Thesis presented March 11, 2024

Abstract:
The working principle of a supercapacitor is based on the electro-adsorption of electrolytic species on the surface of polarized porous carbon electrode materials. Graphene is being studied as a material to try to achieve higher storage capacities (C_SP), because – in theory – it presents a large specific surface area maximizing the number of adsorbed ions, as well as a hierarchical porosity that could promote ions diffusion. However, experimentally, the CSP obtained remains limited because the graphene sheets reaggregate. One path followed to limit this problem is to place a “host” molecule between two sheets of graphene, leading to the production of pillared graphene (PGM). Previous work has indirectly (electrochemically) demonstrated that the structural parameter d, which corresponds to the inter-sheet distance, can be modulated to achieve higher C_SPs. This information is particularly important, and need to be further addressed by direct characterization of these samples.
Therefore during this PhD work, the local structure of these materials (graphene oxide GO, chemically or hydrothermally reduced graphene oxide rGO, pillared graphene PGM) was probed by carrying out WAXS analyses, which highlighted that PGMs exhibit pillared graphene domains that coexist with partially stacked graphene sheets. Studies of the mesostructure, carried out by SANS, revealed that the structure of GO evolves significantly during the functionalization and reduction steps. Indeed, GO presents a very large sheet size with rough interfaces, which transforms into sheets of smaller dimensions post-reduction, exhibiting 3D structure post-pillaring. These SANS analyses also showed the presence of characteristic structural features within the samples, such as the bending length R or the persistence length Σ. A porosity SANS study using the invariant demonstrated that the mesoporosity of an rGO is two times greater than that of a PGM. Despite this difference, PGM showed higher electrochemical performances, highlighting the importance of micropores in the electrochemical process.
It is important to track how this structure is impacted when these materials are subjected to rapid and repetitive charge and discharge cycles. For this, electrochemical tests as well as in-situ/operando characterizations were carried out in organic electrolyte comprising cations with a diameter smaller (TEA⁺) or larger (THA⁺) than the inter-graphene sheet distance. The WAXS study showed that the pillared sample is much more stable in polarization and cycling than rGO which suffers from significant structural evolution.
To go further in the study of the adsorption processes involved in these materials, a study of SANS in-situ, based on contrast modulations linked to relative modifications in the quantity of cations (containing H atoms) and anions (containing B atoms) was carried out. It was observed that within the bridged sample the THA⁺ ions are concentrated in the porosity range around 16 nm, indicating that these ions cannot access the smallest porosity. This blocking effect is not observed in the case of cycling in TEA BF₄, since the TEA⁺ cations are distributed within the entire porosity range. Thus, these studies showed that the meso-porosity of the bridged samples is active in the transport and storage mechanisms.
This thesis work allowed to deepen the understanding of the structure and porosity of GO-based materials, as well as to characterize the electro-diffusion and -adsorp​tion within the meso-porosity of the pillared samples. This work also constitutes a characterization methodology that could be applied to the study of other materials, thus contributing to advancing work on the optimization of supercapacitors.

Keywords:
Supercapacitor, pillared graphene, graphene oxide, storage capacities, electochemistry, functionalization, advanced characterization​​