高性能三元复合Fe-Nx/C非铂氧还原催化剂的制备及活性位结构研究

The development of non-Pt catalysts for oxygen reduction reaction (ORR) is vital for the large scale application of fuel cells. Currently, Fe-Nx/C system is the most active non-Pt catalyst for ORR. However, its activity is not yet high enough for practical fuel cell application, and its active site structure is not clearly. In this project, an Fe-N doped ternary nanocarbon composite (Fe-Nx/C) catalyst with high activity was prepared through pyrolyzing a precursor mixture of graphene oxide, carbon black, monomers of graphitic carbon nitride (g-C3N4) and Fe ions. In order to explore the morphology, surface valence and the structure of the as-prepared composites, especially the internal relationship between the in situ growth nanostructre and its ORR performance, a variety of characterizations, such as TEM, HRTEM, SEM, XPS, BET, and XRD, were carried out. Furthermore, in combination with CNˉ ions, which was utilized as ORR inhibitors to provide insight into the exact nature and chemistry of the catalytically active sites, in situ FTIR spectroscopy was adopted to study the active site structure of Fe-Nx/C. This project may provide high-performance Fe-Nx/C catalysts for ORR, and some guidelines for industrial preparation of non-Pt catalysts.

发展非铂氧还原催化剂对燃料电池大规模商业化具有重要意义。当前性能最好的非铂催化剂是Fe-Nx/C 体系，但其活性仍不够高，且活性中心不明确。本项目拟采用类石墨相氮化碳 (g-C3N4) 为N源，在Fe盐存在下，通过热解方法制备具有高活性的石墨烯/碳黑/碳纳米管三元复合纳米结构Fe-Nx/C非铂催化剂；运用TEM、HRTEM、SEM、XPS、BET、XRD等方法分析Fe-Nx/C纳米电催化剂的形貌、表面价态和结构特征，重点分析原位生长的纳米结构与催化剂ORR性能间的内在关系；采用电化学原位红外等谱学技术，结合CNˉ分子探针，从微观层次和分子水平认识该Fe-Nx/C催化剂的活性位结构，研究Fe物种与活性位间的关系。本项目有望获得高性能Fe-Nx/C 氧还原催化剂，并为其工业化制备提供理论指导。

本项目研究主要探索如何提高燃料电池中非贵金属催化剂的电催化性能。通过以Fe盐、Co盐、金属有机框架聚合物(MOF)为金属前驱体，以类石墨相氮化碳(g-C3N4)、烟酰胺(niacinamide)为氮前驱体，以超导电碳黑(KJ600)、还原氧化石墨烯(rGO)、碳纳米管(CNT)、泡沫CNT(CNT- sponges)为碳载体，通过高温热解法制备了一系列的Fe-N-C催化剂。同时，在酸性体系中研究Fe-N-C催化剂的ORR活性和稳定性，结合电化学研究方法和XRD、XPS、TEM等表征手段探究催化剂结构与电催化性能之间的关系，并进一步探索催化剂可能的活性中心。主要进展如下：1）若以碳纳米管和石墨烯为复合碳载体，含氮丰富的g-C3N4为前驱体制备的Fe-N-C催化剂，优化后半坡电位可达0.78 V vs. RHE；2）通过优化孔道分布后，由金属有机框架聚合物制备的Fe-N-C催化剂半坡电位可提高至0.82 V vs. RHE；3）以g-C3N4和碳黑为前驱体制备的Fe-N-C催化剂，经过4000圈加速衰减测试后，电流仅衰减12%；4）利用SCN-离子作为探针，观察到Fe-N-C催化剂大部分活性位与探针离子配位，但探针离子清洗后，绝大部分活性位能够恢复，表明Fe-N-C催化剂的Fe中心活性位极有可能是由至少两种以上的结构存在的。