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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Enhancing the Work Capacity of Electrochemical Artificial Muscles by Coiling Plies of Twist-Released Carbon Nanotube Yarns Keon Jung Kim,† Jae Sang Hyeon,† Hyunsoo Kim,† Tae Jin Mun,† Carter S. Haines,‡ Na Li,‡ Ray H. Baughman,‡ and Seon Jeong Kim*,† †
Center for Self-Powered Actuation, Department of Biomedical Engineering, Hanyang University, Seoul 04763, Korea Alan G. MacDiarmid NanoTech Institute, University of Texas at Dallas, Richardson, Texas 75080, United States
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ABSTRACT: Twisted-yarn-based artificial muscles can potentially be used in diverse applications, such as valves in microfluidic devices, smart textiles, air vehicles, and exoskeletons, because of their high torsional and tensile strokes, high work capacities, and long cycle life. Here, we demonstrate electrochemically powered, hierarchically twisted carbon nanotube yarn artificial muscles that have a contractile work capacity of 3.78 kJ/kg, which is 95 times the work capacity of mammalian skeletal muscles. This record work capacity and a tensile stroke of 15.1% were obtained by maximizing yarn capacitance by optimizing the degree of inserted twist in component yarns that are plied until fully coiled. These electrochemically driven artificial muscles can be operated in reverse as mechanical energy harvesters that need no externally applied bias. In aqueous sodium chloride electrolyte, a peak electrical output power of 0.65 W/kg of energy harvester was generated by 1 Hz sinusoidal elongation. KEYWORDS: carbon nanotube yarn, artificial muscle, electrochemical actuator, high work capacity, energy harvesting
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INTRODUCTION The application of artificial muscles, from macro- to microscale, requires large-strokes, fast responses, high cycle life, and high mechanical work capacity. Many types of artificial muscles have been reported, such as piezoelectric ceramics,1,2 shapememory alloys,3,4 conducting polymers,5 and ionic-polymer metal composites.6,7 Inserting twist into yarns comprising carbon nanotubes (CNTs)8−21 or polymer,22−27 graphene,28 or metal29 fibers has resulted in muscles that provide both tensile and torsional actuation. Torsional CNT artificial muscles8 fabricated by twisting CNT yarn have provided a similar specific torque as commercial electrical motors. By overtwisting CNT yarns, coiled muscles can be fabricated that provide large tensile strokes. Guest-filled, thermally powered coiled CNT yarn artificial muscle can generate ∼80 times higher mechanical work and power than natural muscle,9,22 but have a cycle rate that is limited by the cooling needed to reverse actuation. To improve muscle performance, we have developed supertough, hierarchically twisted-yarn muscles that have a similar structure to straw ropes and elevator cables.30 By using such muscles, which are able to lift heavy loads, we obtained very high gravimetric contractile work capacities. This coiled multiply structure introduces a new parameter that can be tuned to optimize muscle performance, the twist within the individual yarn plies.31 In this communication, we demonstrate electrochemically driven, hierarchically twisted tensile artificial muscles that are © XXXX American Chemical Society
fabricated from CNT yarns. We call the coiled, multiply CNT yarn a “hierarchically twisted CNT artificial muscle (HTAM)”. The HTAM has a high mechanical toughness (53 J/g), which is 3.5 times higher than for conventional single-ply coiled CNT yarn. A high contractile work capacity of 3.78 kJ/kg was demonstrated, which is 95 times higher than the 0.04 kJ/kg of mammalian skeletal muscle32 and 1.72 times higher that of previous electrochemical muscles.21 A tensile stroke of 15.1% was obtained using a low input voltage (3.25 V), by increasing capacitance by ∼30%, by optimizing the twist inserted in individual yarn plies. By operating the artificial muscle in reverse to convert mechanical energy to electrical energy, a new type of “Twistron”33 energy harvester was demonstrated, which generated 0.65 W/kg of peak electrical power without the need for an externally applied bias voltage. As shown in the schematic images of Figure 1a, the HTAM contains three levels of the hierarchical structure; dualArchimedean spun-twisted yarns that are plied and then coiled. Figure 1b−e shows the scanning electron microscope (SEM) images of a HTAM and its hierarchical components. The HTAM comprises dual-Archimedean spun-twisted CNT yarns that are fabricated from spinnable CNT forests, which were synthesized by chemical vapor deposition.34 DualArchimedean CNT yarns (Figure 1b,c) were twisted under a Received: December 11, 2018 Accepted: March 20, 2019
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DOI: 10.1021/acsami.8b21417 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Muscle configuration and morphology. (A) Schematic images of the hierarchical structure of a HTAM, showing the dual-Archimedean cross-section, the plying, and the yarn coiling. SEM images of (B) a twisted carbon nanotube yarn, which is in torque-balanced state, that has a bias angle of 9.4°; (C) a fully twisted precursor yarn for the torque-balanced yarn, which has a bias angle of 44.9° (scale bar of (B) and (C): 50 μm); (D) a twisted multiply carbon nanotube yarn; and (E) a coiled multiply HTAM (scale bars of (D) and (E): 100 μm).
Figure 2. Characterization of muscle performance. (A) The scan rate dependence of tensile stroke for a 200 μm diameter HTAM for an applied stress of 36.4 MPa. The inset shows the applied −3.25 V triangular wave voltage and the corresponding current for a scan rate of 500 mV/s. (B) Tensile stroke versus time for scan rates of 20 mV/s (top) and 500 mV/s (bottom). (C) The dependence of tensile stroke and contractile work capacity on applied load. (D) Tensile stroke during 1000 cycles of actuation. A 0.2 M THA·PF6/PC electrolyte, a counter electrode of Pt mesh/ CNT buckypaper, and an Ag/AgCl reference electrode were used for the above experiments.
50 μm, and 6340 turns/m of twist. These twisted CNT yarns having a bias angle of 9.4° were used to fabricate nine-ply CNT yarn (Figure 1d), which was then fully coiled by twist insertion (Figure 1e). This yarn is homochiral, since the same chirality was used for yarn twist and yarn plying. The nine-ply CNT yarn had a diameter of 120 μm, and the coiled multiply CNT yarn has an outer diameter of 180 μm, an inner diameter of 102 μm, and a spring index (average coil diameter/fiber diameter)27 of 0.76. A magnified SEM image of the coiled
constant load of 18.1 MPa. As elaborated later, performance was optimized by varying the amount of initially inserted twist and then allowing untwist until a torque-balanced state was realized. Optimal performance was obtained for a bias angle of 9.4° for the torque-balanced state of a 67 μm diameter CNT yarn. Using the yarn equation (α = tan−1(πDT), where α is the bias angle, D is the yarn diameter, and T is the turns/m of twist insertion),8,9 780 turns/m of twist produced an observed bias angle of 9.4° (Figure 1b). Figure 1c shows the initial fully twisted CNT yarn having a bias angle of 44.9°, a diameter of B
DOI: 10.1021/acsami.8b21417 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 3. Mechanical and electrochemical characterizations. (A) The stress−strain curve of a HTAM having a spring index of 0.77. (B) The dependence of HTAM capacitance, tensile stroke, and contractile work capacity on the inserted twist in the yarns that are torque-relaxed for use in HTAMs. These HTAMs have a similar spring index (0.75).
Figure 4. Harvesting electricity by using a HTAM. (A) Illustration of the three-electrode system used for characterizing the HTAM twistron, which includes a torsionally tethered HTAM working electrode, a counter electrode of Pt mesh/CNT buckypaper, and an Ag/AgCl reference electrode. All experiments were conducted in an aqueous 0.6 M NaCl electrolyte. (B) The open-circuit voltage (OCV) and short-circuit current (SCC) that resulted from applying a sinusoidal tensile strain of 40%. (C) The dependence of OCV on applied strain and (inset) cyclic voltammetry scans for strains of 0% (black) and 40% (red). (D) Peak voltage (black square) and peak power (blue circle) versus load resistance for 40% strain at 1 Hz frequency.
in the inset. Because the THA+ cation has a larger van der Waals volume than the PF6− anion, a negative voltage was applied to the muscle electrode to maximize muscle stroke.35 A 20 mV/s voltage scan rate provided a tensile stroke of 15.1% (top of Figure 2b). Even at a voltage scan rate of 500 mV/s, a tensile stroke of 5.67% was obtained, as shown in Figure 2b (bottom). The dependence of muscle stroke on applied stress and the corresponding work capacity is shown in Figure 2c. At a scan rate of 20 mV/s, a tensile stroke of 15.1% was achieved for an applied stress of 36.4 MPa. A contractile work capacity of 3.78 kJ/kg was obtained at this scan rate when using a stress of 63.9 MPa. This work capacity is 1.72 times the previously reported record work capacity of 2.2 kJ/kg for an electrochemical artificial muscle.21 Figure 2d shows that the HTAM provides high cycle life. In fact, the muscle stroke increased by
multiply CNT yarn, showing the aligned CNT bundles and the porous surface, is provided in Figure S1. The tensile actuation of a HTAM having a spring index of 0.75 is shown in Figure 2. Electrochemical experiments were conducted by cyclic voltammetry (CV) in 0.2 M tetrahexylammonium hexafluorophosphate (THA·PF6)/propylene carbonate (PC) electrolyte, using a three-electrode system. A large volume ion was used to increase muscle stroke.21 In addition, we minimized the voltage drop across the counter electrode by maximizing the capacitance of this electrode (using Pt mesh with CNT buckypaper, which has a large surface area relative to the muscle electrode). Tensile actuation was characterized for scan rates between 20 and 500 mV/s (Figure 2a). The applied triangular wave of −3.25 V and the corresponding current for a scan rate of 500 mV/s are shown C
DOI: 10.1021/acsami.8b21417 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
stretched to 40% strain (inset). Figure 4d shows the dependence of peak voltage and peak output power on load resistance. The HTAM provided a peak electrical power of 0.65 W/kg at 1 Hz and an energy conversion efficiency of
