Electrochemical actuation of multiwall carbon nanotube fiber with embedded carbide-derived carbon particles

Accepted Manuscript Electrochemical actuation of multiwall carbon nanotube fiber with embedded carbide-derived carbon particles Margo Plaado, Friedrich Kaasik, Robert Valner, Enn Lust, Rando Saar, Kristjan Saal, Anna-Liisa Peikolainen, Alvo Aabloo, Rudolf Kiefer PII: DOI: Reference:

S0008-6223(15)30100-7 http://dx.doi.org/10.1016/j.carbon.2015.07.077 CARBON 10145

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Carbon

Received Date: Revised Date: Accepted Date:

23 April 2015 9 July 2015 23 July 2015

Please cite this article as: Plaado, M., Kaasik, F., Valner, R., Lust, E., Saar, R., Saal, K., Peikolainen, A-L., Aabloo, A., Kiefer, R., Electrochemical actuation of multiwall carbon nanotube fiber with embedded carbide-derived carbon particles, Carbon (2015), doi: http://dx.doi.org/10.1016/j.carbon.2015.07.077

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Electrochemical actuation of multiwall carbon nanotube fiber with embedded carbide-derived carbon particles

Margo Plaado1, Friedrich Kaasik2, Robert Valner2, Enn Lust3, Rando Saar1, Kristjan Saal1, Anna-Liisa Peikolainen2, Alvo Aabloo2 and Rudolf Kiefer2,* 1

Institute of Physics, Faculty of Science and Technology, University of Tartu, Ravila

14C, 50411, Tartu, Estonia 2

Intelligent Materials and Systems Lab, Faculty of Science and Technology,

University of Tartu, Nooruse 1, 50411 Tartu, Estonia 3

Institute of Chemistry, Faculty of Science and Technology, University of Tartu,

Ravila 14A, 50411 Tartu, Estonia

Abstract Novel multiwall carbon nanotube (MWCNT) fibers and carbide-derived carbon (CDC) MWCNT composite fibers were prepared using dielectrophoresis method. Obtained fibers were investigated as an actuator material in organic electrolytes using an isotonic and isometric electrochemical-mechanical

*

Corresponding author: Tel: 372 7374826. E-mail: [email protected] (Rudolf

Kiefer)

deformation (ECMD) technique. Adding CDC particles to the fiber based electrode, physical properties (density, tensile strength, Young’s modulus and conductivity) changed slightly (all changes remain in the order of 10-15%) but specific capacitance increased by 2.3 times (from 62 to 145 F g-1). Also the ECMD measurements (charging/discharging in balance) reveal intriguing effects, as the CDC addition also leads to 2-3 times higher stress and strain values compared to that of neat MWCNT fiber based electrodes. These results show that the CDC-MWCNT fibers have great potential for application in actuator material or in energy storage devices like supercapacitors.

1. Introduction Nanocarbon based materials such as the multiwall carbon nanotube (MWCNT) and the carbide-derived carbon (CDC) exhibit remarkable physical and chemical properties[1]. Micro and nano fibers of SWCNT (single wall carbon nanotubes) and MWCNT have exceptional physical properties: a tensile strength of 3.3 GPa and a Young’s modulus of 263 GPa[2,3] (for comparison, the tensile strength of high-tensile-strength steel is 2-2.5 GPa and Young’s modulus ~200 GPa)[4]). However, significant improvements are still possible, as individual MWCNT have a tensile strength of ~100 GPa and a Young’s modulus of ~1 TPa[5]. Carbon nanotube (CNT) materials have been proposed for many applications[6] including sensors[7– 9], actuators[10,11], composites for antibacterial materials[12], nanoelectronics[13], supercapacitors[14] and many more. Most CNT-fiber preparation methods lie in drawing the CNT from a solid state[15, 16] (i.e. twisted yarns of CNT-forest or aerogel); this method allows for inter-nanotube interactions and alignment that improves the mechanical characteristics of the fibers[17]. Electrospinning is often applied for fabrication of CNT composite materials[18] such as MWCNT-Nylon[19], MWCNT-DNA[20] and MWCNT-

polyethyleneoxides[21]; due to the feasibly controlled parameters, the method produces nano and micro fibers of moderate conductivity, high strength, low density and low porosity[22]. Another method for CNT-fiber preparation is dielectrophoresis (DEP), where the fibers are drawn from a droplet of a CNT-suspension on a substrate with an ultra-sharp metal needle at an applied AC voltage[23,24]. The mats of CNT-fibers have been investigated from the perspective of their application as actuators. The actuation mechanism, investigated in aqueous electrolytes under isometric electrochemical-mechanical deformation (ECMD) conditions, revealing quantum-chemical and electrostatic double layer interactions[10,25]. The double-layer charge injection actuation mechanism is the primary non-faradic mechanism of the electrochemical charging/discharging process, changing the charge of carbon atoms and resulting in a length change of C-C bonds, which reflects in the expansion or contraction of the CNT[25]. Another investigation with SWCNT electrodes indicates that for electrolyte concentrations higher than 10 -3 M, the actuation mechanism is a faradic process—that is, is due to chemical transformations driven by current[26]; the same has been concluded for SWCNT actuators operating in an ionic liquid medium[27]. CDC belongs to the large family of carbon materials that are derived from carbide precursors[28]. One of the most distinguished characteristic properties of CDC is the highly nanoporous structure (pore Ø0.8−1.2 nm) specifically designed for energy applications such as batteries/gas storage[29,30] and supercapacitors[31]. The main drawback of CDC electrodes in supercapacitors and batteries applications is related to the dimensional changes during charging/discharging cycles, which can lead to structural degradation of the material[32]. On the other hand, this volumetric change of CDC (also related to its structure, like its energy storage capability) points to its usability as an actuator material[33,34]. For this, the CDC is suspended in a polymer solution, such as polyvinylideneflouridecohexafluoropropylene (PVdF(HFP)) and spray coated or laminated onto a flexible membrane

material, such as PVdF(HFP)[35]. The actuation mechanism of the CDC material is based on the charge induced electrical double layer (EDL) formation, including processes like charge injection and ion migration[36]. Usually the CDC polymeric actuators have been applied in a trilayer arrangement (a counter and working electrode separated by a flexible ion-conductive membrane) and actuated (the electrolyte being an ionic liquid embedded in the trilayer sandwich structure) at an applied voltage range between ±2.0 V with an obtained bending strain in the range of 0.6% and stress in the range of 0.3 MPa[35]. To obtain higher conductivity, SWCNT[34] or gold foil[37] have been combined with CDC polymeric actuators, resulting in an increase of strain by 0.85% for the addition of SWCNT and 2.2% for gold foil, respectively. Similar bending actuators have been constructed also using different carbon materials like activated carbons[38], graphene[39, 40] and carbon aerogels[41]. The application of CDC with PVdF(HFP) polymer actuator material in linear actuation was demonstrated recently in an aqueous electrolyte (sodium dodecylbenzenesulfonate NaDBS) at an applied voltage of ±0.8 V under isometric and isotonic ECMD measurement conditions with an obtained strain in the range of 0.5% and stress in the range of 13 kPa[42]. The CDC polymeric actuator showed cation dominated actuation, which was explained by the size difference of the Na+ cations and DBS− anions. In this work, actuation properties and a specific capacitance of dielectrophoretic MWCNTfibers and MWCNT-fibers with embedded CDC particles (30 wt%) was investigated in organic electrolytes under isotonic ECMD measurement conditions. To determine the actuation caused only by an EDL formation, the electrochemically stable potential window was selected and various electrochemical methods (cyclic voltammetry, chronoamperometry and

chronopotentiometry)

were

applied

under

(charging/discharging processes under equilibrium).

balanced

charging

conditions

2. Experimental section 2.1. Materials Amorphous titanium carbide (TiC) derived carbon (CDC TiC-800) was purchased from Skeleton Technologies Ltd. MCWNT were purchased from Sigma-Aldrich (O.D. × I.D. × L = 7–15 nm × 3–6 nm × 0.5–200 µm). Propylene carbonate (PC, 99%) and tetrabutylammonium trifluoro-methanesulfonate (TBACF3SO3, 99%) were obtained from Sigma-Aldrich and used as received. 2.2 Preparation of MWCNT and CDC-MWCNT fibers A dielectrophoresis method was used for preparing fibers. First, dispersions of MWCNT and MWCNT-CDC in deionized water were prepared. The MWCNT (and CDC in the case of MWCNT-CDC fibers) were dispersed in deionized water containing polyvinylpyrrolidone as surfactant (PVP, average mol. wt. 40000, Sigma-Aldrich) and sonicated with a UPS200S (Hielscher) ultrasonic processor for 30 minutes at 50% amplitude. PVP, CDC, MWCNT, and water were mixed at a ratio of 1:2(or 0):4:1500 (wt%), respectively. The surfactant was added to stabilize the dispersion for extended periods of time. Fibers were prepared according to the method described by Tang et al[43]. Briefly, the tip of a chemically etched ∅0.1 µm tungsten needle was inserted into a droplet of MWCNT suspended in deionized water deposited onto a stainless steel plate (∅2 cm). Then, an AC voltage was applied between the tungsten tip and the metal plate (0−350 Vpp, 2 MHz), and the tungsten tip was withdrawn until a MWCNT fiber of the desired length was formed. The initial spacing between the needle and plate electrode (i.e., the height of the droplet) was 3 mm. The retraction of the needle was performed with an M-413.3PD precision stage (Physik Instrumente). 2.3 ECMD (isotonic) measurements

The fibers (Ø150 µm) were cut 1 cm in length and fixed on the force sensor (TRI202PAD, Panlab) connected with a fixed arm that served as a working electrode on the linear muscle analyser set up (Figure 1).

Figure 1: Scheme of the linear muscle analyser with a force sensor and integrated self-written software, potentiostate (Portable Electrochemical Interface & Impedance Analyser CompactState of Ivium) and three-electrode measurement cell. Precision stage (1), beaker with electrolyte solution (2), working electrode (3), Pt-sheet counter electrode (4), Ag/AgCl reference electrode (5), fiber sample (6), force sensor (7). Three electrode measurements were carried out using the following setup: the fiber was immersed in 0.1 M TBACF3SO3 in propylene carbonate electrolyte in a cell with a Pt-sheet counter electrode and an Ag/AgCl (3M KCl) reference electrode. The initial length of the fiber between the clamps was 1 mm. The force and length changes and the applied electrical

signal were measured in real time with self-written software. Within a voltage range from 0.6 V to -0.55 V, the cyclic voltammetry (scan rate 5 mV s-1), the chronoamperometry and the chronopotentiometry at the constant charge density 42.5 mC cm-3 (frequencies 0.0025 Hz, 0.005 Hz, 0.01 Hz, 0.025 Hz, 0.05 Hz and 0.1 Hz) were performed.

2.4 Characterization of fibers: SEM analysis, conductivity and density data The prepared fibers were examined with a scanning electron microscopy method (Helios NanoLab 600, FEI). The electrical conductivity of the fibers was measured by the two-point probe method. The electrical contacts with a graphite conductive adhesive (Electron Microscopy Sciences) were applied. The fiber densities based on their dimensions (obtained using microscopy) and masses (with an analytical balance) were calculated. For reliable estimation, a suitable amount (1 mg) of the fiber material has been weighed.

3. Results 3.1 Fiber properties 3.1.1 Stress-strain measurements To evaluate the Young modulus of the MWCNT and CDC-MWCNT fibers, each sample was investigated at least 3 times in the dry state and in electrolytes with an average standard regression of 12%. The fibers were stretched until their breaking point, and the results are presented in Figure 2.

Figure 2: Strain stress curves of MWCNT fibers (red) and CDC-MWCNT fiber (black) under (a) dry and (b) immersed into the electrolyte (0.1 M TBACF3SO3 in propylene carbonate) state. In the dry state (Figure 2a), the CDC-MWCNT fiber demonstrated higher elasticity (strain before breaking point was found in the range of 10.3% and stress equal to 3.6 MPa) in comparison to the MWCNT data (7.8% strain shows stress of 3.4 MPa at breaking point). Based on the slope value of the stress vs. strain curve, the modulus of the MWCNT fiber was calculated to be 43.5 MPa in comparison to the CDC-MWCNT with a 10% decrease in modulus in the range of 39.5 MPa. The stresses vs. strain curves are different in the electrolyte and in the dry state. The breaking point found in the electrolyte solution was 100 kPa (7.3% strain) for the MWCNT fiber and 130 kPa (11.9% strain) for the CDC-MWCNT fiber, respectively. However the stress vs. strain curve in the electrolyte solution (Figure 2b) shows different behavior, as for the initial 4% strain, the stress increases slightly with the calculated modulus being 1.1 MPa for the MWCNT fiber and 0.68 MPa for the CDCMWCNT fiber. From the 4% strain onward to the breaking point, the modulus increases to 2.6 MPa for the MWCNT fiber and to 1.4 MPa for the CDC-MWCNT fiber. The fiber is weak due to the porous structure[24], and we assume that the electrolyte and the solvent interactions lead to the fiber destabilization (Figure 2b, red curve) that can be caused by the sliding of nanotubes in the bundles[44]. Incorporation of CDC particles into the fibers

increases the slipping effect and leads to a nearly 50% lower Young modulus value (Figure 2b, black curve).

3.1.2 SEM images of CDC-MWCNT and MWCNT fibers The length and diameter of the tested MWCNT and CDC-MWCNT fibers were similar (1 cm in length and 150 µm in diameter). The resulting densities of the MWCNT fibers were in the range of 0.31 g cm-3 and increased up to 0.35 g cm-3 after CDC implementation. The added CDC particles enhanced slightly (~17%) the conductivity from 11.9 S cm-1 (MWCNT fiber) to 13.9 S cm-1 (CDC-MWCNT fiber). The SEM images of MWCNT and CDC-MWCNT fibers are presented in Figure 3.

Figure 3: SEM images of (a) a CDC-MWCNT fiber (inset: cross section), (b) a cross section of the fiber at a high resolution, (c) a MWCNT fiber (inset: cross section) and (d) cross section of the fiber at a high resolution. The morphologies of the fiber materials, including cross sections in Figures 3a and c, reveal only little differences between CDC-MWCNT and MWCNT electrode materials. Both fibers are highly porous, reflecting in lower Young modulus and higher specific capacitance values (discussed later) in comparison to other CNT fibers studied before[45]. A slightly rougher surface can be observed for CDC-MWCNT fibers. In Figures 3b and d, the cross section surface is given at a higher resolution, and it can be seen in Figure 3b that the CDC particles (grey solid features) are surrounded by MWCNT, revealing that MWCNT is acting as a binder for the CDC particles, providing higher mean porosity and better conductivity (14 S cm-1) in comparison to the CDC-PVdF(HFP) films (0.4 S cm-1)[42]. To investigate the actuation properties of the fibers in the organic electrolyte solution, the cyclic voltammetry was conducted under isotonic ECMD measurement conditions.

3.2 Cyclic voltammetry at charging/discharging in balance at isotonic ECMD measurements The comparison of the actuation properties of the MWCNT and CDC-MWCNT fibers has been studied by linear actuation under the isotonic cyclic voltammetry (CV) ECMD measurement conditions presented in Figure 4. At isotonic measurement conditions, the actuator shows expansion under a decrease of stress and an increase of stress at contraction, thus in opposite directions established under isotonic ECMD measurements.

Figure 4: Cyclic voltammetric measurements (at a potential scan rate 5 mV/s, the counter electrode is the platinum sheet, and the reference electrode is Ag/AgCl (3 M KCl) between 0.6 V to -0.55 V) of MWCNT fiber (length: 1 cm, ∅ ~150 µm) and CDC-MWCNT fiber (length: 1 cm, ∅~150 µm) in 0.1 M TBACF3SO3 in propylene carbonate as an electrolyte under ECMD measurement conditions: (a) stress σ (kPa) vs. potential E, (b) strain ∆l/l (%) vs. potential E, (c) current density j vs. potential E, (d) charge density Q vs. potential E and (e): charge density Q vs. stress σ (kPa) curves.

Within the potential window from 0.6 V to -0.55 V the steady state conditions (charging/discharging in balance) for the fiber actuators have been established. For the related

stress and strain curves, both samples revealed different actuation mechanisms. For the MWCNT fibers (red curve, Figure 4a, b), well exposed expansion at a negative potential of 0.55 V (2.3 kPa, 0.13%) and a minor expansion at a positive potential of 0.6 V (0.5 kPa, 0.01%) were observed. In the literature, there have been proposed the possible actuation mechanisms of CNT actuators involving electrostatic double layer effects[10], quantum mechanical induced C-C length change[10], and also faradic processes[25]. Based on the analysis of our results, shown in Figure 4a, b, the correlated actuation mechanism is caused by the movement of ions and EDL effects. If 30 wt% of CDC particles were added into the MWCNT fiber solution, the isotonic CV ECMD results (black curve in Figure 4a) and isometric CV ECMD results (black curve in Figure 4b) in maximum expansion at a cation adsorption of -0.55 V with a double layer stress output of 5 kPa and strain output of 0.3% compared to 2.3 kPa (0.13%) for pure MWCNT fiber. CDC materials are well known for their defined pore size distribution applicable for supercapacitors[31]. Integration of CDC particles into MWCNT fibers increases the current density (Figure 4c), shown in the responding CV curve, where the MWCNT fiber has a charge density of 43 mC cm-2 at 0V in comparison to 183 mC cm-2 calculated for CDC-MWCNT, with the latter being 4 times higher. Within the region of ideal polarization, the main charging/discharging mechanism of CDC electrodes is contributed to the EDL formation. observed in earlier research in aprotic electrolytes, where the expansion at a positive potential will appear first above 0.5 V[32], considering that the electrochemical window in this study was determined in the potential range from 0.6 V to 0.55 V to maintain charging/discharging processes in balance (Figure 4d) and to avoid irreversible faradic processes at higher negative and positive potentials. The charge density vs. stress curves given in Figure 4e revealed that for MWCNT fibers, the small actuations at negative potentials lead to a different position of starting and end points that can be related to the higher creep during continuous actuation cycles (Figure 5c).

3.3 Chronoamperometry of MWCNT and CDC-MWCNT fibers The relation between the charge densities and stress have been investigated at different frequencies from 0.0025 to 0.1 Hz for MWCNT and CDC-MWCNT fibers, and the results obtained are presented in Figure 5.

Figure 5: Square wave potential (chronoamperometry, 0.6 V to -0.55 V) data for MWCNT fiber (red curve) and CDC-MWCNT fiber (black curve) in 0.1 M TBACF3SO3 in propylene carbonate solution is presented as plots of (a) stress σ vs. time (frequency 0.0025 Hz), (b) stress ∆σ vs. charge density (normalized at discharging), (c) stress σ data collected during 50 cycles at frequency 0.1 Hz and (d) stress σ vs time curves for cycles 20-21. The stress curves at square wave potentials (from 0.6 V to -0.55 V) measured at a fixed 0.0025 Hz frequency for MWCNT (red curve) and CDC-MWCNT (black curve), given in Figure 5a, show that the calculated stress values are in the range of 6 kPa for CDC-MWCNT fiber and up to 2 kPa for the MWCNT fiber. The MWCNT stress curve (in Figure 5a, red curve) shows that for the potential jump from 0.6 V to -0.55 V, a small increase in stress appears that correlates to fiber contraction followed by continuous fiber expansion at -0.55 V. The calculated charge densities at negative potentials (normalized) for both fibers at different frequencies (0.1 Hz -0.0025 Hz) with the related stress values are presented in Figure 5b. The strain values in this frequency range (data not shown for shortness) are found for MWCNT fiber in the range between 0.16% (0.1 Hz) to 0.59% (0.0025 Hz) and for CDC-MWCNT fiber strain in the range of 0.24% at 0.1 Hz and 0.77% at 0.0025 Hz. For CDC-MWCNT (Figure 5b, black), there is a linear dependence of stress on charge, and at higher charge densities the

stress increases from 3 kPa (17 mC cm-2 at 0.1 Hz) to 6 kPa (278 mC cm-2 at 0.0025 Hz). For MWCNT fiber (red), the stress increases only slightly from 1.87 kPa (11 mC cm-2 at 0.1 Hz) to 2.5 kPa (118 mC cm-2 at 0.0025 Hz). Thus, from CDC-MWCNT data, it can be concluded that influence of the charge density on the actuation properties (stress) is concurrent with the EDL charging/discharging mechanism for CDC based actuators. For MWCNT fiber, the actuation properties can be seen in Figure 5a and 5d (red curve), where at the potential jump from positive to negative polarization, a peak appears with a time delay that shows a small increase of stress before the stress starts to decrease. Similar processes have been observed in earlier research for CDC in ionic liquid using electrochemical dilatometry[46]. The appearance of this peak is still unclear, but it may refer to the re-orientation of ions or electrostatic repulsion. A similar experiment has been carried out with MWCNT yarn in the organic electrolyte, where contraction and expansion have been found when the square wave potential was switched. A faradic process was excluded and the double charge injection discussed as a reason for changing C-C bond length within the nanotubes and the outer bundle leading to some asymmetric changes[47]. Other studies conducted on SWNT fibers in aqueous electrolytes revealed that faradic processes are involved[26]. Therefore we assume that this peak for MWCNT is related mainly to the EDL charging mechanism, rather than to a complex process where both faradic and non-faradic processes are presented. After 50 cycles, a creep was observed in actuation stress (Figure 5c, red curve) for MWCNT fiber, but for CDC-MWCNT fiber, no creep was detected. The creep could be related to the debundling of MWCNT in the fiber under study.

3.4 Chronopotentiometry of MWCNT and CDC-MWCNT fibers To investigate how the stress changes at constant charge density conditions (42.5 mC cm-3), the chronopotentiometric measurements (different current density vs. time conditions) at frequencies from 0.1 Hz to 0.0025 Hz have been performed. Thus, chronopotentiometric measurements at a frequency 0.025 Hz and at a current density 2.125 mA cm-3 have been conducted, and the results obtained are presented in Figure 6.

Figure 6: Chronopotentiometry measurements data collected at a constant charge density (42.5 mC cm-3) in 0.1 M TBACF3SO3 solution in propylene carbonate at a frequency 0.025 Hz (2.125 mA cm-3) of (a) MWCNT fiber stress σ vs. time (solid line) and potential E vs. time (dotted line) plots and (b) CDC-MWCNT fiber stress σ vs. time (solid line) and potential E vs. time (dotted line) data.

For MWCNT fiber in the electrolyte solution system, two continuous stress vs time curve cycles (0.025 Hz, 2.125 mA cm-3) are shown in Figure 6a, demonstrating that the stress change depends on the charge density applied. The stress vs time curves for MWCNT fibers reveal that applying constant current (Figure 6a) for 8s, the maximum stress peak appears, indicating that the fiber attempts to contract but the length is kept constant. However, a decrease of stress appears when holding the positive (charging) current constant that relates to a relaxation of the MWCNT fiber. At the transition state from positive current to negative charging current, the fiber attempts to contract again, and the stress value increases to 2.5 kPa; however, the reasons for that is unclear, but we consider that the effect is caused by an EDL actuation mechanism. The relaxation may occur mainly for MWCNT fiber based electrodes and not for the CDC-MWCNT fiber based electrodes, because the capacitance of MWCNT fiber is almost two times lower than for the CDC-MWCNT fiber. Due to the higher capacitance also the applied voltage is lower for the CDC-MWCNT (to achieve the same charge) and therefore no relaxation is observed. For CDC-MWCNT composite fiber (Figure 6b), relaxation does not appear during charging, and only an increase in stress at charging the fiber towards positive potential and a decrease at discharging has been found. We assume that if CDC particles have been added into MWCNT fiber, the double layer charging (non-faradic) processes increase the stress to 1.8 kPa at a

constant charge density of 42.5 mC cm-3. To evaluate the specific capacitance Cs (F g-1) of the CDC-MWCNT composite fiber and the MWCNT fiber, the slope (∆V/∆t) values at the chronopotentiometric curves at discharging (after the IR drop correction) were used accordingly to the equation: Cs = i /(-slope * m)

(1)

where i is the applied current (A) and m is the mass (g) of the fibers analyzed [48]. The mass of MWCNT fiber was calculated by multiplying the fiber volume with the density of fiber, and that was found to be 0.31 ± 0.018 g cm-3 for MWCNT and 0.35 ± 0.026 g cm-3 for CDC-MWCNT, with a resulting mass in the range of 55.1 ± 3.2 µg for MWCNT fiber and in the range of 62.3 ± 4.8 µg for CDC-MWCNT fiber. The specific capacitance Cs of the fibers at different applied current densities in chronoapotentiometric measurements (at the applied constant charge density 42.5 mC cm-3 conditions) within frequencies from 0.1 Hz to 0.0025 Hz was determined from the potential vs. time curves following the equation (1). The results are presented in Figure 7.

Figure 7: MWCNT fiber (white rectangles) and CDC-MWCNT composite fiber (black rectangles) specific capacitance Cs (a) and stress σ (b) plots calculated from chronopotentiometric measurements (constant charge) at different applied current densities and current frequencies from 0.0025 to 0.1 Hz of (a) specific capacitance vs. charge density j (A g-1) plots and (b) stress σ vs. charge density plots. The CDC-MWCNT composite fiber (Figure 7a) shows a typical capacitor behavior as at different current densities and charging/discharging times, while the specific capacitance decreases from 144 F g-1 (0.0025 Hz) to 50 F g-1 (0.1 Hz) with the rise of charging current density. CDC powder based electrical double layer capacitors (TiC precursor) showed in the

organic electrolyte (tetraethylammonium-tetrafluoroborate in acetonitrile) capacitance values in the range of 140 F g-1, which reflects a good supercapacitor behavior[28]. MWCNT fibers showed less expressed dependence on the applied charge density with nearly 2.3 times lower specific capacitance ~62 F g-1 at low charging currents (0.0025 Hz), which matches with data for MWCNT in organic electrolytes, where a value around 60 F g-1 was reported[49]. The specific capacitance at higher charging currents stabilizes at 39-45 F g-1. The

isotonic

ECMD

measurements

conducted

at

constant

charge

conditions

(chronopotentiometric studies) (Figure 7b) revealed that in the case of MWCNT fiber, the stress increases with applied higher current densities. The CDC-MWCNT composite fiber demonstrates nearly constant stress in the range of 1.8 kPa at the same fixed charge density 42.5 mC cm-3 (0.12 C g-1). Thus, we assume that the actuation properties relate to the charge densities following the EDL charging/discharging mechanism described in recent research for CDC based polymeric actuators [34]. In the case of MWCNT fiber, the similar charge density of 42.5 mC cm-3 (0.14 C g-1) shows that with increased current density, the stress increased from 0.8 kPa (0.0025 Hz) at an applied current density of 0.2 A g-1 up to 4.2 kPa (0.1 Hz) at 8.7 A g-1. Thus, it should be summarized that the actuation mechanism of MWCNT fiber in non-aqueous electrolytes is not fully understood; however, based on our results, only a nonfaradic actuation process can be drawn.

4. Conclusions MWCNT fiber and CDC-MWCNT composite fibers fabricated by applying dielectrophoresis processes characterized by low modulus, but high porosity, are interesting materials for supercapacitor or ionic actuator applications. Isotonic ECMD have been applied in organic electrolytes by keeping the charging and discharging of the fiber in equilibrium, and it was revealed that for MWCNT fibers, the obtained actuation stress results can be explained solely by the electrical double layer charging, so-called non-faradaic, processes within a selected potential window. After adding 30 wt% of CDC into MWCNT fibers, the actuation mechanism was charge controlled, and the CDC-MWCNT composite showed also typical EDL charging/discharging behavior but with nearly 3 times higher stress and strain in the range of 6 kPa and 0.3 %, respectively. Keeping in mind that the degradation of the supercapacitor material is initiated by the dimensional changes (actuation) during the charging/discharging cycles, it can still be summarized that the novel CDC-MWCNT

composite fiber with a high specific capacitance of 145 F g-1 shows excellent electrical double layer characteristics for application in energy storage devices.

Acknowledgements The research was supported by the European Union through the European Social Fund (MTT76), European Social Fund's (ESF) Doctorial Studies and Internationalization Programme DoRa carried out by Archimedes Foundation. Additional acknowledgements goes to the Estonian Science Foundation (grants no 9007, IUT20-24 and IUT2-25), the European Regional Development Fund (Centre of Excellence „Mesosystems: Theory and Applications“, TK114; and "TRIBOFILM" 3.2.1101.12-0028); „Functionalization of carbon nanomaterials: synthesis, characterization and application“ 3.2.1101.12-0019 and „Hightechnology Materials for Sustainable Developmentand: TK 117 3.2.0101.11-0030 and „Stimuli-Responsive Smart Polymer Composite Materials and their Applications in Lab-on chip Devices and in Robotics“ by the Estonian Nanotechnology Competence Centre.

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