Liberating devices from power cords

Imagine if a laptop’s casing served as its battery, or an electric car powered by energy stored in its chassis, or a home where the dry wall and siding store the electricity that runs the lights and appliances. This could soon be a reality as US researchers have developed a supercapacitor that stores electricity by assembling electrically charged ions on the surface of a porous material, instead of storing it in chemical reactions the way batteries do. As a result, supercaps can charge and discharge in minutes, instead of hours, and operate for millions of cycles, instead of thousands of cycles like batteries.

In a paper appearing online 19 May in the journal Nano Letters, graduate student Andrew Westover and Assistant Professor of Mechanical Engineering Cary Pint at Vanderbilt’s Nanomaterials and Energy Devices Laboratory report that their new structural supercapacitor operates flawlessly in storing and releasing electrical charge while subject to stresses or pressures up to 44 psi and vibrational accelerations over 80 g (significantly greater than those acting on turbine blades in a jet engine).

“These devices demonstrate - for the first time as far as we can tell - that it is possible to create materials that can store and discharge significant amounts of electricity while they are subject to realistic static loads and dynamic forces, such as vibrations or impacts,” said Pint. “Andrew has managed to make our dream of structural energy storage materials into a reality.”

When you can integrate energy into the components used to build systems, it opens the door to a whole new world of technological possibilities. That is important because structural energy storage will change the way in which a wide variety of technologies are developed in the future. Furthermore, the mechanical robustness of the device doesn’t compromise its energy storage capability. “In an unpackaged, structurally integrated state, our supercapacitor can store more energy and operate at higher voltages than a packaged, off-the-shelf commercial supercapacitor, even under intense dynamic and static forces,” Pint said.

Close-up of structural supercapacitor. (Joe Howell/Vanderbilt)

One area where supercapacitors lag behind batteries is in electrical energy storage capability: supercaps must be larger and heavier to store the same amount of energy as lithium-ion batteries. However, the difference is not as important when considering multifunctional energy storage systems. Supercapacitors store 10 times less energy than current lithium-ion batteries, but they can last a thousand times longer.

“Battery performance metrics change when you’re putting energy storage into heavy materials that are already needed for structural integrity,” said Pint. “Supercapacitors store 10 times less energy than current lithium-ion batteries, but they can last a thousand times longer. That means they are better suited for structural applications. It doesn’t make sense to develop materials to build a home, car chassis or aerospace vehicle if you have to replace them every few years because they go dead.”

Westover’s wafers consist of electrodes made from silicon that have been chemically treated so they have nanoscale pores on their inner surfaces and then coated with a protective ultrathin graphene-like layer of carbon. Sandwiched between the two electrodes is a polymer film that acts as a reservoir of charged ions, similar to the role of the electrolyte paste in a battery. When the electrodes are pressed together, the polymer oozes into the tiny pores in much the same way that melted cheese soaks into the nooks and crannies of artisan bread in a panini. When the polymer cools and solidifies, it forms an extremely strong mechanical bond.

“The biggest problem with designing load-bearing supercaps is preventing them from delaminating,” said Westover. The use of silicon in structural supercapacitors is best suited for consumer electronics and solar cells, but Pint and Westover are confident that the rules that govern the load-bearing character of their design will carry over to other materials, such as carbon nanotubes and lightweight porous metals like aluminium.

The intensity of interest in ‘multifunctional’ devices of this sort is reflected by the fact that the US Department of Energy’s Advanced Research Project Agency for Energy is investing $8.7 million in research projects that focus specifically on incorporating energy storage into structural materials. There have also been recent press reports of several major efforts to develop multifunctional materials or structural batteries for use in electric vehicles and for military applications. However, Pint pointed out that there have not been any reports in the technical literature of tests performed on structural energy storage materials that show how they function under realistic mechanical loads.