On the ions diffusion in ion exchange membranes: an ab-initio multiscale model

Process technology has witnessed an exponential increase in the study of ion-exchange membranes (IEMs) in light of their pivotal function for green technologies such as electrodialysis (ED), reverse electrodialysis (RED), and fuel cells. Various computational procedures are currently used to reliably model the IEMs’ properties, as understanding the structure-properties relationships might ultimately lead to the design of novel membranes. As far as this aspect is concerned, multiscale modeling assumes particular importance, though, empirical or adjustable parameters, requiring calibrations, are still used in most of the present approaches.

The proposed ab-initio multiscale methodology focused on the modeling of diffusion in IEMs without resorting to tunable parameters (fitting) which represents an original trait compared to the state of the art. This enhances current predictive capabilities and helps streamline costly and time-consuming trial-and-error procedures. The performance of IEMs is controlled by polymer flexibility, its hydrophobicity and hydrophilicity, nature of functional groups, ion exchange capacity (IEC), and equilibrium water absorption (W_u). We elaborated a molecular dynamics-based protocol to reliably predict water uptake as a function of the above features, considering a tetramethylammonium-functionalized polysulfone polymer as a case study of anion-exchange membrane. The procedure led to a favorable agreement with the experimental data in a wide range of IEC and improved results compared to the DFT-based approach developed in our previous work. The thickness of the membrane model was found to be a critical aspect as it can lead to inaccurate results. The issue was addressed by proposing an alternative simulation setup with respect to those reported in the literature. Finally, we illustrated how the W_u knowledge, evaluated in molecular scale, can be used to compute chloride counter-ion diffusivities within three different theoretical macroscopic frameworks. A reasonable agreement with experiments can be achieved, which confirms the potential of our strategy for the prediction of ion diffusivities in IEMs. In this sense, research efforts have been carried out in the development of biphasic models for ED using bipolar membranes.

Many membrane-based ion separations require selectivity for monovalent cations, although available membranes usually exhibit limited selectivity. In order to provide basic knowledge on the ion selectivity, the confinement effect in narrow pores disrupting the ion hydration was investigated aimed to provide a new alternative to classical Cation Exchange Membranes (CEM). Starting from Na, Ca and Mg hydrated cations, obtained with a quantum approach, single wall carbon nanotubes (SWCNTs) with diameters designed ad hoc were chosen and used as nanochannel models to perform long in silico experiments of permeation of single cations, for which the partial charges at the nanotube inlet were parameterized through ab initio calculations. The results show that 100 % perm-selectivity towards Na+ with respect to Ca2+ and Mg2+ is virtually attainable with SWCNT of 1.33 nm diameter. Interestingly, the origin of this behaviour lays on thermodynamics rather than on size exclusion mechanism as confirmed by the free energies calculations. Opposite to polymeric homogeneous CEM, showing higher affinity for multivalent cations, an inverted behaviour was found for these SWCNTs in terms of hydration free energies. Based on the achieved conclusion, the transport of Li+ and polysulphides through nanochannels is investigated in order to provide insight on membrane based separators for Sulphur-Litium batteries.