Advanced Characterization of Pillared Graphene-Based Materials for Supercapacitors

Abstract

Materials for supercapacitors have to display large surface area to maximize the number of adsorbed ions [1], and they must also possess a hierarchical porosity (Fig. 1) to favour ions diffusion. Graphene-derived materials are studied as they –theoretically- combine all these porosity-related parameters. However experimentally the achieved capacitance (CSP) remains limited because graphene sheets tend to re-aggregate. Our group has selected a method to avoid this issue by creating an expanded graphene structure by bridging and separating the sheets using aliphatic diamine pillars. These assemblies are called Pillared Graphene materials (PGM). Electrochemical results showed that these PGM display higher capacitance than reference non pillared materials [2,3]. However, it was not possible to correlate these storage performances improvements to adsorption active surface area determined with N2 gas sorption (BET), which questions the suitability of this technique for measuring the full porosity of the samples, which is likely to be more accessible in an electrolytic environment. Therefore, to further correlate storage performances of PGM to their structure, structural advanced characterization techniques such as small angle X-ray and neutron scattering (SAXS/SANS) were employed. These methods enable to assess samples porosity at the local scale (~30 to 140nm) as well as the solid phase density. In the frame of this presentation, SAXS/SANS not only have been applied to pristine materials, but also to specifically formulated samples (pellet, electrodes) in order to highlight structural differences arising from synthesis protocol or from molecular pillaring. The ionic charge diffusion and adsorption in specific porosity ranges, analyzed using preliminary Operando scattering techniques, will be evoked. So far the interplay between increased adsorption sites and improved electrochemical performances has only been intuited on the basis of indirect electrochemical studies; the strategy devised herein proposes the use of direct structural techniques to confirm these interpretations.