In October 2019, chemists at the University of Cambridge reported a small device that does something plants have done for billions of years, but with a different end product. Their "artificial leaf" sits in water, absorbs sunlight and uses that energy to turn carbon dioxide and water into syngas, a mixture of hydrogen and carbon monoxide that industry already uses to make fuels and chemicals. The work, led by Professor Erwin Reisner with PhD student Virgil Andrei as first author, appeared in the journal Nature Materials.
The short answer to "what did they make?" is this: a working, bias-free prototype that produces syngas using only sunlight, with no extra electricity and no additional carbon dioxide released in the process. It is not yet a fuel factory. It is a proof that the chemistry can be done in one compact, light-driven unit.
What syngas is and why it matters
Syngas, short for synthesis gas, is a workhorse of the chemical industry. It is a starting material for liquid fuels, plastics, fertilisers and pharmaceuticals. Reisner summed it up neatly in the university's announcement: "You may not have heard of syngas itself but every day, you consume products that were created using it."
The problem is how syngas is usually made. Most of it comes from fossil sources such as natural gas or coal, which means its production adds carbon dioxide to the atmosphere. A route that makes the same mixture from sunlight, water and captured carbon dioxide would cut that link.
nanonov/iStock
How the Cambridge leaf works
The device copies the logic of photosynthesis rather than its biology. A plant leaf captures light and uses that energy to rearrange water and carbon dioxide into sugars. The artificial leaf captures light and uses that energy to rearrange water and carbon dioxide into hydrogen and carbon monoxide.
Two parts do two different jobs:
- Light absorbers. The device combines two absorbers stacked as a tandem: a perovskite layer and bismuth vanadate (BiVO4). Perovskites are crystalline materials that convert light efficiently, and the team said they give a high voltage and current compared with silicon or dye-based absorbers.
- A catalyst. A molecular cobalt catalyst steers the chemistry towards carbon monoxide. The researchers chose cobalt because it is cheaper than platinum or silver and better at producing carbon monoxide.
Some early reports described perovskite as the cobalt catalyst. That is a mix-up: the perovskite captures light, and the cobalt compound drives the reaction. Keeping the roles straight makes the design much easier to understand.
Because no external power is needed, the paper describes the system as "bias-free". All of the energy comes from light falling on the device.
Virgil Andrei
Working on cloudy days
One detail caught attention in 2019. The absorbers keep working in the weaker light of overcast or rainy days, so the device is not restricted to sunny regions or summer months. Andrei made the point directly: the technology need not be limited to warm countries.
That does not mean output is the same in all weather. Less light still means less energy to drive the reaction. What it does mean is that the leaf degrades gracefully instead of stopping when the sky turns grey, which matters for any technology meant to run outdoors for long periods.
Why fuels, not just electricity
Solar panels and wind turbines are now cheap and widespread, but electricity covers only part of the world's energy use. The Cambridge team noted that electricity meets roughly a quarter of global energy demand. Ships, long-haul trucks and aircraft are hard to electrify because batteries are heavy and bulky for the energy they store.
Liquid fuels pack a lot of energy into a small mass, which is why aviation and shipping still rely on them. If such fuels could be made from sunlight and carbon dioxide, they could, in principle, be close to carbon-neutral: the carbon released when they burn would be carbon that was taken from the air or from industrial waste in the first place.
Virgil Andrei and Professor Erwin Reisner.(Chanon Pornrungrog/Cambridge University/PA Wire)
Limits of the 2019 prototype
The researchers were open that efficiency was still low and that the device was a laboratory demonstration. Several questions separate a promising prototype from a useful fuel supply:
- How much product does a square metre make, and for how many hours or days before performance drops?
- How stable are perovskites, which are known to be sensitive to moisture, in long outdoor use?
- Where does the concentrated carbon dioxide come from, and what does it cost to capture?
- What energy and materials go into building the device and converting syngas into a finished fuel?
A full life-cycle accounting is needed before any solar fuel can claim to be truly clean. That caution does not diminish the result; it simply puts it in its place as an important early step.
The team's stated next goals were to skip the syngas stage and produce liquid fuels such as ethanol directly from carbon dioxide and water in one step. The project was co-funded by the Austrian government and the petrochemical company OMV through the Christian Doppler Laboratory for Sustainable SynGas Chemistry, along with UK research councils.
What came next: leaves that float
Chanon Pornrungrog/Cambridge University/PA Wire
In August 2022 the same group reported a much lighter version in Nature. The 2019 leaf relied on thick glass and protective coatings, which made it heavy. The new devices put thin-film perovskite and metal-oxide absorbers on flexible plastic and metal foils, sealed with thin water-repellent carbon layers. They are light enough to float.
The team tested them outdoors on the River Cam in Cambridge, where they split water into hydrogen and oxygen or reduced carbon dioxide to syngas. Andrei said the group could imagine fuel-producing "farms" on water, much as solar farms produce electricity today, while stressing that the leaves still need improvement before any commercial use.
The direction of travel is clear: lighter, cheaper materials that could cover large areas. Whether artificial leaves become a serious fuel source will depend on durability, efficiency and cost, all of which are still being worked out.
Sources: Yusuf Hamied Department of Chemistry, University of Cambridge: Artificial leaf successfully produces clean gas (October 2019); Andrei et al., Nature Materials (2019); University of Cambridge: Floating artificial leaves (August 2022). Photos: nanonov/iStock; Virgil Andrei; Chanon Pornrungrog/Cambridge University/PA Wire.
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