Under certain conditions for some electrochemical cells, especially if they use an aqueous electrolyte, hydrogen ions can be converted into hydrogen atoms and H molecules. In the extreme case, bubbles of hydrogen gas might appear at one of the electrodes. If such a layer of hydrogen or even H gas bubbles appear on the positive plate of a battery, they interfere with the chemical action of the cell. An electrode covered with gases is said to be polarized. Polarization in galvanic cells causes the voltage and thus current to be reduced, especially if the bubbles cover a large fraction of a plate. Depolarizers are substances which are intended to remove the hydrogen, and therefore, they help to keep the voltage at a high level. However, this concept is outdated, since if enough depolarizer is present, it will react directly in most cases by getting electrons from the positive plate of the galvanic cell, i.e. there will be no relevant amount of hydrogen gas present. Therefore, the original concept of polarization does not apply to most batteries, and the depolarizer does not react with hydrogen as H. Still, the term is used today, however, in most cases, it might be replaced with oxidizing agent.
Many different substances have been used as depolarizers; the most notable are listed below.