(318b) Molecular Dynamics Studies of Diffusion and Solvation of Rubidium Bromide Solutions In Nanoconfinement

The need for improved energy storage devices has increased due to demands for renewable energy sources. Supercapacitors, or electrochemical double-layer capacitors (EDLCs), have become a viable option as a leading energy storage device due to their high charge efficiency, charge/discharge times, and long cycle life. While EDLCs have many attractive properties, they cannot store large amounts of energy. Carbide derived carbons (CDCs) have shown an improvement of energy storage in EDLC applications due to a dramatic increase in capacitance in pores ranging from 0.7 to 1.1nm[1]. The increase is thought to occur due to the stripping or deformation of the hydration shell as the ions enter the nanopores[1]. In the present work we analyze the hydration of ions in simple nanopore geometries to gain an improved understanding of the solvation structures inside the EDLC. 

Using molecular dynamics (MD) simulations, we studied the impact of confinement on the solvation of rubidium and bromide ions in aqueous solutions confined in slit-shaped pores ranging from 0.65 to 1.6 nm in size, which encompasses the range typically found in CDCs and other nanoporous EDLC materials[2].  In the MD simulations, the rubidium and bromide ions were modeled as charged Lennard-Jones particles as prescribed by Koneshan, et al.[3].  We investigated 0.1 M, 0.5 M and 1.0 M solutions of these ions using the SPC/E[4] model for water. We found the average number of water molecules hydrating each of the ions and the structure of the solvation shell, which is characterized by the ion-water radial distribution function.  The effect on confinement on the dynamics was also investigated by calculating the self-diffusion coefficients of the ions. Finally, the solvation of ions in an amorphous carbon structure was studied in order to determine the propensity of ions to reside in pores of specific sizes. Our results indicate that although materials with smaller pores have significantly increased capacitances, the diffusion, of ions is hindered by the confinement, which is potentially detrimental to enhancing the power density of EDLCs.