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Green alternative to fluorinated membranes in PEM fuel cells

Polymer electrolyte membrane (PEM) fuel cells have high power density, low operational temperatures. If PEM runs on green hydrogen, it doesn’t even emit carbon. But their fabrication requires perfluorinated sulfonic acid (PFSA) polymers as an electrolyte separator membrane and as an ionomer in the electrode, which is quite expensive. Nafion® is the leading commercial PFSA polymer in the market. However, its manufacturing is costly as well as environmentally hazardous. Therefore, low-cost, environmentally friendly PFSA polymer substitutes are the primary goals for the fuel cell scientific community worldwide.

Researchers of the Texas A&M University and Kraton Performance Polymers Inc. experimented using NEXAR ™ polymer membranes in hydrogen fuel cells, studying different ion exchange capacities. NEXAR ™ polymer membranes are commercially available sulfonated pentablock terpolymers. They published the results in the Journal of Membrane Science . Previous studies showed that changing the ion exchange capacity, that is, the degree of sulfonation of NEXAR ™ membranes can alter the nanoscale morphology and significantly affect mechanical properties. This may influence fuel cell performance. Hence, this polymer may be used as a membrane alternative to Nafion® in fuel cells.

Experimental procedure

  1. Materials under consideration: three different variants of the polymer were taken up each with different Ion Exchange Capacities (IECs: 2.0, 1.5, and 1.0 meq / g), which were named NEXAR ™ -2.0, NEXAR ™ -1.5, and NEXAR ™ – 1.0 respectively.
  2. NEXAR ™ membrane preparation: NEXAR ™ membranes were fabricated by casting the NEXAR ™ solutions onto a silicon-coated Mylar PET film using an automatic film applicator under ambient conditions. Two different sizes were manufactured for measuring mechanical properties and conductivity.
  3. NEXAR ™ membrane characterization: mechanical properties using the size 25 mm (L) x 0.5 mm (W) membrane as test pieces were determined and for proton conductivity test pieces of size 30 mm (L) x 10 mm (W) were tested upon.
  4. Nafion® electrode fabrication: conventional Nafion® electrodes were also fabricated as controls to conduct simultaneous tests.
  5. NEXAR ™ electrode fabrication: NEXAR ™ electrodes were prepared in 2 ways for the 2-part study, each with a different composition.
  6. Electrode characterization: electrode profiling was done using a scanning electron microscopy (SEM).
  7. Membrane electrode assembly (MEA) and fuel cell tests: MEAs were fabricated by placing the membrane in between two catalyst-coated gas diffusion layers (anode and cathode) and heat pressing. The entire fuel cell assembly consisted of an MEA, two gaskets, and two flow plates placed between copper current collectors followed by end plates all held together by bolts. Fuel cell performance tests were conducted under ambient pressure with saturated (100% RH) anode and cathode flow rates of 0.43 L / min hydrogen and 1.02 L / min oxygen respectively.
  8. Electrochemical impedance spectroscopy (EIS): electrochemical impedance spectroscopy was performed after the fuel cell tests and the results analyzed.

Results

NEXAR ™ -2.0 and NEXAR ™ -1.5 had a similar proton conductivity at all temperatures, suggesting that there is a maximum limit in proton conductivity. On the contrary, NEXAR ™ membranes, when compared to Nafion® NR-212 membranes , have sufficient proton conductivity to translate into high power density hydrogen fuel cell performance.

However, NEXAR ™ -2.0 and NEXAR ™ -1.5 membranes (with Nafion® as Ionomer) did not exhibit expected fuel cell performance at all fuel cell operating conditions (temperature, pressure, voltage and humidity). Surprisingly, the NEXAR ™ -1.0 membrane (with Nafion® as Ionomer) showed expected fuel cell performance across all fuel cell operating conditions and comparable power densities to Nafion® , suggesting that NEXAR ™ -1.0 may be a viable alternative to Nafion® in hydrogen fuel cells.

During fuel cell operation the NEXAR ™ -1.0 / NEXAR ™ -1.0 membrane-ionomer was thermally and mechanically stable. These results were supported by the power density results, where MEAs with NEXAR ™ -1.0 membrane-ionomers performed better than all the other MEAs.

From the above-mentioned results it became evident that the NEXAR ™ -1.0 variant was the optimal contender to substitute current state-of-the-art PFSA polymers.

Further, to understand the impact of the NEXAR ™ -1.0 ionomer on fuel cell performance, the composition of the ionomer and solvent mixture ratios in the catalyst ink solution were modified and investigated. Results suggested that NEXAR ™ -1.0 as an ionomer behaves similarly to Nafion® ionomers in fuel cell electrodes.

SEM analysis suggested that the amount of ionomer has a significant impact on the binding of ionomer to the catalyst particles, and consequently on the catalyst layer morphology. Therefore, there is an optimum catalyst / ionomer ratio of 2/1 for the Pt / C ionomer using NEXAR ™ -1.0 in fuel cell electrodes.

Conclusions

Ultimately, NEXAR ™ -1.0 is a potentially commercially viable greener substitute to Nafion® as a membrane and ionomer in PEM Fuel cell applications due to its high conductivity, however; alternative block compositions may improve the properties of the polymer to minimize resistances within the fuel cell to match the performance of Nafion® .

Overall Nafion® / Nafion® MEAs still showed the highest fuel cell performance when overall performance was taken into account but alternative hydrocarbon-based polymer compositions for the NEXAR ™ Polymer might provide a future non-fluorinated polymer as a  Nafion® substitute for PEM fuel cells .

Way forward

More analysis is required to perhaps get an accurate approximation of what variant of the NEXAR ™ polymer might cut the mark, future research may be focused upon exploring variants of Ion Exchange Capacities ranging from say 1 meq / g to 1.5 meq / g. But for now, it can be said that NEXAR ™ polymer shows promise as a viable replacement as a non-fluorinated membrane, and perhaps further research with more iterations of mechanical specs as well as chemical specs of the material we might witness a breakthrough.

Reference: https://doi.org/10.1016/j.memsci.2021.119330 : Sulfonated pentablock terpolymers as membranes and ionomers in hydrogen fuel cells , Journal of Membrane Science, 2021, 119330

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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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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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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)

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A Graphene Membrane Becomes a Nano-Scale Water Gate

Biological systems can control water flow using channels in their membranes. This has many advantages, for example when cells need to regulate their osmotic pressure. Also artificial systems, e.g. in water treatment or in electrochemical cells, could benefit from it. Now, a group of materials researchers behind Dr. Zhou at the University of Manchester in the United Kingdom have developed a membrane that can electrically switch the flow of water.

As the researchers reported in the journal Nature, a sandwiched membrane of silver, graphene, and gold was fabricated. At a voltage of more than 2 V channels it opens its pores and water is immediately channeled through the membrane. The effect is reversible. To do this, the researchers used the property of graphene to form a tunable filter or even a perfect barrier to liquids and gases. New ‘smart’ membranes, developed using a low-cost form of graphene called graphene oxide, allow precise control of water flow by using an electrical current. The membranes can even be used to completely block water when needed.

To produce the membrane, the research group has embedded conductive filaments in the electrically insulating graphene oxide membrane. An electric current passed through these nanofilaments created a large electric field that ionizes the water molecules and thus controls the water transport through the graphene capillaries in the membrane.

At Frontis Energy we are excited about this new technology and can imagine numerous applications. This research makes it possible to precisely control water permeation from ultrafast flow-through to complete shut-off. The development of such smart membranes controlled by external stimuli would be of great interest to many areas of business and research alike. These membranes could, for instance, find application in electrolysis cells or in medicine. For medical applications, artificial biological systems, such as tissue grafts, enable a plenty of medical applications.

However, the delicate material consisting of graphene, gold, and silver nano-layers is still too expensive and not as resistant as our Nafion™ membranes. But unlike Nafion™ you can tune them. We stay tuned to see what is coming next.

(Illustration: University of Manchester)