JP2005190813A - Electrolyte material for fuel cell electrode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrolyte material for a fuel cell electrode which is superior in gas permeability and vapor permeability, and especially superior in thermal resistance and chemical stability at high temperatures. <P>SOLUTION: This is an electrolyte material for the fuel cell electrode which is composed of an organic silicon polymer having as a main frame a linking group having Si-O bonds of ≤2. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an electrode electrolyte material suitable for forming an electrode constituting a power generation cell of a fuel cell, such as a polymer electrolyte fuel cell (PEFC) or a direct methanol fuel cell (DMFC).

  In recent years, fuel cells that generate electricity by electrochemical reaction between hydrogen and oxygen have attracted attention as an energy supply source. In a fuel cell or the like using an ion exchange resin membrane, the ion exchange resin membrane is sandwiched between an anode diffusion electrode and a cathode diffusion electrode, and the electrolyte material used for forming these electrodes includes Perfluorosulfonic acid polymers (Nafion electrolytes) including Nafion are generally used (see, for example, Patent Document 1).

  This Nafion-based electrolyte is generally also used for the ion exchange resin membrane that conducts ions ionized by the electrode, and the electrolyte material for the electrode and the polymer electrolyte constituting the ion exchange resin membrane are the same component, It is said that the familiarity between the electrode and the ion exchange resin membrane is good. However, there are many disadvantages, and specifically, there are the following problems.

  Since the glass transition point of the Nafion-based electrolyte is in the vicinity of 100 ° C., the electrode portion (electrode reaction field) that is expected to be higher than the set temperature of the cell in which the ion exchange resin membrane is disposed has heat resistance. There is a big shortage. The electrode reaction field is expected to tend to be higher than the temperature inside the cell (for example, when set to 80 ° C., it is expected to exceed 120 ° C. at the electrode interface). It becomes more noticeable. In the future, it is considered that the set temperature during the operation of the fuel cell tends to be higher.

  Further, in the electrode portion, the electrolyte material exists so as to enclose a catalyst such as platinum-supported carbon. For this reason, in an environment where water is excessively generated during power generation, the water discharge performance is reduced, Nafion swells, gas diffusion in the electrode does not progress, and gas cannot reach the electrode reaction field, resulting in Cause a large voltage drop (concentration overvoltage).

  However, at present, there is no development report on electrode electrolyte materials other than Nafion, and there is no research case, so there is no choice but to rely on Nafion-based electrolyte materials.

On the other hand, some reports have been made of silicone-based electrolyte materials as having heat resistance (for example, see Non-Patent Documents 1 and 2). However, in general, the electrolyte material is configured to be non-gas permeable, and knowledge about high gas permeability is not recognized.
JP 2000-228206 A I.G.-Luneau et al. Electrochim. Acta 37 (9) 1615-1618 (1992) M. Popall et al. Electrochim. Acta 43 (10, 11) 1155-1161 (1998)

  As described above, no electrolyte material suitable for a fuel cell electrode other than a Nafion-based electrolyte has been provided so far, and a technology that can stably improve the heat resistance and power generation performance of a fuel cell has not been established. is the current situation.

  The present invention has been made in view of the above, and an object thereof is to provide an electrolyte material for a fuel cell electrode that is particularly excellent in gas permeability and water vapor permeability, heat resistance at high temperatures, and chemical stability. An object is to achieve the object.

  In order to achieve the above object, the fuel cell electrode electrolyte material of the present invention is composed of an organosilicon polymer having a linking group having 2 or less Si-O bonds (siloxane bonds) in the main skeleton. The electrolyte material for a fuel cell electrode of the present invention includes a linking group having 2 or less Si-O bonds, that is, one or two oxygen atoms bonded to one Si atom and 2 or less Si-O bonds. It is a silicone polymer in which the main skeleton is formed by Si—O bonds.

  The organosilicon polymer according to the present invention has a smaller rotational energy barrier for Si—O bonds than other bond groups such as carbon-carbon bonds, and has a gas permeability as compared with fluorine resins generally used in Nafion electrolytes and the like. In addition, since the water vapor permeability is two orders of magnitude higher, the gas diffusibility and the diffusive / exhaust properties of the generated water are remarkably improved, and as a result, the electrochemical reaction at the electrode can proceed without rate. In addition, the Si—O bond has higher interatomic bond energy than the carbon-carbon bond and is advantageous in terms of heat resistance.

  An electrode of a fuel cell is generally structured to be coated with a Nafion-based electrolyte solution added to platinum-supported carbon (electrode catalyst). The structure is coated with the polymer using the organosilicon polymer solution of the present invention. As a result, fuel (hydrogen gas, methanol, etc.) and oxidant (air) can be supplied to the electrode catalyst at a rateless rate, and the reaction at the electrode reaction field is promoted with high temperature resistance. Concentration overvoltage is suppressed, and a high current can be stably obtained.

  In addition, since the organosilicon polymer of the present invention has a main skeleton with a linking group having 2 or less Si-O bonds as described above, three Si-O bonds in which three oxygen atoms are bonded to Si atoms. It can exhibit superior flexibility, gas permeability, and water vapor permeability than conventional silicone electrolyte materials bonded together.

  The organosilicon polymer according to the present invention is composed of a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2), or a structure having both in the main skeleton. Can do. At this time, the main skeleton is formed by bonding each formula with bonds at both ends of the Si—O bond.

In the formula (1) or (2), R ¹ and R ² each independently represent a hydrogen atom, an aliphatic group, or an aromatic group. Moreover, two R < ¹ > in Formula (1) may be same or different. R ¹ and R ² may each contain an amino group. X and y in each structural unit represent the number of bonds, and each independently represents an arbitrary number.

When configured to include the structural unit represented by the above formula (1) and / or the structural unit represented by the formula (2), one organosilicon polymer includes at least two types of structural units, One of the two structural units may have a sulfo group (SO ₃ H), and the other may have an aliphatic group having 1 to 20 carbon atoms or an aromatic group having 6 to 20 carbon atoms. . Moreover, the structure comprised using the structural unit which an organosilicon polymer has a sulfo group, a C1-C20 aliphatic group, and / or a C6-C20 aromatic group is also suitable. If there is a sulfo group in the polymer, excellent ion conduction (protons, etc.) can be obtained, and if a group with a large total carbon number such as a long chain, in addition to preventing dissolution in water, the film can be applied by coating or screen printing. The shape can be easily formed, and the electrode can be formed easily.

  The electrolyte material for a fuel cell electrode of the present invention is suitable for forming a fuel electrode (anode) and an oxidant electrode (cathode) constituting a cell of a fuel cell, and a polymer electrolyte membrane (ion) sandwiched between the anode and the cathode. While the exchange resin membrane) is gas impermeable, both electrodes can be configured to be gas / water vapor permeable, and heat resistance can be secured. A fuel cell having excellent power generation performance and high heat resistance performance can be configured.

  According to the present invention, it is possible to provide an electrolyte material for a fuel cell electrode that is particularly excellent in gas permeability and water vapor permeability, heat resistance at high temperatures, and chemical stability.

Hereinafter, the electrolyte material for fuel cell electrodes of the present invention will be described in detail.
The electrolyte material for a fuel cell electrode of the present invention is composed of an organosilicon polymer having a linking group having 2 or less Si—O bonds (siloxane bonds) in the main skeleton. Specifically, the main skeleton can be constituted by either one or both of the following siloxane bonds (a) and (b).

  Since the main skeleton of the organosilicon polymer of the present invention is composed of a portion having a Si—O bond, the rotational energy barrier is smaller than the conventional carbon-carbon bond constituting the Nafion main chain, and the permeability of gas and water vapor is reduced. It has excellent interatomic bond energy and heat resistance, and one or two oxygen atoms are bonded to one Si atom and are linked by a structural unit containing 2 or less Si-O bonds. The structure is flexible and more transparent. Therefore, it is possible to improve the diffusion of fuel (hydrogen gas, methanol, etc.) and the diffusion / exhaust of produced water, obtain stable power generation performance, and electrodes that are expected to be significantly higher than the set temperature in the cell. The heat resistance in the inside can be dramatically improved.

  The organosilicon polymer of the present invention is preferably configured to have either one of the structural unit represented by the following formula (1) and the structural unit represented by the following formula (2), or both in the main skeleton. be able to. At this time, the main skeleton is formed by bonding with bonds at both ends of the Si—O bond.

In the formula (1) or (2), R ¹ and R ² each independently represent a hydrogen atom, an aliphatic group, or an aromatic group. Moreover, two R < ¹ > in Formula (1) may be same or different, and R < ¹ > and R < ² > may each contain an amino group. X and y in each structural unit are the number of bonds, and each independently represents an arbitrary integer.

Examples of the case where R ¹ and R ² have an amino group include — (CH ₂ ) ₃ NH ₂ , — (CH ₂ ) ₄ NH ₂ , — (CH ₂ ) ₃ NH (CH ₂ ) ₂ NH ₂ , _{- (CH 2) 3 N (} CH 3) 2, include like.

The aliphatic group represented by R ¹ or R ² is preferably an aliphatic group having 1 to 20 carbon atoms, and specifically includes an alkyl group, an alkenyl group, an alkynyl group, an aralkyl group, and the like. These groups may further have a substituent. These aliphatic groups may be a chain aliphatic group or a cyclic aliphatic group, and the chain aliphatic group may further have a branch. Among these, an alkyl group or a substituted alkyl group is particularly preferable.

  Examples of the alkyl group include linear, branched, and cyclic alkyl groups, and the total carbon number of the alkyl group is preferably 1-20, and more preferably 2-10. The same applies to the carbon number of the alkyl moiety of the substituted alkyl group. Increasing the total number of carbon atoms such as a long-chain alkyl group has an advantage that a film state can be easily formed by coating or the like when prepared in a solution state. As examples of the alkyl group or the alkyl moiety, ethyl, propyl, butyl, pentyl, hexyl, octyl, 2-ethylhexyl and the like are preferable.

In addition, the aromatic group represented by R ¹ or R ² is preferably an aromatic group having 6 to 20 carbon atoms in total, and examples thereof include an aryl group and a substituted aryl group, and these groups are further substituted. You may have. As carbon number of this aryl group, 6-20 are preferable and 6-10 are more preferable. The same applies to the number of carbon atoms in the aryl moiety of the substituted aryl group. Preferred examples of the aryl group or aryl moiety include phenyl, naphthyl, tolyl, xylyl and the like.

  Furthermore, as a substituent in the substituted alkyl group and the substituted aryl group, a sulfo group, a carboxyl group, a phosphite group, a phosphonic acid group, and the like are preferable.

  In the present invention, in particular, the organosilicon polymer includes at least two kinds of structural units, one of these two kinds of structural units has a sulfo group, and the other has an aliphatic group having 1 to 20 carbon atoms (particularly, (Substituted) alkyl group) or an aromatic group having 6 to 20 carbon atoms (particularly, (substituted) aryl group) is preferable.

  Specific examples of the structural units represented by the formulas (1) to (2) [Exemplary units (1) to (12)] are shown below. However, the present invention is not limited to these.

  The organosilicon polymer according to the present invention can be constituted by appropriately combining units arbitrarily selected from the exemplified units belonging to the above formula (1) or formula (2) with an arbitrary number of bonds.

  Examples of the fuel cell according to the present invention include a polymer electrolyte fuel cell (PEFC) and a direct methanol fuel cell (DMFC). For example, an anode diffusion electrode, a cathode diffusion electrode, a membrane electrode assembly having a polymer electrolyte membrane (ion exchange resin membrane) sandwiched between the anode diffusion electrode and the cathode diffusion electrode, and the membrane electrode junction And a pair of separators that form a fuel flow path through which fuel passes between the anode diffusion electrode and an oxidizing gas flow path through which oxidizing gas passes between the cathode diffusion electrode A structure including a single cell and a stack structure in which a plurality of the single cells are stacked as desired is included. In this case, the anode diffusion electrode and the cathode diffusion electrode are composed of a catalyst layer responsible for an electrochemical reaction and a diffusion layer functioning as a current collector, and among these, the catalyst layer is preferably formed using the organosilicon polymer according to the present invention. Can be formed. A specific example is shown below.

  That is, the catalyst layer is coated with platinum or an alloy of platinum and other metals as a catalyst on the surface of a fluorine-based ion exchange resin membrane such as a perfluorosulfonic acid membrane (for example, Nafion membrane manufactured by DuPont). Etc. are provided. Specifically, carbon powder carrying platinum or an alloy composed of platinum and another metal is prepared, and the carbon powder is dispersed in an appropriate organic solvent. An appropriate amount of electrolyte solution (Solution) can be added to form a paste, which can be formed on the ion exchange resin film by coating or screen printing. Further, the paste containing the carbon powder can be formed into a sheet to form a sheet, and the sheet can be pressed onto the ion exchange resin film. Alternatively, platinum or an alloy made of platinum and another metal may be applied to the surface of the diffusion layer facing the ion exchange resin film instead of the ion exchange resin film.

  The organosilicon polymer according to the present invention is obtained by using, for example, a sol-gel method, one or more alkoxides (preferably alkoxides having sulfo groups obtained by oxidation treatment and alkoxides having a large total carbon number such as long chains. Can be obtained as an electrolyte solution by subjecting it to hydrolysis in a single system or a mixed system and further subjecting it to an oxidation treatment.

  When obtaining an electrolyte solution from two or more kinds of alkoxides, it may be hydrolyzed in a state where two or more kinds of alkoxides are mixed in advance, and may be an organic silicon polymer electrolyte solution that is randomly copolymerized by oxidation treatment. It is good also as the electrolyte solution of the organosilicon polymer which mixed by block hydrolysis after hydrolyzing and oxidizing each.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

(Example 1)
According to the scheme I shown below, an electrolyte solution of an organosilicon polymer according to the present invention was obtained by a sol-gel method.

  The above compound (1) and compound (2) are added to a mixed solution of 0.01N hydrochloric acid and 2-propanol in a ratio of 1: 3, heated and hydrolyzed to form the structural unit (3). And the polymer which consists of (4) was obtained. Thereafter, hydrogen peroxide necessary to oxidize the SH group is added to this polymer in an amount of about 3 times moles, and a plurality of structural units (5) and (6) each derived from the compound (1) or (2) are obtained. A siloxane electrolyte 2-propanol solution (an electrolyte solution of the organosilicon polymer according to the present invention) in which an organosilicon polymer formed by random bonding (polymerization ratio a: b = 1: 3) was dispersed in 2-propanol was obtained. . In addition, there is no restriction | limiting in particular about said polymerization ratio, It is possible to select suitably in the range of 1: 9-9: 1.

-Measurement of ionic conductivity-
The above-obtained siloxane electrolyte 2-propanol solution was applied on glass so as to have a dry film thickness of 50 μm, dried to form an electrolyte membrane, and both ends of this electrolyte membrane were sandwiched between platinum electrodes. The ionic conductivity was measured by the AC impedance method. The measurement results are shown in Table 1 below.

  As shown in Table 1 above, it was confirmed that the electrolyte membrane made of the organosilicon polymer according to the present invention had good ionic conductivity.

-Heating test-
As a result of visually observing and evaluating the state of the electrolyte membrane obtained when the electrolyte membrane obtained in the same manner as in “-Measurement of ion conductivity” was heated to 400 ° C., no change was observed at this temperature. Therefore, it was evaluated that it was useful for improving heat resistance when used as an electrode electrolyte.

-Power generation test-
Next, a single cell 10 having the configuration shown in FIG. 1 was produced and a power generation test was performed.
First, a DuPont Nafion membrane (perfluorosulfonic acid membrane) is prepared as the polymer electrolyte membrane 11, and a dispersion solution (solid content) in which carbon powder carrying platinum is dispersed in 2-propanol on the surface of the Nafion membrane. 6 parts by mass) A catalyst layer forming solution prepared by adding 5 parts of the siloxane electrolyte 2-propanol solution (electrolytic solution of organosilicon polymer) as an electrolyte solution to 5 parts, The catalyst layers 12 and 13 were provided by coating so as to be 30 μm.

  Further, diffusion layers 14 and 15 made of carbon paper are provided on the surfaces of the catalyst layers 12 and 13, and a membrane electrode joint in which the polymer electrolyte membrane 11 is sandwiched between the anode diffusion electrode 16 and the cathode diffusion electrode 17. The body 20 is formed, the membrane electrode assembly 20 is further sandwiched, and the hydrogen gas passage 23 for supplying and discharging hydrogen gas to and from the anode diffusion electrode 16 and the air ( A single cell 10 was obtained by providing a pair of separators 21 and 22 forming an air flow path (oxidizing gas flow path) 24 for supplying and discharging air.

  For the single cell obtained as described above, the cell set temperature is 80 ° C., the bubbler temperature is anode / cathode = 80 ° C./80° C., hydrogen gas and air are supplied to both electrodes, and the current is limited. The value was determined.

As a result of the above, in the single cell provided with the electrode made of the organosilicon electrolyte according to the present invention, a limit current value of 1.3 A / cm ² could be obtained, and good power generation performance was exhibited. Moreover, the IV curve at this time is shown in FIG. Furthermore, although this single cell was operated intermittently for 10 hours, it was stable without causing problems such as voltage drop, and was excellent in heat resistance.

It is a schematic sectional drawing for demonstrating the single cell produced in the Example. It is an IV curve which shows the current-voltage characteristic in a single cell.

Explanation of symbols

10 ... Single cell (fuel cell)
12, 13 ... catalyst layer

Claims (3)

  An electrolyte material for a fuel cell electrode, comprising an organosilicon polymer having a linking group having 2 or less Si-O bonds in the main skeleton. The electrolyte material for a fuel cell electrode according to claim 1, wherein the linking group is at least one of a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2).

[Wherein, R ¹ and R ² each represent a hydrogen atom, an aliphatic group, or an aromatic group. Two R ¹ may be the same or different. R ¹ and R ² may each contain an amino group. ]   The organosilicon polymer includes at least two structural units, one of the two structural units has a sulfo group, and the other has an aliphatic group having 1 to 20 carbon atoms or an aromatic group having 6 to 20 carbon atoms. The electrolyte material for fuel cell electrodes according to claim 2, comprising:

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