Impact of Ionic Liquids on the Exfoliation of Graphite Oxide

Abstract

The fabrication of high performance, graphene-based electrochemical energy storage devices, such as ultracapacitors, depends on the reduction of graphite oxide (GO) and its interaction with ionic liquids (Its), which may be used as the conductive electrolyte. To explore the physical and chemical interactions between Its and thermally reduced GO (TRG) as a function of annealing temperature, three ILs with ammonium based structures were selected to differentiate the role of their anions and cations in the exfoliation process. Intercalation was accompanied by either covalent or noncovalent bonding, as supported by thermogravimetric analysis (TGA) and infrared (IR) absorption spectroscopy performed in situ during thermal annealing and by X-ray diffraction (XRD) analysis. Upon IL intercalation, covalent bonding between the IL and TRG prevented exfoliation, while noncovalently physisorbed ILs were readily removed and therefore facilitated exfoliation of the reduced GO. The anion and cation moieties of the ILs in GO-IL intercalation compounds investigated were found to affect the decomposition temperature as well as the degree of thermal stabilization. Indeed, the ammonium-based cations bearing long alkyl carbon chains did not functionalize the TRG and therefore promoted both sheet expansion and thermal exfoliation. The solvent-dependency of these properties was also investigated by forming GO-IL intercalation compounds from both deionized (DI) water and propylene carbonate (PC). In contrast to DI water, PC was found to decrease the thermal decomposition temperature of GO by about 100 degrees C in the presence of intercalated ILs, thus enabling highly efficient oxygen removal in GO-IL intercalation compounds.