A team of researchers at EMPA have developed a water-activated disposable paper battery. The researchers suggest that it could be used to power a wide range of low-power, single-use disposable electronics, such as smart labels for tracking objects, environmental sensors, and medical diagnostic devices, and minimize their environmental impact.
The battery, created by Gustav Nyström and his team, is made of at least one cell measuring one centimeter squared and consisting of three inks printed onto a rectangular strip of paper. Salt, in this case simply sodium chloride or table salt, is dispersed throughout the strip of paper 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 onto 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 onto the reverse side 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 makes up the current collectors connecting the positive and negative ends of the battery to two wires, which are located at the wax-dipped end of the paper.
When a small amount of water is added, the salts within the paper dissolve and charged ions are released, thus making the electrolyte ionically conductive. These ions activate the battery by dispersing through the paper, resulting in zinc in the ink at the anode being oxidized thereby releasing electrons.
By closing the (external) circuit these electrons can then be transferred from the zinc-containing anode — via the graphite- and carbon black-containing ink, the wires and the device — to the graphite cathode where they are transferred to, and hence reduce, oxygen from ambient air. These redox reactions (reduction and oxidation) thus generate an electrical current that can be used to power an external electrical device.
To demonstrate the ability of their battery to run low-power electronics, Nyström’s team combined two cells into one battery to increase the operating voltage and used it to power an alarm clock with a liquid crystal display. Analysis of the performance of a one-cell battery revealed that after two drops of water were added, 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 one hour, the one-cell battery’s performance decreased significantly due to the paper drying. However, after the researchers added two extra drops of water, the battery maintained a stable operating voltage of 0.5 volts for more than one additional hour.
The researchers propose that the biodegradability of paper and zinc could enable their battery to minimize the environmental impact of disposable, low-power electronics. “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 to add only the amount of zinc to the ink that is actually needed for the specific application,” said Nyström. Metal foils were more difficult to control and not always fully consumed leading to a waste of materials. So, the more zinc the ink contains, the longer the battery is able to operate.
A more critical point of the battery’s current design with water activation, Nyström added, is the time it takes for the battery to dry out. “But I am sure this can be engineered differently to get around this problem,” he said. For environmental sensing applications at a certain humidity or in wet environments, however, the drying of the paper would not be an issue.