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 authors of this work would like to acknowledge the financial support from the LIFE program (LIFE19 ENV/ES/000143). The UC team wants to thank J.A. Abarca and F.J. Rodríguez-Oria for their help in impedance measurements and SGP-RED experiences. This work was also facilitated by REDstack BV in the Netherlands. REDstack BV aims to develop and market the ED and the RED technology. J.V. would like to thank his colleagues from the REDstack company for the fruitful discussions

High Efficiency in Energy Generation from Salinity Gradients with Reverse Electrodialysis

Renewable energy can be captured from the mixing of salt and fresh water in reverse electrodialysis. This paper investigates the energy efficiency of this process for feed waters that pass a reverse electrodialysis cell once and waters that pass multiple cells or electrode segments. So far, the maximum theoretical energy efficiency was considered to be 50% when the feed waters pass a single cell once; significantly higher efficiencies could only be obtained when the waters were recirculated or passed multiple electrodes. In this study, we show that the ion transport corresponding to the obtained energy and the electromotive force mutually influence each other, which enables capture of more than 50% (even up to 95%) of the theoretical energy, even when the feedwater streams pass a reverse electrodialysis cell only once

Concepts and Misconceptions Concerning the Influence of Divalent Ions on the Performance of Reverse Electrodialysis Using Natural Waters

Divalent ions have a negative effect on the obtained power and efficiency of the reverse electrodialysis (RED) process when using natural waters. These effects can largely be attributed to the interaction between the various ions and the membranes, resulting in a decreased membrane voltage, an increased membrane resistance, and uphill transport of divalent ions. The aim of this study was to investigate the causes of these differences and, if possible, to find underlying causes. The approach mainly followed that in literature articles that specifically focused on the effect of divalent ions on RED. It transpired that seven publications were useful because the methodology was well described and sufficient data was published. I found two widely shared misconceptions. The first concerns the role of the stack voltage in uphill transport of divalent ions; itis often thought that the open circuit voltage (OCV) must be taken into account, but it is plausible that the voltage under working conditions is the critical factor. The second debatable point concerns the methodology used to make a series of solutions to study the effect of divalent ions. Typically, solutions with a constant number of moles of salt are used; however, it is better to make a series with a constant ratio of equivalents of those salts. Moreover, it is plausible that the decreased voltage can be explained by the inherently lower Donnan potential of multi-charged ions and that increased resistance is caused by the fact that divalent ions—with a lower mobility there than the monovalent ions—occupy relatively much of the available space in the gel phase of the membrane. While both resistance and voltage play a decisive role in RED and probably also in other membrane processes like electrodialysis (ED), it is remarkable that there are so few publications that focus on measurements on individual membranes. The implications of these results is that research on the effect of divalent ions in RED, ED and similar processes needs to be more structured in the future. Relatively simple procedures can be developed for the determination of membrane resistance in solutions of mixtures of mono- and divalent salts. The same applies to determining the membrane potential. The challenge is to arrive at a standard method for equipment, methodology, and the composition of the test solutions

Inorganic Pseudo Ion Exchange Membranes—Concepts and Preliminary Experiments

Reverse electrodialysis (RED) is a method to produce electricity from the reversible mixing of two salt solutions with different concentrations. RED was first employed for energy generation using sea and river water. New fields of application are energy storage and heat-to-power conversion. In energy storage applications, a stack operates in ED mode during charge and in RED mode during discharge. In a heat-to-power system, the RED stack produces electricity and the outgoing solutions are returned to their original concentrations in a heat-driven regenerator. In both new applications, the salt solutions are circulated and there is a free choice of the combination of salt and membranes for optimal performance. However, classical polymer-based membranes have some disadvantages: they are less suited for operation at higher temperatures, have reduced permselectivity at higher concentrations, and are rather permeable to water, causing an imbalance of the feed waters. We developed a new concept of pseudo-membrane (PM): a metal sheet (sometimes covered with an insoluble salt) on which opposite electrochemical reactions occur at each side of the metal surface. Because a PM is dissolving at one side and growing at the other side during operation, the current should be inverted periodically. We tested a zinc sheet as a pseudo cation exchange membrane for Zn2+ ions and a silver chloride⁻covered silver plate as a pseudo anion exchange membrane for Cl− ions in three steps. First, a stack was built with Ag/AgCl membranes in combination with normal cation exchange membranes and operated with NaCl solutions. The next stack was based on Zn membranes together with normal anion exchange membranes. This stack was fed with ZnCl2 solutions. Finally, we tested a stack with zinc and Ag/AgCl pseudo-membranes with a ZnCl2 solution. The latter RED system worked; however, after standing for one night, the stack did not function and appeared to be damaged by redox reactions. This failure was the basis for general considerations about the possibilities of ED and RED hybrid stacks, consisting of a combination of classical and pseudo ion exchange membranes. Finally, we consider the possibility of using intercalation electrodes as a pseudo-membrane

Electrical Power from Sea and River Water by Reverse Electrodialysis: A First Step from the Laboratory to a Real Power Plant

Electricity can be produced directly with reverse electrodialysis (RED) from the reversible mixing of two solutions of different salinity, for example, sea and river water. The literature published so far on RED was based on experiments with relatively small stacks with cell dimensions less than 10 × 10 cm2. For the implementation of the RED technique, it is necessary to know the challenges associated with a larger system. In the present study we show the performance of a scaled-up RED stack, equipped with 50 cells, each measuring 25 × 75 cm2. A single cell consists of an AEM (anion exchange membrane) and a CEM (cation exchange membrane) and therefore, the total active membrane area in the stack is 18.75 m2. This is the largest dimension of a reverse electrodialysis stack published so far. By comparing the performance of this stack with a small stack (10 × 10 cm2, 50 cells) it was found that the key performance parameter to maximal power density is the hydrodynamic design of the stack. The power densities of the different stacks depend on the residence time of the fluids in the stack. For the large stack this was negatively affected by the increased hydrodynamic losses due to the longer flow path. It was also found that the large stack generated more power when the sea and river water were flowing in co-current operation. Co-current flow has other advantages, the local pressure differences between sea and river water compartments are low, hence preventing leakage around the internal manifolds and through pinholes in the membranes. Low pressure differences also enable the use of very thin membranes (with low electrical resistance) as well as very open spacers (with low hydrodynamic losses) in the future. Moreover, we showed that the use of segmented electrodes increase the power output by 11%.

Een molecuul als detector van sub-golflengte veldverdelingen

Optische velden op nanometer schaal zijn slechts te adresseren door bronnen/detectoren met nanometer dimensies. Enerzijds kan men bron- of detector-dimensies verkleinen tot subgolflengte: nabije-veld optische probes; anderzijds kan men moleculen of nanokristallen inzetten om het locale veld in detecteerbare emissie om te zetten. Hier beschrijven wij de combinatie: individuele moleculen in het locale optische veld van een nabije-veld probe. Nabije-veld optische microscopie (near-field scanning optical microscopy, NSOM) vereist een nabije-veld probe van hoge kwaliteit met een bronopening die substantieel kleiner is dan de gebruikte golflengte. Recentelijk hebben we hoge definitie NSOM-probes ontwikkeld door middel van etsen met gefocusseerde ionen bundel (focused ion beam, FIB) (Veerman et al., 1998). Deze probes geven een “grote” helderheid (~1 microWatt), bij < 80 nm opening (~ 104 W/cm2), polarisatie extinctie ratio > 1:100, een goede rand-scherpte (6 nm) en een vlakker oppervlak dan conventionele NSOM-probes. Een eerste indruk van deze optische nano-bronnen verkrijgen wij door beelden te maken met behulp van een FIB, met verre-veld optische metingen en met een schuif-kracht (shear force) microscoop (SFM). De FIB-beelden verschaffen geometrische informatie, en de verre-veld optische metingen leveren transmissie- en polarisatiekarakteristieken op. Shear force microscopie is een niet eerder gebruikte methode om de grootte van de opening en het oppervlak van de probe te bestuderen (met een ruwheid kleiner dan 1.5 nm). De locale optische velden hebben wij bepaald met single molecule imaging in het nabije veld. Hier demonstreren we het gebruik van individuele moleculen om de volledige driedimensionale optische nabije veld-distributie van de probe te meten met een ruimtelijke resolutie op moleculaire schaal. De single molecule images vertonen verschillende intensiteitspatronen, afhankelijk van de oriëntatie van de moleculen ten opzichte van de probe. Uit de patronen is zowel de positie als de dipool-orientatie van de individuele moleculen direct te bepalen. De optische resolutie van de moleculaire patronen wordt niet bepaald door de grootte van de opening, maar door de hoge gradiënt van het optische veld aan de rand van de opening