고분자구동체의 전기화학적 특성

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

노태근,탁용석,남재도,최혁렬

인하대학교 화학공학과,성균관대학교 고분자공학과,기계공학과

Electrochemical Characterization of Polymer Actuator

Tae-Geun Noha, Yongsug Taka,*, Jae-Do Namb and Hyoukryeol Choic

화학공학의 이론과 응용 제8권 제1호 2002년

aDepartment of Chemical Engineering, Inha University, Inchon, 402-751, Korea.

bDepartment of Polymer Science and cSchool of Mechanical Engineering,

Sung Kyun Kwan University, Suwon, 440-746, Korea.

INTRODUCTION

Recently, there are demands for highly active sensors, flexible and miniaturized actuators in many fields such as medical equipments and micro-robots [1,2]. Because of these requirements, polymer gel, ionic polymer metal composite (IPMC) and conducting polymer have been discussed in the point of various types such as sensors and actuators. IPMC had larger deformation at low voltage than other polymers, but it has some problems. 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[3]. 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 density per volt. Although the lithium ion has the smallest crystal radius, the effective radius of the charged particle is greater than K+, Na+, etc., because of the solvation and the migration of hydrated lithium ions under electric field induces larger deformation.  This work is intended to prepare a large surface nafion-metal composite actuator with a replication technique and to investigate the effect of the increase of electrode/electrolyte interfacial area on deformation.

EXPERIMENTAL

IPMC was prepared with two different methods. Large surface electrode was manufactured by a replication technique. The assembled cells are pressed with 2000 kg/m2 at 75□ and then, etched aluminum was dissolved in a hot chloride solution. Pt electrode was prepared with Pt(NH3)4Cl2 with NaBH4 reducing agent and the morphology was investigated with scanning electron microscopy (Hitachi S-4200).

Before the experiments, IPMC was immersed in different 0.1M solutions (HCl, LiCl, NaCl, KCl, MgCl2 and CaCl2) for a couple of days to exchange cations. Applied potential or current was modulated with galvanostat/potentiostat (PAR 273) and electrochemical behaviors of IPMC were interpreted in air. Deformation of IPMC was measured by a laser displacement sensor. Impedance spectra were recorded by using a frequency response analyzer (Zahner IM6).

RESULTS            AND         DISCUSSION

Enlargement of the composite electrodes surface area --- Both sides of nafion electrolyte were modified with the replication technique. After the replication, membrane surfaces were composed of a flat area and rectangular shaped regions, and its structure result in the increase of surface area. Figure 1 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 picture of Figure 1(a) and (b) indicates that Pt has a uniform and thin layer. On the other hand, Figure 1(c) and (d) prepared with the replicated nafion show anirregularly 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 larger deformation. The interfacial area can be indirectly estimated by measuring either a double layer charging current or electrochemical behaviors under electric field.

Figure 2 shows the current variations when potential is initially stepped up to 2V for 5s and then, reduced to -2V by step.  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. Figure 3 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, water electrolysis can be detrimental in the performance of IPMC since the structure-constructing water around cations are removed.

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. The analysis of impedance spectra was carried out on the base of equivalent circuit. Lower resistance of Rct in the replicated IPMC can be attributed to the higher electrode/electrolyte interface area. At the same applied potential, its charge transferreaction take place, and the amount of current flowing in the replicated nafion is not as different as in plain nafion. It proposes that the current density in the replicated nafion is lower and the charge transfer resistance can be reduced.

Effect of cations on the deformation --- 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 4 shows the potential transients when 1 mA/cm2 was applied. Potential has a rapid initial increase and the slow potential increase is followed. Under the 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 the measured potential is related to ohmic drop, i.e., ionic mobility inside membrane. The potential drop due to ionic mobility is the highest for lithium ions. Na+ causes less potential drop, K+ is next, and H+ is the 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+ is 349.65, 38.66, 50.08 and 73.48 respectively. That is, the effective radius of Li+ is greatest because of hydration. It results in the lowest mobility and requires the highest potential. In the 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 possessed the lowest mobility since the number of univalent cations was supposed to be the same inside membrane.

Ionic mobility is influenced by both the ion size and ion-ion interaction. Under the 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). 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 4.

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+ is 59.47 and 53, respectively and the value is greater than Na+, but Figure 8 indicates that, for a region II, the movement of Ca2+ requires more energy than Na+, Mg2+. The result asserts that ion size cannot give a 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, the effect of interionic forces on ionic motion cannot be negligible.

Impedance characteristics were obtained with different cations inside membrane. For univalent cations, the low frequency resistance is higher for Li+than other cations and for bivalent cations, Ca2+ has higher resistance than Mg2+. The observed increase in resistance can be explained by the difficulties in ionic mobility caused by ion-size and ion-ion interaction.

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

Electrochim. Acta 46, 737 (2000).

Fig. 1. SEM picture of IPMC prepared on different membrane surfaces.

(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.

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

Fig. 2 Current transients when potential square   Fig.3 Deformation of IPMC with the of

waveform is applied. Potential is initially      applied potential. Potential is initially

stepped to 2V and after 10s of holding       stepped to 2V and after 10s of                       time, reduced to -2V.                       holding  time, reduced to -2V.

Fig. 4 The effect of cations inside membrane on potential transients. 1 mA/cm2 of constant current is applied.