Ionic conductivity enhancement due to coguest inclusion in the pure ionic clathrate hydrates

Abstract

In this study, we present the unique role of the coguest when it is additionally included in a pure ionic clathrate hydrate. First, the ionic conductivities of xTHF-TPAOH-32H(2)O hydrates at various coguest THF concentrations (x = 2, 1, 0.5, 0.25, 0.13, 0) were measured in a temperature range from -40 to -10 degrees C and at ambient pressure. The double 2THF(.)TPAOH(.)32H(2)O hydrate (sigma = 1.06 x 10(-3) S(.)cm(-1)) exhibits ionic conductivity 2 orders of magnitude higher than that of THF-free TPAO(.)32H(2)O hydrate (sigma = 6.01 x 10(-6) S(.)cm(-1)) at -30 degrees C. This considerably different ionic conductivity behavior strongly implies that the inclusion of coguest THF induces a structural transformation via host-water lattice distortion, providing such high conductivity values for the mixed (THF + TPAOH) hydrate. We found a maximum conductivity of 0.0184 S(.)cm(-1) at 1.49 THF mol % and -10 degrees C The present results provide strong evidence that THF serves as a promoter for greatly enhancing the ionic conductivity in ionic clathrate hydrates. The structure-II (sII), host lattices formed by THF inclusion can provide an effective pathway for moving the charge carriers. Furthermore, the channel pattern of sII small cages seems to contribute to a further increase in the ionic conductivity. The double tetramethylammonium hydroxide (TMAOH + TPAOH) hydrate structured with TMA(+) cation in the 5(12)6(4) cage and TPA(+) cation in the four 5(12) cages was, observed to maintain its solid state up to 31 degrees C, while the pure TPAOH and TNIAOH hydrates melt below 0 degrees C. The physical characteristics of high ionic conductivity as well as high melting temperature of the double ionic clathrate hydrates might contribute to their use as solid proton conductors.