JP6814087B2 - Electrode catalyst for air electrode - Google Patents
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- 239000003054 catalyst Substances 0.000 title claims description 167
- 239000002245 particle Substances 0.000 claims description 185
- 229910001260 Pt alloy Inorganic materials 0.000 claims description 55
- 239000010953 base metal Substances 0.000 claims description 13
- 239000000203 mixture Substances 0.000 claims description 10
- 229910052742 iron Inorganic materials 0.000 claims description 6
- 229910052759 nickel Inorganic materials 0.000 claims description 6
- 229910052720 vanadium Inorganic materials 0.000 claims description 4
- 229910052782 aluminium Inorganic materials 0.000 claims description 3
- 229910052804 chromium Inorganic materials 0.000 claims description 3
- 229910052745 lead Inorganic materials 0.000 claims description 3
- 229910052748 manganese Inorganic materials 0.000 claims description 3
- 229910052721 tungsten Inorganic materials 0.000 claims description 3
- 229910052727 yttrium Inorganic materials 0.000 claims description 3
- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Substances [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 description 106
- 230000000694 effects Effects 0.000 description 45
- 230000007423 decrease Effects 0.000 description 30
- 238000006243 chemical reaction Methods 0.000 description 20
- 239000000446 fuel Substances 0.000 description 20
- 238000010828 elution Methods 0.000 description 16
- 229910045601 alloy Inorganic materials 0.000 description 14
- 239000000956 alloy Substances 0.000 description 14
- 229910052697 platinum Inorganic materials 0.000 description 13
- 238000004090 dissolution Methods 0.000 description 11
- 239000007789 gas Substances 0.000 description 11
- 230000000052 comparative effect Effects 0.000 description 8
- 229910002837 PtCo Inorganic materials 0.000 description 7
- 238000000034 method Methods 0.000 description 7
- 238000001226 reprecipitation Methods 0.000 description 7
- 229910000531 Co alloy Inorganic materials 0.000 description 6
- 230000014759 maintenance of location Effects 0.000 description 6
- 229910052751 metal Inorganic materials 0.000 description 6
- 230000006866 deterioration Effects 0.000 description 5
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 4
- 238000005275 alloying Methods 0.000 description 4
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 4
- 239000003792 electrolyte Substances 0.000 description 4
- 238000012423 maintenance Methods 0.000 description 4
- 239000012528 membrane Substances 0.000 description 4
- 229910052760 oxygen Inorganic materials 0.000 description 4
- 239000001301 oxygen Substances 0.000 description 4
- 238000006722 reduction reaction Methods 0.000 description 4
- 239000002184 metal Substances 0.000 description 3
- 239000005518 polymer electrolyte Substances 0.000 description 3
- 238000002360 preparation method Methods 0.000 description 3
- 238000012360 testing method Methods 0.000 description 3
- 229910052723 transition metal Inorganic materials 0.000 description 3
- 238000013459 approach Methods 0.000 description 2
- 229910052799 carbon Inorganic materials 0.000 description 2
- 230000003197 catalytic effect Effects 0.000 description 2
- 150000001768 cations Chemical class 0.000 description 2
- 238000011109 contamination Methods 0.000 description 2
- 238000009792 diffusion process Methods 0.000 description 2
- 238000009826 distribution Methods 0.000 description 2
- 239000010419 fine particle Substances 0.000 description 2
- 239000012535 impurity Substances 0.000 description 2
- 230000001629 suppression Effects 0.000 description 2
- 238000010998 test method Methods 0.000 description 2
- 229910052719 titanium Inorganic materials 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- 229910001339 C alloy Inorganic materials 0.000 description 1
- YCKRFDGAMUMZLT-UHFFFAOYSA-N Fluorine atom Chemical compound [F] YCKRFDGAMUMZLT-UHFFFAOYSA-N 0.000 description 1
- 229910006404 SnO 2 Inorganic materials 0.000 description 1
- 230000009471 action Effects 0.000 description 1
- 229910052787 antimony Inorganic materials 0.000 description 1
- 239000002041 carbon nanotube Substances 0.000 description 1
- 229910021393 carbon nanotube Inorganic materials 0.000 description 1
- 239000000919 ceramic Substances 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 239000013078 crystal Substances 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 239000010411 electrocatalyst Substances 0.000 description 1
- 230000008030 elimination Effects 0.000 description 1
- 238000003379 elimination reaction Methods 0.000 description 1
- 229910052731 fluorine Inorganic materials 0.000 description 1
- 239000011737 fluorine Substances 0.000 description 1
- 239000006232 furnace black Substances 0.000 description 1
- 229910052733 gallium Inorganic materials 0.000 description 1
- 229910052737 gold Inorganic materials 0.000 description 1
- 229910052738 indium Inorganic materials 0.000 description 1
- 230000000977 initiatory effect Effects 0.000 description 1
- 239000000543 intermediate Substances 0.000 description 1
- 229920000554 ionomer Polymers 0.000 description 1
- 229910052741 iridium Inorganic materials 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 229910052750 molybdenum Inorganic materials 0.000 description 1
- 229910052758 niobium Inorganic materials 0.000 description 1
- 229910052762 osmium Inorganic materials 0.000 description 1
- 229910052763 palladium Inorganic materials 0.000 description 1
- 229920000642 polymer Polymers 0.000 description 1
- -1 polytetrafluoroethylene Polymers 0.000 description 1
- 229920001343 polytetrafluoroethylene Polymers 0.000 description 1
- 239000004810 polytetrafluoroethylene Substances 0.000 description 1
- 238000010248 power generation Methods 0.000 description 1
- 238000012887 quadratic function Methods 0.000 description 1
- 230000009467 reduction Effects 0.000 description 1
- 230000000717 retained effect Effects 0.000 description 1
- 229910052703 rhodium Inorganic materials 0.000 description 1
- 229910052707 ruthenium Inorganic materials 0.000 description 1
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- 239000007787 solid Substances 0.000 description 1
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- 229910052715 tantalum Inorganic materials 0.000 description 1
- 229910052718 tin Inorganic materials 0.000 description 1
- 238000012546 transfer Methods 0.000 description 1
- 238000004841 transmission electron microscopy energy-dispersive X-ray spectroscopy Methods 0.000 description 1
- 229910052725 zinc Inorganic materials 0.000 description 1
- 229910052726 zirconium Inorganic materials 0.000 description 1
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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
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- Inert Electrodes (AREA)
- Fuel Cell (AREA)
Description
本発明は、空気極用電極触媒に関し、さらに詳しくは、Ｐｔ合金を主成分とし、初期活性及び耐久性に優れた空気極用電極触媒に関する。 The present invention relates to an electrode catalyst for an air electrode, and more particularly to an electrode catalyst for an air electrode, which contains a Pt alloy as a main component and has excellent initial activity and durability.
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 solid polymer 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。
例えば、非特許文献１には、３ｄ金属（Ｔｉ、Ｖ、Ｆｅ、Ｃｏ、Ｎｉ）を含むＰｔ合金の酸素還元反応（ＯＲＲ）が開示されている。
同文献には、
（ａ）３ｄ金属を含むＰｔ合金は、Ｐｔよりも良好な触媒である点、及び、
（ｂ）Ｐｔ3Ｍ（Ｍ＝Ｆｅ、Ｃｏ、Ｎｉ）は、純Ｐｔに比べて活性が大きく改善される点
が記載されている。
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.
For example, Non-Patent Document 1 discloses an oxygen reduction reaction (ORR) of a Pt alloy containing a 3d metal (Ti, V, Fe, Co, Ni).
In the same document,
(A) Pt alloy containing 3d metal is a better catalyst than Pt, and
(B) It is described that the activity of Pt 3 M (M = Fe, Co, Ni) is significantly improved as compared with pure Pt.
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 alloy surface 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、
（ａ）Ｐｔ／Ｃ（Ｐｔの平均粒径：約２．５ｎｍ、担持量約４７質量％）からなる電極触媒と、アイオノマとを含む触媒層インクＡを調製し、
（ｂ）Ｐｔ／Ｃ（Ｐｔの平均粒子径：約３．２ｎｍ、担持量約４７質量％）からなる電極触媒：０．８重量部と、ＰｔＣｏ合金／Ｃ（ＰｔＣｏ合金の平均粒子径：約４ｎｍ、担持量約５０質量％、Ｐｔ：Ｃｏ質量比３：１（４７．５ａｔ％Ｐｔ））からなる電極触媒：０．２重量部と、アイオノマとを含む触媒層インクＢ’を調製し、
（ｃ）Ｐｔ／Ｃ（Ｐｔの平均粒子径：約４．５ｎｍ、担持量約４７質量％）からなる電極触媒：０．１重量部と、ＰｔＣｏ合金／Ｃ（ＰｔＣｏ合金の平均粒子径：約５．２ｎｍ、担持量約５０質量％、Ｐｔ：Ｃｏ質量比３：１（４７．５ａｔ％Ｐｔ））からなる電極触媒：０．９重量部と、アイオノマとを含む触媒層インクＣ’を調製し、
（ｄ）電解質膜の表面であって、ガス供給路の入口側に触媒層インクＢ’、ガス供給路の出口側に触媒層インクＣ’をそれぞれ塗布して触媒層（１）を形成し、
（ｅ）触媒層（１）の表面であって、ガス供給路の入口側に触媒層インクＡ、ガス供給路の出口側に触媒層インクＢ’をそれぞれ塗布して触媒層（２）を形成する
ことにより得られる固体高分子形燃料電池が開示されている。
Therefore, in order to solve this problem, various proposals have been made conventionally.
For example, in Patent Document 1,
(A) A catalyst layer ink A containing an electrode catalyst composed of Pt / C (average particle size of Pt: about 2.5 nm, supported amount of about 47% by mass) and ionoma was prepared.
(B) An electrode catalyst composed of Pt / C (average particle size of Pt: about 3.2 nm, supported amount of about 47% by mass): 0.8 parts by mass and PtCo alloy / C (average particle size of PtCo alloy: about 47% by mass). A catalyst layer ink B'containing 0.2 parts by weight of an electrode catalyst consisting of 4 nm, a loading amount of about 50% by mass, and a Pt: Co mass ratio of 3: 1 (47.5 at% Pt)) and ionoma was prepared.
(C) An electrode catalyst composed of Pt / C (average particle size of Pt: about 4.5 nm, supported amount of about 47% by mass): 0.1 parts by mass and PtCo alloy / C (average particle size of PtCo alloy: about 47% by mass). Prepare a catalyst layer ink C'containing 0.9 parts by weight of an electrode catalyst consisting of 5.2 nm, a loading amount of about 50% by mass, and a Pt: Co mass ratio of 3: 1 (47.5 at% Pt)) and ionoma. And
(D) On the surface of the electrolyte membrane, the catalyst layer ink B'is applied to the inlet side of the gas supply path and the catalyst layer ink C'is applied to the outlet side of the gas supply path to form the catalyst layer (1).
(E) The surface of the catalyst layer (1), the catalyst layer ink A is applied to the inlet side of the gas supply path, and the catalyst layer ink B'is applied to the outlet side of the gas supply path to form the catalyst layer (2). The polymer electrolyte fuel cell obtained by the above is disclosed.
同文献には、
（ａ）触媒層においては、ガス供給路の入口側から出口側に向かって滞留する水分量が多くなり、水分量の多いガス供給路の出口側で触媒粒子の溶解が起きやすい点、
（ｂ）ガス供給路の出口側の触媒粒子の粒子径を大きくすると、触媒粒子の溶解を抑制することができる点、
（ｃ）白金粒子は触媒活性が高いのに対して、白金合金粒子は耐溶解性に優れているので、触媒粒子の溶解が生じがたい部位に白金粒子を配置し、触媒粒子の溶解が生じやすい部位に白金合金粒子を配置すると、高い触媒活性を維持できる点、及び
（ｄ）Ｐｔ粒子の粒径が異なるＰｔ／Ｃのみを用いて作製した燃料電池（実施例１）の耐久性（１０００サイクル後のセル電圧の変化）は０．０２３Ｖ＠１Ａ／ｃｍ2であるのに対し、触媒粒子の粒径が異なるＰｔ／Ｃ及びＰｔＣｏ合金／Ｃを用いて作製した燃料電池（実施例２）の耐久性は０．０４４Ｖ＠１Ａ／ｃｍ2である点（すなわち、ＰｔＣｏ合金／Ｃを用いた燃料電池の方が耐久性に劣る点）、
が記載されている。
In the same document,
(A) In the catalyst layer, the amount of water retained from the inlet side to the outlet side of the gas supply path increases, and the catalyst particles are likely to dissolve on the outlet side of the gas supply path having a large amount of water.
(B) By increasing the particle size of the catalyst particles on the outlet side of the gas supply path, dissolution of the catalyst particles can be suppressed.
(C) Platinum particles have high catalytic activity, whereas platinum alloy particles have excellent solubility resistance. Therefore, platinum particles are arranged in a portion where dissolution of the catalyst particles is difficult to occur, and the catalyst particles are dissolved. By arranging platinum alloy particles in easy-to-use sites, high catalytic activity can be maintained, and (d) the durability (1000) of the fuel cell (Example 1) produced using only Pt / C having different particle sizes of Pt particles. The change in cell voltage after the cycle) is 0.023 V @ 1 A / cm 2 , whereas the fuel cell produced using Pt / C and PtCo alloy / C having different particle sizes of the catalyst particles (Example 2). The durability is 0.044V @ 1A / cm 2 (that is, the fuel cell using PtCo alloy / C is inferior in durability).
Is described.
粒径の大きな触媒粒子は、比表面積が小さいので、溶出速度をある程度遅くすることができる。しかし、電極触媒全体の反応面積が低下するので、効率点性能が低下する。また、粒径の大きな触媒粒子を用いた場合であっても、燃料電池の作動環境下では、触媒粒子の溶解が進行する。
この問題を解決するために、特許文献１に記載されているように、ガス供給路の入口側にＰｔ／Ｃ粒子を多量に配置し、ガス供給路の出口側にＰｔＣｏ合金／Ｃ粒子を多量に配置することも考えられる。しかしながら、この方法により得られた燃料電池は、Ｐｔ粒子の粒径が異なるＰｔ／Ｃのみを用いて作製された燃料電池よりも耐久性が劣っており、触媒粒子の溶出を抑制する効果は低い。
Since the catalyst particles having a large particle size have a small specific surface area, the elution rate can be slowed down to some extent. However, since the reaction area of the entire electrode catalyst is reduced, the efficiency point performance is reduced. Further, even when the catalyst particles having a large particle size are used, the dissolution of the catalyst particles proceeds in the operating environment of the fuel cell.
In order to solve this problem, as described in Patent Document 1, a large amount of Pt / C particles are arranged on the inlet side of the gas supply path, and a large amount of PtCo alloy / C particles are arranged on the outlet side of the gas supply path. It is also possible to place it in. However, the fuel cell obtained by this method is inferior in durability to the fuel cell produced using only Pt / C having different particle sizes of Pt particles, and the effect of suppressing the elution of the catalyst particles is low. ..
本発明が解決しようとする課題は、初期の効率点性能及び出力点性能が高く、かつ、電位変動回数の多い環境下で使用した場合であっても、効率点性能及び出力点性能の低下を抑制することが可能な空気極用電極触媒を提供することにある。 The problem to be solved by the present invention is that the efficiency point performance and the output point performance are deteriorated even when used in an environment where the initial efficiency point performance and the output point performance are high and the number of potential fluctuations is large. An object of the present invention is to provide an electrode catalyst for an air electrode that can be suppressed.
上記課題を解決するために本発明に係る空気極用電極触媒は、以下の構成を備えていることを要旨とする。
（１）前記空気極用電極触媒は、
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、
前記第１触媒粒子及び前記第２触媒粒子を担持するための担体と
を備え、
前記第１触媒粒子及び前記第２触媒粒子は、同一の前記担体表面に担持されている。
（２）前記Ｐｔ合金は、ＰｔxＭ（１≦ｘ≦４、Ｍは卑金属元素）で表される原子組成比を持つ。
In order to solve the above problems, it is a gist that the electrode catalyst for an air electrode according to the present invention has the following configurations.
(1) The electrode catalyst for the air electrode is
The first catalyst particles made of Pt alloy and
A second catalyst particle made of pure Pt having an average particle size smaller than that of the first catalyst particle ,
With the carrier for supporting the first catalyst particles and the second catalyst particles
With
The first catalyst particles and the second catalyst particles are supported on the same carrier surface.
(2) The Pt alloy has an atomic composition ratio represented by Pt x M (1 ≦ x ≦ 4, M is a base metal element).
粒径の大きなＰｔ合金粒子は、相対的に大きな面積活性を持ち、かつ、電位変動が生じる環境下においても反応面積の減少（平均粒径の増大）は少ないが、比表面積は小さい。このような粒径の大きな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する。その結果、初期の効率点性能及び出力点性能が向上する。
また、電位変動を伴う環境下においてこのような電極触媒を使用する場合において、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。そのため、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。また、使用中の効率点性能及び出力点性能の低下が抑制され、効率点性能及び出力点性能の双方を高いレベルで維持することができる。但し、ＰｔよりもＭが原子比で多い場合は、Ｍの溶出が止められず、このような効果は得られない。従って、Ｐｔ：Ｍ＝１：１（原子比）〜Ｐｔ：Ｍ＝４：１が好適となる。
Pt alloy particles having a large particle size have a relatively large area activity, and the reaction area does not decrease (increase in the average particle size) even in an environment where potential fluctuation occurs, but the specific surface area is small. When such 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 increases. As a result, the initial efficiency point performance and output point performance are improved.
Further, when such an electrode catalyst is used in an environment accompanied by potential fluctuation, when the pure Pt particles are arranged in the vicinity of the Pt alloy particles, the pure Pt particles are preferentially eluted, and the eluted Pt is the Pt alloy. Reprecipitates on the surface of the particles. Therefore, elution of alloying elements from the surface of Pt alloy particles is suppressed as compared with an electrode catalyst composed of only Pt alloy particles. In addition, deterioration of efficiency point performance and output point performance during use is suppressed, and both efficiency point performance and output point performance can be maintained at a high level. However, when M is larger in atomic ratio than Pt, the elution of M cannot be stopped and such an effect cannot be obtained. Therefore, Pt: M = 1: 1 (atomic ratio) to Pt: M = 4: 1 are preferable.
以下、本発明の一実施の形態について詳細に説明する。
［１． 空気極用電極触媒］
本発明に係る空気極用電極触媒（以下、単に「電極触媒」ともいう）は、以下の構成を備えている。
（１）前記空気極用電極触媒は、
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
を備えている。
（２）前記Ｐｔ合金は、ＰｔxＭ（１≦ｘ≦４、Ｍは卑金属元素）で表される原子組成比を持つ。
Hereinafter, an embodiment of the present invention will be described in detail.
[1. Electrode catalyst for air electrode]
The electrode catalyst for an air electrode (hereinafter, also simply referred to as “electrode catalyst”) according to the present invention has the following configurations.
(1) The electrode catalyst for the air electrode is
The first catalyst particles made of Pt alloy and
It includes a second catalyst particle made of pure Pt having an average particle size smaller than that of the first catalyst particle.
(2) The Pt alloy has an atomic composition ratio represented by Pt x M (1 ≦ x ≦ 4, M is a base metal element).
［１．１． 第１触媒粒子］
［１．１．１． 組成］
第１触媒粒子は、Ｐｔ合金からなる。本発明において、Ｐｔ合金の組成は特に限定されるものではなく、Ｐｔを含むあらゆる合金に対して本発明を適用することができる。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、かつ、Ｐｔ合金粒子と純Ｐｔ粒子を近接して配置し、これらを電位変動が生じる環境下に曝すと、純Ｐｔ粒子が優先的に溶解し、Ｐｔ合金粒子表面にＰｔが再析出する。このようなＰｔの溶解及び再析出は、Ｐｔ合金の組成や結晶構造によらず起こる。
[1.1. 1st catalyst particle]
[1.1.1. composition]
The first catalyst particles are made of Pt alloy. In the present invention, the composition of the Pt alloy is not particularly limited, and the present invention can be applied to any alloy including Pt. When Pt alloy particles with a large particle size and pure Pt particles with a small particle size coexist, and Pt alloy particles and pure Pt particles are placed close to each other and exposed to an environment where potential fluctuations occur, pure Pt The particles are preferentially dissolved and Pt is reprecipitated on the surface of the Pt alloy particles. Such dissolution and reprecipitation of Pt occurs regardless of the composition and crystal structure of the Pt alloy.
Ｐｔ合金は、次の式（１）で表される原子組成比を持つ。
ＰｔxＭ ・・・（１）
但し、Ｍは卑金属元素、１≦ｘ≦４。
The Pt alloy has 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、卑金属元素を表す。「卑金属元素」とは、Ａｕ、Ａｇ、Ｐｔ、Ｐｄ、Ｒｈ、Ｉｒ、Ｒｕ、及びＯｓ以外の金属元素をいう。
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 formula (1), M represents a base metal element. The "base metal element" refers to a metal element other than Au, Ag, Pt, Pd, Rh, Ir, Ru, and Os.
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。
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.
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 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 pH 1. Therefore, x is preferably 1 or more (that is, 50 at% Pt or more).
Ｐｔ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 alloy reaches a maximum at a certain value of x and then starts 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 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ｄm1］
第１触媒粒子の平均粒径ｄm1は、電極触媒のＯＲＲ活性に影響を与える。一般に、ｄm1が小さくなるほど、第１触媒粒子は、溶解しやすくなる。第１触媒粒子の溶解が進行するすると、第１触媒粒子の反応面積（粒径）が減少する。また、第１触媒粒子から元素Ｍが溶出し、出力点性能が低下する。従って、ｄm1は、４ｎｍ以上が好ましい。
[1.1.2. Average particle size d m1 ]
The average particle size dm1 of the first catalyst particles affects the ORR activity of the electrode catalyst. In general, the smaller d m1 , the easier it is for the first catalyst particles to dissolve. As the dissolution of the first catalyst particles progresses, the reaction area (particle size) of the first catalyst particles decreases. In addition, the element M is eluted from the first catalyst particles, and the output point performance is deteriorated. Therefore, d m1 is preferably 4 nm or more.
一方、ｄm1が大きくなるほど、比表面積が減少する。本発明では、第１触媒粒子中に含まれるＰｔ比を適度に下げることで高い面積活性を得ているため、比表面積の減少に起因する効率点性能の低下をある程度相殺することができる。しかしながら、ｄm1が大きくなりすぎると、比表面積の減少を面積活性の増大で補うことができなくなり、効率点性能が低下する。従って、ｄm1は、６ｎｍ以下が好ましい。 On the other hand, as d m1 increases, the specific surface area decreases. In the present invention, since high area activity is obtained by appropriately lowering the Pt ratio contained in the first catalyst particles, the decrease in efficiency point performance due to the decrease in specific surface area can be offset to some extent. However, if d m1 becomes too large, the decrease in specific surface area cannot be compensated for by the increase in area activity, and the efficiency point performance deteriorates. Therefore, d m1 is preferably 6 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ｄm1±σ1となる確率は、６８．２７％となる。
（ｄm１−σ1）未満の粒径を持つ微細な第１触媒粒子は、元素Ｍ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、（ｄm1＋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 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 m1 ± sigma 1 is 68 It will be .27%.
(D m1 - [sigma] 1) less than the first catalyst particles fine having a particle size, since the element M is likely to elute, it causes a decrease in the output point performance of the overall electrocatalyst. On the other hand, the coarse first catalyst particles having a particle size exceeding ( dm1 + 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］
第２触媒粒子は、純Ｐｔからなる。「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。
[1.2. Second catalyst particle]
[1.2.1. composition]
The second catalyst particle is made of pure Pt. “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.
［１．２．２． 平均粒径ｄm2］
第２触媒粒子の平均粒径ｄm2は、電極触媒のＯＲＲ活性に影響を与える。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、粒径の小さな純Ｐｔ粒子が優先的に溶解し、Ｐｔ合金粒子の表面にＰｔが再析出することによって、Ｐｔ合金粒子からの元素Ｍの溶出を抑制する。そのためには、ｄm2は、少なくともｄm1未満である必要がある。
[1.2.2. Average particle size d m2 ]
The average particle size dm2 of the second catalyst particles affects the ORR activity 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, pure Pt particles having a small particle size are preferentially dissolved, and Pt is reprecipitated on the surface of the Pt alloy particles, thereby suppressing elution of the element M from the Pt alloy particles. .. For that purpose, d m2 must be at least less than d m1 .
一般に、ｄm2が小さくなるほど、Ｐｔの溶解・再析出が起きやすくなる。しかし、ｄm2が小さくなりすぎると、Ｐｔの面積活性が顕著に低下することが知られており、その結果、効率点性能を向上させる効果に欠ける。従って、ｄm2は、１ｎｍ以上が好ましい。ｄm2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、ｄm2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、Ｐｔ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、ｄm2は、４ｎｍ以下が好ましい。ｄm2は、好ましくは、３．５ｎｍ以下、さらに好ましくは、３．０ｎｍ以下である。
In general, the smaller d m2 , the easier it is for Pt to dissolve and reprecipitate. However, it is known that when d m2 becomes too small, the area activity of Pt is remarkably lowered, and as a result, the effect of improving the efficiency point performance is lacking. Therefore, d m2 is preferably 1 nm or more. dm2 is preferably 1.5 nm or more, more preferably 2.0 nm or more.
On the other hand, if d m2 becomes too large, the reaction area of the entire electrode catalyst decreases, and the dissolution / reprecipitation rate of Pt also decreases. As a result, the effect of improving the initial performance or the durability becomes insufficient. Therefore, d m2 is preferably 4 nm or less. dm2 is preferably 3.5 nm or less, more preferably 3.0 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ｄm2±σ2となる確率は、６８．２７％となる。
（ｄm2−σ2）未満の粒径を持つ微細な第２触媒粒子は、Ｐｔの面積活性が低く、効率点性能を向上させる効果に欠ける。一方、（ｄm2＋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、Ｐｔ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 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 dm2 ±. The probability of becoming σ 2 is 68.27%.
(D m2 -σ 2) less fine second catalyst particles having a particle diameter, low surface area activity Pt, lacks effect of improving the efficiency point performance. On the other hand, the coarse second catalyst particles having a particle size exceeding ( dm2 + σ 2 ) have a small specific surface area and an excessively low Pt dissolution / reprecipitation rate. 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． 重量比］
第２触媒粒子の重量（Ｗ2）に対する前記第１触媒粒子の重量（Ｗ1）の比（＝Ｗ1／Ｗ2）は、電極触媒のＯＲＲ活性に影響を与える。電極触媒全体に含まれる第１触媒粒子の量が過度に少なくなると、合金の活性向上効果が十分に得られないために、効率点性能が低下する。従って、Ｗ1／Ｗ2比は、２／１以上が好ましい。Ｗ1／Ｗ2比は、好ましくは、２．５／１以上、さらに好ましくは、３／１以上である。
一方、第１触媒粒子の量が過度に多くなると、より多くの元素Ｍが溶出しやすくなる。従って、Ｗ1／Ｗ2比は、９／１以下が好ましい。Ｗ1／Ｗ2比は、好ましくは、８／１以下、さらに好ましくは、６／１以下である。
[1.3. Weight ratio]
The ratio (= W 1 / W 2 ) of the weight (W 1 ) of the first catalyst particles to the weight (W 2 ) of the second catalyst particles affects the ORR activity of the electrode catalyst. If the amount of the first catalyst particles contained in the entire electrode catalyst is excessively small, the effect of improving the activity of the alloy cannot be sufficiently obtained, so that the efficiency point performance is lowered. Therefore, the W 1 / W 2 ratio is preferably 2/1 or more. The W 1 / W 2 ratio is preferably 2.5 / 1 or more, more preferably 3/1 or more.
On the other hand, when the amount of the first catalyst particles is excessively large, more elements M are likely to be eluted. Therefore, the W 1 / W 2 ratio is preferably 9/1 or less. The W 1 / W 2 ratio is preferably 8/1 or less, more preferably 6/1 or less.
［１．４． 担体］
第１触媒粒子及び第２触媒粒子は、Ｐｔの溶解・再析出が生じる限りにおいて、そのままの状態で使用しても良く、あるいは、担体表面に担持された状態で使用しても良い。
担体としては、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]
The first catalyst particles and the second catalyst particles may be used as they are, or may be used while being supported on the surface of the carrier, as long as Pt is dissolved and reprecipitated.
Examples of the carrier include carbon black, furnace black, carbon nanotubes, mesoporous carbon, and electron conductive ceramics (TiO x , Sb-SnO 2 ).
［１．５． 第１触媒粒子と第２触媒粒子との間の距離］
Ｐｔの溶解・再析出を生じさせるためには、第２触媒粒子は、第１触媒粒子の溶出を抑制することが可能な位置に近接して配置されているのが好ましい。Ｐｔの溶解・再析出が効率良く進行するように、第１触媒粒子と第２触媒粒子とが近接して配置されている態様としては、例えば、
（ａ）微細な担体表面に第１触媒粒子が担持されたものと、微細な担体表面に第２触媒粒子が担持されたものとが均一に混合されている態様、
（ｂ）第１触媒粒子と第２触媒粒子が、同一の担体表面に担持されている態様、
（ｃ）第２触媒粒子が、第１触媒粒子の表面に担持されている態様、
などがある。
[1.5. Distance between the 1st catalyst particle and the 2nd catalyst particle]
In order to cause dissolution / reprecipitation of Pt, it is preferable that the second catalyst particles are arranged close to a position where the elution of the first catalyst particles can be suppressed. As a mode in which the first catalyst particles and the second catalyst particles are arranged close to each other so that the dissolution / reprecipitation of Pt proceeds efficiently, for example,
(A) An embodiment in which the first catalyst particles are uniformly supported on the fine carrier surface and the second catalyst particles are uniformly supported on the fine carrier surface.
(B) A mode in which the first catalyst particles and the second catalyst particles are supported on the same carrier surface.
(C) A mode in which the second catalyst particles are supported on the surface of the first catalyst particles.
and so on.
［２． 空気極用電極触媒の製造方法］
本発明に係る空気極用電極触媒は、種々の方法により製造することができる。本発明に係る空気極用電極触媒は、例えば、
（ａ）特許文献１と同様に、２種類の触媒を混合する方法、
（ｂ）純Ｐｔ粒子（又は、Ｐｔ合金粒子）が担持されている担体表面に、さらにＰｔ合金粒子（又は、純Ｐｔ粒子）を担持させる方法、
（ｃ）Ｐｔ合金粒子の表面に純Ｐｔ粒子を担持させる方法、
などにより製造することができる。
[2. Manufacturing method of electrode catalyst for air electrode]
The electrode catalyst for an air electrode according to the present invention can be produced by various methods. The electrode catalyst for an air electrode according to the present invention is, for example,
(A) A method of mixing two types of catalysts, as in Patent Document 1.
(B) A method of further supporting Pt alloy particles (or pure Pt particles) on the surface of a carrier on which pure Pt particles (or Pt alloy particles) are supported.
(C) A method of supporting pure Pt particles on the surface of Pt alloy particles,
It can be manufactured by such means.
［３． 作用］
電位変動回数の多い環境下において、効率点性能の低下と出力点性能の低下の双方を抑制するためには、（１）面積活性の低下の抑制、（２）反応面積の低下の抑制、及び（３）卑金属元素の溶出の抑制、の３つを同時に達成する必要がある。
効率点性能は、面積活性と反応面積の積で表されるため、面積活性又は反応面積の一方が低下しても、これらの積の低下が抑制されれば、効率点性能の低下を抑制することができる。一方、出力点性能の低下の抑制には、反応面積の低下と、卑金属元素の溶出の双方を抑制する必要がある。
[3. Action]
In an environment where the number of potential fluctuations is large, in order to suppress both the decrease in efficiency point performance and the decrease in output point performance, (1) suppression of decrease in area activity, (2) suppression of decrease in reaction area, and (3) It is necessary to simultaneously achieve the three requirements of suppressing the elution of base metal elements.
Since the efficiency point performance is expressed by the product of the area activity and the reaction area, even if either the area activity or the reaction area decreases, if the decrease in these products is suppressed, the decrease in the efficiency point performance is suppressed. be able to. On the other hand, in order to suppress the decrease in output point performance, it is necessary to suppress both the decrease in the reaction area and the elution of the base metal element.
粒径が比較的大きく、かつ、Ｐｔ比の小さいＰｔ合金触媒は、反応面積の低下（溶解・再析出による平均粒径の増大）の抑制に対して有効であり、結果として効率点性能の低下の抑制には有用と考えられる。しかし、Ｐｔ比が小さいため、卑金属元素の溶出の抑制を達成することは困難である。そのため、出力点性能の低下が大きいと考えられる。
一方、粒径が比較的小さく、かつ、Ｐｔ比の大きいＰｔ合金触媒は、卑金属元素の溶出の抑制に対しては有効であるが、反応面積の低下の抑制に対しては有効ではない。そのため、効率点と出力点の双方の性能低下が大きいと考えられる。
A Pt alloy catalyst having a relatively large particle size and a small Pt ratio is effective in suppressing a decrease in the reaction area (increase in the average particle size due to dissolution / reprecipitation), resulting in a decrease in efficiency point performance. It is considered to be useful for suppressing. However, since the Pt ratio is small, it is difficult to suppress the elution of base metal elements. Therefore, it is considered that the output point performance is greatly deteriorated.
On the other hand, a Pt alloy catalyst having a relatively small particle size and a large Pt ratio is effective in suppressing the elution of base metal elements, but is not effective in suppressing a decrease in the reaction area. Therefore, it is considered that the performance deterioration of both the efficiency point and the output point is large.
これに対し、第１触媒粒子としてＰｔ合金触媒を用いる場合において、平均粒径を大きくすると総表面積は低下するが、Ｐｔ比を下げると面積活性は向上する。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、Ｐｔ総量を一定として平均粒径の大きなＰｔ合金触媒と平均粒径の小さな純Ｐｔ触媒とを共存させると、総表面積が増加する。すなわち、平均粒径の小さな純Ｐｔ触媒は、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。 On the other hand, when a Pt alloy catalyst is used as the first catalyst particles, the total surface area decreases when the average particle size is increased, but the area activity is improved when the Pt ratio is decreased. As a result, the initial efficiency point performance, which is the product of the area activity and the reaction area, can be guaranteed. Further, when the Pt alloy catalyst having a large average particle size and the pure Pt catalyst having a small average particle size coexist with the total amount of Pt constant, the total surface area increases. That is, the pure Pt catalyst having a small average particle size plays a role of complementing the efficiency point performance and the output point performance of the initial Pt alloy catalyst.
電位変動を伴う環境下において純Ｐｔ粒子とＰｔ合金粒子とが共存している電極触媒を使用する場合において、Ｐｔ合金粒子に近接して純Ｐｔ粒子を配置すると、平均粒径の小さな純Ｐｔ粒子が優先的に溶出し、これが平均粒径の大きなＰｔ合金粒子の表面に再析出する。その結果、純Ｐｔ粒子が共存しない場合に比べて、Ｐｔ合金粒子からの合金元素の溶出が抑制される。また、効率点性能の低下及び出力点性能の低下は、純Ｐｔ粒子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。さらに、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。これと同時に、Ｐｔ合金触媒の反応面積の減少、及び合金元素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。 When using an electrode catalyst in which pure Pt particles and Pt alloy particles coexist in an environment with potential fluctuations, if pure Pt particles are placed close to the Pt alloy particles, pure Pt particles with a small average particle size Is preferentially eluted, and this is reprecipitated on the surface of Pt alloy particles having a large average particle size. As a result, elution of alloying elements from Pt alloy particles is suppressed as compared with the case where pure Pt particles do not coexist. Further, the deterioration of the efficiency point performance and the deterioration of the output point performance are suppressed as a whole, although they occur to some extent due to the elution of pure Pt particles. Furthermore, due to the high area activity characteristic of Pt alloy catalysts, efficiency point performance is maintained at a high level. At the same time, the reduction of the reaction area of the Pt alloy catalyst and the elution of the alloying elements are suppressed, so that the output point performance is also maintained at a high level.
従来の触媒設計では、合金元素の溶出を抑制するためにＰｔ合金触媒のＰｔ比を上げて面積活性を少し改良すること、及び、比較的小さな平均粒径とすることで総表面積を増やすことが行われていた。そのため、従来のＰｔ合金触媒は、初期性能は高いが耐久性が低い。これに対し、本発明に係る電極触媒は、従来と同等以上の初期の効率点性能及び出力点性能と、従来よりも高い耐久性とを併せ持つ。 In the conventional catalyst design, the Pt ratio of the Pt alloy catalyst is increased to slightly improve the area activity in order to suppress the elution of alloying elements, and the total surface area is increased by making the average particle size relatively small. It was done. Therefore, the conventional Pt alloy catalyst has high initial performance but low durability. On the other hand, the electrode catalyst according to the present invention has both initial efficiency point performance and output point performance equal to or higher than those of the conventional one, and higher durability than the conventional one.
（参考例１、実施例２、比較例１〜２）
［１． 試料の作製］
［１．１． 電極触媒の作製］
［１．１．１． 比較例１］
旧知の知見として、平均粒径が４〜６ｎｍになると、２〜３ｎｍの触媒より電位変動に対する耐久性が顕著に改善されることがわかっている。また、ＰｔxＭ合金において、卑金属Ｍ（Ｍ＝Ｃｏ、Ｎｉ、Ｆｅなど）に対するＰｔの比（ｘ）が１未満（５０ａｔ％Ｐｔ未満）になると、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であるとの知見もある。
そこで、平均粒径６．０ｎｍのＰｔ2Ｃｏ（６６．７ａｔ％Ｐｔ）をＰｔ合金触媒の代表例（比較例１）として選定した。また、比較として、平均粒径３．１ｎｍのＰｔ7Ｃｏ（８７．５ａｔ％Ｐｔ）（比較例２）も試験に供した。いずれも、カーボン担体に３０ｗｔ％Ｐｔ量で担持したものを用いた。
(Reference Example 1, Example 2, Comparative Examples 1 and 2)
[1. Preparation of sample]
[1.1. Preparation of electrode catalyst]
[1.1.1. Comparative Example 1]
It is known from the old knowledge that when the average particle size is 4 to 6 nm, the durability against potential fluctuation is significantly improved as compared with the catalyst of 2 to 3 nm. Further, in the Pt x M alloy, when the ratio (x) of Pt to the base metal M (M = Co, Ni, Fe, etc.) is less than 1 (less than 50 at% Pt), the number is several under a fuel cell environment of about pH ~ 1. There is also a finding that it is difficult to produce a fine particle alloy that can exist stably for a day.
Therefore, Pt 2 Co (66.7 at% Pt) having an average particle size of 6.0 nm was selected as a representative example (Comparative Example 1) of the Pt alloy catalyst. For comparison, Pt 7 Co (87.5 at% Pt) having an average particle size of 3.1 nm (Comparative Example 2) was also subjected to the test. In each case, a carbon carrier supported on a carbon carrier in an amount of 30 wt% Pt was used.
［１．１．２． 参考例１］
比較例１のカーボン担持Ｐｔ合金触媒（Ｐｔ合金／Ｃ触媒）に対して、平均粒径２．６ｎｍのＰｔ／ＣをＰｔ重量換算で２３ｗｔ％混合した（参考例１）。
［１．１．３． 実施例２］
比較例１のＰｔ合金／Ｃ触媒の担体表面に、Ｐｔ重量換算で２３ｗｔ％相当のＰｔ粒子を追加担持した（実施例２）。追加担持された純Ｐｔの平均粒径は、ＴＥＭ／ＥＤＸ解析より、１．４ｎｍであった。
[1.1.2. Reference example 1]
23 wt% of Pt / C having an average particle size of 2.6 nm was mixed with the carbon-supported Pt alloy catalyst (Pt alloy / C catalyst) of Comparative Example 1 in terms of Pt weight (Reference Example 1) .
[1.1.3. Example 2]
Pt particles equivalent to 23 wt% in terms of Pt weight were additionally supported on the carrier surface of the Pt alloy / C catalyst of Comparative Example 1 (Example 2). The average particle size of the additionally supported pure Pt was 1.4 nm according to the TEM / EDX analysis.
［１．２． ＭＥＡの作製］
上記の触媒に、旧知の方法に従い溶媒とアイオノマ溶液とを加え、十分に混合してインクを作製した。このインクをポリテトラフルオロエチレンシートに塗布することで空気極用の触媒層キャスト膜を得た。それぞれのＰｔ目付量は、約０．２ｍｇ／ｃｍ2であった。また、純Ｐｔ粒子のみを用いた以外は、上記と同様にして、燃料極用の触媒層キャスト膜を得た。フッ素系電解質膜の両面に、それぞれ、空気極用触媒層及び燃料極用触媒層を転写し、ホットプレスを行い、電極面積４ｃｍ2のＭＥＡを作製した。
[1.2. Preparation of MEA]
To the above catalyst, a solvent and an ionomer solution were added according to an old method, and the ink was sufficiently mixed to prepare an ink. By applying this ink to a polytetrafluoroethylene sheet, a catalyst layer cast film for an air electrode was obtained. The basis weight of each Pt was about 0.2 mg / cm 2 . Further, a catalyst layer cast film for the fuel electrode was obtained in the same manner as described above except that only pure Pt particles were used. The catalyst layer for the air electrode and the catalyst layer for the fuel electrode were transferred to both sides of the fluorine-based electrolyte membrane, respectively, and hot-pressed to prepare MEA having an electrode area of 4 cm 2 .
［２． 試験方法］
ＭＥＡを拡散層とともに燃料電池単セルに組み込み、電位サイクル試験を行った。耐久試験条件は、以下の通りである。
（ａ）セル温度：８０℃
（ｂ）空気極Ｎ2流量：０．５Ｌ／ｍｉｎ、フル加湿
（ｃ）燃料極Ｈ2流量：０．５Ｌ／ｍｉｎ、フル加湿
（ｄ）電位サイクル：０．８５Ｖ（３ｓ）−０．１Ｖ（３ｓ）の電位サイクルを５万回
[2. Test method]
The MEA was incorporated into a fuel cell single cell together with the diffusion layer, and a potential cycle test was performed. The durability test conditions are as follows.
(A) Cell temperature: 80 ° C.
(B) Air electrode N 2 flow rate: 0.5 L / min, full humidification (c) Fuel electrode H 2 flow rate: 0.5 L / min, full humidification (d) potential cycle: 0.85 V (3s) -0.1 V (3s) potential cycle 50,000 times
［３． 結果］
図１に、各種触媒の反応面積維持率と活性維持率との関係を示す。高Ｐｔ比の触媒（比較例２）は、活性維持率が高いものの、反応面積維持率が低いことがわかる。これは、出力性能の低下が促進されていることを示している。他方、低Ｐｔ比の触媒（比較例１）は、反応面積維持率が高いものの、活性維持率が低いことがわかる。これは、Ｃｏの溶出が一因と考えられる。これらに対して、Ｐｔ合金粒子に近接して純Ｐｔ粒子を共存させた触媒（参考例１、実施例２）では、反応面積維持率及び活性維持率の両方が改善されることが明らかとなった。
[3. result]
FIG. 1 shows the relationship between the reaction area maintenance rate and the activity maintenance rate of various catalysts. It can be seen that the catalyst having a high Pt ratio (Comparative Example 2) has a high activity retention rate but a low reaction area retention rate. This indicates that the decrease in output performance is promoted. On the other hand, it can be seen that the catalyst having a low Pt ratio (Comparative Example 1) has a high reaction area retention rate but a low activity retention rate. This is thought to be due in part to the elution of Co. On the other hand, it was clarified that both the reaction area retention rate and the activity retention rate were improved in the catalyst in which the pure Pt particles coexisted in the vicinity of the Pt alloy particles (Reference Example 1 and Example 2). It was.
（実施例３）
［１． 試験方法］
Ｃｏ組成の異なるＰｔ−Ｃｏ合金粒子を作製し、Ｐｔ−Ｃｏ合金の表面積当たりの活性（ＳＡ）を回転電極法で評価した。
(Example 3)
[1. Test method]
Pt-Co alloy particles having different Co compositions were prepared, and the activity (SA) per surface area of the Pt-Co alloy was evaluated by the rotating electrode method.
［２． 結果］
図２に、Ｐｔ−Ｃｏ合金中のＣｏ量に対するＰｔ−Ｃｏ合金の表面積当たりの０．９Ｖ vs ＲＨＥにおける活性（ＳＡ）依存性を示す。図２中、実線は、実験値を２次関数でフィッティングしたものである。図２より、以下のことがわかる。
（ａ）Ｃｏ量が２０ａｔ％以上（８０ａｔ％Ｐｔ以下）になると、ＳＡは純Ｐｔの約１．７倍以上になる。
（ｂ）Ｃｏ量が５０ａｔ％近傍のところで、ＳＡは極大値を取る。
[2. result]
FIG. 2 shows the activity (SA) dependence at 0.9 V vs RHE per surface area of the Pt-Co alloy with respect to the amount of Co in the Pt-Co alloy. In FIG. 2, the solid line is the experimental value fitted by a quadratic function. From FIG. 2, the following can be seen.
(A) When the amount of Co is 20 at% or more (80 at% Pt or less), SA becomes about 1.7 times or more of pure Pt.
(B) SA takes a maximum value when the amount of Co is around 50 at%.
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.
本発明に係る空気極用電極触媒は、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 air electrode electrode catalyst according to the present invention can be used as an air electrode electrode catalyst for a fuel cell used in a power source for an automobile, a stationary small generator, or the like.
Claims (4)
（１）前記空気極用電極触媒は、
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、
前記第１触媒粒子及び前記第２触媒粒子を担持するための担体と
を備え、
前記第１触媒粒子及び前記第２触媒粒子は、同一の前記担体表面に担持されている。
（２）前記Ｐｔ合金は、ＰｔxＭ（１≦ｘ≦４、Ｍは卑金属元素）で表される原子組成比を持つ。 An electrode catalyst for an air electrode having the following configuration.
(1) The electrode catalyst for the air electrode is
The first catalyst particles made of Pt alloy and
A second catalyst particle made of pure Pt having an average particle size smaller than that of the first catalyst particle ,
With the carrier for supporting the first catalyst particles and the second catalyst particles
With
The first catalyst particles and the second catalyst particles are supported on the same carrier surface.
(2) The Pt alloy has an atomic composition ratio represented by Pt x M (1 ≦ x ≦ 4, M is a base metal element).
（３）前記第１触媒粒子は、平均粒径ｄm1が４ｎｍ以上６ｎｍ以下であり、かつ、前記平均粒径の標準偏差σ1が２ｎｍ以下である。
（４）前記第２触媒粒子は、平均粒径ｄm2が１ｎｍ以上４ｎｍ以下であり、かつ、前記平均粒径の標準偏差σ2が２ｎｍ以下である。 The electrode catalyst for an air electrode according to claim 1 or 2, further comprising the following configuration.
(3) The first catalyst particles have an average particle size dm1 of 4 nm or more and 6 nm or less, and a standard deviation σ 1 of the average particle size of 2 nm or less.
(4) The second catalyst particles have an average particle size dm2 of 1 nm or more and 4 nm or less, and a standard deviation σ 2 of the average particle size of 2 nm or less.
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