Controllable Assembly Of Fe-based Nanocrystals And Their Application In Electrochemistry

Human demand for energy is increasing with socio-economicdevelopment and progress while traditional fossil energy sources can no longer meet the needs of development.Renewable energy storage and conversion technologies have evolved rapidly in response to this challenge.Electric power is favored as an economical,clean,and easily controlled and converted energy source.Metal-air batteries,a candidate material for electrical energy storage,are widely used in railroad signals,radio communications,beacon lights,watches,hearing aids,and calculators,vehicle power sources and so on by their high energy conversion efficiency,low environmental pollution,high reliability,and easy recycling.However,the air cathode is the key factor to determine the performance of such batteries.The cathode in metal-air batteries mainly occurs in the oxygen reduction reaction（ORR）,which requires catalysts to drive the ORR due to its inherent high overpotential,slow kinetics and so on.Therefore,the main research focus is to obtain the optimal catalyst to overcome the above-mentioneddifficulties.Currently the most utilized and effective catalysts for ORR are precious metal platinum and its alloys,but the high price and low reserves have limited their industry application.Therefore,the development of low-cost and high-activity non-precious metal catalysts applied to replace the role of precious metals in ORR is an important direction of current research in the field of new energy.Transition metals have been widely investigated because of theirabundant reserves and low costs.Although transition metal catalysts are easy to synthesize and low prices,single transition metal compounds have poor electrical conductivity,insignificant catalytic effect,and unclear catalytic mechanism in the ORR process.Based on this,this thesis aims to design and prepare inexpensive and easily available transition metal carbon-based catalysts for ORR reactions,and intends to solve the above-mentioned problems of transition metal catalysts.The main research of this paper is as follows:（1）Nitrogen-doped iron carbide nanoparticles grown on reduced graphene oxide（N-Fe₃C/r GO40 NP）were synthesized by pyrolysis with iron chloride,oleic acid and r GO,and applied to ORR tests as well as assembled into zinc-air battery.The doping of r GO carrier successfully solved the poor conductivity of Fe₃C catalysts and provided structural support for Fe₃C nanoparticles.The doping of N element into Fe₃C due to its strong electronegativity greatly increased the active sites on the surface of the material.The prepared N-Fe₃C/r GO40 NP has high ORR catalytic activity,stability in an alkaline electrolyte.The preparation of is of great significance for the application of alternative conventional non-precious metal catalysts in ORR.（2）Carbon coated N-doped iron carbide nanocrystals assembled on graphene（N-Fe₃C/r GOx NCs,x=80,100,120）were prepared by hydrochloric acid etching.Tested and compared the difference ORR performance between the prepared catalysts and Pt/C.Hydrochloric acid etching caused the catalysts to develop carbon hollow shells and pores which greatly facilitated oxygen adsorption during ORR and exposure of active sites in the material formation.The stability at an applied voltage of0.86 V（vs.RHE）is superior to that of Pt/C.The results of RRDE test and K-L model fitting further indicate that the catalyst catalyzes ORR as a four-electron pathway.