Electrostatic Spinning Construction Of Bifunctional Oxygen Electrocatalyst And Its Application To Zinc Air Batteries

Metal-air batteries are the most popular class of energy storage devices at present.Among them,zinc-air batteries（ZABs）possess high theoretical specific energy,good safety and simple preparation process,and have wide application prospects.Air electrodes carry out oxygen reduction and oxygen precipitation reactions（ORR/OER）during the charging and discharging of ZABs,respectively.These two reaction processes are complex,with high energy barriers and slow reaction rates,and usually rely on noble metal-based catalysts such as Pt/C and Ru O₂ to lower the reaction energy barriers and increase the reaction rates.Scare source,high price and poor stability of such noble metal-based catalysts limit the large-scale application of ZABs.In recent years,graphitic carbon co-doped with transition metals and nitrogen elements has become a research hotspot for non-precious metal catalysts as an efficient,cheap and stable carbon-based catalytic material.Rational design and construction of porous tubular structure of carbon materials can not only improve the kinetics of catalytic reaction process,but also greatly improve the efficiency of active site utilization.Herein,the coaxial electrostatic spinning technique was used to prepare polyacrylonitrile（PAN）fiber precursors containing metal ions.Through adjusting the core-shell composition in spinning,three kinds of structures of porous carbon-based bifunctional oxygen electrocatalysts were prepared by controlling pyrolysis of spun fibers.Besides,the relationship and mechanism between their composition-structure-performance were explored.Finally,these catalysts were applied in charge-discharge ZABs.The details of the study are as follows:（1）Zn Co-ZIF and Fe³⁺and Ni²⁺were introduced into the inner and outer axes of PAN fibers by coaxial electrostatic spinning technique,pyrolysis of which yileded porous graphitic carbon tubes embedded with Fe Ni nanoparticles（Fe Co Ni@HCNFs）.Fe Co Ni@HCNFs exhibited Pt/C comparable ORR performance and better OER performance than Ru O₂.The introduction of Zn Co-ZIF could effectively increase the mesoporous and microporous structures of carbon materials,increase the exposure of active sites,and accelerate the material transfer between the electrode/catalyst/electrolyte three-phase interface.The graphitized degree of good Fe Co Ni@HCNFs has an excellent ultimate current density of-5.06 m A cm^-2.（2）Fe³⁺,Co²⁺,and Ni²⁺were introduced into the outer axis of PAN fibers by coaxial electrostatic spinning technique.Then,ZIF-67 was loaded onto the fiber surface to form precursors,pyrolysis of which yileded Fe Co Ni nanocrystal-embedded carbon nanotubes secondary growing on the surface of porous graphitic carbon tubes（Fe Co Ni NCHNs）.As-prepared carbon tube meshwork exhibited excellent bifunctional oxygen electrocatalytic performance and good cycling stability in the charge-discharge ZABs.This hierarchical carbon structure prevented the agglomeration and encapsulation of metal particles in the carbon material as present in the first part（1）.Besides,the loading of ZIF-67 on the fiber surface secondly developed into carbon nanotubes with the catalysis of Fe Co Ni nanocrystals.The Fe Co Ni NCHNs-based ZABs achieved a peak power density of 160.5 m W cm^-2,a specific capacity of 792 m A g _Zn^-1,and a cycling stability of more than 200 h.（3）PAN was employed as the inner axis,while metal organic salts and PAN/PVP were injected as the outer axis.Pyrolysis of such electrostatic spun fibers yielded Fe Co Ni nanocrystal-embedded carbon nanotubeson the surface of solid carbon fibers（Fe Co Ni FCNFs）,which exhibited excellent bifunctional oxygen electrocatalytic performance and good self-supporting flexibility.Notably,the aqueous ZABs assembled with Fe Co Ni FCNFs as catalysts has an open-circuit voltage of 1.50 V,a peak power density of 120m W cm^-2 and a cycling stability of more than 630 h.In addition,the self-supported flexible all-solid-state ZABs assembled with Fe Co Ni FCNFs as air electrodes has an open-circuit voltage of 1.46 V and a cycling stability of 10,000 s.