A separator plate and its production as well as production of a precursor, and a fuel cell with such separator

Abstract

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Description

A separator plate and its production as well as production of a precursor, and a fuel cell with such separatorFIELD OF THE INVENTIONThe present invention relates to an electroconductive separator plate and a method of producing it, as well as a method of producing a precursor therefore, using an aqueous dispersion of a powder of a polymeric binder and a powder of an electroconductive filler and providing a dry malleable compound of the electroconductive filler and polymeric binder as a precursor after evaporation of water and other liquids. The precursor is used for hot-compacting a separator plate.BACKGROUND OF THE INVENTIONOne of the key components of energy storage / conversion devices, such as batteries and supercapacitors or fuel cells, are separator plates. In fuel cell stacks, electroconductive separator plates separate single cells in the stack and play a role of current collectors. If the energy storage / conversion device has a bipolar design, such separator plates are, typically, called bipolar plates (BPPs). As an alternative to a BPP in a fuel cell, two monopolar plates (MPP) are used as two halves of a BPP, optionally with a coolant chamber in between. In particular for fuel cells the separator plates deliver inlet gases and heat to the electrodes as well [Ref. 1],References that are mentioned herein are listed at the end of the detailed description.Graphite-based BPPs have superior chemical stability compared to metal BPPs. This is especially important if they are used in proton-exchange membrane (PEM) fuel cells, especially in high-temperature PEM (HT-PEM) fuel cells, where the separator between the electrode chambers is an ion-conducting membrane that is doped by strong mineral acids, for example, orthophosphoric acid, for obtaining high electrical conductivity [Ref. 2],Not only the acidic environment but also the operation of HT-PEM fuel cells at elevated temperatures in the range of 120-180 °C puts challenges on the selection of materials for the BPPs, especially the polymeric binding agents for the electroconductive filler, as the BPP should remain mechanically stable [Ref. 3], High-performance thermoplastic polymers, such as polyamide-imide (PAI), polyetheretherketone (PEEK), polyetherimide (PEI), polyethersulfone (PES) polyphenylene sulfide (PPS), are good candidates for this role [Ref. 4], These polymers as binding agents for graphite or other electroconductive fillers in BPPs have high resistant to mechanical stress and correspond at the same time to the strict requirements to thermal and chemical stability addressed to components of HT-PEM fuel cell stack. PPS seems the most used binder for BPPs in high-temperature PEM fuel cells [Ref. 5-8], probably because of its relatively low price as compared to, for instance, PAI and PEEK [Ref. 9], However, om the other hand, PPS has a relatively low elongation at break [Ref. 10], Therefore, a mix of PPS with other thermoplastic polymers is advisable for obtaining BPPs with enhanced mechanical properties and processability.For example, mixing polyetherimide (PEI) with PPS may increase tensile strength and crystallization temperature of such polymer mixture compared to pristine PPS [Ref. 11, 12], which is beneficial in terms of producing mechanically improved BPPs.Polytetrafluoroethylene (PTFE) due to its unique properties, namely low friction and high viscoelasticity [Ref. 13], is also good candidate in the role of a co-binder to PPS in precursors for separator plates. Moreover, due to PTFE’s ability of fibrillation, it can form dough-like structures for precursors, which is advantageous for producing selfsupported precursor films as a part of the process in separator plate production. Such film helps reaching uniform distribution of the precursor inside the molding tool and makes it easier to provide a continuous manufacturing process.There are different techniques for obtaining fibrillation of PTFE in polymer mixtures, for example, as reported in [Ref. 14-16], PTFE particles are mixed with active materials in dry, solvent-free, conditions, whereas in [Ref. 17-22], different organic liquids like ethanol, iso-propanol (IP A), N-methyl pyrrolidone (NMP), propylene carbonate and others are used as dispersing media for more uniform distribution of particles of binding agents and electroconductive fillers or powdered compounds.Having in mind that PTFE is widely available on the market as aqueous dispersions, where the PTFE particles have lower particle size as compared to dry particle sizes available as powder [Ref. 23-26], it seems reasonable to carry out a mixing process in liquid phase if the dispersing medium is water. Aqueous processes, where the water content in liquid phase is above 90 wt.%, were realized in [Ref. 22], However, if a polymer binder like PPS is used, its uniform distribution between graphite or other elec- troconductive fillers is a challenge because PPS has a low ability to adsorb water [Ref. 27], Therefore, graphite or other carbon particles are recommended to pre-mix with PPS in a hot melted process and form pellets, which then are grinded into the composite powder to be mixed with water, as proposed in [Ref. 22],Significant simplification of production processes may be reached if all raw powdered components have good wettability in water for the wet mixing stage. This is a necessary requirement for producing separator plates with extra-ordinary characteristics, for example ultra-low electrical resistivity or ultra-high toughness.It should be noted there that the US Department of Energy (DoE) set ambitious targets for 2020-2025 regarding BPPs for application in PEM fuel cells, among them a minimal value for flexural strength in the range of 25-40 MPa and a maximal value for areal specific resistance not higher than 10 mQ cm2[Ref. 28],However, from a practical point of view, it is better to consider not only flexural strength as a primary mechanical characteristic of BPPs, but also its flexibility. Therefore, toughness is one of the key characteristics of BPPs, since it includes mechanical stress and strain [Ref. 29], Lower flexural strength can be compensated by higher flexibility, to that a parameter of toughness gives a better picture of the mechanical capabilities of a separator plate than the flexural strength seen in isolation. A lower acceptance limit for toughness is realistically 0.1 Nmm / mm3, i.e., separator plates having toughness above this value are useful as BPPs in high-temperature PEM fuel cells.As it appears from the above, production of separator plates, such as BPPs, for HT-PEM fuel cells is a challenge when trying to reach the targets given by the DoE. Accordingly,there is a steady aim for improvements with respect to material selection and production steps. At the current stage in the art, there is still room for improvement.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 improved production steps and material selections for separator plates. This objective and further advantages are achieved with a method for producing a malleable graphite-based precursor from an aqueous slurry, forming a self-supported film from such precursor, and producing separator plates from such film, especially for fuel cell applications, as well as a separator plate produced by such methods and a fuel cell comprising such separator plate, as described below and in the claims.Herein, a wet, organic solvent-free process is presented for production of separator plates, for example, monopolar plates, MPP, or bipolar plates, BPPs. Such separator plates are produced via intermediate stages that include preparation of an aqueous slurry with uniformly dispersed fine powders of an electroconductive filler and at least one polymeric binder with hydrophobic properties. For easing fabrication and overcoming the challenges of the hydrophobic nature of the blend, a polymer that contains hydroxylgroups is added as a sacrificial additive for temporally reducing hydrophobicity of the polymeric binder during the mixing. A candidate for such temporarily hydrophilizing polymeric additive is polypropylene glycol, PPG. From the aqueous slurry of powders, a malleable precursor is provided as well as a self-supported precursor film, which is then shaped into precursor slabs used to provide separator plates after compression moulding. These separators are useful in fuel cells, especially in high-temperature proton-exchange membrane (HT-PEM) fuel cells. Based on these principles, in experiments, separator plates have been produced, containing combinations of polymer thermoplastic binders, in particular polyphenylene sulfide (PPS), polytetrafluoroethylene (PTFE), and polyetherimide (PEI). The produced separator plates had a high level of toughness as well as low porosity, despite relatively high mass loading of graphite, reaching up to 94 wt.%. Moreover, it has been shown that separator plates could be manufactured with such high graphite with ultra-low areal specific resistance, namely, in the order of 1 mQ cm2.Further details are explained in the followingThe invention relates to a method of producing a precursor for a separator plate, in which there is provided an aqueous dispersion of a powder of a polymeric binder having hydrophobic properties and a powder of electroconductive filler, for example graphite or a mix of carbon powders. For example, the polymeric binder has hydrophobic properties due to a content of hydrophobic PPS as part of the polymeric binder.Advantageously, the dispersion also contains a water-soluble hygroscopic polymer, which is a sacrificial additive that temporally reduces the hydrophobicity of the hydro- phobic polymeric binder. For example, the water-soluble hygroscopic polymer is selected among polymers with hydroxyl-groups for causing hydrogen bonds via OH- groups with water molecules in the dispersion. In addition, the water-soluble hygroscopic polymer provides chemical anchors between the polymeric binder and the water- soluble hygroscopic polymer. This way, the otherwise hydrophobic powder of the polymeric binder, obtains a modified surface, causing increased wetting of the powder particles of the polymeric binder, which is improving proper dispersing of the powder in the aqueous medium.The dispersion is mixed, for example by stirring, and then heated to temperatures above decomposition and evaporation temperatures of the water-soluble hygroscopic polymer but below the melting temperature of the polymeric binder for causing evaporation of liquids from the aqueous dispersion and decomposition and evaporation of the water- soluble hygroscopic polymer, hence, justifying the term “sacrificial” that is used for the water-soluble hygroscopic polymer.Options for such sacrificial water-soluble hygroscopic polymers are found among polyalkylene glycols, for example polyethylene glycol, PPG, or polypropylene glycol, PEG. In some embodiments, the polyalkylene glycol in the dispersion is provided at a weight concentration, relatively to the total mass of the electroconductive filler, in the range of 0.1-0.5%, if the polyalkylene glycol is polypropylene glycol, and in the range of 1-5%, if the polyalkylene glycol is polyethylene glycol.Advantageously, the molar weight of the PPG or PEG is no more than 1000 g / mol, for example no more than 500 g / mol. Good results have been obtained with PPG and PEG having a weight in the range of 400-500 g / mol.Due to the evaporation, a dry malleable compound of electroconductive filler and pliable polymeric binder remains as a precursor for further handling and hot-compression into a separator plate.It is an advantage if the aqueous dispersion is entirely free of organic solvents. Particularly advantageously, the aqueous dispersion is free of alcohol. Alternatively, the aqueous dispersion is almost free of organic solvents so as to contain no more than 1 wt.% solvent relatively to the weight of the entire dispersion, including the powders.In useful embodiments, the polymeric binder comprises a first binder polymer, for example PPS, and a second binder polymer, for example PTFE, wherein the second but not the first binder polymer is fluorinated polymer.Optionally, PPS is provided as the first binder polymer with a weight of 0.5-30% and PTFE as the second binder polymer with a weight of 0.2-3% relatively to the mass of the electroconductive filler in the dispersion.Advantageously, the method comprises kneading of the dry malleable compound at a kneading temperature above the glass transition temperatures of the first and second binder polymer but below the melting temperatures of the first and second polymer in order to provide the compound malleable but not molten and for causing fibrillation of the second binder polymer by the kneading.For example, extraction from a kneader is made by extrusion of the kneaded compound, optionally onto a conveyor for transport of the extruded compound to subsequent production stages.For producing a separator plate from the precursor, it is advantageous if the method further comprising hot-calendering the malleable precursor into a precursor film, especially a self-supported precursor film. For example, multiple subsequent calender rollersare used for step-wise reduction of the thickness of the precursor film. For example, the temperature during the calendering is above the glass transition temperatures of the first and second polymer but below the melting temperature of the first and second polymer. For example, the calendering is made in the range of 116-282 °C for a polymeric binder containing PPS and PTFE.The precursor film can be used for immediate further preparation to produce separator plates or, alternatively, the precursor film is rolled onto a roller for storage and later unrolling for the further preparation in the production of separator plates. The latter is useful for production of the precursor at one location and hot-compaction of the precursor film for the production of separator plates at another locationFor the production of separator plates, at least a portion of the precursor film is shaped into a precursor slab, for example by cutting out a slab from the film, the slab having predetermined dimensions. Optionally, the shaping of the slab into predetermined dimensions for the separator plate comprises removal of surplus-material from edges of the film, for example by cutting. Such surplus material can be recycled for minimizing waste.The precursor-slab is positioned in a press-form and hot-compacted into a separator plate.For example, the polymeric binder comprises PPS and PTFE, and the method comprises hot-compaction of the precursor- slab in a press-form at a hot-compacting temperature above a melting temperature of PPS but below a melting temperature of PTFE. For example, for a polymeric binder containing PPS and PTFE, the press-molding temperature is above the melting point of the PPS but below the melting point of the PTFE. As an option, the pressing is made in the range of 285-344 °C.It is advantageous to perform hot compaction close to this upper temperature limit, where it reach or is near at the thermodynamic melting point of PPS, which is around 340 °C according to [Ref. 30], because there are no crystals of PPS at this temperature, i.e., the polymer is completely melted. It should have positive effect on mechanical properties of separator plates, but, unfortunately, higher temperature for hot compactionmay lead to sticking issues when the hot compacted separator plates are getting demolded. In order to counteract sticking issues, an improvement has been found, as discussed further below, to add PEI to the mix of PPS and PTFE.The hot-compaction at high pressure may change the density of the material so that the final separator plate is thinner than the initial precursor slab.Optionally, the hot-compaction molding modifies the shape of the precursor slab into the separator plate, for example with a flow field structure impressed into the material, for flow of fluid along the separator plate. For example, the electroconductive separator plate is an MPP or a BPP with fluid flow fields on at least one of opposite sides, in particular a flow field for oxygen or for hydrogen. Alternatively, the separator plate has a flow field on both sides, especially if the separator plate is a BPP.As an option, for forming a BPP from two separator plates, a first separator plate for a fuel cell stack comprises on one side a flow field for oxygen gas, such as air, and is attached with its opposite side, for example attached back-to-back, to a second separator plate that contains a flow field for hydrogen-gas, such as hydrogen-rich gas, for example, reformate gas or syngas. Optionally, there is provided a cooling flow field for coolant between the two separator plates for the fuel gases, for example by insertion of a corresponding separator plate with a coolant flow field on one or both of its sides.Optionally, the separator plate has a flow field for oxygen on one side and a flow field for coolant on its opposite side.As it appears from the above, depending on the need, the method as explained herein can be used to produce a variety of different separator plates, be it with a fluid flow field only on one side or on both sides, be it for oxygen, hydrogen, or coolant in the respective flow field.An advantage of hot-compaction, also called hot-compression, is a short hot-compac- tion time at high pressure, advantageously in the range of no more than 1-30 seconds and optionally shorter than 1 second.While under pressure in the press-form, the hot-compacted separator plate is cooled to a temperature below a crystallization temperature of the polymeric binder, such as below a crystallization temperature of PPS, for causing rigidity of the separator plate prior to removing the rigidly solidified electroconductive separator plate from the press-form.In some embodiments, the polymeric binder also comprises a third binder polymer, wherein the third binder polymer is PEI. For example, PEI is added to the aqueous dispersion at a concentration in the range of 0.1-2% relatively to the mass of the electro- conductive filler in the dispersion. PEI has turned out to reduce sticking of the hot- compacted separator plate from the press-form and, thus, shorten the time until removal of the separator plate from the press-form, which is advantageous for fast, large-scale production.If PEI is added to the dispersion as part of the polymeric binder, kneading and potential calendering is better performed above the glass transition temperature of PEI, i.e., above 207 °C [Ref. 31] but below 282 °C, in order do not exceed the melting point of PPS. PEI as an amorphous polymer does not have a melting point, unlike PPS and PTFE, and therefore, its addition in minor quantities to the precursor helps reaching higher temperatures for hot compaction of separator plates with minimized risk of their sticking to the press-form.This anti-sticking advantage of PEI is achieved with or without the adding of the sacrificial water-soluble hygroscopic polymer.In experiments, separator plates were produced comprising a weight X of electroconductive filler as well as PPS at a weight concentration in the range of 0.05X-0.3X, PTFE at a weight concentration in the range of 0.002X-0.03X, and PEI at a weight concentration in the range of 0.001X-0.02X. For example, such separator plate has a toughness of no less than 0.17 N mm / mm3and an areal specific resistance of no more than 8.9 Q cm2. In experiments, the produced separator plates had a toughness of no less than 0.17 N mm / mm3at 6 wt.% of binder content, and an areal specific resistance was obtained of no more than 8.5 mQ cm2at 18 wt.% of binder content, where binder mixture includes PPS, PTFE and PEI.Due to the separator plate being formed as a single-layer plate, the method is fast and useful for large-scale, low-cost production. In particular, handling and stacking of multiple ultra-thin sheets in order to be molded into a single separator plate is avoided.Such separator plate, for example BPP, is useful for high temperature polymer electrolyte membrane fuel cell, (HT-PEM), which operates in the range of 120-200 °C, for example in the range of 150-180°C. Such HT-PEM fuel cells are advantageous for compact fuel cell systems, for example for automobile industry.All percentages herein are by weight, unless indicated otherwise.In the following, a number of interdependent aspects are described, each of which can be electively combined with the features of the invention described above and in the claims.Aspect 1. A method of producing a precursor for a separator plate, the method comprising providing an aqueous dispersion of powders of an electroconduc- tive filler and a polymeric binder that is thermoplastic and has hydrophobic properties , wherein the dispersion also contains a water-soluble hygroscopic polymer as sacrificial additive for temporally reducing hydrophobicity of the hydrophobic polymeric binder; wherein the method comprises heating the dispersion to temperatures above decomposition and evaporation temperatures of the water-soluble hygroscopic polymer but below the melting temperature of the polymeric binder and causing evaporation of liquids from the aqueous dispersion as well as decomposition and evaporation of the water-soluble hygroscopic polymer and, due to the evaporation, providing a dry malleable compound of the electroconductive filler and polymeric binder as a precursor.Aspect 2. The method according to aspect 1, wherein, for the temporal reduction of the hydrophobicity of the hydrophobic polymeric binder by the water-soluble hygroscopic polymer, the method comprises selecting the water-soluble hygroscopic polymer among polymers with hydroxyl-groups for causing hydrogen bonds via OH- groups with water molecules in the dispersion in addition to chemical anchors between the polymeric binder and the water-soluble hygroscopic polymer, and causingincreased wetting of the powder particles of the polymeric binder for improving proper dispersing of the powder in the aqueous medium.Aspect 3. The method according to aspect 1 or 2, wherein the method comprises selecting the water-soluble hygroscopic polymer among polyalkylene glycols.Aspect 4. The method according to aspect 3, wherein the method comprises selecting the water-soluble hygroscopic polymer among polyethylene glycol, PEG, and polypropylene glycols, PPG.Aspect 5. The method according to aspect 4, wherein the method comprises providing the polyalkylene glycol in the dispersion at a weight concentration relatively to the total mass X of the electroconductive filler in the range of 0.1-0.5% if the polyalkylene glycol is polypropylene glycol and in the range of 1-5% if the polyalkylene glycol is polyethylene glycol.Aspect 6. The method of aspect 4 or 5, wherein the PEG and / or PPG have a molecular weight no higher than 2000 g / molAspect 7. The method according to aspect any preceding aspect, wherein the polymeric binder comprises a first binder polymer and a second binder polymer, wherein the second but not the first binder polymer is fluorinated polymer, and wherein the method comprises kneading the dry malleable compound at a kneading temperature above the glass transition temperatures but below the melting temperatures of the first and second binder polymer and causing fibrillation of the second binder polymer by the kneading.Aspect 8. The method according to aspect 7, wherein the method comprises providing PPS as the first binder polymer with a relative weight of 0.05X-0.3X, and PTFE as the second binder polymer having a relative weight of 0.002X-0.03X, wherein X is weight of the electroconductive filler in the dispersion.Aspect 9. The method according to aspect 8, wherein the polymeric binder also comprises a third binder polymer, wherein the third binder polymer is PEI, and wherein the method comprises adding PEI to the aqueous dispersion at a concentration in the range of 0.1-2% relatively to the mass X of the electroconductive filler in the dispersion.Aspect 10. The method of any preceding aspect, wherein the water concentration in liquid phase of the dispersion is no less than 99 wt.%, and the dispersion is free from organic solvents.Aspect 11. Method of producing a separator plate from a precursor obtained by a method according to any preceding aspect, the method further comprising hot- calendering the malleable precursor into a precursor film and shaping at least a portion of the precursor film into a precursor slab and positioning the precursor slab in a press mold and hot-compacting the precursor slab into a separator plate and cooling the so formed separator plate to a temperature below a crystallization temperature of the polymeric binder and causing rigidity of the separator plate, and only then removing the rigid separator plate from the press mold.Aspect 12. The method of aspect 11, wherein the polymeric binder comprises PPS and PTFE, and the method comprises hot-compaction of the slab at a hot-compacting temperature above a melting temperature of PPS.Aspect 13. A separator plate comprising a weight X of electroconductive filler and PPS at a concentration in the range of 0.05X-0.3X, PTFE at a concentration in the range of 0.002X-0.03X, and PEI at a concentration in the range of 0.001X- 0.02X, wherein the separator plate has a toughness of no less than 0.17 N mm / mm3 and an areal specific resistance of no more than 8.9 Q.cm2.Aspect 14. A fuel cell comprising a separator plate according to aspect 13.Aspect 15. The fuel cell of aspect 14, wherein the fuel cell is part of a HT- PEM fuel cell stack, and the separator plate is a BPP.Aspect 16. A separator plate produced by a method according to any one of the aspects 1-12.Aspect 17. The separator plate according to aspect 16 comprising a weight X of electroconductive filler and PPS at a concentration in the range of 0.05X-0.3X, PTFE at a concentration in the range of 0.002X-0.03X, and PEI at a concentration in the range of 0.001X-0.02X, wherein the separator plate has a toughness of no less than 0.17 N mm / mm3 and an areal specific resistance of no more than 8.9 Q.cm2.SHORT DESCRIPTION OF THE DRAWINGSThe invention will be explained in more detail with reference to the drawing, where FIG. 1 illustrates a process for manufacturing separator plates with A) showing the left side of the process and B) showing the right side of the process;FIG. 2 illustrates wetting of a PPS surface by water via PPG;FIG. 3 shows experimental results of dispersing PPS powder in water;FIG. 4 shows experimental results of dispersing PPS by adding PPGP400 as compared to adding PEG P400;FIG. 5 shows thermogravimetric curves of (1) PPG P400, (2) 1 wt.% solution of PPG P400 in water and (3) graphite-based precursor pre-dried from all volatile components including PPG P400;FIG. 6 illustrates experimental results showing a dependency of toughness and areal specific resistance of MPPs on total polymer binder content including PPS, PTFE and PEI.DETAILED DESCRIPTION / PREFERRED EMBODIMENTFIG. 1 illustrates an example of a multi-stage process for manufacturing graphite-based separator plates from powdered electroconductive filler and thermoplastic polymer binder. For the production, raw materials 1 are provided, such as graphite or graphitebased powder, for example a mixture of graphite with graphene, carbon black, carbon fibers and carbon nanotubes, as well as some thermoplastic polymer binders, exemplified as PPS and PEI powders. These are added to a water-diluted aqueous dispersion of PTFE.The dispersion (2) also contains, for example premixed, at least one water-soluble polymer having hygroscopic properties, that improves the water-wettability of the hydro- phobic PPS powder in mixer 6. For achieving the desired wetting effect, this added water-soluble advantageously comprises functional hydroxyl groups. When it is adsorbed on the surface of PPS or any other hydrophobic thermoplastic polymer, it forms hydrogen bonds via its OH-groups with water molecules, thus, playing a role of connectors to the water, which helps wetting the particles of PPS and dispersing them properly in the aqueous medium. Examples of such water-soluble hygroscopic polymers are polyalkylene glycols, in particular polyethylene glycol (PEG) and polypropylene glycol (PPG), polyvinyl alcohol (PVA), as well as polysaccharides like starch and hydroxypropyl cellulose (HPC).FIG. 2 illustrates a simplified adsorption process of water-soluble hygroscopic polymer, exemplified as PPG, on a hydrophobic PPS surface, leading to its wetting by water viaformation of hydrogen bonds. In particular, a first portion of the PPG molecule is adsorbed as an anchor on the surface of the PPS, while another portion of the PPG molecule forms hydrogen bonds with water molecules via its OH-groups, as illustrated. This way, PPG forms anchors between PPS and the water, which helps wetting the particles of PPS and dispersing them properly in the aqueous medium, despite PPS itself being hydrophobic.It should be mentioned here that a process of providing a fine dispersion of PPS is disclosed in [Ref. 32, 33], but such dispersions include NMP and IPA as dispersing medias as well as some surfactants. In contrast thereto, the preparation of a water-based dispersion, as in the case presented herein, is beneficial not only because of being environmental-friendly but also at the production site due to its non-flammability and relatively low toxicity.In order to achieve a satisfied dispersibility of PPS in water, certain ratios between the PPS and the water-soluble hygroscopic polymer, such a PEG or PPG, should be reached. This ratio depends on type of hygroscopic polymer used as well as its molecular weight.Wetting abilities of some polyalkylene glycols, in particular PPG and PEG, with the same molecular weight (in the order of 400 g / mol for P400 specs) are presented in FIG 3, illustrating the effect of improving dispersibility of PPS powder in water by increase of PPG P400 or PEG P400 contents in the respective systems. As illustrated in FIG. 3, adding relatively small amount of PPG as water-soluble hygroscopic polymer, for example, in mass ratio PPS:PPG as 1000 : 1, leads to formation of a quasi-stable dispersion after few minutes of intensive stirring. In comparison to PEG as water-soluble hygroscopic polymer, a more than tenfold higher amount of water-soluble hygroscopic polymer is required to get the same degree of dispersibility. This is believed to be due to a smaller area of PPS getting covered by PEG than PPG.The different effect between PPG and PEG is illustrated in more detail in FIG. 4A and 4B at a mass ratio of 100 : 1 for PPS : PPG as compared to the same mass ratio for PPS : PEG. The difference in wettability of PPS by water in the presence of PPG or PEG, respectively, become very obvious in that PPG provides a uniform dispersion, see FIG.4A, while with PEG added at the same concentration still results in agglomerates of PPS particles, see FIG. 4B.However, nevertheless, also PEG is a useful candidate for hydrophilizing the binder polymer.As enhanced wettability of PPS is required only during the stages of mixing raw powders, it is preferable to remove the water-soluble hygroscopic polymer from the graphite-based precursor before its hot-compaction into separator plates in order to avoid undesirable porosity and increased electrical resistivity. From this point of view, PEG and PPG are useful because they have an onset of thermal decomposition at relatively low temperature. Moreover, use of PEG and / or PPG with low molecular weight, for example below 2000 g / mol or even below 1000 g / mol, is preferable due to the lower thermal stability and higher solubility in water [Ref. 34, 35],Good miscibility of polyalkylene glycols with water, for example PPGP400 (molecular weight is 446 g / mol) [Ref. 36], helps evaporating them together at lower temperatures compared to pristine polymer.FIG. 5 illustrated thermogravimetric curves of (1) PPG P400, (2) a 1 wt.% solution of PPG P400 in water, and (3) a graphite-based precursor pre-dried from all volatile components including PPG P400. As observed by curve (1) in FIG. 5, pristine PPG P400 starts to evaporate / decompose after 174 °C, while its 1 wt.% solution in water at the same temperature, see curve (2) has already been removed completely when heating up to 174 °C. As seen by the horizontal line (3), the thermogravimetric analysis of the graphite-based precursor, pre-dried at 250 °C, does not show mass loss up to 400 °C. This implies that no water-soluble hygroscopic polymer, e.g., PPG, was left in the precursor. Accordingly, the PPG can be considered here as a sacrificial polymer additive for temporal modification of PPS surface to make it hydrophilic. The terms “temporal” and “sacrificial” are used here to express that the water-soluble hygroscopic polymer is used temporarily during the mixing steps for wetting the PPS, leading to a fine dispersion of PPS in an aqueous medium, but the water-soluble hygroscopic polymer is not maintained in the final product but removed (sacrificed) by heating it to a decomposition and evaporation temperature.After mixing and stirring the polymers and carbon powders, a viscous slurry is obtained. By controlling the solid content, stirring speed, and torque value in the mixer 6, it is possible to combine mixing with kneading in a single process in one machine, for example, in mixer 6 of FIG.1, and reach different degrees of fibrillation of PTFE. As a result, dough-like structure is formed. However, it is also possible to perform fibrillation of PTFE in a kneading machine separate from the mixer 6. Kneading can take offset with a wet slurry and cause gradual evaporation of the liquids or take offset after drying of the slurry.When the compound is in a final kneading process for causing fibrillation, it should be dried from all liquids. Such liquids include mainly water, but also PPG, which is sacrificed, as well as minor amounts of optional surfactants, for example, from the PTFE dispersion, because it is mentioned by the corresponding suppliers that such aqueous dispersion of PTFE may contain 1-7 wt.% of surfactants, typically non-ionic surfactants [Ref. 23-26],In FIG. 1, the drying is exemplified by a drying step in heating station 9. For example, production can be done automized with a conveyor 7, with transport of the fibrillated slurry through a heating station 9. Optionally, before drying the slurry, it may be uniformly distributed on the belt of the conveyor 7 by a distributor roller 8. Vapours formed during drying of the slurry in heating station 9 may be collected, and water may be separated by condenser 10 and recycled into the mixing process.Optionally, a kneader is provided downstream of the heating station 9 for kneading of the dried compound, a kneading optionally stated in the mixer 6.Notice that the heater station 9 in such process may be omitted if the kneader is heating the slurry during kneading, leading to evaporation of liquids, decomposition and evaporation of the water-soluble hygroscopic polymer. It may also include fibrillation of PTFE, if PTFE is part of the blend, or of another polymer with the ability for fibrillation is added to the blend of binder polymer and electroconductive powder. Various options are possible.The dried precursor on the conveyor 7 downstream of the heating station 9 has a dough- like structure and contains binder polymers and electroconductive powder. For example, the precursor contains graphite, PPS, PTFE and PEI.As exemplified in FIG. 1, the precursor continues to move on conveyor 7 belt to a calender station with at least one roller station 11 comprising calender rollers and heating option. Hot calendering leads to further and final fibrillation of PTFE and formation of a self-supported film from the precursor 3. The temperature for calendering should be at least 116 °C or higher to reach a point where PTFE is in a form that is termed “rigid amorphous” in [Ref. 13], in which it is largely solid but can be deformed when applying shear forces.The temperature for calendering has an upper limit by the melting point of the binder polymer with the lowest melting temperature of the polymers in the precursor after evaporation of any sacrificial polymer. This is so because the dough-like structure should be maintained and stickiness minimized. In the present exemplified case with PPS and PTFE, the upper limit is determined by the melting temperature of PPS, i.e., by 282 °C [Ref. 37], The number of roller stations 11, 12, 13 in the calender station can be more than one, for example, comprising a second roller station 12 and a third roller station 13 or even more roller stations, depending on initial robustness of the formed self-supported film and its final thickness. For example, the thickness is in range from 0.1 to 10 mm.Once the self-supported quasi-endless precursor film has attained the desired thickness in the roller stations 11, 12, 13, it is travelling on the shaping conveyor 14 and cut into slabs 4 of desired shape and size by cutting tool 15, the slabs 4 having width and length similar or close to the dimensions of the finally produced separator plates 5.Moving on conveyor 14, the slabs 4 reach a molding station with a press form 16 having a compression tool 17, for high-pressure hot-compaction after heating of the slab 4 to a temperature above the melting point of PPS, but preferably below the melting point of PTFE [Ref. 38], i.e., within range from 282 to 344 °C. It should be also noted that due to the feature of PTFE to exhibit a lowering of its melting point after a first melting in the temeprature range of 344 to 327 °C [Ref. 38], there is a risk for separator platessticking to the press-form if PTFE, used in the precursor, was pre-melted, for example, by a supplier, and the temperature applied for hot compaction is above 327 °C. In order to minimize this risk, an amorphous thermoplastic polymer, e.g., PEI, which is nonmelted state, is utilized as a part of binder mixture in the precursor. In other words, partial melting PTFE may happen during hot-compaction of precursor-based slabs into the separator plates because of a dependence of the melting point for PTFE on its heat treatment history. This can lead to formation of local areas on the surface of the separator plate, which have increased adhesion to the press-form. Uniform distribution of fine powder of PEI in the bulk of the precursor creates antisticking “spots” on the entire area of the separator plate, preventing or reducing the risk for sticking to the press-form, especially if PTFE has started to melt. The hot-compaction process shapes the slabs 4 into the desired dimensions with a predetermined surface structure, given by the templates in the press form 16.For example, the hot-compaction time for manufacturing 0.5-2 mm thick separator plates can be shorter than 1 min. The pressure during compression molding can vary from 20 to 400 MPa depending on the temperature of the slab 4 in the press form 16 and the hot-compaction time.It should be mentioned here that producing such dough-like self-supporting film 3 for shaping slabs 4 and hot-compressing these into separator plates 5 is advantageous in contrast to compressing powder or pellets, as it results in separator plates 5 with more uniform thickness / density and provides shorter process time in the press form 16.The demolding process occurs when the slab 4 is cooled down below the crystallization temperature of PPS, i.e., below 253 °C [Ref. 39],PTFE and / or PEI as co-binder in addition to PPS also function as antisticking agents. For this reason, co-binders should have a melt-flow index (MFI) lower than PPS when used in a precursor for separator plates as described.Exemplary approximate quantities of raw components used in production of precursors for such separator plates are collected in table 1, normalised to the mass X of the graphite powder.Table 1. Example of quantities for raw components related to X, which is the mass of the electroconductive filler, which is exemplified as graphite powder, for the separator plate.The wet process of making separator plates from powdered materials described in here and exemplified in Table 1 is water-based. The water content in the liquid phase of the slurry is above 99 wt.%, or even above 99.5 wt.%, which also include water-soluble hygroscopic polymer, for example, PPG P400 and small amount of non-ionic surfactants from aqueous PTFE dispersion.Separator plates produced by means of the process shown in FIG. 1 and with the above weight parameters in the material mixture of PPS, PEI and PTFE, using PPG as sacrificial wetting agent, demonstrate excellent mechanical and electrical characteristics.FIG. 6 shows experimental values of measured toughness and areal specific resistance in dependence on the total polymeric binder content in separator plates. Surprisingly, it is observed that an almost linear relationship is found between toughness and the total binder content, as well a between areal specific resistance and the total binder content. For low binder content, the areal specific resistance is also low because the content of electroconductive powder is high.It is observed that so produced separator plates have a relatively low porosity. An acceptable porosity is indicated by region B in Fig. 6, defined according to internal test procedures and experiments for fuel cells. Notice that a satisfactory low porosity isachieved despite high mass loading of electroconductive filler in its composition, up to 94 wt.% of graphite. Separator plates with graphite content above 94 wt.%, corresponding to region A in Fig. 2 show porosities above acceptance levels if applied for BPPs in HT-PEM fuel cells.It should be mentioned here that separator plates produced from commercially available graphite-PPS-based compound for HT-PEM fuel cells in [Ref. 40] according to a process as disclosed in [Ref. 22] (reference sample) has a toughness of only 0.13 Nmm / mm3and an areal specific resistance close to 7 mQ cm2. As an improvement thereto, separator plates made by the herein described process according to FIG. 6 at 15 wt.% binder content have a flexural strength close to the reference sample, 45 MPa vs. 47 MPa, which corresponds to DoE’s 2025 target according to Ref. 28, but the toughness is 92 % higher and its areal specific resistance is much better being less than 5 mQ cm2.In Table 2, some parameters of MPPs produced according to the method taught herein are compared to similar parameters for MPPs made from a commercially available compound of [Ref. 40] by means of a method disclosed in [Ref. 22] (reference sample).TABLE 2*Porosity is defined by an internal quality control procedure with air flow through one side of the MPP at 1000 mbar and registration of time between air bubbles in a cylinder with water from the opposite side of the plate (120 seconds per bubbles is set as lower acceptance limit for porosity).**Flexural strength from technical data sheet (supplier’s data) [Ref. 40],With reference to Table 2, when comparing the reference sample with MPPs having approximately the same areal specific resistance, namely ca. 7 mflcm2at 18 wt.% binder content, a separator plate produced according to the method taught herein demonstrates a substantially higher flexural strength, namely 60 MPa as compared to 47 MPa. Such outstanding characteristics are reached due to the more uniform distribution of polymer binders between particles of electroconductive filler.Moreover, as also seen from Table 2, porosities of separator plates produced according to the method disclosed herein have similar or lower porosities than the reference sample. It is expected that separator plates become more leak tight when the polymer content increases, however, the advantageously low porosity despite low binder content in the separator plates produces as disclosed herein is surprising.It is pointed out and confirmed by the data in Table 2 that the time for demolding shortens significantly for separator plates containing PEI as co-binder. Furthermore,decreasing the total binder content also reduces the demolding time for separator plates, making their manufacturing process fast and useful for automated large scale production.As a conclusion, the production method taught herein results in improved separator plates as compared to the prior art.The following features of the process described herein can be highlighted as follows:1) the mixing method utilizing the water-soluble hygroscopic polymer as wetting agent for the binder or binders, which is (are) polymer in powdered form, in an aqueous media, leads to uniform distribution of fine powders of polymer binders, for example, one of them is highly hydrophobic PPS, without use of toxic and highly flammable organic solvents as dispersing media, such as NMP and IP A;2) the water-soluble hygroscopic polymer, for example PPG, as wetting agent, is a sacrificial water-soluble polymer adsorbing on the surface of hydrophobic polymer binder and having functional hydroxyl groups, which are able to form hydrogen bonds with water molecules;3) the main polymer binder, for example, PPS, is mixed with at least one thermoplastic polymer, as co-binder, having lower MFI than PPS, for example, PTFE and / or PEI;4) at least one of these co-binders is fluor-containing polymer, for example, PTFE;5) at least one of these co-binders is able to form fibrils when applying shear forces, i.e., during its kneading process and / or calendering;6) at least one of these co-binders has longer elongation at break than PPS, for example PTFE and / or PEI;7) at least one of these co-binders is an amorphous thermoplastic polymer, for example PEI, while another (other) one(s) is (are) semicrystalline or crystalline thermoplastic polymer(s), for example PPS or / and PTFE;8) separator plates produced by use of these materials and this process can contain high mass loading of electroconductive filler, for example graphite, up to 94 wt.%, and be tough enough for operation in high temperature PEM fuel cells;9) mixing PPS with other thermoplastic polymers, for example PTFE and PEI, provides separator plates faster due to ability to form a self-supported film and slabsbased on such as well as easier demolding from the hot-compression molding tool.As an alternative to the exemplified ternary mixture of thermoplastic polymers PPS- PEI-PTFE, other thermoplastic polymers with similar thermal and mechanical properties can be utilized as binding agents in various combinations with PTFE as co-binder and fibrillation agent. Examples include polyphenylene oxide (PPO) as well as polymers from polyetherketone (PEK) and polysulfone (PSU) families.References[1] Hermann A, Chaudhuri T, Spagnol P. Bipolar plates for PEM fuel cells: a review. International Journal of Hydrogen Energy 30 (2005) 1297[2] Haque MA, Sulong AB, Loh KS, Maj lan EH, Husaini T, Rosli RE. Acid doped polybenzimidazoles based membrane electrode assembly for high temperature proton exchange fuel cell: a review. International Journal of Hydrogen Energy 42 (2017) 9156[3] Zhang J, Xie Z, Zhang J, Tang Y, Song C, Navessin T, Shi Z, Song D, Wang H, Wilkinson DP, Liu ZS, Holdcroft S. High temperature PEM fuel cells. Journal of Power Sources 160 (2006) 872[4] Spoerk M, Holzer C, Gonsalez-Guttierez J. Material extrusion-based additive manufacturing of polypropylene: a review on how to improve dimensional inaccuracy and warpage. Journal of Applied Polymer Science (2019) 48545[5] Zhang H, Yang X, Liu X, Wang T. Preparation of CF reinforced PPS / graphite conductive composite for bipolar plate. Advanced Materials Research 875 (2014) 1245[6] Zhang H, Guan S, Li T, Yang X. Technology of polyphenylene sulfide (PPS) resin / graphite conductive composite for bipolar plate. Key Engineering Materials 519 (2012) 49[7] Huang J, Baird DG, McGrath JE. Development of fuel cell bipolar plates from graphite filled wet-lay thermoplastic composite materials. Journal of Power Sources 150 (2005) 110[8] Boyaci San FG, Tekin G. A review of thermoplastic composites for bipolar plate applications. International Journal of Energy Research 37 (2013) 283[9] https: / / precisionpunch.com / wp-content / pdf / PropertyComparisonChart.pdf

[0010] https: / / www.solvay.com / sites / g / files / srpend221 / files / 2018-08 / Ryton-PPS- Processing-Guide-HP-Fibers_EN-319619_0.pdf

[0011] Akhtar S, White JL. Characteristics of binary and ternary blends of poly (P- phenylene sulfide) with poly (bis phenol A) sulfone and polyetherimide. Polymer Engineering and Science 31 (1991) 84

[0012] Wu H, Wei H, Zhu J, Fang X. Study of thermal properties and crystallization behavior of blends of poly (phenylene sulfide) / poly (ether imide). Polymer-Plastics Technology and Engineering 49 (2010) 1506

[0013] Calleja G, Jourdan A, Ameduri B, Habas JP. Where is the glass transition temperature of poly(tetrafluoroethylene)? A new approach dynamic rheometry and mechanical tests. European Polymer Journal 49 (2013) 2214

[0014] Lu Y, Zhao CZ, Yuan H, Hu JK, Huang JQ, Zhang Q. Dry electrode technology, the rising star in solid-state battery industrialization. Matter 5 (2022) 876

[0015] Choi H, Moon D, Sheem J, Koo JK, Hong S, Oh SM, Kim YJ. A solvent-free process enabled by polytetrafluoroethylene / carbon black composites for fabricating electrodes for lithium-ion batteries with a high volumetric energy. Journal of the Electrochemical Society 170 (2023) 090511

[0016] Zhang Y, Huld F, Lu S, Jektvik C, Lou F, Yu Z. Revisiting polytetrafluoroethylene binder for solvent-free lithium-ion battery anode fabrication. Batteries 8 (2022) 57

[0017] Gromadskyi DG. Characterization of Ag / Ag2SOr system as reference electrode for in-situ electrochemical studies of advanced aqueous supercapacitors. Journal of Chemical Society 128 (2016) 1011

[0018] 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 Energy 159 (2015) 39

[0019] Gromadskyi DG, Hromadska LI. Bivariant mechanical tuning of porous carbon electrodes for high-power and high-energy supercapacitors. Surface Engineering and Applied Electrochemistry 52 (2016) 584

[0020] Gromadskyi D. Method for producing a separator plate for a fuel cell and method for producing a fuel cell stack with such separator. US Patent 11,404,704 B2

[0021] Gromadskyi DG, Hromadska LI. Separator plate for a fuel cell, precursor therefore and its method of production. WO2021 / 244719A1

[0022] Gromadskyi D. Method of producing separator plates. US2022 / 0293969A1

[0023] https: / / www.fluorochernie.com / product / ptfe-aqueous-dispersion-tpd-600e

[0024] Gardiner J. Fluoropolymers: origin, production, and industrial and commercial applications. Australian Journal of Chemistry 68 (2014) 13

[0025] https: / / fluorogistx.com / wp-content / uploads / 2022 / l l / PTFE-Fluoroplastic-Dispersion-DISP-30-SDS.pdf

[0026] https: / / daikin-america.eom / wp-content / uploads / 2013 / 07 / TDS-PTFE-D-001- D-210.pdf

[0027] Zuo P, Tcharkhtchi A, Shirinbayan M, Fitoussi J, Bakir F. Overall investigation of poly (phenylene sulfide) from synthesis and process to applications - a review. Macromolecular Materials and Engineering 304 (2019) 1

[0028] Hala M, Malis J, Paidar M, Bouzek K. Characterization of commercial polymer-carbon composite bipolar plates used in PEM fuel cells. Membranes 12 (2022) 1050

[0029] Sakai M, Bradt RC. Fracture toughness testing of brittle materials. International Materials Reviews 38 (1993) 53

[0030] Nohara LB, Nohara EL, Moura A, Goncalves MRP, Costa ML, Rezende MC. Study of crystallization of poly(phenylene sulfide). Polimeros: Ciencia e Tecnologia 16 (2006) 104

[0031] Echeverria I, Su PC, Simon SL, Plazek DJ. Physical aging of a polyetherimide: creep and DSC measurements. Journal of Polymer Science: Part B: Polymer Physics 33 (1995) 2457

[0032] Makita K, Ono T, Takahashi T, Akasaka H, Sakane T. Process for producing fine polyphenylene sulfide resin particles, fine polyphenylene sulfide resin particles, and dispersion thereof. US 8,563,681 B2

[0033] Sakane T, Akasaka H, Makita K. Dispersion of polyphenylene sulfide resin fine particle and method for producing the same. JP 2011 122108 A

[0034] Sadeghpour M, Yusoff R, Aroua MK, Tabandesh M. Modification of polyethylene glycol with choline chloride and evaluation of the CO2 absorption capacity of their aqueous solutions. Greenhouse Gases Science and Technology 0 (2017) 1

[0035] Li R, Wu Y, Guo J, Chen X. Effect of molecular weight of polyethylene glycol on crystallization behaviors, thermal properties and tensile performance of polylactic acid stereocomplexes. RSC Advances 10 (2020) 42120

[0036] https: / / www.sigmaaldrich.com / DK / en / sds / aldrich / 81350?userType=red

[0037] https: / / laminatedplastics.com / pps.pdf

[0038] Christakopoulos F, Troisi E, Tervoort TA. Melting kinetics of nascent polytetrafluoroethylene) powder. Polymers 12 (2020) 791

[0039] Jog JP, Bulakh N, Nadkarni VM. Crystallization of polyphenylene sulfide. Bulletin of Material Science 17 (1994) 1079

[0040] https: / / www.ensingerplastics.com / en / compounds / tecacomp-pps-hte-pw-black

Claims

CLAIMS1. A method of producing a precursor for a separator plate, the method comprising providing an aqueous dispersion of a powder of an electroconductive filler at a relative weight of X and powders of- PPS at a relative weight in the range of 0.05X-0.3X,- PTFE at a relative weight in the range of 0.002X-0.03X,- PEI at a relative weight in the range of 0.001X-0.02X; wherein the method further comprises heating the dispersion to temperatures above evaporation temperatures of liquids in the dispersion but below melting temperatures of the PPS, and causing evaporation of liquids from the dispersion and, due to the evaporation, providing a dry, malleable compound containing the electroconductive filler, PPS, PTFE, and PEI as a precursor.

2. The method of claim 1, comprising kneading the dry, malleable compound at a kneading temperature above the glass transition temperatures but below the melting temperatures of PPS, PTFE and PEI and causing fibrillation of PTFE by the kneading.

3. The method of any preceding claim, wherein the water concentration in the aqueous dispersion is no less than 99 wt.%.

4. The method according to any preceding claim, wherein the dispersion is free from organic solvents.

5. The method of any preceding claim, wherein the electroconductive filler is graphite.

6. Method of producing a separator plate from a precursor obtained by a method according to any preceding claim, the method further comprising hot-calendering the malleable precursor into a precursor film and shaping at least a portion of the precursor film into a precursor slab and positioning the precursor slab in a press mold and hot-compacting the precursor slab into a separator plate and cooling the so formed separator plate to a temperature below a crystallization temperature of the polymeric binder andcausing rigidity of the separator plate, and only then removing the rigid separator plate from the press mold.

7. The method of claim 6, wherein the method comprises hot-compaction of the slab at a hot-compacting temperature above a melting temperature of PPS.

8. A separator plate made by a method according to claim 6 or 7.

9. A fuel cell comprising a separator plate according to claim 8.

10. The fuel cell of claim 9, wherein the fuel cell is part of a HT-PEM fuel cell stack, and the separator plate is a BPP.