电化学原位XAFS/PDF技术研究三维自支撑单原子催化剂中“短-中程”的结构演化

Replacing the rare and precious noble-based electrocatalysts with earth-abundant materials is of great interest in promoting the commercialization of flexible Zinc-air batteries. Currently, non-precious transition metal compounds have emerged as a promising alternative to Pt-based catalysts but still facing some tough challenges, including (i) the poor utilization of active species, influenced by the agglomeration of active components during intense pyrolysis process and (ii) the unclear catalytic active site confused by such heterogeneity features. To solve the mentioned critical issues, 3D self-supporting M-N-C catalysts derived from Metal Organic Frameworks (MOFs), integrated unique atomic-level basic units with three dimensional MOFs superstructures, are firstly fabricated. To deeply investigate the high performance catalysts, in-situ electrochemical technique combined with synchrotron-based XAFS and PDF are developed to observe the catalysts structural evolution during ORR/OER process at a “short-medium-range” atomic scale. Combined with DFT calculation, the anchoring and synergistic mechanism of the doped carbon layers with single atoms are explored. It is expected that this project will provide new perspective in enriching the complex structural characterization based on XAFS/PDF techniques.

采用非贵金属材料代替贵金属催化剂对于降低柔性锌空电池成本，促进其产业化具有重要的意义。但是，目前非贵金属催化剂面临的主要问题是催化活性密度太低及催化机理不明确。为解决以上两个难题，本项目首先以金属有机框架材料(Metal Organic Frameworks, MOFs)为起点制备高性能三维自支撑结构的单原子M-N-C催化剂，该催化剂不但具有优越的结构整体效应，还具有最大化的金属原子利用率。针对这种高效催化剂，发展并利用同步辐射XAFS和PDF相结合的电化学原位技术从“短-中程”原子尺度观测其在氧还原/析出条件下的结构演化，阐明活性中心的化学状态。结合DFT计算深入揭示掺杂碳层对单原子金属的锚定机制，进而明确其协同催化机理。该项目对于推动XAFS和PDF相结合的实验技术在复杂结构表征中的应用有着积极意义。

围绕项目研究目标，在项目执行中首先创新性的制备具有三维分级结构的金属前驱体，并通过精准调控其与有机配体的相互作用原位制备了具有三维中空分级结构的框架材料；借助框架材料独特的分子孔道为“笼子”实现了金属节点均匀空间分布；同时创新性的利用真空封管技术制备具有三维阵列结构的单原子催化剂，并将以上工艺技术延伸至具有自支撑作用的基底上，实现了在基底材料上原位构筑具有不同单原子阵列的催化剂；其中Co基单原子催化剂在柔性锌空电池器件中展现出优异的电化学性能。在催化剂结构表征方面，创造性地利用同步辐射XAFS技术和PDF技术分别研究了金属活性位点和氮掺杂碳载体的微观结构特征，发现具有MN4Cx构型的催化剂具有优异的电化学性能。在此基础上，进一步拓展研究内容，尝试利用操作简便、成本低的纺丝工艺一步法制备Fe基单原子催化剂、利用同步辐射多技术联合探究多金属组分相互协同作用以及原位煅烧工艺对碳材料缺陷构型的影响等。在项目执行期共发表相关SCI论文10篇。