Investigating the effect of membrane pore size on the permeability of carbon nanotubes in reverse electrodialysis using molecular dynamics simulation.

Reverse electrodialysis (RED) is an emerging technology that converts the salinity gradient between seawater and freshwater into clean electrical energy. The efficiency of RED is strongly governed by ion transport through the membrane interface, where nanoscale geometry plays a decisive role. This study investigates, through molecular dynamics simulations, how the pore size of carbon nanotube (CNT) membranes influences water and ion permeability under an applied electric field. The simulation framework involved a two-stage procedure (30 ns equilibration at 300 K followed by dynamic evaluation of electrokinetic transport) to achieve stable atomic configurations and reliable transport data. Results revealed that as membrane pore size increased from 20 Å to 30 Å, both water flux and electric current increased significantly, indicating enhanced permeability and reduced flow resistance. However, this improvement was accompanied by a decline in hydrogen bond density from 103 to 86, implying weaker intermolecular cohesion and reduced structural confinement within larger pores. The mean-squared displacement of water molecules also increased, confirming increased molecular mobility and collision frequency. Although electric flow density decreased with pore enlargement due to charge dilution across a wider cross section, the overall voltage output rose from 21.02 to 24.54 μV. These findings demonstrate that optimizing pore geometry enables a balance between fluid transport efficiency and charge density, offering a molecular-scale understanding of how nanopore design can enhance RED performance, improve ion selectivity, and guide the development of next-generation nanoengineered membranes for sustainable energy harvesting.