JP3625633B2 - Gas diffusion electrode structure and manufacturing method thereof - Google Patents

Description

であり、ガス拡散電極を使用する電解反応は、

となり、理論的に約１．２ Ｖ、実際には電極過電圧が加わって約１．５ Ｖの差が出ており、実際の電解電圧でも５０％以上の電圧削減が可能になっている。
これらの新プロセスを工業的に実現するためには高性能かつ上記電解系で十分な安定性を有する酸素ガス拡散陰極（酸素を供給ガスとするガス拡散陰極）又は水素ガス拡散陽極（水素を供給ガスとするガス拡散陽極）の開発が不可欠になる。
現在最も一般的に行なわれている酸素ガス拡散陰極を用いた食塩電解槽の概略図を図１に示す。
【０００６】
この電解槽１では、陽イオン交換膜２により該電解槽１が陽極室３と陰極室４に区画され、更に該陰極室４は酸素ガス拡散陰極５により溶液室６とガス室７に区画されている。原料となる酸素ガスはガス室７側から酸素ガス拡散陰極５のガス相面に供給され、酸素ガス拡散陰極５内を拡散し該陰極５内の触媒層で水と反応して水酸化ナトリウムを生成する。従ってこの電解法に用いられる陰極は、酸素のみを十分に透過し、かつ水酸化ナトリウムの溶液室からガス室への透過を妨げ、いわゆる気液分離型のガス拡散電極でなければならない。このような要求を満たす電極として現在食塩電解用として提案されている酸素ガス拡散陰極は、カーボン粉末とＰＴＦＥを混合しシート状に成形した電極基体に銀、白金等の触媒を担持させたガス拡散電極が中心となっている。
従来の食塩電解における陽極反応及び陰極反応はそれぞれ次の通りであり、理論分解電圧は２．１９Ｖとなる。
陽極反応：２Ｃｌ⁻ →Ｃｌ_２ ＋ ２ｅ （１．３６Ｖ）
陰極反応：２Ｈ_２ Ｏ ＋ ２ｅ → ４ＯＨ⁻ ＋ Ｈ_２ （−０．８３Ｖ）
【０００７】
ここで陰極に酸素を供給しながら電解すると、水素が供給酸素で消費されて陰極反応は次のようになる。
陰極反応：２Ｈ_２ Ｏ＋Ｏ_２ ＋４ｅ → ４ＯＨ⁻ （０．４０Ｖ）
従って理論的には１．２３Ｖ、実用的電流密度範囲でも０．８ Ｖ程度の電力消費を低減でき、水酸化ナトリウム１トン当たり７００ ｋＷｈの節減になる。このような省エネルギー化の観点から１９８０年代以降、ガス拡散電極を利用する食塩電解の実用化が検討されているが、このタイプの電極には次のような欠点があった。
【０００８】
▲１▼ 電極材料として用いられるカーボンが高温で水酸化ナトリウム及び酸素の共存下では容易に劣化し、電極性能を著しく低下させる。
▲２▼ 液圧の上昇及び電極の劣化に伴い発生する水酸化ナトリウムのガス室側へのリークを防止することが困難である。
▲３▼ 実用レベルで必要な大きさ（１ｍ^２ 以上）の電極の作製が困難である。
▲４▼ 槽内の圧力は高さによって変化し、それを補償する供給酸素ガス圧分布を与えることが困難である。
▲５▼ 陰極液の溶液抵抗損失があり、又溶液の攪拌の動力を必要とする。
▲６▼ 実用化に際し、既存の電解設備の大幅な改良が必要になる。
▲７▼ 酸素ガスとして空気を利用すると、空気中の炭酸ガスが水酸化ナトリウムと反応して炭酸ナトリウムとしてガス拡散電極の細孔に析出するため、ガス拡散能が低下してしまう。
【０００９】
これらの問題点を解決する電解法が図２に示す電解槽を使用するセロギャップ型電解法である。この電解法では、電解槽８の酸素ガス拡散陰極９とイオン交換膜１０を密着することにより図１の溶液室を無くし、原料である酸素ガス及び水を供給し、又生成物である水酸化ナトリウムも同じ側から回収することを特徴としている。
この電解法を用いると、溶液室とガス室との間のガスリークが無くなるため、前記問題点▲２▼が解消し、又電極とイオン交換膜が密着した構造であるため従来のイオン交換膜法の電解設備をあまり改良することなく使用できるため、前記問題点▲５▼▲６▼も解決される。
この電解プロセスに適した酸素ガス拡散陰極に要求される性能は、ガス透過性が高いこと、水酸化ナトリウムによる湿潤を避けるために必要な疎水性が高いこと、及び水酸化ナトリウムが電極内を移動するのに必要な透過性が高いことである。このような目的のために前記酸素ガス拡散陰極はニッケルや銀等の耐久性金属で作製されており，前記問題点▲１▼が解決されて、長時間の電解が期待できる。
【００１０】
又この電解プロセスでは、酸素供給側に透過してきた水酸化ナトリウムを回収するので，従来のように陰極により溶液室とガス室に区画することが不要になる。従って電極は液が透過性しても問題が起こらず、大型化も比較的に容易になると考えられ、問題点▲３▼が解決される。溶液室が存在せず、従って高さ方向による液圧変化を受けないため、当然問題点▲４▼は起こりえない。又生成した水酸化ナトリウムが、必然的に電極内部を通って酸素供給側に移動するため、問題点▲７▼が起こりにくくなる。
このようにガス拡散電極を工業電解系に適合させる試みは継続的に行なわれ、種々の改良が施され、成果が上がっている。しかし高さが１ｍにも達する既存の電解槽を利用する場合には、上述の構造を有するガス拡散電極でも本来の電解性能が充分に得られない。その理由として、酸素供給側に移動するアルカリ溶液に加えて、重力により高さ方向に移動した液が、電極内部に滞留するので、ガス供給が阻害されることが挙げられる。
この欠点を解消するために、ガス拡散電極に凹凸を付けあるいは一定高さごとに庇状の案内板を取り付けて電解液の電解表面からの離脱を促進したりする試みが成されているが、特に大型電解槽の場合には十分な液離脱が達成できずに電圧が高くなる傾向があり、完全な解決策が提示されていないという問題点が残っていた。
【００１１】
【発明の目的】
本発明は、前述の従来技術の問題点、つまりガス拡散電極方式の電解、例えば酸素ガス拡散電極をイオン交換膜に密着させて電解を行なうゼロギャップ型の食塩電解、水素ガス拡散電極を有する３室法電解槽を使用する塩分離や過酸化水素生成電解におけるガス拡散電極表面へのガス供給が円滑でないという問題点を解決し、低電解電圧下で水酸化ナトリウムや過酸化水素等を製造できるガス拡散電極構造体とその製造方法を提供することを目的とする。
【００１２】
【問題点を解決するための手段】
本発明に係わるガス拡散電極構造体は、親水性多孔層、該親水性多孔層の片面に形成された液及びガス透過性の電極物質層及び前記親水性多孔層の他面に密着するイオン交換膜を含んで成ることを特徴とするガス拡散電極構造体であり、本発明に係わるガス拡散電極構造体の製造方法は、親水性多孔層の片面の少なくとも一部に電極物質を含有するペーストを塗布し、加熱及び焼付けを行って前記親水性多孔層の片面に液及びガス透過性の電極物質層を形成し、かつ前記親水性多孔層の他面にイオン交換膜を密着させることを特徴とするガス拡散電極構造体の製造方法であり、前記親水性多孔層の焼結又は焼付けを電極物質層の焼付けと同時に行っても良い。
【００１３】
以下本発明を詳細に説明する。
従来からガス拡散電極の食塩電解等の工業電解への適用は検討され報告されている。例えば陰極室を酸素ガス拡散陰極により溶液室とガス室とに区画するタイプの電解槽では、イオン交換膜と陰極間の液による液抵抗は無視できないほど大きい。
イオン交換膜と陰極を密着させるゼロギャップタイプは、この液抵抗を低減させるために開発された技術である。例えば食塩電解の場合、前述した陰極反応：２Ｈ_２ Ｏ＋２ｅ→４ＯＨ⁻ ＋Ｈ_２ がイオン交換膜と陰極との界面で生じ、生成した水酸化ナトリウムは溶液として酸素ガス拡散陰極を透過して該陰極のガス相側から取り出される。この場合水酸化ナトリウムの流れ方向と酸素含有ガスの流れ方向が逆であるため、溶液が酸素電極内に滞留したり、ガス供給速度が遅くなったりする。
【００１４】
例えば酸素ガス拡散陰極を食塩電解に使用する場合とガス発生電極を食塩電解に使用する場合における電流密度の増加に対する電解電圧の上昇は、前者の方が後者の約１．５ 〜２倍であることが知られている。これは酸素ガス拡散陰極の特性として捉えられ、その主要因は反応の種類ではなく、電極反応以外の過電圧に依るものであることが判っている。その過電圧上昇の原因の１つが酸素ガス拡散陰極に対する供給ガス不足であり、例えば食塩電解の場合、ガス源を空気とする場合と純酸素とする場合では前者の方が約２００ ｍＶ過電圧が高くなることが知られている。又供給量を増加した方が過電圧が低くなるが、生成物の取り出しに支障を来たし、結局円滑なガス供給もできなくなる。
【００１５】
本発明は、この酸素ガス拡散陰極を食塩電解に使用する場合以外にも、水素ガス拡散陽極を３室法の塩分離に使用する場合、及びガス拡散電極を過酸化水素の電解製造に使用する場合等における電解生成物を含む溶液の取り出しと原料ガスの供給を共に円滑に行ない得る電解槽で使用するガス拡散電極構造体とその製造方法を提供することも目的とし、これによりガス拡散電極を使用する工業電解槽の実現の可能性が高くなる。
本発明では、イオン交換膜とガス拡散電極の電極物質層を密着させて設置するゼロギャップ型電解槽の前記イオン交換膜と電極物質層間に親水性多孔層を設けて、イオン交換膜−親水性多孔層−電極物質層によりガス拡散電極構造体を構成する。この親水性多孔層は、イオン交換膜で生成する水酸化ナトリウムや過酸化水素を溶解した溶液の全部又は一部を、該親水性多孔層を通して電極室の周囲、特に下部に抜き出して前記溶液がイオン交換膜と電極物質層間に滞留する時間を短くし、これにより電極物質層背面からの酸素含有ガスや水素含有ガス等の原料ガスの供給を円滑に行なうようにしたものである。従って本発明によると生成物を溶解した溶液の円滑な抜き出しと原料ガスの円滑な供給という方向の異なる操作を最大効率で行ない、電解電圧を従来以上に低減してガス拡散電極を工業電解へ適用する道を大きく開くことを可能にする。
このガス拡散電極構造体は、食塩電解の酸素ガス拡散陰極としても、塩分離の水素ガス拡散陽極としても、過酸化水素製造用ガス拡散電極としても使用でき、各種電解のガス拡散電極として有用である。
【００１６】
液抵抗の面から見れば、イオン交換膜と電極物質層との間には何も存在しないことが好ましいので、本発明の親水性多孔層を両者間に挿入しないほうが良いことになり、挿入すれば電解電圧は上昇する。しかし純水電解のようなイオン交換膜を固体電解質として利用する場合以外はイオン交換膜と電極とが密着しなければならない必然性はなく、前記親水性多孔層の挿入による電解電圧の上昇分以上の効果が現れれば、全体としての省エネルギー化が達成できる。
本発明はまさにこの効果を狙ったもので、前述の溶液を親水性多孔層を通して取り出すことにより、ガス供給の円滑化を達成し、これにより前記親水性多孔層の挿入による上昇分以上の電解電圧の低減を行い、全体として省エネルギー化を図ろうとするものである。
【００１７】
又親水性多孔層が連続した液層であると、この液層の高さ方向に前記電極物質層へ掛かる圧力差が生じ、大型化へのネックになる可能性がある。
本発明に係わるガス拡散電極構造体は、前述の通りイオン交換膜−親水性多孔層−電極物質層により構成され、例えばこの構造体を食塩電解槽の酸素ガス拡散陰極としてイオン交換膜と密着状態でガス室のみから成る陰極室に収容すると、該陰極室には溶液室がなく酸素ガス拡散陰極の背面側にはガス圧が等しく掛かっていること、及び前記溶液は前記親水性多孔層から実質的に液滴として抜き出され、該親水性多孔層内には連続的な液層が生じているのではなく途中で途切れた液膜状になっていると考えることが妥当であることから、高さ方向の圧力変化を酸素ガス拡散陰極が受けることはない。これは本発明のガス拡散電極構造体を３室法電解槽の水素ガス拡散陽極あるいは過酸化水素製造用ガス拡散電極に適用した場合も同様である。
【００１８】
本発明で使用する電極物質層は従来のガス拡散電極の特徴を活かしたまま使用できる。例えばチタン、ニオブ、タンタル、ステンレス、ニッケル、ジルコニウム、カーボン、銀などの耐食性材料から成る金網、粉末焼結体、金属繊維焼結体、発泡体等の材料を、必要に応じて前処理洗浄して電極物質層とするか、又はその表面に、白金、パラジウム、ルテニウム、イリジウム，銅、銀、コバルト、鉛等の金属又はそれらの酸化物等を担持した導電性グラファイトやカーボンブラック粉末、又は金属銀粉末単独あるいは白金や白金族金属合金を担持した酸化チタン等のセラミックをバインダーであるフッ素樹脂と混練してペーストとし、このペーストを１５０ 〜３００ ℃で加熱又はホットプレスにより焼付けを行って電極物質層とする。なお前記金網等の表面に、混練物を塗布し焼き付けても良い。電極物質層の多孔性を更に高めるためには前記ペースト中にアルコールやエチレングリコール等の加熱により分解又は揮発する化合物を添加しすれば良い。又このような分解性又は揮発性物質でなく、発泡剤を添加しても良いことは勿論である。
【００１９】
反応ガスの物質移動を速やかに行なうために、疎水性材料を、前記電極物質層や集電体に分散担持することが好ましい。疎水性材料としては、フッ化ピッチ、フッ化黒鉛、フッ素樹脂等が望ましく、特にフッ素樹脂は均一かつ良好な性能を得るために、２００ から４００ ℃の温度において焼成することも好ましい。フッ素成分の粉末の粒径は０．００５ 〜１００ μｍとすることが好ましい。疎水性や親水性の部分は電極断面方向に沿ってそれぞれ連続していることが望ましい。
耐食性や経済性の観点から、前記電極物質層に貴金属めっき特に銀めっきを施すことが望ましい。疎水性銀めっき浴は、例えば、チオシアン化銀１０〜５０ｇ／リットル、チオシアン化カリウム２００ 〜４００ ｇ／リットルの水溶液中へ、ＰＴＦＥ粒子１０〜２００ ｇ／リットル、及び界面活性剤１０〜２００ ｇ／（ｇ／ＰＴＦＥ）を添加して調製し、適度に攪拌しながら、室温にて電流密度０．２ 〜２Ａ／ｄｍ^２ で電着させる。めっき厚としては１〜３００ μｍのときに良好な疏水性及び耐食性を発現する。めっき後はアセトン等で充分に洗浄することが好ましい。
【００２０】
本発明においてイオン交換膜とガス拡散電極間に位置する親水性多孔層は電子の移動には寄与しないため、導電性は無くても良い。その材質は特に限定されないが、例えば食塩電解では高濃度の水酸化ナトリウムと約１００ ℃で接触するため十分な耐性を有することが必要である。又塩分離用陽極として使用する場合には電位はほぼ同じであるが電解液が強酸になることが多いため、耐酸性の材料であることが必要である。更に電解槽内に設置されるイオン交換膜は必ずしも完全な平面ではないので、前記電極物質層もそれに沿ってある程度変形できるフレキシビリティを有することが望ましく、換言すると圧力の不均一が生ずる場合に変形して前記圧力を吸収する材料であることが望ましい。
前記親水性多孔層の材質としては、例えば酸化ジルコニウム、酸化珪素及び酸化チタン等の金属酸化物、カーボン、炭化珪素等のセラミックス、親水性化したＰＴＦＥ、ＥＥＰ等の樹脂、ニッケル、ステンレス、銀等の金属や合金などがあり、前記した樹脂以外の材料の場合はバインダーである３０％以下のフッ素樹脂とともに焼き付け、必要に応じて前処理洗浄して親水性多孔層とする。
【００２１】
前記親水性多孔層の形状は厚さが０．０１〜１０ｍｍ、好ましくは０．１ 〜１ｍｍのシート状とすることが好ましく、更に電解液を常に保持し得る材料及び構造であることが好ましく、例えばその構造としては、網、織物、不織物、発泡体があり、特に粉末を原料として孔形成剤と各種バインダーでシート状に成形した後、溶剤により孔形成粒子を除去した焼結板又はそれを重ねた物が好ましい。
親水性多孔層を構成する粉末の平均気孔率はこれに限定されるものではないが５０〜９０％、望ましくは７０〜９０％とする。気孔率が９０％を越えると物理強度が弱くなって取扱い上の問題が生じ、又５０％未満では保水率及び透水率が低くなり更に電解時の電気抵抗が大きくなる。他の条件にも依るが、気孔率を５０〜９０％にするための親水性多孔層材料の平均粒径は５〜１０μｍ程度であり、粒度分布はできるだけ小さくすることが好ましい。
【００２２】
該親水性多孔層の厚さは、イオン交換膜から該電極物質層に向かって流れる電解液の量、つまり該親水性多孔層により除去されるべき電解液の量に応じて加減すれば良いが、前記厚さを５００ 〜１０００μｍと厚くする場合には、使用する粒子の平均粒径もそれに対応して大きくすることが好ましい。該親水性多孔層が厚いと電解電圧が高くなり電力原単位が悪くなることがある。
又該親水性多孔層を前述した金属酸化物等の粒子とバインダーのみで構成すると強度が不十分になり取扱いが不便になることがある。この場合には金属フォームや金属メッシュあるいは炭素繊維やフッ素樹脂等を心材として使用し、この心材の両面に前記粒子及びバインダーによる多孔層を形成すれば良い。但しこの心材も前述した親水性多孔層の材質に関して述べた特性が必要であり、例えば食塩電解の酸素ガス拡散陰極として使用する場合には、金属銀や銀めっきを施した銅やニッケルを使用することが、又塩分離の水素ガス拡散陽極として使用する場合には、耐酸性のあるチタンやジルコニウム又はハステロイを使用することが望ましい。この心材も親水性多孔層と同様に電子の移動には寄与しないため、導電性でも非導電性でも良いが、電極としての作用はできるだけ小さくすることが好ましい。
【００２３】
このような心材を有する親水性多孔層を製造するには、心材の両面に上述した原料粉末とフッ素樹脂の混練物を塗布し、僅かに圧力を掛けながら例えば２００ 〜３５０ ℃程度の温度でホットプレスして成型することができる。
この親水性多孔層をイオン交換膜と電極物質層間に配置するには、前記イオン交換膜と電極物質層間に挟み、電解液の液高さによる水圧差０．１ 〜３０ｋｇｆ／ｃｍ^２ 程度の圧力で一体化することができる。又前記親水性多孔層を予め電極物質層の膜側表面又はイオン交換膜の電極物質層側表面に形成し、該イオン交換膜及び電極物質層を密着させて所定位置に配置するようにしても良い。
しかしながら液及びガス透過性を有する前記電極物質層は、前記親水性多孔層の片面の少なくとも一部に該電極物質層を構成する成分である電極物質の混練物を含有するペーストを塗布し、加熱及び焼付けを行って形成することが望ましい。この加熱は例えば空気中２００ 〜３５０ ℃で行うことができる。２００ ℃未満でも加熱焼付けは可能であるが、その場合は添加する化合物をより分解又は揮発しやすいものとする。加熱温度が３５０ ℃を越えるとバインダーであるフッ素樹脂の分解が生じ始めるため短時間での処理が必要になり、又高温度では触媒粒子同士の焼結や不活性化が進行しやすくなる。このようにペースト塗布による電極物質層の形成条件は電極物質の種類やバインダーの種類に応じて適宜設定する必要がある。
【００２４】
更に親水性多孔層を前述の該多孔層形成粒子とバインダーであるフッ素樹脂から成る加熱焼結前の親水性多孔層前駆体とし、この前駆体の片面の少なくとも一部に前述した通り電極物質層を構成する成分である電極物質の混練物を含有するペーストを塗布し、これを加熱焼結することにより親水性多孔層の焼結と電極物質層の焼結を一度の加熱処理操作で行い、作業性の向上を図ることもできる。
このように構成されるこの親水性多孔層付き電極物質層は電解槽内に収容される際に常にイオン交換膜に接触するように設置され、イオン交換膜−親水性多孔層−電極物質層から成るガス拡散電極構造体となる。
【００２５】
食塩電解に本発明のガス拡散電極構造体を使用する場合、イオン交換膜としてはフッ素樹脂系の陽イオン交換膜が耐食性の面から好適である。陽極は通常のＤＳＡと呼ばれるチタン製の不溶性電極を使用することが望ましく、他の電極の使用も可能である。
電解条件は、例えば温度６０〜９０℃、電流密度１０〜１００ Ａ／ｄｍ^２ とすることが好ましく、必要に応じて供給酸素含有ガスを加湿する。加湿方法としては、電解槽入口に７０〜９５℃に加湿された加湿装置を設け、前記酸素含有ガスを通すことにより制御する。現在市販されている膜の性能では、陽極液の濃度を２００ ｇ／リットル以下、特に１７０ ｇ／リットル付近に維持すると、酸素含有ガスの加湿は不要になる。得られる水酸化ナトリウム濃度は２５〜４０％程度が適当であるが、基本的にはイオン交換膜の性能により決定される。
【００２６】
本発明のガス拡散電極構造体を設置した電解槽を使用して食塩電解を行なうと、酸素ガス拡散陰極のイオン交換膜側表面近傍で主として生成する水酸化ナトリウムを前記親水性多孔層を通してつまり酸素ガス拡散陰極を通さずに抜き出すことができる。その際に該親水性多孔層がシード状であると、前記水酸化ナトリウムがその周縁に達しなければ抜き出されず、抜き出しまでに比較的長時間を要することがある。この問題点を解決するために、本発明では、例えばシートを複数に分割して各分割シートの一端を、例えば１〜５ｍｍ幅のスリットやガイドを形成した酸素ガス拡散陰極のこれらの隙間から電極背面に達するように配置すると、生成水酸化ナトリウムが周縁に達する前に、短時間でイオン交換膜と酸素ガス拡散陰極間から抜き出される。
【００２７】
図３は、本発明に係わる酸素ガス拡散陰極を使用する食塩電解用電解槽の一例を示す縦断面図である。
電解槽本体１１は、イオン交換膜１２により陽極室１３と陰極室１４に区画され、前記イオン交換膜１２の陽極室１３側にはメッシュ状の不溶性陽極１５が密着し、該イオン交換膜１２の陰極室１４側にはシート状の親水性多孔層１６が密着し更に該親水性多孔層１６の陰極室側には液透過型酸素ガス拡散陰極１７が密着し、該酸素ガス拡散陰極１７にはメッシュ状の陰極集電体１８が接続され該集電体１８により給電されるようになっている。
なお１９は陽極室底部近傍の側壁に形成された陽極液（飽和食塩水）導入口、２０は陽極室上部近傍の側壁に形成された陽極液（未反応食塩水）及び塩素ガス取出口、２１は陰極室上部近傍の側壁に形成された（加湿）酸素含有ガス導入口、２２は陰極室底部近傍の側壁に形成された水酸化ナトリウム及び過剰酸素の取出口である。
【００２８】
この電解槽１１の陽極室１３に陽極液である飽和食塩水を供給しかつ陰極室１４に加湿した酸素含有ガス例えば純酸素や空気を供給しながら両電極１５、１７間に通電すると、イオン交換膜１２の陰極室１４側表面で水酸化ナトリウムが生成する。通常の電解槽ではこの水酸化ナトリウムは水溶液として酸素ガス拡散陰極を透過してその陰極室側表面に達する。しかし図示の電解槽１１ではイオン交換膜１２と酸素ガス拡散陰極１７の間に親水性多孔層１６が存在するため、前記水酸化ナトリウム水溶液は前記陰極１７内を透過するよりも抵抗が小さくなる、前記親水性多孔層１６内を分散し、特に重力により下降して該親水性多孔層１６の下端に達して液滴として陰極室１４底部に落下して貯留される。
この電解槽を図２等の従来の電解槽と比較すると、図２の従来型電解槽では、生成する水酸化ナトリウム水溶液は密度の高い酸素ガス拡散陰極内を透過しなければならず、従って電極内での滞留時間が長くなり、供給される酸素含有ガスの円滑な透過を阻害し、反応を律速するガス供給が不十分になるため見掛け上の陰極過電圧が上昇し、槽電圧が高くなる。それに比べ図３の電解槽では、生成する水酸化ナトリウム水溶液の反応サイトからの取り出しが比較的抵抗の小さい親水性多孔層内の分散により行なわれ、陰極内に殆ど滞留しないため、反応ガスの供給が円滑に行なわれ、従って低い電圧が維持される。
【００２９】
図４は、生成する水酸化ナトリウム水溶液を更に円滑に取り出すことのできる図３の電解槽の一部を改良した要部斜視図で、図４ａは陰極を複数に分割した例、図４ｂは陰極にスリットを形成した例を示す。
図４ａでは、酸素ガス拡散陰極１７ａを複数に分割して陰極片１７ｂとし、かつ親水性多孔層１６ａも対応する数の親水性多孔層片１６ｂに分割している。各親水性多孔層片ｂの下端は前記陰極１７ｂ方向に折り曲げられ上下に隣接する陰極１７ｂ間を通って該陰極１７ｂの背面に達し、折曲片１６ｃを形成している。
【００３０】
この電解槽を使用して電解を行なうと、図３の電解槽の場合と同様に、イオン交換膜の陰極室側表面で生成する水酸化ナトリウム水溶液が親水性の親水性多孔層片１６ｂ内を透過する。該親水性多孔層１６ｂが分割されているので、前記水酸化ナトリウム水溶液は周縁部まで移動せずに各親水性多孔層片１６ｂ内をその下端部までの比較的短い距離を移動すれば陰極１７ｂ方向に折り曲げられた折曲片１６ｃから液滴として落下する。それ故図３の電解槽よりも円滑に液抜きを行なうことができる。
図４ｂは陰極を複数に分割せず、陰極１７ｃに横長の四角形の形状のスリット２３を形成した例である。図４ａのように陰極を複数に分割すると各分割片ごとに給電する必要があって煩雑であるが、図４ｂのように陰極１７ｃにスリット２３を形成し、このスリット２３を通して陰極１６ｂの折曲片１６ｃを陰極背面に位置させるようにすると、陰極への給電を単一の集電体で行なえるため、更に好都合である。
【００３１】
【実施例】
次に本発明に係わるガス拡散電極構造体の製造方法及び該ガス拡散電極構造体を使用する食塩電解及び塩分離の実施例を記載するが、該実施例は本発明を限定するものではない。
【００３２】
【実施例１】
見掛け厚さ０．２ ｍｍの炭素繊維の織物を心材とし、該心材の両面に、平均粒径７μｍの酸化ジルコニウム粉末と、固形分として該酸化ジルコニウムの３０重量％に相当するフッ素樹脂を含むフッ素樹脂の水分散体であるデュポン社の３０Ｊ（バインダー）の混練物を塗布し、見掛け厚さ０．５ ｍｍの板状体を作製し、この板状体に１ｋｇ／ｃｍ^２の圧力を掛けながら２５０ ℃で１５分間ホットプレスして焼結して、親水性多孔層とした。
この親水性多孔層の片面に、平均粒径１μｍの銀粒子表面に白金を焼き付けた触媒物質の分散体を塗布した後、平均粒径３μｍの銀粒子を前記３０Ｊをバインダーとして見掛け厚さ０．１ ｍｍとなるように塗布した（白金担持量は３ｇ／ｍ^２ ）。これを１ｋｇ／ｃｍ^２の圧力を掛けながら２００ ℃で１５分間ホットプレスして、前記親水性多孔層の片面に電極物質層を作製した。
【００３３】
この親水性多孔層付き電極物質層の該親水性多孔層を、電解面積が１００ ｃｍ^２ （１０ｃｍ×１０ｃｍ）である２室法の小型イオン交換膜電解槽の陰極室にデュポン社製ナフィオン９６１ であるイオン交換膜と密着するように設置し陰極を構成した。陰極室の集電体は厚さ１ｍｍの銅の穴明き板の表面に銀をめっきしたものを用いた。陽極として、酸化ルテニウム系の電極物質を被覆したチタン製のエクスパンドメッシュ基体から成る不溶性金属電極を用い、前記陰極とともに前記イオン交換膜を相互に密着するように挟み込んだ。陰極室にはドレーンを設け、生成した陰極液は全て下方に抜くようにした。
陽極室には１８０ ｇ／リットルの食塩水を循環し、陰極室にはＰＳＡ法により酸素富化した酸素濃度９０％のガスを理論量の１．２ 倍供給しながら食塩電解による水酸化ナトリウムの製造を行ったところ、電流密度が３０Ａ／ｄｍ^２ で電解電圧が２．０１Ｖであり、１００ 時間以上の連続運転を行っても安定した電解が可能であった。
【００３４】
【比較例１】
実施例１で使用した炭素繊維織物の表面に、平均粒径１μｍの銀粒子表面に白金を焼き付けた触媒物質の分散体を塗布した後、平均粒径３μｍの銀粒子を前記３０Ｊをバインダーとして見掛け厚さ０．１ ｍｍとなるように塗布して電極物質層とした（白金担持量は３ｇ／ｍ^２ ）。
この電極物質層を陰極とし、直接イオン交換膜に密着させたこと以外は実施例１と同一条件で電解を行ったところ、初期電圧は２．０ Ｖであったが、徐々に電圧が上昇し、８時間後には１００ ｍＶ上昇した。電解を中断して３０分後に再開したところ、電圧は２．０ Ｖに戻っており、電圧時間の経過とともに再度徐々に電圧上昇が起こった。これは電極物質層裏面に透過した電解液が酸素供給を阻害しているからと推測できる。
【００３５】
【実施例２】
実施例１の見掛け厚さ０．５ ｍｍの板状体を６０℃で乾燥して親水性多孔層とした。
この親水性多孔層の片面に、平均粒径０．３ μｍの銀の超微粉とデキストリン及びエチレングリコールから成るペーストを見掛け厚さ３μｍとなるようにドクターブレード法で塗布し乾燥した後、更に平均粒径７μｍの銀粒子とフッ素樹脂分散液デュポン社製３０Ｊとの混練物を塗布し、１ｋｇ／ｃｍ^２の圧力を掛けながら３００ ℃で３０分間ホットプレスして、前記親水性多孔層の片面に電極物質層を作製して親水性多孔層付き電極物質層とした。
この親水性多孔層付き電極物質層の該親水性多孔層を、２室法イオン交換膜電解槽の陰極室内にデュポン社製ナフィオン９６１ であるイオン交換膜と密着するように設置し陰極を構成した。陽極として、酸化ルテニウム系の電極物質を被覆した不溶性金属電極を用い、前記陰極とともに前記イオン交換膜を相互に密着するように挟み込んだ。
陽極室には１８０ ｇ／リットルの食塩水を循環し、陰極室には水蒸気を飽和させた酸素ガスを送りながら温度９０℃、電流密度３０Ａ／ｄｍ^２ で電解を行ったところ、電解電圧は２．０５Ｖであり、安定した電解が継続できた。
【００３６】
【実施例３】
実施例１で使用した１０ｃｍ×１０ｃｍの電極物質層はそのままにし、これに密着している親水性多孔層の縦方向の長さを下向きに１ｃｍ延ばして１１ｃｍとし、下端の余っている長さをイオン交換膜の反対側に引出して導液部とし、集電体の隙間を通して裏側に導いた。この親水性多孔層付き電極物質層を１０枚用意し縦方向に連結して高さ１００ ｃｍの２室法イオン交換膜電解槽に組み込んだ。
このような構成の親水性多孔層付き電極物質層を使用したこと以外は実施例１と同一条件で電解を行ったところ、電流密度３０Ａ／ｄｍ^２ で電解電圧２．００Ｖであり、電解液は前記導液部を通って電極物質層の外側に滴下していることが観察され、前記電極物質層を直接透過する該電極物質層の裏側への滲み出しは殆ど見られずその量は液全体の２０％未満と推測できた。
１００ 時間の連続電解でも電圧変化は見られず、電流密度を４０Ａ／ｄｍ^２ まで上昇させたところ電解電圧は２．１５Ｖまで上昇したが、経時的な電圧変化は見られなかった。
【００３７】
【実施例４】
線径０．１ ｍｍのチタン製の線を織って作製したメッシュを心材としてその表面全体に平均粒径１０μｍの酸化チタン粉末とフッ素樹脂分散液３０Ｊを混練したペーストを塗布し３００ ℃で焼き付けて親水性多孔層とした。該親水性多孔層の気孔率は約８０％であった。
該親水性多孔層の片面に、塩化白金酸のエチレングリコール溶液を塗布し、水素気流中で３００ ℃に３０分間保持することにより白金金属を析出させて電極物質層とした。
【００３８】
この親水性多孔層付き電極物質層を次のようにして３室法による塩分離装置の陽極として使用した。つまり陽極室と中間室を陰イオン交換膜で、又中間室と陰極室を陽イオン交換膜で区画した電解槽の陽極として、前記ガス拡散電極構造体を親水性多孔層が前記陰イオン交換膜と密着するように設置し、対極である陰極としてはニッケルメッシュを使用した。
中間室に硝酸ナトリウム水溶液を供給し、陽極室には陰極室で発生した水素とその１０重量％に相当する水素をボンベから供給し、陰極室には脱イオン水を滴下して生成する水酸化ナトリウム濃度が１５％に保持されるよう調節しながら、温度４５℃、電流密度２０Ａ／ｄｍ^２ で電解を行った（なお陽極室には水蒸気を加えて硝酸濃度が１０％となるように調節した）。この電解により、電解電圧２．４ Ｖで安定した塩分離が進行し、陽極室で硝酸が、陰極室で水酸化ナトリウムが電流効率８０％で得られた。
【００３９】
【比較例２】
陽極として酸素発生型の不溶性金属電極としたこと以外は実施例４と同一条件で電解を行ったところ、電解電圧は３．９ Ｖであった。
【００４０】
【比較例３】
親水性多孔層を形成していない電極物質層を使用したこと以外は実施例４と同一条件で電解を行ったところ、一部の硝酸が分解するためか、陽極の電圧が不安定で最大電流密度が１０Ａ／ｄｍ^２ に制限され、それ以上では電解が不可能であった。
【００４１】
【実施例５】
線径０．１ ｍｍのチタン製の線を織って作製したメッシュを心材とし、その両面に平均粒径５μｍのシリカ粉末とシリコン系発泡材並びにフッ素樹脂水分散剤を混練した物を塗布し乾燥して親水性多孔層とした。
親水性カーボンブラックとフッ素樹脂水分散剤３０Ｊの混練物を平面状に成型し、該成型体の片面に、塩化白金酸のエチレングリコール溶液を塗布し、水素気流中で３００ ℃に３０分間保持することにより白金金属を析出させて電極物質層とした。前記親水性多孔層と電極物質層の白金析出側を密着させ、１ｋｇ／ｃｍ^２の圧力を掛けながら２５０ ℃でホットプレスして、前記親水性多孔層の片面に電極物質層を作製して親水性多孔層付き電極物質層とした。この親水性多孔層の気孔率は約８５％であった。
【００４２】
この親水性多孔層付き電極物質層を３室法塩分離電解装置の陽極として使用し、実施例４と同様の装置を構成した。
中間室に２００ ｇ／リットルの硫酸ナトリウムを供給し、陰極室には陰極液（水酸化ナトリウム）濃度が１００ ｇ／リットルとなるように脱イオン水を供給した。又陽極供給ガスとして、陰極発生水素に水を再飽和したガスを使用した。温度４５℃、電流密度１０Ａ／ｄｍ^２ で電解を行ったところ、電解電圧２．７５Ｖ、電流効率８５％であり、１００ 時間の連続電解後も安定した運転が継続できた。
【００４３】
【発明の効果】
本発明のガス拡散電極構造体は、親水性多孔層、該親水性多孔層の片面に形成された液及びガス透過性の電極物質層及び前記親水性多孔層の他面に密着するイオン交換膜を含んで成ることを特徴とするガス拡散電極構造体である。
従来のガス拡散電極を使用する電解槽特にガス拡散電極をイオン交換膜に密着させるゼロギャップタイプの電解槽では、イオン交換膜表面で生ずる目的生成物が比較的密度の高い前記ガス拡散電極を透過してつまり供給される反応ガスの供給方向と反対方向に、換言すると反応ガスの供給を阻害しながら前記ガス拡散電極を透過しなければならず、生成物が増加するほど反応ガスの反応サイトへの供給が阻害されて電解電圧が上昇するという問題点があった。
【００４４】
これに対し本発明のガス拡散電極構造体では、電極物質層とイオン交換膜の間に親水性多孔層を配置したため、従来はその殆ど全てが前記ガス拡散電極を透過して取り出されなければならなかった水酸化ナトリウム水溶液等の生成物がガス拡散電極（電極物質層）を透過せずに前記親水性多孔層を通って反応ガスの供給方向は対向することなくイオン交換膜表面から取り出すことができる。従って生成物量が増加しても、反応ガス供給には殆ど影響がなく、電圧を低く維持したまま、所定の電解反応を継続できる。
親水性多孔層は該多孔層構成粒子とバインダーとの混練ペーストを焼き付けることにより製造できるが、これのみでは強度が十分でない場合があり、その際には心材を用い該心材の両面に前記ペーストを塗布し加熱することにより機械的強度の高いガス拡散電極構造体とすることができる。
【００４５】
本発明方法は、親水性多孔層の片面の少なくとも一部に電極物質を含有するペーストを塗布し、加熱及び焼付けを行って前記親水性多孔層の片面に液及びガス透過性の電極物質層を形成し、かつ前記親水性多孔層の他面にイオン交換膜を密着させることを特徴とするガス拡散電極構造体の製造方法である。
前述した本発明に係わるガス拡散電極構造体は、別途製造した親水性多孔層と電極物質層を圧力を加えることにより機械的に密着させることにより構成しても良いが、前記方法のように親水性多孔層表面に電極物質含有ペーストを塗布し加熱すると両者が強く結合して機械的な強度が増強される。更に親水性多孔層と電極物質層が一体化するため運搬等が簡単になり、取扱いが容易になる。
更に焼結前の親水性多孔層前駆体の片面の少なくとも一部に前記方法と同様にペーストを塗布し加熱焼結することにより親水性多孔層の焼結と電極物質層の焼結を一度の加熱処理操作で行うことができ、これにより作業性の向上を図ることができる。
【図面の簡単な説明】
【図１】従来の食塩電解槽の一例を示す概略図。
【図２】従来の食塩電解槽の他の例を示す概略図。
【図３】本発明に係わるガス拡散電極構造体を使用する食塩電解用電解槽の一例を示す縦断面図。
【図４】本発明に係わるガス拡散電極構造体を使用する食塩電解用電解槽の他の例を示す縦断面図で、図４ａは陰極を複数の分解した例を、図４ｂは陰極にスリットを形成し例を示す。
【符号の説明】
１１・・・電解槽本体 １２・・・イオン交換膜 １３・・・陽極室 １４・・・陰極室 １５・・・不溶性陽極 １６・・・親水性多孔層 １７・・・酸素ガス拡散陰極
１８・・・集電体[0001]
[Industrial application fields]
The present invention relates to a gas diffusion electrode structure that can smoothly supply a gas and a method for manufacturing the same, and more particularly, can smoothly supply a gas to achieve a large energy saving effect in electrolysis for sodium hydroxide production or hydrogen peroxide production. The present invention relates to a gas diffusion electrode structure and a manufacturing method thereof.
[0002]
[Prior art and its problems]
Electrolysis is a widely used technology in the chemical industry for reasons such as good product selectivity, very few co-produced substances, relatively simple processes that are easy to handle, and high product quality. Furthermore, it is also attracting attention because there are many cases that lead to environmental purification rather than environmental pollution regarding environmental problems. Furthermore, the operating conditions are relatively mild, the utilization rate of raw materials is high, and the waste is extremely small.
Therefore, the electrolysis plays an important role in the material industry represented by chloralkali electrolysis. Although it has such an important role, energy consumption required for chloralkali electrolysis is large, and energy saving is a big problem in countries with high energy costs such as Japan. For example, in chloralkali electrolysis, in order to solve environmental problems and achieve energy savings, the mercury method has been switched to the ion exchange membrane method via the diaphragm method, and energy savings of about 40% have been achieved in about 25 years. However, even this energy saving is insufficient, and the power cost of energy accounts for 50% of the total manufacturing cost. No further power savings are possible as long as current methods are used. In order to achieve further energy saving, it is necessary to drastically change such as using an electrode reaction different from the conventional one. As an example, the use of a gas diffusion electrode adopted in a polymer solid oxide fuel cell that can be operated under almost the same conditions as ordinary aqueous electrolysis is the most probable and is a means of saving power. is there.
[0003]
The gas diffusion electrode has a property of easily supplying a gas as a reactant to the electrode surface, and has been developed based on applications such as a fuel cell. Recently, the use of gas diffusion electrodes for industrial electrolysis has been studied. For example, in an on-site production apparatus for hydrogen peroxide, a hydrophobic cathode for performing an oxygen reduction reaction is used (Industrial Electrochemistry (2nd Edit). .) P279-, 1991). In addition, in the alkali production and various recovery processes, hydrogen oxidation at the anode or oxygen reduction reaction at the cathode is performed using a gas diffusion electrode as a substitute for the generation of oxygen at the anode or hydrogen at the cathode as a counter electrode reaction, thereby reducing power consumption. I am trying. It has also been reported that depolarization with a hydrogen anode is possible as a counter electrode for metal recovery such as zinc extraction or zinc plating.
However, these industrial electrolytic systems have a problem in that the life and performance of the electrode cannot be sufficiently obtained because the composition or operating conditions of the solution and gas are not simple as compared with the case of the fuel cell.
[0004]
An example in the sodium hydroxide manufacturing process by salt electrolysis will be described. Sodium hydroxide and chlorine, which are important as industrial raw materials, are mainly produced by salt electrolysis. This electrolysis process has gone through the transition as described above, and has shifted to an ion exchange membrane method using an ion exchange membrane as a diaphragm and an activated cathode with a small overvoltage. During this time, the power consumption rate for the production of 1 ton of sodium hydroxide decreased to about 2000 kWh. Furthermore, if oxygen reduction reaction without hydrogen generation is performed instead of hydrogen generation at the cathode as in the conventional method, the theoretical decomposition voltage is reduced from 2.19V to 0.96V, which can be reduced to 1.23V. Therefore, significant energy savings can be expected.
As an application technique of the salt electrolysis, there is use of a gas diffusion electrode for electrolytic separation of sodium sulfate, which is a neutralized salt, into acid and alkali. Conventionally, an oxygen generating electrode was used as an anode and a hydrogen generating electrode was used as a cathode. However, a hydrogen gas diffusion electrode was used as an anode, and hydrogen generated at the cathode was used as the hydrogen supplied to the gas diffusion electrode. Realizes a low voltage drop.
[0005]
Conventional electrolytic reaction

The electrolytic reaction using a gas diffusion electrode is

Theoretically, the difference is about 1.2 V, and in actuality, an electrode overvoltage is applied to give a difference of about 1.5 V, and the actual electrolytic voltage can be reduced by 50% or more.
Oxygen gas diffusion cathode (gas diffusion cathode using oxygen as supply gas) or hydrogen gas diffusion anode (supplying hydrogen) which has high performance and sufficient stability in the above electrolytic system to industrially realize these new processes Development of gas diffusion anode) is essential.
FIG. 1 shows a schematic view of a salt electrolysis cell using an oxygen gas diffusion cathode that is most commonly performed at present.
[0006]
In this electrolytic cell 1, the electrolytic cell 1 is partitioned into an anode chamber 3 and a cathode chamber 4 by a cation exchange membrane 2, and the cathode chamber 4 is partitioned into a solution chamber 6 and a gas chamber 7 by an oxygen gas diffusion cathode 5. ing. Oxygen gas used as a raw material is supplied from the gas chamber 7 side to the gas phase surface of the oxygen gas diffusion cathode 5, diffuses in the oxygen gas diffusion cathode 5, reacts with water in the catalyst layer in the cathode 5, and forms sodium hydroxide. Generate. Therefore, the cathode used in this electrolysis method must be a so-called gas-liquid separation type gas diffusion electrode that sufficiently transmits only oxygen and prevents the sodium hydroxide from passing from the solution chamber to the gas chamber. The oxygen gas diffusion cathode currently proposed for salt electrolysis as an electrode that meets these requirements is a gas diffusion in which a catalyst such as silver or platinum is supported on an electrode substrate formed by mixing carbon powder and PTFE into a sheet shape. The electrode is the center.
The anodic reaction and the cathodic reaction in conventional salt electrolysis are as follows, respectively, and the theoretical decomposition voltage is 2.19V.
Anodic reaction: 2Cl⁻→ Cl₂+ 2e (1.36V)
Cathode reaction: 2H₂O + 2e → 4OH⁻  + H₂  (-0.83V)
[0007]
Here, when electrolysis is performed while supplying oxygen to the cathode, hydrogen is consumed by the supplied oxygen, and the cathode reaction is as follows.
Cathode reaction: 2H₂O + O₂+ 4e → 4OH⁻  (0.40V)
Therefore, theoretically 1.23V, power consumption of about 0.8V can be reduced even in a practical current density range, and 700 kWh is saved per ton of sodium hydroxide. From the standpoint of energy saving, practical use of salt electrolysis using a gas diffusion electrode has been studied since the 1980s, but this type of electrode has the following drawbacks.
[0008]
(1) Carbon used as an electrode material easily deteriorates in the presence of sodium hydroxide and oxygen at a high temperature, and the electrode performance is remarkably lowered.
{Circle around (2)} It is difficult to prevent leakage of sodium hydroxide to the gas chamber side that occurs due to an increase in fluid pressure and electrode deterioration.
▲ 3 ▼ Required size for practical use (1m²It is difficult to produce the above electrode.
(4) The pressure in the tank varies depending on the height, and it is difficult to provide a supply oxygen gas pressure distribution that compensates for it.
(5) There is a solution resistance loss of the catholyte, and the power for stirring the solution is required.
(6) For practical use, it is necessary to significantly improve existing electrolysis equipment.
(7) When air is used as oxygen gas, carbon dioxide gas in the air reacts with sodium hydroxide and precipitates as sodium carbonate in the pores of the gas diffusion electrode, so that the gas diffusing ability is lowered.
[0009]
An electrolysis method for solving these problems is a cell gap type electrolysis method using the electrolytic cell shown in FIG. In this electrolysis method, the oxygen gas diffusion cathode 9 and the ion exchange membrane 10 in the electrolytic cell 8 are brought into close contact with each other, thereby eliminating the solution chamber of FIG. 1, supplying oxygen gas and water as raw materials, and hydroxylating as a product. Sodium is also recovered from the same side.
When this electrolysis method is used, gas leakage between the solution chamber and the gas chamber is eliminated, so that the above problem (2) is solved, and the electrode and ion exchange membrane are in close contact with each other, so that the conventional ion exchange membrane method is used. Therefore, the above problems (5) and (6) can be solved.
The performance required for an oxygen gas diffusion cathode suitable for this electrolysis process is high gas permeability, high hydrophobicity necessary to avoid wetting by sodium hydroxide, and sodium hydroxide moving through the electrode. The permeability required for this is high. For this purpose, the oxygen gas diffusion cathode is made of a durable metal such as nickel or silver, and the above problem (1) can be solved and long-term electrolysis can be expected.
[0010]
Further, in this electrolysis process, sodium hydroxide that has permeated to the oxygen supply side is recovered, so that it is not necessary to partition into a solution chamber and a gas chamber by a cathode as in the prior art. Therefore, it is considered that the electrode does not cause a problem even if the liquid is permeable, and that it is relatively easy to increase the size, and the problem (3) is solved. Since the solution chamber does not exist, and therefore the hydraulic pressure is not changed in the height direction, the problem (4) cannot naturally occur. Further, since the generated sodium hydroxide inevitably moves to the oxygen supply side through the inside of the electrode, the problem (7) is less likely to occur.
In this way, attempts to adapt the gas diffusion electrode to an industrial electrolytic system are continuously made, various improvements have been made, and results have been achieved. However, when an existing electrolytic cell having a height of 1 m is used, the original electrolytic performance cannot be sufficiently obtained even with the gas diffusion electrode having the above-described structure. The reason for this is that, in addition to the alkaline solution that moves to the oxygen supply side, the liquid that has moved in the height direction due to gravity stays inside the electrode, so that the gas supply is hindered.
In order to eliminate this drawback, attempts have been made to promote the detachment of the electrolytic solution from the electrolytic surface by providing irregularities on the gas diffusion electrode or attaching a bowl-shaped guide plate at a certain height. In particular, in the case of a large electrolytic cell, sufficient liquid detachment cannot be achieved and the voltage tends to increase, and there remains a problem that a complete solution has not been presented.
[0011]
OBJECT OF THE INVENTION
The present invention has the above-mentioned problems of the prior art, that is, a gas diffusion electrode type electrolysis, for example, a zero gap type salt electrolysis in which an oxygen gas diffusion electrode is brought into close contact with an ion exchange membrane and a hydrogen gas diffusion electrode 3 Solves the problem that the gas supply to the surface of the gas diffusion electrode is not smooth in salt separation and hydrogen peroxide generation electrolysis using a chamber method electrolytic cell, and can produce sodium hydroxide, hydrogen peroxide, etc. under low electrolysis voltage An object of the present invention is to provide a gas diffusion electrode structure and a manufacturing method thereof.
[0012]
[Means for solving problems]
The gas diffusion electrode structure according to the present invention comprises a hydrophilic porous layer, a liquid formed on one side of the hydrophilic porous layer, a gas permeable electrode material layer, and an ion exchange closely contacting the other side of the hydrophilic porous layer. A gas diffusion electrode structure comprising a membrane, wherein the method for producing a gas diffusion electrode structure according to the present invention comprises a paste containing an electrode substance on at least a part of one side of a hydrophilic porous layer. It is applied, heated and baked to form a liquid and gas permeable electrode material layer on one side of the hydrophilic porous layer, and an ion exchange membrane is adhered to the other side of the hydrophilic porous layer. In the method for producing a gas diffusion electrode structure, the hydrophilic porous layer may be sintered or baked simultaneously with the electrode material layer.
[0013]
The present invention will be described in detail below.
Conventionally, application of gas diffusion electrodes to industrial electrolysis such as salt electrolysis has been studied and reported. For example, in an electrolytic cell in which the cathode chamber is partitioned into a solution chamber and a gas chamber by an oxygen gas diffusion cathode, the liquid resistance due to the liquid between the ion exchange membrane and the cathode is so large that it cannot be ignored.
The zero gap type in which the ion exchange membrane and the cathode are in close contact is a technique developed to reduce this liquid resistance. For example, in the case of salt electrolysis, the cathode reaction described above: 2H₂O + 2e → 4OH⁻+ H₂Is generated at the interface between the ion exchange membrane and the cathode, and the produced sodium hydroxide passes through the oxygen gas diffusion cathode as a solution and is taken out from the gas phase side of the cathode. In this case, since the flow direction of sodium hydroxide is opposite to the flow direction of the oxygen-containing gas, the solution stays in the oxygen electrode or the gas supply rate is slow.
[0014]
For example, when the oxygen gas diffusion cathode is used for salt electrolysis and when the gas generating electrode is used for salt electrolysis, the increase in electrolysis voltage with respect to the increase in current density is about 1.5 to 2 times that of the latter in the former. It is known. This is regarded as a characteristic of the oxygen gas diffusion cathode, and it has been found that the main factor is not the type of reaction but the overvoltage other than the electrode reaction. One of the causes of the overvoltage rise is a shortage of supply gas to the oxygen gas diffusion cathode. For example, in the case of salt electrolysis, the former has a higher overvoltage of about 200 mV when the gas source is air and pure oxygen. It is known. In addition, although the overvoltage becomes lower as the supply amount is increased, the product take-out is hindered, and a smooth gas supply cannot be achieved after all.
[0015]
In the present invention, in addition to the case where this oxygen gas diffusion cathode is used for salt electrolysis, the hydrogen gas diffusion anode is used for salt separation in the three-chamber method, and the gas diffusion electrode is used for electrolytic production of hydrogen peroxide. Another object of the present invention is to provide a gas diffusion electrode structure used in an electrolytic cell that can smoothly take out a solution containing an electrolytic product and supply of a raw material gas, and a method for manufacturing the same. The possibility of realizing the industrial electrolytic cell to be used is increased.
In the present invention, a hydrophilic porous layer is provided between the ion exchange membrane and the electrode material layer of the zero gap type electrolytic cell in which the ion exchange membrane and the electrode material layer of the gas diffusion electrode are installed in close contact, and the ion exchange membrane-hydrophilic property is provided. A gas diffusion electrode structure is constituted by the porous layer-electrode material layer. The hydrophilic porous layer is formed by extracting all or part of a solution in which sodium hydroxide or hydrogen peroxide formed in an ion exchange membrane is dissolved, around the electrode chamber through the hydrophilic porous layer, particularly in the lower part. The residence time between the ion exchange membrane and the electrode material layer is shortened, thereby smoothly supplying the source gas such as oxygen-containing gas or hydrogen-containing gas from the back surface of the electrode material layer. Therefore, according to the present invention, the gas diffusion electrode can be applied to industrial electrolysis by performing the operation with different directions of the smooth extraction of the solution in which the product is dissolved and the smooth supply of the raw material gas at the maximum efficiency and reducing the electrolysis voltage more than before. It is possible to open the way to do.
This gas diffusion electrode structure can be used as an oxygen gas diffusion cathode for salt electrolysis, a hydrogen gas diffusion anode for salt separation, a gas diffusion electrode for hydrogen peroxide production, and is useful as a gas diffusion electrode for various electrolysis. is there.
[0016]
From the viewpoint of liquid resistance, it is preferable that nothing exists between the ion exchange membrane and the electrode material layer. Therefore, it is better not to insert the hydrophilic porous layer of the present invention between the two. The electrolysis voltage will increase. However, there is no necessity that the ion exchange membrane and the electrode need to be in close contact except when an ion exchange membrane such as pure water electrolysis is used as the solid electrolyte, and it is more than an increase in electrolytic voltage due to the insertion of the hydrophilic porous layer. If the effect appears, energy saving as a whole can be achieved.
The present invention aims exactly at this effect, and by taking out the above solution through the hydrophilic porous layer, smoothing of the gas supply is achieved. To reduce energy consumption as a whole and to save energy.
[0017]
Further, if the hydrophilic porous layer is a continuous liquid layer, a pressure difference applied to the electrode material layer in the height direction of the liquid layer is generated, which may be a bottleneck in increasing the size.
The gas diffusion electrode structure according to the present invention is composed of an ion exchange membrane, a hydrophilic porous layer, and an electrode material layer as described above. For example, the structure is used as an oxygen gas diffusion cathode of a salt electrolytic cell and is in close contact with the ion exchange membrane. And the cathode chamber does not have a solution chamber, the gas pressure is equally applied to the back side of the oxygen gas diffusion cathode, and the solution is substantially free from the hydrophilic porous layer. Since it is reasonable to think that it is in the form of a liquid film interrupted in the middle of the hydrophilic porous layer instead of being continuously extracted as a droplet, The oxygen gas diffusion cathode does not receive a pressure change in the height direction. The same applies when the gas diffusion electrode structure of the present invention is applied to a hydrogen gas diffusion anode of a three-chamber electrolytic cell or a gas diffusion electrode for hydrogen peroxide production.
[0018]
The electrode material layer used in the present invention can be used while taking advantage of the characteristics of the conventional gas diffusion electrode. For example, materials such as wire mesh, powder sintered bodies, metal fiber sintered bodies, and foams made of corrosion resistant materials such as titanium, niobium, tantalum, stainless steel, nickel, zirconium, carbon, and silver are pretreated and washed as necessary. Conductive graphite, carbon black powder, or metal carrying a metal such as platinum, palladium, ruthenium, iridium, copper, silver, cobalt, lead or oxides thereof on the surface of the electrode material layer A silver powder alone or a ceramic such as titanium oxide carrying platinum or a platinum group metal alloy is kneaded with a fluorine resin as a binder to form a paste, and this paste is heated at 150 to 300 ° C. or baked by hot pressing to form an electrode material. Layer. The kneaded material may be applied and baked on the surface of the wire mesh or the like. In order to further increase the porosity of the electrode material layer, a compound that decomposes or volatilizes by heating, such as alcohol or ethylene glycol, may be added to the paste. Of course, a foaming agent may be added instead of such a decomposable or volatile substance.
[0019]
In order to perform the mass transfer of the reaction gas quickly, it is preferable to disperse and carry a hydrophobic material on the electrode substance layer or the current collector. The hydrophobic material is preferably fluorinated pitch, fluorinated graphite, fluororesin, or the like. In particular, the fluororesin is preferably fired at a temperature of 200 to 400 ° C. in order to obtain uniform and good performance. The particle size of the fluorine component powder is preferably 0.005 to 100 μm. It is desirable that the hydrophobic and hydrophilic portions are continuous along the electrode cross-sectional direction.
From the viewpoint of corrosion resistance and economy, it is desirable to apply noble metal plating, particularly silver plating, to the electrode material layer. The hydrophobic silver plating bath is, for example, an aqueous solution of 10 to 50 g / liter of silver thiocyanide, 200 to 400 g / liter of potassium thiocyanide, 10 to 200 g / liter of PTFE particles, and 10 to 200 g / (surfactant). g / PTFE), and the current density is 0.2-2 A / dm at room temperature with moderate stirring.²Electrodeposit with. When the plating thickness is 1 to 300 μm, good water repellency and corrosion resistance are exhibited. After plating, it is preferable to thoroughly wash with acetone or the like.
[0020]
In the present invention, since the hydrophilic porous layer located between the ion exchange membrane and the gas diffusion electrode does not contribute to the movement of electrons, it does not have to be conductive. Although the material is not particularly limited, for example, in salt electrolysis, it is necessary to have sufficient resistance because it contacts high concentration sodium hydroxide at about 100 ° C. When used as an anode for salt separation, the potential is almost the same, but the electrolyte solution often becomes a strong acid, so it is necessary to be an acid-resistant material. Furthermore, since the ion exchange membrane installed in the electrolytic cell is not necessarily a perfect plane, it is desirable that the electrode material layer also has the flexibility to deform to some extent along it, in other words, it deforms when pressure non-uniformity occurs. It is desirable that the material absorbs the pressure.
Examples of the material of the hydrophilic porous layer include metal oxides such as zirconium oxide, silicon oxide and titanium oxide, ceramics such as carbon and silicon carbide, hydrophilic resins such as PTFE and EEP, nickel, stainless steel and silver. In the case of a material other than the above-described resin, it is baked with a fluororesin of 30% or less as a binder, and pretreated and washed as necessary to form a hydrophilic porous layer.
[0021]
The hydrophilic porous layer preferably has a thickness of 0.01 to 10 mm, preferably 0.1 to 1 mm, and is preferably a material and a structure that can always hold an electrolyte solution. For example, the structure includes a net, a woven fabric, a non-woven fabric, and a foam. In particular, a sintered plate or a sintered plate in which pore-forming particles are removed with a solvent after being formed into a sheet with a pore-forming agent and various binders using powder as a raw material. The thing which piled up is preferable.
The average porosity of the powder constituting the hydrophilic porous layer is not limited to this, but is 50 to 90%, preferably 70 to 90%. If the porosity exceeds 90%, the physical strength becomes weak, causing problems in handling. If the porosity is less than 50%, the water retention and water permeability are lowered, and the electric resistance during electrolysis is further increased. Although depending on other conditions, the average particle size of the hydrophilic porous layer material for setting the porosity to 50 to 90% is about 5 to 10 μm, and the particle size distribution is preferably as small as possible.
[0022]
The thickness of the hydrophilic porous layer may be adjusted depending on the amount of the electrolyte flowing from the ion exchange membrane toward the electrode material layer, that is, the amount of the electrolyte to be removed by the hydrophilic porous layer. When the thickness is increased to 500 to 1000 μm, it is preferable that the average particle diameter of the particles used is correspondingly increased. If the hydrophilic porous layer is thick, the electrolysis voltage may increase and the power consumption rate may deteriorate.
Further, if the hydrophilic porous layer is composed only of the aforementioned metal oxide particles and binder, the strength may be insufficient and handling may be inconvenient. In this case, metal foam, metal mesh, carbon fiber, fluororesin, or the like is used as a core material, and a porous layer made of the particles and the binder may be formed on both surfaces of the core material. However, this core material also needs the characteristics described with respect to the material of the hydrophilic porous layer described above. For example, when used as an oxygen gas diffusion cathode for salt electrolysis, metallic silver, silver-plated copper or nickel is used. However, when used as a hydrogen gas diffusion anode for salt separation, it is desirable to use acid-resistant titanium, zirconium or hastelloy. Since this core material does not contribute to the movement of electrons as well as the hydrophilic porous layer, it may be conductive or non-conductive, but it is preferable that the action as an electrode is made as small as possible.
[0023]
In order to produce a hydrophilic porous layer having such a core material, the above-mentioned raw material powder and fluororesin kneaded material are applied to both sides of the core material, and hot at a temperature of about 200 to 350 ° C., for example, with slight pressure. It can be molded by pressing.
In order to dispose this hydrophilic porous layer between the ion exchange membrane and the electrode material layer, the water pressure difference between the ion exchange membrane and the electrode material layer, depending on the height of the electrolyte, is 0.1 to 30 kgf / cm.²They can be integrated at a certain pressure. Alternatively, the hydrophilic porous layer may be formed in advance on the membrane-side surface of the electrode material layer or the electrode material layer-side surface of the ion exchange membrane, and the ion exchange membrane and the electrode material layer may be placed in contact with each other at a predetermined position. good.
However, the electrode material layer having liquid and gas permeability is coated with a paste containing a kneaded product of the electrode material, which is a component constituting the electrode material layer, on at least a part of one surface of the hydrophilic porous layer, and heated. It is desirable to form by baking. This heating can be performed at 200 to 350 ° C. in air, for example. Although baking with heat is possible even below 200 ° C., the compound to be added should be more easily decomposed or volatilized. When the heating temperature exceeds 350 ° C., the fluororesin serving as a binder starts to decompose, so that a treatment in a short time is required, and at high temperatures, sintering and inactivation of the catalyst particles tend to proceed. As described above, the conditions for forming the electrode material layer by applying the paste need to be appropriately set according to the type of the electrode material and the type of the binder.
[0024]
Further, the hydrophilic porous layer is a hydrophilic porous layer precursor before heating and sintering comprising the aforementioned porous layer forming particles and a fluororesin as a binder, and an electrode material layer is formed on at least a part of one side of the precursor as described above. The paste containing the kneaded product of the electrode material, which is a component constituting, is heated and sintered to sinter the hydrophilic porous layer and the electrode material layer in a single heat treatment operation, Workability can also be improved.
The electrode material layer with the hydrophilic porous layer configured as described above is always placed in contact with the ion exchange membrane when accommodated in the electrolytic cell. From the ion exchange membrane-hydrophilic porous layer-electrode material layer The gas diffusion electrode structure is formed.
[0025]
When the gas diffusion electrode structure of the present invention is used for salt electrolysis, a fluororesin-based cation exchange membrane is preferable as an ion exchange membrane from the viewpoint of corrosion resistance. As the anode, it is desirable to use an insoluble electrode made of titanium called ordinary DSA, and other electrodes can be used.
The electrolysis conditions are, for example, a temperature of 60 to 90 ° C., a current density of 10 to 100 A / dm.²Preferably, the supplied oxygen-containing gas is humidified as necessary. As a humidifying method, a humidifier humidified at 70 to 95 ° C. is provided at the electrolytic cell inlet, and the oxygen-containing gas is controlled to pass through. In terms of the performance of membranes currently on the market, if the concentration of the anolyte is kept below 200 g / liter, especially around 170 g / liter, humidification of the oxygen-containing gas becomes unnecessary. The concentration of sodium hydroxide obtained is suitably about 25 to 40%, but is basically determined by the performance of the ion exchange membrane.
[0026]
When salt electrolysis is performed using the electrolytic cell in which the gas diffusion electrode structure of the present invention is installed, sodium hydroxide mainly produced near the ion exchange membrane side surface of the oxygen gas diffusion cathode passes through the hydrophilic porous layer, that is, oxygen. It can be extracted without passing through the gas diffusion cathode. At this time, if the hydrophilic porous layer is in a seed form, the sodium hydroxide cannot be extracted unless it reaches the periphery thereof, and it may take a relatively long time to extract. In order to solve this problem, in the present invention, for example, the sheet is divided into a plurality of sheets, and one end of each divided sheet is formed from these gaps of the oxygen gas diffusion cathode in which, for example, a slit or guide having a width of 1 to 5 mm is formed. If it arrange | positions so that it may reach a back surface, before the production | generation sodium hydroxide reaches | attains a periphery, it will be extracted from between an ion exchange membrane and an oxygen gas diffusion cathode in a short time.
[0027]
FIG. 3 is a longitudinal sectional view showing an example of an electrolytic cell for salt electrolysis using an oxygen gas diffusion cathode according to the present invention.
The electrolytic cell body 11 is divided into an anode chamber 13 and a cathode chamber 14 by an ion exchange membrane 12, and a mesh-like insoluble anode 15 is in close contact with the anode chamber 13 side of the ion exchange membrane 12. A sheet-like hydrophilic porous layer 16 is in close contact with the cathode chamber 14 side, and a liquid-permeable oxygen gas diffusion cathode 17 is in close contact with the cathode chamber side of the hydrophilic porous layer 16. A mesh-like cathode current collector 18 is connected and supplied with power by the current collector 18.
Reference numeral 19 denotes an anolyte (saturated saline) inlet formed on the side wall near the bottom of the anode chamber, 20 denotes an anolyte (unreacted saline) and chlorine gas outlet formed on the side wall near the top of the anode chamber, 21 Is a (humidified) oxygen-containing gas inlet formed on the side wall near the top of the cathode chamber, and 22 is an outlet for sodium hydroxide and excess oxygen formed on the side wall near the bottom of the cathode chamber.
[0028]
When a saturated saline solution as an anolyte is supplied to the anode chamber 13 of the electrolytic cell 11 and an oxygen-containing gas such as pure oxygen or air is supplied to the cathode chamber 14 and energized between the electrodes 15 and 17, ion exchange is performed. Sodium hydroxide is generated on the surface of the membrane 12 on the cathode chamber 14 side. In a normal electrolytic cell, this sodium hydroxide passes through the oxygen gas diffusion cathode as an aqueous solution and reaches the cathode chamber side surface. However, in the illustrated electrolytic cell 11, since the hydrophilic porous layer 16 exists between the ion exchange membrane 12 and the oxygen gas diffusion cathode 17, the resistance of the aqueous sodium hydroxide solution is smaller than that passing through the cathode 17. The inside of the hydrophilic porous layer 16 is dispersed, in particular, descends due to gravity, reaches the lower end of the hydrophilic porous layer 16 and drops as a droplet to the bottom of the cathode chamber 14 and stored.
When this electrolytic cell is compared with the conventional electrolytic cell of FIG. 2 and the like, in the conventional electrolytic cell of FIG. 2, the sodium hydroxide aqueous solution to be produced must permeate through the dense oxygen gas diffusion cathode. The residence time in the chamber becomes longer, the smooth permeation of the supplied oxygen-containing gas is hindered, and the gas supply for rate-limiting the reaction becomes insufficient, so that the apparent cathode overvoltage increases and the cell voltage increases. In contrast, in the electrolytic cell shown in FIG. 3, since the aqueous sodium hydroxide solution produced is taken out from the reaction site by dispersion in the hydrophilic porous layer having a relatively low resistance and hardly stays in the cathode, the reaction gas is supplied. Is carried out smoothly and thus a low voltage is maintained.
[0029]
FIG. 4 is a perspective view of the essential part of a part of the electrolytic cell of FIG. 3 in which the aqueous sodium hydroxide solution can be taken out more smoothly. FIG. 4a is an example in which the cathode is divided into a plurality of parts, and FIG. Shows an example in which slits are formed.
In FIG. 4a, the oxygen gas diffusion cathode 17a is divided into a plurality of cathode pieces 17b, and the hydrophilic porous layer 16a is also divided into a corresponding number of hydrophilic porous layer pieces 16b. The lower end of each hydrophilic porous layer piece b is bent in the direction of the cathode 17b and passes between the upper and lower cathodes 17b and reaches the back surface of the cathode 17b to form a bent piece 16c.
[0030]
When electrolysis is performed using this electrolytic cell, the aqueous sodium hydroxide solution produced on the cathode chamber side surface of the ion exchange membrane passes through the hydrophilic porous layer piece 16b as in the case of the electrolytic cell of FIG. To Penetrate. Since the hydrophilic porous layer 16b is divided, the aqueous solution of sodium hydroxide does not move to the peripheral portion, but moves within each hydrophilic porous layer piece 16b through a relatively short distance to the lower end portion thereof, so that the cathode 17b. It falls as a droplet from the bent piece 16c bent in the direction. Therefore, the liquid can be drained more smoothly than the electrolytic cell of FIG.
FIG. 4B shows an example in which a horizontally long rectangular slit 23 is formed in the cathode 17c without dividing the cathode into a plurality of pieces. When the cathode is divided into a plurality of pieces as shown in FIG. 4A, it is necessary to supply power to each divided piece, which is troublesome. However, as shown in FIG. 4B, a slit 23 is formed in the cathode 17c, and the cathode 16b is bent through the slit 23. If the piece 16c is located on the back surface of the cathode, it is more convenient because the power supply to the cathode can be performed by a single current collector.
[0031]
【Example】
Next, although the manufacturing method of the gas diffusion electrode structure concerning this invention and the Example of salt electrolysis and salt separation which use this gas diffusion electrode structure are described, this Example does not limit this invention.
[0032]
[Example 1]
Fluorine containing a carbon fiber woven fabric having an apparent thickness of 0.2 mm as a core material, zirconium oxide powder having an average particle diameter of 7 μm on both sides of the core material, and a fluorine resin corresponding to 30% by weight of the zirconium oxide as a solid content A kneaded product of DuPont 30J (binder), which is an aqueous dispersion of resin, is applied to produce a plate-like body having an apparent thickness of 0.5 mm, and 1 kg / cm is applied to the plate-like body.²The mixture was sintered by hot pressing at 250 ° C. for 15 minutes while applying a pressure of 2 to obtain a hydrophilic porous layer.
After applying a dispersion of a catalytic substance obtained by baking platinum on the surface of silver particles having an average particle diameter of 1 μm on one surface of the hydrophilic porous layer, the apparent thickness of the silver particles having an average particle diameter of 3 μm is set to a thickness of 0. It was applied to 1 mm (platinum loading was 3 g / m²). This is 1kg / cm²The electrode material layer was produced on one side of the hydrophilic porous layer by hot pressing at 200 ° C. for 15 minutes while applying the pressure of
[0033]
The hydrophilic porous layer of the electrode material layer with the hydrophilic porous layer has an electrolysis area of 100 cm.²A cathode was constructed by placing it in close contact with an ion exchange membrane that is Nafion 961 manufactured by DuPont in a cathode chamber of a two-chamber small ion exchange membrane electrolytic cell (10 cm × 10 cm). The cathode chamber current collector was a 1 mm thick copper perforated plate with silver plated on the surface. As an anode, an insoluble metal electrode made of an expanded mesh substrate made of titanium coated with a ruthenium oxide electrode material was used, and the ion exchange membrane was sandwiched together with the cathode. A drain was provided in the cathode chamber, and all the produced catholyte was drawn downward.
180 g / liter of saline was circulated in the anode chamber, and the sodium hydroxide by sodium chloride electrolysis was supplied to the cathode chamber while supplying oxygen 90% oxygen enriched by PSA method 1.2 times the theoretical amount. When manufactured, the current density is 30 A / dm.²The electrolytic voltage was 2.01 V, and stable electrolysis was possible even after continuous operation for 100 hours or more.
[0034]
[Comparative Example 1]
After applying a dispersion of a catalyst material obtained by baking platinum on the surface of silver particles having an average particle diameter of 1 μm on the surface of the carbon fiber fabric used in Example 1, silver particles having an average particle diameter of 3 μm are seen with the 30J as a binder. The electrode material layer was coated to a thickness of 0.1 mm (platinum loading was 3 g / m²).
When electrolysis was performed under the same conditions as in Example 1 except that this electrode material layer was used as a cathode and directly adhered to the ion exchange membrane, the initial voltage was 2.0 V, but the voltage gradually increased. After 8 hours, it increased by 100 mV. When the electrolysis was interrupted and resumed 30 minutes later, the voltage returned to 2.0 V, and gradually increased again over time. This can be inferred from the fact that the electrolyte that has permeated the back surface of the electrode material layer inhibits oxygen supply.
[0035]
[Example 2]
The plate-like body having an apparent thickness of 0.5 mm in Example 1 was dried at 60 ° C. to obtain a hydrophilic porous layer.
On one side of this hydrophilic porous layer, a paste composed of ultrafine silver powder with an average particle size of 0.3 μm, dextrin and ethylene glycol was applied by a doctor blade method so as to have an apparent thickness of 3 μm, and dried. A kneaded product of silver particles having a particle diameter of 7 μm and fluororesin dispersion 30J made by DuPont was applied, and 1 kg / cm²The electrode material layer was formed on one side of the hydrophilic porous layer by hot pressing at 300 ° C. for 30 minutes while applying a pressure of 1 to obtain an electrode material layer with a hydrophilic porous layer.
The hydrophilic porous layer of the electrode material layer with the hydrophilic porous layer was placed in the cathode chamber of a two-chamber ion exchange membrane electrolytic cell so as to be in close contact with the ion exchange membrane which is Nafion 961 manufactured by DuPont, thereby constituting a cathode. . An insoluble metal electrode coated with a ruthenium oxide electrode material was used as the anode, and the ion exchange membrane was sandwiched together with the cathode.
A 180 g / liter saline solution is circulated in the anode chamber, and an oxygen gas saturated with water vapor is sent to the cathode chamber at a temperature of 90 ° C. and a current density of 30 A / dm.²As a result of electrolysis, the electrolysis voltage was 2.05 V, and stable electrolysis could be continued.
[0036]
[Example 3]
The electrode material layer of 10 cm × 10 cm used in Example 1 is left as it is, and the length of the hydrophilic porous layer in close contact therewith is extended 1 cm downward to 11 cm, and the remaining length of the lower end is set to 11 cm. It was drawn out to the opposite side of the ion exchange membrane to make a liquid conducting part, and led to the back side through the gap of the current collector. Ten electrode material layers with a hydrophilic porous layer were prepared, connected in the vertical direction, and incorporated into a two-chamber ion exchange membrane electrolytic cell having a height of 100 cm.
When electrolysis was performed under the same conditions as in Example 1 except that the electrode material layer with a hydrophilic porous layer having such a structure was used, the current density was 30 A / dm.²The electrolytic voltage is 2.00 V, and it is observed that the electrolytic solution is dripped to the outside of the electrode material layer through the liquid introduction part, and directly passes through the electrode material layer to the back side of the electrode material layer. There was almost no oozing and the amount was estimated to be less than 20% of the total liquid.
No voltage change was observed even after 100 hours of continuous electrolysis, and the current density was 40 A / dm.²The electrolytic voltage increased to 2.15 V when the voltage was increased to 2, but no change in voltage over time was observed.
[0037]
[Example 4]
A mesh prepared by weaving a titanium wire having a wire diameter of 0.1 mm is used as a core material, and a paste prepared by kneading titanium oxide powder having an average particle size of 10 μm and a fluororesin dispersion 30J is applied to the entire surface and baked at 300 ° C. A hydrophilic porous layer was formed. The porosity of the hydrophilic porous layer was about 80%.
One side of the hydrophilic porous layer was coated with an ethylene glycol solution of chloroplatinic acid and kept at 300 ° C. for 30 minutes in a hydrogen stream to precipitate platinum metal.An electrode material layer was obtained.
[0038]
This electrode material layer with a hydrophilic porous layer was used as an anode of a salt separation apparatus by a three-chamber method as follows. That is, as the anode of an electrolytic cell in which the anode chamber and the intermediate chamber are partitioned by an anion exchange membrane, and the intermediate chamber and the cathode chamber are partitioned by a cation exchange membrane, the hydrophilic porous layer is formed of the anion exchange membrane. Nickel mesh was used as a cathode as a counter electrode.
A sodium nitrate aqueous solution is supplied to the intermediate chamber, hydrogen generated in the cathode chamber and hydrogen corresponding to 10% by weight are supplied from the cylinder to the anode chamber, and dehydration water is added dropwise to the cathode chamber to generate the hydroxide. While adjusting the sodium concentration to be maintained at 15%, the temperature is 45 ° C., the current density is 20 A / dm²The water was added to the anode chamber to adjust the nitric acid concentration to 10%. By this electrolysis, stable salt separation proceeded at an electrolytic voltage of 2.4 V, and nitric acid was obtained in the anode chamber and sodium hydroxide was obtained in the cathode chamber with a current efficiency of 80%.
[0039]
[Comparative Example 2]
When electrolysis was performed under the same conditions as in Example 4 except that an oxygen-generating insoluble metal electrode was used as the anode, the electrolysis voltage was 3.9 V.
[0040]
[Comparative Example 3]
When electrolysis was performed under the same conditions as in Example 4 except that an electrode material layer that did not form a hydrophilic porous layer was used, some of the nitric acid was decomposed or the anode voltage was unstable and the maximum current Density is 10A / dm²Electrolysis was not possible beyond that.
[0041]
[Example 5]
A mesh made by weaving a titanium wire having a wire diameter of 0.1 mm is used as a core material, and a silica powder having an average particle size of 5 μm, a silicon foam material and a fluororesin water dispersant are applied to both sides and dried. To obtain a hydrophilic porous layer.
A kneaded product of hydrophilic carbon black and fluororesin water dispersant 30J is molded into a flat shape, and an ethylene glycol solution of chloroplatinic acid is applied to one side of the molded body and kept at 300 ° C. for 30 minutes in a hydrogen stream. Was used to deposit platinum metal to form an electrode material layer. Adhering the hydrophilic porous layer to the platinum deposition side of the electrode material layer, 1 kg / cm²An electrode material layer was prepared on one surface of the hydrophilic porous layer by applying hot pressing at 250 ° C. while applying a pressure of 1 to obtain an electrode material layer with a hydrophilic porous layer. The porosity of this hydrophilic porous layer was about 85%.
[0042]
This electrode material layer with a hydrophilic porous layer was used as an anode of a three-chamber salt separation electrolysis apparatus, and an apparatus similar to Example 4 was constructed.
200 g / liter of sodium sulfate was supplied to the intermediate chamber, and deionized water was supplied to the cathode chamber so that the catholyte (sodium hydroxide) concentration was 100 g / liter. Further, as the anode supply gas, a gas obtained by re-saturating water with cathode-generated hydrogen was used. Temperature 45 ° C, current density 10A / dm²When electrolysis was performed, the electrolysis voltage was 2.75 V, the current efficiency was 85%, and stable operation could be continued even after 100 hours of continuous electrolysis.
[0043]
【The invention's effect】
The gas diffusion electrode structure of the present invention includes a hydrophilic porous layer, a liquid formed on one side of the hydrophilic porous layer, a gas-permeable electrode material layer, and an ion exchange membrane that adheres to the other side of the hydrophilic porous layer. A gas diffusion electrode structure comprising:
In a conventional electrolytic cell using a gas diffusion electrode, particularly a zero gap type electrolytic cell in which the gas diffusion electrode is in close contact with the ion exchange membrane, a target product generated on the surface of the ion exchange membrane passes through the gas diffusion electrode having a relatively high density. In other words, it must pass through the gas diffusion electrode in a direction opposite to the supply direction of the reaction gas to be supplied, in other words, while inhibiting the supply of the reaction gas, and as the product increases, the reaction gas reaches the reaction site. There is a problem in that the supply voltage is hindered and the electrolysis voltage rises.
[0044]
On the other hand, in the gas diffusion electrode structure of the present invention, since the hydrophilic porous layer is disposed between the electrode material layer and the ion exchange membrane, almost all of the conventional materials must be taken out through the gas diffusion electrode. The product such as sodium hydroxide aqueous solution that has not been passed through the hydrophilic porous layer without passing through the gas diffusion electrode (electrode material layer) can be taken out from the surface of the ion exchange membrane without facing the supply direction of the reaction gas. it can. Therefore, even if the amount of the product increases, the reaction gas supply is hardly affected, and the predetermined electrolytic reaction can be continued while the voltage is kept low.
The hydrophilic porous layer can be produced by baking a kneaded paste of the porous layer-constituting particles and a binder, but this alone may not provide sufficient strength. In this case, the paste is applied to both sides of the core using a core. By applying and heating, a gas diffusion electrode structure with high mechanical strength can be obtained.
[0045]
In the method of the present invention, a paste containing an electrode material is applied to at least a part of one surface of a hydrophilic porous layer, and a liquid and gas permeable electrode material layer is formed on one surface of the hydrophilic porous layer by heating and baking. It is a method for producing a gas diffusion electrode structure, characterized in that an ion exchange membrane is adhered to the other surface of the hydrophilic porous layer.
The gas diffusion electrode structure according to the present invention described above may be configured by mechanically adhering a separately prepared hydrophilic porous layer and an electrode material layer by applying pressure. When the electrode substance-containing paste is applied to the surface of the porous porous layer and heated, the two are strongly bonded and the mechanical strength is enhanced. Furthermore, since the hydrophilic porous layer and the electrode material layer are integrated, transportation and the like are simplified, and handling is facilitated.
Furthermore, the hydrophilic porous layer and the electrode material layer are sintered at a time by applying a paste to at least a part of one surface of the hydrophilic porous layer precursor before sintering and heating and sintering in the same manner as described above. This can be performed by a heat treatment operation, thereby improving workability.
[Brief description of the drawings]
FIG. 1 is a schematic view showing an example of a conventional salt electrolyzer.
FIG. 2 is a schematic view showing another example of a conventional salt electrolyzer.
FIG. 3 is a longitudinal sectional view showing an example of an electrolytic cell for salt electrolysis using a gas diffusion electrode structure according to the present invention.
FIG. 4 is a longitudinal sectional view showing another example of an electrolytic cell for salt electrolysis using the gas diffusion electrode structure according to the present invention, FIG. 4a shows an example in which a plurality of cathodes are disassembled, and FIG. An example is shown.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... Electrolyzer main body 12 ... Ion exchange membrane 13 ... Anode chamber 14 ... Cathode chamber 15 ... Insoluble anode 16 ... Hydrophilic porous layer 17 ... Oxygen gas diffusion cathode
18 ... current collector

Claims (4)

A gas comprising a hydrophilic porous layer, a liquid formed on one surface of the hydrophilic porous layer, a gas-permeable electrode material layer, and an ion exchange membrane that is in close contact with the other surface of the hydrophilic porous layer. Diffusion electrode structure. The gas diffusion electrode structure according to claim 1, wherein the hydrophilic porous layer has a core material. Applying a paste containing an electrode substance on at least a part of one side of the hydrophilic porous layer, heating and baking to form a liquid and gas permeable electrode substance layer on one side of the hydrophilic porous layer, and A method for producing a gas diffusion electrode structure, comprising: adhering an ion exchange membrane to the other surface of a hydrophilic porous layer. The hydrophilic porous layer precursor is coated with a paste containing an electrode material on at least a part thereof and heated to sinter or bake the hydrophilic porous layer and bake the electrode material layer at the same time. A method for producing a gas diffusion electrode structure, comprising forming a liquid and gas permeable electrode material layer on one side of a layer and adhering an ion exchange membrane to the other side of the hydrophilic porous layer.

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