Research Projects Supported by HKU

The aim of this simulation is to investigate the properties of ions inside a nanopore, which is a geometry commonly seen in a selectively membrane widely used in electrochemistry for separation of cathode and anode compartments during electrolysis. The transport behaviour of electrolytes confined in this nanoscale was thought to be different from the bulk state and it is important to understanding the relations between pore size in a membrane and mass transport of various species of particles, which helps to improve the quality of the membranes. Since our research is doing research on fuel cell using glucose as reagent and its performance relies on the selectivity and transport properties of the cell membrane. The simulation results can provide a fundamental data on that aspect.

The orientation of water dipole around ions in the pore with radius equal 4.8 � 10^-10 m shows the arrangements of water molecules are different in a narrow pore (Figure 2). Self diffusion coefficients of electrolytes and electrical conductivity increases with increasing pore radius, but there is a large discrepancy in the conductivity in pores with small radii. Ion-ion pair correlation functions show that more ion-pairings (Figure 3) were formed in an applied external field. The results suggested that using the Nernst-Einstein equation to study transport properties of electrolytes confined in a nanopore would fail.

Figure 1. A visual program generated picture of SPC/E electrolytes in a nanopore. (red, green, yellow and blue spheres represent cations, anions, oxygen and hydrogen in water molecules respectively).

Research Projects Supported by HKU's High Performance Computing Facilities

Researcher:
Dr Yuk-wai Tang, Department of Chemistry

Project Title:
Nonequilibrium Structural and Transport Properties of SPC/E Electrolyte in Nanopores

Project Description:
Molecular dynamics simulations were carried out to study electrolytes confined in a nanopore using a smooth and hydrophobic wall. An 0.5M KCl electrolytes was modeled with water molecules represented by the extended simple point charge model. Cylindrical pores with sizes varied from 4.8 � 10^-10 to 1.6 � 10^-9 m were chosen and temperature of the system was maintained constant by Gaussian thermostat. The effect of wall on the interactions and among molecules and their transport properties were investigated.

Project Duration:
1.5 year

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Project Significance:
The aim of this simulation is to investigate the properties of ions inside a nanopore, which is a geometry commonly seen in a selectively membrane widely used in electrochemistry for separation of cathode and anode compartments during electrolysis. The transport behaviour of electrolytes confined in this nanoscale was thought to be different from the bulk state and it is important to understanding the relations between pore size in a membrane and mass transport of various species of particles, which helps to improve the quality of the membranes. Since our research is doing research on fuel cell using glucose as reagent and its performance relies on the selectivity and transport properties of the cell membrane. The simulation results can provide a fundamental data on that aspect.

Results Achieved:
The orientation of water dipole around ions in the pore with radius equal 4.8 � 10^-10 m shows the arrangements of water molecules are different in a narrow pore (Figure 2). Self diffusion coefficients of electrolytes and electrical conductivity increases with increasing pore radius, but there is a large discrepancy in the conductivity in pores with small radii. Ion-ion pair correlation functions show that more ion-pairings (Figure 3) were formed in an applied external field. The results suggested that using the Nernst-Einstein equation to study transport properties of electrolytes confined in a nanopore would fail.
Figure 1. A visual program generated picture of SPC/E electrolytes in a nanopore. (red, green, yellow and blue spheres represent cations, anions, oxygen and hydrogen in water molecules respectively).
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Figure 2. Orientation of water dipole along the wall surface for SPC/E electrolytes in a pore with (a) R = 4.8 � 10^-10 m; (b) R = 9.5 � 10^-10 m. Direction of dipole is shown by a white arrow for some particles.
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Figure 3. Snap shot of a SPC/E electrolyte in a narrow pore shows an ion pair (red spheres for cations and green for anions)
Figure 4. CPU time required to calculate 50,000 simulation steps of electrolytes confined in a nanopore. The model simulated is a pore with 1.3 � 10^-9 m in radius, 8.6 � 10^-9 m in length and consists of 1122 particles.

Video 1. The ions and water molecules are moving inside the nanopore. The red, green and yellow spheres are cations, anions and water molecules respectively.

Remarks on the Use of High Performance Computing Cluster:
The calculation of interaction potential among particles in the system is a time consuming process. By parallelizing the program and run in the HPC Cluster, the computation time is reduced by about 70% (Figure 4). The simulation takes 1 year to finish in HPC Cluster, which means it will take more than 7 years to complete if I run the jobs in a single processor.

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