Ion conduction and relaxation mechanism in ionogels embedded with imidazolium based ionic liquids

Abstract

The impact of physicochemical properties of imidazolium based different ionic liquids such as BDMIMBF4, BMIMBF, and EMIMBF4 on the ion conduction and relaxation mechanisms in ionogels is investigated using broadband dielectric spectroscopy. The complex conductivity isotherms of these ionogels are analyzed using a universal power law coupled with a modified Poisson-Nernst-Planck model for the contribution of electrode polarization dominated in the low frequency region. The effect of electrode polarization is analyzed by using the Macdonald-Coelho model to determine free-ion diffusivity and number density in these ionogels. The relaxation process of ions is also systematically studied using electric modulus spectroscopy over wide frequency and temperature ranges. The temperature dependence of the ionic conductivity, free-ion diffusivity, and relaxation times follows the Vogel-Tammann-Fulcher relationship, indicating existence of coupling between the ion transport and segmental dynamics in these ionogels. The EMIMBF4 ionic liquid doped ionogel shows the highest ionic conductivity, lowest relaxation time, highest free-ion diffusivity and highest number density due to the smallest size of cations, highest static dielectric constant, and lowest viscosity of the EMIMBF4 ionic liquid. It is observed that the stretched exponents for different ionogels obtained from Havriliak-Negami and Kohlrausch-Williams-Watts fits of electric modulus are lower than unity, indicating highly nonexponential relaxation in the investigated ionogels.