Study On Pseudo-capacitive Properties Of Binary Metal Oxides And Their Composite Materials

Supercapacitor is a new type of energy storage equipments which can fill the gap between conventional dielectric capacitors and batteries, it characterized as high-power density, excellent cycle stability, and quick charge-discharge capability. It aroused considerable interest in applications in military field, electric vehicles, wind power generation, solar cells and electronic products. In the range of reported electrode materials for supercapacitor, metal oxides exhibit higher specific capacitance than carbon materials as well as better stability than conducting polymer materials, is one of the most important materials for next generation supercapacitor with high energy density. In this thesis, binary metal oxides and their composites are designed as novel electrode materials according to the reported pseudo-capacitive characteristics of NiO and CO3O4. The binary metal oxides are divided into AB2O4, ABO4, and A3B2O8type according to their molecular formula, and then the pseudo-capacitive properties of them and their composites are investigated.(1) NiO and CO3O4are synthesized by a sol-gel process and the pseudo capacitive properties of them are studied (Though the specific capacitance of NiO is high, its rate capability and cycle stability are inferior. However, CO3O4shows excellent rate capability and cycle stability, but its specific capacitance is much lower than that of NiO.). in order to combine the advantages of both NiO and CO3O4, the NiCo2O4(AB2O4type binary metal oxide) which based on Ni and Co is designed and synthesized. By controll the calcination temperature, NiCo2O4and NiO/NiCo2O4/Co3O4composite are obtained. The prepared NiCo2O4combines the high specific capacitance, excellent rate capability and good cycle stability, and the NiO/NiCo2O4/Co3O4composite also exhibits superior pseudo-capacitive properties than NiCo2O4. The results indicate that the design of binary metal oxides can combine the advantages of both metal oxides, and design of composites can imropve the pseudo-capacitive properties of the metal oxides. Meanwhile, MnCo2O4(another AB2O4type binary metal oxide) is synthesized by replacing Ni by Co in NiCO2O4, it shows lower specific capacitance and rate capability than NiCo2O4, indicating that the element located in A position has important effects on specific capacitance and rate capability of AB2O4type binary metal oxides.(2) To further study the contribution of the A and B elements to pseudo-capacitive properties of the binary metal oxides, the ABO4type binary metal oxides of NiMoO4, CoMoO4, NiWO4, and COWO4are designed and synthesized. By study the pseudo-capacitive properties of these binary metal oxides, we found that the element located in A position has important effects on specific capacitance, rate capability, and cycle stability of binary metal oxides. The pseudo-capacitive properties of these binary metal oxides are on the verge of metal oxides which located in A position. The change of the element located in B position has minor influence on pseudo-capacitive properties of binary metal oxides, indicating that the influence of element on pseudo-capacitive properties of binary metal oxides is lower than that of A element.(3) To confirm the decisive influence effect of the element located in A position to the pseudo-capacitive properties of binary metal oxide, and the effects of A and B ratios on pseudo-capacitive properties of binary metal oxide, the A3B2O8type binary metal oxides (Ni3V2O8and Co3V2O8) are designed and synthesized. The pseudo-capacitive properties of Ni3V2O8and CO3V2O8indicate the decisive effect of the element located in A position to the pseudo-capacitive properties of binary metal oxide, and the effects of A and B ratios on pseudo-capacitive properties of binary metal oxides. That means the pseudo-capacitive properties of binary metal oxides are mainlg determined by the A element, thus the specific capacitance increasing with at higher A/B ratios. At the same time, we also found that the Ni based binary metal oxides show higher specific capacitance but inferior rate capability and cycle stability (except NiCo2O4). Howerver, the Co based binary metal oxides exhibit excellent rate capability and cycle stability but lower specific capacitance.(4) To improve the pseudo-capacitive properties of binary metal oxides based electrode materials, binary metal oxides composites (CoMoO4/NiMoO4and Ni3V2O8/Co3V2O8nanocomposites) are designed according to the advantages/disadvantages of both Ni and Co based binary metal oxides. By controlling the mass ratios of Ni based binary metal oxides and Co based binary metal oxides in the composites, we found that the high specific capacitance of the composites is contributed by the Ni based binary metal oxides, and the excellent rate capability of the composites is contributed by the Co based binary metal oxides.Meanwhile, by tail the structure and sizes of Ni and Co based binary metal oxides in the composites, we found that nanosize is more useful to improve the pseudo-capacitive properties of binary metal oxides based composites, and the Ni based binary metal oxides with high specific capacitance are more suitable ro serve as skeleton structure of the composite, but the Co based binary metal oxides with excellent rate capability and cycle stability are suitable to serve as coating structure of the composite.