Graphene micro-supercapacitors to replace batteries for microelectonics devices

Will power biomedical implants, active RFID tags, embedded micro-sensors, and flexible electronics
February 27, 2013

Sheet of laser-scribed graphene micro-supercapacitors (credit: UCLA)

UCLA researchers have developed a groundbreaking technique that uses a DVD burner to fabricate miniature graphene-based supercapacitors — devices that can charge and discharge a hundred to a thousand times faster than standard batteries.

These micro-supercapacitors, made from a one-atom–thick layer of carbon, can be easily manufactured and readily integrated into small devices, such as next-generation pacemakers.

The new cost-effective fabrication method holds promise for the mass production of these supercapacitors, which have the potential to transform electronics and other fields.

“The integration of energy-storage units with electronic circuits is challenging and often limits the miniaturization of the entire system,” said Richard Kaner, a member of the California NanoSystems Institute at UCLA, a professor of chemistry and biochemistry, materials science, and engineering at UCLA’s Henry Samueli School of Engineering and Applied Science. “This is because the necessary energy-storage components scale down poorly in size and are not well suited to the planar geometries of most integrated fabrication processes.”

How it works

Fabrication process for laser-scribed graphene micro-supercapacitors. (a) A grapene-based film supported on a sheet is placed on a DVD media disc. The disc is inserted into a LightScribe DVD drive and a computer-designed microcircuit is etched onto the film at precise locations to produce graphene circuits. (b) Copper tape is applied along the edges to improve the electrical contacts, and the interdigitated area is defined by polyimide (Kapton) tape. (c) An electrolyte overcoat is then added. Result is (d,e) a planar micro-supercapacitor. (Credit: UCLA)

“Traditional methods for the fabrication of micro-supercapacitors involve labor-intensive lithographic techniques that have proven difficult for building cost-effective devices, thus limiting their commercial application,” said Maher El-Kady, a graduate student in Kaner’s laboratory.

“Instead, we used a consumer-grade LightScribe DVD burner to produce graphene micro-supercapacitors over large areas at a fraction of the cost of traditional devices. Using this technique, we have been able to produce more than 100 micro-supercapacitors on a single disc in less than 30 minutes, using inexpensive materials.”

The process of miniaturization often relies on flattening technology, making devices thinner and more like a geometric plane that has only two dimensions. In developing their new micro-supercapacitor, Kaner and El-Kady used a two-dimensional sheet of carbon, known as graphene, which only has the thickness of a single atom in the third dimension.

Kaner and El-Kady took advantage of a new structural design during the fabrication. For any supercapacitor to be effective, two separated electrodes have to be positioned so that the available surface area between them is maximized. This allows the supercapacitor to store a greater charge.

In their new design, the researchers placed the electrodes side by side using an interdigitated pattern, akin to interwoven fingers. This helped to maximize the accessible surface area available for each of the two electrodes while also reducing the path over which ions in the electrolyte would need to diffuse. As a result, the new supercapacitors have more charge capacity and rate capability than their stacked counterparts.

The researchers found that by placing more electrodes per unit area, they boosted the micro-supercapacitor’s ability to store even more charge.*

Could be made at home with DVD burner and graphite oxide in water

Kaner and El-Kady were able to fabricate these intricate supercapacitors using an affordable and scalable technique that they had developed earlier. They glued a layer of plastic onto the surface of a DVD and then coated the plastic with a layer of graphite oxide.

Then, they simply inserted the coated disc into a commercially available LightScribe optical drive — traditionally used to label DVDs — and took advantage of the drive’s own laser to create the interdigitated pattern. The laser scribing is so precise that none of the “interwoven fingers” touch each other, which would short-circuit the supercapacitor.

“The process is straightforward, cost-effective and can be done at home,” El-Kady said. “One only needs a DVD burner and graphite oxide dispersion in water, which is commercially available at a moderate cost.”

Flexible electronics uses

The new micro-supercapacitors are also highly bendable and twistable, making them potentially useful as energy-storage devices in flexible electronics like roll-up displays and TVs, e-paper, and even wearable electronics.

The micro-supercapacitors can also be fabricated directly on a chip using the same technique, making them highly useful for integration into micro-electromechanical systems (MEMS) or complementary metal-oxide-semiconductors (CMOS).

Laser-scribed graphene micro-supercapacitors (LSG-MSC) exhibit ultrahigh power and energy densities compared with a commercially available AC-SC, an aluminum electrolytic capacitor and a lithium thin-film battery. LSG micro-devices can deliver ultrahigh power density comparable to those of an aluminum electrolytic capacitor, while providing three orders of magnitude higher energy density. (Credit: UCLA. Data is from Pech, D. et al. Ultrahigh-power micrometre-sized supercapacitors based on onion-like carbon. Nat. Nanotech. 5, 651–654 (2010).)

These micro-supercapacitors show excellent cycling stability, an important advantage over micro-batteries, which have shorter lifespans and which could pose a major problem when embedded in permanent structures — such as biomedical implants, active radio-frequency identification tags and embedded micro-sensors — for which no maintenance or replacement is possible.

As they can be directly integrated on-chip, these micro-supercapacitors may help to better extract energy from solar, mechanical and thermal sources and thus make more efficient self-powered systems. They could also be fabricated on the backside of solar cells in both portable devices and rooftop installations to store power generated during the day for use after sundown, helping to provide electricity around the clock when connection to the grid is not possible.

* 2.1 mWh cm-3 energy density (max), 141 W cm-3 power density

UPDATE Mar. 9, 2013: Added charts showing energy and power density for laser-scribed graphene micro-supercapacitors compared to lithium thin-film batteries and other devices. 

UPDATE April 20, 2013: Added values of energy density and power density in footnote