超低铂氧还原电催化剂活性和稳定性调控的同步辐射XAFS研究

Platinum-based cathodic oxygen reduction (ORR) catalysts exist the problem of high platinum loading and insufficient catalyst life, which restrict the commercialization of proton exchange membrane fuel cells (PEMFCs). The key to solving two major problems depends on the design of the ultra-low platinum catalyst with desirable activity and stability. The ORR performance of catalysts is closely related to its physicochemical structure. Therefore, it is of vital importance how to regulate the ORR activity and stability of ultra-low platinum catalysts by the real-time observation of the electron and geometry structures. The synchrotron radiation X-ray absorption fine structure (XAFS) can obtain the real-time structural information, such as the electronic structure of absorbed atoms, the type of coordination atoms and the distance between atoms, which plays an important role in studying the real-time evolution of catalyst structure. In this project, we intend to synthesize a series of PdM@Pt/C and IrM@Pt/C electrocatalysts by the advanced wet chemistry combined with the underpotential deposition methods. Characterized the ORR activity and stability of catalysts by electrochemical tests, we intend to perform an in-situ XAFS method to investigate the degradation mechanism of ORR activity and stability of the ultra-low platinum catalyst with different alloying elements, element ratios and crystal structures. The goal is to establish the "Structure-Activity" relationship between the structure and the activity and stability of ultra-low platinum catalysts, which instructs the development of the new ultra-low platinum electrocatalysts and provides a scientific theoretical basis to speed up the commercial development of PEMFC.

铂基阴极ORR催化剂中Pt的负载量高、催化剂寿命不足是制约PEMFC商业化的重要因素。解决问题的关键在于设计出活性和稳定性兼顾的超低铂催化剂。催化剂的性能与其物理化学结构密切相关，实现催化剂电子和几何结构的实时监测来调控超低铂催化剂ORR活性和稳定性至关重要。同步辐射X射线吸收精细结构谱学(XAFS)可以实时地获知吸收原子的电子结构，配位原子种类及原子间距离等信息，对研究催化剂结构的实时演变发挥着重要作用。本项目拟采用先进的湿化学法结合欠电位沉积法制备一系列铂单层核壳结构催化剂PdM@Pt/C和IrM@Pt/C；借助电化学测试表征催化剂ORR活性和稳定性，采用原位XAFS方法研究合金元素种类、元素比例和晶体结构等因素对催化剂ORR性能及活性衰减机制的影响。建立超低铂电催化剂结构与其活性和稳定性间的构效关系，指导新型超低铂基电催化剂的开发，为加快PEMFC的商业化发展提供理论依据。

质子交换膜燃料电池(PEMFC)因能量转换效率高，运行条件温度低及绿色环保等优点备受广泛关注，是轻型汽车供能的首选。铂基阴极氧还原催化剂存在Pt的负载量高、催化剂寿命不足的问题，设计出活性和稳定性兼顾的超低铂催化剂，并建立催化剂结构与其活性和稳定性间的“构效”关系，对推动PEMFC的商业化发展至关重要。同步辐射X射线吸收精细结构谱学(XAFS)可以实时地获知吸收原子的电子结构，配位原子种类及原子间距离等结构信息，对研究催化剂结构的实时演变发挥着重要作用。本项目采用原位同步辐射XAFS研究PEMFC中阴极Pt/C和PtCo/C催化剂的电催化活性及其活性衰减机理，结果表明PtCo/C催化剂具有高的5d空穴数量，低的含氧物种覆盖度和短的Pt-Pt键长是引起其ORR活性较Pt/C高的原因。进一步获知两种催化剂活性衰减的原因是：在Pt/C催化剂中Pt的氧化从表层延伸到亚表层，引起催化剂结构从部分覆盖的PtO层向全覆盖表层和部分覆盖亚表层PtO结构转变，从而导致Pt活性位点减少；而在PtCo/C催化剂中Co的氧化引起应力效应和配体效应的衰减以及CoO的溶解流失引起催化剂颗粒表面Pt层厚度的增加。因此，开发铂基核壳催化剂能够有效避免过渡金属的溶解流失。进一步，采用原位同步辐射XAFS研究Pd@Pt/C和PdNi@Pt/C核壳结构催化剂。XAFS结果显示Ni的加入，Pt-Pt键长缩短，Pt-Pt和Pd-Pd配位数减少，Pt-Pd配位数增加。说明Ni的加入，优化了表面Pt的电子和几何结构，以此获得合适的Pt-O结合能，从而抑制了催化剂表面Pt的氧化。另外，Pt的电子结构受电势影响，而Pd的电子结构几乎不受电势的影响，也进一步证明了以Pd为核以Pt为壳的核壳结构催化剂的形成。