Anion-Dependent Structure, Dehydration, and Hydroxide Ion Conductivity of Magnesium Aluminum Layered Double Hydroxides

Inorganic layered double hydroxides (LDHs) that provide effective pathways for hydroxide ion conduction at intermediate temperatures (80–140 °C) are promising materials for fully inorganic and hybrid polymer–inorganic alkaline membranes. In this study, we explore the structure–property relationships of several MgAl LDH compositions intercalated with doubly and singly charged anions (CO32–, SO42–, ClO4–, Cl–, NO3–). The intercalated anion was found to have a significant effect on interlayer spacing, OH– conductivity, and interlayer water retention. At intermediate temperatures (100–140 °C), LDHs intercalated with ClO4– or NO3– anions showed sustained enhancement of in-plane OH– conductivity up to 12 mS/cm, despite decreasing relative humidity with increasing temperature. Variable-temperature X-ray diffraction and thermogravimetric data revealed different degrees of interlayer contraction caused by water loss during heating. Changes in interlayer gallery heights were shown not to affect the hydroxide conductivity, indicating that the conductivity is dominated by OH– ions on the external surfaces of the LDH platelets. FTIR spectra suggest that the intercalated anions disrupt the hydrogen-bonding network of water within the LDH galleries. The hydroxide ion conductivities of LDH compositions intercalated with various anions followed a trend consistent with the lyotropic series: ClO4– ≥ NO3– > Cl– > SO42– > CO32–. The connection between the lyotropic series and intermediate temperature conductivity suggests a design principle for robust LDH-based anion-exchange membranes (AEMs) for electrochemical applications.