JP2007027032A - Stainless steel separator for solid polymer type fuel cell, and fuel cell - Google Patents
Stainless steel separator for solid polymer type fuel cell, and fuel cell Download PDFInfo
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- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
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Abstract
Description
本発明は、低温稼動が可能で、メンテナンスも容易な固体高分子型燃料電池に組み込まれるステンレス鋼製セパレータ及びステンレス鋼製セパレータを組み込んだ固体高分子型燃料電池に関する。 The present invention relates to a stainless steel separator incorporated in a polymer electrolyte fuel cell that can be operated at a low temperature and is easy to maintain, and a polymer electrolyte fuel cell incorporating a stainless steel separator.
固体高分子型燃料電池は、１００℃以下の低温で稼動でき、短時間で起動する長所を備えている。しかも、各部材が固体からなる簡素な構造のため、メンテナンスが容易で、振動や衝撃に曝される用途にも適用できる。出力密度や燃料効率が高く、小型化に適し騒音の少ないことも有望な電力源と受け取られている所以である。
１セル当りの発電量が極僅かな燃料電池から実用に供せられる電力量を取り出すには、固体高分子膜をセパレータで挟んだセルを一単位とし、多数のセルをスタックする必要がある。固体高分子膜を挟むセパレータには導電性が良好で接触抵抗が低いことが要求されるので、従来から黒鉛質のセパレータが使用されている。
The polymer electrolyte fuel cell can be operated at a low temperature of 100 ° C. or less and has an advantage of starting in a short time. In addition, since each member is a simple structure made of a solid, maintenance is easy, and it can be applied to applications where it is exposed to vibration and impact. High power density and fuel efficiency, suitable for downsizing, and low noise are also regarded as promising power sources.
In order to extract the amount of electric power that can be put to practical use from a fuel cell with a very small amount of power generation per cell, it is necessary to stack a large number of cells with a unit of a solid polymer membrane sandwiched between separators. Since a separator sandwiching a solid polymer film is required to have good conductivity and low contact resistance, a graphite separator has been conventionally used.
黒鉛質セパレータは脆く、過度な振動や衝撃が加えられると割れやすいので、車両用電源としての適用には工夫を要する。脆くて加工性に劣る黒鉛を素材としているので、複雑形状品の作製に切削加工が余儀なくされ、厚肉製品にならざるを得ない。そのため、車載用途等で強く求められているコンパクト化が制約され、作製コストも高くなる。そこで、黒鉛に代えてステンレス鋼をセパレータ素材に使用することが検討されている(特許文献１，２)。
ステンレス鋼は、高強度で延性に優れているため薄肉化が可能で、プレス成形等の安価な加工法で目標形状に容易に成形できることが長所である。また、ステンレス鋼の構成成分であるＣｒ，Ｍｏ，Ｆｅ等の酸化物，水酸化物から形成される不動態皮膜で鋼板表面が覆われ、不動態皮膜のバリア効果で下地鋼の腐食が防止される。
不動態皮膜は、耐食性の向上に有効であるものの半導体的な特性を呈し、下地鋼に比較すると電気伝導性に劣っている。そのため、通常の不動態皮膜が生成しているステンレス鋼をセパレータに使用すると、電極との接触抵抗が大きく、電池反応で生じた電気エネルギーがジュール熱として消費され、燃料電池の発電効率が低下する。
Since stainless steel has high strength and excellent ductility, it can be thinned, and it can be easily formed into a target shape by an inexpensive processing method such as press molding. In addition, the steel plate surface is covered with a passive film formed from oxides and hydroxides of Cr, Mo, Fe, etc., which are constituents of stainless steel, and the barrier effect of the passive film prevents corrosion of the underlying steel. The
Although the passive film is effective in improving the corrosion resistance, it exhibits semiconducting properties and is inferior in electrical conductivity compared to the base steel. Therefore, when stainless steel with a normal passive film is used for the separator, the contact resistance with the electrode is large, the electric energy generated by the cell reaction is consumed as Joule heat, and the power generation efficiency of the fuel cell decreases. .
優れた耐食性を活用しながらステンレス鋼をセパレータに適用するには、ステンレス鋼表面の接触抵抗を下げる必要がある。接触抵抗の低減手段として、貴金属コーティング，ステンレス鋼板の粗面化等が検討されている。
しかし、高価な貴金属コーティングは、燃料電池のコストを上昇させ、経済面から燃料電池の普及に制約を加える。しかも、貴金属コーティングでは、孔食を引き起こすピンホールが皮膜に形成されやすいので厳重な製品管理が必要になる。厚膜化によってピンホールのない皮膜を形成できるが、厚膜の貴金属皮膜は高価な貴金属の多量消費を意味し、コスト低減のネックになる。
In order to apply stainless steel to the separator while utilizing excellent corrosion resistance, it is necessary to reduce the contact resistance of the stainless steel surface. As means for reducing contact resistance, precious metal coating, roughening of a stainless steel plate, and the like have been studied.
However, the expensive noble metal coating increases the cost of the fuel cell, and imposes restrictions on the spread of the fuel cell from the economical aspect. In addition, with noble metal coating, pinholes that cause pitting corrosion are likely to be formed in the film, so strict product management is required. Although a film without pinholes can be formed by increasing the film thickness, a thick noble metal film means a large consumption of expensive noble metal, and becomes a bottleneck in cost reduction.
ステンレス鋼の接触抵抗を下げる粗面化処理では、塩化第二鉄浴中での交番電解法が採用されている。しかし、電解処理であるため、大掛かりな設備を必要とする。
ショットブラスト，酸洗，エッチング等によるステンレス鋼の粗面化(特許文献３，４)も知られているが、そのために専用の工程を追加する必要があり製造コストが高くなる。
Although roughening of stainless steel by shot blasting, pickling, etching, etc. (Patent Documents 3 and 4) is also known, a dedicated process needs to be added for this purpose, resulting in an increase in manufacturing cost.
マトリックスにクロム系炭化物を析出させた後でショットブラスト，酸洗等によるクロム系炭化物の頭出し処理で接触抵抗を低減する方法(特許文献５)も知られている。この場合でも、クロム系炭化物を析出させる工程の追加が必要になる。しかも、クロム系炭化物の析出に伴い耐食性付与に必要なＣｒが消費され、燃料電池内の過酷な腐食環境下では耐食性が十分確保されない虞もある。
電解粗面化でステンレス鋼の接触抵抗を低減する方法も知られている(特許文献６)。電解粗面化でステンレス鋼表面に多数のピットが形成され、ピットの周縁に突起が林立した表面に改質される。林立した突起が電極と接触することで接触面積が増大し、接触抵抗が低減する。しかし、電解粗面化による凹凸はピッチが大きく、カーボンや金めっきなみの低接触抵抗を得るには不十分である。電解処理のため大掛かりな設備を必要とすることも、不利な点である。
ステンレス鋼製セパレータの接触抵抗がステンレス鋼の表面状態で大きく変わることは前述の通りであるが、カーボンペーパにセパレータを加圧接触させた固体高分子型燃料電池の使用を前提にすると、カーボンペーパに対するステンレス鋼表面のマッチング性も重要である。そこで、本発明者等は、単にRa，Rz等で表される表面粗さではなく、カーボンペーパを構成する微細なカーボン繊維との接触に有効な表面状態を探求した。その結果、非酸化性酸液を用いて浸漬処理されたステンレス鋼表面には極めて薄いが緻密な不動態皮膜が形成されており、該不動態皮膜を介してステンレス鋼にカーボンペーパを接触させると、従来にない低接触抵抗化が可能になることを見出した。
本発明は、かかる知見をベースとし、ステンレス鋼表面に極めて薄い不動態皮膜を形成することにより、耐食性確保，低接触抵抗化を両立させたステンレス鋼製セパレータを提供することを目的とする。
As described above, the contact resistance of a stainless steel separator varies greatly depending on the surface condition of the stainless steel. However, assuming the use of a polymer electrolyte fuel cell in which the separator is in pressure contact with carbon paper, the carbon paper The matching of the surface of the stainless steel with respect to is also important. Accordingly, the present inventors have sought a surface state effective for contact with fine carbon fibers constituting carbon paper, not simply the surface roughness represented by Ra, Rz or the like. As a result, a very thin but dense passive film is formed on the surface of the stainless steel immersed using the non-oxidizing acid solution. When carbon paper is brought into contact with the stainless steel through the passive film, The present inventors have found that it is possible to reduce the contact resistance, which is unprecedented.
An object of the present invention is to provide a stainless steel separator that achieves both corrosion resistance and low contact resistance by forming an extremely thin passive film on the stainless steel surface based on such knowledge.
本発明の固体高分子型燃料電池用ステンレス鋼製セパレータは、ステンレス鋼製であり、電極と接触するセパレータの表面部分で膜厚：４ｎｍ以下の不動態皮膜がステンレス鋼表面に形成されていることを特徴とする。不動態皮膜は、塩酸，硫酸等の非酸化性酸液を用いた浸漬処理で形成される。
なお、不動態皮膜の膜厚は、(1)未浸漬材，(2)接触抵抗の低下度が最も大きな浸漬時間で酸浸漬された材料，(3)更に長い浸漬処理で接触抵抗が増大した材料について不動態皮膜をTEM観察し、TEM観察の結果から求められた不動態皮膜の膜厚をＴ1とした。
The stainless steel separator for a polymer electrolyte fuel cell of the present invention is made of stainless steel, and a passive film having a film thickness of 4 nm or less is formed on the surface of the stainless steel at the surface portion of the separator in contact with the electrode. It is characterized by. The passive film is formed by a dipping process using a non-oxidizing acid solution such as hydrochloric acid or sulfuric acid.
The film thickness of the passive film was as follows: (1) unimmersed material, (2) material immersed in acid for the largest decrease in contact resistance, and (3) contact resistance increased with longer immersion treatment. the passive film then TEM observations for material, the thickness of the passive film obtained from the results of TEM observation were as T 1.
また、TEM観察で使用した材料と同じ試験片をGDSにかけ、不動態皮膜の厚みの指標となる酸素強度の半減位置までのスパッタ時間を測定した。スパッタ時間の測定値から求められる不動態皮膜の膜厚をＴ2とした。そして、Ｔ2をＴ1に一致させることにより、GDSによる膜厚測定結果から酸浸漬で異なる表面状態の影響を排除した。また、(1)−(2)間及び(2)−(3)間の任意浸漬時間における不動態皮膜の膜厚は、それぞれのGDSスパッタ時間をＴ1，Ｔ2で補間してＴ2を算出した。 In addition, the same specimen as the material used in TEM observation was subjected to GDS, and the sputtering time to the half-position of oxygen intensity, which is an index of the thickness of the passive film, was measured. The thickness of the passive film obtained from the measured values of the sputtering time was set to T 2. Then, by making T 2 coincide with T 1 , the influence of the different surface conditions by acid immersion was excluded from the film thickness measurement result by GDS. Further, (1) - a T 2 by interpolating the thickness of passive film is a respective GDS sputtering time T 1, T 2 in (3) any soaking time between - (2) and between (2) Calculated.
セパレータの素材には、フェライト系，オーステナイト系何れのステンレス鋼も使用できる。
フェライト系ステンレス鋼は、Ｃｒ：１０〜４０質量％(好ましくは、１５〜３５質量％)，Ｍｏ：０〜５質量％(好ましくは、１〜３質量％)含んでいる。Ｃｒは耐食性に必須の合金成分であり、低ｐＨ値で腐食性の強い燃料電池内の環境を想定すると１０質量％以上のＣｒが必要である。Ｃｒ含有量が多くなるほど耐食性が向上する反面、加工性が低下するので、上限を４０質量％とした。Ｍｏも耐食性に有効な成分であり、Ｃｒとの共存によって孔食を防ぐ作用を呈する。そのため、単にＭｏを増量するのではなく、Ｃｒ含有量と関連させてＭｏ含有量を調整する。しかし、過剰なＭｏ含有はステンレス鋼の硬質化，加工性低下をもたらすので、Ｍｏ含有量の上限を５質量％とした。
The separator material can be either ferritic or austenitic stainless steel.
Ferritic stainless steel contains Cr: 10-40 mass% (preferably 15-35 mass%), Mo: 0-5 mass% (preferably 1-3 mass%). Cr is an alloy component indispensable for corrosion resistance, and 10% by mass or more of Cr is necessary assuming an environment in a fuel cell having a low pH value and strong corrosivity. While the corrosion resistance improves as the Cr content increases, the workability decreases, so the upper limit was set to 40% by mass. Mo is also an effective component for corrosion resistance, and exhibits the effect of preventing pitting corrosion when coexisting with Cr. For this reason, the Mo content is not simply increased, but the Mo content is adjusted in relation to the Cr content. However, excessive Mo content causes hardening of stainless steel and deterioration of workability, so the upper limit of Mo content was set to 5 mass%.
Ｃｒ，Ｍｏ以外の成分としては、Ｃ，Ｎ，Ｍｎ，Ｓｉ，Ｐ，Ｓ，Ｎｉ，Ｃｕ，Ｔｉ，Ｎｂ，Ａｌ，Ｖ等が挙げられる。
Ｃ，Ｎはフェライト系ステンレス鋼の加工性，低温靭性を低下させるので可能な限り低減する必要があり、本成分系では上限を共に０.０２質量％とすることが好ましい。Ｍｎは、耐食性低下の原因となるので、好ましくは２.０質量％以下に規制する。Ｓｉは硬質化，加工性低下の原因となるので、好ましくは０.５質量％以下に規制する。Ｐは、セパレータが曝される燃料電池の内部環境における耐食性向上に有効な成分であるが過剰添加は加工性に悪影響を及ぼすので、Ｐを添加する場合には０.０３〜０.０８質量％の範囲でＰ含有量を選定する。Ｓは、加工性，耐食性に有害な成分であるので可能な限り低減する必要があり、本成分系では上限を０.００５質量％とした。
Examples of components other than Cr and Mo include C, N, Mn, Si, P, S, Ni, Cu, Ti, Nb, Al, and V.
C and N reduce the workability and low-temperature toughness of ferritic stainless steel, so it is necessary to reduce them as much as possible. In this component system, the upper limit is preferably set to 0.02% by mass. Since Mn causes a decrease in corrosion resistance, it is preferably regulated to 2.0% by mass or less. Since Si causes hardening and workability deterioration, it is preferably regulated to 0.5% by mass or less. P is an effective component for improving the corrosion resistance in the internal environment of the fuel cell to which the separator is exposed. However, excessive addition has an adverse effect on processability. Therefore, when P is added, 0.03 to 0.08 mass%. The P content is selected within the range. Since S is a component harmful to workability and corrosion resistance, it is necessary to reduce it as much as possible. In this component system, the upper limit is set to 0.005% by mass.
Ｎｉ，Ｃｕは、溶出しやすい元素であるので多量含有を避け、好ましくはＮｉ：０.５質量％，Ｃｕ：０.８質量％を上限とする。なかでも、溶出したＮｉイオンが触媒層に到達すると、触媒が被毒し電池性能が低下する。しかし、少量の添加は酸性雰囲気での耐全面腐食性を改善するので、添加する場合にはＮｉ：０.１５〜０.３５質量％，Ｃｕ：０.２０〜０.５０質量％の範囲で含有量を選定する。 Ni and Cu are elements that are easily eluted, so that they are not contained in large amounts, and preferably Ni: 0.5% by mass and Cu: 0.8% by mass are the upper limit. Among these, when the eluted Ni ions reach the catalyst layer, the catalyst is poisoned and the battery performance is deteriorated. However, addition of a small amount improves the general corrosion resistance in an acidic atmosphere, so when added, Ni: 0.15 to 0.35 mass%, Cu: 0.20 to 0.50 mass% Select the content.
鋼中のＣ，Ｎを固定し加工性を改善するＴｉ，Ｎｂを添加する場合、共に０.０３〜０.２５質量％の範囲で含有量を調整する。Ｎの固定にＡｌを使用する場合、０.０４〜０.２５質量％の範囲で含有量を調整する。燃料電池の内部環境における耐食性の改善に有効なＶは、必要に応じ０.２〜１.０質量％の範囲で添加される。更に特性を大きく変化させない限り、その他の合金成分を添加することも可能である。 When adding Ti and Nb for fixing C and N in steel and improving workability, the content is adjusted in the range of 0.03 to 0.25% by mass. When using Al for fixing N, the content is adjusted in the range of 0.04 to 0.25% by mass. V effective for improving the corrosion resistance in the internal environment of the fuel cell is added in the range of 0.2 to 1.0% by mass as necessary. Further, other alloy components can be added as long as the characteristics are not greatly changed.
オーステナイト系では、フェライト系と同様な理由からＣｒ：１０〜４０質量％(好ましくは、１５〜３５質量％)，Ｍｏ：０〜６質量％(好ましくは、５質量％以下)を含む成分系のステンレス鋼がセパレータ素材に使用される。オーステナイト系ステンレス鋼は、オーステナイト相を形成し、酸性雰囲気下での耐食性を付与するため少なくとも５.０質量％のＮｉを含んでいる。しかし、過剰量のＮｉは、コスト的に不利なだけではなく、含有Ｃｒの影響もあって加工性を低下させ、更には電池内部に溶出するＮｉイオン量の増加，ひいては触媒の劣化を促進させる原因となるので、３０質量％を上限とすることが好ましい。 In the austenite system, for the same reason as the ferrite system, a component system containing Cr: 10 to 40% by mass (preferably 15 to 35% by mass), Mo: 0 to 6% by mass (preferably 5% by mass or less) is used. Stainless steel is used for the separator material. The austenitic stainless steel contains at least 5.0% by mass of Ni in order to form an austenitic phase and impart corrosion resistance in an acidic atmosphere. However, an excessive amount of Ni is not only disadvantageous in terms of cost, but also deteriorates workability due to the influence of Cr contained, and further promotes an increase in the amount of Ni ions eluting into the battery, and thus deterioration of the catalyst. Since it becomes a cause, it is preferable to make 30 mass% into an upper limit.
Ｃｒ，Ｍｏ，Ｎｉ以外の成分として、Ｃ，Ｎ，Ｍｎ，Ｓｉ，Ｐ，Ｓ，Ｃｕ，Ｔｉ，Ｎｂ，Ａｌ，Ｖ，Ｂ等が挙げられる。
Ｃ，Ｎ，Ｍｎ，Ｓｉ，Ｐ，Ｓは、フェライト系と同様な理由からそれぞれＣ：０.０２質量％以下，Ｎ：０.０２質量％以下，Ｍｎ：２.０質量％以下，Ｓｉ：０.５質量％以下，Ｐ：０.０３〜０.０８質量％以下，Ｓ：０.００５質量％以下とする。
Examples of components other than Cr, Mo, and Ni include C, N, Mn, Si, P, S, Cu, Ti, Nb, Al, V, and B.
C, N, Mn, Si, P and S are respectively C: 0.02 mass% or less, N: 0.02 mass% or less, Mn: 2.0 mass% or less, Si: 0.5 mass% or less, P: 0.03 to 0.08 mass% or less, S: 0.005 mass% or less.
Ｃｕは、必要に応じて添加される成分であり、酸性雰囲気下の耐全面腐食性を改善し、オーステナイト系ステンレス鋼の低温靭性を向上させる作用を呈する。しかし、溶出しやすい元素であるので過剰添加を避け、添加する場合も上限を６質量％とすることが好ましい。 Cu is a component added as necessary, and improves the overall corrosion resistance in an acidic atmosphere and exhibits the effect of improving the low temperature toughness of the austenitic stainless steel. However, since it is an element that easily elutes, it is preferable to avoid excessive addition, and when adding it, the upper limit is preferably 6% by mass.
必要に応じＴｉ，Ｎｂを添加して鋼中Ｃ，Ｎを固定すると、オーステナイト系ステンレス鋼の加工性が向上する。しかし、過剰添加は却って鋼材の硬質化を招くので、添加する場合には共に上限を１.０質量％とする。Ｎの固定にＡｌを使用する場合、０.０１〜３.０質量％の範囲で含有量を調整する。燃料電池の内部環境における耐食性の改善に有効なＶは、必要に応じ０.０１〜１.０質量％の範囲で添加される。更に特性を大きく変化させない限り、その他の合金成分を添加することも可能である。 If Ti and Nb are added as necessary to fix C and N in the steel, the workability of the austenitic stainless steel is improved. However, excessive addition leads to hardening of the steel material, so when adding, the upper limit is 1.0 mass%. When Al is used for fixing N, the content is adjusted in the range of 0.01 to 3.0% by mass. V effective in improving the corrosion resistance in the internal environment of the fuel cell is added in the range of 0.01 to 1.0% by mass as necessary. Further, other alloy components can be added as long as the characteristics are not greatly changed.
所定組成に調整されたステンレス鋼を酸液に浸漬すると、鋼板表面が全面的に溶解し新生面が露出する。浸漬処理に非酸化性酸液を使用すると、表面の変質層が除去され、薄く緻密な不動態皮膜が形成される。非酸化性酸液を用いた浸漬処理で形成される不動態皮膜は、硝酸，濃硫酸，王水等の酸化性酸液で形成される不動態皮膜に比較して、Ｃｒ濃縮に伴う膜厚増加の抑制された皮膜になるので、耐食性向上，接触抵抗低減の双方が達成される。 When stainless steel adjusted to a predetermined composition is immersed in an acid solution, the surface of the steel plate is completely dissolved and the new surface is exposed. When a non-oxidizing acid solution is used for the immersion treatment, the altered layer on the surface is removed, and a thin and dense passive film is formed. Passive film formed by immersion treatment using non-oxidizing acid solution is a film thickness accompanying Cr concentration compared to passive film formed by oxidizing acid solution such as nitric acid, concentrated sulfuric acid, aqua regia, etc. Since the coating is suppressed from increasing, both corrosion resistance improvement and contact resistance reduction are achieved.
浸漬処理では、ステンレス鋼の種類に応じて酸の種類，濃度，温度，浸漬時間等の処理条件が選定される。たとえば、３０Ｃｒ-２Ｍｏ鋼では、濃度：１０〜２０質量％，液温：４０〜６０℃の塩酸浴に１〜１０分浸漬する条件が採用される。硫酸を使用する場合、濃度：１０〜２０質量％，液温：５０〜８０℃の硫酸浴にステンレス鋼を０.５〜２０分浸漬する。 In the immersion treatment, treatment conditions such as acid type, concentration, temperature, and immersion time are selected according to the type of stainless steel. For example, in 30Cr-2Mo steel, conditions of immersing in a hydrochloric acid bath having a concentration of 10 to 20% by mass and a liquid temperature of 40 to 60 ° C. for 1 to 10 minutes are employed. When sulfuric acid is used, stainless steel is immersed in a sulfuric acid bath having a concentration of 10 to 20% by mass and a liquid temperature of 50 to 80 ° C. for 0.5 to 20 minutes.
３０Ｃｒ-２Ｍｏ鋼等の高耐食性フェライト系ステンレス鋼を浸漬処理する場合、Ｍｏが最も溶解しにくく、酸浸漬の時間経過に伴って不動態皮膜の表層部に濃化し、不動態皮膜が厚膜化する。Ｍｏ濃化は、燃料電池の内部環境(酸性の湿潤雰囲気)においてステンレス鋼の腐食を抑制し、接触抵抗の上昇を抑制する。Ｍｏ濃化により耐食性が改善されるので、不動態皮膜を薄くしても燃料電池内の腐食雰囲気に十分耐える特性が付与される。 When immersing high corrosion resistance ferritic stainless steel such as 30Cr-2Mo steel, Mo is the least soluble and thickens in the surface layer of the passive film as the acid immersion time elapses. To do. Mo enrichment suppresses corrosion of stainless steel in the internal environment (acidic humid atmosphere) of the fuel cell and suppresses an increase in contact resistance. Since the corrosion resistance is improved by the Mo concentration, even if the passive film is thinned, the characteristic of sufficiently withstanding the corrosive atmosphere in the fuel cell is imparted.
不動態皮膜の薄膜化が可能なことは、接触抵抗の低減にも有効である。一般的に不動態皮膜が薄くなるほどステンレス鋼表面を流れる電流が多くなり接触抵抗の低下傾向がみられるが、膜厚が４ｎｍ以下になると接触抵抗が格段に低下する。これは、膜厚：４ｎｍ以下でトンネル効果が顕著になり、ステンレス鋼表面の見かけ上電気伝導性が高くなったことに由来すると考えられる。実際、膜厚：４ｎｍ以下の不動態皮膜が形成されたステンレス鋼では１０ｍΩ・ｃｍ2以下の低い接触抵抗を示す。他方、不動態皮膜の膜厚が４ｎｍを超えるようになると接触抵抗が急激に上昇し、トンネル効果に起因すると思われる高い電気伝導性(低い接触抵抗)が現れない。このようにして接触抵抗を低減したステンレス鋼をセパレータに使用すると、電池反応で生じた電気エネルギーがジュール熱として消費される割合が少なくなり、発電効率の高い燃料電池が得られる。 The ability to reduce the thickness of the passive film is effective in reducing contact resistance. Generally, the thinner the passive film, the more current flowing on the surface of the stainless steel and the lowering of the contact resistance is observed. However, when the film thickness is 4 nm or less, the contact resistance is remarkably reduced. This is considered to be because the tunnel effect becomes remarkable when the film thickness is 4 nm or less, and the apparent electrical conductivity of the stainless steel surface is increased. Actually, stainless steel having a passive film with a film thickness of 4 nm or less exhibits a low contact resistance of 10 mΩ · cm 2 or less. On the other hand, when the thickness of the passive film exceeds 4 nm, the contact resistance rapidly increases, and the high electrical conductivity (low contact resistance) that seems to be caused by the tunnel effect does not appear. When stainless steel with reduced contact resistance is used for the separator in this way, the rate at which the electric energy generated by the cell reaction is consumed as Joule heat decreases, and a fuel cell with high power generation efficiency is obtained.
フェライト系ステンレス鋼(３０Ｃｒ-２Ｍｏ，２２Ｃｒ-１.２Ｍｏ，１８Ｃｒ-２Ｍｏ，１８Ｃｒ-１Ｍｏ)をセパレータ素材に使用した。
濃度：５質量％，液温：６０℃のオルソケイ酸ソーダ溶液にステンレス鋼を１０分間浸漬して脱脂した後、濃度：１０質量％，液温：５０℃の塩酸浴で浸漬処理した。浸漬処理後、直ちに水洗，乾燥した。なお、表面形態が接触抵抗に及ぼす影響を調査するため、塩酸浴への浸漬時間を種々変更した。
Ferritic stainless steel (30Cr-2Mo, 22Cr-1.2Mo, 18Cr-2Mo, 18Cr-1Mo) was used as a separator material.
The stainless steel was immersed in a sodium orthosilicate solution having a concentration of 5% by mass and a liquid temperature of 60 ° C. for 10 minutes to degrease, and then immersed in a hydrochloric acid bath having a concentration of 10% by mass and a liquid temperature of 50 ° C. Immediately after the immersion treatment, it was washed with water and dried. In order to investigate the influence of the surface form on the contact resistance, the immersion time in the hydrochloric acid bath was variously changed.
浸漬処理前後のステンレス鋼表面にある不動態皮膜を次の二法で算出した。
〔TEM観察結果から膜厚の算出〕
電界放射型透過電子顕微鏡(JEM-2010F：日本電子製)を用いてステンレス鋼表面を観察し、倍率：１９０万倍，明視野のSTEM像について不動態皮膜の厚みを計測した。計測点を任意に三箇所選定し、各計測点における計測結果を平均化して不動態皮膜の膜厚とした。求められた膜厚をＴ1で表す。
The passive film on the stainless steel surface before and after the immersion treatment was calculated by the following two methods.
[Calculation of film thickness from TEM observation results]
The surface of the stainless steel was observed with a field emission transmission electron microscope (JEM-2010F: manufactured by JEOL Ltd.), and the thickness of the passive film was measured for a STEM image with a magnification of 1.9 million. Three measurement points were arbitrarily selected, and the measurement results at each measurement point were averaged to obtain the film thickness of the passive film. The obtained film thickness expressed by T 1.
〔GDS分析結果から膜厚の算出〕
浸漬処理材をグロー放電発光分光法分析(GDS)にかけ、放電電力：３０Ｗ，Ａｒガス流量：１００ｃｃ／分，アノード径：４ｍｍの条件下でステンレス鋼の表面から深さ方向に沿った元素分布を求めた(図１)。不動態皮膜／基材の境界は、酸素の濃度分布を示す曲線上で(ピーク強度−バックグラウンド強度)／２の強度が示されるステンレス鋼表面からの深さ(換言すれば、ステンレス鋼表面から所定深さＺまでのスパッタ時間)をもって不動態皮膜の膜厚を求める指標とした。そして、TEM観察で得られた不動態皮膜の膜厚とスパッタ時間との相関関係から、任意の処理時間における不動態皮膜の膜厚を算出した。求められた膜厚をＴ2で表す。
[Calculation of film thickness from GDS analysis results]
The immersion treatment material was subjected to glow discharge emission spectroscopy analysis (GDS), and the element distribution along the depth direction from the surface of stainless steel under the conditions of discharge power: 30 W, Ar gas flow rate: 100 cc / min, anode diameter: 4 mm (Fig. 1). The boundary between the passive film and the substrate is the depth from the stainless steel surface where the intensity of (peak intensity−background intensity) / 2 is shown on the curve indicating the oxygen concentration distribution (in other words, from the stainless steel surface). Sputtering time to a predetermined depth Z) was used as an index for determining the thickness of the passive film. And the film thickness of the passive film in arbitrary processing time was computed from the correlation of the film thickness of the passive film obtained by TEM observation, and sputtering time. The obtained film thickness expressed by T 2.
また、ステンレス鋼から切り出された試験片に荷重：１ＭＰａでカーボンペーパを加圧接触させ、ステンレス鋼／カーボンペーパの接触抵抗を測定した。
３０Ｃｒ-２Ｍｏ鋼の２Ｂ仕上げ材について、TEM観察結果から求められた不動態皮膜の膜厚Ｔ1を表１に、GDSプロファイルを図１に示す。５分浸漬処理材では、TEM観察結果から求められた不動態皮膜の膜厚Ｔ1が未処理材より小さくなっているが、不動態皮膜の膜厚を示すGDSのスパッタ時間は未処理材より酸浸漬材の方が長くなっており、膜厚Ｔ2が未処理材より大きくなっている。
Further, the carbon paper was brought into pressure contact with a test piece cut out from stainless steel at a load of 1 MPa, and the contact resistance of the stainless steel / carbon paper was measured.
Table 2 shows the thickness T 1 of the passive film obtained from the TEM observation results for the 2B finish of 30Cr-2Mo steel, and FIG. 1 shows the GDS profile. In the 5-minute immersion treatment material, the thickness T 1 of the passive film obtained from the TEM observation results is smaller than that of the untreated material, but the GDS sputtering time indicating the thickness of the passive film is shorter than that of the untreated material. The acid immersion material is longer and the film thickness T 2 is larger than that of the untreated material.
TEM観察から求められる膜厚Ｔ1とGDSのスパッタ時間とで逆の結果になることは、未処理ステンレス鋼表面にある不動態皮膜が比較的凹凸のある見掛け膜厚の大きな皮膜であるのに対し、５分浸漬材では不動態皮膜が緻密化していることが原因と考えられる。すなわち、非酸化性酸液を用いた浸漬処理でステンレス鋼の表面変質層が除去され、ステンレス鋼表面全域に薄くて緻密な不動態皮膜が形成される。 It is the opposite result in the sputtering time of the film thickness T 1 and GDS obtained from TEM observation, though passive film on the untreated stainless steel surface is large film apparent thickness with relatively uneven On the other hand, in the case of the 5-minute immersion material, it is considered that the passive film is dense. That is, the surface altered layer of stainless steel is removed by immersion treatment using a non-oxidizing acid solution, and a thin and dense passive film is formed over the entire surface of the stainless steel.
そこで、５分酸浸漬材，２０分酸浸漬材それぞれの膜厚Ｔ1と膜厚Ｔ2との相関をとり、得られた相関関係から他の浸漬時間で酸浸漬したステンレス鋼の不動態皮膜の膜厚を算出した。表２の結果にみられるように、Ｔ1とＴ2との相関から求めた不動態皮膜の膜厚Ｔ2は、１０分浸漬材より３分浸漬材の方が薄く、したがって接触抵抗も３分浸漬材の方が低い値になっている。 Therefore, the film thickness T 1 and the film thickness T 2 of each of the 5-minute acid immersion material and the 20-minute acid immersion material are correlated, and the passive film of stainless steel that has been acid-immersed in another immersion time from the obtained correlation. The film thickness was calculated. As can be seen from the results in Table 2, the film thickness T 2 of the passive film determined from the correlation between T 1 and T 2 is thinner for the 3-minute immersion material than for the 10-minute immersion material, and thus the contact resistance is 3 The fraction soaking material has a lower value.
同じ鋼種のBA仕上げ材についての調査結果を表３に示す。BA仕上げ材は、そのままでは不動態皮膜が厚く、大きな接触抵抗を示す。しかし、非酸化性酸液で浸漬処理すると、不動態皮膜が接触抵抗の低い皮膜に改質され、膜厚Ｔ2自体も薄くなっている。 Table 3 shows the survey results for BA finishes of the same steel type. As it is, the BA finish has a thick passive film and a large contact resistance. However, when the immersion treatment is performed with the non-oxidizing acid solution, the passive film is modified to a film having a low contact resistance, and the film thickness T 2 itself is also thinned.
３０Ｃｒ-２Ｍｏ鋼のTEM測定結果を基に、２２Ｃｒ-１.２Ｍｏ鋼，１８Ｃｒ-２Ｍｏ鋼，１８Ｃｒ-１Ｍｏ鋼についても不動態皮膜と接触抵抗との関係を調査した。表４の調査結果にみられるように、何れの鋼種においても非酸化性酸液を用いた浸漬処理で不動態皮膜を膜厚：４ｎｍ以下に調整するとき接触抵抗が低位に保たれることが判る。 Based on the TEM measurement results of 30Cr-2Mo steel, the relationship between the passive film and the contact resistance was also investigated for 22Cr-1.2Mo steel, 18Cr-2Mo steel, and 18Cr-1Mo steel. As can be seen from the investigation results in Table 4, the contact resistance can be kept low when the thickness of the passive film is adjusted to 4 nm or less by immersion treatment using a non-oxidizing acid solution in any steel type. I understand.
２５Ｃｒ-２０Ｎｉのオーステナイト系ステンレス鋼を用いた以外は、実施例１と同様に非酸化性酸液で浸漬処理し、酸浸漬前後の接触抵抗を調査した。表５，６に示すように、この場合もフェライト系ステンレス鋼と同様に浸漬処理で不動態皮膜が薄くなり接触抵抗が低下した。
実施例１，２で酸浸漬したステンレス鋼製セパレータを燃料電池の燃料極側，酸化極側に組み込み、燃料電池を100時間連続運転した後で接触抵抗の増加，出力の変化を調査した。
表８の調査結果にみられるように、膜厚：４ｎｍ以下の不動態皮膜を設けたステンレス鋼製セパレータを備えた燃料電池では、０.３Ａ／ｃｍ2の電流密度で０．６Ｖのセル電圧が得られた。
The stainless steel separators soaked in acid in Examples 1 and 2 were assembled on the fuel electrode side and the oxidation electrode side of the fuel cell, and after the fuel cell was continuously operated for 100 hours, the increase in contact resistance and the change in output were investigated.
As can be seen from the results of the investigation in Table 8, the cell voltage of 0.6 V at a current density of 0.3 A / cm 2 in a fuel cell equipped with a stainless steel separator provided with a passive film with a film thickness of 4 nm or less. was gotten.
他方、膜厚が４ｎｍを超える不動態皮膜をもつステンレス鋼製セパレータを装着した燃料電池は電圧が低く、なかでも不動態皮膜が厚い試験No.６では０.５５Ｖのセル電圧しか得られず電池出力が不足していた。
この対比から、不動態皮膜の膜厚を４ｎｍ以下に規制したステンレス鋼は、低接触抵抗で高出力の燃料電池に適したセパレータとなることが確認される。
On the other hand, a fuel cell equipped with a stainless steel separator having a passive film with a film thickness exceeding 4 nm has a low voltage, and in particular, in test No. 6 where the passive film is thick, only a cell voltage of 0.55 V can be obtained. The output was insufficient.
From this comparison, it is confirmed that the stainless steel in which the film thickness of the passive film is regulated to 4 nm or less is a separator suitable for a fuel cell with low contact resistance and high output.
以上に説明したように、非酸化性酸液を用いた浸漬処理でステンレス鋼表面を改質することにより、ステンレス鋼本来の優れた耐食性を活用しながら接触抵抗の低減が可能となり、セパレータの要求特性を満足するステンレス鋼となる。得られたステンレス鋼製セパレータは、燃料電池を長時間連続運転した後でも接触抵抗が増加しないため、高い発電効率を維持する燃料電池に適した部材となる。 As explained above, by modifying the stainless steel surface by immersion treatment using a non-oxidizing acid solution, it becomes possible to reduce contact resistance while utilizing the excellent corrosion resistance inherent in stainless steel. Stainless steel that satisfies the characteristics. The obtained stainless steel separator is a member suitable for a fuel cell that maintains high power generation efficiency because the contact resistance does not increase even after the fuel cell is continuously operated for a long time.
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