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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)

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Bio-electric systems help PFAS removal

Per- and polyfluoroalkyl substances (PFAS) have been manufactured for various applications for many decades. These include medical applications, such as implants and catheters, or consumer products for firefighting, plastics, cookware, and cosmetics. Likewise, PFAS are required in countless industrial applications, such as in the automotive industry, the chemical industry and the energy sector, including hydrogen electrolysis and fuel cells (e.g. Nafion™). They help apparatuses to function properly, reduce wear and the risk of accidents. The widespread use of PFAS has led to traces of these substances entering the environment worldwide. Typical sites with higher environmental PFAS concentrations include airports, chemical plants, fire brigades, military facilities etc.

The long-term health effects of these substances are currently a matter of controversy, particularly with regard to their chemical stability (a desired property).

In addition to completely avoiding their entry into the environment, PFAS can also be eliminated from it. For example, activated charcoal is often used to adsorb PFAs onto it. However, this method is not efficient in soils. Ideally, the activated carbon itself would have to be further processed in order to reuse PFAS. This process is very energy intensive.

As for many treatment processes, microbes can be used also for PFAS. Such biological methods are called bioremediation. However, the carbon-fluorine (C-F) bonds in PFAS are among the strongest covalent bonds in organic chemistry. In addition, there are very few naturally occurring C-F bonds in nature. They only occur in small concentrations. A prominent example is fluoroacetic acid, a highly toxic compound produced by the South African poisonous gifblaar. Few microorganisms with the ability to break the C-F bond have been identified. Thus, bioremediation of PFAS is possible but a slow process.

As already described in our previous articles, bio-electrical systems can accelerate microbial conversion processes. With bio-electrical systems it is possible to offer microbes a greater electrochemical potential gradient. Since this leads to larger energy gain in microbial metabolism, such metabolic rates can be accelerated. This process is successfully employed to clean industrial waste water.

In bio-electrical systems, microorganisms along with contaminants are placed in an electrochemical apparatus. The electrodes of such a system serve as electron donors or acceptors. The biodegradation is can be measured via the electric current.

Indeed, bio-electric systems have been used to degrade fluorinated alkanes. For example, the anti-inflammatory drug dexamethasone was successfully eliminated using such an apparatus. As proposed for bio-electrical liquid fuel, designer microbiomes could also be studied for PFAS. Other drug residues, such as Prozac™, should also be examined to ensure absence from the environment.

At Frontis Energy we are looking forward to new developments for PFAS removal in bio-electrical systems.

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Decentralized waste energy systems produce biogas where it is needed

Among others, the current European energy crisis was caused by a surge in demand after the pandemic, the embargo on Russia, the reluctance of investors to finance fossil energy projects and the throttling of production by the OPEC countries. In this complex situation, European countries are forced to develop alternatives and renewable energy sources. At the same time, however, natural gas is difficult to replace in many industries. One exception is the food and beverage industry, which sits enormous untapped resources of biogas in their wastewater.

Wastewater is a resource of which 380 billion m³ are produced worldwide. It contains valuable nutrients and energy. Global production is projected to increase by 51% by 2050. Wastewater treatment consumes about 3-4% of the energy generated globally. The full reeovery of the energy that is contained in this wastewater would completely offset the energy consumption of its treatment and in many cases even produce a surplus. In addition, the entire global water treatment is estimated to account for up to 5% of man-made CO2 production. Unfortunately, many businesses and municipalities do not invest in complex and expensive wastewater treatment technologies and continue to waste this valuable resource. The European Biogas Association estimates that by 2050, a maximum of 65% of gas requirements (~167 billion m³) could be covered by biogas.

Europe is the largest cheese maker in the world. More than 9 million tons of cheese are produced annually. With every ton of cheese, 9 m³ of cheese whey remain. Despite its high nutritional value, whey is often treated like wastewater for various reasons. Yet, the very high organic load in the whey makes it difficult to treat. Wasted whey can also be used for biogas production. In addition to whey, regular wastewater is also produced by cheese makers. For example, a medium-sized cheese factory pays 1.5 million euros a year for its waste water. Reducing these costs by producing biogas would turn dairy industry wastewater into a valuable resource.

This situation is similar in many other food and beverage sectors such as breweries, distilleries, winemakers, bakeries etc. All of these sectors have high energy requirements. Renewable electrical energy cannot meet this need. The market for wastewater treatment in Europe and the US is around 12 billion euros.

Traditional wastewater treatment is a cascaded process including aeration and anaerobic sludge digestion followed by incineration. These methods often consume more than 70% of the energy in a wastewater treatment plant. If contaminants such as high-energy total organic carbon or ammonia were converted into biogas before the process, at least 80% of the energy needed for wastewater treatment could be saved. It is absurd that this energy is removed from the wastewater using even more energy.

An ever-increasing number of sewage treatment plants already recover the resources contained in their wastewater, apart from the water itself. The oldest recivered products are biogas and fertilizers made from sewage sludge. Due to its heavy metal content such as copper and mercury, sewage sludge is no longer used as fertilizer but incinerated.

Biogas is particularly popular in Europe as the produced volumes and prices are high enough to compete with natural gas. Biogas is also a green alternative to natural gas as no additional CO2 is emitted. (Hence, it is often called Renewable Natural Gas in North America.) A disadvantage of classic biogas is the CO2 and sulfide content. Another disadvantage is that anaerobic digestion is the terminal treatment step, wasting valuable wastewater resources in the preceding treatment. Finally, the size and complexity of current digestion requires significant commitment from users when it comes to capital expenditures. Most food manufacturers prefer to focus on making food rather than cleaning their wastewater.

Novel high-performance biogas reactors solve these problems through miniaturization. A 20-fold size reduction is achieved compared to conventional systems. The new technology used was developed in Japan in the early 1990s and is called microbial electrolysis. The electrolysis of wastewater is catalyzed by electroactive microorganisms on the anode (the positive electrode). The reaction products are CO2 (from organic matter) and nitrogen gas (N2 from ammonia).

Principle of a microbial electrolysis reactor. On the left anode, the organic material is oxidized to CO2. The free electrons are absorbed by the anode and transported to the cathode. Hydrogen gas (H2) is released there. CO2 and hydrogen form methane, the final microbial reaction product.

At the same time, hydrogen gas (H2) is generated at the cathode (the negative electrode). This hydrogen reacts with CO2 to form methane. The final methanation step completes the biocatalytic treatment of the wastewater. Gas grid injection is one possible use. But for cheese makers, the gas would be used on site to generate electricity and/or heat.

The reaction is accelerated using an applied voltage and is based on the laws of thermodynamics. As a result, the reactor volume can be reduced. The size reduction has several advantages. First, it makes biogas accessible in markets where it was previously not possible due to the high investment costs. Second, it enables higher throughput at a lower cost. Smaller units are mobile and can be shared, moved or rented. After all, food manufacturers want to do what they do best, which is to make food.

 

Image: Pixabay

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Nanostructured membranes improve the gas separation of carbon dioxide

To reduce greenhouse gas emissions, various technologies are in development requiring the separation of mixed gases, such as  CO2 and methane or CO2 and nitrogen gas (CO2/CH4 and CO2/N2). Compared to other separation technologies, polymer membranes are  good candidates for industrial use. This is due to their low operating costs, high energy efficiency and simple scalability.

The gas permeability and selectivity, as well as the cost of these polymer membranes are the crucial criteria for their industrial use. These criteria are influenced by molecular order processes during polymerization at nano- and micrometer levels. However, the processes regulating the molecular order of most common membranes do not occur on these levels. Hence, there is little control over them during manufacturing. Not much is known about materials with self organizing properties and their influence on molecular order and gas separation.

Chemists at the Technical University of Eindhoven in the Netherlands examined the effects of the layer distance within the membrane and its halogenation on the gastrunge and published their results in the MDPI Membranes journal. They focused on the separation of helium, CO2 and nitrogen. The researchers used liquid crystal membranes for their investigation. Liquid crystal molecules can align in various nanostructures. These structures vary depending on the manufacturing process and can therefore be controlled. As a result, liquid crystal membranes are ideal in order to investigate the influence of nanostructures on gas separation.

A frequently used manufacturing method is to commence the self organization of the reactive liquid crystal molecules in a cell with spacers. This helps to better control the membrane thickness and alignment and ultimately control the molecular orientation. The final network of the liquid crystal molecules and their fixation in nanostructures is required to achieve mechanical strength. For example, high ordered crystal membranes (i.e. not liquid crystals) have a lower gas permeability. Nonetheless, they also are characterized by a higher selectivity for helium and CO2 compared to nitrogen.

A lamellar morphology and the flow direction of the gas also have a great influence on selectivity and permeability of the membrane. It is also known that halogen atoms such as chlorine or fluorine improve CO2 permeability and selectivity by affecting both gas solubility and diffusion.

In the presented experiments, all liquid crystal membranes with similar chemical compositions, but different halogenated alkyl chains, were aligned. The CO2 sorption and the entire gas permeation were better if their layers were further apart. The gas solubility itself had no impact. This was confirmed by the increased gas diffusion coefficients, which were also determined in the experiments.

Bulky halogens had only limited influence on gas permeability and selectivity. The CO2 permeability of all halogenated liquid crystal membranes increased due to a slightly higher CO2 solubility and diffusion coefficients, which led to improved selectivity for CO2. The layer distance in particular was a crucial factor that directly influenced the diffusion coefficient. The researchers recommended that future investigations should focus on improving separation performance, for example by reducing the membrane thickness.

At Frontis Energy, we are looking forward to a good commercial product that can separate CO2 from gas mixtures, such as biogas, effectively and cheap.

Photo: Pixabay / SD-Pictures

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Novel membranes from plant waste filter heavy metals from water

Unfortunately, water pollution is still an issue in many places. Heavy metals are a group of water pollutants that can accumulate in the human body and causing cancer and other diseases. Existing technologies for heavy metal removal, however, are very energy intensive.

Scientists from the Nanyang Technological University in Singapore and the Swiss Federal Institute of Technology Zurich (ETHZ) have created a new membrane out of byproducts from the vegetable oil industry. The membrane removes heavy metals from contaminated water. The team discovered that proteins, which originated from peanut or sunflower oil production bind heavy metal ions very effectively. In their tests, they showed that this adsorption process can purify contaminated water so much that it fulfills drinking water quality standards.

The researchers see their membranes as an inexpensive, simple, sustainable and scalable solution for heavy metal removal from water. Their results were published in the Chemical Engineering Journal.

The new protein based membranes were generated by an environmentally friendly process and needed little energy for their use. This makes them a promising water purification solution for industrialized nations as well as less developed countries.

The production of commercial vegetable oils generates protein rich waste products. These remnants remain from the raw plant after the oil extraction. For their membranes, the research team used sunflower and peanut oils. After the proteins had been extracted, they were transformed into nano-amyloid fibrils. These are rope-like structures built from tightly intertwined proteins. The protein fibrils strongly attract heavy metals and act like a molecular sieve. In the published experiments, the membranes removed up to 99.89 percent of heavy metals.

Among the three metals tested, lead and platinum were filtered most effectively, followed by chrome. Since platinum is often used as a catalyst in fuel cells or electrolyzers, the new membrane would be an elegant and cheap method to recover this metal.

The researchers combined the extracted amyloid fibrils with activated carbon. Due to the high surface volume ratio of the amyloid fibrils, they are particularly suitable for adsorption large amounts of heavy metals. The filter can be used for all types of heavy metals. In addition, organic pollutants such as perfluoralkyl and polyfluoralkyl compounds are filtered as well. These chemicals are used for a variety of consumer and industrial products, as well as in nafion membranes of fuel cells.

The concentration of heavy metals in contaminated water determines how much volume the membrane can filter. A hybrid membrane made of sunflower amyloids requires only 16 kg of protein to filter a swimming pool contaminated with 400 parts lead per billion. One kilogram of sunflower extract yields about 160 g of protein. The protein-rich sunflower and peanut oils are inexpensive raw materials. Since this is the first time that amyloid fibrils were obtained from sunflower and peanut proteins, the process must still be scaled and industrialized.

However, due to its simplicity and minimal use of chemical reagents, the process seems easy to scale. This makes it possible to recycle the waste product for further applications and to fully exploit such industrial food waste. The filtered metals can also be extracted and further recycled. After filtration, the membrane with the captured metals can simply be burned and leaving behind only the metals.

While toxic metals such as lead or mercury need safe disposal, other metals such as platinum can be re-used in the production of electronics and other high value devices, such as fuel cells. The recovery of the precious platinum, which costs 30,000 euros per kg, only requires 32 kg of protein, while the recovery of gold, which corresponds to almost 55,000 euros per kg, only requires 16 kg of protein. In view of the costs of less than 1 euro per kg of protein, the advantages are enormous.

The co-author of the article, Raffaele Mezzenga, had already discovered in 2016 that whey proteins from cow milk had similar properties. Back then, the researchers noticed that proteins from plant oil seeds could also have similar properties.

Another great advantage is that, unlike other methods such as reverse osmosis, this filtration does not require electricity. Gravity is completely sufficient for the entire filtration process. The method is also suitable for water purification in poorly developed areas.

Photo: Pixabay

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Transition between double-layer and Faradaic charge storage in porous carbon nano-material

In electrochemical cells, such as fuel cells or electrolyzers, electric double-layer (EDL) formation occurs on their electrode surfaces. These EDL act as both, capacitors and resistors and impact therefore the performance of electrochemical cells. Understanding the structure and dynamics of EDL formation could significantly improve the performance of, electrochemical systems, for example in energy storage and conversion, including supercapacitors, water desalination, sensors and so forth.

On a planar electrode, electrolyte ions and the solvent are adsorbed at the electrode surface. The resulting capacitance depends on charge, solvation state and concentration. Traditionally, the capacitance of electrochemical interfaces can be divided into two types:

  1. Double-layer capacitance: ions are adsorbed based on their charge. Ion adsorption is non-specific.
  2. Faradaic pseudocapacitance: specific ions are adsorbed, for example through chemical interactions the electrode surface. This may involve charge transfer.

The electrode interface in the most energy application-based technology is, however, not planar but porous. Layer materials in such situations have various degrees of electrolyte confinement and thus different capacitive adsorption mechanisms. Understanding electrosorption in such materials requires a refined view of electrochemical capacitance and charge storage.

A team of researchers from the North Carolina State University, the Paul Sabatier University in Toulouse and the Karlsruhe Institute of Technology reported new insights in electrolyte confinement at the non-planar interfaces in the journal Nature Energy.

Electric double-layer at planar electrodes

The degree of ion solvation (the process of reorganizing solvent and solute molecules) at ideal (planar) electrochemical interfaces determines the ions interaction with the electrodes. There are two distinct cases:

  1. Ions are non-specifically adsorbed: this is the case with strong ion solvation. The electrode’s interactions are primarily electrostatic. This type of interactions can be considered as the induction – charge is induced but not transferred.
  2. Ions are specifically adsorbed: in this case, ions are not solvated and can undergo specific adsorption and chemical bonding to the electrode. This process can be described as charge transfer reaction between the electrode and the adsorbed ion. However, the charge transfer reaction depends on the bonding between the ion and the electrode. This correlates with the state of ion solvation.  Thus, it can be expected that the ion solvation is crucial for understanding the ion-electrode interactions in a nano-confined environment such as porous materials.

Carbon based EDL capacitor – the confinement effect

There is a great interest for understanding the relationship between the porosity of carbon nano-materials and their specific capacitance.

When electric double-layer formation occurs in a nano-confined micro-environment, the EDL capacitor in porous carbon materials deviates from the classic EDL model on flat interfaces. The degree of the ion solvation under confinement is determined by the pore size in nano-porous materials and by the inter-layer distance in layered materials that is, 2D-layer materials.

Confinement of ions in sub-nanometer pores results in their desolvation, leading to the capacitance increase and deviation from the typical linear behavior on the surface area. During negative polarization of porous carbon materials with the pore sizes <1 nm, a decrease of capacitance  is observed. This is due to the ion selection limiting ion transport.

These insights are important for effectively tailoring carbon pore structures and for increasing their specific capacitance. Since carbon material is not an ideal conductor, it is important to consider its specific electric structure. For graphite materials, the availability of the charge carriers increases during the polarization which leads to increased conductivity.

Unified model of electrochemical charge storage under confinement

Since the electrochemical interface in the most technological application is non-planar, the researchers proposed a detailed evaluation and different concept of electrochemical capacitance on such non-ideal interfaces. The team evaluated electrosorption on 2D surfaces and 3D porous carbon surfaces with a continuous reduction in pore size in a step-by-step approach of increasing complexity.

The example provided relates to the charge storage characteristics of lithium ions (Li+) in the graphene sheets of organic lithium-containing electrolytes depending on the number of graphene layers. In a single graphene layer, the capacitive response is potential independent due to the absence of specific adsorption. However, with an increase of graphene sheets, redox peaks emerged that are associated with the intercalation of desolvated lithium ions. Lithium intercalation is responsible for battery wear. The team’s hypothesis was that the transition of solvated lithium ion adsorption on a single graphene sheet into subsequent intercalation of desolvated lithium ions occurs with a continuous charge storage behavior. There can be a seamless transition based on the increased charge transfer between an electrolyte ion and host associated with the extent of desolvation and confinement.

In the presented research, a unified approach was proposed that involves the continuous transition between double-layer capacitance and Faradaic intercalation under confinement. This approach excludes the traditional “single” view of electrochemical charge storage in nano-materials regarded as purely electrostatic or purely Faradaic phenomenon.

The increasing degree of ion confinement is followed by decreasing degree of ion solvation thus the increase ion-host intercalation. This results in a continuum from EDL formation through transitioning state to Faradaic intercalation, typical for EDLC nanomaterial.

Image: Pixabay

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Bio-electrical system removes nitrogen from the wastewater

Hazardous compound removal from sewage such as organic matter and nitrogen makes wastewater treatment an energy intensive process. For example, treating activated sludge requires blowing oxygen or air into raw, unsettled sewage. This aeration significantly increases the cost of the wastewater treatment. About 5 kWh per kilogram nitrogen are required for aeration depending on the plant. The cost associated with energy consumption makes uof approximately EUR 500,000 per year in an average European wastewater treatment plant. This is up to one-third of the total operational costs of WWTP. It is therefore obvious that nitrogen removal from wastewater must become more economical.

Alternative approach: Microbial electrochemical technology

The conventional way of removing nitrogen is a cascade of nitrification and denitrification reactions. Nitrification that is, aerobic ammonium oxidation to nitrite and nitrate is carried out by ammonia-oxidizing bacteria. Subsequent denitrification is the reduction of nitrate to nitrogen gas (N2). In addition to the costly aeration process, the remaining intermediate products as nitrite and nitrate require further effluent treatment.

Instead of expensive pumping of oxygen into the wastewater, bioelectrical systems could accomplish the same result at a much lower cost. In such systems, an electron accepting anode is used as electron acceptor for microbial ammonium oxidation instead of oxygen, making aeration obsolete.

Complete conversion of ammonium to nitrogen gas

We previously reported the use of such an bio-electrical system to remove ammonia from wastewater in fed-batch reactors. Now, researchers of the University of Girona reported proof-of-concept on a novel technology. Their bioelectrical system is a complete anoxic reactor that oxidizes ammonium to nitrogen gas in continuous mode. The dual-chamber reactor nitrifies and denitrifies and ultimately removes nitrogen from the system.

The electricity-driven ammonium removal was demonstrated in continuously operated one-liter reactor at a rate of ~5 g / m3 / day. A complex microbial community was identified with nitrifying bacteria like Nitrosomonas as key organism involved anoxic ammonium oxidation.

From an application perspective, comparison between bioelectrical systems and aeration in terms of performance and costs is necessary. The researchers reported that the same removal range and treatment of the similar amounts of nitrogen was achieved but that their bioelectrical system converted almost all ammonium to dinitrogen gas (>97%) without accumulation of intermediates. Their system required about 0.13 kWh per kilogram nitrogen energy at a flow rate of 0.5 L / day. Using a bioelectrical system consumes 35 times less energy compared with classic aeration (~5 kWh per kilogram). At the same time, no hazardous intermediates like nitrite or NOx gases are formed.

Unveiling microbial-electricity driven ammonium removal

The new article also indicated potential clues for microbial degradation pathway that may lead to better understanding of the underlying processes of anoxic ammonium removal in bioelectrical systems.

The proposed nitrogen removal pathway was the bioelectrical oxidation of ammonia to nitrogen monoxide, possibly carried out by a microbe named Achromobacter. That was supposedly followed by the reduction of the nitrogen monoxide to nitrogen gas, a reaction that could have been performed by Denitrasisoma. Alternatively, three other secondary routes were considered: bioelectrical oxidation followed by anammox, or without nitrogen monoxide directly to N2. Some sort of electro-anammox may also be possible.

At Frontis Energy, we believe that the direct conversion of ammonium to nitrogen gas through the reversal of nitrogen fixation is a possibility as nitrogen fixation genes are ubiquitous in the microbial world and it would generate the universal bio-currency ATP rather than consuming it.

It was shown that Achromobacter sp. was the most abundant microbe (up to 60%, according to sequence reads) in the mixed community. However, anammox species (Candidatus Kuenenia and Candidatus Anammoximicrobium) and denitrifying bacteria (Denitratisoma sp.) have been also detected in the reactor.

Two possible electroactive reactions were identified: hydroxylamine and nitrite oxidation, reinforcing the role of the anode as the electron acceptor for ammonium oxidation. Data obtained from nitrite and nitrate tests suggested that both, denitrification and anammox based reactions could take place in the system to close the conversion.

As a result, ammonium was fully oxidized to nitrogen gas without accumulated intermediates. Taking it all together, it has been shown that ammonium can be removed in bioelectrical system operated in continuous flow. However, further reactor and process engineering combined with better understanding of the underlying microbial and electrochemical mechanisms will be needed for process scale up.

Experimental system set-up

  • The inoculum consisted of a 1:1 mix of biomass obtained from nitritation reactor and an aerobic nitrification reactor of an urban treatment plant
  • The reactor design was constructed of two 1 L rectangular chambers comprising an anode and cathode compartment
  • The separator, an anion exchange membrane,  was used to minimize the diffusion of ammonium to the cathode compartment
  • The anode and cathode chambers were filled with granular graphite as electrode support
  • Ag/AgCl reference electrode was used in the anode compartment
  • Two graphite rods were placed as current collectors in each chamber
  • The system was operated in batch and semi-continuous mode

Image: 5056468 / Pixabay

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Humidity-resistant composite membranes for gas separation

Hydrogen (H2) is a lightweight alternative fuel with a high energy density. However, its environmental impact and life cycle efficiency are determined by how it is produced. Today, the main processes of hydrogen production is either by coal gasification or steam reforming of natural gas where in the last step the produced carbon dioxide (CO2) is produced. Usually, this CO2 is released to the environment. The hydrogen produced by these processes lead is called black/brown or grey hydrogen. To improve its carbon footprint, CO2 capture is necessary. This hydrogen is then call blue hydrogen. However, to obtain zero-emission green hydrogen, electrolysis of water using renewable energy is necessary. During the electrolysis process, hydrogen and oxygen are produced on two electrodes (download our more about hydrogen production and utilization as fuel can be found in our latest DIY FC manual).

Climate-related economic pressure for more efficient gas separation processes

The produced hydrogen is not pure in any of the mentioned instances. For example, using steam methane reforming reaction there are many byproduct gases like carbon monoxide, CO2, water, nitrogen and methane gas.

Typically, the CO2 of hydrogen gas is up to 50% contributing to the greenhouse effect caused by burning fossil fuels. Currently, around 80% of CO2 emissions come from fossil fuels. It has been predicted that the concentration of CO2 in the atmosphere will increase up to 570 ppm in 2,100 which increases the global temperature of about 1.9°C.

The traditional processes of gas separation such as cryogenic distillation and pressure swing adsorption have certain disadvantages, for example high energy consumption. Therefore, developing high-quality and low-cost technologies for gas separation is an important intermediate step to produce cheap hydrogen while reducing CO2 emissions.

Application of 2D material towards gas separation

Finding low cost alternatives like membrane-based separation methods for hydrogen-CO2 separation is a potentially lucrative research and it is therefor not surprising that numerous publications have investigated the matter. The various membrane materials for gas separation range from polymeric membranes, nano-porous materials, metal–organic frameworks and zeolite membranes. The goal is to reach a good balance between selectivity and permeance of gas separation. Both are key parameters for hydrogen purification and CO2 capture processes.

A study published the journal Nature Energy by researchers of the National Institutes of Japan, offered a material platform as advanced solution for the separation of hydrogen  from humid gas mixtures, such as those generated by fossil fuel sources or water electrolysis. The authors showed that the incorporation of positively charged nanodiamonds into graphene oxide (GO/ND+) results in humidity repelling and high performance membranes. The performance of the GO/ND+ laminates excels particularly in hydrogen separation compared with traditional membrane materials.

Strategy and performance of new membrane materials

Graphene oxide laminates are considered as step-change materials for hydrogen-CO2 separation as ultra permeable (triple-digit permeance) and ultra-selective membranes. Still, graphene oxide films lose their attractive separation properties and stability in humid conditions.

After lamination, graphene oxide sheets have an overall negative charge and can be disintegrated due to the electrostatic repulsion if exposed to water. The strategy to overcome this obstacle was based on the charge compensation principle. That is, the authors incorporated positively and negatively charged fillers as stabilizing agents, and tested different loadings as well as graphene oxide flake sizes. So-prepared membranes were tested for stability in dry and humid conditions while separating either hydrogen from CO2 or oxygen.

The GO/ND+ composite membranes retained up to 90% of their hydrogen selectivity against CO2 exposure to several cycles and under aggressive humidity test. A GO30ND+ membrane with 30% positively charged nano-diamond particles exhibited exceptional hydrogen permeance with more than 3,700  gas permeatin units (GPU) and high hydrogen-CO2 selectivity. Interestingly, incorporation of negatively charged nano-diamond particles had no stabilizing effect. The researcher attributed this mostly to the generation of macro scale voids in ND systems resulting in the loss of selectivity. This phenomenon is commonly observed in polymer-based nano-composite membranes with poor interfacial interactions

The gas separation properties of the composite membranes were also investigated using an equimolar hydrogen-CO2 feed mixture. The hydrogen permeance decreased by 6% and hydrogen-CO2 selectivity of the GO30ND+ membrane by 13%.

The stability test of the membranes exposure to wet and dry feeds of the equimolar hydrogen-CO2 mixture  and hydrogen-oxygen mixture showed that GO/ND+ membranes were reversible membrane properties. On the other hand, graphene oxide-only membranes could not survive a single complete cycle exposure, becoming fully permeable to both gases. The researchers explained that the advantages of GO/ND+ membranes over graphene oxide-only membranes were caused by changes of the pore architecture such as dimensions and tortuosity, which could be improved by optimizing the nano-diamond loading. This results in better permeability without any notable loss of selectivity.

X-ray diffraction analysis showed that the incorporation of nanodiamonds has two major effects on the membrane microstructure: increasing the overall pore volume and reducing the average lateral size. Both make the membrane structure more accessible for molecular transport.

Nevertheless, this relatively new class of humid-resistant membranes still needs more optimization to compete with current industrial separation processes.

Image: Pixabay / seagul