Electrodialysis, reverse electrodialysis, and related electrochemical processes are increasingly important technologies for water purification and renewable energy generation and storage. The electrical efficiency of these processes is directly related to the permselectivity of the ion exchange membranes (IEMs) – defined as the extent to which the membrane permits the passage of counter-ions (ions of opposite charge to the membrane, e.g., cations for a cation exchange membrane) while blocking passage of co-ions. Permselectivity is not a material constant, but rather depends on the concentration and composition of the electrolyte solutions in contact with the IEM. Thus, even though permselectivity is routinely measured at standardized conditions (usually 0.5 M/0.1 M NaCl or KCl), the practical utility of such data is limited because we lack an accurate, quantitative way of using it to predict permselectivity under relevant process conditions. Moreover, the concentration dependence of IEM permselectivity has historically been studied primarily by evaluating the performance of (reverse) electrodialysis stacks rather than individual membranes, which has made it difficult to relate the concentration dependence of permselectivity to specific membrane characteristics. In this study, we measured the permselectivity of four commercial IEMs in six different concentration gradients employing 4 M and 0.5 M NaCl as the high salt concentration. We then constructed a predictive model of membrane permselectivity based on the extended Nernst-Planck equation and investigated how accounting for convection and electrostatic effects (via Manning’s counter-ion condensation theory) affected model accuracy. We demonstrate that accurate, quantitative predictions of IEM permselectivity as a function of external salt concentrations are possible and require knowledge of only four easily measured membrane properties: water uptake, water permeability, charge, and thickness.

Reverse osmosis (RO), nanofiltration (NF), and ion exchange (IX) membranes are becoming increasingly important in water treatment, waste recovery, industrial product purification, renewable energy generation, and energy storage. While all three types of membranes are charged, dense polymers, each has historically been characterized using different methods relevant to their respective applications. This bifurcated characterization approach has obscured similarities among dense membranes that could potentially be exploited to advance membrane development. For example, we recently showed that the water and salt transport properties of commercial IX membranes, which are not frequently reported, are generally on the same order of magnitude as those of other desalination polymers. These findings beg the question whether IX membrane polymers might offer any advantages over RO/NF membranes for pressure-driven desalination, and invite further comparisons among the two different classes of membrane polymers (e.g., IX and RO/NF membranes). In this study, we used the solution-diffusion model as a common framework to compare the permeability, partition and diffusion coefficients, water permeance, and salt rejection of twenty commercial IX membranes with those of the active layers of commercial RO/NF membranes and other membrane polymers. Our analysis shows that the characteristics of all membranes fall within similar ranges, despite differences in intended use (e.g. pressure-driven vs. electric field-driven separations). Thus, the low water permeance of IX membranes compared to RO/NF membranes can be explained primarily by differences in thickness rather than permeability. We also show that IX membranes have excellent water/salt partitioning selectivity, while RO/NF active layers have superior diffusion selectivity, and discuss the implications of this comparison for ongoing membrane research.

Ion exchange membranes (IEMs) are versatile materials relevant to a variety of water and waste treatment, energy production, and industrial separation processes. The defining characteristic of IEMs is their ability to selectively allow positive or negative ions to permeate, which is referred to as permselectivity. Measured values of permselectivity that equal unity (corresponding to a perfectly selective membrane) or exceed unity (theoretically impossible) have been reported for cation exchange membranes (CEMs). Such nonphysical results call into question our ability to correctly measure this crucial membrane property. Because weighing errors, temperature, and measurement uncertainty have been shown to not explain these anomalous permselectivity results, we hypothesized that a possible explanation are junction potentials that occur at the tips of reference electrodes. In this work, we tested this hypothesis by comparing permselectivity values obtained from bare Ag/AgCl wire electrodes (which have no junction) to values obtained from single-junction reference electrodes containing two different electrolytes. We show that permselectivity values obtained using reference electrodes with junctions were greater than unity for CEMs. In contrast, electrodes without junctions always produced permselectivities lower than unity. Electrodes with junctions also resulted in artificially low permselectivity values for AEMs compared to electrodes without junctions. Thus, we conclude that junctions in reference electrodes introduce two biases into results in the IEM literature: (i) permselectivity values larger than unity for CEMs and (ii) lower permselectivity values for AEMs compared to those for CEMs. These biases can be avoided by using electrodes without a junction.

Reverse electrodialysis has long been recognized as a tool for harnessing free energy from salinity gradients but has received little attention for its potential in energy storage applications. Here we present the experimental and modeled performance of a rechargeable electrodialytic battery system developed for the purpose of energy storage. Experimental round-trip energy efficiency ranged from 21.2% to 34.0% when cycling the system between 33% and 40-90% state of charge. A mass transport model based on chemical thermodynamics is also proposed to describe the system’s performance. Results indicate that, upon model calibration, the model effectively predicts experimental values. Experimental and modeled results suggest that the membrane resistance and osmosis are the primary sources of ohmic and faradaic energy losses, respectively. The results demonstrate that a functioning battery can be constructed using typical reverse electrodialysis stack components. Future improvements in membrane technology and optimization of the system chemistry offer promising avenues to improve the power density, energy density, and round-trip energy efficiency of the process.