Unraveling the Hydroxide Ion Transportation Mechanism along the Surface of Two-Dimensional Layered Double Hydroxide Nanosheets

Traditional polymeric anion exchange membranes (AEMs) suffer from longstanding issues such as low ionic conductivities, poor stability, and high toxic preparation procedures. Recent experiments demonstrated that exfoliated two-dimensional layered double hydroxide (2D-LDH) could provide a super high hydroxide ion conductivity of about 0.1 S/cm, which is 1 to 2 orders of magnitude higher than that of commercial AEMs. However, the hydroxide ion conduction mechanism of this material is still unclear. Our ab initio molecular dynamics (AIMD) simulation results reveal that the positively charged 2D-LDH slab (Mg₂Al­(OH)₆⁺) can induce a quasi-two-dimensional hydroxide ion transportation behavior along the surface of 2D-LDH with a diffusivity comparable with that in the bulk water environment. When restacking 2D-LDH nanosheets, the spatial confinement will destroy the connectivity of the hydrogen bonding network and hamper the hydroxide ion conduction capability. The hydroxyl functional groups on 2D-LDH can mediate the hydroxide ion transportation process between non-adjacent water molecules with a relatively high energy barrier, which can only be activated in low humidity conditions. Our results shed light on future designs of stable anion exchange membranes based on inorganic 2D materials.