The international dialogue about climate change and the “net zero” world now includes a focus on hydrogen as an abundant source of energy useful in myriad ways (refining, fertilizer production, motive power, etc.) with potentially zero climate-harmful emissions. For those of us who took Chemistry 101 in high school, we know that hydrogen is by far the most abundant element in the Universe, representing an estimated 70 to 75 percent of all known matter. While that is an awesome number (giving rise to the common student question “How do we know that?”), the fact is that, here on Earth, hydrogen does not exist as a “free” gas—it is present here only in combination with other elements, notably oxygen, carbon, and nitrogen. Thus, to capture hydrogen for use in multiple applications, it must be separated from the paired substances (most commonly water (H₂O) and natural gas (CH₄)).

In the discourse around capturing hydrogen as a free gas, a color-coding terminology has emerged that rivals a paint store. I call it the “hydrogen color wheel” and it is the source of much confusion. While each of the color “codes” involves a somewhat detailed explanation, my goal in this post is to keep the detail short and within understandable limits.

The nine colors currently on the “wheel” are gray, black, brown, blue, green, yellow, aqua, turquoise, and pink. One by one, here are the explanations:

Gray: Gray hydrogen is typically produced from natural gas (methane, CH₄) by a process called steam methane reformation. Basically, steam (super-heated water) is applied under pressure to methane. The byproducts of the process are carbon monoxide and carbon dioxide, both of which are universally considered serious climate pollutants. The production of gray hydrogen is by far the most common form of hydrogen production today and has for decades been the source of hydrogen for application to petroleum refining, fertilizer production, and production of pharmaceuticals.

Black: Black hydrogen is produced by turning black coal (pure carbon) into methane using steam, air, and oxygen, and then extracting hydrogen from the methane. As with gray hydrogen, the byproducts are carbon monoxide and carbon dioxide—climate pollutants.

Brown: Brown hydrogen is the same as black hydrogen except that the feedstock is a lighter form of coal, called brown coal, or bitumen. The process is the same as for black hydrogen and the byproducts are the same undesirable climate pollutants.

Blue: Blue hydrogen is hydrogen produced by the same processes as gray, black, and brown hydrogen, but the carbon monoxide and carbon dioxide climate pollutants are captured for use or storage without release into the atmosphere. Carbon capture, use, and storage (“CCUS”) is an area of rapidly developing technologies, which can apply to streams of carbon pollutants well beyond hydrogen production.

Green: Green hydrogen is the most commonly used term in the discussion of net zero emissions policies. Green hydrogen is hydrogen produced from water (H₂O) by a process called electrolysis, powered by electricity produced from a renewable energy source (e.g., wind, solar or hydro power). Electrolyzers, powered by “green” electricity, separate the hydrogen and oxygen. Pure oxygen is a byproduct of producing green hydrogen, and the oxygen can itself be captured and used for industrial, pharmaceutical, and medical purposes. The equipment and processes for producing green hydrogen are the subject of significant attention and capital investment, with many tax and other economic incentives.

Yellow: Yellow hydrogen is a subdivision of green hydrogen and refers to hydrogen produced using electrolysis driven by solar power.

Aqua: Aqua hydrogen is hydrogen produced using electrolysis, but where the power sources for the electrolyzers are a combination of renewable energy and non-renewable energy. Green hydrogen production faces the issue of the intermittency of wind and solar power (the wind doesn’t always blow, the sun doesn’t always shine). Because yellow hydrogen relies in part on electricity from the utility grid or from natural gas-powered generators to fill in the renewable power gaps, the “green” label is not entirely appropriate.

Turquoise: Turquoise hydrogen is produced from natural gas using a process called methane pyrolysis. That method involves subjecting methane to very high temperatures (for example, from super-heated steam) with the result that the hydrogen and carbon are separated, and the carbon byproduct is a solid, not a gas. The solid (usually referred to as carbon black, itself a very valuable commodity) is captured and there are no carbon emissions to the atmosphere. Turquoise hydrogen is viewed as climate friendly because there are no gaseous carbon byproducts, but it is not technically “green” hydrogen because the process is not usually powered by renewable energy.

Pink: Pink hydrogen is produced by electrolysis, the same as green hydrogen, but the source of electricity is nuclear power. In the United States (unlike other parts of the world), nuclear power is generally not considered renewable energy, and the “pink” color distinguishes it from “green” hydrogen, even though the positive climate impacts are the same.

There is little doubt that hydrogen as a fuel and a feedstock has the potential to make a significant contribution to the zero-emissions goal, provided that it can be produced economically and in a climate-friendly manner (i.e., as blue, green, yellow, pink, and possibly aqua and turquoise hydrogen). The industries that operate ocean-going ships, trains, over-the-road trucks and buses, public transportation vehicles, and even aircraft, as well as the industries that produce refined petroleum products, fertilizers, and pharmaceuticals, are at the forefront of hydrogen’s future.