Hydrogen gas is made using several industrial methods. Most hydrogen produced today comes from fossil fuels. The majority of this hydrogen is called gray hydrogen, which is created through a process called steam methane reforming. This process uses a chemical reaction between steam and methane, the main part of natural gas, to make hydrogen. Making one tonne of hydrogen through this method releases 6.6 to 9.3 tonnes of carbon dioxide. When carbon capture and storage technology is used to remove much of these emissions, the result is called blue hydrogen.
Green hydrogen is typically made using renewable electricity through a process called electrolysis of water. Sometimes, green hydrogen is also defined as hydrogen made from other low-emission sources, such as biomass. Producing green hydrogen is currently more expensive than making gray hydrogen, and the process of converting energy into hydrogen is not very efficient. Other methods of making hydrogen include biomass gasification, methane pyrolysis, extracting natural hydrogen from underground, and creating hydrogen through in situ synthesis.
As of 2023, less than 1% of hydrogen made specifically for use is low-carbon, including blue hydrogen, green hydrogen, and hydrogen from biomass.
In 2020, about 87 million tons of hydrogen were produced worldwide for various purposes, such as oil refining, making ammonia through the Haber process, and producing methanol by reducing carbon monoxide. The global hydrogen production market was valued at 155 billion US dollars in 2022 and is expected to grow at a compound annual rate of 9.3% from 2023 to 2030.
Overview
Molecular hydrogen was found in the Kola Superdeep Borehole. Scientists do not yet know how much molecular hydrogen exists in natural sources, but one company focuses on drilling wells to collect hydrogen. Most hydrogen in Earth's crust is combined with oxygen in water.
Creating elemental hydrogen requires using a hydrogen carrier, such as a fossil fuel or water. Using fossil fuels as a carrier consumes the fuel and produces carbon dioxide during the steam methane reforming (SMR) process. However, a newer method called methane pyrolysis does not produce carbon dioxide. These methods usually need no extra energy beyond the fossil fuel itself.
Breaking water into hydrogen requires electricity or heat, which comes from energy sources like fossil fuels, nuclear power, or renewables. Hydrogen made by splitting water with renewable energy, such as wind or solar power, is called green hydrogen. When hydrogen is made from natural gas using a process that releases no greenhouse gases, it is called turquoise hydrogen.
Hydrogen made from fossil fuels with greenhouse gas emissions is called grey hydrogen. If most of the carbon dioxide is captured during production, it is called blue hydrogen. Hydrogen made from coal is sometimes called brown or black hydrogen.
Hydrogen is often described using color names to show its source. This may be because gray is associated with "dirty" hydrogen, but the colors are only labels for classification.
Current production methods
Hydrogen is made in factories using a process called steam reforming, which uses natural gas. The hydrogen made has about 74% of the energy from the original fuel because some energy is lost as heat. This process creates carbon dioxide, a type of greenhouse gas, and is called gray hydrogen. If the carbon dioxide is captured and stored, the hydrogen is called blue hydrogen.
Steam methane reforming (SMR) uses natural gas, mostly methane (CH₄), and water to make hydrogen. It is the most common way to produce hydrogen, providing about half of the world's hydrogen. The process involves heating the gas to temperatures between 700 and 1,100 degrees Celsius (1,300 to 2,000 degrees Fahrenheit) in the presence of steam. A nickel catalyst helps the reaction occur. This creates carbon monoxide and hydrogen gas (H₂).
In the water-gas shift reaction, carbon monoxide reacts with steam to create more hydrogen. This reaction also needs a catalyst, such as iron oxide. The byproduct is carbon dioxide (CO₂). For every ton of hydrogen made, between 9 and 12 tons of CO₂ are created, depending on the type of fuel used.
In the first step of the process, high-temperature steam (H₂O) reacts with methane (CH₄) to create syngas. In the second step, more hydrogen is made through a lower-temperature reaction called the water-gas shift reaction. This reaction happens at about 360 degrees Celsius (680 degrees Fahrenheit). During this step, oxygen from steam is used to change carbon monoxide into CO₂. This process also provides energy to keep the reaction going. Additional heat is usually added by burning some of the methane.
In May 2019, a study showed that using a tin catalyst heated by electricity can reduce natural gas use and CO₂ emissions by one-third. This method also improves the overall efficiency of the process. Traditional steam reforming uses 4.2 kilowatt-hours per cubic meter of hydrogen, while the new method uses 3.6 kilowatt-hours (2.6 kilowatt-hours from natural gas and 1.0 kilowatt-hour from electricity).
Hydrogen can also be made without using fossil fuels by splitting water (H₂O) into hydrogen and oxygen. If the energy used for splitting water comes from renewable or low-carbon sources, the hydrogen is called green hydrogen. This method is more expensive than using fossil fuels.
Hydrogen can be made using high-pressure or low-pressure electrolysis, or other methods like high-temperature electrolysis. However, current electrolysis methods are about 70–80% efficient. Producing 1 kilogram of hydrogen requires 50–55 kilowatt-hours of electricity.
Steam methane reforming costs between $1 and $3 per kilogram of hydrogen, not including pressurization costs. Electrolysis can be competitive in some areas because of lower costs, as noted by companies like Nel Hydrogen and reports from the International Energy Agency (IEA).
In 2019, about 2% of hydrogen was made using electrolysis, which uses electricity and water. This process requires about 50–55 kilowatt-hours of electricity per kilogram of hydrogen.
Water electrolysis uses electricity to split water into hydrogen and oxygen. As of 2020, less than 0.1% of hydrogen was made this way. Electrolysis is about 70–80% efficient, while steam reforming is 70–85% efficient. Electrolysis efficiency is expected to reach 82–86% by 2030.
Water electrolysis can operate at 50–80 degrees Celsius (120–180 degrees Fahrenheit), while steam reforming needs much higher temperatures (700–1,100 degrees Celsius). The main difference is the energy source: electricity for electrolysis or natural gas for steam reforming. Electrolysis is attractive because water is widely available. Renewable energy sources are being explored to lower hydrogen production costs.
There are three main types of electrolytic cells: solid oxide electrolyser cells (SOECs), polymer electrolyte membrane cells (PEM), and alkaline electrolysis cells (AECs). Alkaline cells are cheaper but less efficient. PEM cells are more expensive but more efficient and can handle higher current levels. SOECs work at high temperatures (about 800 degrees Celsius) and can use heat from other sources, like industrial waste or solar plants, to reduce electricity needs. PEM cells work below 100 degrees Celsius and are good for renewable energy sources like solar panels. AECs work best at high temperatures and with strong electrolytes like potassium hydroxide.
Efficiency is measured by how much energy is used to make hydrogen. A 100%-efficient electrolyser would use 39.4 kilowatt-hours per kilogram of hydrogen. Practical electrolysis uses about 50 kilowatt-hours per kilogram. If hydrogen is compressed for use in cars, it requires an additional 15 kilowatt-hours.
Conventional alkaline electrolysis is about 70% efficient, but newer models can reach 8
Natural hydrogen
Hydrogen is also found naturally underground. This type of hydrogen, known as white hydrogen or gold hydrogen, can be removed from wells in a way similar to how oil and natural gas are extracted.
White hydrogen may be found or created in large amounts in the Mid-continental Rift System. This could support a renewable hydrogen economy. Water can be pumped into hot, iron-rich rock to collect the hydrogen.
Experimental production methods
Pyrolysis of methane (natural gas) involves passing methane through a molten metal catalyst in a single step. This method, called a "no greenhouse gas" approach, was tested in a lab in 2017 and is now being tested on a larger scale. The process occurs at very high temperatures (1065 °C). Producing 1 kg of hydrogen requires about 18 kWh of electricity to provide heat. The chemical reaction for methane pyrolysis is as follows.
The solid carbon produced during this process can be sold as a material for manufacturing, mixed into asphalt for roads, or disposed of in landfills.
As of 2023, several companies are developing methane pyrolysis technology. However, challenges remain before this method can be used widely in industry.
Biological hydrogen production uses bacteria to convert organic materials into hydrogen. This process involves three steps similar to anaerobic conversion. Dark fermentation does not require light and can produce hydrogen continuously day and night. Photofermentation, in contrast, only occurs in the presence of light. Electrohydrogenesis uses microbial fuel cells to generate hydrogen from organic matter.
Hydrogen can also be produced in algae bioreactors. In the late 1990s, scientists discovered that removing sulfur from algae causes it to produce hydrogen instead of oxygen during photosynthesis. Biological hydrogen can also be made using waste materials as feedstocks. Bacteria break down hydrocarbons and release hydrogen and carbon dioxide as byproducts.
Another method is biocatalyzed electrolysis, which uses microbes and aquatic plants like reed sweetgrass, cordgrass, rice, tomatoes, lupines, and algae. Regular electrolysis splits water into hydrogen and oxygen using electricity. High-pressure electrolysis achieves this at pressures of 120–200 bar, eliminating the need for external compressors. The largest high-pressure hydrogen production plant in Europe, located in Kokkola, Finland, produces 1,400,000 kg of hydrogen annually using alkaline technology.
High-temperature electrolysis (HTE) uses heat and electricity to split water into hydrogen and oxygen. Since some energy is provided as heat, less energy is needed to convert it twice (from heat to electricity, then to hydrogen). Nuclear heat could be used to split water into hydrogen. High-temperature gas-cooled nuclear reactors may achieve this using thermochemical methods. HTE has been tested in labs but not at a commercial scale. The hydrogen produced is of lower quality and not suitable for fuel cells.
Using electricity from photovoltaic systems is a clean way to make hydrogen. Electrolysis through a photoelectrochemical cell (PEC) process, also called artificial photosynthesis, splits water into hydrogen and oxygen. In 1983, William Ayers developed a system using amorphous silicon and catalysts to produce hydrogen directly from water. This method, known as an "artificial leaf," uses a thin film of silicon and a Nafion membrane to transport ions. Research continues to improve the efficiency of these systems.
In 2015, Panasonic developed a photocatalyst based on niobium nitride that absorbs 57% of sunlight to split water into hydrogen. The company aims to commercialize this technology soon. Thomas Nann’s team at the University of East Anglia used a gold electrode with indium phosphide nanoparticles and an iron-sulfur complex to produce hydrogen with 60% efficiency when exposed to light and a small electric current.
Very high temperatures are needed to split water into hydrogen and oxygen. A catalyst helps make this process work at manageable temperatures. Concentrating solar power can heat water to 800–1200 °C. The Hydrosol-2 pilot plant in Spain uses sunlight to achieve this and has operated since 2008. Its modular design allows scaling up to larger systems.
Over 352 thermochemical cycles can split water into hydrogen and oxygen without electricity. Examples include the iron oxide cycle, sulfur-iodine cycle, and others. These processes are being tested and may be more efficient than high-temperature electrolysis. However, none have been demonstrated at commercial production levels.
The Kværner process, developed in the 1980s, produces hydrogen from hydrocarbons like methane and natural gas. About 48% of the energy from the feedstock becomes hydrogen, 40% becomes activated carbon, and 10% becomes superheated steam.
As of 2019, hydrogen is mainly used in industry for making ammonia, methanol, and in petroleum refining. Although hydrogen gas was once thought to not occur naturally in large deposits, it is now known to exist in small amounts.
Environmental impact
Most hydrogen is made from fossil fuels, which releases carbon dioxide into the air. When emissions are released directly into the atmosphere, this type of hydrogen is called grey hydrogen. If emissions are captured using technology called carbon capture and storage (CCS), the hydrogen is called blue hydrogen. Studies suggest that blue hydrogen produces about 20% more greenhouse gases than burning natural gas or coal for heat and 60% more than burning diesel for heat. These estimates assume hydrogen is made using steam methane reformers (SMR) with CCS and typical methane leakage rates during production. The amount of greenhouse gases from blue hydrogen depends heavily on how much methane leaks during production. For blue hydrogen to help reduce climate change, methane leakage must stay below 0.3–3.2%, depending on assumptions.
Using a technology called autothermal reformers (ATR) with integrated carbon capture can capture more carbon dioxide efficiently. Studies show that ATR plants with carbon capture produce fewer greenhouse gases than SMR plants with carbon capture. In Europe, ATR technology with carbon capture has been found to reduce greenhouse gas emissions compared to burning natural gas. For example, the H21 project reported a 68% reduction in emissions due to lower carbon dioxide levels in natural gas and better reactor designs for capturing carbon dioxide.
Hydrogen made from renewable energy sources, such as wind or solar power, is called green hydrogen. Two common methods include using electricity to split water into hydrogen and oxygen (a process called power to gas) and using landfill gas in a steam reformer. Hydrogen made from renewable energy is considered a renewable fuel. Hydrogen produced from nuclear energy through electrolysis is sometimes called pink hydrogen. In January 2022, the Oskarshamn Nuclear Power Plant signed an agreement to produce commercial pink hydrogen at a rate of kilograms per day.
As of 2020, the estimated cost to produce grey hydrogen and blue hydrogen was $1–1.80 per kilogram, while green hydrogen cost $2.50–6.80 per kilogram.
In 2022, about 94 million tonnes of grey hydrogen were produced globally using fossil fuels, mostly natural gas. This makes grey hydrogen a major source of greenhouse gas emissions.
Hydrogen uses
Hydrogen is used to change heavy petroleum parts into lighter ones through a process called hydrocracking. It is also used in other processes, such as aromatization, hydrodesulfurization, and the production of ammonia through the Haber process. This process is the main way industries make synthetic nitrogen fertilizer, which helps grow 47 percent of the food eaten worldwide.
Hydrogen can be used in fuel cells to create electricity or may be used as a fuel for transportation.
Hydrogen is made as a by-product during the industrial production of chlorine using electrolysis. Even though it requires costly technology, hydrogen can be cooled, compressed, and purified for use in other processes at the same location or sold to customers through pipelines, cylinders, or trucks. Finding less expensive ways to produce large amounts of hydrogen is important for creating a hydrogen economy.