METHOD FOR PRODUCING A POLYMER-ELECTROLYTE MEMBRANE FOR A FUEL CELL

Abstract

A method for producing a polymer electrolyte membrane, comprising: dissolving polyamic acid in a spin solvent to produce a spin solution; electro-spinning the spin solution to produce a porous nanoweb, comprising nanofibers with an average diameter of 0.005 to 5 µm; post-treating the porous nanoweb by imidization of the polyamic acid to obtain a porous polyimide nanoweb insoluble in an organic solvent of NMP, DMF, DMA, or DMSO; dissolving an ionomer, comprising sulfonated polyimide (S-PI), sulfonated polyarylethersulfone (S-PAES), sulfonated polyetheretherketone (S-PEEK), sulfonated polybenzimidazole (S-PBI), sulfonated polysulfone (S-PSU), sulfonated polystyrene (S-PS), sulfonated polyphosphazene, or mixtures thereof, in the organic solvent to obtain an ionomer solution.Filling the pores of the post-treated porous polyimide nanoweb with the ionomer solution by immersing the porous polyimide nanoweb in the ionomer solution, and subsequently removing the organic solvent from the ionomer solution, thereby enabling the loading of the pores of the porous polyimide nanoweb with the ionomer, such that a thickness ratio of the nanoweb, measured by the following equation, has 20% or more: Nanoweb thickness ratio = [A / (B+C)]x100, where A is an average thickness of the nanoweb, B is an average thickness of an upper ionomer, and C is an average thickness of a lower ionomer.

Description

Technical FieldThe present invention relates to a method for producing a polymer electrolyte membrane for a fuel cell.Background ArtA fuel cell refers to a battery that directly converts chemical energy produced by oxidizing a fuel into electric energy (i.e., electricity) and has high energy efficiency and environmental-friendly properties such as reduced pollutant emission, and thereby has gained popularity as a next generation energy source.The structure of the fuel cell generally includes an oxidation electrode (anode) and a reduction electrode (cathode) with an electrolyte membrane interposed therebetween, such a structure being referred to as a membrane-electrode assembly (MEA).As for types of electrolytes, the fuel cell may include an alkaline electrolyte membrane fuel cell, a polymer electrolyte membrane fuel cell (PEMFC), etc., among which the polymer electrolyte membrane fuel cell has favorable properties such as low operating temperature of less than 100° C., fast starting and responding properties, excellent durability, etc., and thus receives particular attention as a power source for portable applications, automobiles, and / or household applications.A representative example of such a polymer electrolyte membrane fuel cell is a proton exchange membrane fuel cell (PEMFC) using hydrogen gas as a fuel.A reaction occurring in the PEMFC may be briefly described as follows. First, since a fuel such as hydrogen gas is supplied to an oxidation electrode (hereinafter, "anode"), protons (H +) and electrons (e -) are formed at the anode by oxidation of the hydrogen. The formed protons (H +) are supplied to a reduction electrode (hereinafter, "cathode") through a polymer electrolyte membrane, while the formed electrons (e -) migrate to the cathode via an external circuit. The cathode may be provided with oxygen and the oxygen may be combined with protons (H +) and electrons (e -) to produce water by reduction of the oxygen.The polymer electrolyte membrane is a passage through which the protons (H +) are transferred to the cathode, which thus must have a high conductivity for protons (H +). In addition, the polymer electrolyte membrane should have excellent separation properties to separate the hydrogen gas supplied to the anode and the oxygen supplied to the cathode. Further, in addition to the above characteristics, the polymer electrolyte membrane requires superior mechanical strength, dimensional stability, chemical resistance, etc., while reducing ohmic loss at high current densities.The polymer electrolyte membrane currently used in the related art may include, for example, a fluororesin such as perfluorosulfonic acid resin (hereinafter referred to as "fluoroionomer"). However, the fluoroionomer has low mechanical strength, and pinholes may form during long-term use, which in turn decreases energy conversion efficiency. Although it has been attempted to enhance the mechanical strength by increasing the membrane thickness of the fluoroionomer, it not only increases the ohmic loss but also increases the consumption of expensive raw material and thus has lower economics.In order to solve the above problems, there has been proposed a polymer electrolyte membrane having improved mechanical strength, which is produced by impregnating a porous polytetrafluoroethylene resin (trade name: Teflon) as a fluororesin (hereinafter referred to as "Teflon resin") with a fluoroionomer in a liquid state.In this regard, as compared with a polymer electrolyte film comprising only a fluoroionomer, the membrane produced above has a relatively high mechanical strength, thereby decreasing the thickness of the electrolyte membrane and ultimately achieving advantages such as a decrease in ohmic loss, although proton conductivity may be slightly reduced.However, since the teflon resin has very low adhesive force, the selection of ionomers is limited. In addition, a fluoroionomer-using product involves the drawback of high crossover (phenomenon) as compared with hydrocarbon-based products. Further, since the porous teflon resin and the fluoroionomer are expensive, there is a demand for development of inexpensive and new materials for mass production.Polymer electrolyte membranes are described in the prior art, for example in U.S. Pat. No. 6,248,469 B1. The production of nanowebs for polymer electrolyte membranes is described in the prior art, for example, in KR 10 2007 0 098 157 A, WO 2003 / 087443 A1 and WO 2007 / 034233 A1.DisclosureTechnical ProblemTo overcome the above-described conventional problems, the present invention relates to providing a polymer electrolyte membrane having a structure in which an ionomer is charged in a high melting point nanoweb which is insoluble in an organic solvent and has excellent pore properties under optimum conditions, to thereby reduce the total thickness, which in turn reduces ohmic loss and at the same time reduces material cost, and has excellent heat resistance and low thickness expansion rate. Consequently, the above-mentioned polymer electrolyte membrane can maintain the desired proton conductivity for a long period of time.Technical SolutionIn order to solve the aforementioned problems, according to an aspect of the present invention, there is provided a method for producing a polymer electrolyte membrane as defined in the claims.Advantageous EffectsThe present invention achieves the following effects:First, the polymer electrolyte membrane according to the present disclosure has a structure of ionomers charged in pores of the nanoweb, wherein the nanoweb has excellent thermal resistance and is insoluble in an organic solvent under optimum conditions. Thereby, a total thickness of the electrolyte membrane can be reduced, thereby improving proton conductivity while reducing ohmic loss and reducing a thickness expansion rate. Consequently, the performance of the aforementioned membrane can be successfully maintained for a long time.Second, the polymer electrolyte membrane of the present disclosure may include a nanoweb and an ionomer, both of which include hydrocarbon polymer materials to improve adhesion thereof, thereby achieving excellent durability.Third, the polymer electrolyte membrane of the present disclosure is relatively inexpensive to manufacture using hydrocarbon polymer materials without the use of common fluorine-based ionomers or teflon resin. Accordingly, the present invention is economically preferred for reasons of cost and mass production.DESCRIPTION OF THE DRAWINGSThe accompanying drawings are provided to provide an understanding of the present invention and form a part of the detailed description, an illustrative subject matter of the present invention, and explain the principles of the present invention in which: FIG. 1 is a cross-section illustrating a polymer electrolyte membrane according to an aspect of the present disclosure.Best ModeIt would be obvious to those skilled in the art that various modifications and variations are possible within the technical spirit and scope of the present invention. The present invention therefore encompasses such modifications and variations within a scope of the inventions defined by the claims and their equivalents.Hereinafter, the present invention will be described in greater detail.The polymer electrolyte membrane according to the present invention produced by the method of the present invention may comprise a porous nanoweb and an ionomer charged in the pores of the porous nanoweb.The porous nanoweb of the present disclosure may improve mechanical strength of the polymer electrolyte membrane and inhibit moisture-induced volume expansion, thereby improving morphological stability. In addition, the porous nanoweb may comprise hydrocarbon polymers with economic advantages in terms of cost.In particular, because the porous nanoweb is insoluble in an organic solvent such as NMP, DMF, DMA or DMSO at room temperature, a method of filling the nanoweb pores with the ionomer can be easily performed. Thus, in order to load the ionomer into the nanoweb pores, the ionomer is first dissolved in an organic solvent to prepare an ionomer solution, and this solution is charged into the pores of the porous nanoweb. In this regard, when the porous nanoweb is dissolved in the organic solvent, the nanoweb is dissolved during filling the pores of the nanoweb with the ionomer, which in turn leads to a problem in manufacturing a polymer electrolyte membrane having a desired structure. Thus, the porous nanoweb may comprise a hydrocarbon material insoluble in an organic solvent.The porous nanoweb preferably comprises a hydrocarbon material having a melting point of 300°C or more. As such, the porous nanoweb has a high melting point and therefore can maintain a stable shape during less insulation from an electrode even in a high temperature environment, thereby advantageously maintaining the performance for a long period of time.Porous nanowebs that meet the requirements described above may comprise, for example, polyimide, polybenzoxazole, or a copolymer or mixture thereof.The porous nanoweb produced by the method according to the invention essentially comprises a net shape, wherein the desired fibers are three-dimensionally connected, and a fiber thickness here is 0.005 to 5 μm. When the nanoweb forming fiber has a thickness of less than 0.005 μm, the mechanical strength of the porous nanoweb may be lowered. In contrast, when the thickness of the fiber exceeds 5 μm, the porosity of the porous nanoweb cannot be easily controlled.The porous nanoweb may be formed to a thickness of 5 to 20 μm. When the thickness of the porous nanoweb is less than 5 μm, the mechanical strength and morphological stability of the polymer electrolyte membrane may be deteriorated. On the other hand, when the thickness of the porous nanoweb exceeds 20 μm, the polymer electrolyte membrane may have increased ohmic loss.The porous nanoweb may have a porosity of 70 to 98%. When the porosity of the porous nanoweb is less than 70%, the proton conductivity of the polymer electrolyte membrane may be lowered. In contrast, when the porosity of the porous nanoweb exceeds 98%, the mechanical strength and morphological stability may be deteriorated.The ionomer may contribute ionic conductivity as an essential function of the polymer electrolyte membrane, and hydrocarbon polymers excellent in proton conductivity and economic advantages may be used as the ionomer. In order to easily fill the pores of the porous nanoweb with the ionomer as described above, hydrocarbon materials soluble in the organic solvent may be contained. Such ionomeric hydrocarbon polymers include sulfonated polyimide (S-PI), sulfonated polyarylethersulfone (S-PAES), sulfonated polyetheretherketone (S-PEEK), sulfonated polybenzimidazole (S-PBI), sulfonated polysulfone (S-PSU), sulfonated polystyrene (S-PS), sulfonated polyphosphazene, or mixtures thereof. Here, the term "soluble" in an organic solvent means a property of being soluble in an organic solvent such as NMP, DMF, DMA or DMSO at room temperature.The ionomer may be charged into pores of the porous nanoweb. In the case where operating conditions such as temperature, humidity, etc. are changed during operation of a fuel cell, this may deteriorate adhesion between the ionomer and the porous nanoweb. Since both the ionomer and the porous nanoweb according to the present invention comprise hydrocarbon polymers, according to the present invention, the adhesion between them is substantially excellent. A hydrocarbon material contained in the ionomer and another hydrocarbon material contained in the porous nanoweb may be substantially identical. In particular, when S-PI is used as the ionomer while the porous nanoweb comprises polyimide, the adhesion between the ionomer and the porous nanoweb can be improved to a great extent.Thus, since the adhesion between the porous nanoweb and the ionomer is excellent, an electrolyte membrane containing the aforementioned materials can inhibit moisture-induced three-dimensional expansion, thereby relatively reducing length and thickness expansion rates.As described above, a polymer electrolyte membrane including the porous nanoweb having excellent pore properties and an ionomer sufficiently charged in pores of the porous nanoweb may have a thickness expansion rate of 10% or less. Accordingly, the thickness expansion rate, which is a degree of deformation in a thickness direction relative to the water content of the polymer electrolyte membrane, is preferably not more than 10%. The polymer electrolyte membrane used for a fuel cell is repeatedly expanded and contracted upon exposure to high humidity. Therefore, if the thickness expansion rate thereof is too high, it may be peeled from the electrode. When the electrode and the partition wall separate from each other, the performance of the fuel cell may significantly deteriorate.The thickness expansion rate of the polymer electrolyte membrane can be determined by the following equation: wherein T0is the average thickness of a polymer electrolyte membrane before its expansion in water, and T1is the average thickness of a polymer electrolyte membrane after its expansion in water.Such a thickness expansion rate is largely influenced not only by structural materials of an electrolyte membrane but also by the structure thereof. For example, when the electrolyte membrane is a singular film including an ionomer, it may have a high water content based on the properties of the ionomer. By these properties, the singular film can be directly influenced by the water content, which in turn considerably increases the thickness expansion rate.Further, an electrolyte membrane formed by filling the web with the ionomer in which a plurality of fibers are three-dimensionally entangled to form a web can three-dimensionally inhibit expansion caused by moisture, thereby relatively decreasing a length and thickness expansion rate. In particular, when the web acting as a support is strongly adhered to the ionomer, the thickness expansion rate of the electrolyte membrane is further decreased. This is considered because a web acting as a support is generally hydrophobic and less expanded by moisture, and an ionomer strongly adhering to such a web is influenced by moisture expanding properties of the web, thereby further reducing the thickness expansion rate.In addition, when the web comprises fibers having a small diameter, the surface area and porosity of the web are increased and an optimum three-dimensional structure may be formed, whereby an electrolyte membrane manufactured using the above materials may have a very low thickness expansion rate.The polymer electrolyte membrane according to the present disclosure is substantially manufactured using a nanoweb which is formed by entanglement of nanofibers under optimum conditions and has excellent porosity and preferable surface area. Since the membrane produced by the method of the present invention comprises the above-described nanoweb and an ionomer strongly adhering to the nanoweb, the membrane can have a low thickness expansion rate of not more than 10%.The polymer electrolyte membrane produced by the method of the present invention has a thickness ratio of the thickness ratio measured according to the following equation of 20% or more: wherein A is the average thickness of nanoweb, B is the average thickness of an upper ionomer, and C is the average thickness of a lower ionomer.When the thickness ratio of the nanoweb is less than 20%, a supporting part of the electrolyte membrane is too much decreased, and mechanical properties of the membrane are greatly deteriorated. This can considerably deteriorate the durability while increasing the thickness expansion rate, which in turn deteriorates the performance of the (fuel) cell.Since the polymer electrolyte membrane of the present disclosure has a structure of filling pores of the porous nanoweb with an ionomer, the mechanical strength may improve to 10 MPa or more. Such improved mechanical strength can additionally reduce the total thickness of the polymer electrolyte membrane to 80 μm or less, thereby accelerating ionic conductivity (i.e., improving an ionic conductivity rate), while reducing ohmic loss and reducing material costs.The present invention uses a porous nanoweb and an ionomer used for manufacturing a polymer electrolyte membrane, the aforementioned two components comprising hydrocarbon-based polymer materials to obtain excellent adhesion properties, thereby achieving excellent durability. Further, relatively inexpensive hydrocarbon polymers can be used without using an expensive fluoroionomer or teflon resin, thereby achieving an advantage such as economical merit in mass production.The following description serves to explain a method for producing a polymer electrolyte membrane according to an aspect of the present invention. For some items substantially as described above, such as types (or kinds) of polymers, weight ratios, etc., detailed description will be omitted.A method for manufacturing a polymer electrolyte membrane includes: preparing a porous nanoweb including an organic solvent insoluble hydrocarbon material; and preparing an ionomer solution by dissolving an ionomer including an organic solvent soluble hydrocarbon material in the organic solvent.As for the production of nanoweb and the production of ionomer solution, the order of the process steps is not particularly defined.Since the porous nanoweb contains a hydrocarbon material insoluble in an organic solvent, the nanoweb is formed using a precursor soluble in the organic solvent, followed by a desired reaction to produce a porous nanoweb.Specifically, a precursor is dissolved in a spin solvent to prepare a spin solution, and the prepared spin solution is electrically spun to spin a porous nanoweb comprising nanofibers having an average diameter of 0.005 to 5 μm, followed by post-treatment of the nanoweb to obtain a desired porous nanoweb as a final product.The porous nanoweb is produced by electric spinning to obtain high porosity, microfine pores and a thin film (i.e., reduced thickness).The porous nanoweb insoluble in an organic solvent cannot be directly produced by electric spinning. That is, it is difficult to prepare a spin solution because polyimide used for the porous nanoweb cannot be dissolved in a solvent such as NMP, DMF, DMA or DMSO.First, a precursor nanoweb is formed using a precursor well soluble in an organic solvent. The formed precursor nanoweb is then post-treated to prevent its dissolution in an organic solvent described above, thereby obtaining an organic solvent insoluble porous nanoweb.The precursor may have a water content of 0.5% or less. This is considered because, when the water content of the precursor exceeds 0.5%, the viscosity of a spin solution is lowered by moisture, and a filament may be cleaved by moisture after spinning, thereby degrading operational characteristics. The above may result in defect or degrade physical properties (nanoweb).Post-treatment of the nanoweb precursor to form an insoluble porous nanoweb may include thermal treatment or chemical treatment. Thermal treatment can be performed particularly using a hot press set at a high temperature and a high pressure.Hereinafter, a method for producing a porous polyimide nanoweb according to an aspect of the present invention will be described in detail.After preparing a nanoweb precursor by electrically spinning a polyamic acid precursor, the nanoweb precursor may be subjected to imidization using a hot press to form a porous polyimide nanoweb.Specifically, polyamic acid is dissolved in tetrahydrofuran (THF) to prepare a precursor solution, the prepared precursor solution is ejected through spin jetting at a temperature of 20 to 100° C. during application of a high voltage of 1 to 1,000 kV thereto to form a polyamic acid nanoweb on a current collector. Subsequently, the formed polyamic acid nanoweb is heated in a hot press preset to 80 to 400°C to obtain a porous polyimide nanoweb.The high melting point polyimide porous nonwoven fabric insoluble in an organic solvent can improve heat resistance, chemical resistance and / or mechanical properties of an electrolyte membrane.Further, the ionomer solution may be charged into pores of the porous nanoweb as described above.A method of filling pores of the porous nanoweb with the ionomer solution may include, but is not limited to, immersion. Instead, various other methods such as lamination, spraying, screen printing, doctor-coating, etc., which are well known in the art, may be employed.Immersion may be carried out by repeated two to five times of immersion at room temperature for 5 to 30 minutes.After charging the ionomer solution, the organic solvent content in the ionomer is removed, thereby enabling the pores of the porous nanoweb to be charged with the ionomer. The removal of the organic solvent can be carried out by drying the nanoweb in an oven at 60 to 150° C. for two to five hours.Example 1A polyamic acid / THF spin solution having a concentration of 12 wt% was electrically spun under application of 30 kV, and then thermally treated (i.e., heated) in an oven for 350° C. for 5 hours to form a porous polyimide nanoweb having an average thickness of 15 μm. In this regard, the electric spinning was carried out at 25° C. using a pressure jet nozzle upon application of a voltage of 30 kV.Sulfonated polyetheretherketone (S-PEEK) was dissolved in N-methyl-2-pyrrolidone (NMP) to prepare a 10 wt% ionomer solution.The porous nanoweb was impregnated with the ionomer solution. Specifically, immersion was carried out for 20 minutes three times at room temperature. To remove microfine bubbles, reduced pressure was applied for about one hour. To remove NMP, after each immersion for 3 hours, drying was carried out in a hot blast furnace at 80° C., and finally a polymer electrolyte membrane having an average thickness of 45 μm was obtained.Example 2A polymer electrolyte membrane was produced by the same method as in Example 1, except that the average diameter of the porous nanoweb was changed to 10 μm by controlled electric spin conditions, and the average thickness of an electrolyte film was changed to 50 μm by setting an immersed amount of the ionomer.Comparative Example 1Sulfonated polyetheretherketone (S-PEEK) was dissolved in N-methyl-2-pyrrolidone (NMP) to prepare a 15 wt% ionomer solution. Using a doctor blade, a film was formed on a glass plate, followed by drying it in a hot blast furnace at 80° C. to remove NMP, thereby obtaining a singular film-type polymer electrolyte membrane having a thickness of 50 μm.Comparative Example 2After mixing and dissolving 25 wt% polysulfone and 5 wt% PVP in a DMAc solution, a film was formed on a glass plate using a doctor blade, which was then immersed in water at room temperature. Subsequently, the obtained product was immersed in ultra-pure water overnight to remove residual solvent, followed by drying it in a hot blast furnace at 80 °C for 24 hours to remove NMP, thereby obtaining a porous polysulfone film having a thickness of 30 μm. An alcohol-dispersed nafion solution was impregnated into a porous polysulfone film and dried according to the same method as described for Example 1, thereby obtaining a polymer electrolyte membrane having an average thickness of 45 μm.Comparative Example 3A polymer electrolyte membrane was prepared by the same method as described in Example 1, except that the average diameter of the porous nanoweb was changed to 8 μm by controlling the electric spin conditions, and the average thickness of an electrolyte film was changed to 50 μm by adjusting an amount of ionomer immersed.For the porous nanowebs prepared in the above-described Examples and Comparative Examples and the polymer electrolyte porous membranes, physical properties were measured according to the following methods shown in Table 1.Thickness (μm) of each porous nanoweb and polymer electrolyte membraneFor each porous nanoweb and polymer electrolyte membrane, 10 sample points were selected and their thicknesses measured using a micrometer. Based on the average of the obtained values, the thicknesses of the porous nanoweb and the polymer electrolyte membrane were determined.Nanoweb Thickness Ratio (%)As shown in FIG. 1, a nanoweb thickness ratio was obtained according to the following equation from the average thickness (A) of the nanoweb and the average thickness (B+C) of the ionomer through a cross section of the polymer electrolyte membrane using an electron microscope:Mechanical Strength (MPa) of Polymer Electrolyte MembraneAccording to ASTM 638, a mechanical strength (MPa) of the polymer electrolyte membrane was measured. Detailed measurement conditions were as follows:Drawing speed: 25 cm / minPicking interval: 6.35 cmTemperature and Humidity: 25 °C x 50 %Thickness Expansion Rate (%) of Polymer Electrolyte MembraneEach of the polymer electrolyte membranes prepared according to the above Examples and Comparative Examples was prepared into a sample having the dimensions of 10 cm x 10 cm, and the sample was subjected to vacuum drying at 80°C for 3 hours, and its thickness (T0) was measured. Following this, the sample was immersed in water at room temperature for 3 hours and then taken out. After removing the water from the surface of the sample, a thickness (T1) of the sample was measured. Subsequently, the sample thicknesses before and after the expansion (swelling) were used to determine a thickness expansion rate (%) of the polymer electrolyte membrane by the following equation. Table 1 Table 1Example 150255Example 225189Comparative Example 10837Comparative Example 2502011Comparative Example 3191213Industrial applicabilityThe electrolyte membrane produced by the method of the present invention has excellent durability and proton conductivity and a low thickness expansion rate, and is applied in a variety of applications such as a partition wall of a fuel cell.

Claims

A method for manufacturing a polymer electrolyte membrane, comprising: dissolving polyamic acid in a spin solvent to prepare a spin solution; electrically spinning the spin solution to prepare a porous nanoweb comprising nanofibers having an average diameter of 0.005 to 5 μm; after-treating the porous nanoweb by imidation of the polyamic acid to obtain a porous polyimide nanoweb insoluble in an organic solvent of NMP, DMF, DMA, or DMSO; dissolving an ionomer comprising sulfonated polyimide (S-PI), sulfonated polyarylethersulfone (S-PAES), sulfonated polyetheretherketone (S-PEEK), sulfonated polybenzimidazole (S-PBI), sulfonated polysulfone (S-PSU), Sulfonated polystyrene (S-PS), sulfonated polyphosphazene, or mixtures thereof, in the organic solvent to obtain an ionomer solution, filling pores of the post-treated porous polyimide nanoweb with the ionomer solution by immersion of the porous polyimide nanoweb in the ionomer solution, and then removing the organic solvent from the ionomer solution, thereby allowing the pores of the porous polyimide nanoweb to be charged with the ionomer so that a thickness ratio of the nanoweb measured by the following equation is 20% or more, and: nanoweb thickness ratio=[A / (B+C)]x100, wherein A is an average thickness of the nanoweb, B is an average thickness of an upper ionomer and C is an average thickness of a lower ionomer.The method according to claim 1, wherein the polyamic acid has a water content of 0.5 wt% or less.The method of claim 1, wherein the post-treatment comprises thermally treating the porous nanoweb.

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