Electrolysers: Engineering methods

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

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)³⁷⁹¹⁸	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)¹⁸	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)¹⁸¹⁹²⁵	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	No benchmark in the cited sources	Small units	Promises to avoid iridium and platinum	Membrane durability	Early commercial
Chlor-alkali (membrane cell)¹⁰¹⁴	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)¹²¹⁶¹⁷	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 electrowinning³¹	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

Technical Targets for Proton Exchange Membrane Electrolysis US Department of Energy, Hydrogen and Fuel Cell Technologies Office, 2023
Green H2 electrolyser sector enters consolidation: IEA (reporting IEA Global Hydrogen Review 2026; cost figures not on the IEA's free web pages) Argus Media, 2026
Water Electrolyzers and Fuel Cells Supply Chain: Supply Chain Deep Dive Assessment US Department of Energy, 2022
How Can India Indigenise Hydrogen Electrolyser Manufacturing? (Annexure) Council on Energy, Environment and Water (CEEW), 2024
Critical Iridium Demands arising from future Expansion of Proton Exchange Membrane Electrolysis (Wortmann, Stolten, Heinrichs), preprint arXiv 2509.05357, 2025
thyssenkrupp nucera General Investor Presentation, May 2026 (company claims) thyssenkrupp nucera, 2026
The right chlor-alkali electrolysis technology for your needs thyssenkrupp nucera, 2024
Factsheet: Aluminium smelting with use of low-carbon electricity GIZ, 2025
Why Europe needs local production of chlorine Euro Chlor, 2026
AP60 and APXe smelting technology factsheet (company data) Rio Tinto AP Technology, 2015
History and Recent Developments in Aluminum Smelting in China (Gao, Wang, Shi, Hu), 35th ICSOBA Conference ICSOBA, 2017
The Role of E-fuels in Decarbonising Transport (revised January 2024) International Energy Agency, 2024
Bloom Energy Corp Form 10-K for fiscal year 2025 Bloom Energy / US SEC, 2026
Sunfire 2025: scaling, partnerships and European strength (company news) Sunfire, 2025
CODATA value: Faraday constant US National Institute of Standards and Technology, 2022