Tag: cation exchange membrane

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Polyelectrolyte coatings for ion-exchange membranes in electrodialysis

Reverse electrodialysis water purification

DOI: 10.13140/RG.2.2.20145.13929

Ion exchange membranes are key components for various electrochemical technologies in water treatment and energy storage, such as electrodialysis, membrane electrolysis, and flow batteries. These membranes are characterized by a high concentration of charged groups, which can be either cationic (positively charged) or anionic (negatively charged). The function of an ion exchange membrane is to facilitate the transport of counterions while limiting the loss of water and co-ions.

The efficiency of cation exchange membranes is affected by unwanted co-ion and water transport. The transport of hydroxide ions (OH−) through cation exchange membranes is of particular interest. Depending on the application, cation exchange membranes are designed either to selectively facilitate hydroxide transport or to minimize hydroxide loss. Therefore, improved ion exchange membranes must support such additional functionalities.

Researchers at Wageningen University have characterized ion exchange and water transport through both coated and uncoated cation exchange membranes. The scientists published their findings in the Journal of Membrane Science. In their study, they examined cation exchange membrane coating with polyelectrolytes made of polyallylamine and polystyrene sulfonic acid.

The researchers coated one side of commercial cation exchange membranes with double layers of these two polymers. They then studied ion and water transport in diffusion dialysis and electrodialysis. Diffusion dialysis involves passive ion transport driven by concentration gradients, while in electrodialysis, ion transport occurs actively and is powered by an applied current.

The coatings were evaluated for their selectivity for monovalent and divalent ions. This selectivity affects hydroxide transport and water permeability. Both are key factors for the efficiency of bipolar membrane electrodialysis, where solutions containing multivalent cations such as magnesium and calcium are treated.

Magnesium and calcium transport was significantly limited by the coatings, while sodium ion transport remained largely unaffected. This selectivity was attributed to the Donnan exclusion mechanism and differences in hydration shells, as multivalent ions have a higher resistance within the cation exchange membrane.

Orientation is crucial in this context. Coating alignment affected performance. Resistance increased in the direction of multivalent ion flow, which reduced the flow of magnesium ions. This finding is impotant for the design of devices for bipolar membrane electrodialysis.

Surprisingly, the coatings did not reduce water crossover. Denser layers remained the bottleneck. The hydroxide flow was somewhat higher in coated membranes exposed to extreme pH values. This was likely due to structural changes during the coating process.

The combination of a low-water-content cation exchange membrane with a coating could enable the direct use of untreated salt solutions in bipolar membrane electrodialysis. This would reduce pretreatment costs and improve sustainability. The Fuji CEM-12 proved to be a promising candidate for future designs with coatings.

Salt diffusion through uncoated cation exchange membranes was mainly determined by the type of anion, such as chloride, sulfate, or hydroxide. In addition, membrane properties, including water content and ionic charge density, had a significant influence. The ionic charge density determined the anion distribution within the cation exchange membranes.

In summary, the researchers coated various commercial cation exchange membranes multiple times on one side with polyelectrolytes. For uncoated cation exchange membranes, water permeability correlated well with  ionic membrane resistance. This correlation was due to both parameters being dependent on the water content of the membrane. Moreover, permeability for co-ions increased with a higher volume fraction of water in the membranes.

Osmotic water transport in cation exchange membranes was not affected by the multiple layers of polyallylamine and polystyrene sulfonic acid. The researchers recommended single layer coating of low-water-content cation exchange membranes to minimize the transport of hydroxides and problematic multivalent cations.

This work demonstrates that surface modification using polyelectrolyte layers can enhance the functionality of conventional membranes without significant trade-offs. Water transport remained a challenge but the ability to block multivalent ions while maintaining conductivity for sodium ions represented a major step toward more efficient and cost-effective dialysis systems.

At Frontis Energy, we are excited about the future application of multilayered membranes on an industrial scale.

Elozeiri et al. 2026, Water and co-ion transport across ion-exchange membranes coated with PAH/PSS polyelectrolyte multilayer in electrodialysis and diffusion dialysis, Journal of Membrane Science,741, 125072, DOI: 10.1016/j.memsci.2025.125072

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Advancing wastewater sustainability: Nafion-powered ammonium recovery

Wastewater treatment plant Bern

DOI: 10.13140/RG.2.2.24268.68488

With global population growth and the resulting increase in environmental stress, the need for sustainable wastewater treatment is becoming ever more urgent. Traditional methods focus on removing pollutants but often overlook the opportunity to recover valuable resources. One such resource is ammonium. This nitrogen-containing molecule promotes growth and is a key component of fertilizers. When mishandled, such as through over-fertilization, ammonium becomes one of the main contributors to nitrogen pollution.

A promising solution lies in bioelectrical systems. This umbrella term refers to innovative technologies that not only purify wastewater but also recover resources like ammonium. At the same time, bioelectrical systems generate clean energy such as electricity or biogas. The technology is based on galvanic cells, where the two cell chambers are often separated by a membrane. High-performance cation exchange membranes enable precise ion transport and system stability. The premium product among cation exchange membranes is Nafion, such as our Nafion 115 membrane.

At Frontis Energy, we have demonstrated that bioelectrochemical systems can remove ammonium from wastewater, offering an energy-efficient alternative to the energy-intensive Haber-Bosch process. To validate this concept, we developed microbiological electrolysis cells populated with microorganisms from oxygen-deprived marine sediments off the coast of Namibia. These sediments are naturally rich in ammonia and low in organic carbon, ideal conditions for microbes capable of anaerobic ammonium oxidation. For comparison, we also used conventional municipal wastewater to populate the electrodes.

Maintaining anoxic conditions was crucial to avoid nitrification, a process that transfers electrons directly to oxygen, bypassing the anode and resulting in energy loss and reduced hydrogen production. Instead, we regulated the anode potential between +150 mV and +550 mV, well below the redox potential required for water oxidation (+820 mV). This configuration enabled the oxidation of ammonium to nitrogen gas (N₂) at the anode, while hydrogen (H₂) or methane gas was produced at the cathode.

Central to this process is Nafion 115, a membrane made of perfluorosulfonic acid polymers (PFSA polymers). Its exceptional proton conductivity, chemical resistance, and mechanical robustness make it ideal for demanding wastewater environments. Nafion 115 acts like a selective gate, allowing ammonium ions (NH₄⁺) to migrate from the anode to the cathode while blocking competing ions and maintaining anoxic conditions. This selective transport, driven by electric field gradients and concentration differences, ensures efficient nutrient recovery and stable performance of the bioelectrical system.

A practical validation of this technology comes from our earlier report, in which researchers developed a two-chamber, anoxic bioelectrical reactor that continuously removed ammonium at a rate of about 5 g/m³/day. Their system converted over 97% of the ammonium directly into nitrogen gas. This transformation occurred without the formation of harmful byproducts like nitrite or NOx gases. Particularly impressive was the energy consumption, just 0.13 kWh per kilogram of nitrogen removed. That is a 35-fold reduction compared to conventional aeration, which typically requires around 5 kWh/kg.

These results highlight the transformative potential of bioelectrical systems. As mentioned earlier, significant energy is used to remove nitrogen from wastewater, only to make it available again via the Haber-Bosch process, accounting for 1–2% of global energy consumption. Bioelectrical systems offer a circular alternative: by coupling ammonium oxidation with hydrogen production, wastewater treatment plants could become net energy producers. The generated hydrogen and biogas can be used directly for electricity generation and ultimately to reduce greenhouse gas emissions.

With the right biofilms, well-controlled electrode potentials, and robust membranes like Nafion 115, ammonium can serve as a clean, resource-efficient alternative to water electrolysis. This underscores the potential of bioelectrical systems to build a circular water economy, where waste is treated as a resource.

This technology reflects Frontis Energy’s commitment to promoting clean, efficient, and circular solutions that turn ecological challenges into sustainable opportunities.

Siegert and Tan, 2019, Electric stimulation of ammonotrophic methanogenesis, Frontiers in Energy Research 7:17, DOI: 10.3389/fenrg.2019.00017

Image: Wastewater treatment plant Bern

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Improved membrane configurations for capacitive flow-electrode desalination

DOI: 10.13140/RG.2.2.11561.25447

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.

Image: Shutterstock

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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)