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Rotary motors of conventional design can be rather complex and are therefore difficult to miniaturize; previous carbon nanotube artificial muscles provide contraction and bending, but not rotation. We show that an electrolyte-filled twist-spun carbon nanotube yarn, much thinner than a human hair, functions as a torsional artificial muscle in a simple three-electrode electrochemical system, providing a reversible 15,000° rotation and 590 revolutions per minute. A hydrostatic actuation mechanism, as seen in muscular hydrostats in nature, explains the simultaneous occurrence of lengthwise contraction and torsional rotation during the yarn volume increase caused by electrochemical double-layer charge injection. The use of a torsional yarn muscle as a mixer for a fluidic chip is demonstrated.

Supercapacitors operating in aqueous solutions are low cost energy storage devices with high cycling stability and fast charging and discharging capabilities, but generally suffer from low energy densities. Here, we grow Ni(OH) 2 nanoplates and RuO 2 nanoparticles on high quality graphene sheets in order to maximize the specific capacitances of these materials. We then pair up a Ni(OH) 2 /graphene electrode with a RuO 2 /graphene electrode to afford a high performance asymmetrical supercapacitor with high energy and power density operating in aqueous solutions at a voltage of ~1.5 V. The asymmetrical supercapacitor exhibits significantly higher energy densities than symmetrical RuO 2 -RuO 2 supercapacitors or asymmetrical supercapacitors based on either RuO 2 -carbon or Ni(OH) 2 -carbon electrode pairs. A high energy density of ~48 W·h/kg at a power density of ~0.23 kW/kg, and a high power density of ~21 kW/kg at an energy density of ~14 W·h/kg have been achieved with our Ni(OH) 2 /graphene and RuO 2 /graphene asymmetrical supercapacitor. Thus, pairing up metal-oxide/graphene and metal-hydroxide/graphene hybrid materials for asymmetrical supercapacitors represents a new approach to high performance energy storage.

We report on actuation in high tensile strength yarns of twist-spun multi-wall carbon nanotubes. Actuation in response to voltage ramps and potentiostatic pulses is studied to quantify the dependence of the actuation strain on the applied voltage. Strains of up to 0.5% are obtained in response to applied potentials of 2.5 V. The dependence of strain on applied voltage and charge is found to be quadratic, in agreement with previous results on the actuation of single-wall carbon nanotubes, with the magnitude of strain also being very similar. The specific capacitance reaches 26 F g−1. The modulus of the yarns was found to be independent of applied load and voltage within experimental uncertainty.

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