Zou ruqiang group of Peking University and its collaborators have made important progress in the preparation of three-dimensional porous carbon matrix composites with single atom metal sites and their oxygen reduction catalysis

Recently, Zou ruqiang's research group of Beijing University of technology published a research paper entitled "purchasing up energetic metal organic frameworks to large carbon networks with hierarchical poverty and atomically dispersed metal sites" on angel Chem Int ed., reporting on three-dimensional porous carbon skeleton composite materials loaded with single atomic metal sites Preparation and application in fuel cell oxygen reduction Catalysis (DOI: 10.1002/anie.201811126) As a new energy device, fuel cell attracts more and more attention because of its high energy density and zero product pollution At present, the development and promotion of fuel cell are facing many severe challenges Among them, the occurrence of the oxygen reduction reaction on the cathode of the battery needs a high over potential In order to improve the efficiency of oxygen reduction reaction, it is very important to design and synthesize catalysts with high catalytic activity and catalytic stability Pt, a precious metal catalyst, has a high activity of oxygen reduction catalysis, but it can not be industrialized and commercialized because of its high price and poor stability Therefore, the research and preparation of non noble metal based catalysts has become the focus and hot spot in this field The catalytic activity of metal particles depends on their size As the smallest unit of catalysis, monoatomic catalyst has high catalytic activity and atom utilization ratio, which has been paid more and more attention in the field of catalysis However, due to its high surface energy and easy to agglomerate, the activity of monatomic catalysts decreases, so the preparation of high-efficiency and stable monatomic catalysts is a challenge in this field Metal organic frameworks (MOFs) are a new type of porous materials formed by periodic coordination of metal sites and organic ligands The material has high specific surface area, regular pore structure and controllable composition, and is widely used in the fields of gas adsorption and separation, sensing, catalysis, etc Through high temperature heat treatment, MOFs can be further transformed into derivatives with specific physical and chemical properties, including carbon derived from MOFs, metal / metal compound derived from MOFs, and composite materials composed of the two In recent years, MOFs derivatives have been widely used as catalysts in fuel cell oxygen reduction During the heat treatment in inert atmosphere, the organic ligands in MOFs will decompose and turn into carbon bases, and the metal sites will undergo oxidation-reduction reaction and turn into metal / metal compound particles Due to the agglomeration of metal / metal compound particles caused by high temperature, there are few reports on MOFs derived monoatomic catalyst materials At the same time, the reported MOFs will collapse to a certain extent in structure and shrink to a certain extent in volume after they are transformed into MOFs derived materials Most of them are hundreds of nano carbon particles with low efficiency in conduction and mass transfer Relatively speaking, the large-scale three-dimensional carbon materials with graded pore structure have continuous electron conduction and material conduction rate due to their high specific surface area and abundant pores, which have very important application value in the field of catalysis Combining the advantages of single atom metal sites (high activity) with the advantages of large-scale three-dimensional carbon materials (high conductivity and mass transfer), the preparation of large-scale three-dimensional MOFs derived oxygen reduction materials with single atom metal sites is of great significance Zou ruqiang's research group selected a MOFs formed by coordination of high-energy ligand (1h-1,2,3-triazole) and Zn 2 + metal site as the precursor, and introduced transition metal ions (CO 2 + and Fe 2 + into MOFs by impregnation method During high temperature heat treatment (1000 ℃) in inert atmosphere, the high energy ligands in MOFs decompose and release a large amount of gas to form carbon substrate The Zn2 + metal sites in MOFs are reduced to elemental Zn and volatilized At the same time, the CO 2 + and Fe 2 + mixed in MOFs are transformed into monoatomic metal sites and scattered on the carbon substrate It is remarkable that after high temperature heat treatment, the MOFs (150 nm ~ 450 nm) in nanometer scale is expanded to the carbon network structure (> 100 μ m) in micrometer scale, and the network structure has developed and graded pore structure The researchers found that heat treatment temperature plays an important role in the transformation At 700 to 800 degrees, the intermediate product may have a "fusion and growth" process, and at the same time, the morphology is accompanied by the "foam" transformation Finally, a large-scale carbon skeleton with high porosity was formed With the help of high-resolution transmission electron microscope pictures and X-ray absorption fine structure (XAFS) spectrums corrected by spherical aberration, the existing forms and coordination environment of CO and Fe sites were explored The results show that the metal sites CO and Fe are highly dispersed in the three-dimensional carbon skeleton in the form of single atom, and coordinate with the nitrogen element in the carbon skeleton It can be seen that three-dimensional MOFs derivatives with single atom metal sites have been successfully prepared When the MOFs derivative material is used as catalyst for oxygen reduction reaction in fuel cell, the material shows better performance than commercial Pt / C The first author of this paper is Zhao Ruo, Ph.D student, Liang Zibin, Ph.D student, Professor Zou ruqiang, corresponding author, Professor Xiang Xu, from Japan Institute of industrial technology, CO corresponding author This research is supported by NSFC, national key basic research development plan and other funds.