Preparation Of Heteroatom-doped Carbon Foam And Its Electrocatalytic Performance

Energy supply is one of the key factors to determine the economic development.As the third cutting-edge technology after the genome project and super materials,the fuel cell carries the dream of high-efficiency and zero-emission power generation for human because of its characteristics of environmental friendliness,high energy conversion efficiency,quick start and stable operation.Because of the broad market in the field of mobile power and distributed power plants,proton exchange membrane fuel cell（PEMFC）attracts much attention.As researchers,we should find a kind of non-platinum catalyst with a wide range of sources,low prices,good oxygen reduction catalytic performance and strong stability,to remove the obstacles to large scale commercial application of PEMFC.Carbon materials are widely studied in the field of energy conversion and storage due to its excellent stability,controlled pore structure,high surface area and excellent electronic conductivity.It is an efficient method to optimize the internal structure,electronic properties and geometric properties of carbon and result in a large number of active sites for adsorption and catalysis by chemical modification of introducing heteroatoms.Based on this,different heteroatom-doped carbon foam materials were prepared in this paper.The effects of their micro-morphology and bonding components on the catalytic performance of oxygen reduction reaction（ORR）were explored by a series of tests of physical properties,microstructures and electrochemical performance.The possible reasons for the excellent ORR performance ware also discussed.In this thesis,the following researches have been carried out:1.In order to achieve self-foam during the carbonization process,hydrolyzed vinyl imidazolium nitrate（[Hvim]NO₃）was used as the blowing agent and primary nitrogen source,to prepare nitrogen-doped carbon foam material（NCF）.SEM images show that the material has a typical carbon foam morphology.The spherical pores are connect with irregularly shaped windows which diameter is 1-1.5μm.The pore walls are relatively smooth and stacked on top of each other where thickened.2.In order to increase the amount of nitrogen doping in the material,melamine（C₃H₆N₆）as a secondary nitrogen source is creatively introduced.A series of NCF samples have been prepared by precise adjustment of the proportion of primary and secondary nitrogen sources and carbonization temperature.SEM images show that the H1M1-1000 pyrolyzed at 1000°C with a mass ratio of primary and secondary nitrogen source of 1:1 has a more abundant layered fold structure on the basis of retaining the original foam carbon interconnected micron-level pores,which is beneficial to mass transferring and carrying more active sites.XPS results show that the sample has the highest active nitrogen content,with pyridine N and graphite N of 22.23%and55.59%,respectively.Electrochemical testing results show that the limiting current density and initial potential of the sample is 6.25 mA?cm^-2 and 0.914V（vs.RHE）respectively.But its half-wave potential is 0.834V（vs.RHE）,which is only 14mV lower than that of Pt/C.The average number of electron transferred is 3.89,which means four-electron process is dominant.And the H1M1-1000 possess superior methanol resistance and stability than those of commercial Pt/C.3.Using polytetrafluoroethylene micro-powder[（C₂F₄）_n,PTFE]as the fluorine source to prepare nitrogen-fluorine co-doped carbon foam material（NFCF）by precise adjustment of the proportion of nitrogen and fluorine sources and carbonization temperature.SEM images show that the H1M1F2-900 pyrolyzed at 900°C with a mass ratio of primary,secondary nitrogen source and fluorine source of 1:1:2 has a more abundant microstructure with a diameter of 10-20nm,which is of great significance for expanding the number of active sites.A series of structural characterization results show that the sample has most structural defects and disordered arrangement of carbon foams.The active nitrogen and semi-ionic C-F bonds content of the sample are up to 78.80%and 67.12%,respectively,which is the possible reason of excellent oxygen reduction performance.Electrochemical test results show that the sample has the best catalytic performance for ORR.The limiting current density and initial potential are further increased to6.57 mA?cm^-2 and 0.926V（vs.RHE）,which is only 4mV lower than that of Pt/C.The average number of electron transferred is 4,and the methanol resistance and stability are superior to commercial Pt/C.