Method for preparing proton-conducting particles suitable for catalysing oxygen reduction or hydrogen oxidation by grafting specific proton-conducting polymers to the surface of same

Abstract

A method for preparing particles comprising a material suitable for catalysing oxygen reduction or hydrogen oxidation, the particles being grafted by grafts consisting of at least one specific polymer comprising at least one repeating styrene unit bearing at least one proton-conducting group.

Claims

1. ATRP initiator meeting following formula (I): ##STR00021## where: the R.sup.1 groups are each independently an organic spacer group; the Z groups are each independently an alkylene group; and the R.sup.2 groups are each independently a halogen atom.

2. The ATRP initiator according to claim 1 meeting following formula (VI): ##STR00022##

3. Method for preparing particles comprising a material able to catalyse oxygen reduction or hydrogen oxidation, said particles being grafted with grafts composed of at least one polymer comprising at least styrene repeating unit carrying at least one proton-conducting group, said method comprising: a) a step to prepare at least one styrene polymer via ATRP polymerisation of a styrene monomer with an ATRP initiator meeting following formula (I): ##STR00023## where: the R.sup.1 groups are each independently an organic spacer group; the Z groups are each independently a single bond or an organic spacer group; the R.sup.2 groups are each independently a halogen atom; the at least one styrene polymer meeting following formula (II): ##STR00024## where Y corresponds to the styrene repeating unit carrying at least one proton-conducting group and n.sub.1 to a number of repeats of the repeating unit between round brackets, R.sup.1, R.sup.2 and Z being such as defined above; b) a step to place the particles comprising the material able to catalyse oxygen reduction or hydrogen oxidation in contact with the polymer obtained at a), after which particles are obtained grafted with grafts of following formula (III): ##STR00025## the brace indicating the point at which the grafts are covalently bonded to the particles, and R.sup.1, R.sup.2, Z, Y and n.sub.1 being such as defined above.

4. The method according to claim 3, wherein R.sup.1 and Z are each independently an alkylene group.

5. The method according to claim 3, wherein the ZR.sup.2 groups are at para position relative to the COO groups.

6. The method according to claim 3, wherein the ATRP initiator is a compound of following formula (IV): ##STR00026##

7. The method according to claim 3, wherein the proton-conducting group is a sulfonic acid group SO.sub.3H, carboxylic acid group CO.sub.2H or phosphonic acid group PO.sub.3H.sub.2, these groups optionally being present in salt form.

8. The method according to claim 3, wherein the styrene monomer is a monomer of following formula (V): ##STR00027## where: Z.sup.1 corresponds to a phenylene group; and E corresponds to a proton-conducting group, optionally in salt form.

9. The method according to claim 3, wherein the styrene monomer is a sodium styrenesulfonate monomer.

10. The method according to claim 3, wherein the particles are platinum particles.

11. The method according to claim 3, wherein the particles are metal particles.

12. The method according to claim 11, wherein the metal particles are particles comprising a noble metal or mixture thereof.

13. The method according to claim 3, wherein the particles are additionally bound to a carbon material.

14. The method according to claim 13, wherein the carbon material is selected from among graphite, carbon black, carbon fibres, carbon tubes, graphene.

15. The method according to claim 13 also comprising before step a) and/or b), a step to prepare particles comprising the material able to catalyse oxygen reduction or hydrogen oxidation.

16. Polymer meeting following formula (II): ##STR00028## where R.sup.1, R.sup.2, Z, Y and n.sub.1 meet the same definition as those given in claim 3.

17. The polymer according to claim 16, meeting following formula (VII): ##STR00029## where n.sub.1 corresponds to the definition given in claim 16 and X is a sulfonic acid group SO.sub.3H, a carboxylic acid group CO.sub.2H or phosphonic acid group PO.sub.3H.sub.2, these groups optionally being present in salt form.

18. Particles able to be obtained with a method according to claim 3, said particles comprising the material able to catalyse oxygen reduction or hydrogen oxidation, said particles being grafted with grafts of following formula (III): ##STR00030## where R.sup.1, R.sup.2, Z, Y and n.sub.1 meet the same definition as those given in claim 3.

19. The particles according to claim 18, wherein Y is a repeating unit resulting from the repeat of a sodium styrenesulfonate monomer.

20. The particles according to claim 18, that are platinum particles.

21. Electrode comprising particles such as defined in claim 18.

22. Fuel cell comprising at least one electrode-membrane-electrode assembly, wherein at least one of the electrodes is an electrode such as defined in claim 21.

Description

BRIEF DESCRIPTION OF THE SINGLE FIGURE

(1) The single FIGURE is a voltammogram illustrating the trend in intensity I (in A) as a function of the potential E (in V) vs. RHE obtained with a device comprising an electrode conforming to the invention such as defined in Example 4.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

Example 1

(2) This example illustrates the preparation of an ATRP initiator conforming to the invention: disulfanediyldiethane-2,1-diyl bis[4-(chloromethyl)benzoate] of following formula (VI):

(3) ##STR00017##

(4) For this preparation, a 100 mL two-neck flask is charged under an inert atmosphere with 2-hydroxyethyldisulfide (1.53 g; 9.9 mmol; 1 eq.), chloroform (30 mL) and triethylamine (4.22 g; 41.7 mmol; 4.2 eq.). The two-neck flask is sealed under argon then immersed in an ice bath at 0 C.

(5) 4-chloromethylbenzoyl chloride (2.06 g; 10.9 mmol; 1.1 eq.) is added dropwise. The mixture is left to return to ambient temperature overnight. The resulting reaction mixture is washed 4 times (one acid wash, one neutral wash, one basic wash followed by a neutral wash). The organic phases are combined and dried. The organic solvent is removed using a rotary evaporator. The solid product obtained is then dried overnight in an oven at 60 C.

(6) The resulting product (with a yield of 97%) corresponded to the expected product having the above formula as shown by .sup.1H NMR analyses and elementary analysis, the results of which are given below.

(7) .sup.1H NMR (400 MHz, CDCl.sub.3, =7.26 ppm): 7.9 (m, 2H, H.sub.aromatic), 7.4 (m, 2H, H.sub.aromatic), 4.5 (m, 4H, Ph-CH.sub.2CI and OCH.sub.2CH.sub.2), 3.0 (t, 2H, OCH.sub.2CH.sub.2S).

(8) Elementary analysis (in %): (C.sub.20H.sub.20Cl.sub.2O.sub.4S.sub.2), C: 52.1; H: 4.4; CI: 15.5; O: 14; S: 14.

Example 2

(9) This example illustrates the preparation of a polymer able to be schematised by the following formula below:

(10) ##STR00018##
with n.sub.1 corresponding to the number of repeats of the unit between brackets.

(11) For this preparation, first a 100 mL two-neck flask is subjected to vacuum heat treatment comprising 3 cycles with a heating phase and a cooling phase to ambient temperature.

(12) Ultrapure water sold under the trademark MILLIQ (48 mL) is placed in the two-neck flask and vacuum degassed by bubbling argon (15 minutes). Sodium styrenesulfonate (18.8 g; 90 mmol; 1000 eq.) is then added under a stream of argon and vacuum degasification continued with argon.

(13) In parallel, methanol (16 mL) is vacuum degassed by bubbling argon (15 minutes) in a 25 mL conical flask. The initiator prepared in Example 1 (50 mg; 0.09 mmol; 1 eq.) is then added under argon.

(14) When the monomer is fully dissolved in water, bipyridine (116 mg; 0.74 mmol; 8 eq.) and copper chloride (37 mg; 0.37 mmol; 4 eq.) are added under a stream of argon.

(15) Argon is bubbled through the system whilst applying a vacuum.

(16) The initiator solution in methanol is added using a syringe (20 mL) taking care that it is conditioned under argon. Finally, three vacuum-argon cycles are performed.

(17) The two-neck flask is placed in an oil bath previously heated to 45 C. After a polymerisation time of about 21 hours, the reaction is halted by placing the system in air. The solution changes from a brown colour to a green-blue colour.

(18) The reaction mixture is filtered over silica gel to remove the chloride ions contained in the catalytic system and trapped in the polymer.

(19) The filtrate is concentrated in vacuo to increase the polymer concentration and hence facilitate precipitation.

(20) Finally, the polymer is precipitated in cold methanol.

(21) The polymer obtained is a tacky, whitish solid and is placed in an oven overnight at 65 C.

(22) The resulting polymer corresponded to the expected product of the above formula as shown by .sup.1H NMR analyses, the results of which are given below.

(23) .sup.1H NMR (D.sub.2O) : 7.5 (broad s, aromatic proton), 6.6 broad s, aromatic proton), 1.4 (broad s, methyl proton).

Example 3

(24) This example illustrates the preparation of platinum particles bound to a carbon material of carbon black type (called VULCAN XC72 in the formula below), represented by the following formula:

(25) ##STR00019##
with a method involving a so-called water-in-oil emulsion.

(26) Heptane (37.4 g) and a polyoxyethylene (4) Lauryl Ether sold under the trademark BRIJ 30 (8.6 g) are poured into a reactor. Immediately, the reactor is vigorously and manually agitated to prevent the precipitation of BRIJ 30, until the mixture is fully translucent.

(27) In parallel, a hexahydrate platinum salt H.sub.2PtCl.sub.6.6H.sub.2O (257.6 mg; 0.5 mmol, 1 eq.) is dissolved in 2 mL of ultrapure water sold under the trademark MILLIQ in a pill bottle. The solution is strongly agitated until it is fully homogeneous.

(28) 1.6 mL of the metal salt solution are added to the above-mentioned reactor and manually agitated until the mixture becomes limpid. This mixture is of orangish-yellow colour.

(29) The resulting mixture is left to stand for a time of 15 minutes so that it stabilises.

(30) Sodium borohydride (152 mg; 4 mmol; 15 eq.) is rapidly added to the mixture in a single time and the mixture immediately agitated vigorously (the sodium borohydride having to reduce the metal before the water). The mixture changes to an intense black colour.

(31) The mixture is left to stand for 1 to 2 hours so that the platinum is fully reduced and the NaBH.sub.4 fully deactivated.

(32) After two hours, the mixture is placed under ultrasound for 5 minutes, with agitation from time to time, so that there is no longer any deposit at the bottom of the reactor.

(33) VULCANXC 72 carbon black (previously finely ground) is added to the mixture whilst it is still subjected to ultrasound which is continued for 5 minutes after this addition, maintaining sporadic agitation. After 5 minutes, the reactor is strongly agitated manually and, after verifying that the carbon black does not deposit at the bottom of the reactor, it is again subjected to ultrasound for 5 minutes before being manually agitated a further time. These operations are repeated until the carbon no longer deposits at the bottom of the reactor.

(34) Once the carbon black is well suspended, the reactor is left in the ultrasound bath. Acetone (1 volume of acetone per one volume of microemulsion) is added gradually, agitating manually with each addition phase. The resulting mixture is left for 5 to 10 minutes in the ultrasound bath after the addition is completed.

(35) The particles are isolated by vacuum ultrafiltration on a hydrophilic membrane in polyvinylidene fluoride (PVDF), sold under the trademark DURAPORE (0.22 m; GVWP 04700). The platinum particles supported on the carbon material (carbon black) are washed by filtration with cycles of 4*30 mL acetone, 3*30 mL ethanol and 4*30 mL of ultrapure water sold under the trademark MILLIQ (with agitation between each wash). The particles obtained are placed for 2 hours in an oven at a temperature of 135 C., to remove the last traces of BRIJ 30.

(36) The yield is quantitative.

(37) The particles obtained were analysed by elementary analysis confirming the presence of carbon (60%) and platinum (40%), indicating that the platinum particles were supported on the carbon material.

Example 4

(38) This example illustrates the preparation of platinum particles prepared according to Example 3 and grafted with the polymer prepared in Example 2, these particles therefore being grafted with grafts of the following formula:

(39) ##STR00020##
n.sub.1 indicating the number of repeats of the repeating unit between round brackets.

(40) For this preparation, the particles prepared in Example 3 are placed in a 25 mL round-bottom flask with hexylamine. The flask is placed in an ultrasound bath for 15 minutes to obtain a homogeneous suspension of particles. The polymer obtained in Example 2 is placed in solution in a water/hexylamine mixture (50:50 by volume) and then added to the flask. The whole is placed under magnetic stirring for 12 hours.

(41) The functionalised platinum particles are precipitated in acetone and subjected to different wash steps (3*30 mL acetone, 3*30 mL ethanol and 3*30 mL water).

(42) These wash steps allow the removal of traces of any polymers that were not grafted onto the particles.

(43) The particles are placed in an oven overnight at 65 C.

Example 5

(44) In this example, the particles obtained in Example 4 are subjected to different analyses to analyse: electrochemical characterization of these particles; and in-cell testing of the particles.

(45) c) Electrochemical Characterization of the Particles

(46) Characterization of the resistance of the organic ring was performed in a supporting medium (argon) with a cell having three electrodes. In this cell, the reference electrode was a reversible hydrogen electrode (RHE) having a fixed, known electrochemical potential. The second electrode was an auxiliary electrode called counter-electrode (CE) composed of an inert material, in our case a plate of glassy carbon used for current collection. The third electrode was a working electrode (WE) on which the catalyst was examined. A gas input/output system (GI/GO) was added to operate under a controlled atmosphere. Measurements were taken by cycling the electrode potential between 0.05 V vs. RHE and an upper potential limit of successively 1 V vs. RHE (potential positioned after the start of the oxidation reaction on the platinum surface and corresponding to the potential of the cathode of a PEMFC at open-circuit), 1.1 and 1.2 V vs. RHE (strongly oxidizing potential), the results being given in the voltammogram in the single FIGURE.

(47) On successive scans passing through electrode potentials higher than 1.0 V vs. RHE, the recorded currents were stable and characteristic of a modified platinum surface, confirming the presence after several cycles of the organic ring.

(48) By way of comparison, for particles grafted with grafts of formula (I) such as defined above, these exhibited instability after 1 V vs. RHE.

(49) Catalytic activity and selectivity are major properties when choosing a catalyst.

(50) Characterization of the materials in an oxygen-saturated acid medium allowed study of their catalytic behaviour with regard to the oxygen reduction reaction. The trace of the voltammograms was equivalent to that obtained with catalysts of a powdered carbon black sold under the trademark VULCAN XC72/Pt type. For all the characterized materials, the total number of exchanged electrons was 4 at between 0.7 and 0.4 V vs. RHE. Oxygen reduction was therefore complete for water formation.

(51) d) In-Cell Testing

(52) For this testing, cells were prepared comprising a NAFION NR212 membrane and two gas diffusion electrodes (anode and cathode) respectively containing at the anode and cathode 0.4 mg/cm of particles of the invention (Cell 1) and, for comparison, particles grafted with grafts of formula (I) such as defined above (Cell 2).

(53) The gas diffusion electrodes were prepared simply by pouring a catalytic ink (comprising the particles in an ethanol/water mixture (3:1)) onto a fabric followed by evaporation at 50 C. for 4 hours.

(54) The tests were conducted in a 5 cm.sup.2 mono-cell under H.sub.2/O.sub.2 at a pressure of 3 bars, 60 C. and 21% humidity.

(55) With Cell 1 of the invention, better open-circuit voltage (OCV) was obtained (1 V vs. 0.91 V for Cell 2), better activation at between 0 and 0.1 A/cm.sup.2 (in other words, indicating faster set-up of redox reactions within the cell) as well as lesser ohmic drop.

(56) Under these conditions, the maximum power delivered by Cell 1 is 250 mW/cm.sup.2 compared with 140 mW/cm.sup.2 for Cell 2, which corresponds to an improvement of 64%.