JP2005235444A - Membrane-electrode junction and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the durability of the membrane-electrode junction of a solid polymer fuel cell. <P>SOLUTION: Membrane-electrode junctions are manufactured by spray coating electrolyte films with catalytic ink consisting of a catalyst material, a support, and a solvent. In the manufacturing, spray coating of the catalytic ink is conducted by using the catalytic ink so that the particles of the catalytic ink are adjusted to a predetermined particle size or less (preferably 1 μm or less). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a membrane electrode assembly for a polymer electrolyte fuel cell and a method for producing the same.

  A membrane electrode assembly is used as an electrode of a polymer electrolyte fuel cell. A membrane / electrode assembly is a carrier (eg, carbon particles) carrying a catalyst material (eg, gold or platinum) (hereinafter referred to as “catalyst ink particles” in the present invention) dispersed in an aqueous or organic solvent. The formed catalyst ink is directly applied to the electrolyte membrane and dried, or the catalyst ink is once applied to the transfer body and dried, and then hot pressed (thermal transfer) to the electrolyte membrane. Patent Document 1 (Japanese Patent Laid-Open No. 2001-68119) describes that the catalyst ink is applied by spray application.

  In Patent Document 2 (Japanese Patent Laid-Open No. 11-16484), in the membrane electrode assembly as described above, the electrolyte membrane is pierced by the irregularities on the surface of the catalyst layer, which is a catalyst ink, and a pinhole is generated. It is described that an electrolyte layer made of an electrolyte material is further formed between the catalyst layer and the electrolyte membrane in order to avoid the performance degradation and shortening of the life of the electrode assembly, which may be avoided.

JP 2001-68119 A JP-A-11-16684

  When a catalyst layer is formed on an electrolyte membrane using a catalyst ink that has been used in the past, the electrolyte membrane is damaged by the fine irregularities present on the surface of the catalyst layer, and the performance and life of the membrane / electrode assembly are reduced. It is known to cause crystallization. For example, when a membrane electrode assembly is manufactured by transferring a transfer body coated with a catalyst layer to an electrolyte membrane by hot pressing, fine irregularities (projections) existing on the surface of the catalyst layer bite into the electrolyte membrane during transfer. It causes scratches and cracks in the electrolyte membrane. And it grows with time and leak defects increase, leading to performance degradation and shortening of life of the membrane electrode assembly. Even when the catalyst ink is applied directly to the electrolyte membrane by spray coating or ink jet coating, the unevenness formed on the opposite side of the catalyst layer from the electrolyte membrane is extruded by the diffusion layer when the diffusion layer is joined and pressed. Cause scratches and cracks.

  Conventionally, the occurrence of such irregularities is accepted as unavoidable, or additional measures as described in Patent Document 2 are applied to avoid damage. Further forming an electrolyte layer as a boundary layer, although slightly, increases the thickness of the membrane electrode assembly and increases the number of processes, and is not a practical solution.

  The present invention has been made in view of the above circumstances, and smoothness of the coated surface is eliminated by eliminating fine irregularities that are naturally present on the surface of the catalyst layer formed of the catalyst ink. Thus, it is possible to prevent the electrolyte membrane from being damaged by the unevenness formed in the catalyst layer as in the past, and effectively avoid the performance degradation and shortening of the life of the membrane electrode assembly. An object of the present invention is to provide a membrane electrode assembly and a method for producing the same.

  In order to solve the above problems, the present inventor analyzed the surface properties of the catalyst layer formed by applying the catalyst ink with a micrograph or a laser microscope, and determined the surface properties and the durability performance of the membrane electrode assembly. An empirical experiment was conducted on the correlation. As a result, it is possible to reduce the size of the unevenness and reduce the number of unevenness to some extent by devising various methods for adjusting and applying the ink, but depending on the type of ink, It has been found that it is unavoidable that protrusions having a height of about 5 μm and a diameter of about 30 μm are formed on the surface of the coated surface. Further, the number of occurrences per unit area was different depending on the type of ink. In the present invention, such a protrusion (unevenness) is expressed as “catalyst dama” or “aggregate”.

FIG. 1 is a view from a laser micrograph showing a coated surface with a large amount of catalyst lumps (FIG. 1a) and a coated surface with a relatively small amount (FIG. 1b). The catalyst dams having a height of about 5 μm and a diameter of about 30 μm are shown in FIG. those in the 70/1 mm ² of about FIG. 1a, is that of FIG. 1b is expressed in 60/1 mm ² approximately. The present inventor conducted an analytical test using several catalyst inks on how the catalyst lumps (aggregates) are specifically developed. As a result, when the catalyst material is a noble metal and the carrier is a carbon carrier, it has been found that if the amount of impurities contained in the carbon carrier is increased, the amount of catalyst dust in the ink is increased as a result. This is understood that the impurity-rich portion of the catalyst carrier (carbon carrier) produces a hard carbon agglomerate, which remains in the ink as a catalyst dam that cannot be avoided.

  Therefore, when the types of impurities contained in different types of carbon carriers were investigated by the XRF method, S, V, Fe, Ni, and Na were detected as the main impurities, and the amounts differed depending on the carbon carriers. It was. Moreover, the maximum particle size of each carbon support was also different. It is shown in Table 1.

  The expression amount of the catalyst dama in each is as shown in Table 2, and it was found that the amount of impurities (% by weight) as a whole and the expression rate of the catalyst dama are clearly correlated. It was also found that depending on the type of impurities, the amount of impurities greatly affects the rate of catalyst dams (V, Fe, Ni), and that which does not significantly affect (S, Na). . Furthermore, when the amount of impurities is small, the particle size of the carrier species tends to be small, when the amount of impurities is large, the particle size of the carrier species tends to be large, and when the particle size of the carrier species is small, the unevenness of the coating surface In the case of overcoating for the purpose of improving smoothness, the catalyst lumps formed on the coating surface applied earlier may cause the adhesion of aggregates in the next application. It was easy to provide a basis, and it was confirmed that increasing the number of coating layers increased the catalyst lumps proportionally.

  The present invention is based on the above new knowledge obtained by the present inventor. That is, the first invention is a method of manufacturing a membrane electrode assembly by spray-coating a catalyst ink comprising a catalyst substance, a support and a solvent on an electrolyte membrane and drying it. It is characterized by using the catalyst ink adjusted so that it may become a catalyst ink particle of a particle size or less.

  The second invention is a method for producing a membrane electrode assembly by spraying a catalyst ink comprising a catalyst substance, a carrier and a solvent on a transfer body, drying the transferred ink, and then transferring it to an electrolyte membrane. The catalyst ink is spray-coated using a catalyst ink adjusted to have catalyst ink particles having a predetermined particle size or less.

  The first invention relates to a method for directly applying a catalyst ink for an electrode to an electrolyte membrane in a method for producing a membrane electrode assembly, and the second invention is to apply the ink to a transfer body once and apply it to the electrolyte membrane. The present invention relates to a method of hot pressing. In any case, the new knowledge obtained by the present inventors, that is, the fine protrusions that cannot be avoided in the conventional catalyst ink, is the presence of impurities contained in the support (catalyst ink particles). Is the main factor, and the size of the fine protrusions is also due to the size of the carrier carrying such impurities. By using the catalyst ink adjusted so as to become the catalyst ink particles, it is possible to suppress the catalyst ink particles from growing unnecessarily, and it is possible to stack a catalyst layer having no lumps, and durability (cross leak amount is reduced). A membrane electrode assembly with improved discharge time until reaching a limit value can be obtained.

  In the experiments of the present inventors, in each invention of the present application, it is particularly preferable that the maximum particle diameter of the catalyst ink particles is 1 μm or less, and as shown in the following examples, the catalyst ink particles having a maximum particle diameter of 5 μm. Compared with the case of using the catalyst ink in which is dispersed, the life of the membrane electrode assembly is greatly improved.

  As a specific method for adjusting the catalyst ink to be catalyst ink particles having a predetermined particle size or less, a carrier having a low content of impurities that cause aggregate formation is used as the carrier constituting the catalyst ink particles. Can be mentioned. This method is based on the new knowledge obtained by the present inventor, that is, the fine protrusions that cannot be avoided in the conventional catalyst ink are mainly caused by the presence of impurities contained in the support. Based on this knowledge, the catalyst ink is adjusted using a carrier having a low content of impurities from the beginning, thereby reducing the occurrence of aggregates (catalyst lumps) of the catalyst layer formed by spray coating. be able to. As a result, the particle size of the catalyst ink particles becomes a predetermined value or less, the smoothness of the coated surface is improved, and damage to the electrolyte membrane is further suppressed. Thereby, the performance of the membrane electrode assembly is further improved.

  In the case where the support is a carbon support, among the contained impurities, V, Fe, and Ni are greatly involved in the expression of catalyst lumps (aggregates), as described above, the knowledge obtained by the present inventors for the first time. Therefore, it is particularly preferable to use a carbon support having a low content of at least one of V, Fe, and Ni among the impurities as the catalyst support.

  Then, as shown in Tables 1 and 2, there is a marked difference in the amount of catalyst dams appearing on the basis of the respective values of the weight percentages of impurities V, Fe, and Ni (support types A, B, C). Is a new finding obtained by the present inventor. Therefore, particularly when the impurity is V, it is about 0.01% by weight or less, and when it is Fe. It is also a preferred embodiment of the present invention to use a carbon support containing a small amount of impurities, such as about 0.05% by weight or less, and in the case of Ni, about 0.05% by weight or less.

  Also, spray coating is a non-contact type coating, and the solvent can be evaporated while the ink droplet reaches the coating film from the nozzle, so that the electrolyte membrane can be prevented from being damaged by the solvent. . However, if the distance (coating distance) until the ink droplet reaches the coating film from the nozzle is too long, the fluidity of the ink droplet after coating becomes poor, and undulation (unevenness) occurs on the coated surface It may happen that it remains as it is. Therefore, it is preferable that the coating distance when the catalyst ink is applied by spraying is a distance at which the ink after application can flow, so that the ink droplets after application can flow and the catalyst ink particles are fine. In combination with this, the smoothing of the coated surface further proceeds. Although depending on the electrode ink to be used, in the experiments by the present inventors, it was particularly preferable that the coating distance of the catalyst ink be between 15 mm and 50 mm.

  Furthermore, it is preferable to perform spray coating as overcoating to obtain a highly smooth coating surface, but in the experiments of the present inventors, as described above, when obtaining a catalyst layer having the same thickness, It was experienced that when the number of times of overcoating was too many, the expression of aggregates (catalyst lumps) increased and increased. Therefore, it is preferable that the number of times of coating is small on condition that the smoothness is satisfied. Specifically, the overcoating of 10 times or less is suitable.

  The present invention is also a membrane electrode assembly obtained by spray-coating a catalyst ink composed of a catalyst substance, a carrier and a solvent on both surfaces of an electrolyte membrane, as a membrane electrode assembly obtained by the above production method, The catalyst ink is a catalyst ink adjusted so that the maximum particle diameter of the catalyst ink particles is 1 μm or less, and the carrier constituting the catalyst ink particles is a carbon carrier, and the carbon carrier includes at least 0.01. There is also disclosed a membrane electrode assembly characterized in that it contains V of about wt% or less, Fe of about 0.05 wt% or less, and / or Ni of about 0.05 wt% or less as impurities.

  According to the present invention, it is possible to eliminate fine irregularities that are supposed to be present on the surface of the catalyst layer formed by spray coating of the catalyst ink. Thereby, in the membrane electrode assembly, it is possible to prevent the electrolyte membrane from being damaged by the unevenness formed in the catalyst layer as in the past, and it is effective to reduce the performance and shorten the life of the membrane electrode assembly. Can be avoided.

  Hereinafter, the present invention will be described with reference to examples and comparative examples. Of course, the present invention is not limited to this.

[Example 1]
As a carbon support of the cathode catalyst, the maximum particle size is 1 μm or less, the impurity S is 0.034 wt%, V is 0.0 wt%, Fe is 0.004 wt%, Ni is 0.002 wt%, Na Containing 0.0% by weight (carbon carrier type E in Table 1 described above). A platinum catalyst for a cathode was prepared by supporting 60% of platinum as a catalyst on a carbon support (amount of catalyst: 0.45 mg / cm ² ). Using the platinum catalyst for cathode, water, ethanol, propylene glycol, and 10% electrolyte solution were prepared (electrolyte ratio 0.6) to prepare a cathode catalyst ink.

The same carbon support as that of the cathode catalyst was used as the carbon support of the anode catalyst, and 30% of platinum of the catalyst was supported on the carbon support to produce a platinum catalyst for the anode (catalyst amount: 0.15 mg / cm 2). ² ). Using the anode platinum catalyst, water, ethanol, propylene glycol, and a 10% electrolyte solution were prepared (electrolyte ratio 1.0) to prepare an anode catalyst ink.

  The electrolyte membrane was set on a heating and suction platen (temperature: 40 ° C.). Cathode catalyst ink was spray coated on the membrane under the coating conditions shown in Table 3, and anode catalyst ink was spray coated on the opposite side of the membrane. . It was dried (drying temperature: 100 ° C.) to obtain a membrane electrode assembly.

  A diffusion layer, which is a carbon cloth, is transferred to both sides of the cathode catalyst layer and the anode catalyst layer of the obtained membrane electrode assembly (100 ° C., 4 minutes at 3 MPa) to form an electrode, and then a firing separator is assembled to did. The module was subjected to a discharge durability evaluation test. The results are shown in FIGS. 2 shows the relationship between time and the amount of cross leak, and FIG. 3 shows the relationship between time and voltage drop of each cell.

[Comparative Example 1]
As the carbon support for the cathode catalyst and the carbon support for the anode catalyst, both have a maximum particle size of 5 μm, impurities S is 0.29 wt%, V is 0.53 wt%, Fe is 0.18 wt%, Ni is Cathode catalyst ink and anode catalyst ink were prepared in the same manner as in Example 1 except that 0.30% by weight and Na containing 0.025% by weight (carbon carrier type A in Table 1 described above) were used. Created. Using this, a membrane electrode assembly was prepared in the same manner as in Example 1, and a discharge durability evaluation test was conducted as a module. The results are shown in FIGS. 2 shows the relationship between time and the amount of cross leak, and FIG. 3 shows the relationship between time and voltage drop of each cell.

[Evaluation]
As shown in FIGS. 2 and 3, the module of Example 1 has higher module durability than that of the comparative example, indicating the effectiveness of the present invention. This is mainly due to the fact that the use of carbon carrier species with few impurities and a small maximum particle size (1 μm or less compared to 5 μm) suppresses the occurrence of lumps and improves the surface smoothness. It is thought that. Another factor is that the distance between the spray nozzle and the electrolyte membrane during spray coating is reduced (20 mm compared to 90 mm), resulting in the flow of droplets after coating and further smoothing. It is thought that. Further, by reducing the number of coating layers (5 layers compared to 25 layers), the catalyst dust formed on the previously coated surface becomes the basis for the adhesion of aggregates in the next coating. It was thought that the fact that it was possible to suppress the large growth of the film also contributed to the improvement of smoothness.

[Example 2]
As a carbon support of the cathode catalyst, the maximum particle size is 1 μm or less, the impurity S is 0.034 wt%, V is 0.0 wt%, Fe is 0.004 wt%, Ni is 0.002 wt%, Na Containing 0.0% by weight (carbon carrier type E in Table 1 described above). A platinum catalyst for a cathode was prepared by supporting 45% of platinum as a catalyst on a carbon support (amount of catalyst: 0.45 mg / cm ² , electrolyte ratio 0.6).

As the carbon support of the anode catalyst, the maximum particle size is 1 μm or less, the impurity S is 0.870 wt%, V is 0.0 wt%, Fe is 0.001 wt%, Ni is 0.002 wt%, Na Was used (carbon support type F in Table 1 described above), and 30% of platinum as a catalyst was supported on the carbon support to prepare a platinum catalyst for an anode (amount of catalyst: 0.15 mg / cm ² , electrolyte ratio 0.6).

  Cathode catalyst ink and anode catalyst ink having the compositions shown in Table 4 were prepared using a cathode platinum catalyst and an anode platinum catalyst. The boiling point of propylene glycol is 188.2 ° C., and the boiling point of ethanol is 78.3 ° C.

  The electrolyte membrane was set on a heating and suction platen (temperature: 40 ° C.). Cathode catalyst ink was spray coated on the membrane under the coating conditions shown in Table 5, and anode catalyst ink was spray coated on the opposite side of the membrane. . When the coated surface state was examined closely, the surface unevenness was 5 μm or less on both the cathode side and the anode side, and a fine coated surface was obtained. It was dried (drying temperature: 100 ° C.) to obtain a membrane electrode assembly.

  A diffusion layer, which is a carbon cloth, is transferred to both sides of the cathode catalyst layer and the anode catalyst layer of the obtained membrane electrode assembly (100 ° C., 4 minutes at 3 MPa) to form an electrode, and then a firing separator is assembled to did. The module was subjected to a discharge durability evaluation test in the same manner as in Example 1.

[Comparative Example 2]
Cathode catalyst inks and anode catalyst inks having the compositions shown in Table 6 were prepared using the same carbon carrier as the cathode catalyst and anode catalyst as in Example 2. It was applied to the electrolyte in the same manner as in Example 2. After coating, the coated surface state was examined in the same manner as in Example 1. As a result, the surface unevenness was 20 μm or more on both the cathode side and the anode side, and the coated surface was rough compared to Example 2. It was dried (drying temperature: 100 ° C.) to form a membrane electrode assembly, and a module was further obtained. The module was subjected to a discharge durability evaluation test in the same manner as in Example 1.

[Discussion]
The results of the discharge durability evaluation test were almost the same as those shown in FIGS. 2 and 3 in both the relationship between time and the amount of cross leak and the relationship between time and voltage drop of each cell. This is because, as in Example 2, by using a high boiling alcohol such as glycols in the catalyst ink, the volatility of the ink is suppressed and fluidization is facilitated, and the smoothness of the coating surface is further improved. In the case of Example 2, it can be estimated that high discharge durability was obtained. On the other hand, in the case of Comparative Example 2, since propylene glycol was not added and only ethanol having a low boiling point of 78.3 ° C. was used as a solvent, the surface asperity was as coarse as 20 μm or more, which showed discharge durability. It seems to be the cause of the problem.

  Further, in Example 2, the solid content concentration is 3.5%, while in Comparative Example 2, it is 7.0%. Therefore, it can be assumed that this also affects the quality of the surface smoothness.

The figure by the laser micrograph of the coating-film surface of catalyst ink. The graph which shows the relationship between the time in the module of Example 1 and the comparative example 1, and the amount of cross leaks. The graph which shows the relationship between the time in the module of Example 1 and the comparative example 1, and a cell voltage drop.

Claims (10)

  A method for producing a membrane electrode assembly by spray-coating a catalyst ink comprising a catalyst substance, a carrier and a solvent on an electrolyte membrane and drying the catalyst ink, wherein the catalyst ink spray-coating comprises catalyst ink particles having a predetermined particle size or less and A method for producing a membrane electrode assembly, which is carried out using a catalyst ink adjusted as described above.   A catalyst ink comprising a catalyst material, a support and a solvent is spray-coated on a transfer body and dried, and then transferred to an electrolyte membrane to produce a membrane electrode assembly. A method for producing a membrane electrode assembly, which is performed using a catalyst ink adjusted to have catalyst ink particles having a predetermined particle size or less.   The method for producing a membrane / electrode assembly according to claim 1 or 2, wherein the catalyst ink is used so that the maximum particle diameter of the catalyst ink particles is 1 µm or less.   4. The membrane electrode bonding according to claim 1, wherein a catalyst ink using a carrier having a low content of impurities causing the generation of aggregates is used as the carrier constituting the catalyst ink particles. 5. Body manufacturing method.   5. The method for producing a membrane electrode assembly according to claim 4, wherein the carrier is a carbon carrier and the impurity is at least one of V, Fe or Ni.   A carbon support containing 0.01 wt% or less when the impurity is V, 0.05 wt% or less when the impurity is Fe, and 0.05 wt% or less when the impurity is Ni is used. The manufacturing method of the membrane electrode assembly of Claim 5.   The method for producing a membrane / electrode assembly according to any one of claims 1 to 6, wherein the coating distance when the catalyst ink is applied by spraying is a distance at which the applied ink can flow.   8. The method for producing a membrane electrode assembly according to claim 7, wherein the coating distance of the catalyst ink is between 15 mm and 50 mm.   The method for producing a membrane / electrode assembly according to any one of claims 1 to 8, wherein the spray coating is performed as a repeated coating of 10 times or less.   A membrane electrode assembly in which a catalyst ink comprising a catalyst substance, a support and a solvent is spray-coated on both surfaces of an electrolyte membrane, and the catalyst ink is adjusted so that the maximum particle diameter of the catalyst ink particles is 1 μm or less. Catalyst ink, and the carrier constituting the catalyst ink particles is a carbon carrier, and the carbon carrier has at least 0.01 wt% or less of V, 0.05 wt% or less of Fe, and / or 0. A membrane / electrode assembly comprising 05% by weight or less of Ni as an impurity.

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