High yields of graphene nanoplatelets by liquid phase exfoliation using graphene oxide as a stabilizer

Abstract

The exfoliation of graphene nanoplatelets (GNP) in aqueous media is challenging due to their restacking tendency. The sheets need to overcome the van der Waals attractive forces as well as the π−π interactions to form stable dispersions. The use of surfactants can resolve this issue but compromises the properties of GNP and therefore requires additional processing steps. In this work, we utilize graphene oxide as a dispersing agent in the liquid phase exfoliation of GNP from raw graphite. We first analyze the exfoliation process and determine that the relevant process parameters are graphene oxide and graphite concentration, graphite sheet size, dynamic viscosity of the dispersion, shear rate, and exfoliation time. Applying dimensional reasoning reduces the problem to four non-dimensional groups. A set of experiments, guided by a factorial design, is undertaken to analyze the exfoliated content using UV–Vis spectroscopy. We achieve a high GNP concentration of 8.6 mg ml−1, corresponding to a yield of 23 wt%, after a relatively-short exfoliation time of around 3 h. This is a considerably better performance compared to other liquid phase exfoliation processes. Experimental results are used to derive a functional correlation between the dimensional groups, which can be used for optimization. Additionally, we use exfoliated graphene dispersions to synthesize graphene hydrogels as electrode material for flexible aqueous supercapacitors. We measure a capacitance of 206 F g−1 and 143 F g−1 at a current density of 1 A g−1 and 10 A g −1, respectively. The supercapacitor retains 87% of the initial capacitance over 1000 charge and discharge cycles at a current density of 10 A g −1.