Patent Publication Number: US-2018051380-A1

Title: Reinforced composite membrane for water electrolysis and membrane electrode assembly having the same

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates generally to a reinforced composite membrane for water electrolysis, and a membrane electrode assembly having the same. More particularly, the present invention relates to a reinforced composite membrane for water electrolysis, and a membrane electrode assembly (MEA), wherein a two-dimensionally woven fabric base layer that minimizes swelling of the membrane, in the X-axis and Y-axis directions is covered on a hydrogen electrode to which high pressure is applied or on hydrogen and oxygen electrodes by three-dimensionally electrospun reinforcing fiber layers, the reinforcing fiber layers minimizing swelling of the membrane in the Z-axis direction, thereby reducing permeation of oxygen (O 2 ) via the hydrogen electrode and enabling operation under high pressure due to high dimensional stability. 
     DESCRIPTION OF THE RELATED ART 
     Generally, in a water electrolysis system, due to characteristics of a polytetrafluoroethylene-based polymer known by the name of Nafion (a trade name, E. I. DuPont de Nemours and Co., Inc.), which is mainly used as an electrolyte membrane, an oxygen electrode (anode) electrode is continuously in contact with water, and thus the membrane is operated in a wet state. 
     Here, the electrolyte membrane allows the transport of the protons from the oxygen electrode to a hydrogen electrode through a conductive path having irregularly formed porous networks, wherein pore size is typically about 5-20 Å. 
     The membrane swelling due to solutions becomes severe when the membrane is continuously in contact with water or alcohol. For example, the swelling rate of a conventional Nafion membrane having a thickness of 180 μm is greater than or equal to 10% in the x and y directions. As the membrane swells, spaces between the clusters expand, and thus oxygen that should not be allowed to permeate to the oxygen electrode permeates through porous path. Therefore, hydrogen purity and dimensional stability decrease. 
     Under high pressure operation, as pressure of the hydrogen electrode increases, permeation of oxygen generated in the oxygen electrode is decreased. However, since pressure of the hydrogen electrode is high, which causes swelling and contraction of the membrane in one direction, pore space expands. 
     In addition, cracks are formed in the catalyst layer due to swelling and contraction of the membrane in one direction, and thus the catalyst layer is separated from the membrane. Consequently, durability of the membrane is decreased. 
     Generally, in the case of diffusion layers composed of mesh or sintered powder of titanium, fibers arid uneven elements having a size at a micrometer scale cause cracks and pinholes in the membrane. Such cracks and pinholes cause malfunction in the electrolyte membrane cell stack and degradation of system stability. In order to solve these problems and secure water electrolyte stability, technology development for dimensionally stable membrane is required. 
     As a document of related art, Korean Patent No. 10-2014-0000700 discloses a method of forming electrodes of electrochemical device, the method including a process of pressing a mat of electrospun nanofibers having three-dimensional structure onto an electrolyte membrane. In this method, electrospun fibers containing PAA, etc., Platinum catalyst particles, and ionomers are discharged together to form a catalyst layer, which is applied to PEMFC (polymer electrolyte membrane fuel cell). However, since the electrospun fibers having a thickness from nm scale to μm scale are a polymer, a Nafion ionomer having low mechanical strength is used as a main material. 
     Therefore, while having weakness against dimensional stability and swelling, the method may increase the performance by organic connection of three-dimensional networks of ionomers for conducting hydrogen and uniformly distributed catalyst. However, in water electrolysis of PBM (polymer electrolyte membrane), water feed to the anode side is needed. Thus, the electrolyte membrane having high dimensional stability and low swelling rate is required. 
     Further, in the a hydrogen fuel cell system, it is required to secure dimensional stability and swelling resistance of the electrolyte membrane capable of being operated under pressure of 350 bar or 700 bar, at a temperature of 80□, otherwise hydrogen crossover may occur, and thus potential risks and performance degradation, for example, reduction in hydrogen purity and the amount of generated hydrogen. Therefore, it is required to develop a reinforced electrolyte membrane having three-dimensional networks structure of the membrane and the catalyst layer so as to secure dimensional stability thereof. 
     Korean patent No. 10-1285709 discloses a method of manufacturing the reinforced electrolyte membrane as a multi-layer having more than three layers. However, the method uses the multi-layer having more than three layers such that additional processes may be included. In addition, as the reinforced electrolyte membrane has many layers, contact, interface may act as a large resistance. Consequently, hydrogen ion resistance and mass transfer resistance, etc. increase. 
     In order to resolve such two technical problems, in Korean patent No. 10-0897104, dimensional stability and swelling degree can be controlled by using sheets having various patterns and utilizing various materials as the support, wherein the sheet may be ePTFE (expanded PTFE, expanded polytetrafluoroethylene), etc. However, in this case, the controlling of the dimensional stability and swelling degree is typically possible in the x and y directions. In addition, after slightly applying a Nafion ionomer an the sheet or immersing the sheet in a Nafion ionomer solution, the Nafion electrolyte membrane is pressed. Therefore, there are still problems with, interface resistance and dimensional stability in the z direction (in the thickness direction). 
     DOCUMENTS OF RELATED ART 
     (Patent document 1) Korean Patent Application Publication No. 10-2014-0000700 (2014 Jan. 3); 
     (Patent document 2) Korean Patent No. 10-1285703 (2013 Jul. 8); 
     (Patent document 3) Korean Patent No. 10-0897104 (2009 May 4). 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made keeping in mind the above problems occurring in the related art, and the present invention is intended to propose a reinforced composite membrane for water electrolysis, the reinforced composite membrane being configured to maintain an ion-exchange property via a polymer electrolyte matrix, contain a woven base layer therein that prevents the membrane from swelling two-dimensionally in the X-axis and Y-axis directions, and include electrospun fibers that prevent the membrane from swelling three-dimensionally in the Z-axis direction, thus improving the purity of hydrogen (H 2 ) by minimizing swelling of the membrane and permeation of water and oxygen (O 2 ) thereto due to such swelling. 
     Further, another object of the present invention is to propose a reinforced composite membrane for water electrolysis, the reinforced composite membrane having high dimensional stability of an oxygen electrode and a hydrogen electrode under high-pressure operating conditions, thereby having improved thermal, mechanical, and chemical durability. 
     Moreover, a further object of the present invention is to propose a reinforced composite membrane for water electrolysis, the reinforced composite membrane capable of reducing manufacturing costs by using substrates that are less expensive than a conventional Nation membrane, and being mass-produced by selecting raw materials suitable for easy application to a mass-production process. 
     Furthermore, a still further object of the present invention is to propose a reinforced composite membrane having a low coefficient of swelling and low hydrogen crossover, and a membrane electrode assembly (MEA) having the reinforced composite membrane, the membrane electrode assembly being applied to a PEM fuel cell stack. The membrane electrode assembly (MEA) is produced using the reinforced composite membrane of the present invention as a polymer electrolyte membrane (PEM) , wherein the membrane electrode assembly is produced through a process of blending catalytic particles, and electrospinning the blended compound, or through an additional process of forming a catalyst layer. 
     The foregoing is intended merely to aid in the understanding of the background of the present invention, and is not intended to mean that the present invention falls within the purview of the related art that is already known to those skilled in the art. 
     The reinforced composite membrane for water electrolysis according to the present invention is manufactured by the following method: electrospinning fibers from a polyelectrolyte solution to opposite surfaces of a polymer-woven fabric base layer, thus forming an electrospun reinforcing fiber layer; and integrating the electrospun fiber reinforced layer with a polyelectrolyte solution. 
     Further, in the reinforced composite membrane for water electrolysis according to the present invention, the polymer-woven fabric base layer may be made of one or a mixture of two or more selected from the group of polyetheretherketone, polytetrafluoroethylene, polyimide, polybenzimidazole, and polystyrene. 
     Further, in the reinforced composite membrane for water electrolysis according to the present invention, the polyelectrolyte solution that forms the electrospun reinforcing fiber layer or with which the electrospun reinforcing fiber layer is impregnated may include a fluorinated ionomer. 
     Further, in the reinforced Composite membrane for water electrolysis according to the present invention, the polyelectrolyte solution forming the electrospun reinforcing fiber layer is a mixed electrolyte solution that may further include a copolymer. 
     Further, in the reinforced composite membrane for water electrolysis according to the present invention, the mixed-electrolyte solution forming the electrospun reinforcing fiber layer, the polyelectrolyte solution, and the copolymer may be mixed at a ratio of 4:1 to 1:1. 
     Further, in the reinforced composite membrane for water electrolysis according to the present invention, the polymer-woven fabric base layer may be made by weaving or laminating polymer fibers. 
     Meanwhile, a membrane electrode assembly according to the present invention is characterized in that, in the membrane electrode assembly formed by bonding electrodes and a polymer electrolyte membrane into a single body, the polymer electrolyte membrane is a reinforced composite membrane for water electrolysis of the present invention. 
     The above-described reinforced composite membrane according to the present invention has advantages of maintaining an ion-exchange property (ion conductivity) due to the woven fabric base layer that prevents the membrane from swelling two-dimensionally in the X-axis and Y-axis directions, and the electrospun fibers that prevent the membrane from swelling three-dimensionally in the Z-axis direction, thus improving the purity of hydrogen (H 2 ) by minimizing the swelling of the membrane and the permeation of water and oxygen (O 2 ) therethrough due to swelling. 
     Further, the reinforced composite membrane according to the present invention has an advantage of securing high dimensional stability of an oxygen electrode and a hydrogen electrode under high-pressure operating conditions, thereby having improved thermal, mechanical, and chemical durability. 
     Further, the reinforced composite membrane according to the present invention has an advantage of reducing manufacturing costs by using substrates that are less expensive than a conventional Nafion membrane, and being mass-produced by selecting raw materials suitable for easy application to a mass-production process. 
     Meanwhile, the present invention provide a membrane electrode assembly (MEA) produced through a process of blending catalytic particles, and electrospinning the blended compound, or through an additional process of forming a catalyst layer using the reinforced composite membrane of the present invention as a polymer electrolyte membrane (PEM), whereby the present invention further provides a PEM fuel cell stack having the membrane electrode assembly (MEA). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic view showing a reinforced composite membrane of the present invention; 
         FIG. 2  is a schematic view showing the rate of swelling of a reinforced composite membrane of the present invention in the X, Y, and Z directions; 
         FIG. 3  is a SEM image showing a reinforced composite membrane of the present invention; 
         FIG. 4  is a graph showing the diameter of electrospun fibers depending on a mix ratio of a Nafion ionomer to a copolymer; 
         FIG. 5  is a photo showing an actual image of a reinforced composite membrane according to the present invention; 
         FIG. 6  is a cross-sectional SEM image showing a membrane electrode assembly (MEA) that is manufactured using a reinforced composite membrane according to the present invention; and 
         FIG. 7  is a graph showing the water electrolysis performance of a membrane electrode assembly (MEA) that is manufactured using a reinforced composite membrane according to the present invention, the graph showing cell voltage depending on applied current density. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Example embodiments of the present invention are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments of the present invention; however, example embodiments of the present invention may be embodied in many alternate forms and should not be construed as limited to example embodiments of the present invention set forth herein. 
       FIG. 1  is a schematic view showing a reinforced composite membrane of the present invention,  FIG. 2  is a schematic view showing the rate of swelling of a reinforced composite membrane of the present invention in the X, Y, and Z directions,  FIG. 3  is a SEM image showing a reinforced composite membrane of the present invention, and  FIG. 4  is a graph showing the diameter of electrospun fibers depending on a mix ratio of a Nafion ionomer to a copolymer. Further,  FIG. 5  is a photo showing an actual image of a reinforced composite membrane according to the present invention,  FIG. 6  is a cross-sectional SEM image showing a membrane electrode assembly (MEA) manufactured using a reinforced composite membrane according to the present invention, and  FIG. 7  is a graph showing the water electrolysis performance of a membrane electrode assembly (MEA) manufactured using a reinforced composite membrane according to the present invention, the graph showing cell voltage depending on applied current density. 
     The reinforced composite membrane according to the present invention is configured by forming an electrospun reinforcing fiber layer  102  by electrospinning fibers from a polyelectrolyte solution on opposite surfaces of a polymer-woven fabric base layer  101  and impregnating the electrospun reinforcing fiber layer  102  with the polyelectrolyte solution. 
       FIG. 1  is a cross-sectional schematic view showing the reinforced composite membrane according to the present invention. As shown in  FIG. 1 , the reinforced composite polymer electrolyte membrane for water electrolysis is prepared by electrospinning polymer fibers on the polymer-woven fabric base layer  101 , such as a PTFE (Polytetrafluoroethylene) sheet, so as to form the electrospun reinforcing fiber layer  102 , and impregnating the electrospun reinforcing fiber layer  102  with a polyelectrolyte solution, such as a PFSA-based polymer matrix, so as to form a polyelectrolyte impregnation layer  103 . 
     Here, the polymer-woven fabric base layer  101  such as PTFE, plays the role of reducing a phenomenon of swelling of the polymer electrolyte membrane in the X and Y directions. Whereas, the electrospun reinforcing fiber layer  102  plays the role of reducing the phenomenon of swelling of the polymer electrolyte membrane in the Z direction. The electrospun reinforcing fiber layer  102  is prepared by electrospinning a fiber from a mixed electrolyte solution composed of the polyelectrolyte solution of a perfluorinated compound or a hydrocarbon compound, and a copolymer on the polymer-woven fabric base layer  101 . 
       FIG. 2  is a schematic view showing the rate of swelling of the reinforced composite membrane of the present invention in the X, Y, and Z directions. Referring to  FIG. 3 , how the polymer-woven fabric base layer  101  of  FIG. 1  and the reinforced, composite membrane are together able to prevent the swelling phenomenon in three dimensions will be understood. 
     The polymer-woven fabric base layer  101  is made of one or a mixture of two or more selected from the group of polyetheretherketone, polytetrafluoroethylene, polyimide, polybenzimidazole, and polystyrene. Further, the polyelectrolyte solution for forming the electrospun reinforcing fiber layer  102  or impregnating the electrospun reinforcing fiber layer  102  may include a fluorinated ionomer. 
     Further, in the mixed electrolyte solution forming the electrospun reinforcing fiber layer  102 , the polyelectrolyte solution, and the copolymer are mixed at a ratio of 4:1 to 1:1. 
     Meanwhile, a membrane electrode assembly is formed by bonding electrodes and a polymer electrolyte membrane into a single body, wherein the reinforced composite membrane for water electrolysis is adapted for the membrane electrode assembly according to the present invention. 
     Hereinafter, the reinforced composite membrane for water electrolysis according to the present invention is described in more detail through examples, but the examples described below are merely used to illustrate the present invention, and the scope of the present invention is not limited thereby. 
     EXAMPLE 1 
     1. Preparation of an electrospun reinforcing fiber layer 
     The electrospun reinforcing fiber layer  102  is formed on an ePTFE (expanded PTPE) film, having a thickness of 20 μm and serving as the polymer-woven fabric base layer  101 , through an electrospinning method. Here, conditions suitable for electrospinning fibers from the polyelectrolyte solution and the copolymer are given in the following Table 1. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Applied 
                   
                 Spray 
                   
                 Spray 
               
               
                   
                 Voltage 
                 Nozzle # 
                 Distance 
                 X-Y jog 
                 Speed 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Condition 
                 8 kV 
                 #27 
                 140 mm 
                 47 mm 
                 200 μl/min 
               
               
                   
               
            
           
         
       
     
     Specifically, the polymer electrolyte membrane having the PTFE wen fabric base layer  101  of a thickness of 20 μm is placed on a conductive metal plate at a temperature of 90□, and a Nafion solution is loaded into an electrospinning syringe while the spray distance between a nozzle and the plate is maintained at 140 mm. Further, an x-y jog is set to 47 mm, and the Nafion solution is sprayed at a speed of 20 μl/min. 
     Here, the diameter of the electrospun fibers in the electrospun reinforcing fiber layer  102  according to the present invention is observed under the following conditions. In order to prepare the electrospinning solution, the Nafion ionomer, which is the main ingredient, and a copolymer, such as (poly acetic acid) or PEO (polyethylene oxide), is mixed at a mix ratio of 4:1 to 1:1. When the mix ratio is adjusted from 4:1 to 1:1, the diameter of an electrospun fiber of the electrospun reinforcing fiber layer  102  is variable. In Example 1, in order to obtain the electrospun fiber having a diameter of 0.8 μm as shown in Table 2 and  FIG. 4 , showing the diameter of an electrospun fiber depending on the mix ratio of the Nafion ionomer to a copolymer, the electrospinning solution is prepared by adjusting a mix ratio to 2:1, and the total solid content of the composition is adjusted to 1.875 wt % using an IPA solvent. 
     Table 2 and  FIG. 4  show the diameter of an electrospun. fiber depending on a mix ratio of the Nafion ionomer as the polyelectrolyte solution, to PEO (polyethylene oxide), as the copolymer. As the content of PEO is increased, the diameter of the electrospun fiber was increased to micrometer scale. On the other hand, as the content of the Nafion ionomer is increased, the diameter of the electrospun fiber is decreased. If the diameter of the electrospun fiber is too great, the ion conductivity of the reinforced composite membrane may be affected. On the other hand, if the diameter of the electrospun fiber is too small, the mechanical strength of the electrospun fiber is reduced. Thus, it is preferred that the electrospun reinforcing fiber layer  102  may be formed by using an electrospun fiber having a diameter of about 1 μm. 
     The thickness and the image of the electrospun reinforcing fiber layer  102  prepared through the above-described method are observed through SEM (scanning electron microscope) . As shown in  FIG. 3 , the polymer electrolyte membrane having a relatively uniform thickness on the micrometer scale is prepared using the electrospun fibers forming three-dimensional networks. 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Diameter of Fiber 
               
            
           
           
               
               
               
               
            
               
                   
                 0.4 μm 
                 0.8 μm 
                 5 μm 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Nafion/PEO ratio 
                 4:1 
                 2:1 
                 1:1 
               
               
                   
                 Total Weight % 
                 1.2 wt % 
                 1.875 wt % 
                 1.75 wt % 
               
               
                   
                   
               
            
           
         
       
     
     2. Impregnation with the polymer electrolyte solution 
     A PTFE woven fabric base layer  101  having a thickness of 10 to 200 μm and the electrospun reinforcing fiber layer  102  thereon is impregnated with the Nafion solution in an amount sufficient to immerse all layers (1.2 times of pore surface area) on a flat glass sheet that is maintained level. After that, distilled water and the IPA solvent are completely dried in a vacuum drying machine at a temperature of about 80-90° C. for at least 24 hours. Thus, the reinforced composite membrane for water electrolysis is produced. 
     EXAMPLE 2 
     A reinforced composite membrane for water electrolysis according to the present invention is prepared by the same process of Example 1 except that a PFTE woven fabric having a thickness of 4,000 μm is used as a polymer-woven fabric base layer  101 . 
     As could be confirmed in the photo showing an actual reinforced composite membrane for water electrolysis in  FIG. 5 , the reinforced composite membrane for water electrolysis produced under the above-mentioned conditions is a polymer electrolyte membrane in which the PTFE woven fabric base layer  101  shows little opacity. 
     In the case of a polymer electrolyte ionomer used in the present invention, the polymer electrolyte ionomer is a transparent polymer material. The impregnated and dried membrane has a polymer-woven fabric base layer  101  that appears transparent when observed with the naked eye. Particularly, in Example 1, an actual e-PTFE film is a translucent white film. As the Nafion ionomer is impregnated into the membrane, the pores in the opaque parts of the layers are filled therewith. 
     Test Example 
     1. Evaluation of Properties of Polymer Electrolyte Membrane 
     Table 3 shows properties of Nafion 117 by comparing Comparative Example 1 and Examples 1 and 2. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TAELE 3 
               
               
                   
                   
               
               
                   
                   
                 Proton 
                   
                   
                 Swelling 
               
               
                   
                 Thickness 
                 Conductivity 
                 Tensile 
                 Tensile 
                 rate 
               
               
                   
                 (μm) 
                 (S/cm) 
                 Strength 
                 elongation 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Comparative 
                 180 
                 0.09 
                 330 
                 250 
                 14 
               
               
                 Example 1 
               
               
                 (Nafion 117) 
               
               
                 Example 1 
                 50 
                 0.10 
                 460 
                 121 
                 &lt;5 
               
               
                 Example 2 
                 430 
                 0.05 
                 840 
                 2 
                 &lt;5 
               
               
                   
               
            
           
         
       
     
     (1) Proton Conductivity 
     The proton conductivity depending on the temperature of the polymer electrolyte membrane according to Examples 1 and 2 and Comparative Example 1 was measured using a Solarton 1260 impedance/gain phase analyzer. Here, proton conductivity (S/cm) is given by measuring a resistance value (Ω), then multiplying the surface area by the result obtained by dividing the thickness of the polymer electrolyte membrane by the resistance value. 
     (2) Evaluation of Mechanical Property 
     The tensile strength and tensile elongation of the polymer electrolyte membrane according to Examples 1 to 2 and Comparative Example 1 were measured in accordance with ASTM D882 using a universal testing machine (UTM). The measurement speed was 50 mm/min. A 200N load cell was used, and the span (gauge length) was set to be 100 mm under standard temperature and humidity conditions ((25±2)° C., (45±5)% R.H.). 
     Examples 1 and 2 show that the polymer electrolyte membrane has much higher tensile strength than the conventional Nafion 117 membrane, which is a very encouraging result for the polymer electrolyte membrane having a small thickness in accordance with Example 1. Due to the relatively small thickness, the membrane of the present invention exhibits excellent water electrolysis performance and high tensile strength, thereby suggesting that the membrane of the present invention is highly applicable as a polymer electrolyte membrane that is able to bear high pressure. 
     (3) Evaluation of Swelling Rate 
     The swelling rate of the polymer electrolyte membrane prepared in accordance with Examples 1 and 2 and Comparative Example 1 was measured. For evaluation, after the prepared membrane was soaked in distilled water for a day, the swelling rate was calculated using the following equation. 
       Swelling rate (%)=[( x  or  y  length of membrane after swelling,  x  or  y  length of membrane before swelling)/ x  or  y  length of membrane before swelling]×100   Equation 1
 
     (4) Results 
     As shown in Table 3, Example 1 provides a membrane having proton conductivity that is higher than the proton conductivity of the conventional Nafion 117 membrane, a thickness of 50 μm, which is significantly less than the thickness of a conventional Nafion 117 membrane, high tensile strength, and a swelling rate less than or equal to 5%. 
     Examples 1 and 2 show polymer electrolyte membranes having much higher tensile strength than the conventional Nafion 117 membrane, which is a very encouraging result for the polymer electrolyte membrane, which has a small thickness in accordance with Example 1. Due to the relatively small thickness, the membrane of the present invention exhibits excellent water electrolysis performance and high tensile strength, thereby enabling the membrane of the present invention to be highly applicable as a polymer electrolyte membrane that is able to bear high pressure. 
     In other words, while a conventional Nafion 212 membrane having a thickness of 50 μm which is significantly less than the conventional Nafion 117 membrane of Example 1, has tensile strength of less than 330 bar, Example 1 according to the present invention exhibits higher tensile strength of 460 bar, thereby enabling operation under high pressure. 
     As a result, Examples 1 and 2 exhibit the swelling rate of less than 5%, which is a very low value compared with the conventional Nafion 117 membrane of Comparative Example 1. The PTFE woven fabric base layer  101  and the electrospun reinforcing fiber layer  102  have dimensional stability in water by supporting the Nafion polymer matrix in the x, y, and z directions. 
     2. Fabrication of Membrane Electrode Assembly and Evaluation of Performance of Water Electrolysis Cell 
     The membrane electrode assembly (MEA) is prepared through the decal transfer method, which is considered a representative method for preparing an MEA. Here, Pt/C (30 wt % on Vulcan XC-72) and IrRuO2 nanoparticles (homemade) are respectively used as an oxygen electrode catalyst and a hydrogen electrode catalyst. The catalyst loading rates are respectively set to 2 mg/cm 2  and 4 mg/cm 2 . 
     The membrane, from which impurities have been removed through a pretreatment process, is dried. Thereafter, a thermal compression is carried out on catalyst layers that are formed on the PTFE sheet through electro-spraying or doctor blading, at 5 MPa pressure and a temperature of 120-130° C. for 2 minutes in the directions of zero, 120, and 240 degrees. Thereafter, the PTFE sheet is removed from surfaces of the membrane, and carbon paper and Ti fiber electrodes are prepared for the hydrogen electrode and the oxygen electrode by cutting them, and the carbon paper and Ti fiber electrodes are processed through thermal compression at 120° C., under 5 Mpa, and for 2 minutes, thereby forming a membrane electrode assembly (MEA). 
       FIG. 6  is a cross-sectional SEM image showing a membrane electrode assembly (MEA) that is manufactured using a reinforced composite membrane according to the present invention. To prepare the membrane electrode assembly (MEA), the ePTFE film is used as the polymer-woven fabric base layer  101 , the electrospun reinforcing fiber layer  102  is formed on opposite surfaces of the polymer-woven fabric base layer  101  through electrospinning, and thus the polymer electrolyte impregnated layer  103  is formed on both sides of electrodes by impregnation with the Nafion ionomer, which is one of the perfluorinated polymer electrolyte materials. Thereafter, the oxygen electrode layer  104  and the hydrogen electrode layer  105  are bonded through thermocompression bonding, whereby the membrane electrode assembly (MEA) is prepared. 
       FIG. 7  is a graph measuring a voltage value as a function of applied current density, and shows the results of evaluation of water electrolysis performance of the membrane electrode assembly (MEA) prepared in accordance with the above-described method of the present invention. The water electrolysis of the membrane electrode assembly (MEA) according to the present invention was evaluated by comparing a conventional Nafion 212 membrane having a thickness of 50 μm with a conventional Nafion membrane 115 (N115) membrane having a thickness of 125 μm. Here, the performance difference was confirmed by comparing the membranes only under the same catalyst layer preparation conditions. 
     The performance of reinforced electrode membrane according to the present invention can be seen in  FIG. 7 , Example 1 (sample #1 in  FIG. 5 ), in which an ePTFE film having a thickness of 50 μm was used as the polymer-woven fabric base layer  101 , exhibits cell voltage of 1 mA/cm 2  and voltage of 95%, compared with the conventional Nafion 212 (N212) membrane having the same thickness of 50 μm. Also, Example 2 (sample #2 in  FIG. 5 ), using an ePTFE film having a thickness of 4,000 μm as the polymer-woven fabric base layer  101 , exhibits performance similar to N115 and N212 under low-current density conditions. However, as current density increases, Example 2 (sample #2 in  FIG. 5 ) exhibits better performance than N212. In particular, Example 1 (sample #1 in  FIG. 5 ), using the membrane having the same thickness as that of N212, exhibits better performance than N115, having a thickness of 125 μm, as well as N212. 
     When considering the results pertaining to the tensile strength of the membrane electrode assembly (MEA), shown in Table 3, and the water electrolysis performance thereof, shown in  FIG. 7 , the tensile strength of N212 is lower than N117, which has a thickness of 180 μm. Based on the conventional Nafion membranes, which show a great difference in tensile strength depending on the thicknesses of the membranes, the superiority of the present invention is thus clearly exhibited. 
     Although a preferred embodiment of the present invention has 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.