Electrolysis of water to produce hydrogen

Three different processes can be used in the electrolysis of water to produce hydrogen: AEL electrolysis, PEM electrolysis and HTE electrolysis. The processes for water electrolysis and the corresponding systems differ in aspects such as the technologies and materials used, but also in terms of current density and energy efficiency. The latter vary between around 60 and 85 %. However, all methods are united by the basic principle of producing hydrogen through the electrolysis of hydrogen-containing compounds.

The answer to the question of whether hydrogen can ultimately be produced in an environmentally friendly way through the electrolysis of water depends on the electricity used. If it comes from renewable energies, the hydrogen produced by electrolysis is also considered to be free of carbon dioxide (CO2) and sustainable. In this case, it is green hydrogen. The production of green hydrogen therefore also benefits from the expansion of renewable energies. The cheaper it is to produce sustainable electricity, the cheaper it is to produce green hydrogen using water electrolysis. A distinction can be made between the following processes for producing hydrogen by electrolysis:

Depending on its design, a high-temperature electrolyser (HTE) operates in the temperature range between 100 °C and approx. 900 °C, with the efficiency increasing in line with the temperature. Compared to AEL and PEM electrolysers, it does not require any precious metal components. In HTE electrolysis for hydrogen production, the water vapour meets a solid oxide electrolysis cell. In this cell, the water molecules are split using nickel-cermet vapour-hydrogen electrodes and mixed oxides of lanthanum, strontium and cobalt-oxygen electrodes.

HTE electrolysis guarantees high efficiency: With efficiencies of up to 90 %, high-temperature electrolysis is considered a particularly efficient method of industrial hydrogen production. Its advantages lie in energy-intensive sectors such as the steel industry, where large amounts of waste heat are available and can be utilised in high-temperature electrolysis. HTE electrolysis is currently still in the pilot phase.

The chemical industry in Germany currently produces around three terawatt hours (TWh) of hydrogen every year using water electrolysis. To put this into perspective: in 2020, Germany's gross electricity consumption totalled around 545 TWh. The biggest challenge at present is that a lot of energy has to be used to electrolyse water to produce hydrogen. However, a lot is being done in Germany in terms of electrolysis: the PEM hydrogen electrolysis plant at Shell's Rhineland refinery (Wesseling) is the world's largest PEM hydrogen electrolysis plant in Germany.

In the 2020 published National Hydrogen Strategy published by the German government in 2020 contains an action plan that describes the steps required to make the strategy a success. It states that the possibilities for new business models and cooperation between electrolyser operators and electricity and gas network operators will be analysed. This is to take place in compliance with regulatory unbundling. In addition, one or two pilot projects will be announced to strengthen the hydrogen market in Germany. High-potential approaches are to be trialled here - provided that there is significant relief for the electricity and gas grids at reasonable prices and there is no breach of competitive neutrality. Competitive neutrality" means that the state as a market participant does not enjoy any advantages. It is also being examined whether the framework conditions need to be adapted in order to promote corresponding concepts relating to electrolysis and hydrogen.