Effect of ion exchange capacity and water uptake on hydroxide transport in PSU-TMA membranes: A DFT and molecular dynamics study

Anion Exchange Membranes (AEMs) are nowadays used in a very wide range of different applications, from electrodialytic systems for water treatment, to Fuel Cells for energy generation. In many of these cases, the AEM is in contact with water molecules, either in aqueous or gas phase, which can strongly interact with the AEM polymer, thus affecting membrane properties. The role of water in fully swollen AEM and its effect on the diffusion of hydroxide is the focus of this work. Density Functional Theory (DFT) based calculations were merged with Molecular Dynamics (MD) simulations coming to a more accurate description of the ionic exchange capacity effect on the hydroxide ion diffusion in AEM. Hydration of the polysulfone-tetramethylammonium (PSU-TMA), one of the most studied systems in Alkaline anion exchange Membrane Fuel Cells (AMFC), was determined by DFT developing an efficient algorithm. The ab-initio calculations predicted water uptake, at different Ion Exchange Capacity (IEC), resulted in good agreement with the experimental data. The theory-experiment comparison identified a threshold range of 1.2-1.7 meq/g as critical IEC at which over-swelling of functionalized polymer matrix occurs. From this comparison, three IEC values were chosen to perform MD simulations in order to evaluate the hydroxyl diffusion coefficients as a function of the different IEC where the ab-initio water uptake and partial charges, utilized in the force-field, were adopted. Diffusion coefficients in the bulk of the membrane were calculated through the Mean Square Displacement (MSD) over 100 ns MD trajectories, and then compared with the values obtained by the analytical Mackie-Meares's model, finding good agreement between the two computational methodologies. In addition, the computed diffusion coefficients were compared with the experimental conductivity data with a fair agreement. It is worth noting that, thanks to the minimization of adjustable parameters, the present work can be easily adapted to study any type of Ion Exchange Membrane (IEM) and counter-ions diffusion.