低鉑催化劑Pd@Pt/C的制備及其電催化活性的研究

陳　容，黃琦杰

(華南理工大學化學與化工學院，廣東　廣州　510641)

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低鉑催化劑Pd@Pt/C的制備及其電催化活性的研究

陳容，黃琦杰

(華南理工大學化學與化工學院，廣東廣州510641)

燃料電池陰極氧還原動力學緩慢，需要使用大量的鉑催化劑，導致電池高昂的成本，制約了質子交換膜燃料電池的大規模產業化。解決這個瓶頸的關鍵在于研究與制備高性能、低鉑載量、耐久性好的燃料電池催化劑。而核殼結構催化劑因其特殊的結構可以使得Pt的分散度、利用率、活性得到很大的提高。本文采用脈沖電流沉積的方法制備了Pd@Pt/C催化劑。電化學測試結果表明，Pd@Pt/C催化劑的氧還原活性可媲美商品的20% Pt/C催化劑，Pd@Pt/C催化劑的Pt質量活性可達JM Pt/C催化劑的3.1倍。

燃料電池；催化劑；氧還原

低溫質子交換膜燃料電池(PEMFC)作為一種潔凈能源技術，具有能量轉換效率高、工作條件溫和、啟動速度快等優點，是一種理想的新能源汽車動力電源[1-4]。目前，該燃料電池需要在陰極附載大量的鉑碳催化劑以加快緩慢的氧還原反應[5-6]，然而，鉑作為一種貴金屬，價格昂貴，資源稀缺，這嚴重制約了燃料電池的商業化發展[7-10]。核殼結構低鉑納米粒子能夠有效減少Pt的使用量、提高鉑的利用率，且在酸性介質中表現出良好的氧還原性能，對于推進質子交換膜燃料電池的發展及大規模商業化具有重要意義[11-13]。本文采用脈沖沉積技術，制備Pd@Pt/C催化劑，并與商業催化劑Pt/C(Johnson Matthey, 20% Pt)的電化學催化性能進行比較。

1　實　驗

實驗所使用的氯鉑酸(H2PtCl4·6H2O)和氯化鈀(PdCl2)等試劑，沈陽金科廠；乙二醇(EG)，上海強順有限公司；無水乙醇，南京化學試劑公司；Nafion溶液，美國杜邦公司。所有試劑均為分析純。

1.1催化劑的制備

將適量PdCl2和檸檬酸鈉溶于少量乙二醇中，檸檬酸鈉與Pd的摩爾比為2.5:1，加入處理過的Vulcan XC-72炭粉，攪拌10 min，超聲1 h。用5%的KOH/EG溶液調節PH>10，將漿液轉移至高壓反應釜，120 ℃反應6 h，過濾，二次去離子水洗滌，使用氯化鋇檢驗至濾液不含氯離子，70 ℃烘干，研磨待用，催化劑中Pd的質量分數為20%。在5 mM的氯鉑酸鍍液中，含少量聚乙烯吡咯烷酮，加入所制備的Pd/C作基底催化劑100 mg，施加脈沖電流沉積，制得Pd@Pt/C催化劑，其中Pt的載量為7.1%。

1.2催化劑表征及性能評價

使用丹東通達儀器有限公司TD-3500X射線衍射儀對制備的催化劑進行XRD分析，TD-3500的測試條件如下，電壓40 kV，電流：30 mA，物相分析掃描步徑：0.025 °，掃描速率為4 °/min，掃描范圍從20°～80°。電化學性能測試在荷蘭IVIUM電化學工作站上進行。采用三電極體系，鉑絲為對電極，Ag/AgCl為參比電極，工作電極為玻碳電極，工作電極使用前需分別使用0.1微米和0.05微米的氧化鋁打磨超聲清洗。5 mg催化劑分散于1 mL質量濃度0.25%的nafion溶液中，取5 μL漿液涂在5 mm直徑的玻碳電極上，室溫下自然干燥后測試。電解液為0.1 MHClO4的溶液，循環伏安曲線掃描范圍-0.2 V～0.8 V(Ag/AgCl)，掃描速率0.01 V/s。

2　結果與討論

圖1是Pd/C和Pd@Pt/C的XRD譜圖。兩個樣品在25°左右均出現了碳的002衍射峰，而出現在2θ40.1°、46.6°、68.2°附近的衍射峰分別歸屬于面心立方晶體(fcc)Pd的(111)、(200)和(220)晶面衍射峰，這說明使用高壓有機溶膠成功制備了Pd/C基底催化劑。相對于Pd/C，Pd@Pt/C催化劑的半峰寬變寬且峰形變得不對稱，在40.1°的主峰位偏移了約0.1°，可能是鉑主要覆蓋在鈀的表層，形成一個薄的覆蓋層，這可以作為鉑覆蓋在鈀表面的間接證據。薄薄的一層活性物質有利于增加催化劑的活性比表面，提高鉑的利用率。Jade軟件計算的Pd@Pt/C和Pd/C晶粒大小分別為6.6 nm、4.7 nm，與Pd/C晶粒尺寸相比，Pd@Pt/C粒徑增大的原因很可能是鉑在鈀表面覆蓋的緣故，間接證明了鉑覆蓋在鈀表面。鉑的覆蓋使得晶粒的尺寸增大了1.9 nm，即Pd@Pt/C催化劑鉑殼層厚度約為3個Pt原子層。

圖1　Pd/C和Pd@Pt/C納米粒子的XRD衍射圖Fig.1　XRD patterns of Pd/C and Pd@Pt/C nanoparticles

圖2為JM Pt/C與Pd@Pt/C納米粒子在轉速為1600轉/分鐘，氧氣飽和下的0.1 MHClO4溶液中的ORR極化曲線，掃描速率為10 mv/S。典型的氧還原極化曲線可以分為三個部分：動力學控制區，混合動力區，擴散控制區。動力學控制區和混合動力區交界處的起始電位、混合動力區最大電流密度一半位置的半波電位及擴散控制區電流密度平臺的極限電流密度，這三個參數的值越大，表明催化劑的氧還原性能越好[14-15]。從該圖可以看出Pd@Pt/C催化劑表現出良好的ORR催化活性，其電化學催化性能可媲美商品的20%Pt/C催化劑，Pd@Pt/C催化劑的起始電位更高，極限電流密度相比于商業Pt/C催化劑高出約0.5 mA/cm2，半波電位比JM Pt/C催化劑高出約30 mV，Pd@Pt/C催化劑的Pt質量活性可達JM Pt/C催化劑的3.1倍。這表明Pd@Pt/C催化劑具有更高的活性比表面，單位質量鉑的催化活性增大，提高了鉑的利用率。

圖2　在0.1 M HClO4溶液中JM Pt/C與Pd@Pt/C催化劑的氧還原 性能線性掃描(A)和JM Pt/C與Pd@Pt/C催化劑的質量活性對比(B)Fig.2　The LSV carves of JM Pt/C and Pd@Pt/C catalysts(A) and the comparison of mass activities of JM Pt/C and Pd@Pt/C catalysts(B)

3　結　論

采用脈沖沉積技術，兩步法制備了Pd@Pt/C低鉑催化劑(鉑載量為7%)，XRD結果揭示了Pd@Pt/C低鉑催化劑自身的特殊結構。電化學測試結果顯示，與商業JM Pt/C催化劑比較，Pd@Pt/C低鉑催化劑的起始電位、半波電位、極限電流密度都有一定程度上的提升，表明這是一種高活性低鉑催化劑，有良好的應用前景。

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Preparation and Electro-catalytic Activity of Pd@Pt/C Catalyst

CHENRong,HUANGQi-jie

(School of Chemistry and Chemical Engineering, South China University of Technology,Guangdong Guangzhou 510641, China)

The kinetics of the oxygen reduction reaction in fuel cell cathodes is sluggish that needs using large amounts of Pt to compensate, which mainly leads to the high cost of fuel cell, as well as hider the large scale application of proton exchange membrane fuel cell. In order to overcome these problems, it needs to investigate high performance, low platinum loading, excellent durability electrocatalysts. Core-shell structure catalyst, because of its special structure which can make the Pt dispersion, utilization, and activity be greatly improved as well as reduce Pt loading, has been widely recognized as being among the most promising candidates to achieve the commercialization of proton exchange membrane fuel cell. A novel pulse deposition method was used to prepare a low platinum catalyst Pd@Pt/C. For the cathodic reduction of oxygen, Pd@Pt/C catalyst demonstrated three times higher mass activity towards the cathodic reduction of oxygen than commercial Pt/C catalyst, exhibiting competitive performance compared with commercial Pt/C catalyst.

fuel cell; catalyst; oxygen reduction

陳容(1990-)，女，碩士，研究方向是燃料電池低鉑催化劑。

TM911.4

A

1001-9677(2016)015-0085-03