Over the last few years, hydrogen has become increasingly popular with the general public, and is represented by a multitude of colours depending on its production method: pink for nuclear power, turquoise for methane pyrolysis, blue for steam reforming with carbon sequestration, and green for renewable energies. There are as many colors of hydrogen as there are colors of the rainbow.

In this article, we will clarify the different names for hydrogen by presenting the molecule in terms of the European taxonomy.

First of all, what is hydrogen? Hydrogen is a storage energy, an energy carrier that offers a solution for storing and transporting energy that is stable over time in the form of dihydrogen (H2).

So how is hydrogen produced? There are two main methods of producing hydrogen:

• The “carbon” method, based on the steam reforming of natural gas, coal or oil. This method involves the partial combustion of fossil fuels with the addition of steam or air to extract synthesis gases.
• Methods based on the electrolysis of water, which can be described as “renewable or low-carbon” depending on the origin of the electricity used. This method consists of separating water molecules using an electrolyser and an electric current to recover dihydrogen. The term “low-carbon” can be added if the carbon-based hydrogen production method is coupled with carbon sequestration technologies. The term is also applicable when the production of hydrogen by electrolysis uses electricity from the grid in a country where the energy mix is low in carbon.

At present, hydrogen production is very unevenly distributed, dominated by carbon-based methods, which account for 90-95% of global hydrogen production. The remaining 5% is covered by water electrolysis.

It is important to stress the existence of hydrogen in its natural state, known as native hydrogen, which exists in underground cavities and is the result of chemical reactions between water and various minerals in the deep layers of the Earth’s mantle. However, the relevance of native hydrogen to decarbonisation remains to be demonstrated.

The market for renewable hydrogen is growing rapidly, with the installation of electrolysis capacity expected to reach between 170 and 365 GW by the end of the decade, according to the International Energy Agency. An essential element in the development of the hydrogen economy is the ability to certify the origin and production method of hydrogen through certification mechanisms. These do not use a colour system, but are based on the notion of carbon intensity, which is calculated in kilograms of CO2 equivalent per kilogram of hydrogen (CO2eq/kgH2).

The carbon intensity of hydrogen is strongly influenced by two factors: The presence or absence of CO2 sequestration technology for carbon-based production methods.

The type of electricity used for electrolysis. The energy mix of the producer country has a major impact on hydrogen produced from grid electricity. According to the European regulations currently being drawn up, the carbon intensity of low-carbon hydrogen (blue hydrogen) is below 3 kg CO2eq/kgH2. For renewable hydrogen (green hydrogen), it is below 2 kg CO2eq/kgH2.