JP2004071360A

JP2004071360A - Fuel electrode material for solid oxide fuel cells

Info

Publication number: JP2004071360A
Application number: JP2002229212A
Authority: JP
Inventors: Hibiki Ito; 伊藤　響
Original assignee: Central Research Institute of Electric Power Industry
Current assignee: Central Research Institute of Electric Power Industry
Priority date: 2002-08-06
Filing date: 2002-08-06
Publication date: 2004-03-04
Anticipated expiration: 2022-08-06
Also published as: JP4201247B2

Abstract

【課題】製造コストを低廉としつつ一層の高性能化・高出力化と長寿命化を実現する。
【解決手段】酸化ニッケルＮｉＯと８モル％のイットリアで結晶構造を安定化させたジルコニア８ＹＳＺの重量混合比と粒径比を最適化することにより燃料極の性能を長時間維持させ併せて性能を向上させる。具体的には、粗粒相当ＹＳＺ粉末：ＮｉＯ粒子粉末：微粒相当ＹＳＺ粉末の重量混合比を４：６：１とする。また、この重量混合比の下で粗粒相当ＹＳＺ粉末、ＮｉＯ粒子粉末、微粒相当ＹＳＺ粉末の各粉末粒子の粒径をそれぞれ２７．０μｍ、１．０μｍ、０．４μｍとする。
【選択図】　　　　図１An object of the present invention is to achieve higher performance, higher output, and longer life while reducing manufacturing costs.
Kind Code: A1 Abstract: The performance of an anode is maintained for a long time by optimizing the weight mixing ratio and particle size ratio of zirconia 8YSZ whose crystal structure is stabilized by nickel oxide NiO and 8 mol% yttria. Improve. Specifically, the weight mixing ratio of coarse-grain YSZ powder: NiO particle powder: fine-grain YSZ powder is 4: 6: 1. Under the weight mixing ratio, the particle diameters of the coarse YSZ powder, NiO particle powder, and fine YSZ powder are set to 27.0 μm, 1.0 μm, and 0.4 μm, respectively.
[Selection diagram] Fig. 1

Description

【０００１】
【発明の属する技術分野】
本発明は、固体酸化物形燃料電池用燃料極材料に関する。さらに詳述すると、本発明は固体酸化物形燃料電池（ＳＯＦＣ）用のニッケル−安定化ジルコニア燃料極粉体、特に、ＳＯＦＣの燃料極に用いることによりその長寿命化と高性能化を図ることができる燃料極材料の改良に関するものである。
【０００２】
【従来の技術】
固体酸化物形燃料電池の燃料極材料としては、酸化ニッケル（ＮｉＯ、但し燃料電池作動時には金属ニッケルＮｉ）とジルコニア（ＺｒＯ_２）の微粒子を混合して得たニッケル−ジルコニアサーメットが、高い触媒活性（水素の還元能力）を有し、かつ室温から１０００℃までの高温でも導電率（電気抵抗の逆数）が高いことから適していると考えられていた。しかしながら、燃料極中のニッケルの含有量が多いと、熱膨脹係数の違いから熱応力が発生し、セル破壊につながる可能性があり、ニッケルの含有量をあまりふやすことができず、反面、ニッケルの量が少ないと、電極特性はあまり良くなく、電流を取り出すことが困難になり、更に、焼結性が高く緻密化しやすいなどの問題があった。そこで、従来、ジルコニアとして８モル％のイットリアで結晶構造を安定化させたジルコニア（以下８ＹＳＺと記する。）を用いたものが採用されるようになってきている。
【０００３】
（１）このように、従来の固体酸化物形燃料電池用燃料極材料は、細かい粉末のＮｉＯと８ＹＳＺを混合して得られたものであった。しかし、この燃料極は初期特性が優れているものの、発電開始後数十時間で劣化し、発電が不可能な状態になる。この原因を解明したところ、電池動作条件下において、燃料極の緻密化と体積収縮ならびにＮｉ粒子の凝集が原因であることがわかった（非特許文献１参照）。なお、Ｎｉの凝集や緻密化については他の報告もある（非特許文献２参照）。
【０００４】
（２）さらに、Ｎｉ（Ｍｇ）Ｏ−８ＹＳＺを用いることによって、Ｎｉ粒子の高分散化と燃料極の長寿命化を図ろうとする報告がある（非特許文献３〜５参照）。
【０００５】
（３）また粒径の大きいＮｉに粒径の小さいＹＳＺを被覆させて、性能の向上を図ろうとした報告もある（非特許文献６参照）。
【０００６】
（４）さらに金属ルテニウムにＹＳＺを電気化学蒸着した材料（非特許文献７参照）や、金属Ｎｉに気相法にてＹＳＺを付着させた材料についても検討されている（非特許文献８参照）。
【０００７】
（５）また、固体酸化物形燃料電池用燃料極材料およびその製造方法において、ＮｉＯが１０μｍ程度、８ＹＳＺが２０〜４０μｍと０．６μｍ程度の各粒子粉末を用いたものは既に本件出願人によって開示されている（特許文献１、非特許文献９参照）。
【０００８】
【特許文献１】
特開平８−３０６３６１号公報
【非特許文献１】
電力中央研究所報告　Ｗ９３０１９「ＳＯＦＣ用燃料極の高性能化に関する研究−カレント・インターラプション法によるＳＯＦＣの劣化現象の解明−」，財団法人電力中央研究所，平成６年５月
【非特許文献２】
「電気化学協会第６０回大会講演要旨集」，平成５年４月１〜３日，ｐ．２６９
【非特許文献３】
「第３３回電池検討会講演要旨集」，平成４年９月１６〜１８日，ｐ．３５−３６
【非特許文献４】
「電気化学協会第５９回大会講演要旨集」，平成４年４月２〜４日，ｐ．１９７
【非特許文献５】
「電気化学協会第６０回大会講演要旨集」，平成５年４月１〜３日，ｐ．２７０
【非特許文献６】
「電気化学協会第５９回大会講演要旨集」，平成４年４月２〜４日，ｐ．１９８
【非特許文献７】
「第１８回固体イオニクス討論会講演要旨集」，平成４年１０月１２〜１３日，ｐ．５−８
【非特許文献８】
「電気化学協会第５９回大会講演要旨集」，平成４年４月２〜４日，ｐ．１９９
【非特許文献９】
電力中央研究所報告　Ｗ９４０１６「ＳＯＦＣ燃料極の高性能化に関する研究−電極ミクロ構造の改良による長寿命化」，財団法人電力中央研究所，平成７年５月
【０００９】
【発明が解決しようとする課題】
しかしながら、上記（１）〜（４）の従来技術は、特に燃料極の性能の向上のみが目的であり、長時間作動時の劣化についての検討が不十分であるとともに、製造コストに直接影響する製造工程に関する配慮がなされておらず、あるいは製造工程の複雑さから製造コストが高価になると考えられる。
【００１０】
また、（４）の製法の場合は、特殊な雰囲気を必要とするために設備コストが高くなる、原料の歩留まりまたは収率が低い、完全なバッチ処理（連続製造には不向き）を必要とする、等の理由からコストが高くなると考えられる。
【００１１】
また、（５）の段階では、簡便な製造技術を用いて、低コストに大量生産出来る作製法を採用しており、さらにＳＯＦＣの燃料極に用いた場合では、従来材料と同等の性能を上記（１）に述べられているような劣化を起こさずに維持することが達成されている。しかし、上記（５）の段階では性能向上の可能性が未知数であったため、粒径比の検討によって最も優れた材料を決定することが望まれていた。
【００１２】
そこで、本発明は、より一層の高性能化・高出力化と長寿命化を実現でき、しかも製造コストが低廉で済む固体酸化物形燃料電池用燃料極材料を提供することを目的とする。
【００１３】
【課題を解決するための手段】
かかる目的を達成するため、本願発明者は、ＳＯＦＣ用燃料極材料に用いる各粉末の粒径を変更し、その微細構造を改良するという試験と検討を行った。従来の固体酸化物形燃料電池用燃料極材料は細かいＮｉＯと８ＹＳＺとの混合粉体であり、その改善のために開発された材料としては、例えば５〜２０μｍ程度の粒径のＮｉＯ、２０〜４０μｍ程度の粒径の８ＹＳＺ（粗ＹＳＺ）、０．１〜１μｍ程度の粒径の８ＹＳＺ（微ＹＳＺ）が用いられたもの等がある。これに対し、本願発明者は、さらに各粉末の粒径比を変化させることによって評価項目毎の評価を行って適用可能な粒径比の範囲を明らかにすることを試みた。そして、その結果、粒径比ならびに重量混合比という条件を絞り込み最適化することにより燃料極の性能を長時間維持することが可能となり、かつその性能の向上を達成させ得ることを知見するに至った。
【００１４】
本発明はこのような各種原料粉末の処理条件や粒径比の最適値に関する知見に基づくものであり、請求項１記載の固体酸化物形燃料電池用燃料極材料は、酸化ニッケルＮｉＯと８モル％のイットリアで結晶構造を安定化させたジルコニア８ＹＳＺの重量混合比と粒径比を最適化することにより燃料極の性能を長時間維持させ併せて性能を向上させたことを特徴とするものである。
【００１５】
さらに、本願発明者は、本材料の原料粉末である粗ＹＳＺ粉末、ＮｉＯ粉末、微ＹＳＺ粉末の混合比と各材料の収縮挙動、気孔率、導電率変化、電極性能等で定義した劣化要因との各データを収集した関係解明により、燃料極として適用性の高い混合領域が存在するかどうか種々試験と検討をし、本材料を適用したＳＯＦＣの製造時や発電条件下における劣化挙動を把握した。この結果、劣化要因の各許容範囲を例えば収縮率≦５．０％、気孔率≧３０．０％、導電率≧１００Ｓ／ｃｍとした場合において、粗ＹＳＺ粉末、ＮｉＯ粉末、微ＹＳＺ粉末を４：６：１の重量比で混合した場合に燃料極性能の長時間維持が可能であると共に最も優れた電極性能（つまり過電圧が最小の状態）を達成出来ることが知見された。例えば、各種ＹＳＺ支持形燃料極の過電圧経時変化について試験した結果（図４参照）、重量比４：６：１の場合に最小過電圧となり、尚かつ試験時間２５００時間経過の際にも燃料極性能の維持が可能であることが確認された。
【００１６】
請求項２記載の発明はかかる知見に基づくもので、請求項１記載の固体酸化物形燃料電池用燃料極材料において、８ＹＳＺには、予め第一の仮焼温度で仮焼された８ＹＳＺ仮焼粉末粒子から成る粗粒相当ＹＳＺ粉末と、８ＹＳＺ微粉粒子およびこの８ＹＳＺ微粉粒子から造粒された８ＹＳＺ造粒粉末粒子から成る微粒相当ＹＳＺ粉末とが用いられ、ＮｉＯにはＮｉＯ微粉粒子および造粒後に第二の仮焼温度で仮焼されたＮｉＯ仮焼粉末粒子から成るＮｉＯ粒子粉末が用いられ、粗粒相当ＹＳＺ粉末：ＮｉＯ粒子粉末：微粒相当ＹＳＺ粉末の重量混合比が４：６：１であることを特徴とするものである。
【００１７】
この場合、粗ＹＳＺ粉末に相当する８ＹＳＺ仮焼粉末粒子と、微ＹＳＺ粉末に相当する８ＹＳＺ微粉粒子および８ＹＳＺ造粒粉末粒子と、ＮｉＯ粉末であるＮｉＯ微粉粒子およびＮｉＯ仮焼粉末粒子とが、燃料極としての適用性が高くなる領域において混合されることにより、燃料極性能の長時間維持が可能となると共に過電圧が最小の状態になるという最も優れた電極性能が達成される（図４参照）。８ＹＳＺ微粉粒子から８ＹＳＺ造粒粉末粒子を造粒するにはスプレードライヤで造粒することが一般的だが特にこの手段に限られることはなく、例えば噴霧熱分解法でも構わない。
【００１８】
なお、燃料極材料の最適な粒径を明らかにすることを目的とした今回の試験では、各粉末粒子の粒径を変化させる必要があり、その結果として粗ＹＳＺ粒子と微ＹＳＺ粒子の粒径が逆転する場合が生じた。ただし、これらの粉末には元々機能（役割）が付加されており、その機能は原料粉末を大気中仮焼温度（例えば１４００℃）で仮焼きしているかどうかで決定されることから、このように粒径が逆転したか否かに関わらず本来の粗（微）ＹＳＺの範囲に含めるため、本明細書では「粗粒相当ＹＳＺ粉末」、「微粒相当ＹＳＺ粉末」というように「相当」という用語を用いることとした。
【００１９】
また、今回の試験では第一の仮焼温度を１４００℃、第二の仮焼温度を１０００℃というように最も好ましいと考えられた温度に設定したが、これら温度は好適な一例であってこのような値に限定されるものではない。すなわち、所定の劣化要因許容範囲や電極性能を得るためには最終的に出来上がりの燃料極ミクロ構造を概念通りのものにする必要があり、このような視点からすれば予め行う熱処理温度は特に限定されることがない上、ある程度の誤差が許容される。ただ、ボールミルを用いた粉混ぜ法で作製している以上、特に粗ＹＳＺを壊さないようにするにはある程度の熱処理は必要である。
【００２０】
さらに本願発明者は、本材料を用いたＳＯＦＣの一層の高性能化には各原料粉末の粒径比を最適化する必要があると考え、粗ＹＳＺ粉末、ＮｉＯ粒子粉末、微ＹＳＺ粉末の重量混合比が上述のように４：６：１であって、粒径が異なる各原料粉末を用いたＹＳＺ支持形燃料極材料について、劣化要因と発電状態における電極性能を評価してＳＯＦＣ用燃料極として最も適した原料粉末の粒径比を明らかにする試験を行った。ここでは、粒径を制御した粗ＹＳＺ、ＮｉＯ、微ＹＳＺの各粒子粉末を混合した３４種類の材料をサンプルとし、これら粒径が異なる原料粉末を用いたＹＳＺ支持形燃料極の各劣化要因評価試験を行った。そして、これにより電極微細構造は図１に示すように変化することを知見した。図中の３４箇所の○がこれら３４種類の材料を表している。ここでの各試料は、プレス成形後に大気中で第一の仮焼温度にて焼成して得られたものである。上述したように、粗ＹＳＺ粉末（粗粒相当ＹＳＺ粉末）と微ＹＳＺ粉末（微粒相当ＹＳＺ粉末）とでは仮焼の有無に違いがある。ここでは、粗ＹＳＺ、ＮｉＯ、微ＹＳＺの各粉末粒径をａ，ｂ，ｃとし、試料として用いた材料をａ−ｂ−ｃの形で表している。また図１において、粗ＹＳＺ粉末〜ＮｉＯ粉末の辺では、ＮｉＯの方に行くに従ってＮｉＯ粒径に対する粗ＹＳＺ粒径の比が小さくなり、逆に粗ＹＳＺの方に行くに従って粗ＹＳＺ粒径に対するＮｉＯ粒径の比が小さくなる。同様にＮｉＯ粉末〜微ＹＳＺの辺では、微ＹＳＺの方に行くに従って微ＹＳＺ粒径に対するＮｉＯ粒径の比が小さくなり、逆にＮｉＯの方に行くに従ってＮｉＯ粒径に対する微ＹＳＺ粒径の比が小さくなる。更に微ＹＳＺ〜粗ＹＳＺの辺では、粗ＹＳＺの方に行くに従って粗ＹＳＺ粒径に対する微ＹＳＺ粒径の比が小さくなり、逆に微ＹＳＺの方に行くに従って微ＹＳＺ粒径に対する粗ＹＳＺ粒径の比が小さくなる。三角形の各点は、上記の法則に従って三種類の粒子の粒径比を表す。また、これらの中で斜線で示した範囲に入るものが上述した収縮挙動、気孔率、導電率変化から判断される適用可能な材料ということになり、それ以外は適用できない材料であるということがわかった。
【００２１】
このような評価試験の結果、以下のことが明らかになった。
１．劣化要因（図１参照）
（１）１．０μｍ以下のＮｉＯ・微ＹＳＺ粒子粉末は大気中焼成時の収縮が大きく、これらを用いた材料の線収縮率は許容値（５．０％）を上回った。一方、還元雰囲気中での線収縮率は全ての材料で１．０％以下と小さく、発電時の収縮挙動は劣化に影響しないと考えられた。
【００２２】
（２）気孔率は、全ての材料で許容範囲（３０．０％以上）を満足するが、粗ＹＳＺ粒径≧６６．０μｍ、ＮｉＯ≧粒径３４．０μｍを用いた材料は、粗ＹＳＺ・微ＹＳＺによるＹＳＺ骨格の強度低下によって脆くなる。
【００２３】
（３）導電率は、ＮｉＯ粒径＝１．０μｍを用いた材料、およびＮｉＯ粒径：微ＹＳＺ粒径比がほぼ１：１の材料において許容範囲（１００Ｓ／ｃｍ以上）を満たし、経時変化もほとんど生じなかった。
【００２４】
２．電極性能の評価（図２参照）
電極性能評価を実施し、安定性が最も高く、過電圧が最も小さい材料を知見した（図２参照）。つまり、燃料極として適用性が高いと判断された各試料の過電圧（燃料極での電圧損失）は、粗ＹＳＺ、ＮｉＯおよび微ＹＳＺの粒径がそれぞれ２７．０μｍ、１．０μｍ、０．４μｍの粒子粉末を用いた場合に最小となった。なお、図２の説明中のＱ_Ｈ２、Ｑ_ａｉｒはそれぞれ水素ガス、空気の流量を示している。
【００２５】
以上より、粗ＹＳＺ・ＮｉＯ・微ＹＳＺ粒子粉末の重量混合比を４：６：１とした材料では、ＮｉＯと微ＹＳＺを同一範囲内の粒径にすれば燃料極の劣化要因許容範囲を満足することがわかった。さらに、粗ＹＳＺに対するＮｉＯ・微ＹＳＺ粒径比を３０：１（粗ＹＳＺ粒径の約１／３０）にすると、過電圧が小さく、長期安定性に優れたＹＳＺ支持形燃料極材料が得られることがわかった。なお、ここでいう「同一範囲」とは粒径が一致する場合のみならず、上述した収縮挙動、気孔率、導電率変化から判断される適用可能な材料であって図１の斜線で示した適用可能領域の範囲内に収まる場合をも含む意味である。
【００２６】
請求項３記載の発明はかかる知見に基づくもので、請求項２記載の固体酸化物形燃料電池用燃料極材料において、重量混合比４：６：１の場合における粗粒相当ＹＳＺ粉末、ＮｉＯ粒子粉末、微粒相当ＹＳＺ粉末の各粉末粒子の粒径がそれぞれ２７．０μｍ、１．０μｍ、０．４μｍであることを特徴としている。この場合、より高い電極性能を有する燃料極材料が得られることが確認された。
【００２７】
【発明の実施の形態】
本発明に係る燃料極材料は、比較的大きな粒径を有するジルコニア粗粒子群（すなわち粗粒相当ＹＳＺ粉末）と、比較的小さな粒径を有するジルコニア微粒子群（すなわち微粒相当ＹＳＺ粉末）と、酸化ニッケルないしニッケル粒子群（すなわちＮｉＯ粒子粉末）との混合物からなる。図３は、本発明に係る燃料極材料を用いて、燃料極を形成した場合におけるその微細構造を示す概念図であり、図中符号１はジルコニア粗粒子、符号２は酸化ニッケル粒子、符号３はジルコニア微粒子を示す。
【００２８】
図３に示すように、ジルコニア粗粒子１は、燃料極中において骨格をなし、かつ粒子間にできる隙間（粒間細隙）によって気孔４を形成する。これらによって電解質（安定化ジルコニア製）との熱的整合性を図るとともに、焼結の進行による燃料極の収縮ならびに気孔の閉塞を防止する。またジルコニア微粒子３は、粒径の大きいジルコニア粗粒子１どうしをより強固に接着したり、電解質と燃料極の密着性をより良好にしたりする。そして大小のジルコニア粒子１，３によって電極全体の焼結性が制御され、ニッケルの凝集防止と電極反応場の増加が図られる。また、酸化ニッケル粒子２は、粒径の大きなジルコニア粗粒子１の周囲に分散され、電池作動時にニッケルに変化する。これによって、燃料極の電流パスを形成し、かつジルコニア粒子１，３と気孔４との界面において、電極反応を生じる。
【００２９】
本発明の燃料極材料は、各種の形態の固体電解質燃料電池の燃料極の作製に好適に用いることができ、燃料電池、あるいは燃料極の形状等に何ら限定されることなく、いずれの場合であっても優れた性能を発揮し得る。
【００３０】
本発明の燃料極材料の製造方法としては、特に限定されるものではないが、上述したような各粒子の所定の粒径、殊にジルコニア粗粒子１の粒径を維持して、安定に混合することができるように、ボールミルを用い、乾式条件にて攪拌混合することが望ましい。なおボールミルとしては、ポリ軟こう瓶とナイロン製ボールの組合せといった、比較的軟質の表面を有する装置とすることが望まれる。
【００３１】
攪拌混合は、最初に８ＹＳＺ粗粒子とＮｉＯ粒子とを例えば、４８〜６０時間程度混合し、次いでこの混合物に８ＹＳＺ微粒子群を添加してさらに４８時間程度混合することにより行なわれる。
【００３２】
【発明の効果】
以上の説明より明らかなように、請求項１記載の固体酸化物形燃料電池用燃料極材料によると、空気中ならびに電池動作雰囲気中においても体積収縮や気孔率の減少といった緻密化が起こりにくいことから、従来の材料に対して長時間の安定性と電極性能が優れている。したがって、この燃料極材料を用いれば、ＳＯＦＣ発電に関してより一層の高性能化・高出力化と長寿命化が可能となる。粉末混合という簡便な方法で作製可能である点においては従来技術と異ならないため余計なコストがかからず、低廉なままで済む。
【００３３】
また請求項２記載の発明によると、種々の試験と検討によって明らかになった知見に基づき、粗ＹＳＺ、ＮｉＯ、微ＹＳＺの各粒子粉末を最適な重量比で混合することにより、燃料極性能を長時間維持すると共に、最も優れた電極性能（つまり過電圧が最小であること）を達成することが可能となる。
【００３４】
さらに請求項３記載の発明によると、粗ＹＳＺ、ＮｉＯ、微ＹＳＺの各粒子粉末を最適な重量比で混合することに加え、各粉末に最適値の粒径のものを用いることで、より高い電極性能を有する燃料極材料が得られる。
【図面の簡単な説明】
【図１】本発明に係る評価試験に用いた各粉末粒径比、代表的なミクロ構造、ＳＯＦＣ用燃料極としての適用可能領域を示す図である。
【図２】試験に用いた材料の各粒子の粒径分布の測定結果を表すグラフである。
【図３】本発明に係る燃料極材料を固体酸化物形燃料電池に用いた際における燃料極の微細構造を示す概念図である。
【図４】各種ＹＳＺ支持形燃料極の過電圧経時変化の一例を示すグラフである。
【符号の説明】
１　ジルコニア粗粒子（粗粒相当ＹＳＺ粉末）
２　酸化ニッケル粒子（ＮｉＯ粒子粉末）
３　ジルコニア微粒子（微粒相当ＹＳＺ粉末）[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a fuel electrode material for a solid oxide fuel cell. More specifically, the present invention aims to extend the life and improve the performance of a nickel-stabilized zirconia fuel electrode powder for a solid oxide fuel cell (SOFC), and particularly to a fuel electrode of an SOFC by using the fuel electrode. The present invention relates to improvement of a fuel electrode material that can be used.
[0002]
[Prior art]
The fuel electrode material for solid oxide fuel cell, a nickel oxide (NiO, where the fuel during cell operation metallic nickel Ni) nickel was formed by mixing fine particles of zirconia (ZrO ₂₎ - zirconia cermet, high catalytic activity (Reduction ability of hydrogen) and high conductivity (reciprocal of electrical resistance) even at a high temperature from room temperature to 1000 ° C. was considered to be suitable. However, if the content of nickel in the fuel electrode is large, thermal stress is generated due to a difference in thermal expansion coefficient, which may lead to cell destruction, and the content of nickel cannot be increased so much. If the amount is small, the electrode characteristics are not so good, it is difficult to take out a current, and there are problems such as high sinterability and easy densification. Thus, conventionally, zirconia using zirconia whose crystal structure is stabilized with 8 mol% yttria (hereinafter referred to as 8YSZ) has been adopted.
[0003]
(1) As described above, the conventional fuel electrode material for a solid oxide fuel cell was obtained by mixing fine powders of NiO and 8YSZ. However, although this fuel electrode has excellent initial characteristics, it deteriorates several tens of hours after the start of power generation, and becomes in a state where power generation is impossible. As a result of elucidating the cause, it was found that the cause was densification and volume shrinkage of the fuel electrode and aggregation of Ni particles under the operating condition of the battery (see Non-Patent Document 1). In addition, there are other reports on aggregation and densification of Ni (see Non-Patent Document 2).
[0004]
(2) Furthermore, there is a report that attempts are made to increase the dispersion of Ni particles and extend the life of the fuel electrode by using Ni (Mg) O-8YSZ (see Non-Patent Documents 3 to 5).
[0005]
(3) There is also a report that attempts to improve performance by coating Ni having a large particle size with YSZ having a small particle size (see Non-Patent Document 6).
[0006]
(4) Further, a material in which YSZ is electrochemically vapor-deposited on metal ruthenium (see Non-Patent Document 7) and a material in which YSZ is attached to metal Ni by a vapor phase method are also being studied (see Non-Patent Document 8). .
[0007]
(5) Further, in the fuel electrode material for a solid oxide fuel cell and the method for producing the same, those using each particle powder of NiO of about 10 μm and 8YSZ of about 20 to 40 μm and about 0.6 μm have already been disclosed by the present applicant. It is disclosed (see Patent Document 1 and Non-Patent Document 9).
[0008]
[Patent Document 1]
JP-A-8-306361 [Non-Patent Document 1]
Central Research Institute of Electric Power Industry W93019 “Study on high performance of fuel electrode for SOFC-Elucidation of deterioration phenomenon of SOFC by current interruption method-”, Central Research Institute of Electric Power Industry, May 1994 [Non-Patent Document 2]
"Abstracts of the 60th Meeting of the Electrochemical Society of Japan," April 1-3, 1993, p. 269
[Non-Patent Document 3]
"Summary of the 33rd Battery Study Group Lectures", September 16-18, 1992, p. 35-36
[Non-patent document 4]
"Abstracts of the 59th Annual Meeting of the Electrochemical Society," April 2-4, 1992, p. 197
[Non-Patent Document 5]
"Abstracts of the 60th Meeting of the Electrochemical Society of Japan," April 1-3, 1993, p. 270
[Non-Patent Document 6]
"Abstracts of the 59th Annual Meeting of the Electrochemical Society," April 2-4, 1992, p. 198
[Non-Patent Document 7]
"Summary of the 18th Symposium on Solid State Ionics," Oct. 12-13, 1992, p. 5-8
[Non-Patent Document 8]
"Abstracts of the 59th Annual Meeting of the Electrochemical Society," April 2-4, 1992, p. 199
[Non-Patent Document 9]
Report of Central Research Institute of Electric Power Industry W94016 "Study on high performance of SOFC fuel electrode-Long life by improvement of electrode microstructure", Central Research Institute of Electric Power Industry, May 1995 [0009]
[Problems to be solved by the invention]
However, the prior arts (1) to (4) described above only aim at improving the performance of the fuel electrode in particular, and do not sufficiently consider deterioration during long-term operation, and directly affect the manufacturing cost. No consideration is given to the manufacturing process, or the manufacturing cost is considered to be high due to the complexity of the manufacturing process.
[0010]
Further, in the case of the production method (4), a special atmosphere is required, so that the equipment cost increases, the yield or the yield of the raw material is low, and complete batch processing (not suitable for continuous production) is required. It is considered that the cost is increased for the reasons described above.
[0011]
At the stage of (5), a production method that enables mass production at low cost by using a simple production technique is employed, and when used for a fuel electrode of an SOFC, the same performance as that of a conventional material is obtained. It has been achieved to maintain without causing deterioration as described in (1). However, at the stage (5), since the possibility of performance improvement was unknown, it was desired to determine the most excellent material by studying the particle size ratio.
[0012]
Therefore, an object of the present invention is to provide a fuel electrode material for a solid oxide fuel cell which can achieve higher performance, higher output, and longer life, and can be manufactured at low cost.
[0013]
[Means for Solving the Problems]
In order to achieve this object, the inventors of the present application have conducted tests and studies on changing the particle size of each powder used for the fuel electrode material for SOFC and improving the microstructure thereof. A conventional fuel electrode material for a solid oxide fuel cell is a mixed powder of fine NiO and 8YSZ. As a material developed for the improvement, for example, NiO having a particle size of about 5 to 20 μm, There are 8YSZ (coarse YSZ) having a particle size of about 40 μm and those using 8YSZ (fine YSZ) having a particle size of about 0.1 to 1 μm. On the other hand, the inventor of the present application attempted to clarify the range of applicable particle size ratios by further evaluating each evaluation item by changing the particle size ratio of each powder. As a result, it has been found that the performance of the anode can be maintained for a long time by optimizing the conditions of the particle size ratio and the weight mixing ratio, and the performance can be improved. Was.
[0014]
The present invention is based on the knowledge about the processing conditions of such various raw material powders and the optimum value of the particle size ratio, and the fuel electrode material for a solid oxide fuel cell according to claim 1 comprises nickel oxide NiO and 8 mol % Of zirconia 8YSZ whose crystal structure has been stabilized with yttria by optimizing the weight mixing ratio and the particle size ratio, thereby maintaining the performance of the anode for a long time and improving the performance. is there.
[0015]
Further, the present inventor has determined the mixing ratio of the coarse YSZ powder, NiO powder, and fine YSZ powder, which are the raw material powders of the present material, and the shrinkage behavior of each material, the porosity, the change in conductivity, the deterioration factor defined by the electrode performance, etc. Based on the relationship elucidation that collected the above data, various tests and examinations were performed to determine whether there was a highly applicable mixed region as a fuel electrode, and the deterioration behavior of SOFCs using this material during production and under power generation conditions was determined. . As a result, when the respective allowable ranges of the deterioration factors are, for example, shrinkage ≦ 5.0%, porosity ≧ 30.0%, and conductivity ≧ 100 S / cm, the coarse YSZ powder, the NiO powder, and the fine YSZ powder are reduced by 4%. : When mixed at a weight ratio of 6: 1, it has been found that the fuel electrode performance can be maintained for a long time and the most excellent electrode performance (that is, a state in which overvoltage is minimum) can be achieved. For example, as a result of testing over-time change of overvoltage of various YSZ-supported fuel electrodes (see FIG. 4), the minimum overvoltage was obtained when the weight ratio was 4: 6: 1, and the fuel electrode performance was improved even after 2500 hours of test time. It was confirmed that it was possible to maintain
[0016]
The invention according to claim 2 is based on this finding, and in the fuel electrode material for a solid oxide fuel cell according to claim 1, 8YSZ includes 8YSZ calcined at a first calcining temperature in advance. Coarse-grain YSZ powder composed of powder particles, YSZ powder equivalent to 8YSZ fine powder particles and fine YSZ powder composed of 8YSZ granulated powder particles granulated from the 8YSZ fine powder particles are used, and NiO fine powder particles and after granulation are used for NiO. NiO particle powder composed of NiO calcined powder particles calcined at the second calcination temperature is used, and the weight mixing ratio of YSZ powder equivalent to coarse particles: NiO particle powder: YSZ powder equivalent to fine particles is 4: 6: 1. It is characterized by having.
[0017]
In this case, 8YSZ calcined powder particles corresponding to coarse YSZ powder, 8YSZ fine powder particles and 8YSZ granulated powder particles corresponding to fine YSZ powder, NiO fine powder particles and NiO By being mixed in a region where the applicability as the electrode is high, it is possible to maintain the anode performance for a long time and to achieve the best electrode performance in which the overvoltage is minimized (see FIG. 4). . Granulation of 8YSZ granulated powder particles from 8YSZ fine powder particles is generally performed by a spray drier, but is not particularly limited to this means, and may be, for example, a spray pyrolysis method.
[0018]
In this test, which aimed to clarify the optimal particle size of the fuel electrode material, it was necessary to change the particle size of each powder particle, and as a result, the particle size of the coarse YSZ particles and the fine YSZ particles May be reversed. However, a function (role) is originally added to these powders, and the function is determined by whether or not the raw material powder is calcined at the atmospheric calcining temperature (for example, 1400 ° C.). Regardless of whether or not the particle size has been reversed, it is included in the original range of coarse (fine) YSZ, so in this specification, it is referred to as “equivalent” such as “Coarse-grain equivalent YSZ powder” and “Fine-grain equivalent YSZ powder”. Terms were used.
[0019]
In this test, the first calcination temperature was set to 1400 ° C. and the second calcination temperature was set to 1000 ° C., which were considered to be the most preferable. However, these temperatures are preferable examples. It is not limited to such values. That is, in order to obtain a predetermined deterioration factor allowable range and electrode performance, it is necessary to finally make the finished fuel electrode microstructure as conceptual, and from such a viewpoint, the heat treatment temperature performed in advance is particularly limited. In addition, some error is allowed. However, as long as the powder is produced by a powder mixing method using a ball mill, a certain amount of heat treatment is necessary to prevent the coarse YSZ from being broken.
[0020]
Furthermore, the inventor of the present application considers that it is necessary to optimize the particle size ratio of each raw material powder in order to further improve the performance of the SOFC using this material, and the weight of the coarse YSZ powder, the NiO particle powder, and the fine YSZ powder is considered. As described above, for the YSZ-supported fuel electrode material using the respective raw material powders having the mixing ratio of 4: 6: 1 and different particle diameters, the deterioration factors and the electrode performance in the power generation state were evaluated to evaluate the fuel electrode for SOFC A test was carried out to clarify the particle size ratio of the raw material powder most suitable as the above. Here, 34 types of materials obtained by mixing coarse YSZ, NiO, and fine YSZ particle powders whose particle diameters were controlled were used as samples, and evaluation of each deterioration factor of the YSZ-supported fuel electrode using raw material powders having different particle diameters was performed. The test was performed. Then, it was found that the electrode microstructure changes as shown in FIG. The circles at 34 places in the figure represent these 34 types of materials. Each sample here was obtained by firing at the first calcination temperature in the air after press molding. As described above, there is a difference in the presence or absence of calcination between the coarse YSZ powder (coarse-grain YSZ powder) and the fine YSZ powder (fine-grain YSZ powder). Here, the particle diameters of the coarse YSZ, NiO, and fine YSZ powders are a, b, and c, and the material used as the sample is represented by abc. Also, in FIG. 1, on the sides of the coarse YSZ powder to the NiO powder, the ratio of the coarse YSZ particle size to the NiO particle size decreases toward NiO, and conversely, the NiO ratio relative to the coarse YSZ particle size increases toward coarse YSZ. The particle size ratio becomes smaller. Similarly, on the sides from NiO powder to fine YSZ, the ratio of the NiO particle size to the fine YSZ particle size decreases toward the fine YSZ, and conversely, the ratio of the fine YSZ particle size to the NiO particle size increases toward the NiO. Becomes smaller. Further, on the side of fine YSZ to coarse YSZ, the ratio of the fine YSZ particle size to the coarse YSZ particle size decreases toward the coarse YSZ, and conversely, the ratio of the coarse YSZ particle size to the fine YSZ particle increases toward the fine YSZ. Becomes smaller. Each point of the triangle represents the particle size ratio of the three types of particles according to the above rule. Among these, those that fall within the range indicated by hatching are applicable materials determined from the above-described shrinkage behavior, porosity, and change in conductivity, and other materials are not applicable. all right.
[0021]
As a result of such an evaluation test, the following became clear.
1. Deterioration factor (see Fig. 1)
(1) The NiO / fine YSZ particles having a particle size of 1.0 μm or less greatly shrink during firing in the air, and the linear shrinkage of the material using these particles exceeds the allowable value (5.0%). On the other hand, the linear shrinkage in the reducing atmosphere was as low as 1.0% or less for all the materials, and it was considered that the shrinkage behavior during power generation did not affect the deterioration.
[0022]
(2) The porosity of all the materials satisfies the allowable range (30.0% or more), but the material using coarse YSZ particle size ≧ 66.0 μm and NiO ≧ 34.0 μm has coarse YSZ · It becomes brittle due to a decrease in the strength of the YSZ skeleton due to the fine YSZ.
[0023]
(3) The electrical conductivity satisfies the allowable range (100 S / cm or more) in a material using NiO particle size = 1.0 μm and a material having a NiO particle size: fine YSZ particle size ratio of approximately 1: 1 and changes with time. Hardly occurred.
[0024]
2. Evaluation of electrode performance (see Fig. 2)
The electrode performance was evaluated, and a material having the highest stability and the lowest overvoltage was found (see FIG. 2). That is, the overvoltage (voltage loss at the fuel electrode) of each sample determined to have high applicability as a fuel electrode is such that the coarse YSZ, NiO, and fine YSZ particle diameters are 27.0 μm, 1.0 μm, and 0.4 μm, respectively. Was minimized when the particle powder was used. Note that Q _H2 and Q _{air in} the description of FIG. 2 indicate the flow rates of hydrogen gas and air, respectively.
[0025]
As described above, in the material in which the weight mixing ratio of the coarse YSZ / NiO / fine YSZ particles is 4: 6: 1, if the particle size of NiO and fine YSZ is within the same range, the permissible range of the deterioration factor of the fuel electrode is satisfied. I found out. Further, when the particle size ratio of NiO / fine YSZ to coarse YSZ is 30: 1 (approximately 1/30 of the coarse YSZ particle size), a YSZ-supported fuel electrode material having small overvoltage and excellent long-term stability can be obtained. I understood. Note that the “same range” here is not only when the particle diameters match, but also applicable materials determined from the above-described shrinkage behavior, porosity, and change in conductivity, and are indicated by hatching in FIG. The meaning includes the case where it falls within the range of the applicable area.
[0026]
The invention according to claim 3 is based on such knowledge, and in the fuel electrode material for a solid oxide fuel cell according to claim 2, YSZ powder equivalent to coarse particles and NiO particles in a case of a weight mixing ratio of 4: 6: 1. It is characterized in that the particle diameter of each of the powder particles of the powder and the YSZ powder equivalent to fine particles is 27.0 μm, 1.0 μm and 0.4 μm, respectively. In this case, it was confirmed that a fuel electrode material having higher electrode performance was obtained.
[0027]
BEST MODE FOR CARRYING OUT THE INVENTION
The fuel electrode material according to the present invention comprises a group of zirconia coarse particles having a relatively large particle size (namely, YSZ powder corresponding to coarse particles), a group of zirconia fine particles having a relatively small particle size (namely, YSZ powder corresponding to fine particles), and an oxide. It consists of a mixture with nickel or nickel particles (that is, NiO particles). FIG. 3 is a conceptual diagram showing a fine structure of a fuel electrode formed by using the fuel electrode material according to the present invention, wherein reference numeral 1 denotes coarse zirconia particles, reference numeral 2 denotes nickel oxide particles, and reference numeral 3 Represents zirconia fine particles.
[0028]
As shown in FIG. 3, the coarse zirconia particles 1 form a skeleton in the fuel electrode, and form pores 4 by gaps (intergranular gaps) formed between the particles. With these, thermal compatibility with the electrolyte (made of stabilized zirconia) is ensured, and at the same time, shrinkage of the fuel electrode and blockage of pores due to progress of sintering are prevented. Further, the zirconia fine particles 3 more firmly bond the zirconia coarse particles 1 having a large particle diameter, and further improve the adhesion between the electrolyte and the fuel electrode. The sinterability of the entire electrode is controlled by the large and small zirconia particles 1 and 3, thereby preventing nickel agglomeration and increasing the electrode reaction field. Further, the nickel oxide particles 2 are dispersed around the zirconia coarse particles 1 having a large particle diameter, and change to nickel when the battery is operated. As a result, a current path for the fuel electrode is formed, and an electrode reaction occurs at the interface between the zirconia particles 1 and 3 and the pores 4.
[0029]
The fuel electrode material of the present invention can be suitably used for producing a fuel electrode of various forms of solid electrolyte fuel cells, and is not limited in any way to the shape of the fuel cell or the fuel electrode. Even if it does, it can exhibit excellent performance.
[0030]
Although the method for producing the fuel electrode material of the present invention is not particularly limited, it is possible to stably mix the particles while maintaining the predetermined particle diameter of each particle as described above, particularly the particle diameter of the zirconia coarse particles 1. It is desirable to use a ball mill and stir and mix the mixture under dry conditions so that the mixture can be prepared. It is desired that the ball mill be a device having a relatively soft surface, such as a combination of a plastic ointment bottle and a nylon ball.
[0031]
The stirring and mixing are performed by first mixing the 8YSZ coarse particles and the NiO particles for, for example, about 48 to 60 hours, and then adding the 8YSZ fine particles to the mixture and mixing for about 48 hours.
[0032]
【The invention's effect】
As is apparent from the above description, according to the fuel electrode material for a solid oxide fuel cell according to the first aspect, densification such as volume shrinkage and decrease in porosity hardly occurs even in the air and in the cell operating atmosphere. Therefore, the long-term stability and the electrode performance are superior to conventional materials. Therefore, if this fuel electrode material is used, higher performance, higher output, and longer life can be achieved for SOFC power generation. It is not different from the prior art in that it can be manufactured by a simple method of powder mixing, so that no extra cost is required and the cost can be kept low.
[0033]
According to the second aspect of the present invention, the fuel electrode performance is improved by mixing each of the coarse YSZ, NiO, and fine YSZ particle powders at an optimal weight ratio based on the knowledge revealed by various tests and examinations. While maintaining for a long time, it is possible to achieve the best electrode performance (that is, the minimum overvoltage).
[0034]
Further, according to the third aspect of the present invention, in addition to mixing each of the coarse YSZ, NiO, and fine YSZ powder particles at an optimum weight ratio, the use of powder having an optimum particle size for each of the powders enables a higher value. A fuel electrode material having electrode performance can be obtained.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram showing a ratio of each powder particle size, a typical microstructure, and an applicable area as a fuel electrode for SOFC used in an evaluation test according to the present invention.
FIG. 2 is a graph showing a measurement result of a particle size distribution of each particle of a material used in a test.
FIG. 3 is a conceptual diagram showing a fine structure of an anode when the anode material according to the present invention is used in a solid oxide fuel cell.
FIG. 4 is a graph showing an example of a change with time in overvoltage of various YSZ-supported fuel electrodes.
[Explanation of symbols]
1 Zirconia coarse particles (Coarse-grain equivalent YSZ powder)
2 Nickel oxide particles (NiO particle powder)
3 Fine particles of zirconia (YSZ powder equivalent to fine particles)

Claims

By optimizing the weight mixing ratio and particle size ratio of zirconia 8YSZ whose crystal structure has been stabilized with nickel oxide NiO and 8 mol% yttria, the performance of the fuel electrode has been maintained for a long time and the performance has been improved. A fuel electrode material for a solid oxide fuel cell, comprising:

The 8YSZ includes coarse YSZ powder equivalent to 8YSZ calcined powder particles calcined in advance at the first calcining temperature, 8YSZ fine powder particles, and 8YSZ granulated powder particles granulated from the 8YSZ fine powder particles. YSZ powder is used, and NiO is NiO particle powder composed of NiO fine powder particles and NiO calcined powder particles calcined at a second calcining temperature after granulation. 2. The fuel electrode material for a solid oxide fuel cell according to claim 1, wherein the weight mixing ratio of the powder: NiO particle powder: fine equivalent YSZ powder is 4: 6: 1.

The particle diameter of each of the coarse YSZ powder, NiO particle powder, and fine YSZ powder corresponding to the weight mixing ratio of 4: 6: 1 is 27.0 μm, 1.0 μm, and 0.4 μm, respectively. 3. The fuel electrode material for a solid oxide fuel cell according to claim 2, wherein: