Preparation And Performance Of Transition Metal-nitrogen/carbon Oxygen Reduction Electrocatalysts

With the continuous development of economy and society,people are constantly pursuing the quality of living,and the demand for energy is continuously increasing.Energy consumption based on fossil fuels has brought a series of environmental problems.Therefore,it is necessary to seek renewable energy and develop renewable energy conversion devices.Fuel cells and zinc-air batteries have received extensive attention because of their high energy conversion rate and environmental friendliness among all clean energy conversion devices.However,the sluggish and complex kinetics of oxygen reduction reaction at the cathode greatly restricts their development.At present,although the Pt-based catalyst can improve this problem,its scarcity and high cost limit its application in large scale.Hence,the design and development of nonprecious metal catalysts with low cost,high activity,durability and good methanol resistance have paramount importance to the the development of fuel cells and zinc-air batteries.M-N-C(M is transition metal)catalysts are very outstanding among many other non-precious metal catalysts owing to their good activity,long-lasting stability and excellent methanol resistance,and thereby are deemed as the most potential catalysts to replace precious metal Pt.However,the preparation is complicated and the cost is high.In this thesis,I designed and prepared catalysts with uniform morphology and excellent performance based on low price and environmental friendliness,and studied their electrocatalytic oxygen reduction performance.The main results are as follows:(1)Fe,Co,and N co-doped carbon nanotubes was prepared by a zinc-assisted twostep pyrolysis method with dicyandiamide as nitrogen source and ferric chloride and cobalt chloride as metal precursors.The first pyrolysis step is to anchor the metal atoms in the cavity structure of g-C3N4.The second pyrolysis step converts g-C3N4 into nitrogen-doped graphite to provide a good conductivity for the electrocatalytic reaction.The Fe,Co,and N elements are distributed evenly in the catalysts and the presence of nanoparticles(Co3Fe7,Fe4N)effectively reduces the aggregation of carbon nanotubes.Fe,Co,N-CNTs have good ORR activity(E1/2=0.84 V),cycle stability(after 5000 cycles,?E1/2=1 mV)and methanol resistance(there is no current fluctuation after adding 3 M methanol).(2)The Fe,N co-doped carbon black with superior ORR performance was prepared.Commercial carbon black with high specific surface area and low price was used as the carbon substrate.To facilitate the adsorption of Fe ions,the surface of carbon black was oxidized and modified firstly.Then,melamine and ferric chloride were used as precursor for doping the carbon lack through calcination.A large amount of Fe in the Fe,N-CB nanostructure exists in the form of atoms,and only a trace amount of Fe exists in the form of Fe3C.Because of their synergistic effect,the catalyst exhibits much better activity(E1/2=0.86 V),stability(after 10000 s chronocurrent test,it still keeps 90%activity)and methanol resistance(there is no current fluctuation after adding 3 M methanol)than Pt/C.Further study indicated that Fe3C and a large number of Fe atoms coordinated with N are the active centers of the catalytic reaction.Therefore,the catalyst is expected to be further improved and developed,and it has great potential to replace Pt/C.