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'Sandwiching' atomic layers leads to durable energy storage materials

16 August 2015

Drexel University researchers are on the verge of creating new materials, comprising layers of 2D materials, that are super-strong and capable of storing energy.

Drexel materials researchers have developed a method that allows two-dimensional layers of disparate elements to be stacked together to provide durable materials with energy storage capabilities

Using a method they invented for joining disparate elemental layers into a stable material with uniform, predictable properties, Drexel University researchers are testing an array of new combinations that may vastly expand the options available to create faster, smaller, more efficient energy storage, advanced electronics and wear-resistant materials.

Led by postdoctoral researcher Babak Anasori, a team from Drexel’s Department of Materials Science and Engineering created the material-making method that can sandwich 2D sheets of elements that otherwise couldn’t be combined in a stable way. And they proved its effectiveness by creating two entirely new, layered two-dimensional materials using molybdenum, titanium and carbon.

“By ‘sandwiching’ one or two atomic layers of a transition metal like titanium, between mono-atomic layers of another metal, such as molybdenum, with carbon atoms holding them together, we discovered that a stable material can be produced,” says Anasori. “It was impossible to produce a 2D material having just three or four molybdenum layers in such structures, but because we added the extra layer of titanium as a connector, we were able to synthesise them.”

The discovery, which was recently published in the journal ACS Nano, is significant because it represents a new way of combining elemental materials to form the building blocks of energy storage technology such as batteries, capacitors and supercapacitors, as well as super-strong composites. Each new combination of atom-thick layers presents new properties and researchers suspect that one, or more, of these new materials will exhibit energy storage and durability properties disproportional to its size. 

“Due to their structure and electric charge, certain elements just don’t ‘like’ to be combined,” Anasori says. “It’s like trying to stack magnets with the poles facing the same direction - you’re not going to be very successful and you’re going to be picking up a lot of flying magnets.”

But Drexel researchers came up with a way to circumvent this chemistry challenge. It starts with a material called a MAX phase, which was discovered by Professor Michel Barsoum, head of the MAX/MXene Research Group, more than two decades ago. A MAX phase is like the primordial ooze that generated the first organisms; all the elements of the finished product are in the MAX phase, waiting for the researchers to impose some order.

That order was imposed by Barsoum and Professor Yury Gogotsi who heads the Drexel Nanomaterials Group, when they first created a stable, two-dimensional, layered material called MXene in 2011.

To create MXenes, the researchers selectively extract layers of aluminium atoms from a block of MAX phase by etching them out with an acid.

Microscopic images reveal the ordered configuration of Drexel's new layered MXene

As far as energy storage materials go, MXenes were a revelation. Prior to their discovery, graphene was the first two-dimensional material to be touted for its potential energy storage capabilities. But, as it was made up of only one element, carbon, graphene was difficult to modify in form and therefore had limited energy storage capabilities. The new MXenes have surfaces that can store more energy.

Four years later, the researchers have worked their way through the section of the Periodic Table with elements called 'transition metals', producing MAX phases and etching them into MXenes of various compositions all the while testing their energy storage properties.

Anasori’s discovery comes at a time when the group has encountered an obstacle on its progress through the table of elements.

“We had reached a bit of an impasse, when trying to produce molybdenum containing MXenes,” Anasori says. “By adding titanium to the mix we managed to make an ordered molybdenum MAX phase, where the titanium atoms are in the centre and the molybdenum on the outside."

Now, with the help of theoretical calculations done by researchers at the FIRST Energy Frontier Research Centre at the Oak Ridge National Laboratory, Drexel’s team knows that, in principle, it can use this method to make as many as 25 new materials with combinations of transition metals, such as molybdenum and titanium, that previously wouldn’t have been attempted.

“Having the possibility to layer different elements at the thinnest form of material known to the scientific community leads to exciting new structures and allows unprecedented control over materials properties,” Barsoum says. “This new layering method gives researchers an unimaginable number of possibilities for tuning materials’ properties for a variety of high-tech applications.”

Anasori plans to make more materials by replacing titanium with other metals, such as vanadium, niobium, and tantalum, which could unearth a vein of new physical properties that support energy storage and other applications.

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