A Cambridge-built artificial tree designed to make syngas from sunlight, water and carbon dioxide produced far less than expected during an outdoor demonstration in Ispra, Italy. Overcast, stormy and cold conditions contributed to low output and underpressure; a faulty valve then allowed corrosive potassium hydroxide solution into the device, degrading it. The failure shows why promising laboratory results are not enough to establish that an integrated solar-fuel system will work reliably outdoors.
What is an artificial tree for solar fuel?
The Cambridge prototype was a photoelectrochemical device: a 100-leaf array covering 0.35 m², designed to use sunlight to convert carbon dioxide and water into syngas, a mixture of carbon monoxide and hydrogen. Each artificial leaf had an area of 0.001 m² and combined light-absorbing materials with catalysts that drive the reactions.
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Each leaf paired a bismuth vanadate photoanode and a cobalt-based water-oxidation catalyst with a perovskite photocathode and a copper–indium catalyst for carbon-dioxide reduction. The system operated in potassium bicarbonate buffer saturated with carbon dioxide. Its gas-collection arrangement used valves and two tanks: potassium hydroxide solution absorbed excess carbon dioxide, while the carbon monoxide and hydrogen bubbled through and collected in a tank’s headspace. These are reported features of a research prototype, not instructions for building or operating one. Chemistry World’s 2025 report describes the device and demonstration.
What happened to the Cambridge device in Italy?
The Cambridge team was one of three invited to demonstrate a device during a three-day event at the European Commission’s Joint Research Centre in Ispra. The weather was mainly overcast, and a thunderstorm occurred on the first day. Low syngas production, combined with low overnight temperatures, created underpressure in the system. A faulty valve then drew corrosive potassium hydroxide into the device. The alkaline solution degraded the device and depleted carbon dioxide, further reducing syngas production.
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The 10 × 10 array produced 220 ml of carbon monoxide and 910 ml of hydrogen—quantities the report identifies as predictions based on previous tests, not the actual measured output in Ispra. The article does not state the volume the device ultimately produced, so the available figures cannot be used to calculate a field-test shortfall. Nor does the report quantify how much rain fell on the device. Chemistry World’s account links the failure to weather, low temperatures, underpressure and the valve problem, rather than establishing rainfall alone as the cause.
Why did smaller tests not reveal the same problems?
Tests at different scales and in different conditions exposed different risks. Laboratory tests of individual leaves had shown promise. A few weeks before the Ispra final, the team also tested a smaller 3 × 2 array outdoors on a sunny day in Cambridge; some potassium hydroxide leaked, but the report characterizes that leak as minor compared with the Ispra failure.
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| Test | Scale and conditions | What it established |
|---|---|---|
| Individual-leaf laboratory tests | Single leaves in laboratory conditions; exact test conditions are not stated in the report. | The leaves showed promise, but these tests did not demonstrate reliability of the complete outdoor system. |
| Cambridge outdoor test | 3 × 2 leaves on a sunny day, a few weeks before the final. | Some potassium hydroxide leaked; the report describes the problem as minor. |
| Ispra demonstration | 10 × 10 leaves at an outdoor event with mainly overcast weather and a first-day thunderstorm, followed by low overnight temperatures. | Low production and underpressure, a faulty valve, and corrosive solution entering the device degraded the integrated system. |
A larger outdoor system has to work as a whole, not just as a set of promising leaves. The Ispra event brought together gas production, fluid handling, valves, temperature changes and materials exposed to an alkaline solution. The contrast with the sunny, smaller Cambridge test illustrates how an integrated field demonstration can reveal weaknesses that controlled component tests or a smaller array may not expose. It does not, by itself, show that every design using artificial leaves will fail in similar weather.
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What does the failure say about scale and readiness?
The prototype took five postgraduate students and postdoctoral researchers two months to assemble. The team chose vacuum processing because it offered a route they considered scalable. Erwin Reisner said the experience shifted attention toward manufacturing as well as materials: “I think for me this puts the emphasis now much more on scalable processing methods, rather than solely on materials … if you cannot make these square metres quickly and cheaply it will be very difficult to make things at the required scale,” Chemistry World reported.
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The published-paper context is promising but should be kept distinct from the Ispra result. A surfaced description of V. Andrei et al.’s 2025 paper in Energy & Environmental Science reports modular perovskite–BiVO4 artificial leaves made using vacuum processing and a 10 × 10 reactor measuring 0.7 × 0.5 m², demonstrated at the EIC Horizon Prize event. It also reports 10 cm² photovoltaic devices sustaining a 1 V photovoltage and tandem photoelectrochemical devices operating for over 36 hours in tests. These abstract-level details do not establish commercial-scale output or resolve the specific outdoor reliability problems observed in Ispra. The paper is identified by DOI 10.1039/d4ee05780e.
Reisner argued that prototypes matter for attracting engagement and investment, saying, “If we don’t build prototypes, we probably shouldn’t expect others to engage and invest in this type of technology.” That is his view of the value of demonstrations, not evidence that the technology is ready for commercial production. A team from Japan won the competition with an artificial-photosynthesis system that generated methane and used it to power an engine, according to the event report. Chemistry World also quoted Jingshan Luo of Nankai University, who was not involved in the project, saying the work “provides valuable lessons on the discrepancies between ideal laboratory settings and real-world challenges” and could encourage further work on robustness.
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