JP2020202056A - Electrode catalyst - Google Patents
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/30—Hydrogen technology
- Y02E60/50—Fuel cells
Abstract
Description
本発明は、電極触媒に関し、さらに詳しくは、多孔質担体の細孔内に大粒径の第１触媒粒子が選択的に担持され、かつ、多孔質担体の外表面に小粒径の第２触媒粒子が選択的に担持された電極触媒に関する。 The present invention relates to an electrode catalyst, and more specifically, a first catalyst particle having a large particle size is selectively supported in the pores of the porous carrier, and a second catalyst particle having a small particle size is formed on the outer surface of the porous carrier. The present invention relates to an electrode catalyst in which catalyst particles are selectively supported.
固体高分子形燃料電池は、電解質膜の両面に触媒を含む電極（触媒層及びガス拡散層）が接合された膜電極接合体（Membrane Electrode Assembly，ＭＥＡ）を備えている。ＭＥＡの両面には、さらに、ガス流路を備えた集電体（セパレータ）が配置される。固体高分子形燃料電池は、通常、このようなＭＥＡと集電体からなる単セルが複数個積層された構造（燃料電池スタック）を備えている。 The polymer electrolyte fuel cell includes a membrane electrode assembly (MEA) in which electrodes (catalyst layer and gas diffusion layer) containing a catalyst are bonded to both sides of an electrolyte membrane. Further, current collectors (separators) provided with gas flow paths are arranged on both sides of the MEA. The polymer electrolyte fuel cell usually has a structure (fuel cell stack) in which a plurality of single cells composed of such an MEA and a current collector are stacked.
固体高分子形燃料電池の電極触媒には、Ｐｔ触媒、Ｐｔ合金触媒、カーボンアロイ触媒、酸化物触媒などが用いられている。これらの内、Ｐｔ合金触媒は、純Ｐｔ触媒よりも高い効率点性能（低電流密度・高電圧作動条件）が得られることが広く知られている。 As the electrode catalyst of the polymer electrolyte fuel cell, a Pt catalyst, a Pt alloy catalyst, a carbon alloy catalyst, an oxide catalyst and the like are used. Of these, it is widely known that the Pt alloy catalyst can obtain higher efficiency point performance (low current density and high voltage operating conditions) than the pure Pt catalyst.
しかしながら、燃料電池自動車のような電位変動回数の多い環境では、Ｐｔですら溶解する。このような環境下でＰｔ合金触媒を使用すると、比較的小さなＰｔ合金粒子からのＰｔの溶解と比較的大きなＰｔ合金粒子へのＰｔの再析出によって反応面積が減少（平均粒径が増大）すると共に、合金表面に存在する卑金属は、Ｐｔよりも容易に溶出する。さらに、合金組成が純Ｐｔに近づいていくために、触媒の面積当たりの活性（面積活性）も低下する。その結果、その表面積と面積活性との積である効率点性能が低下する。 However, even Pt dissolves in an environment with a large number of potential fluctuations such as a fuel cell vehicle. When a Pt alloy catalyst is used in such an environment, the reaction area decreases (the average particle size increases) due to the dissolution of Pt from relatively small Pt alloy particles and the reprecipitation of Pt into relatively large Pt alloy particles. At the same time, the base metal present on the surface of the alloy elutes more easily than Pt. Further, as the alloy composition approaches pure Pt, the activity per area (area activity) of the catalyst also decreases. As a result, the efficiency point performance, which is the product of the surface area and the area activity, decreases.
他方、反応面積が低下すると、発電集中が生じて酸素輸送抵抗が増加するため、出力点性能（高電流密度・低電圧作動条件）も低下する。さらに、溶出した卑金属は、カチオンコンタミとして電解質のプロトン移動抵抗をも増加させるため、さらに出力性能が低下する。このカチオンコンタミの影響は、特に自動車用途に求められる高温・低加湿作動時に顕著である。 On the other hand, when the reaction area decreases, power generation concentration occurs and oxygen transport resistance increases, so that the output point performance (high current density and low voltage operating conditions) also decreases. Further, the eluted base metal also increases the proton transfer resistance of the electrolyte as cation contamination, so that the output performance is further lowered. The effect of this cation contamination is particularly remarkable during high temperature and low humidification operation required for automobile applications.
そこでこの問題を解決するために、従来から種々の提案がなされている。
例えば、特許文献１には、
ＰｔxＭ（１≦ｘ≦４、Ｍは卑金属元素）で表される原子組成比を持つＰｔ合金からなる第１触媒粒子と、
前記第１触媒粒子よりも平均粒径が小さい純Ｐｔからなる第２触媒粒子と
を備えた空気極用電極触媒が開示されている。
Therefore, in order to solve this problem, various proposals have been made conventionally.
For example, in
First catalyst particles made of Pt alloy having an atomic composition ratio represented by Pt x M (1 ≦ x ≦ 4, M is a base metal element), and
An electrode catalyst for an air electrode including a second catalyst particle made of pure Pt having an average particle size smaller than that of the first catalyst particle is disclosed.
同文献には、
（ａ）粒径の大きなＰｔ合金粒子と粒径の小さな純Ｐｔ粒子とを共存させると、電極触媒全体の比表面積が増大し、初期の効率点性能及び出力点性能が向上する点、及び、
（ｂ）電位変動を伴う環境下においてＰｔ合金粒子に近接して純Ｐｔ粒子を配置すると、純Ｐｔ粒子が優先的に溶出し、溶出したＰｔがＰｔ合金粒子の表面に再析出するために、Ｐｔ合金粒子のみからなる電極触媒に比べて、Ｐｔ合金粒子表面からの合金元素の溶出が抑制される点、
が記載されている。
In the same document,
(A) When Pt alloy particles having a large particle size and pure Pt particles having a small particle size coexist, the specific surface area of the entire electrode catalyst is increased, and the initial efficiency point performance and output point performance are improved.
(B) When pure Pt particles are placed close to the Pt alloy particles in an environment accompanied by potential fluctuations, the pure Pt particles are preferentially eluted, and the eluted Pt is reprecipitated on the surface of the Pt alloy particles. Compared to an electrode catalyst consisting of only Pt alloy particles, elution of alloy elements from the surface of Pt alloy particles is suppressed.
Is described.
特許文献１においては、Ｐｔ合金触媒のＰｔ比を下げることで高い面積活性を得る一方で、Ｐｔ合金触媒の平均粒径を大きくすることで反応面積を減らしている。その結果、面積活性と反応面積の積である初期の効率点性能を担保することができる。また、共存させている小粒径の純Ｐｔ触媒は、反応面積を増加させ、初期のＰｔ合金触媒の効率点性能及び出力点性能を補完する役割を果たす。
In
さらに、電位変動を伴う作動環境下においては、小粒径の純Ｐｔ触媒が優先的に溶出し、これが大粒径のＰｔ合金触媒上に再析出することで、Ｐｔ合金触媒からの卑金属元素の溶出が抑制される。その結果、効率点と出力点の性能低下は、純Ｐｔ触媒の溶出に起因する程度は生じるものの、全体としては抑制される。また、Ｐｔ合金触媒の特徴である高い面積活性に起因して効率点性能が高いレベルで維持されると共に、Ｐｔ合金触媒の反応面積の減少及び卑金属の溶出が抑制されるため、出力点性能も高いレベルで維持される。
しかしながら、燃料電池性能をさらに向上させるためには、触媒成分の溶解・再析出、並びに、これに起因する性能低下をさらに抑制することが望まれる。
Further, in an operating environment accompanied by potential fluctuation, the pure Pt catalyst having a small particle size is preferentially eluted, and this is reprecipitated on the Pt alloy catalyst having a large particle size, so that the base metal element from the Pt alloy catalyst is separated. Elution is suppressed. As a result, the performance deterioration of the efficiency point and the output point is suppressed as a whole, although the degree of deterioration due to the elution of the pure Pt catalyst occurs. In addition, the efficiency point performance is maintained at a high level due to the high area activity characteristic of the Pt alloy catalyst, and the reduction of the reaction area of the Pt alloy catalyst and the elution of base metals are suppressed, so that the output point performance is also improved. Maintained at a high level.
However, in order to further improve the fuel cell performance, it is desired to further suppress the dissolution / reprecipitation of the catalyst component and the performance deterioration caused by this.
本発明が解決しようとする課題は、電位変動を伴う作動環境下において使用した場合であっても、触媒成分の溶解・再析出、並びに、これに起因する性能低下を抑制することが可能な電極触媒を提供することにある。 The problem to be solved by the present invention is an electrode capable of suppressing dissolution / reprecipitation of catalyst components and performance deterioration due to this even when used in an operating environment accompanied by potential fluctuation. To provide a catalyst.
上記課題を解決するために本発明に係る電極触媒は、以下の構成を備えている。
（１）前記電極触媒は、
多孔質の担体と、
前記担体の細孔内に担持された第１触媒粒子と、
前記担体の外表面に担持された第２触媒粒子と
を備えている。
（２）前記第１触媒粒子の平均粒径（粒度分布の中央値）Ｄ1は、前記第２触媒粒子の平均粒径Ｄ2より大きい。
The electrode catalyst according to the present invention in order to solve the above problems has the following configurations.
(1) The electrode catalyst is
Porous carrier and
The first catalyst particles supported in the pores of the carrier and
It includes a second catalyst particle supported on the outer surface of the carrier.
(2) The average particle size (median value of particle size distribution) D 1 of the first catalyst particles is larger than the average particle size D 2 of the second catalyst particles.
電極触媒として、多孔質担体の細孔内に平均粒径の大きい第１触媒粒子を優先的に担持させ、かつ、多孔質担体の外表面に平均粒径の小さい第２触媒粒子を優先的に担持させたものを用いると、電位変動を伴う作動環境下において使用した場合であっても、触媒成分の溶解・再析出、並びに、これに起因する性能低下が抑制される。 As the electrode catalyst, the first catalyst particles having a large average particle size are preferentially supported in the pores of the porous carrier, and the second catalyst particles having a small average particle size are preferentially supported on the outer surface of the porous carrier. When a supported material is used, dissolution / reprecipitation of the catalyst component and deterioration of performance due to the dissolution and reprecipitation of the catalyst component are suppressed even when the catalyst component is used in an operating environment accompanied by potential fluctuation.
これは、
（ａ）第１触媒粒子が担体の細孔内に担持されているために、プロトン伝導体（触媒層アイオノマ）による第１触媒粒子の被毒が抑制されるため、及び、
（ｂ）電位変動を伴う作動環境下において、担体の外表面に存在する小粒径の第２触媒粒子が優先的に溶出し、溶出した触媒成分が細孔内に拡散するために、第１触媒粒子の溶出が抑制されるため、
と考えられる。
this is,
(A) Since the first catalyst particles are supported in the pores of the carrier, poisoning of the first catalyst particles by the proton conductor (catalyst layer ionoma) is suppressed, and
(B) In an operating environment accompanied by potential fluctuations, the second catalyst particles having a small particle size existing on the outer surface of the carrier are preferentially eluted, and the eluted catalyst components are diffused into the pores. Since the elution of catalyst particles is suppressed,
it is conceivable that.
以下、本発明の一実施の形態について詳細に説明する。
［１． 電極触媒］
本発明に係る電極触媒は、以下の構成を備えている。
（１）前記電極触媒は、
多孔質の担体と、
前記担体の細孔内に担持された第１触媒粒子と、
前記担体の外表面に担持された第２触媒粒子と
を備えている。
（２）前記第１触媒粒子の平均粒径（粒度分布の中央値）Ｄ1は、前記第２触媒粒子の平均粒径Ｄ2より小さい。
Hereinafter, an embodiment of the present invention will be described in detail.
[1. Electrode catalyst]
The electrode catalyst according to the present invention has the following configurations.
(1) The electrode catalyst is
Porous carrier and
The first catalyst particles supported in the pores of the carrier and
It includes a second catalyst particle supported on the outer surface of the carrier.
(2) The average particle size (median value of particle size distribution) D 1 of the first catalyst particles is smaller than the average particle size D 2 of the second catalyst particles.
［１．１． 第１触媒粒子］
［１．１．１． 組成］
本発明において、第１触媒粒子の組成は、特に限定されるものではなく、目的に応じて最適な組成を選択することができる。
第１触媒粒子の材料としては、例えば、
（ａ）貴金属（Ａｕ、Ａｇ、Ｐｔ、Ｐｄ、Ｒｈ、Ｉｒ、Ｒｕ、Ｏｓ）、
（ｂ）２種以上の貴金属元素を含む合金、
（ｃ）１種又は２種以上の貴金属元素と、１種又は２種以上の卑金属元素との合金、
などがある。
[1.1. 1st catalyst particle]
[1.1.1. composition]
In the present invention, the composition of the first catalyst particles is not particularly limited, and the optimum composition can be selected according to the intended purpose.
As a material for the first catalyst particles, for example,
(A) Precious metals (Au, Ag, Pt, Pd, Rh, Ir, Ru, Os),
(B) Alloys containing two or more precious metal elements,
(C) An alloy of one or more noble metal elements and one or more base metal elements,
and so on.
第１触媒粒子は、特に、Ｐｔ、又は、Ｐｔと１種若しくは２種以上の卑金属元素Ｍとの合金（以下、これを単に「Ｐｔ−Ｍ合金」ともいう）が好ましい。これは、Ｐｔ及びＰｔ−Ｍ合金のいずれも燃料電池の電極反応に対して高い活性を示すためである。 The first catalyst particles are particularly preferably Pt or an alloy of Pt and one or more base metal elements M (hereinafter, this is also simply referred to as “Pt—M alloy”). This is because both Pt and Pt—M alloys show high activity for the electrode reaction of the fuel cell.
Ｐｔ−Ｍ合金は、特に、次の式（１）で表される原子組成比を持つものが好ましい。
ＰｔxＭ ・・・（１）
但し、Ｍは卑金属元素、１≦ｘ≦４。
The Pt—M alloy is particularly preferably one having an atomic composition ratio represented by the following formula (1).
Pt x M ・ ・ ・ (1)
However, M is a base metal element, 1 ≦ x ≦ 4.
式（１）中、Ｍは、卑金属元素（貴金属元素以外の金属元素）を表す。
元素Ｍとしては、例えば、
（ａ）Ａｌ、Ｇａ、Ｐｂ、Ｓｎ、Ｓｂ、Ｉｎなどの典型金属元素、
（ｂ）Ｔｉ、Ｖ、Ｃｒ、Ｍｎ、Ｆｅ、Ｃｏ、Ｎｉ、Ｃｕ、Ｚｎなどの３ｄ遷移金属元素、
（ｃ）Ｙ、Ｚｒ、Ｎｂ、Ｍｏなどの４ｄ遷移金属元素、
（ｄ）Ｗ、Ｔａなどの４ｆ遷移金属元素、
などがある。
In the formula (1), M represents a base metal element (a metal element other than a noble metal element).
The element M is, for example,
(A) Main group elements such as Al, Ga, Pb, Sn, Sb, and In,
(B) 3d transition metal elements such as Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, etc.
(C) 4d transition metal elements such as Y, Zr, Nb, Mo, etc.
(D) 4f transition metal elements such as W and Ta,
and so on.
これらの中でも、元素Ｍは、Ｃｏ、Ｎｉ、Ｆｅ、Ｗ、Ｐｂ、Ｃｒ、Ｍｎ、Ｖ、Ｍｏ、Ｇａ、Ｙ、及びＡｌからなる群から選ばれるいずれか１種以上の元素が好ましい。また、元素Ｍは、特に、Ｃｏ、Ｎｉ、及び／又は、Ｆｅが好ましい。
これらは、Ｐｔの電子状態をわずかに貴にする効果があると考えられている群であり、これらを含むことによって酸素還元反応の中間体の脱離がしやすくなる。
Among these, the element M is preferably any one or more elements selected from the group consisting of Co, Ni, Fe, W, Pb, Cr, Mn, V, Mo, Ga, Y, and Al. The element M is particularly preferably Co, Ni, and / or Fe.
These are groups that are thought to have the effect of slightly noble the electronic state of Pt, and the inclusion of these facilitates the elimination of intermediates in the oxygen reduction reaction.
式（１）中、ｘは、元素Ｍに対するＰｔの比率を表す。元素Ｍは、通常、単独では酸素還元反応（ＯＲＲ）活性を示さない。一方、元素Ｍを含むＰｔ−Ｍ合金は、ＯＲＲ活性を示す。一般に、ＯＲＲは、触媒粒子表面において起こるので、表面に露出しているＰｔ原子の量が少なくなるほど、ＯＲＲ活性が低下する。また、ｘが小さ過ぎると、元素Ｍの溶出を抑制することができない。さらに、ｘが過度に小さくなると、ｐＨ〜１程度の燃料電池環境下で数日間、安定に存在できる微粒子合金の作製が困難となる場合がある。従って、ｘは、１以上（すなわち、５０ａｔ％Ｐｔ以上）が好ましい。
In the formula (1), x represents the ratio of Pt to the element M. Element M usually does not exhibit oxygen reduction reaction (ORR) activity by itself. On the other hand, the Pt-M alloy containing the element M exhibits ORR activity. In general, ORR occurs on the surface of catalyst particles, so that the smaller the amount of Pt atoms exposed on the surface, the lower the ORR activity. Further, if x is too small, the elution of the element M cannot be suppressed. Further, if x becomes excessively small, it may be difficult to produce a fine particle alloy that can stably exist for several days in a fuel cell environment of about
Ｐｔ−Ｍ合金のＯＲＲ活性は、あるｘの値で極大となり、それ以降は減少に転じる。ＯＲＲ活性が極大値となるときのｘは、元素Ｍの種類により異なるが、通常、１．０±α（５０ａｔ％Ｐｔ±１５ａｔ％）の範囲内にある。そのため、ｘが大きくなるに従い、Ｐｔ−Ｍ合金のＯＲＲ活性は、やがて純Ｐｔのそれに近づく。相対的に大きなＯＲＲ活性を得るためには、ｘは、４以下（すなわち、８０ａｔ％Ｐｔ以下）が好ましい。 The ORR activity of the Pt-M alloy reaches a maximum at a certain value of x and then begins to decrease. The x at the maximum value of the ORR activity varies depending on the type of the element M, but is usually within the range of 1.0 ± α (50 at% Pt ± 15 at%). Therefore, as x increases, the ORR activity of the Pt—M alloy eventually approaches that of pure Pt. In order to obtain a relatively large ORR activity, x is preferably 4 or less (that is, 80 at% Pt or less).
［１．１．２． 平均粒径Ｄ1］
本発明において、第１触媒粒子は主として担体の細孔内に担持され、第２触媒粒子は主として担体の外表面に担持される。この場合、第１触媒粒子の平均粒径Ｄ1は、電極触媒のＯＲＲ活性及び耐久性に影響を与える。
ここで、「粒径」とは、電子顕微鏡観察下で測定される触媒粒子の最大寸法をいう。
また、「平均粒径」とは、粒度分布の中央値（メディアン径Ｄ50）をいう。
[1.1.2. Average particle size D 1 ]
In the present invention, the first catalyst particles are mainly supported in the pores of the carrier, and the second catalyst particles are mainly supported on the outer surface of the carrier. In this case, the average particle size D 1 of the first catalyst particles affects the ORR activity and durability of the electrode catalyst.
Here, the "particle size" refers to the maximum size of the catalyst particles measured under electron microscope observation.
The "average particle size" means the median value of the particle size distribution (median diameter D 50 ).
担体の細孔内に第１触媒粒子が担持され、担体の外表面に第２触媒粒子が担持されている場合において、第１触媒粒子の平均粒径Ｄ1が第２触媒粒子の平均粒径Ｄ2より小さい時には、触媒成分が細孔内から細孔外に向かって溶出する。細孔内から細孔外への触媒成分の溶出は、触媒性能を劣化させる原因となる。
一方、Ｄ1がＤ2より大きい時には、細孔内から細孔外への触媒成分の溶出を抑制することができる。従って、Ｄ1は、少なくとも、Ｄ2より大きいことが必要である。
When the first catalyst particles are supported in the pores of the carrier and the second catalyst particles are supported on the outer surface of the carrier, the average particle size D 1 of the first catalyst particles is the average particle size of the second catalyst particles. When it is smaller than D 2 , the catalyst component elutes from the inside of the pore to the outside of the pore. Elution of the catalyst component from the inside of the pore to the outside of the pore causes deterioration of the catalyst performance.
On the other hand, when D 1 is larger than D 2 , the elution of the catalyst component from the inside of the pore to the outside of the pore can be suppressed. Therefore, D 1 needs to be at least greater than D 2 .
Ｄ1＞Ｄ2の条件を満たす限りにおいて、Ｄ1の大きさは、特に限定されない。一般に、Ｄ1が小さくなりすぎると、第１触媒粒子が溶解しやすくなる。従って、Ｄ1は、５ｎｍ以上が好ましい。Ｄ1は、好ましくは、５．５ｎｍ以上、さらに好ましくは、６．０ｎｍ以上である。
一方、Ｄ1が大きくなりすぎると、第１触媒粒子の比表面積が減少し、効率点性能が低下する。従って、Ｄ1は、７ｎｍ以下が好ましい。Ｄ1は、好ましくは、６．５ｎｍ以下である。
The size of D 1 is not particularly limited as long as the condition of D 1 > D 2 is satisfied. In general, if D 1 becomes too small, the first catalyst particles tend to dissolve. Therefore, D 1 is preferably 5 nm or more. D 1 is preferably 5.5 nm or more, more preferably 6.0 nm or more.
On the other hand, if D 1 becomes too large, the specific surface area of the first catalyst particles decreases, and the efficiency point performance deteriorates. Therefore, D 1 is preferably 7 nm or less. D 1 is preferably 6.5 nm or less.
［１．１．３． 平均粒径の標準偏差σ1］
第１触媒粒子の平均粒径の標準偏差σ1は、電極触媒のＯＲＲ活性及び耐久性に影響を与える。第１触媒粒子の粒度分布が正規分布であると仮定し、第１触媒粒子の平均粒径の標準偏差をσ1とすると、ある粒子の粒径がＤ1±σ1となる確率は、６８．２７％となる。
（Ｄ１−σ1）未満の粒径を持つ微細な第１触媒粒子は、触媒成分が溶出しやすいため、電極触媒全体の出力点性能を低下させる原因となる。一方、（Ｄ1＋σ1）を超える粒径を持つ粗大な第１触媒粒子は、比表面積が小さいため、電極触媒全体の効率点性能を低下させる原因となる。そのため、σ1は、小さいほど良い。出力点性能と効率点性能を高い次元で両立させるためには、σ1は、２ｎｍ以下が好ましい。σ1は、好ましくは、１．５ｎｍ以下、さらに好ましくは、１．０ｎｍ以下である。
[1.1.3. Standard deviation of average particle size σ 1 ]
The standard deviation σ 1 of the average particle size of the first catalyst particles affects the ORR activity and durability of the electrode catalyst. The particle size distribution of the first catalyst particles are assumed to be normally distributed, when the standard deviation of the average particle diameter of the first catalyst particles and sigma 1, the probability that the particle diameter of a particle is D 1 ± sigma 1 is 68 It will be .27%.
Fine first catalyst particles having a particle size of less than (D 1 − σ 1 ) tend to elute the catalyst component, which causes a decrease in the output point performance of the entire electrode catalyst. On the other hand, the coarse first catalyst particles having a particle size exceeding (D 1 + σ 1 ) have a small specific surface area, which causes a decrease in the efficiency point performance of the entire electrode catalyst. Therefore, the smaller σ 1 is, the better. In order to achieve both output point performance and efficiency point performance at a high level, σ 1 is preferably 2 nm or less. σ 1 is preferably 1.5 nm or less, more preferably 1.0 nm or less.
［１．２． 第２触媒粒子］
［１．２．１． 組成］
本発明において、第２触媒粒子の組成は、特に限定されるものではなく、目的に応じて最適な組成を選択することができる。
第２触媒粒子の材料としては、例えば、
（ａ）貴金属（Ａｕ、Ａｇ、Ｐｔ、Ｐｄ、Ｒｈ、Ｉｒ、Ｒｕ、Ｏｓ）、
（ｂ）２種以上の貴金属元素を含む合金、
（ｃ）１種又は２種以上の貴金属元素と、１種又は２種以上の卑金属元素との合金、
などがある。
[1.2. Second catalyst particle]
[1.2.1. composition]
In the present invention, the composition of the second catalyst particles is not particularly limited, and the optimum composition can be selected according to the intended purpose.
As a material for the second catalyst particles, for example,
(A) Precious metals (Au, Ag, Pt, Pd, Rh, Ir, Ru, Os),
(B) Alloys containing two or more precious metal elements,
(C) An alloy of one or more noble metal elements and one or more base metal elements,
and so on.
第２触媒粒子は、特に、Ｐｔ、又は、Ｐｔと１種若しくは２種以上の卑金属元素Ｍ'との合金（以下、これを単に「Ｐｔ−Ｍ'合金」ともいう）が好ましい。また、Ｐｔ−Ｍ'合金は、特に、元素Ｍ'がＣｏ、Ｎｉ、及び／又は、Ｆｅであるものが好ましい。
さらに、カチオンコンタミを防ぐためには、第２触媒粒子は、純Ｐｔなどの貴金属又は貴金属元素のみを含む合金が好ましい。
「純Ｐｔ」とは、９９．９ａｔ％以上のＰｔを含み、残部が不可避的不純物からなるものをいう。不可避的不純物は、燃料電池の作動環境下で溶出し、出力点性能を低下させる原因となるので、少ないほど良い。
The second catalyst particles are particularly preferably Pt or an alloy of Pt and one or more base metal elements M'(hereinafter, this is also simply referred to as "Pt-M'alloy"). Further, the Pt—M'alloy is particularly preferably one in which the element M'is Co, Ni, and / or Fe.
Further, in order to prevent cation contamination, the second catalyst particles are preferably a noble metal such as pure Pt or an alloy containing only a noble metal element.
“Pure Pt” refers to one containing 99.9 at% or more of Pt and the balance of which is unavoidable impurities. Inevitable impurities elute in the operating environment of the fuel cell and cause deterioration of output point performance, so the smaller the amount, the better.
また、第２触媒粒子の組成は、第１触媒粒子の組成と同一であっても良く、あるいは、異なっていても良い。
第２触媒粒子の組成に関するその他の点については、第１触媒粒子と同様であるので、説明を省略する。
Further, the composition of the second catalyst particles may be the same as or different from the composition of the first catalyst particles.
Other points regarding the composition of the second catalyst particles are the same as those of the first catalyst particles, and thus the description thereof will be omitted.
［１．２．２． 平均粒径Ｄ2］
第２触媒粒子の平均粒径Ｄ2は、電極触媒のＯＲＲ活性及び耐久性に影響を与える。第２触媒粒子は、電極触媒全体の反応面積を増加させて、初期の第１触媒粒子の効率点性能及び出力点性能を補完する役割を果たす。また、電位変動を伴う環境下では、粒径の小さな触媒粒子が優先的に溶解し、粒径の大きな触媒粒子の表面に触媒成分が再析出する。
この時、上述したように、細孔内から細孔外への触媒成分の溶出を抑制するためには、少なくともＤ1＞Ｄ2の条件を満たしている必要がある。
[1.2.2. Average particle size D 2 ]
The average particle size D 2 of the second catalyst particles affects the ORR activity and durability of the electrode catalyst. The second catalyst particles play a role of increasing the reaction area of the entire electrode catalyst and complementing the efficiency point performance and the output point performance of the initial first catalyst particles. Further, in an environment accompanied by potential fluctuation, the catalyst particles having a small particle size are preferentially dissolved, and the catalyst component is reprecipitated on the surface of the catalyst particles having a large particle size.
At this time, as described above, in order to suppress the elution of the catalyst component from the inside of the pore to the outside of the pore, it is necessary to satisfy at least the condition of D 1 > D 2 .
Ｄ1＞Ｄ2の条件を満たす限りにおいて、Ｄ2の大きさは、特に限定されない。一般に、Ｄ2が小さくなるほど、触媒成分の溶解・再析出が起きやすくなる。しかし、Ｄ2が小さくなりすぎると、触媒粒子の面積活性が顕著に低下することが知られている。その結果、効率点性能を向上させる効果に欠ける。従って、Ｄ2は、１ｎｍ以上が好ましい。Ｄ2は、好ましくは、１．５ｎｍ以上、さらに好ましくは、２．０ｎｍ以上である。
一方、Ｄ2が大きくなりすぎると、電極触媒全体の反応面積が低下し、かつ、触媒成分の溶解・再析出速度も低下する。その結果、初期性能を向上させ、あるいは、耐久性を向上させる効果が不十分となる。従って、Ｄ2は、３ｎｍ以下が好ましい。Ｄ2は、好ましくは、２．５ｎｍ以下である。
The size of D 2 is not particularly limited as long as the condition of D 1 > D 2 is satisfied. In general, the smaller D 2 is, the easier it is for the catalyst component to dissolve and reprecipitate. However, it is known that when D 2 becomes too small, the area activity of the catalyst particles is significantly reduced. As a result, it lacks the effect of improving efficiency point performance. Therefore, D 2 is preferably 1 nm or more. D 2 is preferably 1.5 nm or more, more preferably 2.0 nm or more.
On the other hand, if D 2 becomes too large, the reaction area of the entire electrode catalyst decreases, and the dissolution / reprecipitation rate of the catalyst component also decreases. As a result, the effect of improving the initial performance or the durability becomes insufficient. Therefore, D 2 is preferably 3 nm or less. D 2 is preferably 2.5 nm or less.
［１．２．３． 平均粒径の標準偏差σ2］
第２触媒粒子の平均粒径の標準偏差σ2は、電極触媒のＯＲＲ活性及び耐久性に影響を与える。第１触媒粒子と同様に、第２触媒粒子の粒度分布が正規分布であると仮定し、第２触媒粒子の平均粒径の標準偏差をσ2とすると、ある粒子の粒径がＤ2±σ2となる確率は、６８．２７％となる。
（Ｄ2−σ2）未満の粒径を持つ微細な第２触媒粒子は、面積活性が低く、効率点性能を向上させる効果に欠ける。一方、（Ｄ2＋σ2）を超える粒径を持つ粗大な第２触媒粒子は、比表面積が小さく、かつ、触媒成分の溶解・再析出速度も過度に小さくなる。そのため、σ2は、小さいほど良い。出力点性能と効率点性能を高い次元で両立させるためには、σ2は、２ｎｍ以下が好ましい。σ1は、好ましくは、１．５ｎｍ以下、さらに好ましくは、１．０ｎｍ以下である。
[12.3. Standard deviation of average particle size σ 2 ]
The standard deviation σ 2 of the average particle size of the second catalyst particles affects the ORR activity and durability of the electrode catalyst. As with the first catalyst particles, assuming that the particle size distribution of the second catalyst particles is normal and the standard deviation of the average particle size of the second catalyst particles is σ 2 , the particle size of a certain particle is D 2 ±. The probability of becoming σ 2 is 68.27%.
Fine second catalyst particles having a particle size of less than (D 2- σ 2 ) have low area activity and lack the effect of improving efficiency point performance. On the other hand, the coarse second catalyst particles having a particle size exceeding (D 2 + σ 2 ) have a small specific surface area and an excessively low dissolution / reprecipitation rate of the catalyst component. Therefore, the smaller σ 2 is, the better. In order to achieve both output point performance and efficiency point performance at a high level, σ 2 is preferably 2 nm or less. σ 1 is preferably 1.5 nm or less, more preferably 1.0 nm or less.
［１．３． 第１触媒粒子の重量比率］
「第１触媒粒子の重量比率」とは、次の式（１）で表される値をいう。
第１触媒粒子の重量比率（％）＝Ｗ1×１００／(Ｗ1＋Ｗ2) …（１）
但し、
Ｗ1は、前記電極触媒に含まれる前記第１触媒粒子の重量、
Ｗ2は、前記電極触媒に含まれる前記第２触媒粒子の重量。
[1.3. Weight ratio of first catalyst particles]
The "weight ratio of the first catalyst particles" means a value represented by the following formula (1).
Weight ratio (%) of the first catalyst particles = W 1 × 100 / (W 1 + W 2 )… (1)
However,
W 1 is the weight of the first catalyst particles contained in the electrode catalyst.
W 2 is the weight of the second catalyst particles contained in the electrode catalyst.
第１触媒粒子の重量比率は、電極触媒のＯＲＲ活性及び耐久性、並びに、湿度による性能変化に影響を与える。第１触媒粒子の重量比率が小さくなりすぎると、ＯＲＲ活性が高い第１触媒粒子の反応面積が低下するため、初期及び耐久後の出力点・効率点のいずれの性能も低下する。従って、第１触媒粒子の重量比率は、５０％以上が好ましい。重量比率は、好ましくは、６０％以上、さらに好ましくは、７０％以上である。
一方、第１触媒粒子の重量比率が大きくなりすぎると、低湿での初期及び耐久後の性能が著しく低下し、広い湿度範囲でロバストな性能が求められる車両用途には使用できなくなる。従って、第１触媒粒子の重量比率は、９０％以下が好ましい。重量比率は、好ましくは、８７．５％以下、さらに好ましくは、８５％以下である。
The weight ratio of the first catalyst particles affects the ORR activity and durability of the electrode catalyst and the performance change due to humidity. If the weight ratio of the first catalyst particles becomes too small, the reaction area of the first catalyst particles having high ORR activity decreases, so that the performances of both the initial and post-durability output points and efficiency points decrease. Therefore, the weight ratio of the first catalyst particles is preferably 50% or more. The weight ratio is preferably 60% or more, more preferably 70% or more.
On the other hand, if the weight ratio of the first catalyst particles becomes too large, the performance at the initial stage and after durability at low humidity is remarkably deteriorated, and it cannot be used for vehicle applications where robust performance is required in a wide humidity range. Therefore, the weight ratio of the first catalyst particles is preferably 90% or less. The weight ratio is preferably 87.5% or less, more preferably 85% or less.
［１．４． 担体］
［１．４．１． 担体の材料］
本発明において、担体は、多孔質体からなる。また、第１触媒粒子は、担体の細孔内に担持され、第２触媒粒子は担体の外表面に担持される。この点が、従来とは異なる。
本発明において、担体の材料は、所定の細孔径を持つ多孔質体である限りにおいて、特に限定されない。担体の材料としては、例えば、カーボンブラック、ファーネスブラック、カーボンナノチューブ、メソポーラスカーボン、電子伝導性セラミックス（ＴｉＯx、Ｓｂ−ＳｎＯ2）などがある。
[1.4. Carrier]
[1.4.1. Carrier material]
In the present invention, the carrier is made of a porous body. Further, the first catalyst particles are supported in the pores of the carrier, and the second catalyst particles are supported on the outer surface of the carrier. This point is different from the conventional one.
In the present invention, the material of the carrier is not particularly limited as long as it is a porous body having a predetermined pore diameter. Examples of the carrier material include carbon black, furnace black, carbon nanotubes, mesoporous carbon, and electron conductive ceramics (TiO x , Sb-SnO 2 ).
［１．４．２． 細孔径］
担体の細孔径は、所定の大きさの第１触媒粒子を細孔内に担持することが可能な大きさであれば良い。担体の細孔径が小さくなりすぎると、細孔内に担持される第１触媒粒子の平均粒径Ｄ1が過度に小さくなる。従って、担体の細孔径は、５ｎｍ以上が好ましい。細孔径は、好ましくは、５．５ｎｍ以上、さらに好ましくは、６．０ｎｍ以上である。
一方、担体の細孔径が大きくなりすぎると、細孔内に担持される第１触媒粒子が細孔内に侵入したアイオノマに被毒され、ＯＲＲ活性が低下する。従って、担体の細孔径は、７ｎｍ以下が好ましい。細孔径は、好ましくは、６．５ｎｍ以下である。
[1.4.2. Pore diameter]
The pore diameter of the carrier may be any size as long as it can support the first catalyst particles of a predetermined size in the pores. If the pore diameter of the carrier becomes too small, the average particle size D 1 of the first catalyst particles supported in the pores becomes excessively small. Therefore, the pore diameter of the carrier is preferably 5 nm or more. The pore diameter is preferably 5.5 nm or more, more preferably 6.0 nm or more.
On the other hand, if the pore size of the carrier becomes too large, the first catalyst particles supported in the pores are poisoned by ionomers that have invaded the pores, and the ORR activity decreases. Therefore, the pore diameter of the carrier is preferably 7 nm or less. The pore diameter is preferably 6.5 nm or less.
［１．４．３． 細孔容量］
担体の細孔容量は、所定量の第１触媒粒子を細孔内に担持することが可能な大きさである限りにおいて、特に限定されない。
[1.4.3. Pore capacity]
The pore volume of the carrier is not particularly limited as long as it is large enough to support a predetermined amount of the first catalyst particles in the pores.
［１．５． 用途］
本発明に係る電極触媒は、特に固体高分子形燃料電池の空気極触媒として好適であるが、固体高分子形燃料電池の燃料極触媒として用いることもできる。
[1.5. Use]
The electrode catalyst according to the present invention is particularly suitable as an air electrode catalyst for a polymer electrolyte fuel cell, but can also be used as a fuel electrode catalyst for a polymer electrolyte fuel cell.
［２． 電極触媒の製造方法］
［２．１． 概要］
一般に、担体表面に触媒粒子を担持させる方法は、
（ａ）触媒金属イオン、あるいは、触媒金属錯体などの触媒金属源を溶解させた水溶液中に担体を分散させ、触媒金属源を還元処理するウェットプロセス、及び、
（ｂ）蒸着法、スパッタリング法、ＣＶＤ法、ＡＬＤ法などを用いて担体表面に触媒金属を付着させるドライプロセス、
に大別される。
[2. Method of manufacturing electrode catalyst]
[2.1. Overview]
Generally, the method of supporting the catalyst particles on the surface of the carrier is
(A) A wet process in which the carrier is dispersed in an aqueous solution in which a catalyst metal source such as a catalyst metal ion or a catalyst metal complex is dissolved to reduce the catalyst metal source, and
(B) A dry process in which a catalyst metal is attached to the surface of a carrier by using a vapor deposition method, a sputtering method, a CVD method, an ALD method, or the like.
It is roughly divided into.
これらの内、ウェットプロセスは、多孔質担体の細孔内及び外表面の双方に触媒粒子が非選択的に担持されやすい。そのため、ウェットプロセスのみを繰り返しても、大粒径の第１触媒粒子が担体の細孔内に優先的に担持され、かつ、小粒径の第２触媒粒子が担体の外表面に優先的に担持されている担持構造を得ることはできない。
むしろ、細孔内の空間は、外部に比べて狭いため、細孔内には外部に比べて小粒径の触媒粒子が析出しやすい。そのため、ギブス−トムソン効果により、内部から外部への触媒成分の流出が促進されやすい。
Of these, in the wet process, the catalyst particles are likely to be non-selectively supported both inside and outside the pores of the porous carrier. Therefore, even if only the wet process is repeated, the first catalyst particles having a large particle size are preferentially supported in the pores of the carrier, and the second catalyst particles having a small particle size are preferentially supported on the outer surface of the carrier. It is not possible to obtain a supported structure that is supported.
Rather, since the space inside the pores is narrower than the outside, catalyst particles having a smaller particle size are more likely to precipitate in the pores than outside. Therefore, the Gibbs-Thomson effect tends to promote the outflow of the catalyst component from the inside to the outside.
また、ドライプロセスの内、蒸着法やスパッタリング法は、多孔質担体の外表面に触媒粒子が選択的に担持されやすい。一方、ＣＶＤ法やＡＬＤ法は、供給ガスの種類、還元速度などを制御することにより、触媒粒子を析出させる位置をある程度制御できるが、プロセス条件の制御による触媒粒子の析出位置の制御には限界がある。そのため、ドライプロセスのみを繰り返しても、上述した本願特有の担持構造を得ることはできない。 Further, among the dry processes, in the vapor deposition method and the sputtering method, the catalyst particles are likely to be selectively supported on the outer surface of the porous carrier. On the other hand, in the CVD method and the ALD method, the position where the catalyst particles are deposited can be controlled to some extent by controlling the type of supply gas, the reduction rate, etc., but there is a limit to the control of the deposition position of the catalyst particles by controlling the process conditions. There is. Therefore, the supported structure peculiar to the present application cannot be obtained by repeating only the dry process.
これに対し、異なる担持プロセスを組み合わせて用いると、本願特有の担持構造を備えた電極触媒を得ることができる。本発明に係る電極触媒は、具体的には、以下のような方法により製造することができる。 On the other hand, when different supporting processes are used in combination, an electrode catalyst having a supporting structure peculiar to the present application can be obtained. Specifically, the electrode catalyst according to the present invention can be produced by the following method.
［２．２． 具体例］
［２．２．１． 細孔内への第１触媒粒子の担持］
まず、担体の細孔内に第１触媒粒子を優先的に担持させる。上述したように、単なるウェットプロセスでは、細孔内だけでなく、担体の外表面にも第１触媒粒子が析出する。そのため、ウェットプロセスを用いて細孔内に大粒径の第１触媒粒子を優先的に担持させるためには、追加の特別な処理（前処理又は後処理）が必要となる。
[2.2. Concrete example]
[2.2.1. Supporting the first catalyst particles in the pores]
First, the first catalyst particles are preferentially supported in the pores of the carrier. As described above, in the mere wet process, the first catalyst particles are precipitated not only in the pores but also on the outer surface of the carrier. Therefore, in order to preferentially support the first catalyst particles having a large particle size in the pores by using the wet process, an additional special treatment (pretreatment or posttreatment) is required.
例えば、通常のウェットプロセスを行った後、担体を乾燥させ、表面張力が高く細孔内に侵入しない液体（例えば、水）中での酸化等の反応により、担体の外表面に付着している余分な触媒粒子を取り除いても良い。 For example, after performing a normal wet process, the carrier is dried and adheres to the outer surface of the carrier by a reaction such as oxidation in a liquid (for example, water) having a high surface tension and not penetrating into the pores. Excess catalyst particles may be removed.
あるいは、ウェットプロセス前に、担体の外表面をプラスに帯電させても良い。担体の外表面をプラスに帯電させると、細孔内において触媒粒子を選択的に成長させることができる。担体の外表面をプラスに帯電させる方法としては、例えば、酸等により担体の外表面を酸化させて官能基を付ける方法がある。担体表面に官能基を付ける際に表面張力の高い溶媒を用いると、担体の外表面にのみ官能基を付けることができる。触媒金属イオンはプラスに帯電しているため、プラスに帯電している担体の外表面ではなく、細孔内部において選択的に還元される。 Alternatively, the outer surface of the carrier may be positively charged prior to the wet process. When the outer surface of the carrier is positively charged, the catalyst particles can be selectively grown in the pores. As a method of positively charging the outer surface of the carrier, for example, there is a method of oxidizing the outer surface of the carrier with an acid or the like to attach a functional group. When a solvent having a high surface tension is used when attaching a functional group to the surface of the carrier, the functional group can be attached only to the outer surface of the carrier. Since the catalytic metal ions are positively charged, they are selectively reduced inside the pores rather than on the outer surface of the positively charged carrier.
あるいは、ウェットプロセス前に、担体の細孔内に還元剤、あるいは、還元の起点となる構造や材料を挿入しても良い。この状態でウェットプロセスを実施すると、細孔内において触媒金属イオンを選択的に還元することができる。 Alternatively, a reducing agent or a structure or material serving as a starting point of reduction may be inserted into the pores of the carrier before the wet process. When the wet process is carried out in this state, the catalytic metal ions can be selectively reduced in the pores.
［２．２．２． 担体の外表面への第２触媒粒子の担持］
担体の外表面に第２触媒粒子を選択的に担持するためには、予め担体の細孔内を封止材で封止しておくのが好ましい。封止材としては、担体の細孔内に侵入しやすい低表面張力の溶媒、例えば、フッ素系溶媒（フロリナート（登録商標））などがある。この状態でドライプロセス又はウェットプロセスを適用すると、担体の外表面にのみ第２触媒粒子を担持させることができる。ドライプロセス又はウェットプロセス終了後、封止材を除去すれば、本発明に係る電極触媒が得られる。
[2.2.2. Supporting the second catalyst particles on the outer surface of the carrier]
In order to selectively support the second catalyst particles on the outer surface of the carrier, it is preferable to seal the inside of the pores of the carrier with a sealing material in advance. Examples of the sealing material include a solvent having a low surface tension that easily penetrates into the pores of the carrier, for example, a fluorine-based solvent (Fluorinert (registered trademark)) and the like. When a dry process or a wet process is applied in this state, the second catalyst particles can be supported only on the outer surface of the carrier. The electrode catalyst according to the present invention can be obtained by removing the encapsulant after the completion of the dry process or the wet process.
［３． 作用］
一般に、触媒粒子が電位変動を伴う環境下に曝されると、触媒成分の溶解及び再析出が繰り返される。また、粒径の小さい触媒粒子と粒径の大きい触媒粒子が近接している状態において、触媒粒子が電位変動を伴う環境下に曝されると、粒径の小さい触媒粒子が優先的に溶解し、溶出した触媒成分が粒径の大きい触媒粒子の表面に再析出する（オストワルド成長）。
[3. Action]
Generally, when the catalyst particles are exposed to an environment with potential fluctuations, the dissolution and reprecipitation of the catalyst components are repeated. Further, when the catalyst particles having a small particle size and the catalyst particles having a large particle size are in close proximity to each other and the catalyst particles are exposed to an environment with potential fluctuation, the catalyst particles having a small particle size are preferentially dissolved. , The eluted catalyst component is reprecipitated on the surface of the catalyst particles having a large particle size (Ostwald growth).
そのため、図１（Ａ）に示すように、担体の細孔内に小粒径の第１触媒粒子が担持され、担体の外表面に大粒径の第２触媒粒子が担持されている場合において、電位変動が生じた時には、触媒成分が細孔内から細孔外に向かって溶出する。触媒成分が細孔内から細孔外に向かって溶出すると、細孔外に存在する触媒粒子の比表面積がオストワルド成長により低下するだけでなく、触媒粒子がアイオノマで被毒される確率が高くなる。その結果、触媒の性能が劣化する。 Therefore, as shown in FIG. 1A, when the first catalyst particles having a small particle size are supported in the pores of the carrier and the second catalyst particles having a large particle size are supported on the outer surface of the carrier. When the potential fluctuation occurs, the catalyst component elutes from the inside of the pore to the outside of the pore. When the catalyst component elutes from the inside of the pores to the outside of the pores, not only the specific surface area of the catalyst particles existing outside the pores decreases due to Ostwald growth, but also the probability that the catalyst particles are poisoned by ionoma increases. .. As a result, the performance of the catalyst deteriorates.
これに対し、図１（Ｂ）に示すように、担体の細孔内に大粒径の第１触媒粒子が担持され、担体の外表面に小粒径の第２触媒粒子が担持されている場合において、電位変動が生じた時には、外表面にある小粒径の第２触媒粒子からより多くの触媒成分が溶出（ギブス−トムソン効果）し、溶出した触媒成分が細孔外から細孔内に向かって拡散する。そのため、細孔内にある第１触媒粒子からの触媒成分の溶出を抑制することができる。
この場合、細孔外にある第２触媒粒子が溶出することに伴う性能低下は発生する。しかしながら、細孔内には高い活性を持つ第１触媒粒子が保持されているので、従来の担持構造に比べて触媒性能の低下を抑制することができる。
On the other hand, as shown in FIG. 1 (B), the first catalyst particles having a large particle size are supported in the pores of the carrier, and the second catalyst particles having a small particle size are supported on the outer surface of the carrier. In this case, when the potential fluctuation occurs, more catalyst components are eluted from the second catalyst particles having a small particle size on the outer surface (Gibbs-Thomson effect), and the eluted catalyst components are discharged from outside the pores to inside the pores. Diffuse towards. Therefore, it is possible to suppress the elution of the catalyst component from the first catalyst particles in the pores.
In this case, the performance deteriorates due to the elution of the second catalyst particles outside the pores. However, since the first catalyst particles having high activity are retained in the pores, it is possible to suppress a decrease in catalyst performance as compared with the conventional supported structure.
（実験１： 第２触媒粒子の平均粒径（１））
［１． 試験方法］
担体の細孔内に大粒径の第１触媒粒子（平均粒径Ｄ1＝５ｎｍ）が担持され、かつ、担体の外表面に小粒径の第２触媒粒子（平均粒径Ｄ2＝１〜５ｎｍ）が担持されている電極触媒について、シミュレーションにより初期性能と耐久後の性能の合計値を求めた。
ここで、「初期性能」とは、燃料電池製造時の触媒性能（ＯＲＲ活性）をいう。
「耐久後の性能」とは、燃料電池が搭載された製品の製品寿命（例えば、運転時間やサイクル数）後の触媒性能をいう。
さらに、初期性能と耐久後の性能の「合計値」を評価指標に用いたのは、製品として販売するためには、初期性能と耐久後性能のいずれもある目標値を満たす必要があるためである。厳密には、初期性能と耐久後性能の重要度に応じて比率を変えるべきであるが、今回は、初期性能：耐久後性能＝１：１として合計値を算出した。
(Experiment 1: Average particle size of the second catalyst particles (1))
[1. Test method]
The first catalyst particles having a large particle size (average particle size D 1 = 5 nm) are supported in the pores of the carrier, and the second catalyst particles having a small particle size (average particle size D 2 = 1) are supported on the outer surface of the carrier. For the electrode catalyst carrying ~ 5 nm), the total value of the initial performance and the performance after durability was obtained by simulation.
Here, the "initial performance" refers to the catalytic performance (ORR activity) at the time of manufacturing the fuel cell.
"Performance after durability" refers to the catalytic performance after the product life (for example, operating time and number of cycles) of a product equipped with a fuel cell.
Furthermore, the reason why the "total value" of the initial performance and the performance after durability is used as the evaluation index is that both the initial performance and the performance after durability must meet certain target values in order to be sold as a product. is there. Strictly speaking, the ratio should be changed according to the importance of initial performance and post-durability performance, but this time, the total value was calculated with initial performance: post-durability performance = 1: 1.
［２． 結果］
図２に、第２触媒粒子の平均粒径Ｄ2と性能向上率との関係を示す。図２中、縦軸の「性能向上率」とは、Ｄ2が５ｎｍであるときの「合計値」で規格化した各触媒の「合計値」を表す。図２より、以下のことが分かる。
（１）細孔内の第１触媒粒子の平均粒径Ｄ1が５ｎｍである場合において、外表面の第２触媒粒子の平均粒径Ｄ2が１．７ｎｍ超５．０ｎｍ未満である時には、性能向上率が１００％を超える。
（２）Ｄ2が２．５〜３．０ｎｍである時に、性能向上率が最大となる。
（３）この結果は、内外の粒径を適切なサイズにすることで、性能が向上することを示している。
[2. result]
FIG. 2 shows the relationship between the average particle size D 2 of the second catalyst particles and the performance improvement rate. In FIG. 2, the “performance improvement rate” on the vertical axis represents the “total value” of each catalyst standardized by the “total value” when D 2 is 5 nm. From FIG. 2, the following can be seen.
(1) When the average particle size D 1 of the first catalyst particles in the pores is 5 nm and the average particle size D 2 of the second catalyst particles on the outer surface is more than 1.7 nm and less than 5.0 nm, Performance improvement rate exceeds 100%.
(2) When D 2 is 2.5 to 3.0 nm, the performance improvement rate is maximized.
(3) This result shows that the performance is improved by adjusting the inner and outer particle sizes to an appropriate size.
（実験２： 第２触媒粒子の平均粒径（２））
［１． 試験方法］
［１．１． 実施例１］
大粒径（平均粒径ｄ1＝６ｎｍ）の触媒粒子を細孔内に選択的に担持し、小粒径（平均粒径ｄ2＝１〜６ｎｍ）の触媒粒子を細孔外に選択的に担持した電極触媒について、シミュレーションにより初期性能と耐久後の性能の合計値を求めた。第１触媒粒子の重量比率は、７０％とした。この場合、第１触媒粒子の平均粒径Ｄ1＝ｄ1、第２触媒粒子の平均粒径Ｄ2＝ｄ2となる。
(Experiment 2: Average particle size of the second catalyst particle (2))
[1. Test method]
[1.1. Example 1]
Catalytic particles having a large particle size (average particle size d 1 = 6 nm) are selectively supported in the pores, and catalyst particles having a small particle size (average particle size d 2 = 1 to 6 nm) are selectively supported outside the pores. The total value of the initial performance and the performance after durability was obtained by simulation for the electrode catalyst carried on the above. The weight ratio of the first catalyst particles was 70%. In this case, the average particle size of the first catalyst particles is D 1 = d 1 , and the average particle size of the second catalyst particles is D 2 = d 2 .
［１．２． 比較例１］
大粒径（平均粒径ｄ1＝６ｎｍ）と小粒径（平均粒径ｄ2＝１〜６ｎｍ）の２種類の触媒粒子を、細孔内及び細孔外にそれぞれ非選択的に担持した電極触媒について、シミュレーションにより初期性能と耐久後の性能の合計値を求めた。第１触媒粒子の重量比率は、７０％とした。この場合、第１触媒粒子の平均粒径Ｄ1＝第２触媒粒子の平均粒径Ｄ2＝０．７ｄ1＋０．３ｄ2となる。
[1.2. Comparative Example 1]
Two types of catalyst particles, a large particle size (average particle size d 1 = 6 nm) and a small particle size (average particle size d 2 = 1 to 6 nm), were non-selectively supported in and out of the pores, respectively. For the electrode catalyst, the total value of the initial performance and the performance after durability was obtained by simulation. The weight ratio of the first catalyst particles was 70%. In this case, the average particle size of the first catalyst particles D 1 = the average particle size of the second catalyst particles D 2 = 0.7d 1 + 0.3d 2 .
［２． 結果］
図３に、小粒径の触媒粒子の平均粒径ｄ2と性能向上率との関係を示す。図３中、縦軸の「性能向上率」とは、小粒径の触媒粒子の平均粒径ｄ2が６ｎｍであるときの「合計値」で規格化した各触媒の「合計値」を表す。図４に、小粒径の触媒粒子の平均粒径ｄ2と劣化後の触媒活性との関係を示す。図４中、縦軸の「劣化後の触媒活性」とは、燃料電池が搭載された製品の製品寿命後の触媒性能を表す。さらに、図５に、実施例１及び比較例１で得られた触媒（ｄ1＝６ｎｍ、ｄ2＝２．５ｎｍ）の性能向上率を示す。図５中、縦軸の「性能向上率」は、ｄ1＝６ｎｍ、ｄ2＝２．５ｎｍである比較例１の触媒の「合計値」で規格化した各触媒の「合計値」を表す。図３〜図５より、以下のことが分かる。
[2. result]
FIG. 3 shows the relationship between the average particle size d 2 of the catalyst particles having a small particle size and the performance improvement rate. In FIG. 3, the “performance improvement rate” on the vertical axis represents the “total value” of each catalyst normalized by the “total value” when the average particle size d 2 of the small particle size catalyst particles is 6 nm. .. FIG. 4 shows the relationship between the average particle size d 2 of the catalyst particles having a small particle size and the catalytic activity after deterioration. In FIG. 4, the vertical axis “catalytic activity after deterioration” represents the catalytic performance after the product life of the product on which the fuel cell is mounted. Further, FIG. 5 shows the performance improvement rate of the catalysts (d 1 = 6 nm, d 2 = 2.5 nm) obtained in Example 1 and Comparative Example 1. In FIG. 5, the “performance improvement rate” on the vertical axis represents the “total value” of each catalyst standardized by the “total value” of the catalysts of Comparative Example 1 in which d 1 = 6 nm and d 2 = 2.5 nm. .. The following can be seen from FIGS. 3 to 5.
（１）実施例１及び比較例１のいずれも、ｄ2＝２．５ｎｍの時に性能向上率が最大となった。
（２）実施例１は、比較例１に比べて性能向上率が大きい。これは、大粒径の触媒粒子を細孔内に選択的に担持させることにより、耐久後においても細孔内にある触媒粒子の消失が抑制され、触媒性能が維持されたためと考えられる。
(1) In both Example 1 and Comparative Example 1, the performance improvement rate was maximized when d 2 = 2.5 nm.
(2) Example 1 has a larger performance improvement rate than Comparative Example 1. It is considered that this is because the catalyst particles having a large particle size are selectively supported in the pores, so that the disappearance of the catalyst particles in the pores is suppressed even after the durability, and the catalyst performance is maintained.
（実験３： 第１触媒粒子の重量比率）
［１． 試験方法］
担体の細孔内に大粒径の第１触媒粒子（平均粒径Ｄ1＝６ｎｍ）が担持され、かつ、担体の外表面に小粒径の第２触媒粒子（平均粒径Ｄ2＝２．５ｎｍ）が担持されている電極触媒について、シミュレーションにより初期性能と耐久後の性能の合計値を求めた。第１触媒粒子の重量比率は、１０〜１００％とした。
(Experiment 3: Weight ratio of first catalyst particles)
[1. Test method]
The first catalyst particles having a large particle size (average particle size D 1 = 6 nm) are supported in the pores of the carrier, and the second catalyst particles having a small particle size (average particle size D 2 = 2) are supported on the outer surface of the carrier. For the electrode catalyst supporting .5 nm), the total value of the initial performance and the performance after durability was obtained by simulation. The weight ratio of the first catalyst particles was 10 to 100%.
［２． 結果］
図６に、第１触媒粒子の重量比率と性能向上率との関係を示す。図６中、縦軸の「性能向上率」は、大粒径の触媒粒子と小粒径の触媒粒子が内外に非選択的に担持された場合（従来プロセス）に対する、大粒径の第１触媒粒子が細孔内に、小粒径の第２触媒粒子が細孔外に選択的に担持されている場合の比率を表す。図６より、以下のことが分かる。
[2. result]
FIG. 6 shows the relationship between the weight ratio of the first catalyst particles and the performance improvement rate. In FIG. 6, the “performance improvement rate” on the vertical axis is the first large particle size when the large particle size catalyst particles and the small particle size catalyst particles are non-selectively supported inside and outside (conventional process). It represents the ratio when the catalyst particles are selectively supported in the pores and the second catalyst particles having a small particle size are selectively supported outside the pores. From FIG. 6, the following can be seen.
（１）第１触媒粒子の重量比率が増加するに伴い、性能向上率が向上した。第１触媒粒子の重量比率を５０％以上にすると、性能向上率は１００％以上となった。
（２）第１触媒粒子の重量比率が８０％を超えると、性能向上率は減少に転じた。性能向上率を１００％以上にするためには、第１触媒粒子の重量比率は９０％以下が好ましいことが分かった。
(1) As the weight ratio of the first catalyst particles increased, the performance improvement rate improved. When the weight ratio of the first catalyst particles was 50% or more, the performance improvement rate was 100% or more.
(2) When the weight ratio of the first catalyst particles exceeded 80%, the performance improvement rate began to decrease. It was found that the weight ratio of the first catalyst particles is preferably 90% or less in order to increase the performance improvement rate to 100% or more.
（実験４： ウェットプロセスのみによる触媒の担持）
［１． 試料の作製］
ウェットプロセスのみを用いて、担体表面に触媒粒子を担持させた（比較例２）。触媒粒子の担持方法は、以下の通りである。
すなわち、白金錯体を溶媒に溶解させた溶液に担体を入れ、溶媒中の白金を還元することで、担体上に白金触媒を担持させた。
(Experiment 4: Supporting catalyst only by wet process)
[1. Preparation of sample]
The catalyst particles were supported on the surface of the carrier using only the wet process (Comparative Example 2). The method for supporting the catalyst particles is as follows.
That is, the carrier was put into a solution in which the platinum complex was dissolved in a solvent, and the platinum in the solvent was reduced to support the platinum catalyst on the carrier.
［２． 試験方法］
サンプル（触媒を担持した担体）を回転させながら、透過型電子顕微鏡（３Ｄ−ＴＥＭ）で観察した。得られた触媒サイズ、担体との位置関係を用いて、細孔内に担持されている第１触媒粒子の粒度分布、及び、細孔外に担持されていている第２触媒粒子の粒度分布を測定した。
[2. Test method]
The sample (carrier carrying the catalyst) was observed with a transmission electron microscope (3D-TEM) while rotating. Using the obtained catalyst size and the positional relationship with the carrier, the particle size distribution of the first catalyst particles supported in the pores and the particle size distribution of the second catalyst particles supported outside the pores can be determined. It was measured.
［３． 結果］
図７に、比較例２で得られた電極触媒の細孔内に担持された第１触媒粒子の粒度分布と細孔外に担持された第２触媒粒子の粒度分布を示す。図７より、以下のことが分かる。
（１）従来のウェットプロセスで触媒粒子を担持させた場合、細孔内と細孔外では粒度分布に明確な差異は認められなかった。
（２）細孔外に比べて、細孔内の方が小粒径となる傾向があることが分かった。
[3. result]
FIG. 7 shows the particle size distribution of the first catalyst particles supported in the pores of the electrode catalyst obtained in Comparative Example 2 and the particle size distribution of the second catalyst particles supported outside the pores. From FIG. 7, the following can be seen.
(1) When the catalyst particles were supported by the conventional wet process, no clear difference was observed in the particle size distribution between the inside and outside the pores.
(2) It was found that the particle size inside the pores tends to be smaller than that outside the pores.
以上、本発明の実施の形態について詳細に説明したが、本発明は上記実施の形態に何ら限定されるものではなく、本発明の要旨を逸脱しない範囲内で種々の改変が可能である。 Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention.
本発明に係る電極触媒は、自動車用動力源、定置型小型発電機等に用いられる燃料電池の空気極の電極触媒として用いることができる。 The electrode catalyst according to the present invention can be used as an electrode catalyst for the air electrode of a fuel cell used in a power source for an automobile, a stationary small generator, or the like.
Claims (7)
（１）前記電極触媒は、
多孔質の担体と、
前記担体の細孔内に担持された第１触媒粒子と、
前記担体の外表面に担持された第２触媒粒子と
を備えている。
（２）前記第１触媒粒子の平均粒径（粒度分布の中央値）Ｄ1は、前記第２触媒粒子の平均粒径Ｄ2より大きい。 An electrode catalyst having the following configuration.
(1) The electrode catalyst is
Porous carrier and
The first catalyst particles supported in the pores of the carrier and
It includes a second catalyst particle supported on the outer surface of the carrier.
(2) The average particle size (median value of particle size distribution) D 1 of the first catalyst particles is larger than the average particle size D 2 of the second catalyst particles.
前記Ｄ2は、１ｎｍ以上３ｎｍ以下である
請求項１又は２に記載の電極触媒。 The D 1 is 5 nm or more and 7 nm or less.
The electrode catalyst according to claim 1 or 2, wherein D 2 is 1 nm or more and 3 nm or less.
前記第２触媒粒子は、Ｐｔ、又は、Ｐｔ−Ｍ'合金（Ｍ'＝Ｃｏ、Ｎｉ、及び／又は、Ｆｅ）からなる
請求項１から３までのいずれか１項に記載の電極触媒。 The first catalyst particles are made of Pt or a Pt—M alloy (M = Co, Ni, and / or Fe).
The electrode catalyst according to any one of claims 1 to 3, wherein the second catalyst particles are made of Pt or a Pt—M'alloy (M'= Co, Ni, and / or Fe).
第１触媒粒子の重量比率（％）＝Ｗ1×１００／(Ｗ1＋Ｗ2) …（１）
但し、
Ｗ1は、前記電極触媒に含まれる前記第１触媒粒子の重量、
Ｗ2は、前記電極触媒に含まれる前記第２触媒粒子の重量。 The electrode catalyst according to any one of claims 1 to 5, wherein the weight ratio of the first catalyst particles represented by the following formula (1) is 50% or more and 90% or less.
Weight ratio (%) of the first catalyst particles = W 1 × 100 / (W 1 + W 2 )… (1)
However,
W 1 is the weight of the first catalyst particles contained in the electrode catalyst.
W 2 is the weight of the second catalyst particles contained in the electrode catalyst.
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