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Fabrication and performance of ionic polymer-metal composites for biomimetic applications
Sensors and Actuators A 299 (2019) 111613
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Fabrication and performance of ionic polymer-metal composites for biomimetic applications Wuxian Peng a , Yajing Zhang a,∗ , Jinhai Gao b , Yiming Wang a , Yang Chen b , Yiran Zhou b a b
School of Materials Science & Engineering, Northeastern University, Shenyang 110819, China School of Mechanical Engineering and Automation, Northeastern University, Shenyang 110819, China
a r t i c l e
i n f o
Article history: Received 2 May 2019 Received in revised form 11 September 2019 Accepted 12 September 2019 Available online 21 September 2019 Keywords: Ionic polymer-metal composites (IPMC) Cu-Pt electrode Bending deformation Surface roughness (Sa) Microstructure Encapsulate
a b s t r a c t Ionic Polymer-Metal Composites (IPMCs) consisting of ionic polymer membrane sandwiched between platinum and copper (Cu-Pt) electrodes have been synthesized via electroless plating and electroplating. The tip displacement of Cu-Pt coated IPMC reached about 10.7 mm when a sinusoidal potential of 3V, 0.6Hz was applied through the membrane. The surface roughness of Pt coated IPMC, Cu-Pt coated IPMC and Cu-Pt coated IPMC after bending deformation tests were 1.21, 0.77 and 0.71 m respectively, indicating that Cu electrode could reduce surface roughness. It was also found that a new generated Cu layer healed the cracks of Pt electrode, was formed through using voltage polarity. Butyl rubber and polydimethysiloxane (PDMS) were employed to encapsulate Cu-Pt coated IPMC to prevent Cu electrode from oxidation. The output deformation of packaged Cu-Pt coated IPMCs were stable under various amplitude of potential. The deflection of Cu-Pt coated IPMCs kept in air for 3, 7, 15 and 22 days respectively had no significant changes and were all about 10.1 mm by applying the potential of 3V, 0.6Hz, revealing that Cu electrode is durable and Cu-Pt coated IPMCs exhibit remarkable promise as biomimetic actuators. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Great efforts have been made to develop ionic polymer-metal composites (IPMCs) as biomimetic actuators owing to their good tenacity, driven at low voltages and large deflection [1–4]. IPMCs are composed of polymer electrolyte plated with metal electrodes (commonly with Pt or Au) [5]. The hydrate cations in ionomer transport toward cathode under applied voltages, which is hailed as electro osmotic phenomenon, leading to the expansion of cathode and the shrinking of anode [6]. The distinct expansion ratio of cathode and anode sides induces bending motion of IPMCs. The actuation mechanisms of them are similar to biological muscles, hence IPMCs are referred as artificial muscles [7,8]. However, cracks and delamination would occur in metal layers when voltage was applied over the membrane since considerable difference of Young’s modulus between polymer electrolyte and electrodes, causing increased resistance [9,10]. What’s worse, water molecules would leak from cracks and be electrolyzed owing to the electrolysis voltage of water is 1.23 V [11], enormously shortening the lifetimes of IPMC [12–14]. Therefore, many researches are
∗ Corresponding author. E-mail address: [email protected] (Y. Zhang). https://doi.org/10.1016/j.sna.2019.111613 0924-4247/© 2019 Elsevier B.V. All rights reserved.
committed to improving the service life of IPMC with stable actuation performance [15,16]. Composite coating is an effective method to enhance IPMC bending deformation of IPMC compared to single coating, for instance sulfonated poly (vinyl alcohol)/aluminium oxide/graphene based IPMC [17], Polypyrrole-Ag based IPMC [18], Cu-Ni coated IPMC [19] and Cu-Pt IPMC [10]. Cu-Pt coated IPMC has advantages of good actuation property, simple preparation process, short preparation cycle and low cost. Nevertheless, Cu layer is oxidizable and facilely peels off from the Pt layer, which vastly impedes the application of Cu-Pt coated IPMC [10,14,20,21]. Hence, it is vital to put forward a preparation method for Cu-Pt coated IPMC that possesses satisfactory bending deformation, meanwhile, enhanced oxidation resistance of Cu layer. In this work, a Cu-Pt coated IPMC has been developed through electroless platinum deposition and then electroplating copper. In order to overcome the problem of effortless oxidation of Cu layer and excessive electrolysis of water in IPMC, a new encapsulation method was proposed in this paper, which combined butyl rubber and polydimethylsiloxane [22–24]. After encapsulation, the oxidation rate of the Cu layer was significantly slowed down and the surface remained reddish brown after encapsulating 22 days. Moreover, the actuation performance of encapsulated Cu-Pt coated IPMC was stable and has great value for biomimetic applications.
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Fig. 1. The preparation process of Cu-Pt coated IPMC.
2. Experimental 2.1. Electroless plating Before electroless plating, Nafion 117 membrane was roughened by #1200 sandpaper in order to increase the surface area and roughness. Then, the membrane was boiled for 30 min in a 2 M aqueous HCl solution and deionized water respectively, after which the specimen was immersed in [Pt(NH3 )4 ]Cl2 (aq) for 24 h and NaBH4 (aq) was used as reducing agent. Subsequently, the sample was boiled in 0.1 M HCl (aq) and soaked in 2 M CuSO4 (aq) for cation exchange. 2.2. Electroplating Cupric pyrophosphate was employed for electroplating to acquire uniform Cu electrode. The primary electrode reactions are as follows: 6−
Anodereaction : [Cu(P2 O7 )2 ]
2−
Cathodereaction : (CuP2 O7 )
2-
(CuP2 O7 ) +P2 O7
+2e- Cu + P2 O7
4-
4-
Fig. 2. The Schematic diagram of testing bending deformation [25].
(1) (2)
The membrane was cut into strips of 10 mm × 35 mm prior to electroplating. The strip was clamped in negative electrode and positive electrode was connected to fine copper plate. Temperature and pH of the solution during electroplating ranged from 40 to 60 ◦ and 7.3 to 9.6, respectively. 2.3. Encapsulation Migration of cations in ionomer ought to be carried by water molecules. When the amplitude of applied potential is over 1 V (1 V refers to the effective value of sinusoidal voltage, the maximum value of which can reach about 1.4 V), water in membrane would be electrolyzed, leading to unstable bending motion. Thus,A we use butyl rubber and polydimethysiloxane (PDMS) for encapsulation, which does not cause contamination to Cu layer and has low stiffness. The preparation process of Cu-Pt coated IPMC is depicted in Fig. 1. The original Nafion film is transparent. After electroless plating, the surface of Pt IPMC has a metallic luster. The black part is unreduced Pt and cannot be used for subsequent electroplating. After electroplating, the IPMC surface is reddish brown, finally the Cu-Pt IPMC is packaged. 2.4. Properties characterization and measurement The morphology and composition were characterized by X-Ray Electron Probe Microanalyzer (EPMA, JXA-8530 F). Surface rough-
Fig. 3. EPMA images of Pt coated IPMC, (a) Surface morphology of Pt coated IPMC; (b) Cross-section Pt coated IPMC.
ness (Sa) was investigated using 3D laser measuring instrument (OLS4100). Before testing the surface topography and Sa, the butyl rubber and PDMS on the IPMC surface were carefully wiped off with alcohol. As shown in Fig. 2, bending deformation test was carried out with Laser displacement sensor. IPMCs were actuated by PC LabVIEW DAC-board (PCI6221) [25]. All actuation performance tests were performed in air. To reduce experimental error, there were at least 5 parallel samples per group. 3. Results and discussion 3.1. Morphology of IPMC The surface morphology of Pt coated IPMCs by primary electroless plating were shown in Fig. 3(a), and it could be seen that many cracks and deep trenches appeared due to the uneven surfaces of membrane after roughening and inhomogeneous depositing owing
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achieved 10.7 mm deflection (Fig. 6(b)), owing to the electrode reactions as follows [10,14,20].
Fig. 4. EPMA images of Cu-Pt coated IPMC, (a) Surface morphology of Cu-Pt coated IPMC; (b) Cross-section Cu-Pt coated IPMC.
to limited Pt particles during chemical deposition. By examining the cross-section morphology of Pt coated IPMCs (Fig. 3(b)), it was determined that the thickness of Pt layer was about 7.0 m. Non uniform distributed Pt electrode could lead to increased surface resistance and decreased output response [16]. The morphology of Cu-Pt coated IPMC is presented in Fig. 4, and it shows that the electrodes were even and uniform. The thickness of Cu-Pt electrodes was about 9.2 m. 3.2. Surface roughness of IPMC In surface roughness testing (Sa), the overall height difference of ˜ m, of which Pt coated IPMC fluctuated greatly, ranging from 013 Sa was 1.21 m (Fig. 5(a)). However, the average height difference of Cu-Pt coated IPMC decreased distinctly with some bumps locally, and its Sa was smaller, being 0.77 m as Fig. 5(b) shows. This demonstrates that Cu electrode could reduce cracks of Pt layer, producing smoothed electrodes with lower Sa. 3.3. Actuation performance of IPMCs Fig. 6 shows the tip displacement of packaged IPMCs under a sinusoidal signal of 3 V, 0.6 Hz. The deflection of the encapsulated Pt coated IPMC fabricated by electroless plating (Fig. 6(a)) was about 1.7 mm. The uneven metal layer (Fig. 3) and large surface roughness (Fig. 5(a)) resulted in lower bending motion. However, the deflection of encapsulated Cu-Pt coated IPMC with uniform electrodes
Anodereaction : Cu(S) + 5H2 O → Cu2+ ·5H2 O + 2e−
(3)
Cathodereaction : Cu2+ ·5H2 O + 2e− → Cu(S) + 5H2 O
(4)
Since the standard electrode potential of Cu is +0.34 V (Cu2e− →Cu2+ ), when a sinusoidal voltage of 3 V is applied to IPMC, Cu atoms on the anode side could be oxidized to Cu2+ ions and the generated Cu2+ migrates into the Nafion film and toward the cathode side, leading to the amount of ions that can migrate in film is increased. Hence, Cu-Pt coated IPMC possesses larger bending deformation. Because Cu2+ is reduced to Cu atoms on the cathode side, the total amount of Cu in IPMC remains constant [20]. The Xray line-scan profiles of Cu-Pt coated IPMC cross-section before and after bending deformation are presented in Fig. 7 which confirms this process. After the energization, the position of Pt peak and Cu peak were shifted, indicating that there is newly generated Cu layer in Cu-Pt coated IPMC. The surface morphology and Sa of Cu-Pt coated IPMC after bending deformation are shown in Fig. 8. The metal layer of Cu-Pt coated IPMC was uniform with a reduced number of cracks and bumps (Fig. 8(a)), with Sa being 0.71 m (Fig. 8(b)). Furthermore, the Cu-Pt coated IPMC overcame the problem associated with the formation of cracks in electrodes under applied voltages, which often occurs with other types of IPMCs, resulting in a reduced output response. The positions of Cu and Pt peaks were exchanged before and after the bending deformation (Fig. 7), which was further pronounced when an asymmetric potential was applied as Urmas Johanson reported [10]. It proved that continuous electrode reactions would heal the cracks of Pt layer by applying voltages, resulting in more stable bending motion. Fig. 9(a) exhibited the output displacement of unpackaged Cu-Pt coated IPMC, of which the maximum deflection was about 9.0 mm. However, the oxidation of Cu layer had adverse effects on the stability of IPMCs. The bending motion of unpackaged Pt coated IPMC with two-step reduction is shown in Fig. 9(b). Although the maximum displacement was 17.0 mm, its output response was unstable owing to poor water-retention [26,27]. It was observed that the
Fig. 5. Surface roughness (Sa) of IPMC, (a) Pt coated IPMC; (b) Cu-Pt coated IPMC.
Fig. 6. Bending deformation of IPMC under sinusoidal potential of 3 V, 0.6 Hz. (a) encapsulated Pt coated IPMC, (b) encapsulated Cu-Pt coated IPMC.
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Fig. 7. X-ray line-scan profiles of IPMC cross-sections: (a) Cu-Pt coated IPMC before bending deformation test, and (b) Cu-Pt coated IPMC after bending deformation test.
Fig. 8. Morphology and Sa of Cu-Pt coated IPMC after testing bending deformation, (a) Morphology of Cu-Pt coated IPMC; (b) Sa of Cu-Pt coated IPMC.
Fig. 9. Bending deformation of unpackaged IPMC under sinusoidal potential of 3 V, 0.6 Hz, (a) Cu-Pt coated IPMC; (b) Pt coated IPMC.
surface of Pt coated IPMC after testing became desiccated due to evaporation of water in ionomer by applying voltages. In the initial stage of voltage application, the water content inside the membrane was adequate. When the hydrated cations migrated to cathode, excess water molecules in anode hindered IPMC from bending toward anode, leading to small deflection of Pt coated IPMC. After energization for a period, water molecules in membrane were electrolyzed. When an equal quantity of hydrated cations accumulated in cathode, the bending resistance near anode decreased, which enhanced output displacement of IPMC. However, water molecules were exorbitantly electrolyzed after 80 s, making the media to be insufficient for transporting cations. As a result, the deflection of the Pt coated IPMC was reduced and unstable. An open-loop control system is employed in the actuation tests, while a closed-loop control system is utilized in practical applica-
tion. It is necessary to feedback and adjust the deflection of IPMCs for stable output response. Therefore, the unstable output response, as shown in Fig. 9, undoubtedly increases the difficulties of system controlling. In summary, it is crucial to adopt proper encapsulation process for Cu-Pt coated IPMC. In this work, we took advantage of butyl rubber and PDMS to the encapsulation of IPMC. When sinusoidal potentials with amplitudes of 1 V and 2 V were applied over the Cu-Pt coated IPMCs after encapsulation, the tip displacements were about 3.2 and 5.2 mm respectively (Fig. 10), indicating an enhanced operating lifetime, more controllable output response and admirable repeatability. The deflection of packaged Cu-Pt coated IPMCs kept in air for 3,7,15 and 22 days respectively, had no apparent changes and were all about 10.1 mm by applying a potential of 3 V, 0.6 Hz (Fig. 11), which further validates that the encapsulation process used in this
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Fig. 10. Actuation performance of encapsulated Cu-Pt coated IPMC under sinusoidal potential of 0.6 Hz with different amplitude, (a) 1 V; (b) 2 V.
Fig. 11. Bending deformation of encapsulated Cu-Pt coated IPMC kept in air for various days under sinusoidal potential of 3 V, 0.6 Hz, (a) 3 days; (b) 7 days; (c) 15 days; (d) 22 days.
study significantly improved the durability of the Cu electrode. The encapsulated Cu-Pt coated IPMCs exhibited steady actuation performance after 22 days. 4. Conclusion A durable Cu-Pt coated IPMC has been fabricated by electroless plating and electroplating. Comparing with coating morphology of Pt coated IPMCs, metal layers of the Cu-Pt coated IPMC are more uniform in agreement with the results of surface roughness tests. X-ray line-scan profiles of the IPMC cross-section indicated that electrode reactions could heal the cracks, leading to larger deflection. The tip displacement of Cu-Pt coated IPMC reached 10.7 mm by applying a sinusoidal potential of 3 V, 0.6 Hz. A novel encapsulation process using butyl rubber and PDMS for the Cu-Pt coated IPMC has been studied due to the poor adhesion of Cu electrode. The output response of the packaged Cu-Pt coated IPMC became more controllable with enhanced operating lifetime. Cu electrode can be maintained for more than 22 days, manifesting that the CuPt coated IPMC in this study is of promising values as biomimetic devices. Acknowledgements This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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[11] C.-Y. Yu, Y.-W. Zhang, Su G-DJ, Reliability tests of ionic polymer metallic composites in dry air for actuator applications, Sens. Actuators A Phys. 232 (2015) 183–189. [12] C. Bonomo, L. Fortuna, S. Graziani, et al., A circuit to model an ionic polymer-metal composite as actuator, 2004 IEEE International Symposium on Circuits and Systems (ISCAS 2004) vol. 4 (2004) 864–867. [13] V.K. Nguyen, J.W. Lee, Y. Yoo, Characteristics and performance of ionic polymer–metal composite actuators based on Nafion/layered silicate and Nafion/silica nanocomposites, Sens. Actuators B Chem. 120 (2) (2007) 529–537. [14] M. Uchida, M. Taya, Solid polymer electrolyte actuator using electrode reaction, Polymer 42 (22) (2001) 9281–9285. [15] U. Johanson, U. Mäeorg, V. Sammelselg, et al., Electrode reactions in Cu–Pt coated ionic polymer actuators, Sens. Actuators B Chem. 131 (1) (2008) 340–346. [16] O.C. Yılmaz, I. Sen, B.O. Gurses, O. Ozdemir, L. Cetin, M. Sarıkanat, et al., The effect of gold electrode thicknesses on electromechanical performance of Nafion-based Ionic Polymer Metal Composite actuators, Compos. Part B Eng. 165 (2019) 747–753. [17] A. Punning, A. Aabloo, Self-healing properties of Cu-Pt coated ionic polymer actuators, in: Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series. Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, 2008, 69271Y-69271Y-11. [18] J. Liu, Y. Li, B. Liang, Y. Du, Z. Sun, P. Zhang, Research on the synthesis and properties of PPY modified electrode IPMC, in: IOP Conference Series: Materials Science and Engineering, 2019, pp. 493, 012048. [19] U. Schmid, H.-K. Lee, N.-J. Choi, S. Jung, K.-H. Park, J. Kim, Ionic polymer-metal composites (IPMCs) containing Cu/Ni electrodes and ionic liquids for durability, Int. Soc. Opt. Photonics 7362 (2009), 73620I. [20] Y. Bar-Cohen, U. Johanson, A. Punning, M. Kruusmaa, A. Aabloo, Self healing properties of Cu-Pt coated ionic polymer actuators, Proc. SPIE - Int. Soc. for Opt. Eng. 6927 (2008), 69271Y.
[21] E. Esmaeli, M. Ganjian, H. Rastegar, M. Kolahdouz, Z. Kolahdouz, G.Q. Zhang, Humidity sensor based on the ionic polymer metal composite, Sens. Actuators B Chem. 247 (2017) 498–504. [22] C. Xu, M.L. Guilly, M. Taya, Design of Nafion actuator with enhanced displacement, Proc. SPIE - Int. Soc. for Opt. Eng. 4695 (2002) 57–66. [23] J. Barramba, J. Silva, P.J. Costa Branco, Evaluation of dielectric gel coating for encapsulation of ionic polymer–metal composite (IPMC) actuators, Sens. Actuators A Phys. 140 (2) (2007) 232–238. [24] M.L. Guilly, M. Uchida, M. Taya, Nafion-based smart membrane as an actuator array, Int. Soc. Opt. Photonics 4695 (2002) 78–84. [25] L. Hao, Y. Chen, Z. Sun, The sliding mode control for different shapes and dimensions of IPMC on resisting its creep characteristics, Smart Mater. Struct. 24 (4) (2015), 045040. [26] H. Rasouli, L. Naji, M.G. Hosseini, Electrochemical and electromechanical behavior of Nafion-based soft actuators with PPy/CB/MWCNT nanocomposite electrodes, RSC Adv. 7 (6) (2017) 3190–3203. [27] A. Khan, Jain R.K. Inamuddin, A.M. Asiri, Thorium (IV) phosphate-polyaniline composite-based hydrophilic membranes for bending actuator application, Polym. Eng. Sci. 57 (3) (2017) 258–267.
Biography Zhang Yajing had her BS, MS and PhD degree from Northeastern University in 1987, 1998 and 2001, respectively. Her doctoral research interest is related to investigation of magnetic properties for new nanocrystalline soft magnetic Fe-Nb-B alloys and mechanical alloying for Fe-Ni nanocrystalline soft magnetic alloys. Since 2002, she became an associate-professor at Northeastern University. Her current researches include Biomedical materials and biomimetic materials.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Fabrication and performance of ionic polymer-metal composites for biomimetic applications Wuxian Peng a , Yajing Zhang a,∗ , Jinhai Gao b , Yiming Wang a , Yang Chen b , Yiran Zhou b a b
School of Materials Science & Engineering, Northeastern University, Shenyang 110819, China School of Mechanical Engineering and Automation, Northeastern University, Shenyang 110819, China
a r t i c l e
i n f o
Article history: Received 2 May 2019 Received in revised form 11 September 2019 Accepted 12 September 2019 Available online 21 September 2019 Keywords: Ionic polymer-metal composites (IPMC) Cu-Pt electrode Bending deformation Surface roughness (Sa) Microstructure Encapsulate
a b s t r a c t Ionic Polymer-Metal Composites (IPMCs) consisting of ionic polymer membrane sandwiched between platinum and copper (Cu-Pt) electrodes have been synthesized via electroless plating and electroplating. The tip displacement of Cu-Pt coated IPMC reached about 10.7 mm when a sinusoidal potential of 3V, 0.6Hz was applied through the membrane. The surface roughness of Pt coated IPMC, Cu-Pt coated IPMC and Cu-Pt coated IPMC after bending deformation tests were 1.21, 0.77 and 0.71 m respectively, indicating that Cu electrode could reduce surface roughness. It was also found that a new generated Cu layer healed the cracks of Pt electrode, was formed through using voltage polarity. Butyl rubber and polydimethysiloxane (PDMS) were employed to encapsulate Cu-Pt coated IPMC to prevent Cu electrode from oxidation. The output deformation of packaged Cu-Pt coated IPMCs were stable under various amplitude of potential. The deflection of Cu-Pt coated IPMCs kept in air for 3, 7, 15 and 22 days respectively had no significant changes and were all about 10.1 mm by applying the potential of 3V, 0.6Hz, revealing that Cu electrode is durable and Cu-Pt coated IPMCs exhibit remarkable promise as biomimetic actuators. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Great efforts have been made to develop ionic polymer-metal composites (IPMCs) as biomimetic actuators owing to their good tenacity, driven at low voltages and large deflection [1–4]. IPMCs are composed of polymer electrolyte plated with metal electrodes (commonly with Pt or Au) [5]. The hydrate cations in ionomer transport toward cathode under applied voltages, which is hailed as electro osmotic phenomenon, leading to the expansion of cathode and the shrinking of anode [6]. The distinct expansion ratio of cathode and anode sides induces bending motion of IPMCs. The actuation mechanisms of them are similar to biological muscles, hence IPMCs are referred as artificial muscles [7,8]. However, cracks and delamination would occur in metal layers when voltage was applied over the membrane since considerable difference of Young’s modulus between polymer electrolyte and electrodes, causing increased resistance [9,10]. What’s worse, water molecules would leak from cracks and be electrolyzed owing to the electrolysis voltage of water is 1.23 V [11], enormously shortening the lifetimes of IPMC [12–14]. Therefore, many researches are
∗ Corresponding author. E-mail address: [email protected] (Y. Zhang). https://doi.org/10.1016/j.sna.2019.111613 0924-4247/© 2019 Elsevier B.V. All rights reserved.
committed to improving the service life of IPMC with stable actuation performance [15,16]. Composite coating is an effective method to enhance IPMC bending deformation of IPMC compared to single coating, for instance sulfonated poly (vinyl alcohol)/aluminium oxide/graphene based IPMC [17], Polypyrrole-Ag based IPMC [18], Cu-Ni coated IPMC [19] and Cu-Pt IPMC [10]. Cu-Pt coated IPMC has advantages of good actuation property, simple preparation process, short preparation cycle and low cost. Nevertheless, Cu layer is oxidizable and facilely peels off from the Pt layer, which vastly impedes the application of Cu-Pt coated IPMC [10,14,20,21]. Hence, it is vital to put forward a preparation method for Cu-Pt coated IPMC that possesses satisfactory bending deformation, meanwhile, enhanced oxidation resistance of Cu layer. In this work, a Cu-Pt coated IPMC has been developed through electroless platinum deposition and then electroplating copper. In order to overcome the problem of effortless oxidation of Cu layer and excessive electrolysis of water in IPMC, a new encapsulation method was proposed in this paper, which combined butyl rubber and polydimethylsiloxane [22–24]. After encapsulation, the oxidation rate of the Cu layer was significantly slowed down and the surface remained reddish brown after encapsulating 22 days. Moreover, the actuation performance of encapsulated Cu-Pt coated IPMC was stable and has great value for biomimetic applications.
2
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Fig. 1. The preparation process of Cu-Pt coated IPMC.
2. Experimental 2.1. Electroless plating Before electroless plating, Nafion 117 membrane was roughened by #1200 sandpaper in order to increase the surface area and roughness. Then, the membrane was boiled for 30 min in a 2 M aqueous HCl solution and deionized water respectively, after which the specimen was immersed in [Pt(NH3 )4 ]Cl2 (aq) for 24 h and NaBH4 (aq) was used as reducing agent. Subsequently, the sample was boiled in 0.1 M HCl (aq) and soaked in 2 M CuSO4 (aq) for cation exchange. 2.2. Electroplating Cupric pyrophosphate was employed for electroplating to acquire uniform Cu electrode. The primary electrode reactions are as follows: 6−
Anodereaction : [Cu(P2 O7 )2 ]
2−
Cathodereaction : (CuP2 O7 )
2-
(CuP2 O7 ) +P2 O7
+2e- Cu + P2 O7
4-
4-
Fig. 2. The Schematic diagram of testing bending deformation [25].
(1) (2)
The membrane was cut into strips of 10 mm × 35 mm prior to electroplating. The strip was clamped in negative electrode and positive electrode was connected to fine copper plate. Temperature and pH of the solution during electroplating ranged from 40 to 60 ◦ and 7.3 to 9.6, respectively. 2.3. Encapsulation Migration of cations in ionomer ought to be carried by water molecules. When the amplitude of applied potential is over 1 V (1 V refers to the effective value of sinusoidal voltage, the maximum value of which can reach about 1.4 V), water in membrane would be electrolyzed, leading to unstable bending motion. Thus,A we use butyl rubber and polydimethysiloxane (PDMS) for encapsulation, which does not cause contamination to Cu layer and has low stiffness. The preparation process of Cu-Pt coated IPMC is depicted in Fig. 1. The original Nafion film is transparent. After electroless plating, the surface of Pt IPMC has a metallic luster. The black part is unreduced Pt and cannot be used for subsequent electroplating. After electroplating, the IPMC surface is reddish brown, finally the Cu-Pt IPMC is packaged. 2.4. Properties characterization and measurement The morphology and composition were characterized by X-Ray Electron Probe Microanalyzer (EPMA, JXA-8530 F). Surface rough-
Fig. 3. EPMA images of Pt coated IPMC, (a) Surface morphology of Pt coated IPMC; (b) Cross-section Pt coated IPMC.
ness (Sa) was investigated using 3D laser measuring instrument (OLS4100). Before testing the surface topography and Sa, the butyl rubber and PDMS on the IPMC surface were carefully wiped off with alcohol. As shown in Fig. 2, bending deformation test was carried out with Laser displacement sensor. IPMCs were actuated by PC LabVIEW DAC-board (PCI6221) [25]. All actuation performance tests were performed in air. To reduce experimental error, there were at least 5 parallel samples per group. 3. Results and discussion 3.1. Morphology of IPMC The surface morphology of Pt coated IPMCs by primary electroless plating were shown in Fig. 3(a), and it could be seen that many cracks and deep trenches appeared due to the uneven surfaces of membrane after roughening and inhomogeneous depositing owing
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achieved 10.7 mm deflection (Fig. 6(b)), owing to the electrode reactions as follows [10,14,20].
Fig. 4. EPMA images of Cu-Pt coated IPMC, (a) Surface morphology of Cu-Pt coated IPMC; (b) Cross-section Cu-Pt coated IPMC.
to limited Pt particles during chemical deposition. By examining the cross-section morphology of Pt coated IPMCs (Fig. 3(b)), it was determined that the thickness of Pt layer was about 7.0 m. Non uniform distributed Pt electrode could lead to increased surface resistance and decreased output response [16]. The morphology of Cu-Pt coated IPMC is presented in Fig. 4, and it shows that the electrodes were even and uniform. The thickness of Cu-Pt electrodes was about 9.2 m. 3.2. Surface roughness of IPMC In surface roughness testing (Sa), the overall height difference of ˜ m, of which Pt coated IPMC fluctuated greatly, ranging from 013 Sa was 1.21 m (Fig. 5(a)). However, the average height difference of Cu-Pt coated IPMC decreased distinctly with some bumps locally, and its Sa was smaller, being 0.77 m as Fig. 5(b) shows. This demonstrates that Cu electrode could reduce cracks of Pt layer, producing smoothed electrodes with lower Sa. 3.3. Actuation performance of IPMCs Fig. 6 shows the tip displacement of packaged IPMCs under a sinusoidal signal of 3 V, 0.6 Hz. The deflection of the encapsulated Pt coated IPMC fabricated by electroless plating (Fig. 6(a)) was about 1.7 mm. The uneven metal layer (Fig. 3) and large surface roughness (Fig. 5(a)) resulted in lower bending motion. However, the deflection of encapsulated Cu-Pt coated IPMC with uniform electrodes
Anodereaction : Cu(S) + 5H2 O → Cu2+ ·5H2 O + 2e−
(3)
Cathodereaction : Cu2+ ·5H2 O + 2e− → Cu(S) + 5H2 O
(4)
Since the standard electrode potential of Cu is +0.34 V (Cu2e− →Cu2+ ), when a sinusoidal voltage of 3 V is applied to IPMC, Cu atoms on the anode side could be oxidized to Cu2+ ions and the generated Cu2+ migrates into the Nafion film and toward the cathode side, leading to the amount of ions that can migrate in film is increased. Hence, Cu-Pt coated IPMC possesses larger bending deformation. Because Cu2+ is reduced to Cu atoms on the cathode side, the total amount of Cu in IPMC remains constant [20]. The Xray line-scan profiles of Cu-Pt coated IPMC cross-section before and after bending deformation are presented in Fig. 7 which confirms this process. After the energization, the position of Pt peak and Cu peak were shifted, indicating that there is newly generated Cu layer in Cu-Pt coated IPMC. The surface morphology and Sa of Cu-Pt coated IPMC after bending deformation are shown in Fig. 8. The metal layer of Cu-Pt coated IPMC was uniform with a reduced number of cracks and bumps (Fig. 8(a)), with Sa being 0.71 m (Fig. 8(b)). Furthermore, the Cu-Pt coated IPMC overcame the problem associated with the formation of cracks in electrodes under applied voltages, which often occurs with other types of IPMCs, resulting in a reduced output response. The positions of Cu and Pt peaks were exchanged before and after the bending deformation (Fig. 7), which was further pronounced when an asymmetric potential was applied as Urmas Johanson reported [10]. It proved that continuous electrode reactions would heal the cracks of Pt layer by applying voltages, resulting in more stable bending motion. Fig. 9(a) exhibited the output displacement of unpackaged Cu-Pt coated IPMC, of which the maximum deflection was about 9.0 mm. However, the oxidation of Cu layer had adverse effects on the stability of IPMCs. The bending motion of unpackaged Pt coated IPMC with two-step reduction is shown in Fig. 9(b). Although the maximum displacement was 17.0 mm, its output response was unstable owing to poor water-retention [26,27]. It was observed that the
Fig. 5. Surface roughness (Sa) of IPMC, (a) Pt coated IPMC; (b) Cu-Pt coated IPMC.
Fig. 6. Bending deformation of IPMC under sinusoidal potential of 3 V, 0.6 Hz. (a) encapsulated Pt coated IPMC, (b) encapsulated Cu-Pt coated IPMC.
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Fig. 7. X-ray line-scan profiles of IPMC cross-sections: (a) Cu-Pt coated IPMC before bending deformation test, and (b) Cu-Pt coated IPMC after bending deformation test.
Fig. 8. Morphology and Sa of Cu-Pt coated IPMC after testing bending deformation, (a) Morphology of Cu-Pt coated IPMC; (b) Sa of Cu-Pt coated IPMC.
Fig. 9. Bending deformation of unpackaged IPMC under sinusoidal potential of 3 V, 0.6 Hz, (a) Cu-Pt coated IPMC; (b) Pt coated IPMC.
surface of Pt coated IPMC after testing became desiccated due to evaporation of water in ionomer by applying voltages. In the initial stage of voltage application, the water content inside the membrane was adequate. When the hydrated cations migrated to cathode, excess water molecules in anode hindered IPMC from bending toward anode, leading to small deflection of Pt coated IPMC. After energization for a period, water molecules in membrane were electrolyzed. When an equal quantity of hydrated cations accumulated in cathode, the bending resistance near anode decreased, which enhanced output displacement of IPMC. However, water molecules were exorbitantly electrolyzed after 80 s, making the media to be insufficient for transporting cations. As a result, the deflection of the Pt coated IPMC was reduced and unstable. An open-loop control system is employed in the actuation tests, while a closed-loop control system is utilized in practical applica-
tion. It is necessary to feedback and adjust the deflection of IPMCs for stable output response. Therefore, the unstable output response, as shown in Fig. 9, undoubtedly increases the difficulties of system controlling. In summary, it is crucial to adopt proper encapsulation process for Cu-Pt coated IPMC. In this work, we took advantage of butyl rubber and PDMS to the encapsulation of IPMC. When sinusoidal potentials with amplitudes of 1 V and 2 V were applied over the Cu-Pt coated IPMCs after encapsulation, the tip displacements were about 3.2 and 5.2 mm respectively (Fig. 10), indicating an enhanced operating lifetime, more controllable output response and admirable repeatability. The deflection of packaged Cu-Pt coated IPMCs kept in air for 3,7,15 and 22 days respectively, had no apparent changes and were all about 10.1 mm by applying a potential of 3 V, 0.6 Hz (Fig. 11), which further validates that the encapsulation process used in this
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Fig. 10. Actuation performance of encapsulated Cu-Pt coated IPMC under sinusoidal potential of 0.6 Hz with different amplitude, (a) 1 V; (b) 2 V.
Fig. 11. Bending deformation of encapsulated Cu-Pt coated IPMC kept in air for various days under sinusoidal potential of 3 V, 0.6 Hz, (a) 3 days; (b) 7 days; (c) 15 days; (d) 22 days.
study significantly improved the durability of the Cu electrode. The encapsulated Cu-Pt coated IPMCs exhibited steady actuation performance after 22 days. 4. Conclusion A durable Cu-Pt coated IPMC has been fabricated by electroless plating and electroplating. Comparing with coating morphology of Pt coated IPMCs, metal layers of the Cu-Pt coated IPMC are more uniform in agreement with the results of surface roughness tests. X-ray line-scan profiles of the IPMC cross-section indicated that electrode reactions could heal the cracks, leading to larger deflection. The tip displacement of Cu-Pt coated IPMC reached 10.7 mm by applying a sinusoidal potential of 3 V, 0.6 Hz. A novel encapsulation process using butyl rubber and PDMS for the Cu-Pt coated IPMC has been studied due to the poor adhesion of Cu electrode. The output response of the packaged Cu-Pt coated IPMC became more controllable with enhanced operating lifetime. Cu electrode can be maintained for more than 22 days, manifesting that the CuPt coated IPMC in this study is of promising values as biomimetic devices. Acknowledgements This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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Biography Zhang Yajing had her BS, MS and PhD degree from Northeastern University in 1987, 1998 and 2001, respectively. Her doctoral research interest is related to investigation of magnetic properties for new nanocrystalline soft magnetic Fe-Nb-B alloys and mechanical alloying for Fe-Ni nanocrystalline soft magnetic alloys. Since 2002, she became an associate-professor at Northeastern University. Her current researches include Biomedical materials and biomimetic materials.