Electrochemical oxidation and exfoliation of graphite

2017-02-08T03:33:55Z (GMT) by Zhiming Tian
Graphene, a monolayer of graphite, possesses a combination of many superlative properties, such as excellent electrical conductivity, great mechanical strength, and large specific surface area. These remarkable properties provide for widespread applications, ranging from structural materials to flexible electronics. To meet graphene’s increasing demands from both academia and industry, research has been focused on the issue of mass producing graphene at low cost. Among all the graphene production methods, the liquid phase exfoliation (LPE) route is the simplest route to produce defect-free graphene. However, the obtained graphene suffers from low yield and poor dispersibility, which limits its further applications. Moreover, the structure and properties of bulk LPE prepared graphene assemblies are rarely investigated, and it has not been established whether the outstanding nanoscale properties of LPE prepared graphene could be scaled to macroscopic level in bulk LPE prepared graphene. <br>    This thesis is arranged into two parts. In the first section, the structure of bulk LPE prepared graphene assembly was investigated, with particular attention paid to the evaluation of restacking behaviour when bulk LPE prepared graphene is assembled. In particular, the dynamic electrosorption analysis (DEA) approach was employed for this purpose. As graphene has high tendency to restack, due to the van der Waals force, many of the superlatives of graphene cannot be fully harnessed in bulk assemblies. It was found that the restacking of bulk LPE prepared graphene assembly is severe, which makes it difficult for LPE prepared graphene to retain the outstanding nanoscale properties in bulk form. It was also found that sonication on its own is insufficient to corrugate the resultant graphene sheets. Consequently, the van der Waals forces between the graphene sheets cannot be overcome when the LPE prepared graphene sheets were assembled. Therefore, in order to obtain corrugated graphene with high yield, a modification step has to be made to graphite before it is sonicated. <br>    In the second section of this thesis, electrochemical modification (electrochemical route) was chosen to increase the yield and corrugation of graphene. This method is environmentally friendly and easy to operate, and large quantities of graphene can be obtained within a short time. However, one of the major challenges associated with this route lies in the difficulty of applying a continuous current to the graphite electrode, which has previously limited the use of this approach for industrial applications. Two methods in this thesis were employed to overcome this limitation. In the first method, a novel mechanically-assisted electrochemical method was developed to produce graphene oxide using loose graphite flakes. The challenge of maintaining a continuous electrical connection between the graphite and current collector was solved with the help of centrifugal force. Notably, the electrochemically exfoliated graphene oxide contains 84 % of monolayer and bilayer graphene, and exhibits reasonable dispersibility in water with less physical defects than the chemically derived graphene oxide (CDGO). Due to the generation of functional groups and corrugation on the basal plane of the obtained graphene product, the restacking of its bulk form is largely overcome. <br>    The second method involves graphite pellets prepared by compressing the loose graphite flakes to act as the working electrode for the production of electrochemically derived graphite oxide (EGO). The above-mentioned challenges were overcome by the spatial confinement of graphite in the Tee-cell set-up. EGO with tuneable chemistry can be obtained by adjusting the acid concentration and reaction time. In particular, EGO contained fewer carboxyl groups and hydroxyl groups, and predominantly more epoxy groups than chemically derived graphite oxide (CGO). Most importantly, despite the high degree of oxidation, the amount of oxygen functional groups on EGO could be as high as 30 wt.%. However, the degree of oxidation did not proceed beyond the generation of carbonyl species. The controllable oxidation level of the EGO makes it an attractive precursor for many applications, such as electronics and nanocomposites. Additionally, by further understanding the mechanism of EGO formation, it was found that graphite can be directly converted into a porous carbon electrode for supercapacitor applications by electrochemical redox reaction. This method eliminates the complicated procedure of re-dispersion and re-assembly of the formed graphene sheets as required in CGO. It is expected that this direct electrochemical modification method will significantly reduce the cost for graphene-based supercapacitors.