Electrochemical and Electromechanical Study of Carbon-Electrode

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Electrochemical and electromechanical study of carbon electrode - based ionic soft actuators Haleh Rasouli, Leila Naji, and Mir Ghasem Hosseini Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03030 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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Industrial & Engineering Chemistry Research

Electrochemical and electromechanical study of carbon electrode - based ionic soft actuators Haleh Rasouli1, Leila Naji*1, Mir Ghasem Hosseini2, 3

1

Department of Chemistry, Amirkabir University of Technology, 424 Hafez Avenue, Tehran P.O

Box: 15875-4413, Iran. 2

Electrochemistry Research Laboratory, Department of Physical Chemistry, Chemistry Faculty,

University of Tabriz, Tabriz, Iran. 3

Engineering Faculty, Department of Materials Science and Nanotechnology, Near East

University, 99138 Nicosia, North Cyprus, Mersin 10, Turkey.

* Corresponding author to whom all correspondence should be directed. Tel: +98 (21) 64542767; Fax: +98 (21) 64542762; e-mail: [email protected] (Leila Naji)

Keywords: Metal-free ionic actuators; Vulcan carbon; Multi-walled carbon nanotube; Electrochemical and electromechanical study.

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ABSTRACT In the current work, metal-free Vulcan carbon/MWCNT (V/M) – based soft actuators were fabricated by physical deposition of V/M ink on both faces of Nafion membrane. The effects of the weight percentage of MWCNTs on the electromechanical and electrochemical behaviors of actuators were studied through extensive experiments. Morphological characterization showed that the electrode structure of actuators changed considerably by changing the MWCNT content of the electrode layer. Electrochemical studies revealed V/M electrodes improve the ionic conduction and capacitive behavior of the Nafion-based actuators. The electro-chemomechanical performance of actuators was compared by measuring maximum tip displacement, rate of displacement, durability, waveform analysis and specific electro-mechanical energy efficiency. The highest tip displacement (44.5 mm) was obtained in actuators comprising of 20 wt% MWCNTs. These actuators showed considerably higher ionic conductivity (26.9 mS/cm) and capacitive characteristic (45.2 µFcm-2) compared to the Pt-based actuators.


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1. INTRODUCTION Smart materials which can perceive external stimuli and respond accordingly have been applied for fabrication of actuators1-3. Ionic electroactive polymers (EAPs) have been considered as promising materials for fabrication of soft electromechanical actuators since they are flexible, lightweight, inexpensive and can be formed to any shape and size4-7. Ionic polymer-metal composites (IPMCs), which are classified as ionic EAPs, undergo large bending deformations as small voltages (1-5 V) are applied across their thickness. The actuation performance of IPMCs is relatively quick and it is not associated with production of electromagnetic waves or heat8. Considering these interesting properties, IPMCs are attractive materials in fabrication of soft actuators for medical, industrial and military applications3, 9, 10. Conventional IPMC actuators are composed of cation exchange polymers sandwiched between two metal-plated electrodes such as Au, Pt or Ni11-13. Perfluorinated sulphonic acid cation exchange membranes such as Aciplex, Flemion and Nafion are commonly used for preparation of IPMCS. These polymers can imbibe ions and polar solvents such as water due to the presence of fixed ionic groups at the end of their side chains14,

15

. The metallic electrodes are deposited on both faces of the cation exchange

membrane via an electroless metal electroding process3. Through this chemical process, metal nanoparticles form a dense layer at the top surface of the membrane and they also penetrate into the subsurface of the membrane and form a dendritic metallic structure at these regions. This process may be repeated many times to improve the electrode surface structure and surface conductivity3,

16

. In general, the actuation mechanism of IPMCs is described considering the

strong impact of hydraulic and electrostatic effects17-19. Therefore, the hydration level, ionic content, the surface conductivity and the flexibility of the electrode layers of IPMCs play key roles in the intensity of their response20.



Researchers have shown that carbon-based nanostructure materials including carbon nanotubes (CNTs)21-23, fullerenes24, 25, graphene26, 27, graphene oxide 28, 29 and carbon blacks (CBs)30, 31 can be applied as electrode materials for fabrication of metal-free actuators since they are relatively inexpensive and have high electrical conductivity, unique mechanical properties and large surface area32. It has been established that using carbon nanostructures as electrode materials improves significantly the response speed, actuation tip displacement and force output of actuators10, 33-35. The positive impact of carbon nanostructures on the actuation performances of ionic actuators has been attributed to higher porosity of carbon-based electrode layer which can facilitate the diffusion of hydrated ions within the membrane and promote the electric double layer formation at these regions6, 36. Carbon nanostructures-electrodes can also undergo volume changes by intercalation of ions, leading to a larger swelling imbalance at the electrode regions and so larger bending deformations37. CNTs possess unique properties such as excellent morphological, electrical, optical, thermal, chemical, and mechanical properties and nanometer size and they have been considered as promising materials in fabrication of supercapacitors38-40. Multi-walled carbon nanotubes (MWCNTs) could provide mechanical strength stronger (up to 15-20 times) than steel while they are significantly lighter (five times ) and have thermal conductivity

comparable to that of diamond and

higher than copper13. The electrical

conductivity of heavily doped MWCNTs can support comparable current densities to copper on a per weight basis41. Single-walled carbon nanotubes (SWCNTs) can generally outperform MWCNTs in applications such as double-layer capacitor electrodes42. However, IPMCs represent a special case where greater bundling between SWCNTs can result in a lower effective capacitance and may ultimately fall short of the performance compared to MWCNTs43. It has been established that


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functionalized MWCNT- based actuators exhibit higher actuation performance compared to nonactivated MWCNT44,

45

owning to their higher specific surface area, higher hydrophilicity32.

However, one of the most important challenges for application of MWCNTs in the electrode structure of ionic actuators is increasing their conductivity to a higher level. The outer wall of MWCNTs is conducting while the inner wall is not involved in conductivity. To overcome this problem, combination of MWCNTs with other materials such as PANI, activated carbon powders and mesoporous silica have been reported46, 47. Palmre et al. has shown that combining CNTs with activated carbon nanoparticles improves significantly the capacitive characteristic of the electrode layer over the pure CNTs electrode47. CBs have already been applied as electrode material in fabrication of fuel cells48, capacitors49 and as additives for

metal oxide nanoparticles in order to enhance

capacitance50. With low cost, high surface area and high ionic conductivity, CBs can be considered as a good alternative material for conventional noble metal electrodes49, 51. Vulcan carbon is well known as industrial standard CBs since it has low sulfur content and ionic contamination and is well suited for use in anti-static and conductive applications. There is also a good correlation between the conductivity and the graphitic character of Vulcan carbon surface. The main drawback of vulcan carbon as electrode material is its low electrical double layer capacitance52. Compositing vulcan carbon with other conductive materials such as conducting polymers48, metal oxides53 and other carbon nanostructures including MWCNTs54 has been suggested as a suitable method to diminish this problem. In our previous study, Pt electrodes were replaced with PPy/CB/MWCNT electrodes. The electrodes were

prepared by

electrodeposition of PPy on both surfaces of CB/MWCNT-coated Nafion membranes. The



prepared actuators showed ionic conductivity of 29.88 mS/cm and capacitive characteristic of 39.23 µF which were higher compared to the Pt-based actuators37. In the current work, we demonstrate the fabrication of metal-free Vulcan carbon/MWCNT (V/M) – based electrode ionic actuators using Nafion. Functionalized MWCNTs was added to vulcan carbon to facilitate the diffusion of hydrated ions within the electrode layers by introducing channels and cavities in these regions. The electromechanical performance of actuators was compared by measuring their maximum tip displacement, rate of displacement, durability and specific electro-mechanical energy efficiency. Waveform analysis of applied voltage and current was also carried out to follow the effect of MWCNT content on the capacitive characteristic of actuators. Current-voltage, displacement-voltage and displacementfrequency characteristics of actuators were studied as a funcion of the MWCNT weight percentage in the electrode layer and compared with vulcan carbon- and Pt-based actuators as standards. To describe the actuation performance of the prepared actuators their morphological, physicochemical and electrochemical properties of actuators were compared. The highest tip displacement (44.5 mm) was obtained in V/M-based actuators comprising of 20 wt% MWCNT which possessed ionic conductivity of (26.9 mS/cm) and capacitive characteristic (45.2 µFcm-2) of about 10 and 44 times higher than that of Pt-based actuators, respectively. Furthermore, the specific electro-mechanical energy efficiency and the maximum tip displacement of these metalfree actuators were two times higher.

2. MATERIALS AND METHODS 2.1. Pretreatment of Nafion membrane


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Prior to fabrication of actuators, Nafion membrane (DuPont) was cut in 3×4 cm size, roughened at both faces using an abrasive paper and cleaned using an intensive cleaning procedure described elsewhere3, 55.

2.2. Preparation of V/M ink solutions Five V/M ink solutions were prepared by adding 0, 10, 20, 30 and 40 mg of MWCNTs (Neutrino) to 60 mg Vulcan carbon (Cobat) and 30 mg of Nafion solution in 3:1 v/v of isopropanol: distilled water. The mixture was put in the ultrasonic for 1.5 h.

2.3. Fabrication of V/M-based actuators The fully-dehydrated membrane (dried in an oven at 70oC) was placed in a Plexiglas holder shown in our previous work37 and then put on the hot plate at 60℃. Then, a same volume of the prepared V/M ink was carefully sprayed on both faces of the Nafion membrane using a jet spray. The membranes were then cut in pieces by dimension of 4 cm × 0.5 cm and stored in deionized water for further experiments. Schematic representation of the fabrication process of V/M-based actuators is shown in Figure 1. Using the same procedure five types of actuators was prepared which differed in the MWCNT content of the electrode layers. The actuators were named as IPV/Mx which x denotes the applied weight percentage of MWCNT. Actuators containing 0 wt% of MWCNT were named as IP-V. Pt-electrode based IPMC actuator (named as IP in this work) prepared using an electroless plating method described in our previous work11. Pt-electrode IP actuator had the same dimension (4 cm × 0.5 cm) as the V/M-electrode based actuators.

2.4. Physicochemical and morphological characterization of actuators



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Field emission scanning electron microscopy (FE-SEM, MIRA3FEG-SEM, Tescan) and transmission electron microscopy (TEM) were used to investigate the morphology of the prepared actuators. A dilute suspension of the carbon black in isopropyl alcohol was employed for preparing samples for TEM imaging. TEM images was obtained by LEO 906 E (100 kV) at 150 nm and 50 nm magnification levels. ATR-FTIR analysis was performed to determine the extent and nature of chemical interactions between V/M electrodes and Nafion membranes. The hydrated thickness of the actuators was measured at multiple points using a digital micrometer. The Water uptake (WUP) and ionic-exchange capacity (IEC) of the prepared actuators were measured using methods described elsewhere11, 37.

2.5. Electrochemical characterization of actuators The electrochemical behaviors of the prepared actuators were investigated using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The measurements were carried out using a Princeton Applied Research, EG&G PARSTAT 2263 system Advanced Electrochemical system run by Power Suite software. The impedance measurements were performed by applying a.c. voltage of ± 5 mV in amplitude over a frequency range of 100 kHz to 10 mHz at ambient temperature. Z-view software was used to analyze the results. The ionic conductivity (σ) of actuators in S/cm unit was obtained using Equation (1), where, Rct is the charge transfer resistance (Ω), l is the thickness of actuators (cm), and A is the area of the actuators (cm2)37. σ = l /Rct A

Equation (1)

CV scans were performed in the potential range of -0.2 to 1.0 V at a scan rate of 30 mV s-1.

2.6. Electromechanical characterization of actuators


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To measure the maximum tip displacement of the prepared actuators, they were immersed in a saturated solution of LiCl for 5 d to allow population of all the ion exchange sites (the -SO3groups) with the lithium cations11. Maximum tip displacement, rate of tip displacement, durability and lifetime of the prepared actuators were measured at ambient conditions as described in our previous work3. A digital camera was used to capture video images of the bending deformation of the actuator as the d.c. or a.c. voltages were applied. The maximum tip displacement was calculated by measuring the distance between the starting point and end point of the free end of the prepared actuators, using image analysis software3. From the resulting displacement signal, strain difference (Ɛ) between the electrodes was calculated using Equation (2)56. Ɛ=

Equation (2)

where Ɛ, σ, d and L are the strain difference, half of the peak to peak displacement, the hydrated thickness of the actuator and the length of the actuator, respectively57. Displacement rate and water loss of the actuators was determined using methods described elsewhere3, 11. To determine the double layer capacitance and specific electro-mechanical energy efficiency (SEEE) of actuators, current–voltage curves were obtained using a digital storage oscilloscope (GPS- 2105S) and a waveform function generator (Yokogawa Electric, FC 200). To this end, one end of the actuator was clamped horizontally in the sample holder, described above and connected to the two terminals of the function generator. A sinusoidal voltage was applied across the sample under study and the sensing voltage and the output current of the actuators were monitored58. Schematic depiction of the set-up used for the current–voltage measurements is shown in Scheme S1 (see Supplementary Information). With the oscilloscope in the x-y mode,



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the voltage across the actuator is VA and the input voltage is VB. The current, I, can be determined from the voltage drop across the resistance, RM, using following equation: I=

Equation (3)

The SEEE of the prepared actuators can be defined using Equation (4) with the reference value of the IP actuator24. ɳ IP-V/M =

ɳ / ɳ

=(

/

) (

!"

!" /

)

Equation (4)

)*+',- are the SEEE of actuators, the maximum tip displacement of the where ɳ , W%&' and E actuators at the applied frequency and the dissipated electrical input energy per area of the ) &+',- of actuators was calculated using an equation described electrode surface, respectively. E elsewhere59.

3. RESULTS AND DISCUSSION 3.1. Surface morphology of actuators TEM images of Vulcan carbon inks containing (a) 0 wt% and (b) 20 wt% of MWCNT are shown in Figure 2. To facilitate the comparison of the samples, TEM images with two different magnifications located in each column. As can be seen in Figure 2 (a and a'), Vulcan carbon form different sized aggregates made of 10-20 nm primary nanoparticles. However, adding MWCNTs limited the aggregation of carbon nanoparticles and a finer nanostructure was formed in the V/M ink sample. Cross-sectional SEM images of IP, IP-V and IP-V/M20 actuators in Figure 3- parts (a-c) confirm the formation of two parallel electrodes on both faces of Nafion membranes. As can be seen, the electrode thickness in IP-V (10.51 µm) and IP-V/M20 (10.57 µm) actuators was considerably controlled using the definite number of spraying and applying


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constant volume of ink solutions. Figure 3 - parts (a'-c') and (a"-c") shows longitudinal SEM images of the electrode region in IP, IP-V and IP-V/M20 actuators at two magnifications. The surface morphology of actuators is an important parameter since it affects the capacitive characteristic, the magnitude and the speed of the bending deformation. The SEM images revealed that the surface morphology of the prepared actuators changed considerably as MWCNTs was added to Vulcan carbon and a finer electrode structure was formed in the IPV/M20 actuator. This was attributed to the barrier effect of MWCNTs in aggregation of Vulcan carbon nanoparticles as observed in TEM images in Figure 2.

3.2. Physicochemical characterization of actuators ATR-FTIR spectra of the prepared actuators are compared in Figure 4. Nafion membranes have strong bands at 1216and 1198 cm-1, which correspond to the C–F stretching modes, whereas peaks between 1090–1038 cm-1 and 960–938 cm-1 are assigned to S–O stretching and C–O–C stretching, respectively. The weak band between 623–615 cm−1is attributed to the C–S stretching vibration31. The strongest band at 3400 cm-1 is related to the to the O-H symmetric stretching in SO3H groups of Nafion backbone. As Figure 4 shows, the intensity of all peaks in IP-V actuator decreased extremely compared to bare Nafion membrane due to the deposition of Vulcan carbon nanoparticles at the surfaces of the membrane. The intensity of (C=O) and (O-H) bands increased by increasing the MWCNT content in the electrode layer from IP-V/M10 to IP-V/M40. The major difference between the ATR spectra of the actuators is the appearance of the broad band between 3600 and 3100 cm−1 indicating the interaction of –OH groups of MWCNTs with Nafion membranes. The band assignments were summarized in Table S1 (see Supplementary Information).



IEC, WUP and the hydrated thickness of the prepared actuators are shown in Table 1. IP-V/M10 actuator with the lowest MWCNTs content exhibited IEC and WUP of about 14.67% and 9.37% higher than that considered for IP-V actuator, respectively. As the MWCNT content was increased, significant enhancement in IEC, WUP and hydrated thickness of samples occurred, in which IP-V/M40 actuator showed the highest values. The variation in IEC and WUP of the prepared actuators versus MWCNTs content is presented in Figure S1. As can be seen, WUP and IEC reached a threshold at 71% and 9.13 (meq/g) respectively for IP-V/M20 actuator and afterward adding higher amount of MWCNTs affected these parameters negligibly. This was attributed to the formation of denser electrode structures in actuators comprising of higher (>20%) MWCNT content. It is interesting to note that the IP-V/M20 actuator appeared to have WUP and IEC of approximately three and eight times higher compared to Pt-based IP actuator, respectively. This emphasize that the deposition of V/M on the Nafion membrane promote water and hydrated ion absorption into the Nafion membrane due to the higher porosity and hydrophilicity of the electrode layer in the IP-V/M actuators.

3.3. Electrochemical characterizations of actuators It has been shown that the capacitance of electrodes directly influences the magnitude of the bending deformation and response time of ionic actuators58. The capacitive characteristic of actuators is strongly dependent on the porosity, hydrophilicity and chemical affinity of the applied electrode material. CV measurements were applied to follow the effects of MWCNTs wt% on the capacitive behavior of the V/M-based actuators. Cyclic voltammograms of the prepared actuators are compared in Figure 5- part (a). The symmetric shape of voltammograms implies that two comparable electrode layers have been formed via physical deposition of V/M


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ink on the Nafion membrane, providing excellent charge distribution in the electrode region of the prepared actuators3, 58. The current response of the prepared metal-free actuators increased by increasing the loading levels of MWCNTs from 0 to 20 wt% while an insignificant current difference was observed between the IP-V/M20, IP-V/M30 and IP-V/M40 actuators. In other words, the current remained approximately constant as MWCNT increased from 20 to 40 wt%. This was attributed to the negligible differences in WUP and IEC of these actuators as shown in Figure S1. Comparing the CV voltammograms of actuators indicates that compositing Vulcan carbon with MWCNTs leads to higher current responses and so higher capacitive characteristics. EIS measurements were carried out to determine the effects of MWCNT content on the ionic conductivity, double layer capacitance, ionic diffusion and charge transfer resistance of the prepared actuators. The Nyquist plots and the fitted equivalent circuits are shown in Figure 5parts (b) and (c), respectively. Nyquist plots of all metal-free actuators appeared linear in the low frequency region, indicating capacitive behavior of actuators at this region. For these actuators, a semicircle was also observed in the high frequency region which was attributed to the resistor behavior of actuators. For Pt-based IP actuators, only a small semicircle appeared at high frequency region which was considerably smaller than that obtained for metal-free actuators, implying that the IP sample possess the lowest charge resistance37. Amongst the prepared actuators, the steepest line in low frequency region was observed for IP-V/M40 actuators (see Figure 5 (b)). This indicates that the diffusion processes occurred at higher rate in this sample, resulting to higher charge storage capacity. Comparing Nyquist plots of metal-free actuators also revealed that compositing Vulcan carbon with MWCNT results in lower ionic resistances. This was deduced from the observation of the semicircle at higher frequencies for IP-V/M actuators compared to the IP-V actuator. The inset in Figure 5 (b) illustrates that by increasing the



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MWCNTs content of the electrode layer the intercepts of the Nyquist plots on the x-axis11 (which is used to determine the ionic and electrical resistance) moves steadily to higher frequencies. IP-V/M40 actuator appeared to have the lowest electrical and ionic resistances. The ionic conductivity is described by diffusion (vehicle) and hopping (Grotthuss) mechanisms60, 61. EIS results revealed that diffusion and hoping processes happen easier in IP-V/M actuators compared to IP-V actuator since they showed to have greater IEC and WUP content (see data shown in Table 1). It was also found that adding MWCNTs to Vulcan carbon in the electrodes could improve the capacitive characteristic and ionic conduction. EIS data was further analyzed by fitting to equivalent electrical circuit model (see Figure 5 (c)) consisting of RS, CPEdl, Rct and W that represent the solution resistance, double layer capacitance, charge transfer resistance of double layer and Warburg diffusion element, respectively37. The CPEdlRct component originates from the charging-discharging process at the electrolyte/electrode interface62. The CPE is defined by two frequency-independent parameters, Tdl and ϕ, in the following equation63: ZCPE = 1/Tdl (jω)ϕ

Equation (5)

Tdl is comparable to admittance and ϕ represents the roughness of the electrode layer which can vary between 0 to 1, implying resistive to ideal CPE behavior37, 64. The mean error for metal-free actuators is less than 1%, indicating a good fitting of the EIS data. The obtained results in Table 2 show that the Φ values decreased from IP-V to IP-V/M10 due to a decrease in the electrode surface homogeneity. Φ value appeared to be almost constant for all Vulcan/MWCNT-based actuators. Pt-based IP actuator showed the lowest ionic conductivity (σ) and the Tdl. The ionic conductivity of the actuators improved by increasing wt% of MWCNTs. IP-V actuator with no MWCNTs showed the lowest Tdl and σ among metal-free actuators. For all actuators, the real capacitance (C) was calculated using Equation (6)65. 14 ACS Paragon Plus Environment

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C=

(/01 . 3" )4/5

Equation (6)

3"

The variation in the solution resistance (Rs), ionic conductivity (σ) and real capacitance (C) of the prepared actuators are compared in Figure 6. The C and σ values increased by increasing MWCNTs content and a reverse trend was observed for the Rct in which the metal free IP-V actuator containing no MWCNTs exhibited the highest Rct among all actuators. IP-V/M40 comprising of 40 wt% of MWCNTs showed the highest σ and C which were about 47 and 11 orders of magnitude greater than that considered for Pt-based IP actuator. The surface resistance, Rs, of the IP-V/M40 actuator was also about ~44% lower. This was attributed to the highest water and ionic content of this actuator as shown in Table 1. This suggests that the V/M nanocomposite contributes in the formation of larger double layer due to its more porous and flexible structure which facilitate the insertion of ions at the electrode regions and imbibing of a higher amount of electrolyte in the actuators.

3.5. Electro-chemo-mechanical characterizations of actuators The actuation performance of the prepared actuators is outcome of the migration of Li ions through the Nafion membrane3. As mentioned earlier, upon application of an electric field, hydrated Li ions migrates toward cathode electrode. The accumulation of Li ions near the cathode electrode results in an imbalance in volume within the actuators and thus a bending deformation occurs37,

66

. The tip displacements of actuators in response to d.c. voltages are

presented in Figure 7 (a). On application of d.c. voltages, the prepared actuators bent instantly towards the positively charged electrode (anode), and as the amplitude of the applied voltage was increased from 1 to 6 V larger tip displacements were recorded for all actuators. The maximum



tip displacement in all input voltages was significantly larger for IP-V/M compared to IP-V, indicating the positive influence of MWCNTs on the actuation response of actuators. For the metal-free actuators, the magnitude of the tip displacement increased from IP-V to IP-V/M20 and then decreased gradually as shown in Figure 7 (a). This was in good agreement with the observed differences in WUP, IEC, capacitive characteristics and also the surface morphology of the prepared actuators11. Figure 7 - parts (b) shows the overlaid digital images for the metal- free actuators in response to the d.c. voltages, captured from the starting point to the end point. On application of 6 V d.c. potential IP-V/M20 actuator (shown in Figure (b3)) showed ~ 44.5 mm tip displacement which was approximately two times higher than that of the conventional Pt- based IP actuator (~ 23 mm)3. Surprisingly, the tip displacement of IP-V/M40 actuator with the highest capacitive characteristic and ionic conductivity was about 1.5 times lower than that observed for IP-V/M20 while it was still higher (94%) than that of Pt-based IP actuator. Figure 8 demonstrates the tip displacements response of the prepared actuators to the a.c. voltage of 4 V in amplitude over a frequency range of 0.01-1 Hz. Similar variations in the measured tip displacement against amplitude and frequency of the applied voltages were recorded for all metal-free actuators and Pt-based IP actuator. An inverse relationship between tip displacement and frequency was observed. The tip displacement of the IP-V/M20 and IP-V actuators differed significantly at 0.01Hz frequency. This was attributed to the concentration of a higher number of ions near the oppositely charged electrodes at low frequencies since the polarity of the electrodes changes slowly3,

67

. The measured tip displacement of the IP-V/M20 actuator comprising of 20 wt%

MWCNTs was also about three orders of magnitude higher than that considered for the IP actuator at 0.01 Hz.


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In Figure 9- part (a), the displacement rates (in mms-1) of the prepared actuators are compared. In response to a 4 V d.c. voltage, the IP-V/M20 actuator exhibited the fastest displacement rate while the IP-V actuator represented the lowest response under application of similar conditions. IP-V/M30 and IP-V/M40 actuators which appeared to have larger capacitances (see Figure 6) compared to the IP-V/M20 actuator, their displacement rate was significantly lower than that considered for IP-V/M20 actuator. This was assigned to the observed differences in the surface morphology and rigidity of these actuators. The displacement rate of IP-V/M20 actuator was respectively about 1.6 and 6 times greater compared to the IP and IP-V actuators. This indicates the promoting effect of V/M nanocomposite on the performance of the Nafion-based soft actuators. Figure 9(a) also illustrates that the displacement rate of the IP-V actuator is much slower than that of Pt-based IP actuator, while this sample showed to have higher WUP, IEC and capacitive characteristic. This was attributed to the different electrode structure of these sample formed through dissimilar routes. Pt electrodes were deposited on both faces of Nafion through a chemical electroless plating process while the deposition of Vulcan electrodes was carried out using a physical method. Pt electrodes are formed by immersing the Nafion membrane in a solution of Pt salts and reducing the absorbed Pt cations to Pt0 using a reducing agent67. This leads to the formation of two distinct Pt layers at each surface of the Nafion membrane: a thin dense layer (~ 1–2 µm) and a porous layer (~ 320 µm) extending into the membrane67. However, in Vulcan- and Vulcan/MWCNT- based actuators the non-metallic electrodes are formed only at the surfaces of the membrane. The penetration of Pt at the sub-surfaces of the polymeric membrane improves the charge transfer processes at the interface of Pt electrode and the Nafion membrane. This affects the rate of double layer formation in vicinity of electrode regions and so the rate of displacement.



In response to the applied 5 V d.c potential, the variation in the tip displacement of the prepared actuators over time was recorded and presented in Figure 9- part (b). The tip displacement of all actuators decayed gradually over time from a maximum value to zero. However, the deactivation of actuators occurred at different time range. The IP-V/M20 actuator became inactive after approximately 180 min. This time was significantly (~ 2.4 times) higher than the time required for IP-V (with no MWCNTs) actuator to become inactive. Therefore, the obtained results suggest that the dispersion of appropriate amount of MWCNTs (here 20 wt%) in Vulcan carbon significantly reduces water loss of the metal-free actuators11. The variation in water loss, tip displacement and the strain difference between the electrodes of the prepared actuators as a function of electrode composition are shown in Figure 10. The tip displacement and the strain difference between the electrodes increased from IP-V to IP-V/M20 and afterward decreased gradually while the variation in the tip displacement was more severe. This trend was reversed for water loss of the actuators in which IP-V/M20 actuator showed the lowest water loss (20%). Amongst all actuators, Pt-based IP actuator exhibited the highest water loss (~59%). Pt-based IP actuator showed a lower tip displacement (~3 times) and strain difference between the electrodes (9%) compared to the IP-V/M20 actuator and its water loss was about 159% higher, probably due to the higher rate of electrolysis and evaporate of water. To evaluate the effect of MWCNTs content of electrodes on the electrical characteristics of the prepared actuators, the current, I, and the applied sinusoidal voltage, V, with amplitude of 4V at frequency of 10 Hz, as a function of time were recorded The waveforms obtained for the prepared actuators are shown in Figures 11. The generated current appeared sinusoidal for all actuators while it was out of phase to the original applied voltage. The phase difference of the IP, IP-V, IP-V/M10, IP-V/M20, IP-V/M30 and IP-V/M40 actuators was estimated to be about 24°,


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36°, 54°, 81°, 72° and 63°, respectively. Results shows that the phase difference between I and V the prepared actuators increased with increasing MWCNT content to 20% in which the IPV/M20 showed the highest phase difference. Furthermore, the current generated in IP-V/M20 was considerably higher (~ 2 times) compared to IP-V. However, the current density and phase difference of IP actuator was found to be approximately 28% and 4 times lower than that considered for IP-V/M20 actuator, respectively, indicating lower charge storage capacity of the Pt-based actuators. To study the influence of the magnitude of applied frequency on the phase difference of I and V, the waveform analysis was repeated for the IP-V/M20 actuator at frequencies lower than 10 Hz (0.5 and 4 Hz) while the applied a.c. voltage was 4 V. As results presented in Figure S2– parts (a) to (c) (see Supplementary Information) show, the amplitude of I increased by 3 order of magnitude as the frequency was increased from 0.5 Hz to 10 Hz, indicating a decrease in the electrical impedance, Z, of the IP-V/M20 actuator at higher frequencies. The phase difference between V and I decreased from 81° to 46° as the frequency of the applied a.c. voltage increased from 0.5 Hz to 4 Hz and reached to 25° at frequency of 10 Hz. These results are in good agreement with data obtained from EIS measurements shown in Figure 5- part (b). As discussed earlier, at low frequencies the diffusion process occurs easier and the prepared metal free-actuators behave like capacitors, while at high frequencies actuators behave mostly like resistors due to fast polarity change of electrodes. At low frequencies, a greater number of hydrated Li+ ions can diffuse towards the cathode, leading to higher capacitive characteristic and so larger tip displacement (see Figure 8). For an ideal capacitor, Z changes inversely corresponding to the magnitude of frequency. Therefore, from results presented in Figure S2, it can be deduced that the prepared metal-free IP-V/M20 actuator acts mainly like capacitors.



Figure 12 shows the hysteresis behaviors of actuators in the potential range of −4 to +4 V and the excitation frequency of 0.5 Hz (at a constant resistance of 1.3 Ω) As can be seen, the V/M- based actuators showed higher current density and dynamic lag compared to IP-V and IP actuators. The variation of current density and dynamic lag as a function of the weight percentage of MWCNTs was similar to the results obtained from the tip displacement measurements. The highest current density and dynamic lag was seen in IP-V/M20 actuator with the highest tip displacement of 44.5 mm. The calculated SEEE values of actuators are listed in Table 3. The electrical input density energy required to activate the actuators increased from IP-V to IP-V/M20 and afterward decreased gradually while it was still higher than that of the Pt-based actuator. This was consistent with the trend observed for the tip displacement of the corresponding actuators. The highest SEEE was observed in the IP-V/M20 actuator which was about 50.4% higher compared to the IP-V actuator, implying that MWCNTs strongly affects the SEEE and reduces the required input power. The IP-V/M20 actuator showed twice the tip displacement and the SEEE compared to Pt-based actuators. This could be explained by the remarkably higher IEC, WUP, Cdl andσ of IP-V/M20 actuator.

4. CONCLUSION The fabrication and characterization of metal- free actuators with V/M nanocomposite as electrode material was demonstrated. V/M-coated Nafion membranes were prepared successfully via physical deposition. The effect of MWCNTs on the morphology of the electrode layers, C, σ and the actuation performance of the actuators were investigated. SEM and TEM analysis showed that V/M electrodes are deposited uniformly on both faces of the Nafion membrane and


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that the surface morphology of the electrode regions varies by wt% of MWCNT. Adding MWCNT gave rise to formation finer electrode structures in V/M-based actuators which contributed to their higher WUP and IEC. It was found that due to higher flexibility and porosity of metal-free electrodes the V/M-based actuators containing 20 wt% of MWCNTs can imbibe higher amount of hydrated ions and water compared to Pt-based actuators. CV studies of actuators showed that using V/M ink as electrode material leads to achieve higher current responses compared to the conventional Pt-based actuators. As MWCNT content of the electrode layers increased, C and σ of the actuators increased, while Rct changed inversely. This was attributed to the higher ionic and water content of actuators comprising of higher MWCNT wt%. For all actuators, the tip displacement was significantly improved as the magnitude of applied d.c. potential was increased. The actuation performance of the pure Vulcan- based actuator was enhanced (~ 2 times) by adding 20 wt% MWCNTs. Water loss of the V/M-based actuators decreased significantly as the wt% of MWCNT increased up to 20%. This led to higher durability of these actuators in open air. Furthermore, the SEEE of metal-free actuators was considerably higher than that of the conventional Pt-based actuators, indicating the lower power consumption of V/M-based actuators.



AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (Leila Naji)

ASSOCIATED CONTENT Supporting information The band assignments of FTIR spectroscopy of the synthesized materials (Table S1); schematic depiction of the set-up used for the current–voltage measurements (Figure S1); the variation in IEC and WUP of the prepared actuators as a function of MWCNTs content (Figure S1); the influence of frequency on V and I, for the IP-V/M20 actuator with the applied voltage of 4 V at frequencies of 0.5, 4 and 10 Hz (Figure S2). The supporting Information is available free of charge on the ACS Publications website at

Author Contributions All authors have given approval to the final version of the manuscript.

Conflict of Interest Disclosure The authors declare no competing financial interest.

ACKNOWLEDGMENT Authors gratefully acknowledge Nanotechnology Initiative Council, Amirkabir University of Technology and Tabriz University for all funding supports through this project. Authors have not used any other funding sources and there is no grant/award numbers. 22 ACS Paragon Plus Environment

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Table 1. WUP, IEC and the hydrated thickness of the prepared actuators Sample

Hydrated thickness (mm)

WUP (%)

IEC (meq/g)

IP

0.25

26

1.11

IP-V

0.238

55.17

5.45

IP-V/M10

0.257

60.34

6.25

IP-V/M20

0.287

71.15

9.13

IP-V/M30

0.292

71.30

9.22

IP-V/M40

0.298

71.43

9.34


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Table 2. Equivalent circuit parameters of the prepared actuators obtained from EIS analysis RS

Rct

Tdl×10-4

C

(ohm.cm2)

(ohm.cm2)

(ohm-1cm-2.Sϕ)

(µF)

IP

2.6

110

-

IP-V

2.2

35.6

IP-V/M10

2.0

IP-V/M20

Sample

Φ

Error (%)

1.1

-

1%

8.5

29.4

0.57

0.17

24.0

14

33.5

0.53

0.26

1.9

17.7

20.5

45.2

0.52

0.42

IP-V/M30

1.8

16.9

21

46.9

0.52

0.97

IP-V/M40

1.8

16.0

21.5

48.8

0.52

0.35



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Table 3. Specific electro-mechanical energy efficiency of the actuators under the sinusoidal electric input with amplitude of 4 V and excitation frequency of 0.5 Hz Actuators Electrical energy Specific electro-mechanical Maximum tip Name

input density,

displacement, DE?A (mm)

efficiency, ɳ

) ?@ABC (mJ.mm-2) > IP

1.49

1.62

1.00

IP-V

1.26

1.76

1.39

IP-V/M10

1.54

2.1

1.78

IP-V/M20

3.52

3.6

2.09

IP-V/M30

2.22

2.7

1.86

IP-V/M40

1.79

2.4

1.82


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Figure captions Figure 1. Schematic representation of the fabrication process of V/M-based actuators. Figure 2. TEM images of Vulcan carbon inks containing (a, a') 0 and (b, b') 20 wt% of MWCNT, at two magnifications. Figure 3. Cross sectional and longitudinal SEM images of electrode region in actuators (a-a") IP, (b-b") IP-V and (c-c") IP-V/M20. Figure 4. Comparison of ATR-FTIR spectra of the prepared Vulcan carbon-based actuators with bare Nafion membrane. Figure 5. Comparison of the (a) CV voltammograms, (b) Nyquist plots of all prepared actuators and (c) obtained equivalent circuit for actuators. Figure 6. The variation in the solution resistance (Rs), real capacitance (C) and ionic conductivity (σ) of the prepared actuators. Figure 7. (a) Voltage dependence of the tip displacement of the prepared actuators and (b) the corresponding overlaid digital images captured at the starting point and end point for actuators (b1) IP-V, (b2) IP-V/M10, (b3) IP-V/M20, (b4) IP-V/M30 and (b5) IP-V/M40 in response to d.c. voltage of 6 V in amplitude. Figure 8. Comparison of the tip displacement of the actuators against frequency in response to the sinusoidal voltages of 4 V in amplitude. Figure 9. Comparison of the (a) displacement rate (in mms-1) of the prepared actuators, in response to a 4 V d.c. input and (b) operating life of the actuators; IP, IP-V, IP-V/M10, IPV/M20, IP-V/M30 and IP-V/M40 actuators as they subjected to 5 V d.c. potential with time interval of 10 min in open air. Figure 10. The average maximum tip displacement (mm), strain difference between the electrodes (%) and water loss (%) of the actuators, in response to the same d.c. voltage of 5 V in amplitude. Figure 11. The current, I, and the applied voltage, V, versus time for the actuators; (a) IP, (b) IPV, (c) IP-V/M10, (d) IP-V/M20, (e) IP-V/M30 and (f) IP-V/M40, using sinusoidal applied voltages with amplitude of 4V at frequency of 10 Hz. Figure 12. J–V curves of the prepared actuators in the potential range of -4 to +4 V at a constant resistance of 1.3 Ω and the excitation frequency of 0.5 Hz.



Figure 1


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Figure 2



Figure 3


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Figure 4



Figure 5


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Figure 6



Figure 7


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Figure 8



Figure 9


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Figure 10



(a)

(b)

(c)

(d)

(e)

(f)

Figure 11


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Figure 12



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Electrochemical and Electromechanical Study of Carbon-Electrode

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