Understanding the hydrogen spectrum

Posted: May 15, 2025

Hydrogen, the most abundant chemical element in the universe, produces only water when converted into usable energy in a fuel cell. This versatile element is gaining traction across diverse sectors including refining, ammonia synthesis, chemical processing, transportation, and electricity generation and storage.

But only rarely does it exist freely on Earth in gas form. Almost always, hydrogen occurs in compounds with other elements like oxygen (as in water or H2O) and carbon (as in hydrocarbons like methane or CH4), so it needs to be separated out to produce the hydrogen gas or liquid that can be used for energy.^[1]

There are many separation methods available, each offering distinct benefits and costs while having different impacts on decarbonization efforts. Consequently, over the last six years, a colorful shorthand has been developed to distinguish between hydrogen produced in different ways.  While different colors of hydrogen may come from different sources and be isolated in different ways, it’s important to note that these color designations do not refer to any difference in the resulting hydrogen’s characteristics or uses.

What began primarily as a way to mark the distinction between emissions-intensive grey hydrogen and renewables-based green hydrogen has since expanded into a veritable Skittles bowl of colors. Rather than a universally agreed-upon code, hydrogen’s rainbow spectrum is a convention, one likely to continue changing as new technologies are developed and new distinctions prove necessary.

Why grey and blue hydrogen dominate today’s market

By far the most common color of hydrogen today is grey hydrogen, which accounts for 95% of the volume currently produced in the United States and roughly two-thirds of the volume produced worldwide. Grey hydrogen is primarily used for ammonia production and in the petrochemical industry, but its applications lie beyond that as well.

Dedicated producers of grey hydrogen typically deploy an energy-intensive technique called steam methane reforming whereby high-temperature steam reacts with the methane in natural gas to release hydrogen (and carbon dioxide). In another process known as “partial oxidation," the hydrocarbons in natural gas react with a limited amount of oxygen, ultimately producing hydrogen and carbon monoxide.

Accordingly, grey hydrogen has been criticized for its ecological impact. It was estimated in 2023 to be responsible for about 2% of carbon dioxide emissions worldwide.

Hydrogen can also be produced through the gasification of brown (lignite) or black (bituminous) coal to make what is called—you guessed it—brown or black hydrogen. Gasification is a process that uses heat to convert carbon-rich materials into a gas, thus capturing hydrogen while releasing both carbon monoxide and carbon dioxide into the atmosphere. This approach generates even more emissions than grey hydrogen.

The IEA reported in 2024 that around 20% of hydrogen production worldwide was coal-based. Especially large amounts of hydrogen are produced via coal gasification in China, which alone accounts for 90% of global coal use for hydrogen production.

As we have seen, grey, brown, and black hydrogen all produce carbon byproducts. If those byproducts are captured—say, by use of a carbon capture, utilization, and storage (CCUS) system—then the hydrogen produced is reclassified as blue.

Theoretically, blue hydrogen is carbon-neutral, making it an attractive climate-friendly intermediate solution for countries undergoing an energy transition: fossil fuels are still consumed, but the carbon footprint is lower^[2]. Blue hydrogen has proven particularly attractive in North America and Asia. And while use of CCUS increases the price of blue compared to grey, brown, or black hydrogen, it remains cheaper and more widely adopted than renewables-based alternatives, in part because it leverages existing natural gas infrastructure.

A final carbon-based production method is known as turquoise hydrogen. Here a process called “pyrolysis” converts methane into hydrogen plus carbon byproducts that are solid—meaning they may not need CCUS and can instead be used in other applications. This method remains in the early stages of development, however, with high heat requirements posing a particular challenge. 

Electrolysis: The future of green hydrogen production

It is also possible to produce hydrogen through electrolysis, a process that uses electricity to split water molecules into hydrogen and oxygen. When that process is performed using electricity from renewable sources like solar, geothermal, or wind, then the hydrogen is categorized as green.

The environmental advantages of green hydrogen are clear. Using water rather than fossil fuels as an input means there are no carbon byproducts. Estimates for grey hydrogen’s emissions intensity range between nine to 13 kilograms of carbon dioxide per kilogram of hydrogen produced. The process of electrolysis, by contrast, emits no carbon dioxide, although the overall system including maintenance of infrastructure like renewables power plants, water supply, and the storage and transportation of hydrogen could mean green hydrogen still requires the equivalent of up to three kilograms of CO2 for each kilogram of hydrogen.

The more efficient the electrolysis process becomes, the greater green hydrogen’s potential to become widely used at a viable price point. That moment has not yet come, however. In 2021, electrolysis-based hydrogen accounted for some 0.1% of global production; while that figure has increased somewhat since, it remains a tiny proportion of the whole. Installed electrolyzer capacity doubled from 2022 to 2023, but in doing so it only reached 1.4 gigawatts, a far cry from the 560 gigawatts considered necessary by the IEA for its Net Zero Emissions by 2050 scenario.

In recent years, public willpower has grown around green hydrogen, especially in Europe and the Asia-Pacific region. The EU’s 2022 REPowerEU Strategy set a target of both producing and importing 10 million tons of renewable hydrogen by 2030, with green hydrogen slated to meet around 10% of energy needs by 2050. Germany, the United Kingdom, Canada, and India have all introduced schemes offering incentives to make electrolysis-based hydrogen economically feasible. Electrolyzer capacity looks set to continue growing worldwide, with major projects recently approved in the EU, China, and India as well as Saudi Arabia and Oman.

Still, green hydrogen production faces major challenges in terms of high cost, energy losses, and supply chain requirements. To succeed at scale, it would require massive increases in wind and solar power generation. It also depends on government support. In the US, federal subsidies have largely proven favorable to blue hydrogen instead, with a slowdown in enthusiasm for hydrogen in general. According to a recent Belfer Center report, the inability to stimulate demand for green hydrogen has been the primary policy sticking point on both sides of the Atlantic; high costs and technological uncertainty present demand-side difficulties, as does the need for further supply chain and infrastructure development.

Exploring yellow, pink, and white hydrogen opportunities

As new hydrogen projects get underway around the world, the hydrogen spectrum continues to evolve to reflect new variations in production methods.

The color yellow has been bestowed upon the subset of green hydrogen produced by electrolysis powered by solar energy in particular,  while so^[3] Another related method, known as photocatalytic water splitting, uses only sunlight and a specialized catalyst to trigger the splitting of water into hydrogen and oxygen, thus avoiding the step where light is turned to solar power for use in electrolysis.

Pink, red, and purple hydrogen all refer to nuclear-based production techniques. To make pink hydrogen, nuclear energy is used to power the electrolysis process; red hydrogen production uses the heat generated by nuclear facilities to catalyze the splitting of water. Purple hydrogen is produced by electrolysis that uses both nuclear power and heat.

Advocates of nuclear-based hydrogen argue that it has the potential to generate low-emissions hydrogen from a notably stable electricity supply while making use of existing infrastructure. They also point to the ways that red and purple hydrogen can take advantage of reactors’ high temperatures. The development of pink hydrogen is still in early stages, but it has been backed by countries like France and the UAE. Meanwhile, a pink hydrogen facility is already up and running in the U.S. at the Nine Mile Point nuclear facility.

Finally: White hydrogen is the name given to naturally-occurring hydrogen gas formed underground and trapped in subterranean deposits. There may well be tens of trillions of metric tons of hydrogen currently available in geologic stores—not all of it will be commercially exploitable, and the economics of tapping it remain to be seen, but using just 2% of that amount would be enough to meet global hydrogen demand for some 200 years. Speculators are already searching for the stuff in Australia, Europe, and the Americas. If it proves feasible, white hydrogen could prove to be a game-changer.

^{[1]Technically, hydrogen is not considered a source but rather a carrier of energy.
[2] Blue hydrogen’s carbon neutrality is not completely true in practice, however. The WEF reckoned in 2021 that 85-95% of the carbon byproducts are captured; researchers have sharply debated exactly how low-emission blue hydrogen really is, with methane leakage and carbon-capture efficiency key drivers of ecological footprint.
[3]me sources use the color yellow to describe electrolysis using mixed-origin and/or grid electricity instead.}