IONOMER DISPERSION HAVING HIGH DISPERSION STABILITY, METHOD FOR PRODUCING SAME, AND POLYMER ELECTROLYTE MEMBRANE PRODUCED USING SAME

Abstract

Disclosed are: an ionomer dispersion having high dispersion stability while also containing high content of ionomer solids, thus optimizing the ionomer morphology in a polymer electrolyte membrane to allow both the ion conductivity and durability of the polymer electrolyte membrane to be improved; a method for producing the ionomer dispersion; and a polymer electrolyte membrane produced using the method.

Claims

1. An ionomer dispersion comprising: a dispersion medium; and an ionomer solid in the dispersion medium, wherein a concentration of the ionomer solid in the ionomer dispersion is 20% by weight or more, and when a viscosity of the ionomer dispersion is measured using a rheometer while a shear rate is increasing from 0.001 s.sup.−1 to 1,000 s.sup.−1 and then decreasing from 1,000 s.sup.−1 to 0.001 s.sup.−1, a viscosity ratio of the ionomer dispersion defined by the following Equation 1 is 1.7 or less:
Viscosity ratio=η2/η1   Equation 1: wherein η1 is a first viscosity of the ionomer dispersion, measured while the shear rate is increasing, when the shear rate is 1 s.sup.−1, and η2 is a second viscosity of the ionomer dispersion, measured while the shear rate is decreasing, when the shear rate is 1 s.sup.−1.

2. The ionomer dispersion according to claim 1, wherein the first viscosity and the second viscosity are 1 Pa.Math.s or less.

3. The ionomer dispersion according to claim 1, wherein, when a shear stress of the ionomer dispersion is measured using a rheometer while the shear rate is increasing from 0.001 s.sup.−1 to 1,000 s.sup.−1 and then decreasing from 1,000 s.sup.−1 to 0.001 s.sup.−1, a shear stress ratio of the ionomer dispersion defined by the following Equation 2 is 1.5 or less:
Shear stress ratio=σ2/σ1   Equation 2: wherein σ1 is a first shear stress which is a shear stress of the ionomer dispersion, measured while the shear rate is increasing, when the shear rate is 1 s.sup.−1, and σ2 is a second shear stress which is a shear stress of the ionomer dispersion, measured while the shear rate is decreasing, when the shear rate is 1 s.sup.−1.

4. The ionomer dispersion according to claim 3, wherein the first shear stress and the second shear stress are 1 Pa.Math.s or less.

5. The ionomer dispersion according to claim 1, wherein the ionomer solid comprises a fluorinated ionomer, a hydrocarbon-based ionomer, or a mixture thereof.

6. The ionomer dispersion according to claim 1, wherein the ionomer solid comprises a perfluorinated sulfonic acid-based ionomer (PFSA-based ionomer).

7. The ionomer dispersion according to claim 1, wherein the concentration of the ionomer solid in the ionomer dispersion is 20 to 50% by weight.

8. A method for preparing an ionomer dispersion comprising: adding an ionomer to a dispersion medium such that a content of an ionomer solid is adjusted to 20% by weight or more; and mixing the dispersion medium with the ionomer using a resonant acoustic method.

9. The method according to claim 8, wherein the mixing comprises applying acoustic energy with a frequency of 10 to 100 Hz to the dispersion medium and the ionomer, whereby subjecting the dispersion medium and the ionomer to resonant vibration at an acceleration of 50 G to 100 G, wherein G is gravitational acceleration.

10. The method according to claim 8, wherein the mixing is performed for 5 to 60 minutes.

11. The method according to claim 8, further comprising applying a pressure of 500 to 1,000 bar to a mixture of the dispersion medium and the ionomer, obtained through the mixing.

12. The method according to claim 11, wherein the pressure is applied to the mixture when the mixture is flowing.

13. A polymer electrolyte membrane comprising: a porous support having a plurality of pores; and an ionomer formed by impregnating and coating the porous support with the ionomer dispersion according to claim 1, and then removing the dispersion medium from the ionomer dispersion.

14. A membrane-electrode assembly comprising: an anode; a cathode; and the polymer electrolyte membrane according to claim 13, interposed between the anode and the cathode.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0042] The accompanying drawings, which are provided for better understanding of the present disclosure and constitute a part of the present specification, are given to exemplify the embodiments of the present disclosure and describe the principles and features of the present disclosure with reference to the following detailed description, in which:

[0043] FIG. 1 is a graph showing the viscosity and shear stress of the ionomer dispersion of Example 1a measured while performing a flow sweep;

[0044] FIG. 2 is a graph showing the viscosity and shear stress of the ionomer dispersion of Example 2a measured while performing a flow sweep; and

[0045] FIG. 3 is a graph showing the viscosity and shear stress of the ionomer dispersion of Comparative Example la measured while performing a flow sweep.

BEST MODE

[0046] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. However, the following embodiments are illustratively provided merely for clear understanding of the present disclosure, and do not limit the scope of the present disclosure.

[0047] The ionomer dispersion of the present disclosure contains a dispersion medium and an ionomer solid dispersed therein.

[0048] The dispersion medium may be water, an organic solvent, or a mixture thereof.

[0049] The organic solvent contains, as a backbone (main chain), a linear or branched saturated or unsaturated hydrocarbon having 1 to 12 carbon atoms, and at least one functional group selected from the group consisting of alcohol, isopropyl alcohol, ketone, aldehyde, carbonate, carboxylate, carboxylic acid, ether, and amide. The backbone may include at least a part of an alicyclic or aromatic cyclic compound.

[0050] The organic solvent may also be N-methylpyrrolidone, dimethyl sulfoxide, tetrahydrofuran, or a mixture thereof.

[0051] According to an embodiment of the present disclosure, the dispersion medium may be a mixture of a hydrophilic organic solvent and water. More specifically, the dispersion medium may be a mixture of C.sub.1-C.sub.12 alcohol and water at a volume ratio of 1:2 to 2:1.

[0052] The ionomer solid may contain an ionomer having, in a side chain, at least one ion-conductive group selected from the group consisting of a sulfonic acid group, a carboxyl group, a boronic acid group, a phosphoric acid group, an imide group, a sulfonimide group, a sulfonamide group, and a sulfonic acid fluoride group. For example, the ionomer may be a cation conductor having a sulfonic acid group and/or a carboxyl group in a side chain.

[0053] The ionomer may be a fluorinated ionomer, a hydrocarbon-based ionomer, or a mixture thereof.

[0054] For example, the ionomer may be a fluorinated ionomer such as poly(perfluorosulfonic acid) or poly(perfluorocarboxylic acid).

[0055] The hydrocarbon-based ionomer may be a hydrocarbon-based polymer having the ion conductive group in the side chain [for example, sulfonated polyimide (S-PI), sulfonated polyarylethersulfone (S-PAES), sulfonated polyetheretherketone (SPEEK), sulfonated polybenzimidazole (SPBI), sulfonated polysulfone (S-PSU), sulfonated polystyrene (S-PS), sulfonated polyphosphazene, sulfonated polyquinoxaline, sulfonated polyketone, sulfonated polyphenylene oxide, sulfonated polyether sulfone, sulfonated polyether ketone, sulfonated polyphenylene sulfone, sulfonated polyphenylene sulfide, sulfonated polyphenylene sulfide sulfone, sulfonated polyphenylene sulfide sulfone nitrile, sulfonated polyarylene ether, sulfonated polyarylene ether nitrile, sulfonated polyarylene ether ether nitrile, sulfonated polyarylene ether sulfone ketone, or the like].

[0056] According to an embodiment of the present disclosure, the ionomer solid may contain a perfluorinated sulfonic acid-based ionomer (PFSA-based ionomer).

[0057] According to the present disclosure, in order to manufacture as dense a polymer electrolyte membrane as possible, the concentration of the ionomer solid in the ionomer dispersion is 20% by weight or more. For example, the concentration of the ionomer solid in the ionomer dispersion may be 20 to 50% by weight.

[0058] Optionally, the ionomer dispersion may further contain a radical scavenger capable of decomposing and/or removing peroxides (e.g., hydrogen peroxide) and radicals (e.g., hydrogen radicals) that are produced during redox reactions in fuel cells and cause degradation of polymer electrolyte membranes (more specifically, ionomers) and catalyst electrodes.

[0059] For example, the ionomer dispersion may further contain at least one radical scavenger selected from the group consisting of: transition metals such as Ce, Ni, W, Co, Cr, Zr, Y, Mn, Fe, Ti, V, Mo, La, and Nd; noble metals such as Au, Pt, Ru, Pd, and Rh; ions thereof; oxides thereof; and salts thereof.

[0060] According to the present disclosure, when a flow sweep is performed by measuring the viscosity of the ionomer dispersion using a rheometer while a shear rate is increasing from 0.001 s.sup.−1 to 1,000 s.sup.−1 and then decreasing from 1,000 s.sup.−1 to 0.001 s.sup.−1, the viscosity ratio of the ionomer dispersion, defined by the following Equation 1, is 1.7 or less, preferably 0.5 to 1.5, more preferably 0.7 to 1.3, and still more preferably 0.9 to 1.1:

Viscosity ratio=η2/η1   Equation 1:

wherein η1 is a first viscosity of the ionomer dispersion, measured while the shear rate is increasing, when the shear rate is 1 s.sup.−1, and η2 is a second viscosity of the ionomer dispersion, measured while the shear rate is decreasing, when the shear rate is 1 s.sup.−1.

[0061] That is, the viscosity ratio is a parameter indicating the dispersion stability of the ionomer dispersion, and as the viscosity ratio becomes closer to 1 (that is, as the difference between the first viscosity and the second viscosity decreases), the dispersion stability of the ionomer dispersion increases. It is ideal for the viscosity ratio to be 1.

[0062] An example of the rheometer used to perform the flow sweep to measure the viscosity ratio is Discovery HR-3, produced by TA Instruments, and the measurement conditions are as follows.

[0063] Temperature: 25° C.

[0064] Soak Time: 0.0 s

[0065] Wait For Temperature: Off

[0066] Logarithmic sweep

[0067] Shear rate: 1.0.Math.e.sup.−3˜10.sup.3 (s.sup.−1)

[0068] Points per decade: 5

[0069] Equilibration time: 1.0 s

[0070] Averaging time: 1.0 s

[0071] According to an embodiment of the present disclosure, both the first and second viscosities may be 1 Pa.Math.s or less.

[0072] In addition, according to an embodiment of the present disclosure, when the shear stress of the ionomer dispersion is measured while performing the flow sweep, the shear stress ratio of the ionomer dispersion, defined by the following Equation 2, may be 1.5 or less:

Shear stress ratio=σ2/σ1   Equation 2:

[0073] wherein σ1 is a first shear stress which is a shear stress of the ionomer dispersion, measured while the shear rate is increasing, when the shear rate is 1 s.sup.−1, and σ2 is a second shear stress which is a shear stress of the ionomer dispersion, measured while the shear rate is decreasing, when the shear rate is 1 s.sup.−1.

[0074] That is, the viscosity ratio is a primary parameter indicating the dispersion stability of the ionomer dispersion, whereas the shear stress ratio is a secondary parameter indicating the dispersion stability of the ionomer dispersion. That is, as the shear stress ratio becomes closer to 1 (that is, as the difference between the first shear stress and the second shear stress decreases), the dispersion stability of the ionomer dispersion increases. It is ideal for the shear stress ratio to be 1.

[0075] According to an embodiment of the present disclosure, both the first shear stress and the second shear stress may be 1.Math.Pa or less.

[0076] Hereinafter, a method for preparing an ionomer dispersion according to an embodiment of the present disclosure will be described in detail.

[0077] The method for preparing an ionomer dispersion of the present disclosure includes adding an ionomer to a dispersion medium and mixing the dispersion medium with the ionomer.

[0078] As described above, the ionomer is generally introduced into the porous support in the form of a dispersion rather than a solution and coated thereon, since the hydrophobic backbone and hydrophilic side chain of the ionomer have different solubilities. Therefore, according to the present disclosure, a dense polymer electrolyte membrane can be manufactured by adding the ionomer to the dispersion medium such that the ionomer dispersion contains an ionomer solid in a large amount of 20% by weight or more.

[0079] However, in order to overcome the problem in which the dispersion stability of the dispersion decreases as the concentration of the solid in the dispersion increases, the dispersion medium and the ionomer are mixed using a resonant acoustic method.

[0080] The mixing based on the resonant acoustic method may be performed by applying acoustic energy with a frequency of 10 to 100 Hz to the dispersion medium to which the ionomer is added to subject the dispersion medium and the ionomer to resonant vibration at an acceleration of 50 G to 100 G, wherein G is gravitational acceleration.

[0081] The mixing based on the resonant acoustic method is performed using, for example, a mixer (model name: LabRAM II) commercially available from Resodyn Acoustic Mixers, Inc.

[0082] The mixing based on the resonance sound wave method may be performed for, for example, 5 to 60 minutes.

[0083] By adding the ionomer to the dispersion medium, followed by mixing using the resonant acoustic method, an ionomer dispersion having high dispersion stability despite having high content of ionomer solids can be prepared.

[0084] According to an embodiment of the present disclosure, the method may further include applying a high pressure of 500 to 1,000 bar to the mixture of the dispersion medium and the ionomer, obtained through the mixing. For example, the pressing may be performed using a homogenizer, and the high pressure described above may be applied to the mixture flowing in the homogenizer.

[0085] Optionally, in order to further increase the dispersion stability of the ionomer dispersion, high-pressure dispersion using a homogenizer may be repeated 2 to 5 times.

[0086] As the particle size becomes smaller and more uniform due to the pressing, the dispersion stability of the ionomer dispersion is further increased.

[0087] In addition, owing to the small and uniform particle size, the ionomer is imparted with a larger reaction surface area, so the crystallinity of the ionomer can be effectively increased through the drying and/or annealing process, and as a result, the hydrogen permeability of the polymer electrolyte membrane can be minimized. Therefore, radicals may be generated due to the permeation of hydrogen gas into the polymer electrolyte membrane and the degradation of the polymer electrolyte membrane can be suppressed, and as a result, the durability of the polymer electrolyte membrane and electrochemical components including the same (e.g., a membrane-electrode assembly, fuel cell, etc.) can be improved.

[0088] The polymer electrolyte membrane of the present disclosure can be manufactured by impregnating and coating a porous support having a plurality of pores with the ionomer dispersion of the present disclosure prepared as described above, followed by removing the dispersion medium.

[0089] Hereinafter, a method for manufacturing a polymer electrolyte membrane according to an embodiment of the present disclosure will be described in detail.

[0090] The porous support may be formed of polytetrafluoroethylene (PTFE) or a copolymer of tetrafluoroethylene and CF.sub.2=CFC.sub.nF.sub.2n.sub.+1 (wherein n is an integer of 1 to 5) or CF.sub.2=CFO—(CF.sub.2CF(CF.sub.3).sub.mC.sub.nF.sub.2n+1 (wherein m is an integer of 0 to 15 and n is an integer of 1 to 15).

[0091] For example, an e-PTFE porous support in the form of an expanded film may be formed by extrusion-molding PTFE on a piece of tape in the presence of a lubricant, followed by expansion and thermal treatment. Additional expansion and thermal treatment may be further performed after the thermal treatment. By controlling the expansion and thermal treatment, e-PTFE porous supports having various microstructures can be formed. For example, the e-PTFE porous support may have a microstructure in which nodes are connected to one another through fibrils or a microstructure consisting only of fibrils.

[0092] Alternatively, the porous support may be a nonwoven web. The nonwoven web may be formed with a support-forming liquid containing at least one hydrocarbon-based polymer selected from the group consisting of polyolefin (e.g., polyethylene, polypropylene, polybutylene, etc.), polyester (e.g. PET, PBT, etc.), polyamide (e.g., nylon-6, nylon-6,6, aramid, etc.), polyamic acid (converted to polyimide through imidization after being molded into a web), polyurethane, polybutene, polylactic acid, polyvinyl alcohol, polyphenylene sulfide (PPS), polysulfone, fluid crystalline polymer, polyethylene-co-vinyl acetate, polyacrylonitrile, cyclic polyolefin, polyoxymethylene, and polyolefin-based thermoplastic elastomer elastomers.

[0093] The nonwoven web may be produced by any one method selected from the group consisting of wet-laying, electrospinning, carding, garneting, air-laying, melt blowing, spunbonding, and stitch bonding.

[0094] The porous support that can be used in the present disclosure may have an average pore diameter of 0.1 to 0.2 μm and a porosity of 60 to 95%.

[0095] When the average pore diameter is less than 0.1 μm or the porosity is less than 60%, due to the excessively small amount of the ionomer in the porous support, the resistance of the polymer electrolyte membrane is increased and the ionic conductivity is decreased. On the other hand, when the average pore diameter is higher than 0.2 μm or the porosity is higher than 95%, subsequent processing may not proceed smoothly due to the deteriorated shape stability of the polymer electrolyte membrane.

[0096] Optionally, in order to further improve the mechanical strength of the polymer electrolyte membrane and to allow all pores in the porous support to be sufficiently filled with the ionomer, two or more relatively thin porous supports may be used instead of one thick porous support.

[0097] When the porous support is prepared, the porous support is impregnated and coated with the ionomer dispersion of the present disclosure. The impregnation and coating may be performed by (i) casting the ionomer dispersion on a substrate and then soaking the porous support with the ionomer dispersion, or (ii) coating the porous support with the ionomer dispersion. The coating may be performed, for example, using bar coating, comma coating, slot die coating, screen printing, spray coating, doctor blade coating, or the like.

[0098] Then, drying to remove the dispersion medium from the ionomer dispersion and annealing to increase the crystallinity of the ionomer are sequentially performed.

[0099] The drying may be performed at 60° C. to 150° C. for 30 minutes to 2 hours. When the drying temperature is less than 60° C. or the drying time is less than 30 minutes, the dispersion medium may not escape and thus a dense polymer electrolyte membrane may be not formed.

[0100] Optionally, the drying may be performed in multiple steps while elevating the temperature within the drying temperature range.

[0101] The annealing may be performed at 150° C. to 190° C. for 3 minutes to 1 hour. When the annealing temperature is higher than 190° C. or the annealing time is longer than 1 hour, there are problems in which the ion-conductive group of the ionomer is decomposed and the performance of the polymer electrolyte membrane is deteriorated.

[0102] According to an embodiment of the present disclosure, provided are a membrane-electrode assembly including the polymer electrolyte membrane manufactured as described above and a fuel cell including the same.

[0103] Specifically, the membrane-electrode assembly includes an anode, a cathode, and the polymer electrolyte membrane interposed therebetween. The membrane-electrode assembly is the same as a conventional membrane-electrode assembly for a fuel cell except that it uses the polymer electrolyte membrane according to the present disclosure as the polymer electrolyte membrane, and thus a detailed description thereof will be omitted herein.

[0104] In addition, the fuel cell is the same as a conventional fuel cell except that it includes the membrane-electrode assembly of the present disclosure, and thus a detailed description thereof will be omitted herein.

[0105] Hereinafter, the present disclosure will be described in more detail with reference to specific examples. However, the following examples are provided only for better understanding of the present disclosure, and should not be construed as limiting the scope of the present disclosure.

Preparation of Ionomer Dispersion

Example 1a

[0106] A PFSA-based ionomer (EW 725) was added to a dispersion medium in which 1-propanol and water were mixed at a weight ratio of 55:45 such that an ionomer solid content was adjusted to 20% by weight. Then, the dispersion medium and the ionomer were mixed by inducing resonance at an acceleration of 90 G using a mixer (Resodyn Acoustic Mixers, Inc., LabRAM II) at room temperature and humidity for 20 minutes (i.e., mixed using a resonant acoustic method) to prepare an ionomer dispersion.

Example 2a

[0107] A PFSA-based ionomer (EW 725) was added to a dispersion medium in which 1-propanol and water were mixed at a weight ratio of 30:40 such that an ionomer solid content was adjusted to 30% by weight. Then, the dispersion medium and the ionomer were mixed by inducing resonance at an acceleration of 90 G using a mixer (Resodyn Acoustic Mixers, Inc., LabRAM II) at room temperature and humidity for 20 minutes. Then, high-pressure dispersion of applying a high pressure of 600 bar to the mixture of the dispersion medium and the ionomer using a homogenizer was repeated three times to prepare an ionomer dispersion.

Example 3a

[0108] An ionomer dispersion was prepared in the same manner as in Example 1a except that a PFSA-based ionomer (EW 725) was added to a dispersion medium in which 1-propanol and water were mixed at a weight ratio of 49:51 such that an ionomer solid content was adjusted to 30% by weight.

Comparative Example 1a

[0109] A PFSA-based ionomer (EW 725) was added to a dispersion medium in which 1-propanol and water were mixed at a weight ratio of 55:45 such that an ionomer solid content was adjusted to 20% by weight. Then, the dispersion medium and the ionomer were mixed by inducing resonance at 250 rpm using a magnetic stirrer at room temperature and humidity for 48 hours to prepare an ionomer dispersion.

Comparative Example 2a

[0110] A PFSA-based ionomer (EW 725) was added to a dispersion medium in which 1-propanol and water were mixed at a weight ratio of 47:53 such that an ionomer solid content was adjusted to 25% by weight. Then, the dispersion medium and the ionomer were mixed at 250 rpm using a magnetic stirrer at 50° C. for 48 hours to prepare an ionomer dispersion.

Evaluation of Dispersion Stability of Ionomer Dispersions

[0111] The dispersion stability of the ionomer dispersions prepared in Examples and Comparative Examples was evaluated while performing a flow sweep. Specifically, the viscosity and shear stress of the ionomer dispersion were measured under the following conditions using a “Discovery HR-3” rheometer from TA Instruments, while the shear rate was increasing from 0.001 s.sup.−1 to 1,000 s.sup.−1 and then decreasing from 1,000 s.sup.−1 to 0.001 s.sup.−1.

[0112] Temperature: 25° C.

[0113] Soak Time: 0.0 s

[0114] Wait For Temperature: Off

[0115] Logarithmic sweep

[0116] Shear rate: 1.0.Math.e.sup.'31˜10.sup.3 (s.sup.−1)

[0117] Points per decade: 5

[0118] Equilibration time: 1.0 s

[0119] Averaging time: 1.0 s

[0120] The viscosity ratio of the ionomer dispersion defined by the following Equation 1 and the ionomer shear stress ratio defined by the following Equation 2 were calculated, and the results are shown in Table 1.

Viscosity ratio=η2/η1   Equation 1:

Shear stress ratio=σ2/σ1   Equation 2:

[0121] wherein η1 and σ1 respectively represent the viscosity and shear stress of the ionomer dispersion when the shear rate is 1 s.sup.−1, measured while the shear rate is increasing, and η2 and σ2 respectively represent the viscosity and shear stress of the ionomer dispersion when the shear rate is 1 s.sup.−1, measured while the shear rate is decreasing.

TABLE-US-00001 TABLE 1 η1 η2 η2/ σ1 σ2 σ2/ (Pa .Math. s) (Pa .Math. s) η1 (Pa) (Pa) σ1 Example 1a 0.402 0.507 1.26 0.400 0.434 1.09 Example 2a 0.226 0.228 1.01 0.226 0.228 1.01 Example 3a 0.620 0.965 1.56 0.625 0.875 1.40 Comparative 0.515 3.000 5.83 0.523 1.305 2.50 Example 1a Comparative 0.610 1.067 1.75 0.605 0.968 1.60 Example 2a

[0122] FIGS. 1 to 3 are graphs respectively illustrating the viscosity and shear stress of the ionomer dispersions of Example 1a, Example 2a, and Comparative Example 1a, measured while performing a flow sweep.

[0123] As can be seen from the graph of FIG. 1 and Table 1, in the ionomer dispersion of Example 1a, prepared by a resonant acoustic method, the viscosity and shear stress measured while the shear rate was increasing were very similar to those measured while the shear rate was decreasing, and the viscosity ratio and shear stress ratio respectively defined by Equation 1 and Equation 2 were less than 1.3, which means that the ionomer dispersion of Example 1a exhibited very high dispersion stability.

[0124] As can be seen from the graph of FIG. 2 and Table 1, in the ionomer dispersion of Example 2a, prepared by additionally performing a high-pressure dispersion process following the mixing process based on the resonant acoustic method, the viscosity and shear stress measured while the shear rate was increasing were substantially the same as those measured while the shear rate was decreasing, and the viscosity ratio and shear stress ratio respectively defined by Equations 1 and 2 reached about 1, which means that the ionomer dispersion of Example 2a exhibits the maximum dispersion stability.

[0125] As shown in Table 1, the ionomer dispersion of Example 3a, containing a higher amount of ionomer solid, exhibited a higher viscosity ratio and shear stress ratio compared to Example la, but exhibited a low viscosity ratio less than 1.7 (i.e., 1.56) and a low shear stress ratio less than 1.5 (i.e., 1.40) because it was prepared through the mixing process based on the resonant acoustic method, as in Example 1a.

[0126] Meanwhile, as can be seen from the graph of FIG. 3 and Table 1, in the ionomer dispersion of Comparative Example la prepared through a mixing process using a conventional magnetic stirrer, the viscosity and shear stress measured while the shear rate was increasing were remarkably different from those measured while the shear rate was decreasing, and the dispersion stability of the ionomer dispersion was so low that the viscosity and shear stress ratios, respectively defined by Equations 1 and 2, were higher than 2.

[0127] On the other hand, the ionomer dispersion of Comparative Example 2a, prepared by performing mixing at a higher temperature, exhibited a lower viscosity ratio and shear stress ratio compared to Comparative Example 1a, but exhibited a high viscosity ratio exceeding 1.7 (i.e., 1.75) and a high shear stress ratio exceeding 1.5 (i.e., 1.60) because it was prepared through a mixing process using a conventional magnetic stirrer, as in Comparative Example 1a.

Production of Polymer Electrolyte Membrane

Example 1b

[0128] Two pieces of e-PTFE porous supports each having an average pore diameter of 0.15 μm and a porosity of 75% were laminated, and the ionomer dispersion of Example 1a was applied to both the upper and lower surfaces thereof. Subsequently, the resulting structure was sequentially subjected to drying and annealing processes to manufacture a reinforced-composite-membrane-type polymer electrolyte membrane. The drying process was performed at 80° C. for 1 hour, and the annealing process was performed at 180° C. for 10 minutes.

Example 2b

[0129] A polymer electrolyte membrane was manufactured in the same manner as in Example 1b, except that the ionomer dispersion of Example 2a was used instead of the ionomer dispersion of Example 1a.

Example 3b

[0130] A polymer electrolyte membrane was manufactured in the same manner as in Example 1b, except that the ionomer dispersion of Example 3a was used instead of the ionomer dispersion of Example 1a.

Comparative Example 1b

[0131] A polymer electrolyte membrane was manufactured in the same manner as in Example 1b, except that the ionomer dispersion of Comparative Example la was used instead of the ionomer dispersion of Example la.

Comparative Example 2b

[0132] A polymer electrolyte membrane was manufactured in the same manner as in Example 1b, except that the ionomer dispersion of Comparative Example 2a was used instead of the ionomer dispersion of Example 1a.

Measurement of Properties of Polymer Electrolyte Membrane

[0133] The (i) in-plane ionic conductivity, (ii) water uptake, (iii) through-plane resistance, and (iv) hydrogen permeability of the polymer electrolyte membranes of Examples and Comparative Examples were measured according to the following methods, and the results are shown in Table 2.

In-Plane Ionic Conductivity & Water Uptake

[0134] The in-plane ionic conductivity and water uptake of the polymer electrolyte membrane at 80° C. and 50 RH % were each measured using a magnetic suspension balance device (Bell Japan).

Through-Plane Resistance

[0135] The through-plane resistance of the polymer electrolyte membrane at 80° C. and 50 RH % was measured using a membrane test system from Scribner (model name: MTS 740).

Hydrogen Permeability

[0136] The hydrogen permeability of the polymer electrolyte membrane at 65° C. and 50 RH % was measured using gas chromatography.

TABLE-US-00002 TABLE 2 In-Plane ionic Through-Plane Water Hydrogen conductivity resistance uptake permeability (S/cm) (Ω) (%) (cm.sup.2/sec) Example 1b 0.049 0.25 15 6.40E−05 Example 2b 0.055 0.19 15 4.47E−05 Example 3b 0.050 0.27 16 7.00E−05 Comparative 0.040 0.34 15 2.02E−04 Example 1b Comparative 0.041 0.35 16 1.58E−04 Example 2b

[0137] As can be seen from Table 2, the polymer electrolyte membranes of Examples 1b and 2b manufactured using ionomer dispersions having high dispersion stability exhibited the same water uptake as the polymer electrolyte membrane of Comparative Example 1b, manufactured using an ionomer dispersion having low dispersion stability, but exhibited (i) higher in-plane ionic conductivity, (ii) lower through-plane resistance, and (iii) lower hydrogen permeability.

[0138] In addition, the polymer electrolyte membrane of Example 2b, manufactured using the ionomer dispersion of Example 2a imparted with maximized dispersion stability by additionally performing a high-pressure dispersion process in addition to the mixing process based on a resonant acoustic method of the present disclosure, exhibited the best in-plane and through-plane ionic conductivity as well as the best hydrogen permeability (that is, the lowest hydrogen permeability).

[0139] On the other hand, the polymer electrolyte membrane of Example 3b, manufactured using the ionomer dispersion of Example 3a, having a viscosity ratio slightly lower than 1.7 (i.e., 1.56) and a shear stress ratio slightly lower than 1.5 (i.e., 1.40), also exhibited excellent physical properties in terms of ion conductivity and hydrogen permeability, albeit lower than those of the polymer electrolyte membranes of Examples 1b and 2b.

[0140] In contrast, the polymer electrolyte membrane of Example 3b, manufactured using the ionomer dispersion of Example 3a having a viscosity ratio slightly higher than 1.7 (i.e., 1.75) and a shear stress ratio slightly higher than 1.5 (i.e., 1.60), exhibited physical properties inferior to those of the polymer electrolyte membrane of Example 3b. In particular, the hydrogen permeability of the polymer electrolyte membrane of Example 3b was at least double that of the polymer electrolyte membrane of Example 3b. This indicates that the polymer electrolyte membrane manufactured using the ionomer dispersion having a viscosity ratio higher than 1.7 and/or a shear stress ratio higher than 1.5 did not exhibit satisfactory ion conductivity or hydrogen permeability.