Solar hydrogen from pure water

A particulate photocatalyst, as a sheet or a suspension in one vessel, splits pure water into hydrogen and oxygen at ambient pressure under one-sun illumination, with no applied bias and no sacrificial reagent, at a solar-to-hydrogen efficiency past the pure-water sheet results cited here, for six continuous hours.

Hydrogen from sunlight and pure water avoids an electrolyser and a purified feed. Photovoltaic electrolysis already reaches high efficiencies and is a different device. The photocatalyst version is still near one percent in the sheet experiments that define the field, and several of those efficiencies were measured below atmospheric pressure or above room temperature. The grade is hydrogen and oxygen from pure water, at 1 bar, under a measured one-sun flux, for an afternoon.

Wang, Hisatomi, Domen, and co-workers report a SrTiO₃:La,Rh / Au / BiVO₄:Mo sheet that splits pure water at pH 6.8 with a solar-to-hydrogen efficiency of 1.1% and an apparent quantum yield above 30% at 419 nm. The body of that paper places the 1.1% result at 331 K and 10 kPa, and a related point of 1.0% at 318 K and 5 kPa. A carbon-conductor sheet, SrTiO₃:La,Rh / C / BiVO₄:Mo, is reported at 1.2% at 331 K and 10 kPa, retaining 80% of that efficiency at 91 kPa, and the paper states that 1.0% was then the highest value among Z-scheme pure-water systems operating at ambient pressure. A 2024 review from the same line of work still treats photocatalyst-sheet efficiencies above 1%, and practical solar-to-hydrogen efficiencies of about 1% at most, as the state of that approach. A 100 m² array of SrTiO₃:Al panels in that review is a scale demonstration, not a higher one-sun efficiency.

Two higher numbers are a different experiment and do not clear this problem. Fu, Wu, and co-workers report 2.47 ± 0.03% for a two-cell system bridged by an iodide redox couple, with the hydrogen cell in aqueous hydrohalic solution. Hansora and co-workers report a perovskite photoelectrode module with a module-level solar-to-hydrogen efficiency of 11.2% under one sun, unbiased, on a 16 cm² artificial leaf. That is a wired photoelectrode module, not a particulate sheet in pure water. This problem does not grade it.

The reactor holds pure water with a resistivity of at least 1 MΩ·cm at the start. No buffer, salt, redox mediator, or sacrificial donor is added. The photocatalyst is a powder, a printed sheet, or a film of particles, in that one vessel. No electrical bias is applied. The headspace is 1.00 ± 0.05 bar. The water is not externally heated above 40 °C. The water temperature is recorded.

Illumination is continuous for 6 hours. The spectrum is AM1.5G. The irradiance at the sample plane is 100 ± 5 mW cm⁻², measured the same day with a calibrated thermopile or reference cell. The illuminated geometric area is at least 1.00 cm² and is fixed before the run.

Hydrogen and oxygen are quantified by gas chromatography against a calibration mixture. The amount of each gas uses the measured headspace volume or the measured outlet flow, not a bubble count. Solar-to-hydrogen efficiency is

(moles of H₂) × 237 kJ mol⁻¹ / (incident energy in kilojoules),

expressed as a percent. The 237 kJ mol⁻¹ is the Gibbs energy of H₂O(l) → H₂ + ½ O₂ at 298 K, fixed here so that every attempt uses the same numerator. A paper that used a different numerator is not recomputed.

The graded efficiency is the value over the full 6 hours. The efficiency over the first hour and over hours 2 through 6 are both reported. Hours 2 through 6 must be at least 80% of the first-hour value.

The 6-hour solar-to-hydrogen efficiency is at least 5.0%. The molar ratio of hydrogen to oxygen over those 6 hours is between 1.90 and 2.10. The dark control and the no-catalyst control pass. The efficiency over hours 2 through 6 is at least 80% of the first hour. The water stays at or below 40 °C, and the headspace stays at 1 bar.

Five percent, the ratio window, the 80% hold, 40 °C, 1 bar, 1.00 cm², and 6 hours are pass marks defined here. Five percent is above the 1.0%, 1.1%, and 1.2% sheet results cited above, and those results were not all taken at 1 bar and at or below 40 °C.

• A dark run of the same reactor, same catalyst, and same duration, with the lamp off, produces less than 5% of the hydrogen measured in the illuminated run.
• An illuminated run with no photocatalyst, same water and same lamp, produces less than 5% of that hydrogen.
• The hydrogen-to-oxygen ratio is inside the window above. Oxygen missing, or hydrogen from a sacrificial donor, fails the attempt.
• The irradiance measurement is reported. A nominal lamp setting with no measurement at the sample plane fails the attempt.

Any particulate semiconductor, cocatalyst, or sheet architecture, including a conductive underlayer that is not connected to a power supply. A suspension stirred in one vessel. A solar simulator.

An applied voltage or a wired photoanode and photocathode. A photovoltaic cell plus an electrolyser. A sacrificial donor, including methanol, triethanolamine, or silver ion. A redox mediator that carries charge between two vessels. A hydrohalic acid as the hydrogen source. A pressure below 0.95 bar. External heating of the water above 40 °C. Quoting an apparent quantum yield in place of the solar-to-hydrogen efficiency. An illuminated area chosen after the run, or an area below 1.00 cm². Using 286 kJ mol⁻¹, or any numerator other than 237 kJ mol⁻¹, in the graded efficiency.

The 6-hour solar-to-hydrogen efficiency is at least 2.0% under the same water, pressure, temperature, area, ratio, and control rules, and hours 2 through 6 hold at least 80% of the first hour. Two percent is a pass mark for this lesser bar. It is above the 1.0–1.2% sheet results and below the 5.0% mark. The 2.47% two-cell iodide system does not meet it, because the electrolyte and the two vessels are excluded. Partial success is not a solution.

Sheet efficiency of 1.1% for pure water, with the 331 K and 10 kPa condition in the body: Wang, Hisatomi, Domen, and co-workers, Nature Materials, 2016, doi:10.1038/nmat4589 (opens in a new tab). Carbon-conductor sheet at 1.2% at 331 K and 10 kPa, 80% of that efficiency retained at 91 kPa, and the statement that 1.0% was the highest ambient-pressure Z-scheme pure-water result then: Journal of the American Chemical Society, 2017, doi:10.1021/jacs.6b12164 (opens in a new tab). The 2024 review treating sheet efficiencies as above 1% and practical values as about 1% at most: Frontiers in Science, 2024, doi:10.3389/fsci.2024.1411644 (opens in a new tab). Two-cell 2.47 ± 0.03% result with an iodide couple: Fu, Wu, and co-workers, Nature Communications, 2025, doi:10.1038/s41467-025-56314-x (opens in a new tab). Perovskite module at 11.2%: Hansora and co-workers, Nature Communications, 2025, doi:10.1038/s41467-025-59597-2 (opens in a new tab). The 5.0%, 2.0%, 237 kJ mol⁻¹, 40 °C, 1 bar, and 6-hour requirements are pass marks defined here. Open point: a pure-water, one-vessel, one-sun result above 2% at 1 bar may exist and was not found in the 2024 review or the papers opened for this draft. If it does, the partial bar moves.