System is designed to mimic key functions of the photosynthetic center in green plants to convert solar energy into chemical energy stored by hydrogen fuel

June 2, 2017

Figure: Etsuko Fujita and Gerald Manbeck of Brookhaven Lab's Chemistry Division carried out a series of experiments to understand why their molecular system with six light-absorbing centers (made of ruthenium metal ions bound to organic molecules) produced more hydrogen than the system with three such centers. This understanding is key to designing more efficient molecular complexes for converting solar energy into chemical energy—a conversion that green plants do naturally during photosynthesis.

Photosynthesis is one of nature's most important processes. Aside from producing oxygen, this natural process converts solar to chemical energy by transforming atmospheric carbon dioxide and water into sugar molecules to provide plants with needed food and energy to survive.

Scientists have tried to artificially replicate this energy conversion process to produce environmentally friendly and sustainable fuels such as hydrogen and methanol. Mimicking the process has been challenging to scientists as artificial photosynthesis requires the creation of a molecular system that absorbs light, transports and separates electrical charge, and catalyzes fuel-producing reactions. All are complicated processes that must operate synchronously to achieve high energy-conversion efficiency.

A team of researchers led by chemists from the U.S. Department of Energy's (DOE) Brookhaven National Laboratory and Virginia Tech has designed two photocatalysts that combine individual components for light absorption, charge separation, or catalysis into a single "supramolecule." Each supramolecule is made up of multiple light-harvesting ruthenium (Ru) metal ions connected to a single catalytic center of rhodium (Rh) metal ions. The researchers found that the supramolecule with six Ru centers and one Rh center was seven times more efficient than the other, cycling 300 times to produce hydrogen for 10 hours. The larger of the supramolecules was slightly electron-deficient, making it more receptive to electrons needed for synthetic photosynthesis.

For more details, read the article at Brookhaven National Laboratory.