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.