Tag: Wageningen University

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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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Alternating catholyte flow improves microbial electrosynthesis start-up

Microbial electrolysis is a technology that uses living microorganisms as electro-catalysts in electrolysis cells. The technology can be used for wastewater treatment. Earlier, we proposed that microbial electrolysis be used to decentralize wastewater treatment and biogas production. Since this is a process that converts CO2 into organic compounds using electricity it can also be used for CO2 valorization. Besides methane, such electrolysis cells produce compounds such as acetic acid (vinegar), caproic acid, and others. It is then called microbial electrosynthesis.

However, the main problem with microbial electrolysis and electrosynthesis is the long start-up time. The start-up time is the time required for the microorganisms to form a biofilm on the electrode surface and to start producing the desired products. It can range from several weeks to several months, depending on the operating conditions and the type of microorganisms. This long start-up time limits the feasibility and the scalability of microbial electrosynthesis, as well as its economic and environmental benefits.

Now, scientists of the Wageningen University in the Netherlands presented new research, which aimed to reduce the start-up time of microbial electrosynthesis. By using a novel technique that involves alternating the direction of the catholyte flow through a three-dimensional electrode they were able to reduce the startup time to only ten days. They hypothesized that this technique enhances mass transfer and biofilm formation, and thus accelerates the CO2 reduction and the product formation. This was a start-up time reduction of 50%, compared to a conventional flow-through electrode.

 

The alternating electrolyte flow also reduced the power consumption to 136 kWh per kg of hydrogen. After 60 days, the local hydrogen concentration at the cathode was at a maximum of 600 μM, which indicates a better mass transport and thus a more active biofilm. The researchers speculated that the alternating catholyte flow improved mass transport, because the hydrogen could be better distributed over the cathode layers. In addition, the researchers think that alternating the flow refreshed potential “dead zones” in the cathode chamber.

The pH in the catholyte was 5.8–6.8 and in optimal range for electrosynthetic microorganisms. Production of short and medium chain fatty acids was linked to the presence of microorganisms identified as Peptococcaceae and Clostridium sensu stricto 12 species. Hydrogenotrophic methanogenesis was also observed and was linked to Methanobrevibacter. The latter is a typical constituent of microbial electrolysis cells that use higher intermediate hydrogen concentrations for electrosynthesis at the cathode.

However, there are limitations of the technique, such as the energy efficiency, the product selectivity, and the scalability of microbial electrosynthesis. Such limitations are typical for bench top experiments. We are therefore looking forward to see an industrial application of this new method.

 

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Trace metals accelerate hydrogen evolution reaction of biocathodes in microbial electrolysis cells

It has been known that microbial biofilms on biocathodes improve the productions rates of hydrogen evolution reaction (HER). This is the process of producing hydrogen gas from water using electricity. The hydrogen evolution was even accelerated when the biofilm colonizing a biocathode was killed. Different types of bacteria, such as exoelectrogenic (Geobacter sulfurreducens), non-exoelectrogenic (Escherichia coli), and a hydrogenotrophic methanogen (Methanosarcina barkeri) accomplished the feat but Geobacter was the fastest. Even cell debris and metalloproteins catalyzed HER. Therefore, living cells are not required for enhanced HER, and biocathodes could be a cheap and environmentally friendly alternative to precious metal catalysts. While the authors back then speculated on the role of metalloproteins, a new publication in Electrochimica Acta by researchers of Wageningen University shows that indeed trace metals in the growth medium are responsible for the observed rate acceleration.

The authors used a mixture of metal compounds present in the microbial medium such as cobalt, copper, iron, manganese, molybdenum, nickel and zinc salts as well as the metal chelating agent ethylenediaminetetraacetic acid (EDTA) as the catalyst for the HER under microbial compatible conditions (near-neutral pH, mesophilic temperature, aquous electrolyte).

They performed a series of experiments to investigate the effect of different parameters on the catalytic activity and stability of the trace metal mix medium. These parameters included the concentration of the metal compounds, the presence or absence of EDTA, the type of electrode material, and the type of electrolyte. Various techniques to measure the cathodic current, the hydrogen production rate, the overpotential, and the exchange current density of the HER were used.

The results show that the trace metal mix medium increased the cathodic current and the electron recovery into hydrogen significantly, and that copper and molybdenum were the most active compounds in the mix. This is surprising because the previous publication found mostly cobalt and iron compounds on the surface of the biocathodes. Both of which are good hydrogen catalysts as well, whereas molybdenum sulfide for example, did not increase production rates in methanogenic microbial electrolysis cells. HER is the rate determining reaction in methanogenic electrolysis cells because it is the intermediate:

4 H2 + CO2 → CH4 + 2 H2O

The results also showed that removing EDTA from the mix improved the catalyst performance further, as EDTA acted as a complexing (chelating) agent that reduced the availability of metal ions for HER. The results also showed that carbon-based electrodes were more suitable than metal-based electrodes for HER, possibly because they have a higher surface area. This is an interesting result because it was previously thought that the mechanism behind the better performance of carbon electrodes was the microbial preference to adhere to carbon than to metal surfaces. The results also showed that using microbial growth medium as the electrolyte did not affect the catalyst performance significantly, as compared to using phosphate buffer solution.

The authors concluded that their method was a simple, cheap, and environmentally friendly way to prepare effective catalysts for HER using trace metals from microbial growth media. They suggested that these catalysts could be integrated in biological systems for in situ hydrogen production in bio-electrochemical and fermentation processes. Indeed, it is inevitable not to use trace metals in microbial electrolysis cells as they are essential to sustain growth.

Both articles demonstrate that trace metals can play an important role in the HER, and that they can be derived from biological sources. However, they also have some limitations and challenges, such as the stability, selectivity, and scalability of the catalysts. Therefore, further research is needed to optimize the performance and applicability of trace metal-based catalysts for HER.

(Image: US National Science Foundation)