Ion Distribution in Ionic Electroactive Polymer Actuators

Transcription

Ion Distribution in Ionic Electroactive Polymer Actuators
Yang Liu*a, Caiyan Lub, Stephen Twiggc, Jun-hong Lind, Gokhan Hatipoglua,
Sheng Liue, Nicholas Winogradb, Q. M. Zhanga,d
a
Department of Electrical Engineering; bDepartment of Chemistry, Pennsylvania
State University, University Park, PA 16802
c
Department of Electrical Engineering, Villanova University, Villanova, PA
19085
d
Department of Materials Science and Engineering, Pennsylvania State
University, University Park, PA 16802;
e
Strategic Polymer Sciences, Inc., State College, PA 16803
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-2,3-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 0.3mS/cm at 100°C and 2.9μS/cm at
25°C, respectively. The electrochemical window of the device is 3V 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
* [email protected]
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: 0277-786X/11/$18 · doi: 10.1117/12.880528
Proc. of SPIE Vol. 7976 79762O-1
Downloaded from SPIE Digital Library on 29 Nov 2011 to 128.118.169.161. Terms of Use: http://spiedl.org/terms
morphing aircraft skin, and energy harvesting.[1-3] Traditional inorganic EMAs such as the piezoceramic PZTs suffer
from very low actuation strain (~ 0.1 %) and high operation voltage (>300 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.[5-13] 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.[11-13] 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 C60
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. EXPERIMENTAL METHODS
2.1 Sample preparation
Figure 3. (a) Device configuration and (b) molecular structure of BMMI-Cl and (c) EMI-Tf studied.
Figure 3 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
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 2273). 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.
3. EXPERIMENTAL RESULTS
3.1 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.
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.
3.2 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
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 3V. Applying a voltage above 3V 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.
Charge/Area = 9.22 × 10 −4 C/mm 2
(1)
Ion/Area = 9.57 × 10 −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 = 0.716 × 10 −3 g/mm 3
Mw BMMICl = 188.70g/mol
(IonPair/Volume) EMITf =
ρ BMMICl
= 3.79 × 10 −6 mol/mm3
Mw BMMICl
(2)
VolumeIonPair = 2.64 × 10 5 mm 3 /mol
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 = 4.01×10 −6 mol/mm3
Cl/volume = 6.67 ×10 −5 mol/mm3
(3)
With 45% of BMMICl swelled in Aquivion, the volume% of BMMICl is calculated from the density of BMMICl
(0.716g/mm3) and Aquivion (1.98g/mm3), 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-.
9.57 × 10 −9 mol/mm 2
= 0.26um
6.67 × 10 −5 mol/mm3 * 55.4%
9.57 × 10 −9 mol/mm 2
=
= 4.31um
4.01×10 −6 mol/mm3 * 55.4%
Cl
t SpaceCharg
e =
BMMI
t SpaceCharg
e
4.2 ToF SIMS Data Analysis
Figure 10. (a) Cation concentration distribution normalized by its middle region concentration; (b) Anion
concentration distribution normalized by its middle region concentration
(4)
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 0.0926V/um and
the electrical field between cation and anion is calculated as 1036V/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/C0 of Cl- is much larger than that of BMMI+ and it takes a much deeper
etching to reach the plateau C0Ca region than to the plateau C0An 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); 3) 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-07-10452 Ionic Liquids in Electro-Active Devices (ILEAD) MURI and by NSF under the Grant No. CMMI 0709333.
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Ion Distribution in Ionic Electroactive Polymer Actuators

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