Ion Distribution in Ionic Electroactive Polymer Actuators

Transcription

1 Ion Distribution in Ionic Electroactive Polymer Actuators Yang Liu* a, Caiyan Lu b, Stephen Twigg c, Jun-hong Lin d, Gokhan Hatipoglu a, Sheng Liu e, Nicholas Winograd b, Q. M. Zhang a,d a Department of Electrical Engineering; b Department of Chemistry, Pennsylvania State University, University Park, PA c Department of Electrical Engineering, Villanova University, Villanova, PA d Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802; e Strategic Polymer Sciences, Inc., State College, PA 1680 ABSTRACT Ionic electroactive polymer (i-eap) actuators with large strain and low operation voltage are extremely attractive for applications such as MEMS and smart materials and systems. In-depth understanding of the ion transport and storage under electrical stimulus is crucial for optimizing the actuator performance. In this study, we show the dominances of ion diffusion charge and we perform direct measurements of the steady state ion distribution in charged and frozen actuators by using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). High temperature actuators that consist Aquivion ionomer membrane and high melting temperature ionic liquid 1-butyl-2,-dimethylimidazolium chloride (BMMI-Cl]) served in this study. Electrical impedance, I-V characteristics, and potential step charging of the actuator are characterized at 25 C and 100 C. The conductivity of the actuator is 0.mS/cm at 100 C and 2.9μS/cm at 25 C, respectively. The electrochemical window of the device is V and a 2mm tip displacement is observed under 2.5V 0.2Hz at 100 C. A semi-quantitative depth profile of the relative ion concentration in charged and frozen actuators is measured by ToF-SIMS. The result shows that, unlike semiconductors, ions do not deplete from the electrodes with same signs. Due to a strong cluster effect between the ions, Cl - and BMMI + accumulate near both cathode and anode. Furthermore, the profile indicates that the ion size difference causes the BMMI + space charge layers (~6um) much thicker than those of Cl - (~0.5um). Keywords: Ion distribution, Ionic Liquids, Electoactive Devices * yul165@psu.edu 1. INTRODUCTION Figure 1. Schematic of an i-eap bending actuator under an electric signal. Solid-state electromechanical actuators (EMAs) with large strain, high elastic energy density, and low operation voltage are in great demand for applications such as MEMS, large-scale tunable antennas, smart materials and systems, Electroactive Polymer Actuators and Devices (EAPAD) 2011, edited by Yoseph Bar-Cohen, Federico Carpi, Proc. of SPIE Vol. 7976, 79762O 2011 SPIE CCC code: X/11/$18 doi: / Proc. of SPIE Vol O-1

2 morphing aircraft skin, and energy harvesting.[1-] Traditional inorganic EMAs such as the piezoceramic PZTs suffer from very low actuation strain (~ 0.1 %) and high operation voltage (>00 volts).[4] Among various EMAs, ionic electroactive polymers (i-eaps) are extremely attractive due to the advantages from both the ionic electroactive polymers and the ionic liquids electrolytes.[5-1] i-eaps allow the device to operate under only a few volts [10,11], while the negligible vapor pressure and high thermal stability of ionic liquid (ILs) enable these electroactive devices to operate at ambient atmosphere with long life cycles. ILs are salts in the liquid state which contain mobile cations and anions.[11-1] The large variety of cations and anions with different sizes, mobility and other properties makes it possible to tailor ILs to suit for the i-eap actuators. Qualitatively, the bending mechanism of a typical i-eap actuator with ionic liquids electrolyte is due to redistribution of ions near the electrodes. Ionic liquids (ILs) are organic salts in the liquid state. Under applied electrical field, the cations and anions drift and diffuse towards cathode and anode respectively, therefore swell the electrode regions and cause bending of the actuator. Although simulation groups modeled the molecular dynamics on the electrolyte/electrode interface from electrical signal, [14, 15] an in-depth experimental study of how ions transport and accumulate with ionomer matrix is absent for understanding and improving the devices. Figure 2. Schematic of the bending mechanism of an i-eap actuator. In this study, we show the dominances of diffusion charge in bending actuation and a direct measurement of the steady state ion distribution in actuators by using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) with C 60 Primary Ion Beam. ToF-SIMS allows etching the surface during the data collection, so the ion concentration change in the film thickness direction could be probed. [16] Since this technique requires ultra high vacuum (UHV) condition, high melting temperature BMMI-Cl (99 C) ionic liquid served in this study, so that at low temperature, the vapor pressure of the sample is negligible. Actuators was firstly fabricated and operated under DC bias above IL melting temperature. After actuation reaches steady state, the actuator was fast frozen with dry ice and sent for ToF-SIMS tests. 2.1 Sample preparation 2. EXPERIMENTAL METHODS Figure. (a) Device configuration and (b) molecular structure of BMMI-Cl and (c) EMI-Tf studied. Figure shows the schematic of the i-eap actuator and the chemical formula of the ionic liquids served in this study. Aquivion (EW790) ionomer membrane was purchased from Solvay Solexis. BMMI-Cl and EMI-Tf were purchased Proc. of SPIE Vol O-2

3 from Aldrich. All the materials were dried in vacuum at 80 C to remove moisture before processing. Aquivion membranes swollen with 40wt% EMI-Tf were prepared by soaking the Aquivion membranes in EMI-Tf at 60 C. Aquivion membranes swollen with 40-45wt% BMMI-Cl uptake were prepared by soaking the Aquivion membranes in BMMI-Cl : Ethanol 1:1 solution at 60 C. The films are then dried again in vacuum oven at 60 C to remove ethanol. 50nm thick of gold foils (L.A.Gold Leaf) were hot-pressed on both side of the membrane as electrodes. The uptake of ionic liquids within Aquivion membrane was calculated by measuring the weight gain after swelling. Density of the ionic liquid is calculated by weighing the ionic liquids and measuring the volume with a graduated cylinder. 2.2 Experiment setup Figure 4(a) is the ToF-SIMS setup used in this study and figure 4(b) is the typical schematic of a ToF-SIMS instrument. High energy C60 Primary ions are supplied by an ion gun and focused on to the target sample, which ionizes and sputters some atoms off the surface. These secondary ions are then collected by ion lenses and filtered according to atomic mass, then projected onto an MCP detector. ToF-SIMS allows collecting data while etching into the Aquivion samples. The etching rate in this study is 1.48±0.15nm/s, calculated from the etching depths and etching times. The etching depths are measured by KLA Tencor 16+ profilometer. Figure 4. (a) ToF-SIMSserved in this study. (b) Schematics of ToF-SIMS To collect the semi-quantitative ion concentration data, the actuator was firstly fabricated and operated under 2.5V bias at 100 C. After 5 minutes, when actuation reaches steady state, the actuator was fast frozen with dry ice and sent for ToF-SIMS tests. The ToF-SIMS used in this study assembled with a cooling stage. After the actuator sample is loaded into the SIMS, it was cooled by liquid nitrogen during the data collection process. The electrical measurement was carried out in a sealed box with desiccant inside to prevent the absorption of moisture and measured by a potentiostat (Princeton Applied Research 227). The electromechanical measurement was taken in a VWR hotplate and recorded by using a probe station (Cascade Microtech M150) equipped with a Leica microscope and a CCD camera (Pulnix 6740CL). The temperature was controlled by Tenney Enviromental Chamber Versa Tenn III.. EXPERIMENTAL RESULTS.1 Electrical and mechanical characterization of actuators with EMI-Tf To illustrate the importance of ion transport on actuation, a series of tests have been done on actuators with room temperature ionic liquids. [16-21] Shown in Figure 5(a) and (b) are the charging data based on actuators with EMI-Tf. It shows that there are two portions of current contribution to the total current measured, drift current and diffusion current. In Figure 5(a), the fast but large current is the drift current and the slowly decaying smaller current is due to the diffusion. From Figure 5(b), we could also observe a fast increase in charge within short time; however, when compared to the charges at later time, it is not substantial. Proc. of SPIE Vol O-

4 Figure 5. (a) Current versus time under different voltage. (b) Charge versus time under different voltage. [16] To further display the relation between ion diffusion and actuation, Figure 6(a) and (b) show the mechanical and electrical response of an actuator with EMI-Tf under 4V step voltage. When linking Figure 6 and Figure 5, it can be seen that the actuation is mainly caused by the slow diffusion ions and how these ions pack in the actuator dominates the actuation. Therefore, in the next section, steady state ion distribution is studied and analyzed, by utilizing high temperature ionic liquid BMMI-Cl. Figure 6. (a) Bending curvature of Nafion and Aquivion with 40wt% EMI-Tf versus time under 4V. (b) Charge of Nafion and Aquivion with 40wt% EMI-Tf versus time under 4V..2 Electrical and mechanical characterization of actuators with BMMI-Cl Figure 7. (a) Electrochemical window measurement by linear sweep voltammetry. (b) Actuation magnitude under 2.5V 0.2Hz at 100 C Proc. of SPIE Vol O-4

5 The electrochemical window of the device is characterized by linear sweep voltammetry. As shown in Figure 7 (a), the electrochemical window of the sample is about V. Applying a voltage above V will introduce oxidation and reduction of ionic liquids at two electrodes. Usually, for a voltage slightly larger than the electrochemical window, one cannot observe obvious change in the actuation and the reaction slowly affects the bending magnitude over thousands of cycles. However, with relatively high voltage, the reduction in actuator lifetime becomes visible that the actuator dies within a minute. Therefore, in this study, a voltage of 2.5V is applied to the device to avoid electrochemical reactions. Shown in Figure 7(b) is the image of actuation magnitude under 2.5V 0.2Hz at 100 C. Above the melting point of the high temperature electrolyte, the actuation magnitude of the actuator is comparable to the ones with room temperature ionic liquids operated at room temperature, such as EMI-Tf, which have been extensively studied by our group. [16-21] Figure 8. (a) Ionic DC conductivity test at 25 C and 100 C. (b) Charger versus time at 25 C and 100 C Figure 8 (a) shows that the conductivity of the actuator with BMMI-Cl at 25 C is about two orders of magnitude smaller than that at 100 C. It indicates that at room temperature, which is below the melting point of BMMI-Cl, the ions could barely move. Figure 6(b) illustrates that the charge under 2.5 V at 25 C is also about 2 orders of magnitude lower than the charge at 100 C. Therefore, by freezing the samples after applying electrical field, ions will not rapidly move back to equilibrium state, since the conductivity and charging (discharging) ability dramatically reduce with temperature. 4. DATA ANALYSIS 4.1 Space charge thickness estimation from electrical measurement Figure 9. Schematic of extra ion packing at steady state One could estimate the space charge thickness at steady state from the electrical measurement results, by assuming the ions are densely packed near the electrodes. From the charging data at 100 C, ion number per area could be calculated as below. Proc. of SPIE Vol O-5

6 Charge/Area Ion/Area = = C/mm (1) 9 mol/mm 2 For the pure ionic liquid, the volume of each ion pair could be calculated from the density of the ionic liquids and the molecular weight of the ionic liquids. ρ BMMICl Mw BMMICl (IonPair/Volume) Volume = g/mm = g/mol IonPair EMITf ρ = Mw BMMICl BMMICl 5 = mm /mol = mol/mm (2) Since the size of Cl is well known as 181pm, from the volume of ion pair and the volume of Cl, the volume of BMMI and the number of ions in per unit volume could be known. BMMI/volume Cl/volume = = mol/mm () 5 mol/mm With 45% of BMMICl swelled in Aquivion, the volume% of BMMICl is calculated from the density of BMMICl (0.716g/mm ) and Aquivion (1.98g/mm ), which is 55.4 vol%. Therefore, by taking the above information into consideration, one could estimate the space charge thickness for Cl and BMMI with the assumption that ions are densely packed near the electrodes, where the space charge layer of BMMI + is 17 times larger than that of Cl ToF SIMS Data Analysis t t Cl SpaceCharge BMMI SpaceCharge mol/mm = = 0.26um mol/mm *55.4% mol/mm = = 4.1um mol/mm *55.4% (4) Figure 10. (a) Cation concentration distribution normalized by its middle region concentration; (b) Anion concentration distribution normalized by its middle region concentration Proc. of SPIE Vol O-6

7 Under electrical field, the ions are accumulated near the electrodes, as shown in Figure 10 (a) and (b). Since the sample was charged for 5mins at 100 C, the sample condition is considered as steady state, therefore, the electric field is totally screened by the extra ions. Figure 10 (a) and (b) are the relative ion distribution of the cation and anion. The cation and anion concentrations are normalized by their concentrations from the middle screened region, respective. Figure 10 (a) and (b) indicate that near the two electrodes, both cation and anion concentration increase. The increases of cations in anode and anions in cathode show that a strong correlation exists between cations and anions. The ions form clusters, therefore when one type of ions accumulates; it drags certain amount of counter ions with it. Some former studies [17, 21] indicated some indirect proof of ion cluster effects, while our study directly prove the existence of either ionic cluster, or ion layering near the electrodes. Furthermore, the electric field applied to the sample is V/um and the electrical field between cation and anion is calculated as 106V/um from the diameter of cation and anion. It is understandable that the local electric field plays an important role on the ion transport and storage in i-eap actuators. Besides clustering effect, by comparing Figure 10 (a) and (b), one could also obtain information of the ion size effect. Near the surface, the concentration ratio C/C 0 of Cl - is much larger than that of BMMI + and it takes a much deeper etching to reach the plateau C 0 Ca region than to the plateau C 0 An region. It means that the Cl - is highly compacted near the surface while the BMMI + spreads into certain depth. From observation, BMMI + space charge layers (~6um) are much thicker than those of Cl - (~0.5um), which agrees with the prediction from the electrical measurement. 5. CONCLUSION In conclusion, we showed the slow diffusion ion dominate the actuation and applied the ToF-SIMS to reveal the steady state ion distribution pattern near the electrodes under electrical field. This study provided us detailed comparison of ion accumulation with and without field and at different electrodes. The results show 1) the actuation is generated by the space charge with micron thickness, not the nano-meter thick electrical double layer charges; 2) Cl - has a smaller space charge thickness due to its smaller size when compare to BMMI + (17 times larger in volume); ) Ions form clusters that under electrical field, ion concentration increase near all electrode regions. This material is based upon work supported in part by the U.S. Army Research Office under Grant No. W911NF Ionic Liquids in Electro-Active Devices (ILEAD) MURI and by NSF under the Grant No. CMMI REFERENCES [1] Polla, D. L. and Francis, L. F., Ferroelectric Thin Films in Microelectromechanical Systems Application, MRS Bulletin 21, (1996) [2] Martin, J. W., Redmond, J. M., Barney, P. S., Henson, T. D., Wehlburg, J. C. and Main, J. A., Distributed Sensing and Shape Control of Piezoelectric Bimorph Mirrors, J. Intell. Mater. Syst. Struct., 11, (2000) [] Liu, Y. M., Ren, K. L., Hofmann, H. F. and Zhang, Q. M., Investigation of Electrostrictive Polymer for Energy Harvesting, IEEE Trans. Ultrason. Ferroelect. Freq. Contr. 52, (2005). [4] Moulson, A.and Herbert, J., [Electroceramics], Chapman&Hall, London, UK (1990). [5] Lu, W., Fadeev, A.G., Qi, B.H., et al., "Use of ionic liquids for π-conjugated polymer electrochemical devices," Science 297(558), (2002) [6] McEwen, A.B., Ngo, H. L., LeCompte, K., Goldman, J. L., "Electrochemical properties of imidazolium salt electrolytes for electrochemical capacitor application," J. Electrochem. Soc. 146(5), (1999) [7] Ue, M., Takeda, M., Toriumi, A., Kominato, A., Hagiwara, R., Ito, Y., "Applicaiton of low-viscosity ionic liquid to the electrolyte of double-layer capacitors," J. Electrochem. Soc. 150(4), A499-A502 (200) [8] Galinski, M., Lewandowski, A., Stepniak, I., "Ionic liquids as electrolytes," Electrochimica Acta 51(26), (2006) Proc. of SPIE Vol O-7

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