Research On Preparation Of Nitrogen-doped Carbon Tubes By Coaxial Electrospinning And Their Activity For Oxygen Reduction Reaction

Platinum remains the best catalyst material for oxygen reduction reaction inpresent fuel cells, but the prohibitive cost of the catalyst imposes significant hindranceon the large scale commercialization of fuel cells. Recently, it has been reported thatnitrogen doped carbn materials exihibit excellent electrocatalytic activity comparable tothat of platinum and better stability for ORR in alkaline solutions and these advantagesmake them the most promising alternative as the catalysts for ORR. However, theactivity of these nitrogen doped carbon materials for ORR in acidic media is notsatisfied and needs further improvement. Thus, the research and development of newkinds of catalysts with enhanced catalytic activity for oxygen reduction reaction inacidic electrolyte is of great importance.In this thesis, the coaxial compound fibers were prepared by coaxialelectrospinning, and then these electrospun fibers were heat treated in N₂or NH₃tomanufacture nitrogen doped carbon tubes. In addition, so as to obtain carbon tubes withenhanced catalytic activity in acidic solutions, impregnation with RuCl₃solution andco-electrospinning with iron oxalate were utilized. SEM, TEM, XPS, XRD and Ramanwere used to characterize the morphology, structure, composition and the degree ofgraphitization of these nitrogen doped carbon tubes. Besides, the electrocatalyticactivtity of the catalysts were investigated in0.5mol/L H₂SO4using cyclic voltammetry（CV） and the rotating disk electrode （RDE） method.Firstly, coaxial compound fibers were prepared by coaxial electrospinning, usingPAN/DMF or the mixed polymer solution of PAN and PVDF as shell solution andsilicone as core solution. The optimum processing parameters for the coaxialelectrospinning were investigated. It turned out that the mixed solution of PVDF andPAN with the ratio of3to10as shell solution was the most favorable to the formationof coaxial compound fiers in the present experiments. Subsequently, these fibers wereheat treated in N₂or NH₃to manufacture porous nitrogen doped carbon tubes. Theelectrochemical measurements of these carbon tubes were conducted in0.5mol/L H₂SO4using cyclic voltammetry and the rotating disk electrode method. The results illustratedthat the porous nitrogen-doped carbon tubes heat treated in NH₃had better catalyticactivity for ORR than the ones treated in N₂. This may be attributed to the fact that NH₃has an intense etching effect on the surface of carbon tubes, and this effect may makemore edge planes exposed.In order to intensify the etching reaction of NH₃, the metallic rutheniumnanoparticels which has significant effect on facilitating NH₃dissociation wasintroduced. The ruthenium nanoparticles decorated nitrogen doped carbon tubes were synthesized employing incipient wetness impregnation method with RuCl₃solution,after which these samples were heat-treated in the mixture of H₂and N₂or NH₃. Notonly did the ruthenium nanoparticles distribute on the suface or the inside of the tubeshomogeneously, but also many micropores appeared around these particles. Theappearance of these micropores can be explained by the intense etching effect of NH₂,NH or other active radicals produced by the synthesis of N₂and H₂or thedecomposition of NH₃catalyzed by the ruthenium nanoparticles. This thesis researchedthe effect of acid treatment （the volunm ratio of concentrated sulfuric acid to nitric acidis3:1）, loading of ruthenium nanoparticles and the atmosphere used in heat treatment onthe catalytic activity of carbon tubes for ORR. The results showed that acid treatment,increasing the loading of ruthenium nanoparticles and conducting heat treatment in NH₃can contribute to the improvement of electrocatalytic activity of rutheniumnanoparticles decorated porous nitrogen doped carbon tubes.The catalytic activity of ruthenium nanoparticles decorated nitrogen doped carbontubes for ORR still cann’t meet the requirement of practical application. It has beendemonstrated that introducing iron species into carbon materials can improve thecatalytic activity for ORR tremendously. Thus, in the present work, Fe containingcoaxial compound carbon fibres were elecrtospun by coaxial electrospinning. The outersolution was prepared by dissolving iron oxalate in the mixed solution of PVDF andPAN, and the inner solution was silicone. The effect of the concentration of iron oxalateon the catalytic activity of the carbon tubes was studied. It was found that with theincrease of iron oxalate, the catalytic activity of the prepared carbon tubes wasenhanced.