Research On Conductivity Of Transition Metal Oxide Electrode For Supercapacitors

Supercapacitors with high power density,fast charge-discharge and long cycle life are being considered recently as energy storage devices for use in hybrid electric vehicles,modern electronic equipment and high-power military equipment.Transition metal oxides of low cost and high theoretical capacity have been widely studied because they can be facilely prepared.However,the low conductivities of the transition metal oxides limit their practical applications.The electrical conductivity of the electrode materials is closely related to the specific capacity of electrodes and energy and power densities of the supercapacitors.Therefore,one of the hot issues is to develop strategies for enhancing the conductivity of transition metal electrode materials in order to improve the electrochemical performance of supercapacitors.This thesis is focused on the investigation of several methods to improve the conductivity of transition metal oxide based electrode materials.Here in,the electrochemical performance of the supercapacitors is improved by adjusting the structure of electrode materials and modifying the conductivity of electrode materials for supercapacitors.Thus,this thesis research is related to the preparation of spinel structured binary transition metal oxides using different methods followed by their conductivity evaluation as follows:?1?Mesoporous textured NiCo₂O₄ with three-dimensional network was synthesized through a sol-gel method by using silica as a template.The as-prepared Ni Co₂O₄ showed an ultra-high specific surface area of 438.3 m² g^-1 and exhibited a long-term cyclic stability?only 2.87%capacity reduced after 5,000 cycles?.The Faradaic interfacial charge-transfer resistance(R_ct)and the bulk solution resistance?R_s?were 1.39 and 0.48?,respectively,after the long cycling experiment was completed.A flow atomization method was employed to fabricate the NiCo₂O₄ hollow microspheres with mesoporous shell structure using silica as the template.The Ni Co₂O₄ electrode prepared from hollow microspheres delivered a 92.5%capacity retention and showed R_ct and R_s values of 1.39and 0.48?,respectively after 50,000 cycles.?2?Inorganic conductive salt was integrated with the transition metal oxide by different methods,which markedly improved the conductivity of the electrodes of LaNiO₃ of perovskite structure.The mesoporous LaNi_0.5Co_0.5O₃/0.333Co₃O₄ composite was synthesized via a sol-gel method by using silica as a template and the resulting composite showed a specific surface area of 372 m² g^-1 with pore volume of 1.49 cm³ g^-1.The composite exhibited an excellent cycling performance by retaining 92.6%of its specific capacitanceafter60,000chargeanddischargecycles.TheR_ctof LaNi_0.5Co_0.5O₃/0.333Co₃O₄ electrode was only 1.87?after cycling test.The atomization route incorporating colloidal silica as a template was also employed to synthesize LaNi_0.5Co_0.5O₃/0.333Co₃O₄ hollow microspheres with highly mesoporous shells.The hollow microspheres with mesoprous shell showed a high specific surface area of 247 m²g^-1 along with a mesopore-size of about 2.53 nm.A hybrid supercapacitor assembled with LaNi_0.5Co_0.5O₃/0.333Co₃O₄ hollow spheres as the positive electrode and N-doped mesoporous carbon?NMC?as the negative electrode showed a very high energy density of 42.8 Wh kg^-1 at a power density of 424 W kg^-1.The hybrid supercapacitor also exhibited a long-term cycling life of up to 30,000 cycles with a specific capacitance retention of 90.4%.By comparing the equivalent series resistance?ESR?of the LaNi_0.5Co_0.5O₃/0.333Co₃O₄,LaNi_0.5Co_0.5O₃ and Co₃O₄ phases,it was determined that the excellent electrical conductivity of LaNi_0.5Co_0.5O₃ played a key role in improving the electrochemical performance of LaNi_0.5Co_0.5O₃/0.333Co₃O₄ electrode materials.?3?Modification with a conductive polymer was done to improve the conductivity of the MnO₂ transition metal oxide electrode material.High-porosity MnO₂ with a mesoporous structure was first synthesized through a sol-gel method followed by the growth of polypyrrole?PPy?nanofilms on the synthesized mesoporous MnO₂ by chemical vapor deposition to form a 3D nanocomposite structure.The as-prepared MnO₂/PPy nanocomposite showed a 91.4%capacitance retention after 5,000 charge-discharge cycles.MnO₂/PPy and NMC were employed to assemble an ASC,which exhibited a high energy density of 38.6 Wh kg^-1 at a power density of 900 W kg^-1,and also maintained 25.1 Wh kg^-1 at 9 kW kg^-1.In addition,mesoporous NiCo₂O₄ was directly coated on an ultrafine nickel wire to fabricate a battery-type electrode by a facile process involving electrodepsition of Ni/Co/Zn alloy,dealloying and oxidation steps.The binder-free electrode with flexible and wearable features delivered a high specific capacity and an excellent cycling performance.An assembled asymmetric supercapacitor?ASC?employing NiCo₂O₄/ultrafine nickel wire along with Fe₃O₄/ultrafine nickel wire showed a very high energy density of 32.6 Wh kg^-1 and power density of 35 kW kg^-1 and 94.8%retention of initial capacitance after 20,000 cycles.The R_ct values of supercapacitor before and after long GCD experiment only increased from 0.485 to 0.612?.Furthermore,PPy coatings were deposited on porous NiCo₂O₄/Ni wire to fabricate PPy/NiCo₂O₄/Ni structure,which was found to be superior to NiCo₂O₄/Ni material in electrochemical performance as supercapacitor.?4?Different methods were employed to prepare doped porous carbon materials for improving the conductivity,which was reflected in the electrochemical performance of an asymmetric supercapacitor?ASC?using transition metal oxides as positive electrode and doped porous carbon materials as negative electrode.An N-doped porous carbon was synthesized employing pyrrole as carbon source to prepare PPy and by carbonizing the PPy with KOH.A facile incomplete phase separation strategy unlike the tedious synthesis approaches of the past was proposed to fabricate P and N co-doped porous carbon with a specific surface area of 1920 m² g^-1 and an average pore size of 2.29 nm.This porous carbon delivered a high specific capacitance of 318 F g^-1 at 1 A g^-1 and a remarkable cycling stability,i.e.,96.2%initial capacitance after 10,000 cycles in 6 M KOH electrolyte solution.In addition,an ASC employing LaNi_0.5Co_0.5O₃/0.333Co₃O₄ hollow spheres with a mesoporous shell as a positive electrode and P/N co-doped porous carbon as a negative electrode showed a maximum specific capacitance of 109.6 Fg^-1 at 1 A g^-1.