JP5922552B2 - Diffusion layer production method - Google Patents

Description

  The present invention relates to a diffusion layer of a fuel cell.

  In order to improve the drainage of the diffusion layer constituting the fuel cell, a technique is known in which a through hole is formed in the diffusion layer by pressing a mold against the diffusion layer (for example, Patent Document 1).

JP 2005-174621 A

  The problem of the prior art is that the cost increases due to the step of pressing the mold against the diffusion layer. In addition, resource saving, easy production, and improved usability have been desired.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms.

(1) According to one aspect of the present invention, a method for producing a diffusion layer for a fuel cell is provided. In this production method, a diffusion layer is produced using a first step of producing a paste containing carbon, a second step of aging the paste produced in the first step, and a paste aged in the second step. And a third step. According to this production method, the performance of the fuel cell can be improved by a technique that does not increase the cost much. The term “ripening” as used herein means agglomeration of carbon by being left standing. When a paste in which carbon is aggregated is used, the number of cracks generated in the diffusion layer increases or becomes large. When the number of cracks increases or becomes larger, the drainage performance is improved, and as a result, the performance of the fuel cell is improved. Moreover, aging for agglomerating carbon is low cost.

According to the following form, aging can be performed appropriately.
(2) In the above production method, the aging period is 30 to 60 days.
(3) In the production method, the aging increases the bottom solid content of carbon in the paste produced in the first step so that the increment is 10% to 25%.
(4) In the above production method, the aging increases the bottom solid content of carbon in the paste produced in the first step so that the increased value is from 22% to 25%.

  The present invention can be realized in various forms other than the above. For example, it can be realized as a paste production method or a fuel cell production method.

1 is a cross-sectional view of a part of a fuel cell. The flowchart which shows the production process of a diffused layer. Relationship between the aging period, the proportion of the bottom solids, and the size of the cracks. The graph which shows the relationship between the hydraulic pressure of a diffused layer, and the ratio of bottom solid content.

  FIG. 1 shows a partial cross section of a fuel cell 10. The fuel cell 10 includes an MEA (membrane electrode assembly) 100, an anode side separator 200a, and a cathode side separator 200c. The anode side separator 200a and the cathode side separator 200c sandwich the MEA 100. The fuel cell 10 has a stack structure of a plurality of single cells.

  The MEA 100 includes a catalyst coated film (CCM: Catalyst Coated Membrane) 130, an anode side gas diffusion layer 140a, and a cathode side gas diffusion layer 140c. The catalyst coating film 130 includes an electrolyte film 110, an anode side catalyst layer 112a, and a cathode side catalyst layer 112c. The anode side gas diffusion layer 140a includes an anode side gas diffusion base material layer 114a and an anode side MPL (Micro Porous Layer) 116a.

  The anode side MPL 116a is formed on the anode side gas diffusion base material layer 114a and then joined to the catalyst coating film 130. The anode side catalyst layer 112a and the anode side gas diffusion layer 140a constitute the anode side electrode 120a.

  The cathode side gas diffusion layer 140c includes a cathode side gas diffusion base material layer 114c and a cathode side MPL 116c, and has the same configuration as the anode side gas diffusion layer 140a. Hereinafter, the anode-side gas diffusion layer 140a and the cathode-side gas diffusion layer 140c are collectively referred to as “bipolar diffusion layers 140a, c”. In addition, those existing on the anode side and the cathode side are referred to as “bipolar”. Say it with a head.

  FIG. 2 is a flowchart showing an outline of the production process of the bipolar diffusion layers 140a and 140c. As shown in FIG. 2, first, a paste is produced (step S400). The main composition of the paste is as follows: carbon is 70 to 90% by mass (total solids ratio), PTFE is 15 to 25% by mass (total solids ratio), and the dispersant (surfactant) is 5 to 15% by mass (carbon). Solid content ratio). As carbon conditions, a particle diameter of 20 to 150 nm, conductivity that can be used for battery applications, and almost no metal contamination (mixing of metals as impurities) were adopted. PTFE functions as a water repellent treatment agent. The physical properties of the paste were that the solid content was 15 to 25% by mass, the viscosity (when the shear rate was 50 / s) was 500 to 2500 mPa · s, and the storage modulus was 500 to 5500 Pa.

  Next, the paste is aged (step S410). The aging of the paste referred to here is to agglomerate carbon contained in the paste by leaving the produced paste as it is. Three types of paste maturation periods were adopted: zero days, 30 days, and 60 days. The paste having a ripening period of zero days means a paste immediately after production. The ratio of the bottom solid content of the paste (hereinafter referred to as “bottom solid ratio”) during the aging period of zero day, 30 days, and 60 days was 20 mass%, 22 mass%, and 25 mass%, respectively. The bottom solid ratio will be described with reference to FIG.

  FIG. 3 shows the relationship between the aging period, the bottom solid ratio, and the size of cracks (described later). The bottom solid ratio is the ratio of the solid content in the paste at the bottom. Carbon and PTFE occupy most of the solid content. The bottom is the bottom of the container 300 that stores the paste. The range of the bottom can be arbitrarily set according to the size and shape of the container, for example, from the bottom to 1/4. The measurement range of the bottom solid content may be a partial range of the bottom or the entire bottom. When a part is adopted as the measurement range, it is preferable to target as close to the bottom as possible. The measurement in the case where a part is adopted as the measurement range is performed, for example, by collecting the paste in the measurement range with a dropper or the like and measuring the ratio of the solid content of the collected paste. Alternatively, the ratio of the solid content may be estimated by measuring the viscosity of the paste in the measurement range while in the container 300.

  Subsequently, the aged paste is stirred and then die-coated on the bipolar gas diffusion base material layers 114a and 114c (step S420). The coating conditions are a discharge rate per unit time of 1.5 ml / s and a coating speed of 2 m / min.

  As the material of the bipolar gas diffusion base material layers 114a and 114c, for example, a carbon porous body such as carbon paper or carbon cloth, or a metal porous body such as a metal mesh or foam metal is adopted. The bipolar gas diffusion base material layers 114a and 114c are subjected to water repellent treatment. This water-repellent treatment is performed by immersing in a dispersion liquid such as PTFE (polytetrafluoroethylene) or tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA).

  Finally, the paste applied to the bipolar gas diffusion layers 114a and 114c is baked (step S430). The firing time was 4 min. The firing temperature was determined from a range of 250 to 350 ° C.

  FIG. 4 is a graph showing the relationship between the hydraulic pressure of the diffusion layer and the bottom solid ratio. The water permeation pressure of the bipolar diffusion layer is a pressure required for water to permeate from the catalyst coating film 130 to the bipolar MPLs 116a, c. A lower value is preferable for the performance of the fuel cell. This is because a low hydraulic pressure value means good drainage. The better the drainage, the better the power generation performance and durability.

  As shown in FIG. 4, when the bottom solid ratio was 20%, the water permeation pressure was 67 ± 1.8 (mean ± standard deviation) kPa. When the bottom solid ratio was 22%, the water permeation pressure was 25 ± 0.9 kPa. When the bottom solid ratio was 25%, the water permeation pressure was 13 ± 1.4 kPa. The measurement was performed 10 times in each case.

  Thus, it is thought that the reason why the hydraulic pressure decreases as the bottom solid content increases is that the cracks of the bipolar MPLs 116a and 116c increase as the bottom solid content increases as shown in FIG. . The reason why the cracks increased in this way is considered to be because the aggregation of carbon contained in the paste progressed as aging progressed, as shown in FIG. The carbon in the aging zero day state is not so agglomerated by the dispersing agent and stirring, and is dispersed throughout the container. As ripening progresses from this state, the action of the dispersant is weakened, and carbon is precipitated at the bottom by its own weight. As a result, carbon aggregation proceeds at the bottom.

  Carbon agglomeration does not disappear so much even if the paste is stirred again in preparation for coating. As a result, the paste that has been aged for 30 days or more is applied in a state where carbon is agglomerated. As a result, as shown in FIG. 3, when the paste is aged for 30 days or more, the carbon constituting the bipolar MPLs 116a, c tends to be unevenly distributed. If carbon is distributed inhomogeneously, cracks increase or grow. When cracks increase or increase, drainage improves. Such cracks were observed by SEM.

  The technique for improving the drainage as in this embodiment hardly increases the cost. In order to improve the durability of the fuel cell, when the bipolar MPLs 116a and 116c are formed thick (for example, 80 to 100 μm), the drainage performance is likely to deteriorate, so that the drainage performance can be improved at a low cost. It is. On the other hand, as a means for improving drainage by a technique other than this embodiment, a thickener may be added to promote carbon aggregation. However, the thickener is consumed, which is contrary to resource saving and increases the cost. In addition, when a thickener is added, the cost also increases due to an increase in firing load.

  The present invention is not limited to the above-described embodiments, examples, and modifications, and can be realized with various configurations without departing from the spirit of the present invention. For example, the technical features in the embodiments, examples, and modifications corresponding to the technical features in each embodiment described in the summary section of the invention are to solve some or all of the above-described problems, or In order to achieve part or all of the above-described effects, replacement or combination can be performed as appropriate. If the technical feature is not described as essential in the present specification, it can be deleted as appropriate.

  For example, the aging period and the bottom solid content are not limited to those employed in the above-described embodiments. For example, the bottom solids value may be 25-30%. On the other hand, if the bottom solid content is too large, the cracks become too large. If the crack becomes too large, through holes may be formed in the bipolar MPLs 116a, c, which is not preferable. In addition to this, if the solid content at the bottom becomes large and the agglomeration proceeds excessively, it may be difficult to stir the paste before coating. In consideration of such a phenomenon, it is preferable to appropriately determine the value of the bottom solid content, the aging period, and the temperature of the paste during aging. It is considered that the temperature of the paste at the time of aging affects the progress of carbon aggregation.

DESCRIPTION OF SYMBOLS 10 ... Fuel cell 100 ... Membrane electrode assembly 110 ... Electrolyte membrane 112a ... Anode side catalyst layer 112c ... Cathode side catalyst layer 114a ... Anode side gas diffusion base material layer 114c ... Cathode side gas diffusion base material layer 116a ... Anode side MPL
116c ... cathode side MPL
DESCRIPTION OF SYMBOLS 120a ... Anode side electrode 120c ... Cathode side electrode 130 ... Catalyst coating film 140a ... Anode side gas diffusion layer 140c ... Cathode side gas diffusion layer 200a ... Anode side separator 200c ... Cathode side separator 300 ... Container

Claims (5)

A method for producing a diffusion layer for a fuel cell, comprising:
A first step of producing a paste containing carbon;
A second step of aging the paste by agglomerating carbon contained in the paste by leaving the paste produced in the first step;
A third step of producing a diffusion layer using the paste aged in the second step.   The production method according to claim 1, wherein the aging period is 30 to 60 days. In the second step, the paste is stored in a container,
By the aging, the pastes are produced in a first step, the height direction of the container have you in the range of from the bottom up to 1/4 increased proportion of solid content is occupied by carbon, the first step The production method according to claim 1 or 2, wherein an increment of the ratio with respect to immediately after is 10% to 25%. In the second step, the paste is stored in a container,
By the aging, the pastes are produced in a first step, the height direction of the container have you in the range of from the bottom up to 1/4 increased mass ratio of solid content is occupied by carbon, the mass ratio the method of production according to claim 1 or claim 2 values after the second step is 25% by mass 22% by mass.   In the second step, the paste is stored in a container,
  As a result of the aging, the mass produced by the solid content of the paste produced in the first step in the range from the bottom to ¼ in the height direction of the container is increased immediately after the first step. The increment of the mass ratio is 10% to 25%, and the value of the mass ratio after the second step is 22 mass% to 25 mass%.
  The production method according to claim 1 or claim 2.

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