Optimal design of graphene nanopores for seawater desalination

Extensive molecular dynamics simulations are employed to optimize nanopore size and surface charge density in order to obtain high ionic selectivity and high water throughput for seawater desalination systems. It is demonstrated that with the help of surface charge exclusion, nanopores with diameter as large as 3.5 nm still have high ionic selectivity. The mechanism of the salt rejection in a surface-charged nanopore is mainly attributed to the ion concentration difference between the cations and anions induced by the surface charges. Increasing surface charge density is beneficial to enhance ionic selectivity. However, there exists a critical value for the surface charge density. Once the surface charge density exceeds the critical value, charge inversion occurs inside a nanopore. Further increasing the surface charge density will deteriorate the ionic selectivity because the highly charged nanopore surface will allow more coions to enter the nanopore in order to keep the whole system in charge neutrality. Besides the surface charge density, the nanopore length also affects the ionic selectivity. Based on our systematic simulations, nanopores with surface charge density between −0.09 C/m2 and −0.12 C/m2, diameters smaller than 3.5 nm, and membrane thickness ranging between 8 and 10 graphene layers show an excellent performance for the ionic selectivity.