Synthesis of carbon-based nanomaterials and composites toward energy storage applications

Abstract

In this dissertation work, we mainly study the carbon-based nanomaterials and composites for energy storage applications, like anode materials for Li-ion materials and electrode materials for supercapacitors. Urchin-like MnO2/carbon nanofiber composites was prepared for anode materials of Li-ion battery, The carbon nanofibers (CNFs) are uniformly deposited on u-MnO2 to improve the electrical conductivity and to utilize the hierarchical architecture of u-MnO2. As the anode electrode of Li-ion batteries, the u-MnO2/CNFs nanocomposites exhibit an comparable cycle performance of 988 mAh·g−1 after 100 cycles with a good rate capability. The superior electrochemical performances of the u-MnO2/CNFs nanocomposites can be attributed to the hierarchical urchin-like structures and the superior electrical conductivity of the nanocomposites, which can facilitate fast electron and ion transport and accommodate a large volume change during charge/discharge. Then obtained q-MC//CC composite is employed as a binder-free electrode of supercapacitor. The g-C3N4 quantum dots could effectively enhance the interface electrical conductivity and ion transportation of MnCO3 electrode, which results in superior electrochemical performances. The q-MC//CC electrode delivers a high specific capacity of 1001 F·g-1 at a current density of 1 A·g-1 and a good cycling stability of 96% capacity retention after 5000 cycles. Moreover, asymmetric flexible supercapacitor (ASC) are assembled with q-MC//CC as a positive electrode and carbon cloth as a negative electrode, which exhibits a high energy density of 27.1 Wh·kg-1 at a power density of 500 W·kg-1. Also, the fabricated ASC device demonstrates to power the light-emitting diode effectively under mechanical bending. Also, as electrode materials of supercapacitors, GH-CN (graphene hydrogel-g-C3N4 quantum dots) composites were prepared by hydrothermal method. The results demonstrated that the addition of g-C3N4 QDs into graphene hydrogel improved the electrochemical performance distinctly, because the nitrogen-riched quantum dots embedded in graphene hydrogel which has high theoretically surface area, the quantum dots can provide more active sites for faradic reactions, then promote ion diffusion\transport capability at the electrode/electrolyte interface and enhance the faradic reaction and electron transfer, leading to the observed increased capacitance. The symmetric supercapacitor (SSC) was assembled using GH-2.5CN as positive electrode and negative electrode. The assembled SSC exhibited a high energy density of 22.5 Wh kg-1 at 250 W kg-1, and illuminated a red light-emitting diode (LED). It also exhibited excellent flexibility and reached 83.3% capacitance retention after 15000 cycles at 5 A g-1. These results demonstrate the potential GH-CN as a next-generation electrode material for energy storage system. The novel hierarchical N-GQDs/MnCO3/ZnMn2O4 (N/MC/ZM) composite was grown directly on Ni foam by simple hydrothermal method followed by calcinations treatment for applications as high rate electrodes in supercapacitors. The N/MC/ZM composites exhibited excellent electrochemical properties, including a high specific capacitance of 960.6 F g-1, as well as better cycling stability than MC/ZM (450.3 F g-1). The homogenously dispersed N-GQDs can increase the conductivity and activity sites. Results clearly indicate that the combination of N-GQDs and MC/ZM can constitute a promising binary metal-based electrode material with superior supercapacitive performance, which can provide an alternative to design the quantum dots decorated novel binary metal compounds materials with sample methods for a wide variety of energy storage devices such as supercapacitors, batteries.