Preparation Of Nitrogen-doped Carbon-supported Transition Metal Alloy Nanoparticles And Its Electrocatalytic Properties On The Oxygen Reduction Reaction

Oxygen reduction reaction is the rate control step of the fuel cell,which requires a large amount of precious metals such as platinum?Pt?as an efficient catalyst to reduce the overpotential.However,the precious metal Pt catalysts are expensive,low reserves,and easy to poison,which fundamentally limit the large-scale commercial application of fuel cells.Thus,the development of low-cost,high-activity,high-stability non-noble metal ORR catalysts is the key to promoting the industrial development of fuel cells.In recent years,nitrogen-doped carbon-supported non-noble metal?M-N/C?catalysts have attracted extensive attention from researchers due to their good oxygen reduction activity and stability.According the existence form of metal elements,there are three different types of M-N/C catalysts,i.e.,monoatoms,clusters,and nanoparticles.There are generally two strategies to improve the activity of M-N/C catalysts.On one hand,people increase the active site density of M-N/C catalysts through strategies such as decreasing particle sizes,increasing pore structure,and increased specific surface area,thereby increasing the oxygen reduction activity of the catalyst.On the other hand,M-N/C catalysts with higher intrinsic activity were prepared by strategies of doping heterogeneous elements and constructing multimetal catalysts to enhance oxygen reduction activity.For nitrogen-doped carbon-supported transition metal alloy nanoparticle catalysts,crystal structure and particle size of alloy nanoparticles play an important role in ORR electrocatalytic performance.This thesis takes nitrogen-doped carbon-supported transition metal alloy catalyst as the research object.The relationship between alloy composition,crystal structure,particle size,etc.of alloy nanoparticles in the catalyst and the ORR activity will be fully studied,and the possible active sites in the catalyst.Based on this,the research work of this thesis will be divided into the following three parts:The first part is the in situ growth of nitrogen-doped carbon nanotube-loaded FeNi nanoparticles(Fe_1-xNi_x-N/C,x=0,0.05,0.1,0.25,0.5,1)by direct pyrolysis of a mixture of metal salts and melamine in bamboo nodules.By adjusting the atomic ratios of Fe and Ni,it was found that Fe_0.75Ni_0.25-N/C and Fe_0.50Ni_0.50-N/C exhibited superior catalytic activity against ORR.Their catalytic activity is closely related to the crystal structure?cell parameters?of Fe_1-xNi_x nanoparticles.It is suggested that besides the generally proposed synergetic effect between the Fe1-xNi_x-alloyed nanoparticles and the N-doped carbon nanotubes,the magnetic ordering is likely to be an important factor for the enhancement of catalytic properties of Fe_1-xNi_x-N/C catalysts.In addition,bimetallic?-Fe_1-xNi_x nanoparticles were found to show better chemical stability than?-Fe_l-xNi_x nanoparticles.The second part is the direct pyrolysis of metal salt and melamine mixtures for the in situ synthesis of nitrogen-doped carbon nanotube-like Fe _XNi_yCo_Z-N/C catalysts.The results showed that the ternary catalysts Fe₂NiCo-N/C,Co₂FeNi-N/C,and Ni₂CoFe-N/C had higher ORR performance compared with the binary and single metal catalysts.Among them,Ni₂CoFe-N/C has the highest onset potential of 0.98 V?vs.RHE?and half-wave potential of 0.81 V?vs.RHE?.The catalytic activity and intermediate product content are closely related not only to the crystal structure of the nanoparticles in the catalyst,but also to the particle size,specific surface area,pore size distribution and defect concentration in the catalyst.In the third part,direct pyrolysis of metal salt and melamine mixtures was carried out for the in situ synthesis of nitrogen-doped carbon nanotube loaded cobalt-zinc nanoparticle catalyst Co?Zn_x?-N/C?x=0.33,0.5,1,2,4,6?.It has been shown that Co nanoparticles can be effectively regulated by adjusting the zinc content in precursors,and ORR activity increases roughly with increasing zinc content in precursors.The effect of the zinc content in the precursor on the limiting current density is more important relative to onset potential and half-wave potential.Among them,Co?Zn_x?-N-C?x=6?showed the best activity with a onset potential of 0.971 V,a half-wave potential of 0.834 V and a limiting current density of 5.42 mA cm^-2 in a 0.1 M KOH electrolyte,similar to commercial Pt/C catalysts.