetska S, Samakoski B, Stevanovska N. Mathematical model on flexural properties of composite laminates. International Journal of Engineering Research & Technology 6 (2017) 526

[0277]

[44] Freudenberg gas diffusion layers: https: / / www.fuelcellstore.com / spec-sheets / freudenberg-gdl-technical-data.pdf

[0278]

[45] Yang T, Shi P. Study on the mesocarbon microbeads / polyphenylenesulfide composite bipolar plates applied for proton exchange membrane fuel cells. Journal of Power Sources 175 (2008) 390

[0279]

[46] Zhang H, Guan, Li T, Yang X. Technology of polyphenylene sulfide (PPS) resin / graphite conductive composite for bipolar plate. Key Engineering Materials 519 (2012) 49

[0280]

[47] Zhang H, Yang X, Liu X, Wang T. Preparation of CF reinforced PPS / graphite conductive composite for bipolar plate. Advanced Materials Research 875-877 (2014) 1245

[0281]

[48] Xia LG, Li AJ, Wang WQ, Yin Q, Lin H, Zhao YB. Effects of resin content and preparing conditions on the properties of polyphenylene sulfide resin / graphite composite for bipolar plate. Journal of Power Sources 178 (2008) 363

[0282]

[49] Annual Book of ASTM Standards 43 (1975) F84

[0283]

[50] Wang H, Sweikart MA, Turner JA. Stainless steel as bipolar plate material for polymer electrolyte membrane fuel cells. Journal of Power Sources 115 (2003) 243

[0284]

[51] Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Material: https: / / www.astm.org / Standards / D790

[0285]

[52] US2006 / 0084750A1 Huang J, Baird DG, McGrath JE. Compression moldable composite bipolar plates with high through-plane conductivity.

[0286]

[53] WO2018 / 072803A1 Gromadskyi, Denys. Method of producing separator plates by compaction and a production facility.

[0287]

[54] US6544680 Shigeru Takano, Kenichi Uehara, Yasunobu Iizuka, Hitomi Hatano. Fuel cell separator, a fuel cell using the fuel cell separator, and a method for making the fuel cell separator

[0288]

[55] US6803139 Kazuo Saito, Atsushi Hagiwara, Yasuo Imashiro, Naofumi Horie, Fumio Tanno, Tsutomu Uehara. Electrically conductive resinous composition, fuel cell separator and production thereof, and polymer electrolyte fuel cell

[0289]

[56] EP1758185 Kazuhito Hatoh, Teruhisa Kanbara, Soichi Shibata, Eiichi Yasumoto, Shigeyuki Unoki, Tatsuo Nagasaki, Masatoshi Teranishi, Tsutomu Kawashima, Toru Sukawa, Yasuo Takebe. Polyelectrolyte fuel cell-use separator, polyelectrolyte fuel cell, method of evaluating polyelectrolyte fuel cell-use separator, and production method of polyelectrolyte fuel cell-use separator

[0290]

[57] US2005 / 0042496 Mukesh Bisara, Yuqi Cai, Divya Chopra, Alistair Mollison, John Fisher. Method for manufacturing fuel cell separator plates under low shear strain.

[0291]

[58] CN1 11048800A.A bipolar plate and method of preparing thereof and fuel cell.

[0292]

[59] US7887927 Jang Boz Z,Guo Jiusheng,Zhama Aruna.Highly conductivemulti-layer composite precursor composition to fuel cell flow field plate orbipolar plate.

[0293]

[60] US2014 / 087287 Shunya Suzuki,Tadashi Iino,Zenichiro Izumi.Methodfor man-ufacturing fuel cell separator.

[0294]

[61] US2019 / 341630 Hitoshi Ando,Tsutomu Suzuki,Masaru Yoneyama,AkiraOkada,Akihiro Koizumi.Separator for fuel cells and method for producing same.

[0295]

[62] US2009 / 152105 Anthony B.LaConti,William A.Titterington,LarryL.Swette,Ricardo Leon,Kwang S.Kim.Proton exchange membrane(PEM)electrochemical cell having an integral electrically-conductive,resilientlycompressible,porous pad.

[0296]

[63] CN101174695A Congsheng Guan;Huayong Zhang;Ziqiang Li.Double polarplate used for phosphoric acid type fuel cell and method for producing thesame.

[0297]

[64] WO2008 / 075812 Sung Jun Kim,Chang Min Hong.Hydrophilic carbonblack ag-gregate,its preparation process,hydrophilic composite material andbipolar plate for fuel cell comprising it.

[0298]

[65] US2018 / 0358630 Woo,Jong Seok;Lee,Mun Hee;Oh,Sung Moon;Ju,HongBeom;Park,Kwang Sang;Park,Sung Hoon.Bipolar plate for fuel cell having con-trolled structure of carbon materials and method for manufacturing the same.

[0299]

[66] US2004 / 033413 YOON JONG-JIN,;LIM IL-JI,;SAITOH AKIHISA:Polymerelectrolyte membrane fuel cell.

[0300]

[67] US2017298200 YI Jee Sung; LEE Hyun Chul; YOON Joon Young; CHO Eun Jeong; KANG Chung Seock; Yi, Jee Sung; Lee, Hyun Chul; Yoon, Joon Young; Cho, Eun Jeong; Kang, Chung Seock. Thermoplastic prepreg intermediate material for fuel cell separation plate and method for manufacturing thermoplastic prepreg for fuel cell separation plate by using the same.

Claims

1. A rigid, rolled, and pressed-molded separator for a fuel cell, wherein the separator is formed from a single-layer precursor sheet into a single layer, wherein the single layer has a thickness X2 in the range of 0.05 mm to 0.6 mm and is provided as a polymer matrix, the polymer matrix comprising only a thermoplastic polymer blend in which carbon fibers and conductive carbon particles are dispersed; wherein the thermoplastic polymer blend comprises PTFE and a thermoplastic polymer other than PTFE; wherein the average length L of the carbon fibers is in the range of 0.1 mm to 1 mm and is greater than the thickness X2 of the layer; wherein the weight concentration of PTFE is at least 0.05 wt% but less than 0.5 wt% PTFE, and wherein the weight concentration of the thermoplastic polymer other than PTFE is 5 wt% to 30 wt%, and wherein the weight concentration of the carbon fibers is in the range of 2 wt% to 20 wt%, the weight concentration being relative to the total weight of the polymer blend, the carbon fibers, and the conductive carbon particles.

2. The partition according to claim 1, wherein the average length L of the carbon fiber is at least twice the thickness x 2 of the layer.

3. The separator according to claim 1 or 2, wherein the weight concentration of the carbon fiber is in the range of 5% to 20% by weight relative to the total weight of the polymer blend, carbon fiber and conductive carbon particles.

4. The separator according to claim 1, wherein the conductive carbon particles in the thermoplastic polymer blend comprise at least a first portion and a second portion, wherein the carbon particles in the first portion are graphite particles with an average size in the range of 10 μm to 100 μm, and wherein the carbon particles in the second portion have a size in the range of 0.1 μm to 10 μm, wherein the weight concentration of the first portion is in the range of 50% to 90% by weight relative to the total weight of the polymer blend, carbon fibers and 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.

5. A fuel cell having a separator according to any one of claims 1 to 4.

6. A method for producing a partition according to any one of claims 1-4, the method comprising: A thermoplastic polymer powder, a carbon fiber powder, and conductive carbon particles are mixed in a dispersion to provide a thermoplastic polymer blend in which carbon fibers and conductive carbon particles are dispersed, wherein the average length L of the carbon fibers is in the range of 0.1 mm to 1 mm, wherein the thermoplastic polymer is a blend comprising polytetrafluoroethylene (PTFE) and a second thermoplastic polymer different from PTFE, wherein the weight concentration of PTFE is at least 0.05 wt% but less than 0.5 wt% PTFE, and wherein the weight concentration of the thermoplastic polymer different from PTFE is 5 wt% to 30 wt%, and wherein the weight concentration of the carbon fibers is in the range of 2 wt% to 20 wt%, the weight concentration being relative to the total weight of the polymer blend, the carbon fibers, and the conductive carbon particles; The mixture is formed into a moldable, stretchable single-layer precursor sheet of thickness X1 by calendering and rolling in a forming station and by said rolling such that the carbon fibers are at least partially parallel to the slab; The single-layer precursor sheet is hot-pressed and molded into a single-layer partition with a thickness X2 in the range of 0.05mm-0.6mm; Where X2 is less than the average length L of the carbon fiber, where L is greater than the thickness X1 of the layer and is in the range of 0.1 mm to 1 mm, wherein the single-layer separator is composed of a polymer matrix comprising only a thermoplastic polymer blend in which carbon fibers and conductive carbon particles are dispersed; wherein the molding is performed at a temperature between the melting temperature of the PTEE and the melting temperature of the second thermoplastic polymer.

7. The method of claim 6, wherein the method comprises providing the carbon fibers with an average length L that is at least twice the thickness x 2 of the layer.

8. The method of claim 6, wherein the second thermoplastic polymer is polyphenylene sulfide (PPS).

9. A method according to any one of claims 6 to 8, the method comprising: An aqueous dispersion is provided, the aqueous dispersion comprising PTFE particles and carbon fibers; A solvent dispersion is provided, the solvent dispersion comprising carbon black particles dispersed in an organic solvent and particles of the second thermoplastic polymer; Stir the two dispersions to prevent the particles from settling; The two dispersions are combined and mixed together; Only after the two dispersions are mixed together, a portion of the graphite particles is mixed with the two dispersions to provide a mixture, wherein the average size of the graphite particles in the portion is in the range of 10 μm to 100 μm; wherein the weight of the portion is 5 to 20 times the weight of the carbon black. After the graphite particles are added, the mixture is kneaded in a kneader; During kneading in the kneader, the temperature is raised to a sufficiently high level to evaporate the organic solvent and water from the mixture, wherein the high temperature level is above the glass transition temperature of PTFE. After the organic solvent and water have evaporated and while the second thermoplastic polymer, rather than the PTEE, is in a molten state, the mixture is formed into a precursor sheet in the molding station.

10. The method of claim 9, wherein the portion of graphite particles added only after the two dispersions are mixed is a second portion of graphite particles, and wherein the method includes providing the aqueous dispersion containing a first portion of graphite particles before the two dispersions are mixed; wherein the average particle size of the graphite particles in the first portion is in the range of 0.1 μm to 10 μm.

11. The method of claim 9, wherein the high temperature level during the kneading process in the kneader is below the melt temperature of the PTFE and the second thermoplastic polymer; wherein the method comprises extracting the mixture from the kneader, then raising the temperature of the mixture to a level high enough to melt the second thermoplastic polymer, and then forming the sheet into a precursor sheet of thickness X1 in the forming station.

12. The method of claim 11, wherein providing the aqueous dispersion comprises adding a surfactant to the aqueous dispersion, wherein the boiling point temperature of the surfactant is higher than that of water and higher than that of the organic solvent; wherein the method comprises extracting the mixture from the kneader when the mixture contains the surfactant but not both the solvent and water, then raising the temperature of the mixture to a level high enough to evaporate the surfactant, and then forming the sheet into a blank of thickness X1 in the forming station.

13. The method according to any one of claims 6 to 8, wherein the method comprises calendering the precursor sheet in at least two different directions to align the carbon fibers in different directions.

Images