Control Synthesis And Capacitive Properties Of Ni - Based Supercapacitor Electrode Materials

Supercapacitor (also called as ultracapacitor, or electrochemical capacitor) as a promising electrochemical energy storage device has attracted much attention due to its long-term cycle stability, fast charge-discharge rate, high power density, and so on. The performance of supercapacitors largely depends on the electrode materials. Among various electrode materials, transition metal hydroxides and sulfides are attractive candidates for supercapacitors due to their low cost, high theoretical specific capacitance, excellent electrochemical redox activity, and environmental sustainability. In the present dissertation, nickel-based functional materials such as α-Ni(OH)2, Ni7Se and NiCo2S4 were applied as the electroactive materials for supercapacitors. The synthesis condition, morphology modification, and combination with other functional material such as graphene oxide (GO) and reduced graphene oxide (RGO) have been systematically studied to improve the electrochemical performance of the electrode materials. The main research work was carried out from five aspects as follows.1. The Anion Exchange Strategy Towards Mesoporous α-Ni(OH)2 Nanowires with Multinanocavities for High-Performance SupercapacitorsA facile and novel strategy was applied to synthesis of mesoporous a-Ni(OH)2 nanowires with multinanocavities based on the anion exchange reaction between SO42- anions and S2-anions. The mesoporous α-Ni(OH)2 nanowires with multinanocavities were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), energy dispersive spectrometry (EDS) mapping, scanning transmission electron microscopy (STEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and N2 adsorption-desorption isotherms. After anion exchange reaction, numerous nanocavities were produced because the spatial volume of SO42-anion is larger than that of S2- anion. And the pore diameter increases with the increasing Na2S concentration. When applied as electrode material for supercapacitors, the as-prepared mesoporous α-Ni(OH)2 nanowires with multinanocavities exhibited higher pseudocapacitances than that of pristine α-Ni(OH)2 nanowires. In addition, the specific capacitance increases with the size of nanocavities on the surface of a-Ni(OH)2 nanowires. The excellent capacitive performance and cycle stability are attributed to their suitable pores improving the interface contact between active sites and electrolytes, and providing spaces for volume expansion during continuous cycle processes.2. In-situ Controllable Growth of a-Ni(OH)2 with Different Morphologies on Reduced Graphene Oxide Sheets and Capacitive Performance for SupercapacitorsThe a-Ni(OH)2 nanowires (α-Ni(OH)2 NWs) and nanoparticles (α-Ni(OH)2 NPs) were grown on RGO sheets by adjusting the reduction degree of the substrates. The α-Ni(OH)2 NWs/RGO composite was achieved by using GO as the substrate. In contrast, the α-Ni(OH)2 NPs/RGO composite was obtained when RGO was applied as the support. The difference in morphology can be attributed to the fact that the hydrophilic function of GO is larger than that of RGO, and sp2 carbon atom concentration in α-Ni(OH)2 NPs/RGO is higher than that in α-Ni(OH)2 NWs/RGO. When α-Ni(OH)2 NWs/RGO and a-Ni(OH)2 NPs/RGO composites were used as electrode materials for supercapacitors, the α-Ni(OH)2 NPs/RGO composite exhibits superior electrochemical performance than α-Ni(OH)2 NWs/RGO, which is attributed to the fact that the specific surface area, conductivity, and sp2 carbon atom concentration of a-Ni(OH)2 NPs/RGO are larger than those of α-Ni(OH)2 NWs/RGO. The a-Ni(OH)2 NPs/RGO composite is a promising candidate as electrode material for high performance supercapacitor.3. Template-free Synthesis of Ni7S6 Hollow Spheres with Mesoporous Shells for High Performance SupercapacitorsNi7S6 hollow spheres with mesoporous shells were successfully synthesized by a novel and facile hydrothermal process without any template or surfactant using nickel chloride hexahydrate (NCH) and sodium thioglycolate (STC) as starting materials. Morphology and microstructure of the samples were examined by XRD, TEM, HRTEM, SAED, EDS mapping, and N2 adsorption-desorption isotherm. It was found that the composition and morphology strongly depend on the molar ratio of NCH/STC, reaction time, and reaction temperature in our system. A bubble template-based ripening process was proposed for the formation of Ni7S6 hollow spheres with mesoporous shells. When applied as electrode material for supercapacitors, the as-prepared Ni7S6 hollow spheres with mesoporous shells exhibited tremendous pseudocapacitance of 2329.5 F/g at 2 mV/s and 2283.2 F/g at 1 A/g, respectively. Capacity retention of 97.1% was achieved even after 1000 cycles. The maximum energy density is 50.7 Wh/kg at a current density of 1 A/g. The excellent capacitive performance is attributed to their unique hollow structure with mesoporous shells providing fast ion and electron transfer, and high electronic conduction. These results suggest the Ni7S6 hollow spheres with mesoporous shells are highly promising candidate materials for supercapacitor electrodes.4. O/W Interface Assisted Hydrothermal Synthesis of NiCo2S4 Hollow Spheres for High Performance SupercapacitorsThe NiCo2S4 hollow spheres were prepared by a O/W (CS2/H2O) interface assisted hydrothermal process using CoCl2·6H2O, NiCl2·6H2O, ethanediamine and CS2 as raw materials. UV-vis absorption spectrum, XRD, TEM, HRTEM, SAED, EDS mapping, N2 adsorption-desorption isotherm, and XPS were measured to characterize the morphology and microstructure of the prepared NiCo2S4 hollow spheres. CS2 droplets were not only used as sulfur source, but also as soft templates. The specific surface area of the NiCo2S4 hollow spheres is 19.2 m2/g. And the pore size distribution is in the range of 2-5 nm. When applied as the electrode material for supercapacitors, the NiCo2S4 hollow spheres show outstanding capacitive performances. The specific capacitance of the NiCo2S4 hollow spheres is 1753.2 F/g at a current density of 1 A/g.77.8% of capacity was retained when the current density increased from 1 A/g to 10 A/g. At a current density of 3 A/g, the specific capacitance of the NiCo2S4 hollow spheres electrode is about 1350.5 F/g after suffering 1000 continuous cycles. At a power density of 200 W/kg, the energy density is 39.0 Wh/kg. The as-prepared NiCo2S4 hollow spheres electrode exhibits high specific capacitance, rate capacity, energy density and good cycle stability, making it a promising electrode material for high performance supercapacitor.5. NiCo2S4 Nanoparticles Anchored on Reduced Graphene Oxide Sheets:In-situ Synthesis and Enhanced Capacitive PerformanceA facile hydrothermal process is developed for the synthesis of NiCo2S4/RGO hybrid, where thiosemicarbazide is chosen for both in-situ formation of NiCo2S4 nanoparticles and reduction of GO to RGO. The morphology and microstructure were characterized by XRD, TEM, HRTEM, SAED, Raman spectrum, and EDS mapping. NiCo2S4 nanoparticles with the diameter of about 20～30 nm were in-situ grown on RGO sheets. NiCo2S4 hollow spheres were obtained with the diameter of about 300～400 nm and the thickness of sheel in the range of 30-40 nm in the absence of GO. GO as a substrate material can offer abundant active sites for nucleation of NiCo2S4 and can be reduced to RGO, providing excellent electron transfer path and high conductivity, which enable the fast surface redox reaction. Supercapacitor based on NiCo2S4/RGO hybrid shows a high specific capacitance of 1804.7 F/g at a current density of 0.5 A/g, the maximum energy density of 40.1 Wh/kg at a power density of 100 W/kg as well as the highest power density of 4.0 kW/kg at an energy density of 27.1 Wh/kg. NiCo2S4/RGO hybrid delivers higher specific capacitance, energy and power densities than the bare NiCo2S4 hollow spheres. NiCo2S4/RGO hybrid can serve as a promising electrode material for supercapacitor.