Water electrolysis

Water electrolysis uses electrical energy to split water molecules into hydrogen gas and oxygen gas. It takes place at two electrodes connected through an ion-conducting electrolyte or membrane.

Hydrogen evolution reaction: The cathodic electrochemical reaction that reduces protons or water to hydrogen gas. This is the half-reaction that produces hydrogen at the cathode.
Electrolyte: A medium containing mobile ions that carries ionic current between electrodes. Water electrolysis needs an ion-conducting path to complete the electrical circuit.
Alkaline water electrolysis: Water electrolysis using a liquid alkaline electrolyte, commonly potassium hydroxide, and porous separators. It is a mature electrolyzer design that uses alkaline-compatible electrodes.
Steam methane reforming: An industrial process that produces hydrogen by reacting methane with steam, usually over a catalyst. It makes hydrogen from fossil methane rather than splitting water with electricity.
Hydrogen economy: An energy system in which hydrogen serves as a carrier, fuel, or industrial feedstock across multiple sectors. Large-scale electrolysis could supply hydrogen for sectors that are difficult to electrify directly.
Oxygen evolution reaction: The anodic electrochemical reaction that oxidizes water or hydroxide ions to oxygen gas. This is the half-reaction that produces oxygen at the anode.
Proton-exchange membrane: A polymer membrane that conducts protons while separating gases and electrodes in an electrochemical device. It transports protons from the anode while keeping product gases apart in PEM electrolysis.
Proton-exchange membrane electrolysis: Water electrolysis using a proton-conducting polymer membrane as both electrolyte and gas separator. Its compact design can respond rapidly to variable electricity supply.
Autothermal reforming: A process that produces hydrogen from hydrocarbons using both partial oxidation and steam reforming within one reactor. It is another hydrocarbon-based hydrogen route, with a different heat and reactor arrangement.
Levelized cost of hydrogen: An estimate of hydrogen's average production cost over a facility's lifetime, including investment and operating expenses. Electrolyzer cost and electricity prices strongly shape the cost of electrolytic hydrogen.

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