JP2016035869A - Method for manufacturing cathode catalyst layer for fuel battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a cathode catalyst layer, which never causes the increase in the amount of platinum over a conventional amount thereof, and when used for a fuel battery, enables the rise of an output of the fuel battery.SOLUTION: A method for manufacturing a cathode catalyst layer for a fuel battery, which includes metal-supporting carbon including platinum or a platinum cobalt alloy as a catalyst component, and an ionomer comprises the step of preparing a cathode catalyst ink by mixing the metal-supporting carbon, the ionomer, a copper compound and a dispersant so that the mass mof copper and the mass mof the metal-supporting carbon satisfy a mixing ratio given by the expression (1) below, and the step of forming the cathode catalyst layer by use of the cathode catalyst ink: 0 mass%<{m/(m+m)}<0.117 mass% Expression (1).SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing a cathode catalyst layer that can improve the output of a fuel cell when used in a fuel cell without increasing the amount of platinum than in the prior art.

  For the cathode of a fuel cell, platinum-supported carbon is usually used as an oxygen reduction catalyst. Patent Document 1 includes a supported catalyst obtained by supporting at least one of platinum and a platinum alloy on a particulate carbon support and a proton conductive solid electrolyte, and a part of the supported catalyst has a predetermined characteristic. A cathode catalyst layer for a polymer electrolyte fuel cell in which an aggregate is formed is disclosed.

JP 2005-056593 A

However, in the cathode catalyst layer for a polymer electrolyte fuel cell disclosed in Patent Document 1, it is only necessary to increase the amount of platinum in order to increase the output, but the increase in the amount of platinum is a factor that increases the manufacturing cost. It becomes.
The present invention has been accomplished in view of the above circumstances, and the production of a cathode catalyst layer capable of improving the output of the fuel cell when used in a fuel cell without increasing the amount of platinum than in the prior art. It aims to provide a method.

The method for producing a cathode catalyst layer for a fuel cell according to the present invention is a method for producing a cathode catalyst layer for a fuel cell comprising metal-supported carbon and ionomer containing platinum or a platinum-cobalt alloy as catalyst components, and the mass of copper m _Cu The cathode metal is prepared by mixing the metal-supported carbon, the ionomer, the copper compound, and the dispersion medium so that the mass ratio of the metal-supported carbon m _{M / C} satisfies the following formula (1). And a cathode catalyst layer is formed using the cathode catalyst ink.
0% by mass <{m _Cu / (m _Cu + m _{M / C} )} <0.117% by mass Formula (1)

According to the present invention, the copper mass ratio (m _Cu / (m _Cu + m _{M / C} )) in the cathode catalyst ink can be appropriately expressed by the formula (1) without increasing the content of expensive platinum or platinum alloy. When the obtained cathode catalyst layer is used in a fuel cell, the discharge characteristics of the fuel cell can be improved by mixing the raw materials such as metal-supported carbon so that the ratio becomes a proper ratio.

4 is a graph showing the IV curves of the membrane-electrode assemblies of Example 1 and Comparative Example 1 to Comparative Example 3 in an overlapping manner. 6 is a graph showing the relationship between the mass ratio of copper in the fuel cell cathode catalyst layer and the cell voltage for the membrane / electrode assembly of Example 1 and Comparative Example 1 to Comparative Example 3. 5 is a graph in which complex impedance curves for membrane / electrode assemblies of Comparative Example 1 and Comparative Example 3 are superimposed. 4 is a bar graph comparing gas diffusion resistances for membrane / electrode assemblies of Comparative Example 1 and Comparative Example 3. FIG.

The method for producing a cathode catalyst layer for a fuel cell according to the present invention is a method for producing a cathode catalyst layer for a fuel cell comprising metal-supported carbon and ionomer containing platinum or a platinum-cobalt alloy as catalyst components, and the mass of copper m _Cu The cathode metal is prepared by mixing the metal-supported carbon, the ionomer, the copper compound, and the dispersion medium so that the mass ratio of the metal-supported carbon m _{M / C} satisfies the following formula (1). And a cathode catalyst layer is formed using the cathode catalyst ink.
0% by mass <{m _Cu / (m _Cu + m _{M / C} )} <0.117% by mass Formula (1)

  In order to improve the high current characteristics of the fuel cell by increasing the surface area of the catalyst and improving the gas diffusibility of the catalyst layer, measures such as an increase in the surface area due to atomization of the catalyst and an increase in the basis weight of the catalyst can be considered. However, these countermeasures may have disadvantages such as a decrease in catalyst durability and an increase in manufacturing cost.

  For example, as a conventional technique, a technique is known in which a hydrophobic layer is formed by producing a catalyst layer using a foaming agent subjected to a water repellent treatment to increase the surface area of the catalyst. However, this prior art requires a step of removing the foaming agent, which increases the number of manufacturing steps, and further has the demerit that the structure of the hydrophobic pores collapses in the process of removing the foaming agent.

As a conventional technique, a technique for increasing the specific surface area by atomizing a platinum catalyst is also known. However, in general, the surface energy of platinum particles increases due to atomization, and the ionization reaction of platinum (Pt → Pt ²⁺ + 2e ⁻ ) is promoted. As a result, platinum becomes easier to elute, and the catalyst durability may be reduced.

  As a result of diligent efforts, the present inventor can improve the output of a fuel cell when the obtained cathode catalyst layer is used in a fuel cell by adding a specific amount of a copper compound to a conventional catalyst. The present invention was completed.

The production method according to the present invention includes (1) a step of preparing a cathode catalyst ink, and (2) a step of forming a cathode catalyst layer. However, the present invention is not necessarily limited to the above two steps.
Hereinafter, the steps (1) and (2) will be described in order.

1. Step of preparing cathode catalyst ink This step is a step of preparing cathode catalyst ink by mixing metal-supporting carbon, ionomer, copper compound, and dispersion medium at a mixing ratio that satisfies the following formula (1).
0% by mass <{m _Cu / (m _Cu + m _{M / C} )} <0.117% by mass Formula (1)
(In the above formula (1), m _Cu represents the mass of copper, and m _{M / C} represents the mass of the metal-supported carbon.)

  The metal-supported carbon used in the present invention is platinum-supported carbon or platinum-cobalt-supported carbon. These metal-supported carbons may be commercially available or may be prepared in advance by a known method.

  The ionomer used in the present invention is a polymer electrolyte used in a fuel cell. In addition to a fluorine-based polymer electrolyte such as perfluorocarbon sulfonic acid resin represented by Nafion (trade name), polyether ether ketone Hydrocarbon polymers such as engineering plastics such as polyetherketone, polyethersulfone, polyphenylene sulfide, polyphenylene ether, and polyparaphenylene, and general-purpose plastics such as polyethylene, polypropylene, and polystyrene. Examples thereof include hydrocarbon polymer electrolytes into which proton acid groups (proton conductive groups) such as acid groups and boronic acid groups have been introduced.

The copper compound used in the present invention is not particularly limited as long as a sufficient amount of copper ion is ionized (ion dissociated) in water. For example, CuSO ₄ and hydrates thereof can be used.

When {m _Cu / (m _Cu + m _{M / C} )} (hereinafter sometimes referred to as a copper mass ratio) represented by the above formula (1) is 0.117% by mass or more, copper ions are so-called Causes cation contamination, copper ions poison platinum and platinum cobalt, and copper ions ionically bind to anionic functional groups such as ionomer sulfonate groups (—SO ₃ ⁻ ) to inhibit proton conductivity. Phenomenon occurs. As a result, the proton resistance of the ionomer in the cathode catalyst layer increases.
The mass ratio of copper is preferably 0.110 mass% or less, and more preferably 0.100 mass% or less.

On the other hand, when the mass ratio of copper is 0% by mass, that is, when copper is not contained at all, the cation contamination is not generated, and the discharge characteristics are not improved. The reason why the discharge performance of the fuel cell is improved by including even a small amount of copper is as follows. Anionic functional groups such as sulfonate groups (—SO ₃ ⁻ ) in ionomers have an affinity for water, but as a result of copper ions forming ionic bonds with the anionic functional groups, water becomes less compatible with ionomers, Improved drainage in ionomers. Therefore, the inclusion of copper improves the gas diffusibility in the cathode catalyst layer, thereby improving the output of the fuel cell.
By adding the copper compound so that the mass ratio of copper satisfies the above formula (1), the effect of improving the gas diffusibility due to the increase in the catalyst surface area is greater than the effect due to the cation contamination, and the discharge performance of the fuel cell is improved. improves. As shown in Example 1 described later, when a copper compound was added so that the mass ratio of copper was 0.08 mass%, the voltage in the high current region (current density was 2.5 A / cm ² ) was higher than before. Was also found to improve by 3%.
The mass ratio of copper is preferably 0.02 mass% or more, and more preferably 0.05 mass% or more.

  In addition, as shown in Example 1 described later, the difference in performance compared to the conventional cathode catalyst layer (Comparative Example 1) does not occur even in the low current region because the mass ratio of copper is the above formula (1). This is because copper poisoning is moderately trapped by the ionomer, and as a result, catalyst poisoning by copper ions hardly poses a problem.

  As described above, by mixing the metal-supporting carbon, the ionomer, the copper compound, and the dispersion medium in such a mixing ratio that the copper mass ratio satisfies the above formula (1), the content of expensive platinum or platinum alloy The performance of the fuel cell can be improved without increasing the value.

  The dispersion medium used in the present invention may be appropriately selected. For example, alcohols such as methanol, ethanol and propanol, organic solvents such as N-methyl-2-pyrrolidone (NMP) and dimethyl sulfoxide (DMSO), or A mixture of these organic solvents or a mixture of these organic solvents and water can be used. In addition to the metal-supported carbon, ionomer, and copper compound, the catalyst ink may contain other components such as a binder and a water-repellent resin as necessary.

2. Step of forming cathode catalyst layer The method of forming the cathode catalyst layer is not particularly limited. For example, the cathode catalyst layer may be formed on the surface of the gas diffusion sheet by applying catalyst ink to the surface of the gas diffusion sheet and drying. Alternatively, the cathode catalyst layer may be formed on the surface of the polymer electrolyte membrane by applying a catalyst ink on the surface of the polymer electrolyte membrane and drying it. Alternatively, a transfer sheet is prepared by applying a catalyst ink to the surface of the transfer substrate and drying, and the transfer sheet is bonded to the polymer electrolyte membrane or the gas diffusion sheet by thermocompression bonding or the like. The cathode catalyst layer may be formed on the surface of the polymer electrolyte membrane or the cathode catalyst layer may be formed on the surface of the gas diffusion sheet by a method of peeling the material film.

Examples of the polymer electrolyte membrane include a membrane containing the polymer electrolyte described above.
As the gas diffusion sheet for forming the gas diffusion layer, a gas diffusion property capable of efficiently supplying fuel to the catalyst layer, conductivity, and a strength required as a material constituting the gas diffusion layer, for example, Examples thereof include a carbonaceous porous body such as carbon paper and a metal porous body such as titanium.

  The method for applying the catalyst ink, the drying method, and the like can be selected as appropriate. For example, the application method includes a spray method, a bar coating method, a screen printing method, a doctor blade method, a gravure printing method, a die coating method, and the like. Examples of the drying method include reduced pressure drying, heat drying, and reduced pressure heat drying. There is no restriction | limiting in the specific conditions in reduced pressure drying and heat drying, What is necessary is just to set suitably. The thickness of the cathode catalyst layer is not particularly limited, but may be about 1 to 50 μm.

  The cathode catalyst layer produced according to the present invention is preferably formed on the cathode portion corresponding to the vicinity of the downstream end of the oxidant gas flow path in the gas flow direction. By forming the cathode catalyst layer in the portion closest to the downstream in the gas flow direction, it is considered that the influence of catalyst poisoning (cation contamination) by copper ions is suppressed as much as possible, and the discharge performance in a high current region is further improved.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples, but the present invention is not limited only to these examples.

1. Production of cathode catalyst layer for fuel cell [Example 1]
First, a platinum-cobalt-supported carbon catalyst (PtCo / C), copper sulfate pentahydrate (CuSO ₄ .5H ₂ O), and 14.24 g of water were mixed by centrifugal stirring, and the catalyst and water were mixed. At this time, the mass ratio of copper represented by the following formula (1a) is 0.08 mass%, and the mass of PtCo / C is 0.9 g, so that the mass of PtCo / C and CuSO ₄ .5H ₂ O is 0.9 g. The amount was adjusted.
(Mass ratio of copper) = m _Cu / (m _Cu + m _{PtCo / C} ) Formula (1a)
(In the above formula (1a), m _Cu represents the mass of copper in the copper sulfate pentahydrate, and m _{PtCo / C} represents the mass of platinum-cobalt-supported carbon.)
Next, 8.16 g of ethanol was added to the mixture, and the whole mixture was homogenized by centrifugal stirring. Furthermore, 1.9 g of Ionoma (DuPont, DE2020CS) was added to the mixture, and the mixture was similarly homogenized by centrifugal stirring to obtain a catalyst ink raw material.
Under a dry atmosphere, 20 mL of the catalyst ink raw material and 60 g of crushing PTFE balls (φ = 2.4 mm) were placed in a PTFE pot and sealed. Thereafter, the container was attached to a planetary ball mill apparatus, and mechanical milling was performed under conditions of a base plate rotation speed of 600 rpm and a temperature of 20 ° C. for a treatment time of 1 hour. After completion of the mechanical milling, the mixture in the container was filtered through a mesh to remove the balls, thereby obtaining a catalyst ink.
The catalyst ink was filled in a spray gun (manufactured by Nordson, Spectrum S-920N), and a catalyst amount of 300 to 500 μg / cm ^{2 was} applied to one surface (cathode side) of the electrolyte membrane (manufactured by DuPont, NR211). It dried as it was, and the cathode catalyst layer for fuel cells of Example 1 was formed on the electrolyte membrane surface.

[Comparative Example 1]
First, 0.9 g of PtCo / C and 14.24 g of water were mixed by centrifugal stirring to adjust the catalyst and water. That is, in Comparative Example 1, CuSO ₄ · 5H ₂ O was not used.
Thereafter, a catalyst ink was prepared in the same manner as in Example 1, and the catalyst ink was applied to one surface of the electrolyte membrane and then dried to form the cathode catalyst layer for the fuel cell of Comparative Example 1 on the electrolyte membrane surface. did.

[Comparative Example 2]
In Example 1, PtCo / C and CuSO ₄ .5H ₂ were used so that the mass ratio of copper represented by the above formula (1a) was 0.15 mass% and the mass of PtCo / C was 0.9 g. Except that the amount of O was adjusted, a catalyst ink was prepared in the same manner as in Example 1, and the catalyst ink was applied to one surface of the electrolyte membrane and then dried, whereby the fuel of Comparative Example 2 was applied to the electrolyte membrane surface. A cathode catalyst layer for a battery was formed.

[Comparative Example 3]
In Example 1, PtCo / C and CuSO ₄ .5H ₂ were used so that the mass ratio of copper represented by the above formula (1a) was 0.5 mass% and the mass of PtCo / C was 0.9 g. Except that the amount of O was adjusted, a catalyst ink was prepared in the same manner as in Example 1, the catalyst ink was applied to one surface of the electrolyte membrane, and then dried, so that the fuel of Comparative Example 3 was applied to the electrolyte membrane surface. A cathode catalyst layer for a battery was formed.

2. IV Evaluation Using MEA For the electrolyte membranes on which the fuel cell cathode catalyst layers of Example 1 and Comparative Examples 1 to 3 were formed, commercially available platinum-supported carbon (on the anode side) ( Ink was prepared and applied in the same manner as the cathode side except that the amount of platinum per electrode area was 0.1 mg. In this way, a membrane / electrode assembly having an electrode area of 20 cm ² was obtained. Hereinafter, the membrane / electrode assemblies produced using the fuel cell cathode catalyst layers of Example 1 and Comparative Examples 1 to 3 were used as the membrane / electrode assemblies of Example 1 and Comparative Examples 1 to 3, respectively. Sometimes called body.
The obtained membrane / electrode assembly was subjected to IV evaluation under the following conditions.
・ Fuel gas: Hydrogen gas, dew point 60 ℃
Oxidant gas: air, dew point 60 ° C
-Cell temperature: 60 ° C
-Humidification: 100%
Measurement was performed by repeatedly sweeping between OCV and 0.2 V at a sweep rate of 1 A / sec. The stable IV curve during the measurement was adopted as the steady IV curve for the fuel cell cathode catalyst layer.

3. Evaluation of Fuel Cell Cathode Catalyst Layer FIG. 1 is a graph in which IV curves of membrane-electrode assemblies of Example 1 and Comparative Example 1 to Comparative Example 3 are superimposed. FIG. 1 is a graph in which the vertical axis represents voltage (V) and the horizontal axis represents current density (A / cm ² ). In FIG. 1, the black square plot shows the data of Example 1, the black circle plot shows the data of Comparative Example 1, the white small square plot shows the data of Comparative Example 2, and the white large square plot shows the data of Comparative Example 3. Data are shown respectively. As can be seen from FIG. 1, the membrane / electrode assembly of Example 1 exhibits a higher voltage than the membrane / electrode assembly of Comparative Example 1 to Comparative Example 3 over almost the entire range of current density.
FIG. 2 is a graph showing the relationship between the mass ratio of copper in the fuel cell cathode catalyst layer and the cell voltage for the membrane / electrode assembly used in this experiment. FIG. 2 is a graph in which the vertical axis represents the cell voltage (V) at a current density of 2.5 A / cm ² and the horizontal axis represents the mass ratio (mass%) of copper in the fuel cell cathode catalyst layer. is there. Here, the cell voltage (V) on the vertical axis is obtained from the IV curve in FIG. 1, and the mass ratio of copper on the horizontal axis is calculated from the above formula (1a). The plots of the copper mass ratios of 0 mass%, 0.08 mass%, 0.15 mass%, and 0.5 mass% are Comparative Example 1, Example 1, Comparative Example 2, or Comparative Example 3, respectively. The data are shown respectively. Moreover, the broken line in FIG. 2 shows the cell voltage (0.625V) when the mass ratio of copper is 0 mass%.
As shown in FIG. 2, when the mass ratio of copper is 1% by mass (Example 1), the cell voltage is 0.645 V, which is 0 than when CuSO ₄ .5H ₂ O is not added (Comparative Example 1). .020V is also high. This indicates that by adding an appropriate amount of CuSO ₄ .5H ₂ O, the effect of improving drainage by copper ions has exceeded the effect of cation contamination by copper ions.
On the other hand, when the mass ratio of copper is 0.15 mass% (Comparative Example 2), the cell voltage is 0.59 V, which is 0.06 V lower than that of Comparative Example 1, and when the copper mass ratio is 0.5 mass%. The cell voltage of (Comparative Example 3) further decreased to 0.54V. This is because the catalytic performance of PtCo / C was lowered as a result of the progress of cation contamination by copper ions due to the excessive addition of CuSO ₄ .5H ₂ O.
In addition, the graph of FIG. 2 (however, when the mass ratio of copper is in the range of 0 to 0.15 mass%) can be represented by the following formula (2).
y = −6.6036x ² + 0.7745x + 0.6245 Equation (2)

FIG. 3 is a graph in which complex impedance curves for the membrane / electrode assemblies of Comparative Example 1 and Comparative Example 3 are superimposed. FIG. 3 is a graph plotting the impedance when the frequency was changed by supplying hydrogen gas (H ₂ ) to the anode side and nitrogen gas (N ₂ ) to the cathode side of these membrane-electrode assemblies. . FIG. 3 is a graph in which the ordinate represents the imaginary component (−Im Z) (Ω) of the impedance and the abscissa represents the real component (Re Z) (Ω) of the impedance. In addition, the relative humidity at the time of measurement is 165% (RH165%, excessive humidification condition).
FIG. 4 is a bar graph comparing gas diffusion resistance for the membrane / electrode assemblies of Comparative Example 1 and Comparative Example 3. FIG. 4 is data obtained from each complex impedance curve of FIG. As shown in FIG. 4, the gas diffusion resistance of Comparative Example 1 (the mass ratio of copper is 0% by mass) is 91.2 (sec / m), while the Comparative Example 3 (the mass ratio of copper is 5% by mass). ) Is a low result of 88.3 (sec / m). This indicates that the greater the amount of CuSO ₄ .5H ₂ O added, the more the copper ions and the anionic functional groups on the ionomer are ionically bonded, resulting in improved drainage in the cathode catalyst layer. Accordingly, the membrane / electrode assembly of Example 1 is also considered to have higher drainage in the cathode catalyst layer than the membrane / electrode assembly of Comparative Example 1, and the gas diffusion resistance is lower than that of Comparative Example 1. Presumed to be.

  From the above, the copper ion in the cathode catalyst layer is adjusted to PtCo / C by adjusting the mass ratio of copper represented by the above formula (1a) to an appropriate amount exceeding 0 mass% and less than 0.117 mass%. As a result, it became clear that the effect of improving gas diffusivity by improving drainage in the cathode catalyst layer was superior to the effect of increasing the proton resistance of the ionomer present in the cathode catalyst layer. . As a result, it can be seen that the cathode catalyst layer containing copper having an appropriate mass ratio improves the discharge characteristics of the fuel cell when used in a fuel cell.

Claims (1)

A method for producing a cathode catalyst layer for a fuel cell comprising a metal-supported carbon and an ionomer containing platinum or a platinum-cobalt alloy as a catalyst component,
The metal-carrying carbon, the ionomer, the copper compound, and the dispersion medium are mixed so that the mass m _{Cu of copper} and the mass m _{M / C} of the metal-carrying carbon have a mixing ratio satisfying the following formula (1). Preparing a cathode catalyst ink,
A method for producing a cathode catalyst layer for a fuel cell, comprising forming a cathode catalyst layer using the cathode catalyst ink.
0% by mass <{m _Cu / (m _Cu + m _{M / C} )} <0.117% by mass Formula (1)