A new battery, consisting of a one-centimetre-square cell and three inks printed on a rectangular strip of paper, shows how little it takes to generate electricity and produce electricity.
Gustav Nyström and his team in Switzerland developed the water-activated paper battery for disposable electronics with a low environmental impact.
They explain the process: “Salt, in this case simply sodium chloride or table salt, is spread over the paper strip and one of its shorter ends has been dipped in wax. An ink containing graphite flakes, which acts as the positive 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 is printed on the back of the paper. Yet 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 the current collectors that connect the positive and negative ends of the battery to two wires located on the wax-dipped end of the paper.
“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 to oxidize at the anode, releasing electrons.
“By closing the (external) circuit, these electrons can then be transferred from the zinciferous anode – through the graphite and carbon black ink, wires and device – to the graphite cathode, where they are transferred – and from there reduce – oxygen from the surrounding air.” . These redox reactions (reduction and oxidation) thus produce an electrical current that can be used to power an external electrical device.”
To demonstrate their battery’s ability to power low-power electronics, Nyström’s team combined two cells into one battery to increase the operating voltage and used it to power a liquid crystal display alarm clock. 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 an energy-consuming device, reached a stable voltage of 1.2 volts. The voltage of a standard AA alkaline battery is 1.5 volts.
After an hour, the performance of the single-cell battery dropped significantly due to the paper drying. However, after the researchers added two additional drops of water, the battery maintained a stable operating voltage of 0.5 volts for more than an additional hour.
The researchers suggest that the biodegradability of paper and zinc could allow their batteries to minimize the environmental impact of low-power, disposable electronics.
“What makes our new battery special is that, unlike many metal-air batteries that use a metal foil that gradually depletes 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,” says Nyström. Metal foils were more difficult to control and were not always fully consumed, resulting in material waste. So the more zinc in the ink, the longer the battery will last.
A more critical issue with current water-activated battery designs, Nystrom says, is the time it takes for the battery to dry out. “But I’m sure this can be constructed differently to circumvent this problem.”
However, for environmental sensing applications at a certain humidity or in wet environments, drying the paper would not be a problem.
Previously, Nyström’s team had 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 around ten times lower – while at the same time a power density that is around ten to a hundred times higher. Supercapacitors can therefore be charged and discharged much faster. They can also survive many more charge and discharge cycles.
“So the two devices actually complement each other,” says Nyström. The idea behind the new water-activated battery was to be able to make devices that are fully charged and only release that energy after triggering a stimulus, in this case simply a drop of water.