Cheng Chi, Jiang Gengping, Simon George Philip, Liu Jefferson Zhe, Li Dan
Department of Chemical Engineering, University of Melbourne, Parkville, Victoria, Australia.
Department of Materials Science and Engineering and New Horizons Research Centre, Monash University, Clayton, Victoria, Australia.
Nat Nanotechnol. 2018 Aug;13(8):685-690. doi: 10.1038/s41565-018-0181-4. Epub 2018 Jul 2.
Ion transport in nanoconfinement differs from that in bulk and has been extensively researched across scientific and engineering disciplines. For many energy and water applications of nanoporous materials, concentration-driven ion diffusion is simultaneously subjected to a local electric field arising from surface charge or an externally applied potential. Due to the uniquely crowded intermolecular forces under severe nanoconfinement (<2 nm), the transport behaviours of ions can be influenced by the interfacial electrical double layer (EDL) induced by a surface potential, with complex implications, engendering unusual ion dynamics. However, it remains an experimental challenge to investigate how such a surface potential and its coupling with nanoconfinement manipulate ion diffusion. Here, we exploit the tunable nanoconfinement in layered graphene-based nanoporous membranes to show that sub-2 nm confined ion diffusion can be strongly modulated by the surface potential-induced EDL. Depending on the potential sign, the combination and concentration of ion pairs, diffusion rates can be reversibly modulated and anomalously enhanced by 4~7 times within 0.5 volts, across a salt concentration gradient up to seawater salinity. Modelling suggests that this anomalously enhanced diffusion is related to the strong ion-ion correlations under severe nanoconfinement, and cannot be explained by conventional theoretical predictions.
纳米限域中的离子传输不同于体相中的离子传输，并且已经在多个科学和工程学科中得到了广泛研究。对于纳米多孔材料的许多能源和水应用而言，浓度驱动的离子扩散同时会受到表面电荷或外部施加电势所产生的局部电场的影响。由于在极端纳米限域（<2纳米）下存在独特的密集分子间力，离子的传输行为会受到表面电势诱导的界面双电层（EDL）的影响，具有复杂的影响，从而产生异常的离子动力学。然而，研究这种表面电势及其与纳米限域的耦合如何操纵离子扩散仍然是一项实验挑战。在此，我们利用基于层状石墨烯的纳米多孔膜中可调节的纳米限域，表明亚2纳米限域的离子扩散可被表面电势诱导的EDL强烈调制。根据电势符号、离子对的组合和浓度，在高达海水盐度的盐浓度梯度下，扩散速率可在0.5伏电压范围内可逆地调制，并异常增强4至7倍。模型表明，这种异常增强的扩散与极端纳米限域下强烈的离子-离子相关性有关，无法用传统理论预测来解释。
