JP7152992B2 - Electrodes for fuel cells - Google Patents

Description

TECHNICAL FIELD The present invention relates to a fuel cell electrode, and more particularly to a fuel cell electrode containing a catalyst (ionomer-coated catalyst) in which the surface of the electrode catalyst is coated with a highly oxygen-permeable ionomer.

A polymer electrolyte fuel cell is based on a membrane electrode assembly (MEA) in which electrodes are joined to both sides of a solid polymer electrolyte membrane. Moreover, in polymer electrolyte fuel cells, electrodes generally have a two-layer structure of a diffusion layer and a catalyst layer. The diffusion layer is for supplying reaction gas and electrons to the catalyst layer, and is made of carbon paper, carbon cloth, or the like. The catalyst layer is a reaction field for electrode reaction, and is generally composed of a composite of carbon supporting an electrode catalyst such as platinum and a solid polymer electrolyte (catalyst layer ionomer).

A catalyst layer used in a polymer electrolyte fuel cell is generally
(a) preparing a catalyst ink containing an electrocatalyst and an ionomer and having a solids concentration of about 10%;
(b) forming a catalyst layer on the substrate surface by applying a catalyst ink to the substrate surface and evaporating the solvent in the coating film using various methods;
(c) It is manufactured by transferring the catalyst layer on the substrate surface to the electrolyte membrane.
There is also a method of applying the catalyst ink directly to the solid polymer electrolyte membrane instead of the base material.

Known methods for applying the catalyst ink to the substrate surface include, for example, a spray method, a blade coating method using a doctor blade or an applicator, a die coating method, a reverse roll coater method, and an intermittent die coating method. Whichever method is used, the properties of the catalyst ink affect the soundness and productivity of the catalyst layer. Therefore, various proposals have been conventionally made for the purpose of improving the soundness and productivity of the catalyst layer.

For example, in Patent Document 1,
(a) After dispersing platinum-supported carbon (Pt / C) in a Nafion (registered trademark) solution, the surface of Pt / C is coated with Nafion (registered trademark) by removing the solvent,
(b) heat treating the Nafion®-coated Pt/C at 140° C. in a nitrogen atmosphere to render Nafion® insoluble in the solvent;
(c) A fuel cell obtained by further dispersing Pt/C coated with insolubilized Nafion (registered trademark) in a Nafion (registered trademark) solution to form a paste, spreading the paste on carbon paper, and drying it. disclosed are electrodes for

In the same document,
(A) When Pt/C coated with a polymer electrolyte is redispersed in a solvent as it is to form a paste, the polymer electrolyte is eluted into the solvent and the ionic conductivity of the electrode is reduced;
(B) It is described that elution of the polymer electrolyte into the solvent can be suppressed by insolubilizing the polymer electrolyte covering the Pt/C before dispersing it in the paste.

In Patent Document 2,
(a) preparing a dispersion containing a highly oxygen-permeable ionomer and platinum-supported carbon, and evaporating the solvent from the dispersion to obtain a dry powder;
(b) A fuel cell electrode is disclosed that is obtained by preparing a dispersion containing a dry powder and a Nafion® solution and applying the dispersion onto a sheet.
The document describes that by making the catalyst layer ionomer a two-layer structure, oxygen permeability is improved even if the amount of high oxygen permeability material used is small, and the amount of catalyst used can be reduced. .

In Patent Document 3,
(a) platinum-supported carbon and a highly oxygen-permeable ionomer are dispersed in a solvent having a water ratio of 0.8 or more to form a dispersion;
(b) drying the dispersion to a powder;
(c) An ionomer-coated catalyst obtained by heat-treating a powder at a heat-treatment temperature of 130° C. or more and 200° C. or less is disclosed.
The document describes that the use of such an ionomer-coated catalyst makes it possible to produce a catalyst ink having a low viscosity and a high solid content concentration.

In Patent Document 4,
(a) preparing a catalyst ink comprising platinum-supported carbon, a high oxygen solubility ionomer, and a low oxygen solubility ionomer;
(b) A cathode electrode catalyst layer obtained by applying a catalyst ink to the electrolyte membrane surface is disclosed.
The document describes that the activation overvoltage can be reduced without lowering the proton conductivity by using two types of ionomers with different oxygen solubility.

Patent Literature 5 discloses a fuel cell using a highly oxygen-permeable ionomer as a catalyst layer ionomer.
The document describes that the amount of platinum used can be reduced by using a highly oxygen-permeable ionomer for the catalyst layer ionomer.

In Patent Document 6,
(a) platinum-supporting carbon, an ionomer with an EW of 910, and an ionomer with an EW of 1030 are dispersed in a solvent to form a paste;
(b) A catalyst sheet obtained by applying a paste to a sheet and drying the sheet is disclosed.
The document describes that by using two types of ionomers with different EW as the catalyst layer ionomers, the ability to discharge water in the catalyst layer is improved and a stable battery voltage can be obtained even in a high current density region. ing.

In Patent Document 7,
(a) platinum-supporting carbon, an ionomer having an EW of 700, and an ionomer having an EW of 1000 are dispersed in a solvent to prepare a catalyst ink;
(b) An electrode catalyst layer obtained by applying a catalyst ink onto a transfer sheet is disclosed.
In the same document,
(A) Cracking of the coating film is suppressed by coating the surface of platinum-supported carbon with an ionomer having a low EW, and
(B) It is described that flooding can be suppressed by adding an ionomer with a high EW.

In Patent Documents 8 and 9,
(a) platinum-supported carbon, a high oxygen-permeable ionomer (e.g., a cyclic compound having a cyclic structure), and a low oxygen-permeable ionomer (e.g., Nafion (registered trademark)) are dispersed in a solvent to form a catalyst ink;
(b) A cathode-side electrode obtained by applying a catalyst ink to the surface of an electrolyte membrane is disclosed.
In the same document,
(A) If an electrode is formed using only a high oxygen permeability ionomer, cracks are likely to occur in the electrode, and
(B) It is described that cracking of the electrode is suppressed by using both a high oxygen permeability ionomer and a low oxygen permeability ionomer.

Further, Patent Literature 10 discloses an electrode catalyst layer using an ionomer having an expansion rate (swelling rate) of 140% or more and 200% or less.
This document describes that as the coefficient of expansion (swelling rate) increases, the high temperature performance (dry performance) increases, while the low temperature performance (wet performance) decreases.

When a polymer electrolyte fuel cell is used in an automotive fuel cell system, the catalyst layer of the fuel cell is required to exhibit high power generation performance under a wide range of operating conditions, from low temperature and high humidity conditions to high temperature and low humidity conditions. . In general, under low-temperature and high-humidity conditions, swelling of the ionomer in the catalyst layer and retention of generated water reduce the number of voids in the catalyst layer, so that power generation performance tends to decrease. On the other hand, under high-temperature and low-humidity conditions, the water content of the ionomer in the catalyst layer decreases, so the proton transfer resistance in the catalyst layer tends to increase.

However, the conventional catalyst layer suppresses elution of ionomer (Patent Document 1), improves performance using ionomer with high oxygen permeability and reduces the amount of platinum used (Patent Documents 2 to 5, 8, 9), high current density Example of a catalyst layer proposed for the purpose of improving performance in a wide range of operating conditions from low temperature and high humidity conditions to high temperature and low humidity conditions (Patent Documents 6 and 7). is less.
For example, Patent Document 10 describes that both high-temperature performance and low-temperature performance are achieved by optimizing the expansion coefficient of the catalyst layer ionomer. is not shown.

JP-A-07-254419 JP 2014-216157 A JP 2018-152333 A Japanese Unexamined Patent Application Publication No. 2011-113739 JP 2010-251086 A JP-A-10-284087 JP 2018-006265 A JP 2013-037940 A JP 2018-081853 A JP 2013-089447 A

The problem to be solved by the present invention is to provide a fuel cell electrode that exhibits high power generation performance under both high-temperature and low-humidity conditions and low-temperature and high-humidity conditions.
Another problem to be solved by the present invention is that the amount of catalyst used can be reduced without sacrificing the power generation performance under high temperature and low humidity conditions and / or the power generation performance under low temperature and high humidity conditions. It is an object of the present invention to provide an electrode for a fuel cell which is capable of
Furthermore, another problem to be solved by the present invention is to shorten the manufacturing time (drying time of the catalyst ink) of a fuel cell electrode having such performance.

In order to solve the above problems, the fuel cell electrode according to the present invention has the following configuration.
(1) The fuel cell electrode is
an ionomer-coated catalyst in which the surface of the electrode catalyst is coated with a first layer made of a first ionomer;
a second layer comprising a second ionomer in contact with the first ionomer;
It has
(2) The first ionomer is made of a high oxygen permeability ionomer (A) having an ion exchange capacity of 1.1 meq/g or more and 1.25 meq/g or less.
(3) The highly oxygen-permeable ionomer (A) consists of a polymer compound containing an acid group and a cyclic structure in its molecular structure.

When the surface of the electrode catalyst is coated with the highly oxygen permeable ionomer (A), optimizing the ion exchange capacity of the highly oxygen permeable ionomer (A) results in high power generation performance under both high temperature and low humidity conditions and low temperature and high humidity conditions. is obtained. this is,
(a) By setting the ion exchange capacity of the high oxygen permeation ionomer (A) to 1.1 meq/g or more, an increase in proton transfer resistance in the catalyst layer is suppressed even in a high temperature and low humidity environment, and
(b) By setting the ion exchange capacity of the highly oxygen permeable ionomer (A) to 1.25 meq/g or less, excessive swelling of the ionomer is suppressed and the surface of the ionomer becomes more water repellent. Sufficient space is secured inside the
it is conceivable that.

FIG. 4 is a diagram showing the relationship between the ion exchange capacity and current density ratio of a high oxygen permeability ionomer.

An embodiment of the present invention will be described in detail below.
[1. Fuel cell electrodes]
A fuel cell electrode according to the present invention has the following configuration.
(1) The fuel cell electrode is
an ionomer-coated catalyst in which the surface of the electrode catalyst is coated with a first layer made of a first ionomer;
a second layer comprising a second ionomer in contact with the first ionomer;
It has
(2) The first ionomer is made of a high oxygen permeability ionomer (A) having an ion exchange capacity of 1.1 meq/g or more and 1.25 meq/g or less.
(3) The highly oxygen-permeable ionomer (A) is composed of a polymer compound containing an acid group and a cyclic structure in its molecular structure.

[1.1. Ionoma-coated catalyst]
The ionomer-coated catalyst comprises an electrode catalyst and a first layer made of a first ionomer in contact with the surface of the electrode catalyst. By using the method described later, a catalyst (ionomer-coated catalyst) in which the surface of the electrode catalyst is coated with the first layer is obtained. A fuel cell electrode according to the present invention comprises a composite of such an ionomer-coated catalyst and a second ionomer.

[1.1.1. Electrocatalyst]
In the present invention, the electrode catalyst is not particularly limited as long as it exhibits oxygen reduction reaction activity or hydrogen oxidation reaction activity. Specific examples of electrode catalysts include platinum, platinum alloys, palladium alloys, metal oxynitrides, and carbon alloys.
The electrode catalyst may consist of only catalyst particles, or may have catalyst fine particles supported on the surface of an appropriate carrier. The material of the carrier is not particularly limited, and various materials can be used depending on the purpose. Examples of the carrier include carbon materials (eg, carbon, activated carbon, fullerene, carbon nanophone, carbon nanotube), metal oxides, and the like.

The electrode catalyst preferably comprises a carbon carrier and fine catalyst particles carried on the surface of the carbon carrier. In general, it is difficult to produce a low-viscosity, high-solid-content catalyst ink using an electrode catalyst containing a carbon carrier. This is probably because the higher the solid content concentration of the catalyst ink, the stronger the cohesive force between carbon atoms in the solvent.
In contrast, when the present invention is applied to an electrode catalyst containing a carbon carrier, it is possible to produce a catalyst ink having a low viscosity and a high solid content concentration.

[1.1.2. 1st ionomer]
In the present invention, the first ionomer constituting the first layer is made of a high oxygen permeability ionomer (A) having an ion exchange capacity of 1.1 meq/g or more and 1.25 meq/g or less. This point is different from the conventional one.

[A. definition]
"High oxygen permeability ionomer (A)" refers to a polymer compound containing an acid group and a cyclic structure in its molecular structure.
Since the highly oxygen permeable ionomer (A) contains a cyclic structure in its molecular structure, it has a high oxygen permeability coefficient, and when used as a catalyst layer ionomer, the oxygen migration resistance at the interface with the catalyst is relatively small.
In other words, "high oxygen permeability ionomer (A)" refers to an ionomer having a higher oxygen permeability coefficient than polyperfluorocarbon sulfonic acid represented by Nafion (registered trademark).

In general, fuel cell performance is rate-determined by the diffusion of oxygen to the catalyst surface. On the other hand, when the surface of the electrode catalyst is coated with a highly oxygen-permeable ionomer, the oxygen permeability of the catalyst layer is improved, and the performance of the fuel cell is improved.
The molecular structure of the highly oxygen permeable ionomer (A) is not particularly limited as long as it exhibits relatively low resistance to oxygen transfer. In particular, an ionomer containing a cyclic structure (aliphatic ring structure) in its molecular structure has lower oxygen transfer resistance than an ionomer not containing a cyclic structure, and is therefore suitable as the first ionomer constituting the first layer.

As the high oxygen permeability ionomer (A), for example,
(a) an electrolyte polymer containing a perfluorocarbon unit having an alicyclic structure and an acid group unit having perfluorosulfonic acid as a side chain;
(b) an electrolyte polymer containing a perfluorocarbon unit having an alicyclic structure and an acid group unit having perfluoroimide as a side chain;
(c) an electrolyte polymer containing a unit in which a perfluorosulfonic acid is directly bound to a perfluorocarbon having an alicyclic structure;
etc. (see References 1-4).
[Reference 1] JP 2003-036856 [Reference 2] International Publication No. 2012/088166 [Reference 3] JP 2013-216811 [Reference 4] JP 2006-152249

[B. Oxygen permeability coefficient]
"Oxygen permeability coefficient" refers to a value measured by a chronoamperometry method using a platinum microelectrode (reference document 5).
[Reference 5] Electrochimica Acta, 209 (2016) 682-690

In order to obtain high fuel cell characteristics, the higher the oxygen permeability coefficient of the high oxygen permeability ionomer (A), the better. Specifically, the high oxygen permeability ionomer (A) preferably has an oxygen permeability coefficient of 2.2×10 ⁻¹⁴ mol/(m·s·Pa) or more at a temperature of 80° C. and a relative humidity of 30% RH. . The oxygen permeability coefficient under the same conditions is preferably 3.0×10 ⁻¹⁴ mol/(m·s·Pa) or more, more preferably 5.0×10 ⁻¹⁴ mol/(m·s·Pa). That's it.

[C. Ion exchange capacity]
The ion exchange capacity of the highly oxygen permeable ionomer (A) affects both the power generation performance under high temperature and low humidity conditions and the power generation performance under low temperature and high humidity conditions. In general, when the ion exchange capacity becomes excessively small, the proton transfer resistance in the catalyst layer increases in a high-temperature, low-humidity environment. Therefore, the ion exchange capacity of the high oxygen permeability ionomer (A) should be 1.1 meq/g or more. The ion exchange capacity is preferably 1.15 meq/g or more.

On the other hand, if the ion exchange capacity of the highly oxygen-permeable ionomer (A) is too large, it swells greatly in a low-temperature, high-humidity environment. If the ionomer swells excessively, the number of voids in the catalyst layer decreases, which tends to lower power generation performance. In addition, the ionomer surface becomes hydrophilic, and the drainage performance of the electrode deteriorates. Therefore, the ion exchange capacity of the high oxygen permeability ionomer (A) should be 1.25 meq/g or less. The ion exchange capacity is preferably 1.23 meq/g or less.

[1.1.3. Survival rate]
The term "residual ratio" refers to the ratio of the ionomer remaining on the surface of the electrode catalyst without dissolving in the mixed solvent of water/ethanol when the ionomer-coated catalyst is dispersed in the mixed solvent. Specifically, the residual rate R (%) is represented by the following formula (1).
R (%)=(x ₀ −x)×100/x ₀ (1)
however,
x is the concentration of the first ionomer dissolved in the mixed solvent when the ionomer-coated catalyst is added to a mixed solvent of water:ethanol = 70:30 (weight ratio) to form a dispersion;
_x0 is the theoretical value of the concentration of the first ionomer in the mixed solvent, assuming that all of the first ionomer is dissolved in the mixed solvent.

When measuring the residual rate, it is preferable to add an excess amount of the mixed solvent to the ionomer coat catalyst. Specifically, it is preferable to add 40 g or more of the mixed solvent to 1 g of the ionomer.
The temperature and treatment time of the mixed solvent during measurement can be arbitrarily selected according to the purpose. In the present invention, the temperature of the mixed solvent is 0 to 10°C. In order to sufficiently dissolve the soluble component in the mixed solvent, the dispersion liquid is subjected to dispersion treatment for 9 minutes using an ultrasonic homogenizer or the like.

When a catalyst ink is prepared by dispersing an ionomer-coated catalyst in a solvent, the higher the residual ratio of the first ionomer, the more difficult it becomes for the first ionomer to dissolve in the solvent. Therefore, the concentration of the first ionomer dissolved in the solvent in the catalyst ink decreases. This also suppresses agglomeration of the electrode catalyst dispersed in the solvent.

Furthermore, when the residual ratio is high, even when the solid content concentration in the catalyst ink is increased, it is possible to suppress an excessive increase in the viscosity of the catalyst ink. As a result, the drying time of the coating film can be shortened without impairing the coatability of the catalyst ink.
In order to obtain such effects, the residual rate is preferably 70% or more. The residual rate is preferably 80% or more.

[1.2. second ionomer]
As will be described later, the fuel cell electrode according to the present invention is obtained by further adding a second ionomer to a catalyst ink containing an ionomer-coated catalyst and coating the catalyst ink on the substrate surface. Therefore, the fuel cell electrode further comprises a second layer of a second ionomer in contact with the first ionomer.

The second ionomer is
(a) reduction in the amount of expensive high-oxygen-permeable ionomer (A) used;
(b) imparting functions other than high oxygen permeability (for example, high proton conductivity);
(c) suppression of cracks in fuel cell electrodes;
(d) improving the strength of fuel cell electrodes;
It is added for the purpose of
Therefore, the second ionomer can be selected optimally according to the purpose.

As the second ionomer, for example,
(a) high oxygen permeability ionomer (B),
(b) polyperfluorocarbon sulfonic acids such as Nafion®, Aciplex®, Flemion®, Aquivion®;
and so on.
When the high oxygen permeability ionomer (B) is used as the second ionomer, the high oxygen permeability ionomer (B) may be the same material as the high oxygen permeability ionomer (A), or may be a different material. .

[1.3. Content]
[1.3.1. Content of first ionomer]
The content of the first ionomer contained in the fuel cell electrode is not particularly limited, and an optimum content can be selected depending on the purpose. The optimum content of the first ionomer varies depending on the type of material that constitutes the fuel cell electrode.

For example, when the electrode catalyst comprises a carbon support and fine catalyst particles supported on the surface of the carbon support (that is, when the electrode catalyst is catalyst-supporting carbon), the content of the first ionomer is I ₁ /C ratio.
Here, the "I ₁ /C ratio" refers to the ratio (=W ₁ /W _c ) of the weight (W ₁ ) of the first ionomer to the weight (W _c ) of the carbon support.

In general, if the I ₁ /C ratio becomes too small, the oxygen permeability of the fuel cell electrode will decrease. Therefore, the I ₁ /C ratio is preferably 0.2 or more. The I ₁ /C ratio is preferably 0.3 or higher, more preferably 0.4 or higher.
On the other hand, if the I ₁ /C ratio becomes too large, the electronic conductivity of the fuel cell electrode will decrease. Therefore, the I ₁ /C ratio is preferably 0.7 or less. The I ₁ /C ratio is preferably 0.6 or less, more preferably 0.5 or less.

[1.3.2. Content of second ionomer]
The content of the second ionomer contained in the fuel cell electrode is not particularly limited, and the optimum content can be selected depending on the purpose. The optimum content of the second ionomer varies depending on the type of material that constitutes the fuel cell electrode.

For example, when the electrode catalyst is catalyst-supporting carbon, the content of the second ionomer can be expressed by the I ₂ /C ratio.
Here, the "I ₂ /C ratio" refers to the ratio (=W ₂ /W _c ) of the weight (W ₂ ) of the second ionomer to the weight (W _c ) of the carbon support.

In general, when the I ₂ /C ratio becomes too small, it becomes difficult to connect proton paths between ionomer-coated catalysts. Therefore, the I ₂ /C ratio is preferably 0.2 or more. The I ₂ /C ratio is preferably 0.4 or higher, more preferably 0.6 or higher.
On the other hand, if the I ₂ /C ratio becomes too large, the second ionomer will reduce the voids in the electrode, degrading power generation performance. Therefore, the I ₂ /C ratio is preferably 1.0 or less. The I ₂ /C ratio is preferably 0.9 or less, more preferably 0.8 or less.

[1.3.3. Total content of first ionomer and second ionomer]
The total content of the first ionomer and the second ionomer contained in the fuel cell electrode is also not particularly limited, and the optimum content can be selected according to the purpose.
For example, when the electrode catalyst is catalyst-supporting carbon, the total content of the first ionomer and the second ionomer is "I ₁ /C ratio + I ₂ /C ratio (hereinafter also referred to as "I / C ratio" ). The details of the "I ₁ /C ratio" and "I ₂ /C ratio" are as described above, so the explanation is omitted.

In general, when the I/C ratio becomes too small, the proton conduction resistance increases. Therefore, the I/C ratio is preferably 0.5 or more. The I/C ratio is preferably 0.7 or higher, more preferably 0.9 or higher.
On the other hand, if the I/C ratio becomes too large, the ionomer will reduce the voids in the electrode, resulting in a decrease in power generation performance. Therefore, the I/C ratio is preferably 1.4 or less. The I/C ratio is preferably 1.2 or less, more preferably 1.0 or less.

[1.4. Application]
The fuel cell electrode according to the present invention is suitable as an air electrode for polymer electrolyte fuel cells, but can also be used as a hydrogen electrode.

[2. Method for producing ionomer-coated catalyst]
The method for producing an ionomer-coated catalyst according to the present invention comprises:
A first step of preparing a dispersion containing an electrode catalyst, a first ionomer, water, and alcohol as necessary, and having a water ratio of 0.8 or more;
a second step of drying the dispersion to obtain a powder;
and a third step of heat-treating the powder at a heat-treating temperature of 130° C. or more and 200° C. or less to obtain an ionomer-coated catalyst.

[2.1. First step]
First, a dispersion containing an electrode catalyst, a first ionomer, water, and, if necessary, an alcohol and having a water ratio of 0.8 or more is prepared (first step).
[2.1.1. Electrocatalyst and first ionomer]
Since the details of the electrode catalyst and the first ionomer are as described above, the description thereof is omitted.

[2.1.2. Dispersion medium]
Water or a mixed solvent of water and alcohol is used as a dispersion medium for dispersing the electrode catalyst and the first ionomer.
Alcohol is not particularly limited as long as it can dissolve the first ionomer and disperse the electrode catalyst. Alcohols include, for example, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutanol, tert-butanol, and the like.
Moreover, the amount of the dispersion medium is not particularly limited as long as the first ionomer can be dissolved and the electrode catalyst can be uniformly dispersed.

[2.1.3. Water ratio]
"Water ratio" refers to the ratio (=W _w /W _t ) of the weight of water (W _w ) to the total weight (W _t ) of water and alcohol contained in the dispersion medium.

The water ratio affects the survival rate of the first ionomer. In general, the higher the water ratio, the higher the survival rate of the first ionomer. In order to make the residual rate 30% or more, the water ratio needs to be 0.8 or more. The water ratio is more preferably 0.85 or more, more preferably 0.88 or more. In particular, the water ratio is preferably 0.87 or more in order to obtain a residual rate of 70% or more.

[2.2. Second step]
Next, the dispersion is dried to obtain a powder (second step). The method and conditions for drying the dispersion are not particularly limited, and various methods and conditions can be selected according to the purpose.

[2.3. Third step]
Next, the powder is heat treated at a heat treatment temperature of 130° C. or higher and 200° C. or lower (third step). This gives an ionomer-coated catalyst.

The heat treatment temperature affects the survival rate of the first ionomer. In general, the higher the heat treatment temperature, the higher the residual rate of the first ionomer. In order to obtain a residual rate of 30% or higher, the heat treatment temperature must be 130° C. or higher. The heat treatment temperature is preferably 150° C. or higher, more preferably 160° C. or higher. In particular, the heat treatment temperature is preferably 150° C. or higher in order to obtain a residual rate of 70% or higher.
On the other hand, if the heat treatment temperature is too high, the first ionomer may decompose. Therefore, the heat treatment temperature is preferably 200° C. or less. The heat treatment temperature is preferably 190° C. or lower, more preferably 180° C. or lower.

[3. Method for manufacturing fuel cell electrode]
A method for manufacturing a fuel cell electrode according to the present invention comprises:
a step of producing an ionomer-coated catalyst;
dispersing an ionomer-coated catalyst and a second ionomer in a dispersion medium to obtain a catalyst ink;
and a step of applying the catalyst ink to the substrate surface and drying the catalyst ink.

[3.1. Ionoma-coated catalyst manufacturing process]
First, an ionomer-coated catalyst in which the surface of the electrode catalyst is coated with a first ionomer is manufactured (ionomer-coated catalyst manufacturing step). Since the details of the method for producing the ionomer-coated catalyst are as described above, the explanation is omitted.

[3.2. Catalyst ink manufacturing process]
Next, the ionomer-coated catalyst and the second ionomer are dispersed in a dispersion medium to obtain a catalyst ink (catalyst ink manufacturing step).
[3.2.1. second ionomer]
The details of the second ionomer are as described above, so the description is omitted.

[3.2.2. Dispersion medium]
The dispersion medium is not particularly limited as long as it can disperse the ionomer-coated catalyst and the second ionomer. Examples of dispersion media include alcohols, ketones, tetrahydrofuran, dimethylsulfoxide, N-methylpyrrolidone, glycols, esters and carboxylic acids.

[3.2.3. Solid content concentration, viscosity]
"Solid content concentration" refers to the ratio of the weight of the electrode catalyst and ionomer to the total weight of the catalyst ink. In general, the higher the solid content concentration of the catalyst ink, the higher the viscosity of the catalyst ink. If the viscosity of the catalyst ink is too high, uniform coating properties and ink dispersibility will deteriorate, making it difficult to obtain a good coated surface. Therefore, the blade coating method cannot be used for coating, and the coating method is limited to a reverse roll coater method or the like.

In contrast, when the ionomer-coated catalyst according to the present invention is used, a catalyst ink having a low viscosity and a high solid content concentration, which can be coated by a blade coating method or an intermittent die coating method, can be obtained. By optimizing the production conditions, a catalyst ink having a solid content concentration of 14 wt % or more and 40 wt % or less and a shear viscosity of 0.13 Pa·s or less at a shear rate of 562 s ⁻¹ can be obtained.

[3.3. Coating process]
Next, the catalyst ink is applied to the substrate surface and dried (coating step). Thereby, the fuel cell electrode according to the present invention is obtained.
A method for applying the catalyst ink is not particularly limited, and an optimum method can be selected according to the purpose. Also, the base material to which the catalyst ink is applied may be a resin sheet for transfer, or may be an electrolyte membrane.

[4. action]
By coating the surface of the electrode catalyst with the highly oxygen-permeable ionomer (A), a catalyst ink with a high solids concentration and low viscosity can be produced. Therefore, when a fuel cell electrode (catalyst layer) is produced using this, the drying time can be shortened. Moreover, since the surface of the electrode catalyst is coated with the highly oxygen-permeable ionomer (A), high performance can be obtained even when the amount of catalyst used is relatively small.

Furthermore, when the surface of the electrode catalyst is coated with the highly oxygen permeable ionomer (A), if the ion exchange capacity of the highly oxygen permeable ionomer (A) is optimized, it will be high under both high temperature and low humidity conditions and low temperature and high humidity conditions. A fuel cell electrode exhibiting power generation performance is obtained. this is,
(a) By setting the ion exchange capacity of the high oxygen permeation ionomer (A) to 1.1 meq/g or more, an increase in proton transfer resistance in the catalyst layer is suppressed even in a high temperature and low humidity environment, and
(b) By setting the ion exchange capacity of the highly oxygen permeable ionomer (A) to 1.25 meq/g or less, excessive swelling of the ionomer is suppressed and the surface of the ionomer becomes more water repellent. Sufficient space is secured inside the
it is conceivable that.

(Example 1, Comparative Examples 1 to 3)
[1. Preparation of sample]
[1.1. Preparation of ionomer-coated catalyst]
A mixed solution was obtained by mixing platinum-supported carbon, water, ethanol, and a highly oxygen-permeable ionomer solution. The weight ratio (I ₁ /C ratio) of the highly oxygen permeable ionomer to the carbon weight was 0.4, the solid content concentration was 6 wt %, and the water ratio was 94 wt %.
An electrolyte polymer composed of a perfluorocarbon unit having an alicyclic structure and an acid group unit having perfluorosulfonic acid as a side chain was used for the highly oxygen-permeable ionomer. The ion exchange capacity (hereinafter also referred to as “IEC”) of the high oxygen permeability ionomer is 1.0 meq/g (Comparative Example 1), 1.2 meq/g (Example 1), 1.3 meq/g (Comparative Example 2), or 1.5 meq/g (Comparative Example 3). The oxygen permeability coefficients of the high oxygen permeability ionomers were all 2.2×10 ⁻¹⁴ mol/(m·s·Pa) or more regardless of IEC.

Next, the mixed solution was subjected to dispersion treatment using a high-pressure homogenizer (Starburst Mini manufactured by Sugino Machine Co., Ltd.). The nozzle diameter was 100 μmφ, and the processing pressure was 75 MPa.
The obtained dispersion solution was heated at 80° C. for 3 hours in an explosion-proof drying oven to evaporate the solvent. Then, it was heat-treated at 160° C. for 2 hours under reduced pressure in a vacuum dryer. Further, the heat-treated powder was pulverized with an electric mill to obtain an ionomer-coated catalyst. When the residual rate of the ionomer-coated catalyst was measured using the method described in Patent Document 3, the residual rate was 70% or more in all cases.

[1.2. Preparation of catalyst ink]
A pulverized ionomer coat catalyst, water, and ethanol were mixed in a predetermined ratio to obtain a mixed liquid. The mixed liquid was subjected to dispersion treatment using a high-pressure homogenizer (Starburst Mini manufactured by Sugino Machine Co., Ltd.) to obtain catalyst ink (A). The nozzle diameter was 150 μmφ, and the processing pressure was 75 MPa.
Next, a second ionomer was added to and mixed with the catalyst ink (A) subjected to dispersion treatment to obtain a catalyst ink (B). A Nafion (registered trademark) solution was used for the second ionomer. The weight ratio (I ₂ /C) of the second ionomer to the carbon weight was set to 0.7, and the solid content concentration of the catalyst ink (B) was set to 20 wt %.

[1.3. Fabrication of fuel cell]
[1.3.1. Preparation of air electrode]
The catalyst ink (B) was applied onto a polytetrafluoroethylene (PTFE) sheet using a Baker applicator and dried to obtain a catalyst layer. The platinum basis weight was 0.15 mg/cm ² . This catalyst layer was transferred to one surface of the electrolyte membrane by a hot press to form an air electrode. The electrode area was 1 cm ² .

[1.3.2. Preparation of hydrogen electrode]
A catalyst ink (C) containing only platinum-supported carbon and Nafion® as an ionomer was prepared. The weight ratio (I/C) of the ionomer to the weight of the carbon was set to 1.0, and the solid content concentration of the catalyst ink (C) was set to 10 wt%.
The catalyst ink (C) was applied onto PTFE and dried to obtain a catalyst layer. This catalyst layer was thermally transferred to the other surface of the electrolyte membrane to form a hydrogen electrode.

[1.3.3. Preparation of evaluation cell]
Both surfaces of the electrolyte membrane on which the catalyst layer was formed were sandwiched between carbon papers with a water-repellent layer to form an evaluation cell.

[2. Test method]
A power generation test was conducted using the evaluation cell. The bubbler temperature was 55°C and the back pressure was 50 kPaG. Humidified air was supplied to the cathode and humidified hydrogen was supplied to the anode. The cell temperature was 45° C. (low temperature and high humidity conditions) or 82° C. (high temperature and low humidity conditions).

[3. result]
Among the four electrodes tested, the current density (I ₀ ) of the electrode with the maximum current density at a cell voltage of 0.6 V is used as a reference, and the ratio of the current density (I) of each electrode to it ( Hereinafter, the "current density ratio (I/I ₀ )") was determined. FIG. 1 shows the relationship between the ion exchange capacity (IEC) and the current density ratio of high oxygen permeability ionomers.

It can be seen that under low temperature and high humidity conditions, the larger the IEC, the smaller the current density ratio. This is probably because the greater the IEC, the greater the swelling of the ionomer under low-temperature and high-humidity conditions. Moreover, as the IEC increases, the hydrophilicity of the ionomer surface increases due to the sulfonic acid groups, making it easier for water to accumulate in the catalyst layer.

It can be seen that under high temperature and low humidity conditions, the higher the IEC, the higher the current density ratio. This is probably because the higher the IEC, the lower the proton transfer resistance in the catalyst layer.
From FIG. 1, the range of IEC showing a high current density ratio (specifically, 0.95 or more) under both high-temperature and low-humidity conditions and low-temperature and high-humidity conditions is 1.1 meq/g or more. It turns out that it is 25 meq/g or less. That is, it can be seen that an ionomer-coated catalyst using an IEC ionomer within this range can obtain high power generation performance in a wide range of temperature and humidity environments from low humidity and high temperature to high temperature and low humidity.

From the above results, by using a catalyst coated with a highly oxygen-permeable ionomer, it is possible to reduce the oxygen transfer resistance even with a low platinum basis weight. Moreover, by using an ionomer-coated catalyst, the solid content concentration of the catalyst ink can be increased, and the drying time of the catalyst ink can be shortened. Furthermore, by optimizing the IEC, it is possible to provide electrodes with high power generation performance in a wide range of operating environments from low temperature and high humidity conditions to high temperature and low humidity conditions, and polymer electrolyte fuel cells using the same.

Although the embodiments of the present invention have been described in detail above, the present invention is by no means limited to the above-described embodiments, and various modifications are possible without departing from the gist of the present invention.

The fuel cell electrode according to the present invention can be used as an air electrode for polymer electrolyte fuel cells.

Claims (6)

A fuel cell electrode having the following structure.
(1) The fuel cell electrode is
an ionomer-coated catalyst in which the surface of the electrode catalyst is coated with a first layer made of a first ionomer;
a second layer comprising a second ionomer in contact with the first ionomer;
It has
(2) The first ionomer is made of a high oxygen permeability ionomer (A) having an ion exchange capacity of 1.1 meq/g or more and 1.25 meq/g or less.
(3) The highly oxygen-permeable ionomer (A) consists of a polymer compound containing an acid group and a cyclic structure in its molecular structure.
(4) The second ionomer is made of polyperfluorocarbonsulfonic acid.
(5) The ionomer-coated catalyst has a residual rate R (%) represented by the following formula (1) of 70% or more.
R (%)=(x ₀ −x)×100/x ₀ (1)
however,
x is the concentration of the first ionomer dissolved in the mixed solvent when the ionomer-coated catalyst is added to a mixed solvent of water:ethanol = 70:30 (weight ratio) to form a dispersion;
x0 is the theoretical value of the concentration of the first _ionomer in the mixed solvent, assuming that all of the first ionomer is dissolved in the mixed solvent. 2. The ionomer according to claim 1, wherein the high oxygen permeability ionomer (A) comprises an ionomer having an oxygen permeability coefficient of 2.2×10 ⁻¹⁴ mol/(m·s·Pa) or more at a temperature of 80° C. and a relative humidity of 30% RH. An electrode for a fuel cell as described. The electrode catalyst is
a carbon carrier;
3. The fuel cell electrode according to claim 1 , further comprising fine catalyst particles supported on the surface of said carbon support. 4. The fuel cell electrode according to claim 3, wherein the I1 _/ C ratio is 0.2 or more and 0.7 or less.
However, the “I ₁ /C ratio” refers to the ratio (=W ₁ /W _c ) of the weight (W ₁ ) of the first ionomer to the weight (W _c ) of the carbon support. 5. The fuel cell electrode according to claim 3 , wherein the _I2 /C ratio is 0.2 or more and 1.0 or less.
However, the “I ₂ /C ratio” refers to the ratio (=W ₂ /W _c ) of the weight (W ₂ ) of the second ionomer to the weight (W _c ) of the carbon support. 6. The fuel cell electrode according to any one of claims 3 to 5, wherein the I1 _/ C ratio + _I2 /C ratio is 0.5 or more and 1.4 or less.
however,
“I ₁ /C ratio” means the ratio of the weight (W ₁ ) of the first ionomer to the weight (W _c ) of the carbon support (=W ₁ /W _c ),
“I ₂ /C ratio” refers to the ratio (=W ₂ /W _c ) of the weight (W ₂ ) of the second ionomer to the weight (W _c ) of the carbon support.

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