The battery made by Volta is credited as one of the first electrochemical cells. It consists of two electrodes: one made of zinc, the other of copper. The electrolyte is either sulfuric acid mixed with water or a form of saltwater brine. The electrolyte exists in the form 2H+ and SO42−. Zinc metal, which is higher in the electrochemical series than both copper and hydrogen, is oxidized to zinc cations (Zn2+) and creates electrons that move to the copper electrode. The positively charged hydrogen ions (protons) capture electrons from the copper electrode, forming bubbles of hydrogen gas, H2. This makes the zinc rod the negative electrode and the copper rod the positive electrode.