Just add a drop of water

The battery, imagined by Gustav Nyström and his team, is composed of at least one cell measuring one square centimeter and composed of three inks printed on a rectangular strip of paper . Salt, in this case simply sodium chloride or table salt, is scattered all over the strip of paper and one of its shorter ends has been dipped in wax. An ink containing flakes of graphite, which acts as the good end of the battery (the cathode), is printed on one of the flat sides of the paper while an ink containing zinc powder, which acts as the negative end of the battery (the anode), is printed on the back of the paper. Yet another ink containing flakes of graphite and carbon black is printed on both sides of the paper, above the other two inks. This ink constitutes the current collectors connecting the constructive and negative ends of the battery to two wires, which are located at the end of the paper dipped in wax.

When a small amount of water is added, the salts in the paper dissolve and charged ions are released, making the electrolyte ion-conductive. These ions activate the battery by dispersing through the paper, causing the zinc in the ink at the anode to oxidize, releasing electrons. By closing the (external) circuit, these electrons can then be transferred from the zinc-containing anode—by way of the graphite- and carbon-black-containing ink, wires, and device—to the graphite cathode where they are transferred to — and therefore reduce — the oxygen in the ambient air. These redox reactions (reduction and oxidation) thus generate an electric current which can be used to power an external electrical device.

Proof of principle: a long-lasting energy source for the low-power electronics

To demonstrate their battery’s ability to run low-power electronics, the Nyström team combined two cells into a single battery to increase the operating voltage and used it to power an alarm clock with an LCD display. Performance analysis of a single cell battery revealed that after adding two drops of water, the battery activated in 20 seconds and, when not connected to a power-consuming device, reached a pressure steady of 1.2 volts. The voltage of a regular AA alkaline battery is 1.5 volts.

After one hour, the performance of the single cell battery has decreased significantly due to the paper drying out . However, after the researchers added two more drops of water, the battery maintained a stable operating stress of .5 volts for over an additional hour.

The researchers propose that the biodegradability of paper and zinc could enable their battery to minimize the environmental impact of low-power disposable electronic devices. “What’s special about our new battery is that unlike many metal-air batteries that use metal foil which gradually wears out as the battery wears out, our design allows only the amount of zinc to be added to the ink that is actually needed for the software,” says Nyström. Metal foils were more difficult to control and not always fully consumed, resulting in wasted materials. Thus, the more zinc the ink contains, the longer the battery is able to operate.

A more critical place in the current battery design with water activation , adds Nyström, is the time it takes for the battery to dry out. “But I’m sure it can be designed differently to get around this problem.” For environmental sensing programs at some humidity or in humid environments, however, paper drying would not be a problem.

Two complementary systems

Previously, the Nyström team had already developed a degradable supercapacitor with paper base that could be loaded and unloaded thousands of times without loss of efficiency. Compared to batteries of the same weight, supercapacitors have an energy density approximately 10 times lower, while having a power density approximately ten to one hundred times superior. Supercapacitors can therefore be charged and discharged much faster. They can also withstand many more charge and discharge cycles. “So the two devices are actually complementary,” says Nyström. The idea behind the new water-activated battery was to be able to make devices that are fully charged and only release this energy after a stimulus is triggered, in this case simply a drop of water.

Related Articles

Back to top button