Particles suitable for catalyzing oxygen reduction or hydrogen oxidation and being proton-conducting by grafting specific proton-conducting fluorinated polymers to the surface thereof

Abstract

Proton-conducting, fluorinated polymer grafted particles for use in the preparation of catalytic layers for fuel cells, such as H.sub.2/air cells or H.sub.2/O.sub.2 cells. The grafted particles include a particle made of a material for catalyzing oxygen reduction or hydrogen oxidation, such as a platinum particle, that has been grafted with a proton-conducting, fluorinated polymer graft. The proton-conducting, fluorinated polymer graft includes an organic spacer group, a single bond or an organic spacer group, a repeating unit resulting from polymerization of a fluorinated styrenic monomer, and a repeating unit resulting, from polymerization of a non-fluorinated styrenic monomer bearing at least one proton-conducting group.

Claims

1. Particles comprising a material for catalysing oxygen reduction or hydrogen oxidation and being grafted by grafts having the following formula (III): ##STR00044## a curly bracket indicating the location at which the grafts are bound in a covalent manner to the particles in which: R.sup.1 represents an organic spacer group; Z represents a single bond or an organic spacer group; R.sup.2 represents a halogen atom; Y.sup.1 corresponds to a repeating unit resulting from polymerisation of a fluorinated styrenic monomer, and n.sub.1 corresponds to a number of repetitions of the repeating unit placed within parentheses, this number of repetitions being a positive integer that is at least equal to 2; Y.sup.2 corresponds to a repeating unit resulting from polymerisation of a non-fluorinated styrenic monomer bearing at least one proton-conducting group, and n.sub.2 corresponds to a number of repetitions of the repeating unit placed within parentheses, this number of repetitions being equal to 0 or being a positive integer that is at least equal to 2; and the group —Z—(Y.sup.1).sub.n1—(Y.sup.2).sub.n2—R.sup.2, which intersects a carbon-carbon bond of the phenyl group, signifying that it can be bound to any one of the carbon atoms of the phenyl group.

2. Particles according to claim 1, in which the group —Z—(Y.sup.1).sub.n1—(Y.sup.2).sub.n2—R.sup.2 is in the para-position relative to the group —CO—O—.

3. Particles according to claim 1, in which R.sup.1 and Z represent an alkylene group and R.sup.2 represents a halogen atom.

4. Particles according to claim 1, in which Y.sup.1 represents a repeating unit having the following formula (XV): ##STR00045## E.sup.1 corresponds to a single bond or an organic spacer group; and E.sup.2 corresponds to a proton-conducting group.

5. Particles according to claim 1 in which, when Y.sup.2 exists, Y.sup.2 is a repeating unit resulting from the polymerisation of a styrenic monomer having the following formula (XII): ##STR00046## wherein: Z.sup.2 corresponds to a phenylene group; and E.sup.3 corresponds to a proton-conducting group.

6. Particles according to claim 1, in which the particles are platinum particles bound to a carbonaceous material selected from graphite, carbon black, carbon fibers, carbon tubes, and graphene.

7. Particles according to claim 1, in which n.sub.2 is equal to 0.

8. Particles according to claim 1, which are platinum particles grafted with grafts having the following formulas: ##STR00047## wherein R is a hydrogen atom or a cation and n.sub.1 corresponds to the number of repetitions of the repeating unit placed within parentheses, this number of repetitions being a positive integer that is at least equal to 2.

9. An electrode comprising particles as defined according to claim 1.

10. A fuel cell comprising at least one electrode-membrane-electrode assembly, in which at least one of the electrodes thereof is an electrode as defined in claim 9.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a polarisation curve illustrating the evolution of the voltage E (in V) as a function of the current density D (in A/cm.sup.2).

(2) FIG. 2 represents the evolution curves showing evolution of the voltage E (in V) as a function of the current density D (in A/cm.sup.2) (curve a′ for Cell 1, curve b′ for Cell 2 and curve c′ for Cell 3).

(3) FIG. 3 represents the evolution curves showing evolution of the voltage E (in V) as a function of time T (in hours) (curve a′ for Cell 1, curve b′ for Cell 2 and curve c′ for Cell 3).

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

Example 1

(4) This example illustrates the preparation of platinum particles bound to a carbon black-type carbonaceous material (in the following formula referred to as “Vulcan XC72”) according to the following reaction scheme:

(5) ##STR00034##

(6) During an initial stage, the salt H.sub.2PtCl.sub.6*6H.sub.2O (267 mg) is dissolved in 100 mL of ethylene glycol. The pH is then about 0.8. It is adjusted to 11 by adding of a solution of sodium hydroxide in the ethylene glycol.

(7) The previously finely ground carbon (Vulcan XC72; 0.145 mg) is then added to the solution as prepared here above and the resulting admixture is placed under ultrasound until total dispersion of the carbon is obtained.

(8) The mixture is then heated by microwave irradiation and under an inert atmosphere of nitrogen (heat-up time 5 minutes, 5 minutes at 100° C., power of 1600 W and impulsion/pulse at 80%).

(9) The pH obtained at the end of the synthesis is equal to 11 at a temperature of 18° C. This pH is adjusted to 2 by adding of a solution of hydrochloric acid and then 50 mL of Milli-Q ultra pure water are added in order to homogenise the mixture. The solution is then placed under ultrasound for a period of 5 minutes.

(10) The particles obtained are isolated by ultrafiltration then rinsed abundantly with Milli-Q ultra pure water and then dried at 60° C. in an oven before being heat treated at 200° C. for a period of 2 hours in an oven.

Example 2

(11) This example illustrates the preparation of a polymer by means of the ATRP process with a specific ATRP initiator which may be represented schematically by the following formula:

(12) ##STR00035##
with n.sub.1 corresponding to the number of repetitions of the repeating unit placed within parentheses,
the reaction scheme of the polymerisation being as follows:

(13) ##STR00036##

(14) Two tests (referred to as Test 1 and Test 2) were carried out with different quantities for the monomer, the other ingredients being used based on the same quantities for these two tests.

(15) In order to do this, during an initial stage, a 50 mL two neck flask is subjected to a heat treatment under vacuum comprising of 3 cycles with a heating phase and a cooling phase for cooling at room temperature.

(16) Then a quantity of dimethylsulfoxide (DMSO) degassed under vacuum by inducing bubbling of argon (15 minutes) is introduced into the two neck flask. 2,3,4,5,6-pentafluorostyrene (6.7 mL, 1000 eq for Test 1 and 16.8 mL, 2500 eq for Test 2) and the ARTP initiator (23 mg, 0.1 mmol, 1 eq) are introduced under an argon stream and the argon is returned to bubble under vacuum.

(17) When the initiator is completely dissolved in the reaction mixture, bipyridine (58 mg, 8 eq) and copper chloride (19 mg, 4 eq) are introduced under the flow of an argon stream. Three vacuum-argon cycles are finally performed.

(18) Then the two neck flask is set in place in an oil bath heated in advance to 80° C. After a period of about 5 hours of polymerisation, the reaction is stopped by allowing the system to be aired. The solution thus changes colour going from a brown to a green-blue colour.

(19) The polymer is finally precipitated in isopropanol and is recovered in the form of a sticky white solid before drying.

(20) The polymer is dried overnight in an oven at 60° C.

(21) The resulting polymer corresponds to the expected product having the formula noted here above according to the .sup.1H NMR and .sup.19F NMR spectroscopic analyses, the results of which are given here below.

(22) .sup.1H NMR (300, 13 MHz, THF-d8, δ=1.73 ppm) δ: 2.9 (s large, CH); 2 (s large, CH.sub.2 of the repeating unit)

(23) .sup.19F NMR (282.40 MHz, THF-d8, ppm): −143 (m, F ortho), −157 (m, para); —164 (m, meta)

(24) The .sup.1H NMR analysis also demonstrates that the degree of conversion of the monomers is comprised between 60 and 70%.

(25) Steric exclusion chromatography in tetrahydrofuran made it possible to determine the molar masses by number (Mn) and by weight (Mw), the polydispersity indices (Ip). The results are shown in the table here below (the first line being for Test 1 and the second line being for Test 2).

(26) TABLE-US-00001 DP.sub.n theoretical M.sub.n theoretical Conversion .sup.a M.sub.n experimental.sup.b M.sub.w experimental.sup.b I.sub.p 1000 194 600 68 132 300 158 800 1.20 2500 485 700 63 306 000 385 600 1.26 .sup.a calculated by .sup.1H NMR; .sup.bcalculated by SEC analysis in THF

Example 3

(27) This example illustrates the preparation of a sulfurised polymer obtained by means of sulfurisation of the polymers obtained in the Tests 1 and 2 of Example 2.

(28) The reaction scheme is as follows:

(29) ##STR00037##

(30) In a 25 mL flask the polymer obtained in Example 2 (2 g) is dispersed in dimethylsulfoxide (DMSO) (20 mL) at room temperature (RT) for a period of 1 hour. Then hydrated sodium hydrogen sulphate (1.2 eq in relation to the number of moles of monomer units) is introduced slowly into the flask at room temperature (RT). It is observed that there is a change in colour from pale yellow to blue and then disappearance of the blue colouration. At the end of 30 minutes the reaction mixture takes on a blue colour and then, at the end of the reaction (after a period of 2 hours), the medium constitutes a homogeneous system that is blue in colour and exhibits a high viscosity.

(31) The crude reaction mixture is diluted with water and is precipitated two times in isopropanol.

(32) The resulting polymer is a yellow solid, which is dried overnight in the oven at 60° C.

(33) The resulting polymer corresponds to the expected product having the formula noted here above according to the .sup.19F NMR spectroscopic analysis, the results of which are given here below.

(34) .sup.19F NMR (282.40 MHz, D.sub.2O): −138 (m, F meta), −151 (m, F ortho)

(35) They provide substantiation, in particular, of the disappearance of the signal relative to fluorine in the para-position, this position being now occupied by a group —SNa.

Example 4

(36) This example illustrates the preparation of a sulfonated polymer obtained by means of sulfonation of the polymer obtained in Example 3.

(37) The reaction scheme is as follows:

(38) ##STR00038##

(39) In a 100 ml flask, the polymer obtained in this Example (2 g) is placed in suspension in formic acid for a period of 30 minutes. The flask is then placed in an ice bath. After 15 minutes, hydrogen peroxide (2 eq in relation to the number of monomer units) is introduced drop by drop. The mixture is then placed at room temperature for a period of 18 hours and then set to be refluxed for a period of 5 hours.

(40) The polymer is precipitated in isopropanol and then added into an aqueous solution of sodium hydroxide (1 M). The mixture is then agitated for a period of 24 hours. The polymer is filtered and rinsed with isopropanol. The polymer is placed in an oven at 60° C. overnight.

(41) The resulting polymer corresponds to the expected product having the formula noted here above according to the IR and .sup.19F NMR spectroscopic analyses, the results of which are given below.

(42) IR (cm.sup.−1): 1160 and 1025 (signals corresponding to the group O═S═O)

(43) .sup.19F NMR (282.40 MHz, D.sub.2O): −138 (m, F meta), −142 (m, F ortho)

Example 5

(44) This example illustrates the preparation of a sulfonated polymer having a sulfur-containing organic spacer group obtained by means of sulfonation according to the following reaction scheme:

(45) ##STR00039##

(46) In a 25 mL flask, the polymer obtained in Example 3 (2 g) is placed in suspension in 1M sodium hydroxide at room temperature for a period of 1 hour. 1,3-Propanesultone (1.2 eq in relation to the number of moles of monomer units) is diluted in dioxane and then added drop by drop at room temperature. The mixture is then set to be refluxed for a period of 24 hours.

(47) The mixture is filtered and washed with water. A colourless gel is obtained.

(48) The polymer obtained is dried in an oven at 60° C. overnight.

(49) The resulting polymer corresponds to the expected product having the formula noted here above according to the .sup.19F NMR spectroscopic analysis, the results of which are given here below.

(50) .sup.19F NMR (282.40 MHz, D.sub.2O, ppm): −150 (m, F meta), −164 (m, F ortho).

Example 6

(51) This example illustrates the preparation of a sulfonated polymer comprising an oxygenated organic spacer group obtained by means of atom transfer radical polymerisation (ATRP) of a specific monomer according to the following reaction scheme:

(52) ##STR00040##

(53) In order to do this, it is necessary during an initial stage to proceed with the preparation of the specific monomer: sodium 3-(2,3,5,6-tetrafluoro-4-vinylphenoxy) propane-1-sulfonate (step a) followed by polymerisation of the monomer with an ATRP initiator.

a) Synthesis of sodium 3-(2,3,5,6-tetrafluoro-4-vinylphenoxy) propane-1-sulfonate

(54) This step illustrates the synthesis of the monomer sodium 3-(2,3,5,6-tetrafluoro-4-vinylphenoxy) propane-1-sulfonate having the following formula:

(55) ##STR00041##

(56) In order to do this, hydroxylated tetrafluorostyrene (the group —OH being in the para-position relative to the ethylene group) (1.5 g, 5 mmol, 1 eq) is dissolved in methanol and then sodium hydroxide (10 mmol, 2 eq) is introduced. The mixture is placed under magnetic agitation at room temperature until complete dissolution of the base is achieved. A solution of 1,3-propanesultone (0.73 g, 6 mmol, 1.2 eq) in dioxane is introduced drop by drop. The mixture is agitated at room temperature for a period of one hour and then set to be refluxed for a period of 24 hours. The solvents are eliminated by rotary evaporation and the residue is washed with dichloromethane three times. The product is then recrystallised two times in a mixture of methanol/water (2:1).

(57) The resulting monomer corresponds to the expected product having the formula noted here above according to the IR, .sup.1H NMR and .sup.19F NMR spectroscopic analyses, the results of which are given below.

(58) IR (cm.sup.−1): 1183 and 1060 (signals corresponding to the group O═S═O)

(59) .sup.19F NMR (282.40 MHz, D.sub.2O, ppm): −145 (2F, m, F meta), −160 (2F, m, F ortho)

(60) .sup.1H NMR (300.13 MHz, DMSO-d6, δ=4.75 ppm): 6.68 (dd, 1H, alkenyl CH), 5.80 (dd, 2H, alkene CH.sub.2), 4.42 (t, 2H, O—CH.sub.2—CH.sub.2—CH.sub.2—SO.sub.3Na), 3.14 (t, 2H, O—CH.sub.2—CH.sub.2—CH.sub.2—SO.sub.3Na), 2.22 (m, 2H, —CH.sub.2—CH.sub.2—CH.sub.2—SO.sub.3Na).

b) Polymerisation of Sodium 3-(2,3,5,6-tetrafluoro-4-vinylphenoxy) propane-1-sulfonate

(61) In order to do this, during an initial stage, a 100 mL two neck flask is subjected to a heat treatment under vacuum comprising of 3 cycles with a heating phase and a cooling phase for cooling at room temperature.

(62) Then the Milli-Q ultra pure water degassed under vacuum by inducing bubbling of argon (15 minutes), is introduced into the two neck flask. Sodium 3-(2,3,5,6-tetrafluoro-4-vinylphenoxy) propane-1-sulfonate (2500 eq) is introduced under the flow of an argon stream and the argon is again set to bubble under vacuum.

(63) In parallel, in a 25 mL piriform flask, methanol (16 mL) is degassed under vacuum by inducing bubbling of argon (15 minutes) and the ATRP initiator (1 eq) is then introduced under the flow of an argon stream.

(64) When the monomer is fully dissolved in water, bipyridine (116 mg) and copper chloride (37 mg) are introduced under the flow of an argon stream.

(65) The argon is set to bubble through the system while drawn under vacuum.

(66) The initiator solution in the methanol is introduced with a syringe (20 ml, being careful to condition the syringe under argon) while maintaining a flow of argon. Three vacuum-argon cycles are finally performed.

(67) The two neck flask is set in place in an oil bath heated in advance to 45° C. After a period of about 21 hours of polymerisation, the reaction is stopped by allowing the system to be aired. The solution thus changes colour going from a brown to a green-blue colour.

(68) The mixture is filtered on silica gel in order to remove the copper II ions (Cu.sup.2+) contained in the catalyst system and trapped by the polymer.

(69) The polymer is finally precipitated in cold methanol and is recovered in the form of a sticky before drying.

(70) The polymer is then placed in an oven at 65° C. for 1 night.

(71) The resulting polymer corresponds to the expected product having the formula noted here above according to the IR and .sup.19F NMR spectroscopic analyses, the results of which are given here below.

(72) .sup.19F NMR (282.40 MHz, D.sub.2O, ppm): −150 (m, F in meta), −165 (m, F in ortho)

(73) IR (cm.sup.−1): 1183 and 1040 (signals corresponding to the group O═S═O)

Example 7

(74) This example illustrates the preparation of platinum particles prepared according to Example 1 grafted with the polymer prepared in Example 5, these particles thus being grafted with grafts having the following formula:

(75) ##STR00042##
n.sub.1 indicating the number of repetitions of the repeating unit placed within parentheses.

(76) In order to do this, the particles prepared in Example 1 (100 mg) and hexylamine (10 ml) are introduced into a 25 ml flask. The flask is placed for 15 minutes in an ultrasonic bath in order for the suspension of particles to become homogeneous. The polymer obtained in Example 5 (2 mg) is dissolved in a water/hexylamine mixture (50/50 by volume) and is then introduced into the flask. The latter in its entirety is then placed under magnetic agitation for a period of 12 hours.

(77) The functionalised platinum particles are precipitated in acetone and then subjected to various different steps of washing (3*30 ml of acetone, 3*30 ml of ethanol and 3*30 ml of water).

(78) These washing steps make it possible to eliminate the traces of polymers that are likely to not have been grafted on to the particles.

(79) The particles are then placed overnight in an oven at 65° C.

(80) A similar preparation may easily be envisaged with the polymers prepared as in Examples 4 and 6.

Example 8

(81) In this example, the particles obtained in Example 7 are incorporated in the electrodes and then subjected to in-cell (referred to as Cell 1) tests intended for purposes of comparison with respect to a cell that is similar but including an electrode comprising platinum particles supported on carbon dispersed in Nafion® (referred to as Cell 2) and a similar cell including an electrode comprising particles grafted with grafts having the following formula (I′).

(82) ##STR00043##
with n.sub.1 indicating the number of repetitions of the repeating unit placed within parentheses, referred to as cell 3.

(83) In order to do this, each cell includes an electrode-membrane-electrode assembly comprising: an anode of the gas diffusion electrode type comprising 0.2 mg/cm.sup.2 of commercially available platinum particles bound to a carbonaceous material (these particles being ungrafted); a cathode of the gas diffusion electrode type comprising 0.4 mg/cm.sup.2 of specific platinum particles; a membrane made of Nafion® NRE 211 disposed between the anode and the cathode.

(84) For Cell 1, the cathode comprises the platinum particles obtained in Example 7.

(85) For Cell 2, the cathode comprises the platinum particles supported on carbon dispersed in Nafion®.

(86) For Cell 3, the cathode comprises platinum particles supported on carbon grafted with grafts having the formula (I′) noted here above.

(87) The electrode-membrane-electrode assemblies are produced according to the following operating protocol.

(88) Regardless of whether they are for the anode or the cathode, these are prepared by simply casting a catalytic ink comprising the platinum particles concerned in a mixture of ethanol/water (3:1) on a Sigracet® 24BC gas diffusion layer (GDL).

(89) Prior to being placed in the assembly, the Nafion® membrane is previously treated by hot pressing by having a reinforcement backing pressed on to each side thereof at a temperature of 110° C. and a pressure of 3 MPa for 90 seconds.

(90) Finally, the gas-diffusion electrodes (anode and cathode) are pressed on to each side of the Nafion® membrane thus treated in advance, at a temperature of 115° C., and then at a pressure of 3.5 MPa for 150 seconds at a temperature of 135° C.

(91) The tests are carried out in a single cell of 5 cm.sup.2 under H.sub.2/O.sub.2 (stoichiometry λ.sub.O2=1.5 and λ.sub.H2=1.5) under a pressure of 2 bar, at 60° C. and at 21% humidity.

(92) The evolution curves showing evolution of the voltage E (in V) as a function of the current density (in A/cm.sup.2) are reported in FIG. 2 (curve a′ for Cell 1, curve b′ for Cell 2 and curve c′ for Cell 3).

(93) The curve b′ represents the behaviour of a conventional electrode constituted of platinum nanoparticles supported on carbon and dispersed in Nafion®. At high current densities, an inflection of the curve that is characteristic of a start of flooding of the electrodes is observed.

(94) The curve c′ represents the behaviour of an electrode constituted of platinum nanoparticles supported on carbon and grafted with grafts having the formula (I′). At high current densities, it is also observed that there is a slight inflection of the curve.

(95) The curve a′ represents the behaviour of an electrode according to the invention. At high current densities, it is observed that there is no more inflection of the curve, which is a reflection of the absence of flooding of the electrodes.

(96) The durability of such cells has also been tested, this test consisting in following the evolution as a function of time of the cell voltage at fixed current density (1 A.Math.cm.sup.−2).

(97) The tests were conducted at 36° C., which represents the equilibrium temperature of an unheated cell.

(98) The results are reported in FIG. 3, with the curve a′ for the Cell 1, the curve b′ for the Cell 2 and the curve c′ for the Cell 3.

(99) The cell according to the invention exhibits a lesser decline, which reflects a lower sensitivity to aging. This phenomenon is explained by the better chemical stability of the particles of the invention in the corrosive medium of the cell.