Hydrogen is recognized as a flexible carrier of renewable energy: it can be produced from any energy source and also converted into various forms. The main challenges with hydrogen are related to its production, distribution, and storage. Due to its small molecule size, hydrogen embrittles some metals and generally, hydrogen needs to be handled properly. Gaseous hydrogen has low energy content by volume, and thus its storage is challenging. 

Most of the hydrogen today is produced from fossil sources, 50% from natural gas, 30% as a by-product from the petroleum refining, 18% from coal, and only 4% by water electrolysis.

Hydrogen can be stored and transported as compressed (CH2), liquefied (LH2) or reversibly bonded into solid materials or liquid organic hydrogen carriers (LOHC).

Hydrogen can also be converted into synthetic liquid electro-fuels (e-fuels), such as e-methanol, e-methane, liquid e-hydrocarbons, e-formic acid, e-ammonia and its derivatives (Table 1). With electro-fuel concept, using renewable hydrogen and atmospheric CO₂ or nitrogen, the total cycle can be carbon-neutral. E-fuels resembling conventional fossil fuels are readily compatible with internal combustion engines of today.