Tracking mobile ion interactions with fixed charge groups in cation exchange membranes using Raman spectroscopy

Understanding the mechanism behind the permeation of multivalent ions through ion exchange membranes remains an ongoing challenge. Here we address this knowledge gap by presenting a molecular theory to elucidate the influence of counterion condensation and electrosteric effects on the (multivalent) divalent counterion permeation through such membranes. Building upon published experimental data (Galizia et al., 2017), we analyze the energetic tendencies of divalent counterions to associate with the fixed charge groups and their entropic preferences to disassociate to gain translational freedom, allowing us to quantitatively assess the partitioning of divalent ions at two distinct levels. (i) The global partitioning between the external 2:1 salt solution and the membrane, and (ii) the local partitioning within the interstitial solution and the crosslinked polymeric chains within the membrane. Notably, our observations reveal that the global partitioning of divalent counterions does not differ when the membranes operate separately in magnesium chloride (MgCl2) and calcium chloride (CaCl2) solutions. However, at the local partitioning level, we identify a higher preference for calcium counterions to associate with the fixed charge groups than magnesium ions. This preference arises from calcium ions’ more favorable charge density (physical size), facilitating their association/pairing with the fixed charge groups. The key takeaway message from this work is that when dealing with multivalent ions, we must downlevel our investigation, looking at how the counterions relax between the interstitial solution and the crosslinked polymeric chains and, in return, comprehending how it ultimately controls the global transport property of these membranes (such as their Donnan potential and water uptake).

Understanding the effects of polymer chemistry on membrane ion transport properties is critical for enabling efforts to design advanced highly permselective ion exchange membranes for water purification and energy applications. Here, the effects of fixed charge group type on anion exchange membrane (AEM) apparent permselectivity and ion transport properties were investigated using two crosslinked AEMs. The two AEMs, containing a similar acrylonitrile, styrene and divinyl benzene-based polymer backbone, had either trimethyl ammonium or 1,4-dimethyl imidazolium fixed charge groups. Membrane deswelling, apparent permselectivity and ion transport properties of the two AEMs were characterized using aqueous solutions of lithium chloride, sodium chloride, ammonium chloride, sodium bromide and sodium nitrate. Apparent permselectivity measurements revealed a minor influence of the fixed charge group type on apparent permselectivity. Further analysis of membrane swelling and ion sorption, however, suggests that less hydrophilic fixed charge groups more effectively exclude co-ions compared to more hydrophilic fixed charge groups. Analysis of ion diffusion properties suggest that ion and fixed charge group enthalpy of hydration properties influence ion transport, likely through a counter-ion condensation, ion pairing or binding mechanism. Interactions between fixed charge groups and counter-ions may be stronger if the enthalpy of hydration properties of the ion and fixed charge group are similar, and suppressed counter-ion diffusion was observed in this situation. In general, the hydration properties of the fixed charge group may be important for understanding how fixed charge group chemistry influences ion transport properties in anion exchange membranes.

Quantifying the coupled monovalent and divalent ions sorption in dense ion-exchange membranes

Molecular simulation of diffusion behavior of counterions within polyelectrolyte membranes used in electrodialysis

Ion exchange membranes are used in various membrane-based processes (e.g., electrodialysis, fuel cells). Charged solute transport is largely governed by the charged groups on the polymer backbone. In this review, fundamental relationships describing salt permeability and ionic conductivity, as well as water permeability, in charged polymers are developed within the framework of the Nernst-Planck and solution-diffusion models. The influence of fixed charge groups and polymer structure on water sorption and diffusion is discussed. Current understanding of ion partitioning in charged polymers, focusing on the use of thermodynamic models (i.e., Donnan theory) to describe such phenomena, is summarized. Ion diffusivity data from the literature are interpreted using a model developed by Mackie and Meares to assess relative and absolute effects of the polymer and fixed charge groups on ion diffusivity. Furthermore, membrane requirements for several important technologies are listed. Knowledge gaps and opportunities for fundamental research are also discussed.

Cation exchange membranes (CEMs) are an ion exchange polymer which selectively shuttles cations through their structure. CEMs are used for a wide range of electrically driven processes, including electrolysis, fuel cells, and electrodialysis. Historically, fluorinated polymers have been used as CEMs, with the most prevalent being Chemours’ Nafion polymer. These materials and their synthesis can produce harmful, long-lived environmental toxins that may provide challenges to scaling to meet material demand. Therefore, significant research is being directed towards finding non-fluorinated CEMs with similar functionality. One promising candidate has been sulfonated poly-ether-ether-ketone (SPEEK), which has similar ion exchange capabilities as Nafion, while being low cost and non-fluorinated.In electrochemical reactors, the effect of different cations in electrolyte solutions has shown high importance for overall conductivity and can affect reaction selectivity, activity, and durability. The cations which move through the CEM (mobile ions) can interact with the fixed sulfonate functional groups on the polymer backbone. While there is significant research on mobile ion effects on the performance of Nafion1, far less work has been done in this area with SPEEK2. Additionally, there are no direct comparisons between Nafion and any non-fluorinated polymer on how the polymer backbone affects these mobile ion interactions.One method to study CEM-ion interactions is with Raman Spectroscopy, which uses a monochromatic light to initiate and observe Raman scattering from matter. While Raman spectra have been obtained for Nafion, we have not found Raman spectra of SPEEK in the presence of different electrolyte compositions. Here, we use Raman spectroscopy paired with water uptake and conductivity measurements to compare the differences of how a variety of mobile cations (H+, NH4+, Li+, Na+, K+, Rs+, Cs+, Mg2+, Ca2+, Sr2+) interact with the fixed sulfonate functional groups found in Nafion and SPEEK. We report that the two polymers show differences in the strength of the interactions between the mobile ions and fixed groups, which leads to changes in physical properties, including water uptake and conductivity. With these findings, more informed decisions of the applications of fluorinated vs non-fluorinated polymers for various electrochemical purposes can be made.Key

Ion sorption in highly charged, highly swollen membranes is influenced by fixed charge group concentration, as well as distribution of fixed charge groups.

The paper discusses the electrodialysis treatment of mixed nickel sulfate and sulfuric acid solution using polyaniline surface-modified cation exchange membranes. The modified membranes are obtained on the basis of industrial cation-exchange MK-40 heterogeneous and MF-4SK homogeneous membranes by in situ oxidative polymerization of aniline under electrodialysis conditions. The conductive and diffusion characteristics of the initial and modified membranes in solutions of sulfuric acid and nickel sulfate are studied. It is shown that the modification of membranes with polyaniline leads to a decrease in their electrical conductivity and diffusion permeability while maintaining high selectivity. The diffusion permeability of cation-exchange membranes is higher in nickel sulfate solutions compared to sulfuric acid one, while an inverse dependence is found for anion-exchange membranes. The competitive transfer of sulfuric acid and nickel sulfate during electrodialysis separation and concentration of their mixture using initial and modified cation-exchange membranes paired with anion-exchange MA-41 membrane are studied. It is shown that the applying a polyaniline layer with positively charged groups on the surface of the MK-40 or MF-4SK cation-exchange membranes suppresses the transport of doubly charged nickel ions both in the separation and concentration modes over the entire range of current densities. The greatest repulsion effect is observed for homogeneous modified membranes, where the selective permeability coefficient P(H2SO4/NiSO4) increases from 0.7–1.7 to 32.5–19.7 depending on the current density. It is established that the use of polyaniline modified cation-exchange membranes permits to concentrate the solution containing 0.1 mol-eq/L (4.9 g/L) H2SO4 and 0.1 mol-eq/L (7.7 g/L) NiSO4, with simultaneous separation into sulfuric acid with concentration about 2.4 mol-eq/L (120 g/L) and nickel sulfate solutions. The content of nickel sulfate in the concentrate does not exceed 0.13 mol-eq/L (10 g/L).

Proton and potassium ions countertransport across perfluorocarbon-type (Nafion) and hydrocarbon-type (K-101) cation-exchange membranes was studied, and the effect of proton transport on potassium ion transport was examined. It is considered that the fixed charge groups are distributed heterogeneously in the former and homogeneously in the latter. The experimental results were compared with calculated values using the Teorell−Meyer−Sievers's theory, which formulated transport phenomena based on the Donnan equilibrium and the Nernst−Planck flux equation. In the case of the K-101 membrane, the experimental results comparatively agreed with theoretical predictions. The deviation is attributed to the low estimation of the membrane effective charge density. On the other hand, the experimental results did not agree with theoretical predictions in the case of the Nafion membrane. It might be attributed not only to the low estimation of the membrane effective charge density but also to the high proton mobility cau...

A study of the effective charge density of a charged membrane with a low water content was carried out. The membrane potentials across charged membranes, which have various water contents and an inhomogeneous fixed charge distribution, were measured. The experimental results were analyzed on the basis of the Donnan equilibrium theory and the Nernst−Planck transport equation considering the fixed charge inhomogeneities in the membrane. The results show that the value of the charge effectiveness will depend on the membrane water content, the ionic radii of the counterions, and the valence of the counterion. To interpret the variation in the value of the charge effectiveness, we attempted to introduce the concept of an ion pair between the fixed charge group and counterion inside the membrane. Compared with the experimental results, the prediction of the ion pair theory qualitatively agreed with the experimental tendency. This suggests that ion pairs cause the difference between the effective charge density and the fixed charge density, especially for a low water content condition.

Water orientation and ion solvation effects during multicomponent salt partitioning in a Nafion cation exchange membrane

Selective transport of neutral amino acids across a double-membrane system comprising cation and anion exchange membranes

In this report, poly ether ether ketone (PEEK)-based anion and cation exchange membranes were assembled separately in two identical and compact microbial fuel cells (MFCs) without water and gas trapping between membrane and the electrode. We examined the use of these two membranes with the same electrode surface area as well as anode and cathode volumes. A maximum power density of 603 and 458 mW m−2 with a columbic efficiency of 76 and 61% were observed for quaternized poly ether ether ketone (QPEEK) and sulfonated poly ether ether ketone (SPEEK), respectively. A few solution chemistry parameters, which are leading factors behind the cell performance, such as variation in conductivity and solution pH in the two chambers were observed. In addition, the mobility of anions and cations (other than protons and hydroxyl ions in the case of cation and anion exchange membranes, respectively) under the inoculated conditions was also investigated. Further, the membrane charge-based biofilm growth was also analyzed by SEM and impedance spectroscopy to find its influence on the MFC performance.

Reverse electrodialysis converts electrochemically into energy conversion technology due to the difference in salinity between diluted salt solutions such as seawater and dilute salt solutions such as fresh water. The RED stack has a cation exchange membrane and anion exchange membrane alternately arranged between two electrodes that selectively transmit anions and cations. Ion exchange membranes are easily contaminated with multivalent ions and organic matter, which affects the performance of RED. In this study, the fouling phenomena of cation and anion exchange membranes for various foulants possibly found in seawater and river water were investigated. Ion exchange membranes were contaminated with NaCl solution containing foulants, and the electrical resistance and the degree of contamination with contamination time were measured. The cation exchange membrane is contaminated with a cationic foulants and the anion exchange membrane is contaminated with anionic foulants. The fouled membranes were regenerated, and the electric resistance and the degree of contamination with regeneration time were measured. Acknowledgment This work was conducted under the framework of Research and Development Program of the Korea Institute of Energy Research (KIER) (B8-2441).

Novel ecofriendly cation exchange membranes for low-cost electrodialysis of brackish water: Desalination and antiscaling performance