High-performance ionic and non-ionic fluoropolymer/ionic liquid gel hybrid actuators based on single-walled carbon nanotubes

Abstract

The electrochemical and electromechanical properties of actuators based on an ionic fluoropolymer (Nafion™) and non-ionic fluoropolymer (poly(vinylidene fluoride-co-hexafluoropropylene) [PVdF(HFP)]) gel fabricated using a single-walled carbon nanotube (SWCNT)-an ionic liquid (IL) gel electrode, were compared with those of actuators based on a non-ionic fluoropolymer (PVdF(HFP)). The ionic conductivity of the Nafion™-PVdF(HFP)-IL gel electrolyte was lower than that of the PVdF(HFP)-IL gel electrolyte. We assume that an ion complex exists between the Nafion™ SO3- anions and imidazolium cations but not between PVdF(HFP) and the imidazolium cations. This Nafion™-PVdF(HFP)-IL gel hybrid actuator mechanism resembled that of an ionic-polymer-metal composite actuator, where the IL molecules move with the IL cations and anions. The maximum strain and maximum generated stress for the Nafion™-PVdF(HFP)-SWCNT actuator with a PVdF(HFP) : Nafion™ ratio of 1 : 3 and a bis(trifluoromethanesulfonyl)imide-containing IL were approximately 1.6 and 1.5 times higher, respectively, than the corresponding values for the PVdF(HFP)-SWCNT-IL actuator. These results indicate that ionic and non-ionic fluoropolymer-based actuators outperform non-ionic fluoropolymer-based actuators and are highly suitable for practical applications. In addition, the frequency dependence of the displacement response of the ionic and non-ionic fluoropolymer-SWCNT-IL actuator was successfully simulated using an electrochemical kinetic model. The results yielded the strain in the low-frequency limit, which was related to the electromechanical mechanism involved, and the time constant of the response, which was represented by an equivalent circuit with the ionic resistance and double-layer capacitance in series, in contrast to the non-ionic fluoropolymer-SWCNT-IL actuator represented by the electronic and ionic resistance and double-layer capacitance.