Synthesis of Graphene Quantum Dots for Energy Applications

Abstract

Recently, with the increased demand in energy resources, great efforts have been devoted to developing advanced energy storage and conversion systems. The increasing consumption and the rapid depletion of fossil fuels has driven the major research focus to exploitation and utilization of renewable energy such as wind energy, tidal energy and solar energy for the past few decades. To provide widespread usage of renewable energies, efficient energy storage and conversion technologies are required. Recently, graphene and graphene-based materials have attracted great attention owing to their unique properties of high mechanical flexibility, large surface area, chemical stability, superior electric and thermal conductivities that render them great choices as alternative electrode materials for electrochemical energy storage systems, i.e., lithium batteries, supercapacitors, water splitting systems, and solar systems. The specific objectives of my thesis are as follow: (1) this dissertation is the results of an effort to develop the solar-to-hydrogen (STH) conversion efficiency using nitrogen doped graphene and graphene quantum sheets as a catalyst of cathode materials for hydrogen evolution reaction (HER) (2) graphene quantum dots with oxygen rich functional groups were incorporated into sulfur cathode materials of lithium sulfur battery to enhance the battery performance. The main results of my dissertation research can be summarized as follows. In Part Ⅰ, firstly, a simple one-step method to prepare nitrogen doped graphene quantum sheets (N-GQSs) was introduced by directly applying nitrogen plasma to as-grown graphene on Cu foil. Secondly, monolayer graphene (Gr) and nitrogen doped graphene (N-Gr) were applied as a catalyst on silicon wafer for HER, which strongly shifted toward the anodic direction without a change in the saturation current density as well as exhibits the strong potential as the passivation layer in neutral pH 7. Finally, N-doped graphene quantum sheets were decorated on an optimized silicon nanowires as a catalyst for the solar-driven HER, which can boost the catalytic activity toward the photoelectrochemical HER. The results showed that the N-GQSs electrodes exhibits higher applied bias photon-to-current efficiency (ABPE) than that of any other carbon-based photoelectrochemical HER catalysts reported to date. In part Ⅱ, to study the role of oxygen functional groups in improving the cyclability, graphene oxide wrapped on sulfur composites were synthesized and used as a cathode electrode in a lithium sulfur battery (Li-S battery). For Further study on structure and functional groups of cathode electrode, graphene quantum dots with oxygen functional groups is found to promote the structural integration of densely packed carbon black (CB) shells surrounding sulfur particles, which enables faster charge transfer and minimal loss of lithium polysulfides through the tightly packed sulfur-carbon morphology and the carbon-sulfur (C-S) bonding. Finally, the mechanism study of N-GQDs assisted Li-S battery that leads to the excellent cycling and rate performance were thoroughly investigated by detailed electrochemical analysis guided by theoretical modeling. This dissertation provides the details of my work on all projects related to synthesize and characterization of functionalized graphene and graphene quantum dots for energy system: hydrogen evolution reaction and lithium sulfur batteries. These contents are reported in Energy Environ. Sci. 8, 1329 (2015), Adv. Mater. 26, 3501 (2014), Energy Environ. Sci. 6, 3658 (2013), and Nanoscale (DOI:10.1039/c5nr01951f).