Zn2+ substitution effects in layered double hydroxide (Mg(1−x)Znx)2Al: textural properties, water content and ionic conductivity

Abstract

Recently, layered double hydroxides (LDHs) have been studied as anion conductors for fuel cell applications because of their anion exchange properties and better thermal stability (up to 150 °C) than those of anion exchange polymers. However, the conductivity of the LDHs is very low, under 10−3 S cm−1 at room temperature, and the mechanism of anion conduction has not been explained clearly. In the present study, we investigate the physical properties of LDHs affecting anion conductivity by synthesizing trimetallic LDHs, (Mg(1−x)Znx)2Al–CO32− with different ratios of Mg2+ and Zn2+. The amount of anion in these LDHs and the particle size were kept the same in all LDHs by controlling the amount of Al3+ and the synthesis methods, respectively. The LDHs synthesized were characterized using techniques such as XRD, ICP-AES, TEM, zeta-potential measurements, XPS and TG and also by measuring the adsorbed water content in a range of temperatures and humidities. The ratio of Zn2+ to divalent metal in the LDH framework affected the interlayer distance and the water content. Among the series of LDHs of (Mg(1−x)Znx)2Al–CO32−, the ones with a Zn2+ : divalent metal ratio of around 0.5 presented the largest interlayer distance, the largest amount of adsorbed water and the highest ion conductivity. Interestingly, the anion conductivity showed a good correlation with the interlayer distance and the adsorbed water content. The anion conductivity increased as the interlayer distance increased and also as the amount of adsorbed water increased. These results imply that the anion conduction may occur primarily in the interlayer space and the adsorbed water assists the anion conduction.