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

Counter-ion Conductivity and Selectivity Trade-Off for Commercial Ion-Exchange Membranes at High Salinities

The growing interest in using ion-exchange membranes (IEMs) for high-salinity applications such as brine concentration and produced water treatment necessitates a better understanding of their properties under relevant conditions. In this study, we examine the ion transport properties of 40 different commercial membranes contacted by either 1 or 5 m NaCl solutions. To sample a broad materials space, we selected 28 membranes marketed toward desalination applications and 12 membranes marketed toward energy applications. We quantified the equilibrium ion concentrations, salt permeability coefficients, and ionic conductivities of these membranes. Using these results, we derived the effective counter-ion and co-ion diffusion coefficients in the membranes and calculated the counter-ion conductivity and counter-ion/co-ion selectivity. There is a clear trade-off between the counter-ion conductivity and selectivity, with desalination membranes generally exhibiting combinations of high selectivity/low conductivity and energy IEMs generally exhibiting combinations of high conductivity/low selectivity. We decoupled the total selectivity into the partition and diffusion selectivity and correlated these parameters with membrane structural properties to establish structure/property relationships. The results of this study highlight shortcomings in the performance of these membranes and identify gaps in our fundamental understanding of ion transport in IEMs at high salinities

Counter-ion Conductivity and Selectivity Trade-Off for Commercial Ion-Exchange Membranes at High Salinities

The growing interest in using ion-exchange membranes (IEMs) for high-salinity applications such as brine concentration and produced water treatment necessitates a better understanding of their properties under relevant conditions. In this study, we examine the ion transport properties of 40 different commercial membranes contacted by either 1 or 5 m NaCl solutions. To sample a broad materials space, we selected 28 membranes marketed toward desalination applications and 12 membranes marketed toward energy applications. We quantified the equilibrium ion concentrations, salt permeability coefficients, and ionic conductivities of these membranes. Using these results, we derived the effective counter-ion and co-ion diffusion coefficients in the membranes and calculated the counter-ion conductivity and counter-ion/co-ion selectivity. There is a clear trade-off between the counter-ion conductivity and selectivity, with desalination membranes generally exhibiting combinations of high selectivity/low conductivity and energy IEMs generally exhibiting combinations of high conductivity/low selectivity. We decoupled the total selectivity into the partition and diffusion selectivity and correlated these parameters with membrane structural properties to establish structure/property relationships. The results of this study highlight shortcomings in the performance of these membranes and identify gaps in our fundamental understanding of ion transport in IEMs at high salinities

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 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

Nanoscale Transport Enables Active Self-Assembly of Millimeter-Scale Wires

Active self-assembly processes exploit an energy source to accelerate the movement of building blocks and intermediate structures and modify their interactions. A model system is the assembly of biotinylated microtubules partially coated with streptavidin into linear bundles as they glide on a surface coated with kinesin motor proteins. By tuning the assembly conditions, microtubule bundles with near millimeter length are created, demonstrating that active self-assembly is beneficial if components are too large for diffusive self-assembly but too small for robotic assembly

Electrosorption Integrated with Bipolar Membrane Water Dissociation: A Coupled Approach to Chemical-free Boron Removal

Boron removal from aqueous solutions has long persisted as a technological challenge, accounting for a disproportionately large fraction of the chemical and energy usage in seawater desalination and other industrial processes like lithium recovery. Here, we introduce a novel electrosorption-based boron removal technology with the capability to overcome the limitations of current state-of-the-art methods. Specifically, we incorporate a bipolar membrane (BPM) between a pair of porous carbon electrodes, demonstrating a synergized BPM–electrosorption process for the first time. The ion transport and charge transfer mechanisms of the BPM–electrosorption system are thoroughly investigated, confirming that water dissociation in the BPM is highly coupled with electrosorption of anions at the anode. We then demonstrate effective boron removal by the BPM–electrosorption system and verify that the mechanism for boron removal is electrosorption, as opposed to adsorption on the carbon electrodes or in the BPM. The effect of applied voltage on the boron removal performance is then evaluated, revealing that applied potentials above ∼1.0 V result in a decline in process efficiency due to the increased prevalence of detrimental Faradaic reactions at the anode. The BPM–electrosorption system is then directly compared with flow-through electrosorption, highlighting key advantages of the process with regard to boron sorption capacity and energy consumption. Overall, the BPM–electrosorption shows promising boron removal capability, with a sorption capacity >4.5 μmol g-C–1 and a corresponding specific energy consumption of –1

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

Membranes capable of differentiating between similarly charged ions could enable applications such as resource recovery from naturally occurring waters and industrial wastewaters. Understanding the factors that govern ion transport in these materials is crucial for designing such membranes. This study investigates the impact of membrane water content on the diffusion of monovalent cations in negatively charged membranes by using absolute reaction rate theory. The ion activation energy and entropy of diffusion in the membrane both increase substantially when most of the water is structurally bound. The increase in activation energy of diffusion is predicted by a model incorporating Coulombic interactions between the membrane fixed charges and counter-ions. The activation entropy of diffusion in the low water content membranes increases with increasing size of the hydrated cations, suggesting possible rearrangement in the primary hydration shells of strongly hydrated cations, such as Li+ and Na+, during diffusion. These results suggest that polymer tortuosity, Coulombic interactions, and water structure govern monovalent cation diffusion in negatively charged membranes

Chemically Enhancing Block Copolymers for Block-Selective Synthesis of Self-Assembled Metal Oxide Nanostructures

We report chemical modification of self-assembled block copolymer thin films by ultraviolet light that enhances the block-selective affinity of organometallic precursors otherwise lacking preference for either copolymer block. Sequential precursor loading and reaction facilitate formation of zinc oxide, titanium dioxide, and aluminum oxide nanostructures within the polystyrene domains of both lamellar- and cylindrical-phase modified polystyrene-<i>block</i>-poly(methyl methacrylate) thin film templates. Near-edge X-ray absorption fine structure measurements and Fourier transform infrared spectroscopy show that photo-oxidation by ultraviolet light creates Lewis basic groups within polystyrene, resulting in an increased Lewis base–acid interaction with the organometallic precursors. The approach provides a method for generating both aluminum oxide patterns and their corresponding inverses using the same block copolymer template