(712f) Preparation of Expandable Graphite with a Taylor-Couette Flow Reactor

Expandable graphene (EG), which consists of stacked, flexible graphene sheets capable of expanding upon heating, has proven to be an instrumental material for a wide range of electronic applications. The production of EG has been limited to batch processes using numerous intercalating agents such as halogens, alkali metals, sulfates, nitrates, organic acids, and metal oxides. Intercalation affects the physical and chemical properties of graphene, which results in graphite material comprising hybrid properties that are a function of the intercalating material. Intercalating agents can penetrate the interlayer spacing in between graphene sheets using ultrasound or microwave irradiation, which can result in exfoliating single- and few-layered graphene sheets upon exposure to heat. High temperatures cause the expansion and gasification of the intercalating agents resulting in sufficient pressure to break the van der Waals forces that hold the graphene sheets together.

In this study, we used a batch Taylor-Couette reactor setup and an aqueous solution to expand natural graphite flakes and produce EG. Conventional Taylor-Couette setups consist of two coaxial cylinders with the inner cylinder rotating while the outer cylinder is still, creating Toroidal, or Taylor, vortices that enhance the mass transfer of the reaction medium. The Taylor-Couette setup used in this study included the rotation of the outer cylinder while the inner cylinder is still. Laminar Couette flow structure and high shear rates were achieved via the rotation of the outer cylinder, which dampened vortex formation due to the absence of centrifugal forces acting on the reaction medium. To study the effect of residence time on graphite expansion, different residence times were employed and investigated.

Our results reveal that expanding graphene sheets could be achieved in an aqueous solution using a dispersant and a stabilizing agent. In addition, the level of expansion using an aqueous solution and a Taylor-Couette reactor is comparable to commercial EG synthesized by intercalating sulfuric acid, as revealed by x-ray diffraction. More importantly, the resultant EG flakes were more structurally homogeneous than commercial EG, and residence time exceeding three hours resulted in structural defects on the EG flakes. The resulting solution could be used for electrode fabrication in lithium-ion battery setups and graphene fiber applications.