LOS ANGELES – A novel graphene-based electrode, produced with a standard LightScribe DVD optical drive, ends the search for an optimal electrochemical capacitor. The discovery could pave the way for a new class of flexible energy-storage devices.
Electrochemical capacitors, also called ultracapacitors or supercapacitors, store higher amounts of charge than regular capacitors and have garnered attention as energy-storage devices because they charge and discharge faster than batteries. However, unlike batteries, they are limited by low energy densities.
Researchers have sought an electrochemical capacitor that combines a battery’s high energy density and a capacitor’s power performance.
Now, scientists from the University of California, Los Angeles, have developed high-performance electrochemical capacitors with graphene electrodes.
“At the early stages of this research, we invented the method of converting graphite oxide to graphene using the LightScribe DVD drives found in our computers,” said Maher El-Kady, a UCLA graduate student and lead author of the study. “Microscopic analysis showed that the produced graphene is well exfoliated without any sticking together.”
Schematic showing the structure of laser-scribed graphene supercapacitors. Courtesy of UCLA.
The LightScribe laser simultaneously produces electrodes with an open network structure reducing the electrolyte ions’ diffusion path and allowing a fast, high-power charge.
“We also measured an interesting specific surface area of over 1500 m2
/g, potentially useful for high charge capacity,” El-Kady said.
“Additionally, the electrical conductivity of this graphene, which is another key factor for high-power supercapacitors, was very decent. We thought this could be the perfect material for making high-performance supercapacitors.”
To demonstrate the idea to his group leader, professor Richard B. Kaner, El-Kady developed a device and used it to light an LED.
“I needed some electrolyte to make the device, and since our lab is not primarily set for making batteries and supercapacitors, I placed a purchase order for a new electrolyte,” he said. “However, I was so excited that I wanted to make it right away, so I looked around the lab for some useful electrolyte. I found an old bottle of an electrolyte that dates back to maybe 10 years ago, which I thought might work. I made the device and charged it for a few seconds and, interestingly enough, it was able to light the LED.”
Evaluations of the device demonstrated ultrahigh energy-density values in various electrolytes, while the high power density and cycle stability of supercapacitors are retained.
“We have tested the device for over 10,000 charge/discharge cycles, and the device maintains about 97 percent of its performance,” El-Kady said. “This contrasts with a lifetime of less than 1000 cycles for conventional rechargeable batteries.”
The team also tested the device’s shelf life over four months and discovered that there was no sign of decrease in performance, he said.
“We believe that our device will pave the way to a completely new class of flexible energy-storage devices,” El-Kady said. “This may find applications as a flexible power supply for roll-up computer displays, keyboards, wearable electronics that harvest and store energy produced by body movements, and as energy-storage systems to be combined with flexible solar cells.”
Commercial batteries and supercapacitors are considered hazardous because of their toxic, corrosive materials. The liquid electrolytes within batteries are known to catch fire under certain conditions, which makes them difficult to discard.
To address this, “we further replaced liquid electrolytes used in commercial electrochemical capacitors with a polymer gelled electrolyte, which also acts as a separator, further reducing the device thickness and weight, and simplifying the fabrication process,” El-Kady said. “This means that electrochemical capacitors are immune from leaking problems.”
To test the solid-state device for flexible storage under real conditions, they placed it under constant mechanical stress and analyzed its performance.
“The supercapacitor continues to function with no degradation, even after bending and straightening multiple times,” he said. “We also tried applying a load on the device and, interestingly enough, the device stored more charge.”
Next, the scientists hope to demonstrate that the materials and devices can be scaled up in a cost-effective manner.
“Our initial calculations show that the price of the precursor, graphite oxide, and the whole process is viable for commercial applications,” El-Kady said. “We are also trying to use this technique to build a number of different devices such as sensors. The combined properties of the laser-scribed graphene make it potentially useful as flexible, all-plastic and inexpensive sensors.”
The work appeared in Science