M-N/C催化剂活性中心密度和耐久性调控策略及氧还原性能研究

The transition metal and nitrogen co-doped carbon material (M-N/C) as the low-cost electrocatalyst for oxygen reduction reaction (ORR) plays an important role for the large-scale applications of fuel cell. The poor thermostability of the precursors usually results in the decomposition of the precursors and the massive loss of active components under atmospheric pressure and high temperature, leading to the problems of low active site density, unclear structure and formation mechanism and poor durability of the M-N/C catalyst. This proposal will start from the design and controllable preparation of covalent organic polymers (COPs) with specific structure, high thermostability and high density of ordered M (Fe,Co)-N sites as precursors to improve the active site density and the durability. The 3-D porous structure will be constructed through electrostatic spinning and hybrid support. The effects of the coordination structures on the formation mechanism of active sites and electrocatalytic performance will be investigated by constructing various M-N2, M-N4 and dual-metal sites. Moreover, this proposal will develop novel high pressure or N2/NH3 plasma assistant heat treatment methods to lower the temperature of M-N/C formation, and to reduce the decomposition of the precursors and the loss of active components, hence improve the active site density. Furthermore, the effects of pressure and temperature on the durability will also be studied. In the end, the proposal will be able to develop new strategies to enhance the active site density and durability of the M-N/C catalysts for ORR, and contribute to the practical applications of M-N/C in fuel cells.

过渡金属与氮共掺杂的碳材料(M-N/C)氧还原电催化剂对降低燃料电池成本、推广其应用有重要意义。传统合成方法中前驱体的热稳定性较差，常压高温下易分解，活性组分大量流失，导致催化剂活性中心密度低、结构和形成机制不明确、稳定性差。本项目从催化剂源头设计与可控制备入手，合成结构明确、高稳定性、高密度(Fe、Co)-N位点有序排列的共价有机聚合物为前驱体，通过静电纺丝或复合载体构筑高效传质的三维多孔结构，提高活性中心密度和结构稳定性；构建不同的M-N2、M-N4和双金属位点，研究配位结构对活性中心形成和活性及耐久性的影响。发展气氛保护高压热处理和等离子体辅助等特殊热处理方法，降低M-N/C形成温度，减少对前驱体的结构破坏和活性组分流失，提高活性中心密度；研究高压和温度对活性中心结构稳定性的影响，提高催化剂耐久性。发展M-N/C催化剂活性中心密度和耐久性提升新策略，为M-N/C走向实用奠定科学基础。

过渡金属和氮共掺杂的碳(M-N-C)被认为是最有希望替代贵金属铂作为质子交换膜燃料电池(PEMFC)阴极氧还原(ORR)催化剂的材料之一。本项目围绕M-N-C氧还原催化剂的活性与稳定性提升策略，取得如下进展：1、构筑了Fe2N@Fe-Nx复合活性位点，Fe2N纳米粒子可减弱ORR中间物种在Fe-Nx位点上的吸附强度，降低反应自由能，促进ORR活性；2、通过在ZIF-8表面包覆聚多巴胺，制得Fe-N共掺杂的碳纳米管串联碳纳米空心球催化剂，有效促进传质和电子传输，提高ORR催化活性；3、源头设计含有N4配位点的共价三嗪骨架，形成Fe-N4模型结构，调控Fe-N4位点的电子结构和平面弯曲度，提高了*OOH在吡啶N相邻的C原子上的吸附，实现ORR反应由2电子路径转变为4电子路径；4、发展了水蒸气辅助热处理法，显著提高Fe-N4位点原子级分布的有序多孔碳催化剂的活性位点密度，其活性位点密度比未经水蒸气活化的样品提高了4.6倍，质量活性提高了1.8倍，应用于PEMFC中的最高功率密度可达到780 mW cm-2，并具有良好的稳定性；5、通过Fe、Co掺杂的ZIF-8制得具有内凹多面体结构的FeCo-NC催化剂，Co的掺杂使Fe原子周围的电子云密度提高，更多低自旋的非平面FeⅡ-N4位点形成，其ORR半波电位可达0.84 V。本项目的工作为理性设计和制备高活性、高稳定性的M-N-C氧还原催化剂，构筑高效传质的孔道结构提供了新的思路和方法，为M-N-C氧还原催化剂在质子交换膜燃料电池中的实际应用奠定了基础。