Method for producing proton-conducting platinum particles with a large active surface area and surface-grafted with specific, proton-conducting polymers

Abstract

A specific method for preparing platinum particles grafted with proton-conducting polymers and use of these particles as catalysts for oxygen reduction.

Claims

1. Method for preparing platinum particles bonded to a carbon material, said particles being grafted with grafts consisting of at least one polymer comprising at least one styrene repeating unit having at least one proton-conducting group, said method comprising: a) a step of preparing said platinum particles bonded to a carbon material comprising an operation of heating under microwaves of a mixture comprising a platinum salt, said carbon material and at least one polyol compound, subject to which said particles are obtained; b) a step of preparing at least one ethylene polymer by ATRP polymerisation of an ethylene monomer with an ATRP initiator having the following formula (I): ##STR00017## wherein: the R.sup.1 groups represent, independently of one another, an organic spacer group; the Z groups represent, independently of one another, a single bond or an organic spacer group; the R.sup.2 groups represent, independently of one another, a halogen atom; the resulting polymer having the following formula (II): ##STR00018## wherein Y corresponds to the styrene repeating unit having at least one proton-conducting group and n.sub.1 to the repetition number of the repeating unit taken in parentheses, the R.sup.1, R.sup.2 and Z being such as defined hereinabove; c) a step of contacting particles obtained in a) with the polymer obtained in b), subject to which particles are obtained grafted with grafts having the following formula (III): ##STR00019## the brace indicating the location where the grafts are bonded, covalently, to the particles and the R.sup.1, R.sup.2, Z, Y and n.sub.1 being such as defined hereinabove.

2. Method according to claim 1, wherein the platinum salt is a platinum halide salt.

3. Method according to claim 1, wherein the carbon material is graphite, carbon black, carbon fibres, carbon tubes or graphene.

4. Method as claimed in claim 1, wherein the carbon material is carbon black.

5. Method as claimed in claim 1, wherein the polyol compound is a hydrocarbon compound comprising at least two carbon atoms each having at least one OH group.

6. Method as claimed in claim 1, wherein the polyol compound is ethylene glycol.

7. Method as claimed in claim 1, wherein the step a) is carried out at a basic pH.

8. Method as claimed in claim 1, wherein R.sup.1 and Z represent, independently of one another, an alkylene group.

9. Method as claimed in claim 1, wherein the ZR.sup.2 groups are located in para position with respect to the COO groups.

10. Method as claimed in claim 1, wherein the ATRP initiator is a compound having the following formula (IV): ##STR00020##

11. Method as claimed in claim 1, wherein the proton-conducting group is a sulphonic acid group SO.sub.3H, a carboxylic acid group CO.sub.2H or a phosphonic acid group PO.sub.3H.sub.2, these groups being able to be present optionally in the form of salts.

12. Method as claimed in claim 1, wherein the styrene monomer is a monomer having the following formula (V): ##STR00021## wherein: Z.sup.1 corresponds to a phenylene group; and E corresponds to a proton-conducting group, optionally in the form of a salt.

13. Method as claimed in claim 1, wherein the styrene monomer is a sodium styrene sulphonate monomer.

14. Platinum particles being able to be obtained by a method according to claim 1, said platinum particles being bonded to a carbon material and being grafted with grafts having the following formula (III): ##STR00022## wherein R.sup.1, R.sup.2, Z, Y and n.sub.1 have the same meaning as those given in claim 1.

15. Particles according to claim 14, wherein Y is a repeating unit coming from the repetition of a sodium styrene sulphonate monomer.

16. Electrode comprising particles such as defined according to claim 14.

17. Fuel cell comprising at least one electrode-membrane-electrode assembly, wherein at least one of its electrodes is an electrode such as defined in claim 16.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The FIG. 1 shows polarisation curves representing the change in the voltage E (in V) according to the current density D (in A/cm.sup.2) for the Cell 1 (curve a) and the Cell 2 (curve b) of the example 5 and of the power curves representing the change in the power density P (in kW/cm.sup.2) according to the current density D (in A/cm.sup.2) (respectively curve a) and b) for the Cells 1 and 2).

(2) The FIG. 2 shows polarisation curves representing the change in the voltage E (in V) according to the current density D (in A/cm.sup.2) (respectively curves a), b), c) and d) for cells comprising particles of tests 2, 3, 4 and 7) and the power curves representing the change in the power density P (in kW/cm.sup.2) according to the current density D (in A/cm.sup.2) (respectively curves a), b), c) and d) for cells comprising particles of tests 2, 3, 4 and 7).

(3) The FIG. 3 shows polarisation curves representing the change in the voltage E (in V) according to the current density D (in A/cm.sup.2) (respectively curves a), b), c) and d) for cells comprising particles of tests 6, 7, 8 and 9) and the power curves representing the change in the power density P (in kW/cm.sup.2) according to the current density D (in A/cm.sup.2) (respectively curves a), b), c) and d) for cells comprising particles of tests 6, 7, 8 and 9).

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

Example 1

(4) This example shows the preparation of an ATRP initiator in accordance with the invention: disulfanediyldiethane-2,1-diyl bis[4-(chloromethyl)benzoate] having the following formula (IV):

(5) ##STR00013##

(6) To do this, in a 100 mL bicol are introduced, under an inert atmosphere, 2-hydroxyethyldisulphide (1.53 g; 9.9 mmol; 1 eq.), chloroform (30 mL) and triethylamine (4.22 g; 41.7 mmol; 4.2 eq.). The bicol is sealed under argon then immersed into an ice bath at 0 C.

(7) Then, 4-chloromethylbenzoyl chloride (2.06 g; 10.9 mmol; 1.1 eq.) is introduced drop-by-drop. Then the mixture is allowed to return to ambient temperature for one night. The resulting reaction mixture is washed 4 times (an acid wash, a neutral wash, a basic wash then a neutral wash). The organic phases are gathered together and dried. The organic solvent is then removed in the rotary evaporator. The solid product obtained is then dried in the oven at 60 C. for one night.

(8) The resulting product (with an output of 97%) corresponds to the product expected having the formula hereinabove according to the .sup.1H NMR spectroscopy and elemental analysis, of which the results are provided hereinbelow.

(9) .sup.1H NMR (400 MHz, CDCl.sub.3. =7.26 ppm): 8.5 (s, 1H, NH); 7.5-7.4 (m, 4H, H aromatic); 2.0 (s, 6H, CH.sub.3)

(10) .sup.13C NMR (100 MHz, CDCl.sub.3, =77.0 ppm): 170 (s, CO); 137.2 (s, C.sub.qS); 132.8 (s, C.sub.qNH); 130.1 (d, HNC.sub.qC.sub.HC.sub.HS); 120.5 (d, HN, C.sub.qC.sub.HC.sub.HS); 63.0 (s, BrC(CH.sub.3).sub.2); 32.5 (q, CH.sub.3)

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

Example 2

(12) This example shows the preparation of a polymer that can be diagrammed by the following formula hereinbelow:

(13) ##STR00014##
with n.sub.1 corresponding to the repetition number of the unit taken in parentheses.

(14) Various tests were implemented with different quantities of monomers (X).

(15) To do this, in a first time, a 100 mL bicol is subjected to a thermal treatment under vacuum comprising 3 cycles with a heating phase and a cooling phase at ambient temperature.

(16) The MilliQ water (48 mL) is introduced into the bicol and is degassed under vacuum by bubbling argon (15 minutes). Sodium styrenesulfonate (X g; Y mol; Z eq.) is then introduced under a flow of argon and argon is again bubbled in a vacuum.

(17) In parallel, methanol (16 mL) is degassed under vacuum by bubbling argon (15 minutes) in a 25 mL pear-shaped flask. The initiator prepared in example 1 (50 mg; 0.09 mmol; 1 eq.) is then introduced under a flow of argon.

(18) When the monomer is perfectly dissolved in the water, bipyridine (116 mg; 0.74 mmol; 8 eq.) and copper chloride (37 mg; 0.37 mmol; 4 eq.) are introduced under a flow of argon.

(19) Argon is set to bubble in the system while drawing under vacuum.

(20) The solution of initiator in the methanol is introduced with a syringe (20 mL) ensuring that the latter is conditioned under argon. Three vacuum-argon cycles are finally carried out.

(21) The bicol is finally set in place in an oil bath heated beforehand to 45 C. After approximately 21 hours of polymerisation, the reaction is stopped by venting the system. The solution changes from a brown colour to a green-blue colour.

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

(23) The filtrate is then concentrated under vacuum, in order to increase the concentration in polymer and therefore facilitate precipitation.

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

(25) The polymer obtained is a tacky white solid then is placed in the oven at 65 C. for 1 night.

(26) The resulting polymer corresponds to the product expected having the formula hereinabove according to the .sup.1H NMR analyses, of which the results are provided hereinbelow.

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

(28) The quantities of monomers used are summarised in the following table:

(29) TABLE-US-00001 Degree of Monomer Number Number of polymerisation weight (g) of moles equivalents sought (DP) X Y (in moles) Z 500 9.4 0.045 500 1,000 18.8 0.09 1,000 1,500 28.2 0.135 1,500 2,000 37.6 0.18 2,000 2,500 47 0.225 2,500

(30) The characteristics of the polymers obtained for each one of the tests are listed in the following table.

(31) TABLE-US-00002 Degree of Molar weight of the Degree of polymerisation polymer experimental sought (DP) obtained (in g) polymerisation 500 76,000 370 1,000 140,000 680 1,500 222,000 1,080 2,000 300,000 1,460 2,500 359,000 1,740

Example 3

(32) This example shows the preparation of platinum particles bonded to a carbon material of the carbon black type (denoted, in the formula hereinbelow Vulcan XC72), represented by the formula hereinbelow:

(33) ##STR00015##
according to two methods: a method that is not in accordance with the invention involving a microemulsion referred to as water-in-oil (referred to as method 3a); and a method in accordance with the invention involving a polyol medium and a heating by microwave radiation (referred to as method 3b).

(34) a) Synthesis of the Particles Via Microemulsion Referred to as Water-in-Oil

(35) Heptane (37.4 g) and Brij 30 (8.6 g) are poured into a reactor. The reactor is then manually stirred and immediately vigorously, in order to prevent the precipitation of the Brij 30, until the mixture is translucent.

(36) In parallel, a platinum salt hexahydrate H.sub.2PtCl.sub.6.6H.sub.2O (257.6 mg; 0.5 mmol, 1 eq.) is dissolved in 2 mL of milliQ water in a beaker. The solution is stirred well, until the solution is homogenous.

(37) 1.6 mL of the solution of metallic salt is added to the abovementioned reactor then it is stirred manually, until the mixture is limpid. This mixture has an orange yellow colour.

(38) The resulting mixture is left to rest for a period of 15 minutes, so that it can stabilise.

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

(40) The mixture is left at rest for a period from 1 to 2 hours, in such a way that the platinum is entirely reduced and that the NaBH.sub.4 is entirely deactivated.

(41) After two hours, the mixture is placed under ultrasounds for 5 minutes, stirring from time to time, in such a way that there is no longer any deposit at the bottom of the reactor.

(42) Vulcan XC 72 carbon black (finely ground beforehand) is added to the mixture, while the latter is still subjected to ultrasounds, the latter being maintained for 5 minutes, once the adding is carried out, while still maintaining a sporadic stirring. After 5 minutes, the reactor is strongly stirred manually then, after verification that the carbon black is not being deposited at the bottom of the reactor, the latter is again subjected to ultrasounds for 5 minutes before being, once again, manually stirred. This manipulation is reiterated, until the carbon is no longer deposited at the bottom of the reactor.

(43) Then, once the carbon black is in suspension, the reactor is left in the ultrasound bath. Acetone (1 volume of acetone for one volume of microemulsion) is added little by little, by stirring manually at each phase of the adding. The resulting mixture is left 5 to 10 minutes in the ultrasound bath after the end of the adding.

(44) The particles are then isolated by ultrafiltration on a hydrophilic membrane made of polyvinylidene fluoride (PVDF) Durapore (0.22 m; GVWP 04700) under vacuum. The platinum particles supported on the carbon material (the carbon black) are washed by filtration by cycles of 4*30 mL of acetone, 4*30 mL of ethanol and 4*30 mL of MilliQ water (with a stirring between each washing). The particles obtained are then placed for 2 hours in the oven at a temperature of 135 C., in order to remove the last traces of Brij 30.

(45) The output is quantitative.

(46) The particles obtained are analysed via elemental analysis attesting to the presence of carbon (at a rate of 60%) and of platinum (at a rate of 40%), which demonstrates that the platinum particles are supported on the carbon material.

(47) b) Synthesis of Particles in a Polyol Medium Via Microwaves

(48) In a first step, a platinum salt hexahydrate having formula H.sub.2PtCl.sub.6.6H.sub.2O (257 mg) is dissolved in 100 mL of ethylene glycol. The pH is then approximately 0.8. It is adjusted to 11 by adding a solution of soda.

(49) Vulcan XC 72 carbon black (finely ground beforehand) (0.145 mg) is then added to the solution obtained beforehand and the resulting mixture is placed under ultrasounds, until complete dispersion of the carbon black.

(50) The mixture is heated by microwave radiation in a MARSXPress microwave oven from CEM under an inert nitrogen atmosphere (Rise in temperature from 20 C. to 100 C. at atmospheric pressure and at a power of 1,600 W; Maintaining for 5 minutes at 100 C. at 1,600 W; Pulse: 80%).

(51) The pH obtained at the end of the synthesis is equal to 11 at a temperature of 18 C. The pH is adjusted to 2 by adding a solution of hydrochloric acid, then 50 mL of milliQ water are added in order to homogenise the mixture. The resulting mixture is then placed under ultrasound for 5 minutes.

(52) The particles are then isolated via ultrafiltration then rinsed abundantly with milliQ water and finally dried at 60 C. in the oven before being placed at 200 C. for 2 hours in the oven.

Example 4

(53) This example shows the preparation of platinum particles prepared according to the modes of preparing of the example 3 grafted with the polymer prepared in the example 2, with these particles being as such grafted with grafts having the following formula:

(54) ##STR00016##
n.sub.1 indicating the repetition number of the unit taken in parentheses.

(55) Whether for the particles prepared according to the method 3a) or particles prepared according to the method 3b), the preparation protocol is as follows.

(56) The particles prepared in the example 3 and hexylamine are introduced into a 25 mL flask. The flask is placed 15 minutes in an ultrasound bath, so that the suspension of particles is homogeneous. The polymer obtained in the example 2 is put into in a water/hexylamine mixture (50/50 by volume) then is introduced into the flask. The whole is placed under magnetic stirring for 12 hours.

(57) The functionalised platinum particles are isolated by ultrafiltration then are subjected to various steps starting with a precipitation in acetone then subjected to various steps of washing (3*30 mL of acetone, 3*30 mL of ethanol and 3*30 mL of water).

(58) These washing steps make it possible to remove the traces of polymers that may not have been grafted on the particles.

(59) The particles are then placed for one night in the oven at 65 C. The table hereinbelow groups together the results of the various tests conducted according to the particulars of the protocol disclosed hereinabove.

(60) TABLE-US-00003 Molar Quantity Quantity Theoretical Experimental weight of of polymer polymer Number of of the polymer particles weight weight moles of polymer engaged engaged* ratio ratio* polymer/g of Test (g) (in mg) (in mg) (in %) (in %) particles 1 76,000 1.7 100 1.69 1.69 5.6*10.sup.7 (Method 3b) 2 140,000 3.1 100 3.05 3.05 5.6*10.sup.7 (Method 3b) 3 222,000 5.0 100 4.76 4.76 5.6*10.sup.7 (Method 3b) 4 300,000 6.8 100 6.33 6.33 5.6*10.sup.7 (Method 3b) 5 359,000 2.8 100 2.75 2.75 2.0*10.sup.7 (Method 3b) 6 359,000 4.0 100 3.89 3.88 2.8*10.sup.7 (Method 3b) 7 359,000 8.1 100 7.48 7.48 5.6*10.sup.7 (Method 3b) 8 359,000 16.2 100 13.92 13.92 1.1*10.sup.6 (Method 3b) 9 359,000 24.3 100 19.52 19.52 1.7*10.sup.6 (Method 3b) 10 359,000 32.3 100 24.44 24.43 2.3*10.sup.6 (Method 3b) 11 0 0 100 0 0 0 (Method 3a) 12 0 0 100 0 0 0 (Method 3b) 13 140,000 3.1 100 3.05 3.05 5.6*10.sup.7 (Method 3a) *measured by elemental analysis

Example 5

(61) In this example, particles obtained in the example 4 are subjected to electrochemical analyses and tests in cells.

(62) a) Electrochemical Characterisation of Particles

(63) The electrochemical properties (accessible surface area, activity and selectivity) of the catalysts are important factors in the choice of the material to be used in the active cathode layers of fuel cells of the PEMFC type.

(64) The presence of an organic crown could substantially modify its catalytic properties, which are, in theory, strongly linked to the surface condition of the catalyst.

(65) The results of the electrochemical analyses (active surface area and catalytic activity) for particles obtained according to the two methods for synthesising nanoparticles are summarised in the table hereinbelow.

(66) TABLE-US-00004 Active surface Activity (j.sub.k) (0.9 V) Particles area (in m.sup.2/g) (in mA/cm.sup.2) Test 11 30 2.29 Test 12 80 4.59 Test 13 22 2.90 Test 2 67 4.03

(67) In this table, it can be observed that there is a major difference in the active surface area between the particles obtained by the method 3a) (namely, the method involving a water-in-oil microemulsion) and the particles obtained by the method 3b (namely, the method involving a polyol and a heating via microwaves).

(68) Indeed, this results in that the particles obtained by the method 3b have an active surface area approximately 2.5 times greater than the particles obtained by the method 3a. For information, the active surface area is determined by cyclic voltammetry with a linear variation in the potential in the support medium (HClO.sub.4 0.1 M, deaerated by an inert gas)

(69) In addition, it can be considered that the particles obtained by the method 3b have a better activity (j.sub.k) with regards to the oxygen reduction reaction. For information, the catalytic activity of the catalyst is determined from the Koutecky-Levich method applied to the measurements of the polarisation curves carried out using an electrode with a rotating disc. The rotating disc makes it possible to record the reduction of oxygen in the liquid electrolyte at different rotating speeds of the electrode.

(70) The characterisations are carried out in the following conditions: rotating speeds of the electrode: 2,500, 2,000, 1,500, 1,000 and 500 rpm.sup.1; speed of linear variation in potential set to 1 mV/s; HClO4 0.1 M medium saturated with oxygen.

(71) b) Test in Cells

(72) To do this, cells were carried out (respectively Cell 1 and Cell 2) with each one comprising an electrode-membrane-electrode assembly each comprising: an anode of the gas diffusion electrode type comprising 0.2 mg/cm.sup.2 of commercial particles of platinum bonded to a carbon material (with these particles not being grafted); a cathode of the gas diffusion electrode type comprising 0.4 mg/cm.sup.2 of platinum particles obtained by one of the tests of the example 4; a Nafion NRE 211 membrane arranged between the anode and the cathode.

(73) For the Cell 1, the cathode comprises platinum particles obtained in the test 2 of the example 4.

(74) For the Cell 2, the cathode comprises platinum particles obtained in the test 13 of the example 4.

(75) The electrode-membrane-electrode assemblies are carried out according to the following operating protocol.

(76) Whether for the anode or for the cathode, the latter are prepared by simple pouring of a catalytic ink comprising the platinum particles concerned into an ethanol/water mixture (3:1) on a Sigracet 24BC gas diffusion layer (GDL).

(77) Before being placed in the assembly, the Nafion membrane is treated beforehand by hot pressing by pressing a reinforcement on either side of the latter at a temperature of 110 C. and at a pressure of 3 MPa pendant 90 seconds.

(78) Finally, the gas diffusion electrodes (anode and cathode) are pressed on either side of the Nafion membrane treated beforehand as such at a temperature of 115 C. and at a pressure of 3.5 MPa for 150 seconds.

(79) The tests are conducted in a single cell of 5 cm.sup.2 under H.sub.2/O.sub.2 (stoichiometry .sub.O2=1.5 and .sub.H2=1.5) under a pressure of 2 bars, at 60 C. and at 21% humidity.

(80) Curves showing the change in voltage E (in V) according to the current density (in A/cm.sup.2) are shown in FIG. 1 (curve a for the Cell 1 and curve b for the Cell 2) as well as the power curves (curve a for the Cell 1 and curve b for the Cell 2).

(81) The tests in cells demonstrate that the catalyst prepared by the method for synthesising polyol via microwaves has better activity. This difference in activity can be observed in the activation zone corresponding to the low current densities, i.e. a current density between 0 and 0.1 A/cm.sup.2. Indeed, in this zone, the curve a has a slope much less steep than that of curve b, which shows the better catalytic activity and as such a better starting of the reactions on the fuel cell leading to a lesser drop in voltage.

(82) Also note that the catalyst prepared by the method of synthesising polyol via microwaves is not as affected by the problems of the diffusion of reagents, which results in a curve a) which does not look like a curve corresponding to a flooding at high current densities, contrary to curve b). This phenomenon of flooding can be explained by the substantial production of water which act as a barrier to the diffusion of reagents.

Example 6

(83) In this example, the particles obtained in the tests 1, 2, 3, 4 and 7 of the example 4 were tested in order to determine their active surface area and their electrochemical activity with regards to the oxygen reduction reaction.

(84) The results are listed in the table hereinbelow.

(85) TABLE-US-00005 Number of Activity Degree of moles of (j.sub.k) (0.9 polymerisation polymer/g of Active surface V) Particles in number DP.sub.n particles area (in m.sup.2/g) (in mA/cm.sup.2) Test 1 500 5.6*10.sup.7 55 1.43 Test 2 1,000 5.6*10.sup.7 63 4.03 Test 3 1,500 5.6*10.sup.7 69 3.53 Test 4 2,000 5.6*10.sup.7 73 3.74 Test 7 2,500 5.6*10.sup.7 86 4.16

(86) These results show particularly interesting properties in terms of active surface area and of activity for the particles obtained by the method of polyol synthesis via microwaves and, in particular, those grafted with polymers with a high molecular weight.

Example 7

(87) In this example, the particles obtained in the tests 2, 3, 4 and 7 of the example 4 were tested in cells in the same assembly and test conditions as those specified in the example 5 hereinabove.

(88) The results are listed in FIG. 2 provided in the appendix, which shows, on the one hand, the polarisation curves showing the change in the voltage E (in V) according to the current density D (in A/cm.sup.2) (respectively curves a), b), c) and d) for cells comprising particles of tests 2, 3, 4 and 7) and the power curves showing the change in the power density P (in kW/cm.sup.2) according to the current density D (in A/cm.sup.2) (respectively curves a), b), c) and d) for cells comprising particles of tests 2, 3, 4 and 7).

(89) Particularly interesting results are observed with the particles obtained by the method of synthesising polyol via microwaves and, in particular, those grafted with polymers with a high molecular weight and with grafting rates of 5.6*10-7.

Example 8

(90) In this example, the particles obtained in the tests 12, 5, 6, 7, 8, 9 and 10 of the example 4 were tested in order to determine their active surface area and their electrochemical activity with regards to the oxygen reduction reaction.

(91) The results are listed in the table hereinbelow.

(92) TABLE-US-00006 Number of Activity Degree of moles of (j.sub.k) (0.9 polymerisation polymer/g of Active surface V) Particles in number DP.sub.n particles area (in m.sup.2/g) (in mA/cm.sup.2) Test 12 80 4.59 Test 5 2,500 2.0*10.sup.7 89 3.86 Test 6 2,500 2.8*10.sup.7 98 8.63 Test 7 2,500 5.6*10.sup.7 86 4.16 Test 8 2,500 1.1*10.sup.6 83 2.93 Test 9 2,500 1.7*10.sup.6 73 1.34 Test 10 2,500 2.3*10.sup.6 68 1.31

(93) These results show particularly interesting properties in terms of active surface area and of activity for the particles obtained by the method of synthesising polyol by microwaves and, in particular, those with intermediate grafting rates.

(94) Surprisingly, the maximum active surface areas obtained are obtained with grafted particles (cf. test 6, namely the test where the particles are grafted with a polymer comprising a molecular weight of 359,000 g/mol and a grafting mass rate of 3.88%) comparatively to the non-grafted particles (cf. test 12), with a gain in the active surface area of 19%. Concerning the electrochemical activity with regards to the reduction of oxygen (j.sub.k), the latter is 8.63 mA/cm.sup.2 for the grafted particles of the test 6 compared to 4.59 mA/cm.sup.2 for the non-grafted particles of the test 12, which represents an increase of 88%.

(95) This contributes in demonstrating that the grafting of polymer not only does not reduce the accessibility of the platinum surface but, on the contrary, compounds the latter.

Example 9

(96) In this example, the particles obtained in the tests 6, 7, 8 and 9 of the example 4 were tested in cells in the same assembly and test conditions as those specified in the example 5 hereinabove.

(97) The results are listed in FIG. 3 provided in the appendix, which shows, on the one hand, the polarisation curves showing the change in the voltage E (in V) according to the current density D (in A/cm.sup.2) (respectively curves a), b), c) and d) for cells comprising particles of tests 6, 7, 8 and 9) and the power curves representing the change in the power density P (in kW/cm.sup.2) according to the current density D (in A/cm.sup.2) (respectively curves a), b), c) and d) for cells comprising particles of tests 6, 7, 8 and 9).

(98) Particularly interesting results are observed with the particles obtained by the method of synthesising polyol via microwaves and, in particular, those grafted with polymers of high molecular weight and with relatively low grafting rates.

(99) The maximum power obtained with the particles with the best performance is nearly 400% higher than in the case of the particles obtained by the water-in-oil synthesis. Indeed, by referring to curve a) (obtained with particles of the test 6), the maximum power density obtained is approximately 1 kW/cm.sup.2, while the maximum power density obtained is approximately 0.25 kW/cm.sup.2 (obtained with the particles of the test 13, as shown on curve b of FIG. 1).