Equivalent circuit modeling of ionomer and ionic polymer conductive network composite actuators containing ionic liquids

In this study, we demonstrate electrical equivalent circuits that model the complex frequency-dependent impedance of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMI-Tf) containing electro-active polymer membranes and ionic polymer conductor network composite (IPCNC) devices. The devices include Nafion membrane actuators, Nafion coated with layer-by-layer (LbL) Au nanoparticle/poly(allylamine hydrochloride) (PAH) composite actuators, and Nafion with vertically aligned carbon nanotube (VA-CNT)/Nafion composite actuators. It is found that the low frequency responses of these devices indicate Warburg diffusion. Therefore, Warburg impedance is utilized to model the low frequency diffusion behavior of the devices, while the electric double layer capacitance (Cdl) represents the storage of drifting ions under electric field at high frequencies. It is found that Cdl for Nafion with 40wt% EMI-Tf is 7.5μF/cm2 and increases to 11.4μF/cm2 with increasing surface area of the LbL composite electrode. Cdl increases further to above 3×103μF/cm2 for an actuator with 12μm VA-CNT/Nafion composite electrodes, while the Warburg coefficient AW remains nearly the same for all the devices. As a result, the actuation magnitude and speed increase with charges accumulated due to higher Cdl, without much increase in the contribution from the slow ion diffusion process.