What is hydrogen and how is it made?

Hydrogen is a simple element. What’s amazing is that our very existence depends on it. On these pages, you can read all about hydrogen and its properties, and the options and risks associated with its use. We also tell you about how hydrogen makes life possible. Hydrogen is virtually non-existent in its isolated form, so to obtain hydrogen you have to separate it from the chemical compounds containing hydrogen. We explain the technology used to do this and the aspects that are important in the process.

What is hydrogen?

The hydrogen atom is the most abundant, lightest and simplest element in the universe.

The Latin name for hydrogen is hydrogenium. Hydrogen is the first element listed in the periodic table and has the chemical symbol H. The hydrogen atom contains one proton in the nucleus around which one electron moves.

Composition of a hydrogen atom

Hydrogen is a molecule (H₂) composed of two hydrogen atoms (H). Hydrogen can be found in abundance on earth, but it always bonds with other atoms.

Composition of a hydrogen molecule

Hydrogen is a non-toxic, colourless, tasteless, odourless substance. Under normal circumstances, such as at room temperature and under normal pressure conditions, hydrogen is a gas. When hydrogen is cooled to -253°C it becomes liquid. In addition to cooling, you can also put hydrogen under pressure. Increasing the pressure increases the density and with this the amount of energy per mass (m³). At room temperature and 1 bar, the density is approximately 0.09kg/m³; at 200 bar the density increases to around 16kg/m³.

Hydrogen: the source of life

More than 90% of all atoms in the universe are hydrogen atoms. We find hydrogen in stars, and also between stars and in large planets (gas giants) like Jupiter and Saturn. Liquid metallic hydrogen forms the bulk of the interior of gas giants.

Hydrogen can be found in abundance on Earth too: about two-thirds of all molecules contain one or more hydrogen atoms. There’s water, for example, which is a molecule made up of 2 hydrogen atoms and 1 oxygen atom (H₂O), and there’s methane (the main component of natural gas), which has a single carbon atom surrounded by 4 hydrogen atoms (CH₄). All known life on Earth is made up of hydrogen compounds and cannot live without these. However, unless actively converted from a source, you'll hardly ever encouter hydrogen in its isolated form here on earth.

The energy content of hydrogen

Why and when does the energy content of hydrogen matter? Well, it matters when it comes to hydrogen supply in a fuel tank, for example. In cars that run on hydrogen, the hydrogen is compressed in a fuel tank to 350 or 700 bar so that the tank can carry as many kilograms of hydrogen as possible. It’s also possible to compact hydrogen even more: you just have to cool it down – a lot, to around -253°C – to turn it into its liquid form. That costs a lot of money and energy, which is why liquid hydrogen is mainly used in the aerospace industry (as rocket fuel for example). In the future, you can expect to see an increase in the transport of liquid hydrogen. For example, Shell is currently working with Japan-based Kawasaki Heavy Industries to develop technologies for transporting liquid hydrogen by sea.

The density of the gas is less of an issue when hydrogen is transported by pipeline. The amount of gas transported through a pipeline is expressed in Nm³. As gas flows through a pipeline the pressure naturally drops, but because hydrogen is so light, the flow rate at the same pressure drop is almost three times as high as that of natural gas. This helps to put hydrogen and natural gas on more even footing. Ultimately, the pipeline used for hydrogen transports only about 20% less energy than if natural gas were to flow through it (based on the volumetric flow rate).

As can be seen from the bar chart below showing the calorific values, a cubic metre of hydrogen contains about a third of the energy of a cubic metre of natural gas. So if you burn 1m³ of hydrogen, this will only generate one-third of the energy you get if you burn 1m³ of natural gas.

Comparison of the energy density and calorific value of hydrogen and Groningen-quality natural gas  
Source: DNVGL

One cubic metre of hydrogen is much lighter than the same volume of natural gas. However, when you compress hydrogen, a kilogram of this contains a lot more molecules than the molecules of natural gas contained in a kilogram of compressed natural gas. This can be inferred from the energy density bar chart. If you burn 1kg of hydrogen, this will generate three times as much energy as you get when you burn 1kg of natural gas.

Nm³ stands for normal cubic metre: the amount of gas that occupies 1m³ at a temperature of 0°C and at a pressure of 1 bar.

The calorific value indicates how much energy is produced by the complete combustion of a specified quantity of the gas.

Does burning hydrogen produce carbon emissions?

The big advantage of hydrogen is that it does not produce CO₂ emissions when burned with oxygen: there is no carbon in the hydrogen molecule after all. If you burn hydrogen with pure oxygen, you only get pure water as a waste product. Atmospheric air is generally used for the combustion process, and since this contains nitrogen, nitrogen oxide (NOX) is released. So to limit emissions, hydrogen burners will have to be developed further, as was done with natural gas burners.

If using hydrogen as a fuel really catches on, it is inevitable that some hydrogen will be released into the atmosphere, through leaks for example. Hydrogen does not, in itself, contribute directly to climate change, since it is not a greenhouse gas like methane or carbon dioxide. All the same, it could indirectly affect the environment, but the impact would be minimal.

Is hydrogen dangerous?

Hydrogen is highly flammable and so must be handled with care. Hydrogen is eight times lighter than natural gas and 14 times lighter than air, meaning if there is a leak the hydrogen will rise quickly, mix with air and be removed by ventilation. However, in a room where ventilation is not an option, the risk of explosion would increase considerably under certain conditions.

A hydrogen flame is virtually invisible in daylight, especially when small. The flame speed of hydrogen is greater than that of natural gas, and with an explosion, hydrogen can generate higher pressures. So a hydrogen explosion can be more intense than an explosion caused by the same volume of natural gas, for example.

A mixture of hydrogen and air can easily ignite. If hydrogen is present in a space in a concentration of between 4% and 75% (its flammability range), mixed with air this is combustible. To make a comparison: the flammability range of natural gas is 4% to 15%. What’s more, the energy required to initiate hydrogen combustion is much lower, meaning even a small spark will ignite it. That’s why, with hydrogen, it’s even more important than with natural gas that leaks be prevented and rooms well ventilated.

On the other hand, thanks to the lack of carbon, with hydrogen there’s no danger of carbon monoxide poisoning.

So you can see that the risks of using hydrogen are different to those when using natural gas, for example. And that’s something everyone who works with hydrogen has to take into account.

How is hydrogen made?

Extracting hydrogen

Since hydrogen molecules like to bond to other molecules, there is practically no free hydrogen (naturally occurring isolated hydrogen atoms) on Earth. That’s why you have to extract hydrogen from compounds that contain hydrogen, like from water or methane for example. In the case of hydrogen extraction, three aspects are important: efficiency, the purity of the hydrogen produced and the amount of carbon emissions.

Extraction is a way to separate a desired substance from a mixture, causing the substance to enter a different phase.


Extracting hydrogen takes energy. The amount of energy required varies greatly depending on the production process. The energy required to convert hydrogen from an energy source determines the efficiency of the process. Ideally, the efficiency should be as high as possible so that the energy from the energy source can actually be converted into the energy carrier hydrogen.


Hydrogen purity requirements vary depending on the end-use applications. If you are using the hydrogen for heating in a boiler, for example, the hydrogen does not need to be as pure as it does for certain chemical processes. If the hydrogen is to be used in a fuel cell on the other hand, this demands a very high purity level (99.999%). Some end-use applications are very sensitive when it comes to specific types of contaminants in the hydrogen, like sulphur-containing compounds, for example, which fuel cells and catalysts are not able to tolerate. Where necessary, the hydrogen produced is purified further. There are various ways to do this, the most common being pressure swing adsorption, or using membranes. These purification steps cost money and energy though.

Carbon emissions

When we produce hydrogen from fossil fuels, CO₂ is emitted and this is generally released into the atmosphere at the moment. In the future, it may be possible to capture CO₂ on a large scale and store it or reuse it. Naturally, sustainable hydrogen production without carbon emissions would be ideal – and essential in the future. An example of sustainable production is electrolysis using sustainably generated electricity. With yet other means of production, carbon monoxide (CO) or carbon is released, which can be used in other processes.

Methods used

Hydrogen can be produced using various methods and using a range of energy sources.

The key energy sources and conversion methods for producing hydrogen.

Below we briefly describe the main conversion methods, starting with the most commonly applied ones first. Please find more details though the various links.

Steam Methane Reforming (SMR)

SMR is a reaction in which steam is mixed with methane, which then ‘reforms’ into a mixture of hydrogen, carbon monoxide and carbon dioxide. The carbon monoxide can then be converted into carbon dioxide and additional hydrogen in a ‘water-gas shift reaction’ (WGSR). This happens at a temperature of between 500 and 900ºC; the heat is generated by burning fossil fuels. SMR is currently the cheapest and most efficient way to produce hydrogen.

  • Efficiency: 75–80%
  • Purity of the hydrogen produced: this depends greatly on how pure the hydrogen needs to be for the downstream process. This generally varies between 95% and 99.999% (higher purity can be achieved using additional process steps).
  • CO₂ emissions: 9kg CO₂ per kg H₂
  • Carbon capture: 66%

Autothermal Reforming (ATR)

ATR is a combination of two reaction steps, the first being the same as occurs with SMR. The hydrogen produced in the first reaction is further reformed using pure oxygen. Since this oxidation process takes place in a reactor, the CO₂ produced remains in the reactor (unlike with SMR). The advantage of this is that virtually all the CO₂ can be captured. This process requires high temperatures, from 900 to 1500ºC.

  • Efficiency: 75–80%
  • Purity of the hydrogen produced: this depends greatly on how pure the hydrogen needs to be for the downstream process. This generally varies between 95% and 99.999% (higher purity can be achieved using additional process steps).
  • CO₂ emissions: 10kg CO₂ per kg H₂.
  • Carbon capture: 95%

Partial Oxidation (POX)

In the case of POX, fossil fuel is mixed with oxygen and partially combusted in a reformer, creating a mix of carbon monoxide, carbon dioxide and hydrogen. The oxygen used in this process must be purified of all traces of nitrogen. In the Netherlands, the industrial sector mainly uses POX to produce hydrogen for refining oil.

  • Efficiency: 70–80%
  • Purity of the hydrogen produced: this depends greatly on how pure the hydrogen needs to be for the downstream process. This generally varies between 95% and 99.999% (higher purity can be achieved using additional process steps).
  • CO₂ emissions: depends on the raw material in the reactor. Up to 20kg CO₂ per kg H₂ when using coal.


Electrolysis is a process that splits water into oxygen and hydrogen by bringing the water into contact with electricity. Though this process does not directly produce CO₂ emissions, it is only sustainable if we use sustainably generated electricity. In this process, about nine litres of water is required to produce one kilogram of hydrogen. The required purity of this water depends on the technique used.

  • Efficiency: 65–80%
  • Purity of the hydrogen produced: close to 100%.
  • CO₂ emissions: using the current energy mix for producing electricity, approx. 25kg CO₂ per kg H₂. When using exclusively sustainably generated electricity this is 0kg CO₂ per kg H₂.

The Dutch Hydrogen Coalition’s electrolysis ambitions
Source: Waterstof Coalitie Manifest (Hydrogen Coalition Manifesto)

Scaling up and cost reduction of electrolysis technology

The Hydrogen Coalition states that between now and 2030 the investment costs of electrolysis could possibly be reduced by roughly two-thirds. This means that an established capacity of 3 to 4 gigawatts of green hydrogen can potentially be achieved. The reasoning behind this is that, thanks to the greater demand for electrolysers, manufacturers will develop more of these – as happened during the development of wind turbines. Steps for various electrolysis projects that will produce anywhere from 1 to 250MW have already been announced. You can see a few of these below.

The Hydrogen project Hystock1 MW

Nouryon and Gasunie study scale-up of green hydrogen project to meet aviation fuels demand

BP, Nouryon en HbR onderzoeken bouw grootschalige elektrolyser [BP, Nouryon and Port of Rotterdam research construction of large-scale electrolyser] (in Dutch)

Gasunie en ENGIE gaan samenwerken om groene waterstof op grote schaal mogelijk te maken [Gasunie and ENGIE collaborate to enable green hydrogen on a large scale] (in Dutch)

Gasunie Deutschland met TenneT en Thyssengas aan de slag met grote 'power-to-gas'-installatie [Gasunie Deutschland, TenneT and Thyssengas join up to build major power-to-gas plant] (in Dutch)

Gigawatt Electrolysis Factory

With the Gigawatt Electrolysis Factory project, a large consortium of players in the Dutch market is aiming to pave the way to the green hydrogen economy. The Dutch Institute for Sustainable Process Technology (ISPT) is playing a leading role in this. The objective of the project is to identify the key technological barriers that need to be overcome when the number of electrolysis cells is hugely increased. The partners in the Gigawatt Electrolysis Factory project will jointly investigate what it would take to build such an electrolysis plant in the Netherlands in the period 2025 to 2030. ISPT estimates that, with the current state of technology and current market prices, the investment for a 1GW electrolysis plant would be about one billion euros. This figure needs to be brought down drastically. The institute says that a gigawatt-scale green hydrogen plant costing €350 million would be a competitive alternative to conventional fossil hydrogen technology.

Electrolysis at sea

Wind over the North Sea is an enormous potential source of energy for the Netherlands. The big challenge is figuring out how to bring all that wind energy ashore. This challenge was the incentive behind the formation of a major international partnership. Electricity and gas companies are investigating how they can solve this transport issue. This is possible, for example, by converting offshore wind energy into hydrogen on location using offshore electrolysis platforms and then bringing the energy ashore in that form. In other words, using hydrogen as a bridge between the world of wind and gas. This project is called the North Sea Wind Power Hub.

Other methods

It’s also possible to produce carbon-neutral hydrogen using other technologies, by applying various pyrolysis processes for example. Pyrolysis is a process in which natural gas or biomass is converted into hydrogen and pure carbon by heating it in the absence of oxygen. The carbon can then be used as a raw material for certain industries, or it can be stored. However, this process requires a lot of input energy.

An alternative process, plasmalysis, uses microwaves. The advantage of this process is that it requires relatively little energy, is energy-compact, and is easy to scale up. However, plasmalysis is still in its infancy.

Carbon-neutral hydrogen can also be produced from biomass, the same way it is using POX. For example, the biomass is gasified into a mixture of carbon monoxide, carbon dioxide and hydrogen in the presence of pure oxygen, water or atmospheric air.

A very promising technique is supercritical water (SCW) gasification, a process that converts wet biomass waste streams like manure, green waste, and sewage sludge into sustainable energy and reusable raw materials. A demo plant for SCW gasification has been built in Alkmaar (Netherlands) in collaboration with Gasunie.

And lastly, you can use microorganisms to produce ‘biohydrogen’. In 1997, a team of scientists discovered that if you deprive microalgae of sulphur, the algae will start producing hydrogen rather than oxygen during photosynthesis. In 2007, it was found that the same result could be achieved by adding copper.

Another possibility is the use of ‘artificial photosynthesis’, a process that mimics natural photosynthesis to produce hydrogen directly from sunlight. In 2015, thanks to research carried out in Eindhoven (Netherlands), this process got an enormous efficiency boost. This technology is being studied extensively around the globe. Advances are being made, but there is no commercially viable system available so far.

Hydrogen as a by-product

In addition to production processes where the main aim is to produce hydrogen, excess hydrogen is also recovered as a by-product from various chemical processes, like during the production of chlorine from sodium chloride (salt), for example, and of raw materials for plastic, such as ethylene and styrene.

Grey, green and blue hydrogen

The various hydrogen production methods produce varying levels of carbon emissions, and the level of these emissions determines in part whether the hydrogen produced is considered grey, blue or green. If the carbon emitted is not captured, the result is commonly called grey hydrogen. If the source is a fossil fuel and the CO₂ released is captured, this is called blue hydrogen. If hydrogen has been obtained from a renewable source (such as biomass) or from sustainably generated electricity, this is referred to as green hydrogen.

To simplify matters in this long read, we mainly use the term carbon-neutral hydrogen, by which we mean both blue and green hydrogen.


Indications of hydrogen. Source Gasunie