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Wednesday, August 15, 2007

A small sheet of paper could pave the way for a new generation of environmentally friendly energy sources.

A battery that resembles a small sheet of paper could pave the way for a new generation of extremely flexible, cheap, and environmentally friendly energy sources.
A traditional battery has three key elements: an electrolyte solution made up of positive and negative ions, two electrodes made of different materials between which the ions flow, and a separator membrane through which positive and negative ions pass in opposite directions. Attempts to make batteries less bulky and more flexible have met with limited success, largely because of the challenges of combining these elements into thinner materials. A team of scientists headed by chemist Robert Linhardt, materials scientist Pulickel Ajayan, and engineer Omkaram Nalamasu of Rensselaer Polytechnic Institute in Troy, New York, wondered whether paper might be the answer.

To make the new battery, the researchers dissolved cellulose, a plant material used to make paper, in a liquid salt solution. They then added microscopic carbon nanotubes and let the mixture dry. Those steps yielded a thin film that resembled a piece of paper, which was white on one side and black with nanotubes on the other. To complete the battery, the team soaked the cellulose with a lithium hexafluorophosphate solution and covered the white side of the film with lithium metal. The carbon nanotubes served as one electrode and the lithium metal the other. The solution provided the electrolyte, and the cellulose worked as the spacer.

Each gram of paper produces about 10 milliamps of current at 2 volts, and the researchers were able to use the batteries to power a fan and LED light. Stacking multiple sheets increases the power, the team reports online this week in the Proceedings of the National Academy of Sciences. Unlike other flexible batteries, the paper battery is completely integrated, says Linhardt.

The battery has other advantages. It works in temperatures as high as 150°C and as low as -70°C, it retains the flexibility of paper, and, because it's made from 90% cellulose, it's cheap to manufacture. Its low toxicity also makes it an attractive power source for medical devices such as pacemakers and insulin pumps, Linhardt says.

The initial results are "very encouraging," says electrical engineer Sandipan Pramanik of the University of Alberta in Edmonton, Canada. In addition to medical applications, he thinks the technology will provide a better way to charge cell phones and laptops. Before that happens, however, Pramanik says engineers will have to find a way to manufacture the paper batteries on a large scale.

How batteries work

How a Battery Works

A battery stores electricity for future use. It develops voltage from the chemical reaction produced when two unlike materials, such as the positive and negative plates, are immersed in the electrolyte, a solution of sulfuric acid and water. In a typical lead-acid battery, the voltage is approximately 2 volts per cell, for a total of 12 volts. Electricity flows from the battery as soon as there is a circuit between the positive and negative terminals. This happens when any load that needs electricity, such as the radio, is connected to the battery.

learn how a battery works from BCI

Most people don't realize that a lead-acid battery operates in a constant process of charge and discharge. When a battery is connected to a load that needs electricity, such as the starter in your car, current flows from the battery. The battery begins to be discharged.

In the reverse process, a battery becomes charged when current flows back into it, restoring the chemical difference between the plates. This happens when you're driving without any accessories and the alternator puts current back into the battery.

As a battery discharges, the lead plates become more chemically alike, the acid becomes weaker, and the voltage drops. Eventually the battery is so discharged that it can no longer deliver electricity at a useful voltage.

You can recharge a discharged battery by feeding electrical current back into it. A full charge restores the chemical difference between the plates and leaves the battery ready to deliver its full power.

This unique process of discharge and charge in the lead-acid battery means that energy can be discharged and restored over and over again. This is what's known as the cycling ability in a battery.

If the battery won't start your car, you usually refer to it as "dead," even though that's not technically correct. A battery that's merely discharged - from leaving your headlights on or from a damaged alternator -- can be recharged to its full capacity. But a battery that's at the end of its service life can't be recharged enough to restore it to a useful power level. Then it truly is dead, and must be replaced.

If the battery is discharged and not dead, you can jump-start it from another fully charged battery. About 30 minutes of driving should allow the alternator to fully charge the battery. But if the alternator or another part of the electrical system in your car is damaged, the battery will not recharge and a mechanic or service station also will not be able to recharge it. So if your battery keeps discharging, have your electrical system checked before you replace it. What looks like a bad battery could be an electrical system problem. If you have a bad component in the electrical system, it will keep draining a new battery, and you'll be stranded again and again

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