Influences of Structural Differences of Layered Double Hydroxides on Their Hydroxide Ion Conductivities

Anion-exchange membrane fuel cells (AEMFCs) have attracted much attention since they can potentially overcome the stumbling blocks of proton-exchange membrane fuel cells. There are many research approaches to improve the performance of AEMFCs, and one of them is to seek new ion conductors that effectively transport hydroxide ions at the ambient temperature. Ion conductors with not only high ion conductivities but also high robustness are strongly required to realize the high-efficiency and long-life performance of AEMFCs. Among such ion conductors, we have focused on layered double hydroxides (LDHs). The basic layer structure of LDHs is based on that of brucite [Mg(OH)₂]. In the brucite lattice, magnesium ions are surrounded octahedrally with hydroxide ions, and these octahedral units form infinite plane of the 2D layers. LDHs are derived by the substitution of a fraction of divalent cations in brucite by trivalent cations, which lead to positive charges in hydroxide layers. These charges are balanced by intercalation of anions and water between the positively charged layers. The possibility of varying the kinds and relative proportions of the di- and trivalent cations as well as the interlayer anions gives rise to the large variety of LDHs with a general formula [M²⁺_1−xM³⁺_x(OH)₂]^x+[Aⁿ⁻]_x/n·yH₂O, where M²⁺ and M³⁺ are di- and trivalent metal cations respectively, and Aⁿ⁻ is an anion. This flexibility in composition of LDHs has induced an increase in interest for many applications, such as catalyst supports, ion-exchange and adsorption, drug-delivery system, etc. In addition, some research groups including our group recently reported that LDHs operated as an OH⁻conductor, which could be applied for alkaline fuel cells, metal-air secondary batteries, and alkaline secondary batteries.

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.