Electrospun Porous Carbon Nanofibers Derived From MOF And Its Application In The Supercapacitors

Nanoporous flexible carbon nanofibers are promising candidates for electrode materials in supercapacitors,especially in energy storage for wearable devices.Conventional carbon nanofibers tend to exhibit low capacitance because of their low specific surface area and single pore structure.The high specific surface area and hierarchically porous structure are beneficial to improve the electrochemical performance of the electrode materials;however,its preparation process is still a great challenge today.In this paper,flexible hierarchically porous carbon nanofibers are prepared by electrospinning zeolitic imidazolate framework?ZIF-8,ZIF-C?suspended in polyacrylonitrile?PAN?solution,followed by a combined heating and acid treatment.Combining scanning electron microscope,transmission electron microscope,X-ray powder diffractometer,Raman spectrometer,and specific surface analyzer,the obtained material were analyzeds including the morphology,phase composition and pore structure,and its electrochemical performance was systematically investigated.?1?The porous carbon nanofibers electrode material based on pyrolysis of ZIF-8/PAN composite fiber was successfully prepared by electrospinning technology.Their properties including morphology,specific surface area,nitrogen content,pore structure were systemcally investigated by controlling the ratio of ZIF-8/PAN and pyrolysis temperature,and their electrochemical performanceswere also studied.The results show that the porous carbon nanofiber electrode material exhibit characteristics of hierarchical pores,nitrogen doping,flexibility,and high specific surface area.The biggest specific surface area are 1349 m²·g^-1,the highest N content are up to 16.72 wt%.The optimum electrochemical performance of this system is obtained by pyrolysis of 66.7 wt%ZIF-8 composite fibers at 800?C,and the specific capacitance can reach 302 F·g^-11 at the current density of 0.5 A·g^-1.Even at a high current density of 10 A·g^-1,a capacitance of 172 F·g^-1 is still obtained.After 12,000 cycles at a current density of 5 A·g^-1,the capacity retention rate is maintained at 84%.?2?By Co equivalent substitution of Zn metal clusters in ZIF-8 crystals,a bimetallic coordination metal organic framework?ZIF-C?was synthesized.Combined with electrospinning and pyrolysis processes,porous carbon nanofiber network electrode materials were prepared.Their properties including morphology,specific surface area,nitrogen content,graphitization and pore structure were systemcally investigated by controlling the ratio of Co content and pyrolysis temperature,and their electrochemical performances were also studied.Moreover,studies have shown that the introduction of Co accelerates the graphitization process of the material,and at the same time,it also results in the loss of a large amount of doped heteroatoms.Further electrochemical studies on the material show that when the Co content in the ZIF-C is 5%and the pyrolysis temperature is 800?C,the carbon nanofiber electrode material has the best electrochemical performance,which shows a high specific capacitance(326 F·g^-1 at 0.5 A·g^-1),excellent rate performance(40 A·g^-11 for 170 F·g^-1)and good cycle stability?after 14,000 cycles,the capacity retention maintained at 81%?.These excellent performance can attribute to the synergistic effect of high specific surface area(623m²·g^-1),high nitrogen content?13.83 wt%?,hierarchically porous structure and conductive fiber network.?3?The obtained flexible carbon fiber material was assembled into a supercapacitor device,and their electrochemical behavior was studied in different electrolytes.The results show that best specific capacitance can be obtained in the aqueous electrolyte,and in terms of energy density,it achieves superior performance in 1 mol·L^-1 tetraethylammonium tetrafluoroborate?TEABF₄/AN?electrolyte,with a energy density of 9.61 Wh·kg^-1.In addition,the experimentally assembled supercapacitor device?button type?successfully lit LEDs of different voltages and powered mini fans.These results illustrate the usability of the prepared porous carbon fiber flexible electrode in terms of energy storage.