Synthesis Of Iron/Cobalt/Nitrogen Doped Carbon-based Materials And Their Application For Oxygen Reduction Reaction

The rapid growth of global energy consumption,the increasing depletion of traditional fossil energy,and the environmental impact of fossil energy have brought tremendous challenges to people's health,energy security,and environmental protection.The fuel cells?FCs?with the advantages of simple structure,extensive and renewable fuel resources,are quite eco-conscious and highly efficient.The first application of the FCs in the Apollo Moon Project made people see the dawn,where the two major issues of energy and environment can be addressed,since then,FCs have been greatly concerned.Unfortunately,in practice,FCs do not exhibit the ideal thermodynamic efficiency mainly because of the sluggish oxygen reduction reaction?ORR?at the cathode.Platinum?Pt?has long been the most efficient catalyst for ORR.On the way to the large-scale commercialization of FCs,there are some pending issues of Pt-based ORR electrocatalysts,i.e.,the very limited availability of Pt,the high price of Pt accounting for approximately50%cost of a fuel-cell stack,the poor stability of Pt under operating conditions?such as dissolution,sintering,and agglomeration resulting in the loss of performance?,and poor tolerance to methanol and further leading to a mixed potential at the cathode.Thus,it is eager and urgent to find alternative catalysts,which not only are efficient,abundant,low-cost,durable,but also show comparable even better catalytic performances to/than those of Pt-based catalysts.Recently,the study of ORR catalysts is focused on the substitution or reduction of Pt-based catalysts and the development of non-precious metals and metal-free catalytic materials.This thesis is to explore the ORR catalyst with low cost and excellent performance.And the ORR catalytic activity of Fe/Co/N doped carbon-based materials in alkaline medium was investigated.The main contents are as follows:?1?Fe-MIL-101 coated with polyaniline composites are synthesized in the presence of APS via an in situ polymerization reaction of aniline monomers.Then,the composites are converted into Fe₃O₄coated with nitrogen doped carbon?NC@Fe₃O₄?after carbonization.The specific surface area and pore size were controlled by adjusting the mass ratio of aniline monomer/Fe-MIL-101 and pyrolysis temperature.We found that the mass ratio and the pyrolysis temperature of the optimal catalyst were 1.5 and 900? respectively.The ORR peak current density of NC@Fe₃O₄-900-1.5 achieves 1.13 mA cm^-2,which is much larger than that of Pt/C(0.68 mA cm^-2).The potential at the current density of 3.0 mA cm^-2 is-0.072 V,which is much higher than that of Pt/C?-0.107 V?.The number of electrons transferred per oxygen molecule from-0.5 to-0.3 V is calculated to be 3.66³.81 from the K-L equation,indicating that NC@Fe₃O₄-900-1.5 favors a four-electron ORR pathway.Besides,NC@Fe₃O₄-900-1.5 exhibits high electrocatalytic activity,superior long-term durablity,and methanol crossover resistance to a commerical Pt/C catalyst in alkaline solution.Due to its unique composite structure and uniform atom doping,it creates a bright future as ORR catalyst.?2?As one type of metal-organic frameworks?MOFs?,ZIF-8 with high specific area,abundant nitrogen atoms,excellent thermostability and chemical stability,is immersed in methanol containing cobalt?II?nitrate hexahydrate.The Co-doped ZIF-8 is obtained by centrifugation and dried at 60? for 12 h.Then the Co-doped ZIF-8 carbonized under N₂ and then the cobalt,nitrogen co-doped nanopolyhedrons carbon is obtained.The as-prepared sample with high Brunauer-Emmett-Teller surface area(905.486 m²g^-1)possesses abundant mesopores.Meanwhile,Co and N are uniformly doped into the carbon framework.Besides,the electrochemical measurements in alkaline solution indicate the samples exhibit high electrocatalytic activity,excellent long-term durablity,and resistance to crossover effect of methanol.The stable framework structure and the isolated metallic Zn lead to uniform doping of cobalt and nitrogen in the carbon skeleton,resulting in its excellent ORR catalytic performance.?3?Biomass-based nitrogen-doped carbon materials are synthesized using the abundant and renewable biomolecule as nitrogen and carbon sources via high temperature carbonization process.X-ray photoelectron spectroscopy shows that the nitrogen element is successfully doped into carbon materials and exists with different chemical states.The electrochemical measurements indicate that nitrogen-doped carbon synthesized at 800? has superior electrocatalytic activity and proves to be a promising alternative for costly Pt-based electrocatalysts in fuel cells in terms of excellent electrocatalytic performance,long-term stability,CO poisoning and tolerance to crossover effect of methanol.