Hydrogen – a versatile gas and source of hope for the future
Hydrogen plays a key role as an energy carrier and as base material in various production lines on the way to a climate-neutral economy. The gas is highly versatile and can also be used in emissions-intensive industrial processes, e.g. in steel production or the raw materials and chemical industries.
In road, rail and air transport, hydrogen can be used as an alternative fuel either in combustion engines or in fuel cells in which electricity is generated to drive electric motors. Maritime transport cannot be electrified directly, so the detour must be taken via the combustion of hydrogen or the use of H2-powered fuel cells. The fossil feedstocks and energy carriers currently used will then be replaced by renewable electricity-based alternatives produced by the power-to-X process.
Hydrogen is also in demand as an indirect electricity storage medium. The electricity-generating renewable energies of sun and wind require reliable energy storage methods due to their intermittent nature. To close the time gap between power generation and power consumption, the electrolysis process is used in which water is split into its components hydrogen and oxygen and the hydrogen is stored in tanks in compressed form (power-to-gas process). If required, the hydrogen is then converted back into electricity in a fuel cell, which practically reverses the electrolysis process.
Onward hydrogen transport
After having been produced in the electrolysis process, hydrogen can be transported directly for further use via pipelines or in freight transport. However, due to its low volumetric energy density, it is often not transported in its pure form as a gas, but either compressed, converted into a liquid by cryogenic temperatures, or bound in chemicals such as ammonia or methanol.
In Germany, the well-developed natural gas network infrastructure with connected gas storage facilities is used in part for the transport and distribution of hydrogen. The existing networks are first tested for hydrogen suitability. However, the extensive expansion of dedicated hydrogen networks is also planned.
To make hydrogen and its downstream products a central part of the decarbonization strategy, its entire value chain - technologies, production, storage, long-distance transport, distribution and use - must be analyzed in detail for economic viability and a high, seamless level of safety. Design, quality, and material durability requirements are particularly high due to the risk of hydrogen embrittlement and the gas's large explosive capability. Recently, at Germany's request, the United Nations adopted size-dependent pressure limits for containers to ensure safety in the transport and storage of hydrogen.
Electrolysis is a proven process for the production of hydrogen. The electrolysis cell has two chambers. It consists of an anode (positively charged), cathode (negatively charged) and the ion conductor (electrolyte). The water is taken up in the cathode chamber and broken down into its basic components hydrogen and oxygen in two partial reactions with the aid of electrical voltage. Hydrogen electrolysis requires energy, which is provided either chemically, electrically or thermally, or in a climate-neutral manner by using solar, hydro or wind energy. Depending on where the power for the splitting comes from, hydrogen is sorted by color.
Alkaline electrolysis (AEL)
Conventional alkaline electrolysis uses nickel-based electrodes, a hydroxide ion-permeable membrane and an aqueous potassium hydroxide solution as the electrolyte.
Water is fed into the electrolyzer as the starting material. Hydroxide ions and hydrogen are then formed at the cathode with electron uptake. Due to their negative charge, the hydroxide ions migrate through the membrane to the anode, where they react to form oxygen and water while releasing electrons. The AEL takes place in a temperature range between 40 °C and 90 °C.
Alkaline electrolysis uses non-precious metals as catalysts, making this technology available at comparatively low cost. Therefore, it is particularly suitable for larger plants in the 10 MW class or higher.
Polymer electrolyte membrane elektrolysis (PEM / PEMEL)
While alkaline electrolysis works with an alkaline solution and the separation of water and oxygen is achieved by a permeable membrane, PEM analysis uses an acidic environment and a gas-tight membrane. This selectively allows only the hydrogen protons to pass, while the electrons are transferred from the cathode to the anode chamber to the outside.
The hydrogen protons and the electrons recombine in the anode chamber and form hydrogen gas. The PEM takes place in a temperature range between 20 °C and 100 °C.
Larger current densities are generated with the PEM. Another advantage is the operation with fluctuating load profiles, so that here is a good possibility to couple electrolysis in a climate-neutral way with renewable energies, which are known to be dependent on solar intensity, water flow or wind occurrence. However, the acidic environment in PEM electrolysis requires very robust materials. For this reason, the catalysts are made of precious metals, such as platinum or iridium.
For PEM electrolysis, the commercialization of large systems is only just beginning, so that considerable cost reduction potential can still be assumed. In the long term, expectations are that PEM technology will even be cheaper to be manufactured than alkaline electrolysis.
High-temperature electrolysis (HT / HTEL)
In contrast to the other processes, high-temperature electrolysis, or steam electrolysis, does not split liquid water but water vapor into its components hydrogen and oxygen. The conversion process takes place at temperatures between 200 °C and 1,000 °C. This process can therefore be incorporated into energy-intensive industries, e.g. the steel industry, where large amounts of waste heat are usually available unused. Electricity for heating is thus saved and higher efficiencies can be achieved than with low-temperature electrolysis.
A solid oxide electrolyzer cell (SOEC) is used to separate the half cells. The vapor is fed into the porous cathode, then split into pure H2 and oxygen ions at the cathode-electrolyte interface. The hydrogen gas diffuses back through the cathode, while the oxygen ions pass through the solid electrolyte. This allows only the oxygen ions to pass through, no vapor and no hydrogen gas. At the interface between the electrolyte and the anode, the oxygen ions oxidize and form pure oxygen gas, which is collected at the surface of the anode.
No precious metal components are required in the electrolytic cell, resulting in considerable cost reduction potential. However, the high operating temperatures stress the material and reduce the service life of the electrolytic cells, in some cases considerably.