JP2005005257A - Air electrode catalyst for fuel cell, and manufacturing method therefor - Google Patents
Air electrode catalyst for fuel cell, and manufacturing method therefor Download PDFInfo
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Abstract
Description
本発明は、電解質として、例えばフッ素樹脂系高分子のようなプロトン伝導性固体高分子膜を用いる固体高分子形燃料電池（ＰＥＦＣ：Ｐｏｌｙｍｅｒ Ｅｌｅｃｔｒｏｌｙｔｅ Ｆｕｅｌ Ｃｅｌｌ、ＰＥＭＦＣ：Ｐｒｏｔｏｎ Ｅｘｃｈａｎｇｅ Ｍｅｍｂｒａｍｅ Ｆｕｅｌ Ｃｅｌｌ）や、リン酸溶液を含浸させた絶縁性マトリックスを用いるリン酸形燃料電池（ＰＡＦＣ：Ｐｈｏｓｐｈｏｒｉｃ Ａｃｉｄ Ｆｕｅｌ Ｃｅｌｌ）に用いられる燃料電池用空気極触媒と、その製造方法に関するものである。 The present invention relates to a polymer electrolyte fuel cell (PEFC) using a proton conductive solid polymer membrane such as a fluororesin polymer as an electrolyte, a phosphorus polymer membrane fuel cell (PEMFC), phosphorus, and the like. The present invention relates to an air electrode catalyst for a fuel cell used in a phosphoric acid fuel cell (PAFC) using an insulating matrix impregnated with an acid solution, and a method for producing the same.
燃料電池は、電極反応による生成物が水であるため、地球環境への悪影響がほとんどないクリーンな発電システムである。
このような燃料電池は、自動車等の移動体用電源や定置用電源としての利用が試みられているが、いずれの場合においても長期に亘って所望の発電性能を維持することが求められている。
また、各種の燃料電池のうち、例えば溶融炭酸塩形燃料電池（ＭＣＦＣ）では、作動温度が６００℃以上、固体酸化物形燃料電池（ＳＯＦＣ）では１０００℃近くの高温であるのに対して、固体高分子形燃料電池では１００℃以下、リン酸形燃料電池でも２００℃程度での比較的低温での作動が可能であるという利点を備えている。
A fuel cell is a clean power generation system with little adverse effect on the global environment because the product of the electrode reaction is water.
Such a fuel cell has been tried to be used as a power source for a moving body such as an automobile or a stationary power source. In any case, it is required to maintain a desired power generation performance over a long period of time. .
In addition, among various fuel cells, for example, in the molten carbonate fuel cell (MCFC), the operating temperature is 600 ° C. or higher, and in the solid oxide fuel cell (SOFC), the temperature is close to 1000 ° C., Solid polymer fuel cells have the advantage that they can be operated at a relatively low temperature of 100 ° C. or lower, and phosphoric acid fuel cells at about 200 ° C.
このような燃料電池における酸素還元電極においては、酸素還元過電圧が大きく、これが燃料電池の効率を低下させる主な原因となっており、プロトンと酸素の反応触媒として白金（Ｐｔ）などの貴金属触媒が用いられるが、これら貴金属単独の電極触媒においては、酸素還元活性が不十分であって、さらに高活性の酸素還元電極触媒が求められている。
そして、このような触媒の活性化手段のひとつとして、貴金属、なかでもＰｔと卑金属の合金、あるいは金属間化合物が、Ｐｔ単独の電極触媒よりも高い酸素還元活性を示すことが知られており、このようなＰｔ系合金触媒を燃料電池用の電極触媒として用いることが提案されている（例えば、特許文献１参照）。
As one of the means for activating such a catalyst, it is known that a noble metal, especially an alloy of Pt and a base metal, or an intermetallic compound exhibits higher oxygen reduction activity than an electrode catalyst of Pt alone, It has been proposed to use such a Pt-based alloy catalyst as an electrode catalyst for a fuel cell (see, for example, Patent Document 1).
しかしながら、上記のような白金系合金触媒は、白金単独の触媒よりも高い活性を示すものの、第２成分としての卑金属が強酸性、貴電位環境で溶出し、電解質膜にダメージを与えることから、燃料電池の性能低下を招くという問題点がある。また、燃料電池の長期間運転を行うと、白金（Ｐｔ）のシンタリングが発生することにより、性能低下が生じるという問題点もあった。 However, although the platinum-based alloy catalyst as described above exhibits higher activity than the catalyst of platinum alone, the base metal as the second component elutes in a strongly acidic, noble potential environment, and damages the electrolyte membrane. There is a problem that the performance of the fuel cell is reduced. In addition, when the fuel cell is operated for a long period of time, platinum (Pt) sintering occurs, resulting in a problem that performance is deteriorated.
本発明は、貴金属合金を用いた従来の電極触媒における上記課題に着目してなされたものであって、合金成分の溶出や貴金属成分のシンタリングを防止して、高い酸素還元活性を長期に亘って維持することができる燃料電池用空気極触媒と、その製造方法を提供することを目的としている。 The present invention has been made by paying attention to the above-mentioned problems in conventional electrode catalysts using noble metal alloys, and prevents elution of alloy components and sintering of noble metal components and provides high oxygen reduction activity over a long period of time. It is an object of the present invention to provide a fuel cell air electrode catalyst that can be maintained and a method for producing the same.
本発明の燃料電池用空気極触媒は、貴金属に少なくとも１種の格子定数調整金属を添加した合金から成ると共に、導電性担体に担持された状態の貴金属合金電極触媒であって、その表面の一部が当該電極触媒を構成する貴金属合金に含まれている格子定数調整金属のうちの少なくとも１種の金属の酸化物で被覆されている構成としたことを特徴としている。 The air electrode catalyst for a fuel cell according to the present invention is a noble metal alloy electrode catalyst made of an alloy obtained by adding at least one kind of lattice constant adjusting metal to a noble metal and supported on a conductive carrier. The portion is coated with an oxide of at least one of the lattice constant adjusting metals contained in the noble metal alloy constituting the electrode catalyst.
また、本発明の燃料電池用空気極触媒の製造方法においては、合金化熱処理によって格子定数調整金属の一部を貴金属に固溶させる第１の工程と、当該合金化熱処理によって固溶せずに残存した格子定数調整金属を酸化処理することによって当該格子定数調整金属の酸化物を形成する第２の工程を含む構成としたことを特徴としている。 In the method for producing an air electrode catalyst for a fuel cell according to the present invention, the first step in which a part of the lattice constant adjusting metal is dissolved in a noble metal by alloying heat treatment, and the alloying heat treatment does not cause solid solution. It is characterized by including a second step of forming an oxide of the lattice constant adjusting metal by oxidizing the remaining lattice constant adjusting metal.
そして、本発明の燃料電池は、固体高分子形燃料電池あるいはリン酸形燃料電池であって、上記した本発明の燃料電池用空気極触媒を備えたことを特徴としている。 The fuel cell of the present invention is a polymer electrolyte fuel cell or a phosphoric acid fuel cell, and is characterized by including the above-described fuel cell air electrode catalyst of the present invention.
以上説明したように、本発明の燃料電池用空気極触媒は、導電性担体の上に、貴金属と格子定数調整金属との合金からなる触媒粒子が担持され、この貴金属合金電極触媒粒子の表面にこの触媒粒子に含まれる合金成分である格子定数調整金属の酸化物を付着させて、その表面を部分的に被覆したものであるから、合金成分の溶出や貴金属成分のシンタリングが抑制され、高い酸素還元活性を長期に亘って維持することができるという極めて優れた効果をもたらすものである。 As described above, in the fuel cell air electrode catalyst of the present invention, catalyst particles made of an alloy of a noble metal and a lattice constant adjusting metal are supported on a conductive support, and the surface of the noble metal alloy electrode catalyst particles is supported. Since the oxide of the lattice constant adjustment metal, which is an alloy component contained in the catalyst particles, is attached and the surface is partially covered, the elution of the alloy component and the sintering of the noble metal component are suppressed, which is high. This brings about an extremely excellent effect that the oxygen reduction activity can be maintained over a long period of time.
また、本発明による燃料電池用空気極触媒の製造方法は、合金化熱処理によって格子定数調整金属の一部を貴金属に固溶させる第１の工程と、固溶せずに残存した格子定数調整金属を酸化処理することによって当該格子定数調整金属の酸化物を形成する第２の工程を含むものであるから、貴金属中に含まれる格子定数調整金属成分と表面に形成されたその酸化物とが連通し、貴金属合金の表面に合金成分の酸化物が密着した燃料電池用空気極触媒を確実に得ることができる。 The method for producing an air electrode catalyst for a fuel cell according to the present invention includes a first step in which a part of a lattice constant adjusting metal is dissolved in a noble metal by alloying heat treatment, and a lattice constant adjusting metal remaining without being dissolved. Since the second step of forming an oxide of the lattice constant adjusting metal by oxidizing is performed, the lattice constant adjusting metal component contained in the noble metal communicates with the oxide formed on the surface, A fuel cell air electrode catalyst in which the oxide of the alloy component is in close contact with the surface of the noble metal alloy can be obtained with certainty.
本発明の燃料電池用空気極触媒は、図１に示すように、導電性担体１の上に、貴金属と格子定数調整金属との合金からなる触媒粒子２が担持され、この貴金属合金電極触媒粒子２の表面には、当該触媒粒子２に含まれる合金成分である格子定数調整金属の酸化物３が付着しており、触媒粒子２の表面を部分的に被覆している。
なお、触媒粒子２に含まれる格子定数調整金属は１種類のみに限定されず、複数種の格子定数調整金属が合金成分として添加されている場合には、その酸化物３には、必ずしも全ての合金成分の酸化物が含まれている必要はなく、これら複数の格子定数調整金属のうちの少なくとも１種の金属の酸化物が含まれていればよい。
As shown in FIG. 1, the air electrode catalyst for a fuel cell of the present invention carries catalyst particles 2 made of an alloy of a noble metal and a lattice constant adjusting metal on a conductive support 1, and the noble metal alloy electrode catalyst particles. On the surface of 2, an oxide 3 of a lattice constant adjusting metal, which is an alloy component contained in the catalyst particle 2, adheres and partially covers the surface of the catalyst particle 2.
In addition, the lattice constant adjusting metal contained in the catalyst particle 2 is not limited to only one type. When a plurality of types of lattice constant adjusting metals are added as alloy components, not all of the oxides 3 are necessarily included in the oxide 3. It is not necessary for the oxide of the alloy component to be included, and it is sufficient that the oxide of at least one of the plurality of lattice constant adjusting metals is included.
このような構造を有する燃料電池用空気極触媒において、触媒粒子２を構成する貴金属としては、例えばＰｔ、Ｐｄ、Ｉｒ又はＡｇを単独で、あるいはこれらの２種以上を任意に組合わせた合金として使用することができる。
これら貴金属は、担体あるいは、合金として高い酸素還元活性を有し、酸素還元活性の高い電極触媒を得ることができる。これらの貴金属のうち、貴金属単体については、特に、Ｐｔ、Ｐｄ、Ｉｒなどが硫酸水溶液中での酸素還元活性に関して高い活性を示す。また、これらＰｔ、Ｐｄ、Ｉｒをベースとした貴金属合金についても高い酸素還元活性を示す。
In the fuel cell air electrode catalyst having such a structure, as the noble metal constituting the catalyst particle 2, for example, Pt, Pd, Ir, or Ag is used alone, or an alloy in which two or more of these are arbitrarily combined. Can be used.
These noble metals have high oxygen reduction activity as a support or alloy, and an electrode catalyst having high oxygen reduction activity can be obtained. Among these noble metals, as for the noble metal simple substance, in particular, Pt, Pd, Ir, etc. show high activity regarding oxygen reduction activity in sulfuric acid aqueous solution. In addition, these noble metal alloys based on Pt, Pd, and Ir also show high oxygen reduction activity.
また、触媒粒子２を構成する上記貴金属に添加される格子定数調整金属としては、貴金属の格子定数を小さくする方向に調整する作用を有する金属であれば特に限定されず、例えば、Ｔｉ、Ｖ、Ｃｒ、Ｃｏ、Ｇａ、Ｗ、Ｙ、Ｚｒ、Ｎｂ、Ｍｏ又はＩｒを単独で、あるいはこれら金属の２種以上を任意に組合わせて使用することができる。
これらの金属元素を貴金属と合金化することによって、貴金属の格子定数を小さくし、さらに酸素還元活性を向上することができ、酸素還元活性の高い電極触媒を得ることができる。なお、Ｉｒについては、上記の貴金属中にＩｒが含まれる場合には格子定数調整金属としての選択肢群から除外されることは言うまでもない。
Further, the lattice constant adjusting metal added to the noble metal constituting the catalyst particle 2 is not particularly limited as long as it has a function of adjusting the noble metal lattice constant in the direction of decreasing, for example, Ti, V, Cr, Co, Ga, W, Y, Zr, Nb, Mo, or Ir can be used alone, or two or more of these metals can be used in any combination.
By alloying these metal elements with a noble metal, the lattice constant of the noble metal can be reduced, the oxygen reduction activity can be improved, and an electrode catalyst having a high oxygen reduction activity can be obtained. Needless to say, when Ir is contained in the above-mentioned noble metal, Ir is excluded from the option group as a lattice constant adjusting metal.
なお、貴金属、例えばＰｔに格子定数調整金属を固溶させることによる酸素還元活性向上メカニズムについては、必ずしも明らかになっていないが、Ｐｔに格子定数調整金属が固溶することによって格子定数が小さくなると、Ｐｔ−Ｐｔ間距離が短くなるため、酸素分子の触媒表面へのｓｉｄｅ−ｏｎ吸着が促進され、酸素の水への還元が促進されるものと考えられる。この場合ｓｉｄｅ−ｏｎ吸着が起こり、酸素の水への還元が促進されるためには、Ｐｔ−Ｐｔ間距離が酸素のＯ＝Ｏ結合距離に近くなるほど良いとされており、Ｐｔ−Ｐｔ間距離はＯ＝Ｏ結合距離と比較してかなり大きいので、Ｐｔ−Ｐｔ間距離を格子定数調整金属との合金化により短くすることによって、酸素還元活性を向上させることができる。
さらにＰｔ以外のＰｄ、Ｉｒなどの貴金属をベースとし、格子定数調製金属を合金化させた合金触媒の酸素還元電極触媒の場合にも同様な機構で活性が向上すると考えられる。
The mechanism for improving oxygen reduction activity by dissolving a lattice constant adjusting metal in a noble metal such as Pt is not necessarily clarified. However, when the lattice constant is reduced by dissolving the lattice constant adjusting metal in Pt, Since the distance between Pt and Pt is shortened, it is considered that side-on adsorption of oxygen molecules on the catalyst surface is promoted and reduction of oxygen to water is promoted. In this case, side-on adsorption occurs, and in order to promote the reduction of oxygen to water, it is considered that the Pt-Pt distance is closer to the O = O bond distance of oxygen, and the Pt-Pt distance. Is considerably larger than the O = O bond distance, and therefore the oxygen reduction activity can be improved by shortening the Pt-Pt distance by alloying with a lattice constant adjusting metal.
Furthermore, it is considered that the activity is improved by a similar mechanism in the case of an oxygen reduction electrode catalyst of an alloy catalyst based on a noble metal such as Pd or Ir other than Pt and alloyed with a lattice constant adjusted metal.
そして、上記触媒粒子２を担持する導電性担体１としては、カーボンブラックなどの炭素質材料を用いるのが一般的であるが、この他には、十分な電子導電率を有し（およそ０．０１Ｓ／ｃｍ以上）、触媒微粒子を分散担持するために十分な比表面積を有するもので（およそ３０ｍ２／ｇ以上）、なおかつ燃料電池の作動環境下で急速な劣化を示すことのない材料であれば利用することが可能であり、グラファイト、非晶質炭素、導電性金属酸化物、導電性金属窒化物、導電性金属炭化物などが例示できる。 In general, a carbonaceous material such as carbon black is used as the conductive carrier 1 that supports the catalyst particles 2. However, in addition to this, the conductive carrier 1 has a sufficient electronic conductivity (approximately 0. 01 S / cm or more), a material having a specific surface area sufficient for dispersing and supporting catalyst fine particles (approximately 30 m 2 / g or more), and a material that does not show rapid deterioration under the operating environment of the fuel cell. Examples thereof include graphite, amorphous carbon, conductive metal oxide, conductive metal nitride, and conductive metal carbide.
このような構造を備えた本発明の燃料電池用空気極触媒においては、貴金属合金の表面が部分的に金属酸化物で覆われていることから、合金中の格子定数調整金属成分の溶出が抑制され、合金の活性が保たれると同時に金属酸化物による活性向上効果によって、さらに酸素還元活性が向上する。また、酸化物により表面の一部が保護されていることにより、燃料電池運転中の貴金属合金のシンタリングが抑えられる。そして、表面の酸化物は、貴金属中に固溶している合金成分と共通であって、貴金属合金との密着性が高いため長期安定性に優れ、高い酸素還元活性が長期間に亘って維持されることになる。
なお、金属酸化物による酸素還元活性の向上メカニズムについても、必ずしも明らかではないが、金属酸化物から高活性な吸着酸素が貴金属合金に供給されるため、活性が向上するものと考えられている。
In the air electrode catalyst for fuel cells of the present invention having such a structure, the surface of the noble metal alloy is partially covered with the metal oxide, so that the elution of the lattice constant adjusting metal component in the alloy is suppressed. In addition, the activity of the alloy is maintained, and at the same time, the oxygen reduction activity is further improved by the activity improvement effect of the metal oxide. Further, since a part of the surface is protected by the oxide, sintering of the noble metal alloy during operation of the fuel cell can be suppressed. And the oxide on the surface is the same as the alloy component that is dissolved in the noble metal, and has high adhesion to the noble metal alloy, so it has excellent long-term stability and maintains high oxygen reduction activity over a long period of time. Will be.
In addition, although the improvement mechanism of the oxygen reduction activity by a metal oxide is not necessarily clear, it is considered that the activity is improved because highly active adsorbed oxygen is supplied from the metal oxide to the noble metal alloy.
また、電気化学的酸素還元反応は、主に貴金属合金表面に酸素が供給されることにより進行し、合金表面に存在する格子定数調整金属酸化物は、多孔質であっても緻密であっても差し支えない。 The electrochemical oxygen reduction reaction proceeds mainly by supplying oxygen to the surface of the noble metal alloy, and the lattice constant adjusting metal oxide existing on the alloy surface may be porous or dense. There is no problem.
本発明の燃料電池用空気極触媒においては、電極触媒を構成する貴金属合金中における格子定数調整金属の固溶率を５〜６０原子％の範囲とすることが望ましい。すなわち、格子定数調整金属の固溶率が５原子％に満たない場合には、このような第２成分を添加した効果が現れ難く、貴金属単独の電極触媒並みの活性しか得られず、６０原子％を超えた場合には、貴金属量が相対的に減少することによって貴金属触媒としての活性が失われ、貴金属単独の電極触媒よりもむしろ活性が低下する傾向があることによる。
なお、格子定数調整金属の「固溶率」とは、貴金属成分に対して固溶した格子定数調整金属成分の原子％で定義され、この固溶率は、Ｘ線回折により格子定数を求めて、Ｖｅｇａｒｄ則にあわせて判断する。
In the air electrode catalyst for a fuel cell of the present invention, it is desirable that the solid solution rate of the lattice constant adjusting metal in the noble metal alloy constituting the electrode catalyst is in the range of 5 to 60 atomic%. That is, when the solid solution ratio of the lattice constant adjusting metal is less than 5 atomic%, the effect of adding such a second component hardly appears, and only the activity equivalent to the electrode catalyst of noble metal alone can be obtained. In the case of exceeding%, the activity as a noble metal catalyst is lost due to a relative decrease in the amount of noble metal, and the activity tends to decrease rather than the electrode catalyst of noble metal alone.
The “solid solution rate” of the lattice constant adjusting metal is defined by atomic% of the lattice constant adjusting metal component dissolved in the noble metal component, and this solid solution rate is obtained by obtaining the lattice constant by X-ray diffraction. Judgment according to the Vegard law.
また、添加する格子定数調整金属のうちの１０〜８０原子％を貴金属に固溶させ、残りの９０〜２０原子％を酸化物とすることが望ましく、これによって、貴金属合金触媒の高い活性が長期間維持されることになる。
すなわち、固溶量が添加した格子定数調整金属のうちの１０原子％未満であると、表面酸化物の割合が多くなり、合金触媒の表面が完全に被覆されてしまいために合金触媒表面の酸素還元活性サイトが減少し、８０原子％を超えると、表面酸化物が少なくなるため、溶出抑制および保護効果が十分に得られなくなる傾向があることによる。ここで、固溶量とは、添加した全格子定数調整金属成分のうち貴金属に固溶した割合（原子％）で定義される。
Further, it is desirable that 10 to 80 atomic% of the added lattice constant adjusting metal is dissolved in the noble metal, and the remaining 90 to 20 atomic% is made into an oxide, whereby the high activity of the noble metal alloy catalyst is long. The period will be maintained.
That is, when the amount of the solid solution is less than 10 atomic% of the added lattice constant adjusting metal, the ratio of the surface oxide is increased and the surface of the alloy catalyst is completely covered. When the reduction active site decreases and exceeds 80 atomic%, the surface oxide is decreased, and thus the elution suppression and protection effects tend not to be sufficiently obtained. Here, the solid solution amount is defined as the ratio (atomic%) in which the total lattice constant adjusting metal component is dissolved in the noble metal.
また、本発明の上記燃料電池用電極触媒は、合金化熱処理によって格子定数調整金属の一部を貴金属に固溶させる第１の工程と、固溶せずに残存した格子定数調整金属を酸化処理することによって当該格子定数調整金属の酸化物を形成する第２の工程を含む工程によって製造することができる。
このような工程において、貴金属に固溶した格子定数調整金属のうち、合金表面近傍のものは酸化され、表面の金属酸化物と連通するため、表面を被覆する金属酸化物は貴金属合金触媒粒子表面に好適に保持されることになる。そして、貴金属合金の表面の一部が金属酸化物に被覆されることによって、合金成分の溶出が抑制され、合金の活性が保たれ、同時に金属酸化物による活性向上効果によりさらに酸素還元活性が向上する。また、酸化物により表面の一部が保護されていることにより、燃料電池運転中の貴金属合金のシンタリングが抑えられることになる。
The electrode catalyst for a fuel cell according to the present invention includes a first step in which a part of a lattice constant adjusting metal is dissolved in a noble metal by alloying heat treatment, and an oxidation treatment of the lattice constant adjusting metal remaining without being dissolved. By doing so, it can be manufactured by a process including a second process of forming the oxide of the lattice constant adjusting metal.
In such a process, among the lattice constant adjusting metals dissolved in the noble metal, those near the alloy surface are oxidized and communicate with the metal oxide on the surface, so the metal oxide covering the surface is the surface of the noble metal alloy catalyst particle surface. It will be suitably held. And, by covering a part of the surface of the noble metal alloy with the metal oxide, the elution of the alloy components is suppressed, the activity of the alloy is maintained, and at the same time, the oxygen reduction activity is further improved by the activity improving effect by the metal oxide. To do. Further, since a part of the surface is protected by the oxide, sintering of the noble metal alloy during operation of the fuel cell can be suppressed.
このとき、第１の工程における合金化熱処理については、４００〜１０００℃の温度範囲で行うことが望ましく、これによって所望の固溶率を備えた合金が作製でき、高い活性を示す触媒が得られようになる。
すなわち、４００℃に満たない温度では格子定数調整金属の合金化が進まず、一方１０００℃を超えた場合には、貴金属合金の粒成長が進んで触媒粒子が粗大化するために、酸素還元活性が低下する傾向がある。なお、当該合金化熱処理は、窒素やアルゴンのような不活性雰囲気中、あるいはこのような不活性ガスに水素を含有させた還元雰囲気中で行うことがさらに望ましい。
At this time, the alloying heat treatment in the first step is desirably performed in a temperature range of 400 to 1000 ° C., whereby an alloy having a desired solid solution rate can be produced, and a catalyst exhibiting high activity can be obtained. It becomes like this.
That is, when the temperature is less than 400 ° C., the alloying of the lattice constant adjusting metal does not proceed. On the other hand, when the temperature exceeds 1000 ° C., the grain growth of the noble metal alloy proceeds and the catalyst particles become coarse. Tends to decrease. The alloying heat treatment is more preferably performed in an inert atmosphere such as nitrogen or argon, or in a reducing atmosphere in which hydrogen is contained in such an inert gas.
また、上記第２の工程における酸化処理、すなわち合金化熱処理によっても合金化されずに残存した格子定数調整金属の酸化処理については、０〜３００℃の温度範囲で行うことが望ましい。これは、０℃未満の処理温度では、格子定数調整金属の酸化速度が遅く、３００℃を超えた処理温度では、導電性担体として一般的に用いられるカーボンが燃焼してしまう傾向があることによる。 The oxidation treatment in the second step, that is, the oxidation treatment of the lattice constant adjusting metal remaining without being alloyed even by the alloying heat treatment, is preferably performed in a temperature range of 0 to 300 ° C. This is because, at a processing temperature of less than 0 ° C., the oxidation rate of the lattice constant adjusting metal is slow, and at a processing temperature exceeding 300 ° C., carbon generally used as a conductive carrier tends to burn. .
すなわち、Ｐｔ担持カーボンの熱重量−示差熱分析を行った結果、酸素含有雰囲気中では３２０℃付近でカーボンの燃焼が起こることが確認できた。なお、通常カーボンブラックのみの場合には、その燃焼温度は６００℃を超えるが、Ｐｔ担持カーボンの場合には、Ｐｔが燃焼触媒として作用するため、上記燃焼温度よりもかなり低温においてもカーボン担体の燃焼が起こるものと認められる。
したがって、第２の工程における酸化処理温度を上記範囲とすることにより、貴金属合金触媒やカーボン担体を変質させることなく、合金化されずに残存した格子定数調整金属成分のみを金属酸化物にすることができる。
That is, as a result of performing thermogravimetric-differential thermal analysis of Pt-supported carbon, it was confirmed that carbon combustion occurred in the vicinity of 320 ° C. in an oxygen-containing atmosphere. In the case of carbon black alone, the combustion temperature exceeds 600 ° C. However, in the case of Pt-supported carbon, Pt acts as a combustion catalyst. It is recognized that combustion occurs.
Therefore, by making the oxidation treatment temperature in the second step within the above range, only the lattice constant adjusting metal component remaining without being alloyed is made into a metal oxide without altering the noble metal alloy catalyst or the carbon support. Can do.
以下、本発明を実施例に基づいて具体的に説明する。なお、本発明は、これらの実施例のみに限定されることはない。また、当該実施例において、「％」は特記しない限り質量百分率を表わすものとする。 Hereinafter, the present invention will be specifically described based on examples. In addition, this invention is not limited only to these Examples. In the examples, “%” represents a mass percentage unless otherwise specified.
［１］実施例Ｉ
（発明例１）
導電性カーボンブラック（Ｃａｂｏｔ社製 ＶｕｌｃａｎＸＣ−７２）３ｇを０．４％のＰｔを含んだ塩化白金酸水溶液２５０ｇ中にホモジナイザを用いて十分に分散させた後、これにクエン酸ナトリウム３ｇを加え、還流反応装置を用いて８０℃に加熱し、Ｐｔの還元担持を行った。そして、室温まで放冷した後、Ｐｔが担持されたカーボンを濾別することによりＰｔ担持カーボンを得た。
このＰｔ担持カーボンを０．２２５ｇのコバルト（Ｃｏ）を含んだ塩化コバルト水溶液２００ｍｌ中によく分散させ、加熱攪拌により水分を蒸発させてＣｏを含浸させた。次いで、これを５％Ｈ２を含んだ窒素気流中に９００℃×４時間保持して合金化処理を行い、一旦室温まで試料温度を低下させた後、大気中で２００℃×２４時間焼成することによって、合金化せずに残ったＣｏ成分を酸化させ、酸化コバルトを形成した。
[1] Example I
(Invention Example 1)
After 3 g of conductive carbon black (Vulcan XC-72 manufactured by Cabot) was sufficiently dispersed in 250 g of an aqueous chloroplatinic acid solution containing 0.4% Pt using a homogenizer, 3 g of sodium citrate was added thereto, The mixture was heated to 80 ° C. using a reflux reactor to carry out reduction loading of Pt. And after standing to cool to room temperature, Pt carrying | support carbon was obtained by separating the carbon carrying Pt by filtration.
This Pt-supported carbon was well dispersed in 200 ml of an aqueous cobalt chloride solution containing 0.225 g of cobalt (Co), and water was evaporated by heating and stirring to impregnate Co. Next, this is held in a nitrogen stream containing 5% H 2 at 900 ° C. for 4 hours to perform alloying treatment, and once the sample temperature is lowered to room temperature, it is fired in the atmosphere at 200 ° C. for 24 hours. As a result, the Co component remaining without being alloyed was oxidized to form cobalt oxide.
このようにして調製したＰｔ−Ｃｏ合金−Ｃｏ３Ｏ４担持カーボンについて、Ｘ線回折の結果、Ｐｔ−Ｃｏ合金におけるＣｏの固溶率が２３原子％であって、全Ｃｏ成分においてＰｔに固溶したＣｏと、酸化物化したＣｏ成分の比（モル比）が４：６であると見積もられることが確認された。つまり、この実施例１に係る触媒においては、固溶率が２３原子％であり、固溶量が４０原子％である。 As a result of X-ray diffraction, the solid solution ratio of Co in the Pt—Co alloy was 23 atomic% with respect to the Pt—Co alloy—Co 3 O 4 supported carbon thus prepared. It was confirmed that the ratio (molar ratio) between the dissolved Co and the oxidized Co component was estimated to be 4: 6. That is, in the catalyst according to Example 1, the solid solution rate is 23 atomic% and the solid solution amount is 40 atomic%.
（比較例１）
格子定数調整金属としてのＣｏを添加することなく、上記実施例１と同様の方法によって、Ｐｔのみをカーボン担体に担持させ、比較例１の電極触媒を得た。
(Comparative Example 1)
Only Pt was supported on the carbon support by the same method as in Example 1 without adding Co as a lattice constant adjusting metal, and the electrode catalyst of Comparative Example 1 was obtained.
（比較例２）
発明例１と同様の方法により作製したＰｔ担持カーボンを０．１ｇのＣｏを含む塩化コバルト水溶液２００ｍｌ中によく分散させ、加熱攪拌により水分を蒸発させてＣｏを含浸させた。これを５％Ｈ２を含んだ窒素気流中に３５０℃×６時間保持して塩化コバルトの熱分解処理を行った。この電極触媒をＸ線回折により分析した結果、ＣｏのＰｔへの固溶は確認できなかった。この電極触媒を大気中で２００℃２４時間焼成した。再びＸ線回折により分析した結果、Ｃｏ成分は酸化されてＣｏ３Ｏ４になっていることがわかった。つまり、比較例２の電極触媒はカーボン担体上にＰｔ微粒子とＣｏ３Ｏ４が混合状態として担持されているといえる。
(Comparative Example 2)
Pt-supported carbon produced by the same method as in Invention Example 1 was well dispersed in 200 ml of a cobalt chloride aqueous solution containing 0.1 g of Co, and water was evaporated by heating and stirring to impregnate Co. This was held in a nitrogen stream containing 5% H 2 at 350 ° C. for 6 hours to perform a thermal decomposition treatment of cobalt chloride. As a result of analyzing this electrode catalyst by X-ray diffraction, solid solution of Co in Pt could not be confirmed. This electrode catalyst was calcined in the atmosphere at 200 ° C. for 24 hours. As a result of X-ray diffraction analysis again, it was found that the Co component was oxidized to Co 3 O 4 . That is, it can be said that the electrode catalyst of Comparative Example 2 carries Pt fine particles and Co 3 O 4 in a mixed state on a carbon support.
ここで、発明例１の電極触媒と、比較例２の電極触媒とは、前者が格子定数調製金属成分を一度に含浸させ、続いて合金化処理と酸化処理を行ったのに対し、後者は格子定数調製金属の合金化処理は行わず、それに続いて酸化物形成処理を行ったことにより、Ｐｔ合金を形成することなくＰｔと金属酸化物の混合状態である点で相違する。 Here, the electrode catalyst of Invention Example 1 and the electrode catalyst of Comparative Example 2 were impregnated with the lattice constant adjusting metal component at the same time, followed by alloying treatment and oxidation treatment, whereas the latter was The alloying treatment of the lattice constant adjusting metal is not performed, and the subsequent oxide forming treatment is different in that it is a mixed state of Pt and metal oxide without forming the Pt alloy.
（電極触媒の性能評価）
ＭＥＡ（Ｍｅｍｂｒａｎｅ Ｅｌｅｃｔｒｏｄｅ Ａｓｓｅｍｂｌｙ：膜−電極接合体）の作製については、以下のような手順で行った。
まず、カソードとして各発明例および比較例に係る電極触媒に精製水とイソプロピルアルコールを加え、さらには所定量のＮａｆｉｏｎ（登録商標）を含んだ溶液を加えてホモジナイザでよく分散させ、さらに脱泡操作を加えることによって触媒スラリーを作製した。これをガス拡散層（ＧＤＬ）であるカーボンペーパー（東レ製 ＴＧＰ−Ｈ）の片面にスクリーン印刷法によって所定量印刷し、６０℃で２４時間乾燥させた後、触媒層を塗布した面を電解質膜に合わせて１２０℃、０．２ＭＰａで、３分間ホットプレスを行うことによって、それぞれのＭＥＡを作製した。
一方、アノードとしては同様な方法を用いて電極触媒として５０％Ｐｔ担持カーボンを用いてＭＥＡを作製した。
(Performance evaluation of electrode catalyst)
The production of MEA (Membrane Electrode Assembly) was performed by the following procedure.
First, as a cathode, purified water and isopropyl alcohol are added to the electrode catalysts according to the invention examples and comparative examples, and a solution containing a predetermined amount of Nafion (registered trademark) is added and dispersed well with a homogenizer. Was added to prepare a catalyst slurry. A predetermined amount is printed on one side of carbon paper (TGP-H manufactured by Toray Industries, Inc.), which is a gas diffusion layer (GDL), and dried at 60 ° C. for 24 hours, and then the surface on which the catalyst layer is applied is applied to the electrolyte membrane. Each MEA was produced by performing hot pressing at 120 ° C. and 0.2 MPa for 3 minutes.
On the other hand, an MEA was produced using 50% Pt-supported carbon as an electrode catalyst using the same method as the anode.
これらのＭＥＡは、アノード、カソードともにＰｔ使用量を見かけの電極面積１ｃｍ２あたり０．５ｍｇとし、電極面積は３００ｃｍ２とした。また、電解質膜としてＮａｆｉｏｎ１１２を用いた。 In these MEAs, the amount of Pt used for both the anode and the cathode was 0.5 mg per 1 cm 2 of the apparent electrode area, and the electrode area was 300 cm 2 . Further, Nafion 112 was used as the electrolyte membrane.
そして、このようにして形成された燃料電池単セルの性能測定を行った。測定に際しては、アノード側に燃料として水素を供給し、カソード側には空気を供給した。両ガスとも供給圧力は大気圧とし、水素は８０℃、空気は６０℃で飽和加湿し、燃料電池本体の温度は８０℃に設定し、水素利用率は７０％、空気利用率は４０％として、電流密度−セル電圧特性を調べた。その結果として、各実施例及び比較例に係る電極触媒を用いた単セルの質量活性を表１に示す。 And the performance measurement of the fuel cell single cell formed in this way was performed. In the measurement, hydrogen was supplied as fuel to the anode side, and air was supplied to the cathode side. Supply pressure for both gases is atmospheric pressure, hydrogen is 80 ° C, air is saturated and humidified at 60 ° C, fuel cell body temperature is set to 80 ° C, hydrogen utilization is 70%, and air utilization is 40%. The current density-cell voltage characteristics were examined. As a result, the mass activity of the single cell using the electrode catalyst according to each example and comparative example is shown in Table 1.
なお、表１において、「質量活性」とは燃料電池における電極触媒性能を示す指標のひとつであり、一般にセル電圧０．９ＶにおいてＰｔ１ｇあたりの電流値で定義される。つまり、質量活性の値が大きいほど高性能電極触媒といえる。 本発明の電極触媒を用いたセルと従来型の電極触媒を用いたセルの質量活性値を比較すると、従来のＰｔ単独の電極触媒（比較例１）よりも、格子定数調製金属成分を固溶させた合金触媒を含んだ電極触媒（発明例１）の方が高い活性を示すことが確認された。 In Table 1, “mass activity” is one of the indexes indicating the electrocatalytic performance of a fuel cell, and is generally defined as a current value per 1 g of Pt at a cell voltage of 0.9V. In other words, the higher the mass activity value, the higher the performance of the electrode catalyst. When the mass activity values of the cell using the electrode catalyst of the present invention and the cell using the conventional electrode catalyst are compared, the metal component of the lattice constant adjustment is more solid solution than the conventional Pt-only electrode catalyst (Comparative Example 1). It was confirmed that the electrode catalyst (Invention Example 1) containing the alloy catalyst thus produced showed higher activity.
次に、これら発明例及び比較例に係る電極触媒を用いた固体高分子電解質型燃料電池を０．５Ａ／ｃｍ２の一定電流密度で連続運転させた場合における各電池のセル電圧の変化を調査した。その結果を図２に示す。
図２から明らかなように、比較例１の電極触媒を用いた燃料電池においては、運転開始当初からセル電圧が低く、運転時間に対するセル電圧の低下が大きい。これに対し、発明例１の電極触媒を用いた燃料電池における初期のセル電圧は、両比較例に係る電極触媒を用いた燃料電池よりも高い値を示しており、運転時間の経過によるセル電圧の低下速度も低く、特に運転時間が長くなるほど比較例電極触媒を用いた燃料電池に較べて格段に高いセル電圧を示すことが確認された。
Next, the change in the cell voltage of each battery when the solid polymer electrolyte fuel cell using the electrode catalyst according to the inventive example and the comparative example is continuously operated at a constant current density of 0.5 A / cm 2 is investigated. did. The result is shown in FIG.
As is apparent from FIG. 2, in the fuel cell using the electrode catalyst of Comparative Example 1, the cell voltage is low from the beginning of operation, and the cell voltage is greatly reduced with respect to the operation time. On the other hand, the initial cell voltage in the fuel cell using the electrode catalyst of Invention Example 1 shows a higher value than the fuel cell using the electrode catalyst according to both comparative examples, and the cell voltage due to the passage of operating time. It was confirmed that the lowering rate of the battery was lower, and in particular, the longer the operation time, the higher the cell voltage compared to the fuel cell using the comparative example electrode catalyst.
この結果から、本発明の電極触媒を用いた電極では、触媒の合金化や金属酸化物の効果により電池効率が高くなるばかりでなく、格子定数調整金属の溶出や触媒粒子のシンタリングが抑制されたため、長時間の連続運転によっても性能の劣化が小さく抑えられたともの考えられる。
これに対し、比較例１の電極触媒においてはＰｔのシンタリングによりセル性能の劣化が進み、比較例２の電極触媒においてはセル性能の劣化速度は比較例１の電極触媒よりも低かったが、初期のセル性能が低いため十分な性能を示さなかった。
From this result, in the electrode using the electrode catalyst of the present invention, not only the battery efficiency is increased due to the alloying of the catalyst and the effect of the metal oxide, but also the elution of the lattice constant adjusting metal and the sintering of the catalyst particles are suppressed. For this reason, it is considered that the deterioration in performance is suppressed to a small extent even by continuous operation for a long time.
In contrast, in the electrocatalyst of Comparative Example 1, cell performance degradation progressed due to Pt sintering, and in the electrocatalyst of Comparative Example 2, the cell performance degradation rate was lower than that of the electrocatalyst of Comparative Example 1, Since the initial cell performance was low, it did not show sufficient performance.
［２］実施例II
（発明例２）
上記発明例１と同様の方法によって、Ｐｔのみをカーボン担体に担持させ、このＰｔ担持カーボン４ｇを０．５ｇのイリジウム（Ｉｒ）を含んだ塩化イリジウム水溶液２００ｍｌ中によく分散させ、加熱攪拌により水分を蒸発させてイリジウムを含浸させた。これを５％Ｈ２を含んだ窒素気流中で９００℃×２時間合金化処理を行い、一旦室温まで試料温度を低下させた後、大気中で２５０℃×２４時間焼成することにより、合金化せずに残ったイリジウム成分を酸化させ、酸化イリジウムを形成した。
[2] Example II
(Invention Example 2)
In the same manner as in Invention Example 1 above, only Pt was supported on a carbon carrier, 4 g of this Pt-supported carbon was well dispersed in 200 ml of an iridium chloride aqueous solution containing 0.5 g of iridium (Ir), and water was stirred by heating and stirring. Was evaporated and impregnated with iridium. This is alloyed by performing alloying treatment at 900 ° C. for 2 hours in a nitrogen stream containing 5% H 2 , once lowering the sample temperature to room temperature, and then firing in air at 250 ° C. for 24 hours. The remaining iridium component was oxidized to form iridium oxide.
このようにして調製したＰｔ−Ｉｒ合金−ＩｒＯ２担持カーボンについて、Ｘ線回折の結果、図３に示すようにＰｔ−Ｉｒ合金の形成とＩｒＯ２の生成が確認され、Ｐｔ−Ｉｒ合金におけるＩｒの固溶率は２０原子％であることがわかった。さらに、誘導結合プラズマ発光分光法によってＩｒ成分の定量分析を行った結果を考慮すると、全イリジウム成分においてＰｔに固溶したイリジウムと酸化物化したイリジウム成分の比（モル比）は、５４：４６であると見積もられる。つまり、この発明例２に係わる電極触媒におけるＰｔに対するＩｒの固溶率は２０原子％、全ＩｒのＰｔへの固溶量は５４原子％である。
なお、図３には、上記比較例１のＸ線回折の結果をも示しているが、当然のことながら、Ｐｔのピークしか認められない。
As a result of X-ray diffraction of the Pt—Ir alloy-IrO 2 -supported carbon prepared as described above, formation of Pt—Ir alloy and generation of IrO 2 were confirmed as shown in FIG. 3, and Ir in the Pt—Ir alloy was confirmed. The solid solution ratio of was found to be 20 atomic%. Furthermore, considering the result of quantitative analysis of the Ir component by inductively coupled plasma emission spectroscopy, the ratio (molar ratio) of iridium dissolved in Pt and oxidized iridium component in all iridium components is 54:46 It is estimated that there is. That is, the solid solution rate of Ir with respect to Pt in the electrode catalyst according to the inventive example 2 is 20 atomic%, and the solid solution amount of all Ir in Pt is 54 atomic%.
FIG. 3 also shows the result of X-ray diffraction of Comparative Example 1 above, but only the peak of Pt is recognized as a matter of course.
（比較例３）
発明例１と同様の方法により作製したＰｔ担持カーボン４ｇを０．３ｇのイリジウムを含んだ塩化イリジウム水溶液１５０ｍｌ中によく分散させ、加熱攪拌により水分を蒸発させてイリジウムを含浸させた。これを５％Ｈ２を含んだ窒素気流中に７００℃×４時間保持した後、続いて９５０℃×４時間合金化処理を行った。
このようにして調製したＰｔ担持カーボンについて、X線回折の結果、Ｐｔ−Ｉｒ合金の形成が確認され、当該Ｐｔ−Ｉｒ合金におけるＩｒの固溶率は２４原子％であることがわかった。
(Comparative Example 3)
4 g of Pt-supported carbon produced by the same method as in Invention Example 1 was well dispersed in 150 ml of an iridium chloride aqueous solution containing 0.3 g of iridium, and water was evaporated by heating and stirring to impregnate iridium. This was held in a nitrogen stream containing 5% H 2 at 700 ° C. for 4 hours, and then alloyed at 950 ° C. for 4 hours.
As a result of X-ray diffraction, the formation of a Pt—Ir alloy was confirmed for the Pt-supported carbon thus prepared, and it was found that the solid solution rate of Ir in the Pt—Ir alloy was 24 atomic%.
（電極触媒の性能評価・耐久性評価）
上記発明例２、比較例１及び比較例３に係わる電極触媒について、以下のような手法を用いて空気極触媒としての性能評価及び耐久性評価を行った。
(Performance evaluation and durability evaluation of electrode catalyst)
About the electrode catalyst concerning the said invention example 2, the comparative example 1, and the comparative example 3, the performance evaluation and durability evaluation as an air electrode catalyst were performed using the following methods.
まず、電極触媒の耐久性評価を行うために、電気化学セルを用いた評価を行った。金メッシュ（１００メッシュ、１０ｍｍ×１０ｍｍ）を集電体とし、この金メッシュに、発明例及び比較例に係わる上記電極触媒に精製水とイソプロピルアルコールを加え、さらには所定量のＮａｆｉｏｎを含んだ溶液を加えてホモジナイザでよく分散させ、さらに脱泡操作を加えることによって作製した触媒スラリーをマイクロピペットを用いて２０μｌ塗布し、室温で減圧乾燥後、１２０℃で乾燥させた。塗布触媒量は金メッシュの塗布前後の重量変化と触媒スラリーの組成から触媒重量を算出した。
触媒塗布後の金メッシュは直径０．５ｍｍの金線の先にスポット溶接により取り付け、これを試料極とし、従来の３電極型の電気化学セルを用いて室温（２２〜２５℃）で電流−電圧特性を測定した。対極としては白金黒、参照極としては水銀／硫酸水銀参照極を用い、電解液は０．５ｍｏｌ／Ｌの硫酸水溶液を使用した。電極触媒の性能評価として、酸素還元活性を評価した。この場合、電解液を酸素で３０分バブリングさせた後、酸素をそのまま電解液にバブリングさせたまま自然電位から０．８５Ｖ ｖｓ．ＳＨＥ（標準水素電極）まで１ｍＶ／ｓの速度で電位を掃引し、測定試料に含まれる白金の単位重量あたりの０．９０Ｖ ｖｓ．ＳＨＥにおける電流値を質量活性（Ａ／ｇ−Ｐｔ）と定義し、この値で電極触媒の酸素還元活性を評価した。
First, in order to evaluate durability of the electrode catalyst, evaluation using an electrochemical cell was performed. A solution containing a gold mesh (100 mesh, 10 mm × 10 mm) as a current collector, purified water and isopropyl alcohol added to the electrode catalyst according to the invention and the comparative example, and a solution containing a predetermined amount of Nafion. Was added and dispersed well with a homogenizer. Further, 20 μl of the catalyst slurry prepared by adding a defoaming operation was applied using a micropipette, dried at room temperature under reduced pressure, and then dried at 120 ° C. The amount of catalyst applied was calculated based on the weight change before and after the gold mesh coating and the composition of the catalyst slurry.
The gold mesh after the application of the catalyst is attached to the tip of a gold wire with a diameter of 0.5 mm by spot welding, and this is used as a sample electrode. Using a conventional three-electrode type electrochemical cell, current is applied at room temperature (22 to 25 ° C.). Voltage characteristics were measured. Platinum black was used as the counter electrode, a mercury / mercury sulfate reference electrode was used as the reference electrode, and a 0.5 mol / L sulfuric acid aqueous solution was used as the electrolyte. As the performance evaluation of the electrode catalyst, the oxygen reduction activity was evaluated. In this case, after bubbling the electrolytic solution with oxygen for 30 minutes, 0.85 V vs. natural potential from the natural potential while bubbling oxygen into the electrolytic solution as it is. The potential was swept up to SHE (standard hydrogen electrode) at a rate of 1 mV / s, and 0.90 V per unit weight of platinum contained in the measurement sample. The current value in SHE was defined as mass activity (A / g-Pt), and the oxygen reduction activity of the electrode catalyst was evaluated based on this value.
一方、耐久性の評価は以下のように行った。上記の性能評価装置をそのまま用いて測定試料に図４に示すような電位サイクル（０．６０Ｖ ｖｓ．ＳＨＥと１．０Ｖ ｖｓ．ＳＨＥの矩形波、それぞれの電位で１０秒保持）を印加し、電極触媒金属の劣化を促進させ、一定サイクル数を印加した後、上記の性能評価を行い、その前後での質量活性の低下幅から、電極触媒の劣化を評価し、耐久性の比較を行った。 On the other hand, the durability was evaluated as follows. Using the above performance evaluation apparatus as it is, a potential cycle (0.60 V vs. SHE and 1.0 V vs. SHE rectangular waves, held at respective potentials for 10 seconds) is applied to the measurement sample, After accelerating the degradation of the electrocatalyst metal and applying a certain number of cycles, the above performance evaluation was performed, and the degradation of the electrocatalyst was evaluated from the decrease in mass activity before and after that, and the durability was compared. .
表２は、各発明例及び比較例の電極触媒を用いた電解液による電気化学セルの質量活性を示している。「質量活性」は、酸素還元電極触媒性能を示す一つの指標であり、質量活性の値が大きいほど高性能電極触媒といえる。
発明例２の電極触媒と比較例１及び３の電極触媒の質量活性値を比較すると、従来タイプであるＰｔ単独の電極触媒（比較例１）よりも、イリジウムを固溶させた合金触媒を含んだ比較例３の電極触媒の方が高い活性を示し、合金化処理後に続いてさらに酸化処理を行うことにより酸化イリジウムが共存する発明例２の電極触媒がさらに高い活性値を示すことが確認された。
Table 2 shows the mass activity of the electrochemical cell by the electrolytic solution using the electrode catalyst of each invention example and comparative example. “Mass activity” is one index indicating the performance of the oxygen reduction electrocatalyst, and the higher the mass activity value, the higher the performance of the electrocatalyst.
When the mass activity values of the electrode catalyst of Invention Example 2 and the electrode catalysts of Comparative Examples 1 and 3 are compared, an alloy catalyst containing iridium as a solid solution is included rather than the conventional Pt-only electrode catalyst (Comparative Example 1). However, the electrocatalyst of Comparative Example 3 showed higher activity, and it was confirmed that the electrocatalyst of Invention Example 2 in which iridium oxide coexists showed a higher activity value by performing further oxidation treatment after alloying treatment. It was.
図５は、発明例２、比較例１及び比較例３の電極触媒について、電位サイクル数に対する質量活性値の低下率の変化を示すグラフであって、図に示すように、比較例１及び比較例３の電極触媒と比較して発明例２の電極触媒の質量活性低下速度は小さく、耐久性が大幅に改善されていることが確認された。 FIG. 5 is a graph showing the change in the rate of decrease in mass activity value with respect to the number of potential cycles for the electrode catalysts of Invention Example 2, Comparative Example 1 and Comparative Example 3, and as shown in FIG. Compared with the electrode catalyst of Example 3, the mass activity decrease rate of the electrode catalyst of Invention Example 2 was small, and it was confirmed that the durability was greatly improved.
以上の結果から、発明例２の電極では触媒の合金化や金属酸化物の効果により酸素還元活性が大幅に改善されたばかりでなく、格子定数調整金属の溶出や触媒粒子のシンタリングが抑制されたため、従来の電極触媒では性能低下が著しい電位サイクル試験においても性能の劣化が小さく抑えられたものと考えられる。 From the above results, in the electrode of Invention Example 2, not only the oxygen reduction activity was greatly improved by the alloying of the catalyst and the effect of the metal oxide, but also the elution of the lattice constant adjusting metal and the sintering of the catalyst particles were suppressed. Thus, it is considered that the degradation of the performance is suppressed to a small level even in the potential cycle test in which the performance degradation is significant in the conventional electrode catalyst.
１ 導電性担体
２ 触媒粒子（貴金属−格子定数調整金属合金）
３ 格子定数調整金属の酸化物
DESCRIPTION OF SYMBOLS 1 Conductive support | carrier 2 Catalyst particle (precious metal-lattice constant adjustment metal alloy)
3 Lattice constant adjusting metal oxide
Claims (9)
合金化熱処理によって格子定数調整金属の一部を貴金属に固溶させる第１の工程と、固溶せずに残存した格子定数調整金属を酸化処理することによって当該格子定数調整金属の酸化物を形成する第２の工程を含むことを特徴とする燃料電池用空気極触媒の製造方法。 A method for producing an air electrode catalyst for a fuel cell according to any one of claims 1 to 5,
A first step in which a part of the lattice constant adjusting metal is dissolved in the noble metal by alloying heat treatment, and an oxide of the lattice constant adjusting metal is formed by oxidizing the remaining lattice constant adjusting metal without being dissolved. The manufacturing method of the air electrode catalyst for fuel cells characterized by including the 2nd process to do.
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