Separator plate, and production method using polymeric binders with high and low mfi and a fuel cell with such separator plate

Abstract

Separator plate, and production method using thermoplastic pol- ymer with high and low-MFI as binder and a fuel cell with such separator plate For producing separator plates having useful characteristics of polyphenylene sulfide (PPS) with high melt flow index (MFI), but also safeguard easy de-molding after com- pression-molding, combinations are provided with PPS having low MFI. For example, high-MFI PPS is mixed with low-MFI PPS for the separator plate, or a central layer of high-MFI PPS is sandwiched between layers of low-MFI PPS. Other water insoluble thermoplastic non-fluoro polymers than PPS can be used as alternatives.

Description

SEPARATOR PLATE, AND PRODUCTION METHOD USING POLYMERIC BINDERS WITH HIGH AND LOW MFI AND A FUEL CELL WITH SUCH SEPARATOR PLATEFIELD OF THE INVENTIONThe present invention relates to separator plates, for example for use in fuel cells, and methods of their production. In particular, is relates to a method of production in which a separator plate is produced by compression molding of a mix of a thermoplastic waterinsoluble polymeric binder (TWIP) and an electroconductive filler, such as a mix of polyphenylene sulfide (PPS) and carbon powder with a minor addition of polytetrafluoroethylene (PTFE).BACKGROUND OF THE INVENTIONHigh-temperature proton exchange membrane (HT-PEM) fuel cells are attractive alternative to its low-temperature version, as they operate at temperatures 120-200°C, where adsorption of carbon monoxide on the platinum-based catalysts is relatively low. Therefore, such HT-PEM fuel cells may operate with non-purified hydrogen gas as fuel, for example obtained from reforming processes, for example, methanol reforming, see [Ref. 1], But these relatively high operating temperatures are challenging with respect to the materials, as the materials need to be thermally stable at such conditions. Moreover, materials used in HT-PEM fuel cell stacks need to be chemically resistant to concentrated mineral acids, because phosphoric acid-doped polybenzimidazole (PBI) is used for the membrane.The bipolar plates (BPPs) that separate single cells in the fuel cell stack have multiple roles, including functioning as current collectors and delivering fuel gases as well as distributing heat transfer fluids. For these reasons, materials such as stainless steel and nickel are not suitable, unless having special protective coatings, see [Ref. 2], Graphitebased composites with different thermoplastic and thermoset polymers are better alternatives for HT-PEM applications, see [Ref. 3], Other carbon materials, such as carbon black (CB), carbon nanotubes (CNT), carbon fibers (CF), and graphene, may be addedto the composites to improve some of their characteristics, for example electrical conductivity and flexural strength [Ref. 4],Polyphenylene sulfide (PPS) is a good candidate as a polymeric binding agent in graphite-based composites for BPPs that are used in HT-PEM fuel cells, as this polymer has a high enough continuous operation temperature, excellent chemical stability and good mechanical properties [Ref. 5], Furthermore, PPS is commercially available at lower prices as compared to other high-temperature performance polymers, like polyetherether ketone (PEEK), poly ether imide (PEI), poly sulfone (PSU) and is also only slightly more expensive than polyphenylene oxide (PPO) [Ref. 6, 7], Some thermal and mechanical properties of PPS, which are important for its processing in separator plates, are given in Table 1.Table 1. Selected thermal and mechanical properties of PPS [Ref. 8-10]In order to produce separator plates fast and continuously, it is advantageous to operate with graphite-based composites which have flexible self-supported film-like structures during production prior to compression molding. International patent applications W02018 / 072803A1 [Ref. 17] and WO2021 / 027999A1 [Ref. 18] disclose fabrication of BPPs through an intermediate stage where a malleable compound is produced which has dough-like mechanical properties. This formation of a malleable precursor structure also has the advantage of uniform distribution of such material within the mold as compared to molding dry powder or pellets.As disclosed in WO2021 / 027999A1, the malleable precursor structure is formed by means of fibrillization agent, for example polytetrafluoroethylene (PTFE), which bindspowders of electroconductive filler and PPS together, making is possible to manufacture a self-supporting precursor film, which is then compression molded at elevated temperature.Some thermal and mechanical properties of PTFE which are important in terms of its fibrillization and taking part in the compression molding process are given in Table 2.Table 2. Some thermal and mechanical properties of PTFE [Ref. 18-23]Another important characteristic of polymers for BPPs is the melt flow index (MFI) which shows how easily the polymer can flow in melted state. The MFI is inversely proportional to viscosity of the melt and strongly depends on the molecular weight of the polymer [Ref. 11], Based on the properties of the specific thermoplastic polymers, different molding methods are used for producing BPPs, but it is mainly injection or compression molding [Ref. 4, 5], Typically, processes of injection molding require polymers with higher MFI as compared to other molding methods including compression molding [Ref. 12], In order to obtain BPPs with low electrical resistance, the content of electroconductive filler in the composite should be high, namely about 70-80 wt.% [Ref. 13, 14], which, however, makes applying injection molding very challengeable because of the low flowability of such composites. Therefore, the process of compression molding appears as a good choice when having production speed as a criterion.However, even when using compression molding, prior art production speed still needs improvements in order to be useful for fast, large scale production . Especially, the continuously growing needs in fuel cell-based solutions [Ref. 15] require large-scale production method for the components. For BPPs, this implies that production rates for a BPP should be in the order of seconds or even shorter.It would be desirable to provide a method that takes offset in the prior art but which brings about improvements that fulfils the needs for fast production methods.DESCRIPTION / SUMMARY OF THE INVENTIONIt is therefore an objective of the invention to provide an improvement in the art. In particular, it is an objective to provide a method for large-scale production of separator plates. This objective and further advantages are achieved with method for production as described below and in the claims.In short, the invention can be described as follows. In order to produce separator plates from compounds with useful characteristics of polyphenylene sulfide (PPS) with high- melt flow index (MFI) but in order to also safeguard easy de-molding after compression-molding, combinations are provided including PPS with low MFI. For example, high-MFI PPS is mixed with low-MFI PPS for the separator plate. Alternatively, a central layer with dominantly or only high-MFI PPS is sandwiched between outer layers with dominantly or only low-MFI PPS. Other thermoplastic water insoluble non-fluoro polymers (TWIPs) can be used as alternatives to PPS. Fluoropolymers are not TWIPs by definition in this disclosure.As it has been already mentioned, the MFI of polymer can be used as indirect measure of its molecular weight. Therefore, the current approach of mixing TWIPs having different MFI is analogous to TWIPs having different molecular weight. For example, high MFI corresponds to low molecular weight, while low MFI corresponds to high molecular weight.Various embodiments and details are described in the following.In experiments directed towards high-speed large-scale production of BPPs, the inventor has compared and combined the properties of PPS with high-MFI and low-MFI with respect to the production capabilities.Herein high-MFI is defined as MFI above 30 g / 10 min, and low-MFI as MFI below 10 g / 10 min.As alternative to PPS, other thermoplastic water-insoluble non-fluoro polymers (TWIP) can be used. Also, other fluoropolymers than PTFE can be used for the purpose of fi- brillization to facilitate forming dough-like structure of a precursor for the separator plate. This will be explained in more detail below. However, for illustrative purposes and due to its usefulness, PPS is used as an example of a TWIP, and PTFE as an example of a fluoropolymer. A PTFE is a fluoropolymer, it is not a TWIP in the sense of this disclosure.The separator plate is advantageously a bipolar plate (BPP) and will be exemplified as such herein, but it is optionally a monopolar plate (MPP), for example as one half of a BPP. The main objective is its use for a fuel cell, typically as part of a fuel cell stack. However, despite is primary usefulness for fuel cells, in particular HT-PEM fuel cells, the separator plate should be understood as having more general usage, as it may potentially also find useful application in other technical fields.As mentioned in the introduction, PPS is a useful candidate as polymeric binder for BPPs with high content of electroconductive carbon filler, for example at least 70%, especially when a small amount of fluoropolymer, especially PTFE, is added.In particular high-MFI PPS has been found to be a very attractive candidate due to its ease of flow and due to the final BPP having high density, low porosity, and high flexural strength. Unfortunately, however, in practice, high-MFI PPS, similar to other high- MFI TWIPS, has turned out to be very difficult to de-mold, as it sticks to the metal of the press-mold, which not only slows the production method but also decreases the success rate in the production. Accordingly, BPPs made of a compound with high-MFI PPS are not optimum, despite the otherwise highly attractive parameters. On the other hand, for fast large-scale production of BPP, use of low-MFI PPS brings problems with respect to speed in melting and flow, which is also not desired. This provides a dilemma with respect to fast, large scale production.In this relation, it is pointed out that the contained PTFE assists in anti-sticking additive and friction-reducing agent in the compound, but fluoropolymer is, typically, only added in small amounts, for example in the order of 1 wt. % or even less, so that the effect of PTFE is not sufficient to overcome the unfortunate sticking properties of the compound based on high-MFI PPS.According to the invention, this problem of sticking of the compound to the mold can be overcome by using high-MFI PPS in combination with low-MFI PPS in different technical solutions, either as a blend or in different layers. This is described in detail in the following. Similar considerations apply for other TWIP than PPS or in mixtures where PPS is mixed with other TWIP. Accordingly, the invention is in the following described in more general terms before presenting specific examples.For producing a separator plate, one or more malleable compounds are provided, each containing a homogenous blend of an electroconductive filler and binder polymers. The binder polymers comprise at least one fluoropolymer. For example, only one fluoropolymer is used for the compound. Alternatively, a blend of different fluoropolymers is provided. The binder polymers also comprise at least one thermoplastic water insoluble non-fluoro (not fluorine based) polymer, TWIP, for example PPS.Notice that the term TWIP is used herein only as an abbreviation for non-fluoro polymers for differentiation from fluoropolymers. The short term TWIP is used instead of the more characteristic term NF-TWIP for non-fluoro thermoplastic water insoluble polymers.The TWIP or the plurality of TWIPs dominate relatively to the fluoropolymer or fluoropolymers. Typically, the ratio between the total weight of all TWIP relatively to the weight of all fluoropolymer in the blend is at least 10.The compound is kneaded at a temperature above all glass temperatures of the binder polymers but below all melting temperatures of the binder polymers in order to cause fibrillization of the fluoropolymer by the kneading and in order for the compound to attain a malleable structure having mechanical properties similar to a dough for bread prior to balking.The compound in its malleable form is shaped, for example by calender rollers, into a precursor slab. In a press mold, the slab is undergoing hot compression molding. After such hot-compaction, the compressed slab is solidified into a separator plate prior to removing the so-formed separator plate from the press-mold.A typical thickness for separator plates after compression molding is in the range of 0.5 mm to 5 mm, for example 0.5 mm to 2 mm. Good experimental results were obtained with separator plates having a thickness in the order of 1 mm.For example, the compound is provided from an aqueous dispersion of a homogeneous mix of carbon powders as electroconductive filler in addition to powders of the at least one fluoropolymer and at least one TWIP. An aqueous dispersion is advantageous is avoiding organic solvents, being beneficial for environment and working conditions. The environmental benefit is obtained if the aqueous dispersion contain no organic solvents at all or at least only in small amounts, for example less than 1% by weight.In a first aspect of the invention, the pre-curser slab is provided as a single layer of a homogeneous compound comprising fluoropolymer, the high-MFI TWIP, and the low- MFI TWIP. In this single layer embodiment with a mix of high-MFI and low-MFI with a weight ratio in the range of 1:1 to 1:5.5 between the total weight of high-MFI TWIP relatively to the total weight of low-MFI TWIP. For example, the weight percentage is in the range 15 wt.% to 49.9 wt.% for the high-MFI TWIP and 50.1 wt.% to 85 wt.%. for the low-MFI TWIP, the two weight percentages adding up to 100 wt.%;Optionally, but not necessarily, the high-MFI TWIP and the low-MFI TWIP are made from the same monomers. For example, high-MFI TWIP is high-MFI PPS and the low- MFI TWIP is low MFI-PPS. However, also other thermoplastic polymers may be used as high-MFI TWIP or low-MFI TWIP.In a further aspect of the invention, the precursor slab is provided as a multi-layer comprising a central layer sandwiched between two outer layers that are forming outer sides of the slab. In some embodiment, a triple layer precursor slab and separator plate isproduced. However, optionally, one or more further layers are provided between the outer layers and the central layers.For example, the central layer is shaped of a first homogeneous compound that comprises fluoropolymer and high-MFI TWIP but not low-MFI TWIP. Alternatively, the central layer is shaped of a first homogeneous compound which comprises fluoropolymer and both high-MFI TWIP and low-MFI TWIP, but more high-MFI TWIP than low- MFI TWIP, for example at least twice as much.For example, the outer layers are shaped from a further homogeneous compound, which comprises fluoropolymer and low-MFI TWIP but not high-MFI TWIP. Alternatively, each of the outer layers is made from a further homogeneous compound, which comprises fluoropolymer and more low-MFI TWIP than high-MFI TWIP, for example at least twice as much. In a further alternative embodiment, each of the outer layers contain less than 10 wt. % high-MFI TWIP and the central layer contains less than 10 wt. % low-MFI TWIP.Optionally, the two opposite outer layers are made from the same further compound, which is advantageous production-wise, but is not strictly necessary. Typically, but not necessarily, the outer layers have identical thickness.For example, the separator plate is provided with a thickness in the range of 0.5 to 5 mm, optionally in the range of 0.5 mm to 2 mm. This thickness is useful for single-layer separators but also for as a multi-layers with three or more layers.Advantages of the production method become more apparent from the following discussion of experiments with PPS and PTFE, which is for illustration and proof, but the considerations can be also be applied to other TWIP / fluoropolymer combinations.In practical experiments leading to the invention, in the following described as technical solutions A, B, and C, separator plates were produced from a compound comprising 23 wt.% PPS, 0.25 wt.% PTFE and 76.25 wt.% graphite. The mix was heated to above the glass temperature of PPS and PTFE, kneaded for fibrillization of the PTFE, then calender rolled for achieving a pre-curser sheet for final hot compression molding at atemperature above the melting temperature of PPS but below decomposition temperatures of PPS and PTFE. Also, the PTFE is advantageously not molten.Tests on flexural strength (FS) and maximal strain (MS) were performed according to standard method at 3-point bending [Ref. 54], Tests on areal specific resistance (ASR) was carried out with a separator plate located between two sheets of carbon paper under 2 MPa of clamping load and constant current of 1 Amp, similar as described in [Ref. 55], Tests on porosity (<|>) was conducted according to an internal procedure by means of air under pressure of 1 bar (0.1 MPa), which flows through the separator plate, and the amount of bubbles, as porosity characteristic, was registered for 2 minutes.In the technical solution A, the compound contains PPS, PTFE and electroconductive carbon filler. However, both high-MFI and low-MFI PPS is used as a blend with a ratio of 25:75 in the compound. It turned out experimentally that mixing low-MFI PPS with a minor amount high-MFI PPS resulted in a separator plate having the beneficial physical features of separator plates where only high-MFI was used, but allowed an easy demolding of the compression-molded separator plate.This was a very positive result, but even more surprising were the findings of the very positive mechanical and electroconductive parameters measured for the final separator plates. Measured physical parameters are found in Table 3 below.Table 3. Experimental resultsFor example, a separator plate produced with such a compound comprising low-MFI PPS as well as a minor portion of high-MFI PPS had a flexural strength of 54-60 MPa. This value is close to the corresponding flexural strength of 64-68 MPa found for a corresponding separator produced only with high-MFI PPS but far from the flexuralstrength of 29-34 MPa for a corresponding separator plate produced with only low-MFI PPS. The fact that the high strength was found, despite the much smaller amount of high-MFI as compared to low-MFI was surprising.As it is also observed from table 3, the maximal strain (MS) of the high / low-MFI separator plate was closer to the maximal strain the high-MFI separator than the low-MFI separator. The areal specific resistance (ASR) was largely intermediate between the separator plates with only high-MFI PPS or only low-MFI PPS. All parameter values for the low / high-MFI separator plate were very satisfactory.Accordingly, it can be concluded that the low / high-MFI blend is important for obtaining useful parameter values in combination with easy de-molding.In the technical solution B, a triple-layer separator plate was produced having a central layer sandwiched between two outer layers. The ratio between the thickness of the central layer to the thickness of each of the outer layers was 2: 1 : 1. All layers were produced in the same way with the same composition, apart from the fact that high-MFI PPS was used for the central layer and low-MFI PPS was used for the two outer layers. Obviously, as the outer layers were made with low-MFI PPS, the separator plate had the beneficial property of easy de-molding.Table 4 shows the experimental results in comparison with the separator plate described under technical solution A.Table 4. Experimental resultsAs is readily observed, this triple layer separator plate of the technical solution B combines the ease of demolding with the beneficial parameters of the separator plate with the high-MFI PPS. Accordingly, despite a more complex production method due to the production of two types of precursor sheets and the combination of three of such precursor sheets prior to molding, this method has also proven to be highly useful for fast, large-scale production due to the de-molding being smooth and fast.In the technical solution C, a triple-layer separator plate was produced having a central layer sandwiched between two outer layers. The ratio between the thickness of the central layer to the thickness of each of the outer layers was 2: 1 : 1. All layers were produced in the same way with the same composition, apart from the fact that the PPS used for the central layer was a mix of high-MFI PPS and low-MFI PPS in a weight ratio of 75:25, and a different mix of high-MFI PPS and low-MFI PPS in a weight ratio of 25:75 was used for the two outer layers. Accordingly, the high-MFI PPS was dominant in the central layer, and the low-MFI PPS was dominant in the outer layers. Due to the outer layers being made predominantly with low-MFI PPS, the separator plate had the beneficial property of easy de-molding.Table 4 shows the experimental results for this technical solution C in comparison with the separator plate described under technical solution A and B. It is observed that the parameters are similar to the single layer high / low-MFI separator plate in technical solution A, apart from the surprisingly much lower porosity, which is similar to the technical solution B.As it appears from the above, by using low-MFI PPS in combination with high-MFI PPS, be it in a PPS-mix or in a combination of various layers, it is possible in a simple way to obtain easily de-molding of the compression molded separator plate, while achieving satisfying physical parameters with respect to strength, stability, resistivity, and porosity. The variations of the method as per the invention are highly useful low- cost, large-scale production.Thus, combination of PPS with different MFI values can be utilized to get benefits from both low- and high-MFI PPS. Similar approach may also be applied with other thermoplastic water-insoluble non-fluoro polymers (TWIP), for example PEEK, PEI, PPO, andPSU, which have suitable thermal and mechanical characteristics to be binder in BPPs for HT-PEM fuel cells [Ref. 16],As mentioned above, also PTFE is useful as an additive for the sake of fibrillization for providing pristine carbon and TWIP powders as a dough-like structure and forming self- supported precursor films for producing separator plates based on it. As alternatives fluoropolymer to PTFE, ethylene chlorotrifluoroethylene (ECTFE), fluorinated ethylene propylene (FEP), perfluoro alkoxy alkanes (PF A), polychlorotrifluoroethylene (PCTFE) and polyvinylidene fluoride (PVDF) may be used as fibrillization agents because these have similar useful properties as PTFE, namely high elongation at break as well as good thermal and chemical stability [Ref. 24-28],In the process of making graphite-based separator plates, PPS-PTFE has been exemplified above as a pair of binders with PPS being the dominant binder. But as it has already been mentioned other combinations of TWIP with fibrillization agents can be used, especially for application in HT-PEM fuel cell. Examples of such binary combinations are PEEK-PTFE, PEI-PTFE, PPO-PTFE, PSU-PTFE, PPS-ECTFE, PEEK-ECTFE, PEI-ECTFE, PPO-ECTFE, PSU-ECTFE, PPS-FEP, PEEK-FEP, PEI-FEP, PPO- FEP, PSU-FEP, PPS-PFA, PEEK-PFA, PEI-PFA, PPO-PFA, PSU-PFA, PPS- PCTFE, PEEK-PCTFE, PEI-PCTFE, PPO-PCTFE, PSU-PCTFE, PPS-PVDF, PEEK-PVDF, PEI-PVDF, PPO-PVDF or PSU-PVDF. Moreover, use of more than two TPWIP and fibrilization agents from this list, for instance, three or four are also possible for graphite-based composites with improved properties.Exemplary contents for the dry, malleable compound, are- electroconductive filler: at least 79.5 wt.%,- TWIPS at least 15 wt.% but typically at least 20 wt.%- PTFE at most 0.5 wt.%, but typically at most 0.3 wt. %.On order to achieve proper wetting of the otherwise hydrophobic TWIP, a surfactant is typically added. Various surfactants are known and disclosed in the prior art cited herein.SHORT DESCRIPTION OF THE DRAWINGSThe invention will be explained in more detail with reference to the drawing, where FIG. 1 illustrates a fabrication process for graphite-based separator plates with a mix of high-MFI and low-MFI PPS;FIG. 2 illustrates a fabrication process for triple-layer separator plates;FIG. 3 illustrates three principles for low / high-MFI separator plates.DETAILED DESCRIPTION / PREFERRED EMBODIMENTIn the following, the invention is explained in more detail with PPS as thermoplastic water-insoluble non-fluoro polymer (TWIP) and PTFE as fluoropolymer that is used as polymeric fibrillization agent to produce a self-supported film as a precursor for the separator plate. However, it is understood that other TWIPs than PPS can be used, as explained above, as well as other fluoropolymers than PTFE for the purpose of fibrillization. Also, graphite will be exemplified as electroconductive filler, although, also other carbon particles can be used in addition or alternatively.FIG. 1 illustrates a production line 1 for producing separator plates 17. The production line 1 is useful as an automated fabrication process for large-scale production of separator plates 17. A number of ingredients for the compound are added, as indicates by arrow 2, into a mixing container 3.As an example, particulate PPS, PTFE, and graphite are added to a dispersion medium, for example water, typically by adding a surfactant, potentially also involving solvents, and mixed by stirring into a homogeneous suspension, optionally stirred at ambient temperature and ambient pressure. For example, the amount of liquid, such as water, by weight is in the order of the weight of the added ingredients.For example, the total stirring time to obtain a homogenous slurry does not exceed 15 minutes. However, it can be between 10 minutes and 60 minutes, which is significantly faster than mixing powders in a dry process, for example, referring to the 12 hours as reported in [Ref. 31],Notice that the initial mixing in the mixing container 3 is to be understood as a principle, only, and more than one container may be used for initial mixing of parts of the ingredients prior to combining them in the final mixing container 3.Various mixing processes can be used, for example as described in international patent application W02018 / 072803A1 [Ref. 17], where iso-propanol (IPA) is used, or processes as described in international patent application WO2021 / 244719A1 [Ref. 32], where N-methyl-2-pyrrolidone (NMP) solution in water is used to wet PPS. Processes of making fine PPS dispersions are described in Japanese patents JP5369645B2 and JP5589373B2 [Ref. 35, 36] which also includes NMP and IPA as solvent and dispersing media, besides nonionic surfactant containing phenyl groups and polymeric surfactant, in particular polyvinyl pyrrolidone (PVP).In order to avoid NMP and IPA, due to flammability and for health and environmental reasons, an aqueous suspension can be used as an advantageous alternative. However, as PPS has a low ability (< 0. 1 wt.%) for water adsorption, see [Ref. 37], a surfactant is useful as a main wetting agent. Advantageously, the final dispersion contains no more than 1% organic solvents.The resulting mix, which is in the form of a slurry, is transferred from the mixing container 3 to a kneader 4, for example Z-type kneader, in which the temperature is raised to above the glass temperatures of the binder polymers but below the melting temperatures, and in which fibrillization of the fluoropolymer, such as PTFE, is achieved by the kneading for forming a soft and malleable compound, with a texture as a dough.The achievement of a malleable texture of the compound requires evaporation of the liquids, for example water and other liquid additives, such as surfactants and / or organic solvents, from the mix. This evaporation can be done entirely before the slurry enters the kneader 4 so that a dried mix enters the kneader. Alternatively, the liquid is partially evaporated from the mix before entering the kneader 4, depending on the heating of the slurry upstream of the kneader 4, and partially from the kneader 4 during the kneading process, in which the temperature has to be raised to above the boiling point of water for the kneader. Alternatively, the evaporation of the entire liquid is achieved by heating in the kneader 4. Liquids, such as water, evaporated from the compound, may berecycled, as indicated by arrow 5, via condenser 6. As a further alternative, mixing and kneading may be combined to simplify the process.PTFE in this recipe is used as fibrillization agent because of its ability to form fibers, which are nanosized in diameter but relatively long, during application of shear force, when it is heated above glass transition temperature transferred to rigid amorphous, so- called viscoelastic, state as also discussed in international patent applications W02018 / 072803A1 [Ref 17], WO2021 / 027999A1 [Ref 18], WO2021 / 244719A1 [Ref.32] as well as in [Ref. 19], As PTFE is widely available on the market in form of aqueous dispersion [Ref. 47-50], it may be directly used in the process of slurry preparation, merely requiring dilution with the main fraction of water. In order to minimize risks for spontaneous agglomeration of PTFE nanoparticles during dilution of the aqueous dispersion by water, the water can be alkalized to the same pH values as the raw PTFE dispersion, typically to a pH of 8.5-10.5, for example, by adding ammonium hydroxide. Such synchronization of pH is not strictly necessary but may be desirable for obtaining a more uniform distribution of PTFE nanoparticles in the slurry. Moreover, use of basic water reduces formation of foam from the intensive stirring of surfactants -contained solutions [Ref. 51], The amount of dry PTFE in the compound (water- free slurry) is less than 0.5 wt.%, and typically in the range between 0.1 and 0.3 wt.%. Accordingly, the content of PTFE is almost two orders of magnitude less than the content of the TWIP, such as PPS, and the electroconductive filler, such as graphite. Addition of PTFE to the compound also improves its anti-sticking properties, which is beneficial. However, as PTFE decreases its electrical conductivity, it is advantageous to keep the concentration of PTFE low.The TWIP, exemplified herein as PPS, is used as a main binder and accordingly, its content in the separator plate is relatively high, namely in the range of 15 to 40 wt.%, typically in the range of 20 to 30 wt.%. Advantageously, PPS is provided as a fine powder. For example, the D50 value for its particles is less than 70 pm or even less than 50 pm, optionally less than 30 pm. The MFI for PPS, for example defined at 300°C, may differ more than a factor of 3 between low-MFI and high-MFI. Herein, PPS with MFI below 10 g / 10 min is identified as low-MFI polymers, while PPS having MFI above 30 g / 10 min is identified as high-MFI polymers.In the current process, two types of PPS were used, one with MFI values of 4g / 10min and one with 50g / 10min. The ratio in weight between the two types of PPS in the blend in some experimental studies for the separator plate was 75:25 between low-MFI PPS and high-MFI-PPS. Thus, the content of low-MFI PPS was dominating.Useful mass ratios are in the range of 50:1 to 1:50 depending on which layer they are added in and the total number of layers.The electroconductive filler is, advantageously, graphite in powder form, although other carbon powder types may be added. Advantageous particle sizes, as defined by the D50 value, are smaller than 60 pm or smaller than 40 pm or even smaller than 20 pm. Other carbon materials like CB, CF, CNT, graphene, etc. can be optionally used as additives to graphite in order to improve, for example, flexural strength and / or electrical conductivity of the separator plate, but their content does, typically, not exceed of 20 wt.% or 15 wt.% or even 10 wt.% relatively to the amount of graphite. Consequently, the content of graphite in the dry compound, i.e. when all liquids have been already evaporated from the slurry, may be in the range of about 40 wt.% to about 85 wt.%, for example 39.5 to 84.9 wt.%, depending on the amount of TWIP, PTFE and other carbon additives utilized therein.It should be noted here that total concentration of all solids, i.e. graphite, PPS and PTFE in liquid phase, which includes mainly water, is typically between 20 and 50 wt.%.In the case of using PPS and PTFE as binder, the maximal temperatures for the stage of kneading should be limited to 218°C, in order do not exceed continuous service temperature for PPS. Although increase of temperature up to 260°C or even up to 279°C for short time is acceptable, for example in order to improve flowability of the compound and make its extrusion from the kneader 4 by the extruder 7 easier.As illustrated stylistically in FIG. 1, the extruded malleable compound 8 is formed into a self-supporting precursor film 10 in a rolling station 9, in which the heated graphite- TWIP-PTFE compound is calender-rolled into a specified thickness, for example, within range of 0.1 to 10 mm or even in the range of 0.5 to 5 mm, for example in the range of 0.5 mm to 2 mm. Temperatures, at which proper calendering occurs, may varyfrom 55°C to 279°C, but is typically in the range of 116°C to 260°C or even from 121°C to 218°C. Typically, the number of calendering stations is at least two and potentially higher, depending on the final desired thickness of the precursor film.When the precursor film 10 of the graphite-TWIP-PTFE compound has been formed, it is transferred to conveyor 11, where this precursor film is cut, for example using a knife 12, into slabs 13 having dimensions in width and length as the finally compression molded separator plate 17. The cut off portion from the precursor film 10 may be recycled, giving product yield close to 100 %.Then slab 13 is situated in the press-mold 14 for compression molding between the base 16 and the press block 15, which are heated to temperatures above the melting point of the main polymer binder but below its decomposition temperature, which for PPS is in the range of 279°C to 430°C. In order to reach the thermodynamical melting point for PPS, where all crystals are transformed into molten state [Ref. 52], but at the same to avoid melting of PTFE, which should protect the compound from the sticking to the mold, the preferable range of compression molding temperature is in the range of 300°C to 380°C. The pressure during compression molding should be within range from 10 to 400 MPa, but advantageously in the range of 50 to 250 MPa. Using maximal values of temperature and pressure helps obtaining separator plates 17 with higher density and lower porosity, but at the same time it creates some difficulties with their extraction from the tool (demolding). The time for hot-compaction in the press-mold is in the range of 0.1 to 120 seconds, typically from 0.2 to 60 second, for example from 0.5 to 20 second, depending on the temperature at which the demolding is needed to occur. It is advantageous to carry out the demolding process at temperatures in the range of 124°C to 244°C, i.e. when (re)crystallization of PPS occurs. It leads to lower values of shrinkage for PPS as compared to samples demolded near or below the glass transition temperature [Ref. 53],Note that the separator plates 17 produced in such way may be compression molded with or without imprinted structures, such as gas-flow channels and / or coolant channels.FIG. 2 illustrates a manufacturing process for producing triple-layer separator plates 17 containing high-MFI TWIP in the middle layer and low-MFI TWIP in top / bottomlayers. In this case, two different types of precursor sheets have to be produced, namely first precursor sheets 13A with high-MFI TWIP and second precursor sheets 13B with low-MFI TWIP. The production lines for each of the precursor-sheets 13 A, 13B are similar to the production line explained in FIG. 1, so that it is not explained here in detail once again.The main difference of this process in FIG. 2 as compared to the previous one described in connection with FIG. 1, is separated production of two batches 2A, 2B in two different mixing containers 3A, 3B with one slurry containing only high-MFI TWIP and one slurry only low-MFI TWIP. These slurries are also kneaded and extruded and cut into the correctly dimensioned precursor-sheets 13A, 13B. For a continuous production, the speed of the rolling stations 9B for the second production line IB is twice as fast as the for the rolling station 9 A in the first production line 1A in order to have possibility to produce two low-MFI TWIP-based precursor sheets 13B as top and bottom layers while producing one high-MFI TWIP-based precursor sheet 13 A for the middle layer. Alternatively, one line as in FIG. 1 is used for each of each of the two outer precursor sheets 13B, which also allows varying the content of the first relatively to the second of the outer precursor sheets 13B. The two low-MFI TWIP precursor-sheets 13B are combined to sandwich one high-MFI TWIP precursor-sheet 13 A to form a precursor slab that is inserted into the press-mold 14 for compression molding. In such process, a distinct border is observed between the middle layers with high-MFI and the outer layers with low-MFI.This is illustrated in FIG. 3B, which is in contrast to the homogeneous single layer separator plate made by the principles of FIG. 1 with a mix of high-MFI PPS and low-MFI PPS and illustrated in FIG. 3A. For example, for the three layer separator plate 17 in FIG. 3B, the thickness ratio between the bottom or top layer relatively to the middle layer varies in the range of 0.1 to 1. In case that the bottom and top layers 13B are substantially thinner than the middle player 13 A, it is useful to have more subsequent calendering rollers in the second roller station 9B than in the first roller station 9A, which is illustrated in FIG. 2. However, it is also possible that the same thickness is used for all three layers.It is pointed out that the triple-layer layer principle, as illustrates in FIG. 3B, can be extended to multi-layer principles with more than 3 layers, as illustrated in FIG. 3C. In this case, more than the two illustrated production lines 1A, IB would be used but following the same principle. In order to produce separator plates 17 containing more than3 layers, for example, 5, 7 or 9 layers, more batches with slurry are needed, namely 3,4 or 5, respectively, as well as corresponding production lines 1.For example, the middle layer 13A is produced with a ratio of 25:75 between low-MFI and high-MFI TWIP, thus containing predominantly high-MFI TWIP, whereas the outer layers 13B are produced with a ratio of 75:25 between the low-MFI and the high- MFI TWIPs in the mix, thus containing predominantly low-MFI TWIP. Alternatively, the middle layer contains more high-MFI TWIP, or only high-MFI TWIP, and the outer layers less high-MFI TWIP, optionally only low-MFI TWIP.For example, precursor-slabs 13A, 13B for the middle and the top / bottom layers, respectively, are produced from batches containing high-MFI TWIP and low-MFI TWIP, as explained in relation to FIG. 2, and further layers 13C, 13D, as illustrated in FIG. 3C, are produced with mixed of both high-MFI and low-MFI in varying amounts, as explained in connection with FIG. 1. For example, further layer 13C has a ratio of 25:75 and further layer 13D a ratio of 75:25 between the high-MFI and the low-MFI TWIPs used in the mix.Alternatively, all layers 13A, 13B, 13C, 13D, as illustrated in FIG. 3C are produced from mixes of high-MFI and low-MFI TWIPs but in different ratios. This yields possibilities for fabricating at least three-layered separator plates 17 with smoother transitions of MFI between the layers, resembling a gradual transition between the layers with respect to MFI.Such combination of multi-layer principles are useful for adjusting mechanical and electrical characteristics of separator plates 17, such as flexural strength and electrical conductivity, although, it makes the manufacturing process more complex as compared to the process described in FIG. 1.With the specific process described herein, separator plates were produced, the characteristics having been summarized and discussed above in connection with Table 4. The separator plates produced by the process described herein, namely by mixing low- and high-MFI TWIP (PPS as an example) demonstrate enhanced performance and processability compared to separator plates made of single PPS with either only low- or only high-MFI PPS.It should be also noted that US Department of Energy (DOE) defines following 2025 targets for bipolar plates used in PEM fuel cells [Ref. 56], namely flexural strength should be above 40 MPa, while specific resistance should be less than 10 m cm2. All separator plates fabricated by the current method correspond to these targeted values.In conclusion, the process described herein has a number of advantages. By combining low- and high-MFI TWIPs, such as PPS, as mixes or in layers, improvements of separator plate performance has been achieved as compared to prior art separator plates. The described method provides separator plates with low degree of porosity and makes demolding stage easy as compared to separator plates having high-MFI as sole material. When using PPS as sole binder in combination with the fibrillization polymer PTFE, a triple-layer structure was superior to a single layer with mixed high- and low-MFI PPS.References[1] C Zhang, W Zhou, MM Ehteshami, Y Wang, SH Chan. Determination of the optimal operating temperature range for high temperature PEM cells considering its performance, CO tolerance and degradation. Energy Conversion and Management 105 (2015) 433[2] SS Araya, F Zhou, V Liso, SL Sahlin, JR Vang, S Thomas, S Jeppesen, SKKser. A comprehensive review of PBI-based high temperature PEM fuel cells. International Journal of Hydrogen Energy 41 (2016) 21310[3] A Hermann, T Chaudhuri, P Spagnol. Bipolar plates for PEM fuel cells: a review. International Journal of Hydrogen Energy 30 (2005) 1297[4] A Adloo, M Sadeghi, M Masoomi, HN Pazhooh. 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Claims

CLAIMS1. A method of producing a separator plate (17), wherein the method comprises providing at least one dry, malleable compound (8, 8A, 8B) containing a homogenous blend of an electroconductive filler and binder polymers, wherein the binder polymers comprise at least one fluoropolymer and at least one thermoplastic water-insoluble nonfl uoro polymer, TWIP, wherein the ratio between the total weight of TWIP and the total weight of fluoropolymer in the blend is at least 10, and kneading the compound (8, 8 A, 8B) at a temperature above all glass temperatures of the binder polymers but below all melting temperatures of the binder polymers and causing fibrillization of the fluoropolymer by the kneading, shaping the compound (8, 8 A, 8B) into a precursor slab (13, 13 A, 13B) and hot compression molding the precursor slab (13, 13 A, 13B) in a press-mold and solidifying the compressed slab (13, 13A, 13B) into a separator plate (17) prior to removing the separator plate (17) from the press-mold (14); characterized in that the method comprises providing a high-MFI TWIP with a melt flow index, MFI, above 30 g / 10 min and a low-MFI TWIP with a melt flow index, MFI, below 10 g / 10 min; wherein the method comprises A, B or C, wherein in A), the pre-curser slab (13) is provided as a single layer (13A) of a homogeneous compound (8) comprising fluoropolymer, the high-MFI TWIP, and the low-MFI TWIP, wherein the weight percentage is in the range 15 wt.% to 49.9 wt.% for the high-MFI TWIP and 50.1 wt.% to 85 wt.%. for the low-MFI TWIP, the two weight percentages adding up to 100 wt.%; wherein in B), the precursor slab is provided as a multi-layer (13A, 13B, 13C, 13D) comprising a central layer (13A) sandwiched between two outer layers (13B) that are forming outer sides of the slab (13), wherein the central layer (13A) is shaped of a first homogeneous compound (8A), which comprises fluoropolymer and high-MFI TWIP but not low-MFI TWIP, and each of the two outer layers (13B) is shaped from a further homogeneous compound (8B), which comprises fluoropolymer and low-MFI TWIP but not high-MFI TWIP; wherein in C) the precursor slab is provided as a multi-layer (13A, 13B, 13C, 13D) comprising a central layer (13A) sandwiched between two outer layers (13B) that are forming outer sides of the slab (13), wherein the central layer (13A) is shaped of a first homogeneous compound (8A) which comprises fluoropolymer and more high-MFITWIP than low-MFI TWIP, and each of the two outer layers (13B) is made from a further homogeneous compound (8B), which comprises fluoropolymer and more low- MFI TWIP than high-MFI TWIP.

2. The method of claim 1 , wherein the method comprises providing the further homogeneous compound (8B) for the outer layers (13B) of the multi-layer (13A, 13B, 13C, 13D) with 15 wt.% to 49.9 wt.% high-MFI TWIP and 50.1 wt.% to 85 wt.%. low-MFI TWIP, the two weight percentages adding up to 100 wt.%.

3. The method of any preceding claim, wherein the method comprises providing the first homogeneous compound (8A) for the central layer (13A) of the multi-layer (13A, 13B, 13C, 13D) with at least twice as much high-MFI TWIP than low-MFI TWIP.

4. The method of any preceding claim, wherein the method comprises providing the further homogeneous compound (8B) for the outer layers (13B) ofthe multi-layer (13 A, 13B, 13C, 13D) with at least twice as much low-MFI TWIP than high-MFI TWIP.

5. The method of any preceding claim, wherein the method comprises providing each of the outer layers (13B) with less than 10 wt. % high-MFI TWIP and the central layer (13 A) with less than 10 wt. % low-MFI TWIP.

6. The method of any one of claims 1-6, wherein the multi-layer is provided as a triplelayer with identical outer layers (13B) and wherein the separator plate has a total thickness in the range of 0.5 to 5 mm.

7. The method of any one of claims 1-6, wherein the multi-layer comprises at least five layers comprising intermediate layers (13C, 13D) between the central layer (13A) and the outer layers (13B), wherein the intermediate layers (13C, 13D) contain more high- MFI TWIP and less low-MFI TWIP than the outer layers but less high-MFI TWIP and more low-MFI TWIP than the central layer (13 A), wherein the separator plate has a total thickness in the range of 0.5 to 5 mm.

8. The method of any one of the preceding claims, wherein the compound comprises high-MFI PPS as a high-MFI TWIP and low-MFI PPS as a low-MFI TWIP.

9. A separator plate produced by a method according to any preceding claim.

10. A HT-PEM fuel cell comprising a separator plate according to claim 7.

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