Electrolysers for low-carbon hydrogen

In plain language

An electrolyser uses electricity to split water into hydrogen and oxygen. If it is powered by low-carbon electricity, it can make hydrogen with lower production emissions than conventional fossil-fuel routes. Hydrogen can serve as a chemical feedstock or fuel in applications where direct electricity use is difficult, but making and using hydrogen involves energy conversions and infrastructure of its own.

There is more than one kind of electrolyser. Alkaline systems use a liquid alkaline solution and are an established commercial design. Proton-exchange-membrane (PEM) systems use a solid membrane and can respond quickly to changing power. Solid-oxide electrolysers run hot and can use heat as well as electricity, but their high operating temperature creates materials and durability challenges. Anion-exchange-membrane systems are an emerging design that uses a solid membrane to conduct hydroxide ions. These differences matter when matching a plant to an electricity source and industrial site.

A low electrolyser price or a large project pipeline does not guarantee low-cost hydrogen. Plants need suitable water, electricity connections, power conversion, gas drying and compression, storage or transport, and a buyer able to use hydrogen. If the electrolyser draws on a carbon-intensive grid or causes fossil generation to increase, the emissions benefit can be reduced. The IEA reports that installed capacity and low-emissions hydrogen production remain small relative to announced ambitions, with policy certainty affecting investment decisions.

Technical explanation

Water electrolysis uses electric current to drive hydrogen evolution at the cathode and oxygen evolution at the anode. The electrolyte and ion-conducting separator distinguish the main cell families. Alkaline water electrolysers circulate liquid potassium- or sodium-hydroxide electrolyte across a porous separator. PEM electrolysers conduct protons through a solid polymer membrane. AEM electrolysers conduct hydroxide ions through a polymer membrane, aiming to combine membrane-based operation with less expensive catalyst options, though the technology is less mature. Solid-oxide electrolyser cells (SOECs) conduct oxide ions through a ceramic electrolyte at high temperature and can use process heat to reduce the electrical input required.

Each design has different operating constraints. The US Department of Energy describes alkaline and PEM units as established lower-temperature approaches, AEM as an emerging membrane approach, and SOEC as a high-temperature technology. PEM systems can support rapid response and compact, high-pressure designs; alkaline systems have a longer industrial history; SOECs may benefit from integrated heat but must manage thermal cycling and material durability. These are tendencies, not universal specifications: efficiency, start-up time, turndown, pressure and lifetime depend on the stack and balance-of-plant design.

System boundaries matter when quoting efficiency. Stack efficiency excludes some equipment such as water treatment, power electronics, cooling, gas purification and compression. Plant-level energy use and hydrogen cost therefore cannot be inferred from cell voltage or a catalyst result alone. Water must meet system purity requirements, and output gas must be separated and conditioned for its intended use. Electricity carbon intensity, plant utilisation, equipment manufacture and hydrogen delivery also affect lifecycle emissions.

Kumar and Lim (2023) review recent advances in PEM electrolysis; the full text and cost data were not reviewed for this profile. IEA data report project and capacity status rather than proving that every announced project will be built. In October 2024, the IEA reported 20 GW of electrolyser capacity at final investment decision and more than 6 GW reaching that stage in the prior twelve months. Its June 2026 data page labels 2026 values as estimates.

What the evidence says

Each statement is typed by the strength of its evidence and linked to the source that supports it.

Limitations and open problems

• The lifecycle emissions benefit depends on electricity carbon intensity, plant utilisation and the wider equipment and delivery chain; the label 'electrolytic' alone does not establish low emissions.
• Plant efficiency depends on the system boundary. Stack figures do not include every energy demand for water treatment, power conversion, gas purification, cooling, compression or transport.
• Water quality and reliable water supply are operating requirements; hydrogen output also needs drying, purification, compression or conversion for many end uses.
• Alkaline and PEM designs are commercial, while SOEC and AEM designs have different maturity and durability constraints; one efficiency or flexibility figure cannot be applied to all types.
• Installed electrolyser capacity and low-emissions hydrogen output remain low relative to announced ambitions (IEA, 2024).
• Many announced projects have not reached final investment decision. The IEA notes its 2030 output outlook holds only if all announced projects are realised (IEA, 2024).
• The IEA's most recent capacity figures are estimates, so precise current totals are not confirmed by the sources reviewed here (IEA, 2026).

Possible future uses

Timeline

1. Jul 2023Kumar and Lim publish a review of recent advances in PEM water electrolysis in Sustainable Energy and Fuels. Sources: [1]
2. 2 Oct 2024The IEA reports that 20 GW of electrolyser capacity has reached final investment decision globally. Sources: [3]
3. Jun 2026The IEA publishes updated electrolyser installed capacity and low-emissions hydrogen production charts, with 2026 values marked as estimates. Sources: [4]

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Glossary

Electrolyser

A device that uses electricity to split water into hydrogen and oxygen.

Final investment decision

The point at which a company commits money to build a project, as opposed to only announcing it.

Electrocatalyst

A material that speeds up the chemical reactions at an electrode, often made from precious metals in electrolysers.

Low-emissions hydrogen

Hydrogen produced with much lower greenhouse gas emissions than conventional methods, usually through electrolysis powered by low-carbon electricity.

Sources

1. review article
   Recent advances in hydrogen production through proton exchange membrane water electrolysis - a review
   S. Shiva Kumar, Hankwon Lim. Sustainable Energy & Fuels, Jul 2023. DOI: 10.1039/d3se00336a. Accessed 21 Oct 2026.
2. government report
   Hydrogen Production: Electrolysis
   US Department of Energy, 2024. Accessed 22 Oct 2026.
3. other
   Low-emissions hydrogen projects grow as policy support races to catch up
   International Energy Agency, 2 Oct 2024. Accessed 21 Oct 2026.
4. government report
   Hydrogen - Production and electrolyser installed capacity
   International Energy Agency, Jun 2026. Accessed 21 Oct 2026.

Revision history

4 changes since this profile was created.