A fundamental chemical reaction pathway for plant biology has been adapted to form the backbone of a new process that converts water into hydrogen fuel using the sun's energy.
In a recent study by the Argonne National Laboratory of the Department of Energy of the United States (DOE), scientists combined two membrane-related protein complexes to perform a complete conversion of water molecules into 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 the electrons to an inorganic catalyst that produces hydrogen. This part of the reaction, however, represents only half of the overall process required for the generation of hydrogen.
Using a second protein complex that uses energy from light to separate water and takes the electrons from it, called Photosystem II, chemist Lisa Argonne Utschig and her colleagues were able to take the electrons from the Water and feed them to Photosystem I.
"The beauty of this design is in its simplicity: you can self-assemble the catalyst with the natural membrane to make the chemistry you want" -Lisa Utschig, chemist of Argonne
In a previous experiment, the researchers provided Photosystems with electrons from a sacrificial electron donor. "The trick was how to get two electrons on the catalyst in quick succession," Utschig said.
The two protein complexes are incorporated into the thylakoid membranes, such as those found inside the chloroplasts that create oxygen in the higher plants. "The membrane, which we took directly from nature, is essential for the association of the two photosystems," Utschig said. "It structurally supports both simultaneously and provides a direct path for the transfer of inter-protein electrons, but does not prevent the link of the catalyst to the Photosystem I."
According to Utschig, the Z-schema is the technical name for the electron transport chain triggered by the light of natural photosynthesis that occurs in the tachacoid membrane and the synthetic catalyst combine quite elegantly. "The beauty of this design is in its simplicity: you can self-assemble the catalyst with the natural membrane to make the chemistry you want," he said.
A further improvement involved the replacement of cobalt or nickel-containing catalysts for the expensive platinum catalyst that had been used in the previous study. The new cobalt or nickel catalysts could drastically reduce potential costs.
The next step for the research, according to Utschig, is to incorporate the membrane-bound Z schema into a living system. "Once we have a system in vivo where the process is happening in a living organism, we will actually be able to see the rubber hit the road in terms of hydrogen production," he said.
New research sheds light on photosynthesis and the creation of solar fuel
Lisa M. Utschig et al, splitting solar water from Z-scheme by self-assembly of hybrid photo-catalysts into photoclab membranes, Chemical Science (2018). DOI: 10.1039 / c8sc02841a