Ion Transport and Storage in Ionic Electroacive Polymer Membranes

Open Access
Author:
Zhao, Ran
Graduate Program:
Electrical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 05, 2012
Committee Members:
  • Qiming Zhang, Thesis Advisor
  • Shizhuo Yin, Thesis Advisor
Keywords:
  • electroactive polymer actuator
  • nonlinear effect
  • ionic liquid
Abstract:
A great deal of research efforts have been denoted to ionic electroactive devices such as ionic electroactive polymer (i-EAP) actuators and supercapacitors. I-EAP actuators are attractive because relatively large electromechanical actuation can be generated under low voltage (a few volts). Hence they can be directly integrated with microelectronic controlling circuits, which have operation voltage of several volts, to perform complex actuation functions, and the low operation voltage also makes them safe to use. Thus they hold promise for a broad range of applications. Supercapacitors, because of their reasonable energy density, relatively high charge/discharge time and long cycle life, provide important energy storage devices besides batteries. The performance of these ionic electroactive polymer devices all combine an understanding of the ion and charge transfer processes that occur in the systems. For real devices, the structures can be very complicated. Yet from fundamental principle, the system can be abstracted to a simple one: metal-ionic conductor-metal sandwich structure. It provides a good study platform for ion transport and storage on ionic electroactive polymers. In general, the response of ionic devices to external applied voltage is slow. For example, major actuation of i-EAP actuators occurs at >0.1s seconds after the application of electric stimulus. In earlier studies[1], it is observed that there is substantial non-linear increase of charge accumulated for ionic liquid (IL) of 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate (EMI-TF) in Aquivion membrane. In this thesis, further investigations are developed to determine whether the effect is limited to this combination of IL/ionomers or a more general phenomenon. Polymer matrix and ionic liquid are always the two basic elements to build up the ionic devices. Along this train of thought, hydrophobic IL 1-Ethyl-3-methylimidazolium bis(rtrifluoromethylsulfonyl)imide (EMI-TFSI) is adopted, comparing with the original hydrophilic IL EMI-TF. On the other hand, P(VDF-CTFE) based polymer matrix group is characterized, which have a drastically different morphology with ionomer Aquivion previous study used. Both the charge response and bending actuation of membrane actuators from these polymer/ILs combinations are investigated, especially at longer time scale (>0.1s). The results show that the nonlinear response of the total charge storage in i-EAPs with applied voltage also occurs in these polymer/IL systems and suggest that the nonlinear charge response at long time scale is a general phenomenon for ionic devices. The time dependence of the bending actuation vs. the charge stored is also studied and the results reveal that charge stored at different periods lead to curvature to various degree.