JP4832670B2 - High ion conductive solid electrolyte and electrochemical system using the solid electrolyte - Google Patents

Description

【００３４】
ポリビニルアルコールに対してＳｉＯ_２換算重量比０．０３以上の珪酸化合物を複合化しているこれらの試料は何れも７０℃以上の熱水中で１時間洗浄しても溶解せず、耐水性に優れている。比較のため珪酸化合物を含まないポリビニルアルコールのみの試料も同様な方法で成膜してみたが、熱水中ですぐに溶解してしまう耐水性の低いものしかできなかった。これらの結果からわかるとおり珪酸化合物との複合化はポリビニルアルコールの耐水性を顕著に向上させる。
【００３５】
一方、ポリビニルアルコールに対して珪酸化合物をＳｉＯ_２換算重量比で３以上加えてみたところ非常に脆くなり、強度の高い膜が得られなかった。従ってポリビニルアルコールに対するＳｉＯ_２換算の珪酸化合物重量比は０．０３〜３であることが必要である。
【００３６】
比較のため上記の作成過程において中和後の水溶液のｐＨを８より高い値にした試料膜を作製したところ、試料は熱水中で洗浄する際に溶解してしまい、複合化による耐水効果が発現されなかった。従って複合化による耐水効果を十分に発現させるためには中和後のｐＨを８以下にする必要がある。
【００３７】
作成した試料膜のイオン伝導度の測定は以下の方法により行った。先ず試料膜を直径３０ｍｍの円形に切り抜き、直径２８ｍｍの２枚の白金円板と、該白金円板の外側に配置した真ちゅうの円板で挟み、更に絶縁されたクリップで挟み込んで固定する。真ちゅうの円板に取り付けたリード線にＬＣＲメータを使って電圧１０ｍＶの交流電圧を周波数５ＭＨｚから５０Ｈｚまで変えながら印加し、電流と位相角の応答を測定した。イオン伝導度は一般的に行われているＣｏｌｅ−Ｃｏｌｅプロットの半円の直径から求めた。尚、この測定は試料を恒温恒湿槽の中に入れて湿度５０℃、相対湿度９０％に制御しながら行った。イオン伝導度の測定結果は表１中に記載している。
【００３８】
これらの試料はプロトンサイトのより完全なプロトン化あるいはアルカリ化の操作を特に行っていなくても多くのものが１０^−７〜１０^−６の比較的高いイオン伝導度を示した。しかし珪酸化合物量の増加とともにイオン伝導度が低下する傾向があり、ＳｉＯ_２換算の珪酸化合物重量の対ポリビニルアルコール比が０．２を超えた場合には、試料Ｎｏ３４のように１０^−８オーダーのきわめて低いイオン伝導度しか示さなかった。従ってＳｉＯ_２換算の珪酸化合物重量の対ポリビニルアルコール比は０．０３〜０．２にすることが好ましい。
【００３９】
上記の製法において原料水溶液中にリン酸ナトリウムの代わりにホウ酸ナトリウム、アルミン酸ナトリウムを添加した結果を表１の試料Ｎｏ１９，２８，２９に示した。なお、表の「ポリビニルアルコールに対する添加物の重量比」欄において記号（Ｂ）はホウ酸塩の添加、記号（Ａ）はアルミン酸塩の添加を意味しており、ホウ酸、アルミン酸化合物量は表中にそれぞれＢ_２Ｏ_３，Ａｌ_２Ｏ_３重量に換算し、ポリビニルアルコールに対する重量比の形で示している。これら結果からホウ酸化合物の添加、アルミン酸化合物の添加はリン酸化合物の添加と同様またはそれ以上のイオン伝導度を示した。
【００４０】
上記の製法において、中和時に添加する酸が塩酸ではなく、０．６Ｍ濃度の硫酸を用いた場合の結果を表１の試料Ｎｏ２に、１．２Ｍ濃度の塩酸か０．６Ｍ濃度の硫酸を添加しｐＨ３まで中和した後、ｐＨ２になるまで８．５重量％のリン酸水溶液を添加した場合の結果を表１の試料Ｎｏ３，４にそれぞれ示した。なお、表の「加えた酸の種類」の欄の記号Ｃは塩酸、記号Ｃ＋Ｐは塩酸＋リン酸、記号Ｓは硫酸、記号Ｓ＋Ｐは硫酸＋リン酸を添加したことを意味する。これら結果からわかるように、添加する酸として硫酸を用いた場合、塩酸を用いた場合よりもイオン伝導度は少し低下するが、その差は大きいものではない。また、塩酸あるいは硫酸をリン酸と合わせて用いた場合には少し高いイオン伝導度を示す。
【００４１】
〔実施例２〕
上記実施例１の方法で作成した電解質膜についてプロトン化を促進する目的で酸浸漬処理を行った。電解質膜を１．２Ｍ濃度の塩酸に常温で５時間浸漬し、その後十分に水洗いした試料の結果を表１の試料Ｎｏ１８に、８５重量％の濃リン酸溶液に浸漬処理した試料の結果を試料Ｎｏ５，７，８，１０，１２，１６，２２，２４，２６，３１に、３０重量％の濃リン酸を含む１．２Ｍ濃度の塩酸に浸漬処理した試料の結果を試料Ｎｏ１７に、濃リン酸浸漬処理を施す前に５Ｍ濃度の水酸化ナトリウム溶液に１時間浸漬する処理を施した試料の結果を試料Ｎｏ２７に示した。なお、表の「処理」欄の記号Ｃは１．２Ｍ濃度塩酸浸漬処理を、記号Ｐは８５重量％リン酸浸漬処理を、記号Ｐ＋Ｃは３０重量％リン酸を含む１．２Ｍ濃度塩酸浸漬処理を、記号Ｎ→Ｐは水酸化ナトリウム溶液浸漬後濃リン酸に浸漬する処理を夫々意味している。これらの結果からわかるとおり、塩酸浸漬処理によってイオン伝導度は向上し、濃リン酸浸漬処理によってイオン伝導度の向上が更に顕著となる。更に３０重量％リン酸を含む１．２Ｍ濃度塩酸の浸漬処理によっても濃リン酸処理に近い結果が得られる。また、酸浸漬処理を施す前にアルカリ溶液浸漬処理を行なうことによってイオン伝導度の向上効果が大きくなることが確認された。
【００４２】
試料Ｎｏ３，５，６，７，９，１０のように酸浸漬処理の効果はリン酸化合物添加量が多いほど効果的であり、特にポリビニルアルコール重量に対するＰ_２Ｏ_５に換算したリン酸化合物の重量比が０．０３５以上の電解質膜では１０^−３〜１０^−４の高いイオン伝導性を示した。
【００４３】
酸浸漬処理とは逆にアルカリ型化を促進するため、５Ｍ濃度水酸化ナトリウム及び５Ｍ濃度水酸化カリウム溶液に浸漬する処理を行った結果を表１の試料Ｎｏ１３，１４，２０，３２，３３に示した。なお、表の「処理」欄の記号Ｎは５Ｍ濃度水酸化ナトリウム溶液浸漬処理を、記号Ｋは５Ｍ濃度水酸化カリウム浸漬処理を意味している。これらの結果ではアルカリ浸漬処理においても大きなイオン伝導度の向上効果が認められ、アルカリ型電解質膜としても機能することが判明した。なお、アルカリ型においても酸型と同レベルのイオン伝導度性を示すことから、アルカリ型においてもアルカリ金属イオンの拡散よりも、酸型同様水分子を媒介としたプロトンの高速イオン拡散が主に行なわれていると考えられる。なお、試料Ｎｏ２０に示されているとおり、電解質がホウ酸化合物を含んでいる方がアルカリ浸漬処理によるイオン伝導度の向上効果が大きく、アルカリ型において高いイオン伝導度が得られる。
【００４４】
リン酸化合物を含む電解質膜の場合は原料水溶液として第３リン酸ナトリウムのかわりに第２リン酸ナトリウムを用いた場合の結果を表１の試料Ｎｏ８に示した。なお、表の「ポリビニルアルコールに対する添加物の重量比」欄において記号（ＨＰ）は第２リン酸塩の使用、記号（Ｐ）は第３リン酸塩の使用を意味している。この結果からわかるとおり、第２リン酸塩を用いると第３リン酸塩を用いた場合よりもイオン伝導がやや低下するが大きな差異はない。
【００４５】
〔実施例３〕
電解質膜作成工程における熱処理条件を変えた例を以下に示す。ポリビニルアルコールに対する珪酸化合物重量比及びリン酸化合物重量比が夫々０．０６５，０．０３８の組成の電解質膜について実施例１の製法の加熱処理条件のみ変えた試料を作成した。表２はポリビニルアルコール／シリカ複合電解質膜のイオン伝導度に対する加熱処理条件の影響の結果を示している。
【００４６】
【表２】

【００４７】
表２によれば、試料Ｎｏ３５のように実施例１と同じ真空中の加熱において、温度を１３０℃から１００℃に下げて３時間加熱処理してもイオン伝導度はほとんど変化しなかった。また、試料Ｎｏ３６のように実施例１と同じ１３０℃で空気中３時間の加熱処理を行った場合には、少しイオン伝導度が低下したが、大きな差異は生じなかった。
【００４８】
更に比較のために全然加熱処理を行わない試料を作成したところ、熱水中での洗浄によって強度がかなり低下してしまった。従って１００℃以上の温度での加熱処理を行なうのが好ましい。加熱雰囲気は空気中でも不活性ガス雰囲気もしくは真空中でも良く、イオン伝導度を維持する観点からは不活性ガス雰囲気もしくは真空中で行うことが好ましい。
【００４９】
〔実施例４〕
ポリマー成分としてポリエチレングリコールあるいは低重合度のポリビニルアルコールを添加した。作成は実施例１と同様に行い、原料水溶液中の平均分子量１２０，０００〜１９０，０００のポリビニルアルコールの一部を平均分子量４００及び７５００のポリエチレングリコールあるいは平均分子量１８，０００〜２６，０００の低分子量ポリビニルアルコールで置き換えた。中和はリン酸及び塩酸で行い、成膜後は実施例２の方法で８５重量％の濃リン酸処理を施した。表３はポリビニルアルコール／シリカ複合電解質膜のイオン伝導度に対するポリマー成分の影響の結果を示しており、前記により得られた電解質膜の組成とイオン伝導度は表３中に記載してある。
【００５０】
【表３】

【００５１】
表３中の「ポリビニルアルコールに対する珪酸化合物重量比」とは、珪酸化合物をＳｉＯ_２として換算した重量／ポリビニルアルコール重量であり、「ポリビニルアルコールに対するリン酸化合物重量比」はリン酸化合物をＰ_２Ｏ_５として換算した重量／ポリビニルアルコール重量、「全ポリマーに対する添加ポリマーの重量％」の欄のＰＥＧ（４００）は平均分子量４００のポリエチレングリコール、ＰＥＧ（７５００）は平均分子量７５００のポリエチレングリコール、ＰＶＡ（１８，０００〜２６，０００）は平均分子量１８，０００〜２６，０００のポリビニルアルコールである。
【００５２】
ポリエチレングリコール（ＰＥＧ）及び低分子量ポリビニルアルコール（ＰＶＡ）を添加したものは、高分子量ポリビニルアルコールのみの場合（前記試料Ｎｏ５）よりもイオン伝導度が向上し、いずれも１０^−３〜１０^−４Ｓ／ｃｍという高い値を示している。但しポリエチレングリコールの量が５０重量％を超えると耐水性が極端に低下してしまい、熱水中での洗浄中に形状を維持することができない。また、低分子量のポリビニルアルコール量が９０重量％を超えると、電解質膜が極めて脆くなってしまうという結果になる。
【００５３】
上記ポリビニルアルコールは完全なものである必要がなく、本質的にポリビニルアルコールとして機能するものであれば使用することができる。例えばヒドロキシル基の一部が他の基で置換されているもの、一部分に他のポリマーが共重合されているものもポリビニルアルコールとして機能することができる。また、本発明の反応過程でポリビニルアルコールを経由すれば同様な効果が得られるので、ポリビニルアルコールの原料となるポリ酢酸ビニルなどを出発原料とすることができる。
【００５４】
珪酸のアルカリ金属塩は水に溶解するものであれば該アルカリ金属塩の種類とかアルカリ金属イオン，珪素，酸素の比率、含水率はどのようなものでも良く、例えば水ガラスとして知られる一般的な珪酸ナトリウムを使用することができる。
【００５５】
本発明における水溶液とは本質的に水が溶媒となっていることを意味しており、水分含有量よりも小量の他の溶媒成分が存在することもある。材料中にリン酸化合物、ホウ酸化合物、アルミン酸化合物を加えることによってイオン伝導性を高めることができるが、これらの化合物の添加は原料水溶液中にリン酸、ホウ酸、アルミン酸のアルカリ金属塩を溶解させることによって行われる。リン酸塩としては第１，第２，第３リン酸塩の何れをも使用することができるが、第１リン酸塩は原料水溶液中に入れる時点で既に珪酸塩を中和してしまうため好ましくない。特に酸型のプロトン伝導性材料を製造する場合には、材料中に多量のリン酸化合物を導入してイオン伝導度を高めるため第３リン酸塩を選択することが良い。ホウ酸塩、アルミン酸塩についてはアルカリ金属イオンの種類とか組成などはどのようなものでも使用することができる。これらの塩は二種以上混合して添加することもできる。
【００５６】
原料水溶液に加える酸の種類は珪酸塩の中和が行えるものであればどのような酸でもよく、塩酸、硫酸、リン酸等が使用可能である。特に酸型のプロトン伝導性材料を製造する場合、材料中により多くのリン酸化合物を導入し、高イオン伝導性を得るには加える酸がリン酸を含んでいることが好ましい。また、添加する酸にホウ酸を含ませることによって材料中にホウ酸化合物を導入することができる。
【００５７】
酸による中和はｐＨが８以下になるまで行う。ｐＨが８よりも高いと珪酸化合物の縮重合反応が十分に進行しないため、生成した複合化合物の耐水性が低くなる。酸によって中和された水溶液は例えば加温乾燥によって溶媒としての水を除去し、薄膜等の所望の形状に加工する。加工後の複合化合物は１００℃以上の温度で加熱処理することによって珪酸化合物自体の縮重合反応やリン酸、ホウ酸、アルミン酸化合物及びポリビニルアルコールとの結合生成を促進し、強度、耐水性、高温安定性を増大することができる。加熱処理を行わない場合には、熱水中で強度が低下する等の問題が生じる。加熱処理における加熱雰囲気は空気中でも不活性ガス雰囲気または真空中でも性能差がなく、何れの加熱雰囲気も採用可能である。空気中での加熱は不活性ガス雰囲気中での加熱よりも若干イオン伝導性が低くなるが、加熱に要する設備が簡単に得られて低コストであるという利点がある。
【００５８】
加熱処理工程の前段あるいは後段で、複合化合物を水などの溶媒で洗浄することにより、該複合化合物中の不要塩を除去する。固体電解質を使用した電気化学システムは何れも電極において酸化還元反応が起きるが、中和過程で酸によって導入される陰イオンのうち固体電解質に固定されていない遊離のイオンが酸化還元反応に悪影響を及ぼすので、遊離のイオンである不要塩は洗浄により除去する必要がある。
【００５９】
ＳｉＯ_２に換算された複合化合物中の珪酸化合物重量のポリビニルアルコール重量に対する比は０．０３〜３の範囲に限定する。重量比が０．０３よりも低いと耐水性が低下し、熱水中で溶解したり強度が極端に低下するという問題が生じる。重量比が３よりも高くなると材料が脆くなり、加工性が低下するという問題が生じる。また、珪酸化合物の比率が増加するとイオン伝導性が低下する傾向があり、その点からは重量比は０．２以下であることが望ましい。
【００６０】
酸型のプロトン伝導性固体電解質を得る場合には、生成した複合化合物を酸に浸漬する処理を施すことによって材料中のプロトンサイトを完全にプロトン化し、プロトン濃度を増やすことによってイオン伝導度を高めることができる。この時に浸漬する酸が３０重量％以上のリン酸を含むことによって材料中に多くのリン酸化合物を導入することができて、より効果的にイオン伝導度を高めることができる。
【００６１】
生成した複合化合物を酸浸漬処理する前段で一旦アルカリ溶液に浸漬する処理を施すことで酸処理の効果を高めることができる。即ち、アルカリ溶液に浸漬することにより、余分の珪酸化合物が除去されるとともに結合の開裂によって新たなプロトンサイトが生じたり、酸処理によるリン酸化合物の導入が促進されるという作用が得られる。更にエッチング効果によって水及びリン酸化合物が導入される微細孔が生じるという利点がある。
【００６２】
アルカリ型のプロトンあるいは水酸化物イオン伝導性固体電解質を得る場合には、生成した複合化合物をアルカリ溶液に浸漬する処理を施すことによってより完全にアルカリ型化され、イオン伝導度を高めることができる。
【００６３】
上記酸あるいはアルカリ浸漬する処理は必ずしも水溶液である必要はない。酸型のプロトン伝導性固体電解質はイオン伝導性が複合化合物中のリン酸化合物量に依存し、Ｐ_２Ｏ_５換算のリン酸化合物重量のポリビニルアルコール重量に対する比を０．０３５以上にすることによって高いイオン伝導性が得られる。
【００６４】
また、ポリビニルアルコールが平均分子量２６，０００以下の低分子量のものを含む場合には高いイオン伝導度を得ることができる。但し平均重合度が２６，０００以下のものを９０重量％よりも多く含むと、複合化合物の柔軟性が低くなり、脆くなってしまうという問題が生じる。更にポリマー成分の一部がポリエチレングリコールを含む場合にも高いイオン伝導度を得ることができるが、ポリエチレングリコール含有量が５０重量％を超えると十分な耐水性を維持することができない。
【００６５】
本発明により得られた高イオン伝導性固体電解質は化合物中のプロトンサイトをアルカリ金属イオンにすることによって容易にアルカリ型にすることができるとともにアルカリ型においても高いプロトンあるいは水酸化物イオン伝導性を示す。また、アルカリ型にすることによって電極その他のシステム構成材料としてニッケルなど比較的安価な素材を用いることが可能となり、システム全体の低コスト化をはかることができる。
【００６６】
更にアルカリ型にすることによって一次電池、二次電池への応用が可能であり、従来のアルカリ電解液を本発明の電解質材料にすることで漏液の惧れをなくすことができる。このアルカリ型固体電解質を使用することで従来から実用化が困難であった二次電池、例えば２価以上の多価金属を負極に用いたような高エネルギー密度電池の実用化をも可能にする。例えば負極に酸化亜鉛、正極にニッケル水素電池と同じ水酸化物ニッケルを用いたニッケル亜鉛電池を例に挙げることができる。ニッケル亜鉛電池は、下記の（１２）式（１３）式に示すように負極では充電時に酸化亜鉛が還元されて金属亜鉛となり、放電時には逆に亜鉛が電気化学的に酸化されて酸化亜鉛に戻る。
〔充電〕 ＺｎＯ ＋ Ｈ_２Ｏ ＋２ｅ⁻ → Ｚｎ＋２ＯＨ⁻ ………（１２）
〔放電〕 Ｚｎ＋２ＯＨ⁻ → ＺｎＯ＋Ｈ_２Ｏ＋２ｅ⁻ ………（１３）
【００６７】
ニッケル亜鉛電池は亜鉛が２価であるため高い貯蔵エネルギー密度を持つが、酸化亜鉛がアルカリ電解液に溶解しやすく、電極から亜鉛イオンが溶出したり、溶出した亜鉛イオンが充電時に還元される際に針状金属亜鉛（デンドライト）が生成し、これがセパレータを貫通して短絡を引き起こすという問題がある。更に亜鉛の酸化還元電位が水素よりも低いため、充電状態での放置中に亜鉛が水によって酸化されて自己放電を起こしやすく、また充電時に亜鉛極から水素を発生して充電効率が低下する等の難点があり、特に液性の電解質を用いた電池の実用化が難しいという問題があるが、本発明の高イオン伝導性固体電解質を用いると金属イオンの溶解が抑制され、僅かに溶解しても金属イオンが電極から拡散していく速度が遅いため、金属のデンドライトが生成する可能性は低く、仮にデンドライトが生じても固体電解質自体が負極から正極への貫通を防ぐことができる。更に固体電解質中の水は反応性に乏しく、水素よりも酸化還元電位の低い金属に対しても自己放電の問題が起こりにくくなり、金属の還元反応と競合する水の電気分解、即ちプロトンの還元反応も起こりにくいため、充電効率も改善される。上記金属イオンの溶解と拡散の抑制作用、デンドライトの生成防止作用は一次電池あるいはニッケル水素電池に対しても同様な作用効果を及ぼすことができる。更に正極として空気極を用いた空気亜鉛電池に上記と同様なメリットがあり、酸素の亜鉛極への拡散が抑制されて容易に充電可能な電池が得られる。
【００６８】
２価以上の金属は亜鉛以外にも銅，コバルト，鉄，マンガン，クロム，バナジウム，錫，モリブデン，ニオブ，タングステン，珪素，ホウ素，アルミニウム等多数存在するので、本発明にかかる電解質の適用によって上記金属を用いた二次電池の実用化が可能となる。
【００６９】
ニッケル水素電池などアルカリ二次電池では、従来多孔性セパレータに染みこませたアルカリ電解液が使用されているが、本発明にかかる電解質は電解液とセパレータとの両方の機能を兼ね備えているので、電解液が不要となるかあるいはその量を軽減することが可能となり、その分だけ電池のエネルギー密度を向上させることができる。また、多孔性のセパレータとは異なって薄膜にしても短絡を防止することができるので、薄膜型で表面積が大きい電極を使用することができる。
【００７０】
本発明の固体電解質は安価な原料を使用しており、簡単な水溶液プロセスを基本としているため、既存のパーフルオロスルホン酸系電解質よりも大幅に安価である。更に無機固体材料とは異なって柔軟性があるため、薄膜加工がし易い。従来試みられているポリエチレンオキシドと珪素化合物の複合化を選択した場合には、本発明の方法を適用しても耐熱水性のある複合化合物を作製することができず、ゾルゲル製法のようなコストの高い方法を用いる必要がある。しかし本発明のようにポリビニルアルコールを選択することにより、製造が容易で低コストの水溶液製法が適用可能である。
【００７１】
本発明にかかる固体電解質はプロトン伝導性であることにより、従来のパーフルオロスルホン酸系イオン交換膜と同様に燃料電池、除湿機、空調機器、エレクトロクロミックデバイス、電気分解型水素生成装置、電解過酸化水素製造装置、湿度センサ、水素センサに応用することができる。この固体電解質材料はアルカリ型でも高いイオン伝導性を示すため、一次電池、二次電池、光スイッチシステム等の電気化学システムあるいは多価金属を用いた電池システムに適用することができる。
【００７２】
【発明の効果】
以上詳細に説明したように、本発明によればポリビニルアルコールの共存する水溶液中で珪酸塩の酸による中和反応を行い、溶媒の水を除去した後、不要塩を除去して複合化合物を作成することが大きな特徴となっており、吸水性と耐水性を両立させた有機無機複合化合物を水溶液による製法で容易に作成することができるため、低価格な高イオン伝導性固体電解質と、該高イオン伝導性固体電解質を用いた電気化学システムを提供することができる。
【００７３】
特に珪酸塩水溶液の中和反応だけならば単なるシリカゲルの一般的製法に一致して該シリカの縮重合反応が起きるだけであるが、ポリビニルアルコールが共存することによってポリビニルアルコールとシリカとのミクロレベルでの絡み合いにより複合化され、このシリカの縮重合反応が加熱によって促進されて強固な複合化合物が得られる。しかも単独のポリビニルアルコールは熱水に溶解するにも関わらず複合化合物は珪酸化合物との強固な絡み合いにより熱水に溶解せず、高温湿潤環境下でも安定に物性を保つことが可能であり、かつ、ポリビニルアルコールと珪酸化合物はともに高親水性であるため、複合化合物は耐水性であるにも関わらず多量の水分を吸収することができて高いイオン伝導性を付与することができる。よって、珪酸化合物とポリビニルアルコールの複合化合物に内包された内包水がプロトンあるいは水酸化物イオンの高速拡散の媒体となるのである。
【００７４】
また、珪酸塩とポリビニルアルコールでなる原料水溶液にリン酸塩、ホウ酸塩、アルミン酸化合物などを共存させておくか、あるいは原料水溶液に加える酸の成分としてリン酸、ホウ酸を加えておくことにより、リン酸化合物、ホウ酸化合物アルミン酸化合物も容易に複合化させることができる。同様に原料水溶液に異種高分子のポリエチレングリコールを共存させておくことにより、容易に複合化させることもできて、これら第３，第４の成分を複合化させることによって特性を改善することができる。また、ポリマー成分への低分子量ポリビニルアルコール又はポリエチレングリコールの添加に伴ってポリマーの分子運動を活発化させたり不規則性を増大してイオンの拡散が促進されたり、リン酸化合物等の添加化合物の結合サイトが増加してイオン伝導性を高めることができる。
【００７５】
本発明の高イオン伝導性固体電解質は、燃料電池、除湿機、空調機器、エレクトロクロミックデバイス、電気分解型水素生成装置、電解過酸化水素製造装置、湿度センサ、水素センサ、一次電池、二次電池、光スイッチシステムあるいは多価金属を用いた電池システム等の各種の電気化学システムに応用可能であり、これら電気化学システムの低価格化に寄与することができる。更に電解質材料をアルカリ型にすることにより、上記電気化学システムに用いる電極等の周辺部材の素材としても低価格のものを使用することができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a proton (hydrogen ion) highly conductive solid electrolyte or hydroxide ion highly conductive solid electrolyte applicable to a fuel cell and the like, and a fuel cell and other electrochemical system using the highly ion conductive solid electrolyte. Is.
[0002]
[Prior art]
Conventionally, as an electrochemical system using a proton conductive solid electrolyte, a fuel cell, an electrolysis-type hydrogen generator, a dehumidifier, and the like have been put into practical use. For example, a solid polymer fuel cell includes the following (1) As shown in the formula, it consists of the electrochemical oxidation reaction of hydrogen supplied to the negative electrode, as shown in the formula (2), the electrochemical reduction reaction of oxygen supplied to the positive electrode, and the proton transfer in the electrolyte therebetween. A current flows through the reaction, and electric energy is extracted.
H₂ → 2H⁺+ 2e⁻      ………………………… (1)
1 / 2O₂ + 2H⁺+ 2e⁻ → H₂O ……………… (2)
[0003]
There are fuel cells that use a fuel other than hydrogen such as methanol as the fuel supplied to the negative electrode, but in this case as well, the reaction in which the fuel is electrochemically oxidized at the negative electrode to release protons is performed in the same way. It can be operated using a conductive solid electrolyte.
[0004]
The electrolysis-type hydrogen generator generates hydrogen by a reaction opposite to the reaction of the above formulas (1) and (2) in the fuel cell, and high purity hydrogen is generated on-site only with water and electric power. Since it is obtained, there is an advantage that a hydrogen cylinder is unnecessary. Moreover, electrolysis can be easily performed simply by introducing fresh water not containing an electrolyte by using a solid electrolyte. In the field of paper industry, an attempt has been made to produce hydrogen peroxide for bleaching on-site by an electrolytic method using the following formula (3) by a similar system (electrochemistry, 69, No. 3,154-159). (See 2001).
O₂+ H₂O + 2e⁻ → HO₂ ⁻+ OH⁻ …………… (3)
[0005]
The dehumidifier has a structure in which a proton-conducting solid electrolyte membrane is sandwiched between positive and negative electrodes in the same manner as a fuel cell and hydrogen generator. When a voltage is applied between the positive and negative electrodes, water is oxygenated by the reaction of the following formula (4) at the positive electrode: The protons decomposed into protons and moved to the negative electrode through the solid electrolyte are combined with oxygen in the air again by the reaction of formula (5) and returned to water. As a result of these reactions, water is transferred from the positive electrode side to the negative electrode side. Is dehumidified on the positive electrode side.
H₂O → 1 / 2O₂+ 2H⁺+ 2e⁻      ………………… (4)
1 / 2O₂ + 2H⁺+ 2e⁻ → H₂O ………………… (5)
[0006]
Water can be decomposed and dehumidified using the same operating principle as the electrolysis-type hydrogen generator, and air conditioners combined with water-evaporating coolers have also been proposed (Proceedings of the Annual Conference of the Institute of Electrical Engineers of Japan, 2000) , P3373 (2000)).
[0007]
In any of the systems put into practical use as described above, a perfluorosulfonic acid ion exchange membrane represented by a Nafion membrane (Nafion) is used as a solid electrolyte. In addition, various sensors, electrochromic devices, and the like are systems based on the same operating principle as described above, and operate by movement of protons in the electrolyte between two different types of redox pairs of positive and negative electrodes. Therefore, a proton conductive solid electrolyte can be used. Currently, empirical studies of systems using these proton-conducting solid electrolytes are also being conducted.
[0008]
The hydrogen sensor uses a change in electrode potential due to the hydrogen concentration, and as an electrolyte material, for example, a solid electrolyte mainly composed of polyvinyl alcohol has been proposed (Sensors and Actuators, 11, 377-386 (1987)). See). Further, it can be applied to a humidity sensor by utilizing a change in electrode potential or a change in ionic conductivity.
[0009]
For example, electrochromic devices are₃When an electric field is applied using the above, it utilizes the fact that color develops by the reaction of the following formula (6), and is considered to be used for display devices and light-shielding glass. In this system, Sn (HPO) which is an inorganic compound as a solid electrolyte is used._Four) ・ H₂O and the like have been proposed (Bull. Chem. Soc. Jpn., 60, 747-752 (1987)).
WO₃+ XH⁺+ Xe⁻ → HxWO₃(Color development) ……………… (6)
[0010]
In addition, primary batteries, secondary batteries, optical switches, and the like can be given as electrochemical systems that operate using a proton conductive solid electrolyte in principle. For example, a nickel metal hydride battery as a secondary battery uses a hydrogen storage alloy as a negative electrode, nickel hydroxide as a positive electrode, and an alkaline electrolyte as an electrolyte, and is charged as shown in the following formulas (7) and (8). At the time of discharge, electrochemical oxidation-reduction of protons and storage of hydrogen in the hydrogen storage alloy occur at the negative electrode.
[Charging] H₂O + e⁻ → H (Occlusion) + OH⁻  ............ (7)
[Discharge] H (Occlusion) + OH⁻→ H₂O + e⁻ ............ (8)
[0011]
In the positive electrode, an electrochemical redox reaction of nickel hydroxide occurs as shown in the following formulas (9) and (10).
[Charging] Ni (OH)₂+ OH⁻ → NiOOH + H₂O + e⁻ ...... (9)
[Discharge] NiOOH + H₂O + e⁻ → Ni (OH)₂+ OH⁻  (10)
The charge / discharge reaction of this battery is established by the movement of protons or hydroxide ions in the electrolyte. In principle, a proton-conducting solid electrolyte can be used, but conventionally, an alkaline electrolyte that is not a solid electrolyte is used. Is used.
[0012]
For example, an optical switch using yttrium as a negative electrode has been proposed (see J. Electrochem. Soc., Vol. 143, No. 10, 3348-3353 (1996)). By applying an electric field, yttrium is hydrogenated and transmits light as shown in the following formula (11), so that transmission and non-transmission of light can be switched by the electric field. In principle, this system can also use a proton-conducting solid electrolyte, but an alkaline electrolyte is conventionally used.
Y + 3 / 2H₂O + 3e → YH₃+ 3OH ……………… (11)
[0013]
[Problems to be solved by the invention]
However, perfluorosulfonic acid-based electrolytes used in the fuel cells, electrolysis-type hydrogen generators, dehumidifiers and the like that have been put into practical use as electrochemical systems using proton conductive solid electrolytes are mainly used. There is a problem of high price due to the complexity of the manufacturing process. Due to the mass production effect of these electrolytes, there is a limit to what is expected to reduce the price to some extent, and the present situation is that there is a demand for the appearance of inexpensive alternative materials.
[0014]
By the way, the proton conducting solid electrolyte operating at room temperature is required to have sufficient water absorption even for an alternative material because protons are transported at a high speed by the action of water contained in the solid. In particular, many are used in wet environments, so they must also have water resistance. In conventional perfluorosulfonic acid electrolytes, water absorbed around the highly hydrophilic sulfonic acid group carries ions at a high speed, and the polyfluoroethylene skeleton has water resistance, chemical stability, high temperature durability, etc. Plays the role of maintaining.
[0015]
The polyvinyl alcohol is an example of a highly hydrophilic and inexpensive hydrocarbon-based polymer material. A material in which phosphoric acid is mixed to provide proton conductivity can be used for applications such as a hydrogen sensor. In this example, the high water absorption of polyvinyl alcohol makes it possible to move protons at high speed. However, since polyvinyl alcohol is soluble in water, there is a problem that the material stability in a wet environment is low.
[0016]
Another example of a highly hydrophilic and water-resistant material is an inorganic hydrous compound. For example, P by sol-gel manufacturing method₂O₅-ZrO₂-SiO₂Hydrous glass exhibits high proton conductivity by absorbing a large amount of water, and is not dissolved in water. It has the advantage of high temperature stability unique to inorganic compounds (J. Electrochem. Soc , Vol. 144, No. 6, 2175-2178 (1997)).
[0017]
However, a common weak point of these water-containing compounds is a fragile point, and there is a problem that it is difficult to process a thin film particularly required for solid electrolyte applications. Further, the sol-gel manufacturing method uses expensive metal alkoxide as a raw material, and is manufactured using an organic solvent such as alcohol, so that there is a problem that it is difficult to reduce the cost in terms of equipment. Sn (HPO) being tested in electrochromic devices_Four) ・ H₂O and the like can be processed by applying a powder, but it is difficult to process a film having a high strength and a gas diffusion suppressing function required for fuel cells and other applications.
[0018]
As a method for overcoming the drawbacks of these hydrophilic organic polymers and inorganic compounds, a means for combining them can be considered. For example, JP-A-2000-90946 proposes a proton conductive material in which a silicon compound and polyethylene oxide are chemically bonded at the nano level. Polyethylene oxide, like polyvinyl alcohol, is an inexpensive and highly hydrophilic organic polymer, but is soluble alone in water. However, it is supposed that by combining with a silicon compound by a sol-gel manufacturing method, water resistance can be imparted and a material having a high temperature strength can be obtained. However, it is difficult to obtain the composite material by a method other than the sol-gel manufacturing method, and other methods are not disclosed. Therefore, there is a problem that the effect of reducing raw materials or manufacturing costs is low. Japanese Patent Application Laid-Open No. 2001-35509 proposes an ion conductive material using an inorganic / organic composite material, but only a composite method using a sol-gel method of a silicon compound and polyethylene oxide is disclosed as a manufacturing method.
[0019]
All of the above conventional solid electrolytes are acidic, and the materials that can be used as electrodes and other system constituent materials are limited to acid-resistant materials such as noble metals, and it is difficult to reduce the cost of the entire system. The fact that the solid electrolyte is acidic also makes it difficult to apply it to some uses such as a primary battery, a secondary battery, and an optical switch due to the deterioration action of the electrode and the active material. On the other hand, the alkaline liquid electrolyte conventionally used in these primary batteries and secondary batteries has a drawback that there is a risk of liquid leakage.
[0020]
Therefore, the present invention solves the problems of the above-mentioned ion conductive solid electrolyte, and a low-cost high ion conductive solid electrolyte using an organic-inorganic composite compound that achieves both water absorption and water resistance. An object of the present invention is to provide various electrochemical systems using the high ion conductive solid electrolyte.
[0021]
[Means for Solving the Problems]
  In order to achieve the above object, the present invention provides a high ion conductive solid electrolyte comprising a composite compound of a silicate compound containing water and polyvinyl alcohol.Wherein the composite compound comprises a silicic acid compound made of SiO. ₂ As a basis, a high ion conductive solid electrolyte having a weight / polyvinyl alcohol weight ratio in the range of 0.03 to 3 is provided. Also,By adding an acid to an aqueous solution in which an alkali metal salt of silicic acid and polyvinyl alcohol are dissolved to neutralize it to pH 8 or less, removing water as a solvent, and then performing a heating step and a step of removing unnecessary salts. The resulting silicic acid compound, polyvinyl alcohol and waterWithCompound compoundConsist ofHigh ion conductive solid electrolyteWherein the composite compound comprises a silicic acid compound made of SiO. ₂ The weight ratio of polyvinyl alcohol / weight ratio of polyvinyl alcohol is between 0.03 and 3High ion conductive solid electrolysisQualityprovide.
[0022]
  Further, the silicic acid compound is changed to SiO. ₂ A weight / polyvinyl alcohol weight ratio in the range of 0.03 to 0.2 is provided.The raw material aqueous solution contains at least one alkali metal salt selected from phosphoric acid, boric acid, and aluminate, and the generated composite compound contains at least one compound of phosphoric acid compound, boric acid compound, and aluminate compound. Yes. And,P₂O₅The weight ratio of the phosphoric acid compound weight converted to the polyvinyl alcohol is 0.035 or more.
[0023]
  At least one acid selected from hydrochloric acid, sulfuric acid, phosphoric acid, and boric acid is used as an acid added to the raw material aqueous solution composed of an alkali metal salt of silicic acid and polyvinyl alcohol. In addition, composite compounds consisting of silicic acid compounds and polyvinyl alcohol,Immersion treatment is performed in at least one acid solution selected from hydrochloric acid, sulfuric acid, and phosphoric acid. The acid solution to be dipped contains 30% by weight or more of phosphoric acid.
[0024]
  Composite compounds consisting of silicic acid compounds and polyvinyl alcohol,At least one selected from sodium hydroxide or potassium hydroxideofImmerse in the solution. In a complex compound comprising a silicic acid compound and polyvinyl alcohol in at least one acid solution selected from hydrochloric acid, sulfuric acid and phosphoric acid after dipping in a compound selected from sodium hydroxide or potassium hydroxide So as to obtain a high ion conductive solid electrolyte. The average molecular weight of about 90% by weight or less of the polyvinyl alcohol is 26,000 or less, and about 50% by weight or less of the polyvinyl alcohol may be replaced with polyethylene glycol.
[0025]
  And the electrochemical system which has arrange | positioned the said high ion conductive solid electrolyte between electrodes using any one of above-mentioned high ion conductive solid electrolyte is provided. Also,Fuel cell as an electrochemical system,Using dehumidifier, air conditioner, electrochromic device, electrolysis hydrogen generator, electrolytic hydrogen peroxide production device, humidity sensor, hydrogen sensor, primary battery, secondary battery, optical switch system or polyvalent metalPowerPond systemOne ofIt is applicable to.
[0026]
According to such a high ion conductive solid electrolyte and an electrochemical system using the solid electrolyte, the high ion conductive solid electrolyte is obtained by performing a simple process of neutralizing silicate in an aqueous solution in which polyvinyl alcohol coexists. It can be prepared, and the polycondensation reaction of silica occurs by the neutralization reaction and the subsequent heat treatment, so that micro-level entanglement with the polyvinyl alcohol occurs and can be combined. Since polyvinyl alcohol and silicic acid compound are hydrophilic, both composite compounds have the ability to encapsulate a large amount of water, and this encapsulated water becomes a medium for rapid diffusion of protons or hydroxide ions.
[0027]
Polyvinyl alcohol and silicic acid compound are strongly bonded by hydrogen bonding or dehydration condensation, and are not dissolved in hot water despite being hydrophilic, and can maintain stable physical properties even in a high temperature and humid environment. Furthermore, phosphoric acid compounds, boric acid compounds, and aluminate compounds increase the water absorption of the composite compound, promote the liberation of protons or hydroxide ions, promote the ion diffusion, and can increase the ionic conductivity. . In addition, since silica itself exhibits strong alkalinity when converted to alkali type, high ion conductivity can be obtained even in the alkali type.
[0028]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, specific embodiments of a highly ion conductive solid electrolyte and an electrochemical system using the solid electrolyte according to the present invention will be described. The present invention is based on a composite compound of polyvinyl alcohol and a silicate compound produced in an aqueous solution at a micro level, and is a proton or hydroxide ion conductive solid that is inexpensive and has high ionic conductivity and has sufficient water resistance. The basic means is to provide an electrolyte and an electrochemical system using the solid electrolyte.
[0029]
In the present invention, an acid is added to an aqueous solution in which an alkali metal salt of silicic acid and polyvinyl alcohol are dissolved to neutralize it to pH 8 or less, and after removing water as a solvent, heating at a temperature of 100 ° C. or higher is unnecessary By performing the step of removing the salt, a composite compound composed of a silicate compound, polyvinyl alcohol and water, specifically, a composite compound of a silicate compound containing water and polyvinyl alcohol was obtained to produce a solid electrolyte. It has become.
[0030]
Specific embodiments of the present invention will be described below. In addition, this invention is not limited to the description content of these example embodiments.
[0031]
[Example 1]
In order to prepare an electrolyte membrane, a raw material aqueous solution is prepared by dissolving a predetermined amount of sodium silicate and, if necessary, a predetermined amount of tribasic sodium phosphate in 80 cc of a 2% by weight aqueous solution of polyvinyl alcohol having an average molecular weight of 120,000 to 190,000. While stirring the aqueous raw material solution, 1.2M hydrochloric acid was added dropwise until the pH of the aqueous solution became 2-3, and neutralized. Thereafter, the aqueous solution was poured into two petri dishes having a diameter of 90 mm and dried at 50 ° C. to remove solvent water. After drying, the film remaining on the petri dish was peeled off and subjected to a heat treatment at 130 ° C. for 3 hours in a vacuum oven, and washed sufficiently in hot water at 70 to 100 ° C.
[0032]
The samples prepared in this way are sample Nos. 1, 11, 15, 21, 23, 25, 30, 34 of Table 1, and the amounts of silicic acid compound, phosphoric acid, and compound are SiO 2 in the table.₂, P₂O₅, Converted to weight, and expressed as a weight ratio to polyvinyl alcohol.
[0033]
[Table 1]

[0034]
SiO against polyvinyl alcohol₂Any of these samples in which a silicic acid compound having a converted weight ratio of 0.03 or more is combined does not dissolve even after being washed in hot water at 70 ° C. or more for 1 hour, and is excellent in water resistance. For comparison, a sample of only polyvinyl alcohol not containing a silicic acid compound was also formed by the same method, but only a low water resistance that was readily dissolved in hot water was produced. As can be seen from these results, the compounding with the silicate compound significantly improves the water resistance of the polyvinyl alcohol.
[0035]
On the other hand, silicic acid compound with respect to polyvinyl alcohol is SiO.₂When adding 3 or more in terms of weight ratio, it became very brittle and a high strength film could not be obtained. Therefore SiO against polyvinyl alcohol₂The converted silicic acid compound weight ratio is required to be 0.03 to 3.
[0036]
For comparison, a sample film was prepared in which the pH of the aqueous solution after neutralization was set to a value higher than 8 in the above preparation process. The sample dissolved when washed in hot water, and the water resistance effect due to the combination was reduced. It was not expressed. Therefore, the pH after neutralization needs to be 8 or less in order to fully develop the water resistance effect by the combination.
[0037]
The ion conductivity of the prepared sample film was measured by the following method. First, the sample film is cut into a circle having a diameter of 30 mm, and is sandwiched between two platinum disks having a diameter of 28 mm and a brass disk disposed outside the platinum disk, and is further sandwiched and fixed by an insulated clip. An AC voltage with a voltage of 10 mV was applied to a lead wire attached to a brass disc using a LCR meter while changing the frequency from 5 MHz to 50 Hz, and the response of current and phase angle was measured. The ionic conductivity was determined from the diameter of a semicircle in a commonly performed Cole-Cole plot. This measurement was performed while the sample was placed in a constant temperature and humidity chamber and controlled to a humidity of 50 ° C. and a relative humidity of 90%. The measurement results of ionic conductivity are shown in Table 1.
[0038]
Many of these samples are 10 to 10% even if the proton sites are not completely protonated or alkalized.^-7-10^-6Showed relatively high ionic conductivity. However, the ionic conductivity tends to decrease as the amount of silicate compound increases, and SiO₂When the ratio of the converted silicic acid compound weight to polyvinyl alcohol exceeds 0.2, 10^-8It showed only very low ionic conductivity of the order. Therefore SiO₂The ratio of the converted silicic acid compound to polyvinyl alcohol is preferably 0.03 to 0.2.
[0039]
The results of adding sodium borate and sodium aluminate to the raw material aqueous solution in place of sodium phosphate in the above-mentioned production method are shown in Sample Nos. 19, 28 and 29 in Table 1. In addition, in the “weight ratio of additive to polyvinyl alcohol” column in the table, symbol (B) means addition of borate, symbol (A) means addition of aluminate, boric acid, amount of aluminate compound Is B in the table.₂O₃, Al₂O₃In terms of weight, it is shown in the form of a weight ratio to polyvinyl alcohol. From these results, addition of boric acid compound and addition of aluminate compound showed ionic conductivity similar to or higher than that of addition of phosphoric acid compound.
[0040]
In the above production method, the acid added at the time of neutralization is not hydrochloric acid, but the result of using 0.6M sulfuric acid is the sample No. 2 in Table 1, with 1.2M hydrochloric acid or 0.6M sulfuric acid. After adding and neutralizing to pH 3, the results when adding 8.5 wt% phosphoric acid aqueous solution until pH 2 are shown in Sample Nos. 3 and 4 of Table 1, respectively. The symbol C in the column of “type of added acid” in the table means that hydrochloric acid, symbol C + P is hydrochloric acid + phosphoric acid, symbol S is sulfuric acid, and symbol S + P means that sulfuric acid + phosphoric acid is added. As can be seen from these results, when sulfuric acid is used as the acid to be added, the ionic conductivity is slightly lower than when hydrochloric acid is used, but the difference is not great. Further, when hydrochloric acid or sulfuric acid is used in combination with phosphoric acid, it shows a slightly higher ionic conductivity.
[0041]
[Example 2]
The electrolyte membrane prepared by the method of Example 1 was subjected to acid immersion treatment for the purpose of promoting protonation. The electrolyte membrane was immersed in 1.2 M hydrochloric acid at room temperature for 5 hours, and then washed thoroughly with water. The result of the sample was immersed in 85% by weight concentrated phosphoric acid solution as sample No. 18 in Table 1. No. 5, 7, 8, 10, 12, 16, 22, 24, 26, 31 were immersed in 1.2M hydrochloric acid containing 30% by weight of concentrated phosphoric acid. Sample No. 27 shows the result of the sample that was subjected to the treatment of immersion in a 5M sodium hydroxide solution for 1 hour before the acid immersion treatment. The symbol C in the “treatment” column of the table is a 1.2 M hydrochloric acid immersion treatment, the symbol P is an 85 wt% phosphoric acid immersion treatment, and the symbol P + C is a 1.2 M hydrochloric acid immersion treatment containing 30 wt% phosphoric acid. The symbol N → P means the treatment of immersing in concentrated phosphoric acid after immersion in a sodium hydroxide solution. As can be seen from these results, the ionic conductivity is improved by the hydrochloric acid immersion treatment, and the improvement of the ionic conductivity is further remarkable by the concentrated phosphoric acid immersion treatment. Furthermore, a result close to that of concentrated phosphoric acid treatment can be obtained by immersion treatment with 1.2 M hydrochloric acid containing 30% by weight phosphoric acid. Moreover, it was confirmed that the effect of improving the ionic conductivity is increased by performing the alkaline solution immersion treatment before the acid immersion treatment.
[0042]
As in sample Nos. 3, 5, 6, 7, 9, and 10, the effect of the acid dipping treatment is more effective as the amount of the phosphoric acid compound added is larger.₂O₅10 for an electrolyte membrane in which the weight ratio of the phosphoric acid compound converted to 0.035 or more^-3-10^-4High ionic conductivity.
[0043]
Contrary to the acid immersion treatment, in order to promote alkalinization, the results of the treatment of immersion in 5M sodium hydroxide and 5M potassium hydroxide solution are shown in Sample Nos. 13, 14, 20, 32 and 33 in Table 1. Indicated. Note that the symbol N in the “treatment” column of the table represents a 5M sodium hydroxide solution immersion treatment, and the symbol K represents a 5M potassium hydroxide immersion treatment. From these results, it was found that even in the alkaline immersion treatment, a great effect of improving the ionic conductivity was recognized, and it also functions as an alkaline electrolyte membrane. In addition, since the alkali type also exhibits the same level of ionic conductivity as the acid type, the alkaline type ion diffusion is mainly due to the fast ion diffusion of protons mediated by water molecules as in the acid type. It is thought that it is done. Note that, as shown in Sample No. 20, when the electrolyte contains a boric acid compound, the effect of improving the ionic conductivity by the alkali immersion treatment is larger, and high ionic conductivity is obtained in the alkaline type.
[0044]
In the case of the electrolyte membrane containing a phosphoric acid compound, the result when the second sodium phosphate was used in place of the third sodium phosphate as the raw material aqueous solution is shown in Sample No. 8 of Table 1. In the “weight ratio of additive to polyvinyl alcohol” column in the table, the symbol (HP) means the use of the second phosphate, and the symbol (P) means the use of the third phosphate. As can be seen from the results, when the second phosphate is used, the ionic conduction is slightly lower than when the third phosphate is used, but there is no significant difference.
[0045]
Example 3
The example which changed the heat processing conditions in an electrolyte membrane preparation process is shown below. Samples were prepared by changing only the heat treatment conditions of the manufacturing method of Example 1 for electrolyte membranes having compositions of weight ratio of silicic acid compound to polyvinyl alcohol and weight ratio of phosphoric acid compound of 0.065 and 0.038, respectively. Table 2 shows the results of the influence of the heat treatment conditions on the ionic conductivity of the polyvinyl alcohol / silica composite electrolyte membrane.
[0046]
[Table 2]

[0047]
According to Table 2, in the same heating as in Example 1 as in sample No. 35, the ionic conductivity hardly changed even when the temperature was lowered from 130 ° C. to 100 ° C. and the heat treatment was performed for 3 hours. Further, when the heat treatment at 130 ° C. for 3 hours in air was performed as in sample No. 36, the ionic conductivity was slightly reduced, but no significant difference was caused.
[0048]
Further, for comparison, a sample that was not subjected to any heat treatment was prepared, and the strength was considerably lowered by washing in hot water. Therefore, it is preferable to perform heat treatment at a temperature of 100 ° C. or higher. The heating atmosphere may be air, an inert gas atmosphere, or a vacuum, and it is preferably performed in an inert gas atmosphere or vacuum from the viewpoint of maintaining ionic conductivity.
[0049]
Example 4
Polyethylene glycol or low-polymerization polyvinyl alcohol was added as a polymer component. Preparation was carried out in the same manner as in Example 1, and a part of polyvinyl alcohol having an average molecular weight of 120,000 to 190,000 in the raw material aqueous solution was reduced to polyethylene glycol having an average molecular weight of 400 and 7500 or an average molecular weight of 18,000 to 26,000. Replaced with molecular weight polyvinyl alcohol. Neutralization was performed with phosphoric acid and hydrochloric acid, and after film formation, 85 wt% concentrated phosphoric acid treatment was performed by the method of Example 2. Table 3 shows the results of the influence of the polymer component on the ionic conductivity of the polyvinyl alcohol / silica composite electrolyte membrane. The composition and ionic conductivity of the electrolyte membrane obtained as described above are shown in Table 3.
[0050]
[Table 3]

[0051]
In Table 3, “weight ratio of silicate compound to polyvinyl alcohol” means that the silicate compound is SiO 2₂The weight of polyvinyl alcohol converted as follows: “weight ratio of phosphoric acid compound to polyvinyl alcohol” is the ratio of phosphoric acid compound to P₂O₅PEG (400) in the column of “weight% of added polymer with respect to the total polymer” is polyethylene glycol having an average molecular weight of 400, PEG (7500) is polyethylene glycol having an average molecular weight of 7500, PVA (18, 000-26,000) is a polyvinyl alcohol having an average molecular weight of 18,000-26,000.
[0052]
What added polyethyleneglycol (PEG) and low molecular weight polyvinyl alcohol (PVA) improved ionic conductivity compared with the case of only high molecular weight polyvinyl alcohol (the said sample No. 5), and all 10^-3-10^-4A high value of S / cm is shown. However, when the amount of polyethylene glycol exceeds 50% by weight, the water resistance is extremely lowered, and the shape cannot be maintained during washing in hot water. Moreover, when the amount of low molecular weight polyvinyl alcohol exceeds 90% by weight, the result is that the electrolyte membrane becomes extremely brittle.
[0053]
The polyvinyl alcohol does not need to be complete, and can be used as long as it essentially functions as polyvinyl alcohol. For example, those in which part of the hydroxyl group is substituted with another group and those in which another polymer is partially copolymerized can function as polyvinyl alcohol. Moreover, since the same effect is acquired if it passes through polyvinyl alcohol in the reaction process of this invention, the polyvinyl acetate etc. which are the raw materials of polyvinyl alcohol can be used as a starting material.
[0054]
As long as the alkali metal salt of silicic acid is soluble in water, the kind of alkali metal salt, the ratio of alkali metal ions, silicon and oxygen, and the water content may be any, for example, a common known as water glass Sodium silicate can be used.
[0055]
The aqueous solution in the present invention means that water is essentially a solvent, and there may be other solvent components smaller than the water content. Ionic conductivity can be increased by adding phosphoric acid compounds, boric acid compounds, and aluminate compounds to the material, but these compounds are added to the alkali metal salts of phosphoric acid, boric acid, and aluminate in the raw material aqueous solution. Is performed by dissolving. As the phosphate, any of the first, second and third phosphates can be used. However, since the first phosphate already neutralizes the silicate when it is put into the raw material aqueous solution. It is not preferable. In particular, when producing an acid type proton conductive material, it is preferable to select a third phosphate in order to increase the ionic conductivity by introducing a large amount of a phosphoric acid compound into the material. Any type or composition of alkali metal ions can be used for borates and aluminates. Two or more of these salts may be mixed and added.
[0056]
The acid added to the raw material aqueous solution may be any acid as long as it can neutralize the silicate, and hydrochloric acid, sulfuric acid, phosphoric acid and the like can be used. In particular, when an acid-type proton conductive material is produced, it is preferable that an acid to be added contains phosphoric acid in order to introduce more phosphoric acid compound into the material and obtain high ion conductivity. Moreover, a boric acid compound can be introduce | transduced into a material by including a boric acid in the acid to add.
[0057]
Neutralization with acid is performed until the pH is 8 or less. If the pH is higher than 8, the polycondensation reaction of the silicic acid compound does not proceed sufficiently, so that the water resistance of the produced composite compound is lowered. The aqueous solution neutralized with the acid is processed into a desired shape such as a thin film by removing water as a solvent by, for example, heating and drying. The composite compound after processing is heat-treated at a temperature of 100 ° C. or higher to promote the condensation polymerization reaction of the silicate compound itself and the bond formation with phosphoric acid, boric acid, aluminate compound and polyvinyl alcohol, strength, water resistance, High temperature stability can be increased. When heat treatment is not performed, problems such as a decrease in strength in hot water occur. The heating atmosphere in the heat treatment has no performance difference even in air, in an inert gas atmosphere, or in vacuum, and any heating atmosphere can be adopted. Heating in air is slightly lower in ionic conductivity than heating in an inert gas atmosphere, but has the advantage that the equipment required for heating can be obtained easily and at low cost.
[0058]
By washing the composite compound with a solvent such as water before or after the heat treatment step, unnecessary salts in the composite compound are removed. In any electrochemical system using a solid electrolyte, an oxidation-reduction reaction occurs at the electrode. Of the anions introduced by the acid during the neutralization process, free ions that are not fixed to the solid electrolyte have an adverse effect on the oxidation-reduction reaction. Therefore, it is necessary to remove unnecessary salts which are free ions by washing.
[0059]
SiO₂The ratio of the weight of the silicic acid compound in the composite compound converted to 1 to the weight of polyvinyl alcohol is limited to the range of 0.03 to 3. When the weight ratio is lower than 0.03, the water resistance is lowered, and there arises a problem that it is dissolved in hot water or the strength is extremely lowered. If the weight ratio is higher than 3, the material becomes brittle and the workability deteriorates. Moreover, when the ratio of a silicic acid compound increases, there exists a tendency for ionic conductivity to fall, and it is desirable for the weight ratio to be 0.2 or less from that point.
[0060]
When obtaining an acid-type proton-conducting solid electrolyte, the resulting composite compound is immersed in an acid to completely protonate the proton sites in the material and increase the ionic conductivity by increasing the proton concentration. be able to. When the acid immersed at this time contains 30% by weight or more of phosphoric acid, many phosphoric acid compounds can be introduced into the material, and the ionic conductivity can be increased more effectively.
[0061]
The effect of the acid treatment can be enhanced by performing a treatment of immersing the produced composite compound in an alkaline solution once before the acid immersion treatment. That is, by immersing in an alkaline solution, an effect is obtained that an extra silicic acid compound is removed and a new proton site is generated by bond cleavage or introduction of a phosphoric acid compound by acid treatment is promoted. Furthermore, there exists an advantage that the micropore into which water and a phosphoric acid compound are introduce | transduced arises according to an etching effect.
[0062]
When obtaining an alkali-type proton or hydroxide ion-conducting solid electrolyte, it can be made more completely alkali-type by immersing the resulting composite compound in an alkali solution, and ion conductivity can be increased. .
[0063]
The treatment in which the acid or alkali is immersed is not necessarily an aqueous solution. In the acid type proton conductive solid electrolyte, the ion conductivity depends on the amount of the phosphate compound in the composite compound, and P₂O₅High ionic conductivity can be obtained by setting the ratio of the converted phosphate compound weight to the polyvinyl alcohol weight to be 0.035 or more.
[0064]
Moreover, when polyvinyl alcohol contains a low molecular weight having an average molecular weight of 26,000 or less, high ionic conductivity can be obtained. However, if the average degree of polymerization is 26,000 or less, the composite compound becomes less flexible and brittle when it contains more than 90% by weight. Further, high ionic conductivity can be obtained even when a part of the polymer component contains polyethylene glycol, but if the polyethylene glycol content exceeds 50% by weight, sufficient water resistance cannot be maintained.
[0065]
The high ion conductive solid electrolyte obtained by the present invention can be easily made into an alkali type by converting the proton site in the compound to an alkali metal ion, and also has high proton or hydroxide ion conductivity even in the alkali type. Show. In addition, by using an alkaline type, it is possible to use a relatively inexpensive material such as nickel as an electrode or other system constituent material, and the cost of the entire system can be reduced.
[0066]
Furthermore, application to a primary battery and a secondary battery is possible by using an alkaline type, and fear of leakage can be eliminated by using a conventional alkaline electrolyte as the electrolyte material of the present invention. By using this alkaline solid electrolyte, it becomes possible to put into practical use a secondary battery that has been difficult to put into practical use, for example, a high energy density battery using a divalent or higher polyvalent metal as a negative electrode. . For example, a nickel zinc battery using zinc oxide as the negative electrode and nickel hydroxide as the nickel metal hydride battery as the positive electrode can be cited as an example. In the nickel-zinc battery, as shown in the following formulas (12) and (13), zinc oxide is reduced to metal zinc at the time of charging at the negative electrode, and zinc is electrochemically oxidized to return to zinc oxide at the time of discharging. .
[Charging] ZnO + H₂O + 2e⁻ → Zn + 2OH⁻  ......... (12)
[Discharge] Zn + 2OH⁻→ ZnO + H₂O + 2e⁻  ……… (13)
[0067]
Nickel-zinc batteries have high storage energy density because zinc is divalent, but zinc oxide is easily dissolved in alkaline electrolyte, and zinc ions are eluted from the electrode, or when the eluted zinc ions are reduced during charging. There is a problem in that acicular metal zinc (dendrites) is formed in the metal, which penetrates the separator and causes a short circuit. Furthermore, since the oxidation-reduction potential of zinc is lower than that of hydrogen, zinc is easily oxidized by water during standing in a charged state, and self-discharge is likely to occur. In addition, hydrogen is generated from the zinc electrode during charging, and charging efficiency is reduced. In particular, there is a problem that it is difficult to put a battery using a liquid electrolyte into practical use. However, when the high ion conductive solid electrolyte of the present invention is used, dissolution of metal ions is suppressed and slightly dissolved. However, since the rate at which metal ions diffuse from the electrode is slow, the possibility of metal dendrite formation is low, and even if dendrite occurs, the solid electrolyte itself can prevent penetration from the negative electrode to the positive electrode. Furthermore, the water in the solid electrolyte is poor in reactivity and it is less likely to cause a self-discharge problem even for a metal having a lower redox potential than hydrogen. Electrolysis of water competing with the metal reduction reaction, ie, proton reduction Since the reaction hardly occurs, the charging efficiency is also improved. The action of inhibiting the dissolution and diffusion of metal ions and the action of preventing the formation of dendrite can have the same effect on the primary battery or the nickel metal hydride battery. Furthermore, an air zinc battery using an air electrode as a positive electrode has the same merit as described above, and diffusion of oxygen to the zinc electrode is suppressed, and a battery that can be easily charged is obtained.
[0068]
In addition to zinc, there are many other metals such as copper, cobalt, iron, manganese, chromium, vanadium, tin, molybdenum, niobium, tungsten, silicon, boron, and aluminum. A secondary battery using a metal can be put into practical use.
[0069]
Alkaline secondary batteries such as nickel metal hydride batteries have conventionally used an alkaline electrolyte soaked in a porous separator, but the electrolyte according to the present invention has both functions of an electrolyte and a separator. It becomes possible to eliminate the amount of electrolyte or reduce the amount of the electrolyte, and the energy density of the battery can be improved accordingly. Moreover, since a short circuit can be prevented even if it is a thin film unlike a porous separator, a thin film type electrode having a large surface area can be used.
[0070]
Since the solid electrolyte of the present invention uses an inexpensive raw material and is based on a simple aqueous solution process, it is much cheaper than an existing perfluorosulfonic acid electrolyte. Further, unlike an inorganic solid material, it has flexibility, so that thin film processing is easy. When the compound of polyethylene oxide and silicon compound, which has been attempted in the past, is selected, a complex compound having hot water resistance cannot be produced even if the method of the present invention is applied, and the cost of the sol-gel manufacturing method is low. It is necessary to use an expensive method. However, by selecting polyvinyl alcohol as in the present invention, an aqueous solution manufacturing method that is easy to manufacture and low in cost can be applied.
[0071]
  Since the solid electrolyte according to the present invention is proton-conducting, the fuel cell is similar to the conventional perfluorosulfonic acid ion exchange membrane.,The present invention can be applied to a dehumidifier, an air conditioner, an electrochromic device, an electrolysis-type hydrogen generator, an electrolytic hydrogen peroxide production apparatus, a humidity sensor, and a hydrogen sensor. Since this solid electrolyte material exhibits high ionic conductivity even in the alkaline type, an electrochemical system such as a primary battery, a secondary battery, or an optical switch system, or a polyvalent metal is used.PowerIt can be applied to the pond system.
[0072]
【The invention's effect】
As described above in detail, according to the present invention, neutralization reaction with silicate acid is performed in an aqueous solution in which polyvinyl alcohol coexists, and after removing water of the solvent, unnecessary salts are removed to form a composite compound. The organic-inorganic composite compound having both water absorption and water resistance can be easily prepared by a method using an aqueous solution. An electrochemical system using an ion conductive solid electrolyte can be provided.
[0073]
In particular, if only the neutralization reaction of the silicate aqueous solution, the condensation polymerization reaction of the silica only occurs in accordance with the general production method of silica gel, but the presence of polyvinyl alcohol makes it possible at the micro level of polyvinyl alcohol and silica. By the entanglement, the polycondensation reaction of the silica is promoted by heating to obtain a strong composite compound. Moreover, despite the fact that single polyvinyl alcohol dissolves in hot water, the composite compound does not dissolve in hot water due to strong entanglement with the silicic acid compound, and can stably maintain physical properties even in a high-temperature and humid environment, and Since both the polyvinyl alcohol and the silicic acid compound are highly hydrophilic, the composite compound can absorb a large amount of water despite the water resistance, and can impart high ionic conductivity. Therefore, the water contained in the composite compound of the silicate compound and polyvinyl alcohol becomes a medium for high-speed diffusion of protons or hydroxide ions.
[0074]
In addition, phosphate, borate, aluminate compound, etc. should coexist in the raw material aqueous solution consisting of silicate and polyvinyl alcohol, or phosphoric acid and boric acid should be added as an acid component added to the raw material aqueous solution. Thus, a phosphoric acid compound and a boric acid compound aluminate compound can be easily combined. Similarly, by coexisting polyethylene glycol of a different polymer in the raw material aqueous solution, it can be easily combined, and the characteristics can be improved by combining these third and fourth components. . In addition, with the addition of low molecular weight polyvinyl alcohol or polyethylene glycol to the polymer component, the molecular motion of the polymer is activated or the irregularity is increased to promote the diffusion of ions. Binding sites can be increased to increase ionic conductivity.
[0075]
  The high ion conductive solid electrolyte of the present invention is a fuel cell.,Using dehumidifier, air conditioner, electrochromic device, electrolysis hydrogen generator, electrolytic hydrogen peroxide production device, humidity sensor, hydrogen sensor, primary battery, secondary battery, optical switch system or polyvalent metalPowerThe present invention can be applied to various electrochemical systems such as a pond system, and can contribute to the price reduction of these electrochemical systems. Furthermore, by using an alkaline electrolyte material, a low-priced material can be used as a material for peripheral members such as electrodes used in the electrochemical system.

Claims (16)

A high ion conductive solid electrolyte composed of a composite compound of a silicate compound containing water and polyvinyl alcohol ,
The composite compounds, high ionic conductive solid electrolyte weight / polyvinyl alcohol weight ratio was calculated as SiO ₂ of the silicate compound is characterized in that is between 0.03 to 3. The high ion conductive solid electrolyte according to claim 1 , wherein a weight ratio of the silicic acid compound converted to SiO ₂ / polyvinyl alcohol is between 0.03 and 0.2. An aqueous solution in which an alkali metal salt of silicic acid and polyvinyl alcohol are dissolved is added with an acid to neutralize it to a pH of 8 or less, and after removing water as a solvent, a silicic acid compound obtained by removing unnecessary salts, polyvinyl alcohol, and A high ion conductive solid electrolyte composed of a composite compound with water ,
The composite compounds, high ionic conductive solid electrolyte weight / polyvinyl alcohol weight ratio was calculated as SiO ₂ of the silicate compound is characterized in that is between 0.03 to 3. 4. The high ion conductive solid electrolyte according to claim 3 , wherein a treatment of heating at a temperature of 100 [deg.] C. or higher is performed before or after removing unnecessary salts. High ionic conductive solid electrolyte according to claim 3 or 4 w / polyvinyl alcohol weight ratio was calculated as SiO ₂ of the silicate compound is between 0.03 to 0.2. The raw material aqueous solution contains at least one kind of alkali metal salt selected from phosphoric acid, boric acid, and aluminate, and the produced composite compound contains at least one compound of phosphoric acid compound, boric acid compound, and aluminate compound Item 6. The high ion conductive solid electrolyte according to Item 3, 4 or 5 . The high ion conductive solid electrolyte according to claim 6 , wherein a weight ratio of the phosphate compound weight converted to P ₂ O _{5 to} polyvinyl alcohol is 0.035 or more. The acid according to claim 3 , 4, 5, 6 or 7, wherein at least one acid selected from hydrochloric acid, sulfuric acid, phosphoric acid and boric acid is used as the acid added to the raw material aqueous solution comprising an alkali metal salt of silicic acid and polyvinyl alcohol. The high ion conductive solid electrolyte described. Silicate compound and complex compound consisting of polyvinyl alcohol, hydrochloric acid, sulfuric acid, according to claim 3, immersed in at least one acid solution selected from phosphoric acid, high ionic according to 4, 5, 6, 7 or 8 Conductive solid electrolyte.   The high ion conductive solid electrolyte according to claim 9, wherein the acid solution to be dipped contains 30% by weight or more of phosphoric acid. The high compound according to claim 3 , 4, 5, 6, 7 or 8, wherein a composite compound comprising a silicic acid compound and polyvinyl alcohol is immersed in at least one solution selected from sodium hydroxide and potassium hydroxide. Ion conductive solid electrolyte. In a complex compound comprising a silicic acid compound and polyvinyl alcohol in at least one acid solution selected from hydrochloric acid, sulfuric acid and phosphoric acid after dipping in a compound selected from sodium hydroxide or potassium hydroxide The high ion conductive solid electrolyte according to claim 3 , wherein the high ion conductive solid electrolyte is subjected to an immersion treatment. The high ion conductive solid electrolyte according to claim 3 , 4, 5, 6, 7, 8, 9, 10, 11 or 12, wherein the average molecular weight of about 90% by weight or less of polyvinyl alcohol is 26,000 or less. 14. The high ion conductive solid electrolyte according to claim 3 , 4, 5, 6, 7, 8, 9, 10, 11 , 12, or 13 wherein about 50% by weight or less of polyvinyl alcohol is replaced with polyethylene glycol. Electrochemistry using a high ion conductive solid electrolyte, wherein the high ion conductive solid electrolyte is disposed between electrodes using the high ion conductive solid electrolyte according to any one of claims 1 to 14. system. The electrochemical system is a fuel cell , a dehumidifier, an air conditioner, an electrochromic device, an electrolysis-type hydrogen generator, an electrolytic hydrogen peroxide production device, a humidity sensor, a hydrogen sensor, a primary battery, a secondary battery, an optical switch system, or electrochemical system using the high ion conductive solid electrolyte according to claim 15 is any one of the batteries system using a polyvalent metal.