The Assembly And Investigation Of High-performance Nickel Hydroxide-based Supercapacitors

With the rapid development of science and technology, new generation of green energy storage device play an increasingly important role in daily life. Compared with the lithium-ion batteries which have achieved market application, supercapacitor shows its great potential in the practical application because of its high power density, long cycle life, rapid charge-discharge property, wide operation temperature range and environment-friendly and so on. And among all the candidate materials, nickel hydroxide becomes an ideal electrode material for supercapacitors due to its high theoretical capacitance value and good electrochemical redox active. In this thesis, we chose the nickel hydroxide as the electrode material, studied the influence of some factors, such as substrate materials and calcination temperature, on its capacitive properties, and assembled it as a complete device. The obtained result provides data support for the further research of the nickel hydroxide based supercapacitor. And the main contents are as follows:(1) The influences of substrate materials on the performance of nickel hydroxide based electrode have been studied. Ni(OH)2 nanosheets/3D graphene(3DG) frameworks hybrid materials have been prepared by combining chemical vapor deposition(CVD) and hydrothermal method. The obtained Ni(OH)2 nanosheets/3DG structure brings the composite an electrochemical capacitance as high as 2061 F/g at a current density of 2 A/g, which can still maintain 1260 F/g at 30 A/g. Moreover, the composite electrode also exhibits a high specific capacitance of 2049 F/g at a scan rate of 5 mV/s. Compared to the Ni(OH)2 nanosheets/Ni foam and Ni(OH)2 nanosheets/carbon fiber cloth composite electrodes, the capacitance value of this composite electrode increased by 77% and 245% respectively. Such advantage is derived from the good electrical contact between the active material and 3DG, which avoids all drawbacks caused by the inert binder and increase the collection efficiency of electrons greatly. Meanwhile, the ultralight graphene reduces the total mass of the electrode largely, which effectively improves the practical value of the capacitors.(2) The influences of calcination temperature on the capacitive properties of nickel hydroxide-base electrode have been investigated. Although 3DG substrate exhibited excellent performance, the purely physical contact between Ni(OH)2 and graphene nanosheets led to poor cyclic stability because there are little surface defects existed on the surface of CVD graphene. Therefore, the nickel foam substrate has been chosen as the substrate material to study the effect of active materials on the capacitance performance of the electrode in the follow-up study. According to the literatures, nickel oxide has higher theoretical capacitance value than nickel hydroxide. Therefore, in order to further improve the capacitance performance and cycle stability of the electrode, we tried to convert the nickel hydroxide to nickel oxide by annealing treatment. Experimental results show that the nickel hydroxide would gradually turn into nickel oxide during the annealing process. And its crystallinity increased with the annealed temperature increased, which affect the overall capacitance properties. The sample annealed at 100°C in air(sample Ni-100) brings an electrochemical capacitance as high as 1437 F/g at a current density of 2 A/g(with the mass of active material is 1.48 mg/cm2), which is higher than that of other samples, especially sample Ni-300(539 F/g) and sample Ni-400(380 F/g). The increase of crystallinity maybe is the main factor that the supercapacitance performances of the samples decrease with the temperature increase. The sample Ni-100 also exhibits high specific capacitance of 1439 F/g at a scan rate of 2 mV/s and excellent cycling stability(86% capacitance retention after 2000 cycles at a scan rate of 10 A/g). This result enriches the data foundation for the design of high performance nickel based supercapacitors.(3) Assembly and performance testing of nickel hydroxide-based supercapacitor. Mn3O4 nanosheets/nickel foam composite materials have been manufactured by a simple and controllable electrochemical deposition method. This composite has excellent electrochemical performance as the negative electrode. Then, the asymmetric capacitor has been assembled with the Ni(OH)2 nanosheets/nickel foam composites and Mn3O4 nanosheets/nickel foam composite as the positive and negative electrode respectively. And the related test and characterization have been analysed. This assembled capacitor device has good charge storage performance and provides research foundation for the preparation of high-performance electronic energy storage device.