Fabrication of Macroporous Naﬁon Membrane from Silica Crystal for Ionic Polymer-Metal Composite Actuator

: Naﬁon membrane with macropores is synthesized from silica crystal and composited with Pt nanoparticles to fabricate macroporous ionic polymer-metal composite (M-IPMC) actuator. M-IPMC shows highly dispersed small Pt nanoparticles on the porous walls of Naﬁon membrane. After the electromechanical performance test, M-IPMC actuator demonstrates a maximum displacement output of 19.8 mm and a maximum blocking force of 8.1 mN, far better than that of IPMC actuator without macroporous structure (9.6 mm and 2.8 mN) at low voltages (5.8–7.0 V). The good electromechanical performance can be attributed to interconnected macropores that can improve the charge transport during the actuation process and can allow the Pt nanoparticles to ﬁrmly adsorb, leading to a good electromechanical property.


Introduction
Since Oguro's group investigated bending behavior under an applied electric field for the first time in 1992, ionic polymer-metal composite (IPMC) is considered as a unique electro-active material [1]. Owing to its light weight and large bending deformation under low driving voltage, IPMC has been attracted much attention in smart robot field like artificial muscle [2]. A typical IPMC actuator is composed of an ion-change polymer membrane (e.g., Nafion or Flemion) and metal (Pt or Au), coated on both sides of the polymer, that are used as working electrodes. Under the electric field, the blocking force and displacement are attributed to the migration of cations (normally Li + ) in the channels of the ion-change membrane. The electrochemical performance of IPMC is depending on various actors, such as the size of metal particles [3], types of polymer [4], and the surface structure of the polymer [5]. Based on above actuation mechanism, the structure of the polymer may play an important role in enhancing the electrochemical performance of IPMC because it can greatly affect the transmission of cations. Furthermore, Pt or Au cannot be plated well on the surface of the polymer without surface treatment. To improve the electrochemical properties of IPMC, one of the most commonly used structure modification methods is sandblasting [6]. However, it is difficult to control the morphology of polymer, and it modifies the surface of the polymer and cannot provide more transport channels for cations. Herein, a kind of Nafion membrane with interconnected macroporous structure is prepared from silica crystal as template. After depositing Pt nanoparticles, M-IPMC is produced. The macroporous structure in Nafion membrane is in favor of the formation of the small Pt nanoparticles, increasing the interface area between Pt and polymer, leading to a good electromechanical property. In addition, the interconnected macropores can provide the fast transport channels of Li ions [7].

Preparation of IPMC with Macroprous Structure (M-IPMC)
To deposit the Pt nanoparticles on the surface of Nafion film, the prepared Nafion film was firstly immersed in 20 mL solution with 2 M of [Pt(NH 4 ) 2 ]Cl 4 (Pt 52%). Then, 10 mL of HCOOH (88%) solution was added in the above solution and stand still for three days. After reduction for 3 days, the film was ultrasonic washed in the DI water and immersed in 20 mL of 2 M LiCl (98%, Shanghai Aladdin Biochemical Technology Co., Shanghai, China) solution at room temperature for 3 days. Finally, M-IPMC was obtained after ionic exchange for 3 days.

Characterization of M-IPMC
The morphologies of the samples were characterized by field-emission scanning electron microscopy (JEOL Ltd., Akishima, Japan, FESEM, JSM-7100F) at 200 kV. Pt particles were attached to Nafion membrane in the IPMC actuators. And samples were adhesive by conductive tape as the substrate. Before using SEM, the samples were coated with a thin layer of gold, approximately 10 nm. Powder X-ray diffraction analysis was conducted on a Rigaku D/max-2500PC X-ray diffractometer (Rigaku, Tokyo, Japan) with Cu-Kα radiation (λ = 1.5418 Å). The samples were characterized by glass as the substrate. X-ray photoelectron spectroscopy (XPS) analysis was carried out on ESCALAB 250 X-ray photoelectron spectrometer (UZONELAB, Shanghai, China) using Al Kα X-rays as the excitation source. FT-IR spectra were acquired using a VERTEX V80 (BRUKER Ltd., Bremen, German) series spectrometer as transmittance data, with KBr as reference, to certify the presence of oxygen-containing functional groups in Nafion membrane.

Results and Discussion
XRD is applied to further investigate the Pt particle sizes on the Nafion film with or without macropores ( , is~2 nm, much smaller than that of IPMC (9 nm) and the reported IPMC actuators [11][12][13][14]. XRD results shows Pt nanoparticles of M-IPMC are small resulting in wide FWHM, which has also been reported in many literatures [14] in the Figure 2. The XRD results confirm the effect of the macroporous structure of Nafion membrane, preventing the growth of the Pt nanoparticles [7,15]. Figure 2b shows the FT-IR spectra of the pure Nafion membranes. FT-IR results present Nafion membrane peaks at 1132 and 1203 cm −1 , which shows the perfluoroethylen backbone of Nafion film. The characteristic peaks of the side chains in Nafion are showed at 970 cm −1 , 1052 cm −1 , 1130-1200 cm −1 , which were assigned, respectively, to the symmetric vibration of C-O-C bonds and S-O stretching vibration of -SO 3 -groups [16,17]. XPS measurement is employed to further characterize the surface chemical states of elements in the prepared IPMC with microporous structure (M-IPMC) (Figure 2c). Figure 2c indicates that XPS spectrum of M-IMPC shows signals of Pt, C, N, O and F elements, demonstrating that Pt nanoparticles have been successfully deposited on Nafion film [16,18,19]. Figure 3a-c show the SEM images of macroporous Nafion film and the uniform macropores over tens of microns length scale can be observed. From Figure 3c, the mean macropore size of Nafion membrane is ca. 400 nm, which is smaller than that of the diameter of original silica spheres (450 nm), owing to the condense of the polymer [8]. Figure 3d-f display the SEM image of M-IPMC after depositing Pt nanoparticles on the surface of Nafion film, demonstrating small and highly dispersed Pt nanoparticles on macroporous Nafion film. In contrast, the Pt nanoparticles tend to aggregation when deposited on the pure Nafion film (Figure 3g-i), indicating that the macroporous structure on the Nafion film plays an important role in controlling the growth and agglomeration of Pt nanoparticles. Under the SEM images of typical IPMC samples, Pt particles show agglomeration, which is due to the smoothness of Nafion membrane surface. From SEM images of typical IPMC samples, agglomerated Pt nanoparticles can be observed, which is due to the smoothness of the Nafion membrane surface. It is well known that support with porous structure would prevent the growth of the Pt nanoparticles, resulting in dispersed Pt nanoparticles [14]. Also, the size of Pt particles in M-IPMC is smaller than typical IPMC. So, the macroporous structure can inhibit the growth of platinum particles [7,14].   [20]. The tip displacement test platform is construed, IPMC is fixed to the clip of the iron rack, and two copper wires are pasted with adhesive tape on both sides of the IPMC [20,21]. The signal generator is connected to the copper wire and the AC voltage of 5.8-7.0 V is loaded, respectively. At room temperature, IPMC was immersed in DI water for 5 min before installing [20,21]. The displacement variation of IPMC was measured by a laser displacement sensor. A high precision force sensor was placed at the tip of IPMC to test the blocking force of IPMC. Both sensors use a PC to collect data. The durability of IPMC is tested by loading different voltages by a signal generator because of IPMC leakage in air, water will gradually be lost and output performance will be gradually reduced [20,21]. Due to the uncertainty of IPMC, the two samples were measured 20 times for each voltage and the highest displacement or force output of five groups (5.8 V, 6.0 V, 6.4 V, 6.8 V and 7.0 V) were calculated the mean as results in Figure 4. Figure 4a shows the maximum displacement outputs of 19.8 mm under 5.8 V and 15.3 mm under 6.4 V for M-IPMC after 300 s, followed by a fast relaxation that causes an abrupt decay of displacement after 400 s. The displacement curves of M-IPMC exhibits stable working time of 100 s. In Figure 4b, IPMC actuator shows the stable displacement of 9.6 mm under 6.4 V and 6.5 mm under 7.0 V after 300 s. And there is the sign of relaxation observed in the IMPC actuator after 300 s. This suggests two actuators show different displacement outputs due to the different structure of Nafion film. Figure 4c exhibits a corresponding maximum displacement of two actuators at various voltages, clearly showing the maximum displacement of M-IPMC is much larger than that of IPMC. Furthermore, the maximum displacement is larger than that of the reported IPMCs (10 mm, 14 mm, 3.0 mm) as is shown in Table 1 [22][23][24].  The blocking forces of the prepared samples are tested by a force sensor (LRS-20g, 196.1 mN, MANTON). As shown in Figure 4d, the stable blocking force of M-IPMC is 8.1 mN under 6.0 V after 400 s and 5.5 mN under 6.4 V after 200 s, followed by a fast relaxation that causes an abrupt decay in the blocking force after 400 s and 200 s, respectively. For IPMC, in Figure 4e, the stable blocking force is 2.8 mN under 6.0 V after 300 s and 2.4 mN under 6.4 V after 250 s. Also, there is a sign of abrupt decay after 300 s under 6.0 V and 250 s under 6.4 V, respectively. This finding indicates that the response time of M-IPMC is faster than that of IPMC, which can be attributed to the smaller Pt nanoparticles and slower water loss caused by macroporous structure [25,26]. As is shown in Figure 4f, the maximum force of M-IPMC actuator is far larger than that of the IPMC actuator. Also, the maximum blocking force of the M-IPMC actuator is higher than that of the reported actuator (0.64 mN, 1.6 mN and 4.2 mN) as is shown in Table 1 [4][5][6]. Since the peak blocking force of M-IPMC is decayed as voltages decreased, the blocking force of actuators can be controlled under certain low voltages (5.8-6.0 V). The superior mechanical performance of M-IPMC is attributed to the macroporous structure that can improve the rapid transportation of Li ions and highly dispersed small Pt nanoparticles that can enhance water-saving capacity of IPMC actuators and conductivity [7].

Conclusions
Nafion film with macroporous structure has been successfully prepared by using silica crystal as a template. After depositing Pt nanoparticles on the macroporous Nafion membrane and Li+ exchanging, M-IPMC was formed. The M-IPMC actuators exhibit enhanced electromechanical performance (the maximum displacement output is 19.8 mm and the maximum blocking force is 8.1 mN) compared with typical IPMC actuator. The good electromechanical properties of M-IPMC can be ascribed to the macroporous structure in Nafion membrane that is in favor of the improvement of the interface area between Pt and polymer, leading to a good electromechanical property and can provide the fast transport channels of Li ions.