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Hydroxyl terminated MXene Ti3C2 with monolayers of hydrazine molecules between the MXene layers. Credit: Vadym Mochalin, Drexel University.
The cover story of the April issue of the ACerS Bulletin described the interesting family of carbides and nitrides known as the MAX phases. Their name refers to their composition, where M is a transition metal, A is an ‘A’ group element from the periodic table (specifically the subset of elements 13-16), and X is carbon or nitrogen. Investigators discovered the materials about 40 years ago, and research on these materials really picked up in the mid-1990s. As the article details, the layered-structure MAX phases have properties typical of metals, as well as properties typical of ceramics.
The locus of activity on the resurgent research on the MAX phases is Drexel University in the laboratories of Yury Gogotsi and Michel Barsoum (and extends now to Texas A&M in Barsoum protégé Miladin Radovic’s group).
In 2004, when they were about 10 years into their work on the MAX phases, graphene was “discovered” by a British and Russian team of physicists, setting off a flood of research on two-dimensional materials. The Drexel team thought they, too, might be able to make 2D materials from the MAX phases by selectively removing the ‘A’ constituent. They named these compounds MXenes as a reference to their M and X constituents and structural similarity to graphene.
Like graphene, the materials have good electrical properties and can be intercalated. If the layers can be made thin enough, a host of interesting applications opens up, such as lithium-ion battery electrodes and electrochemical capacitors (supercapacitors). In a new paper in Nature Communications, they report new work on several titanium carbide compounds-Ti3C2, Ti3CN, and TiNbC-that were synthesized by selectively removing aluminum from the corresponding MAX phases.
Computer simulations indicated it might be possible to store a large amount of charge by delaminating (or exfoliating) the MXene layers, but large-scale delamination had been elusive, according to a university press release. Recently, the Drexel team successfully delaminated MXene layers by intercalating with organic molecules. Gogotsi, an author on the paper, explains in an email, “Intercalation reactions, like the one shown [in the image], establish MXenes as full-fledged members of the growing family of 2D materials.”
They were able to make paper like MXene by filtering flakes from the solution. The “paper” is reportedly flexible and electrically conductive. According to the press release, “Critically, this work demonstrates that such material can be synthesized on a large scale.”
Even more, the lithium-ion storage capacity of the paper like MXene was four times that of typical MXene at, according to the abstract, extremely high charging rates. Results also show that the 2D material’s charge-discharge cycle performs better than graphite, which is the material now used for lithium-ion battery anodes.
Gogotsi says in the press release, “By demonstrating chemical intercalation of organic molecules between MXene layers, we have substantially altered properties of MXenes. By separating MXene sheets via intercalation, we produced excellent materials for electrodes of batteries and electrochemical capacitors. “
The team also thinks that 2D MXenes made by intercalation delamination may be used in composites, sensors, catalysis, and other applications.
Lots of interesting work happening out there:
A team of researchers from Drexel University’s College of Engineering has developed a new method for quickly and efficiently storing large amounts of electrical energy. The researchers are putting forward a plan to integrate into the grid an electrochemical storage system that combines principles behind the flow batteries and supercapacitors. The team’s research yielded a novel solution that combines the strengths of batteries and supercapacitors while also negating the scalability problem. The “electrochemical flow capacitor” consists of an electrochemical cell connected to two external electrolyte reservoirs—a design similar to existing redox flow batteries that are used in electrical vehicles. This technology is unique because it uses small carbon particles suspended in the electrolyte liquid to create a slurry of particles that can carry an electric charge. Uncharged slurry is pumped from its tanks through a flow cell, where energy stored in the cell is then transferred to the carbon particles. The charged slurry can then be stored in reservoirs until the energy is needed, at which time the entire process is reversed in order to discharge the EFC. The main advantage of the EFC is that its design allows it to be constructed on a scale large enough to store large amounts of energy, while also allowing for rapid disbursal of the energy when the demand dictates it. “By using a slurry of carbon particles as the active material of supercapacitors, we are able to adopt the system architecture from redox flow batteries and address issues of cost and scalability,” says Yury Gogotsi, director of the A.J. Drexel Nanotechnology Institute and the lead researcher on the project. “A liquid storage system, the capacity of which is limited only by the tank size, can be cost-effective and scalable. …However, we will need to increase the energy density per unit of slurry volume by an order of magnitude, and achieve it using very inexpensive carbon and salt solutions to make the technology practical.”
(PNAS) Compressed sensing is a method that allows a significant reduction in the number of samples required for accurate measurements in many applications in experimental sciences and engineering. In this work, we show that compressed sensing can also be used to speed up numerical simulations. We apply compressed sensing to extract information from the real-time simulation of atomic and molecular systems, including electronic and nuclear dynamics. We find that, compared to the standard discrete Fourier transform approach, for the calculation of vibrational and optical spectra the total propagation time, and hence the computational cost, can be reduced by approximately a factor of five.
Bioengineered replacements for tendons, ligaments, the meniscus of the knee, and other tissues require re-creation of the exquisite architecture of these tissues in three dimensions. These fibrous, collagen-based tissues located throughout the body have an ordered structure that gives them their robust ability to bear extreme mechanical loading. One popular approach has involved the use of scaffolds made from nano-sized fibers, which can guide tissue to grow in an organized way. Unfortunately, the fibers’ widespread application in orthopaedics has been slowed because cells do not readily colonize the scaffolds if fibers are too tightly packed. Researchers at University of Pennsylvania have developed and validated a new technology in which composite nanofibrous scaffolds provide a loose enough structure for cells to colonize without impediment, but still can instruct cells how to lay down new tissue. Via electrospinning, the team made composites containing two distinct fiber types: a slow-degrading polymer and a water-soluble polymer that can be selectively removed to increase or decrease the spacing between fibers. Increasing the proportion of the dissolving fibers enhanced the ability of host cells to colonize the nanofiber mesh and eventually migrate to achieve a uniform distribution and form a truly three-dimensional tissue. The team is currently testing these novel materials in a large animal model of meniscus repair and for other orthopedic applications.
(Gizmag) A solution containing skin cells and proteins has been shown to speed the healing of venous leg ulcers. While the ulcers can be quite resistant to treatment, a team of scientists is now reporting success in using a sort of “spray-on skin” to heal them. Developed by Texas-based Healthpoint Biotherapeutics, the spray-on solution consists of neonatal keratinocytes and fibroblasts (skin cells), which are suspended in a liquid made up of various proteins associated with blood clotting. It was tested using a group of 228 patients afflicted with the ulcers, all of whom were also treated with compression bandages. It was found that when compared that control group, patients receiving the optimum dosage experienced a 16 percent greater reduction in wound area after seven days. After 12 weeks, they were 52 percent more likely to have achieved wound closure. Not only were the ulcers on patients receiving the optimum dosage more likely to heal, but they also healed quicker - in the control group, ulcers that did heal took an average of 21 days longer to do so. It has been suggested that the spray-on solution may also be useful in treating other types of chronic wounds, such as ischemic and diabetic foot ulcers.
(Tech Beat) A recent paper from the National Institute of Standards and Technology argues that before lab-on-a-chip technology can be fully commercialized, testing standards need to be developed and implemented. Standardized testing and measurement methods, paper author Samuel Stavis writes, will enable MEMS LOC manufacturers at all stages of production-from processing of raw materials to final rollout of products-to accurately determine important physical characteristics of LOC devices such as dimensions, electrical surface properties, and fluid flow rates and temperatures. To make his case for testing standards, Stavis focuses on autofluorescence. Stavis states that multiple factors must be considered in the development of a testing standard for autofluorescence, including: the materials used in the device, the measurement methods used to test the device and how the measurements are interpreted. “All of these factors must be rigorously controlled for, or appropriately excluded from, a meaningful measurement of autofluorescence,” Stavis writes.
NASA’s Space Technology Program has selected Deployable Space Systems of Goleta, Calif. and ATK Space Systems Inc., of Commerce, Calif., for contract negotiation to develop advanced solar array systems. High-power solar electric propulsion, where the power is generated with advanced solar array systems, is a key capability required for extending human presence throughout the solar system. The selected proposals offer innovative approaches to the development of next-generation, large-scale solar arrays and associated deployment mechanisms. These advanced solar arrays will drastically reduce weight and stowed volume when compared to current systems. They also will significantly improve efficiency and functionality of future systems that will produce hundreds of kilowatts of power. These advanced solar arrays could be used in future NASA human exploration and science missions, communications satellites and a majority of other future spacecraft applications.
The Department of Energy’s National Renewable Energy Laboratory recently completed a seven-year project to demonstrate and evaluate hydrogen fuel cell electric vehicles and hydrogen fueling infrastructure in real-world settings. The National Fuel Cell Electric Vehicle Learning Demonstration Final Report shows progress in extending vehicle driving ranges and increasing fuel cell durability and discusses NREL’s key findings from the demonstration project. This effort, funded by DOE’s Office of Energy Efficiency and Renewable Energy, supports the Department’s broader strategy to advance U.S. leadership in hydrogen and fuel cell technological innovation and help the industry bring these technologies into the marketplace at lower cost.
Researchers from the University of Toronto have invented a new device that may allow for the uniform, large-scale engineering of tissue. Scientists manipulate biomaterials into the microdevice through several channels. The biomaterials are then mixed, causing a chemical reaction that forms a “mosaic hydrogel”—a sheet-like substance compatible with the growth of cells into living tissues, into which different types of cells can be seeded in very precise and controlled placements. Unique to this new approach to tissue engineering, however, and unlike more typical methods (for instance, scaffolding), cells planted onto the mosaic hydrogel sheets are precisely incorporated into the mosaic hydrogel sheet just at the time it’s being created, generating the perfect conditions for cells to grow. The placement of the cells is so precise can precisely mimic the natural placement of cells in living tissues. And, by collecting these sheets around a drum, the machine is able to collect layers of cells in thicknesses made to measure: in essence, three dimensional, functional tissues.
A typical mid-size family sedan carries a 200 hp engine. How meaningful is that metric for modern, non-agrarian drivers? Similarly, old ways of measuring battery utility might not be all that useful for comparing new energy storage technologies. Credit: Marchal: Wikipedia.
I drive a 2005 Chevy Malibu LS, as do many thousands of other people. I don’t know what the overall satisfaction with the car is, but I love mine. When I was shopping for it my criteria were simple: I wanted a four-door sedan with a sunroof. Because I bought it used, I also got a remote start, heated seats and a big engine. I may never have a sunroof again, but I will always have a remote start, heated seats and, especially, a big engine.
My car owner’s manual reports engine size three ways: 3.5 L, V6, 200 hp. The first two make sense; I know how big a liter is and I can count to six. But, the horsepower spec is harder to grasp. I know what a horse is, but how powerful is one? And, how powerful are 200?
My guess is that most people these days understand horsepower in terms of cars, not in terms of horses. Which begs the question of whether horsepower is still a meaningful unit of measure, or is it obsolete and ready to be put to pasture?
An article in the March 16 issue of Science asks a similar question. The article, “Valuing Reversible Energy Storage,” by John Miller is actually a perspective piece on another article in the same issue describing a new approach to making flexible graphene electrochemical capacitors (supercapacitors) for energy storage.
Miller observes that new energy storage technology capacities are reported as specific energies — “watt-hours per kilogram” — which dates back to a time when heavy batteries were the primary means of storing electrochemical energy. However, new energy storage methods are fundamentally different from batteries, and Miller challenges whether specific energy is the best, or even a sensible, metric, saying,
Despite, and almost in defiance of, the emergence of newer energy storage technologies, however, specific energy continues to be referenced without further consideration as the most important characteristic of any new energy storage technology, the gold standard of its worth or value. This is all wrong — specific energy is only one of many metrics by which the value of storage technology can be measured. Indeed, it may be one of the least important when it comes to assessing its value for use in today’s newest and most innovative applications.
He eschews the other most common figure of merit, “cost per unit of energy ($/kWh).” Presenting cost data comparing electrostatic capacitors ($2.5 million per kWh) and electrolytic capacitors ($1 million per kWh), he says, “Curiously despite such extremely high costs, both technologies are found in almost every piece of electronics available today.” In comparison, lithium ion batteries cost about $1,000 per kWh, leading Miller to conclude that the cost metric “is not actually a very important metric in some decision-making.”
What metrics does Miller suggest for energy storage technologies? His starting recommendations are reversibility, cycle life and storage system shape.
All energy storage devices have loss — you never get back all the energy you put in, but you want to get back as much as possible. The system’s efficiency is a function of the storage media, rate at which energy is loaded into the device (charge), the storage time span, and the rate at which energy is extracted (discharge). Electrochemical capacitors have the advantage over batteries on this basis because they can be charged and discharged with high efficiency, while batteries undergo losses, especially during charging. Miller says, “Energy reversibility is often a most important factor in establishing the value of a storage technology for many of today’s energy conservation applications.”
Cycle life complements reversibility, especially for conservation applications. Miller points to the example of regenerative energy capture during the stopping of a hybrid city transit bus, which could mean more than one million charge/discharge cycles during its service life. With batteries, there is a trade-off between decaying discharge efficiency and replacement cost. Electrochemical capacitors, because they store charge on physical basis rather than a chemical one, can last for the entire service life of an application (like a bus), and therefore can be made the right size from the start.
The shape of the storage system could be an important metric for depending on the application. Miller says that cubic shapes are best for exploiting energy density, and sheet morphologies are best for maximizing power density. He notes that very thin energy storage devices could lead to flexible systems and open up some interesting new applications like smart garments, camouflage, electronic wall paper and more.
Miller is not alone in his call for metrics that make sense for supercapacitors. Late last year Gogotsi and Simon published a short paper also in Science calling for a consistent and meaningful way of reporting device performances, noting that weight-based performance metrics are not useful points of reference for lightweight (carbon) or nanostructured components of storage devices.
Electrochemical capacitors—also called supercapacitors or ultracapacitors—have received a lot of attention in the past decade or so for energy storage because of their ability to deliver large amounts of energy quickly. Batteries, on the other hand, store large amounts of energy, but delivery it slowly.
In a short paper in Science, researchers Gogotsi and Simon, note that some recent papers report energy densities for ECs that are near or even better than the energy densities of batteries. In their paper, they warn that it is important to be careful about comparing metrics. It matters, they say, because “even when some metrics seem to support these claims, actual device performance may be rather mediocre.”
Energy storage devices can be compared using Ragone plots, which map power density (the rate of charge/discharge) against energy density (the storage capacity). The densities are usually presented as weight-based quantities, which Gogotsi and Simon observe may not be accurate for assembled devices “because the weight of the other device components also needs to be taken into account.” Those other components are the same as the components that comprise a Li-ion battery-current collectors, electrolyte, separator, binder, connectors, packaging and carbon-based electrodes.
The carbon electrodes contribute about 30 percent of the total mass of a commercial EC, which means “the energy density of 20Wh/kg of carbon will translate to about 5 Wh/kg of packaged cell.” However, carbon electrodes that are thinner or lighter will reduce energy density even more. For example, an electrode made of the same carbon material as a commercial electrode, but 10 times thinner or lighter, reduces energy density by about one-third, so the 5Wh/kg is reduced to 1.5 Wh/kg.
Gogotsi and Simon make the case that comparing energy and power densities on a volumetric basis would eliminate uncertainty and confusion about performance metrics for ECs. Nanomaterials have a very low packing density, leaving a lot of empty space which the electrolyte can flood, thus increasing the weight of the device without contributing any capacitance. Also, they note that the smaller the device, the less meaningful weight-based metrics are, simply for lack of mass. For example, a carbon nanotube coating electrode contributes negligible weight to the device.
“These systems may show a very high gravimetric power density and discharge rates, but those characteristics will not scale up linearly with the thickness of the electrode, i.e., the devices cannot be scaled up to power an electric car,” they say in the paper.
The authors also caution against relying too heavily on Ragone plots because they convey no information about other important device performance metrics such as cycle lifetime, energy efficiency, in-service temperature range, cost, etc.
The authors conclude with a call to action to the electrochemical energy storage device research community to present energy and power density data in a consistent manner. And, they recommend setting up national and international testing facilities for benchmarking electrodes and EES devices similar to those that exist for evaluating photovoltaics.
“Clear rules for reporting the performance of new materials for EES devices would help scientists who are not experts in the field, as well as engineers, investors, and the general public, who rely on the data published by the scientists, to assess competing claims,” they conclude in the paper.
The paper is “True Performance Metrics in Electrochemical Energy Storage,” Y. Gogotsi and P. Simon, Science, 18 Nov. 2011 (doi: 10.1126/science.1213003)
Graphene, a two-dimensional sheet of carbon, has been the subject of much research since it was discovered in 2004. Its basic properties are fairly well documented, and papers are appearing about possible applications, for example, as supercapacitor electrodes or composite reinforcement. Some novel ideas are emerging as a recent paper on graphene-based artificial muscles illustrates.
Graphene is interesting stuff, but its range of properties is limited by its super-simple chemistry. In multilayer form, weak van der Waals bonding between layers is a limiting factor, too.
However, if two-dimensional materials with more complex chemistries could be made, the door would be opened to tune properties and engineer materials for specific applications.
Well, count among the door-openers Drexel University professors and ACerS Fellows Yury Gogotsi and Michel Barsoum, who describe a process for synthesizing such materials in a new paper, “Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2,” in Advanced Materials (doi: 10:1002/adma.201102306).
As their paper states, “Complex, layered structures that contain more than one element may offer new properties because they provide a larger number of compositional variables that can be tuned for achieving specific properties.”
Barsoum was among the first to work on the so-called MAX phases, which are layered ternary carbides or nitrides. MAX refers to the material’s chemistry: “M” is an early transition metal (Ti, Ta, etc.), “A” is an A-group metal (Al, In, Si, etc.) and “X” is carbon or nitrogen. So far, more than 60 MAX compounds have been identified.
The layered morphology gives them some interesting physical properties that can be metal-like or ceramic-like, but their structure and chemistry also make them good precursor materials for carbide-derived carbons, which are nanostructured porous materials. CDCs are synthesized by removing the “M” and “A” with hydrofluoric acid, and we wrote about some of their anomalous supercapicitance properties in an earlier post.
Wondering whether a hybrid MAX-CDC material could be synthesized, the Drexel team began experimenting with selective removal of “A” elements. MX compounds are chemically stable, and the “A” elements tend to be weakly bonded and are more reactive.
The process was surprisingly simple: They synthesized Ti3AlC2 by first ball milling, and then immersing the resultant powders in a concentrated HF solution at room temperature; next, they rinsed and centrifuged the material. Finally, they used cold pressing to align flakes. They characterized the flakes with XRD, SEM and TEM, and determined the chemistries with X-ray energy dispersive spectrometry in the TEM.
By removing the aluminum (”A” element), they discovered they had formed a new two-dimensional material with the composition Ti3C2. Because its morphology is similar to graphene, the team refers to this class of materials as “MXene.” They report having formed nanosheets (a few layers thick) and conical scrolls.
Gogotsi says they have demonstrated the ability to synthesize MX compounds through exfoliation on a wide range of MAX compounds, including carbo-nitrides. According to the paper they already have “solid evidence for the exfoliation of Ta4AlC3 into Ta4C3 flakes,” but offered no information on the material properties of these latter two compounds.
Given that the MAX compounds comprise a well-defined family of materials, they seem to be good candidates for the Materials Genome Initiative concept. Gogotsi confirmed that they are. “These materials are a perfect case for computational materials engineering. It’s a much better and more efficient way to go after the structures of this family of materials.”
In a NanoWerk article, Gogotsi says, “We are talking about a large family of 2D metal carbides and nitrides, so exploring different structures to find the optimum chemistry for each application is the next step in our work,” plus property characterization and controlling the surface chemistries.
The potential applications of MXene materials is wide. Ab initio simulations predict that they will have large elastic moduli. By varying their surface chemistries (for example, in the paper, the surfaces are terminated by hydroxl and/or fluorine groups) interfaces and bandgaps can be tuned. The large surface areas and layered structure make these materials interesting candidates for Li-ion battery electrodes, pseudocapacitors, polymer composite fillers and other energy and electronic devices.