Prediction of equilibrium water uptake and ions diffusivities in ion-exchange membranes combining molecular dynamics and analytical models

Abstract

In recent years, the field of process engineering has witnessed an exponential increase in the usage of ion-exchange membranes (IEM) in light of their pivotal function in green technologies such as electrodialysis (ED), reverse electrodialysis (RED) and fuel cells. One of the key parameters in IEMs performance is the equilibrium water uptake (wu) as this has been shown to have a prominent effect on some of their fundamental properties such as ionic diffusivities. In this work, we have elaborated a molecular dynamics (MD) protocol to reliably predict the water uptake of IEMs by considering a polysulfone (PSU) functionalized with tetramethylammonium (TMA) anion-exchange membrane (AEM) compensated with chloride anions as case study. The procedure led to good agreement with reported experimental data in a wide range of ion-exchange capacities (IEC) and improved the results with respect to the DFT-based approach developed in our previous work. The issue of considering too thin membrane models compared to the actual IEM was found to be relevant and was addressed by proposing an ad-hoc simulation setup; this allowed to reconcile results accuracy with reasonable computational costs. Finally, the computed wu were used to evaluate chloride counter-ion diffusivities within three different theoretical frameworks (the Mackie-Meares, Yasuda-Lamaze-Ikenberry and Manning-Meares models) and it was found that a satisfactory agreement with experiments can be achieved. This confirmed the potential of the current strategy to predict the wu and ion diffusivities in IEM without resorting to experimental data, thus paving the way towards a computationally driven design of new membranes.