用于氧气还原反应的新型铂基纳米催化剂的结构设计、可控合成与机理探索

The proton exchange membrane fuel cells (PEMFC) are considered to be one of the most promising clean energy technologies that can help resolve the energy crisis and problems of environmental pollution. However, the promise of their widespread application is seriously hindered by the necessarily high content of Pt in the cathode catalysts and the slow kinetics of oxygen reduction reaction (ORR) on the best available Pt-based catalysts. New catalysts are needed that reduce considerably the Pt content while affording the possibility of enhanced catalytic activity and durability. To this end, here we propose to address this problem by taking advantages of the well-controlled synthesis of Pt-based metallic nanocatalysts with core-shell and hollow structures. ..We plan to use nanocrystals made of other cheaper metals (denoted as M) involving Ag, Pd, Cu, and Ni as a supporting material and/or a sacrificial template to fabricate M-Pt core-shell nanocrystals and/or PtxMy alloy hollow nanocrystals. Since the substantial core will be either made of cheaper metals or removed during the formation of hollow structures, these two kinds of nanocatalysts are expected to have significantly enhanced mass activity for ORR.. .Furthermore, we will evaluate the electrocatalytic properties of the M-Pt core-shell and PtxMy hollow nanocatalysts with both well-defined facets on the surface and tightly controlled thickness and alloy ratios for the shells. Specifically, we will use an electrochemical half cell method to measure the ORR activity and durability. We will address the following scientific issues: i) how does the facet or arrangement of atoms on the surface affect the ORR activity; ii) how do the thickness and morphology of the Pt coating affect the coupling between the substantial core and the Pt overlayers; iii) how do the thickness of PtxMy alloy shell and the alloy ratio affect the activity of PtxMy hollow nanocatalysts; iv) how does the incorporation of the substantial core fight against CO poisoning (a common problem for PEMFC); v) how to use the techniques of localized surface plasmon resonance (LSPR) and surface enhanced Raman spectra (SERS) to monitor the kinetic process of adsorption, desorption, and poisoning of nanocatalysts. ..The ultimate goal of this research is to build a scientific basis for understanding the design method of modern nanocatalysts. Basically, this work will bring significant advances to the field by unraveling the essential knowledge and design rules for the Pt-based nanocatalysts with relatively high activity and durability for ORR. The new nanocatalysts created in this research are also expected to find commercial use.

质子交换膜燃料电池因其高能量转换率、零污染等优势被认为是最有前景的洁净能源技术之一，但其发展却受到了一些现有技术缺陷的阻碍，主要技术瓶颈在于缺乏催化电池阴极端氧气还原反应的高效铂基催化剂。为了改善这一现状，我们在本项目中提出借助贱金属和次贵金属纳米晶为支撑材料，发展一套在其表面沉积薄层甚至单层铂原子的技术和一套原位电置换取代技术，用以构造M-Pt核壳结构和PtxMy合金空心结构的纳米催化剂，进而提高催化剂中铂原子的利用率和其单位质量活性；并精确调控催化剂的表面晶面、棱角、化学组分等特征，筛选出高效且廉价的新型纳米催化剂，使其单位质量活性达到0.66 A/mg，即为美国能源部2015年既定目标的1.5倍；并利用表面等离子共振和表面增强拉曼等光谱手段研究其催化反应机理。本项目的成功实施，不仅能在理论和实验上取得原创性成果，而且能得到拥有自主知识产权的新型纳米催化剂，并有望应用于商业用途。

近年来，随着化石能源的消耗与环境污染的加剧，以质子交换膜燃料电池为代表的新型清洁能源的研究和应用受到广泛关注。但是，该项技术还存在着不小的技术瓶颈，主要表现在传统的阴极氧还原反应催化剂如铂碳成本过高，催化效果不理想。针对这一挑战，我们在本项目中做了一系列原创性的研究工作。首先，我们对液相晶种介导生长法中的关键问题进行了深入探讨，详细研究了氧化刻蚀对晶体结构的影响、晶种缺陷对纳米晶生长的诱导作用、表面包裹分子对特殊晶面的选择性吸附、还原动力学调控非贵金属合金的表面晶面、沉积和扩散的相互竞争、共沉积速率对产物表界面结构的调控等。基于完善之后的液相晶种介导生长法，可控合成了Pd@PtNi核壳结构纳米催化剂和CuPt合金分形纳米框架结构。值得指出的是，我们制备的Pd@PtNi核壳结构新型铂基催化剂，其在氧气还原反应中的单位质量活性上高达0.79 A/mg，超过了当初设定的0.66 A/mg的预期目标，该催化剂在稳定性方面也得到了极大的改善。此外，我们对该催化剂活性和耐久性的调控机制也做出了一定的研究。该项研究成果为高效铂基金属纳米催化剂的开发提供了新的研究思路，对设计和构筑其它高效纳米催化剂也具有重要的指导意义。