限域空间调控制备高密度活性位M-N-C类氧还原催化剂及其反应机制研究

The development of high-performance and low-cost oxygen reduction reaction (ORR) catalysts is the key to the commercialization of fuel cells. Among them, the transition metal-nitrogen doped carbon (M-N-C) catalysts are considered as an ideal material to replace the traditional-metal platinum-based catalysts, but they still suffer from several key issues, such as low doping-efficiency and active site density and unclear catalytic mechanism. In previous study, we found that the use of “space-confined effect” was an effective strategy to promote the doping-efficiency and catalytic activity of M-N-C catalysts. For this reason, here we use the water-removable and thermostable self-assembled sodium chloride crystallines as a space-confined reactor in calcination process in order to avoid the nitrogen loss and improve the nitrogen-doping efficiency. Meanwhile, we further choose the transition metal-coordinated pyridine-based compounds with strong thermal stability as the precursors to further reduce the thermal loss of active nitrogen species, which can facilitate to better synergistically prepare the M-N-C catalysts for ORR with high active site density. Besides, the relationship between microstructure, electron distribution state and catalytic performance of different catalysts is revealed via structural characterization and performance evaluation. The active site centers of M-N-C catalysts and their catalytic oxygen reduction mechanism are also investigated by means of in situ spectroscopy measurement and theoretical calculation. The aims of this project are expected to obtain highly efficient, inexpensive and stable non-platinum electrocatalysts and help to deeply understand the electrocatalytic reaction mechanism of carbon-based catalysts.

发展高性能、低成本的氧还原反应（ORR）催化剂是实现燃料电池商业化的关键。其中，过渡金属-氮掺杂碳（M-N-C）类催化剂是取代传统金属铂催化剂的理想材料，但其仍存在掺杂效率低、活性位密度不高、催化机制不明等问题。前期研究中，我们发现利用“限域空间效应”能有效提升M-N-C类催化剂的掺杂效率及催化活性。为此，本项目拟采用易水溶移除、耐高温的自组装氯化钠作为煅烧过程中的限域空间反应器，以避免氮烧失、提高掺杂效率；并采用热稳定性强的过渡金属-吡啶基配合物为前驱体，进一步减少活性氮的热损失，进而协同制备出高密度活性位的M-N-C类ORR催化剂。通过结构表征与性能评价，以揭示不同催化剂的微观结构、电子分布状态与催化性能的内在关联，并运用原位光谱和理论计算等方法深入研究催化剂中的活性位中心及其催化反应机制。本项目的开展有望得到高效、廉价、稳定的新型非铂电催化剂，有助于对碳基催化剂电催化反应机制的认识。

开发高性能、低成本的氧还原反应（ORR）催化剂是实现燃料电池商业化的关键。其中过渡金属-氮掺杂碳（M-N-C）类催化剂是取代传统金属铂催化剂的理想材料，但其仍存在掺杂效率低、活性位密度不高、催化机制不明等问题。针对上述问题，本项目主要研究了高性能碳基ORR催化剂的设计制备、性能评价与优化、定量构效关系、活性位点测定和电催化反应机制等内容，取得如下进展：.一是采用易水溶移除、耐高温的自组装氯化钠作为煅烧过程中的限域空间反应器，并采用热稳定性强的过渡金属（Fe或Co）-三吡啶基三嗪为前驱体，以更好避免氮烧失、提高掺杂效率，同时筛选优化了气体活化造孔剂种类和高温热处理工艺等条件，进而制备了具有多孔类石墨烯结构的高活性位密度Co-N-C或Fe-N-C类ORR催化剂。二是进一步利用了具有确定孔道尺度的纳米分子筛、纳米粘土以及层状结构-石墨氮等作为空间限域主体，在有限的空间内利用金属-有机配合物（Fe-N4配合物、Mn基MOF材料等）为前驱体，定向精准构筑了可用于金属-空气电池的几种高性能M-N-C类催化剂。三是从分子尺度层面探究了键合限域单FePc分子、高温热气体冲蚀等策略制备单Fe原子或Mn原子碳基ORR催化剂，并对其ORR途径和催化机制进行了理论建模和计算研究。项目得到的多种M-N-C类催化剂的ORR催化活性在碱性条件下可媲美于商业Pt/C催化剂，其电催化稳定性和组装的金属-空气电池性能远远高于商业催化剂，部分催化剂的性能是目前同类催化剂中最优的催化剂。总之，本项目研究达到了预期的科学目标，发展的多种限域空间方法对开发廉价高效的M-N-C类氧还原催化剂有重要意义，理论与实验结果有助于对碳基催化剂ORR机制的深刻认识。.在本项目经费资助下，共发表SCI论文12篇，申请国家发明专利3件，培养省级人才1人、正高1人、中级1人，联合培养博士生1人、培养硕士生10人（毕业3人）。