摘要
高性能低成本的担载型铂基催化剂是直接甲醇燃料电池(DMFC)实用化过程中的一大挑战.利用高比表面积、高稳定性、容易负载金属的载体实现Pt颗粒的高度分散,既可提高催化剂的催化性能,又可提高Pt的利用率以降低成本,是担载型Pt基催化剂实用化的有效途径.碳材料是一种常用的催化剂载体,近年来我们课题组发展了一种高性能的碳纳米笼材料,并可通过异原子掺杂调变其表面性能,提高其活性和负载能力.我们采用原位氧化镁模板法制备氮掺杂碳纳米笼:以具有多级结构的碱式碳酸镁作为氧化镁模板的前体,吡啶为碳源和氮源,经高温热解沉积,在原位形成的氧化镁模板表面形成氮掺杂的石墨化碳纳米薄层;经稀盐酸浸泡并洗涤,获得高纯度的氮掺杂碳纳米笼.氮掺杂碳纳米笼具有分等级的微纳米结构、高导电性、高比表面积和可调变的孔结构,结合表面氮原子的锚钉作用,氮掺杂碳纳米笼有望成为电化学催化剂Pt的优良载体.在前期研究基础上,本文探索多级结构氮掺杂碳纳米笼(hNCNC)作为新型载体负载Pt的能力,并评价所构建的负载型催化剂Pt/hNCNC的电催化性能.通过简便的微波辅助多元醇还原法,将氯铂酸还原成Pt纳米粒子负载于hNCNC的表面.为了揭示氮掺杂的效应,我们对比研究了具有相似分级结构但无掺杂的碳纳米笼(hCNC)以及商业化活性炭(Valcan XC-72)作为载体的情况.经热重(TG)和X射线光电子能谱(XPS)分析,三种催化剂Pt/hNCNC、Pt/h CNC和Pt/XC-72的负载量均接近理论负载量(23.1 wt%),都主要以金属态存在.然而,扫描电子显微镜(SEM)和透射电子显微镜(TEM)结果表明,Pt/hNCNC的Pt分散状态优于Pt/h CNC,更远优于Pt/XC-72.Pt/hNCNC的平均Pt粒径最小,仅约3.3 nm.这种良好的分散状态主要得益于氮原子掺杂,高负电性的氮原子改变了局域的表面极性,有利于Pt颗粒的成核,也有利于固定Pt颗粒.由于hNCNC对Pt的优异分散能力,Pt/hNCNC表现出高的电化学活性面积.氢吸附和一氧化碳溶出伏安曲线表明,Pt/hNCNC的电化学活性面积高于Pt/h CNC和Pt/XC-72,这与显微观察和X射线衍射(XRD)结果相吻合.Pt/hNCNC展现出优异的甲醇电催化氧化活性和高稳定性,其催化电流明显高于Pt/h CNC和Pt/XC-72,电流衰减亦慢于Pt/h CNC和Pt/XC-72.hNCNC的分级微纳米结构有利于孔内传质和电子输运,从而提高反应速度.hNCNC的氮掺杂有利于Pt在载体表面的分散,增强了载体-金属相互作用,提高了电化学活性面积和催化活性.为了进一步考察hNCNC对Pt的负载能力,本文还考察了高负载量Pt/hNCNC的性能.在负载量高达60 wt%时,Pt/hNCNC中的Pt颗粒仍无明显聚集,其甲醇氧化电流增加了30%,可以有效提高DMFC的输出电流密度.综上可见,hNCNC可以有效分散并稳定Pt颗粒,从而提高电化学活性面积和甲醇电催化氧化活性,优于未掺杂的碳纳米笼和传统碳材料,展示了hNCNC高分散Pt颗粒用作DMFC的高效阳极催化剂的重要前景,也表明hNCNC有望成为应用广泛的新型载体.
Hierarchical nitrogen-doped carbon nanocages （hNCNC） with large specific surface areas were used as a catalyst support to immobilize Pt nanoparticles by a microwave-assisted polyol method. The Pt/hNCNC catalyst with 20 wt% loading has a homogeneous dispersion of Pt nanoparticles with the average size of 3.3 nm, which is smaller than 4.3 and 4.9 nm for the control catalysts with the same loading supported on hierarchical carbon nanocages （hCNC） and commercial Vulcan XC-72, respec- tively. Accordingly, Pt/hNCNC has a larger electrochemical surface area than Pt/hCNC and Pt/XC-72. The Pt/hNCNC catalyst exhibited excellent electrocatalytic activity and stability for methanol oxidation, which was better than the control catalysts. This was attributed to the en- hanced interaction between Pt and hNCNC due to nitrogen participation in the anchoring function. By making use of the unique advantages of the hNCNC support, a heavy Pt loading up to 60 wt% was prepared without serious agglomeration, which gave a high peak-current density per unit mass of catalyst of 95.6 mA/mg for achieving a high power density. These results showed the potential of the Pt/hNCNC catalyst for methanol oxidation and of the new hNCNC support for wide applications.
基金
supported by the National Natural Science Foundation of China(21473089,51232003,21373108,51571110,21573107)
the Nation-al Basic Research Program of China(973 Program,2013CB932902)
Suzhou Science and Technology Projects(ZXG2013025)
Changzhou Science and Technology Projects(CE20130032)
supported by a Project Funded by the Technology Support Priority Academic Program Development of Jiangsu Higher Education Institutions
关键词
甲醇氧化
燃料电池
铂催化剂
分级结构氮掺杂碳纳米笼
高性能
Methanol oxidationFuel cellsPlatinum catalystHierarchical nitrogen-doped carbon nanocagesHigh performance
