What is water electrolysis ?



Water electrolysis is an electrochemical process aimed at breaking down water (H2O) into its main elements – dihydrogen (H2) and oxygen (O) – by applying a direct electric current and voltage to water, in liquid or vapour form.

The water electrolysis process


Electrolysis consists of three levels of architecture: the cell, the stack, and the production unit. The electrolyser cell contains two conductive metal electrodes (anode and cathode), connected to a direct current generator, and separated by an electrolyte which can be an aqueous solution or membrane.


When assembled, the cells form stacks, often built in series which, supplemented by auxiliary equipment (electrical controls, water treatment, piping, compressor, etc.) form the electrolysis production units.


The different types of water electrolysis

Alkaline electrolysis


In use since the early 20th century, for ammonia and chlorine production, alkaline electrolysers are the most mature. In the absence of expensive materials, investment costs are relatively limited. Despite advances in reactivity and pressurised production, the alkaline method offers less flexibility than its alternatives, has lower cost reduction potential and requires more maintenance.  

PEM electrolysis


The PEM technology was developed in the 1960s in response to the operational limits of alkaline electrolysers, particularly their size, and to the potash recycling issue. Very flexible and compact, PEMs also reduce maintenance costs. More expensive than their alkaline counterparts, PEMs will benefit from economies of scale, mass production and improved knowledge to become more competitive, in particular to improve the use of precious metals and increase the service life of electrolysis cells.

High-temperature electrolysis


In the presence of a heat source, part of the reaction’s energy requirements is provided by water vapour. Using ceramic electrolysis, SOECs reduce investment costs, open the possibility of water and CO2 co-electrolysis for e-fuel production, and can also be used in reverse mode to generate electricity. However, SOECs have a very limited-service life and must demonstrate their ability to provide a market solution.



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