低铂催化剂Pd@Pt/C的制备及其电催化活性的研究

陈　容，黄琦杰

(华南理工大学化学与化工学院，广东　广州　510641)



低铂催化剂Pd@Pt/C的制备及其电催化活性的研究

陈容，黄琦杰

(华南理工大学化学与化工学院，广东广州510641)

燃料电池阴极氧还原动力学缓慢，需要使用大量的铂催化剂，导致电池高昂的成本，制约了质子交换膜燃料电池的大规模产业化。解决这个瓶颈的关键在于研究与制备高性能、低铂载量、耐久性好的燃料电池催化剂。而核壳结构催化剂因其特殊的结构可以使得Pt的分散度、利用率、活性得到很大的提高。本文采用脉冲电流沉积的方法制备了Pd@Pt/C催化剂。电化学测试结果表明，Pd@Pt/C催化剂的氧还原活性可媲美商品的20% Pt/C催化剂，Pd@Pt/C催化剂的Pt质量活性可达JM Pt/C催化剂的3.1倍。

燃料电池；催化剂；氧还原

低温质子交换膜燃料电池(PEMFC)作为一种洁净能源技术，具有能量转换效率高、工作条件温和、启动速度快等优点，是一种理想的新能源汽车动力电源[1-4]。目前，该燃料电池需要在阴极附载大量的铂碳催化剂以加快缓慢的氧还原反应[5-6]，然而，铂作为一种贵金属，价格昂贵，资源稀缺，这严重制约了燃料电池的商业化发展[7-10]。核壳结构低铂纳米粒子能够有效减少Pt的使用量、提高铂的利用率，且在酸性介质中表现出良好的氧还原性能，对于推进质子交换膜燃料电池的发展及大规模商业化具有重要意义[11-13]。本文采用脉冲沉积技术，制备Pd@Pt/C催化剂，并与商业催化剂Pt/C(Johnson Matthey, 20% Pt)的电化学催化性能进行比较。

1　实　验

实验所使用的氯铂酸(H2PtCl4·6H2O)和氯化钯(PdCl2)等试剂，沈阳金科厂；乙二醇(EG)，上海强顺有限公司；无水乙醇，南京化学试剂公司；Nafion溶液，美国杜邦公司。所有试剂均为分析纯。

1.1催化剂的制备

将适量PdCl2和柠檬酸钠溶于少量乙二醇中，柠檬酸钠与Pd的摩尔比为2.5:1，加入处理过的Vulcan XC-72炭粉，搅拌10 min，超声1 h。用5%的KOH/EG溶液调节PH>10，将浆液转移至高压反应釜，120 ℃反应6 h，过滤，二次去离子水洗涤，使用氯化钡检验至滤液不含氯离子，70 ℃烘干，研磨待用，催化剂中Pd的质量分数为20%。在5 mM的氯铂酸镀液中，含少量聚乙烯吡咯烷酮，加入所制备的Pd/C作基底催化剂100 mg，施加脉冲电流沉积，制得Pd@Pt/C催化剂，其中Pt的载量为7.1%。

1.2催化剂表征及性能评价

使用丹东通达仪器有限公司TD-3500X射线衍射仪对制备的催化剂进行XRD分析，TD-3500的测试条件如下，电压40 kV，电流：30 mA，物相分析扫描步径：0.025 °，扫描速率为4 °/min，扫描范围从20°～80°。电化学性能测试在荷兰IVIUM电化学工作站上进行。采用三电极体系，铂丝为对电极，Ag/AgCl为参比电极，工作电极为玻碳电极，工作电极使用前需分别使用0.1微米和0.05微米的氧化铝打磨超声清洗。5 mg催化剂分散于1 mL质量浓度0.25%的nafion溶液中，取5 μL浆液涂在5 mm直径的玻碳电极上，室温下自然干燥后测试。电解液为0.1 MHClO4的溶液，循环伏安曲线扫描范围-0.2 V～0.8 V(Ag/AgCl)，扫描速率0.01 V/s。

2　结果与讨论

图1是Pd/C和Pd@Pt/C的XRD谱图。两个样品在25°左右均出现了碳的002衍射峰，而出现在2θ40.1°、46.6°、68.2°附近的衍射峰分别归属于面心立方晶体(fcc)Pd的(111)、(200)和(220)晶面衍射峰，这说明使用高压有机溶胶成功制备了Pd/C基底催化剂。相对于Pd/C，Pd@Pt/C催化剂的半峰宽变宽且峰形变得不对称，在40.1°的主峰位偏移了约0.1°，可能是铂主要覆盖在钯的表层，形成一个薄的覆盖层，这可以作为铂覆盖在钯表面的间接证据。薄薄的一层活性物质有利于增加催化剂的活性比表面，提高铂的利用率。Jade软件计算的Pd@Pt/C和Pd/C晶粒大小分别为6.6 nm、4.7 nm，与Pd/C晶粒尺寸相比，Pd@Pt/C粒径增大的原因很可能是铂在钯表面覆盖的缘故，间接证明了铂覆盖在钯表面。铂的覆盖使得晶粒的尺寸增大了1.9 nm，即Pd@Pt/C催化剂铂壳层厚度约为3个Pt原子层。

图1　Pd/C和Pd@Pt/C纳米粒子的XRD衍射图Fig.1　XRD patterns of Pd/C and Pd@Pt/C nanoparticles

图2为JM Pt/C与Pd@Pt/C纳米粒子在转速为1600转/分钟，氧气饱和下的0.1 MHClO4溶液中的ORR极化曲线，扫描速率为10 mv/S。典型的氧还原极化曲线可以分为三个部分：动力学控制区，混合动力区，扩散控制区。动力学控制区和混合动力区交界处的起始电位、混合动力区最大电流密度一半位置的半波电位及扩散控制区电流密度平台的极限电流密度，这三个参数的值越大，表明催化剂的氧还原性能越好[14-15]。从该图可以看出Pd@Pt/C催化剂表现出良好的ORR催化活性，其电化学催化性能可媲美商品的20%Pt/C催化剂，Pd@Pt/C催化剂的起始电位更高，极限电流密度相比于商业Pt/C催化剂高出约0.5 mA/cm2，半波电位比JM Pt/C催化剂高出约30 mV，Pd@Pt/C催化剂的Pt质量活性可达JM Pt/C催化剂的3.1倍。这表明Pd@Pt/C催化剂具有更高的活性比表面，单位质量铂的催化活性增大，提高了铂的利用率。

图2　在0.1 M HClO4溶液中JM Pt/C与Pd@Pt/C催化剂的氧还原 性能线性扫描(A)和JM Pt/C与Pd@Pt/C催化剂的质量活性对比(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　结　论

采用脉冲沉积技术，两步法制备了Pd@Pt/C低铂催化剂(铂载量为7%)，XRD结果揭示了Pd@Pt/C低铂催化剂自身的特殊结构。电化学测试结果显示，与商业JM Pt/C催化剂比较，Pd@Pt/C低铂催化剂的起始电位、半波电位、极限电流密度都有一定程度上的提升，表明这是一种高活性低铂催化剂，有良好的应用前景。

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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

陈容(1990-)，女，硕士，研究方向是燃料电池低铂催化剂。

TM911.4

A

1001-9677(2016)015-0085-03