KR20110129110A - Polyimide porous nanofiber web and method for manufacturing the same - Google Patents

Abstract

PURPOSE: A polyimide porous nanofiber web and a method for fabricating the same are provided to ensure shape stability at high temperature and high humidity. CONSTITUTION: A polyimide porous nanofiber web has 50-99% of porosity and is formed in a fiber aggregate. The volume variation rate of an electrolyte film is less than 15% in case of applying an equation, volume variation rate(%) = [(Va - Vb)/Vb]x100. The electrolyte film is fabricated by immersing hydrocarbon ion conductor in the porous nanofiber web. The polyimide porous nanofiber web comprises fiber(4) with 0.01-5 um of average diameter.

Description

Polyimide porous nanofiber web and method for manufacturing the same

The present invention relates to a polyimide porous nanofiber web and a method for manufacturing the same, and more particularly, to a polyimide porous nanofiber web and a method for manufacturing the same, which can be used in an electrolyte membrane for a fuel cell or a separator for a secondary battery.

Porous nanofiber webs have a wide surface area and excellent porosity, and thus can be used for various purposes. For example, the porous nanofiber webs can be used for water purification filters, air purification filters, composite materials, and battery separators. In particular, such a porous nanofiber web can be usefully applied to the electrolyte membrane of a fuel cell for automobiles.

A fuel cell is a battery that converts chemical energy generated by oxidation of a fuel into electrical energy directly, and has been in the spotlight as a next-generation energy source due to its high energy efficiency and eco-friendly features with low emission of pollutants. Such a fuel cell generally has a structure in which an anode and a cathode are formed on both sides of an electrolyte membrane.

Representative examples of automotive fuel cells include a hydrogen ion exchange membrane fuel cell using hydrogen gas as a fuel. Since the electrolyte membrane used in the hydrogen ion exchange membrane fuel cell is a passage through which hydrogen ions generated from the anode are transferred to the cathode, the conductivity of the hydrogen ions should be excellent. In addition, the electrolyte membrane must have excellent separation ability to separate hydrogen gas supplied to the anode and oxygen supplied to the cathode, and also have excellent mechanical strength, shape stability, and chemical resistance, and have low resistance loss at high current density. Such characteristics are required. In particular, the fuel cell for automobiles should be excellent in heat resistance so as not to rupture the electrolyte membrane when used for a long time at high temperature.

As an electrolyte membrane for fuel cells currently used, a perfluorosulfonic acid resin (Nafion, Nafion, trade name) (hereinafter referred to as 'nafion resin') is a fluorine resin. However, Nafion resins have a weak mechanical strength, so that when used for a long time, pinholes are generated, thereby lowering energy conversion efficiency. Attempts have been made to increase the film thickness of Nafion resin in order to reinforce mechanical strength. However, in this case, the resistance loss is increased, and there is a problem in that the economy is inferior as expensive materials are used.

In order to solve such a problem, an electrolyte membrane having improved mechanical strength by impregnating a porous polytetrafluoroethylene resin (Teflon, Teflon, trade name) (hereinafter referred to as 'Teflon resin') with an ion conductor as an ion conductor has been proposed. There is a bar. In this case, the mechanical strength is relatively higher than that of the electrolyte membrane composed of Nafion resin alone, but there is a problem that the ionic conductivity is somewhat reduced. In addition, since the Teflon resin is very low adhesion, there is a disadvantage in that the separation between hydrogen and oxygen decreases as the adhesion between the Teflon resin and the Nafion resin decreases with the change of operating conditions such as temperature or humidity during fuel cell operation. . In addition, since the price of not only Nafion resin but also porous Teflon resin is expensive, there is a problem in that economic feasibility in mass production.

In order to solve this problem, general-purpose hydrocarbon resins such as polyethylene, polypropylene, polysulfone, polyethersulfone, polyvinylidene fluoride are used instead of teflon resin, and sulfonic acid groups are used instead of Nafion resin as an ion conductor. An electrolyte membrane has been proposed to reduce the production cost by using a low-cost hydrocarbon resin.

However, in this case, although the production cost can be reduced, the economical efficiency can be improved, but there is a problem that durability can not be guaranteed as the hydrocarbon-based resin is dissolved in the organic solvent or the heat resistance is poor.

In addition, the non-woven fabric prepared by the general method or a hydrocarbon-based membrane in the form of a film prepared by a conventional separation membrane manufacturing method has a problem that it is difficult to thin film, the porosity is poor, and the pore size is not easy to control.

In particular, the membrane impregnated with a hydrocarbon-based ion conductor is easily changed in shape in a high temperature and high humidity environment, there is a problem that the performance is degraded or bursts when severe.

The present invention is easy to control the thin film and pore size, excellent porosity and heat resistance, excellent in form stability in the environment of high temperature and high humidity, and in the form of a fiber aggregate having a non-soluble to a conventional organic solvent, fuel cell electrolyte membrane or An object of the present invention is to provide a polyimide porous nanofiber web and a method of manufacturing the same that can be used in various fields such as secondary battery separators.

As an aspect of the present invention for achieving the above object, the present invention is a porous nanofiber web, the porous nanofiber web has a porosity of 50 to 99%, and forms a fiber aggregate form; It provides a polyimide porous nanofiber web, characterized in that the volume change rate of the electrolyte membrane prepared by impregnating the hydrocarbon-based ion conductor in the porous nanofiber web is 15% or less when measured by the following equation.

(%) = [(Va-Vb) / Vb] x 100 (wherein Vb is the volume of the electrolyte membrane before the high humidity treatment, Va is 4 hours under conditions at a temperature of 80 ℃ and 90% relative humidity) Volume of the electrolyte membrane after high humidity treatment for a while)

As another aspect of the present invention for achieving the above object, the present invention comprises the steps of preparing a spinning solution by dissolving a polyimide precursor (precusor) in an organic solvent; Electrospinning the spinning solution to prepare a polyimide precursor nanofiber web made of fibers having an average diameter of 0.01 to 5 μm; And it provides a method for producing a polyimide porous nanofiber web comprising the step of producing a polyimide nanofiber web by imidating the polyimide precursor nanofiber web at a temperature of 200 to 400 ℃.

The present invention has the following effects.

First, the polyimide porous nanofiber web according to the present invention has an effect of easily thinning and easily adjusting the pore size.

Second, since the polyimide porous nanofiber web according to the present invention has a high melting point without being dissolved in a conventional organic solvent, it has an excellent chemical resistance and heat resistance.

Third, the polyimide porous nanofiber web according to the present invention has an effect of having excellent morphological stability in an environment of high temperature and high humidity when the hydrocarbon-based ion conductor is impregnated as it is imidized under optimum conditions.

The polyimide porous nanofiber web having such excellent physical properties can be used for electrolyte membranes for fuel cells or secondary membranes for secondary batteries requiring lightweight, high efficiency, and high stability.

1 is a schematic diagram of an electrospinning apparatus according to an embodiment of the present invention.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the invention. Therefore, the present invention encompasses all changes and modifications that come within the scope of the invention as defined in the appended claims and equivalents thereof.

Hereinafter, a polyimide porous nanofiber web according to an embodiment of the present invention will be described in detail.

The polyimide porous nanofiber web of the present invention comprises a polyimide having an imidation ratio of at least 90%. That is, it means that the polyimide precursor is changed to the imide group by the cyclization reaction occurs through an imidization process such as heat treatment. As such, the polyimide porous nanofiber web has an imidation ratio of 90% or more, thereby having excellent heat resistance and chemical resistance. If the imidation ratio is less than 90%, it may be difficult to maintain the required physical properties for a long time because the nanofiber web may be dissolved in the organic solvent.

The polyimide porous nanofiber web can easily obtain a fiber 4 having the required diameter as the process is smoothly controlled during electrospinning, and thus can have a porosity of 50% or more. As described above, as having a porosity of 50% or more, the porous nanofiber web has a large specific surface area, and thus the ion conductivity is easily impregnated as the ion conductor is easily impregnated. On the other hand, the porous nanofiber web may have a porosity of 99% or less. If the porosity of the porous nanofiber web exceeds 99%, the morphological stability may be lowered, and thus the subsequent process may not proceed smoothly. The porosity was calculated by the ratio of the air volume to the total volume as shown in the following equation. In this case, the total volume was calculated by measuring the width, length, and thickness of a sample in the form of a rectangular shape, the air volume was obtained by subtracting the volume of the polymer inverted from the density after measuring the mass of the sample from the total volume.

Porosity (%) = (air volume / total volume) × 100

The polyimide porous nanofiber web may have an average thickness of 5 to 40 μm. When the thickness of the polyimide porous nanofiber web is less than 5 μm, the mechanical strength and shape stability may be remarkably inferior, whereas when the thickness of the polyimide porous nanofiber web is more than 40 μm, the weight and integration may be reduced.

The polyimide porous nanofiber web may comprise fibers 4 having an average diameter of 0.01 to 5 μm.

In order for the polyimide porous nanofiber web to have the porosity and thickness as described above, it may be preferable that the fibers 4 constituting the fiber aggregate have an average diameter in the range of 0.01 to 5 μm. If the average diameter of the fiber 4 is less than 0.01 μm, the mechanical strength may be lowered. On the other hand, if the average diameter of the fiber 4 is more than 5 μm, the porosity may be significantly decreased and the thickness may be thick.

The polyimide porous nanofiber web may have a melting point of 400 ° C. or higher as it is imidized under optimum conditions to be described later with an imidization ratio in the range as described above. If the melting point of the polyimide porous nanofiber web is less than 400 ° C., the heat resistance may be easily deformed at high temperatures, and thus the fuel cell or the secondary battery manufactured using the polyimide porous nanofiber web may be degraded. In addition, when the heat resistance of the polyimide porous nanofiber web is lowered, the shape is deformed by abnormal heat generation, thereby degrading the performance and, in severe cases, may burst and explode.

When used in the manufacture of the electrolyte membrane of the fuel cell or the separator of the secondary battery that requires high stability as described above, it may be preferable that the polyimide porous nanofiber web has a melting point of 400 ~ 800 ℃. If the melting point of the polyimide porous nanofiber web exceeds 800 ℃, the manufacturing process may not proceed smoothly, it may be economical.

When the polyimide porous nanofiber web is used in an electrolyte membrane for a fuel cell, the fuel cell is exposed to a high temperature and high humidity environment, and the electrolyte membrane used in such an environment is required to have excellent shape stability. That is, when the electrolyte membrane is poor in shape stability in an environment of high temperature and high humidity, the interface between the electrolyte membrane and the electrode may be separated, thereby degrading power generation performance.

In order to confirm the stability of the electrolyte membrane for fuel cells under high temperature and high humidity, the shape stability is evaluated by measuring the rate of change of volume and thickness in an environment similar to that of the fuel cell. The volume change rate may be a criterion for determining performance degradation and stability due to breakage or separation of the electrolyte membrane in a high temperature and high humidity environment. Meanwhile, the thickness change rate may be a criterion for determining the durability and stability of the fuel cell stack including tens to hundreds of layers of electrolyte membranes.

Accordingly, the electrolyte membrane prepared by impregnating the polyimide porous nanofiber web with a hydrocarbon-based ion conductor has a volume change rate of 15% or less. The volume change rate can be measured using the following equation.

Volume change rate (%) = [(Va-Vb) / Vb] × 100

However, Vb is the volume of the electrolyte membrane before the high humidity treatment, and Va is the volume of the electrolyte membrane after the high humidity treatment for 4 hours under conditions at a temperature of 80 ° C. and a relative humidity of 90%.

The volume change rate is described in more detail. The volume change rate is 4 hours in which the electrolyte membrane cut at 10 cm × 10 cm is placed in a constant temperature and humidity chamber maintained at a temperature of 80 ° C. and a relative humidity of 90%, which is a general operating environment of a fuel cell. The volume obtained by measuring the x, y, z axis length of the changed film after standing and the volume measured before the humidification treatment described above are obtained by applying the above formula.

In addition, an electrolyte membrane prepared by impregnating a polyimide porous nanofiber web with a hydrocarbon-based ion conductor has a thickness change rate of 8% or less. The thickness change rate can be measured using the following equation.

% Change in thickness = [(Ta-Tb) / Tb] x 100

However, Tb is the thickness of the electrolyte membrane before the high humidity treatment, and Ta is the thickness of the electrolyte membrane after the high humidity treatment for 4 hours under conditions at a temperature of 80 ° C. and a relative humidity of 90%.

In more detail, the thickness change rate is obtained by applying to the above formula by measuring the thickness of the electrolyte membrane before and after the treatment after treating in the environment of high temperature and high humidity same as the measurement conditions of the volume change rate described above.

As such, the electrolyte membrane, which may have excellent shape stability even in an environment of high temperature and high humidity, may obtain high reliability from customers when used in a fuel cell requiring high stability.

Next, referring to FIG. 1, a method of manufacturing a polyimide porous nanofiber web according to an embodiment of the present invention will be described. 1 is a schematic diagram of an electrospinning apparatus according to an embodiment of the present invention.

First, a polyimide precursor (precusor) is dissolved in an organic solvent to prepare a spinning solution.

Since the polyimide porous nanofiber web of the present invention is produced through electrospinning, the porosity may be high and the thin film may have excellent characteristics.

On the other hand, porous nanofiber webs that are insoluble in organic solvents cannot be directly manufactured through an electrospinning process. In other words, polyimide having insolubility in organic solvents is difficult to prepare a spinning solution because it does not dissolve in organic solvents.

Therefore, polyimide having insolubility in an organic solvent can be prepared by preparing a fiber assembly using a polyimide precursor that is well soluble in an organic solvent and then imidating the fiber assembly.

As the polyimide precursor, polyamic acid may be used. The polyamic acid may be prepared by mixing diamine in a solvent, adding dianhydride thereto, and polymerizing the diamine. The dianhydride may be pyromellyrtic dianhydride (PMDA), benzophenonetetracarboxylic dianhydride (BTDA), ODPA (4,4′-oxydiphthalic anhydride), BPDA (biphenyltetracarboxylic dianhydride), or SIDA (bis (3,4-dicarboxyphenyl) dimethylsilane dianhydride It may be used to include at least one of). In addition, the diamine may be one containing at least one of ODA (4,4'-oxydianiline), p-PDA (p-penylene diamine), or o-PDA (openylene diamine).

Solvents for dissolving the polyamic acid include m-cresol, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), acetone, A solvent including at least one of diethyl acetate, tetrahydrofuran (THF), chloroform, and γ-butyrolactone may be used.

The spinning solution may have a concentration of 5 to 50%. If the concentration of the spinning solution is less than 5%, since spinning does not proceed smoothly, the fiber 4 may not be formed or a fiber 4 having a uniform diameter may not be manufactured, whereas the concentration of the spinning solution is When 50% is exceeded, as the discharge pressure increases rapidly, spinning may not be performed or processability may decrease.

Subsequently, the spinning solution is prepared using the electrospinning apparatus as shown in FIG. 1 to prepare a polyimide precursor nanofiber web made of fibers 4 having an average diameter of 0.01 to 5 μm. That is, the spinning solution is supplied in a predetermined amount to the spinning unit by using a metering pump in the solution tank in which the spinning solution is stored, and the spinning solution is discharged through the nozzle 2 of the spinning unit and coagulated simultaneously with scattering to disperse the fibers (4). The polyimide precursor nanofiber web may be manufactured by forming a polyimide precursor and focusing the formed fibers 4 on the collector 5.

In this case, the intensity of the electric field between the radiator and the collector 5 applied by the high voltage generator 3 may be 3 to 80 kW. If the strength of the electric field is less than 3 kW, the spinning solution is not continuously discharged, and thus, the nanofiber web of uniform thickness cannot be manufactured, and the fiber 4 formed after spinning is smoothly applied to the collector 5. Fabrication of nanofiber webs can be difficult because they cannot be focused. On the other hand, when the strength of the electric field exceeds 80 kPa, the scattered fibers 4 may be concentrated in the state in which the solvent is not removed from the collector 5, so that a nanofiber web made of nano-sized fibers may not be obtained. .

The polyimide precursor nanofiber web is then imidated to produce a polyimide nanofiber web. Production of the polyimide nanofiber web may be carried out through a process using heat imidization, chemical imidization, or a combination of heat imidization and chemical imidization.

The chemical imide process may be performed by treating the polyimide precursor nanofiber web with an imidation catalyst such as acetic anhydride or pyridine.

In addition, the thermal imide process may be performed through a process of heat-treating the polyimide precursor fiber assembly 10 at a high temperature. In particular, the thermal imide process has an advantage that the precursor can be easily imidized. The heat treatment process may be carried out at a temperature of 200 ~ 400 ℃ to obtain a polyimide porous nanofiber web having an imidation rate of 90% or more and a high melting point. If the heat treatment temperature is less than 200 ° C., since imidization does not proceed smoothly and the processing time is long, economic efficiency may be reduced. On the other hand, when the heat treatment temperature exceeds 400 ℃, the imidization rate is not greatly improved, but rather, the physical properties may be lowered, thereby degrading performance.

The imidization process may be performed in a draw ratio of 0.1 or less. That is, it may be preferable to set the draw ratio to 0.1 or less in order to obtain a nanofiber web having uniform and excellent porosity. If the draw ratio is more than 0.1, the shape of the nanofiber web is deformed and a uniform thickness may not be obtained, and the tensile strength may be lowered as the cohesive force is lowered. On the other hand, the lower limit value of the draw ratio may be a state not stretched or over-supplied + 0.1% in order to prevent a smooth process and a decrease in porosity and to maintain an appropriate thickness.

Through the imidization process, the polyimide porous nanofiber web may have an imidation ratio of 90% or more. If the imidation ratio of the polyimide is less than 90%, heat resistance and chemical resistance may be degraded, so it may be difficult to use in a product requiring such characteristics.

Hereinafter, the effects of the present invention will be described in more detail with reference to Examples and Comparative Examples. These examples are merely to aid the understanding of the present invention and do not limit the scope of the present invention.

Example  One

After dissolving the polyamic acid in dimethylformamide solvent to prepare a 12 wt% spinning solution, it is spun through a metering pump (1) through a nozzle (2) installed in the electrospinning apparatus and then to the high voltage generator (3) By scattering and solidifying the electric field is applied to form a fiber (4), and the formed fibers (4) to focus on the collector (5) to prepare a polyimide precursor nanofiber web having an average thickness of 30 ㎛. At this time, the applied voltage was 15 kV and the spinning distance was 15 cm.

Subsequently, the polyimide precursor nanofiber web was heat-treated with a hot press maintained at a temperature of 200 ° C. for 30 minutes and then imidized to prepare a polyimide porous nanofiber web.

Example  2

In Example 1 described above, a polyimide porous nanofiber web was prepared by the same method as Example 1 except that the temperature of the hot press is 400 ° C.

Comparative example  One

The sulfonated polyimide was dissolved in dimethylformamide to prepare a 12% by weight of the composition, which was filmed and dried to prepare a porous membrane having an average thickness of 30 μm.

Comparative example  2

A commercially available ePTFE porous film (W.L.Gore, Teflon) with a thickness of 30 μm was used as the porous membrane.

Comparative example  3

In Example 1 described above, a polyimide porous nanofiber web was prepared by the same method as Example 1 except that the temperature of the hot press is 80 ° C.

The physical properties of the porous nanofiber webs and porous membranes prepared by Examples and Comparative Examples are shown in Table 1 below, measured by the following method.

Volume change rate (%) measurement

The rate of change of volume was changed into x, y, z after leaving the electrolyte membrane cut at 10 cm × 10 cm in a constant temperature and humidity chamber maintained at a temperature of 80 ° C. and a relative humidity of 90%, which is a general operating environment of a fuel cell, for 4 hours. The volume (Va) obtained by measuring the shaft length and the volume (Vb) measured before the humidification treatment were measured by applying the following equation.

Volume change rate (%) = [(Va-Vb) / Vb] × 100

In this case, the electrolyte membrane was prepared by dissolving sulfonated polyimide in N-methyl-2-pyrrolidone solvent to prepare an ion conductor solution and impregnating the prepared ion conductor solution with the porous nanofiber web and the porous membrane twice for 20 minutes. After leaving for 1 hour under reduced pressure it was prepared by drying for 3 hours in a hot air of 80 ℃.

Thickness change rate (%) measurement

The thickness change rate was measured by applying the thickness (Ta) of the electrolyte membrane measured after treatment under the same conditions of high temperature and high humidity as the volume change rate described above, and the thickness (Tb) of the electrolyte membrane measured before the high temperature and high humidity treatment to the following equation. .

% Change in thickness = [(Ta-Tb) / Tb] x 100

At this time, the electrolyte membrane was the same as the sample used for the measurement of the volume change rate mentioned above.

division Nanofiber web type Heat treatment temperature (℃) Volume change rate (%) Thickness change rate (%) Example 1 PI 200 13.2 6.6 Example 2 PI 400 12.9 5.9 Comparative Example 1 S-PI - 20.2 12.1 Comparative Example 2 ePTFE - 15.1 10.8 Comparative Example 3 PI 80 20.3 11.9

With the proviso that PI is a polyimide and S-PI is a sulfonated polyimide.

1: metering pump 2: nozzle
3: high voltage generating unit 4: fiber
5: collector

Claims (8)

In the porous nanofiber web,
The porous nanofiber web has a porosity of 50 to 99% and forms a fiber aggregate;
Polyimide porous nanofiber web, characterized in that the volume change rate of the electrolyte membrane prepared by impregnating the hydrocarbon-based ion conductor into the porous nanofiber web is measured by the following equation.
(%) = [(Va-Vb) / Vb] x 100 (wherein Vb is the volume of the electrolyte membrane before the high humidity treatment, Va is 4 hours under conditions at a temperature of 80 ℃ and 90% relative humidity) Volume of the electrolyte membrane after high humidity treatment for a while) The method of claim 1,
Polyimide porous nanofiber web, characterized in that the rate of change of the thickness of the electrolyte membrane prepared by impregnating the hydrocarbon-based ion conductor in the porous nanofiber web is 8% or less.
The thickness change rate (%) = [(Ta-Tb) / Tb] x 100 (wherein Tb is the thickness of the electrolyte membrane before the high humidity treatment, and Ta is 4 hours under conditions at a temperature of 80 ° C and a relative humidity of 90%). Is the thickness of the electrolyte membrane after high humidity treatment) The method of claim 1,
The polyimide porous nanofiber web has a melting point of 400 ℃ or more polyimide porous nanofiber web. The method of claim 1,
The polyimide porous nanofiber web is a polyimide porous nanofiber web, characterized in that consisting of fibers having an average diameter of 0.01 to 5 ㎛. The method of claim 1,
The polyimide porous nanofiber web has an average thickness of 5 to 40 ㎛ polyimide porous nanofiber web. Preparing a spinning solution by dissolving a polyimide precursor (precusor) in an organic solvent;
Electrospinning the spinning solution to prepare a polyimide precursor nanofiber web made of fibers having an average diameter of 0.01 to 5 μm; And
Method of producing a polyimide porous nanofiber web comprising the step of producing a polyimide nanofiber web by imidating the polyimide precursor nanofiber web at a temperature of 200 to 400 ℃. The method of claim 6,
The process for producing the polyimide porous nanofiber web is a method for producing a polyimide porous nanofiber web, characterized in that the imidation is made at a draw ratio of 0.1 or less. The method of claim 6,
The polyimide porous nanofiber web is a method for producing a polyimide porous nanofiber web, characterized in that it has an imidation ratio of 90% or more.

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