Mechanism of monovalent and divalent ion mobility in Nafion membrane: An atomistic simulation study

Abstract

Polymer cation exchange membranes (CEMs) are widely used in water treatment processes. The fundamental factors that control the separation and selectivity of cations with different valences in CEMs are not fully understood. In this study, we use atomistic molecular dynamics simulations to investigate the underlying molecular mechanisms that control the mobility of cations with different valences in Nafion membranes. Our results indicate structural differences in binding of monovalent and divalent cations, which gives rise to differences in ion diffusion in Nafion. Monovalent cations are shown to be “territorially” bound, i.e., bound while partially hydrated, to the fixed charge groups whereas divalent cations are shown to be “site” bound, i.e., bound while fully dehydrated, to the charge groups on the polymer. This difference in binding structure gives rise to differences in transport characteristics of cations in Nafion.

Understanding Monovalent Cation Diffusion in Negatively Charged Membranes and the Role of Membrane Water Content

“…45,64,88 Contact ion pairing in IEMs results in a lower number of charge carriers, which can notably impact electric-field-driven ion transport in such materials. 39,40,42,44,88,89 Thus, discerning and understanding the speciation of ions in IEMs is important for the design of novel IEMs with improved transport properties. 87,88 Nuclear magnetic resonance (NMR) spectroscopy is suitable for detecting contact ion pairs by probing the local environment and mobility of NMR active ionic nuclei.…”

“…39,40,42,44,88,89 Thus, discerning and understanding the speciation of ions in IEMs is important for the design of novel IEMs with improved transport properties. 87,88 Nuclear magnetic resonance (NMR) spectroscopy is suitable for detecting contact ion pairs by probing the local environment and mobility of NMR active ionic nuclei. 45,83,90,91 Electronic changes in the immediate environment of the ions will result in changes in the NMR chemical shifts.…”

“…In contrast, the ions that form a contact ion pair do not have any water molecules between them. If the water content of an IEM decreases to the extent that there is insufficient water to satisfy the primary hydration shell of the counter-ions and the fixed charged groups, these ionic species can potentially undergo solvent-shared or contact ion pair formation. ,, Contact ion pairing in IEMs results in a lower number of charge carriers, which can notably impact electric-field-driven ion transport in such materials. ,,,,, Thus, discerning and understanding the speciation of ions in IEMs is important for the design of novel IEMs with improved transport properties. , …”