Green hydrogen can be made from seawater without first desalinating it, and the approach has reached large pilot and trial settings. But it has not yet shown a clear general advantage over desalinating seawater and then using established electrolysis. The central challenge is not whether seawater can be split; it is whether a direct system can do so selectively, safely, reliably and economically over long periods.
What “direct from seawater” means
Direct seawater electrolysis feeds seawater to an electrolyzer without first desalinating it or substantially purifying it. The indirect route removes salts and other impurities first, then uses established water-electrolysis technology. That treatment requires equipment and energy and produces a concentrated brine stream, but it also gives the electrolyzer the controlled water quality it needs.
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So the comparison is not free ocean water versus costly freshwater. It is a comparison between complete systems: water treatment, electrolysis, electricity supply, maintenance, waste handling and hydrogen output all matter. The European Commission Joint Research Centre (JRC) identifies desalination followed by electrolysis as the benchmark because both water purification and water electrolysis are established technologies.
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Why seawater is hard to electrolyse directly
Chloride competes with oxygen production
In an electrolyzer, the anode is intended to produce oxygen from water. Seawater contains chloride ions, which can instead participate in chlorine-evolution reactions. This competing chemistry can reduce the share of electricity going toward the desired reaction and may produce chlorine or related compounds that need to be contained and managed safely. Chloride exposure can also corrode electrodes and accelerate catalyst degradation.
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Whether a design produces problematic chlorine compounds depends on its operating conditions and materials; the evidence here does not establish that every direct system has the same output. But controlling chlorine chemistry is a core engineering requirement, not a secondary detail.
Mineral deposits and fouling threaten uptime
Under alkaline operating conditions, calcium- and magnesium-containing deposits can build up on electrodes and block active sites. Fouling and mineral accumulation create maintenance needs and can undermine stable operation. A catalyst that performs well in a brief laboratory experiment therefore does not, by itself, demonstrate a durable, safe electrolyzer.
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Promising design ideas still need system-level proof
Researchers have investigated selective catalysts, protective or chloride-repelling layers, membranes, membraneless designs and hybrid approaches. These strategies target real obstacles, but a materials improvement is not equivalent to commercial readiness. A credible assessment has to include long-duration performance, degradation, cleaning and replacement, as well as safe management of chlorine-related chemistry.
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The International Energy Agency’s Global Hydrogen Review 2025 reports two Chinese milestones from December 2024:
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- Sinopec completed a 100 kW pilot at its Qingdao refinery using chlorine-resistant electrodes.
- China National Offshore Oil Corporation began trial operation of a megawatt-scale seawater electrolyzer and completed a test run.
These projects show that direct seawater electrolysis has progressed beyond small laboratory demonstrations. The cited milestones do not establish sustained availability, delivered hydrogen cost, lifecycle emissions or routine commercial operation. Nameplate capacity and a test run are not substitutes for published operating records and full-system cost data.
Does it save energy or money compared with desalination first?
The JRC review by Marco Serafini, Eveline Weidner and Julien Bolard, published on 20 January 2025, concluded: “There is currently no research or industrial project demonstrating clear benefits of using direct seawater electrolysis over indirect seawater electrolysis.” The review also allows that direct electrolysis could become viable for specific applications. Its conclusion is not that direct systems can never work; it is that a general advantage had not been demonstrated in the projects and research it assessed.
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Some water and desalination figures help frame the question, but do not settle it. A 2025 technical review by A. Augustin and T. Wintgens gives a stoichiometric minimum of 9 kg of water per kg of hydrogen and says practical use can reach 13 kg/kg because of evaporation, leaks, oxygen-stream losses, cleaning or flushing. The same review reports that seawater reverse osmosis accounts for 70% of globally installed desalination capacity and typically uses 3.0–5.0 kWh/m³, depending on feedwater and system design.
Those desalination figures are not a complete energy comparison between hydrogen pathways. A fair calculation would also account for electrolyzer efficiency and lifetime, pretreatment, brine management, maintenance and the integration of renewable electricity. The available project milestones do not provide enough site-specific operating and cost information to rank direct and indirect systems across those factors.
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Where direct seawater electrolysis might make sense
The strongest case to investigate is site-specific: a location may have limited freshwater, access to seawater and nearby renewable electricity, making a direct system attractive if it can avoid enough treatment burden without creating greater costs or reliability problems. The JRC says specific target applications may become viable, but the evidence reviewed does not establish a proven winning application.
That means a project should be judged against the best practical alternative at its actual site, rather than against an assumption that freshwater is unavailable or desalination is prohibitively costly. Relevant questions include:
- How much hydrogen does the complete system deliver per unit of electricity over sustained operation?
- How often must electrodes, catalysts or membranes be cleaned, repaired or replaced?
- How are chlorine and hypochlorite formation monitored, contained and handled?
- What pretreatment, scaling control and waste or brine management does each route require?
- What are the total costs under the site’s water, power, maintenance and permitting conditions?
What would change the outlook
The prospects will become clearer when direct systems publish independently assessable, long-duration operating data alongside comparable indirect-system data. Particularly useful evidence would include hydrogen output and energy use at a clearly defined full-system boundary; electrode and membrane lifetimes; maintenance frequency; chlorine selectivity and handling; and site-specific costs. Until those results demonstrate an advantage for a particular application, the established route of desalination followed by electrolysis remains the practical benchmark.
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