A chemical pathway that is central to plant biology has been adapted to form the backbone of a new process that converts water into hydrogen fuel using energy from the sun.
In a recent study by the US Department of Energy (DOE) Argonne National Laboratory, researchers have combined two membrane-bound protein complexes to perform a complete conversion of water molecules to hydrogen and oxygen.
The work is based on a previous study that examined one of these protein complexes, called Photosystem I, a membrane protein that can use energy from light to feed electrons into an inorganic catalyst that makes hydrogen. However, this part of the reaction represents only half of the total process needed for hydrogen production.
Using a second protein complex that uses energy from light to shattered water and takes electrons from it, called Photosystem II, Argonne chemist Lisa Utschig and her colleagues could take electrons from water and feed them to Photosystem I.
" The beauty of this design is in its simplicity ̵
In a previous experiment, the researchers provided Photosystem I with electrons from a sacrificial electron donor. "The trick was how to get two electrons to the catalyst in rapid succession," Utschig said.
The two protein complexes are embedded in tylakoid membranes, such as those found in the oxygen-forming chloroplasts of higher plants. "The membrane, which we have taken directly from nature, is important to pair the two photo systems," Utschig said. "It structurally supports both simultaneously and provides a direct path for electron protein transfer, but does not prevent catalyst binding to Photosystem I."
According to Utschig, the Z-scheme, which is the technical name for the light-triggered electron transport chain of natural photosynthesis occurring in the thylakoid membrane and the synthetic catalyst, comes together quite elegantly. "The beauty of this design is in its simplicity – you can mount the catalyst yourself with the natural membrane to make the chemistry you want," she said. A further improvement involved the substitution of cobalt or nickel-containing catalysts for the expensive platinum catalyst used in the previous study. The new cobalt or nickel catalysts can dramatically reduce potential costs.
The next step for the research, according to Utschig, is that the membrane-bound Z-system is incorporated into a living system. "Once we have a system in vivo – where the process takes place in a living organism – we will certainly be able to see the rubber beating the way of hydrogen production," she said.
New research highlights light on photosynthesis and the creation of solar fuel
Lisa M. Utschig et al., The solar system's solar water division via self-assembly of hybrids of photosystem I catalysts in thylakoid membranes, [ChemicalScience (2018). DOI: 10,1039 / c8sc02841a