Printing energy storage at the touch of a button

The ink, developed by a collaboration between Drexel University in Philadelphia and Trinity College, Dublin, is based on a highly-conductive two-dimensional material called MXene, which was invented by Yury Gogotsi and colleagues at Drexel in 2011. According to Gogotsi, who has also led the current research, it is more conductive and more easily applied to a range of different surfaces than previous conductive inks.

MXenes are a family of two-dimensional transition metal carbides and nitrites. In Gogotsi’s earlier work, he produced planar nanocrystals by exfoliating titanium aluminium carbide, which is a typical member of the family showing a layered structure known as a MAX phase, a kind of ductile and machineable ceramic.

"So far only limited success has been achieved with conductive inks in both fine-resolution printing and high charge storage devices," Gogotsi said. "But our findings show that all-MXene printed micro-supercapacitors, made with an advanced inkjet printer, are an order of magnitude greater than existing energy storage devices made from other conductive inks."

A major advantage of MXenes over inks made from materials such as nanoparticle silver, graphene and gallium is that it requires no additives to hold the particles together in the ink to ensure printing quality. Because the materials can mix with liquids such as water and organic solvents while retaining their conductive properties, no additional treatments are needed to remove additives after it is printed. It can just be left to dry with no additional steps.

In a paper in Nature Communications, Gogotsi and colleagues explain how they worked with printing experts at Trinity College to produce a series of MXene ink printouts, including a simple circuit, a micro-super capacitor and some text on substrates including paper, plastics and glass. They found they could print lines of constant thickness and the ink’s ability to conduct current varied with its thickness. All the printouts maintained electric conductivity greater than inks containing carbon nanotubes and graphene. They claim that this means that it could be used for components to keep electronic devices running when the battery dies, protecting from electrical surges, and speeding up the charging process. Completely new technologies may also be possible.

"Compared to conventional manufacturing protocols, direct ink printing techniques, such as inkjet printing and extrusion printing, allow digital and additive patterning, customisation, reduction in material waste, scalability and rapid production," said Babak Anasori, co-author of the MXene ink research. "Now that we have produced a MXene ink that can be applied via this technique, we're looking at a world of new opportunities to use it."