Hydrous Proton Transfer through Graphene Interlayer: An Extraordinary Mechanism under Magnifier

Balancing ionic selectivity against permeability in filters made from graphene remains a challenge today. Interlayer distance, as the most important factor, dominates nearly all aspects of the flow inside the channel, from the formation of water molecules to the hydration shell of the ions. Unraveling the effects of the interlayer distance on the proton diffusion process helps lay a foundation for the cutting-edge proton conduction technology. Here, the reactive molecular dynamics simulations are used to probe the proton flow through a series of hydrated graphene channels with different interlayer distance values. The results show that the proton-selectivity experiences a sharp increase when the channel height is reduced to values under 8 Å, which is near the end of the hydration radii range of the monovalent and divalent cations. Reducing the interlayer distance also decreases the number of confined water molecules, consequently reducing the proton diffusion rate as the hopping platform fades. This way, spatial hindrance combined with the proton-selective Grotthuss mechanism provide a proton-exclusive membrane. The outputs of this work can be used for the optimization of proton-exclusive nanochannels and to serve affordable proton-exchange membranes (PEMs) for technological advancement in diverse fields from PEM fuel cells to storing liquid hydrogen

New molecular understanding of hydrated ion trapping mechanism during thermally-driven desalination by pervaporation using GO membrane

© 2019 Elsevier B.V. The graphene oxide (GO)-based membranes have shown effective salt separation from the brine solution. Although several experimental works have been done to study the salt rejection in the thermal-driven desalination system, the behavior of ions inside the GO nanochannels during the pervaporation process remains mostly unelucidated. Moreover, the previous theoretical studies on the driving force for ion transport within the GO membrane only focused on pressure, voltage, or concentration gradient. Here, we investigated the transport of water molecules and hydrated cations through the GO membrane in a temperature-assisted system by using dispersion-corrected density functional theory (DFT) calculations at PBE/Grimme with the ions under extreme confinement, reactive molecular dynamics (MD) simulations as well as validated by experimental methods. The water permeation increases with temperature, while the transport of cations remains minimal, which was not observed in other non-thermal desalination approaches. We have shown that high temperature eases the binding between the hydrated divalent cations and the oxygen functional groups on the GO nanosheets by reducing the hydration shell. Furthermore, the cross-linked cations inside the GO nanochannel create an accessible corridor for water molecules and block other cations such as Na+. Our simulation results provide a new mechanism of ion transport in the GO membranes by the thermal-driven process, which can be tailored by using other cations with higher charge density and high temperature to speed up the process

Insight from perfectly selective and ultrafast proton transport through anhydrous asymmetrical graphene oxide membranes under Grotthuss mechanism

Protons transport profoundly affects diverse fields from proton-exchange membrane fuel cells to storing liquid hydrogen. Recent advances have extended proton-exclusive transport to the two-dimensional channels that use hydrous mechanisms for fast proton transport, where the main challenge is the limited selectivity. However, the physical and chemical properties of 2D nanosheets like GO have the potential to implement full selective and ultrafast proton transport. Here, we uncover the physical potential of anhydrous proton transfer mechanism inside two-dimensional space between graphene nanosheets to exploit the exceptional full proton-selective ability and ultrafast conveyance speed of the Grotthuss mechanism. Reactive molecular dynamics simulations illustrate that the interlayer space between two graphene oxide nanosheets, carpeted with hydroxyl functional groups as additional hopping stages to enable the Grotthuss mechanism, can convey protons without water. Further, we dissect three essential factors that provide a deeper insight into ultrafast proton transport: (i) transitional phase to full anhydrous transport, (ii) outlet size for containing undesired species, and (iii) elastic behavior of the membranes under external strain. Our results show that changes in surface geometry can dramatically increase the diffusion rate in the presence of a small electric field by ~70% compared to hydrous transport. These findings can be used not only to guide the efforts in manufacturing a new generation of sustainable nanochannels but also to advance the pioneering technologies revolving around hydrogen