Electrolysis of water in the context of Hofmann voltameter

Water splitting is the endergonic chemical reaction in which water is broken down into oxygen and hydrogen:

Efficient and economical water splitting would be a technological breakthrough that could underpin a hydrogen economy. A version of water splitting occurs in photosynthesis, but hydrogen is not released but rather used ionically to drive the Calvin cycle. The reverse of water splitting is the basis of the hydrogen fuel cell. Water splitting using solar radiation has not been commercialized.

Green hydrogen (GH2 or GH₂) is hydrogen produced by the electrolysis of water using renewable electricity. Production of green hydrogen causes significantly lower greenhouse gas emissions than production of grey hydrogen, which is derived from fossil fuels without carbon capture.

Green hydrogen's principal purpose is to help limit global warming, reduce fossil fuel dependence by replacing grey hydrogen, and provide for an expanded set of end-uses in specific economic sectors, sub-sectors and activities. These end-uses may be technically difficult to decarbonize through other means such as electrification with renewable power. Its main applications are likely to be in heavy industry (e.g. high temperature processes alongside electricity, feedstock for production of green ammonia and organic chemicals, as direct reduction steelmaking), shipping, and long-term energy storage.

Carbon Recycling International (CRI) is an Icelandic limited liability company which has developed a technology designed to produce renewable methanol, also known as e-methanol, from carbon dioxide and hydrogen, using water electrolysis or, alternatively, hydrogen captured from industrial waste gases. The technology is trademarked by CRI as Emissions-to-Liquids (ETL) and the renewable methanol produced by CRI is trademarked as Vulcanol. In 2011, CRI became the first company to produce and sell liquid renewable transport fuel produced using only carbon dioxide, water and electricity from renewable sources.

Electrofuels, also known as e-fuels, are a class of synthetic fuels which function as drop-in replacement fuels for internal combustion engines. The production process involves the use of captured carbon dioxide or carbon monoxide, together with hydrogen obtained from water splitting. Electrolysis is a viable option with both conventional fossil fuel energy sources and low-carbon electricity sources, including wind, solar, and nuclear power.

The process uses carbon dioxide during manufacturing, and the release of carbon dioxide into the atmosphere is equivalent to the amount released during the combustion of the fuel, resulting in an overall low carbon footprint. Electrofuels emerge as a viable solution for mitigating greenhouse gas emissions from transport sectors, particularly in the context of long-distance freight, marine, and air transport.