The Investigation of the Charge Transport Properties of Ionic Liquids in Response to Step Voltages in Ionic Polymer Actu
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The Investigation of the Charge Transport Properties of Ionic Liquids in Response to Step Voltages in Ionic Polymer Actuators Jun-Hong Lin Department of Mold and Die Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan, Republic of China. ABSTRACT Developing advanced ionic electroactive devices such as ionic actuators and supercapacitors requires the understanding of charge drifting and diffusion processes, which depends on the distances over which the ions travel. The charge dynamics of Aquivion membrane actuators with EMI-Tf ionic liquid are investigated over a broad film thickness (d) range. A time domain charge dynamic method based on Poisson-Nernst-Planck relation is employed to evaluate the charge transport behaviors in the actuators. It is found that for the initial charging process the double layer time τDL is linearly proportional to the film thickness (d). However, for the later charging process under a high applied voltage (>0.5V ) where the substantial electromechanical reaction occurs, the charge transport behavior does not follow the d2 dependence as predicted by the random walk diffusion model. For comparison the charge dynamics of BMI-PF6 ionic liquid films without polymer was also investigated. INTRODUCTION Ion transport and storage in electrolyte containing films are of great interest for ionic electroactive devices, such as actuators, sensors, energy harvesting devices, and supercapacitors [1-3]. In general, charge transport is a result of drift and diffusion, described by the ion mobility μ, diffusion coefficient D, and mobile ion concentration n, e.g. the Nernst-Planck equation ∂n ψ ± = ± μn± E − D ± where ψ+ and ψ- are the fluxes of positive and negative charges. The ∂x first term on the right hand side of the equation is the drift current and second term describes the diffusion current. μ and D are related through the Einstein equation, D = μkT / q , where k is Boltzmann’s constant, T is temperature, and q is the charge ions carry [4-6] . During charging, ions in the electrolyte moves towards electrodes of opposite polarity due to electric field between charged electrodes created by applied potential which leads to the screening of the voltage and potential drop at the two electrodes as illustrated in Fig. 1(a). For the metal-ionic conductormetal (MIM) system of figure 1(a) under a step voltage (from 0 at t < 0 to V volts at t > 0), the initial transient current follows the charging of electric double layer capacitors CD in series with a bulk resistor Rbulk (see figure 1(b)) [4-6], (1) I(t) = I0exp (-t/τDL) Where τDL= d λD/2D = RCD/2, describes the typical charging time for the electric double layer which has a thickness λD, the Debye length, λD = (εε 0 kT / Z 2e2 n)1 / 2 (2) where Ze=q is the mobile ion charge (Z=1 for the ionic liquids investigated in this paper) and e is the electron charge, ε is the relative dielectric permittivity, and ε0 is the vacuum permittivity (ε0= 8.854×10−12 Fm–1). I0==σ V S/d, where σ (=qnμ) is the conductivity, d is the electrolyte fi
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