UCLA Team Develops Graphene-Based Supercapacitor
Researchers at University of California at Los Angeles (UCLA) have developed a supercapacitor or electrochemical capacitor (EC) composed of an expanded network of graphene — a one-atom-thick layer of graphitic carbon. The team demonstrated excellent mechanical and electrical properties as well as exceptionally high surface area.
Richard B. Kaner, UCLA professor of chemistry & materials science and engineering
The California team used a standard LightScribe DVD optical drive to produce their devices. They coated the DVD disc with a film of graphite oxide, and then laser treated it inside a LightScribe DVD drive to produce graphene electrodes.
"Our study demonstrates that our new graphene-based supercapacitors store as much charge as conventional batteries, but can be charged and discharged a hundred to a thousand times faster," said Richard B. Kaner, UCLA professor of chemistry & materials science and engineering.
Electrochemical capacitors (ECs), also known as supercapacitors or ultracapacitors, store substantially higher amounts of charge than traditional capacitors found in consumer devices, such as TVs or PCs. They also charge and discharge faster than batteries. But, despite these advantages, ECs are still limited by low energy densities, which are only a fraction of the energy density of regular batteries. The hunt in on for ECs that can combines the power performance of capacitors with the high energy density of batteries.
Today’s commercial EC layered architecture allow possible harmful leakage of electrolytes, and make it difficult to use them for practical flexible electronics. The California team replaced the EC’s liquid electrolyte with a polymer gelled electrolyte which reduced the device’s thickness and weight, and simplified the fabrication process by eliminating the need for special packaging materials.
"Here, we present a strategy for the production of high-performance graphene-based ECs through a simple all solid-state approach that avoids the restacking of graphene sheets," said Maher F. El-Kady, the lead author of the study and a graduate student in Kaner's lab.
Devices made with Laser Scribed Graphene (LSG) electrodes exhibit ultrahigh energy density values in different electrolytes and also maintain the high power density and excellent cycle stability. This approach holds promise for high power, flexible electronics.
The research team succeeded in fabricating LSG electrodes that overcome the limited performance of today’s commercial ECs. The LightScribe laser caused the simultaneous reduction and exfoliation of graphite oxide and produces an open network of LSG with substantially higher and more accessible surface area. This resulted in a sizable charge storage capacity for the LSG supercapacitors.
Further the “open network” structure of the electrodes, enabled in large part by the flat graphene sheets, helped minimize the diffusion path of electrolyte ions. This was crucial for charging the device. The team concluded that LSG supercapacitors have the ability to deliver ultrahigh power in a short period of time whereas activated carbon cannot.
Additionally, LSG electrodes are mechanically robust and show high conductivity (>1700 S/m) compared to activated carbons (10-100 S/m). This means that LSG electrodes can be directly used as supercapacitor electrodes -- without the need for binders or current collectors as commercial ECs require.
The method improves the mechanical integrity and increases the life cycle of the device even when tested under extreme conditions. Since this high-level of performance has yet to be realized in commercial devices, these LSG supercapacitors could lead the way to ideal energy storage systems for next generation flexible, portable electronics.
"We attribute the high performance and durability to the high mechanical flexibility of the electrodes along with the interpenetrating network structure between the LSG electrodes and the gelled electrolyte," Kaner said. "The electrolyte solidifies during the device assembly and acts like glue that holds the device components together."