Nafion-based IPMC soft actuators with sea anemone-like Pt electrodes and enhanced actuation performance

Accepted Manuscript Fabrication of SGO/Nafion-based IPMC Soft Actuators with Sea Anemone-like Pt Electrodes and Enhanced Actuation Performance Leila Naji, Maryam Safari, Shiva Moaven PII:

S0008-6223(16)30020-3

DOI:

10.1016/j.carbon.2016.01.020

Reference:

CARBON 10651

To appear in:

Carbon

Received Date: 5 October 2015 Revised Date:

4 December 2015

Accepted Date: 6 January 2016

Please cite this article as: L. Naji, M. Safari, S. Moaven, Fabrication of SGO/Nafion-based IPMC Soft Actuators with Sea Anemone-like Pt Electrodes and Enhanced Actuation Performance, Carbon (2016), doi: 10.1016/j.carbon.2016.01.020. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

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Fabrication of SGO/Nafion-based IPMC Soft Actuators with Sea Anemone-like Pt Electrodes and Enhanced Actuation Performance

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Leila Naji,*1 Maryam Safari,1 Shiva Moaven2

Department of Chemistry, AmirKabir University of Technology, 424 Hafez Avenue, Tehran

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P.O Box: 15875-4413, Iran. 2

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Department of Chemistry and Biochemistry, Texas Tech University, 2500 Broadway Lubbock, TX 79409, USA

* Corresponding author to whom all correspondence should be directed. Tel: +98 (21) 64542767;

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Fax: +98 (21) 64542762; e-mail: [email protected] (Leila Naji)

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tip displacement.

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Keywords: Soft Actuators; IPMCs; sulfonated graphene oxide (SGO); electrochemical behavior,

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Abstract In the current work, graphene oxide (GO) and sulfonated graphene oxide (SGO) were synthesized and composited with Nafion to increase the water uptake (WUP) and ion exchange

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capacity (IEC) of the cast membranes. Ionic polymer-metal composite (IPMC) soft actuators in protonated form were fabricated based on the cast pure Nafion, Nafion/GO and Nafion/SGO membranes. The effects of incorporating GO and SGO in IPMC actuators were followed using

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physicochemical, electrochemical and electromechanical measurements and were compared with the corresponding behavior of pure Nafion-based IPMC actuators. Morphology analysis of

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IPMCs showed effective incorporation of GO and SGO and clarified the dependency of Pt electrode structure on the SGO content of the Nafion membranes. The addition of SGO resulted in dramatic increases in the WUP, IEC and hydrated thickness of the Nafion membranes and also in the capacitance, ionic conduction and in the tip displacement of the IPMC actuators. The small

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tip displacement of pure Nafion-based actuator was enhanced by eight times by adding 1 wt% SGO. The results suggested that the dispersion of SGO throughout the continuous polymeric matrix significantly reduce water loss by increasing the obstructions within the Nafion

1. Introduction

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membrane and also by formation of denser Pt grains in the electrode regions.

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Ionic polymer-metal composite (IPMCs) soft actuators are smart materials which change shape in response to an external electrical stimulus.[1] They are of great interest for application in robotics [2] and in medicine [3,4] for example, in replacement valves and artificial muscles. A typical IPMC consists of an ion-exchange polymer membrane with metal electrodes (generally platinum or gold), chemically plated on opposite faces. Generally, perfluorinated polymers such as Nafion are employed for fabrication of IPMC actuators. Nafion has a Teflon-like molecular

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backbone and short side-chains terminated by sulfonic acid group and possesses unique characteristic including light weight, flexibility, large bending deformation, quick bending response and low actuating response.[1] When an adequate potential is applied across the

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thickness of the hydrated IPMC, it shows fast and reversible bending motion towards its positively charged surface, followed by a slow relaxation towards the negatively charged surface. [3] The magnitude and speed of the bending motion of water-based IPMCs may depend on the

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ionic conductivity and thickness of the polymer, the structure and capacitance of the electrodes, the water content of the polymer, the mobility of the counter cations,[5] and the specific

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interactions between the electrode and the cations.[6,7,8] However, conventional Nafion – based IPMC actuators have a serious drawback of poor durability under long-term actuation in open air, mainly because of the leakage of the inner electrolyte and hydrated cations through cracks in the metallic electrodes.[9] To overcome this problem three main strategies have been employed

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by researchers; 1) replacing water with non-volatile solvents such as ethylene glycol [10,11] and ionic liquids [12,13], 2) improving the hydrophilic characteristic of the IPMC actuators by blending Nafion with nanostructure hygroscopic materials such as TiO2 [14] and SiO2 [15], and

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3) replacing conventional electrode materials with flexible conducting materials such as graphene (Gr) [16,17], fullerene [18] and conducting polymers.[19,20] Gr consists of a single

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atomic layer of sp2 hybridized carbon atoms arranged in a honeycomb structure [21] and possesses exceptional properties including high stiffness and strength, excellent thermal and mechanical stability, high conductivity and promising biocompatibility.[22] It can also retain its conductivity in an acceptable level when is subjected to external bending or stretching forces.[23] It has been shown that Gr electrode – based IPMC actuators have higher durability compared to conventional actuators. This has been attributed to the hydrophobic nature of Gr

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which can prevent the leakage of the vaporized or liquid electrolyte and mobile ions during electrical stimuli. [9] Graphene oxide (GO) can be made from chemical exfoliation of graphite using hummer modified method. [23] Throughout this reaction oxygenated functional groups are

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added to the Gr sheets and break the pi-conjugated electronic network, resulting in electrically insulating but highly water-dispersible GO sheets.[24] The thickness of the functionalized Gr sheet is approximately 1 nm, but the lateral dimensions can range from a few nanometers to

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hundreds of micrometers. GO can be considered as an unconventional soft material, such as a 2D polymer, since it consists of nano-graphitic patches surrounded by largely disordered oxygenated

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domains.[24] GO have potential applications in fuel cells,[25,26] photovoltaic devices,[27] supercapacitors,[28] actuators [29,30,31] and sensors [32,33] since it can guide material assembly through π-π stacking and hydrogen bonding.[23,24] Homogenous dispersion of the carbon-based nano materials such as Gr and GO and the use of their large surface area to interact

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with the polymer matrix are the key points to prepare polymer nanocomposites with improved mechanical, thermal, electrical properties.[34] Appropriate functionalization of GO with hydrophilic acid groups such as sulfonic acid, is expected to be an effective method to improve

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the proton transport by promoting the Grotthuss (hopping) and vehicle (diffusion) mechanisms.[35] It has been established that dispersing GO [36] and sulfonated-GO (SGO)

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[26,34] in Nafion increase the hydrophilic and water retaining characteristics of the Nafion membrane. This is prerequisite characteristic for practical applications of Nafion in fabrication of proton exchange membrane fuel cells (PEMFCs) and flexible soft IPMC actuators. In the current work, GO and SGO were synthesized and composited with Nafion to increase the water uptake (WUP) and ion exchange capacity (IEC) of the cast Nafion membranes. IPMC soft actuators in protonated form were fabricated based on the cast pure Nafion, Nafion/GO and

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Nafion/SGO membranes using absorption/reduction process of Pt ions. To the best of our knowledge, no studies have reported on methods to fabricate IPMC actuators based on Nafion/GO and Nafion/SGO composites. The effects of incorporating GO and SGO were

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followed using physicochemical, electrochemical and electromechanical measurements and were compared with the corresponding behavior of pure Nafion – based IPMC actuators. Morphology analysis of IPMCs showed effective incorporation of SGO and clarified the dependency of Pt

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electrode structure on the SGO content of the Nafion membranes. A finer and more ordered Pt electrode structure was formed in SGO – containing IPMC actuator. IPMC actuators

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incorporating 1 wt% SGO showed sea anemone-like Pt electrode structure. The addition of SGO resulted in dramatic increases in the WUP, IEC and hydrated thickness of the Nafion/SGO membranes and also in the capacitance, ionic conduction and in the tip displacement of the Nafion/SGO – based IPMC actuators. The small tip displacement of pure Nafion – based

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actuator was enhanced by eight times by adding 1 wt% SGO. Water loss of IPMC actuators decreased significantly as the SGO content of Nafion membrane increased. Water loss of IPMC actuators incorporating 1 wt% SGO decreased about 31% compared to that of pure Nafion –

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based actuators, indicating a higher durability under electrical stimuli. This strategy is therefore of considerable interest for increasing the applicability of IPMC-based soft actuators in medicine

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and robotics.

2. Experimental 2.1. Materials

Graphite flakes, sodium nitrate (NaNO3), sulfuric acid (H2SO4), sulfanilic acid (C6H7NO3S), nitric acid (HNO3), dimethyl formamide (DMF), deionized water (DI) and absolute ethanol were purchased from Merck chemical company. Potassium permanganate (KMnO4) and sodium

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borohydride (NaBH4) were obtained from Scharlau. Dupont liquid Nafion solution (5 wt%), in a mixture of lower aliphatic alcohols and water (45 wt%) with an equivalent weight of 1100 was purchased from Aldrich. Tetraamineplatinum (II) chloride, [Pt(NH3)4]Cl2, was obtained from

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Alfa Aesar. All materials were used as received. 2.2. Preparation of graphene oxide (GO)

GO was synthesized via oxidizing graphite flakes using modified Hummers method. [37,38] In

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this method 1 g of graphite flakes and 1 g of NaNO3 were added to 30 ml H2SO4 (98%) in a flask

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and stirred for an hour in an ice bath. Then, 3 g of KMnO4 were added gradually over 2 h to the flask and the mixture was stirred at 40ºC for 3 h. Over 30 min, 30 ml of DI water was added to the flask and then diluted with 40 ml of DI water. In order to eliminate the ions formed through the reaction 2 ml of H2O2 (31%) was added to the flask and the mixture was centrifuged for 30 min at 6000 rpm. The resulting brown residues, GO, was rinsed and centrifuged five times with

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water and HCl (5% v/v) solution. The purified GO was then dried in an oven at 50oC and stored as powder. The prepared GO was applied as starting material for synthesis of sulfonated graphene oxide.

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2.3. Preparation of sulfonated graphene oxide (SGO)

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In order to prepare SGO, 50 mg of GO powder was well dispersed in 30 ml DI water using an ultrasonic bath and then heated to 70ºC while stirring (at 750 rpm). Then, 12 mg of sulfanilic acid was added to the dispersion and stirred for 5 h. At the end, the resultant dispersion was centrifuged for 20 min at 6000 rpm and washed with 15 ml of DI water. This was repeated three times to eliminate all impurities. Then, centrifuged residue was dried in an oven at 50oC for 4 h and stored as powder. 2.4. Casting of Nafion membranes

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Three types of Nafion membrane were prepared using the same volume of Nafion solution (5 wt %); pure Nafion, Nafion/SGO and Nafion/GO. Schematic depictions of the fabrication procedure of the three membranes are shown in Figure 1. To prepare pure Nafion membranes, 6.6 ml of the

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Nafion solution were heated to remove excess solvent and reduce the volume to 3.3 ml. To this was added 1.3 ml of DMF to prevent the formation of cracks in the membrane on drying.[39] The mixture was poured into a Teflon mould and left at room temperature for 24 h to dry. The o

C for 5 h and ramped to 150 o C, the temperature was

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sample was heated in an oven at 70

maintained at 150 o C for 1 h and then allowed to cool. The product of this process was a 2 cm ×

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2 cm sheet of Nafion membrane with a yellowish, translucent appearance and dry thickness of 108 µm. This yellowish color was removed during an extensive cleaning process modified from that of MacMillan et al.[40,41 ] The clean, cast Nafion membrane was stored in DI water for use in IPMC preparation.

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In order to study the influence of the loading level of SGO on the physical, electrochemical and electromechanical characteristics of Nafion-based IPMC actuators, nanocomposite membranes comprising of different weight ratio of SGO to Nafion were

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prepared. To this end, predetermined weight of SGO (corresponding to the desired weight ratio) was added to the 6.6 ml of Nafion solution and the mixture was put in an ultrasonic bath at 70 oC

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to reduce the volume to 3.3 ml. DMF (1.3 ml) was then added and the mixture homogenized using an ultrasonic probe for 5 min before casting. The mixture was poured into the Teflon mold and left at room temperature for 24 h to dry. The membrane was then annealed and cleaned using the same process described above for the pure Nafion membrane. Nafion/GO membranes containing 1 wt% of GO was prepared using similar fabrication method described above for

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preparation of Nafion/SGO membranes. The product of these processes was two 2 cm × 2 cm sheet of Nafion membrane with a black appearance. All cleaned membranes were then protonated by immersing in 1 M solution of HNO3 in 80°C for

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1 h and then washed with and stored in DI water for further experiments. All prepared membranes were given a name according to the applied weight ratio of SGO and GO to Nafion. Table 1 gives the names, the detailed process information and the fully hydrated thickness for all

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membranes. The thickness of the membranes was measured at multiple points using a digital

2.5. Fabrication of IPMC actuators

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micrometer.

IPMC actuators were prepared in 5 mm × 20 mm size from pure Nafion, Nafion/SGO and Nafion/GO in their protonated form using a technique presented by Pak et al.[42] After roughening the surfaces using abrasive paper to increase the metal-polymer interface area, the

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membrane was washed using an established method [43,44] to remove all impurities remaining from the manufacturing process. The clean Nafion was soaked in an aqueous solution of [Pt(NH3)4]Cl2 for 24 h and a few drops of NH4OH solution was added to neutralize the solution.

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The amount of [Pt(NH3)4]Cl2 added was calculated to correspond to the number of ion exchange sites available in the Nafion membranes. After rinsing with DI water to remove the excess

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platinum salt solution, the membranes were placed in a small amount of DI water in a crystallization dish under stirring. An aqueous solution (100 ml) of 1 wt% NaBH4 (as reducing agent) was added gradually to the membrane. A silvery layer of Pt was formed on the surfaces of the membranes. This plating procedure was repeated twice to improve the surface conductivity and Pt electrode morphology. The membranes were then rinsed with DI water and immersed in 1 M HNO3 solution for 12 h to neutralize the residual ammonium, rinsed again and

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stored in DI water in protonated form. The actuators were named to indicate the type of the Nafion membrane used in the fabrication of the IPMC actuators; IP-SGO-0.1, IP-SGO-0.25 to IP-SGO-4 for actuators prepared based on the Nafion/SGO membranes (listed in Table 1), IP-

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GO-1 for actuators prepared based on Nafion/GO membrane (see Table 1) and IP for actuators prepared based on the pure Nafion membrane. The digital images of pure Nafion – and N-SGO-1 – based IPMC actuators are shown in Figure 2.

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2.6. Physicochemical characterization of synthesized materials

To evaluate physical and chemical characteristic of GO and SGO, Fourier transform infrared

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spectroscopy (FTIR) and thermogravimetric analysis (TGA) were applied. FTIR spectra were acquired from BRUKER-ALPHA series spectrometer as transmittance data, with KBr as reference, to certify the presence of oxygen-containing functional groups in GO and SGO. The samples were dried in an oven at 70oC for 2 h before FTIR tests. The thermal stability of the GO

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and SGO samples was compared using a Rheometric Scientific STA 1500 at a ramp rate of 10 °C/min up to 600 °C in nitrogen flow of 50 mL/min.

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2.7. Physicochemical characterization of membranes ATR-FTIR analysis was applied to study the changes occurring in the percentage of sulfonate

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groups at the surface of the Nafion membrane as a function of the loading level of SGO. The spectra were acquired from BRUKER-ALPHA series spectrometer as transmittance data. Prior to ATR-FTIR experiments all membranes were put in an oven at 80 oC for 4 h to become fully dehydrated.

Fully hydrated proton-exchanged Nafion, Nafion/GO and Nafion/SGO membranes were vacuum dried in an oven at 80°C overnight and their dry mass, Wdry, measured and subtracted from their wet mass, Wwet and the percentage of water uptake (WUP) calculated using Equation 1. 9

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% WUP = 100× (Wdry-Wwet/Wdry)

Equation 1

The ionic-exchange capacity (IEC) of Nafion, Nafion/GO and Nafion/SGO membranes was

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measured by classical titration method. The proton-exchanged Nafion membranes (in acid form) were dried in an oven at 80oC for 4 h and then immersed in a saturated solution of NaNO3 for 5 d to allow population of all the ion exchange sites (the -SO3- groups) with the sodium cation. The membranes were removed and the remaining solutions were titrated using 0.0238 M NaOH as

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standardized solution. Phenolphthalein was used as indicator for these titrations. The IEC values

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were calculated using equation 2, where IEC is the ion exchange capacity, expressed in meq.g-1, M is the molarity of NaOH, V is the consumed volume of NaOH for each titration and Wdry is the weight of dried membranes. IEC = (M×VNaOH) / Wdry

Equation 2

2.8. Physical and morphological characterization of IPMC actuators

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Field emission scanning electron microscopy (FE-SEM) was employed to study the morphology of the platinum electrode of IPMC actuators using a TESCAN-MIRA3 SEM instrument. All IPMC actuators were coated with a thin layer of gold (10 nm) prior to SEM imaging.

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The hydrophilic character of the surfaces of IPMCs was determined using contact angle

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measurements. Dry IPMC actuators have a tendency to curl or be pushed sideways when absorb water. This limits the accuracy of the contact angle measurements. To avoid the effects of actuator deformation, the measurements were carried out on hydrated IPMC actuators mounted onto a stiff substrate. To this end, prior to contact angle measurements, all IPMC actuators were soaked in DI water at room temperature for 24 h. The fully hydrated IPMC actuators were then removed from DI water and the excess water was wiped off from their surface using a paper tissue. Contact angle was measured using double-sided tape to attach the IPMC actuator (Pt

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electrode surface facing up) onto a platform underneath a syringe and in front of a digital camera. The syringe was programmed to deliver a drop of DI water at 25 µL onto the IPMC actuator and an image was recorded after 2 sec. The frames were then analyzed for the point at

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which the drop stabilized and remained symmetrical on the IPMC actuator. The contact angle was measured in the digital photo with image analysis software. The results were the average of three measurements.

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2.9. Electrochemical characterization of IPMC actuators

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In order to study the effect of compositing Nafion with GO and SGO on the ionic conduction and capacitive characteristics of Nafion-based IPMCs, electrochemical impedance spectroscopy (EIS) and cyclic voltammetery (CV) were used. Square elements of 5 mm × 5 mm were cut from the IPMCs and placed horizontally in a sample holder consisting of a pair of platinum contacts embedded within a Plexiglas clamp. These contacts had the same surface area as the actuator.

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The measurements were performed on a potentiostat/galvanostat (Autolab) controlled by NOVA software at ambient temperature. The impedance measurements were performed with an applied a.c. voltage of 10 mV in amplitude over a frequency range of 100 kHz to 0.01Hz. Z-view

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software was used to analyze the results. Measurements were carried out at room temperature and proton conductivity (σ) was obtained using as Equation 3, where σ is the proton conductivity

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(S/cm), l the measured thickness of the actuators (cm), R is the resistance (Ω) and A is the area of the IPMC actuators (cm2). σ = l /R A

Equation 3

CV scans were performed at room temperature in the potential range of ±1V at a scan rate of 0.1 V s-1. The capacitance (C) was calculated from the CV results using Equation 4, where I+ and I-

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are currents in A at 0 V in the two scanning directions and dV/dt is the potential scan rate (Vs-1). [45]

2.10. Actuation measurements of IPMC actuators

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Equation 4

Maximum tip displacement were measured at room temperature and in ambient air using the

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arrangement shown in our previous work. [46] The IPMC actuator was held at one end in a sample holder consisting of a pair of platinum electrical contacts fixed within an insulating

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clamp. The electrical contacts were connected to the two terminals of a d.c. power supply. All actuators were subjected to d.c. voltages from 3 to 6 V while they were in an open air environment (23◦C) of 40% humidity. A digital camera was used to capture video images of the bending deformation of the actuator as the d.c. voltages were applied. The maximum tip

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displacement was calculated by measuring the linear distance between the starting point and end point of the free end of the actuators, using image analysis software. During the time intervals, the IPMC actuators were kept still in the sample holder. Water loss of the fabricated IPMC

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actuators was determined as water remaining in the Nafion-, Nafion/GO- and Nafion/SGO-based IPMC actuators after being subjected to a d.c. voltage. Water loss of IPMCs indicates the

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performance lifetime of these actuators. [47]. For water loss measurements, each fully hydrated IPMC actuators were subjected to 5 V d.c. potential for 5 min using the same set-up described above and then weighted (We). Water loss was calculated using Equation 5, where Wwet and We denote the weights of the wet IPMC actuators before application and right after applying potential, respectively. % Water Loss =100× (Wwet-We/Wwet )

Equation 5

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The durability of the IPMC actuators was studied by applying 5 V d.c. every 10 min and measuring the maximum tip displacement as a function of time (over 180 min). During the time

open air. 3. Results 3.1. Physicochemical characterization of synthesized materials

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intervals, the IPMC actuators were kept still in the sample holder while they were exposed to

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The FT-IR spectra of GO and SGO samples are compared in Figure 3. The broad band

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between 3600-3200 cm-1 shows the stretching vibrations of O-H groups of alcohol and carboxylic acids in GO. This band is also observable in SGO spectrum with a higher intensity and lower broadness. This suggests the substitution of -OH functionalities with sulfanilic acid molecules in SGO according to the proposed mechanism depicted in Figure 4. Sulfanilic acid molecules react with –OH functional groups present on the GO nanosheets. This results in

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generation of C-N junction between GO and sulfanilic acid molecules while water molecules are formed. The small bands between 2800 cm-1 and 3000 cm-1 correspond to C-H stretching of sp2 hydrogen present on the samples. These bands are more visible in FTIR spectrum of SGO

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sample due to the presence of sulfanilic acid groups. The sharp band at 1728 cm-1 is due to C=O

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stretching of carbonyl groups present on GO and SGO sheets. The band at 1625 cm-1 is the result of C=C stretching which appeared much sharper for SGO compared to GO due to the presence of sulfanilic acid groups in SGO containing benzene ring. For SGO a sharp band at 1400 cm-1 was observed which corresponds to the S=O stretching bonds. Two absorption bands at 1054 cm1

and 1030 cm-1 are the result of C-O and S-O stretching vibrations which appeared stronger for

SGO.

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TGA result of GO and SGO are compared in Figure 5. Both samples start to lose about 15%, weight at temperature range of 70oC -100oC, primarily due to the loss of physically absorbed water molecules. For GO, a sudden weight loss (approximately 75%) occurs at 200oC which is

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attributed to the elimination of unstable oxygen functionalities, including hydroxyl, carbonyl and epoxy groups as CO, CO2 and H2O gases. SGO shows different thermal behavior compared to GO. As discussed before, in SGO hydroxyl groups are mainly substituted with sulfanilic acid

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groups. This seems to have an improving impact on the thermal stability of SGO compared to GO. The decomposition of the oxygen functionalities which did not react with sulfanilic acid

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groups showed a roughly 22% quality loss at 200-240oC for SGO. SGO remained stable up to 500oC. Finally, a 45% quality loss occurred at 500oC was mainly due to the combustion of the carbon skeleton of SGO. TGA results indicate the excellent thermal stability of SGO compared to GO.

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3.2. Physicochemical characterization of the Nafion membranes Figure 6 presents the ATR spectra of the pure Nafion, Nafion/GO (N-GO-1) and Nafion/SGO (N-SGO-1) membranes. ATR is a surface technique and spectra are representative of the sample

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surface. For all membranes peaks at 557, 628, 641, 1132 and 1203 cm-1 were observed which

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assigned to the perfluoroethylen backbone of Nafion membranes. [48] The peaks at 970, 1052, 1130-1200 cm-1 are characteristic peaks of the side chains in Nafion and are assigned to the symmetric vibration of C–O–C bonds and S–O stretching vibration of –SO3 groups, respectively.[48] As can be seen, compositing Nafion with GO (N-GO-1 sample) and SGO (NSGO-1 sample) is accompanied by an intensity increase of all peaks, especially for the characteristic peaks of the side chains at 970-1500 cm-1 region. C–O–C and -SO3 groups are highly polarizable and it is obvious that the change of the number and species of the acido-basic 14

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entities present in the cluster-network influences their band intensity.[48] For N-GO-1 and NSGO-1 new peaks appeared at 1617 and 2900 cm-1 which assigned to functional groups introduced on the membrane surface. The former is attributed to the presence of C-O bonds and

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the latter is due to the stretching vibration of C-S bonds.[49] The peak at 1617 may also contain components from the skeletal vibration of un-oxidized graphitic domains. The region of 15003500 depends on the protonic species content of the studied membrane [48] which is the highest

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for N-SGO-1 sample. This was confirmed with the data obtained from the evaluation of WUP and IEC of the membranes presented in Table 2. As can be seen, the N-SGO-0.1 sample with the

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lowest SGO loading exhibited IEC and WUP of about 52% and 64.3% higher than that considered for pure Nafion sample, respectively. The increase in the SGO content of the Nafion membranes was also accompanied with significant enhancement in IEC, WUP and hydrated thickness of the sample, in which N-SGO-4 showed the highest values. Furthermore, the N-

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SGO-1 sample showed to have a higher IEC (~ 3 times) and WUP (~ 1.7 times) compared to that of the N-GO-1 sample while they contained the same weight percentage of SGO and GO. This indicates that the –SO3- groups substituted on the GO sheets can increase the hydrophilicity and

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also IEC of the Nafion membranes. These two factors play important role in the improvement of the actuation performance of the Nafion-based IPMC actuators. The variation in IEC and WUP

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of Nafion/SGO samples as a function of SGO content can be followed more clearly in Figure 7. This figure shows that the increasing amount of SGO content leads to a similar increase in IEC and WUP of the membranes, while it affects less the IEC. This suggests that the –SO3 groups on GO sheets contribute in the formation of larger ionic clusters within the Nafion/SGO membranes which can absorb higher amount of water. [44]

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3.3. Surface morphology of IPMC actuators As discussed before, the magnitude and speed of deformation of Nafion-based IPMC actuators depends on the structure and capacitance of the electrodes and also the nature of the interface

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between the electrode and the polymer.[46] Figure 8 - parts (a)-(e) shows longitudinal SEM images of Pt electrode region of the IPMC actuators (a) IP, (b) IP-SGO-0.1, (c) IP-SGO-1, (d) IP-SGO-4 and (e) IP-GO-1 and, to facilitate the comparison, the images of the actuators at three

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different magnifications are presented in each row. A uniform deposition of Pt electrode can be seen for all actuators. Comparison of the actuators in each column reveals the effect of the

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compositing Nafion with SGO and GO on the nanostructure of the Pt electrode region. As the images show, the surface morphology of the actuators changes considerably as a function of the amount of the added SGO. The most even surface morphology is observed in pure Nafion-based IPMC actuator (IP) with no filler addition (Figure 8(a)). It is important to note that the size of the

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Pt grains was reduced in IP-SGO-0.1, IP-SGO-1 actuators shown in Figure 8(b) and (c), respectively, compared to IP actuator. Furthermore, the random fine structure of Pt observed for IP actuator (Figure 8(a‘ and a“)) became more ordered as the SGO content of the actuators

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increased from 0.1 to 1 wt%, in which in IP-SGO-1 actuator (Figure 8(c‘ and c“)) uniformly distributed sea anemone-like Pt islands was observed. This can be attributed to the nucleating

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effect of the SGO due to its favorable surface structure during absorption/reduction process of platinum ions and also the contribution of –SO3 groups in the formation of larger ionic clusters in this actuator. [47] However, the anemone-like Pt islands did not form in IP-SGO-4 actuators which contained 4 wt% of SGO (see Figure 8(d and d“). For IP-SGO-4, it seems that the application of higher amount of SGO increases the thickness of the solid layer at the surface of the membrane. [46] This shows that the promoting effect of SGO in the growth of Pt fine

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structure reaches a percolation threshold at 1 wt% of SGO. This might be attributed to the penetration of a higher concentration of Pt ions within the IP-SGO-4 actuators during the IPMC fabrication process because of the favorable electrostatic interaction between these ions and the –

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SO3- groups. This leads to the formation of larger coagulated Pt particles compared to those in the actuators containing a lower amount of SGO. Comparison of the IP-SGO-1 and IP-GO-1 actuators (shown in Figure 8 (c‘ and c“) and (e‘ and e“) respectively) reveals that applying the

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same amount of GO (1 wt%) to the Nafion membranes results in the formation of randomly distributed Pt fine structure at the surface of the IP-GO-1 actuators. In other words, a much

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disordered electrode region was obtained in the IP-GO-1 actuators than in the matching IP-SGO1 actuators. The surface structure of IP-GO-1 actuators is more similar to IP actuator which was prepared based on pure Nafion membrane. However, in comparison with Pt particles formed in the IP actuator (shown in Figure 8 (a‘ and a“)) a finer Pt structure was formed in the IP-GO-1-

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based IPMC actuator.

The contact angle (wetting angle) is a measure of the wetability of a solid by a liquid (mostly water). In the case of complete wetting, the contact angle is 0°. Between 0° and 90°, the solid is

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wettable and above 90° it is not wettable. Figure 9 illustrates the image of a sessile drop at the surface of actuators (a) IP, (b) IP-GO-1, (c) IP-SGO-0.1 and (d) IP-SGO-1 actuators. Point of

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intersection between the drop contour and the projection of the surface was used for calculation of the contact angles. As the images show, the surface of IPMC actuators showed different wetting behavior. The IP actuators (Figure 9(a)) showed the highest contact angle of 72o. Adding GO and SGO to the Nafion membrane resulted in a significant increase in the hydrophilicity of the IPMC actuators. As can be seen in Figure 9-parts (b) to (d), the contact angle was considerably reduced to lower values of 68o, 58o and 50o in IP-GO-1, IP-SGO-0.1 and IP-SGO-1

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actuators. The lowest contact angle was observed for IP-SGO-1. This indicates that the actuator with the finest Pt electrode structure (Figure 8(c-c”)) possesses the highest hydrophilic character.

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3.4. Electrochemical characterizations of IPMC elements The performance of conventional IPMCs is directly related to the capacitance of the electrodes. The higher capacitive characteristic of the Pt electrodes in IPMC actuators containing GO and SGO was confirmed by comparing the CV voltamograms of the IP, IP-GO-1, IP-SGO-0.1, IP-

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SGO-1 and IP-SGO-4 actuators, as shown in Figure 10. All CV voltamograms appear similar in

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shape but differ in the magnitude of the current. The symmetric shape of the CV curves can be assigned to excellent charge distribution in the electrode region of the IPMCs. This indicates that the platinum plating process has given rise to two equivalent electrodes with similar thicknesses in all actuators.[46] The CV curves of the IPMCs are close to rectangular, indicating a capacitive behavior and a low contact resistance. Nafion-based IPMC actuators can be considered as

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polyelectrolyte-based capacitors [50] since they consist of a pair of conductive electrodes sandwiching the cation exchange Nafion membrane. Nafion chemical structure consists of nonpolar tetrafluoroethylene segments and the polar perfluorosulfonic vinyl ether segments. Data

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presented in Table 2, revealed that Nafion/GO and Nafion/SGO membranes possess higher IEC

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and WUP and can absorb higher amount of water compared to pure Nafion membrane. This was attributed to the formation of larger hydrophilic ionic clusters within these actuators. [7] The hydrated thickness of the Nafion membrane was also increased as the weight percentage of SGO within the Nafion membranes increased. Thus, it can be concluded that the incorporation of SGO (and GO) cause the polymer to swell macroscopically and microscopically [51,52] to a higher extent compared to pure Nafion membrane. Therefore, it is expected that the polarization mechanisms happen easier in Nafion/GO- and Nafion/SGO-based actuators. The more easily the

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polarization mechanisms can act, the larger the current response and capacitive characteristic of the actuators will be. [53] As can be seen in Figure 10, pure Nafion- based IP actuators showed the narrowest CV loop and the current response increases in the order: IP-SGO-4>IP-SGO-1>

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IP-GO-1>IP-SGO-0.1>IP. This emphasize that compositing Nafion with GO and SGO generally led to a higher current response - and therefore higher characteristic capacitance. Except for IPSGO-0.1, all prepared Nafion/SGO-based IPMC actuators showed higher current responses

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compared to that considered for IP-GO-1- the highest of all being for IP-SGO-4. The IP-GO-1 actuator showed a lower current response compared to that of the IP-SGO-1 actuator, while they

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contained the same weight percentage of GO and SGO, respectively. This was attributed to the higher involvement of –SO3- groups (present on SGO) in the formation of larger ionic clusters compared to that of proton-exchange functional groups available on GO. It is also important to note that the higher current response of Nafion/SGO-based IPMC actuators reflects their denser

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and finer Pt-electrode-structure compared to IP actuators as shown in Figure 8. The CV results were confirmed with the data obtained from EIS studies on the IPMC actuators. Impedance is the response of an electrochemical system to an applied alternating voltage. Here,

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the electrochemical system is an IPMC actuator sandwiched between two platinum electrodes, so

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the alternating current flows through the Nafion membrane. The impedance of the IPMCs is a function of three variables; the mobility of ions in Nafion membrane, the polarization of Nafion structure and double-layer capacitance at the platinum electrodes/polymer. The frequency dependence of impedance can reveal the underlying processes in IPMC actuators. The complex response of the electrochemical system is usually displayed in Nyquist form; the plot of the real part of impedance against the imaginary part. Nyquist plot appears as a vertical line for ideal capacitors. Moreover, in these plots a semicircle is fitted as a resistor and capacitor in parallel. In

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highly conductive systems, the semicircle in Nyquist plot is not observable; however, for less conductive systems the semicircle is present in a distorted form. Figure 11 –parts (a) and (b) shows the Nyquist plots obtained by EIS for pure Nafion- and Nafion/SGO-based IPMC

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actuators. As can be seen, plots shown for the Nafion/SGO-based IPMC actuators are nearly linear in the low frequency region and have a semicircle in the high frequency region. This indicates that at low frequencies these actuators behave like capacitors while at high frequencies

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they behave like resistors. However, for pure Nafion-based IP actuators only large semicircles were observed which extended over a wide frequency range. Moreover, amongst all IPMC

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actuators, the largest semicircle region was observed for the pure Nafion-based IP actuator, indicating the highest impedance (charge transfer resistance) in this actuator. For Nafion/SGObased IPMC actuators the semicircle appeared in a higher frequency range compared to the IP actuator. This shift is related to the incorporating SGO causing a decrease in the ionic resistance

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of the IPMC actuators. The high frequency intercepts on the real axis (x-axis) were used to calculate the ionic resistance of IPMC actuators. Figure 11(b) shows that the intercepts of the plots on the x-axis shifts gradually to higher frequencies as the loading content of SGO in

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Nafion/SGO-based IPMCs increases, indicating an increase in ionic conduction of the IPMC actuators. IP-SGO-4 with the highest SGO content showed to possess the highest ionic

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conductivity. The ionic conductivity is closely related to the size and number of ionic clusters formed within the swollen Nafion membrane and it is described by Grotthuss (hopping) and vehicle (diffusion) mechanisms. [54] Both mechanisms can occur easier in the case of the Nafion/SGO-based IPMC actuators compared to pure Nafion-based IP actuator since they showed to have higher IEC and WUP content (see Table 2). IP-SGO-4 actuator which was fabricated based on N-SGO-4 membrane showed the lowest ionic resistance. This was attributed

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to the highest IEC and WUP of N-SGO-4 membrane among all Nafion membranes (see Table 2). The steepest vertical line was also observed for IP-SGO-4 (see Figure 11 (b)), indicating the highest charge storage capacity of this actuator. This was in good agreement with the CV

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response of the IP-SGO-4 actuator presented in Figure 10. As was seen in the SEM images (Figure 8) increasing the SGO content of Nafion membrane led to the formation of denser Pt electrode at the surface of IPMCs – the densest Pt structure was observed for IP-SGO-4 (Figure

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8(d)). In previous work, it was found that the platinum particles penetrate the polymer to a depth of ~10 µm and provide a porous electrode structure at both surfaces of the IPMC.[46] The

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penetration depth of Pt particles could be increased by repeating plating process and also increasing the Pt ions concentration within the polymer. As discussed before, the N-SGO-4 membrane showed to have the highest IEC and so the highest number of fixed ion exchange sites. Thus, this membrane could absorb a higher amount of Pt ions during IPMC fabrication

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process, leading to the formation of denser and probably thicker [46] Pt electrodes at the surfaces of the N-SGO-4 – based IP-SGO-4 actuators. EIS results reveal that increasing SGO content of IPMC actuators have an enhancing impact not only on the capacitive characteristic but also on

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the ionic conduction of the IPMC actuators. EIS data are commonly analyzed by fitting them to an equivalent electrical circuit model. The Randles model is one of the simplest and most

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common models. This model includes a solution resistance, Rs, a double layer capacitor, Cdl and a charge transfer resistance, Rct. The variation in the Rct, Cdl and ionic conductivity () of the Nafion/SGO-based IPMCs as a function of SGO content can be followed more clearly in Figure 12. Ionic conductivity of IPMCs was calculated using  = L/ARct equation, where L and A are the hydrated thickness (in cm) and the surface area (in cm2) of the IPMC actuators, respectively. Figure 12 demonstrates that pure Nafion – based IP actuator has the highest Rct and the lowest

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Cdl and  among all actuators. Cdl and  of IPMCs increases as a function of SGO content, moving from IP-SGO-0.1 to IP-SGO-4 actuator, while a reverse trend is observed for Rct of the actuators. Cdl and  of the IP-SGO-4 actuator are about twenty eight and seven times higher than

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that of IP actuator, respectively. Moreover, Cdl and  of IP-SGO-1 actuator appeared to be about three and two times higher than that of the IP-GO-1 actuators while they contain the same weight percentage of SGO and GO, respectively. This was attributed to the enhancing effect of sulfonate

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3.5. Actuation performance of IPMC elements

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groups on the Pt electrode structure, IEC and WUP of the Nafion membrane as discussed earlier.

The average maximum tip displacement of the free end of IPMC elements was measured as a function of the amplitude of the applied d.c. voltage. This was done for IPMCs made of pure Nafion, N-GO-1 and all N-SGO membranes shown in Table 1. The tip displacements of IPMC actuators (a) IP, (b) IP-GO-1, (c) IP-SGO-0.1 and (d) IP-SGO-1 (with their corresponding SEM

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images) in response to d.c. voltages of 5 V over time range of 0 to 60 sec are shown in Figure 13. It is observed that IP and IP-GO-1 actuators reached the maximum tip displacement after

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approximately 30 sec and then a fast relaxation occurred. However, IP-GO-1 showed a higher tip displacement in the same time range. IP-SGO-0.1 and IP-SGO-1 actuators showed slow back

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relaxation after approximately 45 sec. When an electric field is applied across the thickness of a Nafion-based IPMC actuator, fast bending motion towards the positively charged electrode occurs which is followed by a slow relaxation of the actuator towards its original position. This bending motion is the result of the migration of hydrated cations and diffusion of water across the thickness of Nafion membrane. The results presented in Figure 13 show that compositing Nafion with SGO can effectively delay the slow relaxation of the IPMC actuator, probably due to the presence of –SO3 groups on the GO nano-sheets. This can be advantageous in improving the 22

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overall performance of the IPMC actuators. Figure 14 (a) demonstrates that all IPMC actuators exhibited qualitatively similar dependences on the amplitude of the applied voltage in that tip displacement increased as a function of input voltage. Tip displacements were much larger for

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IP-GO-1 than IP, indicating a significant increase in the actuation response on compositing Nafion with GO and much larger for Nafion/SGO – based IPMCs than IP-GO-1, demonstrating the improvement due to functionalization of GO with –SO3- groups. The comparison of the tip

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displacement of SGO containing IPMC actuators in Figure 14 (a) revealed that the magnitude of tip displacement were dependent on the weight percentage of SGO within Nafion membrane.

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The variation in the maximum tip displacement of IPMCS as a function of SGO content (under 5 V) can be followed in Figure 14 – part (b). This figure reveals that the positive effect of SGO on increasing the tip displacement of IPMCs reach a percolation threshold for IP-SGO-1. The IPSGO-4 sample which showed to have the highest ionic conductivity and electrode capacitance

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presented lowest tip displacements among Nafion/SGO – based IPMCs, probably due to higher rigidity of Pt electrode structure (see Figure 8 – part (d-d“). IP-SGO-1 showed the highest tip displacement (~ 22.5 mm) under 5 V d.c. potential which was about eight and two order of

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magnitude higher than that considered for IP and IP-GO-1 actuator. Water loss of the fabricated IPMC actuators was determined after being subjected to 5 V d.c. voltage for 5 min. As Figure 14

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– part (b) shows the water loss of IPMC actuators decreased significantly as the SGO content of Nafion membrane increased, in which IP and IP-SGO-4 actuators showed the highest (~ 86%) and the lowest (~ 40%) water loss. In Figure 15 the performance over time of the IP, IP-GO-1 and IP-SGO-1 actuators is presented in terms of the variation in tip displacement on application of 5 V d.c. every 10 min. All three actuators exhibited very similar behavior; i.e. the tip displacement increased to a maximum value after 10 min and then decayed gradually over time

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to zero. This can be attributed to evaporation of water from the samples and possibly also to loss of water by electrolysis. However, the three actuators became inactive over different time range. The tip displacement of the IP-SGO-1 actuator reached zero value after approximately 150 min

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which was about 5 and 1.7 times higher than the time required for IP and IP-GO-1to become inactive. Consequently, it is reasonable to consider the IP-SGO-1 actuator to be an IPMC with improved tip displacement and durability due to its higher water content and water retention. The

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results suggest that the dispersion of SGO throughout the continuous polymeric matrix significantly reduce water loss by increasing the obstructions within the Nafion membrane and

a good protection against water loss.

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also the formation of denser aggregation of fine Pt grains in the electrode regions which provides

Various efforts have been made to efficiently incorporate graphene [16], CNT [8,55] and nanoscale materials [14,15] into the Nafion matrix and also as electrode of the IPMC actuator [9,16-

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20]. Although there are recent reports on the actuation performance of GO electrode – based IPMC actuators [56] the Nafion/GO and Nafion/SGO composite – based IPMC actuator has not been reported previously. Jung et al. provided evidence that a minute loading of Gr greatly

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improves the harmonic responses, the blocking force and the mechanical stiffness of Nafion/Gr – based IPMC actuators.[16] However, compositing Nafion with Gr resulted in a significant

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decrease in WUP and IEC of the prepared IPMC actuators. This was attributed to a significant decrease in the degree and extent of ion clustering and cluster diameters due to hydrophobic nature and platelet morphology of Gr. [16] As mentioned earlier, the magnitude and speed of the bending deformation of water-based IPMCs depends strongly on the IEC, water content of the polymer and the mobility of the hydrated counter cations. Furthermore, the bending deformation is the result of an electro-osmotic pressure induced by the migration of hydrated cations and

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diffusion of water between ionic clusters. Thus, high water content and water retention are prerequisite characteristics for fabrication and practical applications of rapid response Nafionbased IPMC actuators with high durability in open air condition. Compared with the work

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presented by Jung et al., the current work proposes a simple method for manufacturing highperformance IPMC actuators using Nafion/SGO and Nafion/GO composite with improved hydrophilic, IEC, ionic conductivity, Cdl characteristic. More importantly, our results show

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the possibility of operation of these actuators in open air for a longer period of time due to their higher water absorption and water retention. This paper also provides new insights in the

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relationship between the Pt electrode nanostructure and the loading content of the nanofiller (SGO). Moreover, the tip displacement of the Nafion/SGO (IP-SGO-1) actuators in the protonated form was approximately 29% higher than that measured for Li+-exchanged pure Nafion-based IPMC actuator [57] while it was close to the tip displacement of ionic liquid based

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EMIM+-exchanged Nafion-based IPMC actuator, [57] in response to the d.c. voltage of 5 V in amplitude. Our results indicate that SGO and GO are advantageous compared to other carbonaceous nanostructure materials such as MWNCTs and Gr and can greatly improve the

Conclusion

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durability and actuation performance of Nafion-based IPMC actuators.

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We have demonstrated for the first time the fabrication and characterization of Nafion – based IPMC actuators comprising different weight percentage of SGO. GO was synthesized using modified Hummers method and functionalized with sulfonic acid groups (SGO) via a chemical method. The synthesized materials were characterized using FTIR and TGA. Pure Nafion, Nafion/GO and Nafion/SGO membranes were prepared by casting and compared using ATRFTIR, WUP and IEC methods. IPMC soft actuators were fabricated based on the above-

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mentioned Nafion membranes and the impact of GO and SGO on the morphology of Pt electrodes and also the hydrophilicity of the actuators was studied using SEM and contact angle methods, respectively. Furthermore, CV responses, EIS data and actuation performance of

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IPMCs were compared to follow the impact of SGO on the Pt electrode structure and Rct, Cdl and

 of IPMCs.

FTIR and TGA data confirmed the successful synthesis of GO and SGO materials. Generally,

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SGO-containing Nafion membranes showed higher WUP and IEC compared to pure Nafion

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membrane. The Nafion membrane with the lowest SGO content exhibited IEC and WUP of about 52% and 64.3% higher than that considered for pure Nafion, respectively. The increase in the SGO content of the Nafion membranes was also accompanied with significant enhancement in IEC, WUP and hydrated thickness of the sample. The Nafion membranes incorporated with the same weight percentage of SGO and GO (respectively) showed different WUP and IEC –

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higher values were obtained for SGO-containing membrane. SEM analysis showed that Pt electrode structures had good surface coverage and that the surface

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morphology of the Pt regions was dependent on the SGO content of the Nafion membrane. The SGO gave rise to more ordered and denser Pt-impregnated regions in Nafion/SGO-based

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actuators. The finest and the most ordered Pt surface structure was observed in IPMC actuator with 1wt% SGO content. Comparison of the surface morphology of the IPMC actuators containing the same weight percentage of SGO and GO (respectively) revealed that a finer and more ordered Pt electrode structure was formed in SGO – containing IPMC actuators. Compositing Nafion with SGO generally led to a higher current response - and therefore higher characteristic capacitance compared to pure Nafion – based IPMCs. Nafion/GO – based IPMC actuator showed a lower current response compared to that of the Nafion/SGO – based actuator 26

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with the same filler content. Pure Nafion – based IPMC actuator showed the highest Rct and the lowest Cdl and  among all actuators. Cdl and  of IPMCs increased as a function of increasing content of SGO, while a reverse trend was observed for Rct. Moreover, Cdl and  of Nafion/SGO

Nafion/GO (1 wt %) – based actuator, respectively.

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(1 wt %) – based actuator appeared to be about three and two times higher than that of the

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In electromechanical tests the maximum tip displacement and water loss were recorded for actuators. For all actuators, displacement increased with increasing applied voltage. The small tip

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displacement of pure Nafion – based actuator was enhanced by eight times by adding SGO (1 wt%) and it also showed delayed back relaxation over time. IPMC actuators containing the highest wt% of SGO, which possessed the highest Cdl and, showed the lowest tip displacement among Nafion/SGO – based actuators, due to its denser electrode structure. Moreover, tip displacement of Nafion/SGO (1 wt%) – based actuator appeared to be about two times higher

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than that of the Nafion/GO (1 wt%) – based actuator, respectively. Water loss of IPMC actuators decreased significantly as the SGO content of Nafion membrane increased, in which pure Nafion - and Nafion/SGO (4 wt%) – based actuators showed the highest (~ 86%) and the lowest (~

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40%) water loss. Furthermore, Nafion/SGO (1 wt%) – based actuators showed the highest

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durability upon applying electric potential in open air. Results showed that water sustainability, ionic conductivity, Pt electrode capacitance and tip displacement of IPMCs increases with increasing SGO content of the actuators, while the tip displacement of IPMCs is mainly limited by Pt electrode structure and its rigidity.

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Table 1.Naming system of Nafion membranes Volume of

SGO

GO

Weight ratio of

Dry

Name

Nafion

(mg)

(mg)

SGO (or

thickness

GO)/Nafion (%)

(µm)

6.6

-

-

-

108

N-SGO-0.1

6.6

0.3

-

0.1

110

N-SGO-0.25

6.6

0.75

-

0.25

114

N-SGO-0.5

6.6

1.5

-

0.5

117

N-SGO-1

6.6

3.0

-

1.0

122

N-SGO-2

6.6

6.0

-

2.0

130

N-SGO-4

6.6

12

-

4.0

139

N-GO-1

6.6

-

3.0

1.0

118

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Pure Nafion

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solution (ml)

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Sample

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Hydrated

IEC

WUP

Name

Thickness (µm)

(meq/g)

(%)

Pure Nafion

130

0.58

N-SGO-0.1

135

0.88

N-SGO-0.25

148

N-SGO-0.5

153

N-SGO-2

2.05

35

166

2.62

41

185

3.45

52

205

5.10

60

0.86

24

145

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N-GO-1

23 30

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N-SGO-4

14

1.45

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N-SGO-1

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Sample

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Table 2.Hydrated thickness, ion exchange capacity (IEC) and water uptake (WUP) of Nafion membranes

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Figure captions Figure 1.Schematic illustration of fabrication process of pure Nafion, N-GO-1 and N-SGO-1 membranes

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Figure 2.Digital images of (a) pure Nafion – and (b) N-SGO-1 – based IPMC actuators. Figure 3.Comparison of FT-IR spectra of the synthesized GO and SGO Figure 4 Schematic depiction of sulfonation mechanism of GO

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Figure 5.Comparison of TGA results of the synthesized GO and SGO

Figure 6.Comparison of ATR spectra of pure Nafion, N-GO-1 and N-SGO-1membranes

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Figure 7.The variation in IEC and WUP of Nafion/SGO membranes as a function of SGO content

Figure 8.Longitudinal SEM images of Pt electrode region of the IPMC actuators (a-a“) IP, (b-b“) IP-SGO-0.1, (c-c“) IP-SGO-1, (d-d“) IP-SGO-4 and (e-e“) IP-GO-1, at three magnifications.

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Scale bars shown at left, middle and right images are respectively 10 µm , 500 nm and 200 nm. Figure 9.Digital images obtained from contact angle measurements of actuators (a) IP, (b) IPGO-1, (c) IP-SGO-0.1 and (d) IP-SGO-1

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Figure 10.Comparison of the CV voltamograms of IPMC actuators Figure 11.Comparing Nyquist plots of IPMC actuators (a) in full view and (b) in the selected

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region shown in part (a)

Figure 12.The variation in charge transfer resistance (Rct), double layer capacitance (Cdl) and ionic conductivity () of IPMCs as a function of SGO content Figure 13.The tip displacements of IPMC actuators (a) IP, (b) IP-GO-1, (c) IP-SGO-0.1 and (d) IP-SGO-1 (with the corresponding SEM images) in response to d.c. voltages of 5 V over time range of 0 to 10 s

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Figure 14.Comparison of (a) the dependence of tip displacement on amplitude of the input d.c. voltage for fabricated IPMC actuators and (b) the average maximum tip displacement and water loss (%) of IPMC actuators, in response to the same d.c. voltage of 5 V in amplitude.

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Figure 15. Comparison of the operating life of the IPMC actuators; IP, IP-GO-1 and IP-SGO-1

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actuators as they subjected to 5 V d.c. potential with time interval of 10 min in open air.

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Figure 1

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Figure 2

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Figure 3

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Figure 5

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1.000

Transmittance

0.990

Pure Nafion N-GO-1 N-SGO-1

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0.995

0.985

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0.980

0.975

0.965

0.960

4000

3500

3000

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0.970

2500

2000

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Wavenumber (cm-1)

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Figure 6

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1500

1000

500

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6

70 IEC water uptake

60

5

3

30

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2

0

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1

Pure Nafion N-SGO-0.25 N-SGO-0.25 N-SGO-0.5

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Figure 7

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IEC (meg/g)

40

38

N-SGO-1 N-SGO-2 N-SGO-4

20

10

0

WUP (%)

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50

4

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Figure 8

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2.5 2 1.5 1

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IP IP-SGO-0.1 IP-SGO-1 IP-SGO-4 IP-GO-1

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I(mA)

0.5

0

-1 -1.5 -2 -1

-0.5

0 E(V)

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-1.5

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Figure 10

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1.5

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Figure 11

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Rct (Ω)

30

Cdl (μf)

σ (mS/cm)

25.6

18.8

11.9

8.57

8.4

11.9

5.1 6.9

5

10.7

9.27

6.42

10

19.2

14.3

12.4

6.6 6.2

IP-SGO-0.25

IP-SGO-0.5

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4.35

IP-SGO-0.1

IP-SGO-1

IP-SGO-2

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Figure 12

43

9.7

9.1

2.71

6.2

0

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15

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Magnitude

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28.8

25

IP-SGO-4

IP

1.03

6.1

σ (mS/cm) Cdl (μf)

Rct (Ω)

IP-GO-1

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References:

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