Membrane-electrodes assembly for proton exchange fuel cells (PEMFC), and manufacturing method

Abstract

A membrane-electrode assembly (MEA) including a membrane and two electrodes, and further at least one layer located at the interface of the membrane and of an electrode. The layer contains a proton conductive polymer which has a glass transition temperature lower than or equal to, advantageously lower than, that of the proton conductive polymer contained in the membrane.

Claims

1. A membrane-electrode assembly (MEA) comprising a membrane and two electrodes, said assembly further comprising at least one layer located at the interface of the membrane and of an electrode, wherein: the membrane and the electrodes contain a same proton conductive polymer; said layer contains a proton conductive polymer which has a glass transition temperature lower than that of the proton conductive polymer contained in the membrane and in the electrodes; the proton conductive polymers contained in the membrane and the electrodes and in the layer are perfluorosulfonated ionomers (PFSA).

2. The membrane-electrode assembly (MEA) of claim 1, wherein the polymer of the layer has a longer dangling chain than the polymer of the membrane and of the electrodes.

3. The membrane-electrode assembly (MEA) of claim 1, wherein the difference between the glass transition temperatures of the polymer of the layer and of the polymer of the membrane and of the electrodes is greater than or equal to 20 C., advantageously greater than 20 C.

4. The membrane-electrode assembly (MEA) of claim 3, wherein the polymer of the layer has a glass transition temperature in the order of 100 C. and the polymer of the membrane and of the electrodes has a glass transition temperature in the order of 140 C.

5. The membrane-electrode assembly (MEA) of claim 1, wherein the polymer of the layer has the following structure: ##STR00008## and the polymer of the membrane and of the electrodes has the following structure: ##STR00009##

6. The membrane-electrode assembly (MEA) of claim 1, wherein the layer has a thickness in the range from 1 to 5 micrometers, advantageously equal to 2 micrometers.

7. A fuel cell (PEMFC) comprising the membrane-electrode assembly (MEA) of claim 1.

8. A method of manufacturing the membrane-electrode assembly (MEA) of claim 1, comprising the steps of: depositing on at least one of the surfaces of the membrane and/or of the electrodes a layer containing a proton conductive polymer; assembling the membrane and the electrodes; raising the temperature of the assembly to a temperature higher than or equal to the glass transition temperature of the polymer of the layer but lower than or equal to the glass transition temperature of the polymer of the membrane and of the electrodes; possibly pressurizing the assembly.

9. The membrane-electrode assembly (MEA) manufacturing method of claim 8, wherein the layer is deposited on one of the surfaces of the two electrodes, the assembly being formed by placing opposite each other the layer deposited on the electrodes and the membrane.

10. The membrane-electrode assembly (MEA) manufacturing method of claim 8, wherein, before the assembly, advantageously before the deposition of the layer, the electrodes are submitted to a thermal treatment, advantageously at a temperature higher than or equal to the glass transition temperature of the polymer contained in the electrodes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The foregoing features and advantages will now be discussed in the following non-limiting description of a specific embodiment, in relation with the accompanying drawings, among which:

(2) FIG. 1 shows the diagram of the operating principle of a PEMFC-type fuel cell.

(3) FIG. 2 shows a MEA according to the invention.

(4) FIG. 3 compares the performances of a device according to the invention and of a device according to prior art in automobile conditions (A) and in drowning conditions (B).

EXAMPLES OF EMBODIMENT

(5) The present invention will be further illustrated in relation with a specific embodiment, that is, a MEA comprising: an Aquivion-type membrane (3);

(6) As a reminder, Aquivion, for example, commercialized by Solvay, has a glass transition temperature, noted Tg, in the range from 140 to 150 C. and the following structure (k being an integer):

(7) ##STR00006## an anode and a cathode (2) also comprising Aquivion.

(8) The electrodes typically contain: 1/ 72% of platinum-type catalyst dispersed on a carbonaceous support, for example, carbon black. The assembly is formed of 50% of platinum and of 50% of carbon; 2/ 28% of ionomer, in the case in point, Aquivion.

(9) The % should be understood as mass percentages in the dry catalytic layer: a Nafion-based layer (4);

(10) As a reminder, Nafion, for example, commercialized by Dupont, has a glass transition temperature, noted Tg, in the order of 90-100 C. and the following structure (k being an integer):

(11) ##STR00007##

(12) 1/ Forming of the Device According to the Invention:

(13) A/ Electrode Forming:

(14) The first step comprises preparing a catalytic ink comprising the polymer, in the case in point Aquivion, and the carbonaceous support in a solvent. A water-based catalytic ink is thus prepared to obtain the above-mentioned concentrations. According to the manufacturing method, the dry extract varies from 1 to 20%. A thermal treatment enables to dry the deposited ink.

(15) Thus, the second step is the manufacturing of the electrodes (2) by deposition of the ink on the gas diffusion layers (5), typically formed by coating (or inkjet, silk-screening, spraying), thus forming the GDE.

(16) The GDE thus formed, based on Aquivion, having a Tg of 140-150 C., is then submitted to a thermal treatment at 180 C. for 30 minutes so that the ionomer particles coalesce, which favors the forming of triple contact areas necessary for a good catalytic operation. It should be noted that this step is not necessary in the conventional assembly manufacturing method, since this high-temperature treatment is carried out at the time of the assembly of the electrodes with the membrane.

(17) The electrodes (2) thus formed typically have a thickness in the order of 10 m with a mass in the order of 3 mg/cm.sup.2.

(18) B/ Forming of the Interface Layer:

(19) The next step comprises depositing the interface layer (4) on at least one of the electrodes (2), advantageously on the two electrodes.

(20) According to the invention, this layer is formed of an ionomer having a glass transition temperature lower by at least 20 C. than that contained in the membrane and in the case in point in the electrodes (Aquivion having a Tg of 140-150 C.). The ionomer selected to form the interface layer (4) is an ionomer from Dupont's Nafion range, advantageously in the form of an aqueous dispersion. Thus, the water/alcohol phase Nafion D2020 and Nafion D520 dispersions may be used to form the layer.

(21) In practice, the dispersion is deposited on the free surface of the electrode, that is, on the surface which is not in contact with the GDL (5) and which will be the surface placed opposite the membrane (3). The deposition may be performed by different conventional techniques such as spraying, coating, silk-screening. In practice, a spraying process is advantageously implemented.

(22) The interface thickness is typically in the range from 1 to 5 micrometers, advantageously 2 micrometers. In practice, during the deposition and the drying, the ionomer of the interface layer penetrates into the active area.

(23) The interface thickness should be in the range from 1 to 5 m, that is, a mass in the range from 0.2 to 1 mg. The Nafion D2020 suspension having a 22% dry extract, the wet mass to be deposited is thus in the range from 0.908 to 4.540 mg, per cm.sup.2 of active area.

(24) C/ Assembly:

(25) The electrode surface comprising the interface layer is then placed opposite the Aquivion membrane (3) to form the assembly.

(26) The compression of the assembly is performed in a hot press in the following conditions: at a temperature higher than the Tg of the polymer of interface layer 4 (90-100 C.) but lower than that of the polymers forming the electrodes (2) and the membrane (3) (140-150 C.). According to another constraint, to provide a good membrane-electrode interface, the assembly should advantageously be performed at a temperature higher by 40 C. than the glass transition temperature of the polymer present. In the case in point, a temperature in the range from 100 C. to 140 C., advantageously higher than 120 C., or even just lower than 140 C., should thus be applied: at a pressure typically in the range from 1 to 5 MPa, for example, 3.5 MPa; for a time period capable of ranging from a few seconds to a few minutes, for example, 3 minutes.

(27) At the end of the assembly, the stack has a structure such as schematized in FIG. 2. Such a structure (apart from the GDL), dry and finished, typically has a thickness in the ranee from 20 to 50 micrometers.

(28) 2/ Performance of the Device According to the Invention:

(29) The MEA thus obtained is mounted in a single cell to test the performances. The single cell is fitted with monopolar plates allowing the arrival of gases, with current collection plates, and with clamping plates.

(30) The performance of the device according to the invention has been compared with that of a reference MEA (formed of Nafion compressed at 135 C.), in two different conditions: in conditions close to those recommended for an automobile use: operating temperature=80 C.; relative humidity=50%; gas pressure=1.5 bar. in drowning conditions: operating temperature=80 C.: relative humidity=100%; gas pressure=1.5 bar.

(31) The results, disclosed in FIG. 3, correspond to a voltage-vs.-current density curve representative of the cell performance. The performance gain is particularly significant in drowning conditions. Thus, the optimizing of the membrane-electrode interface in a compressed MEA, such as provided in the context of the invention, provides a better management of the water generated by the cell.

(32) An improvement of the cell durability is also expected.