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Plastic Batteries

In today's world everything is run on batteries: portable computers, battlefield equipment, satellites, portable radios, etc. The limitations of this battery technology is that the batteries produced are heavy, just ask anyone who has carried a portable electronic device. Researchers at John Hopkins University in conjunction with the Rome Laboratory have made an all-polymer battery. The numerous advantages include relative insensitivity to temperature, are flexibility, thin size, ability to be shaped into small spaces, and the lack of dangerous heavy metals. These benefits give plastic batteries, and those who control its patent, a very bright future.

All batteries have three main, important parts: the anode, cathode, and a substance which allows ions to move through. The anode by definition is the electrode in a galvanic cell at which oxidation occurs, the loss of electrons. The cathode is the electrode in a galvanic cell at which reduction occurs, the gain of electrons. There are many substances that can transfer electrons. One such substance is the electrolyte, a material that dissolves in water to produce a solution that conducts an electric current.

Let us take a lead acid battery and see how it works. The lead storage battery were first used in automobiles around 1915. In this battery the anode is composed of lead (Pb) while the cathode is composed of lead dioxide (PbO2). These two electrodes are submersed into an electrolytic solution of sulfuric acid (H2SO4). The reaction that occurs at the anode is:

Pb + HSO4- PbSO4 + H+ + 2e

The reaction that happens at the cathode is:

PbO2 + HSO4- + 3H+ + 2e- PbSO4 + H2O

These two combined create the cell reaction. At the anode, Pb is converted to PbSO4 after the break down of HSO4- into SO42- and H+ has occured. This reaction produces an extra 2 electrons, because lead gives up two electrons to bond with the sulfate ion. The cathode at the same time converts PbO2 into PbSO4 by taking apart the PbO2 into its constituents, Pb4+ and O24-. and bonding it to the SO42- ion that is meanwhile breaking down from the HSO4- leaving H+. The sulfate ion now bonds with lead and the hydrogen along with three extra hydrogen, made in the anode reaction, form to make water. The two electrons made by the reaction of the anode are used in the formation of PbSO4. The typical lead storage battery consists of 6 cells which can produce together 12 V of potential. This is just one type of battery, there are many others, such as: Dry Cell, Lithium Ion, NiCad, and NiMh, to name a few.

Modern developments have allowed electrochemists to substitute gels and polymers for the electrolytes and one of the metal plates. It was thought that it would be impossible to design an all polymer battery without using any metals.

Part of the problem with the early uses of polymers as both anodes and cathodes was that they did not have large enough potentials, the tendancy to either reduce or oxidize at the electrode. Most polymers can be p-doped, a process that increases the tendencies to collect electrons, allowing them to become better cathodes. However, polymers have not been formulated to be n-doped, a process that increases the tendency to give up electrons, making the best polymer a neutral one. The pairing of the neutral polymer and a p-doped polymer would not give a useful voltage. A different approach was necessary.

Rome Laboratories, a research facility run by the U.S. Air Force, teamed up with John Hopkins University to formulate an anode could would be n-doped and have enough potential to be useful. Their basic approach was to pseudo n-doped the polymer. They placed a thin layer polystyrenesulfonate. Thus they made the anode n-doped. After fabricating the anode on a graphite fiber base with a polypyrrole conductor (see figure 1). The pseudo-doped anode allowed researchers to build a 2.5 volt cell.

They used lithium perchlorate in a polyacrylonitrile, ethylene carbonate, acetonitrile polymer gel. The fabrication of the polymer cathode involves electropolymerization of pyrrole and lithium perchlorate. The construction of anode involves electropolymerization with pyrrole and polystyrenesulfonate (see figure 2).

The pyrrole double bond chain of carbon atoms creates a resonant electron bond. See Figure 3. This allows free electrons to roam up and down, thus you could have an electrical current as well as free electrons. See Figure 4 for justification.

Anode:

[PPY0/PSS-] Li+ [PPY+/PSS-] + Li+ + e-
See Figure 5

PPY = Polypyrrole

PPY/PPS = Polypyrrole/polystyrenesulfonate

Cathode:

[PPY+] ClO4- + e- [PPY0] + ClO4-

See Figure 5

PPY = Polypyrrole

PPY/PPS = Polypyrrole/polystyrenesulfonate

The battery was fabricated onto 2cm by 10cm graphite fiber substrate. The polymer gel was only 100 micrometers. The whole cell is only 1mm thick and stands through 100 cycles with little loss in capacity. The flat sheets may be rolled up into a cylinder like an AA battery. Drawbacks to the polymer battery include the need for specialized charging equipment, potential use as weapons of terrorism as they are hermetically sealed and are invisible to x-ray scanners. These batteries have been shown to operate under extreme conditions. According to researchers, the batteries are easy to fabricate. The polymer battery can become an efficient energy storage solution in the future.

"All Plastic Battery Unveiled." Success Stories. [A Rome Laboratories Web Page] April 1997.

Killian, J. G. et al. "Polypyrrole Composite Electrodes in an All-Polymer Battery System." Journal Electrochemical Society. Vol. 143, No 3. March 1996.

"Popular Science Magazine Selects All-Plastic Battery for Award." Rome Laboratory Press Release. RL 96-74. Nov. 12, 1996.

"Power from Plastics: Hopkins Scientists Create All-polymer Battery." John Hopkins University Press Release. Nov. 12, 1996.

"Plastic Power." Scientific American. April 1997.

Wolf, Irvine. Interview with authors. Oct. 8, 1996.