One of the first conjugated polymers that I learned about was named polypyrrole. I’ve written a sequence of papers before here at Associated Content about the subject of conjugated polymers, and how they are plastics that conduct electricity. It’s a curious notion, as often we think of plastics as being electrical insulators – after all, we wrap our power lines with plastic to protect against accidental electrocution. However, with the right molecular design, plastics can be convinced to carry an electric charge. Some of them are beginning to rival copper metal in terms of their performance. I recently read an article in the American Chemical Society journal Nano Letters which described how polypyrrole, a greenish-black conjugated polymer, was being put to use in new battery designs.
The researchers who published the paper were from Uppsala University in Sweden, and decided to try to design a battery that could be made from the simplest possible components. Normally batteries require sulfuric acid and metals such as lead or lithium. The Swedish researchers decided to completely overhaul this way of thinking, and settled on an unusual combination of materials: river algae, salt-water, and the polypyrrole. The salt-water would act as an electrolyte – a conductive liquid medium. The dissolved salts help to transfer the electric charge through the water. The algae was used as a source of cellulose, which is a rigid starch present in the cell walls of the plant cells. The cellulose could then be used as a structural component to provide stiffness to the battery. As for the electrodes of the battery, they were made up of the polypyrrole.
To assemble the new battery, the researchers fashioned two strips out of cellulose and soaked them in the salt water. One of the strips was coated with a very thin layer of polypyrrole that had been oxidised, or made electron-deficient; this gave the strip a very small positive charge. The other strip was coated with polypyrrole that had been reduced, or made electron-rich; this gave the strip a very small negative charge. The difference in electronic potential between the two strips (which acted as the electrodes) gives rise to the voltage of the battery. When the battery is used as a source of electricity, chloride ions present in the salt-water move from one electrode to the other. When the battery is drained, it can be recharged, which moves chloride ions in the opposite direction. While these batteries can only supply a low voltage, they are flexible, light-weight, and very low cost.
After all, the batteries consist mainly of (naturally-sourced) paper and salt water. This makes the battery design very simple and both the production and disposal of the batteries is environmentally-friendly, unlike metal-based batteries which represent a source of heavy-metal contamination and are often difficult to dispose of due to the high concentrations of acid present.
Applications of this new battery will most likely take advantage of the batteries thin design and light-weight, for example putting batteries in odd places such as a newspaper or in labels. We already have the technology for ultra-thin flexible computer displays, but mass production of those devices has lagged behind progress because of the lack of lightweight power supplies. With these new batteries, the displays could begin popping up all over the place. Once researchers have optimized this design and have increased the power density, they should be able to be mass-produced (due to the low cost) and we will start to see them making an appearance in our everyday life.