This hydrogen machine could be the ultimate guide to self-improvement

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 as carbonless hydrogen for fuel cells with twice the efficiency and stability of some previous technologies.

Today, scientists from the Department of Energy’s (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) – in collaboration with the University of Michigan and the Lawrence Livermore National Laboratory (LLNL) – have discovered a surprising and self-improving property in Si / GaN which contributes to the highly efficient and stable performance of the material in converting light and water to carbonless hydrogen. Their findings, reported in the newspaper Materials from nature, 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 scientist in the Chemical Sciences Division of the Lawrence Berkeley National Laboratory in the Department of Energy (Berkeley Lab). Usually, materials in solar fuel systems degrade, become less stable, and therefore produce hydrogen less efficiently, she said. “But we have discovered an unusual property in Si / GaN that somehow allows it to become more efficient and more stable. I have never seen such stability.”

Previous artificial photosynthetic materials are either excellent light absorbers which lack 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 like 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 the Si / GaN artificial photosynthesis devices ten years ago.

When Mi’s Si / GaN device achieved a record 3% efficiency between solar and hydrogen, he wondered how such ordinary materials could perform so extraordinarily well in an exotic artificial photosynthesis apparatus – it is therefore turned to Toma for help.

HydroGEN: Adopting a Team Science approach to solar fuels

Mi had learned of Toma’s expertise in advanced microscopy techniques to probe the nanoscale (billionths of a meter) properties of artificial photosynthetic materials from HydroGEN, a consortium of laboratories in five countries supported by the Office of Hydrogen Technologies. and fuel cells from DOE, and led by the National Renewable Energy Laboratory to facilitate collaborations between national laboratories, universities and industry for the development of advanced water separation materials. “These interactions between industry and academia that support advanced water separation materials and the capabilities of national laboratories are precisely the reason HydroGEN was formed, so that we can advance the production technology of ‘clean hydrogen,’ said Adam Weber, Hydrogen and Program Manager of the Fuel Cell Technologies Laboratory and Deputy Co-Director of HydroGEN.

Toma and lead author Guosong Zeng, a postdoctoral researcher in the chemical sciences division of the Berkeley lab, suspected that GaN might play a role in the device’s unusual potential for efficient and stable hydrogen production. .

To find out, Zeng conducted a photoconductive atomic force microscopy experiment at 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 the material begins to degrade and become less. stable and efficient.

They expected to see a sharp drop in photon absorption efficiency and material stability after just a few hours. To their amazement, they observed a 2-3 order of magnitude improvement in the photocurrent of the material from tiny facets along the “side wall” of the GaN grain, Zeng said. Even more puzzled, the material has increased its efficiency over time, although the overall surface area of ​​the material hasn’t changed 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 Electron Microscopy Center at the Berkeley Lab Molecular Foundry, and angle-dependent X-ray photon spectroscopy (XPS).

These experiments revealed that a 1-nanometer layer mixed with gallium, nitrogen, and oxygen – or gallium oxynitride – had formed along some of the sidewalls. A chemical reaction had taken place, adding “active catalytic sites for hydrogen production reactions,” Toma said.

Density Functional Theory (DFT) simulations performed by co-authors Tadashi Ogitsu and Tuan Anh Pham at LLNL confirmed their observations. “By calculating the change in distribution of chemical species at specific parts of the material surface, we have successfully found a surface structure that correlates with the development of gallium oxynitride as the site of the release reaction. hydrogen, ”Ogitsu said. “We hope that our findings and approach – a tightly integrated theory-experiment collaboration made possible 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 has helped identify the fundamental mechanisms that explain why it becomes more robust and efficient instead of degrading. . The results of this work will help us build more efficient artificial photosynthesis devices at a lower cost. “

Going forward, Toma said she and her team would like to test the Si / GaN photocathode in a water-separated photoelectrochemical cell, and that Zeng will experiment with similar materials to better understand how nitrides contribute to device stability. of artificial photosynthesis. – which they never thought 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 results will help us design even better artificial photosynthesis devices.”

“This was an unprecedented collaborative network between national laboratories and a research university,” Toma said. “The HydroGEN consortium brought us together – our work demonstrates how the approach of the national laboratory science team can help solve big problems that affect the whole world.