Download Response and Driving Mechanism of an EAP Actuator based on an

Response and Driving Mechanism of an EAP Actuator
based on an Ion-Gel Electrolyte
Satoru Imaizumi, Yuichi Kato, Hisashi Kokubo,
Masayoshi Watanabe
Department of Chemistry and Biotechnology
Yokohama National University
79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501,
Japan
Polymer gels swollen by ionic liquids, named “iongels”, are expected to be applied in several solid
electrochemical devices. As one of the applications, we
have studied electroactive polymer (EAP) actuators that
can change their shapes with the application of electronic
stimulus.
EAPs have attracted much attention as one of artificial
muscles, since their motion is very similar to that of
biological systems. Ionic EAP, driven by diffusion or
migration of ions, can exhibit relatively large bending
deformation by applying a low voltage. However, typical
ionic EAP actuators are required to maintain their wetness,
therefore, they have a problem in durability under
atmospheric condition due to evaporation of a solvent.
In contrast, an ion-gel actuator enables to operate in
an open atmosphere without evaporation of electrolyte
solutions, because of negligible volatility of ionic liquids.
A structure of the actuator and chemical structure of the
ionic liquid and polymer used in this study are shown in
Fig. 1. The actuator has a trilaminar structure, where an
ion-gel film was sandwiched between two flexible carbon
electrodes consisting of activated carbon and ion-gel.
observed after 500 s from the application of voltage, as
shown in Fig. 3. By shorting the circuit after 3000 s, the
actuator bends toward to the cathodic side and then turns
back gradually.
Based on these observations, we will propose two
driving mechanisms of the ion-gel actuator.
1. Electrostatic repulsive force between ions charged in
electric double layer.
The cathodic capacitance is larger than the anodic one,
determined by constant current charge-discharge
measurements using a three-electrode cell. This result
suggests that the charge density at the cathode is larger
than that of anodic side, because the electric double layer
at the cathode becomes thinner, and thus the Coulombic
repulsive force at the cathode becomes larger. As a result,
the actuator bends toward to the anodic side.
However, the back relaxation and the back actuation
by the short circuit can not be explained by this
electrostatic repulsive mechanism.
2. Asymmetrical volume change with ion transport.
In typical ionic liquids, the cationic transference
number is larger than 0.5. By applying voltage to the iongel actuator, the cathode and anode electrode layers are
swollen and shrunken, respectively, because of the
imbalance of the ion transport. After a long-time voltage
application, however, back-diffusion of the ionic liquid
may occur to offset the concentration gradient.
Consequently, the back relaxation takes place because of
the reduction in the swelling difference of both electrodes.
We have estimated the displacement of the actuator
considering the transference number.
Carbon electrode
Ion-gel electrolyte
0.0
-0.5
Voltage ON
CF3
N
N
C2H5
O O
S
O
N
CH3
CF3
S
O
[C2mim][NTf2]
( CH 2 CF 2 )
0.88
( CF 2 CF )0.12
CF 3
P(VDF/HFP)
2
0
-2
50
0
-50
0
20
Fig. 1 A structure of an ion-gel actuator.
By applying a voltage between two electrodes, the
actuator bends toward to the anodic side with charging the
electric double layers. In this study, the response to the
potential and the actuation mechanism are presented.
In the short time scale (less than 102 s), the
displacement of the actuator is increased by charging the
electric double layers (Fig. 2). As a result, the
displacement decreases by applying voltage with a higher
frequency. The resistance of the carbon electrode film
toward the tip of the actuator is larger than that of the iongel electrolyte toward thickness direction. Thus, the tip of
the actuator is hard to be charged because of a large IRdrop of the electrodes.
On the other hand, back relaxation behavior was
40
60
80
Time / s
n
Fig. 2
Current and displacement responses of an ion-gel
actuator with application of 0.5 V rectangular waveform
voltage at a cycle of 40 s.
Displacement / mm
Ionic liquid
Displacement / µm
Current / mA
Ionic liquid
Polymer
Polymer
0.5
Voltage / V
Ionic liquid
Carbon materials
Polymer
0.2
0.1
0.0
-0.1
0
2000
4000
6000
8000
Time / s
Fig. 3
Displacement responses of an ion-gel actuator with
application of 0.5 V step voltage for 3000 s, followed by the
short circuit.