In 1875 the French Novelist Jules Verne published The Mysterious Island in which the protagonist Cyrus Smith envisioned a solution to the perceived inevitable exhaustion of coal:
“Water, decomposed into its primitive elements, by electricity. (…) I believe that water will one day be employed as fuel, that hydrogen and oxygen which constitute it, used singly or together, will furnish an inexhaustible source of heat and light, of an intensity of which coal is not capable.”
Hydrogen has continued to capture the imagination of scientists, entrepreneurs, and decision-makers because electrolysis is a well-established technology and because hydrogen has three times the energy density (MJ/kg) of gasoline and six times the energy density of coal. Large-scale commercialization of hydrogen as a fuel source has remained elusive due to a combination of economic, technological, safety, regulatory, and end-use compatibility issues.
Hydrogen can be produced in many ways, resulting in a hydrogen “color” classification to compare pathways. Different colors depend on the initial molecule being broken down, the energy source used to take hydrogen from it, and the byproducts of the chemical reaction.1 Given the imperative to reduce greenhouse gas emissions, the hydrogen color discussion centers on carbon dioxide and methane emissions.
An important caveat: The hydrogen industry’s color classification is evolving, and not all terms have universally agreed-upon definitions. This can lead to variations in how terms are used and understood.2
Gray hydrogen is the dominant method of producing hydrogen via steam methane reforming (SMR) that relies on natural gas as a feedstock. In SMR, methane from natural gas is reacted with steam (H₂O) at high temperatures (700–1,000°C) in the presence of a catalyst such as nickel. The reaction produces hydrogen (H₂) and carbon monoxide (CO). The carbon monoxide is further processed to produce more hydrogen and carbon dioxide. The SMR process is an energy-intensive process because it requires high temperatures and often uses natural gas as both a feedstock and an energy source. It also is carbon-intensive due to its release of carbon dioxide.
Black hydrogen and brown hydrogen are produced by the gasification of bituminous and brown (lignite) coal, respectively. In gasification, pulverized coal is fed into a gasifier, a high-temperature (800-1,500°C), high-pressure reactor. This produces syngas, a mixture of hydrogen (H₂), carbon monoxide (CO), carbon dioxide (CO₂), methane (CH₄), and some impurities. The syngas is processed to yield additional hydrogen and carbon dioxide. Coal gasification is a very carbon-intensive process.
Blue hydrogen refers to hydrogen produced as black, brown, or gray hydrogen that is coupled with carbon capture, utilization, and storage (CCUS) technologies. A wide range of CCUS technologies are under investigation, but none are close to commercial viability at scale and are projected to be 90% efficient at removing carbon dioxide. They are accurately described as “low carbon” and not “zero carbon.”
Green hydrogen uses electricity from renewable sources (wind, solar, hydro) to drive the electrolysis of water. In water electrolysis, an electric current is passed through water to separate the hydrogen and oxygen atoms. Green hydrogen is frequently described as “zero carbon” because there are no carbon emissions at the point of production. Small quantities of carbon dioxide and other greenhouse gases (GHG) are emitted across the supply chain of renewable electricity.3 Overall green hydrogen has much lower GHG emissions compared to gray, black, or brown hydrogen
Turquoise hydrogen is an attractive but unproven technology that uses natural gas as a feedstock. It is made in a process called methane pyrolysis in which methane is heated to high temperatures (800-1,500°C) in the absence of oxygen, which prevents combustion and CO₂ formation. The heat causes methane molecules to break apart (pyrolysis) into their elemental components: hydrogen gas (H₂) and solid carbon (C). Solid carbon has a variety of industrial applications such as printer ink, tires, steel production, hydrogen fuel cells, and the fabrication of batteries and solar panels.4
Electricity from nuclear power can produce three colors of hydrogen:
- (b)Pink hydrogen(/b) is created using electrolysis with electricity from a nuclear power plant. The electrolysis process is the same as green hydrogen.
- (b)Purple hydrogen(/b) is produced by a process called nuclear thermochemical water splitting or high-temperature electrolysis that leverages both the heat and electricity generated by nuclear reactors to increase efficiency and reduce carbon emissions. This distinguishes it from pink hydrogen, which is created via traditional electrolysis using electricity from nuclear reactors.
- (b)Red hydrogen(/b) is a novel proposed technology that uses the heat produced in a nuclear reactor to drive the thermolysis of water, the process of splitting water molecules into hydrogen and oxygen gas by applying heat. This process requires extremely high temperatures (typically above 2500°C) to “crack” the water molecules.
The European Union (EU) has taken a different approach to classifying hydrogen.5 The EU places a heavy emphasis on the sustainability of the hydrogen production supply chain, especially GHG emissions, and it distinguishes between fossil fuel and renewable energy sources. The “Climate Delegated Act” of 2021 sets specific greenhouse gas thresholds relating to hydrogen production and other criteria for other hydrogen-related activities. The table above maps the EU system to the color-based system.
There are many feasible technical pathways to produce hydrogen. But the vast majority (99.6%) of hydrogen currently comes from fossil fuels. About 71% is gray hydrogen produced by the SMR of natural gas. Most of the rest is brown hydrogen from coal via gasification.6 For hydrogen to play an important role in a climate-friendly world the non-fossil fuel production and consumption pathways must rely on low-carbon technologies that are safe and affordable.
1 Mills, Ryan. “Clean Energy 101: The Colors of Hydrogen.” RMI, April 13, 2022. https://rmi.org/clean-energy-101-hydrogen/.
2 Barregren, Thomas. “Tech Brief: The Colors of Hydrogen.” Smoltek, January 16, 2024. Link
3 Valente, Antonio, Diego Iribarren, and Javier Dufour. “Harmonised Life-Cycle Global Warming Impact of Renewable Hydrogen.” Journal of Cleaner Production 149 (April 15, 2017): 762–72. https://doi.org/10.1016/j.jclepro.2017.02.163.
4 Kay, Adam. “It’s Time to Pay Attention to Turquoise Hydrogen.” American Gas Association, June 23, 2023. https://www.aga.org/its-time-to-pay-attention-to-turquoise-hydrogen/.
5 European Commission, “A hydrogen strategy for a climate-neutral Europe,” August 7, 2020, https://tinyurl.com/2kux9v3n
6 Wood Mackenzie, “The rise of the hydrogen economy,” accessed October 6, 2024, https://www.woodmac.com/market-insights/topics/hydrogen-guide/
