Hydrogen

The three available types of electrolyzers differ in terms of their applications, possible system sizes, and costs:

Alkaline Electrolyzers (AEL):

• Market leader (largest installed capacity to date)
• Highest technological maturity
• Lower equipment costs compared to other electrolyzers

Proton Exchange Membrane Electrolyzers (PEMEL):

• Available since the early 21st century (high technological maturity)
• Promising for use with fluctuating renewable energy due to high load flexibility
• High system costs (high investment costs), partly due to the use of precious metal catalysts

High-Temperature (Solid Oxide) Electrolyzers (SOEL):

• Lowest technological maturity due to high-temperature stress and shorter operating lifetimes
• Allows the use of waste heat to reduce electricity demand
• System and maintenance costs are still higher than those for low-temperature electrolysis

The Electrolyte 

The electrolyte contains free ions and can be either a liquid or a solid. Alkaline electrolyzers (AEL), for example, use a liquid potassium hydroxide solution as the electrolyte. Alkaline water electrolysis is a widely used process that has been commercially available for 100 years. PEM electrolyzers use a solid polymer electrolyte that is semipermeable to protons and surrounded by water. The membrane consists of an ionomer with high ion conductivity (a thermoplastic polymer, specifically a perfluorinated sulfonic acid). SOEL electrolyzers use a solid oxide electrolyte that conducts oxygen ions at high temperatures.

The Electrodes

AEL electrodes are typically made of nickel or stainless steel metal plates. PEM electrolyzers have graphite cathodes, coated with precious metals like platinum, iridium, ruthenium, or palladium. Iridium oxide is commonly used for the anode. Research is aiming to reduce the use of iridium (e.g., through doping) due to concerns about resource scarcity and high costs if PEM electrolysis is scaled up.

A significant advantage of alkaline electrolyzers is the ability to use non-precious metals like nickel for the electrodes, keeping investment costs low. The use of other non-precious metals, such as stainless steel, is also being explored. Additionally, alloys like aluminum, iron, cobalt, or molybdenum are used as catalyst materials.

Solar Thermochemical Hydrogen Production

This novel process is being researched and is based on the idea of using high temperatures from concentrated sunlight to split water into oxygen and hydrogen. Chemicals like cerium or copper chloride are required for the process but are reused in each cycle, creating a closed loop.

For more information on the process, follow the links below: