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 is colorless, odorless, and non-toxic substance; the lightest and smallest of the elements. Due to its small molecule size, hydrogen embrittles some metals and generally, hydrogen needs to be handled properly. Hydrogen has high energy content by weight, but low energy content by volume, and thus its storage is challenging.  Hydrogen can be stored and transported as compressed (CH2), liquefied (LH2) or reversivebly binded into solid materials or liquid organic hydrogen carriers (LOHC). Stored hydrogen can be used later in turbines, in internal combustion engines (ICEs), in high-efficiency fuel cells (FCs) or for chemicals. Hydrogen is also important in refinery processes to upgrade raw fossil and bio-based fuels to final products.  Hydrogen can also be used as blending component in methane (hytane).

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.