Paper battery offers future power

The black piece of paper can power a small light Flexible paper batteries could meet the energy demands of the next generation of gadgets, says a team of researchers. They have produced a sample slightly larger than a postage stamp that can store enough energy to illuminate a small light bulb. But the ambition is to produce reams of paper that could one day power a car. Professor Robert Linhardt, of the Rensselaer Polytechnic Institute, said the paper battery was a glimpse into the future of power storage. The team behind the versatile paper, which stores energy like a conventional battery, says it can also double as a capacitor capable of releasing sudden energy bursts for high-power applications. Graphic: How a paper battery works While a conventional battery contains a number of separate components, the paper battery integrates all of the battery components in a single structure, making it more energy efficient. Integrated devices The research appears in the Proceedings of the National Academy of Sciences (PNAS). "Think of all the disadvantages of an old TV set with tubes," said Professor Linhardt, from the New York-based institute, who co-authored a report into the technology. "The warm up time, power loss, component malfunction; you don't get those problems with integrated devices. When you transfer power from one component to another you lose energy. But you lose less energy in an integrated device." You can implant a piece of paper in the body and blood would serve as an electrolyte Professor Robert Linhardt The battery contains carbon nanotubes, each about one millionth of a centimetre thick, which act as an electrode. The nanotubes are embedded in a sheet of paper soaked in ionic liquid electrolytes, which conduct the electricity. The flexible battery can function even if it is rolled up, folded or cut. Although the power output is currently modest, Professor Linhardt said that increasing the output should be easy. "If we stack 500 sheets together in a ream, that's 500 times the voltage. If we rip the paper in half we cut power by 50%. So we can control the power and voltage issue." Because the battery consists mainly of paper and carbon, it could be used to power pacemakers within the body where conventional batteries pose a toxic threat. "I wouldn't want the ionic liquid electrolytes in my body, but it works without them," said Professor Linhardt. "You can implant a piece of paper in the body and blood would serve as an electrolyte." But Professor Daniel Sperling at University of California, Davis, an expert on alternative power sources for transport, is unconvinced. 'More difficult' "Batteries and capacitors are being steadily improved, but electricity storage is much more difficult and expensive than liquid fuels and probably will be so forever," he said. "The world is not going to change as a result of this new invention any time soon." Professor Linhardt admitted that the new battery is still some way from the commercial market. "The devices we're making are only a few inches across. We would have to scale up to sheets of newspaper size to make it commercially viable," he said. But at that scale, the voltage could be large enough to power a car, he said. However, carbon nanotubes are very expensive, and batteries large enough to power a car are unlikely to be cost effective. "I'm a strong enthusiast of electric vehicles, but it is going to take time to bring the costs down," said Professor Sperling. But Professor Linhardt said integrated devices, like the paper battery, were the direction the world was moving. "They are ultimately easier to manufacture, more environmentally friendly and usable in a wide range of devices," he said. The ambition is to produce the paper battery using a newspaper-type roller printer. Electricity is the flow of electrical power or electrons 1. Batteries produce electrons through a chemical reaction between electrolyte and metal in the traditional battery. 2. Chemical reaction in the paper battery is between electrolyte and carbon nanotubes. 3. Electrons collect on the negative terminal of the battery and flow along a connected wire to the positive terminal 4. Electrons must flow from the negative to the positive terminal for the chemical reaction to continue. Further Readings At Stanford, nanotubes + ink + paper = instant battery Dip an ordinary piece of paper into ink infused with carbon nanotubes and silver nanowires, and it turns into a battery or supercapacitor. Crumple the piece of paper, and it still works. Stanford researcher Yi Cui sees many uses for this new way of storing electricity. BY JANELLE WEAVER Stanford scientists are harnessing nanotechnology to quickly produce ultra-lightweight, bendable batteries and supercapacitors in the form of everyday paper. Simply coating a sheet of paper with ink made of carbon nanotubes and silver nanowires makes a highly conductive storage device, said Yi Cui, assistant professor of materials science and engineering. "Society really needs a low-cost, high-performance energy storage device, such as batteries and simple supercapacitors," he said. Like batteries, capacitors hold an electric charge, but for a shorter period of time. However, capacitors can store and discharge electricity much more rapidly than a battery. Cui's work is reported in the paper "Highly Conductive Paper for Energy Storage Devices," published online this week in the Proceedings of the National Academy of Sciences. "These nanomaterials are special," Cui said. "They're a one-dimensional structure with very small diameters." The small diameter helps the nanomaterial ink stick strongly to the fibrous paper, making the battery and supercapacitor very durable. The paper supercapacitor may last through 40,000 charge-discharge cycles – at least an order of magnitude more than lithium batteries. The nanomaterials also make ideal conductors because they move electricity along much more efficiently than ordinary conductors, Cui said. L.A. Cicero Bing Hu, a post-doctoral fellow, prepares a small square of ordinary paper with an ink that will deposit nanotubes on the surface that can then be charged with energy to create a battery. Cui had previously created nanomaterial energy storage devices using plastics. His new research shows that a paper battery is more durable because the ink adheres more strongly to paper (answering the question, "Paper or plastic?"). What's more, you can crumple or fold the paper battery, or even soak it in acidic or basic solutions, and the performance does not degrade. "We just haven't tested what happens when you burn it," he said. The flexibility of paper allows for many clever applications. "If I want to paint my wall with a conducting energy storage device," Cui said, "I can use a brush." In his lab, he demonstrated the battery to a visitor by connecting it to an LED (light-emitting diode), which glowed brightly. A paper supercapacitor may be especially useful for applications like electric or hybrid cars, which depend on the quick transfer of electricity. The paper supercapacitor's high surface-to-volume ratio gives it an advantage. "This technology has potential to be commercialized within a short time," said Peidong Yang, professor of chemistry at the University of California-Berkeley. "I don't think it will be limited to just energy storage devices," he said. "This is potentially a very nice, low-cost, flexible electrode for any electrical device." Cui predicts the biggest impact may be in large-scale storage of electricity on the distribution grid. Excess electricity generated at night, for example, could be saved for peak-use periods during the day. Wind farms and solar energy systems also may require storage. "The most important part of this paper is how a simple thing in daily life – paper – can be used as a substrate to make functional conductive electrodes by a simple process," Yang said. "It's nanotechnology related to daily life, essentially." Cui's research team includes postdoctoral scholars Liangbing Hu and JangWook Choi, and graduate student Yuan Yang. Janelle Weaver is a science-writing intern at the Stanford News Service. Media Contact Dan Stober, Stanford News Service: (650) 721-6965, dstober@stanford.edu