基于配位聚合物网络的M-N/C催化剂的制备及其氧还原催化性能研究

Among numerous non-Pt cathodic ORR catalysts, non-noble metal nitrogen-doped carbon materials (M-N/C) are considered as one of the best candidates, but the problems of low density and the “exposure” level of active sites (M-Nx) are existed in many M-N/C catalysts，leading to the lack of catalytic activity. Be aimed at these problems, a novel method for the preparation of such M-N/C catalysts from a single precursor is proposed by direct pyrolysis of coordination polymeric networks. The goal of increasing the density and the “exposure” level of active sites is expected to be achieved in this project. First, coordination polymeric networks can be synthesized via the coordination between transition metal (M) ions and nitrogen-rich multidentate ligand. Due to the strong interaction between the nitrogen-rich multidentate ligand and M ions, they can sufficiently react with each other and form active sites during high temperature pyrolysis process of coordination polymeric networks, thus increase the density of active sites. Second, the fixation of the metal atoms within the polymeric networks, which prevent their agglomeration and loss in the pyrolysis process, is benefit to the formation and homogeneously distribution of the catalytic active sites, can also increase the density of active sites. Moreover, the introduction of rigid ligands could endow the coordination polymeric networks with porous structure and higher skeleton strength, enable to prevent the collapse of pores during high temperature pyrolysis process; ensure the as-prepared M-N/C catalysts with relatively high specific surface area and the abundant pore structures, which are favorable to increase the “exposure” of catalytic active sites.

非贵金属氮掺杂碳催化剂是目前最具发展潜力的一类氧还原反应催化剂，但普遍存在活性位点密度偏低以及活性位点“暴露”程度不够等问题，导致ORR催化活性欠佳。针对这些问题，本项目提出以配位聚合物网络为单一前驱体制备自支撑型M-N/C催化剂，以期同时提高催化剂的活性位点密度及其“暴露”程度。首先设计合成富含氮原子的多齿配体，并使之与过渡金属（M）离子配位形成配位聚合物网络。由于富氮配体和M离子之间存在较强的相互作用，在热解该配位聚合物网络时，二者可以充分反应形成活性位点，故可以提高活性位点密度；其次M被均匀固定在聚合物网络中，能有效防止其在热解过程中迁移或流失，有利于活性位点均匀形成，也可以提高活性位点密度。此外，本项目还将刚性富氮配体引入配位聚合物网络中，可赋予配位聚合物网络多孔结构并提高其骨架强度，以避免热解过程中骨架坍塌，确保催化剂具有丰富的孔隙结构和较大的比表面积，使活性位点能充分“暴露”。

杂原子掺杂多孔碳（H/C）及杂原子/过渡金属掺杂多孔碳（M-N/C）材料具有较大的比表面积、良好的导电性和优异的稳定性，一直是能量储存与转换领域的研究热点。本项目以系列多孔有机聚合物（POP）及生物质为前驱体制备H/C及M-N/C材料，并结合N掺杂量及其键合形式（吡啶N、M-Nx、吡咯N和石墨N）和材料形貌调控，进一步提高了材料的氧还原（ORR）电催化性能或超级电容器性能并探究了其催化或储能机理。主要研究内容包括三嗪基、四嗪基以及生物质基H/C和(或)M-N/C材料的制备及电化学性能研究。其中所制备的球形TPOP-900在碱性介质中的起始电位(Eo)、半波电位(E1/2)分别为0.976V、0.875V，其ORR催化性能已远超研究目标，同时还兼具出色的稳定性和抗甲醇/CO性能。所制备的类似大脑结构的TBPF-900也具有出色的ORR催化活性，其Eo、E1/2分别为0.969V、0.843V。此外，所制备的类石墨烯纳米片也具有优异的ORR催化性能，其Eo、E1/2分别为0.970V、0.829V。用该纳米片组装的锌-空气电池的功率密度可达113.8mW/cm2，比容量高达821mAh/gZn。同时我们还定量评估了该催化剂中N-C键合形式（吡啶N、石墨N和吡咯N）分别对ORR的催化贡献。上述催化剂优异的催化活性主要归因于独特形貌（球形、脑沟回结构及纳米片）有效的杂原子掺杂以及分级多孔结构。同时我们还发现含有三嗪环、四嗪环以及吡啶环的前驱体，在热解过程中有利于形成吡啶N和石墨N。此外，前驱体如果能通过多齿配体与金属配位，则有利于含金属的活性中心（M-Nx）的形成及均匀分布，使催化性能获得显著提升。