Electrochemical and electromechanical properties of high-performance fluoropolymer/ionic liquid (with wide electrochemical window of 6 V) gel hybrid actuators based on single-walled carbon nanotubes

Abstract

In this study, various ionic liquids (ILs) with electrochemical windows as wide as 6 V were synthesized and their electrochemical and electromechanical properties were evaluated for their application in actuators. These actuators were based on an ionic fluoropolymer (Nafion™)/non-ionic fluoropolymer (poly(vinylidene fluoride‑co‑hexafluoropropylene) [PVdF(HFP)]) gel fabricated using a single-walled carbon nanotube (SWCNT) containing an ionic liquid (IL) gel electrode, which was in turn composed of aliphatic or cyclic quaternary cations and perfluoroalkyltrifluoroborate anions. The ionic conductivity of the gel electrolyte layer was dependent on the IL species employed. In addition, the maximum strains of the ±3-V actuators were 1.5–2-times larger than those of the ±2-V actuators. These results indicated that the Nafion™/PVdF(HFP)-based hybrid actuators, containing quaternary cations and perfluoroalkyltrifluoroborate anions, are ideal for practical applications and could be used as electrochemical materials in wearable and energy conversion devices. After determining the frequency dependences of the displacement responses of the above-mentioned Nafion™/PVdF(HFP)–SWCNT–IL gel hybrid actuators, these results were simulated using a double-layer charging kinetic model. Surprisingly, the simulated curves were similar and comparable to the experimentally obtained ones. The response time constant was determined and was represented by an equivalent circuit comprising a series combination of the ionic resistances and the double-layer capacitance. The Nafion™/PVdF(HFP)–SWCNT–IL (imidazolium-cation-type) gel hybrid actuator was represented by the ionic resistance and double-layer capacitance, in contrast to the PVdF(HFP)–SWCNT–IL (imidazolium-cation-type) actuator, which was represented by the electronic resistance and double-layer capacitance.