SEPARATOR PLATE FOR A FUEL CELL, PRECURSOR THEREFORE AND ITS METHOD OF PRODUCTION
20230197977 · 2023-06-22
Assignee
Inventors
Cpc classification
Y02P70/50
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
Y02E60/50
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
International classification
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 or multi-layer structure or multi-layer structure, where the layer thickness is less than the length of the carbon fibers.
Claims
1. A moldable, malleable, precursor sheet for hot-compression molding into an electrically conductive, rigid separator plate for a fuel cell, wherein the precursor sheet is formed as a multilayer structure with a plurality of layers in stacked condition or as a single layer, wherein the single layer or each layer in the plurality of layers of the multilayer structure has a thickness X1 and is provided as a polymer matrix that comprises a thermoplastic polymer blend in which carbon fibers and electroconductive carbon particles are dispersed; wherein the thermoplastic polymer blend comprises polytetrafluoroethylene, PTFE, and a thermoplastic polymer different from PTFE, for example PPS; wherein the carbon fibers have an average length that is larger than the thickness X1 of the layer.
2. Precursor sheet according to claim 1, wherein the carbon fibers have an average length L that is at least twice as large as the thickness X1 of the layer.
3. Precursor sheet according to claim 2, wherein the precursor sheet is formed as a single layer with a thickness X1 is in the range of 0.05-1 mm.
4. Precursor sheet according to claim 1 or 2, wherein the precursor sheet is formed as a multilayer structure, wherein the thickness X1 is in the range of 0.05-0.5 mm, optionally 0.05-0.3 mm.
5. Precursor sheet according to any one of the preceding claims, wherein the weight concentration of PTFE is at least 0.05 wt. % but less than 0.5 wt. % PTFE and wherein the weight concentration of the thermoplastic polymer different from PTFE, for example PPS, is 5-30 wt. %, relatively to the total weight of the polymer blend, carbon fibers and electroconductive carbon particles.
6. Precursor sheet according to any one of the preceding claims, wherein the electroconductive carbon particles in the thermoplastic polymer blend comprises at least a first and a second portion, wherein the carbon particles of the first portion are graphite particles with an average size in the range of 10-100 μm, and wherein the carbon particles of the second portion have a size in the range of 0.1-10 μm, wherein the weight concentration of the first portion is in the range of 50-90 wt. % relatively to the total weight of the polymer blend, carbon fibers and electroconductive carbon particles, and wherein the weight ratio between the first and the second portion of electroconductive carbon particles is in the range of 5-20.
7. Precursor sheet according to any one of the preceding claims, wherein the thermoplastic polymer is PPS.
8. Precursor composition according to any preceding claim, wherein the carbon fibers have an average length in the range of 0.1-1 mm.
9. Method of producing a separator plate by mixing thermoplastic polymer, carbon fibers, and electroconductive carbon particles in a dispersion, wherein the carbon fibers have an average length of L in the range of 0.1-1 mm; forming a moldable malleable precursor sheet of thickness X1 from the mix by calender rolling in a forming station; hot-compression molding the precursor into a single layer separator plate into a thickness of X2 in the range of 0.05-0.6, for example 0.05-0.3 mm; or cutting the sheet into slabs and stacking multiple of such slabs of thickness X1 one on top of the other in molten state into a stack and hot-compression molding the stack, wherein each layer of the plurality of layers after molding has a thickness X2 in the range of 0.05-0.2 mm; wherein X2 is less than the average length L of the carbon fibers.
10. Method according to claim 9, wherein the method comprises providing the carbon fibers with an average length L that is at least twice as large as the thickness X2 of the layer.
11. Method according claim 9 or 10, wherein the thermoplastic polymer is a blend that comprises PTFE and a thermoplastic polymer different from PTFE
12. Method according to claim 11, wherein thermoplastic polymer different from PTFE 15 polyphenylene sulfide, PPS.
13. A method according to anyone of the claims 9-12, the method comprising: providing an aqueous dispersion, the aqueous dispersion comprising PTFE particles, carbon fibers; providing a solvent dispersion, the solvent dispersion comprising carbon black partides and particles of a second thermoplastic polymer that is different from PTFE, for example PPS, dispersed in an organic solvent; stirring both dispersions for preventing sedimentation of the particles; combining and mixing the two dispersions; mixing a portion of graphite particles with the two dispersions, the graphite particles of the portion having an average size in the range of 10-100 μm; wherein the portion has a weight which is 5-20 times larger than the weight of the carbon black; kneading the mix in a kneader; during kneading in the kneader, raising the temperature to elevated temperature levels sufficiently high for evaporating the organic solvent and water from the mix, wherein the elevated temperature levels are above the glass transition temperature of PTFE, after evaporation of the organic solvent and water and while the second thermoplastic polymer is in a molten state, forming the mix into a precursor sheet in the forming station.
14. Method according to claim 13, wherein the portion of graphite particles is a second portion of graphite particles, and wherein the method comprises providing the aqueous dispersion containing a first portion of graphite particles in addition to the PTFE and carbon fibers; wherein the graphite particles of the first portion have an average particle size in the range of 0.1-10 μm.
15. Method according to any one of the claims 13-14, wherein the elevated temperature levels during the kneading in the kneader are below the melting temperatures of the PTFE and the second thermoplastic polymer; wherein the method comprises extracting the mix from the kneader, and then raising the temperature of the mix to a level sufficiently high to melt the second thermoplastic polymer before forming the sheet into a precursor sheet with thickness X1 in the forming station.
16. Method according to claim 15, wherein the providing of the aqueous dispersion comprises adding a surfactant to the aqueous dispersion, wherein 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 extracting the mix from the kneader while the mix contains the surfactant but neither the solvent nor water, and then raising the temperature of the mix to a level sufficiently high to evaporate the surfactant prior to forming the sheet into a slab with thickness X1 in the forming station.
17. Method according to any one of the claims 13-16, wherein the weight concentration of PTFE in the mix is at least 0.05 wt. % but less than 0.5 wt. % PTFE and the weight concentration of the thermoplastic polymer different from PTFE is in the range of 5-30 wt. %, wherein the weight concentration of the carbon fibers is 2-20 wt. %, the weight percentages being relatively to the total weight of the carbon fibers and carbon particles, the PTFE, and the thermoplastic polymer.
18. Method according to any one of the claims 9-17, wherein the method comprises calender rolling the precursor sheet in at least two different directions for aligning the carbon fibers in different directions.
19. A rigid, rolled and press-molded separator plate for a fuel cell, wherein the separator plate is formed as a multilayer structure with a plurality of layers in stacked condition or as a single layer from a single layer precursor sheet, wherein the single layer or each layer in the plurality of layers of the multilayer structure has a thickness X2 in the range of 0.05-0.6, for example 0.05-0.3 mm, and is provided as a polymer matrix that comprises a thermoplastic polymer blend in which carbon fibers and electroconductive carbon particles are dispersed; wherein the thermoplastic polymer blend comprises PTFE and a thermoplastic polymer different from PTFE, for example PPS; wherein the carbon fibers have an average length L that is in the range of 0.1-1 mm and larger than the thickness X2 of the layer.
20. Separator plate according to claim 19, wherein the carbon fibers have an average length L that is at least twice as large as the thickness X2 of the layer.
21. Separator plate according to claim 19 or 20, wherein the weight concentration of the carbon fibers is in the range of 5-20 wt. % relative to the total weight of the polymer blend, carbon fibers, and electroconductive carbon particles, wherein the weight concentration of PTFE is at least 0.05 wt. % but less than 0.5 wt. % PTFE and wherein the weight concentration of the thermoplastic polymer different from PTFE, for example PPS, is 5-30 wt. %, relatively to the total weight of the polymer blend, carbon fibers and electroconductive carbon particles.
22. Separator plate according to claim 19, 20, or 21, wherein the electroconductive carbon particles in the thermoplastic polymer blend comprises at least a first and a second portion, wherein the carbon particles of the first portion are graphite particles with an average size in the range of 10-100 μm, and wherein the carbon particles of the second portion have a size in the range of 0.1-10 μm, wherein the weight concentration of the first portion is in the range of 50-90 wt. % relatively to the total weight of the polymer blend, carbon fibers and electroconductive carbon particles, and wherein the weight ratio between the first and the second portion of electroconductive carbon particles is in the range of 5-20.
23. Separator plate according to anyone of the claims 19-22, wherein the separator is provided as a multilayer structure comprising a plurality of layers in stacked condition, wherein each layer of the plurality of layers has a thickness X2 in the range of 0.05-0.2 mm.
24. Separator plate according to claim 23, wherein the separator plate has an area specific resistance of at most 2 mΩ.Math.cm.sup.2 per thickness unit of 0.3 mm.
25. Separator plate according to claim 23 or 24, wherein the separator plate has a flexural strength of more than 180 MPa per thickness unit of 0.3 mm.
26. A fuel cell with a separator plate according to any one of the claims 19-25 or with a separator plate provided as a hot-press-molded plate from a precursor sheet according to anyone of the claims 1-8.
Description
SHORT DESCRIPTION OF THE DRAWINGS
[0143] The invention is explained in more detail with reference to the drawings, wherein:
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DETAILED DESCRIPTION/PREFERRED EMBODIMENT
[0150] In the production method described herein, a few partial processes are combined, namely compounding of raw powdered materials, followed by their kneading and calendering 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 advantage is believed to be achieved by this method for BPPs.
[0151] In
[0152] 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), perfluoroalkoxy alkane (PFA), polyamideimide (PAI), polychlorotrifluoroethylene (PCTFE), polyether ether ketone (PEEK), polyetherketone (PEK), polyetherimide (PEI), polyethersulfone (PES), polyphenylsulfone (PPSU), polysulfone (PSU), polyvinylidene fluoride (PVDF), see also [Ref. 25].
[0153] 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 be further diluted to a suitable concentration by mixing with deionized water. In addition, surfactants may be added.
[0154] As illustrated in
[0155] Non limiting example of surfactants are Tergitol™ 15-S Series from Dow Chemicals®, Triton® X Series from Union Carbide Corporation® or Tween® Series from Croda International®. For example, TritonX-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 C.sub.14H.sub.22O(C.sub.2H.sub.4O).sub.n, where n is 9-10.
[0156] 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 maintaining a proper dispersion.
[0157] 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.
[0158] 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 with average particle sizes in the range 0.1-10 μm, for example in the range of 0.5-2 μm. In experiments, the average size of the graphite particles of the SFG was 1 μm.
[0159] 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 dimensionally-averaged sizes have a FWHM lower than or in the order of ±20%.
[0160] In this first container 1, all ingredients are intensively stirred.
[0161] 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.
[0162] 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 μm, for example in the range of 20-30 μm, such as approximately 25 μm.
[0163] 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 approximately 40 nm.
[0164] 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.
[0165] Some other solvents can be used as alternative to NMP for this purpose, for example N,N-dimethyl acetamide, N,N-dimethylformamide, and dimethyl sulfoxide. The use of 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.
[0166] 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 consumption 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 production method takes into consideration the minimizing of the amounts of organic solvents 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.
[0167] In our case, sedimentation and agglomeration of fine PPS particles in the second container 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 impossible, 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.
[0168] After separate preparation of these two dispersions in the first container 1 and the second container 2, they are mixed, for example in a third container 3, as illustrated in
[0169] 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 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.
[0170] 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 concentration of MFG was 66.25 wt. % of the dry mass.
[0171] Optionally, this graphite for the MFG has an average particle size in the range of 10-100 μm, for example in the range of 10-30 μm. In experiments, the average size of the graphite particles of the MFG was 20 μm. Notice that the particle size in MFG is an order of magnitude larger than in SFG.
[0172] The MFG is the main carbon component in the composite.
[0173] 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 powdered materials.
[0174] 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 solvent, 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. % solubility 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 problematic to further remove it from the compound before its molding.
[0175] 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 formation.
[0176] The kneading process takes typically 10-30 min, however, depending on the speed of the temperature increase and the evaporation of the liquids.
[0177] 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.
[0178] 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.
[0179] 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.
[0180] It is brought forward that PTFE undergoes a phase change at the glass transition temperature, 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.
[0181] 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 experimental 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.
[0182] 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 manufacturing process. It is pointed out that potential small rest amounts of surfactants dissolved therein does not interfere with this.
[0183] The soft compound is after kneading extracted from the kneader 5 as a dough-like malleable material. It is extruded onto a heated Conveyor 9.
[0184] 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 T5 that causes evaporation of the surfactants. For example, the fifth temperature level T5 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 T5 of 270° C., which removed the used surfactants but did not melt the PPS.
[0185] 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. Accordingly, the temperature is increased to a level T5 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 temperature 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 viscosity, as it was done in the experiment. The final temperature, however, is depending on the molding parameters that are chosen.
[0186] 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. The process is illustrated in greater detail in
[0187] 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 malleable structure, a cutting tool 11 cuts it into slabs.
[0188] In some performed experiments, thicknesses of 0.1 and 0.6 mm were used for comparison, 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 precursor thickness, which is due to a decrease in thickness by the compression. In this connection, the following is pointed out. The slabs have a specified density, normally in the range of 0.5 to 1.5 g/cm.sup.3, such as approximately 1 g/cm.sup.3, as used in the experiments. However, the compression molded BPPs have a density slightly more than 2 g/cm.sup.3. 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.
[0189] Experimental dimensions of the slabs, apart from the thickness, are 400×100 mm, but by this method the slabs of any dimensions can be produced.
[0190] 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 advantageously in the range between the melting point of PPS (271-292° C.) and the melting point of PTFE (320-347° C.). Optionally, the temperature is in the range of 300-320° C.
[0191] Alternatively, the temperature is slightly higher than the melting point of PTFE. However, it should be below the decomposition temperature of PPS, which is around 475° C. [Ref. 37].
[0192] 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].
[0193] 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.
[0194] During molding, the applied pressure is typically in the range of 25-225 MPa, for example in the range of 75-175 MPa. In the experiments, a pressure of 125 MPa was used.
[0195] 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 temperature of PPS, i.e. less than 93° C. [Ref. 38].
[0196] 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 condition, 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.
[0197] 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.
[0198] Percentage ratios between all these components in the final mixture can be varied within some constraints. For example, with all percentages being by weight: [0199] 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. [0200] The minimal amount of PPS is 5 wt. % and will typically be less than 30%. In the experiment, 20 wt. % is was used. [0201] The total content of additives, SFG, CFs, CB, PTFE in the final compound is typically less than 45 wt. %. [0202] 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. %. [0203] 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.
[0204] All weight percentage are for given relatively to the polymer blend with the particles and fibers, thus, after removal of the liquids.
[0205] Experimentally, optimal electro-mechanical properties were demonstrated by the final compound without liquids containing the following quantities of individual components: MFG (66.25 wt. %), PPS (17.50 wt. %), CFs (10.00 wt. %), SFG (5.00 wt. %), CB (1.00 wt. %), PTFE (0.25 wt. %).
[0206] 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
[0207] 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
[0208] For ultra-thin slabs and BPPs based thereon, there appears not enough space for perpendicular 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 structure with the demonstrated exponential grow of flexural strength towards small thickness, as reflected in
[0209] 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]. Accordingly, strength should be considered in terms of distributed load as well, which is illustrated in
[0210] 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 multilayer 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
[0211] 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.
[0212] 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.
[0213] 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-US-00001 TABLE 1 Mechanical and electrical properties for a 6-layer BWT BPP, tested comparative ones and BPPs from literature sources Areal In-plane Flexural specific electrical Thickness strength resistance conductivity Sample name [mm] [MPa] [mΩ .Math. cm.sup.2] [S/cm] BWT - 6 layers 0.3 186* 2** 158*** Comparative #1 1.5 40* 7** 125*** Comparative #2 2.0 52* 24** 152*** [Ref. 15] 0.4 N/A N/A 50 [Ref. 19] 3.0 60 25 209 [Ref. 45] 2.0 39 N/A 134 [Ref. 46] N/A 65 N/A 78 [Ref. 47] N/A 56 N/A 125 [Ref. 48] 5.0 52 N/A 119 *ASTM D790-17 Standard [Ref. 49] **DOE's testing protocol [Ref. 50] ***Four-probe method [Ref. 51]
[0214] 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 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.
[0215] 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 discussed above would not be obtained.
[0216] A highlight of the features achieved by this manufacturing process, are presented by the list given below. [0217] 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]. [0218] 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]. [0219] 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. [0220] 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. %, i.e. the negative effect of the polymer on the electrical properties of BPPs is reduced. [0221] 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 μm (MFG)+25 μm (PPS). [0222] 6. The process of making slabs leads to formation of self-organized laminate structure 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. [0223] 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. [0224] 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. [0225] 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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