Investigation of the Physics of Transduction in Nafion / Ionic Liquid Composite Membranes
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Investigation of the Physics of Transduction in Nafion / Ionic Liquid Composite Membranes Matthew Bennett, Barbar Akle, and Donald Leo Center for Intelligent Material Systems and Structures Virginia Tech, Department of Mechanical Engineering 310 Durham Hall Blacksburg, VA 24061, USA ABSTRACT Ionomeric polymer actuators based on Nafion membranes exhibit large bending motion (1%) under the application of small voltages (1-5 V). Actuation in these materials is believed to arise from the field-induced motion of mobile charges when a voltage is applied. In order for this charge motion to occur, the material must be swollen with a diluent, typically water. However, dehydration of the water limits the lifetime of these actuators in non-aqueous environments. Recently, highly stable ionic liquids have been demonstrated as viable diluents for these actuators [1]. In the current paper, the physics of transduction in these ionic liquidswollen Nafion membranes are investigated. Small-angle X-ray scattering reveals that the structure and properties of the ionic liquid have a strong influence on the morphology of the composites. Infrared spectroscopy is used to probe the ion associations within the films and shows that the ionic liquids are able to effectively mobilize the counterions of the Nafion membrane. Nuclear magnetic resonance spectroscopy is also used to investigate the composites and reveals that the mobility of the counterions increases as the content of ionic liquid within the membrane is increased. The results of these characterizations are compared to an experimental investigation of transduction in Nafion / ionic liquid composites to form an interpretation of the mechanisms of actuation. This comparison reveals that the counterions of the Nafion membrane are the primary charge carriers and that it is the motion of these mobile charges that gives rise to the actuation behavior of the films. INTRODUCTION / EXPERIMENTAL Water-swollen Nafion membranes have been used as ion conductors in fuel cells, chloralkali cells, and water electrolysis cells. Recently, these membranes have shown promise as electromechanical transducers [2,3]. However, the use of water as the diluent for these devices has limited their long-term stability in non-aqueous environments. Bennett and Leo have demonstrated the use of ionic liquids to replace water as the diluent for electromechanical transducers based on Nafion membranes [1]. The low vapor pressure of the ionic liquid allows these transducers to operate for over 250,000 cycles in air as compared to about 3,000 cycles for a similar water-swollen device. The high thermal stability of the ionic liquids also permits these devices to withstand high temperature processing not previously possible. For the current work, two different ionic liquids were studied: 1-ethyl-3-methylimidazolium trifluoromethanesulfonate and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide. These will be abbreviated as EMI-Tf and EMI-Im, respectively, throughout the remainder of this p
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