Study On Fe,N-co-doped Carbon-based Oxidation Pro-electrocatalysts

In contemporary society,the excessive use of fossil fuels has caused a series of serious problems such as environmental pollution,ecological imbalance and resource shortage.The development of clean energy and energy conversion devices has become an urgent task.Fuel cell and metal-air battery as a kind of efficient and environmentally friendly energy conversion device are attracting more and more attention.However,the slow kinetics of cathodic oxygen reduction reaction seriously affects the efficiency of fuel and metal-air battery.Up to now,the most effective catalyst for oxygen reduction reaction is the precious metal Pt and its alloy,but it is different to make fuel cell large-scale commercial application owing to its characteristics of high price,less natural reserves,poor durability and easy corrosion.Therefore,the development of non-noble metal catalysts with lower cost,better stability and excellent catalytic performance is of great significance for the commercialization of fuel cells and metal-air batteries.Non-precious metal iron-nitrogen-carbon based catalysts are considered as the most potential materials to replace Pt based catalysts,but their preparation process is complex and cost is high.In terms of the questions,I develop a simple two-step pyrolysis method for the preparation of iron-nitrogen-carbon based catalysts with different morphologies,and study their performance of electrocatalytic oxygen reduction in my thesis.The main research results are as follows:(1)I develop a cost-effective two-step pyrolysis method for the preparation of FeN/C-based hybrids using ferrous chloride and urea.During the first pyrolysis step,urea polymerizes into g-C3N4,with Fe in the sixfold cavities of g-C3N4.In the second pyrolysis,g-C3N4 with Fe ions convers into Fe-N/C materials with Fe and Fe3N nanoparticles encapsulated with defect-free N-doped carbon shells.The obtained hybrids have multiple types of ORR active components,including homogeneously distributed coordinating Fe moieties(FeCxNy or FeNx)and Fe and Fe3N nanoparticles.Therefore,the hybrids display a superior electrocatalytic performance for ORR,which are comparable to Pt/C at the same catalyst loading.At the same time,because these active sites are covered by defectfree N-doped carbon shells,the stability and methanol resistance of the catalyst are improved,and the catalyst is more suitable for commercial application than Pt/C catalyst.(2)(Fe,N-codoped CNT)/(Fe-based nanoparticle)(Fe,N-CNT/FeNP)hybrid nanostructures are prepared by the carbonization of Fe-doped g-C3N4 as a Fe-and N-rich precursor with the assistance of Zn powder.The prepared Fe,N-CNT/FeNP hybrid nanostructures combine the advantages of CNTs and the nanoparticles.On the one hand,the loading of nanoparticles improves the dispersion of CNTs.On the other hand,the CNTs enhance the stability of the FeNP during ORR.Owing to the multiple types of ORR active components,including homogeneously distributed coordinating Fe moieties(FeCxNy or FeNx)and Fe,CFe15.1 and Fe4N nanoparticles encapsulated by Fe,N-CNTs,the Fe,N-CNT/FeNP nanostructures exhibit a superior catalytic activity for ORR and excellent stability and resistance to methanol crossover.(3)Fe,N-hollow mesoporous carbon spheres are prepared for ORR by use of silica as template.Since no nanoparticles are observed on the prepared Fe,N-HMCS,Fe may exist in the carbon matrix in the form of atoms or tiny clusters,that is FeCxNy or FeNx active substances are formed.In order to further optimize the performance,Fe,N-hollow mesoporous carbon spheres are combined with graphene with excellent conductivity to obtain Fe,N-HMCS/Graphene composites.In comparison with Fe,N-HMCS,the catalytic performance of Fe,N-HMCS/Graphene composites is further improved.In order to verify the universality of the method,Co,N-HMCS/Graphene and Fe,Co,N-HMCS/Graphene composites are also prepared.It is found that the ORR performance of bimetal-based catalyst is better than that of single-based catalyst,and the catalytic activity of metal Fe is better than that of metal Co.