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.