Potato battery digital clock. A potato battery is a type of electrochemical cell. Potatoes contain a mild phosphoric acid (H3PO4). When a zinc negative electrode (the anode) is inserted into a potato a chemical reaction occurs which releases electrons that are captured by the copper positive electrode (the cathode). Each potato added in series increases the voltage. This flow of electrons between the anode and the cathode generates an electric current in the opposite direction, from the cathode to the anode. The current passes through the clock and makes it work for as long as the current is sufficiently powerful. Eventually, the electrodes undergo chemical reactions that block the flow of electricity.