JP4346860B2

JP4346860B2 - Method for producing membrane electrode assembly for polymer electrolyte fuel cell

Info

Publication number: JP4346860B2
Application number: JP2002108261A
Authority: JP
Inventors: 靖菅原; 誠内田; 慎也古佐小; 栄一安本; 堀　　喜博; 安男武部; 伸一有坂
Original assignee: Panasonic Corp; Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Corp; Panasonic Holdings Corp
Priority date: 2002-04-10
Filing date: 2002-04-10
Publication date: 2009-10-21
Anticipated expiration: 2022-04-10
Also published as: JP2003303596A

Description

【０００１】
【発明の属する技術分野】
本発明は、高分子電解質型燃料電池用膜電極接合体の製造方法に関する。さらに詳しくは、高分子電解質膜を挟む電極の改良に関する。
【０００２】
【従来の技術】
高分子電解質型燃料電池は、水素などの燃料ガスと空気などの酸化ガスをガス拡散電極によって電気化学的に反応させて、電気と熱とを同時に発生させるものである。このような高分子電解質型燃料電池の一般的な構成を図１に示す。１１は水素イオンを選択的に輸送する高分子電解質膜を表す。この電解質膜１１の両面には、白金族金属触媒を担持したカーボン粉末を主成分とする触媒層１２が、ホットプレスなどにより密着して配置されている。触媒層１２の外面には、通気性と導電性を兼ね備えた一対の拡散層１３が、ホットプレスなどにより触媒層に密着して配置されている。この拡散層１３と触媒層１２により、電極１４が構成される。電極１４の外側には、導電性セパレータ板１６が配置されている。導電性セパレータ板１６は、電極１４と高分子電解質膜１１とで形成される膜−電極接合体（ＭＥＡ）を機械的に固定するとともに、隣接するＭＥＡ同士を互いに電気的に直列に接続し、さらに電極に反応ガスを供給し、かつ反応により発生したガスや余剰のガスを運び去るための、ガス流路１７を電極と対向する面に有する。
【０００３】
電極に反応ガスを供給するガス流路は、セパレータ板１６と別に設けることもできるが、セパレータ板の表面に溝を設けてガス流路とする方式が一般的である。セパレータ板１６の他方の面には、電池温度を一定に保つための冷却水を循環させる冷却流路１８が設けられる。このように冷却水を循環させることにより、反応により発生した熱エネルギーは、温水などの形で利用することができる。このような積層型の電池では、ガスの供給孔および排出孔、並びに冷却水の供給孔および排出孔を、積層電池内部に確保したいわゆる内部マニホルド型が一般的である。
【０００４】
電極１４の周縁部には、それぞれ対極へのガス漏れあるいは外部へのガスの漏れを防止するために、シール機能を有するガスケット１５が設けられる。ガスケットには、Ｏリング、ゴム状シート、弾性樹脂と剛性樹脂との複合シートなどが用いられる。ＭＥＡの取り扱い性の観点からは、ある程度剛性を有する複合材系のガスケットをＭＥＡと一体化させることが多い。上記のような高分子電解質型燃料電池スタックでは、バイポーラ板等の構成部品の電気的接触抵抗を低減するため、電池全体を恒常的に締め付けることが必要である。このためには、多数の単電池を一方向に積み重ね、その両端にそれぞれ端板を配置し、その２つの端板の間を、締結用部材を用いて固定することが効果的である。締め付け方式としては、単電池を面内でできるだけ均一に締め付けることが望ましい。機械的強度の観点から、端板等の締結用部材にはステンレス鋼などの金属材料が通常用いられる。
【０００５】
【発明が解決しようとする課題】
これまで、接触抵抗を低下し、ガスのシール性の保持のため、ＭＥＡ接合はホットプレスで行なわれていた。ところが、電池の積層方向に恒常的に締結圧をかけていると、図３のように電極基材として通常用いられるカーボンペーパーやカーボンクロスの針状突起物２２が、高分子電解質膜１１を貫通して微小短絡が発生する。微小短絡した電池は、微小短絡部での局所発熱、水素リークによる燃焼反応などにより、経時劣化しやすく、耐久性が低い。また、図４のように、全く電極基材の微小突起がめり込まない触媒層を作ると、ホットプレスするときの接合性が悪く、積層電池を作成することが非常に困難である。
【０００６】
【課題を解決するための手段】
以上の課題を解決するため本発明の高分子電解質型燃料電池用膜電極接合体の製造方法は、
高分子電解質膜と、
前記高分子電解質膜側に配置されている触媒層、及び、前記触媒層の前記高分子電解質膜と反対側に配置されているガス拡散層を有し、前記高分子電解質膜を挟むように配置されている一対の電極と、を有する高分子電解質型燃料電池用膜電極接合体の製造方法であって、
前記一対の電極のうちの少なくとも一方の前記触媒層は、
前記高分子電解質膜上に、第１触媒層を熱転写により形成する第１工程と、前記第１触媒層上に、前記第１触媒層よりも空隙率の高い第２触媒層を形成する第２工程と、により形成され、
前記第１工程は、
触媒と、当該触媒を担持した触媒担持粒子と、高分子電解質と、前記触媒を担持した触媒担持粒子を分散させる分散媒と、を含む第１触媒インクを樹脂フィルムに塗布する工程と、
前記触媒インクを前記樹脂フィルムに塗布した塗布層を前記高分子電解質膜に張り合わせる工程と、
前記高分子電解質膜及び前記塗布層を加熱及び加圧する工程と、
前記樹脂フィルムを剥離する工程と、を有し
前記第２工程は、
触媒を担持した触媒担持粒子に高分子電解質の溶液を噴霧しながら乾燥し、前記触媒担持粒子の表面に前記高分子電解質を被覆したドライプロセス粉末を得る工程と、
前記ドライプロセス粉末を分散媒と混合して第２触媒インクを調製する工程と、
前記第１工程により形成された第１触媒層の上に、前記調製された第２触媒インクを塗布する工程と、を有し、
前記一対の電極のうちの、前記触媒層が前記第１触媒層および前記第２触媒層からなる電極は、前記ガス拡散層の一部が前記触媒層の途中までめり込むように、前記高分子電解質膜に熱圧着される、高分子電解質型燃料電池用膜電極接合体の製造方法を提供する。
ここにおいては、前記第２工程において、第１触媒層の単位面積あたりの触媒量と前記第２触媒層の単位面積あたりの触媒量との比が７：３となるように前記第２触媒インクが塗布されるのが好ましい。
【０００８】
【発明の実施の形態】
上記のように本発明の高分子電解質型燃料電池は、少なくとも一方の電極の触媒層が２層以上の多層から構成されている。
図２は本発明によるＭＥＡの要部の構成を模式的に示す。触媒層１２は、ガス拡散層１３に接する層（第２触媒層）３１と、電解質膜１１に接する層（第１触媒層）３２との２層からなる。電解質膜１１に接する層３２は、触媒を担持したカーボン粒子４２とこれに被着した電解質５２とが比較的密に詰まっている層であることが好ましい。一方、ガス拡散層１３に接する層３１は、触媒を担持し電解質５１を被着したカーボン粒子４１が比較的疎に集合している層が好ましい。そのような層は、ガス拡散層を構成するカーボンペーパーやカーボンクロスの針状突起が容易に侵入しやすい。
【０００９】
触媒層３２は、カーボンペーパーやカーボンクロスの針状突起が容易には侵入しない構成とするのがよい。そのような比較的密な層を得るには、触媒材料とその分散媒とで調製した触媒インクを転写フィルムとなる樹脂フィルムに塗布し、その塗布層を高分子電解質膜に張り合わせ、加熱加圧した後、前記樹脂フィルムを剥離することで形成するのが好ましい。従来の図３に示す触媒層は、層３１のみから構成され、図４に示す触媒層は層３２のみから構成されている例である。
【００１０】
このように触媒層１２を疎な層３１と緻密な３２で構成し、ガス拡散層の電極基材の一部が触媒層の途中まで、例えば層３１にめり込んでいるのが好ましい。触媒層の途中まで電極基材がめり込んでいることで、ＭＥＡの接合力が向上し、積層電池を作成する際のハンドリングが非常に良好である。
また、電極基材は高分子電解質膜にまで到達していないことから、微小短絡が起こりにくく、耐久性の高いＭＥＡを実現できる。多層触媒層において、電極基材側から高分子電解質膜側に向かって触媒層の空隙率を減少させることにより、上記の構造を容易に実現できる。また、膜に接する触媒層は、誘電率が高いかあるいは高分子電解質に対する溶解度パラメーターが大きい分散媒で調製された触媒インクから、樹脂基材に一旦形成することにより、高分子電解質のネットワークが発達した緻密で電極基材が貫通しにくい触媒層が実現できる。
【００１１】
【実施例】
本発明に好適の実施例を、図面を参照しながら詳細に説明する。
実施例１
３０ｎｍの平均一次粒子径を持つ導電性カ−ボン粒子であるケッチェンブラックＥＣ（オランダ国、ＡＫＺＯＣｈｅｍｉｅ社）に、平均粒径約３０Åの白金粒子を５０重量％担持したものを触媒担持粒子ａとした。ついで、この触媒担持粒子を高分子電解質溶液に高分子電解質とカーボンとの重量比が１：１となるように分散させ、スラリー化した。ここで高分子電解質溶液は、１０重量％濃度のパーフルオロカーボンスルホン酸（デュポン社製ＳＥ１００７２）の水溶液ｂを用いた。
上記スラリーを厚さ５０μｍのポリプロピレンシートに、白金量が０．３５mg/ｃｍ²となるようバーコーターで塗工し、室温で乾燥した。次に、上記塗工シートを打ち抜き型で、所定の電極サイズ（６０ｍｍ角）に切断した。切断したシートの触媒層面を高分子電解質膜（ゴア社製Gore select膜）の裏表両面に重ね合わせ、１３０℃に加熱し、５０ｋｇ／ｃｍ²の圧力で加圧した後、ポリプロピレンシートを剥がすことで、高分子電解質膜に触媒層を熱転写した。
【００１２】
一方、スプレードライ装置を用い、触媒担持粒子ａの表面に高分子電解質の溶液ｂを噴霧しながら乾燥し、触媒担持粒子の表面に高分子電解質を被覆した触媒体（ドライプロセス粉末：以下DP粉末という）を得た。この触媒体を窒素雰囲気中でエチレングリコ−ルと混合し、電極触媒層用のペースト状のインクを調製した。次に、熱転写により裏表両面に触媒層を形成した高分子電解質膜の裏表両面に、上記電極触媒層用ペーストをスクリーン印刷法により塗布した。形成後の電極中に含まれる総白金量は、０．５ｍｇ／ｃｍ²となるよう調製した。
【００１３】
電極となる厚さ４００μｍのカーボンクロス（日本カーボン（株）製、ＧＦ−２０−３１Ｅ）を、フッ素樹脂の水性ディスパージョン（ダイキン工業（株）製、ネオフロンＮＤ１）に含浸した後、乾燥し、４００℃で３０分加熱することで、撥水性を与えた。さらに、このカーボン織布の一方の面に、導電性カーボン粉末とポリテトラフルオロエチレン（ＰＴＦＥ）微粉末を分散させた水とを混合したインクを、スクリーン印刷法を用いて塗布することで撥水層を形成した。このとき、撥水層の一部を、カーボン織布の中に埋め込んだ。これを電極サイズ（６０ｍｍ角）の打ち抜き型で打ち抜いた。この打ち抜いた２枚のカーボン織布により、上記の両面に触媒層を備えた高分子電解質膜を、撥水層を内側にして挟持し、電極基材の外周部には、シリコンゴム／ポリエチレンテレフタレート／シリコンゴムの３層に積層した複合材料ガスケットを位置合わせした。そして、これらを１３０℃、５０ｋｇｆ／ｃｍ²で１０分間熱圧着してＭＥＡを得た。このＭＥＡを用いて構成した単電池を電池Ａとする。
【００１４】
《比較例１》
実施例１と同じく触媒担持粒子ａを高分子電解質溶液ｂに高分子電解質とカーボンとの重量比が１：１となるように分散させ、スラリー化した。このスラリーを厚さ５０μｍのポリプロピレンシートに白金量が０．５mg／ｃｍ²となるようバーコーターで塗工し、室温で乾燥した。次に、上記塗工シートを打ち抜き型で所定の電極サイズ（６０ｍｍ角）に切断し、切断したシートを実施例１と同じ高分子電解質膜の裏表両面に１３０℃、５０kg／ｃｍ²で触媒層をポリプロピレンシートから高分子電解質膜に熱転写した。
次に、電極サイズ（６０ｍｍ角）に打ち抜いた撥水層を備えた撥水化カーボンクロスで、上記の両面に触媒層を備えた高分子電解質膜を両側より挟持し、電極基材の外周部には、シリコンゴム／ポリエチレンテレフタレート／シリコンゴムの３層に積層した複合材料ガスケットを位置合わせし、１３０℃、５０ｋｇｆ／ｃｍ²で１０分間熱圧着してＭＥＡを得た。このＭＥＡを用いて構成した単電池を電池Ｂとする。
【００１５】
《比較例２》
上記DP粉末を窒素雰囲気中でエチレングリコ−ルと混合し、電極触媒層用のペースト状のインクを調製した。次に、実施例１と同じ高分子電解質膜の裏表両面に前記電極触媒層用ペーストをスクリーン印刷法により塗布した。形成後の電極中に含まれる総白金量は、０．５ｍｇ／ｃｍ²となるよう調製した。
次に、電極サイズ（６０ｍｍ角）に打ち抜いた撥水層を備えた撥水化カーボンクロスで、上記の両面に触媒層を備えた高分子電解質膜を両側より挟持し、電極基材の外周部には、シリコンゴム／ポリエチレンテレフタレート／シリコンゴムの３層に積層した複合材料ガスケットを位置合わせし、１３０℃、５０ｋｇｆ／ｃｍ²で１０分間熱圧着してＭＥＡを得た。このＭＥＡを用いて構成した単電池を電池Ｃとする。
【００１６】
以上の単電池Ａ、ＢおよびＣについて、電池温度７５℃において、アノードに露点が７０℃となるように加湿した水素を１気圧で供給し、カソードに露点が６５℃となるように加湿した空気を１気圧で供給し、水素利用率７０％、酸素利用率５０％で初期の電流−電圧特性を測定した。その結果を、図５に示す。
図５より、単電池ＡおよびＢの開回路電圧Ｖｏｃはそれぞれ０．９９５Ｖおよび１．０００Ｖと高いことが確認された。これと比較して、電池Ｃは０．８９５Ｖと低かった。これは図３に示したように、電極基材の針状突起物２２が高分子電解質膜１１を貫通して、微小短絡が発生しているためである。初期の電流−電圧特性においては、開回路電圧Ｖｏｃ以外、電流を流しているときの電圧値に大きな差は見られなかった。
次に、各電池の耐久試験を行った。電流密度を０．３Ａ／ｃｍ²とした他は前記と同様の条件で測定した電圧の経時変化を図６に示す。微小短絡が発生していた電池Ｃは、徐々に電圧が低下していき、１００００時間ぐらいから急激に電圧が低下した。電池ＡおよびＢは１００００時間経過後も電圧の低下は増加せず、電圧低下率は０．５ｍＶ/１０００ｈで推移した。
【００１７】
次に、実施例１および比較例１で作成したＭＥＡを、それぞれ単電池に組まない状態で、湿度の変化に対する形状保持力を測定した。雰囲気温度を７５℃に設定し、相対湿度２５％（露点４５℃相当）に２時間、相対湿度８０％（露点７０℃相当）に２時間を１サイクルとしてサイクル試験を行った。比較例１のＭＥＡは１５サイクル目に剥離が生じたが、実施例１のＭＥＡは１００サイクルを越えても剥離は観察されなかった。この結果から実施例１のＭＥＡの接合力が高いことがわかった。実際の積層電池作成時のハンドリング性および積層電池での間欠作動試験、振動試験などでも実施例１のＭＥＡが優れていると考えられる。
【００１８】
【発明の効果】
以上のように本発明によれば、微小短絡がなく耐久性が高く、電解質膜と触媒層との接合力の強い高分子電解質型燃料電池が実現できる。
【図面の簡単な説明】
【図１】高分子電解質型燃料電池の代表的な構成を示す縦断面図である。
【図２】本発明の高分子電解質型燃料電池のＭＥＡの実施の形態を示す要部の断面図である。
【図３】従来例のＭＥＡの要部の断面図である。
【図４】他の従来例のＭＥＡの要部の断面図である。
【図５】実施例および比較例の高分子電解質型燃料電池の初期の電流−電圧特性を示す図である。
【図６】実施例および比較例の高分子電解質型燃料電池の定電流密度での電圧の経時変化を示す図である。
【符号の説明】
１１高分子電解質膜
１２触媒層
１３拡散層
１４電極
１５ガスケット
１６セパレータ板
１７ガス流路
１８冷却水の流路
２２針状突起物
３１疎な触媒層
３２緻密な触媒層
４１、４２触媒担持粒子
５１、５２高分子電解質[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a polymer electrolyte fuel cell membrane electrode assembly. More specifically, the present invention relates to an improvement in electrodes sandwiching a polymer electrolyte membrane.
[0002]
[Prior art]
A polymer electrolyte fuel cell generates electricity and heat simultaneously by electrochemically reacting a fuel gas such as hydrogen and an oxidizing gas such as air with a gas diffusion electrode. A general structure of such a polymer electrolyte fuel cell is shown in FIG. 11 represents a polymer electrolyte membrane that selectively transports hydrogen ions. On both surfaces of the electrolyte membrane 11, a catalyst layer 12 mainly composed of carbon powder carrying a platinum group metal catalyst is disposed in close contact by hot pressing or the like. On the outer surface of the catalyst layer 12, a pair of diffusion layers 13 having air permeability and conductivity are disposed in close contact with the catalyst layer by hot pressing or the like. The diffusion layer 13 and the catalyst layer 12 constitute an electrode 14. A conductive separator plate 16 is disposed outside the electrode 14. The conductive separator plate 16 mechanically fixes a membrane-electrode assembly (MEA) formed by the electrode 14 and the polymer electrolyte membrane 11, and electrically connects adjacent MEAs to each other in series. Further, a gas flow path 17 is provided on the surface facing the electrode for supplying the reaction gas to the electrode and carrying away the gas generated by the reaction or excess gas.
[0003]
The gas flow path for supplying the reaction gas to the electrode can be provided separately from the separator plate 16, but a system in which a groove is provided on the surface of the separator plate to form a gas flow path is common. On the other surface of the separator plate 16 is provided a cooling flow path 18 for circulating cooling water for keeping the battery temperature constant. By circulating the cooling water in this way, the heat energy generated by the reaction can be used in the form of hot water or the like. In such a stacked battery, a so-called internal manifold type in which a gas supply hole and a discharge hole, and a cooling water supply hole and a discharge hole are secured inside the stacked battery is generally used.
[0004]
A gasket 15 having a sealing function is provided at the peripheral edge of the electrode 14 in order to prevent gas leakage to the counter electrode or gas leakage to the outside. As the gasket, an O-ring, a rubber-like sheet, a composite sheet of elastic resin and rigid resin, or the like is used. From the viewpoint of handling of the MEA, a composite material gasket having a certain degree of rigidity is often integrated with the MEA. In the polymer electrolyte fuel cell stack as described above, in order to reduce the electrical contact resistance of components such as a bipolar plate, it is necessary to permanently tighten the entire battery. For this purpose, it is effective to stack a large number of single cells in one direction, dispose end plates at both ends thereof, and fix between the two end plates using a fastening member. As a tightening method, it is desirable to tighten the cells as uniformly as possible in the plane. From the viewpoint of mechanical strength, metal materials such as stainless steel are usually used for fastening members such as end plates.
[0005]
[Problems to be solved by the invention]
Until now, MEA bonding has been performed by hot pressing in order to reduce contact resistance and maintain gas sealing performance. However, when a fastening pressure is constantly applied in the stacking direction of the battery, the carbon paper or carbon cloth needle-like protrusions 22 normally used as the electrode substrate penetrate the polymer electrolyte membrane 11 as shown in FIG. As a result, a short circuit occurs. A micro short-circuited battery is likely to deteriorate over time due to local heat generation at the micro short circuit part, a combustion reaction due to hydrogen leak, and the like, and has low durability. Further, as shown in FIG. 4, when a catalyst layer in which the minute protrusions of the electrode base material are not recessed at all is formed, the bonding property at the time of hot pressing is poor and it is very difficult to produce a laminated battery.
[0006]
[Means for Solving the Problems]
In order to solve the above problems, a method for producing a membrane electrode assembly for a polymer electrolyte fuel cell according to the present invention comprises:
A polymer electrolyte membrane;
A catalyst layer disposed on the polymer electrolyte membrane side, and a gas diffusion layer disposed on the opposite side of the catalyst layer from the polymer electrolyte membrane, and disposed so as to sandwich the polymer electrolyte membrane a process for producing a polymer electrolyte fuel cell membrane electrode assembly to have a, a pair of electrodes which are,
The catalyst layer of at least one of the pair of electrodes is
A first step of forming a first catalyst layer on the polymer electrolyte membrane by thermal transfer; and a second step of forming a second catalyst layer having a higher porosity than the first catalyst layer on the first catalyst layer. Formed by a process,
The first step includes
Coating a resin film with a first catalyst ink containing a catalyst, catalyst-carrying particles carrying the catalyst, a polymer electrolyte, and a dispersion medium for dispersing the catalyst-carrying particles carrying the catalyst;
Bonding the coating layer obtained by applying the catalyst ink to the resin film to the polymer electrolyte membrane;
Heating and pressurizing the polymer electrolyte membrane and the coating layer;
And a step of peeling the resin film.
The second step includes
Drying while spraying a polymer electrolyte solution onto catalyst-carrying particles carrying a catalyst to obtain a dry process powder having the surface of the catalyst-carrying particles coated with the polymer electrolyte;
Mixing the dry process powder with a dispersion medium to prepare a second catalyst ink;
Applying the prepared second catalyst ink on the first catalyst layer formed in the first step, and
Of the pair of electrodes, the electrode in which the catalyst layer is composed of the first catalyst layer and the second catalyst layer has the polymer electrolyte so that a part of the gas diffusion layer is embedded in the middle of the catalyst layer. Provided is a method for producing a membrane electrode assembly for a polymer electrolyte fuel cell, which is thermocompression bonded to a membrane.
Here, in the second step, the second catalyst ink is set such that the ratio of the catalyst amount per unit area of the first catalyst layer to the catalyst amount per unit area of the second catalyst layer is 7: 3. Is preferably applied .
[0008]
DETAILED DESCRIPTION OF THE INVENTION
As described above, in the polymer electrolyte fuel cell of the present invention, the catalyst layer of at least one electrode is composed of two or more layers.
FIG. 2 schematically shows the configuration of the main part of the MEA according to the present invention. The catalyst layer 12 includes two layers: a layer (second catalyst layer) 31 in contact with the gas diffusion layer 13 and a layer (first catalyst layer) 32 in contact with the electrolyte membrane 11. The layer 32 in contact with the electrolyte membrane 11 is preferably a layer in which the carbon particles 42 supporting the catalyst and the electrolyte 52 deposited thereon are relatively densely packed. On the other hand, the layer 31 in contact with the gas diffusion layer 13 is preferably a layer in which the carbon particles 41 supporting the catalyst and adhering the electrolyte 51 are gathered relatively sparsely. In such a layer, the needle-like protrusions of the carbon paper or carbon cloth constituting the gas diffusion layer are easily invaded.
[0009]
The catalyst layer 32 is preferably configured such that the needle-like protrusions of carbon paper or carbon cloth do not easily enter. In order to obtain such a relatively dense layer, a catalyst ink prepared with a catalyst material and its dispersion medium is applied to a resin film serving as a transfer film, and the applied layer is bonded to a polymer electrolyte membrane and heated and pressurized. Then, it is preferable to form by peeling off the resin film. The conventional catalyst layer shown in FIG. 3 is composed of only the layer 31, and the catalyst layer shown in FIG. 4 is an example composed of only the layer 32.
[0010]
Thus, it is preferable that the catalyst layer 12 is composed of the sparse layer 31 and the dense 32, and a part of the electrode base material of the gas diffusion layer is embedded in the layer 31 to the middle of the catalyst layer, for example. Since the electrode base material is embedded in the middle of the catalyst layer, the joining force of MEA is improved, and the handling at the time of producing a laminated battery is very good.
In addition, since the electrode base material does not reach the polymer electrolyte membrane, a micro short circuit hardly occurs and a highly durable MEA can be realized. In the multilayer catalyst layer, the above structure can be easily realized by reducing the porosity of the catalyst layer from the electrode substrate side toward the polymer electrolyte membrane side. In addition, the catalyst layer in contact with the membrane is formed once on a resin substrate from a catalyst ink prepared with a dispersion medium having a high dielectric constant or a high solubility parameter for the polymer electrolyte, thereby developing a polymer electrolyte network. Thus, a dense catalyst layer that does not easily penetrate the electrode substrate can be realized.
[0011]
【Example】
Preferred embodiments of the present invention will be described in detail with reference to the drawings.
Example 1
Catalyst-supported particles a were obtained by supporting 50% by weight of platinum particles having an average particle size of about 30 mm on Ketjen Black EC (AKZO Chemie, Netherlands), which is conductive carbon particles having an average primary particle size of 30 nm. It was. Next, the catalyst-supported particles were dispersed in a polymer electrolyte solution so that the weight ratio of the polymer electrolyte and carbon was 1: 1, and slurried. Here, as the polymer electrolyte solution, an aqueous solution b of 10% by weight perfluorocarbon sulfonic acid (SE10072 manufactured by DuPont) was used.
The slurry was applied to a polypropylene sheet having a thickness of 50 μm with a bar coater so that the amount of platinum was 0.35 mg / cm ² and dried at room temperature. Next, the coated sheet was cut into a predetermined electrode size (60 mm square) with a punching die. By superposing the catalyst layer surface of the cut sheet on both sides of the polymer electrolyte membrane (Gore select membrane manufactured by Gore), heating to 130 ° C., pressing at a pressure of 50 kg / cm ² , and then peeling off the polypropylene sheet The catalyst layer was thermally transferred to the polymer electrolyte membrane.
[0012]
On the other hand, a catalyst body (dry process powder: hereinafter referred to as DP powder), which is dried by spraying the polymer electrolyte solution b on the surface of the catalyst-carrying particles a using a spray-drying apparatus and coating the surface of the catalyst-carrying particles with the polymer electrolyte. I got). This catalyst body was mixed with ethylene glycol in a nitrogen atmosphere to prepare a paste-like ink for an electrode catalyst layer. Next, the electrode catalyst layer paste was applied by screen printing on both sides of the polymer electrolyte membrane on which the catalyst layers were formed on both sides by thermal transfer. The total amount of platinum contained in the electrode after formation was adjusted to 0.5 mg / cm ² .
[0013]
A carbon cloth having a thickness of 400 μm to be an electrode (manufactured by Nippon Carbon Co., Ltd., GF-20-31E) was impregnated with an aqueous dispersion of fluororesin (manufactured by Daikin Industries, Ltd., Neoflon ND1), and then dried. Water repellency was imparted by heating at 400 ° C. for 30 minutes. Furthermore, water repellency is achieved by applying an ink in which conductive carbon powder and water in which polytetrafluoroethylene (PTFE) fine powder is dispersed is applied to one surface of the carbon woven fabric using a screen printing method. A layer was formed. At this time, a part of the water repellent layer was embedded in the carbon woven fabric. This was punched with a punching die having an electrode size (60 mm square). The two carbon woven fabrics punched out sandwich the polymer electrolyte membrane having the catalyst layers on both sides with the water repellent layer inside, and on the outer periphery of the electrode substrate, silicon rubber / polyethylene terephthalate. / Composite gaskets laminated in 3 layers of silicon rubber were aligned. These were thermocompression bonded at 130 ° C. and 50 kgf / cm ² for 10 minutes to obtain MEA. A single battery configured using this MEA is referred to as a battery A.
[0014]
<< Comparative Example 1 >>
As in Example 1, the catalyst-carrying particles a were dispersed in the polymer electrolyte solution b so that the weight ratio of the polymer electrolyte to carbon was 1: 1 and slurried. This slurry was applied to a polypropylene sheet having a thickness of 50 μm with a bar coater so that the amount of platinum was 0.5 mg / cm ² and dried at room temperature. Next, the coated sheet was cut into a predetermined electrode size (60 mm square) with a punching die, and the cut sheet was applied to the catalyst polymer layer at 130 ° C. and 50 kg / cm ² on both sides of the same polymer electrolyte membrane as in Example 1. Was thermally transferred from a polypropylene sheet to a polymer electrolyte membrane.
Next, a polymer electrolyte membrane provided with a catalyst layer on both sides is sandwiched from both sides with a water-repellent carbon cloth provided with a water-repellent layer punched into an electrode size (60 mm square), and the outer periphery of the electrode substrate The composite material gasket laminated in three layers of silicon rubber / polyethylene terephthalate / silicone rubber was aligned and thermocompression bonded at 130 ° C. and 50 kgf / cm ² for 10 minutes to obtain MEA. A single battery configured using this MEA is referred to as a battery B.
[0015]
<< Comparative Example 2 >>
The DP powder was mixed with ethylene glycol in a nitrogen atmosphere to prepare a paste-like ink for an electrode catalyst layer. Next, the electrode catalyst layer paste was applied by screen printing on both sides of the same polymer electrolyte membrane as in Example 1. The total amount of platinum contained in the electrode after formation was adjusted to 0.5 mg / cm ² .
Next, a polymer electrolyte membrane provided with a catalyst layer on both sides is sandwiched from both sides with a water-repellent carbon cloth provided with a water-repellent layer punched into an electrode size (60 mm square), and the outer periphery of the electrode substrate The composite material gasket laminated in three layers of silicon rubber / polyethylene terephthalate / silicone rubber was aligned and thermocompression bonded at 130 ° C. and 50 kgf / cm ² for 10 minutes to obtain MEA. A cell constituted by using this MEA is referred to as a battery C.
[0016]
For the above cells A, B, and C, at a battery temperature of 75 ° C., humidified hydrogen was supplied to the anode so that the dew point was 70 ° C. at 1 atm, and air was humidified so that the dew point was 65 ° C. Was supplied at 1 atm, and the initial current-voltage characteristics were measured at a hydrogen utilization rate of 70% and an oxygen utilization rate of 50%. The result is shown in FIG.
From FIG. 5, it was confirmed that the open circuit voltages Voc of the cells A and B were as high as 0.995 V and 1.000 V, respectively. Compared with this, the battery C was as low as 0.895V. This is because, as shown in FIG. 3, the needle-like protrusion 22 of the electrode base material penetrates the polymer electrolyte membrane 11, and a micro short circuit occurs. In the initial current-voltage characteristics, there was no significant difference in voltage values when current was passed, except for the open circuit voltage Voc.
Next, each battery was subjected to a durability test. FIG. 6 shows changes with time in voltage measured under the same conditions as described above except that the current density was set to 0.3 A / cm ² . The voltage of the battery C in which the micro short circuit had occurred gradually decreased, and the voltage rapidly decreased from about 10,000 hours. In batteries A and B, the decrease in voltage did not increase even after 10000 hours had elapsed, and the voltage decrease rate remained at 0.5 mV / 1000 h.
[0017]
Next, the shape retention force with respect to a change in humidity was measured in a state where the MEAs prepared in Example 1 and Comparative Example 1 were not assembled into single cells. The cycle test was performed with the ambient temperature set at 75 ° C., with a relative humidity of 25% (corresponding to a dew point of 45 ° C.) for 2 hours and a relative humidity of 80% (corresponding to a dew point of 70 ° C.) for 2 hours. The MEA of Comparative Example 1 was peeled at the 15th cycle, but the MEA of Example 1 was not peeled even after exceeding 100 cycles. From this result, it was found that the bonding strength of the MEA of Example 1 was high. It is considered that the MEA of Example 1 is excellent also in handling properties at the time of actual multilayer battery creation, intermittent operation test, vibration test and the like in the multilayer battery.
[0018]
【The invention's effect】
As described above, according to the present invention, it is possible to realize a polymer electrolyte fuel cell having no short-circuit and high durability and having a strong bonding force between the electrolyte membrane and the catalyst layer.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view showing a typical configuration of a polymer electrolyte fuel cell.
FIG. 2 is a cross-sectional view of an essential part showing an embodiment of MEA of a polymer electrolyte fuel cell of the present invention.
FIG. 3 is a cross-sectional view of a main part of a conventional MEA.
FIG. 4 is a cross-sectional view of a main part of another conventional MEA.
FIG. 5 is a diagram showing initial current-voltage characteristics of polymer electrolyte fuel cells of Examples and Comparative Examples.
FIG. 6 is a graph showing changes with time in voltage at a constant current density of polymer electrolyte fuel cells of Examples and Comparative Examples.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 Polymer electrolyte membrane 12 Catalyst layer 13 Diffusion layer 14 Electrode 15 Gasket 16 Separator board 17 Gas flow path 18 Cooling water flow path 22 Needle-like protrusion 31 Sparse catalyst layer 32 Dense catalyst layers 41 and 42 Catalyst support particles 51 , 52 Polyelectrolyte

Claims

A polymer electrolyte membrane;
A catalyst layer disposed on the polymer electrolyte membrane side; and a gas diffusion layer disposed on the opposite side of the catalyst layer from the polymer electrolyte membrane, and disposed so as to sandwich the polymer electrolyte membrane A membrane electrode assembly for a polymer electrolyte fuel cell having a pair of electrodes,
The catalyst layer of at least one of the pair of electrodes is
A first step of forming a first catalyst layer on the polymer electrolyte membrane by thermal transfer; and a second step of forming a second catalyst layer having a higher porosity than the first catalyst layer on the first catalyst layer. Formed by a process,
The first step includes
Coating a resin film with a first catalyst ink containing a catalyst, catalyst-carrying particles carrying the catalyst, a polymer electrolyte, and a dispersion medium for dispersing the catalyst-carrying particles carrying the catalyst;
Bonding the coating layer obtained by applying the catalyst ink to the resin film to the polymer electrolyte membrane;
Heating and pressurizing the polymer electrolyte membrane and the coating layer;
Peeling the resin film, and having the second step,
Drying while spraying a polymer electrolyte solution onto catalyst-carrying particles carrying a catalyst, and obtaining a dry process powder having the surface of the catalyst-carrying particles coated with the polymer electrolyte;
Mixing the dry process powder with a dispersion medium to prepare a second catalyst ink;
Applying the prepared second catalyst ink on the first catalyst layer formed in the first step, and
Of the pair of electrodes, the electrode in which the catalyst layer is composed of the first catalyst layer and the second catalyst layer has the polymer electrolyte so that a part of the gas diffusion layer is embedded in the middle of the catalyst layer. A method for producing a membrane electrode assembly for a polymer electrolyte fuel cell, which is thermocompression bonded to a membrane.

In the second step, the second catalyst ink is applied so that the ratio of the catalyst amount per unit area of the first catalyst layer to the catalyst amount per unit area of the second catalyst layer is 7: 3. The manufacturing method of the membrane electrode assembly for polymer electrolyte fuel cells of Claim 1.