High performance polymer actuators based on single-walled carbon nanotube gel using ionic liquid with quaternary ammonium or phosphonium cations and with electrochemical window of 6 V

Abstract

This study focused on poly(vinylidene fluoride-co-hexafluoropropylene) based actuators containing single-walled carbon nanotubes. The actuators used composite gel electrodes and electrolytes that included an ionic liquid (IL) having an electrochemical window of 6V. A comparison was made of the electrochemical and electromechanical properties of actuators with ILs composed of different quaternary ammonium or phosphonium cation species. Either bis(trifluoromethanesulfonyl)imide (TFSI) or tetrafluoroborate (BF4) was used as the anion species.For TFSI based electrode layers, the double-layer capacitance was found to depend on the type of IL cations present. At an applied voltage of ±2V, the strain showed a dependence on the self-diffusion coefficient in both the kinetic and static regions of the strain–frequency curve. This was also the case for the kinetic region at an applied voltage of ±3V. The maximum strain and stress were produced by an actuator with an IL composed of N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide and were approximately 2 times larger than those for an actuator with an IL composed of 1-ethlyl-3-methylimidazolium tetrafluoroborate, with an electrochemical window of just 4V. This indicates that actuators with an electrochemical window of about 6V are promising candidates for real-world applications.It was found that the ion size, self-diffusion coefficient and ion transport in the electrodes and electrolyte are important for realizing low-voltage electroactive polymer actuators. A simple model for the actuation mechanism was proposed, based on the product of the ionic transport number and the ionic volume, and it was shown to successfully explain the actuator behavior.