Hydrogen compounds are only required to contain carbon. However, most of them also contain hydrogen, and because it is the carbon-hydrogen bond which gives this class of compounds most of its particular chemical characteristics, carbon-hydrogen bonds are required in some definitions of the word "organic" in chemistry. Millions of hydrocarbons are known, and they are usually formed by complicated synthetic pathways that seldom involve elementary hydrogen. Compounds of hydrogen are often called hydrides, a term that is used fairly loosely. The term "hydride" suggests that the H atom has acquired a negative or anionic character, denoted H, and is used when
Hydrogen storage to convert carbon dioxide and hydrogen to methane, (see natural gas) using electrolysis and the Sabatier reaction. A third option is to combine the hydrogen via electrolysis with a source of carbon (either carbon dioxide or carbon monoxide from biogas, from industrial processes or via direct air-captured carbon dioxide) via biomethanation, where biomethanogens (archaea) consume carbon dioxide and hydrogen and produce methane within an anaerobic environment. This process is highly efficient, as the archaea are self-replicating and only require low-grade (60°C) heat to perform the reaction. Another process has also been achieved by SoCalGas to convert the carbon dioxide in