MASc Oral Exam| Enhancing Graphene-Based Supercapacitor with Ionic Liquid/Surfactant Electrolyte by Structural Characterization and Modification by Tianyu Liu

You are welcome to attend Tianyu Liu MASc oral exam, where they will discuss their research in Enhancing Graphene-Based Supercapacitor with Ionic Liquid/Surfactant Electrolyte by Structural Characterization and Modification.

Abstract:

Supercapacitors are advanced energy storage devices designed to deliver frequent, short-duration energy bursts due to their distinct charge storage mechanisms. Significant research efforts have been directed toward enhancing their capacitance and energy density. Preliminary investigations have demonstrated promising outcomes through the incorporation of high-surface-area reduced graphene oxide (rGO) as the active electrode material, combined with ionic liquids (ILs) whose broad electrochemical stability window maximizes the energy density and non-ionic surfactant to bind them together while boosting the double layer capacitance. This mixture of surfactant and IL simultaneously facilitates as the electrolyte and a spacer material, preventing restacking of rGO sheets and maximizing the ion accessible surface area. However, the self-assembling process of this composite is not yet fully studied.

It is challenging to characterize the nano-scale structure of the composite specifically at the double-layer. Common characterization techniques at this resolution range, such as atomic force microscopy (AFM) and scanning electron microscopy (SEM), probe only the surface. Hence, this thesis investigates the self-assembled structures of GO, IL, and surfactant through small angle X-ray and neutron scattering (SAXS, SANS). These techniques provide bulk structural information on structures in the range of 1 – 100 nm, enabling precise determination of size, shape, and internal organization. The dispersion of each component and their mixtures was studied progressively, from simple to more complex systems, to understand their individual and collective behavior.

The surfactant was found to form stable spherical micelles with a radius of 5.9 nm when the concentration reaches 20 mg/mL. The addition of IL to the surfactant allows the formation of an amphipathic micelle by attaching to the hydrophilic tails of the surfactant while also lowering the minimum micelle formation concentration to 0.8 mg/mL due to the strong ion-dipole interaction. However, at higher concentrations, large aggregates were found in the dispersion. This is not favorable for the self-assembly of proposed lamellar structure with rGO in the composite electrode, as it would result in inhomogeneous distribution of electrolyte. The surfactant flattens GO sheets by attaching its hydrophobic segment, promoting the formation of ordered layered structures. A new model of the complete self-assembled structure is proposed based on contrast-varied SANS profiles. The GO is sandwiched by the hydrophobic layer, then the hydrophilic layer decorated by spherical IL droplets. When compressed to dense solid electrodes, these droplets spread and fill up the rGO interlayer spaces. If the electrolyte is insufficient, voids may form, reducing the ion-accessible surface area and leading to poor performance or long work-in cycles observed in previous studies.

In addition to the structural characterization, vertical ion bombardment to suppress the work-in cycles and improve rate performance was found to be selectively damaging the electrolyte rather than perforating rGO, resulting in deteriorating capacitance. However, the performance was able to partially recover from cycling. Since long-term electrochemical performance of the electrolyte along was found to negatively impacts its capacitive behavior, this recovery suggests the replenishing of fresh IL, added while assembling the device, driven by scanning potential.

Supervisor: Professor Pope