Equilibrium Partitioning of Monovalent/Divalent Cation-Salt Mixtures in Nafion Cation-Exchange Membranes

Energy harvesting from water streaming at charged surface

Water flowing at a charged surface may produce electricity, known as streaming current/potentials, which may be traced back to the 19th century. However, due to the low gained power and efficiencies, the energy conversion from streaming current was far from usable. The emergence of micro/nanofluidic technology and nanomaterials significantly increases the power (density) and energy conversion efficiency. In this review, we conclude the fundamentals and recent progress in electrical double layers at the charged surface. We estimate the generated power by hydrodynamic energy dissipation in multi-scaling flows considering the viscous systems with slipping boundary and inertia systems. Then, we review the coupling of volume flow and current flow by the Onsager relation, as well as the figure of merits and efficiency. We summarize the state-of-the-art of electrokinetic energy conversions, including critical performance metrics such as efficiencies, power densities, and generated voltages in various systems. We discuss the advantages and possible constraints by the figure of merits, including single-phase flow and flying droplets.

Single and binary ion sorption equilibria of monovalent and divalent ions in commercial ion exchange membranes

The co-ion and counter-ion sorption of monovalent (Na⁺, K⁺, Cl^- and NO₃^-) and divalent ions (Ca²⁺ and SO₄^2-) in commercial Neosepta ion exchange membranes were systemically studied in both single and binary salt systems. The new generation of Neosepta cation exchange membrane (CSE) showed a significant difference in water uptake and co-ion sorption compared to the earlier generation (CMX). Use of the Manning model confirmed that there were significant differences between these membranes, with the estimated value of the Manning parameter changing from 1.0 ± 0.1 for CMX to 2.8 ± 0.5 for CSE. There were fewer differences between the two Neosepta anion exchange membranes, AMX and ASE. In single salt solutions, potassium sorbed most strongly into the cation exchange membranes, but in binary salt mixtures, calcium dominated due to Donnan exclusion at low concentrations. While these trends were expected, the sorption behaviour in the anion exchange membranes was more complex. The water uptake of both AMX and ASE was shown to be the greatest in Na₂SO₄ solutions. This strong water uptake was reflected in strong sorption of sulphate ions in a single salt solution. Conversely, in a binary salt mixture with NaCl, sulphate sorption fell significantly at higher concentrations. This was possibly caused by ion pairing within the solution, as well as the strongly hydrophobic nature of styrene in the charged polymer. Water uptake was lowest in NaNO₃ solutions, even though sorption of the nitrate ion was comparable to that of chloride in these single salt solutions. In the binary mixture, nitrate was absorbed more strongly than chloride. These results could be due to the low surface charge density of this ion allowing it to bond more strongly with the hydrophobic polymeric backbone at the exclusion of water and other ions.

Enhancing Performance of Capacitive Deionization with Polyelectrolyte-Infiltrated Electrodes: Theory and Experimental Validation

The energy efficiency of capacitive deionization (CDI) with porous carbon electrodes is limited by the high ionic resistance of the macropores in the electrodes. In this study, we demonstrate a facile approach to improve the energy efficiency by filling the macropores with ion-conductive polyelectrolytes, which is termed polyelectrolyte-infiltrated CDI (pie-CDI or πCDI). In πCDI, the filled polyelectrolyte effectively turns the macropores into a charged ion-selective layer and thus increases the conductivity of macropores. We show experimentally that πCDI can save up to half of the energy consumption compared to membrane CDI, achieving identical desalination during the charging step. The energy consumption can be even lower if the process is operated at a smaller average salt adsorption rate. Further energy breakdown analysis based on a theoretical model confirms that the improved energy efficiency is largely attributed to the increased conductivity in the macropores.

Ion Transport in Laser-Induced Graphene Cation-Exchange Membrane Hybrids

Ion-exchange membranes hybridized with laser-induced graphene (LIG) might lead to membranes with functional surface effects such as antifouling, antibacterial, or joule heating effects; however, understanding the change in the electrical properties of the membrane is essential. Here we studied LIG-modified ion-exchange polymeric membranes using electrochemical impedance spectroscopy (EIS). The conductivity of the anionic sulfonated poly(ether sulfone) membranes and the effective capacitance of the membrane-electrolyte interface were obtained by fitting the EIS spectra to an electrochemical equivalent circuit and compared with LIG-modified nonionic poly(ether sulfone) films. The transport selectivity (as the relative permeability) of counterions (K⁺, Na⁺, Mg²⁺, Ca²⁺) across the membrane was quantified using the membrane's conductivities obtained from the EIS measurements. The total ohmic resistance of the membrane was directly correlated to the polymer thickness (with negligible contribution from the conductive LIG layer), thereby establishing EIS as a rapid, low-cost, and noninvasive method to accurately probe substrate usage in LIG modification.

Effect of Film Diffusion on the Ion-Exchange Kinetics of a Tracer Ion in Nafion-117 Membrane from a Mixture of Salt Solution

The ion-exchange kinetics of a tracer ion (Cs⁺ and Ba²⁺) in presence of a bulk ion (Na⁺/H⁺) has been measured in Nafion-117 membrane for a range of concentrations of NaCl/HNO₃ using the nonstationary radiotracer method. A systematic increase in the ion-exchange rate and decrease in the partition coefficients of the tracer ions between membrane and solution have been observed with the increase in bulk ion concentration. The sigmoidal nature of experimental profiles indicates film-diffusion-controlled kinetics even for well-stirred solutions. In the absence of an existing analytical or numerical solution, a simple empirical approach has been proposed to find the variable membrane surface concentration and has been used to solve the membrane diffusion equation by the finite difference method. The fitting of the experimental curves with a single diffusion coefficient for Cs⁺/Ba²⁺ has been achieved. The exchange rate has been found to be independent of the stirring speed beyond a limiting speed.

Transport Properties of Multivalent Cations in Nafion-117 Membrane with Mixed Ionic Composition

The transport characteristics of multivalent cations like Ba(2+) and Eu(3+) have been studied in bi-ionic form of the Nafion-117 membrane. The membranes have been prepared by loading different proportions of H(+)-Ba(2+)/Mg(2+)-Ba(2+)/Ba(2+)-Eu(3+)/H(+)-Eu(3+)/Na(+)-Eu(3+). The cationic compositions of the membranes have been determined from the measured ion exchange isotherms. Results show that the self-diffusion coefficient of Ba(2+) (D(Ba)) in H-Ba/Mg-Ba systems as well as the self-diffusion coefficient of Eu(3+) (D(Eu)) in H-Eu/Na-Eu systems are strongly dependent on the membrane ionic compositions and decreased continuously with increasing concentration of the highly hydrated ions (H(+)/Na(+)/Mg(2+)) in the membrane. Increase in the proportion of H(+)/Na(+)/Mg(2+) ions in the membrane increases the effective charge on the membrane matrix. This causes stronger electrostatic interaction of the less hydrated multivalent ions (Ba(2+)/Eu(3+)) with the membrane matrix charges, which ultimately results in their slower self-diffusion coefficients. The higher the valence, the stronger the electrostatic interaction is with the fixed ionic charges; hence, in general, D(Eu) is affected more as compared to D(Ba). On the basis of the free-volume theory for polymers, the effective interaction potential (Φ) of the Ba(2+) with the fixed ionic sites in the membrane has been calculated and found to be on the order of approximately millivolts. The higher the proportion of hydrated ion in the membrane, the higher the Φ is and the stronger the ion pair formation is with the fixed ionic sites in the membrane. However, in the Ba-Eu system, as the electrostatic interactions of the two ions with the membrane matrix are close, D(Ba) and D(Eu) are independent of the membrane ionic composition. The ionic composition dependence of D(Ba) in the H-Ba system is reflected in the transport rate of Ba(2+), showing the importance of such measurements in understanding the transport characteristics of the membrane.

Characterization of a membrane-based, electrochemically driven pumping system using aqueous electrolyte solutions

Electrokinetic flow provides a mechanism for a variety of fluid pumping schemes. The design and characterization of an electrochemically driven pump that utilizes porous carbon electrodes, iodide/triiodide redox electrolytes, and Nafion membranes is described. Fluid pumping by the cell is reversible and controlled by the cell current. Chronopotentiometry experiments indicate that the total available fluid that can be pumped in a single electrolysis without gas evolution is determined solely by the initial concentration of electrolyte and the applied current. The magnitude of the fluid flow at a given current is determined by the nature of the cation in the electrolyte and by the water absorption properties of the Nafion membrane. For 1 M aqueous electrolytes, pumping rates ranging from 1 to 14 microL/min were obtained for current densities of 10-30 mA/cm2 of membrane area. Molar volume changes for the I3-/I- redox couple and for the alkali cation migration contribute little to the observed volumetric flow rates; the magnitude of the flow is dominated by the migration-induced flow of water.

Buffer salt effect on pH in the interior of an anion exchange resin

The internal pH of Q Sepharose Fast Flow anion exchange resin in equilibrium with a bis-tris acetate buffer solution is investigated as a function of buffer salt concentration. Direct evidence of a resin phase pH shift is presented. At low buffer salt concentrations of 20 mM NaCl the resin phase pH is found to be as much as 1.1 pH units greater than that of the buffer phase, approaching to within 0.1 units of the buffer phase at salt concentrations greater than 250 mM. An ideal model with no adjustable parameters based on the Boltzmann distribution and the electroneutrality condition provides excellent agreement with experimental observations. The model assumes that small ions do not bind to the resin fixed charge sites and the agreement between the model predictions and observed resin internal pH suggests that strong electrolytes do not form ion pairs with the resin fixed charge sites.