Description of Hydroxide Ion Structural Diffusion in a Quaternized SEBS Anion Exchange Membrane Using Ab Initio Molecular Dynamics

The transport via structural diffusion of hydroxide ions through a segment of the functionalized polystyrene-block-poly(ethyleneran-butylene)-block-polystyrene (QSEBS) membrane is analyzed at two hydration levels using ab initio molecular dynamics (AIMD). First, dynamic simulations are carried out to identify and describe the characteristics of structural diffusion with respect to (a) hydration of the conductive polymer and (b) location and solvation pattern of hydroxide ions. Then, hydroxide diffusivity and conductivity are estimated and compared with data from simulations in pure water and experimental conductivity for hydrated QSEBS. A strong influence was found of the hydration and location of hydroxide ions in the polymeric system in the characteristics and frequency of charge-transfer events. As an example of this, hydroxide ions having high coordination numbers or located in dry zones are the molecules with lowest number of structural diffusion events that contribute effectively to hydroxide displacement (nonrattling events) and have the highest mean-lifetimes. Calculated diffusion coefficients and hydroxide conductivities showed a coherent tendency and magnitude with respect to hydration and conductivity in bulk water. Also, simulations show that as hydration increases, so does the number of nonrattling charge-transfer events and contribution to hydroxide mobility by structural diffusion. This not only corroborates the consistency of the developed simulations with respect to proposed physical models for anion- exchange membranes in the literature, but also represents an important contribution to the detailed understanding of transport phenomena at the atomic scale and the role of cationic functional chains of the polymer in the development of transport mechanisms at such scale and their contribution to hydroxide mobility.