Advanced Janus membrane (MXene/CoAl-LDH) for efficient asymmetric ion transport and nanofluidic energy harvesting

Abstract

Two-dimensional (2D) material membranes with nanometer or sub-1 nanometer confined space present enormous potential for water permeation, ion sieving and transport. In addition, electrolyte flowing through the asymmetric channels exhibits ion asymmetric transport behavior which provides a basis to control the ion flow. Therefore, it is an opportunity to develop Janus membranes by 2D materials for alleviating the concentration polarization and enhancing the performance in nanofluidic energy harvesting. Whereas the previously reported 2D material-based Janus laminated membranes were adjusted interlayer charge distribution by modifying positively charged polymer, due to the intrinsically negative surface charge of these 2D nanosheets. Thus, design of 2D/2D Janus membrane from positive and negative changed 2D materials will have great opportunity for efficient asymmetric ion transport and nanofluidic energy harvesting. In this work, Janus MXene/LDH (MXLDH) membranes containing negatively charged MXene and positively charged CoAl-LDH were designed without additional modifying process for efficient ion asymmetric transport and neutralization energy harvesting. The ratio of MXene/LDH draws a significant effect on ion rectification factor, osmotic voltage, ion selectivity and energy conversion efficiency. The Janus membrane with 86 wt% LDH (MXLDH-86) revealed the highest rectification factor of ∼6.2 in 0.1 mM NaCl aqueous solution. And the osmotic power density of MXLDH-86 under 1 M HCl/ 1 M NaOH can reach ∼8.75 W/m2, ∼17.7 % higher than that of the pristine MXene membrane, indicating that the Janus membrane with asymmetric charges is a promising material for harvesting neutralization energy.