JP4595297B2 - Fuel cell system - Google Patents

Description

【００４９】
ただし、Ｔ[Ｋ]は膜の温度、Ｅ_Aは定数、Ａ（λ）は電解質膜２１の含水率に依存した定数である。セルの内部抵抗Ｒ₁は、以下のように表すことができる。
【００５０】
【数２】

【００５１】
ただし、ｒは電解質膜２１の抵抗Ｒｍ以外のガス拡散層２３、セパレータ２４の抵抗、触媒層２２・ガス拡散層２３・セパレータ２４間の接触抵抗の和とする。ここでｒは温度依存性があるが、電解質膜２１の抵抗Ｒｍの変化に比べてこの依存性はかなり小さく、温度によらず一定と近似することができる。
【００５２】
一方、触媒層２２の抵抗である反応抵抗Ｒ₂は以下のように示される。
【００５３】
【数３】

【００５４】
ただし、ηは触媒活性化過電圧、ｉは電流密度である。なお、触媒活性化過電圧ηと電流密度ｉは、以下のターフェル式の関係がある。
【００５５】
【数４】

【００５６】
ただし、αは通過係数、ｚは価数、Ｆはファラデー定数、Ｒは気体定数、ｉ₀は交換電流密度である。なお、交換電流密度ｉ₀は、以下の温度依存性がある。
【００５７】
【数５】

【００５８】
ただし、ｋ₀は反応定数、Ｅｃは活性化エネルギであり、これらは温度に依存しない。式（３）〜（５）から反応抵抗Ｒ₂を算出することができる。
【００５９】
拡散抵抗Ｚｗについては、基本的には温度依存性がなく、予め電流１Ａ／ｃｍ²付近の電流電圧特性を測定しておくことにより実験的に求められる。あるいは、通常運転時の電流と電圧の特性値の履歴 から推定してもよい。この場合には、運転毎の変化を反映してより正確な値として設定することができる。
【００６０】
取り出し電流Ｉを、セル電圧Ｖｃが許容電圧範囲Ｖ₂より小さい値とならないように設定する。ここでは、例えば所定電圧範囲Ｖ₂の下限値Ｖ_2i（＝０．１Ｖ）を下回らない範囲に設定するには、取り出し電流Ｉの電流密度ｉを次式の範囲に設定する。
【００６１】
【数６】

【００６２】
ここで、Ｅｒは可逆電位で、１．２〜１．３Ｖ程度である。各定数を、予め実験等により求めておき記憶しておく。ここでは、Ａ（λ）、Ｅ_A、ｒ、α、ｚ、ｋ₀、Ｅ_c、Ｚｗを予め求めておき、記憶しておく。セル温度Ｔｃを計測して電流密度ｉが（６）式となる範囲での最大電流Ｉ_maxを求め、取り出し電流Ｉをこれに設定する。
【００６３】
すなわち、上述の式の各定数を実験的に求め、セルの内部抵抗Ｒ₁と反応抵抗Ｒ₂の温度依存性を予め関連付けておけば、起動時のセル温度Ｔｃに基づいて、最大電流Ｉ_maxを求めることができる。これにより、燃料電池１が劣化するのを避けつつ、つまり、セル電圧Ｖｃが下限電圧Ｖ_2iを下回るのを避けつつ、取り出し電流Ｉを最大に設定することができる。これにより、効率良く暖機を行って、セル温度Ｔｃが氷点以上となるまでの時間を短縮することができる。
【００６４】
次に、燃料電池システムの起動時の制御方法を図９のフローチャートを用いて説明する。
【００６５】
ステップＳ２１〜Ｓ２４においては、ステップＳ１〜Ｓ４と同様とする。反応ガスの供給を開始したら、ステップＳ２５において、セル温度Ｔｃを計測する。ステップＳ２６において、上述したようにセル温度Ｔｃに基づいて、最大電流Ｉ_maxを算出する。ステップＳ２７〜Ｓ２９においては、ステップＳ５〜Ｓ７と同様に、セル電圧Ｖｃが所定電圧Ｖ₁以上となるまで待機する。
【００６６】
セル電圧Ｖｃが所定電圧Ｖ₁以上となったら、ステップＳ３０において、電流の取り出しを開始する。ここでは、取り出し電流Ｉを、ステップＳ２６において算出した最大電流Ｉ_maxに設定する。ステップＳ３１において、セル温度Ｔｃを計測する。ステップＳ３２において、セル温度Ｔｃが氷点以上であるか否かを判断する。氷点以上でない場合には、セル内で凍結が生じる可能性がある。そこで、ステップＳ３３に進み、ステップＳ３１で計測したセル温度Ｔｃに基づいて、最大電流Ｉ_maxを再び算出し、取り出し電流Ｉを再設定する。次に、ステップＳ３４において、所定時間待機したら、ステップＳ３１に戻り、再びセル温度Ｔｃを計測する。これを繰り返して、ステップＳ３２において、セル温度Ｔｃが氷点以上となったら、セル内の凍結の可能性がないので、本フロ−を終了して通常の運転モードに移行する。
【００６７】
このように制御することで、電流取り出し開始直後から、燃料電池１を劣化しない範囲でできるだけ多くの電流を取り出し、発電反応を促進することができる。そのため、暖機時間を短縮することができるので、セル内で凍結が生じる頻度を低減することができる。
【００６８】
なお、ステップＳ３１以降の制御として、ステップＳ９〜Ｓ１４の制御を行っても良い。この場合には、第１の実施形態と同様に、セルの劣化を引き起こさないように、セル電圧Ｖｃを監視しつつ、取り出し電流Ｉを最大に設定することができる。
【００６９】
図１０に、セル温度Ｔｃに応じた電圧・電流特性を示す。ここでは、セル温度Ｔｃが−５℃、−１０℃、−１５℃の場合を示す。図１０に示すように、セル温度Ｔｃが小さくなるほど性能が低下する。そこで最大電流Ｉ_maxをセル電圧Ｔｃに応じて設定し、これを取り出し電流Ｉとすることで、セル電圧Ｖｃを過剰に低下することによりセルが劣化するのを避けることができ、かつ、暖機を促進させることができる。
【００７０】
例えば、上述した式（１）〜（６）における定数Ａ、Ｅ_A、ｒ、α、ｚ、ｋ₀、Ｅｃ、Ｚｗを予め実験で求め、Ｔｃ＝−１０℃とした場合に、セルの性能を低下させない最大電流Ｉ_maxは、例えば３００ｍＡ／ｃｍ²と算出された。
【００７１】
次に、本実施形態の効果を説明する。以下、第１の実施形態と異なる効果のみを説明する。
【００７２】
一つあるいは複数のセルの内部抵抗Ｒ₁と反応抵抗Ｒ₂とから、セル電圧Ｖｃが所定電圧Ｖ_2iを下回らない範囲で、最大電流Ｉ_maxを求める最大電力検出手段（Ｓ２６）を備える。これにより、燃料電池１の劣化を避けつつ、燃料電池１から取り出す電流を最大に設定することができる。これにより、暖機時間を短縮することができ、生成水が再凍結する頻度を低減することができる。
【００７３】
通常運転時の電流と電圧の特性値からセルの拡散抵抗Ｚｗを推測し、内部抵抗Ｒ₁と反応抵抗Ｒ₂と拡散抵抗Ｚｗとから、セル電圧Ｖｃが所定電圧Ｖ_2iを下回らない範囲で、最大電流Ｉ_maxを求める。拡散抵抗Ｚｗは温度依存性がそれほどないので、通常運転時の電流電圧特性から求めて記憶しておき、これによる電圧降下分を最大電流Ｉ_maxの決定の際に反映させることにより、正確に最大電流Ｉ_maxを求めることができる。
【００７４】
また、一つあるいは複数のセル温度を計測する温度センサ９を備える。内部抵抗Ｒ₁と反応抵抗Ｒ₂の温度依存性を予め求めておき、起動時のセル温度Ｔｃに基づいて、最大電流Ｉ_maxを求める。このように、温度依存性を予め求めておき、セル温度Ｔｃに応じて内部抵抗Ｒ_１および反応抵抗Ｒ_２を設定することで、簡単な計測により最大電流Ｉ_maxを求めることができる。
【００７５】
次に、第３の実施形態について説明する。燃料電池システムの概略構成を図１１に示す。以下、第２の実施形態と異なる部分を中心に説明する。
【００７６】
燃料電池システムを、第１の実施形態と同様に構成する。ただし、セルの内部抵抗Ｒ₁を計測する内部抵抗計測器を備える。例えば、高周波電流を印加する発振器と、電圧応答を検出する検出器からなる高周波インピーダンス計測器３１を備える。ここでは、周波数１ｋＨｚ、電流振幅７ｍＡの高周波インピーダンス計測器３１を備える。
【００７７】
高周波電流を流しての計測は燃料ガスを流していないときにも計測できる。なお、高周波電流の周波数は５００Ｈｚ以上１０ｋＨｚ以下が好ましい。５００Ｈｚ以下の場合には触媒層２２の影響が現れ、１０ｋＨｚ以上の場合には、計測リード線のインダクタンスが現れてしまうためである。
【００７８】
式（１）において、定数Ａ（λ）は、電解質膜２１の湿潤状態に応じて変化する。そこで、ここでは、内部抵抗Ｒ₁を計測することにより、内部抵抗Ｒ₁を、定数Ａ（λ）の変動を考慮した値とする。
【００７９】
また、反応抵抗Ｒ₂、ここでは交換電流密度ｉ₀をセル間で検出する交換電流密度計測器を備える。交換電流密度計測器としては、前述した高周波インピーダンス計測器３１と、電圧センサ３２、電流センサ３３を備える。高周波インピーダンスとともに、電流電圧特性をモニタすることにより交換電流密度ｉ₀を検出する。ここでは電流センサ３３をモニタし、所定の低電流密度領域で、数点のセル電圧Ｖｃを計測することにより、電流電圧特性を検出する。
【００８０】
式（５）において、反応定数ｋ₀は、触媒の劣化状態に応じて変化する。そこで、交換電流密度ｉ₀を計測することにより、反応抵抗Ｒ₂を反応定数ｋ₀の変動を考慮した値とする。
【００８１】
つまり、内部抵抗Ｒ₁と交換電流密度ｉ₀を計測することにより、セル温度Ｔｃの変化に加えて、Ａ（λ）、ｋ₀のセル状態に応じた変動を考慮した最大電流Ｉ_maxを求めることができる。
【００８２】
次に、停止・起動時の制御方法を図１２に示したフローチャートを用いて説明する。図１２（ａ）を停止時、図１２（ｂ）を起動時のフローチャートとする。
【００８３】
停止信号を検知したら図１２（ａ）の停止フローを開始する。このとき、取り出し電流Ｉおよびセル電圧Ｖｃの値は常時モニタしておく。ここでは、停止時に反応抵抗Ｒ₂を求めるための計測、つまり交換電流密度ｉ₀の計測を行う。
【００８４】
ステップＳ３１において、取り出し電流Ｉの低下を開始する。ステップＳ３２において、電流値２００ｍＡ／ｃｍ²となった時点で電圧値を記憶する。ステップＳ３３において、電流値５０ｍＡ／ｃｍ²となった時点で電圧値を記憶する。ステップＳ３４において、電流値２０ｍＡ／ｃｍ²となった時点で電圧値を記憶する。ステップＳ３５において、電流値５ｍＡ／ｃｍ²となった時点で電圧値を記憶する。ステップＳ３６において、高周波インピーダンスを計測する。ステップＳ３７において、セル温度Ｔｃを計測する。
【００８５】
ステップＳ３８において電流を停止する。ステップＳ３９において、ステップＳ３２〜Ｓ３７の検出結果を用いて、交換電流密度ｉ₀を近似的に算出する。後述するようなターフェルプロットを行うことにより交換電流密度ｉ₀を求める。さらに、交換電流密度ｉ₀から反応定数ｋ₀を算出し、記憶しておく。ステップＳ４０において、セルの乾燥を行う。ここでは、乾燥した、もしくは、低湿度の空気および水素ガスを所定時間、例えば１０分間流通させることにより、各電極１ａ、１ｃを乾燥させる。その後、ステップＳ４１において空気および水素ガスの供給を停止して燃料電池システムを停止する。
【００８６】
このように、停止制御においては、ステップＳ３６において高周波インピーダンスを計測すると共に、ステップＳ３２〜Ｓ３５において、低電流密度領域の電圧電流特性をモニタする。この検出結果を用いて、ステップＳ３９において、交換電流密度ｉ₀を求める。
【００８７】
例えば、温度７０℃で通常運転している状態から燃料電池システムを停止した場合の交換電流密度ｉ₀を算出する際の電流電圧特性を図１３に示す。
【００８８】
図１３（ａ）には、低電流領域の電圧電流特性を、（ｂ）には、電流の対数をＸ軸、電圧をＹ軸としてプロット（ターフェルプロット）したグラフを示す。破線を検出電圧値、実線を検出電圧値に内部抵抗Ｒ₁による電圧降下分を補正した値とする。ステップＳ３６で計測した高周波インピーダンスから、それぞれの電流密度における内部抵抗Ｒ₁による電圧降下を算出して補正する。なお、電流密度が２００ｍＡ／ｃｍ²から５ｍＡ／ｃｍ²に低下する際の温度は一定とし、ステップＳ３６においては、この温度における内部抵抗Ｒ₁を計測している。
【００８９】
このように、ターフェルプロットした図１３（ｂ）において、交換電流密度ｉ₀は切片から求めることができる。ただし、反応ガスが十分存在する状態で計測する。なお、拡散抵抗Ｚｗによる電圧降下量は低電流領域で小さく、無視できる。
【００９０】
ここでは電流密度２００ｍＡ／ｃｍ²までの数点によりターフェルプロットを行ったが、この限りではない。さらに低い電流領域、例えば１ｍＡ／ｃｍ²〜１０ｍＡ／ｃｍ²でプロットを行ってもよい。この場合には、電解質膜２１やガス拡散層２３の抵抗に相当するセルの内部抵抗Ｒ₁による電圧降下は無視できる。よって、内部抵抗Ｒ₁による電圧降下の補正を行う必要はなく、電圧電流特性をモニタする際に高周波インピーダンスを計測する（Ｓ３６）必要はない。なお、電流密度２００ｍＡ／ｃｍ²以上では拡散抵抗Ｚｗの影響がでてくる。そのため、図１３（ｂ）に示すように直線から電圧値が外れてくるので、交換電流密度ｉ₀の計測には好ましくない。
【００９１】
また、セル電圧Ｖｃをモニタしておき、例えば、０．７Ｖ、０．７５Ｖ、０．８Ｖのときの電流Ｉを計測し、ターフェルプロットして交換電流密度ｉ₀を算出してもよい。
【００９２】
このように、運転停止時に交換電流密度ｉ₀を検出した後、燃料電池システムを起動する際の制御方法を図１２（ｂ）のフローチャートを用いて説明する。
【００９３】
燃料電池システムの起動信号を検知したら本フローを開始する。ステップＳ５１〜Ｓ５４を、Ｓ１〜Ｓ４と同様とする。反応ガスの供給を開始したら、ステップＳ５５において、内部抵抗Ｒ₁の検出を行う。ここでは、１ｋＨｚの高周波インピーダンスを計測して内部抵抗Ｒ₁とする。
【００９４】
次に、ステップＳ５６において、最大電流Ｉ_maxを算出する。具体的には、ステップＳ３９で求めた反応定数ｋ₀と、起動時のセル温度Ｔｃから反応抵抗Ｒ₂を算出し、これと内部抵抗Ｒ₁とから最大電流Ｉ_maxを算出する。
【００９５】
ステップＳ５７〜Ｓ５９において、ステップＳ５〜Ｓ７と同様に、セル電圧Ｖｃが所定電圧Ｖ₁以上となるまで待機する。所定電圧Ｖ₁以上となったら、ステップＳ６０において電流の取り出しを開始する。このときの取り出し電流Ｉを、ステップＳ５６で算出した最大電流Ｉ_maxに設定する。これにより、電流取り出し開始時点から、燃料電池１の劣化を引き起こさない範囲内で、最大の電流を取り出すことができ、暖機時間を短縮することができる。なお、拡散抵抗Ｚｗを予め求めておき、これを考慮して最大電流Ｉ_maxを算出してもよい。
【００９６】
以下、ステップＳ６１〜Ｓ６６においては、ステップＳ９〜Ｓ１４と同様に、セル電圧Ｖｃが所定電圧範囲Ｖ₂以内に維持されるように取り出し電流Ｉを設定することにより、より多くの反応を生じて燃料電池１の暖機を促進する。または、ステップＳ６１以降を、ステップＳ３１〜Ｓ３４と同様とし、セル温度Ｔｃに応じて設定した最大電流Ｉ_maxを取り出し電流Ｉに設定してもよい。ただし、この場合には、ステップＳ５６において最大電流Ｉ_maxを求めた際の内部抵抗Ｒ₁、Ｒ₂に応じた定数ｋ₀、Ａ（λ）を用いて、セル温度Ｔｃに応じた内部抵抗Ｒ₁、反応抵抗Ｒ₂を再度算出し、最大電流Ｉ_maxを設定する。
【００９７】
このように、停止時に計測した交換電流密度ｉ₀から反応定数ｋ₀を算出し、起動時に内部抵抗Ｒ₁を計測して最大電流Ｉ_maxを求める。反応定数ｋ₀は、主に触媒の劣化に応じて変化するため、停止時と起動時とでその値はほぼ一致する。おこで、停止時に測定した反応定数ｋ₀を用いて、起動時のセル温度Ｔｃに応じた交流電流密度ｉ₀、ひいては、反応抵抗Ｒ₂を求める。なお、ここでは停止時に交換電流密度ｉ₀の計測を行ったが、起動時に行うこともできる。
【００９８】
一方、内部抵抗Ｒ₁は、セル温度Ｔｃに応じて変化するため、停止時と起動時でその値は一致しない。このため、ここでは、起動時の計測値を用いている。または、停止時に内部抵抗Ｒ₁を計測し、これから電解質膜２１の湿度に応じて変化する定数Ａ（λ）を求め、この定数Ａ（λ）を用いて起動時のセル温度Ｔｃに応じた内部抵抗Ｒ１を求めても良い。ここで、定数Ａ（λ）は、停止時と起動時とで、電解質膜２１の湿潤度に差がない場合には、一致する。
【００９９】
また、ステップＳ３９において、反応定数ｋ₀を求めているがこの限りではない。例えば、各計測を停止時に行って、反応定数ｋ₀は、起動時の交換電流密度ｉ₀、ひいては反応抵抗Ｒ₂を算出する際に計算しても良い。
【０１００】
ここで、内部抵抗Ｒ₁および反応抵抗Ｒ₂を求めるための計測を行うタイミングの例を、図１４に示す。
【０１０１】
図１４（Ａ）には、上記本実施形態におけるタイミングを示す。つまり、停止時に、交換電流密度ｉ₀、ひいては反応抵抗Ｒ₂を求めるための計測を行う。起動時には、内部抵抗Ｒ₁を求めるための計測を行う。これにより、セル温度Ｔｃおよびセル状態、ここでは電解質膜２１の湿潤度、触媒の劣化の程度を考慮して、適切な最大電流Ｉ_maxを設定することができる。
【０１０２】
図１４（Ｂ）には、起動時に、内部抵抗Ｒ₁と反応抵抗Ｒ₂を求めるための計測を行う。ここでは、反応ガスの供給開始後、電流の取り出しを開始するまでに計測を行う。これにより、停止動作を速やかに終了できる。このとき、反応抵抗Ｒ₂を求める計測においては、セル温度Ｔｃについての補正を行う必要がないので、ターフェルプロットから求めた交換電流密度ｉ₀から反応定数ｋ₀を求めずに、直接、反応抵抗Ｒ₂を求めることができる。
【０１０３】
図１４（Ｃ）には、通常運転中に反応抵抗Ｒ₂を求めるための計測を行う場合を示す。なお、内部抵抗Ｒ₁の計測は（Ａ）と同様に起動時に行う。これにより、停止操作を速やかに行うことができる。なお、反応定数ｋ₀は、触媒の劣化状態により変化する。触媒の劣化状態は経年的には変化するが、通常運転中と運転停止時ではさほど変化しないので、起動時の最大電流Ｉ_maxの算出にさほど影響を与えることはない。通常運転中の計測値から反応定数ｋ₀を求め、これから、起動時のセル温度Ｔｃに応じた反応抵抗Ｒ₂を求める。
【０１０４】
図１４（Ｄ）には、停止時の乾燥操作後に、内部抵抗Ｒ₁を計測する場合を示す。なお、反応抵抗Ｒ₂を求めるための計測は、（Ａ）と同様に停止時とする。乾燥後と起動直後の電解質膜２１の湿潤度が大きく変化しない場合には、乾燥後に計測した内部抵抗Ｒ₁から定数Ａ（λ）を求め、起動時にセル温度Ｔｃに応じた内部抵抗Ｒ₁を算出する。これにより、起動時に、２〜３秒を要する高周波インピーダンスの計測を行う必要がなくなり、起動時間を短縮することができる。
【０１０５】
ここで、破線矢印で示すように、（Ｄ）において、さらに外気温度を計測する計測器を備え、運転停止後、セル温度Ｔｃが外気まで下がって、燃料電池１に残留する水分の凍結が開始すると予測される所定時間後に内部抵抗Ｒ₁を算出してもよい。これにより、起動時の電解質膜２１の湿潤状態により近い定数Ａ（λ）を計測することができ、内部抵抗Ｒ₁を正確に算出することができる。
【０１０６】
図１４（Ｅ）には、燃料電池システム停止後に温度が所定温度、例えば氷点まで低下した時に、内部抵抗Ｒ₁と反応抵抗Ｒ₂を求めるための計測を行う場合を示す。この場合には、計測するのはその後氷点下から起動するときに限られるので、氷点以上の環境から起動するときには計測する必要がなくなり、計測のための消費電力を抑えることができる。
【０１０７】
次に、本実施形態の効果を説明する。以下、第２の実施形態とは異なる効果のみを説明する。
【０１０８】
一つあるいは複数のセルの内部抵抗Ｒ₁を求めるための計測を行う内部抵抗計測手段と、一つあるいは複数のセルの反応抵抗Ｒ₂を求めるための計測を行う反応抵抗計測手段と、を備える。少なくとも氷点下からの起動直後の最大電流Ｉ_maxを、内部抵抗計測手段および反応抵抗計測手段を用いて求める。定数Ａ（λ）については電解質膜２１の湿潤状態により、反応定数ｋ₀については触媒の劣化状態により変動する。そこで、内部抵抗Ｒ₁と反応抵抗Ｒ₂とを計測により求めることで、セル状態の変化に伴う内部抵抗Ｒ₁および反応抵抗Ｒ₂の変動を考慮して最大電流Ｉ_maxを求めることができる。よって、より正確に取り出し電流Ｉを、劣化を避ける範囲で略最大に設定することができる。
【０１０９】
ここでは内部抵抗計測手段として、高周波電流を印加する発振器と、電圧応答を検知する検出器から構成される高周波インピーダンス計測器３１を用いる。これにより、内部抵抗Ｒ₁を計測することができる。また、反応抵抗計測手段として、燃料電池１の直流電流を計測する電流センサ３３と、セル電圧Ｖｃを計測する電圧センサ３２と、高周波インピーダンス計測器３１を用いる。燃料ガスと酸化剤ガスが燃料電池内１に存在している時に、反応抵抗Ｒ₂を求めるための計測を行うことができる。
【０１１０】
また、停止時に、セル内を乾燥させるための乾燥操作を行う。停止時に、反応抵抗Ｒ₂を求める計測を行い、停止時の乾燥操作後に内部抵抗Ｒ₁を求めるための計測を行う。氷点下で発電を開始する前、ここでは停止運転中にセル内の水を取り除いておかなければならず、この操作によって電解質膜２１や触媒層２２の状態が変化し、内部抵抗Ｒ₁が変化することがある。よって乾燥後に内部抵抗Ｒ₁を求めるための計測をすることにより、より正確に最大電流Ｉ_maxを求めることができる。
【０１１１】
または、外気温度を計測する外気温度計測手段を備え、運転停止後、燃料電池１の温度が外気温度まで低下し、セル内に残っていたガス中の水分が凝結する所定時間後に、内部抵抗Ｒ₁を求めるための計測を行う。運転停止後、しばらく放置しておくと、燃料電池１の温度が下がるにつれて、セル内に残留したガスに含まれる水分が凝結する。これにより、触媒層２２に水分が付着し、電解質膜２１に吸収されて、定数Ａ（λ）が変動する。そこで、スタック運転終了後に所定時間が経過した後に計測することにより、計測時と起動時の電解質膜２１の湿潤状態を同じとすることができ、より正確に最大電流Ｉ_maxを求めることができる。
【０１１２】
または、一つあるいは複数のセルの温度を計測する温度センサ９を備える。運転停止後に、次に起動するまでに温度が低下して所定温度となったときに、内部抵抗Ｒ₁を求めるための計測と反応抵抗Ｒ₂を求めるための計測とを行う。ここでは所定温度を凍結が生じる可能性がある温度とする。これにより、計測を行うのは、その後氷点下からの起動を行う場合に限ることができる。つまり、氷点以上の環境から起動させるときには計測の必要がなく、消費電力を抑えることができる。
【０１１３】
または、起動時に、燃料ガスおよび酸化剤ガスの燃料電池１への供給を開始した後、かつ、電流の取り出しを開始する前に、内部抵抗Ｒ₁を求めるための計測と反応抵抗Ｒ₂を求めるための計測を行う。このように、起動直前に計測を行うことにより正確な判定ができ、複雑な温度換算を行う必要がない。
【０１１４】
次に、第４の実施形態について説明する。燃料電池システムの概略構成を図１５に示す。以下、第３の実施形態と異なる部分を中心に説明する。
【０１１５】
セルの内部抵抗Ｒ₁を計測する内部抵抗計測器としては、高周波電流を印加する発振器と、電圧応答を検出する検出器からなる高周波インピーダンス計測器３１ａを備える。ここでは、周波数１ｋＨｚ電流振幅７ｍＡの高周波インピーダンス計測器３１ａを備える。
【０１１６】
また、反応抵抗Ｒ₂を求めるのに用いる交換電流密度ｉ₀を計測するための交換電流密度計測器を備える。ここでは、第３と同様に、前述した高周波インピーダンス計測器３１ａと、電圧センサ３２、電流センサ３３を備える。また、低周波電流を印加する発振器と電圧応答を検出する検出器とからなる低周波インピーダンス計測器３１ｂを備える。なお、発振器の周波数が広領域について可変の場合には、高周波インピーダンス計測器３１ａと低周波インピーダンス計測器３１ｂとを同一としてもよい。
【０１１７】
ここでは、高周波インピーダンス計測器３１ａの計測から内部抵抗Ｒ₁を計測する。また、高周波インピーダンス計測器３１ａ、低周波インピーダンス計測器３１ｂ、および、電圧センサ３２、電流センサ３３によりモニタした電流電圧特性から、交換電流密度ｉ₀を計測し、反応抵抗Ｒ₂を算出する。
【０１１８】
高周波電流と低周波電流の振幅と位相差を検出し、横軸をインピーダンスの実部に、縦軸をインピーダンスの虚部に−１をかけた値でプロットする（コールコールプロット）と、図１６のような半円になる。この半円の直径に相当する抵抗値に膜のガスクロスオーバによる過電圧に対応する抵抗値を加えたものが反応抵抗Ｒ₂となる。この反応抵抗Ｒ₂から交換電流密度ｉ₀を求める。この計測方法は、ガスは供給しなければならないが、直流電流は微小値あるいは流さなくてもよい。なお、低周波電流の周波数は０．１Ｈｚから１０Ｈｚの間で数点であることが好ましい。また、高周波電流に周波数を１ｋＨｚとする。
【０１１９】
次に、図１７に示したフローチャートを用いて起動時の制御方法を説明する。なお、停止時には計測を行わず、通常の制御を行う。燃料電池システム起動の信号を検知したら、本フローを開始する。
【０１２０】
ステップＳ７１〜Ｓ７７をステップＳ１〜Ｓ７と同様とする。セル電圧Ｖｃが、セルから電力を取り出すことができる所定電圧Ｖ₁以上となったら、セル電圧Ｖｃを記録してステップＳ７８に進む。可逆電位Er（ここでは１．２Ｖとする）との差を過電圧Ｖ_Bとする。このＶ_B は膜のクロスオーバにより低下する過電圧である。
【０１２１】
ステップＳ７８において、低電流密度の電流を取り出す。ここでは取り出し電流Ｉの電流密度ｉを５ｍＡ／ｃｍ²に設定する。ステップＳ７９において、低周波インピーダンスの計測を行う。ここでは、１Ｈｚ〜１０Ｈｚの周波数を混合したパルス電流をセルに供給し、電圧応答波形を高速フーリエ変換解析（FFT）を行う。例えば、周波数を変えた２０点の低周波インピーダンスを計測する。これにより、Ｒ₁＋Ｒ₂値による電圧降下分を算出する。次に、ステップＳ８０において、高周波インピーダンス、例えば１ｋＨｚの高周波インピーダンスを計測する。これによりＲ₁値による電圧降下分を計算する。これらから、反応抵抗Ｒ₂による電圧降下分を求め、これを過電圧Ｖ_Aとする。また、高周波インピーダンスを内部抵抗Ｒ₁とする。
【０１２２】
過電圧Ｖ_A、Ｖ_Bの和を触媒活性化過電圧ηとして、式（３）、（４）に従って交換電流密度ｉ₀を算出する。
【０１２３】
なお精度は低下するが、ステップＳ８１として、低周波インピーダンスの計測を行う代わりに低電流密度（ここでは、５ｍＡ／ｃｍ²）に設定した場合におけるセル電圧Ｖｃを計測して可逆電位Er（１．２Ｖ）との差を触媒活性化過電圧ηとしても構わない。
【０１２４】
さらに無負荷電圧Ｖｃと可逆電位Er（１．２Ｖ）との差を触媒活性化過電圧η、このときの電流を膜のガスクロスオーバ量から仮定した値としても構わない。
【０１２５】
さらに、ステップＳ８２において、交換電流密度ｉ₀、内部抵抗Ｒ₁から最大電流Ｉ_maxを決定する。ステップＳ８３において、取り出し電流Ｉを最大電流Ｉ_maxとして、電流の取り出しを開始する。以下、第３の実施形態と同様とする。
【０１２６】
なお、ここでは、内部抵抗Ｒ₁を求めるための計測と、反応抵抗Ｒ₂を求めるための計測とを、起動時に行ったがこの限りではない。例えば、図１８に示すようなタイミングで行うこともできる。
【０１２７】
図１８（Ａ）には、停止時に反応抵抗Ｒ₂を求めるための計測を行う。起動時に内部抵抗Ｒ₁を計測する。図１８（Ｂ）においては、停止時に、高周波インピーダンス、低周波インピーダンスを計測する。高周波インピーダンスより内部抵抗Ｒ₁を求め、定数Ａ（λ）を求める。また、起動時に、無負荷電圧を計測して、交換電流密度ｉ₀を求める。
【０１２８】
さらに、図１８（Ｃ）においては、停止時に、低周波インピーダンスを計測する。また、セル温度Ｔｃが外気温度程度になってから、さらに氷点下になるまでの間に、高周波インピーダンスを計測する。起動時に無負荷時電圧を計測する。図１８（ｄ）では、本実施形態と同様に、起動時の反応ガス供給開始から電流取り出し開始までの間に、高周波インピーダンス、低周波インピーダンス、無負荷電圧を計測して最大電流Ｉ_maxを設定する。
【０１２９】
次に本実施形態の効果について説明する。以下、第３の実施形態の効果とは異なる効果のみを説明する。
【０１３０】
反応抵抗計測手段として、燃料電池１の直流電流を計測する電流センサ３３と、セル電圧Ｖｃを計測する電圧センサ３２と、高周波電流と低周波電流を印加する発振器と電圧応答を検知する検出器とから構成される交流インピーダンス計測器３１ａ、３１ｂと、を用いる。燃料ガスと酸素剤ガスが燃料電池１内に存在している時に、反応抵抗Ｒ₂を求めるための計測を行う。これにより、直流電流は微小値あるいは流さなくても計測ができる。
【０１３１】
次に、第５の実施形態について説明する。燃料電池システムの構成を図１９に示す。以下、第３の実施形態と異なる部分を中心に説明する。
【０１３２】
ここでは燃料電池１として、複数の単位セルを積層することにより構成したスタックを用いる。内部抵抗Ｒ₁の計測と反応抵抗Ｒ₂を求めるための計測を、スタックの中央部と、端部の二つのセルについて行う。つまり、スタック中央部近傍のセルに、高周波インピーダンス計測器３１Ａ、電圧センサ３２Ａ、温度センサ９Ａを備える。また、スタック端部近傍のセルに、高周波インピーダンス３１Ｂ、電圧センサ３２Ｂ、温度センサ９Ｂを備える。最大電流Ｉ_maxを算出する際には、このスタック中央部と端部のセルのうち、性能の劣る方の値を用いて行う。
以下、第３の実施形態と同様とする。
【０１３３】
単位セルの場合、セル電圧Ｖｃが０Ｖ以下になると電力が取り出せなくなるが、スタックの場合には、例えば１０セルのうち６セルは ＋０．５Ｖ、残り４セルは−０．３Ｖとなった場合、スタック電圧は＋１．８Ｖとなり電力を取り出すことができる。スタック内の一部のセルにおいて生成水の凍結が生じた場合にも、他のセルからの伝熱により凍結したセルが解凍されてセル内の電流電圧特性が回復した時点で、通常運転に移行することができる。
【０１３４】
しかしこの場合、−０．２０７Ｖ以下になっているセルがあると、このセルで触媒層２２の劣化が進んでしまう。よって、スタックを用いる場合には、外部に取り出せるスタック電圧だけでなく 単位セル毎の電圧をモニタして取り出し電流Ｉを調節する必要がある。例えば、スタック端部のセルを含む複数箇所にセル電圧を計測する電圧センサを備える。電圧を計測するセルは、少なくともスタック端部のエンドプレート２５近傍の、生成水を生じ易いセルとするのが好ましい。性能が最も低いセル電圧を基準にして取り出し電流値Ｉを決定する。また、性能が最も低いセル温度Ｔｃに基づいて通常運転に移行するかどうかを判断する。
【０１３５】
なお、ここでは、第３実施形態についてスタックを採用した場合について説明したが、この限りではない。第２〜４実施形態のいずれの燃料電池１について、スタックを用いてもよい。なお、第１の実施形態についてもスタックを用いることはできるが、上述したように、いずれのセルについても許容電圧範囲以下とならないように制御する必要がある。
【０１３６】
次に、本実施形態の効果について説明する。以下、第３の実施形態と異なる効果のみを説明する。
【０１３７】
燃料電池１を、複数の前記セルを積層するスタック構造により構成し、前記スタック端部のセルを含むスタックの複数の箇所のセルのうち、性能が最も悪いセルに基づいて、取り出し電流または電圧を設定する。このように、最も起動時の温度が上昇し難いスタック端部のセルを含む複数のセルのうち最も性能の悪いセルを基準として取り出し電流Ｉを設定するので、取り出し電流Ｉを大きくし過ぎることにより燃料電池１が劣化するのを確実に防ぐことができる。
【０１３８】
なお、上記実施の形態においては、取り出し電流について設定したが、取り出される電力が電圧によって調整される場合には、取り出し電圧について同様に設定することができる。
【０１３９】
このように、本発明は上記実施の形態に限定されるわけではなく、特許請求の範囲に記載の技術思想の範囲内で様々な変更が為し得ることは言うまでもない。
【図面の簡単な説明】
【図１】第１の実施形態に用いる燃料電池システムの概略構成図である。
【図２】第１の実施形態に用いる燃料電池の概略構成図である。
【図３】第１の実施形態に用いる起動制御のフローチャートである。
【図４】取り出し電流を変化させた場合の電圧電流特性である。
【図５】第１の実施形態に用いる起動制御の別の例を示すフローチャートである。
【図６】通常運転時への移行可能性を示す電圧電流特性である。
【図７】第１の実施形態における電力制御とそれによる温度変化を示す図である。
【図８】第２の実施形態におけるセルの等価回路図である。
【図９】第２の実施形態に用いる起動制御のフローチャートである。
【図１０】セル温度が変化した場合の電圧電流特性である。
【図１１】第３の実施形態に用いる燃料電池システムの概略構成図である。
【図１２】第３の実施形態における停止・起動制御のフローチャートである。
【図１３】低電流領域における電圧電流特性とターフェルプロットを示す図である。
【図１４】第３の実施形態における計測のタイミングの例を示す図である。
【図１５】第４の実施形態に用いる燃料電池システムの概略構成図である。
【図１６】反応抵抗検出方法におけるコールコールプロットを示す図である。
【図１７】第４の実施形態における起動制御のフローチャートである。
【図１８】第４の実施形態における計測のタイミングの例を示す図である。
【図１９】第５の実施形態に用いる燃料電池システムの概略構成図である。
【符号の説明】
１ 燃料電池
６ 定電流制御回路（電気回路）
９ 温度センサ（温度計測手段、セル温度計測手段）
１０ 電圧センサ（電圧計測手段）
３１ 高周波インピーダンス計測器（高周波インピーダンス計測手段）
３１ａ 高周波インピーダンス計測器（交流インピーダンス計測手段）
３１ｂ 低周波インピーダンス計測器（交流インピーダンス計測手段）
３２ 電圧センサ（電圧計測手段）
３３ 電流センサ（電流検出手段）
Ｓ１１、Ｓ３３ 電力設定手段
Ｓ２６、Ｓ５６、Ｓ８２ 最大電力検出手段[0001]
[Industrial application fields]
The present invention relates to a fuel cell system. In particular, the present invention relates to a fuel cell system having a structure for quickly starting in a low temperature environment.
[0002]
[Prior art]
A conventional fuel cell system using a polymer electrolyte fuel cell that operates while adding water to a fuel gas is known. For example, the control unit measures the open circuit voltage of the battery body and determines whether or not the measured value is equal to or higher than a preset open circuit voltage. If it is equal to or higher than the set open circuit voltage, a load for detection is applied to take out the current, and the voltage of the battery body is measured with a voltmeter. And it is judged whether a measured value is more than a setting load voltage. Until the measured value exceeds the set load voltage, only the fuel gas is supplied to the anode side channel. When the measured value is equal to or higher than the set load voltage, supply of the gas-liquid mixture to the anode side channel is started, supply of power to the external load is started, and normal operation is started. Thus, there is a known start-up method that shortens the start-up time while preventing damage to the anode layer by quickly eliminating even if the anode-side channel is clogged with water during start-up (for example, (See Patent Document 1).
[0003]
[Patent Document 1]
JP-A-8-315843
[0004]
[Problems to be solved by the invention]
However, in such a solid polymer electrolyte fuel cell starting method, when current is taken out when starting the stack at a low temperature below freezing, water generated by power generation starts to freeze in the channel of the stack, and the reaction gas There is a problem that hinders reaching the electrocatalytic reaction zone. In other words, if the ice blocks the supply path to the catalyst when starting the fuel cell in a low temperature environment, the electrochemical reaction will not proceed even if fuel gas is supplied, and power generation will not be possible. Cannot start. In other words, once the channel is blocked with water, there is a problem that it is difficult to eliminate it.
[0005]
Therefore, an object of the present invention is to provide a fuel cell system capable of suppressing freezing of generated water at the time of startup by shortening the warm-up time.
[0006]
[Means for solving problems]
  A fuel cell including one or a plurality of cells that generate electric power using a fuel gas and an oxidant gas, and an electric circuit that adjusts at least a current or a voltage extracted from the fuel cell. Also,From the internal resistance and reaction resistance of one or more cells,When starting from below freezing point, the voltage of the cellCorrosion potential at which oxidation reaction of carbon proceedsIn a range not less thanFind the maximum extraction current.
[0007]
[Action and effect]
  From the internal resistance and reaction resistance of one or more cells,When starting from below freezing, the cell voltageCorrosion potential at which oxidation reaction of carbon proceedsBy setting the extraction current to the maximum within a range not lower than, the power generation reaction is promoted regardless of the required power. As a result, the self-heating of the fuel cell can be promoted and the warm-up time can be shortened, so that the generated water at the start-up can be suppressed from freezing.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
A first embodiment will be described. A schematic configuration of the fuel cell system will be described with reference to FIG.
[0009]
A fuel cell 1 is provided that generates power by supplying hydrogen as fuel gas to the anode 1a and air as oxidant gas to the cathode 1c. Further, an anode system 2 for supplying hydrogen to the anode 1a is provided, and the anode system 2 is provided with a hydrogen control device 3 for adjusting the hydrogen gas flow rate and pressure. Moreover, the cathode system 4 which supplies air to the cathode 1c is provided, and the cathode system 4 is provided with the air control apparatus 5 which adjusts an air flow rate and a pressure. In a controller 11 or other control means described later, hydrogen and air necessary for generating a desired electromotive force are supplied to the fuel cell 1. The exhausted hydrogen and the exhausted air after power generation are combusted in a not-shown combustor or the like and then discharged outside the system. Alternatively, it may be reused for power generation reaction or used for warming up.
[0010]
Further, the fuel cell 1 is electrically connected to the load 8 via the constant current control circuit 6 and the inverter 7. As the constant current control circuit 6, a circuit that can extract current (actually supply current) so that the fuel cell 1 has a negative voltage is used. A controller 11 is provided as means for controlling the constant current control circuit 6. Moreover, the temperature sensor 9 which measures the temperature of the fuel cell 1 and the voltage sensor 10 which measures a voltage are provided. As the temperature sensor 9, for example, a thermocouple is used in which a groove is formed in the separator 24 constituting the cell as described later and the gas diffusion layer 23 is touched in the groove.
[0011]
In the controller 11, the current extracted from the fuel cell 1 is adjusted by controlling the constant current control circuit 6 according to the temperature and voltage of the fuel cell 1. Further, the current taken out from the fuel cell 1 is converted into alternating current in the inverter 7 and supplied to the load 8. Instead of the constant current control circuit 6, a constant voltage control circuit may be provided to instruct an extraction voltage. Further, as will be described later, since there is a possibility that electric power higher than the required electric power may be taken out from the fuel cell 1 at the time of low temperature startup, a battery or the like for storing this electric power may be provided. Moreover, you may provide the heater which converts this electric power into heat and warms a stack.
[0012]
Next, the configuration of the fuel cell 1 will be described with reference to FIG. In the present embodiment, the fuel cell 1 is shown as having one cell, but may be constituted by a stack in which a plurality of cells are stacked. The voltage sensor 10 measures the cell voltage Vc of the cells constituting the fuel cell 1.
[0013]
The anode 1a and the cathode 1c are composed of a catalyst layer 22 and a gas diffusion layer 23, respectively. The catalyst layer 22 is configured by supporting a platinum catalyst on carbon using an electrolyte (Nafion) solution as a binder. A membrane electrode assembly (hereinafter referred to as MEA) is formed by sandwiching the electrolyte membrane 21 with the catalyst layer 22 and further sandwiching with the gas diffusion layer 23 from the outside. A seal 26 for preventing gas leakage is formed along the outer periphery of the MEA laminated surface.
[0014]
A separator 24 is arranged further outside the MEA in the stacking direction. A hydrogen gas flow path 27 is configured by a groove formed in a surface facing the MEA of the separator 24a on the anode 1A side. The air gas flow path 28 is constituted by a groove formed on the surface facing the MEA of the separator 24c on the cathode 1c side. Further, a groove is provided on the contact surface of the separator 24 with the gas diffusion layer 23, and the temperature sensor 9 is provided as described above.
[0015]
An end plate 25 is provided on the outer side of the separator 24 in the stacking direction. The fuel cell 1 is assembled by providing bolts 29 penetrating in the stacking direction and applying a predetermined pressure in the stacking direction from the outside of the end plate 25.
[0016]
During power generation, hydrogen supplied from the anode system 2 is distributed to the hydrogen gas flow path 27 and supplied to the anode 1a. Hydrogen diffuses to the power generation surface in the gas diffusion layer 23a and reaches the catalyst layer 22a. Here, catalytic reaction using platinum (H₂→ 2H⁺+ 2e^-) Is generated. Protons generated by this reaction move through the electrolyte membrane 21 from the anode 1a toward the cathode 1c. The air supplied from the cathode system 4 is distributed to the air gas flow path 28 and supplied to the cathode 1c. Air diffuses to the power generation surface in the gas diffusion layer 23c and reaches the catalyst layer 22c. Here, catalytic reaction using platinum (1 / 2O₂+ 2H⁺+ 2e^-→ H₂O). As described above, in the cathode 1c, protons that have moved through the electrolyte membrane 21 react with oxygen in the air to generate water. When such a fuel cell 1 is started at a low temperature below freezing, water generated by power generation may freeze in the fuel cell 1 and hinder the reaction gas from reaching the catalyst layer 22.
[0017]
Therefore, at the time of low-temperature startup, the power generation of the fuel cell 1 is prevented from being stopped by raising the temperature of the cell above the freezing point before the performance deteriorates due to the freezing of the generated water. Here, regardless of the required load, the maximum current (hereinafter referred to as the maximum current I) is within a range where deterioration of the fuel cell 1 can be avoided._max) To promote self-heating of the fuel cell 1 and shorten the warm-up time. Hereinafter, the control method at the time of starting will be described using the flowchart shown in FIG.
[0018]
When a signal instructing activation of the fuel cell system is detected, this control is started. At the time of stopping, a drying operation is performed in advance before the cell temperature Tc reaches the freezing point, so that water does not remain in the piping, the hydrogen gas passage 27, and the air gas passage 28 to be frozen. .
[0019]
In step S1, the temperature of the fuel cell 1 is measured. Here, the cell temperature Tc is measured using the temperature sensor 9. Next, in step S2, it is determined whether or not the measured cell temperature Tc is equal to or higher than the freezing point. That is, it is determined whether or not there is a possibility of freezing in the fuel cell 1. The determination of the possibility of freezing may be performed according to the outside air temperature. In step S2, if the temperature is above the freezing point, the process proceeds to step S3 to start up in the normal startup mode. Here, the normal startup mode is omitted.
[0020]
On the other hand, in the case of below freezing point, the cold start mode is started. Hereinafter, the low temperature startup mode will be described.
[0021]
In step S4, supply of hydrogen to the anode 1a and air to the cathode 1c is started. In step S5, the cell voltage Vc is measured. Next, in step S5, the cell voltage Vc is set to the predetermined voltage V.₁It is determined whether this is the case. Here, the predetermined voltage V₁Is a lower limit value of a voltage indicating a cell state from which power can be extracted, or a value higher than that. For example, the predetermined voltage V₁Is 0.9V. In the case of the stack structure, the stack voltage may be measured and it may be determined whether or not power can be taken out. Cell voltage Vc is a predetermined voltage V₁If not, the process waits for a predetermined time in step S7, returns to step S5 again, and measures the cell voltage Vc. Cell voltage Vc is a predetermined voltage V₁Is repeated until a predetermined voltage V is reached.₁If it becomes above, it will progress to step S8.
[0022]
In step S8, extraction of current from the fuel cell 1 is started. The current to be extracted at this time is an initial value of the extraction current I, and is set in advance with respect to the temperature. Alternatively, a fixed value such as 15A may be used.
[0023]
In step S9, the cell voltage Vc is measured again. In step S10, the measured cell voltage Vc is a predetermined voltage range V.₂It is determined whether or not. Here, the predetermined voltage range V₂Is a range that includes a lower limit value of the voltage that can avoid carbon deterioration of the catalyst layer 22 or a range that is higher than the lower limit value. The cell voltage Vc is within this predetermined voltage range V₂In this case, the maximum current is extracted from the cell while avoiding carbon deterioration. That is, the cell voltage Vc is within a predetermined voltage range V₂In this case, the extraction current I is the maximum current I within a range where deterioration of the fuel cell 1 can be avoided._maxIs set to
[0024]
Here, the predetermined voltage range V₂Therefore, the extraction current I at this time is a value within a current value range including the upper limit of the current that can be extracted while avoiding deterioration. Here, the predetermined voltage range V₂Is set to 0.1V to 0.2V. Predetermined voltage range V₂A predetermined voltage value, for example, a predetermined voltage range V₂Lower limit V_2iIn this case, the extraction current I is the upper limit value of the current that can be extracted while avoiding deterioration.
[0025]
The cell voltage Vc is within a predetermined voltage range V₂If not, the process proceeds to step S11. In step S11, the extraction current I is reset according to the cell voltage value Vc. The cell voltage Vc is within a predetermined voltage range V₂If it is larger, the extraction current I is set to be large within the range allowed by the system. Thereby, since a power generation reaction increases, the self-heating of the fuel cell 1 can be promoted. On the other hand, the predetermined voltage range V₂If it is lower, the extraction current I is set small. As a result, the cell voltage Vc falls within the predetermined voltage range V₂Therefore, the carbon deterioration of the catalyst layer 22 can be avoided. By controlling in this way, the extraction current I is the maximum current I within the current range in which deterioration can be avoided._maxTherefore, the warm-up time can be shortened while preventing the fuel cell 1 from deteriorating.
[0026]
When the extraction current I is set in step S11, the process returns to step S9 and the cell voltage Vc is measured again. The cell voltage Vc is within a predetermined voltage range V₂Repeat until a predetermined voltage range V₂If it becomes inside, it will progress to step S12. The cell voltage Vc is within a predetermined voltage range V₂When it is detected that the current is within, it is preferable to maintain the current I at that time until the next current I is set or to perform general control such as PID.
[0027]
In step S12, the cell temperature Tc is measured. In step S13, it is determined whether or not the cell temperature Tc is equal to or higher than the freezing point. That is, it is determined whether there is a possibility of refreezing of the generated water. Although the temperature at which the ice in the catalyst layer 22 thaws is considered to be approximately 0 ° C., it is necessary to consider the temperature at which the possibility of freezing is determined based on the location to be measured. For example, in MEA, the temperature generally rises before the separator 24. In consideration of this, when the temperature of the separator 24 is measured, a control method of shifting to normal operation when the temperature of the separator 24 is lower than 0 ° C., for example, −0.5 ° C. is examined. Is preferred.
[0028]
If it is determined in step S13 that there is a possibility of refreezing, after waiting for a predetermined time in step S14, the process returns to step S9, and the extraction current I is changed to the maximum current I_maxContinue the control set to. On the other hand, if it is determined in step S13 that there is no possibility of refreezing, the start control is terminated and the operation mode is shifted to the normal operation mode.
[0029]
As described above, when starting from a low temperature environment, the cell voltage Vc is within the predetermined voltage range V.₂The power generation reaction is performed by taking out the maximum current while controlling to be inside. Thereby, at the time of low temperature startup, the extraction current can be set to a substantially maximum within a range where deterioration of the fuel cell 1 is avoided, so that the temperature of the fuel cell 1 can be quickly raised. As a result, it is possible to increase the frequency at which the cell can be warmed up to or above the freezing point before the generated water is frozen and the performance is reduced, and power generation can be started smoothly at low temperature start.
[0030]
Here, the predetermined voltage range V₂Is 0.1 to 0.2 V, but is not limited thereto. Predetermined voltage range V₂Is preferably −0.207 V or more in order to prevent carbon deterioration, and is set to be high so as to be within a range of 0.1 to 0.2 V for safety.
[0031]
When the cell voltage Vc falls below −0.207 V during power generation, the carbon oxidation reaction (C + 2H) occurs at the anode 1a.₂O → CO₂+ 4H⁺+ 4e^-) Occurs. As a result, the carbon supporting the catalyst disappears, and the three-phase interface (consisting of carbon (electron passage), polymer electrolyte membrane (proton passage), and gas (hydrogen gas passage)) composed of the catalyst portion) Disappears. As a result, the number of catalyst sites that cause the power generation reaction in the anode 1a is reduced, and as a result, the performance of the fuel cell 1 is permanently lowered. The corrosion potential at which the oxidation reaction of carbon proceeds is described in, for example, K. Kinoshita, Caarbon, john & Wiley (1998) p.316. Further, when the cell voltage Vc is −0.207 V or less, the cathode electrode 1c is corroded by an oxidation reaction of carbon, and there is a possibility that the sintering between the catalysts becomes remarkable.
[0032]
FIG. 4 shows the deterioration characteristics of the cell according to the cell voltage Vc. FIG. 4 shows deterioration characteristics when the extraction current I is controlled so that the cell voltage Vc becomes 0.3V, 0V, and −0.3V. Here, the current is taken out for about 30 seconds so that the unit cells maintain the respective voltages, and then the current-voltage characteristics of the unit cells are examined. As shown in FIG. 4, when the cell voltage Vc is maintained at -0.3V, the deterioration of the cell is confirmed. Predetermined voltage range V₂Is set higher than the voltage (−0.207 V) at which the deterioration of the carbon proceeds, and the cell voltage Vc is set to a predetermined voltage range V₂By suppressing the extraction current I so as to be within the range, deterioration of the cell can be prevented.
[0033]
Furthermore, here, in step S13, when the cell temperature Tc becomes higher than the freezing point, the operation shifts to the normal operation, but this is not restrictive. For example, as in the flow shown in FIG. 5, a voltage / current region that can be shifted to the normal operation as shown in FIG. 6 is set in advance, and it can be determined that the normal operation is shifted to the voltage / current region.
[0034]
That is, in step S12-2, the voltage / current value of the cell is confirmed. The current value I is the extraction current I set in the constant current control circuit 6. Alternatively, a current sensor that measures the extraction current I may be provided. The cell voltage Vc may be the cell voltage Vc measured in step S9 or may be measured again by the voltage sensor 10. In step S13-2, it is determined whether or not the voltage / current region can be shifted to normal operation. That is, a map as shown in FIG. 6 is stored in advance, and it is determined whether or not the current I and the voltage Vc confirmed in step S12-2 apply to the hatched portion in FIG. If determined to be the case, normal operation is started.
[0035]
Next, FIG. 7 shows temporal changes in current, voltage, and cell temperature when the fuel cell system is started by the control shown in FIG. Here, the MEA area is 25cm²The change in current, voltage, and cell temperature when the fuel cell 1 is started after being left in an environment of 10 ° C. below freezing point is shown.
[0036]
FIG. 7 (a) shows changes with time of each value when the fuel cell 1 is started according to the control shown in the present embodiment. That is, the cell voltage Vc is changed to a predetermined voltage range V₂(= 0.1V to 0.2V), the maximum current I from the fuel cell 1_maxTake out. By controlling in this way, the cell temperature Tc exceeded 0 ° C. after about 1 minute.
[0037]
FIG. 7B shows a comparative example. Here, after startup, the extraction current I is constant, here 3A. In this case, the cell voltage Vc reached 0V before the cell temperature Tc became higher than the freezing point, and power generation could not be continued. This is because the generated water was frozen before the cell temperature Tc reached 0 ° C., and the supply of hydrogen or air was shielded.
[0038]
Next, the effect of this embodiment will be described.
[0039]
A fuel cell 1 including one or a plurality of cells that generate power using fuel gas and oxidant gas is provided. In addition, a constant current control circuit 6 that adjusts at least the current taken out from the fuel cell 1 is provided. Further, it includes extraction power setting means (S11) for setting the extraction current I to a substantially maximum value within a range where the cell voltage Vc does not fall below a predetermined voltage related to cell degradation when starting from below freezing point. Here, the predetermined voltage is a predetermined voltage range V₂And In this way, by setting the extraction current I to a substantially maximum within a range where the fuel cell 1 does not deteriorate, the current can be extracted regardless of the power demand from the load 7 and the self-heating of the fuel cell 1 can be promoted. . As a result, the cell can be warmed up rapidly and the frequency of warming above the freezing point before the performance drops due to freezing of the product water can be increased.
[0040]
Predetermined voltage (predetermined voltage range V₂) To be −0.207V or more. Here, if the extraction current I is increased too much, the cell voltage will be inverted, that is, 0 V or less. Furthermore, when it becomes -0.207V or less, corrosion by the oxidation reaction of carbon will arise in the cathode 1c, and the sintering of catalysts may become remarkable. Therefore, cell deterioration can be avoided by setting the predetermined voltage to −0.207V or higher.
[0041]
Moreover, the temperature sensor 9 which measures the temperature of the fuel cell 1 is provided, and when the temperature of the fuel cell 1 exceeds a predetermined temperature, the operation is shifted to normal operation. The predetermined temperature is a predetermined temperature at which it is determined that refreezing can be avoided. If it becomes more than predetermined temperature, freezing can be avoided even if it shifts to normal driving | operation after that, and efficiency fall can be suppressed.
[0042]
Further, when the relationship between the current and voltage of the fuel cell 1 or the cell enters a predetermined region, the normal operation is started. If the ice of the catalyst layer 22 in the cell is thawed, the current-voltage characteristics are improved, so that the ice thaw can be detected by measuring the voltage-current characteristics.
[0043]
Moreover, the voltage sensor 10 which measures the voltage of one or several cells is provided. The cell voltage Vc measured by the voltage sensor 10 is a predetermined voltage range V.₂The extraction current I is set so as to be within the range. Thereby, the extraction current I can be set to the maximum within a range where the fuel cell 1 does not deteriorate. As a result, the warm-up time can be shortened, and the frequency with which the produced water is re-frozen can be reduced.
[0044]
Next, a second embodiment will be described. The configurations of the fuel cell system and the fuel cell 1 are shown in FIGS. 1 and 2 as in the first embodiment. Hereinafter, a description will be given centering on differences from the first embodiment.
[0045]
Here, the internal resistance R of the cell₁And reaction resistance R₂Is experimentally determined in advance, and the maximum current I is determined based on the cell temperature Tc at startup._maxAsk for. At low temperature startup, the extraction current I is the maximum current I_maxThe constant current circuit 6 is controlled so that
[0046]
First, the maximum current I_maxThe calculation method of will be described. An equivalent circuit of the unit cell is shown in FIG. 8 (for example, Shiro Haruyama, Electrochemistry for Surface Engineers Maruzen 2001 p.256). Here, the equivalent voltage of the unit cell is expressed as the internal resistance R of the cell.₁Reaction resistance R₂The diffusion resistance Zw is shown.
[0047]
Here, the combined resistance of the respective resistances of the electrolyte membrane 21, the gas diffusion layer 23, and the separator 24 and the respective contact resistances between the catalyst layer 22, the gas diffusion layer 23, and the separator 24 results in the internal resistance R of the cell.₁Is determined. Among these, the resistance Rm of the electrolyte membrane 21 varies depending on the degree of wetting and the temperature. About temperature, there exists dependence like following Formula (1).
[0048]
[Expression 1]

[0049]
Where T [K] is the film temperature, E_AIs a constant, and A (λ) is a constant depending on the moisture content of the electrolyte membrane 21. Cell internal resistance R₁Can be expressed as:
[0050]
[Expression 2]

[0051]
However, r is the sum of the resistance of the gas diffusion layer 23 and the separator 24 other than the resistance Rm of the electrolyte membrane 21, and the contact resistance between the catalyst layer 22, the gas diffusion layer 23, and the separator 24. Here, r has temperature dependence, but this dependence is considerably smaller than the change in the resistance Rm of the electrolyte membrane 21, and can be approximated to be constant regardless of the temperature.
[0052]
On the other hand, the reaction resistance R which is the resistance of the catalyst layer 22₂Is shown as follows.
[0053]
[Equation 3]

[0054]
However, (eta) is a catalyst activation overvoltage and i is a current density. The catalyst activation overvoltage η and the current density i have the following Tafel relationship.
[0055]
[Expression 4]

[0056]
Where α is a passage coefficient, z is a valence, F is a Faraday constant, R is a gas constant, i₀Is the exchange current density. The exchange current density i₀Has the following temperature dependence.
[0057]
[Equation 5]

[0058]
Where k₀Is the reaction constant and Ec is the activation energy, which is independent of temperature. Reaction resistance R from Formulas (3) to (5)₂Can be calculated.
[0059]
The diffusion resistance Zw basically has no temperature dependence and has a current of 1 A / cm in advance.²This can be obtained experimentally by measuring the current-voltage characteristics in the vicinity. Alternatively, it may be estimated from a history of current and voltage characteristic values during normal operation. In this case, it can be set as a more accurate value reflecting changes in each operation.
[0060]
The extraction current I is determined as the cell voltage Vc is within the allowable voltage range V.₂Set the value so that it is not smaller. Here, for example, the predetermined voltage range V₂Lower limit V_2iIn order to set a range that does not fall below (= 0.1 V), the current density i of the extraction current I is set to the range of the following equation.
[0061]
[Formula 6]

[0062]
Here, Er is a reversible potential and is about 1.2 to 1.3V. Each constant is obtained and stored in advance by experiments or the like. Here, A (λ), E_A, R, α, z, k₀, E_c, Zw is obtained in advance and stored. The maximum current I in the range where the cell temperature Tc is measured and the current density i is in the formula (6)_maxAnd the extraction current I is set to this.
[0063]
That is, each constant of the above formula is experimentally obtained, and the internal resistance R of the cell₁And reaction resistance R₂If the temperature dependence of the current is related in advance, the maximum current I is determined based on the cell temperature Tc at the start-up._maxCan be requested. This prevents the fuel cell 1 from deteriorating, that is, the cell voltage Vc is lower than the lower limit voltage V._2iIt is possible to set the extraction current I to the maximum while avoiding falling below. Thereby, warming up can be performed efficiently and the time until the cell temperature Tc becomes equal to or higher than the freezing point can be shortened.
[0064]
Next, a control method at the start of the fuel cell system will be described with reference to the flowchart of FIG.
[0065]
Steps S21 to S24 are the same as steps S1 to S4. When the supply of the reaction gas is started, the cell temperature Tc is measured in step S25. In step S26, the maximum current I is determined based on the cell temperature Tc as described above._maxIs calculated. In steps S27 to S29, the cell voltage Vc is set to the predetermined voltage V as in steps S5 to S7.₁Wait until the above is reached.
[0066]
Cell voltage Vc is a predetermined voltage V₁If it becomes above, extraction of an electric current will be started in step S30. Here, the extraction current I is the maximum current I calculated in step S26._maxSet to. In step S31, the cell temperature Tc is measured. In step S32, it is determined whether or not the cell temperature Tc is equal to or higher than the freezing point. If it is not above freezing, freezing may occur in the cell. Therefore, the process proceeds to step S33, and based on the cell temperature Tc measured in step S31, the maximum current I_maxIs calculated again, and the extraction current I is reset. Next, after waiting for a predetermined time in step S34, the process returns to step S31 and the cell temperature Tc is measured again. This process is repeated, and if the cell temperature Tc becomes equal to or higher than the freezing point in step S32, there is no possibility of freezing in the cell, so the present flow is terminated and the normal operation mode is entered.
[0067]
By controlling in this way, it is possible to take out as much current as possible within a range not deteriorating the fuel cell 1 immediately after the start of taking out the current and promote the power generation reaction. Therefore, since the warm-up time can be shortened, the frequency of freezing in the cell can be reduced.
[0068]
In addition, you may perform control of step S9-S14 as control after step S31. In this case, similarly to the first embodiment, the extraction current I can be set to the maximum while monitoring the cell voltage Vc so as not to cause deterioration of the cell.
[0069]
FIG. 10 shows voltage / current characteristics according to the cell temperature Tc. Here, the case where the cell temperature Tc is −5 ° C., −10 ° C., and −15 ° C. is shown. As shown in FIG. 10, the performance decreases as the cell temperature Tc decreases. So the maximum current I_maxIs set according to the cell voltage Tc, and this is taken out as the current I, so that the cell can be prevented from being deteriorated by excessively reducing the cell voltage Vc, and warm-up can be promoted. it can.
[0070]
For example, the constants A and E in the above formulas (1) to (6)_A, R, α, z, k₀, Ec, and Zw are experimentally obtained in advance, and when Tc = −10 ° C., the maximum current I that does not deteriorate the cell performance_maxIs, for example, 300 mA / cm²And calculated.
[0071]
Next, the effect of this embodiment will be described. Only the effects different from those of the first embodiment will be described below.
[0072]
Internal resistance R of one or more cells₁And reaction resistance R₂The cell voltage Vc is a predetermined voltage V_2iThe maximum current I within a range not less than_maxThe maximum power detection means (S26) for obtaining. Thereby, the current taken out from the fuel cell 1 can be set to the maximum while avoiding the deterioration of the fuel cell 1. Thereby, the warm-up time can be shortened, and the frequency with which the generated water is re-frozen can be reduced.
[0073]
The diffusion resistance Zw of the cell is estimated from the characteristic values of current and voltage during normal operation, and the internal resistance R₁And reaction resistance R₂And the diffusion resistance Zw, the cell voltage Vc is a predetermined voltage V_2iThe maximum current I within a range not less than_maxAsk for. Since the diffusion resistance Zw is not so temperature dependent, it is obtained and stored from the current-voltage characteristics during normal operation, and the voltage drop due to this is stored in the maximum current I_maxIs accurately reflected in the determination of the maximum current I_maxCan be requested.
[0074]
Moreover, the temperature sensor 9 which measures one or several cell temperature is provided. Internal resistance R₁And reaction resistance R₂Is determined in advance, and the maximum current I is determined based on the cell temperature Tc at startup._maxAsk for. As described above, the temperature dependency is obtained in advance, and the internal resistance R is determined according to the cell temperature Tc.₁And reaction resistance R₂By setting, the maximum current I can be measured by simple measurement._maxCan be requested.
[0075]
Next, a third embodiment will be described. A schematic configuration of the fuel cell system is shown in FIG. Hereinafter, a description will be given centering on differences from the second embodiment.
[0076]
The fuel cell system is configured similarly to the first embodiment. However, the internal resistance R of the cell₁It has an internal resistance measuring instrument that measures For example, a high-frequency impedance measuring device 31 including an oscillator that applies a high-frequency current and a detector that detects a voltage response is provided. Here, a high frequency impedance measuring instrument 31 having a frequency of 1 kHz and a current amplitude of 7 mA is provided.
[0077]
Measurement with high-frequency current can be performed even when fuel gas is not flowing. The frequency of the high frequency current is preferably 500 Hz or more and 10 kHz or less. This is because the influence of the catalyst layer 22 appears when the frequency is 500 Hz or lower, and the inductance of the measurement lead wire appears when the frequency is 10 kHz or higher.
[0078]
In the formula (1), the constant A (λ) changes according to the wet state of the electrolyte membrane 21. Therefore, here, the internal resistance R₁By measuring the internal resistance R₁Is a value that takes into account variations in the constant A (λ).
[0079]
Reaction resistance R₂Here, the exchange current density i₀Is provided with an exchange current density measuring device for detecting between the cells. The exchange current density measuring instrument includes the high-frequency impedance measuring instrument 31 described above, a voltage sensor 32, and a current sensor 33. By monitoring the current-voltage characteristics along with the high-frequency impedance, the exchange current density i₀Is detected. Here, the current sensor 33 is monitored, and the current-voltage characteristics are detected by measuring several cell voltages Vc in a predetermined low current density region.
[0080]
In equation (5), the reaction constant k₀Changes according to the deterioration state of the catalyst. Therefore, the exchange current density i₀By measuring reaction resistance R₂Is the reaction constant k₀It is a value that takes into account fluctuations.
[0081]
That is, the internal resistance R₁And exchange current density i₀In addition to the change in cell temperature Tc, A (λ), k₀Maximum current I considering fluctuations according to cell state_maxCan be requested.
[0082]
Next, a control method at the time of stop / start will be described using the flowchart shown in FIG. FIG. 12A is a flowchart at the time of stopping, and FIG. 12B is a flowchart at the time of starting.
[0083]
When the stop signal is detected, the stop flow of FIG. At this time, the values of the extraction current I and the cell voltage Vc are constantly monitored. Here, reaction resistance R at the time of stop₂Measurement to determine the value, that is, the exchange current density i₀Measure.
[0084]
In step S31, the extraction current I starts to decrease. In step S32, the current value is 200 mA / cm.²At that time, the voltage value is stored. In step S33, the current value is 50 mA / cm.²At that time, the voltage value is stored. In step S34, the current value is 20 mA / cm.²At that time, the voltage value is stored. In step S35, the current value is 5 mA / cm.²At that time, the voltage value is stored. In step S36, the high frequency impedance is measured. In step S37, the cell temperature Tc is measured.
[0085]
In step S38, the current is stopped. In step S39, using the detection results of steps S32 to S37, the exchange current density i₀Is approximately calculated. By performing Tafel plot as described later, the exchange current density i₀Ask for. Furthermore, the exchange current density i₀To reaction constant k₀Is calculated and stored. In step S40, the cell is dried. Here, the electrodes 1a and 1c are dried by flowing dry or low-humidity air and hydrogen gas for a predetermined time, for example, 10 minutes. Thereafter, in step S41, the supply of air and hydrogen gas is stopped and the fuel cell system is stopped.
[0086]
As described above, in the stop control, the high-frequency impedance is measured in step S36, and the voltage-current characteristics in the low current density region are monitored in steps S32 to S35. Using this detection result, in step S39, the exchange current density i₀Ask for.
[0087]
For example, the exchange current density i when the fuel cell system is stopped from the normal operation at a temperature of 70 ° C.₀FIG. 13 shows current-voltage characteristics when calculating the current.
[0088]
FIG. 13A shows the voltage-current characteristics in the low current region, and FIG. 13B shows a graph in which the logarithm of current is plotted on the X axis and the voltage is plotted on the Y axis (Tafel plot). Internal resistance R with broken line as detection voltage value and solid line as detection voltage value₁This is the value corrected for the voltage drop due to. From the high frequency impedance measured in step S36, the internal resistance R at each current density.₁Calculate and correct the voltage drop due to. The current density is 200 mA / cm.²To 5 mA / cm²The temperature when the temperature decreases to a constant value is constant, and in step S36, the internal resistance R at this temperature is₁Is measured.
[0089]
Thus, in FIG. 13B in which the Tafel plot is performed, the exchange current density i₀Can be determined from the intercept. However, the measurement is performed in a state where the reaction gas is sufficiently present. The amount of voltage drop due to the diffusion resistor Zw is small in the low current region and can be ignored.
[0090]
Here, the current density is 200 mA / cm.²The Tafel plot was performed with the above several points, but this is not the case. Lower current region, eg 1 mA / cm²-10 mA / cm²You may plot with. In this case, the internal resistance R of the cell corresponding to the resistance of the electrolyte membrane 21 and the gas diffusion layer 23 is used.₁The voltage drop due to can be ignored. Therefore, the internal resistance R₁There is no need to correct the voltage drop due to, and there is no need to measure the high-frequency impedance when monitoring the voltage-current characteristics (S36). Note that the current density is 200 mA / cm.²As described above, the influence of the diffusion resistance Zw appears. Therefore, the voltage value deviates from the straight line as shown in FIG.₀It is not preferable for measurement.
[0091]
Further, the cell voltage Vc is monitored, for example, the current I at 0.7 V, 0.75 V, and 0.8 V is measured, and the exchange current density i is measured by Tafel plotting.₀May be calculated.
[0092]
In this way, the exchange current density i is₀A control method for starting the fuel cell system after detecting the above will be described with reference to the flowchart of FIG.
[0093]
This flow is started when a start signal of the fuel cell system is detected. Steps S51 to S54 are the same as S1 to S4. When the supply of the reaction gas is started, in step S55, the internal resistance R₁Detection is performed. Here, the internal resistance R is measured by measuring the high frequency impedance of 1 kHz.₁And
[0094]
Next, in step S56, the maximum current I_maxIs calculated. Specifically, the reaction constant k obtained in step S39.₀And the reaction resistance R from the cell temperature Tc at startup.₂And this and the internal resistance R₁To the maximum current I_maxIs calculated.
[0095]
In steps S57 to S59, the cell voltage Vc is set to the predetermined voltage V as in steps S5 to S7.₁Wait until the above is reached. Predetermined voltage V₁If it becomes above, extraction of an electric current will be started in step S60. The extracted current I at this time is the maximum current I calculated in step S56._maxSet to. As a result, the maximum current can be extracted within a range that does not cause deterioration of the fuel cell 1 from the current extraction start time, and the warm-up time can be shortened. The diffusion resistance Zw is obtained in advance and taking this into account, the maximum current I_maxMay be calculated.
[0096]
Thereafter, in steps S61 to S66, the cell voltage Vc is set to a predetermined voltage range V as in steps S9 to S14.₂By setting the extraction current I so as to be maintained within the range, more reactions occur and the warm-up of the fuel cell 1 is promoted. Alternatively, step S61 and subsequent steps are the same as steps S31 to S34, and the maximum current I set according to the cell temperature Tc._maxMay be taken out and set to the current I. However, in this case, the maximum current I is determined in step S56._maxInternal resistance R when calculating₁, R₂Constant k depending on₀, A (λ) and the internal resistance R corresponding to the cell temperature Tc₁Reaction resistance R₂Is calculated again and the maximum current I_maxSet.
[0097]
Thus, the exchange current density i measured at the time of stoppage₀To reaction constant k₀And the internal resistance R at startup₁To measure the maximum current I_maxAsk for. Reaction constant k₀Changes mainly according to the deterioration of the catalyst, so that the values are almost the same at the time of stopping and at the time of starting. The reaction constant k measured at the time of stopping₀And the alternating current density i corresponding to the cell temperature Tc at the start-up₀, By extension, reaction resistance R₂Ask for. Here, the exchange current density i is₀Was measured, but can also be done at startup.
[0098]
On the other hand, the internal resistance R₁Changes depending on the cell temperature Tc, so that the values do not coincide between the stop time and the start time. For this reason, the measured value at the time of starting is used here. Or internal resistance R when stopped₁, And a constant A (λ) that changes according to the humidity of the electrolyte membrane 21 is obtained, and the internal resistance R1 corresponding to the cell temperature Tc at the start-up is obtained using this constant A (λ). Here, the constant A (λ) matches when there is no difference in the wetness of the electrolyte membrane 21 between the stop time and the start time.
[0099]
In step S39, the reaction constant k₀This is not the case. For example, each measurement is performed at a stop, and the reaction constant k₀Is the exchange current density i at startup₀, Eventually reaction resistance R₂You may calculate when calculating.
[0100]
Where internal resistance R₁And reaction resistance R₂FIG. 14 shows an example of the timing for performing the measurement for obtaining.
[0101]
FIG. 14A shows the timing in the present embodiment. That is, at the time of stop, the exchange current density i₀, Eventually reaction resistance R₂Measure to find out. At startup, internal resistance R₁Measure to find out. Thus, the appropriate maximum current I is considered in consideration of the cell temperature Tc and the cell state, here, the wetness of the electrolyte membrane 21 and the degree of deterioration of the catalyst._maxCan be set.
[0102]
FIG. 14B shows the internal resistance R at the start-up.₁And reaction resistance R₂Measure to find out. Here, the measurement is performed after starting the supply of the reaction gas and before starting the extraction of the current. As a result, the stop operation can be quickly terminated. At this time, reaction resistance R₂In the measurement for determining the cell temperature Tc, it is not necessary to correct the cell temperature Tc, so the exchange current density i determined from the Tafel plot is obtained.₀To reaction constant k₀Reaction resistance R directly₂Can be requested.
[0103]
FIG. 14C shows reaction resistance R during normal operation.₂The case where the measurement for calculating | requiring is performed is shown. The internal resistance R₁The measurement of is performed at the time of start-up as in (A). Thereby, stop operation can be performed rapidly. Reaction constant k₀Changes depending on the deterioration state of the catalyst. Although the deterioration state of the catalyst changes over time, it does not change so much during normal operation and when operation is stopped._maxIt does not affect the calculation of. Reaction constant k from measured values during normal operation₀From this, the reaction resistance R according to the cell temperature Tc at startup₂Ask for.
[0104]
FIG. 14D shows the internal resistance R after the drying operation at the stop.₁The case where is measured is shown. Reaction resistance R₂The measurement for obtaining is performed at the time of stop similarly to (A). When the wetness of the electrolyte membrane 21 after drying and immediately after startup does not change significantly, the internal resistance R measured after drying₁A constant A (λ) is obtained from the internal resistance R corresponding to the cell temperature Tc at the start-up.₁Is calculated. As a result, it is not necessary to measure a high-frequency impedance requiring 2 to 3 seconds at the time of activation, and the activation time can be shortened.
[0105]
Here, as indicated by a broken line arrow, in (D), a measuring instrument for measuring the outside air temperature is further provided. After the operation is stopped, the cell temperature Tc is lowered to the outside air, and freezing of water remaining in the fuel cell 1 is started. The internal resistance R after a predetermined time₁May be calculated. As a result, a constant A (λ) closer to the wet state of the electrolyte membrane 21 at the time of startup can be measured, and the internal resistance R₁Can be calculated accurately.
[0106]
FIG. 14E shows the internal resistance R when the temperature drops to a predetermined temperature, for example, the freezing point after the fuel cell system is stopped.₁And reaction resistance R₂The case where the measurement for calculating | requiring is performed is shown. In this case, since the measurement is limited only when starting from below freezing point, it is not necessary to perform measurement when starting from an environment above freezing point, and power consumption for measurement can be suppressed.
[0107]
Next, the effect of this embodiment will be described. Hereinafter, only effects different from those of the second embodiment will be described.
[0108]
Internal resistance R of one or more cells₁An internal resistance measuring means for measuring to obtain the reaction resistance R of one or a plurality of cells₂Reaction resistance measuring means for performing measurement for obtaining. Maximum current I immediately after starting from at least below freezing_maxIs determined using internal resistance measuring means and reaction resistance measuring means. The constant A (λ) depends on the wet state of the electrolyte membrane 21 and the reaction constant k₀Varies depending on the deterioration state of the catalyst. Therefore, the internal resistance R₁And reaction resistance R₂Is obtained by measurement, so that the internal resistance R accompanying the change of the cell state₁And reaction resistance R₂The maximum current I_maxCan be requested. Therefore, the extraction current I can be set to a substantially maximum value within a range where deterioration is avoided more accurately.
[0109]
Here, as the internal resistance measuring means, a high frequency impedance measuring device 31 including an oscillator for applying a high frequency current and a detector for detecting a voltage response is used. As a result, the internal resistance R₁Can be measured. Further, as the reaction resistance measuring means, a current sensor 33 that measures the direct current of the fuel cell 1, a voltage sensor 32 that measures the cell voltage Vc, and a high-frequency impedance measuring instrument 31 are used. Reaction resistance R when fuel gas and oxidant gas are present in the fuel cell 1₂Can be measured.
[0110]
Moreover, when stopping, a drying operation for drying the inside of the cell is performed. Reaction resistance R when stopped₂The internal resistance R is measured after the drying operation at the stop.₁Measure to find out. Before starting power generation below the freezing point, the water in the cell must be removed during the stop operation, and this operation changes the state of the electrolyte membrane 21 and the catalyst layer 22 and causes an internal resistance R.₁May change. Therefore, internal resistance R after drying₁By measuring to find the maximum current I more accurately_maxCan be requested.
[0111]
Alternatively, an outside air temperature measuring means for measuring the outside air temperature is provided, and after the operation is stopped, the internal resistance R is reached after a predetermined time when the temperature of the fuel cell 1 decreases to the outside air temperature and the moisture in the gas remaining in the cell condenses.₁Measure to find out. If the operation is stopped for a while, the moisture contained in the gas remaining in the cell condenses as the temperature of the fuel cell 1 decreases. Thereby, moisture adheres to the catalyst layer 22 and is absorbed by the electrolyte membrane 21, and the constant A (λ) varies. Therefore, by measuring after a predetermined time has elapsed after the stack operation is completed, the wet state of the electrolyte membrane 21 at the time of measurement and at the time of starting can be made the same, and the maximum current I can be more accurately determined._maxCan be requested.
[0112]
Alternatively, a temperature sensor 9 that measures the temperature of one or a plurality of cells is provided. When the temperature decreases to a predetermined temperature after the operation is stopped, the internal resistance R₁Measurement and reaction resistance R₂Measurement to find Here, the predetermined temperature is a temperature at which freezing may occur. As a result, the measurement can be performed only when starting from below freezing point. That is, when starting up from an environment above the freezing point, there is no need for measurement and power consumption can be suppressed.
[0113]
Alternatively, at the start-up, after the supply of the fuel gas and the oxidant gas to the fuel cell 1 is started and before the extraction of the current is started, the internal resistance R₁Measurement and reaction resistance R₂Measure to find out. Thus, accurate determination can be made by performing measurement immediately before activation, and there is no need to perform complicated temperature conversion.
[0114]
Next, a fourth embodiment will be described. FIG. 15 shows a schematic configuration of the fuel cell system. Hereinafter, a description will be given centering on differences from the third embodiment.
[0115]
Cell internal resistance R₁As an internal resistance measuring instrument for measuring a high frequency impedance, a high frequency impedance measuring instrument 31a comprising an oscillator for applying a high frequency current and a detector for detecting a voltage response is provided. Here, a high-frequency impedance measuring instrument 31a having a frequency of 1 kHz and a current amplitude of 7 mA is provided.
[0116]
Reaction resistance R₂Exchange current density i used to determine₀An exchange current density measuring instrument for measuring the current is provided. Here, similarly to the third, the high-frequency impedance measuring instrument 31a, the voltage sensor 32, and the current sensor 33 are provided. Moreover, the low frequency impedance measuring device 31b which consists of an oscillator which applies a low frequency current and a detector which detects a voltage response is provided. When the frequency of the oscillator is variable over a wide region, the high frequency impedance measuring instrument 31a and the low frequency impedance measuring instrument 31b may be the same.
[0117]
Here, the internal resistance R is determined from the measurement by the high frequency impedance measuring instrument 31a.₁Measure. Further, from the current-voltage characteristics monitored by the high-frequency impedance measuring device 31a, the low-frequency impedance measuring device 31b, the voltage sensor 32, and the current sensor 33, the exchange current density i₀Measure reaction resistance R₂Is calculated.
[0118]
When the amplitude and phase difference between the high-frequency current and the low-frequency current are detected, the horizontal axis is plotted with the real part of the impedance, and the vertical axis is plotted with the imaginary part of the impedance multiplied by −1 (Cole-Cole plot). It becomes a semicircle like this. The reaction resistance R is obtained by adding a resistance value corresponding to the overvoltage due to the gas crossover of the film to the resistance value corresponding to the diameter of the semicircle.₂It becomes. This reaction resistance R₂To exchange current density i₀Ask for. In this measuring method, the gas must be supplied, but the direct current does not have to be a minute value or flow. The frequency of the low-frequency current is preferably several points between 0.1 Hz and 10 Hz. The frequency is set to 1 kHz for the high-frequency current.
[0119]
Next, a control method at startup will be described using the flowchart shown in FIG. It should be noted that normal control is performed without performing measurement when the vehicle is stopped. When the signal for starting the fuel cell system is detected, this flow is started.
[0120]
Steps S71 to S77 are the same as steps S1 to S7. The cell voltage Vc is a predetermined voltage V that can extract power from the cell.₁If it becomes above, cell voltage Vc will be recorded and it will progress to step S78. The difference from the reversible potential Er (1.2 V here) is the overvoltage V_BAnd This V_BIs an overvoltage that decreases due to crossover of the membrane.
[0121]
In step S78, a low current density current is extracted. Here, the current density i of the extraction current I is 5 mA / cm.²Set to. In step S79, low frequency impedance is measured. Here, a pulse current mixed with a frequency of 1 Hz to 10 Hz is supplied to the cell, and the voltage response waveform is subjected to fast Fourier transform analysis (FFT). For example, 20 low frequency impedances with different frequencies are measured. As a result, R₁+ R₂Calculate the voltage drop by value. Next, in step S80, a high frequency impedance, for example, a high frequency impedance of 1 kHz is measured. R₁Calculate the voltage drop by value. From these, reaction resistance R₂The voltage drop due to is obtained, and this is overvoltage V_AAnd Also, the high frequency impedance is changed to the internal resistance R₁And
[0122]
Overvoltage V_A, V_BIs the catalyst activation overvoltage η, and the exchange current density i according to equations (3) and (4)₀Is calculated.
[0123]
Although the accuracy is reduced, as a step S81, instead of measuring the low frequency impedance, a low current density (here, 5 mA / cm²), The cell voltage Vc is measured, and the difference from the reversible potential Er (1.2 V) may be used as the catalyst activation overvoltage η.
[0124]
Further, the difference between the no-load voltage Vc and the reversible potential Er (1.2 V) may be a catalyst activation overvoltage η, and the current at this time may be a value assumed from the amount of gas crossover of the membrane.
[0125]
In step S82, the exchange current density i₀, Internal resistance R₁To maximum current I_maxTo decide. In step S83, the extraction current I is changed to the maximum current I._maxThen, the extraction of current starts. Hereinafter, it is the same as that of the third embodiment.
[0126]
Here, the internal resistance R₁Measurement to determine the reaction resistance R₂However, this is not the case. For example, it can be performed at the timing shown in FIG.
[0127]
FIG. 18 (A) shows the reaction resistance R when stopped.₂Measure to find out. Internal resistance R at startup₁Measure. In FIG. 18B, high frequency impedance and low frequency impedance are measured at the time of stop. Internal resistance R from high frequency impedance₁And a constant A (λ). Also, at the time of start-up, the no-load voltage is measured and the exchange current density i₀Ask for.
[0128]
Further, in FIG. 18C, the low frequency impedance is measured at the time of stopping. Further, the high frequency impedance is measured during the period from when the cell temperature Tc becomes about the outside air temperature until it becomes below freezing point. Measure no-load voltage at startup. In FIG. 18D, as in the present embodiment, the maximum current I is measured by measuring the high frequency impedance, the low frequency impedance, and the no-load voltage between the start of the reaction gas supply at the start and the start of the current extraction._maxSet.
[0129]
Next, the effect of this embodiment will be described. Hereinafter, only effects different from those of the third embodiment will be described.
[0130]
As reaction resistance measuring means, a current sensor 33 that measures the direct current of the fuel cell 1, a voltage sensor 32 that measures the cell voltage Vc, an oscillator that applies high-frequency and low-frequency currents, and a detector that detects the voltage response AC impedance measuring instruments 31a and 31b composed of When the fuel gas and the oxygen agent gas are present in the fuel cell 1, the reaction resistance R₂Measure to find out. As a result, the direct current can be measured even if it is a minute value or not.
[0131]
Next, a fifth embodiment will be described. The configuration of the fuel cell system is shown in FIG. Hereinafter, a description will be given centering on differences from the third embodiment.
[0132]
Here, as the fuel cell 1, a stack formed by stacking a plurality of unit cells is used. Internal resistance R₁Measurement and reaction resistance R₂Measurement is performed for two cells at the center and end of the stack. That is, the cell near the center of the stack is provided with a high frequency impedance measuring instrument 31A, a voltage sensor 32A, and a temperature sensor 9A. The cell near the stack edge is provided with a high frequency impedance 31B, a voltage sensor 32B, and a temperature sensor 9B. Maximum current I_maxIs calculated using the value of the inferior one of the cells at the center and end of the stack.
Hereinafter, it is the same as that of the third embodiment.
[0133]
In the case of a unit cell, power cannot be taken out when the cell voltage Vc becomes 0 V or less. However, in the case of a stack, for example, when 6 cells out of 10 cells are +0.5 V and the remaining 4 cells are −0.3 V, The stack voltage becomes + 1.8V, and power can be taken out. Even if the generated water freezes in some of the cells in the stack, normal operation starts when the frozen cells are thawed and the current-voltage characteristics in the cells are restored due to heat transfer from other cells. can do.
[0134]
However, in this case, if there is a cell that is −0.207 V or less, the deterioration of the catalyst layer 22 proceeds in this cell. Therefore, when using the stack, it is necessary to adjust the extraction current I by monitoring not only the stack voltage that can be extracted to the outside but also the voltage for each unit cell. For example, voltage sensors that measure cell voltages are provided at a plurality of locations including cells at the stack end. The cell for measuring the voltage is preferably a cell that is likely to generate water at least near the end plate 25 at the stack end. The extraction current value I is determined based on the cell voltage having the lowest performance. Further, it is determined whether or not to shift to normal operation based on the cell temperature Tc having the lowest performance.
[0135]
In addition, although the case where the stack is employed in the third embodiment has been described here, this is not restrictive. A stack may be used for any of the fuel cells 1 of the second to fourth embodiments. Although the stack can be used also in the first embodiment, as described above, it is necessary to perform control so that any cell does not fall below the allowable voltage range.
[0136]
Next, the effect of this embodiment will be described. Hereinafter, only effects different from those of the third embodiment will be described.
[0137]
The fuel cell 1 is configured by a stack structure in which a plurality of the cells are stacked, and the extraction current or voltage is calculated based on the cell having the worst performance among the cells at a plurality of locations of the stack including the cells at the stack end. Set. As described above, since the extraction current I is set based on the cell having the worst performance among the plurality of cells including the cell at the stack end where the temperature at the start-up is difficult to rise, by making the extraction current I too large It is possible to reliably prevent the fuel cell 1 from deteriorating.
[0138]
In the above embodiment, the extraction current is set. However, when the extracted power is adjusted by the voltage, the extraction voltage can be set similarly.
[0139]
Thus, it goes without saying that the present invention is not limited to the above-described embodiment, and various modifications can be made within the scope of the technical idea described in the claims.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a fuel cell system used in a first embodiment.
FIG. 2 is a schematic configuration diagram of a fuel cell used in the first embodiment.
FIG. 3 is a flowchart of activation control used in the first embodiment.
FIG. 4 shows voltage-current characteristics when the extraction current is changed.
FIG. 5 is a flowchart showing another example of activation control used in the first embodiment.
FIG. 6 is a voltage-current characteristic showing the possibility of transition to normal operation.
FIG. 7 is a diagram illustrating power control and a temperature change caused by the power control in the first embodiment.
FIG. 8 is an equivalent circuit diagram of a cell in the second embodiment.
FIG. 9 is a flowchart of activation control used in the second embodiment.
FIG. 10 shows voltage-current characteristics when the cell temperature changes.
FIG. 11 is a schematic configuration diagram of a fuel cell system used in a third embodiment.
FIG. 12 is a flowchart of stop / start control in the third embodiment.
FIG. 13 is a diagram showing a voltage-current characteristic and a Tafel plot in a low current region.
FIG. 14 is a diagram illustrating an example of measurement timing in the third embodiment.
FIG. 15 is a schematic configuration diagram of a fuel cell system used in a fourth embodiment.
FIG. 16 is a diagram showing a Cole-Cole plot in the reaction resistance detection method.
FIG. 17 is a flowchart of activation control in the fourth embodiment.
FIG. 18 is a diagram illustrating an example of measurement timing in the fourth embodiment.
FIG. 19 is a schematic configuration diagram of a fuel cell system used in a fifth embodiment.
[Explanation of symbols]
1 Fuel cell
6 Constant current control circuit (electric circuit)
9 Temperature sensor (temperature measuring means, cell temperature measuring means)
10 Voltage sensor (voltage measurement means)
31 High-frequency impedance measuring instrument (high-frequency impedance measuring means)
31a High frequency impedance measuring instrument (AC impedance measuring means)
31b Low frequency impedance measuring instrument (AC impedance measuring means)
32 Voltage sensor (voltage measuring means)
33 Current sensor (current detection means)
S11, S33 Power setting means
S26, S56, S82 Maximum power detection means

Claims (11)

A fuel cell comprising one or more cells for generating power using fuel gas and oxidant gas;
An electric circuit that adjusts at least a current or a voltage extracted from the fuel cell ,
From the internal resistance and reaction resistance of one or more cells,
A fuel cell system , wherein a maximum extraction current is obtained in a range where the voltage of the cell does not fall below a corrosion potential at which a carbon oxidation reaction proceeds during startup from below freezing point. Estimating the diffusion resistance of the cell from the current and voltage characteristic values during normal operation,
Wherein the internal resistance and the reaction resistance and the diffusion resistance, to the extent that the cell voltage does not fall below the corrosion potential of the oxidation reaction of the carbon progresses, the maximum of the current extracted, the fuel cell system according to claim 1. Cell temperature measuring means for measuring the one or more cell temperatures,
The temperature dependence of the internal resistance and the reaction resistance is obtained in advance,
The fuel cell system according to claim 1 , wherein a maximum extraction current is obtained based on a cell temperature at startup. Internal resistance measuring means for measuring to determine the internal resistance of the one or more cells;
Reaction resistance measuring means for measuring to determine the reaction resistance of the one or more cells,
2. The fuel cell system according to claim 1 , wherein at least a maximum extraction current immediately after starting from below freezing point is obtained using the internal resistance measurement unit and the reaction resistance measurement unit. 5. The fuel cell system according to claim 4 , wherein the internal resistance measuring means uses a high frequency impedance measuring means comprising an oscillator for applying a high frequency current and a detector for detecting a voltage response. As the reaction resistance measuring means, using a current measuring means for measuring the direct current of the fuel cell, a voltage measuring means for measuring the voltage of the cell, and the internal resistance measuring instrument,
The fuel cell system according to claim 4 or 5 , wherein measurement for obtaining the reaction resistance is performed when fuel gas and oxidant gas are present in the fuel cell. As the reaction resistance measuring means, a current measuring means for measuring the direct current of the fuel cell, a voltage measuring means for measuring the voltage of the cell, an oscillator for applying a high frequency current and a low frequency current, and detection for detecting a voltage response AC impedance measuring means composed of a container,
6. The fuel cell system according to claim 4 , wherein measurement for obtaining the reaction resistance is performed when fuel gas and oxygen agent gas are present in the fuel cell. When stopping, perform a drying operation to dry the inside of the cell,
5. The fuel cell system according to claim 4 , wherein measurement for obtaining the reaction resistance is performed at the time of stopping, and measurement for obtaining the internal resistance is performed after a drying operation at the time of stopping. An outside temperature measuring means for measuring outside temperature is provided,
5. The measurement according to claim 4 , wherein after the operation is stopped, the temperature of the fuel cell is lowered to the outside air temperature, and the measurement for obtaining the internal resistance is performed after a predetermined time when moisture in the gas remaining in the cell condenses. Fuel cell system. Cell temperature measuring means for measuring the temperature of the one or more cells,
5. The fuel according to claim 4 , wherein after the operation is stopped, when the temperature decreases to a predetermined temperature until the next activation, the measurement for obtaining the reaction resistance and the measurement for obtaining the internal resistance are performed. Battery system. To start measurement of the internal resistance and to determine the reaction resistance after starting the supply of fuel gas and oxidant gas to the fuel cell at the time of starting the fuel cell system and before starting the extraction of current The fuel cell system according to claim 4 , wherein the measurement is performed.

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