Magnesium phosphate cement based bipolar plate composite material

Abstract

This invention provides a composite material for bipolar plates for fuel cells including cemented by a MPC binder and electrically conductive fillers, and a method of manufacturing the same. The resulting bipolar plate achieves low gas permeability, high electrical conductivity, high flexural strength and good corrosion resistance. The flexural strength and corrosion resistance can further be enhanced by the incorporation of macro-reinforcement and a polymer based surface treatment, respectively.

Claims

1. A method that produces a composite bipolar plate, the method comprising: mixing magnesium phosphate cement (MPC) raw materials, electrically conductive fillers, and water to produce a wet powder; placing a macro-reinforcement net in a steel mold; transferring the wet powder into the steel mold; hot-pressing the wet powder in the steel mold to produce a plate; and curing the plate by air to produce the composite bipolar plate, wherein a flexural strength of the composite bipolar plate is greater than 25 MPa.

2. The method of claim 1, wherein the macro-reinforcement net is an acrylonitrile butadiene styrene (ABS) co-polymer net.

3. The method of claim 1, wherein mixing of the MPC raw materials, the electrically conductive fillers, and the water are conducted in an automatic mechanical grinding setup.

4. The method of claim 1 further comprising: designing the steel mold according to a shape and flow field of a fuel cell device that is made of the composite bipolar plate.

5. The method of claim 1 wherein the hot-pressing is conducted under a compressive pressure of 70 MPa and a temperature of up to 140 C.

6. The method of claim 1, wherein the MPC raw materials include magnesia, potassium di-hydrogen phosphate, borax, fly ash, and water, and the electrically conductive fillers include graphite, carbon fibers, and carbon nanotubes.

7. A method that produces a composite bipolar plate, the method comprising: mixing magnesium phosphate cement (MPC) raw materials, electrically conductive fillers, and water to produce a wet powder; transferring the wet powder into a steel mold; hot-pressing the wet powder in the steel mold to produce a plate; converting the plate into an organic-inorganic interpenetrated product; and curing the organic-inorganic interpenetrated product by air to produce the composite bipolar plate, wherein a corrosion current density of the composite bipolar plate is 110.sup.6 A/cm.sup.2.

8. The method of claim 7, further comprising: heating the plate to a mold temperature to produce a heated plate; and cooling the heated plate to produce the organic-inorganic interpenetrated product, wherein the mold temperature is higher than a melting point of an ultra-high molecular weight polyethylene.

9. The method of claim 7, wherein mixing of the MPC raw materials, the electrically conductive fillers, and the water are conducted in an automatic mechanical grinding setup.

10. The method of claim 7 further comprising: designing the steel mold according to a shape and flow field of a fuel cell device that is made of the composite bipolar plate.

11. The method of claim 7 wherein the hot-pressing is conducted under a compressive pressure of 70 MPa and a temperature of up to 140 C.

12. The method of claim 7 further comprising: placing a macro-reinforcement net in the steel mold, wherein a flexural strength of the composite bipolar plate is greater than 25 MPa.

13. The method of claim 7, wherein the MPC raw materials include magnesia, potassium di-hydrogen phosphate, borax, fly ash, water, and ultra-high molecular weight polyethylene, and the electrically conductive fillers include graphite, carbon fibers, and carbon nanotubes.

14. A method that produces a composite bipolar plate, the method comprising: mixing magnesium phosphate cement (MPC) raw materials, fillers, and water to produce a wet powder; placing a macro-reinforcement net in a steel mold; transferring the wet powder into the steel mold; replacing 30% of the powder in surface layers by ultra-high molecular weight polyethylene powder; hot-pressing the wet powder in the steel mold to produce a plate; heating the plate to a mold temperature for at least 10 minutes to produce a heated plate; cooling the heated plate to produce an organic-inorganic interpenetrated product; and curing the organic-inorganic interpenetrated product by air to produce the composite bipolar plate, wherein a flexural strength of the composite bipolar plate is greater than 25 MPa, and a corrosion current density of the bipolar plate composite material is 110.sup.6 A/cm.sup.2.

15. The method of claim 14, wherein mixing of the MPC raw materials, the electrically conductive fillers, and the water are conducted in an automatic mechanical grinding setup.

16. The method of claim 14, wherein the macro-reinforcement net is an acrylonitrile butadiene styrene (ABS) co-polymer net.

17. The method of claim 14 further comprising: designing the steel mold according to a shape and flow field of a fuel cell device that is made of the composite bipolar plate.

18. The method of claim 14 wherein the hot-pressing is conducted under a compressive pressure of 70 MPa and a temperature of up to 140 C.

19. The method of claim 14, wherein the mold temperature is higher than a melting point of an ultra-high molecular weight polyethylene.

20. The method of claim 14, wherein the MPC raw materials include magnesia, potassium di-hydrogen phosphate, borax, fly ash, water, and ultra-high molecular weight polyethylene, and the electrically conductive fillers include graphite, carbon fibers, and carbon nanotubes.

Description

BRIEF DESCRIPTION OF FIGURES

(1) FIGS. 1A and 1B shows a comparison of the filler volumetric efficiency in a MPC-carbon filler composite and a polymer-carbon filler composite with the same filler loading: MPC-carbon composite in FIG. 1A and polymer-carbon composite in FIG. 1B. Black dots, matrix in light gray and circles in grey represent the carbon fillers, the binder phase and unreacted magnesia grains, respectively.

(2) FIG. 2 shows the automatic mechanical grinding setup for the mixing of raw materials.

(3) FIG. 3 shows the acrylonitrile butadiene styrene co-polymer macro-reinforcement produced by 3D printing for the purpose of enhancing the flexural strength.

(4) FIG. 4 shows a sketch of the surface-treated sandwich composite structure for the purpose of enhancing the corrosion resistance.

(5) FIG. 5 shows the compression plate component of the hot-press for bipolar plate production, with counter channels and lands patterned on the surface.

(6) FIG. 6 shows the effect of graphite volume fraction on electrical conductivity, corrosion current density and flexural strength of the composite (data from G35-G50).

(7) FIG. 7 shows the effect of carbon fiber on electrical conductivity, corrosion current density and flexural strength of the composite (data from G45, CF0.5, Ex. 2, CF2 and Ex. 3).

(8) FIG. 8 shows the effect of carbon black on electrical conductivity, corrosion current density and flexural strength of the composite (data from Ex. 2, CB2, Ex. 4, CB6 and CB10).

(9) FIG. 9 shows the effect of CNT on electrical conductivity, corrosion current density and flexural strength of the composite (data from Ex. 2, CNT0.5, CNT1, Ex. 5, CNT3 and Ex. 6).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(10) As used herein and in the claims, comprising means including the following elements but not excluding others.

(11) As used herein and in the claims, acidic cement based binder refers to any cement based binders that can work in a typical fuel cell acidic environment (<pH4) without being dissolved or decomposed by the fuel cell environment in affecting the performance of the fuel cell during normal operation of the fuel cell.

(12) Magnesium phosphate cement (MPC) is a type of low-pH cement, and it can be kept stable under acidic environment. Using MPC as the matrix, and carbon-based materials (graphite powder, carbon fiber, CNT, etc.) as fillers, it is possible to produce high performance bipolar plate that fulfils all technical targets set forth by the US Department of Energy (DOE) and achieves the goal of lowering cost.

(13) The present invention provides a bipolar plate composite material for fuel cells containing, for example, MPC, an inorganic binder, and electrically conductive carbon fillers including graphite powder, carbon black, CNTs, carbon fibers, etc. The production of bipolar plate composite material for fuel cells is also provided in which the bipolar plate composite material is produced. The bipolar plate composite material prepared according to the present invention have desired properties according to US DOE, such as high electrical conductivity, flexural strength and corrosion resistance, while the production cost thereof can be greatly reduced.

The Bipolar Plate Composite Material

(14) The bipolar plate composite material contains MPC binders and electrically conductive fillers. In one exemplary embodiment, the electrically conductive fillers are cemented by the MPC binder to form the bipolar plate composite material. MPC is a multi-component inorganic binder consisting of dead burnt magnesia, potassium di-hydrogen phosphate (KDP) and borax, in which the magnesia-to-KDP molar ratio (M/P) is 6-12:1; in another exemplary embodiment, MPC also contains fly ash. Borax is added as a reaction retarder at 5% of the weight of magnesia; in another exemplary embodiment, fly ash is added to replace 20-40% of the magnesia.

(15) In one exemplary embodiment, the electrically conductive fillers (or fillers) are loaded at 40-56 vol. % based on the total volume of the MPC binder. In another exemplary embodiment, the fillers include 35-50% graphite powder, and/or 2-10% carbon black, and/or 0.5-4% carbon nanotubes (CNTs), and/or 0.5-3% carbon fibers, and/or any combination thereof.

Process

(16) The composite material in the following examples of the present invention are prepared by the following steps: (a) mixing MPC raw materials and electrically conductive fillers according to a particular mix proportion; (b) mixing the mixed product from step (a) with a particular amount of deionized water to form wet powder; (c) transferring the product from step (b) into a steel mold; (d) using hot-press to convert the wet powder in the mold into a plate; (e) curing the plate to achieve the properties of interest and/or until such properties has become stable.

(17) Preferably, the mixing of step (a) and step (b) are conducted in an automatic mechanical grinding setup to achieve a homogeneous distribution of the fillers in the binder phase. The automatic mechanical grinding setup includes a fixed mortar and an automatic pestle. When mixing, room temperature water is added at a water-to-cement ratio (W/C) of 0.2-0.3 by weight, allowing the formation of magnesium potassium phosphate hexahydrate (MKP, or struvite-K) as the main cementing phase according to the following reaction:
MgO+KH.sub.2PO.sub.4+5H.sub.2O.fwdarw.MgKPO.sub.4.6H.sub.2O(I)

(18) The steel mold for preparing the bipolar plate in step (c) can be designed according to the shape and flow field demanded by the fuel cell.

(19) The hot-press in step (d) is a process involving the conversion of the wet powder into a plate under a compressive pressure of 70 MPa and a temperature of up to 140 C., with a loading duration up to 60 min depending on the composition of the binder.

(20) The curing in step (e) indicates an air curing process for the resulting plate sandwiched between two steel plates for one day, which allows the release of the stress and the completion of the chemical process as indicated in equation (I) in the pressed plates.

(21) To further enhance the flexural strength of the resulting composite material, macro-reinforcement is incorporated in a further step (f). The macro-reinforcement could be a thin acrylonitrile butadiene styrene (ABS) co-polymer net produced by 3D printing.

(22) A polymer based surface treatment is adapted in a further step (g) to further enhance the corrosion resistance of the bipolar plate composite material. The polymer could be a type of ultra-high molecular weight polyethylene powder, with a molecular weight of 3,500,000 and a melting point of 142 C., which is used to replace 30% of the binder in the surface layers of the composite material. After a normal hot-press process as in step (d), the mold temperature is increased to 160 C. to allow the melting of the polymer and the formation of an organic-inorganic interpenetrated structure, which is followed by a cooling process to allow the formation of a polymer enhanced surface layer for the composite material.

(23) In the following examples, the raw materials for making MPC include dead burnt magnesia powder (calcined under 1600 C. for 5 hours, passed a 300 m sieve) with a purity of 95.1%, powder KDP (which is grounded and passed a 250 m sieve before mixing), fly ash (with a mean particle size of 18 m) and deionized water. Micro-sized powder graphite (>95% particles are smaller than 30 m), nano-sized powder carbon black (mean particle size <100 nm), short carbon fiber (2 mm pitched carbon fiber) and industrial CNTs were employed as functional fillers, to provide conductive pathways in the composite material.

(24) In a MPC, the normal optimized magnesia-to-KDP molar ratio (M/P) for the chemical reaction of equation (I) is 1:1. However, in the present invention, the M/P is 612:1, so that the degree of reaction will be very low. In such cases, a large volume fraction of unreacted magnesia particles remain in the binder phase of the composite material. The magnesia particles are 1300 m, while graphite powder is <30 m; the characteristic size of carbon black powder is 100 nm. The diameters of CNT and carbon fiber are 30 nm and 2 m, respectively. As compared with the filler particles, the unreacted magnesia particles are much larger. The distribution of filler particles is thus restricted in a narrower space as limited by the unreacted magnesia grains, so that the volumetric efficiency of the fillers in forming a percolated conductive pathway is improved. In this way, the target electrical conductivity, i.e. 100 S/cm, can be achieved at a relatively low filler volume fraction (50%). As a comparison, in a polymer/carbon filler bipolar plate composite material, the filler particles are distributed in a homogeneous binder phase, as the polymer molecules are much smaller than the filler particles. In the case of polymer-graphite composite materials, typically 70% (volume fraction) of carbon materials are mixed with 30% of polymeric binder to achieve conductivities between 50 and 100 S/cm. Larger amount of graphite will significantly reduce the strength of the composite material. This comparison is clearly shown in FIG. 1. The lowered carbon filler volume fraction of the composite material of this invention not only limits the raw material cost, but also avoid the severe strength reduction due to the incorporation of the fillers.

(25) In the following examples, the mixing of raw materials is conducted by an automatic mechanical grinding setup, which includes a fixed mortar and an automatic pestle as shown in FIG. 2. The mixed wet mixture is transferred into a steel mold, and pressed into a 100 mm50 mm3 mm plate.

(26) Key properties of the bipolar plate composite material include electrical conductivity, flexural strength, corrosion resistance and gas permeability. All materials obtained in the following examples were tested for gas permeability, using a commercial gas permeability tester designed according to ASTM D1434. The permeabilities of all the MPC/carbon filler composites are lower than the testing capacity of the tester, which means that the equivalent hydrogen gas permeation coefficient of the composites is much lower than the target value, i.e. 10.sup.5 cm.sup.3/(s.Math.cm.sup.2). Besides, the electrical conductivity was measured by a four-point probe resistivity meter, the flexural strength was tested according to ASTM D790-10, and the corrosion resistance was evaluated under conditions recommended by US DOEanode corrosion current density test conditions: pH 3, 0.1 ppm HF, 80 degree, potentiodynamic test at 0.1 mV/s, 0.4V to +0.6V (Ag/AgCl), de-aerated with Ar purge; cathode corrosion current density test conditions: pH 3, 0.1 ppm HF, 80 degree, potentiostatic test at +0.6V (Ag/AgCl) for >24 h, aerated solution (US DOE 2012).

Examples 1-6

(27) Raw materials used in examples 1-6 are dead burnt magnesia, KDP, borax, fly ash, water, graphite powder, carbon black, CNTs and carbon fiber. In the binder of Example 1, the magnesia-to phosphate molar ratio is 8, the water-to-cement mass ratio is 0.25 and dosage of borax is 5% of the mass of magnesia. In the binder of Examples 2-6, fly ash is used to replace 30% of the mass of magnesia, while other ratios remain the same as those in Example 1. Correspondingly, the mass based compositions of 100 g binders of Examples 1-6 are listed in Table 1. Electrically conductive fillers take different volume fractions in the 6 examples as shown in Table 2.

(28) The raw materials are mixed in the setup as shown in FIG. 2 for 20 min, and placed for another 10 min. The wet mixture is then transferred into the steel mold, and processed in a hot-press under 100 C. The pressure is applied in two steps, i.e. a pre-loading at 5 MPa for 15 min, followed by a loading at 70 MPa for 30 min. The loading process is followed by unloading, de-molding and curing to obtain the bipolar plate composite material. The resulting composite material is then tested for properties of interest such as electrical conductivity, flexural strength and corrosion current density, and the results thereof are shown in Table 3.

(29) The compositions in all of Examples 1-6 contain 45% graphite powder based on the total volume of the produced composite material, with different amounts of other components. Graphite powder is used as the dominant conductive filler, due to its low cost and high performance. In view of the test results, partial replacement of the magnesia in the binder by fly ash can improve the flexural strength and corrosion resistance of the composite material, with a slightly negative effect on the electrical conductivity. The incorporation of a small amount of carbon fiber is intended to improve all of the properties of the resulting composite material, but when the carbon fiber loading reaches 3%, all of the properties are reduced due to the conglomeration of carbon fibers. The adding of carbon black, at a relatively small dosage, can significantly increase the electrical conductivity, but would lead to a much poorer corrosion resistance at the same time. CNTs can play a similar role of carbon black in enhancing the electrical conductivity, without resulting in a negative effect on the corrosion resistance. Surprisingly, CNTs at a volume fraction of 2% can also enhance the flexural strength. However, CNTs at higher volume fraction will also conglomerate and lead to the overall degradation of the composite properties. The effect of graphite volume fraction, carbon fiber, carbon black and CNTs on the electrical conductivity, corrosion current density and flexural strength of the composite are studied and shown in FIGS. 6-9.

(30) TABLE-US-00001 TABLE 1 Composition of binders (unit: g/100 g binder) Materials Magnesia KDP Borax Fly ash Water Ex. 1 54.35 22.93 2.72 0 20 Exs. 2-6 38.05 22.93 2.72 16.3 20

(31) TABLE-US-00002 TABLE 2 Composition of composite material (unit: %) Filler Carbon Carbon Materials Binder Graphite fiber black CNTs Ex. 1 54 45 1 G35 65 35 G40 60 40 G45 55 45 G50 50 50 CF0.5 54.5 45 0.5 Ex. 2 54 45 1 CF2 53 45 2 Ex. 3 52 45 3 CB2 52 45 1 2 Ex. 4 51 45 1 3 CB6 48 45 1 6 CB10 44 45 1 10 CNT0.5 53.5 45 1 0.5 CNT1 53 45 1 1 Ex. 5 52 45 1 2 CNT3 51 45 1 3 Ex. 6 50 45 1 4

(32) TABLE-US-00003 TABLE 3 Achieved properties of the composites Corrosion Electrical Flexural current conductivity strength density Properties (S/cm) (MPa) (10.sup.6 A/cm.sup.2) Ex. 1 124 19.7 2.7 G35 64 23.1 0.73 G40 85 21.9 1.2 G45 105 21.3 1.9 G50 135 21 3.1 CF0.5 108 22.9 1.7 Ex. 2 107 23.1 0.93 CF2 115 23.3 1.1 Ex. 3 99 18.6 2.9 CB2 127 23.2 2.1 Ex. 4 146 22.9 3.5 CB6 183 21.8 5.4 CB10 207 20.9 11.7 CNT0.5 111 24.3 0.89 CNT1 125 25.1 0.67 Ex. 5 139 25.9 0.53 CNT3 145 19.8 1.4 Ex. 6 123 17.7 3.1

Example 7

(33) As shown in Table 3, the flexural strengths of the composite material of some Examples are lower than the US DOE technical target, i.e. 25 MPa. It has been proven that, for bipolar plate composite material with a thickness of several millimeters, a slightly lower flexural strength will not lead to plate breaking under normal cell assembling and operating conditions. However, it is preferred to have the target flexural strength achieved reliably. For this purpose, in the present Example, an ABS reinforcement is used to enhance the flexural strength of the resulting composite material.

(34) The ABS reinforcement is produced by 3D printing, which is 84 mm long, 34 mm wide and 0.8 mm thick, and the rib width is 5 mm, as shown in FIG. 3. The surface of the reinforcement can be processed to a very rough surface to enhance the mechanical locking between the reinforcement and the matrix, which can be easily achievable by adjusting the printing settings.

(35) Also, in the present Example, the composition of the binder is the same as that in Example 5, and the hot-press process is also the same as that used in Examples 1-6, except that the ABS reinforcement is placed in the composite material matrix when filling the mold. As compared with Example 5, although there are no obvious differences to the electrical conductivity (141 S/cm) and the corrosion current density (0.5910.sup.6 A/cm.sup.2) by the application of the macro-reinforcement, the flexural strength (29.9 MPa) of the composite material with macro-reinforcement is improved.

Example 8

(36) The US DOE corrosion current density target for bipolar plate is <110.sup.6 A/cm.sup.2. As shown by the previous test results, this target can be achieved in Examples 2, 5 and 7. To guarantee the durability of the bipolar plate as well as the fuel cell, the bipolar plate has to have satisfactory corrosion resistance.

(37) In the present Example, composite material with compositions of Example 7 is used, but the 3 mm thick plate is divided into 3 layers, i.e. two 0.6 mm thick surface layers and a 1.8 mm central layer, as shown in FIG. 4. In the two surface layers, an ultra-high molecular weight (3,500,000) polyethylene with a melting point of 142 C. is used to replace 30% of the binder phase. After a normal hot-press process as used in previous Examples, the mold temperature is raised to 160 C. and held for 10 min, and then the plate is cooled down. This increased mold temperature is higher than the melting point of the polymer, so that the melting of the polymer in the surface layers are allowed. After cooling, an organic-inorganic interpenetrated binder structure can be formed to protect the functional fillers from being oxidized. As compared with Examples 5 and 7, this surface treatment brings negative effect on the electrical conductivity (116 S/cm) and positive effect on the flexural strength (31.1 MPa) to the composite material of Example 8, and significantly reduces the resulting corrosion current density (0.1510.sup.6 A/cm.sup.2).

(38) In Examples 1-8, the composite materials are processed into flat plates, for the ease of properties testing. However, the steel mold for hot-press can be tailor-designed according to the properties, for example shape and flow field, as demanded by the fuel cell. Two compression plate samples of the mold, with counter channels and lands, are shown in FIG. 5. With the use of the steel mold, bipolar plates composite material with flow field can be produced in the light of the aforementioned hot-press process.

(39) The exemplary embodiments of the present invention are thus fully described. Although the description referred to particular embodiments, it will be clear to one skilled in the art that the present invention may be practiced with variation of these specific details. Hence this invention should not be construed as limited to the embodiments set forth herein.

(40) For example, the automatic mechanical grinding setup for mixing the materials afore-described could be replaced by any setup or machines which can achieve the same results.