Molecular dynamics study of ion transport in nanostructures

Abstract

Ion transport in nanoporous materials plays a fundamental role in a wide range of applications across diverse scientific fields, such as energy storage and conversion, nanofluidic, and physical, chemical, and biological systems. Nanoporous materials, including nanoporous graphene and metal-organic frameworks (MOFs), have emerged as promising nanostructures for desalination, ion extraction, and electrode materials for batteries. They offer unique settings for the selective separation and controlled transport of molecules and ions. These materials can be engineered with precise pore sizes and shapes that enable targeted interactions with fluids that transport through them, making them highly suitable for various applications such as catalysis, gas separation, drug delivery, and environmental remediation. Although nanoporous materials like MOFs and nanoporous graphene provide notable benefits, their practical deployment in diffusion and transport applications is limited by many obstacles. The key challenges include undefined material structures and unknown diffusion coefficients. How factors, such as porosity, pore size, temperature, particle size, and concentration, affect molecular transport remains poorly understood and requires extensive investigation. This thesis utilizes molecular dynamics (MD) simulations to provide unique insights into nanoscale ion transport within porous materials, particularly focusing on the diffusion of aqueous electrolytes in nanoporous graphene structures. In addition, ion separation as one of the applications of MOFs is also examined through MD simulations. In the study of ions diffusion through 2D nanoporous graphene, the transport of K⁺ and Cl^- across nanopores in graphene sheets is investigated and the diffusion coefficient, D is calculated for various pore sizes and porosities. It is observed that D is sensitive to changes in pore size, especially when the diameter of the pore is smaller than 3 nm. In contrast, for larger pores, the value of D does not significantly change with variations in pore size. A general scaling law for the D is proposed, which enhances the understanding of ion transport in 2D nanoporous structures. Ion diffusion in 3D nanoporous graphene structures (3D-NGS) is also studied using MD simulations. The D of water, Li⁺ ions, and Cl^- ions are calculated in 3D-NGS with different porosities and surface charge densities across varied temperatures and concentrations of LiCl. The results show that the D conforms to the Årrhenius Equation, indicating a dependence on temperature, and adhere to power laws in relation to porosity. The D decrease as the salt concentration increases. Nevertheless, the impact of surface charge density on D is limited when the surface charge density is low. General scaling laws for the D of water, Li⁺, and Cl^- are proposed. Finally, the separation of Li⁺ and Mg²⁺ using MOF membranes is explored through MD. Five MOFs with various pore sizes are studied. It is discovered that MOFs with a pore size less than 7.08 Å have a critical pressure, below which, Li⁺ and Mg²⁺ can be fully separated. The critical pressure rises as the pore size decreases. When subjected to practical pressures below 50 MPa, MOFs with a pore size of around 6.5-7.0 Å are the most effective for completely separating Li⁺ and Mg²⁺. If high pressures can be achieved, MOF with a pore size of 5.48 Å is more effective as it guarantees excellent separation and a high flux of Li⁺. The hydration energy, potential of mean force, and density distribution of Li⁺ and Mg²⁺ are calculated to clarify the separation mechanisms. The results of this study provide important insights into the transport of ions in nanostructures, which will have an impact on the development of electrodes and various energy systems.

Date of Award	2024
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Zhigang LI (Supervisor) & Jinglei YANG (Supervisor)