MEMBRANE ELECTRODE ASSEMBLY

Abstract

Membrane electrode assembly for PEM fuel cells, including a proton exchange membrane, two catalyst layers (anode and cathode catalyst layer), and two gas diffusion layers, the anodic one of which is based on a carbon fiber paper and is provided with a microporous layer including graphite, carbon nanotubes or carbon nanofibers, and PTFE, whereas the cathodic gas diffusion layer is based on a carbon fiber structure and is provided with a microporous layer based on carbon black, carbon nanotubes and/or carbon nanofibers, and PTFE.

Claims

1-11. (canceled)

12. Membrane electrode assembly for PEM fuel cells, comprising: a proton exchange membrane, two catalyst layers (anode and cathode catalyst layer) and two gas diffusion layers, wherein the anodic GDL is based on a carbon fibre paper and is provided with a microporous layer (MPL), comprising graphite, carbon nanotubes or carbon nanofibres, and PTFE, and in which the cathodic GDL is based on a carbon fibre structure and is provided with a microporous layer based on carbon black, carbon nanotubes and/or carbon nanofibres, and PTFE.

13. Membrane electrode assembly according to claim 12, wherein the porosity of the anodic carbon fibre structure is 84% and/or less, and the density thereof is at least 0.25 g/cm3.

14. Membrane electrode assembly according to claim 12, wherein the graphite content of the microporous layer of the anodic GDL is 50 to 90%.

15. Membrane electrode assembly according to claim 12, wherein the content of carbon nanotubes in the microporous layer of the anode and cathode GDL is 8 to 25%.

16. Membrane electrode assembly according to claim 12, wherein the content of fluoropolymer of the microporous layer of the anode MPL is 15 to 30 percent by weight and the fluoropolymer content of the cathode MPL is 10 to 25 percent by weight.

17. Membrane electrode assembly according to claim 12, wherein the porosity of the carbon fibre structure of the cathodic GDL is 85% or more and the density thereof is at most 0.2 g/cm3.

18. Membrane electrode assembly according to claim 12, wherein the content of activated carbon, carbon black or mesoporous carbon of the cathodic GDL is 50 to 90 percent by weight.

19. Membrane electrode assembly according to claim 12, wherein the cathodic GDL additionally comprises mesoporous carbon or activated carbon.

20. Membrane electrode assembly according to claim 12, wherein the microporous layers of the anodic GDL and the cathodic GDL comprise two types of carbon materials which differ with respect to the average pore diameter of the carbon particles.

21. Membrane electrode assembly according to claim 20, wherein the average particle diameter of the carbon particles here is in the range of from 20 nm to 20 μm.

22. Fuel cell which comprises a membrane electrode assembly according to claim 12 and a pair of separators which are arranged such that the membrane electrode assembly is arranged between them.

Description

EMBODIMENT AND REFERENCE EXAMPLES

[0046] To produce various GDLs 2 types of standard carbon fibre papers, Sigracet® GDL 24 BA (porosity=83%, carbon fibre substrate 51) and Sigracet® 25 BA (porosity=88%, carbon fibre substrate S2) were used. The substrates were each hydrophobised with 5 percent by weight of polytetrafluoroethylene (PTFE). Microporous layers were produced on the abovementioned carrier papers by knife coating by means of paste-like carbon-PTFE dispersions.

[0047] For this, various, optionally preground carbons were dispersed in water with 2 percent by weight of polyvinyl alcohol under the action of shearing forces. In each case 1.25 percent by weight of hydroxyethylcellulose was added as an additive and the required amount of polytetrafluoroethylene (PTFE) in the form of an aqueous dispersion (Dyneon TF5035) and 1 percent by weight of polyethylene glycol were then added. In the case of the coating dispersion for the cathodic MPL 1 to 3 percent by weight of pore-forming agent (ammonium carbonate, oxalic acid, hexamethylenetetramine) are optionally also added.

[0048] After homogenising for several hours the viscous dispersions were pumped via a particle filter (<30 μm) on to a knife coating unit and coated on to a web of the carrier substrate.

[0049] The amount of the MPL applied to the carrier substrate was in each case 3 to 3.7 mg/cm.sup.2. After the coating the materials, which had been dried in a flow-through oven, were sintered at 350° C. for 15 minutes. Acetylene black (BET=65 m.sup.2/g), synthetic graphite (BET=30 m.sup.2/g) and multiwalled carbon nanotubes (BET=260 m.sup.2/g) were used as the carbon components of the dispersions.

[0050] Overview of the composition of the various GDL/MPL recipes

TABLE-US-00001 Carbon fibre carrier substrates, hydrophobised M1 GDL 24 BA, thickness 190 μm, density 0.25 g/cm.sup.3, PTFE 5 ± 1 wt. % M2 GDL 25 BA, thickness 190 μm, density 0.20 g/cm.sup.3, PTFE 5 ± 1 wt. % Microporous layers (MPL) M1 77 wt. % of acetylene black, 23 wt. % of PTFE M2 62.5 wt. % of acetylene black, 22.5 wt. % of MWCNT, 15 wt. % of PTFE M3 61.5 wt. % of synthetic graphite, 20.5 wt. % of MWCNT, 18 wt. % of PTFE

[0051] Membrane electrode assemblies were produced by means of various GDLs (substrate/MPL combinations S/M) and a catalyst-coated perfluorosulphonic acid-(e-PTFE) composite membrane (dry thickness 18 μm, noble metal loadings of 0.1 mg/cm.sup.2 of platinum on the anode and 0.4 mg/cm.sup.2 of platinum on the cathode).

TABLE-US-00002 Anode Cathode Membrane electrode assembly GDL/MPL GDL/MPL MEA 1 (reference example) S1/M1 S1/M1 MEA 2 (reference example) S2/M1 S2/M1 MEA 3 (reference example) S1/M3 S1/M3 MEA 4 (reference example) S2/M2 S2/M2 MEA 5 (MEA according to S1/M3 S2/M2 the invention)

[0052] Fuel cell tests on the various membrane electrode assemblies (MEA 1 to MEA 5) were carried out by means of an individual cell (active area 25 cm.sup.2) with current collectors of graphite with milled-in flow channels.

[0053] FIG. 2 shows the cell voltages measured (in millivolt) using various membrane electrode assemblies at a cell temperature of 80° C. for relative humidities of the gases of 25%, 50% and 100% at current densities of in each case 1.5 A/cm.sup.2 (FIG. 2, A) and 2 A/cm.sup.2 (FIG. 2, B). The supply pressure was 150 KPa and the stoichiometry for the anode was 1.5 and for the cathode 2.0 respectively. The contact pressure of the MEA on the graphite plates was 1 MPa.

[0054] The membrane electrode assembly (5) according to the invention renders possible at least comparable or significantly higher cell voltages than the symmetric membrane electrode assemblies (MEA 1 to 4) under all operating conditions.

LEGEND TO FIG. 1

[0055] (10) Carbon fibre structure of the anodic GDL [0056] (15) MPL of the anodic GDL [0057] (20) Anodic catalyst layer [0058] (25) Proton exchange membrane [0059] (30) Cathodic catalyst layer [0060] (35) MPL of the cathodic GDL [0061] (40) Carbon fibre structure of the cathodic GDL [0062] (50) Carbon nanotubes [0063] (60) Separators