Three years ago, scientists at the University of Michigan discovered an artificial photosynthesis device made of silicon and gallium nitride (Si / GaN) that harnesses sunlight in carbon-free hydrogen for fuel cells with twice the efficiency and stability of some older technologies.
Now, scientists at the Department of Energy (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab), in collaboration with the University of Michigan and Lawrence Livermore National Laboratory (LLNL), have discovered a surprising and self-improving property in Si / GaN that contributes to the highly efficient and stable performance of the material in converting light and water into carbon-free hydrogen. Their findings, published in the journal Nature materials, could help radically accelerate the commercialization of artificial photosynthesis technologies and hydrogen fuel cells.
“Our discovery is a real game changer,” said lead author Francesca Toma, a staff scientist in the Department of Energy’s Lawrence Berkeley National Laboratory, Division of Chemical Sciences (Berkeley Laboratory). Typically, materials in solar fuel systems degrade, become less stable, and therefore produce hydrogen less efficiently, he said. “But we discovered an unusual property in Si / GaN that somehow allows it to become more efficient and stable. I have never seen such stability.”
The above artificial photosynthesis materials are excellent light absorbers lacking in durability; or they are durable materials that lack light absorption efficiency.
But silicon and gallium nitride are abundant and cheap materials that are widely used as semiconductors in everyday electronics, such as LEDs (light-emitting diodes) and solar cells, said co-author Zetian Mi, professor of electrical and computer engineering. at the University of Michigan, who invented Si / GaN artificial photosynthesis devices a decade ago.
When Mi’s Si / GaN device achieved a record 3 percent solar-to-hydrogen efficiency, he wondered how these ordinary materials could perform so extraordinarily in an exotic artificial photosynthesis device, so he turned to Toma for help.
HydroGEN: Taking a Team Science Approach to Solar Fuels
Mi had learned from Toma’s experience in advanced microscopy techniques to probe the nanoscale (billionths of a meter) properties of artificial photosynthesis materials through HydroGEN, a consortium of laboratories from five countries supported by the Office of Cell Technologies. DOE Fuel and Hydrogen Department, and led by the National Renewable Energy Laboratory to facilitate collaboration between national laboratories, academia, and industry for the development of advanced materials for water division. “These interactions of supporting industry and academia in advanced water-splitting materials with the capabilities of national laboratories are precisely why HydroGEN was formed, so that we can move the needle towards clean hydrogen production technology. “said Adam Weber, Berkeley Laboratory and Hydrogen Laboratory. Head of the Fuel Cell Technologies Laboratory Program and Deputy Co-Director of HydroGEN.
Toma and lead author Guosong Zeng, a postdoctoral fellow in the Berkeley Laboratory’s Division of Chemical Sciences, suspected that GaN could be playing a role in the device’s unusual potential for hydrogen production efficiency and stability.
To find out, Zeng conducted a photoconductive atomic force microscopy experiment in Toma’s lab to test how GaN photocathodes could efficiently convert absorbed photons into electrons, and then recruit those free electrons to split water into hydrogen, before that the material began to degrade and become less. stable and efficient.
They expected to see a sharp decrease in the material’s photon absorption efficiency and stability after just a few hours. To their amazement, they observed a 2-3 orders of magnitude improvement in the material’s photocurrent from tiny facets along the “side wall” of the GaN grain, Zeng said. Even more puzzling was that the material had increased in efficiency over time, even though the overall surface of the material didn’t change that much, Zeng said. “In other words, instead of getting worse, the material got better,” he said.
To gather more clues, the researchers recruited scanning transmission electron microscopy (STEM) at the National Center for Electron Microscopy at Berkeley Lab’s Molecular Foundry, and angle-dependent X-ray photon spectroscopy (XPS).
Those experiments revealed that a 1-nanometer layer mixed with gallium, nitrogen and oxygen, or gallium oxynitride, had formed along some of the side walls. A chemical reaction had occurred, adding “active catalytic sites for hydrogen production reactions,” Toma said.
Density functional theory (DFT) simulations carried out by co-authors Tadashi Ogitsu and Tuan Anh Pham at LLNL confirmed their observations. “By calculating the change in distribution of chemical species in specific parts of the material’s surface, we successfully found a surface structure that correlates with the development of gallium oxynitride as a hydrogen evolution reaction site,” said Ogitsu. “We hope that our findings and our approach, a tightly integrated theory-experiment collaboration enabled by the HydroGEN consortium, will be used to further improve renewable hydrogen production technologies.”
Mi added: “We have been working on this material for over 10 years; we know it is stable and efficient. But this collaboration helped identify the fundamental mechanisms behind why it becomes more robust and efficient rather than degrading. This work will help us build more efficient artificial photosynthesis devices at a lower cost. “
Looking ahead, Toma said that she and her team would like to test the Si / GaN photocathode in a water-splitting photoelectrochemical cell, and that Zeng will experiment with similar materials to better understand how nitrides contribute to stability in devices. artificial photosynthesis. “Which is something they never thought was possible.”
“It was totally surprising,” Zeng said. “It didn’t make sense, but Pham’s DFT calculations gave us the explanation we needed to validate our observations. Our findings will help us design even better artificial photosynthesis devices.”
“This was an unprecedented collaborative network between National Labs and a research university,” Toma said. “The HydroGEN consortium brought us together; our work demonstrates how National Labs’ team science approach can help solve big problems that affect the entire world.”
Water splitting: nanoscale images provide key information
Development of an efficient and durable photoelectrochemically self-improving Si / GaN photocathode for Htwo production, Nature materials (2021). dx.doi.org/10.1038/s41563-021-00965-w
Provided by Lawrence Berkeley National Laboratory
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