Edges facilitate water evaporation through nanoporous graphene

Understanding molecular processes of evaporation at the liquid-vapor interfaces is of critical importance for development of phase-change-related applications. The interfacial behaviors are defined by liquid-vapor equilibrium following thermodynamic rules, while the process through nanopores can be modulated by spatial confinement and intermolecular interaction with the pore. Based on molecular dynamics simulations, we explore water evaporation across nanoporous graphene membranes, which have been recently fabricated by, for example, ion or beam irradiation. The simulation results suggest that the molecular outflow can be facilitated by the graphene edges, boosting the overall evaporative flux by more than 100%. Free-energy analysis shows that the affinity of the graphene edge for water molecules provides a 'hub'-like function in the path of molecular effusion, reducing the free energy barrier for evaporation across the liquid-vapor interface. This prominent edge effect can be further engineered by modifying the atomic charges. Our findings demonstrate the feasibility of nanoengineering for the liquid-vapor phase-change processes using nanoporous graphene as a model system, which can find applications in heat transfer and energy conversion with high efficiency.