While hydrogen is the most abundant element in the universe, here on earth it is almost always bonded with other elements – as in water, where two hydrogen atoms bond with one oxygen atom to form a molecule of H2O.
There are different ‘colours’ of hydrogen, with the colour referring to the way in which the hydrogen is produced, or extracted, and what it is extracted from.
Separating hydrogen from oxygen in water uses a process called electrolysis which – as the term suggests – requires electricity. When the electricity comes from a renewable source such as solar, water or wind power, the hydrogen is green hydrogen, also referred to as clean hydrogen.
When it comes from nuclear energy it is called pink hydrogen and when it comes from fossil fuel burning generators it is called yellow. There are no shades of yellow in the description, but obviously the dirtier the source of the electricity, the dirtier the hydrogen.
Most hydrogen in use right now does not come from water but is produced by extracting it from natural gas /methane. This is a much more complicated process involving high temperature steam and chemical catalysts and produces carbon dioxide and carbon monoxide along with the hydrogen.
If the carbon emissions and byproducts are captured and stored, usually underground, it is called blue hydrogen. If they are not stored it is called grey hydrogen. The advocates for blue hydrogen say it should be considered carbon neutral, but there is debate about how that terminology is misleading and whether it is just a marketing play. Turquoise hydrogen is at an experimental stage but is extracted using a thermal /methane method which removes the carbon in a solid form instead of as CO2 gas.
There is no debate about black and brown hydrogen. They are not clean sources of hydrogen. Through a process called gasification, solid coal or other high carbon material is broken down into its chemical constituents, which includes methane gas – from which the hydrogen is subsequently extracted.
And then there is White hydrogen: naturally occurring.
Editor’s note: There is not universal agreement about the definition of each colour. The information here is distilled from: Enapter/H2View, H2 bulletin, National Grid, World Economic Forum, CNBC.