Nonlinear ion transport in the supercooled ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide: Frequency dependence of third-order and fifth-order conductivity coefficients

摘要

We have carried out nonlinear ion conductivity measurements on the supercooled ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide[C(6)mim][NTf2] by applying ac electric fields with amplitudes up to about 200 kV/cm. At these field amplitudes, 3 omega and 5 omega harmonic components in the current response were detected, and the higher-order conductivity coefficients sigma(1)(3), sigma(3)(3), sigma(3)(5), and sigma(5)(5) were determined. The frequency and temperature dependence of these conductivity coefficients was analyzed in detail. The most important findings were the following: (i) The third-order spectra sigma(1)(3) and sigma(3)(3) exhibit very similar values in the dc plateau regime but differ considerably in the dispersive regime. The same was observed for the fifth order spectra sigma(3)(5) and sigma(5)(5). (ii) In the dispersive regime, the third-order spectra display a minimum, while the fifth-order spectra display a maximum. (iii) The frequencies of these minima and maxima are thermally activated with the same activation energy as the low-field dc conductivity sigma(1,dc), whereas the dc values of the higher-order conductivity coefficients, sigma(3,dc) and sigma(5,dc), are characterized by lower activation energies. (C) 2015 AIP Publishing LLC.