Pt composites

Journal Pre-proof Counter-ion and humidity effects on electromechanical properties of Nafion®/Pt composites Matheus Colovati Saccardo, Ariel Gustavo Zuquello, Kaique Afonso Tozzi, Roger Gonçalves, Laos Alexandre Hirano, Carlos Henrique Scuracchio PII:

S0254-0584(20)30056-0

DOI:

https://doi.org/10.1016/j.matchemphys.2020.122674

Reference:

MAC 122674

To appear in:

Materials Chemistry and Physics

Received Date: 5 November 2019 Revised Date:

14 January 2020

Accepted Date: 16 January 2020

Please cite this article as: M.C. Saccardo, A.G. Zuquello, K.A. Tozzi, R. Gonçalves, L.A. Hirano, C.H. Scuracchio, Counter-ion and humidity effects on electromechanical properties of Nafion®/Pt composites, Materials Chemistry and Physics (2020), doi: https://doi.org/10.1016/j.matchemphys.2020.122674. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Counter-ion and humidity effects on electromechanical properties of Nafion®/Pt composites Matheus Colovati Saccardoa, Ariel Gustavo Zuquelloa, Kaique Afonso Tozzia, Roger Gonçalvesa,*, Laos Alexandre Hiranob and Carlos Henrique Scuracchioa a

Materials Engineering, Federal University of São Carlos (UFSCar), São Carlos, SP – Brazil

b

Institute of Science and Technology, Federal University of Alfenas (UniFAl), Poços de Caldas, MG –

Brazil

* Corresponding author: Roger Gonçalves, PhD Department of Materials Science, Federal University of São Carlos, São Carlos-Brazil Tel: +55 16 3351 9452. E-mail: [email protected]

ABSTRACT Ionomeric polymer/metal composites (IPMCs) are smart materials that deform in response to electrical stimuli and vice versa. Its electromechanical performance depends on several factors, such as the electrical stimulus, environment humidity, counterions, and the number of actuation cycles. For this reason, in this paper, the electromechanical response of Pt-Nafion® IPMC samples was evaluated using several counterion types (H+, Li+, Na+, K+ and Ionic Liquid) and relative humidities. Results showed that the electromechanical behaviour of the IPMC was strongly influenced by the counterion type and polymer matrix hydration level (RH). Blocking force, Electric Charge Storage and the Coulombic Efficiency of the devices increased with the reduction of counterion ionic radius and increase of the hydration level of the polymeric matrix. An atypical current response, associated with water electrolysis, was observed for samples incorporated with H+ counterion. The mechanical performance decreased with the number of cycles, showing a limited life cycle for the device. SEM images presented that Pt surface cracks density increases after several actuations, harming the performance of the IPMC. As the main conclusion, this work shows that the hydration level and the counterion type exert a great influence on IPMC electromechanical properties, being the hydration level more prevailing than counterion.

KEYWORDS IPMC, Electromechanical, Hydration level, Counterion, Performance.

1. INTRODUCTION Nafion® is the most investigated ionic conductor polymer for fuel cell applications [1–3], batteries [4–6], sensors [7,8], and actuators [9–11]. The amphiphilic characteristic of Nafion® arises from its structural duality: while the sulfonated side groups agglomerate in hydrophilic sites, the perfluorinated main chain forms the hydrophobic matrix, with small crystalline domains, where the hydrophilic sites are embedded. Because of these structural characteristics, Nafion® is an interesting choice for an electromechanical device: in the form of films, it has great ionic conductivity and enough toughness to be a soft actuator. The major drawback comes from its inherent dependence on the ambient condition, mainly on the hydration level, which is directly related to the relative humidity (RH) of the operating environment [12]. Mostly due to the properties of Nafion®, ionomeric polymer/metal composite (IPMCs) made with it as the membrane is one of the most exploited types of soft actuators. Besides, IPMCs presents several properties such as flexibility, lightweight [13], biocompatibility [14] and miniaturization capability [15]. Once the structure type of the Nafion® is responsible for the most part of its properties, several studies were made in order to elucidate it. However, despite so many efforts, there is still no consent about how its actual structure is. In one of the most accepted models, Gierke et al. [16] put forth the so-called ionic cluster network model, where spherical shape ion clusters with a diameter within 30–50 Å have inverted micelles configuration and are arranged in a cubic structure. These micelles are interconnected through hydrophilic narrow channels of diameter length near 10 Å, which explains its high ionic conductivity. Gebel et al. [17] using data obtained by SAXS came up with the theory that the ionic clusters have aleatory distribution, disagreeing with the cubic structure proposed by Gierke and collaborators. Schmidt-Rohr and Chen [18] have recently associated the characteristic peak of SAXS measurements, heretofore related to the ionic clusters, with cylindrical inverted micelles parallelly with each other, being these structures known as ionomeric channels. These channels are randomly packed, which their size directly related to the hydration level, with an average diameter of 24 Å. Although the morphology cannot be fully explained yet by the deeply discussed SAXS data obtained so far, the statement of inverted cylindrical micelles forming ionomeric channels, where solvated ions can move through, is important to comprehend the electromechanical mechanism of Nafion-based IPMC. Apart from that, some papers [19,20] suggest that the counterion type has a great influence on electrical and electromechanical properties. The IPMC electromechanical mechanism is based on the movement of solvated ionic charges inside the polymeric matrix driven by an electric potential. This movement generates a cationic concentration gradient within the matrix, causing internal stresses and leading to the IPMC deformation. For this reason, to exploit IPMC device potential, blocking force [21–23] and back relaxation [24,25] parameters can be used. For example, IPMC microgrippers require high blocking

force and no back relaxation to ensure an object grasping for a long period without the need to increase the applied voltage [26]. It is also important for artificial muscles [27] and soft robotic [14,28] applications. So, avoid back relaxation mechanism is important [29,30]. Notwithstanding, back relaxation is widely discussed on Nafion-based IPMC papers, there is no agreement of this phenomenon origin. However, the most accepted theory says that the moving solvated ions toward anode saturate this electrode, and the IPMC slightly moves in the opposite direction [31,32]. Several papers have studied IPMC performance changing counterions and external RH. Onishi et al. [33] conclude that the mechanical behaviour of IPMC is closely related to the counterion ionic radius. Vunder et [34] studied the humidity and temperature influence on IPMC behaviour by conducting tests at a humidity and temperature range of 30-58% and 23-29°C, respectively. They concluded that the most significant condition factor for good performance is the relative humidity. A recurrently described way in the literature to get around the moisture problem is to incorporate ionic liquids into the membranes [35]. In this sense, Hong et al. [36] investigated the electromechanical response of doped IPMC with different concentrations of 1-ethyl-3methylimidazolium trifluoromethanesulfonate ionic liquid. They found that the maximum electromechanical deformation, about 1,4%, was achieved with 22% weight of content ionic liquid. In this paper, it was investigated the influence of the counterion species and the hydration level of the polymeric matrix on the electrical and electromechanical properties of a Nafion based IPMC with platinum electrodes (Pt-IPMC). The influence of counterions with distinct sizes and chemical nature (H+, Li+, Na+, K+) and 1-butyl-3-methyl-imidazolium (BMIM+) was investigated in association with the hydration level. Thus, it was also possible to infer how each of these ions behaves both electromechanically and electrochemically in the IPMC at different ambient conditions. Measurements of electrical current and blocking force during the uninterrupted 18 hours IPMC operation was carried out. Furthermore, Scanning Electron Microscopy (SEM) was obtained after the exhaustive actuation time in order to identify morphological changes on the platinum electrodes.

2. EXPERIMENTAL 2.1 Reactants and solutions Hydrochloric acid (HCl), lithium chloride (LiCl), sodium chloride (NaCl), and potassium chloride

(KCl)

were

used

as

received,

all

analytical

grade

from

Sigma-Aldrich.

Tetraammineplatinum chloride ([Pt(NH3)4]Cl2]) - 98% and sodium borohydride (NaBH4) – 90% were purchased from Sigma-Aldrich. HCl, LiCl, NaCl, KCl (0.5 mol L-1) solutions were prepared using ultrapure water (Milli-Q purification method).

2.2 IPMC Sample Preparation Commercially available Nafion® N117 (178 µm thickness at a relative humidity of 50%) produced by Dupont, was used as the electroactive polymer. The preparation of the IPMC followed the procedure by Oguro [37]. In this method, platinum cations are absorbed by the membrane and then reduced on its surface, forming the metallic electrodes. This procedure is divided into 3 steps. Membrane surface cleaning, ion diffusion (absorption), and primary deposition (reduction). Hydrochloric acid (HCl), tetraammineplatinum chloride ([Pt(NH3)4]Cl2]) and sodium borohydride (NaBH4) were used as precursors for the cleaning, ion diffusion, and reduction processes, respectively. After preparation, samples were cut and sized 35 x 4 mm.

Figure 1 - IPMC sample preparation process. a) The cleaning process, b) absorption and reduction and c) cation (C+) ion-exchange process, in which C+ may be H+, Li+, Na+, K+ or BMIM+.

2.3 Counterion Incorporation IPMC samples were immersed in HCl (0.5 mol L-1) solution for 24 hours to exchange H+ ions into the actuator. The same was done with lithium chloride (LiCl), sodium chloride (NaCl) and potassium chloride (KCl) solutions in the same concentration, to exchange Li+, Na+ and K+ ions into the samples, respectively. The ionic liquid was incorporated to the IPMC by immersion in a mixture of deionized water and ionic liquid for 48 hours at room temperature, allowing counterion exchange. Figure 1 shows a scheme of the sample preparation and cation impregnation process. After cation impregnation, samples were dried in a vacuum oven at 80 oC for 24 hours to ensure complete drying [38].

Table 1 - IPMC sample exchanged with different ions, showing their ionic radii. Sample name IPMC-H IPMC-Li IPMC-Na

Counterion

Size (Å)

+

0.380

+

0.691

+

0.102

H

Li

Na

IPMC-K IPMC-BMIM

K+ BMIM

0.138 +

10.7

2.4 Characterization The characterization was performed using the instrumentation described in the Supplementary Information, section SI.I and SI.II and the system represented in Figure 2. In order to control and avoid any variation of the hydration level during the tests, the IPMC samples were kept in an environment with controlled RH for 6 hours. Five different samples, as described in Table 1, were kept in environments in five different relative humidities (RH): 30, 45, 60, 75, and 90%. A square wave signal of 2.75 V amplitude and frequency of 62.5 mHz, which corresponds to a 32 seconds cycle, was applied on the conditioned samples and the electrical and the blocking force (as an electromechanical response) were monitored for 18 hours. Additionally, the IPMC samples surface was investigated using an InspectTM Scanning Electron Microscope (SEM) model FEI S50 in order to analyse if morphology changes after the cycle of actuation.

Figure 2 - System to analyze the electromechanical behaviour under controlled temperature and humidity.

RESULTS AND DISCUSSIONS 3.1 Mechanical Behavior Figures 3a and 3b show the blocking force of the IPMC versus time when a 2.75 V bias is applied in an environment with RH = 30% and RH = 90%, respectively. At RH = 30%, there is no noticeable back- relaxation for IPMC samples during the experiment interval. On the other hand, at RH = 90%, the IPMC blocking force increases over time until it reaches the maximum value. IPMC-H and IPMC-Li samples reached maximum values faster than the other samples (20 and 22 seconds, respectively). Then, they exhibited back relaxation, that is, a reduction in the intensity of force after its maximum. IPMC-Na and IPMC-K samples reached the maximum blocking force

slower (60 and 80 seconds, respectively). Besides, less pronounced back relaxation was observed. On the other hand, the IPMC-BMIM sample showed no noticeable back relaxation during the experiment time. Furthermore, it was observed that the maximum blocking force follows the sequence: IPMC-H > IPMC-Li > IPMC-Na > IPMC-K > IPMC-BMIM, independent of hydration level.

Figure 3 – a) IPMC blocking force in RH = 30%; b) IPMC blocking force in RH = 90%; c) maximum blocking force as function of RH; d) IPMC actuation rate movement as function of RH.

This result agrees with Shahimpoor [39] statement after voltage application, solvated ions migrate towards the anode, bringing together free water molecules. Back relaxation occurs when these H2O molecules migrate in the opposite direction, causing the “relaxation”. Following this premise, back relaxation can be attenuated by reducing H2O molecules content. In this way, Bennett and Leo [40] showed that Nafion®-based IPMC containing IL as solvent did not show back relaxation. According to the author, this effect could be explained by the higher viscosity of ionic liquids compared to water. Moreover, near-zero vapour pressure solvents allow longer operation time, less environment dependence, and higher operating voltage [41,42] without back relaxation. It means that, although more absorbed water leads to better electromechanical responses, the greater the drawbacks will be, such as the back relaxation. Figure 3c presents the maximum blocking force versus RH. Figure 3d illustrates IPMC force rate versus RH, obtained through the regression of the linear part of the blocking force versus

time graph. In Figure 3c, it is evident that the blocking force increases with RH increasing. According to several references, the RH is directly related to the amount of H2O molecules inside the membrane and, consequently, the size of the ionomeric channels [14, 31–34]. For this reason, solvated ions can migrate/diffuse easily and in larger amounts inside the channels, which in turn explains both observed effects, the increase in force, and the increase in actuation rate (Figure 3d). As observed, in Figure 3d, the actuation rate is also dependent on the humidity. Due to the actuation mechanism, it is expected that the cationic diffusion rate strongly affects the actuation rate. Hirano et al. [47] exchanged IPMC samples with Na+ and performed a blocking force study at different RH conditions. They showed that blocking force and back relaxation increased with RH increasing. It was inferred that in wetter conditions, the sample is softer, and more water molecules allow greater ion diffusivity. Shoji and Hirayama [48] tested a perfluorinated ionomerplatinum/Li+-based actuator performance in different RH conditions. Displacement at 90% RH was nearly 10 times larger than the displacement at 30% RH. Nemat-Nasser and Yongxian Wu [49], prepared IPMC samples exchanged with various alkali metal cations Li+, Na+, K+, Rb+, and Cs+, and alkyl-ammonium cations, tetramethylammonium (TMA+) and tetrabutylammonium (TBA+) and studied IPMC displacement. It was shown that the IPMC displacement response was higher and faster for Li+ with appreciable back relaxation. However, TBA+ showed considerable displacement with no back relaxation. Thus, it can be inferred that high RH and small cations combination is the ideal scenario for high displacement speed and high blocking force requirements, but with great back relaxation. In contrast, large cations combined with low RH result in low blocking force and slow displacement speed, but no noticeable back relaxation. These combinations are key information for designing and producing an actuator for the desired function. In addition, some other values can be obtained from these blocking force data. Table 2 presents another electromechanical characteristic: the blocking force variation (∆F) and the increase in the blocking force (dF/dRH), both obtained from Figure 3c data. The blocking force variation was obtained from the difference between the minimum (RH = 30%) and the maximum blocking force (RH = 90%). The increase in the blocking force was obtained from the slope of the Figure 3c plot. These two values are different ways of expressing the same data, the force variation as a function of the environment relative humidity. Table 2 – IPMC incorporated with H+, Li+, Na+, K+ and BMIM+ eletromechanical characteristics. Cation Blocking force variation / mN Blocking force increase* / µN/RH% H+

0.727

12.7

+

0.749

11.6

+

0.394

6.43

0.353

5.90

Li

Na K

+

BMIM+

0.294

5.06

* The slope of the blocking force versus humidity plot

It is observed that small ions suffer sharp variation with the humidity, pointing out that they are more dependent on the humidity. Instead, as expected, the variation suffered by BMIM+ is smaller, i.e., it is less sensitive to humidity, as well as the cation K+. The relation of the blocking force and the hydration level (water uptake) is shown in Figure 4. The maximum water content absorbed was registered after 6 hours of equilibrium at each humidity, thereby each point on the graph for the same sample corresponds to humidity from 30% to 90%. At the highest RH, the sample with H+ absorbed more water (9.0%wt), while the sample with BMIM+ absorbed less (2.3%wt). As observed for the maximum blocking force, the relation IPMC-H > IPMC-Li > IPMCNa > IPMC-K > IPMC-BMIM is also valid for the water uptake maximum. Komoroski and Mauritz [50], affirmed that water uptake depends on the cations type. Yoshida and Miura [51] studied the nature of water in Nafion® exchanged with different cations, using Differential Scanning Calorimetry (DSC). They showed that the water “hydration radius” decreases in the order K+ < Na+ < Li+. In other words, the state of a water molecule is a function of the cation type. Gavach et al. [52] using an ac impedance analyser, studied the electrical resistance of Nafion® membranes. They observed that specific conductivity versus the number of moles of water follows K+ < Na+ < Li+ < H+. Chen and co-workers [53] showed that water uptake is dependent on the temperature, and IPMC performance could vary with hydration. Accordingly, Figure 4 data confirms this, showing that the smaller cation radius, more water is absorbed.

Figure 4 –IPMC mechanical behaviour response in different hydration levels (water absorbed). A spline function was used to connect the data points.

3.2 Electrical behaviour As described, the IPMC samples' performance was measured by applying a square-wave of 2.75 V of amplitude with 62.5 mHz frequency for several cycles. Concomitantly, the current flow

over the IPMC was collect. Figure 5 shows the first cycle chronoamperometry for each sample. As soon as there is the voltage application, a current peak is generated. Maintaining the voltage constant, the current decreases exponentially over time until it reaches a quasi-steady state value. When the polarity of the applied voltage is reversed the same phenomenon is observed, which is a characteristic capacitor discharging behaviour. In a typical RC circuit, as the charge on the capacitor decreases, the current through the resistor increases exponentially. When the IPMC is triggered, hydrated ions migrate from the cathode to the anode in response to an electrical field generated between the metal electrodes [54,55], forming a double layer near the electrodes [12,56]. Even with the voltage kept constant, the ion migration rate decreases over time due to counterion saturation in this region, i.e., the charging is followed by the reorganization of the double-layer capacitance. As a consequence, the current also decreases with time exponentially [14]. This behaviour is similar for all samples, regardless of RH and counterion. According to Grahame [57], when an electrified electronic conductor is placed in contact with an ionic conductor (ionic solution), occurs the formation of a region full of charged species, with charge opposite from the electrified surface. Then, a second layer is formed next to the first one, which has charge species equal to that of the electrified surface; this pair of layers is known as the electric double layer [58]. Additionally, in this model, the charge distribution of the ions decreases exponentially as a function of the distance from the electrode to the ionic solution bulk. Some ions may penetrate the first layer, and then, they lose their solvation shell. Thus, it is possible to distinguish three regions. The inner Helmholtz plane (IHP), which corresponds to the first layer formed by H2O molecules and non-solvated ions, the outer Helmholtz plane (OHP), which corresponds to the region of solvated ions and, finally, the diffuse layer [59]. The double layer form between the IHP and the OHP. It is important to note that, according to Gierke et al. [16], ion transport mechanisms in Nafion® can be described by aqueous diffusion laws. Gierke's model also includes polarity effects, dielectric constant, and inhomogeneous spatial orientations. These considerations are necessary because water molecules interacting with the sulfonic groups assume a higher level of orientation and spatial order than the molecules located far from these groups, i.e., at the centre of the channels and pores [60]. This characteristic of ion interaction inside the ionomeric channels has a great influence on the capacitor-like behaviour of the device. However, the IPMC-H sample showed a distinct current profile when a positive voltage is applied, as shown in the detail of Figure 5. This is not observed for low RH (30% and 45%) and occurs sharply with increasing RH. It is known that, when a voltage above 1,23V is applied, water electrolysis may occur, especially in a low pH media [61,62], so, this atypical current response can be associated with the electrolysis process [63]. Water electrolysis is a physical phenomenon that is

characterized by the decomposition of water into gaseous products: hydrogen (H2) and oxygen (O2). H2 molecules are formed on the cathode and O2 molecules on the anode. This process causes the intensive drying of the IPMC and results in a higher current draw than is necessary for IPMC actuation [64].

Figure 5 - Square wave chronoamperometries for IPMC with H+, Li+, Na+, K+ e BMIM+ counterion at different RH (30, 45, 60, 75 and 90%). In detail, IPMC-H current response. Square-wave type DC voltage applied the amplitude of 2.75V and 62.5 mHz frequency.

From the chronoamperometry curves, it is possible to obtain the total electrical charge spend to trigger the device. It is also possible to observe that the accumulated electric charge (Q) during the test time is different for each sample, influenced by the variation of humidity and counterion used. Q can be calculated by integrating the current versus time curve. Another important parameter that can be calculated is the Coulombic Efficiency (CE), which is the ratio between (Q+) and (Q-), i.e., the ratio between the charge spend when a positive bias is applied and when a negative one is applied. Both parameters, electrical charge, and CE in the function of the RH are present, respectively, in Figure 6a and Figure 6c. In general, the charge consumed increases with the hydration level. In addition, it can be observed that the electric charge consumed is strongly influenced by the counterion used. For alkali metal cations, the charge consumption decay is proportional to the increase of the ionic radius. This may be associated with a reduction in their solvation layer. Although the discussion here is based mainly on the size of the ionic radius for metal cations, it is important to say that associated with the ionic there are some other important parameters, such as: i) hydration radius , which is inversely proportional to the ionic radius; ii) cation acidity, also inversely proportional to the ionic radius and; iii) the ionic mobility in front of

an electric field, proportional to the size of the hydration radius [65,66]. Especially the ionic mobility and the hydration radius are important parameters for IPMC because they are closely related to the maximum blocking force and actuation speed, respectively. On the other hand, H+ presented an exceptionally large charge consumption, which did not increase with humidity; once again, as explained before, this fact is associated with the electrolysis reaction. Finally, IL presents a charge consumption above that of K+. In this case, it is expected that dipole effects may lead to this behaviour [67].

Figure 6 – a) Electrical charge as a function of RH, b) electromechanical efficiency and c) IPMC Coulombic Efficiency in the function of RH. A spline function was used to connect the data points.

On the other hand, the Coulombic Efficiency describes the charge consumption efficiency in reversible devices. CE close to 100% means that the device uses the same amount of charge to be operated on both sides. Likewise, efficiency greater than 100% means that there are a higher positive charge consumption, and vice versa [68]. In general, IPMC samples showed a good CE, up to 92% in the worst case (IPMC-K at RH = 30%). As can be seen, CE is also dependent on RH and the counterion size. CE increases when RH increases, reaching values above 99% when the hydration level is 90% for all IPMC samples. However, as RH decreases, a greater variation in efficiency is observed between samples. For RH = 90% CE varies about 1%, while for RH = 30% CE varies about 5%. It indicates that there is ion migration preference during discharging instead charging cycle, and it is more pronounced in low RH. Levitsky et al. [69] demonstrated that the actuation process leads to the asymmetry in the electrochemical properties of the cathodic and anodic interfaces. It results in different

electrochemistry properties along with the actuator. It has also been reported that electrical current is associated with two major competing processes of electrostatic movement of ions and water molecules [70]. For his reason, ion migration preference during the discharging cycle was observed. Zhu et al. [71] related IPMC current response with the quantities of the accumulated cations in the electrode/ionomer interface. It was observed that as high is the current response higher will be the electrical charge accumulated at the interface. Thus, larger ions migration is more difficult, probably due to the ratio between these and the ionomeric channels. Moreover, when we compare the coulombic efficiency and the electromechanical responses (Figure 6b), it is evident that both the hydration degree and the counterion type have a great influence on these properties. On the other hand, it can be observed for CE, the counterion influence is attenuated with the increase in the membrane hydration degree. That is, when the membrane is in a humid environment, the properties' differences are less evident, it means that the membrane hydration degree influences more the IPMC properties.

3.3 Performance Analysis As already described, in order to evaluate their performance, the samples were submitted to strenuous actuation cycles for 18 hours. Each complete cycle lasted about 32 seconds. The test was performed in a controlled environment with a constant temperature of 24 °C and relative humidity of 50%. Thus, in Figure 7a, blocking force variation versus actuation time is presented, while Figure 7b shows a performance loss versus time. As observed, the blocking force decreased over time for all samples. So, IPMC's performance also decreased. IPMC-Li, IPMC-K and IPMCBMIM samples showed the worst performance after 18 hours of actuation. On the other hand, IPMC-H

and

IPMC-Na presented better results. Working with this type of composite in the air has always been a challenge to overcome, even though some works [72–75] have already tested alternatives to improve IPMC performance, especially about solvent evaporation and excess electrode fractures [76]. Park et al. [77] conducted a study that investigated the effect of solvent evaporation and IPMC electrodes degradation after various cyclic bending movements. Tests were performed in aqueous solution and in air. They concluded that blocking force resulting from flexions decreased rapidly when it was acting in air. The rapid loss of water to the environment is generally associated with both solvent electrolysis and the appearance of cracks and cracks in the metal electrode. Kin et al. [78] developed a new electroactive actuator composed of ionic sulfonated block copolymers. The authors present a significant improvement in working performance (13,500 cycles) in the air when a voltage below 1V is applied, which ensures that no solvent decomposition reaction occurs.

Figure 7 – IPMC performance for 18 hours of operation. a) Blocking force variation with time; b) IPMC performance after 18 hours. RH = 50%. A spline function was used to connect the data points.

Finally, to analyse the morphological change of the platinum electrodes, Scanning Electron Microscopy micrographs were obtained from the newly synthesized IPMC and from the different samples after 18 hours of operation (Figure 8). The region of greatest deflection in the sample was chosen for image acquisition. As can seem, the IPMC surface before testing exhibits no cracks at the surface. After 4000 cycles, a higher crack density was observed. As samples were tested in a controlled environment (RH = 50%), IPMC polymer matrix expansion and contraction were avoided. It has been reported that bending actuation creates cracks on the surface of the electrode, and it deteriorates surface conductivity [79]. This drawback is more pronounced when bending amplitude and number of cycles are increased [80]. Also, under continuous bending cycles, cracks are generated along the grain boundaries, further increasing resistivity [81]. Since electrodes resistance increases, the electric field generated between then decreases. So, cations and water molecules migration are hindered, lowering IPMC performance. Since it was conducted in a controlled environment, it can be assumed that these cracks did not originate from IPMC expansion and contraction. In fact, solvent evaporation affects surface electrodes resistance [81]; however, it is an issue under air operation conditions. Also, it is interesting to note that each sample presented different crack densities. IPMC-Li sample showed

higher cracks density and high-performance loss (Figure 7b). On the other hand, a few cracks were observed at the surface of the IPMC-BMIM sample.

Figure 8 – IPMC SEM images. a) IPMC sample before actuation; b) IPMC-H after 18 hours of actuation; c) IPMC-Li after 18 hours of actuation; d) IPMC-Na after 18 hours of actuation; e) IPMC-K after 18 hours of actuation; f) IPMC-BMIM after 18 hours of actuation. RH = 50%.

Considering that water electrolysis is present during this experiment, and cracks density is different for each sample, it seems that the cracks are more result of samples displacement than water evaporation. Moreover, solvent evaporation is not the most pronounced effect that causes cracks and performance loss, because IPMC-BMIM has a better capacity against water loss than other counterions used in this work but showed greater performance loss. The importance of conducting new experiments with longer test time is emphasized. In future work, further testing will be performed over long working hours with variations in relative humidity to more accurately assess the performance of the IPMC as an actuator.

3. CONCLUSIONS It is noticeable that due to the properties of the Nafion membrane, the IPMC has a large water absorption capacity, which is very dependent on the counterion used, as larger is the cation, lower is the polymer water absorption. Furthermore, as the hydration level increases, the average separation between the ionic clusters increases with a simultaneous reduction in the number of clusters, justifying a high diffusivity between the channels. Thus, IPMC devices blocking force and

the actuation rate followed the same order, indicating that the smallest the counterion, the greatest is ion diffusion/migration. In addition, no back relaxation was observed for low polymer matrix hydration level, and when it was incorporated with IL, this observation corroborates the proposal that the back relaxation occurs when non-bonded, i.e., free H2O molecules migrate in the opposite direction after anode saturation. Also, IPMC containing IL as a counterion has been shown to be an excellent option to avoid variations in membrane hydration level and, consequently, variations in electromechanical behaviour. Thus, high RH and small cations combination promotes higher displacement speed and high blocking force, but with greater back relaxation. In contrast, larger cations combined with low RH result in lower blocking force and slow displacement speed, but no noticeable reverse relaxation. The complex behaviour of ions inside of the ionomeric chains causes the IPMC to acts as an RC circuit. Thus, the current response showed similarities with the charging/discharging of a capacitor. The energy expenditure (electric charge) for the device's operation was coherent for alkali metal cations. On the other hand, H+, due to intense parallel reaction and BMIM+ due to an organic cation, showed peculiar behaviours. Although CE is also dependent on RH and the counterion size. in general, IPMC samples showed a good CE, up to 92% for RH = 30%. Just like any other electrochemical device, the IPMC performance decreased over time. Furthermore, IPMC-Li showed an excessive performance loss due to high cracks density after actuation. On the other hand, IPMC-H was the best sample tested, with low-performance loss, even with a high solvent loss due to electrolysis. Therefore, after the results of this work, it is possible to evidence that RH > 75% and small counterions (H+ and Li+) are the best combinations for IPMC actuation. It is noteworthy that new trials with a larger number of cycles and for a longer time will be performed in future works in order to corroborate more clearly the evidence obtained in this work.

4.

ACKNOWLEDGEMENTS This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de

Nível Superior - Brasil (CAPES) - Finance Code 001. The authors would also like to thank the CNPq and FAPESP (process #2018/07001-6, #2018/10843-9, and #2018/09761-8) funding agencies.

APPENDIX A. SUPPLEMENTARY INFORMATION Supplementary information is available.

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HIGHLIGHTS •

Nafion/Pt composites with several cations were tested in different humidities;

•

High humidity and small cations combination promote better device performance;

•

H+-samples showed atypical current response associated with electrolysis process;

•

Hydration level exerts more influence than counterion type on device properties;

Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: