Stony Brook University graduate student Lei Wang is part of a team of scientists at Brookhaven National Laboratory (BNL) who have doubled the efficiency of a chemical combo that captures light and splits water molecules so the building blocks can be used to produce hydrogen fuel.
Their study, selected as an American Chemical Society “Editors’ Choice” that will be featured on the cover of the Journal of Physical Chemistry C, provides a platform for developing revolutionary improvements in so-called artificial photosynthesis—a lab-based mimic of the natural process aimed at generating clean energy from sunlight.
In natural photosynthesis, green plants use sunlight to transform water (H2O) and carbon dioxide (CO2) into carbohydrates such as sugar and starches. The energy from the sunlight is stored in the chemical bonds holding those molecules together.
Many artificial photosynthesis strategies start by looking for ways to use light to split water into its constituents, hydrogen and oxygen, so the hydrogen can later be combined with other elements—ideally the carbon from carbon dioxide—to make fuels. But even getting the hydrogen atoms to recombine as pure hydrogen gas (H2) is a step toward solar-powered clean-fuel generation.
To achieve water splitting, scientists have been exploring a wide range of light-absorbing molecules (also called chromophores, or dyes) paired with chemical catalysts that can pry apart water’s very strong hydrogen-oxygen bonds. The new approach uses molecular “tethers”—simple carbon chains that have a high affinity for one another—to attach the chromophore to the catalyst. The tethers hold the particles close enough together to transfer electrons from the catalyst to the chromophore—an essential step for activating the catalyst—but keeps them far enough apart that the electrons don’t jump back to the catalyst.
“You just dip the electrode coated with the chromophores into a solution in which the catalyst is suspended and the tethers on the two types of molecules find one another and link up,” said Lei Wang, a coauthor on the current paper and lead author on a paper published earlier this year [https://pubs.acs.org/doi/10.1021/jacs.9b01044] that described the self-assembly strategy.
“Electrons move fast, but chemical reactions are much slower. So, to give the system time for the water-splitting reaction to take place without the electrons moving back to the catalyst, you have to separate those charges,” explained Brookhaven Lab chemist Javier Concepcion, who led the project.
In the complete setup, the chromophores (tethered to the catalyst) are embedded in a layer of nanoparticles on an electrode. Each nanoparticle is made of a core of tin dioxide (SnO2) surrounded by a titanium dioxide (TiO2) shell. These different components provide efficient, stepwise shuttling of electrons to keep pulling the negatively charged particles away from the catalyst and sending them to where they are needed to make fuel.
Here’s how it works from start to finish: Light strikes the chromophore and gives an electron enough of a jolt to send it from the chromophore to the surface of the nanoparticle. From there the electron moves to the nanoparticle core, and then out of the electrode through a wire. Meanwhile, the chromophore, having lost one electron, pulls an electron from the catalyst. As long as there’s light, this process repeats, sending electrons flowing from catalyst to chromophore to nanoparticle to wire.
Each time the catalyst loses four electrons, it becomes activated with a big enough positive charge to steal four electrons from two water molecules. That breaks the hydrogen and oxygen apart. The oxygen bubbles out as a gas (in natural photosynthesis, this is how plants make the oxygen we breathe!) while the hydrogen atoms (now ions because they are positively charged) diffuse through a membrane to another electrode. There they recombine with the electrons carried by the wire to produce hydrogen gas—fuel!