Category: Paper of the Month

At Frontis Energy we believe that even in times of hyperactive publishing, there are still gems in the haystack. Here is what we believe matters.

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Advances in Ammonium Recovery with Bioelectrical Systems

DOI: 10.13140/RG.2.2.16014.22082

In recent decades, the focus of wastewater treatment has shifted from mere disposal to the recovery of valuable resources. This approach aims to harness energy and nutrients found in wastewater. Among emerging technologies are bioelectrical systems, which can recover not only energy and carbon but also valuable compounds like ammonium. Nitrogen bound in ammonium is a key component of fertilizers. Today, two-thirds of this resource are produced through the highly energy-intensive Haber-Bosch process using natural gas extracted from air. Fertilizer production significantly contributes to anthropogenic CO₂ emissions and, ultimately, to global warming.

Bioelectrical systems for ammonium recovery are classified into microbial fuel cells and microbial electrolysis cells. In both, electrogenic microorganisms oxidize organic matter in wastewater into carbon dioxide and protons. Exoelectrogens, also known as anode-respiring bacteria, use the anode as an electron acceptor instead of oxygen, thereby gaining energy for their metabolic processes.

By combining microbial activity with electrochemical processes, chemical reactions in wastewater treatment are catalyzed efficiently. This novel biocatalytic application still faces challenges in terms of optimization for practical use. At Frontis Energy, we have already demonstrated through a patented process that ammonium can be effectively removed from wastewater using bioelectrical systems. We are currently working on scaling this method for industrial deployment. However, a comprehensive understanding of the underlying processes and recovery mechanisms is still lacking.

A new study conducted by the Autonomous University of Barcelona investigated the development and optimization of bioelectrical systems aimed at recovering ammonium from wastewater in an energy-efficient and concentrated form. The findings were recently published in Bioelectrochemistry. Using a three-chamber configuration with a hydrophobic membrane, the researchers systematically examined the influence of different levels of electric current and ammonium concentrations on recovery efficiency. The system achieved its highest ammonium recovery rate of 55 g/m²/day at a current of 75 mA. Overall, a 97% removal of ammonium from a 0.3% solution was attained.

Notably, electrons flow from the anode to the cathode via an external circuit, where they react with an electron acceptor. In ammonium recovery systems, ammonium ions migrate from the anode to the cathode through a cation exchange membrane, driven by concentration gradients and the electric field, allowing them to accumulate in the cathode chamber.

While this ion transport mechanism supports efficient ammonium recovery, the researchers found that high-performance operation led to material wear at the cathode. This highlights the need to balance operational intensity with material durability. Consequently, the team explored different cathode materials and voltages. Stainless steel electrodes operated at 1.4 V yielded the best results, achieving a removal rate of 21 g/m²/day and a recovery rate of 17 g/m²/day, primarily due to enhanced cation migration resulting from higher current density.

Long-term experiments revealed that higher ammonium concentration in the anolyte significantly improves selective migration of ammonium ions through the cation exchange membrane, further boosting system performance. Operating at 1.4 V increased recovery efficiency and reduced energy consumption per gram of nitrogen—making the process more cost-effective and environmentally friendly.

These insights underscore the practical potential of bioelectrical systems as a pioneering solution for sustainable nitrogen recovery. By fine-tuning material selection, system design, and operational parameters, high ammonium removal and recovery rates can be achieved with minimal energy input.

From an industrial perspective, this study represents a scalable advance in resource recovery within existing wastewater treatment infrastructures. If scaled, the technology could reduce reliance on the energy-intensive Haber-Bosch process.

Since concentrated ammonium is a commercially viable product, its recovery reduces operational costs for wastewater treatment plants. If the scaled-up process maintains its long-term stability and low energy demand, these would be compelling arguments for adopting bioelectrical technologies as part of a circular economy.

At Frontis Energy, we see great potential in scaling this technology and making a meaningful contribution to sustainable wastewater treatment.

Image: Shutterstock

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Improved membrane configurations for capacitive flow-electrode desalination

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.

Image: Pixabay

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Electrolytic water splitting for binders in building material

The oceans are rich in magnesium resources, which that could be used in the production of construction materials. Sorel cement (magnesium cement), for example, can be used in interior building applications as an alternative to screed. Extracting magnesium from seawater traditionally requires a highly energy-intensive calcination process to isolate magnesium oxide (magnesia). The innovative method of electrolysis-controlled water splitting can bypass this process, significantly reducing CO₂ emissions.

To obtain the precursor of magnesia, magnesium hydroxide (Mg[OH]₂), an alkaline solution must be produced. While previous research has investigated electrochemical methods for hydroxide production, few studies have combined efficient alkali synthesis with the direct precipitation of magnesium hydroxide to make magnesia for low-carbon cement. This critical knowledge gap in optimizing energy and material efficiency has now been addressed.

A new study led by a research team at Columbia University used electrochemical water splitting at low voltages (1.6–2.0 V). Hydroxide ions (OH⁻) were generated from seawater through hydrogen production. This led to the direct precipitation of magnesium hydroxide. The findings were recently published in the journal Desalination. This new approach reduces energy intensity by 52–78%. Normally, the energy consumption per ton of MgO is 0.56 MWh. With the new method, carbon emissions per ton of magnesia can be reduced by up to 0.41 tons of CO₂.

To further improve production efficiency, the nanostructure of magnesium hydroxide was optimized using urea as a crosslinker. This enhanced its reactivity, porosity, and specific surface area. At an optimal urea concentration of 0.2 mol/L, magnesia particles exhibited excellent binding properties. The researchers attributed this to the sealing effects of rosette-shaped dypingite and rod-shaped nesquehonite. According to the authors, the formation of these minerals facilitates CO₂ incorporation and enhances carbonate hardening.

Advances in symmetric electrochemical systems, as demonstrated in this study, result in up to a 78% reduction in energy demand for the production of alkaline solutions. This gives these methods the potential to serve as viable alternatives to traditional processes. The further optimization of electrodes and electrolytes represents a pioneering approach to the carbon-neutral production of building materials and alkalis. Additionally, this method highlights how construction material manufacturing can efficiently lead to large-scale CO₂ mineralization. As a result, the greenhouse gas can be permanently removed from the atmosphere.

The industrial scaling of electrochemical alkali production can reduce operating costs, minimize environmental impact, and improve the properties of low-carbon building materials. The economic aspects of this manufacturing process are particularly noteworthy, as the demand for efficient binding materials continues to grow.

At Frontis Energy, we are committed to promoting sustainable and economically viable energy solutions. Research like this provides valuable insights and innovations to support such sustainable advancements.

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From waste heat to ultrapure water: A new technology transforming renewable hydrogen

Hydrogen (H₂), produced using renewable energy, has emerged as a possible alternative to fossil fuel. This versatile molecule can serve as an energy carrier, an efficient storage solution, and a sustainable feedstock for transportation, chemical processing, and energy systems worldwide.

Unlike fossil fuels, hydrogen produces no harmful emissions when used. It can be generated using electrolyzers running on renewable energy and abundant water as feedstock. It then becomes a renewable and sustainable energy source, reducing reliance on depleting fossil fuel reserves, helping combat climate change. Consequently, hydrogen production has become a key priority on the political agenda of numerous countries.

However, the water used in electrolyzers must be ultrapure in order to protect the electrodes of electrolyzers from poisoning and avoid chloride oxidation to chlorine. Abundant seawater adds several challenges when directly fed to electrolyzer plants for hydrogen production, making highly pure water, specifically ultrapure water, an expensive necessity. Ultrapure water is produced in a series of steps, including pretreatment to remove suspended solids and desalination to eliminate salts, organics, and colloidal particles. Polishing techniques such as deionization, degasification, and ultraviolet treatment are then used to achieve the required quality. Among these processes, desalination is particularly critical for removing most impurities.

Reverse osmosis, especially seawater reverse osmosis, is a widely used desalination technology but has notable drawbacks, such as requiring high-pressure operation (high energy consumption), intensive pretreatment, and producing concentrated brine, which can harm marine ecosystems when discharged. Membrane distillation has gained attention as an alternative for producing high-quality water and supporting recovery applications. It operates at lower temperatures and has the ability to utilize waste heat.

Membrane distillation is a thermal separation process where a vapor pressure difference across a hydrophobic membrane causes liquid particles to phase change and pass through as gas. Operating at ambient pressure and utilizing low-temperature heat sources (<90 °C), membrane distillation offers significant advantages. However, research on membrane distillation as a viable alternative to reverse osmosis for ultrapure water production remains limited, particularly in areas such as module design and techno-economic analysis.

A group of researchers at the Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg, Germany, has explored the potential of membrane distillation as a cost- and energy-efficient alternative to reverse osmosis for producing ultrapure water for proton exchange membrane (PEM) electrolyzers. The findings were recently published in the Desalination Journal. They introduced membrane distillation as a possible alternative to reverse osmosis for ultrapure water production. But here is the twist: the membrane distillation system ingeniously taps into waste heat from a 5 MW proton exchange membrane electrolyzer, transforming what would typically be an efficiency liability into an asset for sustainability. So far, their results are impressive—membrane distillation not only produces exceptional distillate (<3 μS/cm) but does so at a cost ranging from €2.33 to €2.85 per ton of distillate, compared to reverse osmosis’s €2.80 to €5.51. Using membrane distillation, seawater desalination could be 50% or more cheaper.

Economic analyses highlight that membrane distillation’s cost-effectiveness is driven by its low electrical energy requirements and optimized short-channel module design. Its impressive energy efficiency, enabled using low-grade thermal energy, establishes membrane distillation as a highly versatile and environmentally friendly solution that aligns seamlessly with the vision for renewable hydrogen production. This study positions membrane distillation as more than just an alternative to reverse osmosis: it is a smarter and greener approach to ultrapure water production.

Their findings have the potential to reshape the industrial approach to ultrapure water production. By demonstrating an efficient use of waste heat and providing a more economical solution, it offers industries a pathway to lower operational costs while advancing sustainability. This aligns particularly well with sectors striving for greener operations, such as renewable hydrogen production and other energy-intensive applications. Moreover, the adoption of membrane distillation could catalyze innovation in system design and integration, encouraging industries to optimize processes and reduce dependence on traditional, energy-intensive methods. This shift can contribute to broader sustainability goals and improve the economic feasibility of renewable energy initiatives.

At Frontis Energy, we are committed to advancing sustainable and green energy solutions by embracing innovative technologies like membrane distillation, bringing us closer to a sustainable future.

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Unlocking the Potential of Conducting Polymers for Sustainable Water Treatment and Energy Solutions

Carbon based materials have a broad range of applications such as energy storage and conversion, electronics, nanotechnology, water purification, and catalysis. They are made of an element which is available everywhere.

In recent times, the electrochemical features of carbon-based electrodes are being enhanced by using conducting polymers. Carbon cloth, woven from carbon microfibers, serves as a promising carbon-based electrode, which acts as a durable and cost-effective medium for facilitating electrochemical reactions that degrade pollutants and improve water quality. These electrodes, notable for their mechanical flexibility, strength, and cost-effectiveness, are employed in processes such as electrochemical oxidation, microbial fuel cells, and other advanced wastewater treatment technologies.

Due to a few limitations of pristine carbon cloth electrodes such as low specific capacitance and limited wettability associated with its inherent hydrophobicity, scientists conduct research to improve the modern electrodes. For instance, since wettability is crucial for for immersing the electrode surface in liquid and ensuring interaction with contaminants, enhancing it is always beneficial for the process. Improving the performance of carbon cloth electrodes could lead to more efficient treatment, faster reaction times, and better overall performance.

A research group at San Diego State University undertook the task of addressing these limitations by making conformal conducting polymer films on carbon fibers via oxidative chemical vapor deposition (oCVD) method. They recently published their results in the Advanced Material Interface Journal. With antimony pentachloride (SbCl5) as the oxidant, they developed a highly uniform coating of poly(3,4-ethylenedioxythiophene) (PEDOT) on three-dimensional porous fibers. The oCVD technique ensures uniform coatings while preserving the geometric and functional properties of the carbon cloth, making it a promising approach for enhancing electrochemical performance.

The PEDOT-coated carbon cloth electrodes achieved a remarkable improvement in specific capacitance and pseudocapacitance compared to pristine carbon cloth. Depending on the deposition temperature, the oCVD PEDOT-coated electrodes showed a 1.5- to 2.3-fold enhancement in specific capacitance. Notably, the electrode fabricated at a deposition temperature of 80 °C exhibited the highest specific capacitance and superior electrochemical performance. Adjusting the deposition temperature to optimize performance can help tailor carbon cloth electrodes for specific wastewater treatment needs.

The investigation underscores the effectiveness of the oCVD method in addressing the limitations of carbon cloth electrodes and expanding their potential applications in wastewater treatment and electrochemical energy storage devices. Furthermore, the researchers showed that PEDOT-coated carbon cloth can be applied as supercapacitors, where flexibility and high capacitance are critical. It should be noted that the study not only showcases significant advancements in material design but also open new avenues for optimizing electrode performance for diverse applications.

Overall, the findings emphasize the growing potential of advanced electrode technologies in addressing industrial challenges. By improving the functionality of carbon-based electrodes through novel material coatings, industries can achieve more efficient and tailored solutions for both wastewater treatment and energy storage. The ability to fine-tune electrode properties to meet specific requirements offers a pathway toward the development of highly efective and cost-efficient technologies, which could be a game-changer for sectors focused on sustainability and resource management. As these innovations continue to evolve, they have the potential to significantly improve operational efficiency and environmental impact across various industries. For example, in wastewater treatment, electrochemical processes such as electrocoagulation, electrooxidation, or electroreduction are often used to remove contaminants.

At Frontis Energy, we believe that improvements and customization can aid in designing electrodes tailored to specific contaminants or types of wastewater.

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Complex interaction between nitrogen emissions and global warming

Nitrogen compounds are essential for life on Earth. The use of fossil fuels and artificial fertilizers have led to a significant increase in reactive nitrogen available to the biosphere. This increase has far-reaching and well-researched effects on ecosystems, biodiversity and health. Air pollution can lead to premature deaths and nitrogen compounds may play an important role. Previous studies have only inadequately researched the effects of reactive nitrogen on the global climate system since industrialization.

A new study by the Max Planck Institute for Biogeochemistry in Jena is now closing this knowledge gap. The researchers modeled the terrestrial biosphere and global atmospheric distribution of nitrogen. They then combined the results with findings from atmospheric chemistry. This combination enabled them to come up with a new and comprehensive assessment of the climate impact of anthropogenic reactive nitrogen. The results were recently published in the renown scientific journal Nature.

Man emits a number of nitrogen compounds. Some of these, such as nitrous oxide (N2O, laughing gas), are greenhouse gases. Others, such as fine dust particles that reflect solar radiation, have a cooling effect on the climate. These effects were also described in the present the study. Significant warming due to increasing concentrations of the greenhouse gases nitrous oxide  and ozone (O3) were reported. In contrast, several processes that contribute to the cooling effect of nitrogen were also described. In addition to particulate matter, these processes include chemical reactions that lead to a shorter residence time of the greenhouse gas methane in the atmosphere, as well as an increased uptake of carbon dioxide (CO2) by the terrestrial biosphere due to the fertilizing effect of nitrogen.

If all global warming and cooling processes caused by the reactive nitrogen are combined, a net cooling effect is the result. This new result suggests that nitrogen emissions have compensated for about one sixth of the global warming to date caused by the increase in CO2 over the industrial period.

The new results are also important for future strategies for nitrogen regulation in the context of climate protection policy. In most scenarios, nitrous oxide emissions from farming remained high due to the continued use of fertilizers in agriculture and thus the warming influence of this gas. Scenarios that are compatible with the climate goals of the Paris Agreement require an end to CO2 emissions from fossil fuels. This also reduces the release of reactive nitrogen from fossil sources and its harmful effects on health and biodiversity, but also eliminates its cooling effect. The researchers therefore expect a slightly warming contribution from total nitrogen for these climate protection scenarios, but this is far less than the warming from the unchecked consumption of fossil fuels.

The study underlines the urgency of finally stopping emissions from fossil fuels and using fertilizers more specifically. This would not only slow down global warming, but also reduce the burden of harmful ozone and particulate matter concentrations for all of us in rural areas and in cities. New technologies are needed to reduce harmful nitrogen emissions while making good use of beneficial nitrogen. At Frontis Energy we have developed such a technology. Our patented process removes ammonia from wastewater while producing useful carbon neutral biogas. Using this technology, harmful nitrous oxide emissions can be reduced.

Picture: Smog over Guangzhou, China

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Terrestrial vegetation and soils absorb up to 30% more CO2

A significant part of the scientific community is investigating in impact of GHG emissions, and in particular CO2, on our past, present, and future climate. To model our future climate, researchers use CO2 emissions to estimate the future climate of our planet. They predict how much CO2 industry and households can emit to remain within the set 1.5°C target. The central question therefore is: How much of the emitted CO2 actually remains in the atmosphere? How much is metabolized or otherwise bound?

It seems the models have so far underestimated the capacity of biomass to incorporate CO2. That is because the models have now received a major upgrade from an analysis of radiocarbon data from nuclear bomb testing in the 1960s, which suggests that terrestrial ecosystems are capable of absorbing more CO2 than previously thought. Does that mean that the earth copes with more emissions?

Researchers  at the Worcester Polytechnic Institute in Massachusetts found that plants are currently absorbing 80 million tons of CO2 each year. They published their finding in a recent article in the renown magazine Science. That is 30% more than previously assumed.

The initial idea was to take a closer look at the remnants of the nuclear tests in the 1950s and 1960s. Carbon dioxide molecules are made of carbon and oxygen. The radioactive substances that atomic bombs have left all over the world is also a radioactive carbon (14C). Like ordinary carbon (12C), the radioactive version is a possible component in CO2. After the atomic bombing tests, 14C got into the terrestrial biosphere through the photosynthesis of plants along with 12C. So one studies how 14C was enriched, the rates of the CO2 uptake in plants reveal how much carbon is captured.

Animals that feed on plants take in the same CO2 into their organism, as well as fungi and soil bacteria. Through decaying biomass in soil, carbon is then again released into the atmosphere in the form of CO2 and the cycle closes.

However, the question of is how much CO2 from the air goes into the ground and in is bound in biomass long term is anything but trivial. The analyzes of radioactive carbon now show slightly less 14CO2 in the atmosphere shortly after the nuclear tests. That is, plants worldwide would bind 80 gigatons carbon per year. So far, climate researchers assumed that storage performance was 43 to 76 gigatons.

The current assumptions were wrong due to the fact that mainly tree trunks were used for the calculation. Carbon stored in wood has been bound for many decades or centuries. The new study looked at non-wood biomass such as leaves, which also store a large proportion of carbon. Plus, extensive  underground plant biomass received hitherto too little attention. As a rule, underground biomass is comparable to what’s found above ground. In particular, only the woody roots were taken into account, but not the fine rhizosphere which has even more mass and accordingly binds more carbon.

Eighty gigatons are not only significantly more than is used in the common climate change forecasts. It is also twice as much as the annual man-made CO2 emissions (37.2 billion tons).

Unfortunately, this does not mean that there is nothing to worry about. Living creatures also use biomass as a fuel. Almost the same amount of CO₂ is released again when plants lose their leaves in autumn, which then serve soil creatures as a source of food. These natural CO2 emissions from the biosphere have accelerated more since the 1960s.

Overall, photosynthesis can get only 30% of the total man-made CO2 emissions from the atmosphere and can therefore not make up for fossil fuel consumption. With large natural areas available for photosynthesis more CO2 could be extracted from air. In consequence, more and not fewer forests and meadows need to be re-naturalized.

Some climate models are based on wrong assumptions. And yet, the more important carbon storage is not on land, but in the water. CO2 dissolves better there and countless micro-algae in the oceans also carry out photosynthesis. Carbon is very popular as building material for shells. In total, the oceans 16 times more carbon than the biosphere on land.

The key findings include:

  • Terrestrial vegetation and soils might absorb up to 30% more CO2 than what was estimated by earlier models.
  • The carbon storage in these ecosystems is more temporary than once believed, implying that man-made CO2 may not remain in the terrestrial biosphere as long as current models suggest.
  • The discrepancy in the models is due to underestimating the carbon stored in short-lived or non-woody plant tissues, as well as the extensive underground parts of plants, like fine roots.

The implications of these findings are significant for climate predictions and crafting effective climate policies. It highlights the need for more accurate representation of the global carbon cycle in climate models. While this increased uptake of CO2 by vegetation is a positive sign, it does not negate the urgency of reducing carbon emissions to combat climate change.

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Poly-electrolyte layers determine the efficiency of desalination membranes

Increasing water scarcity and pollution with micropollutants are responsible for the increasing cost of drinking water. Desalination of sea water and better wastewater treatment are necessary to counter this trend. Membranes can desalinate and remove most wastewater pollution. However, a lot of energy is required. Therefore, modern membranes must be more efficient in order to achieve satisfactory results.

Nano-filtration membranes consisting of poly-electrolytic layers are a promising approach to treat water more efficiently. Accordingly, the composition of poly-electrolytic layers has stirred up much interest in the production of nano-filtration membranes. Such membranes are manufactured layer by layer, which enables a good tuning of membrane properties for different purposes.

Commercially available nano-filtration membranes are usually a trade-off between high water permeability and good salt retention (desalination). This trade-off impacts either the quality or the volume of the cleaned water. Nano-filtration membranes that are produced in layer by layer can have a positive impact on this trade-off balance due to the formation of nano scaled layers. It is important to know which component plays a crucial role in the layer forming process.

A research group of the Technical University Eindhoven in the Netherlands had therefore undertaken the task of examining these layer components more closely. They specifically investigated the poly-electrolyte concentration. It is known that a higher poly-electrolyte concentration produces thicker layers. However, their impact on the membrane performance has so far been unknown. They now published their work, in which the researchers used two well-known strong poly-electrolytes: PDADMAC and PSS (polydiallyldimethylammonium chloride and poly(sodium-4-styrene sulfonate)). The membrane output was examined with regard to water permeability, the molecular weight cutoff and salt retention.

In the first double layer, the membranes made with a 50 mM saline solution showed a lower water permeability and molecular weight cutoff, as well as better salt retention (magnesium sulfate) due to the higher poly-electrolyt concentration. After a certain number of double layers, the molecular weight cutoff and the salt retention efficiency for all poly-electrolyte concentration leveled off. The higher the poly-electrolyte concentration, the sooner the plateau value was reached.

The membranes prepared with a 1 M salt concentration had a lower or comparable salt retention efficiency with one exception. The scientists concluded that the poly-electrolyte concentration significantly changed the membrane properties. A plateau was reached with seven or more double layers. The thicker layers showed a lower water permeability than those that were coated with poly-electrolyte solutions using a 50 mM salt concentration. Due to the reduced swelling of these membranes, they all had better salt retention efficiency, with the exception of magnesium chloride.

The results showed that increasing the poly-electrolyte concentration also increased the amount of poly-electrolyte adsorption. Due to a higher coating thickness, this led to lower permeability with pure water. However, this did not lead to a lower molecular weight cutoff or salt retention. The additional poly-electrolyte adsorption resulted in fewer links between the individual layers. The higher diffusivity of PDADMAC compared to PSS resulted in highly positively charged membranes, which in turn led to a better salt retention of magnesium and sodium chloride.

Overall, increased poly-electrolyte concentration and the salt concentration influenced the membrane charge exclusion significantly due to a higher charged surface, which led to better salt retention. However, the membrane size exclusion has not changed, which led to the same plateau values. The study presented here will allow chemists to produce better tuned desalination membranes, which will reduce the energy requirement and raw material requirements during production.

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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)