Desalination Design Rules for Nanoporous 2D Materials: Low Edge Charges Sieve Ions, While Positive Edge Charges Exclude Boron

Abstract Molecular and ionic transport through nanopores in two-dimensional (2D) materials, which govern seawater desalination, are complex phenomena influenced by material chemistry, functional groups, and electrostatic interactions. We present a unified picture of nanoscale transport in 2D nanopores, demonstrating a pervasive principle valid across graphene, hexagonal boron nitride (hBN), and molybdenum disulfide (MoS2). That is, low edge charge effectively sieves ions, while positive edge charge excludes boron, which is regarded as the Achilles heel of seawater desalination. Quantum-mechanical calculations reveal that functional groups, such as hydrogen, hydroxyl, and oxo, modulate edge charges, tuning salt/boron rejection and water flux. Molecular simulations indicate that hydrogen-functionalized boron-terminated nanopores and oxygen-functionalized nitrogen-terminated hBN nanopores demonstrate superior salt rejection of close to 100%, due to roughly neutral edge atoms, which offer balanced ion rejection. This finding is found to persist with MoS2 and graphene nanopores, wherein low edge charge correlates with better salt rejection. Furthermore, hydrogen-functionalized nitrogen-terminated nanopores completely reject boron, a neutral contaminant that is difficult to remove from seawater, due to their positive edge charge. However, excessive positive charge compromises anion rejection and water permeance, despite repelling boric acid. These charge- and chemistry-mediated mechanisms provide a predictive basis and design rules for nanoporous 2D desalination membranes with high selectivity.