Solid-state functionalization of graphene with amino acids toward water-dispersity: implications on a composite with polyaniline and its characteristics as a supercapacitor electrode material

A new and facile modification method to endow graphene with water-dispersity is reported. The remanent epoxide group on graphene prepared by the thermal reduction of graphite oxide is effectively utilized for the modification reaction with the amine group of potassium 6-aminocaproate using a simple process of heating the well-mixed solid state mixture. The graphene modified to have negatively charged carboxylate moieties disperses easily and stably in water. A graphene/polyaniline (PANI) composite is prepared efficiently in an aqueous system by the simple physical mixing of aqueous dispersions of negatively charged graphene and positively charged micron-size PANI particles, followed by drying. The composites exhibit higher energy storage capacities as supercapacitor electrodes compared to those of either graphene or PANI themselves. The synergistic effect is most evident at a graphene/PANI weight ratio of 20/80. The composite exhibits more than twice the specific capacitance of either PANI or graphene. This suggests that the scaffold structure of hydrophilic graphene enclosing PANI particles reduces not only the contact resistance at the electrode/electrolyte interface but also the diffusion length for effective charge transfer.