JP2017188335A - Method for manufacturing membrane-electrode assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a power generation performance.SOLUTION: A method for manufacturing a membrane-electrode assembly to be used for a solid polymer fuel cell comprises the steps of: preparing a mixture solution arranged by mixing carbon carriers 22 with a catalyst 20 supported thereon, an ionomer 24 consisting of a perfluoro sulfonic acid polymer including an acid functional group and a cyclic group, and a dispersant including water and alcohol, in which the percentage of a mass of the water to a mass of the dispersant is 57-86%; preparing a catalyst ink by dispersing the carbon carriers and the ionomer in the mixture solution so that the coverage on the catalyst by the ionomer becomes 62-88%; and forming a catalyst electrode layer on a surface of an electrolyte membrane by using the catalyst ink.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a method for producing a membrane electrode assembly.

  A polymer electrolyte fuel cell includes a membrane electrode assembly in which catalyst electrode layers are provided on both surfaces of an electrolyte membrane having proton conductivity. The catalyst electrode layer includes a carbon support carrying a catalyst and an ionomer having proton conductivity. Protons reach the catalyst through the ionomer. For this reason, it is known to improve the power generation performance by increasing the coverage of the catalyst with ionomer (for example, Patent Document 1).

JP 2006-127895 A

  However, in patent document 1, the coverage of the catalyst by an ionomer is adjusted by adjusting the usage-amount of an ionomer. If the amount of ionomer used is increased in order to increase the coverage of the catalyst with the ionomer, the ionomer covering the catalyst becomes thick and the gas diffusion resistance increases, which may reduce the power generation performance.

  The present invention has been made in view of the above problems, and an object thereof is to suppress an increase in gas diffusion resistance and improve power generation performance.

  The present invention relates to a method for producing a membrane electrode assembly used in a polymer electrolyte fuel cell, comprising a carbon carrier carrying a catalyst, and an ionomer comprising a perfluorosulfonic acid polymer containing an acidic functional group and a cyclic group. A step of preparing a mixed liquid of a dispersion medium composed of water and alcohol, wherein the dispersion medium has a ratio of the mass of the water to the mass of the dispersion medium of 57% or more and 86% or less; A step of producing a catalyst ink by dispersing the carbon carrier and the ionomer in the mixed solution so that a coverage of the catalyst by the ionomer is 62% or more and 88% or less; and an electrolyte using the catalyst ink And a step of forming a catalyst electrode layer on the surface of the membrane.

  According to the present invention, power generation performance can be improved.

FIG. 1A is a cross-sectional view of a membrane electrode assembly used in a polymer electrolyte fuel cell, and FIG. 1B is an enlarged view of a cathode catalyst layer. FIG. 2 is a flowchart showing a method for manufacturing a membrane electrode assembly. FIG. 3A is a diagram showing a cyclic voltammogram obtained by cyclic voltammetry measurement, and FIG. 3B is an enlarged view of a one-dot chain line region of FIG. FIG. 4 is a diagram showing the relationship between the water ratio, the coverage, and the activity.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1A is a cross-sectional view of a membrane electrode assembly used in a polymer electrolyte fuel cell, and FIG. 1B is an enlarged view of a cathode catalyst layer. Since the anode catalyst layer has the same structure as the cathode catalyst layer, an enlarged view and description of the anode catalyst layer are omitted. As shown in FIG. 1A, a membrane electrode assembly (MEA) 10 includes an electrolyte membrane 12, and an anode catalyst layer 14a and a cathode catalyst layer 14c that are a pair of catalyst electrode layers sandwiching the electrolyte membrane 12. ,including. The electrolyte membrane 12 is a solid polymer membrane formed of a fluorine resin material or a hydrocarbon resin material, and has good proton conductivity in a wet state.

  As shown in FIG. 1B, the cathode catalyst layer 14c includes a carbon support 22 that supports a catalyst (for example, platinum) 20 that proceeds with an electrochemical reaction, and an ionomer 24 that has proton conductivity. As the ionomer 24, a perfluorosulfonic acid polymer containing an acidic functional group and a cyclic group can be used. The carbon carrier 22 is porous carbon particles. As the porous carbon particles, acetylene black-based carbon black can be used.

The catalyst 20 is supported on the outer surface of the carbon support 22 and in the pores. The ionomer 24 is provided on the outer surface of the carbon support 22 or in the pores so as to cover the catalyst 20. Since the ionomer 24 has proton conductivity, the proton H ⁺ generated by the chemical reaction of H ₂ → 2H ⁺ + 2e ^{− in} the anode catalyst layer 14a is conducted to the cathode catalyst layer 14c through the electrolyte membrane 12 and then ionomer. It moves through 24 and reaches the catalyst 20. Therefore, under operating conditions where there is not enough moisture in the cathode catalyst layer 14c, the catalyst 20 covered with the ionomer 24 contributes to power generation, but the catalyst 20 not covered with the ionomer 24 cannot contribute to power generation.

  Next, an experiment conducted by the inventor will be described. First, a method for manufacturing the MEA 10 used in the experiment will be described. FIG. 2 is a flowchart showing a method for manufacturing the MEA 10. As shown in FIG. 2, first, pure water was added to the carbon carrier 22 supporting the catalyst 20 and stirred (step S10). As the carbon support 22 for supporting the catalyst 20, an acetylene black carbon black support having 50 wt% (weight percent) of platinum supported thereon was used.

Next, 1-propanol was further added and stirred (step S12). Next, the ionomer 24 is further added and stirred so that the ratio (I / C) of the weight I of the ionomer 24 to the weight C of the carbon support 22 supporting the catalyst 20 is 1.0, and a mixed solution is prepared. (Step S14). As the ionomer 24, a perfluorosulfonic acid polymer containing an acidic functional group and a cyclic group was used. Table 1 shows the preparation amounts of the catalyst, ionomer, pure water, and 1-propanol, and the water ratio that is the ratio of the mass of pure water to the total mass of the dispersion medium (pure water + 1-propanol).

  As shown in Table 1, mixed liquids A to F in which the addition amounts of pure water and 1-propanol were changed were produced. In all of the mixed liquids A to F, the amount of the catalyst was 4.0 g, and the amount of the ionomer was 8.91 g. The ionomer is composed of solid content: 20 wt%, water: 60 wt%, and 1-propanol: 20 wt%. Therefore, the water ratio is calculated in consideration of the amount of water and 1-propanol contained in the ionomer.

  Next, the carbon carrier 22 and the ionomer 24 in the mixed liquids A to F were dispersed with an ultrasonic homogenizer to prepare a catalyst ink (step S16). Dispersion with an ultrasonic homogenizer was performed using 20 cycles (total 40 minutes) of output ON: 60 seconds and output OFF: 60 seconds using US-600AT manufactured by Nippon Seiki Seisakusho. The catalyst inks prepared with the mixed liquids A to F are referred to as catalyst inks A to F, respectively.

Next, each of the catalyst inks A to F is directly applied to the electrolyte membrane 12 by spray coating so that the amount of platinum applied is 0.4 mg / cm ^2, and the cathode catalyst layer 14c and the anode are formed on the surface of the electrolyte membrane 12. A catalyst layer 14a was formed (step S18). The MEA 10 manufactured using the catalyst ink A is referred to as sample A, and similarly, the MEA 10 manufactured using the catalyst inks B to F is referred to as samples B to F.

  The inventor measured the coverage of the catalyst 20 with the ionomer 24 on the produced samples A to F. The coverage is the ratio of the area covered with the ionomer 24 in the surface area of the catalyst 20. The method for measuring the coverage performed by the inventors will be described below. In the following, a case where the coverage ratio in the cathode catalyst layer 14c is measured will be described as an example. However, the anode catalyst layer 14a can also be measured by the same method.

First, the temperature of the MEA 10 is set to the first temperature (for example, 45 ° C.). Thereafter, the humidified hydrogen gas is supplied to the anode catalyst layer 14a, and the nitrogen gas humidified so as to have a dew point at a second temperature (for example, 55 ° C.) equal to or higher than the first temperature is supplied to the cathode catalyst layer 14c. Thereby, proton H ⁺ is generated in the anode catalyst layer 14a. Proton H ⁺ is conducted to the cathode catalyst layer 14 c through the electrolyte membrane 12.

Since the temperature of the cathode catalyst layer 14c is set to a temperature (first temperature) equal to or lower than the dew point temperature (second temperature) of the humidified nitrogen gas, water droplets are generated by the humidified nitrogen gas in the cathode catalyst layer 14c. When water droplets are generated in the cathode catalyst layer 14c, the catalyst 20 that is not covered with the ionomer 24 is covered with water droplets. Since water has proton conductivity, the proton H ⁺ conducted to the cathode catalyst layer 14 c through the electrolyte membrane 12 moves not only in the ionomer 24 but also in the water droplets and reaches the catalyst 20. A state where water droplets are generated in the cathode catalyst layer 14c is referred to as a wet state.

  Next, CV (cyclic voltammetry) measurement in a wet state is performed by sweeping a potential in a range set in advance in the cathode catalyst layer 14c with reference to the potential of the anode catalyst layer 14a. Then, the first electric double layer capacity is obtained from the cyclic voltammogram obtained by the CV measurement in a wet state.

  FIG. 3A is a view showing a cyclic voltammogram obtained by CV measurement, and FIG. 3B is an enlarged view of a one-dot chain line region of FIG. In FIGS. 3A and 3B, sweeping is performed at a speed of 50 mV / sec in the positive direction from the start potential 0.05 V to the end potential 1.2 V, and then from the start potential 1.2 V to the end potential. An example of a cyclic voltammogram when sweeping in the negative direction up to 0.05 V at a speed of 50 mV / sec is shown. In addition to the cyclic voltammogram obtained by CV measurement in a wet state, a cyclic voltammogram obtained by CV measurement in a dry state described later is also shown.

As shown in FIG. 3A, when the potential is swept in the positive direction, two peaks A and B are observed. The peak A is derived from a change in current due to desorption of the proton H ⁺ adsorbed on the catalyst 20. The peak B is derived from a change in current accompanying the formation of an oxide film on the catalyst 20. When the potential is swept in the negative direction, two peaks C and D are observed. The peak C is derived from a change in current accompanying reduction of the oxide film formed on the catalyst 20. The peak D is derived from a change in current accompanying proton H ⁺ adsorption on the catalyst 20.

  In the forward sweep, a region where the change in the current value is small (a region in the circle of the alternate long and short dash line) is observed between the peak A accompanying proton desorption and the peak B accompanying oxide film formation. Similarly, even in a negative sweep, a region where the current value change is small (a region in the circle of the alternate long and short dash line) is observed between peak C accompanying reduction of the oxide film and peak D accompanying proton adsorption. . This region is an electric double layer region. Therefore, as shown by the solid line arrow in FIG. 3B, the electric double layer capacity can be measured by obtaining the current value width in this region. Since the catalyst 20 that is not covered with the ionomer 24 in a wet state is covered with water droplets, the electric double layer is covered with the surface of the ionomer 24 or the carbon carrier 22 covered with water droplets and the ionomer 24 or water droplets. Formed on the surface of the catalyst 20.

  After obtaining the first electric double layer capacity, the supply of nitrogen gas and hydrogen gas is temporarily stopped. Next, the temperature of the MEA 10 is set to a third temperature (for example, 80 ° C.) higher than the second temperature (for example, 55 ° C.). Thereafter, the humidified hydrogen gas is supplied to the anode catalyst layer 14a, and the nitrogen gas humidified so as to be a dew point at the second temperature is supplied to the cathode catalyst layer 14c.

Since the temperature of the cathode catalyst layer 14c is set to a temperature (third temperature) higher than the dew point temperature (second temperature) of the humidified nitrogen gas, no water droplets are generated in the cathode catalyst layer 14c. Since no water droplets are generated in the cathode catalyst layer 14 c, the proton H ⁺ conducted to the cathode catalyst layer 14 c through the electrolyte membrane 12 moves only in the ionomer 24 and reaches only the catalyst 20 covered with the ionomer 24. Become. A state where no water droplets are generated in the cathode catalyst layer 14c is referred to as a dry state.

  Next, CV measurement in a dry state is performed by sweeping a potential in a range set in advance in the cathode catalyst layer 14c with reference to the potential of the anode catalyst layer 14a. And the 2nd electric double layer capacity is acquired from the cyclic voltammogram obtained by CV measurement in a dry state.

  As described above, in the wet state, the electric double layer is formed on the surface of the carbon support 22 and the surface of the catalyst 20 covered with the ionomer 24 or water droplets. On the other hand, in the dry state, the electric double layer is formed only on the surface of the carbon support 22 and the surface of the catalyst 20 covered with the ionomer 24. Therefore, at the same voltage (for example, 0.4 V), the first electric double layer capacity in the wet state and the second double layer capacity in the dry state are different in size. By calculating the ratio between the first electric double layer capacity and the second electric double layer capacity, the coverage of the carbon carrier 22 by the ionomer 24 can be calculated. Since the catalyst 20 is considered to be uniformly distributed on the surface of the carbon support 22, it can be said that the coverage of the carbon support 22 by the ionomer 24 and the coverage of the catalyst 20 by the ionomer 24 are equivalent. Therefore, the coverage of the catalyst 20 by the ionomer 24 can be calculated by calculating the ratio of the first electric double layer capacity and the second electric double layer capacity. Therefore, from the percentage of the second electric double layer capacity with respect to the first electric double layer capacity ((second electric double layer capacity / first electric double layer capacity) × 100), the coverage of the catalyst 20 by the ionomer 24 is determined. Is calculated.

Table 2 shows measurement results obtained by measuring the coverage of the catalyst 20 with the ionomer 24 using the above method.

Next, the inventor performed activity evaluation on the samples A to F by current-voltage measurement (IV measurement). In addition, activity evaluation evaluated the voltage value in 0.01 A / cm < ² > of IV characteristic as activity. This is because the decrease in the output voltage in the region where the current density is low is dominated by the decrease in the activity, so the output voltage in the region where the current density is low can be used as an index for determining whether the activity is good or bad. It is. The IV measurement was performed by setting the temperature of the MEA 10 to 80 ° C. and the dew point temperature to 55 ° C., supplying air (back pressure: 150 kPa) to the cathode catalyst layer 14 c and supplying hydrogen (back pressure: 150 kPa) to the anode catalyst layer 14 a. . And it swept at 5 mV / sec in the range of 0.4V from OCV, and the voltage value in 0.01 A / cm < ² > at the time of raising a voltage from 0.4V to OCV was calculated | required.

  FIG. 4 is a diagram showing the relationship between the water ratio, the coverage, and the activity. In FIG. 4, the horizontal axis represents the water ratio (ratio of the mass of water to the total mass of the dispersion medium (water + 1-propanol) used in the preparation of the catalyst ink). The left vertical axis represents the coverage of the catalyst 20 by the ionomer 24, and the right vertical axis represents the activity of the MEA 10. As can be seen from FIG. 4, the coverage increases as the water ratio increases. That is, it can be seen that the coverage can be adjusted by adjusting the water ratio. Thus, it is thought that it is based on the following reasons that a coverage changes with water ratios.

  That is, the carbon support 22 has hydrophobicity, while the ionomer 24 has both a hydrophilic group and a hydrophobic group. For this reason, when the water ratio is high, the ionomer 24 is preferentially adsorbed on the carbon carrier 22, and as a result, the coverage of the catalyst 20 by the ionomer 24 is increased. On the other hand, when the alcohol ratio is high, the alcohol is also hydrophobic, so that the wettability with the carbon carrier 22 is good. As a result, the coverage of the catalyst 20 by the ionomer 24 is reduced. Moreover, the ionomer 24 in Example 1 is a perfluorosulfonic acid polymer containing an acidic functional group and a cyclic group, and has higher rigidity than Nafion (registered trademark) or the like generally used as an ionomer. For this reason, the ionomer 24 made of a perfluorosulfonic acid polymer containing an acidic functional group and a cyclic group can be closely attached to the carbon carrier 22 having a smaller particle diameter than an ionomer such as Nafion (registered trademark). It seems difficult. Since ionomers such as Nafion (registered trademark) are easy to coat the carbon support 22, it is difficult to change the coverage even if the water ratio is adjusted. Since the ionomer 24 is difficult to coat the carbon carrier 22, it is considered that the covering rate can be adjusted by adjusting the water ratio.

  Further, as shown in FIG. 4, the activity of the catalyst 20 covered with the ionomer 24 is low, whether it is low or high. The reason why the activity decreases as the coverage decreases is that fewer catalysts can conduct protons. The reason why the activity decreases as the coverage increases is that the influence of poisoning of the catalyst 20 by the ionomer 24 increases. Therefore, there is an optimum range of coverage in terms of improving power generation performance. In view of improving the power generation performance, the activity is preferably 0.9 V or more, and therefore the coverage is preferably 62% or more and 88% or less. That is, the water ratio is preferably 57% or more and 86% or less.

  From the above, according to Example 1, the MEA 10 is manufactured by the following manufacturing method. A carbon support 22 supporting the catalyst 20, an ionomer 24 made of a perfluorosulfonic acid polymer containing an acidic functional group and a cyclic group, a dispersion medium made of water and alcohol and having a water ratio of 57% to 86%. To prepare a mixed solution. A catalyst ink is produced by dispersing the carbon carrier 22 and the ionomer 24 in the mixed solution so that the coverage of the catalyst 20 by the ionomer 24 is 62% or more and 88% or less. Then, a cathode catalyst layer 14c and an anode catalyst layer 14a are formed on the surface of the electrolyte membrane 12 using the produced catalyst ink. According to this manufacturing method, since the coverage can be adjusted to the optimum range by adjusting the water ratio without increasing the amount of ionomer 24 used, it is possible to suppress an increase in the gas diffusion resistance of the catalyst electrode layer. As described above, the power generation performance can be improved.

  In Example 1, the case where the water ratio is 57% to 86% and the coverage is 62% to 88% is shown as an example so that the activity is 0.9 or more. Not limited. From the viewpoint of improving the power generation performance, it is preferable that the water ratio is 60% to 83% and the coverage is 64% to 86% so that the activity is 0.902 or more. More preferably, the water ratio is 65% to 80% and the coverage is 68% to 83% so that the activity is 0.904 or more.

  In Example 1, the case where acetylene black carbon black was used as the carbon carrier 22 was shown as an example. However, furnace black carbon black, graphite, carbon fiber, activated carbon, etc., pulverized products thereof, carbon nano Carbon compounds such as fibers and carbon nanotubes may be used. Although the case where platinum (Pt) is used as the catalyst 20 is shown as an example, a platinum-cobalt alloy or the like may be used.

  In Example 1, the coverage of the catalyst with the ionomer is measured by comparing the first electric double layer capacity and the second electric double layer capacity obtained from the cyclic voltammogram by CV measurement. Although shown in the example, it may be measured by other methods.

  In Example 1, the dispersion of the carbon carrier 22 and the ionomer 24 in the mixed solution is performed using an ultrasonic homogenizer, but may be performed using a ball mill or a bead mill.

  In Example 1, the case where the catalyst electrode layer is formed on the surface of the electrolyte membrane 12 by directly applying the catalyst ink to the electrolyte membrane 12 is shown as an example, but by other methods using the catalyst ink, A catalyst electrode layer may be formed on the surface of the electrolyte membrane 12.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and changes can be made within the scope of the gist of the present invention described in the claims. It can be changed.

DESCRIPTION OF SYMBOLS 10 Membrane electrode assembly 12 Electrolyte membrane 14a Anode catalyst layer 14c Cathode catalyst layer 20 Catalyst 22 Carbon support 24 Ionomer

Claims (1)

A method for producing a membrane electrode assembly used in a polymer electrolyte fuel cell,
A carbon carrier carrying a catalyst, an ionomer comprising a perfluorosulfonic acid polymer containing an acidic functional group and a cyclic group, and a dispersion medium comprising water and alcohol, wherein the ratio of the mass of the water to the mass of the dispersion medium is A step of producing a mixed liquid in which the dispersion medium is 57% or more and 86% or less;
A step of producing a catalyst ink by dispersing the carbon support and the ionomer in the mixed solution so that a coverage of the catalyst by the ionomer is 62% or more and 88% or less;
Forming a catalyst electrode layer on the surface of the electrolyte membrane using the catalyst ink, and a method for producing a membrane electrode assembly.

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