Systematic study of ionic conduction in silver iodide/mesoporous alumina composites 1: effects of pore size and...

A systematic study of Ag⁺-ion conducting behavior in Ag⁺-loaded porous materials was conducted over the entire sub-10 nm region for the first time. The effects of the pore diameter of mesoporous aluminas (MPAs) and the amount of silver iodide (AgI) loaded into MPAs were investigated using N₂ gas adsorption/desorption, powder X-ray diffraction, differential scanning calorimetry, and electrochemical impedance spectroscopy measurements. Confinement of AgI in the mesoporous space lowers the phase transition temperature between the β/γ- and α-phases relative to that of bulk AgI. The AgI-loading into the MPAs with smaller pores led to a more significant decrease in the transition temperature, possibly because the smaller AgI nanoparticles in the pores must have a higher surface energy to stabilize the high-temperature phase. The room-temperature ionic conductivity exhibits a volcano-type dependence on the pore diameter with the highest value when AgI was loaded into MPA with a pore diameter of 7.1 nm (7.2 × 10^-4 S cm^-1 at room temperature). Concerning the 7.1 nm-MPA, the room-temperature ionic conductivity was the highest for the nearly fully occupied composite, which is more than three orders of magnitude higher than that of the bulk AgI. The present study reveals that the Ag⁺-ion conductivity in AgI/MPA composites can be controlled by optimizing the pore diameter of MPA and the AgI-loading ratio.