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High-Performance Electroactive Polymer Actuators Based on Ultrathick Ionic Polymer−Metal Composites with Nanodispersed Metal Electrodes Hyuck Sik Wang,†,‡,⊥ Jaehyun Cho,†,⊥ Dae Seok Song,† Jong Hyun Jang,§ Jae Young Jho,*,† and Jong Hyuk Park*,∥ †
School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Republic of Korea Department of Advanced Materials Engineering for Information and Electronics, Kyung Hee University, Gyeonggi-do 17104, Republic of Korea § Fuel Cell Research Center and ∥Photo-Electronic Hybrids Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea ‡
S Supporting Information *
ABSTRACT: Ionic polymer−metal composites (IPMCs) have been proposed as biomimetic actuators that are operable at low applied voltages. However, the bending strain and generating force of the IPMC actuators have generally exhibited a trade-off relationship, whereas simultaneous enhancement of both the qualities is required for their practical applications. Herein, a significant improvement in both the strain and force of the IPMC actuators is achieved by a facile approach, exploiting thickness-controlled ion-exchange membranes and nanodispersed metal electrodes. To guarantee a large generating force of the IPMC actuators, ultrathick ion-exchange membranes are prepared by stacking pre-extruded Nafion films. Metal electrodes with a nanodispersed structure are formed on the membranes via alcohol-assisted electroless plating, which allows increased capacitance and facilitated ion transport. The resulting actuators exhibit greatly enhanced electromechanical properties, including an approximately four times larger strain and two times larger force compared to those of actuators having the conventional structure. Moreover, the ability to lift 16 coins (a weight of 124 g) has been successfully demonstrated using ultrathick IPMC actuators, which shows great promise in realizing artificial muscles. KEYWORDS: electroactive polymer, actuator, ionic polymer−metal composite, nanodispersed metal electrode, alcohol-assisted electroless plating ing the dimensions and shapes of the membranes.23,24 In addition, metal electrodes for IPMCs with improved electrical and electrochemical properties were prepared by thickening via multiple plating and additional metal deposition,23,25,26 manipulating the structure and morphology,27,28 and applying novel materials such as carbon materials and conducting polymers.29−33 Despite some progress being achieved, most IPMCs based on the previous approaches have had difficulty in providing sufficiently large strain and force simultaneously. One smart strategy to address this issue is to combine the previous methods. In other words, exploiting predeveloped membranes and electrodes together can generate a strong synergistic effect if they are compatible with each other. IPMC actuators based on thick ion-exchange membranes have demonstrated large force generation, whereas their strain
1. INTRODUCTION Ionic polymer−metal composites (IPMCs) are electroactive polymers that can show a large strain and prompt response under low driving voltages.1−3 IPMCs have a layered structure consisting of a metal electrode−polymer electrolyte (ionexchange membrane)−metal electrode configuration. When an electrical potential is applied to the metal electrodes, the hydrated mobile ions in the ion-exchange membrane migrate by means of electrostatic force, resulting in the actuation of the IPMCs.3−6 The promising properties demonstrated by IPMCs suggest their application as biomimetic actuators for artificial muscles.7−10 However, the electromechanical performance of conventional IPMC actuators has been considered insufficient for practical applications.11 Many efforts have been undertaken to enhance the actuation characteristics of IPMCs. 12,13 First, the chemical and mechanical properties of ion-exchange membranes in IPMCs were upgraded by synthesizing novel membranes, 14−17 incorporating membranes with nanoparticles,18−22 and optimiz© 2017 American Chemical Society
Received: April 5, 2017 Accepted: June 8, 2017 Published: June 8, 2017 21998
DOI: 10.1021/acsami.7b04779 ACS Appl. Mater. Interfaces 2017, 9, 21998−22005
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic diagram for preparing ultrathick IPMC actuators based on thickness-controlled ion-exchange membranes and nanodispersed metal electrodes. (b) Scanning electron micrograph and schematic diagram of the metal electrode−polymer electrolyte interfaces in IPMCs. The nanodispersed electrodes increased the capacitance and facilitated ion transport in the resulting IPMCs, thereby simultaneously improving both the strain and force.
strain.23,24,34 The improvement in the strain of thick IPMCs was pursued in this study because guaranteeing a large force generation of IPMC actuators is important for their practical applications. Figure 1a describes the procedure used to control the thickness of ion-exchange membranes via stacking preextruded Nafion films. On the basis of previous research,24 the ion-exchange membranes for thick IPMCs were designed as 5L membranes, comprising a stack of five layers of Nafion 117 films. The thickness of the hydrated 5L membranes was about 950 μm. Metal electrodes compatible with the 5L membranes were sought to increase the strain of thick IPMCs without decreasing the force. Typically, the metal electrode−polymer electrolyte interfaces in IPMCs have three distinct layers, including electrode, intermediate, and polymer layers (Figure 1b).8,37 Previous approaches, such as simple thickening of the electrode layer via repeated plating or metal deposition,23,25,26 may not assist in enhancing the actuation performance of thick IPMCs because it makes the IPMCs stiff, presumably lowering their strain. The intermediate layer consists of dispersed metal nanoparticles in the polymer electrolyte. A properly designed intermediate layer can be a solution to obtaining optimum metal electrodes for the 5L membranes because metal electrodes with a nanodispersed structure can provide a high capacitance,37,38 facilitate ion transport, and have a relatively small stiffness (Figure S1 in the Supporting Information). The role of nanodispersed metal electrodes in thick IPMCs is further discussed below. The nanodispersed electrodes were constructed in the 5L membranes via alcohol-assisted electroless plating of platinum (Pt) to produce ultrathick (millimeterthick) IPMC actuators. To develop the nanodispersed morphology in the metal electrodes, the swelling behaviors of the 5L membranes in aqueous solutions containing methanol (MeOH), ethanol
has decreased significantly because of the increased stiffness.23,24,34 To increase the strain without compensating the force, tailored metal electrodes for thick IPMC actuators are required that can improve the electrical and electrochemical properties. Metal electrodes containing nanodispersed particles have been developed via electroless plating in alcohol solutions.35 In general, hydroxide ions (OH−), a reducing agent for metals, penetrate with difficulty into negatively charged membranes such as Nafion.27,35 However, alcohol swelling can moderate the electrostatic repulsion, allowing the formation of nanodispersed metal particles inside the IPMCs. The large surface area and high capacitance of the nanodispersed electrodes are favorable to draw hydrated ions under electric potential and facilitate ion transport,27,36,37 especially in thick IPMCs that contain a large number of mobile ions. Moreover, the increase in the stiffness of IPMCs by thickening the electrodes can be alleviated through the dispersed morphology of the metal nanoparticles. Herein, we report a convenient approach to achieve a significant improvement in both the strain and force of IPMC actuators by exploiting thickness-controlled ion-exchange membranes and nanodispersed metal electrodes. Ultrathick ion-exchange membranes were prepared by stacking preextruded Nafion films. The nanodispersed electrodes were constructed in the membranes via alcohol-assisted electroless plating. The two components were highly compatible with each other, allowing great improvement of the electromechanical properties of the resulting IPMC actuators.
2. RESULTS AND DISCUSSION The thickness of IPMCs is a critical factor that determines their actuation properties; thin IPMCs exhibit large strain but small force, whereas thick ones show large force but small 21999
DOI: 10.1021/acsami.7b04779 ACS Appl. Mater. Interfaces 2017, 9, 21998−22005
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) Swelling ratio of the 5L membranes in aqueous alcohol solutions with different alcohol contents at room temperature. The swelling ratio of the membrane in deionized water was about 22 wt %. (b) Small-angle X-ray scattering (SAXS) profiles of the 5L membranes swollen with deionized water and different alcohol solutions. The volume ratio of each alcohol to water was fixed at 1:3.
Figure 3. Cross-sectional images of the IPMCs plated in (a) water and (b) H2O−iPrOH solution. The scale bars in the inset images correspond to 50 nm. (c) Pt profiles in the through-thickness direction of the IPMCs.
clusters with a size of about 4.1 nm. When the membranes were swollen with the H2O−MeOH, H2O−EtOH, and H2O−iPrOH solutions, the peak shifted to 2θ = 1.99, 1.66, and 1.46°, respectively. The diffraction peaks at small angles indicate largesized ionic clusters and thus the membranes swollen with the H2O−iPrOH solution have the largest ionic clusters (ca. 6.0 nm), which can be favorable for the penetration of hydroxide ions. IPMCs were fabricated with the 5L membranes via alcoholassisted electroless plating. The IPMCs prepared in H2O− MeOH, H2O−EtOH, and H2O−iPrOH solutions are denoted as H2O−MeOH-, H2O−EtOH-, and H2O−iPrOH-IPMCs, respectively. The morphology of the metal electrodes in the IPMCs was observed using transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) and compared to that of the IPMC plated in water (denoted as H2O−IPMC). Figure 3 shows the cross-sectional images and Pt profiles of the H2O- and H2O−iPrOH-IPMCs. For the H2OIPMC, Pt particles were mostly located at the surfaces and the intermediate layer consisting of isolated nanoparticles was relatively thin (ca. 10 μm thick). In contrast, the H2O−iPrOHIPMCs contained broadly dispersed Pt nanoparticles inside the membranes, leading to thick electrodes, including an approximately 30 μm thick intermediate layer (Figure 3c). The thick metal electrodes resulted from the deep penetration of hydroxide ions into the largely swollen membranes in the H2O−iPrOH solution. Moreover, as shown in the magnified
(EtOH), and isopropyl alcohol (iPrOH) were investigated. Figure 2a shows the swelling ratio in each solution, which was determined by measuring the amount of the solution absorbed into the membrane. The membranes swollen in water− isopropanol (H2O−iPrOH) solutions exhibited larger swelling ratios compared to those of membranes swollen in water− methanol (H2O−MeOH) and water−ethanol (H2O−EtOH) solutions. Additionally, the swelling ratio increased with an increase in the volume ratio of the alcohol. The large swelling ratio can be beneficial for the penetration of hydroxide ions into the membranes, leading to facile formation of nanodispersed metal electrodes.35 However, the membranes swollen with H2O−EtOH and H2O−iPrOH solutions with an alcohol volume ratio of 1:1 showed severely wrinkled surfaces, resulting in a deteriorated morphology of the metal electrodes. Thus, membranes swollen in solutions with an alcohol volume ratio of 1:3 were chosen for fabrication of the IPMCs. In general, ionic groups in the Nafion membranes gather together and thus form microphase-separated domains, which are known as ionic clusters.39,40 Because the ionic clusters function as a pathway for hydroxide ions during the metal reduction process for IPMCs, their structures can significantly affect the morphology of the metal electrodes. The structure of ionic clusters in the membranes swollen with deionized water and with the alcohol solutions was characterized by SAXS measurements (Figure 2b). The H2O-swollen membrane had a diffraction peak at 2θ = 2.14°, implying the existence of ionic 22000
DOI: 10.1021/acsami.7b04779 ACS Appl. Mater. Interfaces 2017, 9, 21998−22005
Research Article
ACS Applied Materials & Interfaces
Figure 4. Electrical and electrochemical properties of the IPMCs prepared via alcohol-assisted electroless plating. The volume ratio of each alcohol to water was fixed at 1:3. (a) Surface resistance and metal content of the IPMCs. (b) Ionic resistance between the metal electrodes of the IPMCs. (c) Cyclic voltammetry (CV) curves for the IPMCs. The scan rate was 100 mV s−1. (d) Frequency dependence of the differential capacitance for the IPMCs. The current density in (c) and capacitance in (d) were calculated using the nominal area of the IPMC electrodes.
Figure 5. (a) Equivalent circuit model of the IPMCs consisting of two double-layer capacitances, C1, and ohmic resistance, R. The double-layer capacitance can be represented by C = C1/2. (b) Electrical and electrochemical parameters of the IPMCs obtained from the CV and EIS analyses of Figure 4.
constant at high frequencies (104−106 Hz). Over this frequency range, the ohmic resistance of the H2O-, H2O−MeOH-, H2O− EtOH-, and H2O−iPrOH-IPMCs was 3.49 ± 0.02, 2.60 ± 0.01, 2.19 ± 0.01, and 1.65 ± 0.01 ohms, respectively. The decreased ohmic resistance of H2O−iPrOH-IPMC was attributed to the combined effect of facilitated transport of hydrated ions and larger metal content. It is noteworthy that the ohmic resistance of every layer, such as the polymer membrane, intermediate layer, and electrode layer, is an important factor in determining the electrochemical properties of IPMCs.37 While the interfaces between the Pt electrodes and Nafion electrolytes in the IPMCs can be interpreted as electrochemical capacitors, their double-layer capacitance reflects the ability to
images, the individual metal particles formed in the H2O− iPrOH-IPMCs had large surface areas, thereby accommodating more charges and increasing the capacitance.28,37 Figure 4 shows the electrical and electrochemical properties of the prepared IPMCs. Compared with H2O-IPMC, the IPMCs prepared via alcohol-assisted electroless plating had a lower surface resistance and higher metal content (Figure 4a). In particular, the H2O−iPrOH-IPMCs exhibited the lowest surface resistance (1.67 ohm sq−1) and highest metal content (15.5%). These results indicate that large-sized ionic clusters in largely swollen membranes promoted the formation of metal electrodes. The Bode plot of impedance data for IPMCs in Figure 4b shows that the real part of the impedance (Z′( f)) was 22001
DOI: 10.1021/acsami.7b04779 ACS Appl. Mater. Interfaces 2017, 9, 21998−22005
Research Article
ACS Applied Materials & Interfaces
Figure 6. Actuation performances of the IPMCs prepared via alcohol-assisted electroless plating. Time−displacement curves of the IPMCs measured (a) at a point 10 mm away from the grip at a potential of 3 V DC and (b) at a point 20 mm away from the grip at a potential of 3 V AC in the form of a step wave at a frequency of 0.1 Hz. For comparison, the displacement of the conventional thin H2O-IPMC (ca. 210 μm thick, 1L) fabricated with a Nafion 117 film under a potential of 3 V DC is shown. (c) Blocking force of the IPMCs, recorded with a load cell at a point 30 mm away from the grip under a potential of 4 V DC for 30 s. (d) Actuation test of the H2O- and H2O−iPrOH-IPMCs with dimensions of 20 × 60 × 1.0 mm3 under a potential of 4 V DC for 20 s. The weight of the coins was about 15.4 g.
accommodate mobile ions at the electrode−electrolyte interfaces.28 This strongly affects the actuation behavior. CV measurements were performed to quantify the capacitance of the prepared IPMCs; the capacitance values were 4.8, 17.3, 23.9, and 32.2 mF cm−2 for the H2O-, H2O−MeOH-, H2O− EtOH-, and H2O−iPrOH-IPMCs, respectively (Figure 4c). The IPMCs prepared in the alcohol solutions exhibited much a greater capacitance compared to that of the H2O-IPMC. In particular, the capacitance of the H2O−iPrOH-IPMC was more than six times greater than that of the H2O-IPMC. This implies that a large number of charges can migrate to form the electrical double layer in the H2O−iPrOH-IPMC. The differential capacitance of the IPMCs, which was analyzed by electrochemical impedance spectroscopy (EIS), is shown in Figure 4d. The trend in the data obtained from the EIS measurements was consistent with the CV results, confirming a significantly enhanced capacitance of the H2O−iPrOH-IPMC (20.0 mF cm−2 at 0.1 Hz) compared to that of the H2O-IPMC (3.3 mF cm−2 at 0.1 Hz). These results demonstrate that the thick metal electrodes with a highly nanodispersed structure in H2O− iPrOH-IPMC can provide a high capacitance with a large interfacial area, which is expected to be beneficial in improving the actuation performance. Figure 5a shows that the electrochemical characteristics of IPMCs can be described by a serial connection of two capacitors and one resistance, which represent the Pt electrode/ Nafion interface and the ohmic resistance, respectively.32
Assuming that the IPMCs in this study have a symmetrical structure, the two capacitances (Pt electrodes) have identical capacitances (C1) and so can be merged into a single capacitor with a capacitance of C (=C1/2). On the basis of the equivalent circuit model of IPMCs, the amount of transported charges during IPMC actuation is expected to be proportional to C, whereas the small R indicates rapid responses to voltage perturbation. Figure 5b summarizes the capacitance values and ohmic resistances of the IPMCs. In addition to the enhanced capacitance, the IPMCs prepared in the alcohol solutions provided smaller ohmic resistances, which were attributed to the nanodispersed Pt particles in the intermediate layer. The amount of transported charges in the IPMCs under the electric potentials was quantified via chronoamperometry techniques (Figure S2). Indeed, the area in the chronoamperometry curve of the H2O−iPrOH-IPMC was about 1.9 times larger than that in the chronoamperometry curve of the H2O-IPMC. This result implies that a larger amount of charges is transported and stored at the electrode of the H2O−iPrOH-IPMC. Therefore, IPMC actuation behaviors will be highly improved when they are prepared in alcohol solutions compared to those of IPMCs prepared in water. Figure 6 describes the actuation behaviors of the IPMCs. When a direct current (DC) voltage of 3 V was applied to the IPMCs for 60 s (Figure 6a), the maximum displacements were as follows: 1.24, 3.15, 4.18, and 5.80 mm for the H2O-, H2O− MeOH-, H2O−EtOH-, and H2O−iPrOH-IPMCs, respectively. 22002
DOI: 10.1021/acsami.7b04779 ACS Appl. Mater. Interfaces 2017, 9, 21998−22005
Research Article
ACS Applied Materials & Interfaces Surprisingly, the H2O−iPrOH-IPMC showed a larger displacement compared to that of the conventional thin H2O-IPMC (ca. 210 μm thick) fabricated with the Nafion 117 film. The conventional thin IPMC deformed promptly under applied voltages, but back-relaxation was observed after 18 s because of back-diffusion of water.41 Even though the H2O−iPrOH-IPMC exhibited a relatively slow response at the beginning (
