A Scientific Q&A on the Two Leading Methods of Green Hydrogen Production
By Kathy Ayers, Ph.D., SVP Research & Development, Nel Hydrogen, and Egil Rasten, Ph.D., lead scientist at Alkaline Applied Research, Nel Hydrogen
What are the primary technologies used in water electrolysis today?
The two most established technologies for hydrogen production that use water electrolysis are Alkaline (ALK) electrolyzers and Proton Exchange Membrane (PEM) electrolyzers. Both systems use electricity to split water into hydrogen and oxygen, but they have different electrolyte types, system designs, and applications.
How do ALK electrolyzers work?
The electrolyte in ALK electrolyzers is a hot alkaline solution (25-30 wt% KOH at 60-80 °C) where hydroxyl ions (OH-) are the charge carriers between the cathode and anode. The hydrogen and oxygen gases are separated by a porous diaphragm that fills with electrolyte and enables charge transport.
The electrolyte can easily be pushed out from the pores and lead to gas mixtures, so the ALK electrolyzers do not allow for much pressure difference or fluctuation in differential pressure between the hydrogen and oxygen sides. Gas bubbles in the electrolyte limit the ALK electrolyzer, meaning it operates at lower current densities. This mature technology has a long history of industrial use and is widely regarded for its robustness and reliability.
Atmospheric Alkaline Electrolyser
Image Credit: NEL Hydrogen
How do PEM electrolyzers work?
The electrolyte in PEM electrolyzers is a solid polymer membrane that conducts protons (H+), enabling charge transport between the cathode and the anode. They operate with only pure water as a liquid. The polymer membrane is also the gas separator. It is highly gas-tight, allowing for much higher pressure differences and fluctuations in differential pressure between the hydrogen and oxygen sides than the ALK electrolyzer.
With its solid-state electrolyte, the PEM electrolyzer is not limited by gas bubbles and typically operates at much higher current densities. This enables a more compact system and a smaller footprint than the ALK electrolyzer. This technology has also, to a lesser extent, been proven on a larger scale.
PEM Electrolyser – PSM Series
Image Credit: NEL Hydrogen
How do the two technologies compare in terms of efficiency?
Both Alkaline and PEM electrolyzers can achieve electrical efficiencies of 55 to 70 %. PEM systems typically maintain higher efficiency at elevated current densities and can react faster to variable load conditions. Alkaline electrolyzers, on the other hand, deliver stable efficiencies in steady-state operation and are well-suited to baseload applications.
What are the differences in capital and operational costs?
Alkaline electrolyzer stacks generally have lower capital costs due to already established supply chains and the use of inexpensive materials, such as non-precious metal catalysts. PEM stacks usually involve higher upfront investment because they use noble metals like platinum and iridium in their catalysts.
However, PEM electrolyzer systems can offer operational savings in other ways because of their smaller footprint, higher current densities, and potential integration with intermittent power sources. PEM systems are also often shipped to the site with a more integrated setup, reducing installation time.
What advantages does Alkaline technology offer?
Alkaline electrolyzer technology has a proven industrial track record. It's because of its low equipment and maintenance costs and non-precious metal components that it's ideal for large-scale, steady-state hydrogen production, such as in ammonia or methanol synthesis.
What advantages does PEM technology?
PEM electrolyzer technology has some other distinct advantages. It rapidly responds to dynamic power input, has been proven to be reliable in smaller installations, and has a high current density and system compactness. It also has modular scalability and can be easily integrated into other tech, as well as being operable at higher pressures (up to 30 bar or more), which reduces its compression needs.
These features make PEM a strong candidate for renewable-powered hydrogen production, mobility applications, and energy storage.
What challenges does each technology face?
Alkaline systems are limited by lower start-up and load-following responses, the need for larger system size for equivalent output, and limited dynamic operation capabilities. The challenges faced by PEM electrolyzers are slightly different: they have higher material costs due to precious metals, need high-purity water, and have a shorter operational track record in large-scale installations.
Both technologies are continuing to evolve, with active research addressing their respective limitations.
Which system is better suited for coupling with renewable energy?
PEM electrolyzers are generally more compatible with variable renewable energy sources like wind and solar due to their quick start-up times and ability to manage fluctuating power inputs. Alkaline electrolyzers, while slower to respond, can be integrated with renewable systems when paired with appropriate buffering or hybrid energy strategies. Both technologies can be integrated with more consistent renewable energy sources such as hydropower.
How are these technologies scaling up to meet future demand?
Alkaline electrolyzers are already deployed at a gigawatt scale and are well understood in industrial applications. PEM electrolyzers are rapidly gaining ground in decentralized and flexible systems, and continued investment is driving down their costs while improving durability. Both technologies are expected to play complementary roles in the emerging hydrogen economy.
Conclusion
Rather than a case of one technology replacing the other, PEM and Alkaline electrolyzers each offer distinct strengths that suit different use cases. As global hydrogen demand grows, a balanced deployment of both technologies will be key to enabling efficient, cost-effective, and scalable green hydrogen production.
About Nel Hydrogen
Nel traces its history back to 1927 and has grown into a leading pure-play hydrogen technology company with a global presence. The company specializes in PEM and Alkaline electrolyser technology for renewable hydrogen production. Its product portfolio plays a vital role in enabling a green hydrogen economy, supporting the decarbonization of industries including transportation, refining, steel, and ammonia.
This information has been sourced, reviewed, and adapted from materials provided by Nel Hydrogen.
For more information on this source, please visit Nel Hydrogen.
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