Method for manufacturing a composite bipolar plate, composite bipolar plate, uses thereof and fuel cell comprising such a composite bipolar plate

Abstract

A method for manufacturing a composite bipolar plate from a composition including at least one lamellar graphite and at least one thermoplastic polymer. This method includes dry sieving of the composition with a sieve for which the mesh diameter is less than or equal to 1,000 m, dry blending of the sieved composition, deposition of the blended composition in a mold, this mold preferably being pre-heated, molding by thermocompression of the blended composition with induction heating of the mold, and removal from the mold of the thermocompressed composition leading to the obtaining of the composite bipolar plate. A composite bipolar plate manufactured by this method, to the use of this composite bipolar plate as well as to a fuel cell including such a composite bipolar plate.

Claims

1. A method for manufacturing a composite bipolar plate from a composition comprising at least one lamellar graphite and at least one thermoplastic polymer, said method comprising the following successive steps: (a.sub.0) forming a composition comprising the at least one lamellar graphite and the at least one thermoplastic polymer, (a) dry sieving of the composition with a sieve for which the mesh diameter is less than or equal to 1,000 m, (b) dry blending of the sieved composition, (c) deposition of the blended composition in a mold, (d) molding by thermocompression of the blended composition with induction heating of the mold, and (e) removal from the mold of the thermocompressed composition leading to obtaining of the composite bipolar plate.

2. The manufacturing method according to claim 1, wherein the mesh diameter of the sieve is comprised between 100 m and 600 m.

3. The manufacturing method according to claim 1, wherein step (d) is carried out by applying a pressure comprised between 5.Math.10.sup.7 Pa and 15.Math.10.sup.7 Pa, the mold being heated to a temperature above the melting temperature of the thermoplastic polymer.

4. The manufacturing method according to claim 1, wherein the composition comprises, the mass percentages being based on the total mass of the composition: at least 70% of said at least one lamellar graphite, and at least 10% of said at least one thermoplastic polymer.

5. The method according to claim 4, wherein the mass percentage of lamellar graphite(s) is comprised between 75% and 90%, based on the total mass of the composition.

6. The method according to claim 4, wherein the mass percentage of thermoplastic polymer(s) is comprised between 11% and 25%, based on the total mass of the composition.

7. The method according to claim 1, wherein each lamellar graphite appears as particles for which at least 90% in number have an average particles size d.sub.90 comprised between 1 m and 300 m.

8. The method according to claim 1, wherein each thermoplastic polymer is selected from a polyolefin, a fluoropolymer and a poly(phenylene sulfide) (PPS).

9. The method according to claim 8, wherein the fluoropolymer is a polyvinylidene fluoride (PVDF) or a poly(vinylidene fluoride-hexafluoropropylene) (PVDF HFP).

10. The method according to claim 1, wherein each thermoplastic polymer appears as particles for which at least 90% in number have an average particles size d.sub.90 comprised between 1 m and 200 m.

11. The method according to claim 1, wherein each lamellar graphite and each thermoplastic polymer appear as particles for which at least 90% in number have an average particles size d.sub.90 comprised in the same interval.

12. The method according to claim 1, wherein the composition further comprises at least one non-metal electrically conductive additive.

13. The method according to claim 12, wherein each additive is selected from among an expanded graphite, a lamellar graphite, carbon black, active coal, carbon fibers, carbon nanotubes and mixtures thereof and is.

14. The method according to claim 12, wherein the mass percentage of additive(s) represent up to 10% of the total mass of the composition, is comprised between 2% and 5% based on the total mass of the composition.

15. A composite bipolar plate obtained by the method according to claim 1.

16. A fuel cell comprising at least one composite bipolar plate according to claim 15.

17. The manufacturing method according to claim 1, wherein said mold is pre-heated.

18. The method according to claim 10, wherein each thermoplastic polymer appears as particles for which at least 90% in number have an average particles size d.sub.90 comprised between 2 m and 100 m.

19. The method according to claim 10, wherein each thermoplastic polymer appears as particles for which at least 90% in number have an average particles size d.sub.90 comprised between 3 m and 50 m.

20. The method according to claim 11, wherein each lamellar graphite and each thermoplastic polymer appear as particles for which at least 90% in number have an average particles size d.sub.90 comprised between 1 m and 300 m.

21. A fuel cell according to claim 16, said fuel cell being a polymeric electrolyte membrane fuel cell (PEMFC).

22. A fuel cell according to claim 16, said fuel cell being a direct methanol fuel cell (DMFC).

Description

SHORT DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates the curves expressing the time-dependent change in the temperature (noted as T and expressed in C.) and in the pressure (noted as P and expressed in Pa) depending on time (noted as t and expressed in s), during phases A to G of the manufacturing cycle of a composite bipolar plate from the composition 6, subject beforehand to dry sieving and then to dry blending.

(2) FIG. 2 illustrates the time-dependent change in the planar electric conductivity (noted as .sub.// and expressed in S/cm) of bipolar plates manufactured from compositions 1 to 4, depending on the mass percentage of lamellar graphite (noted as P.sub.G and expressed in %) present in said compositions 1 to 4.

(3) FIG. 3 illustrates the time-dependent change in the planar electric conductivity (noted as .sub.// and expressed in S/cm) of the bipolar plates manufactured from compositions 5 to 7, depending on the mass percentage of additive (noted as P.sub.A and expressed in %) present in said compositions 5 to 7.

(4) FIG. 4 illustrates the time-dependent change in the planar electric conductivity (noted as .sub.// and expressed in S/cm) and of the rejection level to the gas imperviousness test (noted as Rejected and expressed in %) of the composite bipolar plates manufactured from compositions 8 to 12, depending on the mass percentage of lamellar graphite (noted as P.sub.G and expressed in %) present in said compositions 8 to 12.

DETAILED DISCUSSION OF PARTICULAR EMBODIMENTS

(5) I Compounds Used

(6) Within the scope of the present example, the following compounds were used: the lamellar graphites are synthetic lamellar graphites so-called primary synthetic graphites, marketed by Timcal under references Timrex KS 75 and Timrex KS 150, for which at least 90% of the particles in number have an average particles size, noted as d.sub.90, of 55.8 m for Timrex KS 75 (noted as KS 75 hereafter) and of 180 m for Timrex KS 150 (noted KS 150 hereafter), the additive is an expanded graphite, marketed by Timcal under reference Timrex BNB90 (noted as BNB90 hereafter), which has an average particles size d.sub.90 of 85 m, and the thermoplastic polymer is a polyvinylidene fluoride, marketed by Arkema under reference Kynar301-F (noted as PVDF hereafter), which is characterized by a viscosity in the molten condition comprised between 28.0 kP and 34.0 kP at a temperature of 232 C. and under a shearing rate of 100/s (measured according to the ASTM D3835 standard), with a hot fluidity index comprised between 2.0 g/10 min and 6.0 g/10 min at a temperature of 232 C. and under a load of 21.6 kg (measured according to the ASTM D1238 standard) as well as with a melting point comprised between 155 C. and 165 C., this polymer further appearing as a powder for which the particles size d.sub.90 is of about 5.0 m.
II Preparation of the Compositions and then of the Blended Compositions

(7) Compositions 1 to 12 were prepared from compounds described in chapter I, in mass proportions indicated in Tables 1 and 2 below:

(8) TABLE-US-00001 TABLE 1 Composition 1 2 3 4 5 6 7 PVDF (in %) 15 15 10 10 14 14 14 KS 75 (in %) 85 90 KS 150 (in %) 85 90 84 82.5 81 BNB90 (in %) 2 3.5 5

(9) TABLE-US-00002 TABLE 2 Composition 8 9 10 11 12 PVDF (in %) 15 14 13 12 10 KS 150 (in %) 80 81 82 83 85 BNB90 (in %) 5 5 5 5 5

(10) The blended compositions obtained from the compositions 1 to 12 as detailed in Tables 1 and 2 above were prepared according to a same procedure which is described hereafter, with reference to the preparation of the blended composition 6.

(11) To produce 10 kg of the blended composition 6, the following amounts of compounds were weighed: 8,250 g (0.5 g) of KS 150, 350 g (0.5 g) of BNB90, and 1,450 g (0.5 g) of PVDF.

(12) After manual pre-blending for a period of 1 min of the amounts mentioned above, it was proceeded with dry sieving of the composition 6 by means of an automatic sieve Ro-Tap RX 29-10 available from Tyler. This automatic sieve is equipped with a sieve having a mesh diameter of 300 m as well as with a unclogging device. This dry sieving step lasted for 45 min.

(13) After the dry sieving step, it was proceeded with dry blending of the sieved composition 6 by means of a blender Turbula T50 A available from Wab, the dry blending step being conducted under a speed of rotation of 40 revolutions/minute, for a period of 20 mins. It is specified that this duration of 20 mins gives the possibility of giving an optimum dispersion of the three compounds in the corresponding blended composition 6 thus obtained, notably as compared with a shorter duration which would be of 10 min for a same obtained amount of blending composition (in this case, 10 kg).

(14) At the end of this dry blending step, a blended composition 6 was obtained appearing as a homogenous dry blend, this homogeneity being observable at a sub-millimeter scale as defined above.

(15) III Manufacturing of Composite Bipolar Plates from the Compositions

(16) The composite bipolar plates made from the compositions 1 to 5 and 7 to 12 were manufactured according to an identical manufacturing cycle with the one which is described hereafter for the manufacturing of a composite bipolar plate from composition 6.

(17) The various phases A to G of the manufacturing cycle are described in detail hereafter, with reference to FIG. 1. FIG. 1 moreover illustrates the time-dependent change in the temperature and in the applied pressure versus time.

(18) The blended composition 6, as obtained at the end of the dry sieving and then dry blending steps, is deposited in the imprint of a mold brought beforehand to a temperature of 80 C. by means of induction heating. This deposition phase corresponds to the phase A illustrated in FIG. 1.

(19) During the manufacturing cycle described here, the deposition of an amount weighed beforehand of the blended composition 6 in the mold was carried out manually.

(20) During phase B, it is preceded with the spreading of the blended composition 6 so as to obtain a regular distribution of this composition in the cavity of the mold.

(21) The mold is then closed and brought, within 40 s, to a temperature of 210 C., i.e. at a temperature 55 C. above the melting temperature of PVDF. This rise in temperature of the mold, which corresponds to phase C of FIG. 1, is carried out as rapidly as possible facing the configuration of the inductors, the shape of the composite bipolar plate to be manufactured and the power of the generators.

(22) It is specified that the temperature applied to the mold should remain compatible with the maximum admissible temperature which may be imposed by a surface treatment, such as an anti-adherent treatment, practiced at the imprint of the mold. In the case of the mold used during the present manufacturing cycle, the maximum admissible temperature is 220 C.

(23) During phase D, which follows phase C, an isothermal plateau is achieved with a duration of 1 s at 210 C. before applying pressure to the mold as well as to the composition 6 which it contains, which corresponds to phase E.

(24) During phase E, it is preceded with a gradual rise in the applied pressure up to a rated applied pressure value of 9.4.10.sup.7 Pa, the temperature being still maintained at 210 C. This gradual rise is achieved within 5 s, by increasing the applied pressure at a rate of 300 kN/s.

(25) During phase F, which follows phase E, an isothermal and isobaric plateau is achieved with a duration of 10 s at 210 C. and under the rated pressure value of 9.4.10.sup.7 Pa.

(26) At the end of phase F, which is located at a time of 150 s after the beginning of phase A for depositing the blended composition 6 into the cavity of the mold, the applied pressure is released and the temperature of the mold is lowered down to a value of 100 C., a temperature at which the obtained composite bipolar plate from the blended composition 6 is extracted from the mold, for example by ejection. This phase G, which begins with the release of the applied pressure and ends with the extraction of the composite bipolar plate, corresponds to the cooling step for the mold and to a duration of about 110 s.

(27) It should be noted that for the composite bipolar plates manufactured from the compositions 1 to 12, it is necessary to lower the temperature of the mold until a value of 100 C. is attained, at most, for allowing extraction of the composite bipolar plates without deformation of these plates, in particular when it is proceeded with this extraction by ejection.

(28) It is observed that the manufacturing cycle, formed by the successive phases A to G, is achieved within a duration of less than 5 min, it being specified that the overall time of the manufacturing method taking into account the pre-blending, dry sieving and dry blending steps of the composition is established to less than 75 min.

(29) IV Evaluation of the Planar Electric Conductivity

(30) The evaluation of the planar electric conductivity was carried out by the 4-point method by applying the semi-infinite model, the thickness being considered as infinite relatively to the spacing of the tips. The planar electric conductivity, noted as .sub.// and expressed in S/cm, is inferred from the following formula:

(31) ${}_{//}=\frac{I}{2\mathrm{sU}}$
wherein I is the intensity

(32) s is the spacing between the tips, and

(33) U is the voltage.

(34) For evaluating this planar electric conductivity, composite bipolar plates were manufactured, according to the manufacturing cycle described earlier in chapter III, from the compositions 1 to 7 prepared according to the procedure described in chapter II above.

(35) FIG. 2 illustrates the time-dependent change in the planar electric conductivity, noted as .sub.//, of the composite bipolar plates manufactured from the compositions 1 to 4, depending on the mass percentage of lamellar graphite present in said compositions 1 to 4.

(36) In addition to confirming the expected increase in the planar electric conductivity of the composite bipolar plates with the mass percentage of lamellar graphite, the bar diagrams of FIG. 2 show that, for a same mass percentage of lamellar graphite, better planar electric conductivity is obtained for the composite bipolar plates manufactured from the compositions 2 and 4 comprising the lamellar graphite Timrex KS 150 than for the composite bipolar plates manufactured from the compositions 1 and 3 which comprise the lamellar graphite Timrex KS 75.

(37) FIG. 3 illustrates the time-dependent change in the planar electric conductivity .sub.// of the composite bipolar plates manufactured from the compositions 5 to 7, depending on the mass percentage of expanded graphite present in said compositions 5 to 7. It is specified that these compositions 5 to 7 all comprise a total mass percentage of 86% of a mixture of lamellar graphite (Timrex KS 150) and of expanded graphite (Timrex BNB90), with variable respective percentages of lamellar graphite and of expanded graphite, as indicated in table 1 above.

(38) The observation of the curve of FIG. 3 shows values of planar electric conductivity for the composite bipolar plates manufactured from the compositions 5 to 7 which range between 77 S/cm and 84 S/cm, such values being greater than the value of 74 S/cm which is obtained for the planar electric conductivity of the composite bipolar plate manufactured from composition 2 which only comprises lamellar graphite Timrex KS 150, in a comparable mass percentage of 85%.

(39) This comparison shows the fact that by substitution of expanded graphite for lamellar graphite, even in a reduced mass percentage, and notably comprised between 2% and 5% based on the total mass of the composition, it is possible to optimize the planar electric conductivity properties. Indeed, the presence of expanded graphite promotes the formation of electrically conducting bridges within the composite bipolar plates manufactured from compositions comprising such an additive.

(40) FIG. 3 moreover shows that the maximum planar electric conductivity value is attained for a mass percentage of 3.5% of expanded graphite. In other words, by a substitution of the order of 3.5% by mass of the lamellar graphite with expanded graphite it is possible to manufacture a composite bipolar plate having optimized electric conductivity properties, in particular of planar electric conductivity.

(41) V Evaluation of the Planar Electric Conductivity and of the Sealing Properties

(42) The evaluation of the planar electric conductivity was achieved according to the method specified in Chapter IV above.

(43) The gas imperviousness properties were evaluated by means of a sealed cell in which each plate was inserted, so as to define two chambers, an upstream chamber and a downstream chamber. By means of a manometer, a pressure of 10.sup.5 Pa of dinitrogen N.sub.2 is applied in the upstream chamber and then the time-dependent change in the difference of the respective pressures of the upstream and downstream chambers is tracked. It is considered that a plate is impervious, i.e. that it does not have any network of open porosities, when this difference of respective pressures of the upstream and downstream chambers is less than or equal to 10.sup.3 Pa. Beyond this threshold value, the plate is rejected. A rejection level is thus determined which corresponds to the number of non-impervious plates based on the total number of tested plates, it being specified that for each composition 8 to 12, a total number of 20 plates was subject to the present imperviousness test.

(44) For evaluating the planar electric conductivity as well as the gas imperviousness properties, composite bipolar plates were manufactured according to the manufacturing cycle described earlier in chapter III, from compositions 8 to 12, prepared according to the procedure described in chapter II above.

(45) FIG. 4 illustrates the time-dependent change in the planar electric conductivity, noted as .sub.//, as well as the rejection level to the gas imperviousness test, noted as Rejected, of the composite bipolar plates manufactured from the compositions 8 to 12, depending on the mass percentage of lamellar graphite present in said compositions 8 to 12.

(46) As already observed above for FIG. 2, FIG. 4 shows that the planar electric conductivity of the composite bipolar plates gradually increases with the mass percentage of lamellar graphite. On the other hand, it is observed that the rejection level to the gas imperviousness test also increases gradually according to this same mass percentage of lamellar graphite. In other words, if the increase in the mass percentage of lamellar graphite and of expanded graphite gives the possibility of improving the planar electric conductivity, this increase is accomplished to the detriment of the gas imperviousness properties of the composite bipolar plate.

(47) Therefore, it appears necessary to limit the mass percentage of lamellar graphite and, if necessary, of electrically conductive additive(s) like expanded graphite, in order to meet the whole of the specifications of the requirements, notably those imposed by the planar electric conductivity and for the rejection level to the gas imperviousness test for which a maximum value of 5% is conventionally retained.

(48) The inventors were thus able to show, that from the compounds listed in chapter I, a composition comprising mass percentages of 14% of PVDF and of 86% of lamellar graphite Timrex KS150 alone, or for which a mass percentage ranging up to 10% may be substituted with expanded graphite Timrex BNB90, gives the possibility of manufacturing composite bipolar plates which simultaneously meet the planar electric conductivity specifications and of the rejection level to the gas imperviousness test conventionally set by the requirements.

(49) VI Optimization of the Parameters of the Manufacturing Cycle

(50) Tests were conducted for determining which are the parameters of the manufacturing cycle which allow optimization of the planar electric conductivity and of the rejection level to the gas imperviousness test of a composition according to the invention.

(51) The parameters (a) to (g) evaluated during these tests are the following: (a) the temperature for pre-heating the mold as established before starting the manufacturing cycle and the deposition of the composition into the imprint of the mold, (b) the temperature for removal from the mold which corresponds to the temperature attained by the mold during the extraction of the composite bipolar plate and which corresponds to the term of phase G, (c) the temperature of the isothermal plateau of phase D, (d) the duration of this isothermal plateau of phase D, (e) the rate of the increase in the applied pressure during phase E, (f) the duration of the isothermal and isobaric plateau of phase F, and (g) the applied rated pressure at the term of phase E and during phase F.

(52) These evaluations showed the fact that the parameters of the pre-heating temperature conducted at 80 C. and at 100 C. (a), of the mold-removal temperature conducted at 80 C. and at 100 C. (b) and of the rate of increase in the applied pressure (e) does not have any actual influence, whether this is on the planar electric conductivity values or on the rejection level values to the gas imperviousness test of the composite bipolar plates manufactured from composition 6.

(53) On the contrary, as shown by the results reported in Table 3 below, the parameters which are the isothermal plateau temperature of phase D (c), the duration of this isothermal plateau of phase D (d), the duration of the isothermal and isobaric plateau of phase F (f) as well as the rated pressure applied at the term of phase E and during phase F (g) have an impact on these planar electric conductivity values (noted as .sub.//) and on the rejected level to the gas imperviousness test.

(54) TABLE-US-00003 TABLE 3 Parameter (c) Parameter (f) Temperature Parameter (d) Duration of of the Duration of the isobaric Parameter (g) isothermal the isothermal and isothermal Applied rated plateau plateau plateau pressure .sub.ll Rejection phase D ( C.) phase D (s) phase F (s) phase F (Pa) (S/cm) (%) 210 30 60 9 .Math. 10.sup.7 95 10 190 30 60 9 .Math. 10.sup.7 73 0 210 10 60 9 .Math. 10.sup.7 85 0 190 10 60 9 .Math. 10.sup.7 87 0 210 30 10 9 .Math. 10.sup.7 88 40 190 30 10 9 .Math. 10.sup.7 83 0 210 10 10 9 .Math. 10.sup.7 97 0 190 10 10 9 .Math. 10.sup.7 82 0 210 30 60 7.5 .Math. 10.sup.7 89 10 190 30 60 7.5 .Math. 10.sup.7 80 40 210 10 60 7.5 .Math. 10.sup.7 93 0 190 10 60 7.5 .Math. 10.sup.7 86 0 210 30 10 7.5 .Math. 10.sup.7 100 0 190 30 10 7.5 .Math. 10.sup.7 80 30 210 10 10 7.5 .Math. 10.sup.7 97 30 190 10 10 7.5 .Math. 10.sup.7 80 30

(55) Table 3 shows that the planar electric conductivity of the composite bipolar plates manufactured from the composition 6 is, quasi systematically, much higher when the temperature of the isothermal plateau applied during the phase D (c) is of 210 C. rather than 190 C., i.e. when this temperature is itself a high temperature.

(56) The other parameters (d), (f) and (g), as for them, have a less pronounced impact on the planar electric conductivity values. However, Table 3 shows, that in order that the rejection level to the gas imperviousness test be lower, these parameters are, by order of decreasing importance: an applied rated pressure (g) during phase F of 9.10.sup.7 Pa rather than 7.5.10.sup.7 Pa, i.e. higher, a isothermal plateau duration (d) during phase D, of 10 s rather than 30 s, i.e. shorter, and an isothermal and isobaric plateau duration (f) during phase F, of 60 s rather than 10 s, i.e. longer.

(57) In order to take into account the specifications of the requirements as well as the more demanding productivity constraints, with the composition 6, the optimal operating conditions are: an isothermal plateau temperature (c) during phase D of 210 C., an applied rated pressure (g) during phase F of 9.10.sup.7 Pa, an isothermal plateau duration (d) during phase D, of 10 s, and an isothermal and isobaric plateau duration (f) during phase F, of 10 s.

BIBLIOGRAPHY

(58) [1] U.S. Pat. No. 7,494,740