几何效应与电子效应对Pt-M/C催化剂氧还原比表面活性衰减的作用机理

The distinguishing features of PEMFCs include their low operating temperature, high power density, quick start-up, and quick match to shifting demands for power. They are being developed for transport applications as well as stationary and portable applications.However, the obvious degradation of the specific activity of Pt-M/C (M=Co,Ni,Y) catalysts obstructs the large scale application of PEMFCs.In the proposed project, we study the geometric effect and electronic effect on the specific activity degradation mechanism of Pt-M/C catalysts for oxygen reduction reaction, by measuring the d-band structure and the geometric structure ,and calculating the catalyst stability against dissolution and degradation with DFT and electrode kinetics models. Here, the Pt-M/C catalysts are prepared through wet-chemistry method developed by ourselves. An improved electrochemical cell for in-situ XAS measurement is constructed to measure the d-band vacancy of Pt in different stages of attenuation. And TEM-EELS and XPS are used to measure the geometric construction of alloy and the d-band center of Pt. Moreover, the stability against dissolution of alloys in highly acidic solution is evaluated through the calculation. Through the research on attenuation process of the Pt-M/C specific activity during operation, we hope to find out the geometric effect and the electronic effect on the specific activity degradation mechanism of the Pt-M/C catalysts for oxygen reduction reaction,and provide a guideline for the design of Pt-M/C catalysts with a balance between the specific activity and stability.

氢气/空气燃料电池（PEMFC）因其运行温度低、启动快、功率匹配速度快等优点受到广泛关注，是轻型汽车和建筑物供能的首选。然而具有初始性能优势的阴极催化剂在工作过程中比表面活性发生显著衰减是亟待解决的问题。本课题将开发先进的湿化学法来制备大小、形貌和成分可控的Pt-M/C（M=Co，Ni，Y）催化剂，并搭建在线XAS测试电化学池装置，应用TEM-EELS，XPS以及XAS技术测定Pt-M/C比表面氧还原活性衰减过程中的几何结构以及电子结构,并结合密度泛函理论（DFT）和电极动力学模型计算不同合金的溶解稳定性，探索几何效应和电子效应对Pt-M/C比表面氧还原活性衰减的作用机理，旨在为开发比表面活性与稳定性兼顾的Pt-M/C催化剂提供理论支持。 本课题通过对Pt-M/C衰减过程的系统研究，有望探明合金比表面活性衰减的机理，并开发出寿命与比表面活性兼顾的Pt-M/C催化剂。

经过本项目的实施，使用同步辐射X射线吸收精细结构谱学（XAFS）测试手段对经加速衰减测试后的Pt-Co/C催化剂进行表征分析研究。结果显示，随着加速衰减循环次数的增加，Pt-Co/C催化剂内Pt-Pt键变长，表明整个过程中存在Co的溶解流失，这是ORR活性降低的关键因素。在此基础上，成功制备出一系列具有特殊结构的Pt-Ni/C和PdxNi1-x@Pt/C铂基催化剂。其中，Pt-Ni/C为“富铂核-富镍夹层-富铂壳”组成的“三明治”结构正二十面体，且表面为活性较高的Pt（111）晶面包裹，其比表面活性，Pt质量比活性为0.91 mA cm-2，0.32 A mg-1Pt @ 0.9V (vs. RHE)，分别为商业Pt/C的4.1和2.5倍，且具有较高的稳定性。PdxNi1-x@Pt/C具有以成分渐变结构的PdxNi1-x球形纳米颗粒为核，以单原子层或多原子层Pt为壳的核壳结构。首先借助一种创新的基于Pd置换Ni的化学反应方法合成出一系列单分散的、成分比例以及颗粒尺寸可控的PdxNi1-x球形纳米颗粒，其具有富Pd表层及富Ni内核的成分渐变结构。然后，结合Cu欠电位沉积技术和Pt化学置换技术，成功将单原子层或多原子层Pt沉积到上述PdxNi1-x球形纳米颗粒表面，该种新型催化剂极大地提高了Pt的原子利用率。由于PdxNi1-x纳米球内核对表层Pt壳的晶格间距（几何效应）及电子结构（电子效应）的调节作用及其成分渐变的特殊结构，经过成分优化后的Pd0.42Ni0.58@Pt/C的比表面活性为0.61 mA cm-2，贵金属质量比活性为0.42 A mg-1Pd+Pt，Pt质量比活性为1.45 A mg-1Pt @ 0.9 V (vs RHE)，分别为商业Pt/C的2.8，3.3和11.2倍。在经过12000次加速衰减循环后仍保持了其超高的氧还原活性，展现出优异的稳定性。该项目拓展了同时具有高氧还原活性及高稳定性Pt合金催化剂的结构设计思路以及催化剂衰减过程中氧还原性能“构-效”关系的理解。