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Direct seawater electrolysis has no demonstrated general efficiency or cost advantage over desalinating seawater and using a conventional electrolyzer. In cases analyzed in a 2023 study, reverse-osmosis water supply accounted for just 0.05–0.14% of hydrogen-production energy. Direct seawater systems also face chloride-driven side reactions, corrosion, fouling and component-lifetime challenges. The fair comparison is between complete systems—not simply the water entering the electrolyzer.
What “seawater electrolysis” means
The phrase can describe two different system designs. That distinction matters because one adds a water-treatment stage to established electrolysis, while the other asks the electrolyzer to operate with seawater’s dissolved salts and other constituents.
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- Freshwater electrolysis: Electrolysis using water from a freshwater source, treated as needed. Commercial systems commonly require highly purified or deionized feed water, so ordinary tap water should not be assumed suitable.
- Indirect seawater electrolysis: Seawater is desalinated—often by reverse osmosis—and purified further if required before it enters a conventional electrolyzer.
- Direct seawater electrolysis (DSE): Seawater is used directly as the water source and electrolyte, bypassing a separate desalination stage. “Direct” does not necessarily mean untreated: research designs may still use filtration, pretreatment, membranes, selective catalysts or pH management.
A sound comparison includes water treatment, electrolyzer and balance-of-plant equipment, electricity use, hydrogen output, maintenance and replacement intervals, and any safety controls for chlorine-containing products.
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Is seawater electrolysis more efficient?
The energy needed to desalinate seawater is small compared with the electricity used to make hydrogen in the cases assessed by Lee and co-authors in their 2023 Energy & Environmental Science analysis. Their compiled examples of seawater reverse-osmosis supply for PEM electrolysis put desalination at 0.05–0.14% of hydrogen-production specific energy. The paper uses 3.5 kWh/m³ for seawater reverse osmosis; the cases below are compiled examples, not a guarantee for every site or system.
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| Case cited in the 2023 analysis | Water demand (kg/kg H₂) | Desalination energy (kWh/kg H₂) | Hydrogen-generation energy (kWh/kg H₂) | Water-supply share of hydrogen-production energy |
|---|---|---|---|---|
| Beswick et al. | 9.0 | 0.03 | 63.97 | 0.05% |
| Lampert et al. | 25.7 | 0.09 | 63.97 | 0.14% |
| Hydrogenics case | 11.126 | 0.04 | 74.54 | 0.05% |
| Siemens case | 16.689 | 0.06 | 61.86 | 0.09% |
These figures isolate the energy for water supply against the cited hydrogen-generation values; they are not a universal efficiency rating for desalination or electrolysis. The underlying assumptions and cases are described in Lee et al.’s 2023 analysis.
In that paper, the authors conclude: “Even with major technological improvements, such as pretreatment, to address all the challenges, DSE would save only ∼0.1% of the cost and energy usage over a combination of water purification and conventional electrolyzers.” This is the authors’ modeled conclusion, not a universal measurement of every possible design.
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Why direct seawater electrolysis is difficult
Seawater contains chloride as well as other dissolved ions, including sulfate, magnesium and calcium. At the anode, the desired oxygen evolution reaction competes with chlorine evolution. Although oxygen evolution has a more favorable equilibrium potential, chlorine evolution can have faster kinetics and may prevail under relevant operating conditions.
- Chlorine chemistry: Chloride oxidation can produce chlorine or hypochlorite, creating hazardous by-products and corrosion risks.
- Materials and component degradation: Chloride can corrode cell hardware; dissolved ions can poison or detach catalysts; and seawater can degrade membranes.
- Fouling and precipitation: Suspended material and sparingly soluble salts can foul components or form deposits.
- Product crossover: Oxygen and chlorine products crossing a membrane can interfere with hydrogen evolution at the cathode.
A 2024 review surveys electrolyte, electrode and electrolyzer-geometry approaches intended to suppress chlorine evolution, while a 2025 review identifies corrosion, chlorine-related side reactions, metal precipitates, catalyst performance, membrane degradation and limited lifetime as continuing challenges. Reducing chlorine formation alone does not establish long stack life or favorable economics. See the 2024 ACS Materials Letters review and the 2025 Nature Reviews Materials review.
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NREL’s June 2022 report described DSE as having low technology readiness at that time. It reported that chlorine evolution dominated over desired oxygen evolution at normal current densities in the work reviewed, and identified selective membranes or separators, catalysts and pH control as development areas. Its conclusion was that more research and development was needed before reliable commercial deployment at scale. This is a dated readiness assessment, not a definitive survey of the 2026 market. Read the NREL report.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the cost comparison includes
Desalination requires equipment and energy, but Lee and co-authors argue that these contribute little to total energy and operating cost in their modeled hydrogen-production cases. DSE avoids a separate desalination stage, yet adds challenges that can affect capital costs, maintenance and component replacement.
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The 2023 analysis reports DSE capital costs above $6,000/kW, compared with cited estimates of $920–$1,725/kW for alkaline electrolyzers and $1,610–$2,668/kW for PEM electrolyzers. These are figures reported by the paper from underlying estimates—not current vendor quotations or universal present-day prices. The paper also identifies chlorine crossover and corrosion as factors that can lead to more frequent replacement of membranes, catalysts and other components. See the analysis and its cited estimates.
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For a project-level comparison, evaluate the same system boundary and operating assumptions for both options:
- Compare complete process chains. Include seawater intake and treatment, water polishing, electrolysis, balance-of-plant equipment and any safety systems—not just the electrolyzer stack.
- Separate energy uses. Compare electricity per kilogram of hydrogen and report water-treatment energy separately from electrolysis energy.
- Account for useful output. Consider operating voltage and current, Faradaic selectivity and the hydrogen actually produced over time.
- Model maintenance and downtime. Include corrosion, scaling, fouling, membrane and catalyst replacement, and lost production during servicing.
- Match the site conditions. Consider freshwater access, coastal location, footprint, integration with marine infrastructure, discharge and specialized operating needs.
When might direct seawater electrolysis make sense?
The 2023 analysis identifies ships and submarines as possible specialized settings where seawater is available and space for water purification is extremely limited. That is a potential niche, not evidence of broad commercial readiness or a general cost advantage. For nearly every other case considered in that analysis, the authors favor conventional water purification or desalination followed by established electrolysis.
Why not make hydrogen directly from seawater?
Because avoiding a desalination stage does not automatically make the whole hydrogen system cheaper or more efficient. The cited 2023 cases put reverse-osmosis water supply at a small share of hydrogen-production energy, while direct seawater operation must contend with chloride reactions, corrosion, fouling, lifetime and replacement costs. Direct seawater designs may be worth exploring where space is unusually constrained, but their value has to be demonstrated for the particular site and complete system.
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