Reinforced composite electrolyte membrane for fuel cell

Abstract

Disclosed is a composite electrolyte membrane comprising a microporous polymer substrate and a sulfonated polymer electrolyte. The composite electrolyte membrane comprises: a first polymer electrolyte layer formed of a first non-fluorinated or partially-fluorinated sulfonated polymer electrolyte; a non-fluorinated or partially-fluorinated microporous polymer substrate stacked on the first polymer electrolyte layer, wherein pores of the microporous polymer substrate are impregnated with a second non-fluorinated or partially-fluorinated sulfonated polymer electrolyte, and the first polymer electrolyte and the second polymer electrolyte are entangled with each other on an interface thereof; and a third polymer electrolyte layer formed on the microporous polymer substrate impregnated with the second polymer electrolyte by a third non-fluorinated or partially-fluorinated sulfonated polymer electrolyte, wherein the second polymer electrolyte and the third polymer electrolyte are entangled with each other on an interface thereof. A method for manufacturing the composite electrolyte membrane, and a membrane-electrode assembly (MEA) and a fuel cell comprising the composite electrolyte membrane are also disclosed.

Claims

1. A membrane-electrode assembly comprising an anode, a cathode and a composite electrolyte membrane provided between the anode and the cathode, wherein the composite electrolyte membrane including a microporous polymer substrate and a sulfonated polymer electrolyte, the composite electrolyte membrane comprising: a first polymer electrolyte layer formed of a first non-fluorinated sulfonated polymer electrolyte; a non-fluorinated microporous polymer substrate stacked on the first polymer electrolyte layer, wherein pores of the microporous polymer substrate are impregnated with a second non-fluorinated sulfonated polymer electrolyte, and the first polymer electrolyte and the second polymer electrolyte are entangled with each other on an interface thereof, a third polymer electrolyte layer formed on the microporous polymer substrate impregnated with the second polymer electrolyte by a third non-fluorinated sulfonated polymer electrolyte, wherein the second polymer electrolyte and the third polymer electrolyte are entangled with each other on an interface thereof, the first non-fluorinated sulfonated polymer electrolyte, the second non-fluorinated sulfonated polymer electrolyte, and the third non-fluorinated sulfonated polymer electrolyte are each independently selected from the group consisting of homopolymers, alternating copolymers, random copolymers, block copolymers, multiblock copolymers and graft copolymers, and the first non-fluorinated sulfonated polymer electrolyte, the second non-fluorinated sulfonated polymer electrolyte, and the third non-fluorinated sulfonated polymer electrolyte independently comprises at least one sulfonated hydrocarbon-based polymer selected from the group consisting of sulfonated poly(arylene ether)s, sulfonated poly(amide)s, sulfonated polyphosphazene, sulfonated radiation-grafted FEP-g-polystyrene, sulfonated radiation-grafted ETFE-g-polystyrene, sulfonated radiation-grafted LDPE-g-polystyrene, sulfonated radiation-grafted PVDF-g-polystyrene, wherein each of the first polymer electrolyte layer and the third polymer electrolyte layer independently has a thickness of 1m-50 m.

2. The membrane-electrode assembly according to claim 1, wherein the composite electrolyte membrane is manufactured in accordance with a method comprising: a first step of applying a solution of the first non-fluorinated sulfonated polymer electrolyte onto a base to form the first polymer electrolyte layer; a second step of stacking the non-fluorinated microporous polymer substrate on the first polymer electrolyte layer and allowing a solution of the second non-fluorinated sulfonated polymer electrolyte to infiltrate into pores of the microporous polymer substrate; and a third step of applying a solution of the third non-fluorinated sulfonated polymer electrolyte onto the microporous polymer substrate impregnated with the second polymer electrolyte to form the third polymer electrolyte layer.

3. The membrane-electrode assembly according to claim 1, wherein the microporous polymer substrate is formed of at least one material selected from the group consisting of polyethylene, polypropylene, polyethylene terephthalate, polyimide and polyamide.

4. The membrane-electrode assembly according to claim 1, wherein the microporous polymer substrate has a thickness ranging from 3m to 1,000 m, a porosity ranging from 20% to 95%, and a pore size ranging from 0.05m to 20m.

5. The membrane-electrode assembly according to claim 1, which has a thickness of 10 m-100 m.

6. The membrane-electrode assembly according to claim 1, wherein the second polymer electrolyte layer is formed by using a surfactant, while the first polymer electrolyte layer and the third polymer electrolyte layer are formed by using no surfactant.

7. The membrane-electrode assembly according to claim 1, wherein the microporous polymer substrate is impregnated with the second polymer electrolyte via a solution pouring process using a solution of the second polymer electrolyte.

8. A fuel cell comprising the membrane-electrode assembly composite electrolyte membrane as defined in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The foregoing and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

(2) FIG. 1 is a two-dimensional schematic view of the composite electrolyte membrane that may be obtained according to the present invention;

(3) FIG. 2 is a schematic sectional view of the composite electrolyte membrane that may be obtained according to a preferred embodiment of the present invention;

(4) FIG. 3 is a sectional image of the composite electrolyte membrane according to Example 1, taken by SEM (scanning electron microscopy);

(5) FIG. 4 is a sectional image of the composite electrolyte membrane according to Example 2, taken by SEM; and

(6) FIG. 5 is a sectional image of the composite electrolyte membrane according to Comparative Example 2, taken by SEM.

MODE FOR CARRYING OUT THE INVENTION

(7) Reference will now be made in detail to the preferred embodiments of the present invention. However, the following examples are illustrative only, and the scope of the present invention is not limited thereto.

EXAMPLE 1

(8) Manufacture Of Reinforced Composite Electrolyte Membrane Using First And Third Polymer Electrolyte Solutions Containing No Surfactant, Second Polymer Electrolyte Solution Containing Surfactant, and a Non-Fluorinated Microporous Polymer Substrate

(9) First, 10 g of a sulfonated poly(ether ketone) block copolymer, prepared according to examples disclosed in Korean Patent Application No. 10-2004-0110487, was dissolved in 90 g of dimethyl formamide (DMF), and the resultant solution was filtered through a BORU glass filter (pore size 3) to remove dust, etc. Next, 30 g of the above solution was separately used to form a second polymer solution to which 4% of Triton X-100 (a surfactant) was added. The remaining 70 g of the solution was used to provide a first polymer solution and a third polymer solution. Then, a polyethylene-based microporous film was washed with ethanol, followed by drying. The first polymer solution was poured onto a glass base and the copolymer solution applied onto the glass base was leveled by using a film applicator. The copolymer solution was dried in an oven at 80 C. for 2 hours or more, and then the polyethylene-based microporous film was stacked smoothly onto the resultant first polymer layer. The second polymer solution containing a surfactant added thereto was poured onto the microporous film and the copolymer solution applied onto the microporous film was leveled by using a film applicator. The copolymer solution was dried in an oven at 80 C. for 2 hours or more. Further, the third polymer solution was poured onto the microporous film containing the second polymer. Then, the copolymer solution was dried in an oven at 80 C. for 2 hours or more to provide a reinforced composite electrolyte membrane having a thickness of 50 m. FIG. 3 is the sectional image of the reinforced composite electrolyte membrane obtained as described above, taken by SEM (scanning electron microscopy).

COMPARATIVE EXAMPLE 1

(10) Manufacture of Electrolyte Membrane Using Sulfonated Block Copolymer

(11) The sulfonated poly(ether ketone) block copolymer solution as described in Example 1 was poured onto a glass base and the copolymer solution applied on the glass base was leveled by using a film applicator. Then, the copolymer solution was dried in an oven at 80 C. for 2 hours or more to provide an electrolyte membrane having a thickness of 50 m.

EXAMPLE 2

(12) Manufacture of Reinforced Composite Electrolyte Membrane Using No Surfactant in First, Second and Third Polymer Electrolyte Solutions and Using a Non-Fluorinated Microporous Polymer Substrate

(13) An electrolyte membrane having a thickness of 50 m was provided in the same manner as described in Example 1, except that the same sulfonated poly(ether ketone) block copolymer solution was used as the first polymer solution, the second polymer solution and as the third polymer solution (i.e. the second polymer solution contained no surfactant). FIG. 4 is the sectional image of the reinforced composite electrolyte membrane obtained as described above, taken by SEM.

COMPARATIVE EXAMPLE 2

(14) Manufacture of Reinforced Composite Electrolyte Membrane Using Neither Surfactant Nor Multi-Coating Process

(15) A polyethylene-based microporous film was washed with ethanol, followed by drying. The polyethylene-based microporous film was stacked smoothly onto a glass base, and the sulfonated poly(ether ketone) block copolymer solution used in Example 1 was poured onto the microporous film and the copolymer solution applied onto the microporous film was leveled by using a film applicator. The copolymer solution was dried in an oven at 80 C. for 2 hours or more to provide an electrolyte membrane having a thickness of 50 m. FIG. 5 is the sectional image of the reinforced composite electrolyte membrane obtained as described above, taken by SEM.

EXAMPLE 3

(16) Manufacture of Reinforced Composite Electrolyte Membrane Using First and Third Polymer Electrolyte Solutions Containing No Surfactant, Second Polymer Electrolyte Solution Containing Surfactant, and a Partially-Fluorinated Microporous Polymer Substrate

(17) An electrolyte membrane having a thickness of 50 m was provided in the same manner as described in Example except that a polyvinyldifluoroethylene microporous membrane, which is a partially-fluorinated microporous membrane, was used instead of a polyethylene microporous membrane, which is a non-fluorinated microporous membrane, and that fluorinated surfactant was used instead of the surfactant used in Example 1, i.e., Triton X-100 in order to increase the affinity between the microporous membrane and the sulfonated poly(ether ketons) block copolymer solution used in Example 1.

EXPERIMENTAL EXAMPLES

(18) The following tests were performed to determine IEC (ion exchange capacity), mechanical properties, methanol crossover and dimensional stability of the reinforced composite electrolyte membrane according to Example 1 and the electrolyte membrane according to Comparative Example 1.

(19) (A) Ion exchange capacity (IEC) and mechanical properties

(20) First, 0.5 g of each of the electrolyte membranes according to Example 1 and Comparative Example 1 was hydrated in ultrapure water at 100 C. for 2 hours and dipped into 100 mL of saturated aqueous NaCl solution for at least 10 hours to substitute protons (H.sup.+) with sodium ions (Na.sup.+). The concentration of protons substituted with sodium ions was titrated with 0.1N NaOH standard solution. Then, IEC value of each membrane was calculated according to the following Mathematical Formula 1 by using the volume of NaOH used for the titration. The results are shown in the following Table 1. IEC value of Nafion 112 available from Dupont Co. is also shown as a reference value.
IEC(SO.sub.3H meqiv./g)=(volume (mL) of consumed NaOH standard solution0.1N)/weight (g) of dried thin film[Mathematical Formula 1]

(21) Mechanical strength of each electrolyte membrane was measured by using Zwick UTM. Under the conditions of room temperature and a humidity of 25%, each of the electrolyte membranes according to Example 1 and Comparative Example 1 was converted into a dog bone-shaped film satisfying the requirements defined by ASTM D-882 (standard test method for tensile properties of thin plastic sheeting). Then, tensile strength of each film was measured five times under a crosshead speed of 50 mm/min. The average of the measured values for each film is shown in the following Table 1.

(22) TABLE-US-00001 TABLE 1 Apparent Tensile IEC physical strength Elongation Item (meq./g) properties (Mpa) (%) Ex. 1 1.38 transparent 76 17 excellent Comp. Ex. 1 1.41 transparent 70 14 excellent Nafion 112 0.91 transparent 43 225 excellent

(23) As can be seen from Table 1, the reinforced composite membrane of Example 1 according to the present invention shows a slight drop in IEC when compared to the electrolyte membrane of Comparative Example 1. On the contrary, the reinforced composite electrolyte membrane of Example 1 shows tensile strength and elongation improved by 10% and 20%, respectively, when compared to the electrolyte membrane of Comparative Example 1.

(24) (B) Methanol (MeOH) crossover

(25) Methanol crossover of each of the electrolyte membranes according to Example 1 and Comparative Example 1 was measured by using a diffusion cell system. First, 10M aqueous methanol solution and pure water were introduced into the left cell and the right cell, respectively, and each electrolyte membrane was inserted in the middle of both cells. Then, methanol crossover was obtained by calculating a variation in methanol concentration (C.sub.i(t)) in the right cell over time (t) while sampling the solution in the right cell. Herein, methanol crossover (D.sub.i.Math.K.sub.i) was calculated by using the thickness (L) of an electrolyte membrane, exposed area (A) of the corresponding membrane, the volume (V) of the right cell, and the initial methanol concentration (C.sub.i0) of the left cell according to the following Mathematical Formula 2. The results are shown in the following Table 2, and methanol crossover of Nafion 112 is also shown as a reference value.
C.sub.i(t)={(A.Math.D.sub.i.Math.K.sub.i.Math.C.sub.io)/V.Math.L}t[Mathematical Formula 2]

(26) TABLE-US-00002 TABLE 2 Item Ex. 1 Comp. Ex. 1 Nafion 112 Methanol 1.05 1.8 2.4 crossover (10.sup.6*cm.sup.2/sec)

(27) As can be seen from Table 2, the reinforced composite electrolyte membrane according to Example 1 shows a lower methanol crossover when compared to the electrolyte membrane according to Comparative Example 1, and has improved methanol barrier characteristics when compared to the currently used polymer membrane, Nafion.

(28) (C) Dimensional Stability

(29) Each of the electrolyte membranes according to Example 1 and Comparative Example 1 was cut into a size of 44 cm, hydrated in ultrapure water at 80 C. for 4 hours, and then measured for variations in size of each electrolyte membrane. The results are shown in the following Table 3, and a variation in size of Nafion 112 available from Dupont Co. is also shown as a reference value.

(30) TABLE-US-00003 TABLE 3 Ex. 1 Comp. Ex. 1 Nafion 112 Item width length width length width length Before hydration 4.0 4.0 4.0 4.0 4.0 4.0 After hydration 4.4 4.4 6.2 6.2 4.6 4.6

(31) As can be seen from Table 3, the reinforced composite electrolyte membrane of Example 1 according to the present invention shows significantly improved dimensional stability when compared to the electrolyte membrane according to Comparative Example 1.

(32) <Discussion>

(33) As can be seen from FIG. 3, the reinforced composite electrolyte membrane according to Example 1 includes polymer electrolyte layers formed on the top and bottom surfaces of a layer comprising a microporous polymer substrate and the pores of the microporous polymer substrate are sufficiently filled with a polymer electrolyte. Meanwhile, as can be seen from FIG. 4, the reinforced composite electrolyte membrane according to Example 2 includes polymer electrolyte layers formed on the top and bottom surfaces of a layer comprising a microporous polymer substrate, but the pores of the microporous polymer substrate are partially vacant because the pores are not sufficiently filled with a polymer electrolyte.

(34) Additionally, as can be seen from FIG. 5, the reinforced composite electrolyte membrane according to Comparative Example 2 includes a layer comprising a microporous polymer substrate at the top thereof. Therefore, it is problematic in that the microporous polymer substrate may be in direct contact with an electrode layer.

INDUSTRIAL APPLICABILITY

(35) As can be seen from the foregoing, the reinforced composite electrolyte membrane comprising a microporous polymer film and a non-fluorinated or partially-fluorinated sulfonated polymer electrolyte has excellent mechanical properties and dimensional stability when compared to an electrolyte membrane comprising the sulfonated polymer electrolyte alone. Therefore, it is possible to provide a composite electrolyte membrane by using a non-fluorinated or partially-fluorinated hydrocarbon-based polymer electrolyte having high proton conductivity, which was not applicable according to the prior art due to its low mechanical properties and dimensional stability, in combination with a microporous film. By doing so, it is possible to provide a reinforced composite electrolyte membrane having improved proton conductivity and mechanical properties.

(36) Additionally, according to the present invention, it is possible to provide various reinforced composite electrolyte membranes having a desired structure and/or physical properties while preventing a phase separation phenomenon and limiting the amount of a surfactant by using a non-fluorinated or partially-fluorinated substrate as a microporous polymer substrate and by filling and coating pores and both surfaces of the substrate with a non-fluorinated or (fluorine content-limited) partially-fluorinated sulfonated hydrocarbon-based polymer electrolyte via a multilayer coating process.

(37) Although several preferred embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.