**3. Fabrication methods**

#### **3.1. IPMC**

The current IPMC preparation technique involves two distinct steps: initial pretreatment, impregnation–reduction (IR) and chemical deposition. In our lab, we improve the technique by combining impregnation electroplating (IEP) [19]. The detailed process is as follows:

w = 2R sin2

the measured displacement by the following Eq. (2).

in an increase, the maximum displacements are increased.

to explain the deformation behavior of IPMC as shown in **Figure 6**.

where *d* is the distance between the measuring point and the fixed point.

\_\_1

**Figure 4.** Schematic of the bending deformation.

( \_\_\_*l*

where *l* is the length of the free part of IPMC strip. The radius of curvature R is evaluated from

The currents and deformations under DC voltage were measured more than 20 s. The voltage range was set from 0.5 to 1.7 V with an interval of 0.3 V. For the electrochemical system composed of water, palladium, Nafion, and Na + cations, the electrolytic voltage of water is 1.75 V even higher [35]. So the voltages higher than 1.7 V were not employed in order to avoid the electrolysis process of water. To facilitate the analysis, the averages and errors of the peak currents and maximum deformations of the samples, testing three times for each sample, were extracted and recorded as shown in **Figure 5**. It can be observed that the peak currents increase with the applied voltage increasing (**Figure 5(a)**). Under the voltage of 2 V, the current response fluctuates in some degree due to the electrolysis of water. From **Figure 5(b)**, the maximum displacements of sample exhibit significant differences. With the applied voltages

To further investigate the relationship between the electrode morphologies, physical and electrical parameters and the electromechanical responses, an electrical component is introduced

2R), (1)

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45

Ionic Polymer Actuators: Principle, Fabrication and Applications

<sup>R</sup> <sup>=</sup> <sup>2</sup>δ/(*d*<sup>2</sup> <sup>+</sup> *<sup>δ</sup>*2) (2)

1) Nafion 117 was used as the interlayer roughened by sandblasting process. The diameter size of powders 200# is 0.0750 mm and the sandblasting time is 30 s. 2) Immerse the pre-treated Nafion in a 160 mL ammonia solution of [Pd (NH3)4]Cl2 with 140 mg Pd and 20 mL ammonia of 25% for 2 h with low-speed stirring. Then soak the pre-exchanged Nafion with the Pd complex cations in an alkaline solution of NaBH<sup>4</sup> (2–5%, PH > 13) under an ultrasonic environment at a continuous raising temperature (i.e. from 30 to 50°C). Repeat the first two steps for 3 times. 3) The pretreated Nafion membrane was soaked in Pd complex solution again for over 2 h and then placed in the apparatus to electroplate for over 30 s for both sides. Repeat the third step for 3 times. Immerse the IPMC in an aqueous solution of NaOH (0.1–0.5 mol/L) for 2 h.

#### **3.2. BGA**

A typical preparation method to fabricate BGAs is described below. The electrode film was obtained from SWCNT (HiPco–SWCNT, purified grade), PVDF-HFP (Kynar FlexⓇ2801), and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) as an ionic liquid. 20 wt% of SWCNT, 32 wt% of PVDF-HFP, and 48 wt% of EMIBF4 were dissolved into 9 mL of N, N-dimethylacetamide (DMAC) and stirred for more than 1 day at room temperature, then sonicated for 24 hours in an ultrasonic bath. A gelatinous black solution was obtained after sonication. Obtained gelatinous solution was cast into a Teflon mold (25 × 25 mm<sup>2</sup> ) and dried on a hotplate at 50°C for 12 hours and dried DMAC furthermore at 80°C in vacuo for 3 days. As a result, a black self-standing electrode film was obtained. The electrolyte film was obtained from similar procedure. 50 wt% of PVDF-HFP and 50 wt% of EMIBF4 were mixed into the solvent mixture of 4-methyl-2-pentanone and propylene carbonate anhydrous and cast into the mold. The solvents were dried on a hot-plate then an opaque self-standing gel electrolyte was obtained. One electrolyte film was sandwiched by two electrode films with a hot-pressing technique to obtain the three-layered BGA. Super-growth SWCNT is also good nanocarbon for the electrode of BGAs [34]. The more detail fabrication process is described elsewhere [25, 26].

#### **4. Electromechanical responses**

To evaluate the effect of parameters, the responses of IPMC, mainly including current and deformation, are measured in fully hydrated state. The performances of strip sample were tested for comparison. **Figure 4** shows the testing schematic. The strip sample with 40 mm in length and 5 mm in width is clamped by two copper disks. The displacement at the point 25 mm away from the fixed point is measured by a laser displacement sensor (Keyence, LK-G80). The applied voltage and current are simultaneously measured. The tip displacement w of samples can be calculated from the measured displacement *δ* by the following Eq. (1).

$$\mathbf{w} = \text{ZR}\sin^2\left(\frac{l}{2\mathcal{R}}\right),\tag{1}$$

where *l* is the length of the free part of IPMC strip. The radius of curvature R is evaluated from the measured displacement by the following Eq. (2).

$$\frac{1}{R} = 2\Im / \{d^2 + \delta^2\} \tag{2}$$

where *d* is the distance between the measuring point and the fixed point.

**3. Fabrication methods**

plex cations in an alkaline solution of NaBH<sup>4</sup>

**4. Electromechanical responses**

The current IPMC preparation technique involves two distinct steps: initial pretreatment, impregnation–reduction (IR) and chemical deposition. In our lab, we improve the technique by combining impregnation electroplating (IEP) [19]. The detailed process is as follows:

1) Nafion 117 was used as the interlayer roughened by sandblasting process. The diameter size of powders 200# is 0.0750 mm and the sandblasting time is 30 s. 2) Immerse the pre-treated Nafion in a 160 mL ammonia solution of [Pd (NH3)4]Cl2 with 140 mg Pd and 20 mL ammonia of 25% for 2 h with low-speed stirring. Then soak the pre-exchanged Nafion with the Pd com-

at a continuous raising temperature (i.e. from 30 to 50°C). Repeat the first two steps for 3 times. 3) The pretreated Nafion membrane was soaked in Pd complex solution again for over 2 h and then placed in the apparatus to electroplate for over 30 s for both sides. Repeat the third step

A typical preparation method to fabricate BGAs is described below. The electrode film was obtained from SWCNT (HiPco–SWCNT, purified grade), PVDF-HFP (Kynar FlexⓇ2801), and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) as an ionic liquid. 20 wt% of SWCNT, 32 wt% of PVDF-HFP, and 48 wt% of EMIBF4 were dissolved into 9 mL of N, N-dimethylacetamide (DMAC) and stirred for more than 1 day at room temperature, then sonicated for 24 hours in an ultrasonic bath. A gelatinous black solution was obtained after

on a hotplate at 50°C for 12 hours and dried DMAC furthermore at 80°C in vacuo for 3 days. As a result, a black self-standing electrode film was obtained. The electrolyte film was obtained from similar procedure. 50 wt% of PVDF-HFP and 50 wt% of EMIBF4 were mixed into the solvent mixture of 4-methyl-2-pentanone and propylene carbonate anhydrous and cast into the mold. The solvents were dried on a hot-plate then an opaque self-standing gel electrolyte was obtained. One electrolyte film was sandwiched by two electrode films with a hot-pressing technique to obtain the three-layered BGA. Super-growth SWCNT is also good nanocarbon for the electrode of BGAs [34]. The more detail fabrication process is described elsewhere [25, 26].

To evaluate the effect of parameters, the responses of IPMC, mainly including current and deformation, are measured in fully hydrated state. The performances of strip sample were tested for comparison. **Figure 4** shows the testing schematic. The strip sample with 40 mm in length and 5 mm in width is clamped by two copper disks. The displacement at the point 25 mm away from the fixed point is measured by a laser displacement sensor (Keyence, LK-G80). The applied voltage and current are simultaneously measured. The tip displacement w of samples can be calculated from the measured displacement *δ* by the following Eq. (1).

for 3 times. Immerse the IPMC in an aqueous solution of NaOH (0.1–0.5 mol/L) for 2 h.

sonication. Obtained gelatinous solution was cast into a Teflon mold (25 × 25 mm<sup>2</sup>

(2–5%, PH > 13) under an ultrasonic environment

) and dried

**3.1. IPMC**

44 Actuators

**3.2. BGA**

The currents and deformations under DC voltage were measured more than 20 s. The voltage range was set from 0.5 to 1.7 V with an interval of 0.3 V. For the electrochemical system composed of water, palladium, Nafion, and Na + cations, the electrolytic voltage of water is 1.75 V even higher [35]. So the voltages higher than 1.7 V were not employed in order to avoid the electrolysis process of water. To facilitate the analysis, the averages and errors of the peak currents and maximum deformations of the samples, testing three times for each sample, were extracted and recorded as shown in **Figure 5**. It can be observed that the peak currents increase with the applied voltage increasing (**Figure 5(a)**). Under the voltage of 2 V, the current response fluctuates in some degree due to the electrolysis of water. From **Figure 5(b)**, the maximum displacements of sample exhibit significant differences. With the applied voltages in an increase, the maximum displacements are increased.

To further investigate the relationship between the electrode morphologies, physical and electrical parameters and the electromechanical responses, an electrical component is introduced to explain the deformation behavior of IPMC as shown in **Figure 6**.

**Figure 4.** Schematic of the bending deformation.

**Figure 5.** (a). Peak current of IPMC strip under different voltage; (b) Maximum displacements of samples versus voltages.

**Figure 6.** Equivalent circuit of IPMC.

The interlayer of IPMC can be viewed as an ion conductive material and modeled by a capacitor and a resistor in parallel. The electrode can be modeled by two resistors. Then the peak current *i peak* (total current) can be figured out by Eq. (3). The first item of the Eq. (3) describes the steady-state current from the resistance components, while the second item reflects the transient current from the capacitive element. Eq. (4) shows the qualitative relations between deformation, peak current and bending stiffness based on experimental test.

$$\mathbf{i}\_{peak} = \frac{\mathbf{U}}{R\_{pl} + R\_m} + \frac{\varepsilon \,\mathbf{S}\_i}{4kd} \cdot \frac{d\mathbf{U}}{dt} \,\tag{3}$$

$$\mathbf{D} \sim \frac{i\_{\text{peak}}}{K\_{\text{effloss}}} \,\,\,\,\,\tag{4}$$

the condition of constant voltage. Eq. (3) and (4) can be employed to interpret the deformation behaviors of IPMC. From the perspective of the fabrication process, different fabrication process will exert an important effect on the surface resistances of the samples. Roughening increases the surface resistance while chemical plating can reduce it. Meanwhile, the decrease of surface resistance largely increases the bending stiffness. Although the bending stiffness does not contribute to peak current directly, it is also a key factor to affect the deformation of IPMC as shown in Eq. (3). So it is necessary to optimize the roughening process and chemical plating process. The impregnation-reduction process mainly forms a penetration electrode to increase the area of interface electrode. But it is difficult to further improve the interface due

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As we described before, the capacitive current is generated in BGAs during applying voltages. This means that the electrolyte (ionic liquid (IL)) plays a very important role for the actuation of BGAs. So, we studied the influence of ILs on the actuation mechanism of BGAs. We investigated the electrochemical and electromechanical properties of BGAs by using seven kinds of ILs [36]. The chemical structures of ILs are shown in **Table 1**. Some physicochemical properties, such as melting point (*T*mp), viscosity (ƞ) at 25° C, and electric conductivity (*κ*) are also summarized in **Table 1**, especially for imidazoliumtype ILs [37]. We measured the displacement of bending by a laser displacement meter at different frequencies of applied voltages in order to investigate actuation properties of BGAs. The three-layered BGA sample (normally, the actuator sample has a size of 1 (width) x 10 mm (length)) was clipped by two gold current collectors to apply voltages. Then, alternative voltages were applied to actuator sample. A Potentio/Galvanostat with a wave generator was used to apply voltages. The voltage, current, and displacement were simultaneously monitored by an oscilloscope. The detail experimental setup is

to the blocking effect of previous plated layer.

described elsewhere [25, 38].

**Table 1.** Chemical structure and physicochemical property of IL [37].

where *U, RPd, SI , D* and *Kstiffness* represent applied voltage, surface resistance, area of interface electrode, deformation and bending stiffness, respectively. Other parameters, such as *Rm,* ɛ*, π, k* and *d*, are considered to be constants.

It shows that the peak current depends on the electrode resistance, surface resistance, membrane resistance and the area of interface electrode closely related to dielectric modulus under the condition of constant voltage. Eq. (3) and (4) can be employed to interpret the deformation behaviors of IPMC. From the perspective of the fabrication process, different fabrication process will exert an important effect on the surface resistances of the samples. Roughening increases the surface resistance while chemical plating can reduce it. Meanwhile, the decrease of surface resistance largely increases the bending stiffness. Although the bending stiffness does not contribute to peak current directly, it is also a key factor to affect the deformation of IPMC as shown in Eq. (3). So it is necessary to optimize the roughening process and chemical plating process. The impregnation-reduction process mainly forms a penetration electrode to increase the area of interface electrode. But it is difficult to further improve the interface due to the blocking effect of previous plated layer.

As we described before, the capacitive current is generated in BGAs during applying voltages. This means that the electrolyte (ionic liquid (IL)) plays a very important role for the actuation of BGAs. So, we studied the influence of ILs on the actuation mechanism of BGAs. We investigated the electrochemical and electromechanical properties of BGAs by using seven kinds of ILs [36]. The chemical structures of ILs are shown in **Table 1**. Some physicochemical properties, such as melting point (*T*mp), viscosity (ƞ) at 25° C, and electric conductivity (*κ*) are also summarized in **Table 1**, especially for imidazoliumtype ILs [37]. We measured the displacement of bending by a laser displacement meter at different frequencies of applied voltages in order to investigate actuation properties of BGAs. The three-layered BGA sample (normally, the actuator sample has a size of 1 (width) x 10 mm (length)) was clipped by two gold current collectors to apply voltages. Then, alternative voltages were applied to actuator sample. A Potentio/Galvanostat with a wave generator was used to apply voltages. The voltage, current, and displacement were simultaneously monitored by an oscilloscope. The detail experimental setup is described elsewhere [25, 38].


**Table 1.** Chemical structure and physicochemical property of IL [37].

The interlayer of IPMC can be viewed as an ion conductive material and modeled by a capacitor and a resistor in parallel. The electrode can be modeled by two resistors. Then the peak

**Figure 5.** (a). Peak current of IPMC strip under different voltage; (b) Maximum displacements of samples versus voltages.

the steady-state current from the resistance components, while the second item reflects the transient current from the capacitive element. Eq. (4) shows the qualitative relations between

*RPd* + *Rm*

*peak* \_\_\_\_\_\_ *Kstiffness*

electrode, deformation and bending stiffness, respectively. Other parameters, such as *Rm,* ɛ*, π, k*

It shows that the peak current depends on the electrode resistance, surface resistance, membrane resistance and the area of interface electrode closely related to dielectric modulus under

deformation, peak current and bending stiffness based on experimental test.

<sup>i</sup>*peak* <sup>=</sup> \_\_\_\_\_\_ *<sup>U</sup>*

D~ *<sup>i</sup>*

and *d*, are considered to be constants.

*peak* (total current) can be figured out by Eq. (3). The first item of the Eq. (3) describes

<sup>+</sup> *<sup>ε</sup> <sup>S</sup>* \_\_\_\_1 <sup>4</sup>*kd* <sup>∙</sup> \_\_\_ *dU*

*, D* and *Kstiffness* represent applied voltage, surface resistance, area of interface

*dt* , (3)

, (4)

current *i*

46 Actuators

**Figure 6.** Equivalent circuit of IPMC.

where *U, RPd, SI*

The observed displacement (*δ*) was converted to the strain difference (*ε*) between the two electrodes of BGAs in order to normalize the size differences of BGAs by using the following equation on the assumption that there is no distortion of the cross-sections in the actuator during bending.

$$
\varepsilon = \text{'2D}\delta / (\text{L2} + \delta \text{2}) \tag{5}
$$

where *D* is the thickness of actuator sample and *L* is the free length of actuator sample from the fixed end of gold current collectors.

The frequency dependence of strain (*ε*) for BGAs including seven kinds of ILs is shown in **Figure 7**. The BGA with EMIBF4 as an electrolyte shows the best actuation (strain) in all the frequency range. The strain becomes worse with increasing the length of alkyl chain in the imidazoliumcation. This is the reason why the ionic conductivity decreases with increasing the length of alkyl chain related to increasing the viscosity. The frequency dependence of strain can be successfully fit to an electrochemical kinetic model (a double-layer charging kinetic model). In this model, the electrode of BGA is fully charged at low frequencies of applied voltage. However, there is not enough time to be fully charged for the electrode at higher frequencies. In other words, the stored charge (*Q*) decreases with increasing the frequencies of applied voltage. In this consideration, the strain (*ε*) is proportional to the stored charge (*Q*). A simple equivalent circuit model was used to make the electrochemical kinetic model as shown in **Figure 8**. The double-layer capacitance of each electrode (*C*1) is replaced by the capacitance (C); C = C1/2. *R* is resistance of ionic gel electrolyte. *R* is calculated from the ionic conductivity *κ* (=thickness/*R* × area). The stored charge *Q*( *f* ) at a frequency ( *f* Hz) is represented by the following equation:

$$Q\left(f\right)|\text{Q0} = 1 - 4\text{CR}\sharp[1 - \exp\{-1/4\text{CR}\sharp\}]\tag{6}$$

where *Q*0 is the stored charge at a limit of low frequency. And the strain (*ε* ( *f* )) is given by following equation:

$$
\varepsilon(f) = \varepsilon 0 \mathbf{Q}(f) / \mathbf{Q} \mathbf{0} \tag{7}
$$

densities of BGAs are of the same order of magnitude as those of natural muscles. Similar ionic polymer actuators with carbonaceous electrodes are introduced in a review paper

**Table 2.** Gravimetric capacitance of SWCNT(*C*SWCNT) and double-layer capacitance per electrode area (*C*) of BGA electrode, ionic conductivity (*κ*) and ionic resistance (*R*) of ionic gel electrolyte, strain difference at a limit of low

frequency (*ε*0) and time constant (*CR*) (reproduced with permission [36]).

**Figure 7.** Frequency dependence of strain difference (e) for BGAs with seven kinds of ILs (reproduced with permission [36]).

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by Asaka et al. [41].

**Figure 8.** Equivalent circuit model of BGA.

where *ε*0 is the strain at a limit of low frequency.

The obtained parameters such as the double layer capacitance of the electrode (*C*), the ionic conductivity of the ionic gel electrolyte (*κ*), the calculated resistance of the ionic electrolyte (*R*), the strain at a limit of low frequency (*ε*0), and the time constant (*CR*) are summarized in **Table 2**. Base on the equivalent circuit model analysis, it was found that the frequency dependence of actuation of BGAs is depended on the electrochemical time constant (*CR*) that is mainly related to the ionic conductivity. Furthermore, the actuation of BGAs is affected by the size difference between cation and anion included as an internal electrolyte because the volume changes of cathode and anode are caused by the sorption/desorption of cations and anions. More detail studies on the impedance analysis for the porous structure in the electrode of BGAs [39] and further studies on electrochemical energy and power density of BGAs [40] were reported by Randriamahazaka and Asaka et al. It was found that BGA behaves as supercapacitors and the electrochemical energy

Ionic Polymer Actuators: Principle, Fabrication and Applications http://dx.doi.org/10.5772/intechopen.75085 49

**Figure 7.** Frequency dependence of strain difference (e) for BGAs with seven kinds of ILs (reproduced with permission [36]).

**Figure 8.** Equivalent circuit model of BGA.

The observed displacement (*δ*) was converted to the strain difference (*ε*) between the two electrodes of BGAs in order to normalize the size differences of BGAs by using the following equation on the assumption that there is no distortion of the cross-sections in the actuator

*ε* = 2*D*/(*L*2 + *δ*2) (5)

where *D* is the thickness of actuator sample and *L* is the free length of actuator sample from

The frequency dependence of strain (*ε*) for BGAs including seven kinds of ILs is shown in **Figure 7**. The BGA with EMIBF4 as an electrolyte shows the best actuation (strain) in all the frequency range. The strain becomes worse with increasing the length of alkyl chain in the imidazoliumcation. This is the reason why the ionic conductivity decreases with increasing the length of alkyl chain related to increasing the viscosity. The frequency dependence of strain can be successfully fit to an electrochemical kinetic model (a double-layer charging kinetic model). In this model, the electrode of BGA is fully charged at low frequencies of applied voltage. However, there is not enough time to be fully charged for the electrode at higher frequencies. In other words, the stored charge (*Q*) decreases with increasing the frequencies of applied voltage. In this consideration, the strain (*ε*) is proportional to the stored charge (*Q*). A simple equivalent circuit model was used to make the electrochemical kinetic model as shown in **Figure 8**. The double-layer capacitance of each electrode (*C*1) is replaced by the capacitance (C); C = C1/2. *R* is resistance of ionic gel electrolyte. *R* is calculated from the ionic conductivity *κ* (=thickness/*R* × area). The

stored charge *Q*( *f* ) at a frequency ( *f* Hz) is represented by the following equation:

*Q* ( *f* )/*Q*0 = 1 − 4*CRf*(1 − exp(−1/4*CRf* )) (6)

where *Q*0 is the stored charge at a limit of low frequency. And the strain (*ε* ( *f* )) is given

ε( *f* ) = *ε*0*Q*( *f*)/*Q*0 (7)

The obtained parameters such as the double layer capacitance of the electrode (*C*), the ionic conductivity of the ionic gel electrolyte (*κ*), the calculated resistance of the ionic electrolyte (*R*), the strain at a limit of low frequency (*ε*0), and the time constant (*CR*) are summarized in **Table 2**. Base on the equivalent circuit model analysis, it was found that the frequency dependence of actuation of BGAs is depended on the electrochemical time constant (*CR*) that is mainly related to the ionic conductivity. Furthermore, the actuation of BGAs is affected by the size difference between cation and anion included as an internal electrolyte because the volume changes of cathode and anode are caused by the sorption/desorption of cations and anions. More detail studies on the impedance analysis for the porous structure in the electrode of BGAs [39] and further studies on electrochemical energy and power density of BGAs [40] were reported by Randriamahazaka and Asaka et al. It was found that BGA behaves as supercapacitors and the electrochemical energy

during bending.

48 Actuators

by following equation:

where *ε*0 is the strain at a limit of low frequency.

the fixed end of gold current collectors.


**Table 2.** Gravimetric capacitance of SWCNT(*C*SWCNT) and double-layer capacitance per electrode area (*C*) of BGA electrode, ionic conductivity (*κ*) and ionic resistance (*R*) of ionic gel electrolyte, strain difference at a limit of low frequency (*ε*0) and time constant (*CR*) (reproduced with permission [36]).

densities of BGAs are of the same order of magnitude as those of natural muscles. Similar ionic polymer actuators with carbonaceous electrodes are introduced in a review paper by Asaka et al. [41].
