The Study On Ordered Mesoporous Carbons Modified By Transitionmetal Oxides Nanoparticlesand Their Electrochemical Properties

Transition metal oxides with superior charge-discharge capacity and electrochemical activity have gained great interests.Among them,cobalt oxides and molybdenum oxides are considered the most promising supercapacitor electrode materials due to their high theoretical specific capacitance,abundant reserves,low cost,high electrochemical activity and environmental friendliness.However,these oxides have large band gaps and low specific surface areas.The electrode polarization and volume expansion in electrochemical reactions limit their performance.Based on the above deficiencies,CoO,MoO₃ and CoMoO₄ nanoparticles with high specific capacitances are assembled inside ordered mesoporous carbon CMK-3 channels,and multi-layer nanostructure hybrids are designed and prepared.The hybrids can take full advantages of each component to form the synergistic effect,improving the electron transmission rate,alleviating the electrode polarization and volume expansion of the hybrids in the electrochemical reaction.Therefore,the electrode hybrids can exhibit good electrochemical properties.In this thesis,the phases,morphologies,pore structures and electrochemical properties of the noval hybrids are characterized and tested,and the relationship between performance and structure is also explored.CoO_x modified CMK-3 hybrids are synthesized using CMK-3 as the carbon support and cobalt nitrate hexahydrate as the Co precursor.The acid pretreatment of the carbon surface and the ammonia post-treatment of the intermediate achieve effective control of the particle size and dispersion of the cobalt oxides nanoparticles.The hybrids possess similar microstructure with CMK-3,ordered mesoporous structure,high specific surface area(620-1160 m²/g),well-dispersed uniform cobalt oxides nanoparticles(5-20nm).The dispersion,sizes and phases of the nanoparticles heat-treated at various temperatures have been investigated through FESEM,XRD,XAS and other techniques.It demonstrates that Co₃O₄ nanoparticles are formed in Co/C_A-500 with size of?5 nm.The energy and power density of the symmetric supercapacitors fabricated with Co/C_A-500 are 14.3 Wh/kg and 750.2 W/kg.Moreover,the capacitance retention reaches 84.6%after 1000 CV cycles.This multi-layer nanostructure with high specific surface area can alleviate the expansion of the electrode material and provide more electrochemically active sites,thereby enhancing the specific capacitance and cycle stability of the hybrid.The MoO₂ nanoparticles are successfully loaded into the mesporous structure of CMK-3 with good dispersion by a temperature-controlled solvent evaporation-induced assembly method.The hybrids all retain high specific surface area(400-900m²/g)and large pore volume(0.4-1.1 cm³/g).The specific capacitance of 20Mo@C₄₀₀is composed of electric double layer capacitance(EDLC)of CMK-3 and the pseudocapacitance of MoO₂ nanoparticles.The pseudocapacitance contributed by MoO₂ nanoparticles is 245.3 F/g.After 200 cyclic voltammetry(CV)tests,the capacitance retention of 20Mo@C₄₀₀ is 99.6%.This is because CMK-3 exhibits a large specific surface area for ion adsorption and rich active sites for Faradic redox reaction,which is beneficial for achieving high specific capacitance of the hybrids.Furthermore,the specific capacitance at the current density of 1.0 A/g is 88.5%of the specific capacitance at 0.2 A/g.This is due to the small size,moderate content and good dispersion of the MoO₂ nanoparticles.The special three-dimensional ordered channel structure builds a fast network for charge transfer and ion transport,promoting the adsorption and desorption process of electrolyte ions and alleviating the hybrid electrode polarization in the electrochemical reactions,so that the electrode material shows good rate performance.The nitrogen-doped ordered mesoporous carbons(NOMC)have been prepared and subsequently modified with molybdenum oxide nanoparticles by a temperature-controlled solvent evaporation-induced assembly method.The heat treatment temperature and molybdenum precursor content are changed to obtain hybrids with different molybdenum oxide and specific surface area.All hybrids retain the ordered mesopore structure of NOMC with high specific surface area(400-900 m²/g)and large pore volume(0.4-1.1 cm³/g).The main phase in the hybrid after heat treatment at 400?is orthorhombic MoO₃.20Mo@NOMC₄₀₀ exhibits a quasi-rectangular CV curve within the enlarged potential window compared with the pristine NOMC together with pairs of redox peaks.The MoO₃nanoparticles encapsulated in the ordered mesoporous channels exhibit significant electrochemical activity with the pseudocapacitance up to 410 F/g,and the capacitance retention of 20Mo@NOMC₄₀₀is 86%after 1000 cycles.According to the analysis,the specific capacitance of20Mo@NOMC₄₀₀ is composed of EDLC of CMK-3 and the pseudocapacitance of MoO₃ nanoparticles,and the pseudocapacitance of 20Mo@NOMC₄₀₀ includes not only the adsorption/desorption pseudocapacitance on the surface of MoO₃nanoparticles,but also the intercalation pseudocapacitance generated by the intercalation/deintercalation.Moreover,the three-dimensional network pore structure can make full use of active materials for electrochemical reactions and fully alleviate the volume expansion of electrolyte ions during the intercalation/deintercalation of the electrode material,resulting in excellent cycle stability.Monodispersed CoMoO₄ nanoclusters on CMK-3 are successfully fabricated by a temperature-controlled solvent evaporation-induced assembly method.The synthesized hybrids possess improved wettability,high specific surface area(?700m²/g)and regular mesoporous channels(?4 nm).The well-dispersed CoMoO₄nanoclusters exhibit a significant specific capacitance up to 367 F/g and the capacitance retention reaches 99.8%after 200 CV cycles.The energy density and power density of the symmetrical supercapacitor fabricated by CoMoO₄@C₃₀₀ are12.3 Wh/kg and 451.8 W/kg,respectively.Through fitting calculation,most of the specific capacitance of CoMoO₄@C₃₀₀ is derived from the EDLC and the surface pseudocapacitance of CoMoO₄ nanoclusters,the other specific capacitance is derived from the intercalation pseudocapacitance generated by the intercalation/deintercalation of electrolyte K⁺in CoMoO₄ nanoclusters through fitting calculation.Furthermore,the surface pseudocapacitance and intercalation pseudocapacitance of CoMoO₄@C₃₀₀ in neutral electrolyte are derived from the redox reactions on the surface of MoO₃ and the processes of intercalation/deintercalation,respectively.