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

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·32H₂O 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 °C and at ambient pressure. The double 2THF·TPAOH·32H₂O hydrate (σ = 1.06 × 10^-3 S·cm^-1) exhibits ionic conductivity 2 orders of magnitude higher than that of THF-free TPAO·32H₂O hydrate (σ = 6.01 × 10^-6 S·cm^-1) at -30 °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 °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 5¹²6⁴ cage and TPA⁺ cation in the four 5¹² cages was, observed to maintain its solid state up to 31 °C, while the pure TPAOH and TMAOH hydrates melt below 0 °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.