JP2022142887A - Electrode catalyst, manufacturing method thereof, electrode for fuel cell, and fuel cell - Google Patents

Abstract

To provide a novel electrode catalyst in which the surfaces of catalyst particles are coated with a coating that satisfies a specific condition, a manufacturing method thereof, and an electrode for a fuel cell including the electrode catalyst, and a fuel cell.SOLUTION: A electrode catalyst includes catalyst particles and a nitrogen-containing carbon film covering the surface of the catalyst particles, and the nitrogen-containing carbon film contains micropores. An electrode for a fuel cell and the fuel cell is composed by using such an electrode catalyst. Such an electrode catalyst is obtained by forming an electrode catalyst precursor by coating the surface of the catalyst particles with a coating containing polymelamine and polydopamine, heat-treating the electrode catalyst precursor, and thermally decomposing the coating.SELECTED DRAWING: Figure 3

Description

TECHNICAL FIELD The present invention relates to an electrode catalyst, a method for producing the same, an electrode for a fuel cell, and a fuel cell. The present invention relates to a fuel cell electrode and a fuel cell provided with an electrode catalyst.

A polymer electrolyte fuel cell includes a membrane electrode assembly (MEA) in which electrodes containing a catalyst are joined to both sides of an electrolyte membrane. Current collectors (separators) having gas flow paths are further disposed on both sides of the MEA. A polymer electrolyte fuel cell usually has a structure (fuel cell stack) in which a plurality of single cells each including such an MEA and a current collector are stacked.

A fuel cell electrode is usually composed of a laminate of a catalyst layer arranged on the electrolyte membrane side and a diffusion layer arranged on the gas channel side. The catalyst layer is generally composed of a mixture of an electrode catalyst in which catalyst particles such as platinum or platinum alloy are supported on the surface of a carrier, and a catalyst layer ionomer. Electrode reactions occur primarily on the surface of catalyst particles. Therefore, efforts have been made to reduce the amount of platinum used per unit area of the electrode by making the catalyst particles as fine as possible.
However, under the operating environment of a fuel cell that involves potential fluctuations, the finer the catalyst particles, the more likely they are to dissolve, coarsen due to agglomeration, and/or detach from the carrier. As a result, there is a problem that the activity of the catalyst layer is gradually lowered.

In order to solve this problem, various proposals have been conventionally made.
For example, in Non-Patent Document 1,
(a) supporting irregular fcc-PtFe nanoparticles on the carbon particle surface;
(b) coating the nanoparticle surface with polydopamine by treating it with an aqueous solution of dopamine hydrochloride;
(c) Carbon-supported and dopamine-coated fcc-PtFe nanoparticles (fcc-PtFe/C) were heat-treated at 700° C. to obtain a N-doped carbon-coated ordered fct-PtFe nanoparticle catalyst (fct -Pt/Fe/C) is disclosed.

In the same document,
(A) Heat treatment of PtFe/C without dopamine coating increases the size of the nanoparticles to tens of nanometers, whereas heat treatment of dopamine-coated PtFe/C increases the size of the nanoparticles compared to before heat treatment. maintaining approximately the same size (6.5 nm);
(B) fct-PtFe/C catalyst coated with N-doped carbon shell exhibits 11.4 times higher mass activity and 10.5 times higher specific activity than commercial Pt/C catalyst;
(C) It is stated that coating Pt/C with an N-doped carbon shell has a negligible effect on activity.

Patent Document 1 discloses an electrode catalyst comprising catalyst particles and a carbon film covering the surfaces of the catalyst particles, wherein the content of chloride ions per unit surface area of the catalyst particles is less than 12.5 μg/m ² . is disclosed.
In the same document,
(a) By coating the surface of the catalyst particles with a film derived from an organic substance and heat-treating it, an electrode catalyst in which the surfaces of the catalyst particles are coated with a carbon film can be obtained;
(b) when the obtained electrode catalyst is washed, the chloride ions adsorbed on the surface of the catalyst particles are desorbed, so that the initial activity is improved compared to the electrode catalyst without washing;
(c) The carbon film suppresses dissolution, aggregation, and/or detachment of the catalyst particles, and suppresses ionomer poisoning of the catalyst particles, thereby improving initial activity and durability. there is

As-synthesized PtFe alloy nanoparticles have an irregular face-centered cubic (fcc) structure and low catalytic activity. Heat treating it at 700° C. transforms it into a regular face-centered tetragonal (fct) structure, improving the area specific activity. However, since the nanoparticles are coarsened in the heat treatment step, there is a problem that the mass activity is remarkably lowered.

On the other hand, Non-Patent Document 1 describes that when the surfaces of PtFe alloy nanoparticles are coated with dopamine and heat-treated at 700° C., the increase in particle size of the nanoparticles is suppressed. This is explained by the fact that the thin carbon film (dopamine pyrolysis product) covering the surface of the nanoparticles inhibited the aggregation of the nanoparticles during the heat treatment process.
In Non-Patent Document 1, the fct-PtFe/C catalyst subjected to such treatment has higher area activity, mass activity and durability than pure Pt/C catalyst and irregular fcc-PtFe/C catalyst. It states that the performance is improved. It is explained that the durability was improved because the carbon film covering the Pt surface suppressed the dissolution, aggregation, and detachment of Pt that occurred during the durability test.

However, the effects of dopamine modification and the carbon film itself formed by heat treatment on the performance of the catalyst have not been sufficiently clarified. In fact, Non-Patent Document 1 reports that even if a pure Pt/C catalyst was coated with dopamine and heat-treated at 700° C., the initial performance was not improved.

On the other hand, Patent Document 1 describes that the initial activity and durability of the electrode catalyst are improved by washing the electrode catalyst after coating the surface of the pure Pt/C catalyst with a dopamine-derived carbon film. However, in order to further improve the performance of fuel cells, it is desired to further improve the initial activity and durability of the electrode catalyst.

Japanese Patent Application Laid-Open No. 2020-136109

D. Y. Chung, et al., J. Am. Chem. Soc., 2015, 137, 15478-15485

The problem to be solved by the present invention is to provide a novel electrode catalyst in which the surfaces of catalyst particles are coated with a coating that satisfies specific conditions, and a method for producing the same.
Another object of the present invention is to provide a novel electrode catalyst with excellent initial activity and durability, and a method for producing the same.
Furthermore, another problem to be solved by the present invention is to provide a fuel cell electrode and a fuel cell having such an electrode catalyst.

In order to solve the above problems, the electrode catalyst according to the present invention is
catalyst particles;
and a nitrogen-containing carbon film covering the surface of the catalyst particles,
The gist is that the nitrogen-containing carbon film includes micropores.

A fuel cell electrode according to the present invention comprises a catalyst layer containing the electrode catalyst according to the present invention and a catalyst layer ionomer.
The fuel cell according to the present invention is
an electrolyte membrane made of a solid polymer electrolyte;
and electrodes bonded to both sides of the electrolyte membrane,
At least one of the electrodes is composed of the fuel cell electrode according to the present invention.

The method for producing an electrode catalyst according to the present invention comprises:
a coating step of coating the surface of the catalyst particles with a coating containing polymelamine and polydopamine to obtain an electrode catalyst precursor;
a thermal decomposition step of thermally decomposing the film by heat-treating the electrode catalyst precursor to obtain an electrode catalyst in which the surfaces of the catalyst particles are coated with a nitrogen-containing carbon film.

By coating the surface of the catalyst particles with polymelamine and polydopamine and thermally decomposing the polymelamine and polydopamine, an electrode catalyst in which the surfaces of the catalyst particles are coated with a nitrogen-containing carbon film containing micropores is obtained. The electrode catalyst thus obtained may have improved initial activity and durability as compared with an electrode catalyst coated with a polydopamine-derived carbon film. this is,
(a) Since the nitrogen-containing carbon film suppresses grain growth of catalyst particles during heat treatment,
(b) Since the nitrogen-containing carbon film has micropores, even if the thickness of the nitrogen-containing carbon film increases, the transport of substances to the surface of the catalyst particles is less likely to be inhibited;
(c) Since the nitrogen-containing carbon film suppresses dissolution, aggregation, and/or detachment of the catalyst particles and ionomer poisoning of the catalyst particles,
it is conceivable that.

1 shows current-voltage curves of electrode catalysts obtained in Examples 1 and 3 and Comparative Example 1 under high humidity. FIG. 2 is an enlarged view of the low load region of the current-voltage curve shown in FIG. 1; 1 shows the results of endurance tests of electrode catalysts obtained in Example 1 and Comparative Examples 1 and 2. FIG. 1 shows mass activity retention rates of electrode catalysts obtained in Example 1 and Comparative Examples 1 and 2. FIG. 1 is an HRTEM image of an electrode catalyst obtained in Example 1. FIG.

An embodiment of the present invention will be described in detail below.
[1. Electrocatalyst]
The electrode catalyst according to the present invention is
catalyst particles;
and a nitrogen-containing carbon film covering the surface of the catalyst particles.
The electrode catalyst may further include a carrier that supports the catalyst particles.

[1.1. catalyst particles]
[1.1.1. composition]
In the present invention, the material of catalyst particles is not particularly limited. Materials for catalyst particles include:
(a) noble metals (Pt, Au, Ag, Pd, Rh, Ir, Ru, Os),
(b) an alloy containing two or more precious metal elements;
(c) alloys containing one or more noble metal elements and one or more base metal elements (e.g., Fe, Co, Ni, Cr, V, Ti, etc.);
and so on.

Among these, the catalyst particles are preferably Pt or Pt alloy. This is because it has high activity for the electrode reaction of the fuel cell.
Pt alloys include, for example, Pt--Fe alloys, Pt--Co alloys, Pt--Ni alloys, Pt--Pd alloys, Pt--Cr alloys, Pt--V alloys, Pt--Ti alloys, Pt--Ru alloys, Pt--Ir alloys, etc.

[1.1.2. Particle size]
The particle size of the catalyst particles is not particularly limited, and an optimum particle size can be selected according to the purpose. In general, when the particle size of the catalyst particles is too small, the catalyst particles tend to dissolve. Therefore, the particle size of the catalyst particles is preferably 1 nm or more.
On the other hand, if the particle size of the catalyst particles becomes too large, the mass activity will decrease. Therefore, the particle size of the catalyst particles is preferably 20 nm or less. The particle size of the catalyst particles is preferably 10 nm or less, more preferably 5 nm or less.

[1.2. carrier]
[1.2.1. material]
The catalyst particles may be used for various purposes as they are, or may be used while being supported on the surface of a carrier. When the catalyst particles are carried on the surface of the carrier, fine catalyst particles can be stably dispersed, so that the amount of catalyst used can be reduced.
Examples of carriers include carbon black, carbon nanotubes, carbon nanohorns, activated carbon, natural graphite, mesocarbon microbeads, and glassy carbon powder.

[1.2.2. Amount of catalyst supported]
When the catalyst particles are supported on the surface of the carrier, the amount of catalyst supported is not particularly limited, and an optimum supported amount can be selected according to the purpose. In general, if the amount of catalyst supported is too small, sufficient activity cannot be obtained. On the other hand, even if the amount of catalyst supported is increased more than necessary, there is no difference in the effect and there is no practical benefit.
For example, when catalyst particles made of Pt or a Pt alloy are supported on the surface of a carbon support, the catalyst support amount is preferably 5 wt % to 70 wt %.

[1.3. Nitrogen-containing carbon film]
[1.3.1. composition]
The surfaces of the catalyst particles are coated with a nitrogen-containing carbon film. A "nitrogen-containing carbon film" refers to a film obtained by coating the surfaces of catalyst particles with a film containing polymelamine and polydopamine and thermally decomposing the film.

Pyrolysis of a film containing only polydopamine results in a carbon film that is dense and consists essentially of carbon. On the other hand, when a film containing polymelamine and polydopamine is thermally decomposed, a nitrogen-containing carbon film containing micropores and having a higher amount of nitrogen than the carbon film derived from polydopamine can be obtained. Nitrogen contained in the nitrogen-containing carbon film is derived from polymelamine. The amount of nitrogen contained in the nitrogen-containing carbon film is usually 0.1 mass % to 35 mass %, depending on the manufacturing method.

The nitrogen-containing carbon film is
(a) obtained by coating the surface of catalyst particles with a coating comprising a mixture of polymelamine and polydopamine and thermally decomposing the coating (simultaneous modification method), or
(b) Those obtained by coating the surfaces of the catalyst particles with polymelamine, further coating the surfaces of the catalyst particles with polydopamine, and then thermally decomposing the laminated film of polymelamine and polydopamine (sequential modification method). ,
It may be either.
In particular, the nitrogen-containing carbon film obtained by the sequential modification method exhibits higher properties than the nitrogen-containing carbon film obtained by the simultaneous modification method.

[1.3.2. Micropore capacity]
"Micropores" refer to pores with a diameter of 2 nm or less.
The “capacity of micropores” refers to a value obtained by measuring the amount of nitrogen adsorbed on an electrode catalyst and analyzing it by the t-plot method using solid carbon as a reference isotherm.

As described above, when polydopamine is thermally decomposed in the presence of polymelamine, a nitrogen-containing carbon film having micropores can be obtained. If the volume of the micropores becomes too small, mass transfer to the surface of the catalyst particles may be hindered, resulting in a decrease in initial activity. Therefore, the volume of micropores is preferably 0.001 cc/g or more.
On the other hand, excessive micropore volume may result in dissolution, agglomeration, and/or detachment of the catalyst particles, or ionomer poisoning of the catalyst particles. Therefore, the volume of micropores is preferably 0.3 cc/g or less.

[1.3.3. thickness]
The thickness of the nitrogen-containing carbon film affects the stability and activity of the catalyst particles. If the thickness of the nitrogen-containing carbon film is too thin, dissolution, agglomeration and/or detachment of the catalyst particles tend to occur. Further, when such an electrode catalyst is applied to a fuel cell electrode, if the thickness of the nitrogen-containing carbon film is too thin, the catalyst particles are likely to be poisoned by the catalyst layer ionomer. Therefore, the thickness of the nitrogen-containing carbon film is preferably 0.2 nm or more. The thickness of the nitrogen-containing carbon film is preferably 0.5 nm or more.
On the other hand, when the thickness of the nitrogen-containing carbon film is too thick, the transport resistance of the reactants increases, and the activity may decrease. Therefore, the thickness of the nitrogen-containing carbon film is preferably 20 nm or less.

[1.4. Characteristic]
[1.4.1. Initial activity]
"Initial activity" refers to the current density at 0.86 V in the initial state (after break-in) of a polymer electrolyte fuel cell using the electrode catalyst according to the present invention as a cathode catalyst.
In the electrode catalyst according to the present invention, the initial activity is 0.02 A/cm ² @0.86 V or more when the manufacturing conditions are optimized.

[1.4.2. Activity after durability test]
"Activity after endurance test" refers to the current density at 0.86 V after endurance test of a polymer electrolyte fuel cell using the electrode catalyst according to the present invention as a cathode catalyst.
“After endurance test” refers to the state after 10,000 cycles of potential fluctuation of 0.6 V (held for 3 seconds) and 1.0 V (held for 3 seconds) to the fuel cell.
In the electrode catalyst according to the present invention, if the manufacturing conditions are optimized, the activity after the endurance test will be 0.02 A/cm ² @0.86 V or more.

[1.4.3. ECSA maintenance rate]
“ECSA retention rate (%)” refers to the ratio of ECSA (x) after durability test to ECSA (x ₀ ) before durability test (=x×100/x ₀ ).
“After endurance test” refers to the state after 10,000 cycles of potential fluctuation of 0.6 V (held for 3 seconds) and 1.0 V (held for 3 seconds) to the fuel cell.
In the electrode catalyst according to the present invention, when the manufacturing conditions are optimized, the ECSA retention rate becomes 80% or more. If the production conditions are further optimized, the ECSA retention rate will be 90% or more, or 95% or more.

[2. Fuel cell electrode]
A fuel cell electrode according to the present invention comprises a catalyst layer containing the electrode catalyst according to the present invention and a catalyst layer ionomer. The fuel cell electrode may further include a diffusion layer arranged on the current collector side.
The content of the catalyst layer ionomer contained in the catalyst layer is not particularly limited, and the optimum content can be selected according to the purpose. For example, when catalyst particles are supported on a carbon support, the ratio of the weight (I) of the ionomer in the catalyst layer to the weight (C) of carbon (=I/C) is preferably 0.3 or more and 2.0 or less.

[3. Fuel cell]
The fuel cell according to the present invention is
an electrolyte membrane made of a solid polymer electrolyte;
and electrodes bonded to both surfaces of the electrolyte membrane.

[3.1. Electrolyte membrane]
The electrolyte membrane consists of a solid polymer electrolyte. The composition of the solid polymer electrolyte is not particularly limited, and an optimum material can be selected according to the purpose. Examples of solid polymer electrolytes include Nafion (registered trademark), Flemion (registered trademark), Aciplex (registered trademark), and Aquivion (registered trademark).

[3.2. electrode]
Electrodes are bonded to both sides of the electrolyte membrane. In the fuel cell according to the present invention, at least one of the electrodes is composed of the fuel cell electrode according to the present invention. The fuel cell electrode according to the present invention may be used on either the anode side or the cathode side.
The fuel cell electrode according to the present invention is particularly preferably used as a cathode. This is because the presence or absence of a coating has a greater effect on the initial activity and durability of the fuel cell on the cathode than on the anode.

[4. Electrocatalyst manufacturing method]
The method for producing an electrode catalyst according to the present invention comprises:
a coating step of coating the surface of the catalyst particles with a coating containing polymelamine and dopamine to obtain an electrode catalyst precursor;
a thermal decomposition step of thermally decomposing the film by heat-treating the electrode catalyst precursor to obtain an electrode catalyst in which the surfaces of the catalyst particles are coated with a nitrogen-containing carbon film.

[4.1. Coating process]
First, the surface of the catalyst particles is coated with a coating containing polymelamine and polydopamine to obtain an electrode catalyst precursor (coating step).
In the coating step, the surfaces of the catalyst particles may be coated with a coating comprising a mixture of polymelamine and polydopamine (simultaneous modification method).
Alternatively, the coating step is
a first coating step of coating the surface of the catalyst particles with polymelamine;
and a second coating step of further coating the surface of the catalyst particles with polydopamine (sequential modification method).

[4.1.1. catalyst particles]
The catalyst particles may be supported on the surface of the carrier, or may not be supported on the surface of the carrier. Other points regarding the catalyst particles are the same as described above, so the description is omitted.

[4.1.2. coating]
The coating contains polymelamine and polydopamine. The coating may be a mixture of polymelamine and polydopamine, or a laminate film of polymelamine and polydopamine. In order to obtain an electrode catalyst excellent in initial activity and durability, the film is preferably a laminated film of polymelamine and polydopamine.

When forming a coating film comprising a mixture of polymelamine and polydopamine, for example, dopamine hydrochloride is added to a solution in which polymelamine is dissolved, and then catalyst particles (or catalyst particles supported on a carrier) are added. Stir for a predetermined time and dry. As a result, dopamine is polymerized to polydopamine, and at the same time, the surface of the catalyst particles is coated with a mixture of polymelamine and polydopamine (simultaneous modification method).

On the other hand, when forming a film made of a laminated film, for example, first, catalyst particles (or catalyst particles supported on a carrier) are added to a solution in which polymelamine is dissolved, stirred for a predetermined time, and dried. This yields catalyst particles coated with polymelamine. Then, the polymelamine-coated catalyst particles are added to a solution containing dopamine hydrochloride, stirred, and dried. As a result, dopamine is polymerized into polydopamine, and at the same time, the surface of the polymelamine-coated catalyst particles is further coated with polydopamine (sequential modification method).

[4.1.3. Addition amount of polymelamine and polydopamine]
The amount of polymelamine and the amount of polydopamine to be added are not particularly limited, and the optimum amount to be added can be selected according to the purpose. The thickness and nitrogen content of the nitrogen-containing carbon film can be controlled by changing the amount of these additives.

Here, "addition amount of polymelamine or polydopamine" refers to a value represented by the following formula (1).
Content (mass%) = y x 100/(y + z) (1)
however,
y is the mass of polymelamine or polydopamine contained in the electrode catalyst precursor;
z is the mass of the catalyst particles contained in the electrode catalyst precursor (when the catalyst particles are supported on a carrier, the total mass of the catalyst particles and the carrier);

If the amount of polymelamine added is too small, micropores will not form and the durability will be reduced. Therefore, the amount of polymelamine to be added is preferably 10% by mass or more. The amount of polymelamine added is more preferably 50 mass % or more.
On the other hand, if the added amount of polymelamine is excessive, the durability is lowered. Therefore, the amount of polymelamine to be added is preferably 99 mass % or less. The amount of polymelamine added is more preferably 93 mass % or less.

If the amount of polydopamine added is too small, micropores will not form and the durability will be reduced. Therefore, the amount of polydopamine to be added is preferably 5 mass % or more. The amount of polydopamine to be added is more preferably 10 mass % or more.
On the other hand, if the amount of polydopamine added is excessive, the durability will rather decrease. Therefore, the amount of polydopamine to be added is preferably 50 mass % or less. The amount of polydopamine to be added is more preferably 24 mass % or less.

[4.2. Thermal decomposition process]
Next, the film is thermally decomposed by heat-treating the electrode catalyst precursor (thermal decomposition step). As a result, an electrode catalyst in which the surfaces of catalyst particles are coated with a nitrogen-containing carbon film can be obtained.
The heat treatment conditions are not particularly limited as long as the coating can be carbonized. The heat treatment is preferably carried out at a temperature of 600° C. or more and 1000° C. or less in an inert atmosphere for 0.5 to 10 hours.

[5. action]
A carbon film can be formed on the surface of the catalyst particles by modifying the surface of the catalyst particles only with polydopamine and thermally decomposing it. However, since the polydopamine-derived carbon film is dense, the thicker the carbon film, the more impeded mass transfer to the surface of the catalyst particles, and the lower the performance.
On the other hand, when the surface of the catalyst particles is modified only with polymelamine and used without being thermally decomposed, the effect of improving the performance in the low load range is higher than when modified with a polydopamine-derived carbon film. However, when the polymelamine film is heat-treated, most of it is decomposed and only a very small amount remains as a carbon film. In addition, since the slightly remaining carbon film derived from polymelamine is as dense as the carbon film derived from polydopamine, the thicker the carbon film, the lower the performance.

On the other hand, when the surfaces of the catalyst particles are coated with polymelamine and polydopamine, and the polymelamine and polydopamine are thermally decomposed, an electrode catalyst in which the surfaces of the catalyst particles are coated with a nitrogen-containing carbon film containing micropores is obtained. be done. The electrode catalyst thus obtained may have improved initial activity and durability as compared with an electrode catalyst coated with a polydopamine-derived carbon film. this is,
(a) Since the nitrogen-containing carbon film suppresses grain growth of catalyst particles during heat treatment,
(b) Since the nitrogen-containing carbon film has micropores, even if the thickness of the nitrogen-containing carbon film increases, the transport of substances to the surface of the catalyst particles is less likely to be inhibited;
(c) Nitrogen-containing carbon film dissolves, aggregates, and/or desorbs catalyst particles, and ionomer poisoning of catalyst particles (a phenomenon in which sulfonic acid groups of ionomers adsorb to catalyst particle surfaces to reduce active sites) to suppress
it is conceivable that.

(Examples 1-3, Comparative Examples 1-2)
[1. Preparation of sample]
[1.1. Preparation of electrode catalyst]
[1.1.1. Examples 1 and 2: Sequential modification material]
30 mass% Pt/Vulcan (registered trademark) (TEC10V30E) (hereinafter also simply referred to as "Pt/C catalyst") was used as the catalyst. This was washed with hot (100° C.) Milli-Q (registered trademark) water (hereinafter also simply referred to as “water”) and dried. The washed Pt/C catalyst was dispersed in a mixed solvent of water and 2-propanol, to which a polymelamine (PME) solution was added and stirred at room temperature for 3 hours. After stirring, the powder A was collected by suction filtration and vacuum dried at 100°C. The amount of PME added (=the ratio of the mass of PME to the total mass of the Pt/C catalyst and PME) was 93 mass% (Example 1) or 19 mass% (Example 2). The following formula (2) shows the molecular structure of polymelamine (poly(melamine-co-formaldehyde)).

Next, the dried powder A, dopamine hydrochloride, and Tris-hydrochloride buffer adjusted to pH 8.5 were placed in a beaker and ultrasonically dispersed. The amount of dopamine hydrochloride added (=the ratio of the mass of dopamine hydrochloride to the total mass of the Pt/C catalyst and dopamine hydrochloride) was 30% by mass. The mass of dopamine excluding hydrochloric acid is 24 mass%. The dispersion was then stirred at room temperature in the air for 6 hours using a stirrer. This caused dopamine to polymerize into polydopamine (PDA). The following formula (3) shows the molecular structure of polydopamine.

After stirring, powder B was collected by suction filtration and washed with water at room temperature. Further, the powder B after washing was vacuum-dried at 100°C. The obtained powder B was heat-treated at 700° C. for 2 hours while flowing Ar in a tubular furnace to carbonize PME and PDA. The heat-treated powder C was washed with high-temperature (100° C.) water to obtain an electrode catalyst.

[1.1.2. Example 3: Simultaneous modification material]
The above Pt/C catalyst was dispersed in a mixed solvent of water and 2-propanol. A PME solution (PME addition amount: 93 mass%) and dopamine hydrochloride (mass of dopamine excluding hydrochloric acid: 24 mass%) were added thereto, and the mixture was stirred at room temperature for 3 hours. After stirring, the powder D was collected by suction filtration and vacuum dried at 100°C. The obtained powder D was heat-treated at 700° C. for 2 hours while flowing Ar in a tubular furnace to carbonize PME and PDA. The heat-treated powder D was washed with high-temperature (100° C.) water to obtain an electrode catalyst.

[1.1.3. Comparative Example 1: Unmodified Material]
The above Pt/C catalyst was used for the test as it was.
[1.1.4. Comparative Example 2: PDA Modifier]
An electrode catalyst coated with a PDA-derived carbon film was obtained in the same manner as in Example 1, except that the PME coating on the Pt/C catalyst surface was omitted.

[1.2. Fabrication of fuel cell]
The above electrode catalyst was dispersed in water/ethanol/Nafion (registered trademark) ionomer solution (D-2020) to prepare a catalyst ink. The water/alcohol mass ratio in the catalyst ink was about 1. This catalyst ink was applied onto a polytetrafluoroethylene sheet to prepare a cathode catalyst layer. The Pt basis weight of the cathode catalyst layer was set to 0.1 mg/cm ² , and the ratio (I/C) of the mass of the ionomer to the mass of the carbon support was set to about 1.

The anode catalyst layer contains 60 mass% Pt/Ketjen (registered trademark) as an electrode catalyst, contains Nafion (registered trademark) (D ^- 2020) as an ionomer, has a Pt basis weight of 0.2 mg/cm /C of 1.0 was used.

The cathode catalyst layer and the anode catalyst layer were thermally transferred (120° C., 50 kgf/cm ² , 5 min) to both sides of Nafion (registered trademark) membrane (NR211) to prepare a membrane electrode assembly (MEA). The electrode area was 1 cm ² . A cell was constructed by sandwiching this MEA between paper diffusion layers (GDL) with a water-repellent layer.
[2. Test method]
[2.1. Micropore capacity]
The nitrogen adsorption amount of the obtained electrode catalyst was measured. The obtained data were analyzed by the t-plot method using solid carbon as a reference isotherm, and the pore inner surface area, pore outer surface area, and micropore volume were determined.

[2.2. Initial power generation performance and power generation performance after endurance]
Using the obtained cell, power generation performance after break-in (initial power generation performance) and power generation performance after potential cycle test (durability power generation performance) were examined. The power generation performance was evaluated under high humidity (cell temperature 60° C./humidity 80% RH). Further, the potential cycle endurance test was carried out by applying a potential change of 0.6 V (held for 3 seconds) and 1.0 V (held for 3 seconds) for 10,000 cycles.

[2.3. Observation with a high-resolution transmission electron microscope]
High-resolution transmission electron microscopy (HRTEM) observation was performed to examine the state of the PME- and PDA-modified electrocatalysts after heat treatment.

[3. result]
[3.1. Micropore capacity]
Table 1 shows the pore inner surface area, pore outer surface area, and micropore volume analyzed by the t-plot method for each electrode catalyst. Both the unmodified material (Comparative Example 1) and the PDA-modified material (Comparative Example 2) had zero inner pore surface area and zero micropore volume. On the other hand, the sequential modification materials (Examples 1 and 2) and the simultaneous modification material (Example 3) had positive values for both the pore inner surface area and the micropore volume. In addition, the higher the amount of modification by PME, the higher these values. It is considered that this is because the film became porous due to the partial decomposition of PME due to the heat treatment at 700°C. Further, from the weight analysis after the heat treatment, it is considered that the remaining PME remains in the film together with the PDA.

[3.2. Initial power generation performance and power generation performance after endurance]
FIG. 1 shows current-voltage curves of the electrode catalysts obtained in Examples 1 and 3 and Comparative Example 1 under high humidity. From Figure 1,
(a) Example 1 has less performance deterioration due to the durability test than Comparative Example 1, and
(b) It can be seen that the initial power generation performance of Example 3 was lower than that of Example 1.
FIG. 2 shows an enlarged view of the low load region of the current-voltage curve shown in FIG. As can be seen from FIG. 2, in the low-load range, Example 1 has higher initial power generation performance and higher power generation performance after endurance than Comparative Example 1, and has improved catalytic activity.

FIG. 3 shows the durability test results of the electrode catalysts obtained in Example 1 and Comparative Examples 1 and 2. As shown in FIG. From FIG. 3 , Example 1 shows a higher ECSA retention rate than Comparative Examples 1 and 2, indicating that the durability is high.
FIG. 4 shows the mass activity retention ratios of the electrode catalysts obtained in Example 1 and Comparative Examples 1 and 2. As shown in FIG. From FIG. 4, it can be seen that the mass activity retention rate of Example 1 is remarkably improved as compared with Comparative Examples 1 and 2. FIG.

[3.3. Observation with a high-resolution transmission electron microscope]
5 shows an HRTEM image of the electrode catalyst obtained in Example 1. FIG. The thickness of the nitrogen-containing carbon film covering the platinum surface was about 20 nm at the thickest point. Although not shown, it was observed that the surface of the catalyst was covered with a carbon film even when modified only with PDA (see Reference 1). The reason why the electrode catalyst of Example 1 is more durable than the electrode catalyst of Comparative Example 2 is that the chelate effect of nitrogen contained in co-adsorbed PME suppresses the movement of dissolved platinum ions to the electrolyte. , it is thought that the deterioration of the catalyst was suppressed.
[Reference 1] H. Yamada et al., J. Electrochem. Soc. 167 (2020) 084508

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 electrode catalyst according to the present invention can be used as a catalyst contained in a catalyst layer of a polymer electrolyte fuel cell.

Claims (14)

catalyst particles;
and a nitrogen-containing carbon film covering the surface of the catalyst particles,
The nitrogen-containing carbon film is an electrode catalyst containing micropores. 2. The electrode catalyst according to claim 1, wherein the nitrogen-containing carbon film has a capacity of the micropores of 0.001 cc/g or more and 0.3 cc/g or less. 3. The electrode catalyst according to claim 1, wherein the nitrogen-containing carbon film has a thickness of 0.2 nm or more and 20 nm or less. 4. The electrode catalyst according to any one of claims 1 to 3, further comprising a carrier that supports said catalyst particles. 5. Electrocatalyst according to any one of claims 1 to 4, wherein the catalyst particles are Pt or a Pt alloy. The nitrogen-containing carbon film is obtained by coating the surface of the catalyst particles with polymelamine, further coating the surfaces of the catalyst particles with polydopamine, and then thermally decomposing the laminated film of the polymelamine and the polydopamine. 6. The electrocatalyst according to any one of claims 1 to 5, comprising: 7. The electrocatalyst according to any one of claims 1 to 6, having an initial activity of 0.02 A/cm< ² > @ 0.86 V or more. 8. The electrode catalyst according to any one of claims 1 to 7, which has an activity of 0.02 A/ ^cm2 @ 0.86 V or more after an endurance test. 9. The electrode catalyst according to any one of claims 1 to 8, which has an ECSA retention rate of 80% or more after an endurance test. A fuel cell electrode comprising a catalyst layer comprising the electrode catalyst according to any one of claims 1 to 9 and a catalyst layer ionomer. an electrolyte membrane made of a solid polymer electrolyte;
and electrodes bonded to both sides of the electrolyte membrane,
11. A fuel cell, wherein at least one of the electrodes is the fuel cell electrode according to claim 10. a coating step of coating the surface of the catalyst particles with a coating containing polymelamine and polydopamine to obtain an electrode catalyst precursor;
and a thermal decomposition step of thermally decomposing the film by heat-treating the electrode catalyst precursor to obtain an electrode catalyst in which the surfaces of the catalyst particles are coated with a nitrogen-containing carbon film. The coating step includes
a first coating step of coating the surface of the catalyst particles with polymelamine;
13. The method for producing an electrode catalyst according to claim 12, further comprising a second coating step of further coating the surfaces of the catalyst particles with polydopamine. The method for producing an electrode catalyst according to claim 12 or 13, wherein the thermal decomposition step thermally decomposes the coating at a temperature of 600°C or higher and 1000°C or lower.

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