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.

中文翻译：

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沿二维层状双氢氧化物纳米片表面解开氢氧根离子的迁移机理

传统的聚合物阴离子交换膜（AEM）长期存在问题，例如离子电导率低，稳定性差和高毒性的制备程序。最近的实验表明，剥落的二维层状双氢氧化物（2D-LDH）可以提供约0.1 S / cm的超高氢氧根离子电导率，比商用AEM的电导率高1至2个数量级。但是，该材料的氢氧根离子传导机理仍不清楚。我们的从头算分子动力学（AIMD）模拟结果表明，带正电的2D-LDH平板（Mg ₂ Al（OH）₆ ⁺）可以沿2D-LDH表面诱导准二维氢氧根离子迁移行为，其扩散率与散装水环境中的扩散率相当。当重新堆叠2D-LDH纳米片时，空间限制将破坏氢键网络的连通性并妨碍氢氧离子的传导能力。2D-LDH上的羟基官能团可以介导具有较高能量垒的非相邻水分子之间的氢氧根离子传输过程，该过程只能在低湿度条件下被激活。我们的结果为基于无机二维材料的稳定阴离子交换膜的未来设计提供了启示。