【０００１】
【発明の属する技術分野】
本発明は、固体高分子型燃料電池用電極触媒に関する。
【０００２】
【従来の技術】
従来、固体高分子型燃料電池は、電解質としてイオン交換樹脂膜を用いるものであり、電極触媒としては、アノード、カソードともに、導電性担体に担持された白金系貴金属が使用されている。このような固体高分子型燃料電池またはそれに使用される触媒の性能を向上させるために、従来から種々の工夫がなされている。例えば、特許文献１には、白金の担持量が増加しても白金の結晶子径を小さく抑制することにより、触媒活性を向上させる手法が開示されている。この特許文献１では、酸素を所定の割合で結合させた導電性カーボンを触媒担体として用いている。ここでは、白金と結合した酸素は検知されないとされている。
【０００３】
【特許文献１】
特開２００１−３２５９６４号公報
【０００４】
【発明が解決しようとする課題】
固体高分子型燃料電池用触媒としては、その活性の向上もさることながら、電池に組み込んだときに、長期にわたって電池の電圧が低下しないという耐久性も要求される。固体高分子型燃料電池を長期間運転した場合、電極触媒の触媒粒子の凝集や成分の溶出が生じる結果、触媒が劣化し、電池の寿命が短縮される。
【０００５】
したがって、本発明は、長期運転しても劣化が少なく安定な固体高分子型燃料電池用電極触媒を提供することを目的とする。
【０００６】
【課題を解決するための手段】
本発明によれば、導電性担体に担持された白金系貴金属触媒を含み、該貴金属触媒の表面に、該担体担持貴金属触媒の全重量の１〜１０重量％の割合で酸素が保持されていることを特徴とする固体高分子型燃料電池用電極触媒が提供される。
【０００７】
【発明の実施の形態】
以下、本発明をより詳しく説明する。
【０００８】
本発明の固体高分子型燃料電池用電極触媒は、導電性担体に担持された白金系貴金属触媒を含むものであり、カソード電極触媒、アノード電極触媒のいずれにも、それら両者にも適用され得る。
【０００９】
使用する導電性担体は、通常、導電性カーボンブラック粒子からなる。本発明の固体高分子型燃料電池用電極触媒をカソード電極触媒に適用する場合、導電性担体は、約７００〜約１３００ｍ２／ｇのＢＥＴ比表面積を有することが好ましい。他方、本発明の電極触媒をアノード電極触媒に適用する場合、導電性担体は、約５０〜約５００ｍ２／ｇのＢＥＴ比表面積を有することが好ましい。
【００１０】
導電性担体に担持される白金系貴金属触媒には、白金および白金合金が含まれる。白金合金としては、白金−鉄合金、白金−ニッケル合金、白金−コバルト合金、白金−銅合金等の白金−遷移金属合金、白金−パラジウム合金、白金−ロジウム合金等の白金−貴金属合金を例示することができる。白金系貴金属触媒の担持割合（導電性担体と白金系貴金属触媒の合計重量における白金系貴金属触媒の割合）は、約２０〜８０重量％であることが好ましい。
【００１１】
さて、本発明の固体高分子型燃料電池用電極触媒において、白金系貴金属触媒の表面には、担体担持白金系貴金属触媒の全重量（導電性担体と白金系貴金属触媒の合計重量）の１〜１０％に相当する割合で酸素が保持・固定されている。このような酸素保持は、担体担持白金系貴金属触媒を空気中５０〜９０℃、好ましくは８０℃の温度で保持することによるアニール処理により達成することができる。酸素の保持割合は、アニール処理時間の調節により調節することができる。この酸素保持割合は、ＴＧ−ＤＴＡ測定により測定することができる。その際、１０体積％の水素と９０体積％のヘリウムからなる雰囲気中、５℃／分の昇温速度で、室温から３００℃まで加熱する。なお、導電性カーボンブラック担体のみを同様にアニール処理しても、かかるＴＧ−ＤＴＡ測定では吸着水以外の重量変化がほとんどみられないことが確認されている。また、ＸＰＳ分析では、ごく微量ではあるが、白金系貴金属触媒の酸化物のピークが確認されている。すなわち、酸素は、化学吸着により白金系貴金属触媒表面に保持・固定されているということができる。酸素保持割合が１重量％未満であると、長期間運転後の電池の電圧が、初期電池電圧に比べて大幅に低下する傾向にある。他方、酸素保持割合が１０重量％を超えると、触媒活性が低下する結果、電池の初期電圧が大幅に低下する。
【００１２】
本発明の電極触媒は、通常の湿式法により導電性担体に白金系貴金属触媒（白金合金の場合には、白金と合金化用金属）を担持させ、真空中で乾燥した後、白金合金触媒の場合には還元処理および白金と合金化用金属の合金化処理を行ってから、上記アニール処理を行うことによって製造することができる。
【００１３】
上に述べたように、本発明の固体高分子型燃料電池用電極触媒は、カソード電極触媒として、またはアノード電極触媒とし使用することができるが、カソード電極触媒およびアノード電極触媒の両者を本発明の固体高分子型燃料電池用電極触媒により構成することができる。なお、カソード電極触媒およびアノード電極触媒の一方のみを本発明の固体高分子型燃料電池用電極触媒で構成する場合、他方の電極触媒は、通常の電極触媒を利用することができる。また、本発明の固体高分子型燃料電池用電極触媒の使用量は、通常と同様であり、例えば、白金量として、０．３〜０．５ｍｇ／ｃｍ２程度で十分である。
【００１４】
本発明の固体高分子型燃料電池用電極触媒は、長期にわたって劣化の少ない、安定した性能を示す。
【００１５】
【実施例】
以下本発明を実施例により説明するが、本発明はそれらに限定されるものではない。
【００１６】
実施例１
本実施例では、カソード電極触媒を以下の方法により調製した。
【００１７】
まず、市販の比表面積約１０００ｍ２／ｇのカーボン粉末４．０ｇを純水０．２Ｌに分散させた。この分散液に白金６．０ｇを含有するヘキサヒドロキソ白金硝酸水溶液を滴下し、カーボン粉末と十分に混合した。この混合物に純水１Ｌを滴下した後、ろ過し、得られたケーキを十分に水洗した。このケーキを再び純水１Ｌに均一に分散させ、この分散液に、還元剤として水素化ホウ素ナトリウム４ｇを純水中に溶解した溶液を滴下した。この混合物をろ過し、得られたケーキを十分に水洗し、真空中、１００℃で１０時間乾燥させた。この乾燥ケーキを空気中に８０℃で１０時間保持（アニール処理）して、所望のカソード電極触媒粉末（Ａ−１）を得た。
【００１８】
得られた触媒粉末Ａ−１について一酸化炭素（ＣＯ）の吸着量を測定したところ、白金グラム当たり３２ｍＬであり、これに基づき白金担持割合を算出したところ、６０．０重量％であった。また、触媒粉末Ａ−１のＸ線回折（ＸＲＤ）を測定し、３９°付近のＰｔ（１１１）面のピークの半価幅から白金の平均粒径を算出したところ、１．７〜２．０ｎｍであった。さらに、触媒粉末Ａ−１のＢＥＴ比表面積は、４００ｇ／ｍ２であった。また、上記測定条件下でのＴＧ−ＤＴＡ分析の結果、触媒粉末Ａ−１の酸素保持量は４重量％であった。触媒粉末Ａ−１の各種物性を表１に示す。また、触媒粉末Ａ−１のＸ線光電子分光（ＸＰＳ）スペクトルを図１に示す。図１から、白金表面上に酸素が保持されていることがわかる。
【００１９】
なお、上記導電性カーボンのみを上記アニール処理に供したが、同様のＴＧ−ＤＴＡ分析により、吸着水以外の重量変化がほとんどみられなかった。
【００２０】
実施例２〜４、比較例１
乾燥ケーキのアニール時間をそれぞれ１時間、５時間、１５時間、２０時間に変えた以外は実施例１と同様の手法により所望のカソード電極触媒粉末（それぞれ、Ａ２〜Ａ−５）を得た。得られた触媒粉末の物性を表１に示す。
【００２１】
実施例５
本実施例では、アノード電極触媒を以下の方法により調製した。
【００２２】
まず、市販の比表面積約２５０ｍ２／ｇのカーボン粉末７．０ｇを純水０．２Ｌに分散させた。この分散液に白金３．０ｇを含有するヘキサヒドロキソ白金硝酸水溶液を滴下し、カーボン粉末と十分に混合した。この混合物に純水１Ｌを滴下した後、ろ過し、得られたケーキを十分に水洗した。このケーキを再び純水１Ｌに均一に分散させ、この分散液に、還元剤として水素化ホウ素ナトリウム４ｇを純水中に溶解した溶液を滴下した。この混合物をろ過し、得られたケーキを十分に水洗し、真空中、１００℃で１０時間乾燥させた。この乾燥ケーキを空気中に８０℃で２０時間保持（アニール処理）して、所望のアノード電極触媒粉末（Ｂ−１）を得た。
【００２３】
得られた触媒粉末Ｂ−１についてＣＯの吸着量を測定したところ、白金グラム当たり３０ｍＬであり、これに基づき白金担持割合を算出したところ、３０．０重量％であった。また、触媒粉末Ｂ−１のＸＲＤを測定し、３９°付近のＰｔ（１１１）面のピークの半価幅から白金の平均粒径を算出したところ、２．２〜２．６ｎｍであった。さらに、触媒粉末Ｂ−１のＢＥＴ比表面積は、１７５ｇ／ｍ２であった。また、ＴＧ−ＤＴＡ分析の結果、触媒粉末Ｂ−１の酸素保持量は４重量％であった。触媒粉末Ｂ−１の各種物性を表１に示す。
【００２４】
実施例６〜７、比較例２〜３
空気中での乾燥ケーキのアニール時間をそれぞれ１時間、５時間、１５時間、２０時間に変えた以外は実施例１と同様の手法により所望の触媒粉末（それぞれ、Ｂ２〜Ｂ−５）を得た。得られた触媒粉末の各種物性を表１に示す。
【００２５】
実施例８
本実施例では、カソード電極触媒を以下の方法により調製した。
【００２６】
まず、市販の比表面積約１０００ｍ２／ｇのカーボン粉末４．０ｇを純水０．２Ｌに分散させた。この分散液に白金６．０ｇを含有するヘキサヒドロキソ白金硝酸水溶液を滴下し、カーボン粉末と十分に混合した。この混合物に純水１Ｌを滴下した後、ろ過し、得られたケーキを十分に水洗した。このケーキを再び純水１Ｌに均一に分散させ、この分散液に、硝酸鉄４．５ｇを純水中に溶解した溶液を滴下した。この混合物をろ過し、得られたケーキを十分に水洗し、真空中、１００℃で１０時間乾燥させた。この乾燥ケーキを電気炉内に置き、水素雰囲気中７００℃で２時間還元処理した後、アルゴン雰囲気中８５０℃で２時間加熱して白金と鉄を合金化させた。得られたケーキを空気中に８０℃で１０時間保持（アニール処理）して、所望のカソード電極触媒粉末（Ｃ−１）を得た。
【００２７】
得られた触媒粉末Ｃ−１についてＣＯの吸着量を測定したところ、白金グラム当たり１７．５ｍＬであり、これに基づき白金担持割合および鉄担持割合を算出したところ、それぞれ、６０．０重量％および４．０重量％であった。また、触媒粉末Ｃ−１のＸＲＤを測定し、３９°付近のＰｔ（１１１）面のピークの半価幅からＰｔ合金の平均粒径を算出したところ、３．８〜４．０ｎｍであり、同じくピークシフト量からＦｅの固溶度を算出したところ、２１重量％であった。さらに、触媒粉末Ｃ−１のＢＥＴ比表面積は、３８０ｇ／ｍ２であった。また、ＴＧ−ＤＴＡ分析の結果、触媒粉末Ｃ−１の酸素保持量は４重量％であった。触媒粉末Ｃ−１の各種物性を表１に示す。
【００２８】
比較例４
アニール処理を行わなかった以外は実施例１と同様の手法により比較のカソード電極触媒（Ａ−０）を調製した。この触媒の各種物性を表１に示す。
【００２９】
比較例５
アニール処理を行わなかった以外は実施例５と同様の手法により比較のアノード電極触媒（Ｂ−０）を調製した。この触媒の各種物性を表１に示す。
【００３０】
＜性能評価＞
以上のように調製したカソード電極触媒粉末とアノード電極触媒粉末を用いて、下記要領で固体高分子型燃料電池（単電池）を作製した。
【００３１】
すなわち、カソード電極触媒粉末とアノード電極触媒粉末をそれぞれ有機溶媒に分散させ、これら分散液をそれぞれテフロンシートに塗布して触媒層を形成した。電極面積１ｃｍ２当りの触媒粉末の量は、０．４ｍｇであった。これらの電極を高分子電解質膜（デュポン社製商品名ナフィオン）を介してホットプレスにより貼り合わせ、各テフロンシートを剥離した後、両側にカーボンクロスからなる拡散層を設けて単電池を作製した。なお、カソード電極触媒粉末として触媒粉末Ａ−０〜Ａ−５およびＣ−１を用いた場合は、いずれも、アノード電極触媒粉末として触媒粉末Ｂ−０を用いた。また、アノード電極触媒粉末として触媒粉末Ｂ−０〜Ｂ−５を用いた場合は、いずれも、カソード触媒電極粉末として触媒粉末Ａ−０を用いた。
【００３２】
作製した単電池のカソード電極側に空気を１Ｌ／分、アノード電極側に水素を０．５Ｌ／分の流量で供給した。その際、単電池を８０℃に維持し、０．５Ａ／ｃｍ２の電流密度で運転し、初期の電池電圧特性と５００時間経過後の電流電圧特性を測定した。なお、運転時の加湿条件（フル加湿）は、カソード側バブラ温が７５℃であり、アノード側バブラ温が８５℃であった。また、５００時間経過後の白金の平均粒径も上記と同様に測定した。白金の平均粒径の測定結果は表１に併記する。
【００３３】
【表１】
【００３４】
カソード電極触媒粉末として触媒粉末Ａ−１を用い、アノード電極触媒として触媒粉末Ｂ−０を用いた場合（本発明）と、カソード電極触媒粉末として触媒粉末Ａ−０を用い、アノード電極触媒として触媒粉末Ｂ−０を用いた場合（比較）における電池電圧の経時変化をそれぞれ図２に示す。図２に示す結果から、本発明のカソード電極触媒は、５００時間経過後でも、電池電圧の低下をほとんど生じさせず、安定な電池運転が行えることがわかる。
【００３５】
カソード電極触媒における酸素保持割合と単電池の初期電圧の関係を図３に示す。図３に示す結果から、酸素保持割合が１０重量％を超えると、触媒活性が低下し、初期電圧が大幅に低下することがわかる。また、図４に、カソード電極触媒における酸素保持割合と５００時間経過後の電圧低下量の関係を示す。図４に示す結果から、酸素保持割合が１重量％以上の場合には、長時間運転しても電圧低下が実用上許容できる−５ｍＶの範囲内にあり、安定であることがわかる。以上のことから、カソード電極触媒における酸素保持割合が１〜１０重量％であると、初期電圧が高く、長時間運転しても安定な電圧を示す固体高分子型燃料電池を提供できることがわかる。
【００３６】
次に、アノード電極触媒における酸素保持割合と単電池の初期電圧の関係を図５に示す。図５に示す結果から、酸素保持割合が１０重量％を超えると、触媒活性が低下し、初期電圧が大幅に低下することがわかる。また、図６に、アノード電極触媒における酸素保持割合と５００時間経過後の電圧低下量の関係を示す。図６に示す結果から、酸素保持割合が１重量％以上の場合には、長時間運転しても電圧低下が実用上許容できる−５ｍＶの範囲内にあり、安定であることがわかる。以上のことから、アノード電極触媒にあってもその酸素保持割合が１〜１０重量％であると、初期電圧が高く、長時間運転しても安定な電圧を示す固体高分子型燃料電池を提供できることがわかる。
【００３７】
図７は、カソード電極触媒として触媒粉末Ｃ−１を用い、アノード電極として触媒粉末Ｂ−０を用いた場合における電池電圧の経時変化を示す。図７に示す結果から、白金合金を用いた場合でも、その酸素保持割合が本発明の範囲内にあれば、電池電圧の低下をほとんど生じさせず、安定な電池運転が行えることがわかる。
【００３８】
なお、表１に示すように、酸素を保持した電極触媒は、５００時間経過後も貴金属触媒の粒径が大幅に変化することがなく、凝集等が生じ難いことがわかる。
【００３９】
【発明の効果】
以上述べたように、本発明によれば、長期運転しても劣化が少なく安定な固体高分子型燃料電池用電極触媒が提供される。
【図面の簡単な説明】
【図１】実施例１で得た触媒のＸ線光電子分光スペクトル図。
【図２】カソード電極として本発明の電極触媒を用いた場合におけるる電池電圧の経時変化を比較例のそれとともに示すグラフ。
【図３】カソード電極触媒における酸素保持割合と単電池の初期電圧の関係を示すグラフ。
【図４】カソード電極触媒における酸素保持割合と５００時間経過後の電池電圧の低下量の関係を示すグラフ。
【図５】アノード電極触媒における酸素保持割合と単電池の初期電圧の関係を示すグラフ。
【図６】アノード電極触媒における酸素保持割合と５００時間経過後の電圧の低下量の関係を示すグラフ。
【図７】カソード電極触媒として本発明の他の電極触媒を用いた場合における電池電圧の経時変化を示すグラフ。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an electrode catalyst for a polymer electrolyte fuel cell.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, a polymer electrolyte fuel cell uses an ion exchange resin membrane as an electrolyte, and a platinum-based noble metal supported on a conductive carrier is used for both an anode and a cathode as an electrode catalyst. In order to improve the performance of such a polymer electrolyte fuel cell or the catalyst used therein, various devices have hitherto been devised. For example, Patent Literature 1 discloses a technique for improving the catalytic activity by suppressing the crystallite diameter of platinum to be small even if the amount of supported platinum is increased. In Patent Document 1, conductive carbon to which oxygen is bonded at a predetermined ratio is used as a catalyst carrier. Here, it is assumed that oxygen combined with platinum is not detected.
[0003]
[Patent Document 1]
JP 2001-325964 A
[Problems to be solved by the invention]
The catalyst for a polymer electrolyte fuel cell is required to have not only improved activity but also durability such that the voltage of the battery does not decrease for a long time when incorporated into the battery. When the polymer electrolyte fuel cell is operated for a long period of time, the catalyst particles of the electrode catalyst are aggregated and the components are eluted, so that the catalyst is deteriorated and the life of the battery is shortened.
[0005]
Therefore, an object of the present invention is to provide an electrode catalyst for a polymer electrolyte fuel cell that is stable with little deterioration even after long-term operation.
[0006]
[Means for Solving the Problems]
According to the present invention, a platinum-based noble metal catalyst supported on a conductive carrier is included, and oxygen is retained on the surface of the noble metal catalyst at a rate of 1 to 10% by weight based on the total weight of the carrier-supported noble metal catalyst. An electrode catalyst for a polymer electrolyte fuel cell is provided.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in more detail.
[0008]
The polymer electrolyte fuel cell electrode catalyst of the present invention contains a platinum-based noble metal catalyst supported on a conductive carrier, and can be applied to both the cathode electrode catalyst and the anode electrode catalyst, and to both of them. .
[0009]
The conductive carrier used usually consists of conductive carbon black particles. When the electrode catalyst for a polymer electrolyte fuel cell of the present invention is applied to a cathode electrode catalyst, the conductive support preferably has a BET specific surface area of about 700 to about 1300 m 2 / g. On the other hand, when the electrocatalyst of the present invention is applied to an anode electrocatalyst, the conductive support preferably has a BET specific surface area of about 50 to about 500 m 2 / g.
[0010]
The platinum-based noble metal catalyst supported on the conductive carrier includes platinum and a platinum alloy. Examples of the platinum alloy include platinum-precious metal alloys such as platinum-iron alloys, platinum-nickel alloys, platinum-cobalt alloys, platinum-copper alloys, and other platinum-transition metal alloys, platinum-palladium alloys, and platinum-rhodium alloys. be able to. The loading ratio of the platinum-based noble metal catalyst (the ratio of the platinum-based noble metal catalyst to the total weight of the conductive support and the platinum-based noble metal catalyst) is preferably about 20 to 80% by weight.
[0011]
Now, in the electrode catalyst for a polymer electrolyte fuel cell of the present invention, the surface of the platinum-based noble metal catalyst has a surface area of 1 to 1 of the total weight of the carrier-supported platinum-based noble metal catalyst (total weight of the conductive support and the platinum-based noble metal catalyst). Oxygen is held and fixed at a rate corresponding to 10%. Such oxygen retention can be achieved by an annealing treatment by keeping the carrier-supported platinum-based noble metal catalyst at a temperature of 50 to 90 ° C, preferably 80 ° C, in air. The oxygen holding ratio can be adjusted by adjusting the annealing time. This oxygen retention ratio can be measured by TG-DTA measurement. At that time, heating is performed from room temperature to 300 ° C. in an atmosphere composed of 10% by volume of hydrogen and 90% by volume of helium at a rate of 5 ° C./min. It should be noted that even if only the conductive carbon black carrier is annealed in the same manner, it has been confirmed by such TG-DTA measurement that there is almost no change in weight other than the absorbed water. In the XPS analysis, a very small amount of an oxide peak of the platinum-based noble metal catalyst was confirmed. That is, it can be said that oxygen is held and fixed on the surface of the platinum-based noble metal catalyst by chemical adsorption. If the oxygen holding ratio is less than 1% by weight, the battery voltage after long-term operation tends to be significantly lower than the initial battery voltage. On the other hand, when the oxygen holding ratio exceeds 10% by weight, the catalyst activity is reduced, and as a result, the initial voltage of the battery is significantly reduced.
[0012]
The electrode catalyst of the present invention is prepared by supporting a platinum-based noble metal catalyst (platinum and a metal for alloying in the case of a platinum alloy) on a conductive carrier by a usual wet method, and drying in a vacuum. In this case, it can be manufactured by performing the above-mentioned annealing treatment after performing the reduction treatment and the alloying treatment of platinum and the alloying metal.
[0013]
As described above, the polymer electrolyte fuel cell electrode catalyst of the present invention can be used as a cathode electrode catalyst or as an anode electrode catalyst. Of the polymer electrolyte fuel cell electrode catalyst. When only one of the cathode electrode catalyst and the anode electrode catalyst is formed of the electrode catalyst for a polymer electrolyte fuel cell of the present invention, a normal electrode catalyst can be used as the other electrode catalyst. Further, the amount of the electrode catalyst for a polymer electrolyte fuel cell of the present invention is the same as usual, and for example, a platinum amount of about 0.3 to 0.5 mg / cm 2 is sufficient.
[0014]
The electrode catalyst for a polymer electrolyte fuel cell of the present invention shows stable performance with little deterioration over a long period of time.
[0015]
【Example】
Hereinafter, the present invention will be described with reference to Examples, but the present invention is not limited thereto.
[0016]
Example 1
In this example, a cathode electrode catalyst was prepared by the following method.
[0017]
First, 4.0 g of a commercially available carbon powder having a specific surface area of about 1000 m 2 / g was dispersed in 0.2 L of pure water. An aqueous solution of hexahydroxoplatinic nitric acid containing 6.0 g of platinum was dropped into this dispersion, and mixed sufficiently with carbon powder. After 1 L of pure water was dropped into this mixture, the mixture was filtered, and the obtained cake was sufficiently washed with water. This cake was again uniformly dispersed in 1 L of pure water, and a solution in which 4 g of sodium borohydride as a reducing agent was dissolved in pure water was added dropwise to the dispersion. The mixture was filtered, and the obtained cake was sufficiently washed with water and dried in vacuum at 100 ° C. for 10 hours. This dried cake was kept in air at 80 ° C. for 10 hours (annealing treatment) to obtain a desired cathode electrode catalyst powder (A-1).
[0018]
The amount of carbon monoxide (CO) adsorbed on the obtained catalyst powder A-1 was measured and found to be 32 mL per gram of platinum. Based on this, the platinum carrying ratio was calculated to be 60.0% by weight. Further, the X-ray diffraction (XRD) of the catalyst powder A-1 was measured, and the average particle size of platinum was calculated from the half width of the peak of the Pt (111) plane at around 39 °. It was 0 nm. Further, the BET specific surface area of the catalyst powder A-1 was 400 g / m 2 . As a result of TG-DTA analysis under the above measurement conditions, the amount of oxygen retained in the catalyst powder A-1 was 4% by weight. Table 1 shows various physical properties of the catalyst powder A-1. FIG. 1 shows an X-ray photoelectron spectroscopy (XPS) spectrum of the catalyst powder A-1. FIG. 1 shows that oxygen is retained on the platinum surface.
[0019]
Although only the conductive carbon was subjected to the annealing treatment, the same TG-DTA analysis showed that there was almost no change in weight other than the absorbed water.
[0020]
Examples 2 to 4, Comparative Example 1
Desired cathode electrode catalyst powders (A2 to A-5, respectively) were obtained in the same manner as in Example 1 except that the annealing time of the dried cake was changed to 1 hour, 5 hours, 15 hours, and 20 hours, respectively. Table 1 shows the physical properties of the obtained catalyst powder.
[0021]
Example 5
In this example, an anode electrode catalyst was prepared by the following method.
[0022]
First, 7.0 g of commercially available carbon powder having a specific surface area of about 250 m 2 / g was dispersed in 0.2 L of pure water. An aqueous solution of hexahydroxo platinum nitrate containing 3.0 g of platinum was added dropwise to the dispersion, and mixed sufficiently with carbon powder. After 1 L of pure water was dropped into this mixture, the mixture was filtered, and the obtained cake was sufficiently washed with water. This cake was again uniformly dispersed in 1 L of pure water, and a solution in which 4 g of sodium borohydride as a reducing agent was dissolved in pure water was added dropwise to the dispersion. The mixture was filtered, and the obtained cake was sufficiently washed with water and dried in vacuum at 100 ° C. for 10 hours. This dried cake was kept in air at 80 ° C. for 20 hours (annealing treatment) to obtain a desired anode electrode catalyst powder (B-1).
[0023]
The amount of CO adsorbed on the obtained catalyst powder B-1 was measured and found to be 30 mL per gram of platinum. Based on this, the platinum carrying ratio was calculated to be 30.0% by weight. Further, the XRD of the catalyst powder B-1 was measured, and the average particle size of platinum was calculated from the half-value width of the peak of the Pt (111) plane at around 39 °, to be 2.2 to 2.6 nm. Further, the BET specific surface area of the catalyst powder B-1 was 175 g / m 2 . Moreover, as a result of TG-DTA analysis, the oxygen retention amount of the catalyst powder B-1 was 4% by weight. Table 1 shows various physical properties of the catalyst powder B-1.
[0024]
Examples 6-7, Comparative Examples 2-3
Desired catalyst powders (B2 to B-5, respectively) were obtained in the same manner as in Example 1 except that the annealing time of the dried cake in the air was changed to 1 hour, 5 hours, 15 hours, and 20 hours, respectively. Was. Table 1 shows various physical properties of the obtained catalyst powder.
[0025]
Example 8
In this example, a cathode electrode catalyst was prepared by the following method.
[0026]
First, 4.0 g of a commercially available carbon powder having a specific surface area of about 1000 m 2 / g was dispersed in 0.2 L of pure water. An aqueous solution of hexahydroxoplatinic nitric acid containing 6.0 g of platinum was dropped into this dispersion, and mixed sufficiently with carbon powder. After 1 L of pure water was dropped into this mixture, the mixture was filtered, and the obtained cake was sufficiently washed with water. This cake was again dispersed uniformly in 1 L of pure water, and a solution in which 4.5 g of iron nitrate was dissolved in pure water was added dropwise to the dispersion. The mixture was filtered, and the obtained cake was sufficiently washed with water and dried in vacuum at 100 ° C. for 10 hours. The dried cake was placed in an electric furnace, reduced in a hydrogen atmosphere at 700 ° C. for 2 hours, and then heated in an argon atmosphere at 850 ° C. for 2 hours to alloy platinum and iron. The obtained cake was kept in air at 80 ° C. for 10 hours (annealing treatment) to obtain a desired cathode electrode catalyst powder (C-1).
[0027]
The amount of CO adsorbed on the obtained catalyst powder C-1 was measured and found to be 17.5 mL per gram of platinum. Based on this, the ratio of supported platinum and the ratio of supported iron were calculated to be 60.0% by weight and It was 4.0% by weight. Further, when the XRD of the catalyst powder C-1 was measured and the average particle size of the Pt alloy was calculated from the half width of the peak of the Pt (111) plane at around 39 °, it was 3.8 to 4.0 nm, Similarly, when the solid solubility of Fe was calculated from the peak shift amount, it was 21% by weight. Further, the BET specific surface area of the catalyst powder C-1 was 380 g / m 2 . Moreover, as a result of TG-DTA analysis, the oxygen retention amount of the catalyst powder C-1 was 4% by weight. Table 1 shows various physical properties of the catalyst powder C-1.
[0028]
Comparative Example 4
A comparative cathode electrode catalyst (A-0) was prepared in the same manner as in Example 1 except that the annealing treatment was not performed. Table 1 shows various physical properties of this catalyst.
[0029]
Comparative Example 5
A comparative anode electrode catalyst (B-0) was prepared in the same manner as in Example 5, except that the annealing treatment was not performed. Table 1 shows various physical properties of this catalyst.
[0030]
<Performance evaluation>
Using the cathode electrode catalyst powder and anode electrode catalyst powder prepared as described above, a polymer electrolyte fuel cell (unit cell) was produced in the following manner.
[0031]
That is, the cathode electrode catalyst powder and the anode electrode catalyst powder were each dispersed in an organic solvent, and these dispersions were respectively applied to a Teflon sheet to form a catalyst layer. The amount of the catalyst powder per 1 cm 2 of the electrode area was 0.4 mg. These electrodes were bonded by a hot press via a polymer electrolyte membrane (trade name: Nafion, manufactured by DuPont), and after peeling off each Teflon sheet, a diffusion layer made of carbon cloth was provided on both sides to produce a unit cell. When the catalyst powders A-0 to A-5 and C-1 were used as the cathode electrode catalyst powder, the catalyst powder B-0 was used as the anode electrode catalyst powder. When the catalyst powders B-0 to B-5 were used as the anode catalyst powder, the catalyst powder A-0 was used as the cathode catalyst electrode powder.
[0032]
Air was supplied at a flow rate of 1 L / min to the cathode electrode side and hydrogen was supplied at a flow rate of 0.5 L / min to the anode electrode side of the produced unit cell. At that time, the cell was maintained at 80 ° C., operated at a current density of 0.5 A / cm 2 , and the initial battery voltage characteristics and the current-voltage characteristics after 500 hours had elapsed were measured. The humidification conditions during operation (full humidification) were such that the cathode-side bubbler temperature was 75 ° C and the anode-side bubbler temperature was 85 ° C. The average particle size of platinum after 500 hours had elapsed was measured in the same manner as above. The measurement results of the average particle size of platinum are also shown in Table 1.
[0033]
[Table 1]
[0034]
When the catalyst powder A-1 is used as the cathode electrode catalyst powder and the catalyst powder B-0 is used as the anode electrode catalyst (the present invention), the catalyst powder A-0 is used as the cathode electrode catalyst powder, and the catalyst is used as the anode electrode catalyst. FIG. 2 shows the time-dependent changes in the battery voltage when powder B-0 was used (comparison). From the results shown in FIG. 2, it can be seen that the cathode electrode catalyst of the present invention hardly causes a decrease in battery voltage even after 500 hours, and can perform a stable battery operation.
[0035]
FIG. 3 shows the relationship between the oxygen holding ratio in the cathode electrode catalyst and the initial voltage of the cell. From the results shown in FIG. 3, it is found that when the oxygen retention ratio exceeds 10% by weight, the catalyst activity decreases, and the initial voltage drops significantly. FIG. 4 shows the relationship between the oxygen retention ratio in the cathode electrode catalyst and the amount of voltage drop after 500 hours. From the results shown in FIG. 4, it can be seen that when the oxygen holding ratio is 1% by weight or more, the voltage drop is within a practically allowable range of -5 mV even when the operation is performed for a long time, and it is stable. From the above, it can be seen that when the oxygen retention ratio in the cathode electrode catalyst is 1 to 10% by weight, a polymer electrolyte fuel cell having a high initial voltage and exhibiting a stable voltage even when operated for a long time can be provided.
[0036]
Next, the relationship between the oxygen retention ratio in the anode electrode catalyst and the initial voltage of the unit cell is shown in FIG. From the results shown in FIG. 5, it can be seen that when the oxygen holding ratio exceeds 10% by weight, the catalyst activity decreases, and the initial voltage significantly decreases. FIG. 6 shows the relationship between the oxygen retention ratio in the anode electrode catalyst and the amount of voltage drop after 500 hours. From the results shown in FIG. 6, it can be seen that when the oxygen holding ratio is 1% by weight or more, the voltage drop is within a practically allowable range of -5 mV even when the operation is performed for a long time, and it is stable. From the above, if the oxygen retention ratio of the anode electrode catalyst is 1 to 10% by weight, a polymer electrolyte fuel cell having a high initial voltage and exhibiting a stable voltage even when operated for a long time is provided. We can see that we can do it.
[0037]
FIG. 7 shows the change over time of the battery voltage when the catalyst powder C-1 was used as the cathode electrode catalyst and the catalyst powder B-0 was used as the anode electrode. From the results shown in FIG. 7, it can be seen that even when a platinum alloy is used, if the oxygen holding ratio is within the range of the present invention, the battery voltage hardly decreases and stable battery operation can be performed.
[0038]
As shown in Table 1, the particle size of the noble metal catalyst in the electrode catalyst holding oxygen does not change significantly even after 500 hours, and aggregation and the like hardly occur.
[0039]
【The invention's effect】
As described above, according to the present invention, there is provided an electrode catalyst for a polymer electrolyte fuel cell that is stable with little deterioration even after long-term operation.
[Brief description of the drawings]
FIG. 1 is an X-ray photoelectron spectroscopy spectrum of the catalyst obtained in Example 1.
FIG. 2 is a graph showing time-dependent changes in battery voltage when using the electrode catalyst of the present invention as a cathode electrode, together with those of a comparative example.
FIG. 3 is a graph showing a relationship between an oxygen retention ratio in a cathode electrode catalyst and an initial voltage of a unit cell.
FIG. 4 is a graph showing the relationship between the oxygen retention ratio in a cathode electrode catalyst and the amount of decrease in battery voltage after 500 hours.
FIG. 5 is a graph showing a relationship between an oxygen retention ratio in an anode electrode catalyst and an initial voltage of a unit cell.
FIG. 6 is a graph showing the relationship between the oxygen retention ratio in an anode electrode catalyst and the amount of decrease in voltage after 500 hours.
FIG. 7 is a graph showing a change over time in battery voltage when another electrode catalyst of the present invention is used as a cathode electrode catalyst.
