Preparation And Electrochemical Properties Of Transition Metal Oxide-based Composite Materials

Recent years, supercapacitors have attracted extensive research interest due to their intrinsic characteristics such as high power density, fast charge–discharge rate,long life and low maintenance cost. These outstanding advantages make supercapacitors very promising candidates for applications in numerous fields,including portable electronic devices, electric vehicle, electric Toy, backup power sources, instrumentation, etc. However, the major application bottleneck of commercial supercapacitors based on carbonaceous materials which have low energy density. Therefore, it is imperative to develop new electrode materials with both high energy and high power densities. Among various electrode materials, transition metal oxides are the most attractive candidates due to their high energy density arising from the fast and reversible faradaic redox reactions at the electrode/electrolyte interfaces.Ru O2 has been the most extensively studied pseudocapacitance materials due to the perfect electric properties of them, but the cost of Ru O2-based materials limit their application in daily life. In this thesis, with the use of different synthetic methods,including hydrothermal method, in situ chemical polymerization methods, chemical bath deposition(CBD) and electrospinning method, we obtained several kinds of pseudocapacitance complex materials based on cheap transition metal oxides,including Titanium dioxide(Ti O2), zinc oxide(Zn O), and nickel oxide(Ni O). The structures, morphologies and element composition were investigated by IR, XRD,SEM, TEM and EDX. In addition, the electrochemical performance has been investigated by galvanostatic charge-discharge(gcd), cyclic voltammetry(CV) and electrochemical impedance spectra(EIS). The main work is the following:1. As an electrode material, Ti O2 has high chemical stability, polyaniline(PANI)nanomaterials have lots of advantages, such as high conductivity and capacitance, low cost, etc. However, PANI shows an obvious volume change during the charge and discharge process, which has largely decreased its mechanical stability. In this chapter,by the combination of hydrothermal and in situ chemical polymerization methods, we obtained ordered vertical arrays of a Ti O2@PANI core–shell nanorods on FTO substrate. The electrochemical performance were investigated, the performance of nanocomposite Ti O2@PANI is improved obviously, and the cycle stability is also improved, the specific capacitance of the Ti O2@PANI nanocomposite is as high as820 F/g at a charge and discharge current density of 1 A/g, and the corresponding energy density and power density are 102.5 W h /kg and 4.62 k W/ kg, respectively.The speci?c capacitance retention of the nanocomposite is over 85% after 1000 charge and discharge cycles at a current density of 10 A/g, suggesting good cycling stability.2. Among the different transition metal oxides, manganese dioxide(Mn O2) and titanium dioxide(Ti O2) have been considered as potential pseudocapacitive materials,due to their low cost, low toxicity, etc. Mn O2 has high theoretical speci?c capacitance(1370 F/g); however, it suffers from low electrical conductivity. In contrast, Ti O2 has higher electrical conductivity and electrochemical stability compared with Mn O2. In this chapter, by the use of a simple hydrothermal method and calcination process, we fabricate novel hierarchical Ti O2@Mn O2 core/shell arrays composite on FTO substrate, where Ti O2 are the ‘‘core’’ and ultrathin Mn O2 nanoflakes the ‘‘shell’’ layer,by changing the concentration of KMn O4, the morphology and electric performance were also changed. The results showed that when the concentration of KMn O4 was0.09 M, the obtained composites have the best performance. The specific capacitance of nanocomposite is 34.79 m F/cm2 at the scan rate of 5 m V/s. The speci?c capacitance retention of the nanocomposite is over 91% after 1000 charge and discharge cycles at a current density of 0.2 m A/cm2, suggesting good cycling stability.3. Aligned Ti O2 nanorods have high chemical stability in alkaline electrolyte and can be easily fabricated by hydrothermal methods. Meanwhile, Ti O2 arrays structure having a high specific surface area, the short diffusion path, so that the electrolyte ions can easily diffuse into them. In this chapter, we prepared Ti O2@Ni O nanocomposite film by combination of hydrothermal method and chemical bath deposition(CBD), in which Ti O2 as the core, and Ni O nanoflakes as the shell. The excellent electrochemical properties was performed in 1M KOH electrolyte solution.4. For the same material, compared with compact 2D counterparts, 3D architectures take advantage of the higher surface/body ratios, larger surface areas,more active sites, and accordingly enlarged areal capacity. In this charper, with the combination of electrospinning, low temperature hydrothermal and chemical bath method, three-dimensional composites Ti O2/Zn O@Ni O were prepared. First through electrospinning technique for preparing titanium dioxide nanofibers, then by low temperature hydrothermal process for preparing a loose dendritic titanium dioxide and zinc oxide composites, and then by CBD method, nickel oxide nanoflakes were coated on the surface of the titanium oxide and zinc surfaces. In order to prove thesuperiority of the 3D material properties, we prepared a 2D Zn O@Ni O core-shell arrays, and the results showed that the three-dimensional material capacitance doubled.