低铂修饰TM-N-C催化剂界面结构调控及多活性位协同机制研究

The structure design and modulation of the active sites in electrocatalysts are of far-reaching scientific significance and important research value in improving the performance of non-precious metal electrocatalysts. This project focuses on the in-sufficient performance of transition metal-nitrogen-carbon (TM-N-C) electrocatalysts in acidic media, aiming to improve the performance through introducing trace high-active Pt on TM-N-C. In order to achieve the controllable preparation of the high-efficient electrocatalysts, it is critical to disclose the synergistic mechanism between Pt and the TM-Nx sites through the design and modulation of the interfacial structure. Herein, the graphitic carbon nitride (g-C3N4) with highly ordered framework and unique coordinated local structure is adopted to construct the active-site uniformly dispersed TM-N-C electrocatalysts. After introducing Pt on TM-N-C through a Polyol method, an effective modulation of the active-site structure will be achieved through the optimization of technique parameters. Moreover, combining with the physiochemical characterization, electrochemical measurements and the First Principle Calculation, the effect of the active-site density on the interfacial configuration as well as the chemical states and electron structure of the components will be analyzed. With a deep exploration of the inner relationship between the interfacial structure and the catalytic performance, the synergistic effect between the active sites of Pt and TM-N-C will be elucidated, and the enhancement mechanism of the multi-active sites in the acidic media will also be demonstrated. Based on these investigations, this project will provide experimental and theoretical supports for designing high performance electrocatalysts.

催化剂活性位结构设计及调控对提高非贵金属催化剂性能具有深远的科学意义及重要的研究价值。本项目针对过渡金属-氮-碳（TM-N-C）在酸性体系活性不高的问题，拟利用微量高活性铂（Pt）修饰TM-N-C构筑复合催化剂。通过催化剂界面结构调控，探究多活性位协同增强机制，实现酸性体系高效氧还原反应催化剂的可控制备。为此，我们选择结构高度有序且易配位的类石墨相氮化碳（g-C3N4）锚定过渡金属，构建活性位分布均匀的TM-N-C。通过多元醇法将Pt修饰于TM-N-C表面，优化制备工艺实现活性位结构的有效调控。结合物性表征、电化学测试及第一性原理计算，通过分析TM-N-C活性位密度变化对催化界面结构、组元化学状态及电子结构的影响，探究催化界面结构与活性之间的内在联系，揭示酸性体系Pt与TM-N-C活性位之间的协同作用，阐明氧还原反应过程中多活性位增强机制，为高性能、低铂催化剂的构筑提供实验和理论基础。

降低催化剂中铂的使用量是燃料电池商业化过程具有挑战的一项工作。本项目提出双活性位协同作用机制，利用Pt元素促进酸性电解液中TM-N-C催化剂表面氧还原反应4电子过程的进行。项目采用葡萄糖和类石墨相氮化碳（g-C3N4）单体（尿素、二氰二胺和三聚氰胺）为碳源和氮源，利用葡萄糖脱水形成的醛基和氮源中的胺基之间的席夫碱反应构造配位结构锚定过渡金属，并通过调控热解温度以及碳源和氮源比例分别获得氮掺杂碳载四氧化三钴（Co3O4/NPC）复合催化剂和多孔碳包覆钴(Co@PC)复合催化剂。测试结果表明，葡萄糖中丰富的含氧官能团保证了800 oC下Co3O4的稳定存在。同时，碳热还原氛围促进了Co3O4中氧空位的产生，使所制备催化剂表现出良好的双功能氧电极（氧析出和氧还原）反应活性。降低葡萄糖加入量并增加二氰胺和三聚氰胺加入量， 利用高温下钴对含碳分子的催化作用形成碳纳米管以调控Co@PC孔结构和活性位点密度，将Co@PC表面ORR半波电位提高30 mV。进一步引入盐模板，促进分级孔结构中大孔的产生并抑制金属团聚，制备得到Co(Fe)-N-C氧还原反应催化剂。其中，热解升温速率对Co(Fe)-N-C催化剂中缺陷浓度和催化剂组成产生重要影响。升温速率从2 oC min-1提高到10 oC min-1，分子芳构化程度降低，催化剂缺陷增加，活性呈现先增加后降低的趋势。在Co(Fe)-N-C催化剂表面修饰Pt，分别构造Pt、Co共掺杂氮碳（Pt-Co-N-C）、Pt负载于Co-N-C（Pt/Co-N-C）表面以及PtCo负载于Co-N-C（PtCo/N-C-DZ）表面三种不同Pt与Co-N-C相对位置的复合催化剂。结果表明，Co-N-C表面修饰Pt有效提高复合催化剂对ORR的活性。当Pt担载量为5 %时，所得复合催化剂在酸性电解液中的质量比活性是Pt/C （20 %）的2.2倍。综上所述，本项目研究结果不仅有利于促进酸性体系高效低铂催化剂的开发，同时为基于葡萄糖以及类石墨相氮化碳单体构筑、优化碳载四氧化三钴和碳包覆钴复合催化剂提供了实验依据。