Proton-conducting inorganic particles, method for the preparation thereof, and use thereof to form a fuel cell membrane

Abstract

The invention relates to inorganic particles which are covalently bonded to first polymer chains made up of at least one polymer carrying proton exchange groups, optionally in the form of salts, and bonded to second polymer chains made up of at least one fluorinated polymer that does not carry any proton exchange groups, the second chains being bonded to the particles via organic spacer groups, or the second chains being bonded to first chains via organic spacer groups, or some of the second chains being bonded to the particles via organic spacer groups while the remaining second chains are bonded to first chains via organic spacer groups.

Claims

1. Proton-conducting membrane for fuel cell formed of inorganic particles bonded, covalently, to first polymer chains constituted of at least one polymer carrying proton-exchanging groups, optionally in the form of salts, said particles also being bonded to second polymer chains constituted of at least one fluorinated polymer not carrying proton-exchanging groups, with the bond between the particles and said second polymer chains corresponding to one of the following alternatives: said second polymer chains are bonded to the particles via organic spacer groups; or said second polymer chains are bonded to first polymer chains via organic spacer groups; or a portion of said second polymer chains is bonded to the particles via organic spacer groups while the other portion of said second polymer chains is bonded to first polymer chains via organic spacer groups.

2. Membrane according to claim 1, wherein the proton-exchanging groups are groups having formulas —PO.sub.3H.sub.2, —CO.sub.2H, or —SO.sub.3H, with these groups optionally able to be in the form of salts.

3. Membrane according to claim 1, wherein the polymer or polymers carrying proton-exchanging groups, optionally in the form of salts, forming first polymer chains are polymers comprising one or several repetitive units coming from the polymerisation of one or several vinyl monomers, said monomers have at least one proton-exchanging group, optionally in the form of salts.

4. Membrane according to claim 3, wherein the vinyl monomer or monomers are styrenic monomers comprising at least one proton-exchanging group, optionally in the form of salts.

5. Membrane according to claim 1, of which the polymer carrying proton-exchanging groups is poly(sodium 4-vinylbenzenesulfonate) or poly(4-vinylbenzenesulfonic acid).

6. Membrane according to claim 1, wherein all of the second polymer chains are bonded to the particles via organic spacer groups.

7. Membrane according to claim 1, wherein the fluorinated polymer or polymers not carrying proton-exchanging groups, forming said second polymer chains, are polymers comprising one or several repetitive units, with each one of these repetitive units comprising at least one fluorine atom, with these polymers being chosen from: the polymers comprising a single repetitive unit coming from a fluorinated monomer, such as polytetrafluoroethylenes (PTFE), polyvinylidene fluorides (known as the abbreviation PVDF); copolymers comprising two repetitive units coming from two fluorinated monomers, such as copolymers of vinylidene fluoride and of hexafluoropropene (poly(VDF-co-HFP)).

8. Membrane according to claim 1, wherein the organic spacer group is a non-polymeric hydrocarbon group, having the form of a linear or branched chain, into which can be inserted one or several groups (referred to as linking groups) chosen from —(C═O)—, —(C═O)O—, —SO.sub.2, amide groups, amine groups, silanoxy groups.

9. Membrane according to claim 8, wherein the organic spacer group has the following formula (III): wherein: X.sup.1 is the group covalently bonded to a particle or to a first polymer chain, with this group being one of the linking groups defined previously; R.sup.1 is an organic group forming a bridge between X.sup.1 and X.sup.2; and X.sup.2 is the group covalently bonded to at least one second polymer chain, with this group being one of the linking groups defined previously.

10. Inorganic particici Membrane according to claim 9, wherein the group R.sup.1 is an alkylene group.

11. Inorganic particici Membrane according to claim 1, which are particles of zeolites, particles of zirconium phosphates, particles of zirconium phosphonates, particles of clays, particles of oxides such as silica, alumina, zirconia, titanium oxide.

12. Inorganic particici Membrane according to claim 1, which are particles of silica.

13. A fuel cell comprising at least one electrode-membrane-electrode assembly, wherein the membrane is such as defined in claim 1.

14. Method for the preparation of inorganic particles as defined in claim 1, said method comprising a step of putting into contact of inorganic particles covalently bonded to first polymer chains formed by at least one polymer carrying proton-exchanging groups, optionally in the form of salts, with at least one fluorinated polymer not carrying proton-exchanging groups in the presence of a reagent comprising at least one group able to react with the inorganic particles and/or the first polymer chains in order to form a covalent bond and at least one second group able to react with at least one group of said fluorinated polymer in order to form a covalent bond and a reaction step between the reagent, the particles and the fluorinated polymer which results in inorganic particles covalently bonded, to first polymer chains constituted of at least one polymer carrying proton-exchanging groups, optionally in the form of salts, and being bonded to second polymer chains constituted of at least one fluorinated polymer not carrying proton-exchanging groups, wherein either said second polymer chains are bonded to the particles via organic spacer groups; or said second polymer chains are bonded to first polymer chains via organic spacer groups; or a portion of said second polymer chains being bonded to the particles via organic spacer groups while the other portion of said second polymer chains is bonded to first polymer chains via organic spacer groups.

15. Inorganic particles bonded, covalently, to first polymer chains constituted of at least one polymer carrying proton-exchanging groups, optionally in the form of salts, said particles also being bonded to second polymer chains constituted of at least one fluorinated polymer not carrying proton-exchanging groups, with the bond between the particles and said second polymer chains corresponding to one of the following alternatives: said second polymer chains are bonded to the particles via organic spacer groups; or said second polymer chains are bonded to first polymer chains via organic spacer groups; or a portion of said second polymer chains is bonded to the particles via organic spacer groups while the other portion of said second polymer chains is bonded to first polymer chains via organic spacer groups; wherein the inorganic particles form a membrane in a fuel cell.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a graph showing the change in the voltage E (in V) as a function of the current density D (in A.Math.cm.sup.−2) for the cell comprising the membrane 1 (curve a) and for the cell comprising the membrane 2 (curve b).

(2) FIG. 2 is a graph showing the change in the voltage E (in V) as a function of the time T (in hours) for the cell comprising the membrane 1 (curve a) and for the cell comprising the membrane 2 (curve b).

DETAILED EXPOSURE OF PARTICULAR EMBODIMENTS

Comparative Example

(3) This example shows the preparation of a mixture in order to produce a composite material that is compliant with what is proposed in prior art, i.e. a composite material comprising a matrix made from a copolymer of vinylidene fluoride and of hexafluoropropene and a filler formed by nanoparticles of silica grafted by poly(sodium 4-vinylbenzenesulfonate).

(4) The copolymer is supplied by the company Solvay under the reference Solef® 21216.

(5) The grafted particles have a composition by weight of silica and of poly(sodium 4-vinylbenzenesulfonate) respectively of 15% and of 85%.

(6) The preparation is carried out in the following way.

(7) In a 50-mL Erlenmeyer, 1 g of copolymer is weighed. Then, the nanoparticles (1.631 g) and dimethylsulfoxide (26 g) are added to the copolymer. The proportions of the ingredients were chosen in order to obtain a dry extract of 9.2% and in order to obtain a good compromise between the viscosity and the solubility of the copolymer.

(8) The mixture is heated to 60° C., under moderate stirring, in order to accelerate the solubilisation of the copolymer and of the nanoparticles, with the non-solubilised nanoparticles able to be crushed using a spatula.

(9) Then the resulting mixture is processed with a mixer of the “Speedmixer” type from the brand Hauschild, model DAC 400.1 FHZ, which makes it possible to obtain a degassed mixture with a rotation of 2000 rpm.sup.−1 in 2 minutes.

Example 1

(10) This example shows the preparation of a mixture in order to produce particles in accordance with the invention.

(11) To do this, a protocol is applied similar to the one from the comparative example except that the compound APTES is added (of which the formula is defined in the description hereinabove) for 0.068 g into the mixture after the step of solubilisation of the nanoparticles and of the copolymer of vinylidene fluoride and of hexafluoropropene.

Example 2

(12) This example shows the preparation of membranes from mixtures prepared in the comparative example and in the example 1.

(13) To do this, the aforementioned mixtures are poured under a laminar flow hood onto a glass plate cleaned beforehand respectively with acetone, with methanol then with acetone, with the pouring being carried out using a manual applicator of the “Hand coater” type provided with an air-gap of 500 μm. The glass plate, on which the solution was poured, is placed on a plate heated to 110° C., still under a laminar flow hood, for 2 hours in order to evaporate the solvent. Use of the laminar flow hood for the pouring and the evaporation is justified in order to prevent the introduction of dusts into the membranes.

(14) At the end of the step of evaporating on a hot plate, the membranes are brought to 150° C. for 2 hours in order to increase the yield of the reaction of the reagent APTES for the membrane poured and evaporated from the mixture of the example 1.

(15) The membranes obtained have a thickness of 25 μm.

Example 3

(16) In this example, the membranes obtained in the example 2 are subjected to a step of acidification in order to transform the sodium sulfonate groups into sulphonic acid groups, with the resulting membranes thus being proton-conducting membranes.

(17) To do this, the membranes are put into contact with a solution of concentrated sulphuric acid at 98% at ambient temperature for 4 days.

(18) The membranes are then rinsed three times with water and dried at ambient temperature. These membranes are, hereinbelow, respectively named “Membrane 1” (for the one coming from the mixture of the comparative example) and “Membrane 2” (for the one coming from the mixture of the example 1).

Example 4

(19) In this example, the physical-chemical properties of membranes were tested. The results are provided in the table hereinbelow.

(20) TABLE-US-00001 Solubility Solubility Solubility Swelling Proton in in in in conductivity Membrane DMSO.sup.(1) DMF.sup.(2) NMP.sup.(3) water (%) (mS .Math. cm.sup.−1) Membrane Yes Yes Yes 33 68 1 Membrane No No No 12 89 2 .sup.(1)DMSO corresponding to dimethylsulfoxide .sup.(2)DMF corresponding to N,N-dimethylformamide .sup.(3)NMP corresponding to N-methylpyrrolidone

(21) The membrane 2 obtained with the particles of the invention is insoluble in organic solvents, including polar solvents. This property demonstrates the effectiveness of the coupling reaction between the reagent APTES, the nanoparticles and the copolymer of vinylidene fluoride and of hexafluoropropene.

(22) Moreover, the results obtained with the membrane 2 show a substantial drop in the swelling in water (often at the origin of the ageing of cells) and a significant increase in the proton conductivity (which is an essential criterion for the operation of a cell).

Example 5

(23) In this example, the membranes of the example 3 are placed in a fuel cell H.sub.2/O.sub.2 and were characterised, in this cell environment, at 2 bars and 60° C.

(24) It was determined, in a first step, the change in the voltage E (in V) as a function of the current density D (in A.Math.cm.sup.−2), with the results being listed in the FIG. 1 (curve a) for the cell comprising the membrane 1 and curve b) for the cell comprising the membrane 2.

(25) The open circuit voltage obtained for the two cells is greater than 1 V (respectively, 1.019 V for the cell comprising the membrane 1 and 1.009 V for the cell comprising the membrane 2). This result indicated that there is no “cross-over” or leakage current for the cell comprising the membrane 2.

(26) Then, at a low current density, the curves a) and b) are superimposed, which indicates identical catalytic activity for the two membranes.

(27) Finally, at a higher density current, the voltages obtained for the cell comprising the membrane 2 are greater than those obtained for the cell comprising the membrane 1. For example, the current density obtained in order to reach 0.7 V is 1.4 A.Math.cm.sup.−2 for the cell comprising the membrane 2, although it is only 1.2 A.Math.cm.sup.−2 for the cell comprising the membrane 1, which represents an increase in performance of 16% for the cell comprising the membrane 2.

(28) It was determined, in a second step, the change in the voltage E (in V) as a function of the time (in hours) at 80° C. for 150 hours for a fixed current density of 1 A.Math.cm.sup.−2, with the results being listed in the FIG. 2 (curve a) for the cell comprising the membrane 1 and curve b) for the cell comprising the membrane 2. This entails in fact observing the voltage drop as a function of the time at a fixed current density.

(29) The decline in voltage obtained with the cell comprising the membrane 2 is 2.5 times lower than that obtained with the cell comprising the membrane 1. This attests to better stability of the nanoparticles in the membrane 2 and validates the principle of the stabilisation of the membranes in accordance with the invention.