A team of chemists have designed a unique catalyst that expedites the rate of a crucial step in artificial photosynthesis, laying the groundwork for next-generation solar-to-fuel conversion devices.
The researchers from the Brookhaven National Laboratory produced a “single-site” catalyst that can initiate an entire reaction sequence on a single catalytic site for one molecule.
This particular catalyst was engineered to speed up a part of the photosynthesis process called water oxidation, which releases protons and electrons from water molecules yielding oxygen as a byproduct.
Computer modeling helped the researchers analyze these reactions at the theoretical level in order to discern which elements would work or not before they performed experiments in the lab.
Metal sits at the core of the molecule, where it is surrounded by other components that the scientists can choose to imbue the catalyst with certain properties.
The reaction is triggered by oxidizing the metal to pull electrons away from the oxygen on a water molecule. A “positively charged” or “activated” oxygen and two positively charged hydrogen protons are left behind.
“Taking electrons away makes the protons easier to release. But you need those protons to go somewhere. And it’s more efficient if you remove the electrons and protons at the same time to prevent the build-up of excess charges,” said Brookhaven chemist and research leader Javier Concepcion, in a statement.
However, the scientists still had find a way to activate the H2O molecule by binding it to the metal atom at the center of the catalyst.
Previous experiments indicated phosphonate groups that served as ligands to the metal could act as a base to accept these protons, therefore making it easier to oxidize the metal so the electrons could be removed in the first place. The groups were so strongly bound to the metal that they hindered the water molecule from binding to the catalyst early enough to maintain a smooth process.
A substitution for this experiment was then created where one phosphonate group was kept to act as the base, while a less-tightly bound carboxylate was added.
“The carboxylate group can more easily adjust its coordination to the metal center to allow the water molecule to come in and react at an earlier stage,” said lead author and Brookhaven research associate David Shaffer, in a statement.
Ultimately, adding this additional chemical element transformed a catalyst from one that generated two or three oxygen molecules per second to one that produced more than 100 per second. It also led to a corresponding increase in the production of protons and electrons that can be used to produce hydrogen fuel.
Next, the new catalyst needs to be tested in devices incorporating electrodes and other components for converting protons and electrons to hydrogen fuel. Another experiment will follow where the engineers will add light-absorbing compounds to provide energy to drive the whole reaction.
Findings from this investigation were published in the Journal of the American Chemical Society.