Hydrogen production today and in the future

A large share of current hydrogen production takes place via steam reforming of natural gas. In this process, natural gas reacts with steam at high temperatures to form carbon monoxide (CO) and hydrogen (H₂). To increase the hydrogen yield, steam reforming can be coupled with a water-gas shift reaction. In this reaction, hydrogen and carbon dioxide (CO₂) are additionally produced from CO and water vapour. The resulting CO₂ can be used in subsequent processes.

This process is similar to steam reforming, but in addition to steam and natural gas, pure oxygen is also used here. This allows for higher temperatures. This is advantageous as the efficiency of hydrogen production in steam reforming depends on the reaction temperature.

Coal can be converted with steam to hydrogen and CO at elevated temperatures. Analogous to steam reforming, this reaction can also be combined with a water-gas shift reaction to increase the hydrogen yield.

In the chemical industry, hydrogen can also be produced as a by-product, for example in the production of chlorine and caustic soda by means of chlor-alkali electrolysis. The climate impact of these processes depends on whether petroleum is used as a feedstock and whether electricity and heat originate from fossil fuels.

Even when using conventional technologies based on fossil fuels (steam reforming or coal gasification), hydrogen can be produced with a lower CO₂ footprint if the resulting CO₂ is captured. The separated CO₂ can either serve as a raw material (via CCU: carbon capture and utilisation) or be injected underground (via CCS: carbon capture and storage/sequestration).

Existing natural gas-based plants could also run with biomethane. If the resulting CO₂ is captured, negative emissions are possible, as CO₂ is also bound during the growing process of the plants from which biomass or biogas originates. However, the widespread use of biomass or biogas is limited by its land use requirements.

At very high temperatures, methane can be split into carbon and hydrogen. This produces solid carbon, which is easier to handle than CO₂, does not escape into the atmosphere and can also serve as a raw material.

Experts consider the electrolytic splitting of water into pure hydrogen and oxygen to be the most climate-friendly production technology. However, its CO₂ footprint depends on the electricity mix used - only electricity from renewable energies produces truly climate-neutral hydrogen. Additionally, significantly less water is consumed compared to hydrogen production from fossil fuels. To produce 1 kg of hydrogen, water electrolysis consumes a total of about 10 kg of water. Of this, 9 kg of water is used for the chemical reaction alone. The remaining water is needed for the production of the electrolysers. In contrast, natural gas and coal-based processes consume much more water in total at 13-18 kg and 40-85 kg, respectively (IEA, Global Hydrogen Review, 2021). There are different processes to electrolytically extract hydrogen from water:

• Alkaline electrolysis (AEL):
  AEL accounts for about 60% of the current electrolysis capacity worldwide. Alkaline electrolysis has the advantage that it does not require precious metal electrodes. This makes it cheaper than other types of electrolysers. In addition, it is characterised by a high long-term stability. However, alkaline electrolysis has a comparatively poor load-following capability. This could lead to problems in coupling with the volatile electricity supply from renewable energy sources.
• Proton exchange membrane (PEM):
  PEM accounts for about 30% of the electrolysis capacity worldwide. PEM electrolysers use precious metal electrodes, which makes them more expensive than other electrolysers. However, PEM electrolysis is characterised by excellent load-following capability.
• Solid oxide electrolysis (SOEC):
  This type of electrolysis is currently in the demonstration phase for large-scale use. It combines the use of steam and ceramics as electrolytes at high temperature. This keeps the material costs very low. However, the high temperatures result in long start-up times for the electrolysers. Solid oxide electrolysers are very efficient and, unlike AEL and PEM electrolysers, can also be used as fuel cells.
• Anion exchange membrane  (AEM):
  In the future, AEM electrolysers could also play an important role, as they combine the advantages of AEL and PEM electrolysis. They use transition metal complexes instead of precious metals. In addition, the membrane acts as a solid-state electrolyte. However, AEM electrolysers are still in the demonstration phase.