Process for producing ion exchange membranes by melt-processing of acidic PFSA ionomers

Abstract

A process for producing an ion exchange membrane involves melt-processing a mixture of a perfluorosulfonic acid ionomer in its acid form and a specific azole additive. The additive may be a triazole, alkyl triazole, vinyl triazole, fluoro-alkyl triazole, fluoro-vinyl triazole, pyrazole, alkyl pyrazole, vinyl pyrazole, fluoro-alkyl pyrazole, fluoro-vinyl pyrazole, benzimidazole, alkyl benzimidazole, vinyl benzimidazole, fluoro-alkyl benzimidazole, fluoro-vinyl benzimidazole or any mixture thereof to form a film having a thickness of from 3 to 200 microns. Ion exchange membranes so produced have reduced in-plane-swelling, improved dimensional stability and mechanical properties, and are useful as electrolytes in proton exchange membrane fuel cells.

Claims

1. A process for producing an ion exchange membrane comprising: melt-processing a mixture of a perfluorosulfonic acid ionomer in its acid form and an additive comprising triazole, alkyl triazole, vinyl triazole, fluoro-alkyl triazole, fluoro-vinyl triazole, pyrazole, alkyl pyrazole, vinyl pyrazole, fluoro-alkyl pyrazole, fluoro-vinyl pyrazole, benzimidazole, alkyl benzimidazole, vinyl benzimidazole, fluoro-alkyl benzimidazole, fluoro-vinyl benzimidazole or any mixture thereof that is non-volatile, and thermally stable at a temperature of the melt-processing, to form a film having a thickness of from 3 to 200 microns.

2. The process according to claim 1, wherein the additive comprises 1,2,4-triazole, benzimidazole or a mixture thereof.

3. The process according to claim 1, wherein the additive comprises 1,2,4-triazole.

4. The process according to claim 1, further comprising removing the additive from the film after melt-processing.

5. The process according to claim 4, wherein the additive is removed by dissolving it in a solvent.

6. The process according to claim 5, wherein the solvent comprises water or an aqueous solution.

7. The process according to claim 5, wherein the aqueous solution is a solution of sulfuric acid.

8. The process according to claim 1, wherein the melt-processing comprises melt-extrusion, melt-casting or melt-blowing.

9. The process according to claim 1, wherein the additive is first dissolved in an aqueous solution before melt-processing with the perfluorosulfonic acid ionomer.

10. The process according to claim 1, wherein the perfluorosulfonic acid ionomer is pre-swollen before melt-processing with the additive.

11. A process according to claim 1, wherein the perfluorosulfonic acid ionomer is a perfluorosulfonic acid-polytetrafluoroethylene copolymer of Formula (II): ##STR00003## wherein the number of repeat units x and y are such that there are less than 15 x units for each y and the value of m and n are integers between 0 and 5.

12. An ion exchange membrane made by the process of claim 1.

13. The membrane according to claim 12, wherein the film has a thickness of from 3 to 95 microns.

14. The membrane according to claim 12, wherein the length and/or width changes by less than 7.5%.

15. The membrane according to claim 12, wherein the film consists of the perfluorosulfonic acid ionomer.

16. An electrochemical device comprising an electrolyte comprising the membrane as defined in claim 12.

17. The device according to claim 16 which is a polymer electrolyte membrane fuel cell.

18. The membrance according to claim 12, wherein the perfluorosulfonic acid ionomer is a perfluorosulfonic acid-polytetrafluoroethylene copolymer of Formula (II): ##STR00004## wherein the number of repeat units x and y are such that there are less than 15 x units for each y and the value of m and n are integers between 0 and 5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order that the invention may be more clearly understood, embodiments thereof will now be described in detail by way of example, with reference to the accompanying drawings, in which:

(2) FIG. 1 provides rheological data measured on acid PFSA (Nafion NR-40) with 10 wt % of three different azole additives (Tz: 1,2,4-Triazole, Bz: Benzimidazole and Im: Imidazole) using a rotational rheometer (Advanced Rheometric Expansion System (ARES)) in dynamic mode. Frequency sweep at 260 C. under dry nitrogen atmosphere.

(3) FIG. 2 provides rheological data measured on PFSA precursor (Nafion R-1000 from Ion-Power) using a rotational rheometer (Advanced Rheometric Expansion System (ARES)) in dynamic mode. Frequency sweep at 260 C. under dry nitrogen atmosphere.

(4) FIG. 3 provides a picture of extruded membranes based on Nafion NR40 and 15 wt % of 1,2,4-triazole (left) and imidazole (right) additives.

(5) FIG. 4 provides Transmission Electron Microscopy (TEM) images with two magnifications (55.5 k and x 555 k) on lead acetate stained membranes as extruded (FIG. 4A) and after activation using protocol A1 (FIG. 4B)

(6) FIG. 5 is a graph depicting dry/wet volume change (VC) and water uptake (WU) measured for acid extruded Nafion NR40 with 15 wt % of 1,2,4-triazole additive activated with protocols A1 (MB-NR40/15% Tz A1) and A2 (MB-NR40/15% Tz A2) as well as NRE-211 and N111-IP references.

(7) FIG. 6 is a graph depicting dry/wet changes in thickness and linear expansion in the machine direction (MD) and transverse direction (TD) measured for acid extruded Nafion NR40 with 15 wt % of 1,2,4-triazole additive activated with protocols A1 (MB-NR40/15% Tz A1) and A2 (MB-NR40/15% Tz A2) as well as NRE-211 and N111-IP references.

(8) FIG. 7 provides Young modulus in machine direction (MD) and transverse direction (TD) determined from stress-strain curves for MB-NR40/15% Tz A2 membrane and NRE-211 and N111-IP references.

(9) FIG. 8 provides fluoride release per gram of membranes for acid extruded Nafion NR40 with 15 wt % of 1,2,4-triazole additive as prepared (MB-NR40/15% Tz) and after activation with protocols A1 (MB-NR40/15% Tz A1) and A2 (MB-NR40/15% Tz A2) as well as N111-IP references.

DESCRIPTION OF PREFERRED EMBODIMENTS

(10) Methods and Materials:

(11) Materials: For the examples described herein, two PFSA ionomers in the acid form were used: a long side chain PFSA; Nafion NR-40 with an Equivalent Weight EW=1000 g/eq, a short side chain PFSA Aquivion with EW=830 g/eq.

(12) Rheology: To determine melt viscosities of polymer/additive blends, dynamic rheological measurements were performed in an ARES (Advance Rheometric Expansion System) rotational rheometer under dry nitrogen atmosphere at T=260 C. Prior to testing, samples were dried 24 h under active vacuum at 60 C. The test consists on a frequency sweep over a range spanning from 100 down to 0.1 rad/s. Small deformations (10% or 15%) oscillatory motions were imposed on the samples for all time and frequency sweeps to avoid any irreversible damage of the structure of the material. The measurements allow to evaluate the response of the materials tested in term of elastic or storage modulus (G), viscous or loss modulus (G), and the dynamic complex viscosity (*).

(13) Melt-processing: PFSA Nafion NR-40/additive blends were directly extruded by melt-casting or melt-blowing at a bench-top microextruder (DSM), and scaled-up to pilot-scale extruders (LabTech). Processing temperature was 260 C.

(14) Activation: Membranes obtained with the process described herein, could be used directly, or could be activated to remove the additive for low temperature operation. The activation consists of soaking the membranes in de-ionized (DI) water or a H.sub.2SO.sub.4 solution in water (1:5 vol %) for 1 hour at 80 C.

(15) Proton conductivity: In-plane proton conductivities were measured using a Solartron 1260. A strip of membrane (in H.sup.+ form) was set between 2 Pt electrodes and an alternating current was passed through the plane of the sample. In the case of room temperature and liquid water conditions, the samples were immersed in Millipore water. Nyquist plots between 5 MHz to 10 Hz were collected and membrane resistance was extrapolated by fitting the semi-circle part of the data to equivalent circuits. Proton conductivities were calculated from the equation below:

(16) $=\frac{d}{RS}$
where is proton conductivity, d is the distance between the Pt electrodes, R is membrane resistance and S is the cross-sectional area of the sample.

(17) Water Uptake (WU) and Volume Change (VC): After measuring mass of wet and dried membranes (in H.sup.+ form), WU was calculated from the equation below:

(18) $WU=\frac{\mathrm{Wet}\mathrm{mass}-\mathrm{dry}\mathrm{mass}}{\mathrm{dry}\mathrm{mass}}100\%$
The mass of dried membranes were obtained after drying them in a vacuum oven at 80 C. overnight. For VC measurements, thickness, width and length of wet and dried membranes were determined. Wet/dry volume change was calculated from the equation below:

(19) $VC=\frac{\mathrm{Volume}\mathrm{of}\mathrm{wet}\mathrm{membranes}-\mathrm{volume}\mathrm{of}\mathrm{dried}\mathrm{membranes}}{\mathrm{Volume}\mathrm{of}\mathrm{dried}\mathrm{membranes}}100\%$
Dry measurements were obtained after drying the membranes in a vacuum oven at 80 C. overnight. Wet/dry dimensional changes were measured for the membranes and compared to reference solution cast (Nafion NRE-211) and melt extruded (Nafion N-111-IP) commercial membranes.

(20) Transmission Electron Microscopy (TEM): Membranes in the acid form were previously immersed in a saturated lead acetate solution during 2 h at room temperature to stain the ionic domains. The samples were then microtomed into thin samples of 50 nm thickness using a diamond knife. TEM of ultrathin sections of the samples were obtained with a Philips CM 200 instrument with an acceleration voltage of 200 kV.

(21) Mechanical micro-tensile tests: The tensile mechanical properties of films were measured according to standard ASTM D1708 in an Instron 5548 microtester. The test specimens were drawn at a speed of 5 mm/min. Each reported value is the average of five measurements.

(22) Fenton test: Fluoride ion release using the Fenton's test was conducted using 10 wt % hydrogen peroxide and FeSO.sub.4.7H.sub.2O solution (800 ppm Fe.sup.2+). Around 100-150 mg of dried membrane was weighed and placed in a vial. 20 mL of the H.sub.2O.sub.2 10 wt % solution and 1.0 mL Fe.sup.2+ solution were added to each bottle. Then the membrane was exposed to the Fenton's reagent for 6 hours at 80 C. After treatment, the membrane was repeatedly rinsed with de-ionized water. 2.5 mL of the Total Ionic Strength Adjustor Buffer (TISAB) were added into each bottle to adjust the PH of the solution, decomplex fluoride and provide a constant background ionic strength, the resulting solution including rinses was weighted. A Fluoride Combination Electrode was used to analyze the concentration of fluoride ions. Both the concentration of fluoride ions and the volume of recovered solutions were used to determine the total number of moles of fluoride released.

Example 1

Preparation of PFSA/Additive Blends and their Rheological Characterization

(23) Two additives useful in the present invention (1,2,4-triazole (Tz) and benzimidazole (Bz)) and one comparative additive from the prior art (imidazole (Im)) were selected. Nafion/additive (10 wt %) blends were prepared by dissolving the additive in water followed by addition of the solution to Nafion NR40 and stirring to allow a homogeneous swelling of the ionomer. The blends were then dried at 60 C. overnight under vacuum to remove the water. The dried solid mixture was then analyzed in a rheometer. FIG. 1 shows the results of the frequency sweep test at 260 C., where the dynamic complex viscosity (Eta* or *) is represented as a function of the oscillation frequency. While the Nafion NR40 sample was impossible to analyze due to its very high viscosity and visible thermal degradation of the acidic groups, the Nafion/additive samples show a flow behavior as the additive forms a conjugated acid with the ionic groups of the ionomer, shielding the strong sulfonic acid interactions. The melt viscosity increases as the shear rate is decreased, showing the non-Newtonian properties of these blends as the linear dependence of complex viscosity at low frequency was not observed. NafionNR40/1,2,4-triazole blend show higher viscosity than imidazole and benzimidazole based blends.

Example 2

Effect of the Concentration of Additive on the Melt-Viscosity

(24) NafionNR40/1,2,4-triazole blends were prepared as described in Example 1, with additive loadings of 10, 15 and 20 wt % corresponding to 1.6, 2.6 and 3.6 mol of 1,2,4-triazole per mol of SO.sup.3 respectively. FIG. 2 shows the results of the frequency sweep test at 240 C., where the dynamic complex viscosity is represented as a function of the oscillation frequency. NafionNR40/1,2,4-triazole blends show a decrease in the viscosity as the concentration of additive increases.

Example 3

Melt-Processability of the Acid Ionomer/Additive Blends Prepared in Examples 1 And 2 to Produce Practical Thin Membranes

(25) A blend based on Aquivion short side chain PFSA ionomer (EW=830 g/eq) and 15 wt % 1,2,4-triazole was also prepared as described in Example 1. Melt-processing was carried out at 260 C., using a 5 cc bench-top micro-extruder (DSM-Explore) equipped with a film line. The die used for thin film preparation has an opening gap of 0.1 mm and a width of 3.5 cm. The screws RPM, the calender rolls speed and torque was varied to achieve the required thickness. The strips of membranes obtained had a final width of approximately 2.5 cm and a thickness in the range of 5 to 50 microns.

(26) Melt-processing to produce PFSA-based thin and transparent membranes was successful with triazole and benzimidazole additives. However, it is particularly noteworthy that melt-processing using imidazole as the additive completely failed to produce PFSA-based membranes, despite the fact that it was successfully used in the prior art to form polysulfone-based membranes (US 2009-1315444). Imidazole based membranes show clear signs of degradation (yellow-brown coloration) and the presence of holes as illustrated in FIG. 3, since the processing temperature of PFSA ionomer (260 C.) is close to imidazole boiling point (256 C.).

Example 4

Process Scale-Up

(27) To demonstrate the feasibility of the procedure developed herein at an industrial level, the manufacturing process was scaled-up from bench-top to pilot-scale level. The process described in U.S. provisional patent application U.S. Ser. No. 61/577,138 based on a multilayer melt-blowing process was used successfully for the prototyping of 15 to 20 cm wide rolls of polymer electrolyte membranes with a thickness ranging from more than 200 microns down to 3 microns directly from NafionNR40/15 wt %1,2,4-triazole blend.

Example 5

Activation Protocols and Membrane Characterization

(28) The membranes obtained in Example 4 with the pilot-scale process were used.

(29) Two activation protocols were used:

(30) Protocol A1 comprises soaking the membranes in DI water for 1 h at 80 C.;

(31) Protocol A2 comprises soaking the membranes for 1 h at 80 C. in a solution H.sub.2SO.sub.4:H.sub.2O (1:5 vol %), followed by DI water for 1 h at 80 C.

(32) Properties of acid extruded membranes activated with protocols A1 and A2 were measured and compared with NRE-211 a solution-cast Nafion membrane, and N111-IP an extruded commercial Nafion membrane. For comparison purposes, the thickness of the membranes selected was about 255 microns.

(33) Transmission Electron Microscopy (TEM) technique was used to examine the morphology and the arrangement of the hydrophobic/hydrophilic phase separation within the ionomeric materials produced. High resolution TEM images on lead acetate stained membranes prepared by melt-blowing with 15 wt % 1,2,4-triazole as additive are presented in FIG. 4. FIG. 4A corresponds to the membrane as extruded and FIG. 4B to the membrane after activation using protocol A1. Two magnifications are shown for each membrane. The fine phase separation of hydrophilic and hydrophobic domains characteristic of PFSA ionomers is visible in all cases. The dark regions correspond to the ionic domains and the clear domains to the hydrophobic backbone of PFSA. The low magnification micrograph corresponding to the as extruded sample (FIG. 4A), shows dark spots related to the excess additive still present.

(34) After activation according to the protocols A1 and A2, ion exchange capacity (IEC) was determined by titration, and proton conductivity was measured by impedance spectroscopy at room temperature in water, and at 80 C. at 50% and 30% relative humidity (RH). The results, presented in Table 1, show a higher IEC and conductivity for the membranes activated according to protocol A2. Those activated according to protocol A1 show values in the same range as commercial references. These results suggest that a simple activation in water at 80 C. allows a complete dissociation of the additive in water and a recovery of the full ion exchange capacity of the membranes.

(35) TABLE-US-00001 TABLE 1 IEC and Conductivity Conductivity (S/cm) IEC RT @ 80 C. @ 80 C. @ Sample (mmol/g) in water 50% RH 30% RH NRE-211 0.835 6.87E02 2.81E02 3.61E03 N111-IP 0.872 6.65E02 2.30E02 6.02E03 MB-NR40/15% Tz A1 0.887 6.91E02 1.57E02 3.02E03 MB-NR40/15% Tz A2 0.939 1.02E01 5.59E02 1.43E02

(36) FIG. 5 shows water uptake (WU) and dry/wet volume change (VC) measured for acid extruded membranes, NRE-211 and N111-IP references. Membranes activated according to protocol A1 show reduced volume change but higher water uptake compared to NRE-211 and N111-IP references, while membranes activated with protocol A2 show higher WU and VC due to its higher IEC.

(37) The membranes prepared according to the process described in this patent show a very low linear expansion independently of the activation protocol used. They swell preferentially in the thickness direction as shown in FIG. 6 and Table 2. This is an extremely interesting property in PEMFC. This dimensional stability behavior has not been observed before to our knowledge and is desirable for minimizing stresses that develops in an operating fuel cell during humidity cycling, thus leading to enhanced mechanical durability.

(38) TABLE-US-00002 TABLE 2 Dry/Wet Dimensional Changes Dimensional changes (%) Sample TD MD Thickness Area Volume NRE-211 15.56 22.74 13.33 41.92 60.84 N111-IP 12.08 13.46 13.09 27.18 43.81 MB-NR40/15% Tz A1 1.19 5.72 25.36 7.01 34.10 MB-NR40/15% Tz A2 1.54 2.33 71.30 3.88 77.92

(39) The micro-tensile properties of membranes activated with protocol A2 were measured and compared to commercial references. FIG. 7 provides Young modulus in machine direction (MD) and transverse direction (TD) determined from stress-strain curves. The results show a higher elastic modulus for MB-NR40/15% Tz A2; 94 MPa and 98 MPa in TD and MD, respectively.

(40) Ex-situ Fenton's aging tests are not representative of in-situ fuel cell degradation as the presence of Fe.sup.2+ catalyses hydroxyl radical generation at levels much higher than those present in an operating fuel cell. However, they can be used to compare chemical durability of materials to Fenton reagents. Membranes as extruded in the acid form with the process described in this patent, and after activation with protocols A1 and A2 were tested and compared to commercial extruded N111-IP. The results in FIG. 8 show milligrams of fluoride ions released per gram of membrane. In a comparative basis, these results suggest that the membranes of the present invention show a much higher stability to Fenton's aging tests, as the fluoride released in almost half of the N111-IP reference. These results demonstrate that an improvement in mechanical properties and dimensional stability translate to improved chemical stability.

REFERENCES

(41) The contents of the entirety of each of which are incorporated by this reference. Lai Y-H, et al. (2009) Journal of Fuel Cell Science and Technology. 6 (2), 1-13. Mokrini A, et al. U.S. Provisional Patent Application Ser. No. 61/577,138 filed Dec. 19, 2011. Sanchez J-Y, et al. (2009) United States Patent Publication US 2009-1315444 published May 21, 2009. Sen U, et al. (2008) Anhydrous proton conducting membranes for PEM fuel cells based on Nafion/Azole composites. International Journal of Hydrogen Energy. 33, 2808-2815.

(42) The novel features of the present invention will become apparent to those of skill in the art upon examination of the detailed description of the invention. It should be understood, however, that the scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the specification as a whole.