Voltage-gated multilayer graphene nanochannel for K+/Na+ separation: A molecular dynamics study

Understanding the K+/Na+-Selectivity-Enabled Osmotic Power Generation: High Selectivity May Not Be Indispensable

By mixing ionic solutions, considerable energy can be harvested from entropy change. Recently, we proposed a concept of potassium-permselectivity enabled osmotic power generation (PoPee-OPG) by mixing equimolar KCl and NaCl solutions via artificial potassium ion channels (APICs, Natl. Sci. Rev. 2023, 10, nwad260). However, a fundamental understanding of the relationship between the K+/Na+ selectivity and optimal performance remains unexplored. Herein, we establish a primitive molecular thermodynamic model to investigate the energy extraction process. We find PoPee-OPG differs from previous charge-selectivity-based techniques, such as the salinity gradient power generation, in two distinct ways. First, the extractable energy density and efficiency positively depend on concentration. More surprisingly, a very high potassium selectivity is not indispensable for satisfactory efficiency and energy density. An optimal K+/Na+ selectivity region of 3 to 10 is found. This somewhat counterintuitive discovery provides a renewed understanding of the emerging PoPee-OPG, and it predicts a broad applicability among existing APICs.

Designing artificial ion channels with strict K+/Na+ selectivity toward next-generation electric-eel-mimetic ionic power generation

A biological potassium channel is >1000 times more permeable to K+ than to Na+ and exhibits a giant permeation rate of ∼108 ions/s. It is a great challenge to construct artificial potassium channels with such high selectivity and ion conduction rate. Herein, we unveil a long-overlooked structural feature that underpins the ultra-high K+/Na+ selectivity. By carrying out massive molecular dynamics simulation for ion transport through carbonyl-oxygen-modified bi-layer graphene nanopores, we find that the twisted carbonyl rings enable strict potassium selectivity with a dynamic K+/Na+ selectivity ratio of 1295 and a K+ conduction rate of 3.5 × 107 ions/s, approaching those of the biological counterparts. Intriguingly, atomic trajectories of K+ permeation events suggest a dual-ion transport mode, i.e. two like-charged potassium ions are successively captured by the nanopores in the graphene bi-layer and are interconnected by sharing one or two interlayer water molecules. The dual-ion behavior allows rapid release of the exiting potassium ion via a soft knock-on mechanism, which has previously been found only in biological ion channels. As a proof-of-concept utilization of this discovery, we propose a novel way for ionic power generation by mixing KCl and NaCl solutions through the bi-layer graphene nanopores, termed potassium-permselectivity enabled osmotic power generation (PoPee-OPG). Theoretically, the biomimetic device achieves a very high power density of >1000 W/m2 with graphene sheets of <1% porosity. This study provides a blueprint for artificial potassium channels and thus paves the way toward next-generation electric-eel-mimetic ionic power generation.

Water purification modeling by functionalized hourglass-shape multilayer nano-channel

In this study, inspired by the overall structure and operation of the aquaporin channel, graphene-based nanochannels are proposed to be used as potential membranes for the water purification process. To this end, an hourglass-shaped channel has been designed using the three-layer porous graphene sheets and the effects of some main channel's elements, such as the channel bending angle and attached functional groups to it, on the filtration performance have been examined by using molecular dynamics simulations. We find that a suitable bending channel shape can improve the channel efficiency, i.e. both the water permeability and the ion rejection rate of the suitable bent channels were more than for the straight channels. In addition, regarding the different functionalized channels, the half-functionalized channels were more efficient than the completed functionalized ones. Furthermore, by monitoring the dynamics of water molecules as they pass through the narrowest part of the channels, it was found that water molecule rotation assists water transport.

Biomimetic Nanochannels: From Fabrication Principles to Theoretical Insights

Biological nanochannels which can regulate ionic transport across cell membranes intelligently play a significant role in physiological functions. Inspired by these nanochannels, numerous artificial nanochannels have been developed during recent years. The exploration of smart solid-state nanochannels can lay a solid foundation, not only for fundamental studies of biological systems but also practical applications in various fields. The basic fabrication principles, functional materials, and diverse applications based on artificial nanochannels are summarized in this review. In addition, theoretical insights into transport mechanisms and structure-function relationships are discussed. Meanwhile, it is believed that improvements will be made via computer-guided strategy in designing more efficient devices with upgrading accuracy. Finally, some remaining challenges and perspectives for developments in both novel conceptions and technology of this inspiring research field are stated.

Control one-dimensional length of rectangular pore on graphene membrane for better desalination performance

At present, there is a general contradiction between permeability and selectivity of reverse osmosis (RO) membranes for desalination; a membrane with higher water permeability will give a lower salt rejection or selectivity, and vice versa. In this work, single-layer nanoporous graphene is used as RO membrane to investigate the effects of pore shape to reduce this contradiction by molecular dynamics simulations. Two kinds of pores (round and rectangular pores) with different sizes are simulated. For round pore, although the water permeability increases with the increase of the pore size, the salt rejection rate drops rapidly. For rectangular pore, reasonable designed pore structure can achieve improved water permeability and high salt rejection of graphene membrane by keeping one-dimensional length (i.e. the width) of the pore less than the size of the hydrated ions and increasing the other dimensional length. The restriction of one dimension can prevent the passage of hydrated ions through the pore effectively. This 'one-dimensional restriction' provides a simple strategy for designing RO membrane with variable pore structures to obtain a better desalination performance.

Self-assembly of graphene oxide sheets: the key step toward highly efficient desalination

Lamellar graphene oxide (GO) membranes are new membrane materials for seawater desalination due to their selective sub-nanometer interlayer two-dimensional channels. In general, the reliable and precise desalination of GO membranes is still heavily dependent on thick membranes that usually have a low water flux. The trade-off between the water flux and ion rejection is a long-lasting problem that restricts the development of highly efficient desalination membranes. In this work, we theoretically predicted that this trade-off can be broken by the self-assembly of GO sheets during the membrane preparation. Our molecular dynamics (MD) simulations indicate that the high-water permeability of the GO membrane is due to the frictionless flow of water in the 2D nanochannels enclosed by the non-oxidized regions of neighboring GO sheets, while the oxidized regions are responsible for the high ion rejection rate. Meanwhile, the MD simulations of the self-assembly processes of GO sheets in aqueous solutions just demonstrate that the oxidized regions of neighboring GO sheets are prone to stacking with each other, while the non-oxidized regions of neighboring GO sheets are inclined to matching with each other. Therefore, more interlayer nanochannels for fast water flow and ion rejection will be formed, respectively, after the full assembly of GO sheets during membrane preparation. Finally, based on our results, a new but simple method has been proposed to prepare GO membranes with superior desalination performance via deposition rate control.