Ion sorption and transport in ion exchange membranes : importance of counter-ion condensation

Due to their ability to selectively transport charged species (e.g., ions), ion exchange membranes (IEMs) are required for efficient operation of various membrane-based technologies for producing clean water and energy. Examples include electrodialysis, reverse electrodialysis, and fuel cells, among others. IEMs are also actively explored for use in processes that have not traditionally used them (e.g., reverse osmosis, forward osmosis, batteries, etc.). A molecular understanding of the relationship between polymer structure and water/ion transport properties could lead to new strategies for rational design of high performance membranes, improving efficiencies of membrane-based technologies and catalyzing their use in novel applications. However, despite the sustained relevance of ion exchange membranes, such molecular level understanding remains largely incomplete. This study is aimed at elucidating the main factors governing ion transport in ion exchange membranes. Fundamental models for equilibrium ion sorption and concentration gradient driven ion transport (i.e., salt permeability coefficients) in IEMs were developed using ideas from the polyelectrolyte literature (i.e., Manning’s counter-ion condensation theory). The framework presented in this dissertation accurately predicted, for the first time to the best of our knowledge, experimental equilibrium ion sorption and salt permeability results in a series of commercial IEMs with no adjustable parameters. The modeling results were used to establish a connection between polymer structure and ion transport properties. The influence of fixed charge group concentration on equilibrium ion sorption in IEMs was also investigated. A series of cation and anion exchange membranes having different fixed charge group concentrations but similar water content were synthesized. Equilibrium membrane ion concentrations were experimentally measured. Co-ion sorption in the membranes decreased with increasing fixed charge group concentration, presumably due to enhanced Donnan exclusion. However, the extent of co-ion sorption decrease in the cation exchange membranes was greater than that in the anion exchange membranes, despite similar changes in fixed charge group concentration, presumably due to polymerization induced phase separation in the cation exchange membranes. The experimental ion sorption data were interpreted within the framework of the ion sorption model presented in this study, and relations between membrane properties and equilibrium ion sorption in such materials were developed.Chemical Engineerin

Reformulating the permselectivity‐conductivity tradeoff relation in ion‐exchange membranes

Polymer membranes used in separation applications exhibit a tradeoff between permeability and selectivity. That is, membranes that are highly permeable tend to have low selectivity and vice versa. For ion‐exchange membranes used in applications such as electrodialysis and reverse electrodialysis, this tradeoff is expressed in terms of membrane permselectivity (i.e., ability to selectively permeate counter‐ions over co‐ions) and ionic conductivity (i.e., ability to transport ions in the presence of an electric field). The use of membrane permselectivity and ionic conductivity to illustrate a tradeoff between counter‐ion throughput and counter‐ion/co‐ion selectivity in ion‐exchange membranes complicates the analysis since permselectivity depends on the properties of the external solution and ionic conductivity depends on the transport of all mobile ions within a membrane. Furthermore, the use of these parameters restricts the analysis to ion‐exchange membranes used in applications in which counter‐ion/co‐ion selectivity is required. In this study, the permselectivity‐conductivity tradeoff relation for ion‐exchange membranes is reformulated in terms of ion concentrations and diffusion coefficients in the membrane. The reformulated framework enables a direct comparison between counter‐ion throughput and counter‐ion/co‐ion selectivity and is general. The generalizability of the reformulated tradeoff relation is demonstrated for cation‐exchange membranes used in vanadium redox flow batteries.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/171020/1/pol20210304-sup-0001-Supinfo.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/171020/2/pola30065.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/171020/3/pola30065_am.pd

Manning condensation in ion exchange membranes: A review on ion partitioning and diffusion models

The rational design of ion exchange membranes (IEMs) is becoming more pertinent as their usage becomes broader and as their staple applications (i.e., electrodialysis, flow batteries, and fuel cells) improve in commercial viability. Such efforts would be catalyzed by an improved fundamental understanding of ion transport in IEMs. This review discusses recent progress in modeling ion partitioning and diffusion in IEMs in an effort to relate IEM performance metrics to fundamental membrane properties over which researchers and membrane manufacturers possess direct and sometimes precise control. Central focus is given to the Donnan-Manning model for ion partitioning and the Manning-Meares model for ion diffusion in IEMs. These two frameworks, which are derived from Manning’s counter-ion condensation theory for polyelectrolyte solutions, have been widely used within the IEM literature since their recent introduction. To explore this topic, the mathematical derivation of both models is revisited, followed by a survey of experimental and computational discussions of counter-ion condensation in IEMs. Alternative models which fulfill similar roles in predicting IEM transport properties are compared. This review concludes by highlighting the uniquely favorable positions of the Donnan-Manning and Manning-Meares models and discussing their prospects as leading predictors of IEM partitioning and diffusive properties.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/175058/1/pola30290_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/175058/2/pola30290.pd

Predicting the Conductivity–Selectivity Trade-Off and Upper Bound in Ion-Exchange Membranes

Ion-exchange membranes (IEMs) are integral to electrochemical technologies utilized in water purification, energy generation, and energy storage. The effectiveness of these technologies is contingent upon the selective and rapid permeation of ions through IEMs. However, like most synthetic membranes, IEMs exhibit a trade-off between selectivity and permeability. Understanding the fundamental basis for this trade-off is essential for developing membranes that overcome this limitation. In this study, we present and validate a model that predicts the conductivity–selectivity trade-off in IEMs. We use this framework to assess the membrane structural properties that yield membranes at the frontier of this trade-off and then explore the potential for advancements in IEM design. Notably, the model predicts that preparing materials with higher charge densities could enhance performance by several orders of magnitude. This analysis unfolds a blueprint for substantial advancements in membrane design, potentially catalyzing breakthroughs in technologies for clean water and energy

Ion Activity Coefficients in Ion Exchange Polymers: Applicability of Manning’s Counterion Condensation Theory

Manning’s counterion condensation theory, originally developed for polyelectrolyte solutions, was used to predict ion activity coefficients in charged (i.e., ion exchange) membranes with no adjustable parameters. Equilibrium sodium and chloride ion concentrations in negatively and positively charged membranes were determined experimentally as a function of external NaCl concentration, and ion activity coefficients in the membranes were obtained via a thermodynamic treatment. Theoretical values for membrane ion activity coefficients obtained via Manning’s model were compared with those obtained experimentally. Good agreement was observed between the experimental and theoretical values for membrane ion activity coefficients, especially at higher external NaCl concentrations. However, some deviation between experimental and theoretical values was observed in the dilute regime. Manning’s model was also used to obtain activity coefficients for various electrolytes in ion exchange resins using ion sorption data from the literature, and these values were compared to those obtained experimentally

Ion-capture electrodialysis using multifunctional adsorptive membranes.

Technologies that can efficiently purify nontraditional water sources are needed to meet rising global demand for clean water. Water treatment plants typically require a series of costly separation units to achieve desalination and the removal of toxic trace contaminants such as heavy metals and boron. We report a series of robust, selective, and tunable adsorptive membranes that feature porous aromatic framework nanoparticles embedded within ion exchange polymers and demonstrate their use in an efficient, one-step separation strategy termed ion-capture electrodialysis. This process uses electrodialysis configurations with adsorptive membranes to simultaneously desalinate complex water sources and capture diverse target solutes with negligible capture of competing ions. Our methods are applicable to the development of efficient and selective multifunctional separations that use adsorptive membranes

Ion Diffusion Coefficients in Ion Exchange Membranes: Significance of Counterion Condensation

This study presents a new framework for extracting single ion diffusion coefficients in ion exchange membranes from experimental ion sorption, salt permeability, and ionic conductivity data. The framework was used to calculate cation and anion diffusion coefficients in a series of commercial ion exchange membranes contacted by aqueous NaCl solutions. Counterion diffusion coefficients were greater than co-ion diffusion coefficients for all membranes after accounting for inherent differences due to ion size. A model for ion diffusion coefficients in ion exchange membranes, incorporating ideas from counterion condensation theory, was proposed to interpret the experimental results. The model predicted co-ion diffusion coefficients reasonably well with no adjustable parameters, while a single adjustable parameter was required to accurately describe counterion diffusion coefficients. The results suggest that for cross-linked ion exchange membranes in which counterion condensation occurs condensed counterions migrate along the polymer backbone and contribute to a current in the presence of an externally applied electric field. Moreover, diffusion coefficients for condensed counterions were approximately 2–2.5 times greater than those for uncondensed counterions

Enhancing Water Splitting Activity and Chemical Stability of Zinc Oxide Nanowire Photoanodes with Ultrathin Titania Shells

Zinc oxide nanowire photoanodes are chemically stabilized by conformal growth of an ultrathin shell of titania through atomic layer deposition, permitting their stable operation for water splitting in a strongly alkaline solution. Because of the passivation of zinc oxide surface charge traps by titania coating, core/shell nanowire arrays supply a photocurrent density of 0.5 mA/cm² under simulated AM1.5G sunlight at the thermodynamic oxygen evolving potential, demonstrating 25% higher photoelectrochemical water splitting activity compared to as-grown zinc oxide wires. By thermally annealing the zinc oxide wire arrays prior to surface passivation, we further increase the photocurrent density to 0.7 mA/cm²the highest reported value for doped or undoped zinc oxide photoanodes studied under similar simulated sunlight. Photoexcitations at energies above the zinc oxide band gap are converted with efficiency greater than 80%. Photoluminescence measurements of the best-performing nanowire arrays are consistent with improved water splitting activity from removal of deep trap states