The battery, developed by Gustav Nyström and his team, consists of at least one cell measuring one square centimeter and consists of three inks printed on a rectangular strip of paper. Salt, in this case simply sodium chloride or table salt, is spread across the entire strip of paper, with one of its shorter ends dipping into the wax. Ink containing graphite flakes acting as the positive end of the battery (cathode) is printed on one of the flat sides of the paper, while ink containing zinc powder acting as the negative end of the battery (anode) is printed on the reverse side of the paper . Another ink containing graphite flakes and carbon black is printed on both sides of the paper on top of the other two inks. This ink forms current collectors that connect the positive and negative ends of the battery to the two wires on the wax-soaked end of the paper.
When a small amount of water is added, the salts inside the paper dissolve and the charged ions are released, making the electrolyte ionic. These ions activate the battery by scattering across the paper, causing the zinc in the ink on the anode to oxidize, releasing electrons in the process. When the (external) circuit is closed, these electrons can be transferred from the anode containing zinc, through the ink containing graphite and carbon black, the wires and the device, to the graphite cathode, where they are transferred to… and thus reduce — oxygen from the surrounding air. Therefore, these redox reactions (reduction and oxidation) create an electrical current that can be used to power an external electrical device.
Proof of concept: a sustainable power source for low-power electronics
To demonstrate their battery’s ability to work with low-power electronics, Nyström’s team combined the two cells into a single battery to increase the operating voltage and used it to power an alarm clock with a liquid crystal display. An analysis of the performance of a single cell battery showed that after adding two drops of water, the battery activated within 20 seconds and, when not connected to a power-hungry device, reached a stable voltage of 1.2 volts. The voltage of a standard AA alkaline battery is 1.5 volts.
After one hour, the performance of the single-cell battery was significantly reduced due to drying of the paper. However, after the researchers added two additional drops of water, the battery maintained a stable operating voltage of 0.5 volts for more than one hour.
The researchers believe that the biodegradability of paper and zinc could allow their battery to minimize the environmental impact of disposable low-power electronics. “What’s special about our new battery is that, unlike many metal air batteries that use a metal foil that gradually discharges as the battery discharges, our design allows us to add only the amount of zinc that is actually needed to the ink specific,” Nyström says. Metal foil is more difficult to control and doesn’t always fully consume, leading to wasted materials. So the more zinc the ink contains, the longer the battery can last.
A more critical point with the current water-activated battery design, Nyström adds, is the time it takes for the battery to dry out. “But I’m sure it could be designed differently to get around that problem.” However, for environmental sensing applications at a certain humidity or in a humid environment, paper drying will not be a problem.
Two complementary technologies
Previously, Nyström’s team had already developed a degradable paper-based supercapacitor that can be charged and discharged thousands of times without losing efficiency. Compared to batteries of the same weight, supercapacitors have an energy density about 10 times lower – and at the same time a power density of about ten to a hundred times higher. Therefore, supercapacitors can be charged and discharged much faster. They can also withstand much more charge and discharge cycles. “So the two devices are actually complementary,” Nyström says. The idea behind the new water-activated battery was to be able to produce fully charged devices that release this energy only when triggered by a stimulus, in this case just drops of water.
Materials is provided Swiss Federal Laboratories for Materials Science and Technology (EMPA). Originally written by Michael Hagman. Note: Content can be edited for style and length.