Tag: Journal of Membrane Science

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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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Green alternative to fluorinated membranes in PEM fuel cells

Polymer electrolyte membrane (PEM) fuel cells have high power density, low operational temperatures. If PEM runs on green hydrogen, it doesn’t even emit carbon. But their fabrication requires perfluorinated sulfonic acid (PFSA) polymers as an electrolyte separator membrane and as an ionomer in the electrode, which is quite expensive. Nafion® is the leading commercial PFSA polymer in the market. However, its manufacturing is costly as well as environmentally hazardous. Therefore, low-cost, environmentally friendly PFSA polymer substitutes are the primary goals for the fuel cell scientific community worldwide.

Researchers of the Texas A&M University and Kraton Performance Polymers Inc. experimented using NEXAR ™ polymer membranes in hydrogen fuel cells, studying different ion exchange capacities. NEXAR ™ polymer membranes are commercially available sulfonated pentablock terpolymers. They published the results in the Journal of Membrane Science . Previous studies showed that changing the ion exchange capacity, that is, the degree of sulfonation of NEXAR ™ membranes can alter the nanoscale morphology and significantly affect mechanical properties. This may influence fuel cell performance. Hence, this polymer may be used as a membrane alternative to Nafion® in fuel cells.

Experimental procedure

  1. Materials under consideration: three different variants of the polymer were taken up each with different Ion Exchange Capacities (IECs: 2.0, 1.5, and 1.0 meq / g), which were named NEXAR ™ -2.0, NEXAR ™ -1.5, and NEXAR ™ – 1.0 respectively.
  2. NEXAR ™ membrane preparation: NEXAR ™ membranes were fabricated by casting the NEXAR ™ solutions onto a silicon-coated Mylar PET film using an automatic film applicator under ambient conditions. Two different sizes were manufactured for measuring mechanical properties and conductivity.
  3. NEXAR ™ membrane characterization: mechanical properties using the size 25 mm (L) x 0.5 mm (W) membrane as test pieces were determined and for proton conductivity test pieces of size 30 mm (L) x 10 mm (W) were tested upon.
  4. Nafion® electrode fabrication: conventional Nafion® electrodes were also fabricated as controls to conduct simultaneous tests.
  5. NEXAR ™ electrode fabrication: NEXAR ™ electrodes were prepared in 2 ways for the 2-part study, each with a different composition.
  6. Electrode characterization: electrode profiling was done using a scanning electron microscopy (SEM).
  7. Membrane electrode assembly (MEA) and fuel cell tests: MEAs were fabricated by placing the membrane in between two catalyst-coated gas diffusion layers (anode and cathode) and heat pressing. The entire fuel cell assembly consisted of an MEA, two gaskets, and two flow plates placed between copper current collectors followed by end plates all held together by bolts. Fuel cell performance tests were conducted under ambient pressure with saturated (100% RH) anode and cathode flow rates of 0.43 L / min hydrogen and 1.02 L / min oxygen respectively.
  8. Electrochemical impedance spectroscopy (EIS): electrochemical impedance spectroscopy was performed after the fuel cell tests and the results analyzed.

Results

NEXAR ™ -2.0 and NEXAR ™ -1.5 had a similar proton conductivity at all temperatures, suggesting that there is a maximum limit in proton conductivity. On the contrary, NEXAR ™ membranes, when compared to Nafion® NR-212 membranes , have sufficient proton conductivity to translate into high power density hydrogen fuel cell performance.

However, NEXAR ™ -2.0 and NEXAR ™ -1.5 membranes (with Nafion® as Ionomer) did not exhibit expected fuel cell performance at all fuel cell operating conditions (temperature, pressure, voltage and humidity). Surprisingly, the NEXAR ™ -1.0 membrane (with Nafion® as Ionomer) showed expected fuel cell performance across all fuel cell operating conditions and comparable power densities to Nafion® , suggesting that NEXAR ™ -1.0 may be a viable alternative to Nafion® in hydrogen fuel cells.

During fuel cell operation the NEXAR ™ -1.0 / NEXAR ™ -1.0 membrane-ionomer was thermally and mechanically stable. These results were supported by the power density results, where MEAs with NEXAR ™ -1.0 membrane-ionomers performed better than all the other MEAs.

From the above-mentioned results it became evident that the NEXAR ™ -1.0 variant was the optimal contender to substitute current state-of-the-art PFSA polymers.

Further, to understand the impact of the NEXAR ™ -1.0 ionomer on fuel cell performance, the composition of the ionomer and solvent mixture ratios in the catalyst ink solution were modified and investigated. Results suggested that NEXAR ™ -1.0 as an ionomer behaves similarly to Nafion® ionomers in fuel cell electrodes.

SEM analysis suggested that the amount of ionomer has a significant impact on the binding of ionomer to the catalyst particles, and consequently on the catalyst layer morphology. Therefore, there is an optimum catalyst / ionomer ratio of 2/1 for the Pt / C ionomer using NEXAR ™ -1.0 in fuel cell electrodes.

Conclusions

Ultimately, NEXAR ™ -1.0 is a potentially commercially viable greener substitute to Nafion® as a membrane and ionomer in PEM Fuel cell applications due to its high conductivity, however; alternative block compositions may improve the properties of the polymer to minimize resistances within the fuel cell to match the performance of Nafion® .

Overall Nafion® / Nafion® MEAs still showed the highest fuel cell performance when overall performance was taken into account but alternative hydrocarbon-based polymer compositions for the NEXAR ™ Polymer might provide a future non-fluorinated polymer as a  Nafion® substitute for PEM fuel cells .

Way forward

More analysis is required to perhaps get an accurate approximation of what variant of the NEXAR ™ polymer might cut the mark, future research may be focused upon exploring variants of Ion Exchange Capacities ranging from say 1 meq / g to 1.5 meq / g. But for now, it can be said that NEXAR ™ polymer shows promise as a viable replacement as a non-fluorinated membrane, and perhaps further research with more iterations of mechanical specs as well as chemical specs of the material we might witness a breakthrough.

Reference: https://doi.org/10.1016/j.memsci.2021.119330 : Sulfonated pentablock terpolymers as membranes and ionomers in hydrogen fuel cells , Journal of Membrane Science, 2021, 119330