Graphene-Based Assemblies As Electrode Materials for Supercapacitors

Abstract

Reduced graphene oxide (RGO) based electrochemical double-layer capacitors (EDLC), have been extensively studied as good power capability can be achieved but they suffer from low capacitances (~100-150 F/g), as the reduced graphene sheets partially restack through π- π interactions limiting the resulting adsorption active surface area.[1] To avoid this restacking different paths have been followed in the literature: using graphene aerogels or expanded graphene structures.[2] In this latter case, an intercalate or pillar is used to space out the graphene layers and recover active surface area. Different nature of pillars or intercalates such as carbon nanotubes, carbon blacks, diamine, aromatic, metallic ions or big macrocyles have been described.[2,3] The results, that will be presented, depict the researches devoted to the development and study of graphene based expanded assemblies designed to be tested as electrode materials for supercapacitors. The material development involves the bridging of graphene oxide derived sheets using alkyl diamine as linkers (Fig. 1a).[3] The production of graphene galleries of height matching that of different alkyl chain length pillars was evidenced, notably with XRD (Fig. 1b). The purpose of this design was to enhance the specific capacitance by limiting graphene sheet restacking and optimizing the inter-sheet distance (d-spacing) or pore size with respect to the ion size, as was previously evidenced with other carbon materials.[4] These materials (named RPs) have been tested electrochemically with various tetraalkylammonium tetrafluoroborate salts in acetonitrile. This approach resulted in adsorption active surface area tuning, yielding clear evidence of the electrolytic ions entering the inter-layer galleries. Indeed only ions with diameter smaller than the d-spacing access the inter-layer galleries.[5a] Ex-situ solid-state NMR analysis were performed to further demonstrate the ionic species adsorption at the material surface. Further optimization dealing with pillar amount or nature and material density tuning showed the high interest of this re-aggregation limitation method, as gravimetric and volumetric capacitances 4 times higher than that of RGO (230 F/g and 210 F/cm3 respectively) have been achieved (Fig. 1c).[5b] Figure 1: a) Scheme differentiating reduced graphene oxide RGO and pillared graphene structures; b) XRD diffractograms obtained for pillared graphene prepared diamines with different chain lengths (5, 6 and 8 C atoms); c) Volumetric capacitance and power capability tests obtained on a pillared material compared to RGO.