What's the Difference Between Gray, Blue, and Green Hydrogen?

Jessica Shea Choksey | Sep 27, 2021

As the world looks to new and more sustainable forms of energy, hydrogen has been on a steady climb to the top. This alternative fuel, when consumed in a fuel cell system, only produces water as its byproduct. In mobility applications, this means hydrogen-powered vehicles emit water vapor from their tailpipes instead of greenhouse gases that are harmful to the environment. But does that mean zero-emissions motoring from hydrogen has no carbon footprint?

Hydrogen Colors Appolinary Kalashnikova Unsplash

Hydrogen's Carbon Footprint

Although hydrogen-powered cars operate with zero emissions, producing hydrogen as a fuel can be a very different story. As the earth's most abundant element, hydrogen is found everywhere, from fossil fuels to water to plants, but it never appears naturally in pure form.

Procuring hydrogen requires its separation from other elements through chemical processes that require energy. These processes create varying degrees of environmental pollutants. As such, there are three main categories of hydrogen: gray, blue, and green.

Gray Hydrogen

Gray hydrogen is derived from natural gas and produced from fossil fuels, making it the least renewable form of hydrogen. Most of the hydrogen produced today is gray hydrogen. It is relatively inexpensive and commonly used in the chemical industry to make fertilizer and for refining oil.

Gray hydrogen is produced by reforming natural gas, a processing technique used to rearrange the molecular structure of hydrocarbons. In this process, methane—the primary element in natural gas—is mixed with steam at a high temperature to yield hydrogen and carbon dioxide through a catalytic chemical reaction.

Unfortunately, almost 10 kg of carbon dioxide releases into the atmosphere for every 1 kg of gray hydrogen produced. This high ratio of CO2 generation gives this form of hydrogen its "gray" designation.

Gray hydrogen is viewed as a "bridging" energy alternative as the world weans off fossil fuels. But its impact on climate is negative overall, much like drilling for oil or mining coal.

Blue Hydrogen

The same chemical processing technique used to make gray hydrogen is also used to produce blue hydrogen. The big difference, however, is the management of CO2. With blue hydrogen, the CO2 produced does not escape into the environment. Instead, it is captured at the production facility and stored separately. This technology is known as Carbon Capture and Storage (CCS). However, storage is costly and has logistical challenges.

Blue hydrogen is currently attracting attention as a realistic alternative because it has a significantly lower CO2 impact on the environment than gray hydrogen, making it more sustainable overall. However, the blue hydrogen process does not eliminate carbon emissions into the atmosphere entirely. It may be more ecologically friendly than gray hydrogen but is by no means perfect.

Green Hydrogen

Closer to real sustainability is green hydrogen. This form of hydrogen follows an entirely different production process than that of gray or blue hydrogen. The technique employs electrolysis—the separation of hydrogen and oxygen molecules by applying electrical energy to water. Renewable sources such as wind and solar power generate the electricity for this process.

Utilizing renewable sources instead of fossil fuels is the key to making green hydrogen. This technique yields a closed loop of sustainable energy in which no harmful gases come into existence at any point in the production chain, making it the ultimate goal in the hydrogen fuel space.

Although green hydrogen appears to be the ideal choice for mobility and other applications, producing it presents certain challenges. The machines used to carry out electrolysis are costly. Furthermore, clean electricity from solar and wind sources is limited in supply. The environmental advantages of green hydrogen are potentially far-reaching, but producing this clean energy source is not yet practical.


Although hydrogen may play a significant role in our clean energy future, it still has a long way to go. To reach its zero-emissions potential, there will need to be a gradual shift from gray hydrogen to green hydrogen in the years ahead. How long that will take and its overall feasibility is still to be determined.

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