Over the years, the telephone has gone mobile, from the house to the car to the pocket. The University of South Carolina's Xiaodong Li envisions even further integration of the cell phone -- and just about every electronic gadget, for that matter -- into our lives.
He sees a future where electronics are part of our wardrobe.
"We wear fabric every day," said Li, a professor of mechanical engineering at USC. "One day our cotton T-shirts could have more functions; for example, a flexible energy storage device that could charge your cell phone or your iPad."
Li is helping make the vision a reality. He and post-doctoral associate Lihong Bao have just reported in the journal Advanced Materials how to turn the material in a cotton T-shirt into a source of electrical power.
Starting with a T-shirt from a local discount store, Li's team soaked it in a solution of fluoride, dried it and baked it at high temperature. They excluded oxygen in the oven to prevent the material from charring or simply combusting.
The surfaces of the resulting fibers in the fabric were shown by infrared spectroscopy to have been converted from cellulose to activated carbon. Yet the material retained flexibility; it could be folded without breaking.
"We will soon see roll-up cell phones and laptop computers on the market," Li said. "But a flexible energy storage device is needed to make this possible."
The once-cotton T-shirt proved to be a repository for electricity. By using small swatches of the fabric as an electrode, the researchers showed that the flexible material, which Li's team terms activated carbon textile, acts as a capacitor. Capacitors are components of nearly every electronic device on the market, and they have the ability to store electrical charge.
Moreover, Li reports that activated carbon textile acts like double-layer capacitors, which are also called a supercapacitors because they can have particularly high energy storage densities.
But Li and Bao took the material even further than that. They then coated the individual fibers in the activated carbon textile with "nanoflowers" of manganese oxide. Just a nanometer thick, this layer of manganese oxide greatly enhanced the electrode performance of the fabric. "This created a stable, high-performing supercapacitor," said Li.
This hybrid fabric, in which the activated carbon textile fibers are coated with nanostructured manganese oxide, improved the energy storage capability beyond the activated carbon textile alone. The hybrid supercapacitors were resilient: even after thousands of charge-discharge cycles, performance didn't diminish more than 5 percent.
"By stacking these supercapacitors up, we should be able to charge portable electronic devices such as cell phones," Li said.
Li is particularly pleased to have improved on the means by which activated carbon fibers are usually obtained. "Previous methods used oil or environmentally unfriendly chemicals as starting materials," he said. "Those processes are complicated and produce harmful side products. Our method is a very inexpensive, green process."
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