Electrolyte membrane for membrane-electrode assembly comprising self-assembled block copolyer

Abstract

The present invention relates to an electrolyte membrane for a membrane-electrode assembly comprising a block copolymer composed of a hydrophilic domain and a hydrophobic domain.

Description

Technology Field

[0001] The present invention relates to an electrolyte membrane for a membrane-electrode assembly comprising a block copolymer composed of a hydrophilic domain and a hydrophobic domain. Background Technology

[0002] A proton exchange membrane fuel cell (PEMFC) basically comprises an anode, which serves as the hydrogen fuel, a cathode, which supplies oxygen, and a polymer electrolyte membrane placed between the two electrodes; this configuration is referred to as a membrane-electrode assembly (MEA). In the reaction for electricity generation in the fuel cell, hydrogen supplied to the anode is separated into hydrogen ions (protons) and electrons. The hydrogen ions then move through the membrane toward the cathode, which is the reduction electrode, while the electrons move toward the cathode through an external circuit. At the cathode, oxygen molecules, hydrogen ions, and electrons react together to generate electricity and heat, while simultaneously producing water (H2O) as a reaction byproduct.

[0003] Here, the polymer electrolyte membrane plays the role of transferring hydrogen ions generated at the anode to the cathode and acts as a barrier to prevent hydrogen fuel from directly contacting oxygen. Electrolyte membranes typically composed of perfluorinated sulfonic acid ionomers (PFSA) are the most commonly used in the field of polymer electrolyte membrane fuel cells due to their high proton conductivity and high performance and stability under various humidification conditions. However, pure perfluorinated sulfonic acid ionomer membranes present many problems, such as susceptibility to thermal degradation at temperatures above 100°C and a rapid decrease in mechanical and dimensional stability due to their low proton conductivity. For these reasons, the operation of fuel cells utilizing conventional perfluorinated ionomer membranes is typically limited to a range below 100°C, preferably below 80°C. In addition, since hydrogen ion conduction depends on the exchange of hydrogen ions through sulfonic acid functional groups (-SO3H groups) in the presence of moisture, it is necessary to maintain the hydration level of the polymer electrolyte membrane at an optimal level.

[0004] Generally, hydrogen and oxygen from the air, which are the reaction gases of fuel cells, crossover through the electrolyte membrane to promote the generation of hydrogen peroxide (HOOH). This hydrogen peroxide generates highly reactive oxygen-containing radicals, such as hydroxyl radicals (·OH) and hydroperoxyl radicals (·OOH). These radicals attack the ionomers in the perfluorosulfonic acid-based electrolyte membranes and electrodes, causing chemical degradation of the membranes and electrodes and ultimately having an adverse effect that reduces the durability of the fuel cell.

[0005] Conventionally, methods involving the addition of various types of antioxidants have been proposed as techniques to mitigate such chemical degradation. These antioxidants include primary antioxidants with radical scavenger or quencher functions and secondary antioxidants with hydrogen peroxide decomposer functions, which can be used individually or in combination. In the general polyolefin-based plastics industry, primary antioxidants include phenolic antioxidants, monophenolic, bisphenolic, and polymeric phenolic antioxidants, as well as amine-based antioxidants. As secondary antioxidants for peroxide decomposition, sulfur-based and phosphorus-based antioxidants have been reported. For example, since polypropylene is more susceptible to oxidation than polyethylene, 0.1–1.0% of 2.6-di- tIt is known that using the phenolic antioxidant -Butyl-4-methylphenol (BHT) in combination with secondary antioxidants such as dilauryl thiodipropionate and distearyl thiodipropionate is practical.

[0006] Representative primary antioxidants used in perfluorosulfonic acid-based electrolyte membranes and ionomers for fuel cells include cerium-based (Cerium (III) Nitrate Hexahydrate) and cerium oxide (or Ceria). Additionally, secondary antioxidants include manganese-based materials such as manganese oxide and transition metal catalysts such as platinum (Pt).

[0007] However, if the above primary or secondary antioxidant is used in the form of a metal salt, metal ions bind to the sulfonic acid terminals of the perfluorosulfonic acid-based ionomer, blocking the pathway for hydrogen ion movement. Additionally, metals or metal oxides hinder the movement of hydrogen ions by blocking the hydrated microchannels of the electrolyte membrane with particles ranging in size from tens to hundreds of nanometers. Therefore, generally, the use of metal salts or metal-type antioxidants improves the chemical durability of the electrolyte membrane, but conversely, it can reduce the proton conductivity of the electrolyte membrane.

[0008] JP 4876407 invented an antioxidant for fuel cells other than the aforementioned metal or metal salt, utilizing an organic oxidation-reduction compound with a standard oxidation-reduction potential in the range of 0.68[V] to 1.00[V]. Representatively, the compound TEMPO ((2,2,6,6-Tetramethylpiperidin-1-yl)oxyl), which has a nitroxide radical (NO·) group, converts the hydroxyl radical into a hydroxide (OH) group as shown in [Reaction Scheme 1] below. - It is a primary and secondary complex organic antioxidant that acts as a primary antioxidant capable of being converted into ) and acts as a secondary antioxidant as a hydrogen peroxide decomposer as shown in [Reaction Scheme 2] below.

[0009] [Reaction Equation 1]

[0010]

[0011] [Reaction Equation 2]

[0012]

[0013] However, the above-mentioned organic redox compounds with low molecular weight have the disadvantage that they are not immobilized on the electrolyte membrane during fuel cell operation and can easily diffuse and elute through hydration channels. Prior art literature

[0014] JP 4876407 B2JP 5023475 B2JP 4910310 B2 The problem to be solved

[0015] The present invention aims to provide an additive that can improve durability while maintaining the performance of an electrolyte membrane.

[0016] The present invention aims to provide an additive capable of simultaneously improving the hydrogen ion conductivity and antioxidant properties of an electrolyte membrane.

[0017] The present invention aims to provide an additive that does not leach out of an electrolyte membrane and can maintain its function for a long time.

[0018] The objectives of the present invention are not limited to those mentioned above. The objectives of the present invention will become more apparent from the following description and will be realized by the means and combinations thereof described in the claims. means of solving the problem

[0019] The electrolyte membrane for a membrane-electrode assembly according to the present invention comprises an ionomer and an additive dispersed in the ionomer, and the additive may comprise a block copolymer comprising a hydrophilic domain and a hydrophobic domain.

[0020] The above hydrophilic domain may include a cationic conductive repeating unit.

[0021] The above cation-conducting repeating unit may include at least one of the repeating units represented by the following chemical formulas 1-1 to 1-5.

[0022] [Chemical Formula 1-1]

[0023]

[0024] [Chemical Formula 1-2]

[0025]

[0026] [Chemical Formula 1-3]

[0027]

[0028] [Chemical Formula 1-4]

[0029]

[0030] [Chemical Formula 1-5]

[0031]

[0032] The above hydrophobic domain may include an antioxidant repeating unit.

[0033] The above antioxidant repeating unit may include at least one of the repeating units represented by the following chemical formulas 2-1 to 2-10.

[0034] [Chemical Formula 2-1]

[0035]

[0036] [Chemical Formula 2-2]

[0037]

[0038] [Chemical Formula 2-3]

[0039]

[0040] [Chemical Formula 2-4]

[0041]

[0042] [Chemical Formula 2-5]

[0043]

[0044] [Chemical Formula 2-6]

[0045]

[0046] [Chemical Formula 2-7]

[0047]

[0048] [Chemical Formula 2-8]

[0049]

[0050] [Chemical Formula 2-9]

[0051]

[0052] [Chemical Formula 2-10]

[0053]

[0054] The above block copolymer may have a ratio (n : m) of the number of repeating units (n) of the hydrophilic domain and the number of repeating units (m) of the hydrophobic domain of 20 : 80 to 70 : 30.

[0055] The above block copolymer may have a number average molecular weight (Mn) of 25,000 or less.

[0056] The block copolymer may be in the form of a micelle comprising a core portion and a shell portion surrounding the core portion, wherein the core portion may include a hydrophobic domain and the shell portion may include a hydrophilic domain.

[0057] The above block copolymer may have a particle radius of 4 nm to 6 nm.

[0058] The above electrolyte membrane may contain 1 to 10 parts by weight of the additive based on 100 parts by weight of the ionomer.

[0059] A membrane-electrode assembly according to one embodiment of the present invention may include the electrolyte membrane and a pair of electrodes located on both sides of the electrolyte membrane.

[0060] The above membrane-electrode assembly can be used in fuel cells and / or water electrolysis devices. Effects of the invention

[0061] Using the additive according to the present invention can simultaneously improve the hydrogen ion conductivity and antioxidant properties of the electrolyte membrane.

[0062] The additive according to the present invention can maintain its function for a long time without leaching out from the electrolyte membrane.

[0063] The effects of the present invention are not limited to those mentioned above. It should be understood that the effects of the present invention include all effects that can be inferred from the following description. Brief explanation of the drawing

[0064] Figure 1 illustrates a block copolymer according to the present invention. Figure 2 illustrates a state in which a block copolymer according to the present invention is self-assembled in the form of micelles. Figure 3 shows the results of the antioxidant evaluation according to Experimental Example 2. Specific details for implementing the invention

[0065] The above objects, other objects, features, and advantages of the present invention will be easily understood through the following preferred embodiments associated with the accompanying drawings. However, the present invention is not limited to the embodiments described herein and may be embodied in other forms. Rather, the embodiments introduced herein are provided to ensure that the disclosed content is thorough and complete and to ensure that the spirit of the invention is sufficiently conveyed to a person skilled in the art.

[0066] In describing each drawing, similar reference numerals have been used for similar components. In the attached drawings, the dimensions of the structures are depicted enlarged from their actual size for clarity of the invention. Terms such as "first," "second," etc., may be used to describe various components, but said components should not be limited by said terms. These terms are used solely for the purpose of distinguishing one component from another. For example, without departing from the scope of the invention, the first component may be named the second component, and similarly, the second component may be named the first component. A singular expression includes a plural expression unless the context clearly indicates otherwise.

[0067] In this specification, terms such as "comprising" or "having" are intended to specify the existence of the features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof. Furthermore, when a part such as a layer, film, region, or plate is described as being "on" another part, this includes not only the case where it is "immediately above" the other part, but also the case where there is another part in between. Conversely, when a part such as a layer, film, region, or plate is described as being "below" another part, this includes not only the case where it is "immediately below" the other part, but also the case where there is another part in between.

[0068] Unless otherwise specified, all numbers, values, and / or expressions used herein to represent amounts of ingredients, reaction conditions, polymer compositions, and formulations should be understood to be modified by the term “approximately” in all cases, as these numbers are essentially approximations reflecting the various uncertainties of measurement that occur in obtaining these values ​​among other things. Furthermore, where numerical ranges are disclosed herein, such ranges are continuous and, unless otherwise indicated, include all values ​​from the minimum value of such range to the maximum value including said maximum value. Moreover, where such ranges refer to integers, they include all integers from the minimum value to said maximum value including said maximum value, unless otherwise indicated.

[0070] The electrolyte membrane for a membrane-electrode assembly according to the present invention comprises an ionomer and an additive dispersed in the ionomer.

[0071] The above ionomer is a component that serves to transport hydrogen ions within the electrolyte membrane.

[0072] The above ionomer may include a perfluorosulfonic acid-based polymer having a functional group capable of transferring hydrogen ions, such as Nafion.

[0073] The above additive may include a block copolymer as shown in FIG. 1. The block copolymer may include a hydrophilic domain (A) and a hydrophobic domain (B).

[0074] The above hydrophilic domain (A) may include a cation-conducting repeating unit.

[0075] The above-mentioned cation-conducting repeating unit is a repeating unit comprising a functional group such as a sulfonic acid group capable of transferring hydrogen ions, and may include at least one of the repeating units represented by the following chemical formulas 1-1 to 1-5.

[0076] [Chemical Formula 1-1]

[0077]

[0078] [Chemical Formula 1-2]

[0079]

[0080] [Chemical Formula 1-3]

[0081]

[0082] [Chemical Formula 1-4]

[0083]

[0084] [Chemical Formula 1-5]

[0085]

[0086] The block copolymer containing the above hydrophilic domain (A) provides a new pathway for hydrogen ions other than the ionomer within the electrolyte membrane, so the hydrogen ion conductivity of the electrolyte membrane is greatly improved.

[0087] The above hydrophobic domain (B) may include an antioxidant repeating unit.

[0088] The above antioxidant repeating unit may have a partial structure capable of converting hydroxyl radicals into hydroxide or decomposing hydrogen peroxide through the reaction pathways of Reaction Scheme 1 and Reaction Scheme 2 below.

[0089] [Reaction Equation 1]

[0090]

[0091] [Reaction Equation 2]

[0092]

[0093] The above antioxidant repeating unit may include at least one of the repeating units represented by the following chemical formulas 2-1 to 2-10.

[0094] [Chemical Formula 2-1]

[0095]

[0096] [Chemical Formula 2-2]

[0097]

[0098] [Chemical Formula 2-3]

[0099]

[0100] [Chemical Formula 2-4]

[0101]

[0102] [Chemical Formula 2-5]

[0103]

[0104] [Chemical Formula 2-6]

[0105]

[0106] [Chemical Formula 2-7]

[0107]

[0108] [Chemical Formula 2-8]

[0109]

[0110] [Chemical Formula 2-9]

[0111]

[0112] [Chemical Formula 2-10]

[0113]

[0114] The above block copolymer may have a ratio (n : m) of the number of repeating units (n) of the hydrophilic domain to the number of repeating units (m) of the hydrophobic domain of 20 : 80 to 70 : 30. If the ratio of the number of repeating units (m) of the hydrophobic domain exceeds 80, the particle radius of the block copolymer becomes too large, and the hydrogen ion conductivity may not be improved.

[0115] In addition, the number average molecular weight (Mn) of the block copolymer may be 25,000 or less, 10,000 or less, or 8,000 or less. The lower limit of the number average molecular weight (Mn) is not specifically limited. If the number average molecular weight of the block copolymer exceeds 25,000, the particle radius of the block copolymer becomes too large, and the hydrogen ion conductivity may not be improved.

[0116] The above electrolyte membrane exists in a humidified state, and since the block copolymer contains both hydrophilic and hydrophobic domains within a single molecule, it is self-assembled within the electrolyte membrane to form a micelle shape including a core part (10) and a shell part (20) surrounding the core part (10) as shown in FIG. 2.

[0117] The above block copolymer may have a particle radius of 4 nm to 6 nm. In this specification, "particle radius" refers to the straight-line distance from the center point of a micelle to the surface of a shell portion when the block copolymer is self-assembled in a micelle form. Additionally, the particle radius refers to the particle radius when the block copolymer is in a hydrated state. According to the Cluster-network model, which is the micromolecular structure of hydrated Nafion, sulfonic acid groups (-SO3 -The absorbed water of the block copolymer forms spherical clusters with a diameter of about 4 nm, and the movement path of hydrogen ions is known to be a narrow channel 1 nm wide connecting continuous clusters. Therefore, to increase hydrogen ion conductivity, it is preferable that the particle radius of the block copolymer be 4 nm to 6 nm, or 4 nm to 5 nm.

[0118] The above electrolyte membrane may contain 1 to 10 parts by weight of the additive based on 100 parts by weight of ionomer. If the content of the additive is less than 1 part by weight, the degree of improvement in hydrogen ion conductivity and antioxidant properties is negligible, and if it exceeds 10 parts by weight, the amount is excessive and may actually reduce the hydrogen ion conductivity of the electrolyte membrane.

[0120] Other forms of the present invention will be described in more detail through the following examples. The following examples are merely illustrative to aid in understanding the present invention and do not limit the scope of the present invention.

[0122] Preparation Examples 1 to 3

[0123] A block copolymer was prepared in the following manner.

[0124] 2,2,6,6-Tetramethyl-4-piperidinyl methacrylate, represented by the following chemical formula 3, was used as the monomer of the hydrophobic domain. For convenience of explanation, this is referred to as the hydrophobic monomer.

[0125] [Chemical Formula 3]

[0126]

[0127] Sodium 4-vinylbenzenesulfonate, represented by the following chemical formula 4, was used as the monomer of the hydrophilic domain. For convenience of explanation, this is referred to as the hydrophilic monomer.

[0128] [Chemical Formula 4]

[0129]

[0130] The block copolymer was synthesized using the reversible addition-fragmentation chain transfer (RAFT) method as described below.

[0131] First, 10 g (0.04 moles) of hydrophobic monomer, 0.146 g (0.8 moles) of 2,2'-Azobis(2-methylpropionitrile) (AIBN), and 1.117 g (0.01 moles) of 4-Cyano-4-(phenylcarbonothioylthio)pentanoic acid were added to 20 mL of anhydrous toluene, and after removing dissolved oxygen, argon purging was performed. After reacting at 55–75°C for 5 hours, the polymerization was terminated by cooling. The reaction mixture was precipitated in hexane solvent, and the precipitate was obtained by centrifugation. The precipitate was then dried in a vacuum oven for one day to obtain an intermediate as shown in Chemical Formula 5 below.

[0132] [Chemical Formula 5]

[0133]

[0134] 0.02 moles of the above intermediate, the above hydrophilic monomer, and 0.146 g (0.8 moles) of AIBN were added to 10 ml of a mixed solvent of water and methanol. At this time, samples were prepared by adjusting the amount of the above hydrophilic monomer added to 0.01 moles (Preparation Example 1), 0.02 moles (Preparation Example 2), and 0.04 moles (Preparation Example 3), respectively.

[0135] After reacting each sample at 55–75°C for 5 hours, the polymerization was terminated by cooling. The reaction mixture was precipitated in hexane solvent, the precipitate was obtained by centrifugation, and the copolymer was obtained by drying in a vacuum oven for one day.

[0136] 5 g of the copolymer and 17.25 g (0.1 moles) of meta-Chloroperoxybenzoic acid (mCPBA) were added to 50 mL of dichloromethane, and the copolymer was oxidized by stirring at room temperature for 12 hours. The reaction mixture was precipitated in hexane solvent, a precipitate was obtained by centrifugation, and the precipitate was dried in a vacuum oven for one day to obtain a block copolymer according to the present invention represented by the following chemical formula 6.

[0137] [Chemical Formula 6]

[0138]

[0139] The physical properties of the block copolymers of Preparation Example 1, Preparation Example 2, and Preparation Example 3 were measured. The results are shown in Table 1 below.

[0140] division m / n 1) Number average molecular weight 2 ) Particle radius [nm] Preparation Example 1 43 / 57 6,000 2.3 Preparation Example 2 31 / 69 8,000 3.4 Preparation Example 3 73 / 27 23,000 5.2

[0141] 1) The ratio (m / n) of the number of repeating units of the hydrophilic domain (n) and the number of repeating units of the hydrophobic domain (m), 1 Measured by H-NMR

[0142] 2) Measured by DOSY-NMR

[0143] 3) Measured using Dynamic Light Scattering (DLS)

[0145] Examples 1 to 4 and Comparative Example

[0146] A Nafion solution was prepared. Based on 100 parts by weight of Nafion (ionomer) contained in the above Nafion solution, the block copolymer of Preparation Example 1 was added in amounts of 1 part by weight (Example 1), 3 parts by weight (Example 2), 5 parts by weight (Example 3), and 10 parts by weight (Example 4), respectively, to prepare a mixture.

[0147] Each mixture was applied onto a release liner, and an electrolyte membrane was prepared by drying and heat treatment.

[0148] An electrolyte membrane was prepared using only Nafion solution without the addition of block copolymers and was set as a comparative example.

[0150] Experimental Example 1 - Measurement of Hydrogen Ion Conductivity

[0151] The hydrogen ion conductivity of the electrolyte membranes according to Examples 1 to 4 and Comparative Example was measured in-plane at 80°C and 50% relative humidity. The results are shown in Table 2 below.

[0152] division Block copolymer content Electrolyte membrane thickness Hydrogen ion conductivity Comparative example 0 parts by weight 28㎛ 45.1 mS / cm Example 1 1 part by weight 24㎛ 46.5 mS / cm Example 2 3 parts by weight 28㎛ 48.6 mS / cm Example 3 5 parts by weight 26㎛ 51.4 mS / cm Example 4 10 parts by weight 31㎛ 38.2 mS / cm

[0153] Referring to Table 2, Example 3 showed the highest hydrogen ion conductivity, which is an increase of about 6 mS / cm compared to the comparative example.

[0155] Experimental Example 2 - Evaluation of Antioxidant Properties

[0156] Antioxidant properties were evaluated by measuring the change in fluoride ion discharge over time of the electrolyte membranes according to Examples 1 to 4 and Comparative Example. The results are shown in Fig. 3.

[0157] Referring to this, Examples 1 to 4 showed significantly lower fluoride ion emission rates compared to the comparative example, and through this, it can be seen that the chemical durability of the electrolyte membrane can be greatly improved by adding the block copolymer according to the present invention as an additive.

[0159] As the experimental examples and embodiments of the present invention have been described in detail above, the scope of the present invention is not limited to the aforementioned experimental examples and embodiments, and various modifications and improvements by those skilled in the art using the basic concept of the present invention as defined in the following claims are also included within the scope of the present invention. Explanation of the symbols

[0160] 10: Center 20: Shell

Claims

Claim 1 An electrolyte membrane for a membrane-electrode assembly comprising: an ionomer; and an additive dispersed in the ionomer, wherein the additive comprises a block copolymer having a hydrophilic domain and a hydrophobic domain, wherein the hydrophobic domain comprises an antioxidant repeating unit, and the antioxidant repeating unit comprises a nitrooxide radical. Claim 2 An electrolyte membrane for a membrane-electrode assembly according to claim 1, wherein the hydrophilic domain comprises a cation-conducting repeating unit. Claim 3 An electrolyte membrane for a membrane-electrode assembly according to claim 2, wherein the cation-conducting repeating unit comprises at least one of the repeating units represented by the following Chemical Formulas 1-1 to 1-5. [Chemical Formula 1-1] [Chemical Formula 1-2] [Chemical Formula 1-3] [Chemical Formula 1-4] [Chemical Formula 1-5] Claim 4 delete Claim 5 An electrolyte membrane for a membrane-electrode assembly according to claim 1, wherein the antioxidant repeating unit comprises at least one of the repeating units represented by the following chemical formulas 2-1 to 2-10. [Chemical Formula 2-1] [Chemical Formula 2-2] [Chemical Formula 2-3] [Chemical Formula 2-4] [Chemical Formula 2-5] [Chemical Formula 2-6] [Chemical Formula 2-7] [Chemical Formula 2-8] [Chemical Formula 2-9] [Chemical Formula 2-10] Claim 6 An electrolyte membrane for a membrane-electrode assembly according to claim 1, wherein the block copolymer has a ratio (n : m) of the number of repeating units of the hydrophilic domain (n) to the number of repeating units of the hydrophobic domain (m) of 20 : 80 to 70 :

30. Claim 7 In claim 1, the block copolymer is an electrolyte membrane for a membrane-electrode assembly having a number average molecular weight (Mn) of 25,000 or less. Claim 8 An electrolyte membrane for a membrane-electrode assembly according to claim 1, wherein the block copolymer is in the form of a micelle comprising a core portion and a shell portion surrounding the core portion, wherein the core portion comprises a hydrophobic domain and the shell portion comprises a hydrophilic domain. Claim 9 In claim 1, the block copolymer is an electrolyte membrane for a membrane-electrode assembly having a particle radius of 4 nm to 6 nm. Claim 10 An electrolyte membrane for a membrane-electrode assembly according to claim 1, comprising 1 to 10 parts by weight of the additive based on 100 parts by weight of the ionomer. Claim 11 An electrolyte membrane according to any one of claims 1 to 3 and claims 5 to 10; and a membrane-electrode assembly comprising a pair of electrodes located on both sides of the electrolyte membrane. Claim 12 A fuel cell comprising the membrane-electrode assembly of claim 11. Claim 13 A water electrolysis device comprising the membrane-electrode assembly of claim 11.

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