JP4719979B2 - Polymer electrolyte fuel cell - Google Patents

Description

【００１４】
また、モノマーＡとしては、重合すると環化重合して環状ポリマーとなるモノマー又は環状モノマーであって、上記モノマーＢの前駆体であるＣＦ₂＝ＣＦ（ＯＣＦ₂ＣＦＸ）_mＯ_p（ＣＦ₂）_nＳＯ₂Ｆと重合することにより主鎖に脂肪族環構造を有する含フッ素ポリマーとなるモノマーが好ましく、この場合カソード樹脂の溶媒への溶解性が良好となる。ここで、「主鎖に脂肪族環構造を有する」とは、繰り返し単位中の脂肪族環構造の炭素原子の１つ以上がポリマーの主鎖に共有されていることをいう。具体的な好ましい例としては、下記のモノマーが挙げられる。
【００１５】
ペルフルオロ（３−ブテニルビニルエーテル）（以下、ＢＶＥという。）、ペルフルオロ（アリルビニルエーテル）、ペルフルオロ（３，５−ジオキサ−１，６−ヘプタジエン）等の、環化重合しうるモノマー。
ペルフルオロ（２，２−ジメチル−１，３−ジオキソール）（以下、ＰＤＤという。）、ペルフルオロ（１，３−ジオキソール）、ペルフルオロ（２−メチレン−４−メチル−１，３−ジオキソラン）、ペルフルオロ（４−メトキシ−１，３−ジオキソール）等の、環状モノマー。
【００１６】
本発明においてカソード樹脂であるモノマーＡに基づく繰り返し単位とモノマーＢに基づく繰り返し単位とを含む共重合体は、ランダム共重合体、交互共重合体、ブロック共重合体、グラフト共重合体のいずれであってもよい。比表面積の高い樹脂を得やすく、重合が容易であるという観点から、特にランダム共重合体又はブロック共重合体が好ましい。
【００１７】
上記共重合体がブロック共重合体又はグラフト共重合体である場合は、モノマーＡに基づく繰り返し単位を含む単独重合体又は共重合体からなるセグメントａと、モノマーＢに基づく繰り返し単位を含む単独重合体又は共重合体からなるセグメントｂとからなることが好ましい。特に、上述のスルホン酸基を有するペルフルオロビニル化合物の前駆体はラジカル反応性が低いので、セグメントｂは上記ペルフルオロビニル化合物に基づく繰り返し単位と他のモノマーに基づく繰り返し単位とを含む共重合体であることが好ましい。
【００１８】
当該他のモノマーとしては、テトラフルオロエチレン（以下、ＴＦＥという。）、ヘキサフルオロプロピレン等のペルフルオロレフィン、ペルフルオロ（アルキルビニルエーテル）等が挙げられる。また、他のモノマーとして上述の環状モノマーや環化重合するモノマーも使用できる。セグメントｂは、特に、ＴＦＥ／（式１〜４のいずれかで表される化合物）共重合体からなることが好ましい。また、セグメントａは、特にＰＤＤ単独重合体（以下、ＰＰＤＤという。）、ＴＦＥ／ＰＤＤ共重合体、ポリ（ペルフルオロ（１，３−ジオキソール））、ポリ（ペルフルオロ（２−メチレン−４−メチル−１，３−ジオキソラン））等からなることが好ましい。なお、本明細書において、Ａ／Ｂ共重合体とは、Ａに基づく繰り返し単位とＢに基づく繰り返し単位とからなる共重合体をいう。
【００１９】
カソード樹脂がランダム共重合体からなる場合も、スルホン酸基を有するペルフルオロビニル化合物に基づく繰り返し単位と環状モノマー又は環化重合しうるモノマーに基づく繰り返し単位のほかに、ＴＦＥ、ヘキサフルオロプロピレン等のペルフルオロオレフィン、ペルフルオロ（アルキルビニルエーテル）等に基づく繰り返し単位が含まれていてもよい。
【００２０】
カソード樹脂がグラフト共重合体である場合は、ブロック共重合体同様にセグメントａとセグメントｂからなることが好ましく、セグメントａが主鎖で、スルホン酸基を有するセグメントｂが側鎖であることが好ましい。
【００２１】
本発明において、カソード樹脂のイオン交換容量（以下、Ａ_Rという。）は、０．５〜２ミリ当量／ｇ乾燥樹脂であることが好ましい。Ａ_Rが０．５ミリ当量／グラム乾燥樹脂未満では、プロトン導電性が低いため燃料電池の抵抗が高くなり高出力が得られないおそれがある。また、２．０ミリ当量／グラム乾燥樹脂を超えると、樹脂自体の結晶性が低下するため、固体状態の維持が困難となりやすい。カソード樹脂中のモノマーＢの含有割合の範囲は、カソード樹脂が上記イオン交換容量となるように選択することが好ましい。
【００２２】
カソード樹脂がブロック共重合体又はグラフト重合体である場合、セグメントａ／セグメントｂの割合は、モル比で１０／９０〜６０／４０、特に２０／８０〜４０／６０であることが好ましい。また、カソード樹脂の分子量は５×１０³〜５×１０⁶、特に１×１０⁴〜３×１０⁶であることが好ましい。また、セグメントａ、セグメントｂそれぞれの分子量はいずれも１×１０³〜５×１０⁶、特に２×１０³〜２×１０⁶であることが好ましい。また、ブロック共重合体である場合、ＡＢ型であってもＡＢＡ型であってもよい。
【００２３】
また、カソード樹脂がランダム共重合体又は交互共重合体である場合、モノマーＡに基づく繰り返し単位／モノマーＢに基づく繰り返し単位の割合はモル比で１４／８６〜８２／１８、特に２０／８０〜５２／４８であることが好ましい。また、分子量は、５×１０³〜５×１０⁶、特に１×１０⁴〜３×１０⁶であることが好ましい。
【００２４】
触媒層を作製する場合、通常、カソード樹脂と触媒とを含む塗工液により作製するので、カソード樹脂の分子量が大きすぎるとカソード樹脂が溶媒又は分散媒に溶解又は良好に分散しにくくなるので好ましくない。またカソード樹脂の分子量が小さすぎると、高温でクリープしやすく、発電中に樹脂が触媒を被覆した状態が維持できないことがあるので好ましくない。
【００２５】
上記塗工液は、通常の手法に従って作製でき、例えばカソード樹脂の溶液又は分散液に触媒を分散させて得られる。ここで触媒としては、白金又は白金合金等の金属触媒も使用できるが、特に白金微粒子又は白金合金微粒子を導電性のカーボン粉末に担持させた担持触媒が好ましい。カソードの触媒層は、上記塗工液を用いて、以下の２つのいずれかの方法で膜−電極接合体を得ることが好ましい。
【００２６】
第１の方法は上記塗工液を、好ましくは表面にフッ素樹脂とカーボン粉末とからなる撥水性のカーボン粉末層が形成されたカーボンクロス又はカーボンペーパーからなるガス拡散層上に塗布して乾燥後、陽イオン交換膜にホットプレス法などにより密着させる方法である。ここでガス拡散層は、集電体の役割を有しかつ触媒層にガスをより均一に供給できるように配置されるものであって、本発明における電極はアノード、カソードともに触媒層とガス拡散層とから構成されていることが好ましい。
【００２７】
第２の方法は、ＰＴＦＥやポリエチレンテレフタレート（ＰＥＴ）シート等の基材に上記塗工液を塗布し乾燥後、塗工面に陽イオン交換膜を積層しホットプレスした後基材を剥離することにより塗工層を陽イオン交換膜上に転写して触媒層とする方法である。この場合、上述のガス拡散層を触媒層に積層して配置させるか、又は積層した後ホットプレスして触媒層と接合させることが好ましい。
【００２８】
また、その他の方法として、ガス拡散層上に形成した触媒層を接着法（特開平７−２２０７４１、特開平７−２５４４２０）等により陽イオン交換膜と接合する方法などもある。また、陽イオン交換膜に直接上記塗工液を塗工する方法も採用できる。
【００２９】
本発明において、カソードに含まれる触媒と含フッ素樹脂とは、質量比で触媒：含フッ素樹脂＝４０：６０〜９５：５、特に５５：４５〜８０：２０であることが、触媒層の導電性、触媒層内の反応サイトの数、及び生成水等の水の排出性を考慮すると好ましい。なお、ここでいう触媒の質量は、カーボン等の担体に白金等の貴金属が担持された担持触媒の場合は該担体の質量も含む。
【００３０】
本発明における固体高分子電解質膜は特には限定されないが、例えば、スルホン酸基、リン酸基又はフェノール系水酸基等の陽イオン交換基を有する樹脂からなることが好ましい。具体的には、スルホン酸基を有するペルフルオロカーボン重合体からなることが好ましく、例えばＴＦＥ／（式１〜４のいずれかで表される化合物）共重合体が好ましい。また、本発明におけるカソード樹脂からなる膜も使用できる。
【００３１】
上記共重合体からなる膜の場合、熱流動性のある前記共重合体の前駆体（例えば末端に−ＳＯ₃Ｈ基ではなく−ＳＯ₂Ｆ基を有する共重合体）を熱プレス成形、ロール成形、押出し成形等の公知の方法で膜状に成形し、加水分解、酸型化処理することにより得られる。また、前記共重合体をアルコール等の溶媒に溶解した溶液から、溶媒キャスト法で得ることもできる。
【００３２】
また、固体高分子電解質膜として、スルホン酸基やリン酸基等を有する炭化水素系樹脂又は部分フッ素化された炭化水素系樹脂からなる膜も使用できる。さらに、上記の陽イオン交換樹脂を補強材と複合化した膜からなるものでもよい。当該補強材としては、例えばポリエチレン、ＰＴＦＥ、ＴＦＥ／ペルフルオロ（プロピルビニルエーテル）共重合体やＴＦＥ／ヘキサフルオロプロピレン共重合体等からなるフィブリル状、織布状、不織布状及び多孔体の形態の補強材が使用できる。
【００３３】
固体高分子電解質膜の厚さは、例えば２０〜１５０μｍのものが使用される。２０μｍより薄いと水素ガスリークが無視できない程度に増えるためカソード上で酸素と反応しセル電圧が大きく低下する。と同時にホットプレス時や電極押し付け時に局所的に膜が薄くなり、極端に多いガス漏れやショートのおそれがある。１５０μｍより厚いと膜の電気抵抗が高くなるとともに、膜内の水移動に時間がかかるため燃料電池の出力特性が低下する。特には２０〜８０μｍの厚さが好ましい。
【００３４】
本発明におけるアノードは、カソードと同じであってもよいが、従来より使用されている非多孔性のイオン交換樹脂を用いた触媒層を有するガス拡散電極等からなっていてもよい。アノードはカソードと同様に形成でき、固体高分子電解質膜の片面にアノード、もう一方の面にカソードが配置され接合されることにより膜−電極接合体が得られる。
【００３５】
得られた膜−電極接合体は、アノードには水素を含む燃料ガス、カソードには酸素を含む酸化剤ガス（空気、酸素等）がそれぞれ供給されるように、ガスの通路となる溝が形成された導電性カーボン板等からなるセパレータの間に挟まれ、セルに組み込まれることにより本発明の固体高分子型燃料電池が得られる。
【００３６】
【実施例】
以下に、本発明を実施例（例１〜２）及び比較例（例３）により具体的に説明するが、本発明はこれらに限定されない。なお、以下の例において、カソード樹脂の原料に用いるものを下記の略号で表す。
【００３７】
ＰＳＶＥ：ＣＦ₂＝ＣＦＯＣＦ₂ＣＦ（ＣＦ₃）ＯＣＦ₂ＣＦ₂ＳＯ₃Ｈ、
ＩＰＰ：（ＣＨ₃）₂ＣＨＯＣ（＝Ｏ）ＯＯＣ（＝Ｏ）ＯＣＨ（ＣＨ₃）₂、
ＨＣＦＣ１４１ｂ：ＣＨ₃ＣＣｌ₂Ｆ、
ＨＣＦＣ２２５ｃｂ：ＣＣｌＦ₂ＣＦ₂ＣＨＣｌＦ。
【００３８】
〔例１〕
まず、カソードの触媒層に含有させる樹脂として、ＰＤＤ／ＰＳＶＥ共重合体を以下のように、製造した。
内容積１Ｌのオートクレーブを真空脱気後、窒素置換し、再び真空脱気した。これにＰＤＤを４５．０ｇ、ＣＦ₂＝ＣＦＯＣＦ₂ＣＦ（ＣＦ₃）ＯＣＦ₂ＣＦ₂ＳＯ₂Ｆを１２３．３ｇ、ＨＣＦＣ２２５ｃｂを４３７．９ｇ吸入させ、さらに０．５ｇのＩＰＰを１０ｇのＨＣＦＣ２２５ｃｂに溶解した溶液を吸入させた。オートクレーブ内を窒素で０．１ＭＰａとし、３０℃に加熱、撹拌し、重合を開始した。１０．５時間後、冷却、パージして重合を止め、精製物をヘキサンに投入することで再沈殿させ、ＨＣＦＣ１４１ｂで洗浄した。ろ過後、８０℃で１６時間、真空乾燥することにより、１４．５ｇの白色のポリマーを得た。
【００３９】
このポリマーについて、元素分析で硫黄の含有量を求め、ＰＤＤに基づく繰り返し単位とＣＦ₂＝ＣＦＯＣＦ₂ＣＦ（ＣＦ₃）ＯＣＦ₂ＣＦ₂ＳＯ₂Ｆに基づく繰り返し単位との比を求めたところ、７７．４／２２．６（モル比）であった。また、この共重合体はランダム共重合体であった。次に、得られたポリマーの−ＳＯ₂Ｆ基を加水分解、酸型化してスルホン酸基に変換してＰＤＤ／ＰＳＶＥ共重合体を得て、さらにこれをエタノールに溶解して９質量％の溶液を得た。
【００４０】
なお、上記ＰＤＤ／ＰＳＶＥ共重合体の比表面積をＢＥＴ法により測定した。測定方法は、ＰＤＤ／ＰＳＶＥ共重合体を数ｍｍ〜数百ｍｍ程度にまで小さく砕き、さらに液体窒素温度にて凍結粉砕装置により平均粒径が約１０μｍ以下となるまで粉砕し、乾燥してＢＥＴ装置に組み込み窒素吸着させて、比表面積を測定した。結果を表１に示す。
【００４１】
次にカソード用の触媒層形成用の塗工液を次のようにして調製した。白金を５４質量％担持した白金担持カーボンと上記ＰＤＤ／ＰＳＶＥ共重合体とを質量比で６：４となるようにして、上記のＰＤＤ／ＰＳＶＥ共重合体を含むエタノール溶液に分散させた。得られた液を撹拌した後、蒸発乾固して容器の壁面にこびり付いた固形分（触媒と樹脂）を掻き出して粉砕した。その後、得られた粉末に２，２，３，３，３−ペンタフルオロ−１−プロパノールを加えて固形分濃度が５質量％となるように分散させ、これをカソード用の触媒層形成用の塗工液とした。
【００４２】
電極のガス拡散層には、カソード、アノードともに撥水性カーボンクロス（織布）の片方の表面にカーボンブラックとＰＴＦＥとからなるカーボン粉末層が形成されていてカーボンクロスの表面の細孔が部分的に上記カーボン粉末層で目詰めされたものを、ホットプレスし、表面を平坦化して用いた（厚さ約３４０μｍ）。次にカソード用の触媒層形成用の塗工液を、上記ガス拡散層のカーボン粉末層側に、白金付着量が０．８ｍｇ／ｃｍ²となるように１回塗布して乾燥させ、この層をカソード触媒層とし、カソードを得た。
【００４３】
一方、アノードの触媒層は次のようにして形成した。イオン交換容量１．１ミリ当量／ｇ乾燥樹脂のＴＦＥ／ＰＳＶＥ共重合体を、９質量％の濃度となるようにエタノールに溶解した。
【００４４】
アノード用の触媒層形成用の塗工液としては、白金が４０質量％担持された白金担持カーボンと上記ＴＦＥ／ＰＳＶＥ共重合体とを質量比で７：３となるようにＴＦＥ／ＰＳＶＥ共重合体を９質量％含むエタノール溶液を添加し、次いで固形分濃度が５質量％となるようにさらにエタノールと水を加えて得た。なお、塗工液中のエタノール／水の割合は質量比で１：１とした。この塗工液を用いた以外はカソードと同様にして上記ガス拡散層のカーボン粉末層側に白金付着量が０．５ｍｇ／ｃｍ²となるように１回塗布して乾燥させ、これをアノード触媒層とし、アノードを得た。カソード及びアノードは、それぞれ有効電極面積が２５ｃｍ²となるように切り出した。
【００４５】
固体高分子電解質膜としては、スルホン酸基を有するペルフルオロカーボン重合体からなるイオン交換膜（商品名：フレミオンＨＲ、旭硝子社製、イオン交換容量１．１ミリ当量／ｇ乾燥樹脂、乾燥膜厚５０μｍ）を使用し、上記で得られたカソード及びアノードと接合した。具体的には、カソードとアノードとをそれぞれの触媒層を内側に向けて対向させ、その間に上記膜を挟み込んだ状態でホットプレスし、膜・電極接合体を得た。
【００４６】
得られた膜・電極接合体を、ガスの流路が形成された２枚のカーボン製のセパレータで挟み込み、セルに組み込んで燃料電池特性の測定を行った。電流−電圧特性の測定は、電子負荷（高砂製作所製、ＦＫ４００Ｌ）と直流電源（高砂製作所製、ＥＸ７５０Ｌ）を用いて行った。セル駆動条件は、電流密度１Ａ／ｃｍ²にて水素入口圧力０．１５ＭＰａ、空気入口圧力０．１５ＭＰａで、水素利用率７０％、空気利用率４０％、セル作動温度８０℃とした。セルへの通電開始から約２４０分後の電流−電圧特性及び内部抵抗（ｉＲ損失）を測定した。セルの電流−ｉＲフリー電圧特性（ｉＲ損失の影響を除いた特性）の測定結果を図１に示す。
【００４７】
〔例２〕
カソードの触媒層に含有させる樹脂として、（ＰＰＤＤ）−（ＴＦＥ／ＰＳＶＥ共重合体）−（ＰＰＤＤ）ブロックポリマーを用い、２，２，３，３，３−ペンタフルオロ−１−プロパノールに分散する前に使用する溶媒としてエタノールのかわりにエタノールと１Ｈ−ペルフルオロヘキサンの混合溶媒（質量比で１：１）を用いた以外は、例１と同様にしてカソードを作製し、該カソードを用いた以外は例１と同様にして膜・電極接合体を作製してセルに組み込んだ。カソードに使用した樹脂のＢＥＴ法による比表面積測定結果を表１に、セルの電流−ｉＲフリー電圧特性の測定結果を図１に示す。
【００４８】
なお、ここで使用した（ＰＰＤＤ）−（ＴＦＥ／ＰＳＶＥ共重合体）−（ＰＰＤＤ）ブロックポリマーは以下のように合成した。
まず、ＴＦＥ／ＰＳＶＥ共重合体セグメントを重合した。脱気した内容積１Ｌのオートクレーブに、４．１５ｇのＩ（ＣＦ₂）₄Ｉと７７８．５ｇのＰＳＶＥを吸入させた後に４０℃に加熱した。５８ｇのＴＦＥを圧入後、７．５３ｇのＩＰＰを７８．２ｇのＨＣＦＣ２２５ｃｂに溶解した溶液６ｍＬを圧入し重合を開始した。圧力を一定（ゲージ圧で０．４５ＭＰａ）に保ちながら重合を継続し、重合速度の低下に伴い、上記ＩＰＰの溶液をさらに添加して重合を続けた。添加したＩＰＰの総量は１．６６ｇであった。ＴＦＥが８０ｇ入ったところで加熱を止めてＴＦＥをパージし、重合を止めた。得られた溶液をＨＣＦＣ１４１ｂに注いで凝集後、洗浄、ろ過、乾燥することにより室温においてエラストマー状のポリマー２３５ｇを得た。
【００４９】
次に、ＰＤＤをブロック共重合させた。５００ｍＬのガラス製のフラスコに上記で得られたＴＦＥ／ＰＳＶＥ共重合体を５０ｇと、２５０ｇの１Ｈ−ペルフルオロヘキサンと２５ｇのＰＤＤを入れて撹拌し、ＴＦＥ／ＰＳＶＥ共重合体を溶解させた。これに０．０８２ｇのＩＰＰを５ｇのＣ₆Ｆ₁₃Ｈに溶解した溶液を加え、３０℃で６５時間重合した。この反応液をＨＣＦＣ１４１ｂに注いで凝集後、洗浄、ろ過、乾燥することにより、白色のポリマー６２．９ｇを得た。次にこのポリマーを水酸化カリウム及び硫酸を用いて処理し、酸型化した。
【００５０】
〔例３（比較例）〕
カソード樹脂として、例１においてアノードの触媒層に用いた樹脂と同じものを用いた以外は、例１と同様にしてカソード、アノード、及び膜・電極接合体を作製した。カソード樹脂のＢＥＴ法による比表面積測定結果を表１に、セルの電流−ｉＲフリー電圧特性の測定結果を図１に示す。
【００５１】
【表１】

【００５２】
【発明の効果】
本発明によれば、カソードの触媒層に含まれるカソード樹脂として比表面積が大きいものを使用しているため、過電圧の小さなカソードが得られているものと考えられる。その結果、高出力密度の固体高分子型燃料電池が提供できる。
【図面の簡単な説明】
【図１】実施例及び比較例のセルの電流−ｉＲフリー電圧特性を示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a polymer electrolyte fuel cell, and more particularly to a catalyst layer of a cathode thereof.
[0002]
[Prior art]
Hydrogen / oxygen fuel cells are attracting attention as a power generation system that has almost no adverse effect on the global environment because its reaction product is in principle only water. Polymer electrolyte fuel cells were once installed on spacecraft under the Gemini and Biosatellite programs, but the battery power density at that time was low. Since then, higher performance alkaline fuel cells have been developed, and alkaline fuel cells have been adopted for space use up to the current space shuttle.
[0003]
However, in recent years, solid polymer fuel cells have attracted attention again due to technological advances. There are two reasons for this. (1) A highly conductive membrane has been developed as a solid polymer electrolyte. (2) The catalyst used for the gas diffusion electrode layer is supported on carbon, and further coated with an ion exchange resin, an extremely large activity can be obtained. Many studies have been made on a method for producing a joined body of an electrode of a polymer electrolyte fuel cell and a solid polymer electrolyte membrane (hereinafter simply referred to as an electrode / membrane joined body).
[0004]
The polymer electrolyte fuel cell currently being studied has a drawback that it is difficult to effectively use the exhaust heat for auxiliary power of the fuel cell because the operating temperature is as low as 50 to 120 ° C. In order to compensate for this, the polymer electrolyte fuel cell is required to have a particularly high power density. Further, as an issue for practical use, development of an electrode / membrane assembly capable of obtaining high energy efficiency and high power density even under operating conditions with high fuel and air utilization is required.
[0005]
In order to increase the power density of the fuel cell, it is particularly important to reduce the overvoltage of the oxygen reduction of the cathode accompanied by a great energy loss. For this purpose, in particular, increase the reaction sites of the cathode (where the catalyst and the ion-exchange resin that covers the catalyst are in contact with each other and supply protons, electrons, and oxygen), diffusion of oxygen-containing gas to the reaction sites, It is effective to speed up the supply.
[0006]
[Problems to be solved by the invention]
In order to increase the reaction site in order to increase the power density, it is effective to increase the ratio of the resin covering the catalyst in the catalyst layer. In this case, the following problems occur. That is, when the proportion of the resin increases, a network of the resins is built and a proton passage is secured, but the thickness of the coating layer made of the resin increases. For this reason, the diffusion of the fuel gas (the gas containing hydrogen or the gas containing oxygen) in the resin is delayed, the supply of the fuel gas onto the catalyst is delayed, and the overvoltage tends to increase. Moreover, when there are too many said resins, the pore of a catalyst layer is filled with the said resin, and porosity is easy to be lost.
[0007]
Accordingly, the present invention provides a polymer electrolyte fuel cell having a cathode or anode with a small activation overvoltage, particularly an activation overvoltage by increasing the supply rate of the reaction gas to the catalyst in the coating resin layer in the catalyst layer. An object of the present invention is to provide a polymer electrolyte fuel cell having a high output density with reduced activation overvoltage of a cathode that tends to be large.
[0008]
[Means for Solving the Problems]
The present invention relates to an anode and a cathode comprising a gas diffusion electrode having a catalyst layer containing a fluorinated ion exchange resin having a sulfonic acid group and a catalyst, and a solid polymer electrolyte disposed between the anode and the cathode in the solid polymer fuel cell and a membrane, wherein the fluorinated ion exchange resin contained in the catalyst layer of the cathode state, and are measured specific surface area of 3m ² / g or more by the BET method, an ion-exchange group And a perfluorovinyl compound represented by CF ₂ ═CF (OCF ₂ CFX) _m O _p (CF ₂ ) _n SO ₃ H (wherein X is a fluorine atom or a trifluoromethyl group) in and, m is an integer of 0 to 3, n is an integer from 1 to 12, p is a copolymer der Rukoto comprising the repeating units based on a 0 or 1.) To provide a polymer electrolyte fuel cell according to claim.
[0009]
When the specific surface area of the fluorine-containing ion exchange resin (hereinafter referred to as cathode resin) contained in the catalyst layer of the cathode is 3 m ² / g or more, the resin itself is usually porous and the surface of the resin is uneven. . When the cathode resin is porous, the gas permeation rate is high in the coating layer of the catalyst made of the resin, and the reaction gas is considered to reach the catalyst surface quickly. For this reason, it is considered that the reaction rate is substantially increased and the activation overvoltage of the electrode is decreased. Further, when the surface of the cathode resin is uneven, the contact area between the resin and the catalyst increases, so that the number of reaction sites increases, and as a result, the cell voltage seems to increase.
[0010]
On the other hand, if the specific surface area of the cathode resin is too large, the resin becomes extremely porous, and it becomes difficult to maintain the shape of the resin itself, so that the catalyst layer tends to be structurally unstable. Therefore it is preferable that the specific surface area of the cathode resin is less than 200m ² / g, it is preferable in particular 5 to 50 m ² / g. In addition, the specific surface area of resin in this specification means the specific surface area measured with the following method. That is, after the resin was crushed to several mm to several hundred mm, it was further pulverized using a freeze pulverizer at a liquid nitrogen temperature until the average particle size of the resin became several μm to several tens of μm. The powder is used as a sample, dried, and the specific surface area is measured by the BET method.
[0011]
In the present invention, the cathode resin is composed of a repeating unit based on a fluorine-containing monomer substantially having no ion exchange group (hereinafter referred to as monomer A), CF ₂ ═CF (OCF ₂ CFX) _m O _p (CF ₂ ) _n. A perfluorovinyl compound represented by SO ₃ H (wherein, X is a fluorine atom or a trifluoromethyl group, m is an integer of 0 to 3, n is an integer of 1 to 12, and p is 0 or 1. Hereinafter, it is preferably a copolymer containing a repeating unit based on the monomer B). In particular, a perfluoropolymer is preferable from the viewpoint of durability. Here, the perfluoropolymer refers to a polymer in which all hydrogen atoms (excluding sulfonic acid groups) are substituted with fluorine atoms.
[0012]
In particular, the perfluorovinyl compound is preferably a compound represented by any one of formulas 1 to 4. However, in the following formula, q is an integer of 1 to 8, r is an integer of 1 to 8, s is an integer of 1 to 8, and t is an integer of 1 to 3.
[0013]
[Chemical 1]

[0014]
The monomer A is a monomer or a cyclic monomer that undergoes cyclization polymerization to form a cyclic polymer upon polymerization, and is a precursor of the monomer B CF ₂ ═CF (OCF ₂ CFX) _m O _p (CF ₂ ) _A monomer that becomes a fluorine-containing polymer having an aliphatic ring structure in the main chain by polymerizing with _{n 2} SO ₂ F is preferable. In this case, the solubility of the cathode resin in a solvent is good. Here, “having an aliphatic ring structure in the main chain” means that one or more carbon atoms of the aliphatic ring structure in the repeating unit are shared by the main chain of the polymer. Specific preferred examples include the following monomers.
[0015]
Monomers capable of cyclopolymerization such as perfluoro (3-butenyl vinyl ether) (hereinafter referred to as BVE), perfluoro (allyl vinyl ether), perfluoro (3,5-dioxa-1,6-heptadiene) and the like.
Perfluoro (2,2-dimethyl-1,3-dioxole) (hereinafter referred to as PDD), perfluoro (1,3-dioxole), perfluoro (2-methylene-4-methyl-1,3-dioxolane), perfluoro ( Cyclic monomers such as 4-methoxy-1,3-dioxole).
[0016]
In the present invention, the copolymer including the repeating unit based on the monomer A and the repeating unit based on the monomer B as the cathode resin is any of a random copolymer, an alternating copolymer, a block copolymer, and a graft copolymer. There may be. From the viewpoint of easily obtaining a resin having a high specific surface area and easy polymerization, a random copolymer or a block copolymer is particularly preferable.
[0017]
When the copolymer is a block copolymer or a graft copolymer, a segment a consisting of a homopolymer or copolymer containing a repeating unit based on the monomer A and a homopolymer containing a repeating unit based on the monomer B The segment b is preferably composed of a polymer or a copolymer b. In particular, since the precursor of the above-mentioned perfluorovinyl compound having a sulfonic acid group has low radical reactivity, the segment b is a copolymer containing a repeating unit based on the perfluorovinyl compound and a repeating unit based on another monomer. It is preferable.
[0018]
Examples of such other monomers include tetrafluoroethylene (hereinafter referred to as TFE), perfluoroolefins such as hexafluoropropylene, and perfluoro (alkyl vinyl ether). Further, as the other monomer, the above-mentioned cyclic monomer or a monomer that undergoes cyclopolymerization can also be used. The segment b is particularly preferably made of a TFE / (compound represented by any one of formulas 1 to 4) copolymer. In addition, the segment a includes PDD homopolymer (hereinafter referred to as PPDD), TFE / PDD copolymer, poly (perfluoro (1,3-dioxole)), poly (perfluoro (2-methylene-4-methyl-) 1,3-dioxolane)) and the like. In this specification, the A / B copolymer refers to a copolymer composed of a repeating unit based on A and a repeating unit based on B.
[0019]
Even when the cathode resin is made of a random copolymer, in addition to a repeating unit based on a perfluorovinyl compound having a sulfonic acid group and a repeating unit based on a cyclic monomer or a monomer capable of cyclopolymerization, a perfluorocarbon such as TFE or hexafluoropropylene is used. Repeating units based on olefins, perfluoro (alkyl vinyl ethers) and the like may be included.
[0020]
When the cathode resin is a graft copolymer, it is preferably composed of a segment a and a segment b as in the block copolymer, and the segment a is a main chain and the segment b having a sulfonic acid group is a side chain. preferable.
[0021]
In the present invention, the ion exchange capacity of the cathode resin (hereinafter. Referred A _R) is preferably 0.5 to 2 meq / g dry resin. If the _AR is less than 0.5 meq / g dry resin, the proton conductivity is low, so the resistance of the fuel cell is high and high output may not be obtained. On the other hand, if it exceeds 2.0 milliequivalents / gram dry resin, the crystallinity of the resin itself is lowered, so that it is difficult to maintain a solid state. The range of the content ratio of the monomer B in the cathode resin is preferably selected so that the cathode resin has the ion exchange capacity.
[0022]
When the cathode resin is a block copolymer or a graft polymer, the ratio of segment a / segment b is preferably 10/90 to 60/40, particularly 20/80 to 40/60 in terms of molar ratio. The molecular weight of the cathode resin is preferably 5 × 10 ^{3 to} 5 × 10 ⁶ , particularly preferably 1 × 10 ^{4 to} 3 × 10 ⁶ . The molecular weight of each of the segment a and the segment b is preferably 1 × 10 ^{3 to} 5 × 10 ⁶ , particularly preferably 2 × 10 ³ to 2 × 10 ⁶ . Moreover, when it is a block copolymer, it may be AB type or ABA type.
[0023]
When the cathode resin is a random copolymer or an alternating copolymer, the ratio of the repeating unit based on monomer A / the repeating unit based on monomer B is 14/86 to 82/18, particularly 20/80 to Preferably it is 52/48. The molecular weight is preferably 5 × 10 ^{3 to} 5 × 10 ⁶ , particularly preferably 1 × 10 ^{4 to} 3 × 10 ⁶ .
[0024]
When preparing the catalyst layer, it is usually prepared with a coating solution containing the cathode resin and the catalyst. Therefore, if the molecular weight of the cathode resin is too large, it is difficult to dissolve the cathode resin in the solvent or dispersion medium or to disperse it well. Absent. Moreover, if the molecular weight of the cathode resin is too small, it is not preferable because creep is likely at high temperatures and the state in which the resin is coated with the catalyst during power generation may not be maintained.
[0025]
The coating liquid can be prepared according to a normal method, and can be obtained, for example, by dispersing a catalyst in a cathode resin solution or dispersion. Here, a metal catalyst such as platinum or a platinum alloy can be used as the catalyst, but a supported catalyst in which platinum fine particles or platinum alloy fine particles are supported on conductive carbon powder is particularly preferable. For the cathode catalyst layer, it is preferable to obtain a membrane-electrode assembly by using the above-mentioned coating solution by one of the following two methods.
[0026]
The first method is to apply the above coating liquid onto a gas diffusion layer made of carbon cloth or carbon paper, preferably having a water-repellent carbon powder layer made of fluororesin and carbon powder on the surface, and then drying. This is a method of adhering to a cation exchange membrane by a hot press method or the like. Here, the gas diffusion layer has a role of a current collector and is arranged so that gas can be supplied more uniformly to the catalyst layer. In the present invention, the anode and the cathode are both the catalyst layer and the gas diffusion layer. It is preferable that it is comprised from the layer.
[0027]
The second method is to apply the above coating liquid to a base material such as PTFE or polyethylene terephthalate (PET) sheet, dry it, laminate a cation exchange membrane on the coated surface, hot press, and then peel off the base material. In this method, the coating layer is transferred onto a cation exchange membrane to form a catalyst layer. In this case, it is preferable that the gas diffusion layer described above is stacked on the catalyst layer, or is hot-pressed after being stacked and bonded to the catalyst layer.
[0028]
As another method, there is a method in which a catalyst layer formed on a gas diffusion layer is bonded to a cation exchange membrane by an adhesion method (Japanese Patent Laid-Open Nos. 7-220741, 7-254420) or the like. Moreover, the method of coating the said coating liquid directly on a cation exchange membrane is also employable.
[0029]
In the present invention, the catalyst and the fluorine-containing resin contained in the cathode have a mass ratio of catalyst: fluorine-containing resin = 40: 60 to 95: 5, particularly 55:45 to 80:20, In view of the property, the number of reaction sites in the catalyst layer, and the discharge of water such as product water. The mass of the catalyst here includes the mass of the carrier in the case of a supported catalyst in which a noble metal such as platinum is supported on a carrier such as carbon.
[0030]
The solid polymer electrolyte membrane in the present invention is not particularly limited, but is preferably made of a resin having a cation exchange group such as a sulfonic acid group, a phosphoric acid group, or a phenolic hydroxyl group. Specifically, it is preferably composed of a perfluorocarbon polymer having a sulfonic acid group, for example, a TFE / (compound represented by any one of formulas 1 to 4) copolymer. Also, a membrane made of a cathode resin in the present invention can be used.
[0031]
In the case of a film made of the above-mentioned copolymer, the precursor of the copolymer having heat fluidity (for example, a copolymer having —SO ₂ F groups instead of —SO ₃ H groups at the end) is subjected to hot press molding, roll It can be obtained by forming into a film by a known method such as molding or extrusion, followed by hydrolysis and acidification. Moreover, it can also obtain by the solvent casting method from the solution which melt | dissolved the said copolymer in solvents, such as alcohol.
[0032]
Further, as the solid polymer electrolyte membrane, a membrane made of a hydrocarbon resin having a sulfonic acid group, a phosphoric acid group or the like or a partially fluorinated hydrocarbon resin can be used. Further, it may be made of a membrane obtained by combining the above cation exchange resin with a reinforcing material. Examples of the reinforcing material include reinforcing materials in the form of fibrils, woven fabrics, non-woven fabrics and porous materials made of polyethylene, PTFE, TFE / perfluoro (propyl vinyl ether) copolymer, TFE / hexafluoropropylene copolymer, and the like. Can be used.
[0033]
The thickness of the solid polymer electrolyte membrane is, for example, 20 to 150 μm. If it is thinner than 20 μm, the hydrogen gas leak increases to a level that cannot be ignored, so that it reacts with oxygen on the cathode and the cell voltage is greatly reduced. At the same time, the film is locally thinned during hot pressing or electrode pressing, and there is a risk of excessive gas leakage or short circuit. If it is thicker than 150 μm, the electrical resistance of the membrane becomes high, and it takes time to move water in the membrane, so that the output characteristics of the fuel cell deteriorate. In particular, a thickness of 20 to 80 μm is preferable.
[0034]
The anode in the present invention may be the same as the cathode, but may also comprise a gas diffusion electrode having a catalyst layer using a non-porous ion exchange resin that has been conventionally used. The anode can be formed in the same manner as the cathode, and the membrane-electrode assembly can be obtained by arranging and bonding the anode on one side of the solid polymer electrolyte membrane and the cathode on the other side.
[0035]
In the obtained membrane-electrode assembly, a groove serving as a gas passage is formed so that a fuel gas containing hydrogen is supplied to the anode and an oxidant gas containing oxygen (air, oxygen, etc.) is supplied to the cathode. The polymer electrolyte fuel cell of the present invention is obtained by being sandwiched between separators made of a conductive carbon plate or the like and incorporated into a cell.
[0036]
【Example】
Hereinafter, the present invention will be specifically described with reference to Examples (Examples 1 to 2) and Comparative Examples (Example 3), but the present invention is not limited to these. In the following examples, materials used for the cathode resin material are represented by the following abbreviations.
[0037]
PSVE: CF ₂ = CFOCF ₂ CF (CF ₃ ) OCF ₂ CF ₂ SO ₃ H,
_{IPP: (CH 3) 2 CHOC} (= O) OOC (= O) OCH (CH 3) 2,
HCFC141b: CH ₃ CCl ₂ F,
HCFC225cb: CClF ₂ CF ₂ CHClF.
[0038]
[Example 1]
First, a PDD / PSVE copolymer was manufactured as follows as a resin to be contained in the cathode catalyst layer.
The autoclave with an internal volume of 1 L was vacuum degassed and then purged with nitrogen, followed by vacuum degassing again. 45.0 g of PDD, 123.3 g of CF ₂ = CFOCF ₂ CF (CF ₃ ) OCF ₂ CF ₂ SO ₂ F, 437.9 g of HCFC225cb were sucked into this, and 0.5 g of IPP was dissolved in 10 g of HCFC225cb. The solution was inhaled. The inside of the autoclave was adjusted to 0.1 MPa with nitrogen, heated and stirred at 30 ° C., and polymerization was started. After 10.5 hours, the polymerization was stopped by cooling and purging, and the purified product was re-precipitated by adding it to hexane and washed with HCFC141b. After filtration, it was vacuum-dried at 80 ° C. for 16 hours to obtain 14.5 g of a white polymer.
[0039]
About this polymer, the sulfur content was determined by elemental analysis, and the ratio of the repeating unit based on PDD to the repeating unit based on CF ₂ = CFOCF ₂ CF (CF ₃ ) OCF ₂ CF ₂ SO ₂ F was determined. 4 / 22.6 (molar ratio). Further, this copolymer was a random copolymer. Next, the —SO ₂ F group of the obtained polymer was hydrolyzed and converted into an acid form to convert it into a sulfonic acid group to obtain a PDD / PSVE copolymer, which was further dissolved in ethanol to obtain 9% by mass. A solution was obtained.
[0040]
The specific surface area of the PDD / PSVE copolymer was measured by the BET method. The measurement method is to pulverize the PDD / PSVE copolymer to a few millimeters to several hundreds of millimeters, further pulverize it with a freeze pulverizer at a liquid nitrogen temperature until the average particle size is about 10 μm or less, and dry the BET. The specific surface area was measured by incorporating nitrogen into the apparatus and adsorbing nitrogen. The results are shown in Table 1.
[0041]
Next, a coating solution for forming a catalyst layer for the cathode was prepared as follows. A platinum-supported carbon carrying 54% by mass of platinum and the PDD / PSVE copolymer were dispersed in an ethanol solution containing the PDD / PSVE copolymer in a mass ratio of 6: 4. After stirring the obtained liquid, the solid content (catalyst and resin) stuck to the wall surface of the container by evaporation to dryness was scraped and pulverized. Thereafter, 2,2,3,3,3-pentafluoro-1-propanol is added to the obtained powder and dispersed so that the solid concentration is 5% by mass, and this is used for forming a catalyst layer for the cathode. It was set as the coating liquid.
[0042]
In the gas diffusion layer of the electrode, a carbon powder layer composed of carbon black and PTFE is formed on one surface of a water-repellent carbon cloth (woven fabric) for both the cathode and the anode, and the pores on the surface of the carbon cloth are partially What was packed with the above carbon powder layer was hot-pressed and the surface was flattened (thickness: about 340 μm). Next, a coating solution for forming a catalyst layer for the cathode is applied once to the carbon powder layer side of the gas diffusion layer so that the amount of platinum deposited is 0.8 mg / cm ² and dried. Was used as a cathode catalyst layer to obtain a cathode.
[0043]
On the other hand, the catalyst layer of the anode was formed as follows. A TFE / PSVE copolymer having an ion exchange capacity of 1.1 meq / g dry resin was dissolved in ethanol to a concentration of 9% by mass.
[0044]
As a coating solution for forming a catalyst layer for the anode, a TFE / PSVE copolymer is used so that a mass ratio of platinum-supported carbon on which 40% by mass of platinum is supported and the TFE / PSVE copolymer is 7: 3. An ethanol solution containing 9% by mass of the coalescence was added, and then ethanol and water were further added so that the solid content concentration was 5% by mass. The ratio of ethanol / water in the coating solution was 1: 1 by mass ratio. Except for using this coating solution, it was applied once to the carbon powder layer side of the gas diffusion layer and dried so that the amount of platinum deposited was 0.5 mg / cm ² except that this coating solution was used. A layer was obtained as an anode. The cathode and the anode were each cut out so that the effective electrode area was 25 cm ² .
[0045]
As the solid polymer electrolyte membrane, an ion exchange membrane made of a perfluorocarbon polymer having a sulfonic acid group (trade name: Flemion HR, manufactured by Asahi Glass Co., Ltd., ion exchange capacity 1.1 meq / g dry resin, dry film thickness 50 μm ) And bonded to the cathode and anode obtained above. Specifically, the cathode and the anode were opposed to each other with the respective catalyst layers facing inward, and hot pressing was performed with the membrane sandwiched therebetween to obtain a membrane / electrode assembly.
[0046]
The obtained membrane / electrode assembly was sandwiched between two carbon separators in which gas flow paths were formed and incorporated in a cell to measure fuel cell characteristics. The measurement of the current-voltage characteristics was performed using an electronic load (manufactured by Takasago Seisakusho, FK400L) and a DC power source (manufactured by Takasago Seisakusho, EX750L). The cell driving conditions were a current density of 1 A / cm ^{2, a} hydrogen inlet pressure of 0.15 MPa, an air inlet pressure of 0.15 MPa, a hydrogen utilization factor of 70%, an air utilization factor of 40%, and a cell operating temperature of 80 ° C. The current-voltage characteristics and internal resistance (iR loss) about 240 minutes after the start of energization of the cell were measured. FIG. 1 shows the measurement results of the cell current-iR free voltage characteristics (characteristics excluding the influence of iR loss).
[0047]
[Example 2]
(PPDD)-(TFE / PSVE copolymer)-(PPDD) block polymer is used as the resin to be contained in the catalyst layer of the cathode, and dispersed in 2,2,3,3,3-pentafluoro-1-propanol. A cathode was prepared in the same manner as in Example 1 except that a mixed solvent of ethanol and 1H-perfluorohexane (mass ratio of 1: 1) was used instead of ethanol as the solvent used before, and the cathode was used except that the cathode was used. Prepared a membrane / electrode assembly in the same manner as in Example 1 and incorporated it into the cell. Table 1 shows the measurement result of specific surface area of the resin used for the cathode by the BET method, and FIG. 1 shows the measurement result of the current-iR free voltage characteristic of the cell.
[0048]
The (PPDD)-(TFE / PSVE copolymer)-(PPDD) block polymer used here was synthesized as follows.
First, the TFE / PSVE copolymer segment was polymerized. A degassed 1 L autoclave was inhaled with 4.15 g of I (CF ₂ ) ₄ I and 778.5 g of PSVE, and then heated to 40 ° C. After injecting 58 g of TFE, 6 mL of a solution prepared by dissolving 7.53 g of IPP in 78.2 g of HCFC225cb was injected to initiate polymerization. Polymerization was continued while keeping the pressure constant (gauge pressure 0.45 MPa), and the polymerization was continued by further adding the IPP solution as the polymerization rate decreased. The total amount of IPP added was 1.66 g. When 80 g of TFE was added, the heating was stopped and the TFE was purged to stop the polymerization. The obtained solution was poured into HCFC141b, aggregated, washed, filtered, and dried to obtain 235 g of an elastomeric polymer at room temperature.
[0049]
Next, PDD was block copolymerized. In a 500 mL glass flask, 50 g of the TFE / PSVE copolymer obtained above, 250 g of 1H-perfluorohexane, and 25 g of PDD were placed and stirred to dissolve the TFE / PSVE copolymer. A solution prepared by dissolving 0.082 g of IPP in 5 g of C ₆ F ₁₃ H was added thereto, and polymerized at 30 ° C. for 65 hours. The reaction solution was poured into HCFC141b, aggregated, washed, filtered, and dried to obtain 62.9 g of a white polymer. The polymer was then treated with potassium hydroxide and sulfuric acid to acidify.
[0050]
[Example 3 (comparative example)]
A cathode, an anode, and a membrane / electrode assembly were produced in the same manner as in Example 1 except that the same resin as that used in the anode catalyst layer in Example 1 was used as the cathode resin. Table 1 shows the measurement result of the specific surface area of the cathode resin by the BET method, and FIG. 1 shows the measurement result of the current-iR free voltage characteristic of the cell.
[0051]
[Table 1]

[0052]
【The invention's effect】
According to the present invention, since a cathode resin having a large specific surface area is used as the cathode resin contained in the cathode catalyst layer, it is considered that a cathode with a small overvoltage is obtained. As a result, a polymer electrolyte fuel cell with a high output density can be provided.
[Brief description of the drawings]
FIG. 1 is a graph showing current-iR free voltage characteristics of cells of an example and a comparative example.

Claims (3)

An anode and a cathode comprising a gas diffusion electrode having a catalyst layer containing a fluorinated ion exchange resin having a sulfonic acid group and a catalyst, and a solid polymer electrolyte membrane disposed between the anode and the cathode in the solid polymer fuel cell, wherein the fluorinated ion exchange resin contained in the catalyst layer of the cathode state, and are measured specific surface area of 3m ² / g or more by the BET method, perfluoro no ion-exchange group A monomer-based repeating unit and a perfluorovinyl compound represented by CF ₂ ═CF (OCF ₂ CFX) _m O _p (CF ₂ ) _n SO ₃ H (wherein X is a fluorine atom or a trifluoromethyl group, m is an integer of 0 to 3, n is an integer from 1 to 12, p is to said copolymer der Rukoto comprising the repeating units based on a 0 or 1.) Solid polymer electrolyte fuel cell. The polymer electrolyte fuel cell according to claim 1 , wherein the repeating unit based on a perfluoromonomer having no ion exchange group has an aliphatic ring structure in the main chain. The polymer electrolyte fuel cell according to claim 2 , wherein the perfluoromonomer having no ion exchange group is perfluoro (2,2-dimethyl-1,3-dioxole).

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