Influences of Structural Differences of Layered Double Hydroxides on Their Hydroxide Ion Conductivities
In this study, we focus on the nature of ion conductivities in LDHs. To understand the mechanism of ion conductivities in LDHs in detailed, we synthesized Mg-Al and Mg-Ga LDHs with different relative proportions of di- and trivalent metal cations, and evaluated the influence of the proportion of trivalent cation on their ion conductivities by electrochemical measurements. We chose Mg-Al and Mg-Ga LDHs since they are widely used in LDH studies. We shed light on the structural relationship, which could guide the design of improved or novel hydroxide-conductive materials.
Using the co-precipitated method, we successfully synthesized Mg-Al-CO3 and Mg-Ga-CO3 LDHs with various proportions of trivalent cations. In the results of X-ray diffraction, as the Mg/Al ratio decreased from 4 to 2, the 003 diffraction peak positively shifted, which indicates that layer height between metal hydroxide layers were diminished. This decrease in the layer height was due to an increase in electrostatic attraction between positive and negative layers. In contrast, resulted LDHs were characterized in similar BET surface areas and mean diameter in spite of changing the proportion of trivalent cations. Both series of LDHs did not displayed a linear increase in ion conductivities as a function of the proportion of trivalent cations, but a sharp raise appeared only for Mg2+/Al3+ = 2 and Mg2+/Ga3+ = 3. Based on the results of electron-diffraction analysis, Mg-Al and Mg-Ga LDHs, showing the highest ion conductivities had the ordered honeycomb cation arrangement within some parts of (0001) hydroxide layers. Other LDHs had complete random cation distribution. The ordered honeycomb structure was a cation distribution that can elude direct contact of M3+−M3+ and diminish the distance of the nearest trivalent cations; “neither near nor far” for individual interlayer anions. These things for the ordered cation arrangements not only demonstrate a helpful strategy for improving ion conductivities of LDHs, but also present new insights into structural relationship between immobile cationic charge centers and mobile interlayer anions.