Ion Transport and Storage in Ionic Polymer Bending Actuators
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Ion Transport and Storage in Ionic Polymer Bending Actuators Jun-Hong Lin1,, Yang Liu2,, Gokhan Hatipoglu2, and Qiming Zhang1,2 1 2
Department of Materials Science and Engineering, The Penn State University, PA16801, U.S.A. Department of Electrical Engineering, The Penn State University, PA16801, U.S.A.
ABSTRACT The actuation of ionic polymer actuators is mainly caused by the ion transport and excess ions storage in the membrane and electrodes. To quantify the charge transport behavior, a time domain method based on Poisson-Nernst-Planck equations was applied. The time domain transient current in response to a step voltage can provide insights on the charge transport and storage behaviors in the membranes. In this study, we investigate the charge transport behavior of Aquivion ionomer with different uptakes of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMI-Tf). A critical uptake and voltage independent of charge transport behavior were observed. The results also show that bending actuations of the Aquivion membrane with 40wt% EMI-Tf is much larger than that of Nafion, indicating that the shorter flexible side chain ionomer possesses a better electromechanical coupling between the excess ions and the membrane backbones, while not affect the actuation speed. INTRODUCTION Ion transport and storage in ionomer membranes are of great interest for electroactive polymer (EAP) devices, such as actuators, sensors, energy harvesting devices, and supercapacitors.[1-3] Ionic liquids (ILs), which are a class of salt in liquid form that containing both ions and neutral molecules, possess many interesting properties that make them very attractive to be employed as electrolytes in electroactive polymer devices.[4-6] For example, the vapor pressure of ionic liquids is negligibly low and as a result they will not evaporate out of the EAP devices when operated in ambient condition. It has been demonstrated that comparing to water the use of ILs as solvent for EAP actuators can dramatically increase the lifetime of transducer.[12,13] Their high mobility leads to the potential of fast response of EAP devices while the wide electro-chemical window allows for higher applied voltages.[8-14] This paper investigates charge dynamics of room temperature ionic liquid EMI-Tf in Aquivion® (Hyflon®) membranes swelled with different uptakes of ILs and the actuations of the membranes swelled with ILs above the critical uptake. EMI-Tf served in this study due to its comparably high conductivity (0.86 S/m), low viscosity (45 cP at 25oC) and larger electrochemical window(4.1V).[12] Hyflon® is known in the literature as short-side-chain ionomer (in comparison to Nafion® that is indicated as long-side-chain ionomer) and was originally developed by Dow Chemicals Company at the beginning of the ’80 but now named Aquivion® by Solvay Solexis.[17-20] When swelled with ILs the clusters expand. Above a certain IL uptake (critical uptake), these clusters will be connected with narrow channels forming percolation pathways for easy ion conduction, res
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