Commercial electrochemical cells are classified into primary, secondary, and fuel cells.
Primary cells, like the dry cell and mercury cell, are single-use and cannot be recharged. A dry cell uses a zinc anode and a graphite cathode surrounded by MnO₂, while the mercury cell uses zinc amalgam and HgO with a KOH-ZnO electrolyte. Secondary cells, such as the lead storage battery, are rechargeable and commonly used in vehicles. During discharge, lead and lead dioxide react with sulfuric acid to form lead sulfate and water. This reaction is reversed during charging. Fuel cells, like the H₂–O₂ fuel cell, convert the chemical energy of fuels directly into electricity with high efficiency (~70%) and minimal pollution. They use porous carbon electrodes with platinum or silver catalysts and KOH as electrolyte.