Synthesis And Study Of NiCo₂O₄with Controllable Morphology For Supercapacitors

Supercapacitors is becoming a promising electrochemical energy storage conversion systems because of their high power density, excellent pulse charge-discharge characteristics and long cycle life. Pseudocapacitive ternary nickel cobaltite (N1CO2O4) has recently been investigated as a high-performance electrode material for supercapacitors due to its high theoretical specific capacitance, good electrical conductivity and electrochemical activity environmental friendliness. In this thesis, the electroactive NiCo2O4were grown on conductive substrates to be directly used as integrated electrodes, which not only simplifies the preparations for electrodes but also enhances the utilization of active material and short electron-and ion-transport resistance, thus resulting in high specific capacitance and good rate performance, and cycle stability. The concrete content of the thesis are as follows:In Ultrathin mesoporous NiCo2O4nanosheets were directly grown on the low-cost carbonfiber paper (CFP) with large surface area, high porosity and good electric conductivity by a facile solvothermal method combined with a post thermal treatment. The well interconnected ultrathin NiCoiCU nanosheets could allow for easy diffusion of the electrolyte, shorten the transport path of ion and electron and accommodate the strain during cycling. As a result, superior pseudocapacitive performance is achieved with large specific capacitance of1422and999F g-1at the current density of1A g-1and20A g-1, respectively. The capacitance loss is15.6%after3000cycles at a current density of10A g-1and the morphology and structure of the nanosheets are well-retained, which display good cycle ability and high rate capability.2%Hierarchical N1CO2O4nanowires and nanosheets were grown on CFP through the elaborately control and adjust of experiment condition. When the reaction time and temperature is120℃and12h, respectively, whether the hexamethylenetetramine or urea are used as capping agent, the morphology of NiCo2O4can be easily controlled to be nanosheets or nanowires by just changing the solvent from methanol to water, namely the solvent can guide morphological variation to a large extent. It is presumed that the formation of nanowires in water can be expressed as a kinetically controlled nucleation-dissolution-recrystallization mechanism and, because the high dielectric constant of water features with high solubility, and thus the nanosheets may be dissolved to form new nuclei. While, in methanol, the new nuclei is difficult to form because of its low dielectric constant leads to a low solubility, so the nanosheets is stable with increasing the duration. DMF is selected as solvent to replace methanol owing to its similar dielectric constant as that of methanol.3n Eectrochemical measurements revealed that the CFP supported NiCo2O4nanosheets exhibits high value of specific capacitance、 rate performance and cycling stability. Many characterization techniques including BET、SEM、TEM and EIS indicates that the difference may be attributed to:(1) the nanosheets morphology with more BET than nanowires (231m2g-1for nanosheets vs97m2g-1for nanowires), thus nanosheets can offer more electroactive sites to participate in the electrochemical reaction,(2) the nanosheets is more favorable for the transport of ion and electron since the thickness of nanosheets is smaller than the diameter of nanowires,(3) the nanosheets can better accommodate the strain during cycling.4、In order to enhance the electrochemical performance of prepared electrodes, two different core-shell structure of NiCo204@Mn02and NiCo2O4@ppy were successfully constructed. Eectrochemical measurements revealed that the NiCo2O4@MnO2shows not too much advance in electrochemical performance compared with MCO2O4, and the NiCo2O4@ppy manifests excellent electrochemical performance only within the first cycle. Therefore, we attempt to coat the MnO2to the surface of NiCo2O4@ppy, thus achiving NiCo2O4@ppy@MnO2.