Deciphering the electronic-level mechanism of Na+ transport in a graphdiyne desalination membrane with periodic nanopores

Abstract

With the advancement of industry and population growth, the crisis of freshwater shortage has turned into a formidable dilemma, with nearly one billion people worldwide experiencing an urgent requirement for healthy freshwater. Recently, two-dimensional porous carbon materials with pore sizes of 0.45–0.55 nm have been confirmed to demonstrate a rejection rate of 100 %. However, there are still challenges in fabricating periodic defect-free nanopores in monolayers, so the development of these desalination membranes has somewhat stagnated at the investigation stage. Here, we have conducted a thorough study of desalination performance using molecular dynamics (MD) and density functional theory (DFT) with a view to delineating the electronic-level mechanism of Na+ transport in periodic nanopores with a self-cleaning effect. The results further suggested that synthesizable graphdiyne was one of the most promising potential desalination membranes by virtue of its periodic pore structure, electronic properties, and structural stability. Graphdiyne membranes have a salt rejection rate close to 100 % and require less pressure compared to other 2D materials. We then found that the transition-state energy of Na+ in a graphdiyne membrane was satisfactory (0.03 eV) due to significant charge-transfer between the carbon atoms in the periodic pores and Na+ in the pz orbitals. This blocked the filtration and agglomeration of Na+, while imparting the graphdiyne membrane with metallicity. Therefore, in striking contrast to conventional polymeric thin films, the periodic conductive network is expected to enable the reuse of reverse osmosis membranes through the application of an electric field, thereby increasing their service life. We present herein a new approach for the design and screening of next-generation desalination membranes, which is only one step away from solving the problem of introducing periodic nanopores in two-dimensional carbon materials.