Hydrogen electrolysis – Benefits and goals

The electrolysis of hydrogen is used for energy storage and conversion and is enjoying increasing importance in the energy industry. For example, the use of hydrogen fuel cells is currently being tested in industries such as automotive and marine.7, 8

To produce pure, unbound hydrogen, the naturally occurring hydrogen in the form of compounds must be separated from its various starting materials by hydrogen electrolysis. The possible starting materials include water, biomass, natural gas (methane) and other hydrocarbons such as crude oil.1, 2

Energy is required for hydrogen electrolysis. Whereas chemical, electrical or thermal energy has been used primarily for separation to date, hydrogen production using renewable energies is becoming increasingly popular. About 96 percent of the hydrogen produced worldwide still comes from fossil fuels, especially natural gas. Only four percent is produced with hydrogen electrolysis devices.10

With the use of solar or hydro energy, climate-neutral production of "green hydrogen" is possible. Other CO2-neutral processes include the gasification and fermentation of biomass and the reformation of biogas. However, many alternative processes for hydrogen production – for example, the production of hydrogen from green algae – are still in their infancy.2

Process: Hydrogen electrolysis

How is hydrogen produced by electrolysis? Hydrogen electrolysis is a process for splitting hydrogen and oxygen. In addition to hydrogen electrolysis using water, there are other proven processes for obtaining hydrogen. These include alkaline electrolysis, PEM electrolysis (proton exchange membrane) and high-temperature electrolysis (SOEC, solid oxide). While alkaline electrolysis uses an alkaline solution and achieves the separation of water and oxygen by means of a permeable membrane, PEM analysis uses an acidic environment and a gas-tight membrane. High-temperature electrolysis uses a solid oxide for the separation of the half-cells, through which the oxygen ions can diffuse. In another process, the direct methanol fuel cell, methanol reacts with water to form carbon dioxide and hydrogen.1, 5

In hydrogen electrolysis with water, the latter is enriched with a liquid that makes ion transport possible. Electrical energy is then supplied to the mixture via two electrodes. The water then splits at one electrode into the components hydrogen and oxygen. While the oxygen rises, the hydrogen ions migrate to the other electrode. There they are converted into gaseous hydrogen. The electrical energy is converted into chemical energy and stored in the hydrogen. Fuel cell technology uses the opposite principle: Here, the energy stored chemically in the hydrogen is converted back into electrical energy.3

Hydrogen electrolysis in the future

Hydrogen has the potential to replace fossil fuels, which is why hydrogen electrolysis is seen as the method of the future for producing energy carriers. Experts see water as the only truly sustainable source of hydrogen. But what about the efficiency of hydrogen electrolysis with water? There is still a lack of efficient, safe and affordable processes for the production of hydrogen.4, 9

Hydrogen electrolysis with water poses several problems: Precious metals, which are comparatively rare and therefore expensive, are still mostly used as catalysts. In addition, efforts are being made to reduce energy losses at the electrodes. Researchers are thus looking for more cost-effective materials and ways to increase efficiency in order to improve the established process of hydrogen electrolysis.4

One solution could be special catalysts in the electrode material. Due to their atomic arrangement, so-called perovskite oxides offer the advantage that they can be replaced very easily. Through intensive testing in the context of hydrogen electrolysis, researchers are currently still finding out which chemical elements are best suited for electrolysis. It is now considered proven, for example, that a perovskite structure of cobalt, barium and iron provides very good electrode material.4

Functionality of hydrogen electrolysis plants

How does a hydrogen electrolysis plant work? A modern hydrogen electrolysis plant, also known as a power-to-gas plant, uses various types of green electricity, for example from hydropower plants. Furthermore, hydrogen electrolysis requires energy storage by a hydrogen storage tank, an electrolyzer, as well as a plant for compression.6

First, the electricity for hydrogen electrolysis must be fed into the plant from the power grid. In the so-called electrolyzer, a device for the separation, the water is then divided by electrolysis into its basic components water and oxygen. The hydrogen obtained is then fed into a compressor. This exposes the hydrogen to a higher pressure (up to 300 bar). Compressed in this way, the gas is easier to transport. The hydrogen is stored in an intermediate storage facility until it can be transported further. Usually, hydrogen is stored in gaseous form in pressure vessels, but in some cases it is also stored in liquid form or at the molecular level in metals.6

Further information on hydrogen electrolysis

What else should you know about hydrogen electrolysis? Hydrogen has numerous advantages – for example, it can be stored and transported excellently. However, there are still a number of problems that need to be addressed if the hydrogen economy is to function smoothly. For one thing, the production of hydrogen from fossil fuels produces the greenhouse gas carbon dioxide. Secondly, the comparatively high production costs of hydrogen electrolysis with water, the high weight of some storage systems and the lack of hydrogen filling stations pose problems. Without a widespread supply of hydrogen, the technology for automobiles will be difficult to implement. At the same time, the hydrogen filling station network will probably only be expanded when a number of production-ready fuel cell cars come onto the market.2

Conclusion

Various processes exist for the production of hydrogen. Green hydrogen is obtained by hydrogen electrolysis with water, i.e. the splitting of water into oxygen and hydrogen, using energy from renewable resources. However, due to the high costs involved, hydrogen electrolysis with water currently accounts for only about four percent of hydrogen production. Hydrogen therefore holds great potential as an energy carrier for fuel cell technology, but it also still faces a number of hurdles.