Exploring Tiny Technologies with the Biggest Impact

Masdar Institute Researchers Use Carbon Nanotubes to Develop Lighter and Stronger Materials, Improved Energy Storage Devices, and Novel Water Purification Technologies

Carbon Nanotubes
MSc student Zainab Karam holds an anode for a lithium-ion battery made of carbon nanotubes (CNT) coated with metal oxides. She believes CNTs could significantly improve a lithium-ion battery's energy density. Photo: Tahra Al Hammadi, Masdar Institute News

Researchers at Masdar Institute are contributing to solving some of the world’s biggest challenges with the tiniest technologies.

Carbon nanotubes (CNTs), which are about 50,000 times smaller than the width of a human hair, are at the heart of several Masdar Institute research projects – focused on applications ranging from energy storage to wastewater treatment – because of their unique ability to make materials stronger, lighter, and much better at conducting heat and electricity.

RATIONALE

CNTs are tiny cylindrical tubes made of tightly bonded carbon atoms, measuring just one atom thick. Despite their miniscule size, nanotubes are more than 100 times stronger than steel, but only one-sixth as heavy, and are extremely stretchy and flexible.

It is the cylindrical shape of CNTs that contributes to their unique thermal, electrical and mechanical properties, making them highly desirable for a growing number of materials applications. Additionally, fabrication of CNTs has become easier and cheaper over its 25-year lifetime, enabling researchers to discover  innovative, commercially viable applications for them.

In fact, the market for CNTs is significantly higher than graphene, the other carbon-based wonder material. According to a recent Lux Research report, the carbon nanotube market is expected to grow to US$560 million by 2025, while the graphene market is expected to grow to US$305 million in the same time.

Researchers at Masdar Institute have been capitalizing on CNTs exceptional properties in a range of research activities, with a focus on making materials stronger, energy storage cheaper, and water purification more sustainable.

ADVANCED COMPOSITES

Nanotubes’ extraordinary mechanical properties make them ideal for reinforced composites, which are strong, lightweight materials commonly used today in aircraft components, car bumpers, and building materials to reduce their weight.

One of the most commonly used composites combines carbon fibers with plastic, creating carbon fiber reinforced polymer composites; a material that is strong and light, but also vulnerable. The many layers in carbon fiber reinforced polymers are very weak at bonded interfaces, which is why researchers at MIT have developed a way to infuse CNTs into a polymer glue (between the fibers) that bonds the layers strongly without disrupting the intrinsic strength of the carbon fibers.

The nano-engineered composites are much less vulnerable to failure. However, engineers still must have robust and low-cost means of understanding the mechanical properties of such materials in order to develop commercial applications. Masdar Institute Assistant Professor of Mechanical and Materials Engineering Dr. Kumar Shanmugam has developed a computer model that offers a much faster approach to determining the strength of nano-engineered multi-scale composites than the traditional approach, which would involve manufacturing hundreds of different composites and putting them through physical tests. This trial and error approach to assessing a composite’s breaking point costs significant time and money and hinders its eventual commercialization.

“Nano-engineered composites can be architectured in hundreds of different ways, as the nanotubes can be dispersed in the matrix around the fiber in many different arrangements, which affect the material’s strength and toughness. It would be extremely difficult and expensive to independently fabricate and test each of these different architectures in order to determine the resulting strength of each arrangement. The computer models developed at Masdar Institute can overcome these limitations,” Dr. Shanmugam explained.

His team’s computer model helps predict the mechanical response of hundreds of different nano-engineered composite architectures without having to physically fabricate the composites. The model even helps to improve the overall performance of nano-reinforced composites by determining ideal arrangements, or dispersions, of CNTs in the matrix. A paper on his research was recently published in the journal Mechanics of Materials.

Dr. Shanmugam’s work leverages computational materials engineering to advance the application of CNTs in advanced composites, which could bolster key UAE sectors that rely on advanced composites, such as transportation, defense and construction.

ENERGY STORAGE

CNTs are also being explored for their potential to help store renewable energy. Efficient energy storage technologies are critically needed if the UAE is going to achieve its renewable energy generation target of 24% by 2021, which is why Assistant Professor of Mechanical and Materials Engineering Dr. Saif Al Mheiri is using nanotubes to improve electrochemical energy storage.

Dr. Al Mheiri is developing nanostructure-coated CNTs to improve a lithium-ion battery’s power density and in turn, its longevity. Currently, lithium-ion batteries average a lifetime of around 300 to 500 full charge-discharge cycles before the battery capacity falls sufficiently to warrant replacement. While this may be acceptable for selected applications, a longer lifetime is desirable for energy storage applications that require very long lifetimes with repeated charge and discharge cycles. .

A major reason behind the lithium-ion battery’s current low power density and short lifetime is that the graphite-based anode traditionally used is unable to store many ions, which are the electrically charged atoms that carry electric current through the battery. CNTs have a much higher  energy capacity than graphite, thanks in part to its tubular and porous shape, which gives it a larger surface area, and in turn more space to hold ions. Their unique shape also allows for better contact with the electrolyte (the liquid or solid that separates the electrodes and allows the  lithium ions to pass through the battery to the electrodes), giving the anode better thermal stability.

However, while CNTs’ unique structural, mechanical and electrical properties help improve the anode’s energy density, when used as an anode material alone, the anode is unable to produce a constant stream of ions, resulting in a phenomenon known as voltage instability.

“The lithium ions that shuttle electric charge between the electrodes get permanently lodged on the CNT anode, preventing ions from getting back through the anode to the cathode during the charging cycle. This leads to irreversible capacity loss and voltage instability,” Dr. Al Mheiri explained.

That challenge presents an opportunity for innovation. To overcome the voltage instability issue caused by CNTs, Dr. Al Mheiri’s team, which includes MSc student Zainab Karam and Post-Doctoral Researcher Dr. Rahmat Agung Susantyoko, is coating CNTs with low-cost metal oxides. These metal oxides will flatten or stabilize the voltage by helping to store and release the lithium ions, thereby preventing the ions from getting lodged onto the anode.

In another project, Dr. Al Mheiri is leveraging CNTs to improve the electrodes used in vanadium redox flow batteries (VRFB). VRFBs are another type of electrochemical energy storage device that generates an electrical current when certain liquids (called electrolytes) flow next to each other while separated by a membrane. Dr. Al Mheiri is researching the possibility of using CNTs to improve the transfer of electrons between the electrolytes through the membrane, which would make the VRFB both cheaper and more efficient, bringing the VRFB technologies one step closer to commercialization.

WATER TREATMENT

CNTs are also being explored for their use as a solution for water purification. Masdar Institute’s Dr. Farrukh Ahmad, Associate Professor of Chemical and Environmental Engineering, believes CNTs may help sustainably remove micro-pollutants from treated wastewater while reducing the UAE’s heavy economic and environmental freshwater production costs.

“Seawater desalination provides over 70% of Abu Dhabi City’s freshwater, but results in high energy demand, a strong carbon footprint and high economic costs. Recycling wastewater offers a much more sustainable and affordable approach to meeting the UAE’s freshwater needs. However, because conventional wastewater treatment does not completely remove residual organic contaminants from water, we think carbon nanotubes could play a critical role in making wastewater treatment technologies extremely efficient and affordable,” he explained.

Dr. Ahmad is leading a team of researchers, which includes PhD student Qammer Zaib, to investigate the use of multi-walled carbon nanotubes (MWCNTs) – which are several concentric CNTs – to remove organic micro-pollutants, such as pharmaceutical ingredients, present in wastewater.

CNTs are particularly useful for water purification because their carbon backbone attracts a wide variety of synthetic organic pollutants. Additionally, their high thermal stability and electrical conductivity allow CNTs to efficiently be regenerated after adsorbing pollutants.

Dr. Ahmad’s team capitalizes on the MWCNTs unique properties as well as the catalytic properties of metal oxides to develop novel nano-composite materials that can be fashioned into coatings, membranes, and filters that allow multiple cycles of treatment, thereby making them sustainable over long-term operation. They are currently studying the ability of MWCNTs to adsorb synthetic organic compounds – which are a class of compounds to which most pharmaceuticals belong – because of the prevalence of pharmaceutical micro-pollutants in treated municipal wastewater effluent not only in the UAE but globally.

The team is in the process of developing optimal methods to make a variety of nano-composite materials that incorporate CNTs and different metal oxides. They have already published their results in two journal papers, filed a patent with the US Patent & Trademark Office (USPTO) on the concept and have another three journal papers in preparation.

Dr. Ahmad believes CNT-based nano-composites could be used to provide the final polishing for treated wastewater, potentially making the water clean enough to be re-injected back into the ground to help replenish the UAE’s groundwater reserves. This could help increase the UAE’s depleting water table and support the country’s goal of establishing a resilient water infrastructure.

CONCLUSION

With these projects and others, Masdar Institute researchers are expanding the applications of CNTs while pushing forward the cutting edge of science. Their efforts to leverage this advanced material to improve energy storage devices, develop sustainable water purification technologies and provide efficient computer models to test materials that utilize CNTs are providing a critical boost to the UAE’s innovation and sustainability goals.