Optimization of Aqueous Electric Double-Layer Capacitors

This thesis examines three approaches to optimize the performance of aqueous electric double layer capacitors (EDLCs). First, a number of approaches to determining the maximum working voltage (MVW) of aqueous EDLCs are tested. A reliable method for MWV determination is proposed: three-electrode cyclic voltammetry, complemented by galvanostatic charge/discharge cycles, resulting in a MWV of 1.2 V for a standard activated carbon EDLC with an aqueous 6 M KOH electrolyte. Next, the use of shear-exfoliated few-layer graphene (FLG) as a conductive additive for activated carbon (AC) electrodes is explored. A novel `vacuum infiltration' approach to incorporating the FLG is shown to increase the specific capacitance (Csp) by 50 %, reaching 142.3 0.1 F/g in 6 M KOH, without compromising equivalent series resistance (ESR) or stability compared with conventional carbon black additives. Finally the impact of surfactants, particularly Triton X-100, on EDLCs with an aqueous 6 M KOH electrolyte, is examined. Triton X-100 reduces the ESR but also lowers Csp by over 10 %. Soaking the AC electrodes in Triton X-100 increases the MWV to 1.4 V but substantially reduces Csp. When the AC was annealed prior to soaking in Triton, the MWV was increased further to 1.6 V, indicating that electrode surface chemistry can significantly alter the effect of surfactants. Unfortunately, the enhanced MWV again came at the cost of a reduced Csp, to the extent that the specific energy is lowered relative to untreated AC operating at 1.2 V. This research demonstrates that while significant gains in specific energy from increases to the MWV are difficult to realise, there is still scope to improve specific capacitance by developing new electrode materials.