1. What alkaline water electrolysis is (the short version)

It is a way of splitting water in a strongly alkaline solution, using nickel-based electrodes and a porous diaphragm, and it has a history of more than a century. IRENA writes that from the 19th century to the first half of the 20th, alkaline electrolysis was the only technology used, with asbestos as the diaphragm separating the gasesSourced.

• Electrolyte: the review by Brauns and Turek names the most-used electrolyte as 20 to 30 wt% KOH in water, with conductivity best at 50–80 °CSourced. IRENA’s 2020 table gives an operating temperature of 70–90 °C and 5–7 mol/L KOHSourced (the ranges differ a little between sources)
• Electrodes and diaphragm: in IRENA’s example the electrodes are nickel-coated perforated stainless steel and the diaphragm is a polyphenylene sulfide (PPS) mesh carrying a ZrO₂ (zirconium oxide) layerSourced. No precious-metal catalyst is used
• The weak point: when the current is cut, the share of hydrogen in the oxygen (or the reverse) rises toward the level at which the plant is shut down for safety (2 vol%). Brauns and Turek call this the main problem when coupling to renewable energySourced

PEM electrolysis is covered in our explainer on PEM water electrolysis; AEM electrolysis and solid oxide electrolysis cells have their own explainers. For the parts of the whole stack see our explainer on electrolyzer stacks and materials, and for the cost of hydrogen our explainer on the cost of hydrogen production.

2. Reactions and cell structure — the zero-gap cell

At the cathode, water becomes hydrogen and hydroxide ions (OH⁻); at the anode, hydroxide ions become oxygen and water. Current flows because OH⁻ moves through the diaphragm from the cathode to the anode (the reactions and the movement are textbook material, our commentary). Older cells left a fixed gap between the electrodes. Later the “zero-gap” layout, with electrodes pressed directly against the diaphragm, became the norm, cutting resistance through the electrolyteSourced (Brauns and Turek).

All Sourced (IRENA [Source 1] Table 1, Brauns and Turek [Source 2], Fujita et al. [Source 3]). Materials differ widely between manufacturers.

3. A materials engineer’s view (1): electrolyte and diaphragm — passing ions but not gas in strong KOH

The electrolyte: why KOH

Brauns and Turek report a conductivity of about 95 S/m for KOH at 50 °C against about 65 S/m for the cheaper NaOH, so that KOH is 40 to 50% more conductiveSourced (95 ÷ 65 = about 1.46, consistentOur calculation). Gas solubility, on the other hand, falls as electrolyte concentration rises (salting out), and the effect is slightly stronger in NaOH, so the product gases dissolve more readily in KOHSourced. Choosing for conductivity costs something in gas solubility, which bears on purity — a small trade-off here too (our commentary).

The diaphragm: from asbestos to composites

• History: IRENA writes that asbestos was used until its health hazards became known late in the 20th century, then replaced by ZrO₂-based composite separators such as ZIRFON®Sourced
• Construction: IRENA describes the diaphragm as ZrO₂ stabilised with a PPS mesh. Brauns and Turek give Zirfon™ Perl UTP 500 (Agfa) as an example of a porous separatorSourced
• What is required: the Asahi Kasei paper says a diaphragm needs a high gas barrier, low electrical resistance and excellent thermal and chemical durability, and that porous membranes of aromatic hydrocarbons, or composites of them with inorganic oxides, are commonly usedSourced. The company has also developed a diaphragm surface-modified with hydrophilic inorganic particles and a fluororesin coating
• The thickness tug of war: IRENA explains that thicker diaphragms and spacers used to prevent gas mixing raise resistance and lower efficiencySourced
• A manufacturer’s claim: Agfa says ZIRFON UTP 500+ gives a 2.5% higher hydrogen yield than its UTP 500, with the note that this was measured at 10 kA/m² (1 A/cm²)Sourced (manufacturer’s stated value). In March 2022 Agfa announced that thyssenkrupp nucera had signed a purchase contract for a large volume of ZIRFON membranesSourced (Agfa announcement)

4. Electrodes — nickel at the core, no precious metals

The great attraction of alkaline electrolysis is that its electrodes use no precious metals. Brauns and Turek say nickel is the most-used electrode material, but its overpotentials for oxygen and hydrogen evolution are very high, so catalytic additions are made: iron is a low-cost catalyst for oxygen evolution and molybdenum lowers the overpotential for hydrogen evolutionSourced.

The Asahi Kasei paper says it draws on its chlor-alkali (salt electrolysis) experience, and that for the anode it chose a mixture of transition metal oxides, guided by the Pourbaix diagram (the map of stable potential and pH regions)Sourced. It adds that a reduction treatment by heating in flowing hydrogen raised the conductivity and the specific surface area of the catalyst layer (it hypothesises that the added pore area comes from cracks forming in the layer)Sourced. On the cathode side an elastic metal mattress presses the whole diaphragm evenly.

5. Following variable power — what happens at low current density

Hydrogen purity is normally above 99.9 vol% and oxygen purity 99.0–99.5 vol%Sourced (Brauns and Turek). Because hydrogen and oxygen each form explosive mixtures when the other gas is between 4 and 96 vol%, the technical limit for an emergency shutdown is set at 2 vol%Sourced.

Gas can mix by two routes: permeation through the diaphragm and gas dissolved in the electrolyte and carried by the circulating liquidSourced (Asahi Kasei paper). The second grows in proportion to the electrolyte flow, so a lower flow is preferable; at high current, though, a certain flow is needed to hold down the temperature rise from Joule heating. Asahi Kasei therefore says it introduced automatic control of the electrolyte flow according to the current density, which suppresses overheating at high current and gas contamination at low currentSourced (the manufacturer’s report).

In the measurements cited by Brauns and Turek, with separate electrolyte loops for the anode and cathode sides, the hydrogen concentration in the anode gas stayed below 0.7 vol% in every condition tested (0.05–0.7 A/cm², 1–20 bar). With the loops mixed, it was slightly above 2 vol% even at 0.05 A/cm² and atmospheric pressure, reached 2% at 0.5 A/cm² at 10 bar, and was 2.5% even at 0.7 A/cm² at 20 barSourced (60 °C, KOH about 32 wt%, flow 0.35 L/min). At low current the amount of contaminating gas barely changes while the amount of gas produced falls, so the concentration rises (the review’s own explanation). That is why the part-load floor of most alkaline electrolyzers is held at 10–25% of rated loadSourced.

In IRENA’s 2020 summary the load range is 15–100% for alkaline and 5–120% for PEMSourced. This is, however, a summary as of 2020. Asahi Kasei says it has widened the operating range through flow control according to current density (see above). IRENA writes that some companies claim to have resolved the trade-off between gas permeation and efficiency of the diaphragm with new-generation diaphragmsSourced.

6. A materials engineer’s view (2): every shutdown damages the electrodes — reverse current

7. Scale-up so far — separating demonstration, commercial and plan

In alkaline cells a single electrode can be made large. IRENA says electrode areas reach up to 3 m²Sourced. The Asahi Kasei paper describes a system at FH2R in Fukushima that stacks 170 cells of 2.7 m² electrode area in seriesSourced. The table below separates, from published material, demonstration, commercial specifications and orders and plans.

The “commercial scale” figures are the manufacturer’s statement. The page does not let us tell installed, delivered and ordered apart, nor alkaline water electrolysis from chlor-alkali. The operating status of NEOM and others has not been confirmed in this article.

8. Our calculation: back-calculating the current density at FH2R

The result sits somewhat above IRENA’s nominal range (0.2–0.8 A/cm²) at the top, and the minimum is 15% of the maximum — the same level as the part-load floor given by Brauns and Turek (10–25%) and IRENA’s 15%Our calculation. Against PEM (IRENA 1–2 A/cm², DOE target of 3.0 A/cm²), the gap is several-fold. The thick diaphragm and the resistance of a liquid electrolyte can be read as the main reason (our commentary). DOE’s alkaline targets are 1.0 A/cm² for 2026 and 2.0 A/cm² as the ultimate targetSourced.

9. Reading the numbers, evaluation and standards

All Sourced (DOE [Source 4], IRENA [Source 1], Fujita et al. [Source 3]). Numbers at different levels are set side by side; this is not a ranking. IRENA’s stack lifetime of 60,000 h is from its table, and “over 30 years” from its text.

• The international safety standard: ISO 22734:2019 has been withdrawn and replaced by ISO 22734-1:2025 (Part 1: Safety), published in July 2025Sourced (ISO, checked 5 October 2026). The scope of the old edition included ion-transport media of aqueous bases. The text is paid, so we limit ourselves to the title and scope
• Testing for grid services: ISO/WD TS 22734-2 (testing guidance for grid services) is under development, focusing on alkaline and PEM electrolysersNot yet confirmed
• Test methods for performance and durability: the JRC report (EUR 30752 EN, 2021) sets out harmonised operating conditions and testing protocols for low-temperature water electrolysis including alkaline (AWE)Sourced

10. Challenges and limits

(1) Long-term data under variable power are limited

Within the scope of this article we could not confirm a case in which a manufacturer has published, as primary information, a long-term degradation rate for operation following renewable energy. The figures from Brauns and Turek and the Asahi Kasei paper come from tests with stated conditions and from a manufacturer’s report.

(2) Constraints on pressure and purity

In IRENA’s summary the operating pressure of alkaline electrolysis is below 30 bar. Higher pressure means more dissolved gas and a worse mixing problem (Brauns and Turek), so large differential-pressure operation like PEM’s is hardSourced.

(3) Manufacturers’ scale figures cannot be broken down

thyssenkrupp nucera’s “over 10 GW installed” and similar figures cannot be split, from the page, into alkaline water electrolysis and chlor-alkali, or into installed, delivered and ordered. The operating status of NEOM and others is not confirmed.

(4) What is not covered

Comparison with other technologies (cost, installed base), resource questions such as nickel, the state of deployment in China, AEM and cell structures other than zero-gap, diaphragm manufacturers’ technical data sheets (Agfa’s must be requested and could not be checked), and the handling of hydrogen and electrolyte are not covered.

11. Glossary

12. Primary sources

1. IRENA “Green hydrogen cost reduction: Scaling up electrolysers to meet the 1.5°C climate goal”, December 2020 (PDF) — irena.org
2. Brauns J, Turek T “Alkaline Water Electrolysis Powered by Renewable Energy: A Review”, Processes 8(2), 248 (2020, peer-reviewed) — doi.org/10.3390/pr8020248
3. Fujita Y et al. (Asahi Kasei) “Development and Demonstration of Large-scale Alkaline Water Electrolysis System ‘Aqualyzer’”, Electrochemistry 93(11), 117007 (2025, peer-reviewed) — doi.org/10.5796/electrochemistry.25-00129
4. U.S. Department of Energy (DOE) “Technical Targets for Liquid Alkaline Electrolysis” (viewed 5 October 2026) — energy.gov
5. U.S. Department of Energy (DOE) “Technical Targets for Proton Exchange Membrane Electrolysis” (viewed 5 October 2026) — energy.gov
6. thyssenkrupp nucera “Alkaline Water Electrolysis” technology page (viewed 6 October 2026) — thyssenkrupp-nucera.com
7. thyssenkrupp nucera “thyssenkrupp nucera Supplies Electrolyzers for Moeve to Build Southern Europe’s Largest Green Hydrogen Project”, 18 March 2026 — thyssenkrupp-nucera.com
8. Agfa “Agfa sets a new efficiency standard in the production of green hydrogen with its ZIRFON UTP 500+ electrolysis membrane” — agfa.com
9. Agfa “Agfa’s ZIRFON separator membrane selected by thyssenkrupp nucera for large-scale green hydrogen projects”, 9 March 2022 — agfa.com
10. ISO “ISO 22734-1:2025 Hydrogen generators using water electrolysis — Part 1: Safety” — iso.org
11. ISO “ISO 22734:2019 Hydrogen generators using water electrolysis — Industrial, commercial, and residential applications” (withdrawn) — iso.org
12. European Commission JRC Tsotridis G, Pilenga A, “EU harmonised protocols for testing of low temperature water electrolysers”, EUR 30752 EN (2021) — publications.jrc.ec.europa.eu

13. Claim-to-source audit

Last updated 6 October 2026. Sources are limited to primary material (official documents of international bodies, governments, standards bodies and manufacturers, and peer-reviewed papers). Because the article includes structural readings and materials-design interpretations, those are marked as Commentary and kept separate from sourced fact. Long-term degradation under renewable-power operation, diaphragm manufacturers’ data sheets, the breakdown of thyssenkrupp nucera’s figures and the operating status of NEOM and others could not be confirmed and are not stated. The table in Chapter 9 and Fig. 6 set side by side numbers of different origin and level and are not a ranking. Fig. 2, Fig. 5, Fig. 6 and Fig. 4 are vector drawings and Fig. 1 and Fig. 3 are AI-generated images; none of them shows a real cross-section, equipment or product.