Influence of the electrolyte film thickness on charge dynamics of ionic liquids in ionic electroactive devices

Developing advanced ionic electroactive devices such as ionic actuators and supercapacitors requires the understanding of ionic diffusion and drifting processes, which depend on the distances over which the ions travel, in these systems. The charge dynamics of [C ₄mim][PF ₆] ionic liquid films and Aquivion membranes with 40 wt % [C ₂mim][TfO] were investigated over a broad film thickness (d) range. It was found that the double layer charging time τ _DL follows the classic model τ _DL = λ _Dd/(2D) very well, where D is the diffusion coefficient and λ _D the Debye length. In the longer time regimes (t ≫ τ _DL) where diffusion dominates, the charge dynamics become voltage dependent. For low applied voltage, the later stage charge process seems to follow the d ² dependence. However, at high voltages (>0.5 V) in which significant device responses occur, the charging process does not show d ² dependence so that τ _diff = d ²/(4D), corresponding to the ion diffusion from the bulk region, was not observed.