‘Artificial leaf’ for carbon neutral syngas production

Virgil Andrei, University of Cambridge

RESEARCHERS at the University of Cambridge, UK, have demonstrated simple and sustainable syngas production using a carbon neutral “artificial leaf” device, setting a new benchmark in the field of solar fuels.

Syngas, or synthetic gas, is a mixture primarily consisting of carbon monoxide (CO) and hydrogen (H₂), used in the production of a variety of products such as fuels, pharmaceuticals, plastics, and fertilisers. Typically, syngas is produced by reforming fossil fuels, requiring high temperatures and pressures, and biomass gasification, which can introduce contaminants. Additionally, current industrial processes emit carbon dioxide (CO₂) into the environment.

The artificial leaf developed at Cambridge only uses sunlight, carbon dioxide, and water for syngas production and the process doesn’t release any additional CO₂ into the atmosphere. It could eventually be used to develop a sustainable liquid fuel alternative to petrol.

The device was inspired by photosynthesis, a process in which plants use energy from the sun to convert CO₂ and water into glucose and oxygen.

The artificial leaf device comprises two light absorbers, similar to plant photosystems which convert light into chemical energy, and a cobalt catalyst immobilised on carbon nanotubes.

According to PhD student Virgil Andrei, the leaf operates in aqueous solution containing dissolved CO₂. “The first light absorber, BiVO₄, uses blue light to oxidise water to oxygen, while the second light absorber, the perovskite, reduces CO₂ to CO and water to hydrogen using a molecular catalyst,” he explained.

Other artificial leaves developed so far have usually only produced hydrogen. According to the researchers, their device is capable of sustainable syngas production due to the combination of materials and catalysts used. The state-of-the-art perovskite light absorbers provide high voltage and electrical current for the CO₂ reduction reaction in comparison to light absorbers made from silicon or dye-sensitised materials. Additionally, they used a molecular catalyst containing a cobalt atom. The cobalt centre is responsible for catalysis. Cobalt is cheaper than silver and platinum alternatives, and is better at producing CO.

Currently, the device is capable of solar-to-H₂ and solar-to-CO conversion efficiencies of 0.06% and 0.02%, respectively.

The researchers believe they could improve the conversion efficiencies by using better light absorbers, device architectures, and catalysts. According to Andrei, improvements could allow for “solar-to-fuel conversion efficiencies of up to 0.5-2% with a CO:H₂ product ratio above 2:1”. He added that efficiencies of more than 5% could be achieved by modifying the device architecture.

According to Andrei, conversion efficiencies of 5–10% and above are considered to be commercially viable.

Under 1 sun light intensity, defined as the nominal intensity of sunlight on a bright clear day – 1,000 W/m² – the artificial leaf device can sustain syngas production linked to water oxidation for three days. Furthermore, the perovskite photocathode, which is responsible for CO₂ reduction, can operate for one day under low light conditions of 0.1 sun, equivalent to those observed on cloudy or overcast days. According to Andrei, this is due to the “excellent optical properties of the perovskite light absorber, combined with the high selectivity of the molecular catalyst towards CO₂ reduction”.

“This means you are not limited to using this technology just in warm countries, or only operating the process during the summer months,” said Andrei, adding: “You could use it from dawn until dusk, anywhere in the world.”

Additionally, the amount and composition of syngas produced can be tuned by adjusting the amount of CO₂ reduction catalyst used and changing the light intensity and applied electrical potential.

Andrei said: “The total amount of products obtained is proportional with the light intensity, while a higher applied electrical bias improves both product formation and CO₂ reduction selectivity.”

Andrei noted three major aspects of the researchers’ technology that must be addressed; the stability, scalability, and performance. Whilst the technology can operate for three days, practical application would require “weeks or months” of operation. Additionally, “the devices need to be scaled up to square-meter panels to enable household- or industrial-scale operation”.

Andrei said the researchers are continuing to work on the device architecture to achieve higher product rates and better selectivity towards CO₂ reduction, “which could soon even enable direct liquid fuel production”.

The researchers are investigating technology to produce a sustainable liquid fuel alternative to petrol. Though syngas is already used in liquid fuel production, the researchers are aiming to produce liquid fuel in one step from carbon dioxide and water, eliminating syngas as an intermediate.

Andrei said: “We are aiming at sustainably creating products such as ethanol, which can readily be used as a fuel.”

“It’s challenging to produce it in one step from sunlight using the carbon dioxide reduction reaction. But we are confident that we are going in the right direction, and that we have the right catalysts, so we believe we will be able to produce a device that can demonstrate this process in the near future.”

Advances are being made in generating electricity from renewable energy sources but according to Erwin Reisner, Professor in Cambridge’s Department of Chemistry, the development of synthetic petrol is vital as electricity can only satisfy 25% of total global energy demand. He added that “there is a major demand for liquid fuels to power heavy transport, shipping and aviation sustainably.”

Nature Materials: http://doi.org/dgsh