Construction Of Doped Fe-N-C Catalyst Based On ZIF-8 For Oxygen Reduction Reaction

Oxygen reduction reaction（ORR）is closely related to human production and life.Most energy conversion devices require the participation of ORR,especially fuel cells and metal-air batteries.However,due to the insufficient cathode reaction power,a large number of precious metal catalysts are required to promote ORR,which seriously hinders the development of electrochemical devices.Among the various non-precious metal ORR catalysts,iron-nitrogen-carbon catalysts（Fe-N-C）have attracted much attention,because they have the ORR activity closest to commercial platinum-carbon.In order to further promote the development of Fe-N-C catalysts,in this paper,the zeolite imidazolate framework material（ZIF-8）,which can be easily obtained by stirring at room temperature,is used as the carbon source and nitrogen source,combining a variety of iron sources with cost-efficient and facile methods to prepare Fe-N-C catalysts,and explore the ORR activity of Fe-N-C catalysts from the perspectives of structural design,heteroatom doping,and active site construction.The specific research work and innovative results obtained are as follows:（1）The choice of iron source is closely related to the preparation cost,morphology and activity of Fe-N-C catalysts.In this work,lithium iron phosphate（Li Fe PO₄）is selected as the Fe and P sources at first time,which are directly ground and mixed with ZIF-8.Through atomic recombination at high temperature,Fe,N,and P co-doped coral-like carbon nanotubes array coated with Fe₂P carbon-based catalyst（C-ZIF/LFP）,which is rich in Fe-N_x sites,is successfully obtained.Benefiting from the special coral-like carbon nanotube arrays morphology of C-ZIF/LFP and the multiple active sites（C-N,C-P,Fe₂P and Fe-N_x）,the half-wave potentials of C-ZIF/LFP in acid and alkaline electrolytes are 0.74 V and 0.88 V,respectively,and the ORR activity and stability in alkaline media and primary zinc-air battery are higher than commercial 20%Pt/C.This study provides a new research approach for the high-value recycling of lithium iron phosphate.（2）In the solution co-precipitation method,weakening Fe³⁺damage to ZIF is necessary and also a great challenge.In this paper,the reducing agent hydrazine hydrate is introduced into the ZIF-8 synthesis system containing Fe SO₄·7H₂O,and Fe,N,S co-doped carbon-based catalyst with large amounts of carbon nanotubes（Fe/N/S-CNTs）is obtained by high temperature heat treatment.However,Fe/N/S-C catalyst prepared without hydrazine hydrate only has a few carbon nanotubes.In addition,S-doping in the catalyst is due to the instability of SO₄^2-adsorbed on the ZIF surface can react with C at high temperatures and form C-S-C and SO_x-C.Benefiting from the combined effect of the conductive networks and transmission channels provided by carbon nanotubes in Fe/N/S-CNTs and the active sites of C-N,Fe/Fe₃C,C-S-C and Fe-N_x,Fe/N/S-CNTs has the half-wave potentials of 0.767 V and 0.887 V in acid and alkaline electrolytes,respectively,which not only exceeds the ORR activity of Fe/N/S-C catalyst,but also exceeds the ORR performance of commercial 20%Pt/C in alkaline solutions and primary zinc-air batteries.（3）Both polyhedron carbon and carbon nanotubes play an important role in catalyst research.Therefore,this paper uses impregnation method to introduce Fe SO₄·7H₂O and hydrazine hydrate on the surface of ZIF-8 crystal,and Fe,N,S co-doped polyhedral carbon matrix and carbon nanotube composite（Fe-N-S CNN）is obtained in-situ by one-step calcination.The impregnation method can maintain the cubic octahedron morphology of the synthesized precursor,and different amounts of iron sources can lead to different amounts of carbon nanotubes generation in the pyrolysis product,so as to controllably prepare a composite catalyst coexisting with polyhedral carbon matrices and carbon nanotubes.Benefiting from the porous structure provided by ZIF-8-derived carbon,the good electrical conductivity and material transport channels provided by carbon nanotubes,and the rich Fe-N_x and C-S-C active sites,6%Fe-N-S CNN has the half-wave potentials of 0.78 V and 0.91 V in acid and alkaline electrolytes,respectively.And 6%Fe-N-S CNN exhibits better ORR performance than commercial 20%Pt/C in alkaline solutions and primary zinc-air batteries,and shows the comparable ORR activity with commercial 20%Pt/C in acidic media.（4）On the basis of the efficient oxygen reduction activity of both Fe-N and Fe-P sites,how to make full use of Fe-N and Fe-P sites has become a difficult problem in current research.In this regard,this paper designs and constructs a catalyst with new type of active site where P is directly coordinated to Fe-N sites.This paper introduces triphenylphosphine and Fe SO₄·7H₂O,step by step into the ZIF-8 synthesis system to make iron ions through the strong metal coordination ability of triphenylphosphine to connect with ZIF-8.Then,porous cubic octahedral carbon-based catalyst（CPFe NPC）with novel P-Fe-N_x sites is obtained by high temperature pyrolysis.The formation of P-Fe-N_x sites is due to the Fe atoms that bond with P atoms are also anchored by N atoms at high temperature.Benefiting from the porous 3D structure provided by ZIF-derived carbon,the high pyridinic N and graphitic N content caused by P doping,and the rich P-Fe-N_x and P-C active sites,CPFe NPC catalyst has high half-wave potentials of 0.791 V in acidic electrolytes and 0.923 V in alkaline electrolytes.More importantly,CPFe NPC exhibits superior ORR performance in alkaline solutions and primary zinc-air batteries than commercial 20%Pt/C,and in hydrogen/oxygen proton exchange membrane single cells（PEMFCs）can output higher power density than Fe NC catalyst.This research result provides a new research direction for the application of traditional Fe-N-C catalyst.（5）Nowadays,the influence of Bi elements of the same family as P on the oxygen reduction activity of Fe-N-C catalysts is less involved.This paper chooses the unstable C₃N₄ at high temperature as template,a precursor C₃N₄@ZIF-8 assembled with ZIF-8crystals outside and C₃N₄ inside is prepared,Then,by introducing metal elements（Fe,Bi）into the design rod-like C₃N₄,metal atoms are released and doped into the ZIF-8-derived carbon structure at high temperature,thereby the porous rod-like carbon-based catalyst（Fe/Bi-RNC）with bi-metal atoms doping is obtained.Benefiting from the rod-like structure with multiple mesopores,rich Fe-N_x active sites,and special Bi-O bonds,the half-wave potentials of Fe/Bi-RNC catalyst in acidic and alkaline electrolytes are0.766 V and 0.899 V,respectively,which not only exceeds the ORR activity of Fe-RNC in both acidic and alkaline media,but also shows better ORR performance in alkaline conditions and primary zinc-air batteries than commercial 20%Pt/C.