Preparation And Electrochemical Properties Of Hypercrosslinked Polymer-Derived Carbon-Based Materials

With the rapid development of modern society,traditional fossil fuels are increasingly depleted and environmental problems are becoming more serious.It is urgent to explore new energy technologies with zero pollution,renewable and high energy conversion efficiency.As an economical,efficient and stable energy conversion device,fuel cell has received more attention and is regarded as"one of the most ideal power generation devices in the 21st century".However,due to the over-reliance on platinum catalysts for the slow cathodic oxygen reduction reaction,fuel cells are still unable to achieve large-scale commercial applications.The development of cathode catalysts with low price,high catalytic activity and high stability is of great significance to the sustainable development of fuel cells.Hypercrosslinked polymers（HCPs）have mild and controllable synthesis conditions,a wide range of monomer sources,low cost of reagents,and strong designability.Based on the above advantages of HCPs,the control of their structure,morphology and chemical composition can be achieved by adjusting the structure and chemical composition of monomer molecules.In this paper,different nitrogen-containing monomers were used as building blocks to prepare N-C-type hypercrosslinked polymers through Friedel-Crafts alkylation reaction.Through carbonization treatment and metal element doping,a new type of oxygen reduction electrocatalyst was prepared.The main research contents are as follows:（1）Using 5,10,15,20-4’-chlorophenylporphyrin（TPP）as monomers,ferric chloride（Fe Cl₃）as catalysts,formaldehyde dimethyl acetal（FDA）as crosslinking agent,1,2-Dichloroethane（DCE）as solvent,two N-containing HCPs were synthesized by one-step Friedel-Crafts alkylation.After carbonization at 900℃,two carbon-based catalyst materials（TPP-HCP-1 and TPP-HCP-2）were prepared.The surface morphology and physical structure of TPP-HCP-1 and TPP-HCP-2 functional carbon-based materials were characterized by scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）and Raman spectroscopy（Raman）.The results show that TPP-HCP-1 has a large specific surface area and good porosity,which provides the possibility for the exposure of active sites and the rapid transport of oxygen atoms.Due to the high content of pyridine N in TPP-HCP-1,the lone pair of electrons of pyridine N has a strong affinity for oxygen atoms,thus improving the oxygen reduction activity of the catalyst.In addition,the presence of ferric chloride can not only serve as a catalyst for functionalized hypercrosslinked polymer precursors,but also as a metal dopant,and the synergistic effect of Fe and N double doping further enhances the ORR performance.In alkaline electrolyte,the half-wave potential and onset potential of TPP-HCP-1 can reach 0.85V and 0.96V,which shows better stability and methanol resistance than platinum carbon catalysts.（2）Using carbazole as monomers,ferric chloride as catalysts,formaldehyde dimethyl acetal as crosslinker,and 1,2-dichloroethane as solvent,the N-containing HCPs were prepared.Three different carbon-based oxygen reduction catalysts,TSP-HCP-800,TSP-HCP-900 and TSP-HCP-1000,were obtained by pyrolysis treatment at different temperatures of 800°C,900°C and 1000°C.Through characterization and analysis,TSP-HCP-900 has the largest specific surface area(755.80 m² g^-1)and ORR performance,the half-wave potential is 0.78V,the onset potential is 0.87V,the electron transfer number is 3.85,and hydrogen peroxide yield is 7%.Better stability and methanol resistance than commercial platinum carbon in alkaline electrolyte.（3）Using TSP-HCP as precursor,Co-TSP-HCP-900 and Fe-TSP-HCP-900 were prepared by doping cobalt metal and iron into the carbon framework through co-mixing and high-temperature treatment.The SEM,TEM,EDS,BET and XPS were used to characterize the structure,specific surface area and composition of the materials.It was found that Co and Fe were evenly distributed in the carbon framework.The synergistic effect between N and metal made Co-TSP-HCP-900 and Fe-TSP-HCP-900 exhibit excellent ORR properties.Among them,the half-wave potential of Co-TSP-HCP-900 is 0.80V,initial potential of Co-TSP-HCP-900 is 0.90V,limit current density of Co-TSP-HCP-900 is 4.75 m A cm^-2,transfer electron number is3.94,and hydrogen peroxide yield is 3%in alkaline electrolyte.