Resistance Of Ion-exchange Membranes Research Articles

Resistance of Ion Exchange Membranes in Aqueous Mixtures of Monovalent and Divalent Ions and the Effect on Reverse Electrodialysis

Salinity gradient energy has gained attention in recent years as a renewable energy source, especially employing reverse electrodialysis technology (RED), which is based on the role of ion exchange membranes. In this context, many efforts have been developed by researchers from all over the world to advance the knowledge of this green source of energy. However, the influence of divalent ions on the performance of the technology has not been deeply studied. Basically, divalent ions are responsible for an increased membrane resistance and, therefore, for a decrease in voltage. This work focuses on the estimation of the resistance of the RED membrane working with water flows containing divalent ions, both theoretically by combining the one-thread model with the Donnan exclusion theory for the gel phase, as well as the experimental evaluation with Fumatech membranes FAS-50, FKS-50, FAS-PET-75, and FKS-PET-75. Furthermore, simulated results have been compared to data recently reported with different membranes. Besides, the influence of membrane resistance on the overall performance of reverse electrodialysis technology is evaluated to understand the impact of divalent ions in energy generation. Results reflect a minor effect of sulfate on the gross power in comparison to the effect of calcium and magnesium ions. Thus, this work takes a step forward in the knowledge of reverse electrodialysis technology and the extraction of salinity gradient energy by advancing the influence of divalent ions on energy recovery.

The Influence of Concentration and Temperature on the Membrane Resistance of Ion Exchange Membranes and the Levelised Cost of Hydrogen from Reverse Electrodialysis with Ammonium Bicarbonate.

The ohmic resistances of the anion and cation ion-exchange membranes (IEMs) that constitute a reverse electrodialysis system (RED) are of crucial importance for its performance. In this work, we study the influence of concentration (0.1 M, 0.5 M, 1 M and 2 M) of ammonium bicarbonate solutions on the ohmic resistances of ten commercial IEMs. We also studied the ohmic resistance at elevated temperature 313 K. Measurements have been performed with a direct two-electrode electrochemical impedance spectroscopy (EIS) method. As the ohmic resistance of the IEMs depends linearly on the membrane thickness, we measured the impedance for three different layered thicknesses, and the results were normalised. To gauge the role of the membrane resistances in the use of RED for production of hydrogen by use of waste heat, we used a thermodynamic and an economic model to study the impact of the ohmic resistance of the IEMs on hydrogen production rate, waste heat required, thermochemical conversion efficiency and the levelised cost of hydrogen. The highest performance was achieved with a stack made of FAS30 and CSO Type IEMs, producing hydrogen at 8.48× 10 kg ms with a waste heat requirement of 344 kWh kg hydrogen. This yielded an operating efficiency of 9.7% and a levelised cost of 7.80 € kg.

Impact of Solution Composition on the Resistance of Ion Exchange Membranes

Introduction: Resistance to ion transport in ion exchange membranes (IEMs) is detrimental to the performance of IEM-based processes such as electrodialysis (ED) and reverse electrodialysis (RED). IEM resistance depends on solution composition. However, such understanding is currently limited and of a qualitative nature (e.g., IEM resistance decreases with salt concentration in solution, multivalent ions increase IEM resistance). Therefore, a quantitative understanding of this dependency is needed to predict ED/RED performance. Accordingly, we measured the resistance of representative IEMs in single-solute solutions and used our results to draw quantitative and mechanistic conclusions. Methods: IEM resistance values were obtained using electrochemical impedance spectroscopy with the membrane mounted in a diffusion cell. The resistance of one representative cation (CEM) and one anion (AEM) exchange membrane was investigated in 15 single-solute solutions. Results: In general, membrane resistance for both the CEM and AEM decreased with increasing solution concentration. Membrane resistance was on the order of tens to hundreds of ohm.cm2 in dilute (<0.01M) solutions and a few to tens of ohm.cm2 in concentrated (>0.1M) solutions. The primary findings include: -Membrane resistance was sensitive to salt identity only in the case of the CEM, for which it depended on the counter-ion identity. -Membrane resistance increased with increasing hydration free energy of the counter-ion in bulk solution, indicating that steric effects are important determinants of membrane ionic resistance. -Membrane resistance had a strong inverse correlation with solution concentration below 0.1 M, and remained approximately constant at higher concentrations. -The dependence of membrane resistance on salt concentration in bulk solution was fitted well by a semi-empirical expression containing a concentration-independent term and a concentration-dependent term; the latter followed a non-linear dependence on bulk solution concentration. The non-linear dependence on bulk solution concentration suggests that the interconnectedness of the structural domains within the membrane that contribute to ionic resistance has a certain level of randomness. Future studies are required to fully understand the dependence of membrane ionic resistance on bulk solution concentration at low salt concentrations, including why membrane resistance depends on the co-ion identity only at relatively low salt concentrations (0.001-0.05 M). Conclusions: Overall, our analysis produced a quantitative understanding of the dependence of IEM resistance on the identity and properties of single-salt solutions, and contribute necessary information towards the development of a complete theory for IEM resistance. Specifically, we found that steric effects are a primary determinant of membrane resistance and that the structural domains are arranged randomly (as opposed to in series or parallel). Thus, future work to develop a complete IEM resistance theory should include (i) understanding how the hydration free energy of ions relates to their interactions with water and the membrane, (ii) elucidating how these interactions relate to ion mobility, and (iii) developing a mathematical model describing the random interconnectedness of structural membrane domains. Figure 1

Impact of solution composition on the resistance of ion exchange membranes

Resistance to ion transport in ion exchange membranes (IEMs) is detrimental to the performance of IEM-based processes. In this work we measured the resistance of representative IEMs, i.e. one cation (CEM) and one anion (AEM) exchange membrane, in 15 single-salt solutions using electrochemical impedance spectroscopy. Resistance was sensitive to solute identity only in the case of the CEM for which it depended on the counter-ion identity; the resistance of the CEM was mostly insensitive to the co-ion identity, and the resistance of the AEM was mostly insensitive to both the counter-ion and co-ion identity. For all solutes, membrane resistance decreased sharply with increasing solution concentration below 0.1 M, and remained approximately constant above 0.1 M. An empirical mathematical model comprising a concentration-dependent term and a concentration-independent term successfully described membrane resistance as a function of solution concentration. The model builds on that previously proposed by Galama et al. (JMS 467 (2014), 279–291). We found that for both membranes, the concentration-dependent and concentration-independent terms of the resistance increased with increasing counter-ion hydration free energy. This was rationalized as the energy barrier to counter-ions having to shed/reorient water molecules of hydration, due to steric effects, when permeating the membranes. Also for both membranes, the concentration-dependent term of the resistance generally had a non-linear relationship with salt concentration. This result suggests that the concentration-dependent term is not attributable to bulk solution, and that there is a degree of randomness to the interconnectedness between the different membrane regions that contribute to ionic resistance. Our findings improve the understanding of the relationships between electrolyte properties and IEM resistance, and provide tools for assessing IEM resistance. This improved understanding is critical to establishing a complete IEM resistance theory and to evaluating new applications for IEMs.

Sharp rise in resistance of ion exchange membranes in low concentration NaCl solution

Abstract Using a modified impedance measurement technique, ohmic resistance of commercial ion exchange membranes (Selemion CMV and AMV as well as Neosepta CMX and AMX) was measured in low concentration aqueous NaCl solutions. The ohmic resistance increased by an order of magnitude as the NaCl concentration was reduced from 0.5 M to 0.017 M Resistance of cation exchange membranes increased more than the anion exchange membranes. Resistance increased monotonically with a decrease in the NaCl concentration and the variation of resistance could be explained very well using a model based on the Kohlrausch law of conductivity. The model also provided value of the tortuosity for the membranes. Tortuosity of the membranes was between 3.4 and 4.5.

Properties and structure of heterogeneous ion-exchange membranes after exposure to chemical agents

This work deals with resistance of ion-exchange membranes in selected chemical solutions. Specimens of heterogeneous membranes were analyzed after exposure to chemically aggressive agents such as nitric acid, sodium hydroxide, etc. The analyses were carried out after period up to 6 months. The chemical resistance was evaluated from the mechanical and electrochemical property changes’ point of view before and after exposure. The tested materials were not only the membranes themselves but also their individual components, i.e., ion-exchange resins from various manufacturers and polyester reinforcing fabrics. The results show that exposure to NaOH solution has the most damaging effect especially on reinforcing fabric and a membrane as whole due mainly to dimension changes. The same stands for electrochemical resistance of the membrane and permselectivity. Ion-exchange capacity remains almost the same after exposure.

Transport limitations in ion exchange membranes at low salt concentrations

In this work we show that the electrical resistance of ion exchange membranes strongly depends on the solution concentration: especially at low solution concentrations (<0.1 M NaCl) we observe a very strong increase in electrical resistance of the membrane with decreasing concentration. To understand and clarify this behavior we systematically investigate the influence of the solution concentration on ion transport phenomena in two anion exchange membranes (Neosepta AMX and Fumasep FAD) and two cation exchange membranes (Neosepta CMX and Fumasep FKD) in the concentration range from 0.017 M to 0.5 M NaCl and for different hydrodynamic conditions. The results are highly valuable for processes that operate in the low concentration range (<0.5 M) such as reverse electrodialysis, electrodialysis, microbial fuel cells and capacitive deionization, where the standard membrane characterization values as usually determined in 0.5 M NaCl solutions do not represent the practical application.