Effect of Mg/Al Ratio on Hydroxide Ion Conductivity for Mg–Al Layered Double Hydroxide and Application to Direct Ethanol Fuel Cells

Mg–Al layered double hydroxides (LDHs) intercalated with carbonate anions were prepared by the co-precipitation process with Mg/Al = 2, 3 and 4, and the effect of Mg/Al ratio on the ionic conductivity was examined. It is shown that the Mg/Al ratio influences ionic conductivities of Mg–Al CO 3 2− LDH. The conductivity of Mg-Al CO 3 2− LDH with Mg/Al = 2 and 3 exhibited almost the same ionic conductivity of about 1 × 10 −3 S cm −1 at room temperature under 80% relative humidity, while that of LDH with Mg/Al = 4 was lower by one order of magnitude. Thermal analyses showed that Mg-Al CO 3 2− LDH with Mg/Al = 2 or 3 keeps interlayer water molecules at higher temperatures compared with Mg/Al = 4, suggesting that the interlayer water molecules are strongly held in the interlayer. The strongly held interlayer water molecules are assumed to contribute the high ionic conductivity in Mg-Al CO 3 2− LDH with Mg/Al = 2 and 3. The alkaline type direct ethanol fuel cells using Mg/Al CO 3 2− LDHs as hydroxide conducting electrolyte were confirmed to operate, and larger power was obtained in Mg-Al CO 3 2− LDH with smaller Mg/Al ratio.