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J. Phys. Chem. C 2010, 114, 17982–17988
Electromechanical Analysis by Means of Complex Capacitance of Bucky-Gel Actuators Based on Single-Walled Carbon Nanotubes and an Ionic Liquid Hyacinthe Randriamahazaka*,† and Kinji Asaka‡ Interfaces, Traitements, Organisation et Dynamique des Syste`mes (ITODYS) CNRS-UMR 7086, UniVersite´ Paris Diderot, Paris 7, Baˆtiment LaVoisier, 15 rue Jean-Antoine de Baı¨f, 75205 Paris Cedex 13, France and National Institute of AdVanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan ReceiVed: July 6, 2010; ReVised Manuscript ReceiVed: September 2, 2010
The displacement caused by an applied sinusoidal voltage and electrochemical impedance spectroscopy are used to analyze the electromechanical behavior of bucky-gel single-walled carbon nanotube actuators containing an ionic liquid. This behavior can be understood in terms of complex capacitance diagrams and the frequency dependence of the real and imaginary components of the capacitance. The complex power allows analyzing the relationship between energy storage and energy dissipation during the operation of the electrochemical actuator. Bucky-gel SWCNT actuators behave as supercapacitors. Accordingly, the energy-power plot or Ragone plot expresses the useable energy as a function of the power. In order to visualize the relationship between mechanical behavior and electrochemical properties, the strain-power plot is reported. The maximum power Pmax and specific energy densities Emax of the electrochemical actuator are calculated. The energy densities of bucky-gel SWCNT actuators containing an ionic liquid are of the same order of magnitude as those of natural muscles. 1. Introduction Actuators perform physical functions and interact with their environments by altering geometries at the micrometer scale; they are the point at which energy is converted into force and motion.1-3 In many cases, an input signal is converted from the electrical domain to a nonelectrical output signal in the magnetic, thermal, mechanical, or chemical domains. Most notable are piezoelectric, electromagnetic, shape memory alloy, thermal, electrostatic, and electrochemical transducers.4-6 Volume expansion associated with phase changes in materials has been exploited in a large number of actuators.5 Microactuation using thermally driven phase changes to induce volume expansion has been shown to generate high forces in small volumes; both liquid-gas and solid-liquid systems have been explored.4 A somewhat more familiar means of actuation that also makes use of phase transformations is the shape memory effect, which is exhibited by certain alloys.7 However, piezoelectric, shape memory alloy, thermal, and thermo-pneumatic actuation methods often require high voltages or high temperatures for their operation.4,5 An electrostatic actuator employs an insulating layer sandwiched between two electrodes that are attracted to each other when a voltage is applied across them. The voltages need to be quite large (over 100 V) if large forces or actuation lengths are required. Much lower voltages, consistent with standard electronics (
