Preparation And Properties Of Supercapacitor Of Ni,Co/C Nanofibers Based On Coal Group Component

The electrode material is one of the most important factors that determine the electrochemical performance of supercapacitors.The design and preparation of electrode materials with excellent electrochemical performance is a key issue that urgently needs to be solved in the field of energy storage.Among numerous electrode materials,carbon nanofibers（CNF）have several advantages,such as small diameter and large inter-fiber gaps,which could maximize the utilization of specific surface area and are very beneficial for electrolyte transport.Electrospinning is the simplest and most widely used method for preparing CNF,and polyacrylonitrile（PAN）is often used as the raw material for electrospinning to produce CNF due to its high carbon yield and good structural stability.However,CNF prepared from single-component PAN polymer have low specific surface area,and fibers are independent of each other.The structure obtained solely through physical stacking is not conducive to electronic transport.Therefore,improving the electrochemical performance of PAN-based CNF is a topic of great concern to many researchers.This thesis aimed to optimize the performance of PAN-based materials in terms of specific surface area,conductivity,specific capacitance,and rate performance by adding the loose medium component（LMC）,a carbon precursor from coal.Based on this,a composite electrode material with both double-layer and pseudocapacitance characteristics was further prepared by coating with Ni,Co bimetallic compounds.The inherent relationship between the material and its electrochemical performance was studied in depth.（1）LMC could be highly dispersed in solution system composed of N,N-dimethylformamide（DMF）and PAN,owing to its ultra-small size（80-120 nm）and oxygen-containing structure.Then,carbon nanofibers with specific nodular structures（LCNF）were prepared by electrospinning,pre-oxidation and carbonization.This binder-free electrode material not only had a certain flexibility but also exhibited excellent electrochemical performance.LMC is an effective pore-enhancing modifier,which not only maintains the material’s good pore structure and pore size distribution characteristics but also increases its specific surface area and pore volume.Owing to the dominant pore size distribution of 0.9 nm and the unique morphology of the LCNF,its specific surface area utilization efficiency was significantly improved compared to ordinary porous carbon materials.The specific capacitance of a well-performing sample reached 227.4 F g^-1(36.9μF cm^-2)at a current density of 1 A g^-1,and even when the current density increased to 50 A g^-1,the specific capacitance remained at 161.3 F g^-1(26.2μF cm^-2).After 10000 cycles,the capacitance retention rate was 104.3%.At a power density of 258.6 W kg^-1,its energy density reached 5.0 Wh kg^-1.The material exhibited good rate performance and excellent cycling stability.（2）Cross-linked carbon nanofibers（CLCNF）were prepared using PAN and LMC as carbon precursors.By adding PVP,cross-linking structure was achieved between fibers,which solved the problem of poor conductivity caused by the physical stacking of LCNF fibers.The cross-linking structure connected the conductive networks between fibers,reducing the contact resistance and increasing the conductivity to more than 2.2 times,thus significantly improving the electrochemical performance.The electrode material obtained with a PVP/PAN mass ratio of 0.5∶1,final carbonization temperature of 800℃,and carbonization time of 2 hours had a specific capacitance of243.6 F g^-1 at a current density of 1 A g^-1 and a specific capacitance of 172.7 F g^-1 at50 A g^-1.The specific capacitance,especially the specific capacitance per unit area was significantly improved compared to LCNF,with 54.6μF cm^-2 at 1 A g^-1 and 38.7μF cm^-2 at 50 A g^-1,which was a direct effect of the improved conductivity brought by the cross-linking structure.The material demonstrated good cycling stability,with a capacitance retention rate of 95.40%after 10000 cycles at a current density of 5 A g^-1.（3）LCNF was used as a conductive substrate,and 2-methylimidazole（2-MI）was used as a source of base（OH^-）and morphology controlling agent to prepare CNF-structured bimetallic hydroxide composite material Ni Co-LDH@CNF with both double-layer and pseudocapacitive characteristics through a triple-solvent thermal method.Ni Co-LDH was fully covered with folded nanosheets on LCNF.As result of this,it acting as conductive substrate could accelerate the electron transfer in the electrochemical reaction and provide a large surface area for the uniform growth of Ni Co-LDH.It also effectively solved the problem of powder samples easily agglomerating,thus exposing more active reaction sites.The induction of Co²⁺could improve the over-dense covering layer of single-metal Ni-LDH@CNF.The best performance was achieved by the material（Ni Co-LDH@CNF-120）prepared with a Ni/Co ratio of 1∶0.5 and a solvent thermal temperature of 120℃.Meanwhile,the folded nanosheets formed by Ni,Co compounds were curled and interconnected at the edges,and most of the nanosheets were surrounded by nano-sized small pits,which could serve as storage tanks for electrolyte ions,shortening the transmission path of electrolyte ions to the surface active sites during charging and discharging processes.The overall specific capacitance of Ni Co-LDH@CNF-120 reached 609.4 F g^-1 at a current density of 1 A g^-1,and the rate performance was still 71.6%at a current density of 10 A g^-1.Ni Co-LDH@CNF-120 and activated carbon（AC）were assembled into a hybrid supercapacitor（ASC）,and the working voltage could reach 1.6 V,with a maximum energy density of 20.0 Wh kg^-1.（4）LCNF containing Si O₂（Si_x-C）was prepared by incorporating nano-sized Si O₂into LCNF,which cleverly anchored TMOs（Transition Metal Oxides）between LCNF and TMOs,effectively alleviating the problem of TMOs being difficult to load on the surface of CNF and not being firmly attached.The effect of different amounts of Si O₂on the microstructure and electrochemical performance of Ni,Co oxide composite materials（Ni Co@Si_x-C）was studied.Among them,Ni Co@Si₁-C with the best performance had an ultra-thin and porous nanoscale layered structure,which shortens the distance of ion diffusion and provides sufficient active sites for redox reaction.At a current density of 0.5 A g^-1,the specific capacitance of Ni Co@Si₁-C was 518.1 F g^-1,which was 2.25 times that of Ni Co@Si₀-C without Si O₂.When the current density was increased to 50 A g^-1,the specific capacitance only decreased by 3.9%(497.9 F g^-1),demonstrating high-rate-performance.The ASC were assembled with AC（Ni Co@Si₁-C//AC）with a high energy density of 31.1 Wh kg^-1 and still maintained 84.39%of its initial capacitance after 5000 cycles.DFT theoretical calculation showed that the O atoms in the nano-Si O₂ attached to the surface of LCNF formed a strong interaction with Ni²⁺and Co²⁺,which was the cause of the nano-Si O₂ anchoring role of Ni,Co compounds and CNF,making the combination more firmly.This thesis contains 83 figures,23 tables and 244 references.