Preparation Of Cobalt/iron Modified Nitrogen-doped Carbon Materials And Their Electrocatalytic Performances

Cathode reactions of zinc-air batteries include oxygen reduction reaction?ORR?and oxygen eduction reaction?OER?.Due to their slow kinetics,precious metals such as Pt,Ir and Ru are commonly used as catalysts,limiting the large-scale application of zinc-air battery.In recent years,iron/nitrogen-doped carbon materials?Fe/N-C?have received widespread attention as ORR catalysts in alkaline media.Herein,both template method and reverse microemulsion method were utilized to synthesize Fe/N-C materials with high ORR catalytic activity and low cost.Then,cobalt element was introduced into the Fe/N-C materials to form ORR/OER bifunctional electrocatalyst,which was subsequently used in zinc-air batteries.The specific researches are as follows:Nitrogen-doped mesoporous carbon material modified with Fe@FeN_x nanoparticles?Fe@FeN_x/N-C?was templately prepared and systemly explored for their ORR catalytic activity and stability.Fe₃O₄ nanoparticles were supported on carbon black to form templates.Then,pyrrole polymerization was catalyzed by ammonium persulfate to coat the template,carbonization and pyrolysis of which under nitrogen atmosphere yielded Fe@FeN_x/N-C.Synergistic effects between the yolk-shell Fe@FeN_x nanoparticles and nitrogen-doped mesoporous carbon enhanced ORR performance of Fe@FeN_x/N-C.After acid etching,the electrocatalytic activity and electrical conductivity of nitrogen-doped mesoporous carbon materials significantly attenuated,indicating that the yolk-shell Fe@Fe N_x nanoparticles not only provided effective catalytic sites,but also assisted charge transfer in nitrogen-doped mesoporous carbon.Fe/N-C material with nanosphere structure was prepared with a water-in-oil reverse microemulsion.The Fe³⁺ions in the water catalyzed pyrrole polymerization on the interface of water and oil,forming 50 nm Fe-polypyrrole?Fe-PPy?spheres.The concentration of Fe³⁺in the aqueous phase(C_Fe3+=0.5 M)and the carbonization pyrolysis temperature of Fe-PPy?T=850°C?were optimized respectively to prepare an excellent ORR electrocatalyst?FeNC-850?with the E_onset=0.957 V,and E_1/2=0.875 V,even 35 m V higher than E_1/2 of commercial Pt/C in alkaline media.Co@CoO nanoparticles embedded in Fe/N-C sponge?Co@CoO/FeNC?was prepared by pyrolysis of a mixture of cobalt chloride and the Fe-PPy spheres under nitrogen atmosphere.Co@CoO/FeNC materials were explored for their electrocatalytic performance and application in zinc-air batteries.Pyrolysis temperature was adjusted to optimize the ORR/OER performance of sponge-like electrocatalyst.Finially,Co@CoO/FeNC-850 showed sponge embedded with 20 nm Co@CoO nanoparticles,which were tightly wrapped by 3-6 layers of graphitized carbon materials.This special structure was supposed to enhance material stability and activity during the charging and discharging processes of zinc-air batteries.The sponge-like electrocatalyst was analyzed with electron microscopy and X-ray photoelectron spectroscopy.Co@CoO nanoparticles provided the main OER active sites,while Co-N bonds increased the ORR active sites.Therefore,Co@CoO/FeNC-850 showed better ORR/OER acvitities than FeNC-850.As an air electrode for zinc-air batteries,Co@CoO/FeNC-850 demonstrated an open circuit voltage of 1.42 V,a peak power density of 132.8 mW cm²,a specific capacity of 813mAh g_Zn^-1,and a long-term charge and discharge stability.