Aqueous ion partitioning in Nafion: Applicability of Manning's counter-ion condensation theory

Thermodynamics of Mobile Ion in Ion Exchange Membranes: Water-Swollen-Membrane Reference State and Quasi-Regular Solution Model

Thermodynamics of mobile ions in swollen ion exchange membranes (IEM) are essential for understanding their permselectivity. The theoretical modeling of the ion activity coefficient and the ion partitioning of the IEM is challenging. Based on a water-swollen membrane reference state and a quasi-regular solution model, we successfully correlated the ion activity coefficient in various IEMs. For all the studied 59 systems, the correlation coefficient r and determination coefficient R2 are 0.973 and 0.947, respectively. As expected, the water activity in the IEM and the ion partitioning between the IEM and the external salt solution are represented. In the new theory framework, the standard chemical potential of the counterion is different from that of the aqueous solution. Moreover, the ion activity coefficient in the IEM reaches unity for the infinite dilution external salt concentration for all systems. Our quasi-regular solution model only considered the short-range interactions between the co-ions and the "effective free counterion", but it showed excellent correlation ability with the activity coefficient data of extensive systems. A power-law relation between the effective concentration of the free counterion and the apparent concentration of the free counterion was suggested. This indicates that the long-range electrostatic interactions in the swollen IEM are negligible due to abundant counterion condensation shielding the charge of the polymer chain.

Cation Effect on the Electrochemical Platinum Dissolution

Ensuring the stability of electrocatalysts is paramount to the success of electrochemical energy conversion devices. Degradation is a fundamental process involving the release of positively charged metal ions into the electric double layer (EDL) and their subsequent diffusion into the bulk electrolyte. However, despite its vital importance in achieving prolonged electrocatalysis, the underlying causality of catalyst dissolution with the EDL structure remains largely unknown. Here, we show that electrochemical Pt dissolution is strongly influenced by the identity of the alkali metal cation (AM+) in the electrolyte. By monitoring Pt dissolution in real-time, we found a trend of reduced Pt leaching in the sequence Li+ > Na+ > K+ > Cs+. Our computational predictions suggest that interfacial OH- concentration plays a pivotal role in Pt dissolution, where OH- facilitates the outward diffusion of dissolved Pt ions into the bulk electrolyte by neutralizing the Ptz+ species, thereby screening the migration force for their redeposition. Combined with this theoretical result, we verify a strong correlation between the amount of dissolved Pt and the hydrolysis pKa or acidity of AM+, indicating that the AM+ identity determines the local OH- concentration and thereby modifies the amount of Pt dissolution. Our results underscore the need to tune the EDL structure to achieve durable electrocatalysis, a promising area for future research.

Accounting for Ion Pairing Effects on Sulfate Salt Sorption in Cation Exchange Membranes

Ion exchange membranes (IEMs) are frequently used in water treatment and electrochemical applications, with their ion separation properties largely governed by equilibrium ion partitioning between a membrane and contiguous solution. Despite an expansive literature on IEMs, the influence of electrolyte association (i.e., ion pairing) on ion sorption remains relatively unexplored. In this study, salt sorption in two commercial cation exchange membranes equilibrated with 0.01-1.0 M MgSO₄ and Na₂SO₄ is investigated experimentally and theoretically. Association measurements of salt solutions using conductometric experiments and the Stokes-Einstein approximation show significant concentrations of ion pairs in MgSO₄ and Na₂SO₄ relative to those in simple electrolytes (i.e., NaCl), which is consistent with prior studies of sulfate salts. The Manning/Donnan model, developed and validated for halide salts in previous studies, substantially underpredicts sulfate sorption measurements, presumably due to ion pairing effects not accounted for in this established theory. These findings suggest that ion pairing can enhance salt sorption in IEMs due to partitioning of reduced valence species. By reformulating the Donnan and Manning models, a theoretical framework for predicting salt sorption in IEMs that explicitly considers electrolyte association is developed. Remarkably, theoretical predictions of sulfate sorption are improved by over an order of magnitude by accounting for ion speciation. In some cases, good quantitative agreement is observed between theoretical and experimental values for external salt concentrations between 0.1 and 1.0 M using no adjustable parameters.

Ion and Water Transport in Ion-Exchange Membranes for Power Generation Systems: Guidelines for Modeling

Artificial ion-exchange and other charged membranes, such as biomembranes, are self-organizing nanomaterials built from macromolecules. The interactions of fragments of macromolecules results in phase separation and the formation of ion-conducting channels. The properties conditioned by the structure of charged membranes determine their application in separation processes (water treatment, electrolyte concentration, food industry and others), energy (reverse electrodialysis, fuel cells and others), and chlore-alkali production and others. The purpose of this review is to provide guidelines for modeling the transport of ions and water in charged membranes, as well as to describe the latest advances in this field with a focus on power generation systems. We briefly describe the main structural elements of charged membranes which determine their ion and water transport characteristics. The main governing equations and the most commonly used theories and assumptions are presented and analyzed. The known models are classified and then described based on the information about the equations and the assumptions they are based on. Most attention is paid to the models which have the greatest impact and are most frequently used in the literature. Among them, we focus on recent models developed for proton-exchange membranes used in fuel cells and for membranes applied in reverse electrodialysis.