Eco Innovation

Salt, paper, water … battery?

8th August 2022
Sheryl Miles

Scientists based at Empa (the Swiss Federal Laboratories for Materials Science and Technology) have taken crafts to a whole new level by creating a paper battery which is activated by a water ‘switch’.

Researchers suggest that the battery could be used to power a wide range of low-power, single-use disposable electronics such as smart tracking labels, environmental sensors, and medical diagnostic devices.

The ingredients

Take a strip of rectangular paper; some sodium chloride (or table salt); wax; three inks – one infused with graphite, one infused with zinc, and one infused with graphite and carbon black; add a dash of wires; two drops of water; and one part oxygen.

Making a battery

So now all the ingredients are gathered, how is a paper battery made?

The battery is made of at least one cell measuring 1cm2 (for more power, just add more cells).

The shorter end of the paper is dipped in wax, and the sodium chloride is evenly dispersed throughout the paper. To achieve the battery’s positive end, or the cathode, ink infused with graphite flakes is printed on one side of the paper. A second ink, this time infused with zinc is then printed onto the reverse side of the paper, this acts as the battery’s negative end, or the anode. A third and final ink infused with more graphite plus carbon black is then printed onto both sides of the paper, over the top of the other two inks.

This ink makes up the current collectors which connects the positive and negative ends of the battery to two wires, which are at the wax-dipped end of the paper.

How does it work?

Well, this is where the water, or the switch, comes into play.

When a small amount of water is added, the salts within the paper dissolve, releasing the charged ions. These charged ions then activate the battery by spreading through the paper, allowing the zinc in the negative end to release electrons.

When the wires are added to an electrical device, this closes the circuit so the electrons can move from the negative end to the positive end. They are then transferred to oxygen in the surrounding air.

The culmination of these redox (reduction and oxidation) reactions generates an electrical current that can be used to power an electrical device.

The team, led by Gustav Nyström, found that by adding just two drops of water to the battery, when not connected to an energy-consuming device, it activated within 20 seconds and generated 1.2V – only marginally less than a standard AA alkaline battery, which operates at 1.5V.

However, over the period of one hour the paper dries out, which significantly decreases the battery’s performance. The team then applied two additional drops of water to the circuit, which enabled the battery to keep a stable operating voltage of 0.5V for more than an additional hour.

Demonstrating the theory that the battery could operate an electrical device, the team tested it on a small LCD alarm clock. They found that by combining two cells into one battery, it could power the clock.

An advantage of a paper battery is the reduced environmental impact because of the biodegradability of the paper and the zinc.

Nyström said: "What's special about our new battery is that, in contrast many metal air batteries using a metal foil that is gradually consumed as the battery is depleted, our design allows [us] to add only the amount of zinc to the ink that is actually needed for the specific application."

The team have already developed a paper-based degradable capacitor. Who knows, maybe in the future we will be using paper equipment and electronical paper. One thing that is certainly clear is that in the world of technology and electronics – anything is possible.

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