Mixed metallic oxides as scavengers for fluorinated ion exchange polymers

Abstract

A mixed oxide of Si and at least one metal M comprising inorganic groups —SO.sub.3H. The addition of the mixed oxide to fluorinated polymers containing sulfonic acid functional groups increases their stability towards radical degradation when used in fuel cell applications.

Claims

1. A mixed oxide of Si and at least one metal M, said oxide comprising inorganic groups —SO.sub.3H, wherein the inorganic groups —SO.sub.3H are not bound to any moiety containing at least one carbon atom.

2. The mixed oxide according to claim 1 wherein the inorganic groups —SO.sub.3H are bound via the sulphur atom to at least one Si, metal M or oxygen atom of the mixed oxide.

3. The mixed oxide according to claim 1 wherein the weight ratio Si/M is at least 1 and it does not exceed 40.

4. The mixed oxide according to claim 1 wherein the inorganic groups —SO.sub.3H are at least 0.2% with respect to the total amount of atoms of metal M in the mixed oxide.

5. The mixed oxide of claim 1 wherein the at least one metal M is selected from the group consisting of the elements of Group 4, Group 5, Group 6, Group 7, Group 8, Group 9, Group 10, Group 11 of the periodic table, Zn, Al, La and Ce.

6. The mixed oxide of claim 1 wherein the at least one metal M is selected from the group consisting of Ce, Co, Cr, Mn.

7. A process for the preparation of the mixed oxide of claim 1 comprising: reacting an aqueous suspension comprising SiO.sub.2, at least one water soluble salt of metal M and at least one source of inorganic groups —SO.sub.3H at a temperature between 30 and 100° C. to form a gel; and heat treating the gel at a first temperature between 30 and 180° C. and subsequently at a second temperature between 180 and 350° C. to obtain the mixed oxide in solid form.

8. A composition comprising at least one fluorinated polymer comprising —SO.sub.2X functional groups, wherein X is selected from X′ or from OZ and wherein X′ is selected from the group consisting of F, Cl, Br, I and Z is selected from the group consisting of H, alkaline metals, NH.sub.4, and at least one mixed oxide of claim 1.

9. The composition according to claim 8 wherein the at least one mixed oxide is present in an amount of at least 0.1% and not exceeding 20% moles of metal M/moles of —SO.sub.2X functional groups in the fluorinated polymer.

10. A liquid composition comprising the composition of claim 8 dispersed in a liquid medium.

11. The liquid composition of claim 10 wherein in the fluorinated polymer X═OZ and Z═H.

12. A process for the preparation of a composition according to claim 8 comprising blending the at least one mixed oxide and the at least one fluorinated polymer comprising —SO.sub.2X functional groups in solid form or in a solution.

13. An article comprising the composition of claim 8.

14. The article according to claim 13 which is a proton exchange membrane, an electrocatalytic layer or a membrane electrode assembly.

15. A process for the preparation of the article of claim 13 comprising impregnating, casting or coating a liquid composition, wherein the liquid composition comprises at least one fluorinated polymer comprising —SO.sub.2X functional groups and at least one mixed oxide of Si and at least one metal M, said oxide comprising inorganic groups —SO.sub.3H dispersed in a liquid medium, and wherein X is selected from X′ or from OZ and wherein X′ is selected from the group consisting of F, Cl, Br, I and Z is selected from the group consisting of H, alkaline metals, NH.sub.4.

16. A fuel cell comprising the article of claim 13.

Description

EXAMPLES

(1) Characterization

(2) X-ray fluorescence (XRF) analysis was conducted according to standard procedures on the mixed oxide samples to determine the amounts of Si, S, and of the metal M. XRF analysis of the samples was carried out with a Bruker AXS S4 Explorer spectrometer operating at a power of 1 kW and equipped with a Rh X-ray source, a LiF 220 crystal analyzer and a 0.12° divergence collimator.

(3) X-ray photoelectron spectroscopy (XPS) analysis was performed to characterize the surface properties of the mixed oxide in terms of oxidation states and binding energies. XPS measurements were performed by using a Physical Electronics (PHI) 5800-01 spectrometer. A monochromatic Al Kα X-ray source was used at a power of 350 W. Spectra were obtained with pass energies of 58.7 eV for elemental analysis (composition) and 11.75 eV for the determination of the oxidation states. The pressure in the analysis chamber of the spectrometer was 1×10.sup.−9 Torr during the measurements. The Ag 3d5/2 peak of an Ag foil was taken, after argon sputtering, for checking the calibration of the binding energy scale. The quantitative evaluation of each peak was obtained by dividing the integrated peak area by atomic sensitivity factors, which were calculated from the ionization cross-sections, the mean free electron escape depth and the measured transmission functions of the spectrometer. XPS data were interpreted by using the on-line library of oxidation states implemented in the PHI MULTIPAK 6.1 software and the PHI Handbook of X-ray photoelectron spectroscopy. Deconvolution of the XPS spectra was carried out by using the MULTIPAK software.

Example 1

General Procedure for the Preparation of Mixed Oxides [MO] According to the Invention

(4) In a closed vessel SiO.sub.2 (Cab-o-sil® EH-5 supplied by Cabot Corp.), a water soluble inorganic salt of the metal M and (NH.sub.4).sub.2SO.sub.3.H.sub.2O were suspended in water.

(5) The weight ratio SiO.sub.2: salt of metal M: (NH.sub.4).sub.2SO.sub.3.H.sub.2O was 8:1.5:0.5. The slurry was stirred at 80° C. for 10 h providing a gel. The gel thus obtained was heat treated according to the following conditions: from room temperature to 150° C. (1 h ramp, 2.5° C./min); 2 h at 150° C.; from 150° C. to 300° C. (1 h ramp, 2.5° C./min); 2 h at 300° C.

(6) The powder obtained at the end of the heat treatment was cooled down to room temperature and then washed with 0.5M H.sub.2SO.sub.4 solution at 70° C. until no change in the amount of metal M and sulphur was determined by XRF analysis of the sample. The powder was dried under vacuum at 80° C. for 2 h and then ground in a planetary ball mill for 2 h at 200 rpm.

(7) Table 1 lists the mixed oxides prepared as well as their composition.

(8) TABLE-US-00001 TABLE 1 Sample Water soluble salt Si/M % —SO.sub.3H vs M [MO—Ce] Ce(NO.sub.3).sub.3•6H.sub.2O 3.8 1.6 [MO—Co] Co(NO.sub.3).sub.2•6H.sub.2O 3.4 1.2 [MO—Cr] Cr(NO.sub.3).sub.3•9H.sub.2O 3.4 13.3 [MO—Mn] Mn(NO.sub.3).sub.2•4H.sub.2O 8.1 0.7 [MO—Ce—Cr] Ce(NO.sub.3).sub.3•6H.sub.2O 35.6 33.4 Cr(NO.sub.3).sub.3•9H.sub.2O 4.1 3.8

(9) A mixed oxide of Ce and Cr was prepared according to the same general procedure by mixing SiO.sub.2, Ce(NO.sub.3).sub.3.6H.sub.2O, Cr(NO.sub.3).sub.3.9H.sub.2O and (NH.sub.4).sub.2SO.sub.3.H.sub.2O in water in a weight ratio SiO.sub.2: salt of Ce: salt of Cr: (NH.sub.4).sub.2SO.sub.3. H.sub.2O of 8:0.75:0.75:0.5. The ratio Ce:Cr in the resulting mixed oxide [MO-Ce—Cr] was 1:9.

Example 2

Preparation of a Fluorinated Polymer (P1) Comprising —SO3H Functional Groups

(10) In a 22 L autoclave the following reagents were charged: 11.5 L of demineralised water; 980 g of the monomer with formula: CF.sub.2═CF—O—CF.sub.2CF.sub.2—SO.sub.2F 3100 g of a 5% weight solution of CF.sub.2ClO(CF.sub.2CF(CF.sub.3)O).sub.n(CF.sub.2O).sub.mCF.sub.2 COOK in water (average molecular weight=521, ratio n/m=10).

(11) The autoclave, stirred at 470 rpm, was heated at 60° C. A water based solution with 6 g/L of potassium persulfate was added in a quantity of 150 mL. The pressure was maintained at a value of 12 bar (abs) by feeding tetrafluoroethylene.

(12) After adding 1200 g of tetrafluoroethylene in the reactor, 220 g of the monomer CF.sub.2═CF—O—CF.sub.2CF.sub.2—SO.sub.2F were added every 200 g of tetrafluoroethylene fed to the autoclave.

(13) The reaction was stopped after 280 min by stopping the stirring, cooling the autoclave and reducing the internal pressure by venting the tetrafluoroethylene; a total of 4000 g of tetrafluoroethylene were fed.

(14) The latex was then coagulated by freezing and thawing and the recovered polymer was washed with water and dried at 150° C. for 24 hours. The polymer was then treated with fluorine gas in a metallic vessel for 8 hours at 80° C., then purged several hours with nitrogen to remove any residual unstable end-groups.

(15) The polymer thus obtained was immersed in a KOH solution (10% by weight) at 80° C. for 8 hours, followed by washing in demineralised water at room temperature. Immersion in a HNO.sub.3 solution (20% by weight) at room temperature for 2 hours, followed by washing in demineralised water at room temperature converted all functional groups into —SO.sub.3H functional groups.

(16) The resulting fluorinated polymer in —SO.sub.3H form (P1) was then dried in a vacuum oven at 80° C. The equivalent weight of the polymer (EW) was determined (by IR analysis on the precursor polymer) to be 790 g/eq.

Example 3

Liquid Compositions Comprising P1 and the Mixed Oxides Prepared in Example 1

(17) Each one of the mixed oxides prepared in Example 1 was suspended at room temperature in 1-propanol ([MO]/1-propanol=1/50 w/w) and then sonicated for 2 h obtaining complete dispersion of the solid. Solid content in dispersion was determined using a thermobalance (160° C., 45 min). The dispersion of the mixed oxide thus obtained was then added to a water dispersion of P1 (100 g) further comprising 1-propanol (36 g) and N-ethyl pyrrolidone (15.5 g). This mixture was stirred at room temperature for 15 min obtaining a clear solution.

(18) The amount of the mixed oxide [MO] and of P1 added in the preparation of each of the liquid compositions was calculated to obtain a final concentration of the metal in the composition of 1% moles of metal M/moles of —SO.sub.3H groups in the fluorinated polymer P1.

Example 4

Membrane Preparation—General Procedure

(19) Foamed PTFE support (TETRATEX® #3101), having an average pore diameter of 0.2 μm (as specified in the product datasheet) and a thickness of 35±10 μm, mounted on a PTFE circular frame having an internal diameter of 100 mm, was immersed in each of the liquid compositions obtained in Example 3 as well as in a liquid composition containing polymer P1 alone and then dried in a vent oven at a temperature of 65° C. for 1 h, at 90° C. for 1 h and then from 90° C. to 190° C. in 1 h.

(20) The membranes thus obtained were transparent and colourless indicating full occlusion of the pores of the support. The thickness of the resulting membranes was 25±5 μm.

Example 5

Fuel Cell Characterization of Membranes Prepared in Example 4

(21) Membranes obtained as described in Example 4 were assembled in a single cell (Fuel Cell Technology®) with an active area of 25 cm.sup.2 and tested on an Arbin® 50W test stand. The membranes were assembled with E-TEK® LT250EW gas diffusion electrodes (0.5 mg/cm.sup.2 Pt).

(22) After 24 hours conditioning at a fixed voltage of 0.6 V a polarization curve was measured to verify the membrane performance. The conductivity of membranes containing the mixed metal oxides of the invention was found not to differ from the conductivity of reference membrane (M1).

(23) The membranes were tested at the following operating conditions: Anode side flow: 500 sccm pure H.sub.2, 64° C. dew point, 1 bar (abs) Cathode side flow: 500 sccm pure O.sub.2, 64° C. dew point, 1 bar (abs) Cell temperature: 90° C. Open circuit voltage condition (=current zero ampere).

(24) The voltage was monitored during the test. The end of test was set at a voltage below 0.7 V, which is typically assumed to indicate the formation of pinholes in the membrane. The results are reported in Table 2.

(25) TABLE-US-00002 TABLE 2 Time to reach voltage <0.7 V Membrane (hours) M1 (reference) 230 M-Ce 1400 M-Co 450 M-Cr 600 M-Mn 400 M-Ce—Cr >600

(26) With respect to a membrane comprising fluorinated polymer (P1) alone (reference membrane M1) the membranes comprising the mixed oxides of the invention show a significant increase in stability under fuel cell operating conditions.

Example 6 And Comparative Example 1

Membranes Comprising [MO-Ce] and Ce(III)

(27) A liquid composition was prepared as described in Example 3 starting from [MO-Ce] prepared in Example 1 and polymer P1 to obtain a final concentration of cerium in the composition of 2.5% moles of Ce/moles of —SO.sub.3H groups in fluorinated polymer P1.

(28) A second liquid composition containing fluorinated polymer P1 and 2.5% moles of Ce(III) ions per moles of —SO.sub.3H groups in fluorinated polymer P1 was prepared by dissolving Ce(NO.sub.3).sub.3.6H.sub.2O in the liquid composition of P1.

(29) A membrane was prepared from each liquid composition following the general procedure described in Example 4.

(30) The membranes were assembled in a single fuel cell as described in Example 5 and each fuel cell resistance measured under the following operating conditions: Anode side flow: air, Cathode side flow: pure H.sub.2, Cell temperature: 65° C. Reactant Humidity level: 125% (70° C. dew point) Current intensity: from 2 A to 16 A

(31) The test showed that the cell prepared with the membrane containing [MO—Ce] exhibits a lower resistance than the cell prepared with Ce(III) ions (60 vs. 70 mOhm.Math.cm.sup.−2). The lower resistance of the membrane containing the inventive mixed oxide [MO] is associated with the higher number of available conductive —SO.sub.3H groups in fluorinated polymer P1, that is to the lower number of —SO.sub.3H groups in fluorinated polymer P1 which are ionically coordinated with Ce ions. The lower resistance is constant over prolonged period of times as shown by the comparison between 0 h vs. 100 h of fuel cell operation (62 vs 60 mOhm.Math.cm.sup.−2).

(32) Thus, the use of the mixed oxide of the invention is advantageous over the use of prior art systems containing soluble forms of the metal M in that the metal M in the mixed oxide improves the stability of the membrane over time (as shown by the data in Table 2) without increasing the electrical resistance of the membrane. Additionally, as the metal M is contained in the mixed oxide lattice, it is believed to be more stable towards leaching during the operation of the fuel cell for longer periods of time.