Thesis presented October 19, 2018
Abstract: This PhD thesis focuses on developing graphene-based materials for electrochemical energy storage. Graphene-based materials have been extensively studied for applications in supercapacitors due to their favorable material characteristics. However, much of this interest is focused on the bulk porosity of graphene derivatives with a surprisingly little attention given to their unique layered structures. In this thesis, the role and capacitance contributions of the inter-layer galleries were investigated in a model class of pillared graphene materials with varying inter-layer separation (d-spacing). Alkyl diamines with varying chain lengths were used to obtain cross-linked graphene layers with desired d-spacing (7-12 Å). A family of tetraalkylammonium tetrafluoroborate (TAABF4) electrolytes with varying cation (6.8 for tetraethyl to 9.5 Å for tetrahexyl) and constant anion (4.8 Å) sizes were used to investigate these materials properties. A comprehensive study with different d-spacing values and ion sizes has revealed that the ions with smaller naked sizes than the d-spacing access the inter- layer galleries whereas the larger ions are restricted. This direct correlation between naked ion size and the gallery spacing suggests partial desolvation of the electrolyte ions during adsorption. Electrochemical impedance analyses have supported these findings and have also indicated extremely impeded ion transport in the galleries. Finally, through optimization of diamine pillar density and porosity of the pillared materials, high volumetric capacitances (210 F/cm
3) in graphene materials with excellent power density (90 F/cm
3 at 1 V/s) were achieved. The robustness of the pillars has been confirmed indirectly by electrochemical cycling and X- ray diffraction investigation and directly through ss-NMR studies that provided evidence of the covalent link between the organic molecules and graphene.
Keywords: Supercapacitors, ion sorption, pillared graphene materials, Electrochemistry, Assembly, Functionalization, Graphene
On-line thesis.