JP2004259643A - Solid oxide fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solid oxide fuel cell superior in strength and oxidation resistance even in high temperature and oxidizing atmosphere and having a porous metal substrate with a low contact resistance with an electrode. <P>SOLUTION: The solid oxide fuel cell permits both improvement in oxidation resistance and reduction in contact resistance, because a coating containing 20 to 70 wt. % Al is formed on metal surfaces comprising the porous metal substrate. In addition, an oxidizer electrode is formed on the porous metal substrate, and consequently a coating containing Al and a high electrically conductive oxide is formed, and thus the solid oxide fuel cell that can be operated at wide temperature zone is obtained. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００５８】
表１における酸化状態の○は、図１に模式的に示すように多孔質金属基体を構成する粒子１０表面に厚さ数μｍ程度の酸化被膜１２が形成されている正常な酸化状態であり、×（異常酸化）は図２のように酸化が進行し、厚さ数十μｍ以上の酸化物１４が成長している状態である。
【００５９】
異常酸化の状態は、成長した酸化物１４が多孔質金属基体の気孔１６を埋め、ガス拡散性を阻害する。また、酸化物１４の成長により多孔質金属基体が体積膨張し、その上に形成される電極や電解質層に割れや剥離を引き起こす原因になる。異常酸化が進行すると最後には金属粒子１０が全て酸化されてしまう。
【００６０】
多孔質金属基体Ａは１０００℃、８時間の熱処理において、正常な酸化状態を示していたが、１１５０℃、８時間の熱処理後は異常酸化に到った。多孔質金属基体Ｂは１１５０℃、８時間の熱処理において、正常な酸化状態であったが、１２００℃、８時間後には異常酸化に到った。また、多孔質金属基体Ｘは１０００℃、８時間の熱処理で異常酸化していることが確認された。
【００６１】
表１より、正常な酸化状態においては表面酸化被膜１２中のアルミニウムの濃度が２０ｗｔ．％以上であり、異常酸化した状態においては２０ｗｔ．％以下であることがわかる。このことから、良好な耐酸化性を維持するためには、表面酸化被膜１２中に少なくともアルミニウムが２０ｗｔ．％以上存在する必要がある。即ち、アルミニウム２０ｗｔ．％以上の酸化被膜１２を作製すれば、良好な耐酸化性が維持でき、電池作製工程や電池運転時における多孔質金属基体の酸化進行を大幅に抑制できると考えられる。
【００６２】
なお、表面酸化被膜１２中のアルミニウムの濃度は、多孔質金属基体に用いる合金の組成、特にアルミニウムの含有率や微量添加元素の種類や量、さらに電池作製工程における熱処理温度や時間に影響されるため、これらの適正化により、表面酸化被膜中のアルミニウム濃度を２０ｗｔ．％以上に保つことは可能である。
【００６３】
【実施例２】
〔電極の作製〕
電極のスラリーを下記の方法により作製した。
【００６４】
＜スラリー溶媒の作製＞
エタノール、テルピネオール、エチルセルロース、消泡剤、分散剤を重量比で５６：３７：６．８：０．１：０．１で混合し、スラリー溶媒を作製した。
【００６５】
＜酸化剤極スラリーの作製＞
平均粒径２μｍのＬａ_０．８５Ｓｒ_０．１５ＭｎＯ_３粉末と平均粒径０．５μｍのＹＳＺ（８ｍｏｌ％Ｙ_２Ｏ_３−９２ｍｏｌ％ＺｒＯ_２）粉末とを重量比８：２の割合で混合した粉末と、上記スラリー溶媒とを重量比３：７の割合で混合することにより、酸化剤極スラリーを作製した。
【００６６】
＜燃料極スラリーの作製＞
平均粒径３μｍの酸化ニッケル粉末と平均粒径０．５μｍのＹＳＺ粉末を重量比９：１の割合で混合した粉末と、上記スラリー溶媒とを重量比１：１の割合で混合することにより、燃料極スラリーを作製した。
【００６７】
〔酸化剤極材料との接触抵抗測定〕
多孔質金属基体Ａを空気中１０００℃で８時間熱処理した試料Ａ−０、多孔質金属基体Ａを空気中１１００℃で８時間熱処理した試料Ａ−１、多孔質金属基体Ｂを空気中１０００℃で８時間熱処理した試料Ｂ−０、及び、多孔質金属基体Ｂを空気中１１００℃で８時間熱処理した試料Ｂ−１を用い、図３に示す位置に、上記の方法で作製した酸化剤極スラリー１８を塗布し乾燥させた。乾燥後、酸化剤極１８上にＰｔ網（集電体）２０を貼りつけ、集電体２０上に再度酸化剤極スラリー１８を塗布し乾燥させることにより多孔質金属基体抵抗測定用の試料２２を作製した。
【００６８】
作製した４種類の試料を、電気炉で空気中１０００℃に昇温した後、１０００〜５００℃の範囲で接触抵抗を測定した。抵抗値は、Ｐｔ集電体の単位面積当たりの値に換算した。
【００６９】
〔表面酸化被膜中の元素濃度分析〕
上記接触抵抗を測定した４種類の試料と同一条件のサンプルを用いて、表面酸化被膜中の元素濃度を分析した。分析は１サンプルにつき５〜１０箇所測定し、その平均値をとった。表２に４種類の試料の表面酸化被膜中元素濃度を示す。
【００７０】
【表２】

【００７１】
図４に７００℃における表面酸化被膜中のアルミニウム濃度と接触抵抗との関係を示す。表面酸化被膜中の元素濃度には測定個所によりばらつきがあったため、平均値及びばらつきを考慮した誤差を含めてプロットしている。
【００７２】
この結果から、表面酸化被膜１２中のアルミニウム濃度がある値を超えると接触抵抗が急激に上昇することがわかる。特に７０ｗｔ．％程度を超えるものは、酸化剤極材料１８との接触抵抗が非常に大きいことがわかる。
【００７３】
逆に７０ｗｔ．％以下であれば、比較的小さな接触抵抗が得られると考えられる。実施例１において、耐酸化性確保のために２０ｗｔ．％以上必要であることがわかったので、表面酸化被膜１２中のアルミニウム濃度が２０ｗｔ．％から７０ｗｔ．％の範囲内において、優れた耐酸化性と低い接触抵抗の両方が得られることがわかった。
【００７４】
【実施例３】
〔酸化剤極材料との接触抵抗測定〕
多孔質金属基体Ａ及び多孔質金属基体Ｂを空気中１０００℃で８時間熱処理後、図３に示す位置に、〔電極の作製〕に記載した方法で作製した酸化剤極スラリー１８を塗布し乾燥させた。乾燥後、酸化剤極１８上にＰｔ網（集電体）２０を貼りつけ、集電体２０上に再度酸化剤極スラリー１８を塗布し乾燥させることにより多孔質金属基体抵抗測定用の試料２２を作製した。
【００７５】
また、多孔質金属基体Ａと同一材料（ＨＲＥ５^Ｒ）の緻密体からなる板状試料２８について、空気中１０００℃で８時間熱処理後、図５のように、緻密体２８の表裏に〔電極の作製〕に記載した方法で作製した酸化剤極スラリー１８を塗布し乾燥させた。乾燥後、酸化剤極１８上にＰｔ網（集電体）２０を貼りつけ、集電体２０上に再度酸化剤極スラリー１８を塗布し乾燥させることにより緻密体抵抗測定用の試料３０を作製した。
【００７６】
作製した３種類の試料を、電気炉にて空気中１０００℃に昇温した後、１０００〜５００℃の範囲で抵抗測定を行った。抵抗値は、Ｐｔ集電体の単位面積当たりの値に換算した。
【００７７】
抵抗測定結果を図６に示す。多孔質金属基体Ａ、Ｂは、緻密体に比べ１〜２桁小さい抵抗値を示した。これは、多孔質金属基体２６の方が、比表面積が大きいこと、電極材料１８が多孔質金属基体２６内部に浸透（図７参照）すること等の要因により、接触領域が３次元的に広がっているためであると考えられる。
【００７８】
また、多孔質金属基体Ｂよりも多孔質金属基体Ａの接触抵抗が１桁近く小さくなった原因は、表面酸化被膜１２中のアルミニウム濃度の差が影響していると考えられる。
【００７９】
〔燃料極材料との接触抵抗測定〕
多孔質金属基体Ａを空気中１０００℃で２時間熱処理後、図３に示す位置に、〔電極の作製〕に記載した方法で作製した燃料極スラリー１８を塗布し乾燥させた。乾燥後、燃料極１８上にＰｔ網（集電体）２０を貼りつけ、集電体２０上に再度燃料極スラリー１８を塗布し乾燥させることにより、多孔質金属基体抵抗測定用の試料２２を作製した。
【００８０】
また、多孔質金属基体Ａと同一材料（ＨＲＥ５^Ｒ）の緻密体からなる板状試料２８について、空気中１０００℃で８時間熱処理後、図５のように、緻密体２８の表裏に〔電極の作製〕に記載した方法で作製した燃料極スラリー１８を塗布し乾燥させた。乾燥後、燃料極１８上にＰｔ網（集電体）２０を貼りつけ、集電体２０上に再度燃料極スラリー１８を塗布し乾燥させることにより緻密体抵抗測定用の試料３０を作製した。
【００８１】
作製した２種類の試料を、電気炉にて水素雰囲気中３００〜１０００℃の所定温度に加熱して抵抗測定を行った。抵抗値は、Ｐｔ集電体の単位面積当たりの値に換算した。
【００８２】
抵抗測定結果を図８に示す。水素雰囲気での抵抗は温度によりあまり変化せず、多孔質金属基体Ａでは０．２〜０．３Ωｃｍ^２、緻密体では３〜４Ωｃｍ^２であった。多孔質金属基体Ａの接触抵抗が緻密体の試料に比べて１桁以上小さくなった原因は、酸化剤極材料との接触抵抗と同様に、多孔質金属基体２６の方が、比表面積が大きいこと、電極材料１８が内部に浸透すること等により接触領域が３次元的に広がっているためであると考えられる。
【００８３】
また、燃料極材料との接触抵抗は温度にあまり依存せず、温度の上昇により抵抗が若干大きくなった。この結果は、燃料極材料であるニッケルと多孔質金属基体を構成する金属粒子とが直接接触していること、及び、燃料極と多孔質金属基体との間の電子伝導において、この接触部分を介した電子伝導が支配的であることを示唆している。すなわち、還元雰囲気においては表面酸化被膜の成長は遅いので、表面酸化被膜の微細な欠陥部分において金属面が露出し、この部分にニッケル粒子が接触していることが考えられる。
【００８４】
【実施例４】
〔酸化剤極材料との接触抵抗測定〕
空気中１０００℃で８時間熱処理した多孔質金属基体Ｂを用いて、図３に示すような酸化剤極材料との接触抵抗を測定する試料▲１▼を作製した。次に、この試料▲１▼を空気中１１００℃で２時間熱処理（試料▲２▼）し、さらに試料▲２▼を空気中１１００℃で６時間熱処理、即ち、試料▲１▼に対して空気中１１００℃で合計８時間熱処理を行った試料▲３▼、そして試料▲３▼を空気中１１００℃で８時間（合計で１６時間）熱処理を行った試料▲４▼を作製し、試料▲１▼〜▲４▼のそれぞれについて抵抗測定を行った。
【００８５】
抵抗測定結果を図９に示す。１１００℃で熱処理を行うことにより、抵抗が低下した。また、１１００℃での熱処理時間が長くなるほど、直線の傾きが緩やかになり、特に低温側での抵抗が小さくなる傾向が見られた。
【００８６】
〔表面酸化被膜中の元素分析〕
上記抵抗測定における試料▲３▼、▲４▼と同一条件で作製した試料の表面酸化被膜中の元素分布を、実施例１と同一の方法により測定した。この結果を表３に示す。
【００８７】
【表３】

【００８８】
表３より、酸化剤極材料と接触した状態で空気中１１００℃の熱処理を行うと、多孔質金属基体単体に比べ、表面酸化被膜中におけるアルミニウム濃度が低く、ランタン、マンガンの濃度が増加していることが分かる。これは、酸化剤極材料中のランタンマンガナイト（Ｌａ_０．８５Ｓｒ_０．１５ＭｎＯ_３）と多孔質金属基体表面の酸化被膜とが固相反応したことが原因である。
【００８９】
以上の結果より、空気中１１００℃の熱処理により、多孔質金属基体と酸化剤極材料との接触抵抗が低下したのは、多孔質金属基体表面と酸化剤極材料とが固相反応し、相対的に表面酸化被膜中のアルミニウム濃度が低下したことが原因であると考えられる。また、図９における直線の傾きが小さくなり、特に低温側での抵抗が小さくなった原因は、表面酸化被膜に含まれる鉄やクロムがランタンマンガナイトと反応し、表面酸化被膜中に電子伝導の活性化エネルギーが小さい高伝導性のＬａＦｅＯ_３、ＬａＣｒＯ_３、ＬａＭｎＯ_３などのペロブスカイト型酸化物が生成したことによるものと考えられる。
【００９０】
〔Ｐｔペーストを用いた接触抵抗測定〕
比較のため、酸化剤極スラリーの代わりにＰｔペーストを用い、図３に示す接触抵抗を測定する試料▲１▼を作製した。次に、試料▲１▼に対して空気中１１００℃で合計８時間熱処理を行った試料▲２▼、そして試料▲２▼を空気中１１００℃で８時間（合計で１６時間）熱処理を行った試料▲３▼を作製し、試料▲１▼〜▲３▼のそれぞれについて抵抗測定を行った。
【００９１】
この結果を図１０に示す。酸化剤極材料を用いた場合よりも全体的に抵抗値は小さくなったが、１１００℃の熱処理による抵抗の低下は起こらなかった。また、熱処理時間が長くなっても直線の傾きは小さくならなかった。このことからも、図９のように熱処理により直線の傾きが小さくなった原因は、多孔質金属基体表面と酸化剤極材料との反応によるものである可能性が高いことが立証できた。
【００９２】
Ｐｔペーストを用いた場合、酸化剤極スラリーより抵抗が１桁近く小さくなった。これは、Ｐｔの導電率がランタンマンガナイトよりも２桁以上高いこと、またＰｔの焼結性が高いため、多孔質金属基体を構成する粒子とのミクロな接触面積が多いことが原因であると考えられる。
【００９３】
以上から、多孔質金属基体表面の酸化被膜と酸化剤極材料とを高温で固相反応させることにより、表面酸化被膜中のアルミニウム濃度が低下すると同時に高導電性の酸化物が生成することがわかった。
【００９４】
この方法により、多孔質金属基体と酸化剤極との接触抵抗を低減でき、特に、低温側での抵抗が低下することによって、広い温度域で低い接触抵抗を示す実用的な多孔質金属基体を得ることができる。
【００９５】
なお、本実施例では酸化剤極材料にランタンストロンチウムマンガナイト（Ｌａ，Ｓｒ）ＭｎＯ_３を用いたが、その他にも酸化剤極材料として一般的に用いられるランタン系ペロブスカイト型酸化物であってもよい。
【００９６】
また、多孔質金属基体の表面処理のため、酸化剤極材料を塗布する前に、別の材料を塗布し熱処理することも考えられる。この場合、多孔質金属基体表面の酸化被膜と固相反応を起こし、表面酸化被膜中に適度なアルミニウム濃度を保ちつつ、導電性酸化物を生成する材料を用いる。このような材料としては、チタン、クロム、マンガン、鉄、コバルト、ニッケル、銅、イットリウム、マグネシウム、カルシウム、ストロンチウム、バリウム、ランタン、セリウム等を含有する酸化物が考えられる。
【００９７】
熱処理温度は１１００℃の場合を示したが、多孔質金属基体の耐熱温度以下で上記反応が適度に進行する条件であれば、熱処理温度は１１００℃に限定されない。
【００９８】
【実施例５】
〔比表面積の測定〕
多孔質金属基体Ａを空気中１０００ ℃で１時間熱処理後、数ｍｍ程度の大きさに粉砕し、窒素吸着法（ＱＵＡＮＴＡＣＨＲＯＭＥ ＡＵＴＯＳＯＲＢ１）により比表面積を測定した。
【００９９】
その結果、多孔質金属基体Ａの比表面積は０．２６ｍ^２／ｇであった。
【０１００】
〔多孔質金属基体と電極との有効接触面積の計算〕
次に、多孔質金属基体について外径２０ｍｍ、内径１７ｍｍ、長さ１０ｍｍ、気孔率４０％と仮定し、多孔質金属基体と電極との有効な接触面積について計算する。
【０１０１】
上記多孔質金属基体のみかけの体積は、
【０１０２】
【数１】

【０１０３】
体積０．８７１ｃｍ^３に対して、気孔率が４０％であるため、バルク（金属部分）の体積は、
【０１０４】
【数２】

【０１０５】
多孔質金属基体材料はフェライト系ステンレスであり、真密度は約８ｇ／ｃｍ^３であるので、上記多孔質金属基体の質量は、
【０１０６】
【数３】

【０１０７】
多孔質金属基体の比表面積の実測値は０．２６ｍ^２／ｇであったので、上記多孔質金属基体の表面積は、
【０１０８】
【数４】

【０１０９】
外側表面に酸化剤極スラリーを塗布したとき、図７に示すように、スラリーが多孔質金属基体表面から約５０μｍ内部まで染みこんでいることをＳＥＭ観察により確認しているので、微視的に見た多孔質金属基体を構成する粒子と酸化剤極との接触面積は、
【０１１０】
【数５】

【０１１１】
あることになる。
【０１１２】
よって、見かけ上の酸化剤極の塗布面積：２×３．１４×１＝６．２８ｃｍ^２に対して、実際には６０倍近い接触面積があることがわかった。
【０１１３】
以上の計算結果から、多孔質金属基体が多孔質体であり、高い比表面積をもつこと、多孔質金属基体内部に電極が浸透していること等により、緻密体の場合に比べ、約６０分の１まで低い接触抵抗が得られることになる。
【０１１４】
上記計算法により、多孔質金属基体の比表面積、電極の浸透深さをパラメータとして、多孔質金属基体／電極間の有効接触面積（見かけの面積に対する比）を求めたものを図１１に示す。図１１より、有効接触面積を増加させるためには、多孔質金属基体の比表面積を増大させるか、電極の内部への浸透深さを増大させればよいことがわかる。
【０１１５】
多孔質金属基体の比表面積は、構成する粒子の粒子径分布や、焼結度により影響されるが、以下のように概略の値を予想することは可能である。例えば、構成する粒子の表面が滑らかであり、粒径が全て同じであり、焼結度が低いと仮定すると、図１２に示すように粒子径と比表面積の関係が求まる。図１２より、粒子径１００μｍのときには、比表面積は０．０１ｍ^２／ｇ程度、また粒子径１０μｍのときには、比表面積は０．１ｍ^２／ｇ程度であると予想される。粒子径が小さくなれば、比表面積は増大する。また、粒子の表面はかならずしも平滑ではないため、比表面積は予想よりも高めになることが多いと思われる。
【０１１６】
また、本実施の形態以外にも金属繊維により基体を作製することも可能であるが、この場合も同様に繊維径と比表面積の関係を予想することが可能である。基体を構成する繊維の表面が滑らかであり、繊維径が均一であると仮定すれば、繊維径５０μｍのときには、比表面積は０．０１ｍ^２／ｇ程度、繊維径５μｍのときには、比表面積は０．１ｍ^２／ｇ程度になると予想される。繊維径が小さくなれば、比表面積は増大する。また、粒子の場合と同様に、表面はかならずしも平滑ではないため、比表面積は予想よりも高めになることが多いと思われる。
【０１１７】
電極の浸透深さは、多孔質金属基体上に塗布する電極スラリーの溶媒種や溶媒／粉末混合比によって、多孔質金属基体との濡れ性やスラリー粘度を調整することで制御可能である。すなわち、濡れ性を高く、スラリー粘度を低くすることにより、多孔質金属基体内部まで浸透させることができる。
【０１１８】
多孔質金属基体の比表面積については、あまり大きくすると耐酸化性が低下する傾向がある。このため実用的には １ｍ^２／ｇ程度以下が望ましい。また、電極の浸透深さについては、あまり大きすぎると多孔質金属基体中の気孔を塞ぎ、ガス拡散を阻害するため、実用的には５００μｍ程度以下が望ましい。ただし、多孔質金属基体を構成する粒子の表面のみを薄く被覆する場合はこの限りではない。これらの制約条件をふまえて、多孔質金属基体／電極間の接触面積をできるだけ大きくするように、図１１に示した関係から比表面積、電極浸透深さを選定することが望ましい。
【０１１９】
なお本結果は、酸化剤極材料との接触抵抗のみならず、燃料極材料との接触抵抗にも同様に適用できる。また、多孔質金属基体内部に電極材料が浸透する場合を述べたが、集電効果を高めるため、電極材料と類似した導電性の高い材料を予め多孔質金属基体に塗りこむことも可能である。例えば、酸化剤極材料にランタンマンガナイト系酸化物を用いる場合に、集電用材料としてランタンコバルタイト系酸化物を多孔質金属基体に予め塗りこむこと等が可能である。
【０１２０】
【実施例６】
〔粒子内部のアルミニウム濃度測定〕
多孔質金属基体Ｂを１０００℃で８時間熱処理した後の試料Ｂ−１と多孔質金属基体Ｂを１１００℃で８時間熱処理した後の試料Ｂ−２について、多孔質金属基体を構成する粒子内部のアルミニウム濃度をＸ−Ｒａｙ Ｍｉｃｒｏａｎａｌｙｚｅｒ（ＨＯＲＩＢＡ ＥＭＡＸ−３７００）にて測定した。測定に際し、粒子径により内部のアルミニウム濃度に差が見られたため、直径１００μｍ程度の粒子と直径５０μｍ程度の粒子とをそれぞれ５〜１０個ずつ選び、粒子中央部のアルミニウム濃度を測定した。
【０１２１】
測定結果を図１３に示す。熱処理前（４．９ｗｔ．％）に比べ、粒子内部のアルミニウム濃度は、熱処理後には全体的に低く、特に熱処理温度が高いものほど低くなった。また、同一熱処理条件においては、粒子径が小さいものほどアルミニウム濃度が低くなった。
【０１２２】
〔粒子内部のアルミニウム濃度の計算〕
次に、粒子内部のアルミニウム濃度について、下記の仮定に基づいて算出する。熱処理前の多孔質金属基体には、粒子内部に５ｗｔ．％のアルミニウムが均一に含まれている。これを熱処理することにより、粒子内部のアルミニウムは表面へ拡散し、基体表面にアルミニウムを高い濃度で含有する酸化被膜が厚さ数μｍ形成される。
【０１２３】
酸化被膜の厚さはＳＥＭによる観察、アルミニウム濃度はＥＤＸにより測定できているので、表面酸化被膜に含まれるアルミニウム量を酸化被膜の厚さとアルミニウム濃度より求める。この表面酸化被膜に含まれるアルミニウム量を、熱処理前に粒子内部に存在していたアルミニウム量から差し引き、粒子の体積で割れば、熱処理後における粒子内部のアルミニウム濃度を予測することができる。
【０１２４】
多孔質金属基体Ｂを１０００℃で８時間熱処理した試料Ｂ−１の表面酸化被膜の厚さは１．５μｍ、表面酸化被膜中のアルミニウム濃度は５５％であった。
【０１２５】
また、多孔質金属基体Ｂを１１００℃で８時間熱処理した試料Ｂ−２の表面酸化被膜の厚さは２．５μｍ、表面酸化被膜中のアルミニウム濃度は６０％であった。いずれの試料においても、表面酸化被膜の厚さは粒子径には依存しなかった。
【０１２６】
上記の値より、粒子内部のアルミニウム濃度を計算した結果を図１３に合わせて示す。計算結果は測定結果と整合しており、このモデルによるアルミニウム濃度の計算は妥当であると考えられる。
【０１２７】
〔最適アルミニウム含有率の導出〕
上記の結果より、多孔質金属基体を構成する粒子の径が小さくなるほど、粒子の体積に対する表面積が大きくなるので、内部のアルミニウム濃度が低下する。アルミニウム濃度が低下すれば、表面酸化被膜中に十分なアルミニウム濃度を確保することができなくなり、保護被膜として機能しない異常酸化に到る。このため、多孔質金属基体を小さな粒子から構成する場合は、多孔質金属基体に用いる合金のアルミニウム含有率を増加させる必要があると考えられる。
【０１２８】
実施例５で述べた通り、多孔質金属基体と電極との有効接触面積を十分確保するためには、粒子径を小さくし、比表面積を大きくする必要がある。粒子表面に数μｍの酸化被膜が形成されることを考慮すると、平均粒子径は１０〜５０μｍ程度の範囲が適当である考えられる。また、実施例１より、表面酸化被膜中のアルミニウム濃度は４０ｗｔ．％程度確保することが望ましい。
【０１２９】
そこで上記の計算において、表面酸化被膜の厚さを２μｍ、表面酸化被膜中のアルミニウム濃度を４０ｗｔ．％と仮定し、１０〜５０μｍの大きさの粒子について、粒子内部のアルミニウム濃度を０ｗｔ．％以上保持するために必要な多孔質金属基体のアルミニウム含有率を導出した。
【０１３０】
結果を図１４に示す。この結果から、多孔質金属基体を粒子径５０μｍの粒子から構成する場合、アルミニウム含有率は２．５ｗｔ．％以上、粒子径２０μｍの粒子から構成する場合は６．３ｗｔ．％以上、粒子径１０μｍの粒子から構成する場合は１２．６ｗｔ．％以上のアルミニウム含有率が必要であることがわかった。
【０１３１】
逆に、この条件を満たせば、多孔質金属基体を構成する粒子内部においてアルミニウムが欠乏する状態は起こらず、表面酸化被膜中に十分なアルミニウム濃度が確保することができる。従って、このような多孔質金属基体は、良好な耐酸化性を維持することができ、ＨＲＥ５^Ｒの耐熱温度のカタログ値である１２５０℃に近い耐熱性が得られると考えられる。
【０１３２】
【実施例７】
〔単電池の作製〕
電解質のスラリーを下記の方法により作製した。
【０１３３】
＜スラリー溶媒の作製＞
エタノール、テルピネオール、エチルセルロース、消泡剤 、分散剤を重量比で５６：３７：６．８：０．１：０．１で混合し、スラリー溶媒を作製した。
【０１３４】
＜電解質スラリーの作製＞
平均粒径０．５μｍのＹＳＺ粉末と上記スラリー溶媒とエタノールとを重量比２０：５０：３０の割合で混合することにより、電解質スラリーを作製した。
【０１３５】
次に、〔電極の作製〕に記載した方法で作製した電極スラリーと上記電解質スラリーを用いて、下記の方法により図１５に示すようなセル３４を作製した。
【０１３６】
＜単電池の作製＞
多孔質金属基体Ａを酸化剤極スラリー中へ浸漬し引き上げる方法（ディップ法）により表面に酸化剤極スラリーを塗布し、６０℃程度で乾燥後、空気中１０００℃で２時間熱処理を行った。
【０１３７】
多孔質金属基体内部へのスラリーの浸透深さはスラリーの粘度により調整可能であるが、本実施例での浸透深さは５０μｍ程度であった。また、酸化剤極３６の厚さは５０μｍ程度であった。
【０１３８】
次に、酸化剤極３６の上に電解質スラリーを同様のディップ法で塗布し、６０℃程度で乾燥後、空気中１０００℃で２時間熱処理を行った。電解質層３８の厚さは２０μｍ程度であった。
【０１３９】
さらに、電解質層３８の上に燃料極スラリーを同様のディップ法で塗布し、６０℃程度で乾燥後、空気中１０００℃で２時間熱処理を行った。燃料極４０の厚さは５０μｍ程度であった。
【０１４０】
〔電池特性試験法及び結果〕
多孔質金属基体２６の片側の端は、多孔質金属基体２６と同一材料（ＨＲＥ５^Ｒ）からなる緻密な円板（直径２０ｍｍ×厚さ２ｍｍ）を蓋材４２とし、これを、金リング４４を介して熱圧着することにより閉じた。
【０１４１】
金リング４４は多孔質金属基体２６と上記蓋材４２とを接着すると同時に隙間からのガスリークを防止するシール材として機能する。熱圧着は、荷重をかけた状態でセルを運転温度まで上昇させる過程において自然になされる。なお、本実施例では金リング４４を用いたが、多孔質金属基体２６と蓋材４２を溶接しても良い。
【０１４２】
単電池を電気炉中で運転温度（７００℃）まで昇温した後、燃料極側に水素、酸化剤極側に空気を流し、電圧−電流特性を測定した。電池特性測定のための集電は、白金線を用いて行った。
【０１４３】
電池特性を図１６、図１７に示す。７００℃において安定した発電特性が確認され、１０ｍＡ／ｃｍ^２で１０００時間連続発電が可能であった。
【０１４４】
本実施例では、多孔質金属基体２６上に酸化剤極３６、電解質層３８、燃料極４０の順に配置した。本発明による抵抗低減のメリットは、多孔質金属基体２６を酸化剤極３６側に用いることにより最大に発揮されると思われるが、多孔質金属基体２６上に燃料極４０、電解質層３８、酸化剤極３６の順に配置してもよい。また、セルの構造は円筒形の方が、耐熱衝撃性などの点で有利であるが、本発明は平板形セルにも適用できる。
【０１４５】
本実施例では酸化剤極３６材料として、ランタンマンガナイト系酸化物を用いたが、これ以外にも低温での電極活性の高いランタンコバルタイト系、ランタン鉄コバルタイト系などを用いても良い。
【０１４６】
また、電解質層３８材料として８ｍｏｌ％イットリア添加安定化ジルコニアを使用したが、これに限定されるものではない。添加量が３ｍｏｌ％や１０ｍｏｌ％のイットリア添加安定化ジルコニアでも、スカンジア添加ジルコニア、サマリアドープセリア、ランタンガレートなどを用いても良い。
【０１４７】
さらに、燃料極４０材料として、ニッケルとＹＳＺの混合物を用いたが、ニッケルの代わりに鉄、銅やこれらを合金化したものでも、ＹＳＺの代わりにスカンジア添加ジルコニアやセリア系酸化物などを用いてもよい。その他、炭化水素系燃料を用いる場合の特性を向上させるため、微量の白金、金、銀、パラジウムなどの貴金属を分散させてもよい。
【０１４８】
実施例１〜７では、鉄基のフェライト系ステンレス鋼を用いたが、表面にアルミニウムを多く含む保護性の酸化被膜が形成されるものであれば、ニッケル基やコバルト基の合金であっても、本発明は適用できる。
【０１４９】
本発明の多孔質金属基体２６に適用することが可能な合金材料としては、アルミニウムを数ｗｔ．％程度以上含有し、表面にアルミニウムを保護性の酸化被膜が形成されるものが望ましいが、アルミニウムを含まない金属材料に、拡散浸透法により表面からアルミニウムを拡散させ、アルミニウムと合金化してもよい。
【０１５０】
【発明の効果】
本発明の請求項１の固体酸化物燃料電池は、固体電解質層を挟んで酸化剤極と燃料極とが配置されたセルが、多孔質金属基体上に形成される固体酸化物燃料電池において、多孔質金属基体は、多孔質金属基体を構成する金属表面に、アルミニウムを含有する被膜が形成されている金属からなるので、良好な耐酸化性を確保することができる。これにより、電池作製工程を１０００℃以上の高温酸化雰囲気で行うことが可能となる。
【０１５１】
また、多孔質基体にセラミックスを用いる従来の技術に比べ、多孔質基体材料に安価な金属材料を用いるため、材料コストの低減が図れ、且つ、内部抵抗が小さく高い出力密度を示す長期安定性に優れた電池が提供できる。
【０１５２】
さらに、多孔質金属基体は強度、熱伝導性が高いため、電池内の温度分布を均一化し、耐熱衝撃性においても大幅な向上が見こめるので、急激な負荷変動や、電池温度の上昇・下降に対して、耐久性に優れる電池が提供できる。
【０１５３】
本発明の請求項２の固体酸化物燃料電池は、固体電解質層を挟んで酸化剤極と燃料極とが配置されたセルが、多孔質金属基体に酸化剤極が対向するように形成される固体酸化物燃料電池において、多孔質金属基体は、多孔質金属基体を構成する金属表面に、アルミニウムと、基体を構成する金属に含まれる元素と酸化剤極に含まれる元素との固相反応により生成される高導電性酸化物と、を含有する被膜が形成されている金属からなるので、多孔質金属基体と酸化剤極との接触抵抗を低減することができ、特に、低温域での接触抵抗が低下するので、広い温度域で運転可能な固体酸化物燃料電池が得られる。
【０１５４】
本発明の請求項３の固体酸化物燃料電池は、請求項１または請求項２記載の固体酸化物燃料電池において、多孔質金属基体を構成する金属表面に形成される被膜のアルミニウム含有率を２０〜７０ｗｔ．％とするので、電池作製工程や電池運転時における多孔質金属基体の酸化進行を大幅に抑制することができ、且つ、多孔質金属基体と電極との接触抵抗を小さくすることができる。
【０１５５】
本発明の請求項４の固体酸化物燃料電池は、請求項１または請求項２または請求項３記載の固体酸化物燃料電池において、多孔質金属基体の比表面積を０．０１〜１ｍ^２／ｇとするので、電極との有効接触面積を増加させ、接触抵抗を低下させることができ、且つ、多孔質金属基体の耐酸化性を維持することができる。
【０１５６】
本発明の請求項５の固体酸化物燃料電池は、請求項１または請求項２または請求項３または請求項４記載の固体酸化物燃料電池において、多孔質金属基体を構成する金属粒子の平均粒子径を１０〜５０μｍとしているので、理想的な比表面積を有する多孔質金属基体を作製することができる。
【０１５７】
本発明の請求項６の固体酸化物燃料電池は、請求項１または請求項２または請求項３または請求項４または請求項５記載の固体酸化物燃料電池において、多孔質金属基体を構成する合金のアルミニウム含有率を２．５ｗｔ．％以上とするので、表面酸化被膜へ十分なアルミニウムを供給することができ、良好な耐酸化性を維持する多孔質金属基体が得られる。
【図面の簡単な説明】
【図１】多孔質金属基体の金属表面に形成される表面酸化膜の正常な酸化状態を示す断面模式図である。
【図２】多孔質金属基体の金属表面に形成される表面酸化膜の異常な酸化状態を示す断面模式図である。
【図３】多孔質金属基体と電極材料との接触抵抗を測定する試料の模式図である。
【図４】空気雰囲気中７００℃における表面酸化被膜中のＡｌ濃度と接触抵抗との関係を示すグラフである。
【図５】緻密体と電極材料との接触抵抗を測定する試料の模式図である。
【図６】空気雰囲気中における多孔質金属基体と緻密体の温度と接触抵抗との関係を示すグラフである。
【図７】多孔質金属基体に電極材料が浸透している状態を示す断面模式図である。
【図８】水素雰囲気中における多孔質金属基体と緻密体の温度と接触抵抗との関係を示すグラフである。
【図９】空気雰囲気中における各種熱処理試料の温度と接触抵抗との関係を示すグラフである。
【図１０】空気雰囲気中における各種熱処理試料の温度とＰｔペースト接触抵抗との関係を示すグラフである。
【図１１】多孔質金属基体の比表面積と接触面積倍率との関係を示すグラフである。
【図１２】多孔質金属基体の粒子径と比表面積との関係を示すグラフである。
【図１３】多孔質金属基体の粒子径と熱処理後の粒子内部のＡｌ濃度との関係を示すグラフである。
【図１４】多孔質金属基体の粒子径と基体材料として要求されるＡｌ含有率との関係を示すグラフである。
【図１５】作製した単電池の模式図である。
【図１６】作製した単電池における初期発電特性を示すグラフである。
【図１７】作製した単電池における連続運転特性を示すグラフである。
【図１８】従来の円筒形固体酸化物燃料電池の単電池の断面模式図である。
【図１９】従来の円筒形固体酸化物燃料電池の単電池の断面拡大模式図である。
【符号の説明】
１０ …金属粒子
１２ …酸化被膜
１４ …酸化物
１６ …気孔
１８ …電極材料
２０ …集電体
２２ …多孔質金属基体抵抗測定用試料
２６ …多孔質金属基体
２８ …緻密体板状試料
３０ …緻密体抵抗測定用試料
３４ …セル
３６ …酸化剤極
３８ …電解質層
４０ …燃料極
４２ …蓋材
４４ …金リング[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a solid oxide fuel cell, and more particularly to a solid oxide fuel cell provided with a porous metal substrate having excellent strength and oxidation resistance and low contact resistance with an electrode.
[0002]
[Prior art]
Solid oxide fuel cells (SOFC) are characterized by high operating temperatures and high power generation efficiency. Conventionally, SOFCs generally operate at a high temperature of about 1000 ° C. However, in recent years, operating temperatures have been increasing due to the need to use low-cost materials and to improve long-term durability including starting and stopping cells. Developments aimed at lowering the temperature (about 500 to 700 ° C.) have been actively conducted.
[0003]
In order to lower the operating temperature of the SOFC, an electrode and a thin-film (10 to 50 μm) electrolyte are formed on a porous substrate in order to reduce the resistance due to oxide ion conduction in the electrolyte. It is necessary to adopt a “membrane cell”.
[0004]
The same material as the oxidant electrode or the fuel electrode is generally used for the porous substrate of the supporting membrane type cell. The oxidant electrode material is a lanthanum manganite-based oxide, and the fuel electrode material is nickel. / Zirconia cermet (NiO / YSZ) and the like have been used.
[0005]
However, the above materials have insufficient mechanical strength as a porous substrate, particularly insufficient thermal shock resistance, and have a problem that the porous substrate is easily damaged by a rapid temperature change or a temperature gradient.
[0006]
On the other hand, as a means for solving this, a type using a heat-resistant metal for a porous substrate has been developed. For example, Patent Literature 1 proposes using a NiCrAl-based, NiCrFe-based, or NiCrFeAl-based material that forms alumina as a protective film as a base material. Patent Literature 2 proposes using a heat-resistant alloy having a low coefficient of thermal expansion as a base material. Patent Literature 3 discloses that a ferrite-based stainless steel is used as a base material. It has been proposed to use a sintered body of an Fe-Cr-Al-based heat-resistant alloy having excellent mechanical strength and reliability.
[0007]
[Patent Document 1]
JP-A-6-29024
[0008]
[Patent Document 2]
JP-A-6-243879
[0009]
[Patent Document 3]
JP-A-8-138690
[0010]
[Patent Document 4]
JP-A-9-63605
[0011]
[Patent Document 5]
Japanese Patent Application No. 2002-156076
As described above, since the porous metal substrate is required to have excellent heat resistance, it is roughly classified as a candidate material into an alloy that forms a protective film mainly composed of chromia on the surface (a chromia forming alloy) and a An alloy (alumina-forming alloy) that forms a protective film containing alumina as a main component is considered.
[0012]
The chromia-forming alloy is made of Cr having a thickness of about 10 to 20 μm and ₂ O ₃ An oxide film containing a large amount of acts as a protective film and maintains oxidation resistance.
[0013]
However, when the electrode is fired at 1000 ° C. or more in the air and the electrolyte is baked during the production of the battery, the oxide grows abnormally on the surface of the particles constituting the porous metal substrate, causing a problem of volume expansion and a decrease in gas permeability. .
[0014]
In addition, even when the battery manufacturing process is performed in a vacuum or an inert gas atmosphere, the oxidation of the surface of the porous metal substrate proceeds rapidly during the battery operation at about 600 to 800 ° C., and the battery characteristics are greatly deteriorated. There is.
[0015]
Furthermore, chromium oxide evaporates from the surface and diffuses and mixes into the electrode, which may degrade electrode characteristics.If used on the fuel electrode side, oxidation and carburization due to water vapor and carbon components contained in the fuel Due to such issues, there is concern about long-term stability.
[0016]
In contrast, the alumina-forming alloy has a thickness of about 2 to 3 μm formed on the surface. ₂ O ₃ A thin oxide film containing a large amount of shows excellent oxidation resistance and corrosion resistance. Further, since the vapor pressure at a high temperature is small, it is more stable than chromia in terms of evaporation. As an example of the alumina-forming alloy, an alloy based on Fe-Cr, Ni-Cr and CO-Cr is 2 wt. % Or more is generally added.
[0017]
Cabot 214 (Ni: 75.0, Fe: 2.5, Cr: 16.0, Al: 4.5, Y: 0.01 wt.%), Nimonic 105 (Ni: 53. 0, Co: 20.0, Cr: 15.0, Mo: 5.0, Al: 4.7, Ti: 1.2, C: 0.13, B: 0.005, Zr: 0.10 wt. %), Nimonic 115 (Ni: 60.0, Co: 13.2, Cr: 14.3, Al: 4.9, Ti: 3.7, C: 0.15, B: 0.16, Zr: 0) .04 wt.%) And Haynes Alloy 214 (Cr: 16, Al: 4.5 wt.%, Ni balance).
[0018]
In stainless steel containing iron as a main component, SUH21 (C: ０．１0.10 or less, Si: １．５1.50 or less, Mn: １．０1.00 or less, P: ０．０0.040 or less, S: ０0 0.030 or less, Ni: ~ 0.60 or less, Cr: 17.00 to 21.00, Al: 2.00 to 4.00 wt.%), 18SR (C: to 0.05, Si: to 1.0 , Mn: ∼0.5, P: ∼0.02, S: ∼0.01, Ni: ∼0.5, Cr: 18, Al: 2, Ti: 0.4wt.%), FCH-1 ( Iron chromium type 1 for electric heating (C: 〜0.10 or less, Si: １．５1.5 or less, Mn: １．０1.0 or less, Cr: 23 to 26, Al: 4 to 6 wt.%), FCH-2 (2 types of iron chromium for electric heating) (C: up to 1.0, Si: up to 1.5, Mn: up to 1.0, Cr: 17 to 21, Al: 2 to 4 wt.% ), 20Cr-5Al (C: -0.15 or less, Si: -1.00 or less, Mn: -1.0 or less, P: -0.040 or less, S: -0.030 or less, Cr: 19- or less) 21, Al: 4.5-6.0, Ti: 0.10 wt.%), And those having improved oxidation resistance and high-temperature strength by adding a rare earth element or the like thereto. is there.
[0019]
If the above-described alumina-forming alloy can be used as the material of the porous metal substrate, it is possible to realize a porous metal substrate that can withstand a high-temperature oxidizing atmosphere in a battery manufacturing process and has excellent long-term stability during battery operation. Be expected.
[0020]
[Problems to be solved by the invention]
However, the alumina-forming alloy has a problem that the electric resistance of the surface oxide film is high and the contact resistance with the electrode is very large. For this reason, it has been considered that it is difficult to apply to the material of the porous metal substrate.
[0021]
Further, when a porous metal substrate made of an alumina-forming alloy is produced and the battery production process is performed in a high-temperature oxidizing atmosphere of 1000 ° C. or more, the porous metal substrate becomes abnormal at a temperature lower than the heat resistance temperature of the bulk (dense body). There was a problem of oxidation.
[0022]
The present invention has been made in view of the above-described problems, and uses an alloy in which a protective oxide film containing aluminum is formed on the surface as a material of a porous metal substrate, and has excellent oxidation resistance. An object is to achieve both low contact resistance with an electrode.
[0023]
[Means for Solving the Problems]
The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, have found that the problems can be solved by devising a manufacturing method using, for example, stainless steel excellent in strength and oxidation resistance as a base tube material. Having found this, the present invention has been completed.
[0024]
That is, in the solid oxide fuel cell according to claim 1 of the present invention for solving the above-mentioned problems, a cell in which an oxidizer electrode and a fuel electrode are arranged with a solid electrolyte layer interposed therebetween is formed on a porous metal substrate. In the solid oxide fuel cell according to the present invention, the porous metal substrate is made of a metal having a coating containing aluminum formed on the surface of the metal constituting the porous metal substrate.
[0025]
According to the first aspect of the present invention, it is possible to suppress the progress of oxidation of the porous metal substrate during the battery manufacturing process and during battery operation.
[0026]
In the solid oxide fuel cell according to the second aspect of the present invention, the cell in which the oxidizer electrode and the fuel electrode are arranged with the solid electrolyte layer interposed therebetween is formed such that the oxidizer electrode faces the porous metal substrate. In the solid oxide fuel cell, the porous metal substrate is formed by bonding aluminum and an element contained in the metal constituting the porous metal substrate and an element contained in the oxidizer electrode to a metal surface constituting the porous metal substrate. A highly conductive oxide generated by a phase reaction; and a metal having a coating formed thereon.
[0027]
According to the second aspect of the present invention, the contact resistance between the porous metal substrate and the oxidant electrode can be reduced. In particular, since the contact resistance in a low temperature range is reduced, the solid oxide operable in a wide temperature range can be used. Fuel cell is obtained.
[0028]
The solid oxide fuel cell according to claim 3 of the present invention is the solid oxide fuel cell according to claim 1 or 2, wherein the aluminum content of the film formed on the metal surface constituting the porous metal substrate is 20%. ~ 70 wt. %.
[0029]
According to the third aspect of the present invention, the coating formed on the metal surface of the porous metal substrate contains aluminum in an amount of 20 wt. % Or more, the progress of oxidation of the porous metal substrate during the battery manufacturing process and during battery operation can be greatly suppressed, and aluminum is contained in an amount of 70 wt. %, The contact resistance between the porous metal substrate and the electrode can be reduced.
[0030]
A solid oxide fuel cell according to a fourth aspect of the present invention is the solid oxide fuel cell according to the first or second or third aspect, wherein the specific surface area of the porous metal substrate is 0.01 to 1 m. ² / G.
[0031]
According to the invention of claim 4, the specific surface area of the porous metal substrate is 0.01 m ² / G or more, the effective contact area between the porous metal substrate and the electrode can be increased, and the contact resistance can be reduced. Also, in order to maintain the oxidation resistance of the porous metal substrate, 1 m ² / G or less is practical.
[0032]
The solid oxide fuel cell according to claim 5 of the present invention is the solid oxide fuel cell according to claim 1, 2, 3, or 4, wherein the average particle size of the metal particles constituting the porous metal substrate is The diameter is 10 to 50 μm.
[0033]
According to the fifth aspect of the present invention, it is possible to produce a porous metal substrate having an ideal specific surface area by selecting the average particle diameter of the metal particles constituting the porous metal substrate to be 10 to 50 μm. Can be.
[0034]
The solid oxide fuel cell according to claim 6 of the present invention is the solid oxide fuel cell according to claim 1 or 2, or 3 or 4 or 5, wherein Of 2.5 wt. % Or more.
[0035]
According to the invention of claim 6, aluminum is added in an amount of 2.5 wt. % Of a metal alloy can be supplied to the surface oxide film, and a porous metal substrate maintaining good oxidation resistance can be obtained.
[0036]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail.
[0037]
The metal material forming the porous substrate tube of the solid oxide fuel cell of the present invention is a heat-resistant metal. Since this single cell is manufactured at about 800 to 1100 ° C. and is operated at 500 ° C. or more, the metal material forming the porous metal substrate uses a heat-resistant metal that is stable in air at these temperature ranges. There is a need.
[0038]
Specifically, at least during operation of the battery, the mass increase per unit area due to oxidation of the porous substrate tube is 0.01 mg / cm per hour. ² It is necessary to use a heat-resistant metal exhibiting oxidation resistance of a degree or less.
[0039]
In addition, it is desirable that the coefficient of thermal expansion is matched to these values so that the electrodes and the electrolyte layer are not damaged when a unit cell is manufactured or when the temperature changes due to the start / stop of the fuel cell. ^-6 ~ 14 × 10 ^-5 K ^-1 Is desirably within the range.
[0040]
Specific examples of such a heat-resistant metal include austenitic stainless steel, martensitic stainless steel, ferritic stainless steel, and nickel-based heat-resistant alloys such as Inconel and Hastelloy.
[0041]
The coefficient of thermal expansion of austenitic stainless steel is, for example, 16 to 17 × 10 ^-6 K ^-1 The thermal expansion coefficient of martensitic stainless steel is close to that of ferritic stainless steel, for example, 10 to 14 × 10 ^-6 K ^-1 However, the oxidation resistance at high temperatures is rather low, and nickel-based heat-resistant alloys such as Inconel and Hastelloy have a coefficient of thermal expansion of 16 to 17 × 10 ^-6 K ^-1 Ferrite stainless steel has a coefficient of thermal expansion of 12 × 10 ^-6 K ^-1 Among them, they are preferably used in the present invention because they are as low as possible and have excellent oxidation resistance.
[0042]
Examples of ferritic stainless steel that can be preferably used in the present invention will be described below.
[0043]
Ferritic stainless steel can be preferably used because it has a low coefficient of thermal expansion, is excellent in oxidation resistance, and is a low-cost material. The chromium content of the ferritic stainless steel includes 11 to 32% by mass, C, Si, and Mn, and may further include Al, Ti, Mo, and other elements in some cases. As an example of ferritic stainless steel, Cr ₂ O ₃ A protective film mainly composed of ₂ O ₃ And a material on which a protective film made of
[0044]
The former material is mainly composed of iron and contains chromium, and has a Cr surface on a high-temperature oxidizing atmosphere (during fuel cell production or fuel cell operation). ₂ O ₃ This is a material for forming an oxide layer containing as a main component, and this oxide layer has an effect of suppressing the progress of oxidation into the metal. On the other hand, the chromium component diffuses and mixes into the electrode, which may cause deterioration of electrode characteristics due to chromium poisoning.
[0045]
On the other hand, the latter material is mainly composed of iron and contains aluminum, and the surface thereof is made of Al in a high-temperature oxidizing atmosphere (during fuel cell production or fuel cell operation). ₂ O ₃ Fe-Cr-Al-based stainless steel can be given as a material on which an oxide layer mainly composed of is formed. As an example of Fe-Cr-Al stainless steel, for example, based on Fe, Cr24wt. %, Al5 wt. %, C, Si, Mn, etc. as in ordinary stainless steel. This type of material contains several wt. % Or more in the high-temperature oxidizing atmosphere ₂ O ₃ An oxide layer having a high resistance and containing as a main component is formed. While this layer has the effect of suppressing the progress of oxidation into the metal, it has a problem that it is difficult to have a current collecting function on the oxidant electrode side due to high resistance.
[0046]
FIG. 18 shows a conventional solid oxide fuel cell 34 in which an oxidizer electrode 36, a solid oxide electrolyte layer 38, and a fuel electrode 40 are formed in this order on a porous substrate tube 26, and is a high-temperature oxidizing atmosphere (fuel cell FIG. 3 is a schematic cross-sectional view illustrating a cross-section of a solid oxide fuel cell 34 used at the time of creation or operation of a fuel cell). FIG. 19 shows an enlarged schematic diagram of the cross section. Reference numeral 10 denotes a metal particle, and an oxide layer having high resistance is formed around the metal particle 10, the pores 16 formed by the metal particle 10, the interface between the porous substrate tube 26 and the oxidant electrode 36, and the like. .
[0047]
Such a porous substrate tube 26 is formed by mixing a powder having a particle size of about several tens to several hundreds μm made of the above-described heat-resistant metal material with a binder, kneading, extrusion molding, drying, degreasing, and sintering in a vacuum. It is made.
[0048]
Here, control of the degree of sintering is important. If the degree of sintering is high, the gas permeability is low, and the battery characteristics are low. If the degree of sintering is low, the strength of the porous base tube 26 decreases during operation (it becomes easy to break), and a problem of a decrease in thermal conductivity occurs.
[0049]
This is because, when the porous substrate tube 26 is exposed to a high-temperature air atmosphere, an oxide layer having a thickness of several μm to several tens μm is formed on the surface of the material. This is because the material layer spreads over the entire base tube, and the connection between the micro metal particles is cut.
[0050]
For this reason, it has been found that the porosity of the porous base tube 26 is about 30 to 60%, preferably about 40 to 50%, and the average pore diameter is about several μm to several hundred μm. In a single cell manufactured in an example described later, the outer diameter of the porous base tube 26 was 20 mm.
[0051]
Embodiment 1
(Preparation of porous metal substrate)
HRE5 manufactured by Hitachi Metals, Ltd. ^R (Fe-Cr-Al system (by ICP measurement,
Fe: Bal. , Cr: 22.9, Al: 5.0, Ti: 0.22, Si: 0.43, Mn: 0.0039, Ni: 0.20 wt. %) Containing an alloy powder having a particle size of about 10 μm to 100 μm with a binder → kneading → extrusion molding → drying → degreasing → sintering in vacuum, outer diameter: 20 mm, tube thickness: 1.5 mm, porosity : A 40% porous metal substrate A was prepared.
[0052]
Next, HRE5 having a different production lot from the porous metal substrate A was used. ^R (Fe-Cr-Al-based (by ICP measurement, Fe: Bal., Cr: 23.4, Al: 4.9, Ti: 0.22, Si: 0.39, Mn: 0.015, Ni: < Using the powder, a porous metal substrate B was prepared in the same manner as the porous metal substrate A.
[0053]
As a comparative example, ZMG232 manufactured by Hitachi Metals, Ltd. ^R (Fe-Cr system: Cr2
2 wt. %, La and Zr by adding a trace amount thereof, a material having a higher oxidation resistance than ordinary Fe-Cr stainless steel), and a porous metal substrate X is produced by the same production method as the porous metal substrate A. did.
[0054]
(Oxidation resistance test)
The above three kinds of porous metal substrates were heat-treated in air at 1000 to 1200 ° C. for 8 hours.
[0055]
The sample after the heat treatment was embedded in resin and cut, and the fine structure of the cross section was observed by SEM (HITACHI S-2300). Further, the element concentration in the oxide film formed on the surface of the metal particles constituting the porous metal substrate was analyzed with an X-Ray Microanalyzer (HORIBA EMAX-3700). Since there was some variation in the measurement results of the element concentrations, measurement was performed at 5 to 10 points, and the average value was obtained.
[0056]
Table 1 shows the element concentrations and oxidation states in the surface oxide film of each sample.
[0057]
[Table 1]

[0058]
O in the oxidation state in Table 1 indicates a normal oxidation state in which an oxide film 12 having a thickness of about several μm is formed on the surface of the particles 10 constituting the porous metal substrate as schematically shown in FIG. X (abnormal oxidation) is a state in which oxidation progresses as shown in FIG. 2 and an oxide 14 having a thickness of several tens μm or more is growing.
[0059]
In the state of abnormal oxidation, the grown oxide 14 fills the pores 16 of the porous metal substrate and inhibits gas diffusion. In addition, the growth of the oxide 14 causes the volume expansion of the porous metal substrate, which may cause cracking or peeling of the electrode or the electrolyte layer formed thereon. When abnormal oxidation progresses, all the metal particles 10 are finally oxidized.
[0060]
The porous metal substrate A showed a normal oxidation state after heat treatment at 1000 ° C. for 8 hours, but reached abnormal oxidation after heat treatment at 1150 ° C. for 8 hours. The porous metal substrate B was in a normal oxidation state by heat treatment at 1150 ° C. for 8 hours, but reached abnormal oxidation after 1200 ° C. and 8 hours. Further, it was confirmed that the porous metal substrate X was abnormally oxidized by the heat treatment at 1000 ° C. for 8 hours.
[0061]
From Table 1, it can be seen that the concentration of aluminum in the surface oxide film 12 is 20 wt. % Or more, and 20 wt. %. From this, in order to maintain good oxidation resistance, at least 20 wt. % Must be present. That is, aluminum 20 wt. %, It is considered that good oxidation resistance can be maintained, and the progress of oxidation of the porous metal substrate during the battery manufacturing process and battery operation can be significantly suppressed.
[0062]
Note that the concentration of aluminum in the surface oxide film 12 is affected by the composition of the alloy used for the porous metal substrate, particularly the aluminum content, the type and amount of the trace added element, and the heat treatment temperature and time in the battery manufacturing process. Therefore, the aluminum concentration in the surface oxide film is reduced to 20 wt. % Is possible.
[0063]
Embodiment 2
(Preparation of electrode)
An electrode slurry was prepared by the following method.
[0064]
<Preparation of slurry solvent>
Ethanol, terpineol, ethyl cellulose, an antifoaming agent, and a dispersant were mixed at a weight ratio of 56: 37: 6.8: 0.1: 0.1 to prepare a slurry solvent.
[0065]
<Preparation of oxidant electrode slurry>
La with an average particle size of 2 μm _0.85 Sr _0.15 MnO ₃ Powder and YSZ having an average particle size of 0.5 μm (8 mol% Y ₂ O ₃ -92 mol% ZrO ₂ An oxidizer electrode slurry was prepared by mixing the powder mixed with the powder at a weight ratio of 8: 2 and the slurry solvent at a weight ratio of 3: 7.
[0066]
<Preparation of fuel electrode slurry>
By mixing a powder obtained by mixing a nickel oxide powder having an average particle diameter of 3 μm and a YSZ powder having an average particle diameter of 0.5 μm at a weight ratio of 9: 1, and the slurry solvent at a weight ratio of 1: 1, An anode slurry was prepared.
[0067]
(Measurement of contact resistance with oxidizer electrode material)
Sample A-0 in which porous metal substrate A was heat-treated in air at 1000 ° C. for 8 hours, sample A-1 in which porous metal substrate A was heat-treated in air at 1100 ° C. for 8 hours, and porous metal substrate B in air at 1000 ° C. Using the sample B-0 heat-treated for 8 hours at room temperature and the sample B-1 heat-treated the porous metal substrate B in air at 1100 ° C. for 8 hours, the oxidant electrode prepared by the above-described method at the position shown in FIG. The slurry 18 was applied and dried. After drying, a Pt net (current collector) 20 is adhered on the oxidant electrode 18, and the oxidant electrode slurry 18 is applied again on the current collector 20 and dried to obtain a porous metal substrate resistance measurement sample 22. Was prepared.
[0068]
After the four kinds of prepared samples were heated to 1000 ° C. in the air by an electric furnace, the contact resistance was measured in a range of 1000 to 500 ° C. The resistance value was converted to a value per unit area of the Pt current collector.
[0069]
(Analysis of element concentration in surface oxide film)
Using the samples under the same conditions as the four types of samples whose contact resistance was measured, the element concentration in the surface oxide film was analyzed. In the analysis, 5 to 10 points were measured per sample, and the average value was taken. Table 2 shows the element concentrations in the surface oxide films of the four types of samples.
[0070]
[Table 2]

[0071]
FIG. 4 shows the relationship between the aluminum concentration in the surface oxide film at 700 ° C. and the contact resistance. Since the element concentration in the surface oxide film varied depending on the measurement location, it is plotted including an average value and an error considering the variation.
[0072]
From this result, it can be seen that when the aluminum concentration in the surface oxide film 12 exceeds a certain value, the contact resistance sharply increases. In particular, 70 wt. %, The contact resistance with the oxidizer electrode material 18 is very large.
[0073]
Conversely, 70 wt. %, It is considered that a relatively small contact resistance can be obtained. In Example 1, 20 wt. %, It was found that the aluminum concentration in the surface oxide film 12 was 20 wt. % To 70 wt. %, Both excellent oxidation resistance and low contact resistance can be obtained.
[0074]
Embodiment 3
(Measurement of contact resistance with oxidizer electrode material)
After heat treatment of the porous metal substrate A and the porous metal substrate B in air at 1000 ° C. for 8 hours, the oxidant electrode slurry 18 prepared by the method described in [Preparation of Electrode] is applied to the position shown in FIG. 3 and dried. I let it. After drying, a Pt net (current collector) 20 is adhered on the oxidant electrode 18, and the oxidant electrode slurry 18 is applied again on the current collector 20 and dried to obtain a porous metal substrate resistance measurement sample 22. Was prepared.
[0075]
Further, the same material (HRE5) as the porous metal substrate A is used. ^R 5) After heat-treating the plate-shaped sample 28 made of the dense body in air at 1000 ° C. for 8 hours, the oxidant electrode slurry prepared by the method described in [Production of Electrode] on the front and back of the dense body 28 as shown in FIG. 18 was applied and dried. After drying, a Pt net (current collector) 20 is adhered on the oxidant electrode 18, and the oxidant electrode slurry 18 is applied again on the current collector 20 and dried to produce a sample 30 for dense body resistance measurement. did.
[0076]
After raising the temperature of the three kinds of prepared samples to 1000 ° C. in the air in an electric furnace, the resistance was measured in the range of 1000 to 500 ° C. The resistance value was converted to a value per unit area of the Pt current collector.
[0077]
FIG. 6 shows the results of the resistance measurement. The porous metal substrates A and B exhibited resistance values smaller by one to two orders of magnitude than the dense bodies. This is because the porous metal substrate 26 has a larger specific surface area and the electrode material 18 penetrates into the porous metal substrate 26 (see FIG. 7). It is thought that it is because.
[0078]
It is considered that the reason why the contact resistance of the porous metal substrate A is smaller than that of the porous metal substrate B by almost one digit is due to the difference in the aluminum concentration in the surface oxide film 12.
[0079]
[Measurement of contact resistance with fuel electrode material]
After heat treatment of the porous metal substrate A in air at 1000 ° C. for 2 hours, the fuel electrode slurry 18 prepared by the method described in [Preparation of Electrode] was applied to the position shown in FIG. 3 and dried. After drying, a Pt net (current collector) 20 is adhered on the fuel electrode 18, and the fuel electrode slurry 18 is applied again on the current collector 20 and dried to obtain a sample 22 for measuring the resistance of the porous metal substrate. Produced.
[0080]
Further, the same material (HRE5) as the porous metal substrate A is used. ^R 5) After heat-treating the plate-shaped sample 28 made of the dense body at 1000 ° C. for 8 hours in air, as shown in FIG. Was applied and dried. After drying, a Pt net (current collector) 20 was stuck on the fuel electrode 18, the fuel electrode slurry 18 was applied again on the current collector 20, and dried to prepare a sample 30 for dense body resistance measurement.
[0081]
The two types of prepared samples were heated in an electric furnace to a predetermined temperature of 300 to 1000 ° C. in a hydrogen atmosphere to measure a resistance. The resistance value was converted to a value per unit area of the Pt current collector.
[0082]
FIG. 8 shows the resistance measurement results. The resistance in a hydrogen atmosphere does not change much with the temperature, and is 0.2 to 0.3 Ωcm for the porous metal substrate A. ² , 3-4Ωcm for dense body ² Met. The reason why the contact resistance of the porous metal substrate A becomes smaller by one digit or more than that of the dense sample is that the specific surface area of the porous metal substrate 26 is larger than the contact resistance with the oxidizer electrode material. This is considered to be because the contact region is three-dimensionally expanded due to the permeation of the electrode material 18 into the inside.
[0083]
In addition, the contact resistance with the fuel electrode material did not depend much on the temperature, and the resistance increased slightly as the temperature rose. This result indicates that nickel, which is an anode material, and metal particles that constitute the porous metal substrate are in direct contact, and that this contact portion is involved in electron conduction between the anode and the porous metal substrate. It suggests that the electron conduction through is dominant. That is, since the growth of the surface oxide film is slow in the reducing atmosphere, it is considered that the metal surface is exposed at the minute defect portion of the surface oxide film and the nickel particles are in contact with this portion.
[0084]
Embodiment 4
(Measurement of contact resistance with oxidizer electrode material)
Using the porous metal substrate B heat-treated at 1000 ° C. for 8 hours in the air, a sample (1) for measuring the contact resistance with the oxidant electrode material as shown in FIG. 3 was prepared. Next, this sample (1) was heat-treated in air at 1100 ° C. for 2 hours (sample (2)), and the sample (2) was heat-treated in air at 1100 ° C. for 6 hours. Sample (3) heat-treated at 1100 ° C. for 8 hours in total and Sample (4) heat-treated at 1100 ° C. in air for 8 hours (16 hours in total) were prepared. The resistance was measured for each of ▼ to ４4 ▼.
[0085]
FIG. 9 shows the resistance measurement results. By performing the heat treatment at 1100 ° C., the resistance was reduced. Also, as the heat treatment time at 1100 ° C. becomes longer, the slope of the straight line becomes gentler, and the resistance particularly at the low temperature side tends to become smaller.
[0086]
(Elemental analysis in surface oxide film)
The element distribution in the surface oxide film of the samples prepared under the same conditions as those of the samples (3) and (4) in the above resistance measurement was measured by the same method as in Example 1. Table 3 shows the results.
[0087]
[Table 3]

[0088]
As shown in Table 3, when heat treatment was performed in air at 1100 ° C. in contact with the oxidizer electrode material, the aluminum concentration in the surface oxide film was lower, and the concentrations of lanthanum and manganese increased as compared to the porous metal substrate alone. I understand that there is. This is because lanthanum manganite (La) in the oxidant electrode material _0.85 Sr _0.15 MnO ₃ ) And an oxide film on the surface of the porous metal substrate.
[0089]
From the above results, the contact resistance between the porous metal substrate and the oxidant electrode material was reduced by the heat treatment at 1100 ° C. in air because the surface of the porous metal substrate and the oxidant electrode material were subjected to a solid-phase reaction, It is considered that the cause is that the concentration of aluminum in the surface oxide film has been reduced. Further, the reason why the slope of the straight line in FIG. 9 becomes small, and particularly the resistance at the low temperature side becomes small, is that iron and chromium contained in the surface oxide film react with lanthanum manganite, and electron conduction in the surface oxide film is caused. Highly conductive LaFeO with low activation energy ₃ , LaCrO ₃ , LaMnO ₃ This is considered to be due to the generation of a perovskite-type oxide.
[0090]
[Contact resistance measurement using Pt paste]
For comparison, a sample (1) for measuring the contact resistance shown in FIG. 3 was prepared using a Pt paste instead of the oxidant electrode slurry. Next, Sample (1) was heat-treated in air at 1100 ° C. for a total of 8 hours, and Sample (2) was heat-treated in air at 1100 ° C. for 8 hours (16 hours in total). A sample (3) was prepared, and the resistance was measured for each of the samples (1) to (3).
[0091]
The result is shown in FIG. Although the overall resistance was lower than when the oxidizer electrode material was used, the resistance did not decrease due to the heat treatment at 1100 ° C. Further, the slope of the straight line did not decrease even if the heat treatment time was long. From this, it was proved that the reason why the inclination of the straight line was reduced by the heat treatment as shown in FIG. 9 was likely to be due to the reaction between the surface of the porous metal substrate and the oxidant electrode material.
[0092]
When the Pt paste was used, the resistance was reduced by almost one digit compared to the oxidant electrode slurry. This is because the conductivity of Pt is two orders of magnitude higher than that of lanthanum manganite, and the high sinterability of Pt results in a large micro-contact area with the particles constituting the porous metal substrate. it is conceivable that.
[0093]
From the above, it was found that a solid phase reaction between the oxide film on the surface of the porous metal substrate and the oxidizer electrode material at a high temperature lowers the aluminum concentration in the surface oxide film and at the same time generates a highly conductive oxide. Was.
[0094]
By this method, the contact resistance between the porous metal substrate and the oxidant electrode can be reduced, and in particular, the resistance on the low temperature side is reduced, so that a practical porous metal substrate exhibiting low contact resistance in a wide temperature range can be obtained. Obtainable.
[0095]
In this embodiment, lanthanum strontium manganite (La, Sr) MnO is used as the oxidant electrode material. ₃ Was used, but a lanthanum-based perovskite oxide generally used as an oxidant electrode material may also be used.
[0096]
It is also conceivable to apply another material and heat-treat it before applying the oxidizer electrode material for the surface treatment of the porous metal substrate. In this case, a material that causes a solid phase reaction with an oxide film on the surface of the porous metal substrate and generates a conductive oxide while maintaining an appropriate aluminum concentration in the surface oxide film is used. Examples of such a material include oxides containing titanium, chromium, manganese, iron, cobalt, nickel, copper, yttrium, magnesium, calcium, strontium, barium, lanthanum, cerium, and the like.
[0097]
The heat treatment temperature is 1100 ° C., but the heat treatment temperature is not limited to 1100 ° C. as long as the above reaction proceeds moderately below the heat resistant temperature of the porous metal substrate.
[0098]
Embodiment 5
[Measurement of specific surface area]
After heat treatment of the porous metal substrate A in air at 1000 ° C. for 1 hour, it was pulverized to a size of about several mm, and the specific surface area was measured by a nitrogen adsorption method (QUANTACHROME AUTOSORB1).
[0099]
As a result, the specific surface area of the porous metal substrate A was 0.26 m ² / G.
[0100]
[Calculation of effective contact area between porous metal substrate and electrode]
Next, assuming that the porous metal substrate has an outer diameter of 20 mm, an inner diameter of 17 mm, a length of 10 mm, and a porosity of 40%, the effective contact area between the porous metal substrate and the electrode is calculated.
[0101]
The apparent volume of the porous metal substrate is
[0102]
(Equation 1)

[0103]
0.871cm in volume ³ On the other hand, since the porosity is 40%, the volume of the bulk (metal portion) is
[0104]
(Equation 2)

[0105]
The porous metal base material is ferritic stainless steel, and the true density is about 8 g / cm. ³ Therefore, the mass of the porous metal substrate is
[0106]
[Equation 3]

[0107]
Actual measured value of specific surface area of porous metal substrate is 0.26 m ² / G, the surface area of the porous metal substrate is:
[0108]
(Equation 4)

[0109]
When the oxidant electrode slurry was applied to the outer surface, as shown in FIG. 7, it was confirmed by SEM observation that the slurry had penetrated from the surface of the porous metal substrate to the inside of about 50 μm. The contact area between the particles constituting the porous metal substrate and the oxidant electrode seen is
[0110]
(Equation 5)

[0111]
There will be.
[0112]
Therefore, the apparent application area of the oxidant electrode: 2 × 3.14 × 1 = 6.28 cm ² On the other hand, it was found that there was actually a contact area nearly 60 times.
[0113]
From the above calculation results, the porous metal base is a porous body, has a high specific surface area, and the electrode has penetrated inside the porous metal base. As a result, a contact resistance as low as 1 can be obtained.
[0114]
FIG. 11 shows the results of calculating the effective contact area (the ratio to the apparent area) between the porous metal substrate and the electrode by using the specific surface area of the porous metal substrate and the penetration depth of the electrode as parameters by the above calculation method. From FIG. 11, it can be seen that the effective contact area can be increased by increasing the specific surface area of the porous metal substrate or increasing the penetration depth into the inside of the electrode.
[0115]
The specific surface area of the porous metal substrate is affected by the particle size distribution of the constituent particles and the degree of sintering, but it is possible to estimate a rough value as follows. For example, assuming that the surfaces of the constituting particles are smooth, the particle diameters are all the same, and the sintering degree is low, the relationship between the particle diameter and the specific surface area is obtained as shown in FIG. From FIG. 12, when the particle diameter is 100 μm, the specific surface area is 0.01 m. ² / G, and when the particle diameter is 10 μm, the specific surface area is 0.1 m. ² / G is expected. As the particle diameter decreases, the specific surface area increases. Also, since the surface of the particles is not always smooth, the specific surface area is likely to be higher than expected.
[0116]
Further, other than the present embodiment, it is also possible to produce the base with metal fibers, but in this case, it is also possible to predict the relationship between the fiber diameter and the specific surface area. Assuming that the surface of the fiber constituting the substrate is smooth and the fiber diameter is uniform, when the fiber diameter is 50 μm, the specific surface area is 0.01 m. ² / G and a fiber diameter of 5 μm, the specific surface area is 0.1 m ² / G is expected. As the fiber diameter decreases, the specific surface area increases. Also, as in the case of particles, the surface is not always smooth, so that the specific surface area is likely to be higher than expected.
[0117]
The penetration depth of the electrode can be controlled by adjusting the wettability with the porous metal substrate and the slurry viscosity, depending on the solvent type and the solvent / powder mixing ratio of the electrode slurry applied on the porous metal substrate. That is, by increasing the wettability and lowering the slurry viscosity, it is possible to penetrate into the porous metal substrate.
[0118]
If the specific surface area of the porous metal substrate is too large, the oxidation resistance tends to decrease. For this reason, practically 1m ² / G or less is desirable. Further, if the penetration depth of the electrode is too large, pores in the porous metal substrate are blocked and gas diffusion is hindered. However, this does not apply when only the surface of the particles constituting the porous metal substrate is thinly coated. Based on these constraints, it is desirable to select the specific surface area and the electrode penetration depth from the relationship shown in FIG. 11 so that the contact area between the porous metal substrate and the electrode is made as large as possible.
[0119]
This result can be applied not only to the contact resistance with the oxidant electrode material but also to the contact resistance with the fuel electrode material. Although the case where the electrode material permeates into the porous metal substrate has been described, it is also possible to apply a highly conductive material similar to the electrode material to the porous metal substrate in advance to enhance the current collection effect. . For example, when a lanthanum manganite-based oxide is used as the oxidant electrode material, a lanthanum cobaltite-based oxide can be previously applied to a porous metal substrate as a current collecting material.
[0120]
Embodiment 6
(Measurement of aluminum concentration inside particles)
The inside of the particles constituting the porous metal substrate was determined for the sample B-1 after the porous metal substrate B was heat-treated at 1000 ° C. for 8 hours and the sample B-2 after the porous metal substrate B was heat-treated at 1100 ° C. for 8 hours. Was measured with an X-Ray Microanalyzer (HORIBA EMAX-3700). At the time of the measurement, a difference was observed in the internal aluminum concentration depending on the particle diameter. Therefore, 5 to 10 particles each having a diameter of about 100 μm and 50 μm were selected, and the aluminum concentration at the central part of the particles was measured.
[0121]
FIG. 13 shows the measurement results. The aluminum concentration inside the particles was lower as a whole after the heat treatment, especially as the heat treatment temperature was higher, as compared to before the heat treatment (4.9 wt.%). Also, under the same heat treatment conditions, the smaller the particle size, the lower the aluminum concentration.
[0122]
(Calculation of aluminum concentration inside particles)
Next, the aluminum concentration inside the particle is calculated based on the following assumption. Before the heat treatment, 5 wt. % Aluminum is uniformly contained. By heat-treating this, aluminum inside the particles diffuses to the surface, and an oxide film containing aluminum at a high concentration is formed on the surface of the substrate to a thickness of several μm.
[0123]
Since the thickness of the oxide film can be observed by SEM and the aluminum concentration can be measured by EDX, the amount of aluminum contained in the surface oxide film is determined from the thickness of the oxide film and the aluminum concentration. By subtracting the amount of aluminum contained in the surface oxide film from the amount of aluminum existing inside the particles before the heat treatment and dividing by the volume of the particles, it is possible to predict the aluminum concentration inside the particles after the heat treatment.
[0124]
The thickness of the surface oxide film of Sample B-1 in which the porous metal substrate B was heat-treated at 1000 ° C. for 8 hours was 1.5 μm, and the aluminum concentration in the surface oxide film was 55%.
[0125]
Further, the thickness of the surface oxide film of Sample B-2 in which the porous metal substrate B was heat-treated at 1100 ° C. for 8 hours was 2.5 μm, and the aluminum concentration in the surface oxide film was 60%. In each of the samples, the thickness of the surface oxide film did not depend on the particle diameter.
[0126]
The results of calculating the aluminum concentration inside the particles from the above values are also shown in FIG. The calculation results are consistent with the measurement results, and the calculation of the aluminum concentration by this model is considered to be valid.
[0127]
[Derivation of optimal aluminum content]
From the above results, as the diameter of the particles constituting the porous metal substrate becomes smaller, the surface area with respect to the volume of the particles becomes larger, so that the internal aluminum concentration decreases. If the aluminum concentration decreases, it becomes impossible to secure a sufficient aluminum concentration in the surface oxide film, resulting in abnormal oxidation that does not function as a protective film. Therefore, when the porous metal substrate is composed of small particles, it is considered necessary to increase the aluminum content of the alloy used for the porous metal substrate.
[0128]
As described in Example 5, in order to ensure a sufficient effective contact area between the porous metal substrate and the electrode, it is necessary to reduce the particle diameter and increase the specific surface area. Considering that an oxide film of several μm is formed on the particle surface, it is considered that the average particle diameter is suitably in the range of about 10 to 50 μm. Further, from Example 1, the aluminum concentration in the surface oxide film was 40 wt. % Is desirable.
[0129]
Therefore, in the above calculation, the thickness of the surface oxide film was 2 μm, and the aluminum concentration in the surface oxide film was 40 wt. %, And for particles having a size of 10 to 50 μm, the aluminum concentration inside the particles is set to 0 wt. The aluminum content of the porous metal substrate required to maintain the percentage of not less than% was derived.
[0130]
FIG. 14 shows the results. From this result, when the porous metal substrate is composed of particles having a particle diameter of 50 μm, the aluminum content is 2.5 wt. % Or more and 6.3 wt. % Or more and 12.6 wt. It was found that an aluminum content of at least% was required.
[0131]
Conversely, if this condition is satisfied, a state in which aluminum is depleted inside the particles constituting the porous metal substrate does not occur, and a sufficient aluminum concentration can be ensured in the surface oxide film. Therefore, such a porous metal substrate can maintain good oxidation resistance, and HRE5 ^R It is considered that heat resistance close to 1250 ° C., which is a catalog value of the heat resistance temperature, is obtained.
[0132]
Embodiment 7
(Preparation of unit cell)
An electrolyte slurry was prepared by the following method.
[0133]
<Preparation of slurry solvent>
Ethanol, terpineol, ethyl cellulose, defoamer and dispersant were mixed at a weight ratio of 56: 37: 6.8: 0.1: 0.1 to prepare a slurry solvent.
[0134]
<Preparation of electrolyte slurry>
An electrolyte slurry was prepared by mixing YSZ powder having an average particle size of 0.5 μm, the slurry solvent, and ethanol at a weight ratio of 20:50:30.
[0135]
Next, using the electrode slurry prepared by the method described in [Preparation of Electrode] and the above-mentioned electrolyte slurry, a cell 34 as shown in FIG. 15 was prepared by the following method.
[0136]
<Preparation of unit cell>
The oxidant electrode slurry was applied to the surface by dipping the porous metal substrate A into the oxidant electrode slurry and pulled up (dip method), dried at about 60 ° C., and then heat-treated at 1000 ° C. in air for 2 hours.
[0137]
The depth of penetration of the slurry into the porous metal substrate can be adjusted by the viscosity of the slurry, but the depth of penetration in this example was about 50 μm. The thickness of the oxidant electrode 36 was about 50 μm.
[0138]
Next, an electrolyte slurry was applied on the oxidant electrode 36 by the same dipping method, dried at about 60 ° C., and then heat-treated at 1000 ° C. in air for 2 hours. The thickness of the electrolyte layer 38 was about 20 μm.
[0139]
Further, a fuel electrode slurry was applied on the electrolyte layer 38 by the same dipping method, dried at about 60 ° C., and heat-treated at 1000 ° C. for 2 hours in air. The thickness of the fuel electrode 40 was about 50 μm.
[0140]
[Battery characteristics test method and results]
One end of the porous metal base 26 is made of the same material (HRE5) as the porous metal base 26. ^R ) Was used as the lid member 42, which was closed by thermocompression bonding via a gold ring 44.
[0141]
The gold ring 44 functions as a sealing material for bonding the porous metal base 26 and the cover member 42 and at the same time preventing gas leakage from gaps. Thermocompression bonding occurs naturally in the process of raising the cell to operating temperature under load. In this embodiment, the gold ring 44 is used, but the porous metal base 26 and the cover 42 may be welded.
[0142]
After the temperature of the cell was raised to the operating temperature (700 ° C.) in an electric furnace, hydrogen was supplied to the fuel electrode side and air was supplied to the oxidant electrode side, and voltage-current characteristics were measured. Current collection for measuring battery characteristics was performed using a platinum wire.
[0143]
The battery characteristics are shown in FIGS. A stable power generation characteristic was confirmed at 700 ° C., and 10 mA / cm ² For 1000 hours.
[0144]
In this embodiment, the oxidant electrode 36, the electrolyte layer 38, and the fuel electrode 40 are arranged on the porous metal substrate 26 in this order. Although the merit of the resistance reduction according to the present invention is considered to be maximized by using the porous metal substrate 26 on the oxidant electrode 36 side, the fuel electrode 40, the electrolyte layer 38, The electrode 36 may be arranged in this order. A cylindrical cell structure is more advantageous in terms of thermal shock resistance and the like, but the present invention can be applied to a flat cell.
[0145]
In this embodiment, a lanthanum manganite-based oxide is used as the material of the oxidant electrode 36. However, a lanthanum cobaltite-based material or a lanthanum iron cobaltite-based material having high electrode activity at a low temperature may be used.
[0146]
Further, 8 mol% yttria-added stabilized zirconia was used as the material of the electrolyte layer 38, but the material is not limited to this. Yttria-added stabilized zirconia with an addition amount of 3 mol% or 10 mol%, scandia-added zirconia, samarium-doped ceria, lanthanum gallate, or the like may be used.
[0147]
Further, a mixture of nickel and YSZ was used as a material of the fuel electrode 40, but instead of nickel, iron, copper, or an alloy thereof was used instead of YSZ using scandia-added zirconia or ceria-based oxide. Is also good. In addition, a small amount of a noble metal such as platinum, gold, silver, or palladium may be dispersed in order to improve the characteristics when a hydrocarbon fuel is used.
[0148]
In Examples 1 to 7, iron-based ferritic stainless steel was used, but a nickel-based or cobalt-based alloy may be used as long as a protective oxide film containing much aluminum is formed on the surface. The present invention is applicable.
[0149]
As an alloy material applicable to the porous metal substrate 26 of the present invention, aluminum is a few wt. % Or more, and an oxide film that protects aluminum is formed on the surface. However, aluminum may be alloyed with aluminum by diffusing aluminum from the surface by diffusion infiltration into a metal material not containing aluminum. .
[0150]
【The invention's effect】
The solid oxide fuel cell according to claim 1 of the present invention is a solid oxide fuel cell in which a cell in which an oxidizer electrode and a fuel electrode are arranged with a solid electrolyte layer interposed therebetween is formed on a porous metal substrate. Since the porous metal substrate is made of a metal in which a coating containing aluminum is formed on the surface of the metal constituting the porous metal substrate, good oxidation resistance can be ensured. Thus, the battery manufacturing process can be performed in a high-temperature oxidizing atmosphere of 1000 ° C. or higher.
[0151]
In addition, compared to the conventional technology using ceramics for the porous substrate, the use of an inexpensive metal material for the porous substrate reduces the material cost and achieves long-term stability with low internal resistance and high output density. An excellent battery can be provided.
[0152]
Furthermore, since the porous metal substrate has high strength and thermal conductivity, the temperature distribution in the battery is made uniform and the thermal shock resistance can be greatly improved. In contrast, a battery having excellent durability can be provided.
[0153]
In the solid oxide fuel cell according to the second aspect of the present invention, a cell in which an oxidizer electrode and a fuel electrode are arranged with a solid electrolyte layer interposed therebetween is formed such that the oxidizer electrode faces a porous metal substrate. In the solid oxide fuel cell, the porous metal substrate is formed on the surface of the metal constituting the porous metal substrate by a solid phase reaction between aluminum, an element contained in the metal constituting the substrate, and an element contained in the oxidant electrode. Since the film is formed of a metal containing a highly conductive oxide that is formed, the contact resistance between the porous metal substrate and the oxidant electrode can be reduced. Since the resistance is reduced, a solid oxide fuel cell operable in a wide temperature range is obtained.
[0154]
The solid oxide fuel cell according to claim 3 of the present invention is the solid oxide fuel cell according to claim 1 or 2, wherein the aluminum content of the film formed on the metal surface constituting the porous metal substrate is 20%. ~ 70 wt. %, The progress of oxidation of the porous metal substrate during the battery manufacturing process and during battery operation can be greatly suppressed, and the contact resistance between the porous metal substrate and the electrode can be reduced.
[0155]
A solid oxide fuel cell according to a fourth aspect of the present invention is the solid oxide fuel cell according to the first or second or third aspect, wherein the specific surface area of the porous metal substrate is 0.01 to 1 m. ² / G, the effective contact area with the electrode can be increased, the contact resistance can be reduced, and the oxidation resistance of the porous metal substrate can be maintained.
[0156]
The solid oxide fuel cell according to claim 5 of the present invention is the solid oxide fuel cell according to claim 1, 2, 3, or 4, wherein the average particle size of the metal particles constituting the porous metal substrate is Since the diameter is set to 10 to 50 μm, a porous metal substrate having an ideal specific surface area can be manufactured.
[0157]
The solid oxide fuel cell according to claim 6 of the present invention is the solid oxide fuel cell according to claim 1 or 2, or 3 or 4 or 5, wherein Of 2.5 wt. % Or more, sufficient aluminum can be supplied to the surface oxide film, and a porous metal substrate maintaining good oxidation resistance can be obtained.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing a normal oxidation state of a surface oxide film formed on a metal surface of a porous metal substrate.
FIG. 2 is a schematic sectional view showing an abnormal oxidation state of a surface oxide film formed on a metal surface of a porous metal substrate.
FIG. 3 is a schematic diagram of a sample for measuring contact resistance between a porous metal substrate and an electrode material.
FIG. 4 is a graph showing a relationship between an Al concentration in a surface oxide film at 700 ° C. in an air atmosphere and a contact resistance.
FIG. 5 is a schematic diagram of a sample for measuring contact resistance between a dense body and an electrode material.
FIG. 6 is a graph showing the relationship between temperature and contact resistance of a porous metal substrate and a dense body in an air atmosphere.
FIG. 7 is a schematic sectional view showing a state in which an electrode material has penetrated a porous metal substrate.
FIG. 8 is a graph showing a relationship between temperature and contact resistance of a porous metal substrate and a dense body in a hydrogen atmosphere.
FIG. 9 is a graph showing a relationship between temperature and contact resistance of various heat-treated samples in an air atmosphere.
FIG. 10 is a graph showing the relationship between the temperature of various heat-treated samples in an air atmosphere and Pt paste contact resistance.
FIG. 11 is a graph showing the relationship between the specific surface area of the porous metal substrate and the contact area magnification.
FIG. 12 is a graph showing the relationship between the particle size and the specific surface area of a porous metal substrate.
FIG. 13 is a graph showing the relationship between the particle diameter of a porous metal substrate and the Al concentration inside the particles after heat treatment.
FIG. 14 is a graph showing the relationship between the particle diameter of a porous metal substrate and the Al content required as a substrate material.
FIG. 15 is a schematic view of a manufactured unit cell.
FIG. 16 is a graph showing initial power generation characteristics of the manufactured single cell.
FIG. 17 is a graph showing continuous operation characteristics of the manufactured single cell.
FIG. 18 is a schematic sectional view of a unit cell of a conventional cylindrical solid oxide fuel cell.
FIG. 19 is an enlarged schematic cross-sectional view of a unit cell of a conventional cylindrical solid oxide fuel cell.
[Explanation of symbols]
10 ... metal particles
12 ... oxide film
14 ... oxide
16… pores
18 ... electrode material
20 ... current collector
22… Sample for measuring resistance of porous metal substrate
26 ... porous metal substrate
28… Dense plate sample
30… Sample for dense body resistance measurement
34 ... cell
36… oxidizer electrode
38… Electrolyte layer
40 ... fuel electrode
42 ... lid material
44… Gold ring

Claims (6)

In a solid oxide fuel cell in which a cell in which an oxidizer electrode and a fuel electrode are arranged with a solid electrolyte layer interposed therebetween is formed on a porous metal substrate,
The solid oxide fuel cell according to claim 1, wherein the porous metal substrate is made of a metal having a coating containing aluminum formed on a surface of a metal constituting the porous metal substrate. In a solid oxide fuel cell in which a cell in which an oxidant electrode and a fuel electrode are arranged with a solid electrolyte layer interposed therebetween is formed such that the oxidant electrode faces a porous metal substrate,
The porous metal substrate is formed on the surface of the metal constituting the porous metal substrate by a solid phase reaction between aluminum, an element contained in the metal constituting the porous metal substrate, and an element contained in the oxidant electrode. A solid oxide fuel cell comprising a metal on which a film containing a highly conductive oxide to be produced is formed. The coating formed on the surface of the metal constituting the porous metal substrate is made of aluminum in an amount of 20 to 70 wt. 3. The solid oxide fuel cell according to claim 1, wherein 4. The solid oxide fuel cell according to claim 1, wherein the porous metal substrate has a specific surface area of 0.01 to ¹ m ² / g. 5. The solid oxide fuel cell according to any one of claims 1 to 4, wherein the porous metal substrate has an average particle diameter of metal particles constituting the porous metal substrate of 10 to 50 m. The porous metal substrate contains aluminum in an amount of 2.5 wt. The solid oxide fuel cell according to any one of claims 1 to 5, wherein the solid oxide fuel cell is made of an alloy containing at least 10% by weight.

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