Photocatalytic system efficiently converts CO2 into fuel

Researchers from the City University of Hong Kong (CityU), The University of Hong Kong (HKU), Jiangsu University and the Chinese Academy of Sciences have developed a stable artificial photocatalytic system that is said to be more efficient than natural photosynthesis. Described in the journal Nature Catalysis, the system mimics a natural chloroplast to efficiently convert carbon dioxide in water into valuable methane — a breakthrough that could contribute to the goal of carbon neutrality.

Photosynthesis is the process by which chloroplasts in plants and some organisms use sunlight, water and carbon dioxide to create food or energy. In past decades, many scientists have tried to develop artificial photosynthesis processes to turn carbon dioxide into carbon-neutral fuel.

“However, it is difficult to convert carbon dioxide in water because many photosensitisers or catalysts degrade in water,” said CityU’s Professor Ye Ruquan, one of the leaders of the study. “Although artificial photocatalytic cycles have been shown to operate with higher intrinsic efficiency, the low selectivity and stability in water for carbon dioxide reduction have hampered their practical applications.”

The research team overcame these difficulties by using a supramolecular assembly approach to create an artificial photosynthetic system. It mimics the structure of the purple bacteria Rhodobacter sphaeroides’s light-harvesting chromatophores (ie, cells that contain pigment), which are very efficient at transferring energy from the Sun.

The core of the artificial photosynthetic system is a highly stable artificial nanomicelle — a kind of polymer that can self-assemble in water, with both a water-loving (hydrophilic) and a water-fearing (hydrophobic) end. The nanomicelle’s hydrophilic head functions as a photosensitiser to absorb sunlight and its hydrophobic tail acts as an inducer for self-assembly. When it is placed in water, the nanomicelles self-assemble due to intermolecular hydrogen bonding between the water molecules and the tails. Adding a cobalt catalyst results in photocatalytic hydrogen production and carbon dioxide reduction, resulting in the production of hydrogen and methane.

Using advanced imaging techniques and ultrafast spectroscopy, the team unveiled the atomic features of the innovative photosensitiser. They discovered that the special structure of the nanomicelle’s hydrophilic head, along with the hydrogen bonding between water molecules and the nanomicelle’s tail, make it a stable, water-compatible artificial photosensitiser, solving the conventional instability and water-incompatibility problem of artificial photosynthesis. The electrostatic interaction between the photosensitiser and the cobalt catalyst, and the strong light-harvesting antenna effect of the nanomicelle improved the photocatalytic process.

In their experiment, the team found that the methane production rate was more than 13,000 μmol h⁻¹ g⁻¹, with a quantum yield of 5.6% over 24 hours. It also achieved a highly efficient solar-to-fuel efficiency rate of 15%, surpassing natural photosynthesis. Most importantly, the new artificial photocatalytic system is economically viable and sustainable, as it doesn’t rely on expensive precious metals.

“The hierarchical self-assembly of the system offers a promising bottom-up strategy to create a precisely controlled, high-performance artificial photocatalytic system based on cheap, Earth-abundant elements, like zinc and cobalt porphyrin complexes,” Ye said. He believes this discovery will benefit and inspire the rational design of future photocatalytic systems for carbon dioxide conversion and reduction using solar energy, contributing to the goal of carbon neutrality.

Image caption: The hierarchical self-assembly photocatalytic system mimics the natural photosynthesis apparatus of Rhodobacter sphaeroides, achieving 15% solar-to-fuel efficiency when converting carbon dioxide into methane. Image credit: Professor Ye Ruquan’s research group/City University of Hong Kong.