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The paper battery is composed of two electrochemical cells – at both ends of the paper strip – separated by a water barrier (between the letters "m" and "p") and connected in series. Credit: Empa |
A team of researchers at Empa 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 proof-of-principle study has been published in the journal Scientific Reports.
The battery, devised 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.