Preparation And Supercapacitive Performance Of Flexible Carbon Nanofibers Regulated By Metal-organic-framework Derivatives

In recent years,flexible energy strorage devices have received more and more attentions with the emergence and development of flexible,wearable and smart electronics.Flexible supercapacitors?FSCs?have become an ideal erergy storage device for flexible electronics due to their light-weight,high energy density,high reversibility,long cycling-life,safty,reliability and envieronmental-friendliness.As an important component of FSC,the electrochemical performance of flexible electrode material is directly related to electrochemical performance of the deivce.Among the flexible electrode materials,electrospun carbon nanofibers?ECNFs?have become a hotspot flexible electrode material for FSC due to their controable morphology,superior flexibility,low-cost,large-scale production and easiness to incorporate with other materials.However,due to their low specific surface area and micropore structure,the specific capacitance of ECNFs is generally low,and FSC based on ECNFs commonly shows low energy density.Thus,exploring flexible ECNFs with high specific capacitance is neccessary.In this work,we synthesized porous and flexible ECNFs by electrospinning MOFs incorporated nanofibers precursor with subsequent carbonization.The MOFs can not only ameliorate the physical structure,but also improve the electrchemical performances of ECNFs.FSC based on ECNFs were also studied.The contents are summarized as follows:1.N doped flexible ECNFs with high specific capacitance were prepared by electrospinning ZIF-8 incorperated nanofibers precursor with subsequent carbonization and corrosion.The addition of ZIF-8 can not only improve the mesopore volume and specific surface area?SSA?,but also greatly increase the N content of ECNFs,which is beneficial to the improvement of electrochemical performancs.The sample carbonized at 800?shows the best electrochemical performance and its specific capacitance can reach up to 345 F g^-1 at a current density of 1 A g^-1 in 1 mol L^-1 H₂SO₄ electrolyte,much higher than that of unmodified ECNFs(192 F g^-1).Flexible supercapacitor based on the N doped flexible ECNFs exhibits a high energy density of 13.2 Wh kg^-1 at the power density of 250 W kg^-1.A high specific capacitance retention of 102.7%is remained after 3000 charge-discharge cycles.Moreover,a hgih specific capacitance retion of 94.4%is remained after bending for 180°,showing superior cycling stability and outstanding flexiblilty.2.Porous N,S codoped ECNFs were prepared by electrospinning thiourea?TU?and ZIF-67 incorporated nanofiber precursor with subsequent carbonization and etching.The addition of ZIF-67 can greatly improve the SSA and mesoporous volume of ECNFs,and the addition of TU can not only increase the N content,but dope S into ECNFs,which is befinificial to the improvement of electrochemical performances of ECNFs.The electrochemical test shows that carbonization at 800?is the best condition,and the obtained sample can achieve a high specific capacitance of 396 F g^-1 at a current density of 1 A g^-1 in 1 mol L^-1 H₂SO₄ solution,much higher than that of N doped ECNFs(adding ZIF-67 only,365 F g^-1).Flexible supercapacitor based on the N,S codoped ECNFs exhibits a high energy density of 14.3 Wh kg^-1 at the power density of 250 W kg^-1.It also displays a high specific capacitance retention of 107%after 3000 charge-discharge cycles,showing high energy density and superior cycling stability.After bending for 180°,a high capacitance rention of 97.3%is remained,revealing superior flexiblilty.3.Nanographite@Fe₂O₃ nanorod incorporated micro-/mesoporous ECNFs were syntherized by electrospinning MIL-88Fe nanrodes incorporated nanofiber precursor with subsequent carbonation and oxidization in air.It should be noted that Fe₂O₃derived from MIL-88Fe can greatly improve the specific capacitance of ECNFs via Faradic reactions,and the nanographite coating on Fe₂O₃ can supply fast electron transport for Faradic reactions and prevent Fe₂O₃ from structural damage in long-cycling process.The porous ECNFs are not only used as flexible substrate,but also provide fast electron and ion transfer for electrochemical reations.The electrochemical test in 2 mol L^-1 KOH solution shows that the best ratio of PAN:MIL-88Fe in precursor solution is 1:1,and the obtained sample can achieve a high specific capacitance of 523 F g^-1 at a current density of 1 A g^-1,much higher than that of unmodified ECNFs(151 F g^-1).An asymmetric FSC?AFSC?based on nanographite@Fe₂O₃ nanorod incorporated ECNFs anode and PANC cathode was assembled with a maximum operating voltage of 1.7 V.It shows a high energy density of 22.1 Wh kg^-1 at the power density of 425 W kg^-1,and can still remain 5.5 Wh kg^-1at a high power density of 17000 W kg^-1,showing high energy density and superior rate capacity.After 3000 charge-discharge cycles,a high specific capacitance of 96.4%is maintained,revealing excellent cycling stability.Moreover,it also displays a high capacitance retention of 95.6%after bending for 180°,revealing outstanding flexibility.4.Nanographite@NiO nanosphere incorporated ECNFs were synthesized by electrospinning MOF-74Ni nanosphere incorporated nanofiber precursor with subsequent carbonation and oxidization.The porous carbon nanofibers not only act as flexible substrate,but supply fast electron and ion transfer for electrochemical reations.NiO can greatly improve the specific capacitance of ECNFs via Faradic reactions,while the nanographite catalyzed by Ni on NiO can supply fast electron transport for Faradic reactions and prevent NiO from structural damage.The electrochemical test in 2 mol L^-1 KOH soluition shows that the highest specific capacitance of 468 F g^-1 can be achieved when the ratio of PAN:MOF-74Ni in precursor solution is 1:1.An AFSC based on nanographite@NiO nanosphere incorporated ECNFs cathode and nanographite@Fe₂O₃ nanorod incorporated ECNFs anode was assembled with a high operating voltage window of 1.8 V.It exhibits a high energy density of 43.9 Wh kg^-1 at the power density of 412.5 W kg^-1,and still remains 17.2 Wh kg^-1 at the power density of 16500 W kg^-1,showing high energy density and superior rate capacity.High specific capacitance retentions of 94.7%and94.4%are remained after 3000 charge-discharge cycles and bending for 180°,respectively,displaying excellent cycling stability and outstanding mechanical property of the FSC.