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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

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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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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

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Smart fuel cells catalyzed by self-adjusting anodes improve water management

Hydrogen fuel cells are often regarded as a key element in the green energy transition. Their efficiency is double the thermochemical energy conversion of internal combustion engines. Hydrogen fuel cells convert the chemical energy of hydrogen and oxygen directly into electricity and water. Hence, water plays a central role in fuel cells. It supports ion transport and participates is product of the reaction itself. In an anion exchange membrane fuel cell (AEMFC), for the oxygen reduction reaction to take place, the water in the anode catalyst layer (ACL) must diffuse to the cathode catalyst layer (CCL). In summary, water management is required to remove water from the ACL for higher efficiency of hydrogen diffusion and to balance the water in the entire membrane electrode assembly (MEA).

Significant research efforts have been made to achieve conditions that is suitable for both the anode and cathode in AEMFC. Asymmetric humidification of reactant gases was proposed to be beneficial to achieve well equilibrated water balance between the two electrodes. At higher temperatures, excess anode water evaporates. It also causes deficiencies at the cathode which also requires water to function. To counteract this, a new system that controls the back pressure at the anode and cathode was introduced. However, external control mechanisms (active control) increase the complexity of the system control.

This is where a passive control system involving MEA modification comes into the picture. Moisture control in a fuel cells can be achieved by designing a suitable gas diffusion layer. Adopting different types of hydrophobic materials for the anode and hydrophilic for the cathode can improve overall fuel cell performance. Poly ethylene tetrafluoroethylene (PTFE) copolymer ion exchange membranes, such as Nafion™ have high water mobility. This property can help water back diffusion to avoid anode flooding while preventing dehydration of the cathode. Designing a gradient microstructure or ionomer content within the CCL could also be useful to improve cell performance and durability.

Recent research published in the journal Cell Reports Physical Science addresses these questions. The presented study was carried out to assess a multi-layer CCL design with the gradient capillary force which has a driving effect on water to solve the water balance problem of anodes in AEMFC. For the purpose of the study, platinum on carbon and platinum-ruthenium on carbon were selected as anode catalysts. Ruthenium increases the hydrogen oxidation reaction activity and possesses beneficial structural properties. Water management and performance of AEMFC would be influenced by the structure of the ACL.

Microstructure analysis of ACLs

ACLs composed of different layers of Pt/C and PtRu/C and a mixed version with a similar thickness of around 9-10 µm were analyzed with energy-dispersive X-ray spectroscopy (EDX).

Pt/C ACL had pores of less than 150 nm while PtRu/C catalysts pores ranged between 300-400 nm. The mixed ACL had a pore size <200 nm.

The researchers concluded that Pt/C and PtRu/C ACL had a stratified and gradient pore size distribution spanning across the anion exchange membrane and the gas diffusion layer. The mixed ACL, however, had a homogenous pore structure throughout the MEA.

Membrane electrode assembly using a polymer electrolyte membrane

Moisture adsorption and desorption behavior of ACLs

To investigate moisture adsorption and desorption, the change of the fuel cell’s moisture content was checked with regards to different levels of relative humidity.

It was observed that the moisture content level increased by up to 50% mass weight along with an increase in relative humidity from 20% to 80%.

With an extended equilibrium time for a relative humidity of 80%, the moisture content of Pt/PtRu and PtRu/Pt ACL began to decrease. This was evidence for the self-adjusting water management behavior.

Desorption at a relative humidity of 60% was done. The water content in ACL showed rapid adsorption and slow-release properties at each relative humidity setting.

Physical adjustment of water behavior was observed in PtRu/Pt ACLs. This was attributed to gradient nano-pores and promoted water transport when water was generated within the ACLs during the electrochemical reactions. It would facilitate fuel cells operation at high current density.

Fuel cell performance of ACLs

To assess the structural effect on water management during operation, fuel cell performance was investigated at different relative humidity and temperature levels.

With increasing relative humidity from 40% to 80%, an increase of the maximum power density was observed as well while the temperature remained constant at 50°C. This was due to higher ionic conductivity at high membrane hydration.

At relative humidity of 100%, a maximum power density of the Pt/PtRu MEA and the mixed MEA decreased, however. The inverted MEA version using PtRu/Pt an increase to 243 mW/cm2 was observed. This suggested that the moisture desorption ability of PtRu/Pt MEA promoted mass transfer during fuel cell operation.

At a temperature of 60°C and 100% relative humidity, the maximum power density of PtRu/Pt reached 252 mW/cm2.

A durability test was conducted for PtRu/Pt MEA. It showed that after continuous operation for more than 16 hours at 100 mA/cm2 the voltage drop was <4%.

Conclusion

It became clear from the tests that the PtRu/Pt anode catalyst layer with its homogenous layer had a better self-adjustment capability for fuel cell water management. The gradient nanopore structure of the catalyst layer made it possible to transport water through the capillary effect. Excess water at the anode could either be transported towards the cathode where it would be used for reaction or towards the gas diffusion layer for its removal prevented flooding. Moreover, this catalyst layer made from PtRu/Pt showed better performance abilities too.

At Frontis Energy we think that this could resolve the issues faced with water management in the fuel cells. Since it is a passive control system that involves modifying the design of the fuel cells internally, intricate external systems could be replaced or complemented. The study certainly helps future fuel cell automation as an interesting new aspect of fuel cell design was discovered that could make them smarter.

Reference: Self-adjusting anode catalyst layer for smart water management in anion exchange membrane fuel cells, Cell Reports Physical Science, Volume 2, Issue 3, 24 March 2021, 100377

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How infrared radiation influences the behavior of interfacial water

Despite a common belief, very little is known about the structure of water and interfacial interactions. Interfacial water that is adsorbed on the surface of the hydrophilic materials is formed by both water-surface and water-water interactions. It has been discovered that the interfacial water differs from the water in bulk and can exclude solutes and microspheres, and hence it is termed an exclusion zone (EZ). EZ water is known to have a higher refractive index, viscosity, and light adsorption at 270 nm. Charge separation is also caused by water-surface interactions. For example, the water EZ near Nafion™ membranes has an electrical potential of −200 mV.

Studies showed that electromagnetic energy can affect interfacial water. Infrared (IR) energy can cause expansion of the size of the EZ leading to charge separation. This study was conducted by researchers of the University of Washington with IR light of varying intensities and wavelengths to see if they can accelerate the process and bring protons into bulk water. The scientists attempted to shed light on the complex nature of aqueous  interfaces.

Experimental analysis

Materials used:

Deionized (DI) water with the resistivity of 18.2 MΩ × cm was purified with a Barnstead D3750 Nanopure Diamond water system. Other materials were a Nafion™ N117 membrane, a potassium phosphate buffer, a pH dye and carboxylate microspheres (1 µm diameter in a 2.5% suspension)

Sample preparation:

Carboxylate microsphere suspensions with a microsphere-to-water volume ratio of 1:300 and pH-sensitive dye with the dye-to-water volume ratio of 1:20 for better visualization were added.

Due to carbon dioxide absorption the water had a slightly acidic pH of 6.35 and was neutralized. To stabilize the pH, a 1 molar potassium phosphate buffer of pH 7.0 made from equal volumes of 1 molar K2HPO4 and KH2PO4 solutions and added at a final concentration of 1 mM.

A Nafion™ membrane of 3 × 20 mm size was pre-soaked in 1 liter of DI water for 24 hours before use.

Control and irradiation experiments:

A thick plastic block chamber was injected with the 1 mL water the containing buffer solution, pH dye, and microspheres. The chamber consisted of a glass slide and a groove in the central vertical plane of the chamber was used to hold the Nafion™ membrane. This setup was placed on the stage of an inverted microscope for observation over 10 min.

For irradiation experiments, mid-infrared (MIR) LED wavelengths at 3.0 μm, and three near-infrared (NIR) LEDs of different wavelengths were used. It was placed 2 mm above the water level in the chamber. The light was kept as continuous as possible with constant emission power. It shone for 5 mins onto the water surface. The temperature of the water samples was obtained using infrared cameras.

Results

Water zones differ from bulk water

Interfacial water excluded dye and microspheres by forming EZ water next to Nafion™. A red zone with of pH 4 was formed beyond the EZ water called proton zone (PZ). The researchers concluded that the protons accumulated there due to growing interfacial water. With the time of contact between Nafion™ and water progressing, the EZ size was doubled as did the PZ. The microspheres drifted away from Nafion™ with time.

Stability of EZ size and PZ size

It was evident from the observation that EZ water was not caused by the substance flowing out of Nafion™. It is believed that the ice-like structure of interfacial water cause EZ and PZ water. This network of hexagonal structure, several hundred microns. Electrostatic attractions exist between the EZ water layers.

Effect of IR radiation on EZ water and PZ water

The proton concentration in PZ water increased with IR intensity along with the size of EZ and PZ. Higher IR intensities weaken the OH bonds aiding those molecules to participate in EZ expansion. IR radiation also caused thermodiffusion with carboxylate microspheres moving away from the IR light spot with increasing intensity.

Effect of NIR on EZ and PZ waters

The study of the effect of NIR on interfacial water can help to better understand light therapy. Red wavelengths and NIR wavelengths are considered suitable due to their ability to deeply penetrate tissue. Light therapy aids in the synthesis of adenosine tri-phosphate (ATP), the universal biological energy currency. This could have medical benefits. Interfacial water could act as a photoreceptor in light therapy, as cells contain macromolecules and organelles. The use of NIR to establish a proton gradient requires further investigation.

Conclusions

The research showed that the  EZ and PZ zones in interfacial water stabilize after five minutes and that infrared radiation can considerably increase the size of these zones with intensity. This is possibly due to the special nature of water present on hydrophilic material surfaces.

It is also evident that IR radiation can help in building up microsphere-free zones − a phenomenon that in turn creates proton-rich zones. This is also  responsible for charge separation in interfacial water. In summary, some of the mysteries regarding the complexity of interfacial water, EZ, and PZ water zones have been clarified but much remains to be studied.

Outlook

As always, further research to understand the nature of EZ and PZ of water is required. For example the viability and the possibility of the use of NIR for light therapy using interfacial water as a photoreceptor should to be studied. This applications has the potential to make a positive impact on medical applications.

References: https://doi.org/10.1016/j.colcom.2021.100397 : Effect of infrared radiation on interfacial water at hydrophilic surfaces, Colloid and Interface Science Communications, Volume 42 , May 2021, 100397

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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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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