Multifunctional Carbon Nanotube Yarns for Artificial Muscles and Energy Harvesters
The superb mechanical, physical, and chemical properties of carbon nanotube (CNT) yarns have promoted their application as function components that program actuation, sensing, and power management for soft robotics and smart systems. Success in making artificial muscles that are faster, more powerful, and that can provide larger strokes would expand their applications. Efficient conversion of ambient mechanical energy into electrical energy is needed for diverse applications, including self-powered wireless sensors, structural and human health monitoring systems, and the extraction of energy from ocean waves. Herein, the development of CNT yarn artificial muscles and mechanical energy harvesters are first discussed, and the obtained understanding of underlying mechanisms provide guidance for optimizing muscle and harvester performances. Next, unipolar stroke CNT yarn muscles are described, in which muscle stroke changes between extreme potentials are additive and muscle stroke remarkably increases with increasing potential scan rate. The normal decrease in stroke with increasing scan rate, because of decreased capacitance, is overwhelmed by a dramatic increase in effective ion size caused by electroosmotic pumping of solvent. These coiled carbon nanotube yarn muscles contain a yarn guest that shifts the yarn’s potential of zero charge (pzc) by over a volt, either positively or negatively. Such pzc shift agents include ion-exchange membrane polymers, oxidized graphene platelets, and surfactants. Record muscle strokes, contractile work-per-cycle, contractile power densities, and energy conversion efficiencies are obtained for unipolar muscles. Then, powerful CNT yarn mechanical energy harvesters (we call twistrons) are described, which are electrochemical artificial muscles run in reverse. Stretching a coiled CNT yarn can provide large, reversible changes in electrochemical capacitance, which enables conversion of mechanical energy to electrical energy. The performance of these twistron harvesters can be increased by diverse fabrication methods: optimizing the structure of the precursor CNT forest, using stretchinduced alignment, thermal annealing under tension, and incorporating reduced graphene oxide nanoplates. The peak output power at 1 Hz and at 30 Hz for a sinusoidal stretch were 0.73 and 3.19 kW/kg, which are 15- and 13-fold higher than for previous twistron harvesters at these respective frequencies. This performance at 30 Hz was over 12-fold that of other prior-art mechanical energy harvesters for frequencies between 1 Hz and 600 Hz. Last, the opportunities and challenges for future practical applications of CNT yarns are highlighted.