工學碩士學位 請求論文

Electrochemical Characterization of Polymer Actuator

고분자 구동체의 전기화학적 특성 연구

2002 年 2 月

仁荷大學校 大學院

化學工學科 (工業化學 專攻)

魯 台 根

工學碩士學位 請求論文

Electrochemical Characterization of Polymer Actuator

고분자 구동체의 전기화학적 특성 연구

2002 年 2 月

指 導 敎 授 卓 容 奭

이 論文을 碩士學位 論文으로 提出함

Electrochemical Characterization of Polymer Actuator

by

Tae-Geun Noh

A THESIS

Submitted to the faculty of

INHA UNIVERSITY

In partial fulfillment of the requirement

For the degree of

MASTER OF ENGINEERING

Department of Chemical Engineering

February 2002

이 論文을 魯台根의 工學碩士 學位論文으로 確定함

2002 年 2 月

主 審

副 審

委 員

Electrochemical Characterization of Polymer Actuator

by Tae-Geun Noh

Inha University

i

Table of Contents

초 록…………………………………………………………………..iii

ABSTRACT……………………………………………………………….vi

List of Figures…………………………………………………………….vii

List of Tables…………………………………………………………..…ix

1. INTRODUCTION……………………………………………….….….1

2. ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY(EIS)

…..……………….………………………………………………………....4

2.1. Introduction……………………………………..……………..4

2.2. Theoretical advantages………………………………..……….5

2.3. Equivalent circuit elements………………………………..…..7

2.4. Plot analysis……………………………………………..…….10

2.5. The Nyquist plot……………………………………..….…….14

2.6. The Bode plot………………………………………………….16

ii

3. EXPERIMENTAL……………………………………………………..21

4. RESULT AND DISCUSSION………………………….…….…..……26

4.1. Enlargememt of the composite electrodes surface area……….26

4.2. Effect of cations on IPMC…………………………………….39

5. CONCLUSIONS…………………………………………….……..…..44

6. REFERENCE…………………………………………………………..46

7. ACKNOWLEDGEMENTS…………………………………....………48

iii

초 록

최근에 기능성 고분자를 이용한 기술이 급속하게 발전하고 있으며,

이를 이용한 의학분야의 장비들이나 마이크로 로봇등과 같은 많은 분야에서

민감한 센서들이나 유동성있는 축소된 구동체들이 요구되고 있다. 이러한

요구들 때문에 고분자 젤 (Polymer Gel), 이온 고분자 금속 혼합물 (Ionic

Polymer Metal Composite - IPMC), 전도성 고분자 (Conducting Polymer)등

기능성 고분자를 이용한 여러가지 형태의 센서들이나 구동체들이 논의되고

있다.

IPMC 구동체는 다른 기능성 고분자들보다 낮은 전압에서 더 큰 변형을

가진다는 장점이 있다. 그러나 높은 전압에서 구동체 내부에 있는 물의 분해와

건조한 환경에서 물의 증발로 인한 IPMC 구동체의 변형이 감소되는 문제점을

가지고 있다.

IPMC 구동체의 변형은 Nafion 전해질 내에서 움직이는 양이온과 그

주변에 수화된 물분자들, 고분자 전해질과 전극 사이의 표면적 등 여러가지

인자들의 영향을 받는다. IPMC 구동체의 변형에 대해서 양이온들의 영향은

전기장의 작용보다는 수화작용이 더욱 중요한 역활을 하며, IPMC 구동체가

Li+이온을 가지고 있을 때 전압 (V)당 더 큰 변형과 힘 (force)을 갖는다는 연구

보고가 있다. 비록 Li+이온은 가장 작은 이온 크기를 갖지만, 수화된 Li+이온의

iv

크기는 Na+나 K+이온보다는 더욱 크기 때문에 막내의 양이온으로 Li+이온이

있을때 더 큰 변형을 일으킨다. 그러므로 변형 메카니즘은 음극표면의 과잉된

물분자의 양 때문이며, 구동체의 큰 변형을 얻기 위해서는 넓은 전극표면적이

필요로 하게 된다.

이 논문은 넓은 표면적을 갖는 향산된 병형 성능의 Nafion-금속

복합구동체를 만들기 위해 본을 뜨는 방법 (Replication Method)을 사용하였고,

전극과 전해질 사이의 면적 증가에 대한 구동체의 변형 효과를

전기화학적으로 관찰하였다. 또한 Nafion 내의 서로 다른 양이온들에 대한

구동체의 변형을 전기화학적 방법을 통해 해석하였다.

v

ABSTRACT

Actuating and electrochemical behaviors of Nafion-based electrode

were strongly dependent on interfacial area between the electrode and

polymer electrolyte, the hydrated sphere around moving cations inside

polymer electrolyte.

Replication method was utilized to manufacture a large surface-area

composite actuator. Etched aluminum foil was used as a template for

replication using liquid Nafion solution. Measurement of double layer

charging and scanning electron microscopy indicated that interfacial area

could be greatly increased by replication method. Larger interfacial area

induced a better bending performance of ionic polymer metal composite (The

magnitude of IPMC increased about 50% at 3V).

The surface resistance of ionic polymer metal composite (IMPC) and

the effect of cations on IPMC were investigated with constant current

experiment, linear sweep voltammetry and electrochemical impedance

spectroscopy (EIS). We knew that the ionic mobility is influenced by both

the ion size and ion-ion interactions.

vi

List of Figures

Figure 1. Equivalent electronic circuit for a simple electrochemical cell….13

Figure 2. Nyquist plot for a simple electrochemical system……………….19

Figure 3. Bode plot for a simple electrochemical system…………………..20

Figure 4. Schematic diagram of the replication method……………………24

Figure 5. Surface image of Nafion membrane manufactured by aluminium

etching replication technique……………………………………..25

Figure 6. SEM picture of IPMC prepared on different membrane surface.

(a) top view, (b) cross-section view of Pt electrode on plain Nafion

surface; (c) top view, (d) cross-section view of Pt electrode on

replicated Nafion surface…………………………………………29

Figure 7. Current transients when potential square waveform is applied.

Potential is initially stepped to 2 V and after 5 s of holding time,

reduced to –2V……………………………………………………30

Figure 8. Deformation of IPMC with the change of applied potential……..31

Figure 9. Cyclic voltammetry of plain Nafion (solid line) and replicated

Nafion (dotted line) with scan rate of 40 mV/s…………………..32

vii

Figure 10. Nyquist plot of (a) plain Nafion and (b) replicated Nafion. Dotted

line and full circle indicate simulated data and experimental

observation, espectively…………………………………………..33

Figure 11. Bode plot of (a) plain Nafion and (b) replicated Nafion. Dotted

line and full circle indicate simulated data and experimental

observation, respectively…………………………………………34

Figure 12. Equivalent circuit model of IPMC……………………………..35

Figure 13. Bode plot of impedance response of differently prepared

membrane (cation : H+)…………………………………………..37

Figure 14. SEM image of IPMC prepared on different membrane surface.

(a) and (b): Commercial plain Nafion surface (Dupont), (c)

and(d) :Home-made thin Nafion prepared by (University of New

Mexico)……………………………………………………………38

Figure 15. The effect of cations inside membrane on potential transient.

1 mA/cm2 of constant current is applied…………………………42

Figure 16. Bode plot of impedance response of different cations inside plain

Nafion…………………………………..…………………………43

viii

List of Tables

Table 1. Applications amenable to impedance studies………………………6

Table 2. Impedance equation for equivalent circuit elements……………….9

1

1. INTRODUCTION

In recent, there were demands for highly active sensors, flexible and

miniaturized actuators at many fields such as medical equipments and micro-

robots [1-5]. Because of these requirements, polymer gel, ionic polymer

metal composite (IPMC) and conducting polymer had been extensively

considered as various types of sensors and actuators.

IPMC had larger deformation at low cell voltage than other types of

polymers, but there are several problems to be investigated. Since the

deformation was caused by the excess amount of water near the cathode, the

electrolysis of solution at high voltage and the loss of solution in dry

environment decrease the deformation of IPMC. The deformation of IPMC

was affected by several factors such as the hydrated sphere around moving

cations inside Nafion electrolyte and the interfacial area between the

electrodes and polymer electrolyte [6-7].

Investigation of counter-cation effects on the performance of IPMC

actuation indicated that the hydration process is more important than

electrostatic field interaction and IPMC having lithium ion showed a greater

displacement and force dens ity per volt [7]. Although the lithium ion has the

2

smallest crystal radius, the effective radius of the charged particle is greater

than K+, Na+, etc., due to the solvation and the migration of hydrated lithium

ions under electric field induces larger deformation. Since the mechanism of

deformation is due to the water surplus on cathode surface, the enlargement

of electrode surface is required to obtain a large bending motion.

Oguro et al. used a chemical plating method to manufacture gold

electrode and a fractal- like structure was grown inside the membrane [8].

Their method increases the electrode surface area and the larger deformation

was followed. However, in platinum metallization of membranes with

chemical plating, the increase of electrode surface area was limited due to the

following reasons. In using H2PtCl6 as a source of platinum, Pt metallization

is achieved by

)HCOONa(2H4Cl6PtNaOH2HCHO226PtCl +++−+→++− (1.1)

with HCHO reducing agent. In case of Pt(NH3) 4Cl2, Pt electrode was formed

by

( )[ ] 3NH4Pte2243NHPt +→−++ (1.2)

3

−++++−→−+ e8O2H6Na2BOOH84NaBH (1.3)

with NaBH4 reducing agent [9]. In both cases, negatively-charged PtCl62-

(1.1) or OH- (1.3) is required respectively but they cannot be inserted inside

the negatively-charged membrane because of electrostatic repulsion. Pt

metallization reaction is only limited to the surface area and the enlargement

of surface area cannot be achieved. It demands a new electrode preparation

method to have a greater bending performance.

In this thesis, I shell intend to introduce the preparing method,

replication technique of Nafion-metal composite actuator with large surface

area and investigate the effect of the increase of electrode/electrolyte

interfacial area on deformation.

4

2. ELECTROCHEMICAL IMPEDANCE

SPECTROSCOPY (EIS)

2.1. Introduction

When used to study electrochemical system, electrochemical

impedance spectroscopy (EIS) can give us accurate, error- free kinetic and

mechanistic information using a variety of techniques and output formats.

For this reason, EIS is becoming a powerful tool in the study of corrosion,

semiconductors, batteries, electroplating, and electro-organic systhesis. Table

1 summarizes some of the electrochemical phenomena that have been studied

using EIS. In these areas, EIS offers three advantages over D.C. technique.

Small Amplitude : EIS techniques use very small excitation

amplitudes, often in the range of 5 to 10mV peak-to-peak. Excitation

waveforms of this amplitude cause only minimal perturbation of the

electrochemical test system, reducing errors caused by the measurement

technique.

Mechanism Study : Because electrochemical impedance experiments

provide data on both electrode capacitance and charge-transfer kinetics, EIS

technique can provide valuable mechanistic information.

5

Measurement Accuracy : Because the method does not involve a

potential scan, you can make measurements in low conductivity solutions

where D.C. techniques we can use EIS to determine the uncompensated

resistance of an electrochemical cell.

2.2. Theoretical advantages

The main advantage of EIS is that we can use a purely electronic

model to represent an electrochemical cell. An electrode interface undergoing

an electrochemical reaction is typically analogous to an electronic circuit

consisting of a specific combination of resistors and capacitors. We can take

advantage of this analogy by using establishes A.C. circuit theory to

characterise the electrochemical system in terms of its equivalent circuit.

In practice, we can correlate an impedance plot obtain for a given

electrochemical system with one or more equivalent circuits. We can use this

information to verify a mechanistic origin of the system, or at least to rule out

incorrect models. Once we choose a particular model, we can correlate

physical or chemical properties with circuit elements and extract numerical

values by filtering the data to the circuit model.

6

Table 1. Applications amenable to impedance studies.

7

2.3. Equivalent circuit elements

We can now look at impedance expression for some simple electrical

circuits. Table 2 shows that the impedance of a resister has no imaginary

component at all. The phase shift is zero degrees-that is, the current is in

phase with the voltage. Both current and impedance are independent of the

frequency.

Conversely, the impedance of a capacitor has no real component. Its

imaginary component is a function of both capacitor is always 90 degrees out

of phase with the voltage cross it, with current leading the voltage. Because

the impedance of a capacitor varies inversely with frequency, at high

frequencies a capacitor acts as a short circuit – it’s impedance tends toward

zero. At low frequencies (approaching D.C.) a capacitor acts as an open

circuit, and the impedance tends toward infinite.

The third simple electrical component is the inductor. Like a capacitor,

the current through an inductor is always 90 degrees out of phase with the

voltage drop across it. However, the phase shift is in the opposite direction –

the current lags behind the voltage. Also, as the frequency increases, the

impedance of an inductor increases. It acts as a short circuit at low

frequencies and as a large impedance at high frequencies.

8

To determine the total impedance of a combination of simple

elements, we combine the impedance values of the individual components

according to some simple rules. For two circuit elements in series, the

combined impedance is simply the vector sum of the individual impedance

values.

2Z1ZSZ += (2.1)

In complex number representation, the real parts must be added

together to form the real component of the series combination and the

imaginary parts must be added to form the imaginary component of the

combination.

)2Z1Z(j)2ZSZ(SZjSZ ″+″+′+′=″+′ (2.2)

For example, if we have a resister and capacitor in series, the

impedance expression will be the sum of the impedance of the resistor (which

has only a real part – the imaginary part is identically zero and the impedance

part – the real part is zero).

9

Table 2. Impedance equation for equivalent circuit elements.

R

R

C

L

C

10

For the more complex parallel resistor/capacitor network, the

impedance expression becomes more complicated. For circuit elements in

parallel, the admittance must be added together. Thus, for two impedance

values in parallel.

2Z

1

1Z1

PZ1 += (2.3)

2.4. Plot analysis

We can study an equivalent circuit by deriving its impedance equation.

However, it’s simpler to perform a measurement on the circuit and analyze

the resulting plot. We’ll get a good picture of the real and imaginary

impedance components and the phase shift characteristics as a function of

frequency.

The Randles cell (Figure 1) models the electrochemical impedance of

an interface and fits many chemical systems. We can easily equate the circuit

components in the Randles cell with familiar physical phenomena, such as

adsorption or film formation.

11

RΩ is the ohmic or uncompensated resistance of the solution between

the working and reference electrodes. RP is the polarization resistance or

charge-transfer resistance at the electrode/solution interface.

If we know the polarization or charging-transfer resistance, we can

calculate the electrochemical reaction rates. Double- layer capacitance

measurements can provide information on adsorption and desorption

phenomena. In some systems, a CDL measurement may not represent the

double layer capacitance. Rather, it may indicate the degree of film formation

or the integrity an organic coating.

The impedance of a capacitor diminishes as the frequency increases

(see Table 2), while the impedance of a resistor is constant. Thus, above a

certain frequency, the impedance of the capacitor, CDL becomes much

smaller than the impedance of the resistor, RP. Since CDL is in parallel with

RP, the capacitor acts as a short and effectively removes the resistor will also

become much smaller than RΩ. Thus, the high frequency behavior of the

Randles cell is controlled almost entirely by RΩ.

However, at the lowest frequencies, the capacitor acts as an open

circuit and is effectively removed from the circuit. The impedance of the

12

Randles cell is then the combined resistance values of the two series resistor

RΩ and RP.

13

Figure 1. Equivalent electronic circuit for a simple electrochemical cell.

14

Thus, at both the high and low frequency limits, the Randles cell

behaves primarily as a resistor. The imaginary component is very small, the

phase angle is close to zero degrees, and the impedance does not change with

frequency. At intermediate frequencies, the capacitor’s impedance begins to

have an effect and the cell becomes more capacitive. The imaginary

component becomes significant, the phase angle can start to approach 90

degrees, and the cell impedance becomes frequency dependent.

2.5. The Nyquist plot

Figure 2 shows one popular format for evaluation electrochemical

impedance data, the Nyquist plot. This format is also known as a Cole-Cole

plot or a complex impedance plane plot. In our study, we plotted the

imaginary impedance component (Z″) against the real impedance component

(Z′) at each excitation frequency. The plot in Figure 2 illustrates the expected

response of the simple circuit in Figure 1.

We saw that at high frequencies, the impedance of the Randles cell

was almost entirely created by the ohmic resistance, RΩ. The frequency

reaches its high limit at the leftmost end semicircle, where the semicircle

touches the x axis. At the low frequency limit, the Randles cell also

15

approximates a pure resistance, but now the value is (RΩ + RP). The

frequency reaches its low limit at the rightmost end of the semicircles.

The Nyquist plot has several advantages. The primary one is that the

plot format makes it easy to see the effects of the ohmic resistance. If we take

data at sufficiently high frequencies, it is easy to extrapolate the semicircle

toward the left, down to the x axis to read the ohmic resistance. The shape of

the curve (often a semicircle) does not change when the ohmic resistance

changes. Consequently, it is possible to compare the results of two separate

experiments which differ only in the position of the reference electrode.

Another advantage of this plot format is that it emphasizes circuit

components which are in series, such as the ohmic resistance.

The Nyquist plot format also has some disadvantages. For examples,

frequency does not appear explicitly. Secondly, although the ohmic

resistance and polarization resistance can be easily read directly from the

Nyquist plot, the electrode capacitance can be calculated only after the

frequency information is known. As shown in Figure 2, the frequency

corresponding to the top of the semicircles, ω (θ = MAX), can be used to

calculated the capacitance if RP is known.

16

Although the Nyquist format emphasize series circuit elements, if

high and low impedance network are in series, we will probably not see the

low impedance circuit, since the larger impedance controls plot scaling.

2.6. The Bode plot

Figure 3 shows a Bode plot for the same data pictured in the Nyquist

plot in Figure 2. The Bode plot format lets us examine the absolute

impedance, Z, and the phase shift, θ, of the impedance, each as a function

of frequency.

The Bode plot has some distinct advantage over the Nyquist plot.

Since frequency appears as one of the axes, it’s easy to understand from the

plot uses the impedance depends on the frequency. The plot uses the

logarithm of frequency to allow a very wide frequency range to be plotted on

one graph, but with each decade given equal weight. The Bode plot also

shows the magnitude (Z) on a log axis so that you can easily plot wide

impedance ranges on the same set of axes. This can be an advantage when

the impedance depends strongly on the frequency, as is the case with a

capacitor.

17

The logZ vs. logω curve can yield values of RP and RΩ. At the

highest frequencies shown in Figure 3, the ohmic resistance dominates the

impedance and log(RΩ) can be read from the high frequency horizontal

plateau. At the lowest frequencies, polarization resistance also contributes,

and log(RΩ + RP) can be read from the low frequency horizontal plateau. At

intermediate frequencies, this curve should be a straight line with a slope of –

1. Extrapolating this line to the logZ axis at ω = 1 (log ω = 0, f =0.16 Hz)

yield the value of CDL from the relationship.

DLC1

Z = (ω = 2 π f ) (2.4)

The Bode plot format also show the phase angle, θ. At the high and

low frequency limits, where the behavior of the Randles cell is resistor- like,

the phase angle is nearly zero. At intermediate frequencies, θ increases as the

imaginary component of the impedance increases.

The θ vs. logω plot yield a peak at ω (θ = Max), the frequency, in radians,

at which the phase shift of the response is maximum. The double- layer

capacitance, CDL, can be calculated from Equation (2.2)

18

)ÙR/PR1()PRDLC/1()MAXè(ù +== (2.5)

Note that both RP and RΩ appear in this equation. It is important to

remember that this frequency will not be the same as the frequency at which

the Nyquist plot reaches its maximum.

The Bode plot is a useful alternative to the Nyquist plot. It lets us

avoid the longer measurement times associated with low frequency RP

determinations. Furthermore, the logZ vs. logω plot sometimes allows a

more effective extrpolation of data from higher frequencies.

The Bode format is also desirable when data scatter prevents adequate

fitting of the Nyquist semicircle. In general, the Bode plot provides a clear

description of the electrochemical system’s frequency-dependent behavior

than does the Nyquist plot, in which frequency values are implicit rather than

explicit.

In some electrochemical process, there is more than one rate-

determining step. Each step represents a system impedance component and

contributes to the overall reaction rate constant. The electrochemical

impedance experiment can often distinguish among these steps and provide

information on their respective rates or relaxation times.

19

Figure 2. Nyquist plot for a simple electrochemical system.

20

Figure 3. Bode plot for a simple electrochemical system.

- 90°

0°

21

3. EXPERIMENTAL

IPMC was prepared with two different methods. Large surface

electrode was manufactured with a replication technique. As a template,

electrochemically etched aluminum foil (4N high-purity, TOYO) was used.

When Al is etched in acid solution with applying direct current, high density

of etch tunnels are formed and the surface area is significantly increased.

Figure 4 shows the schematic diagram of the replication method. Assembled

cells are pressed with 2000 kg/m2 at 75 and then, etched Al was dissolved

in a hot chloride solution. Surface of membrane has a replicated image of

etch structure, as shown in Figure 5. Pt electrode was prepared with Pt(NH3)

4Cl2 with NaBH4 reducing agent and the morphology was investigated with

scanning electron microscopy (Hitachi S-4200).

Prior to experiments, IPMC was immersed in different solutions

(0.1M HCl, LiCl, NaCl, KCl, MgCl2 and CaCl2) for a couple of days to

exchange cations. A. Lehmani et al. calculated the cation concentration

inside membrane based on Donnam equation [10].

22

( ) 2

12

2

42

⋅++= ±+ eC

XXC γ (3.1)

+C : cation concentration

X : fixed-ion concentration

±γ : mean activity coefficient of salt in water

eC : external salt concentration

Donnan’s equation is in an agreement with experimental results at

low external concentrations. At low concentration such as 0.1M NaCl, γ± =

1.1 and the C+ is the same as the X, which suggests that, in bracket of

equation (1), X2 / 4 is much greater than (γ± Ce)2. In interpreting the

experimental results, the value of γ± does not affect the equilibrium

concentration of cations inside membrane and, therefore, ion size effect can

be discussed based on the equilibrium concentration.

Applied potential or current was modulated with galvanostat

/potentiostat (EG&G, PAR 273A) and electrochemical behaviors of IPMC

were interpreted in air. Counter electrode was short-circuited with the

reference electrode. Deformation of IPMC was measured with a laser

23

displacement sensor. Impedance spectra were recorded by using a frequency

response analyzer (Zahner IM6). The amplitude of the superimposed

sinusoidal potential signal was 10 mV. The frequency range is from 200 mHz

to 100 kHz.

24

Figure 4. Schematic diagram of the replication method.

25

Figure 5. Surface image of Nafion membrane manufactured by

aluminium etching replication technique.

26

4. RESULTS AND DISCUSSION

4.1. Enlargement of the composite electrodes surface area

Both sides of Nafion electrolyte were modified with the replication

technique using an aluminum etched foil which has about 107 number of etch

tunnels per cm2 (1~2 width and 30 length). After replication,

membrane surfaces were composed of a flat area and rectangular shaped

regions, as shown in Figure 5 and its structure result in the increase of surface

area. Figure 6 shows the morphological variation of IPMC with different

electrode preparation methods. Pt metal is chemically plated with an

electroless deposition on flat Nafion surface and SEM image of Figure 6(a)

and 6(b) indicates that thin layer Pt is uniformly deposited. On the other hand,

Figure 6(c) and (d) prepared on replicated Nafion show a irregularly thick

and porous layer. In interpreting the performance of IPMC, the interfacial

area between the electrode and polymer electrolyte is important because the

increase of the interfacial area leads to la rger deformation [8]. Interfacial area

can be indirectly estimated by measuring either a double layer charging

current or electrochemical behaviors under electric field.

27

Figure 7 shows the current variations when pulse potential method

applied. Initial potential is 2 V and duration is 5 s. Then potential jumps into

the –2 V and it is applied for 5 s. Measured current only represents the value

of current flowing through the interface between Pt and electrolyte. Figure 7

suggests that replicated Nafion has a large interfacial area, since higher

charging current is obtained on the replicated Nafion as compared with plain

Nafion. Simultaneous measurements of both current and displacement

indicate that maximum bending occurs during double layer charging and the

degree of bending increases with the increase of current [11]. Figure 8

reveals that replicated Nafion has a large bending performance and the

degree of deformation increases with the increases of applied voltage, which

results in the increase of charging current. After the charging is completed,

water electrolysis takes place and current reaches a steady-state value.

However, the electrolysis of water can be detrimental in the performance of

IPMC since the structure-constructing water around cations could be

removed. Figure 9 shows a cyclic voltammogram in two electrode system

and replicated Nafion has a higher slope in I-E curve, which reflects the

ability of the system to store energy that determine the degree of actuation

[12].

28

Figure 10 and 11 illustrated the impedance spectra at electrode

potential, -0.6 mV. Impedance spectra take the form of semicircle for a

replicated nafion. However, for a plain nafion, the size of the semicircle at

high frequency increases and linear region at low frequency appears.

Analysis of impedance spectra was carried out based on equivalent circuit in

Figure 12.

It was consisted of the resistance for reaction at electrode (RCT), the

resistance for migration of water molecules in Nafion (RS), the capacitance

which was occurred by the double layer charging at electrode (CDL), and the

capacitance between the charge of Nafion chain and the migrated water

molecules (CS).

Lower resistance of Rct in replicated IPMC can be attributed to the

higher electrode/electrolyte interface. At an applied potential, of which

charge transfer reaction take place, the amount of current flowing in

replicated nafion is not as different as in plain nafion, as shown in Figure 4. It

proposes that the current density in replicated nafion is lower and the charge

transfer resistance can be reduced.

29

Figure 6. SEM picture of IPMC prepared on different membrane surface.

(a) top view, (b) cross-section view of Pt electrode on plain Nafion

surface; (c) top view, (d) cross-section view of Pt electrode on

replicated Nafion surface.

(a)

(b)

(c)

(a)

(d)

30

Figure 7. Current transients when potential square waveform is applied.

Potential is initially stepped to 2V

and after 5s of holding time, reduced to –2V.

31

Figure 8. Deformation of IPMC with the change of applied potential.

32

Figure 9. Cyclic voltammetry of plain Nafion (solid line) and replicated

Nafion (dotted line) with scan rate of 40mV/s.

33

Figure 10. Nyquist plot of (a) plain Nafion and (b) replicated Nafion.

Dotted line and full circle indicate simulated data and experimental

observation, respectively.

34

Figure 11. Bode plot of (a) plain Nafion and (b) replicated Nafion.

Dotted line and full circle indicate simulated data and experimental

observation, respectively.

35

Figure 12. Equivalent circuit model of IPMC.

36

Figure 13 shows the bode plot of impedance response measured with

differently prepared membranes; plain Nafion, replicated Nafion and 30

thick Nafion (thin Nafion). A thin Nafion membrane was fabricated with a

liquid Nafion solution. Replicated Nafion, which has a large electrode surface

area, showed the decreased impedance at high frequencies. Enlarged surface

area increases the number of hydrated cations migrating toward the electrode

and the decreased impedance is supposed to be the result of a reduced

resistance. A thin Nafion was expected to have a low resistance because of its

thin thickness. However, contrary to the expectation, a thin Nafion has a low

resistance, it has a higher resistance than others because of its thin layers,.

Figure 14 shows the SEM image of thin Nafion composite electrode which

has a nonuniform morphlogy of Pt metal. It results the reduction of

polymer/metal interface area and induces the higher resistance than plain

Nafion.

37

Figure 13. Bode plot of impedance response of differently prepared

membrane (cation : H+).

38

Figure 14. SEM image of IPMC prepared on different membrane surface.

(a) and (b): Commercial plain Nafion surface (Dupont),

(c) and (d) : Home-made thin Nafion (University of New Mexico).

(a)

(d) (c)

(b)

39

4.2. Effect of cations on IPMC

Counter-ion sites of sulfonic acid group inside membrane are

exchanged with 6 different cations; 4 univalent cations (H+, Li+, Na+, K+) and

2 bivalent cations (Mg2+, Ca2+). Figure 15 shows the potential transients

when 1 mA/cm2 was applied. Potential has a rapid initial increase and the

slow potential increase is followed. Under electric fields, double layer is

charged at an incipient stage and, after then, faradaic reaction takes place.

Faradaic reaction is water decomposition reaction in this system. In two-

electrode system, measured potential includes the overpotential at both anode

and cathode, ohmic drop inside membrane. Since both electrochemical

reaction and applied current density are the same for all cases, the difference

in measured potential is related to ohmic drop, i.e., ionic mobility inside

membrane. Potential drop due to ionic mobility is highest for Li+ ions. Na+

ion causes less potential drop, K+ ion is next, and H+ ion is least. This

observation is in a good agreement with equivalent ionic conductivity.

Considering the degree of solvation at infinite dilution, the equivalent ionic

conductivity of H+, Li+, Na+, K+ ion is 349.65, 38.66, 50.08 and 73.48

respectively [13]. That is, the effective radius of Li+ ion is greatest because of

hydration. It results in lowest mobility and requires highest potential. Inside

40

membrane, the concentration of cations is higher than infinite dilution and

equivalence conductance cannot be so high as infinite dilutions. However, it

can be stated that Li+ ion has still lowest mobility since the number of

univalent cations is supposed to be the same inside membrane.

Ionic mobility is influenced by both the ion size and ion- ion

interactions. Under electric field, cations migrate through hydrophilic

channels and should overcome electrostatic forces felt by sulfonic acid group.

In uni-univalent solution electrolytes, interionic forces become greater with

the increase of cation size; KCl, NaCl, LiCl, HCl (in descending order) [14].

The effect of interionic forces on ionic mobility is not as big as ion size

because the required energy for ionic movement is dependent on the ion size,

as shown in Figure 15.

Compared to univalent cations, bivalent cations show a different

potential transients. After double layer charging, potential variation is divided

into two regions; relatively fast potential increase region (I) and the following

slow increase region (II). Equivalent conductance of Mg2+, Ca2+ ion is 59.47

and 53, respectively and the value is greater than Na+ ion, but Figure 15

indicates that, for a region II, the movement of Ca2+ ion requires more

energy than Na+ and Mg2+ ion. The result asserts that ion size cannot give a

41

complete explanation for an ionic motion inside membrane. A bivalent cation

occupies two-sulfonic sites and moves toward cathode under electric field.

Since interionic forces between bi-bivalent electrolytes are four times greater

than uni-univalent electrolytes [14], the effect of interionic forces on ionic

motion cannot be negligible.

Figure 16 shows the impedance characteristics obtained with

different cations inside membrane. For univalent cations, the low frequency

resistance is higher for Li+ ion than other cations and for bivalent cations,

Ca2+ ion has higher resistance than Mg2+ ion. The observed increase in

resistance can be explained by the difficulties in ionic mobility caused by

ion-size and ion-ion interaction.

42

Figure 15. The effect of cations inside membrane on potential transient.

1mA/cm2 of constant current is applied.

(a) H+, Li+, Na+ and K+ (b) H+, Mg2+ and Ca2+

0 1 0 2 0 3 0 4 0 5 0 6 0-0 ,5

0 , 0

0 , 5

1 , 0

1 , 5

2 , 0

H+

L i+

N a+

Time (s)

Ce

ll p

ote

nti

al

(V)

K+

0 1 0 2 0 3 0 4 0 5 0 6 0-0,5

0,0

0,5

1,0

1,5

2,0

H+

Time (s)

Ce

ll p

ote

nti

al

(V)

M g2 +

C a2 +

(a)

(b)

43

Figure 16. Bode plot of impedance response of different cations

inside plain Nafion.

10 -1 10 0 101 10 2 103 104 105100

101

102

103

Imp

ed

an

ce I

Z I

(

oh

m )

Frequency ( Hz )

H+ Li+

Na+ K

+

Mg2+ Ca 2+

44

5. CONCLUSIONS

Deformation of IPMC is significantly affected by interfacial area

between electrode and polymer electrolyte and induced by the motion of

hydrated cations, electrode preparation method and cations inside membranes.

In this work, a large surface Nafion-metal composite actuator was prepared

with a replication technique and the effect of the increase of the interfacial

area on deformation was investigated. Its bending behavior is also interpreted

with electrochemical aspects of different cations inside the Nafion membrane.

Etched aluminum foil was used as a template for replication using

liquid Nafion solution. Replicated Nafion was chemically coated with Pt

metal. Double layer charging current and morphological investigation

indicated that interfacial area was greatly increased by replication method

and increased area induced a better bending performance of IPMC. At the

analysis of impedance spectra of IPMC, the electrode surface area affected

the impedance and we suggested the model of capacitive and resistive

coupling in IPMC. Based on equivalent ionic conductivity, the effect of

cations on IPMC was interpreted with constant current experiment. For

univalent cations, the required energy to pass the current increases with the

45

increase of ion size. For bivalent cations, ion- ion interaction can be an

important factor for an ionic mobility but the mechanism of ion transfer

inside membranes is not quite clear.

46

6. REFERENCES

[1] M. Shahinpoor, Smart Materials and Structures-International Journal 3,

367 (1994).

[2] Yoshihito Osada and Danilo E. Rossi, Sensors and Actuators B 63, 1

(2000).

[3] Y. Bar-Cohen, T. Xue, M. Shahinpoor, J. O. Simpson and J. Smith,

Proceedings of SPIE- Smart Structures and Materials 1998, 3324 (1998).

[4] J. W. Gardner and P. N. Battlett, Sensors and Actuators A 51, 57(1995).

[5] O. Inganas and Q. Pei, Advanced Maerrials 4, 277(1992).

[6] Kazuo Onishi, Shingo Sewa, Kinji Asaka, Naoko Fujiwara and Keisuke

Oguro, Electrochim. Acta 46, 737 (2000).

[7] M. Shahinpoor and K. J. Kim, Proceedings of SPIE- Smart Structures and

Materials 2000, 110 (2000).

[8] K.Oguro, N. Fujiwara, K. Asaka, K. Onishi and S. Sewa, Proceedings of

SPIE- Smart Structures and Materials 2000, 64 (2000).

[9] M Shahinpoor, Y Bar-Cohen, J O Simpson and J Simith, Smart Materials

and Structures-International Journal 3, R15 (1998).

47

[10] Albert Lehmani, Pierre Turq, Michelle Perie, Jacques Perie and Jean-

Pierre Simonin, Journal of Electroanalytical Chemistry 428, 81 (1997).

[11] Satoshi Tadokoro, Masahiko Fukuhara, Yoseph Bar-Cohen, Keisuke

Oguro and Toshi Takamori, Proceedings of SPIE- Smart Structures and

Materials 2000, 262 (2000).

[12] T. W. Lewis, B. C. Kim, G. M. Spinks and G. G. Wallace, Proceedings

of SPIE- Smart Structures and Materials 2000, 1351 (2000).

[13] D. R. Lide, Handbook of Chemistry and Physics, 71st ed., 5-97, CRC

Press (1990).

[14] M. Paunovic and M. Schlesinger, Fundamentals of Electrochemical

Deposition, 70, The Electrochemical Soc. (1998).

48

7. ACKNOWLEDGEMENTS

지난 2 년 동안 세심하게 학문의 길을 열어주시고, 많은 조언과 관심을

아끼지 않으신 탁용석 교수님께 머리숙여 감사드립니다. 또한 바쁘신

와중에서도 부족한 논문을 심사해주시고 귀중한 조언을 해주신 박동화 교수님,

김건중 교수님께 깊은 감사를 드립니다. 그리고 대학원 생활동안 많은 도움을

주신 화학공학과 교수님들께도 진심으로 감사드립니다.

처음부터 지금까지 깊은 관심과 애정으로 지켜봐주시는 재영

선배에게 특히 감사드리며, 실험실 생활에 쉽게 적응할 수 있게 도와주신 재광,

진식, 성훈 선배에게 고맙다는 말씀을 전하고 싶습니다. 대학원 생활동안 많은

도움과 위로가 되어준 도연형, 재광, 영우 그리고, 이제 막 대학원 생활을

시작하게되는 인화, 재호, 화영에게도 깊은 감사의 말씀 전하고 싶습니다.

힘이들때 항상 곁에서 이야기를 들어준 용민에게 고마움을 전하며,

창훈과 대학원 동기들, 초등학교 동문인 성곤, 용주, 성범과 고등학교 동문인

덕수, 대용, 융희 그리고 종규에게 감사하다는 말 전하고 싶습니다.

어려운 형편에도 불구하고 사랑으로 돌봐주신 부모님과 형, 형수

그리고 사랑스러운 조카 현민과 현주에게 이 작은 결실이 조금이나마 기쁨이

되었으면 좋겠습니다.

끝으로, 부족한 점이 많은 저를 아낌없는 사랑과 믿음으로 지켜봐준

나의 약혼녀, 제 미래의 동반자가 될 진희씨에게 진심으로 고맙다는 말 전하고

싶습니다. 또한 항상 걱정해주시고 힘이 되어주시는 진희씨 어머니와 언니,

형부들께도 진심으로 감사드립니다.

2002 년 1 월

정든 실험실에서...