# Electrolysers: Engineering methods

Alkaline, PEM, solid oxide, and the older chlorine and aluminium cells

Source page: https://www.materialinsights.org/energy/e_electrolysers/methods/

What this tab covers The engineering methods that can do this conversion, compared on efficiency, scale, economics, limitations and reliability.

Four families split water; chlor-alkali, aluminium smelting and plating are older and, for now, far larger users of electrolytic power.

#### Comparison

| Method | Efficiency | Scale | Economics | Limitations | Reliability |
|---|---|---|---|---|---|
| **Alkaline water electrolysis (AWE)**^[3](https://www.argusmedia.com/en/news-and-insights/latest-market-news/2841836-green-h2-electrolyser-sector-enters-consolidation-iea)^[7](https://www.ceew.in/sites/default/files/annexure-how-can-hydrogen-electrolysers-be-made-in-india.pdf)^[9](https://ir-api.eqs.com//media/document/5cf6586c-ca10-433a-9464-0d277fdd9ced/assets/thyssenkrupp_nucera_General_Investor_Presentation_May_2026.pdf)^[18](https://www.iea.org/reports/the-role-of-e-fuels-in-decarbonising-transport) | About 65-70% LHV for large commercial units | 20 MW modules of about 300 cells; GW-scale plants | Chinese systems installed in China 500-1,100 USD/kW (2025) | Bulky; slower to follow fast swings in wind and solar | Longest-lived of the low-temperature types |
| **Proton-exchange membrane (PEM)**^[1](https://www.energy.gov/eere/fuelcells/technical-targets-proton-exchange-membrane-electrolysis)^[8](https://arxiv.org/html/2509.05357v1) | 55 kWh/kg system (61% LHV) in 2022 | MW to hundreds of MW | 1,000 USD/kW uninstalled (2022) | Iridium and titanium supply | 40,000-hour stack life (2022), target 80,000 |
| **Solid-oxide electrolysis (SOEC)**^[18](https://www.iea.org/reports/the-role-of-e-fuels-in-decarbonising-transport)^[19](https://www.sec.gov/Archives/edgar/data/1664703/000162828026006516/be-20251231.htm)^[25](https://sunfire.de/en/news/sunfire-2025-scaling-partnerships-and-european-strength/) | About 90% LHV on electricity | Pilot to multi-MW (largest units at refineries) | No settled market price yet | Needs a heat source and steady running; ceramics degrade faster | Little long-term fleet data |
| **Anion-exchange membrane (AEM)**^[6](https://www.energy.gov/sites/default/files/2022-02/Fuel%20Cells%20%26%20Electrolyzers%20Supply%20Chain%20Report%20-%20Final.pdf) | No benchmark in the cited sources | Small units | Promises to avoid iridium and platinum | Membrane durability | Early commercial |
| **Chlor-alkali (membrane cell)**^[10](https://www.thyssenkrupp-nucera.com/newsroom/blog/the-right-chlor-alkali-electrolysis-technology-for-your-needs)^[14](https://eurochlor.org/app/uploads/2026/03/why-europe-needs-local-production-of-chlorine.pdf) | 1,979 kWh per tonne NaOH at 6 kA/m2 | Plants above 800,000 t NaOH a year | Electricity about 70% of chlorine cost in Europe | Output tied to chlorine demand | Mature |
| **Aluminium smelting (Hall-Heroult)**^[12](https://giz.de/en/downloads/giz2025-en-factsheet-aluminium-smelting-with-use-of-low-carbon-electricity.pdf)^[16](https://www.ap-technology.com/wp-content/uploads/2019/03/AP_Factsheet_AP60-APXe.pdf)^[17](https://icsoba.org/assets/files/publications/2017/KN06S - History and Recent Developments in Aluminum Smelting in China.pdf) | 12.1-13.1 DC kWh/kg in modern cells; world average 14.1 MWh/t AC | Pot lines of hundreds of cells at 400-600 kA | Electricity is the largest cash cost | Burns 0.43 t of carbon per tonne, giving direct CO2 | Mature |
| **Electroplating and electrowinning**^[31](https://physics.nist.gov/cgi-bin/cuu/Value?f) | Set by the share of current that deposits metal rather than making hydrogen | Workshop baths to large copper and zinc refineries | Small energy cost per part | Toxic baths (cyanide, chromium) | Mature |

For hydrogen, alkaline is cheapest per kW and dominant in China; PEM costs more and needs iridium but suits variable power; solid oxide uses least electricity where heat is available but is still early; AEM is a bet on dropping precious metals. In all of them three numbers rule: kWh per kg (voltage), USD per kW (current density and factory scale) and running hours. Chlor-alkali and aluminium show a mature electrolytic industry: the best aluminium cells still use about twice the theoretical minimum, and plants go where power is cheap.

#### How each method works
**Alkaline water electrolysis (AWE)**

Nickel-coated steel electrodes in 30-40% potassium hydroxide, separated by a porous diaphragm (Zirfon) that passes ions but not gas. Cheapest and most mature.
**Proton-exchange membrane (PEM)**

A solid acid polymer membrane with an iridium anode and a platinum cathode. Compact, high current density, fast response.
**Solid-oxide electrolysis (SOEC)**

Ceramic cells split steam at 700-900 C, so industrial or nuclear heat can replace part of the electricity.
**Anion-exchange membrane (AEM)**

A solid membrane that carries hydroxide ions in a dilute alkaline liquid, aiming for PEM-style compactness with nickel catalysts.
**Chlor-alkali (membrane cell)**

Brine gives chlorine at a coated titanium anode; sodium ions cross a membrane to a nickel cathode, where water becomes caustic soda and hydrogen.
**Aluminium smelting (Hall-Heroult)**

Alumina dissolved in molten cryolite is split by current from carbon anodes into a pool of liquid aluminium.
**Electroplating and electrowinning**

Metal ions are reduced onto a part or cathode; Faraday's law gives about 1.19 g of copper or 1.10 g of nickel per amp-hour (calculated).

#### Sources

1. [Technical Targets for Proton Exchange Membrane Electrolysis](https://www.energy.gov/eere/fuelcells/technical-targets-proton-exchange-membrane-electrolysis) US Department of Energy, Hydrogen and Fuel Cell Technologies Office, 2023
2. [Green H2 electrolyser sector enters consolidation: IEA (reporting IEA Global Hydrogen Review 2026; cost figures not on the IEA's free web pages)](https://www.argusmedia.com/en/news-and-insights/latest-market-news/2841836-green-h2-electrolyser-sector-enters-consolidation-iea) Argus Media, 2026
3. [Water Electrolyzers and Fuel Cells Supply Chain: Supply Chain Deep Dive Assessment](https://www.energy.gov/sites/default/files/2022-02/Fuel%20Cells%20%26%20Electrolyzers%20Supply%20Chain%20Report%20-%20Final.pdf) US Department of Energy, 2022
4. [How Can India Indigenise Hydrogen Electrolyser Manufacturing? (Annexure)](https://www.ceew.in/sites/default/files/annexure-how-can-hydrogen-electrolysers-be-made-in-india.pdf) Council on Energy, Environment and Water (CEEW), 2024
5. [Critical Iridium Demands arising from future Expansion of Proton Exchange Membrane Electrolysis (Wortmann, Stolten, Heinrichs), preprint](https://arxiv.org/html/2509.05357v1) arXiv 2509.05357, 2025
6. [thyssenkrupp nucera General Investor Presentation, May 2026 (company claims)](https://ir-api.eqs.com//media/document/5cf6586c-ca10-433a-9464-0d277fdd9ced/assets/thyssenkrupp_nucera_General_Investor_Presentation_May_2026.pdf) thyssenkrupp nucera, 2026
7. [The right chlor-alkali electrolysis technology for your needs](https://www.thyssenkrupp-nucera.com/newsroom/blog/the-right-chlor-alkali-electrolysis-technology-for-your-needs) thyssenkrupp nucera, 2024
8. [Factsheet: Aluminium smelting with use of low-carbon electricity](https://giz.de/en/downloads/giz2025-en-factsheet-aluminium-smelting-with-use-of-low-carbon-electricity.pdf) GIZ, 2025
9. [Why Europe needs local production of chlorine](https://eurochlor.org/app/uploads/2026/03/why-europe-needs-local-production-of-chlorine.pdf) Euro Chlor, 2026
10. [AP60 and APXe smelting technology factsheet (company data)](https://www.ap-technology.com/wp-content/uploads/2019/03/AP_Factsheet_AP60-APXe.pdf) Rio Tinto AP Technology, 2015
11. [History and Recent Developments in Aluminum Smelting in China (Gao, Wang, Shi, Hu), 35th ICSOBA Conference](https://icsoba.org/assets/files/publications/2017/KN06S - History and Recent Developments in Aluminum Smelting in China.pdf) ICSOBA, 2017
12. [The Role of E-fuels in Decarbonising Transport (revised January 2024)](https://www.iea.org/reports/the-role-of-e-fuels-in-decarbonising-transport) International Energy Agency, 2024
13. [Bloom Energy Corp Form 10-K for fiscal year 2025](https://www.sec.gov/Archives/edgar/data/1664703/000162828026006516/be-20251231.htm) Bloom Energy / US SEC, 2026
14. [Sunfire 2025: scaling, partnerships and European strength (company news)](https://sunfire.de/en/news/sunfire-2025-scaling-partnerships-and-european-strength/) Sunfire, 2025
15. [CODATA value: Faraday constant](https://physics.nist.gov/cgi-bin/cuu/Value?f) US National Institute of Standards and Technology, 2022

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_electrolysers/index.md)
- [Overview](https://www.materialinsights.org/energy/e_electrolysers/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_electrolysers/correlation/index.md)
- [Science and limits](https://www.materialinsights.org/energy/e_electrolysers/science/index.md)
- [Value chain](https://www.materialinsights.org/energy/e_electrolysers/valuechain/index.md)
- [Unit economics](https://www.materialinsights.org/energy/e_electrolysers/economics/index.md)
- [Equipment and suppliers](https://www.materialinsights.org/energy/e_electrolysers/equipment/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_electrolysers/links/index.md)