Process for preparing nanoparticles of a catalyst for cathodic reduction of dioxygen in the presence of methanol

Abstract

The invention relates to a process for preparing nanoparticles of a catalyst for cathodic reduction and which is tolerant to methanol, these nanoparticles comprising a metallic center and a submonolayer of a chalcogen.

Claims

1. A method for preparing catalyst nanoparticles for cathode reduction of dioxygen and tolerant to methanol comprising: (a) a metal center comprising at least one transition metal either supported or not supported and selected from the group consisting of platinum, ruthenium, palladium, rhodium and iridium; (b) a sub-monolayer of a chalcogen selected from selenium or sulfur; in a maximum electrocatalytic activity molar ratio R (chalcogen/transition metal) of less than 1, the chalcogen being essentially present on the surface of the metal center; the method comprising: i) preparing catalyst nanoparticles comprising a transition metal either supported or not supported covered with a sub-monolayer of chalcogen in a molar ratio (chalcogen/transition metal) R.sup.1 of less than or equal to 1; ii) preparing an electrochemical cell comprising a working electrode, a reference electrode and an auxiliary electrode, an electrolytic solution comprising methanol and catalyst nanoparticles deposited at the surface of the working electrode in a specific mass ranging from 0.010 to 0.300 mg per cm.sup.2 of working electrode; iii) measuring the electrocatalytic activity of the electrochemical cell by applying an oxidation potential ranging from 0.8 to 1.4V, under an inert atmosphere and for a period ranging from 0 to 40 min; iv) determining the half-wave potential E.sub.1/2 depending on the application time of the oxidation potential; v) determining the residual covering rate of the metal center by the chalcogen for which the half-wave potential E.sub.1/2 is maximum; and vi) determining the value of the ratio R of the catalyst.

2. The method according to claim 1, wherein the ratio R.sup.1 ranges from 0.001 to 1.

3. The method according to claim 1 wherein the metal center comprises at least one transition metal supported and selected from the group consisting of platinum, ruthenium, palladium, rhodium and iridium.

4. The method according to claim 1, wherein the support comprises carbon.

5. The method according to claim 1, wherein the support comprises amorphous carbon, carbon nanotubes or graphene.

6. The method according to claim 1, wherein the support comprises an oxide-carbon composite.

7. The method according to claim 1 wherein the transition metal is platinum and the chalcogen is selenium and the ratio R.sup.1 ranges from 0.5 to 0.7 and the ratio R is less than 0.5.

8. The method according to claim 1, wherein the transition metal is platinum and the chalcogen is sulfur and the ratio R.sup.1 ranges from 0.5 to 0.7 and the ratio R is less than or equal to 0.5.

9. The method according to claim 1, wherein the metal center also comprises an additional metal either supported or not supported selected from the group consisting of gold, titanium, tin, cobalt, nickel, iron and chromium.

10. The method according to claim 1, wherein the working electrode of step ii) comprises a metal selected from the group consisting of gold, titanium, tin, cobalt, nickel, iron, and chromium.

11. The method according to claim 1, wherein the electrolytic solution of step ii) is an acid solution.

12. The method according to claim 1, wherein the electrochemical cell of step ii) comprises methanol in a molar concentration ranging from 0.1M to 20 M.

13. The method according to claim 1, wherein the transition metal is selected from platinum, rhodium, palladium or iridium and the oxidation potential of step iii) ranges from 1 to 1.2V.

14. The method according to claim 1, wherein the size of the catalyst nanoparticles ranges from 1 to 10 nm.

15. The method according to claim 1 further comprising: vii) preparing with stirring a mixture of the transition metal either supported or not supported with a mixture of water and isopropanol in a v/v ratio ranging from 2/1 to 10/1 for a period ranging from 5 to 60 minutes; viii) adding an inorganic compound comprising a chalcogen in a molar ratio R (chalcogen/transition metal); ix) stirring at a temperature ranging from 20 to 50 C. for a period ranging from 5 to 24 hours; x) evaporating the water and the isopropanol; xi) calcining, under an inert atmosphere, at a temperature ranging from 100 to 400 C., for a period ranging from 30 min to 2 hours.

16. The method according to claim 1, wherein the preparation of nanoparticles of catalysts of step i) comprises: i.a) preparing with stirring a mixture of the transition metal either supported or not supported with a mixture of water and isopropanol in a v/v ratio ranging from 2/1 to 10/1 for a period ranging from 5 to 60 minutes; i.b) adding an inorganic compound comprising a chalcogen in a molar ratio R.sup.1 (chalcogen/transition metal) of less than or equal to 1; i.c) stirring at a temperature ranging from 20 to 50 C. for a period ranging from 5 to 24 hours; i.d) evaporating the water and the isopropanol; and i.e) calcining, under an inert atmosphere, at a temperature ranging from 100 to 400 C., for a period ranging from 30 min to 2 hours.

17. A method (P2) for preparing nanoparticles of a catalyst tolerant to methanol comprising: a) a metal center comprising at least one transition metal either supported or not supported and selected from platinum, ruthenium, palladium, rhodium or iridium; (b) a sub-monolayer of a chalcogen selected from selenium or sulfur; the method comprising: i) preparing with stirring a mixture of the transition metal either supported or not supported with a mixture of water and isopropanol in a v/v ratio ranging from 2/1 to 10/1 for a period ranging from 5 to 60 minutes; ii) adding an inorganic compound comprising a chalcogen in a molar ratio R.sup.1 (chalcogen/transition metal) of less than or equal to 1; iii) stirring at a temperature ranging from 20 to 50 C. for a period ranging from 5 to 24 hours; iv) evaporating the water and the isopropanol; and v) calcining, under an inert atmosphere, at a temperature ranging from 100 to 400 C., for a period ranging from 30 min to 2 hours.

18. A cathode for a direct methanol fuel cell or for a microfluidic fuel cell comprising catalyst nanoparticles comprising a metal center comprising platinum either supported or not and covered with a sub-monolayer of selenium or with a sub-monolayer of sulfur in a (selenium/platinum) ratio or in a (sulfur/platinum) ratio ranging from 0.1 to 0.5, which may be obtained by the method according to claim 1.

19. The method according to claim 2, wherein the ratio R.sup.1 ranges from 0.3 to 0.9.

20. The method according to claim 2, wherein the ratio R.sup.1 ranges from 0.5 to 0.7.

21. The method according to claim 6, wherein the oxide-carbon composite is selected from the group consisting of WO.sub.3-carbon and SnO.sub.2-carbon composites.

22. The method according to claim 7, wherein the ratio R ranges from 0.1 to 0.3.

23. The method according to claim 8, wherein the ratio R ranges from 0.1 to 0.3.

24. The method according to claim 9, wherein the additional metal is titanium.

25. The method according to claim 11, wherein the electrolytic solution of step ii) comprises sulfuric acid.

26. The method according to claim 12, wherein the methanol molar concentration ranges from 0.5 M to 5 M.

27. The method according to claim 14, wherein the size of the catalyst nanoparticles ranges from 2 to 3 nm.

28. The method according to claim 15, wherein (ix) stirring at a temperature ranging from 20 to 50 C. is for a period ranging from 10 to 20 hours.

29. The method according to claim 17, wherein molar ratio R.sup.1 is from 0.001 to 1.

30. The method according to claim 17, wherein (iii) stirring is for a period from 10 to 20 hours.

31. The cathode according to claim 17 wherein the selenium/platinum ratio or in a (sulfur/platinum) ratio is from 0.1 to 0.3.

32. The method according to claim 11, wherein the acid in the acid solution is sulfuric acid.

Description

(1) FIG. 1 represents chronoamperometry for different applied stripping durations.

(2) FIG. 2 represents the curves for reducing dioxygen at 900 revolutions per minute measured after the chronoamperometric measurement at 1.1 V for different stripping times.

(3) FIG. 3 represents the half-wave potential E.sub.1/2 versus the stripping duration.

(4) FIGS. 4, 5 and 6 respectively represent the half-wave potential E.sub.1/2 versus the stripping time comprised between 0 and 40 min and for an amount of deposited catalyst of 20 g, 81 g and 162 g.

(5) FIG. 7 simultaneously represents the cell voltage curves and the power density curves versus the current density at temperatures of 30 C., 50 C. and 80 C. for catalyst nanoparticles having a molar ratio R (selenium/platinum) equal to 0.2 (PtSe.sub.0.2/C) in a direct methanol fuel cell (DMFC).

(6) FIG. 8 simultaneously represents the cell voltage curves and the power density curves versus the current density at temperatures of 30 C., 50 C. and 80 C. for Pt/C catalysts in a direct methanol fuel cell (DMFC).

(7) FIG. 9 represents a comparative diagram of the maximum power density values of catalyst nanoparticles having a molar ratio R (selenium/platinum) equal to 0.2 (PtSe.sub.0.2/C) and of a catalyst (Pt/C) in a direct methanol fuel cell at temperatures of 30 C., 50 C. and 80 C.

(8) FIG. 10 simultaneously represents the electrode potential curves (cathode and anode) and the power density curves versus the current density at a temperature of 25 C. for nanoparticles of catalysts having a molar ratio R (selenium/platinum) equal to 0.2 (PtSe.sub.0.2/C) in a microfluidic cell of the LFFC (laminar flow fuel cell) type and of the MRFC (mixed-reactant fuel cell) type.

(9) The different objects of the invention and their embodiments will be better understood upon reading the examples which follow. These examples are given as an indication without any limitation.

EXAMPLE 1: Preparation of Catalyst Nanoparticles of Platinum Supported on Carbon and Covered with a Sub-Monolayer of Selenium in a Molar Ratio (Selenium/Platinum) R1 Equal to 0.5 (PtSe0.5/C) According to Step I) of Method (P1) or According to Method (P2)

(10) First of all, a platinum composite supported on carbon (Pt/C) was synthesized by the carbonyl method.

(11) A mixture of sodium hexachloroplatinate of formula Na.sub.2PtCl.sub.6.6H.sub.2O (1 mole) and of sodium acetate (6 moles) in a molar (sodium hexachloroplatinate/sodium acetate) ratio equal to 0.16 was produced under a nitrogen atmosphere for 30 minutes.

(12) The reaction was then activated for 15 minutes at 55 C. in the presence of carbon monoxide with stirring.

(13) A platinum-carbonyl complex was obtained after 24 hours.

(14) Carbon was then added under a nitrogen flow and then the solution was maintained with stirring for 12 h under a nitrogen atmosphere.

(15) The solvent is then evaporated by heating to 80 C. under nitrogen.

(16) The powder of the Pt/C compound was then recovered by washing and filtration with ultrapure water.

(17) Subsequently, the compound (Pt/C) was modified on its surface with selenium atoms by a selenization method.

(18) For this, the compound Pt/C (62.5 mg) and selenium oxide SeO.sub.2 (3.8 mg) were mixed in an aqueous solution of isopropanol (30 ml) in a (water/isopropanol) v/v ratio equal to 5 and stirred for 12 h at room temperature.

(19) The resulting powder is heated to 200 C. for 1 h under a nitrogen atmosphere.

(20) The obtained catalyst consists of platinum supported on carbon and modified at the surface with a sub-monolayer of selenium in a (selenium/platinum) molar ratio R.sup.1 equal to 0.5.

Example 2: Preparation of Catalyst Nanoparticles Tolerant to Methanol Consisting of Platinum Supported on Carbon and Covered with a Sub-Monolayer of Selenium in a (Selenium/Platinum) Molar Ratio R Equal to 0.2 (PtSe0.2/C) According to Method (P1)

(21) In order to end up with catalyst nanoparticles tolerant to methanol, consisting of platinum supported on carbon and covered with a sub-monolayer of selenium in a (selenium/platinum) molar ratio equal to 0.2 (PtSe.sub.0.2/C), catalyst nanoparticles of Example 1 were used as initial nanoparticles.

(22) Catalyst nanoparticles of Example 1 were introduced into a thermostated electrochemical cell comprising: a gold electrode as a working electrode, having a surface area of 0.071 cm.sup.2, a vitreous carbon electrode as an auxiliary electrode, a reversible hydrogen electrode (RHE) as a reference electrode, and an acid electrolytic solution comprising water, sulfuric acid (96%, Merck) in a molar concentration equal to 0.5M and methanol (99.9%, Sigma-Aldrich) in a molar concentration equal to 0.5M.

(23) The catalyst nanoparticles were deposited by nebulization with argon at the surface of the working electrode in a specific mass of 0.27 mg/cm.sup.2, corresponding to a total mass of 20 g of catalyst nanoparticles.

(24) An oxidation potential of 1.1V was then applied within the electrochemical cell by means of a potentiostat.

(25) The oxidation potential was applied under an atmosphere saturated with nitrogen for a period of 10, 15, 20, 25 and 30 minutes, called the stripping period.

(26) It may be observed in FIG. 2 that, when the stripping time increases, the mixed region of the cathode curve for the dioxygen reduction reaction is enlarged.

(27) This is demonstrated by the fact that the half-wave potential E.sub.1/2 increases while the open circuit potential (OCP) remains constant.

(28) For the catalyst comprising platinum supported on carbon or Pt/C, illustrated by a dotted line in FIG. 2, the open circuit potential moves towards a more negative potential. This mixed potential stems from the simultaneous electrochemical oxidation of methanol and from the dioxygen reduction reaction.

(29) This means that the catalyst comprising platinum supported on carbon, but in the absence of a sub-monolayer of selenium, is less tolerant to methanol.

(30) The half-wave potential E.sub.1/2 for each stripping time was determined by measuring the potential for which the current intensity is equal to half the limiting diffusion current intensity.

(31) To each value of half-wave potential, corresponds a specific composition of a catalyst, characterized by a specific (selenium/platinum) molar ratio R.

(32) FIG. 3 shows that the half-wave potential of the catalyst attains a plateau for a stripping period equal to 20 min, corresponding to a value close to 0.8V.

(33) For this stripping period and at this value of the half-wave potential, the selenium covering rate exhibits larger activity for the dioxygen reduction reaction (see curve 3 of FIG. 2) while being more tolerant to poisoning with methanol.

(34) For this stripping period equal to 20 min and at this value of the half-wave potential close to 0.8V, the residual covering rate of the platinum supported by the selenium was determined to have a value ranging from 0.15 to 0.2.

(35) For this, the method for deposition under a potential (underpotential deposition of hydrogen) was used as for example described in Elezovic et al. (Elezovic et al, Int. J. Hydrogen Energy 32 (2007), 1991-1998)

(36) From this residual rate, the molar ratio (selenium/platinum) was determined to be at a value close to 0.2.

(37) For this, determination of the active surface by hydrogen potential or CO-stripping deposition was achieved, combined with physical measurements of transmission electron microscopy by means of X-fluorescence.

(38) Thus, it was determined that the maximum electrocatalytic activity for the reaction of reduction of dioxygen in the presence of methanol combined with improved tolerance to methanol is obtained for a catalyst comprising platinum supported on carbon and covered with a sub-monolayer of selenium in a molar ratio R (selenium/platinum) close to 0.2.

Example 3: Evaluation of the Influence of the Amount of Catalyst PtSe0.5 Deposited at the Surface of the Working Electrode on the Determination of the Maximum Value of the Half-Wave Potential

(39) The method according to Examples 1 and 2 was reproduced for amounts of catalysts PtSe.sub.0.5 deposited at the surface of the working electrode of 20 g, 81 g and 162 g.

(40) The results described in FIGS. 4, 5 and 6, show that the maximum value of the half-wave potential E.sub.1/2 only varies very little according to the amount of catalyst deposited at the surface of the working electrode, this value remaining close to 0.8V.

(41) Moreover, the use of different amounts of catalysts allowed determination of the optimum stripping time used for activating the catalyst.

(42) The optimum stripping time corresponds to the time for which the value of the half-wave potential E.sub.1/2 is maximum, and therefore for which electrocatalytic activity is maximum.

(43) For the catalyst PtSe.sub.0.5, the optimum stripping time (t.sub.strip) is related to the amount of catalyst (m.sub.cat) used through the relationship t.sub.strip*=0.07 m.sub.cat+18.87.

(44) Thus, it is possible to determine, for different catalyst natures, the optimum stripping time for activating the catalyst; this thereby allowing, within the scope of industrial production of this catalyst, simplification and/or optimization of its manufacturing method.

Example 4: Evaluation of the Power Density in (mW.Math.cm2) According to the Current Density (mA.Math.cm2) of Nanoparticles of Catalysts having a Molar Ratio R (Selenium/Platinum) Equal to 0.2 (PtSe0.2/C) in a Direct Methanol Fuel Cell (DMFC) at a Temperature of 30 C., 50 C. and 80 C.

(45) The method according to Example 1 was reproduced for an amount of catalyst PtSe.sub.0.2 deposited at the surface of the cathode in a specific mass of 0.9 mg.Math.cm.sup.2 of Pt.

(46) The direct methanol fuel cell consists of an MEA (membrane electrode assembly) assembly comprising a membrane Nafion N212 (DuPont) inserted between the anode and the cathode. One face of this membrane is covered with catalyst PtRu/C nanoparticles used as an anode in a specific mass of 1.5 mg.Math.cm.sup.2 of Pt; and the other face of this membrane is covered with catalyst PtSe.sub.0.2/C nanoparticles used as a cathode in a specific mass of 0.9 mg.Math.cm.sup.2 of Pt.

(47) The values of the power density (mW.Math.cm.sup.2) versus the current density (mA.Math.cm.sup.2) were obtained under the following experimental conditions: an aqueous phase comprising methanol (99.9% Sigma-Aldrich) in a molar concentration equal to 2M, a flow rate of the aqueous solution of 100 ml.Math.min.sup.1, an oxygen flow at atmospheric pressure.

(48) It may be observed that the maximum value of the power density of the MEA assembly comprising the catalyst nanoparticles for which the molar ratio R (selenium/platinum) is equal to 0.2 (PtSe.sub.0.2/C) is equal to 21 mW.Math.cm.sup.2 at a temperature of 80 C.

Example 5: Comparison of the Power Densities of Catalyst Nanoparticles for which the Molar Ratio R (Selenium/Platinum) is Equal to 0.2 (PtSe0.2/C), and of a Catalyst (Pt/C) in a Direct Methanol Fuel Cell

(49) An amount of catalyst Pt/C was deposited at the surface of the cathode in a specific mass of 1 mg.Math.cm.sup.2 of Pt.

(50) The values of the power density (mW.Math.cm.sup.2) according to the current density (mA.Math.cm.sup.2) were obtained under identical experimental conditions with those of Example 4 for a direct methanol fuel cell.

(51) It may be observed that the maximum value of the power density of the MEA assembly comprising the Pt/C nanoparticles in a direct methanol fuel cell is equal to 7 mW.Math.cm.sup.2, at a temperature of 80 C.

(52) It may be observed that the values of the power density of the catalyst nanoparticles having a molar ratio R (selenium/platinum) equal to 0.2 (PtSe.sub.0.2/C) respectively correspond to 6 mW.Math.cm.sup.2 for a temperature of 30 C., to 12 mW.Math.cm.sup.2 for a temperature of 50 C. and to 21 mW.Math.cm.sup.2 for a temperature of 80 C. These values are always greater than the values of the power density of the Pt/C catalysts regardless of temperature. It may also be observed that the value of the power density of catalyst nanoparticles having a molar ratio R (selenium/platinum) equal to 0.2 (PtSe.sub.0.2/C) is three times greater than the value of the power density of the Pt/C catalyst at a temperature of 80 C.

Example 6: Evaluation of the Power Density (mW.Math.cm2) and of the Potential E of the Cathode and of the Anode (V/RHE) Versus the Current Density (mA.Math.cm2) of the Nanoparticles of Catalysts having a Molar Ratio R (Selenium/Platinum) Equal to 0.2 (PtSe0.2/C) in a Microfluidic Cell of the LFFC (Laminar Flow Fuel Cell) Type or of the MRFC (Mixed-Reactant Fuel Cell) Type, at a Temperature of 25 C.

(53) The method according to Example 1 was reproduced for an amount of catalyst PtSe.sub.0.2 deposited at the surface of the cathode in a specific mass of 0.9 mg.Math.cm.sup.2 of Pt.

(54) The microfluidic cell of the LFFC (laminar flow fuel cell) type and the microfluidic cell of the MRFC (mixed-reactant fuel cell) type operate in a self-humidifying mode for the cathode; this mode being known to one skilled in the art.

(55) The microfluidic cell of the LFFC type consists of: an anode for which the surface is covered with catalyst PtRu/C nanoparticles in a specific mass of 1.5 mg.Math.cm.sup.2 of Pt, a cathode for which the surface is covered with catalyst PtSe.sub.0.2/C nanoparticles in a specific mass of 0.9 mg.Math.cm.sup.2 of Pt, an SU-8 microchannel with 10-750 geometry having a height of 250 m, a width of 750 m and a length of 2,000 m.

(56) The values for the power density (mW.Math.cm.sup.2) and for the potential of the cathode and of the anode (V/RHE) versus the current density (mA.Math.cm.sup.2) for the microfluidic cell of the LFFC type were obtained with the following experimental conditions: a flow of an electrolytic solution comprising sulfuric acid (Sigma-Aldrich) at a concentration of 0.5M and comprising methanol (99.9% Sigma-Aldrich) in a molar concentration equal to 5M, a second flow of an electrolytic solution comprising sulfuric acid (Sigma-Aldrich) at a concentration of 0.5M without any methanol, a flow rate of both electrolytic solutions of 3.4 ml.Math.min.sup.1.

(57) The microfluidic cell of the MRFC type consists of: an anode for which the surface is covered with catalyst PtRu/C in a specific mass of 1.5 mg.Math.cm.sup.2 of Pt, a cathode for which the surface is covered with catalyst PtSe.sub.0.2/C nanoparticles in a specific mass of 0.9 mg.Math.cm.sup.2 of Pt, an SU-8 microchannel of 10-750 geometry.

(58) The values for the power density (mW.Math.cm.sup.2) and for the potential of the cathode and of the anode (V/RHE) versus the current density (mA.Math.cm.sup.2) for the microfluidic cell of the LFFC type were obtained with the following experimental conditions: a flow of an electrolytic solution comprising sulfuric acid (Sigma-Aldrich) at a concentration of 0.5M and comprising methanol (99.9% Sigma-Aldrich) in a molar concentration equal to 5M, a second flow of an electrolytic solution comprising sulfuric acid (Sigma-Aldrich) at a concentration of 0.5M without any methanol, a flow rate of this electrolytic solution of 3.4 ml.Math.min.sup.1.

(59) It may be observed that, the open circuit potential values of the cathode are for the microfluidic cell of the LFFC type and for the microfluidic cell of the MRFC type of 0.8 V and of 0.79V respectively. This small potential difference between both of these types of microfluidic cells shows that the nanoparticles of catalysts according to the invention are highly selective relatively to the electrolytic medium.

(60) It is also possible to observe that, the maximum value of the power density of the nanoparticles of catalysts having a molar ratio R (selenium/platinum) equal to 0.2 (PtSe.sub.0.2/C), in a microfluidic cell of the LFFC type or of the MRFC type is equal to 3 mW.Math.cm.sup.2 and to 3.7 mW.Math.cm.sup.2 respectively, at a temperature of 25 C. This small difference between the power density for a microfluidic cell of the LFFC type comprising catalyst nanoparticles according to the invention and the power density for a microfluidic cell of the MFRC type comprising nanoparticles of catalysts according to the invention, shows that the catalyst nanoparticles according to the invention do not deteriorate in the presence of methanol and they retain constant and long-lasting electrocatalytic activity.

(61) These results show that the catalyst nanoparticles according to the invention have similar behaviors and retain their catalytic efficiency regardless of the type of microfluidic cell (LFFC or MFRC).