FUEL CELL GAS DIFFUSION LAYER MADE UP OF A CARBON SUBSTRATE GRAFTED WITH AN AROMATIC GROUPING

Abstract

The present invention relates to a gas diffusion layer for a fuel cell, made of a carbon substrate grafted with at least one aromatic group having formula (II):

##STR00001##

wherein: the asterisk * designates a carbon atom with no hydrogen and no R.sup.i group, with i=1 to 5, and covalently bonded to the carbon substrate; at least two of the R.sup.1, R.sup.2, R.sup.3, R.sup.4, and R.sup.5 groups are different from a hydrogen atom; at least two of the R.sup.1, R.sup.2, R.sup.3, R.sup.4, and R.sup.5 groups are hydrophobic groups or hydrophilic groups or a hydrophobic group and a hydrophilic group.

Claims

1. A gas diffusion layer for a fuel cell, made of a carbon substrate grafted with at least one aromatic group having formula (II): ##STR00006## wherein: the asterisk * designates a carbon atom with no hydrogen and no R.sup.i group, with i=1 to 5, and covalently bonded to the carbon substrate; at least two of the R.sup.1, R.sup.2, R.sup.3, R.sup.4, and R.sup.5 groups are different from a hydrogen atom; at least two of the R.sup.1, R.sup.2, R.sup.3, R.sup.4, and R.sup.5 groups are hydrophobic groups or hydrophilic groups or a hydrophobic group and a hydrophilic group.

2. The gas diffusion layer of claim 1, wherein the R.sup.1, R.sup.2, R.sup.3, R.sup.4, and R.sup.5 groups are selected independently from one another from the group comprising: C(═O)O.sup.−Y.sup.+; SO.sub.3.sup.−Y.sup.+; CH.sub.2—SO.sub.3.sup.−Y.sup.+; NR.sub.3.sup.+X.sup.−; OH; PO.sub.3H.sup.−Y.sup.+; H; F; C.sub.nF.sub.2n+1; C.sub.nH.sub.2n+1; NO.sub.2; —O—CH.sub.2—O—; imidazole groups; and derivatives of imidazole groups; with Y=H, Na, K, Li, NR′.sub.4, X=F, Cl, Br, I; n being an integer in the range from 1 to 10; R=C.sub.m,H.sub.2m+1; R′=H, C.sub.mH.sub.2m+1 and mixtures thereof, m being an integer in the range from 1 to 10.

3. The gas diffusion layer of claim 2, wherein the R.sup.1, R.sup.2, R.sup.3, R.sup.4 and R.sup.5 groups correspond to one of the following combinations: R.sup.1=R.sup.3=R.sup.5=H and R.sup.2=R.sup.4=CF.sub.3; R.sup.1=R.sup.4=R.sup.5=H and R.sup.2+R.sup.3=—O—CH.sub.2—O—; R.sup.l=R.sup.3=R.sup.5=H; R.sup.2=CF.sub.3 and R.sup.4 =C(═O)O.sup.−Y.sup.+; and R.sup.l=R.sup.3=R.sup.5=H; R.sup.1=SO.sub.3.sup.−Y.sup.+and R.sup.4 =CF.sub.3.

4. The gas diffusion layer of claim 1, wherein the carbon substrate appears in the form of a material selected from the group comprising: carbon nonwoven; carbon fabric; carbon felt; carbon cloth; carbon paper; and graphite; carbon black; carbon nanotubes; and graphene.

5. The gas diffusion layer of claim 1, wherein the carbon substrate has a thickness in the range from 100 to 500 micrometers.

6. The gas diffusion layer of claim 1, wherein said layer is obtained by the placing into contact of a carbon substrate with a solution of diazonium salt having formula (I), and then by reduction of the diazonium salt having formula (I) ##STR00007##

7. The gas diffusion layer of claim 6, wherein the carbon substrate is oxidized prior to its placing into contact with the diazonium salt solution.

8. The gas diffusion layer of claim 6, wherein the reduction of the diazonium salt is performed: electrochemically by electrochemical generation of an electron by a diazonium salt molecule; or chemically.

9. The gas diffusion layer of claim 6, wherein the placing into contact of the carbon substrate with the aqueous solution comprises: grafting a first main surface of the substrate with a first solution of a first diazonium salt having formula (I); and then grafting a second main surface of the substrate with a second solution of a second diazonium salt having formula (I), the first and the second diazonium salts being different from each other.

10. A fuel cell comprising at least one gas diffusion layer of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0101] FIG. 1 corresponds to the cyclic voltammogram obtained for a carbon nonwoven before and after a hydrophobic grafting.

[0102] FIG. 2 corresponds to the XPS spectrums of a carbon substrate before and after the grafting.

[0103] FIG. 3 corresponds to an enlarged view of region C1s of the XPS spectrums of a carbon substrate before and after a grafting.

[0104] FIG. 4 corresponds to an enlarged view of region F1s of the XPS spectrums of a carbon substrate after a grafting.

[0105] FIG. 5 corresponds to the curves of polarization at 60° C. and 100% of relative humidity of a carbon substrate before and after a grafting according to the present invention.

[0106] FIG. 6 corresponds to the curves of polarization at 80° C. and 50% of relative humidity of a carbon substrate before and after a grafting according to the present invention.

[0107] FIG. 7 corresponds to the curves of polarization at 80° C. and 20% of relative humidity of a carbon substrate before and after a grafting according to the present invention.

EMBODIMENTS OF THE INVENTION

[0108] Four gas diffusion layers (GDL) have been prepared. These are counter-examples (CE-1 to CE-3) and a GDL according to the present invention (INV-1). The GDLs have been prepared and tested in a button cell configuration.

TABLE-US-00001 TABLE 1 Characteristics of the studied GDLs hydrophobic Example GDL.sup.(a) treatment grafted molecule CE-1 carbon — — CE-2 carbon (b) — CE-3 carbon (c) 1-CF.sub.3 INV-1 carbon (d) 2-CF.sub.3 .sup.(a)carbon nonwoven (SGL reference GDL 24AA, material with no hydrophobic treatment) (b) hydrophobic treatment obtained by deposition (immersion and then sintering) of PTFE (polytetrafluoroethylene), the commercial reference being: SGL's GDL 24BA comprising 5 wt. % of PTFE. (c) grafting with 1-CF.sub.3 (4-(trifluoromethyl)phenyl)) (c) grafting with 2-CF.sub.3 (3,5-bis(trifluoromethyl)phenyl))

1) Preparation of the GDL According to the Invention (INV-1)

[0109] A carbon nonwoven is grafted with a hydrophobic group of type 2-CF3 (INV-1).

[0110] The grafting comprises the steps of: [0111] following a diazonium salt by addition of an excess of oxidizing agent (NaNO.sub.2, 4 mM, in excess with respect to the amine) in an aqueous solution of hydrochloric acid (0.5 M) and of amine (2 mM), the amine being 3,5-bis(trifluoromethyl)aniline (CAS number: 328-74-5). [0112] electrochemically reducing the diazonium salt in the presence of a carbon nonwoven. At least 50 cycles of linear variations of a potential from 0.2 V to -1.0 v vs Hg/Hg.sub.2SO.sub.4 have been carried out. In such conditions, a larger number of cycles changes nothing to the grafting method. [0113] obtaining a hydrophobic grafted GDL.

2) Detection of the Grafting by Cyclovoltammetry

[0114] The grafting is characterized by cyclic voltammetry of the grafted GDL (CE-3 and INV-1) in the presence of the ferrocyanide/ferricyanide redox couple ([Fe(CN).sub.6].sup.4−/[Fe(CN).sub.6].sup.3−) which is very sensitive to the surface condition of the studied material.

[0115] FIG. 1 shows the results obtained before the grafting (CE-1) and after the grafting (CE-3). It is a cyclic voltammogram obtained at 50 mV/s in the presence of the [Fe(CN).sub.6].sup.4- /[Fe(CN).sub.6].sup.3- couple.

[0116] Before the grafting, the redox peaks of the ferrocyanide/ferricyanide couple are effectively present (CE-1).

[0117] After the grafting (CE-3), the redox peaks of the ferrocyanide/ferricyanide couple are no longer visible. Indeed, the grafted species generate a resistance to charge transfer for the [Fe(CN).sub.6].sup.4- /[Fe(CN).sub.6].sup.3- couple, which confirms the grafting of the hydrophobic group of type 1-CF.sub.3.

3) XPS Characterization

[0118] X-ray photoelectronic spectroscopy (XPS) is a non-destructive technique applied to the extreme surface (analysis depth ˜5 nm) which enables to test the electronic structure and the chemical modifications at the GDL surface after grafting.

[0119] The general spectrum recorded after the grafting of the diazonium salts of compounds 1-CF.sub.3 and 2-CF.sub.3 on gas diffusion layers (GDL) indicates the presence of fluorinated groups (FIGS. 2 and 3).

[0120] The high-resolution analysis of core peaks C1s, F1s and O1s provides information relative to the efficiency of the grafting method (FIGS. 2 to 4)

[0121] Before the grafting, the GDL is similar to graphite-type carbon, with a strong presence of sp2-hybridized carbon atoms and (π-π*) plasmonic oscillation peaks.

[0122] After the grafting, core peak C1s indicates the appearing of a new peak towards high bonding energies (292.45 eV) attributed to carbon atoms bonded to three fluorine atoms.

[0123] The further presence of peaks C 1 s with a sp2 hybridization and of the (π-π*) plasmonic bands indicates that the grafting has effectively occurred. This result is confirmed by the detection of peak F1s at 688.0 eV which is the signature of fluorine and —Ph—CF.sub.3 bond.

[0124] Core peak C1s also shows that the surface of the GDLs is functionalized by other C—OH, —COO, and —Ph—CH.sub.2 groups.

[0125] The presence of —OH, —OOH groups would induce a hydrophilic character at the surface of the carbon GDLs.

[0126] XPS spectroscopy also enables to follow the hydrophobic/hydrophilic character of the samples by quantizing the atomic percentage of the C—F/C—O, —COO at the surface of the GDL after grafting.

4) Cell Tests

[0127] Two membrane/electrode assemblies (MEA) have been prepared by hot pressing of two GDLs (while adding a microporous layer and a catalytic layer common to all MEAs) on each side of a Nafion® proton-exchange membrane. The pressing step enables to provide a good connection between surfaces and a good cohesion between the GDL and the membrane.

[0128] Three types of conditions have been tested to detect the hydrophobic character of GDLs: [0129] humid conditions; 60° C. and 100% RH (relative humidity) (FIG. 5); [0130] automobile conditions: 80° C. and 50% RH (FIG. 6): and [0131] conditions 80° C. +20% RH (FIG. 7).

[0132] FIG. 5 shows that the performance corresponding to the grafted GDL (INV-1) is greater than that of the hydrophobically-treated GDL CE-2.

[0133] Further, no limitation to mass transport can be observed, which indicates that there is no drowning in the cell, particularly at the cathode.

[0134] FIG. 5 also shows that the performance corresponding to grafted GDLs (CE-3 and INV-1) are better than those of a GDL having been submitted to a hydrophobic treatment (CE-2). Further, the performance of the grafted GDL according to the invention (INV-1) is better than that of the GDL grafted with a monosubstituted compound (CE-3). The performance of GDL CE-1 is not shown since it is too low.

[0135] FIG. 6 shows that the performance corresponding to the GDL grafted with a bisubstituted compound (INV-1) is better than all others.

[0136] More generally, FIG. 7 shows that the performance of the tested GDLs may be classified in the following order: INV-1>CE-3>CE-2>CE-1.

[0137] The less advantageous results relate to the GDL having been submitted to no hydrophobic treatment (CE-1). A PTFE-type hydrophobic treatment enables to improve the performance (CE-2). However, the best results relates to GDLs grafted with a mono-(CE-3) or bisubstituted (INV-1) compound.

[0138] Surprisingly, the Applicant has observed that the presence of two substituents enables to improve the performance of the GDL even though such modifications may generate other problems such as an increase of steric constraints.

[0139] Further, the presence of two substituents may enable to simultaneously introduce hydrophobic and hydrophilic properties.