Research On Transition Metal Oxides NiCo₂O₄and Their Composites For Supercapacitors

Supercapacitors are electrochemical energy storage devices between secondary batteries andgeneral physical capacitors. Because of its high power density and better cycling property, in recentyears, supercapacitors have attracted tremendous interests in electrochemical energy storageapplications. NiCo₂O₄, as pseudocapacitive transition metal oxide, has recently been investigated as ahigh-performance electrode material for supercapacitors because of its good electrical conductivityand excellent electrochemical activity. However, the main disadvantage that restricts the applicationof NiCo₂O₄is the low electronic conductivity which results in low capacitance and poor ratecapability. In this thesis, the formation of nanocomposites with carbon materials （e.g., orderedmesoporous carbon） or directly growth on metal were used to improve their electrochemicalperformance.（1） OMC/NiCo₂O₄composite were prepared by a co-precipitation process using orderedmesoporous carbon （OMC） as a carbon support. TEM images showed that NiCo₂O₄nanoparticles wasuniformly coated on the OMC. Cyclic voltammetry and galvanostatic charge-discharge were appliedto investigate the electrochemical performance of the OMC/NiCo₂O₄composite. The specificcapacitances of the OMC/NiCo₂O₄composite with a mass fraction of40%were577.0F·g^-1at acurrent density of1A·g^-1and470.8F·g^-1at8A·g^-1, respectively. The specific capacitance ofOMC/NiCo₂O₄composite remains at508.4F·g^-1after2000cycles at a current density2A·g^-1with acapacitance retention of92.7%.（2） Mesoporous NiCo₂O₄nanowire arrays （NWAs） supported by Ni foam substrate are preparedvia two-step strategy. The average diameter of the NiCo₂O₄NWAs is about80nm and the length is upto10μm. The nanowire arrays has large specific surface area (114m2·g^-1). The three-dimensional （3D）network architecture composed of mesoporous NiCo₂O₄NWAs on Ni substrate process highelectrical conductivity and ion transfer efficiency. Cyclic voltammetry and galvanostaticcharge-discharge are applied to investigate the performance of the NiCo₂O₄NWAs. Electrochemicaltests show that NiCo₂O₄NWAs deliver a specific capacitance of1264F·g1at a current density of0.5A·g1, and even998.4F·g1at16A·g1. After2000continuous charge-discharge cycles at4A·g1, thespecific capacitance degradation is only5.51%, indicating their excellent electrochemical stability.The improved performance is reasonably attributed to their unique3D hierarchical structure.（3） Order NiCo₂O₄nanowire arrays （NWAs） supported by carbon cloth substrate are preparedvia two-step strategy. The large specific surface area of the carbon cloth and vertical growth NiCo₂O₄ nanowire structure not only provides large interfacial area for redox reaction but also be beneficial inreducing the diffusion pathways and for electronic and ionic transport. Cyclic voltammetry,chronopotentiometry, and electrochemical impedance measurements are applied to investigate theperformance of the NiCo₂O₄NWAs. Electrochemical tests show that NiCo₂O₄NWAs deliver aspecific capacitance （SC） of1282.5F·g^-1at1A·g^-1, and even1010F·g^-1at20A·g^-1. Theelectrochemical performance of the composite material is very high.