Microporous and Mesoporous Carbide-Derived Carbons for Strain

Letter pubs.acs.org/Langmuir

Microporous and Mesoporous Carbide-Derived Carbons for Strain Modification of Electromechanical Actuators Janno Torop,*,†,⊥ Mati Arulepp,‡ Takushi Sugino,§ Kinji Asaka,§ Alar Jan̈ es,∥ Enn Lust,∥ and Alvo Aabloo† †

IMS Lab, Institute of Technology, University of Tartu, Nooruse 1, 50411 Tartu, Estonia Skeleton Technologies Ltd., Riia 181a, 51014 Tartu, Estonia § Health Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan ∥ Institute of Chemistry, University of Tartu, Ravila 14a, 50411, Estonia ‡

ABSTRACT: Low-voltage stimuli-responsive actuators based on carbide-derived carbon (CDC) porous structures were demonstrated. Bending actuators showed a differential electromechanical response defined by the porosity of the CDC used in the electrode layer. Highly porous CDCs prepared from TiC (mainly microporous), B4C (micromesoporous), and Mo2C (mainly mesoporous) precursors were selected to demonstrate the influence of porosity parameters on the electromechanical performance of actuators. CDC-based bending-type actuators showed a porosity-driven displacement response over a frequency range of 200 to 0.005 Hz at an applied excitation voltage of ±2 V. The displacement response of the CDC actuators increased with an increasing number of mesopores in the electrode layer, and the generated strain of the bending actuators was proportional to the total porosity (micropores and mesopores) of the CDC. The modifiable electromechanical response that arises from the precise porosity control attained through tailoring the CDC architecture demonstrates that these actuators hold great promise for smart, low-voltage-driven actuation devices. mechanical devices.15 Despite all these advances, the control at the desired actuation levels, especially on the fine scale, is still a challenge for actuators using high-surface-area carbon. This lack of control is caused by the complexity of the electromechanical actuation mechanism, specifically, the fact that charge-induced electromechanical actuation is a combination of both charge injection during electrical double-layer (EDL) charging of highsurface-area carbons and ion migration/accumulation induced by charge distribution.16 The aforementioned effects result in a greater expansion of the negatively polarized electrode compared to that of the positively charged electrode.17 EDL charging is expected to be a fast process where the main limitation is the electronic conductivity of the carbon-based electrode. On the contrary, ion migration is affected by the pore size distribution of the porous carbon and the ionic conductivity of the porous separator.18,19

1. INTRODUCTION Recently, significant attention has been focused on soft and flexible electrochemomechanical and electromechanical actuators that are able to transform electrical energy into mechanical work or motion.1−3 These materials hold great potential for many applications including artificial muscles, lightweight microsensors, and switches.4−6 In the last few decades, conductive polymers,7 dielectric elastomers,8 and ferroelectric polymers9 have been studied as materials for electromechanical actuators. Since 1992,10 ionic electroactive polymer actuators (iEAP) based on an ionic conductive membrane covered with metallic plates and a variety of polymer-supported carbon materials in combination with membranes containing ionic liquid have been extensively studied.11,12 Recent studies showed that by introducing single-walled carbon nanotubes into the electrode layer the actuation speed can reach up to the millisecond range and the generated strain and stress can be improved dramatically. 13,14 Reported methods for thin graphene film synthesis also show the potential for a wide breakthrough of rapid response and energy-efficient electro© 2014 American Chemical Society

Received: March 7, 2013 Revised: February 18, 2014 Published: February 27, 2014 2583

dx.doi.org/10.1021/la404616w | Langmuir 2014, 30, 2583−2587

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Table 1. Selected Pore Characteristics of Carbon Materials

a

carbon

crystal structure of precursor carbide

Tchlor (°C)a

SBET (m2 g−1)b

Vμ (cm3 g−1)c

Vtot (cm3 g−1)d

APS (Å)e

CDC-TiC CDC-B4C CDC-Mo2C

cubic rhombohedral hexagonal

600 800 800

1150 1534 1675

0.49 0.43 0.72

0.53 0.99 1.39

9.2 12.9 16.6

Tchlor − chlorination temperature. bSBET − BET surface area. cVμ − micropore volume. dVtot − total pore volume. eAPS − average pore size.

Figure 1. Schematic depiction of the CDC actuator and SEM micrograph of the structure actuator. (a) Simplified notation of electrical double-layer formation inside porous media of CDC. (b) Typical SEM cross-sectional image of the CDC actuator device.

Previously, we studied mainly microporous TiC-derived carbons with a pore size distribution ranging from 0.6 to 1.1 nm.20 The study suggested that the electromechanical performance of actuators is strongly dependent on the porosity characteristics of the CDC. In this letter, we report electromechanical bending actuators prepared from TiC-, B4C-, and Mo2C-derived carbons with peak pore sizes from 0.9 to 36 nm, respectively. Differences in microporous and mesoporous properties are expected to determine variations in the EDL capacitance and also the ion mobility. Therefore, various CDCs can affect the displacement of actuators and offer the opportunity to control the electromechanical performance of the actuator.21,22

(Figure 1a). Bending displacement and current profiles were continuously monitored by using a laser displacement sensor (Keyence LC2100/2220) at a fixed distance of 5 mm from clamps. Actuators were driven by potentiostat/galvanostat (Hokuto Denko HA-502G) with a waveform generator (Yokogawa Electric FC 200). For each actuator device, the actuation was recorded at fixed frequency values of 200, 100, 50, 10, 5, 1, 0.5, 0.1, 0.05, 0,01, and 0.005 Hz (measurements started from the highest frequency values), and in each fixed frequency, we recorded at least 10 bending cycles. The displacement (δ) of the actuator strip was later expressed as a strain (ε) between two CDC-based electrodes by using the following equation

2. EXPERIMENTAL SECTION

where L is the free length from gold contacts (fixed at 5 mm), d is the thickness of the actuator, and δ is the displacement of the actuator.13,20 This equation allows us to express the strain difference of two layers; therefore, it is possible to discard the direct displacement comparison because there was still variation in thickness of prepared actuators. The stress−strain measurements were carried out to evaluate the Young’s modulus of the electrode film using a Seiko Instruments Inc. TMA/SS6000. The Young’s modulus of an independent CDC is between 32 and 39 MPa. The SEM micrograph of the cross-section of the trilayered actuator element is given in Figure 1b. This micrograph is obtained by using a Helios Nanolab 600 microscope in secondary electron image mode. For sample preparation, the actuator film was cooled in liquid nitrogen and cracked just before observations to get flat, clean surfaces. Difference in the morphology of the membrane located in the middle of the composite and outer regions is clearly visible in Figure 1b. The micrograph also demonstrates good adhesion between the PVdF-based membrane layer and the CDC-based outer layers.

ε=

2.1. Carbide-Derived Carbon. CDC-TiC and CDC-Mo2C powders were synthesized by thermal chlorination as described elsewhere.23,24 CDC-B4C was purchased from Y-Carbon. It can be indicated that both the selection of the precursor carbide and the synthesis temperature (Table 1) have an effect on the average pore size (APS) and on the incremental surface area. Thus, the prevailing pore diameter can be successfully controlled on the nanoscale. 2.2. Actuator Preparation. The ionic liquid containing polymer actuators was fabricated by placing an ion-conductive polymeric membrane between two CDC-based electrode sheets (Figure 1). Actuator electrodes were composed of 20 wt % CDC, 48 wt % 1-ethyl3-methylimidazolium tetrafluoroborate (EMIBF4), and 32 wt % polyvinylidenefluoride-co-hexafluoropropylene (PVdF(HFP)). The electrolyte layer was prepared from PVdF(HFP) and EMIBF4 with a weight ratio of 50/50. Electrode layers were obtained from the casting solution by evaporating solvent completely in a vacuum oven. For each type of CDC actuator, the electrodes were prepared from respective CDC-containing cast solutions. The laminated actuator films were 111−128 μm thick. 2.3. Characterization. The electromechanical actuation experiments were conducted by applying a rectangular wave potential (±2 V) to a 10 × 1 mm2 actuator strip clipped between gold clamps from one end of the actuator strip, leaving the other end freely bending

2dδ (L + δ 2) 2

(1)

3. RESULTS AND DISCUSSION Figure 2A shows laser displacement recordings from different CDC-based bending actuator elements. When the square wave voltage (±2 V) with 1 Hz frequency was applied to the CDC 2584


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presented in Figure 2B also demonstrate the capacitive nature of CDC-based actuators. The charging/discharging process initiating the bending movement was dramatically influenced by the pore content of CDCs. It also determined the electric double-layer capacitance and set up the bending range of the device. Both the electrochemical characteristics (current) and the electromechanical characteristics (bending displacement) retained their shape in time and demonstrated the reversibility and good stability of the actuators. The threshold of high frequency in the case of ionic actuators is defined ambiguously. Currently, there are no valid standards for determining the high-frequency area. In our application, we consider the high-frequency range above 10 Hz, the midfrequency area from 10 to 0.1 Hz, and the low-frequency area below 0.1 Hz. Figure 3A shows the strain output of the mesoporous and microporous CDC-based actuators recorded at a fixed excitation voltage of ±2 V as a function of selected frequencies between 5 mHz and 200 Hz (11 frequencies selected). As can be recognized, the porosity of CDC has a remarkable influence on the bending displacement. Actuators containing pores larger than 2 nm had higher strain values in all frequency regions (except at 200 Hz where strain values were close to the detection limit). Mesoporous and micromesoporous CDC-containing actuators also quickly attained a plateau value where the generated strain of actuators was not affected by the excitation frequency. On the contrary, the strain values of the CDC-TiC actuator were drastically lower in the high-frequency region and also in the midfrequency area. This confirms that electric double-layer formation in higher frequency ranges is incomplete and the amount of stored charge is lessened in addition to the volumetric effects being reduced. Figure 3B displays the charge consumption collected during the bending actuation cycle. The CDC-Mo2C actuator had the largest charge values, which allows for the conclusion that the strain in a CDC actuators is in accordance with the accumulated charge during actuation. As demonstrated before, the characteristic time when actuators reach the plateau value is defined by time constant τR.25 The time constant defines the frontier between the capacitive behavior and the resistive behavior of an electrochemical cell. It also corresponds to the energy dissipation of the actuator by IR drop and by irreversible faradaic charge transfer. Figure 3A show that Mo2C-CDC and B4C-CDC strain values quickly reach the plateau level while the strain of TiCCDC continues increasing even at a 10 mHz frequency. This

Figure 2. Charge-induced electromechanical actuation response of CDC actuators consisting of different carbons prepared from TiC, B4C, and Mo2C precursors. (A) Displacement vs time under a ±2 V square wave voltage of 1 Hz measured 5 mm from the clamps. (B) Simultaneously registered current profiles during actuation.

actuators, the differential bending displacement according to the CDC material used in the electrode was achieved. This is the first instance of mesoporous carbon structures having dominant control of electromechanical actuation in low-voltage electroactive devices. The micropore content of CDC-B4C and CDC-TiC actuator electrodes is similar (Table 1), but their electromechanical response is very different. The different electromechanical response of CDC actuators is related to the mesopore content. The bending displacement amplitude of the mesoporous CDC-Mo2C actuator was about 2 times higher than that of micromesoporous CDC-B4C, reaching the 0.5 mm peak-to-peak value. On the contrary, the displacement of microporous CDC-TiC was lowest among porous carbons selected and barely exceeded the noise signal. Figure 2B displays simultaneously recorded electric current profiles of the CDC-based actuators at 1 Hz using an excitation voltage of ±2 V. These current profiles are in good correlation with differential displacement profiles recorded by the laser displacement sensor. It can be concluded that higher current peaks and a larger amount of stored charge in the electric double-layer lead to higher volumetric effects and result in a higher degree of bending for the device. The current profiles

Figure 3. (A) Generated strain of CDC actuators plotted as a function of the frequency of the driving square wave voltage of ±2 V between 5 mHz and 200 Hz. (B) Charge consumption during actuation as a function of excitation frequency between 5 mHz and 200 Hz at a ±2 V square wave voltage input. 2585


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indicates that micropores of TiC-derived carbon have a long path length and a narrow pore size distribution (APS = 9.2 Å). In addition, the absence of mesoporosity makes the high charging rates and fast bending responses unobtainable. The strain value of the CDC-TiC actuator increased in the lowerfrequency region (below 0.1 Hz), whereas other investigated carbons reached the plateau value at f = 1 Hz. Therefore, the maximal strain (fixed strain at 5 mHz) of CDC-based actuators is related to, and driven by, the total porosity of CDC. Moreover, the actuation response time is controlled by the quantity of mesoporosity available in the CDC-based electrode matrix.

REFERENCES

(1) Otero, T. F. Soft, wet, and reactive polymers. Sensing artificial muscles and conformational energy. J. Mater. Chem. 2009, 19, 681− 689. (2) Shahinpoor, M.; Bar-Cohen, Y.; Simpson, J. O.; Smith, J. Ionic polymer-metal composites (IPMCs) as biomimetic sensors, actuators and artificial muscles-a review. Smart Mater. Struct. 1998, 7, R15−30. (3) Carpi, F.; Smela, E. Biomedical Applications of Electroactive Polymer Actuators; Wiley-VHC: Chichester, U.K., 2010. (4) Bar-Cohen, Y. Electroactive Polymer (EAP) Actuators as Artificial Muscles: Reality, Potential, and Challenges, 2nd ed.; SPIE: Washington, DC, 2004. (5) Shahinpoor, M.; Kim, K. J. Ionic polymer-metal composites. I. Fundamentals. Smart Mater. Struct. 2001, 10, 819−833. (6) Shahinpoor, M.; Kim, K. J. Ionic polymer-metal composites: III. Modeling and simulation as biomimetic sensors, actuators, transducers, and artificial muscles. Smart Mater. Struct. 2005, 14, 197−214. (7) Jager, E. W. H.; Smela, E.; Inganäs, O. Microfabricating conjugated polymer actuators. Science 2000, 290, 1540−1545. (8) Pelrine, R.; Kornbluh, R.; Pei, Q. B.; Joseph, J. High-speed electrically actuated elastomers with strain greater than 100%. Science 2000, 287, 836−839. (9) Zhang, Q. M.; Bharti, V.; Zhao, X. Giant electrostriction and relaxor ferroelectric behavior in electron-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer. Science 1998, 280, 2101−2104. (10) Oguro, K.; Kawami, Y.; Takenaka, H. Bending of an ionconducting polymer film-electrode composite by an electric stimulus at low voltage. J. Micromach. Soc. 1992, 5, 27−30. (11) Akle, B. J.; Bennett, M. D.; Leo, D. J.; Wiles, K. B.; McGrath, J. E. Direct assembly process: a novel fabrication technique for large strain ionic polymer transducers. J. Mater. Sci. 2007, 42, 7031−7041. (12) Punning, A.; Kruusmaa, A.; Aabloo, A. Surface resistance experiments with IPMC sensors and actuators. Sens. Actuators A 2007, 133, 200−209. (13) Fukushima, T.; Asaka, K.; Kosaka, A.; Aida, T. Fully plastic actuator through layer-by-layer casting with ionic-liquid-based bucky gel. Angew. Chem., Int. Ed. 2005, 44, 2410−2413. (14) Li, J.; Ma, W.; Song, L.; Niu, Z.; Cai, L.; Zeng, Q.; Zhang, X.; Dong, H.; Zhao, D.; Zhou, W.; Xie, S. Superfast-response and ultrahigh-power-density electromechanical actuators based on hierarchal carbon nanotube electrodes and chitosan. Nano Lett. 2011, 11, 4636−4641. (15) El-Kady, M. F.; Strong, V.; Dubin, S.; Kaner, R. B. Laser scribing of high-performance and flexible graphene-based electrochemical capacitors. Science 2012, 335, 1326−1330. (16) Hantel, M. M.; Presser, V.; Kötz, R.; Gogotsi, Y. In situ electrochemical dilatometry of carbide-derived carbons. Electrochem. Commun. 2011, 13, 1221−1224. (17) Hahn, M.; Barbieri, O.; Gallay, R.; Kötz, R. A dilatometric study of the voltage limitation of carbonaceous electrodes in aprotic EDLC type electrolytes by charge-induced strain. Carbon 2006, 44, 2523− 2533. (18) Jänes, A.; Permann, L.; Arulepp, M.; Lust, E. Electrochemical characteristics of nanoporous carbide-derived carbon materials in nonaqueous electrolyte solutions. Electrochem. Commun. 2004, 6, 313− 318. (19) Conway, B. E. Electrochemical Supercapacitors. Scientific Fundamentals and Technological Applications; Kluwer Academic Publishers/Plenum: New York, 1999. (20) Torop, J.; Palmre, V.; Arulepp, M.; Sugino, T.; Asaka, K.; Aabloo, A. Flexible supercapacitor-like actuator with carbide-derived carbon electrodes. Carbon 2011, 49, 3113−3119. (21) Thomberg, T.; Jänes, A.; Lust, E. Energy and power performance of electrochemical double-layer capacitors based on molybdenum carbide derived carbon. Electrochim. Acta 2010, 55, 3138−3143. (22) Dash, R. K.; Nikitin, A.; Gogotsi, Y. Microporous carbon derived from boron carbide. Microporous Mesoporous Mater. 2004, 72−78.

4. CONCLUSIONS Low-voltage stimuli-responsive actuators based on porous carbon architectures have been developed and characterized. The importance of the pore size distribution on the electromechanical performance of actuators has been studied. Highly porous carbons derived from TiC, B4C, and Mo2C precursors were used as tunable base materials for the preparation of actuators with diverse strain values. The CDCMo2C-based actuator with remarkable mesoporosity showed the fastest response and generated the largest strain values observed. The improvement in the strain values over those of microporous CDC-TiC-based actuators and micromesoporous CDC-B4C-based actuators has been explained by the increased mesoporosity of Mo2C-derived carbon, particularly in a porosity region larger than 2 nm. Mesoporosity allows for quicker ionic diffusion and overall higher electrolyte ion accessibility of the porous carbon electrode matrix materials and leads to larger dimensional effects. Therefore, the availability of a wide array of precursor carbides in preparing tunable porous structures provides an opportunity to develop actuators with differential displacement/strain characteristics simply by selecting appropriate precursor carbon. This unique property, together with their facile preparation method, flexibility, remarkable specific capacitance, and low driving voltage enable CDC-based actuator applications in many areas such as artificial muscles and hybrid actuator/energy conversion devices.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +372 7374825. E-mail: [email protected]. Present Address ⊥

Biosensors and Bioelectronics Center, Department of Physics, Chemistry and Biology (IFM), Linköping University, 58183 Linköping, Sweden. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been partially supported by Estonian Science Foundation project nos. 8172 and 8553, ESNAM COST Action MP1003, targeted financing from the Estonian Ministry of Education (no. SF0180008s08), and also by the graduate school of “Functional Materials and Processes”, receiving funding from the European Social Fund under project 1.2.0401.09-0079 in Estonia. J.T. is grateful for financial support from the European Science Foundation DoRa programme. We thank Dr. Peter Sherrell (Linkö p ing University) for helpful discussions and comments. 2586


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(23) Leis J.; Arulepp M.; Lätt M.; Kuura H. U.S. Patent 7,803,345, 2010. (24) Leis, J.; Arulepp, M.; Käar̈ ik, M.; Perkson, A. The effect of Mo2C derived carbon pore size on the electrical double-layer characteristics in propylene carbonate based electrolyte. Carbon 2010, 48, 4001− 4008. (25) Torop, J.; Sugino, T.; Asaka, K.; Jänes, A.; Lust, E.; Aabloo, A. Nanoporous carbide-derived carbon based actuators modified with gold foil: prospect for fast response and low voltage applications. Sensor Actuators, B 2012, 161, 629−634.

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Microporous and Mesoporous Carbide-Derived Carbons for Strain

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