Preparation Of Fe-N-C Based Catalyst To Oxygen Reduction And Performance In Zinc-air Batteries

Zinc-air battery is a branch of fuel cells as a new type of cleaning device.This energy conversion technology has become the focus of research because of its high efficiency,stability,and being green and pollution-free.However,the traditional precious metal catalysts,such as Pt/C catalysts,have many disadvantages including high price,low resources,poor stability,which limit the large-scale application of zinc-air batteries or even fuel cells.Therefore,it is crucial to develops non-noble metal-based catalysts to oxygen reduction reaction（ORR）with high efficiency,superior electrochemical stability and sustainable resources.Recently,transition metal and nitrogen-doped carbon-based catalysts（M-N-C,M=transition metal）have been proven as a kind of promising alternative to the Pt-based ORR catalysts.Thereinto,one of the research topics is to construct a catalyst with high-density defect active sites,high specific surface area and high nitrogen content by using template method.Aiming at the shortages of the low density of active sites,low-scale pore distribution,low mass transfer rate and poor electrochemical stability for the carbon-based catalysts derived from pyrolyzed metal chelates,Fe and N co-doped carbon-based catalysts to ORR were prepared in this paper.Typically,the high active sites,high stability and multi-scale pore structure were obtained by using metal-organic framework compounds（MOF）,SiO₂ and F127 as the functional templates.The relationship between the structure and composition of catalysts with the electrochemical performance to ORR was thoroughly investigated.（1）N-doped graphene nanonets grafted with carbon nanotubes（D-Fe NC/MOF）were constructed by gas-etching MOF,which were co-functionalized by coupling Fe₂O₃ nanoparticles with Fe-N_x active sites.The aggregation of Fe²⁺in heme was effectively prevented due to the unique hydrocarbon structure of MOF to facilitate the formation of Fe-N_x active sites.Then,an N-doped catalyst with multi-scale pores and high-density of active sites was prepared by etching the graphene layer using dicyandiamide at a high temperature.The effects of calcination temperature and pore-making agent on the formation of Fe₂O₃ and Fe-N_x active sites were investigated.The charge density of graphene can be regulated by the interaction between Fe₂O₃ with the adjacent Fe-N_x and C-N structures,to enhance the catalytic activity of the active sites.The exfoliation of nanosheets can be promoted by the insertion of Fe²⁺ions in the MOF,to further construct multi-scale channels and expose more active sites.Under the synergetic effect of high-density active sites and multi-size pore distribution,the diffusion resistance of molecules in the channels is reduced to favor the transfer of reactants and products,and the oxygen reduction catalytic efficiency is improved.D-Fe NC/MOF has a more positive half-wave potential of 0.875 V（vs.RHE）than the0.845 V for the commercial Pt/C catalyst,showing the higher ORR activity and stability.Meanwhile,the zinc-air battery assembled with D-Fe NC/MOF as the cathode catalyst can output higher and more stable discharge voltages.（2）An N-doped carbon-based catalyst（DT-Fe-N-C）was prepared by using a dual-template strategy with SiO₂and polyether F127 as hard and soft template,respectively.As a structural directing agent,F127 realizes the dynamic self-assembly with SiO₂ by crosslinking hydrogen bonding with carbon and nitrogen sources.The relation between the structure and catalytic activity was investigated by optimizing the ratio of the two templates and the pyrolyzing temperature.The results indicate that the specific surface area and the amount of mesopore of the catalyst were apparently increased due to the pore-creating effect of SiO₂and F127.The rich edge-located defects is beneficial for the doping of nitrogen with 4.97%of content,and the Fe-N_x and pyridinic nitrogen configurations with better ORR activity were preferentially generated in the DT-Fe-N-C.Electrochemical measurements indicate that the DT-Fe-N-C has 0.855 V of half wave potential,66 m V dec^-1of Tafel slope,57 m F cm^-2of electrochemical active surface area,revealing the excellent ORR activity.The zinc-air battery assembled with DT-Fe-N-C as cathode catalyst delivers 861 Wh kg^-1 of specific energy density,outperforming the 770 Wh kg^-1 of Pt/C-based battery.In conclusion,the ORR catalytic activity of Fe and N co-doped porous carbon-based catalyst can be effectively enhanced by regulating the pore distribution and introducing high-density active sites.The investigations in this paper provide new ideas for the development of cost-effective non-noble metal catalysts with high performance.