Preparation And Oxygen Reduction Properties Of Co-N-Doped Conjugated Polymer-derived Carbon Materials

In order to alleviate the global shortage of fossil energy and solve the problem of environmental pollution caused by its development and use,the development of fuel cell,a new type of energy conversion device with high efficiency,clean and renewable,has become one of the important directions of energy development.At present,precious metal platinum-based catalysts are mostly used as cathodic oxygen reduction catalytic materials in commercial fuel cells.However,due to the high price,low reserves and poor stability of platinum,it is easy to be poisoned by harmful substances in the process of oxygen reduction,which limits the large-scale application of fuel cells.Therefore,the development of new cathodic oxygen reduction catalysts with low price,anti-poisoning,abundant sources and durability has become an important research direction in the development of fuel cells.In this paper,different conjugated polymers were prepared by chemical oxidation method using different nitrogen-containing monomers as raw materials and?sub-group metal elements as doping elements.Using this as the precursor,the doped conjugated polymer derived carbon material was prepared by high temperature carbonization,and its physical structure was characterized,and the oxygen reduction properties and other electrochemical properties of the doped conjugated polymer derived carbon material were tested.The main contents of the study are as follows:?1?One-dimensional nanotubular polypyrrole conjugated polymer was prepared by self-assembly using pyrrole as monomer and fibrous micelle formed by anhydrous ferric trichloride and azo dye methyl orange as soft template.Three kinds of nitrogen-doped polypyrrole nanotube carbon materials PPy-NCNTs-800,PPy-NCNTs-900,PPy-NCNTs-1000 were prepared by high temperature carbonization at 800?,900?and1000?respectively.The morphology,structure and element composition were analyzed by electron microscope test,Raman test and XPS analysis,and they were tested by cyclic voltammetry,linear sweep voltammetry,methanol resistance and stability in 0.1M KOH electrolyte.The results show that when the pyrolysis temperature is 900?,PPy-NCNTs-900 nitrogen-doped polypyrrole nanotube carbon material has the best oxygen reduction performance,its half-wave potential reaches0.826V,and has better stability and methanol resistance than commercial platinum-carbon catalyst.In addition,when polypyrrole nanotubes are formed by oxidative polymerization,different molar concentrations of cobalt nitrate hexahydrate are added to polypyrrole nanotubes and were pyrolyzed and carbonized at the optimum pyrolysis temperature of900?to prepare cobalt-nitrogen co-doped nanotube carbon materials with different Co contents.It is found that it still maintains a good tubular morphology by TEM test,and the defect degree increases with the increase of cobalt content in Raman test.Through cyclic voltammetry and linear sweep voltammetry,it is found that the doping of cobalt can effectively increase the half-wave potential and facilitate the four-electron reaction pathway of oxygen reduction reaction compared with the undoped polypyrrole nanotube carbon materials.And with the increase of cobalt content,the half-wave potential and the number of transferred electrons are further increased,and the lower yield of hydrogen peroxide can prevent the corrosion of electrode materials.Among them,the half-wave potential of 3-PPy-NCNTs-900?nanotube carbon material reaches 0.83V,and it shows better stability and methanol resistance than platinum-carbon catalyst in methanol resistance test and stability test.?2?Using o-phenylenediamine as monomer,cobalt nitrate hexahydrate and o-phenylenediamine can form Co-OPD complexes with high yield in aqueous phase due to the existence of two amino groups in its ortho-position.Three kinds of cobalt-nitrogen double-doped carbon materials were prepared by pyrolysis Co-OPD complex at 700?,800?,900?and obtain the Co-OPD-700,Co-OPD-800,Co-OPD-900respectively.The morphology and structure were analyzed by SEM and BET,and the best defect level was obtained when the pyrolysis temperature was 800?.The results of cyclic voltammetry and linear sweep voltammetry showed that the carbon material of Co-OPD-800 complex had the best catalytic performance for oxygen reduction when the pyrolysis temperature was 800?,and its half-wave potential is 0.845V.Then,on the basis of this,two other catalysts,FeCl₃-OPD-800 and FeCl₃-Co-OPD-800 were designed.The two catalysts use ferric trichloride as oxidant to directly oxidize o-phenylenediamine by chemical oxidation to prepare poly?o-phenylenediamine?conjugated polymers.The difference is that the same amount of cobalt as Co-OPD-800 is added in the polymerization process of FeCl₃-Co-OPD-800.The oxygen reduction properties of Co-OPD-800,FeCl₃-OPD-800,FeCl₃-Co-OPD-800were investigated by cyclic voltammetry,linear sweep voltammetry,methanol resistance and stability tests.Combined with BET test,Raman test and XPS elemental analysis test,it was found that the FeCl₃-OPD-800 catalyst with high specific surface area and maximum pyridine nitrogen content had the best oxygen reduction performance.In addition,the three oxygen reduction catalysts prepared with o-phenylenediamine as monomer showed better methanol tolerance and catalytic stability than platinum-carbon catalysts,which provided a new research direction for the development of platinum-based catalyst substitutes.