Synthesis Of Three-dimensional Nanomaterials And Their Applications As Electrode Materials For High-performance Supercapacitors

Supercapacitor is an electrochemical energy storage device.According to the charge storage mechanism,supercapacitors can be divided into electrical double-layer capacitors and pseudocapacitors.To develop high-performance supercapacitors with a high energy density and high power density,design and engineering of new,effective electrode materials is the key.Towards this end,three-dimensional electrode materials represent a promising candidate which exhibit a high specific surface and multilevel pore structure for facile transport of electrolyte ions.The supercapacitor performance is also dependent on the electrical conductivity of the electrode materials.In this dissertation,various electrode materials were prepared and tested for supercapacitor applications,including porous carbon materials and transition metal compounds with a three-dimensional structure.Furthermore,the relationship between the performances of the prepared electrode materials and their structures and compositions was systematically studied.The research activities are summarized below:（1）Nitrogen-doped porous carbon was synthesized by controlled pyrolysis of surplus sludge,an abundant toxic by-product in microbiological treatment of wastewater,as manifested in scanning and transmission electron microscopic,X-ray diffraction,and Raman spectroscopic measurements.The resulting porous carbon exhibited a large surface area of940 m²/g and pore volume of 3.14 m³/g,as estimated by nitrogen adsorption/desorption isotherm studies.X-ray photoelectron spectroscopic measurements showed that nitrogen was embedded within the graphitic matrix forming pyridine-and pyrrole-like structures.With low-resistance electron-transfer pathways and short ion diffusion channels,the sheet-like porous N-doped carbon exhibited great potentials as a unique electrode material for energy storage.In fact,electrochemical studies showed that the N-doped porous carbon possessed a high specific capacitance of 247 F/g at 1 A/g in 1 mol/L H₂SO₄,and excellent cycle durability without an apparent decrease of the capacitance even after 10,000 cycles of charging and discharging.The strategy presented herein may be exploited for low-cost and large-scale production of N-doped porous carbon materials as electrodes for high-power supercapacitor application.（2）Ultrathin MoO₃ nanocrystals were assembled on 3D graphene oxide frameworks via a solvothermal reaction forming a layered structure by oxygen-bonding interactions at the interface.The structure and morphology of the resulting MoO₃-GAs hybrids were characterized by a range of experimental tools including atomic force microscopy,scanning electron microscopy,transmission electron microscopy,X-ray photoelectron spectroscopy,infrared and Raman spectroscopy.Because of abundant exposed active sites of the ultrathin MoO₃ and rapid ion diffusion and electron transport of 3D graphene frameworks,the resulting MoO₃-GAs hybrids possess the highest specific capacitances and excellent cycling stability in both aqueous（527 F/g at the current density of 1.0 A/g,100%retention after 10,000 cycles）and solid electrolytes（373 F/g at 1.0 A/g,100%retention after 5,000 cycles）among leading literature results of similar systems.（3）In this study,a general and effective phosphorization strategy is successfully demonstrated to enhance supercapacitor performance of various transition metals oxide or hydroxide,such as Ni（OH）₂,Co（OH）₂,MnO₂,and Fe₂O₃.For example,a 3D networked Ni₂P nanosheet array is grown on the surface of a Ni foam via a facile phosphorization reaction of Ni（OH）₂ nanosheets.The Ni foam-supported Ni₂P nanosheet（Ni₂P NS/NF）electrode shows a remarkable specific capacitance of 2141 F/g at a scan rate of 50 mV/s and remains as high as1109 F/g even at the current density of 83.3 A/g.The specific capacitance is much larger than those of Ni（OH）₂ NS/NF（747 F/g at 50 mV/s）.Furthermore,the electrode retains a high specific capacitance of 1437 F/g even after 5000 cycles at a current density of 10 A/g,in sharp contrast with only 403 F/g of Ni（OH）₂ NS/NF at the same current density.Similar enhancement is observed for Ni₂P powders,which eliminates the influence of nickel foam.The enhanced performances are attributed to the 3D porous nanosheets network,good conductivity,and two active components of Ni²⁺and P^δ-with rich valences of Ni₂P.Importantly,an asymmetrical supercapacitor based on the Ni₂P nanosheets/Ni foam and activated carbon achieves a specific capacitance of 96?F/g at the scan rate of 5?mV/s with a stable operational voltage of 1.4 V and a maximum energy density of 26 W h/kg,suggesting a great potential for practical applications.