Plate Nº 70 · recorded October 9, 2026
Chemistry & MaterialsReported finding
Real-Time Oxygen Sensing Exposes Hidden Losses in Solar Water Splitting
Imperial College scientists measured oxygen in real time during solar water splitting and found current can hide losses in hematite — until efficiency jumps to about 80%.
By James Calloway4 min read704 words
In brief
- Oxygen-production efficiency in hematite rises to around 80% once enough positive charge accumulates at the surface.
- The study appears in the Journal of the American Chemical Society (2026), DOI 10.1021/jacs.6c11949.
- A custom photoelectrochemical mass spectrometry (PEC-MS) platform at Royce at Imperial, White City, enabled real-time oxygen tracking under illumination.
- The same 'selectivity switch' behavior appeared in darkness, pointing to a fundamental property of hematite.
- The team plans to extend the method to other metal oxides and to CO2 and nitrogen reduction reactions.
Researchers at Imperial College London have measured oxygen production in real time during solar water splitting and found that electrical current does not always match how much oxygen a material actually makes. In hematite, one of the most widely studied materials in the field, a large share of the electrical charge at low current levels was diverted into competing surface reactions instead of producing oxygen — a limitation that standard measurements had completely hidden.
Once enough positive charge accumulated at the hematite surface, however, oxygen generation became far more efficient, reaching roughly 80% oxygen-production efficiency. The team, led by Dr. Flurin Eisner, formerly a research fellow in Imperial's Department of Materials and now at Queen Mary University of London, published the work in the Journal of the American Chemical Society (2026).
What did the researchers actually find?
Solar water splitting uses sunlight to split water into hydrogen and oxygen. Researchers typically gauge a material's performance by reading the electrical current it produces, assuming that more current means more oxygen — and therefore more useful chemistry. The new measurements break that assumption.
The Imperial team identified what they call a "selectivity switch" — a hidden stage in the reaction:
- At low current levels, much of the electrical charge feeds competing surface reactions rather than oxygen production.
- Once positive charge builds up sufficiently at the surface, the reaction switches track and oxygen generation becomes much more efficient, climbing to around 80% efficiency.
- The same switch appeared when the researchers drove the reaction electrically in complete darkness, which suggests the behavior is a fundamental property of hematite itself, not an artifact of light-driven chemistry.
That last point matters. Because the effect shows up without illumination, researchers cannot blame it on quirks of the solar setup. Any evaluation method that relies on current alone risks misjudging a material's true performance — and, by extension, misdirecting the search for better materials for green hydrogen and solar fuels.
How did they measure it?
The discovery relied on a custom instrument: a photoelectrochemical mass spectrometry (PEC-MS) platform built at the Royce at Imperial facility in White City. Mass spectrometry, in plain terms, identifies and counts individual gas molecules; photoelectrochemistry studies chemical reactions driven by light and electricity. Combining the two let the team watch oxygen molecules emerge one by one as the reaction ran.
The researchers adapted an existing electrochemical mass spectrometry system so it could work under illumination. The finished platform could do three things at once: shine light on a sample, measure the photocurrent, and directly monitor oxygen generation in real time. Comparing those streams side by side exposed the mismatch between current and oxygen that conventional measurements would have missed.
Sensitivity was the key. "Access to the EC-MS equipment at Royce Imperial, together with support to develop a light-compatible cell, made this study possible," Eisner said. "Its sensitivity allowed us to measure oxygen at reaction rates where current alone would have given an incomplete picture."
Professor Ifan Stephens of Imperial's Department of Materials, a co-author, emphasized how the combination of methods changed the picture. "I am very proud to have played a small role in this paper," he said. "It is amazing to see how multiple techniques together, in this case electrochemistry, operando optical spectroscopy and electrochemical mass spectrometry, can make us rethink our mechanistic understanding of a reaction."
The study also involved Dr. Daniele Benetti of Imperial's Department of Chemistry and Professor James Durrant from the same department.
What comes next?
The findings are preliminary in the sense that they concern one material, hematite, under specific conditions. The researchers have not yet shown that the same selectivity switch occurs in other candidates for solar fuel devices. The team now plans to apply the PEC-MS approach to other metal-oxide materials and to test it in a wider range of energy-conversion reactions, including carbon dioxide reduction and nitrogen reduction.
If the method proves general, it could give scientists a more accurate yardstick for evaluating new materials — one that counts the oxygen actually produced rather than the current that merely looks promising. For a field working toward cheap green hydrogen, that correction could redirect effort toward materials that genuinely deliver.
via Phys.org Chemistry (Source)
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