Short answer: Photocatalytic water splitting could use fewer major components by converting sunlight to hydrogen in one light-driven process, but its reported solar-to-hydrogen efficiency has predominantly remained around 1–2% or lower. Electrolytic hydrogen made with solar power is a more established system architecture, yet its economics depend heavily on electricity supply, electrolyser technology, utilization, location and project design. Neither route can be named the universal winner from today’s published figures because the most commonly quoted efficiency and cost numbers use different system boundaries and evidence levels.
The practical comparison is therefore not simply “which percentage is higher?” It is whether a complete project can produce, separate, recover and safely handle hydrogen for years at a competitive cost. The analysis below keeps direct photocatalysis, photovoltaic-powered electrolysis and photoelectrochemical (PEC) systems separate, then compares them on efficiency, cost, durability, scale-up and evidence maturity.
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What the two pathways actually do
Direct photocatalytic water splitting
In direct photocatalysis, light-absorbing semiconductor particles or other photocatalysts absorb sunlight and drive the reactions that split water into hydrogen and oxygen. The light absorber and catalytic reaction are integrated in the same material system, so the process does not first route solar energy through a conventional photovoltaic (PV) array and a separate electrolyser.
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PV-powered electrolysis
PV-electrolysis uses a two-stage energy chain. Solar modules convert sunlight into electricity; an electrolyser then uses that electricity to split water. The electrolyser has separate electrodes and an electrolyte or membrane, while the PV plant, electrical conversion equipment and water-treatment systems sit upstream.
When people say “electrolytic green hydrogen,” they usually mean hydrogen made by electrolysis using renewable electricity. Electrolysis itself is not automatically green: the climate result depends on the electricity source and the emissions associated with operating the complete project. A grid-connected electrolyser can have a very different emissions profile from one supplied by dedicated renewable generation.
Photoelectrochemical systems are related, but distinct
A PEC device uses photoactive electrodes inside an electrochemical cell. The electrodes absorb light and participate directly in the electrochemical reactions, but the architecture is not identical to a suspension of photocatalyst particles and is not the same as PV modules feeding a conventional electrolyser. Treating “photocatalytic,” “PEC” and “PV-electrolysis” as interchangeable obscures which components, interfaces and failure modes are being evaluated.
Reviews published in 2024 describe these as separate solar-water-splitting configurations. See the systematic review in Sustainable Materials and Technologies and the systems-and-economics review in Advanced Materials.
Efficiency: why one percentage cannot settle the comparison
Use the same conversion boundary
A direct solar pathway is normally discussed with a solar-to-hydrogen efficiency: the chemical energy in the produced hydrogen divided by incident sunlight under specified conditions. An electrolyser is commonly characterized by electricity-to-hydrogen efficiency, which starts after electricity has already been generated. If the PV array and its losses are omitted, that figure is not a complete sunlight-to-hydrogen result.
A fair system comparison should state all of the following alongside the percentage:
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- Whether the input is incident sunlight, PV electricity or electricity delivered to the electrolyser.
- The solar spectrum, irradiance and temperature used for the measurement.
- Whether wiring, power conversion, water treatment, gas separation and compression are included.
- Whether the result is a short laboratory measurement, a sustained operating value, a modeled target or a field demonstration.
- Whether hydrogen output is measured after purification and recovery or inferred from an intermediate reaction rate.
What is known about photocatalytic efficiency
The clearest field-level figure in the available recent literature is not a record result but a characterization of the field. A 2024 Advanced Materials review says photocatalytic solar-hydrogen efficiency has predominantly remained at approximately 1–2% or lower. That statement describes the predominant level reported across the field; it is not a universal physical ceiling, and it is not a like-for-like comparison with electricity-to-hydrogen efficiency.
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What can be said about electrolysis efficiency
The supplied evidence does not establish one current, globally representative electricity-to-hydrogen percentage for every electrolyser technology and project. Results vary with electrolyser type, operating point, stack age, balance-of-plant loads and whether the figure is reported on a lower- or higher-heating-value basis. A number quoted without those qualifications cannot be used as a universal benchmark against the 1–2% photocatalytic characterization.
For a complete solar-electrolysis calculation, multiply the performance of the PV and electrical system by the electrolyser’s electricity-to-hydrogen performance, then account for operating conditions and auxiliary loads. That calculation is different from quoting the electrolyser alone.
Efficiency questions to ask before accepting a headline number
| Question | Why it changes the interpretation |
|---|---|
| What is the input boundary? | Sunlight-to-hydrogen includes solar collection; electricity-to-hydrogen begins after electricity production. |
| What is measured? | Hydrogen recovered from the outlet is more useful for project planning than a theoretical reaction rate. |
| How long did the test run? | A brief peak value does not establish sustained output, degradation or maintenance needs. |
| What operating conditions were used? | Spectrum, irradiance, temperature, pressure and water quality affect both pathways. |
| Are auxiliary systems included? | Pumps, controls, gas separation, purification and compression can materially change system-level performance. |
Cost: potential savings versus demonstrated economics
Why photocatalysis could reduce equipment count
A direct photocatalytic plant may combine light capture and chemical conversion in one reactor rather than buying a PV field and a separate electrolyser. Fewer major components could reduce some capital costs, simplify electrical integration and remove conversion steps.
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That is a conditional advantage, not proof of a lower cost per kilogram. At approximately 1–2% or lower solar-to-hydrogen efficiency, more collector or reactor area is needed for a target output. The economics must also include catalyst manufacture and replacement, reactor materials, optical access, mixing, gas separation, hydrogen recovery, monitoring and safety systems. A cheaper-looking process flow can still be expensive when its productive area and maintenance burden are included.
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What the IEA number means
The International Energy Agency’s Global Hydrogen Review 2024 executive summary gives a modeled outlook of approximately USD 2–9 per kilogram of hydrogen by 2030 under deployment consistent with its Net Zero Emissions by 2050 Scenario. The report describes a smaller reduction under its Stated Policies Scenario. These are scenario-based outlooks, not today’s universal market price and not a direct cost estimate for photocatalytic hydrogen.
The range is wide because future cost depends on technology development, manufacturing scale, renewable electricity prices, financing, capacity utilization, local construction conditions and policy. Applying the upper or lower end to every country, electrolyser type or operating profile would overstate what the report establishes.
What the DOE Hydrogen Shot figure means
On December 4, 2024, the U.S. Department of Energy said: “The report shows that electrolysis has the potential to achieve the aggressive DOE Hydrogen Shot™ goal of reducing hydrogen production costs to $1 per kilogram (kg), and it details innovations needed, along with the necessary advancements in manufacturing, technology efficiency, and integrated energy systems.” The statement is an ambition conditioned on further advances, not evidence that electrolysis is already available at $1/kg across the market. Read the announcement at DOE Releases Assessment of Electrolysis Technology—Highlighting Pathways to Cut the Cost of Hydrogen.
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How to read PEC cost targets
DOE’s technical-target page for PEC water splitting lists historical target-year cost values and assumptions for components, replacement lifetime, operation and maintenance. Those figures are technical targets or modeled parameters. They are useful for identifying the performance and lifetime a design would need, but they are not current supplier quotations or proof of commercial production cost.
Evidence status of the main cost figures
| Figure or statement | Evidence status | What it can and cannot answer |
|---|---|---|
| USD 2–9/kg by 2030 | IEA modeled outlook under its Net Zero Emissions by 2050 Scenario | Shows a possible future range for hydrogen production under a specified deployment pathway; it is not a current price or photocatalysis comparison. |
| USD 1/kg | DOE Hydrogen Shot goal | Defines an aggressive target and the need for innovation; it is not an achieved general-market cost. |
| DOE PEC target-year values | Technical targets with stated assumptions | Indicates design requirements for a target case; it is not a quotation or deployed-project result. |
| Photocatalytic lower component count | Potential systems advantage identified in reviews | Suggests where capital savings might arise, but does not establish lower cost at useful scale. |
Photocatalysis: practical limits beyond catalyst activity
Low useful output per area
The reported 1–2% or lower field-level efficiency means that catalyst activity alone cannot be the project’s only design metric. Developers must ask how much hydrogen is produced per square metre of illuminated reactor, after downtime and gas recovery. A material that performs well in a small, optimized test may not retain that output when light penetration, slurry circulation or larger optical paths are introduced.
Stability, corrosion and replacement
Photocatalysts and reactor components operate in an oxidizing, reducing and illuminated environment. Photocorrosion, poisoning, fouling and mechanical loss can reduce activity. Replacement frequency affects both operating cost and the environmental footprint of manufacturing the catalyst. Long-duration stability data are therefore as important as an initial efficiency value.
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Scale-up and reactor engineering
Scaling a photocatalytic suspension or coated surface changes the relationship between light and active material. Large reactors must distribute light, water and catalyst evenly while limiting dead zones and heat buildup. The 2024 review in Frontiers in Science identifies reactor design and gas-management issues as central constraints on large-scale solar-to-chemical conversion.
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Gas separation and hydrogen recovery
Water splitting produces hydrogen and oxygen together. If the gases are allowed to mix, the product can require additional separation and the mixture can create a serious safety hazard. A practical plant needs a controlled way to collect, separate, purify and recover hydrogen at the outlet. Those systems consume equipment, energy and maintenance capacity that a catalyst-only comparison can omit.
Safety and operations
Safe handling requires control of ignition sources, pressure, leaks and oxygen enrichment, as well as procedures for shutdown and maintenance. These obligations apply even when the reactor is at atmospheric pressure and even when the process is described as “direct.”
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Electrolysis: practical limits and project dependencies
Electricity supply determines the economics
Electrolysers convert electricity into hydrogen, so the price, carbon intensity, availability and temporal profile of that electricity are decisive. Dedicated renewable generation can provide a clearer green-hydrogen claim but may leave the electrolyser underused when solar or wind output is low. Grid electricity can improve utilization while creating a more complex emissions accounting problem. The best operating choice depends on the project’s location, power contract, storage strategy and regulatory definition of renewable hydrogen.
Capital and balance of plant
The stack is only one part of an electrolysis project. Water treatment, pumps, power electronics, controls, cooling, gas separation, purification and hydrogen handling all contribute to capital and operating cost. Stack replacement and degradation must be modeled over the project life rather than ignored in a nameplate efficiency calculation.
Utilization and intermittency
Solar-powered electrolysis may run below full capacity or cycle with renewable output unless the project adds storage or another electricity source. Lower utilization spreads fixed capital over fewer kilograms of hydrogen. Conversely, running continuously on high-emissions electricity can undermine the environmental purpose of calling the product green. A credible comparison reports both utilization assumptions and electricity provenance.
Electrolysis is not automatically the cheaper or greener choice
Electrolysis has a clearer commercial architecture than direct photocatalysis, but that maturity does not eliminate cost, durability or emissions questions. The IEA and DOE both describe future reductions as dependent on technology development, manufacturing progress, deployment and integrated energy systems rather than on a single efficiency improvement.
A fair method for comparing a proposed project
- Define the product. Specify hydrogen purity, delivery pressure, annual output and whether oxygen is sold, vented or treated as a coproduct.
- Draw the system boundary. Include sunlight or electricity generation, collection, conversion, water treatment, reactor or stack, separation, purification, compression and storage as appropriate.
- Label the efficiency metric. Use solar-to-hydrogen for a direct solar route and electricity-to-hydrogen for the electrolyser; provide a full solar-to-hydrogen result when comparing an entire PV-electrolysis plant.
- Separate evidence levels. Mark each value as laboratory data, a pilot demonstration, a deployed-project observation, a modeled scenario or a technical target.
- Model lifetime, not just day one. Include catalyst or stack degradation, replacement intervals, downtime, maintenance and end-of-life handling.
- Calculate output under local conditions. Use the site’s solar resource, renewable-power profile, water availability, temperature and operating schedule rather than a generic nameplate value.
- Account for safety and gas management. Cost and energy for separation, leak detection, purification, pressure control and safe shutdown belong in the comparison.
- Test sensitivities. Show how the result changes with electricity price, utilization, component lifetime, financing, replacement cost and hydrogen selling price.
Which pathway is more practical today?
For a project that needs a defined delivery schedule and a bankable equipment chain, PV-powered electrolysis generally presents a more recognizable integration route because PV generation and electrolysers are already distinct engineering categories. That does not guarantee low cost; electricity and utilization can dominate the result.
Direct photocatalysis is better viewed as a potentially simpler long-term architecture whose commercial case depends on simultaneous progress in efficiency, lifetime, reactor scale-up and gas handling. Its lower component count is valuable only if the reactor produces enough hydrogen per unit area and remains stable without frequent replacement.
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What progress would change the balance?
For photocatalytic systems
- Higher sustained solar-to-hydrogen output, not only short peak measurements.
- Photocatalysts that resist corrosion, poisoning and deactivation in real water and sunlight.
- Reactors that maintain uniform illumination and fluid flow at large scale.
- Safe, low-energy hydrogen and oxygen separation with reliable recovery.
- Manufacturing and replacement methods that keep catalyst and reactor costs predictable.
For electrolytic systems
- Lower stack and balance-of-plant costs through manufacturing scale and improved materials.
- Higher durability and less frequent replacement under variable renewable operation.
- More efficient integration with renewable generation, storage and power electronics.
- Lower-cost clean electricity and operating strategies that improve utilization without compromising the green-hydrogen claim.
- Project designs that reduce water-treatment, compression and other auxiliary burdens.
Bottom line
Photocatalytic water splitting offers an appealing direct route from sunlight to hydrogen and may eventually reduce equipment count. Its central obstacle is not merely an inexpensive catalyst: 2024 reviews characterize predominant solar-to-hydrogen efficiency at about 1–2% or lower, while stability, reactor scale-up, gas separation and safety remain unresolved practical requirements.
PV-electrolysis has a more established architecture, but it must be judged on the complete renewable-electricity system. Electricity price, source, utilization, stack lifetime and balance-of-plant costs determine whether a specific installation is competitive. The IEA’s USD 2–9/kg by-2030 range and DOE’s USD 1/kg Hydrogen Shot are respectively a modeled scenario and a policy goal, not universal current prices.
The defensible conclusion is conditional: electrolysis is the nearer-term engineering baseline for many projects, while photocatalysis and PEC could become compelling if they demonstrate durable, high-output operation at scale. Compare them with identical boundaries, lifetime assumptions and safety requirements before making a cost or efficiency claim.
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