CA2480345A1

CA2480345A1 - Ion exchange composite material based on proton conductive silica particles dispersed in a polymer matrix

Info

Publication number: CA2480345A1
Application number: CA002480345A
Authority: CA
Inventors: Philippe Bebin; Marc St-Arnaud
Original assignee: Individual
Current assignee: Sim Composites Inc
Priority date: 2002-03-28
Filing date: 2003-03-26
Publication date: 2003-10-09
Also published as: JP2005521777A; WO2003083985A2; EP1504486A2; KR100759143B1; AU2003212171A1; KR20040111458A; WO2003083985A3; AU2003212171A8

Abstract

The composite material comprises acid functionalized silica dispersed in a polymer matrix that is based on poly(aromatic ether ketones), or poly(benzoyl phenylene), or derivatives thereof. The composite material is characterized by good water retention capabilities due to the acidic functions and the hydrophilicity of the silica particles. Moreover, a good impermeability to gas and liquid fuels commonly used in fuel cell technology, like hydrogen gas or methanol solution, is also obtained due to the presence of silica particles.
Good mechanical properties of the composite material let the material to be formed easily in thin film or membrane form. In that form, the composite material is usable for proton exchange membrane for fuel cells, for drying or humidifying membrane for gas or solvent conditioning, or as acid catalytic membrane.

Description

ION EXCHANGE COMPOSITE MATERIAL BASED ON PROTON CONDUCTIVE
SILICA PARTICLES DISPERSED IN A POLYMER .MATRIX
TECHNICAL FIELD
The present invention relates to a composite material based on proton conductive silica particles dispersed in a polymer matrix. The present invention also relates to a method for producing the above composite material, and forming membranes therewith, that can for example be used for electrochemical devices, particularly for proton exchange membranes in fuel cells, as drying/humidifying membranes, for gas or solvent conditioning, or as acid catalysis membranes.
BACKGROUND ART
Ion exchange materials have numerous uses in several technological fields such as in electrochemical devices, for environmental needs, and in chemical reactions. Among ion exchange materials, proton conductive materials are under considerable studies because of the growing interest in clean power generation for which polymer electrolyte membrane fuel cells (PEMFC) are one of its important representatives.
The proton conductivity of a material can be obtained, for example, by incorporating proton exchange groups in the chemical structure of the material. The sulfonic acid function is one of the most efficient proton exchange group, however carboxylic or phosphonic acid groups or the like can also be used for proton mobility.
Many developments on perfluorinated or partially fluorinated polymers or copolymers bearing sulfonic .acid groups have taken place. This fariiily of materials can be found in the market under the commercial names of, for example, Nafion~
(Du Pont de Nemours and Co.) [US 3,282,875 ; US 4,330,654], Aciplex~ (Asahi Chemical Industry), FlemionT"" (Asahi Glass KK) or Gore-Select~ (W.L. Gore) [US

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5,635,041 ; US 5,547,551 ; US 5,599,614]. A phase separation between the hydrophilic acid regions and the hydrophobic fluorocarbon regions occurs and seems to contribute to the good proton conductivity in the material [T.D. Gierke, G.E. Munn, F.C. Wilson, J. Polym. Sci. Polym. Phys. Ed. 1981, 19, 1687 ; M. Fujimura, T.
Hashimoto, H. Kawai, Macromolecules, 1981, 14, 1309]. Unfortunately, at high temperatures (close to 100°C), water management becomes problematic, mainly because of the hydrophobicity of the fluorinated backbone of the material that causes a rapid dehydration of the membrane.
By comparison, non fluorinated but sulfonated polymers can also present good proton conductivity with less critical dehydration effects. A strong chemical structure, preferably an aromatic based structure, is essential to give the material a good stability at high temperatures. Interesting properties for fuel cell applications have already been demonstrated for polymers based on, for example, poly(aromatic ether ketone)s ([US 6,355,149]), poly(aromatic ether sulfone) or polyphenylene ([US
5,403,675]).
To reduce dimensional variations between the wet and dry states of the material and to enhance its water retention, some inorganic fillers can be added to the sulfonated polymer. In that case, proton conductivity is ensured by the organic phase while the inorganic phase helps retaining water and reduces material expansion ["Proceedings of 1998 Fuel Cell Seminar", November 16-19, Palm Spring, California].
The combination of the advantageous properties of the inorganic and organic phases is encountered in numerous developments of composite material dealing with the formation of a stable continuous proton conductive phase. In these developments, alkoxysilane derivatives are polymerized via sol-gel or co-condensation processes to lead mainly to three-dimensionally cross-linked silicon-oxygen based structures ([EP 1223632A2], [EP 056089981], [US 6,277,304]). Such C

kind of composite materials are promising but the control of their preparations is not easy and is often difficult to achieve. Moreover, such kind of structure does not easily offer some ion exchange capacity. Simpler composite preparations can present interesting solutions for the challenges of electrochemical devices, such as fuel cell membranes.
Japanese Patent Application PH 11-336986 published on June 8, 2001 under Publication Number P2001-155744 and filed in the name of Toyota Central R & D
Labs. Inc. describes a proton conductor based on a high molecular weight electrolyte comprising functionalized silica. Silica functionalized with sulfonic acid, carboxylic acid and phosphonic acid groups are mentioned. With respect to the electrolyte, the description is restricted to perfluoro sulfonic acid type polymers, styrene divinyl benzene sulfonic acid type polymers and styrene - ethylene - butadiene -styrene copolymers. In a specific example using sulfonated silica and a perfluoro sulfonic acid polymer, the membrane obtained has a current density of 0.5 volt at 1 A/cm2, which is not satisfactory. No data is available on the current density of the membrane obtained in the only other example. It has to be presumed that it is substantially the same or inferior to that of the membrane of example 1. There is therefore a need to provide an improved membrane in which the current density will give satisfaction.
Canadian Application No. 2,292,703 published on June 8, 2000 and filed in the name of Universite Laval, discloses an electrolytic membrane made of a polymer matrix and a filler material that contributes to the enhancement of the proton . conductivity of the membrane. In all the examples, the polymer matrix is based on an aromatic polyether ketone (PEEK) or a sulfonated derivative thereof (SPEEK) while the filler is BP04 or a heteropolyacid. This composite will not be time resistant because of the progressive solubilization of the filler into the polymer matrix.
There therefore exists a need for a composite material based on an inorganic phase dispersed in a polymer matrix that has a good proton exchange capacity, that C

can give membranes with excellent current density, that is time resistant, and that can be easily prepared.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome the problems mentioned previously.
It is another object of the invention to provide an ion exchange composite material that presents a relevant proton exchange capacity.
It is another object of the invention to provide a method for producing an ion exchange composite material in a membrane form that can be easily prepared.
It is another object of the present invention to provide a composite material adapted to form a membrane with good current density.
The above and other objects of the present invention may be achieved by providing a composite material comprising acid functionali~ed silica particles, the balance comprising a polymer based on poly(aromatic ether ketones), or poly (benzoyl phenylene), or derivatives thereof, the composite material being capable of providing a membrane with a current density of at least about 1 A/cm2 under 0.6V.
The composite material may be used in membrane form.

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DISCLOSURE OF INVENTION
In the composite material according to the invention, the silica particles are preferably functionalized with sulfonic, carboxylic and/or phosphonic acid groups, sulfonic acid groups being preferred.
According to a preferred embodiment, the composite material of the invention normally comprises at least about 10 weight percent, preferably 20 weight percent of functionalized silica particles.
The polymer used for constitute the polymer matrix may be acid functionalized, for example with sulfonic, carboxylic and/or phosphonic acid groups, or derivatives thereof.
The acid groups may be covalently bonded to the silica particles and/or to the polymer, for example through linear or ramified alkyl chains, linear or ramified aromatic chains, or a combination of alkyl and aromatic chains that are linear or ramified with a linear or ramified alkyl or aromatic chains, the chains optionally comprising heteroatoms and/or halogen atoms.
In the composite material according to the invention, the silica particles are preferably characterized by:
i. a surface area of 10 m2 per gram to 1500 m2 per gram, ii. silica particle dimension from 0,01 ~,m to 500 p,m, iii. silica pore diameter from 0 angstrom to 500 angstroms.
Ion exchange groups are usually present in the silica particles in amounts between 0.1 and 5.0 mmol/g.

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The acid groups are normally present in the polymer in amounts varying between 0 mmol/g and 5.0 mmol/g.
The membrane according to the invention are preferably intended for use in fuel cells, for humidifying or drying, in conditioning gas or solvent, or as an acid catalytic membrane.
The composite material can be easily prepared in a membrane form usable for electrochemical devices like proton exchange membranes for fuel cells, humidifying or drying membranes for gas or solvent conditioning, and acid catalytic membrane.
The silica particles are functionalized with acid moieties and, when dispersed inside the polymer matrix, they constitute an inorganic hydrophilic phase with a proton exchange capacity. The organic phase comprising the polymer matrix may contain ion exchange groups that are initially present in the chemical structure of the polymer, or ion exchange groups bonded to the chemical structure of the polymer to enhance the proton conductivity of the composite material. The proton exchange capacity is achieved by both the functionalized polymer matrix and the dispersed silica particles.
Several functional groups are appropriate to give the material a proton exchange capacity. Preferred functionalities are acid groups, more preferably sulfonic groups (-S03H). Other acid groups can also be grafted to the structures to give an interesting proton conductivity such as carboxylic (-C02H) or phosphonic (-P03H2) acid groups.
The ion exchange groups are preferably covalently bonded to the chemical structures of the organic and the inorganic phases. The chemical bonds are preferably made of alkyl or aromatic chains or a combination of both, linear or ramified, and can contain eventually some heteroatoms or halogen atoms.

As mentioned above, various kinds of silica can be used for the formation of the inorganic phase in the composite material. Preferred silica is porous silica, however other types may be used including but not limited to: amorphous silica, fumed silica, spherical silica, irregular silica, structured silica, molecular sieve silica, silesquioxane derivatives, and mixture thereof. The amount of silica particles and their average size play important roles in the formation of a continuous hydrophilic phase and in the mechanical properties of the material.
In the family of poly(aromatic ether ketones), the preferred polymer is the poly(oxy-1,4-phenylene-oxy-1,4-phenylene-carbonyl-1,4-phenylene) (PEEK) manufactured by Victrex (UK) and having the following formula:

o ~ o ~ c ~n The glass transition temperature of PEEK is typically about 200 °C, and it has the required thermal and chemical resistance to lead to a strong composite.
Sulfonation is a common way to modify a polymer structure by grafting sulfonic acid groups that give the sulfonated material proton exchange capacity. The capacity of proton mobility depends on the amount and on the dispersion of the acid groups in the material.
Several studies are currently available on the sulfonation of this kind of structures. One of the most suitable sulfonation methods for use in the present invention is with a sulfonation in concentrated H2S04, as described in EP 8895 and later in Br. Polym. J., vol. 17, 1985, p. 4. This sulfonation reaction is less damageable for the polymer than the chlorosulfonation route because no significant chain scission or degradation occurs. Once the polymer is sulfonated, the corresponding formula for the sulfonated PEEK is typically:

Q
_g_ O
O ~ 0 ~ C ~ O
S03H ~x ~ n-x The degree of sulfonation corresponds to x/n, with x corresponding to the number of repeat units carrying one sulfonic acid group. Then, PEEK with 100%
sulfonation has one acid group per repeat unit, or one acid group per three aromatic rings. The number of sulfonic acid groups per gram of sulfonated polymer determines the ion exchange capacity (IEC) of the polymer. For example, a 100% sulfonated PEEK has an IEC of 2,9 mmol/g.
The amount of sulfonic acid groups bonded to the aromatic rings depends on several parameters such as temperature, time, concentration of polymer in the acid.
Many properties of the sulfonated PEEK (SPEEK) such as its proton capacity, solubility, water retention, and expansion coefficients vary with its sulfonation rate, i.e. with its ion exchange capacity For the inorganic phase, the use of silica functionalized with sulfonic acid groups presents not only the advantage of the proton conductivity, but also a better efficiency in water retention than the non functionalized silica. Typically, the water retention of acid silica is twice higher than usual silica. For example, the water retention of acid silica is about 30% instead of 15% with usual silica in an environment under 70% of relative humidity.
The structure of silica also plays an important role in water retention. For example, a low bulk density structure increases the water retention in comparison to a high bulk density silica mainly because of its higher specific area.
Typically, a low bulk density structure can take twice more water than a high bulk density structure.
For example, the water retention of silica with a low bulk density structure is about 15% comparatively to 7% for silica with a high bulk density structure under 70% of C

relative humidity. Moreover, a large surface area, as encountered in a low bulk density structure, improves the loading of the acid functionality in the inorganic compound. For example, the loading of a functionalized low bulk density silica is typically 1,7 mmol/g while it is typically twice less with only 0.9 mmol/g for a porous high bulk density silica.
Low bulk density sulfonic acid silica can be typically prepared via a co-condensation process as described, for example, in Chem. Mater. 2000, Vol. 12, p.2448. Sulfonic acid groups can also be grafted on high bulk density silica using, for example, the method described in J. Chromato. 1976, Vo1.117, p.269. Several types of bonding are possible to link the sulfonic acid groups to the silica particles. In the present invention, preferred but not limited bonding deals with a propylphenyl chain.
The link may also comprise any kinds of alkyl derivatives or aromatic derivatives and combination thereof, with or without heteroatoms and/or halogens in the chemical structure.
The composite material is prepared by adding the acid silica particles into the polymer matrix and mixing both homogenously. A preferred method proceeds via a polymer solution in which the silica particles or a silica suspension in the same solvent or in a miscible solvent of the polymer solution are added. The suspension is then homogenized before being spread in a uniform thin layer and dried.
Satisfying mixture may also be obtained without using a solvent such as a melting phase based process.
The mechanical properties of the composite material depend mainly on the ones of the polymer matrix and on the silica content. Mechanical properties determine the lower limit of a film thickness that can be manipulated without breaking. A polymer that is too rigid does not allow enough deformation of a thin film without breaking while structures that are too flexible do not hold the composite material in a thin film form. In the same way, too many inorganic particles prevent a good tear resistance and make,the film particularly brittle.
The solubility properties of the composite material depend particularly on the ones of the polymer matrix. As previously mentioned, the solubility of the polymer depends on the temperature and on its ion exchange capacity. The maximum temperature at which the material may be used in a particular liquid such as water for the hydrated state is directly related to the solubility properties of the polymer.
Sufficient silica in the composite material, that may vary between 10 to 30 weight percent enhances proton conductivity to a degree that depends on the density of the corresponding silica used.
In the present invention, many parameters can easily be changed to adjust the properties of the final composite. Typically, the following parameters have to be considered for the formulation of the composite material: solvent solubility, utilization temperature, thickness of the material in final form, and the expected ion exchange value. The corresponding sulfonation rate of the polymer matrix is then determined.
The characteristics of the silica are subsequently evaluated considering mainly the porosity needs depending on the desired acid loading and water retention.
BRIEF DESCRIPTION OF DRAWINGS
The invention is illustrated by means of the annexed drawings in which FIGURE 1 is a polarization curve of current density versus voltage of a membrane according to the invention.
The invention is also illustrated by means of the following non limiting examples.

Sulfonation of PEEK
SPEEK with 55% of sulfonation is obtained, for example, by stirring 50g of PEEK in 2 I of H2S04 (95-98% in H20) for 48 hours at room temperature. The solution is poured in H20 and the solid phase, corresponding to sulfonated PEEK
(SPEEK), is washed vigorously 2 to 3 times in 5 I of pure water. The isolated solid is firstly dried in an oven at about 70 °C for one night and then, after another washing, it is dried at 100 °C under vacuum for several days. About 40 g of SPEEK
is obtained (yield ~ 80%). Elementary analysis gives the sulfur content of the sulfonated polymer and the corresponding ion exchange capacity (IEC) is then calculated. An IEC
of 1,6~0,1 mmol/g is obtained, corresponding to a sulfonation rate of about 55%.

Composite film preparation a) 1 g of 55% sulfonated PEEK (SPEEK55) is solubilized in 10 ml of dimethylformamide (DMF) at room temperature and filtered on filter paper. A
suspension of 0.2707 g of sulfonic acid grafted silica in 2 ml of DMF is added to the clear polymer solution. After stirring, the homogenous mixture is spread out over a 385 cm2 glass substrate before being dried at 70 °C for several days.
After the complete evaporation of the solvent, the film is easily removed from the glass substrate by immersion in water. Once dried, the thickness of the composite film, made of 80% in weight of a 55% sulfonated PEEK and of 20% in weight of acid silica, is 40~10 wm.
b) 0.1755 g of SPEEK55 is solubilized in 1.7 ml of DMF and filtered. 0.0195 g of sulfonic acid grafted silica is added to the polymer solution. After homogeneization, the mixture is spread out over a 25 cm2 glass substrate. Once dried, the composite film, comprising 90% in weight of a 55% sulfonated PEEK
and 10% in weight of acid silica, has a thickness of 50 pm.
~Yennpi ~ ~
Electrode deposition on composite films for fuel cell testing Commercial Pt/C electrodes (Pt/Vulcan XC-72 from ElectroChem Inc.) are stuck on composite films by spreading a small amount of SPEEK55 10% DMF
solution (w/v) on the side of the two electrodes that sandwich the membrane.
Assemblies are dried under vacuum at room temperature for one day, under vacuum at 60 °C for one night, and at 80 °C for several days.

Performance comparison The performance obtained with a membrane according to the present invention is compared to that obtained with a membrane according to JP 2001-155744 (example 1 ).
The composite material of the Japanese reference contains an inorganic phase mixed inside a polymer solution at 5% (w/v). The inorganic phase is fumed silica grafted with phenylsilane as coupling agent and is thereafter reacted with H2S04 cc. The organic phase is the binding agent of the inorganic phase. In the present case, Nafion~, a perFluorinated polymer bearing sulfonic acid groups, is used. For the experimentation, the fuel cell is operated at 80 °C under an H2/air atmosphere at 22 psig. Under voltage from 0.6 V to 0.7 V, the fuel cell generates a current density of 0.5 A/cm2 while under 0.5 V, it generates 1 A/cm2. It will be noted that the membrane according to the Japanese reference contains 1 weight percent silica, while the membrane according to the present invention contains 20 weight percent silica.
The composite material according to the present invention contains an inorganic phase mixed inside a polymer solution at 10% (w/v). The inorganic phase contains silica obtained by co-condensation and fu.nctionalized by chlorosulfonation.
The organic phase is SPEEK. For the experimentation, the fuel cell is operated at 75 °C under an H2/air atmosphere at 20/30 psig. Under a voltage of 0.7 V, the fuel cell generates a current density of 1 A/cm2, under 0.6 V, it generates 1.7 A/cm2 to 1.8 A/cm2, and under 0.5 V, it generates 2.2 A/cm2 to 2.3 A/cm2.
Under similar operating conditions, the present invention generates a much higher current density than that of the Japanese patent, as will be seen from FIGURE
1 wherein the material used is made of 20 weight percent silica containing 1.4 mmol of sulfonic acid groups per gram and 80 weight percent of SPEEK55 prepared as in example 1.
It is understood that the invention is not restricted to the above embodiments and that many modifications are possible within the scope of the appended claims.

Claims

1. A composite material comprising acid functionalized silica particles, the balance comprising a polymer matrixbased on poly(aromatic ether ketones), or poly (benzoyl phenylene), or derivatives thereof, said composite material capable of providing a membrane with a current density of at least about 1 A/cm2 under 0.6V.

2. Composite material according to claim 1, wherein said functionalized silica particles are dispersed in said polymer matrix.

3. Composite material according to claim 1, wherein said silica particles are functionalized with sulfonic, carboxylic and/or phosphonic acid groups.

4. Composite material according to claim 2, wherein said silica particles are functionalized with sulfonic acid groups.

5. Composite material according to claim 1, which comprises at least about 10 weight percent of acid functionalized silica particles.

6. Composite material according to claim 5, which comprises at least about 20 weight percent of acid functionalized silica particles.

7. Composite material according to claim 1, wherein said polymer is acid functionalized.

8. Composite material according to claim 7, wherein said polymer is functionalized with sulfonic, carboxylic and/or phosphonic acid groups, or derivatives thereof.

9. Composite material according to claim 2 or 8, wherein the acid groups are covalently bonded to the silica particles and/or the polymer.

10. Composite material according to claim 9, wherein said acid groups are covalently bonded through linear or ramified alkyl chains, linear or ramified aromatic chains, or a combination of alkyl and aromatic chains that are linear or ramified with a linear or ramified alkyl or aromatic chains, said chains optionally comprising heteroatoms and/or halogen atoms.

11. Composite material according to claim 1, wherein said silica particles are characterized by:
i. a surface area of 10 m2 per gram to 1500 m2 per gram, ii. silica particle dimension from 0,01 µm to 500 µm, iii. silica pore diameter from 0 angstrom to 500 angstroms.

12. Composite material according to claim 1, wherein ion exchange groups are present in said silica particles in amounts between 0.1 and 5.0 mmol/g.

13. Composite material according to claim 7, wherein said acid groups are present in the polymer in amounts varying between 0 mmol/g and 5.0 mmol/g.

14. Composite material according to claim 1, wherein said silica is selected from the group consisting of amorphous silica or derivatives thereof, fumed silica or derivatives thereof, spherical silica or derivatives thereof, porous irregular silica or derivatives thereof, porous structure silica or derivatives thereof, irregular porous molecular sieve silica or derivatives thereof, spherical porous molecular sieve silica or derivatives thereof, and a silsesquioxane compound or derivatives thereof.

15. Composite material according to claim 1 wherein said polymer is a poly(aryl ether ketone) (PEEK) or derivatives thereof.

16. Composite material according to claim 1 wherein said polymer is a poly(benzoyl phenylene) (PBP) or derivatives thereof.

17. A membrane comprising a composite material according to claims 1 to 16

18. Membrane according to claim 17, for use in fuel cells.

19. Membrane according to claim 17, for use in humidifying or drying.

20. Membrane according to claim 17, for use in conditioning gas or solvent.

21. Membrane according to claim 17, for use as an acid catalytic membrane.