Making hydrogen directly from seawater is difficult because the anode must produce oxygen without letting seawater’s chloride ions drive competing chlorine-related reactions, while the electrodes and cell must withstand corrosion and mineral deposits over time. Researchers are testing selective catalyst surfaces, protective materials, membranes, feedwater treatment and new cell designs. None is yet a universal fix, and direct seawater electrolysis has not demonstrated a clear general advantage over desalinating water first.
What “direct seawater electrolysis” means
Electrolysis uses electricity to split water, producing hydrogen at the cathode and oxygen at the anode. In direct seawater electrolysis (DSE), seawater is the electrolyser’s feed. In the indirect route, water is desalinated before it enters the electrolyser. That distinction matters: desalination-first systems do not expose the electrolyser to the same seawater chemistry.
Using seawater directly might avoid a separate desalination step, but that possibility alone does not establish lower cost or energy use. The relevant comparison is between complete systems—including water treatment, electrolyser performance, maintenance and operating life—not just the water entering the cell. The European Commission Joint Research Centre’s review, dated 20 January 2025, found no research or industrial project demonstrating clear benefits of DSE over the desalination-first route, while noting that DSE could suit particular target applications. Read the JRC review.
Why seawater makes the chemistry harder
Chloride competes with oxygen production
The desired anode reaction is oxygen evolution. But seawater contains abundant chloride, which can also be oxidized. Chloride oxidation competes with oxygen evolution and can produce chlorine-related products. Researchers therefore need an anode that favors oxygen production while suppressing chloride reactions—and continues doing so in chloride-rich conditions.
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This is a selectivity problem as well as a materials problem. A catalyst may show promising oxygen-evolution activity yet still be unsuitable if chloride reactions occur, if its surface degrades, or if it cannot sustain performance in a working cell. The Nature Reviews Materials review identifies chlorine-related side reactions, corrosion and metal precipitates among the challenges associated with poor catalytic activity and limited lifetime. See the 2025 review; a separate perspective surveys approaches to avoiding chlorine evolution. Read the ACS Materials Letters perspective.
That does not mean every seawater electrolyser necessarily releases chlorine. The products depend on the cell’s materials, chemistry and operating conditions; suppressing unwanted chloride reactions is precisely one of the problems researchers are trying to solve.
Corrosion can undo good initial performance
Chloride-rich water creates corrosive conditions that can attack electrode materials and interfaces. A material’s initial activity is not enough to establish that it can work reliably: corrosion can reduce activity or damage the cell, so selectivity and durability have to be considered together.
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Durability claims are most useful when they describe the feedwater, electrolyte composition, operating load, test duration and full-cell configuration. A short material or catalyst test does not, on its own, establish practical service life. The 2024 review on long-term durability discusses the challenge from catalysts through complete systems. Read the durability review.
Mineral deposits can foul electrodes
Seawater contains more than salt. Under operating conditions, dissolved species can form deposits on electrode surfaces. Magnesium- and calcium-containing hydroxides are particular concerns: precipitation and fouling can block active surfaces and interfere with sustained hydrogen production.
The challenge depends on local conditions at the electrode, not simply on the composition of the water before it enters the cell. A design that controls one unwanted reaction may still struggle with mineral deposition, so performance has to be assessed across the complete operating system.
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“Seawater” is not a single, standardized test feed
Results from synthetic seawater, natural seawater, pretreated seawater, alkaline electrolyte containing added salt and desalinated water are not interchangeable. Feed composition affects the chemistry and can change what a test demonstrates. Reviews also emphasize reporting the feedwater and relating catalyst results to membrane and reactor performance. See the 2025 Journal of Power Sources review.
How researchers are trying to solve the problems
Design anodes to favor oxygen evolution
One approach is to engineer catalyst surfaces that favor oxygen evolution while discouraging chloride oxidation. Researchers investigate protective or passivating layers, chloride-blocking surfaces and surface chemistries intended to selectively interact with the species involved in the desired reaction.
A label such as “selective” is not enough to judge a material. The evidence should identify how selectivity was measured and under what feed and operating conditions, including whether the result came from a catalyst test or a complete electrolyser.
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Protect electrodes and interfaces from corrosion
Anti-corrosion materials, protective coatings and interfacial designs aim to keep chloride-rich water from degrading the active components. The practical test is whether the protection preserves performance in the intended cell and feed, not merely whether a material appears stable in a limited trial. Durability reviews emphasize evaluating the system as well as the catalyst. The durability review and the Nature Reviews Materials review discuss these linked concerns.
Use membranes and ion management
Membranes and ion-selective designs can help manage which species reach an electrode or the environment in which reactions occur. They introduce their own engineering questions, including fouling, degradation and compatibility with the cell. Their value must therefore be judged as part of a functioning device rather than as an isolated materials improvement.
Manage precipitation and deposits
Strategies discussed in the durability literature include inhibiting interactions that lead to magnesium- or calcium-containing hydroxide precipitation and preventing deposits from adhering to electrode surfaces. The review also describes a proposed route that combines electrolysis with production of nanoscale magnesium hydroxides. Such approaches address specific deposition challenges; they should not be read as evidence of a single, established solution for every feed or cell.
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Treat the feedwater—or desalinate first
Pretreatment can reduce impurities before water reaches the electrolyser; desalination-first electrolysis avoids direct exposure to seawater. These routes may reduce some direct-contact problems, but they require their own equipment and operating inputs. The meaningful question is whether the full system’s treatment requirements are outweighed by gains in electrolyser performance, reliability or suitability for a particular site.
Redesign the reactor and evaluate the whole device
Cell geometry and reactor design affect how water and ions move, how membranes and electrodes interact, and how products are separated. A catalyst result in a laboratory half-cell does not establish that a complete device will manage those functions under realistic operation. Device-level testing with clearly described feedwater is essential to judge whether a design’s benefits persist beyond a material test. The Journal of Power Sources review highlights the need to connect catalyst performance with membrane and reactor performance.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to judge claims about direct seawater hydrogen
When comparing DSE with desalination-first electrolysis, look for evidence that addresses the full system rather than a single attractive metric. Useful questions include:
- Reaction selectivity: Does the study show oxygen production while measuring or otherwise addressing chlorine-related products?
- Feedwater: Was the test run with natural seawater, synthetic seawater, pretreated water or a salt-containing laboratory electrolyte?
- Durability: How long did the cell operate, at what load, and did the test use a complete device?
- Deposits and corrosion: Does the design manage mineral scaling and chloride-related degradation over the reported operation?
- System comparison: Are treatment, energy, maintenance and operating requirements compared with the desalination-first alternative?
- Cell design: Are membrane and reactor performance demonstrated with the stated feed, or is the claim based only on a catalyst result?
These details matter because a result under one electrolyte or cell configuration may not predict performance with natural seawater or in another device. The available reviews describe active approaches and unresolved engineering challenges, not a universally validated design or representative performance benchmark.
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Not as a general conclusion supported by the cited assessment. The Joint Research Centre’s review record, dated 20 January 2025, states: “There is currently no research or industrial project demonstrating clear benefits of using direct seawater electrolysis over indirect seawater electrolysis.” It also allows that DSE may become viable for specific target applications. This is a dated assessment, not a guarantee about later deployments, and it does not rule out a future or site-specific advantage. Read the JRC assessment.
For now, claims that direct electrolysis automatically saves money or makes hydrogen more sustainable simply because it uses seawater skip the central comparison. Any advantage would need to be demonstrated for a defined feed, complete cell and system, and operating context.
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