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Highly durable platinum-palladium-based alloy electrocatalyst for PEM fuel cells

To decrease the consumption of fossil fuel-derived energy for transportation, proton exchange membrane fuel cells (PEMFCs) are one of the most promising clean power sources. Their performance, however, strongly depends on the efficiency and durability of the electrocatalyst used for the hydrogen and oxygen reactions occurring at the electrodes. Noble metals such as platinum and gold are still considered as the most efficient catalysts. At the same time, their high cost and scarcity are major road blocks for scale commercialization of these energy devices.

Various solutions of catalyst design are intensively investigated in order to make this technology economically successful. Searching for high catalyst activity and durability for fuel cells is in focus of current research and development. To date, state-of-the-art electrocatalysts are based on carbon materials with varying platinum loadings.

Ultra-high active platinum group metal (PGM) alloy catalyst

Although, recent research reported ultra-high activity of some metal alloy catalysts, problems still remain. Some of these issues are related to utilization of high atomic percentages of PGM (sometimes up to 75% Pt), poor durability and performance under industrial conditions. In search for new solutions, researchers of the State University of New York at Binghamton, USA, and their collaborators reported a new design in journal Nature Communication: a highly-durable alloy catalyst was obtained by alloying platinum and palladium at less than 50% with 3d-transition metals (Cu, Ni or Co) in ternary compositions.

They addressed the problem of severe de-alloying of conventional alloy catalysts under the operating conditions, resulting in declining performance. For the first time, dynamic re-alloying as a way to self-healing catalysts under realistic operating conditions has been demonstrated to improve fuel cell durability.

Alloy combination and composition

The wet-chemical method was used for synthesis of Pt20PdnCu80−n alloy nanoparticles with the desired platinum, palladium and copper percentages. The selected set of ternary alloy nanoparticles with tunable alloy combinations and compositions, contained a total content of platinum and palladium of less than 50%, keeping it lower than current PGM-based alloy catalysts. The incorporation of palladium into platinum nanomaterials enabled a lower degree of de-alloying and therefore better stability. Additionally, palladium is a good metal partner to platinum due to their catalytic synergy and their resistance to acid corrosion.

To reduce the need for platinum and palladium core catalysts, a third, non-noble transition metal played a central role in the catalytic synergy of alloying formation. Non-noble metals such as copper, cobalt, nickel or similar were used. The platinum-palladium alloy with base metals allowed the researchers to fine tune the thermodynamic stability of the catalysts.

Morphology and phase structure

The thermochemical treatment of carbon-supported nanoparticles was crucial for the structural optimization. The metal atoms in the catalytic nanoparticles were loosely packed with an expanded lattice constant. The oxidative and reductive treatments of the platinum-palladium alloy (PGM <50%) allowed a thermodynamically stable state in terms of alloying, re-alloying and lattice strains. The re-alloying process not only homogenized the inhomogeneous composition by inter-diffusion upon calcination of nanoparticles, but also provided an effective pathway for self-healing following de-alloying.

Single face-centered cubic type structures were observed in Pt20PdnCu80–n nanoparticles (n = 20, 40, 60, 80) nanoalloys. Copper-doping of platinum-palladium alloys reduced the lattice constant effectively, as shown by high energy X-ray diffraction. Maximized compressive strain and maximized activity of the Pt20Pd20Cu60 catalyst confirmed strong correlation between the lattice constants and the oxygen reduction activity.

The researchers demonstrated that the thermodynamically-stable Pt20Pd20Cu60/C catalyst exhibited not only the largest compressive strain after 20,000 cycles, but also high activity and high durability. The discovery that the alloy catalyst remains alloyed under fuel cell operating condition is in sharp contrast to the fully de-alloyed or phase-segregated platinum skin or platinum shell catalysts reported in almost all current literature.

The significance in understanding of the thermodynamic stability of the catalyst system is a potential paradigm shift of design, preparation, and processing of alloy electrocatalysts.

(Photo: Pixabay)

 

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Self-cleaning membranes for biofouling control and prevention in water treatment

Membrane-based water treatment is critical for obtaining potable water, for example through wastewater treatment and seawater desalination. However, membrane fouling remains a common undesirable phenomenon affecting all membrane-based separation processes. Various efforts have been made to either directly control biofouling or to prevent it.

Ceramic membranes have better thermal and chemical stability along with higher fouling resistance and longer lifetimes when compared to polymeric membranes. These properties render ceramic membranes superior to polymers.

During the filtration process, the amount of water that can pass through a membrane is known as membrane flux. Due to membrane fouling, this flux is reduced and the affected membrane needs to be refurbished. Different membrane cleaning strategies have been researched including self-cleaning conductive polymeric membrane and electrically-assisted filtration but neither of them has shown a satisfactory flux recovery behavior.

Previous researches have suggested the use of ‘nano zeolite’ and carbon nanostructures for water treatment and desalination applications.

  • Zeolites are crystalline aluminosilicates possessing a well-defined inorganic structure, whose microporous 3-D channels and pores act as filters.
  • Carbon nanostructures consist of highly entangled carbon nanotubes which are made through a standardized chemical vapor deposition method.

To investigate the use of ceramic membranes made from nano zeolite and carbon nanostructures, a group of researchers at the New York University Abu Dhabi, United Arab Emirates, developed a new electro-ceramic membrane and evaluated its antifouling performance. Their research findings were published in the Chemical Engineering Journal.

Research Approach:

Zeolite / CNS membrane preparation:

Nano zeolite-Y (nano-Y) membranes were prepared by dispersing the desired amounts of nano-Y, carbon nanostructures, and polyvinylidene fluoride (PVDF) binder in a water-alcohol solution.

The suspension was vacuum filtered through a microfiltration membrane filter and the membrane was peeled off from it before drying it at room temperature.

Three different ratios of zeolite and carbon nanostructures were prepared initially, with 60, 70, and 80 wt% zeolite. The carbon nanostructures and the binder were prepared at a ratio of 1:1.

Membrane characterization:

The electrical conductivity and mechanical properties of the dried membranes were investigated.

The surface morphology of the zeolite carbon nanostructure membrane was studied through scanning electron microscopy and transmission electron microscopy.

Other tests including the membrane contact angle test were also performed on the different labeled membranes.

Membrane cleaning setup and antibacterial assessment:

Two foulants, yeast (200 mg / L) and sodium alginate (30 mg / L) were used as biofoulants.

A custom-made cell was designed and a fresh membrane was used for each electrochemical measurement performed using linear sweep voltammetry.

Antibacterial properties of the nano-Y carbon nanostructure membranes were determined by the disk diffusion method. Different bacteria were cultured overnight at 37°C in a shaking incubator at 100 rpm.

Results:

Membrane cross-sections showed a uniform distribution of nano-zeolite particles with the carbon nanostructure. Decreasing tensile strength was seen interpreted as successful nano zeolite incorporation. These values changed from 3.3 MPa to 2.1, 1.1 or 0.3 MPa, respectively for 60, 70 and 80 weight% nano-Y. In addition, a decrease in water contact angle from 84.7±2° to 18±4° was demonstrated within 4 min.

The composite membrane demonstrated enhanced electrocatalytic activity for hydrogen evolution in two foulants; yeast and sodium alginate.
These MF electro-ceramic self-cleaning, anti-bacterial membranes seem promising for various separation processes such as in wastewater treatment, dye separation and oil / water separation where fouling and bacterial growth are a major concern.

(Photo: WET GmbH, Attribution, via Wikimedia Commons)

Reference: https://doi.org/10.1016/j.cej.2020.128395 Electro-ceramic self-cleaning membranes for biofouling control and prevention in water treatment, Chemical Engineering Journal, Volume 415, 2021

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Making zinc-air batteries rechargeable using developed cobalt(II) oxide as a catalyst

Zinc-air batteries are a promising alternative to expensive lithium-ion batteries. Compared with lithium-ion technology, zinc-air batteries have a greater energy density, very low production cost, and superior safety. However, their fundamental inability to recharge has lowered their wide-scale adoption.

Zinc-air batteries use charged zinc particles to store large amounts of electricity at a time. When electricity is required, the charged zinc is combined with oxygen from the air (and water), releasing the stored electricity and producing zincate. This process is known as oxygen reduction reaction (ORR).

Theoretically, this zincate can again be broken down into oxygen and zinc ions by passing electricity through it. This process, in turn, is called oxygen evolution reaction (OER). Using these reactions, zinc-air batteries can be made rechargeable, competing with lithium-ion batteries.

The major challenge of the recharging process is the sluggish kinetics of the reactions which lead to poor cycle life. These batteries require a catalyst that could potentially enhance the ORR and the OER reactions, making their kinetics fast. Hence, the development of highly efficient catalysts is of paramount importance for rechargeable zinc-air batteries.

Previous studies have suggested transition-metal oxides as great bifunctional ORR / OER catalysts because of their ability to provide sites for the reversible adsorption of oxygen. But the methods involved in creating well-defined defects for reversible adsorption of oxygen in such oxides are challenging.

To investigate the use of cobalt(II) oxide nanosheets deposited on stainless steel or carbon cloth as a bifunctional catalyst, a group of researchers from different universities of China and Canada collaborated and conducted several experiments. Their research findings were published in the journal Nano Energy .

Research approach

Preparation of catalyst

Different nano-structures were prepared using simple heat treatment and electrodeposition to test them as bifunctional electrocatalysts. The type of nano-structures prepared were:

      • Cobalt hydroxide  nanosheets on steel and carbon cloth
      • Layered cobalt (II) oxide nanosheet on steel and carbon cloth
      • Cobalt (II) oxide on steel
      • Layered cobalt tetroxide nanosheet on steel

Material Characterization

To understand the characteristics of the prepared samples, various analyticaland tests were carried out:

Charging and discharging tests

Later discharge and charge cycling tests of single cells were operated by the battery testing system.

Results

The simple heat treatment strategy created oxygen vacancy sites. According to the authors, layered cobalt-oxide nano-sheets exhibited excellent bifunctional ORR / OER performance. Investigations suggested abundant oxygen vacancies and cobalt sites be the reason for enhanced ORR / OER performance. Later, the developed layered cobalt-oxide nanosheets on steel were used as an electrode in a rechargeable zinc-air flow battery and a record-breaking cycle life of over 1,000 hours with nearly unchanged voltage was observed. Galvanostatic discharging-charging cycles also demonstrated long life and high energy efficiency.

This research carried out provides a new method to design highly efficient bifunctional ORR / OER catalysts that could be used to enhance the cycle life of rechargeable zinc-air flow batteries. At Frontis Energy we are looking forward to industrial applications.

(Photo: Engineersforum)

Reference: https://doi.org/10.1016/j.nanoen.2020.105409 Wu et al., Cobalt (II) oxide nanosheets with rich oxygen vacancies as highly efficient bifunctional catalysts for ultra-stable rechargeable Zn-air flow battery, 2021

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Solid oxide fuel cells convert methane gas recovered from groundwater

Solid oxide fuel cells (SOFCs) are highly efficient energy conversion devices and have low operating costs. They work at a temperature range of 800 to 1,000 degrees Celsius. This allows for the possibility of using internal conversion of hydrocarbon fuels into hydrogen. Methane, methanol, petroleum, and other hydrocarbons can be converted to hydrogen (H2) directly within the fuel cell.

SOFCs have a number of additional advantages over traditional combustion engines or other types of fuel cells. For example, the high exhaust heat (over 800 degrees Celsius) makes them a useful application in the industry for cogeneration of electricity and heat. Because of combined cycles, high efficiency for electricity production can be achieved. In addition, due to the modular nature of SOFCs, they offer flexible planning of power generation capacity. This way, the use of SOFCs results in a further reduction of carbon dioxide emission.

The greatest advantage of SOFCs is that they can be operated with hydrocarbon fuels such as methane (CH4, the main component of natural gas). The direct use of methane eliminates the need for pre-reformers, thus reducing the complexity, size, and cost of the overall SOFC system.

Methane can be recovered from the decay of organic waste in municipal solid waste landfills, drinking water treatment plants, etc. The gas can also be recovered from groundwater because of the naturally occurring anaerobic degradation of organic matter in the subsurface or the infiltration of methane from natural gas reservoirs.

A research team from the Delft University of Technology assumed that the gas collected from groundwater treatment can be effectively used as fuel in SOFCs and put their hypothesis to a test. They published their results in the journal Journal of Cleaner Production. Currently, the methane recovered from the Drinking Water Treatment Plant (DWTP) of Spannenburg, Netherlands is either released into the atmosphere or flared, wasting a precious resource and contributing to further greenhouse emission in the form of CO2.

SOFCs provide the cleanest of the viable solutions of converting recovered methane into electrical energy, which, in turn, can be utilized by the DWTP. This process will decrease the power demands and simultaneously reduce the greenhouse gas emissions of the DWTP.

The entire process was divided into the following steps:

  1. Methane was recovered from groundwater: The groundwater was pumped from the deep-wells directly to a system of vacuum towers, which remove 90% of the dissolved gas using a near vacuum of 0.2 bar.
  2. Subsequent treatment by plate aeration was done to remove the remaining 10% of methane in the groundwater.
  3. Tower aeration used to further remove CO2 before pellet softening process to lower hardness.

Recovered gas sampling:

200 mL of the recovered gas enriched in methane was used to determine the concentration of CH4, H2, Oxygen (O2), nitrogen (N2), carbon monoxide (CO), and CO2.

SOFC set up & thermodynamic approach:

A SOFC test station was used to carry out the experiments. The methane rich gas was fed to the anode and the open circuit potential was logged. Methane must be reformed to hydrogen and CO before electricity can effectively be generated in an SOFC.

Results:

The main components in the sampled gas were methane and CO2 with concentrations of 71 and 23 mol%, respectively. Additionally, the recovered gas contained 9 ppm of hydrogen sulphide (H2S), which can permanently reduce the cell performance of an SOFC. Hydrogen sulphide was effectively removed (<0.1 ppm) with impregnated activated carbon

The use of CH4 recovered from the groundwater in an SOFC helps to mitigate the greenhouse gas emissions and improve the sustainability of DWTPs. The recovered methane gas of the Spannenburg DWTP can be used to run a 915 kW SOFC system. This can supply 51.2% of the total electrical power demand of the plant and decreases greenhouse gas emissions by 17.6%, which is around 1794 tons of CO2.

The annual power generation of the SOFC system can be 8 GWh, which is about 3 GWh more than that produced by an internal combustion engine such as a gas turbine or piston engine.

In the future, the researchers will conduct a long-term tests to determine the safe operating condition of SOFC with respect to the carbon deposition issue. These tests will be extended to the SOFC stack level and pilot plant (in the range of a few kW systems)

(Photo: Indiamart)

Reference: https://doi.org/10.1016/j.jclepro.2021.125877 (A solid oxide fuel cell fueled by methane recovered from groundwater, 2021)

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Production of Green Hydrogen through exposure of nano particles to sunlight

The demand for energy is increasing and raw material for the fossil fuel economy is diminishing. Moreover, the emission of gases from fossil fuel usage significantly degrades air quality. The carbon by-products produced from these fossil fuels severely affect the climate.

Hence, there is a need to find a renewable energy resource, that can be produced, stored, and used easily as per requirement. Hydrogen can be a promising energy resource as it is an abundantly available, non-toxic resource, and can be readily used to store excess electrical energy.

Hydrogen when combined with oxygen in a fuel cell produces electricity and the by-products obtained are water and heat. Based on the method of production of hydrogen it is categorized as blue hydrogen and green hydrogen. Blue hydrogen is produced from fossil fuels such as methane, gasoline, coal while green hydrogen is produced from non-fossil fuels / water. The cleanest way to produce eco-friendly hydrogen is via electrolysis of water where water is electrolyzed to separate hydrogen and oxygen. Renewable energy can be used as a power electrolyzer to produce hydrogen from water. Solar driven photo electrochemical (PEC) water splitting is one of the common method used these days. In photo electrochemical (PEC) water splitting, hydrogen is produced from water using sunlight.

PEC cells comprise of a working photoelectrode and a counter electrode. The photoelectrode consists of semiconductor material with a band gap to absorb solar light and generate an electron-hole pair. The photo-generated charges are responsible for the oxidation of water and its reduction into hydrogen. The PEC suffer devices from low stability and efficiency.

The research team from the  Institut national de la recherche scientifique (INRS) along with researchers from the Institute of Chemistry and Processes for Energy, Environment and Health (ICPEES) , a CNRS-University of Strasbourg joint research lab published a way to significantly improve the efficiency of water dissociation to produce hydrogen by the development of sunlight photosensitive-nanostructured electrodes.

A comparative study between cobalt and nickel oxide nanoparticles deposited onto TiO 2 nanotubes prepared through anodization was carried out. The TiO 2 nanotubes were decorated with CoO (cobalt oxide) and NiO (nickel oxide) nanoparticles using the reactive pulsed laser deposition method. The surface loadings of CoO or NiO nanoparticles were controlled by the number of laser ablation pulses. The efficiency of CoO and NiO nanoparticles as co-catalysts for photo-electrochemical water splitting was studied by cyclic voltammetry, under both simulated sunlight and visible light illuminations and by external quantum efficiency measurements

The entire research work was carried out in the following steps:

Catalyzed Green Hydrogen synthesis
Steps followed to improve the efficiency of hydrogen production

(Source: Favet et al ., Solar Energy Materials and Solar Cells , 2020)

In this study Cobalt (CoO) and Nickel (NiO) oxides were considered as effective co-catalysts for splitting water molecules. Both co-catalysts improved photo-electrochemical conversion of ultra violet as well visible light photons.

However, CoO nanoparticles were found to be the best co-catalyst under visible light illumination, with a Photo Conversion Efficiency almost 10 times higher than for TiO 2 . The performance of CoO nanoparticles got enhanced in the visible spectral region (λ> 400 nm). The possible reason can be a consequence of their visible bandgap which enables them to harvest more photon in the 400-500 nm range and transferring effectively the photo-generated electrons to TiO 2 nanotubes.

At Frontis Energy we are exited about these new discovery to improve hydrogen production from sunlight and hope to see an industrial application soon.

(Image: Engineersforum)

Reference: Favet et al ., Solar Energy Materials and Solar Cells , 2020

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Pilot-scale microbial fuel cells produce electricity from wastewater

In wastewater treatment, aeration is an energy-intensive but necessary process to remove contaminants. Pumps blow air into the wastewater to supply the microbes in the treatment tank with oxygen. In return, these bacteria oxidize organic substances to CO2 and hence remove them from the wastewater. This process is the industrial standard and has proven itself for over a century. If the researchers at Washington State University and the University of Idaho have their way, that is changing now.

In their project, the researchers used a unique microbial fuel cell system they developed to replace aeration. Their novel wastewater treatment system cleans wastewater with the help of microorganisms that produce electricity. These microbes are called electrophiles.

The work should one day lead to less dependence on the energy-intensive treatment processes. Most of the energy in such processes is consumed in the activated sludge and its disposal. The energy consumption in water treatment produces around 4-5% of anthropogenic CO2 worldwide. to put that in perspective, according to the Air Transport Action Group in Geneva, international air transport produced 2.1% CO2 in 2019. The researchers published their work in the journal Bioelectrochemistry. In addition to cutting green house gas emissions, lowering the energy consumption of wastewater treatment would save billions in annual operation and maintenance costs.

Microbial fuel cells allow microbes to convert chemical energy into electricity, much like in a battery. In wastewater treatment, a microbial fuel cell can replace aeration while capturing electrons from wastewater organics. These electrons themselves are in turn a waste product of the microbial metabolism. All living organisms strive to discharge their excess electrons. This process is known as respiration or fermentation. The electricity generated the microbes can be used for useful applications in the wastewater treatment plant itself. The technology kills two birds with one stone. On the one hand, the treatment of the wastewater saves energy. On the other hand, it also generates electricity.

Up until now, microbial fuel cells have been used experimentally in wastewater treatment systems under ideal conditions, but under real and changing conditions they often fail. Microbial fuel cells lack regulation that controls the potential of anodes and cathodes and thus the cell potential. This can easily lead lead to a system failure. The entire cell must then be replaced.

To tackle this problem, the researchers added an additional reference electrode to the system that enables them to control their fuel cell. The system becomes more flexible. It can either work as a microbial fuel cell on its own and consume no energy, or it can be converted so that less energy is used for aeration while it purifies the wastewater more intensively. Frontis Energy uses a similar control system for its electrolysis reactors.

The system was operated for one year without major issues in the laboratory as well as a pilot in a wastewater treatment plant in Idaho. It removed contaminants at rates comparable to those in a classic aeration tanks. In addition, the microbial fuel cell could possibly be used completely independent of grid power. The researchers hope that one day it could be used in small wastewater treatment plants, such as cleaning livestock farms or in remote areas.

Despite the progress, there are still challenges to be overcome. They are complex systems that are difficult to build. At Frontis Energy we specialize in such systems and can help with piloting and commercialization.

(Photo: Wikipedia / National University of Singapore)

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Biochar from waste removes pharmaceuticals from wastewater

Biochar is a coal-like substance that is mainly made from agricultural waste products. It can remove contaminants such as pharmaceuticals from treated wastewater. This is the result of research carried out by scientists of the Pennsylvania State University and the Arid Lands Agricultural Research Center in Arizona. The biochar was made from two agricultural residues common in the US: cotton and guayule.

To test the ability of biochar to adsorb pharmaceuticals from treated wastewater, the scientists compared three common compounds. During adsorption, a material like a pharmaceutical adheres to the surface of solid biochar particles. In the case of absorption, in turn, one material is taken up into another, such as in a sponge.

The shrub guayule grows in the dry southwestern US and its waste was used for the biochar tested. Among bonatics, it is also called Parthenium argentatum. The shrub is cultivated as a source of rubber and latex. The plant is chopped to the ground and its branches crushed to extract the latex. The dry, mushy, fibrous residue that remains after the stalks are chopped up to extract the latex is called bagasse.

The results are important as they demonstrate the potential of biochar made from abundant agricultural waste. If it wasn’t re-used, this waste would have to be disposed at a cost. The production of biochar is an inexpensive additional processing step to reduce contamination in treated wastewater used for irrigation.

At the same time, most wastewater treatment plants are currently not equipped to remove emerging contaminants such as pharmaceuticals. If these toxic compounds were removed by biochar, the wastewater could be reprocessed in irrigation systems. This re-use is crucial in regions where water scarcity is a constraint for agricultural production.

The pharmaceutical compounds used in the study were: sulfapyridine, an antibacterial drug commonly used in veterinary medicine; docusate, a widely used laxative and stool softener, and erythromycin, an antibiotic used to treat infections and acne.

The results, published in the journal Biochar, suggest that biochar can effectively adsorb agricultural waste. The biochar obtained from cotton processing waste was a lot more efficient. It adsorbed 98% of the docusate, 74% of the erythromycin and 70% of the sulfapyridine from aqueous solutions. In comparison, the biochar obtained from guayule residues bagasse adsorbed 50% of the docusate, 50% of the erythromycin and only 5% of the sulfapyridine.

Research found that a temperature rise from about 340°C to about 700°C in the oxygen-free pyrolysis process used to convert agricultural waste materials to biochar resulted in a improved capacity for adsorption.

To date, there have been no studies on the use of guayule bagasse to make biochar and remove contaminants, nor are there any for cotton processing waste. Some research has been carried out into the possible removal of other contaminants. However, this is the first study to use cotton gin waste specifically to remove pharmaceuticals from water.

The research is more than theoretical. At Frontis Energy we hope that the technology will soon be available on industrial scale. With cotton gin waste being widespread even in the poorest regions, we believe this source of biochar holds great promise for decontaminating water. The next step would be to develop a mixture of biochar material to adsorb a wider variety of contaminants from water.

(Photo: Wikipedia)

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Highly efficient desalination using carbon nanotubes

Separating liquid compartments is not only important for generating energy in biological cells, respiration that is, but also for electrochemical cells and desalination through reverse osmosis and other processes. Therefore, scientists and engineers intensively research this field. We have already reported in several posts about promising attempts to make membranes cheaper and more effective. New nanomaterials have also been developed.

As a result of climatic changes caused by global warming, water scarcity is increasingly becoming a problem in many parts of the world. Settlements by the sea can secure their supply by desalinating water from seawater and brackish water sources. The process, however, is very energy intensive.

Now, researchers at California’s Lawrence Livermore National Laboratory (LLNL) have developed artificial pores made of carbon nanotubes that remove salt from water so efficiently that they are comparable to already available commercial desalination membranes. These tiny pores are only 0.8 nanometers in diameter. A human hair with a diameter of 60,000 nm. The researchers published the results in the journal Science Advances.

The predominant technology used to remove salt from water is reverse osmosis. A thin-film composite membrane (TCM) is used to separate water from ions. Hitherto the performance of these membranes has, however, been unsatisfactory. There is, for example, always a tradeoff between permeability and selectivity. In addition, exisiting membranes often show insufficient ion repulsion and are contaminated by traces of impurities. This requires additional cleaning stages, which again increase energy costs.

As is so often the case, the researchers got inspired by nature. Biological water channels, also known as aquaporins, are a great model for the structures that can improve performance. These aquaporins have extremely narrow internal pores that compress the water. This enables extremely high water permeability with transport rates of more than 1 billion water molecules per second per pore. Due to the low friction on the inner surfaces, carbon nanotubes represent one of the most promising approaches for artificial water channels.

The research group developed nanotube porins that insert themselves into artificial biomembranes. These engineered water channels simulate the functionality of aquaporin channels. The researchers measured the water and ion transport through their artificial porins. Computer simulations and experiments using the artificial porins in lipid membranes showed improved flux and strong ion repulsion in the channels of carbon nanotubes.

This measurement method can be used to determine the exact value of the water-salt permselectivity in such narrow carbon nanotubes. Atomic simulations provide a detailed molecular view of the novel channels. At Frontis Energy, we are excited about this promising approach and hope to see a commercial product soon.

(Image: Wikipedia)

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Rechargeable PEM fuel cell with hydrogen storage polymer

Energy-converting devices such as fuel cells are among the most efficient and clean alternative energy-producing sources. They have the potential to replace fossil-fuel-based power generators. More specifically, proton exchange membrane fuel cells (PEMFCs) are promising energy conversion devices for residential, transportation and portable applications owing to their high power density and efficiency at low operating temperatures (ca. 60–80 °C). For the complete approach, with electrolytic hydrogen renewable sources, PEM fuel cells can become one of the cleanest energy carriers. This is because water is the final product of such energy conversion systems. Currently, Nafion™ membranes are regularly used as hydrogen barriers in these fuel cells.

A Proton exchange membrane

Sufficient hydrogen gas supply is crucial for practical application of the PEMFC systems. Currently, expensive high-pressure tanks (70 MPa) are state-of-the-art for hydrogen storage. Besides cost, there are other drawbacks such as portability and safety. In order to address these issues, alternative hydrogen storage materials have been extensively investigated. For example, metal hydrides and organic hydride materials, can fix and release hydrogen via covalent bonding.

Now, Dr. Junpei Miyake and colleagues of the University of Yamanashi, Japan, have proposed an “all-polymer” rechargeable PEMFC system (RCFC). The work has been published in Nature Communications Chemistry. Their strategy was to apply a hydrogen-storage polymer (HSP) sheet (a solid-state organic hydride) as a hydrogen-storage medium inside the fuel cell. With this approach, the issues like toxicity, flammability and volatility as well as concerns related to other components such as the fuel reservoir, feed pump and vaporizer were solved. The HSP structure is based on fluorenol/fluorenone groups that take over hydrogen-storage functionality.

In order to test the performance of their HSP-based rechargeable fuel cell, the scientists attached the HSP sheet of the membrane electrode to the catalyst layer of the anode. At the same time, the cathode side was operated as in a regular PEMFC. The researchers reported that an iridium catalyst has been applied to the inside of the HSP sheet to improve the hydrogen-releasing and fixing processes.

Fuel cell operation, cycle performance and durability were tested using cycles of 6 periodic steps. At first, hydrogen was infused into HSP sheet for 2 h, followed by nitrogen gas flushing to remove hydrogen from the anode. Then, heating of the cell up to 80°C to initiated hydrogen release from the HSP sheet. Together with oxygen gas supplied to the cathode side the fuel cell produced constant electrical current.

The team demonstrated that their HSP sheet released 20%, 33%, 51%, or 96% of the total fixed hydrogen gas in 20, 30, 60, or 360 min, respectively. The temperature was 80°C in the presence of the iridium catalyst. Also, the iridium catalyst could absorb up to 58 mol% hydrogen, which was considerably lower than that stored in the HSP. The maximum operation time was approximately 10.2 s / mgHSP (ca. 509 s for 50 mg of HSP) at a constant current density of 1 mA / cm2. The RCFCs reached cycleability of least 50 cycles. In addition, the utilization of a gas impermeable sulfonated poly-phenylene membrane (SPP-QP, another type of PEM) turned out to be a good strategy to enhance the opration time of the RCFC.

The advantageous features of the reported RCFC system include better safety, easier handling and lower weight. These are perfect for example in mobile application such as fuel cell vehicles. However, for the improvement of the RCFC performance, hydrogen storage capacity and kinetics (H2-releasing/fixing reactions) as well as catalyst stability need further improvements.

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Reverse electrodialysis using Nafion™ membranes to produce renewable energy

In the order to address the global need for renewable and clean energy sources, salinity-gradient energy harvested by reverse electrodialysis (RED) is attracting significant interest in recent years. In addition, brine solution coming from seawater desalination is currently considered as a waste; however thanks to its high salinity it can be exploited as a valuable resource to feed RED. RED is an engineered adaptation of nature’s osmotic energy production where ions flow pass the cell membrane in order to produce the universal biological currency ATP. This energy is also harvested by the RED technology.

Now, more than ever there is need for sustainable and environmentally friendly technological solutions in order to keep up with ever growing demand for clean water and energy. The traditional linear way “produce and throw away” does no longer serve the society anymore and the new approach of circular economy has take a place, where any waste can be considered as a valuable resource for another process. In this respect, reverse electrodialysis is a promising electromembrane-based technology to generate power from concentrated solutions by harvesting the Gibbs free energy of mixing the solutions with different salinity. In particular, brine solutions produced in desalination plants, which is currently considered as a waste, can be used as concentrated streams in RED stack.

Avci et al. of the University of Calabria, Italy, have recently published their solution for brine disposal using RED-stack. They have realized that in order to maximize generated power, the high permselectivity and ion conductivity of membrane components in RED are essential. Although Nafion™ membranes are among the most prominent commercial cation exchange membrane solutions for electrochemical applications, no study has been done in its utilization toward RED processes. This was the first reported RED stack using Nafion™ membranes.

A typical RED unit is similar to an electrodialysis (ED) unit, which is a commercialized technology. ED uses a feed solution and the electrical energy, while producing concentrate and dilute, separately. On the other side, RED uses concentrated and dilute solutions that are mixed together in a controlled manner in order to produce spontaneously electrical energy. In a RED stack, repeating cells comprised of alternating cation and anion exchange membranes that are selective for anions and cations. The salinity gradient over each ion exchange membrane creates a voltage difference which is the driving force for the process. The ion exchange membranes are one of the most important components of a RED stack.

The performance of Nafion™ membranes (Nafion™ 117 and Nafion™ 115) have been evaluated under a high salinity gradient conditions for the possible application in RED. In order to simulate the natural environments of RED operation, NaCl solution as well as multicomponent NaCl + MgCl2 have been tested.

Gross power density under high salinity gradient and the effect of Mg2+ on the efficiency in energy conversion have been evaluated in single cell RED using Nafion™ 117, Nafion™ 115, CMX and Fuji-CEM-80050 as cation exchange membranes. Two commercial cation exchange membranes – CMX and Fuji-CEM 80050, frequently used for RED applications, have served as benchmark.

The results show that under the condition of 0.5 M / 4.0 M NaCl solutions, the highest Pd,max was achieved using Nafion™ membrane. This result is attributed to their outstanding permselectivity compared to other CEMs. In the presence of Mg2+ ions, Pd,max reduction of 17 and 20% for Nafion™ 115 and Nafion™ 117 were recorded, respectively. Both membranes maintained their low resistance; however a loss in permselectivity was measured under this condition. Even though, it was reported that Nafion™ membranes outperformed other commercial membranes such as CMX and Fuji-CEM-80050 for RED application.

(Photo: Wikipedia)