Thermal stability and ionic conductivity of the ionic clathrate hydrates incorporated with potassium hydroxide

Abstract

The hydroxide clathrate hydrates exhibit high ionic conductivity even at quite low temperature because of the relatively high mobility of OH− ions in the water host framework. The unique crystalline structure of Me4NOH·10H2O facilitates a transportation of charge carriers, but its low melting temperature of −20 °C decisively hinders practical applications. We confirmed that the co-inclusion of a secondary guest, using KOH in the present study, in Me4NOH clathrate hydrate can induce a drastic increase of the melting temperature from −19.1 °C to +39.8 °C without any occurrence of structural distortion. For this ionic clathrate hydrate the ionic conductivity reaches the value of 0.20 S·cm−1, suggesting a potential super-ionic conductor. For real applications to ionic conductors the electrochemical stability region was checked by cyclic voltammetry (CV) and it was found that Me4NOH·9H2O·KOH showed a wide electrochemical window corresponding to 5.8 V. To synthesize a hydrate-based proton conductor with acceptable thermal stability and electrochemical properties, the specific anionic and cationic species have to be examined with the full consideration of the structural framework and anion/proton migration.