Tag: ion exchange membrane

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Improved membrane configurations for capacitive flow-electrode desalination

DOI: 10.13140/RG.2.2.19476.16002

With the growing global scarcity of drinking water, the demand for practical and energy-efficient desalination methods is on the rise. Among potential solutions are osmotic desalination methods like capacitive deionization and its advanced form, flow-electrode capacitive deionization (FCDI). Flow electrodes are streaming electrodes composed of conductive particles suspended in a liquid. When electrically charged, these particles behave like capacitors and gain capacitive properties.

In flow-electrode deionization, flowable carbon electrodes are combined with ion-exchange membranes. The use of membranes enables continuous and efficient desalination. Membranes induce a selective transport of charged ions, allowing oppositely charged ions (counterions) to pass while repelling similarly charged ions (co-ions). This selective ion transport is essential for targeted salt removal from the feed stream.

Research advancements have improved membrane properties, associated ion selectivity, and the design of galvanic cells, leading to practical applications. For example, flow-electrode deionization was tested for industrial feasibility in a pilot plant in 2023. Performance optimization depends significantly on understanding how ion transport behaves with different membrane configurations. Ion-exchange membranes play a key role in controlling ion movement. Certain membrane arrangements, such as membrane “sandwiches” made of anion and cation exchange membranes, significantly accelerated desalination. While promising results were achieved with simple salt solutions like NaCl and KCl, mixtures of diverse ions, as found naturally in seawater, are more challenging.

Researchers from RWTH Aachen University recently investigated how different ion-exchange membrane arrangements affect selective ion removal from complex salt mixtures, such as those containing carbonate and sulfate ions, in flow-electrode deionization. Their findings were published in the journal Desalination. Two deionization modules with different membrane setups were analyzed. Membrane layers were tested with the cation membrane on the inside and the anion membrane on the outside, and vice versa. Both configurations delivered similar desalination performance, but the time to reach a stable state varied depending on the arrangement and the flow electrode’s buffering capacity.

The tested anion exchange membranes showed a higher affinity for sulfate ions than for carbonate ions, which delayed stabilization in some cases. Strategies like reducing electrode volume and steering specific ions along the electrode path helped reach the steady state more quickly. These findings underscore the importance of membrane selectivity, electrode properties, and system design in enhancing the performance of flow-electrode deionization, especially for mixed-ion water sources.

The effectiveness of the process depends not just on reaching a steady desalination state but also on managing ion selectivity and system adaptation. With saltwater containing multiple cations and anions, membrane arrangement alone is not enough to achieve the desired results. To tackle these challenges, strategies like membrane coatings or modifying electrode properties must be considered.

The researchers also addressed evaluation methods in their study. Interestingly, measuring conductivity alone is not sufficient to assess desalination performance. While it indicates total salt concentration, it does not reflect changes in salt composition. Therefore, more precise evaluation methods are needed to meet specific requirements.

These advances are crucial for optimizing flow-electrode deionization performance and meeting the growing demand for efficient, adaptable water treatment technologies. At Frontis Energy, we are excited to see how this groundbreaking technology will scale in the future.

Mankertz, Theis, Linnartz, Wessling, 2025, Membrane arrangement influences time to steady state in FCDI with multi-ionic salt solutions, Desalination, Volume 613, 118939, DOI: 10.1016/j.desal.2025.118939.

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Highly efficient desalination using carbon nanotubes

Separating liquid compartments is not only important for generating energy in biological cells, respiration that is, but also for electrochemical cells and desalination through reverse osmosis and other processes. Therefore, scientists and engineers intensively research this field. We have already reported in several posts about promising attempts to make membranes cheaper and more effective. New nanomaterials have also been developed.

As a result of climatic changes caused by global warming, water scarcity is increasingly becoming a problem in many parts of the world. Settlements by the sea can secure their supply by desalinating water from seawater and brackish water sources. The process, however, is very energy intensive.

Now, researchers at California’s Lawrence Livermore National Laboratory (LLNL) have developed artificial pores made of carbon nanotubes that remove salt from water so efficiently that they are comparable to already available commercial desalination membranes. These tiny pores are only 0.8 nanometers in diameter. A human hair with a diameter of 60,000 nm. The researchers published the results in the journal Science Advances.

The predominant technology used to remove salt from water is reverse osmosis. A thin-film composite membrane (TCM) is used to separate water from ions. Hitherto the performance of these membranes has, however, been unsatisfactory. There is, for example, always a tradeoff between permeability and selectivity. In addition, exisiting membranes often show insufficient ion repulsion and are contaminated by traces of impurities. This requires additional cleaning stages, which again increase energy costs.

As is so often the case, the researchers got inspired by nature. Biological water channels, also known as aquaporins, are a great model for the structures that can improve performance. These aquaporins have extremely narrow internal pores that compress the water. This enables extremely high water permeability with transport rates of more than 1 billion water molecules per second per pore. Due to the low friction on the inner surfaces, carbon nanotubes represent one of the most promising approaches for artificial water channels.

The research group developed nanotube porins that insert themselves into artificial biomembranes. These engineered water channels simulate the functionality of aquaporin channels. The researchers measured the water and ion transport through their artificial porins. Computer simulations and experiments using the artificial porins in lipid membranes showed improved flux and strong ion repulsion in the channels of carbon nanotubes.

This measurement method can be used to determine the exact value of the water-salt permselectivity in such narrow carbon nanotubes. Atomic simulations provide a detailed molecular view of the novel channels. At Frontis Energy, we are excited about this promising approach and hope to see a commercial product soon.

(Image: Wikipedia)

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Reverse electrodialysis using Nafion™ membranes to produce renewable energy

In the order to address the global need for renewable and clean energy sources, salinity-gradient energy harvested by reverse electrodialysis (RED) is attracting significant interest in recent years. In addition, brine solution coming from seawater desalination is currently considered as a waste; however thanks to its high salinity it can be exploited as a valuable resource to feed RED. RED is an engineered adaptation of nature’s osmotic energy production where ions flow pass the cell membrane in order to produce the universal biological currency ATP. This energy is also harvested by the RED technology.

Now, more than ever there is need for sustainable and environmentally friendly technological solutions in order to keep up with ever growing demand for clean water and energy. The traditional linear way “produce and throw away” does no longer serve the society anymore and the new approach of circular economy has take a place, where any waste can be considered as a valuable resource for another process. In this respect, reverse electrodialysis is a promising electromembrane-based technology to generate power from concentrated solutions by harvesting the Gibbs free energy of mixing the solutions with different salinity. In particular, brine solutions produced in desalination plants, which is currently considered as a waste, can be used as concentrated streams in RED stack.

Avci et al. of the University of Calabria, Italy, have recently published their solution for brine disposal using RED-stack. They have realized that in order to maximize generated power, the high permselectivity and ion conductivity of membrane components in RED are essential. Although Nafion™ membranes are among the most prominent commercial cation exchange membrane solutions for electrochemical applications, no study has been done in its utilization toward RED processes. This was the first reported RED stack using Nafion™ membranes.

A typical RED unit is similar to an electrodialysis (ED) unit, which is a commercialized technology. ED uses a feed solution and the electrical energy, while producing concentrate and dilute, separately. On the other side, RED uses concentrated and dilute solutions that are mixed together in a controlled manner in order to produce spontaneously electrical energy. In a RED stack, repeating cells comprised of alternating cation and anion exchange membranes that are selective for anions and cations. The salinity gradient over each ion exchange membrane creates a voltage difference which is the driving force for the process. The ion exchange membranes are one of the most important components of a RED stack.

The performance of Nafion™ membranes (Nafion™ 117 and Nafion™ 115) have been evaluated under a high salinity gradient conditions for the possible application in RED. In order to simulate the natural environments of RED operation, NaCl solution as well as multicomponent NaCl + MgCl2 have been tested.

Gross power density under high salinity gradient and the effect of Mg2+ on the efficiency in energy conversion have been evaluated in single cell RED using Nafion™ 117, Nafion™ 115, CMX and Fuji-CEM-80050 as cation exchange membranes. Two commercial cation exchange membranes – CMX and Fuji-CEM 80050, frequently used for RED applications, have served as benchmark.

The results show that under the condition of 0.5 M / 4.0 M NaCl solutions, the highest Pd,max was achieved using Nafion™ membrane. This result is attributed to their outstanding permselectivity compared to other CEMs. In the presence of Mg2+ ions, Pd,max reduction of 17 and 20% for Nafion™ 115 and Nafion™ 117 were recorded, respectively. Both membranes maintained their low resistance; however a loss in permselectivity was measured under this condition. Even though, it was reported that Nafion™ membranes outperformed other commercial membranes such as CMX and Fuji-CEM-80050 for RED application.

(Photo: Wikipedia)