Electrochemical Approaches for the Production of Functional Graphene and its Niche Applications

Abstract

Graphene has inspired the intrigue of researchers and industry for its potential to improve the performance of existing materials and create entirely new materials and devices. Although graphene has numerous proposed applications, it has not seen widespread adoption in the marketplace. This is partly due to the limitations of existing graphene synthesis routes, which can be costly, hazardous, low yield, or difficult to scale. Electrochemical approaches to graphene synthesis, however, may allow us to address these challenges. In this thesis, an electrochemical route to graphene is developed and its applications explored. Specifically, a packed bed electrochemical reactor capable of producing electrochemically-derived graphene oxide (EGO) from graphite is introduced. The developed method has several distinguishing features which make it promising for certain applications and larger-scale implementation. In contrast to most existing electrochemical approaches, the current method can use as its input natural flake graphite with no binder, compression, or extensive preprocessing. Low, constant current anodic charging in a dilute sulfuric acid electrolyte produces graphite oxide which can be readily dispersed in polar solvents to predominantly single- to few-layer EGO. The graphite electrode making up the packed bed can be scaled along all of its dimensions for larger scale implementations. The product can be thermally treated in air at 200 °C to increase its conductivity beyond what is possible with conventional, chemically-derived graphene oxide. Throughout the thesis, several key synthesis parameters are explored to improve our fundamental understanding of graphite oxidation and produce a variety of EGO products. It was found that using boron-doped diamond as the conductive interface between the graphite and power source dramatically improved the yield. The dispersibility and degree of oxidation could be increased by using expanded graphite as precursor. Poor electrolyte diffusion throughout the packed bed was overcome by implementing bulk solution diffusion channels inside the bed itself. A systematic study found several relationships between the electrolyte acid concentration and the product. Dilute sulfuric acids (less than or equal to 7.1 M) produced EGO with a less crystalline and less oxidised structure, relative to the more concentrated acid. It was found that 11.6 M sulfuric acid produced optimally oxidised graphene, while 7.1 M acid produced less oxidised, but more conductive material. Two different graphene applications were considered. The utility of EGO as a conductive nanofiller in lithium ion battery cathodes was demonstrated. A thorough investigation also explored EGO as a conductive nanofiller in flexible, wearable tactile sensors. Here, EGO can be readily mixed with aqueous surfactantwrapped polydimethylsiloxane (PDMS), 3D printed, then thermally deoxygenated in situ. The 3D printed sensors have exceptional feature resolution and performance. Ultimately, the current thesis represents a significant step forward for EGO synthesis and application. The experiments demonstrate the utility of electrochemical reactor engineering for producing new processes and unique types of graphene. This type of work will be critical for the eventual larger-scale production of electrochemically-derived graphene.