Electrochemical stability of ionic clathrate hydrates and their structural consideration

Although electrochemical stability is an essential factor in relation to the potential applications of ionic clathrate hydrates to solid electrolytes, most studies regarding the proton conductors have focused on their ionic conductivity and thermal stability. Solid electrolytes in various electrochemical devices have to endure the applied potentials; thus, we examined the linear sweep voltammograms of various tetraalkyl-ammonium hydroxide hydrates in order to shed light on the trend of electrochemical stability depending on the hydrate structure. We revealed that the electrochemical stability of Me4NOH hydrates is mainly affected by both their ionic concentration and cage occupancy. In particular, the true clathrate structures of beta-Me4NOH hydrates are more electrochemically stable than their alpha-forms that possess partially broken hydrogen bonds. We also observed that the binary THF-Pr4NOH and pure Bu4NOH clathrate hydrates exhibit greater electrochemical stability than those of pure Me4NOH hydrates having lower or similar ionic concentrations. These results are considered to arise from the fact that each of the Pr4N+ and Bu4N+ ions occupies an extended space comprising four cages, which leads to stabilization of the larger unit, whereas a Me4N+ ion is completely included only in one cage. (C) 2013 Elsevier Ltd. All rights reserved.