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No. Photocurrent measures how much charge flows through a solar water-splitting electrode, not how much of that charge ends up as molecular oxygen. A study published online in the Journal of the American Chemical Society on September 26, 2026 (Benetti et al.) measured oxygen directly, in real time, from hematite (α-Fe₂O₃) photoanodes while they operated. At low photocurrent, the oxygen-associated share of the charge was close to zero. Oxygen selectivity only rose once the reaction shifted into a different kinetic regime.
Why current alone can mislead
An ammeter reading tells you how many electrons leave the photoanode per second. Water splitting at the anode is supposed to turn those electrons into oxygen, but the electrons can also be consumed by other surface processes, or lost to recombination before they react. A current reading cannot tell these outcomes apart.
The paper handles this with a simple ratio. Oxygen Faradaic efficiency (FE) is the charge associated with oxygen divided by the total anodic charge. An FE of 100% would mean every electron that crossed the interface went into oxygen; a low FE means much of the measured current went elsewhere.
How the team measured oxygen during operation
The authors used operando photoelectrochemical mass spectrometry (PEC-MS). The arrangement was built to keep three things at once: optical access for illumination, electrochemical control of the electrode, and real-time detection of molecular oxygen leaving the hematite surface. The oxygen detection reached picomole-per-second sensitivity.
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They then combined these oxygen data with rate laws derived independently from spectroelectrochemical measurements of surface-hole density. That pairing is what lets the paper connect how much oxygen forms to how the reaction depends on the concentration of reactive holes at the surface.
When comparing measurement approaches, three questions matter:
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- Direct or inferred: Does the method detect molecular oxygen itself, or estimate output from charge?
- Operando or not: Was the electrode measured under the same illumination and bias it operates under?
- Selectivity or total current: Does the result describe what fraction of charge became oxygen, or how much charge flowed?
Current-based estimates answer the last question only. The PEC-MS approach answers all three.
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Oxygen efficiency at low and high photocurrent
Below roughly 0.6 μA·cm⁻²
Oxygen FE fell sharply once photocurrent density dropped below approximately 0.6 μA·cm⁻², and it reached approximately 0% near 0.1 μA·cm⁻². In that range, the electrode was passing measurable current without producing oxygen in any meaningful amount, at least as the PEC-MS measurement could detect it.
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Higher current densities
As current density rose, oxygen FE climbed and approached a plateau of roughly 80%. The authors are careful about what that number means. The non-unity plateau may reflect the particular PEC-MS measurement conditions rather than an intrinsic ceiling for hematite. Readers should not treat 80% as the best oxygen efficiency hematite can reach.
The kinetic shift behind the selectivity change
The oxygen results line up with a change in how the water-oxidation rate depends on surface-hole density. At low hole density, the rate scales roughly first order. As hole density increases, it becomes approximately third order, which the authors describe as a cooperative pathway. The fitted exponents under illumination were 1.08 for the first-order regime and 3.29 for the third-order regime.
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| Condition | Quantity fitted | First-order exponent (m₁) | Third-order exponent (m₃) |
|---|---|---|---|
| Light (illuminated) | Water-oxidation kinetics | 1.08 | 3.29 |
| Dark (electrochemical) | Total current | 1.19 | 3.32 |
| Dark (electrochemical) | Oxygen-evolution current, after FE correction | not stated | 3.42 |
Oxygen selectivity rose across the same transition in which the third-order pathway emerged. The authors interpret this to mean that current in the low-density, first-order regime contributes little to molecular oxygen formation. Their abstract puts it directly: near-zero oxygen FE at low current and hole density indicates that isolated single-hole water oxidation is poorly coupled to molecular oxygen formation.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11The paper also names possible competing routes for that charge, including recombination and single-hole oxidation pathways. These are mechanistic interpretations. The study does not directly measure every competing product, so the explanation should be read as the most consistent account of the data rather than a full accounting of where each electron went.
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Dark operation points to the surface chemistry
The authors also ran the electrode in the dark and observed comparable behavior. Because the same kinetic trend appears without photoexcitation, the effect looks like a property of hematite’s surface chemistry rather than something that only happens under light. That is an interpretation supported by the parallel dark measurements, not a separate proof.
What the result does and does not establish
- Sample: The findings concern hematite photoanodes under the study’s laboratory conditions.
- Device performance: This was not a full solar-hydrogen device or field test. The low-current finding should not be read as the fraction a commercial system would lose.
- Other oxides: Similar rate laws have been reported for other metal-oxide electrodes, and the authors call broader relevance plausible. They also say direct measurements are still needed before the oxygen-selectivity relationship can be extended beyond hematite.
- “Wasted” current: The accurate statement is that some measured charge did not correspond to molecular oxygen. It does not mean all current was lost.
How to read solar water-splitting claims after this study
When a report presents photocurrent as evidence of hydrogen or oxygen production, check whether the authors measured the product directly, whether the measurement was taken under the operating illumination, and whether the efficiency is stated as a selectivity or simply as current. A high current with no oxygen measurement is an incomplete result. Current, oxygen FE, and kinetic order each answer a different question, and this study shows that the answers can diverge sharply at low reaction rates.
The paper is available through the Journal of the American Chemical Society; the authors are listed as Benetti et al., and the study is titled “Linking Oxygen Evolution Selectivity to Water Oxidation Reaction Order on Hematite Photoanodes by Operando Photoelectrochemical Mass Spectrometry.”
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