KR100696460B1 - Proton-conducting polymer - Google Patents

Abstract

The present invention provides a fuel cell employing a heat-resistant aromatic hydrocarbon-based polymer having improved hydrogen ion conductivity, a heat-resistant aromatic hydrocarbon-based polymer electrolyte membrane having improved hydrogen ion conductivity, and a heat-resistant aromatic hydrocarbon-based polymer electrolyte membrane having improved hydrogen ion conductivity. The fluorinated alkylsulfonated polystyrene derivative of the present invention has superior ion conductivity as compared with the sulfonated polystyrene derivative. Therefore, the hydrogen ion conductive polymer of the present invention can be usefully used as an electrolyte membrane for fuel cells.

Description

Proton-conducting polymer

The present invention relates to a fuel cell, and more particularly, to an electrolyte membrane for a fuel cell, and more particularly to a hydrogen ion conductive polymer used in the electrolyte membrane for a fuel cell.

A fuel cell is a device that generates electric energy by electrochemically reacting fuel and oxygen. Unlike thermal power generation, a fuel cell does not undergo a Carno cycle, and thus its theoretical power generation efficiency is very high. Fuel cells can be applied not only to the supply of power for industrial, domestic and vehicle driving, but also to the power supply of small electrical / electronic products, in particular portable electrical / electronic devices.

The operating mechanism of a fuel cell begins by oxidizing fuels such as hydrogen, natural gas, methanol, and the like at the anode to produce electrons and hydrogen ions. Hydrogen ions produced at the anode move to the cathode through the electrolyte membrane, and electrons generated at the anode are supplied to an external circuit through a conductive wire. The hydrogen ions that reach the cathode combine with the electrons that reach the cathode through an external circuit and oxygen or oxygen in the air to generate water.                         

The fuel cell may be a polymer electrolyte membrane fuel cell (PEMFC), a phosphoric acid fuel cell (PAFC), or a molten carbonate fuel cell (MCFC) depending on the type of electrolyte used. And solid oxide fuel cells (SOFC). Depending on the type of fuel cell, the operating temperature, the material of the component, and the like vary.

In addition, according to the fuel supply method to the anode, the fuel cell converts the fuel into hydrogen-enriched gas through a fuel reformer and supplies the anode to the anode, and the fuel directly supplies the gas or liquid fuel directly to the anode. It can be divided into supply type or internal reforming type.

A representative example of a direct fuel supply type is a direct methanol fuel cell (DMFC). In DMFC, an aqueous methanol solution or a mixed vapor of methanol and water is generally supplied to the anode. Since DMFC does not require an external reformer and is easy to handle fuel, it is known that DMFC has the highest possibility of miniaturization among various kinds of fuel cells.

The electrochemical reaction process of DMFC consists of an anode reaction in which fuel is oxidized and a cathode reaction by reduction of hydrogen ions and oxygen.

Anode Reaction: CH ₃ OH ^{_{+ H 2 O → 6 H +}} + 6 e - + CO 2

The cathode ^{_{reaction: 1.5 O 2 + 6 H +}} + 6 e - → 3 H 2 O

Total reaction: CH ₃ OH + 1.5 O ₂ → 2 H ₂ O + CO ₂

As shown in the scheme, the anode reacts with methanol to produce carbon dioxide, six hydrogen ions and six electrons. The generated hydrogen ions move to the cathode using a hydrogen ion conductive electrolyte membrane positioned between the anode and the cathode as a medium. In the cathode, hydrogen ions, electrons and oxygen delivered through an external circuit react to form water. The overall reaction of DMFC is the reaction of methanol and oxygen to produce water and carbon dioxide. In this process, much of the energy corresponding to the heat of combustion of methanol is converted into electrical energy. In order to promote this reaction, the anode and cathode contain a catalyst.

The hydrogen ion conductive electrolyte membrane not only serves as a passage for the hydrogen ions generated by the oxidation reaction at the anode to move to the cathode, but also serves as a separator that separates the anode and the cathode.

In PEMFC and DMFC, a hydrogen ion conductive polymer film is mainly used as a hydrogen ion conductive electrolyte film. In general, hydrogen ion conductive polymer membranes are hydrophilic and exhibit ion conductivity by impregnating an appropriate amount of water.

The hydrogen ion conductive polymer for the electrolyte membrane of a fuel cell is a fluorinated polymer having fluorinated alkylene in the main chain, and partially having a cation exchange group such as a sulfonic acid group, a carboxylic acid group, etc. at the terminal of the fluorinated vinyl ether side chain. polymers) are mainly used. Specific examples of the electrolyte membrane made of such a fluorine-based polymer include a Nafion membrane (product name of DuPont), a Dow membrane (product name of Dow Corporation), an aciplex membrane (product name of Asahi Chemical), and a plemion membrane (Flemion). : Asahi Glass's product name).

Such fluorine-based polymer electrolyte membranes typically operate in a relatively low temperature range, for example, in the range of room temperature to 80 ° C., because such fluorine-based polymers have a glass transition temperature of about 130 ° C. This is because at an excessive temperature, the ion channel structure involved in ion conductivity is destroyed.

Fuel cells using such fluorine-based polymer electrolyte membranes have problems such as low power generation efficiency, poisoning of electrode catalysts by carbon monoxide, etc. resulting from operating in such a low temperature range, and in the case of DMFC, moreover In addition, methanol used as a fuel penetrates such a fluorine-based polymer electrolyte membrane and degrades the performance of the fuel cell.

As one approach to solve this problem, various hydrogen ion conductive polymers that can replace the conventional hydrogen ion conductive fluorine-based polymer have been developed. Representative examples thereof include hydrogen ion conductive polymers based on heat-resistant aromatic hydrocarbon-based polymers such as polystyrene, polybenzimidazole, polyether sulfone, polyether ether ketone and the like.

US 6,110,616 discloses hydrogenated and sulfonated styrene-butadiene copolymers. US Pat. No. 6,087,031 discloses sulfonated polysulfones.                         

However, these hydrocarbon-based polymers have low fuel permeability but at the same time have low ion conductivity. This is due to the fact that the cation exchange groups present in the polymer are benzenesulfonic acids having significantly lower acidity than the fluorinated alkylsulfonic acids used in conventional fluorine-based hydrogen ion conductive polymers such as Nafion. Thus, the ionic conductivity at room temperature of such a heat-resistant aromatic hydrocarbon-based polymer membrane is about 10% to about 50% of the ionic conductivity at room temperature of the Nafion membrane of DuPont, a representative fluorine-based hydrogen ion conductive polymer membrane. Is nothing.

The low ion conductivity of such a heat resistant aromatic hydrocarbon polymer electrolyte membrane is a major obstacle to the practical use of the heat resistant aromatic hydrocarbon polymer membrane as an electrolyte membrane.

The present invention provides a heat resistant aromatic hydrocarbon polymer having improved hydrogen ion conductivity.

The present invention provides a heat resistant aromatic hydrocarbon-based polymer electrolyte membrane having improved hydrogen ion conductivity.

The present invention provides a fuel cell employing a heat resistant aromatic hydrocarbon-based polymer electrolyte membrane having improved hydrogen ion conductivity.

The hydrogen ion conductive polymer provided by the present invention includes a repeating unit represented by the formula (1).                     

<Formula 1>

In Formula 1, R _f is a linear or branched fluorinated alkylene having 1 to 10 carbon atoms, and X is a hydrogen atom or an alkali metal.

Another embodiment of the hydrogen ion conductive polymer provided by the present invention may include a repeating unit represented by Formula 1 and a repeating unit represented by Formula 2, wherein the number of repeating units represented by Formula 1 and Formula 2 The sum of the number of repeating units represented is about 1,000 to about 10,000, and the number of repeating units represented by Formula 1 is about 10% to about the sum of the number of repeating units represented by Formula 1 and the number of repeating units represented by Formula 2 99%.

<Formula 1>

<Formula 2>

In Formula 1, R _f is a linear or branched fluorinated alkylene having 1 to 10 carbon atoms, X is a hydrogen atom or an alkali metal, and in Formula 2, R is an aromatic group including a benzene group.

The hydrogen ion conductive electrolyte membrane provided by the present invention includes the above hydrogen ion conductive polymer according to the present invention.

The fuel cell provided by the present invention includes a cathode in which a reduction reaction of an oxidant occurs; An anode in which oxidation of the fuel occurs; And a hydrogen ion conductive electrolyte membrane according to the present invention located between the cathode and the anode.

In theory, the ion conductivity of the polymer electrolyte membrane is expressed as follows.

σ = D _σ × n × q ² / (T × k)

Where σ is the ion conductivity, D _σ is the diffusion coefficient, n is the concentration of the ion carrier, q is the charge amount, T is the temperature, and k is the Boltzmann constant.

As can be seen from the above formula, the ion conductivity of the polymer electrolyte membrane is proportional to the concentration n of the ion carrier present in the polymer electrolyte membrane. Thus, the hydrogen ion conductivity is proportional to the concentration of the cation transporter, ie, the cation exchange site, in the polymer electrolyte membrane.

Increasing the concentration of the cation carrier in the polymer electrolyte membrane can be achieved by increasing the number of acid groups present in the polymer constituting the polymer electrolyte membrane, and / or increasing the dissociation (or acidity) of the acid groups.

Since increasing the number of acid groups in the polymer leads to a decrease in the mechanical properties of the polymer, the increase in the number of acid groups in the polymer has a certain limit. Thus, the binding of acid groups with high dissociation to the polymer can result in even more advantageous results.

The sulfonic acid group has an excellent acidity superior to that of the phosphoric acid group, the carboxylic acid group and the like in the aqueous solution. The general formula of sulfonic acid is -SO ₃ H. There is also a slight sulfonic acid, but most are eutechonic acids. The sulfonic acid group is represented by -R-SO ₃ H, wherein when R is an alkyl group, it is called an aliphatic sulfonic acid, and when R is an aryl group, it is called an aromatic sulfonic acid.

The acidity of such sulfonic acid groups is known to be greatly affected by R bonded to -SO ₃ H. That is, the higher the electron affinity of R, the higher the acidity (or dissociation degree) of the sulfonic acid group. For example, the acidity (pKa) value of benzenesulfonic acid is only 0.7, whereas in the case of fluorinated methylsulfonic acid, it has a very high acidity of about -14.

The inventor of the present invention combines a fluorinated alkylsulfonic acid group having a very high acidity to a polystyrene polymer to obtain a polymer having excellent hydrogen ion conductivity.

The hydrogen ion conductive polymer provided by the present invention includes a repeating unit represented by the formula (1).

Another embodiment of the hydrogen ion conductive polymer provided by the present invention includes a repeating unit represented by Formula 1 and a repeating unit represented by Formula 2.

<Formula 1>

<Formula 2>

In Formula 1, R _f is a linear or branched fluorinated alkylene having 1 to 10 carbon atoms, X is a hydrogen atom or an alkali metal, and in Formula 2, R is an aromatic group including a benzene group.

There is no particular limitation on the sequence of the repeating unit represented by the formula (1) and the repeating unit represented by the formula (2).

However, the sum of the number (m) of the repeating units represented by the formula (1) and the number n of the repeating units represented by the formula (2) is preferably about 1,000 to about 10,000. The reason is that if the value of m + n is too small, mechanical properties such as thin film formation may be lowered. If the value of m + n is too large, workability may be reduced.

In addition, the number (m) of the repeating units represented by the formula (1) is preferably about 10% to about 99% of the sum of the number of repeating units (m) represented by the formula (1) and the number of repeating units (n) represented by the formula (2) Do. The reason is that if the value of m / (m + n) is too small, the ratio of acid groups may be excessively reduced and the ion conductivity may be lowered. If the value of m / (m + n) is too large, the mechanical strength is excessively large. This is because the polymer film can be easily chipped off.

In the fluorinated alkylsulfonic acid group bonded to the benzene ring of the repeating unit represented by Formula 1, that is, -R _f -SO ₃ X, the R _f is a linear or branched fluorinated alkylene having 1 to 10 carbon atoms. desirable. The reason is that if the said R _f having a carbon number of less than 1, i.e. 0, the sulfonic acid group due to indicate a very low acidity be to take the form of sulfonic acid, that of said R _f having a carbon number greater than 10, the hydrophobic group as compared to the group This is because the ratio of is so high that the ion conductivity may be lowered.

Representative examples of the carbon skeleton of R _f are listed in Tables 1 to 4 according to the number of carbon atoms. However, the carbon skeleton of R _f is not limited to those listed in Tables 1-4.

Carbon skeleton of C 1 R _f -R _f -designation Chemical Formula of -R _f -SO ₃ X  Methylene -C- (SO ₃ X)

Carbon skeleton of R _{f with} 2 carbon atoms -R _f -designation Chemical Formula of -R _f -SO ₃ X  Ethylene -CC- (SO ₃ X)  iso-ethylene

Carbon skeleton of R _{f with} 3 carbon atoms -R _f -designation Chemical Formula of -R _f -SO ₃ X  N-propylene

 1-iso-propylene

 2-iso-propylene

 tert-propylene

Carbon skeleton of R _{f with} 4 carbon atoms -R _f -designation Chemical Formula of -R _f -SO ₃ X  N-butylene

 1-iso-butylene

 2-iso-butylene

 3-iso-butylene

 1,2-iso-butylene

 1-tert-butylene

 2-tert-butylene

In the above R _f , "fluorinated" means that at least a part of the hydrogen atoms bonded to the carbon skeleton of the alkylene constituting R _f is substituted with fluorine. Is defined as fluorine-substituted in a ratio of the number of fluorine atoms bonded to the carbon skeleton of the R _f of the sum of hydrogen atoms and fluorine atoms bonded to the carbon skeleton of the number of the R _f R _f FIG. If the degree of fluorine substitution of R _f is too small, the acidity of the sulfonic acid group may decrease. On the other hand, the greater the degree of fluorine substitution of R _f , the higher the acidity of the sulfonic acid group. In view of this point, the number of fluorine atom bonded to the carbon skeleton of said R _f is preferably in the R _f of the carbon skeleton agreed about 10% to about 100% of the hydrogen atom and fluorine atom number bound to. The degree of fluorine substitution of R _f can be up to 100%, in which case R _f is perfluorinated alkylene.

The hydrogen ion conductive polymer of the present invention can be prepared, for example, in the following manner. After dissolving the hydrocarbon polymer in an organic solvent, introducing a halogenated alkyl group into the hydrocarbon polymer through a reaction such as chloromethylation, and then reacting a mixture of perfluorosultone and silver fluoride with the halogenated alkylated hydrocarbon polymer, Hydrocarbon polymer derivatives can be obtained.

The hydrogen ion conductive electrolyte membrane provided by the present invention includes the hydrogen ion conductive polymer according to the present invention described above. The electrolyte membrane of the present invention can be produced using a conventional polymer film production method such as, for example, spin coating, solvent casting, or the like.

The thickness of the electrolyte membrane of the present invention is not particularly limited. However, if the thickness is too thin, sufficient mechanical strength to be used as the electrolyte membrane of the fuel cell may not be guaranteed, and if the thickness is too thick, the internal resistance of the fuel cell may be excessively increased. In view of this point, the electrolyte membrane of the present invention may have a thickness of, for example, about 10 μm to about 300 μm.

The hydrogen ion conductive electrolyte membrane of the present invention can be used for all types of fuel cells using an electrolyte membrane including a polymer electrolyte, for example, PEMFC, a mixed vapor of methanol and water or a methanol aqueous solution supplied with hydrogen gas or hydrogen enrichment gas to the anode. It can be applied to a DMFC or the like supplied to the anode.

The fuel cell provided by the present invention includes a cathode in which a reduction reaction of an oxidant occurs; An anode in which oxidation of the fuel occurs; And an electrolyte membrane positioned between the anode and the cathode, wherein the electrolyte membrane is a hydrogen ion conductive electrolyte membrane according to the present invention described above.

The cathode includes a catalyst layer for promoting the reduction of oxygen. The catalyst layer comprises a polymer having catalyst particles and a cation exchange group. As the catalyst, for example, a carbon supported platinum catalyst (Pt / C catalyst) can be used.

The anode includes a catalyst layer for promoting oxidation of fuels such as hydrogen, methanol and the like. The catalyst layer comprises a polymer having catalyst particles and a cation exchange group. Specific examples of the catalyst include a carbon supported platinum catalyst and a carbon supported platinum-ruthenium catalyst (Pt-Ru / C). In particular, the carbon-supported platinum-ruthenium catalyst is useful when directly supplying organic fuel other than hydrogen to the anode.

The catalyst used in the cathode and the anode may be a catalyst metal particle itself or a supported catalyst including catalyst metal particles and a catalyst carrier. In the case of the supported catalyst, as the catalyst carrier, for example, solid particles having micropores capable of supporting catalytic metal particles, such as carbon powder, may be used. Examples of the carbon powder include carbon black, ketjen black, acetylene black, activated carbon powder, carbon nanofiber powder, or a mixture thereof. As the polymer having a cation exchange group, the polymer described above can be used.

The catalyst layer of the cathode and the anode is in contact with the electrolyte membrane.

The cathode and the anode may further include a gas diffusion layer in addition to the catalyst layer. The gas diffusion layer comprises a porous material having electrical conductivity. The gas diffusion layer acts as a current collector and as a gateway for reactants and products. The gas diffusion layer may be, for example, carbon paper, more preferably water repellent carbon paper, and even more preferably water repellent carbon paper coated with a water repellent carbon black layer. The water repellent treated carbon paper contains a hydrophobic polymer such as PTFE (polytetrafluoroethylene), and the hydrophobic polymer is sintered. The water repellent treatment of the gas diffusion layer is for securing access passages for the polar liquid reactant and the gas reactant at the same time. A water repellent treated carbon paper having a water repellent treated carbon black layer, wherein the water repellent treated carbon black layer comprises carbon black; And a hydrophobic polymer such as PTFE as a hydrophobic binder, and is attached to one surface of the water repellent treated carbon paper as described above. The hydrophobic polymer of the water repellent treated carbon black layer is sintered.

The preparation of the cathode and the anode can be made in a variety of ways known in the literature, and thus will not be described in detail here.                     

Another aspect of the fuel cell provided by the present invention, the cathode comprising a catalyst layer for the reduction reaction of the oxidant; An anode comprising a catalyst layer in which oxidation of the fuel occurs; And an electrolyte membrane positioned between the anode and the cathode, wherein the catalyst layer of the cathode and the catalyst layer of the anode comprise a hydrogen ion conductive polymer according to the present invention as described above.

As described above, the catalyst layer of the cathode and the anode generally comprises a polymer having catalyst particles and a cation exchange group. At this time, the polymer having a cation exchange group not only serves as a binder to ensure the mechanical strength of the catalyst layer, but also serves as a transfer passage of hydrogen ions in the catalyst layer. Since the hydrogen ion conductive polymer according to the present invention has excellent ion conductivity, it may be usefully used as a polymer having a cation exchange group of the catalyst layer of the cathode and the catalyst layer of the anode.

Hereinafter, an example of a method of manufacturing the hydrogen ion conductive polymer of the present invention will be described in detail through Examples.

<Example>

2.0 g of polystyrene (from Sigma Aldrich) and 100 mL of 1,4-dioxane were charged into a 250 mL two-necked flask and stirred until a clear solution formed. Then, 2.5 mL of 37% by weight aqueous hydrochloric acid solution and 2.5% by weight of formaldehyde aqueous solution were added to the solution, followed by stirring at a temperature of about 60 ° C. for about 8 hours. The solution thus obtained was precipitated in an aqueous methanol solution (volume ratio of methanol to water 1: 1) to give a white solid. This white solid is chloromethylated polystyrene.                     

Into a 250 mL three-necked flask equipped with a stirrer, 2.1 g of silver fluoride and 50 mL of anhydrous diglyme were added, cooled to about -78 ° C, and then sulfonated fluoride (1,2,2). 2.6 g of -trifluoro-2-hydroxy-1-trifluoromethyl ethanesulfonic acid sultone) were added. After standing for about 1 hour, the solution in the three-necked flask turned into a clear liquid was cooled again to about -78 ℃ to prepare a fluorinated sulfonated solution.

The solution obtained by dissolving 3 g of chloromethylated polystyrene in 50 ml of anhydrous diglyme was slowly added to 50 ml of fluorinated sulfonated solution, and then reacted at a temperature of about 50 ° C. for about 16 hours.

The reaction product thus obtained was passed through a filter paper to remove silver chloride, and the solution passed through the filter paper was dropped into water to precipitate a white solid.

Thus, 3.2 g of the white polymer was dissolved in 100 ml of tetrahydrofuran (THF), and then 100 ml of a lithium hydroxide (LiOH) aqueous solution of 3 to 5 wt% concentration was slowly added thereto. The solution thus obtained was neutralized again by slowly dropping 50 ml of a 2M aqueous hydrochloric acid solution. The precipitate obtained by filtration of this neutralized solution was dried to prepare methyl fluorinated polystyrene (F-PSB).

The hydrofluoric acid conductive electrolyte membrane having a thickness of about 150 μm was prepared by solvent casting using the fluorinated methyl sulfonated polystyrene thus obtained, and then ion conductivity was measured using a four-point probe method.

Comparative Example

2.0 g of polystyrene (from Sigma Aldrich) and 100 mL of 1,2-dichloroethane were charged into a 250 mL two-necked flask and stirred until a clear solution formed. To this clear solution, 0.34 g of 28 mM chlorosulphonic acid was slowly added, followed by reaction at a temperature of about 50 ° C. for 8 hours. The reaction product was filtered to remove the precipitate, and the solution passed through the filter paper was precipitated in an aqueous methanol solution (1: 1 by volume of water and methanol), and the resulting white solid was separated and dried. This white solid is sulfonated polystyrene.

Using the sulfonated polystyrene derivative thus obtained, a hydrogen ion conductive electrolyte membrane having a thickness of about 150 μm was prepared by solvent casting, and then ion conductivity was measured using a four-point probe method.

<Comparison Result>

Table 5 shows the ion conductivity measurement results of the hydrogen ion conductive electrolyte membrane of the example and the hydrogen ion conductive electrolyte membrane of the comparative example.

Example Comparative example  30 ℃  0.05 S / cm  0.04 S / cm  75 ℃  0.26 S / cm  0.06 S / cm

From Table 5, it can be seen that the ionic conductivity of the electrolyte membrane obtained in accordance with the present invention in Examples is improved than the ionic conductivity of the electrolyte membrane obtained in Comparative Examples, and the improvement effect thereof becomes very large as the temperature is increased. From this, it can be seen that the fluorinated alkylsulfonated polystyrene derivative of the present invention is a polymer having excellent hydrogen ion conductivity.

The fluorinated alkylsulfonated polystyrene derivative of the present invention has superior ion conductivity as compared with the sulfonated polystyrene derivative. Therefore, the hydrogen ion conductive polymer of the present invention can be usefully used as an electrolyte membrane for fuel cells.

Claims (8)

Hydrogen ion conductive polymer comprising a repeating unit represented by the formula (1). <Formula 1>

In Formula 1, R _f is a linear or branched fluorinated alkylene having 1 to 10 carbon atoms, and X is a hydrogen atom or an alkali metal. The method of claim 1, further comprising a repeating unit represented by Formula 2, wherein the sum of the number of repeating units represented by Formula 1 and the number of repeating units represented by Formula 2 is 1,000 to 10,000, and is represented by Formula 1. The number of repeating units is a hydrogen ion conductive polymer of 10% to 99% of the sum of the number of repeating units represented by the formula (1) and the number of repeating units represented by the formula (2). <Formula 2>

In Formula 2, R is an aromatic group including a benzene group. The method of claim 1 wherein the number of fluorine atom bonded to the carbon skeleton of the R _f, hydrogen ions, characterized in that the R _f of the carbon skeleton of the hydrogen atoms and the agreement of 10% to 100% of the fluorine atom number bound to Conductive polymer. A hydrogen ion conductive electrolyte membrane comprising the hydrogen ion conductive polymer according to claim 1. 5. The hydrogen ion conductive electrolyte membrane according to claim 4, having a thickness of 10 µm to 300 µm. A cathode in which a reduction reaction of the oxidant occurs; An anode in which oxidation of the fuel occurs; And A fuel cell comprising the hydrogen ion conductive electrolyte membrane according to claim 4 positioned between the cathode and the anode. The fuel cell according to claim 6, wherein the hydrogen ion conductive electrolyte membrane has a thickness of 10 µm to 300 µm. A cathode comprising a catalyst layer in which a reduction reaction of the oxidant occurs; An anode comprising a catalyst layer in which oxidation of the fuel occurs; And an electrolyte membrane positioned between the anode and the cathode. The catalyst layer of the cathode and the catalyst layer of the anode comprises a hydrogen ion conductive polymer according to claim 1 or 3.