Flexible 3D graphene supercapacitors may power portables and wearables

Flexible 3D graphene supercapacitors may power portables and wearables

Rice University scientists have advanced their recent development of laser-induced graphene (LIG) by producing and testing stacked, three-dimensional supercapacitors — energy-storage devices that are important for portable, flexible electronics.

The Rice lab of chemist James Tour discovered last year that firing a laser at an inexpensive polymer burned off other elements and left a film of porous graphene, the much-studied atom-thick lattice of carbon.

The researchers viewed the porous, conductive material as a perfect electrode for supercapacitors or electronic circuits.

To prove it, members of the Tour group have since extended their work to make vertically aligned supercapacitors with laser-induced graphene on both sides of a polymer sheet. The sections are then stacked with solid electrolytes in between for a multilayer sandwich with multiple microsupercapacitors.

Combining energy and power density

The flexible stacks show excellent energy-storage capacity and power potential and can be scaled up for commercial applications. LIG can be made in air at ambient temperature, perhaps in industrial quantities through roll-to-roll processes, Tour said.

The research was reported in Applied Materials and Interfaces.

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A schematic shows the process developed by Rice University scientists to make vertical microsupercapacitors with laser-induced graphene. The flexible devices show potential for use in wearable and next-generation electronics. (Credit: Tour Group/Rice University)

Capacitors use an electrostatic charge to store energy they can release quickly, to a camera’s flash, for example. Unlike chemical-based rechargeable batteries, capacitors charge fast and release all their energy at once when triggered.

But chemical batteries hold far more energy. Supercapacitors combine useful qualities of both — the fast charge/discharge of capacitors (power density) and high-energy capacity of batteries (energy density) — into one package.

Tour said that while thin-film lithium ion batteries are able to store more energy, LIG supercapacitors of the same size offer three times the performance in power (the speed at which energy flows). And the LIG devices can easily scale up for increased capacity.

“We’ve demonstrated that these are going to be excellent components of the flexible electronics that will soon be embedded in clothing and consumer goods,” he said.

Flexibility

LIG supercapacitors appear able to do all that with the added benefits of flexibility and scalability. The flexibility ensures they can easily conform to varied packages (they can be rolled within a cylinder, for instance) without giving up any of the device’s performance.

“What we’ve made are comparable to microsupercapacitors being commercialized now, but our ability to put devices into a 3D configuration allows us to pack a lot of them into a very small area,” Tour said.

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An electron microscope image shows the cross section of laser-induced graphene burned into both sides of a polyimide substrate. The flexible material created at Rice University has the potential for use in electronics or for energy storage. (Credit: Tour Group/Rice University)

“The other key is that we’re doing this very simply. Nothing about the process requires a clean room. It’s done on a commercial laser system, as found in routine machine shops, in the open air.”

Ripples, wrinkles and sub-10-nanometer pores in the surface and atomic-level imperfections give LIG its ability to store a lot of energy. But the graphene retains its ability to move electrons quickly and gives it the quick charge-and-release characteristics of a supercapacitor.

In testing, the researchers charged and discharged the devices for thousands of cycles with almost no loss of capacitance. The vertical supercapacitors also showed almost no change in electrical performance when flexed, even after 8,000 bending cycles.

The Air Force Office of Scientific Research and its Multidisciplinary University Research Initiative (MURI) and the Office of Naval Research MURI supported the research.

Abstract of Flexible and stackable laser induced graphene supercapacitors

In this paper, we demonstrate that by simple laser induction, commercial polyimide films can be readily transformed into porous graphene for the fabrication of flexible, solid-state supercapacitors. Two different solid-state electrolyte supercapacitors are described, namely vertically stacked graphene supercapacitors and in-plane graphene microsupercapacitors, each with enhanced electrochemical performance, cyclability, and flexibility. Devices with a solid-state polymeric electrolyte exhibit areal capacitance of >9 mF/cm2 at a current density of 0.02 mA/cm2, over twice that of conventional aqueous electrolytes. Moreover, laser induction on both sides of polyimide sheets enables the fabrication of vertically stacked supercapacitors to multiply its electrochemical performance while preserving device flexibility.

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