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.