JP2012146438A - Catalyst electrode layer, membrane electrode assembly and fuel battery cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve proton conductivity and gas diffusion properties in a catalyst electrode layer used for a fuel battery.SOLUTION: A catalyst electrode layer used for a fuel battery includes: a conductive particle on which a catalyst is supported; and a polymer electrolyte having proton conductivity. In the conductive particle, an acid group density of the catalyst (mmol/g) that is an acid amount per unit weight satisfies an expression: y=0.03x+z [in the expression, x represents a thickness (μm) of the catalyst electrode layer, y represents an acid group density of the catalyst (mmol/g) and 0.25≤z≤0.35 is satisfied].

Description

  The present invention relates to a catalyst electrode layer, a membrane electrode assembly, and a fuel cell used for a fuel cell.

  In general, a fuel cell such as a polymer electrolyte fuel cell is a catalyst electrode including conductive particles (for example, carbon particles) carrying a catalyst such as Pt and a proton conductive polymer electrolyte. With layers.

  Various techniques for improving the proton conductivity and gas diffusibility of this catalyst electrode layer are known (Patent Document 1).

JP 2008-186798 A JP 2008-192490 A JP 2005-235461 A JP 2006-012476 A JP 2007-200855 A

  However, it is difficult to improve both proton conductivity and gas diffusivity in the catalyst electrode layer, and there is still room for improvement in the technology for improving the power generation efficiency of the fuel cell by improving these.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to improve proton conductivity and gas diffusibility in a catalyst electrode layer used in a fuel cell.

  In order to solve at least a part of the above problems, the present invention can be realized as the following aspects or application examples.

[Application Example 1]
A catalyst electrode layer used in a fuel cell,
Conductive particles carrying a catalyst; and
A proton conductive polymer electrolyte, and
The conductive particles have a catalyst acid group density (mmol / g), which is an acid amount per unit weight, of the following formula: y = 0.03x + z
[Wherein x is the thickness (μm) of the catalyst electrode layer, y is the catalyst acid group density (mmol / g), 0.25 ≦ z ≦ 0.35]
Satisfying the catalyst electrode layer.
The catalyst electrode layer.

  According to this configuration, the catalyst acid group density (mmol / g) of the conductive particles is y = 0.03x + z (x is the thickness (μm) of the catalyst electrode layer, and y is the catalyst acid group density (mmol / g). g) and 0.25 ≦ z ≦ 0.35), the proton conductivity and gas diffusibility in the catalyst electrode layer can be improved.

  The present invention can be realized in various modes. For example, the present invention can be realized by a membrane electrode assembly or a fuel cell including the above-described catalyst electrode layer. It can also be realized in the form of a manufacturing method, a manufacturing method of a fuel cell, or the like.

It is explanatory drawing for demonstrating schematic structure of the fuel cell in 1st Example. It is explanatory drawing which illustrated the catalyst electrode layer vicinity of the fuel cell whose thickness of a catalyst electrode layer is comparatively thick. It is explanatory drawing which illustrated the catalyst electrode layer vicinity of the fuel cell whose thickness of a catalyst electrode layer is comparatively thin. It is explanatory drawing which illustrated the relationship between a catalyst acid group density and a voltage value. It is explanatory drawing which illustrated the relationship between the thickness of a catalyst electrode layer, and a catalyst acid group density.

  Next, embodiments of the present invention will be described based on examples.

A. First embodiment:
A-1. Fuel cell configuration:
FIG. 1 is an explanatory diagram for explaining a schematic configuration of a fuel cell in the first embodiment. The fuel cell 10 is a polymer electrolyte fuel cell, and includes a catalyst electrode layer that is a feature of the present invention. Details of the catalyst electrode layer will be described later.

  The fuel cell 10 has a stack structure in which a plurality of single cells 14 are stacked. The single cell 14 is a unit module that generates power in the fuel cell 10 and generates power by an electrochemical reaction between hydrogen gas and oxygen contained in air. Each single cell 14 has a pair of gas diffusion layers 240 (anode side) on both sides of a membrane electrode assembly (also referred to as MEA) 230 in which a catalyst electrode layer 220 (anode 220a and cathode 220c) is formed on each surface of the electrolyte membrane 210. The power generation body 200 is provided with a diffusion layer 240a and a cathode side diffusion layer 240c) and a pair of separators 300 (an anode side separator 300a and a cathode side separator 300c) sandwiching the power generation body 200.

  The electrolyte membrane 210 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. The catalyst electrode layer 220 (the anode 220a and the cathode 220c) is a layer in which an electrochemical reaction occurs in the fuel cell 10, and the configuration and the like will be described later with reference to FIGS. The gas diffusion layer 240 (the anode side diffusion layer 240a and the cathode side diffusion layer 240c) is configured by a member having gas permeability and electronic conductivity. For example, the gas diffusion layer 240 is formed by a porous carbon member such as carbon cloth or carbon paper. Can be formed. The gas diffusion layer 240 may include a water repellent layer on the surface in contact with the MEA 230.

  The separator 300 (the anode-side separator 300a and the cathode-side separator 300c) is configured by a member having gas barrier properties and electronic conductivity. For example, the separator 300 is made of carbon such as dense carbon that is compressed by gas and impervious to gas. It is formed of a member or a metal member such as press-formed stainless steel. The separator 300 has a concavo-convex shape for forming a flow path through which gas or liquid flows on the surface. Between the anode side separator 300a and the anode side diffusion layer 240a, an anode gas flow path AGC through which gas or liquid can flow is formed. The cathode side separator 300c forms a cathode gas flow path CGC through which gas and liquid can flow between the cathode side diffusion layer 240c.

A-2. Configuration of catalyst electrode layer:
2 and 3 are explanatory views for explaining a schematic configuration of the catalyst electrode layer. FIG. 2 is an explanatory diagram illustrating the vicinity of the catalyst electrode layer of a fuel cell having a relatively thick catalyst electrode layer, and FIG. 3 is the vicinity of the catalyst electrode layer of a fuel cell having a relatively thin catalyst electrode layer. It is explanatory drawing which illustrated.

  As shown in FIGS. 2 and 3, the catalyst electrode layer 220 (the anode 220 a and the cathode 220 c) includes catalyst-supported carbon particles 221 and a proton conductive polymer electrolyte (hereinafter, also simply referred to as “electrolyte”) 222. It consists of The catalyst-carrying carbon particles 221 include, for example, a structure in which a catalytic metal (for example, platinum) cat that advances an electrochemical reaction is supported on carbon particles cpa having a hydrophilic group such as a carboxyl group coh on the surface by a hydrophilic treatment. ing. The proton conductive polymer electrolyte 222 is composed of a fluorine resin or the like containing a fluorine atom in the polymer skeleton pol of the main chain and a hydrophilic group such as a sulfonic acid group soh in the side chain. A method for producing the catalyst electrode layer 220 will be described later.

  The catalyst electrode layer 220 has different proton conductivity and gas diffusibility (flooding resistance) depending on its thickness (μm). Specifically, as shown in FIG. 2, as the catalyst electrode layer 220 increases in thickness, the density of generated water decreases in the inside and gas diffusibility (anti-flooding resistance) is improved. Proton movement distance (proton movement distance) of the proton moving through 222 becomes longer, and proton conductivity decreases. On the other hand, as shown in FIG. 3, as the catalyst electrode layer 220 becomes thinner, the proton movement distance (proton movement distance) that moves through the electrolyte 222 during power generation becomes shorter and the proton conductivity is improved. In this case, the density of generated water is increased and gas diffusibility (flooding resistance) is lowered.

  From these facts, the catalyst electrode layer 220 of this example changes the catalyst acid group density (mmol / g), which is the amount (acid amount) of hydrophilic groups per unit weight of the catalyst-supporting carbon particles 221 depending on the thickness. The water retention is changed. Specifically, as the thickness of the catalyst electrode layer 220 of this example increases, the catalyst acid group density of the catalyst-carrying carbon particles 221 is increased to improve water retention, thereby suppressing a decrease in proton conductivity. ing. On the other hand, as the thickness of the catalyst electrode layer 220 decreases, the catalyst acid group density of the catalyst-carrying carbon particles 221 is lowered to reduce water retention, thereby suppressing a decrease in gas diffusibility (flooding resistance). .

In the catalyst electrode layer 220 of this example, the catalyst acid group density (mmol / g) of the catalyst-carrying carbon particles 221 and the thickness (μm) of the catalyst electrode layer 220 are further represented by the following formula (1). It has been adjusted to hold.
y = 0.03x + z (1)
In the above formula (1), x is the thickness (μm) of the catalyst electrode layer 220, y is the catalyst acid group density (mmol / g) of the catalyst-supporting carbon particles 221, and z is 0.25 ≦ z ≦. It represents a value that satisfies 0.35.

When the thickness of the catalyst electrode layer and the catalyst acid group density satisfy the formula (1), the proton conductivity and gas diffusibility (anti-flooding resistance) of the catalyst electrode layer 220 are in a well-balanced state, and the power generation of the fuel cell The performance can be improved. The reason why the power generation performance of the fuel cell can be improved when the thickness of the catalyst electrode layer 220 and the catalyst acid group density satisfy the formula (1) will be described later. In the formula (1), the intercept z has a range of 0.25 ≦ z ≦ 0.35, which is a specific surface area per unit weight of the catalyst-supporting carbon particles 221 (m ² / g). This is due to the difference. That is, when using catalyst-carrying carbon particles having a large specific surface area, even if the catalyst acid group density (mmol / g) is the same, the amount of acid per surface area decreases. It is more desirable to set the value around .35). On the other hand, when using catalyst-carrying carbon particles having a small specific surface area, even if the catalyst acid group density (mmol / g) is the same, the amount of acid per surface area is large, so the value of z is small (0 It is more desirable to set a value around .25).

A-3. Evaluation of catalyst electrode layer:
Here, in order to explain the reason why the power generation performance of the fuel cell can be improved by using the catalyst electrode layer in which the thickness of the catalyst electrode layer and the catalyst acid group density satisfy the formula (1), the catalyst electrode layer The power generation performance of the fuel cells was compared using a plurality of catalyst electrode layers having different thicknesses and different catalyst acid group densities of the catalyst-supporting carbon particles contained in the catalyst electrode layer. Specifically, the catalyst electrode layer thickness is 2 (μm), 6 (μm), 10 (μm), 20 (μm), 30 (μm), and the catalyst acid group density of the catalyst-supporting carbon particles. 0.3 (mmol / g), 0.45 (mmol / g), 0.6 (mmol / g), 0.9 (mmol / g), 1.2 (mmol / g), 1.5 The power generation performance of the fuel cells was compared using 30 catalyst electrode layers obtained by combining 6 types (mmol / g).

In order to compare the power generation performance, a gas diffusion layer (GDL) composed of a carbon base material and a water repellent layer (carbon + PTFE) is disposed outside the MEA including each of the catalyst electrode layers described above to generate a power generator (fuel cell). ) And generated electricity by flowing hydrogen on the anode side and air on the cathode side. The performance evaluation of the fuel cell was performed by detecting the voltage value for each load current. The humidification conditions were 40% RH for both electrodes with respect to a cell temperature of 80 ° C., and the load current was 1.0 A / cm ² .

  FIG. 4 is an explanatory diagram illustrating the relationship between the catalyst acid group density and the voltage value. The vertical axis in FIG. 4 indicates the voltage value (V) of the fuel cell, and the horizontal axis indicates the catalyst acid group density (mmol / g) of the catalyst-supporting carbon particles. FIG. 4 shows a regression curve of a plot for each catalyst electrode layer having the same thickness. As shown in FIG. 4, each regression curve has a mountain shape. This is because the catalyst electrode layer density of the catalyst-supported carbon particles at each thickness is the peak of the regression curve, that is, the catalyst acid group density when the voltage value is highest (hereinafter referred to as “catalyst at peak time”). When the density is higher than the “acid group density”, the catalyst-carrying carbon particles have water retention more than necessary, which indicates that gas diffusibility (flooding resistance) decreases. In addition, when the catalyst acid group density of the catalyst-carrying carbon particles is lower than the peak catalyst acid group density, the catalyst-carrying carbon particles do not have the necessary water retention capacity, indicating that proton conductivity decreases. ing. Therefore, at each thickness of the catalyst electrode layer, as the catalyst acid group density of the catalyst-supported carbon particles is closer to the peak catalyst acid group density, the decrease in gas diffusibility (flooding resistance) and proton conductivity are suppressed. As a result, power generation performance is improved.

FIG. 5 is a plot of the relationship between the peak catalyst acid group density and the thickness of the catalyst electrode layer corresponding to each peak catalyst acid group density shown in FIG. FIG. 5 is an explanatory diagram illustrating the relationship between the thickness of the catalyst electrode layer and the catalyst acid group density. The vertical axis in FIG. 5 indicates the peak catalyst acid group density (mmol / g), and the horizontal axis indicates the thickness (μm) of the catalyst electrode layer corresponding to the peak catalyst acid group density. As shown in FIG. 5, when linear regression is performed on a plot of the peak catalytic acid group density for each thickness of the catalyst electrode layer, the regression equation is expressed by the following equation (2).
y = 0.03x + 0.3 (2)

  In the above formula (2), x represents the thickness (μm) of the catalyst electrode layer, and y represents the catalyst acid group density (mmol / g). Considering the error between this linear regression equation (2) and each plot, in the above equation (2), even if the intercept is in the range of 0.3 ± 0.05, the gas diffusion in the catalyst electrode layer The power generation performance can be improved by sufficiently suppressing the decrease in the performance (flooding resistance) and the proton conductivity. That is, in the catalyst electrode layer, when the thickness (μm) x and the catalyst acid group density (mmol / g) y of the catalyst-supporting carbon particles satisfy the above formula (1), the gas diffusibility ( Reduction in flooding resistance and proton conductivity are sufficiently suppressed, and power generation performance is improved. The above is the description regarding the evaluation of the catalyst electrode layer.

A-4. Preparation method of catalyst electrode layer:
Below, the preparation method of a catalyst electrode layer is demonstrated easily. The production method described below is an example of a method for producing the catalyst electrode layer of this example, and the catalyst electrode layer of this example can also be produced by a method other than the following method.

<Preparation of catalyst-supported carbon particles>
As carbon particles for supporting the catalyst (Pt), ketjen EC (trade name; manufactured by Ketjen Black International), Vulcan (trade name; manufactured by Cabot), or the like can be used. Here, carbon black Vulcan XC72R was used.

  7 g of this carbon black was suspended and stirred in distilled water, and a Pt compound such as chloroplatinic acid was added dropwise for 3 g of Pt weight. Thereafter, a reducing agent such as ethanol was dropped to deposit Pt on the carbon particles. The mixture was filtered and the solid was dried to obtain Pt-supported carbon particles. When the catalytic acid group density was measured for the Pt-supported carbon particles by the back titration method, the catalytic acid group density was 0.3 (mmol / g). Further, the obtained Pt catalyst was immersed in a 0.5N nitric acid aqueous solution, and stirred to perform a hydrophilic treatment while controlling the temperature. By changing the temperature of the aqueous nitric acid solution and the stirring time (acid treatment time), a plurality of Pt-supported carbon particles having different catalyst acid group densities used in the evaluation of the catalyst electrode layer described above were obtained. In addition, a carboxylic acid group is mainly given to Pt carrying | support carbon particle by hydrophilic treatment.

  Specifically, the temperature of the aqueous nitric acid solution was 50 ° C., the stirring time (acid treatment time) was 0.5 hour, and Pt-supported carbon particles having an acid group density of 0.45 (mmol / g) were obtained. Further, Pt-supported carbon particles having an acid group density of 0.6 (mmol / g) were obtained by setting the temperature of the aqueous nitric acid solution to 80 ° C. and the stirring time (acid treatment time) to 0.5 hours. Moreover, the temperature of the nitric acid aqueous solution was 80 ° C., the stirring time (acid treatment time) was 24 hours, and Pt-supported carbon particles having an acid group density of 0.9 (mmol / g) were obtained. Moreover, the temperature of the nitric acid aqueous solution was 90 ° C., the stirring time (acid treatment time) was 24 hours, and Pt-supported carbon particles having an acid group density of 1.2 (mmol / g) were obtained. Moreover, the temperature of the nitric acid aqueous solution was 90 ° C., the stirring time (acid treatment time) was 48 hours, and Pt-supported carbon particles having an acid group density of 1.5 (mmol / g) were obtained.

<Catalyst ink production>
Distilled water was added to the Pt-supported carbon particles that had been subjected to hydrophilic treatment, and then a solvent such as ethanol or 1-propanol was added. As a proton conductive polymer electrolyte, a commercially available Nafion solution (EW1000 manufactured by DuPont) and the like were further added. The input amount at this time was adjusted so that the sulfonic acid density was 0.8 with respect to the Pt-supported carbon particles. These mixtures were sufficiently stirred, and dispersion treatment by ultrasonic irradiation, bead milling, etc. was performed for atomization and uniform dispersion of particles to prepare catalyst ink.

<Production of MEA>
MEA was formed by apply | coating the catalyst ink produced above on the electrolyte membrane (Nafion 112) by the spray system. The thickness of the catalyst electrode layer was controlled by the number of sprayed layers to obtain a catalyst electrode layer of 2 to 30 μm. The above is the description of the method for producing the catalyst electrode layer.

  As described above, according to the catalyst electrode layer 220 of this example, when the thickness (μm) of the catalyst electrode layer is x and the catalyst acid group density (mmol / g) of the conductive particles is y, Since the thickness of the catalyst electrode layer and the catalyst acid group density are adjusted to be y = 0.03x + z (0.25 ≦ z ≦ 0.35), the proton conductivity and gas diffusivity of the catalyst electrode layer are adjusted. Improvements can be made.

  Specifically, as shown in FIG. 4, the regression curve indicating the relationship between the voltage value (V) of the fuel cell and the catalyst acid group density has a mountain shape for each catalyst electrode layer having the same thickness. Yes. That is, the catalyst electrode layer can make the voltage value of the fuel cell higher by bringing the catalyst acid group density closer to the catalyst acid group density at the peak of the regression curve (peak catalyst acid group density). This is because by bringing the catalyst acid group density of the catalyst electrode layer close to the peak catalyst acid group density, the proton conductivity and gas diffusibility (anti-flooding resistance) of the catalyst electrode layer 220 are balanced. .

  Therefore, the catalyst electrode layer has a thickness of the catalyst electrode layer and a catalyst acid group density, as shown in FIG. 5, the peak catalyst acid group density at each thickness of the catalyst electrode layer, and the thickness of the corresponding catalyst electrode layer. The power generation performance of the fuel cell can be improved by satisfying the equation (2) that is a linear regression equation calculated from the above relationship and the equation (1) that considers the difference in specific surface area of the catalyst-supporting carbon particles.

  Conventionally, it is known that in order to improve the performance of a fuel cell, it is necessary to improve proton conductivity and gas diffusibility (flooding resistance) in a catalyst electrode layer. Usually, a catalyst electrode layer is normally comprised by the thickness of 2-30 micrometers. The present invention focuses on the fact that proton conductivity and flooding resistance differ depending on the thickness of the catalyst electrode layer. By optimizing the catalyst acid group density according to the thickness of the catalyst electrode layer, the proton conductivity It is characterized by improving the properties and gas diffusibility.

B. Variations:
The present invention is not limited to the above-described examples and embodiments, and can be implemented in various modes without departing from the gist thereof. For example, the following modifications are possible.

B1. Modification 1:
In this embodiment, the fuel cell 10 has been described as having the catalyst electrode layer 220 according to the present invention described in FIGS. 2 and 3 on both the anode 220a and the cathode 220c. 2 and FIG. 3, the catalyst electrode layer 220 according to the present invention is provided on only one of the anode 220a and the cathode 220c, and the other is provided with a catalyst electrode layer having a configuration different from that of the catalyst electrode layer 220 of the present invention. May be. Even with this configuration, the power generation performance of the fuel cell can be improved. In addition, when the catalyst electrode layer 220 according to the present invention is used for only one of the anode 220a and the cathode 220c of the fuel cell 10, it is more preferable to use the catalyst electrode layer 220 for the cathode 220c.

B2. Modification 2:
In the present embodiment, platinum is used as the catalyst metal cat included in the catalyst electrode layer 220. However, the catalyst metal cat is not limited to platinum, for example, cobalt, palladium, ruthenium, gold, rhodium. Further, it may be a metal such as osmium or iridium, an alloy composed of two or more of the above metals, a complex of a metal and an organic compound or an inorganic compound, or a metal oxide.

B3. Modification 3:
In this example, it is described that carbon black is used as the carrier for supporting the catalyst, but the carrier for supporting the catalyst is not only carbon black but also graphite, carbon fiber, activated carbon, carbon nanotube, acetylene black, etc. There may be.

DESCRIPTION OF SYMBOLS 10 ... Fuel cell 14 ... Single cell 112 ... Nafion 200 ... Power generation body 210 ... Electrolyte membrane 220 ... Catalyst electrode layer 221 ... Catalyst carrying | support carbon particle 222 ... Proton conductive polymer electrolyte 240 ... Gas diffusion layer 300 ... Separator AGC ... Anode gas Channel CGC ... Cathode gas channel cpa ... carbon particles cat ... catalyst metal coh ... carboxyl group soh ... sulfonic acid group pol ... polymer skeleton

Claims (3)

A catalyst electrode layer used in a fuel cell,
Conductive particles carrying a catalyst; and
A proton conductive polymer electrolyte, and
The conductive particles have a catalyst acid group density (mmol / g), which is an acid amount per unit weight, of the following formula: y = 0.03x + z
[Wherein x is the thickness (μm) of the catalyst electrode layer, y is the catalyst acid group density (mmol / g), 0.25 ≦ z ≦ 0.35]
Satisfying the catalyst electrode layer. A membrane electrode assembly used in a fuel cell,
An electrolyte membrane;
A membrane electrode assembly comprising the catalyst electrode layer according to claim 1. A fuel cell,
A fuel cell comprising the membrane electrode assembly according to claim 2.

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