Analysis of non-uniform charge/discharge and rate effects in porous carbon capacitors containing sub-optimal electrolyte concentrations

The effects of internal electrolyte resistance on the acceptance and delivery of charge by a series of 3 V, non-aqueous, double-layer capacitors have been investigated by cyclic voltammetry and dc charging/discharging procedures conducted at different rates. The data obtained enable Ragone plots to be constructed for the performance of the capacitor modules. A series of four capacitor modules have been constructed using 2000 m2 g−1 carbon and containing tetraethylammonium tetrafluoroborate in dry propylene carbonate as electrolyte at four concentrations: 1.0, 0.4, 0.2 and 0.08 M. The lower concentrations are chosen to introduce electrolyte starvation deliberately and thus to be able to investigate the effects of such conditions on the discharge/recharge behaviour of the capacitor modules. In dilute solutions, electrolyte starvation can become enhanced on charge by redistribution of cations and anions of the electrolyte due to adsorption of the ions at respective negative and positive surfaces of the capacitor electrode matrices, from the electrolyte bulk. The magnitude of such effects depends on the adsorption charge capacity in relation to the total electrolyte ion concentrations available in the device and to the potential attained at the termination of charging.