Formation of a hydrogen-bond enhanced nanomicelle and its hydrogen production and carbon dioxide reduction under solar energy. Image: Ye et al, Nature Catalysis

New system outperforms natural photosynthesis

7 August 2023

Hong Kong scientists have created a highly stable and efficient artificial photosynthetic system that mimics nature to produce energy-rich compounds from water and atmospheric CO2. 

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,  scientists have tried to develop artificial photosynthesis processes to turn carbon dioxide into carbon-neutral fuel. The findings were published in Nature Catalysis.

“However, it is difficult to convert carbon dioxide in water because many photosensitisers or catalysts degrade in water,” explained Dr Ye Ruquan of the Department of Chemistry, City University of Hong Kong, one of the leaders of the joint 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.”

In the latest study, researchers from CityU, the University of Hong Kong, Jiangsu University and the Shanghai Institute of Organic Chemistry of the Chinese Academy of Sciences overcame these difficulties by using a supramolecular assembly approach to create an artificial photosynthetic system. It mimics the structure of a purple bacteria’s light-harvesting chromatophores which are very efficient at transferring energy from the sun.

The core of the new artificial photosynthetic system is a highly stable artificial nanomicelle, a kind of polymer that can self-assemble in water, with both a hydrophilic and a hydrophobic end. The nanomicelle’s hydrophilic head functions as a photosensitizer 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 photosensitizer. 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 photosensitizer, solving the conventional instability and water-incompatibility problem of artificial photosynthesis. The electrostatic interaction between the photosensitizer and the cobalt catalyst, and the strong light-harvesting antenna effect of the nanomicelle improved the photocatalytic process.

In the experiment, the team found that the methane production rate was more than 13,000μmol 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 which, typically, has an efficiency rate of 1-2% only.

Importantly, the new artificial photocatalytic system does not 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,” said Ye.

Professor David Lee Phillips of the Department of Chemistry, University of Hong Kong, co-leader of the study, said, "Our research has the potential to advance renewable energy by replicating nature's efficient light-harvesting mechanisms. This could lead to sustainable solutions for our energy needs and the production of carbon-neutral fuels, contributing to a greener future."