Combining the oxygen in the air with the lithium in a battery cell could create batteries with more than five times the energy than those currently powering all our electronics.

Read the Arabic story here:

With the transport sector accounting for 24 percent of direct carbon dioxide emissions from fuel combustion, it has a considerable role to play in the global decarburization effort. According to the International Energy Agency, almost three-quarters of these emissions derive from road vehicles although emissions from aviation and shipping continue to increase as well.

To combat global warming and improve the efficiency of electric vehicles (EVs), researchers from Khalifa University investigated the major breakthroughs achieved in developing a new type of battery, using lithium and air. These batteries are smaller, lighter and more energy-efficient than lithium-ion batteries, but they do have their drawbacks too. PhD candidate Khizar Hayat, Prof. Lourdes Vega, Director of the Research and Innovation Center on CO2 and H2 (RICH), and Dr. Ahmed Al Hajaj, Assistant Professor of Chemical Engineering, used multiscale models to identify how to make improvements to this new battery type. They published their results in.

Currently, lithium-ion (Li-ion) batteries are the solution of choice for powering EVs. This type of battery is everywhere: In the decades since their commercial introduction, they have been powering billions of devices including mobile phones, cameras, laptops, e-scooters, and electric vehicles. This is due to their high energy density relative to other battery technologies and lithium being the lightest of all metals with great electrochemical properties.

“Decarbonizing the economy goes beyond power generation,” Dr. AlHajaj said. “Adopting electric vehicles is one such strategy. Rechargeable lithium-ion batteries are the superior technology available on the market today, thanks to their relatively high storage capacity and energy density, but they are still a heavy component in an electric vehicle and can only offer limited mileage before they need to be recharged.”

A lithium-air (Li-air) battery uses the chemical reactions between oxygen and lithium to produce a current flow during discharge: electrons and ions flow from the lithium anode to the cathode. Pairing lithium and ambient oxygen from air could lead to battery cells with the highest possible specific energy, or the most amount of energy crammed into a cell as possible.

Ions moving between the anode and the cathode store the energy. When the battery is in use, electrons follow the external circuit to power the object, and the lithium ions migrate to the cathode. When the battery charges, the lithium build up on the anode, releasing the oxygen at the cathode. Batteries can use electrolytes that are aqueous, using water, or non-aqueous, where the electrolyte fluid does not react with the lithium. The KU team’s research focuses on highlighting the advances in non-aqueous Li-air batteries.

“Although there has been extensive research over the past two decades in enhancing battery storage capacity, in practice, Li-air battery capacity is still not up to the mark,” Dr. AlHajaj said. “Plus, developing novel cathodes to improve capacity is not straightforward. This is where multiscale modelling could be instrumental. It can be used to simulate how various materials could perform and provide insight to designing new cathode structures.”

The performance of a battery is limited by the reactions at the anode and the cathode. Improving either electrode of the battery cell would improve performance. The theoretical specific energy of a non-aqueous Li-air battery is 40.1 megajoules per kilogram, which is comparable to the theoretical specific energy of gasoline at 46.8 megajoules per kilogram.

In practice, Li-air batteries produce around 6.12 megajoules per kilogram. While this is around five times greater than that of a commercial lithium-ion battery, significant advances are needed to ready Li-air batteries for commercial use.

Capturing the multiscale processes happening in a Li-air battery cathode is very difficult using experimental approaches. Modelling, however, speeds understanding and development, delivering insights to the behavior and properties of the active materials in such complex structures.

Using their modelling techniques, the KU research team found that pore size and shape of the porous air cathode are major factors controlling the discharge capacity of a Li-air battery, along with how many pores the cathode material has. Anode material properties and lithium deposition on the anode during charging were also important.

Different shapes of pores were modelled separately to see exactly how cylindrical, plain and spherical pores impact the discharge capacity of battery cells, with the researchers using these results to control pore size distribution and optimal shape to show the potential for significantly improved storage capacities.

“Multiscale modelling provides valuable insights into the multiphysics/multiscale nature of a porous cathode, helping us to optimize and develop novel electrode structures for lithium-air batteries,” Dr. AlHajaj said. “Using our results, we can design better performing batteries for the electric transport sector of the future.”

Jade Sterling
Science Writer
21 January 2022

The post Modeling Li-Air Batteries to Optimize their Performance for Powering Electric Vehicles appeared first on Khalifa University.

Hydrogels aren’t new but conventional fabrication methods leave much to be desired in controlling their detailed structure and properties. A team of researchers from Khalifa University, led by Prof. TieJun Zhang, has investigated how 3D printing can produce hydrogel devices for myriad advanced applications.��

The soft, pliable and thin material that make up more than 90 percent of contact lenses prescribed in the United States are made possible by hydrogels: water-swollen polymeric materials that maintain a 3D structure.

But hydrogels have far more applications than just correcting vision: They are one of the most promising materials, revolutionizing many applications including artificial organs, drug delivery, soft electronics, and enhanced water evaporation and purification.

The 3D network of hydrophilic polymers that can swell in water while maintaining their structure is dynamic, tuneable, harmless to living tissue, biodegradable, and capable of encapsulating large amounts of water.

Recent advances in additive manufacturing — or 3D printing — allow hydrogel fabrication to overcome the limitations of conventional fabrication methods.��

Light-based stereolithography is one such additive manufacturing technique. A light source — a laser or projector — cures liquid resin into hardened plastic. When these resins are exposed to certain wavelengths of light, short molecular chains join together, polymerizing monomers into solidified rigid or flexible shapes. This technique is highly accurate and offers the sharpest details and smoothest surface finishes of all 3D-printing techniques. The main benefit, however, is its versatility.

Khalifa University PhD student Afra S. Alketbi investigated how curing and ink formulation affect the toughening or hardening of the 3D-printed hydrogels. Ms. Alketbi worked with Dr. Hongxia Li, Postdoctoral Fellow; Dr. Aikifa Raza, Research Scientist; Dr. TieJun Zhang, Professor of Mechanical Engineering; and Prof. Yunfeng Shi from Rensselaer Polytechnic Institute (RPI), USA. The researchers found that a hydrogel’s elasticity and pore formation highly depends on the exposure time, light intensity, and the associated degree of crosslinking. Their results were published in.��

When chemically crosslinked hydrogels are produced by stereolithography, the covalent bonding between the chains offers enhanced mechanical strength. However, this mechanical strength is also strongly related to the precursor solution — the 3D printer ”ink.” This dictates the mechanical properties and physical integrity of the resulting hydrogels, along with other physical properties such as how the hydrogels swell with water and their water content once swollen. The researchers also discuss how the addition of solvent to dilute the precursor solution or to allow for the direct print of hydrogels, when using water as the solvent, imposes a new set of challenges. Solvents can hinder the polymerization process and lead to reduced crosslinking and compromised structural integrity.

“This work provides new molecular insights into the relationship between processing and the resulting structure developed by stereolithographic-hydrogel printing.”

To better understand how solvents, the precursor solution, and the amount and intensity of light affect the production of hydrogels, the researchers used molecular simulation. In collaboration with Prof. Shi from RPI in the USA, they combined simulations with mechanical testing of tangible polymers to gain a molecular-level insight into the photo-crosslinked polymers produced by 3D printing.

Various computational methods were used to reveal key morphological features such as the molecular pores and the extent of curing, while swelling dynamics were monitored using environmental scanning electron microscopy. Their results show that the cross-linking density is vital to the physical properties and mechanical integrity of 3D-printed hydrogels during swelling and deswelling.

Light intensity and exposure time can influence photopolymerized polymers to exhibit different characteristics,  Ms. Alketbi said. “We found that the degree of curing is critical to the structure of the hydrogels produced,” she added, “and the strength of the cross-linking determines the hydrogel’s performance when swollen.”

The researchers also found that when hydrogels are prepared with low exposure to the light source, the molecular network can irreversibly collapse.

The video shows swelling and bending of hydrogel micropillars.

The team’s method of visualizing hydrogel swelling and bending behaviors at the microscale can also be used to characterize hydrogel devices and evaluate their performance in situ. The new molecular insights from this work will assist in developing hydrogels for further applications and can also be applied to other photo-crosslinkable polymeric systems. This work was supported by a 2019 Abu Dhabi Award for Research Excellence.

Jade Sterling
Science Writer
23 September 2021

The post Insights into 3D Printing of Hydrogels appeared first on Khalifa University.

The insights from a paper published by Mechanical Engineering PhD student Xuan Li could guide the material design and performance improvement of direct Z-scheme systems and lead to increased interest in the field 

A paper by Khalifa University Engineering PhD student Xuan Li has been published in, providing a comprehensive and timely review on the mechanisms, material systems, and optimizing strategies of a type of photocatalyst, which is a catalyst that generates a chemical reaction using light.

“The rapid economic development and massive use of fossil fuels has caused the environmental problems we’re seeing today,” explained Li. “Air pollution has already become the fourth leading risk factor for human mortality and contributes to around five million deaths per year around the world. Even at low concentrations, the pollutants released from solvents, paints, building materials and furnishings can cause illness, underscoring how the need to produce clean fuels and protect the environment is also crucial for human health.”

Photocatalysis, Li says, is one way these pollutants could be removed from the atmosphere sustainably. With Dr. Lianxi Zheng, Professor, Dr. Corrado Garlisi, Postdoctoral Fellow, Qiangshun Guan, Graduate Student, Dr. Shoaib Anwer, Postdoctoral Fellow, Dr. Khalid Al-Ali, Assistant Professor, and Dr. Giovanni Palmisano, Associate Professor, Li provides an extensive review and discussion of the design process of direct Z-schemes, a type of photocatalyst inspired by natural processes, to combat atmospheric pollution.

What are Photocatalysts?

Photocatalysts are often made with semiconductors and use solar energy to generate electron-hole pairs on the surface of the catalyst. When exposed to sunlight, the ground-state electrons in the semiconductors become excited and “jump” to a higher energy level, leaving behind positively-charged holes. These electrons and holes then interact with the organic molecules in the atmosphere around them.

Two simultaneous reactions occur during photocatalysis: oxidation (when a molecule loses an electron) from the photogenerated holes and reduction (when a molecule gains an electron) from the photogenerated electrons. As the photocatalyst reduces and oxidizes the water and oxygen molecules in the atmosphere around it, several reactive species are created that can break organic pollutants down into clean end-products.

However, there are some fundamental shortcomings in conventional photocatalysts that are preventing them from being used at an industrial scale, which Li believes Direct Z-scheme photocatalysts can overcome. These shortcomings stem from the semiconductors needed, with only a few materials such as titanium oxide and zinc oxide meeting the requirements for producing the necessary reactive radicals with energy efficiency.

What are Direct Z-Scheme Photocatalysts?

Direct Z-scheme photocatalysts are inspired by natural plant photosynthesis. In photosynthesis, plants use two photosystems to separate electrons and holes for efficient reactions in converting carbon dioxide and water into sugar and oxygen. The Z-scheme is a photosystem coupling layout for electron transfer in the light reactions of photosynthesis, where plants transform light energy into chemical energy.

Direct Z-scheme catalysts attempt to mimic the same charge-transfer pathways that plants follow during photosynthesis by using two-semiconductor structures. They are designed in a way that the pathway travelled by electrons and holes follows a ‘Z-scheme’ pathway. This special pathway enhances the photocatalyst’s redox potential (its ability to carry out the oxidation and reduction reactions) and extends the lifetime of electron-hole pairs, making it more efficient.�� 

While there are three types of Z-scheme photocatalysts, direct Z-schemes can maximize the solar energy harvested and can also be used in both the liquid and gas environments.

However, constructing a direct Z-scheme catalyst remains challenging. Various approaches and numerous materials have been proposed in the hope of achieving a highly efficient photocatalyst with broad practical applications. Li’s work focuses on the driving forces of charge transfer to guide the material design.

Reviewing Direct Z-Scheme Photocatalyst Designs

“The core idea of direct Z-schemes is to leverage the synergistic effects that occur between the two semiconductors of a photocatalyst by regulating the charge transfer direction,” explained Li.

“The formation mechanisms, suitable applications, and their performance are strongly dependent on the properties of the two semiconductors and their interactions. For example, the surface nature of the photocatalysts will affect the adsorption and selectivity of the reactant molecules.”

Because there are so many potential materials for developing these photocatalysts, and because their properties influence their performances so greatly, there is no systematic comparison of all developed materials.

In her paper, Li offers universal guidelines on material design of direct Z-schemes, by identifying the main formation mechanisms, considering emerging materials, and noting modification strategies for performance improvement.

“Considering the fact that new materials and new material properties always play fundamental and promoting roles in technology development, it is vital to provide a materials-focused review for direct Z-scheme photocatalysts,” said Li.

Among all available material systems, wide band-gap semiconductors remain the most popular in building reliable direct Z-schemes. Their high potentials on both reductive and oxidative reactions allow use in a wide range of applications from carbon dioxide reduction to organic pollutant degradation, but they generally show a low efficiency in solar light utilization.

In contrast, some visible-light semiconductors show a much higher energy efficiency but suffer from issues with photocorrosion. The researchers indicate that coupling a wide band-gap semiconductor and a narrow band-gap semiconductor could possibly lead to maximizing the light spectrum and notable photocatalytic efficiency.

Organic materials offer great flexibility in building Z-schemes due to being able to tune their morphology and physiochemical properties. Metal-organic frameworks could boost the effectiveness of the material system by modifying the metal and organic crosslinkers, boosting their surface area and extended light absorbance.

“Direct Z-scheme systems are extremely promising and present unique advantages, but the field is still in its early stages with the main emphasis on concept demonstration and efficient light utilization,” said Li. “Considering the enormous variety of known and unknown pollutants in the environment, future studies should focus more on developing direct Z-scheme catalyst systems with high redox capabilities that can degrade a broad range of organic pollutants.”

The insights discussed in this work could help guide engineers to design better photocatalysts with optimized materials and improved performance. Improved photocatalysts can in turn contribute significantly to global efforts to produce clean energy and clean the air.

Another advantage of Li’s review is that it could attract more material scientists to work in and contribute to this area, so that the maximum potential of direct Z-schemes can be achieved in multiple applications.

Jade Sterling
Science Writer
8 June 2021

The post Khalifa University PhD Student Reviews Material Aspects in Developing Novel Photocatalysts that Could Clean Air More Efficiently appeared first on Khalifa University.

KU Researcher worked with NYUAD to create the first woven calixarene-based covalent organic framework (COF) with plenty of tunable pores for adsorption applications

Synthetic dyes are common ingredients in the textile industry, but because of their general use, they often find their way into waterbodies from industrial wastewater, where they pollute the water and threaten water security.

Removing these polluting dyes can be achieved through adsorption, where the dyes are collected in the pores of highly porous materials that scoop the pollutants from water and trap them in the pores.

Dr. Dinesh Shetty, Assistant Professor of Chemistry, has created a tunable, two-dimensional polymeric network from an organic macrocycle called calixarenes that can selectively adsorb toxic dyes from wastewater.

Dr. Shetty is a member of the Khalifa University Center for Catalysis and Separation (CeCaS), one of KU’s 18 specialized research centers. CeCaS research aims at developing practical solutions to chemical engineering challenges faced by several industries today. In collaboration with researchers from New York University Abu Dhabi, Dr. Shetty has developed a novel structure using calixarenes to remove dyes selectively and efficiently from wastewater. Their work was recently published in the and appeared as a cover article.

Calixarenes are bowl-shaped organic molecules that consist of defined hydrophobic cavities. This unique feature allows host-guest chemistry where calixarenes play the host for small molecules and/or ions.

“Calixarene molecules have been extensively exploited as versatile supramolecular building blocks,” explained Dr. Shetty. “This is due to their ability to adopt different conformations, which refers to the spatial arrangement of atoms in a molecule, and the relative ease with which they can be functionalized, which refers to how easily a calixarene can take on new functions, features, capabilities, or properties by changing its surface chemistry.”

This is particularly true of calixarenes where the ring consists of four aromatic rings. Calixarenes work as excellent adsorbers, but in the monomer form, they can be dissolved in some solvents, which would hinder their practical use.

In a macroscopic architecture, however, calixarenes become insoluble in almost every solvent, especially in water.

To create calixarenes with a macroscopic architecture, Dr. Shetty and his team turned to covalent organic frameworks, or COFs. COFs are a class of materials that form two- or three-dimensional structures through reactions between their organic components, resulting in strong, covalent bonds that create porous, crystalline materials.�� 

“COFs have proven to be an important class of porous materials on account of their well-defined structures, tuneable pore functionality, and good chemical stability,” explained Dr. Shetty.

Giving calixarenes a macroscopic architecture is very challenging but if successful could allow them to be incorporated into practical platforms such as powders in cartridges or membranes.

While few researchers have attempted to build COFs with multiple ringed, or macrocyclic molecules, like calixarenes, Dr. Shetty’s team realized that a calixarene-based COF would be an ideal way to remove toxic dyes from industrial wastewater.

Dr. Shetty, along with Trabolsi research group at NYUAD, developed the first woven structures of calixarene-based COFs, making a 2D network that can be delicately tuned for each application. They joined calixarenes by creating covalent bonds between the organic molecules to link them together, then these calixarene chains were interwoven by slotting one calixarene into the bowl-shape of another, effectively stacking the chains.

The synthesized COFs showed well-defined lattice structures, indicating a highly crystalline nature for both COFs, with plenty of pores for adsorption applications. By varying the concentration of calixarene units in the solution, the stacking orientation in the COFs can be altered, meaning researchers can create both interpenetrated (meaning catenated) and non-interpenetrated frameworks. The resulting COFs featured wavy layers containing the calixarene cavities, making them attractive candidates for adsorbing small molecules.

The researchers validated the materials ability by using them for selective removal of cationic-dyes from aqueous mixtures.

Importantly, creating a structure with organized pores increases the number of molecular interactions between pollutant molecules and adsorbent. Interestingly, the COFs developed by the research team were exceptionally selective for the cationic dyes in the test mixtures without depending on the size of the molecules. The COFs also demonstrated a highly negative surface charge, allowing charge-selective removal of the dye molecules.

“With the inherently hydrophobic cavity, anionic surface, and possibility to develop these COFs in a membrane, our work has the potential to bring calixarene chemistry to an exciting materials science horizon,” said Dr. Shetty.

Jade Sterling
Science Writer
20 April 2021

The post A Tunable 2D Covalent Network for Charge-selective Removal of Toxic Dyes from Wastewater appeared first on Khalifa University.