Hybrid proton exchange membrane

Abstract

The present application relates to a proton exchange membrane for fuel cell, said membrane having self-regenerating properties, to cells comprising said membranes, and to the manufacturing method thereof via electrospinning.

Claims

1. A membrane assembly comprising: a proton exchange membrane for a fuel cell, said membrane having a cathode side and an anode side, a catalytic cathode layer, and a catalytic anode layer; wherein said proton exchange membrane comprises two layers comprising: (a) a conducting and self-regenerating cathodic layer that ensures a cathode surface on the cathode side of the membrane, wherein said cathodic layer comprises at least one polymer (iii) carrying SO.sub.3H functions, at least one polymer (ii) selected from the group consisting of a silicon derivative, a sulfonated fluorinated copolymer, and a sulfonated polyether, and at least one polymer (i) carrying thiol (SH) functions; and (b) a conducting anodic layer that ensures an anode surface on the anode side of the membrane, said anodic layer comprising at least one polymer (iii) carrying SO.sub.3H functions; said polymers (iii) and (iii) being the same or different, wherein said polymers (i), (ii), and (iii) comprised within the cathodic layer are in a mixture or are co-polymerized; wherein said membrane is coated with the catalytic cathodic layer on an outer surface of the cathodic layer of the membrane, and with the catalytic anodic layer on an outer surface of the anodic layer of the membrane, and wherein said cathodic layer of said membrane differs at least from the conducting anodic layer in that it comprises at least one polymer (i) carrying thiol (SH) functions.

2. The membrane assembly according to claim 1, wherein said cathodic layer and/or said anodic layer further comprise(s) an inorganic polymer (ii) and/or (ii) respectively; said polymers (ii) and (ii) being the same or different.

3. The membrane according to claim 1, wherein the polymer (i) carrying SH functions is (3-mercaptopropyl)trimethoxysilane (MPTMS).

4. The membrane according to claim 2, wherein the polymer (ii) and/or (ii) is TEOS (tetraethyl orthosilicate) or TMOS (tetramethyl orthosilicate).

5. The membrane according to claim 1, wherein the polymer (iii) and/or (iii) is CSPTC (2-(4-Chlorosulfonylphenyl)ethyltrichlorosilane).

6. The membrane according to claim 1, wherein within said cathodic layer, the ratio of the concentration of SO.sub.3H functions to the concentration of SH functions is between 0.5 and 4.

7. The membrane according to claim 1, wherein said cathodic layer has a thickness of between 3 and 15 lam.

8. The membrane according to claim 1, wherein said cathodic layer has an increasing concentration gradient of SH functions, from its inner surface with said anodic layer to its outer surface ensuring the interface with a catalytic cathodic layer.

9. The membrane according to claim 2 wherein: in the cathodic layer, the polymers (ii), (iii) and (i) are in a ratio of 1:2:1; and in the anodic layer the polymers (ii) and (iii) are in a ratio of 1:2.

10. A fuel cell comprising the membrane assembly according to claim 1.

11. A method for preparing the membrane assembly according to claim 1, comprising preparing the membrane; wherein preparing the membrane comprises: forming fibres comprising said cathodic layer, from a solution comprising at least one polymer (i) carrying thiol (SH) functions and at least one polymer (iii) carrying SO.sub.3H functions, via electrospinning; forming fibres comprising said anodic layer, from a solution comprising at least one polymer (iii) carrying SO.sub.3H functions, via electrospinning; forming the anodic layer and the cathodic layer respectively from the fibres obtained; and applying said cathodic layer onto said anodic layer.

12. The membrane assembly according to claim 6 wherein within said cathodic layer, the ratio of the concentration of SO.sub.3H functions to the concentration of SH functions is comprised between 1 and 4.

13. The membrane assembly according to claim 7 wherein said cathodic layer has a thickness of between 5 and 10 ?m.

14. The membrane assembly according to claim 7 wherein said cathodic layer has a thickness of between 5 and 7 ?m.

Description

FIGURES

(1) FIG. 1 illustrates the description of the electrospinning process to synthesize the constituent fibres of the membranes of the invention.

(2) FIG. 2 illustrates the structure of a fuel cell comprising a membrane of the invention.

(3) FIG. 3 gives EDX analyses showing the distribution of different elements in the fibres: oxygen (a), silicon (b), sulfur (c) and fluorine (d).

(4) FIG. 4 illustrates the trend in proton conductivity as a function of relative humidity and the presence or absence or self-regenerating SH functions: for a hybrid electrospun membrane comprising the polymers (ii) and (iii) in a ratio of 1:2 (TEOS:CSPTC (1:2)), or comprising the polymers (i), (ii) and (iii), in a (ii):(iii):(i) ratio of 1:2:1 (TEOS:CSPTC:MPTMS (1:2:1)).

(5) FIG. 5 illustrates the trend in proton conductivity as a function of relative humidity for a hybrid electrospun membrane after accelerated degradation tests, and as a function of the presence or absence of self-regenerating SH functions: for a membrane comprising the polymers (ii) and (iii), in a ratio of 1:2 (TEOS:CSPTC (1:2)), or comprising the polymers (i), (ii) and (iii), in a (ii):(iii):(i) ratio of 1:2:1 (TEOS:CSPTC:MPTMS (1:2:1)).

(6) FIG. 6 is a photograph of the two surfaces of the bilayer membrane: the photo on the left shows the anode surface (conductive anodic layer) and the photo on the right shows the cathode surface (self-regenerating cathodic layer).

(7) FIG. 7 gives images taken under scanning electron microscopy (SEM) showing the structure of the conductive anodic layer (a), the self-regenerating cathodic layer (b) and the cross-section of the bilayer membrane (c).

(8) FIG. 8 illustrates the variation in conductivity as a function of relative humidity for the anodic layer (a) and the cathodic layer (b).

EXAMPLES

(9) 1. Synthesis and Characterizations

(10) Cathodic Layer Solution:

(11) A sol-gel solution was prepared by prior dissolution of the non-conductive polymer (PVDF-HFP) in dimethylformamide (DMF) (600 mg PVDF-HFP in 700 ml DMF). The additives (PEG: polyethylene glycol, and OFP: 2,2,3,3,4,4,5,5-octafluoro-1-pentanol) were added thereto (340 mg PEG and 60 mg OFP). The addition was then made of the silicon precursors (TEOS (tetraethylorthosilicate, 310 mg), CSPTC (2-(4-chlorosulfonylphenyl) ethyltrichlorosilane, 2.04 g from a 50 weight % solution in dichloromethane) and MPTMS ((3-Mercaptopropyl)trimethoxysilane, 310 mg), according to the different tested molar ratios, ending with CSPTC which is the most reactive. Maturation of the solution was obtained under agitation for 3 hours at 70? C., to pre-hydrolyse the silicon precursors. A homogeneous solution V2 was obtained containing the SH and SO.sub.3H functions, and having a TEOS/CSPTC/MPTMS ratio of 1:2:1. Solutions with ratios of 1:2:2 and 1:1:1, were also prepared following this operating mode.

(12) Anodic Layer Solution:

(13) The above operating mode was followed from a mixture of precursors not containing MPTMS, to obtain a solution V1 (containing the SO.sub.3H functions).

(14) Electrospinning:

(15) The fibres were prepared by electrospinning each of the solutions V1 and V2 obtained.

(16) The electrospinning installation used (such as illustrated in FIG. 1) comprised a syringe and needle (ID=0.75 mm), ground electrode and commercial high voltage unit (ES1a, Electrospinz). The needle was connected to the high voltage unit which was able to generate positive direct current voltages of up to 40 kV.

(17) Two syringes each containing a volume of solution V1 or solution V2 were prepared. To obtain a total thickness of 20 microns, 0.15 mL for solution V1 and 0.075 mL for solution V2 were used. Each solution V1 and V2 was electrospun at 26 kV, at a rate dependent on ambient humidity. In general, relative humidity must be lower than . . . at 60? C. and the flow rate about 0.025 mL.Math.min.sup.?1. The steel needle was connected to an electrode of the high voltage unit and grounded aluminium foil was placed 15 cm away from the tip of the needle to collect the different layers.

(18) To prepare the second layer of the membrane, the first syringe was substituted by the second syringe.

(19) A white, flexible membrane was obtained. The proton exchange membrane was finally dried in air for twenty-four hours at 70? C. After drying, the EHS separators were carefully detached from the aluminium foil. The thickness of the membrane is dependent on the volume injected into the electric field and can vary between 15 and 40 microns.

(20) The two layers thus obtained are shown in FIG. 6.

(21) Analyses of the Membrane

(22) The structure was investigated under scanning electron microscopy such as illustrated in FIG. 7.

(23) Scanning electron spectroscopy images of the surface and cross-section of the electrospun proton exchange membrane show intermeshed fibres (D=200 nm) randomly organised in the 3D space, delimiting a continuous porous matrix of 85.4%. Membrane thickness was measured to be about 20 ?m. EDX analyses of the fibres (FIG. 3) show homogeneous surface distribution of the elements O, F, S and Si, not detecting any phase separation on micrometric level.

(24) 2. Properties

(25) Calculation of water retention shows that the cathodic layer TEOS:CSPTC:MPTMS (1:2:1) is much less hydrophilic than the corresponding anodic layer TEOS:CSPTC (1:2) (69.6% water vs. 141.9% water).

(26) The self-regenerating properties of the membranes comprising a cathodic layer having SH functions were analysed.

(27) Such as illustrated in FIG. 4, the trend in proton conductivity as a function of humidity shows that the addition of reactive self-regenerating functions has a low impact on conductivity values for a conventional membrane. Nevertheless, this membrane, which comprises SH functions, displays better resistance to oxidation and in particular in the presence of H.sub.2O.sub.2 such as illustrated in FIG. 5 which shows conductivity retention after an accelerated degradation test.

(28) The impact of the ratio of SO3H/SH functions on the physicochemical properties of the membranes was examined for different compositions: For the anodic layer, the following TEOS:CSPTC ratios were tested: 1:2, 1:3, 1:1, and 2:1 For the cathodic layer, the following TEOS:CSPTC:MPTMS ratios were tested: 1:2:1, 1:2:2 and 1:1:1.

(29) The impact on conductivity is illustrated in FIG. 8 which illustrates the variation in conductivity as a function of humidity rate for different compositions of hybrid membrane. The graph on the left shows the variation in conductivity as a function of the ratio TEOS/CSPTC, and the graph on the right shows the variation in conductivity as a function of the quantity of TEOS/CPSPTC/MPTMS. These experiments show that it is the TEOS:CSPTC values of 1:2 and TEOS:CSPTC:MPTMS values of 1:2:1 which afford the best results in terms of conductivity, at 80? C.