Preparation And Electrochemical Properties Of Coal-based Porous Carbon Nanofibers And Their Nickel-based Composites

Xinjiang has abundant reserves of coal resources and the coal quality is fine.It is of great significance to develop coal-based carbon nanomaterials with high added value to promote the efficient utilization of coal resources and develop the advantages of Xinjiang coal.Among many carbon materials,porous carbon nanofibers have many advantages,such as good electrical conductivity,large specific surface area,high aspect ratio,and chemical stability,and so on.Thus they have a broad application prospect in many fields.Especially,in the field of energy storage and conversion,porous carbon nanofibers can not only be used as electrode materials to construct carbon-based electrochemical energy storage devices,but also be compounded with nano-carbon electroactive materials.Porous carbon nanofibers can transport electrons,inhibit the aggregation of active materials and buffer volume expansion in the composites,and play an important role in electrochemical energy storage devices.It is still a research hotspot to design and prepare low-cost porous carbon nanofibers and their composites with high electrochemical performance.This paper developed coal based porous carbon nanofibers by electrospinning method with Heishan bituminous coal as raw material,and the composites were prepared by compounding coal based porous carbon nanofibers with nickel based compounds.The effects of preparation conditions on the morphology,structure and electrochemical properties of the materials were systematically analyzed.The main research contents and results are as follows:（1）Coal-based porous carbon nanofibers（CPCNFs）were prepared by single-nozzle electrospinning with water as solvent,acid-treated coal as carbon source,F127as template and polyvinyl alcohol（PVA）as spinning aid,and followed one-step carbonization.The effects of carbonization temperature and F127 concentration on the morphology,structure and electrochemical properties of CPCNFs were discussed,and the mesoporous formation mechanism was also studied.When the optimum carbonization temperature is 900℃,the mass ratio of acid treated coal and F127 is 2:1,the specific surface area of sample CNFs-F2-900 is as high as 1161 m²g^-1,the total pore volume is 0.76 cm³g^-1,and the mesopore size is mainly 2.5-4 nm.As the electrode material of supercapacitor,the specific capacitance of CNFs-F2-900 can reach 265.2 F g^-1at a current density of 1 Ag^-1,and when the current density increases by 50 times,its capacitance retention rate can reach 73.0%.When the current density is 10 Ag^-1,the capacitance retention rate is almost unchanged after20000 cycles of charge and discharge.CNFs-F2-900 has large specific capacitance and good cycle stability,which are better than most similar materials reported at present.（2）In order to further improve the specific capacitance and power/energy density of CNFs-F2-900,the surface of CNFs-F2-900 was pretreated with mixed acid,and the nanosheet NiS/coal-based porous carbon nanofibers（NiS/PCNFs）composites were synthesized by one-step hydrothermal method.The effects of the amount of Ni and S sources on the morphology,structure and electrochemical properties of the composites were investigated.The results show that changing the amount of Ni and S sources can control the thickness and quantity of NiS nanosheets on surface of NiS/PCNFs,and then affect their electrochemical performance.When the amount of Ni and S sources is 1 mmol and 4 mmol,the loading amount of NiS on the surface of NiS/PCNFs2 is moderate,and the number of NiS nanosheets is thin and the largest,which makes NiS/PCNFs2 provide a larger contact area for electrolyte ions and is conducive to rapid reversible redox reaction.At the same time,thanks to the synergy between NiS and PCNFs,as the electrode material of supercapacitor,the specific capacitance of NiS/PCNFs2 can reach 869.5 F g^-1at a current density of 1 Ag^-1,and the specific capacitance retention rate can reach 92.2%after 5000 cycles.The specific capacitance and cyclic properties of NiS/PCNFs2 are better than that of some electrode materials reported.Moreover,the power density of NiS/PCNFs2 is 348.3 W kg^-1,and energy density is 12.3 W h kg^-1.（3）In order to improve the rate performance and power/energy density of nanosheet NiS/PCNFs2 composites,elm seed-like NiS anchored on the surface of coal-based carbon nanofibers composites（NiS/CNFs）were rationally designed and prepared via a multistep transformation approach.The intermediate elm seed-like nickel silicate were first deposited on the surface of coal-based carbon nanofibers,followed by chemical conversion into elm seed-like NiS.The effects of SiO₂on the morphology,structure and electrochemical properties of the composites were investigated,and the formation mechanism was discussed.The elm seed-like NiS nanosheet on the surface of NiS/CNFs is formed by the self-assembly of small NiS nanosheets,which increases the contact area between electrolyte and material.Moreover,the removal of SiO₂in hydrothermal reaction not only increases the specific surface area of the material,but also produces abundant mesoporous.As a self-supporting electrode material of supercapacitor,NiS/CNFs presents good electrochemical performance.When the current density increases from 0.5 Ag^-1to 10Ag^-1,the specific capacitance retention rate can reach 62.9%.In addition,the power density of NiS/CNFs is 373.8 W kg^-1,and the energy density can reach 13.8 W h kg^-1,which has potential application value.This method can not only avoid the surface treatment of carbon nanofibers,but also prepare NiS/CNFs self-supporting electrode materials with novel morphology.（4）NiMoO₄/necklace-like coal-based carbon nanofibers composites were fabricated by one-step coaxial electrospinning and Kirkendall effect.NiMoO₄/necklace-like coal-based composite carbon nanofibers composites（NiMoO₄/CNFs）were synthesized by coaxial electrospinning DMF solution of nickel acetylacetonate and polyvinylpyrrolidone in the core layer and DMF solution of acid-treated coal,ammonium molybdate,polymethylmethacrylate and polyacrylonitrile in the shell layer,and then carbonized in N₂atmosphere and calcined in air.The effects of different calcining atmosphere on the composition,morphology,structure and electrochemical properties of the materials were discussed,and the formation mechanism of necklace-like structure was discussed.The necklace-like structure of NiMoO₄/CNFs is formed by the connection of hollow carbon nanospheres and NiMoO₄are embedded in the necklace-like CNFs.The necklace-like structure can not only provide a large electrode/electrolyte reaction interface,but also enhance the charge transfer reaction and buffer the volume expansion of NiMoO₄during the charge discharge process.Moreover,the catalytic effect of Ni enhances the graphitization degree of carbon,thus improving the electrical conductivity of the material.In addition,the etching effect of O₂not only increases the specific surface area of the material,but also produces abundant micro/mesoporous.NiMoO₄/CNFs shows excellent electrochemical performance.The initial discharge capacity of NiMoO₄/CNFs is 1028.2 mA hg^-1at the current density of 0.5 Ag^-1,and the reversible capacity can still keep 828.3 mA hg^-1after 400 cycles.When the current density increases to 2 Ag^-1,the reversible capacity of the NiMoO₄/CNFs is still 537.8mA hg^-1after 1000 cycles,and the coulombic efficiency remains 99.5%,which are better than that of some electrode materials reported.