Image: Empa

A drop of water is enough: A paper battery with water switch

A team of Empa researchers has developed a disposable paper battery that can be activated by water. This could be used to power a wide range of small, disposable electronic devices with low power consumption, such as smart tags for tracking objects, environmental sensors or medical diagnostic devices – and minimize their environmental impact to boot. The proof-of-concept study has just been published in Scientific Reports.

The battery developed by Empa researcher Gustav Nyström and his team consists of at least one electrochemical cell measuring around one square centimeter. Three different inks are printed on a rectangular paper strip. Salt, in this case simply sodium chloride or table salt, is distributed throughout the paper strip, and one of the two shorter ends of the strip has been dipped in wax. On one side of the paper, an ink is printed that contains graphite flakes and acts as the positive pole of the battery – the cathode; on the reverse side, a second ink is printed that contains zinc powder and acts as the negative pole of the battery – the anode. A third ink, containing graphite flakes and carbon black, is printed on both sides of the paper over the other two inks. This forms the current collectors that connect the two poles of the battery to two wires located at the end of the paper strip dipped in wax.

If a small amount of water is added, the salt contained in the paper dissolves, charged ions are released, and the electrolyte becomes ionically conductive. This step activates the battery: the ions distribute in the paper, causing the zinc on the anode to oxidize and release electrons. By closing the (external) circuit, these electrons can then flow from the zinc-containing anode – via the graphite- and carbon black-containing ink and wires – to the graphite cathode, where they are transferred to the oxygen from the ambient air, thereby reducing it. These two “redox reactions” (a reduction and an oxidation) produce an electric current that can be used to power an electrical device.

“Proof of concept”: a sustainable energy source for low-power electronics

To demonstrate the viability of their battery for powering low-power electronics, Nyström’s team combined two identical cells – increasing the battery’s operating voltage – and used them to power an alarm clock with a liquid crystal display. When the researchers analyzed the performance of a (single-cell) battery, they found that after adding two drops of water, the battery was activated within 20 seconds and reached a stable voltage of 1.2 volts. For comparison, the voltage of a normal AA alkaline battery is 1.5 volts.

After one hour, the performance of the single-cell battery decreased significantly as the paper dried out. However, when the researchers added two more drops of water, the battery maintained a stable operating voltage of 0.5 volts for more than another hour.

What’s special about the new battery is that because both paper and zinc and the other components are biodegradable, it could significantly minimize the environmental impact of disposable, low-power electronics. “And unlike many metal-air batteries, which use a metal foil that is gradually used up as the battery is used, in our design we only put just the amount of zinc into the ink that is actually needed for the application,” Nyström adds. In other words, the more zinc the ink contains, the longer the battery will last. Metal foils, on the other hand, are much more difficult to “dose”, i.e. they are not always completely used up, which leads to a waste of material.

One minor weakness of the new water-activated battery concept is the amount of time the battery remains wet – and thus functional – as Nyström admits. “But I’m sure we can solve this problem by building it differently.” And for environmental sensing applications above a certain humidity or in wet and humid environments, drying out would not be an issue anyway.

Two complementary mini power storage units

Recently, Nyström’s team had already developed a degradable paper-based supercapacitor that could be charged and discharged thousands of times without losing efficiency. Compared to batteries of the same weight, supercapacitors have an energy density that is about ten times lower – but a power density that is ten to one hundred times higher. Supercapacitors can therefore be charged and discharged much faster. They can also withstand many more charge and discharge cycles. “So the two technologies complement each other very well,” Nyström says. The idea behind the new water-activated battery, he said, was to make small power storage devices that are fully charged and provide that energy only after a stimulus is triggered, in this case simply a drop of water.