CH719959A1 - Bipolar plate for fuel cell and its manufacturing process. - Google Patents

Abstract

The invention relates to a bipolar plate (1) for a fuel cell made from a composite material comprising one or more thermoplastic polymers, a first filler comprising carbon in one or more allotropic forms chosen from graphite, carbon black, fibers of carbon and carbon nanotubes, and a second filler comprising titanium nitride, with a total content by weight for the thermoplastic polymer(s) of between 5 and 40%, preferably between 10 and 30%, more preferably between 15 and 25%, with a total content by weight for the first charge and the second charge of between 60 and 95%, preferably between 70 and 90%, more preferably between 75 and 85%. The invention also relates to the method of manufacturing said bipolar plate.

Description

Technical field of the invention

[0001] The present invention relates to a composite material for a bipolar plate and to the method of manufacturing said plate.

Technology background

[0002] Bipolar plates for fuel cells must meet several criteria including good electrical conductivity and good corrosion resistance. Polymer-based composites are promising materials for bipolar plates due to their low weight and high corrosion resistance. In the literature, the addition of different types of carbon fillers with diverse physical and electrical properties, including graphite (Gr), carbon black (CB), carbon fibers (CF), expanded graphite (EG ) and carbon nanotubes (CNT), has proven to be essential for the development of bipolar plates. The synergistic effect between the different fillers increases the electrical conductivity values to the desired levels while maintaining good mechanical properties. However, these fillers combined with a polymer matrix still exhibit low electrical conductivity. In addition, it requires a high amount of fillers. This high quantity leads to a very viscous composite during processing where a compression molding process is required, which is not suitable for large quantity production.

Summary of the invention

The object of the present invention is to remedy the aforementioned disadvantages by proposing a new composite material for bipolar plates combined with a new and rapid process.

[0004] To this end, the present invention proposes a bipolar plate comprising a polymer matrix, a first electrically conductive filler and a second electrically conductive filler, with said second electrically conductive filler chosen to improve corrosion resistance.

[0005] More precisely, the present invention relates to a bipolar plate for a fuel cell made from a composite material comprising one or more thermoplastic polymers, a first electrically conductive filler comprising carbon in one or more allotropic forms chosen from graphite, carbon black, carbon fibers and carbon nanotubes, and a second electrically conductive filler comprising titanium nitride, with a total content by weight for the thermoplastic polymer(s) of between 5 and 40%, preferably between 10 and 30%, more preferably between 15 and 25%, with a total content by weight for the first electrically conductive filler and the second electrically conductive filler of between 60 and 95%, preferably between 70 and 90%, more preferably between 75 and 85%. Preferably, the polymer is polypropylene, the first filler is graphite and the second filler is titanium nitride.

[0006] These composites meet high requirements in terms of electrical and thermal conductivity, chemical resistance, mechanical properties and processing. In-plane and across-plate electrical conductivity, flexural properties, and corrosion resistance were measured for a composite material comprising a polypropylene matrix and graphite and titanium nitride fillers and very good results were obtained. Additionally, composites with high rates of titanium nitride fillers showed high corrosion resistance.

[0007] These results pushed the properties of the bipolar plates above the target values defined by the US Department of Energy (US-DoE) and showed very good implementation via a compression-injection molding process. having the specificity, according to the invention, of integrating heating and cooling means at the level of the mold to modulate the temperature.

[0008] Indeed, materials with a thermoplastic matrix have considerable advantages such as recyclability, short cycle times and high resistance to breakage. They also offer alternative assembly processes such as welding, making them a very attractive option for automated high-volume production. However, for the composite bipolar plates according to the invention, the filler content is generally greater than 80% by weight, which increases the viscosity of the material and makes it impossible to process with the conventional injection molding process.

[0009] Increasing the temperature of the mold is the best way to solve the problems of molding high viscosity materials. However, by increasing the mold temperature, the cycle time will also increase significantly. Therefore, the rapid thermal cycle molding process is introduced in the present invention to improve the quality of parts within a reasonable cycle time. The variothermal process consists of increasing the temperature of the mold during the injection and compression phase, and cooling the mold at the end of the injection and compression. This technology made it possible to overcome the filling problems due to the high viscosity of the composite materials according to the invention. This variothermal injection-compression molding process is suitable for manufacturing large batches with a high production rate, typically 3 plates per minute.

[0010] More precisely, the manufacturing process comprises the following steps: – Provision of a mixture of powders comprising the thermoplastic polymer(s), the first electrically conductive filler and the second electrically conductive filler having compositions and contents such as described above, – Injection and compression of said mixture in a mold arranged following a feed sheath, said feed sheath being provided with first heating means and said mold being provided with second heating means allowing to increase the temperature of the mixture in a range between +5°C and +20°C, preferably between +5°C and +15°C and more preferably between +7°C and +13°C, relative to at the temperature of the mixture in the feed sheath, said mold also being provided with cooling means making it possible to cool the injected and compressed mixture after heating with said second heating means.

[0011] The desired properties are thus achieved by combining the choice of a specific composite material with a molding process with a variable injection-compression temperature.

Other characteristics and advantages of the present invention will appear in the following description of a preferred embodiment, presented by way of non-limiting example with reference to the appended drawings.

Brief description of the figures

[0013] Figure 1 represents a bipolar plate made with the composite material and the manufacturing process according to the invention.

[0014] Figure 2 schematically represents the device for implementing the manufacturing process according to the invention with in Figure 3 two schematic views of the material being injected and compressed, the heating and cooling means not being shown in Figure 3.

[0015] Figure 4 represents a graph with the results of electrical conductivity measurements across the plate according to the invention.

[0016] Figure 5 represents a graph with the results of electrical conductivity measurements in the plane of the plate according to the invention.

[0017] Figure 6 represents a graph with the results of the bending resistance measurements of the plate according to the invention.

[0018] Figure 7 represents a graph with the results of corrosion resistance measurements.

Detailed description of the invention

The present invention relates to a bipolar plate 1 illustrated in Figure 1 with the channels 1A stamped into the plate which ensure the distribution of gases and the evacuation of water. It is intended to equip a fuel cell. According to the invention, this bipolar plate is made of a composite material comprising by volume a polymer matrix with one or more thermoplastic polymers and two types of electrically conductive fillers such as graphite having an electrical resistivity less than 3.10^-3 Ω.m and titanium nitride with an electrical resistivity less than 2.5.10^-7 Ω.m. Preferably, the composite material comprises one or more polymers chosen from polypropylene (PP), polyphenylene sulfide (PPS), polyamide (PA), polyetherimide (PEI), polyethersulfone (PES) and polyetheretherketone (PEEK). A first electrically conductive charge consists of carbon in one or more allotropic forms chosen from graphite, carbon black, carbon fibers and carbon nanotubes, the term graphite also including expanded graphite. The composite material has a second electrically conductive filler including titanium nitride (TiN). Titanium nitride has a particle size of less than 3 µm, or even less than 1 µm.

The total percentage by weight of polymers is between 5 and 40%, preferably between 10 and 30%, more preferably between 15 and 25%, with a total content by weight for the first electrically conductive filler and the second filler. electrically conductive between 60 and 95%, preferably between 70 and 90%, more preferably between 75 and 85%.

More precisely, the percentage by total weight of carbon in one or more forms is between 20 and 90%, preferably between 35 and 80%. The total weight percentage of titanium nitride is between 2 and 50%, preferably between 4 and 45% by weight. Preferably, the composite comprises one or more polymers chosen from polypropylene (PP), polyphenylene sulfide (PPS), polyamide (PA), polyetherimide (PEI), polyethersulfone (PES) and polyetheretherketone (PEEK), titanium nitride (TiN) and graphite. More preferably, the composite consists of the polymer(s) chosen from polypropylene (PP), polyphenylene sulfide (PPS), polyamide (PA), polyetherimide (PEI), polyethersulfone (PES) and polyetheretherketone (PEEK). , titanium nitride and graphite and possible impurities with a content for the latter less than or equal to 1% by weight. More preferably, the composite comprises polypropylene with a weight percentage of between 5 and 40%, preferably between 10 and 30%, more preferably between 15 and 25%, titanium nitride with a weight percentage of between 2 and 50 %, preferably between 4 and 45% and graphite with a weight percentage of between 20 and 90%, preferably between 35 and 80%. Even more preferably, the composite consists of polypropylene with a percentage by weight of between 5 and 40%, preferably between 10 and 30%, more preferably between 15 and 25%, of titanium nitride with a percentage by weight of between 2 and 50%, preferably between 4 and 45%, graphite with a percentage by weight of between 20 and 90%, preferably between 35 and 80% and possible impurities with a total content for the latter less than or equal to 1 % in weight.

The process for manufacturing the bipolar plate is an injection-compression molding process illustrated in Figures 2 and 3. According to the invention, the injection and compression are carried out with a variable temperature. We speak in English of „Varie thermal injection-compressing molding“. Conventionally, the device 2 comprises heating means 3, called first heating means, upstream of the injection-compression mold 4. The material 5, which will also be described as a mixture, is thus heated continuously as it passes through the sheath 6, at the level of the screw 10. According to the invention, the mold 4 is also provided with second heating means 7 as well as cooling means 8. Typically, the first heating means consist of resistive heating and the second heating means consist of a circuit of coils in which hot oil circulates. According to the invention, the second heating means make it possible to increase the temperature in a range between +5°C and +20°C compared to the temperature obtained with the first heating means. Preferably, said range is between +5°C and +15°C and more preferably between +7°C and +13°C, i.e. around +10°C. Then, after compression, the blank is cooled quickly to reduce cycle time. The cooling means are typically water cooling means. The cooling rate is preferably in the range 5-10°C/s.

[0023] Tests were carried out on the compositions given in table 1 with a polypropylene matrix, titanium nitrides and graphite.

The materials mentioned above were mixed starting from powders (HAAKE™ Rheomix OS Lab Mixer from Thermo Fischer). Polypropylene powders are supplied by Eltex® P and have a particle size of 0.5 mm. TIN particles are supplied by Sigma-Aldrich and are less than 3 µm in size. Graphite is also supplied by Sigma-Aldrich and has a particle size of less than 100 µm. The tests were carried out by injection-compression molding with a mold temperature 10°C higher than that of the temperature at the sheath with respective values of 220°C and 230°C. The product produced is a plate.

[0025] Electrical conductivity measurements on the surface of the plates and through the plates were carried out. Bending strength measurements as well as corrosion resistance tests were also carried out.

Electrical conductivity measurements across the plate

[0026] For the characterization of the conductivity through the plate, a digital micrometer (MI 3250 MicroOhm 10A from METREL) was used to measure the resistance. The cross-sectional area of the gold-coated copper electrodes was 6.25 cm<2>(25 mm × 25 mm) while plates of different thicknesses were tested. A pressure of 1.6 MPa was applied. The measured resistance was Rtot given by equation (1): Rtot= Rapp+ Rplaque(1) where Rplaque is the resistance of the plate and Rapp is the contribution of all the resistances which are not the overall resistance of the plate. This includes the contact resistances between the electrodes and the surface of the plate, as well as the resistance of the device itself. Rplate, the overall resistance of the plate, depends on the resistivity of the material, ρmat, the thickness t of the plate, and the surface A of the electrode (Eq. (2)): Rplate= ρmat- t / A (2)

[0027] By inserting equation (2) into equation (1), we obtain: Rtot= Rapp+ ρmat- t / A (3)

[0028] Thus, according to equation (3), the plot of the total resistance of the plates of different thicknesses as a function of t gives a straight line with slope ρmat/A and an ordinate at the origin of Rapp. The conductivity of the material can then be obtained from the slope according to equation (4): σmat= 1/ρmat(4)

The method presented above is adapted from Cunningham et al. [Cunningham N, Lefèvre M., Lebrun G., Dodelet J.P., Journal of Power Sources 2005;143:93-102] and makes it possible to separate the contributions of resistance as a property of the material from other phenomena. The electrical conductivity of the through plane is presented in Figure 4 with increasing results with the content of titanium nitrides.

Electrical conductivity measurements on the surface of the plate

The measurements were carried out using the 4 pole method. The results are presented in Figure 5 with also an improvement with the increase in the content of titanium nitrides.

Mechanical characterization: resistance to bending

The flexural properties of the plates were measured with the 3-point test according to the DIN EN ISO 178 standard. The results are given in Figure 6 with the best results for the highest graphite content.

Corrosion resistance

The results of the anodic corrosion current values are presented in Figure 7.

[0033] The anodic corrosion current for composites based on polypropylene with graphite and titanium nitride is slightly higher than the US-DoE target (US-DoE target < [10] ^(-6) [A cm] ^( -2)). This result underlines the advantage of a polymeric system based on titanium nitride whose corrosion temperature is greater than 500°C and based on graphite compared to metallic bipolar plates which have low corrosion resistance.

Claims (8)

1. Bipolar plate (1) for a fuel cell made from a composite material comprising one or more thermoplastic polymers, a first filler comprising carbon in one or more allotropic forms chosen from graphite, carbon black, carbon fibers and carbon nanotubes, and a second filler comprising titanium nitride, with a total content by weight for the thermoplastic polymer(s) of between 5 and 40%, preferably between 10 and 30%, more preferably between 15 and 25%, with a total content by weight for the first charge and the second charge of between 60 and 95%, preferably between 70 and 90%, more preferably between 75 and 85%. 2. Bipolar plate (1) according to the preceding claim, characterized in that the weight content for the first charge is between 20 and 90% and the weight content for the second charge is between 2 and 50%. 3. Bipolar plate (1) according to the preceding claim, characterized in that the weight content for the first charge is between 35 and 80% and the weight content for the second charge is between 4 and 45%. 4. Bipolar plate (1) according to one of the preceding claims, characterized in that the polymer(s) are chosen from polypropylene, polyphenylene sulfide, polyamide, polyetherimide, polyethersulfone and polyetheretherketone. 5. Bipolar plate (1) according to one of the preceding claims, characterized in that the composite material comprises polypropylene with a percentage of between 5 and 40%, preferably between 10 and 30%, more preferably between 15 and 25% , titanium nitride with a percentage of between 2 and 50%, preferably between 4 and 45% and graphite with a percentage of between 20 and 90%, preferably between 35 and 80%. 6. Method for manufacturing the bipolar plate (1) for a fuel cell according to one of the preceding claims, comprising the steps of: – Provision of a mixture (5) of powders comprising the thermoplastic polymer(s), the first filler and the second filler having compositions and contents according to one of the preceding claims, – Injection and compression of said mixture (5) in a mold (4) arranged following a feed sheath (6), said feed sheath (6) being provided with first heating means (3) and said mold (4) being provided with second heating means (7) making it possible to increase the temperature of the mixture (5) in a range between +5°C and +20°C, preferably between +5°C and +15 °C and more preferably between +7°C and +13°C, relative to the temperature of the mixture (5) in the feed sheath (6), said mold (4) being further provided with cooling means ( 8) making it possible to cool the mixture (5) injected and compressed after heating with said second heating means (7). 7. Manufacturing process according to the preceding claim, characterized in that the range is between +5°C and +15°C and more preferably between +7°C and +13°C. 8. Manufacturing method according to claim 6 or 7, characterized in that the second heating means consist of a circuit of coils in which oil circulates.