Preparation And Electrocatalytic Performance Of Fe-based Nitrogen Doping Materials

Oxygen reduction reaction（ORR）is the most critical electrochemical reaction at the cathode,which limits the overall reaction rate due to its high overpotential and slow electron transfer kinetics.At present,platinum-based（Pt）catalysts are the most active electrocatalysts for ORR.However,platinum’s high cost and lack of resources have prevented its large-scale commercial application.The aim of this thesis is to develop a low cost iron-based nitrogen doped carbon material with high catalytic activity and high stability,in which a variety of characterization methods and electrochemical testing have been conducted for oxygen reduction reaction electrocatalyst.The main contents and innovations are as follows:1.The development of transition metal based oxygen reduction reaction electrocatalysts with high activity and excellent stability has been a problem in industry.Transition metal-based nanoparticles loaded on the outer surface of carbon materials are susceptible to Oswald curing during the catalytic process,resulting in poor long-term stability^[1-4].Therefore,it is still challenging to construct transition metal based oxygen reduction reaction electrocatalysts with high activity and stability.In this study,Fe based skewer-like carbon nanotubes（Fe-CNTs）with uniform diameter were successfully prepared by a simple one-step pyrolysis strategy.The Fe N_x-based nanoparticles were evenly dispersed at the tip of the carbon nanotubes and the junction of the bamboo segments.Fe-CNTs catalyst has abundant active sites.ORR tests on the material showed that the initial potential and half-wave potential of 0.96 and 0.86 V（vs.RHE）in 0.1 M KOH were comparable to those of commercial 20%Pt/C catalysts.According to the Koutecky-Levich（K-L）curve calculated from LSV,the value of electron transfer number（n）is between 3.81-3.93 in the potential range of 0.3 to 0.7 V,indicating that this is a four-electron transfer path^[5].Fe-CNTs also showed a very small Tafel slope value of 81m V dec^-1,which was comparative to the Tafel slope value of commercial 20%Pt/C(63 m V dec^-1)catalysts.The results showed that Fe-CNTs catalyst had better electron transfer efficiency and more complete reaction.In addition,Fe-CNTs are more stable,making them an excellent electrochemical catalyst for ORR.2.In this study,low-cost Prussian blue was used as the precursor to synthesize the high efficiency oxygen reduction electrocatalyst（ORR）under the condition of high temperature pyrolysis between 700 and 900℃.Among the Fe-N-C-700,Fe-N-C-800 and Fe-N-C-900Fe-N-C-800 has the best ORR catalytic activity,with an initial potential of 1.01 V and a half-wave potential of 0.92 V,both of which are better than commercial 20%Pt/C electrocatalysts.The excellent performance of the Fe-N（C）catalyst is attributed to its larger specific surface area and more abundant active sites.The K-L curve,calculated from LSV,has a value of 3.85-3.92 electron transfer number（n）in the potential range of 0.3 to 0.7 V,and has a near direct four-electron reaction path in alkaline solution.The Taffel slope of Fe-N（C）is 72m V dec^-1,which is almost the same as that of Pt/C.It indicates that our material has a lower overpotential and a more complete reaction at the same kinetic current density,which further indicates that our material has excellent electrochemical catalytic performance.In addition,Fe-N-C-800 has superior long-term stability,with a performance loss of only 18m V after14,000 CV cycles.The excellent performance of Fe-N-C-800 is attributed to the formation of the pyridine Fe-N₄site near this temperature,where critical conductivity of the electrocatalyst occurs.These results show that the prepared Fe-N-C-800 catalyst is a simple and efficient ORR catalyst for fuel cells.