JP3723515B2 - Separator for polymer electrolyte fuel cell and fuel cell - Google Patents

Description

Ｎｏ２〜Ｎｏ１４は被覆層の防食性能を調べた結果である。導電材が高くなるに従って、鉄の溶出速度が高くなる。特に導電材の割合が９０ｗｔ％になると、被覆層を形成していない裸のＳＳ４００（Ｎｏ１）よりは、耐食性の改善が図られているものの、ＳＳ４００の１／１０程度まで腐食量が増加する。
【００８８】
逆に、導電材の割合が小さくなると腐食量が減少するが、別試験で、Ｎｏ３の面抵抗を測定したところ、１Ω・ｃｍ²に達していた。この抵抗値は、例えば、燃料電池の電流値を０.５Ａ／ｃｍ²程度流すとすると、０.５ＶのｉＲドロップに相当し、発電によって得られる電圧の８０％が抵抗のロスとなり、実用的な値ではなかった。
【００８９】
電池の効率を高く保つためには、ｉＲドロップをできる限り小さくすることが肝要で、その際、セパレータの抵抗は３０ｍΩ・ｃｍ²以下とする必要がある。この値を満足する導電材：結着材との比は３５：６５〜５５：４５であった。
【００９０】
以上を纏めると、導電材と結着材との配合比率は１５：８５〜９０：１０で耐食性の向上は期待できるが、セパレータにおける導電材として機能をより引き出すためには導電材：結着材の比は３５：６５〜５５：４５であることが分かった。
【００９１】
被覆層の膜厚は耐食性に大きな影響を及ぼすことも分かった。図２は被覆層の膜厚と腐食速度との関係を表し、膜厚が１０μｍ以上あれば、耐食性の改善効果が現れる。特に４０μｍ以上あれば、基板のＳＳ４００は腐食していない。
【００９２】
逆に膜厚を１００μｍ以上にすると、膜厚の不均一化が生じ、それが乾燥ムラを誘発し、一部の被覆層が風船状に膨らむ現象が発生する。風船状に膨らんだ箇所は被覆層が形成されていなかった。
【００９３】
平均膜厚を７０μｍ以下とすると、この現象を完全に抑えることができた。従って、膜厚は１０〜１００μｍが適当で、好ましくは４０〜７０μｍがよい。
【００９４】
Ｎｏ１２およびＮｏ１５〜Ｎｏ２２は、結着材の影響を調べた結果である。いずれの結着剤を用いてもＳＳ４００（Ｎｏ１）を高耐食化する効果はあるが、最も防食効果の大きい結着剤はＰＶＤＦやＰＴＦＥのフッ素系樹脂である。これらの樹脂は特に疎水性に優れ、水の透過が小さいためと考えられる。
【００９５】
更に、ＰＶＤＦに関しては、金属との密着力を向上できると期待されるＨＦＰとの共重合体についても試験した。Ｎｏ６〜Ｎｏ１１はＰＶＤＦ−ＨＦＰ（フッ化ビニリデン−６フッ化ポリプロピレン）共重合体の組成を０〜５０％まで変化させたときの結果である。効果の差異は微小であるが、ＨＦＰの割合が５〜３０％の範囲で防食性能を向上できる。
【００９６】
Ｎｏ２５は、チタン／銅／チタン無垢の状態、Ｎｏ２６はチタン／銅／チタン金属層に導電材量４５ｗｔ％のＰＶＤＦ系被覆層を形成した場合である。どちらの場合も試験溶液中にチタンは存在せず、最も優れている。しかし、後で詳述するが、チタン／銅／チタンを無垢のままセパレータとして使用すると、酸化皮膜の成長のため、十分な発電性能が得られず、被覆層の形成が必須であることが分かった。
【００９７】
〔実施例 ２〕
本実施例では金属層に外層／中間層／外層の構造を有し、その両面に実施例１で述べた黒鉛系の導電材とＰＶＤＦ系結着材とで構成される被覆層を備えたセパレータについて説明する。
【００９８】
中間層は銅，鉄，アルミニウムおよびマグネシウム、あるいは、これらの合金類が選ばれる。これらの金属は低コストである上に、プレス成形性に優れるためである。これらの金属のいずれでも構わないが、特に、銅は電気伝導性，熱伝導性および組成加工性に優れるため選定した。
【００９９】
また、以下で述べるチタンとの接着性，親和性にも優れる。外層は、チタン，ジルコニウム，タンタル，ニオブ，タングステンが適当であるが、これらの金属の中で最も汎用的なチタンを選んだ。このような外層／中間層／外層を形成した金属層を用いることで、高価なチタン，ジルコニウム，タンタル，ニオブ，タングステンの使用量を抑えられ、低コスト化が可能である。
【０１００】
また、チタン，ジルコニウム，タンタル，ニオブ，タングステンは加工性が乏しく、単体でプレス加工などの塑性加工を施すことが困難であったが、このような外層／中間層／外層の構造とすることで、塑性加工性が向上する。
【０１０１】
例えば、最も効果が現れたのはセパレータの流路をプレス加工した際で、板厚０.３ｍｍのチタン／銅／チタン、あるいは、チタン／アルミニウム／チタンでは、図３に示すようなリブ２１の巾１ｍｍ、溝２２の開口部幅１.５ｍｍ、溝２２の深さ０.５ｍｍを有する波板構造のセパレータ６にプレス成型すると、反りの大きさが１／１００であったのに対し、チタン単体（ＪＩＳ ＴＰ２７０Ｈ）では１／２０と大きかった。また、コーナー部にクラックの発生が認められた。
【０１０２】
このように、金属層１を外層／中間層／外層とすることで、加工性の向上ができた。
【０１０３】
図１は、上記の金属層１として厚さ０.３ｍｍのチタン／銅／チタン材をプレスで直線流路を形成し、これに被覆層２を形成したセパレータ６の断面拡大図である。
【０１０４】
ここで用いたチタン／銅／チタン材は次の特徴を有する。チタン／銅／チタンは冷間圧延で接合した材料である。但し、チタンと銅は変形抵抗が異なるため、それぞれの金属の硬度をできるだけ合わせる必要があった。一般に中間層である銅は軟らかい金属で、外層であるチタンは硬い。中間層と外層との硬度を種々変化させ、出来上がり後の接着強度を板の曲げ試験で評価したところ、外層と中間層との硬度の比が１／３〜３／１の範囲にあると、チタン／銅／チタンは９０度曲げ試験でも剥離することは無かった。
【０１０５】
チタン／鉄／チタンも同じく、剥離は生じなかった。チタン／アルミニウム／チタンやチタン／マグネシウム／チタンは曲げにより剥離が生じた。これを解決する手段として冷間圧延後に４００℃程度まで加熱すると、隣接金属間で金属間化合物が生成し、接着力の向上が図られた。このような効果はチタン／銅／チタンやチタン／鉄／チタンでも同じように得られた。これら金属は上述したように冷間圧延だけでも接着は可能であるが、熱処理を加え、層間に金属化合物を生成させると、より接着力が強くなった。接着力の他に、材料の電気抵抗も小さくなった。層間がより密着するため、接触抵抗も低減できたためと考えられる。
【０１０６】
本実施例では中間層に銅を用いるが、これは軟らかい金属であるため、外層に用いるチタンも変形抵抗差が大きくならない材料を選んだ。チタンおよびチタン合金はＪＩＳおよびＡＳＴＭ規格で規定されているように、１０種類程度存在する。
【０１０７】
これら中で中間層の銅，鉄，アルミニウム，マグネシウムと最も接着力の良い金属は、チタンの不純物濃度として水素が０.０５ｐｐｍ以下、酸素が０.２ｐｐｍ以下、鉄が０.２５ｐｐｍ以下、窒素が０.０５ｐｐｍ以下であるか、あるいはチタンまたはチタン合金の硬度がビッカース硬さで７２〜１１２の範囲にあるチタンであった。
【０１０８】
このチタンは、変形抵抗が銅，鉄，アルミニウム，マグネシウムに近く、そのため、圧延したときに外層のチタンも中間層の銅，鉄，アルミニウム，マグネシウムもほぼ均一に圧延できた。
【０１０９】
チタンの不純物濃度が上記から外れるとチタンが硬くなり、圧延時に中間層だけが優先して圧延され、均一な厚みを有する多層金属が得られなかった。特に、アルミニウムやマグネシウムを中間層とした場合、これが顕著であった。
【０１１０】
チタンが上記組成の範囲にあっても、アルミニウムやマグネシウムを中間層としたときは均一化できないこともあるので、アルミニウム−銅合金やアルミニウム−マグネシウム合金を用いることが好ましい。
【０１１１】
チタン／銅／チタン材におけるチタン層の厚みは１０μｍ以上必要であった。これはチタンの結晶粒径が１０μｍ前後であるためである。チタン層の厚みが結晶粒径の厚みと同程度であると、プレス時の曲げ応力が外層のチタンに加わった際、粒界から破壊し易くなるためである。また、外層であるチタンの厚みを、むやみに厚くしても、加工性の面で好ましくない。
【０１１２】
金属層のうち、外層の厚みの比が３０％以下であれば、中間層の優れた加工性を損なうことはなく、中間層の金属が単体のときと同様に組成加工が可能であった。外層の厚みが３０％を超えると、チタンの機械的性質が顕在化し、塑性変形性が悪化した。従って、外層の厚みは１０μｍ以上あって、金属層の厚みに対する外層の厚みの比が３０％以下であることが好ましい。
【０１１３】
図１に示した被覆層２は、表１に示したＮｏ８の材料で構成されている。結着材としてのＰＶＤＦ−ＨＦＰ（１０ｗｔ％）共重合体をＮＭＰ（Ｎ−メチルピロリドン）に溶解し、これに平均粒径約４μｍの黒鉛と平均粒径０.０２μｍのファーネスブラックを添加し、ロールで分散処理して混合体を得た。黒鉛とファーネスブラックの配合比率は、重量比で８０：２０とした。最終的に混合体に対する導電材＋結着材の質量比（固形分量）が２５ｗｔ％と４０ｗｔ％となる２種類の混合体を調製した。
【０１１４】
図４に示すようにセパレータ６の流路断面は台形で、拡散層９と接触するリブの幅は１ｍｍ，溝の開口部幅は１.５ｍｍ，流路深さは０.５ｍｍである。図１の被覆層２はディップコート法とスクリーン印刷法の２種類とした。
【０１１５】
ディップコート法は、室温で固形分量（導電材＋結着材）２５ｗｔ％の混合体中にセパレータ６を１０秒程度浸漬し、引き上げ速度１０ｍｍ／秒の速度で引き上げる方法で行った。これを真空乾燥器で１４０℃に加熱し、３時間乾燥した。この方法で得られた被覆層２の厚さは平均６０μｍであった。
【０１１６】
スクリーン印刷は、最終的な被覆層２（図１参照）の厚さが６０μｍとなるようなステンレス製メッシュサイズ＃１００のスクリーンを用い、シリコンゴム製ブレードで固形分量４０ｗｔ％の混合体を印刷塗布した。スクリーン印刷方法はディップコート法がセパレータ６の全面に被覆層２が形成されるのに対し、セパレータ６のリブ面（電気が流れる通電面）だけに被覆層２が形成できる。但し、金属層１のうち、板周辺の銅が露出する部分については、はけ塗りで被覆層２を形成した。このセパレータ６を真空乾燥器で１４０℃，３時間加熱乾燥した。被覆層２の厚さは平均６０μｍであった。
【０１１７】
本実施例で例示した導電材は、黒鉛およびファーネスブラックであるが、これに限定されるものではなく、フラーレンやカーボンチューブ等であってもよく、また、タングステンカーバイドや窒化チタンなどの導電性セラミクスであってもよいし、混合体であってもよい。さらにまた、金属紛体であっても同じ効果が得られる。
【０１１８】
〔実施例 ３〕
本実施例を図を用いて説明する。本実施例では実施例２のセパレータを単セル燃料電池に使用した例を示す。
【０１１９】
図４はこのセパレータ６を備えた単電池発電試験セル７の断面模式図である。単電池発電試験セル７は、ゴアテックジャパン製ＭＥＡ（Ｍembran Ｅlectrode Ａssembly；ＰＲＩＭＥＡ５５１０）８を、同社製の拡散層（ＣＡＲＢＥＬ−ＣＦ）９の２枚で挟み、更に、セパレータ６で挟持している。
【０１２０】
セパレータ６は肉厚０.３ｍｍの板材で、発電面（拡散層９との接触面）が３０ｍｍ×３０ｍｍであり、この部位に直線流路が形成されている。さらに、このセパレータ６は緻密黒鉛製のホルダ１０に納められている。
【０１２１】
前記の表面処理層２はセパレータ６の外表面全てに形成されているタイプと、リブ面だけに形成されているタイプ、および、表面処理層２を形成しないタイプの３種類を用いた。
【０１２２】
上記被覆層２は図１に示すように、樹脂５がポリフッ化ビニリデン、導電材３が平均粒径４μｍの黒鉛と平均粒径０.０２μｍのカーボンブラックの混合体で構成されている。
【０１２３】
被覆層２に占める導電材３の質量は４０％で、被覆層２の厚みは平均６０μｍである。
【０１２４】
これらによって形成された単電池発電試験セル７の連続発電試験を実施した。発電試験条件はセル温度７０℃，アノードおよびカソード加湿温度７０℃，電流密度０.５Ａ／ｃｍ²とし、水素利用率７０％，酸素利用率４０％の条件下、出力電圧の経時変化を観察した。なお、燃料ガスには常圧の純水素を、また、酸化剤ガスには乾燥空気を用いた。
【０１２５】
その結果、試験開始初期の電圧は０.６８Ｖであった。比較のために測定した機械切削緻密黒鉛製セパレータと比較すると、チタンでは約０.０３Ｖほど低い値であった。これはセパレータ６の抵抗が緻密黒鉛より大きいためである。
【０１２６】
また、初期電圧は黒鉛セパレータ製より低いものの、チタン／銅／チタン製セパレータでは連続発電時間１,０００時間の時点で電圧低下率が−１５ｍＶ／１，０００時間であり、５,０００時間以上の寿命と見做すことができる。
【０１２７】
発電性能に関しては、表面処理層２がセパレータ６の全面形成タイプとリブ面形成タイプのいずれもほぼ同じであった。このことから表面処理層２はリブ面のみに形成するだけも十分である。被覆層２を必要箇所に限って形成することにより材料の使用量を低く抑えることができる。
【０１２８】
被覆層２を有しない裸のチタン／銅／チタン製のセパレータ６を用いると、初期の電圧が０.６Ｖ以下で、電圧低下率は−１,０００ｍＶ／１,０００時間であった。分解後のセパレータ６の表面は、全体的に薄紫色の干渉色に変化しており、チタンの皮膜が成長して抵抗が増加したことが、大きな低下率を招く要因と考える。
【０１２９】
比較のために、同じ表面処理層を形成したステンレス鋼（ＳＵＳ３０４鋼）についても同じ試験を実施した。ステンレス鋼の場合、出力電圧および電圧低下率は、それぞれチタン／銅／チタン製セパレータとほぼ同じ０.６８Ｖおよび−１５ｍＶ／１,０００時間であったが、発電時間が４００時間を超えると、出力電圧が急激に減少し、０.２Ｖ以下となった。
【０１３０】
ステンレス製セパレータを用いた電池を分解し、表面処理層２を除去して表面を観察したところリブ表面は全面腐食の様相を呈していた。
【０１３１】
更に、ＭＥＡの元素分布をＴＯＦ−ＳＩＭＳで分析した結果、ステンレス鋼の成分であるニッケルが電解質膜中に検出された他に、特に、カソード側電極層において鉄の蓄積が観察された。
【０１３２】
ステンレス鋼をセパレータとして用いた場合、発電に伴う腐食によりステンレス鋼からの溶出成分がＭＥＡに悪害を与え、寿命を短くした。チタン／銅／チタンのセパレータでは若干のチタンがＭＥＡで観察されたものの、その量は少なく、発電性能に影響を及ぼすほどではなかった。
【０１３３】
このように被覆層２を形成したチタン／銅／チタンは電池の寿命を短くさせることなく使用することができる。
【０１３４】
〔実施例 ４〕
本実施例を図を用いて説明する。本実施例では積層型電池の一例を示す。
【０１３５】
図５は本実施例の積層型電池に用いたセパレータ６の模式鳥瞰図である。セパレータ６は金属層１がチタン／アルミニウム／チタンの３層、被覆層２が表１に示したＮｏ８の混合体を前記チタン／アルミニウム／チタン３層の金属層１の両面全体に被覆してある。
【０１３６】
被覆方法は実施例２で述べたディップコート法を用いた。前記セパレータ６の発電面は直線状の流路溝をプレス加工で成型されており、流路溝部８０×１５０ｍｍである。この流路部はリブ２１（図３参照）の幅１ｍｍ、溝２２の開口部幅１.５ｍｍ、溝２２の深さ０.５ｍｍである。前記セパレータ６は反応ガスおよび冷却水の出入り口であるマニホールド１３が形成されている。
【０１３７】
また、前記セパレータ６は反応ガスを流路部に導入するための導入溝が形成されているＰＰＳ製のフレーム１２がシリコン接着剤で接着されている。
【０１３８】
外層／中間層／外層の構造を有する金属層はセパレータ６の周辺部や、マニホールド端面１３において中間層が露出する。そのため、この露出部を何の保護もせず、そのまま使用すると中間層が選択的に腐食し、腐食生成物が発電性能を悪化させる。そのため、実施例２では露出面を保護層２と同じ材料で被覆したが、これはメッキ，樹脂被覆などその他の方法でもよい。チタン／アルミニウム／チタンを金属層１として用いた場合はアルマイト処理が適当である。
【０１３９】
室温，１０ｗｔ％硫酸中で１５分間、１５Ｖの電圧でこれをアノード酸化すると、中間層であるアルミニウムが緻密で耐食性に優れるアルマイト皮膜で覆われる。アルマイト処理後の耐食性は約３桁ほど向上した。
【０１４０】
アルマイト処理に続き純水，９５℃で１０分間封孔処理すると、更に耐食性が向上した。但し、外層のチタンも酸化され、紫色の絶縁性皮膜も成長するので、これを研磨等で除去してからセパレータとした。
【０１４１】
図６に冷却板１５の鳥瞰図を示す。冷却板（冷却セル）１５はセパレータ６と同一構造のものを用いた。セパレータ６と同じ形状を有する冷却板１５の２枚で冷却水用フレーム１６を挟持し、これを一単位の冷却セルとした。冷却水は、冷却板１５−ａと冷却板１５−ｂとの間を流れる構造である。
【０１４２】
冷却板１５−ａおよび冷却板１５−ｂの対向面が冷却水の通り道であるのに対し、それぞれの反対側の面は電極面となる。
【０１４３】
図７は前記セパレータ６を積層した積層型電池の電池の構造を示す。セパレータ６とセパレータ６の間にはゴアテックジャパン製ＭＥＡ（Ｍembran Ｅlectrode Ａssembly；ＰＲＩＭＥＡ５５１０）８と、同社製の拡散層（ＣＡＲＢＥＬ−ＣＦ）９の２枚で挟み込まれている。これを一単位の発電セルとした。
【０１４４】
発電セル３単位に対し、冷却セル１単位を繰り返し積み上げ、トータルで発電セル９単位、冷却セル２単位積層し積層型電池とした。積層体の両端には外側に向かってカーボンペーパ１７／ニッケル製集電材１８／テフロン製絶縁板１９／エンドプレート２０が設置されている。
【０１４５】
上記積層体の両端に設けられた２つのエンドプレート２０間で、ボルトナットと皿バネを使用し積層体を締め付けた。エンドプレート２０には反応ガスおよび冷却水の出入口用コネクタが設けられ、これを介して反応ガスと冷却水が積層型電池に導入される。
【０１４６】
この積層型電池を次の条件で発電した。発電条件は電流密度０.３Ａ／ｃｍ²、電池温度７０℃、アノードおよびカソード加湿水温度７０℃（外部加湿器により反応ガスを加湿）、水素利用率７０％（純水素）、酸素利用率４０％（空気）、ガス排圧大気圧とした。
【０１４７】
開回路電圧は１発電セル当たり平均９００ｍＶ、０.３Ａ／ｃｍ²負荷時、１発電セル当たり平均０.７４Ｖであった。１,０００時間まで発電したところ、出力電圧は１発電セル当たり０.０２Ｖ低下したのみであった。
【０１４８】
発電時の交流抵抗を定期に測定した結果は、発電開始から３,０００時間後まで全く変化せず、９０ｍΩ・ｃｍ²であった。このことから、セパレータ自身は全く劣化が認められず、長寿命のセパレータとすることができた。
【０１４９】
本実施例で用いたセパレータ６の形状並びにそのリブ２１の幅、溝２２の幅、深さは一例であって、これに限定するものではない。
【０１５０】
【発明の効果】
本発明によれば、最外層にチタン，ジルコニウム，タンタル，ニオブ，タングステンを有する金属層にカーボン等の導電材と導電材を固定するための樹脂結着材とで構成される被覆層を形成することにより、長期に渡り金属層の耐食性を保持し、抵抗増大を抑制した固体高分子電解質型燃料電池の金属セパレータ、並びに、固体高分子電解質型燃料電池を提供することが可能となる。
【図面の簡単な説明】
【図１】チタン／銅／チタンに被覆層を形成したセパレータの模式断面拡大図である。
【図２】被覆層の膜厚と腐食速度の関係を表すグラフである。
【図３】プレスセパレータの鳥瞰図と断面拡大図である。
【図４】単電池発電試験セルの模式断面図である。
【図５】積層型電池に用いたセパレータの鳥瞰図である。
【図６】積層型電池に用いた冷却板の鳥瞰図である。
【図７】積層型電池の構成を表す模式断面図である。
【符号の説明】
１…金属層、２…被覆層、３…導電材、５…樹脂、６…セパレータ、７…単電池発電試験セル、８…ＭＥＡ、９…拡散層、１０…ホルダ、１２…フレーム、１３…マニホールド、１５…冷却板、１６…冷却水用フレーム、１７…カーボンペーパ、１８…集電板、１９…絶縁板、２０…エンドプレート、２１…リブ、２２…溝。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a separator and a polymer electrolyte fuel cell which are constituent members of a polymer electrolyte fuel cell.
[0002]
[Prior art]
Separator materials used in polymer electrolyte fuel cells are broadly divided into carbon and metal types. In the case of carbon, for example, a dense graphite material is cut to form a flow path and a manifold. For this reason, there is a problem that not only material costs but also processing costs are high.
[0003]
In order to solve this, for example, there is a method in which a resin is mixed with graphite and a separator is completed by means of heat compression molding or injection molding. Since this method is easy to mold, the cost can be remarkably reduced as compared with the above-described dense graphite cutting.
[0004]
Metals satisfy the impermeability of reactive gases (a general term for fuel gas and oxidant gas), which is one of the functions required of separators, and there are differences in thermal conductivity and electrical conductivity depending on the type of metal. Generally larger than graphite. In addition, the strength, toughness, and ductility of metals are superior to those of graphite materials, and they also excel in functions as structural materials and functions as workpieces.
[0005]
If the metal used as the separator is a general-purpose general-purpose metal, the material can be easily obtained and the material cost is low. Therefore, it is expected that the cost can be further reduced as compared with a separator formed by mixing graphite and resin.
[0006]
However, metals have the disadvantage of being easily corroded, not found in carbon-based materials. Except for precious metals such as platinum and gold, most metals are at risk for corrosion.
[0007]
The polymer electrolyte fuel cell is operated at a temperature of about 70 ° C., and the separator on the fuel electrode side may contain vapor (water) of humidified components such as carbon dioxide gas and a small amount of carbon monoxide gas in addition to hydrogen gas. ). The other air electrode side separator is exposed to air containing steam and moisture.
[0008]
Normally, if the metal has a corrosion resistance higher than that of stainless steel, it will not be corroded in such an environment, but a phenomenon called polarization unique to the battery is imposed on the separator. This is because there is electrical continuity between the electrode and the separator. When materials having different potentials are brought into electrical contact, they are polarized according to the strength of the electrochemical reaction (speed of reaction rate) and the area. At this time, ionic conductivity is required between the separator and the electrode.
[0009]
The substance that controls ionic conductivity is thought to be mainly condensed water. When water is present between the separator and the electrode, an ion passage is formed. Since the shortest distance from the separator surface to the electrode is as short as several hundred μm and the temperature is high, even with pure water, the iR drop is about several tens of mV, so there is a sufficient potential to polarize the separator. It is believed that it can be applied.
[0010]
When the separator is polarized and the potential resulting from the polarization hits the active state region or the hyperpassive region of the metal, the corrosion of the metal is accelerated, the contact resistance between the separator and the diffusion layer increases, or the corrosion product May be trapped by the electrolyte membrane as an ion, and as a result, the ionic conductivity of the ion exchange membrane may be reduced.
[0011]
When the potential at which the separator is polarized is in the passive region, the occurrence of corrosion is very small, but the passive film grows. Ordinary passive films are composed of oxyhydroxides, oxides, etc. starting from hydroxides. Since most of these compounds have poor electrical conductivity, as the passive film of the metal separator grows thicker, the electrical resistance increases, leading to deterioration in battery performance.
[0012]
The corrosion resistance of the metal is maintained by the passive film, but this also causes performance deterioration.
[0013]
In addition, an oxide film may grow or corrode due to a current flowing through the separator. This phenomenon is particularly noticeable when the contact resistance in the separator / diffusion layer is large, and this phenomenon is particularly likely to occur in the separator on the anode side. This phenomenon is similar to a kind of stray current corrosion (electric corrosion) or galvanic corrosion, and the higher the current density, the greater the corrosion.
[0014]
Many inventions related to prevention of high resistance and corrosion when a metal separator is used have been disclosed from the background that electrical resistance increases or corrosion occurs when a metal is used as a separator.
[0015]
Although it is not a separator, Japanese Patent No. 3,211,378 (or Japanese Patent Laid-Open No. 4-159227) uses a foamed metal such as nickel, aluminum or copper as a diffusion layer and coats it with a corrosion-resistant material such as platinum or gold. are doing.
[0016]
The bipolar plate (separator) of Japanese Patent No. 2,953,555 (or Japanese Patent Application Laid-Open No. 6-092637) is aluminum, titanium, zirconium, niobium, tantalum and alloys thereof, or stainless steel, high alloy steel, nickel. -It is made of a chromium alloy and the surface thereof is coated with a conductive material.
[0017]
In JP-A-8-190883, a noble metal layer is formed on a corrugated separator made of stainless steel or titanium alloy to suppress an increase in resistance.
[0018]
In JP-A-8-222237, a large number of protrusions are arranged on the front and back surfaces of a metal separator, and the surface is provided with an electrically conductive coating.
[0019]
In JP-A-9-298064, a water repellent layer is formed on a metal, and this water repellent layer is used as a gold plating layer or the like.
[0020]
In Japanese Patent Application Laid-Open No. 10-228914, a gold plating layer of 0.01 to 0.06 μm is formed on a surface of a metal such as stainless steel in contact with the electrode.
[0021]
In Japanese Patent Laid-Open No. 10-255823, a conductive carbon material is formed on a metal containing 80% by weight or more of aluminum or titanium. Similarly, in Japanese Patent Application Laid-Open No. 11-162479, a metal film in which conductive ceramics are dispersed is formed.
[0022]
In JP-A-11-345618, a conductive coating film is formed on a stainless steel substrate in a thickness of 3 to 20 μm, and this coating film is a mixed powder containing graphite and carbon black.
[0023]
In JP-A-11-144744, a coating material in which carbon is dispersed is applied on a stainless steel substrate, and after rolling, this is heat-treated to form a carbon layer.
[0024]
In Japanese Patent Laid-Open No. 2000-036309, a metal is compositely plated with graphite fluoride particles and a noble metal.
[0025]
In Japanese Patent Laid-Open No. 2000-123850, a noble metal plating layer is provided on any of stainless steel, titanium, and a titanium alloy.
[0026]
In Japanese Patent Laid-Open No. 2000-138067, first and second coat layers are formed on a metal substrate, a conductive material other than carbon is formed on the first coat layer, and a carbon material is formed on the second coat layer. The separator is made of high corrosion resistance.
[0027]
In Japanese Unexamined Patent Publication No. 2000-155742, an anion trapping layer containing an anion trapping substance is formed on the surface of a metal, and the anionic substance is trapped before the corrosive substance reaches the metal to prevent corrosion.
[0028]
In Japanese Patent Laid-Open No. 2000-164228, a plurality of layers of stainless steel, copper, aluminum, titanium, and alloys or composite materials thereof are formed on a separator to obtain peeling resistance, conductivity, and corrosion resistance.
[0029]
In Japanese Patent Laid-Open No. 2000-260441, conductive particles are disposed in a portion of a metal gas flow path plate (separator) that comes into contact with a diffusion layer, and the hardness of the conductive particles is made harder than that of the separator. To form a conductive path.
[0030]
In Japanese Patent Laid-Open No. 2000-307747, a metal coating layer that is melt-formed is provided on a substrate metal, and further a corrosion-resistant coating layer made of a carbon material or the like is formed to improve corrosion resistance.
[0031]
In Japanese Patent Application Laid-Open No. 2000-353531, a metal nitride layer such as titanium, aluminum, chromium, copper, alloys thereof, and stainless steel is formed.
[0032]
[Problems to be solved by the invention]
The above-mentioned invention is mainly intended to improve the corrosion resistance by forming a coating layer on the surface of the metal, which reliably improves the corrosion resistance of the metal and extends the power generation life of the battery compared to when used alone. It was possible.
[0033]
However, the manufacturing process becomes too complicated, continuous production is difficult, or even if the effect of improving the corrosion resistance is great, the cost of materials is high, and thus high costs cannot be avoided. Alternatively, even if the corrosion resistance can be increased at a low cost, the sustainability of the effect may be insufficient.
[0034]
For example, considering that the battery life generally required is 5000 hours or more for in-vehicle use and 40,000 to 90,000 hours for home use or distributed power supply, it is necessary to provide sufficient corrosion resistance. Yes, life extension is a major issue when metals are used.
[0035]
In order to maintain the anti-corrosion and high resistance prevention functions of the coating layer for a long time, it is necessary to block water and air that permeates the coating layer, and to select a metal that does not elute as ions even if the outer surface metal is oxidized. We thought that it was important and came to obtain the present invention.
[0036]
In order to actually form the separator, it is necessary to consider the workability of the substrate metal in addition to the corrosion resistance. The characteristics of metal are sufficient strength and ease of plastic working, but even a number of metals are excellent in plastic working like aluminum and copper, and corrosion resistance such as nickel-chromium-molybdenum alloy and titanium alloy Even if it is good, there is a wide range of metals that are not suitable for plastic working. In this invention, the separator which implement | achieves high corrosion resistance with the metal excellent also in workability is provided.
[0037]
[Means for Solving the Problems]
In order to achieve the above object, the gist of the present invention is as follows.
[0038]
(1) In a polymer electrolyte fuel cell separator comprising a metal layer and a conductive coating layer,
The metal layer is made of at least one metal selected from iron, aluminum, copper, titanium, magnesium, zirconium, tantalum, niobium, tungsten, nickel, chromium, and an alloy of the metal, and the outer surface of the metal is titanium. , Zirconium, tantalum, niobium and alloys thereof, and the coating layer includes at least one conductive material selected from carbon, conductive ceramics, and metal powder, and a resin for fixing the conductive material. A coating layer composed of a binder is formed.
[0039]
(2) The metal layer is formed of a three-layer structure of outer layer / intermediate layer / outer layer, and the intermediate layer is formed of a metal selected from iron, copper, aluminum, magnesium and alloys thereof. It is in the separator for fuel cells.
[0040]
(3) In the polymer electrolyte fuel cell separator, an intermetallic compound composed of adjacent metals is formed between the outer layer / intermediate layer / outer layer of the metal layer.
[0041]
(4) The metal layer is formed of a three-layer structure of outer layer / intermediate layer / outer layer, and a solid whose hardness of the metal of the outer layer is 1/3 to 3/1 with respect to the hardness of the metal of the intermediate layer. It is in the separator for polymer fuel cells.
[0042]
(5) The metal layer is formed of a three-layer structure of outer layer / intermediate layer / outer layer, the intermediate layer is a metal layer made of aluminum or an aluminum alloy, and the portion where the aluminum or aluminum alloy is exposed is anodized Or it exists in the separator for polymer electrolyte fuel cells coat | covered with the boehmite membrane | film | coat.
[0043]
(6) The outer surface of the metal is titanium or a titanium alloy, and the impurity concentration in titanium is 0.05 ppm or less, oxygen is 0.2 ppm or less, iron is 0.25 ppm or less, nitrogen is 0.05 ppm or less, Or it exists in the separator for polymer electrolyte fuel cells which the hardness of the said titanium or titanium alloy exists in the range of 72-112 in Vickers hardness.
[0044]
(7) The separator for a polymer electrolyte fuel cell, wherein the thickness of the outermost layer of the metal layer is 10 μm or more and the ratio of the thickness of the outermost layer to the thickness of the metal layer is 30% or less.
[0045]
(8) The coating layer is in a polymer electrolyte fuel cell separator having a thickness of 10 to 100 μm, preferably 40 to 70 μm.
[0046]
(9) The coating layer includes a carbon-based conductive material containing graphite and carbon black and a binder, and the mixing ratio of the carbon-based conductive material and the binder is 15:85 to 90:10 by weight ratio. Preferably, the ratio is 35:65 to 55:45, and the blend ratio of graphite and carbon black of the carbon-based conductive material is 30:70 to 90:10 by weight ratio in the polymer electrolyte fuel cell separator.
[0047]
(10) The binder of the coating layer is at least one of fluorine resin, silicon resin, phenol resin, epoxy resin, polyimide resin, polyamide resin, polyolefin resin, furan resin, and rubber resin. In the separator for a polymer electrolyte fuel cell.
[0048]
(11) The binder of the coating layer is a copolymer (PVDF-HFP copolymer) of polyvinylidene fluoride (PVDF) and hexafluoropropylene (HFP), and the propylene hexafluoride is a binder. The solid polymer fuel cell separator is in the range of 5 to 30% by weight.
[0049]
(12) In the polymer electrolyte fuel cell having a separator having an electrode, a gas diffusion layer, and a manifold on both surfaces of the polymer electrolyte having ionic conductivity, any one of (9) to (11) only on the current-carrying surface It is in the separator for polymer electrolyte fuel cells which formed the coating layer of description.
[0050]
(13) In a polymer electrolyte fuel cell having a polymer electrolyte having ionic conductivity and electrodes formed on both sides of the polymer electrolyte, and having a gas diffusion layer and a separator, the separators are (1) to (12). The polymer electrolyte fuel cell is a separator according to any one of the above.
[0051]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors first tried to select a material suitable as a metal separator material. As the method, the change in weight before and after the immersion test in a sulfuric acid aqueous solution at room temperature, the current density obtained from the polarization curves of sulfuric acid (pH about 1.2) and aqueous sodium sulfate (pH about 6.8) at 30 ° C., The current density and the amount of eluted metal in a constant potential holding test in a sodium sulfate aqueous solution at 70 ° C. were used.
[0052]
Although there are many unclear points about the actual battery environment to which the separator is exposed, it was considered possible to make a relative comparison of the corrosion resistance between materials by carrying out the above test.
[0053]
As a result of conducting various tests on pure metals, the following knowledge was obtained. The behavior of corrosion resistance in sulfuric acid aqueous solution is roughly classified into three.
[0054]
Classification 1: Both the anode and cathode have excellent corrosion resistance
Category 2: Excellent corrosion resistance at the anode electrode
Classification 3: Corrosion resistance is poor for both anode and cathode
The metals belonging to the above category 1 were as follows.
[0055]
Elements belonging to category 1: titanium, zirconium, tantalum, niobium, tungsten, gold, platinum, lead, silicon, graphite.
[0056]
In these materials, these metal components contained in the test solution after the end of the polarization curve and the test solution for the constant potential holding test were extremely small, and the values were about three orders of magnitude lower than those of other metals. Therefore, it is expected that there is little possibility of adverse effects on the electrode and the electrolyte.
[0057]
However, since a certain amount of current flows during polarization, it is considered that most of this current was used for film growth. Most of the metals mentioned above are valve metals except for noble metals such as gold. Therefore, although corrosion resistance is good, there is a concern that the surface resistance of the metal increases with the passage of time. In fact, in the case of titanium, the initial sheet resistance is 0.3 mΩ · cm. ² (Thickness 4 mm) is 100 mΩ · cm in tens of hours ² It rose to the order. That is, these metals other than noble metals also need to be subjected to some surface treatment.
[0058]
Stainless steel, which is generally called a corrosion-resistant metal, is a widely used metal, is inexpensive, and has excellent workability. Therefore, it is expected as a material for a separator, but the corrosion resistance is particularly poor in the cathode potential region. Stainless steel is thought to be because its components, chromium and nickel, dissolve in a high potential region. Actually, constituent element ions of stainless steel were detected in the test solution after the constant potential holding test.
[0059]
Pure chromium and pure nickel are just equivalent to the hyperpassive region in the cathode potential region of the fuel cell. Therefore, it is considered that the stainless steel (especially austenite type) containing chromium and nickel as main constituents also has an increased current in the cathode potential region and an increased amount of dissolution.
[0060]
Therefore, high chromium, high nickel stainless steel, such as SUS309S and SUS310S, was poorer in corrosion resistance than SUS304. Molybdenum has been added to stainless steel as an element imparting high corrosion resistance, but molybdenum did not show any passivation in an aqueous sulfuric acid environment. Therefore, the SUS310 series alloy to which molybdenum is added is inferior in corrosion resistance to SUS304, though only slightly. Copper as an additive element was the same as molybdenum.
[0061]
For the same reason, the nickel base alloy has poor corrosion resistance on the cathode side. In chemical plants that handle strongly acidic solutions such as hydrochloric acid and sulfuric acid, nickel-based alloys are used as structural materials, but they are considered difficult to use in fuel cell environments.
[0062]
However, since these materials showed excellent corrosion resistance in the anode potential region, there is a possibility that the corrosion resistance can be improved by surface treatment.
[0063]
Considering the price of the material and the corrosion resistance, austenitic SUS304 steel, SUS316 steel, and ferrite SUS430 were excellent overall. Double-layer stainless steel (SUS329 or the like) is excellent from the standpoint of corrosion resistance alone, but this material is also promising if the material cost can be reduced. In addition, titanium is a metal whose price is about one digit higher than that of SUS304 steel. However, if it is a composite metal material in which a thin layer of titanium is coated with an inexpensive metal, it is a metal that can be expected because it is inexpensive.
[0064]
From the above, metals that can be used in the separator as a single metal are difficult except for noble metals. Since the use of noble metals is economically out of the question, some surface treatment means such as suppressing an increase in electrical resistance or imparting corrosion resistance to other inexpensive metals is necessary.
[0065]
Furthermore, the following experiment was conducted, and it was found that the substrate metal needs to be insoluble even when corroded.
[0066]
Specimen is metal / carbon paper / metal (contact area 10cm ² And lead wires were attached to the metal at both ends of the test piece. This test simulates a separator and carbon diffusion layer in a fuel cell. This test piece was neutral 0.05M Na at 70 ° C. ₂ SO _Four Immerse in an aqueous solution (open to the atmosphere, 2 L), and use a galvanostat to force current (0.1 A / cm between both electrodes). ² The voltage between the metals after correcting the iR drop of the lead wire or the like was about 1.0 V).
[0067]
As a result, in an alloy containing Ni or Cr such as stainless steel or nickel-base alloy, the test solution after 100 hours contained Ni or Cr in the order of ppm. The observed surface after the test exhibited both positive and negative corrosion, and the positive electrode was particularly remarkable. When the metal was titanium, elution of the titanium component into the test solution was not observed.
[0068]
As described above, the elution component from the metal causes deactivation of the electrode and a decrease in the ionic conductivity of the polymer electrolyte membrane, so that the dissolved component from the metal must be suppressed. Also from the above experiments, the elements belonging to category 1, ie, titanium, zirconium, tantalum, niobium, tungsten, gold, platinum, lead, silicon, and graphite, are effective as the substrate metal.
[0069]
Among these, practical metals are titanium, zirconium, tantalum, niobium, and tungsten, and it is desirable that the outermost surface of the metal layer be these metals. However, in addition to the disadvantage that these metals are expensive, they have the disadvantage of being inferior in formability.
[0070]
Since the separator plays a role of efficiently supplying the reaction gas to the electrode, a complicated groove shape, dimple processing, or embossing is usually performed. In the present invention, although corrosion resistance such as iron, copper, aluminum, and magnesium is inferior, a metal with good workability is placed in the intermediate layer, and both surfaces thereof are covered with the above-mentioned metal such as thin titanium to form an outer layer / intermediate layer / outer layer.
[0071]
As a method for forming this material, there are an explosion method, a rolling method, an adhesion method, and the like. For this reason, it is necessary to select an appropriate one for the properties of the outer layer and the intermediate layer.
[0072]
Next, various surface treatment methods were examined. In addition, as a test method, in order to quickly determine the effect of the coating layer, various surface treatment methods are applied to carbon steel (SS400) with poor corrosion resistance, and anticorrosion is achieved through polarization curves and constant potential immersion tests (implemented by simple immersion tests). The function was evaluated.
[0073]
Typical plating methods include chrome plating, nickel plating, and gold plating. As described above, since chromium and nickel have poor corrosion resistance, electrolytic gold plating was employed. As a result, the plating was divided into two stages, and unless the thickness of the plating layer was increased to 0.1 μm or more, pinholes could not be eliminated and sufficient corrosion resistance could not be obtained. In addition, gold plating requires many processes such as pretreatment and increases the cost.
[0074]
A material obtained by depositing conductive ceramics by a dry process such as ion plating or sputtering could not obtain satisfactory corrosion resistance. In addition to the fact that the membrane is porous, depending on the ceramic used, the ceramic itself may change and dissolve in an electrochemical reaction.
[0075]
Next, application of a conductive paint was attempted as a surface treatment method. When carbon steel was coated with a paint in which a conductive material such as graphite was mixed with a resin binder, this had the highest anticorrosion effect and resistance increase suppression effect. However, the effect of anticorrosion varies markedly depending on the type of binder (binder) and the concentration of the conductive material, and it is necessary to obtain an optimum composition.
[0076]
[Example 1]
This embodiment will be described with reference to the drawings. In this example, the corrosion resistance of various materials was compared for the coating layer.
[0077]
The substrate metal uses a multiphase metal composed of titanium, zirconium, tantalum, niobium, and tungsten as an outer layer and an intermediate layer made of iron, copper, aluminum, and magnesium. Thereby, a substrate metal excellent in corrosion resistance and workability can be obtained. Here, titanium / copper / titanium is used as an example.
[0078]
As for the titanium / copper / titanium material, three sheets of titanium and copper having an appropriate thickness were stacked and rolled cold or hot to finally obtain a sheet thickness of 0.2 to 0.3 mm. In the case of hot rolling, the temperature was set to 400 to 600 ° C., and the oxide film formed on the surface was removed with emery paper.
[0079]
As the coating layer, a carbon / binder / solvent mixture having various compositions was used. Carbon used was graphite or carbon black added thereto. As the binder, fluorine resin, silicon resin, phenol resin, epoxy resin, polyimide resin, polyamide resin, polyolefin resin, furan resin, rubber resin, and some of these mixtures were used. .
[0080]
The method for preparing the mixture is as follows. The binder is dissolved in various solvents (an appropriate solvent is selected according to the binder material), and graphite having an average particle size of about 4 μm and furnace black having an average particle size of 0.02 μm are added thereto, and dispersed with a roll. To obtain a mixture. The blending ratio of graphite and furnace black was 80:20 by weight.
[0081]
When the mass ratio of graphite: furnace black affecting resistance was tested in another test, the mass ratio of graphite: furnace black was 30:70 to 90:10, and the resistance could be made smaller than that of graphite alone, especially graphite: furnace black. The composition ratio was selected because the resistance was the smallest at a mass ratio of 80:20.
[0082]
This mixture was applied by screen printing on a titanium / copper / titanium material cut to 20 × 50 mm. The screen thickness was selected so that the film thickness after drying was about 50 μm. After coating, it was dried in accordance with the characteristics of each solvent and binder. Furthermore, this test piece left one side of 20 × 50 mm, and the other part was covered with silicon resin so that the exposed surface was finally about 16 mm × 46 mm.
[0083]
The metal used in the test was also applied to an SS400 (carbon steel) metal plate of the same size as titanium / copper / titanium in order to examine only the corrosion resistance of the coating layer, and the anticorrosion performance was compared.
[0084]
The following method was used for the corrosion test. After inserting the test piece into the glass container (volume 2L) of the loop type tester, 0.05M Na ₂ SO _Four After the aqueous solution was introduced and the test solution in the container reached a dissolved oxygen concentration <10 ppb, a dissolved hydrogen concentration of about 30 ppb, and a pH of 6.8 to 7.2, the temperature was raised to 70 ° C. in 30 minutes. The outlet valve was closed.
[0085]
The test solution in the container was sampled periodically, and the metal components contained in the test solution were quantified by ICP. The test time was 100 hours.
[0086]
Table 1 shows the details of the coating layer used in this example and the results of the corrosion test. The conductive material (wt%) in the table represents the ratio of the amount of conductive material after drying.
[0087]
[Table 1]

No2-No14 is the result of investigating the anticorrosion performance of the coating layer. As the conductive material increases, the elution rate of iron increases. In particular, when the proportion of the conductive material is 90 wt%, although the corrosion resistance is improved compared to the bare SS400 (No. 1) in which the coating layer is not formed, the corrosion amount increases to about 1/10 of SS400.
[0088]
Conversely, when the proportion of the conductive material is reduced, the amount of corrosion decreases, but when the surface resistance of No. 3 was measured in a separate test, 1 Ω · cm ² Had reached. This resistance value is, for example, a current value of the fuel cell of 0.5 A / cm. ² When flowing to a certain extent, it corresponds to an iR drop of 0.5 V, and 80% of the voltage obtained by power generation is a loss of resistance, which is not a practical value.
[0089]
In order to keep the efficiency of the battery high, it is important to make the iR drop as small as possible. At that time, the resistance of the separator is 30 mΩ · cm. ² It is necessary to do the following. The ratio of conductive material: binder satisfying this value was 35:65 to 55:45.
[0090]
In summary, the blending ratio of the conductive material and the binder is 15:85 to 90:10, and an improvement in corrosion resistance can be expected. However, in order to bring out more functions as the conductive material in the separator, the conductive material: binder The ratio was found to be 35:65 to 55:45.
[0091]
It was also found that the thickness of the coating layer has a great influence on the corrosion resistance. FIG. 2 shows the relationship between the film thickness of the coating layer and the corrosion rate. If the film thickness is 10 μm or more, the effect of improving the corrosion resistance appears. In particular, when the thickness is 40 μm or more, the SS400 of the substrate is not corroded.
[0092]
On the other hand, when the film thickness is 100 μm or more, the film thickness becomes non-uniform, which causes drying unevenness and a phenomenon that a part of the coating layer swells in a balloon shape. The coating layer was not formed in the balloon-swelled portion.
[0093]
When the average film thickness was 70 μm or less, this phenomenon could be completely suppressed. Accordingly, the film thickness is suitably 10 to 100 μm, preferably 40 to 70 μm.
[0094]
No12 and No15-No22 are the results of examining the influence of the binder. Even if any binder is used, SS400 (No. 1) has the effect of increasing the corrosion resistance, but the binder having the largest anticorrosion effect is a fluororesin such as PVDF or PTFE. It is considered that these resins are particularly excellent in hydrophobicity and have small water permeation.
[0095]
Furthermore, regarding PVDF, a copolymer with HFP, which is expected to be able to improve the adhesion to metal, was also tested. No. 6 to No. 11 are results when the composition of the PVDF-HFP (vinylidene fluoride-6-fluorinated polypropylene) copolymer is changed from 0 to 50%. Although the difference in effect is small, the anticorrosion performance can be improved when the HFP ratio is in the range of 5 to 30%.
[0096]
No25 is a state of pure titanium / copper / titanium, and No26 is a case where a PVDF-based coating layer having a conductive material amount of 45 wt% is formed on a titanium / copper / titanium metal layer. In either case, no titanium is present in the test solution and is the best. However, as will be described later in detail, it is found that when titanium / copper / titanium is used as a solid separator as a separator, sufficient power generation performance cannot be obtained due to the growth of an oxide film, and formation of a coating layer is essential. It was.
[0097]
Example 2
In this example, a separator having a metal layer having an outer layer / intermediate layer / outer layer structure and a coating layer composed of the graphite-based conductive material and the PVDF-based binder described in Example 1 on both surfaces thereof. Will be described.
[0098]
The intermediate layer is selected from copper, iron, aluminum and magnesium, or alloys thereof. This is because these metals are low in cost and excellent in press formability. Any of these metals may be used, but copper was selected because it is particularly excellent in electrical conductivity, thermal conductivity, and composition workability.
[0099]
Moreover, it is excellent also in the adhesiveness and affinity with titanium described below. Titanium, zirconium, tantalum, niobium and tungsten are suitable for the outer layer, but the most versatile titanium among these metals was selected. By using such a metal layer formed with an outer layer / intermediate layer / outer layer, the amount of expensive titanium, zirconium, tantalum, niobium, and tungsten used can be suppressed, and the cost can be reduced.
[0100]
In addition, titanium, zirconium, tantalum, niobium, and tungsten have poor workability, and it was difficult to perform plastic processing such as press working alone. However, by adopting such an outer layer / intermediate layer / outer layer structure, The plastic workability is improved.
[0101]
For example, the most effective effect was when the separator flow path was pressed, and the thickness of the rib 21 as shown in FIG. 3 is 0.3 / mm or titanium / copper / titanium or titanium / aluminum / titanium. When press-molding into a corrugated separator 6 having a width of 1 mm, an opening width of the groove 22 of 1.5 mm, and a depth of the groove 22 of 0.5 mm, the warp size was 1/100, whereas titanium The single unit (JIS TP270H) was as large as 1/20. In addition, cracks were observed at the corners.
[0102]
Thus, the workability could be improved by using the metal layer 1 as the outer layer / intermediate layer / outer layer.
[0103]
FIG. 1 is an enlarged cross-sectional view of a separator 6 in which a straight channel is formed by pressing a 0.3 mm thick titanium / copper / titanium material as the metal layer 1 and a coating layer 2 is formed thereon.
[0104]
The titanium / copper / titanium material used here has the following characteristics. Titanium / copper / titanium is a material joined by cold rolling. However, since titanium and copper have different deformation resistances, it is necessary to match the hardness of each metal as much as possible. In general, copper as an intermediate layer is a soft metal, and titanium as an outer layer is hard. When the hardness of the intermediate layer and the outer layer was variously changed and the adhesive strength after completion was evaluated by a bending test of the plate, the hardness ratio between the outer layer and the intermediate layer was in the range of 1/3 to 3/1. Titanium / copper / titanium did not peel even in the 90-degree bending test.
[0105]
Similarly, no peeling occurred in titanium / iron / titanium. Titanium / aluminum / titanium and titanium / magnesium / titanium were peeled off by bending. As a means for solving this, when the steel was heated to about 400 ° C. after cold rolling, an intermetallic compound was generated between adjacent metals, and the adhesive force was improved. Such an effect was obtained in the same way with titanium / copper / titanium and titanium / iron / titanium. As described above, these metals can be bonded only by cold rolling, but when heat treatment is performed and a metal compound is generated between the layers, the bonding strength becomes stronger. In addition to adhesive strength, the electrical resistance of the material was also reduced. It is considered that the contact resistance could be reduced because the layers were more closely adhered.
[0106]
In this example, copper is used for the intermediate layer. However, since this is a soft metal, titanium used for the outer layer was selected so that the deformation resistance difference does not increase. There are about 10 types of titanium and titanium alloys as defined in JIS and ASTM standards.
[0107]
Among these, the metal having the best adhesive strength with copper, iron, aluminum, and magnesium of the intermediate layer is that the impurity concentration of titanium is 0.05 ppm or less of hydrogen, oxygen is 0.2 ppm or less, iron is 0.25 ppm or less, and nitrogen is It was 0.05 ppm or less, or titanium whose titanium or titanium alloy had a Vickers hardness in the range of 72 to 112.
[0108]
This titanium has a deformation resistance close to that of copper, iron, aluminum, and magnesium. Therefore, when rolled, the outer layer of titanium and the intermediate layers of copper, iron, aluminum, and magnesium could be rolled almost uniformly.
[0109]
When the impurity concentration of titanium deviated from the above, the titanium became hard and only the intermediate layer was preferentially rolled during rolling, and a multilayer metal having a uniform thickness could not be obtained. This was particularly remarkable when aluminum or magnesium was used as the intermediate layer.
[0110]
Even when titanium is in the above composition range, it may be impossible to make uniform when aluminum or magnesium is used as the intermediate layer. Therefore, it is preferable to use an aluminum-copper alloy or an aluminum-magnesium alloy.
[0111]
The thickness of the titanium layer in the titanium / copper / titanium material needs to be 10 μm or more. This is because the crystal grain size of titanium is around 10 μm. This is because when the thickness of the titanium layer is approximately the same as the thickness of the crystal grain size, when the bending stress during pressing is applied to the titanium of the outer layer, the titanium layer is easily broken from the grain boundary. Moreover, even if the thickness of titanium as the outer layer is increased excessively, it is not preferable in terms of workability.
[0112]
If the thickness ratio of the outer layer of the metal layer is 30% or less, the excellent workability of the intermediate layer is not impaired, and composition processing is possible as in the case where the metal of the intermediate layer is a single body. When the thickness of the outer layer exceeded 30%, the mechanical properties of titanium became obvious and the plastic deformability deteriorated. Therefore, the thickness of the outer layer is preferably 10 μm or more, and the ratio of the thickness of the outer layer to the thickness of the metal layer is preferably 30% or less.
[0113]
The covering layer 2 shown in FIG. 1 is composed of the material No. 8 shown in Table 1. PVDF-HFP (10 wt%) copolymer as a binder is dissolved in NMP (N-methylpyrrolidone), and graphite having an average particle diameter of about 4 μm and furnace black having an average particle diameter of 0.02 μm are added thereto. The mixture was obtained by dispersing with a roll. The blending ratio of graphite and furnace black was 80:20 by weight. Finally, two types of mixtures having a mass ratio (solid content) of the conductive material + binder to the mixture of 25 wt% and 40 wt% were prepared.
[0114]
As shown in FIG. 4, the cross section of the flow path of the separator 6 is trapezoidal, the width of the rib in contact with the diffusion layer 9 is 1 mm, the width of the groove opening is 1.5 mm, and the flow path depth is 0.5 mm. The coating layer 2 in FIG. 1 is of two types, a dip coating method and a screen printing method.
[0115]
The dip coating method was performed by dipping the separator 6 in a mixture having a solid content (conductive material + binder) of 25 wt% at room temperature for about 10 seconds and pulling it up at a speed of 10 mm / second. This was heated to 140 ° C. with a vacuum dryer and dried for 3 hours. The coating layer 2 obtained by this method had an average thickness of 60 μm.
[0116]
Screen printing uses a stainless steel mesh size # 100 screen with a final coating layer 2 (see FIG. 1) thickness of 60 μm, and a silicon rubber blade is used to print and apply a 40 wt% solid content mixture. did. In the screen printing method, the coating layer 2 is formed on the entire surface of the separator 6 by the dip coating method, whereas the coating layer 2 can be formed only on the rib surface (the current-carrying surface through which electricity flows) of the separator 6. However, the coating layer 2 was formed by brushing the portion of the metal layer 1 where the copper around the plate was exposed. This separator 6 was dried by heating at 140 ° C. for 3 hours in a vacuum dryer. The average thickness of the coating layer 2 was 60 μm.
[0117]
The conductive materials exemplified in the present embodiment are graphite and furnace black, but are not limited thereto, and may be fullerene, carbon tube or the like, and conductive ceramics such as tungsten carbide or titanium nitride. It may be a mixture. Furthermore, the same effect can be obtained even with a metal powder.
[0118]
[Example 3]
This embodiment will be described with reference to the drawings. In this example, an example in which the separator of Example 2 is used for a single cell fuel cell is shown.
[0119]
FIG. 4 is a schematic cross-sectional view of a unit cell power generation test cell 7 provided with this separator 6. In the single cell power generation test cell 7, a MEA (Membran Electrode Assembly; PRIME 5510) 8 manufactured by Goretech Japan is sandwiched between two sheets of diffusion layers (CARBEL-CF) 9 manufactured by the same company, and further sandwiched by a separator 6.
[0120]
The separator 6 is a plate material having a thickness of 0.3 mm and has a power generation surface (contact surface with the diffusion layer 9) of 30 mm × 30 mm, and a straight flow path is formed in this portion. Further, the separator 6 is housed in a holder 10 made of dense graphite.
[0121]
The surface treatment layer 2 used was of three types: a type formed on the entire outer surface of the separator 6, a type formed only on the rib surface, and a type not formed with the surface treatment layer 2.
[0122]
As shown in FIG. 1, the coating layer 2 is composed of a mixture of a resin 5 of polyvinylidene fluoride, and a conductive material 3 of graphite having an average particle diameter of 4 μm and carbon black having an average particle diameter of 0.02 μm.
[0123]
The mass of the conductive material 3 occupying the coating layer 2 is 40%, and the thickness of the coating layer 2 is 60 μm on average.
[0124]
The continuous power generation test of the single cell power generation test cell 7 formed by these was performed. The power generation test conditions were a cell temperature of 70 ° C, an anode and cathode humidification temperature of 70 ° C, and a current density of 0.5 A / cm. ² The change in output voltage with time was observed under the conditions of a hydrogen utilization rate of 70% and an oxygen utilization rate of 40%. Note that pure hydrogen at normal pressure was used as the fuel gas, and dry air was used as the oxidant gas.
[0125]
As a result, the voltage at the beginning of the test was 0.68V. Compared with a machine-cut dense graphite separator measured for comparison, the value for titanium was about 0.03 V lower. This is because the resistance of the separator 6 is larger than that of dense graphite.
[0126]
In addition, although the initial voltage is lower than that of the graphite separator, the voltage drop rate is -15 mV / 1,000 hours at the time of continuous power generation time of 1,000 hours in the titanium / copper / titanium separator, which is more than 5,000 hours. It can be regarded as a lifetime.
[0127]
Regarding the power generation performance, the surface treatment layer 2 was almost the same for both the entire surface formation type of the separator 6 and the rib surface formation type. Therefore, it is sufficient to form the surface treatment layer 2 only on the rib surface. By forming the coating layer 2 only at the necessary locations, the amount of material used can be kept low.
[0128]
When a bare titanium / copper / titanium separator 6 without the coating layer 2 was used, the initial voltage was 0.6 V or less, and the voltage drop rate was -1,000 mV / 1,000 hours. The surface of the separator 6 after decomposition has changed to a light purple interference color as a whole, and the increase in resistance due to the growth of the titanium film is considered to cause a large reduction rate.
[0129]
For comparison, the same test was performed on stainless steel (SUS304 steel) on which the same surface treatment layer was formed. In the case of stainless steel, the output voltage and the voltage drop rate were 0.68 V and -15 mV / 1,000 hours, respectively, which are almost the same as those of the titanium / copper / titanium separator. The voltage decreased rapidly and became 0.2V or less.
[0130]
When the battery using the stainless steel separator was disassembled, the surface treatment layer 2 was removed, and the surface was observed, the rib surface was in the form of overall corrosion.
[0131]
Furthermore, as a result of analyzing the element distribution of MEA by TOF-SIMS, nickel, which is a component of stainless steel, was detected in the electrolyte membrane, and in particular, accumulation of iron was observed in the cathode side electrode layer.
[0132]
When stainless steel was used as a separator, the elution component from stainless steel caused harmful effects on MEA due to corrosion accompanying power generation, and shortened the life. In the titanium / copper / titanium separator, some titanium was observed by MEA, but the amount was small and did not affect power generation performance.
[0133]
Thus, the titanium / copper / titanium formed with the coating layer 2 can be used without shortening the battery life.
[0134]
[Example 4]
This embodiment will be described with reference to the drawings. In this embodiment, an example of a stacked battery is shown.
[0135]
FIG. 5 is a schematic bird's-eye view of the separator 6 used in the stacked battery of this example. In the separator 6, the metal layer 1 is coated with three layers of titanium / aluminum / titanium, and the coating layer 2 is coated on the both surfaces of the titanium / aluminum / titanium three-layered metal layer 1 with the No8 mixture shown in Table 1. .
[0136]
As the coating method, the dip coating method described in Example 2 was used. The power generation surface of the separator 6 is formed by pressing a straight channel groove and has a channel groove portion of 80 × 150 mm. This flow path portion has a width of 1 mm of the rib 21 (see FIG. 3), an opening width of the groove 22 of 1.5 mm, and a depth of the groove 22 of 0.5 mm. The separator 6 is formed with a manifold 13 which is an inlet / outlet of the reaction gas and the cooling water.
[0137]
The separator 6 has a frame 12 made of PPS in which an introduction groove for introducing a reaction gas into the flow path is formed and bonded with a silicon adhesive.
[0138]
In the metal layer having the structure of outer layer / intermediate layer / outer layer, the intermediate layer is exposed at the periphery of the separator 6 and at the manifold end face 13. Therefore, if this exposed part is not protected and used as it is, the intermediate layer is selectively corroded, and the corrosion product deteriorates the power generation performance. Therefore, in Example 2, the exposed surface was coated with the same material as that of the protective layer 2, but this may be performed by other methods such as plating or resin coating. When titanium / aluminum / titanium is used as the metal layer 1, alumite treatment is appropriate.
[0139]
When this is anodized in a 10 wt% sulfuric acid at room temperature for 15 minutes at a voltage of 15 V, aluminum as an intermediate layer is covered with a dense anodized aluminum film having excellent corrosion resistance. Corrosion resistance after anodizing was improved by about 3 digits.
[0140]
Subsequent to the alumite treatment, the corrosion resistance was further improved by sealing with pure water at 95 ° C. for 10 minutes. However, the outer layer of titanium is also oxidized, and a purple insulating film grows. Therefore, this was removed by polishing or the like to obtain a separator.
[0141]
FIG. 6 shows a bird's-eye view of the cooling plate 15. A cooling plate (cooling cell) 15 having the same structure as the separator 6 was used. The cooling water frame 16 is sandwiched between two cooling plates 15 having the same shape as the separator 6, and this is used as a unit cooling cell. The cooling water has a structure that flows between the cooling plate 15-a and the cooling plate 15-b.
[0142]
The opposing surfaces of the cooling plate 15-a and the cooling plate 15-b are passages for cooling water, whereas the opposite surfaces are electrode surfaces.
[0143]
FIG. 7 shows a battery structure of a stacked battery in which the separator 6 is stacked. Between the separator 6 and the separator 6, two sheets of Gore-Tech Japan MEA (Membran Electrode Assembly; PRIME 5510) 8 and the company's diffusion layer (CARBEL-CF) 9 are sandwiched. This was defined as a unit of power generation cell.
[0144]
One unit of cooling cells was repeatedly stacked on three units of power generation cells, and a total of nine units of power generation cells and two units of cooling cells were laminated to form a stacked battery. A carbon paper 17 / a nickel current collector 18 / a Teflon insulating plate 19 / an end plate 20 are installed at both ends of the laminate.
[0145]
Between the two end plates 20 provided at both ends of the laminate, the laminate was tightened using bolts and nuts and disc springs. The end plate 20 is provided with a reaction gas and cooling water inlet / outlet connector, through which the reaction gas and the cooling water are introduced into the stacked battery.
[0146]
This stacked battery was generated under the following conditions. Power generation conditions are current density 0.3A / cm ² Battery temperature 70 ° C., anode and cathode humidification water temperature 70 ° C. (humidification of reaction gas by external humidifier), hydrogen utilization 70% (pure hydrogen), oxygen utilization 40% (air), gas exhaust pressure atmospheric pressure did.
[0147]
The open circuit voltage is 900 mV on average per power generation cell, 0.3 A / cm ² When loaded, the average was 0.74 V per power generation cell. When power was generated up to 1,000 hours, the output voltage only decreased by 0.02 V per power generation cell.
[0148]
The result of regular measurement of AC resistance during power generation does not change at all until 3000 hours after the start of power generation. ² Met. Therefore, the separator itself was not deteriorated at all, and could be a long-life separator.
[0149]
The shape of the separator 6 used in the present embodiment, the width of the rib 21, the width of the groove 22, and the depth are merely examples, and the present invention is not limited thereto.
[0150]
【The invention's effect】
According to the present invention, a coating layer composed of a conductive material such as carbon and a resin binder for fixing the conductive material is formed on a metal layer having titanium, zirconium, tantalum, niobium, and tungsten as the outermost layer. As a result, it is possible to provide a metal separator for a solid polymer electrolyte fuel cell that maintains the corrosion resistance of the metal layer for a long period of time and suppresses an increase in resistance, and a solid polymer electrolyte fuel cell.
[Brief description of the drawings]
FIG. 1 is an enlarged schematic cross-sectional view of a separator having a coating layer formed on titanium / copper / titanium.
FIG. 2 is a graph showing the relationship between the film thickness of a coating layer and the corrosion rate.
FIG. 3 is a bird's-eye view and an enlarged cross-sectional view of a press separator.
FIG. 4 is a schematic cross-sectional view of a single battery power generation test cell.
FIG. 5 is a bird's-eye view of a separator used in a stacked battery.
FIG. 6 is a bird's-eye view of a cooling plate used in a stacked battery.
FIG. 7 is a schematic cross-sectional view illustrating a configuration of a stacked battery.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Metal layer, 2 ... Coating layer, 3 ... Conductive material, 5 ... Resin, 6 ... Separator, 7 ... Single cell power generation test cell, 8 ... MEA, 9 ... Diffusion layer, 10 ... Holder, 12 ... Frame, 13 ... Manifold, 15 ... cooling plate, 16 ... cooling water frame, 17 ... carbon paper, 18 ... current collecting plate, 19 ... insulating plate, 20 ... end plate, 21 ... rib, 22 ... groove.

Claims (11)

  A separator for a polymer electrolyte fuel cell having a metal layer and a conductive coating layer, wherein the metal layer has a three-layer structure of outer layer / intermediate layer / outer layer, and the outer layer and the intermediate layer An intermetallic compound composed of adjacent metals is formed therebetween, and the intermediate layer is at least selected from iron, aluminum, copper, titanium, magnesium, zirconium, tantalum, niobium, tungsten, nickel, chromium and alloys thereof. The outer layer has a metal selected from titanium, zirconium, tantalum, niobium and alloys thereof, and the coating layer has at least one conductive selected from carbon, conductive ceramics, and metal powder. A separator for a polymer electrolyte fuel cell, comprising a material.   The separator for a polymer electrolyte fuel cell according to claim 1, wherein the hardness of the metal of the outer layer is 1/3 to 3/1 in a ratio to the hardness of the intermediate layer.   2. The polymer electrolyte fuel cell separator according to claim 1, wherein the intermediate layer is a metal layer made of aluminum or an aluminum alloy, and a portion where the aluminum or aluminum alloy is exposed is coated with an alumite or boehmite film. .   2. The separator for a polymer electrolyte fuel cell according to claim 1, wherein the thickness of the outermost layer of the metal layer is 10 μm or more and the ratio of the thickness of the outermost layer to the thickness of the metal layer is 30% or less.   The separator for a polymer electrolyte fuel cell according to claim 1, wherein the coating layer has a thickness of 10 to 100 μm.   The coating layer is a coating layer composed of at least one conductive material selected from carbon, conductive ceramics, and metal powder, and a resin binder for fixing the conductive material. The separator for a polymer electrolyte fuel cell according to claim 1.   The coating layer is composed of a carbon-based conductive material containing graphite and carbon black and a binder, and the mixing ratio of the carbon-based conductive material and the binder is 15:85 to 90:10 by weight, and the carbon-based 2. The solid polymer fuel cell separator according to claim 1, wherein the blending ratio of graphite and carbon black of the conductive material is 30:70 to 90:10 by weight.   The binder of the coating layer includes at least one of fluorine resin, silicon resin, phenol resin, epoxy resin, polyimide resin, polyamide resin, polyolefin resin, furan resin, and rubber resin. Item 10. The polymer electrolyte fuel cell separator according to Item 1.   The binder of the coating layer is a copolymer of polyvinylidene fluoride (PVDF) and propylene hexafluoride (HFP) (PVDF-HFP copolymer), and the propylene hexafluoride is a binder 5 The separator for a polymer electrolyte fuel cell according to claim 1, which is -30% by weight.   The polymer separator having ionic conductivity and electrodes formed on both sides of the polymer electrolyte, and having a gas diffusion layer and a separator, the separator is the separator according to claim 1. A polymer electrolyte fuel cell characterized by the above.   A separator for a polymer electrolyte fuel cell comprising a metal layer and a conductive coating layer, wherein the metal layer is formed in a three-layer structure of outer layer / intermediate layer / outer layer, and the outer layer and the intermediate layer An intermetallic compound composed of adjacent metals is formed therebetween, and the intermediate layer is at least selected from iron, aluminum, copper, titanium, magnesium, zirconium, tantalum, niobium, tungsten, nickel, chromium and alloys thereof. The outer layer is made of a metal selected from titanium, zirconium, tantalum, niobium and alloys thereof, and the coating layer is at least one conductive material selected from carbon, conductive ceramics, and metal powder. And a separator for a polymer electrolyte fuel cell, characterized by comprising a resin binder for fixing the conductive material

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