JP2013051047A - Membrane electrode assembly manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a membrane electrode assembly, by which gas permeability of ionomer contained in an electrode catalyst layer can be improved.SOLUTION: Using a catalyst ink including ionomer and a catalyst carrier that has already carried a catalyst, an anode 52 and a cathode 53 that are electrode catalyst layers are formed on both membrane faces of an electrolyte membrane 51 to produce MEA. Then, the MEA is subject to gas permeability improving processing (step S300). This gas permeability improving processing is a processing to reduce a dense region because the ionomer of the electrode catalyst layer exhibits a crystal phase, and increase a coarse region that is coarse and has high gas permeability because the ionomer exhibits an amorphous phase. In such a method, an inside of the electrode catalyst layer of the MEA is impregnated with water, and the impregnated water is frozen. Thereafter, the water is thawed and dry-removed. An orientation of molecules in the crystal phase is disturbed by expansion of the water during freezing, and the crystal phase is made coarse as is similar to the amorphous phase.

Description

  The present invention relates to a method for producing a membrane electrode assembly in which an electrode catalyst layer is joined to a membrane surface of an electrolyte membrane having proton conductivity.

  A fuel cell that generates electricity by an electrochemical reaction between a fuel gas and an oxidant gas has attracted attention as an energy source. As this fuel cell, there is a solid polymer fuel cell using a solid polymer membrane as an electrolyte membrane. In such a polymer electrolyte fuel cell, generally, a membrane-electrode assembly (MEA) in which an electrode catalyst layer is bonded to both surfaces of an electrolyte membrane having proton conductivity is used.

  A membrane electrode assembly is generally manufactured by applying a so-called catalyst ink on both surfaces of an electrolyte membrane. The catalyst ink includes a conductive catalyst carrier (for example, carbon particles) carrying a catalyst and an ionomer having proton conductivity, and these are dispersed in a dispersion medium.

  The fuel gas and oxidant gas supplied to the anode and cathode of the fuel cell are subjected to an electrochemical reaction by the action of the catalyst in the electrode catalyst layer of the membrane electrode assembly. This electrochemical reaction occurs via the catalyst at a so-called three-phase interface where the gas flow path, the ionomer, and the catalyst carrier on which the catalyst is loaded are in contact. For this reason, in order to increase the chance of gas contact with the catalyst by allowing the catalyst to exist on the three-phase interface, the surface area of the ionomer in the vicinity of the catalyst is increased by forming voids in the electrode catalyst layer through removal of the polymer organic compound particles. Has been proposed (for example, Patent Document 1).

JP 2011-28978 A

  By the way, the polymer organic compound particles blended for forming the voids are covered with an ionomer before the removal, and the covered portions are defined as voids after the removal. For this reason, since the size of the voids formed in the electrode catalyst layer through the removal of the polymer organic compound particles depends on the size of the compound particles, there is a limit to the miniaturization of the voids. In addition, although the voids thus formed contribute to the gas permeability of the entire electrode catalyst layer, the gas permeability of the ionomer covering the catalyst and its support is not so much affected. In the above-mentioned patent documents, since there is no consideration about the improvement of gas permeability in such an ionomer, a technique for improving the gas permeability of the ionomer has been requested.

  In view of the above problems, an object of the present invention is to provide a new method for producing a membrane electrode assembly capable of improving the gas permeability of an ionomer contained in an electrode catalyst layer.

  In order to achieve at least a part of the above object, the present invention can be implemented as the following application examples.

[Application Example 1: Manufacturing method of membrane electrode assembly]
A method for producing a membrane electrode assembly in which an electrode catalyst layer is joined to a membrane surface of an electrolyte membrane having proton conductivity,
In forming the electrode catalyst layer on the membrane surface of the electrolyte membrane using a catalyst ink including a conductive catalyst carrier carrying a catalyst and an ionomer having proton conductivity, the ionomer molecules in the electrode catalyst layer are formed. The gist is to execute a density reduction process for reducing the dense region where the ionomer exhibits a crystalline phase and becomes dense due to the alignment.

  The ionomer contained in the electrode catalyst layer already formed on the membrane surface of the electrolyte membrane using catalyst ink is a dense ionomer that exhibits a crystalline phase by aligning the molecular orientation due to its properties as a polymer resin. In other words, it is divided into ionomers in a sparse and sparse region that exhibits an amorphous phase with little alignment. The ratio of the dense region due to the crystalline phase and the sparse region due to the amorphous phase is determined by the molecular orientation depending on the molecular chain structure of the ionomer. It is necessarily included in the ionomer of the catalyst layer.

  In the manufacturing method of the membrane electrode assembly of Application Example 1, since the ionomer dense region of the electrode catalyst layer is reduced as described above, the sparse and sparse region increases because of the amorphous phase. Since the amorphous phase is superior in gas permeability between the crystalline phase and the amorphous phase, the gas permeability of the ionomer contained in the electrode catalyst layer is obtained according to the method of manufacturing the membrane electrode assembly of Application Example 1. Can be increased.

  The membrane electrode assembly of the high-pressure gas tank of Application Example 1 described above can be configured as follows. For example, it is convenient to perform the density reduction step in a situation where the electrode catalyst layer is already formed on the membrane surface of the electrolyte membrane using the catalyst ink. In this case, one method is to impregnate the inside of the electrode catalyst layer formed on the membrane surface of the electrolyte membrane with water, freeze the thawed impregnated water, and then thaw and dry remove it. The water impregnated in the electrode catalyst layer spreads in or around the crystal phase of the ionomer and freezes at that location. During this freezing, the water expands, so that the orientation of the molecules is disturbed during the freezing process, and the crystal phase is made sparse like the amorphous phase. Then, after thawing and removing the frozen water, fine voids remain in the dense region that exhibited the crystal phase, so that the gas permeability of the ionomer is increased. In this case, since the phenomenon described above also occurs in the amorphous phase, fine voids are formed even in the sparse region, and the gas permeability of the ionomer can be enhanced with high effectiveness.

  Further, the electrode catalyst layer already formed on the membrane surface of the electrolyte membrane is exposed to a gas containing a solvent that causes dissolution of the ionomer, and the solvent contained in the gas is guided to the inside of the electrode catalyst layer. The guided solvent can also be removed. The solvent introduced into the electrode catalyst layer spreads in or around the crystalline phase of the ionomer, dissolves the ionomer at that location, disturbs the molecular orientation, and makes the crystalline phase similar to the amorphous phase. Sparse. For this reason, after the removal of the solvent, the dense region that was dense because it exhibited a crystalline phase becomes sparse, and the gas permeability of the ionomer increases. In this case, if the solvent that causes the ionomer to be dissolved has an affinity for the crystalline quality of the ionomer, the ionomer crystal phase can be made sparse as in the amorphous phase. If the solvent has a low affinity with the crystalline material, the above phenomenon occurs even in the amorphous phase, so the sparse situation increases even in the sparse region, so the ionomer gas permeability is highly effective. Can be increased.

  Further, the electrode catalyst layer already formed on the membrane surface of the electrolyte membrane is impregnated with a solution containing ions that dissolve the crystalline phase of the ionomer, and the impregnated solution is removed together with the ions. You can also. Ions contained in the solution introduced into the electrode catalyst layer are distributed in or around the crystal phase of the ionomer, and the crystal phase of the ionomer at that location is solved. For this reason, after removal of the solution, the dense region that is dense because it exhibits a crystalline phase becomes sparse, so that the gas permeability of the ionomer is increased.

  The present invention can be applied to a fuel cell including a membrane electrode assembly manufactured by the above-described method for manufacturing a membrane electrode assembly, and a method for manufacturing the fuel cell, in addition to the above-described method for manufacturing a membrane electrode assembly.

It is explanatory drawing which shows roughly the fuel cell 40 as one Example of this invention by sectional view. 4 is an explanatory diagram showing an outline of a manufacturing process of the fuel cell 40. FIG. It is explanatory drawing which shows notionally the mode of the crystalline phase and amorphous phase which an ionomer exhibits in the electrode catalyst layer of the membrane-electrode assembly (MEA) obtained using the catalyst ink containing an ionomer. It is explanatory drawing which shows one method of evaluation measurement. Oxygen reduction current values and electrodes for the examples subjected to the gas permeability improvement treatment (electrode catalyst layer pseudo-thin leaf pieces) and the comparative examples (existing MEA / electrode catalyst layer pseudo-thin leaf pieces) not subjected to the gas permeability improvement treatment It is a graph which shows the relationship with an electric potential. It is explanatory drawing which shows roughly the mode of the gas permeability improvement treatment in 2nd Example. X-ray irradiation of the membrane-electrode-diffusion layer assembly (MEGA) of the example subjected to the gas permeability improvement treatment of the second example and the comparative example MEGA (existing MEGA) that has not been subjected to the gas permeability improvement treatment It is a graph which shows the relationship between an angle and its reflection intensity. It is a graph which shows the relationship of the electric current, cell voltage, and cell resistance about Example MEGA processed to the gas-permeability improvement process of 2nd Example, and comparative example MEGA (existing MEGA) which has not performed the gas-permeability improvement process. . It is a graph which shows the relationship between the glass transition temperature and dielectric loss tangent at the time of substituting the fluorine of the fluorine-type ionomer (perfluorosulfonic acid resin) represented by Chemical formula 1 in 3rd Example by various ions.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an explanatory view schematically showing a fuel cell 40 as an embodiment of the present invention in a sectional view.

  As shown in the drawing, the fuel cell 40 is configured as a battery cell unit that generates electricity by an electrochemical reaction between hydrogen and oxygen, and has a stack structure in which a plurality of the units are stacked between a pair of end plates (not shown). . In this case, the number of stacked battery cell units can be arbitrarily set according to the output required for the fuel cell 40.

  The fuel cell 40 sandwiches battery cells 50 serving as a power generation unit with opposing separators 41. As shown in FIG. 1, the battery cell 50 includes both electrodes of an anode 52 and a cathode 53 on both sides of an electrolyte membrane 51. The electrolyte membrane 51 is a proton-conductive ion exchange membrane formed of a solid polymer material, for example, a fluorine-based resin, and exhibits good electrical conductivity in a wet state. The anode 52 and the cathode 53 are configured by coating conductive particles carrying a catalyst such as platinum or a platinum alloy, for example, carbon particles (hereinafter referred to as catalyst-carrying carbon particles) with an ionomer having proton conductivity. The electrode catalyst layer is joined to both membrane surfaces of the electrolyte membrane 51 to form a membrane electrode assembly (MEA) together with the electrolyte membrane 51. Usually, the ionomer is a polymer electrolyte resin (for example, a fluororesin) that is a solid polymer material of the same quality as the electrolyte membrane 51, and has proton conductivity due to its ion exchange group.

  In addition, the battery cell 50 sandwiches the electrolyte membrane 51 with electrodes formed between the anode gas diffusion layer 54 and the cathode gas diffusion layer 55 from both sides. The separator 41 is located outside both the gas diffusion layers, and sandwiches the battery cell 50 including the gas diffusion layers. The anode side gas diffusion layer 54 and the cathode side gas diffusion layer 55 are formed of a conductive member having gas permeability, such as carbon paper or carbon cloth.

  The separator 41 is a member that forms a gas flow path through which a reaction gas (a fuel gas containing hydrogen gas or an oxidizing gas containing oxygen) flows for each battery cell 50, and has low hydrogen permeability and good conductivity. Formed of material. For example, a plate-shaped conductive composite material or metal steel plate formed by mixing a conductive material into resin is used for forming the separator 41. The separator 41 includes an in-cell hydrogen gas flow path 42 and an in-cell air flow path 43 on the front and back surfaces thereof in order to supply and discharge gas to and from each electrode of the battery cell 50. In this embodiment, the in-cell hydrogen gas flow path 42 and the in-cell air flow path 43 are flow paths that are orthogonally arranged in the cell plane (in the electrode plane). It is set as the multi-row linear flow path extended up and down. The battery cell 50 of the fuel cell 40 receives the supply of hydrogen gas and air to the anode 52 and the cathode 53 through these in-cell flow paths, and generates electricity by causing an electrochemical reaction between hydrogen and oxygen.

  Next, the manufacturing process of the fuel cell 40 described above will be described. FIG. 2 is an explanatory diagram showing an outline of the manufacturing process of the fuel cell 40.

  As shown in FIG. 2, in manufacturing the fuel cell 40, in this embodiment, the fuel cell 40 is obtained by proceeding with the manufacture of MEA and the manufacture of MEGA (battery cell 50) using the MEA. In manufacturing the MEA, first, an electrolyte membrane 51 (see FIG. 1, the same applies hereinafter) as a solid polymer electrolyte membrane is prepared (step S100). In this case, as for the electrolyte membrane 51, the electrolyte membrane 51 that has already been formed can be purchased, or can be formed from a polymer electrolyte resin that is a film forming material. In this embodiment, the electrolyte membrane 51 is a Nafion membrane having proton conductivity (Nafion is a registered trademark, the same applies hereinafter).

  Next, the catalyst ink for forming the anode 52 and the cathode 53 which are electrode catalyst layers is prepared (step S120). In this step, carbon particles (catalyst-carrying carbon particles / catalyst carrier) supporting the catalyst particles using platinum alloy as a catalyst, ionomers having proton conductivity, and a dispersion medium (pure water and organic solvent) thereof are used. Are mixed and stirred with an appropriate stirring device to prepare a catalyst ink. Various types of carbon particles can be selected. For example, carbon nanotubes, carbon nanohorns, carbon nanofibers and the like can be used in addition to carbon black and graphite.

  When the catalyst is supported, a generally employed method such as an impregnation method, a coprecipitation method, or an ion exchange method may be performed. Moreover, what is distribute | circulating as the carbon particle by which catalyst support was carried out can also be obtained. An ultrasonic homogenizer was used for dispersing the catalyst-supporting carbon particles. The mixing ratio of pure water and organic solvent in the above dispersion medium can be determined as appropriate.

  As the ionomer, for example, a fluorine-based ionomer (perfluorosulfonic acid resin) represented by the following chemical formula 1 is used. FIG. 3 is an explanatory diagram conceptually showing a state of a crystalline phase / amorphous phase exhibited by an ionomer in an electrode catalyst layer of a membrane-electrode assembly (MEA) obtained using a catalyst ink containing an ionomer.

  This ionomer exhibits proton conductivity and hydrophilicity at a sulfonic acid group in the side chain, and has a repeating unit based on a fluorine-containing vinyl compound as a main chain structure. For this reason, in the mixing / stirring process described above in preparation of the catalyst ink and the film forming process described later, a crystalline phase is exhibited in a region where the main chain molecules are aligned, and an amorphous phase is formed in a region where the alignment is not very uniform. Presents. The ionomer is divided into a dense region because it exhibits a crystalline phase and a sparse region because it exhibits an amorphous phase. In this figure, the region surrounded by sulfonic acid groups in the amorphous phase The state of proton conductivity and hydrophilicity due to the permeation transfer of hydrogen ions is shown. In FIG. 3, the catalyst-carrying carbon particles are not depicted, but the catalyst-carrying carbon particles are covered with the illustrated ionomer exhibiting a crystalline or amorphous phase, or the adjacent carbon particles are the ionomers. It will be bound by.

  Following the preparation of the catalyst ink described above, a membrane-electrode assembly (MEA) is produced using this catalyst ink (step S200). That is, the prepared catalyst ink is applied to the front and back surfaces of the electrolyte membrane 51 obtained in step S100 by a film formation method such as a doctor blade method or a screen printing method, so that electrode catalyst layers are formed on both sides of the electrolyte membrane 51. An anode 52 and a cathode 53 are formed. Alternatively, a sheet may be formed by forming a film using the prepared catalyst ink, and the anode 52 and the cathode 53 may be bonded to the electrolyte film 51 by pressing the sheet on the electrolyte film 51. Further, the prepared catalyst ink is applied to a sheet having releasability (for example, Teflon sheet: Teflon is a registered trademark) and dried to prepare an electrode transfer sheet from the prepared catalyst ink. Then, the two Teflon sheets are sandwiched between the electrolyte membrane 51 so that the electrode transfer sheet is bonded to both sides of the electrolyte membrane 51, heat-pressed at a predetermined temperature and pressure, the Teflon sheet is peeled off, and the electrode transfer sheet is made into an electrolyte. It is also possible to transfer and bond to both sides of the film 51. In the MEA thus obtained, the ionomer covers or binds the catalyst-supporting carbon particles as shown in FIG. 3 at the anode 52 and the cathode 53 which are the electrode catalyst layers.

  Following the production of the MEA, the obtained MEA is subjected to a gas permeability improvement treatment of the electrode catalyst layer (step S300). In this gas permeability improving treatment, first, the obtained MEA is immersed in pure water at room temperature, and water is impregnated into the anode 52 and cathode 53 layers which are MEA electrode catalyst layers. The impregnated water is then frozen by placing the hydrated MEA in a low temperature environment of −30 ° C. for 1 hour. Thereafter, for example, the MEA is subjected to a drying process such as room temperature drying or hot air drying, and water is thawed and removed by drying.

Subsequently, both the gas diffusion layers of the anode side gas diffusion layer 54 and the cathode side gas diffusion layer 55 are prepared (step S400), and the MEA is sandwiched between the gas diffusion layers.
The anode side gas diffusion layer 54 is joined to the anode 52 and the cathode side gas diffusion layer 55 is joined to the cathode 53 (step S500). Thereby, the battery cell 50 as MEAGA is obtained. The gas diffusion layer can be bonded by an appropriate bonding method, for example, hot pressing. As described above, by applying heat and pressure, the catalyst ionomer constituting the anode 52 and the cathode 53 is softened by heat, and the softened ionomer is adapted to the entire porous surface of the gas diffusion layer of the anode / cathode. While increasing, both are pressure-bonded.

  Next, the separator 41 is bonded to the gas diffusion layers on both sides of the battery cell 50 as the obtained MEAGA (step S600), and these are predetermined in a predetermined order (so that the cells in FIG. 1 are repeatedly formed). A stack structure is assembled by stacking several layers, and a predetermined pressing force is applied in the stacking direction to hold the entire structure (step S700). Thereby, the fuel cell 40 is completed.

  Next, the effect obtained by the gas permeability improvement process in step S300 will be described. FIG. 4 is an explanatory diagram showing one method of evaluation measurement. In the evaluation measurement, a film-formed sheet used for forming the anode 52 and the cathode 53, which are the MEA electrode catalyst layers, is prepared using the catalyst ink prepared in step S120 described above. About the produced film-forming sheet, performance evaluation was performed before and after this was subjected to the gas permeability improvement treatment of Step S300 described above. The membrane-molded sheet before being subjected to the gas permeability improving treatment in step S300 described above becomes a thin leaf piece imitating the electrode catalyst layer in the existing MEA, and the membrane-molded sheet subjected to the gas permeability improving procedure in step S300 is This is a thin leaf piece in which the electrode catalyst layers (the anode 52 and the cathode 53) in the MEA of the present example are forged.

  First, as shown in FIG. 4, the electrolyte membrane 51 is disposed on the counter electrode side and the reference electrode side with a space therebetween, and then joined to the platinum (Pt) electrode of each electrode. The electrode catalyst layer pseudo-made thin leaf piece is disposed so as to straddle the adjacent electrolyte membrane 51 and bonded to the Pt electrode of the working electrode before being subjected to the gas permeability improvement treatment. In addition, on the back side of the thin leaf piece, a dry filter paper is disposed so as to block between the adjacent electrolyte membranes 51. On the back side of the filter paper and the left and right electrolyte membranes 51 at 80 ° C. and a relative humidity of 80%. An oxygen-containing gas (for example, air) is supplied by spraying. During this oxygen supply, the oxygen reduction current value at the Pt electrode of the working electrode is measured for each electrode potential of the counter electrode and the reference electrode. The same current value measurement was performed on the electrocatalyst layer pseudo-thin leaf pieces subjected to the gas permeability improvement treatment, and this was compared. FIG. 5 shows oxygen reduction currents of the example (electrode catalyst layer pseudo-thin leaf piece) subjected to the gas permeability improvement treatment and the comparative example (existing MEA / electrode catalyst layer pseudo-thin leaf piece) not subjected to the gas permeability improvement treatment. It is a graph which shows the relationship between a value and electrode potential.

  As shown in FIG. 5, the thin leaf pieces that were made by simulating the example MEA improved the oxygen reduction current value by about 1.5 times in the base low electrode potential region as compared with the thin leaf pieces that were made by simulating the existing MEA. doing. The oxygen reduction current value measured as shown in FIG. 4 increases as the amount of oxygen passing through the electrode catalyst layer pseudo-made thin leaf piece from the filter paper side to the Pt electrode of the working electrode increases. Then, it can be said that the thin leaf piece in which the example MEA is forged is superior in gas permeability compared to the thin leaf piece in which the existing MEA is forged. In addition, since the base electrode potential region corresponds to the high power generation region where the gas supply is rate-determining, the thin leaf piece in which the embodiment MEA is forged is compared with the thin leaf piece in which the existing MEA is forged. It can also be said that the gas permeability is excellent.

  The thin leaf piece that is a pseudo-made example MEA having different gas permeability and the thin leaf piece that is a pseudo-made example of the existing MEA are different only in whether or not they are subjected to the gas permeability improvement treatment in step S300. Therefore, it can be said that the gas permeability improvement treatment of this embodiment is a treatment that brings about the improvement of gas permeability of the electrode catalyst layer, and can be explained as follows.

  In the thin leaf piece imitating Example MEA, the water impregnated inside the thin leaf piece, specifically, the inside of the electrode catalyst layer pseudo thin leaf piece, spreads in or around the crystal phase of the ionomer shown in FIG. Freeze at. During this freezing, the water expands, so that the orientation of molecules in the crystalline phase is disturbed during the freezing process, and the crystalline phase becomes sparse like the amorphous phase. And after thawing / removing frozen water, fine voids remain in the dense region where the crystal phase was present, and it can be said that the gas permeability of the ionomer has increased. In other words, the ionomer dense regions constituting the electrode catalyst layers of the anode 52 and the cathode 53 are reduced, and the sparse region increases due to the amorphous phase. Moreover, since such a phenomenon also occurs in the amorphous phase, fine voids are formed even in a sparse and sparse region because of the amorphous phase. As a result, in the fuel cell 40 having the embodiment MEA that has been subjected to the gas permeability improvement treatment in step S300, the power generation performance is improved due to the high gas permeability of the ionomer itself contained in both the electrode catalyst layers of the anode 52 and the cathode 53. This can also be improved. Then, according to the manufacturing process of the present embodiment, the MEA having high gas permeability, and hence the fuel cell 40 having high power generation performance can be simplified by the gas permeability improvement treatment in step S300 in the state where the MEA has been formed. Can be manufactured.

  Next, another embodiment will be described. This second embodiment is characterized in that the electrode catalyst layers of the anode 52 and the cathode 53 constituting the MEA are exposed to a gas containing a solvent that causes the ionomer to be dissolved in the gas permeability improvement treatment in step S300. FIG. 6 is an explanatory view schematically showing the state of the gas permeability improvement treatment in the second embodiment.

  As shown in the drawing, the MEA to be treated for the gas permeability improvement treatment and another MEA (the treatment target for the next and subsequent times) include a cathode side gas diffusion layer 55 and an anode side gas diffusion layer 54 (not shown). Is prepared in the state of the battery cell 50 (MEGA) that has been joined. In other words, in the second embodiment, the gas permeability improvement treatment in step S300 in the manufacturing process of FIG. 2 is performed after the gas diffusion layer bonding in step S500.

Next, the prepared MEA to be treated and another MEA are connected to a circuit 60 including a power source and a switch. At this time, the cathode-side gas diffusion layer 55 (or anode-side gas diffusion layer 54) of the treatment MEGA including the treatment MEA is connected to the negative electrode side, and the MEGA gas diffusion layer including other MEAs is connected to this. Is connected to the counter electrode side (positive electrode). Then, while supplying the following gases to the cathode gas diffusion layer 55 of the MEGA to be treated and the gas diffusion layer of the MEGA on the counter electrode side at 80 ° C. and a relative humidity of 100%, both gas diffusion layers 1 Current is supplied for 10 min at a current density of 0.0 Acm ² or less. In this case, the gas diffusion layer 55 of the MEGA to be treated is supplied with hydrogen gas containing 10 mg / cm ^{2 in} total of ethanol, which is a solvent for dissolving the ionomer, at a flow rate of 250 cc / min. The gas diffusion layer is supplied with hydrogen gas or inert gas (for example, nitrogen gas) at a flow rate of 250 cc / min.

  In the MEGA to be treated under such a gas supply, the cathode 53, which is the MEA electrode catalyst layer, is exposed to ethanol-containing hydrogen gas through the cathode-side gas diffusion layer 55, and the ethanol is introduced into the cathode layer. It is burned. This phenomenon can be explained as follows. In the MEGA to be treated, ethanol contained in the hydrogen gas is led to the ionomer of the cathode 53 as the electrode catalyst layer via the cathode side gas diffusion layer 55 and diffuses into the layer. It is expected not to go to the 51 side. However, when the switch of the circuit 60 is closed and electrons flow from the power source into the cathode side gas diffusion layer 55, ethanol and hydrogen are decomposed and consumed on the cathode side gas diffusion layer 55 side by the electrons, and the concentration of ethanol is increased. There will be a difference. Since the supply of ethanol-containing hydrogen gas continues in such a situation where there is a difference in concentration, ethanol diffusion in the ionomer at the cathode 53 occurs during the above energization so as to eliminate the concentration difference. Will diffuse to the electrolyte membrane 51 side.

  Since ethanol has an affinity for the crystalline quality of the ionomer, the ethanol diffused in the electrode catalyst layer spreads in or around the crystalline phase of the ionomer shown in FIG. For this reason, in the energization period and the curing period after the energization is stopped, ethanol dissolves the ionomer at the site where the energization has occurred, disturbs the orientation of the molecules, and makes the crystal phase sparse like the amorphous phase. Then, after the curing period, the electrocatalyst layer together with the treated MEGA is washed with pure water to remove residual ethanol, and if dried, a dense region of ionomer that was dense because it exhibited a crystalline phase Therefore, the ionomer gas permeability at the cathode 53 can be increased. In other words, even in the second embodiment in which the ionomer is dissolved by ethanol, the dense region of the ionomer constituting the electrode catalyst layer of the anode 52 and the cathode 53 is reduced, and the amorphous phase is generated accordingly. Will lead to an increase in sparse areas. Moreover, since such a phenomenon can occur even in an amorphous phase, the sparse state increases even in a sparse region, so that the gas permeability of the ionomer can be enhanced with high effectiveness. And this improvement in gas permeability is the result of the dissolution of the ionomer with ethanol and the removal of the ethanol that caused the dissolution, so there were minute gaps between the ionomer molecules where ethanol spread. It can be said that it depends.

  In this case, the cathode side gas diffusion layer 55 is connected to the circuit 60 and the gas supply and energization and curing described above are followed by the anode side gas diffusion layer 54 in the same MEGA to be treated connected to the circuit 60 and the gas supply and the gas supply layer. By performing energization and curing, the gas permeability of the ionomer in both the anode-side and cathode-side electrode catalyst layers of the same MEGA to be treated can be improved. In addition, the MEGA that has already undergone the above-described gas permeability improvement treatment through ionomer dissolution with ethanol is removed from the circuit 60, and a new MEGA to be treated is connected to the circuit 60 to supply and energize the gas. For example, MEGAs having electrode catalyst layers with improved gas permeability can be sequentially produced.

  Next, the effect obtained by the gas permeability improvement treatment of the second embodiment will be described. As an evaluation judgment, X-ray analysis was performed to analyze the crystal structure from the reflection intensity by irradiating small angle X-rays. In this X-ray analysis, an MEA having an anode 52 and a cathode 53, which are electrode catalyst layers formed of the catalyst ink prepared in step S120 of FIG. 2 described above, is produced (step S200). The battery cell 50) is produced (steps S400 to S500). The produced MEGA was subjected to X-ray analysis before and after being subjected to the above-described gas permeability improvement treatment through ionomer dissolution with ethanol. The MEGA before being subjected to the gas permeability improvement treatment corresponds to an existing MEGA, and the MEGA subjected to the gas permeability improvement treatment is the MEGA of the present embodiment. FIG. 7 shows the membrane-electrode-diffusion layer assembly (MEGA) of the example subjected to the gas permeability improvement treatment of the second example and the comparative example MEGA (existing MEGA) in which the gas permeability improvement treatment is not implemented. It is a graph which shows the relationship between an X-ray irradiation angle and its reflection intensity.

  As shown in FIG. 7, in the existing MEGA that has not been subjected to the gas permeability improvement treatment, a remarkable reflection intensity is observed at an irradiation angle corresponding to the crystal phase of the ionomer, whereas in the example MEGA, the above-described irradiation is performed. The reflection intensity greatly decreases before and after the angle. From the X-ray analysis result of FIG. 7, the ionomer crystalline phase constituting the electrode catalyst layer is made sparse as in the amorphous phase by subjecting it to gas permeability improvement treatment that causes ionomer dissolution by ethanol. It can be said that it can be reduced.

  Next, the performance improvement of the battery cell 50 as MEGA of this 2nd Example is demonstrated. FIG. 8 shows the relationship between the current, cell voltage, and cell resistance of Example MEGA subjected to the gas permeability improvement treatment of the second example and Comparative Example MEGA (existing MEGA) not subjected to the gas permeability improvement treatment. It is a graph. In measuring the cell voltage and cell resistance, the battery cell 50 shown in FIG. 1 is used, and the anode 52 is supplied with hydrogen gas at a flow rate of 272 cc / min, and the cathode 53 is supplied with air at a flow rate of 866 cc / min. Power was supplied to generate power, and the cell voltage and cell resistance were measured during the power generation. The gas supply was performed at 80 ° C. and a relative humidity of 100%.

  As shown in FIG. 8, the cell voltage and the cell resistance were improved in Example MEGA subjected to the gas permeability improvement treatment that caused the ionomer dissolution with ethanol. It can be said that this is because the electrochemical reaction is activated by improving the gas permeability in the electrode catalyst layers of the anode 52 and the cathode 53 as described above. The same effects as those of the above-described embodiments can be obtained by the gas permeability improvement treatment of the second embodiment that causes ionomer dissolution by ethanol. In particular, in the second embodiment, since the electrode catalyst layer of the anode 52 and the cathode 53 is exposed to ethanol-containing hydrogen gas, there are the following advantages compared with the method in which MEA or MEGA is immersed in an ethanol solution. In the method of immersing in an ethanol solution, it is feared that the electrolyte polymer resin constituting the electrolyte membrane 51 is dissolved by ethanol, but the method of the second embodiment in which the electrode catalyst layer is exposed to ethanol-containing hydrogen gas. Then, it is possible to prevent the electrolyte polymer resin constituting the electrolyte membrane 51 from being dissolved by ethanol.

  Next, a third embodiment will be described. The third embodiment is characterized in that the ionomer crystal phase is solved by ions in the electrode catalyst layers of the anode 52 and the cathode 53 constituting the MEA in the gas permeability improvement treatment in step S300. FIG. 9 is a graph showing the relationship between the glass transition temperature and the dielectric loss tangent when fluorine of the fluorine ionomer (perfluorosulfonic acid resin) represented by Chemical Formula 1 in the third example is substituted with various ions. FIG. 9 is a graph based on Macromolecules 2000, 33, 6031.

  When the various ions shown in the figure are replaced, the peak value of the dielectric loss tangent based on the ions changes to a low temperature. Since the horizontal axis of FIG. 9 is the glass transition temperature of the ionomer, the glass transition temperature of the ionomer can be shifted to the low temperature side by the substituted ion species. On the other hand, when the crystalline phase of the ionomer is dissolved and the ionomer becomes sparse, the glass transition temperature of the ionomer decreases due to the reduction of the crystalline phase. Therefore, if ions that cause the glass transition temperature of the ionomer to transition to a low temperature side are replaced by, for example, an ion exchange method, the ionomer crystal phase can be solved by the substituted ions to loosen the ionomer crystal phase. Utilizing this property, the third embodiment employs a technique (ion exchange method) in which ions capable of lowering the glass transition temperature of the ionomer are replaced with ionomers in the electrode catalyst layer constituting the MEA.

In the third embodiment, in step S300 in FIG. 2, the MEA is immersed in a solution containing, for example, tetrabutylammonium ion (TBA ⁺ ), or the electrode of the anode 52 or the cathode 53 is sprayed or the like. Adhere to the catalyst layer. Since the tetrabutylammonium ion contained in the solution has an affinity for the crystalline quality of the ionomer, it spreads in or around the crystalline phase of the ionomer and is replaced with the fluorine of the ionomer. The tetrabutylammonium ion substituted by such an ion exchange method is in a solution state by dissolving the crystalline phase of the ionomer, which is the substitution site, until it is removed as described later. The sparseness is the same as the mass phase, and the glass transition temperature of the ionomer is lowered as shown in FIG. And after progress of the curing period after ion substitution, MEA is immersed in an acid solution such as hydrochloric acid, MEA is washed with pure water, and then dried. Thereby, tetrabutylammonium ions are removed from the electrode catalyst layer of MEA.

  In the MEA that has undergone such substitution of tetrabutylammonium ions, the ionomer crystal phase has been solved by the tetrabutylammonium ions as described above, and thus the dense region that was dense because it exhibited the crystal phase is sparse. Situation increases and ionomer gas permeability increases. That is, even in the third embodiment in which the ionomer crystal phase is released with tetrabutylammonium ions to form a solution state, the ionomer dense regions constituting the electrode catalyst layers of the anode 52 and the cathode 53 are reduced, and accordingly, In this case, since the amorphous phase is exhibited, the sparse region increases. And this improvement in gas permeability is the result of the removal of tetrabutylammonium ions caused by the tetrabutylammonium ions dissolving the ionomer crystal phase into a solution state and the resulting solution. It can also be said that the ionomer molecule was dissolved at the place where the ammonium ion was replaced, and a minute gap was formed.

  Although the embodiments of the present invention have been described above, the present invention is not limited to such embodiments, and can be implemented in various modes without departing from the scope of the present invention. For example, in the above embodiment, in both the electrode catalyst layers of the anode 52 and the cathode 53, the gas permeability of the ionomer constituting the catalyst layer is improved, but the gas permeability of either one of the electrode catalyst layers is improved. Treatment can also be performed.

  Moreover, in the gas permeability improvement treatment in the second embodiment, the ionomer is dissolved with ethanol contained in hydrogen gas, but a solvent other than ethanol can also be used. A solvent that does not have high affinity with the crystalline phase of the ionomer can also be used. According to the solvent, the amorphous phase can be made more sparse and gas permeability can be improved. In addition, in the second embodiment, the ionomer can be dissolved by ethanol in the state of MEA. In this case, both side electrodes of the circuit 60 shown in FIG. 6 may be plate-like electrodes having gas permeability and conductivity like the cathode side gas diffusion layer 55, and the MEA may be joined to the electrodes.

  In addition, although MEA or MEGA is subjected to a gas permeability improvement treatment, a method of solving the crystal phase of the ionomer when preparing a catalyst ink containing the ionomer together with the catalyst support is also possible.

DESCRIPTION OF SYMBOLS 40 ... Fuel cell 41 ... Separator 42 ... In-cell hydrogen gas flow path 43 ... In-cell air flow path 50 ... Battery cell (MEA)
DESCRIPTION OF SYMBOLS 51 ... Electrolyte membrane 52 ... Anode 53 ... Cathode 54 ... Anode side gas diffusion layer 55 ... Cathode side gas diffusion layer 60 ... Circuit

Claims (6)

A method for producing a membrane electrode assembly in which an electrode catalyst layer is joined to a membrane surface of an electrolyte membrane having proton conductivity,
In forming the electrode catalyst layer on the membrane surface of the electrolyte membrane using a catalyst ink including a conductive catalyst carrier carrying a catalyst and an ionomer having proton conductivity, the ionomer molecules in the electrode catalyst layer are formed. A method for producing a membrane electrode assembly, wherein a density reduction step is performed in which the ionomer exhibits a crystal phase and the density of dense regions is reduced by aligning the orientation.   The method for producing a membrane electrode assembly according to claim 1, wherein the density reduction step is executed in a state where the electrode catalyst layer is already formed on the membrane surface of the electrolyte membrane using the catalyst ink. It is a manufacturing method of the membrane electrode assembly according to claim 2,
The density reduction step includes:
Impregnating water inside the electrode catalyst layer formed on the membrane surface of the electrolyte membrane; and
Freezing the impregnated water;
A method for producing a membrane electrode assembly, comprising the step of thawing the frozen water and removing it by drying. It is a manufacturing method of the membrane electrode assembly according to claim 2,
The density reduction step includes:
Exposing the electrode catalyst layer formed on the membrane surface of the electrolyte membrane to a gas containing a solvent that causes dissolution of the ionomer, and introducing the solvent contained in the gas into the electrode catalyst layer; ,
A method for producing a membrane electrode assembly, comprising the step of removing the introduced solvent. It is a manufacturing method of the membrane electrode assembly according to claim 2,
The density reduction step includes:
Impregnating the electrode catalyst layer already formed on the membrane surface of the electrolyte membrane with a solution containing ions that dissolve the crystalline phase of the ionomer;
And a step of removing the impregnated solution together with the ions. A method for producing a fuel cell comprising a membrane electrode assembly in which an electrode catalyst layer is joined to a membrane surface of an electrolyte membrane having proton conductivity,
Preparing the membrane electrode assembly;
A step of sandwiching the prepared membrane electrode assembly with a gas diffusion member having gas permeability and conductivity,
The membrane electrode assembly is
In forming the electrode catalyst layer on the membrane surface of the electrolyte membrane using a catalyst ink including a conductive catalyst carrier carrying a catalyst and an ionomer having proton conductivity, the ionomer molecules in the electrode catalyst layer are formed. A method of manufacturing a fuel cell, wherein the ionomer exhibits a crystal phase by aligning the orientation, and is subjected to a density reduction step of reducing a dense area where the ionomer becomes dense.

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