Three years ago, scientists at the University of Michigan discovered an artificial photosynthesis device made of silicon and gallium nitride (Si / GaN) that uses sunlight in carbon-free hydrogen for fuel cells twice the efficiency and stability of some previous technologies.
Now scientists from the Lawrence Berkeley National Laboratory at the Department of Energy (DOE) ̵
“Our discovery really changed the game,” said senior author Francesca Thomas, a scientist in the chemical department at the Lawrence Berkeley National Laboratory at the Department of Energy (Berkeley Laboratory). Typically, materials in solar fuel systems decompose, become less stable and thus produce less efficient hydrogen, she said. “But we found an unusual property in Si / GaN that somehow allows it to become more efficient and stable. I’ve never seen such stability.”
Previous artificial photosynthetic materials are either excellent light absorbers that have no durability; or they are durable materials that do not have light absorption efficiency.
But silicon and gallium nitride are abundant and inexpensive materials that are widely used as semiconductors in everyday electronics such as LEDs and solar cells, said co-author Zetian Mi, a professor of electrical engineering and computer engineering at the University of Michigan who invented devices for artificial photosynthesis Si / GaN a decade ago.
When Mi’s Si / GaN device achieved a record high efficiency of 3% solar to hydrogen, he wondered how such ordinary materials could perform so extraordinarily well in an exotic artificial photosynthesis device – so he turned to Tom for help.
HydroGEN: Adopting a team scientific approach to solar fuels
He had taught me about Thomas’s experience in advanced microscopy techniques for studying the properties of nanoscales (billions of parts per meter) of artificial photosynthetic materials through HydroGEN, a five national laboratory consortium backed by the DOE’s Hydrogen and Fuel Cell Service and led by the National Renewable Energy Laboratory to facilitate collaboration between National Laboratories, academia and industry to develop advanced water separation materials. “These interactions between industry and academia for advanced water-based materials with the capabilities of National Laboratories are the reason for the creation of HydroGEN – so we can move the needle on pure hydrogen production technology,” said Adam Weber, hydrogen at the Berkeley Laboratory. and Manager of the Fuel Cell Technology Laboratory and Deputy Director of HydroGEN.
Thomas and lead author Guosong Zeng, a doctoral student in the chemical sciences department at Berkeley Laboratory, suspected that GaN could play a role in the device’s unusual potential for efficiency and stability in hydrogen production.
To find out, Zeng conducted a photoconductive atomic microscopy experiment in Tom’s lab to test how GaN photocathodes can efficiently convert absorbed photons into electrons and then hire those free electrons to separate water into hydrogen before the material begins. to degrade and become less stable and effective.
They expected to see a sharp drop in the efficiency and stability of photon absorption in just a few hours. To their surprise, they saw an improvement of 2-3 orders of magnitude in the photocurrent of the material coming from small veneers along the “side wall” of the GaN grain, Zeng said. Even more confusing is that the material has increased its efficiency over time, although the overall surface of the material has not changed as much, Zeng said. “In other words, instead of getting worse, the material is improving,” he said.
To gather more clues, the researchers hired a scanning transmission electron microscopy (STEM) at the National Electron Microscopy Center at the Berkeley Laboratory Molecular Foundry and an angle-dependent X-ray photon spectroscopy (XPS).
These experiments revealed that a 1-nanometer layer mixed with gallium, nitrogen and oxygen – or gallium oxynitride – formed on some of the side walls. A chemical reaction took place, adding “active catalytic sites for hydrogen-producing reactions,” Thomas said.
Functional density theory (DFT) simulations conducted by co-authors Tadashi Ogitsu and Tuan Anh Pham of LLNL confirmed their observations. “By calculating the change in the distribution of chemical species in certain parts of the surface of the material, we have successfully found a surface structure that correlates with the development of gallium oxynitride as a reaction site for the evolution of hydrogen,” Ogitsu said. “We hope that our discoveries and approach – a closely integrated collaboration between theory and experimentation provided by the HydroGEN consortium – will be used to further improve renewable hydrogen production technologies.”
He added: “We have been working on this material for more than 10 years – we know that it is stable and effective. But this collaboration has helped to identify the main mechanisms behind why it becomes stronger and more effective instead of deteriorating. The findings of this work will help us build more efficient artificial photosynthesis devices at a lower cost. “
Looking ahead, Thomas said she and her team would like to test the Si / GaN photocathode in a water-separating photoelectrochemical cell, and that Zeng will experiment with similar materials to gain a better understanding of how nitrides contribute to stability in artificial photosynthesis devices — which is something they never thought would be possible.
“It was surprising,” Zen said. “It didn’t make sense, but DF’s DFT calculations gave us the explanation we needed to confirm our observations. Our findings will help us design even better artificial photosynthesis devices.”
“It was an unprecedented network of collaboration between National Labs and a research university,” said Thomas. “The HydroGEN consortium brought us together – our work demonstrates how the National Labs’ Team Science approach can help solve major problems that affect the world.”
Water Separation: Nanoscale imaging provides key insights
Development of photoelectrochemically self-improving Si / GaN photocathode for efficient and durable H2 production, Natural materials (2021). dx.doi.org/10.1038/s41563-021-00965-w
Provided by Lawrence Berkeley National Laboratory
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