New 'clay' could help remold possibilities for renewable energy

As wind and solar power continue to elbow their way into the nation's energy mix, demand is building for better tools to store the electricity they generate for times when the wind isn't blowing or prolonged cloudiness masks the sun.

Now, a research team says it has produced a clay-like material that when dried displays a remarkable capacity for storing an electrical charge – an important trait for electrodes in batteries or in storage devices known as supercapacitors.

The electrically conductive clay is easy to turn into a range of forms – from thin films a few millionths of a meter thick to watch-battery-size disks. And it is produced with chemicals that are cheaper and easier to handle than those used in alternative materials researchers have been exploring.

The work touches on one in a range of components for rechargeable batteries and supercapacitors that scientists and engineers are working on to boost performance in a relatively small package at a low cost while using materials that are more environmentally friendly than materials used in today's storage devices.

Two factors are driving the demand for a new generation of devices to store electricity, explains Yury Gogotsi, a material scientist and director of Drexel University's A.J. Drexel Nanotechnology Institute in Philadelphia.

"One is the storage of renewable energy," he says. "We face a problem: We need to store order of magnitudes more electrical energy than in the past."

The second is the growing interest in distributed energy sources as a means of improving energy security and reliability, says Dr. Gogotsi, a member of a research team reporting the results in Thursday's issue of the journal Nature.

Even the expanding use of mobile devices, electric vehicles, and the Internet to remotely control systems in homes and offices are contributing to the push for smaller, more reliable, and more powerful electrical-storage devices, he adds.

The discovery of the clay-like material grew out of work Gogotsi and Drexel colleague Michel Barsoum spearheaded to explore the properties of a blend of metals and carbon known as titanium aluminum carbide.

In 2011, the team reported that by using hydrofluoric acid to remove the aluminum, the titanium carbide that remained spread out to form ultra-thin layers that could be rolled into tubes only four-billionths of a meter in diameter. This Lilliputian scale rivals other carbon-based structures known as carbon nanotubes, which host valuable electrical and thermal properties.

Carbon nanotubes are formed from graphene, a layer of pure carbon only one atom thick. In a nod to nanotubes, the team dubbed its new material MXene.

For storage devices, the new material held promise. Its capacity to hold an electrical charge was about three times higher than currently available carbon-based electrodes and comparable to the capacity of newer carbon-based electrodes other teams were studying. But among acids, hydrofluoric acid is particularly nasty to handle.

Michael Ghidiu, a PhD student and the lead author of the Nature paper describing the results, was looking for other approaches to removing the aluminum. He tried a mixture of hydrochloric acid and a fluoride-based salt. After treating the titanium aluminum dioxide with the mixture, then rinsing and drying it, all he had left was a powder – but one that, like clay, can be mixed with water and crafted into a range of shapes and thicknesses.

The surprise came with electrical measurements of the titanium dioxide processed this way. Its electrical capacity was triple that of its predecessor and well within striking range of experimental materials that have a higher capacity but use rarer and more expensive minerals.

The new approach dramatically slashed the time to make an electrode – from about a day to about 15 minutes, the researchers say. And it maintains its storage ability through more than 10,000 charge-discharge cycles.

"This is not the end of the story," Gogotsi says, because his team, as well as others in university and in industrial labs, are just beginning to explore the potential of MXenes.

"We believe that with time ... there will be significant improvements in the properties and in applications that go well beyond just energy storage," he says.