JP2017036360A - Electrolyte film, production method of the same, and solid polymer fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrolyte film which exhibits proton conductivity higher than that of perfluorosulfonic acid (PFSA) polymer represented by Nafion, and has high proton conductivity even at low humidification.SOLUTION: By introducing a basic molecule into an acidic polymer such as a PFSA polymer, an excellent conductive path based on an acid-base bond is formed, and subsequently the basic molecule is removed. Thereby, as shown in figure, very large proton conductivity is achieved in a wide temperature range and in a wide relative humidity range compared with a case in which Nafion or the like is used as it is. Also a hydrocarbon-based polymer can be used as the acidic polymer. Also a heterocyclic compound such as 1,2,4-triazole can be used as the basic molecule.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing a perfluorosulfonic acid polymer membrane suitable for an electrolyte membrane for PEMFC (proton exchange membrane fuel cell), and a PEMFC using these membranes as an electrolyte membrane.

  PEMFC using hydrogen gas and oxygen gas is a clean energy system, has a high energy density and high conversion efficiency, and has attracted attention as a next-generation power generation device. Perfluorosulfonic acid ion exchange polymers such as Nafion (registered trademark of EI DuPont de Nemours and Company) over the past several decades (perfluorocarbons with a hydrophobic perfluorocarbon skeleton and sulfonic acid groups) It is a perfluorocarbon material composed of side chains and is a copolymer of tetrafluoroethylene and perfluoro [2- (fluorosulfonylethoxy) propylvinyl ether] .In this application, this copolymer is referred to as “perfluorosulfonic acid (PFSA). ))) Has been used as an electrolyte for PEMFC.

  Although the proton transport properties of these membranes are strongly defined by their water content, there is a need for fuel cells with high proton conductivity even in environments with low water content, which operate at high efficiency over a wide range of temperatures, and also contain them. Research and development of electrolyte membranes having high proton conductivity that does not depend on the amount of water have been conducted. In order to provide high proton conductivity, there are a method of increasing the proton concentration and a method of increasing the proton mobility. In general, as a technique for increasing the proton concentration, there is a technique for increasing the amount of sulfone groups introduced into the polymer unit, and most research belongs to this (for example, Non-Patent Document 1). On the other hand, there is a method of controlling the conduction path of a polymer as a technique for increasing the mobility of protons. However, little research has been done on the development of high proton conducting membranes using the latter method.

  There have been many reports of polymers with high degree of sulfonation of polymer units and high proton conductivity under high water content conditions. However, there are few reports on the development of electrolyte membranes that exhibit high proton conductivity even in a wide temperature range or in a low water content state. In practical use, it is necessary to find an alternative material that can solve the problem of performance degradation at low water content, and can be used as an electrolyte membrane for high-temperature PEMFC that can exhibit high proton conductivity even in a wide temperature range or low water content state. It has been.

  An object of the present invention is to provide a membrane that can be used as an electrolyte membrane for a high-temperature PEMFC that can solve the above-described problems of the prior art and can exhibit high proton conductivity even in a wide temperature range or in a low water content state.

According to one aspect of the invention,
After introducing basic molecules and forming an acid-base conduction path, an electrolyte membrane containing an acidic polymer from which the basic molecules have been removed is provided.
Here, the acidic polymer may be an acidic polymer including a sulfone group.
The acidic polymer may be selected from the group consisting of a perfluorosulfonic acid polymer and a hydrocarbon polymer having a sulfone group.
The perfluorosulfonic acid polymer may be selected from the group consisting of the following chemical structural formulas.

The hydrocarbon polymer may be selected from the group consisting of the following chemical structural formulas.


Alternatively, the hydrocarbon polymer is selected from the group consisting of polyarylsulfide, polyarylether, polyarylsulfone, polyarylketone, and polyaryhexafluoroisopropylidene It may be a polymer or copolymer containing a skeleton structure.
According to another aspect of the present invention, there is provided the method for producing an electrolyte membrane according to any one of the above, wherein a basic molecule is added to the acidic polymer, and then the basic molecule is removed from the acidic polymer.
Here, the basic molecule may be a heterocyclic compound or a surfactant containing an amino group.
The heterocyclic molecule may be selected from the group consisting of 1,2,4-triazole, undecylimidazole, 1,2,3-triazole, benzimidazole, pyrazole, and imidazole.
The removal of the basic molecule is selected from the group consisting of heating in water, holding in hydrogen peroxide, and holding in dilute sulfuric acid to the acidic polymer with the basic molecule added in the form of a film. It is also possible to perform at least one process.
According to still another aspect of the present invention, a polymer electrolyte fuel cell using any one of the above electrolyte membranes is provided.

  According to the present invention, it is possible to provide an electrolyte membrane having high proton conductivity and maintaining this characteristic even when the humidity is low.

Nafion modified electrolyte membrane (Modified Nafion in the figure) and Aquivion modified electrolyte membrane (Modified Aquivion in the figure), and commercially available Nafion 212 membrane (Nafion 212 in the figure) and Aquivion E87-05S membrane of Examples of the present invention The figure which shows the conductivity change with respect to relative humidity RH of (Aquivion E87-05S in the figure). (A), (b), (c) and (d) show the case where the cell temperature (Tcell in the figure) is 40 ° C., 80 ° C., 100 ° C. and 120 ° C., respectively. Relationship between cell current density and voltage when Nafion modified electrolyte membrane of the present invention (Modified Nafion in the figure) and commercially available Nafion 115 membrane (Nafion 115 in the figure) are used as the electrolyte membrane used in the cell FIG. Here, a microporous layer (MPL) was used for the diffusion layer of the cell, and the amount of platinum catalyst used was 0.3 mg / cm ² . (A) When the cell temperature is 80 ° C. and the relative humidity is 100%, (b) When the cell temperature is 100 ° C. and the relative humidity is 50%, and (c) When the cell temperature is 120 ° C. and the relative humidity is 25%.

  Nafion and Aquivion electrolytes, which are fluoropolymers, have high proton conductivity and high stability. However, the conductivity decreases greatly in any electrolyte under low humidification conditions. In the present invention, in order to solve these problems, basic molecules are removed after introducing a basic molecule into an acidic polymer to form a conduction path having good proton conductivity due to an acid-base bond. Proton conductivity is improved by the conduction path thus produced, and an electrolyte membrane that exhibits high proton conductivity even at low temperatures or under low humidification can be provided. Basic molecules are used to form a good conduction path, but are removed after the formation of this conduction path because this treatment improves proton conductivity. In addition, if basic molecules remain, there is a possibility that problems may occur when used as an electrolyte membrane. Here, as explained above, since the PFSA polymer has a sulfone group, it can be used as a kind of acidic polymer. The proton acidity is generally the strongest in the sulfone group, but the group that makes the polymer acidic may be other than the sulfone group.

  Without being limited thereto, examples of PFSA polymers are shown in the following chemical formula.

  Here, the manufacturer name is shown on the structural formula of each PFSA. The compound labeled “Dupont” is Nafion and the compound labeled “Solvay” is Aquivion (a registered trademark of Solvay) used with Nafion in the examples.

  In addition, not only the fluorine-based polymer described above but also a hydrocarbon-based polymer having an acidic group such as a sulfonic acid group can similarly exhibit high proton conductivity under low humidification. . Further, as basic molecules, heterocyclic compounds such as 1,2,4-triazole, undecylimidazole, 1,2,3-triazole, benzimidazole, pyrazole, imidazole, and surfactants containing amino groups can be used. . When a surfactant is used, this functions as a kind of mold for forming a conductive path. Furthermore, silica-based oxide materials, particularly nanoparticles thereof, can also be used.

  Conventionally, in order to increase the strength of this type of electrolyte membrane, the electrolyte is filled into a reinforcing material (porous film) (pore filling). It goes without saying that the conventional pore filling can be applied to the electrolyte of the present invention as it is.

  Examples of hydrocarbon polymers that can exhibit high proton conductivity even under low humidification include polyarylsulfide, polyarylether, polyarylsulfone, polyarylketone, polyarylketone, and polyarylketone. A polymer or copolymer containing a skeleton structure of arylhexafluoroisopropylidene can be preferably used. Chemical formulas of examples of hydrocarbon-based polymers that can be used in the present invention or monomers therefor are shown below. In addition, some of the following chemical structural formulas do not have an acidic group. In that case, the skeleton structure for producing an acidic polymer by modifying with an acidic group such as a sulfone group is shown.

  Other hydrocarbon polymers are shown below.

  The compound represented by the chemical structural formula shown last is a block copolymer disclosed in Non-Patent Document 2.

  In the following, examples of the present invention will be described with reference to the case where two PFSA polymers (Nafion, Aquivion) are used, but it should be noted that the acidic polymer that can be used is not limited thereto.

[Manufacture of Nafion-based modified electrolyte membrane]
5% 1,2,4-triazole was added to Nafion polymer, and N-methylpyrrolidone (NMP) as a solvent was further added and stirred to dissolve in NMP. The solution was put in a container and kept at 80 ° C. for 24 hours, and then kept at 130 ° C. for 24 hours to obtain a membrane. The obtained film was boiled with boiling water for 2 hours, then kept in 1 MH ₂ O ₂ at 80 ° C. for 2 hours, further kept in 1 MH ₂ SO ₄ at 80 ° C. for 2 hours, and boiled in boiling water for 2 hours. went. The Nafion-based modified electrolyte membrane was completed by removing basic molecules (here, 1,2,4-triazole) from the membrane by the above treatment after the initial membrane preparation.

[Manufacture of Aquivion modified electrolyte membrane]
Aquivion polymer was charged with 5% 1,2,4-triazole, and NMP as a solvent was further added and stirred to be dissolved in NMP. This solution was put in a container and kept at 80 ° C. for 24 hours, and then kept at 150 ° C. for 24 hours to obtain a membrane. The obtained film was boiled in boiling water for 2 hours, then kept in 1 MH ₂ O ₂ at 80 ° C. for 2 hours, kept in 1 MH ₂ SO ₄ at 80 ° C. for 2 hours, and boiled in boiling water for 2 hours. As in the above, the basic molecule was removed from the original membrane to complete the Aquivion modified electrolyte membrane.

[Measurement of properties of Nafion and Aquivion modified electrolyte membranes]
The proton conductivity characteristics of both modified electrolyte membranes prepared as described above were measured together with a commercially available Nafion 212 membrane (Ei Dupont de Nemours and Company) and Aquivion E87-05S membrane (Solvay). The result is shown in FIG.

  Furthermore, the current density-voltage characteristics of a single cell using both modified electrolyte membranes were measured together with a commercially available Nafion 115 membrane. The result is shown in FIG. In addition to this, in FIG. 2, the current density-power density characteristic is calculated from the measured current density-voltage characteristic and plotted. In the figure, the numerical value shown in parentheses on the right side of these film names indicates the thickness of the film.

  As can be seen from the results shown in FIG. 1, even when Nafion or Aquivion was used as the material of the electrolyte membrane, basic molecules (here, 1,2,4-triazole) were once introduced and removed. The modified electrolyte membrane has a much higher conductivity over both a wide relative humidity range (20% to 90%) and a wide temperature range (40 ° C to 120 ° C) than the corresponding unmodified electrolyte membrane. In particular, a very high value of 0.3 S / cm could be achieved in a region where the relative humidity was high. Since the internal resistance of the cell is reduced by this high conductivity, the modified electrolyte membrane can extract a larger current as shown in FIG. 2, and therefore the cell using the modified electrolyte membrane is 1 It was confirmed that electric power exceeding 5 to 2 times can be taken out. As can be seen from FIG. 2, this tendency was particularly remarkable at high temperatures and low humidity.

  It is believed that the reason why PFSA polymers represented by Nafion exhibit high proton conductivity is that a better proton conduction path is formed in the polymer by the sulfone group of these polymers. However, it is extremely difficult to directly observe how the proton conduction path actually exists, and it has not been realized yet.

  Therefore, as described above, the modified electrolyte membrane of the present invention showed a remarkable improvement in proton conductivity as described above, as described above. It should be considered that the conduction path better formed by removal contributed to proton conductivity. In addition, research to replace the PFSA polymer with a hydrocarbon polymer having a sulfone group is underway, but it is considered that a proton conduction path similar to that of the PFSA polymer is formed in such a hydrocarbon polymer having a sulfone group. The proton conductivity can be sufficiently explained. In view of this, it should be considered certain that the function of the sulfone group in this type of hydrocarbon polymer is the same as that in the PFSA polymer. Therefore, in the present invention, it is also possible to use a hydrocarbon polymer having a sulfone group already listed as an example instead of the PFSA polymer.

  As described above, according to the present invention, it is possible to provide an electrolyte membrane that has high proton conductivity and maintains this characteristic even when the humidity is low, so that it greatly contributes to the development of a polymer electrolyte fuel cell. There is expected.

R. Devanathan, Recent developments in proton exchange membranes for fuel cells, Energy & Environmental Science, 2008,1, 101-119. Angew. Chem. Int. Ed. 2010, 49, 317-320.

Claims (11)

  An electrolyte membrane comprising an acidic polymer in which a basic molecule is introduced, an acid-base conduction path is formed, and then the basic molecule is removed.   The electrolyte membrane according to claim 1, wherein the acidic polymer is an acidic polymer containing a sulfone group.   The electrolyte membrane according to claim 2, wherein the acidic polymer is selected from the group consisting of a perfluorosulfonic acid polymer and a hydrocarbon polymer having a sulfone group. The electrolyte membrane according to claim 3, wherein the perfluorosulfonic acid polymer is selected from the group consisting of the following chemical structural formulas.
The electrolyte membrane according to claim 3, wherein the hydrocarbon polymer is selected from the group consisting of the following chemical structural formulas.

  The hydrocarbon polymer is selected from the group consisting of polyarylsulfide, polyarylether, polyarylsulfone, polyarylketone, and polyaryhexafluoroisopropylidene. The electrolyte membrane according to claim 3, which is a polymer or copolymer containing a skeleton structure.   The method for producing an electrolyte membrane according to claim 1, wherein after adding a basic molecule to the acidic polymer, the basic molecule is removed from the acidic polymer.   The electrolyte membrane manufacturing method according to claim 7, wherein the basic molecule is a heterocyclic compound or a surfactant containing an amino group.   9. The electrolyte membrane manufacture according to claim 8, wherein the heterocyclic molecule is selected from the group consisting of 1,2,4-triazole, undecylimidazole, 1,2,3-triazole, benzimidazole, pyrazole, and imidazole. Method.   The removal of the basic molecule is selected from the group consisting of heating in water, holding in hydrogen peroxide, and holding in dilute sulfuric acid to the acidic polymer with the basic molecule added in the form of a film. The method for producing an electrolyte membrane according to claim 7, wherein at least one treatment is performed.   A solid polymer fuel cell using the electrolyte membrane according to claim 1.