Graphene membrane, having a staggered multilayer structure, is demonstrated to be a promising membrane for energy storage and liquid separation. The superior property of graphene membrane is owing to the exotic behaviour of fluid confined in the graphene nanochannel (< 10 nm). Unlike the 1-D nanochannel in lab-on-a-chip devices, the graphene membrane has a unique cascading nano-slit system. Understanding of ion transport in graphene membranes is very limited.

In this talk, a quantitative and statistical representative microstructure model is obtained for graphene membranes by correlating diffusion permeation experimental results with continuum simulation results. This task is achieved by taking advantages of the tuneable graphene membrane platform recently developed in our team. The crucial role of the pinhole defects in graphene sheets is revealed. Based on this structure model, comprehensive continuum simulations were performed to study the electrokinetic properties of ion transport inside graphene membranes. Comparison with direct experimental measurements leads to an interesting scaling law that correlates the relative conductance with channel size.

We find some novel electric double layer (EDL) structures, such as EDL caused by external electric field (coined as binary boundary layer (BBL)) and EDLs at the pore aperture regions. Influences of these EDL structures and the channel surface charges on the driving force distribution and ion concentration inside the cascading nanoslit system are carefully studied. With the obtained information, ion transport in graphene membranes are analysed.

To understand the unusual ion transport behaviour observed by the experiments at molecular scale, atomistic simulations for ion electro-kinetic flow through the membranes with and without surface charge were performed. The EDL structures, ion concentrations, and ion transport properties were carefully studied and compared with continuum simulations. For graphene membranes with zero surface charge, our MD simulations showed a strong BBL caused by external electric field. As a result, a novel polarized electro-osmosis flow (EOF) phenomenon is observed. Unlike the conventional EOF, the polarized EOF has two flows in opposite directions next to the two opposite walls of a slit, respectively. With surface charges, the ion transport is a combination of ion electrophoresis, conventional EOF, and the polarised EOF. In small slits, the conventional EOF is dominant for enhancing electrokinetic conductivity. Our MD simulations provide novel physical insights that are missed in continuum simulations, which should be considered in future continuum simulations and models.

 

主讲人简介：刘哲，Jefferson Zhe Liu，¹Department of Mechanical and Aerospace Engineering, Monash University, Clayton, VIC 3800, Australia.Dr. Zhe Liu graduated from Engineering Mechanics department of Tsinghua University in 2002. After that he moved to Northwestern University to get his PhD degree in Materials Science and Engineering Department. In 2008 he joined Mechanical and Aerospace Engineering Department of Monash University to establish his group "virtual laboratory of nanomechanics and nanomaterials". His current research interests include nanoionics (ion dynamic transport in nano-channles), and energy conversion nano-materials.