The electrolysis of water serves to decompose water into its constituents water and oxygen, by exposing it to an electrical voltage. Since the reaction is endothermic, the chemical potential can be released again after storing hydrogen for any duration.
Depending on the process, different process parameters, catalysts and substances dissolved in the water are used. These lead to different costs, efficiencies and peak powers of the systems. PEM electrolyzers (Proton Exchange Membrane) use a proton-permeable membrane and distilled water. Typical efficiencies are 70-80%.