Preparation Of Ldhs. Nanocomposites And Their Capacitance Performance Study

In the recent years, LDHs-based nanocomposites have been paid to wide attention in molecular sieves, selective catalysts, selective absorbents, and supercapacitors due to their particular structures and properties. Manganese oxides are considered as the most potentially useful material for supercapacitors due to their low cost, high electrochemical activity and environmental friendliness. And graphene materials are also conceived as a promising candidate for electrochemical double-layer capacitors because of its large surface area, low resistance and good electrical conductivity. Therefore, the nanocomposites based on Ni2+-Fe3+ layered double hydroxides and graphene (or manganese oxides) are hoped to be prepared. They are expected to have the synergistic properties of LDHs and graphene (or manganese oxides) and can be used as the cathode materials for supercapacitor. In the present thesis, a series of LDHs-based nanocomposites have been prepared by a hydrothermal treatment technology, an ion-exchange/reduction reaction, and an exfoliation/reassembling technology, and their capacitive properties have been also studied.The main research contents are as follows:The delaminated graphite oxide nanosheets are used as the precursor, a hybrid graphene/Ni2+-Fe3+ layered double hydroxide material has been fabricated by the one-step hydrothermal technology and the electrochemical propertie of the obtained material is investigated. Firstly, graphite oxide (GO) is prepared from natural graphite by a Hummers method. Sample GO is treated with an aqueous solution of tetramethylammonium hydroxide (TMAOH) for 7 days, it is delaminated into unilamellar nanosheets. Then the delaminated GO nanosheet suspension is poured into the mixture solution of Ni(NO3)2·6H2O, Fe(NO3)3·9H2O, urea and trisodium citrate, and the obtained mixture suspension is hydrothermally treated at 150℃for 48 h, the hybrid graphene/Ni2+-Fe3+ LDH material has been fabricated. Under the soft hydrothermal treatment condition, the Ni2+-Fe3+ layered double hydroxide platelets are homogeneously grown on the surface of GO nanosheets in company with a reduction process of the pristine GO to graphene. This simple approach to prepare the hybrid graphene/Ni2+-Fe3+ LDH material not only shows simple and low cost, but also shows environmental friendliness. This preparation approach will give a new research idea to fabricate graphene-based nanocomposites with novel structure and property.MnO2-pillared Ni2+-Fe3+ layered double hydroxides nanocomposite has been successfully fabricated using an ion exchange/reduction reaction, and the capacitive property of the obtained material is investigated. Firstly, the precursor, Ni2+-Fe3+-CO32- LDHs with good crystallinity and well-defined hexagonal shapes is synthesized by a complexing-agent-assisted homogeneous precipitation technology. Sample Ni2+-Fe3+-CO32- LDHs is converted to sample Ni2+-Fe3+-Cl--LDHs through an ion exchange reaction in NaCI-HCl salt-acid mixed solution, and then sample Ni2+-Fe3+-Cl--LDHs is converted into sample Ni2+-Fe3+-MnO4-LDHs by treating sample Ni2+-Fe3+-Cl--LDHs with a solution of KMnO4 for 1 day. Secondly, sample Ni2+-Fe3+-MnO4-LDHs is treated with a MnCl2 solution, MnO4- ions intercalated in the interlayer are reduced to MnO2 particles, and MnO2 pillared Ni2+-Fe3+ LDHs nanocomposite is obtained. Thirdly, MnO2-pillared Ni2+-Fe3+ LDHs nanocomposite is heat-treated at 200℃for 4 h, MnO2-Ni2+-Fe3+ LDHs (200) porous layered nanocomposite is obtained. Sample MnO2-Ni2+-Fe3+ LDHs (200) not only has a big specific surface area (202 m2/g), but also has an ideal capacitive behavior and good cycling property. MnO2-Ni2+-Fe3+ LDHs (200) porous layered nanocomposite shows a specific capacitance of 190 F g-1 in 1 mol/L solution of Na2SO4 at a scan rate of 5 mV s-1.MnO2/Ni2+-Fe3+ LDHs layered nanocomposite has been fabricated by using both layer-by-layer self-assembly method and flocculated technology, and the capacitive property of the obtained material is also investigated. Firstly, the precursor, Ni2+-Fe3+-CO32- LDHs with good crystallinity and well-defined hexagonal shapes is synthesized by a complexing-agent-assisted homogeneous precipitation technology. Sample Ni2+-Fe3+-CO32- LDHs is soaked in NaCl-HCl salt-acid mixed solution and a solution of NaClO4, respectively, samples Ni2+-Fe3+-Cl--LDHs and Ni2+-Fe3+-CIO4-LDHs are obtained by a ion exchange reaction. Sample Ni2+-Fe3+-CIO4-LDHs is soaked in formamide for 3 days, it is delaminated into Ni2+-Fe3+ LDHs nanosheet suspension. On the other hand, the Na-type layered manganese oxide is soaked in an HCl solution; it is converted into H-type layered manganese oxide. The H-type layered manganese oxide is treated with an aqueous solution of tetramethylammonium hydroxide (TMAOH) for 7 days, it is delaminated into unilamellar nanosheets. The delaminated Ni2+-Fe3+ LDHs nanosheet suspension and manganese oxide nanosheet suspension are used as precursors, (MnO2/Ni2+-Fe3+ LDHs)n multilayer film nanocomposite is fabricated by a layer-by-layer (LBL) self-assembly method, while the MnO2/Ni2+-Fe3+ LDHs flocculated nanocomposite is obtained when the delaminated Ni2+-Fe3+ LDHs nanosheets is slowly added into the manganese oxide nanosheet suspension and followed by stirring for 24 h. Electrochemical reserch indicates that the MnO2/Ni2+-Fe3+ LDHs flocculated nanocomposite shows good capacitive property, and the initial capacitance value is 104 F/g in 1 mol/L solution of Na2SO4 at a scan rate of 5mV s-1.