Controllable Preparation And Properties Of Metal-Nitrogen-Carbon Oxygen Electrocatalysts

Since gradual depletion of traditional energy sources and growing concerns about environmental issues,it is an increasing demand for environmentally friendly renewable energy sources,driving the development of sustainable energy conversion and subsequent energy storage technologies.Rechargeable metal-air batteries with high energy density,abundant raw materials,reliable safety,and low environmental impact,show great potential for applications.Due to the slow reaction kinetics,the key air electrode reactions including oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）,rely on efficient electrocatalysts.At present,platinum（Pt）,iridium（Ir）/ruthenium（Ru）-based catalysts are still commonly used ORR and OER catalysts.However,the scarcity of raw materials for these noble metal-based catalysts largely limits the large-scale production and application of rechargeable metal-air batteries.Therefore,it is crucial to design and synthsize environmentally friendly,efficient and stable oxygen electrocatalysts.Transition metal and nitrogen codoped carbon electrocatalysts（M/NC）show not only low cost,but also high catalytic activity and good stability.Herein,multi-metal（iron,cobalt,nickel）and nitrogen co-doped porous carbon nanospheres（FeCoNi/NC）,metal nanoparticle@carbon nanotube-decorated polyhedral carbon（Co@CNT/NC）and metal nanoparticle-embedded ultra-flake-like porous carbon（FeCo/NG）were controllably prepared by inverse microemulsion polymerization,ZIFs three-dimensional coating and molten salt-assisted pyrolysis,respectively.The structure-function relationship and mechanism were explored and discussed based on the study between the morphologies,components and oxygen electrocatalytic performances.The specific research contents are as follows:（1）Different metal-doped polypyrrole nanospheres were synthesized by inverse microemulsion polymerization,and then pyrolyzed at high temperature to obtain four different metal-doped M/NC-type porous carbon nanospheres,including Fe/NC,FeCo/NC,Fe Ni/NC and FeCoNi/NC.All the four samples showed high ORR electrocatalytic activity.FeCoNi/NC showed the highest ORR electrocatalytic activity.Compared with Fe/NC,the OER electrocatalytic activities of the other three samples were significantly improved.The illustrated doping of Co and Ni elements can have a significant performance enhancement on the OER performance of Fe/NC.The results showed that multi-metal doping promotes the combination of metal and nitrogen,reduces the loss of metal and nitrogen contents during pyrolysis,and increases the contents of M-N_x active sites,resulting in improved electrical conductivity and enhanced bifunctional oxygen electrocatalysis.Through density functional theory（DFT）modeling and calculations,the effects of multi-component metal doping on the catalytic kinetics of ORR and OER were further simulated.Experimental tests and theoretical calculations have proven that FeCo Ni N₄ is the main contributor to the excellent catalytic performance.（2）Coating of ZIFs prepared ZIF-67-coated Co₃O₄-decorated ZIF-8（Co₃O₄-ZIF-8@ZIF-67）precursors,pyrolysis of which yielded Co nanoparticle@carbon nanotube-decorated carbon polyhedrons（Co@CNT/NC）.The products exhibit good ORR and OER electrocatalytic performances.Meanwhile,pyrolysis of Co₃O₄-ZIF-8 yielded carbon polyhedrons(Co₃O₄-ZIF-8₉₅₀),and pyrolysis of Co₃O₄-ZIF-8@ZIF-8 yielded Co nanoparticle@carbon polyhedrons(Co₃O₄-ZIF-8@ZIF-8₉₅₀).Co@CNT/NC showed much better performances than Co₃O₄-ZIF-8₉₅₀ and Co₃O₄-ZIF-8@ZIF-8₉₅₀.After acid washing,there was obvious loss of Co particles in Co@CNT/NC,resulting in poor ORR and OER performances.So,the Co@CNT/NC retained large specific surface area and porous structure can be retained,and on the other hand,the carbon tube structure and original polyhedron framework.Besides,existence of Co nanoparticle@carbon nanotube provided more active sites and improved the ORR and OER performances.（3）FeCo alloy nanoparticle-embedded ultra-flake porous carbon（FeCo/NG）was prepared by Na Cl molten salt-assisted pyrolysis of Fe and Co-doped ZIF-8 precursor（FeCo/ZIF-8）.In contrast,FeCo alloy nanoparticles decorated carbon spheres（FeCo/NC）were prepared by direct pyrolysis of FeCo/ZIF-8.FeCo/NG with more specific surface area,porosity and defect degrees exhibited better ORR and OER electrocatalytic performances.Using FeCo/NG as the air electrode catalyst,the rechargeable Zn-air battery exhibits a peak power density of 109 m W cm^-2,a specific capacity of 783 m Ah g^-1,and an operating time of 100 h.Therefore,molten salt-assisted pyrolysis of ZIFs the pore-forming and deformation unfolding of the polyhedral structure was realized by the method of,the size distribution and uniform dispersion of metal nanoparticles were controlled,and an efficient and stable ultra-flake bifunctional oxygen electrocatalyst was prepared.Therefore,it provides a feasible synthetic method for the design of high-performance electrocatalysts by elaborating the M/NC structure and using molten salt-assisted carbonization and ZIF unfolding to design high-performance electrocatalysts,which provides a new idea for the synthesis and modification of catalysts.