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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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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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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Improving direct ethanol fuel cells by fluorine doping

Direct ethanol fuel cells (DEFCs) are fuel cells that run on ethanol to directly produce electrical power. Despite having much to offer they have not been forayed into. Ethanol can be made from biomass by yeasts and its oxidation products – CO2 and H2O – are hence environmentally friendly. The application of DEFCs could be a lucrative solution for vehicles due to the energy efficiency if mass-produced. Our current infrastructure for combustion fuels is ready for ethanol. DEFC usage would therefore be a sustainable and environment-friendly alternative to current internal combustion engines. Moreover, ethanol is liquid, which facilitates distribution, storage and use.

According to studies sponsored by  International Energy Agency (IEA), DEFCs deliver high power densities, culminating between 50 to 185 mW / cm2. Currently, DEFCs face multiple challenges such as slow redox kinetics, limited performance, and the high cost of electrocatalysts needed for DEFCs.

In a DEFC, the two key reactions are:

  1. Ethanol Oxidation Reaction (EOR)
  2. Oxygen Reduction Reaction (ORR)

Their sluggish rates have prevented widespread adoption of this technology. State-of-the-art DEFCs require expensive platinum-based materials to catalyze these reactions. Yet, they do not completely oxidize ethanol to CO2 to complete the EOR reaction, limiting the energy efficiency. One way to fix this issue is to separate and re-inject the unreacted ethanol. Since this adds more engineering to the fuel cell, a better solution is to find more efficient catalysts. Hence, to realize the true potential of DEFCs, is to find cheaper and more active catalysts for the two reactions in DEFCs.

The researchers at the University of Central Florida and their colleagues experimented on Pd–N–C catalyst and attempted to improve catalyst performance by introducing fluorine atoms. The team used alkaline membranes and platinum-free catalysts. Not only were these more cost-effective but also produced a high power output.

Previous research on electrocatalytic systems revealed that the local coordination environment (LCE) of the electrode surface is pivotal in tuning the activity of electrocatalysts made of carbon-supported metal nanoparticles. The study showed that introducing fluorine atoms in Pd–N–C catalysts regulated the LCE around the Pd, improving both activity and durability for the two key reactions. This improved the catalytic performance, and ultimately the fuel cell’s performance.

The new study demonstrated that fluorine doping rearranged the electron structure of the fuel cell catalyst. This substantially improved power density and ultimately the performance of the DEFC when compared with present-day benchmark catalysts. The experimental results on long-term stability demonstrated promising advancements towards practical applications of such catalysts in DEFCs.

Results

Upon experimental analysis, it was found that the fluorine atoms in the catalyst weakened carbon-nitrogen bond and pushed the N atoms towards palladium. This electron translocation efficiently regulated the LCE of palladium by forming palladium-nitrogen active sites for catalytic reactions.

The N-rich palladium surface promoted carbon-carbon bond cleavage and enabled complete ethanol oxidation. During the ORR, the N-rich palladium surface surface not only weakened CO2 adsorption but also created more accessible catalytic sites for rapid O2 adsorption.

According to the authors, a commonly occurring problem in DEFCs – the inability to complete the two key reactions – has been resolved. Their catalyst enhanced the overall performance of the fuel cell. The addition of fluorine also enhanced the durability of the catalyst by reducing the corrosion of carbon materials as well as inhibiting palladium migration and aggregation.

When the novel catalyst was tested in a DEFC, an output maximum power density of 0.57 W/cm2 was obtained. The fuel cell was stable for more than 5,900 hours. The proposed strategy, when experimented with using other carbon-supported metal catalysts, also gave improved results in activity and stability.

Outlook

The main shortcoming of DEFCs running in the alkaline condition is their durability. Currently, it is not sufficient for practical applications. Moreover, the anion-exchange membranes in use have two issues:

  • Structural stability of membrane is insufficient for long-term use
  • Carbonation occurs in presence of CO2 due to its reaction with hydroxide ions, ultimately degrading the catalyst.

Albeit stable for remarkable 5,900 hours, the membrane was replaced after 1,200 hours in the presented study. Since replacing membranes require complete disassembly of the cell, this is not a long-term practical solution.

Hence, there must be further research on increasing ionic conductivity and stability of anionic membranes for practical use of DEFC in alkaline conditions. Ideally, the hydroxide solution used to increase ionic conductivity is avoided to preserve energy density and reduce the complexity of the device. Solid oxide fuel cells offer a solution for these problems since the fuel is oxidized in gaseous form but their ceramic membrane are too fragile for mobile applications.

The current experiment makes significant strides in improving power density in DEFCs much more than any state-of-the-art DEFCs. The way ahead is further research to overcome these smaller obstacles in the long-term use of anionic membranes.

Experimental analysis

Materials used

Commercial Pd/C (10%, 8 nm Pd particles on activated carbon), as well as Pt/C (20%, 3 nm Pt particles on carbon black), were used as baseline catalysts. Also, Nafion™ solution (5%), carbon paper (TGP-H-060), and anion-exchange membranes (Fumasep FAS-PET-75)

Synthesis of heteroatom X-doped carbon (X–C, X=N, P, S, B, F)

Carbon black with abundant oxygen functional groups and melamine (C3H6N6) were mixed and ground, and finally pyrolyzed. After cooling to room temperature, N–C was obtained by washing with ethanol and water. The same method was used to synthesize P–C, S–C, B–C, and F–C from sodium hypophosphite anhydrous, sulfur powder, boric acid, and polyvinylidene difluoride.

Synthesis of hetero-atom fluorine-doped carbon catalysts

N–C and polyvinylidene difluoride were mixed and ground before adding them into a solution of acetone and water. After ultra sound treatment, the mixture was refluxed in an oil bath until fully dried. The mixture was then pyrolyzed and after cooling to room temperature, the samples were washed with ethanol and ultrapure water, followed by a vacuum to obtain the fluorinated catalyst support. The same method was used for the other precursors.

A microwave reduction method was used to synthesize palladium catalyst on the catalyst support. The content of palladium in all samples was kept at 1.0%, which was determined and double-confirmed by X-ray spectroscopy and inductively coupled plasma.

Electrochemical characterizations

For the electrical measurements, either a glassy carbon ring-disc electrode or rotating ring-disc electrode were used. The Fumasep membrane was used as an anion-exchange membrane, modified to change it to a hydroxide environment.

Reference

Chang et al., 2021, Improving Pd–N–C fuel cell electrocatalysts through fluorination-driven rearrangements of local coordination environment. Nature Energy 6, 1144–1153 https://doi.org/10.1038/s41560-021-00940-4

Image Source: P_Wei, Pixabay

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Water desalination and fluoride ions removal from water using electrodialysis

Clean freshwater is of the utmost importance for our health. Despite its central role for our lives, progressing global industrialization threatens freshwater resources around the world. Albeit a vital trace element, fluoride is a serious public health threat. Absorbed in larger quantities for a long time, fluoride causes fluorosis, a form permanent poising responsible for irreparable bone damage.

Fluoride bearing rocks are particularly common in India. Fluoride is leached into adjacent aquifers and contaminates the soil. Sometimes, the concentration of fluoride ions in Indian aquifers exceeds 30 mg/L. Toxic concentrations of 20-80 mg / day over a period of 10 to 20 years cause irreparable damage to the human body.

Fluoride ions in groundwater are removed for water treatment using membranes. However, such membranes foul easily, for example by bacteria present in wastewater or other deposits.  Fouling can become a serious threat to public health. Therefore, a particular focus in membrane research is on the development of fluoride removing membranes that prevent fouling. It can be accomplished when bacterial growth is slowed down or inhibited entirely. For water treatment, antimicrobial surface modifications are used in high-quality membranes for ultrafiltration, nanofiltration, reverse osmosis and electrodialysis.

Electrodialysis is often used to remove water contamination, because only little energy is needed for the process. For electrodialysis membranes, salt deposits are an economic risk that is to be avoided. Precipitates can occur when the concentration of bivalent ions in the water is too high. Added to precipitates comes the risk of biofouling caused by microbial growth. Both affect the performance of electrodialysis membranes, causing economic losses as the membranes must be cleaned or replaced. For efficient water treatment, it is therefore important to improve the thermal and mechanical properties of the membranes.

A group of scientists have synthesized a composite anion exchange membrane for water-salt altitude and fluoride ion removal by electrodialysis that has improved antimicrobial properties. She published her results in the journal ACS ES&T Water. The consortium consisted of researchers of the Academy of Scientific and Innovative Research in Ghaziabad, India and the University of Tokyo.

Their anion exchange membranes are based on cross-linked terpolymers with built-in silver nanoparticles to slow microbial growth. The membranes are suitable for water desalination and fluoride ion removal by electrodialysis. The preparation of the terpolymers and polyacrylonitrile copolymers was carried out by N-alkylation using various alkyl halides. N-alkylation of the terpolymer through various alkyl groups affected the water absorption, hydrophobicity, ion transport and ionic conductivity of the membrane. Long alkyl groups increased the effectiveness of fluoride removal as well as the oxidative and physical stability of the membranes. The suitability of the composite membranes was verified by testing removal efficiency of fluoride ions (5.5 and 11 mg/L) from a sodium chloride solution (2 g/L) by electrodialysis at an applied voltage of 2 V.

The incorporation of 0.03% silver nanoparticles in the quaternized polymer caused the desired antimicrobial effect. The uniform distribution of silver nanoparticles in the liquid and solid phases was detected by transmission electron microscopy and atomic force microscopy. The attachment of bacteria was quantified counting colony forming units and 100x lower when silver nanoparticles were present in the membrane. The reduced microbial attachment to the membrane surface is therefore due to the antimicrobial effect of the silver nanoparticles. The small amount of 0.03% silver nanoparticles was sufficient to achieve desired antimicrobial effect in the membrane.

After 15 days and at a water temperature of 50°C, no detectable silver leaching occurred. The novel membranes are thus an improved anion exchange solution with antimicrobial properties for efficient removal of fluorine and desalination by electrodialysis.

Methodology

The entire synthesis was carried out in four steps:

  • Step 1: Silver nitrate was diluted with deionized water to produce a 30 mm solution
  • Step 2: Terpolymer and quaternized terpolymers were prepared by free radical polymerization
  • Step 3: Composite additives were prepared by the reduction of silver nitrate with sodium borohydrite in the presence of dimethylformamide
  • Step 4: The membrane was networked with the silver nanoparticles

Characterization of the anion exchange membrane

The membrane was characterized using several analytical methods:

  • UV-VIS and IR spectroscopy
  • Incorporation of silver nanoparticles by scanning electron microscopy, atomic force microscopy and transmission electron microscopy
  • Thermal stability, tensile properties, solubility and further physicochemical and electrochemical properties of the silver nanoparticle composite
  • Desalination and fluoride removal
  • The effectiveness of silver nanoparticles on microbial attachment
  • Energy consumption and efficiency during water desalination and fluoride removal by the composite membrane
  • Membrane stability with respect to pH, temperature and Fenton’s Reagent was evaluated

Reference:

Pal et al. 2021 “Composite Anion Exchange Membranes with Antibacterial Properties for Desalination and Fluoride Ion Removal” ACS ES&T Water 1 (10), 2206-2216, https://doi.org/10.1021/acsestwater.1c00147

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Multifunctional iridium-based catalyst for water electrolysis and fuel cells

Most of the world’s energy needs are currently served by fossil fuels. The International Energy Agency (IEA) annual projection indicates that the global energy demand will increase twice by 2040, mostly in emerging markets and developing economies.

To meet increasing global energy demands and to replace depleting fossil fuels, policy makers believe that alternative clean and renewable energy sources are the best solution. Such renewable energy sources can be electricity for solar, wind or geothermal energy as well as hydroelectric power. The latter, however, has reached a certain degree of saturation in fully industrialized countries.

While solar and wind energy are available in most places of the world at more or less reasonable cost, their biggest disadvantage is that they are intermittent, difficult to store and transport, and difficult to tank in cars, planes and ships. Converting solar and wind energy in hydrogen gas could be an elegant way out of this dilemma as the fuel’s resource can be abundant water. Diversifying the energy mix by adding hydrogen at acceptable cost may prove more efficient with a lower environmental footprint as compared to other fuels. Hence, the interest for  water electrolysis and fuel cells  is constantly growing.

Most of today’s hydrogen is produced through steam reforming of natural gas. However, it can also be made from water electrolysis. Electrolysis is two-electrode reactions: the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode.

Fuel cells reverse the reaction and harvest electricity produced by fusing the hydrogen and oxygen atoms back together to obtain water. While there are different types of fuel cells, those commonly used with hydrogen as fuel are polymer electrolyte membrane fuel cells, or PEMFC. The PEM acronym is also often used for proton exchange membranes, which can be made of polymers, for example Nafion™.  In PEMFC, energy is liberated through the hydrogen oxidation reaction (HOR) at the anode and oxygen reduction reaction (ORR) at the cathode. To become economically feasible, there are still technical challenges of water electrolyzers and fuel cells to overcome. Some technical problems result in serious system degradation.

Water is pumped into a fuel cell where two electrodes split it into hydrogen (H2) and oxygen (O2)

A study published in Nature Communications by researcher of Technical University Berlin and the Korea Institute of Science and Technology, suggests using a novel iridium electrocatalyst with multifunctional properties and remarkable reversibility. While iridium also is precious and one of the platinum group metals, the novel Ir-catalyst was designed for the processes where electrochemical reactions change rapidly, such as the voltage reversal of water electrolysis and PEMFC systems. This would integrate the two energy conversion systems in one and therefore be a great economical benefit over existing solutions.

Challenges

Unexpectedly changing operating conditions such as a sudden shut-down of water electrolysis result in increased hydrogen electrode potentials which lead to degradation hydrogen producing electrodes.

In fuel cells, fuel starvation can occur at the anode, leading to voltage reversal. Ultimately, this causes degradation of fuel cell components such as the catalyst support, gas diffusion layer and flow field plates. It has been proposed to introduce a water oxidation catalyst to the anode of the PEMFcs in order to promote OER since it is the reaction that competes with the carbon corrosion reaction.

Design of a unique iridium-based multifunctional catalyst

For the study, a crystalline multifunctional iridium nanocatalyst has been designed considering the mentioned challenges in water electrolysis and fuel cell operation.

The reason why an iridium-based material has been selected is its remarkable OER activity as well as good HER and HOR catalytic activity. It is a superior material for anodes and cathodes in electrolyzers and for anodes of PEMFC. For comparisons, the researchers synthesized two catalysts  using the modified impregnation method: carbon-supported IrNi alloy nanoparticles with high crystallinity (IrNi/C-HT) and with low crystallinity (IrNi/C-LT).

The findings indicated that the surface of IrNi/C-HT had reversibly converted between a metallic character and an oxidic IrNiOx character. Under OER operation that is, anodic water oxidation, the crystalline nanoparticles form an atomically-thin IrNiOx layer. This oxide layer reversibly transforms into metallic iridium when returning towards more cathodic potentials. The reversal allows the catalyst to return to its high HER and HOR activity.

The experiments also revealed that the performance of IrNi/C-LT sharply decrease after carrying out the OER. The catalyst degradation was due to the irreversible destruction of the amorphous IrNiOx surface.

In situ/operando X-ray absorption near edge structure (XANES) and depth-resolving X-ray photoelectron spectroscopy (XPS) profiles, suggested that the thin layers of IrNiOx possess an increase in the number of d-band holes during OER, due to which catalyst IrNi/C-HT exhibited excellent OER activity. As expected, under HER conditions, the thin IrNiOx layer was reversibly converted to metallic surface. The mechanistic study of the reversible catalytic activity of the IrNiOx layer has been additionally analyzed by electrochemical flow-cell using inductively coupled plasma-mass spectrometry (ICP-MS). The results demonstrate that the reversible IrNiOx layers come from a dissolution and re-deposition mechanism.

In addition, the performance and catalytic reversibility of synthetized electrocatalysts were used to perform HOR and OER in a real electrochemical device and tested under fuel starvation of the PEMFC. Using voltage reversal, the fuel cell was converted into an electrolyzer.

Fuel starvation experiments were conducted in a single PEM fuel cell built using IrNi/C-HT and IrNi/C-LT as the catalytically active components in the anode catalyst layer. The initial fuel cell performance of IrNi/C-LT and -HT was lower than that of the commercial Pt/C catalyst due to the lower HOR and metal composition.

Further results demonstrate that IrNi/C-HT catalyst retained its bifunctional catalytic activity, reversibing between HER and OER in a real device. This approach promoted the reversibility of nanocatalysts, which enable a variety of electrochemical reactions and can be used as catalysts to resist the reverse voltage in fuel cells and water electrolysis systems.

At Frontis Energy, we are looking forward to adding the novel iridium catalyst to our Fuel Cell Shop as soon as it becomes available.

Photo: Iridium / Wikipedia

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Lead removal from water using shock electrodialysis

Lead was widely used in water pipes during the industrial revolution that triggered urbanization and exponential growth of the population in metropolitan centers. The reason for its popularity was the plasticity of the material used in service lines near the end user. The negative health effects have been known since the 1920s, but infrastructure modernization in industrialized countries remains an enormous economic challenge. Lead service lines therefore continue to circulate water in our supply systems. The city of Flint in the northwest of Detroit, for example, received much press attention due to its long struggle with lead poisoning (e.g. Flint Water Crisis). Dissolved lead is highly toxic in a very small concentration and accumulates in body tissues.

The biggest challenge when removing lead from the water cycle is that it is usually dissolved in very low concentrations. Other compounds “mask” the dissolved lead, which makes its removal difficult. Sodium, for instance, is concentrated ten thousand times higher than lead. While nowadays lead can be removed from water by reverse osmosis or distillation, these processes are not selective and thus ineffective. They consume a lot of energy, which in turn is an environmental issue in itself. High energy consumption makes water treatment also very expensive. At the same time, other minerals dissolved ion water are beneficial and therefore desired ingredients that should not be removed.

MIT engineers have developed a much more energy-efficient method to selectively remove lead from water and published their results in the journal ACS EST Water. The new system can remove lead from water in private households or industrial plants and hence from the water cycle. Through its efficiency, it is economically attractive and offers its users the clear advantage of not being poisoned.

The method is the most recent of a number of development steps. The researchers started with desalination systems and later developed it into radioactive decontamination method. With lead the engineers have found an attractive market. It is the first system that is also suitable for private households. The new approach uses a process that was named shock electrodialysis by the MIT engineers. It is essentially very similar to electrodialysis as we know it, as charged ions migrate into an electric field through the electrolyte. As a result, ions are enriched on one side while being depleted on the other.

The difference of the new method is that the electric field moves as a sort of shock wave through the electrolyte and drags dissolved ions along. The shock wave traverses from one side to the other is the voltage increases. The process leads to a lead reduction of 95%. Today, similar methods are also used to clean up aquifers or soil contaminated by solvents. In principle, the shock wave makes the process much cheaper than existing processes because the electrical energy is targeted to remove specifically lead while leaving other minerals in the water. Hence, a lot less energy is consumed.

As usual for bench top prototypes, shock electrodialysis is still too ineffective to be economically viable. Its up-scaling will take time. But the strong interest of potential users will certainly accelerate its industrialization. For a household whose water supply is contaminated by lead, the system could be placed in the basement and slowly clean the water carried by the supply pipes because high rates occur only during peak hours. For this purpose, a water reservoir is necessary, keeping a stock of purified water. This can be a fast and cheap solution for communities such as Flint.

The process could also be adapted for some industrial purposes. The mining and oil industries produce much heavily contaminated wastewater. One imagine to reclaim dissolved metals and sell them to the market. This would create economic an incentives for wastewater treatment. However, a direct comparison with currently existing methods is difficult because the longevity of the developed system is yet to be demonstrated.

At Frontis Energy we are thrilled by the idea of ​​creating economic incentives to help implementing environmentally friendly processes and are already looking forward to a commercial product.

(Photo: Wikipedia)

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Rapid imaging of ion dynamics in battery materials

Particles in lithium-ion batteries are crucial for releasing positively and negatively charged lithium ions. The migration of these ions is a limiting factor for the batteries’ charge and discharge cycles. To develop fast charging batteries, engineers and scientists need to understand how ions in batteries travel. Now, researchers at the University of Cambridge published in the prestigious journal Nature an imaging approach that follows ion movement in functional battery materials in real-time. This technology helps to better understand how lithium-ion batteries work at sub-micrometer sizes and ultimately to construct batteries that charge in only a few minutes.

Scientists need to understand the ion dynamics of active particles to build better batteries but also other galvanic cells such as fuel cells or electrolyzers. Hitherto, traditional approaches for studying lithium-ion dynamics could not trace the rapid changes that occur in batteries that charge in minutes at sub-micrometer precision.

The problem

In lithium-ion batteries, two porous electrodes (positive and negative) are comprised of active particles: carbon, a metal oxide and a binder. The carbon and metal oxides act as electron conductors, while the binder glues the particles to hold the materials together. An electrolyte separates the two electrodes of the battery and acts as a conduit for ions to travel from one electrode to the other.

Engineers need to image the relevant physical and chemical interactions at least ten times faster than the operation time to track the internal ion dynamics of batteries for each of these processes. This is similar to choosing a camera shutter speed appropriate for filming sports – if the shutter speed is too slow, the camera will generate hazy images. The geometry of the active particles and the structure of the porous electrodes are of particular interest for battery development.

Each battery imaging technique has a unique image capture time, defining which battery functions can be accurately recorded. Previously existing approaches take a few minutes to collect an image; therefore, they can only catch processes that take many hours to complete.

Which is the new concept?

Notably, the researchers’ novel technique takes less than a second to acquire a picture, allowing for considerably faster processes to be studied than previously feasible. As an imaging tool, it is also capable of studying batteries while in use and has a sufficient spatial resolution. This sub micrometer resolution is required to track what happens in an active particle. Furthermore, by comparing the evolution of ion concentration in active particles spatially separated in the electrode, the approach can map ion dynamics at the electrode scale.

Methodology

The research team adapted an optical microscopy approach previously used in biology to follow lithium-ion mobility in active battery materials. This method involves passing a laser beam at electrochemically active battery particles storing or releasing lithium ions and then analyzing the scattered light. As additional lithium is stored, the local concentration of electrons in these particles varies. This changes the scattering pattern. As a result, the local change in lithium concentration correlates with the time development of the scattering signals and can be used to locate the particles.

During charge-discharge cycles, ‘active’ materials in battery electrodes store and release ions. The researchers describe in their publication a real-time imaging approach that uses light scattered from active particles to follow ion concentration changes. The intensity of scattering fluctuates with local ion concentration. In their approach, the evolution of scattering patterns over time indicated the system’s ion dynamics. As additional ions are stored in a particle, the colors of the contours show the change in scattering intensity over the previous 5-second period: red denotes an increase in intensity, while blue suggests a reduction. The shifting patterns correspond to the material’s passage from one phase to the next. When a central domain of one phase shrinks and surrounding domains of another phase grows, broken black lines show phase borders.

Conclusion

The new imaging technique can be used for almost all active materials that store lithium or other ions, suffering electronic changes as the ion concentration changes. Because standard approaches cannot directly track changes in local concentration throughout a particle during fast operation, the time variation of ion concentration in active particles remains poorly understood. The new solution will enable electrochemical engineers to test proposed mechanisms of ion transport in these materials by overcoming the imaging problem.

Limitations to this approach

It should be emphasized that the spatial resolution of the authors’ imaging technique is limited by a basic restriction imposed by the wavelength of the light. Shorter wavelengths are required to resolve finer details. In the presented work, the resolution was around 300 nanometers. Another point to consider is that laser scattering is the result of light interacting with just one object. Another drawback is that scattering results from light interacting with the particle’s first couple of atomic levels. As a result, this method only catches the ion movements in the 2D plane related to these atomic layers. Slower approaches, such as X-ray tomography, can be used to gather 3D information.

Way forward

It will be fascinating to follow up on the authors’ findings for individual particles and investigate porous electrodes under the far-from-equilibrium conditions of fast charging.

This approach could also investigate solid electrolytes, which are intriguing but poorly understood battery materials. Suppose light scattering from solid electrolytes varies with local ion concentration, as it does in active materials. In that case, the approach could be used to map how the ion distribution changes in such electrolytes as electric current travel through them. Other systems involving coupled ion and electron transport, such as catalyst layers in fuel cells and electrochemical gas sensors, could benefit from the optical scattering method as well.

In the future, thorough scattering tests using homogeneous particles could help to quantify the link between the scattering response and lithium-ion concentration. The scattering signals might then be converted to local concentrations using this correlation. However, the link between different materials will not always be the same. Machine-learning approaches could accelerate finding these links and automate light scattering analysis.

The authors’ imaging method also opens up the possibility of measuring chemical and physical (geometric) changes in active particles during battery operation at the same time. The difference between the scattering from a particle and that from other materials in a battery (such as the binder or electrolyte) could be used to determine the particle shape and how it evolves. The time required for light scattering a particle would reveal local changes in lithium concentration. These materials store much more energy than current active materials, and their adoption could reduce battery weight. This would be especially advantageous in electric vehicles, as it would allow for longer driving ranges.

The research provides previously unavailable insights into battery materials working in non-equilibrium situations. Their method for directly monitoring changes in active particles during operation will complement previous approaches that rely on destructive battery tests to infer internal alterations. As a result, it has the potential to transform the battery-design process.

Reference details

Merryweather, et al., 2021 “Operando optical tracking of single-particle ion dynamics in batteries”, Nature, 594, 522–528, doi:10.1038/s41586-021-03584-2

Image: Wikipedia