Why MXenes Matter
These novel nanomaterials tantalize with super-supercapacitance, electromagnetic impermeability, and sensor possibilities
Even though they were discovered only about a dozen years ago, MXenes (max-eens) have delighted materials engineers with their versatility. These conductive 2D layered nanomaterials are exceptionally durable, impermeable to electromagnetic radiation, and can store energy faster that materials currently used in batteries and supercapacitors.
MXenes are a large family of nitrides and carbides of transition metals, arranged into two-dimensional layers. Two or more metal layers (M) are interspersed by a carbon or nitrogen layer (X). To this is added a surface termination layer (T), giving a general formula of Mn+1XnTx, where n = 1, 2, 3 or 4.
MXenes can be fabricated as nanometer-thin flakes that can be dispersed in water and inked onto any surface. They can also be produced as films, fibers, and powders. Scientists have experimentally made a few dozen different types of MXenes, but there are over 100 theoretical combinations. And with mixed-metal combinations and different surface terminations, there are theoretically an infinite number of MXene compositions. Research into MXene-based applications has skyrocketed recently—optoelectronics, electromagnetic interference shielding, wireless antennas, photocatalysis, water purification, biosensing, and more—and here are three particularly interesting directions:
Supercapacitors you can wear
MXenes were discovered in 2011 at Drexel University, in Philadelphia, thanks to some serendipity and the persistence of Ph.D. student Michael Naguib, who was then being cosupervised by Michel Barsoum and Yury Gogotsi. Since then, Gogotsi, a materials scientist, has worked extensively with MXenes, including creating wearable textiles with MXene-coated yarns.
Gogotsi’s team, along with industry collaborators Accenture Labs, have now developed a supercapacitor patch integrated with a fabric. The supercapacitor properties of MXenes have been well documented, but this is the first time it has been applied to textiles, says Andreea Danielescu, a researcher at Accenture and a study coauthor. The patch can power a 6-volt microcontroller for over 90 minutes.
Creating soft wearables, such as an intelligent shirt that can sense or activate in some way, has been one of the lab’s long-term goals, Danielescu says. But wearables need batteries, which tend to be rigid. “One of the reasons that we looked at MXenes was to start to think about how we can replace that [rigid battery] puck with things embedded in the fabric…that don’t take away from the experience of the fabric itself.”
The MXene coating was directly applied onto the fabric in a 25-by-25-centimeter square. The researchers tried different configurations to find what would power the microcontroller longest, and ended up with a five-layer stacked architecture of cells. The patch can be charged just like any other battery, in a period of about 20 minutes.
There is still a lot of work ahead, Danielescu says. “Right now…we’ve been able to prove that it does work…. The textile at the moment does not last very long before it starts to degrade.” They still need to undergo washability tests, for example, for scalability. “A long-term vision is to combine energy harvesting directly in the textile.” Eventually, she says, They want to create functional textiles for health-care settings, for worker safety, in the wellness industry. “And there’s nothing stopping it from being integrated into your couch [as well],” she adds.
Better than lithium-ion batteries
The energy storage capabilities of MXenes make them a potential alternative to the ubiquitous lithium-ion batteries we use today. But MXene tends to oxidize (read: rust) and degrade very quickly in ambient operating conditions. Removing the rust to prolong its shelf life is possible, but involves hours- or even days-long processes, requiring high temperatures and added chemicals.
A research team at the Australian university RMIT, however, have discovered a simple solution—high-frequency acoustic waves. Electromechanical vibrations in the form of surface-reflected bulk waves at a 10-megahertz frequency removes rust from the MXene. In an emailed statement, three of the authors, Hossein Alijani, Amgad Rezk, and Leslie Yeo, say: “These waves chip away the rust from the surface of MXene, while keeping its core intact.”
The entire process takes a minute, and can be done at room temperature. This, the authors say, holds promise to develop future batteries that are more easily recyclable and last more than three to four times as long as present-day Li-on batteries.
The high-frequency vibrations remove about two-thirds of the rust from the surface, restoring the electrical conductivity of the material close to 100 percent. Despite the remaining presence of some oxides, since the MXene core is intact, the electron flow continues.
Current Li-based energy storage materials have limitations. Lithium mining raises environmental concerns, and Li-ion batteries have known safety hazards, especially when not disposed of properly. Recycling of lithium batteries is also expensive. Therefore, to ensure the commercial potential of MXenes in energy storage, prolonging its shelf life is critical.
The RMIT researchers say they need collaborators from industry to scale up the technology to commercial level. “A very good starting point would be adding MXene as a booster to the current battery configurations.”
Sniffing out cancer
MXene’s electrical conductivity, biocompatibility, hydrophilicity, and large surface area make it conducive to biosensor applications, such as for the detection of cancer. Exosomes, tiny extracellular vesicles that ferry molecular information between cells, are considered excellent biomarkers for cancer through liquid biopsies (a noninvasive cancer-detection method done by screening biofluids like blood, saliva, and urine).
Wen-Fei Dong, a biomedical researcher at the Souzhou Institute of Biomedical Engineering and Technology, in China, who has been working on noninvasive cancer detection for close to half a decade, has proposed a MXene-based electrochemical biosensor with gold nanoarrays.
To build the biosensor, a MXene membrane was first fabricated on a substrate, and then cut into a 10-by-10-millimeter square as a working electrode, says Qiannan You of the University of Science and Technology of China, one of the study’s authors. On this, the gold nanoarrays were deposited. Gold is a transition metal with excellent conductivity, and its combination with the MXene membrane results in increased electrocatalytic activity.
The biosensor was modified with a synthetic antibody for the specific recognition of the exosome biomarkers, which were extracted directly from human lung-cancer cells. The exosomes were labeled with another aptamer to enhance the specificity of the biosensor, You continues. “The application of two different aptamers can reduce the interference of other cellular substances.”
Though the results were promising, she adds that there are a few bottlenecks yet to clinical application, such as interference from other biomarkers in cancer cells. There are technical difficulties in the isolation, purity, enrichment, and recovery of exosomes as well.
What’s the future for MXenes?
There is a multitude of possible applications of MXenes, says Alex Inman, a Ph.D. student in Yury Gogotsi’s lab, the first author of the paper on functional wearables. His work focuses on bringing MXenes into the real world. “We will [need to] start teaming up with computational people looking at properties of different MXenes…and start targeting specific applications,” he says.
It’s a bit of a chicken-and-egg situation from a commercial standpoint, he adds. Researchers are reluctant to look at applications with MXenes because there’s no material supply at scale, while manufacturers don’t see an application out there that can take on scale. “There’s plenty of places where [MXenes] could add value over the current material sets, but you have to see if they can make it out of the lab.”
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