Preparation Of Transition Metal Doped ZIF-8 Derived Carbon Materials Catalysts For Orr Performance

The realization of the“dual carbon”goal is inseparable from the large-scale use of environmentally friendly,green,efficient and renewable energy.Fuel cells are considered to be potential new energy conversion and storage devices due to their environmental protection,cleanliness,safety,abundant raw material sources and high energy conversion efficiency.The cathodic oxygen reduction reaction（ORR）is a key reaction in fuel cells,but it is limited by slow kinetics.Therefore,catalysts are needed to promote the kinetics of the ORR reaction.At present,commercial ORR catalysts are mainly platinum（Pt）-based catalysts.However,the high price of Pt and the lack of resources restrict the large-scale industrialization of fuel cells.In addition,Pt-based catalysts have problems such as poor stability and susceptibility to poisoning.The development of high-performance and economically valuable non-noble metal catalysts to replace Pt-based catalysts is of great practical significance for reducing the cost and commercialization of fuel cells.In recent years,zeolitic imidazole frameworks（ZIFs）,one of the important branches of metal-organic frameworks（MOFs）,have been considered as excellent precursors for the preparation of ORR catalysts due to high specific surface area,self-doping nitrogen,open pore structure,mild synthesis conditions and tunable structure and function.ZIFs derived carbon materials-based catalysts can be obtained by high temperature pyrolysis treatment of ZIFs.However,the high temperature process is prone to cause problems such as poor pore structure distribution and loss of catalytic active components,which seriously affects the development of active sites,resulting in poor performance.It’s necessary to obtain ZIFs based composites with the exposition of generous active sites and high-efficiency ORR performance by doping ZIFs derived carbon materials.Therefore,based on the designable structure of ZIF-8 and the tunable types of central metal elements,this paper modifies it by doping two metal atoms,and prepares ORR catalysts with special morphology and structure through high-temperature pyrolysis.The main research contents are as follows:（1）Fe and Co metal salts were added to form the FeCoZIF-8 precursors during the self-assembly of ZIF-8.The precursors were etched with tannic acid to obtain a hollow polyhedron,followed by the first pyrolysis and pickling to obtain the H-FeCoNC sample.Then nitrogen doping（with g-C₃N₄）and the second high-temperature pyrolysis were performed on the sample to obtain Fe,Co co-doped hollow porous nitrogen doped carbon nanocage catalysts（H-FeCoNC/g-C₃N₄）.The morphology,structure,pore size distribution and chemical composition of the sample were analyzed and characterized by SEM,TEM,BET and XPS.The results show that the sample displays an obvious hollow carbon nanocage structure,and a large number of mesopores and micropores in the carbon skeleton lead to a large specific surface area of the sample.This hierarchical pore structure fully exposes a large number of uniformly dispersed Fe-N_X and Co-N_X active sites in the sample.Therefore,the H-FeCoNC/g-C₃N₄ catalyst shows good ORR catalytic activity,long-term stability,and methanol tolerance with a half-wave potential of 0.886 V under alkaline conditions,and the ORR reaction follows a four-electron transfer process.The Zinc-air battery assembled with H-FeCoNC/g-C₃N₄ as the air cathode exhibits high open-circuit voltage,high power density and high specific capacity during discharge,and excellent cycling stability.（2）Ce and Fe metal salts were added to form Ce-Fe-ZIF-8 during ZIF-8 self-assembly,followed by high-temperature pyrolysis to obtain nitrogen-doped porous carbon framework supported perovskite CeFeO₃ nanoparticle catalysts（Ce-Fe/NC）.The morphology,structure,pore size distribution and chemical composition of the sample were analyzed and characterized by SEM,TEM,HAADF-STEM,BET,Raman,XRD and XPS.The results show that the sample is a rhombic dodecahedron structure with a lot of hierarchical pores,and the abundant mesopore-micropore distribution in the carbon skeleton leads to a large specific surface area inside the sample.A great deal of perovskite CeFeO₃ nanoparticles with a size of about 2.9 nm are anchored in mesopores with a size of about 3.79 nm through M-N-C,which reduces the resistance of interfacial electron transfer and improves the conductivity between oxide nanoparticles while preventing the nanoparticles reunion.Therefore,a large number of oxygen vacancies and Fe sites inside CeFeO₃ nanoparticles are fully exposed as catalytic active sites.The Ce-Fe/NC catalyst reveals excellent activity,stability,and methanol tolerance in pH value universal electrolyte.The half-wave potentials under basic and acidic conditions are 0.913 V and 0.791 V,respectively,with fast reaction kinetic rates,low H₂O₂ yields,and following a four-electron transfer pathway.Zinc-air battery using Ce-Fe/NC as air cathode has high open circuit potential,power density and specific capacity,as well as excellent long-term stability.