DK180950B1

DK180950B1 - Single layer separator plate for a fuel cell, precursor therefore and its method of production

Info

Publication number: DK180950B1
Application number: DKPA202001469A
Authority: DK
Inventors: Gromadskyi Denys; Hromadska Larysa
Original assignee: Blue World Technologies Holding ApS
Priority date: 2020-06-04
Filing date: 2020-12-30
Publication date: 2022-08-10
Also published as: DK180730B1; DK202000656A1; DK202001469A1

Abstract

For production of a separator plate in a fuel cell, a malleable precursor sheet is made by mixing thermoplastic polymer, carbon fibers, and electroconductive carbon particles, which is then hot-compression molded as a single layer structure, where the layer thickness is less than the length of the carbon fibers.

Description

DK 180950 B1 1 Single layer separator plate for a fuel cell, precursor therefore and its method of production

FIELD OF THE INVENTION The present invention relates to separator plates, for example bipolar plates, for fuel cells. It also relates to a precursor sheet according to the preamble of claim 1 for hot- compression molding of such separator plate and for its production.

BACKGROUND OF THE INVENTION Bipolar plates (BPPs) are key components for fuel cell stacks, as they separate single membrane-electrode assemblies and deliver fuel to the membrane [Ref. 1]. High-tem- perature proton-exchange membrane (PEM) fuel cells have great advantages as com- pared to low-temperature PEM fuel cells due to their high tolerance to impurities in the input gases, especially tolerance to carbon monoxide residues in hydrogen gas [Ref. 2]. However, the relatively high operating temperatures in the range of 160-200°C and the acidic media set strict limits for those materials that are suitable to be used in bipolar plates [Ref. 3]. Graphite seems very attractive as a material due to its high electrical and thermal conductivity as well as good chemical stability [Ref. 4]. Other carbon materials such as carbon black (CB), carbon fibers (CFs), carbon nanotubes (CNTs), and gra- phene can be utilized as additives to the compound material for the BPP in order to improve the mechanical and electrical properties [Ref. 5, 6]. However, in order to hold the carbon particles together, use of binding materials, e.g. thermoplastic polymers, is needed. Polyphenylene sulfide (PPS) is a good candidate, as it fulfills all strict require- ments for high-temperature PEM fuel cell, including being chemically inert, having a high service temperature of above 200°C, and having good mechanical properties [Ref. 7].

Alternative BPP structures are disclosed in CN111048800A [Ref. 58] and US7887927 [Ref. 59], where multi-layer BPPs are produced by stacked carbon fiber sheets or graph- ite sheets with an electro-conductive polymeric filler in between. The Department of Energy (DoE) of the United States declared high 2020 targets for BPPs utilized in transportation applications, among which is the requirement that the

DK 180950 B1 2 areal-specific resistance should be less than 10 mQcm?, while flexural strength must exceed 25 MPa [Ref. 8]. It should be noted here that, normally, for graphite-based BPPs, the thickness is around 1.5-3 mm [Ref 9-12]. However, the newest research work demonstrates graphite BPP samples based on graphite compounds having thicknesses below Imm, e.g. 0.85 mm in [Ref. 13], 0.6 mm in [Ref. 14] or even 0.4 mm in [Ref.

15], which gives some benefits, such as more compact fuel cell design and cost reduc- tion because of lower material consumption. In order to produce thin BPPs from a pow- dered compound, a multi-step process is disclosed in [Ref. 16]. It would be desirable to provide a simple but efficient process for mass production of BPPs.

There are few patents and research manuscripts that are describing processes for the preparation of carbon/PPS compounds and corresponding BPPs [Ref. 17-20]. For ex- ample, in international patent application WO2014/100082 [Ref. 17], PPS is mixed with carbon nanotubes (CNTs) in a hot extruder in which the polymer is melted. Normally, high polymer loading is needed for an extrusion process, typically more than 50 wt.%, which, unfortunately, has negative impact on the electrical properties of such com- pounds. After extrusion, the PPS-based compound can be formed into the sheets, for example as it is proposed in US patent No. 7,736,786 [Ref. 18].

Chinese patent application CN101174695A [63] discloses a dry method for producing a BPP made of a material containing 45-55% graphite, 5-10% carbon fiber, 30-40% PPS and 5-10% fluoropolymer resin, for example PTFE. The flow field structure has a depth of 0.5-1.5 mm, inside a sheet having a thickness in the order of 5 mm. The carbon fiber length is in the range of 10-200 micrometer. A powder mix is first cold pressed at 3-5 MPa and then slowly heated to 310-330%C and pressed at 15-25 MPa for compaction for 30 minutes, after which it is quickly cooled in water for preventing crystallinity and increase toughness. US2014/087287 [Ref. 60] discloses a dry method for manufacturing separator plates for a fuel cell, wherein the separator plate is made of two sheets, A and B. The sheet A comprises 100 parts of thermoplastic resin and 130-3200 parts of carbonaceous mate- rial, and the sheet B comprises 100 parts of thermoplastic resin, for example polypro- pylene, and 3-280 parts carbonaceous material. Half of the carbonaceous material is fibrous carbon. The fiber length is in the range of 0.001-20 mm, more preferably 1-10

DK 180950 B1 3 mm, and exemplified with 6 mm. The sheets are molded by compression of the powder mix at a temperature of 60 degrees higher than the higher of the melting points of their respective binder components. Groove depths were in the final plate were 0.3 and 0.5 mm in a 1 mm thick separator.

US2019/0341630 [Ref. 61] discloses a production method for fuel cell separators, for example with a thickness of 1 mm. In the method, carbon fibers are dispersed with fibrous resin in water dispersion, after which the slurry is dried to obtain a paper-like composite sheet, into which as a next step carbon particles are pressed at elevated tem- perature. The length of the carbon fibers is not limited but, for example, in the range of 20 micrometer to 6 mm, and exemplified by 3 mm for a 2 mm thick composite sheet. US2009/0152105 [Ref. 62] discloses a production method of a compression pad in a fuel cell, in which I inch long carbon fibers are blended into molten PTFE, which is then hardened and chopped in a coffee grinder. The blend is then mixed with PVDF and heated to 200°C while compressed. Alternatively, the carbon fibers are molten into the PVDF. Carbon fibers are preferably 3 mm long (1/8 inch). Typical thicknesses of the pads are 10-15 mils, roughly corresponding to 0.25-0.40 mm.

WO2008/075812 [Ref 64] discloses a hydrophilic polymer composite for a bipolar plate comprising a carbon black aggregate with hybrid particles embedded on the surface of carbon black particles.

Wet processes, for example as described in [Ref. 19] and in US2019/0341630 [Ref. 20], imply mixing of graphite, carbon fibers, and PPS binder in liquid phase, in particular in water, before the solid mixture is filtered and formed into sheets for their further drying and molding into the BPPs. However, disadvantageously, PPS is very poorly wetted by water, as it can only take up ca. 0.1% water at 23°C after 500 h [Ref. 21]. Accordingly, itis a challenge to disperse PPS in aqueous media without agglomeration of its particles.

Probably, this is why highly diluted dispersions are applied in [Ref. 19, 20], where the total solid content does not exceed 10 wt.%, and is kept preferably in the range of 1-3 wt.%.

DK 180950 B1 4 A process of making sheets from powdered compounds, prepared by dry mixing of graphite and PPS, is also proposed in [Ref. 22], where the mixed compounds are dis- persed together with another thermoplastic polymer, namely polytetrafluoroethylene (PTFE), in iso-propanol, which is good surfactant for wetting hydrophobic surfaces [Ref. 23]. An increase of the temperature up to the boiling point of alcohol leads to a rapid coagulation of PTFE, forming a pliable and malleable material, similar to a dough- like structure, see US2019/0260037 [Ref. 22], because the PTFE has an ability to high elongation compared to other polymers, seeing that it can reach 550% [Ref. 24].

In the patent literature, various processes are disclosed in relation to handling PPS, for example US8563681 [Ref 30], and for forming articles therefrom, for example US5043112 [Ref 27], as well as production of BPPs, for example US2008/0318110 [Ref. 31] and US 2006/0084750 [Ref. 50].

US6544680 [Ref. 54] discloses molded separator plates with carbon and PPS with the addition of a thermosetting resin. US6803139 [Ref. 55] discloses molded separator plates with carbon and a thermoplastic, for example polyphenylene sulfide (PPS) and with the addition of carbodiimide. EP1758185 [Ref. 56] discloses molded separator plates with 84% carbon, 2% PTFE, 14% epoxy that is cured in the hot press. Polyphe- nylene sulfide (PPS) or Polytetrafluoroethylene (PTFE) are mentioned as thermoplastic resins but not exemplified. US2005/0042496 [Ref. 57] discloses a continuous process in which polymer is blended with a filler, for example graphite, kneaded and extruded before transferred into a form, in which it is compacted, for example into a separator plate.

International patent application WO2018/072803 [Ref. 53] discloses a production method for separator plates, in which a powder mix of carbon and a thermoplastic pol- ymer is suspended in alcohol and mixed with a water suspension of PTFE, after which liquid is evaporated, and the final malleable slurry is rolled into a sheet and press molded into a separator plate. Examples of carbon powder includes graphite, carbon black, gra- phene, carbon nanotubes or amorphous carbon.

As compared to pressed powdered compounds for BPPs, the use of malleable sheets or slabs for molding of BPPs appear beneficial in terms of easier storing, handling, and

DK 180950 B1 dosing as well.

However, in order for this method as well as the resulting BPPs to be optimized, further improvements are desirable, in particular with respect to the targets as set in the USA DOE’s 2020 program.

In order to be successful as production method also ease, costs and speed of production are important factors that need consideration. 5 DESCRIPTION / SUMMARY OF THE INVENTION It is an objective of the invention to provide an improvement in the art.

In particular, it is an objective to provide improved separator plates, such as BPPs, and improved fuel cells with such separator plates.

A further objective is to provide an improved method for providing separator plates, such as BPPs, and an improved material mix of carbon and polymer and a corresponding precursor for the pressure molding of separator plates.

This is achieved with a method for production as described in the method claims and in the following, as well as with a precursor, characterized as in claim 1, for hot-compres- sion molding of a separator plate, and further with a separator plate, such as a BPP as described in the product claims.

In particular, this objective is achieved by a process of making a graphite-based com- pound with thermoplastic polymers therein as well as a moldable, malleable, precursor sheet thereof for hot-compression molding into an electrically conductive, rigid separa- tor plate for a fuel cell, such as BPPs.

Although, the invention is targeting production of BPPs for the use in fuel cell stacks, it is equally valid for producing separator plates in general, as the process and material applied equally well.

Especially, the method is likewise useful for the production of single electrode plates for fuel cells as well as end plates for a fuel cell stack.

Examples of configurations are given in WO2018/072803 [Ref. 53]. For example, the separator plate is a bipolar plate with fluid flow fields on opposite sides, in particular a flow field for oxygen on one side and a flow field for hydrogen on the opposite side.

Alternatively, a 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.

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

DK 180950 B1 6 oxygen on one side and a flow field for coolant on its opposite side. Depending on the need, the method as explained herein can be used to produce a variety of different sep- arator 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.

In short, as a general aspects, for production of such separator plate in a fuel cell, a malleable precursor sheet is made by mixing thermoplastic polymer, carbon fibers, and electroconductive carbon particles, where the precursor is then hot-compression molded as a single layer that makes up the separator plate. Advantageously, as will be discussed below, the layer thickness is less than the average length of the carbon fibers. Details are described in the following. The compound is provided as a polymer matrix that comprises a thermoplastic polymer blend in which carbon fibers and electroconductive carbon particles are dispersed. The thermoplastic polymer blend comprises PTFE and a thermoplastic polymer different from PTFE, for example polyphenylene sulfide (PPS). Alternatives for PPS include ethylene tetrafluoroethylene (ETFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy alkane (PFA), polyamideimide (PAT), polychlorotri- fluoroethylene (PCTFE), polyether ether ketone (PEEK), polyetherketone (PEK), pol- yetherimide (PEI), polyethersulfone (PES), polyphenylsulfone (PPSU), polysulfone (PSU), or polyvinylidene fluoride (PVDF). Herein and in the following, all stated weight percentages are given relatively to the total weight of the polymer blend, carbon fibers, and carbon particles. Thus, liquids used in the production process, such as water and organic solvents, as well as liquid additives, for example surfactants, are not taken into regard, as these are evaporated prior to the final production step. For example, the precursor and the final separator plate comprises one or more of the following parameters, of which two, three or more can be combined successfully: PTFE: at least 0.05 wt.% PTFE: less than 0.5 wt.% Thermoplastic polymer different from PTFE: minimum 5-30 wt.%

DK 180950 B1 7 Carbon fibers relative weight: 2-20 wt. %, for example 5-20% Carbon fibers average length L: 0.1-1 mm Carbon particles (different from carbon fibers) relative weight: 25-90 wt.% Carbon particles (different from carbon fibers) average size: 0.1-100 um.

The sizes given herein for polymer particles and carbon particles, including graphite and carbon black, are average sizes, which means averaged over the three dimensions of the particles as well as over the number of particles of this specific group or type.

Typically, for the particles, the statistical distribution related to the dimensionally-av- eraged sizes, including the length of the carbon fibers, have a FWHM lower than or in the order of +20%.

The compound is formed into a moldable, malleable precursor sheet in a forming sta- tion, for example a stationary forming station. However, advantageously, the forming station is a calender rolling station, and the forming comprises press-rolling the sheet by calender rollers the calender rolling station.

The precursor sheet is formed as a single layer slab with a thickness X1 and then molded into a single layer separator plate. Typically, the precursor is cut into shape before the molding, and the resulting slab is then hot-compression molded into a separator plate. The terminology of the thickness X1 of the precursor slab and the corresponding thick- ness X2 of the separator plate is used for differentiation, as the compression stage in- creases the density of the slabs due to the high-pressure compaction. Whereas the thick- ness X1 of a slab applies for the precursor slab before molding, the thickness X2 applies for the separator plate after molding of the slab. For example, the density from the slab to the final separator plate, such as BPP can increase by a factor of 2. However, it is also pointed out that the formation of flow fields during the compression molding may compensate somewhat for the decrease in thickness.

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

DK 180950 B1 8 As will be discussed in more detail below, the thickness X1 is smaller than the average length L of the carbon fibers. This is advantageous, as it causes the carbon fibers to be at least partially parallel with the slab, especially during the rolling process, which in- creases mechanical strength of the final separator plate. In particular, the average length L is at least twice as large as the thickness X1 of the precursor sheet. When comparing the length L of the carbon fibers to the thickness X2 of the separator plate, it is also valid to assume that this thickness X2 should be less than the average length L of the carbon fibers, however, rather at least two times less than L, or even at least four times less than L, which is due to the compaction of the precursor during compression molding.

The selection of the length of the carbon fibers is a balance between positive influence on the strength of the final separator plate and the ability to get the carbon fibers dis- persed, which is easiest for short carbon fibers. From these perspectives, an average length for the carbon fibers has been found useful if in the range of 0.1-1 mm. The selection of the length of the fibers is dependent on the thickness of the final separator plate.

In other words, when the length L is selected to be two times larger than the thickness XI of the precursor, it will typically be in the order of four times larger than the thick- ness X2 of the final separator plate, due to the compaction during molding.

When mixing the carbon fibers in the initial dispersions, the fibers should not be too long, which would make mixing difficult. For example, 6 or 10 mm long fibers, as dis- closed in the prior art US2014/087287, US2019/0341630, and US2009/0152105 would be disadvantageously long when producing a separator plate with a thickness X2 of

0.05-0.3 mm. On the other hand, fiber lengths of 0.01 mm or 0.02 mm as disclosed in the same prior art would be too short. Accordingly, the ranges as suggested by the prior art appear rather arbitrary and do not express the considerations of balancing between proper mixing and with increase of strength.

Also, the amount of carbon fibers in the compound should not be arbitrarily chosen. It has been experimentally verified that strength of the final separator plate increased for

DK 180950 B1 9 increasing amount of carbon fibers until 10 wt.%, however with a slower increase in the range 7-10 wt.%. For higher amounts than 10 wt.%, the flexible strength was found to decrease. For this reason, the amount of 5-20 wt.% , for example 5-15 wt.%, appears most useful, with an optimum range within 6-12 wt.%, for example 6-10%.

As discussed in the introduction, thin separator plates are desirable, as they influence size, weight, performance, and cost of the final fuel cell stack. The thickness X2 of the separator plates is in the range of 0.05-6 mm or even 0.05-0.3 mm. For example, the thickness X1 of the precursor sheet is in the range of 0.05-1 mm, for example 0.1-1 mm or 0.05-0.6 mm, optionally 0.1-0.6 mm or even 0.05-0.3 mm. The following method of production of the precursor sheet has been found advanta- geous. In this method, an aqueous dispersions and a solvent dispersion are provided and mixed. The aqueous dispersion comprises PTFE particles and the carbon fibers, and optionally a first portion of graphite particles, for example having an average particle size in the range of 0.1-10 um. The solvent dispersion comprises the second thermoplastic particulate polymer that is different from PTFE, for example PPS, as well as carbon black particles, which are more hydrophobic than graphite particles in the mix. The organic solvent is optionally N-methyl-2-pyrrolidone, NMP. Alternative solvents are also possible to use, for example N,N-dimethyl acetamide, N,N-dimethylforma- mide, and dimethyl sulfoxide. Both dispersions are stirred for preventing sedimentation of the particles and then com- bined and mixed. Further, graphite particles are mixed into the mix dispersions, for example having an average size in the range of 10—100 um. Typically, the portion of these graphite particles has a weight which is 5-20 times larger than the weight of the carbon black.

DK 180950 B1 10 It has been found advantageous to use two portions of graphite in the mix, with a first portion having a smaller particle size than a second.

As already mentioned, the first portion is advantageously mixed into the first dispersion.

The second portion of graphite particles is advantageously mixed with the mixed two dispersions.

For example, the graphite particles of the second portion are having an average size in the range of 10— 100 um.

This second portion is relatively large as compared to the first portion and the amount of carbon black.

Typically, the second portion has a weight which is 5-20 times larger than the sum of the weight of the first portion and the weight of the carbon black.

This large amount of graphite makes further stirring difficult, why the proper blending is performed in a kneader.

During kneading in the kneader, the temperature is raised to elevated temperature levels sufficiently high for evaporating the organic solvent and water from the mix.

Further- more, by increasing the temperature to levels above the glass transition temperature of PTFE but below the melting temperature of PTFE, the kneading leads to fibridization of the PTFE, which is advantageous for the strength of the final separator plate.

In an advantageous process, the elevated temperature levels during the kneading in the kneader are below the melting temperatures of the PTFE and the second thermoplastic polymer.

Although, it is possible to increase the temperature of the mix in the kneader to above the melting temperature of the second thermoplastic polymer, it has been found more advantageous to melt the second thermoplastic polymer after extraction of the mix from the kneader.

Accordingly, only after extraction, the temperature of the mix is raised to a level sufficiently high to melt the second thermoplastic polymer before form- ing the sheet into a slab with thickness X1 in the forming station.

For example, there is provided a conveyor between the kneader and the rolling station, in which the heating of the compound for the melting of the second thermoplastic pol- ymer is done so that second thermoplastic polymer is in a molten state when the mix is formed into a sheet in a forming station, for example calender rolling station.

DK 180950 B1 11 For example, after the forming station, the sheet is cut into a slab with the right dimen- sions for the hot-compression molding in an inline process which includes the kneader and the forming station.

For very thin precursor sheets, several rolling stages are provided in the rolling station. In experiments, successful production of sheets with a thickness X1 of 0.05 mm has been achieved with a roller station having 6 rolling stages. In such performed experi- ments, the carbon particles have an average size of 20 um, so that the 0.05 mm thickness XI of the sheet was at the theoretical lower limit for a sheet with such particles.

Notice that such thin precursor sheets with a thickness X1 of 0.05-0.1 mm cannot be produced by pressing a powder mix of carbon particles and polymer particles. One of the factors for successfully producing thin plates is the fibridization of the PTFE during the kneading process, which is why the kneading is an important stage for the produc- tion. Alternative, the sheet is provided as a quasi-endless sheet with thickness X1, which is cooled to solidify and then rolled onto a roll for storage until the final shaping, for ex- ample by cutting, prior to hot-compression molding. Calender rolling has proven to be advantageous for aligning the carbon fibers at least partially into a direction parallel to the surface. Furthermore, it has turned out that cal- ender rolling of the slab in different directions tend to align the fibers in different direc- tions. This is surprising in that not only the last calender rolling step determines the direction of the fibers but also the previous steps. It is believed that subsequent calender rolling steps tend to align the outermost carbon fibers mostly, so that the direction of the carbon fibers from previous calendering steps is maintained inside the bulk of the sheet. Accordingly, when a precursor slab after calendering in one direction is calender rolled in a different direction, for example transverse direction, an increase of strength is observed also in this calendering direction.

DK 180950 B1 12 For example, an endless sheet is first calendered into a direction parallel with the endless sheet and then cut into a slab and then calendered further in a different direction, for example transverse direction. The subsequent hot-compression molding transforms the slab into an electrically con- ductive, rigid separator plate for a fuel cell, for example a BPP. For example, the separator plate has an area specific resistance of at most 2 mQ-cm? per thickness unit of 0.3 mm.

Optionally, the separator plate has a flexural strength of more than 180 MPa per thick- ness unit of 0.3 mm. The hot compression molding modifies the shape of the separator plate, for example with a flow field structure for flow of fluid impressed into the material on at least one side of the separator plate, although typically on both sides, especially if the separator plate is a BPP. Such separator plate, for example BPP, is useful for high temperature polymer electro- lyte membrane fuel cell, (HT-PEM), which operates above 120 degrees centigrade, dif- ferentiating HT-PEM fuel cell from low temperature PEM fuel cells, the latter operating at temperatures below 100 degrees, for example at 70 degrees. The normal operating temperature of HT-PEM fuel cells is the range of 120 to 200 degrees centigrade, for example in the range of 160 to 170 degrees centigrade. Such HT-PEM fuel cells are advantageous for compact fuel cell systems, for example for automobile industry.

SHORT DESCRIPTION OF THE DRAWINGS The invention is explained in more detail with reference to the drawings, wherein: Fig. 1 is a scheme of a continuous process for making a graphite-based compound, preforming the compound into slabs, and molding bipolar plates from it; Fig. 2 illustrates (a) flexural strength and (b) areal specific resistance depending on the thickness for MFG/SFG/CFs/CB/PPS/PTFE-based BPPs; Fig. 3 shows microphotographs of a cross section of a slab;

DK 180950 B1 13 Fig. 4 illustrates the distributed load at break in dependence of the thickness of MFG/SFG/CFs/CB/PPS/PTFE-based BPPs (ellipse symbols refer to single-layered BPPs, cross symbols refer to multi-layered BPPs); FIG. 5 is a simplified illustration for flow of electrical current through a composite with electroconductive carbon particles of different sizes with a) only one size of particles, and b) different sized of particles. DETAILED DESCRIPTION / PREFERRED EMBODIMENT In the production method described herein, a few partial processes are combined, namely compounding of raw powdered materials, followed by their kneading and cal- endering into preformed shapes, such as thin slabs with specified density, and further compression molding such slabs to provide separator plates for fuel cells, optionally electrode plates, end plates, or BPPs. In the following, the method will be explained for BPPs, however, the method applies equally well for such variety of plates in a fuel cell or a fuel cell stack. Thus, all partial method processes as described in the following should also be read on such other type of separator plates, although, the highest ad- vantage is believed to be achieved by this method for BPPs. In FIG. 1 shows a scheme that illustrates the fabrication for producing BPPs based on graphite, carbon fibers (CFs), and carbon black (CB) and its binding in a polymer matrix with the polymers PPS and PTFE. It should be mentioned here that other thermoplastic polymers can be also be utilized in the fabrication of BPPs for high-temperature PEM fuel cells. Candidates are, among others, ethylene tetrafluoroethylene (ETFE), fluorinated ethylene propylene (FEP), per- fluoroalkoxy alkane (PFA), polyamideimide (PAI), polychlorotrifluoroethylene (PCTFE), polyether ether ketone (PEEK), polyetherketone (PEK), polyetherimide (PEI), polyethersulfone (PES), polyphenylsulfone (PPSU), polysulfone (PSU), polyvi- nylidene fluoride (PVDF), see also [Ref 25].

PTFE is provided in an aqueous dispersion, for example with a relative concentration in the range of 10-80 wt.%, optionally in the range of 50-70 wt.% PTFE in water, for instance a 60 wt.% aqueous dispersion. Such latter dispersion is commercially available, for example from the company Merck®. The dispersion, as purchased, may optionally

DK 180950 B1 14 be further diluted to a suitable concentration by mixing with deionized water. In addi- tion, surfactants may be added. As illustrated in FIG. 1, PTFE, CFs and graphite as well as other potential ingredients, such as surfactants, are provided in a first container 1. The content of surfactants does not exceed 10 vol.% in the total liquid composition, but typically it is within range from

0.2 to 2 vol.%. It should be noted here that the PTFE dispersion from the supplier, typ- ically, already contains a small amount of surfactants to avoid agglomeration of the polymer particles [Ref. 26].

Non limiting example of surfactants are TergitolTM 15-S Series from Dow Chemicals®, Triton® X Series from Union Carbide Corporation® or Tween® Series from Croda International ®. For example, Triton X-100™ from the Triton X Series has a hydrophilic polyethylene oxide chain and an aromatic hydrocarbon lipophilic or hydrophobic group. The hydrocarbon group is a 4-phenyl group. The formula is CuH220(C2H40)n, where n is 9-10. For example, useful mass ratios between solid and liquid phases in the first container 1 are in the range of 1:4 to 1:6, such as about 1:5. On the one hand, a low volume of water is desired in the process in order to minimize energy and resource consumption in the process, on the other hand, the process needs a sufficient amount of liquid for maintain- ing a proper dispersion. Optionally, CFs are provided with average lengths in the range 0.1-1 mm, for example lengths in the range of 0.2-0.4 mm, such as approximately 0.3 mm in length. The lengths given here are average lengths. For example, the statistical distribution related to the averaged lengths have a FWHM less than or in the order of £20%. As will become more apparent in the following, the average length of the CFs are selected in relation to the layer thickness X1.

Advantageously, a small fraction of graphite (SFG) is added. The term “small fraction” refers to a relatively percentage range of 2—10 wt.% relatively to the total dry weight of the final product, i.e. without liquid. Optionally, the SFG graphite particles are provided

DK 180950 B1 15 with average particle sizes in the range 0.1-10 um, for example in the range of 0.5-2 um. In experiments, the average size of the graphite particles of the SFG was 1 um. As already mentioned, the sizes given herein for polymer particles and carbon particles, including graphite and carbon black, are average sizes, which means averaged over the three dimensions of the particles as well as over the number of particles of this specific group or type. Typically, for the particles, the statistical distribution related to the di- mensionally-averaged sizes have a FWHM lower than or in the order of £20%.

In this first container 1, all ingredients are intensively stirred.

Examples of weight ratios between PTFE, CF, and graphite (SFG) are in the ranges of (0.05-0.5):(0.05—15):(0.05—15). In experiments, the ratios were 0.25 : 10: 5.

In parallel, a second powder mixture is provided in a second container 2. This second powder mix contains PPS powder, optionally having average particle sizes in the range of 10-100 um, for example in the range of 20-30 um, such as approximately 25 um. Further, in this second container 2, CB particles are provided, optionally having average sizes in the range of 10-100 nm, for example in the range of 30-50 nm, such as approx- imately 40 nm.

The particles in the second container 2 are mixed with N-methyl-2-pyrrolidone (NMP) in order to provide a viscous slurry. NMP is provided in container 2 to achieve wetting of the hydrophobic PPS and CB particles before dispersing them in the aqueous media from the first container 1, as NMP has excellent wetting characteristics due to the polar nature and low surface tension [Ref. 27]. It is pointed out that NMP can dissolve ca. 10 wt.% of PPS at 203°C [Ref. 28]. At lower temperatures, NMP probably dissolves only very thin near-surface layers of the polymer particles [Ref. 29]. Being miscible with water at all temperatures [Ref. 30], NMP plays role of a “bridge” for water molecules, delivering them directly to the surface of the hydrophobic particles.

Some other solvents can be used as alternative to NMP for this purpose, for example N,N-dimethylacetamide, N,N-dimethylformamide, and dimethyl sulfoxide. The use of

DK 180950 B1 16 these solvents in combination with surfactants allows production of long-time stable dispersions of PPS, but the process occurs at temperatures in the range of 220-320°C [Ref. 31], which is not optimum.

The distribution of one portion of carbon particles into the first container 1 and another portion of carbon particles into the second container 2 is based on the consideration that the overall amount of liquid should be minimized in order to avoid unnecessary con- sumption of energy for subsequent evaporation of the liquids. In principle, all particles could be added to the second container 2 with the NMP, but in that case the solid content in container 2 would require unnecessarily high amount of NMP in order for the particle concentration be at an acceptable level for an efficient mixing. Furthermore, the pro- duction method takes into consideration the minimizing of the amounts of organic sol- vents that are used, which adds to the method being environmentally friendly, especially when the solvent is recycled. The selection of dispersing the CB and PPS into the second container 2 is due to the fact that these carbon particles are more difficult to be wetted by water than by NMP. On the other hand, SFG and CFs are not so hydrophobic, why these are more suitable to be added to the aqueous dispersion in the first container 1. In our case, sedimentation and agglomeration of fine PPS particles in the second con- tainer 2 is avoided due to the continuous stirring of the dispersion until the main filler, i.e. graphite, is added. After this addition of the graphite at substantial concentration, the viscosity of the system increases so much that sedimentation becomes almost im- possible, even if stirring is stopped for a long time. As stirring is no longer feasible, a kneader is used for the next stages, as described in more detail in the following.

After separate preparation of these two dispersions in the first container 1 and the sec- ond container 2, they are mixed, for example in a third container 3, as illustrated in FIG. 1, for uniform distribution of polymer and carbon particles. Typically, the NMP content in the water after their mixing in the third container 3 does not exceed 25 vol.% rela- tively to the entire mixture volume in container 3. For example, in the third container 3, the concentration of NMP in the mixture is in the range of 10-25 vol. %.

When this mixing process step is completed, the SFG/CFs/CB/PPS/PTFE suspension is transferred from the third container 3 to a kneader 5, together with an amount of graphite

DK 180950 B1 17 from a fourth container 4. As the amount of graphite from the fourth container 4 is relatively large and leads to a relatively large fraction in the final mix, it is termed “main fraction of graphite” (MFG). The ratio between MFG:SFG is at least 2, for example at least 3, but typically at least 5, and typically at most 20, for example in the range of 5—

20. In experiments, a ratio MFG:SFG of 13 has been used. For example, the relative content of the MFG in weight percentages relatively to the dry mass of the mix in kneader 5 is in the range of 40-80 wt.%. In experiments, the con- centration of MFG was 66.25 wt.% of the dry mass.

Optionally, this graphite for the MFG has an average particle size in the range of 10— 100 um, for example in the range of 10-30 um. In experiments, the average size of the graphite particles of the MFG was 20 um. Notice that the particle size in MFG is an order of magnitude larger than in SFG.

The MFG is the main carbon component in the composite. FIG. 5 illustrates in a cross sectional view in simplified manner the advantage of different particle sizes. In FIG. 5a, only one size of carbon particles is provided, namely MFG particles 16 in addition to melted PPS 21, whereas in FIG. 5b with electroconductive additives, it is clearly seen that the smaller SFG particles 18 and CB particles 19 act as electro-conductive bridges for the current route 20 between the larger MFG particles 16, which reduces the overall resistivity of the final composite with the carbon fibers 14. When constructing such composite matrix, also considerations apply with respect to realistic and competitive production costs for commercial products, which is why the amount of relatively ex- pensive nano-sized graphite has to be balanced with the advantage it provides dependent on its concentration. The addition of CB is based on a compromise to obtain a high electrical conductivity and competitive production costs. By the addition of the MFG, the solid content in the mixture inside the kneader 5 is increased, reaching a solid content in the range of 30-70 wt.%, for example in the range of 40-60 wt.%, such as approximately 50 wt.%. Mixing procedures in the containers 1, 2, 3, and 4 are carried out at a first temperature level T1, typically at room temperature, e.g. at in the range of 20-25°C, which is enough for uniform distribution of the pow- dered materials.

DK 180950 B1 18 This is an advantage as compared to processes described in [Ref. 32], where polymers used for binding carbon particles must be completely dissolved in an appropriate sol- vent, which is not so easy for high engineering plastics like PPS because temperatures above 200°C are required. And even worse, in order to obtain more than 50 wt.% solu- bility of PPS, the temperature would have to be higher than 300°C [Ref. 28]. Also, the solvents that are suitable for this, typically, have high boiling point, making it prob- lematic to further remove it from the compound before its molding.

The kneading of the MFG/SFG/CFs/CB/PPS/PTFE mixture in the kneader 5 starts at the first temperature level T1, for example at room temperature, and continues during heating of the mix to elevated temperatures at or above the boiling points of the used liquids in order to remove the liquids from the mix by evaporation. The kneading during evaporation prevents bubble formation or at least minimizes the risk for bubble for- mation.

The kneading process takes typically 10-30 min, however, depending on the speed of the temperature increase and the evaporation of the liquids.

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

In order to make sure that the kneading step is continued without bubble formation due to residue water in the mix, the mixture is heated to a third temperature level T3, well above the boiling point of water. For example, the third temperature level T3 is in the range of 102—-120°C.

In experiments, the kneading was continued at the third temperature level T3, which was at 116°C, which is well above the boiling point of water and therefore makes certain that all water is removed from the mixture.

DK 180950 B1 19 It is brought forward that PTFE undergoes a phase change at the glass transition tem- perature, which in our case was determined to be at 116°C, where the polymer is in a rigid amorphous state [Ref 33] and the tendency for its fibridization from nanoparticles increases. Because of this, the compound becomes softer and malleable for kneading, i.e. the fibridization is useful in that it ultimately leads to enhanced cohesion between the components in the mixture. After having removed water by evaporation, the temperature is increased further to a fourth temperature level T4 in order to remove the solvent by evaporation. In the exper- imental case described herein, where NMP was used as solvent, the fourth temperature T4 was adjusted to 204°C, at the boiling point of NMP [Ref. 30], in order to remove NMP by evaporation. Optionally, for recycling purposes, all evaporated substances can be condensed to liquid phase again in a further container 6 and separated into pure solvents. Useful separation methods include distillation and/or membrane separation [Ref. 34, 35]. Water and NMP are collected in other containers 7 and 8, respectively, to be returned into the manufac- turing process. It is pointed out that potential small rest amounts of surfactants dissolved therein does not interfere with this.

The soft compound is after kneading extracted from the kneader 5 as a dough-like mal- leable material. It is extruded onto a heated Conveyor 9. In order to remove further liquids with higher boiling point from the kneaded MFG/SFG/CFs/CB/PPS/PTFE compound, for example non-ionic surfactants [Ref. 26], the temperature of the mix is raised even further to a fifth temperature level TS that causes evaporation of the surfactants. For example, the fifth temperature level TS is below the melting point of PPS, which is in the range of 271-292. In the experiment, the mix was heated up to a fifth temperature level TS of 270°C, which removed the used surfactants but did not melt the PPS. This temperature rise can be done while the mix is inside the kneader. However, for a smooth extrusion of compounds with relatively low content the polymer, it has been found advantageous if the compound still contains some non-evaporated surfactants.

DK 180950 B1 20 Accordingly, the temperature is increased to a level TS for evaporation of the surfactant after extrusion from the kneader because, after extrusion, the surfactants are not needed anymore. For example, the boiling point of the surfactants Triton® X-100 is 270°C [Ref. 36]). This temperature increase can be done on the conveyor. The temperature of the mix, for example while in the conveyor, is increased even further up to sixth tem- perature level T6 above the melting level of the second thermoplastic polymer, which in the present experiment was a level of 347°C in order to melt PPS and lower the vis- cosity, as it was done in the experiment. The final temperature, however, is depending on the molding parameters that are chosen.

The Conveyor 9 forwards the dough-like structure through at least one rolling station 10 with a gap between calender rollers which is adjusted as needed in order to calender press the structure into a sheet with a specified thickness, normally in the range from

0.05 to 10 mm. However, as will be apparent from the discussion later, a thickness of less than 1 mm is advantageous for BPPs in order to save material and weight and for good performance. For example, the thickness is adjusted to a value in the range of

0.05-1 mm. It should be noted, here, that producing films in the lower end of the thickness range, typically, requires more than one rolling station. After forming the film from the malle- able structure, a cutting tool 11 cuts it into slabs. In some performed experiments, thicknesses of 0.1 and 0.6 mm were used for compar- ison, where the slabs with 0.1 mm thickness were used for a stack in a precursor of 6 slabs, and the 0.6 mm slab was used for a single layer precursor and BPP. The resulting BPP after compression molding had a thickness of 0.3 mm, which is half of the precur- sor thickness, which is due to a decrease in thickness by the compression. In this con- nection, the following is pointed out. The slabs have a specified density, normally in the range of 0.5 to 1.5 g/cm?, such as approximately 1 g/cm”, as used in the experiments. However, the compression molded BPPs have a density slightly more than 2 g/cm’. Accordingly, the precursor slabs should be provided thicker, in some cases up to ca. 2 times thicker. However, it is also pointed out that the formation of flow fields during the compression molding may compensate somewhat for the decrease in thickness.

DK 180950 B1 21 Experimental dimensions of the slabs, apart from the thickness, are 400 x 100 mm, but by this method the slabs of any dimensions can be produced. Finally, the pre-heated graphite-based slabs are formed into BPPs in a hot press machine 12 by means of compression molding. The compression molding is advantageously done at temperatures that are between the melting points of the multiple thermoplastic polymers used. For example, in case of PPS-PTFE , the molding temperature is advan- tageously in the range between the melting point of PPS (271-292°C) and the melting point of PTFE (320-347°C). Optionally, the temperature is in the range of 300-320°C.

Alternatively, the temperature is slightly higher than the melting point of PTFE. How- ever, it should be below the decomposition temperature of PPS, which is around 475°C [Ref. 37].

The temperature for the molding is generally depending on the molecular weights of the polymers and their melting temperatures as well as behavior during heat treatment [Ref. 38-40].

Such temperature range, where not all polymers are melted, helps avoiding the slabs sticking to the mold. Furthermore, PTFE can play the role as an anti-sticking component for the MFG/SFG/CFs/CB/PPS/PTFE compound.

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

The processing time is defined by the cooling speed of the BPP within the mold. Release of pressure occurs, when the temperature of mold is below the glass transition temper- ature of PPS, i.e. less than 93°C [Ref. 38].

It should be mentioned here that the BPP can be fabricated either by utilization of a single slab, resulting in a single-layered BPP, or by using multiple slabs in stacked con- dition, for example 4-8 slabs on top of each other. In the experiment, 6 slabs were used to obtain a multi-layered slab and pressing it into a multi-layered BPP. However, the multilayer option is not part of the invention.

DK 180950 B1 22 Each component in the compound, produced in such way, has a specific purpose. MFG is a main filler for PPS, whereas other additives improve both the mechanical properties, especially influenced by CFs, as well as electrical properties, especially influenced by SFG and CB, in addition to the ability of binding fine powdered materials in malleable dough-like structure, which is especially achieved by PTFE. Percentage ratios between all these components in the final mixture can be varied within some constraints. For example, with all percentages being by weight: - The content of MFG is in the range of 25-90 wt.%, for example 50-90 wt.%. In the experiment slightly less than 70% was used.

- The minimal amount of PPS is 5 wt.% and will typically be less than 30 %. In the experiment, 20 wt.% is was used. - The total content of additives, SFG, CFs, CB, PTFE in the final compound is typically less than 45 wt.%.

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

- Examples of ranges for the components are 25-90 wt.% MFG, 5-30 wt.% PPS, 2-20 wt.% CF, 0.05-15 wt.% SFG, 0.05-10 wt.% CB, 0.05-5 wt.% PTFE, for example at least 0.05 wt.% but less than 0.5 wt.% PTFE.

All weight percentage are for given relatively to the polymer blend with the particles and fibers, thus, after removal of the liquids.

Experimentally, optimal electro-mechanical properties were demonstrated by the final compound without liquids containing the following quantities of individual compo- nents: MFG (66.25 wt.%), PPS (17.50 wt.%), CFs (10.00 wt.%), SFG (5.00 wt.%), CB (1.00 wt.%), PTFE (0.25 wt.%).

Dependences of flexural strength and areal specific resistance on the thickness for BPPs produced from compounds with such percent ratio between components are shown in Fig. 2.

DK 180950 B1 23 As seen from the figure given above, these two dependences are not linear.

Decrease of BPP thickness leads to a reduced areal specific resistance, as seen in FIG. 2b, and a significant growing of flexural strength, as seen in FIG. 2a.

In particular, the flexural strength in dependence of thickness is growing for decreasing thickness and deviates from a quasi-linear shape for thicknesses below 1 mm, and in particular remarkable for thickness below 0.5 mm of the BPP.

The growth of flexural strength for small thickness is believed to be due to a planar orientation of CFs in a near-surface layer.

This near- surface layer is believed to be formed when the slabs are rolled and to have improved mechanical properties as compared to the remaining volume underneath the near-sur- face layer, where almost-perpendicular orientation of CFs is retained.

This explanation implies that a self-organized laminate structure takes place, which was experimentally verified by microtomographic investigation, images of which are reproduced in Fig. 3 and which show existing of two main zones in the slab with different orientation of CFs therein . It is emphasized that this is an important finding, which can be utilized to great advantage as explained in the following.

The alignment of the carbon fibers 14 in the graphite based slab 13 during calender rolling in the roller station 10 is illustrated in FIG. 6. When the polymer of the mix is soft due to the elevated temperature, graphite particles mixed with the softened poly- mers can be considered as a quasi-liquid substance where solid fibers are dispersed.

This is a relatively good approximation, seeing that the particles and the fibers 14 flow in the polymer and also partially due to the different hardness of graphite and carbon fibers 14, the latter being 3—5 times harder.

Accordingly, during the calendering pro- cess, the slab 13 is mainly extended in the direction of rolling, and as the carbon fibers 14 follow to the path of least resistance, they get aligned in the direction 15 of rolling.

For ultra-thin slabs and BPPs based thereon, there appears not enough space for per- pendicular and mechanically weak orientation of CFs, why the CFs have to be at least partially oriented in parallel with the slab, which leads to a superior mechanical struc- ture with the demonstrated exponential grow of flexural strength towards small thick- ness, as reflected in Fig. 2a.

It should be noted here that the flexural strength of PPS is in the range of 125-135 MPa [Ref. 41], which is why such behavior appears to be re- lated to anisotropic properties of the used carbon materials, mainly CFs.

Something

DK 180950 B1 24 similar is shown in [Ref. 42] and there is also good correlation with mathematical mod- els [Ref. 43]. In order to define a practical minimal thickness of the BPP which is acceptable for use in PEM fuel cell stacks, a criterion is used that it needs to withstand a pressure 1 MPa, which is also recommended by one of the gas diffusion layer suppliers [Ref. 44]. Ac- cordingly, strength should be considered in terms of distributed load as well, which is illustrated in Fig. 4 for various BPP thicknesses.

Fig. 4 demonstrates that a minimal thickness for single-layer, i.e. 1 slab based, BPP is 0.38 mm in order to fulfill the 1 MPa criterion.

However, for multi-layered BPPs, the necessary minimum thickness for this criterion is remarkably lower, namely only 0.29 mm.

This smaller necessary minimum thickness reflects the higher relative strengths of the thinner slabs used for fabrication of multi- layer BPPs.

This value of 0.29 mm for the necessary minimum thickness is in agreement with simulations as expressed by the left solid curve in FIG 2. However, the curve ap- pears only valid and reproducing the correct minimum thickness, as long as the thick- ness of the multi-slabs is small.

For larger thicknesses, the theoretical simulation curve in FIG. 4 clearly shows a significant deviation from experimental data obtained for multi-layered BPPs.

This can be explained by an effect of re-orientation of CFs from planar orientation back to the almost-perpendicular orientation during the molding pro- cess.

This becomes possible, when the thickness of the BPPs exceeds the length of the CFs that are used in the material.

This length was 300 um in our experimental case.

In other words, if the thickness of a layer is less than the average length of the embedded carbon fibers, an increase of strength is achieved.

It is pointed out, however, that the effect is especially pronounced in sub-millimeter thickness of layers, why multi-layers are advantages for thicker slabs and corresponding thicker separator plates, such as BPPs.

When each layer in a multi-layer stack has been rolled as a separate slab prior to stacking the rolled layers into a multi-layer slab, the resulting increase of strength by the realignment of the CFs in the near surface layers is correspondingly multiplied.

DK 180950 B1 25 It is noteworthy, that our experimental 6-layer BPP with a total thickness of 0.3 mm, produced by the method described herein, not only is the thinnest graphite-based BPP in the world at the time of writing the current patent application, but also by far the strongest.

Table 1 shows collected data on thickness, flexural strength, areal specific resistance and in-plane electrical conductivity for the 6-layered MFG/SFG/CFs/CB/PPS/PTFE- based BPP as produced experimentally by the method outlined herein (named “BWT” in the table), in comparison to BPPs fabricated from graphite-based compounds with PPS binder as obtained from commercial suppliers for testing, as well as BPPs for which corresponding test data were obtained from literature sources [Ref. 15, 19, 45-48]. Table 1. Mechanical and electrical properties for a 6-layer BWT BPP, tested compara- tive ones and BPPs from literature sources Sample name Thickness | Flexural | Areal specific | In-plane electrical [mm] strength | resistance conductivity [MPa] [mQ2cm/] [S/em] Comparative #1 125" Comparative #2 1527 Fer 5 før [Ja [5 [w Fr E Fr 7 Fer = Fr ™ "ASTM D790-17 Standard [Ref. 49], “DOE ’s testing protocol [Ref. 50], ””Four-probe method [Ref. 51] It is relevant to point out that there has been obtained better results than required by DOE’s 2020 targets [Ref 8] for ultra-thin BWT BPPs. In addition, numerous

DK 180950 B1 26 advantages have been obtained as compared to other graphite-based BPPs, namely higher strength and electrical conductivity, in addition to the BPPs providing higher levels of power density because of the volume of the entire stack is reduced.

It should be pointed out in comparison with the method in WO2018/072803 [Ref. 53] that the carbon powder in this disclosure is mixed with polymer and then ground into a carbon-polymer powder. If carbon fibers would be part of such carbon mix, the grinding process would destroy much of the carbon fibers, so that the beneficial results as dis- cussed above would not be obtained.

A highlight of the features achieved by this manufacturing process, are presented by the list given below.

1. An all-in-one production process, i.e. material compounding, slab rolling, and BPP molding occurs continuously in one manufacturing line with high degree of utilization for raw components, which is unlike the method applied in [Ref. 22].

2. The use of a combination of different solvents, including water and an organic solvent, eases dispensing and compounding the various powdered materials even at room temperatures, which is advantageous relatively to processes where elevated temperatures are required, such as in [Ref. 17, 30].

3. The solid content in suspension is high in comparison with other prior art, in particular [Ref. 20], so that evaporation and drying occurs much faster.

4. The dough-like structure for the slabs is obtained by adding much smaller amount of PTFE as compared to other prior art, in particular [Ref. 22], namely 0.25 vs. 2 wt.%, 1.e. the negative effect of the polymer on the electrical properties of BPPs is reduced.

5. Softness of the slabs allows them to be rolled to and afterwards molded within a wide range of thickness, where the lower limit reaches 0.05 mm, which is very close to a theoretical value set by dimensions of the biggest components in the compound, i.e. 20 um (MFG) + 25 um (PPS).

6. The process of making slabs leads to formation of self-organized laminate struc- ture having enhanced mechanical properties that is beneficial compared to prior art, in particular [Ref. 52], where a similar structure is achievable only by means of additional coatings on a core plate.

DK 180950 B1 27

7. Applying a multi-layer design makes it possible to increase the flexural strength by 40% and, as a consequence, reduce the necessary minimum thickness by 25 % for BPPs accepted for assembly in PEM fuel cell stacks, when taking into account strength criteria.

8. Experimentally produced 0.3 mm thick multi-layer BPPs demonstrate extremely high levels of flexural strength at relatively low polymer content, namely 186 MPa at less than 18 wt.% of PPS.

9. The low amount of polymer binder, combined with the thin design, leads to an insignificant contribution of BPP resistance into the total resistance of the fuel cell.

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

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

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

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

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

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

[58] CN111048800A. A bipolar plate and preparation method thereof and a fuel cell.

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DK 180950 B1 32

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Claims

DK 180950 B1 33 PATENTKR AV

1. A castable, moldable, precursor sheet for hot pressure molding into an electrically conductive rigid separator plate for a fuel cell, wherein the precursor sheet is formed as a single layer, wherein the single layer has a thickness X1 of at least 0.05 mm and is provided as a polymer matrix comprising a thermoplastic polymer blend in which carbon fibers and electrically conductive carbon particles are dispersed; wherein the thermoplastic polymer mixture comprises polytetrafluoroethylene, PTFE, and a thermoplastic polymer other than PTFE; where the carbon fibers have an average length L in the range 0.1-1 mm; characterized in that the carbon fibers have an average length L that is at least twice as large as the thickness X1 of the layer.

2. Pre-product sheet according to claim 1, where the thermoplastic polymer different from PTFE is polyphenylene sulphide, PPS.

3. Pre-product sheet according to any one of the preceding claims, wherein the weight concentration of the carbon fibers is in the range 5-20% by weight in relation to the total weight of the polymer mixture, the carbon fibers and the electrically conductive carbon particles.

4. Preform sheet according to any one of the preceding claims, wherein the weight concentration of PTFE is at least 0.05 weight percent but less than 0.5 weight percent PTFE and wherein the weight concentration of the thermoplastic polymer other than PTFE for example PPS, is 5-30% by weight relative to the total weight of the polymer mixture, the carbon fibers and the electrically conductive carbon particles.

5. Pre-product sheet according to any one of the preceding claims, wherein the electrically conductive carbon particles in the thermoplastic polymer mixture comprise at least a first and a second portion, wherein the carbon particles in the first portion are graphite particles with an average size in the range of 10-100 µm and where the carbon particles in the second portion have a size in the range 0.1-10 µm, where the weight concentration of the first portion is in the range 50-90% by weight in relation to the total weight of the polymer mixture, the carbon fibers and the electrically conductive carbon particles, and wherein the weight ratio between the first and the second proportion of the electrically conductive carbon particles is in the range of 5-20.

DK 180950 B1 34

6. Method for producing a separator plate by: - mixing thermoplastic polymer, carbon fibers and electrically conductive carbon particles in a dispersion, where the carbon fibers have an average length L in the range 0.1-1 mm; - forming a formable single-layer pre-product sheet of thickness X1 from the mixture by calender rolling in a forming station; - hot-pressing the pre-product sheet into a single-layer separator plate with a thickness of X2 in the range from 0.05-0.6, for example 0.05-0.3 mm, where X2 is smaller than the average length L of the carbon fibers, where the method includes providing the carbon fibers with an average length L that is at least twice the thickness X2 of the layer.

7. Method according to claim 6, wherein the thermoplastic polymer mixture comprises PTFE and a thermoplastic polymer different from PTFE.

8. Method according to claim 7, wherein the thermoplastic polymer other than PTFE is polyphenylene sulphide, PPS.

9. Method according to any one of claims 6-8, wherein the method comprises: - providing an aqueous dispersion, wherein the aqueous dispersion comprises PTFE particles, carbon fibers; where the carbon fibers have an average length L; - providing a solvent dispersion, wherein the solvent dispersion comprises carbon-black particles and particles of another thermoplastic polymer different from PTFE, for example PPS, dispersed in an organic solvent; - stirring both dispersions to prevent sedimentation of the particles; - to combine and mix the two dispersions; - mixing a quantity of graphite particles with the two dispersions, the graphite particles in this quantity having an average size in the range of 10-100 µm; wherein the amount has a weight 5-20 times greater than the weight of carbon black; - to knead the mixture in a kneading machine; - during kneading in the kneader, raising the temperature to elevated temperature levels sufficiently high to evaporate the organic solvent and water from the mixture, the elevated temperature levels being above the glass transition temperature of PTFE,

DK 180950 B1 35 - after evaporation of the organic solvent and water and while the second thermoplastic polymer is in a molten state, forming the mixture into a pre-product sheet in a forming station.

10. A method according to claim 9, wherein the quantity of graphite particles is a second proportion of graphite particles, and wherein the method comprises providing the aqueous dispersion containing a first proportion of graphite particles; where the graphite particles of the first portion have an average particle size in the range 0.1-10 µm.

11. A method according to any one of claims 6-10, wherein the elevated temperature levels during kneading in the kneader are below the melting temperatures of PTFE and the second thermoplastic polymer; wherein the method comprises extracting the mixture from the kneader and then raising the temperature of the mixture to a level sufficiently high to melt the second thermoplastic polymer before the sheet is formed into a sheet of thickness X1 in the forming station.

12. Method according to claim 11, where the provision of the aqueous dispersion comprises adding a surfactant to the aqueous dispersion, where the surfactant has a boiling temperature above the boiling temperature of water and above the boiling temperature of the organic solvent; wherein the method comprises removing the mixture from the kneader while the mixture contains the surfactant but neither the solvent nor water, and then raising the temperature of the mixture to a level sufficiently high to vaporize the surfactant before the sheet is formed into a sheet of thickness X1 in the forming station.

13. Method according to any one of claims 6-12, wherein the weight concentration of PTFE in the mixture is at least 0.05 weight percent but less than 0.5 weight percent PTFE and the weight concentration of the thermoplastic polymer other than PTFE is in the range of 5 -30% by weight, where the weight concentration of the carbon fibers is 2-20% by weight, where the weight percentages are relative to the total weight of the carbon fibers and the carbon particles, the PTFE and the thermoplastic polymer.

14. A method according to any one of claims 6-13, wherein the method comprises calendering the pre-product sheet in at least two different directions to align the carbon fibers in the different directions.

DK 180950 B1 36

15. A rigid, rolled and die-cast separator plate for a fuel cell, where the separator plate is formed as a single-layer structure from a single-layer precursor sheet, where this single layer has a thickness X2 in the range 0.05-0.6, for example 0.05-0.3 mm , and is provided as a polymer matrix comprising a thermoplastic polymer mixture in which carbon fibers and electrically conductive carbon particles are dispersed; wherein the thermoplastic polymer mixture comprises PTFE and a thermoplastic polymer other than PTFE, for example PPS; where the carbon fibers have an average length L that is in the range 0.1-1 mm, where L is at least twice as large as the thickness X2 of the layer.

16. Separator plate according to claim 15, where the weight concentration of the carbon fibers is in the range of 5-20% by weight in relation to the total weight of the polymer mixture, the carbon fibers and the electrically conductive carbon particles, where the weight concentration of PTFE is at least 0.05% by weight but less than 0.5% by weight PTFE, and where the weight concentration of the thermoplastic polymer different from PTFE, for example PPS, is 5-30% by weight relative to the total weight of the polymer mixture, the carbon fibers and the electrically conductive carbon particles.

17. Separator plate according to claim 15 or 16, where the electrically conductive carbon particles in the thermoplastic polymer mixture comprise at least a first and a second portion, where the carbon particles in the first portion are graphite particles with an average size in the range from 10-100 µm and where the carbon particles in the second portion has a size in the range of 0.1-10 µm, where the weight concentration of the first portion is in the range of 50-90% by weight relative to the total weight of the polymer mixture, the carbon fibers and the electrically conductive carbon particles, and where the weight ratio of the first and the other proportion of electrically conductive carbon particles is in the range 5-20.