Artificial photosynthesis offers a way to store the energy of sunlight within chemical bonds and could be used to produce renewable solar fuels like hydrogen. Researchers hoping to harness the Sun’s energy in this way tend to mimic the strategy of natural photosynthesis, using separate molecules to capture light and catalyze chemical reactions.
But maybe nature doesn’t always know best. Claudia Turro and colleagues at the Ohio State University have now developed a rhodium catalyst that performs both of these tasks, harvesting light and then using its energy to create hydrogen gas (Nat. Chem. 2020, DOI: 10.1038/s41557-019-0397-4). The catalyst is more efficient than previous examples that combine these two functions in a single molecule, and unlike rival catalysts it can make full use of the Sun’s spectrum, including red and infrared light. “It’s easy to prepare, and it’s also stable in air and water, which is not the case for many previous systems,” Turro says.
Many artificial photosynthesis systems use a mix of molecules to drive water-splitting reactions, making oxygen and hydrogen. The hydrogen side of this process generally relies on a photosensitizer that absorbs light to generate excited electrons. Those electrons are transferred to a hydrogen-evolving catalyst, which brings them together with two protons to make hydrogen gas. “But whenever you have charge transfer from the light absorber to the catalyst, there are going to be energy losses,” Turro says.
Keeping the charge transfer step within the same molecule should avoid those losses. But previous complexes that act as both light absorber and catalyst for hydrogen generation have been unstable, sluggish, and unable to absorb red or infrared light.
Turro’s catalyst contains two rhodium atoms bonded together and flanked by two pairs of ligands, benzo[c]cinnoline and N,N’-diphenylformamidate. These help to shorten the Rh-Rh bond, which alters the complex’s energy levels and extends the lifetime of its excited state—a key factor in its improved performance.
So far, the researchers have done proof-of-principle tests of the catalyst to show that it can efficiently absorb red light to produce hydrogen. Rather than teaming it with a water-oxidizing catalyst, they used 1-benzyl-1,4-dihydronicotinamide (BNAH) to offer a ready source of electrons, along with an acid to supply protons.
Under red light, a single-catalyst molecule could produce up to 28 hydrogen molecules per hour, and over a full day it made an average of 170 hydrogen molecules. “It’s an improvement compared to other systems that combine sensitizer and catalyst,” says Leif Hammarström of Uppsala University, who chairs the Swedish Consortium for Artificial Photosynthesis.
To develop the catalyst into a more practical system, it will be important to put it together with a water-oxidation catalyst. “If we want to make solar fuels on a really large scale, we have to get the electrons from water” rather than convenient electron-donating additives like BNAH, Hammarström says.
Artificial photosynthesis will need to compete with other approaches for making renewable hydrogen, such as using the electricity from photovoltaic panels to run water electrolyzers. In principle, artificial photosynthesis catalysts could offer a much higher efficiency than systems that daisy-chain different technologies like this, but that remains a long-term research goal, Hammarström says. For now, Turro’s team is fine-tuning its catalyst to improve hydrogen output, and hopes to replace the rhodium with more abundant transition metals.