Solution Electrochemical Approach to Synthesis of Low Stage Graphite Bisulphate

Abstract

Since their discovery in 1841, Graphite Intercalation Compounds (GICs) have found tremendous applications in various industrial and research fields such as supercapacitors, superconductors, heterogeneous catalysts, anode materials, hydrogen and lithium-ion storage and notably in graphene synthesis. GICs are a type of graphitic compounds formed by the insertion of a different atomic, ionic or molecular species, known as intercalants between the layers of graphite host. Intercalation occurs in stages, and the phenomenon of staging is characterised by stage index n, which is the number of carbon layers present between two successive intercalate layers. The concentration of intercalant in graphite increases as the stage index decreases.Oxyacids readily intercalate in graphite. Researchers have studied intercalation of graphite in a wide variety of such acids such as H2SO4, HNO3, H2SeO4, HClO4 and H3PO4, and concluded that sulphuric acid is the best intercalant for graphite owing to higher efficiency and lesser reaction times. Intercalation in sulphuric acid gives Graphite Bisulphate (GB). Conventional GB chemical synthesis methods, pose an environmental and safety hazard due to the usage of hazardous and explosive oxidants. Moreover, chemical methods such as Hummers’ are primarily oxidation routes for the synthesis of Graphene Oxide (GO) wherein GB is believed to be the process intermediate, which is difficult to isolate.Electrochemical synthesis of GB is inexpensive, scalable and provides an easy defect-free transfer to the application substrate due to its ability to deliver the product in a suspension. Furthermore, the electrochemical approach provides an environment- friendly solution to the synthesis of GB because it replaces harsh chemical oxidants with an external polarisation potential. The quantity of electricity passed can also be controlled by changing the applied potential to obtain desired stages of GB.However, the synthesis of low stage GB by the electrochemical process in aqueous sulphuric acid solutions is an arduous task as a result of the excess water present in the system. Water molecules react with surface C atoms to form bulky surface groups which hinder the process of intercalation. Furthermore, undissociated water molecules may also co-intercalate with the bisulphate ions from aqueous sulphuric acid. Electrochemical oxidation of this intercalated water causes reduction of bisulphate ions and ultimately results in the vigorous evolution of oxygen and sulphur dioxide, which exfoliates graphite. Thus, GB formed before exfoliation is disintegrated and lost.On the other hand, water also facilitates intercalation by opening up graphitic grain boundaries for intercalation by carrying out a nucleophilic attack on the edge sites. Thus an optimal concentration of water in aqueous sulphuric acid along with an optimal positive polarisation potential is required for the synthesis and stability of GB. The present study aims at the electrochemical synthesis and characterisation of low stage GB and determination of optimal reaction conditions for the process. A pure stage GB synthesised and isolated this way can be particularly useful for the synthesis of layer-specific graphene flakes. Intercalation reduces the force of attraction between layers of graphite and hence makes selective cleaving possible by sonication. The stage index is strongly correlated with the number of graphene layers produced upon electrochemical exfoliation and is thus a key parameter to tune the electrochemical exfoliation of graphite.