News in Science

Salt and paper make disposable batteries

A piece of wet, salty paper doesn't look impressive, but cut it up, stack it up, and it can hold an impressive amount of energy.

With this in mind, scientists in Sweden have created a salt and paper battery that can hold up to one volt of electricity.

The scientists hope their battery will one day power remote, biodegradable sensors.

"We wanted to make a battery that was very simple and used the same material for both electrodes," says Professor Leif Nyholm of Uppsala University in Sweden and co-author of a study that appeared in the journal Nano Letters. "To make it simple we just put NaCl (table salt) in."

The battery is as thin as a single piece of paper, but it is made up of multiple layers of cellulose, coated in a conductive polymer 50 nanometres thick, and sandwiched between layers of filter paper. Water conducts the chlorine ions to the negative electrode, and electrons to the positive electrode.

The whole flexible assembly, several millimetres thick, is wrapped in plastic. The initial salt and paper battery prototype can generate one volt of energy.

Stacking more layers should provide more voltage, says Nyholm.

Gram for gram, the salt and paper battery isn't as powerful as its thin-film cousins that use lithium, cobalt or nickel, but it does charge faster.

The salt and paper battery won't be used to power a laptop or a cell phone. Its environmentally-friendly components will more likely be used for fabrics that heat up or remote sensors that monitor temperature or humidity.

Problem solver

The battery doesn't just degrade into harmless products, it actually helps solve an environmental problem.

The traditional source of paper, wood pulp, doesn't have enough surface area for ion exchange. Instead, the scientists turned to a noxious marine algae that clogs up harbours. The marine algae produces cellulose with 100 times the surface area, compared with terrestrial plants.

"The utilisation of the nanostructure of this cellulose preparation was a bright idea," says Professor Gyorgy Inzelt, an expert on thin films from Eotvos Lorand University in Hungry.

The salt and cellulose assembly could also be used as a capacitor, a device able of dumping all its electrical charge at once, instead of a battery, which releases power more slowly.

As either a battery or a capacitor, Inzelt suggests the device could be useful in pacemakers, nanomotors, optical displays and other devices. But, he says, more exhaustive testing is required before any commercial device can be produced.

"There are so many things we want to do with an actual commercial device," says Nyholm, "But we still have a lot of work to fully characterise its performance."