Multiscale Modeling of Morphology and Proton/Ion Transport in Electrolytes

Understanding structure-function relationships in electrolytes is essential for advancing energy conversion and storage. This dissertation employs multiscale modeling and simulations to study the morphology and proton/ion transport in various electrolytes for electrochemical systems, including anion exchange membranes (AEMs), protic ionic liquids (PILs), pure phosphoric acid (PA) and aqueous acid solutions, ionic liquids (ILs), and polymerized ionic liquids (polyILs).

Mesoscale dissipative particle dynamics (DPD) simulations were employed to study the hydrated morphology of polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS)-based AEMs. The results indicate that the choice of the functional group moderately affects the water distribution and has little influence on the backbone structure. The associated anion seems to have a greater impact on the size of exclusive water domains compared to the cationic groups.

The effect of the base on proton transfer in PA-based PILs was explored by using hybrid density functional theory (DFT) in conjunction with a continuum solvation model. Potential energy scans reveal that the bases with stronger proton affinity significantly influence the energetics of proton transfer between PA molecules. However, this effect diminishes when the potential shifts from a double-well to a single-well. Ab initio molecular dynamics (AIMD) simulations were performed to uncover the underlying mechanisms of proton transport in pure PA and aqueous acid solutions. The findings show that protons move by surprisingly short jumps of only ~0.5-0.7 Å, with these proton jumps being anticorrelated, resulting in suppressed conductivity. Furthermore, acid molecules exhibit different roles in proton transport in their aqueous solutions depending on their molecular structures and acidity.

Classical molecular dynamics (MD) simulations were utilized to elucidate ionic correlations and ion transport mechanisms in ILs and polyILs. For ILs, the relative ion mass significantly impacts distinct ion-ion correlations, swapping the relative amplitudes of distinct cation-cation and anion-anion correlations. In polyILs, ion transport mechanisms were systematically investigated to understand the effects of sidechain flexibility coupled with specific interactions, revealing dynamical heterogeneity and stringlike cooperative motion. Importantly, distinct anion-anion correlations suppress conductivity in a barycentric reference frame but enhance it under a polycation-fixed reference frame. The cage escape emerges as the primary mechanism governing ion transport in these systems.