Molecular Dynamics Insights into Water Flux and Salt Rejection in Nitrogen-Doped Carbon Nanotubes Incorporated with Ag Nanoparticles for Capacitive Deionization

Capacitive deionization (CDI) is an environmentally friendly and energy-efficient method for saline water desalination; however, conventional carbon-based electrodes are often limited by low salt adsorption capacity (SAC). In this study, molecular dynamics (MD) simulations were used to investigate the desalination performance of nitrogen-doped carbon nanotube (N-CNT) and silver-encapsulated N-CNT (Ag/N-CNT) electrodes under pressure-driven flow and varying electric field strengths. The results show that water flux is strongly field-dependent in both systems, with Ag/N-CNT exhibiting significantly higher flux due to the presence of Ag nanoparticles. Notably, increasing the electric field enhances salt rejection in N-CNT electrodes but reduces it in Ag/N-CNT. At high electric fields, high water flux promotes severe Na + -Cl − cluster formation in pristine N-CNT. These clusters experience restricted entry into the nanochannels, which hinders ion capture and prevents ions from reaching the pure-water reservoir, thereby increasing salt rejection at the expense of SAC. In contrast, the Ag/N-CNT system mitigates this effect through Ag-induced selective dehydration of Cl − ions. This process disrupts the symmetry of the hydration shells and suppresses ionic cluster nucleation, allowing dispersed, mobile ions to readily enter the nanochannels. This significantly enhances SAC through efficient ion capture, although a portion of the ions permeates into the pure-water reservoir, leading to a decrease in salt rejection. Consequently, an inherent trade-off between high SAC and salt rejection emerges under high-throughput conditions. Despite this trade-off, the superior water flux and enhanced SAC of Ag/N-CNT highlight its potential for high-throughput CDI applications and provide atomistic insights for electrode design.