Designing graphene supercapacitor electrodes
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Graphene, a two-dimensional material comprised of sp2-hybridised carbon atoms, has significant potential in energy storage as an electrode material for supercapacitors. Unfortunately, strong intermolecular forces between the graphene sheets results in aggregation during assembly and use, reducing the accessible surface area and experimentally available capacitance. Prevention of aggregation during electrode assembly and cycling will allow the development of graphene materials with better energy storage capability. In this thesis work, molecular spacers grafted to few-layer graphene (FLG) were investigated as a way of preventing aggregation of the graphene sheets.
Molecular spacers were grafted to FLG using three strategies: the spontaneous reaction with aryldiazonium salts, the Diels Alder reaction of an aryne, and the addition of an amine. The aryldiazonium reaction was studied using five different salts. The results indicated that at least two reaction pathways are operative for the spontaneous reaction, giving a multilayer film with both -C-C- and -N=N- linkages. Furthermore, the experimental protocol allowed the modified FLG to be collected with the film either sandwiched between the FLG and the substrate, or exposed to the electrolyte. In the sandwiched orientation two nitrophenyl reduction peaks were sometimes seen and larger surface concentrations were measured, behaviour that has not been reported previously for films grafted onto carbon materials from aryldiazonium salts. The Diels- Alder reaction, which relied on the generation of an aryne from an anthranilic acid precursor, provided an efficient route to monolayer growth. The amine addition reaction provided an alternative route modifying FLG, though a Michael-like addition or partial intercalation.
Supercapacitor electrodes were assembled from aryldiazonium modified FLG using a layer-by-layer (LBL) strategy. The grafted film could efficiently separate the FLG sheets during assembly and prevent restacking during cycling, with the full surface area remaining accessible even after 20,000 galvanostatic charge discharge cycles. Furthermore, the grafted film did not diminish the total capacitance of the FLG or hinder ion movement to the surface of the sheets. To further enhance the capacitance of the FLG, pseudocapacitive metal hydroxide films were electrochemically deposited on the FLG sheets prior to LBL assembly, which enhanced the total areal capacitance of the system.
This thesis work successfully developed a novel LBL protocol that allowed electrodes comprised of stacks of FLG to be assembled without diminishing the total accessible surface area and therefore capacitance of each graphene sheet, which is an essential step in the development of energy storage devices from graphene.