One drop is enough: Paper battery with water switch
A team of Empa researchers has developed a disposable paper battery that works with metallic inks and salt - activated by a drop of water. This could be used to power a wide range of small disposable electronic devices with low power consumption.
As Empa reports, the battery consists of at least one electrochemical cell about one square centimetre in size. Three different inks are printed on a rectangular paper strip. Salt is distributed throughout the paper strip, and one of the two shorter ends of the strip has been dipped in wax. One side of the paper is printed with an ink containing graphite flakes, which acts as the positive pole of the battery - the cathode; the reverse side is printed with a second ink containing zinc powder, which acts as the negative pole of the battery - the anode. A third ink, containing graphite flakes and carbon black, is printed on both sides of the paper above the other two inks. This forms the current collectors that connect the two poles of the battery to two wires located at the end of the paper strip dipped in wax.
If you add a small amount of water, the salt contained in the paper dissolves, charged ions are released and the electrolyte becomes ionically conductive. This step activates the battery: the ions disperse in the paper, causing the zinc at the anode to oxidise and release electrons. By closing the (external) circuit, these electrons can then flow from the zinc-containing anode to the graphite cathode, where they are transferred to the oxygen from the ambient air, thereby reducing it. This creates an electric current that can be used to power a device.
More environmentally friendly disposable electronics
What is special about the new battery is that both paper and zinc and the other components are biodegradable. This could significantly minimise the environmental impact of disposable, low-power electronics.
However, the battery still only works for one to a few hours. "But I am sure we can solve this problem by building it differently," said Empa researcher Gustav Nyström.