Hydrogen: A key technology in the energy transition

Hydrogen (chemical symbol H, derived from the Latin ‘hydrogenium’) was discovered in 1766 by the English natural philosopher Henry Cavendish during experiments with metals and acids. The French chemist Antoine Laurent de Lavoisier demonstrated with his parallel scientific work that no mass is lost in the reaction between hydrogen and oxygen, the so-called oxyhydrogen test.

With an abundance of around 75 per cent, hydrogen is the most common element in the universe, followed by helium (23 per cent). All other elements are produced in stars through nuclear fusion and account for only around 2 per cent. On Earth, hydrogen practically never occurs in atomic form as H+, but mainly as H2 molecules. In nature, however, its occurrence as a free gas is also very rare.

Industrially, hydrogen is used as a raw material in the chemical industry, in the production of fertilisers, plastics and fuels, and in technical processes. Global demand stands at around 500 billion cubic metres per year, of which around 19 billion cubic metres are consumed in Germany, with an upward trend.

In the energy sector, hydrogen is regarded as a secondary energy carrier, as it must be produced using energy. Electrolysis of water and its use in fuel cells create a closed – from water to water – cycle that is free of pollutants [1],[2].

Current methods of hydrogen production

A basic distinction is made between fossil and non-fossil hydrogen production, although some processes can be classified in both groups. These include, for example, electrolysis as well as various reforming, gasification and oxidation processes. Electrolysis can be powered by both renewable energy and electricity from fossil fuel power stations. Similarly, both biomass and fossil hydrocarbons can be used in reforming and gasification processes.

In industry, hydrogen is currently used predominantly as a chemical feedstock and is mainly produced from fossil sources, actually. The breakdown is approximately 48 per cent from petroleum fractions, 30 per cent from natural gas and up to 16 per cent from coal. The remaining 6 per cent or so come from other processes, mainly as a by-product of chlor-alkali electrolysis or from water electrolysis.

When water and carbon are used, hydrogen and carbon monoxide are initially produced, which is then further processed into carbon dioxide. In processes involving hydrocarbons, hydrogen and predominantly carbon dioxide are produced directly. Important large-scale processes include steam reforming, plasma reforming, partial oxidation, carbon monoxide conversion and coal gasification.

Non-fossil hydrogen production encompasses processes that are based directly or indirectly on solar energy. These include the use of biomass via fermentation, gasification or reforming processes, as well as the use of electrical energy through water or steam electrolysis. The required energy can be sourced directly from photovoltaics or indirectly from wind and hydro power. Solar thermal processes and direct photochemical processes such as bio-photolysis or photo-electrolysis also play a role. Most of these processes are already established or are currently under technical development [2].

The hydrogen colour classification

To provide a clearer framework, the so-called hydrogen colour classification has been developed. It distinguishes between different types of hydrogen based on raw materials, production methods and environmental impact, and classifies them into three groups:

• Hydrogen from fossil sources with carbon/carbon dioxide emissions into the environment,
• hydrogen from fossil sources without carbon or carbon dioxide emissions, and
• hydrogen from renewable energy sources.

For example, grey, black or brown hydrogen is based on fossil sources with high emissions, whilst green hydrogen comes from renewable energy sources and is considered particularly climate-friendly. Other variants, such as blue or turquoise hydrogen, are regarded as transitional solutions [3],[4].

Life cycle assessment as a basis for evaluation

The environmental impacts of hydrogen production are assessed using life cycle assessments, which, in accordance with standards such as DIN ISO 14040 and 14044, record the environmental impacts from raw material extraction through product use to end-of-life disposal. This involves considering either the product’s life cycle from ‘cradle to grave’ or up to a specific point.

Current hydrogen production generates greenhouse gas (GHG) emissions, although these can be reduced. In Germany, hydrogen is considered climate-friendly if emissions do not exceed 3.0 kg of CO₂-equivalent per kilogram of H₂. Hydrogen-based synthetic fuels must achieve a GHG reduction of 70 per cent.

The EU project Certif-Hy has developed a certification system for sustainably produced hydrogen and e-fuels. However, the problem is that the term “climate-friendly hydrogen” is not uniformly defined and thresholds vary widely internationally [5],[6].

Future use of hydrogen as an energy carrier

Hydrogen is a versatile energy carrier and can be used in all three major energy consumption sectors: in the provision of electric power and heat in the private and public sectors, across all temperature ranges in industry (here also as a chemical feedstock), and in the transport sector as a fuel, including in the form of synthetic hydro-carbons for road, maritime and air transport.

This versatility is referred to as “sector coupling”. Green hydrogen and Power-to-X technologies (PtX) open up new pathways to decarbonisation, particularly where renewable energy cannot be used directly.

As a secondary energy carrier, hydrogen is also suitable for long-term or seasonal energy storage. It can be flexibly produced via electrolysis from renewable energy sources, follows their fluctuating supply and enables the decoupling of energy supply and demand. Hydrogen can be stored safely and with virtually no loss as a gas [3], [7], [8]

[1] A. Brinner, B. Brinner; Das Erklärbuch zum Wimmelbuch „Grüner Wasserstoff für Anfänger“; Verlag Steinbeis-Edition; Stuttgart, 2025
[2] Herausgeber: E-Mobil BW GmbH, ZSW, WBZU, FM BW, UM BW; Energieträger der Zukunft – Potenziale der Wasserstoff-Technologie in Baden-Württemberg; E-Mobil BW GmbH; Stuttgart, 2012
[3] Green Hydrogen Esslingen; Die Farben des Wasserstoffs; green-hydrogen-esslingen.de/wissen/farben-wasserstoff; abgerufen am 02.04.2025
[4] Wien-Energie GmbH; Die Farben des Wasserstoffs; Internet-Artikel; 09.10.2023; https://positionen.wienenergie.at/grafiken/wasserstoff-farbenlehre/; abgerufen am 11.04.2025
[5] R. Lutz, D. Franke, A. Bahr; Klimabilanzierung der Wasserstoffherstellung; Commodity Top News No. 69, Herausgeber: Bundesanstalt für Geowissenschaften und Rohstoffe; Hannover, September 2022
[6] M. Riemer, J. Wachsmuth, B. Pfluger, S. Oberle; Welche Treibhausgasemissionen verursacht die Wasserstoffproduktion?; Herausgeber: Umweltbundesamt; Dessau-Roslau; Oktober 2022
[7] Die Bundesregierung, Die nationale Wasserstoffstrategie, Herausgeber: Bundesministerium für Wirtschaft und Energie (BMWi), Berlin, Juni 2020
[8] Umwelt-Bundesamt; Welche Rolle kann Wasserstoff im künftigen Energiesystem einnehmen?; Veröffentlichung 03.04.2024; www.umweltbundesamt.de/themen/klima-energie/klimaschutz-energiepolitik-in-deutschland; abgerufen am 07.04.2025