On the Temporal Selectivity of Desalination for a Porous Composite Graphenecopper Membrane (gcum): A Molecular Dynamics Study

The concept of the temporal selectivity breaks the spatial selectivity-based paradigm of permeability and selectivity in membrane separation technology, and its realization can provide simultaneous efficient salt rejection and high permeability with large pores. In this work, a rotating centrifuge model made of porous copper foil covered with porous graphene is designed to explore the possibility to realize the temporal selectivity on a porous composite graphene-copper membrane (GCuM) in a molecular dynamics study. The results show that the permeability of porous rotating GCuM increases up to 156 L/cm2/day/MPa with a salt rejection larger than 90 %, which is 2.6 to 1000 times larger than the commercial and other advanced reverse osmosis membranes. The boundary slip velocity between the seawater-GCuM interface inhibits the passage of salt ions, resulting in high salt rejection even in the case of 2-4 nm diameter pores. The stability of the ionic hydration shell and the energy barrier of the ions passing through the pores increase with the increase of the angular velocity, which explains the influence of the boundary slip velocity on the salt rejection from the molecular mechanism. The geometric relation that governs the temporal selectivity principle on porous rotating GCuM has been further derived to explain pore size, membrane thickness, and hydrated ion mass on high permeability and salt rejection. The findings further verify and expand the application scope of the temporal selectivity mechanism, and will promote the application of graphene-based nanoporous centrifuges.