KR20090023994A - Polymer electrolyte membrane comprising sulfated cyclodextrin for direct methanol fuel cell and method of preparing the same and membrane electrode assembly and direct methanol fuel cell using the same - Google Patents

Abstract

A polymer electrolyte membrane for a direct methanol fuel cell is provided to improve selectivity by suppressing increase in methanol permeation and secure high proton conductivity using a sulfation cyclodextrin dispersed in an ion exchange film matrix. A method for manufacturing a polymer electrolyte membrane for a direct methanol fuel cell comprises the following steps of: uniformly mixing a sulfation cyclodextrin in a polymer solution for an ion exchange film; evaporating a solvent of the mixing solution by drying; and heat-processing the solvent removed resultant to obtain a polymer electrolyte membrane.

Description

Polymer electrolyte membrane comprising sulfated cyclodextrin for direct methanol fuel cell and method of preparing the same and Membrane electrode assembly and Direct methanol fuel cell using the same}

The present invention relates to a polymer electrolyte membrane comprising a sulfated cyclodextrin, a method for producing the same, and a membrane electrode assembly and a direct methanol fuel cell using the same. More specifically, the present invention relates to a polymer electrolyte membrane having excellent moisture content, proton conductivity, ion exchange capacity, and selectivity, a method of manufacturing the same, and a membrane electrode assembly and a fuel cell including the same.

Recently, fuel cells are attracting attention as one of alternative energy sources with low emission of environmental pollution sources and high energy efficiency. Among these fuel cells, direct methanol fuel cells (DMFCs) using methanol as fuel have a lower electrode density than fuel cells that directly use hydrogen, and thus have a low output density. However, methanol, a fuel, has a high energy density and is easy to store. And for long time use.

The principle of electricity generation in direct methanol fuel cells is an electrochemical process that converts chemical energy directly into electrical energy. Specifically, referring to Scheme 1, an oxidation reaction of a fuel occurs at an anode electrode to generate hydrogen ions (protons) and electrons, and protons move to the cathode electrode through an electrolyte membrane, and at the cathode electrode, oxygen (oxidant) and Protons and electrons transferred through the electrolyte membrane react with each other to generate water. This reaction causes the movement of electrons in the external circuit.

A fuel electrode _{(anode): CH 3 OH + H 2} O → CO 2 + 6H + + 6e -

An air electrode _{(cathode): 3 / 2O 2 + 6H} + + 6e - → 3H 2 O

Total reaction: CH ₃ OH + H ₂ O → CO ₂ + 3H ₂ O

In particular, perfluorosulfonic acid membranes having excellent chemical stability are mainly used as electrolyte membranes for direct methanol fuel cells, and ion clusters having hydrophilicity formed by sulfonic acid groups at the side chain ends of perfluorosulfonic acid (in the case of perfluorosulfonic acid membranes, the size thereof is weak). 1 to 4 nm) protons are propagated. At this time, when water is insufficient around the ion cluster, proton conductivity is significantly reduced. Therefore, in a fuel cell using a perfluorosulfonic acid membrane, the electrolyte membrane should always be kept wet.

However, since methanol, which is used as a fuel of a direct methanol fuel cell, dissolves well in water and diffuses well in water, a portion of methanol moves from an anode to a cathode through a thin polymer electrolyte membrane (methanol crossover). In this case, methanol is usually oxidized by a catalyst used as a reduction electrode, and when methanol is oxidized, not only fuel is consumed, but also oxidation and reduction reactions occur at the reduction electrode, resulting in a mixed potential, resulting in a decrease in efficiency. have. Therefore, there has been a need for an electrolyte membrane having a low methanol permeability or a high selectivity (proton conductivity divided by methanol permeability).

In order to solve this problem, researches on nanocomposite membranes incorporating organic and inorganic nanoparticles (SiO ₂ , TiO ₂ , montmorillonite, polyphenylene oxide) have been attempted. However, these nanoparticles can reduce methanol permeability, but there is a problem in that proton conductivity is reduced due to a decrease in ion exchange capacity by the amount of nanoparticles added.

In this regard, Ji Hwan Son, Yong Soo Kang, Jongok Won, Poly (vinyl alcohol) -based polymer electrolyte membranes containing polyrotaxane, J. Membr. Sci. 281 (2006) 345-450 discloses an electrolyte membrane in which polyrotaxane, an adhesive of polyethylene glycol and alpha-cyclodextrin, is dispersed in a polyvinyl alcohol matrix, but its effect is insufficient.

Accordingly, the problem to be solved by the present invention is to provide a polymer electrolyte membrane for direct methanol fuel cell having excellent proton conductivity and improved selectivity by suppressing an increase in methanol permeability.

In order to solve the above problems, the polymer electrolyte membrane for a direct methanol fuel cell of the present invention is a polymer electrolyte membrane including an ion exchange membrane, characterized in that sulfated cyclodextrin is dispersed in a polymer matrix forming the ion exchange membrane. . The sulfated cyclodextrin according to the present invention exhibits a tortuous pathway effect in the electrolyte membrane and thus inhibits methanol crossover. In addition, as shown in FIG. 1, since it has 7 to 11 hydrogen sulfate groups (HSO ₄ ⁻ ), it is possible to improve the proton conductivity, thereby exhibiting high selectivity and thus improving the performance of the direct methanol fuel cell.

Since the sulfated cyclodextrin according to the present invention has a hydrophilic hydrogen sulfate group, it is mainly dispersed in the hydrophilic ion cluster of the ion exchange membrane, so that proton conductivity and selectivity may be further excellent.

The content of the sulfated cyclodextrin according to the present invention in the electrolyte membrane may be appropriately adjusted by those skilled in the art, for example, may be 5% by weight or less based on the total weight of the ion exchange membrane polymer matrix and the sulfated cyclodextrin, It is not limited to this.

Method for producing a polymer electrolyte membrane for a direct methanol fuel cell of the present invention, (S1) step of uniformly mixing a sulfated cyclodextrin in a polymer solution for forming an ion exchange membrane; (S2) drying the mixed solution at room temperature to evaporate the solvent; And (S3) heat treating the resultant to obtain a polymer electrolyte membrane.

In the manufacturing method of the present invention, the heat treatment in the step (S3) is performed to evaporate the remaining solvent as much as possible, and to increase the strength of the polymer electrolyte membrane can be appropriately performed by those skilled in the art as needed, for example For example, it may be performed at 80 to 170 ° C. for 5 minutes to 3 days, but is not limited thereto.

The polymer electrolyte membrane of the present invention described above can be used in the membrane electrode assembly for direct methanol fuel cell and direct methanol fuel cell.

The polymer electrolyte membrane for direct methanol fuel cell of the present invention has excellent proton conductivity, ion exchange capacity and selectivity due to sulfated cyclodextrin dispersed in an ion exchange membrane matrix. If the selectivity is excellent, it is possible to manufacture a direct methanol fuel cell having the same or superior performance as in the prior art even with less methanol.

Hereinafter, the polymer electrolyte membrane for a direct methanol fuel cell of the present invention will be described in detail according to the preparation method thereof. The terms or words used in this specification and claims are not to be construed in a general or dictionary sense, and the inventors may appropriately define the concept of terms in order to best describe their invention. It should be interpreted as meaning and concept corresponding to the technical idea of the present invention based on the principle that it can.

First, the sulfated cyclodextrin is mixed with the polymer solution for ion exchange membrane formation and stirred uniformly (S1).

The polymer forming the ion exchange membrane of the present invention may be used without limitation the polymer for forming an ion exchange membrane used in the art, especially if the polymer capable of forming ion clusters can be expected to have a better effect. For example, perfluorosulfonic acid polymer, polyvinylidene fluoride, polyvinylidene fluoride hexafluoropropylene, polyphenylene oxide, polyimide, polyetherketone, polybenzimidazole, polysulfone polymer, polyvinyl alcohol And it may be used a polymer selected from the group consisting of polystyrene, but is not limited thereto.

As the solvent for forming the polymer solution for forming the ion exchange membrane, a solvent used in the art may be used without limitation. For example, any one or a mixture of two or more selected from the group consisting of water, methanol, ethanol, iso propanol, n-propanol and butanol may be preferably used, but is not limited thereto.

Sulfated cyclodextrins according to the invention are sulfur oxides of cyclodextrins. Cyclodextrins are cylindrical structures having different sizes of inlets at both ends, and include alpha-cyclodextrin, beta-cyclodextrin, gamma-cyclodextrin, and the like. In the present invention, cyclodextrin may be used without limitation, and for example, sulfated alpha-cyclodextrin, sulfated beta-cyclodextrin and sulfated gamma-cyclodextrin may be used, but is not limited thereto. In terms of economy, sulfated beta-cyclodextrin may be preferred.

As illustrated in FIG. 1, the sulfated cyclodextrin is introduced with hydrogen sulfate group (HSO ₄ ⁻ ) in the cyclodextrin, and has more hydrophilicity than cyclodextrin. It is known in the art that when hydrophilic particles are introduced into the perfluorosulfonic acid membrane, they are primarily located in the hydrophilic ion cluster rather than the hydrophobic portion of the perfluorosulfonic acid membrane.

Therefore, in the polymer electrolyte membrane of the present invention, it can be seen that the sulfated cyclodextrin may be mainly dispersed in the hydrophilic portion when the ion exchange membrane has a hydrophilic ion cluster. As such, when the sulfated cyclodextrin is mainly dispersed in the ion cluster, since the proton and methanol are mainly conducted by moisture, the effect of the proton conductivity and the flexibility on the methanol molecule may be further improved.

The sulfated cyclodextrin according to the present invention can be mixed with the polymer solution for forming an ion exchange membrane in an appropriate amount as required by those skilled in the art. For example, an effect of the present invention may be excellent when the weight of the ion exchange membrane polymer matrix and the sulfated cyclodextrin is 0.1% by weight or more, and when 1% by weight or more, an optimal effect may be obtained. In addition, it can be mixed so that it is 5% by weight or less, if the 5% by weight or less may be less likely to crack the polymer electrolyte membrane.

The method of uniformly mixing the sulfated cyclodextrin in the polymer solution for forming an ion exchange membrane may be applied to the mixing method used in the art as needed, and for example, a stirring or sonication method may be used. It is not limited to this.

After uniformly mixing the sulfated cyclodextrin in the polymer solution for ion exchange membrane formation, the solvent is evaporated by drying at room temperature (S2).

The above-mentioned mixed solution is evaporated at room temperature to form a film. The drying time at room temperature is until the solvent is evaporated to form a film, so there is no particular limitation, but generally 2 to 4 days may have a film shape.

After drying at room temperature, heat treatment is performed to obtain a polymer electrolyte membrane (S3).

By heat-treating the resultant room temperature, the remaining solvent can be evaporated as much as possible, and the strength of the polymer electrolyte membrane can be increased. Therefore, those skilled in the art can select appropriate heat treatment conditions according to the polymer electrolyte membrane selected in a range that does not cause breakage of the polymer electrolyte membrane due to excessive heat treatment. For example, heat treatment may be performed at 80 to 170 ° C. for 5 minutes to 3 days. Within this range, the polymer electrolyte membrane may be better formed, and thermal decomposition may be prevented.

The above-described polymer electrolyte membrane for direct methanol fuel cell of the present invention may be interposed between the anode electrode and the cathode electrode to be used for the preparation of the membrane electrode assembly for direct methanol fuel cell of the present invention. The membrane electrode assembly of the present invention can be used without limitation the technology used in the art except for using the electrolyte membrane of the present invention described above as the polymer electrolyte membrane.

For example, the anode electrode and the cathode electrode may include a catalyst layer, and the catalyst layer includes a metal catalyst or ionomer polymer supported on a metal catalyst or a carbon-based material.

Platinum, ruthenium, platinum-ruthenium alloy or platinum-palladium alloy may be used as the metal catalyst. Graphite, carbon black, carbon nanotube or carbon nanofiber may be used as the carbon-based material. Sulfonated polymers such as Nafion ionomers may be representatively used, but not limited thereto. In addition, a gas diffusion layer may be formed of a carbon-based material having micropores on the side of the catalyst layer opposite to the electrolyte membrane.

The present invention also provides a direct methanol fuel cell that can be manufactured using any of the techniques in the art, except for using the electrolyte membrane and membrane electrode assembly of the present invention. For example, the direct methanol fuel cell of the present invention includes a stack including one or more membrane electrode assemblies of the present invention, a fuel supply unit supplying methanol to the stack, and an oxidant supply unit supplying oxygen or air to the stack. can do.

Hereinafter, the present invention will be described in detail with reference to Examples. However, embodiments according to the present invention can be modified in many different forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Example 1

To the Nafion solution in which 5% by weight of Nafion was dissolved in water, added sulfated beta-cyclodextrin to 1% by weight based on the total weight of Nafion and sulfated beta-cyclodextrin in the Nafion solution, followed by sonication. Was performed to mix uniformly. The mixed solution was placed in a petri dish and left at room temperature for 3 days to evaporate the solution, followed by heat treatment at 150 ° C. for 10 minutes to prepare a polymer electrolyte membrane.

Examples 2 and 3

Example 1 except that the sulfated beta-cyclodextrin was added to 3% by weight (Example 2) and 5% by weight (Example 3) to Nafion 5% by weight solution A polymer electrolyte membrane was prepared in the same manner.

Comparative example

A polymer electrolyte membrane was prepared in the same manner as in Example 1 except that no sulfated cyclodextrin was added.

The water content, ion exchange capacity, proton conductivity, methanol permeability and selectivity of each polymer electrolyte membrane prepared in Examples and Comparative Examples were measured and shown in Table 1.

In addition, the pore size of the ion clusters of the polymer electrolyte membranes prepared in Examples and Comparative Examples was measured and shown in FIG. 3, and the test results of the unit cells prepared using the polymer electrolyte membranes prepared in Examples and Comparative Examples are shown. 4 is shown.

Moisture Content and Ion Exchange Capacity Measurement

The moisture content was measured by the following formula (1).

Moisture Content (%) = (W _wet -W _dry ) / W _dry × 100 (1)

Here, W _wet denotes the weight of the electrolyte membrane swollen by water, and W _dry denotes the weight of the dried electrolyte membrane.

Ion exchange capacity can be seen that the degree of inclusion of the cation exchange group, which is a hydrophilic group in the prepared electrolyte membrane, was measured by acid-base titration method. The electrolyte membranes prepared according to Examples and Comparative Examples were immersed in 50 ml of 0.1 N aqueous sodium hydroxide solution for 24 hours, for example, so that -OSO ₃ H ⁺ in the electrolyte membrane was replaced with -OSO ₃ Na ⁺ , and then 10 ml was collected. By titrating with 0.1 N hydrochloric acid, the amount of reduction of sodium hydroxide was measured using the following equation (2).

Ion exchange capacity (meq / g) = (V _NaOH × N _NaOH × 5) / W _dry (2)

Where V _NaOH is the amount of sodium hydroxide (NaOH) solution used for titration, N _NaOH is the normal concentration of sodium hydroxide solution, and W _dry is the weight of the dried electrolyte membrane.

Proton Conductivity, Methanol Permeability and Selectivity

Proton conductivity values were measured by impedance analysis, and methanol permeability was measured by measuring the concentration of methanol that passed through the electrolyte membrane using a gas chromatograph. As mentioned above, the selectivity is the measured proton conductivity divided by the methanol permeability.

Moisture content (%) Ion exchange capacity (meq / g) Proton Conductivity (S / cm) Methanol Permeability (cm ² / s) Selectivity (S · s / cm ³ ) Example 1 22.1 0.91 1.19 × 10 ^-2 1.071 × 10 ^-6 1.111 × 10 ⁴ Example 2 23.3 0.94 1.35 × 10 ^-2 1.086 × 10 ^-6 1.243 × 10 ⁴ Example 3 24.4 0.96 1.44 × 10 ^-2 1.093 × 10 ^-6 1.317 × 10 ⁴ Comparative example 21.4 0.89 1.05 × 10 ^-2 1.065 × 10 ^-6 0.986 × 10 ⁴

As shown in the table, the polymer electrolyte membrane containing more sulfated beta-cyclodextrin showed higher ion exchange capacity and proton conductivity.

According to a conventional study, methanol permeability has been reported to increase linearly with increasing water content. However, in the Nafion / Sulfated Beta-cyclodextrin polymer electrolyte membrane according to an embodiment of the present invention, the sulfated beta-cyclo It can be seen that the methanol permeability increases slightly with increasing dextrin content. This may be due to the bypass caused by the sulfated beta-cyclodextrin present in the Nafion ion cluster pores of methanol.

In addition, looking at the selectivity, unlike the comparative example, it can be seen that the selectivity gradually increases as the sulfated beta-cyclodextrin content increases in the electrolyte membrane.

Ion Cluster Pore Size Measurement of Electrolyte Membrane

Proton nuclear magnetic resonance (NMR) cryoporometry and the Gibbs-Thomson equation can be used to measure the size of the ion cluster pores of the polymer electrolyte membrane.

Proton nuclear magnetic resonance (NMR) cryoporometry is based on the principle that the melting point of the condensed organic liquid in the porous material depends on the pore size and the spin-spin relaxation time in liquid and solid phases in nuclear magnetic resonance relaxation. T ₂ ) is remarkably different.

In addition, according to Gibbs-Thompson theory, the smaller the pore size, the larger the melting point drop, so that it starts melting from the water filled in the small pore. That is, as shown in the following equation (3), the liquid and solid phases of the water filled in the pores are present at different fractions depending on the temperature.

ΔT _m = T _m -T _m (x) = (4 · σ · ΔT _m ) / (x · ΔH _f ρ) = k / x (3)

Where ΔT _m is the melting point drop, T _m is the melting point in bulk, T _m (x) is the melting point of the material in the pore with pore size x, σ is the surface energy at the interface between liquid and solid, ΔH _f is a constant of melting enthalpy in bulk, ρ is the density of solids, and k is the type of organic liquid.

Also, the pore volume V (x) is a function of pore radius x. Since the volume between the radius x and x + Δx is given by (dV / dx) x, it can be expressed by the following equation (4).

(dV / dx) = [dV / dT _m (x)] [dT _m (x) / dx] = (dV · k) / [dT _m (x) · x ² ] (4)

Therefore, the pore size distribution can be determined by inserting a substance having x as a pore into the pores and measuring [dV / dT _m (x)].

Accordingly, after impregnating the polymer electrolyte membranes prepared according to the above Examples and Comparative Examples in water, the temperature was lowered to condense all the water in the system and then the temperature was measured while increasing the strength. That is, a relative spin-echo signal intensity graph, which is directly proportional to the volume fraction of water inserted into the ion cluster, was obtained according to the temperature, which is shown in FIG. 2.

Also, through the graph of FIG. 2, the melting point drop, which is a temperature range in which the phase change occurs, was measured, and the size distribution of the cluster can be known by substituting this value into the Gibbs-Thomson equation, which is shown in FIG. 3.

As shown in FIG. 3, it can be seen that as the sulfated beta-cyclodextrin content increases in the polymer electrolyte membrane, the ion cluster size distribution becomes wider, and its size gradually increases. As the sulfated beta-cyclodextrin content increases in the polymer electrolyte membrane, the ion cluster size and its distribution become wider, which means that the sulfated beta-cyclodextrin has an effective effect on improving proton conductivity.

Unit cell test

Using the polymer electrolyte membrane prepared in Examples and Comparative Examples, a membrane electrode assembly was prepared in the art. The anode and cathode used E-TEK's A11STDA and A11STDC products for direct methanol fuel cells. The polymer electrolyte membrane according to the examples and the comparative example was sandwiched between the anode and the cathode, and pressed at 3000 ° C. for 5 minutes at 3000 ° C. to prepare a membrane electrode assembly. A unit cell test was conducted with a 2 M methanol / water solution at the anode and air at the cathode. As shown in FIG. 4, as the sulfated beta-cyclodextrin content increased in the polymer electrolyte membrane, the current density and the maximum power density of the unit cell increased.

1 is a chemical formula of sulfated beta-cyclodextrin.

FIG. 2 is a graph showing relative spin-echo signal intensities directly proportional to the volume fraction of water inserted into ion clusters of polymer electrolyte membranes prepared according to Examples and Comparative Examples.

3 is a graph showing the ion cluster size of the polymer electrolyte membrane prepared according to Examples and Comparative Examples, using the results of FIG.

4 is a graph showing test results of a unit cell manufactured using a polymer electrolyte membrane prepared according to Examples and Comparative Examples.

Claims (12)

In the polymer electrolyte membrane for a direct methanol fuel cell interposed between the anode and the cathode, A polymer electrolyte membrane comprising an ion exchange membrane, wherein a polymer electrolyte membrane for direct methanol fuel cell in which sulfated cyclodextrin is dispersed in a polymer matrix forming the ion exchange membrane. The method of claim 1, The sulfated cyclodextrin is a polymer electrolyte membrane for a direct methanol fuel cell, characterized in that mainly dispersed in the hydrophilic ion cluster of the ion exchange membrane. The method of claim 1, The polymer matrix forming the ion exchange membrane may be a perfluorosulfonic acid polymer, polyvinylidene fluoride, polyvinylidene fluoride hexafluoropropylene, polyphenylene oxide, polyimide, polyether ketone, polybenzimidazole, polysulfone type Polymer electrolyte membrane for a direct methanol fuel cell, characterized in that the polymer matrix formed of a polymer selected from the group consisting of a polymer, polyvinyl alcohol and polystyrene. The method of claim 1, The sulfated cyclodextrin is any one or a mixture of two or more selected from the group consisting of sulfated alpha-cyclodextrin, sulfated beta-cyclodextrin and sulfated gamma-cyclodextrin, the polymer electrolyte for direct methanol fuel cell membrane. The method of claim 1, The content of the sulfated cyclodextrin is a polymer electrolyte membrane for a direct methanol fuel cell, characterized in that less than 5% by weight relative to the total weight of the ion exchange membrane polymer matrix and sulfated cyclodextrin. (S1) uniformly mixing the sulfated cyclodextrin in the polymer solution for forming an ion exchange membrane; (S2) drying the mixed solution at room temperature to evaporate the solvent; And (S3) heat treating the resultant to obtain a polymer electrolyte membrane; Method for producing a polymer electrolyte membrane for direct methanol fuel cell comprising a. The method of claim 6, The sulfated cyclodextrin is a direct methanol fuel cell polymer, characterized in that any one or a mixture of two or more selected from the group consisting of sulfated alpha-cyclodextrin, sulfated beta-cyclodextrin and sulfated gamma-cyclodextrin Method for producing an electrolyte membrane. The method of claim 6, The ion-exchange membrane-forming polymer may be a perfluorosulfonic acid polymer, polyvinylidene fluoride, polyvinylidene fluoride hexafluoropropylene, polyphenylene oxide, polyimide, polyether ketone, polybenzimidazole, polysulfone polymer, Method for producing a polymer electrolyte membrane for a direct methanol fuel cell, characterized in that the polymer selected from the group consisting of polyvinyl alcohol and polystyrene. The method of claim 6, The content of the sulfated cyclodextrin in the prepared polymer electrolyte membrane is a method for producing a polymer electrolyte membrane for a direct methanol fuel cell, characterized in that less than 5% by weight relative to the total weight of the ion exchange membrane polymer matrix and sulfated cyclodextrin. The method of claim 6, The heat treatment in the step (S3) is a method for producing a polymer electrolyte membrane for a direct methanol fuel cell, characterized in that performed for 5 minutes to 3 days at 80 ~ 170 ℃. Electrolyte membrane; In the membrane electrode assembly for a direct methanol fuel cell comprising a; and formed between the electrolyte membrane, each comprising an anode and a cathode comprising a catalyst layer, The electrolyte membrane is a direct methanol fuel cell membrane electrode assembly, characterized in that the polymer electrolyte membrane for direct methanol fuel cell according to any one of claims 1 to 5. A stack comprising one or more membrane electrode assemblies according to claim 11; A fuel supply unit supplying fuel to the stack; And An oxidant supplier for supplying an oxidant to the stack; Direct methanol fuel cell comprising a.

Images