• Hydrogen can be extracted from fossil fuels like oil, gas, or coal through processes such as steam reforming, partial oxidation, pyrolysis, and gasification.
• Renewables and nuclear power can all be used to produce low-carbon hydrogen. Biomass gasification is also being explored as another production pathway.
• The environmental impact of making hydrogen depends on the methods and feedstock used to produce it.

Although hydrogen is the most abundant element in the universe, its propensity to bond with other molecules means that the occurrence of natural deposits of hydrogen is generally uncommon. Consequently, it is necessary to extract (or separate) it from other sources using various methods. Currently, the most common methods are steam methane reforming, gasification, and electrolysis. Other methods include thermochemical processes that convert biomass into gas or liquids to then separate out hydrogen, photolytic processes that use solar energy to split water into hydrogen and oxygen, and biological processes that use microbes, such as bacteria and microalgae.

Today, over 95% of the hydrogen used is produced through natural gas or coal reforming processes that emit generate greenhouse gases since they are rarely paired with carbon capture technologies.

Hydrogen produced from fossil fuels

One common method of producing hydrogen today is steam methane reformation (SMR) of natural gas which combines methane from natural gas and water at very high temperatures (approximately 900°C) to produce a mix of carbon monoxide (CO), carbon dioxide (CO₂) and hydrogen (H₂), so-called synthesis gas (or “syngas”). By controlling the amount of air, water, and methane in the reaction, operators can alter the SMR process and change the amount of energy required and the mix of waste gases produced.

Hydrogen can also be extracted from coal in a process called gasification. In this process, syngas is created using coal and water at high temperatures (above 750°C). The coal is also used to provide the heat needed for the reaction.

A major downside to using fossil fuels for hydrogen production is the associated CO₂ emissions. However, by integrating carbon capture, usage and storage (CCUS) technologies to the process, around 90% of the CO₂ emissions can be captured and either used in the production of other products or processes, or stored underground. Although CCUS currently has high costs, which limit its widespread deployment, ongoing projects aim to advance the technology and reduce these costs.

Hydrogen made using electricity

A more sustainable way of producing hydrogen is through electrolysis powered by renewable electricity, which significantly reduces its carbon footprint. Electrolysis is a process that uses electricity to bring about non-spontaneous chemical reactions. Electrolysers, the devices used for this process, use electricity to split water into its constituent hydrogen and oxygen elements.

Electrolysis operates at temperatures typically between 20-100°C and can produce hydrogen at a range of scales, from small home-based units (e.g. generating hydrogen for residential fuel cell applications) to large industrial systems (e.g. ammonia production, or steel manufacturing). In addition, the purity of hydrogen produced through electrolysis is much higher than that of reformed hydrogen, which makes it more compatible for use in fuel cells without the need for expensive purification processes.

However, due to the high costs of equipment and energy, less than 1% of dedicated hydrogen production uses electrolysis today. Production costs are expected to decrease as the cost and size of electrolysers improve, and the cost of renewable generation continues to fall.

An important consideration in the production of hydrogen through electrolysis is the availability of water. With electrolysis requiring around nine litres of water for every kilogram of green hydrogen produced, freshwater access is likely to be a barrier to widespread deployment in water-stressed regions, though integrating desalination could help address this limitation.

Classifying Methods of Hydrogen Production

Hydrogen is often colour-coded to indicate where it comes from (the feedstock), the source of energy used to drive the process, and whether carbon capture is involved. For example, hydrogen produced via electrolysis powered by electricity generated from renewables such as wind and solar is called “green hydrogen”, whilst hydrogen produced from natural gas reforming is referred to as "grey hydrogen," which becomes "blue hydrogen" if carbon capture and storage (CCUS) is applied. Naturally occurring deposits of hydrogen are classified as “white” or “gold” hydrogen.

Recently, the classification of hydrogen has been looking beyond colour labels and focusing more on carbon intensity, taking into account its full life-cycle emissions, including processing and transportation.